Conventional liquefied biomethane (LBM) production plants consist of upgrading and liquefaction processes. Commonly, the captured CO2 during biogas upgrading is emitted into the atmosphere. One alternative, to produce more LBM, is to combine the conventional LBM production plant with the Power-to-Gas concept. Within this context, green H2 produced from renewable electricity can react with the CO2 in the biogas mixture, producing additional CH4. The present study evaluated the feasibility of integrating methanation with conventional LBM production plants. The performance of the proposed process designs was assessed through exergy and cost analysis considering detailed models. The results illustrated that integrated direct biogas methanation was superior to methanation of captured CO2. The integrated scheme with direct biogas methanation increased the LBM production by 52.2 % compared to the conventional LBM production plant but with lower exergy efficiency. It was indicated the feasibility of the integrated scheme through direct biogas methanation was highly dependent on the price of H2. A H2 price of 1.27 USD/kg H2 was required to produce LBM at a similar price as the conventional LBM production plant.

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.

The microbial diversity in anaerobic digestion (AD) is important because it affects process robustness. High-throughput sequencing offers high-resolution data regarding the microbial diversity and robustness of biological systems including AD; however, to understand the dynamics of microbial processes, knowing the microbial diversity is not adequate alone. Advanced meta-omic techniques have been established to determine the activity and interactions among organisms in biological processes like AD. Results of these methods can be used to identify biomarkers for AD states. This can aid a better understanding of system dynamics and be applied to producing comprehensive models for AD. The paper provides valuable knowledge regarding the possibility of integration of molecular methods in AD. Although meta-genomic methods are not suitable for on-line use due to long operating time and high costs, they provide extensive insight into the microbial phylogeny in AD. Meta-proteomics can also be explored in the demonstration projects for failure prediction. However, for these methods to be fully realised in AD, a biomarker database needs to be developed.

The inevitable nexus between energy use and CO2 emission necessitates the development of sustainable energy systems. The conversion of CO2 to CH4 using green H-2 in power-to-gas applications in such energy systems has attracted much interest. In this context, the present study provides a thermodynamic insight into the effect of water removal on CO2 conversion and irreversibility within a CO2 methanation reactor. A fixed-bed reactor with one intermediate water removal point, representing two reactors in series, was modeled by a one-dimensional pseudo-homogeneous model. Pure CO2 or a mixture of CO2 and methane, representing a typical biogas mixture, were used as feed. For short reactors, both the maximum conversion and the largest irreversibilities were observed when the water removal point was located in the middle of the reactor. However, as the length of the reactor increased, the water removal point with the highest conversion was shifted towards the end of the reactor, accompanied by a smaller thermodynamic penalty. The largest irreversibilities in long reactors were obtained when water removal took place closer to the inlet of the reactor. The study discusses the potential benefit of partial water removal and reactant feeding for energy-efficient reactor design.

Biomethane production from biogas can be increased by methanation of carbon dioxide with hydrogen through the Sabatier reaction. In this work, the performance of the methanation process is investigated under isothermal and adiabatic conditions for different temperature and pressure levels. The processes were modelled assuming equilibrium conditions, minimizing the Gibbs free energy. The results indicate that the exergy of heat removed from the process, and thereby the integration potential, increases with increasing temperature. The internal irreversibility is smaller and the heat integration potential larger for adiabatic reactors than for isothermal reactors.

Production of liquefied biomethane (LBM) from biogas comprises two major energy intensive processes; upgrading to increase the methane concentration and refrigeration to liquefy the upgraded biogas. Amine-based absorption has been considered an attractive option for biogas upgrading in industrial applications. The temperature increase associated with amine regeneration is, however, in conflict with the cooling requirement of the subsequent liquefaction process. Hence, cryogenic biogas upgrading, integrated with liquefaction, has emerged as an interesting alternative. In this paper, a rigorous energy analysis was performed for comprehensive models of the two aforementioned LBM production alternatives. Both processes were modeled using Aspen HYSYS (R) and optimized to minimize the energy use. The results indicate that the integrated cryogenic upgrading process is favorable in terms of both overall energy efficiency and methane utilization. Moreover, the energy analysis implies that the liquefaction process accounts for the major part of the energy input to an LBM plant, demonstrating the significance of improving the energy efficiency of the liquefaction process in order to improve the overall performance of the LBM process.

The present work proposes a methodology for optimization of a liquefied biomethane (LBM) production plant. The LBM production plant comprises amine-based absorption upgrading followed by a single expander refrigeration cycle. The processes were modeled using Aspen HYSYS® and optimized through a Sequential Quadratic Programming algorithm. Any changes in the operating conditions of the upgrading process will affect the cooling demand in the liquefaction, while the opposite is not true. Based on this, a sequential optimization approach starting with the upgrading process is proposed. In order to accommodate the connection between the processes, different objective functions were formulated for the sequential optimization approach. The results from the sequential approach were compared with an overall optimization approach, where the entire LBM plant was optimized simultaneously. The results indicate that the same solution was obtained both for the sequential approach and the simultaneous approach. For the sequential approach, however, the best result was observed when the interaction between the upgrading and liquefaction processes was accounted for by considering the effect of the upgrading process on the exergy requirement in the liquefaction process.