Catalytic effect of alkali carbonates on CO2 gasification of Pingshuo coal

Mining Science and Technology( china )21(2011)587-590Contents lists available at Science DirectMining Science and Technology( China)ELSEVIERjournalhomepagewww.elsevier.com/locate/mstcCatalytic effect of alkali carbonates on CO2 gasification of pingshuo coalMeng Lili, Wang Meijun, Yang Huimin, Ying Hongyan, Chang LipingKey Laboratory of Coal Science and Technology. Taiyuan University of Technology, Taiyuan 030024, ChinaARTICLE INFOA BSTRACTArticle historyNa COj. Li2 CO,. and K2 Co were used as additives to Pingshuo(PS)coal that was subsequently gasifiedReceived 5 December 2010deceived in revised form 2 January 2011under a COz stream. The catalytic gasification of coal samples by Co2 in the presence single or mixedalkali carbonates was investigated by thermogravimetric analysis. the experimental results indicate thatepted 30 January 2011Available online 23 July 2011the catalytic effect of Li2 CO, is significantly larger than that of Na2 CO, or K_ CO]. The catalytic effect of themixed,bi-metal carbonate containing Li2CO, and Na2,, or LiD CO3, and K 2 CO, is related to the compo-sition of the catalyst and the proportion of the two components. The bi-metal carbonates having a moieratio of 9: 1(Li: X)has the largest catalytic effect for PS coal gasification. A synergistic effect between Ligasificationand K or Na, carbonate appears at temperatures greater than 1300 K An un-reacted shrinking core modelAlkali carbonatesis suitable for kinetic analysis of catalytic gasification of coal samples in the presence of alkali carbonates.CatalysisIt is inappropriate. however, to evaluate the catalytic effect only by the activation energy obtained fromthe kinetic calculationso 2011 Published by Elsevier B V. on behalf of China University of Mining Technology1 Introductionthe Pingshuo(PS)mine in Shanxi province of China either singlyor as mixed metal salts. The catalytic effects on COz gasificationCoal is Chinas primary source of energy. However, the utiliza- of the PS coal were then investigated systematically and a kinetiction techniques are relatively backward and this causes serious analysis of catalytic gasification was carried out. An experimentalenvironmental pollution. A clean, efficient, and comprehensive uti- and theoretical basis for clean coal technology, especially the cata-lization of coal is the only way to effectively improve the atmo- lytic gasification of coal, is provided.spheric environment and energy efficiency ( 1]. Coal gasification isthe foundation of, and is key to, successful implementation of sucha technology. However, the development of coal gasification suffers 2. Experimentalfrom several drawbacks including high reaction temperatures andenergy consumption, the difficult purification of the gaseous prodPingshuo raw coal(PSR)was collected, ground and screeneducts formed, stringent requirements on the process equipment, and a particle size of 0.12-0. 15 mm. This was used as the experimentalserious environmental pollution. The catalytic gasification of coal sample. The proximate and ultimate analyses of the raw coal arecan effectively improve the reaction rate and conversion efficiency given in Table 1. The lithium, sodium or potassium carbonate werecompared to conventional gasification techniques. This provides an evenly mixed with the coal at a loading of five weight percent. Theenergy source. Some investigators have shown that hydroxides PSRNa, PSRLi, and PSRK, respectively.and oxides of alkali metals, alkaline earth metals, or partial transiThe gasification tests were carried out using a STA409C thermo-tion metals have catalytic affects on coal gasification (2, 3]. There- gravimetric analyzer (Germany Netzsch Company). Approximatelyfore, using these alkali metal salts as catalysts has become one of 20 mg of sample was put into the crucible and heated from roomthe research hotspots in the coal gasification areatemperature to 1673 K at a 10 K/min heating rate. High purity nitro-In general, phosphates, borates, and silicates will form poly- gen was used as the carrier gas at a flow rate of 80 mL/ min the gasmeric compounds in a glassy state at the gasification temperature, ification agent was CO2 at a flow rate of 20 mL/min.which inhibits their performance Other weakly acidic salts of alka-In this study carbon conversion, X, is expressed byli metals are good catalysts for coal gasification, however 14.5).this study, Na2CO, Li, CO3 and K2CO, were added to coal from x=Wo-Ws%Corresponding author. Tel +86 351 6010482where Wo is theE-mail address: lpchangetyut.edu. cn(L Chang).tion (at the terH中国煤化工ierature of gasifica-1674-5264s- see front matter o 2011 Published by Elsevier B V on behalf of China University of MiningCNMHGdoi:101016/mstc201101001L Meng et aL/ Mining Science and Technology( dhina) 21(2011)587-590Table 1Proximate and ultimate analyses of Lingshou raw coal.Proximate analysis(%Ultimate analysis(‰dan員o一PRK2.23179337.1980415.201.381061195By difference.gasification has not yet begun), mg: W the mass of coal at time t,mg: and woo the mass of residue after gasification is complete mg100000Temperature( K)3. Results and discussionFig 2 Carbon conversion versus gasification temperature of coal samples; with3.1. Efect of single alkali carbonates on the catalytic gasification of and without added alkali carbonatePingshuo coalThe TG and DTG curves of Pingshuo raw coal and the treated of the three carbonates toward CO2 gasification of the PSr is notcoal samples are shown in Fig. 1. Notice that the weight loss of exactly the same. The main factors influencing catalytic activitythe treated coal samples is obviously greater than the weight loss are the melting point of the catalyst and the catalytic activity ofof the raw coal However the loss occurs in a different temperaturethe metal-graphite complex[7]. Molten alkali carbonate has easierrange. The difference is insignificant below the temperature of access to the solid carbon because of its fluidity. The melting points973 K but becomes obvious as the temperature increases. Above of Li, co Na- COj, and K- CO3 are 723, 854, and 891C respectively1600 K the treated coal samples are stable but the weight loss of Thus, Liz CO should show catalytic activity at lower temperaturescompared to the others. At gasification temperatures higher thanthe raw coal continues. Weight loss peaks at the same tempera- the melting point of K- CO, there are very significant catalytictures for all coal samples, both with and without the additive, over effects because the molten K Co, enhances the reactivity of thethe range of 573-973 K This temperature range is considered to bethe temperature of pyrolysis of the coal sample. Hence, these18].results show that the alkali metal carbonates added to psr have little effect on coal pyrolysis3. 2. Efect of mixed alkali carbonates on catalytic gasification ofAbove 973 K the peak weight loss of the treated samples is lower than that of the raw coal. this temperature range is consideredto be the starting point of gasification. Hence, the results show thatLi2CO3(the main active component)and Na2, or K2 CO3 in dif-Li2CO3, Na2 CO3, and K2 CO, have a significant catalytic effect on the ferent proportions were added to coal at a five weight percentCO2 gasification of PS coal to different extents. Li2CO3 has a notable loading. These coal samples are denoted PSRLiNa (m n) and PSRLiKcatalytic effect at about 1000-1200 K. the catalytic effect of (m: n)where m and n are the relative contents of Li2 CO andNa2 CO3 and K, Co, appears around 1300-1400 K and the effect of Na2 CO,, or Li2 CO and K2CO3. The carbon conversion during Co2the K2CO appears to be larger than that from the Na2 CO% withor. conversion of th sented in Fig. 3. It can be seen that the carbonalkali metal carbonates is shown in Fig. 2 as a function of temper- with increasing Li2CO3 content, at the same gasification tempera-ature. In this figure carbon conversion is based on the data starting ture. The catalytic effect of Li2CO3-Na2CO on coal gasificationfrom 973 K, the initial temperature where gasification begins[6]. It has no obvious relationship to the composition of the mixed alkalian be seen that the treated samples have higher carbon conver- carbonate. However, the catalytic effect of PSRliNa (9: 1)is superiorsion than the raw coal over the temperature range from 1000 to to the other ratios.1600 K. Above 1600 K all samples tend to uniform behavior. TheFig 3 also shows that PSRli has the most obvious catalytic effectcatalytic effect of pSRli is larger than that of PSRNa or PSRK at a at temperatures below 1300 K The carbon conversion of the PSrgiven temperature. This suggests that the addition of Li2, to LiK (7: 1 ), PSRLiK(9: 1 ), or PSRLiNa(9: 1)samples does increase rap.raw coal would markedly reduce the gasification temperature of idly as the gasification temperature increasesPSR, while the effects from the potassium and sodium salts are relThese results indicate that there is a synergistic effect from theatively weaker. The carbon conversion of PSRK is a little lower than mixed carbonates related to the metal component and the gasifica-that of sRNA below the temperature of 1330 K but the carbon tion temperature. The synergy is caused by the gradual transfor-conversion of PSRK is clearly higher than that of pSRNa above mation of the metal salt catalyst from a solid to a molten state as1400K. This phenomenon suggests that the catalytic mechanism the gasification temperature increases. During this process the ionsPSRLi中国煤化工CNMHGFig 1. tG and dtG curves of coal samples with and without added alkali carbonates.L Meng et al Mining Science and Technology( China)21(2011)587-590RLIK(I: 1SALiNa(I: I5 60F-- PSRLiK (9: I)PSRNa1001400Fig 3 Carbon conversion versus temperature: Pingshuo raw coal with added alkali carbonatesare strongly activated and cations and anions aggregate allowing Table t istic temperatures and kinetic parameters of catalyzed gasification.ion-exchange to occur by cross contact. This is the basis of the syn- characergistic effect [9]. The mixed salt eutectic melts at a lower meltingpoint and this lower melting point increases the wettability of theSample Activation PreCorrelation Toos Toso Togscatalyst and thereby improves the catalytic activity.energyexponential coefficient (K)(] (Kfactor A3.3. Gasification dynamics of coal containing alkali carbonatesPSRLiPSRNa 87.9925.7509792106013941602A suitable model is required for the analysis of coal gasification219.238513741539dynamics. Homogeneous reaction models, un-reacted shrinking PSRLiK 120.01111996core models, hybrid reaction models, activation energy models,PSRLNa 93.9409748103913221522or random pore models might be used to describe a gas-solid reac-(9:1)tion [10-15 Currently, the un-reacted shrinking core model ismainly used in the coal gasification field. A mathematical expression of this model is:d=A()=3+1-xy2dwhere 2 is the heating rate k/ min: x the fractional carbon conver-(2) sion, % E the activation energy. J/mol:R the universal gas constantT the reaction temperature, K; and a the pre-exponential factorF(x)=/rax)=1-(1-x)2(3) lated for gasification in the presence of different alkali carbonatesare shown in Table 2. PSRLi has a lower activation energy thanThe logarithm, In(Fx), of the integral form of this expression PSR for gasification but the activation energy of the other samplesfor coal gasification and 1/m should have a straight line relation-fication of coal samples with different alkali carbonates is shown in higher than that of the raw coal or coal with a single carbonateand 1/T and that the correlation coefficients are all larger than 0.99. bon conversion is 58, 50%, and 95%, the initial gasification temperThis indicates that the un-reacted shrinking core model is suitablefor the dynamic analysis of catalytic gasification of coal samplesature falls, and the temperature range from To.os to Togs iscontaining alkali carbonates.significantly less, when carbonates are added. The catalytic effectEq(4)is obtained by logarithmic transformation of Eq. (2). observations of the characteristic gasification temperatures of theFrom this e and a can be calculatedcoal samples are inconsistent with the activation energy obtainedfrom the kinetic calculations. Hence, it is considered to be inappro-InK=In3(1-x万可“+mA(4) priate to evaluate the catalytic effect of alkali carbonates on coalgasification by the activation energy only. this is mainly becausethe activation energy is apparent activation energy for the overallgasification process, including the non-catalytic gasification reac-08tion, the gasification, and the melting of the alkali carbonate.124. Conclusions16PSRThe following results were obtained experimentally20F·PSRL▲ PSR Na(1)Li2CO3, Na 2CO3, and K, CO3 loaded into coal have a catalytic24F+ PSR Keffect toward coal gasification. The action of the salts is◆ PSR LiK(91)28· PSR LiNa9related to the gasification temperature. Single alkali carbon-ates show a catalytic effect where the effect of LicO is sigI/T(10/K)中国煤化工KO(2) The catalydonates is differentFlg. 4 In(Rx))versus 1/T for the catalyzed gasification of coal samples containingCNMH Gios of Li2CO, withdifferent alkali carbonates.either Na, CO3 or K, CO3. The catalytic effect of Li2CO3-L Meng et al/Mining Science and Technology( China)21(2011)587-590Na2CO3 and Li2, CO, in the mole ratio of 9: 1 is greater [31 Zhang LX. Huang I. Fang YT. Wang Y. Effect of mineral matter on gasificationthan at other ratios. a synergistic effect is present for bi-tion of typical Chinese anthracite chars. J Chin Univ Min Technolmetal carbonates, with mole ratio of 9: 1. at temperatures (4) Yeboah YD. Xu Y Sheth A. Godavarty A. Agrawal PK Catalytic gasification ofgreater than 1300 K. This is mainly caused by the gradualtransformation of the metal salt catalyst from the solid to (5) Zhang w, Zong ZM. Wang TX Xie RL Ding M]. Cai KY, Huang YG. Gao js, wuthe molten state at increased gasification temperatures.(3)It is inappropriate to evaluate the catalytic effect of alkalidioxide. J Chin Univ Min Technol 2007: 17(2): 197-200carbonates toward coal gasification by only the activation 16 Ahn DH, Gibbs BM, Ko KH, Kim J Gasification kinetics of an Indonesian sutenergy values determined from kinetic calculations.[7 Wen wY. Mechanisms of all18] Pang KL Xiang WG, Zhao CS, Xi B Gasification of coal char-cO in the presenceAcknowledgmentsof potash. J Combust Sci Technol 2007: 13(1): 63-6(In Chinese).[9] Sun XL Wang L Zhang ZT. Study on compound catalyst for gasification and itsmechanism, Coal Convers 2006; 29(1): 15-8(In chinese)The authors gratefully acknowledge the financial supports of [10 Adanez j Dediego RF. Reactivity of lignite chars with CO2: infiuence of thethe National Natural Science Foundation of China (Noneral matter. 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