Reducing Alkali Chloride-Induced High Temperature Corrosion by Sulphur Containing Additives – A Combined Laboratory and Field Study

Doctoral thesis, 2015

In order to achieve a more sustainable society renewable fuels such as biomass and waste are attractive alternatives to fossil fuels for power production. However, fireside corrosion is a serious concern in boilers combusting biomass or waste. The high corrosion rates are often linked to large amounts of alkali chlorides in the flue gas. One way of mitigating the corrosion is to change the corrosive environment in the boiler, e.g. by increasing the availability of SOx(g) in order to sulphate the alkali chlorides to less corrosive alkali sulphates. However, the reaction releases the corrosive gas HCl and the presence of high concentrations of SO2 is often associated with increased corrosion rates. Hence, this thesis aims to investigate the interplay between different corrosion mechanisms by connecting the reaction kinetics to the corrosion attacks. The results showed that the presence of KCl accelerated the corrosion of the stainless steel 304L by two types of corrosion attack: general corrosion and steel grain boundary attack. The general corrosion has been proved to be initiated by chromate formation while the steel grain boundary attack is suggested to be accelerated by HCl, released from the chromate formation reaction. Increasing the availability of SOx(g) in the flue gas was shown to decrease the corrosion rate. If the alkali chlorides were sulphated in the flue gas a non-corrosive deposit was formed on the steel and the corrosion attack was negligible. However, during a simultaneous presence of SOx(g) and alkali chlorides (e.g. KCl(s)) on the steel surface, competing reactions such as sulphation of the alkali chloride and chromate formation occurred. The chromate formation resulted in a general corrosion attack. However, the oxide growth was significantly reduced due to the fast simultaneous sulphation of the alkali chlorides. In contrast, the steel grain boundary attack increased in the presence of SO2. It is proposed that this attack is accelerated by HCl released from the sulphation reaction as well as metal sulphides formation in the steel grain boundaries.