Effect of limestone on physicochemical properties of the residual chars from simulated underground gasification of lignite

Based on the possible effect of limestone on composition and structure of coal chars during underground coal gasification (UCG), gasification of lignite by steam with different dosages of limestone of 0-30% (mass fraction) was performed in a simulated UCG test system. The composition, specific surface area (S_BET) as well as pore structure properties, microcrystal structure and surface functional groups of the obtained residual chars were examined by low-temperature nitrogen adsorption, XRD and FT-IR, etc. The results show that composition of coal char is significantly influenced by limestone. Limestone contributes to development of micro-pore toward meso-pore, increases S_BET and total pore volume (v_t) of residual chars. When dosage of limestone increases from 0 to 30%, the S_BET of residual chars increases by 21.91%, and the ratio of meso-pore volume to total pore volume rises percentage points of 21.49. XRD analysis shows that the presence of calcium destroys aromatic structure of coal char, increases both degree of disorder and interplanar spacing (d₀₀₂), and inhibits graphitization tendency of coal char. FT-IR analysis indicates that hydroxyl group of residual chars is reduced with the presence of limestone.