Ultrafine ash emission control of pulverized coal-fired boilers with electrostatic precipitators

We have studied ash formation during the pulverized combustion of coal both at laboratory and field scale combustion systems, with special emphasis on the formation of ultrafine ash particles below 0.5 µm in diameter. In addition, we have determined ash penetration through the electrostatic precipitator (ESP) as the function of particle diameter at various ESP operation conditions, both at field scale and at pilot-scale ESP. Volatilized ash species, mainly oxides of Al and Si, form agglomerated ultrafine ash particles during char combustion. The fraction of ash volatilized and accordingly the average particle size increases as the function of char surface temperature, i.e. when increasing the oxygen partial pressure surrounding the burning char particle. In addition, the ultrafine particle size decreases when increasing the distance between the burning char particles. The size distribution of ultrafine ash formed at given combustion conditions varied from one coal to another. The maximum penetration value from 3 to 10 % was measured for particles from 0.1 to 0.5 µm in diameter at full scale industrial ESP's. Pilot-scale studies showed that ultrafine ash particle collection efficiency can be significantly improved by reducing the ion concentration within the ESP via pulsed mode operation, by increasing the gas humidity and by reducing the gas temperature within the ESP.