Comparative investigations of coal pyrolysis under inert gas and H2 at low and high heating rates and pressures up to 10 MPa

Five German hard coals of 6–36 wt% volatile matter yield (maf) were pyrolysed at pressures up to 10 MPa, using two different apparatuses, which mainly differ in the heating rates. One consists of a thermobalance where a coal sample of ≈ 1.5 g is heated at a rate of 3 K min ^?1 under a gas flow of 3 I min^?1. The other apparatus is constructed for rapid heating (10²?10³ K s^?1) of a small sample of ≈10 mg of finely-ground coal distributed as a layer between the folded halfs of a stainless-steel screen, heated by an electric current. The product gas composition was determined by quantitatively analysing for H₂, CH₄, C₂H₄, C₂H₆, CO, CO₂ and H₂O. The amounts of tar and char were measured by weighing. The heating rate, pressure and gas atmosphere were varied. Under an inert gas atmosphere, high heating rates result in slightly higher yields of liquid products, e.g. tar. The yields of light hydrocarbon gases remain the same. With increasing pressure, the thermal cracking of tar is intensified resulting in high yields of char and light hydrocarbon gases. Under H₂, pyrolysis is influenced strongly at elevated pressure. Additional amounts of highly aromatic products are released by hydrogenation of the coal itself, particularly between 500 and 700°C. This reaction is less effective at higher heating rates because of the shorter residence time and diffusion problems of H₂. The yield of light gaseous compounds CH₄ and C₂H₆ increases markedly under either heating condition owing to gasification of the reactive char.