High-Temperature Corrosion Chemistry in Oxy-Fuel Combustion
Drastic cuts in global CO2 emissions are needed to mitigate the global warming if limiting global warming to 2ºC. The power generation sector is largely based on fossil fuels and produces a significant share of the global CO2 emissions. Thus, new power generation processes with drastically reduced CO2 emissions need to be employed to mitigate global warming. Two alternatives which may be part of the solution is the replacement of coal with biomass or to apply the concept of carbon capture and storage (CCS). In CCS the CO2 is captured and processed on site and thereafter transported to a storage location. Oxy-fuel combustion, which has been studied in this thesis, has been demonstrated in large-scale pilot plants (30-60 MW). This work investigates the possibilities to co-combust biomass and coal in oxy-fuel combustion for CO2 capture. Biomass combined with CO2 capture has the potential to contribute to negative CO2 emissions. However, the high temperature corrosion (HTC) and the related K-Cl-S chemistry need to be studied in detail in order to determine the potential consequences for corrosion on heat transfer surfaces. This, since the use of biomass in power generation is problematic due to the relative high content of alkali (mainly potassium) and chlorine. Together these compounds form KCl, a salt which causes corrosive deposits and subsequent problems with so called high temperature corrosion (HTC). When sulphur is present, alkali sulphates may form instead of alkali chlorides. Sulphates have a higher melting point and causes less problems with corrosion and sulphates are therefore preferred instead of chlorides.
The work in this thesis is based on experiments performed in a 100 kW combustion unit and modelling of chemical kinetics. Both the experimental and modelling results show that a high SOX concentration is preferable to achieve as high degree of sulphation of the alkali chlorides. In oxy-fuel combustion, the SOX concentration is typically high due to flue gas recycling, which enables almost complete potassium sulphation in some of the studied oxy-combustion atmospheres. This makes oxy-fuel combustion an attractive process for co-combustion of coal and biomass, since alkali chloride formation to large extent may be suppressed. In addition, the effect of KCl on the CO oxidation process has been studied in air-fuel and oxy-fuel environments. The results show that KCl can promote CO-oxidation in a CO2 rich environment. However, no change could be observed for the total burnout time even though the CO concentration was decreased.