Effect Of Equivalent Ratio (ER) On the Flow and Combustion Characteristics in A Typical Underground Coal Gasification (UCG) Cavity

Abstract

Underground Coal Gasification (UCG) is one of the most favourable clean coal technology options from geological-engineering-environmental viewpoint (less polluting and high efficiency) for extracting energy from coal without excavating or burning it on the surface. UCG process requires injecting oxidizing agent (O2 or air with steam) as raw material, into the buried coal seam, at an effective ratio which regulates the performance of gasification. This study aimsto evaluate the influence of equivalent ratio (ER) on the flow and combustion characteristics in a typical half tear-drop shape of UCG cavity which is generally formed during the UCG process. A flow modelling software, Ansys FLUENT is used to construct a 3-D Computational Fluid Dynamics (CFD) model and to solve flow hydrodynamics in the cavity. The boundary conditions are- (i) a mass-flow-inlet passing oxidizer (in this case, air) into the cavity, (ii) a fuel-inlet where the coal volatiles are originated and (iii) a pressure-outlet for flowing the product synthetic gas (syngas) out of the cavity. A steady-state simulation has been run using k-ε turbulence model. The mass flow rate of air varied according to an equivalent ratio (ER) of 0.16, 0.33, 0.49 and 0.82, while the fuel flow rate was fixed. The optimal condition of ER has been identified through observing flow and combustion characteristics, which looked apparently stable at ER 0.33. In general, the flow circulation mainly takes place around the ash-rubble pile. A high temperature zone is found at the air-releasing point of the injection pipe into the ash-rubble pile. This study could practically be useful to identify one of the vital controlling factors of gasification performance (i.e., ER impact on product gas flow characteristics) which might become a cost-effective solution in advance of commencement of any physical operation