A back analysis of the temperature field in the combustion volume space during underground coal gasi

Mining Science and Technology( China)21(2011)581-585Contents lists available at ScienceDirectand I氵家Mining Science and Technology( China)ELSEVIERjournalhomepageswww.elseviercom/locate/mstca back analysis of the temperature field in the combustion volume spaceduring underground coal gasificationChen Liang*, Hou Chaohu, Chen Jiansheng, Xu JitingKey Laboratory of Ministry of Education for Geomechanics and Embankment Engineering Geotechnical Engineering Research Institute, Hohai University, Nanjing 210098, Chinab School of Geoscience and Engineering, Hohai University, Nanjing 210098, ChinaARTICLE INF OABSTRACTThe exact shape and size of the gasification channel during underground coal gasification(UGC) are ofReceived 18vital importance for the safety and stability of the upper parts of the geological formation. In practiceReceived in revised form 5 January 2011existing geological measurements are insufficient to obtain such information because the coal seam isAccepted 31 January 2011Available online 22 July 2011typically deeply buried and the geological conditions are often complex. this paper introduces a cylindrical model for the gasification channel. the rock and soil masses are assumed to be homogeneous and iso-tropic and the effect of seepage on the temperature field was neglected. The theory of heat conductionUnderground coal gasificationwas used to write the equation predicting the temperature field around the gasification channel. The ideaGasification channelof an excess temperature was introduced to solve the equations. Applying this model to UCG in the fieldfor an influence radius, r, of 70 m gave the model parameters. u123., of 2.4, 5.5. 8.7... By adjusting theCombustion space arearadius(2, 4, or 6 m)reasonable temperatures of the gasification channel were found for 4 m. The temper-back analysisature distribution in the vertical direction, and the combustion volume, were also calculated Compansonto field measurements shows that the results obtained from the proposed model are very close topracticee 2011 Published by Elsevier B V, on behalf of China University of Mining& Technology.1 IntroductionResearchers have tried many methods to study the behavior ofthe cavity during UCG. Some, like Sateesh et al. simulated the proUnderground coal gasification(UCG )is one of the research top- cess through laboratory experiments [7]. They thought that duringics of interest to coal miners around the world It is also an impor- UCG the shape and size of the combustion space changed alongtant way to solve a series of technological and environmental with the distance between the air inlet and outlet therefore theproblems that exist in traditional coal mining methods 11]. UCG is simulated shape and size of the space was found by adopting seep-a promising technology as it is a combination of mining, exploita- age CFD software to the laboratory experiments. Britten and Perkinstion, and gasification, that has been shown to be both technically studied the impact of different conditions(e. g the distance be-and economically feasible[2,3]. Highly efficient and green technol- tween the air inlet and outlet or the operation time)on cavityogies are of concern to both domestic and foreign workers [4]. the growth using a mathematical model to determine the shape andsize of the combustion space area is important to the process of volume of the combustion space [8, 9]. In China, Yang establishedunderground gasification Cavity growth is a key factor related to a gasification channel model and a temperature and concentrationcontinuous and steady production ( 5]. However, no effective meth- field model I 10 He thought that the temperature field effect on theods have been suggested for measuring the volume of the combus- heating value of the coal gas was comparatively large. Liu et al.tion space. This information is needed to solve some technological speculated that the occurrence, location, shape and volume of theproblems such as how to prevent ground collapse(e. g, the stability combustion space could be observed by transient electromagneticof the space)and the flow of underground water resulting from the methods(TEM)[11]. This is the observation of the character of ancollapse[6]. Therefore, the long term development of UCG requires induced electromagnetic field generated by a geological body overthe study of serious problems such as maintaining the volume of time. Liu et al. speculated that the scope and volume of the spacethe combustion space and evaluation of the stability of the combus- could be monitored by the movement of underground radon gastion space. An understanding of these points may be obtained among rock cracks[ 12).hrough the study of the underground temperature field.Underground coal gasification is a process accompanied byintense combustion, heat-release, and simple and multiphaseCorresponding author. Tel. +86 25 83787031chemical reaE-mail address: chenliangehhu edu cn(L Chen)ficult to accura中国煤化工entific studies. it is dif-me of the combustionCNMHG1674-5264/S- see front matter e 2011 Published by Elsevier B V. on behalf of China University of Miningdoi:101016jmsc2011.06018L Chen et aL/Mining Science and Technology( China )21(2011)581-585space because the methods are not well calibrated and the geological situation is quite complex. Therefore, the temperature field mod-T2 Initial underground temperatureel of a cylindrical gasification channel is introduced herein. Bystudying the combustion space during underground coal gasifica-tion the distribution of the temperature field around the channeland the volume of the space are estimated. The changing tempera-ture field around the space is determined for a project case withz(m)on-site detectionGasification channel2. Temperature field model of the gasification channelRock mass2.1. Establishing the temperature field modelFig- 2 Longitudinal cross-section of the gasification channelA longitudinal section of the gasification channel is roughlrectangular, ideally. However, in reality this is not so. The effects the heat conduction equation of a rock-soil mass is given byof heating value and the gaseous temperature field during gasifica- [151tion cause combustion in the side wall of the cavity to be incomplete making the shape round or oval. Suppose a circular channel aT(r, -/ T(r, t).1 OT(r, t)ywith a radius rn exists and that the average temperature in the gas-t(1)ification channel is T The radius influenced by high temperature isT2 Forr ?r2 the temperature of the rock is near the initial under- initial condition: T(r, O)-gor)boundary conditions: T(r1 ()*T1,Tr2ground temperature.r)Suppose the heating value inside the channel is uniform and con-sider only temperature changes in the radial direction. Solving this 22 Solution to the temperature field modelproblem is made more convenient by the following assumptionsA convenient solution may be found by introducing the excess(1)The impact of seepage on temperature can be neglected temperaturebecause the top and the bottom of the coal seam are both 6(r, )=T(, t)-Tbounded by an impermeable layer. Hence the seepage coef.ficient is very small [13Now Eq (1)may be written as:(2)The rock surrounding the gasification channel is homoge- 80(r, t)2( 8e(r, t),1 80(r, t)neous and the coefficient of thermal conductivity is(3)Because the coal seam is thin the bottom and top of the reac- a(r, 0)=go(r)-Tetion zone can be considered a floor and a roof 14]. The tem- (i, t =t( 1, t ) -T=Ti-Teperature field around the gasification channel is distributed 0(r2, t=T(2, t)-=T-T=0symmetrically.(4)The temperature inside the gasification channel is the sameT2=Teas that at the inner wallTo solve these differential equations suppose a solution isThe cross sections along the radial and axial directions are e(r, t) =m(r)N(t)shown in Figs. 1 and 2.Substituting this into the above equations givesN(c(cp)N(t)M(r)Setting both sides of this equation to the constant, -B2 gives:N(t)+(x/cp)2N(r)=0M"(r)+=M(r)+M(r)=O(5)The general solution of Eq (4)isN(c)=Cexp(-cp'where C is constant and b is the zero order Bessel equation with the72general solution [ 16]:M={(2)+BY0(z)A and B are constant in Eq. (7)andJ(z)=中国煤化工Initial underground temperatureCNMHGFg 1. Radial cross-section of the gasification channelis a zero order Bessel function of the first kind andL Chen et aL/ Mining Science and Technology(China)21(2011)581-585a(1)m-S,2(6c+厂ct)is a zero order Bessel function of the second kind, where y is Eulersconstant andDn3(un)亏m=1,2,)The size of the gasification channel is smaller than that of the for-mation In Eq (7)as r tends to a minimum value Ydz tends to neg. omitting the first item on the right side changes Eq (15)into:ative infinity. this is not the same as the actual situation. Therefore,set the constant B-O in Eq (7)and it becomes:M=4(2)-(1=D广cb-2m)(6(8Now the solution of Eq (2)is:And thuse(r, t) =M(r)N(t)=DJo(z)exp(-4B2Dnn=1,2,In Eq (9)So now Eq. (12)reads:D- Ac and both d and ac are constant Since 0( 2, t)T(2, t)-T=T2-Tc=0 substituting this into Eq (9)givese(r, t)=D(B2)exp(-2r)=0(m)xcp(eC(月nD≠0Substituting Eq (14)into the expression for the initial temperatureand exp(-o)*o the characteristic equation isJ(所2)=0T(r, r)=O(r, t)+TcThe left side of Eq (10)is a periodic function. The characteristics of2÷(当a periodic function require infinite roots to Eq (10). Therefore, one (, 2(Hm)solution to the differential equation of rock -soil masses can be ob.tained by makingx∞(C-(PmTeNamelyThe series in Eq (18)rapidly converges with the changes in m. Som- 1 satisfies the theoretical solution to the physical model:(r, )=Do(n/)exp (-cp'T(,t)=5(4)Superposing these infinite solutions gives a combination of solu-tions to the differential equations of rock-soil massesx ex做9=?()(12)+to(19)Since:where Ts is the initial underground temperature, C: Ti the temperature at the tunnel inner wall. C: T2 the boundary temperature, oC,e(r,0)=g0(r)-Teequal to Te: A the coefficient of heat conductivity for the rock(w/(moC): c the specific heat of the rock O/(kg C)): p the density ofsubstitution into Eq(12)givesthe rock, kg/m: r the radius of the gasification channel; and T2the radius of some constant temperature boundary, namely thee(r,0)=>DmO(mr)=go(r)-Tradius where temperature has a significant influence, mSince the temperature inside the rock-soil mass changes slowlyer time this may be viewed as a steady-state problem. therefore,rom the initial conditions we havean exponential term containing the time variable is constant andto Eq (19)beco(-1)m(Br)2m(r,0)=go(r)-T=22mr(m+1)(14)Using the orthogonal property of7(时+(20In Eq (20)Ewith time and中国煤化工 as nothing to do60-T(2CNMHGand integrating from 0+T2 givesL Chen et al/Mining Science and Technology( China)21(2011)581-585is a constant that has nothing to do with the radius.Since rer. T- Tl Eq(20)may be writtenMeasurement of 11#Theoretical curveT=山rigo(r)-tlL/,+Te=T1(21)E may then be found and substituted into Eq (18)to give theexpression for temperaturem/()+r(22)16018020022024026Te=To+nhFig 4 Temperature from on-site drill holes.Because the center of the Earth is hot the temperature becomeshigher as depth increases. An increase of 100 m generally causes arsMtemperature increase of 3C[17]. Eq (22)estimates the tempera-ture distribution independent of time around the gasificationr=6m3. A field test caseThe value of any theoretical model is based on its feasibility forse with actual projects. The theoretical model was verified byworking out the volume and shape of the combustion volume fora certain area of an underground coal gasification project. The5. Calculated temperatures versus r.heating value has decreased gradually and the temperature atthe drilling bottom has fallen to 92C in the project area, A simpli-fied schematic diagram of the combustion space within this areaThe calculated temperature distribution shows no influenceand the temperature curve measured by drilling are given in Figs. from the high temperatures in the channel when r2 is 70m. The in-3 and 4verted temperature distribution above the gasification channel isThe temperature increased at a depth of 200 m. The under- close to the values from on-site detection when n, is equalground temperature is unaffected by the high temperature in the 4m. Using the approximate value of 150 m the volume of the com-gasification channel when r is equal to 70 m. The drilling temper- bustion volume can be calculatedature did not increase after a depth of 280 m so drilling was v=ril a 7500 mstopped at that depthThe initial underground temperature is4 ConclusionsToo=25.5Cat a depth of 200 m and the local geothermal gradient is(1)A Bessel function was used to establish a model of a gasifica-tion channel. the average temperature in the gasificationn=25c/100mchannel was 92C and the calculated temperature value.The periodic function in Eq (7)can be used to obtain the modelby back analysis, was close to the measured value whenthe radius, hr1. was 4 m. the volume of the combustionzone calculated from the gases produced during UCG wasH123.≈24,5.5,875000-10000 m which is close to the result obtained fromhe proposed modeL.when r, is 70m. The model parameters and the known conditionsare used to obtain the calculated temperature estimates shown in(2)The calculated combustion volume is crucial to solving stability problems and in providing some theoretical principlesIgon avoiding the collapse of the ground from large scale fallground level: 1377ing of the coal seam roof.(3)Effective combustion and cavity growth during UCG can, tosome extent, be controlled. In fact, the shape of the gasifica-tion channel is not regular or generally oval. However, theshape of the gasification channel was simplified for thenodel introduced here this was done to facilitate calculations and the approximation introduces some limitations.Further studies are suggested to address theseThis work中国煤化工 the major State BasicCombustion space area IResearch andNo.2007CB714102)and the workC NMHGndamental ResearchFi 3, Simplified combustion space.Funds for the Central Universities(No. 2009B00714)study ofL Chen et aL/ Mining Science and Technology(China)21(2011)581-585585formation mechanism of piping central seepage flow channel in 19 Perkins G Sahajwalla V. A Numerical study of the effects of operating conditionty growth in cavity gIin lignite coal blocks inhe context of UCG. Energy 2010: 35: 2374-8E110] Yang LH, Liu SQ, Liang ] Numerical analysis of dynamic temperature field andReferencesoncentration field in the process of underground coal gasification.J China[1] Liang J. Liu sQ Yu L Chang. Method of stable controlling the process of [11 Lu BY Shi xx Qju B. Wang MY. Test on the csa of underground coal[2 Anil K, Mohammed Q SanJay M. Preeti A Urpal gasification: a ne[12] Liu JH, Wang XT, Tian NG wang W. the application of radioactiveclean coal utilization technique for India. Energy 2007: 32: 2061-71the dynamic monitoring of underggasified burned-out area of[3] Liu SQ, ui JG, Mei M, Dong GL Groundwater pollution from underground cocoal, Geophys Geochem Exploration 2000: 24( 2): 92-8 In ChineseL.nJ China Univ Mining Technol 2007: 17(4): 467-72(13] Jim B. Influence of radial seepage on temperature distribution around[4 Yang LH. Liu YG, Jiang G Model test study on underground coal gasification inindrical cavity in a porous medium. Heat Mass Transfer 1998: 41(11inclined coal seams. J China Univ Mining Technol 2002: 31(1): 10-3.1531-41[51 Yang LH. Liang ]. TEM detection method of combustion sp4]Yexperiment and numerical simulation for61s叫 Xx. Mao wL L YL Lg啊山mp:2aan15)M题tmFue2004:83:573-84.opment of underground coal gasification in China. Energy Source EngUniversity Press: 1991. p. 111-4 [In Chinese)2008:1:5-10| n Chinese[161 Chen CS. Differential equations in mathematical physics. Beijing: Scienceowtlignite coal blocks in the context of UCG. Energy 2010: 35: 2374-86[17] Chen JS, Dong HZ The study on new theory and technology of tracer detecting[8] Britten JA Thorsness CB Model for cavity growth and resource recovery duringthe leakages of dams. Beijing: The Science Publishing Company: 2007, p.derground coal gasification In Situ Oil Coal Shale Miner 1989: 13: 1-5351-80 [In Chinesel中国煤化工CNMHG