Modelling and simulation of SOx and NOx formation under oxy-coal combustion conditions

The oxy-coal combustion represents one option for Carbon Capture & Storage to mitigate greenhouse gas emissions from fossil fuel fired power plants. Compared to conventional air-fired combustion, the oxy-coal combustion is characterized by its modified combustion atmosphere consisting mainly of oxygen and recycled flue gas which affects thermo-physical properties and flame characteristics. The work focuses on modelling and simulation of SOx and NOx formation under oxy-coal combustion conditions. In a CFD framework, pollutant models are commonly applied in a post-processing step based on results from a basic combustion simulation. Hence, reliable and accurate results of the basic combustion simulation are an essential precondition for validation of pollutant models. Special attention is thus drawn to the field of turbulence modelling which is of crucial relevance for the prediction quality of the flow field particularly in the near-burner region. The evaluation of different turbulence models reveals that especially for swirl-stabilized flames the choice of the turbulence model may significantly affect the accuracy of the basic combustion simulation (and of the subsequent pollutant simulation as well). Since the pollutant formation mechanisms with regard to SOx and NOx are fundamentally similar at oxy-fuel and air-fired conditions, existing pollutant models developed for conventional air-fired systems are applicable to oxy-fuel systems as well. To ensure reliable predictions at varying operating conditions, the proposed global SOx and NOx models are refined and additional reactions are implemented. Both pollutant models are composed in a similar way involving (a) the release of coal-bound sulphur and nitrogen, respectively, (b) competing gas-phase reactions, and (c) heterogeneous reduction reactions. For validation purposes, two different furnaces of laboratory and semi-industrial scale are considered. The comparison of measured data and simulation results demonstrates that the proposed pollutant models are well suited for predicting concentration profiles of the most relevant pollutant species at both air-fired and oxy-fuel combustion conditions. Discrepancies observed for the intermediate species in the main combustion zone are attributed to the use of simplified global gas-phase reaction schemes. The influence of the presence of SO₂ and NO in the oxidant due to flue gas recycling under oxy-fuel conditions as well as the effect of oxidant staging are accurately reproduced in the modelling. In particular, the predicted furnace exit concentrations consistently match the measured data with high accuracy.