Role of coal surface functional groups in methane cracking over different chars

Methane cracking was studied over a set of Xiaolongtan lignite chars in a fixed bed reactor at 1123K and atmospheric pressure with a mixture of CH₄/N₂ (1:4). The chars were obtained by pyrolysis of raw Xiaolongtan coal in nitrogen at 1173K for 30min in a fixed bed reactor. The main functional groups in coal char were hydroxyl group, carbonyl group and ether bond and so on, which can be protected by dipping the char into solutions of barium hydroxide, phenyl hydrazine and hydrogen iodide, respectively. The catalytic activity of coal chars treated by barium hydroxide was lower than the raw chars, while that of the chars treated by phenyl hydrazine or hydrogen iodide were higher. Hydroxy in the coal char was expended by Ba (OH)₂. The initial methane conversion was achieved at about 90.5% for the Ba (OH)₂ -treated char with equivalent-volume impregnation. The corresponding hydrogen yield was at about 65.2%. The carbonyl group of the coal char was reduced by phenyl hydrazine, with the methane conversion and hydrogen yield about 55.4% and 42.9%, respectively. The methane conversion and hydrogen yield decreased, while the ether bond turned into hydroxy. It was speculated that hydroxyl group in coal char restrain the methane cracking, while carbonyl group and ether bond accelerate it. The methane conversion and hydrogen yield on the different coal chars decreased with increasing reaction duration. The char became deactivated at 123min following its exposure to methane. The main reason was that the carbon from methane cracking was deposited on the char, which is supported by scanning electron microscopy analysis.