Stricter environmental regulation implies the need for further reduction of NOx emissions below the levels found in operating and new coal-fired units. The scope of the present study is to examine the reliability, operating performance and ash toxicity of a 1.2 MWth Circulating Fluidised Bed Combustion (CFBC) installation, when applying the micro-combustion of coal using the fine tuned air control along the riser to decrease NOx emissions. Two coals imported in Europe were tested under the usual combustion configuration and under the multi air staging conditions. Gaseous pollutants were measured at different locations along the furnace height. Fly and bottom ash samples were collected and characterised for their morphology, mineralogy, leaching and toxicity behaviour. Based on the results, the South African coal was perfectly adapted to the CFBC combustion, while the combustion of the Colombian one was troublesome due to its very low ash content. Despite the high concentration of toxic trace elements in the fly ash particles, the latter are acceptable for safe disposal, according to the EPA limits. Additionally, the Microtox toxicity test proved that the baghouse filter ash leachates, which presented the higher heavy metals concentrations due to their low particle sizes, caused the higher toxic effects.

Keywords:Combustion, air staging, emissions, trace elements