Oxy-firing of solid fuels is one of the most relevant technological alternatives aiming at the CO₂ capture in large-scale power plants. If oxy-firing is carried out in a fluidized bed reactor, the possibilities for application are then extended to low-rank coals and other residues, which would be very difficult to fire under pulverized conditions. In addition, the control of other emissions (NOx, SOx) is also improved. Finally, it is also possible to add biomass to the feedstock, thus resulting in a negative balance for the CO₂ emissions. Several on-going national and international initiatives were focused in the research of oxy-fuel related issues, covering lab-scale plants, pilot plants and semi-commercial units. But most of the works addressed oxy-firing under pulverized fuel combustion, and there was a lack of publications as for its application to fluidized bed units. In particular, the field of ash fouling and corrosion under oxy-firing in fluidized beds remained almost completely open.
Although there were available works in relation to the control of deposition rates in fluidized bed units, especially when biomass is the fuel; they were developed under conventional combustion with air but not for the particular conditions occurring with oxy-firing. The objective of this project was to characterize the phenomena and effects of fouling and corrosion due to the deposition of ashes in the tubes, when oxy-firing is carried out in fluidized bed boilers. The influences of different factors on fouling and corrosion were sought: the coal/biomass share in the fuel blend, the O₂/CO₂ atmosphere, the recycling of flue gases to the reactor, the use of limestone as a sorbent and the supply of selected additives in the bed. The research was arranged in a double way, by means of a coordinated project, taking advantage of the own resources and background of the groups proposing the project: part 1) pilot-scale, in an experimental facility reproducing the real conditions of fluidized bed oxy-firing; part 2) lab-scale, under well-controlled conditions for the governing parameters. Thank to this collaboration, the accurate lab-scale characterization was also useful by the comparison to the conditions existing in a real plant.