Decreased corrosion problems by controlled fuel composition
9 May, 2017
Changing fuel source from fossil fuel to more environmental friendly fuels, is not done without introducing new challenges, and, introducing new fuels has shown to increase problems with corrosion. Different sources of, for instance, wood fuels can have a wide spread in composition and this give differences in how corrosive they are. Initial corrosion studies presented in the licentiate thesis(1) “Decreased furnace wall corrosion in fluidised bed boilers – The influence of fuel lead content and sewage sludge additive” have shown that regulating the fuel composition or co-firing an additive with a corrosive fuel can result in less corrosion of metallic materials.
One commonly used fuel in Swedish power plants today is used-wood (also known as recycled or waste-wood). Compared to when firing fossil fuel, used wood fuel has shown to increase corrosion of the metal tubes in the boilers. The corrosion problems have been shown to be related to higher amount of corrosive species in the fuel. In particular, the corrosion caused by hydrogen chloride and alkali chlorides is a problem that has been widely studied. In addition to chlorine, which often is present in high amount in these types of fuel, also the amount of heavy metals like lead and zinc have shown to be high. Important sources of these elements in the fuels are paint remnants and galvanised nails or screws.
Controlling fuel lead content
In a laboratory study2, lead oxide was added in different amounts to a wood Decreased corrosion problems by controlled fuel composition fuel to create synthetically made usedwood fuels mixtures. These fuels were fired in a 10kW laboratory fluidised bed boiler and ring-samples of selected metallic materials were exposed to evaluate the corrosion in relation to the lead content in the fuel up to 1000 mg/kg. Other elements present in used-wood fuel (Cl, K, Zn) were also added to the different fuel blends, but in constant amounts, in order to simulate fuels often used at the power plants.
The results showed that there was a relation between increased lead content in the fuel and increased corrosion, at least for the low alloyed steel 16Mo3. At 350 degrees minor corrosion was observed and at 400 degrees the corrosion was considered severe with respect to the relatively short exposure time of eight hours. This is illustrated in Figure 1.
By controlling the lead content in the fuel and regulate it to lower levels, corrosion problems could consequently be decreased. More research is, however, needed in order to fully understand the role of lead related to corrosion mechanisms.
Co-firing with municipal sewage sludge
Another possible solution in order to decrease corrosion problems is to use an additive. An additive can be used in order to transform aggressive corrosive compounds present in the boiler into less corrosive compounds. Municipal sewage sludge is a promising additive that has been shown to decrease the corrosion when co-fired with used-wood fuel. Comparing the initial corrosion (14 hours) when only using used-wood as fuel, with the corrosion observed when co-firing the used-wood fuel with sewage sludge, decreased corrosion was observed for several materials3,4. An example of the stainless steel 310S is shown in Figure 2. This figure shows that when only used-wood fuel is fired, corrosion of the material results in oxide formation and nickel particles present in the corrosion products, (Figure 2a). When using sewage sludge as an additive, thinner oxide is observed and no nickel particles are present. This is believed to be a result of a slower corrosion process when using the sludge addition.
It is believed that alumina silicates in the sludge are capturing alkali, for instance potassium, and form alkali alumina silicates. By forming this compound less potassium would be free to form potassium chlorides, which have shown to be corrosive, and, consequently, less corrosion is expected. Most studies have however been performed during rather short exposure times, up to 24 hours and more research is needed in order to establish the positive effect municipal sewage sludge can have by decreasing the corrosion problems.
At the moment several field exposures are being performed which are related to both the lead content in used-wood fuel and the use of sewage sludge. In these field tests the materials are exposed to the real environment in a boiler and are thus very useful in order to get information of how specific materials behave when exposed to these conditions. The exposure times are longer than previous exposures and the long-term effect of fuel composition and co-firing with sewage sludge can be evaluated.
Figure 1. Corrosion of low alloyed steel 16Mo3 exposed when firing wood fuel with a lead content of 1000 mg/kg at a) 350 °C and b) 400 °C.
Figure 2. Corrosion of stainless steel 310S when exposed to a) only used-wood fuel and b) used-wood fuel co-fired with municipal sewage sludge.
1) A. Talus, “Decreased furnace wall corrosion in fluidised bed boilers - The influence of fuel lead content and sewage sludge additive”, 2016, Licentiate thesis, Chalmers, Göteborg
2) Talus et al., “Effect of Lead Content in Used Wood Fuel on Furnace Wall Corrosion of 16Mo3, 304L and Alloy 625”, 2017, Oxidation of Metals, pp. 1-12, doi: 10.1007/s11085-017-9727-3
3) Talus et al., “Effect of sewage sludge addition on initial corrosion of 16Mo3 and 310S”, 2016, Materials and Corrosion, 67, 683
4) Alipour et al., “The effect of co-firing sewage sludge with used wood on the corrosion of an FeCrAl alloy and a nickelbased alloy in the furnace region”, 2015, Fuel Processing Technology, 138, 805