The experimental study of fly ash recirculation combustion characteristics on a circulating fluidized bed combustor

Fly ash recirculation combustion (FARC) technology is an effective way to improve the combustion performance due to the improvement of carbon burnout as well as the utilization of calcium oxide. In order to study the effect of FARC on operation and emission characteristics of CFB boilers, a series of experiments were carried out on a lab-scale CFB combustor by using two types of fly ash (desulfurization and non-desulfurization fly ashes). Results show that with FARC, the temperature distribution in the furnace becomes uniform. The difference between the two different types of fly ash is mainly reflected in the emissions of SO2 and NOX. Desulfurization fly ash would increase the NOX emission while decrease the SO2 emission, and the optimum desulfurization temperature is 880 °C. On the contrary, non-desulfurization fly ash would increase SO2 emission while decrease NOX emission. FARC could effectively improve the combustion performance, as the recirculation rate increases from 0 to 0.3, the combustion efficiency increases by 2% for both two fly ashes and the CO emission decreases, but the particulate matter emission increases. For desulfurization fly ash, Ca/S molar ratio should be appropriately reduced for inhibiting the NOX emission when limestone was used for in situ desulfurization.