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

The behaviour of alkali metals, especially of potassium, during circulating fluidised bed combustion of wood-based fuels was studied experimentally in pilot-scale and industrial scale combustors. The fuels included willow, forest residue and waste wood co-combusted with paper mill sludge. As a result of this work, the main chemical and physical transformation mechanisms of potassium compounds in the combustion chamber and in the convective pass are presented in this thesis. Aerosol measurement techniques were used for sampling fly ash and inorganic vapours from the flue gas, upstream and downstream of the convective pass. Samples were collected with cyclones, impactors and filter samplers. Fly ash size distribution was also measured directly in the superheater section with a low-pressure impactor located in-duct in a region where the gas temperature was 650 °C. The method is described and the factors affecting the impactor operation in elevated temperatures are discussed. Elemental analysis methods were used for analysing samples collected by aerosol measurement methods, as well as for conventional samples of fuel, bottom ash and fly ash. Scanning electron microscopy was applied to the fly ash aerosol samples. Up to 40 % of the ash-forming constituents were retained in the bed and were removed with the bottom ash. The reaction of potassium compounds with quartz sand bed material was found to result in enrichment of K in the bottom ash (relative to Ca) when the fuel Si-content was low (< 0.2 %). The high Si-content (2.6 %) in the fuel resulted in depletion of K in the bottom ash, as the amounts of quartz and silicates in the fuel were large enough for efficient capture of potassium to the ash.