OCR Text |
Show The overall chemical reaction for reducing NOx with urea is: The optimum temperature window for NOx reduction with urea is 1850 of to 1950 of. At lower temperatures the reaction slows and requires increasing residence time. Enhancer chemicals and alternatives to urea have been developed which accelerate the reaction rates and extend effectiveness to lower temperatures. At high temperatures equilibrium conditions favor increasing concentrations of NOx.(7) Modeling of reaction kinetics and experience from experimental and field testing provide a basis for quantitative performance predictions. Effective NOx reduction is achievable using additives over the temperature range from 1200 of to 2100 of. The distribution of chemical to fully treat the combustion products in the regions of most favorable reaction conditions is essential. The chemicals used in the NOxOUT process are water soluble. Thus, they are easily injected through spray nozzles with a wide option of available spray patterns. A database of nozzle spray characteristics has been developed using laser equipment to measure droplet size and discharge velocity. Knowledge of the flow conditions within the unit to be treated is important to selecting the best process application conditions and making confident performance predictions. CFD modeling provides information for choosing the chemical formulation, injector locations, and nozzle performance requirements. Among the concerns addressed by the CFD model are the distribution of chemical in the flue gas, the amount of residence time available for reactions, and an approximate temperature profile. Furthermore, the model can warn of situations where sprays would impinge on tubes, which can cause failure. Nozzle placement must be such that droplet impingement is avoided. It was decided early in the development of the NOxOUT Process that benchscale or pilot-scale physical models were too restrictive for routine process engineering. These models required several people working many weeks to assemble the physical unit, and several weeks to run tests under controlled conditions. Once built, these models were difficult to modify, and the data taken from models running at ambient conditions were of questionable value when applied to high temperature droplet evaporation. The commercial availability of accepted computational fluid dynamics programs provided an attractive alternative for process simulation. The programs were adaptable and could incorporate as complicated a model as desired, with the restriction that the greater the computational demands, the longer the calculation time. The CFD model has become a valuable part of the NOxOUT technology by allowing simulations of customers' units on a project by project basis. -3- |