Abstract
As there is a strong pressure in the EU to reduce CO2 emissions and overall fossil fuel consumption in the energy sector, many boilers are burning biomass instead of traditional fuels (coal, natural gas, oil, etc.). This is mainly due to the EU 2030 energy strategy, which commits Member States to reduce fossil fuel emissions by at least 40% (compared to the 1990 level) and to use at least 32% of renewable energy. The combustion of biomass containing aggressive elements such as chlorine or sulfur causes serious damage to various boiler components, with negative impacts such as reduced boiler lifetime, increased investments and maintenance costs, reduced availability, and others. These problems occur mainly in plants/boilers designed to burn coal and redesigned to burn biomass (straw, wood chips, wood pellets, etc.). In this paper, the corrosion resistance of heat coatings determined in long-term laboratory tests in an environment specifically corresponding to biomass flue gas is presented. These results can be used to design a suitable modification of existing coal boilers using conventional materials. The aim was to compare three completely different technologies currently available on local markets for the preparation of these coatings—thin wire arc spray (TWAS), high-velocity oxygen fuel (HVOF), and water-stabilized plasma. These coatings were compared with the base material of the boiler tubes—low alloyed steel 16Mo3 and high alloyed austenitic stainless steel AISI 310 as a more expensive option for retrofit. After 5000 h of exposure in an environment containing HCl and SO2, no cracks or structural defects were observed in any of the coatings, and the substrate material showed no signs of oxidation. All the tested coatings had higher corrosion resistance than the 16Mo3 material, and some of them presented a corrosion behavior close to that of the high alloy AISI 310 steel. Structurally and corrosion-wise, the thermally sprayed coating prepared by HVOF technology was the best of all tested materials.
Highlights
As there is a strong pressure in the EU to reduce CO2 emissions and overall fossil fuel consumption in the energy sector, many boilers are burning biomass instead of traditional fuels
The second reference material was austenitic high alloyed steel AISI 310, which can be used as a replacement for boiler tubes that switched from coal to biomass or waste fuel, as it has significantly higher thermal and corrosion resistance compared to standard ferritic/pearlitic or martensitic boiler tubes, e.g., 16Mo3, P91, T23, and others [8]
Even in the plasma spraying coating case (e in Figure 6), where higher porosity or higher number of oxidized particles were expected in the initial state, the structure was relatively compact in a cross section, and no defects or cracks in the coating structure were observed
Summary
As there is a strong pressure in the EU to reduce CO2 emissions and overall fossil fuel consumption in the energy sector, many boilers are burning biomass instead of traditional fuels (coal, natural gas, oil, etc.). In the case of wooden chips, the flue gas is not corrosive as that from straw or grass/hay but, still, the corrosion rate is much higher than that in coal-fired powerplants using ferritic/pearlitic steels up to 550 ◦ C or austenitic steels in supercritical boiler units This can be problematic for power/heating plants with coal-fired boilers that are switching to wooden chips fuel without major investments or enhancements in superheater materials. In in this case, it is better to use austenitic or nickel-based alloys, but the investments cost is high Another viable option is to use alloyed claddings or thermally sprayed coatings, which can greatly improve the corrosion resistance of conventional materials in chlorine-containing environments [5]
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