Year: 2000
Type: dissertation
Abstract: 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. The fly ash released from the combustion chamber consists of i) coarse particles (1-100 μm) that contain all the non-volatile species, including alkali silicates and CaSO4, as well as of ii) sub-micron K2SO4 particles. KCl is released from the combustion chamber as vapour. The fraction of potassium present as sulphates and chlorides was found to be higher the lower the Si to K ratio in the fuel was. The increased amount of Si in the fuel increases the extent of alkali silicate formation, resulting in a decrease in the mass concentrations of K2SO4 and KCl. The fine particles included 16-32 % of K (including KCl vapour condensed in the sampling system) when the fuel Si content was < 0.2 %, but less than 1 % of K when the Si content was 2.6 %. 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 deposition efficiency of the fine mode particles, including alkali chlorides and sulphates, was less than about 25 %. The deposition efficiencies of particles with variable compositions, but with the same aerodynamic diameter were not found to be different. 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. A majority of the condensed-phase S was present as K2SO4 during combustion of willow, resulting in a deposition efficiency of below 20 %. On the other hand, 80 % of the condensed S was deposited during the combustion of forest residue, when 99 % of it was present in the coarse particles as CaSO4.
Source: Aaltodoc (Aalto University)
Author: T Valmari
Language: English
Subfield: Fuel Technology
Field: Energy
Domain: Physical Sciences
Open Access status: green
Funders: Vattenfall, Tekes
OpenAlex ID: W2183816083