Potassium behaviour during combustion of wood in circulating fluidised bed power plants: Dissertation

The behaviour of potassium during circulating fluidised bed combustion of wood-based fuels was studied experimentally in pilot-scale and industrial scale combustors. Aerosol measurement techniques were used for sampling fly ash and inorganic vapours from the flue gas, upstream and downstream of the convective pass. Elemental analysis methods and scanning electron microscopy were used for sample characterisation. The factors affecting in-duct impactor operation in elevated temperatures are discussed in detail. The fly ash released from the combustion chamber consisted of i) coarse particles (1-100 µm) that contained all the non-volatile species, including alkali silicates and CaSO4, as well as of ii) sub-micron K2SO4 particles. KCl was released from the combustion chamber as vapour, and was condensed on the fly ash particles in the convective pass. The fraction of K ending up in the fly ash as alkali silicates was found to depend on the fuel Si-content. A high fuel Si-content also resulted in depletion of K in the bottom ash. About 60-70 % of the fly ash entering the convective pass was deposited on the heat exchanger surfaces in the convective pass, and was removed during the soot-blowing period. The deposition efficiency correlated clearly with the ash-particle size. The largest particles, including alkali silicates, were deposited most effectively, and the deposition efficiency decreased with decreasing particle size. The physical state (vapour, fine particle or coarse particle) of the ash species was shown to have a remarkable effect on the form and rate of ash deposition in the convective pass. When the different fuels were compared, the variation in the deposition efficiency was the most remarkable for sulphur.