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Show Metals Reactions and Vaporization The heart of the metals partitioning model is the reaction/vaporization submodel which predicts metals chemical reactions and phase behavior using the program CET85[7], developed by NASA's Lewis Research Center. This program utilizes a free-energy minimization approach which is based on two assumptions: -all reactions achieve equilibrium at local conditions; and -all elements (coal and combustion air) are intimately mixed. This equilibrium approach provides conservative upper-limit estimates for uncontrolled metals emissions and allows the impact of various parameters on metals reactions to be assessed[8]. It is recognized that equilibrium may not be maintained throughout a coal combustor; however, the equilibrium assumption is reasonable when considering the high temperatures typical of coal combustion chambers. Sufficient kinetic data are not presently available to develop a detailed kinetic model of metals behavior. Metals Partitioning Model Combustion System Thennal Analysis Aerosol Dynamics Submodels Air Pollution Control Submodel Figure 1. Illustration of the components of EER's metals partitioning model. The equilibrium program can be used to predict the vapor pressure of each metal species and the resulting amount of each metal partitioned in the combustion gas by vaporization. Additionally, the program provides metals speciation informatjon for both the condensed and vapor phases. The ability to predict and analyze the metals vaporization process is largely dependent on the accuracy and the completeness of the data on the thermodynamic properties of metal species. The equilibrium program has to be provided with thermodynamic data on all of the possible combinations of metal compounds which form under combustion conditions. The thermodynamics database normally provided with the NASA code has limited data on toxic metals. For example, metals such as Sb, As, Cd, Se, TI, and many toxic metal-earth element (Ca, 5 |
