1. Introduction
The paradigm of development for the European biogas sector has changed significantly in recent years owing to higher sustainability requirements and cost production reduction as the main drivers of new trends in biogas production and utilisation [
1]. Most biogas plants (especially in less mature biogas systems) still use cultivated energy crops (primarily maize silage), which increase the inefficient use of arable land and compete with food production [
2]. Since such a practice is not in line with the principles of sustainable development, the biogas sectors among the European countries limited the utilisation of maize silage and corn to a share of 30–50% of the total input feedstock [
3,
4], and a further decrease in the use of maize silage is expected.
In 2018, the European Commission adopted a revised version of the Renewable Energy Directive (RED II), which stated that the biomass fraction of municipal waste, biowaste and streams from industry and agriculture [
5] should play a greater role in future biogas production since they have a low indirect land-use change impact to produce biofuel [
6]. In addition, the RED II determined that wastes and residues from agricultural activities and processing shall be considered to have zero life cycle greenhouse gas (GHG) emissions up to the process of the collection of those materials. This opens a space for these materials to enter the biogas sector and contribute to the transition towards more sustainable and efficient waste management systems [
7].
In addition to facing the feedstock-changing policy, biogas plant owners and operators also look towards alternative biogas utilisation pathways compared with producing heat and electricity under subsidy models [
8]. The biogas sector in the EU28 recorded a rapid increase in the number and capacity of installed plants in the period 2009–2012 [
9]. The earliest of these biogas plants are near the completion of their subsidised operation, as these subsidies were granted for 12–20 years, depending on the country [
10]. In the post-subsidy era, the operation of biogas plants on the day-ahead market could only be viable in the case of low-cost feedstocks or by implementing the feedstock gate fee business model [
11]. Among the most likely alternatives after the expiry of subsidies are biogas upgrading, the production of biomethane [
12] and/or the integration of the power-to-gas (P2G) concept using variable electricity from wind and solar to produce e-methane [
13]. Renewable methane with natural gas quality (ca. >96% CH
4 [
14]) can be directly injected into the gas grid or stored on location as compressed or liquified gas [
15].
A geographical information system (GIS) was recognised as a valuable tool for the detailed mapping and planning of new energy projects [
16] and in assessing the environmental and economic benefits of renewable energy source (RES) integration [
17]. In the context of bioenergy, geospatial investigation contributes to the assessment of the physico-chemical properties of biomass, facilitating the choice of the best technologies for application in the studied region [
18]. The geospatial assessment of the energy potential of crop residues and manure for biogas production in the EU showed the yearly availability of 0.7 EJ (ca. 195 TWh) [
19], which was ca. double the EU production of biogas from agricultural feedstocks in 2016. The bottom-up GIS model applied in the assessment of biomass potential from grasslands in Northwest Europe showed that ca. 45% of the sustainable grass could be utilised for energy production purposes in the model region [
20]. Another bottom-up analysis of using animal manure from various husbandry operations in East Croatia showed the potential of feedstock to produce 6.5 GWh of biogas, which could generate double the yearly electricity consumption of that municipality [
21]. The top-down mapping of agricultural residues in Croatia using Quantum GIS (QGIS) software [
22] showed that stover, straw and stalk could generate biogas potential up to 3000 MWh/(km
2∙a) in the extensive agricultural regions of Croatia. ArcGIS software was applied to reveal the potential of renewable electricity generation from municipal solid waste, including organic and dry material in Iran [
23]. The results showed that the studied region could produce ca. 2% of the total household electricity consumption, while achieving the avoidance of 6.7 thousand tons of CO
2eq/year due to the proposed measures. Integrated tools in GIS software allow users to determine important factors when assessing the availability of feedstocks for biogas production, such as the length of transportation routes from the biomass harvesting location to the biogas plant and the optimal location for setting up a new biogas plant. [
24]. In this sense, QGIS was successfully applied when determining the optimum area for establishing the biogas hub in Karditsa, Greece [
25]. The results showed that the optimal distance between the available biomass sources and the planned hub was ca. 20 km in order to maintain a feasible hub operation. ArcGIS was applied when finding an optimal biogas plant sited on the territory of Ohio in the United States for the case of corn stover and wheat straw [
26]. It was found that the average biomass availability radius for that case ranged between 22 and 34 km in the case of 10 newly examined biogas plants. The same software was applied to the case of Southern Finland with the goal of quantifying the relationship between the length of transportation distances to deliver feedstocks to existing biogas plants and an increase in their production capacity [
27]. Increasing the radius of biomass collection from 10 to 40 km could increase biogas plant production capacity by ca. 10–127%. However, the study did not reveal the impact of a capacity extension on the environmental performance of plants.
From environmental and economic points of view, the penetration of bioenergy into energy production systems (especially the ones based on fossil fuels) could bring multiple contributions and benefits [
28]. As the price of the carbon tax increases, so will the switch to carbon-neutral and carbon-sink resources. In the context of this research, the application of a life cycle assessment (LCA) can reveal the actual environmental impact of feedstock-changing policies in biogas production and utilisation related to future energy systems [
29]. It was shown that sugar beet generates ecological effects that are similar to those of maize crops in bioenergy production [
30], while intercropping forage sorghum with maize contributes to a lower environmental impact than a maize monoculture [
30]. Examining the environmental impact assessment of replacing maize silage with marine macroalgal biomass using SimaPro (an LCA software) showed a reduction in the environmental burden in almost all the impact categories that were examined. However, the significantly longer transport route for algae (150 km) compared to maize silage (12 km) resulted in higher values in the global warming potential (GWP) category, from 140 g CO
2-eq/kg(energy crops), to 160 g CO
2-eq/kg(macroalgae). A similar observation was also found by the authors of the present study in the case of applying residue grass from landscape management as a replacement for maize silage in existing biogas production [
31]. Biogas plants that are fed with agro-industry by-products and waste, such as distiller’s waste, rapeseed cake, cheese whey, pulp, seeds, peel, and fruit and vegetable residues, yielded better environmental performances than those fed with cereal silage [
32]. Nevertheless, the overall environmental performance also depends on the variability in terms of the total solids/volatile solids (TS/VS) content, specific biogas yield, origins and other factors [
32]. A comparison of the LCA performance for a biogas plant fed with animal manure and energy crops for various biogas utilisation pathways [
33] showed that biogas for electricity generation saves around 300 kg CO
2/MWh(electricity), while upgrading biogas to biomethane and its injection into the gas grid saves 191 kg CO
2-eq/MWh(biomethane). Another study of LCA claimed that using biogas in cogeneration achieved better overall environmental results compared to biogas upgrading [
34]. In both studies, the details about the considered electricity mix in the study were not provided, and the results were not presented using the same reference point. Projections from the Joint Research Centre (JRC) of the European Commission (EC) showed that by 2030, the further penetration of renewable energy sources (primarily wind and solar photovoltaic) will decrease the overall GHG emissions of the electricity generation sector [
35]. The integration of P2G and methanation in a biogas plant to fully exploit biogenic CO
2 potential yielded better environmental performance, with a projected European electricity mix for 2030 compared to 2016 [
36]. In the case of Ireland, an LCA of a biogas upgrade with P2G integration showed that using an electricity mix with an 85% share of renewables could satisfy the GHG savings of 70% compared to fossil fuels [
37]. Future development of P2G efficiency and the integration of renewable credits from CO
2 valorisation could increase the competitiveness of the biogas sector in future energy systems [
38].
Based on the detailed literature review, there is no reported research that integrally analyses the geospatial availability of novel feedstocks in the replacement of maize silage in biogas production, combined with the environmental impact assessment of feedstock replacement and alternative biogas utilisation pathways in future energy systems operating with a high share of RES. To address this gap in the scientific literature, the research objectives were as follows:
To assess the energy potential for biogas production using lignocellulosic residues, agri-food industry streams and the biodegradable fraction of municipal waste;
To present the geospatial distribution of the energy potential of novel feedstocks using a GIS mapping approach and to determine which existing and newly added biogas plants are suitable to contribute to natural gas decarbonisation;
To estimate the environmental impact via using an LCA of novel operational measures on the biogas production side and the utilisation side, while using actual biogas plants as test cases.
The hypothesis of this research was that applying a holistic approach to biogas plants, on both the production and utilisation sides, can increase environmental benefits over the current operation based on maize silage utilisation and baseload electricity production.
6. Conclusions
The link between the use of maize silage and electricity generation under subsidy models in biogas plants is becoming weaker as the new sustainability requirements are raised. The holistic and comprehensive analysis carried out in this study showed the opportunities and challenges that existing biogas plants would face if alternative measures for both biogas production and its utilisation were implemented to contribute to EU climate and energy targets.
The geospatial availability of alternative feedstocks was shown to have a potential in Northern Croatia that was high enough to replace all maize silage in current biogas production, which is ca. 212 GWh. Moreover, after the expiration of support schemes and guaranteed prices for electricity, the analysis showed that most of the examined biogas plants were well positioned for the injection of renewable gas into the natural gas grid (<2 km of distance). Others would have to consider the installation of a storage system if applying the upgraded technology. The total potential of biomethane from newly planned biogas plants was found to be ca. 191 GWh, out of which, the plants located nearby to the source of biodegradable industry waste would produce ca. 58% of its quantity.
The environmental impact analysis of actual biogas plants showed that an integrated approach to both biogas production and utilisation created synergistic effects in terms of reduced environmental burdens, which directly verified the hypothesis of the study. Scenarios that included feedstock transition and production of renewable gas in the form of biomethane showed reduced environmental burdens by 4 and 36 times compared to baseline scenarios (current operation), respectively. The analysis also showed that the integration of the P2G concept is recognised as a complex process from both the economic and the environmental point of view.
The operation of biogas plants based on sustainable feedstocks in future energy systems with a high share of RES showed that the role of the biogas sector in the energy transition should receive greater emphasis since it generates multiple positive effects for energy and the environment. There is still considerable scope for the improvement of existing processes, applying other biological feedstocks and integrating biogas technologies further in all energy sectors.
Future work in the studied area will be pointed towards engaging important stakeholders of the existing and future biogas sector to the presented concept with the aim to cluster all key variables from technical, financial and social aspects. This would include setting up a dialogue and information exchange with biogas plant owners; feedstock producers; farmers; industry plants; waste management companies; natural gas grid operators; regulators; and local, regional and national policy and decision makers. In this sense, causal loops were recognised as an attractive tool for managing the complex relationships between stakeholders and maintaining biogas technologies as an important factor in energy and environmental transitions.