Development of a dry bottom ash extraction/afterburning system from pulverized fuel co-fired utility boilers

Co-firing of coal with waste/biomass in PF boilers is frequently associated with coarse unconverted waste/biomass particles reporting to the bottom ash. Accordingly, ash postcombustion may still be active during dry bottom ash extraction from utility boilers. This phenomenon has been frequently observed in plants equipped with dry ash belt extraction/conveying systems, such as the MAC (Magaldi Ash Cooler) proprietary technology. In the present work a model is presented that describes the fate of ash particles from a PC boiler co-fired with coal and RDF during their fall-out from the boiler and during the subsequent extraction on the belt-conveyor. The physico-chemical properties and the reactivity of ashes have been characterised experimentally. The model deals with the broad distribution of ash particles falling from the furnace on the belt and the establishment of a polydisperse granular bed on the conveyor. Modelling of heat transfer and combustion in the granular bed of ash is accomplished considering two distinct length- and time-scales: at the macroscale the deposited solids undergo transient heat transfer within the granular bed and with the environment; at the microscale ash particles of different size and temperature exchange heat among themselves, equalising their temperatures. Model results confirm that extensive carbon conversion can be achieved on the belt and that the MAC system can be effectively implemented to operate as a bottom ash afterburner.