Thermodynamic Modelling of the Corrosive Deposits in Oxy-Fuel Fired Boilers

Abstract

The aim to reduce the CO2-emissions has triggered the evaluation of new cycle concepts for power plants. For the coal-fired power plants, the oxy-fuel firing is a promising option for CO2- emission reduction. Here, the combustion takes place in a nitrogen-free atmosphere. The oxygen is separated from the air and burned in near-stoichiometric conditions with the fuel. The gas composition is significantly changed, when the combustion is changed from air-fired to oxy-fuel fired condition. For lignite, the carbon dioxide content is raised from 15 to 59vol% and the watercontent from 10 to ~32%. For the same fuel, the SO2-content in the flue gas increases by a factor of 3-4 to ~0.5%. These changed environmental boundary conditions will affect corrosion life of the materials especially on the water walls and the heat exchanger surfaces. Considering the significant changes in the combustion gas, the composition and the occurrence of the corrosive deposits has been evaluated with the thermodynamic modelling program ‘FactSage’. The chemical compositions of the deposits have been modelled for dried lignite from Germany. The results exhibit that the oxy-fuel firing will give a significant change in the atmosphere as well as in the deposit composition. Consequently, the corrosion rates of current used materials in air fired boilers need to be evaluated for the application in oxy-fuel fired boilers.

Effect of SO2 on oxidation of metallic materials in CO2/H2O‐rich gases relevant to oxyfuel environments

“…The so‐called oxyfuel process is frequently considered the most promising option of the available technologies for CO 2 capture from the exhaust gas in power plants. The principle of this process consists of burning coal or other fossil fuels with pure oxygen and includes a recirculation of the exhaust gas . The main advantage of the oxyfuel technology is the composition of the flue gas, which contains almost only CO 2 and H 2 O.…”

Effect of oxy‐firing on corrosion rates at 600–650 °C

“…The most extreme oxy‐firing conditions, assuming hot gas recirculation and high levels of H 2 O and SO 2 , have been used in some studies . However, warm or cold gas recirculation may be more practical to lower the boiler temperature and these scenarios would reduce the H 2 O and could reduce the SO 2 contents in an oxy‐fired boiler.…”

“…However, warm or cold gas recirculation may be more practical to lower the boiler temperature and these scenarios would reduce the H 2 O and could reduce the SO 2 contents in an oxy‐fired boiler. Therefore, four different environments were investigated with the same synthetic coal ash: the air‐fired and oxy‐fired conditions previously suggested and the oxy‐fired condition with lower H 2 O (10%) and with lower SO 2 (0.15%) . Figure shows the oxide thickness results for several of the specimens exposed at 600 °C as a function of their Cr content.…”

Microscopy of fireside corrosion on superheater materials for oxy-fired pulverised fuel power plants

“…and partial water condensation. All these options raise concerns for the life of the superheateryreheater tubes due to changes in the flue gas compositions and deposition behaviour (deposit compositions and deposition fluxes), coupled with an increase in metal temperature [5,6].…”

“…Oxy-firing of coalybiomass involves the recycling of the flue gases produced, in addition to using oxygen instead of air to burn the fuel. For the system variant based on the recycling of hot flue gases after particle removal, pilot scale testing and modelling indicate typical gas compositions of~60 volume % CO 2 -~30% H 2 O, 4% O 2 together with SO X and HCl enriched by up to~5 times compared to air firing of the same fuel mixes, but with similar deposition rates [5,6]. In both cases, the increase in metal surface temperatures combined with new flue gas compositions and surface deposition (in terms of deposition compositions andyor fluxes) requires a better understanding of fireside corrosion mechanisms and the quantification of such damage with these changes in exposure conditions [5,6].…”