Occurrence of bromine in fluidised bed combustion of solid recovered fuel: Dissertation

Corrosive ash species are the single most important factor limiting the electric efficiency of steam boiler plants fired with waste or biomass. Chlorine has been found to have a central role in the chemistry involved as it reduces the melting temperature of ash, forms corrosive vapour and gas species in the furnace and halogenated deposits on boiler heat transfer surfaces. In this context chlorine has been extensively researched. At the time of writing this thesis there was hardly any published data available on the occurrence of bromine (Br) in the aforementioned context. The objective of this work was to review the occurrence of bromine in solid fuels and characterise the behaviour of bromine in full-scale fluidised bed combustion. The central questions in the context of ash chemistry were: In which fuels bromine can be found and in which form, what is the speciation of bromine in the furnace and fly ash, what role bromine may have (if any), and finally, which are the practical implications? The review on the occurrence of bromine in solid fuels revealed that in anthropogenic wastes bromine is mainly found in connection to flame retarded substances. Several weight percentages of bromine can be found in plastics treated with brominated flame retardants (BFRs). Bromine is typically found some 100-200 mg kg-1 in mixed municipal solid wastes (MSW). Bromine may be enriched in fuels with high share of plastics, such as solid recovered fuel (SRF) or refuse derived fuel (RDF). Up to 2000 mg kg-1 was found as a monthly average in SRF, typical levels being 20-200 mg kg-1. Wastewater sludge from paper mills may contain bromine 20-100 mg kg-1 due the use of bromine based biocides. In other fuels bromine may be found in significant amounts in marine influenced coal deposits and peat as well as in biomass treated with brominated pesticides. In the experimental part SRF, spruce bark and wastewater sludge from a paper mill were co-fired in a full-scale bubbling fluidised bed (BFB) boiler, and the collected fuels, aerosols and waterwall deposits were analysed with the focus on the fate of bromine. Bromine was mainly found to form water soluble high vapour pressure alkali metal halides in the furnace - in the form of KBr(g) and NaBr(g) as estimated by thermodynamic equilibrium modelling. A positive correlation was observed between the halogen (Br, Cl) and the metal (Cu, Pb, and Zn) concentrations in the lower furnace vapours when measured next to the waterwalls. Further, bromine was observed both in the wall deposits and in the corrosion front of the wa-terwall tubes. This was found to take place even if the bromine content in the SRF was less than 60 mg kg-1 in SRF. Laboratory scale oven tests with commercial boiler steels showed that a bromine containing salt induced a higher corrosion rate than the corresponding chloride.