2125_02_11 Petroleum Technology, Volume 1-2

FUELS,SYNTHETIC,GASEOUSSubstitute or synthetic natural gas(SNG)has been known for several centu-ries.When SNG wasfirst discovered,natural gas was largely unknown as a fuel and was more a religious phenomenon(see N ATURAL GAS)(1).Coal(qv) was thefirst significant source of substitute natural gas and in the early stages of SNG production the product was more commonly known under varia-tions of the name coal gas(2,3).Whereas coal continues to be a principal source of substitute natural gas(4)a more recently recognized source is petroleum (qv)(5).1.Gas from CoalCoal can be converted to gas by several routes(2,6–11),but often a parti-cular process is a combination of options chosen on the basis of the pro-duct desired,ie,low,medium,or high heat-value gas.In a very general sense,coal gas is the term applied to the mixture of gaseous constituents that are produced during the thermal decomposition of coal at tempera-tures in excess of5008C(>930 F),often in the absence of oxygen(air)(3).A solid residue(coke,char),tars,and other liquids are also produced in the process:C coalþheatÀ!C charþtar=liquidþCOþCO2þH2The tars and other liquids(liquor)are removed by condensation leaving principally hydrogen,carbon monoxide,and carbon dioxide in the gas phase. This gaseous product also contains low boiling hydrocarbons(qv),sulfur-con-taining gases,and nitrogen-containing gases including ammonia(qv)and hydrogen cyanide.The solid residue is then treated under a variety of condi-tions to produce other fuels which vary from a purified char to different types of gaseous mixtures.The amounts of gas,coke,tar,and other liquid pro-ducts vary according to the method used for the carbonization(especially the retort configuration),and process temperature,as well as the nature(rank)of the coal(3,11).The recorded chronology of the coal-to-gas conversion technology beganin 1670when a clergyman,John Clayton,in Wakefield,Yorkshire,produced in the laboratory a luminous gas by destructive distillation of coal(12).At the same time,experiments were also underway elsewhere to carbonize coal to pro-duce coke,but the process was not practical on any significant scale until1730 (12).In1792,coal was distilled in an iron retort by a Scottish engineer,who used the by-product gas to illuminate his home(13).The conversion of coal to gas on an industrial scale dates to the early nine-teenth century(14).The gas,often referred to as manufactured gas,was pro-duced in coke ovens or similar types of retorts by simply heating coal to Kirk-Othmer Encyclopedia of Chemical Technology.Copyright John Wiley&Sons,Inc.All rights reserved.10.1002/0471238961.0701190519160509.a01vaporize the volatile constituents.Estimates based on modern data indicate that the gas mixture probably contained hydrogen (qv)(ca 50%),methane (ca 30%),carbon monoxide (qv)and carbon dioxide (qv)(ca 15%),and some inert material,such as nitrogen (qv),from which a heating value of approxi-mately 20.5MJ/m 3(550Btu/ft 3)can be estimated (6).Blue gas,or blue-water gas,so-called because of the color of the flame upon burning (10),was discovered in 1780when steam was passed over incandescent carbon (qv),and the blue-water gas process was developed over the period 1859–1875.Successful commercial application of the process came about in 1875with the introduction of the carburetted gas jet.The heating value of the gas was low,ca 10.2MJ/m 3(275Btu/ft 3),and on occasion oil was added to the gas to enhance the heating value.The new product was given the name carburetted water gas and the technique satis fied part of the original aim by adding luminosity to gas lights (10).Coke-oven gas is a by-product fuel gas derived from coking coals by the pro-cess of carbonization.The first by-product coke ovens were constructed in France in 1856.Since then they have gradually replaced the old and primitive method of beehive coking for the production of metallurgical coke.Coke-oven gas is pro-duced in an analogous manner to retort coal gas,with operating conditions,mainly temperature,set for maximum carbon yield.The resulting gas is,conse-quently,poor in illuminants,but excellent as a fuel.Typical analyses and heat content of common fuel gases vary (Table 1)and depend on the source as well as the method of production.In Germany,large-scale production of synthetic fuels from coal began in 1910and necessitated the conversion of coal to carbon monoxide and hydrogen.Water gas reaction C coal þH 2O À!CO þH 2The mixture of carbon monoxide and hydrogen is enriched with hydrogen from the water gas catalytic (Bosch)process,ie,water gas shift reaction,and passed Table 1.Analyses of Fuel GasesGas composition,vol%Heat value,a MJ/m 3Type of fuel gasCO CO 2H 2N 2O 2CH 4Illuminants blast-furnace27.510.0 3.058.0 1.00.5 3.8producer (bituminous)27.0 4.514.050.90.6 3.0 5.6blue-water42.8 3.049.9 3.30.50.511.5carburetted water33.4 3.934.67.90.910.48.9b 20.0retort coal8.6 1.552.5 3.50.331.4 2.2c 21.5coke-oven6.3 1.853.0 3.40.2 1.6 3.7d 21.9naturalmid-continent0.8 3.296.036.1Pennsylvania1.167.631.3e 46.0a To convert MJ/m 3to Btu/ft 3,multiply by 26.86.b 6.7vol%C 2H 4plus 2.2vol%C 6H 6.c 1.1vol%each of C 2H 4and C 6H 6.d 2.7vol%C 2H 4plus 1.0vol%C 6H 6.e 31.3vol%C 2H 6.over a cobalt –thoria catalyst to form straight-chain,ie,linear,paraf fins,ole fins,and alcohols in what is known as the Fisher-Tropsch synthesis.n CO þ2n þ1ðÞH 2ÀÀ!cobaltcatalyst C n H 2n þ2þn H 2O 2n CO þn þ1ðÞH 2ÀÀ!iron catalystC n H 2n þ2þn CO 2n CO þ2n H 2ÀÀ!cobalt catalystC n H 2n þn H 2O CO þn H 2ÀÀ!iron catalystC n H 2n þn CO 2n CO þ2n H 2ÀÀ!cobalt catalystC n H 2n þ1OH þn À1ðÞH 2O 2n À1ðÞCO þn þ1ðÞH 2ÀÀ!iron catalyst C n H 2n þ1OH þn À1ðÞCO 2In Sasolburg,South Africa,a commercial plant using the Fischer-Tropsch process was completed in 1950and began producing a variety of liquid fuels and chemicals.The facility has been expanded to produce a considerable portion of South Africa ’s energy requirements (15,16).In 1948,the first demonstration of suspension gasi fication was success-fully completed by Koppers,Inc.The product gas was of 11.2MJ/m 3(300Btu/ft 3)calori fic value and consisted primarily of a mixture of hydrogen and oxides of carbon.In the United States,so-called second-generation coal gasi fication processes came into being as a result of the recognized need to develop reliable,domestic energy sources to replace the rapidly diminishing supply of conventional fuels (see F UEL RESOURCES )(3,9).More recently,the biological conversion of coal and synthesis gas (carbon monoxide –hydrogen mixtures)into liquid fuels by methanogenic bacteria has received some attention (17–19).1.1.Gas Products.The originally designated names of the gaseous mixtures are used herein,with the understanding that since their introduction there may be differences in means of production and in the make-up of the gas-eous products.Properties of fuel gases are available (20).There are standard tests to determine properties and character of gaseous mixtures.These tests are accepted by the American Society for Testing and Materials (ASTM),by the British Standards Institution (BSI),by the Institute of Petroleum (IP),and by the International Standards Organization (ISO)(1,10,20).Low Heat-Value Gas.Low heat-value (low Btu)gas (7)consists of a mix-ture of carbon monoxide and hydrogen and has a heating value of less than 11MJ/m 3(300Btu/ft 3),but more often in the range 3.3–5.6MJ/m 3(90–150Btu/ft 3).The gas is formed by partial combustion of coal with air,usually in the presence of steam (7).2C coal þO 2À!2CO C coal þH 2O À!CO þH 2CO þH 2O À!CO 2þH 2This gas is of interest to industry as a fuel gas or even,on occasion,as a raw material from which ammonia and other compounds may be synthesized.Thefirst gas producer making low heat-value gas was built in1832.(The product was a combustible carbon monoxide–hydrogen mixture containing ca50vol%nitrogen).The open-hearth or Siemens-Martin process,built in 1861for pig iron refining,increased low heat-value gas use(see I RON).The use of producer gas as a fuel for heating furnaces continued to increase until the turn of the century when natural gas began to supplant manufac-tured fuel gas.The combustible components of the gas are carbon monoxide and hydrogen, but combustion(heat)value varies because of dilution with carbon dioxide and with nitrogen.The gas has a lowflame temperature unless the combustion air is strongly preheated.Its use has been limited essentially to steel(qv)mills, where it is produced as a by-product of blast furnaces.A common choice of equip-ment for the smaller gas producers is the Wellman-Galusha unit because of its long history of successful operation(21).Medium Heat-Value Gas.Medium heat-value(medium Btu)gas(6,7) has a heating value between9and26MJ/m3(250and700Btu/ft3).At the lower end of this range,the gas is produced like low heat-value gas,with the notable exception that an air separation plant is added and relatively pure oxy-gen(qv)is used instead of air to partially oxidize the coal.This eliminates the potential for nitrogen in the product and increases the heating value of the pro-duct to10.6MJ/m3(285Btu/ft3).Medium heat-value gas consists of a mixture of methane,carbon monoxide,hydrogen,and various other gases and is suitable as a fuel for industrial consumers.High Heat-Value Gas.High heat-value(high Btu)gas(7)has a heating value usually in excess of33.5MJ/m3(900Btu/ft3).This is the gaseous fuel that is often referred to as substitute or synthetic natural gas(SNG),or pipeline-quality gas.It consists predominantly of methane and is compatible with natural gas insofar as it may be mixed with,or substituted for,natural gas.Any of the medium heat-value gases that consist of carbon monoxide and hydrogen(often called synthesis gas)can be converted to high heat-value gas by methanation(22),a low temperature catalytic process that combines carbon monoxide and hydrogen to form methane and water.COþ3H2À!CH4þH2OPrior to methanation,the gas product from the gasifier must be thoroughly pur-ified,especially from sulfur compounds the precursors of which are widespread throughout coal(23)(see S ULFUR AND HYDROGEN SULFIDE RECOVERY).Moreover,the composition of the gas must be adjusted,if required,to contain three parts hydro-gen to one part carbon monoxide tofit the stoichiometry of methane production. This is accomplished by application of a catalytic water gas shift reaction.COþH2OÀ!ÀCO2þH2The ratio of hydrogen to carbon monoxide is controlled by shifting only part of the gas stream.After the shift,the carbon dioxide,which is formed in the gasi-fier and in the water gas reaction,and the sulfur compounds formed during gasi-fication,are removed from the gas.The processes that have been developed for the production of synthetic nat-ural gas are often configured to produce as much methane in the gasification step as possible thereby minimizing the need for a methanation step.In addition, methane formation is highly exothermic which contributes to process efficiency by the production of heat in the gasifier,where the heat can be used for the endothermic steam–carbon reaction to produce carbon monoxide and hydrogen.CþH2OÀ!COþH21.2.Carbonization.Next to combustion,carbonization represents one of the largest uses of coal(2,24–26).Carbonization is essentially a process for the production of a carbonaceous residue by thermal decomposition,accompa-nied by simultaneous removal of distillate,of organic substances.C organicÀ!C coke=char=carbonþliquidsþgasesThis process may also be referred to as destructive distillation.It has been applied to a whole range of organic materials,more particularly to natural pro-ducts such as wood(qv),sugar(qv),and vegetable matter to produce charcoal (see F UELS FROM BIOMASS).However,in the present context,coal usually yields coke,which is physically dissimilar from charcoal and appears with the more familiar honeycomb-type structure(27).The original process of heating coal in rounded heaps,the hearth process, remained the principal method of coke production for over a century,although an improved oven in the form of a beehive was developed in the Durham-Newcastle area of England in about1759(2,26,28).These processes lacked the capability to collect the volatile products,both liquids and gases.It was not until the mid-nineteenth century,with the introduction of indirectly heated slot ovens,that it became possible to collect the liquid and gaseous products for further use.Coal carbonization processes are generally defined according to process operating temperature.Terms are defined in Table2.Table2.Coal Carbonization Methods aCarbonization process Final tempe-rature,8C Products Processeslow temperature500–700reactive coke andhigh tar yield rexco(7008C)made in cylindrical vertical retorts;coalite(6508C) made in vertical tubesmedium temperature700–900reactive coke withhigh gas yield,ordomestic briquettes town gas and gas coke (obsolete);phurnacite, low volatile steam coal, pitch-bound briquettes carbonized at8008Chigh temperature900–1050hard,unreactive coalfor metallurgical use foundry coke(9008C); blast-furnace coke (950–10508C)a Ref.29.Low Temperature Carbonization.Low temperature carbonization,when the process does not exceed7008C,was mainly developed as a process to supply town gas for lighting purposes as well as to provide a smokeless(devolatilized) solid fuel for domestic consumption(30).However,the process by-products (tars)were also found to be valuable insofar as they served as feedstocks for an emerging chemical industry and were also converted to gasolines,heat-ing oils,and lubricants(see G ASOLINE AND OTHER MOTOR FUELS;L UBRICATION AND LUBRICANTS)(31).Coals preferred for the low temperature carbonization were usually lignites or subbituminous,as well as high volatile bituminous,coals(see LIGNITE AND BROWN COAL).These yield porous solid products over the temperature range 600–7008C.Certain of the higher rank(caking)coals were less suitable for the process,unless steps were taken to destroy the caking properties,because of the tendency of these higher rank coals to adhere to the walls of the carboniza-tion chamber.The options for efficient low temperature carbonization of coal include vertical and horizontal retorts which have been used for batch and continuous processes.In addition,stationary and revolving horizontal retorts have also been operated successfully,and there are also several process options employingfluidized or gas-entrained coal.Coke production from batch-type carbonization of coal has been supplanted by a variety of continuous retorting processes which allow much greater throughput rates than were previously possible.These processes employ rectangular or cylindrical vessels of sufficient height to carbonize the coal while it travels from the top of the vessel to the bottom and usually employ the principle of heating the coal by means of a countercurrentflow of hot combustion gas.Most notable of these types of carbonizers are the Lurgi-Spulgas retort and the Koppers continuous steaming oven(2).High Temperature Carbonization.When heated at temperatures in excess of7008C(12908F),low temperature chars lose their reactivity through devolatilization and also suffer a decrease in porosity.High temperature carbo-nization,at temperatures>900 C,is,therefore,employed for the production of coke(27).As for the low temperature processes,the tars produced in high tem-perature ovens are also sources of chemicals and chemical intermediates(32).A newer concept has been developed that is given the name mild gasifica-tion(33).It is not a gasification process in the true sense of the word.The process temperature is some several hundred degrees lower,hence the term mild,than the usual gasification process temperature and the objective is not to produce a gaseous fuel but to produce a high value char(carbon)and liquid products.Gas is produced,but to a lesser extent.Documented efforts at cokemaking date from1584(34),and have seen various adaptations of conventional wood-charring methods to the production of coke including the eventual evolution of the beehive oven,which by the mid-nineteenth century had become the most common vessel for the coking of coal(2).The heat for the process was supplied by burning the volatile matter released from the coal and,consequently,the carbonization would progress from the top of the bed to the base of the bed and the coke was retrieved from the side of the oven at process completion.Some beehive ovens,having various improvements and additions of waste heat boilers,thereby allowing heat recovery from the combustion products,may still be in operation.Generally,however,the beehive oven has been replaced by wall-heated,horizontal chamber,ie,slot,ovens in which higher temperatures can be achieved as well as a better control over the quality of the coke.Modern slot-type coke ovens are approximately15m long,approximately6m high,and the width is chosen to suit the carbonization behavior of the coal to be processed. For example,the most common widths are ca0.5m,but some ovens may be as narrow as0.3m,or as wide as0.6m.Several(usually20or more,alternating with similar cells that contain heating ducts)of these chambers are constructed in the form of a battery over a commonfiring system through which the hot combustion gas is conveyed to the ducts.Theflat roof of the battery acts as the surface for a mobile charging car from which the coal(25–40t)enters each oven through three open-ings along the top.The coke product is pushed from the rear of the oven through the opened front section onto a quenching platform or into rail cars that then move the coke through water sprays.The gas and tar by-products of the process are collected for further processing or for on-site use as fuel.Most modern coke ovens operate on a regenerative heating cycle in order to obtain as much surplus gas as possible for use on the works,or for sale.If coke-oven gas is used for heating the ovens,the majority of the gas is surplus to requirements.If producer gas is used for heating,much of the coke-oven gas is surplus.The main difference between gas works and coke oven practice is that,in a gas works,maximum gas yield is a primary consideration whereas in the coke works the quality of the coke is thefirst consideration.These effects are obtained by choice of a coal feedstock that is suitable to the task.For example,use of lower volatile coals in coke ovens,compared to coals used in gas works,produces lower yields of gas when operating at the same temperatures.In addition,the choice of heating (carbonizing)conditions and the type of retort also play a principal role(10,35).1.3.Gasification.The gasification of coal is essentially the conversion of coal by any one of a variety of processes to produce combustible gases (7,8,11,22,36–38).Primary gasification is the thermal decomposition of coal to produce mixtures containing various proportions of carbon monoxide,carbon dioxide,hydrogen,water,methane,hydrogen sulfide,and nitrogen,as well as products such as tar,oils,and phenols.A solid char product may also be pro-duced,and often represents the bulk of the weight of the original coal.Secondary gasification involves gasification of the char from the primary gasifier,usually by reaction of the hot char and water vapor to produce carbon monoxide and hydrogen.C charþH2OÀ!COþH2The gaseous product from a gasifier generally contains large amounts of carbon monoxide and hydrogen,plus lesser amounts of other gases and may be of low, medium,or high heat value depending on the defined use(Table3)(39,40).The importance of coal gasification as a means of producing fuel gas(es)for industrial use cannot be underplayed.But coal gasification systems also haveundesirable features.A range of undesirable products are also produced which must be removed before the products are used to provide fuel and/or to generate electric power (22,41).Chemistry.Coal gasi fication involves the thermal decomposition of coal and the reaction of the carbon in the coal,and other pyrolysis products with oxy-gen,water,and hydrogen to produce fuel gases such as methane by internal hydrogen shiftsC coal þH coal À!CH 4or through the agency of added (external)hydrogenC coal þ2H 2À!CH 4although the reactions are more numerous and more complex as can be seen in Table 4.If air is used as a combustant,the product gas contains nitrogen and,depending on design characteristics,has a heating value of approximately 5:6À11:2Â103MJ =m 3(150–300Btu/ft 3).The use of pure oxygen,although expensive,results in a product gas having a heating value of 11–15MJ/m 3(300–400Btu/ft 3)and carbon dioxide and hydrogen sul fide as by-products.If a high heat-value gas (33.5–37.3MJ/m 3(900–1000Btu/ft 3))ie,SNG,is the desired product,efforts must be made to increase the methane content.Shift conversion reactionCO þH 2O À!CO 2þH 2CO þ3H 2À!CH 4þH 2O2CO þ2H 2À!CH 4þCO 2CO þ4H 2À!CH 4þ2H 2OThe gasi fication is performed using oxygen and steam (qv),usually at elevated pressures.The steam –oxygen ratio along with reaction temperature and Table 3.Gas Composition Requirements for Substitute Natural Gas and for Power GenerationProduct requirementCharacteristicSynthetic natural gas Power generation methane contenthigh,less synthesis required low,probably means no tars H 2/CO ratiohigh,less shifting required low,CO more ef ficient fuel moisture contenthigh,steam required for shift low,lower condensate treatment costs outlettemperaturelow,maximizes methane minimizes sensible heat loss high,precludes tar formation provides for steam generation leads to high cold gas ef ficiency reduces cold gas ef ficiency gasi fier oxidant O 2only,cost of N 2removalexcessive air or O 2,low heating gas valueacceptable fuelpressure determine the equilibrium gas composition.The reaction rates for these reactions are relatively slow and heats of formation are negative.Catalysts may be necessary for complete reaction (2,3,24,42,43).Process Parameters.The most notable effects in gasi fiers are those of pressure (Fig.1)and coal character.Some initial processing of the coal feedstockTable 4.Gasi fication ReactionsReactionD H,kJ a Gasi fication zone,595–12058CC þCO 2À!2CO 9.65b CO þH 2O À!CO 2þH 2À1:93c C þH 2O À!CO þH 27.76d C þ2H 2À!CH 4À5:21C þ2H 2O À!2H 2þCO 2 5.95Combustion zone,>1205 C2C þO 2À!2CO À22:6C þO 2À!CO 2À6:74Methanation reactionsCO þ3H 2À!CH 4þH 2O À12:9CO 2þ4H 2À!CH 4þ2H 2O À11:02C þ2H 2O À!CH 4þCO 20.62Others reactions2H 2þO 2À!2H 2O CO þ2H 2À!CH 3OH 2C þH 2À!C 2H 2CH 4þ2H 2O À!CO 2þ4H 2aTo convert kJ to kcal,divide by 4.184.b Boudouard reaction.c Water gas shift reaction.d Water gas reaction.C o m p o s i t i o n , v o l %50H e a t i n g v a l u e , M J /m 3206040302010015H 2CH 400.5 1.0 1.5 2.0 2.53.0 3.54.0Gasification pressure, MPa CO Fig.1.Variation of (—)gas composition and (ÀÀÀÀÀÀ)heating value with gasi fier pressure.To convert MJ/m 3to Btu/ft 3,multiply by 26.86.To convert MPa to psi,multiply by 145.may be required.The type and degree of pretreatment is a function of the process and/or the type of coal.Depending on the type of coal being processed and the analysis of the gas product desired,some or all of the following processing steps may be required: (1)pretreatment of the coal(if caking is a problem);(2)primary gasification of the coal;(3)secondary gasification of the carbonaceous residue from the primary gasifier;(4)removal of carbon dioxide,hydrogen sulfide,and other acid gases;(5)shift conversion for adjustment of the carbon monoxide–hydrogen mole ratio to the desired ratio;and(6)catalytic methanation of the carbon monox-ide–hydrogen mixture to form methane.If high heat-value gas is desired,all of these processing steps are required because coal gasifiers do not yield methane in the concentrations required.An example of application of a pretreatment option occurs when the coal displays caking or agglomerating characteristics.Such coals are usually not amenable to gasification processes employingfluidized-bed or moving-bed reac-tors.The pretreatment involves a mild oxidation treatment,usually consisting of low temperature heating of the coal in the presence of air or oxygen.This destroys the caking characteristics of coals.Gasification technologies for the production of high heat-value gas do not all depend entirely on catalytic methanation,that is,the direct addition of hydrogen to coal under pressure to form methane.The hydrogen-rich gas for hydrogasification can be manufactured from steam by using the char that leaves the hydrogasifier.Appreciable quantities of methane are formed directly in the primary gasifier and the heat released by methane formation is at a sufficiently high temperature to be used in the steam–carbon reaction to produce hydrogen so that less oxygen is used to produce heat for the steam–carbon reaction.Hence,less heat is lost in the low temperature methanation step,thereby leading to a higher overall process efficiency.There are three fundamental reactor types for gasification processes:(1)a gasifier reactor,(2)a devolatilizer,and(3)a hydrogasifier(Fig.2).The choice of a particular design is available for each type,eg,whether or not two stages should be involved depending on the ultimate product gas desired.Reactors may also be designed to operate over a range of pressure from atmospheric to high pressure.Gasification processes have been classified on the basis of the heat-value of the produced gas.It is also possible to classify gasification processes according to the type of reactor and whether or not the system reacts under pressure.Addi-tionally,gasification processes can be segregated according to the bed types, which differ in the ability to accept and use caking coals.Thus gasification pro-cesses can be divided into four categories based on reactor(bed)configuration:(1)fixed bed,(2)moving bed,(3)fluidized bed,and(4)entrained bed.In afixed-bed process the coal is supported by a grate and combustion gases,ie,steam,air,oxygen,etc,pass through the supported coal whereupon the hot produced gases exit from the top of the reactor.Heat is supplied intern-ally or from an outside source,but caking coals cannot be used in an unmodified fixed-bed reactor.In the moving-bed system(Fig.3),coal is fed to the top of the bed and ash leaves the bottom with the product gases being produced in the hot zone just prior to being released from the bed.Steam andH 2-rich gasCoalHydrogen(a )(b )(c )Fig.2.Chemistry of (a )gasi fier;(b )hydrogasi fier;and (c)devolatization processes.The gaseous product of (a )is oflow heating value;that of (b )and (c )is of intermediate heating value.or air Coalor air GasSteam,oxygen,or airCoal Slag(a )(b )(c )Fig.3.Gasi fier systems:(a ),moving bed (dry ash);(b ),fluidized bed;and (c ),entrained flow.。