Properties of particles in the fly ash of a biofuel-fired circulating fluidized bed (CFB) boiler

A quench/dilution probe was designed and used for sampling of particles and vapors in the flue gas upstream of the convective path in a circulating fluidized bed boiler. Downstream of the convective path, flue gas was sampled by a heated probe. The sampled gas was lead through low-pressure impactors for analysis of the mass size distribution of particles. The particles were analyzed for Cl, S, K, Na, Ca, Mg, P, Al, Fe, Ba, Mn, and Si. A series of tests was carried out to study minimization of problems originating from the alkali content in the fuel, including the effect of additives to the fuel, change of bed material, and co-combustion with sewage sludge. Particle concentration and composition were compared to deposit growth and composition on a deposit probe, which represents a superheater. Alkali chloride in the particles was compared to online measurements of gaseous alkali chlorides. The continuous mass deposition on the heat-transfer surface in the convective path was illustrated by calculation of the reduction of heat transfer by means of the temperature drop of the flue gas. Fine particles (< 1 mu m) contained mostly potassium chloride, except during addition of a sulfur additive when potassium sulfate was dominant. Addition of kaolin or co-combustion with sludge lowered the concentration of fine particles at the same time as the concentration of coarse (> 1 mu m) particles increased. These test cases also showed the lowest deposit growth on the superheater probe. The particles deposited in the convective path were mostly coarse ones. The most favorable case with respect to low deposits was co-combustion with sewage sludge.