Control system for an oxy-fuel combustion fluidized bed with flue gas recirculation

In oxy-fuel combustion it is generally accepted that fed O-2 into the boiler diluted together with CO2 is advantageous when similar combustion conditions as in air-firing case want to be reached. In experimental oxy-fuel combustion plants, auxiliary CO2 from commercial tanks is often used for carrying out experimental tests. However, this fact entails high operational costs in addition to scale prices are not applied in experimental size plants. A reduction of these costs could be achieved by supplying the required CO2 through flue gas recirculation, as it would happen in an industrial oxy-fuel plant. However, transition from air-firing to oxy-firing, and changes in the flue gas composition during operation can vary conditions inside the boiler. This issue becomes particularly important when oxy-fuel combustion takes place inside a fluidized bed boiler, where fluidization conditions must be kept inside a narrow optimum range, i.e. fluidization velocity or oxygen content at inlet, although changes in recirculation rates or flue gas composition could disturb it. With this aim, an efficient recycled flue gas control strategy has been modeled and validated through experiments in a test rig. This system could be easily adapted to any experimental oxy-fuel pilot plant, since it has been designed for controlling conventional instrumentation, valves and drives. (C) 2011 Published by Elsevier Ltd.