Ash formation in circulating fluidised bed combustion of coal and solid biomass: Dissertation

Formation mechanisms of the fly ash and bottom ash during circulating fluidised bed combustion of a bituminous coal and two solid biomass fuels were studied experimentally. The biomass fuels were forest residue and willow (Salix). The experiments were carried out at full-scale boilers. As a result, the main ash formation mechanisms are presented for coal and biomass combustion. Ash formation was studied using the methods of traditional ash sampling and aerosol technology. By traditional ash sampling methods, samples of ash were collected from different ash streams of the boiler. The collection streams were the fuel feed, sorbent or bed material feed, boiler bottom ash, and the fly ash collected in the electrostatic precipitator. The methods of aerosol technology enabled sampling of the particles directly from the flue gas flows. In this way, the size distributions of the particles were determined. In addition, the elemental contents of the particles were analysed with different methods to obtain the concentrations of the different ash compounds in the flue gas. The aerosol instrumentation included the following devices: low-pressure impactors, cyclones, a differential mobility analyser combined with a condensation nucleus counter and an electrical low-pressure impactor. The morphology of the particles was analysed using scanning electron microscopy. In coal combustion, limestone sorbent was used to capture SO2 and no additional bed material was utilised. The bottom ash consisted mainly of coal minerals and unfragmented sorbent particles. The coal minerals agglomerated during combustion forming bottom ash particles with a wide range of compositions. Sorbent particles captured SO2 and were mainly present as CaSO4 and CaO. During biomass combustion, quartz sand was fed into the furnace to maintain an adequate bed inventory. No sorbent was used. The adhesion of ash to the sand particles and the subsequent growth of bed particles formed the bottom ash. The ash compounds adhered to the sand particles by two mechanisms: i) adhesion of the non-volatile ash compounds as particles on the sand surface, and ii) diffusion of the volatile ash compounds into the quartz sand and subsequent chemical reaction. The ash particles formed a sticky layer on the sand particles which in high temperatures or when grown sufficiently thick might have caused agglomeration and deposition problems in the bed as well as in the upper parts of the furnace and in the cyclone loop. The fly ash size distributions were bimodal with all fuels. The fine mode was formed in the submicron size range by volatilisation and subsequent nucleation of the volatilised species. These particles then grew by condensation. The mass of the fine particle mode consisted 0.3 % of the total mass of the fly ash particles during coal combustion, 2 % during forest residue combustion, and 8 % during willow combustion. The major fraction of the mass in the fine mode consisted of HCl with coal, KCl with forest residue, and K2SO4 with willow. The coarse fly ash particles, so-called residual ash, were formed mainly from the ash compounds that did not volatilise during combustion. The coal minerals formed agglomerates with a few minerals in one particle. In addition, a major proportion of the limestone sorbent fragmented and escaped the furnace as fly ash. The coarse fly ash particles from biomass combustion were large agglomerates comprised of mainly submicron-sized primary particles up to several thousand in number. The agglomerate shape of the particles was found to affect the gas-to-particle conversion of the volatilised species. Consequently, the resulting concentration size distributions were different from those for spherical particles. Condensation of the volatilised species resulted in the enrichment of the condensed species in the fine particles whereas the gas-to-particle conversion by a chemical surface reaction resulted in a concentration of the volatilised species that did not depend on the particle size.