Basic equations of channel model for underground coal gasification

PROGRESS IN NATURAL SCIENCEVol. 12, No. 5, May 2002Basic equations of channel model for underground coal gasificationHANG Yongli**, ZHANG Mengtao and SONG WeiyuanLiaoning Technical University, Fuxin 123000, ChinaReceived September 6, 2001 revised October 15, 2001advantages of zero rubbish nonpollution, low cost and high safety. According to the char-acteristics of the gasification the channel model of chemical fluid mechanics is used to set up the fluid equations and chemical equations by somereasonable suppositions in this paper which lays a theoretical foundation on requirements of fluid movement rules in the process of underground coalKeywords: underground coal gasification, channel model, gasification area, constituent diffusionUnderground coal gasification is a process that un- through the ground was carried out in former USSR inrground coal changes into fluid before being excavated, 1942. Now the experimental research has all directed towhich has a clear advantage over coal mining. So, itthe hole-drilled model of undeoal gasificationbelieved to be a revolution of coal mining.used by former USSR and US. In this paper undergroundcoal gasification means the hole-drilled model, unlessCoal gasification in combustion ovens on the ground othenoise annotatedhas been realized and developed to be a coal gasificationndustry. The underground coal gasification proceedsThe process of underground coal gasificationthe natural geological environment, whereby the gasifieshown in Fig. 1. Firstly a pair of holes to the coal layertion process is very complicated. Both theoretical analare drilled ,i. e. the admission port and gas outlet theand experimental research in a laboratory or in situ come distance between the two is 300-400 m. By differentacross great problems. Although an underground gasifica- techniques the coal layer is reached through the admistion oven by drilling has already been set up a prob- sion port so that gas can traverse then the gas has alem in mining -for quality and quantity of gasification chemical reaction with coal and a gasification processmay not satisfy the design requirements may not be finishes in the gasification zone. The gasification issolved in a long time. So, according to the character of thought of as a lluid flowing channel inunderground coal gasification, the mathematical modelt Gas outletestablished which lays a foundation for research on un-derground coal gasification by the computers" virtualsimulation" technology and improvement of gasification industrial processes and early promotion of generation anddevelopment of the gasification industry in China1 Basic theorems of underground coal gasifiCoal layercationround coal gasification experiments were done in the minBecause of the simplici中国煤化工 Ind coal gasificationstudies were conducted through the experiments under theCNMHGin some countries of the world so as to get some use-mpirical conclusions. The first successful underground coal gasification experiment by drilling a holeTo whom correspondence should be addressed. E-mail: zyl238@ sina cof? 9872031)Supported Ional Natural Science Foundation of China( Grant162Progress in Natural Science Vol 12 No 5 2002setting up a mathematical model so it is called the chanIf dry oxygen or air enters an admission port oxidanel modeltion reaction produces CO and CO2, and heat( unit: kJ/mol, the same below),i.eCoal must first be ignited in the process of under-C+03→CO,+395850ground coal gasification by the electric igniter near the ad2C+O3→2C0+219660mission port. The temperature of the coal body increasesgradually and air or oxygen containing vapor pours into theCo has a chemical reaction with oxygen,i.eadmission port. The coal body is first dried in the process2C0+O2→-2CO2+572040(3)of temperature increasing. When the temperature reachesIf there is vapor when buming, the reaction ofabout 400C, the coal body begins pyrogenation and e- bon and vapor will produce H2 and CH4, which will havevaporation and then enters the destructive-distillation a chemical reaction with oxygen and may yield CO2 andstage.When the temperature reaches a burning point of h(the oxidation zone isabout 650-850C of solid carbon - the buming pointC+2H3O→CO,+2H+45402(4of coal changes with different coal quality and ash content2H,+O3→2H2O+232302the coal body is burned and oxidized to release heatand further increase the temperature of the coal bodyCO+H2O→CO2+H2+40530(6)Some heat passes through the side of the gas outlet so thatgenerally the terminal of oxidation zone is deter-the temperature of part of the coal body increases to red mined by oxygen exhaustion in the process of gas flowinheat, and gases produced by burning generate a reduction If only oxygen enters fluid may be considered as havinreaction,i.e. heat-absorbing reaction. When heat trans- two constituents O and cO,; if air enters, there is anothmitted from the gasification zone is less than the absorbed er constituent N2; if vapor enters, H20 and H2 will alsoheat, the temperature of the coal body decreases. When existthe temperature is below the critical point the coal bodycannot generate a chemical reaction but just dehydrate(ii Reduction zoneand distil. The gases produced by dehydration and distillation flow to the outlet then to the ground. For the sameIn fact, gases produced in the oxidation zone may bpart of the coal body the three processes of dehydrationreduced by heating carbon in the same zone i.e. oxidation effect and reduction effect coexist, Which dominatesand distillation reduction and oxidation happen continuously in turn. For the whole coal body the sequence ofof depends mainly on the temperature and in addition, isoxidation, reduction, dehydration and distillation is froalso related to the gas flowing rate and the coal grain sizethe admission port to the gas outletGenerally reduction effect predominates above 1000"Cnd oxidation effect dominates at about 800C2 Chemical reaction equations in differentIf dry air or oxygen enters gas from the oxidationzoneszone into the reduction zone is cO, which is reduced byC1. eCoal gasification belongs to fractional combustionwhich may be called broad sense combustion CombustionC+CO3→2C0-176190is a kind of phenomenon of physics and chemistry. In theIf there is vapor in gases the following reactions wilcombustion processes of gas liquid and solid chemical happen in the reduction zoneprocesses always take place which are caused by the heatC+2H2→CH4-75138(8)and mass transmission of many-constituent fluids, or moreC+H2O→CO+H2-130788(9)specifically the flowing of gases. By many-constituent wemean there is more than one kind of fuel, more than oneC+2H2O→CO2+2H+45402(10)kind of oxidizer and burning production for combustion. in which CO2 is reduced by C once more and the reactionThe process of underground coal gasification is divided in- of Hi中国煤化工to three zones stages), and the chemical reaction in eachone is different The chemical reactions are first re-CNMHGe is determined by temperature. When heat transmitted from an oxidation zone issearched about fluid flowing like that of underground gasl- less than exhausted heat, temperature begins to dropfication, so as to determine different constituents of fluidflowing in each zoneWhen temperature reaches its critical point the terminalof the reduction zone is determined. If only oxygen en-(1)次海aoeteis, CO, is reduced as CO in the reduction zone, in'rogress in Natural Science Vol. 12 No 5 2002which there are only two constituents CO and CO2 i if vapor enters there are also H2, H,O and CH4o1(pF)2bo/+aQ.(13)iii Dehydration and distillation zoneConstituent conservation equation)(14)The dehydration and distillation zone is located between the reduction zone and gas outlet. Dehydration orState equationdistillation happens in the coal layer in this zone. Distilp=p∑(15)lation produces evaporable gases mainly CO, CO2, H2CHa and other hydrocarbons which are a part of gas. DeReaction dynamic equationydration produces vapor, which blows off with gasws s0exP RT)(16)3 Basic flowing equations in different zones The equation( 16 )is known as Arrhenius lawactivation energyColIn the 1950s, Von Karman first put forward that stant, T burning temperature Ws mass varying rate ofntinuous medium mechanics should be used to researchconstituent s caused by chemical reaction in micro concombustion. Many scientists did a lot of work in the field stituent, C, specific heat in constant pressure, k perme-soon afterwards. From that time the fluid flowing prob-ability of coal u gas viscosity ms molar mass of corlem of many constituents, with chemical reaction and heatstituents,Y, relative consistency of constituent s and Dand mass transmission, began to be a branch of fluid me- diffusion coefficientchanics focusing on fluid flowing rule of more constituentreaction , i. e. chemical fluid mechanics. The gasificationThe constituents in fluid field in uneis a process with many kinds of fluid flowing with chemi- gasification vary with fluid constituents from the admissioncal reaction, belonging to a category of chemical fluid me- port, and because the reaction in the three zones is differchanics. Because of many constituents and mutual diffu- ent it is essential to respectively set up the basic gasifision and chemical reaction fluid flowing in the process of cation according to the situation of each zonegasification still abides not only by continuity,movementof fleAssuming that dry oxygen enters the admission portconstituent conservation equationand it is at a steady state and neglecting volume fosome padered as constants with theflowing channels are complicated and can not be clearly are develortionls uent, temperature and pressure, andin pore space and cracks in coal layers the shapes of fluidsthus basiInh zoneunderstood. And area scale and viscous effect are greatThe movement equation cannot be deduced from momen(i)Oxidation zonetum conservation theorem and constituent equation as genWhen oxygen enters the oxidation zone and the proeral viscous fluid flowing. In this paper the seepage ideaduced co enters reduction zone, the fluids in the oxida-of fluid mechanics is introduced for the fluid flowing in tion zone have only two constituents for which the basicunderground coal gasification and Darcy law which isequations arewidely used is used as a movement equationContinuIty equationAccording to chemical fluid mechanics neglectingexternal field forces and varying material features abiding=0,j=123(17)by continuous and differential conditions for a gasificationfield, the basic differential equation group is as followsMovement equation(18)Continuity equation中国煤化工=123+(m)=0HCNMHGMovement equationk ap2(19)(12)Constituent conservation equationEmen棍Pv YsDo164Progress in Natural Science Vol 12 No 5 2002Constituent dynamic equationn constituent conservation and energy equationsEk,( RT12mr)12(2)( 21) in which Ep is the heated and resolved active energy andThe sum of relative consistency of each constituent is B, is the factor before component, which has relation with100%,i.e.2Y=1. Because the sum of the whole each coal sort and pressureconstituent conservation equations is the continuous equation, only(s-1) constituent conservation and continuequations must be solved in the wholefluid field. Same as above the equations must be solvedous equations are independent. So basic equations in thin the whole fluid fieldoxidation zone consist of 1 continuous equation, 1 energyquation,3 movement equations and I constituent conser- 4 Conclusionvation equation with >Ys=I. In the zone, there are 7unknown factors ,i.e. fluid pressure, temperature 3Only considering the simplest situationspeed components consistency distributions of O2 and each parameter is constant, 21 differential equations andCO,. Because the three zones belong to the same fluid 3 algebra equations must be simultaneously solved so as tofield, all the equations in the three zones need to be ascertain fluid pressure speed, temperature and con-solved simultaneously in order to determine the boundarystituent consistency distribution in each zone. The uniqueof gasification. If(ii)Reduction zoneadmission there will be 5 constituents of N2,O2, H2H,O and CO, in the oxidation zone, 5 coWhen dry oxygen enters the admission the gas froCO2, H2,CO and CH in reduction and 6 constituentsthe oxidation zone into the reduction zone is CO2, which of n,, cO2, H2, CO, CH4 and H2O in the dehydrationreduces to co. due to the two constituents of co andand distillation zone in the result of the chemical reacCO2, the basic equations in the reduction zone are thetion. Assuming that each parameter does not vary withame as those in the oxidation zone i. e. 6 differential econstituent, temperature, pressure and fluid is idealjuations and I algebraic equation >y), the there are 27 differential equations and 3 algebraic equaboundary conditions can be determined by a solution of tions to be solved. If variation of the parameters is consid-the whole gasification field so as to get fluid pressureore etemperature, and the consistency distribution of Co and which cannot be solved only by analysis unless by computer aided simulation(iii) Dehydration and distillation zoneReferencesIn the zone, the main evaporant are CO,, COI Zhang, M.T. Research on fluid flow state of underground coal gasification( in Chinese ) Joumal of Liaoning Technical University, 1999, 18H2O, with little CHa H2, C H4 and higher hydrocar5):449bons which may be neglected. As before, I continuous 2 Zhou, L.X. Combustion Theory and Chemical Fluid Mechanics( inequation, 3 movement equations I energy equation ,sChinese ), Beijing: Science Press,1986constituent conservation equations and >Y,=I3 Lewis, B. et al. Combustion, flames and explosions of gases. Newamount to 10 equations. There are 10 unknown numbers 4 Smoot, L D. et al. Pulverized-coal combustion and gasification. Newof fluided, temperature and 5 constitYork Plenum Press, 1979N. Coal Gasificaocess, Park Riconsistencies中国煤化工CNMHG