Full-Scale Investigation of Dry Sorbent Injection for NOx Emission Control and Mercury Retention

An innovative dry SNCR method realized by a sorbent injection applied to a stoker furnace is presented. The process is based on urea powder admixed with halloysite, an aluminosilicate clay mineral. Field tests were performed at an industrial stoker hot water boiler of 30 MWth capacity. A unique nozzle design for injecting powdery sorbents into the combustion zone was implemented. The base NOx emission without SNCR was determined to be 365 mg/Nm3. During the reference test, the emission was reduced to avg. 175 mg/Nm3, which produces a NOx reduction of 52%. NH3 slip in the flue gas was stable and did not exceed 2 ppm. Combining urea and halloysite powders leads to a number of positive effects; not only is NOx emission reduced to values typical for wet SNCR, but also a significant, over ten-fold increase in the concentration of adsorbed mercury in fly ash was observed. When confronted with wet SNCR, dry SNCR has no adverse effect on boiler efficiency because it does not increase the stack heat loss. The presented method can be used in any small- or medium-scale furnace, including waste-to-energy units or medical and hazardous waste incineration units.