Gasification of coal char in H2O/CO2 atmospheres: Evolution of surface morphology and pore structure

Increasing studies focus on gasification of coal char in mixed H2O/CO2 atmospheres, but quite controversial results were obtained on this issue. Char porosity and pore size distribution affect the reactant gas diffusion inside the char, reaction on the char active sites and gas product diffusion out of the particle. Hence, this study aims to understand the surface morphology and pore structure characteristics of the coal chars obtained from gasification in H2O/CO2 atmospheres. The raw coal was heated to 800-1000 °C in a fixed bed reactor to produce coal chars. The resulting chars were gasified under 40% H2O, 40% CO2, and 20% CO2 + 20% H2O atmospheres at the same temperatures to obtain a series of residual chars with different carbon conversion levels. The morphology and pore characteristics of chars were characterized by scanning electron microscope (SEM) and N2 adsorption method. The results show that CO2 char has an uneven surface with irregular potholes, whereas that the surface of H2O char is relatively smooth and distributed on the surface with honeycomb pores. CO2 and H2O play different role in creating the char porous structure. The CO2 chars are mainly micropores and along with some small mesopores. Under H2O and H2O/CO2 atmosphere, the char are rich in micropores, mesopores, and macropores, the pore size distribution is continuous. The total pore volume of gasified chars prepared under H2O/CO2 atmosphere was not significantly changed compared with H2O atmosphere, indicating there has positive interaction between H2O/CO2 in developing the coal char pore structures during gasification. H2O could create a wider porous structure that facilitates the access of CO2 to the coal char matrix to further develop the pore structure.