Modeling NO formation in CFD black liquor recovery boiler models part 1: Development of global reaction mechanism

Nitrogen oxides (NOx) emissions are harmful to humans and the natural environment. These emissions originate mainly from combustion processes, such as the combustion of black liquor in recovery boilers. In addition to increasingly stringent emission control policies, the need to reduce NOx emissions is of great importance. Mathematical models based on computational fluid dynamics (CFD) are crucial in the development of modern black liquor recovery boilers to ensure low NOx emissions while maximizing efficiency. The prediction accuracy of NOx emissions in these models is highly dependent on the chemistry sub-model. In this work, a new global reaction mechanism (referred to as JLNO-GRM) was developed for NO formation in CFD models of recovery boilers. A multi-objective genetic algorithm was used to fit the kinetic parameters of the nitrogen reactions to data from 1-D ideal reactor simulations using detailed chemical kinetics. The global reaction mechanism was tested in a full-scale 3-D CFD recovery boiler model and the produced NOx emissions compared with boiler data. Multiple simulations were run to study the effect of excess air and air distribution on NO formation. Higher oxygen concentrations in the flue gas increased NOx emissions. Decreasing the ratio of tertiary air to secondary air resulted in higher NOx emissions. The modeled NOx emissions ranged from 109 to 186 ppm at 6 vol% oxygen (dry) and were generally in good agreement with boiler data and known trends.