Experimental and Numerical Investigation of Gas-Phase Freeboard Combustion. Part 2: Fuel NO Formation

In part 1 of the present work (10.1021/ef900752a), experimental data and computational fluid dynamics (CFD) modeling predictions for velocity field, temperatures, and major species were compared for a 50 kW axisymmetric, non-swirling natural gas fired combustion setup, constructed to simulate the conditions in the freeboard of a grate-fired boiler.

Here, in part 2, the ability of CFD to predict volatile N oxidation to NO and N2 is evaluated. Trace amounts of ammonia were added to the natural gas, and local measurements of NH3 and NO in the reactor were compared to modeling predictions. Different modeling approaches, including global schemes and analytically reduced mechanisms, were tested in the CFD calculations.

In addition, the simplified schemes were compared to reference calculations with a detailed mechanism under isothermal plug flow reactor conditions. While none of the global ammonia schemes was able to provide satisfactory predictions over a wider range of conditions, an analytically reduced nitrogen scheme generally provided a satisfactory agreement with the detailed mechanism.

Application of the selected schemes in a CFD analysis showed that both the standard Fluent postprocessing approach with the De Soete global scheme and the combination of a skeletal combustion mechanism with the analytically reduced N scheme provided a reasonable agreement with the experimental data.

Most of the tested ammonia oxidation schemes were able to qualitatively predict the trends in NO formation going from one operational case to the other, but the main combustion solution on which the ammonia oxidation was based proved to have a large impact on the quantitative NO prediction.