Hydrogen production by non-catalytic partial oxidation of coal in supercritical water:Explore the way to complete gasi?cation of lignite and bituminous coal
Zhiwei Ge,Simao Guo,Liejin Guo *,Changqing Cao,Xiaohui Su,Hui Jin
State Key Laboratory of Multiphase Flow in Power Engineering,Xi’an Jiaotong University,Xi’an 710049,China
a r t i c l e i n f o
Article history:
Received 11April 2013Received in revised form 19June 2013
Accepted 20June 2013
Available online 14August 2013Keywords:
Supercritical water Coal
Hydrogen production Quartz tube Partial oxidation
a b s t r a c t
Supercritical water gasi?cation (SCWG)of coal is a promising technology for clean coal utilization.In this paper,hydrogen production by non-catalytic partial oxidation of coal was systematically investigated in supercritical water (SCW)with quartz batch reactors for the ?rst time.The in?uences of the main operating parameters including residence time,temperature,oxidant equivalent ratio (ER)and feed concentration on the gasi?cation characteristics of coal were investigated.The experimental results showed that H 2yield and carbon gasi?cation ef?ciency (CE)increased with increasing temperature and decreasing feed concentration.CE increased with increasing ER,and H 2yield peaked when ER equaled 0.1.CE increased quickly within 1min and then tended to be stable between 2and 3min.In particular,complete gasi?cation of lignite was obtained at 950 C when ER equaled 0.1,as for bituminous coal,at a higher temperature of 980 C when ER equaled 0.2.Copyright a2013,Hydrogen Energy Publications,LLC.Published by Elsevier Ltd.All rights
reserved.
1.Introduction
Coal is important as an energy resource and organic chemical feedstock in the 21st century [1].China is the world’s largest producer and user of coal,and coal supplies 75%of energy demand and 95%of thermal electric power generation in China [2].However,the negative effect of coal utilization on environment and related expense for maintaining environ-mental safety restrain the growth in coal consumption [3e 6],and this stimulates the research for clean and ef?cient methods of coal conversion.
SCW (T >374 C and P >22.1MPa)is attracting increasing attention as a medium for organic chemistry for its more “green”or environmentally benign chemical processes.The
properties of SCW are quite different from those of ambient liquid water.The dielectric constant is much lower,and the number and persistence of hydrogen bonds are both dimin-ished.As a result,SCW behaves like organic solvents so that many organic compounds and gases have complete misci-bility with SCW.Therefore,SCW provides a single ?uid phase for chemical reaction,which reduces the mass transfer limi-tation of reaction [7].The solvent power and diffusivity of SCW can be also easily controlled by operation conditions,so it is easy to separate gases from SCW by simply reducing the reaction temperature and pressure [8].
Gasi?cation or oxidation of coal in SCW is a newly devel-oped technology for clean and effective conversion of coal.It is reported that energy ef?ciency of power generation from
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i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 38(2013)12786e 12794
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oxidation of coal in SCW is higher than those of conventional coal power plants at the same steam conditions [9].Moreover,the relatively low temperature of SCWG of coal impedes for-mation of NO x and SO x ,and closeness of the system excludes emissions of ?ne ashes [3,6].Cheng et al.[8]studied conver-sion of Xiaolongtan lignite in sub-and supercritical water with an autoclave in the temperature range of 350e 550 C,and found that the key factor affecting the product distribution was reaction temperature,and about 68%of carbon in the form of residue char was not gasi?ed under 550 C.Li et al.[10]conducted SCWG of coal with a continuous ?ow system.The incomplete gasi?cation was obtained and the increase of coal concentration led to a decrease in CE and H 2yield,and the similar case was also found by Jin et al.[11]and Zhang et al.[12].Yamaguchi et al.[13]investigated the non-catalytic gasi?cation characteristics of Victorian brown coal in SCW by using a novel immersion technique with quartz reactors,and found that the SCWG of coal was not complete due to the residual coal char observed in all the reactors after the SCWG reactions.Incomplete gasi?cation of coal hinders the com-mercial development of SCWG,so an important job is to explore the way to complete gasi?cation of coal at present.
Coal char contains plenty of phenolic structures,which are dif?cult to be gasi?ed and are regarded as the ‘last hurdle’to get over for complete gasi?cation [10,12,14].Therefore,once the feedstock is partially converted into char,it is hardly gasi?ed,resulting in low product gas yield and gasi?cation ef?ciency [13].To decompose the aromatic compounds and realize the complete gasi?cation,radical reaction led by radical derivatives such as hydrogen peroxide will help [15].In addition,oxidant provides an in situ heat resource to heat the gasi?cation medium rapidly through the sensitive tempera-ture range,resulting in less tar and char formation [16,17],because tar-forming and condensation reactions are known to be favored by long residence time at lower temperature [18].Some researches on partial oxidation in SCW for various biomass and model compounds were carried out [19e 23],and it was reported that oxidant could improve the gasi?cation ef?ciency and decrease the production of char.
A novel high-throughput screening technique proposed by Potic et al.[24]was used to conduct SCWG reaction with the quartz reactors.Unlike the autoclave method,the technique enables rapid heating and cooling.Furthermore,the method allows SCWG reactions to be conducted in an environment free of catalytic surfaces.Hence,it is possible to precisely characterize the SCWG reaction.In this work,to realize the complete gasi?cation of coal,non-catalytic partial oxidation
of coal in SCW with quartz reactors was conducted for the ?rst time.The effects of various operating parameters on gasi?-cation characteristics of coal were investigated,and the related reaction mechanism was also clari?ed.
2.
Experimental method
2.1.
Materials
Yimin lignite and Shenmu bituminous coal (air-dried basis)were used as feedstock,and the elemental and proximate analysis is listed in Table 1.Particle sizes of coal were in the range of 100e 150m m.Hydrogen peroxide solution (30wt%),in which the molecule of H 2O 2was used as oxidant,was pur-chased from the Tianjin Chemistry Factory.
2.2.Experimental apparatus and procedure
200mm length of quartz capillaries with inner and outer di-ameters of 1.5and 3mm respectively were employed for the experiment.One end of the quartz capillary was sealed,and the other open.
A certain amount of coal,according to its concentration,was measured by using a precision electronic balance and loaded into a quartz capillary.Then the 30wt%hydrogen peroxide solution was diluted to the desired concentration according to required ER assuming that a molecule of H 2O 2provided half a molecule of O 2[25],and 20mg of the solution was injected into the quartz capillary by using a 50m L micro syringe (If ER equaled 0,only ultrapure water was added into the quartz capillary).The reaction pressure could be calcu-lated according to thermodynamic property of pure water.The predicted inner pressures at each temperature are listed in Table 2.After that,the air inside the capillary was replaced by argon in a chamber coupled with a vacuum pump.At last,the open end of quartz capillary was ?ame-sealed by oxy-hydrogen ?ame.
The sealed quartz reactor was heated by using an electric furnace.The temperature of furnace was measured by a K
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type thermocouple and was maintained at the desired tem-perature by a temperature controller.For each test,one quartz capillary was ?xed by a customized clamping device,and then it was put into the furnace.After a certain contact time,the quartz capillary was lifted out of the furnace and cooled at atmospheric environment.We designated 600,700,800and 900 C as base temperatures and 10wt%as a base coal con-centration.Four to ?ve runs were conducted repeatedly at the each reaction condition.
2.3.Quantitative analysis
Because the solid and liquid products in the mini-scale quartz reactors were hard to be analyzed [13],only gaseous products were analyzed in this study.Herein,a convenient and fast way to analyze the gaseous products inside the small quartz re-actors was used.The quartz reactor after the SCWG reaction was placed inside a PVC tube (inner diameter of 6mm),and then the air in the PVC tube was replaced by argon at the at-mospheric pressure.Afterward,both ends of the PVC tube were tightly sealed by two clips.Then the quartz reactor was crushed by using a vice to release the product gas into the PVC tube completely.At last,the gaseous sample inside the PVC tube was extracted by a 100m L micro syringe and measured by using a gas chromatograph (Agilent 7890A)equipped with a thermal conductivity detector (TCD).
In this paper,CE is used to show the gasi?cation extent of coal,YH 2indicates the hydrogen production capacity of coal,gas fraction (based on mole)demonstrates the selectivity of gas products,and ER (Equivalent Ratio of oxidant)shows the amount of added oxidant.The expressions are as follows (Equations (1)e (4)):CE ?
the mass of carbon in gaseous product
?100;%
(1)
YH 2?the molar number of produced hydrogen
the mass of coal ead T
;mol =kg
(2)
uncertainty ?
CE àthe average value of CE
the average value of CE
?100;%
(5)
Herein,it is noted that there are a lot of factors affecting the calculated value of CE,such as the real inner diameter and measured length of PVC tube (it has an in?uence on the
calculated quantity of argon)and the process of loading and weighing of coal,so there is some deviation between the calculated value and real value.To assure the reliable data,four to ?ve runs were conducted repeatedly in the same re-action condition and error bars were also provided (Figs.2e 6).We also calculated the uncertainty of the CE of coal (Equation (5)).The results showed that most of values were within the range of 5%,and all the values were less than 5.8%,which showed that the method was good enough for gasi?cation characteristics detection [24].
3.Results and discussion
Traditional coal gasi?cation under low pressure is converting coal into natural gas or syngas by the reaction between coal and gasi?cation agent (steam,air or oxygen).Process and mechanism of coal gasi?cation play an important role in analyzing gasi?cation characteristics.Matsumura et al.[26]investigated the gasi?cation characteristics of a coconut shell activated carbon in supercritical water and found that their measurements enjoyed good agreement with the rate law for carbon in steam at subatmospheric pressure (<0.1MPa),which suggested that the reaction mechanism was not strongly in?uenced by pressure.Therefore,there are mainly three processes for coal gasi?cation in SCW just as those at low pressures as follows.(I)Drying process:water is rapidly evaporated and uniformly distributed throughout the reactor,leaving the dry coal.(II)Pyrolysis of coal:coal pyrol-ysis is important because it is the initial step in coal chemical conversion process.The volatile matter (CH 4,CO 2,CO,H 2,tar and so on)is separated out during pyrolysis of coal,leaving coal char.(III)gasi?cation reaction:at last,the gasi?cation or oxidation of volatile matter and coal char will proceed,and the reaction equations are shown in Equations (6)e (14).
D H ?131kJ =mol
(6)
CO eg TtH 2O eg T#CO 2eg TtH 2eg T;D H ?à41kJ =mol (8)CH 4eg TtH 2O eg T#CO eg Tt3H 2eg T;
D H ?t206kJ =mol
(9)
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2O 2eg T/CO eg T;
D H ?à110:5kJ =mol (10)
2O 2eg T/H 2O eg T;
D H ?à242kJ =mol
(12)
CO eg Tt1
O 2eg T/CO 2eg T;
D H ?à283kJ =mol
(13)CH 4eg Tt2O 2eg T/CO 2eg Tt2H 2O eg T;
D H ?à802kJ =mol
(14)
3.1.Effect of residence time
Five residence times (0.5,0.75,1,2and 3min)were employed for evaluating the time dependence of SCWG reaction of lignite.Fig.1a shows that CE ?rstly increases quickly within 1min,and then tends to be stable between 2and 3min at all the base temperatures.In addition,the incomplete gasi?ca-tion of the lignite at the base temperatures was obtained,and it would be because of the residual char observed in all the reactors after the SCWG reaction (it was also found by Yamaguchi et al.[13]).Xu et al.[18]employed various char-coals as catalysts for SCWG of organic compounds,implying the stability of charcoals in SCW.Indeed,they reported a slow gas generation rate (1.0?10à4mol/min)for SCWG of the co-conut shell charcoal at 600 C and 34.5MPa.It is worth mentioning that the increase of CE from 800to 900 C is larger
than that from 700to 800 C,and also much larger than that from 600to 700 C at a certain ?xed time (Fig.1a),which in-dicates that higher temperature plays an important role in reducing coal char and breaking the stable structure of poly-aromatic hydrocarbons in residual char.
A chemical equilibrium model based on minimizing Gibbs free energy was adopted to analyze the gas yields and frac-tions [11,27,28].We also calculated the reaction equilibrium values of CE and gas fractions based on the model at the base temperatures,and the comparison with the experimental values is listed in Table 3.The table shows that CE is almost 100%at equilibrium state at all the base temperatures,and it is easier to approach complete gasi?cation at higher tem-peratures within a certain time.Furthermore,when temper-ature is below 800 C,the experimental values of CO fraction are higher than the equilibrium values (Table 3),indicating that the chemical equilibrium of water e gas shift reaction (Equation (8))is not reached and its rate without catalyst is slow.However,when temperature is higher than or equal to 800 C,its rate is greatly accelerated,and the experimental values of CO fraction are less than and close to the reaction equilibrium ones.CO fraction stays around 5%within 3min at 900 C (Fig.1c).In addition,when temperature is higher than or equal to 700 C,the experimental values of CH 4and CO 2fraction are higher than the reaction equilibrium values (Table 3),illustrating that the steam reforming of CH 4(Equa-tion (9))and the reaction of C and CO 2(Equation (7))without catalyst are slow even at high temperature.When time in-creases from 0.5to 3min at 900 C,CH 4and CO 2fractions
a
C E (%)
time (min)
g a s y i e l d (m o l /k g c o a l )
time (min)
b
c
time (min)
g a s f r a c t i o n (%)
Fig.1e Effect of residence time on gasi?cation characteristic of lignite:(a)CE (10wt%coal,ER [0.1);(b)gas yield (900 C,10wt%coal,ER [0.1);(c)gas fraction (900 C,10wt%coal,ER [0.1).
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decrease slightly from 14%and 41.1%to 11%and 36.9%,respectively (Fig.1c).
It is necessary to accelerate water e gas shift reaction and steam reforming of CH 4to produce high-purity hydrogen by adding proper catalyst in further research.Steam reforming of CH 4always proceeds with the aid of a heterogeneous catalyst (Ni on an oxidic support)at temperatures of 700e 900 C and pressure of up to 50MPa [29].It is reported that alkaline compounds [11,29e 31]such as KOH could catalyze the water e gas shift reaction [12,29,32],and the CO content de-creases to 0.02vol%due to the catalytic activity of KOH in the water e gas shift reaction in the form of Equations (15)and (16)[29].Then H 2and CO 2will be the only products,moreover,the solubility of CO 2in water under pressure is signi?cantly higher than that of H 2,which means that the two components can be readily separated to produce high-purity H 2.KOH tCO #HCO 2K
(15)HCO 2K tH 2O #KOH tCO 2tH 2
(16)
3.2.Effect of temperature
High temperature facilitates the steam reforming reaction of coal (Equation (6)),leading to the decrease of coal char,and it
is a key factor for gasi?cation of coal in SCW.CE increases from only 29%to 99.6%when temperature increases from 600to 950 C at 2min (Fig.2a).Compared with traditional non-catalytic coal gasi?cation,the case using supercritical water is more ef?cient.Wang et al.[33]conducted steam gasi?cation of a bituminous coal char without catalyst at atmospheric pressure,which showed that it took more than 30min to realize the complete gasi?cation even at 1200 C.In addition,CE of lignite is 53%for 10wt%of coal concentration at 800 C when ER equals 0.1in our study,while CE is only 34.7%for 6.8wt%of coal concentration at 800 C in Yamaguchi’s paper.The reason may be not only the lower degree of coal meta-morphism for Yimin lignite than Victorian brown coal,but also oxidant addition that is discussed in Section 3.3.
H 2yield increases greatly from 1.9to 30.9mol/kg when the temperature increased from 600to 950 C (Fig.2a).CO 2is the key component while H 2fraction is low at 600 C (Fig.2b),because CO 2is mainly derived from broken carboxyl group [8]and the steam reforming of coal (Equation (6))is slow at low temperature.With the increase of temperature,the steam reforming of coal will be enhanced greatly,leading to increasing H 2fraction in corresponding with decreasing CO 2fraction.CO fraction decreases from 6.3%to 3.5%when temperature increases from 600to 800 C,and then increases to 6.4%with temperature increasing to 950 C (Fig.2b).As we have mentioned above,the chemical equilibrium of water-e gas shift reaction is not reached below 800 C,and its rate without catalyst is slow and increases with the increasing temperature,leading to decreasing CO fraction below 800 C.And then,when temperature is higher than or equal to 800 C,the experimental values of CO fraction are less than and close to the reaction equilibrium ones and higher temperature is in favor of the reverse of water e gas shift reaction,and the re-action between CO 2and C (Equation (7))also plays an important role [4,5],resulting in the increasing CO fraction.In addition,CH 4fraction maintains almost 15%below 800 C,and then decreases to 12.4%and 10.6%when the temperature is 900 C and 950 C,respectively (Fig.2b).Because steam reforming of CH 4is an endothermic reaction,it is favored at higher temperature,leading to the decreasing CH 4.
Considering commercial reactor fabrication,high temper-ature is a drawback to the SCWG process,because yield stress of metal decreases as temperature increases,leading to
a
C E (%)
C
b
Temperature C
g a s f r a c t i o n (%)
g a s y i e l d (m o l /k g c o a l
Fig.2e Effect of temperature on gasi?cation characteristic of lignite:(a)CE and gas yield;(b)gas fraction (10wt%,ER [0.1,2min;square symbols represent the average value of CE obtained from the duplicated data at each reaction condition while the error bars show the maximum and minimum values obtained).
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thicker wall and thus larger amount of material needed for reactor and high initial cost [20].Hence,catalyst is needed to decrease the reaction temperature in the further research.Wang and Takarada [34]carried out SCWG of low rank coals in an autoclave at 690 C and 30MPa,and found that Ca(OH)2facilitated the extraction of volatile matter from coal with a decrease in the amount of residual char.However,the carbon conversion was still incomplete at a Ca/C ratio of 0.6.Lin et al.[35]almost realized the complete gasi?cation of coal at 973K by using 0.1g of coal,0.6g of Ca(OH)2and 0.05g of NaOH.However,the eutectic melting of Ca(OH)2/CaCO 3was found in the experiment at 973K,which led to the growth of large particles of solid materials [36].This would cause plugging problems in the continuous ?ow system.Other catalysts such as K 2CO 3[10,11],Raney-Ni [10]and KOH [12]have been tested,however,the complete gasi?cation of coal was not realized.Therefore,it is necessary to screen proper catalysts to realize complete gasi?cation of coal in further research.
3.3.Effect of ER
When the oxidant was added,the lignite mass was decreased not only by the steam reforming of coal (Equation (6)),but also by oxidation of coal (Equations (10)and (11))[3].Martino and Savage [15]proposed a general reaction network for oxidation of phenols bearing e CH 3and e CHO substituents in SCW,the
phenolic compounds were decomposed to ring-opening products with the help of oxidant.Wang et al.[37]proposed a phenol hydroxyl oxidation mechanism that polyaromatic hydrocarbons were ?rst oxidized to phenolic hydroxyl and followed by aromatic ring opening with simultaneous for-mation of carboxyl.
Just as mentioned above,hydrogen peroxide is usually used as a source of free radicals to help decompose the phenolic compounds of coal,so CE in the present of oxidant is higher [10,12].In our experiment,CE of lignite increases from 86.6%to 99.6%at 950 C when ER increases from 0to 0.1(Fig.3a).When ER increases further,oxidation reaction of coal is enhanced and lignite is gasi?ed completely.It is noted that the calculated average value of CE equals 100.2%that violates mass balance (the similar situation is present at Figs.4e 6)when ER equals 0.2,due to the uncertainty of the gaseous sampling method just as described at Section 2.3.However,it is good enough for gasi?cation characteristics detection.H 2yield ?rstly increases from 28.3to 30.9mol/kg at 950 C when ER increases from 0to 0.1,and then decreases as ER increases further (Fig.3a).When ER is less than 0.1,less oxidant is in favor of partial oxidation of coal (Equation (10)),and then more H 2is produced by water e gas shift reaction (Equation (8)).However,combustible gas (H 2,CO and CH 4)will be consumed by the excess oxidant (Equations (12)e (14))if ER further in-creases,leading to decreasing combustible gas and increasing
C E (%)
g a s y i e l d (m o l /k g c o a
l
ER
a
b
g a s f r a c t i o n (%)
ER
Fig.3e Effect of ER on gasi?cation characteristic of lignite:(a)CE and gas yield;(b)gas fraction (950 C,10wt%,2min).
C E (%)
g a s y i e l d (m o l /k g c o a l
feed concentration (wt%)
a
b
g a s f r a c t i o n (%)
feed concentration (wt%)
Fig.4e Effect of coal concentration on gasi?cation characteristic of lignite:(a)CE and gas yield;(b)gas fraction (950 C,ER [0.1,2min).
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CO 2.Therefore,there exists proper amount of oxidant to be added [10,12].
3.4.Effect of feed concentration
It was reported that gasi?cation of model compounds (glucose and glycerol)would proceed homogeneously in the quartz tubes,resulting in high carbon gasi?cation ef?ciency at lower temperature [38].However,in the coal case,water was rapidly evaporated and uniformly distributed throughout the reactor,but complete dissolution and uniform distribution of the coal particles did not occur.During our experiment,when coal concentration was larger than or equaled 20wt%,it was found that the quartz reactor top was still clear after the reaction whereas the reactor bottom was black,indicating that the reaction was not taking place uniformly throughout the reactor (it was also observed by Yamaguchi et al.[13]).Nevertheless,the distribution of coal was much better when concentration was less than or equaled 10wt%,so the mass transfer between coal and gasi?cation agent (H 2O and oxidant)would be better for lower concentration.
As mentioned above,coal pyrolysis is important for its initial step in coal chemical conversion process.When the coal concentration in the quartz reactors was larger than or
equaled to 20wt%,volatile matter was ?rstly separated out and uniformly distributed throughout the quartz reactors,however,the residual coal char gathered at the bottom of the quartz reactors,then the contact area between coal char and gasi?cation agent (H 2O and oxidant)was decreased.Steam reforming reaction of coal (Equation (6))and oxidation of carbon (Equations (10)and (11))will be limited if mass transfer between coal char and gasi?cation agent becomes worse,resulting in that the coal char mass is hardly decreased and CE decreases from nearly 100%to 85%when feed concentration increases from 5to 30wt%(Fig.4a).Therefore,it is necessary to enhance mass transfer in further continuous ?ow reactor design,and a supercritical water ?uidized bed reactor may be a good choice [11,19,39,40].In addition,higher coal concen-tration means a lower concentration of water [19],so water-e gas shift reaction and steam reforming of CH 4will be inhibited,leading to increasing CO and CH 4fractions corre-sponding decreasing H 2and CO 2fractions (Fig.4b).
3.5.Effect of coal rank
Since wide variations occur among coals,knowledge of the effect of coal rank on gasi?cation is important for future gasi?cation applications.It has been accepted by most of
C E (%)
g a s y i e l d
(m o l /k g )
C
a
b
C
g a s f r a c t i o n (%)
Fig.5e Effect of temperature on gasi?cation characteristic of bituminous coal:(a)CE and gas yield;(b)gas fraction (10wt%,ER [0.1,2min).
g a s y i e l d (m o l /k g c o a l
ER
a
C E (%)
b
g a s f r a c t i o n (%)
ER
Fig.6e Effect of ER on gasi?cation characteristic of bituminous coal:(a)CE and gas yield;(b)gas fraction (980 C,10wt%,2min).
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scholars that coal reactivity decreases with the increasing coal rank at lower temperature[41e43].Brown et al.[41]studied the effect of coal type for four coals of varying rank in an entrained?ow gasi?er at atmospheric pressure,and obtained an increased carbon conversion with decreasing coal rank at lower reaction temperatures(lower than1400K)where re-actions were partially kinetically-controlled and coal char structure was important.Yang and Watkinson[42]gasi?ed chars from seven Western Canadian coals ranging from lignite to bituminous coal with steam in a stirred-bed reactor at870and930 C,and found that the lignite and subbitumi-nous coals were more reactive than the bituminous coals.Liu et al.[43]also indicated that coal char reactivity with CO2at 1123K decreased as coal rank increased.
In this paper,when temperature increases from600to 950 C and ER equals0.1,CE of bituminous coal increases from 21%to only75%,lower than that of lignite at950 C,so a higher temperature of980 C was tried subjectively,resulting in a higher CE of88%(Fig.5a).When ER is equal to or larger than 0.2,bituminous coal is gasi?ed completely at980 C(Fig.6a). Therefore,the reactivity of bituminous coal was lower than that of lignite,and it needs higher temperature and more oxidant to realize complete gasi?cation of bituminous coal. Herein,SCWG characteristics of bituminous coal(Figs.5and6) are similar with those of lignite(Figs.2and3).There is less moisture and ash for bituminous coal than lignite(Table1),so the quality of bituminous coal is higher.The higher the quality of coal is,the more combustible gas will be produced.When ER equals0.1,H2and CH4yields for bituminous coal are45.8 and12.1mol/kg at980 C(Fig.5a)while those for lignite are only30.9and6.8mol/kg at950 C(Fig.2a),respectively.
4.Conclusion
Non-catalytic partial oxidation of lignite and bituminous coal for hydrogen production in supercritical water was carried out in quartz reactors for the?rst time.The effects of various operating parameters were investigated.
1.CE?rstly increases quickly before1min,and then tends to
be stable between2and3min at all the base temperatures.
The reaction rate of water e gas shift reaction without catalyst is slow below800 C,and the steam reforming of CH4and the reaction of C and CO2without catalyst are slow even at900 C.
2.Temperature is a key factor for complete gasi?cation of coal
in supercritical water,and higher temperature is in favor of more gas production and higher CE.
3.Oxidant facilitates the complete gasi?cation of coal,and CE
increases with increasing ER.H2yield peaks as ER equals
0.1,and it decreases when ER further increases,because the
combustible gas will be consumed by the excess oxidant.
4.Low feed concentration is in favor of H2production,and
mass transfer between coal and gasi?cation agent(H2O and oxidant)has an important effect on complete gasi?cation of coal.
5.The complete gasi?cation of lignite was realized at950 C
when ER equaled0.1,as for bituminous coal,at980 C when ER equaled0.2.It required higher temperature and more
oxidant to realize complete gasi?cation for bituminous coal than lignite.
6.Although lignite and bituminous coal have been gasi?ed
completely,the operation condition is so severe that appropriate catalyst is needed to decrease the reaction temperature in the further research.
Acknowledgments
This work was?nancially supported by the National Basic Research of China(Contract No.2009CB220000)and the Na-tional Natural Science Foundation of China(Contract No. 50821064).
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