Influence of surface ash layer on dual corrosion

Resumo

Oxycombustion is considered one of the most efficient technologies to reduce CO2 emissions caused during energy production. This process consists of burning coal with pure oxygen and recirculated combustion gas. In the boiler the heat exchange tubes are exposed to different atmospheres. This means, the outer side is in contact with the oxyfuel gas and ash deposits, while inside flows water vapor. This type of corrosion under different atmospheres is called dual condition. The objective of this thesis is to evaluate the influence of ash deposition on the corrosion behavior of alloys with different chromium contents under dual condition at 600°C. A setup was designed to carry out the experiments, which allows the opposite surfaces of the sample to be in contact with different atmospheres, and therefore perform experiments under both conditions. Under single condition was evaluated the influence of ash in oxyfuel flue gas atmosphere. For this ash was deposited on one of the faces of the sample and exposed to oxyfuel gas, while the opposite face was exposed to oxyfuel gas without ash deposit. In this way, the influence of the ash deposit is evaluated in terms of the material loss (corresponding to the internal oxidation layer), sulfur transport (from the oxyfuel flue gas and the ash) and carbon penetration in the metal (from the oxyfuel flue gas). Under dual condition was evaluated the impact of water vapor on the opposite side of the sample. One of the faces of the sample with ash deposit was exposed to oxyfuel flue gas, while the opposite face was exposed to water vapor. Under this dual condition, the influence of water vapor - due to the hydrogen transport - on the opposite side, is evaluated in terms of material loss (internal oxidation layer) and changes in the phase composition of the oxide scale. In order to simulate as much as possible the real conditions, the composition of oxyfuel flue gas -27% H2O + 60% CO2 + 1% SO2 + 10% N2 + 2% O2- as well as the phase composition of ash -40% CaSO4 + 20% Fe2 (SO4) 3 + 10% SiO2 + 20% FeSO4 + 10% Brownmillerite (3.3% Fe2O3 + 1.9% Al2O3 + 4.8% CaO) – were taken from the Vattenfall’s Schwarze Pumpe Pilot Power Plant in Spremberg (Germany). The materials selected for this study are alloys with a good corrosion resistance at 600°C. Two technical alloys, T92 and VM12-SHC, with 9% and 12% chromium content respectively, were selected to evaluate the influence of the chromium content; as well as Armco iron (without chromium) to evaluate the absence of such element. In addition, two model alloys were selected, Fe-9Cr and Fe-13Cr, with a chromium content similar to the techniques, 9% and 13% respectively, to evaluate the effect of minor alloying elements (C, Si, Co, Mn, Ni, Mo) on the corrosion resistance. The experiments were performed at 1 bar of pressure and for different annealing times, 24 h, 120 h and 240 h in the case of pure iron, and 24 h, 240 h and 960 h for the rest of alloys. The results obtained indicate that the parameters that influence on corrosion behaviour under the studied conditions are: microstructure, material composition, exposure condition (oxyfuel flue gas, ash deposit and water vapor) and annealing time. Results demonstrate that ash acts as a barrier that reduces the material loss, in addition the dual condition reduces the material loss even more. In addition, this study confirms that technical alloy VM12-SHC displays the best corrosion behaviour under both conditions, simple and dual.