Abstract
Underground coal gasification (UCG) proceeds generally in the presence of an ash layer on coal (or char) surface. The ash layer increases the mass transfer resistance of O2 to the gasification surface, which may become the limiting step of whole process. This paper studies O2 diffusion in ash layer formed on cylindrical char samples using a specially designed one-dimension setup in a thermogravimetric apparatus (TGA). The effective internal diffusion coefficient (De) is found to increase with an increase in ash layer thickness, due to an increase in median pore diameter. Methods are established to correlate De with operating conditions and to estimate the role of internal diffusion resistance in overall mass transfer resistance.
Highlights
Underground coal gasification (UCG) has been viewed as a potential technology because it requires no mining and transportation of coal and leaves the gasification residue underground
These steps can be expressed by Equation (1), where Kc is the mass transfer coefficient of convection, L is the thickness of ash layer, diffusion coefficient (De) is the effective diffusivity in the ash layer, K is the rate constant of C-O2 reaction, while Cf, CS1 and CS2 are O2 concentration in the gas bulk, at the external surface of ash layer, and at the char surface, respectively: NO2 = Kc (C f − CS1 ) =
The gasification front moves along the axial direction of the cylindrical char as expected by the one-dimensional shrinking-core model and the ash remained adheres to the char surface to form a layer of similar thickness
Summary
Underground coal gasification (UCG) has been viewed as a potential technology because it requires no mining and transportation of coal and leaves the gasification residue underground. The CO and H2 can be used as feedstock for many chemical industries and fuels for various purposes [2] This process has been tested for more than one hundred years, such as in the former Soviet Union, and has been studied extensively in recent decades in many countries including Poland, China, Australia and Ukraine [3]. These studies, ranged from field scale to laboratory scale, have advanced this technology significantly. Urych studied UGC in a TGA/DSC system and evaluated pyrolysis of coal, a step prior to gasification, in a temperature range of 298–1173 K [4]
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