LIM Simulations of Mixing and Chemical Reaction in Gas Reburning Applications

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LIM SIMULATIONS OF MIXING AND CHEMICAL REACTION IN GAS REBURNING APPLICATIONS Nallan C. Suresh. Werner J .A. Dahm and Gretar Tryggvason Laboratory for Turbulence and Combustion (LTC) Department of Aerospace Engineering The University of Michigan Ann Arbor. MI 48109-2118 USA Achieving high NOx removal efficiency with gas reburning in any given installation requires tailoring the gas injection and mixing scheme to the distributions of temperature and NOx within the furnace. The injection scheme must also be robust enough to accommodate changes in these distributions under varying furnace load conditions. These considerations suggest that advanced modeling of the reburn gas injeclion and mixing can greatly assist effective implementation of reburn technology in most installations . Results are presented from local integral moment (LIM) simulations of gas injection and mixing for reburning applications. The LIM approach is fundamentally different from classical turbulence models. It uses the fact that molecular mixing processes in turbulent nows are concentrated on universal. self-similar. small-scale structures. The model incorporates this through a local parabolization of the complete time-dependent ~overning transport equalions on the lime-evolvin .~ material surface on which gradients are concenlrated . This leads Lo a closed cL of equation ,govcrning the local integral moments along the layer-normal direction at each point on the su rface. effectively tran forming the ori,ginal partial differential equations to a set of ordinary dill rcnlial equations that can be solved on a time-evolvin,C: surface . Results from validalion cases indicate that LIM simulations can provide accurate insights into the complex now and mixing processes that are essential to successfully implementing gas reburn in any given installation . 1. INTRODUCTION 4 NOx en1issions from practical combustion systems far exceed adiabatic equilibrium levels corresponding to the temperature and oAJ'gen concentration in the flue gas. This is due to non-adiabatic processes that cool the flu e gases along the furnace I ngth. quenching NO equilibrating reactions that would otherwise reduce the oxides of nitrogen produced in high temperature regions of the furnace near the burner. To reduce flue gas emissions to regulatory levels. various NOx reduction processes are employed or under development. These include modifications to burner designs ain1ed at lowering NOx levels produced in the primary conlbustion zone. as well as reburnind" of NOx in the upper furnace by injection and mixing of natural gas into the cOlnbustion products formed in the primary zone. For lar e installations such as utility boilers. or for high-tcnlpe rature industrial pro e e where NOx emissions are lar e. ga reburning can play an essential role in reducing en1issions to meet regulatory leve ls .