CFD modelling of black liquor recovery boiler NOx emission formation—Influence of envelope flame and thermal NO

Fuel NO is considered the main source of pulp mill black liquor recovery boiler NOx emissions. The extent to which (i) NO chemistry when pyrolysis occurs with envelope flame as well as (ii) thermal-NO contribute to the boiler NOx emissions is currently unknown. A CFD model of the recovery boiler was used to study these phenomena in recovery boilers. The CFD model includes fully coupled envelope flame prediction, with different source terms for volatile release with and without envelope flame. A simplified thermal NO expression was used to assess its formation. The recovery boiler modelled is a 600 MW 5300 tds/day boiler. Accounting for the envelope flame did not result in significant changes in the temperature and O ₂ profiles in the boiler. However, nitrogen chemistry was notably affected, with generally a decrease of 20 % in outlet NO when the envelope flame was considered. This decrease in outlet NO occurred due to the reduction of volatile nitrogen to N ₂ in the envelope flame, as well as the re-burning chemistry, which reduced nitrogen species in the bulk gas. Various boiler cases with different levels of excess air as well as cases with different model assumptions concerning the split of fuel nitrogen into volatile and char nitrogen were analysed and the trends in outlet NO were similar whether or not the envelope flame was included. A similar trend was seen with different nitrogen splits. When thermal NO was included in the model, an 18 % increase in outlet NO was observed. Thermal NO formation occurred close to the char bed in regions where the char bed carbon was being depleted.