Numerical optimization of the influence of multiple deep air-staged combustion on the NOx emission in an opposed firing utility boiler using lean coal

Multiple deep air-staged combustion is a promising technology to significantly reduce NOx emission in the opposed firing utility boilers using low-volatile coal. The gas-solid two-phase flow, pulverized coal combustion and NOx emission characteristics of an existing 600 MW coal-fired boiler were numerically simulated to evaluate the influence of excess air coefficient in primary combustion zone (alpha(M)), variational air distribution modes and multiple deep air-staged combustion on the NOx formation and destruction process in the furnace. The detailed NOx formation and reduction models were proposed to consider the reduction reaction between hydrocarbon and NO under fuel rich conditions and well validated by the experimental results in the laboratory and field tests. The results show that the alpha(M) has an important influence on flue gas temperature distribution and forms a reducing atmosphere in the primary combustion zone to significantly reduce NOx concentration. The NOx concentration at the furnace outlet is greatly decreased when the deep air-staged combustion is adopted with alpha(M) of 0.75, and the CO concentration maintains within a lower level. Compared with the balanced air distribution, the pagoda and inverse pagoda air distributions are found to be ineffective to enhance NOx reduction performance under deep air-staged combustion. However, the pagoda air distribution effectively reduces NOx emission under middle air-staged combustion condition. The NOx emission is further reduced by adopting the multiple air-staged combustion due to the higher CO concentration formed in the burnout zone. The results are helpful to the design and operation optimization of the opposed firing utility boilers using lean coal.