Protective Power of Coal and Peat against Alkali Chloride Formation and Cl Deposition in Blends with Forest Biomass in FBC

The capacity of two coals and one peat to protect the exposed metal surface against alkali chloride deposition and to decrease alkali chlorides concentration in the furnace when blended with forest residues were determined in pilot-scale bubbling bed (15 kW) and circulating fluidised bed (60 kW) reactors of VTT by combining advanced measurement techniques in an unique way. Mass impactor (LPI) sampling was followed by analysis of Na, K, S and Cl and Collinear Photofragmentation and Atomic Absorption Spectroscopy (CPFAAS) was used to real-time KCl determination. In addition, deposits were collected on temperature controlled probes (superheater simulators) and analysed afterwards with SEM EDXA at various positions.The highest alkali chloride vapour concentration existing at the upper furnace without measured Cl deposition was obtained. Presence of alkali chlorides at upper parts of the furnace was possible during 100% forest biomass firing due to weak reactions to convert alkali chlorides to HCl. However, some degree of sulphation of alkali chlorides was found.The order of capacity against alkali chlorides formation and Cl deposition was coal 2 > coal 1 > peat. At pilot-plant tests, where the blend composition can be better controlled than in power plants, addition of coal 2 allowed almost 90% forest biomass (on energy basis) to the blends without Cl deposition. Combustion technology (BFB or CFB) did not have measurable effect on alkali chlorides flow in the furnace with identical fuel, but highest Cl concentrations in deposits (up to 13 wt% at side position with 100% forest biomass) were obtained at CFB conditions. Such Cl concentration does not only form firm superheater deposits but what is more critical probably starts metal corrosion