Field Measurements on the Emission and Removal of PM2.5 from Coal-Fired Power Stations: 2. Studies on Two 135 MW Circulating Fluidized Bed Boilers Respectively Equipped with an Electrostatic Precipitator and a Hybrid Electrostatic Filter Precipitator

摘要

This study reports the formation and emission characteristics of particulate matter (PM) with aerodynamic diameters less than 2.5 mu m (PM2.5) from two coal-fired circulating fluidized bed (CFB) boilers (B1 and B2) respectively equipped with electrostatic precipitator (ESP) and hybrid electrostatic filter precipitator (EFP). PM and total fly ash samples were collected at the inlets and outlets of the ESP and the EFP via a low-pressure impactor, a gravimetric impactor, and/or a smoke analyzer, and they are further characterized with X-ray fluorescence microscopy as well as field emission scanning electron microscopy with energy disperse X-ray analysis. Results show that PM2.5 generated from the two CFB boilers is of unimodal mass size distribution, with the only size peak in the coarse mode. Different from PM formed in pulverized coal-fired boilers, coarse mode PM generated in the CFB boilers is primarily irregular in shape, and some small particles are observed to adhere on large ones due to the lower combustion temperature and the more vigorous collision and abrasion in CFB combustion. More PM2.5 is produced from CFB combustion as a result of enhanced fragmentation of minerals in coal and sorbent limestone compared with the conventional pulverized coal combustion. The yield of ultrafine PM is inhibited in CFB combustion due to the lower combustion temperature and fixation of volatile species by sorbent limestone, which lead to the absence of an ultrafine modal peak in PM2.5. Downstream from the boilers, most PM is removed in dust collectors, with PM2.5 removal efficiencies of 98.90% and 99.96% for the ESP and the EFP, respectively. And a significant difference appears in the size range 0.1-2 mu m, where the PM removal efficiency of the ESP is similar to 1.5% lower than that of the EFP.