Abstract
Conventionally, liquefied biomethane (LBM) is produced through biogas upgrading followed by a liquefaction process. In the present study, a detailed model for an LBM production plant including amine-based biogas upgrading and liquefaction was provided to compare thermodynamic and economic optimization for the biogas upgrading. In this context, multiple objective function formulations based on energy, exergy, and economy were examined. Furthermore, their impact on the exergy demand in the liquefaction process and the overall LBM production plant was investigated. The results indicated that optimization of the upgrading process based on exergy and total annualized cost would result in similar solutions, providing both the highest thermodynamic and economic performances, because the operating pressure was forced to be high to meet the strict CO2 limitations for LBM. However, the results also indicated that the exergy demand for the overall LBM production plant would be approximately the same regardless of the objective function formulation used for the upgrading process, as exergy savings in the liquefaction process would compensate higher exergy demand in the upgrading process. Overall, thermodynamic and economic optima of the LBM production plant would be similar if the LBM production plant was optimized based on exergy supply or total annualized cost. It was also illustrated that the selection of a suitable refrigeration cycle would have more impact on the overall performance of the LBM plant than the formulation of the objective function for the optimization.