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Life Cycle Assessment of Biogas from Separated slurry

6 Biogas production from raw pig slurry and fibre fraction from mechanical screw press separation (Scenario H) – results and interpretation

• 6.1 System description
• 6.2 Results of the Impact Assessment
  • 6.2.1 Overall results of the comparison
  • 6.2.2 Global warming
  • 6.2.3 Acidification
  • 6.2.4 Aquatic eutrophication (N)
  • 6.2.5 Aquatic eutrophication (P)
  • 6.2.6 Photochemical ozone formation (“smog”)
  • 6.2.7 Respiratory inorganics (small particles)
  • 6.2.8 Non-renewable energy resources
  • 6.2.9 Consumption of phosphorus as a resource
  • 6.2.10 Carbon stored in soils
• 6.3 Uncertainties
• 6.4 Synthesis of the results for all impact categories assessed

This section presents the results and the interpretation from the life cycle assessment carried out for “Scenario H”, described below. The results from “Scenario H” are compared to those of the reference scenario for fattening pig slurry management, i.e. “Scenario A”. The life cycle assessment is performed in order to answer the research question: “What are the environmental benefits and disadvantages of using the fibre fraction from Samson Bimatechs mechanical separation of pig slurry (see Annex C) for biogas production compared to the reference scenario for pig slurry management?”.

This scenario does not rely on “best available technologies” or “best possible practices” as it was the case for Scenarios F and G.

The detailed description of this scenario, including all mass balances, assumptions and calculations, is presented in Annex H. All life cycle inventory data used for the results presented in this section can therefore be found in Annex H.

The environmental impacts and conclusions in this section build, among others, on data and information delivered by the producer of the technology, Samson Bimatech, and on data measured for Samson Bimatech (laboratory measurements of the slurry composition), combined with data for biogas production based on information from Xergi. The conclusions made in this section rely on this information, and the authors of this study have not had the possibility of verifying these data.

6.1 System description

The system constituting Scenario H, as described in section 2.2.4, consists to produce biogas from a mixture of raw slurry and fibre fraction obtained from a mechanical screw press separation (Samsom Bimatech process), both from fattening pigs. This scenario is highly similar to Scenario F, but some major differences can be highlighted.

The main differences between Scenario H and Scenario F are:

• The separation technology of Scenario H is based on a mechanical screw press separation (the Samson Bimatech separation technology). In Scenario H, polymer is not added to the separation (polymer is added for the separation in Scenario F).
   
• No separation of the degassed biomass mixture resulting from the anaerobic digestion is performed after the biogas plant. This involves that the subsequent storage and field processes concern the degassed biomass only.

A flow diagram for “Scenario H” is shown in figure 6.1. The process numbers in figure 6.1 follows the numbers of the sections in Annex H.

Figure 6.1. Process flow diagram for “scenario H” – Biogas production from raw pig slurry and fibre fraction from mechanical screw press separation. The process numbers follows the numbers of the sections in annex H.

Click here to see Figure 6.1.

6.2 Results of the Impact Assessment

6.2.1 Overall results of the comparison

Table 6.1 presents the overall environmental impacts from “Scenario H” (biogas from raw pig slurry and fibre fraction from mechanical separation), and compare them to the environmental impacts from the reference scenario for pig slurry (described in section 3). Figures 6.2 A and 6.2.B illustrate the results presented in table 6.1. Figures 6.2 A and 6.2.B are identical except for the minimum and maximum at the axis. In the case of figure 6.2.B, the minimum and maximum were adjusted in order to present fully the consumption of non-renewable energy impact category. As in previous sections, results are presented for soil JB3 only (sandy soil).

As explained in section 4.2.1, the results presented are “characterised” results and are expressed, for each impact categories, relative to the result of the reference scenario. The positive values are the contributions to the environmental impacts and resource consumptions by the slurry management scenarios. The negative values are “avoided environmental impacts”.

Results presented in table 6.1 should be interpreted with care in the light of the assumptions and data that were used to obtain them, i.e. the life cycle inventory (LCI) data presented in Annex H. An attempt to discuss these results based on this focus, impact category per impact category, is presented in sections 6.2.2 to 6.2.10.

Table 6.1. Scenario H vs A: Contribution of the different processes to each environmental impact categories selected. Results, for each impact category, are expressed in % of the total positive contributions from the reference scenario (considering the 10 years values, when this applies). Soil JB3 (sandy)^[1]

Click here to see Table 6.1.

Figure 6.2.A Overall environmental impacts for the selected impact categories – scenario H vs scenario A. Fattening pig slurry management. Soil type JB3. 10 and 100 years time horizon for global warming and for aquatic eutrophication (N). Axis ranging from -140 to 120.

Click here to see Figure 6.2.A

Figure 6.2.B Overall environmental impacts for the selected impact categories – scenario H vs scenario A. Fattening pig slurry management. 10 and 100 years time horizon for global warming and for aquatic eutrophication (N). Axis ranging from -450 to 150.

Click here to see Figure 6.2.B

In the following sections, the benefits (and shortcomings) of producing biogas as described in Scenario H instead of the reference slurry management are discussed in details for each impact categories.

Sensitivity analyses were performed for some processes, as described in Annex H. The results of these sensitivity analyses are presented in section 8 of this report. However, as all the sensitivity analyses identified concern the same processes as for Scenario F, the sensitivity analyses are performed with the data of Scenario F only.

6.2.2 Global warming

From table 6.1 as well as figure 6.2.A, it can be observed that two main processes contribute to global warming:

• In-house storage (for both Scenario A and H, this process represents 32 % of the total positive contributions to global warming from the reference scenario).
   
• Field processes.
  • For Scenario A, this represents 44 % of the total positive contributions to global warming from the reference scenario (and 51 % if the 100 years values are considered for C horizon in the field).
  • For Scenario H, this represents 39 % of the total positive contributions to global warming from the reference scenario (and 46 % if the 100 years values are considered for C horizon in the field).

The main contributing substance for in-house storage contribution to global warming is CH₄ (representing, for Scenario H, 84 % of the contribution from this process). The other contributors are N₂O (12%) and CO₂ (4 %). The proportions shown in parenthesis are for a 10-years value as regarding the horizon considered for C in the field. This process is exactly the same for Scenario A, F, H and I. In section 4.2.2, the explanations and discussions about the importance of CH₄ emissions versus the assumptions taken are presented.

The contribution to global warming from field process is less important if the slurry is managed as described in Scenario H than as in the reference scenario. This is mainly due to less biogenic CO₂ emissions from the field in Scenario H (74 kg CO₂ equivalent in Scenario H as compared to 89 kg CO₂ equivalent in Scenario A). The biogenic CO₂ emissions in this case represent the emissions of CO₂ from the applied slurry in the field. Less biogenic CO₂ emissions for Scenario H are due to the anaerobic digestion effect, where an important share of the C was removed from the slurry and used to produce energy. In fact, the emissions of biogenic CO₂ from the liquid fraction (50 kg CO₂ equivalent) are about the double as those from the digested biomass (24 kg CO₂ equivalent). Overall, biogenic CO₂ contribute to 67 % of the contribution to global warming from field processes in Scenario H (considering 10 years values horizon for C in the field). For Scenario A, this is 70 %.

The other important contributing substance to global warming from field processes is N₂O. As explained in section 4.2.2, the emissions of N₂O were estimated based on the IPCC methodology (IPCC, 2006), accordingly, the emissions were estimated as a function of the N content in the applied slurry. As such, the contribution to global warming from the emissions of N₂O of the degassed biomass (6.6 kg CO₂ equivalent) are much lower than from the liquid fraction (26.6 kg CO₂ equivalent) which has a higher N content. However, if the IPCC methodology underestimate N₂O emissions from field as suggested by Crutzen et al. (2008), the global warming from field process could be much more important, as detailed in section 4.2.2.

For both Scenario A and H, outdoor storage appears as the third most important contributing process to global warming. In Scenario A, this process represents 21 % of the total positive contributions to global warming from the reference scenario and the main contributing substance is CH₄ (44.6 kg CO₂ equivalent, representing 73 % of the contributing substances to global warming from this process). In Scenario H, this process represents, when accounting for all fractions to be stored (liquid fraction and degassed biomass), 16 % of the total positive contributions to global warming from the reference scenario. In this case, the main contributing substance is also CH₄, but with 28.5 kg CO₂ equivalent (representing 61 % of the contributing substances to global warming from this process). From this 28.5 kg CO₂ equivalent, 23.5 kg is from the storage of the liquid fraction and 5 kg is from the storage of the degassed biomass. This illustrates the positive effect of digestion for storage of slurry, as most of the VS (whose degradation is the origin for CH₄ emission) were degraded during the anaerobic digestion, leaving a much lower potential for subsequent CH₄ emissions. Methane emissions from the liquid fraction may however be slightly overestimated in this study, as it was assumed that all the VS present in the liquid fraction are easily degradable (see section 4.2.2 of this report and F.5.4 of Annex F). This also applies for N₂O, as described in section F.5.7 of Annex F. Emissions of N₂O actually contribute to 32 % of the contribution to global warming from outdoor storage in Scenario H (i.e. 14.8 kg CO₂ equivalent). Most of it comes from the liquid fraction (12.9 kg CO₂ equivalent).

For Scenario H, the contribution to global warming from co-generation of heat and power does not have an important magnitude as compared to the other biogas scenarios because of the lower amount of biogas produced per functional unit.

Finally, as it can be observed from figure 6.2.A, the contributions to global warming from the use of electricity, the biogas process and the transportation are rather negligible.

Using the slurry as an organic fertiliser allows, for both scenarios, to avoid the same amount of inorganic fertiliser to be produced, and consequently, contribute to avoid the same magnitude of global warming potential to be avoided. However, the biogas production in Scenario H has the additional advantage to avoid heat and electricity to be produced, which allows more global warming potential to be avoided. The avoided wheat production induced by an increased yield in Scenario H does not contribute significantly to avoid global warming potential, as shown in figure 6.2.

Overall, when the “deductions” from the avoided contributions to global warming are accounted for, managing the slurry as described in Scenario H allow a net reduction of 37 kg CO₂ equivalent of the global warming potential as compared to the reference scenario (figure 6.3).

Table 6.2 summarises, for selected processes, the contribution of the main contributing substances to global warming, for both Scenario A and H.

Table 6.2. Scenario H vs A: Contribution of the main contributing substances to global warming for selected processes. All values in kg CO₂ equivalent. Soil JB3 (sandy).^[1]

[1] The number of digits is not an expression of the uncertainty.

[a] The balance is from other global warming contributing substances not presented in this table.

[b] Fossil CO₂

[c] This is not a zero value

The major results as regarding global warming can be summarised as:

• Overall, managing the slurry as in Scenario H allows, based on the reference scenario considered, to reduce the contributions to global warming from slurry management.
   
• There are 2 major hot spots regarding global warming:
  • In-house storage of slurry. The main contributor is CH₄.
  • Field processes. The main contributor is CO₂ due to field processes. The contribution to global warming from field process is lower in the case of Scenario H as compared to Scenario A.
     
• Storing digested slurry has considerable benefits on global warming as compared to storage of raw slurry.
   
• The contributions to global warming from the use of electricity, the biogas process and the transportation are rather negligible in both scenarios.
   
• Both scenarios allow avoiding the contributions to global warming from the production of inorganic N, P and K fertilisers in the same magnitude.
   
• Scenario H allows avoiding the production of marginal heat and electricity, which has considerable benefits on global warming contributions.

This information is summarised in figure 6.3. This figure presents the contribution to climate change of Scenario A and H only for the processes that are not equal between A and H (i.e. in-house storage and avoided N fertiliser are not included). All processes are presented; the category labelled “other processes non equal” represents the aggregation of all processes not presented in the legend.

Figure 6.3. Comparison of Scenario H vs Scenario A for global warming including carbon sequestration, for processes differing between A and F only. Soil JB3, 10 years values.

Click here to see Figure 6.3.

6.2.3 Acidification

As it can be observed from figure 6.2, the major contributor to acidification is in-house storage, for both scenarios. In both cases, it represents 60 % of the total positive contributions to acidification from the reference scenario. This is mostly due to one substance, namely NH₃, as discussed in section 4.2.3.

Field processes represent the other most important contributor to acidification, representing, for Scenario H, about 20 % of the total positive contributions to acidification from the reference scenario (accounting for both liquid fraction and degassed biomass). The main contributing substance in this case is also NH₃ (96 % of the contributions for this process).

The outdoor storage, if the contributions from the liquid fraction and the degassed biomass are aggregated in the case of Scenario H, contributes to acidification in the same magnitude for both scenarios (but it is a little higher for Scenario A). The main contributing substance to acidification for outdoor storage is also NH₃ (77% of the contributions for this process).

For Scenario H, as it can be observed in figure 6.2.A, the contributions to acidification from the biogas production, the co-generation of heat and power and the transportation are rather small.

As explained in section 6.2.1, the amount of fertiliser avoided is the same for both scenarios, and so is the avoided contribution to acidification from the inorganic fertilisers not produced/used.

The production of marginal heat avoided through the use of biogas for heating has a small contribution to acidification avoidance, but the production of marginal electricity avoided as well as the wheat production avoided have insignificant contributions in avoiding acidification.

Overall, when the “deductions” from the avoided contributions to acidification are accounted for, the difference between managing the slurry as described in Scenario H with the reference slurry management amount to 7.1 m² unprotected ecosystem (UES) (figure 6.4). When accounting for uncertainties, this difference is however not significant (figure 6.10.A)

Table 6.3 summarises, for selected processes, the contribution of the main contributing substances to acidification, for both Scenario A and H.

Table 6.3. Scenario H vs A: Contribution of the main contributing substances to acidification for selected processes. All values in m² unprotected ecosystem (UES). Soil JB3 (sandy).^[1]

[1] The number of digits is not an expression of the uncertainty.

[a] The balance is from other acidification contributing substances not presented in this table.

[b] This includes other contributing substances which are not reflected when contributions are presented with 2 decimal places.

[c] This is not a zero value.

The major results as regarding acidification can be summarised as:

• Overall, managing the slurry as in Scenario H does not allow significant benefit over the reference slurry management as regarding acidification.
   
• There are 2 major hot spots as regarding acidification:
  • In-house storage of slurry. The main contributor is NH₃.
  • Field processes. The main contributor is NH₃.
     
• The overall contribution to acidification from outdoor slurry storage is similar for both Scenario A and H, and NH₃ is the main contributor.

This information is summarised in figure 6.4. This figure presents the contribution to acidification of Scenarios A and H only for the processes that are not equal between A and H (i.e. in-house storage and avoided N fertiliser are not included). All processes are presented; the category labelled “other processes non equal” represents the aggregation of all processes not presented in the legend.

Figure 6.4. Comparison of Scenario H vs Scenario A for acidification, for processes differing between A and H only. Soil JB3, 10 years values.

Click here to see Figure 6.4.

6.2.4 Aquatic eutrophication (N)

Aquatic N eutrophication is, as illustrated in figure 6.2.A, mostly due to field processes:

• For Scenario A, this represents 81 % of the total positive contributions to N-eutrophication from the reference scenario;
• For Scenario H, this represents 79 % of the total positive contributions to N-eutrophication from the reference scenario;

The percentages above considers 10 years value as regarding the horizon for C during field processes, these percentages are higher if 100 years values are considered, as presented in table 6.1.

The contribution to N-eutrophication from field processes is therefore similar for both scenarios. In both case, it is essentially caused by N leaching through soils.

Figure 6.2 illustrates that the contribution to N-eutrophication from Scenario H is much alike the contributions from the reference scenario. However, when the 100 years values are taken into account as regarding the horizon time for C during field processes, the gain created by avoided wheat in Scenario H becomes more significant, in which case Scenario H appears slightly more advantageous than the reference scenario.

Overall, when the “deductions” from the avoided contributions to N-eutrophication are accounted for, managing the slurry as described in Scenario H present a difference of 0.07 kg N reaching aquatic recipients as compared to the reference scenario. This is for the 10 years value for C. Taking the 100 years values for C during field processes into account, the difference is 0.08 kg N. However, when accounting for uncertainties, there are no significant benefits (figure 6.10.A).

This information is summarised in figure 6.5. This figure presents the contribution to N-eutrophication of Scenarios A and H only for the processes that are not equal between A and H (i.e. in-house storage and avoided N fertiliser are not included). All processes are presented; the category labelled “other processes non equal” represents the aggregation of all processes not presented in the legend.

Figure 6.5. Comparison of Scenario H vs Scenario A for N-eutrophication, for processes differing between A and H only. Soil JB3, 10 years values.

Click here to see Figure 6.5.

6.2.5 Aquatic eutrophication (P)

As it can be observed from figure 6.2.A and table 6.1, the contribution (positive and negative) to P-eutrophication are about the same for both scenarios. In fact, the only difference is the extra avoided heat, electricity and wheat production avoided in Scenario H, but all together they represent a minor magnitude.

The major positive contribution to this impact is field processes (through P leaching in soils which represents 99 % of the contributions from this process in both scenarios) and the major negative contribution is the inorganic P fertiliser avoided.

For Scenario H, it can be highlighted that, for the field processes, the liquid fraction contribute to about 79 % of the total positive contributions to eutrophication (P) from the reference scenario, while it is 21 % for the degassed biomass mixture. This difference reflects the important difference between the mass flow of these 2 fractions in the system; while there is 888.5 kg liquid fraction ex-storage (including water) per 1000 kg ex-animal, there is only 187.7 kg degassed biomass ex storage (including water) per 1000 kg ex-animal.

For both fractions, it was considered that P leaching to soils corresponds to 10 % of the P applied to field, and 6 % of this has the possibility to reach aquatic recipients (based on Hauschild and Potting, 2005). This is detailed in sections H.7 and H.22 of Annex H. This assumption involves some uncertainties, as discussed in section 4.2.5.

Overall, when the “deductions” from the avoided contributions to P-eutrophication are accounted for, managing the slurry as described in Scenario H does not allow a significant benefit for P-eutrophication over managing the slurry as in the reference scenario. This is illustrated in figure 6.6 and 6.10.A.

Figure 6.6. Comparison of Scenario H vs Scenario A for P-eutrophication, for processes differing between A and H only. Soil JB3, 10 years values.

Click here to see Figure 6.6.

6.2.6 Photochemical ozone formation (“smog”)

In both scenarios, there are 2 main hot spots for photochemical ozone formation:

• In-house storage, representing about 56 % of the total positive contributions to photochemical ozone from the reference scenario (for both Scenario A and H).
   
• Outdoor slurry storage:
  • Scenario A: this process represents 39 % of the positive contributions from the reference scenario;
  • Scenario H: this process represents 27 % of the positive contributions from the reference scenario.

In the case of in-house storage, the main contributor is CH₄, which represents about 95 % of the contribution to ozone formation for this process. Concerns regarding potential overestimation of in-house CH₄ are discussed in section 4.2.6 and 4.2.2.

Methane is also the main contributing substance to ozone formation for the outdoor slurry storage process (81 % in the case of Scenario A and 75 % in the case of Scenario H). The fact that the overall contribution from outdoor storage is lower for Scenario H reflects the effect of the digestion. This is due to the lower VS content of degassed slurry, thus involving a much lower potential for CH₄ emissions. This again highlights the positive effect of slurry digestion as regarding CH₄ emissions during slurry storage.

Another contributing process to this impact category is, for Scenario H, the co-generation of heat and power. The main contributing substance in this case is NO_X (85 % of the total contributions for this process), which is emitted during the combustion of the biogas in the biogas engine.

Avoiding marginal heat and electricity to be produced only have a minor contribution in avoiding the photochemical ozone formation. Avoiding inorganic fertilisers to be used/produced does contribute in reducing the overall ozone formation impact, in similar magnitude for both scenarios.

Overall, when the “deductions” from the avoided contributions photochemical ozone formation are accounted for, managing the slurry as described in Scenario H present a difference of 0.012 pers*ppm*h as compared to the reference scenario. When the uncertainties are taken into account, this benefit is not significant (figure 6.10.A)

This information is summarised in figure 6.7. This figure presents the contribution to photochemical ozone formation of Scenarios A and H only for the processes that are not equal between A and H (i.e. in-house storage and avoided N fertiliser are not included). All processes are presented; the category labelled “other processes non equal” represents the aggregation of all processes not presented in the legend.

Figure 6.7. Comparison of Scenario H vs Scenario A for photochemical ozone formation, for processes differing between A and H only. Soil JB3, 10 years values.

Click here to see Figure 6.7.

6.2.7 Respiratory inorganics (small particles)

Respiratory inorganics is caused by 3 main processes:

• In-house storage of slurry: this process represents 56 % of the total positive contributions to respiratory inorganics from the reference scenario (for both Scenario A and H).
   
• Field processes:
  • Scenario A: this process represents 30 % of the total positive contributions to respiratory inorganics from the reference scenario
  • Scenario H: this process represents 20 % of the total positive contributions to respiratory inorganics from the reference scenario
     
• Outdoor slurry storage:
  • Scenario A and H: this process represents about 10 % of the total positive contributions to respiratory inorganics from the reference scenario

For in-house storage, the main contributor is NH₃ emissions, representing about 97 % of the contribution to respiratory inorganics for this process.

For field process, the main contributing substance to respiratory inorganics is also NH₃ (contributing to about 91 % in Scenario A and 87 % in Scenario H for this process).

For outdoor storage, the contributions are divided between NH₃ and NO_X.

In the case of Scenario H, the co-generation of heat and power, i.e. when the biogas is burnt in the biogas engine, also contributes to “respiratory inorganics” formation, representing 4.5 % of the total positive contributions to respiratory inorganics from the reference scenario.

Avoiding marginal heat and electricity to be produced only have a minor contribution in avoiding respiratory inorganics. This observation also applies for the avoided wheat production induced by the extra yield. Avoiding inorganics fertilisers to be produced does contribute to avoid “respiratory inorganics” formation, in similar magnitude for both scenarios.

Overall, when the “deductions” from the avoided contributions to “respiratory inorganics” are accounted for, managing the slurry as described in Scenario H present a difference of 0.028 kg PM_2.5 equivalent as compared to the reference scenario. This difference is not significant when accounting for uncertainties (figure 6.10.A).

This information is summarised in figure 6.8. This figure presents the contribution to “respiratory inorganics” of Scenarios A and H only for the processes that are not equal between A and H (i.e. in-house storage and avoided N fertiliser are not included). All processes are presented; the category labelled “other processes non equal” represents the aggregation of all processes not presented in the legend.

Figure 6.8. Comparison of Scenario H vs Scenario A for “respiratory inorganics”, for processes differing between A and H only. Soil JB3, 10 years values.

Click here to see Figure 6.8.

6.2.8 Non-renewable energy resources

Both scenarios contribute to the use of the non-renewable energy through the use of marginal electricity, of liquid fuel for transportation (road and tractors) and the use of marginal heat. This is reflected in figure 6.2.B where the contributing processes are those requiring electricity or fuel inputs.

Avoiding marginal electricity but also heat to be produced through the use of biogas for scenario H allows a considerable “avoidance” of non-renewable energy to be used. This is also true for the fertilisers avoided, but the magnitude of this is similar for both scenarios. The wheat production avoided in Scenario H through the yield increase also contributes to avoid, in a rather small magnitude, the use of non-renewable energy resources.

Overall, the difference between Scenario H and the reference scenario lies mostly in the avoided contributions rather than in the positive contributions. This is why, as explained in section 4.2.8, using less electricity or heat input for the biogas process (as suggested in Annex F, section F.15.3) would not contribute to a drastic change of the situation for this impact category, given the larger importance of the avoided contributions.

Managing the slurry as described in Scenario H thus offers a significant advantage over the reference management, as regarding the impact on non-renewable energy use. Overall, when the “deductions” from the avoided contributions to non-renewable energy use are accounted for, managing the slurry as described in Scenario H allows a reduction of 266 MJ of (primary) non-renewable energy use as compared to the reference scenario.

6.2.9 Consumption of phosphorus as a resource

Both scenarios allows about the same amount of P to be preserved as a resource (through the avoidance of inorganic P fertiliser to be produced). The wheat production avoided in Scenario H (induced by the increased yield) is an extra as compared to the reference scenario but has a rather insignificant contribution in P consumption avoidance, as it can be seen it table 6.1 and figure 6.2.A.

6.2.10 Carbon stored in soils

Through Scenario H, a certain amount of C ends up to be stored in soils, which means this C is not emitted as CO₂. This is through the C of the different slurry fractions that is applied to field and not emitted as CO₂.

For the reference scenario, a total of 3.61 kg C per 1000 kg pig slurry ex-animal is stored in soils, corresponding to 13.2 kg CO₂ not emitted per functional unit. This is considering a horizon of 10 years. With the 100 years values, more CO₂ is emitted and consequently less C is stored per functional unit (1.03 kg), resulting to 3.8 kg CO₂ not emitted per 1000 kg pig slurry ex-animal. These values are presented in table 4.1 of Wesnæs et al. (2009).

For Scenario H, less C is added to field per functional unit but also less CO₂ is overall emitted. As a result, the amount of C sequestrated in the soil per functional unit (3.48 kg C considering 10 years values; 0.64 kg C considering 100 years values) is similar to the amount of C sequestrated for the reference scenario. In terms of CO₂ avoided, this correspond to 12.7 kg CO₂ (10 years values) and 2.4 kg CO₂ (100 years values) per 1000 kg pig slurry ex-animal.

6.3 Uncertainties

The uncertainties on the compared results have been estimated by analysing the most important factors that are changed, when comparing scenario H with the reference scenario A. It means that the uncertainties for each scenario is not analysed as such, but only the emissions that are important for the differences. The uncertainties on the comparisons are based on estimates of the uncertainties on those emissions that are most important for the changes.

The uncertainties are related to the total positive contributions from the reference scenario A (i.e. the total that is set to 100% in figures 6.3 and 6.4 as “index”).

The values of the uncertainty ranges are shown in table 6.4.

6.4 Synthesis of the results for all impact categories assessed

Table 6.4 compares the overall characterised results of Scenario A versus Scenario H, for all impacts categories (including carbon stored in soils). It also presents the uncertainty ranges for all impact category results.

Figures 6.9.A and 6.9.B illustrate the results presented in table 6.4, and give an impression of the uncertainty. The difference between these two figures is the axis, which has a greater range in the case of figure 6.9.B in order to capture the whole impacts of non-renewable energy consumption. Figures 6.10.A and 6.10.B present only the net differences between Scenario A and H, including the uncertainties.

Table 6.4. Comparison of the impacts from Scenario A (reference) versus Scenario H (biogas from raw pig slurry + fibre fraction from mechanical separation). The number of digits is not an expression of the uncertainty.

Click here to see Table 6.4.

Figure 6.9.A Comparison of the environmental impacts from Scenario A (reference) versus Scenario H (biogas from raw pig slurry + fibre fraction from mechanical separation). Axis ranging from -180 to 120.

Click here to see Figure 6.9.A

Figure 6.9.B Comparison of the environmental impacts from Scenario A (reference) versus Scenario H (biogas from raw pig slurry + fibre fraction from mechanical separation). Axis ranging from -1000 to 200.

Click here to see Figure 6.9.B

Figure 6.10.A Comparison of the environmental impacts from Scenario A (reference) versus Scenario H (biogas from raw pig slurry + fibre fraction from mechanical separation). Net difference only. Axis ranging from -100 to 100.

Click here to see Figure 6.10.A

Figure 6.10.B Comparison of the environmental impacts from Scenario A (reference) versus Scenario H (biogas from raw pig slurry + fibre fraction from mechanical separation). Net difference only. Axis ranging from -1000 to 100.

Click here to see Figure 6.10.B

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