The exergy optimization of the reverse combusion linking in underground coal gasification

Underground Coal Gasification (UCG) is a gasification process carried on in non-mined coal seams using injection and production wells drilled from the surface, which enables the coal to be converted into product gas. A key operation of the UCG is linking the injection and production wells. Reverse combustion linking (RCL) is a method of linking the process wells within a coal seam, which includes injection of an oxidant into one well and ignition of coal in the other so that combustion propagates towards the source of oxidant thereby establishing a low hydraulic resistance path between the two wells. The new theory of the RCL in typical UCG conditions has been recently suggested. The key parameters of the RCL process are determined using the technique of Intrinsic Disturbed Flame Equations (IDFE). This study is concerned with extending the results of the RCL theory to incorporate hydrodynamics of air injection and flow during RCL operation to derive mass flow rate of air to the combustion front as a function of the injection pressure. The results enabled an optimization procedure maximizing the exergy efficiency of the RCL process. The optimization has been performed on a model case using a quasi-two-dimensional air flow model in the coal seam. The results have been compared to the industrial operational data of RCL in the conditions of the Chinchilla UCG project in Australia. The comparison has indicated a reasonable conformity of the modeling with the operational results. The preliminary outcomes of the study will be further refined to incorporate more realistic air flow models in the coal seams

Underground fire prospective technologies

In this work, we present a brief review of underground fires, which are generally characterized by slow, heterogeneous combustion (or smoldering) of porous combustible materials. Due to low temperatures and small propagation velocities of the smoldering processes, underground fires are difficult to detect, especially in their initial stages. Furthermore, the estimation of the extent of the detected underground fires is problematic as well. Very few diagnostic techniques provide adequate information on these fires. Underground fires are also extremely difficult to extinguish. Such fires can last for a very long period of time, posing a serious safety threat and having substantial adverse environmental and economic consequences. Current technologies used in controlling and extinguishment of underground fires either are costly or usually do not result in satisfactory outcomes. We also discuss the technologies developed over the years in underground coal gasification and possibilities to utilize these technologies for controlling underground fires and decreasing their harmful impact

Modeling of enhanced coal bed methane recovery and CO2 sequestration in coal seams

In this study, the recently proposed Porous Distance Conditioned Moment Closure (PDCMC) model is used to tackle the problem of modelling of adsorbing, desorbing and reacting flows through coal, which is of a particular importance for such technologies technologies as CO geological sequestration and enhanced recovery of Coal Bed Methane (CBM). The model, which is formulated in terms of single-conditioned expectations, simulates complex processes of convective and diffusive transport of species through a complete cascade of pores of different sizes in a continuous and consistent manner. The model addresses the major difficulty of describing sorption process in porous media with fractal properties, where distant transport occurs in the largest pores or fractures, while the adsorbing or desorbing surface is mainly allocated in small pores. Experimental measurements of methane replacement by CO conducted at the University of Queensland reveal that, at specific conditions, the decay of CH concentration in in the outflow gas follows a power-law, rather than to be exponential. The PDCMC model allows consistent treatment of methane diffusion from smaller to larger pores together with a counterflow of CO induced by the pressure gradient. As a result, the model can match the power-law observed in the experiments. Ability of the model to simulate different regimes of methane replacement by CO makes it useful for optimizing the operational parameters for such technologies as coal seam CO storage and enhanced CBM recovery

Conditional model for sorption in porous media with fractal properties

In this study, the conditional moment closure approach, which is proven to be very useful for modelling of reactions in turbulent flows, is extended to characterise adsorbing, desorbing or reacting flows in porous media. A complete specification of the porous distance conditioned moment closure model, which is formulated in terms of single-conditioned expectations, is presented. The closure of the model equations is obtained assuming the diffusion approximation for fluxes of the reactive species. The model simulates complex multi-cascade processes of convective and diffusive transport of species between pores in a continuous and consistent manner and is a generalisation of dual (or triple) porosity concept. The model addresses the major difficulty of describing transport, entrapment and sorption processes in porous media with fractal properties, where distant transport occurs in the largest pores or fractures, while the adsorbing or desorbing surface is mainly allocated in small pores. The model is able to simulate various regimes of methane replacement by CO in a coal sample, which makes it useful for optimising the design and parameters of enhanced coal bed methane recovery operations. It is demonstrated that the power-low decrease in downstream methane concentration, which has been observed experimentally, can be accurately reproduced by the model