Oxy-Fuel Combustion - The Control of Nitrogen Oxides
Oxy-fuel combustion is one of the main candidates for carbon dioxide capture from power plants. In oxy-fuel combustion, oxygen diluted with recycled flue gas oxidizes the fuel. The elimination of air-borne nitrogen generates a flue gas that mainly consists of carbon dioxide and water. The carbon dioxide is compressed and separated from impurities to generate a stream suitable for storage. The emission of nitrogen oxides (NOx), which is the topic of this thesis, is important in the construction of all power plants. In oxy-fuel power plants, NOx may require consideration in the gas entering the flue gas treatment, in the gas ventilated to the atmosphere, and in the storage gas.
The combustion conditions of importance to nitrogen chemistry differ between the state of air and oxy-fuel operation due to the low concentration of air-borne nitrogen and the recycling of flue gases. This work investigates two combustion strategies for controlling the emission of NOx from oxy-fuel combustion: 1) reburning and 2) what we call “high-temperature reduction”. Reburning reactions rapidly reduce NOx, which is recycled back to the flame zone. Reburning is promoted by sub-stoichiometric combustion and by controlling combustion temperatures. The high-temperature reduction is made feasible by the low concentration of nitrogen in oxy-fuel combustion, which may reverse the Zeldovich mechanism (responsible for thermal NOx formation) to reduce the NOx formed from fuel-bound nitrogen. To achieve this result, a combustion strategy with low air-ingress, sub-stoichiometric conditions and high inlet oxygen concentration is required. In contrast with the strategy for reburning reduction, the combustion strategy for high-temperature reduction is not conventional, but benefits from reduced flue gas flow and efficient combustion.
An oxy-fuel power plant also offers expanded opportunities for controlling NOx in the flue-gas treatment, which are reviewed in the present work. For example, the elevated pressure increases the formation of NO2, which may be absorbed in water. To find the optimal organisation of NOx control in oxy-fuel power plants, further work is required to define the limits of NOx, experimentally validate the proposed measures, and then evaluate the corresponding cost. It is of importance not to mimic the air-combustion in the development of oxy-fuel combustion, but rather utilize the broadened combustion conditions to arrive at the optimal performance with respect to emissions, combustion efficiency and investment.