Volatilisation and catalytic effects of alkali and alkaline earth metallic species during the pyrolysis and gasification of Victorian brown coal. Part II. Effects of chemical form and valence

Alkali and alkaline earth metallic (AAEM) species (Na, Mg and Ca) exist in Victorian brown coal mainly as carboxylates forming a part of the coal organic matter or as dissolved salt (NaCl) in the coal moisture. The experimental results in this paper show that the chemical and/or physical form of sodium in the brown coal is an important factor influencing the volatilisation of sodium during pyrolysis. Significant amounts of light species containing carboxyl or carboxylate groups such as formate, acetate and oxalate were found in the volatiles from the pyrolysis of the brown coal. It is believed that the release of AAEM carboxylates is an important mechanism for the volatilisation of AAEM species, particularly at low temperatures (<600°C). The carrier gas flow rate passing through the coal bed can greatly affect the volatilisation of AAEM species through this mechanism. Another mechanism for the volatilisation of AAEM species is the breakage of bonds between AAEM species and char matrix at high temperatures. Under our experimental conditions, the sodium in the form of NaCl in the coal substrate seems to volatilise more easily than the sodium in the form of carboxylate in the coal substrate. The monovalent species (Na) is volatilised much more easily that the divalent species (Mg and Ca) during pyrolysis.