Temporal and spatial availability of cereal straw and biomethane
On national level (Fig. 3), in the years in question the theoretical and the technical potential of straw fluctuate between 27.3 and 38.3 Tg fm a−1 and between 19.1 and 26.8 Tg fm a−1, respectively. The maximum in 2014 and the minimum in 2018 are separated by just four years, but also a difference of up to 11 Tg fm a−1, or 40%. The use of cereal straw, however, exhibits no significant fluctuations. From 2010 to 2014 there is an increase (+ 4.1%) followed by a slight decrease (− 0.9%). In 2018, the level was 5.3 Tg fm a−1, i.e. 3.2% above that of 2010. Due to the relatively constant level of use and the fluctuating supply of raw material, the share of the technical straw potential used ranges between a fifth (2014) and more than a quarter (2018).
The use is dominated by the amounts required for bedding and feed. The highest demand arises in 2018 for the husbandry of horses (41%), pigs (24%) and cattle (22%). The remaining animals account for 8%. Apart from livestock farming, the amounts required for special crops (2%) and industrial use (2%) take up a comparatively small share. One point that should be emphasised, however, is that industrial use increased 35 times from 2010 to around 114,000 Mg a−1 in 2018. This is mainly made up of incineration (59%) and fermentation (35%). The remaining 6% are other industrial uses. It should be taken into account that the primary data collection on industrial use is not claimed to be complete. In total, detailed information was collected on 12 plants. According to FNR , there are several decentralised, smaller plants in the incineration business on which no further information is publicly available. The number is expected to be in the double digits. Neither is there any information on the quantities or locations of straw used as a building material. Despite these uncertainties, on the national level it can be said that the amount of resources used was never higher than the amount supplied. This produces a range between 13.9 and 21.5 Tg fm a−1 for the mobilisable straw potential. Compared to Weiser et al.  (see Chapter 1), this potential is at least 62% above the level discussed previously, as long as the straw is used in biogas plants including returning the digestate.
The regional availability of raw materials is influenced, among other things, by acreage, yield levels, and existing use. The maps in Fig. 4 show the spatial distribution of the technical potential (= raw material supply) including the trend compared to the previous year, raw material utilisation and the mobilisable potential (= availability). The years 2014 and 2018 were selected to illustrate the spatial range of the findings. Although these years are very close together, they are the all-time maximum (2014) and the minimum (2018) since 1994 (see Fig. 1).
The technical potential falls by almost eight million tonnes between 2014 and 2018. Nevertheless, in both years there are clear hotspots in the north (Schleswig-Holstein, Mecklenburg-West Pomerania) and the east (parts of Brandenburg, Saxony, Saxony-Anhalt), along with certain regions in the west (on the border between Lower Saxony and North Rhine-Westphalia) and a few regions in southern Germany. A comparison of the two years shows a relatively high level of change in the regions in the far east (Brandenburg, eastern Saxony) and the far northwest (northern Lower Saxony). The 2017/2018 trend reveals significant losses ranging between over 20% and, sometimes, over 40% for these and numerous other areas. In contrast, the extreme west and south show some significant increases. The situation is contradictory for 2013/2014, with production significantly higher in 2014 than the previous year, 2013. Significant increases extend from the south across the east to the north. The level remained stable in the other regions. In contrast with this considerable dynamism, regional utilisation hardly shows any sign of change in the observation period. Consistently high utilisation rates can be seen for the northwest (western Lower Saxony, northern North Rhine-Westphalia), and utilisation is also comparatively high in the north (e.g. Mecklenburg-Western Pomerania, Prignitz, Uckermark) and certain regions in the south. On the one hand, this is due to the need for straw for livestock farming. On the other hand, especially in western Lower Saxony and Uckermark, there are industrial uses with a high straw requirement.
The mobilisable potential is the difference between the technical potential and utilisation. Compared with the technical potential, the situation is similar, but more nuanced. For example, the high utilisation rate in the northwest significantly reduces the potential that can still be mobilised there. In the weak year, 2018, the situation is similar for northern Lower Saxony, western Brandenburg and eastern Saxony-Anhalt. With regard to the mobilisable potential, the findings have a clear regional focus in eastern Schleswig-Holstein, throughout Mecklenburg-Western Pomerania, in Uckermark, central Saxony, western Saxony-Anhalt and southern Lower Saxony. Despite the higher use of raw materials and the strongly negative trend in 2018, these regions have a large mobilisable potential. In some regions, meanwhile, the amount used is higher than the supply, especially in the Alpine Foreland and a small number of regions in the northwest. The supra-regional situation is evaluated in Chapter 3.2. For some regions, it was not possible to generate any consistent data sets. For 2018, in particular there are a comparatively high number of gaps in the data, e.g. for the metropolitan regions in North Rhine-Westphalia.
To complement the maps, Fig. 5 summarises the quantitative distribution of the mobilisable potential for all NUTS-3 regions. The primary axis shows the potential for each NUTS-3 region sorted in descending order. The secondary axis shows the cumulative potential as a biomass supply curve. In some hotspot regions, especially in Mecklenburg-Western Pomerania, the mobilisable potential is over 500,000 Mg fm a−1 (2014) or over 300,000 Mg fm a−1 (2018), respectively. This presentation of the findings shows that, of a total of 401 regions, a third of the total potential is concentrated in 27 (2014) and 30 (2018) regions. Two thirds of the potential are located in 88 (2014) and 92 (2018) regions.
Table 4 adds context to what these findings mean for the transport sector. It shows the number of possible plants of the selected plant concept (Chapter 2.1, Table 3), the potential amount of biomethane as fuel and two key figures for the selected transport modes passenger cars, heavy goods vehicles and maritime shipping. These relate to the possible GHG mitigation and the substitution of energy requirements in the target market when replacing fossil fuels. As it is unlikely that all the mobilisable potential will be fully tapped, a distinction is also made between the three levels of 66%, 33% and 10%, which can be understood as farmers’ willingness to supply straw. At the same time, this differentiation can also be interpreted as a reduction of the recovery rate from 70 to 46%, 23% and 7%.
The differences in production levels for 2014 and 2018 show clear effects on the strategic relevance of biomethane in the transport sector. If the potential is fully tapped, from the point of view of resource availability, well over 300 plants could still be built even in weak years, and up to 7 Tg CO2-eq. avoided—in good years even up to 12 Tg CO2-eq. With regard to achieving the climate target in the transport sector by 2030 (Chapter 1), this means making possible progress of up to 11–17% through the effective use of straw alone. However, if the minimum values are adopted, the share is more than halved to 5–8%. In both considerations, the different straw availabilities during extreme years reduce the potential GHG mitigation by more than one third. Taking into account a reduced farmers’ willingness to supply straw, the strategic relevance in terms of emissions reduction changes significantly. If only a third of the potential were tapped, well over 100 plants could still be built and in the best case 3–4 Tg CO2-eq. could be avoided. If, by contrast, only one in ten farmers made their straw available for future biomethane production, 35 plants could still be supplied. However, the use of straw could save not more than one million tonnes of CO2 equivalents, which is well below 2% of the sector’s target. However, this level is higher than the GHG savings achieved in the entire transport sector since 1990. Especially in the case of a higher utilisation rate of cereal straw, there are promising opportunities to realise a significant contribution to GHG mitigation.
With regard to the substitution potential for fossil fuels, considerable differences can be identified for the respective modes of transport. Due to the different energy requirements and taking into account the optimum case, the demand for bunkering seagoing vessels could be met in full. Up to a fifth could be substituted for HGVs and up to a tenth of passenger cars could be supplied with a low-emission fuel. If only a third of the mobilisable straw potential was provided as biomethane in the transport sector, the shares would decrease to 7% and 3%, respectively. The shipping sector would still achieve around half. If only 10% of the potential would be utilised, the share for HGVs and passenger cars is in almost all cases well below one percent and between 13 and 15% for the shipping sector.
Hotspots for future biomethane production
In some regions, meanwhile, the amount of raw materials used is higher than the supply. Figure 6 shows the findings for two selected examples as a graph. In the first region, “Grafschaft Bentheim” (western Lower Saxony), Germany’s first plant designed for the industrial use of cereal straw entered operation in 2014. As a result, the use of straw has tripled to over 90,000 Mg a−1 and will significantly exceed the local raw material supply this year. Meanwhile, in the second example “Rosenheim, Landkreis” (Alpine Foreland), straw use is entirely related to livestock farming. The high demand has to be balanced out by other regions, which cannot be assessed at the level of the administrative unit (Fig. 4).
The spatial links between the supply of resources and their use were therefore analysed using a GIS. The results generated take into account both cross-regional compensation for deficits and the regional importance of multiple small neighbouring regions that can be viewed as a network. On this basis, options for the future use of raw materials can be evaluated on a plant-specific basis. In combination with the catchment areas with radii of 20 and 50 km, Fig. 7 shows the areas in which a plant requirement of 40,000 Mg a−1 (Chapter 2.1, Table 3) can be met either fully or multiple times. In line with Table 4, the spatial context is also shown in the case of a willingness to supply straw of 33% and 66% or a recovery rate of 23% and 46%, respectively.
A low willingness to supply straw (33%) and a low transport distance (20 km) set high requirements and generate clearly delineated hotspot regions. In the weak year of 2018 (Map 1), the hotspots run through the fertile Börde lowlands from west to east, from the Jülich-Zülpich Börde west of Cologne, via the Warburg Börde north of Kassel to the Hildesheim Börde south of Hanover, the Magdeburg Börde and the Thuringian Basin. In the strong year of 2014 (Map 2), these regions expand, forming a ribbon extending from west Saxony to North Rhine-Westphalia. In the east of Schleswig-Holstein, the north of Mecklenburg-Western Pomerania, parts of Brandenburg (Uckermark, Oderbruch), around Würzburg and south of Regensburg, there are also very good conditions for a supply of raw material. As these are classic wheat-growing areas with fertile soils, out of all regions they pose the lowest risk of a lack of raw materials. At the same time, the digestate would not have to be transported long distances for spreading. If it is transported over longer distances, the raw material can be supplied almost anywhere. With a catchment area radius of up to 50 km (Maps 3 and 4), plants could be built all over Germany, at least from the point of view of resource availability. The only exceptions are the north and west of Lower Saxony, the Black Forest and the Alpine Foreland, as use in these areas is already relatively high and there is little or no cereal cultivation in those areas. The area in the south of North Rhine-Westphalia is not a gap in the data, but a densely forested mountainous region (the Taunus range). If there is greater willingness to supply straw (66%), the hotspots expand accordingly. Above all, large parts of Bavaria and Hesse also emerge as priority areas (Maps 5 and 6). With a larger catchment area radius, sufficient amounts of raw material can be tapped across the country to operate more or larger plants (Maps 7 and 8). In summary, it can be said that the spatial precision of the findings in Chapter 3.1 can be improved using GIS analysis. This considerably adds to the range of possible interpretations regarding the replication of the plant concept under consideration.