Experimental and CFD investigation of gas phase freeboard combustion

Reliable and accurate modeling capabilities for combustion systems are valuable tools for optimization of the combustion process. This work concerns primary precautions for reducing NO emissions, thereby abating the detrimental effects known as “acid rain”, and minimizing cost for flue gas treatment. The aim of this project is to provide validation data for Computational Fluid Dynamic (CFD) models relevant for grate firing combustion conditions. CFD modeling is a mathematical tool capable of predicting fluid flow, mixing and
chemical reaction with thermal conversion and transport. Prediction of pollutant formation, which occurs in small concentrations with little impact on the general combustion process is in this work predicted by a post-processing step, making it less computationally expensive. A reactor was constructed to simulate the conditions in the freeboard of a grate fired boiler, but under well-defined conditions. Comprehensive experimental data for velocity field, temperatures, and gas composition are obtained from a 50 kW axisymmetric non-swirling natural gas fired combustion setup under two different settings. Ammonia is added to the combustion setup in order to simulate fuel-NO formation during grate firing biomass
combustion conditions. The experimental results are in this work compared to CFD modeling. The modeling results show, that the CFD model captured the main features of the combustion process and flow patterns. The application of more advanced chemical reaction mechanisms does not improve the prediction of the overall combustion process, but do provide additional formation about species (especially H2 and radicals), which is desirable for post-processing pollutant formation. NO formation is post-processed using various ammonia oxidation schemes and different post-processing techniques. The results in some cases provide a
reasonable agreement with the experimental data. In general the application of advanced combustion modeling and more advanced ammonia oxidation mechanisms does not improve the agreement with experimental data compared to the simple eddy dissipation (mixed is burned) approach with post
processing of a global combustion mechanism. The experimental setup does however not serve as a perfect validation case. The Reynolds numbers in the system put the flow regime in the transitional region, where turbulence modeling is difficult. Furthermore, the inclined jets show an affinity towards wall attachment, the entire modeling result is very sensitive to the prediction of these jets.