Industrial Trials of High-Temperature Selective Noncatalytic Reduction Injected in the Primary Combustion Zone in a 50 MWe Tangentially Firing Pulverized-Coal Boiler for Deeper NOx Reduction

This paper aims to demonstrate an industrial retrofit of NOx reduction through use of a new technology, high-temperature selective noncatalytic reduction (abbr. HTSNCR), in a 50 MWe tangentially firing boiler of pulverized coal. Over the last several years, the characteristics of HTSNCR were investigated in the bench-scale experiments, as presenting the temperature window of SNCR can extend to a higher temperature range in the absence of oxygen. Therefore, HTSNCR provides a promising option of greater NOx reduction via injecting urea solution or ammonia into the primary combustion zone specialized with oxygen of nearly zero in furnace. These retrofit experiments in this paper successfully showed the ability of HTSNCR. NOx emission of 68 mg/Nm³ was finally achieved through use of the optimum hybrid application of HTSNCR, SNCR, and OFA. The overall reduction efficiency is approximately 90%, in which 17% is devoted by HTSNCR. The main factors in HTSNCR were studied extensively, including (1) primary stoichiometric ratio (SR₁) of air staging, (2) normalized stoichiometric ratio (NSR) of reagent quantity injected, (3) allocation of injectors, e.g., on the corners or at the middle of side walls, (4) ammonia slip brought about by HTSNCR or SNCR, (5) optimized hybrid configuration about SNCR and HTSNCR. On the basis of the optimum setting of the above factors, two key features of HTSNCR employed in a tangentially firing furnace were obtained. First, there is a critical minimum value of NOx emission in the relationship of NOx emission versus NSR₁ of HTSNCR. More NSR₁ beyond the critical value, i.e., more reagent quantity injected, results in more NOx formation. Secondarily, the injection of reagent near the corners is beneficial to reach higher NOx reduction rather than that injected from the side walls, due to the aerodynamics in the tangentially firing furnace.