Slagging, fouling and fireside corrosion in biomass fuelled boilers and gasifiers are major obstacles that diminish the efficiencies of the energy transformation processes in these systems. High chemical to thermal and thermal to electric conversion efficiencies require high surface temperatures in the equipment construction components. This leads to increased risks of shortening the service life times of the components, when biomass based fuels are used in place of fossil fuels. The inorganic part of biomass fuels are different from fossil fuels and therefore lead to different ash behaviour in the combustors. Sulphur contents in fossil fuels are typically much larger than in biomass fuels, whereas the potassium contents in bio-mass based fuels are typically larger than in fossil fuels. During combustion, potassium is initially released in the gas phase as elemental potassium, potassium hydroxide or potassium chloride. These compounds then react with SiO2(s,l,g), SO2(g)/SO3(g), HCl(g), and CO(g)/CO2(g) in the combustor and play a major role in the slagging, fouling and corrosion processes. Most of the published scientific work done so far to understand these processes conclude that KCl(s,l,g) is the most important potassium compound responsible for the slagging, fouling and corrosion problems. The importance of KOH(s,l,g) in these processes has gained a lot less attention. In this thesis, the possible effect of KOH(s,l,g) in the fouling and corrosion issues was studied. Categorizing biomass and fossil fuels based on the free potassium content (potassium not bound as chloride or sulphate) was found to separate the biomass based fuels from fossil fuels. Biomass has typically free potassium, whereas fossil fuels do not. This difference suggests that during combustion and gasification, the potassium in the gas phase may exist as KOH(g) in the flue gases near the heat transfer surfaces. In addition, a correlation of KOH(g) with the corrosion rates of different steels in a straw fired boilers was found. Furthermore, using in-situ electrochemical galvanic probe measurements in a full-scale biomass fired boiler showed that the electrochemical signal activity increases abruptly at ≈ 400 °C. This may indicate the melting temperature of the condensing layer, which is very close to the melting point of KOH (406 °C), suggesting that condensation of KOH(l) can happen before reaction with CO2(g) to form K2CO3(s). Finally, using laboratory exposures of KOH(s,l) and KCl(s,l) with Cr2O3(s) and Fe2O3(s) it was found out that the formation of K2CrO4(s) could be explained for both salts with similar mechanism: 4KOH(s,l) + Cr2O3(s) + 1½O2(g) → 2K2CrO4(s) + 2H2O(g). This result sheds light on the initial breakdown mechanism of the protective oxides on high temperature steels in biomass combustion and gasification atmospheres. Overall, the conclusion is that KOH(s,l,g) may be a major reaction intermediate taking part in the slagging, fouling and corrosion mechanisms in biomass fired combustors.
|Translated title of the contribution||Kaliumhydroxidin rooli likaantumis– ja korroosioprosesseissa biomassaa polttavissa kattiloissa|
|Publication status||Published - 2020|
|MoE publication type||G5 Doctoral dissertation (article)|
- biomass combustion
- potassium hydroxide
- CO2 emission