Biogas production has been on the rise in Europe in the last two decades as countries seek to reduce their dependence on fossil fuels and meet climate change targets. Biogas plants can range greatly in terms of installed capacity, from micro plants in developing countries, small-scale plants used on farms, and large-scale plants used in centralised systems in cities [1
]. Although mostly used to generate electricity and heat, biogas can also be utilised to create biomethane, a fuel that can replace natural gas in transportation and industrial applications. Biofuels are the only commercially viable alternative already utilized for transport and industrial needs [2
Up to 72% of the feedstocks used for biogas production come from the agricultural sector [3
], primarily maize silage. The competitive use of biogas feedstocks with food and feed production has raised not only environmental but also socioeconomic concerns, reflected in new sustainability requirements defined by European Union (EU) legislation [4
]. The revised Renewable Energy Directive, which came into effect in December 2018, has established sustainability and the greenhouse gas (GHG) emission-savings criteria with which biogas used in transport, electricity, heating, and cooling production, must comply. The new Directive enhances the sustainability requirements for biogas feedstocks and adds new requirements for specific greenhouse gas emission savings from biogas production, which biogas facilities must adhere to in order to contribute to renewable energy goals and qualify for government funding. As one of these criteria, the Directive states that the GHG savings from the use of biomass for heating, cooling, and electricity production must be at least 70% for plants that started to work in 2021 and 80% for plants starting operation in 2026 [5
]. The Directive also defines typical and default values for GHG savings for the three mostly used biogas feedstocks: manure, biowaste, and maize whole plant (maize silage).
A significant amount of sustainable feedstocks and a thorough understanding of the sustainable potential of biomass supply are necessary to achieve GHG savings [6
]. The utilisation of materials previously regarded as waste, such as agricultural and industrial residues and by-products, is receiving increased attention, as it not only improves the sustainability of biogas production but also improves waste management and resource efficiency.
Research Problem and Literature Review
The environmental sustainability of biomass utilisation for energy purposes has raised significant concerns. It has been reported that biomass utilisation may result in unsolved challenges and trade-offs concerning the accounting of GHG and non-GHG emissions [7
]. The environmental sustainability of biomass utilisation is a complex problem which depends on various factors such as the feedstock type, feedstock preprocessing and processing technology, transportation and distribution distance, emissions from the fuel in use, etc. Because of its high complexity, the importance of this problem has increased in the last decades. Hence, a significant number of research papers have investigated various types of environmental sustainability performance of biomass utilisation for energy production. Some of them are presented in the following paragraphs.
Hamelin et al. [8
] performed the life cycle assessment of biogas production based on manure and the following co-substrates: straw, garden waste, food waste, energy crops, and animal urine and faeces. The results, given in kgCO2
eq per functional unit, prioritised source-segregated solid manure as co-substrates, followed by straw and biowastes, while energy crops were identified as co-substrates whose utilisation would result in adverse environmental impacts. In their recent work, Meng et al. [9
] examined the viability of total or partial replacement of peat by maize straw biogas residues and manure biogas residues. The results showed that a biogas plant that produced 10,000 m3
biogas daily could achieve savings of 439.4 tonnes/year of CO2
through the proposed replacement. Den Boer et al. [10
] calculated that using kitchen waste for biogas production could lead to 680,000 tCO2
eq savings per year.
The transport distance of the biomass supply and biomass availability throughout the year have a significant impact on the energy conversion efficiency and GHG reductions in anaerobic digestion (AD) technology [11
]. Anaerobic digestion (AD) is a collection of processes by which microorganisms break down biodegradable material in the absence of oxygen. The results of AD are biogas and digestate. Berglund et al. [12
] performed the energy life cycle analysis of eight feedstocks for biogas production. The results showed that the difference between energy output and input was positive in the cases of transport distances less than 700 km for slaughterhouse waste, 580 km for municipal organic waste, 240 km for straw, 220 for pig manure, and 200 km for cow manure. In their study, Uusitalo et al. [13
] concluded that using biogas to produce heat and electricity leads to greater GHG reductions than composting feedstock, yet not as high as in the case of its utilisation for transport. In the work of Balcioglu et al. [14
], the authors calculated that if 60% of cattle manure and all available chicken manure in Turkey were co-digested with other waste feedstock, this could lead to annual GHG emissions reduction of up to 2.5%. Wąs et al. [15
] assessed the GHG mitigation potential of biogas production that uses agricultural waste and manure as biogas feedstock in Ukraine. Results indicated that the theoretical potential of GHG savings ranged between 5% to 6.14%, while technical potential varied between 2.3% to 2.8% of total GHG emissions. Tamburni et al. [16
] calculated that biogas production from agricultural waste could result in GHG emission savings of up to 3,000,000 MgCO2
eq in the Emilia Romagna region.
As can be seen from the literature review, environmental sustainability and GHG savings have been studied extensively, obtaining results in different forms using a variety of methods. However, there are still numerous feedstocks recognised as novel feedstocks for biogas production (mostly so-called waste materials), but the constraints to achieving required GHG savings still need to be defined, as there are limited data on GHG emissions from the biogas production chain that can serve as typical and defaults limits. To address this gap, the research object of this study was to define the maximal transport distance of various novel biomass feedstock that complies with the GHG savings of 80%, compared with fossil fuels, as required in Directive 2018/2001. This was calculated for agricultural residues, municipal biowaste, and industrial by-products. This work hypothesised that all considered feedstocks would achieve the requested GHG savings (80%) for transport and distribution distances up to 50 km. The method used for the calculation is presented in the section below.