Abstract:
The emission of CO2 to the atmosphere from firing conventional fossil fuels has
become a major concern for the power industry, due to the enhanced
greenhouse effect and global warming predictions. The increasing worldwide
demand for electricity production is another issue. The replacement of fossil
fuels with increasing quantities of biomass is of interest as biomass is
considered to be carbon neutral and is widely distributed. Unfortunately, due to
its composition, the risk of fireside corrosion found on heat exchangers (super-
heaters and re-heaters) is greater than in coal-fired plants. Consequently,
biomass-fired power plants operate at lower steam temperatures and
pressures, leading to their poorer efficiency. Biomass-fired power plants suffer
from alkali chloride-induced corrosion, considered faster and more severe than
alkali sulphate-based corrosion common in traditional coal-fired plants.
The main aim of this project was to develop a range of novel coating
compositions which would be resistant to fireside corrosion found on boiler
tubes in biomass-fired power plants. To accomplish this, studies were carried
out into salt stabilities, coating oxidation and deposit corrosion.
Salt stability experiments have resulted in improved understanding of the
evaporation and sulphidation behaviour of KCl, NaCl, K2SO4 and Na2SO4 at
high temperatures in environments containing HCl and SO2. KCl was chosen as
a deposit for coating screening. Two-target magnetron co-sputtering was
successfully used to deposit a range of coating compositions. These coatings
were analysed at 550°C in corrosion environments containing combinations of
HCl, KCl and water vapour. The addition of gaseous HCl did not have a
significant influence on the coating degradation compared to similar tests in air.
Deposited KCl significantly increased the corrosion rate, whereas adding 10%
moisture to the environment with KCl had little additional effect. The growth of
either protective Cr2O3 or less protective mixed oxides was observed on the
different coating compositions. The best performing coatings had compositions
in the range: 26.2 – 79.4 at% Cr, 12.1 – 62.9 at% Fe, 8.5 – 10.9 at% Al.
