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Show laboratory and field data. The model describes an ideal plug flow and requires measured NOx ' 02' CO, and measured or estimated temperature-residence time profiles as input. Components of the model and the reaction mechanism have been presented elsewhere [12, 13] . Through this model, the effects of residence time, temperature, treatment rate, baseline NOx ' and CO concentrations are determined. In addition, the lowest achievable NOx concentrations, referred to as "critical NOx", have been identified. The model has also been useful in evaluating specific applications and is an integral part of the process technology. CFD Modelling of Furnace Process conditions such as flue gas temperature and flow fields are evaluated using computational fluid dynamics (CFD) modelling techniques. Flow modelling is performed using the PHOENICS CFD program developed by CHAM, with process-specific correlations provided through subroutines proprietary to Nalco Fuel Tech. A process unit is approximated as a set of mathematically defined cells that simulate the unit's physical geometry. In each cell, mass, energy and momentum are balanced to compute density, velocity, species concentration and temperature. This technique, called finite volume method, is widely used and accepted in CFD modelling. The description of the model, its role in the NOxOUT Process, and examples have been presented earlier [14,15]. Improvements have been made through the incorporation of separate convective and radiative heat transfer models, and by the use of a two equation turbulence model. Field test data are used whenever possible to verify the model. Actual performance is often different from the design. Direct temperature measurements reduce uncertainties that can arise from assumptions or estimates of such factors as furnace wall cleanliness, gas emissivity, and fuel characteristics. In operating units, flow distributions through burners, overfire air, and flue gas recirculation often vary from design values. CFD models have been created for more than 75 different units. A database has been gathered for selecting model parameters such as physical property or transfer coefficients. This experience has benefitted the model in establishing a high degree of confidence in its accuracy. CFD Injector Model Trajectories and evaporation of droplets from injectors are simulated through the multi-component droplet dynamics calculations in the CFD model. Sprays are modelled as sources of many individual droplets grouped into classes. Each droplet class has a different droplet size, velocity, or angle relative to the |
