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Review of greenhouse gas__ emissions from crop production systems andfertilizer management__ effects

Review of greenhouse gas__ emissions from crop production systems andfertilizer management__ effects
Review of greenhouse gas__ emissions from crop production systems andfertilizer management__ effects

Review

Review of greenhouse gas emissions from crop production systems and fertilizer management effects

C.S.Snyder a ,*,T.W.Bruulsema b ,T.L.Jensen c ,P.E.Fixen d

a

International Plant Nutrition Institute,P.O.Drawer 2440,Conway,AR 72033,USA

b

International Plant Nutrition Institute,18Maplewood Drive,Guelph,Ontario,Canada N1G 1L8c

International Plant Nutrition Institute,102-411Downey Road,Saskatoon,Saskatchewan,Canada S7N 4L8d

International Plant Nutrition Institute,2301Research Parkway,Suite 126,Brookings,SD 57006,USA

Contents 1.Introduction...........................................

..........................................................2482.

Background .....................................................................................................2482.1.Greenhouse gases ...........................................................................................2482.2.Agricultural share ...........................................................................................2482.3.Agricultural sources ..................

.......................................................................2492.3.1.Emissions of nitrous oxide (N 2O)from cropland ...........................................................2492.3.2.Emissions of CO 2from lime ....

.............

(251)

Agriculture,Ecosystems and Environment 133(2009)247–266

A R T I C L E I N F O Article history:

Received 30May 2008

Received in revised form 28August 2008Accepted 27April 2009

Available online 3June 2009Keywords:

Greenhouse gas Nitrous oxide Fertilizer

Best management practice Nitrogen

Cropping system Climate change Global warming Carbon dioxide Methane

A B S T R A C T

Fertilizer nitrogen (N)use is expanding globally to satisfy food,?ber,and fuel demands of a growing world population.Fertilizer consumers are being asked to improve N use ef?ciency through better management in their ?elds,to protect water resources and to minimize greenhouse gas (GHG)emissions,while sustaining soil resources and providing a healthy economy.A review of the available science on the effects of N source,rate,timing,and placement,in combination with other cropping and tillage practices,on GHG emissions was conducted.Implementation of intensive crop management practices,using principles of ecological intensi?cation to enhance ef?cient and effective nutrient uptake while achieving high yields,was identi?ed as a principal way to achieve reductions in GHG emissions while meeting production demands.Many studies identi?ed through the review involved measure-ments of GHG emissions over several weeks to a few months,which greatly limit the ability to accurately determine system-level management effects on net global warming potential.The current science indicates:(1)appropriate fertilizer N use helps increase biomass production necessary to help restore and maintain soil organic carbon (SOC)levels;(2)best management practices (BMPs)for fertilizer N play a large role in minimizing residual soil nitrate,which helps lower the risk of increased nitrous oxide (N 2O)emissions;(3)tillage practices that reduce soil disturbance and maintain crop residue on the soil surface can increase SOC levels,but usually only if crop productivity is maintained or increased;(4)differences among fertilizer N sources in N 2O emissions depend on site-and weather-speci?c conditions;and (5)intensive crop management systems do not necessarily increase GHG emissions per unit of crop or food production;they can help spare natural areas from conversion to cropland and allow conversion of selected lands to forests for GHG mitigation,while supplying the world’s need for food,?ber,and biofuel.Transfer of the information to fertilizer dealers,crop advisers,farmers,and agricultural and environmental authorities should lead to increased implementation of fertilizer BMPs,and help to reduce confusion over the role of fertilizer N on cropping system emissions of GHGs.Gaps in scienti?c understanding were identi?ed and will require the collaborative attention of agronomists,soil scientists,ecologists,and environmental authorities in serving the immediate and long-term interests of the human population.

?2009Elsevier B.V.All rights reserved.

*Corresponding author at:International Plant Nutrition Institute,P.O.Drawer 2440,Conway,AR 72034,USA.Tel.:+15013368110;fax:+15013292318.E-mail address:csnyder@https://www.doczj.com/doc/0f17295624.html, (C.S.Snyder).Contents lists available at ScienceDirect

Agriculture,Ecosystems and Environment

j o u r n al h o m e p a g e :w w w.e l se v i e r.co m /l oc a t e /a g e e

0167-8809/$–see front matter ?2009Elsevier B.V.All rights reserved.doi:10.1016/j.agee.2009.04.021

2.3.3.Emission of CO2from urea (251)

2.3.4.Emissions of CH4from rice culture (251)

2.4.Agricultural sinks (251)

2.5.Soil carbon stabilization (252)

2.5.1.Role of N in stabilizing soil C (252)

2.6.Factors affecting GHG emissions from soil (253)

2.6.1.Soil physical properties and conditions (253)

2.6.2.Soil compaction (253)

3.Cropping and fertilizer practices and the associated BMPs (253)

3.1.Tillage systems (253)

3.2.Tile drainage (254)

3.3.Cropping systems (254)

3.4.Fertilizer use and crop yield enhancement (255)

3.5.Fertilizer production and transport (256)

3.6.Nitrogen source impact on N2O emissions from soil (257)

https://www.doczj.com/doc/0f17295624.html,mercial fertilizer sources (257)

3.6.2.Biologically?xed legume-derived N (258)

3.6.3.Livestock manures (258)

3.6.4.Inhibitors and controlled-release fertilizer sources or enhanced-ef?ciency fertilizers (258)

3.7.Application management—rate,timing,and placement (259)

3.7.1.N rate impact on N2O emissions from soil (259)

3.7.2.N placement (260)

3.7.3.N timing (261)

3.8.Balanced fertilization including other required nutrients along with N (261)

4.Fertilizer BMPs (262)

4.1.General practices (262)

4.1.1.Equipment,proper application,and application technology (262)

4.1.2.Crop management,system planning,and evaluation (262)

4.1.3.Inhibitors and enhanced-ef?ciency sources (262)

4.1.4.Research and development needs (262)

5.Conclusions (263)

Acknowledgements (263)

References (263)

1.Introduction

The objective of this literature review is to assess best management practices(BMPs)in relation to their potential to mitigate the greenhouse gas(GHG)emissions associated with fertilizer use in cropping systems.

This document is organized as follows:

Background information that describes the global warming potential(GWP)of GHG emissions from agriculture and fertilizer use,and how they compare to other sources.

A description of how crop and fertilizer management practices affect GHG emissions.

A conclusion recommending how the BMPs described might be implemented to mitigate GHG emissions.

2.Background

2.1.Greenhouse gases

Greenhouse gases are those that absorb infrared radiation in the atmosphere,trapping heat and warming the surface of the Earth. The three greenhouse gases(GHGs)associated with agriculture are carbon dioxide(CO2),methane(CH4),and nitrous oxide(N2O). Other important GHGs include water vapor and many halocarbon compounds,but their emissions are not considered to be in?uenced by agriculture.

Fossil fuel combustion is considered responsible for more than 75%of human-caused https://www.doczj.com/doc/0f17295624.html,nd use change(primarily deforestation)is responsible for the remainder.Human activities are thought to have more than doubled the rate of emission of CH4 over the last25years(Denman et al.,2007).Atmospheric concentrations of N2O are reported to have risen from about 270parts per billion(ppb)during the pre-industrial era to319ppb in2005.According to Hirsch et al.(2006),and the United Nations Educational,Scienti?c and Cultural Organization(UNESCO)and the Scienti?c Committee on Problems of the Environment of the International Council on Science(SCOPE)(2007),emissions of N2O from the Earth’s surface have increased by about40–50%over pre-industrial levels as a result of human activity.

2.2.Agricultural share

The U.S.Environmental Protection Agency(EPA,2007)has estimated that agriculture generates less than10%of the total emissions of GHGs in the U.S.,and its share is not increasing (Fig.1).In Canada,the share attributed to agriculture accounts for less than8%of the emissions inventory(Environment Canada, 2007).While the major GHG issue for the total economy is CO2,for agriculture the most important is N2O,mainly from soils and N inputs to crop and soil systems(Fig.2).Emissions of CH4,mainly from livestock,are also substantial in their contribution to GWP.So even though N2O is a small part of the overall GHG issue,it becomes the major focus of this review because agriculture is considered to be its major source,and it is linked to soil management and fertilizer use.Although animal manures, legumes,and other sources are also important,the majority of this report will address fertilizer use.

The emissions shown in Figs.1and2are estimated by the U.S. EPA(2007)to be only partially mitigated by sinks including forests and agricultural soils.The U.S.EPA estimate of these sinks in2005 amounted to a total of829million t(Tg)of CO2equivalents, dominated by forests,with agricultural soils accounting for a sink of only32Tg of CO2equivalents.

C.S.Snyder et al./Agriculture,Ecosystems and Environment133(2009)247–266 248

2.3.Agricultural sources

The three GHGs associated with agriculture –CO 2,CH 4,and N 2O –differ in their effectiveness in trapping heat and in their turnover rates in the atmosphere.Calculating their GWP therefore depends on the timeframe considered.For a 100-year timeframe,unit masses of CH 4and N 2O are considered to have 23and 296times the GWP,respectively,as a unit of CO 2(IPCC,2001).Although older sources (IPCC,1996)may use the GWP CO 2equivalent values of 21for CH 4and 310for N 2O,for the purposes of this review we will use the more recent values of 23and 296,respectively (IPCC,2001).

Carbon dioxide,in comparison to CH 4and N 2O,is cycled in the largest amounts through agricultural cropping systems.Plants consume large amounts of CO 2through photosynthesis to make food,feed,?ber,and fuel—but all these plant products eventually convert back to CO 2when consumed or when they decompose.The net emission of CO 2is small in comparison to its total cycling in agriculture,and is mostly due to energy use on-farm and in the manufacture and transport of agricultural products.

Methane is emitted from rice cultivation and from ruminant livestock.It can be captured from manure using anaerobic digesters,collection from covered anaerobic manure lagoons,and emissions can be reduced by livestock diet modi?cation.Non-rice agricultural soils are a sink for atmospheric CH 4,but to a smaller extent than soils in their native condition.

Nitrous oxide emissions result from two microbial soil processes:nitri?cation and denitri?cation.Both processes gen-erally release small amounts relative to the soil N supply.The N 2O

is produced through the activity of soil microorganisms (Bange,2000).The interaction of controlling factors for nitri?cation and denitri?cation are complex because the amount of N 2O produced depends on the range of oxygen (O 2)concentrations in the soil.Temperature determines the rate at which the soil microorganisms nitrify or denitrify.At cooler temperatures the rate of N species conversion is slow.It increases to a maximum as temperatures rise.The O 2concentration in the soil is in?uenced by its moisture content.Other important factors affecting N 2O emissions are:soil texture,the amount of ammonium (NH 4+)available for nitri?ca-tion,and the amount of nitrate (NO 3à)available for denitri?cation (Granli and B?ckman,1994;Firestone,1982).

Nitri?cation occurs when NH 4+oxidizing bacteria such as Nitrosomonas sp.catabolize NH 4+and transform it to nitrite (NO 2à)which is further changed into NO 3àby Nitrobacter sp.and Nitrospira sp.bacteria (Norton,2008).Nitrous oxide and nitric oxide (NO)are minor by-products of the transformation from nitrite (NO 2à)under oxygen-limited conditions,when nitri?ers use NO 2àas a terminal electron acceptor (IFA/FAO,2001).About three decades ago,Bremner and Blackmer (1978)reported emissions of N 2O arising from nitri?cation under fully aerobic conditions,at rates ranging from 0.04to 0.45%of N added.Nitri?cation determines the form of N present and therefore how N is absorbed,utilized,or dispersed into the environment;which has large implications for plant productiv-ity and environmental quality.During nitri?cation,the relatively immobile NH 4+is converted to the highly mobile NO 3à.Conversion of NH 4+to NO 3àstrongly in?uences N utilization by plants,because the NO 3àformed is highly susceptible to loss from the root zone by leaching and or denitri?cation (Subbarao et al.,2006),but is also highly available to plants and is often the major uptake form of N.

Denitri?cation –as described by Firestone (1982),Firestone and Davidson (1989),and Robertson and Groffman (2007)–occurs when NO 3àis transformed to dinitrogen (N 2)gas as described in the following pathway,NO 3à!NO 2à!NO !N 2O !N 2.The conversion of NO 3àto N 2can be complete,but a small and variable portion of the N is often emitted as N 2O gas.Emissions are sporadic,occurring before,during,and after the growing season.Flushes of N 2O can occur when previously well-aerated soils become moistened or saturated from precipitation or irrigation,or when frozen soils thaw (e.g.during snowmelt).Bedard-Haughn et al.(2006)reported that the composition of the microbial population exerts a dominant control on emissions and remains relatively constant over time,whereas interactions among spatially and temporally variable environmental drivers [NO 3àconcentrations,temperature,water-?lled pore space (WFPS),available carbon (C),etc.]control the magnitude of N 2O.

2.3.1.Emissions of nitrous oxide (N 2O)from cropland

The International Fertilizer Industry Association and the Food and Agriculture Organization of the United Nations (IFA/FAO,2001)provided estimates of emissions of N 2O from cropland in Canada,the U.S.,and the world.These estimates are shown in Table 1.

The IFA/FAO estimates showed that although North American ?eld crop agriculture accounted for roughly 16%of the world ?eld cropland area and 17%of the world N consumption in 1995,its N 2O emissions were smaller,at about 12%.This would tend to indicate that N 2O emissions may be higher in other regions of the world.If one assumes that 1%of fertilizer N is emitted as N 2O-N (as in IPCC,2006)then fertilizer-induced emissions account for about 33%of the estimated total in North America.

The fraction of applied N actually emitted as N 2O varies widely on a site-speci?c basis.Thornton and Valente (1996)reported that coef?cients of variation for N 2O emissions measurements

typically

Fig.1.Greenhouse gas emissions from the U.S.economy by sector,in billion (109)t of CO 2equivalents.Calculated from Table 2-16in U.S.EPA (2007)

.

Fig.2.Distribution of greenhouse gas emissions from U.S.agriculture and total.Calculated from Table 2-16in U.S.EPA (2007).

C.S.Snyder et al./Agriculture,Ecosystems and Environment 133(2009)247–266249

range between 100and 300%.The current Tier 1method emission factor is 0.01with an uncertainty range of 0.003–0.03(IPCC,2006).In terms of GWP,this is equivalent to 4.65kg CO 2per kg of N applied,with an uncertainty range of 1.4–14.0.

The Environment Canada (2007)GHG inventory reported that recent research indicated low N 2O emissions in arid regions.The research found that in the Prairie regions,which account for about 80%of the land area and N applications (Grant and Wu,2008),an average of 0.16and 0.8%of N fertilizer applied was emitted as N 2O in the Brown-Dark Brown and Grey-Black soil zones,respectively,compared to 1.19%in Eastern Canada.The emission coef?cient used in the calculation of Canada’s GHG inventory is thus based on regression to an aridity index linked to these observations.

McSwiney and Robertson (2005)reported that a set emission factor is appropriate only when crops are fertilized at N rates less than or equal to those required for maximum yields,because the percentage of fertilizer N that is emitted as N 2O becomes more variable at higher N rates.Halvorson et al.(2008b),Del Grosso et al.(2008),and Burton et al.(2008)observed that set emission factors,including the IPCC (2006)factor,over-estimated seasonal N 2O emissions.Emission factors may increase when the N rate exceeds the crop and soil uptake capacity (Grant et al.,2006;Halvorson et al.,2008b ).In?uence of fertilizer N rate is further discussed in Section 3.7.

The most important factors affecting N 2O emissions from fertilized ?elds were listed by IFA/FAO (2001)as (1)climate,soil organic C (SOC)content,soil texture,soil drainage,abundance of NO 3-N and soil pH and (2)management related factors including:N application rate per fertilizer type,and type of crop,with major differences between grasses,legumes and other annual crops.The length of measurement period and the frequency (and intensity)of measurements were also cited as key factors in any local or large-scale estimations of N 2O emissions,in agreement with Parkin (2008).Eichner (1990)listed the following factors affecting fertilizer-derived N 2O emissions:(1)management factors—ferti-lizer type,application rate,application technique,application timing,tillage system,use of other chemicals,crop type,irrigation,and residual N and C from crops and fertilizer and (2)environ-mental factors—temperature,precipitation,soil moisture content,SOC content,soil O 2status,soil porosity,soil pH,freezing and thawing cycles,and microorganism abundance and activity.

Del Grosso et al.(2006)pointed out that in addition to increasing direct soil N 2O emissions,agricultural practices can also increase ammonia (NH 3)volatilization and NO 3àleaching.Volatilized N can affect N 2O emissions because a portion of this N will be deposited on agricultural and non-agricultural soils and in water and be subjected to transformations that may result in N 2O emissions.A portion of the NO 3àthat is leached or discharged in drainage can also be denitri?ed and result in N 2O emissions.

Crutzen et al.(2008),considering the global rate of increase in atmospheric N 2O in comparison to the total reactive N produced by human activity,concluded that the sum of direct and indirect emissions could amount to 3–5%of N applied—a total emission three to ?ve times larger than the direct emission coef?cient of IPCC (2006).They stated,‘‘The large difference between the low yield of N 2O in agricultural ?elds,compared to the much larger average value derived from the global N 2O budget,implies considerable ‘‘background’’N 2O production occurring beyond agricultural ?elds but,nevertheless,related to fertilizer use,from sources such as rivers,estuaries and coastal zones,animal husbandry and the atmospheric deposition of ammonia and NO x .’’

In contrast,Nevison et al.(2007)reported that a 2%emission factor for global anthropic N input was suf?cient to explain the atmospheric N 2O increases over the last 150years.Galloway et al.(2004),using a similar analysis,stated that ‘‘of the $156Tg N year à1created by human action in the early 1990s,$2.5%can be accounted for by tropospheric accumulation of N 2O.’’

Assumptions that natural background N 2O emissions have remained relatively constant in natural estuarine and marine systems since the pre-industrial era may be questioned,since Fulweiler et al.(2007)reported decreases in estuarine primary production attributed to climate change.They measured decreased organic matter deposition to coastal marine sediments,and a reversal in net sediment N ?ux in Narragansett Bay,near Rhode Island in the U.S.Fulweiler et al.(2007)suggested that some estuaries may no longer remove N from the water column,but are actually net N contributors through N ?xation.

Crutzen et al.(2008)reported the pre-industrial,natural N 2O sinks and sources were ‘‘equal to 10.2Tg N 2O-N year à1,’’which included marine emissions.They noted that the decrease in global N 2O emissions arising from deforestation since 1860was ‘‘an uncertain 0to 0.9Tg N 2O-N year à1.’’Though it is not the intent of these authors to minimize the importance of N 2O emissions associated with agricultural soil management,it is interesting to note this uncertainty exceeds the 0.47Tg N 2O-N year à1worldwide fertilizer-induced emissions,calculated from the estimate pro-vided in IFA/FAO (2001).

A number of comments can be made in regard to the global N budget approaches (i.e.,Crutzen et al.,2008;Galloway et al.,2004;Nevison et al.,2007).The analyses are subject to uncertainties in the estimates of natural biological N ?xation and natural rates of N 2O emissions.Crutzen et al.(2008)also assume that the fraction of ‘‘new’’anthropogenic N released as N 2O is the same for all four sources:fertilizer N,industrial N,biological N ?xation in agricultural crops,and the N emission resulting from fossil fuel combustion.Their approach attributes all the increase in N 2O production to these new sources,and none to the effect of human activities on the rate of N 2O release from the huge pool of N in soils.Nonetheless,the possibility of this approach’s accuracy must be acknowledged.The indirect emissions arising from N use merit greater consideration.

These N 2O emission estimation uncertainties underscore the importance of appropriate methodology and measurements of N 2O and N 2emissions associated with denitri?cation.‘‘Denitri?cation is dif?cult to measure because of the analytical dif?culty in detecting small increases in N 2concentrations against the large background in the environment.A number of recent advances in approaches to directly quantify denitri?cation in aquatic and

Table 1

Estimates of N 2O emissions from cropland in the U.S.,Canada,and the world in 1995(adapted from IFA/FAO,2001).Region

Area (million ha)

Fertilizer N

applied (million t)

Animal manure N applied (million t)

N 2O-N emitted Total (million t)

Fertilizer-induced a Million t

%of total Canada 46 1.580.210.0670.01624U.S.19011.15 1.580.3160.11235World

143673.4820.66 3.1500.735

23

a

Estimated using IPCC emission factor of 1%.

C.S.Snyder et al./Agriculture,Ecosystems and Environment 133(2009)247–266

250

terrestrial environments have been made,but few people are trained to use these newer approaches.’’(https://www.doczj.com/doc/0f17295624.html,m.with Dr.Eric Davidson,Woods Hole Research Institute,10December2007).

Chapuis-Lardy et al.(2007)noted that while much research has concentrated on net emission,there are numerous reports of net negative?uxes of N2O,?uxes from the atmosphere to the soil.They concluded that the factors regulating N2O consumption are not yet well understood and merit further study.Such study could potentially provide valuable information toward management of soils and soil biology to enhance conditions favorable to the consumption of N2O.

2.3.2.Emissions of CO2from lime

When agricultural lime is added to soil to increase pH,some fraction of its carbonate may be released as CO2.The current IPCC Tier2methodology for the U.S.greenhouse gas inventory(USEPA, 2007)uses emission factors of0.059kg C kgà1of lime and 0.064kg C kgà1of dolomite(West and McBride,2005).Thus,the GWP of lime use would average0.22kg CO2kgà1of limestone. Hamilton et al.(2007),citing evidence from cropping systems in Michigan,concluded that use of lime in agriculture could provide a sink for CO2,through enhanced export of bicarbonate in drainage water.They pointed out that acidity generated from nitri?cation would reduce this sink.However,their assumption regarding the amount of acidity generated by urea and anhydrous ammonia fertilizers exceeds that commonly used in soil fertility literature by more than threefold(Havlin et al.,2005).

The most common N fertilizers(urea,anhydrous NH3, ammonium nitrate)generate at most3.6kg of lime need per kg of N to balance the acidity they https://www.doczj.com/doc/0f17295624.html,ing IPCC Tier2 methodology,this amount of lime works out to an additional GWP of3.6?0.22=0.84kg CO2per kg of N applied—small compared to the estimate of N2O emission resulting from the application.This factor would be of relevance only to N use on soils that have low pH and low pH buffering capacity.The median pH of North American soils sampled in2005was6.3,with69%of the soils testing above pH 6.0(PPI/PPIC/FAR,2005).The majority of sampled soils in North America do not have a large lime requirement.In many states and provinces in North America, soils are calcareous and might bene?t from some acidi?cation that would result in the soils becoming more neutral in pH,enhancing availability of some plant nutrients.

The need for lime is not only due to the use of ammoniacal fertilizer.Soil acidi?cation is also associated with plant root proton excretion(i.e.exudation of H+ions)(Marschner,1991)and removal of basic cations in crop harvests(Jackson and Reisenauer, 1984)and the acidity of rainfall.

One possible way to avoid the emission associated with lime use is to apply oxide(e.g.quicklime or slaked lime)rather than carbonate materials,if they can be produced with CO2recovery.If relatively pure oxides of calcium or magnesium could be used,lime use would likely result in a small GHG sink,since CO2could be precipitated from the air in response to the addition of the material.These materials may be prohibitively expensive,how-ever.These oxide and hydroxide forms of lime may also present unacceptable inhalation,dermal exposure,and eye injury concerns in broad commercial use because of their caustic nature.

2.3.3.Emission of CO2from urea

Applying urea to soils leads to a loss of CO2that was?xed in the industrial production process.Urea(CO(NH2)2)is converted into NH4+,hydroxyl ion(OHà),and bicarbonate(HCO3à),in the presence of water and urease enzymes.Similar to the soil reaction following addition of lime,bicarbonate that is formed evolves into CO2and water.This source category is included because the CO2 removal from the atmosphere during urea manufacturing is estimated in the Industrial Processes and Product Use Sector(IPCC, 2006).

Similar to lime,all C in urea is considered to be emitted as CO2, but a default uncertainty factor ofà50%of the chemical maximum may be applied.Since urea contains12g C for every28g N,this works out to a GWP of1.6kg CO2kgà1of urea-N applied,or0.8if the uncertainty factor is applied.This GWP cost could be similar to that of the lime requirement generated by the ammoniacal fertilizers,but is still small compared to the estimated effect on N2O emission.

2.3.4.Emissions of CH4from rice culture

Flooded rice(Oryza sativa L.)culture is one of the major anthropogenic sources of CH4emissions.Urea and ammonium sulfate are the principal N sources used on the roughly3million ha of rice grown in the U.S.annually,and urea is the most common N source(Snyder and Slaton,2001).Compared to corn(Zea mays L.)at roughly38million ha in2007,and over101million ha of all?eld and forage crops,rice may be considered a relatively insigni?cant contributor to the total GHG emissions from U.S.agriculture.On a global scale,rice culture is a more important GHG contributor than in North America.

Bufogle et al.(1998)cited work reporting that CH4emissions were less when ammonium sulfate rather than urea was the fertilizer N source for rice.As?ooded soils become more reduced,sulfate-reducing bacteria effectively compete with methanogenic bacteria.Research by Jugsujinda et al.(1995)in Louisiana showed that CH4emissions were reduced when NO3-N was applied to rice,because added NO3-N increased the redox potential.Other work with rice in China,where manure and urea were the N sources,showed that CH4emissions occurred at lower redox(more reduced)potentials(<à100mV)than did N2O emissions(>+200mV),and there was a signi?cant inverse relationship between emissions of these two GHGs(Hou et al., 2000).These results indicate the risk of promoting N2O emissions when?ooded rice soils are drained.Hou et al. (2000)stated that maintaining the soil redox potential between à100to+200mV would prevent CH4production and would also be low enough to encourage N2O reduction to N2.Lindau et al. (1990)reported greater CH4emissions in urea-treated rice plots as opposed to unplanted urea-treated plots.They also stated that N2O emissions due to fertilizer N addition were low,and N2O emissions after urea application were not above untreated control levels.The presence of rice plants and the type of fertilizer N applied were reported to affect the emissions of N2, N2O,and CH4.

Immediate wetting and continuous?ooding(‘‘permanent ?ood’’)of soil after application of urea or ammonium-based fertilizer N is an effective measure to reduce nitri?cation and N2O emissions(Hutsch et al.,1999).This is a common best manage-ment practice for drill-seeded,?ood-irrigated rice culture in the U.S.(Wilson et al.,2006).

2.4.Agricultural sinks

Since C is cycled in large amounts in cropping systems,small increases in C capture combined with small decreases in respiration (C release)can result in large changes in the balance—the net emission or sequestration.Essentially,crops(including pastures and rangelands)can capture and store CO2by converting it to organic forms of C that are stored in the soil.When agronomic practices increase soil organic matter(SOM),CO2is removed from the atmosphere in the long-term(Lal et al.,1998,2003;Follett et al., 2001).

Robertson(2004)listed four strategies by which net CO2 emissions from agriculture could be reduced.They include:

C.S.Snyder et al./Agriculture,Ecosystems and Environment133(2009)247–266251

1.gains in energy ef?ciency from improvements in the fuel

ef?ciency of farm machinery,irrigation scheduling,and other farm operations that consume fuel;

2.carbon sequestration in soil from changes in tillage,changes in

crop residue and animal waste management,and changes in the use of cover crops,fallow periods,and other aspects of crop rotation management;

3.the production of biofuels and the emergence of bio-based

materials technology to offset the use of fossil fuels for energy production and industrial feedstocks;and

4.continued gains in the production or yield ef?ciencies for grain,

livestock,and other agricultural products to defray the need to otherwise open new land for agricultural development(Cass-man,1999)and consequent C loss.

Optimized fertilizer management is an important aspect of each of the above four strategies.

1.When fertilizer is used to increase crop yields,it increases the

ef?ciencies of other energy-consuming inputs used in produc-tion.However,since fertilizer use itself involves energy consumption,the importance of applying the optimum rate is underscored.

2.By increasing the net primary productivity of cropland,

fertilizers can increase the return of crop residue C to the soil.

3.Since demand for biofuels increases the need for higher biomass

production per unit of land area,it also increases fertilizer use.If the goal is net energy production or fossil fuel offset,this fertilizer use must be ef?cient.

4.When fertilizer is used to increase crop yields,land for forests

and other natural areas can be spared from conversion to cropland.

2.5.Soil carbon stabilization

Agriculture is considered to have large potential to reduce CO2 emissions and increase C sinks.Cole et al.(1997)estimated that it would be possible to increase the amount of C stored in the world’s agricultural soils by0.44–0.88billion t annually over a50-year period,recovering one-half to two-thirds of the estimated historic loss of C from currently cultivated soils.High-yield agriculture has the potential to increase the annual input of crop residue C to soils. Carbon isotope studies with corn in Ontario(Gregorich and Drury, 1996)showed that fertilization over a35-year period led to a higher level of corn-derived C in soil,while the level of native soil C was the same as in unfertilized soil.Fertilization and crop rotation often increase crop yields,and they can also increase SOM levels.In some environments,with proper management,continuous corn (i.e.no rotation)may also produce high residue yields and increases in SOM(e.g.Dobermann et al.,2007).Adequate fertilization contributes to the increase of SOM and does not alter the turnover of native SOM.‘‘Agricultural intensi?cation through the adoption of scienti?cally proven BMPs can solve,rather than cause,numerous environmental problems,including CO2emis-sion.BMPs can improve soil organic carbon(SOC)content,enhance soil quality,restore degraded ecosystems,increase biomass production,improve crop yield,and encourage investment in soil resources for soil restoration’’(Lal et al.,1998).

2.5.1.Role of N in stabilizing soil C

Nitrogen has been reported to play an important role in soil C storage,both by promoting crop dry matter production and by chemically stabilizing C in the soil(Paustian et al.,1992,1995, 1997).Many experiments have shown that fertilizing crops with N results in higher levels of soil C over time.An example from Minnesota is shown in Table2.In this25-year study,Wilts et al.(2004)reported that total soil organic C declined for all treatments, but at a slower rate in the fertilized treatments than in the unfertilized control.The difference resulted from increased accumulation of C in the soil with the isotopic signature of corn. Much of this corn C was considered to have come from roots and root exudates during growth.

Liang et al.(1996)compared a high and normal rate of N–P–K fertilizer(400–132–332kg haà1vs.170–44–141kg haà1)in Que-bec,Canada,on a sandy loam soil growing corn,which also received4t haà1(dry matter basis)of liquid manure for the?rst5 years.They reported that at the end of a9-year period,there was 12%more soil C to a20-cm depth with the higher fertilizer rate. Paustian et al.(1997)documented20sites worldwide that gained soil C in response to application of N fertilizers over periods ranging from7to120years.

Paustian et al.(1992)reported data from a long-term experiment in Sweden that showed N stabilized C in soil (Fig.3).In this experiment,the addition of N[80kg haà1as Ca(NO3)2]increased the growth of the cereal crop.The increased root growth provided additional C to the soil,but the net storage in the long-term was enhanced even more.Addition of N increased net C stored in response to additions of straw and sawdust as well. The authors speculated that nitrogenous compounds may react with lignin in the process of humus formation,as a mechanism of C stabilization.In addition,most SOM stabilizes with a C:N ratio of approximately10:1,indicating again that if soil C storage is to increase,N is needed.As shown in Fig.4,Schulten and Schnitzer (1997)found N to be integral to the chemical structure of SOM,and to be stabilized within it.

Nitrogen is sometimes perceived as reducing SOM stabilization. New crop residues with C:N ratios greater than20usually decompose more quickly when N fertilizer is applied.However,the net amount of C stabilized does not appear to decrease;rather, decomposition of SOM to stabilized C is more rapid.

Table2

Soil organic carbon(SOC)accumulated from1965to1995in the top30cm of

a soil cropped to continuous corn.

Applied N SOC derived from corn

kg haà1yearà1t haà1% 16817.021 8313.017 010.112 Source:Wilts et al.(2004)

.

Fig.3.Annual change in soil C storage over30years in response to additions of N, the presence of a crop,added straw,and added sawdust.In all plots other than fallow,a cereal crop was grown each year and all aboveground crop residues were removed(adapted from Paustian et al.,1992).

C.S.Snyder et al./Agriculture,Ecosystems and Environment133(2009)247–266 252

Khan et al.(2007)reported an analysis of soil organic C changes over a 51-year period in the Morrow plots in Illinois.Their ?ndings were claimed to ‘‘implicate fertilizer N in promoting the decomposition of crop residues and soil organic matter’’and to show that ‘‘current fertilizer N management practices,if combined with stover removal for bioenergy purposes,exacerbate soil C loss.’’

However,these claims do not appear to be supported by the data provided (Reid,2008).The three N rates compared were zero,168–224,and 336kg ha à1,applied as urea.Only the high rate showed a greater net loss of soil C than the zero rate,and it was confounded with a higher initial soil C content built up by 51years of manure application prior to the period studied.In the corn–oats (Avena sativa L.)-hay rotation,N fertilization at the intermediate (recommended)rate increased soil C stored in the top 46cm of soil by 6600kg ha à1(a relative GWP of about à0.5t ha à1CO 2per year).In the other two rotations (continuous corn and corn–soybean (Glycine max Merr.)[oats])there was no difference in net change in soil C between the same two treatments.

In contrast to the report by Khan et al.(2007),in a 23-year study on a chisel-plowed Mollisol in northwest Illinois (Jagadamma et al.,2007),SOC and total N (TN)increased in the 0–30-cm soil depth with increasing N rates (?ve rates:0–280kg N ha à1year à1).Appropriate N fertilization increased agronomic productivity and crop biomass production,which resulted in higher TN and SOC storage in both corn–corn and corn–soybean systems.

The results reported by Khan et al.(2007)do underscore the fact that many soils engaged in crop production have lost C relative to their native state,and that once soil C is built up it can be more susceptible to decline.The positive role of N fertilization in sequestering C may be offset by N 2O emissions,if care is not taken to properly manage the entire cropping and tillage system.2.6.Factors affecting GHG emissions from soil

2.6.1.Soil physical properties and conditions

According to Mosquera et al.(2007),‘‘The production and consumption of both N 2O and CH 4from soils occurs as a result of different microbial processes,which in turn are controlled by factors that in?uence the growth of microorganisms (soil O 2content,soil temperature,mineral N content in organic matter and pH).Soil management practices (land use,nutrient application via manure and N fertilizer,incorporation of either crops or crop

residues,tillage,reduction of soil compaction),through their effect on these factors,can indirectly in?uence these ?uxes.’’

Alternating wetting and drying cycles that permit nitri?cation to progress,and WFPS above about 60%,but below saturation,contribute to the greatest potential for N 2O emissions (Granli and B?ckman,1994).The magnitude of N loss is controlled by the interaction of soil moisture and N availability,principally NO 3àavailability (McSwiney and Robertson,2005).

Proper irrigation practices to improve water use ef?ciency,and to avoid moisture excesses associated with reductions in air-?lled pore space,may help minimize the potential for N 2O emissions.Increases in soil moisture up to about 60%WFPS tend to encourage nitri?cation and soil CO 2production (Granli and B?ckman,1994;Linn and Doran,1984).The research by Linn and Doran (1984)showed that 90%of the variation in CO 2production between no-tillage and plowed soils was accounted for by differences in WFPS,regardless of the application of N fertilizer.Similarly,research in Canada showed the amount of denitri?cation was controlled primarily by soil O 2supply,as controlled by WFPS and C availability,and the N 2O:(N 2O +N 2)ratio was generally high where there was abundant moisture,but not saturated conditions (Gillam et al.,2008).

Soil position in the landscape,as affected by topography,can greatly affect whether or not emissions of N 2O will occur,and their magnitude.Izaurralde et al.(2004)reported that emissions were greater in portions of the landscape where water tended to gather,such as depressions compared to shoulder positions.Depressions tend to have higher WFPS and SOM compared to upper slope positions,and thus differ in microbial activity,transformations of applied N,and crop nutrient uptake.

2.6.2.Soil compaction

Naturally high bulk density associated with fragipans,and high bulk density associated with compaction by tillage implements and agricultural equipment such as grain carts,combines,and tractors,can result in reduced aeration under moist soil conditions.Disruption of soil compaction,while preserving surface cover by crop residues,may lead to a lower risk of denitri?cation and N 2O emissions,and a lower risk of CH 4emissions.For example,Mosquera et al.(2007)reported that compaction can reduce the ability of soils to consume or oxidize atmospheric CH 4by as much as 30–90%.This same publication indicated that slight soil compaction,resulting in reduced aeration,can increase N 2O emissions by as much as 20%,while severe compaction may double N 2O emissions.The compaction of clay soils was reported to have a greater negative effect on N 2O emissions than in sandy soils.Some of the negative effects of compaction may also be related to reductions in crop root growth and root zone microbial processes.3.Cropping and fertilizer practices and the associated BMPs Many crop management practices can affect GHG emissions;directly by affecting NO 3àavailability,or indirectly by modifying the soil microclimate and cycling of C and N.3.1.Tillage systems

It cannot be taken for granted that a switch from conventional primary tillage to conservation tillage or no-till systems will mitigate GHG emissions.In many regions,soil C storage can be increased and GWP decreased by appropriate conservation tillage (Lal,2003).But,there is an important proviso.Changes to tillage systems must not greatly reduce the cropping system’s net primary productivity,and the impact of the change on all associated GHGs must be considered,as indicated in the following section on systems

comparisons.

Fig.4.Chemical structure of soil organic matter (Schulten and Schnitzer,1997,with permission).The element colors are N—blue,C—cyan,H—white,O—red,S—yellow.(With permission of the Soil Science publishers:Wolters Kluwer/Lippincott Williams &Wilkins).

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There is not a clear response–positive or negative–for the mitigation of GHG emissions using conservation or no-tillage practices compared to conventional tillage,as de?ned by intense, inversion tillage.It appears that in some regions the bene?t of less tillage is an increase in stored soil organic matter,both organic C and organic N,to a greater degree than any potential increase in N2O emissions,so that the net GWP decreases.In other studies the GWP increases slightly from switching from conventional tillage to conservation or no-till(Blanco-Conqui and Lal,2008;Gregorich et al.,2004;Lal,2003).

Venterea et al.(2005)reported that emissions of N2O were higher under no-till and conservation-tillage compared to con-ventional tillage when N was applied post-emergent as broadcast urea,but completely different tillage effects occurred when N was applied pre-plant as either injected anhydrous ammonia or broadcast urea–ammonium nitrate.Fertilizer source and tillage interactions can result from differences in soil water content and bulk density,differences in soil nitrite accumulation among N sources,and may depend on whether nitri?cation or denitri?ca-tion dominates in the crop and soil system(Venterea and Stanenas, 2008).

Combined estimates(spring plus summer)of N2O loss under no-till were equal to or lower than those under intense tillage (Venterea et al.,2005).Halvorson et al.(2008b)observed reduced cumulative growing season N2O emissions under no-till compared to conventional till(plowed),when averaged across years and four N rates.

No-till management systems have the potential for reducing GHG emissions in the Alberta Parkland region(Lemke et al.,1999). Amount of N lost as N2O was higher from conventional tillage than from no-till plots(Malhi et al.,2006).These studies and other results(Pattey et al.,2007)have led to a reduced N2O emission estimate for land under no-till in the Canadian prairie region. Environment Canada’s1990–2005inventory(Environment Canada,2007)of GHG estimates emission from no-till land at 0.68?0.57kg N2O-N haà1yearà1,compared to0.95?0.98for intensively tilled land.

Six et al.(2004)and Lee et al.(2006)reported that the effects of changing from conventional tillage to no-till need to be integrated over time to understand the net effect on GWP.This can vary between ecological regions and within a speci?c agro-ecosystem. Their calculations of the cumulative GHG?uxes and GWP over a 20-year time span indicated that in humid climates,high C sequestration rates and relative decreases in N2O?uxes over time lead to a mean cumulative negative GWP for no-till relative to conventional tillage.In drier environments,they estimated negative or low C sequestration rates were not enough to compensate for increases in N2O emissions,resulting in a zero-balanced mean cumulative GWP.Adoption of no-till in dry climates can be combined with an increase in cropping intensity, which typically increases C sequestration rates and might there-fore lead to a negative GWP.

Strati?cation of C near the soil surface and changes in soil bulk density in reduced tillage systems complicate comparisons and interpretations about C sequestration among tillage systems.A review of the literature by Baker et al.(2006)indicated that many studies have looked at soil C only to a depth of30cm or less,which may have resulted in some over-estimation of soil C sequestration with reduced tillage systems,compared to tilled systems. Contributions of roots to soil C balances below30cm,and changes in soil bulk density,which are often associated with reduced tillage systems,should also be considered in soil C mass balance estimates.

Long-term changes in SOC are dif?cult to assess.In studies comparing tillage systems,soil bulk density can vary considerably from one treatment to another.In many reports on SOC,the mass of C to a speci?c depth is compared to a similar depth in each treatment,by multiplying the concentration of C in g kgà1of soil by the bulk density in kg haà1.Particularly when only shallow depths (7–20cm)are compared,very large errors may result from such relative comparisons.With less tillage,or with less crop residue inputs,bulk density tends to increase(Halvorson et al.,1999, 2002),so a soil with no C gain would appear to have a gain in mass of C if sampled to the same depth.At rates of N application that increase crop residue return to the soil,decreases in soil bulk density can occur.Deep soil sampling is required to accurately monitor the increase in soil organic C that results from different rates of N.

Deen and Pataki(2003)found that25years of zero-tillage under humid conditions in Ontario,Canada increased SOC in the top5cm relative to conventional tillage,but did not change it in the top 60cm.Campbell et al.(1998)reported that the mass per equal depth method of calculation(Ellert and Bettany,1995)showed that6years of no-tillage in the sub-humid region of Saskatchewan, Canada did not increase SOC.The same method clearly showed reductions in SOC in crop rotations without fertilizer.

It would seem that there is a slight long-term gain in reducing GWP by using conservation or no-till cropping systems compared to conventional tillage,depending on the environment and crop management practices.This is due to an increased sequestration of C in SOM and slightly less N2O emissions,along with a greater potential to absorb CH4.The sequestration of C in no-till cannot be assumed to continue in perpetuity.VandenBygaart et al.(2003) reviewed62long-term studies in Canada and found that‘‘the potential to store SOC when NT was adopted decreased with increasing background levels of SOC.’’

3.2.Tile drainage

Denitri?cation is strongly affected by soil moisture and aeration (Coyne,2008).Therefore,soil drainage management may also in?uence emission of N2O and other GHGs.Water table manage-ment(open drains at1m below the surface with subirrigation vs. free drainage)and N management research in Quebec,Canada showed little difference in denitri?cation N2O losses as N rates for corn increased from120to180kg of N haà1(Elmi et al.,2002). However,soil NO3-N levels did increase with the higher N rate.The denitri?cation rate increased with the subirrigation treatment. While this water table and drainage management approach may be desirable to reduce NO3-N discharge,it presents risks of increased N2O emissions.Further work with these systems(Elmi et al.,2005) showed that although denitri?cation rates were greater in the subirrigation treatment compared to free drainage,N2O emissions were similar between the two drainage treatments.These results indicate the combined N2O and N2losses were greater in wetter soils(i.e.strong correlation with WFPS),but that N2O comprised a small part of the combined gaseous N emissions.

3.3.Cropping systems

There are few long-term studies in the literature comparing cropping systems for GHG emissions.Many studies have measured only periodic emissions,rather than on an annual basis.

Koga et al.(2006)used a life-cycle inventory analysis of farming systems in northern Japan and reported that soil-derived CO2 emissions accounted for64–76%of the total GHG emissions.They concluded that GHG emissions from their cropping systems might be mitigated by enhancing soil management practices to enhance C sequestration in the soil.

Adler et al.(2007)used the DAYCENT simulation model to conduct a30-year life-cycle assessment of GWP for?ve bioenergy cropping systems:(1)switchgrass(Panicum virgatum L.),(2)reed

C.S.Snyder et al./Agriculture,Ecosystems and Environment133(2009)247–266 254

canarygrass(Phalaris arundinacea L.),(3)corn–soybean rotation(2 years of corn followed by1year of soybeans),(4)corn–soybean–alfalfa(Medicago sativa L.)rotation(3years corn,1year of soybeans, followed by4years of alfalfa),and(5)hybrid poplar(Populus spp.). No-tillage and conventional tillage were compared within the corn–soybean and corn–soybean–alfalfa rotations.Hybrid poplar and switchgrass provided the greatest net GHG sinks.The largest CO2 sinks were displaced fossil fuel,followed by soil C sequestration, while N2O emissions were the largest GHG source.Adler et al.(2007) found that each of the?ve cropping systems provided net GHG sinks, even when soil C was assumed to reach a new steady state and C sequestration in soil was not counted.

Robertson et al.(2000)measured GHG?uxes and other sources of GWP in cropped and nearby unmanaged ecosystems in southwest Michigan from1991to1999.Their inventory of emissions included CO2from lime application,N fertilizer manufacture,and fuel use in farm operations.It also included the CO2balance associated with change in SOC,and measured emissions of N2O and uptake of CH4(Table3).The largest factor in?uencing the net GWP of the four agricultural cropping systems compared was the change in SOC.Since their no-till system gained the most soil C in the9-year period(estimated only at0–7.5cm soil depth;as noted above in tillage systems,this may have resulted in a strong upward bias in no-till C sequestration estimates),its net emission of GWP was smallest.The greatest single source of GWP in most of the systems monitored was N2O emission.Several other non-agricultural systems showed a net negative GWP(Table3), suggesting that maximum mitigation of GHGs occurs when land is removed from production.The authors concluded that‘‘for productive cropland that must feed the burgeoning global population,mitigation...will depend on policies that address the multiple sources of GWP in cropping systems.’’

In comparison,Adviento-Borbe et al.(2007)monitored GWP on four much more intensive cropping systems.They found the net GWP of high-yielding continuous corn was two to three times higher than the conventionally tilled corn–wheat–soybean rota-tion of Robertson et al.(2000),but the food yield in terms of calories was four to?ve times higher(Table3).As in the previous study,the largest single source of GWP was N2O emission,though CO2emission from fuel was also high because of the use of irrigation.The largest factor mitigating the GWP was soil C storage driven by the large returns of crop residue to the soil.

The above two studies illustrate that many factors vary among cropping systems in determining their net GWP.The importance of assessing cropping systems for their GWP per unit of productivity is underscored by the fact that for net mitigation,land spared from production presents the greatest opportunity.

Mosier et al.(2006)similarly assessed the GWP of no-till and conventional tillage irrigated corn-based cropping systems in Colorado,with a range of N fertilizer input levels,over a3-year period.Their evidence suggested that in no-till continuous corn, GWP decreased as N rates increased from0to134to224kg haà1 and that with N fertilization this system was a net GWP sink.In conventionally tilled corn,N fertilizer increased the return of crop residue to the soil without increasing soil respiration rates and CO2 emissions.Their data also indicated that the rate of SOC accumulation under no-till was slowing,and that the N2O?ux from no-till was increasing relative to that under conventional tillage.In2of3years during2005–2007,N2O emission was signi?cantly greater in the conventional tillage system than in the no-till continuous corn system(Halvorson et al.,2008a,b).

Nitrogen?xing crops may increase the nitrous oxide emissions in cropping systems.The apparent N input from the previous year of soybean production,in a no-till corn–soybean rotation,raised background N2O emissions(i.e.,those occurring when corn is grown without N fertilizer)by approximately90%compared to conventional tilled continuous corn,and more than300% compared to no-till continuous corn(Mosier et al.,2006).In fallow seasons,they found N2O emissions were not affected by previous spring N fertilization.During the fallow season,N2O emissions from conventionally tilled soils were about double those from no-till soils.

Winter cover crops(e.g.,wheat(Triticum aestivum L.),rye (Secale cereal L.))can effectively prevent NO3-N leaching in the winter months on permeable soils,and reduce drainage losses of NO3-N(Feyereisen et al.,2006;Shipley et al.,1992).Reductions in NO3-N leaching/drainage losses are likely to reduce overall N2O emissions from water resources.

3.4.Fertilizer use and crop yield enhancement

Improving the ef?ciency and effectiveness of crop N use can potentially reduce N2O emissions,by reducing the potential for elevated residual NO3-N in the soil pro?le(Dobermann,2007;

Table3

Comparison of selected agricultural cropping systems for net global warming potential(GWP).

Cropping system GWP in CO2equivalents(kg haà1yearà1)Food yield a

(Gcal haà1yearà1)Mean crop yields(t haà1)

Soil C b N fert.c Lime Fuel N2O CH4Net GWP Corn Wheat Soybean

Robertson et al.(2000)—Michigan(9-year study)

Corn–soybean–wheat rotation

Conventional tillage0270230160520à40114012 5.3 3.2 2.1

No-tillà1100270340120560à5014013 5.6 3.1 2.4

Low-input with legume cover cropà40090190200600à5063012 4.5 2.6 2.7 Organic with legume cover cropà29000190560à504109 3.3 1.6 2.7

Perennial crops

Alfalfaà1610080080590à60à200

Poplarà117050020100à50à1050

Late succession forest0000210à250à40

Adviento-Borbe et al.(2007)—Nebraska(6-year study:non-inversion deep till system)

Continuous corn at BMPà161380722015031173à11019804814.0

Continuous corn—intensiveà2273121033018332090à11030805115.0

Corn–soybean rotation at BMP11002932201283917à7337403514.7 4.9 Corn–soybean rotation—intensiveà7366033016131247à3737403715.6 5.0

a Food energy calculated from crop yields and USDA national nutrient database https://www.doczj.com/doc/0f17295624.html,/NDL/index.html.

b Estimate of net soil C storage are based on change in soil C measured to a depth of7.5cm in the Michigan study and30cm in the Nebraska study.Shallower sampling depths tend to upwardly bias the C sequestration estimates in no-till systems.

c Estimate

d GWP associated with fertilizer N manufactur

e and transport was4.51kg CO

2kgà1N in the MI study and4.05in the Nebraska study.

C.S.Snyder et al./Agriculture,Ecosystems and Environment133(2009)247–266255

Snyder and Bruulsema,2007;Dobermann and Cassman,2004;Mosier et al.,2004;Cassman et al.,2002).Better nutrient management,including fertilizer BMPs,can potentially increase crop N recovery and minimize the ‘‘cascade’’to air and water resources (Galloway et al.,2003,2004).Improvements in N use ef?ciency can also lead to greater C sequestration and reductions in CO 2emissions (Paustian et al.,1992).3.5.Fertilizer production and transport

As indicated in Table 3,the production and transport of fertilizer is currently estimated to generate a substantial propor-tion of the GHG emissions associated with crop production.The estimates vary,however,and have a considerable in?uence on the net GWP attributed to a cropping system.Since mitigation efforts may someday include incentives for producers to reduce fertilizer use or select sources with lower ‘‘carbon footprints’’,accuracy in these estimates is important.

The U.S.EPA (2007)describes the Haber–Bosch process for producing anhydrous NH 3from natural gas by conventional steam reforming of CH 4approximately by the following reaction:0:88CH 4t1:26air t1:24H 2O !0:88CO 2tN 2t3H 2N 2t3H 2!2NH 3

Thus a fully ef?cient process would generate 0.88moles of CO 2for every 2moles of NH 3,or 0.88?44/28=1.38kg CO 2kg à1of N.This ?gure represents emission from feedstock or process gas.Additional emission also occurs from the fuel used for heat,gas turbines and other equipment in the ammonia plant (NRCan,2007).A portion of the process emission is often captured for use in urea manufacture.

Pach (2007)estimated the energy used in global N manufacture as 1%of the world’s total energy consumption,and concluded that a typical ammonia plant emits 2.5kg of CO 2kg à1of NH 3-N.Wood and Cowie (2004)listed 10studies estimating GHG emission factors for NH 3production,mainly from ammonia plants in Europe and North America.The emission factors they reported ranged from 1.4to 2.6kg CO 2kg à1of N,but they noted that lack of transparency in the reports they reviewed made it dif?cult to ascertain the reasons for the wide range.

Natural Resources Canada (NRCan,2007)reported on a benchmarking study completed by Plant Surveys International.The study provided ?gures for total CO 2generation from ammonia production.Per tonne of NH 3-N,emissions ranged from 2.2to 2.7t of CO 2,with a world average of 2.6.Canada and Western Europe were reported to be the most ef?cient regions.

Ef?ciency of NH 3production is reported to be on an improving trend (Smil,2002).Replacement of older plants with newer technology can be expected to move emission coef?cients to the lower end of the ranges reported above.However,it is unlikely that companies in regions with high natural gas and environmental compliance costs will make such capital investments.Additional energy is consumed in the conversion of NH 3to other forms of N fertilizer,such as urea and ammonium nitrate,and in the transport of fertilizers to the ?eld.Production of nitric acid also generates N 2O emissions.These associated emissions need to be considered in assigning emission factors.

The Argonne National Laboratory’s life-cycle model GREET (Greenhouse gases,Regulated Emissions,and Energy use in Transportation;Wang,2007)includes emission coef?cients for agricultural inputs.The coef?cients from GREET 1.8a shown in Table 4indicate that N fertilizer sources differ in the net GWP associated with their production and transport.Sources containing NO 3àhave higher GWP owing to N 2O emitted by nitric acid production.Estimates of this emission coef?cient vary widely,but can be reduced by 70–90%by use of non-selective catalytic reduction technology (Wood and Cowie,2004).

Urea production often recaptures much of the process emission of CO 2from ammonia production (NRCan,2007).This can reduce the production emission factor for urea by as much as 1.6kg CO 2per kg of N (i.e.half the ?gure reported in Table 4),but as described previously in this review,some or all of this CO 2would be emitted from the soil after application.

Agronomic studies analyzing the GWP of cropping systems have used varying emission coef?cients.Generally,the coef?-cients for N are higher than those speci?cally for ammonia,since they aim to represent the general mix of N sources used in North America.About 40%of total fertilizer N is applied as urea,26%as anhydrous ammonia,and 21%as ?uid urea–ammonium nitrate (UAN)(based on Association of American Plant Food Control Of?cials (AAPFCO)data and H.Vroomen,https://www.doczj.com/doc/0f17295624.html,m.,August 2007).Ammonium nitrate comprises only 4%of the total,and its use is declining.

Adviento-Borbe et al.(2007)used a coef?cient of 4.05kg CO 2kg à1N in their study of irrigated corn in Nebraska,while Robertson et al.(2000)used 4.51kg CO 2kg à1N for crop rotations in Michigan.Mosier et al.(2006)used a ?gure of 3.0kg CO 2kg à1N,plus 45.5kg CO 2ha à1for application,for UAN in irrigated corn in Colorado.West and Marland (2002)reported the direct CO 2emission from the manufacture and transport of fertilizer as 3.15,0.61,0.44and 0.13kg CO 2kg à1of N,P 2O 5,K 2O,and lime,respectively,not including CH 4or N 2O emissions associated with their production and transport.

The current mix of N sources used in North America,when assessed an emission coef?cient as indicated in Table 4,result in a general GWP coef?cient of about 4kg CO 2kg à1,close to what has been used in the agronomic studies cited above.As use of ammonium nitrate declines,and if the use of anhydrous ammonia and urea increase,a lower emission coef?cient may be justi?ed.Only small increases in ammonia production ef?ciency are expected in the short-term.In the long-term,if a C-free method can be found to generate hydrogen for the Haber–Bosch process,ammonia could be produced with a much smaller ‘‘carbon footprint.’’

Table 4

Energy use and greenhouse gas emissions associated with manufacture and transport of major fertilizer products,calculated from GREET 1.8a (Wang,2007).

Ammonia (N)

Urea a (N)

Ammonium nitrate b (N)

Phosphate (P 2O 5)Potash (K 2O)Lime (CaCO 3)per kg of nutrient as indicated in parentheses above Energy use (MJ)45536514.098CH 4emission (g) 2.5 3.7 4.2 1.8 1.00.9N 2O emission (g)0.020.0319.70.020.010.01CO 2emission (kg) 2.6 3.1 3.8 1.00.70.6GWP (kg CO 2equiv.)

2.6

3.29.7

1.0

0.7

0.6

a Version 1.7deducted CO 2captured in the process of producing urea;version 1.8a does not.

b

Non-selective catalytic reduction technology,installed on an estimated 20%of nitric acid plants worldwide,can reduce the manufacturing N 2O emission by 70–90%(Wood and Cowie,2004).

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3.6.Nitrogen source impact on N2O emissions from soil

https://www.doczj.com/doc/0f17295624.html,mercial fertilizer sources

Tenuta and Beauchamp(2003)reported that three methods of measuring N2O production from granular fertilizer N sources applied to?eld soil demonstrated decreasing magnitudes of production from urea,ammonium sulfate[(NH4)2SO4],ammonium nitrate(NH4NO3),and calcium nitrate[Ca(NO3)2].The relative magnitude of N2O emissions from these N sources in laboratory experiments agreed with those of the?eld study.Under aerobic conditions,emission of N2O was greater with urea than with the other N fertilizers,with fewer differences at higher moisture contents.Under saturated conditions,both monoammonium and diammonium phosphates produced more N2O per unit of N than the other N fertilizers,including urea.The authors concluded that this observation warrants more research since ammonium phosphate fertilizers are commonly used and because of the possible implication of P status impacting N2O emissions from N fertilizers.

Granli and B?ckman(1994)concluded that there is no single fertilizer type that generally contributes more to N2O-N emissions than another.These conclusions are in agreement with earlier work by Eichner(1990),who stated,‘‘Comparing emissions data from experiments with sampling periods of various lengths,using different types and amounts of fertilizer,and different application techniques should be done with caution.’’Scientists in the United Kingdom(Harrison and Webb,2001)stated,‘‘It is dif?cult to say with any certainty if a strategy based on urea or ammonium nitrate would result in the smaller N2O emissions.’’These same scientists proposed a relative emission assessment scheme that suggested N2O emissions from urea under warm,wet conditions may exceed those of NH4-based sources.Relative N2O emissions from NO3-based sources may exceed those from NH4-based sources,and differences may increase with increasing wetness,according to Harrison and Webb(2001).More recent comparisons among urea, NH4-based and NO3-based N fertilizers have shown higher emissions of N2O from urea(Tenuta and Beauchamp,2003),and higher N2O emissions from NH4-based N fertilizers compared to NO3-based fertilizers(Velthoff et al.,2003;Tenuta and Beauchamp, 2003).This higher emission with NH4-based fertilizers may be related to potential NO2àaccumulation or N2O production during nitri?cation(Venterea and Stanenas,2008).

Bouwman et al.(2002)reviewed numerous studies and reported N2O emissions appear to be lower for NO3-based fertilizers compared to NH4-based fertilizers and organic or synthetic-organic sources.Yet,one might expect a potentially higher N loss with an abundance of NO3-N in soil systems from NO3-based fertilizers compared to other N fertilizers,since NO3àand NO2àare essential for denitri?cation(Coyne,2008;Alexander, 1977).However,anhydrous NH3has exhibited higher N2O emissions in several studies comparing it with other N sources (Breitenbeck and Bremner,1986a;Venterea et al.,2005).In contrast,Burton et al.(2008)found no differences in N2O emissions between anhydrous ammonia and urea in Manitoba,Canada.

Higher emissions of N2O with anhydrous NH3application compared to other sources are believed to be related at least in part to the typical knife or coulter injection of this fertilizer,which produces highly alkaline soil zones with a high NH4+concentration (Breitenbeck and Bremner,1986a).Emission factors for broadcast urea,ammonium sulfate,ammonium nitrate,and nitrate-salts ‘‘...depend on the conditions in the period after fertilization’’(Harrison and Webb,2001).

Bouwman et al.(2002)warned against applying any of the estimates from their summarized data,some of which are included in Table5,to individual?elds because they represent gross relative differences among fertilizer sources/types.Perhaps soil tempera-ture and moisture conditions conducive to rapid nitri?cation and denitri?cation dominate the fertilizer source/type effects on N2O emissions.

After balancing for rate of application,crop type,climate,SOC, soil pH,and length of experiment,differences among fertilizer types almost disappear(Stehfest and Bouwman,2006).

Recent work in Colorado reported data on N2O emissions in response to applied N across three cropping systems and over3 years(Mosier et al.,2006).These authors reported that their data suggest an emission rate of0.0074kg N2O-N kgà1of N applied,26% lower than the current IPCC emission factor.Emissions of N2O vary spatially and temporally.Del Grosso et al.(2005)indicated that the margin of uncertainty of N2O emissions,especially in large-scale estimates,may be?50%.Adviento-Borbe et al.(2007)reported that the fraction of the applied N lost as N2O ranged from0.4to3.5%in their3-year study of four cropping systems,with the greatest variability in emissions attributed to year effects.

Table5

Fertilizer-induced N2O-N emissions from fertilized soils with various N sources/types,as summarized by different reports(not including losses in drainage water, groundwater,or streams).

Fertilizer source Report1Report2Report3

n Median a Range Range n Range

kg haà1%of applied N%of applied N%of applied N

Anhydrous ammonia38 1.040.05–19.60.05–6.8120.04–6.84 Ammonium carbonate,chloride,or sulfate740.820.01–36.54110.03–0.90 Ammonium phosphates60.260.06–7

Ammonium sulfate30.08–0.18 Ammonium-based0.03–1.840.04–0.12 Ammonium nitrate131 1.120–30.40.04–1.780.04–1.71 Calcium nitrate70.01–1.75 Calcium ammonium nitrate73 1.560.05–11

Potassium nitrate10.02 Sodium nitrate90–0.50 Calcium nitrate,potassium nitrate,sodium nitrate580.790–41.80

Nitrate-based0.001–1.8

Urea ammonium nitrate400.780.03–16.031 1.57

Urea1310.960.01–46.440.01–2.170.08–0.18‘‘Mix’’of synthetic fertilizers45 1.130–16.78

Organic-synthetic fertilizer mixes480.810–31.732 1.78–1.80 Organic88 1.150.03–560.01–2.05

Note.References:(1)Stehfest and Bouwman(2006);(2)Granli and B?ckman(1994);(3)Eichner(1990).‘‘n’’represents number of studies or observations.

a Median values,balanced for in?uence of rate,soil organic C,soil pH,and length of experiment.

Interactions between N sources and tillage systems often occur, as in the corn study by Venterea et al.(2005).They compared N sources(spring knife placement of anhydrous NH3;UAN solution surface sprayed(both about a week prior to planting);and surface broadcast urea(applied to20cm tall corn)),and tillage systems (conventional till,conservation till,and no-till).Emissions of N2O with urea were higher under no-till and conservation tillage compared to conventional tillage,while no differences among tillage systems were observed with the UAN.With anhydrous NH3, N2O emissions were greater with conventional tillage than the other two tillage systems.As was reported by Breitenbeck and Bremner(1986a),anhydrous NH3resulted in higher(2?to4?) N2O emissions compared to the other two sources and their placement methods.However,in the study by Venterea et al. (2005)and many others,the fertilizer N source effects are often confounded with placement methods.Such confounding prevents valid comparisons of N2O emissions among N sources.

Reducing the loss of N through one pathway may increase the loss through another.For example,banding of N may reduce immobilization,or use of a urease inhibitor with urea may leave more N in the soil subject to potential delayed losses via denitri?cation or leaching.The source,timing and placement combination that produces the greatest yield with the least amount of N is likely to optimize both agronomic and environ-mental goals.Continued in-?eld assessment of new practices for optimum crop N use ef?ciency is essential to these goals.

3.6.2.Biologically?xed legume-derived N

The N originating from biological?xation by Rhizobium sp. bacteria associated with legume crops is generally less available for nitri?cation and subsequent denitri?cation and the associated N2O emissions initially,when the legume crop is actively growing.This is because the bacteria?x the N2gas from the air into the NH4+ion that is largely used by the legume crop to form protein compounds. In a study by Parkin and Kaspar(2006)comparing N2O emissions from soybean and corn crops within a corn–soybean rotation,the authors reported higher emissions from the soils planted to corn than those planted to soybean.They thought the greater availability of mineral N from fertilizer was the controlling factor. Rochette et al.(2004)noted that N2O emissions are usually lower from a non-N fertilized non-legume crop than from an actively growing legume crop.

Once the legume crops are harvested,or the plants senesce and die,the protein compounds in residues above and below ground are susceptible to decomposition and mineralization to NH4+, which can then be nitri?ed and denitri?ed,leading to N2O emissions.Dick et al.(2006)noted that even in perennial plant situations,soil collected under N-?xing tree species emit more N2O compared to soils from under non-N-?xing crop species,and this was correlated with a larger pool of mineral N(NH4+and NO3à) available in the soil(Dick et al.,2006).As noted above(see Section 3.3),Mosier et al.(2006)reported that soybeans in rotation with corn raised background N2O strikingly compared to continuous corn.

3.6.3.Livestock manures

The mineral N in manure and the organic N that is mineralized can be as susceptible to GHG emissions as N originating from mineral N fertilizers(Oenema et al.,2001).For example,Lowrance et al.(1998)reported that in coastal plain soils in Georgia,high rates of liquid manure application produced rates of denitri?cation 10–100times higher than with inorganic N fertilizer.In fact,the manured soils produced N2O at a rate greater than the total denitri?cation rate(N2plus N2O)of the fertilized soils.Higher soil moisture content and dissolved organic C input with the liquid manure may have promoted denitri?cation.Velthoff et al.(2003)found that N2O emissions from manures were higher than from mineral fertilizers when manures were applied to soils with low organic matter,and emissions varied with types of manures and their quality,as affected by manure management and handling.

In North America,recoverable manure(from con?ned animal feeding operations)represents less than6%of the total N(fertilizer, legume,recoverable manure)inputs(Fixen and Johnston,2002). From1990to1996,the USDA-estimated crop acreage receiving manure was as follows:corn17%,soybean6%,winter wheat3%, and cotton4%(Ludwick and Johnston,2002).While locally important,recoverable manure cannot be considered a major nutrient source in North American agriculture.There are many factors preventing greater manure substitution for fertilizer, including synchronization of nutrient release to meet crop demand,spatial coordination and planning at the regional level, and technological advances(Oenema and Pietrzak,2002;Oenema et al.,2001).Transportation and labor costs associated with handling can make manure nutrients less economically attractive than fertilizers.

3.6.

4.Inhibitors and controlled-release fertilizer sources or enhanced-ef?ciency fertilizers

Trenkel(1997),in a review article,cited Shoji and Gandeza (1992)who suggested that an ideal fertilizer should have the following three characteristics,which may be associated with an ability to minimize accumulation of soil NO3-N:

1.Only one application is needed through the entire growing

season to supply the necessary nutrients for optimum growth.

2.Allow a high percentage uptake into the target crop to maximize

yield and return on investment.

3.Have minimum detrimental effects environmentally on the soil,

water and atmosphere portions of the global ecosystem.

Enhanced-ef?ciency fertilizers(slow-and controlled-release fertilizers and stabilized N fertilizers)have been de?ned as products that minimize the potential of nutrient losses to the environment,as compared to‘‘reference soluble’’fertilizers.

There is considerable interest in using new forms of N fertilizer, other than the readily soluble forms presently used(e.g.urea,UAN, ammonium nitrate,ammonium sulfate,etc.),that will be less readily available in the environment and release evenly at a reduced or controlled rate.Weiske(2006)divided these special types of fertilizers into two general categories:

1.Slow-and controlled-release,or encapsulated fertilizers.

2.Fertilizers with nitri?cation and urease inhibitors or stabilized

fertilizers.

Weiske(2006)reported that slow-release and particularly controlled-release as well as stabilized fertilizers can meet these required characteristics to a considerable extent.The delay of initial availability,or extended time of continued availability of slow-and controlled-release fertilizers,might occur through a variety of mechanisms.These include controlled water solubility of the material(by semi-permeable coatings,occlusion,or by inherent water insolubility of polymers,natural nitrogenous organics,protein materials,or other chemical forms),by slow hydrolysis of water-soluble low molecular weight compounds,or by other unknown means(AAPFCO,1995cited in Weiske,2006). Many of the results in the literature indicate that controlled-release fertilizers are useful for the reduction of N2O emission from fertilized https://www.doczj.com/doc/0f17295624.html,e of controlled-release technologies,by affecting the timing of N release from fertilizer(Shaviv,2000),has the potential to reduce leaching losses of NO3à,volatile losses of N as NH3,and N2O emissions.Reductions in these losses may improve N

use ef?ciency and provide greater stability in fertilizer N performance.

Delgado and Mosier(1996)reported that application of a slow-release(polymer-coated urea,PCU)fertilizer resulted in lower initial N2O emissions compared to urea with or without the nitri?cation inhibitor dicyandiamide(DCD),in a study with barley on a clayey soil in Colorado.However,N2O emissions continued 60–80days after fertilization,such that total emissions of N2O from plots treated with the PCU were higher than plots treated with urea alone.This is an area of on-going research(Grant and Wu,2008;Halvorson et al.,2008b;Motavalli et al.,2008).More PCU fertilizers are becoming available to farmers,which may increase the ability to match speci?c PCU release characteristics to crop and soil system requirements.

Maggiato et al.(2000)reported that urea-based(urea,slow-release urea)N fertilizer applied to turfgrass has resulted in lower N2O emissions compared to NH4NO3.Other researchers have shown decreased N2O emissions with slow-or controlled-release fertilizers compared to conventional fertilizers,such as urea,under diverse crops and soils(Motavalli et al.,2008).

These controlled-release fertilizers contrast with soluble fertilizers formulated with inhibitors that reduce or block the conversion of N species by affecting speci?c types of microbes involved.Urease inhibitors prevent,for a certain period of time,the enzymatic hydrolysis of urea,which depends on the enzyme urease(Watson,2005;Rawluk et al.,2001).The action of nitri?cation inhibitors is to block or control conversion of NH4+ to NO2àand subsequently to NO3à.This helps to keep N in the NH4+ form longer,encourage NH4+uptake by crops and prevent N2O emissions from either nitri?cation or denitri?cation.Weiske (2006)summarized studies documenting how the use of different nitri?cation inhibitors can reduce N2O emissions from mineral fertilizers(Table6).He further noted that under German market conditions,the addition of these inhibitors results in higher costs per mass unit of N.The3,4-dimethylpyrazole phosphate(DMPP) nitri?cation inhibitor increases the cost of the fertilizer by about 9%.Application cost may be less because of fewer required?eld operations and thus lower energy and labor requirements when using a nitri?cation inhibitor.

It is important to recognize that a number of the studies cited in Table6were conducted over relatively short time periods. Continuous,daily measurements over long time periods have been recommended by Thornton and Valente(1996),since short-term point estimates are inadequate to accurately detect seasonal or annual N2O emissions.Therefore,it is possible that some of the values cited in Table6may over-estimate seasonal or annual N2O emission reductions.

Shoji et al.(2001)reported that the nitri?cation inhibitor DCD and polyole?n coated urea(POCU/CRF)were capable of reducing N2O emissions by81and35%,when used with urea applied to a barley?eld.In the same study,the average total N fertilizer losses from all potential mechanisms(e.g.NH3volatilization,NO3àleaching,NO and N2O emissions)were15and10%in the DCD and urea treatments,respectively,and only 1.9%in the POCU/CRF treatment.The POCU/CRF showed the highest potential to increase N use ef?ciency.Similar N2O emission patterns were observed with urea and POCU/CRF,with two high emission periods following the basal and top-dressed N applications in corn.Total N2O emission with POCU/CRF was about two-thirds of emissions with urea,while plant recovery of POCU/CRF-N was almost double that of urea N.These results indicate N use ef?ciency may be improved with POCU/CRF and nitri?cation inhibitors(e.g.DCD) and allow possible reductions in the total fertilizer N rate.Wolt (2004)reviewed the effects of the nitri?cation inhibitor–nitrapyrin–in Midwestern U.S.corn production systems and reported a bene?t,in terms of year-long or seasonal inorganic N retention in the root zone,in39out of50studies.The grand mean effect was a28%(?5)increase in root zone inorganic N retention, re?ecting a reduction in loss from the agro-ecosystem.However, these results represent inorganic soil N retention during the crop cycle and they may not necessarily re?ect the long-term fate of seasonally retained N.Bronson et al.(1992)found that N2O emissions were positively correlated with soil NO3àlevels,and that the nitri?cation inhibitors in their study indirectly controlled N2O emissions by preventing NO3àaccumulation in the soil.

Delgado and Mosier(1996)reported that nitri?cation inhibitors can also inhibit CH4oxidation in soils.

3.7.Application management—rate,timing,and placement

3.7.1.N rate impact on N2O emissions from soil

Nitrate-N can accumulate in soils when the N rate exceeds crop demand and the point of crop response,and when crop recovery of the applied N is low(Legg and Meisinger,1982).Use of appropriate N rates can help minimize soil accumulation of NO3-N.Jaynes and Karlen(2005)noted that merely cutting applied N rates to reduce the potential for increases in residual soil NO3-N is not considered an appropriate management action because N rates below the economic optimum could result in‘‘mining’’of SOC and cause a decline in long-term soil productivity.Enhanced and sustained soil productivity can help optimize crop production per hectare,which can reduce overall GHG emission and minimize the land area that must be converted from natural forests,grasslands,and wetlands to meet global food demands.

Table6

Inhibition of N2O emissions after use of different nitri?cation inhibitors(cited in and adapted from Weiske,2006).

Nitri?cation inhibitor or coating Fertilizer Crop N2O reduction

(%)Length of

monitoring

Reference cited in Weiske(2006)

Nitrapyrin Ammonium sulfate Soil only;lab study a9330days Bremner and Blackmer(1978) Nitrapyrin Urea Soil only;lab study a9630days Bremner and Blackmer(1978) Nitrapyrin Urea Corn40–65100days Bronson et al.(1992)

Calcium carbide Urea Corn33–82100days Bronson et al.(1992)

DCD Liquid manure Pasture grass50–8814days De Klein and van Logtestijn(1994) DCD Ammonium sulfate Pasture grass40–9264days Skiba et al.(1993)

DCD Urea Spring barley82–95b90days Delgado and Mosier(1996)

POCU c Urea Spring barley35–71b90days Delgado and Mosier(1996)

DCS d Ammonium sulfate Pasture grass6264days Skiba et al.(1993)

DMPP e Ammonium sulfate

nitrate Spring barley corn

and winter wheat

513years Weiske et al.(2006)

a Conditions set to measure emissions during nitri?cation only.

b Interpretation varies due to relatively large baseline emission.

c POCU=polyole?n coate

d urea.

d DCS=N(2,5-dichlorophenyl)succinic acid monoamide.

e DMPP=3,4-dimethylpyrazole phosphate.

In a3-year study in southwest Michigan with corn comparing split-applied,injected UAN or post-emergence,surface broadcast granular urea at rates from0to292kg N haà1(9rates),N2O emissions were measured biweekly(McSwiney and Robertson, 2005).The authors reported grain yields(5–9t haà1,depending on the year)were maximized with101kg N haà1,which also resulted in moderately low emissions(ca.20g N2O-N/haà1dayà1.At N rates above134kg N haà1,N2O emissions were observed to increase sharply to about450g N haà1dayà1and then decline somewhat once grain yields maximized.The greatest percentage (calculated)of fertilizer N lost as N2O(7%)occurred with 134kg N haà1.The authors found that the percentage of applied N lost as N2O dropped to2–4%with N rates above134kg N haà1. The authors stated,‘‘This threshold N2O response to N fertilization suggests that agricultural N2O?uxes could be reduced with no or little yield penalty by reducing N fertilizer inputs to levels that just

satisfy crop needs.’’The distribution in patterns of N2O emission were governed by soil N availability,and the magnitude of emissions was related to the interaction of soil water content and N availability.

Grant et al.(2006)noted that taken together,the effects of past and current fertilizer use on N2O emissions suggest that emission factors attributed to fertilizer use should rise to the extent applications exceed ecosystem N uptake capacity over time.This capacity could be estimated from pre-planting measurements of residual N,and from annual estimates of gains or losses in soil organic N and of removals in harvest N,much as estimates of N fertilizer requirements are made currently.Applications in excess of uptake capacity would be allocated larger N2O emission factors.

This reference to managing N fertilizer rates to an appropriate amount is challenging because each agro-ecosystem and speci?c growing season will differ as to what is appropriate.While residual soil N estimates have proven useful in some regions,wide ranges in yield response to a given N rate often occur when attempting to calibrate soil nitrate tests or mineralizable N tests(Follett,2001; Dahnke and Johnson,1990;Stanford,1982).Researchers fre-quently observe a scatter or‘‘cloud’’of data points instead of a distinct response curve in calibration efforts because of the large spatial and temporal variability(Meisinger,1984).There are many examples showing that when an agronomic N threshold level is exceeded,N2O emissions can increase dramatically.For example, Malhi et al.(2006)observed that when N rates exceeded 80kg haà1in a speci?c cropping study,N2O emissions increased. Similarly,N2O emissions began to increase signi?cantly compared to unfertilized check treatments with N rates above100kg haà1in an irrigated corn study(Kachanoski et al.,2003;Grant et al.,2006).

In on-farm practical terms,a grower can minimize the potential for N2O emissions by following a nutrient management plan(NMP) which includes soil testing to determine residual NO3àin the soil (where appropriate and calibrated by research),taking into consideration the normal N mineralization potential from SOM for soils in a?eld,and then?lling in the de?cit between the sum of these two N inputs and reasonable crop yields with an appropriate amount of timely and well-placed N fertilizer.

Both the DAYCENT N2O emission model(Del Grosso et al.,2006) and measured data(Bouwman et al.,2002)show a relationship between the amount of N fertilizer added and N2O emissions. Without fertilizer N addition,however,there is a baseline level of N2O emission from N mineralized from SOM(Del Grosso et al., 2006).As fertilizer additions increase,N2O emissions also increase, but the rate of increase may slow at higher rates,as seen in Tennessee corn data(Thornton and Valente,1996).In contrast,N2O emission rates increased linearly with increasing N rates in irrigated cropping system research in Colorado(Halvorson et al., 2008b;Mosier et al.,2006).Generalizing across multiple sites, years,sources,and cropping systems,Bouwman et al.(2002)showed that N2O emissions appear to remain relatively static across a broad range of rates(perhaps near the crop demand levels) and then tend to increase with higher rates(Fig.5).This relationship is in agreement with nonlinear N2O emission response measured by McSwiney and Robertson(2005),but contrasts with the linear emission factor approach assumed by the IPCC(2006) Tier I methodology.

Grant et al.(2006)measured and modeled N2O-N emissions in a temperate humid climate.They reported a nonlinear rise in N2O-N emissions with fertilizer N rate once the rate exceeded crop and soil ecosystem uptake capacities.These results are in agreement with the trendline in Fig.5and results reported by Kachanoski et al. (2003).In contrast,Sehy et al.(2003)found no signi?cant increase in N2O emissions with increasing fertilizer N rates(ranging up to 175kg N haà1yearà1)in high-yielding areas,on a site where corn yields were not responsive to increased N rate.The authors found that site-speci?c application of lower N rates in the low-yielding areas did result in34%less N2O emitted in10months following their differentiated fertilization,which was attributed to differ-ences in soil NO3àcontents in the lower yielding areas of their study site.

3.7.2.N placement

Breitenbeck and Bremner(1986b)found that N2O emissions from soil fertilized with anhydrous NH3(112kg N haà1)injected at a depth of30cm were107and21%greater than injections at depths of10and20cm,respectively.The effect of depth of application of anhydrous NH3on emission of N2O was less when this fertilizer was applied at a rate of225kg N haà1.While it is thought that anhydrous NH3may possibly inhibit nitrifying bacteria and allow the accumulation of nitrite,Venterea and Stanenas(2008)reported no increase in denitri?er enzyme activity or potential N2O emission production rate with increasing soil depth to30cm under aerobic conditions in a recent modeling study using silt loam from a?eld experiment at Rosemount, Minnesota.

Drury et al.(2006)compared N2O emissions following the application of160kg N haà1as ammonium nitrate as a side-dress to corn grown with three tillage treatments.They found low emissions(2.8kg N2O-N haà1yearà1)with fertilizer placed2cm deep,averaged over3years.With placement depth at10cm, however,emissions increased to3.0,3.7and4.8kg N haà1yearà1 for zone-tillage,no-tillage,and moldboard plow tillage,respec-tively.The authors concluded‘‘Zone tillage and shallow N placement consequently appear to be management practices that reduce N2O emissions from corn crops on?ne-textured soils in cool,humid climates.’’Tillage and N placement depth did not affect CO2emissions in this study.Corn yields,however,were4%higher with the deeper placement(7.5t haà1vs.7.2t haà1),and peak levels of soil NO3àwere lower with shallow placement.While

soil Fig.5.Balanced median N2O emission rates as a function of applied N(adapted from Bouwman et al.,2002).

pH data were not provided,it is possible that higher NH3loss with shallow placement provides a partial explanation for these results. In addition,N2O emissions varied more strongly with site-year than with any of the imposed treatments.

When N is applied on the soil surface and not incorporated, particularly in humid environments,a substantial proportion can be lost to the air as NH3,especially with manure or urea as sources. While NH3is not a GHG,its ultimate fate is to be deposited back on the soil elsewhere.It is generally assumed that the proportion emitted as N2O is the same,whether the applied N stays available in the soil for plant uptake or it goes elsewhere as NH3.For this reason,BMPs that reduce NH3volatilization also reduce N2O emission in the same proportion as the amount of N conserved.

Research comparing surface-applied urea to urea placed in a band below and to the side of the seed-row showed that N2O emissions were higher from broadcast compared to band placement,in2years of a3-year study at two sites in Saskatchewan(Hultgreen and Leduc,2003).

3.7.3.N timing

In an intensive wheat production system in the Yaqui Valley near Sonora,Mexico,Matson et al.(1998)evaluated three alternative practices that were based on agronomist recommen-dations.Fertilizer was applied later in the crop cycle,or less fertilizer N,or both,compared to the standard farmer practice of applying250kg N haà1;75%as urea1month before planting,and 25%as anhydrous NH31month after planting.The agronomist-recommended fertilizer BMP involving later applications of N and a reduced N rate(180kg N haà1;33%at planting and67%6weeks after planting)resulted in no reduction in grain yield(6.1t haà1), better economic returns,less residual soil NO3-N,and50%less emission of N2O and NO compared to the standard farmer practice.

In addition to lower N2O emissions observed with band placement of urea near the seed-row compared to surface applications,as noted above in the section on N placement,the study by Hultgreen and Leduc(2003)in Saskatchewan showed

lower N2O emissions from spring compared to fall N fertilizer applications.The closer soluble N fertilizer such as urea can be applied to the time crop N uptake begins,the less potential for losses as N2O emissions.

Zebarth et al.(2008)reported that in a study where no corn grain or silage response was observed to N application(indicating that all N applications were at or in excess of crop N requirement), changing fertilizer application to side-dress and reducing the N rate reduced‘‘nitrate intensity’’,(an index of soil NO3àavailability calculated as the summation of daily soil NO3-N concentration for the0–15cm soil depth).However,they observed no signi?cant effect of N fertility treatment on cumulative N2O emissions,and found that NO3àintensity explained little of the variation in cumulative N2O emissions.This study provides evidence that N rate reductions and split applications may not result in direct reductions of N2O emissions under some conditions.

3.8.Balanced fertilization including other required

nutrients along with N

Research in Kansas by Schlegel et al.(1996)has shown that fertilizer N rates may be increased without causing an accumula-tion of NO3-N by adding P along with N,when P was limiting effective fertilizer N use and limiting yields.Proper P nutrition led to increased yields,improved economic returns,and reduced soil pro?le NO3-N levels(Figs.6and7);thereby reducing the risk for adverse environmental N losses.Johnson et al.(1997)reported that adequate K nutrition also helps improve crop N uptake ef?ciency and NO3-N retention in the upper soil pro?le.High-yield management research in Kansas reported by Gordon(2005)demonstrated that balanced fertilization,using the right rate of other essential nutrients like S in addition to N,P,and K can signi?cantly increase crop apparent recovery of applied N(Fig.8).

Nitrate may accumulate in the soil whenever N is not the factor most limiting crop production.Therefore,all other agronomic factors should be optimized to ensure that applied N in the system is used as ef?ciently as possible,and that it does not accumulate in the soil in forms that are mobile or prone to environmental loss. Balanced fertilization and soil fertility are important major factors under a grower’s control which affect crop yield and nitrogen use ef?ciency(Snyder and Bruulsema,2007;Stewart et al.,2005; Cassman et al.,2002

)

Fig.6.Proper P nutrition improves corn yield and maximizes response to N rate (Schlegel et al.,1996

).

Fig.7.Adequate P nutrition reduces residual soil NO3-N in corn(Schlegel et al., 1996

).

Fig.8.Apparent N recovery by corn using balanced fertilization(assuming25kg of N uptake per tonne of grain,Gordon,2005).

4.Fertilizer BMPs

The foundation of good fertilizer stewardship rests on the principles of using the right source,at the right rate,at the right time,and with the right placement(Roberts,2007).The principal fertilizer BMPs,which can assist in minimizing GHG and NH3 emissions are mentioned below,and include many which were identi?ed by the International Fertilizer Industry Association task force report on reactive N(IFA,2007)and highlighted by Snyder (2008).

4.1.General practices

Choose the N source that best matches the agronomic needs of the speci?c crop and soil system,and which minimizes risks of N loss via all loss pathways,particularly risks of N2O emission. Use appropriate N rates to optimize crop yield and minimize residuals of NO3-N.

Use nutrient management plans that consider soil N supply and the nutrient content of all nutrient sources applied, especially manure.

Account for the N from N-?xing legume crops when deciding N application rates and timing.

Time N applications to coincide,as practically and logistically possible,with crop N uptake demand and to avoid losses to air or water.

Split spring N applications where increased N use ef?ciencies are expected in comparison to single pre-plant or at-planting applications.

Avoid applying N too early or too late,relative to crop uptake demand.

Avoid application of any N sources on wet or waterlogged soils.

High pre-plant rates of NO3-based N sources should be avoided on highly permeable soils,or soils with drainage(tiles,ditches) that may permit rapid transfer of applied NO3àto groundwater or surface water resources.

Use balanced fertilization by supplying all required nutrients to increase crop growth,increase crop N use ef?ciency,and maximize crop capture of CO2.

For drill-seeded rice,immediately wet the soil and establish a continuous?ood(‘‘permanent?ood’’)after application of urea or NH4-based https://www.doczj.com/doc/0f17295624.html,e appropriate management with any midseason N applications to enhance crop N recovery.

Use the right combinations of source,rate,placement,and timing of N to increase the probability of maximizing crop yields and farmer pro?ts,while reducing the net GWP associated with the speci?c local crop and soil system(Roberts,2007;Fixen,2007).

4.1.1.Equipment,proper application,and application technology

Calibrate fertilizer application equipment to ensure accurate delivery of prescribed N rates and proper placement.Avoid application overlaps;if subsurface applied,ensure proper application depth.

Any subsurface placement of N sources should be just deep enough to ensure good soil closure and N retention.Placement of some sources,such as anhydrous ammonia,deeper than10cm has resulted in increased N2O emissions(Breitenbeck and Bremner,1986a).

Where research-proven and properly calibrated,use crop N sensing and variable rate and/or variable N source applications to avoid excessive N applications that may result in increases in residual soil NO3-N and potentially increased risks of N2O emissions.

Incorporate urea and urea-containing N sources(by tillage, irrigation,rainfall)where practical,and use controlled-release

forms or urease inhibitors to minimize NH3loss(volatilization) where it is a manageable risk.

4.1.2.Crop management,system planning,and evaluation

Use continuous,system-level management planning to enhance ef?cient and effective N use by the cropping system or rotation. Intensive management of each crop in the system is essential if N losses from agro-ecosystems are to be minimized.

Conservation practices that complement fertilizer BMPs should also be considered in management plans,for example:

crop rotations matched to speci?c site characteristics,

effective irrigation scheduling and rates to match consumptive water and nutrient demand,

cover crops(where feasible)to retain and recover residual inorganic N,

managed drainage and wetlands to further reduce N2O emissions.

Use consistent,representative sampling and analysis of soils and plant tissues to help identify any nutrient imbalances that may limit crop yields and lead to inef?cient N use.

Where practical,evaluate NO3-N losses in?eld drainage water which may impact groundwater resources or lakes,streams,and downstream areas.Any elevated NO3-N losses identi?ed should result in management adjustments to minimize the potential build-up of residual soil NO3-N and to enhance crop N recovery.

4.1.3.Inhibitors and enhanced-ef?ciency sources

Use urease inhibitors when applying urea-containing N sources, to reduce NH3emissions,especially when surface applied,and to reduce potential transfer of reactive N to unintended resources. Use nitri?cation inhibitors with ammoniacal N sources in environments where there is a high potential for NO3-N leaching losses and/or N2O emissions.For example:

humid,high rainfall(e.g.above60–70cm yearà1)environ-ments,

soils with>60%WFPS within several weeks after fertilizer N application,or sustained throughout much of the year,

when high NH4-N rates are applied outside the period of rapid crop growth and rapid nutrient uptake.

Although preliminary research results appear promising,pub-lished data are limited on N2O and other GHG emissions associated with the use of controlled-release N sources or enhanced-ef?ciency N sources.Enhanced crop recovery of the applied N with these sources has been https://www.doczj.com/doc/0f17295624.html,e of these fertilizers may help minimize GHG emissions and their GWP.

4.1.4.Research and development needs

Future GHG and fertilizer BMP research should include measurement of NH3emissions,NO3-N leaching/runoff/drainage, and effects on long-term soil productivity,SOC storage,and dynamics.Research involving comparisons of different tillage systems and fertilization should include measurement of SOC below the surface,with accounting of potential changes in soil bulk density,to more accurately assess management effects.

Independent,comprehensive agronomic and environmental research–published in public peer-reviewed publications–is needed to evaluate and document the potential reductions in GHG emissions that may be possible with several newer technologies and practices,emerging crops,and cropping systems.

Studies are needed to evaluate the agronomic impact of newer fertilizer forms(e.g.controlled-release sources)and additives that control the rate and timing of N release to better match plant uptake demand,while simultaneously evaluating speci?c N

losses(NO3àleaching/drainage discharge,N2O emissions)and net GWP.

Measurement of environmental N losses should not be restricted to short-term measurement,but extend beyond the period of active crop growth and encompass crop rotations and cropping system changes.

In future experiments,care should be taken in properly planning comparisons among N sources and forms,to consider the potential for confounded interpretations.Practical combinations of sources,placement,and timing are often dictated by the N source.

Crop N sensing and variable rate and/or variable N source applications should be evaluated in large plot or?eld-scale studies,to better gather information about?eld-scale N losses and GHG emissions.

Management of N(rate,source,placement,timing),as well as other nutrients,for biofuel crops should receive immediate research attention.

Decision support tools for N management should be developed that take GHG impacts speci?cally into account.

More investigation is needed in the area of microbiological-biochemical inhibition or catalysis to avoid N2O emission from the soil and possibly enhance its consumption.

5.Conclusions

Considering the context of burgeoning global demand for food,?ber,and fuel,the appropriate strategy to manage GHG emissions must involve ecologically intensive crop management practices (Cassman,1999)that enhance nutrient use ef?ciency while continuing to achieve gains in yield.As was shown by Dobermann et al.(2007),intensi?cation does not necessarily increase the GWP per unit of production of a cropping system.High-yielding crops can mitigate GHG emissions through increased soil C storage, provided they are grown with BMPs.The following factors helped minimize GWP in their irrigated Nebraska cropping system:(1) choice of the right combination of adapted varieties or hybrids, planting date,and plant population to maximize crop biomass production;(2)use of tactical water and N management,including frequent N applications to achieve high N use ef?ciency with minimal opportunity for N2O emissions;and(3)use of residue management approaches that favored a build-up of SOC as a result of large amounts of crop residues returned to the soil.

There are GWP costs associated with the manufacture and use of virtually all fertilizer N sources.For example:

Estimates of total GWP associated with N fertilizer production and transport range from2.6to3.2kg CO2equivalents kgà1of N for anhydrous NH3and urea,but can be much higher for sources containing NO3à,particularly if N2O emission controls are not in place at the production facility.

The default coef?cient(1%of N applied;IPCC,2006)for fertilizer-induced N2O emission from soil equates to a GWP of 4.65kg CO2kgà1of N applied.In some drier climates,it is possible to justify a lower emission factor using local data.In more humid climates where NH3volatilization and NO3àleaching may result in N transfer to non-agricultural soils and water resources,the estimated N2O emission associated with fertilizer N may equate to a GWP several times higher.

Fertilizer N supports primary plant productivity and stabiliza-tion of SOM,upon which soil C storage https://www.doczj.com/doc/0f17295624.html,binations of source,rate,timing,and placement that optimize crop yields minimize the GWP of emissions per unit of production and reduce the need for conversion of natural lands to agriculture.

For many fertilizer BMPs,little is known about their measured impact on GHG emissions.One would expect with improved crop recovery,reduced NH3loss,and reduced NO3-N leaching,there would be considerable potential to reduce fertilizer-associated GHG emissions and GWP.Research is underway in many areas that will likely result in improved con?dence in the effectiveness of certain practices and newer products in reducing GHG emissions associated with fertilizer N use.In spite of these current data limitations,there are many practical opportunities to improve nutrient use ef?ciency and effectiveness through better manage-ment and newer technologies.

Acknowledgements

The authors thank Don Armstrong and Katherine Grif?n for their editorial assistance.

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50万吨年煤气化生产工艺

咸阳职业技术学院生化工程系毕业论文(设计) 50wt/年煤气化工艺设计 1.引言 煤是由古代植物转变而来的大分子有机化合物。我国煤炭储量丰富,分布面广,品种齐全。据中国第二次煤田预测资料,埋深在1000m以浅的煤炭总资源量为2.6万亿t。其中大别山—秦岭—昆仑山一线以北地区资源量约2.45万亿t,占全国总资源量的94%;其余的广大地区仅占6%左右。其中新疆、内蒙古、山西和陕西等四省区占全国资源总量的81.3%,东北三省占 1.6%,华东七省占2.8%,江南九省占1.6%。 煤气化是煤炭的一个热化学加工过程,它是以煤或煤焦原料,以氧气(空气或富氧)、水蒸气或氢气等作气化剂,在高温条件下通过化学反应将煤或煤焦中的可燃部分转化为可燃性的气体的过程。气化时所得的可燃性气体称为煤气,所用的设备称为煤气发生炉。 煤气化技术开发较早,在20世纪20年代,世界上就有了常压固定层煤气发生炉。20世纪30年代至50年代,用于煤气化的加压固定床鲁奇炉、常压温克勒沸腾炉和常压气流床K-T炉先后实现了工业化,这批煤气化炉型一般称为第一代煤气化技术。第二代煤气化技术开发始于20世纪60年代,由于当时国际上石油和天然气资源开采及利用于制取合成气技术进步很快,大大降低了制造合成

气的投资和生产成本,导致世界上制取合成气的原料转向了天然气和石油为主,使煤气化新技术开发的进程受阻,20世纪70年代全球出现石油危机后,又促进了煤气化新技术开发工作的进程,到20世纪80年代,开发的煤气化新技术,有的实现了工业化,有的完成了示范厂的试验,具有代表性的炉型有德士古加压水煤浆气化炉、熔渣鲁奇炉、高温温克勒炉(ETIW)及干粉煤加压气化炉等。 近年来国外煤气化技术的开发和发展,有倾向于以煤粉和水煤浆为原料、以高温高压操作的气流床和流化床炉型为主的趋势。 2.煤气化过程 2.1煤气化的定义 煤与氧气或(富氧空气)发生不完全燃烧反应,生成一氧化碳和氢气的过程称为煤气化。煤气化按气化剂可分为水蒸气气化、空气(富氧空气)气化、空气—水蒸气气化和氢气气化;按操作压力分为:常压气化和加压气化。由于加压气化具有生产强度高,对燃气输配和后续化学加工具有明显的经济性等优点。所以近代气化技术十分注重加压气化技术的开发。目前,将气化压力在P>2MPa 情况下的气化,统称为加压气化技术;按残渣排出形式可分为固态排渣和液态排渣。气化残渣以固体形态排出气化炉外的称固态排渣。气化残渣以液态方式排出经急冷后变成熔渣排出气化炉外的称液态排渣;按加热方式、原料粒度、汽化程度等还有多种分类方法。常用的是按气化炉内煤料与气化剂的接触方式区分,主要有固定床气化、流化床气化、气流床气化和熔浴床床气化。 2.2 主要反应 煤的气化包括煤的热解和煤的气化反应两部分。煤在加热时会发生一系列的物理变化和化学变化。气化炉中的气化反应,是一个十分复杂的体系,这里所讨论的气化反应主要是指煤中的碳与气化剂中的氧气、水蒸汽和氢气的反应,也包括碳与反应产物之间进行的反应。 习惯上将气化反应分为三种类型:碳—氧之间的反应、水蒸汽分解反应和甲烷生产反应。 2.2.1碳—氧间的反应 碳与氧之间的反应有: C+O2=CO2(1)

煤气化工艺流程

煤气化工艺流程 1、主要产品生产工艺煤气化是以煤炭为主要原料的综合性大型化工企业,主要工艺围绕着煤的洁净气化、综合利用,形成了以城市煤气为主线联产甲醇的工艺主线。 主要产品城市煤气和甲醇。城市燃气是城市公用事业的一项重要基础设施,是城市现代化的重要标志之一,用煤气代替煤炭是提高燃料热能利用率,减少煤烟型大气污染,改善大气质量行之有效的方法之一,同时也方便群众生活,节约时间,提高整个城市的社会效率和经济效益。作为一项环保工程,(其一期工程)每年还可减少向大气排放烟尘万吨、二氧化硫万吨、一氧化碳万吨,对改善河南西部地区城市大气质量将起到重要作用。 甲醇是一种重要的基本有机化工原料,除用作溶剂外,还可用于制造甲醛、醋酸、氯甲烷、甲胺、硫酸二甲酯、对苯二甲酸二甲酯、丙烯酸甲酯等一系列有机化工产品,此外,还可掺入汽油或代替汽油作为动力燃料,或进一步合成汽油,在燃料方面的应用,甲醇是一种易燃液体,燃烧性能良好,抗爆性能好,被称为新一代燃料。甲醇掺烧汽油,在国外一般向汽油中掺混甲醇5?15勉高汽油的辛烷值,避免了添加四乙基酮对大气的污染。 河南省煤气(集团)有限责任公司义马气化厂围绕义马至洛阳、洛阳至郑州煤气管线及豫西地区工业及居民用气需求输出清洁能源,对循环经济建设,把煤化工打造成河南省支柱产业起到重要作用。 2、工艺总流程简介: 原煤经破碎、筛分后,将其中5?50mm级块煤送入鲁奇加压气化炉,在炉内与氧气和水蒸气反应生成粗煤气,粗煤气经冷却后,进入低温甲醇洗净化装置,除去煤气中的CO2和H2S净化后的煤气分为两大部分,一部分去甲醇合成系统,合成气再经压缩机加压至,进入甲醇反应器生成粗甲醇,粗甲醇再送入甲醇精馏系统,制得精甲醇产品存入贮罐;另一部分去净煤气变换装置。合成甲醇尾气及变换气混合后,与剩余部分出低温甲醇洗净煤气混合后,进入煤气冷却干燥装置,将露点降至-25 C后,作为合格城市煤气经长输管线送往各用气城市。生产过程中产生的煤气水进入煤气水分离装置,分离出其中的焦油、中油。分离后煤气水去酚回收和氨回收,回收酚氨后的煤气水经污水生化处理装置处理,达标后排放。低温甲醇洗净化装置排出的H2S到硫回收装置回收硫。空分

煤化工产业概况及其发展趋势

煤化工产业概况及其发 展趋势 集团标准化办公室:[VV986T-J682P28-JP266L8-68PNN]

我国煤化工产业概况及其发展趋势 煤化学加工包括煤的焦化、气化和液化。主要用于冶金行业的煤炭焦化和用于制取合成氨的煤炭气化是传统的煤化工产业,随着社会经济的不断发展,它们将进一步得到发展,同时以获得洁净能源为主要目的的煤炭液化、煤基代用液体燃料、煤气化—发电等煤化工或煤化工能源技术也越来越引起关注,并将成为新型煤化工产业化发展的主要方向。发展新型煤化工产业对煤炭行业产业结构的调整及其综合发展具有重要意义。 1 煤化工产业发展概况 1. 1 煤炭焦化 焦化工业是发展最成熟,最具代表性的煤化工产业,也是冶金工业高炉炼铁、机械工业铸造最主要的辅助产业。目前,全世界的焦炭产量大约为~亿t/a,直接消耗原料精煤约亿t/a 。受世界钢铁产量调整、高炉喷吹技术发展、环境保护以及生产成本增高等原因影响,工业发达国家的机械化炼焦能力处于收缩状态,焦炭国际贸易目前为2500万t/ a。 目前,我国焦炭产量约亿t/a,居世界第一,直接消耗原料煤占全国煤炭消费总量的14%。 全国有各类机械化焦炉约750座以上,年设计炼焦能力约9000万 t/a,其中炭化室高度为4m~5.5m以上的大、中型焦炉产量约占80%。中国大容积焦炉(炭化室高≧6m)已实现国产化,煤气净化技术已达世界先进水平,干熄焦、地面烟尘处理站、污水处理等已进入实用化阶段,焦炭质量显着提高,其主要化工产品的精制技术已达到或接近世界先进水平。 焦炭成为我国的主要出口产品之一,出口量逐年上升,2000年达到1500t/a,已成为全球最大的焦炭出口国。 从20世纪80年代起,煤炭行业的炼焦生产得到逐步发展,其中有的建成向城市或矿区输送人工煤气为主要目的的工厂,有的以焦炭为主要产品。煤炭行业焦化生产普遍存在的问题是:焦炉炉型小、以中小型焦炉为主,受矿区产煤品种限制、焦炭质量调整提高难度较大,采用干法熄焦、烟尘集中处理等新技术少,大多数企业技术进步及现代化管理与其他行业同类工厂相比有较大差距。 1.2 煤气化及其合成技术 1.2.1 煤气化 煤气化技术是煤化工产业化发展最重要的单元技术。全世界现有商业化运行的大规模气化炉414台,额定产气量446×106Nm3/d,前10名的气化厂使用鲁奇、德士古、壳牌3种炉型,原料是煤、渣油、天然气,产品是F-T合成油、电或甲醇等。 煤气化技术在我国被广泛应用于化工、冶金、机械、建材等工业行业和生产城市煤气的企业,各种气化炉大约有9000多台,其中以固定床气化炉为主。近20年来,我国引进的加压鲁奇炉、德士古水煤浆气化炉,主要用于生产合成氨、甲醇或城市煤气。

煤气化技术的现状及发展趋势分析

煤气化技术是现代煤化工的基础,是通过煤直接液化制取油品或在高温下气化制得合成气,再以合成气为原料制取甲醇、合成油、天然气等一级产品及以甲醇为原料制得乙烯、丙烯等二级化工产品的核心技术。作为煤化工产业链中的“龙头”装置,煤气化装置具有投入大、可靠性要求高、对整个产业链经济效益影响大等特点。目前国内外气化技术众多,各种技术都有其特点和特定的适用场合,它们的工业化应用程度及可靠性不同,选择与煤种及下游产品相适宜的煤气化工艺技术是煤化工产业发展中的重要决策。 工业上以煤为原料生产合成气的历史已有百余年。根据发展进程分析,煤气化技术可分为三代。第一代气化技术为固定床、移动床气化技术,多以块煤和小颗粒煤为原料制取合成气,装置规模、原料、能耗及环保的局限性较大;第二代气化技术是现阶段最具有代表性的改进型流化床和气流床技术,其特征是连续进料及高温液态排渣;第三代气化技术尚处于小试或中试阶段,如煤的催化气化、煤的加氢气化、煤的地下气化、煤的等离子体气化、煤的太阳能气化和煤的核能余热气化等。 本文综述了近年来国内外煤气化技术开发及应用的进展情况,论述了固定床、流化床、气流床及煤催化气化等煤气化技术的现状及发展趋势。 1.国内外煤气化技术的发展现状 在世界能源储量中,煤炭约占79%,石油与天然气约占12%。煤炭利用技术的研究和开发是能源战略的重要内容之一。世界煤化工的发展经历了起步阶段、发展阶段、停滞阶段和复兴阶段。20世纪初,煤炭炼焦工业的兴起标志着世界煤化工发展的起步。此后世界煤化工迅速发展,直到20世纪中叶,煤一直是世界有机化学工业的主要原料。随着石油化学工业的兴起与发展,煤在化工原料中所占的比例不断下降并逐渐被石油和天然气替代,世界煤化工技术及产业的发展一度停滞。直到20世纪70年代末,由于石油价格大幅攀升,影响了世界石油化学工业的发展,同时煤化工在煤气化、煤液化等方面取得了显著的进展。特别是20世纪90年代后,世界石油价格长期在高位运行,且呈现不断上升趋势,这就更加促进了煤化工技术的发展,煤化工重新受到了人们的重视。 中国的煤气化工艺由老式的UGI炉块煤间歇气化迅速向世界最先进的粉煤加压气化工艺过渡,同时国内自主创新的新型煤气化技术也得到快速发展。据初步统计,采用国内外先进大型洁净煤气化技术已投产和正在建设的装置有80多套,50%以上的煤气化装置已投产运行,其中采用水煤浆气化技术的装置包括GE煤气化27套(已投产16套),四喷嘴33套(已投产13套),分级气化、多元料浆气化等多套;采用干煤粉气化技术的装置包括Shell煤气化18套(已投产11套)、GSP2套,还有正在工业化示范的LurgiBGL技术、航天粉煤加压气化(HT-L)技术、单喷嘴干粉气化技术和两段式干煤粉加压气化(TPRI)技术等。

煤气化工艺流程

精心整理 煤气化工艺流程 1、主要产品生产工艺 煤气化是以煤炭为主要原料的综合性大型化工企业,主要工艺围绕着煤的洁净气化、综合利用,形成了以城市煤气为主线联产甲醇的工艺主线。 主要产品城市煤气和甲醇。城市燃气是城市公用事业的一项重要基础设施,是城市现代化的重要标志之一,用煤气代替煤炭是提高燃料热能利用率,减少煤烟型大气污染,改善大气质量行之 化碳 15%提 作用。 2 。净化 装置。合成甲醇尾气及变换气混合后,与剩余部分出低温甲醇洗净煤气混合后,进入煤气冷却干燥装置,将露点降至-25℃后,作为合格城市煤气经长输管线送往各用气城市。生产过程中产生的煤气水进入煤气水分离装置,分离出其中的焦油、中油。分离后煤气水去酚回收和氨回收,回收酚氨后的煤气水经污水生化处理装置处理,达标后排放。低温甲醇洗净化装置排出的H2S到硫回收装置回收硫。空分装置提供气化用氧气和全厂公用氮气。仪表空压站为全厂仪表提供合格的仪表空气。 小于5mm粉煤,作为锅炉燃料,送至锅炉装置生产蒸汽,产出的蒸汽一部分供工艺装置用汽

,一部分供发电站发电。 3、主要装置工艺流程 3.1备煤装置工艺流程简述 备煤工艺流程分为三个系统: (1)原煤破碎筛分贮存系统,汽运原煤至受煤坑经1#、2#、3#皮带转载至筛分楼、经节肢筛、破碎机、驰张筛加工后,6~50mm块煤由7#皮带运至块煤仓,小于6mm末煤经6#、11#皮带近至末煤仓。 缓 可 能周期性地加至气化炉中。 当煤锁法兰温度超过350℃时,气化炉将联锁停车,这种情况仅发生在供煤短缺时。在供煤短缺时,气化炉应在煤锁法兰温度到停车温度之前手动停车。 气化炉:鲁奇加压气化炉可归入移动床气化炉,并配有旋转炉篦排灰装置。气化炉为双层压力容器,内表层为水夹套,外表面为承压壁,在正常情况下,外表面设计压力为3600KPa(g),内夹套与气化炉之间压差只有50KPa(g)。 在正常操作下,中压锅炉给水冷却气化炉壁,并产生中压饱和蒸汽经夹套蒸汽气液分离器1

煤化工工艺流程

煤化工工艺流程 典型的焦化厂一般有备煤车间、炼焦车间、回收车间、焦油加工车间、苯加工车间、脱硫车间和废水处理车间等。 焦化厂生产工艺流程 1.备煤与洗煤 原煤一般含有较高的灰分和硫分,洗选加工的目的是降低煤的灰分,使混杂在煤中的矸石、煤矸共生的夹矸煤与煤炭按照其相对密度、外形及物理性状方面的差异加以分离,同时,降低原煤中的无机硫含量,以满足不同用户对煤炭质量的指标要求。 由于洗煤厂动力设备繁多,控制过程复杂,用分散型控制系统DCS改造传统洗煤工艺,这对于提高洗煤过程的自动化,减轻工人的劳动强度,提高产品产量和质量以及安全生产都具有重要意义。

洗煤厂工艺流程图 控制方案 洗煤厂电机顺序启动/停止控制流程框图 联锁/解锁方案:在运行解锁状态下,允许对每台设备进行单独启动或停止;当设置为联锁状态时,按下启动按纽,设备顺序启动,后一设备的启动以前一设备的启动为条件(设备间的延时启动时间可设置),如果前一设备未启动成功,后一设备不能启动,按停止键,则设备顺序停止,在运行过程中,如果其中一台设备故障停止,例如设备2停止,则系统会把设备3和设备4停止,但设备1保持运行。

2.焦炉与冷鼓 以100万吨/年-144孔-双炉-4集气管-1个大回流炼焦装置为例,其工艺流程简介如下:

100万吨/年焦炉_冷鼓工艺流程图 控制方案 典型的炼焦过程可分为焦炉和冷鼓两个工段。这两个工段既有分工又相互联系,两者在地理位置上也距离较远,为了避免仪表的长距离走线,设置一个冷鼓远程站及给水远程站,以使仪表线能现场就近进入DCS控制柜,更重要的是,在集气管压力调节中,两个站之间有着重要的联锁及其排队关系,这样的网络结构形式便于可以实现复杂的控制算法。

煤气化工艺流程

煤气化工艺流程 1、主要产品生产工艺 煤气化是以煤炭为主要原料的综合性大型化工企业,主要工艺围绕着煤的洁净气化、综合利用,形成了以城市煤气为主线联产甲醇的工艺主线。 主要产品城市煤气和甲醇。城市燃气是城市公用事业的一项重要基础设施,是城市现代化的重要标志之一,用煤气代替煤炭是提高燃料热能利用率,减少煤烟型大气污染,改善大气质量行之有效的方法之一,同时也方便群众生活,节约时间,提高整个城市的社会效率和经济效益。作为一项环保工程,(其一期工程)每年还可减少向大气排放烟尘1.86万吨、二氧化硫3.05万吨、一氧化碳0.46万吨,对改善河南西部地区城市大气质量将起到重要作用。 甲醇是一种重要的基本有机化工原料,除用作溶剂外,还可用于制造甲醛、醋酸、氯甲烷、甲胺、硫酸二甲酯、对苯二甲酸二甲酯、丙烯酸甲酯等一系列有机化工产品,此外,还可掺入汽油或代替汽油作为动力燃料,或进一步合成汽油,在燃料方面的应用,甲醇是一种易燃液体,燃烧性能良好,抗爆性能好,被称为新一代燃料。甲醇掺烧汽油,在国外一般向汽油中掺混甲醇5~15%提高汽油的辛烷值,避免了添加四乙基酮对大气的污染。 河南省煤气(集团)有限责任公司义马气化厂围绕义马至洛阳、洛阳至郑州煤气管线及豫西地区工业及居民用气需求输出清洁能源,对循环经济建设,把煤化工打造成河南省支柱产业起到重要作用。 2、工艺总流程简介: 原煤经破碎、筛分后,将其中5~50mm级块煤送入鲁奇加压气化炉,在炉内与氧气和水蒸气反应生成粗煤气,粗煤气经冷却后,进入低温甲醇洗净化装置

,除去煤气中的CO2和H2S。净化后的煤气分为两大部分,一部分去甲醇合成系统,合成气再经压缩机加压至5.3MPa,进入甲醇反应器生成粗甲醇,粗甲醇再送入甲醇精馏系统,制得精甲醇产品存入贮罐;另一部分去净煤气变换装置。合成甲醇尾气及变换气混合后,与剩余部分出低温甲醇洗净煤气混合后,进入煤气冷却干燥装置,将露点降至-25℃后,作为合格城市煤气经长输管线送往各用气城市。生产过程中产生的煤气水进入煤气水分离装置,分离出其中的焦油、中油。分离后煤气水去酚回收和氨回收,回收酚氨后的煤气水经污水生化处理装置处理,达标后排放。低温甲醇洗净化装置排出的H2S到硫回收装置回收硫。空分装置提供气化用氧气和全厂公用氮气。仪表空压站为全厂仪表提供合格的仪表空气。 小于5mm粉煤,作为锅炉燃料,送至锅炉装置生产蒸汽,产出的蒸汽一部分供工艺装置用汽,一部分供发电站发电。 3、主要装置工艺流程 3.1备煤装置工艺流程简述 备煤工艺流程分为三个系统: (1)原煤破碎筛分贮存系统,汽运原煤至受煤坑经1#、2#、3#皮带转载至筛分楼、经节肢筛、破碎机、驰张筛加工后,6~50mm块煤由7#皮带运至块煤仓,小于6mm末煤经6#、11#皮带近至末煤仓。 (2)最终筛分系统:块煤仓内块煤经8#、9#皮带运至最终筛分楼驰张筛进行检查性筛分。大于6mm块煤经10#皮带送至200#煤斗,筛下小于6mm末煤经14#皮带送至缓冲仓。 (3)电厂上煤系统:末煤仓内末煤经12#、13#皮带转至5#点后经16#皮

(能源化工行业)我国煤化工产业概况及其发展方向

(能源化工行业)我国煤化工产业概况及其发展方向

我国煤化工产业概况及其发展趋势 煤化学加工包括煤的焦化、气化和液化。主要用于冶金行业的煤炭焦化和用于制取合成氨的煤炭气化是传统的煤化工产业,随着社会经济的不断发展,它们将进壹步得到发展,同时以获得洁净能源为主要目的的煤炭液化、煤基代用液体燃料、煤气化—发电等煤化工或煤化工能源技术也越来越引起关注,且将成为新型煤化工产业化发展的主要方向。发展新型煤化工产业对煤炭行业产业结构的调整及其综合发展具有重要意义。 1煤化工产业发展概况 1.1煤炭焦化 焦化工业是发展最成熟,最具代表性的煤化工产业,也是冶金工业高炉炼铁、机械工业铸造最主要的辅助产业。目前,全世界的焦炭产量大约为3.2~3.4亿t/a,直接消耗原料精煤约4.5亿t/a。受世界钢铁产量调整、高炉喷吹技术发展、环境保护以及生产成本增高等原因影响,工业发达国家的机械化炼焦能力处于收缩状态,焦炭国际贸易目前为2500万t/a。 目前,我国焦炭产量约1.2亿t/a,居世界第壹,直接消耗原料煤占全国煤炭消费总量的14%。全国有各类机械化焦炉约750座之上,年设计炼焦能力约9000万t/a,其中炭化室高度为4m~5.5m之上的大、中型焦炉产量约占80%。中国大容积焦炉(炭化室高≧6m)已实现国产化,煤气净化技术已达世界先进水平,干熄焦、地面烟尘处理站、污水处理等已进入实用化阶段,焦炭质量显著提高,其主要化工产品的精制技术已达到或接近世界先进水平。 焦炭成为我国的主要出口产品之壹,出口量逐年上升,2000年达到1500t/a,已成为全球最大的焦炭出口国。 从20世纪80年代起,煤炭行业的炼焦生产得到逐步发展,其中有的建成向城市或矿区输送人工煤气为主要目的的工厂,有的以焦炭为主要产品。煤炭行业焦化生产普遍存在的问题是:焦炉炉型小、以中小型焦炉为主,受矿区产煤品种限制、焦炭质量调整提高难度较大,采用干法熄焦、烟尘集中处理等新技术少,大多数企业技术进步及现代化管理和其他行业同类工厂相比有较大差距。 1.2煤气化及其合成技术 1.2.1煤气化 煤气化技术是煤化工产业化发展最重要的单元技术。全世界现有商业化运行的大规模气化炉414台,额定产气量446×106Nm3/d,前10名的气化厂使用鲁奇、德士古、壳牌3种炉型,原料是煤、渣油、天然气,产品是F-T合成油、电或甲醇等。 煤气化技术在我国被广泛应用于化工、冶金、机械、建材等工业行业和生产城市煤气的企业,各种气化炉大约有9000多台,其中以固定床气化炉为主。近20年来,我国引进的加压鲁奇炉、德士古水煤浆气化炉,主要用于生产合成氨、甲醇或城市煤气。 煤气化技术的发展和作用引起国内煤炭行业的关注。“九五”期间,兖矿集团和国内高校、科研机构合作,开发完成了22t/d多喷嘴水煤浆气化炉中试装置,且进行了考核试验。 结果表明:有效气体成分达83%,碳转化率>98%,分别比相同条件下的德士古生产装置高1.5%~2%、2%~3%;比煤耗、比氧耗均低于德士古7%。该成果标志我国自主开发的先进气化技术取得突破性进展。 1.2.2煤气化合成氨 以煤为原料、采用煤气化—合成氨技术是我国化肥生产的主要方式,目前我国有800多家中小型化肥厂采用水煤气工艺,共计约4000台气化炉,每年消费原料煤(或焦炭)4000多万t,合成氨产量约占全国产量的60%。化肥用气化炉的炉型以UGI型和前苏联的Д型为主,直径由2.2m至3.6m不等,该类炉型老化、技术落后。加压鲁奇炉、德士古炉是近年来引进用于合成氨生产的主要炉型。

煤气化制甲醇工艺流程

煤气化制甲醇工艺流程 1 煤制甲醇工艺 气化 a)煤浆制备 由煤运系统送来的原料煤干基(<25mm)或焦送至煤贮斗,经称重给料机控制输送量送入棒磨机,加入一定量的水,物料在棒磨机中进行湿法磨煤。为了控制煤浆粘度及保持煤浆的稳定性加入添加剂,为了调整煤浆的PH值,加入碱液。出棒磨机的煤浆浓度约65%,排入磨煤机出口槽,经出口槽泵加压后送至气化工段煤浆槽。煤浆制备首先要将煤焦磨细,再制备成约65%的煤浆。磨煤采用湿法,可防止粉尘飞扬,环境好。用于煤浆气化的磨机现在有两种,棒磨机与球磨机;棒磨机与球磨机相比,棒磨机磨出的煤浆粒度均匀,筛下物少。煤浆制备能力需和气化炉相匹配,本项目拟选用三台棒磨机,单台磨机处理干煤量43~ 53t/h,可满足60万t/a甲醇的需要。 为了降低煤浆粘度,使煤浆具有良好的流动性,需加入添加剂,初步选择木质磺酸类添加剂。 煤浆气化需调整浆的PH值在6~8,可用稀氨水或碱液,稀氨水易挥发出氨,氨气对人体有害,污染空气,故本项目拟采用碱液调整煤浆的PH值,碱液初步采用42%的浓度。 为了节约水源,净化排出的含少量甲醇的废水及甲醇精馏废水均可作为磨浆水。 b)气化 在本工段,煤浆与氧进行部分氧化反应制得粗合成气。 煤浆由煤浆槽经煤浆加压泵加压后连同空分送来的高压氧通过烧咀进入气化炉,在气化炉中煤浆与氧发生如下主要反应: CmHnSr+m/2O2—→mCO+(n/2-r)H2+rH2S CO+H2O—→H2+CO2 反应在6.5MPa(G)、1350~1400℃下进行。 气化反应在气化炉反应段瞬间完成,生成CO、H2、CO2、H2O和少量CH4、H2S等气体。 离开气化炉反应段的热气体和熔渣进入激冷室水浴,被水淬冷后温度降低并被水蒸汽饱和后出气化炉;气体经文丘里洗涤器、碳洗塔洗涤除尘冷却后送至变换工段。 气化炉反应中生成的熔渣进入激冷室水浴后被分离出来,排入锁斗,定时排入渣池,由扒渣机捞出后装车外运。 气化炉及碳洗塔等排出的洗涤水(称为黑水)送往灰水处理。 c)灰水处理 本工段将气化来的黑水进行渣水分离,处理后的水循环使用。 从气化炉和碳洗塔排出的高温黑水分别进入各自的高压闪蒸器,经高压闪蒸浓缩后的黑水混合,经低压、两级真空闪蒸被浓缩后进入澄清槽,水中加入絮凝剂使其加速沉淀。澄清槽底部的细渣浆经泵抽出送往过滤机给料槽,经由过滤机给料泵加压后送至真空过滤机脱水,渣饼由汽车拉出厂外。 闪蒸出的高压气体经过灰水加热器回收热量之后,通过气液分离器分离掉冷凝液,然后进入变换工段汽提塔。 闪蒸出的低压气体直接送至洗涤塔给料槽,澄清槽上部清水溢流至灰水槽,由灰水泵分别送至洗涤塔给料槽、气化锁斗、磨煤水槽,少量灰水作为废水排往废水处理。 洗涤塔给料槽的水经给料泵加压后与高压闪蒸器排出的高温气体换热后送碳洗塔循环

国内煤气化技术评述与展望

2012年 第15期 广 东 化 工 第39卷 总第239期 https://www.doczj.com/doc/0f17295624.html, · 59 · 国内煤气化技术评述与展望 付长亮 (河南化工职业学院,河南 郑州 450042) [摘 要]依据煤气化技术的常用分类标准和评价指标,分析研究了国内所用的煤气化技术的优势与不足。综合考虑原料广泛性、技术先进性、投资成本等因素,认为航天炉干粉煤气化技术具有适应的煤种多、气化效率高、生产能力大、碳转化率高、投资省、操作费用低等优势,在未来的煤化工产品生产中将会得到普遍的应用。 [关键词]煤气化技术;评述;展望 [中图分类号]TQ [文献标识码]A [文章编号]1007-1865(2012)15-0059-02 Review and Prospects of Domestic Coal Gasification Technology Fu Changliang (Henan V ocational College of Chemical Technology, Zhenzhou 450042, China) Abstract: According to common classification standard and evaluation index, advantages and disadvantages of domestic coal gasification technology were analyzed and studied. Considering comprehensively the raw material extensive, technology advanced and investment cost, it was thought that HT-L dry powder coal gasification had the vast potential for future development, because of the more quantity of coal type used, higher gasification efficiency, larger production capacity, higher carbon conversion, lower investment cost. Keywords: coal gasification technology ;review ;prospects 1 煤气化及其评价指标 煤气化指在高温下煤和气化剂作用生成煤气的过程。可简单表示如下: +???→高温 煤气化剂煤气 其中的气化剂主要指空气、纯氧和水蒸汽。煤气化所制得的煤气是一种可燃性气体,主要成分为CO 、H 2、CO 2和CH 4,可作为清洁能源和多种化工产品的原料。因此,煤气化技术在煤化工中处于非常重要的地位。 煤气化反应主要在气化炉(或称煤气发生炉、煤气炉)内进行。不同的煤气化技术主要区别在于所用的气化炉的形式不同。 通常,对煤气化技术的评价主要从气化效率、冷煤气效率、碳转化率和有效气体产率四个方面进行。气化效率衡量原料(煤和气化剂)的热值转化为可利用热量(煤气的热值和产生蒸汽的热值)的情况,是最常用的评价指标,标志着煤气化技术的能耗高低。冷煤气效率衡量原料的热值转化为煤气热值的情况,是制得煤气量多少及质量高低的标志。碳转化率衡量煤中有多少碳转化进入到煤气中,是煤利用率高低的标志。有效气体产率衡指单位煤耗能产出多少有效气体(CO+H 2),是对煤气化技术生产有价值成分效果好坏的评价。这四个指标不完全独立,从不同的方面反映了煤气化技术中人们最关注的问题。 2 煤气化技术的分类 煤气化的分类方法较多,但最常用的分类方法是按煤与气化剂在气化炉内运动状态来分。此法,将煤气化技术分为如下几种。 2.1 固定床气化 固定床气化也称移动床气化,一般以块煤或煤焦为原料。煤由气化炉顶加入,气化剂由炉底送入。流动气体的上升力不致使固体颗粒的相对位置发生变化,即固体颗粒处于相对固定状态。气化炉内各反应层高度亦基本上维持不变。因而称为固定床气化。另外,从宏观角度看,由于煤从炉顶加入,含有残炭的灰渣自炉底排出,气化过程中,煤粒在气化炉内逐渐并缓慢往下移动,因而又称为移动床气化。目前,国内采用此方法的煤气化技术主要有固定床间歇气化法和加压鲁奇气化法。 2.2 流化床气化 流化床煤气化法以小颗粒煤为气化原料,这些细粒煤在自下而上的气化剂的作用下,保持着连续不断和无秩序的沸腾和悬浮状态运动,迅速地进行着混和和热交换,其结果导致整个床层温度和组成的均一。目前,国内属于此方法的煤气化技术主要有恩德粉煤气化技术和ICC 灰融聚气化法。 2.3 气流床气化 气流床气化是一种并流式气化。气化剂(氧与蒸汽)与煤粉一同进入气化炉,在1500~1900 ℃高温下,将煤部分氧化成CO 、H 2、CO 2等气体,残渣以熔渣形式排出气化炉。也可将煤粉制成 煤浆,用泵送入气化炉。在气化炉内,煤炭细粉粒与气化剂经特殊喷嘴进入反应室,会在瞬间着火,发生火焰反应,同时处于不充分的氧化条件下。因此,其热解、燃烧以及吸热的气化反应,几乎是同时发生的。随气流的运动,未反应的气化剂、热解挥发物及燃烧产物裹挟着煤焦粒子高速运动,运动过程中进行着煤焦颗粒的气化反应。这种运动形态,相当于流态化技术领域里对固体颗粒的“气流输送”,习惯上称为气流床气化。属于此类方法的煤气化技术较多,国内主要有壳牌干粉煤气化法、德士古水煤浆气化法、GSP 干粉煤气化法、航天炉干粉煤气化等[1-3]。 3 国内主要煤气化技术评述 3.1 固定床间歇式气化 块状无烟煤或焦炭在气化炉内形成固定床。在常压下,空气和水蒸汽交替通过气化炉。通空气时,产生吹风气,主要为了积累能量,提高炉温。通水蒸汽时,利用吹风阶段积累的能量,生产水煤气。空气煤气和水煤气以适当比例混合,制得合格原料气。 该技术是20世纪30年代开发成功的。优点为投资少、操作简单。缺点为气化效率低、对原料要求高、能耗高、单炉生产能力小。间歇制气过程中,大量吹风气排空。每吨合成氨吹风气放空多达5000 m 3。放空气体中含CO 、CO 2、H 2、H 2S 、SO 2、NO x 及粉灰。煤气冷却洗涤塔排出的污水含有焦油、酚类及氰化物,对环境污染严重。我国中小化肥厂有900余家,多数采用该技术生产合成原料气。随着能源和环境的政策要求越来越高,不久的将来,会逐步被新的煤气化技术所取代。 3.2 鲁奇加压连续气化 20世纪30年代,由德国鲁奇公司开发。在高温、高压下,用纯氧和水蒸汽,连续通过由煤形成的固定床。氧和煤反应放出的热量,正好能供应水蒸汽和煤反应所需要的热量,从而维持了热量平衡,炉温恒定,制气过程连续。 鲁奇加压气化法生产的煤气中除含CO 和H 2外, 含CH 4高达10 %~12 %,可作为城市煤气、人工天然气、合成气使用。相比较于固定床间歇气化,其优点是炉子生产能大幅提高,煤种要求适当放宽。其缺点是气化炉结构复杂,炉内设有破粘机、煤分布器和炉篦等转动设备,制造和维修费用大,入炉仍需要是块煤,出炉煤气中含焦油、酚等,污水处理和煤气净化工艺复杂。 3.3 恩德粉煤气化技术 恩德粉煤气化技术利用粉煤(<10 mm)和气化剂在气化炉内形成沸腾流化床,在高温下完成煤气化反应,生产需要的煤气。 由于所用的原料为粉煤,煤种的适应性比块煤有所放宽,原料成本也得到大幅度降低。得益于流化床的传质、传热效果大大优于固定床,恩德粉煤气化炉的生产能力比固定床间歇制气有较大幅度的提高。由于操作温度不高,导致气化效率和碳转化率都不高,且存在废水、废渣处理困难等问题。此技术多用于替代固定床间歇制气工艺[4-6]。 [收稿日期] 2012-07-21 [作者简介] 付长亮(1968-),男,河南荥阳人,硕士,高级讲师,主要从事化工工艺的教学与研究。

煤气化技术的现状和发展趋势

煤气化技术的现状和发展趋势 1、水煤浆加压气化 1.1 德士古水煤浆加压气化工艺(TGP) 美国Texaco 公司在渣油部分氧化技术基础上开发了水煤浆气化技术,TGP 工艺采用水煤浆进料,制成质量分数为60%~65%的水煤浆,在气流床中加压气化,水煤浆和氧气在高温高压下反应生成合成气,液态排渣。气化压力在2.7~6.5MPa,提高气化压力,可降低装置投入,有利于降低能耗;气化温度在1 300~1 400℃,煤气中有效气体(CO+H2)的体积分数达到80%,冷煤气效率为70%~76%,设备成熟,大部分已能国产化。世界上德士古气化炉单炉最大投煤量为2 000t/d。德士古煤气化过程对环境污染影响较小。 根据气化后工序加工不同产品的要求,加压水煤浆气化有三种工艺流程:激冷流程、废锅流程和废锅激冷联合流程。对于合成氨生产多采用激冷流程,这样气化炉出来的粗煤气,直接用水激冷,被激冷后的粗煤气含有较多水蒸汽,可直接送入变换系统而不需再补加蒸汽,因无废锅投资较少。如产品气用作燃气透平循环联合发电工程时,则多采用废锅流程,副产高压蒸汽用于蒸汽透平发电机组。如产品气用作羟基合成气并生产甲醇时,仅需要对粗煤气进行部分变换,通常采用废锅和激冷联合流程,亦称半废锅流程,即从气化炉出来粗煤气经辐射废锅冷却到700℃左右,然后用水激冷到所需要的温度,使粗煤气显热产生的蒸汽能满足后工序部分变换的要求。 1.2 新型(多喷嘴对置式)水煤浆加压气化 新型(多喷嘴对置式)水煤浆加压气化技术是最先进煤气化技术之一,是在德士古水煤浆加压气化法的基础上发展起来的。2000 年,华东理工大学、鲁南化肥厂(水煤浆工程国家中心的依托单位)、中国天辰化学工程公司共同承担的新型(多喷嘴对置)水煤浆气化炉中试工程,经过三方共同努力,于7 月在鲁化建成投料开车成功,通过国家主管部门的鉴定及验收。2001 年2 月10 日获得专利授权。新型气化炉以操作灵活稳定,各项工艺指标优于德士古气化工艺指标引起国家科技部的高度重视和积极支持,主要指标体现为:有效气成分(CO+H2)的体积分数为~83%,比相同条件下的ChevronTexaco 生产装置高1.5~2.0 个百分点;碳转化率>98%,比ChevronTexaco 高2~3 个百分点;比煤耗、比氧耗均比ChevronTexaco 降低7%。 新型水煤浆气化炉装置具有开车方便、操作灵活、投煤负荷增减自如的特点,同时综合能耗比德士古水煤浆气化低约7%。其中第一套装置日投料750t 能力新型多喷嘴对置水煤浆加压气化炉于2004 年12 月在山东华鲁恒升化学有限公司建成投料成功,运行良好。另一套装置两台日投煤1 150t 的气化炉也在兖矿国泰化工有限公司于2005 年7 月建成投料成功,并于2005 年10 月正式投产,2006 年已达到并超过设计能力,目前运行状况良好。该技术在国内已获得有效推广,并已出口至美国。 2、干粉煤加压气化工艺 2.1 壳牌干粉煤加压气化工艺(SCGP) Shell 公司于1972 年开始在壳牌公司阿姆斯特丹研究院(KSLA)进行煤气化研究,1978 年第一套中试装置在德国汉堡郊区哈尔堡炼油厂建成并投入运行,1987 年在美国休斯顿迪尔·帕克炼油厂建成日投煤量250~400t 的示范装置,1993年在荷兰的德姆克勒(Demkolec)电厂建成投煤量2 000t/d 的大型煤气化装置,用于联合循环发电(IGCC),称作SCGP 工业生产装置。装置开工率最高达73%。该套装置的成功投运表明SCGP 气化技术是先进可行的。 Shell 气化炉为立式圆筒形气化炉,炉膛周围安装有由沸水冷却管组成的膜式水冷壁,其内壁衬有耐热涂层,气化时熔融灰渣在水冷壁内壁涂层上形成液膜,沿壁顺流而下进行分

现代煤气化技术发展趋势及应用综述_汪寿建

2016年第35卷第3期CHEMICAL INDUSTRY AND ENGINEERING PROGRESS ·653· 化工进展 现代煤气化技术发展趋势及应用综述 汪寿建 (中国化学工程集团公司,北京 100007) 摘要:现代煤气化技术是现代煤化工装置中的重要一环,涉及整个煤化工装置的正常运行。本文分别介绍了中国市场各种现代煤气化工艺应用现状,叙述汇总了其工艺特点、应用参数、市场数据等。包括第一类气流床加压气化工艺,又可分为干法煤粉加压气化工艺和湿法水煤浆加压气化工艺。干法气化代表性工艺包括Shell炉干煤粉气化、GSP炉干煤粉气化、HT-LZ航天炉干煤粉气化、五环炉(宁煤炉)干煤粉气化、二段加压气流床粉煤气化、科林炉(CCG)干煤粉气化、东方炉干煤粉气化。湿法气化代表性工艺包括 GE水煤浆加压气化、四喷嘴水煤浆加压气化、多元料浆加压气化、熔渣-非熔渣分级加压气化(改进型为清华炉)、E-gas(Destec)水煤浆气化。第二类流化床粉煤加压气化工艺,主要有代表性工艺包括U-gas灰熔聚流化床粉煤气化、SES褐煤流化床气化、灰熔聚常压气化(CAGG)。第三类固定床碎煤加压气化,主要有代表性工艺包括鲁奇褐煤加压气化、碎煤移动床加压气化和BGL碎煤加压气化等。文章指出应认识到煤气化技术的重要性,把引进国外先进煤气化技术理念与具有自主知识产权的现代煤化工气化技术有机结合起来。 关键词:煤气化;市场应用;气化特点;参数数据分析 中图分类号:TQ 536.1 文献标志码:A 文章编号:1000–6613(2016)03–0653–12 DOI:10.16085/j.issn.1000-6613.2016.03.001 Development and applicatin of modern coal gasification technology WANG Shoujian (China National Chemical Engineering Group Corporation,Beijing100007,China)Abstract:Modern coal gasification technology is an important part of modern coal chemical industrial plants,involving stable operation of the entire coal plant. This paper introduces application of modern coal gasification technologies in China,summarizes characteristics of gasification processes,application parameters,market data,etc. The first class gasification technology is entrained-bed gasification process,which can be divided into dry pulverized coal pressurized gasification and wet coal-water slurry pressurized gasification. The typical dry pulverized coal pressurized gasification technologies include Shell Gasifier,GSP Gasifier,HT-LZ Gasifier,WHG (Ning Mei) Gasifier,Two-stage Gasifier,CHOREN CCG Gasifier,SE Gasifier. The typical wet coal-water slurry pressurized gasification technologies include GE (Texaco) Gasifier,coal-water slurry gasifier with opposed multi-burners,Multi-component Slurry Gasifier,Non-slag/slag Gasifier (modified as Tsinghua Gasifier),E-gas (Destec) Gasifier. The second class gasification technology is fluidized-bed coal gasification process. The typical fluidized-bed coal gasification technologies include U-gas Gasifier,SES Lignite Gasifier,CAGG Gasifier. The third class gasification technology is fixed-bed coal gasification process. The typical fixed-bed coal gasification technologies include Lurgi Lignite 收稿日期:2015-09-14;修改稿日期:2015-12-17。 作者:汪寿建(1956—),男,教授级高级工程师,中国化学工程集团公司总工程师,长期从事化工、煤化工工程设计、开发及技术管理工作。E-mail wangsj@https://www.doczj.com/doc/0f17295624.html,。

煤气化工艺资料

煤化工是以煤为原料,经过化学加工使煤转化为气体,液体,固体燃料以及化学品的过程,生产出各种化工产品的工业。 煤化工包括煤的一次化学加工、二次化学加工和深度化学加工。煤的气化、液化、焦化,煤的合成气化工、焦油化工和电石乙炔化工等,都属于煤化工的范围。而煤的气化、液化、焦化(干馏)又是煤化工中非常重要的三种加工方式。 煤的气化、液化和焦化概要流程图 一.煤炭气化

煤炭气化是指煤在特定的设备内,在一定温度及压力下使煤中有机质与气化剂(如蒸汽/空气或氧气等)发生一系列化学反应,将固体煤转化为含有CO、H2、CH4等可燃气体和CO2、N2等非可燃气体的过程。 煤的气化的一般流程图 煤炭气化包含一系列物理、化学变化。而化学变化是煤炭气化的主要方式,主要的化学反应有: 1、水蒸气转化反应C+H2O=CO+H2 2、水煤气变换反应CO+ H2O =CO2+H2 3、部分氧化反应C+0.5 O2=CO 4、完全氧化(燃烧)反应C+O2=CO2 5、甲烷化反应CO+2H2=CH4 6、Boudouard反应C+CO2=2CO 其中1、6为放热反应,2、3、4、5为吸热反应。 煤炭气化时,必须具备三个条件,即气化炉、气化剂、供给热量,三者缺一不可。 煤炭气化按气化炉内煤料与气化剂的接触方式区分,主要有: 1) 固定床气化:在气化过程中,煤由气化炉顶部加入,气化剂由气化炉底部加入,煤料与气化剂逆流接触,相对于气体的上升速度而言,煤料下降速度很慢,甚至可视为固定不动,因此称之为固定床气化;而实际上,煤料在气化过程中是以很慢的速度向下移动的,比

较准确的称其为移动床气化。 2) 流化床气化:它是以粒度为0-10mm的小颗粒煤为气化原料,在气化炉内使其悬浮分散在垂直上升的气流中,煤粒在沸腾状态进行气化反应,从而使得煤料层内温度均一,易于控制,提高气化效率。 3) 气流床气化。它是一种并流气化,用气化剂将粒度为100um以下的煤粉带入气化炉内,也可将煤粉先制成水煤浆,然后用泵打入气化炉内。煤料在高于其灰熔点的温度下与气化剂发生燃烧反应和气化反应,灰渣以液态形式排出气化炉。 4) 熔浴床气化。它是将粉煤和气化剂以切线方向高速喷入一温度较高且高度稳定的熔池内,把一部分动能传给熔渣,使池内熔融物做螺旋状的旋转运动并气化。目前此气化工艺已不再发展。 以上均为地面气化,还有地下气化工艺。 根据采用的气化剂和煤气成分的不同,可以把煤气分为四类:1.以空气作为气化剂的空气煤气;2.以空气及蒸汽作为气化剂的混合煤气,也被称为发生炉煤气;3.以水蒸气和氧气作为气化剂的水煤气;4.以蒸汽及空气作为气化剂的半水煤气,也可是空气煤气和水煤气的混合气。 几种重要的煤气化技术及其技术性能比较 1.Lurgi炉固定床加压气化法对煤质要求较高,只能用弱粘结块煤,冷煤气效率最高,气化强度高,粗煤气中甲烷含量较高,但净化系统复杂,焦油、污水等处理困难。 鲁奇煤气化工艺流程图

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