Prediction of NOx control by basic and advanced gas reburning using the Two-Stage Lagrangian model

Natural gas reburning for NOx control is currently a mature technology whic h has been successfully demonstrated at full scale on numerous occasions. R ecent reburning research has revealed that advanced reburning, i.e., inject ion of an N-agent into the reburning zone, can significantly improve the ef ficiency of NOx reduction. This paper presents results of the application o f the Two-Stage Lagrangian (TSL) model of Broadwell and Lutz to the basic a nd advanced reburning processes in a 300-kW natural gas fired Boiler Simula tor Facility (BSF). The injection of the reburning fuel and overfire air is modeled as independent deflected jets and the TSL model is applied while e ach jet completely mixes with the main hue gas stream of the boiler. The en trainment rate to the jet, which is required as a model input, is derived f rom control volume analysis using the experimentally determined jet-traject ory. The rest of the facility is modeled as a plug flow reactor (PFR), whil e the droplet injection of N-agent is modeled as being distributed and thus instantaneously mixed. Calculations were conducted with a detailed chemica l mechanism based on GRI-Mech-2.11 with additional reactions characterizing thermal DeNO(x) chemistry and modified HCCO + NO reaction rates. The TSL m odel allows continuous chemical reaction during the mixing while preserving some of the important characteristics of the jet. The model predicts 50 to 80% NOx removal depending on the thermal input of the reburning fuel, init ial concentration of NOx, and injection temperature of the overfire air and N-agent. The comparison with the experimental data shows good agreement fa r relatively high reburn zone stoichiometric ratios (SR similar to 0.99) wh ile the removal of NO is overestimated for richer conditions (SR similar to 0.95 or lower). The model in general follows the trend observed in the exp eriment and is able to quantitatively predict the NO removal in the gas reb urning process. The improvement of these predictions over previous modeling efforts using a PFR and time-distributed instantaneous mixing, suggests th e importance of modeling the mixing in reburn calculations. (C) 1999 by The Combustion Institute.