Seepage Characteristics Of Coal And Pyrolysis Model Optimization In The Context Of Underground Coal Gasification

As an in-situ chemical mining technology,underground coal gasification（UCG）is the strategic direction of coal innocuous mining technology innovation in China’s energy technology revolution and innovation action plan.In the early stage of UCG process,the coal seams are dried and pyrolyzed,in which water and volatiles are released,leading to the formation of char seams.The porous char seams are the reactants in the following gasification and combustion reactions.In that low-temperature heating process,the physical structure,chemical properties and permeability of coal show most dramatic changes,which determines the penetration behavior of gas in the coal seam,and further affects the gasification rate and the propagation of the‘flame’（oxidation surface）in the coal seam.The emission of volatiles is related to development of pores and fissures in coal,thereby affects permeability of char seam.In addition,the pyrolysis gas produced also makes great contribution to final UCG product.Therefore,the study on gas permeable behavior of coal seams under thermal treatment and gas emission behavior of coal in this process are of great theoretical significance to the feasibility evaluation and process control of UCG.In this research,three typical coal samples with different metamorphic degrees,Ulanqab lignite（WL）,Yuanbao long flame coal（YB）,and Qinshui anthracite（QA）,are choosen and systematically studies are carried out on the permeability and gas emission characteristics of coal bodies under low-temperature heating.With the help of the high temperature three-axial permeability testing system,the permeability changes of lignite under thermal action and simulated stress conditions were studied,and the influence of water content,temperature and pore pressure on permeability was investigated.The effect of thermal treatment on pore/fracture structure development as well as the thermal deformation behavior were studied by the medical Computed Tomography（CT）.The low-temperature and slow-heating rates pyrolysis kinetics were studied,and the applicability of three prevalent pyrolysis models were compared.The applicability of the model was compared based on operating efficiency and prediction accuracy.The distribution of gas,liquid,and solid products was calculated and the components of light gas were determined.The yield and composition of volatiles were predicted by using Chemical Percolation Devolatilization（CPD）model.The main conclusions were described as follows:（1）The permeability of WL lignite shows“three zones”with temperature increasing,for parallel and perperdicular to the bedding direction.Two permeable points exsits at around 100～150℃and 400℃respectively,mainly related to moisture and volatile release.There exists water“seepage threshold points”,that is,when the moisture content is lower enough to over this point,the permeability will rise rapidly.Threshold points of moisture contents for samples parallel and perperdicular to the bedding direction are 35%and 21%respectively.For samples perperdicular to the bedding direction,increasing pore pressure can play a permeable effect,especially at 400℃.However,in the range of 250～300℃and permeability is less affected by pore pressure.It is more permeable in the direction parallel to the bedding,and for the water saturated sample,this effect becomes more obvious.The permeability of water saturated sample and original coal sample in the parallel bedding direction are 13 times and 3 times to that in the perperdicular bedding direction respectively.（2）Medical CT on three different metamorphic coal samples show that cracks can be observed in the WL lignite and YB long flame coal at 300℃and 400℃respectively,wherein WL lignite contains branch-like and randomly distributed fissures.The connectivity between these cracks is good.There are mostly axial-through type cracks in YB coal,with a small number of radial small cracks connected to it.QA coal show no obvious cracking phenomenon.Axial pressure changes the form of the fissure,that is,making cracks close or creates a shear cracks or pressure-initiated cracks.The pores in heated samples of YB coal are mainly large pores（60-80%）,and the distribution of pores inside the sample is uneven.According to fractal calculation,large holes above 2um and micropores of 4.5～100 nm have fractal characteristics.The resistance of dense coal body to volatiles is the main reason for its thermal expansion.（3）With the incremental isoconversional method,activated energy distribution of QA,YB and WL are calculated,they are located in the ranges of 234 to 300 k J/mol,132to 206 k J/mol,135 to 291 k J/mol respectively.The prediction performance of 1st order model was poor for the three types of coal,and the fitting correlation coefficients of WL coal,YB coal and QA coal were 0.871,0.906 and 0.866 respectively.Model selection needs to be considered from the prediction accuracy and operation efficiency,the low temperature slow pyrolysis weight loss characteristics of WL coal and YB coal can be described better by the discrete distributed activation energy model（DDAEM）,while single distributed activation energy model（1-DAEM）can greatly improve the operating efficiency of the model under the premise of ensuring accuracy for QA coal.（4）Volatile yields of QA coal,YB coal and WL coal in low temperature at slow heating rate pyrolysis are in the range of 5.9 to 11.6%,18.0 to 39.8%,2 6.7 to 39.4%（wt%）respectively.For QA anthracite,the volatile gas and liquid proportion are close,light gas is the main products for YB long flame coal,the proportion of gas ranges from57 to 78%in the volatiles,while liquid yield are 48 to 74%of volatile for WL lignite.The pyrolysis gas yields of QA coal,YB coal and WL coal were 0.005 to 0.166 Nm³/kg,0.012to 0.202 Nm³/kg and 0.030 to 0.201Nm³/kg,respectively.H₂ accounted for 79%of pyrolysis gas for QA,while the gas products for YB coal are H₂ and CH₄.In the pyrolysis gas of WL coal,contents of CO₂ and H₂are higher.The coefficients in the empirical formula proposed by Genetti are recalculated based on the structure information of 28 Chinese coals,the structural parameters of the WL and YB coal are calculated using the new formula,which were used as the input to the CPD model,and the prediction error of volatiles is 14.3%and 9.0%lower than that from the structure paramters based on the original empirical formula.