A four-fuel-stream flamelet model for large-eddy simulation of piloted pulverized coal/ammonia co-combustion

In advanced industrial applications, multiple fuel streams may be introduced in the combustion burners to improve the combustion efficiency and reduce pollutants formation. In particular, the addition of carbon- free sustainable fuel to the existing hydrocarbon burners becomes a feasible and intermediate step to reduce greenhouse gas emissions, e.g. addition of hydrogen to natural gas burners, addition of ammonia to pulverized coal/biomass power plants, etc. To characterize the complex mixing process in such industrial applications, a four-fuel-stream flamelet model is proposed in this work in the framework of large-eddy simulation (LES). The new flamelet model characterizes the mixing among four fuel streams but preserves model consistency for less number of fuel streams. The performance of the proposed flamelet model is first evaluated for a laminar counterflow coal/ammonia co-combustion flame through an a priori analysis and then applied to a laboratory-scale piloted turbulent pulverized coal flame with ammonia addition. The simulation results are compared to the detailed chemistry solutions of the counterflow flame and the experimental data of the turbulent flame. The results show that the thermo-chemical quantities, including the major species and radicals, can be accurately predicted for the laminar counterflow flame. For the turbulent piloted flame, the predicted flame structure and the NH, OH, and PAH mass fractions qualitatively agree with the experimental LIF data. The flame temperature and species mass fractions are reasonably predicted as well. Compared with pure coal combustion, the NO formation along the central axis of the burner is suppressed with ammonia addition due to the lower combustibility, caused by the reduced oxygen concentration in the co-combustion. It is, however, increased in the annular regions due to the significant fuel-NO contribution from ammonia combustion. The total NOx x formation in the pulverized coal flame with ammonia addition is slightly decreased compared to the pure pulverized coal flame.