Fly ash penetration through electrostatic precipitator and flue gas condenser in a 6 MW biomass fired boiler

The effects of an electrostatic precipitator and a flue gas condenser on size resolved fly ash particle concentration and composition were studied in a 6 MW biomass combustion unit, fired with moist forest residue. The boiler was of moving grate type. The fly ash particles were sampled upstream and downstream of the electrostatic precipitator and flue gas condenser, respectively. Fine particle number size distributions were measured using an electric mobility spectrometer (SMPS) and coarse particle number size distributions were measured using a time-of-flight instrument (APS). The mass size distributions were measured using a multi-jet low pressure cascade impactor (DLPI). For chemical analyses of the impactor substrates particle induced X-ray emission analysis (PIXE) was used. After the flue gas passed the electrostatic precipitator (ESP), the fly ash particle concentration was reduced by approximately 96% by number and 83% by mass. After the particles passed the flue gas condenser, particle number concentration was only marginally altered, whereas the mass concentration was reduced by half. Both the ESP and the condenser showed, size dependent particle separation efficiency. The main elements (Z > 12) in the fine fly ash fraction were K, S, and Cl, whereas the main elements in the coarse fraction were Ca, K, S, and Cl. After passing the ESP the mass ratio of Ca decreased in the coarse fraction, while the ratios of K, S, and Cl increased, indicating transference of fly ash material from the fine to the coarse particle fraction. There was no significant difference in the elemental composition for any particle size fraction sampled upstream or downstream of the condenser.