A thermodynamic study of the gaseous potassium chemistry in the convection sections of biomass fired boilers

Thermodynamic calculations of the gaseous potassium species in boiler flue gases are presented. Results show that if the K(g)/Cl(g) molar ratio is still higher than 1 after the K(2)SO(4)(s) aerosol formation at high gas temperatures, the excess potassium is in the form of KOH(g). K(g)/Cl(g) molar ratios at the inlet of the convection section for biomass fired boilers were estimated from the chemical analysis of the fuels, with the assumption that all the fuel sulfur had reacted to K(2)SO(4)(s) aerosols before the inlet of the convection section. 0-100% conversion of fuel alkali to vaporized alkali was calculated. Generally, if the conversion was more than 60%, the calculated ratios for biomass fuels were higher than 1. Furthermore, due to the thermodynamic instability of K(2)CO(3)(g), it is likely that the formation of K(2)CO(3)(s) on the heat transfer surfaces follows a heterogeneous route where KOH(l,s) is transformed to K(2)CO(3)(s) at the surface, not in the gas phase. This leads to a possible formation of a basic salt melt on the heat transfer surfaces and thus alloying with Cr may be ineffective in improving the hot corrosion resistance of steels in these environments. Based on the literature data of oxide solubilities in basic alkali melts, it is suggested that alloying with Ni or Co will provide better protection in these applications.