Development and Optimization of Processes for Liquefied Biomethane Production

Abstract

The contribution of renewable energies to the globally fast-expanding transport sector is the lowest among the other sectors like power generation. Many alternative fuels have been suggested to boost the green transition towards sustainable transportation. Liquified biomethane (LBM) has recently gained much attention within this context. LBM has similar characteristics as liquefied natural gas. Moreover, the abundance and origin of LBM from biogas make it an exciting energy source in the transport sector. LBM production involves multiple energy-intensive processes. Biogas upgrading to remove CO2 and low-temperature refrigeration to liquefy the final product are the most critical parts of an LBM production plant. For a long time, the development of processes regarding biogas upgrading focused on applications such as compressed gaseous fuel and gas grid injection, where a purity of 90-97 mol% of CH4 is required. Hence, the design of the biogas upgrading process complied with such purity requirements. The emergence of LBM as an alternative transportation fuel has imposed an even more restrictive purity requirement (i.e., CO2 content below 50 ppm in upgraded biogas known as biomethane). The liquefaction process after the biogas upgrading process is the main reason for considering such stringent CO2 requirements; exceeding the CO2 concentration limit in the biomethane can damage low-temperature heat exchangers due to CO2 ice-formation. Developing processes for LBM production that are energy-efficient and cost-efficient requires further considerations for the highly restrictive CO2 content in biomethane. Hence, the focus of this thesis has been to develop and optimize the design of LBM production plants through thermodynamic and cost analyses. Further, a novel process concept has been developed to convert CO2 available in the biogas mixture to additional LBM using the CO2 methanation process fed by renewable hydrogen. In this thesis, detailed process models of state-of-the-art technologies for biogas upgrading, CO2 methanation, and biomethane liquefaction have been simulated with a commercial process simulation tool. Amine-based absorption and cryogenic gas separation have been considered for the upgrading process. Different refrigeration cycles, including N2 expander cycles and single mixed refrigerant cycles, have been used for liquefaction. The CO2 methanation process model has been developed so that it can to be integrated in the LBM production plant. The processes have been optimized using a sequential quadratic programming (SQP) algorithm. Determination of potential synergies and overall energy efficiency improvements of LBM production plants due to integration of the upgrading and liquefaction processes has been performed by comparing LBM production using amine-based biogas upgrading and cryogenic biogas upgrading. The results indicated that integrating biogas upgrading with the liquefaction process using the cryogenic gas separation would reduce the specific energy requirement of the LBM production plant. However, cryogenic gas separation for biogas upgrading was associated with challenges regarding CO2 ice-formation that limit its application in practice, even with a better thermodynamic performance. Optimization studies have aimed to propose alternative approaches to improve the performance of the conventional LBM production plant using amine-based biogas upgrading. The results illustrated that the interaction between the upgrading and liquefaction processes within the conventional LBM production plant was limited to only the pressure level of the biomethane produced from the upgrading process. Hence, a sequential optimization approach was adequate to determine the optimal operating conditions for minimum exergy demand within the plant. Further, the results revealed that the thermodynamic optima obtained from minimizing the exergy supply and the total annualized cost for the upgrading process would be similar since operating at high pressure was required to satisfy the restrictive CO2 content specification. Concerning the total exergy demand within the overall plant, the difference between solutions obtained from different objective function formulations for the upgrading process would be insignificant. In this thesis, a comprehensive investigation has been carried out to design a CO2 methanation reactor considering the improvement of CO2 conversion and irreversibility rate within the reactor. It was observed that a series of methanation reactors with intermediate water removal operating under non-isothermal conditions could provide maximum CO2 conversion with an improved irreversibility rate within the reactor. Further, the required reactor length to perform CO2 methanation was determined. The results indicated that the CO2 methanation reaction could be run in a shorter reactor when the intermediate water removal was considered as the gaining for additional CO2 conversion due to extra length was not significant. Finally, a conceptual process design has been proposed to combine the conventional LBM production plant with the methanation process. Here, the feasibility of such a process concept has been thoroughly studied. The results illustrated that the methanation process could be partly responsible for upgrading; however, an additional polishing step was required to meet the CO2 content specification. The feasibility study concluded that the applicability of the proposed process design was highly dependant on the price of H2. Further, the overall exergy efficiency of the proposed concept could outweigh the exergy efficiency of the conventional LBM production plant if the available exergy of heat was utilized.