Abstract
Biomass is a clean alternative fuel to coal in terms of carbon, NOx, and SO2 emissions in the power generation sector. However, ash deposition problems have been a concern with biomass fuels due to the high alkali and residual sulphur contents in the ash forming particles. In this study, the influence of oxy-fuel firing conditions of wood pellets combustion on ash partitioning and deposit formation, were investigated experimentally on a 250-kW pilot scale pulverized furnace and theoretically through chemical equilibrium modelling using the FactSage program. Oxy-fuel combustion case was compared with air-fuel case in this assessment. The results of this study showed that wood pellets had a low tendency for radiation zone slagging, but, had a high fouling tendency in the convective passes. It is possible that oxy-fuel combustion inhibited the release of volatile elements to the gas phase in the initial stages of the combustion, thus reducing the alkali sulphates slagging, increasing however, the alkali sulphate fouling tendencies on the convective passes. Moreover, the effect of the oxy-fuel environment on the ash formation was significant. The chemical equilibrium modelling showed reasonable predictions of the ash behaviour of wood pellets in terms of alkali behaviour and explained to some extent the influence of the oxy environment on ash deposit formation.
Highlights
Ash deposition is a classic problem in solid fuel combustion power plants
Alkali, sulphur, and alkaline earth metals react to form elemental or poly-silicates that melt at a temperature lower than 1100 C or form alkali sulphates that can condense on the heat transfer surfaces in the combustor leading to unavoidable deposition on the furnace walls or boiler tube surfaces, namely slagging and fouling, and less bothersome soot (Miles et al, 1996)
It can be expected that the biomass can cause high fouling and corrosion problems in the boiler tubes, and low slagging problems in the radiation sections of the furnace
Summary
Ash deposition is a classic problem in solid fuel combustion power plants. Biomass ash depositions can cause heat transfer deficiency and corrosion problems to the boiler tubes. The most notably volatile materials are compounds of alkali, chlorine, sulphur, and phosphorous (Pronobis, 2005; Vamvuka et al, 2008; Sommersacher et al, 2012) Elements such as silicon, alkali, sulphur, and alkaline earth metals react to form elemental or poly-silicates that melt at a temperature lower than 1100 C or form alkali sulphates that can condense on the heat transfer surfaces in the combustor leading to unavoidable deposition on the furnace walls or boiler tube surfaces, namely slagging and fouling, and less bothersome soot (Miles et al, 1996). The release of the fuel sulphur and chlorine can cause severe corrosion problems on the boiler tube surfaces in the presence of water vapour and alkali hydroxides in the flue gas (FG) (Baxter et al, 1998). With the pulverized fuel combustion, caution must be applied due to the difference in the fuel particle size, fuel transport with the primary air, and the full process design
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