Simulation & Optimization Of Underground Coal Gasification Based On A Seepage Model

Energy demand of China is continuously increasing.Coal is the major fossil fuel in China and continues to play a pivotal role in the energy sector.This is caused by the fact that China is “rich coal,lean oil,less gas”.While the traditional utilizations of coal face with many pollution problems.In this context,gasification is considered a cleaning promising option for the future use of coal.Coal gasification generally requires construction of special plants,including large coal storage facilities and gasifiers.Meanwhile,there exists an alternative method,denoted underground coal gasification?UCG?.Underground Coal Gasification can utilize unmineable coal deposits and deeper seams,and reduce pollution.Many countries had researched and developed this technology,include USSR,United State,European Union,China and so on.As the reactions are in the deep coal seam,it's hard to detect and control UCG's process,so the simulation of UCG plays an important role in UCG's development.This paper focuses on the development of a simulation system which is based on a seepage model,and the optimization of the UCG's applications.In this paper,a simulation system of UCG was developed by FORTRAN language.The UCG processes include muti-physics phenomenons,that makes it harder for simulation system development.This paper discussed the simplify of the physics,and mathematical models on heat and mass transfer in the process of UCG are established.Then the discrete methods for the mathematical models were discussed,the second-order upwind scheme and second-order center-difference scheme were used.Specially,this paper has designed a mathematical model which is used in the flow phenomenons.the model is based on the Darcy's law.This model can be used in percolation channel and early free channel.The development of UCG's simulation system is from simple to complication.A 1-D simple UCS's simulation system was developed at the first step.And the 1-D simulation was compared with 2-D simulation by the same case.The results show that 1-D simulation is similar with 2-D simulation.Approximated results can be calculated by one-dimensional in some situations that require quick results Then a 2-D simulation with pure char was developed.This simulation was compared with simulation that includes pyrolysis module.The results show that pyrolysis is an important part in UCG's simulation,it can not be ignored in simplifying for the UCG's model.Besides the forward gasification,this UCG simulation system was developed to simulate the reverse link process in UCG.After the development of UCG's simulation system,the model was verified by comparing with experiment result.The results show that the calculation results of temperature field conform with the measurement results.The relative errors between calculated value and measured value of temperature is 7.7 %.The speed of the thermal front movements between caculated value and measured value were also close.The relative errors are about-4.6 %.The mole fraction of gas products,include H2,CO,CO₂,CH4,were compared with experiment.The result show that the mole fraction of gas products by simulation fit well with experiment at early stage.And the last stages were also close between simulation and experiment,but simulation sustained longer in early stage,comparing with experiment result.The sensitive analysis was also involved to optimize the UCG's applications.It was used to find out the key factors to improve the quality of gas products.The results showed the CO concentration of gas product is sensitive to a change of flow rate when flow rate is less then 32 L/h.Pressure showed an important role to CH4 concentration The O₂ content of inlet gas has significate influence on CO's concentration.While the H2 O content of reaction gas will increase H2's but reduce CO's concentration.The activity of pyrolysis and gasification also showed the importance to the H2 and CO's concentrations,but the activity of combustion has little influence to the result.The char yeild or the coal-ash ratio also show little influence to the concentration of gas product.That's cause by the less important role of coal combustion in UCG.At last,the concentration of gas product of H2,CO,CH4 is very sensitive to the gas production of pyrolysis.