Agricultural biomethane: technology, sustainability and outlook
To obtain biomethane, the upgrading process, i.e. the removal of carbon dioxide from the biogas, is essential. Today, several efficient technological solutions are available which, supported by clearer regulations and incentives, can facilitate its deployment
The need to reduce greenhouse gas emissions accelerates the transition towards renewable energy sources. The decarbonisation of energy and the resulting search for sustainable solutions are at the centre of the energy strategies of many countries, including Italy. Although characterised by the supply of many niche products, in Italy agricultural and agro-industrial activity is focused on several sectors (dairy and livestock, oil, wine, potatoes, etc.) that generate considerable volumes of by-products and organic residues. Before the industrialisation of these sectors, the management of so-called 'waste' took place locally, according to what could be called a circular economy ante litteram. Today, however, their management in most cases accounts for a quite significant share of production costs. Thanks to the advent of new technologies, the evolution of once non-optimised solutions and the updating of energy policies, these by-products and agro-industrial residues are now seen as a resource, which can be converted into clean and sustainable energy.
Biomethane. Also in the light of a regulatory framework that is finally clearer, the biomethane production chain, deriving from the purification of biogas produced as a result of the anaerobic digestion of biomasses of agricultural origin, is becoming increasingly interesting. This is a promising process for several reasons: not only does it offer a sustainable alternative to fossil fuels, thus contributing to the reduction of greenhouse gas emissions, but it can also represent a further source of income, supplementing and diversifying traditional activities. The possible uses of biomethane overlap with those of natural gas. Indeed, biomethane can be used as a fuel for cogeneration (electricity and heat), in centralised plants and/or associated with large thermal or domestic users, and as a fuel for refuelling stations located close to the production plant (taking into account a suitable gas distribution network). Biomethane is thus an energy carrier that is flexible, and therefore more efficient, than the biogas from which it is derived. This efficiency is even more significant if one takes into account that today biomethane can also be used in centralised plants, located exactly where the production of thermal energy can be adequately exploited. Therefore, it is worth going into detail about the peculiarities of these systems, analysing their technological and operational aspects, potential, criticalities, but also their level of economic and environmental sustainability.
Technological and operational aspects. In the anaerobic digestion phase, the basic elements of the biomass (carbohydrates, proteins and fats) are decomposed by specific micro-organisms, in the absence of oxygen, and converted approximately 60 % to methane and 40 % to carbon dioxide. A key aspect for process stability is temperature control: the presence of a cogenerator in the use of biogas to power an endothermic engine coupled to a generator guarantees more than enough heat to heat the digester.
Purification and upgrading. In addition to methane and carbon dioxide, 'raw' biogas contains water vapour and other impurities, such as gaseous ammonia and traces of hydrogen sulphide, which, albeit in a limited percentage, must be removed through a process known as purification and 'upgrading', at the end of which a gas consisting of 95-98% methane is obtained (Figure 1).
Purification involves dehydration, desulphurisation, and removal of gaseous ammonia and dust, and is implemented through the combined use of chillers (for dehumidification), scrubbers (for hydrogen sulphide removal), filters, including activated carbon filters, etc. Upgrading, on the other hand, is aimed at the elimination of carbon dioxide, and can be achieved through various methods, such as compression, adsorption, separation through selective membranes and cryogenic separation. Among the different options, separation by selective membrane does not require the use of chemical additives, as the biogas, previously compressed, simply passes through a polymer membrane, which retains the carbon dioxide and other impurities through selective permeability, with excellent process efficiency (Figure 2). Among the many options on the market, AB's BioCH4nge is available in standardised sizes from 150 to 2,500 Nm³/h, complete with pre-treatment systems that can also be used with already operational biogas plants. The biogas extracted from the gasometer dome or gasometer is first filtered and dehumidified, then compressed and cooled, and finally sent to the next treatment step. In order to remove impurities, the biogas is passed through an activated carbon bed which, thanks to the possibility of reversing flows, the presence of bypasses and the individual filter sectioning, guarantees maximum flexibility. Subsequently, the dehumidified and purified gas is ready for upgrading, which is based on the use of selective membranes. Biomethane is produced at a pressure in a range of 7 to 15 bar to minimise consumption, but also facilitate feeding into distribution networks where necessary.
Biomass supply. One of the operational aspects to be carefully evaluated, already at the plant design and dimensioning stage, is the supply of biomass. As with biogas plants, biomethane plants must also operate continuously, and it is therefore necessary to ensure an adequate supply of organic matrices, possibly to be met with appropriate storage dynamics. For plants fed with agro-industrial by-products (e.g. industrial crop processing residues, wine-making by-products), the seasonality of the different biomasses to be processed must be considered
Possible criticalities. First of all, the initial investment for the construction and start-up of a biomethane plant is certainly significant compared to biogas. However, in the absence of a cogenerator, purification and upgrading systems have to be foreseen, which bring the overall cost per unit of digested biomass to a higher level than is typical for biogas. Another aspect that objectively held back the spread of biomethane in the past was regulatory uncertainty regarding the incentive mechanism; this has now been largely overcome, and there is now legislative certainty in this regard. While from an environmental point of view, and in particular for the emission of climate-altering gases, the production and use of biomethane is more sustainable than natural gas, some local-scale impacts such as land use, digestate management, and the production of acidifying and eutrophying substances can be a problem. In detail, with regard to carbon footprint, biomethane is essentially 'carbon neutral' because during its use, carbon dioxide that was previously fixed in the biomass through photosynthesis is reintroduced into the atmosphere. Thus, even taking into account the emissions associated with biomass transport and storage operations and those associated with the construction and operation of the plant, compared to natural gas of fossil origin, the reductions in carbon footprint are considerable and in the most favourable cases can exceed 70%. In addition, it must be considered that the digestion of biomasses that are not always of local origin results in increased nitrogen and phosphorous loads in the vicinity of the plant. The management of the digestate produced must therefore be rationalised because, while it is true that it is an organic fertiliser that can be easily used, its transport over long distances is uneconomical.