Enhanced catalysis of K2CO3 for steam gasification of coal char by using Ca(OH)2

Enhanced catalysis of K 2CO 3for steam gasification of coal char by using Ca(OH)2in char preparationJie Wang *,Yihong Yao,Jianqin Cao,Mingquan JiangDepartment of Chemical Engineering for Energy,East China University of Science and Technology,No.130Meilong Road,Shanghai 200237,Chinaa r t i c l e i n f o Article history:Received 12January 2009Received in revised form 1September 2009Accepted 1September 2009Available online 13September 2009Keywords:Coal char Gasification Hydrogena b s t r a c tA novel approach has been proposed for mitigating the potassium deactivation in the K 2CO 3-catalyzed steam gasification of coal char by addition of Ca(OH)2in the char preparation.It was experimentally found that the Ca(OH)2-added char had higher reactivity for the catalytic gasification than the raw char.Ca(OH)2played a role in suppressing the interactions of K 2CO 3with acidic minerals in coal during the gas-ification and also probably in forming more active oxygenated intermediate on the char surface.The dis-tribution of gaseous products was examined during the catalytic gasification.An oxygen transfer and intermediate hybrid mechanism is applied for understanding of the rate and selectivity of the catalytic gasification.Ó2009Elsevier Ltd.All rights reserved.1.IntroductionCatalytic performances of alkali and alkaline earth metal salts on steam gasification of coal or coal char have been extensively investigated with scientific and application interests [1–6].Potas-sium carbonate is generally regarded as a most promising catalyst for commercial application by virtue of its superior catalytic prop-erties.This catalyst was employed in the Exxon pilot-scale coal gasification which was demonstrated using a fluidized-bed gasifier in late 70s [7,8].Later,Hauserman et al.at University of North Da-kota also selected this catalyst to develop a process of catalytic coal gasification integrated with fuel cells [9,10].At present,however,none of the catalytic coal gasification tech-nologies has been utilized commercially.As regards the K 2CO 3-cat-alyzed coal gasification,a major obstacle is that the catalyst suffers from deactivation,which lowers the gasification rate and causes a difficulty of recovering the catalyst.It has been revealed that the interaction between potassium and clay minerals in coal is mainly responsible for the deactivation of catalyst [11–13].To reduce the expenditure of catalyst,the Exxon process proposed a subsidiary process of potassium recovery by digesting the ash with Ca(OH)2.An alternative solution is the use of relatively inexpensive potas-sium-based compounds instead of potassium carbonate,as it may become feasible to discard the catalyst together with the ash.For example,Huttinger et al.[14,15]tested the mixtures of KCl and K 2SO 4or the mixture of KCl and FeSO 4for coal gasification.These additives have been manifested to have a similar catalytic activity but there is a disadvantage that KCl is decomposed to cor-rosive HCl.Takarada et al.[16]invented a method to impregnate potassium onto lignite using a solution of KCl under a pH control with NH 3and Ca(OH)2.They claimed that potassium was ion-ex-changed to lignite while chloride was completely removed by water washing,and the ion-exchanged potassium had a strong cat-alytic gasification activity.Wang et al.[17]and Sharma et al.[18]reported the K 2CO 3-catalyzed gasification of the highly demineral-ized coals to overcome the problem of catalyst deactivation.The shortcoming of this method is that the process of coal demineral-ization to a high level is not commercially available.On the other hand,it is well known that some common cal-cium-bearing compounds,such as Ca(OH)2,CaCO 3and Ca(COO)2,are catalytically active for coal gasification,particularly for lignite gasification [19–21].Nevertheless,the gasification of a medium-or high-rank coal char with calcium is not kinetically remarkable at a low-temperature used typically for potassium-catalyzed gasi-fication due to a weaker catalytic activity of calcium [19].Radovic et al.made a comparison between potassium and calcium in the gasification of North Dakota lignite [22].They observed that potas-sium achieved a higher catalytic activity than calcium,no matter how it was added to the lignite or its char.In this study,we have explored an approach for improving the catalytic activity of K 2CO 3for steam gasification of coal char by adding cheaper Ca(OH)2to coal before the char formation.Ca(OH)2functions as an effective promotor towards the K 2CO 3-catalyzed steam gasifica-tion of coal char.We have further discussed how Ca(OH)2influ-ences the performances of the K 2CO 3-catalyzed gasification of coal char.0016-2361/$-see front matter Ó2009Elsevier Ltd.All rights reserved.doi:10.1016/j.fuel.2009.09.001*Corresponding author.Tel./fax:+862164252853.E-mail address:jwang2006@ (J.Wang).Fuel 89(2010)310–317Contents lists available at ScienceDirectFueljournal homepage:www.else v i e r.c o m /l o c a t e /f u el2.Experimental2.1.Coal sample and char preparationThe coal sample used in this study was an Australian Newlands bituminous coal,which was a coal sample from the Coal Bank man-aged by National Institute of Advanced Industrial Science and Tech-nology,Japan.The coal was crushed and ground to the particle sizes of less than0.15mm.The proximate analysis showed that the coal contained13.2%ash,27.7%volatile matter,and59.1%fixed carbon, on the dry basis.The ultimate analysis showed that the coal con-sisted of86.3%C,5.33%H,1.84%N,0.36%S,on the dry ash-free ba-sis.The ash was composed of49.7%SiO2,38.4%Al2O3,4.2%Fe2O3, 1.6%CaO,0.80%MgO,0.27%Na2O,0.78%K2O,and others.The coal char was prepared by a method similar to the standard method of coal devolatilization(ASTM D3175-07).Briefly,the accurately weighed amount of coal sample wasfilled in an alumina crucible which was capped to isolate the sample from air.The cru-cible was then promptly placed into a heated furnace at a predeter-mined temperature for30min.The char was collected after pyrolysis.In preparation of the Ca(OH)2-added char,the coal sam-ple was thoroughly mixed with an analytical grade reagent of pow-dery calcium hydroxide purchased from Shanghai Lingfeng Chemicals Co.(purity:P95%)with agate mortar and pestle,and then the mixture was subjected to pyrolysis in the same manner as described above.The Ca(OH)2loading is referred to as the weight percents of Ca(OH)2in the initial amount of the Ca(OH)2/ coal mixture.For brevity,the char samples prepared without and with Ca(OH)2are named the raw char and the Ca(OH)2-added char (CA char),respectively.The temperature labeled on the sample names,for example,the900°C CA char,refers to the pyrolysis tem-perature.The resultant char was ground to the particle size of less than0.15mm.The pulverized char sample was used for elemental analysis and coal gasification.2.2.Steam gasificationThe experimentation of char gasification was described in detail elsewhere[23].Briefly,the gasification was carried out in a hori-zontal corundum tubular reactor,which was heated by an electric furnace.An analytical grade reagent of powdery anhydrous potas-sium carbonate purchased from Shanghai Lingfeng Chemicals Co. (purity:P99%)was added to the char sample by thoroughly blend-ing the mixture in an agate mortar.The K2CO3loading is referred to as the weight percents of K2CO3in the initial amount of the K2CO3/ char mixture.About0.2g sample of the mixture was thinly spread on a platinum boat,and then heated at a heating rate of10°C/min to a predetermined temperature under a stream of argon(purity: P99.98%).The isothermal gasification started by switching the ar-gon stream to the steam/argon stream(partial pressure of steam: 0.5)at atmospheric pressure.Theflowing rate of argon was 500mL/min,which was controlled by the massflow meters.The outgoing gas passed through a water condenser followed by two moisture trappers.Theflowing rate of desiccated gas at the outlet was measured by afilm volumetric method.By the way,some heat-treatment experiments were carried out using the gasifica-tion reactor under a stream of argon to investigate the interaction between K2CO3or Ca(OH)2and mineral matter in coal.The major gaseous products CO,CH4and CO2were quantita-tively determined online using a rapid gas chromatograph(Agilent Micro3000)equipped with a thermal conductivity detector.This gas chromatograph used helium as the carrier gas to enhance the measurement precision of CO and CH4.H2was only qualitatively observed on this gas chromatograph.In determining the gas com-position,the gas was collected by a sequence of gas bags,and then used for the quantification of H2on a gas chromatograph(Agilent 6820)using argon as the carrier gas and the quantification of the other three gases on the Agilent Micro3000.2.3.Other analysesDissolution experiment was carried out to examine the solubil-ity of potassium in water for the ash or residue sample.The ash or residue sample together with the platinum boat was taken out from the reactor after gasification,and it was immersed in the distilled water for a while.Then the boat was taken out through water wash-ing,and the solution with some suspended matter was stirred at room temperature for30min.Afterfiltration,the amount of potas-sium in the solution was determined with ICP-AES.The dissolution of potassium was defined as the amount of potassium dissolved out, divided by the amount of potassium in K2CO3added in the sample.XRD was performed on a diffractometer(Rigaku D/max 2550VB/PC).Elemental analysis(C,H,and N)was carried out on an elemental analyzer(Elementar Vario EL III).Sulfur analysis was accomplished on a Coulomb sulfur analyzer(CLS-2).Potas-sium metal in a solution was measured by inductive coupled plas-ma atomic emission spectrometry(ICP-AES,model IRIS1000). Microscopic observation of char sample was performed on a scan-ning electron microscopy system that was equipped with energy-dispersive X-ray analysis(SEM-EDX,JSM6360LV).3.Results3.1.Effect of Ca(OH)2addition on the gasification rateFig.1shows typical results of the K2CO3-catalyzed steam gasifi-cation of the raw char and the CA char at750°C.The gas releaseJ.Wang et al./Fuel89(2010)310–317311rate is defined as the amount of gas released per minute per weight of carbon in the char sample fed for gasification.The carbon con-version is defined as the accumulated amount of carbon in all three major carbon-containing gases including CO 2,CO and CH 4,divided by the initial amount of carbon in the char sample fed.The carbon contained in K 2CO 3is presumed to be recycled in the gasification,and the amount of carbon in the catalyst is not taken into account in calculation of the carbon conversion.The data of the replicate experiments are shown in Fig.1b due to a concerned heterogeneity of the physically mixed char/catalyst sample.It was seen that small quantities of CO 2and CO were released during the heat-up,and the carbon conversions were 1.0%and 2.9%for the raw char and the CA char,respectively,indicating that the gasification was insignificant in this stage.After switching to the steam/argon stream at 750°C,the evolution of CO 2became vigorous for either char,with the con-comitant formation of H 2(qualitative observation);simulta-neously,a small quantity of CO was evolved but no CH 4was formed.An interesting observation was that the raw char and the CA char behaved quite differently in the gasification rate.For the raw char,the release rate of CO 2exhibited a gradual decline after a low carbon conversion of 20%,and the carbon conversion re-mained incomplete even after 90min of the gasification,whereas for the CA char,the release rate of CO 2did not decrease until the carbon conversion reached over 80%,and the gasification was com-plete after 60min.A reduction in the gas release rate after a low carbon conversion for the raw coal is attributable to the catalyst deactivation.In contrast,the release rate of CO 2was slightly in-creased for the CA char in a carbon conversion range of 20–50%,and this could be ascribed to the enhanced catalysis due to the con-centrated effective catalysis.Fig.2shows the effect of the Ca(OH)2loading on the char reac-tivity for the K 2CO 3-catalyzed steam gasification.Even with addi-tion of 4%Ca(OH)2,the CA char demonstrated substantially higher reactivity than the raw char.With increasing the Ca(OH)2loading,the CA char showed an increase in the rate of the K 2CO 3-catalyzed steam gasification.Fig.3shows the pyrolysis temperature dependence of the char reactivity.It was observed that the K 2CO 3-catalyzed steam gasifica-tion was completed after 80min of the gasification for all CA chars obtained at different pyrolysis temperatures.The gasification rate is quantified by a reactivity index (R 0.5)[24],defined asR 0:5¼0:5s 0:5where s 0:5is the gasification time (min)taken to reach a carbon conversion of 50%.Reactivity index against pyrolysis temperature showed a valley-shaped trend from 700to 900°C.In addition,the catalytic gasification of three raw chars formed at the pyrolysis temperatures of 700,800,and 900°C was examined to compare with the results of the corresponding CA chars.All three raw chars were not entirely gasified after 80min of the gasification.It was meant that the CA chars obtained at all these three temperatures had higher gasification reactivity than the corresponding raw chars.To clarify how the addition of Ca(OH)2in char preparation af-fects the K 2CO 3-catalyzed steam gasification of char,we examined two different sequences of Ca(OH)2and K 2CO 3addition.One was the co-addition of Ca(OH)2and K 2CO 3to coal,and the other was the co-addition of Ca(OH)2and K 2CO 3to the raw char.These addi-tion sequences are compared to the two-step addition sequence described above,namely,the addition of Ca(OH)2to coal followed by the addition of K 2CO 3to the CA char.The gasification rates ob-tained by the different addition sequences are shown in Fig.4.The gasification rate of the CA char without K 2CO 3is illustrated in the figure,and it was extremely slow.Note that the addition of Ca(OH)2alone to the raw char was also examined,the data were almost superimposed to those for the gasification of the CA char without K 2CO 3,indicating that Ca(OH)2alone had weak catalysis for the gasification of the char at 750°C.The co-addition of Ca(OH)2and K 2CO 3to coal also showed a low gasification rate.The co-addition of Ca(OH)2and K 2CO 3to coal char led to a faster gasification than the co-addition of Ca(OH)2and K 2CO 3to coal or the addition of K 2CO 3alone to coal char (Fig.1a).The two-step addition sequence achieved the greatest promotion towards the gasification rate.Only in this case was the gasification of the coal char completed in 60min.3.2.Characterizations of chars and ashesKaolinite and quartz were assigned to be the major crystalline mineral species in the used coal by XRD analysis (not shown).Fig.5shows the SEM-EDX observations of a sample of the K 2CO 3/char mixture which was experienced by the heat-treatment under argon at 750°C for 30min.The left image shows a charparticle312J.Wang et al./Fuel 89(2010)310–317which embraced less mineral grains.The EDX peaks of some spots (A3–A6)were characteristic of some potassium compounds which had not reacted with mineral matter in coal.The right image illus-trates a mineral-rich char particle.Some spots(B1–B5)were likely to be the reaction products of K2CO3and kaolinite.Fig.6illustrates the SEM-EDX observations of a sample of the char prepared by the coal pyrolysis with10%Ca(OH)2at700°C for30min.The left image shows a mineral-lean char particle,on which spots C2and C4are identified as the unreacted calcium oxide.The right image illustrates a mineral-rich char particle,on which some spots(D2–D5)looked like some associations of illite or kaolinite with calcium oxide.Spot D1was composed of Al and Si but no Ca,indicating that some clay was not touched by calcium.However,SEM-EDX analysis only provides a qualitative obser-vation of the interactions between potassium or calcium with min-eral matter in coal.Since it is known that the potassium interacted with mineral matter in coal became insoluble in water[13],we examined the solubility of potassium present in the ash or residue to quantify the extents of interaction between potassium and min-eral matter in coal.Table1shows the dissolutions of potassium for the ashes derived from the gasification of the different chars with 10%K2CO3.All the ashes derived from the CA chars had appreciably higher dissolution of potassium than the raw chars,strongly sug-gesting the suppressed interactions between K2CO3and minerals in coal by means of the Ca(OH)2addition.It can be envisaged that Ca(OH)2played a role in inactivating some acidic clay minerals be-fore gasification.The clay minerals in coal might be inactivated to a larger extent as the pyrolysis temperature was raised to900°C or higher,leading to a higher dissolution of potassium.3.3.Gas compositionFig.7shows the yields of four major gases(H2,CO2,CO,and CH4)resulting from the complete gasification of three CA chars with10%K2CO3.It should be noted that the comparison in the yields makes sense under the premise of complete gasification.If the yields of four gases are taken to calculate the gas composition, H2and CO2comprise66.7–66.8v%and28.2–29.2v%in the gaseous product,respectively,with a low CO concentration of2.9–4.0v% and virtually no CH4(about0.1v%),for three chars.It was seen that the char prepared at a higher temperature yielded subtly more H2 and less CO.Fig.8shows the molar ratios of H2to2CO2+CO versus carbon conversion for the K2CO3-catalyzed steam gasification of two CA chars.The molar ratio is calculated according to the accumulated amounts of gases at the indicated point of carbon conversion.For the700°C CA char,the molar ratio of H2/(2CO2+CO)was substan-tially larger than1at low carbon conversion;it decreased sharply with increasing carbon conversion to about20%,and then gradu-ally decreased andfinally became close to 1.The900°C char showed a trend similar to the700°C char,but its variation wasweaker. A3A4A6A5A1A2B3B1B5B2 B4J.Wang et al./Fuel89(2010)310–317313Fig.9shows the molar ratio of CO/CO 2versus carbon conversion for two chars obtained with different Ca(OH)2loadings.The molar ratio is calculated according to the cumulative amounts of CO and CO 2at the indicated point of carbon conversion.Both chars ap-peared to have a maximal molar ratio of CO/CO 2at a carbon con-version around 5%.After that the CO/CO 2molar ratio decreased with increasing carbon conversion,implying the subdued release of CO in the later stage of the catalytic gasification.It could be seen that the addition of more Ca(OH)2in char preparation resulted in a higher CO/CO 2molar ratio,especially in the stage of low carbon conversion.2 umC4C3C1C2D2D1D3D4D5Table 1Dissolution of potassium present in the residues or ashes derived from the gasification of the raw chars (obtained with different pyrolysis temperature,without Ca(OH)2and with 10%Ca(OH)2).Gasification conditions:10%K 2CO 3,750°C,80min.Pyrolysis temperature (°C)Dissolution (%)Raw charCA char 70020.547.2750–44.680021.734.1850–47.790026.981.2314J.Wang et al./Fuel 89(2010)310–3174.Discussion4.1.An oxygen transfer and intermediate hybrid mechanismThere is a large body of the literature regarding the mechanism of the alkali catalyzed gasification [2,4,5,25–30].We have recently proposed an oxygen transfer and intermediate hybrid mechanism for elucidating the rate and the selectivity of the catalytic gasifica-tion [23].To explain the experimental results in this study,it is worthwhile remarking on the mechanism briefly again.For conve-nience,Fig.10is duplicated from the previous paper [23].K 2O–C and K 2O 2–C stand for,respectively,a reducing form and an oxidiz-ing form of the K–C–O intercalates,which catalyze the gasification of carbon in a redox cycle way.The core of this mechanism is the introduction of another intermediate,C(O),to the gasification pathway.C(O)is an adsorbed oxygen or oxygenated species on the carbon surface,which is regarded as a key intermediate in a unified mechanism of the non-catalytic gasification and the cata-lytic gasification [30].From the mechanism,the catalytic gasifica-tion rate depends not only on the concentration of K–C–O but also on that of C(O).The gasification selectivity towards the forma-tion of CO and CO 2is governed by the competition between the desorption of C(O)(reaction a in Fig.10)and the reaction of C(O)with K 2O 2–C (reaction b).If a denotes the ratio of the desorption rate of C(O)to the total conversion rate of C(O),namely,r a /(r a +r b ),the overall gasification reaction can be expressed asC þð2Àa ÞH 2O !ð2Àa ÞH 2þa CO þð1Àa ÞCO 2ð1Þ4.2.Effects of the Ca(OH)2additionPotassium carbonate catalytically enables the steam gasifica-tion of coal char to take place at a temperature as low as 700°C with the aid of its high mobility on coal char and its further inter-action with carbon [16,17,25,31].As potassium is chemically com-bined with minerals in coal,it is resistant to the formation of the K–C–O intercalates during the gasification,leading to the deactiva-tion of catalyst.Brono et al.pointed out that some acidic minerals such as kaolinite and illite had a strong affinity to K 2CO 3,while quartz had a weak affinity at 700°C [12].K 2CO 3reacts with kaolin-ite even at 400°C,forming water-insoluble compounds such as kaliophilite (KAiSiO 4),leading to the deactivation of potassium cat-alyst [13].It was reported that the low-temperature ash of New-lands coal contained 78%kaolinite,8%quartz and 3.5%montomorillonite [32].For this kind of coal,kaolinite could be a predominant mineral which caused the deactivation of potassium.A rational reason for the promoted catalytic gasification rate (Fig.1)and higher dissolution of potassium present in the ash (Ta-ble 1)via the Ca(OH)2addition is the interactions between Ca(OH)2and kaolinite during the coal pyrolysis.Although Ca(OH)2may not significantly react with kaolinite forming calcium alumosilicates at a low-temperature,it may alter the chemical properties of kaolin-ite particle surface and thus hinder the interaction between kaolin-ite and potassium.As the temperature was raised to 900°C,some reactions between kaolinite and Ca(OH)2may lead to a sharp in-crease in the dissolution of potassium.In addition,an increase in the gasification rate (Fig.2)with increasing Ca(OH)2loading could be due to the improved contact of mineral grains with Ca(OH)pared to the co-addition of Ca(OH)2and K 2CO 3to coal or the co-addition of Ca(OH)2and K 2CO 3to the raw char,the two-step addition sequence achieved a high gasification rate (Fig.4)because it avoided the reactions between K 2CO 3and reactive minerals in coal to a great extent.It should be kept in mind that apart from the K–C–O interca-lates,C(O)is another critical intermediate for triggering the gasifi-cation.Although the ash derived from the 700°C CA char had lower dissolution of potassium than the 900°C CA char (Table 1),the for-mer char exhibited higher reactivity index than the latter char (Fig.3).This is probably because the low-temperature char is prone to the C(O)formation on the char surface during the gasification than the high-temperature char.The manufacture of more C(O)on the char resulted in higher reactivity for reactions a and b (Fig.10).The valley-shaped variation of the reactivity index against pyrolysis temperature (Fig.3)could be explained by the concurrent impacts of temperature on the interaction of clay mineral with Ca(OH)2and on the formation of C(O)as well.Zhang et al.confirmed that calcium oxide facilitated the forma-tion of oxygen complexes on carbon [33].Miura et al.proposed the mechanism of the C(O)formation promoted by CaO [34].Conse-quently,another reason for the enhanced rate of the K 2CO 3-cata-lyzed gasification is speculated to be the formation of more C(O)on the resultant char by Ca(OH)2.Although Ca(OH)2alone did not significantly facilitate the gasification at 750°C (Fig.4),Ca(OH)2exerts an important concerted catalysis together with K 2CO 3.This is responsible for a higher gasification rate by the co-addition of K 2CO 3and Ca(OH)2to the raw char than that by the addition of K 2CO 3alone (Figs.1and 4).The concerted catalysis may account for the well-known fact that calcium exhibits a signif-icant catalysis for low-rank coals but not for high-rank coals,be-cause lignite is generally enriched in active alkali metals.It could be deduced from reaction 1that the gross molar ratio of H 2to (2CO 2+CO)equals to 1,regardless of the value of a .Never-theless,the molar ratio of H 2to (2CO 2+CO)varied dramatically with the carbon conversion (Fig.8).This is because that in the beginning stage of gasification,the intermediate C(O)may beK 2O –CK 2O 2–CH 2OH 2CC(O)COCO 2H 2OH 2r ar b K 2O –CK 2O 2–CFig.10.An oxygen transfer and C(O)intermediate hybrid reaction scheme of steam gasification of carbon catalyzed by potassium.J.Wang et al./Fuel 89(2010)310–317315accumulated in a significant amount on the char,leading to a H2/ (2CO2+CO)molar ratio of substantially larger than1.Then,the molar ratio gradually became close to1,as C(O)on the char was experienced by a relatively steady state and afinal diminution. The700°C char may host more C(O)than the900°C char,so that the former char showed a higher reactivity(Fig.3)and a more dis-tinct variation of the H2/(2CO2+CO)molar ratio with carbon conversion.The variation of CO/CO2with carbon conversion(Fig.9)is explainable from the postulated formation of C(O)during the gas-ification.With increasing carbon conversion,the concentrated K2O2–C allows an accelerated reaction with C(O)to form CO2,while the desorption of C(O)to CO becomes relatively weak.This leads to a reduction in the CO/CO2molar ratio.Interestingly,the addition of more Ca(OH)2in the char preparation demonstrated a higher CO/ CO2molar ratio especially at low carbon conversion.This implies that the Ca(OH)2addition is favorable for the formation of more C(O),leading to a more rapid desorption of C(O)to CO(reaction a in Fig.10)relative to reaction b especially at low carbon conver-sion.It should be noticed that an increase in the CO/CO2ratio oc-curred at the beginning of gasification,and the reason may be due to a rapid formation in the concentration of C(O)immediately after switching argon to steam but the exact reason remains un-known.By the way,the gross CO/CO2ratios obtained from the cat-alytic gasification of three chars shown in Fig.7were0.13,0.12and 0.10for the700°C CA char,the800°C CA char and900°C CA char, respectively.This is consistent with the aforementioned specula-tion that the low-temperature char has a high concentration of C(O).From reaction1,a decrease in the value of a means the produc-tion of more H2with the formation of less CO.Theoretically,if none of CO is produced,the catalytic gasification of pure carbon with steam yields a maximal concentration of H2in the gaseous product of66.7v%.Since the chars used in Fig.7contained0.71–1.33wt.% hydrogen element which formed H2during the gasification,the concentrations of H2in the gaseous products were close to or slightly higher than66.7v%(Fig.7),despite the presence of some CO.It was reported that the low-temperature steam gasification with potassium carbonate features in the production of a hydro-gen-rich gas[23].The Ca(OH)2addition led to a slight change but not to an essential change in the gas composition formed from the potassium-catalyzed steam gasification of char.5.Conclusions(1)A novel approach has been proposed for mitigating the cat-alyst deactivation in the K2CO3-catalyzed steam gasification of coal char by use of cheaper Ca(OH)2in the char prepara-tion.The Ca(OH)2-added char exhibited a higher gasification rate than the raw char.(2)A major cause of the promoted catalytic gasification byCa(OH)2is the suppressed interactions of K2CO3with miner-als like kaolinite in coal,depending on the pyrolysis condi-tion.For the char obtained by the pyrolysis with addition of10%Ca(OH)2at900°C,about80%of potassium was recov-ered by distilled water leaching from the ash derived from the gasification with10%K2CO3,whereas only26.9%of potassium was recovered from the corresponding ash with-out addition of Ca(OH)2.(3)The formation of C(O)on the char surface is postulated to bean important factor affecting the catalytic gasification.The 700°C CA char was likely to form more C(O)than the 900°C CA char.Consequently,the former char had higher reactivity towards the catalytic gasification,although itwas subjected to the catalyst deactivation to a larger extent during the gasification.The CA char appeared to form more C(O),which exerted an concerted catalysis together with potassium carbonate.AcknowledgementsThis study is sponsored mainly by the National‘‘863”Scientific Research Program(Contract No.2006AA05Z116).Thefirst author expresses thanks to the Program for Changjiang Scholars and Inno-vative Research Team in University in China(IRT0620)and to Dr.O. 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