Low temperature heat and water recovery from supercritical coal plant flue gas
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For this work, I constructed an original thermodynamic model to estimate waste heat and water recovery from the flue gas of a supercritical coal plant burning lignite, subbituminous, or bittuminous coal. This model was written in MATLAB as a list of linear equations based on first and second law analyses of the power plant components. This research is relevant because coal accounted for the largest increase in primary energy consumption worldwide as recently as 2013. Coal-fired electricity generation is particularly water intensive. As populations increase, especially in the developing world, much of the increased demand for electricity will be provided by new coal-fired power plants. One way to improve the efficiency of a coal-fired power plant is to recover the low temperature waste heat from the flue gas and use it to preheat combustion air or boiler feedwater. A low temperature economizer or flue gas cooler can be used for this purpose to achieve overall efficiency improvements as high as 0.4%. However, a side effect of the efficiency improvements is an increase in water consumption factor of nearly 10%. The water consumption factor can be reduced with the addition of a flue gas dryer after the flue gas cooler. The flue gas dryer is a condensing heat exchanger between the flue gas and ambient air. As the flue gas cools, its water content condenses and can be recovered and treated for use within the plant. In general, the results indicate that low temperature waste heat and water recover from boiler flue gas would be more feasible and beneficial for coal plants burning lignite as opposed to higher quality coal. Because these plants already have a lower efficiency, the relative increase in efficiency is somewhat higher. Similarly, the relative increase in water consumption factor is somewhat lower for a lignite plant. The high moisture content and dew point of the flue gas produced from lignite combustion makes it easier to recover water with a flue gas dryer. The higher water recovery factor along with the lower water consumption factor means that a greater percentage of the water evaporated in the cooling tower can be recovered in the flue gas dryer of a lignite plant than for a plant burning higher quality coal.