Fig 1.
Illustration of the method and data sources used.
First, available substrate amounts were estimated using subnational statistics. Second, the substrate amounts were spatially downscaled using land cover and livestock population datasets. Third, economic and technical limitations were analyzed using a model of a biogas plant. The local production potentials were upscaled to the EU level to obtain an overall potential estimate.
Table 1.
Residue-to-crop production ratios, the assumed quotients between total (wet) weight of crop residues and reported crop production (kernel/seed weight).
Table 2.
Assumed excretion and total manure production per animal head for different animals and manure management systems.
Excretion values for cattle and pigs from [27]. Excretion for poultry based on [28]. See the S1 Appendix for details on manure management and bedding.
Table 3.
Dry matter (DM) expressed as fraction of total weight. Volatile solids (VS) expressed as fraction of DM, and carbon (C) and nitrogen (N) expressed as fractions of VS. For conversion of methane volume to higher heating value (HHV) the factor 40 MJ m−3 was used. Compositions are based on data reported in [19, 26, 29–34]. Methane yields based on data from [19, 35–38].
Table 4.
Parameters for the constraints as assumed in the base scenario.
Alternative scenarios were defined by varying two of the constraint parameters at a time, while keeping the others equal to the base scenario values.
Fig 2.
Available and utilized substrates in the base scenario.
Black bars: Estimated available substrate quantities in EU28, excluding Greece, after accounting for the maximum removal rate from fields (40%, see Table 4) and the straw used for animal bedding. Straw used for bedding is excluded from the reported straw amounts and included in the solid manures. Manure fallen on pastures is not available as a substrate. Grey bars: Utilized substrates in the base scenario. 1 Tg DM = 1 million metric dry tonnes.
Fig 3.
Estimated available crop residues and manure in the EU.
The panels show estimated available amounts of (a) crop residues, (b) cattle manure, (c) pig manure, (d) chicken manure, (e) sum of liquid manure, and (f) sum of solid manure. The estimates account for the maximum sustainable removal of crop residues, and exclude manure that falls on pastures. Straw used for bedding is excluded from the crop residues density and included in the solid manure densities. Striped areas are not analyzed.
Fig 4.
DM and C:N ratio of combined crop residues and manure.
(a): DM concentration in the available substrates. (b): C:N ratio of the available substrates. Both maps concern the base scenario (see parameters in Table 4). C and N contents of substrates are given in Table 3. Striped areas are not analyzed.
Fig 5.
Simulated biogas potential when changing parameter values compared to the base scenario.
The potential is expressed as a fraction of the base scenario potential. Each cell in each grid represents a different scenario, with all parameters as in the base scenario except for the two named parameters.
Fig 6.
Biogas potential density in four different scenarios.
Panel (a) shows the base case with production 0.7 EJ/year (see parameters in Table 4). Panel (b) shows that the potential is substantially increased (to 1.1 EJ/year) if drier substrate mixtures are allowed. Panel (c) shows that increasing the minimal C:N ratio of the substrate mixture from 10 to 14 severely diminishes the potential (to 0.1 EJ/year), all other things equal. However, as shown in panel (d), the extra carbon can be supplied (by crop residues) if 25% DM content is allowed, and in this case the potential is 0.9 EJ/year.