Investigation of NOx Chemistry in Oxy-Combustion Flue Gas

Oxy-coal combustion technology can be a promising solution to meet the challenge of reducing carbon emissions from power plants while continuing with the fossil fuel usage. Implementation of such technology requires the removal of nitrogen (N₂) from the oxidizer and introduction of a pure oxygen (O₂) stream and a recycled flue gas stream into the boiler. In absence of N₂, the flue gas stream generated from such system will be low in volume and highly concentrated in carbon dioxide (CO₂) which in turn will facilitate the capture and sequestration process. Moreover, oxy-coal combustion process has shown promise in reducing nitric oxide (NO) emissions. Lower NO emissions are expected in the oxy-combustion process due to the absence of N₂ in the oxidizer and reduction of recycled NO in the flame zone through the reburn mechanism. In the reburn process, recycled NO interacts with fuel fragments forming different intermediates. Depending on the combustion conditions, these intermediates can form N₂and thus reduce the NO emission. To gain a better understanding of this reduction process under oxy-combustion conditions and to evaluate the influences of different combustion parameters, a comprehensive study focused on nitrogen chemistry in oxy-combustion is crucial.

The present study aims to provide speciation data of nitrogen oxides (NO_X) and to reveal the reaction pathways of recycled NO destruction under oxy-combustion conditions. To do so, the oxy-coal combustion environment has been simulated in a lab-scale reactor with a time-temperature history representing a plant boiler. Flue gas samples collected from different temperature points of the reactor have been analyzed by applying Fourier Transform Infrared (FTIR) spectroscopy for NO_X speciation. The influence of different combustion parameters, i.e., equivalence ratio (0.8-0.98), O₂ percentage (28-34%) and recycled NO concentration (800-2000 ppmv) has also been investigated in the course of the study. Moreover, the combustion experiments have been simulated by conducting kinetic modeling with Chemkin to evaluate the performance of reaction mechanisms reported in the literature. Sensitivity analysis has also been performed to identify the destruction routes of recycled NO. The data reported in the present study can aid in predicting NO_x emissions from power plants operating under oxy mode and to evaluate the performance of combustion mechanisms in the literature.