Formation of corrosive compounds from biomass/waste combustion.

Biomass/waste is a renewable energy source which can be fired in combustion power plants. These fuels can be used to replace coal combustion, with its associated environmental impacts, however challenges associated with biomass include accelerated fireside corrosion of heat exchangers (HX) due to producing different corrosive compounds; i.e. fireside corrosion is fuel dependent. As such compromises may be needed. For example, virgin wood fuels have lower fireside corrosion risks, although alternative biomass/waste fuels are cheaper. This thesis compares the fireside corrosion influences of several biomass/waste fuel categories to further understand their attack mechanisms on conventional HX steels, T91 and 374HFG. The corrosive species generated during the fuels’ combustion have been investigated using thermodynamic modelling by MTData. Corrosion damage has been evaluated using high temperature corrosion furnace tests, conducted with isothermal conditions. In contrast to isothermal test conditions, further testing developed an alteration to the furnace setup to address heat gradient impacts on corrosion damage. In both fireside corrosion methodologies, the well-established deposit re-coat technique has been employed to simulate the deposition of representative salts. To evaluate corrosion trends influenced by the biomass/waste fuels with different exposure parameters, temperature, gas and deposition flux conditions have also been varied. Corrosion data evaluation is both quantitative (dimensional and weight changes of metals samples) and qualitative (scanning electron microscopy and energy dispersive X-ray spectroscopy). Thermodynamic modelling shows higher partitioning of alkali metal species into the flue gas occurs for Wood Waste Fuels (WWF) than for Agricultural Plants & Residues (APR) and Herbaceous Grass Biomass (HGB) fuels. However, APR and HGB fuels release higher absolute amounts of these species in combustion. The high chloride percentages in APR and HGB deposits (69-89 mol.%) do not always correlate with higher corrosion rates as determined in the laboratory tests. Indeed, WWF deposits, with chlorides as low as 30 mol.%, have proven to be more destructive under certain operating conditions. Despite this, the increased deposition flux on plant HXs of corrosive species from HGB and APR firing on plant HXs is responsible for their shorter operational lives as compared to WWF. This has implications for the type of biomass/waste fuel combusted for power generation.