Corrosion due to KOH in chemical looping combustion

Chemical looping combustion of biomass and waste streams is a promising technology for cost-effective CO2 capture and increased energy efficiency due to higher steam parameters in the air reactor, because the air reactor should be almost free from corrosive impurities. However, some alkali is transferred to the air reactor with the oxygen carrier and as carry-over with the char and ash. Potassium in these sources can be released as KOH vapor. This may have an impact on the corrosion of heat exchangers. CO2 in the air will eventually carbonate KOH, and both KOH and K2CO3 can coexist in a deposit. In this work, the fate of alkali in CLC was studied using thermodynamic calculations, and the deposition mechanism on heat exchangers from small amounts of impurities released from the oxygen carrier was assessed. Furthermore, corrosion tendencies of three steel qualities (PH235GH, 10CrMo9-10, and AISI347) were studied below and above the solidus temperature of the KOH-K2CO3 system. The solidus of the system is about 360 °C, and the test temperatures in the experiments were 345, 380, and 415 °C. For the lowest temperature, no melt was formed, whereas various amounts of melt can form for the higher temperatures, depending on the degree of KOH carbonation. Thermodynamic calculations were performed to determine the stability of KOH and the melting behavior of KOH-K2CO3 mixtures in air reactor conditions. To assess the corrosion, cross-sections of the steel samples were analyzed with SEM-EDX. The analyses required special attention as both KOH and K2CO3 are highly hygroscopic. This work reveals new insights into corrosion phenomena in the air reactor in CLC combustion.