Oxy-Fuel Combustion in Fluidized Beds

Fluidized beds particular features drive this technology as an appropriate candidate technology to apply oxy-fuel combustion, producing so a highly concentrated CO₂ flue gas stream to be processed and stored. Still, there are several issues differencing the conventional combustion to that with O₂/CO₂ mixtures. This chapter examines the main issues involved in oxy-fuel combustion in fluidized beds through the experimental results obtained in the CIRCE oxy-fuel bubbling fluidized bed.

The fluidization velocity during oxy-firing is in general, below the air-firing case. This is caused by the higher O₂ concentration in the oxidant stream, together with the higher gas density when substituting air-N₂ by CO₂. The lower fluidization velocity affects in opposite ways the combustion and pollutant formation: it increases the residence time of particles in bed, whereas poorer mixing of fuel particles disadvantages reactions. Higher O₂ at inlet obliges to increase the fuel input to maintain proper fluid-dynamics conditions. This is adequate for the combustion efficiency and also, for the in-furnace SO₂ capture. Unlike in conventional combustion, SO₂ capture optimum temperature is higher than 850 ºC, because of the influence of high CO₂ partial pressure.

NO_x emissions showed no significant differences with air-firing case, if concentration is expressed per unit of energy. This is due again to the lower flow of flue gases per thermal fuel input. Plant heat balance of large oxy-fuel fluidized bed boilers will change considerably in oxy-fuel case. Boilers will be more compact and thus, additional heat transfer surface will be essential.

Keywords: Oxyfuel, SO2 and NOx emissions, combustion efficiency, fluidized bed, desulfurization, heat transfer in fluidized beds, pilot plants, power plant.