Chemical Interactions between Potassium, Nitrogen, Sulfur and Carbon Monoxide in Suspension-Fired Systems

Doctoral thesis, 2017

Drastic cuts in global CO2 emissions are needed to mitigate global warming and limit the average temperature increase to well below 2ºC. The power generation sector is largely based on fossil fuels and generates a significant share of the global CO2 emissions. Thus, new power generation processes with substantially reduced CO2 emissions need to be employed to mitigate global warming. Two alternatives that 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 processes the CO2 is captured and processed on the plant 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 with 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 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 that causes corrosive deposits and subsequent issues with so called high temperature corrosion (HTC). When sulfur is present, alkali sulfates may form instead of alkali chlorides. Sulfates have a higher melting point and causes less problems with corrosion and sulfates 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 a high degree of sulfation of the alkali chlorides. In oxy-fuel combustion, the SOX concentration is typically high due to flue gas recycling that enables almost complete potassium sulfation in some of the studied oxycombustion atmospheres. This makes oxy-fuel combustion an attractive process for cocombustion of coal and biomass, since alkali chloride formation can be suppressed. In addition, the effect of KCl and SO2 on the CO oxidation and NO formation has been studied in both experimental and modelling work. The results show that KCl can promote COoxidation in a CO2 rich environment. However, no change was observed for the total burnout time even though the CO concentration was decreased locally. Regarding the nitrogen chemistry, KCl was found to inhibit the formation of NO whereas SO2 promotes the oxidation of already formed NO to NO2.