The Corrosive Effect of Chlorine Containing Species on Waterwalls and Superheater Materials in Waste and Biomass-Fired Power Plants

Doctoral thesis, 2017

To increase power generation from waste- and biomass-fired boilers, it is necessary to increase the temperature and pressure of the steam generated. This implies an increase in the temperature both of the waterwalls and of the steam superheaters in the boiler. Low-alloyed steels are preferred in the waterwalls due to low cost and excellent mechanical properties and weldability. The recent shift from fossil fuels (oil, coal and natural gas) to biofuels and waste for the generation of heat and power has introduced several challenges to the boilers.  One is the rapid corrosion of superheaters and waterwalls. Many observations imply that the corrosivity of the fireside environment in boilers fired by biomass and waste is connected to the relatively high chlorine/sulphur - ratio in the flue gas. Hence, this thesis investigates the role of chlorine compounds on fireside corrosion of both water walls and superheaters.   The waterwall part of this thesis investigates the corrosive effect of alkali chlorides (KCl and NaCl), PbCl2 and ZnCl2, including the effect of KCl+ZnCl2 and NaCl+ZnCl2 mixtures. This work shows that all four salts accelerates the corrosion of waterwall steels in the temperature range of interest. A new mechanism describing alkali chloride-induced corrosion of low-alloyed steels is presented. The other part of the thesis investigates the influence of SO2 on the corrosion of superheater steels in the presence of alkali chlorides in a laboratory setting. Also, complementary corrosion experiments were performed in a commercial waste-fired power plant.  The results show that the corrosivity of chlorides in superheater deposits is mitigated by SO2 in the flue gas. The effect is attributed to the conversion of KCl and NaCl into K2SO4 and Na2SO4, the two sulfates being relatively harmless from a corrosion point of view at the temperatures studied (525 °C and 600 °C). However, the laboratory study indicated that the simultaneous presence of KCl and SO2 can cause steel grain boundary attack beneath the original KCl particle.