Formation of NOx and SOx precursors during the pyrolysis of coal and biomass. Part II. Effects of experimental conditions on the yields of NOx and SOx precursors from the pyrolysis of a Victorian brown coal

A Victorian brown coal was pyrolysed in a quartz reactor. The reactor has some features of a drop-tube reactor and of a fixed-bed reactor, capable of operating at fast and slow heating rates. The yield of HCN was found to change with gas flow rate and coal feeding rate, indicating that HCN and/or its precursors could interact significantly with the nascent char to be incorporated into char as soot or to form N2. Experimental results indicated that HCN does not significantly convert to NH3, either on the char surface or in the gas phase, at least during the pyrolysis of the brown coal in this study. The yields of HCN and NH3 were both sensitive to changes in heating rate. The reduction in the yields of HCN and NH3 with decreasing heating rate is mainly due to the lack of radicals at the slow heating rate, which are required to initiate the opening of the N-containing rings. The carbonisation/condensation reactions also make the N-containing heteroaromatic ring systems increasingly stable during the extended holding at high temperatures at the slow heating rate. Experimental results appear to suggest that there are two types of organic sulphur-containing structures in the brown coal with very different thermal stability. The first type could be converted into H2S at low temperatures (<600°C). The other type was stable at temperatures up to 1000°C. The changes in heating rate or coal feeding rate did not affect significantly the formation of H2S.