Upgrading biogas to biomethane is a very interesting technology to purify biogas to recover methane, which can be used as an alternative to fossil fuel. Biogas is a renewable energy resource that can be generated from the anaerobic digestion of wastewater sludge and from many other industrial and agricultural wastes.
Biogas that comes from anaerobic digestion processes is a gas mixture mainly composed by methane (CH4 50-70%), carbon dioxide (CO2 50-30%) and other trace compounds (H2S, steam, etc.).
The biogas upgrading process consists in the removal of carbon dioxide, so that the resulting gas has a methane content and a calorific value very similar to that of natural gas, meeting the recently published grid standards (Italian Ministerial Decree, 16 May 2018). Thus, biomethane can be used directly as a fuel for transport or injected into the natural gas grid distribution that in Italy is particularly widespread throughout the peninsula.
Currently there are several commercial biogas upgrading technologies, based on chemical-physical processes, that allow the CO2 separation from biogas: water scrubber, chemical scrubber, pressure swing absorption, membranes and cryogenic treatments developed in the oil&gas field. Now these are the technologies used in existing plants (approximately 500 in Europe and 7 in Italy).
For a few years now, several research projects have been underway looking for innovative solutions, including the biological biogas upgrade, which not only allow to carry out the process with a lower energy consumption and a lower environmental impact, but also to make it applicable in a greater variaty of cases, for example when the biogas production potential is not very high, as in many purification plants.
In Italy, as part of the fossil fuel reduction targets agreed at European level, there is a specific target to reach the target of 10% by 2020 of renewable energy consumption in the transport sector, for advanced biomethane and other advanced biofuels of 0.9% by 2020 and 1.5% by 2022.
Biogas converted to biomethane through an upgrading process, makes it usable as an alternative to natural gas, that can be used as fuel or feeded into natural gas distribution networks.
Italy has a high potential for biomethane production, given the large number of existing biogas plants and the diffusion and articulation of the natural gas distribution and transport network that makes it easy to use in pipelines.
In the specific case of wastewater purifiers, the sludge from both primary and secondary water treatment allows the recovery of biogas through the anaerobic digestion process. As a guide, the wastewater from each inhabitant connected to a sewage treatment plant that digests the sludge can produce up to 15 litres of methane per day. Upgrading to biomethane is also very interesting in this area, for example to feed the fleets of vehicles serving Integrated Water Service companies or co-generators directly on-site, thus increasing the self-sufficiency of wastewater plants themselves and their environmental sustainability.
The existing upgrading technologies on the market have, as already mentioned, some limitations (investment costs, high energy expenditure, consumption of chemicals) and scientists are driven to study and propose new solutions. Within the PerFORM WATER 2030 project, two innovative technological solutions for biogas upgrading are being experimented: biologically, through a biochemical process via hydrogenotrophic methanogenesis and chemically, with eco-friendly solvents. In both cases experimentation are held at pilot scale, the purpose being to gather the scientific knowledge and technical informations needed to design full-scale plants, once the feasibility has been confirmed.
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