Unburnt carbon elutriation from pressurised fluidised bed combustion of Australian black coals

A batch-fed bench-scale pressurised fluidised bed combustion (PFBC) facility was built to research the in-bed processes generating fine char particles during PFBC of Australian black coals. Elutriation of such particles is responsible for the combustion inefficiency and their combustion in the filter cake on high-temperature ceramic filters may contribute to the formation of ‘sticky ash’ and consequent cleaning difficulties. Measured carbon elutriation varied from coal to coal and the results agreed qualitatively with large-scale PFBC performance, allowing definition of satisfactory and unsatisfactory performance criteria. Carbon elutriation in PFBC was correlated tentatively with crucible swelling number although the range of values of the crucible swelling number needs to be extended. Carbon elutriation in PFBC did not correlate with volatile matter for the five Australian coals investigated in contrast with previous PFBC pilot plant studies on four Northern Hemisphere coals. The higher swelling coal produced a significantly larger average macro-pore diameter after devolatilisation in PFBC, which caused greater attrition of fine char from the particle surface during the char burnout stage.