Mini plants for biogas production
On small to medium-sized livestock farms, anaerobic digestion holds interesting and largely unexplored opportunities. The technologies available today make the energy conversion plant economically viable, thus helping to reduce dependence on fossil fuels
Beside reducing dependence on fossil fuels, power generation from renewable sources has always been seen as a solution that could diversify operators' income. Given the current geopolitical upheavals and the resulting spikes in the prices of gas and petroleum products, a further effort toward exploiting renewable sources is probably the best way forward to continue meeting energy demand while not negatively impacting the environment.
Particularly in Northern Italy, the anaerobic digestion of fermentable biomass to produce biogas to be exploited for electricity generation or biomethane is undoubtedly the most successful agro-energy chain. The wide availability of biomass (e.g., livestock manure), together with the possibility of producing more of it specifically for digestion (grain silage) and the implementation of (all too generous...) incentive policies, have in the recent past favored the spread of agricultural biogas plants. Moreover, the incentives initially provided were later analytically reshaped and are now granted according to plant size and the type of biomass exploited. This favors less than 300 kW plants fueled with waste biomass and/or by-products. Recently, the "Milleproroghe" decree extended incentives to 2022 for biogas plants below 300 kW, which is most common on livestock farms.
Despite this paradigm shift, a large part of the plants built even after the rate revision still involve supplying the digesters with dedicated biomass (often silomix), albeit in co-digestion with wastewater and/or other agricultural and agro-industrial by-products.
Mini plants
Mini-biogas plants (indicatively of less than 50-100 kW capacity), strictly fed only with wastewater and by-products, are still very few in number. In fact, while the use of by-product matrices reduces feeding costs (especially in the case of short transport distances), it must be considered that these biomasses (and wastewater in particular) have extremely modest specific biogas productions (e.g., for pig slurry only 8-10 m3/t of biogas), thus requiring the construction of medium to large sized digesters, with limited production rates.
However, as of today, a still relatively unexplored share of the biogas market lies in the potential of small- to medium-sized livestock farms (from 70-100 heads of adult cattle). This is still a widespread farm size in central and southern Italy, which the leading producers of biogas plants do not account for because of their limited raw material production. This mismatch between the biomass available on the farm and that required for efficient plant utilization has prevented small agribusinesses from investing in the agro-energy sector.
However, mini-biogas plant solutions have recently been developed, which can best suit farms of limited size with reduced energy supply costs while enhancing local biomass, whose management usually accounts for cost.
Available solutions
Generally speaking, mini plants for biogas production are well suited to the size of the farm, resulting highly effective even in small settings: for smaller plants, it is possible to start with as few as 40-50 cows or 1,000 pigs, although solutions are also available for exploiting wastewater from buffaloes, rabbits, and poultry.
Furthermore, although designed to be supplied mainly with livestock manure, it is also possible to advantageously exploit these plants for other types of by-products, such as whey, residues from winemaking and sugar beet processing, and brewers' grains, etc.
Typically, plants of this type are simpler than those of a few hundred kW in capacity up to 1 MW. This is due both to the need to reduce costs and also because powering is usually done with biomass with a low dry matter content, so the feeding and mixing apparatus for the digesters are rather simple. The area required is also tiny: For instance, the plants proposed by Micro Biogas Italia (Bioeletric), with power ranging from 11 to 44 kW, need an area between 100 and 250 m2.
Even with some differences offered by the different manufacturers, generally, the main parts of the plant are: the digester usually a 2.5 m high tank made of insulated and waterproof prefabricated stainless-steel panels topped by a double or triple membrane gasometer dome for storing the biogas produced and equipped with the mixer and a heating system to maintain the correct process temperature even in the coldest months; a technical room (often a simple container) housing the electrical components; the cogenerator, the digester loading and unloading systems, the biogas treatment devices (filters, desulfurization circuit, dehumidifier), one or more heat exchangers for heat recovery from the cogenerator flue gas; the support boiler; and the biogas quality analyzer.
The service room is usually delivered "turnkey," therefore requiring only to be connected to the digester. The latter can be placed on a simple concrete base: plant installation and set-up take only 5-10 days, while the biogas production can start after a 4–8-week start-up phase.
The results
The investment for the plant differs with its size. Compared to larger ones, the specific cost per power unit is logically significantly higher, although the overall expense is lower. During the production phase, the main costs are related to process monitoring, any minor maintenance to the cogenerator and several feeding and mixing devices. Typically, manufacturers offer the maintenance service at the cost of 0.04-0.05 euros/kWh of energy produced. Moreover, finding the matrices is very often a negligible cost, as they are all waste biomasses.
Revenues depend, obviously, on the amount of electricity that can be harnessed to meet business needs or fed into the national grid and valued according to incentive mechanisms, which currently recognize 0.233 euros/kWh. Thermal energy gained from engine cooling and flue gas heat recovery is partly used to regulate the temperature of the digesters. The thermal surplus is usually wasted, even though it could be conveniently utilized to meet other needs (space heating, drying of fodder). Heat valorization is generally limited due to the opposite seasonality between the time when the heat surplus is highest (the summer) and the period when high demands occur (the winter period).
A practical example
For a 35-kW plant powered by farm-sourced livestock manure, considering: an initial investment of 265,000 euros and additional installation costs of 10% of the base cost; a maintenance fee of 0.04 euro/kWh; an electricity self-consumption of 11%; 500 euro/year of contingencies not included in the service charge; electricity valuation at the incentive rate; a 4% discount rate results in an NPV (Net Present Value) of about 350,000 euros, with a payback time of just over 7 years.
Thus, the results are remarkably interesting, particularly considering that these plants require minimal human resources from the farmer and are fueled only with waste biomass, the traditional management of which would entail not only a cost but also a non-negligible impact on the environment because of the methane, ammonia and nitrous oxide emissions occurring during the storage period.
