Effects of ash fouling on heat transfer during combustion of cattle biomass in a small-scale boiler burner facility under unsteady transition conditions

摘要

Combustion of cattle biomass (CB) as a supplementary fuel has been proposed for reducing emissions of NO_x, Hg, SO_2, and nonrenewable CO_2 in large coal-fired power plants; however, its high ash content resulted in fouling and slagging problems when the CB was co-fired with coals during small- and pilot-scale tests. Ash depositions during combustion of the CB as a reburn fuel were investigated using a 30kW_t (100000 Btuh~-1) boiler burner facility with water-cooled heat exchangers (HEXs) under unsteady transition conditions and short-term operations. Two parameters were used to characterize the effects of the ash deposition: (1) Overall heat transfer coefficient (U) and (2) Burnt fraction (BF). A methodology was presented and empirically demonstrated for the effects of ash deposition on heat transfer under unsteady transition conditions. Experiments involving ash deposition during reburning the CB with coals were compared with experiments involving only ash-less natural gas. It was found that the growth of the ash layer during reburning the CB and coals lowered the heat transfer rate to water in the HEXs. In low-temperature regions, the thin layer of the ash deposition promoted radiation effects, while the thick layer of the ash deposition promoted the thermal resistance of the ash layer. A chemical analysis of the heavy ash indicated that the BF increased when a larger fraction of the CB was used in the reburn fuels, indicating better performance compared with coal combustion alone. However, the results of ash fusion temperature indicated the ash deposited during combustion of the CB and coals was more difficult to remove than the ash deposited during coal combustion alone.