Reducing High Temperature Corrosion when Burning Waste by Adding Digested Sewage Sludge
Conference contribution, 2011
The presence of alkali chlorides are well known to cause high temperature corrosion during combustion of biomass and waste. Low alloyed steels as well as stainless steels are experiencing an accelerated corrosion attack in such environments. Even though more highly alloyed steels (i.e. higher Cr/Fe ratio in the steel) are being used, there is only a small decrease in corrosion rate compared to low alloyed steels. To maintain the corrosion rates at an acceptable level the temperature of the superheaters (used for steam production to the steam turbine) of the boiler have been lowered. However, this causes a decrease in power production when the driving force for waste-to-energy boilers in the future is to increase the power production by increasing the temperatures of the steam from the final superheater stage.
One of the reasons for the corrosive behavior of alkali chlorides towards stainless steels is the formation of alkali chromates. It has been shown that alkali chlorides react with chromium in the initial formed protective oxide on stainless steel:
1/2Cr2O3(s) + 3/4O2(g) + H2O(g) + 2KCl(s) K2CrO4 (s) + 2 HCl(g)
This result in a chromium depleted oxide which is converted into an iron-rich fast-growing oxide. This oxide has much poorer protective properties as it has higher diffusion rates compared to chromium rich oxides. Furthermore, the iron rich oxide is also more susceptible towards chlorine induced corrosion by chlorine ions penetrating the oxide scale. This leads to the formation of transition metal chlorides (e.g. FeCl2) at the metal/oxide interface causing poor scale adherence.
A way to mitigate the alkali chloride induced corrosion is by introducing fuel additives and thus, changing the flue gas chemistry and furthermore the deposit composition. In this study, the effect of digested sewage sludge as fuel additive was investigated at the 12MW circulating fluidized bed (CFB) boiler at Chalmers University of Technology. The initial corrosion attack of the stainless steel 304L(Fe18Cr10Ni exposed at 600°C (material temperature) was investigated during 24h exposure of three different environments. Deposit analysis by means of XRD and IC were carried out using Sanicro 28 (Fe35Cr27Ni31) as sample ring. The exposures were denoted “RDF” (a reference exposure 80%Bark + 20%RDF), “SjöMed” (80%Bark + 20%RDF with sewage sludge from Sjölundaverket (medium dosage)) and “HimHög” (80%Bark + 20%RDF with sewage sludge from Himmerfjärdsverket (high dosage)).
The results showed that the most severe corrosion attack of 304L occurred in the “RDF” exposure. The corrosion attack was characterized by an up to 100µm thick corrosion product layer and signs of internal corrosion of the steel. The deposit in the RDF exposure was dominated by alkali chlorides. The exposures with sewage sludge additions, “SjöMed” and “HimHög”, showed a remarkable decrease in corrosion rate. 304L performed especially well in the “HimHög” exposure, the steel ring was protected by a thin oxide, less than 0.3µm in thickness. Furthermore, the deposit was dominated by sulphate- and phosphate containing compounds. The presence of alkali chlorides was low.
alkali chloride induced corrosion
digested sewage sludge