Biomass is gaining increasing importance as a renewable energy source for the production of heat, electricity and transport fuels. However, corrosion issues are numerous and include accelerated wastage under ash and alkali salt deposits, erosion, and metal dusting in conjunction with gasification. This work focuses on deposit issues and is based on laboratory exposures for a total of 960 hours at 550ºC (1022ºF) and 700ºC (1292ºF) under deposits of 52.4 wt% KCl + 47.6 wt% K2SO4 in a nitrogenbased gaseous atmosphere containing 15% H2O, 5%O2, 13%CO2 and 0.02%HCl. The materials tested include carbon steel, low alloyed steels with 2%Cr or 9%Cr, the austenitic AISI 304 and the high temperature grade 253MA (21%Cr, 11%Ni, 1.6%Si, Ce). Metal loss data obtained from metallographic evaluation show the corrosion rate to decrease in this order and illustrate how materials substitution can permit an increase in process temperature. Examination of the reaction interface underlines the importance of both chlorination and oxidation in the materials corrosion process.
The production of electricity in biomass fired boilers is increasing rapidly due to the dual advantages of renewability and CO2 neutrality. However, corrosion and fouling of superheater tubes is a serious concern in these plants and limits the steam temperature and thus the efficiency of the turbine. Corrosion can also lead to unplanned plant stoppages due to tube rupture. Wood-based fuels typically have high contents of potassium and chlorine. The high corrosivity of the gas environment is usually attributed to the presence of chlorides in the fuel, which are released during combustion and transported by the flue gas to the superheater tubes. These deposited chlorides cause premature failure of normally protective or semi protective oxides on superheater alloys. The dominant theory for the detrimental effect of chlorides on high temperature corrosion in oxidizing atmospheres is the “active oxidation” or chloride-catalysed oxidation process1,2,3,4. This is thought to involve formation of volatile metal chlorides which can oxidise to re-release Cl2 which again reacts with the metal to form more volatile chlorides. Some work has indicated that Cl2 is needed from the beginning and that HCl does not take part in the chlorine induced active corrosion 5,6, others suggest that it is HCl which plays a critical role in maintaining a catalytic molten chloride at the oxide-metal interface 6. The deposits on the superheaters normally involve both a solid ash phase and a molten salt phase, and the presence of a liquid on the surface of a metal strongly increases corrosion rates 7,8. The alkali metal potassium has also been proposed to play an active role in the corrosion process, and it has been demonstrated 9 that potassium salts can react with otherwise protective chromium oxides to form non-protective potassium chromates, which can lead to the initiation of a corrosion process. The present work involves testing in a simulated biomass combustion environment, with HCl present in the gas phase and a regularly renewed deposit of KCl+K2SO4. This type of testing is a useful complement to field tests in boilers.