Control of Ash-Related Operational Problems in BFB Combustion of Biofuels and Waste

When moving towards CO₂ neutral bio fuels and waste derived fuels, new challenges are set for combustion facilities and technical boiler solutions. A common feature for both bio- and waste fuels is a big variety in composition, often high levels of alkali metals, chlorine and moisture which make these fuels difficult to burn in facilities designed for conventional fuels such as coal, peat and wood. The problems that might occur due to high alkali and chlorine levels in the fuels, are slagging, fouling, corrosion and bed sintering. The Fortum BioMAC BFB boilers are designed especially for difficult, unconventional fuels such as rice husk, olive waste, straw, construction residue, de-inking sludge, etc. The design of each individual boiler is made based on advanced theoretical prediction tools and extensive fuel testing in laboratory and in pilot scale combustion facilities. The theoretical tools consist of a multi-phase multi-component chemical equilibrium model that estimates the slagging/fouling, sintering and corrosion propensity of the fuels/fuel mixtures and of a computational fluid dynamics part. CFD calculations are used to optimize the flow pattern and the temperature of the boiler in order to avoid hot temperatures in the vicinity of refractory linings and cooled surfaces. The chemical equilibrium calculations predict the melting behavior of the fuel ash, which is used as an indicator for the placement of the superheaters. The bottom ash removal is controlled for efficient removal of coarse material, screening and recirculation. The ash related problems of important bio and waste fuels, the analytical procedure of the evaluation of the usability of the fuels and the adaptation of the boiler design are discussed in the paper.