Computational fluid dynamic simulation of oxyfuel combustion in gas-fired water tube boilers

Carbon capture is essential to enable the use of fossil fuels while reducing the emissions of CO2 into the atmosphere. Among the methods of CO2 capture and storage, oxyfuel technology provides a promising option for power and steam generation systems. In this technology, the fuel is burned in pure oxygen instead of air, and the flue gas consists primarily of CO2 and H2O that can be easily separated via condensation. In order to moderate the furnace temperature in the absence of N2, a fraction of the flue gas is recycled in the combustion chamber. In this work, the characteristics of oxyfuel combustion are compared to those of air–fuel combustion in a typical natural gas fired package boiler and the results can be applied to large boilers such as industrial and utility boilers. The percentages of recycled CO2 considered in this study are 83.8% and 77% by mass. The first corresponds to 21% O2 and the second corresponds to 29% O2 by volume. Validations for both oxyfuel combustion and air–fuel combustion are conducted through comparison with the available experimental data. Results indicate that the temperature levels are reduced in oxyfuel combustion. As the percentage of recirculated CO2 is increased, the temperature levels are greatly reduced. We found that the fuel and oxygen consumption rates are slower in oxyfuel combustion relative to air–fuel combustion. Heat transfer from the burnt gases to the water jacket along the different surfaces of the furnace is calculated. It is shown that the energy absorbed is much higher in the case of air–fuel combustion along all surfaces except for the end part of the furnace close to the furnace rear wall. The heat transfer in the return chamber (tube bank) was also calculated and the results indicated higher heat transfer in the oxyfuel case in comparison with the air fuel case as a result of ignition delay in the vicinity of the furnace entrance region.

► The thermal characteristics of oxyfuel combustion were investigated and compared to the conventional air–fuel combustion.
► As the percentage of recirculated CO2 is increased, the temperature levels are greatly reduced.
► The fuel and oxygen consumption rates are slower in oxyfuel combustion relative to air–fuel combustion.
► The energy absorbed along all surfaces of the furnace is much higher in the case of air–fuel combustion.