Two Stage Ammonia Combustion in a Gas Turbine like Combustor for Simultaneous NO and Unburnt Ammonia Reductions


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Ammonia is expected not only as a hydrogen energy carrier but also as a carbon free fuel. Recently, ammonia fueled gas turbine combustor was successfully demonstrated. However, large amount of NOx was produced when ammonia burns because ammonia includes nitrogen atom in the ammonia molecule. In addition, unburnt ammonia concentration in exhaust gas also needs to be reduced. In this study, we proposed a combustion concept in order to reduce NO and unburnt ammonia concentrations in the exhaust gas simultaneously in a gas turbine like model swirl combustor. In this concept, two stage (rich – lean) combustion was employed. Two stage (rich – lean) combustion has been already employed in hydrocarbon fueled gas turbine combustors in order to reduce thermal NOx. However, the two stage combustion for ammonia fuel is different from that of hydrocarbon because production path of NO is different each other, i.e., NOx is generated via fuel NOx path in the ammonia flame.

In two stage combustion for ammonia, rich flame is generated in the primary zone, and our previous study showed that NO concentration decreases with an increase in the equivalence ratio in rich flame region. Therefore, the exhaust gas from the primary zone has very low amount of NO, nevertheless, considerable amount of unburnt ammonia and hydrogen are contained. Then, the secondary air is injected at the downstream of the primary zone, and then injected air and the exhaust gas from the primary zone mix and burn in the secondary combustion zone. Consequently, over-all NO emission can be reduced if the ammonia concentration in the exhaust gas from the primary zone is low. Secondary air does not reduce NO and ammonia simultaneously if unburnt ammonia concentration in the exhaust gas from the primary zone is high, i.e., equivalence ratio is too high. Therefore, it was clarified that the equivalence ratio control is important in order to achieve simultaneous NO and ammonia reduction.