Partitioning behaviour of some toxic volatile elements during stoker and fluidised bed combustion of alkaline sub-bituminous coal

During coal combustion, the partitioning behaviour of the volatile toxic trace elements, boron, arsenic and selenium, is influenced by the highly alkaline nature of a typical New Zealand industrial sub-bituminous coal, and the combustion regime (underfeed stoker or fluidised bed) used to burn it. A significant amount of boron is retained in the bottom ash of the underfeed stoker, and some arsenic and selenium retention is also encountered. Mercury partitioning is not influenced by the combustion regime or alkalinity, although the fly ash generated from the stoker combustion appears to possess a significant mercury capture capability. Over 60% sulphur retention may be achieved, without additive, for fluidised bed combustion. Some of the chemical and physical factors responsible for the above partitioning behaviours are identified.