Interpretation of Methane and Hydrogen Evolution in Coal Pyrolysis from the Bond Cleavage Perspective

Abstract

Pyrolysis of 34 coals of different rank with carbon contents of 73.7–91.9% was studied in a TG-MS system to analyze CH4 and H2 evolution. On the basis of the shape of evolution curves and dissociation energy of covalent bonds in coals, the CH4 formation is attributed to the cleavage of a Cal–Cal bond in a low temperature range and that of Cal–Car bond in a high temperature range, while the H2 formation was attributed to the cleavage of H–Cal bond in a low temperature range and that of Cal-Car bond in a high temperature range. The yields of CH4 and H2 corresponding to cleavage of each of these bonds are quantified by deconvolving each of the evolution curves into two subcurves. It is found that the cleavage of bonds containing only aliphatic carbon, such as Cal–Cal and H–Cal peaking at 549 and 540 °C, respectively, contributes to small fractions of CH4 and H2 generation. The cleavage of bonds containing an aromatic carbon, such as Cal–Car and H–Car peaking at temperatures of 618 and 774 °C, respectively, c...