Extensive thermodynamic and economic analysis of the cogeneration of heat and power system fueled by the blend of natural gas and biogas

Due to the finite resources of fossil fuels and their role as the most important factor in air pollution and climate change, the use of renewable energy sources is the best solution to meet the power requirements. One of the appropriate technologies in terms of energy saving and power generation with high efficiency is using the cogeneration of heat and power in which renewable fuels are used instead of fossil fuels. In the present work, a cogeneration system, consisting of a gas turbine cycle with power generation capacity of 30 MW, a steam generator, an anaerobic digester, and a heat exchanger for heating the digester and pretreatment system, has been applied. The purpose is the thermodynamic and the exergo-economic evaluation of a cogeneration of heat and power cycle (CHP), considering the three objective functions of first and second law efficiencies, and the total specific cost of the system. The combined heat and power plant was fueled by the mixture of natural gas and biogas. A parametric investigation was conducted to assess the influences of decision parameters on the performance and the total cost rate of the cycle. The results showed that in the absence of biogas (high mixing ratios) the irreversibilities of the combustion chamber and the heat recovery steam generator are high, and at low mixing ratios those of the combustion chamber and the anaerobic digester are significant. The irreversibility of the combustion chamber at all the mixing ratios is the highest value. The exergy efficiency of the cycle is increased from 46.94% in the case of pure biogas to 50.64% in the case of pure natural gas. Also, the total specific cost of the system is increased from 66.7 $/MWh in the case of pure natural gas to 98.71 $/MWh in the case of the pure biogas. Dual fuel GT-CHP is an interesting option to access for high efficiency, high flexibility and plant reliability at low cost in comparison to only biogas systems, facilitating a blend of fossil fuel and renewable resource systems.