Synergistical reduction of PM and NO formation in preheating co-firing of coal and biomass

Abstract

Preheating co-firing of coal and biomass was proposed for synergetic mitigation of pollutant source emissions. The effects of different operating parameters on particle matter (PM) and nitric oxide (NO) emissions were investigated, including excess air ratio (αp), temperature (Tp), residence time (tp) in the preheating zone and temperature (Tc) in the combusting zone. Coal and biomass particles were fed separately by two feeders, mixed uniformly in a mixing device and then inserted in a high-temperature drop-tube furnace. PM and NO emissions at the furnace outlet were measured by ELPI+ and the infrared gas analyzer. The results showed that as αp increased from 0 to 0.5, PM10 emission declined by 10.65% while NO emission went down first and then rose up with a turning point at αp = 0.3 of 247.93 mg/m3. When Tp increased from 1400 K to 1800 K, PM10 emission increased but NO emission decreased. The increase in the former was more than twice the decrease in the latter. More interestingly, the forming mechanism of ultrafine mode PM0.3 was dominated by “solid-vapor” process at low combusting temperature but transformed into “vapor-particle” process at high temperature. When Tc ascended from 1700 K to 1800K, the dominant “vapor-particle” process was inhibited, leading to a decrease of 13.5% in PM0.3 emission. PM0.3-2.5, PM2.5-10 and NO emissions were boosted by 64.99%, 36.40% and 78.20% respectively with Tc ascending from 1400 K to 1800 K. As tp rose from 0.1 to 0.5, the production of PM0.3 and NO decreased by 29.50% and 44.55% respectively, but that of PM0.3-2.5 and PM2.5-10 increased by 20.04% and 33.83% continuously. Under optimum parameters (Tp = 1300 K, αp = 0.3, Tc = 1400 K and tp = 0.5 s), PM10 and NO emissions could be reduced by 30.88% and 48.01%, respectively.