Industrial & Engineering Chemistry Research, Vol.58, No.42, 19571-19585, 2019
Sustainable Retrofit Design of Natural Gas Power Plants for Low-Grade Energy Recovery
Large amounts of low-grade energy in natural gas power plants are not being efficiently recovered or used. They are often emitted directly to the environment. It is quite challenging to retrofit natural gas power plants for the integration of low-grade energy recovery and further optimize power generation, production and supply of hot and cold energy, and pipeline networks for cold energy distribution as a total site. In this study, we present a process superstructure for the sustainable retrofit of natural gas power plants to recover low-grade energy and deliver it to different users. The waste energy sources include cold energy from liquefied natural gas, pressure energy from pipeline natural gas streams, and low-grade heat from low-pressure steam in power plants. A mixed integer nonlinear programming framework is formulated to optimize the process structure to minimize the total annual cost and maximize the exergy efficiency to obtain the best sustainable retrofit process for low-grade energy recovery. The optimization framework includes models of the existing gas turbines and boilers and the new pipeline network and refrigeration system. The mathematical framework is applied to a real industrial example, and the power consumption for cold energy production is significantly reduced by 32%. The ratio of different energy sources and the distance of cold energy distribution are investigated, and different optimization results are analyzed. Finally, exergy analysis is suggested to evaluate the sustainability of the retrofit. According to the results of the exergy analysis, the total exergy efficiency under the minimum total annual cost is increased by 6.172%.