Application of an In-Furnace NO_x Removal(MACT)System for Recovery Boilers

Abstract

In recent years, the black liquor concentration in pulp recovery boilers has been markedly elevated to its high concentration of about 80% form its conventional level of about 70%, accompanied with improved performance of the black liquor evaporator in response to the requirement for an increased handling capacity of black liquor. This trend raised the combustion temperature in char bed, causing a problem of elevated NO_x concentration level. In our present research using a field boiler, the authors verified that the NO_x level can be decreased by designing improvements of the combustion process utilizing changes of the air distribution into the pulp recovery boiler. As opposed to the current recovery boiler where primary, secondary and tertiary air are used to complete combustion, our research was conducted on the application to the recovery boiler of the in-furnace NO_x removal system where the NO_x reduction process is designed to be enhanced in the char bed zone and in its upper zone by decreasing primary, secondary and tertiary air injected into such zones and by further injecting the fourth air into the upper zone of tertiary air to complete combustion. Results of the field boiler test showed that the NO_x emission level can be decreased by about 30% compared with the conventional level by injecting the fourth air at a ratio of 8%, demonstrating that the in-furnace NO_x removal system can also be effectively applied to the recovery boiler.