Numerical investigation of NO emissions from an entrained flow reactor under oxy-coal conditions

The NO emissions of two anthracitic and three high volatile bituminous coals were experimentally and numerically studied under both air and oxy-fuel combustion conditions in an entrained flow reactor (EFR). Thermal decomposition experiments under N2 and CO2 atmospheres were carried out to determine the distribution of fuel-bound nitrogen between the volatile and char and the results compared with those obtained by means of the network pyrolysis model, FG-DVC (Functional Group-Depolymerisation Vaporisation Cross-linking). This code was also used as a pre-processing stage to predict the evolution of HCN and NH3 during devolatilisation of the coals. A Computational Fluid Dynamic (CFD) model was used to predict NO emissions under different O2/CO2 (21–35% O2) conditions in the EFR.Three different models were used. The first assumed that all of the fuel-bound nitrogen had been converted to HCN. The second assumed that all of the volatile nitrogen would evolve as HCN, and the char–N formed NO by an amount determined by a conversion factor. The third approach was similar to the second but it included NH3 as a precursor of NO as well. The NO emissions predicted with the third approach were in good agreement with the experimental results. A decrease in NO emissions was observed when N2 was replaced by CO2 for the same oxygen concentration for both the experimental and computed results. Higher NO emissions under O2/CO2 conditions were observed when the oxygen concentration was 30 or 35%.

► Experimental and numerical study of NO formation in oxyfuel combustion was performed. ► Experimental study was made using an entrained flow reactor for five coals at 1273 K. ► NO formation was highly dependent on oxygen enrichment level in oxy cases. ► Oxygen concentration of > 30–35% showed an increase in NO formation in oxy cases. ► One of the numerical approaches for modelling NO formation showed the best agreement.