(739a) Superstructure Optimization Approach for the Optimization of Biogas Upgrading into Biomethane

The amount of residues generated by society represents a challenge in terms of treatment but, at the same time, it is also an opportunity towards the production of sustainable resources and energy within the circular economy concept (Korhonen et al., 2018). Among the technologies, anaerobic digestion is deemed as an interesting and promising alternative (Fehrenbach et al., 2008). The biogas generated in the anaerobic digestion has traditionally been used as an energy source. It is possible to directly use biogas in gas turbines (Somehsaraei et al., 2014), or in generators (Reddy et al., 2016). However, the large infrastructure available for the shipping of natural gas in Europe (Entsog, 2015) or the US (EIA, 2018), suggests the purification of the biogas to achieve a composition similar to natural gas, also referred to as upgrading, as an alternative for the generation of renewable energy to be evaluated. There are several paths to achieve this purpose. On the one hand, it is possible to hydrogenate the CO₂ within the biogas into methane (Curto and Martín, 2019). The main issue is the need for large amounts of renewable energy for the generation of hydrogen in order to avoid the generation of negative environmental impacts. On the other hand, CO₂ capture technologies can be used. A number of reviews have been presented comparing different technologies for CO₂ capture in general (Angenlidaki et al., 2018), but no systematic comparison and techno economic analysis has been developed for biogas upgrading.

A hybrid heuristic-mathematical optimization approach has been proposed to select the technology and the operating conditions for the upgrading of biogas into biomethane produced from different wastes sources including manure, urban food waste and sludge. First, a prescreening or heuristic stage evaluates various technologies including water scrubbing, physical and chemical absorption, pressure swing adsorption, cryogenic separation, and membrane separation systems. In a second stage, alkali solvents, adsorbent media and membrane materials are selected. Finally, a superstructure of technologies is formulated to select among three different amines, MEA, DEA and MDEA, two different zeolites, 13X and 4 A and three membrane materials, cellulose acetate, polyimide and polycarbonate. The entire process is modeled from the digester to the production of synthetic biomethane using mass and energy balances, experimental data, thermodynamic models, industrial rules of thumb, etc. We optimized the upgrading process for the different waste sources considered: cattle and pig manure, food waste, and sludge.

The optimization results show that among the amines, DEA is preferred. Among the zeolites, the 13 X and among the membrane materials, the polyimide is selected. The best technology among them all is pressure swing adsorption using zeolite 13X as adsorbent. Food waste is the most promising waste due to the largest organic matter content, resulting in an investment of 67 M€ and a production cost of 0.36 €/Nm³ for the processing of 10 kg/s of waste. The comparison of the upgrading of biogas using CO₂ capture technologies with the direct CO₂ hydrogenation (Curto and Martín, 2019), or the direct production of synthetic methane from CO₂ hydrogenation (Davis and Martin, 2014), is in favour of the direct hydrogenation of biogas for areas of large availability of solar or wind energy. However, for higher electricity costs, CO₂ capture is suggested. Scale up/down studies are carried out to evaluate the effect of city and farm size on the investment and production costs of biomethane production

References

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