Numerical study on the SNCR application of space-limited industrial boiler

Considering that the direct application of SNCR method to existing industrial boiler is usually known as not quite satisfactory mainly due to the insufficient residence time by the limitation of SNCR application space, it is quite interesting to examine whether it is possible to improve the efficiency of SNCR reaction by the adjustment of design and operational parameters. Especially one of novel concepts is the introduction of auxiliary air employed for the increase of turbulence mixing of reduction material in SNCR injection system. To this end a comprehensive computer program is developed using SIMPLE algorithm by Patankar in order to evaluate the efficiency of SNCR system for the boiler with 40 tons of steam/h using heavy oil. The computer program is made in 3-D rectangular coordinate using various phenomenological models. For example, standard k-epsilon turbulence model and typical eddy breakup model are incorporated for the Reynolds stresses and turbulent reaction of major fuel species, respectively. However, detailed data of chemical kinetics are also incorporated for the process of NO formation together with the NH3 reduction reaction of SNCR system. The complex coupling phenomena between combustion chemistry and turbulence are resolved by the strategy of harmonic mean expression assuming proper empiricism. Further, the calculation of droplet trajectory and volatilization is incorporated in Lagrangian frame assuming eight possible trajectories in each grid by the consideration of the calculation efficiency and accuracy. The validation of program developed was made by the comparison with the measured temperature profile and a series of parametric investigations have been performed to enhance the NO removal efficiency. The major variables considered in this study are droplet diameter, the injection location and amount of reduction agent together with the introduction of auxiliary mixing-enhanced air for the reduction material. Based on the results of the calculation, it is found that the removal efficiency of NO was improved to a significant amount by the increase of penetration depth of the reducing agent into the center region of boiler. The increase of penetration distance and thereby the enhancement of mixing efficiency was obtained either by the employment of the mixing air together with the increase of the injection velocity and droplet size, respectively. Based on this study, the successful application of SNCR method even for the space-limited industrial boiler can be achieved by the proper increase of the mixing of reducing agents. (C) 2006 Elsevier Ltd. All rights reserved.