Effect of Stoichiometry and Temperature on NOx Reduction by Reagent Injection in the Fuel-Rich Zone of Pulverized Coal Combustion

In this paper, the characteristics of NOx reduction by reagent injection into the fuel-rich zone (RIFR) in coal combustion were investigated, under high temperatures and reducing atmosphere conditions. The stoichiometry and temperature have a major influence on the chemistry of NO and NH3 reagent (ammonia or urea). Therefore, experiments were conducted on a bench-scale test system with urea solution as the reagent to investigate the key factors influencing NOx reduction, including primary stoichiometric ratio (SR1), temperature in the reaction zone, and normalized stoichiometric ratio (NSR) of the injected reagent. The results indicated that the primary stoichiometric ratio SR1 was the key parameter affecting the reduction of NOx emissions. Better NOx reduction was achieved with a decrease in the SR1 for bare air staging. However, there was no benefit for NOx reduction by reagent injection in the very fuel-rich zone (SR1 <= 0.75), which depended on the distribution of N-intermediates and initial NO concentration. On the other hand, a negative NOx reduction was obtained by reagent injection when SR1 >= 0.95 because the added reagent was oxidized to form NO. The optimum SR1 for RIFR was found to be 0.85 in this study. A higher SR1 greatly improved the NOx reduction by RIFR only when SR1 was less than 1, and high temperatures (1473-1673 K) were required for the generation of more OH free-radicals in the fuel-rich zone, which promoted NOx reduction by NH3 in the absence of oxygen. Therefore, RIFR is different from the traditional selective non-catalytic reduction technology, which has a strong temperature dependency from 1100-1300 K. The NOx reduction efficiency was increased by 21.4% with RIFR compared to the bare air-staged method, under the optimum conditions of SR1 = 0.85, T = 1673 K, and NSR = 2. More urea solution led to greater NOx formation in the burnout zone but with no ammonia slip. This method can be applied as a new alternative technology to further reduce NOx in combination with the existing low NOx combustion technologies.