Reaction Kinetic Equation for Char Combustion of Underground Coal Gasification

Jun.2006J. China Univ of Mining Tech. English Edition)Vol 16 No. 2Reaction Kinetic Equation for Char Combus.tion of Underground Coal gasificationYU Hong-guan, YANG Lan-he2, FENG Wei-min, LIU Shu-qin, SONG Zhen-qiSchool of Chemical and Environmental Engineering, Shandong University of Science and TechnologyQingdao, Shandong 266510, ChinaSchool of Mineral Resource and Geoscience, China University of Mining Technology, Xuzhou, Jiangsu 221008, Chinas Erb A School of Chemical and Environmental Engineering, China University of Mining Technology, Beijing 100083, ChinaEngineering Department, Beizao Coal Mine, Group Companies of Longkou Mining Industry, Longkou, Shangdong 265700, ChinaSchool of Mineral Resource and Environmental Engineering, Shandong University of Science and Technology,Qingdao, Shandong 266510, chinaAbstract: Based on the quasi-steady-state approximation, the dynamic equation of char combustion in the oxidationzone of underground coal gasification(UC)was derived. The parameters of the dynamic equation were determined at900C using a thermo-gravimetric (TG) analyzer connected to a flue gas analyzer and this equation. The equation wasimplified for specific coals, including high ash content, low ash content, and low ash fusibility ones. The results showthat 1)the apparent reaction rate constant increases with an increase in volatile matter value as dry ash-free basis, 2)theeffective coefficient of diffusion decreases with an increase in ash as dry basis, and 3)the mass transfer coefficientindependent of coal quality on the whole. The apparent reaction rate constant, mass-transfer coefficient and effectivecoefficient of diffusion of six char samples range from 7.51x10" m/s to 8.98x10* m/s, 3.05x10 m/s to 3. 23x10'm/ and536×10°msto8.23×10°m3/at9o0c, respective\s, mustin; kinetic equation; thermo-gravimetriKey words: underground coal gasification; char; comlCLC number: TQ 5461 Introductionsition". The objective of the present study is to de-rive the kinetic equation for char oxidation of UCGand to determine the parameters of the kinetic equaUnderground coal gasification (UCG)is a proction experimentallyess in which underground coals react with oxygen(orair)and steam to produce combustible gases withOxidant/ SteamProduct gaslow/ medium calorific value, which is a basic energyconversion process from coal to gas.boreholeboreholeThe basic configuration for UCG is shown in Fi2I which has two boreholes, one for the injection ofthe oxidant(oxygen or air) or steam, and the other foremoval of the product gases. The UCG process canbe approximately divided into three zones, whichare the oxidization zone the reduction zone and thedrying-pyrolysis zone along the gasification tunnelfrom inlet borehole to outlet borehole. The oxidationreaction of char(semi-coke) that mainly takes placeReduction zonein the oxidation zone will produce quantities of heatDrying- Pyrolysis zoneto provide energy for the reduction and pyrolysis re-actions, so the chemical and physical changes duringFig 1 Basic configuration for UCGchar burning have an important influence on UCGIt is highly important that researchers know the中国煤化工etic equationreaction dynamics of char combustion, especially thereaction kinetic equation, to understand the basic cir-cumstances of the oxidation zone's movement, to 2.1CN MHGesscontrol and predict the temperature of the gasificationThe combustion reaction in the oxidization zoneoven, and to improve the stability of coal gas coris mainly oxidation of char, which is the resultantReceived 06 November 2005: accepted 15 January 2006CmF搞nor.Tel:+86-532-86057857:E-mailaddress:yuhongguan65@163.comofChina50276066 and 20207014 supported by National Natural Science FoundationJ. China Univ of Mining& Techh Edition)Vol 16 No. 2after drying and decomposing the coal in the dry- are stoichiometric coefficients of O2, char, CO2 anding-pyrolysis zone, with the oxidant injected through ash, respectivelythe inlet borehole. The char is frequently coveredwith ash, which is formed by mineral matter decomBoundary layer: ash layerposing in the reduction and oxidization zones, sincethe coal is composed of organic matter and mineralmatterProduct layer: ashIn the oxidization zone of UCG the multiphasereactions between oxygen and carbon are as follows1)C+O2→CO2+393.8MJ/kmol;2)2C+O2→2CO+231.4 MJ/kmol; and 3)2C0+02-2C02+5712 MJ/kmol. Among above reactions, only the first reactionis the basic process which controls the changes of theUnreacted nucleus: charoxidization zone in UCG because the rates of theother two reactions are faster than that of the firstFig 2 Sketch of char combustion and oxygen distributionThe physical and chemical changes in the oxidi-3)Char is composed of carbon and mineralzation zone are very complicated; however, the basicter. and there no influence on carbon oxidationprocess is an oxidation reaction between char andthe mineral matter is oxidizedoxygen. According to the characteristics of matter4)The surface's oxygen content is equal to theexchange for spheroidal carbon combustion, the tunnelI's as solid ash is formed; that is, the oxygen iscombustion process of char may be divided into folconstant outside the boundary ash layer.lowing stages: 1)Oxygen transfers from the gasifica- 2.3 Kinetic equationtion tunnel into the ash surface through a boundaryWhen oxygen in the gas tunnel transfers throughash layer, which is called outer mass-transfer of oxy- the boundary ash layer into the char surface, the massgen;2)Oxygen at the ash surface diffuses ontochar interface, which is called inner diffusion of oxof outer mass-transfer of oxygen is k, (Co-C).So,gen;3)Oxygen is adsorbed on the char surface and outer mass-transfer velocity of oxygen may be exreacts with the carbon in char, which is called inter- pressed asface chemical reaction; 4) The gas product of the re-Um=k,.(Co-Cs)action, CO2, diffuses into the ash interface throughdm=l/kash clink, called inner diffusion of CO2; and 5)CO2transfers off the boundary ash layer into the gas tun-nel of UCG called outer mass-transfer of cog. n The oxygen diffusion in ash can be describeds first law of diffusion, so the mass of innerBecause the rates of oxygen adsorption on thedcstages"?, the chemical reaction process in UCG is oro, dt=De(C-C). The inner diffusion velocitychar surface, resultant(CO2) diffusion and mass-transfer off the ash layer are faster than those of otherygen may be expressed as follows:mainly dependent on outer mass-transfer of oxygen,inner diffusion of oxygen, and the reaction betweenU2=S(C-C∥4oxygen and carbon. Thus, the rate-limiting stages areRo(Ro-R)),2)and3)DR2.2 Reaction modelThe chemical reaction velocity on the unreactedchar interface isThe sketch of model for char combustion inUCG is shown in Fig. 218). For simplifying the estab-U=kS(Ce-Ce co, /K)lishment of the kinetic equation, the following ap- where C, Ceco, are concentrations of O2 and Coproximations were maderespectively (g/m), on the char interface when the1)The oxygen diffuses across the product layer chemical reaction is in an equilibrium state. Accord(ash)at a quasi-steady-state during char oxidation ing to quasi-steady-state approximation, we obtainwhen ash is formed, and the content of absorbed中国煤化工∞2oxygen decreases to zero at the oxidation surface2)When the solid layer is formed, the equationCNMHGdefinition of reacfor the oxidant(oxygen)and char isIc Lunucnuauun of CO2 isaA(g)+bb(s)+cC(g)+dD(s)Ceco=C+Ceco,-C=C+KC'-C;where, A(g), B(s), C(g)and D(s)are gaseous oolid char (including carbon and mineral matter)Fe., =(1+K)C-Cgaseous SGra and solid ash, respectively; a, b, c and dThus Eq (6)can be transformed intoYU Hong-guan et al.Reaction Kinetic Equation for Char Combustion of Underground Coal GasificationkSce-x[+ K)C(12)RIfwhere P is the density of char(g/m), x is the11+K( 9) mass fraction of char(dimensionless)then the equation of the reaction rate on the char inrface isRo C-C(13)kso(c-C)So(ce-c")dpx(R)λ++1(10)The initial condition is R=Ro as t=0where, Un, U and v are outer mass-transfer velocSubstituting equations (3), (5)and(9)into equaity of O2, inner diffusion velocity of O2, and reactionion(13)and integrating it according to the initialcondition, the kinetic equation of the relation betweenof char (m,), c,, C, and Ce are concentrations of O2 in oxidation rate x and radius of char (Ro) isthe gas tunnel of UCG at the ash layer boundary andon the char surface respectively (g/m), Ro and re arex+。1+2(1-x)-3(1-x)ash radius and radius of unreacted char respectively(m), kr and k are the mass-transfer coefficient through61-0-2]=2(Cash boundary and the apparent reaction rate constantrespectively(m/s), K is reaction constant of Eq. (1)R(dimensionless), De is the effective coefficient of difwhere x=l-is the conversion ratiofusion(m/s),A, n and a are diffusion resistanceof the char interface, and the mass transport resis- reactant char, dimensionlesstances through the ash layer boundary and throughthe ash layer, respectively( s/m), and C is the equi- 3 Determination of Kinetic Equation Pa-librium concentration of oxygen on char interfacerameters(g/m2)Because Um=U=U at quasi-steady state, the 3.1 Experimentalindetermined terms Cs and Ce can be deleted, so theThe characteristics of different kinds of coals inconsumption rate of oxygen isUCG are diverse. To investigate the influencingSo(co-C)factors of the equation parameters, six coal samples(II) were used, including bituminous coal and anthraciteBasic characteristic indices of the coal samples andConsumption rate of solid char can be expressed chars are shown in Table Ias followsTable 1 Basic characteristic indices of the coal samples and charssampleProximate analysisUltimate analysisTotal Fusibility of coal ash/tProximate analysis/%oMoisture Ash VMFC" C" H"N"0 Sulfur DT ST FT Moisture Ash VM'BS23912514549545179.1749410910.453811050109011400.2122890.12SJL0.57138335.78642286304031.194463461230128013200.1121.58009YZ130329737.56624484355020693.384401030105010900.1852790.07YQ3,7617.56836916490743010.813961.221235>1400>14000.1519.18002LF10823051439856187902610925.262.551080112010.1326.950.11DF1811649346965.31766241811212764441270131013500.1425.230.0is Dry basis(d), **is Dry, ash-free base(daf)M=Volatile matter, FC=Fixed carbon, DT=Deformation temperature, ST=Softening temperature, FT=Fluid temperatureThe char can be obtained with coal pyrolysis. The中国煤化工 be so that the coalcoal samples larger than 50 g were crushed to parti- or chicles less than 3 mm and sealed for preparing charCNMHGmeasured with aThe samples were placed in a high-silica glass tube, density flask, and its average volume was obtainedwhich was inserted into a cold tube furnace chambeusing its true density and mass. The average equiva-The temperature of the furnace was raised to lent diameters of the char particles regarded as sphe-(900=+20)C in 1 h, and kept for an additional 0.5 h, roid were obtained using the average volumeon te edition that a great enough quantity of niA thermo-gravimetric analyzer(TGA) is oftenJ. China Univ, of Mining Tech. English Edition)Vol 16 No. 2used to study the combustion characteristics-2 and 3.2 Apparent reaction rate constantcombustion dynamics"3-4of coal. The parameters ofThe interface oxidation reaction of carbon inthe kinetic equation in this paper were determinedusing a tga connected with a flue analyzer. The ex-char takes place at the beginning of the experimentprimarily, because the ash layer has not yet beenperimental systems include a WRT-2 TGA made in formed. So the kinetic equation isShanghai used to obtain a TG curve of char combustion, and a KM9106 flue analyzer made by Kaneused to determine the oxygen concentration at time t.1-(1-x)“R只kkC-C)t(15)The TGa includes a gas system, weight sensor, elecas the reaction constant of char oxidation K istric thermocouple, sample container and data analysis1+Ksystem. All experiments were carried out at atmosgreat at the beginning, soKpheric pressure, under an air atmosphere with a con-Fig. 4 is constructed by plotting the calculatedtrolled flow rate at 900 C. The samples weight lossand rate of loss were recorded continuously as a func-1-(1-x)versus the t(time)with B-S bituminoustion of timecoal and Y-Q anthracite. The apparent reaction ratAbout 0.1 g of char samples with deteconstant k can be obtained from slope of the line.average equivalent diameters were placed in a crucible suspended at a specified height, and weighed. Theanalyzer was calibrated to balance in the cold state060The furnace was raised to keep the crucible in itschamber when it was heated to 900 C. and thetemperature of furnace chamber was maintained at900℃The TG curve of char combustion is shown inFig. 3. The conversion ratio of char (x) during oxidaFig. 4 Calculation of apparent reaction rate constant ktion can be obtained with the following equation(G-G)3.3 Mass-transfer coefficientG(00A。-Vn)where G is the char massOuter mass-transfer of oxygen mainly controlsobtained from the TG curve at time t(g), G is the char the combustion of char when the ash layer has beenmass used in the experiment(g), Adc, Vae are the ash formed, so the kinetic equation isand volatile matter yield of char as dry basis supplied(16)in Table 1(%).Co-C)tRoPeIt can be seen from Eq.(16) that a lineationship exists between x and t as show in Fig. 5.Themass-transfer coefficient k can be obtained fromslope of the line1216203045}·Bs·YQI(min)Fig 3 TG curve of char combustionThe mass fraction of carbon in experimental3har'is x=1-(.+vae)/100. The initial oxygenconcentration Cb at T K temperature is calculatedusing the Ideal-gas State Equation and the volumeFig. 5 Calculation of mass-transfer coefficientconcentration of oxygen(21%)in air, viz., Ch=32×1×0.21×1038195. g/m. In a similar way3.4 Effective coefficient of diffusion0.082Tbustion reaction process will mainly dethe oxygen concentration Ce at TK temperature at中国煤化工 xygen, and reactionany time t is gained with volume concentration of betvCNMHGnterface by enhanc-oxygen(C, %)being the reading of the flue analyzer gat room temperature, viz., C*= 32xlxCx10influence of outer mass-transfer of oxygenThe following parameters were introduced to0082T3902Csimplify Eq (14): F=l-(1x),=凡2x万方数影6k(C-C")YU Hong-guan et alReaction Kinetic Equation for Char Combustion of Underground Coal GasificationRoP xkRopwhen theof char with low ash fusibility is proportional to theC)(+K)parameters F, t, I, S are put into Eq (14), then the 4.2 Char with high ash yieldfollowing equation is obtained.High ash content leads to a slow velocity of oxy-4)/F=gen diffusion through the ash layer, because the ashlayer is too thick to fall off. Thus the process of charThe influence of the outer mass-transfer of oxy- oxidation will be controlled by oxygen diffusiongen on the combustion reaction process was weakened by raising the velocity of oxygen flow. tr can berough the ash layer, i.e. A, >>am, A, >>A. So, Eqignored when the value of kr is great. So, the follow( 14)can be rewritten as:ing equation is obtained岖R=1(Rc-C/F=+SF-2F2)dr Prr aBased on Eq (17), a linear chart of t/VersusThat is(3F-2F )is drawn, as shown in Fig. 6. According tothe slope of straight line S, the effective diffusion co-1+2(1-x)-3(1-x)R'erCb-C)refficient De can be determinedThus it can be seen that 1+21-x-31-xg is indirect proportion to the reaction time of char withhigh ash4.3 Char with low ash yieldaB.s·YQAs the thickness of the ash layer is too thin forlow ash yield char, the oxygen can diffuse through the0020.40.608ash layer easily, so the combustion process will becontrolled by interface chemical reaction of carbon inFig. 6 Calculation of effective diffusion coefficientchar. As A>>Am, A >>A, the kinetic equation for3.5 Relation between kinetic parameters andchar with low ash content is Eq (15)coal quality5 ConclusionsThe kinetic parameters of char oxidation withsix chars under experimental conditions are shown in1) The reaction kinetic equation for char comTable 2bustion of UGC based on the quasi-steady-state ap-Table 2 Kinetic parameters of char oxidationproximation can be described asinetic parameters B-s SJL Y-z Y-Q L-F D.k(10°ms)3.213.123.233.183053.151x+A1+21-x)-3(-x)ke DD2(10m3s)8.237985.36778689757k(10m/s)8987517.694925.68723k1+k1-(-x)1=6(C-C").tRoP KThere is an incidence relation between apparent2)Under conditions of controlled oxygen flow,reaction rate constant k and volatile matter as dry the apparent reaction rate constant, mass-transfer co-ash-free basis (v aat) in coal, which is that k increases efficient and effective coefficient of diffusion of sixwith an increase in Vdaf. The effective coefficient of char samples measured by TGA connected with a fluediffusion De decreases with a decrease in ash content analyzer range from 7.51x10 m/s to 8.98x10" m/s,as dry basis in coal. The mass-transfer coefficient k is3.05×10°msto3.23×105m/sand536×105m/stondependent of coal quality, and basically constant.8.23×10°m2/sat90℃c, respectively. The apparentreaction rate constant is in direct proportion to vola4 The Concise Kinetic equationstile matter as dry ash-free basis in coal; the effectivecoefficient of diffusion is inversely proportional to4.1 Char with low ash fusibilityash casis and the mass-transfer coeffiIn the oxidation zone. ash with a low fusible cient中国煤化工point will melt at 1 200 C, making oxygen hard toCNMHwith low ash fus-pass through the ash layer, which enable the ibilityto the reaction timemass-transfer through ash boundary to control the The diffusion process through coal ash controls twhole combustion process. For am>>a, am >>A, the oxidation of char with high ash yield; however,kinetic equation of coal with low softening tempera- oxidation of char with low ash content is mainly deture of ash is Eq (16). This shows that oxidation ratio pendent on the chemical reaction rate of charJ. China Univ of Mining Tech (English Edition)VoL 16 No. 2References[l] Liang J, Liu SQ, Yang Z, et al. Exergie analysis of underground coal gasification process, Journal of China University ofMining Technology, 2004, 33 (2): 200-204.(In Chinese)[2] Yang L H, Liang J. Study on mathematical model of underground coal gasification in steep seam. Journal of Fuel Chemicaland Technology, 2003, 31(3): 193-198.(In Chinese)[3] Zhang Y L, Zhang M T, Song WY. Basic equations of channel model for underground coal gasification. Progress in NatureScience,2002,12(5):364-367[4] Yang L H. Study on moving velocity of burning front in underground coal gasification. 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