ABSTRACT

Exergoeconomic modelling and multi-objective optimization of a novel Kalina-based polygeneration system driven by an in-service Brayton-cycle with integrated biomass-gasifier was performed. A double pressure heat recovery steam generator, primarily powered by gas turbine waste was utilized to power five other bottoming cycles. These cycles comprised a steam turbine, a novel organic Rankine cycle for cooling and power production, and a modified Kalina cycle for power production and cooling with an additional vapor absorption system for cooling, powered by the Kalina cycle. In total, the system generates seven products from natural gas and biomass syngas used as a reheater from the gas turbine. To simulate the system performance, very detailed thermodynamic models were developed from both energetic and exergetic approaches across all components of the system. Additionally, the exergoeconomic cost implications of the overall system were developed based on specific exergy costing, as well as development of the environmental impact assessment from exergo-environmental considerations. The exergo-sustainability indicators of the system were also considered, and the plants’ optimum operating parameters obtained, based on a multi-objective optimization of the efficiency and total levelised cost of operation. Consequently, a program source code was developed to accommodate this robust system at varying operating conditions. From the analysis, the results obtained showed that the multigeneration energy system had an enhanced performance. The exergy efficiency was improved from 35.79 % for the topping cycle alone to about 44.26 % for the multigeneration system. This configuration gave reasonably high exergy efficiency with respect to recent multigeneration systems in literature with estimated 17.3 % and 36.13 % exergetic efficiency. Moreover, the multiple output condition of this system resulted in 16.745 MW of power from the steam turbine and 71.162 MW from the topping cycle. Additionally, as much as 42.28 kW of power and 45.36 kW of cooling resulted from the utilization of low pressure steam from the heat recovery steam generator (HRSG) to power the modified organic Rankine cycle (ORC) system. Furthermore, dual refrigerative cooling of 67.72 kW and 130.5 kW were obtained from the utilization of waste heat from the HRSG to power the Kalina and the vapour absorption systems, respectively. This was in addition to Kalina turbine output which was 36.27 kW. The results from the exergetic sustainability demonstrate improvement in the overall system performance in terms of environmental effect factor and exergetic sustainability. From the optimization performed, it indicated that several pareto frontiers including that at a pressure ratio of 8 corresponding to the maximum efficiency, and least cost rate of 39.50 % and 125.8361 $/hr respectively, were produced. The simulated results revealed that provision of this system translated in monetary savings of about 77.0453 $/hr, hitherto lost to exergy destruction and loss from the flue gas in the gas turbine plant. With an estimated 25 years plant life, these amount to lifetime savings of $ 12,327,248 in plant operation from the waste gas of the in-service plant. Overall, this plant generally is economically viable, cost-effective and reliable for production of energy.