Technical review of multi-fuel bubbling fluidised bed boilers for paper industry wastes, with predictive models for metals and dioxin emissions

摘要

The combustion of sludges in fluidised beds is widely and successfully practised in the paper industry. The extent of burnout is good, with a high conversion of carbon. Very wet sludges have been treated with the use of natural gas as auxiliary fuel. No report of furnace degradation has been found in a literature search. Fouling is possible in the downstream heat exchangers due to the deposition of clay-derived calcium compounds, but these can be readily removed. Acid gases are absorbed at ＞90 % efficiency with lime. The SNCR deNO_x system can be expected to give around 50 % reduction in NO emissions under typical operating conditions. The clay particles released by the paper feed tend to agglomerate into multi-micron size aggregates which are readily filtered. As trace metals are found almost exclusively on the particulates, good paniculate removal, as in a fabric filter, will ensure low metal emissions. Gaseous mercury will be adsorbed at 80 to 90 % efficiency by the addition of activated carbon, which also removes PCDD/F. Two simple, empirical mathematical models are described - one for trace metal emissions from a furnace, and the other for PCDD/F formation. The models are applied to two combustion applications relevant to sludge combustion. In each case the output predicts that the operation will meet standard emission limits.