Exergy analysis of waste incineration plant: flue gas recirculation and process optimization

Simulations of two incineration processes, with and without flue gas recirculation, have been carried out performing also an exergy analysis, to investigate on the most critical equipment unit in terms of second-law efficiency. Flue gas from the economizer outlet is employed to partially replace secondary combustion air, to reduce, at the same time, incinerator temperature and oxygen concentration. Conversely, in the proposed configuration the recirculated flue gas flow rate is used to control incinerator temperature, while the air flow rate is used to control the oxygen content of the fumes leaving the incinerator to be as close to 6% as possible, i.e., the minimum allowed for existing plants to ensure completion of the combustion reactions, and determines the corresponding minimum flue gas flow rate. The flue gas recirculation guarantees a larger level of energy recovery (up to +3%) and, at the same time, lower investment costs for the lower flow rate of fumes actually emitted if compared to the plant configuration without flue gas recirulation. Various operating parameters were varied (incinerator's effluent gas superheating temperature, air flowrate, oxygen % in air flowrate, flue gas recirculation flowrate) to investigate on their influence on process exergy efficiency. Exergy analysis allowed to individuate the equipment units characterized by larger exergy destruction, i.e. water condenser (exergy efficiency=0.31) and incinerator (exergy efficiency=0.48), and demonstrated that the flue gas recirculation led to an overall process exergy effiiciency increase of about 2.5%. Basing on the obtained results, a further otpimization of the process was proposed substituting the water condenser with an air-condenser, obtaining an increase of the unit exergy efficiency of about 13%.