Concentrations of inorganic elements in biomass fuels and recovery in the different ash fractions

Inorganic elements and compounds in biomass fuels influence the combustion process and the composition of the ashes produced. Consequently, knowledge about the material fluxes of inorganic elements and compounds during biomass combustion for different kinds of biofuels and their influencing variables is of great importance. The results gained will especially influence the future design and control of biomass furnaces and boilers in order to prevent slagging, fouling and corrosion and to assist in the definition of quality requirements for biofuels as well as the possibilities of a sustainable ash utilization. For this reason, comprehensive test runs were carried out in several biomass combustion plants equipped with different combustion technologies and using various biomass fuels (wood chips, bark, straw and cereals). During continuous observation periods of at least two days, samples of the biomass and the different ash fractions were taken and analysed. Furthermore, the most important operating data of the plants were recorded. The results of the material balances for inorganic elements showed that the concentrations of environmentally relevant heavy metals (especially Cd and Zn) in biomass ashes increase with decreasing precipitation temperature and particle size. This effect is independent of the biofuel used. Consequently, a major requirement for a sustainable ash utilization is a fractionated heavy metal separation, distinguishing between different fly-ash fractions and taking the temperature of fly-ash precipitation into consideration for new furnace technologies. Research has also shown that straw and cereals, as well as their ashes, contain significantly lower amounts of heavy metals than woody biofuels and wood ashes. The same principles pointed out for environmentally relevant heavy metals are also valid for K, Na, Cl and S. The high concentrations of these elements in the filter fly-ash as well as in the boiler fly-ash are of great relevance for reactions that can take place in the boiler section where the flue gas is subjected to a considerable temperature gradient which is accompanied by chemical reactions, phase transitions and precipitation processes that can support or initiate fouling and corrosion. These effects are of special importance for biofuels that are rich in alkali metals and Cl such as straw and cereals