Combustion aerosols from co-firing of coal and solid recovered fuel in a 400 mw pf-fired power plant

摘要

In this work, combustion aerosols (i.e. fine particles 2.5 μm) formed in a 400 MW pulverized coal-fired power plant was sampled with a low-pressure impactor, and analysed by transmission and scanning electron microscopy. The power plant was operated at both dedicated coal combustion conditions and under conditions with cofiring of up to 10% (thermal basis) of solid recovered fuel (SRF). The SRFs were characterized by high contents of Cl, Ca, Na and trace metals, while the coal had relatively higher S, Al, Fe and K content. The mass-based particle size distribution of the aerosols was found to be bi-modal, withan ultrafine (vaporization) mode centered around 0.1 μm, and a coarser (finefragmentation) mode above 2 μm. Co-firing of SRF tended to increase the formation of ultrafine particles as compared with dedicated coal combustion, while the coarse mode tended to decrease. The increased formation of ultrafine particles was probably caused by a relatively higher volatility (and subsequent enhanced homogeneous condensation) of Ca, P and K during co-firing of SRF. The influence of SRF type, thermal fraction, particle size and injection position was however not evident from our data, probably due to the inhomogeneous characteristics of SRF. S was found to be a special case. While the concentration of S was decreased in the ultrafine particles from co-firing (in consistence with a low initial concentration in SRF), the concentration of S in the electrostatic precipitator ash was higher for co-firing as compared with dedicated coal combustion. This implies an increased capture of SO2/SO3 by reaction with CaO in the fly ash. It leads us to suggest that a reduced collection efficiency of the electrostatic precipitator observed during co-firing of SRF may be linked to SO3 deficiency in the flue gas, as small amounts of SO3 are needed for proper ESPperformance.