Fates and roles of alkali and alkaline earth metals during the pyrolysis of a Victorian brown coal

H-, Na- and Ca-form coal samples were prepared from a sample of Loy Yang brown coal and pyrolysed in a wire-mesh reactor. The tars were characterised with UV-absorption and UV-fluorescence spectroscopies. Increases in heating rate (1 to 2000 K s(-1)) and temperature (up to 700 degrees C) were found to facilitate the release of larger ("equivalently" larger than naphthalene) aromatic ring systems from coal during pyrolysis. The presence of alkali and alkaline earth metallic (AAEM) species in the coal samples greatly hindered the release of the larger aromatic ring systems during pyrolysis. The AAEM species also reduced the effects of heating rate on the release on aromatic ring systems at lower temperatures. However, the hindering effect was not proportional to the contents of AAEM species in the coal. In addition, the ion-exchange processes caused irreversible changes to coal structure. Significant proportions of the AAEM species in the coal samples were volatilised during pyrolysis even at temperatures as low as 300 degrees C. The volatilisation of AAEM species was not sensitive to changes in heating rate but was intensified with increasing temperature. The monovalent species (Na) was always volatilised to a much larger extent that the divalent species (Mg and Ca) under similar pyrolysis conditions. At high temperatures (900-1200 degrees C), the drastic volatilisation of Na (up to 80%) and of Ca (up to 40%) was accompanied by the increases in tar yield during the pyrolysis of the Na-form and Ca-form samples. The fates and roles of the AAEM species during pyrolysis are thought to be related to their transformation during pyrolysis. The AAEM species might have been involved in a repeated bond-forming and bond-breaking process between the AAEM species and the coal/char matrix. During this process, tar precursors were repeatedly linked to the coal/char matrix and were thermally cracked. Some of the more aliphatic components and/or smaller aromatic ring systems in a tar precursor were cracked to gas and some of the larger aromatic ring systems were charred. (C) 2000 Elsevier Science Ltd. All rights reserved.