The detection of reactive intermediates in the combustion and pyrolysis of coals, chars and macerals

The temperature-programmed combustion of coal was studied and reactive intermediates were observed by gas sampling directly above the sample in a thermogravimetric analyser-mass spectrometer (t.g.a.-m.s.). Species detected included OCS, OCN, HCN and (CN)(2). NO levels observed at the exit of the t.g.a. differed from those measured directly above the sample, the former usually being higher. This observation can be attributed to the conversion of volatile nitrogen-containing species to NO by homogeneous gas-phase reactions. The temperature-programmed combustion of maceral concentrates was also studied, to assess differences due to heterogeneity in the coal. The differences between the macerals were in species detected and their relative concentrations. Fusinite macerals tended to produce the highest levels of NO measured directly above the sample, compared with vitrinites and semifusinites, and the highest levels of sulfur species. The results are discussed in terms of the release of volatile nitrogen species and the reduction of NO produced in char gasification on the surface and in the pores of the char.