An exergy-based analysis of gasification and oxyburn processes

An exergy analysis on gasification and oxyburn processes has been conducted. Equilibrium modeling in Aspen Plus© was used to develop a methodology for evaluating different fuels for gasification based on exergy analysis. The exergetic efficiency of gasifying a fuel strongly depended on the carbon boundary point, which is the equivalence ratio limit at which all carbon is converted to gaseous products in an adiabatic system. When evaluating a fuel for gasification, it is important to consider if the temperature of the carbon boundary point falls below 1050 K, which is also the temperature that the water-gas shift reaction begins to favor CO₂ and H₂. It was found that the rational efficiency, a common exergetic efficiency used in the literature, remained relatively unchanged at equivalence ratios past the carbon boundary point. A different exergetic efficiency, termed the gas efficiency, was proposed that showed better variance to the equivalence ratio and related better to the desired operation of the gasifier. It is shown that an oxyburn process can be used to decrease the energy requirement of capturing CO₂ if it is run closer to stoichiometric. Flue gas recirculation was investigated as a means to improve gasification efficiency, lower the reactor temperature of coal gasification, and capture CO₂. It was found that a higher percentage of flue gas must be recirculated back to the gasifier if flue gas from combustion with pure O₂ is used instead of air. Using flue gas recirculation allowed the gasifier equivalence ratio to be increased without solid carbon in the products. Increasing the equivalence ratio also resulted in a slight increase in the maximum achievable exergetic efficiency of the gasifier. Finally, an internal combustion engine model was developed based on closed-system thermodynamics and successfully integrated with the open-system realm of Aspen Plus©.