Modeling NO formation in CFD black liquor recovery boiler models part 2: Impacts of nitrogen model parameters

Mathematical models based on computational fluid dynamics (CFD) are helpful tools for understanding complex combustion processes, such as the combustion of black liquor in recovery boilers. One key area of study is the formation of harmful air pollutants, such as nitrogen oxides (NO _x), with the goal being minimizing their formation while maximizing boiler efficiency. To produce the most accurate CFD simulations, its various submodels often need adjusting on a case-by-case basis, which in turn requires an understanding of their behavior under a wide range of conditions. This study investigated how changes to fuel-bound nitrogen (fuel-N) release affect the formation of nitric oxide (NO) in a CFD model of a black liquor recovery boiler. The gas-phase chemistry was modeled using a global reaction mechanism with 9 reactions and 10 species. Four different test series of 30 simulation cases in total were computed and analyzed. The results showed that having the volatile nitrogen released as NO, as opposed to ammonia, significantly increased NO _x emissions. Secondly, NO _x emissions increased further as more of the fuel-N was released as NO via char oxidation. Lastly, NO _x emissions increased when fuel-N content increased, although the in-furnace reduction also increased. The results were generally in line with known trends and boiler data.