Numerical investigation on NOx formation of staged oxy-fuel combustion in a 35 MW large pilot boiler.

• NO x formation in staged oxy-combustion has been numerically studied. • The fuel staged mode reduces NO concentration in furnace outlet by 5.6%. • Oxygen-fuel two-way staged mode reduces NO concentration in furnace outlet by 17%. • The NO formation pathway of HCN → NCO → NO is suppressed, and NO reduction pathway of NO → HCN is enhanced in staged oxy-combustion. In this study, high-fidelity simulations are conducted on a 35 MW pulverized coal oxy-fuel boiler to investigate NO x formation characteristics under staged oxy-fuel combustion, including both fuel-staged and oxygen-fuel two-way staged modes. Detailed volatile components are considered by using the chemical percolation devolatilization model. A skeletal reaction mechanisms consisting of 35 species developed for fuel-NO x formation under oxy-fuel conditions are employed for the gas phase combustion and NO x modeling. The radiative property models are also optimized for oxy-fuel combustion. The results show that in the fuel-staged mode, a reduction atmosphere is established in the reburning zone to reduce the NO x concentration in the flue gas. A reduction of 5.6% in the average NO concentration at the furnace outlet is observed relative to the baseline oxy-fuel case. Moreover, the application of oxygen-fuel two-way staged combustion leads to a further reduction in NO x formation. The average NO concentration at the furnace outlet is decreased by 17% compared to the baseline oxy-fuel case. Pathway analysis indicates a reduction in the NO formation pathway involving HCN → NCO → NO, while the NO reduction pathway of NO → HCN is enhanced. [ABSTRACT FROM AUTHOR]