In this work, a method to study the formation of syngas during the underground coal gasification (UCG) process and its reactive transport in the surrounding strata is proposed. It combines a thermodynamic equilibrium stoichiometric model of the cavity reactions with a coupled thermo-hydraulic-chemical-mechanical (THCM) framework of COMPASS code for the transport of UCG products away from the cavity. With the input information of coal properties obtained from the South Wales coalfield, gasification reagents (air and steam) and thermodynamic conditions (initial temperature and pressure), the thermodynamic equilibrium model developed provides the maximum yield of gasification products and temperature from a UCG system. Gasification results giving the syngas composition with the highest percentage of methane and carbon dioxide, are then used as the chemical (gas) and thermal boundary conditions for the coupled thermo-chemical model of the THCM framework to analyse the variations of temperature and gas concentrations, in strata surrounding the UCG reactor. For that purpose, a set of numerical simulations considering three porous media (coal, shale and sandstone) with different physico-chemical properties is conducted. The gasification results demonstrate that increasing the amount of steam injected in the UCG reactor decreases the temperature of the system as well as the concentration of carbon monoxide and nitrogen, while benefiting the production of hydrogen, methane and carbon dioxide. The numerical simulations performed using the THCM model indicate that multicomponent gas diffusion and advection are competing transport mechanisms in porous media with intrinsic permeability higher than 1 mD (sandstone), while the gas diffusion becomes a dominant transport process in porous media with an intrinsic permeability lower than 1 mD (coal and shale). Moreover, the simulation results of reactive transport of methane and carbon dioxide in different porous media demonstrate the significance of considering the adsorption effect in the gas transport in the overall UCG process. In particular, the retardation of the gas front due to gas sorption is the most pronounced in coal, followed by shale and then sandstone. In conclusion, the model presented in this study demonstrates its potential application in managing the environmental practices, reducing pollution risk and securing greater public and regulatory support for UCG technology.

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Pyro-oxidation and reduction reactions in a downdraft gasifier optimized for improved carbon conversion efficiency and calorific value.

The article is focused on demostrating the combined effects of reaction and insulation parameters on syngas compostion during gasification process. A kinetic model is developed and implemented using FORTRAN program to simulate a downdraft gasifier using rubber wood as feedstock. The gasifier model consists of three zones: the pyrolysis, oxidation zone, and reduction zone; the gas composition and temperature distribution are estimated for each zone taking into account equilibrium ratios and reaction kinetics. To study the effect of heat loss, a novel expression in terms of insulation parameters is included for each zone. The properties of biomass composition and oxidant along with reaction constants is supplied as input parameter to the model.The results obtained from the model have been validated with experimental values. The effect of air supply and moisture content, coupled with properties of thermal insulation on final syngas composition and temperture is discussed. The analysis is useful for understanding the effects of energy interaction and it's losses, chemical reactions and operating conditions to maximize the energy output of the gasification process.

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