Toward understanding the reactivity and catalytic mechanism of coal pyrolysis with metal chloride modification

Abstract

Catalytic pyrolysis of Zhundong coal over three metal chlorides (NaCl, CaCl2, and FeCl3) was investigated via a thermogravimetric analyzer and a fixed bed reactor in this study. The structural features of the chars were examined using X-ray diffraction, scanning electron microscopy and N2 adsorption analyzer, and the pyrolysis mechanisms regarding the cleavage of bonds were proposed. The results demonstrate that all three chlorides have a beneficial effect on the production of CH4/H2 and are ranked according to catalysis ability as follows: NaCl > FeCl3 > CaCl2; the opposite effect is observed for the production of CO. The addition of metal chlorides results in a highly rough surface and a low degree of graphitization. Moreover, the sample with added FeCl3 has a more porous and disintegrated structure than those with added NaCl and CaCl2, and the pores and cracks are caused by gas release during the transformation process of FeCl3 to Fe2O3. In terms of the catalytic mechanism, FeCl3 differs from NaCl and CaCl2 in coal pyrolysis. FeCl3 is converted to Fe2O3 to promote the formation of free radicals during the pyrolysis process, while Na/Ca species are involved in a repeated process of bond formation and bond breakage between the species and the coal matrix.