Fireside corrosion in oxy-fuel environments and the influence of fuel and ash characteristics on corrosion and deposition

The development of advanced techniques (such as carbon capture and storage) for future power plants and the implementation of retrofit technologies to existing ones (like biomass co-firing) in order to reduce pollutant emission, has raised several concerns for the power industry. One such problem, which also forms the basis of this thesis, is the effect of these measures on corrosion and deposition of the boiler heat transfer surfaces. This research work can be divided into two parts. The first part involved studying the corrosion behaviour of a typical waterwall and a superheater material under simulated oxy-fuel environments with and without the influence of an ash deposit. A custom-built, laboratory scale, corrosion rig with the ability to simulate a range of flue gas compositions and temperatures, in addition to generating a heat flux through the specimen, was set up for this purpose. The second part of this work deals with evaluating the properties of a UK power station coal and four biomass samples with the help of laboratory techniques and thermodynamic modelling in order to predict their fusion and deposit forming tendencies in combustion systems. A series of experiments were performed on the corrosion rig to assess the influence of individual variables on the rate of corrosion. The results indicated that the increased concentration of SO2 in oxy fuel combustion due to recycling of the flue gas, can lead to an increase in corrosion rates especially in the presence of reactive alkali containing deposits. Under the conditions studied, the presence of a biomass ash deposit aggravated the corrosive propensity of the environment while coal ash lessened it. With regard to predicting the fusion behaviour of different ashes, the standard ash fusion tests proved inadequate for explaining the relationship between high alkali constituents in biomass ash and the expected higher slagging and fouling tendencies. Simultaneous thermal analysis was more useful in assessing the physical & chemical changes taking place in the ash. Prediction of the fuel behaviour using FactSage thermodynamic analysis showed that ash melting commences at much lower temperatures than those predicted from laboratory techniques. This would help to explain the increased risk of deposition and corrosion linked with burning high alkali containing fuels.