Chemical Properties of FBC Ashes

Doctoral thesis, 1998

In this thesis several aspects of ash chemistry are discussed. An issue of primary concern for boiler operators is the formation of ash deposits. New biomass fuels are introduced, many of which have unfavourable ash melting behaviour due to high levels of alkali metal species and chlorine. In addition to the inferior heat recovery caused by insulating deposits, aggressive ash components may cause corrosion on heat exchanger tube materials. In fluidised bed combustion, ash sintering and melting can cause bed material agglomeration and, at worst, total bed sintering. Furthermore, chemical reactions involving bed ash components may add to the sintering through formation of new products encapsulating the original particles. Reducing conditions, which in combination with high SO2 levels resulted in formation of CaS, was observed in the bottom bed of an FBC boiler. Under reducing conditions, due to insufficient fuel-air mixing and staged combustion, transformations and eutectics in the systems CaS - CaSO4 - alkali species and silicates-iron-rich oxides increase the risk for sintering. Chemical mechanisms involved in bed material agglomeration, deposit formation and fouling of heat exchanger surfaces during coal firing in pressurised fluidised bed combustors were investigated. The results showed that all of the following three chemical systems CaS - CaSO4 - alkali species, silicate - alkali - chlorine and silicate systems containing iron-rich oxides had contributed to the sintering and fouling processes. Furthermore, the use of fuel additives to enhance the ash melting point in biomass combustion has been investigated. Kaolin was found to be a more effective additive than dolomite. This can be assigned partly to chemical reactions between kaolin and alkali salts from the fuel ash and partly to physical adsorption of the ash melt. In the second part of this thesis the environmental effects of ash utilisation and deposition are discussed. Recycling of wood ash to forest soil may be used as a means of maintaining a sustainable forestry and avoiding a mineral nutrient impoverishment in the forest soil. In order to obtain suitable release rates for nutrients, a stabilisation of the ash is recommended. Stabilisation may be accomplished by moistening and agglomeration of the ash. In this so-called hardening process some ash components form secondary hydrated minerals with low solubility. The secondary minerals contribute to inter-particle bonding during agglomeration The most important transformation is that of Ca(OH)2 to CaCO3 since it reduces the solubility of calcium and the alkalinity of the ash. Thus, a pH shock in the forest soil is avoided. The ash particle dissolution rate is dependent on the particle size and the stability of the hardened ash matrix. The results obtained indicate that granulation may be a more efficient agglomeration method than spontaneous self-hardening. Potassium is mainly present in soluble salts and thus the release rate is rather high whereas the release of phosphorous as well as minor and trace metals is very slow due to the high pH of the pore water. Since P is an essential plant nutrient, its slow release from wood ash may be problematic. Granules were observed to retain alkali chlorides better than spontaneously formed agglomerates. Ashes from co-combustion of wood with peat, coal and oil were also investigated. The coal and peat introduce inorganic species such as sulphides, clay minerals and feldspars, giving the ash a higher content of S, Al, Si and Fe than that of wood ash. However, the release of minor and trace metals was equally low as observed for pure wood ashes. Thus, the results obtained in this work indicate that a limited admixture of fossil fuels may not be detrimental to the use of the ash as a forest mineral fertiliser.