Formation and emission of fine particles from two coal-fired power plants

Abstract

The generation and emission of combustion particles from two full-scale coal-fired power plants was studied by field measurements during which particles are sampled for size classification and chemical analysis simultaneously at three positions in the plants: before the electrostatic precipitator, before the desulfurisation plant, and in the stack. The following sampling techniques are used: scanning mobility particle sizer, low pressure cascade impactor, dichotomous PM 2.5 sampler, and total particle filter. The so-called multi-platform method used in this work proves useful for gaining insight into the many particle-affecting processes in a power plant. In the boiler the size of particles extends over four decades, from approximately 20 nm to 200 µm, with the largest mass contained in particles in the size range 10-100 µm. Approximately 99.9% of the particles are removed in the electrostatic precipitator and the desulfurization scrubber. The mass fraction of submicron particles, i.e., PM 1 , increases from ∼0.3% at the exit of the boiler, just before the electrostatic precipitator, to ∼30% in the aerosol emitted via the stack. In the stack aerosol 50-80% of the particles are in the PM 2.5 range. The emitted particles primarily stem from the coal ash with a minor contribution of particles of entrained, dried-out droplets of scrubber slurry. The large emitted particles are compact, almost-spherical single particles originating from the ash mineral inclusions in the coal. The small ones, corresponding in mass approximately to the PM 0.45 fraction--in terms of number concentration by far the dominant fraction--are dendritic clusters, each consisting of several partly fused, even smaller particles. The cluster particles are generated in the burner by volatilization of Si, Al, Ca, and Fe under reducing conditions. Several other ash-contained elements, e.g., P, Ba, Co, Cu, Mn, Ni, Pb, V, and Zn, are partly vaporized in the boiler and enrich the small particles when they condense during cooling of the flue gas. Due to the higher penetration of the small particles through the filter and scrubber, these elements tend to have an enhanced concentration in the emitted particles.