Numerical investigation on ammonia co-firing in a pulverized coal combustion facility: Effect of ammonia co-firing ratio

Recently, the co-firing ammonia (NH3) with coal in boiler has been paid more and more attention. A computational Fluid Dynamics (CFD) simulation approach was built to investigate the NH3 co-firing in a pulverized coal combustion facility with a single swirling burner. The simulation accuracy was evaluated with the experimental data. The differences between coal firing and NH3 co-firing can be well reproduced with simulation. Then, the effects of NH3 co-firing ratio (CR) on the combustion characteristic and NOx emission were investigated. The flame shape is significantly affected by NH3 CR, due to the changes of momentum of NH3 jet from center of burner. When the NH3 CR exceeds 40 cal.%, the internal recirculation zone is completely penetrated by high velocity NH3 flow, which leads to a long flame and much unreacted NH3 in downstream. As the increase of NH3 CR, the overall trend is that the total heat absorbed slightly decreases due to the decrease of particle radiation, and unburnt carbon in fly ash obviously increases due to decrease of flame temperature. In the case with NH3 CR of 10 cal.%, the NO concentration at exit increases due to the more intense combustion and more fuel-NOx, compared with the case of coal-firing. When the NH3 co-firing ratio exceeds 10 cal.%, the NO concentration at exit decreases monotonously due to the DeNOx effect of unreacted NH3. However, once the NH3 co-firing ratio exceeds 40 cal.%, the unreacted NH3 concentration at exit increases rapidly, which requires careful designs of burner and furnace.