2006 Hydrogen production by biomass gasification in supercritical water Aparametric study

International Journal of Hydrogen Energy 31(2006)822–831/locate/ijhydeneHydrogen production by biomass gasification in supercritical water:A parametric studyY .J.Lu,L.J.Guo ∗,C.M.Ji,X.M.Zhang,X.H.Hao,Q.H.YanState Key Laboratory of Multiphase Flow in Power Engineering,Xi’an Jiaotong University,Xi’an,Shaanxi Province 710049,ChinaReceived 26December 2004;received in revised form 27July 2005Available online 13October 2005AbstractHydrogen production by biomass gasification in supercritical water is a promising technology for utilizing high moisture content biomass,but reactor plugging is a critical problem when feedstocks with high biomass content are gasified.The objective of this paper is to prevent the plugging problem by studying the effects of the various parameters on biomass gasification in supercritical water.These parameters include pressure,temperature,residence time,reactor geometrical configuration,reactor types,heating rate,reactor wall properties,biomass types,biomass particle size,catalysts and solution concentration.Biomass model compounds (glucose,cellulose)and real biomass are used in this work.All the biomasses have been successfully gasified and the product gas is composed of hydrogen,carbon dioxide,methane,carbon monoxide and a small amount of ethane and ethylene.The results show that the gas yield of biomass gasification in supercritical water is sensitive to some of the parameters and the ways of reducing reactor plugging are obtained.᭧2005International Association for Hydrogen Energy.Published by Elsevier Ltd.All rights reserved.Keywords:Hydrogen;Supercritical water;Biomass gasification;Effects of parameters1.IntroductionBiomass is a renewable energy paring with fossil fuel,biomass for energy production can lead to zero CO 2emission in a very short life cycle period since carbon in the form of CO 2and energy are fixed by photosynthesis during biomass growth.Biomass has always been a major source of energy for mankind,and it contributes about 10–14%of the world’s energy supply today [1].Conversion of biomass to energy is undertaken using two main process technologies:thermo-chemical and∗Corresponding author.Tel.:+862982663895;fax:+862982669033.E-mail address:lj-guo@ (L.J.Guo).0360-3199/$30.00᭧2005International Association for Hydrogen Energy.Published by Elsevier Ltd.All rights reserved.doi:10.1016/j.ijhydene.2005.08.011bio-chemical/biological.Thermo-chemical gasification of biomass is likely to be a cost-effective process to produce fuel gas.However,a large portion of biomass wastes is wet biomass containing up to 95%water and this wet biomass causes high drying costs if classical gas-phase gasification process is used [2].Supercriti-cal water (SCW)gasification can advantageously avoid high drying costs,and therefore,more attention has been paid to SCW gasification recently.SCW possesses properties very different from those of liquid water.The dielectric constant of SCW is much lower,and the number of hydrogen bonds is much lower and their strength is much weaker.As a result,SCW be-haves like many organic solvents so that organic com-pounds have complete miscibility with SCW.Moreover,gases are also miscible in SCW,thus a SCW reac-tion environment provides an opportunity to conductY.J.Lu et al./International Journal of Hydrogen Energy31(2006)822–831823Fig.1.Main reaction pathways for cellulose liquefaction and/or gasification[2].chemistry in a singlefluid phase that would otherwise occur in a multiphase system under conventional con-ditions[3].Gasification of biomass in SCW have many advantages such as high gasification efficiency,high molar fraction of hydrogen,and no need of a drying process for wet biomass as compared to other biomass conversion methods.In the last10years,a large number of experimen-tal investigations for the gasification of biomass model compounds and real biomass in SCW have been carried out[4–13].However,reactor plugging is still a critical problem for the SCW gasification of biomass.Most of the research conducted in this area has employed tubular reactors,but tubular reactors are susceptible to system shut-downs due to reactor plugging resulting from the formation of char at the heating section and the buildup of ash inside the reactor.Such shut-down presents a se-vere problem for the energy conversion process[14]. To solve this problem,much work has to be done. The intermittent removal of the deposits that caused plugging in the tubular reactors was proposed by Antal [5].Matsumura[14]explored fundamental engineering feasibility of the SCWfluidized bed reactor instead of the tubular reactor.Kruse[2]developed simplified reac-tion pathways for the main constituent of the biomass, i.e.,cellulose.These main reaction pathways are repre-sented in Fig.1.It is obvious that the“higher molecu-lar weight products”is the main constituent of the de-posits on the reactor wall.Furfurals and phenols are very key compounds causing reactor plugging.In this paper,the efforts are focused on the parametric effects, include pressure,temperature,residence time,reactor size,reactor types,heating rate,reactor wall properties, biomass types,particle size,catalysts and solution con-centration,on biomass gasification in SCW.2.Apparatus and experimental procedures2.1.Supercriticalflow reactorA schematic of supercriticalflow reactor is shown in Fig.2.The reactor is made of special stainless steel tubing with14mm o.d.×6mm i.d.×650mm or18mm o.d.×9mm i.d.×650mm length.It is de-signed for temperatures up to923K and pressures up to35MPa.The unique feature of the experimental ap-paratus is its ability to realize the overall continuous reaction by operating the valves.There are two feed tanks in the experimental system.One tank can be kept at high pressure for reaction and at the same time, the other can be fed with the feedstock at atmospheric pressure.The detailed experimental procedures were provided by Hao et al.[12].Briefly,both the feeder and the reactor are pressurized at the beginning of a run.Water is pumped into the reactor tube directly and pressurized in the reactor,and the reactor is heated to bring system to the set temperature and pressure. When the temperature and pressure reach the desired values,the valve between the feeder and the reactor is opened.Then waterflowing to the reactor is replaced with feeding stock.At the exit of the reactor,product is rapidly cooled down by a water jacket.A back-pressure regulator is used to decrease the exit pressure of productfluid to atmospheric pressure.After leaving the back-pressure regulator,the products pass througha gas–liquid separator.2.2.Batch reactorDesigned for temperatures of up to873K and pres-sures of up to25MPa,the batch reactor is made of 316L stainless steel with an inner volume of140ml. One gram of cellulose and10g of water are charged in the reactor.Nitrogen gas is used to purge the residual air in the autoclave,and it is pressurized to4MPa to avoid vaporization of water during reaction.2.3.SampleBiomass model compounds(glucose and cellulose) and real biomass feedstocks including wood sawdust, rice straw,rice shell,wheat stalk,peanut shell,corn stalk,corn cob and sorghum stalk are used in this824Y.J.Lu et al./International Journal of Hydrogen Energy 31(2006)822–831Fig.2.Supercritical flow reactor schematic.Table 1Analysis data of biomass feedstock being used Biomass typesElemental analysis (wt%)Proximate analysis (wt%)Calorific value CH N S O a M A V FC (kJ/kg)Wood sawdust 46.76 5.270.110.0338.478.00 1.3677.1213.5216701.3Rice straw 36.96 4.580.870.1834.767.0115.6461.9515.4013669.7Rice shell 39.71 4.940.490.0834.07 4.0416.6763.3715.9215718.6Wheat stalk 42.80 5.100.680.2137.86 6.217.1468.7517.9016458.8Peanut shell 43.80 4.92 1.520.1734.107.997.5065.8518.6616425.3Corn stalk 41.18 4.960.780.1938.418.02 6.4667.5517.9715580.6Corn cob43.24 5.280.440.0841.027.04 2.9072.6717.3916379.3Sorghum stalk43.325.420.370.2538.205.976.4769.1118.4516797.5a Difference.work.The continuous feeding of multiphase mixture of real biomass feedstock is realized by using sodium carboxymethylcellulose (CMC)[12,15].Real biomass is ground with plant mill to 40mesh before it is mixed into CMC.Water is added to achieve the desired solids loading and the mixture is continually stirred until it is homogeneous.Generally,the concentration of CMC was about 2–3wt%.Table 1shows the results of element and proximate analyses of the biomass feedstock used.2.4.AnalyticalAnalyses of the product gas are accomplished with a Hewlett–Packard model 6890gas chromatograph with thermal conductivity detectors.The carrier gas is helium and carrier gas flow rate is 10ml/min.A Carbon-2000capillary column is used,operating at 333K for 2min,followed by a 30K/min ramp to 423K,and hold 2min at 423K.The total carbon contents of the liquid phase were determined using Elementar High TOC.We have analyzed the uncertainty of experimental data,and the results show that the uncertainty of gas yield is less than 0.5%,uncertainity of pressure is less than 2.85%,uncertainity of temperature is less than 1.84%and uncertainty of gas temperature is less than 2.0%.3.Experimental results and discussionsIn this study,the gas products,which were identified and quantified,were H 2,CO,CO 2,CH 4with a smallY.J.Lu et al./International Journal of Hydrogen Energy 31(2006)822–831825G a s Y i e l d (m o l /k g )Pressure (MPa)8001000120014001600180020002200G E o r C E (%)Pressure (MPa)T O C (p p m )(a)(b)Fig.3.Effects of pressure on wood sawdust gasification in SCW.(a)gas yield;(b)GE,CE and TOC (Temperature,923K;Residence time 27s;Feedstock,2wt%wood sawdust +2wt%CMC).amount of C 2H 4and C 2H 6.In some cases,the color of the liquid effluent became yellow and a thin layer of a dark brown,oil-like tar was observed on the surface of aqueous solution.We regard the GE (gasification effi-ciency =(the total mass of the product gas)/(the total mass of the dry feed))and CE (carbon gasification effi-ciency =(the total carbon in the product gas)/(the total carbon in the dry feed))to be indicative of the hydrogen production efficiency.3.1.Effect of pressureFig.3shows the product gas yields,the unconverted total organic carbon (TOC),the GE and CE as func-tions of pressure during the gasification of a feedstock with 2wt%wood sawdust mixed with 2wt%CMC at 923K.It is shown that the parameters,such as GE and CE,are not monotonic functions of pressure.Hydrogen yield increases with increasing pressure,but CH 4and CO yields have a tendency to decrease with pressure.The results show that higher pressure seems to favorthe water–gas shift reaction,but the unconverted TOC increases with pressure.The influences of pressure on gasification can be ex-plained by roles of water for chemical reaction in SCW [16].Firstly,individual water molecules may participate in elementary reaction steps as reactants.The proper-ties of water,such as density,static dielectric constant and ion product,increase with pressure.As a result,the ion reaction rate increases and free-radical reaction is restrained with an increase of pressure.Secondly,water molecules can participate in elemen-tary reaction steps as catalysts.Many organic com-pounds,both small and polymeric,are hydrolyzed to various degrees in SCW.Hydrolysis reaction plays a significant role in biomass gasification in SCW,but it requires the presence of H +or OH −.The ion concen-tration influences the hydrolysis rate.With increasing pressure,the ion product increases,and therefore the hydrolysis rate increases too.Thirdly,SCW as a reaction medium can influence the biomass gasification.In supercritical gasification,water participates as collision partner in intermolecu-lar energy-transfer steps,which are required for nomi-nally unimolecular reactions.Water is a very effective energy transfer agent.Increasing water density should thus promote these reactions and thereby influence the overall gasification kinetics.Fourthly,water as a solvent can act as physical barrier that retards the progress of a chemical reaction.A com-plete reaction in a solvent involves three steps:(1)diffu-sion of the reactant molecules to each other,(2)chem-ical transformation,and (3)diffusion of the products away from each other.The rate of diffusion of solutes in a given solvent depends on the solvent viscosity.The viscosity of the solvent is a function of pressure and tem-perature.As a result,the reaction rate can be influenced by pressure.Diffusion of solution is hindered by the for-mation of a solvent cage around the solute molecules.Such solvent “cage effect”is strengthened for reactions in SCW,thus the reaction pathways with the proper-ties of the solvent or with water density.Cage effects hinder fission-type reactions (initiation,decomposition)in SCW by detaining the nascent products within the cage.If the products cannot escape the cage,they are more likely to recombine and regenerate the reactants.The solvent cage can also reduce the rate of reaction by isolating the reactant molecules thereby preventing solute–solute reaction,but promoting solute–solvent re-actions,such as hydrolysis and water–gas shift reaction.The cage effects increase with increasing water density.Consequently,high pressure is in favor of water–gas shift reaction,but reduce decomposition reaction rate.826Y.J.Lu et al./International Journal of Hydrogen Energy 31(2006)822–831H 2COCH 4CO 2510152025G a s Y i e l d (m o l /k g )873K 923K100020003000400050006000GECETOC20406080100120G E o r C E (%)873K 923KT O C (p p m )(a)(b)Fig.4.Effects of temperature on wood sawdust gasification in SCW.(a)gas yield;(b)GE,CE and TOC (Residence time,27s;Pressure 30MPa;Feedstock,2wt%wood sawdust +2wt%CMC).Finally,phase behavior of reactants or products in SCW can also influence the SCW gasification.In conclusion,the influences of pressure on gasification results from the integrated actions of the factors men-tioned above.It is believed that the optimal pressure of biomass gasification must exist.3.2.Effects of temperatureFeedstock of 2wt%wood sawdust mixed with 2wt%CMC was gasified at 30MPa with a residence time of 27s at 873and 923K,respectively.It is shown in Fig.4that temperature has a significant effect on wood saw-dust gasification in SCW.The yields of H 2,CO 2and CH 4at 923K are larger than those at 873K.The carbon efficiency of 80.8%is achieved at 923K,while it is only 67.1%at 923K.Similar results were obtained when the model compounds,such as glucose,were gasified in SCW [12].At low temperature the color of the liquid ef-fluent became yellow and a think layer of a dark brown,oil-like tar was observed.High temperature promotes the free-radical reactions [17],which are necessary for gas formations.Therefore the contents of the gases such as H 2and CH 4increase sharply.Antal [6]gasified10152025303540455005101520G a s Y i e l d (m o l /k g )10001500200025003000G E o r C E (%)Residence Time (s)T O C (p p m )Residence time (s)(a)(b)Fig.5.Effects of residence time on wood sawdust gasification in SCW.(a)gas yield;(b)GE,CE and TOC.(Temperature,923K;Pressure 25MPa;Feedstock,2wt%wood sawdust +2wt%CMC).completely the 10.4wt%cornstracth at a reactor peak temperature of 988K and the carbon gasification effi-ciency reached 98%.As compared with a typical tem-perature of 1073–1173K in the conventional biomass gasification [18],SCW gasification needs a lower temperature.3.3.Effects of residence timeBiomass can be decomposed rapidly in SCW.Some-times,only a few seconds was required to finish the gasification process.Fig.5displays the effect of in-creasing residence time on gasification of wood saw-dust at 923K and 25MPa.Feedstock of 2wt%wood sawdust mixed in 2wt%CMC was used.The varia-tion in the reactor residence time at each temperature resulted from the change in the feed flow rate.As shown in Fig.5(a),it is obvious that the H 2and CH 4yields increase sharply as the residence time increases from 9to 46s.At the same time,the CO yield has a tendency to decrease with increasing residence time.Fig.5(b)shows that the GE and CE increase with in-creasing residence time but the unconverted TOC inY.J.Lu et al./International Journal of Hydrogen Energy31(2006)822–831827 Table2Effect of solution concentration on biomass gasification aFeedstock Gas yield(mol/kg)GE(%)CE(%)TOC(ppm)H2CO CH4CO2C2H4C2H61.95wt%Wood sawdust+2.08wt%CMC b18.700.447.3719.840.42 1.4210992.31195.84.12wt%Wood sawdust+2.02%CMC c12.560.45 6.3216.300.30 1.209072.31812.9a Temperature,923K;Pressure,25MPa.b Residence time,46s.c Residence time,50s.the liquid effluent decreases.It is believed that longer residence time is favorable to biomass gasification. When1.95wt%wood sawdust mixed in2.08wt%CMC was gasified at923K,25MPa with a reactor residence time of46s in9mm reactor,the GE reached109%, indicating that water was not only the solvent but also a reactant.3.4.Effects of solution concentrationThe effect of solution concentration on gasification of wood sawdust in SCW was also examined.The re-sults are shown in Table2.It is shown that the concen-tration of feedstock has significant effect on biomass gasification.The CE and GE of low concentration feed-stock gasification are larger than those of high concen-tration feedstock gasification.For the gas products,the yields of H2,CH4and CO2of low concentration feed-stock gasification are higher than those of high con-centration feedstock gasification while the yield of CO is lower.When organic feedstock containing6.19wt% wood sawdust mixed in3.0wt%CMC was gasified at 923K and30MPa with a reactor residence time of62s, plugging took place after about40min.Plugging existed in the entrance region of reactor and a large amount of black solid(char)was obtained during reactor cleaning. The reasons might be as follows.Firstly,the temperature is not high enough in the entrance region of reactor be-cause biomass gasification is an endothermic reaction. At low temperature,biomass is gasified with the forma-tion of char and tar which are difficult to decompose in SCW.These tar and char deposited on the wall of re-actor and then the plugging occurs.Secondly,CMC is gasified in SCW with the formation of inorganic salts. These salts are insoluble under supercritical conditions because of the low density and the small dielectric con-stant of water[19,20].As a result,salts precipitation can cause reactor plugging.Finally,the ash in the biomass can also lead to plugging.Therefore,our existing reac-tion system is difficult to conduct the gasification of high concentration feedstock.According to results from Ref.[6],it is believed that high temperature,high heating rate and catalyst are required in the gasification of high concentration feedstock.At the same time,preventing the formation of inorganic salts is necessary.The work of studying the role of catalyst in biomass gasification and designing new reactor that can be heated quickly to desired high temperature is in process.3.5.Effects of catalystsPrevious researches indicated that some kinds of materials,such as activated carbon,metals and al-kalis,can catalyze biomass gasification in SCW for hydrogen production.Different carbon substrates were used as catalysts for the gasification of1.2M glucose at873K,34.5MPa[5].In all the cases the carbon gasification efficiency increased considerably as com-pared with those without catalyst.The nickel catalyst can catalyze the gasification from aqueous products to gases[7,8,21].Gas yield of glucose and cellulose gasi-fication can increase in SCW with ZrO2catalyst[22]. Sato[23]studied the gasification of alkylphenols as lignin model compounds in the presence of supported noble metal catalysts without hydrogen donor at673K and the activity of the catalyst was tested.The activity of the decomposition of alkylphenols is in the or-der of Ru/ -alumina>Ru/carbon,Rh/carbon>Pt/ −alumina,Pd/carbon,and Pd/ -alumina.A high CH4 yield was obtain in the presence of the Ruthenium catalyst when the cellulose and organosol-lignin were gasified in SCW[24].Alkali[8,9,12,22,25,26]such as K2CO3,KOH,NaOH and Na2CO3can catalyze the water–gas shift reaction with the formation of H2and CO2instead of CO.The effects of alkali salts may be simply the effect of a base[2].Na2CO3lowered the decomposition temperature of cellulose,inhibited the char formation,and produced more aqueous prod-ucts[8].In this work,gasification of cellulose in the presence of metal catalysts,including CeO2,n CeO2,828Y.J.Lu et al./International Journal of Hydrogen Energy 31(2006)822–831510152025G a s Y i e l d (m o l /k g )None222Pd/CRu/CG E o r C E (%)Catalyst222Pd/CRu/CCatalyst(a)(b)parison of gas yield(a),GE and CE (b)for gasification of cellulose with various catalysts in SCW (Batch reactor;water,10g;cellulose,1.0g;catalyst,0.4g;CMC,0.2g;773K;Reaction time,20min;Initial pressure,4.0MPa).n(CeZr )x O 2,Pd/C and Ru/C,was examined in a batch reactor.As shown in Fig.6,the maximal values of hy-drogen yield,GE and CE were obtained with Ru/C as a catalyst.3.6.Effects of reactor geometrical configuration Based on the same running conditions,two reactors were used for our comparison,one is 6mm inner diam-eter and the other is 9mm inner diameter.The experi-mental results are shown in Table 3.It is known that the effect of the reactor geometrical configuration on the gas yield is not obvious.The reactor with larger inner diameter seems to favor the biomass gasification.The possible reason is that the inner diameter influences the heat transfer between feedstock and the reactor.3.7.Effects of reactor typesThree typical reactors,including batch reactors [7–10,13,21,22],tubular flow reactors [4–6,11,12]and continuous stirred tank reactors (CSTR)[2,25],wereused for biomass gasification in SCW.The batch reac-tor has a sample structure,but the reaction processes are not isothermal and the different reaction occurring during the heat-up stages and the cool-down stages.The tubular flow reactor can make up for some disad-vantages of the batch reactor and achieve a continuous gasification of biomass with a short residence time.Tubular reactor is also well suited for reaction analysis,as the assumption of plug flow reactor is well applied to tubular structures,but the key technique needed for the SCW gasification is the prevention of reactor plugging.The continuous stirred tank reactor is an integration of autoclave and tubular flow reactor.The biomass can be mixed uniformly in the reactor and heated abruptly,but the system is more complex.Char/coke is formed in the batch and tubular flow reactor but not in the CSTR,but it is not known if higher biomass contents are gasi-fied in CSTR with formation of Char.Recently,a SCW fluidized bed for biomass gasification was proposed by Matsumura [14].It may avoid reactor plugging,but it is still at the theoretical designing stage.A lot of diffi-culties have to be faced because of the complexity of SCW fluidized bed.A porous reactor,which prevents sticky solids from depositing on the reactor and reduces corrosion,is used in SCW oxidation process [27].It may be used for reference of SCW gasification reactor.3.8.Effects of reactor wall propertiesThe tubular reactor is made of special stainless steel which is composed of Ni,Cr and other metals.As it has been mentioned above,these metals can cat-alyze biomass gasification.Results from experiments of biomass gasification show that carbon buildup can oc-cur on the reactor wall even though the concentration of feedstock is not high.It is believed that carbon on the reactor will reduce the catalytic effects of the metallic wall if the carbon is not eliminated before the beginning of experiment.Table 4presents results of reactor wall properties effect on gasification.It is obvious that gas yields,GE and CE increase when the carbon on reactor wall is eliminated.Also,reactor wall with carbon elim-inated will reduce the probability of reactor plugging because the carbon on the wall is the possible carrier for carbon formation in the next gasification experiment.3.9.Effects of heating rateAt low temperature biomass can be decomposed with formation of an amount of inert compounds,which can only be gasified very slowly even at increased temper-ature.Fast heating can shorten the residence time atY.J.Lu et al./International Journal of Hydrogen Energy31(2006)822–831829 Table3Effect of the reactor geometrical configuration on biomass gasification aInner diameter Gas yield(mol/kg)GE(%)CE(%)TOC(ppm) H2CO CH4CO2C2H4C2H66mm12.21 1.02 4.4115.440.390.7583.669.73255.339mm13.46 1.91 4.2516.440.370.609072.42924.50a Residence time,15s;Pressure,25MPa;Temperature,923K;Feedstock,2.03wt%Sawdust+2.02wt%CMC.Table4The catalytic effects of the reactor wall on biomass gasification aWall conditions Gas yield(mol/kg)GE(%)CE(%)TOC(ppm) H2CO CH4CO2C2H4C2H61b10.050.38 2.8912.270.290.4263.7854.361571.752c13.300.51 3.8516.140.380.6084.1371.751515.88a Temperature,923K;Pressure,25MPa;Residence time,30s;Feedstock,2wt%Rice shell+2wt%CMC.b Carbon was not eliminated.c Carbon was eliminated.Table5The effect of biomass particle size on gasification aParticle size(mesh)Gas yield(mol/kg)GE(%)CE(%)TOC(ppm) H2CO CH4CO2C2H4C2H640–8013.740.11 3.3110.470.270.4356.5150.21455.38<8017.000.34 5.0617.510.460.8293.2277.281673.63a Temperature,923K;Pressure,25MPa;Residence time,30s;Feedstock,2wt%Rice straw+2wt%CMC.every low temperature,so that the inert compound yield decreases sharply and the gas yield increases.Sinag et al.[28]conducted the experiment of hydropyrolysis of 5wt%glucose in SCW at heating rates of1and3K/min. The results show that high yields of hydrogen,methane and carbon dioxide were obtained with high heating rate,but carbon monoxide was low.Consequently,high heating rate is in favor of biomass gasification in SCW.3.10.Effects of biomass particle sizeBiomass was pretreated with mechanical grinding before it was gasified.The experiment of biomass gasification was conducted with different particle sizes.It is believed from the result in Table5that higher hydrogen yield is obtained with gasification of smaller particle sizes.It is well known that mechani-cal grinding requires much energy.If smaller particles are obtained,more energy will be consumed.Even though smaller particles produce more hydrogen when gasified,it is not economical.An optimal particle size should be found with considering economy and feasibility.3.11.Effects of biomass typesThe real biomass including wood sawdust,rice straw, rice shell,wheat stalk,peanut shell,corn stalk,corn cob and sorghum stalk,were gasified under the same conditions.In Fig.7(a),results from different biomass gasification show that the gas compositions are not dis-tinguishable.The molar fraction of hydrogen is about 40%and the molar function of CO is less than1%. There are two reasons for so low CO content.Firstly,the high excess water leads to a preference of the formation of H2and CO2instead of CO.Secondly,the K2CO3 content of real biomass is slightly higher than0.5wt% [25],It was believed that the K2CO3content catalyz-ing the biomass gasification reaction resulted in the low CO content.It is shown in Table6that higher CO con-tent was gained from gasification of glucose without catalyst.In the presence of K2CO3,composition of the830Y.J.Lu et al./International Journal of Hydrogen Energy 31(2006)822–831parison of gas composition (a)GE,CE and TOC (b)for gasification of various biomass feedstocks in SCW (Temperature,923K;Residence time 27s;Pressure,25MPa;Feedstock,2wt%biomass +2wt%CMC).A,rice straw;B,rice shell;C,wheat stalk;D,peanut shell;E,corn stalk;F,corn cob;G,sorghum stalk;H,wood sawdust.Table 6Gasification of glucose in SCW with or without alkali addition FeedstockGas composition (%)GE (%)CE (%)TOC (ppm)H 2CO CH 4CO 2C 2H 4C 2H 6Glucose a 33.514.511.736.30.1 3.9111.486.3—Glucose b39.810.215.044.99——46.9636.8—a Temperature,923K;Pressure,25MPa;residence time,2.9min;feedstock concentration,0.2mol/L [12].b Temperature,773K;Pressure,30MPa;residence time,5.3min;feedstock,5wt%glucose with 0.5wt%K 2CO 3[25].Table 7Recycle of the liquid effluent a FeedstockGas composition (%)GE (%)CE (%)TOC (ppm)H 2CO CH 4CO 2C 2H 4C 2H 6Liquid effluent a45.340.5814.3936.301.042.36—39.81436.6a Temperature,923K;Pressure,30MPa;Residence time 20.8s;Feedstock,liquid effluent of wood sawdust gasification (the value ofunconverted TOC is 2585.9ppm).gas product from glucose gasification is similar to that from the gasification of real biomass.The molar frac-tion of CH 4is more than 10%here,but it is not in agreement with the chemical equilibrium calculation.From the kinetic point of view,it is not clear via which reaction pathway the CH 4is formed.It can be a free-radical mechanism or hydrogenation of CO.The second reaction is only possible in the presence of a suitable catalyst [29].This shows that CH 4is possibly mainly formed by free-radical reactions.Also,it can be seen from Fig.7(b)that wheat stalk,corn cob and sorghum stalk are easier to gasify than the other biomass.The un-converted TOC in liquid effluent is high.It is said that a portion of biomass are converted to liquid product in-stead of gas product.The liquid effluent cannot be let off directly because of environment problem.The liquid effluent of wood sawdust gasification could be recycled in SCW and the results are shown in Table 7.It is can be seen that the unconverted TOC decreases and more gases are gained due to recycling the liquid effluent.4.ConclusionBiomass can be gasified successfully in SCW with a product gas consisting H 2,CO,CH 4,CO 2and a small amount of C 2H 4and C 2H 6.Generally,40%molar fraction of H 2can be reached.Hydrogen。