Ash related problems with biomass combustion: application to co-firing in large scale boilers

CO2-emissions from coal-fired power stations can be limited by co-firing biomass. Operational problems as slagging and fouling can increase and quality of ashes applied in cement production could be compromised regarding Cr(VI). Full-scale air-cooled probe deposition tests have been performed in pf-boilers and ash deposition models have been developed on basis of thermodynamic equilibrium modelling. Superheater deposits are mainly formed by inertial impaction of molten slag and Fe-O-S. Co-firing of already small percentages of biomass (i.e. < 10% on basis of dry mass input) may directly impact slagging characteristics. Slagging reduces with co-firing wood and increases with co-firing MBM-mix. Model calculations show that fouling increases with co-firing, although not linearly and in general steeply increases above 80% co-firing. Alkalis will interact with (molten) aluminosilicates, capturing them from the gas phase and prevent fouling. Alkali dissolution decreases with an increasing ratio of (CaO + MgO) to SiO2 in the aluminosilicate glass phase which, in its turn, increases with biomass co-firing ratio. Volatile alkalis that do not interact with aluminosilicates will condense downstream and may form of molten alkali salts depending on both the S:Cl ratio and the K:Na ratio. The S:Cl ratio decreases with increasing biomass co-firing rate. Full-scale air-cooled probe tests have been performed in a BFB boiler firing wood-based fuel blends. Superheater deposits are rich in Ca, K and S and are mainly formed by thermophoresis and eddy diffusion. Sintering occurs by sulphation of Ca and by liquid flow of molten K-Na-SO4-Cl salt. A fouling prediction model is presented based on the amount of molten salt present according to equilibrium calculations. Possible reactions schemes for chromium are proposed based on thermodynamic calculations and analyses (XAFS, Relative Enrichment factors) of fly ashes resulting from coal firing and biomass co-firing. Organically bound Cr may vaporize during combustion and with sufficient oxygen form CrO3(g). This can stabilize with e.g. alkali (earth) or iron, as leachable Cr(VI) or with Ba as non-leachable Cr(VI). With insufficient oxygen, or when Cr is bound in coal in illite or a trivalent oxide, Cr in fly ash will occur as non-leachable Cr(III).