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Life Cycle Assessment of Biogas from Separated slurry

5 Biogas production from raw cow slurry and fibre fraction from mechanical-chemical separation (Scenario G) – results and interpretation

• 5.1 System Description
• 5.2 Results of the Impact Assessment
  • 5.2.1 Overall results of the comparison
  • 5.2.2 Global warming
  • 5.2.3 Acidification
  • 5.2.4 Aquatic eutrophication (N)
  • 5.2.5 Aquatic eutrophication (P)
  • 5.2.6 Photochemical ozone formation (“smog”)
  • 5.2.7 Respiratory inorganics (small particles)
  • 5.2.8 Non-renewable energy resources
  • 5.2.9 Consumption of phosphorus as a resource
  • 5.2.10 Carbon stored in soil
  • 5.2.11 Polymer
• 5.3 Uncertainties
• 5.4 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 G”, described below. The results from “Scenario G” are compared to those of the reference scenario for dairy cow slurry management, i.e. “Scenario A”. Doing so, it is therefore possible to answer the research question: “What are the environmental benefits and disadvantages of producing biogas from raw cow slurry and the fibre fraction obtained from a mechanical-chemical separation process, as compared to the reference situation for cow slurry management?”

As it was done for Scenario F, Scenario G was built in such a way that it integrates the “best available technologies” as well as the “best possible practices” as much as possible for an optimal environmental performance. This is important to remember in the results interpretation.

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

5.1 System Description

The system constituting Scenario G, as described in section 2.2.3, consists to produce biogas from a mixture of fibre fraction (from mechanically separated slurry, flocculated with polymer) and raw slurry, both from dairy cows. After excretion, raw slurry is stored in-house; part of it is separated and part of it is kept as raw slurry. These fractions do not necessarily come from the same farm (and most probably they do not), but they both end up at the biogas plant. Once at the biogas plant, these fractions are mixed according to their composition and to their degradability in order to achieve realistic production conditions. The separation process used is considered as a “best available technology” as regarding its efficiency to increase the relative fraction of dry matter and nutrients transferred to the fibre fraction. While the separated liquid fraction is stored and used on-field as an organic fertiliser, the separated fibre fraction, as well as the raw slurry, is then used as an input for biogas production. The temporal storage of the fibre fraction before the fibre is used for biogas production is assumed to be rather short (range of 1 to 3 days with 7 days as a maximum), which is considered as a “best management practice”. Similarly, it was assumed that the raw slurry is stored in the pre-tank for a duration of less than 14 days before it is transferred whether to the biogas plant or to the separation process, which is also considered as a “best management practice”.

The biogas produced from the cow raw slurry and the fibre fraction mixture is used for co-production of heat and power. The biogas engine used for the generation of heat and power is also considered as a “best available technology”, as the engine used has conversion efficiencies ranking in the highest available range.

After the anaerobic digestion, the resulting degassed slurry is mechanically separated, but without polymer addition. The degassed liquid fraction resulting from this separation process is then stored until it can be used on-field as a fertiliser. The resulting degassed fibre fraction is stored as air-tight covered heap, the heap being covered by a polyethylene plastic sheet. Other options are available for the management of the degassed fibre fraction (e.g. processing it in order to make fibre pellets), but covering was considered as a “best available technology”, as explained in Annex G (section G.21.1).

The processes described and used for this scenario were built in collaboration with Xergi A/S and some of the data used were obtained directly from Xergi A/S (see Annex F). 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.

Figure 5.1 presents the process flow diagram for “Scenario G”. The process numbers in figure 5.1 follows the numbers of the sections in Annex G.

Figure 5.1. Process flow diagram for “scenario G” – Biogas from raw cow slurry and fibre fraction of mechanical-chemical separation of cow slurry. The process numbers follows the numbers of the sections in annex G

Click here to see Figure 5.1.

5.2 Results of the Impact Assessment

5.2.1 Overall results of the comparison

Table 5.1 presents the overall environmental impacts from “Scenario G” (biogas from raw cow slurry and fibre fraction from mechanical-chemical separation), and compare them to the environmental impacts from the reference scenario for cow slurry (described in section 3). Figures 5.2 A and 5.2.B illustrate the results presented in table 5.1. Figures 5.2 A and 5.2.B are identical except for the minimum and maximum at the axis. In the case of figure 5.2.B, the minimum and maximum were adjusted in order to present the full impacts covered for the impact “consumption of non-renewable energy resources”. As in section 4, results are presented for soil JB3 only (sandy soil), but a sensitivity analysis assesses the differences in the results that are obtained if another soil type (soil JB6: clay soil) is considered (this is however only performed with the data of Scenario F, see section 8).

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 consumption by the slurry management scenarios. The negative values are “avoided environmental impacts”.

Results presented in table 5.1 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 data presented in Annex G. An attempt to discuss these results based on this focus, impact category per impact category, is presented in sections 5.2.2 to 5.2.11.

Table 5.1. Scenario G 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 5.1.

Figure 5.2.A Overall environmental impacts for the selected impact categories – scenario G vs scenario A. Dairy cow slurry management. Soil type JB3. 10 and 100 years time horizon for global warming and for aquatic eutrophication (N). Axis ranging from -180 to 140.

Click here to see Figure 5.2.A

Figure 5.2.B Overall environmental impacts for the selected impact categories – scenario G vs scenario A. Dairy cow slurry management. Soil type JB3. 10 and 100 years time horizon for global warming and for aquatic eutrophication (N). Axis ranging from -1200 to 200.

Click here to see Figure 5.2.B

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

5.2.2 Global warming

As for Scenario F (which is the same as the present scenario, but with pig slurry), there are two main processes contributing to global warming:

• In-house storage of slurry
  • Scenarios A and G: This process represents about 27 % of the total positive contributions to global warming from the reference scenario.
     
• Field processes
  • Scenario A: This process represents 53 % of the total positive contributions to global warming from the reference scenario (and 62 % if the 100 years values are considered for C horizon in the field).
  • Scenario G: This process (for the aggregation of all organic fertilisers: liquid fraction, degassed fibre fraction and degassed liquid fraction) represents 35 % of the total positive contributions to global warming from the reference scenario (and 44 % if the 100 years values are considered for C horizon in the field).

As for Scenario F, the high contribution from the in-house storage is due to CH₄ emissions. There are only three gases contributing to the global warming potential of this process: CH₄ (76 %), N₂O (14 %) and CO₂ (10 %). The proportions shown in parenthesis are for a 10-years value as regarding the C horizon in the field.

Important emissions from CH₄ were expected for this process, as the anaerobic conditions for slurry stored below animal floors favour CH₄ formation more than the formation of other greenhouse gases. Yet, the high absolute contribution from CH₄ is due to a potentially conservative methodological choice, as detailed in section 4.2.2.

For Scenarios A and G, the contribution of field processes to global warming potential is dominated by biogenic CO₂ (due to C applied emitted as CO₂). In the case of Scenario G, biogenic CO₂ represents 73 % of the positive contributions to global warming from field processes, while N₂O represents 25 %. The contribution to global warming from field processes caused by fossil CO₂ emissions (due to diesel combustion) is 2 %. In fact, there are other gases involved, but their relative contribution is rather insignificant (and not reflected when the percentages are expressed with no decimal place). The biogenic CO₂ emitted in the field is lower for Scenario G than Scenario A (83 kg CO₂ equivalent in Scenario G as compared to 132 kg CO₂ equivalent in Scenario A). This is simply because, in the case of Scenario G, this biogenic CO₂ was emitted in earlier stages, mostly during the co-generation of heat and power. In that case, however, heat and electricity were produced together with the emission.

For Scenario G, it can also be highlighted that the biogenic CO₂ emissions from field processes are the highest with the application of the degassed fibre fraction (49 kg CO₂equivalent) as compared to the degassed liquid fraction (24 kg CO₂equivalent) and the liquid fraction (10 kg CO₂equivalent), respectively. This is because the fibre fraction has the highest C content per functional unit (table 5.2), and because it was considered that, on both a 10 and a 100 years horizon for C, most of the “slowly degradable” portion of the C is also degraded, thus contributing to CO₂ emissions.

As explained in section 4.2.2, high contributions to global warming potential from N₂O were expected for field processes, as N₂O has a 100-years global warming potential of 296 kg CO₂ equivalent per kg N₂O, based on the EDIP method (which is in turn based on IPCC, 2001). Because, for all fractions applied to field, the emission of N₂O were estimated based on the IPCC methodology (IPCC, 2006), the N₂O emissions are function of the N content in the applied slurry. Table 5.2 shows the C and N content of the different fractions involved in scenario A and G.

Table 5.2. C and N content of the different organic fractions involved in Scenario A and G

The degassed liquid fraction thus has the highest contribution to N₂O, as it has the highest N content per functional unit, followed by the liquid fraction and the degassed fibre fraction. The contribution of N₂O to global warming from field processes is higher for Scenario A than Scenario G (29 kg CO₂ equivalent in Scenario G and 40 kg CO₂ equivalent in Scenario A). If the emissions of N₂O-N represent 3 to 5 % of the N applied as suggested by Crutzen et al. (2008) rather than the 1 % of the IPCC methodology (IPCC, 2006) as used in this study, the global warming contribution from field processes would be much more important, as a small increment of N₂O has huge impacts on global warming potential.

The co-generation of heat and power from biogas also represents, in the case of Scenario G, a rather significant contribution to global warming, representing 19 % of the total positive contributions to global warming from the reference scenario. This, as explained in section 4.2.2, is mainly due to the combustion gases from burning the biogas (i.e. CH₄ and CO₂) in the biogas engine. The production of biogas itself (process G.15 of Annex G) represents a minor share of the contribution to global warming potential, with only 2% of the total positive contributions to global warming from the reference scenario.

Another interesting observation to highlight from figure 5.2.A is the benefit on global warming contribution obtained through storing the slurry as separated (and degassed) fractions rather than as raw slurry. While storage of raw slurry represents 18 % of the total positive contributions to global warming from the reference scenario, the contribution from separated liquid is 2.4 %, and it is 0.9 % and 4 % for the degassed fibre fraction and the degassed liquid fraction, respectively. This is mainly because of lower CH₄ emissions due to the separation (and digestion) of the VS, as explained in section 4.2.2.

Finally, as it can be observed from figure 5.2.A, the contributions to global warming from the use of electricity, the use of polymer for separation and the transportation are rather negligible.

If both slurry management assessed allow to avoid the use of inorganic fertilisers (N, P and K), the biogas scenario also allow to avoid the production of marginal heat and electricity (see definition section 2.3). Avoiding the production of marginal electricity (a mix of wind, coal and natural gas, see table 2.1) by the use of the electricity produced from the biogas allow considerable benefits in terms of global warming contribution avoided. This corresponds to an “avoidance” of 19 % of the total positive contributions to global warming from the reference scenario. Avoiding the production of marginal heat (i.e. 100 % coal, see table 2.1) through the heat produced from the biogas also has a positive impact on global warming contribution (an avoidance of about 6 % of the total positive contributions to global warming from the reference scenario).

Avoiding the production and use of inorganic fertilisers (particularly N, but also P and K to a lesser extent) through the use of the produced organic fertilisers contribute, for both Scenario A and Scenario G, to the avoidance of global warming potential, and this avoidance is in the same order of magnitude for both scenarios. Avoiding the production and use of inorganic N avoids the production of N₂O which represents the main reason for the magnitude of avoided contribution to global warming for this process. As explained in Annex G (section G.28.2), the amount of N avoided is the same in Scenario G than in the reference scenario as the “rule of conservation” (Gødskningsbekendtgørelsen, 2008) was applied to calculate the amount of inorganic N substituted by each organic fractions. The amount of inorganic P and K substituted in Scenarios A and G are different, but this does not affect the avoided contribution to global warming, as they are in the same order of magnitude for both scenarios. In the case of avoided P and K, the benefits are mostly due to the avoided fossil CO₂.

Similarly, the higher wheat production obtained through higher yields in Scenario G also allows to avoid contribution to global warming (mostly through N₂O), though this is rather small.

Overall, when the “deductions” from the avoided contributions to global warming are accounted for, managing the cow slurry as described in Scenario G allows a net reduction of 107 kg CO₂ equivalent as compared to the reference scenario (figure 5.3).

Table 5.3 summarises, for selected processes, the contribution of the main contributing substances to global warming, for both Scenario A and G.

Table 5.3. Scenario G 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

Managing cow slurry as described in scenario G appears as an interesting mitigation strategy as regarding the efforts to reduce the global warming impacts.

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

• Overall, managing the slurry as in Scenario G allows, based on the reference scenario considered, to reduce significantly the contributions to global warming from cow 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 the application of the different slurry fractions. The contribution to global warming from field process is much lower in the case of Scenario G as compared to Scenario A.
     
• Storing slurry in separated phases (with the separation efficiencies considered in Scenario G) has considerable benefits on global warming contribution as compared to storage of raw slurry.
   
• The contributions to global warming from the use of electricity, the use of polymer for separation 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 similar magnitude.
   
• Scenario G allows avoiding the production of marginal heat and electricity, which has considerable benefits on global warming contributions. Avoided wheat production resulting from yield increases in Scenario G also contribute to additional avoided contributions to global warming, though the magnitude of it is rather small.

This information is summarised in figure 5.3. This figure presents the contribution to climate change of Scenario A and G only for the processes that are not equal between A and G (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 5.3. Comparison of Scenario G vs Scenario A for global warming including carbon sequestration, for processes differing between A and G only. Soil JB3, 10 years values.

Click here to see Figure 5.3

5.2.3 Acidification

The contribution to acidification is dominated by 2 processes:

• Field processes:
  • Scenario A: This represents about 52 % of the total positive contributions to acidification from the reference scenario.
  • Scenario G: This represents about 48 % of the total positive contributions to acidification from the reference scenario.
     
• In-house storage: For both scenarios, this process represents about 37 % of the total positive contributions to acidification from the reference scenario.

The main contributing substance to acidification from field processes is NH₃, accounting for 98 % of the contributions from this process from both scenarios.

In Scenario G, the highest potential to acidification from the field processes comes from the application of the degassed liquid fraction (64 %), followed by the application of the liquid fraction (25 %) and the degassed fibre fraction (11 %). The NH₃ emissions for the degassed liquid fraction were evaluated with the same estimation as for raw slurry, i.e. as a function of the TAN in the raw slurry. This approach was used as digestion affects both factor promoting and reducing the propensity for volatilisation, as discussed in section 4.2.3 of this report and section G.27.3 of Annex G. If factors reducing the propensity for volatilisation predominate over those promoting it, then the estimation for ammonia emissions would be slightly overestimated for the degassed liquid fraction. This would have some influence but it would not change the overall results of the comparison. For example, cutting the actual ammonia emissions from the digested liquid fraction of 50 % would yield a total acidification potential from the field processes of 14 m² unprotected ecosystem, as compared to 21 m² as it is now. As a comparison, the in-house storage has an acidification potential of 16 m² unprotected ecosystem.

As described in Annex G, there are no differences in the in-house storage between Scenario A and G. The main contributing substance to acidification for the in-house storage of slurry is NH₃, contributing to 98 % of the total acidification potential for this process. The other contributor to acidification from in-house slurry storage is NO_x, with the remaining 2 % of the total acidification potential for this process.

Apart from the in-housing slurry storage and the field processes, “co-generation of heat and power from the biogas” and “outdoor storage” (aggregated for all fractions) can be distinguished as contributors to acidification for Scenario G, though their extent is much lower. For the “co-generation of heat and power”, the main contributor (91%) to acidification from the process is NO_X. This is emitted during the combustion of biogas in the biogas engine. The process “co-generation of heat and power” represents 8.0 % of the total positive contributions to acidification from the reference scenario.

The storage of slurry represents 12 % of the total positive contributions to acidification from the reference scenario. The main contributor to acidification from the storage processes is NH₃ (88 %), but there are also some contributions from NO_X (12 %). Among the different fractions to store, the degassed liquid fraction has the highest contribution to acidification followed by the degassed fibre fraction and the liquid fraction. For all fractions, NH₃emissions were estimated as a function of the N content in it (NH₃-N = 2 % of the N content for both liquid fractions and 5.75 % for fibre fraction, see Annex G). The N content per functional unit for stored degassed liquid is higher (3.83 kg N per 1000 kg slurry ex-animal) than for the other fractions (1.47 kg N per 1000 kg slurry ex-animal for the liquid fraction and 1.03 kg N per 1000 kg slurry ex-animal for the degassed fibre fraction), explaining the highest emissions for the storage of that fraction (table 5.2). The estimate used for the fibre fraction (5.75 %) is lower than the estimate recommended by Hansen (2009) (i.e. 13 %: see Annex G). Therefore, the emissions of NH₃ from the fibre fraction and consequently the contribution to acidification may be higher in reality than as estimated in this project.

As discussed in section 4.2.3, the storage of the fibre fraction at the farm (process G.8), just before it is sent to the biogas plant, was assumed to be without any losses, due, among others, to the temporal nature of this process (see section G.8 of Annex F). Yet, as mentioned in section 4.3.2, NH₃ from separated fibre fraction of animal slurries is recognised as a “hot spot” from slurry management involving separation, so this “no losses” assumption may have contributed to underestimate the overall acidification potential of Scenario G.

The contribution to acidification from transport, use of electricity and use of polymer for separation are rather negligible.

Both scenarios allow avoiding the use of inorganic fertilisers and this contributes, in the two cases, to avoid the same magnitude of acidification potential. However, Scenario G also contributes to avoid the production of marginal heat and marginal electricity, which is translated by an additional credit on the acidification potential (see figure 5.2.A). For both avoided marginal heat and electricity, the main avoided contributor to acidification is SO₂. The higher yield in Scenario G contributes to avoid the production of a given amount of crop (here modelled as wheat) and consequently the related contribution to acidification from it. Yet, this is rather small, as it can be seen in table 5.1 and figure 5.2.A.

Overall, when the “deductions” from the avoided contributions to acidification are accounted for, managing cow slurry as described in Scenario G allows a net reduction of 2.36 m² area of unprotected ecosystem (UES) as compared to the reference scenario (figure 5.4).

Table 5.4 summarises, for selected processes, the contribution of the main contributing substances to acidification, for both Scenario A and G.

Table 5.4. Scenario G 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] This includes other contributing substances which are not reflected when contributions are presented with 2 decimal places.

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

[c] This is not a zero value.

Based on these results, it can be concluded that managing slurry as described in Scenario G does not allow significant environmental benefits as regarding acidification potential, as compared to the reference slurry management.

The major results as regarding acidification can be summarised as:

• Overall, managing the slurry as in Scenario G does not allow significant environmental benefits as regarding acidification potential.
   
• There are 2 major hot spots are 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 greater for Scenario G and NH₃ is the main contributor. This is mostly due to the storage of the degassed fibre fraction, even though the degassed fibre fraction is not the highest absolute contributor among the 3 fractions to store (because of the overall amount per functional unit).
   
• The contributions to acidification from the use of electricity, the use of polymer for separation, and the transportation are rather negligible in both scenarios.
   
• Both scenarios allow avoiding the contributions to acidification from the production of inorganic N, P and K fertilisers in similar magnitude. Scenario G allows to avoid the production of marginal heat and electricity, which has additional benefits on acidification contributions. In a much smaller extent, the avoided wheat production resulting from increased yield also contributes to avoid contributions to acidification if Scenario G is implemented as compare to Scenario A.

This information is summarised in figure 5.4. This figure presents the contribution to acidification of Scenarios A and G only for the processes that are not equal between A and G (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 5.4. Comparison of Scenario G vs Scenario A for acidification, for processes differing between A and G only. Soil JB3, 10 years values.

Click here to see Figure 5.4

5.2.4 Aquatic eutrophication (N)

This impact category is, for both scenarios, dominated by field processes. Field processes represent, for Scenario A, 89 % of the total positive contributions to eutrophication (N) from the reference scenario (10 years value for C horizon in the field). For Scenario G, field processes represent 88 % of the total positive contributions to eutrophication (N) from the reference scenario (10 years value for C horizon in the field). In both cases, the main contributing substance is N leaching through soil. For Scenario G, N leaching through soil represents about 88 % of the contributions to N-eutrophication from field processes, and NH₃ represents about 12 % to it.

The use of the degassed liquid fraction in the field has the highest contribution to N-eutrophication as compared to the liquid fraction and the degassed fibre fraction. This is because, there is, for the the degassed liquid fraction, a greater N content per functional unit (3.69 kg N per 1000 kg slurry ex-animal) as compared to the liquid fraction (1.40 kg N per 1000 kg slurry ex-animal) and the degassed fibre fraction (0.97 kg N per 1000 kg slurry ex-animal) (table 5.2).

In-house storage of slurry represents about 8 % of the total positive contributions to eutrophication (N) from the reference scenario (for the 10 years values for C horizon in the field), and this is mostly due to NH₃ (97 %). All other processes are contributing rather insignificantly.

The production (and use) of inorganic N is avoided in both scenarios, and this contribute to a quite important avoidance of N-eutrophication to occur (64 % of the total positive contributions to N-eutrophication from the reference scenario, for both Scenario A and Scenario G and for the 10 years values). The avoided production of heat and electricity in Scenario G has a minor impact on the N-eutrophication potential avoided.

The avoided wheat production resulting from higher yield in Scenario G also contributes to avoid eutrophication-N potential, but the importance of this is rather small, as it can be visualised from figure 5.2.A.

Overall, when the “deductions” from the avoided contributions to N-eutrophication are accounted for, the difference between managing the slurry as described in Scenario G as compared to the reference slurry management allows a net difference of 0.04 kg N reaching aquatic recipients, however, when accounting for uncertainties, this benefit could be reduced to 0 (figure 5.10.A). This is for the 10 years value for C horizon in the field.

The major findings as regarding N-eutrophication potential can be summarised as:

• Managing slurry as described in Scenario G does allow a small benefit as regarding aquatic N-eutrophication, as compared to slurry management described in the reference scenario. This benefit may however be negated considering the values of the uncertainty range.
   
• Field process is the main hot spot as regarding aquatic N-eutrophication: N leaching through soil is the main contributing substance to N-eutrophication for this process.
   
• In-house storage, in both scenarios, also has some contributions to aquatic N-eutrophication, and this is mainly due to NH₃ emissions.
   
• Avoiding inorganic N fertilisers to be produced allows, in both scenarios, to avoid significant contribution to aquatic N-eutrophication.

This information is summarised in figure 5.5. This figure presents the contribution to N-eutrophication of Scenarios A and G only for the processes that are not equal between A and G (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 5.5. Comparison of Scenario G vs Scenario A for aquatic N-eutrophication, for processes differing between A and G only. Soil JB3, 10 years values.

Click here to see Figure 5.5

5.2.5 Aquatic eutrophication (P)

For aquatic P-eutrophication, there is also one major hot spot: field processes, contributing to this environmental impact in approximately the same extent in both Scenario A and Scenario G. In both scenarios, it represents about 99 % of the total positive contributions to eutrophication (P) from the reference scenario, meaning that all other processes contribute rather insignificantly to this environmental impact. For field processes in both scenarios, P leaching to soil contributes to 99 % of the substances contributions to this impact category.

For Scenario G, it can be highlighted that, for the field processes, the degassed fibre fraction contribute to about 56 % of the total positive contributions to eutrophication (P) from the reference scenario, while it is 29 % for degassed liquid fraction and 15 % for the liquid fraction. For all organic fertilisers involved (liquid fraction, degassed fibre fraction and degassed liquid fraction), it was considered that P leaching to soil (the main contributor to P-eutrophication for field process) 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 G.7, G.23 and G.27 of Annex G. The differences obtained between the two liquid fractions therefore reflect the efficiency of the first separation to separate the P in the solid fraction, thus explaining a much lower contribution from the liquid fraction.

The assumption used to estimate P leaching involves some uncertainties, as detailed in Wesnæs et al. (2009) (section 3.4.6 and section A.5.6, among others). In fact, the P actually reaching the aquatic recipients is also a function of the soil type and of the P already present in the soil. However, the purpose of the present study is comparative, and the same assumptions have been applied to both scenarios. In this perspective, the conclusions of this study are reliable, i.e. P leaching through soils is the main contributor to aquatic P-eutrophication for both scenarios. However, the “real” magnitude of P leaching through aquatic recipient (in kg P per 1000 kg slurry ex-animal) may be different than as presented in Annex A and G of the present study.

Avoiding inorganic P fertilisers to be produced and applied contributes, in both scenarios, to an important share of the avoidance of aquatic P eutrophication. This effect is however more important for Scenario G. This is because in Scenario G, the P is not applied in excess as most of it is applied via the degassed fibre fraction, which is applied on a field where P is the limiting nutrient for crop growth. Therefore, all organic P applied in Scenario G corresponds to avoided inorganic P, as compared to the reference scenario where only a share of the organic P substitutes inorganic P, the rest being pure excess. To a much smaller extent, avoiding N and K fertilisers to be used and produced also allow to avoid P-eutrophication potential.

Similarly to N-eutrophication, the avoided wheat production resulting from higher yields in Scenario G has here some importance, the avoidance of P-eutrophication representing about 3 % of the total positive contributions to P-eutrophication from the reference scenario.

Avoiding the production of marginal heat and electricity has, for Scenario G, only a minor effect on the avoidance of P-eutrophication potential.

One interesting point about P eutrophication potential is that it is an impact category where the net contributions are negative. This means that whether the slurry is managed as in Scenario A or G, the fact of avoiding inorganic P fertilisers to be produced/used overcome the contribution to P-eutrophication from managing the slurry itself. This statement, however, is only true if 1 kg P in slurry contributes equally to P-eutrophication as 1 kg P in mineral fertiliser, which might not be the case.

Overall, when the “deductions” from the avoided contributions to P-eutrophication are accounted for, there is a gain in managing the slurry as described in Scenario G as compared to the reference slurry management. This is illustrated in figure 5.6.

Figure 5.6. Comparison of Scenario G vs Scenario A for aquatic P-eutrophication, for processes differing between A and G only. Soil JB3, 10 years values.

Click here to see Figure 5.6

The major results as regarding P-eutrophication potential can be summarised as:

• Managing slurry as described in Scenario G results in a gain as compared to Scenario A.
• Field process is the main hot spot as regarding aquatic P-eutrophication: P leaching is the main contributor to this process.

5.2.6 Photochemical ozone formation (“smog”)

In both scenarios, there are 2 main hot spots for photochemical ozone formation. Common to both scenarios is:

• In-house storage, representing about 56 % of the total positive contributions to photochemical ozone from the reference scenario.

For Scenario A, outdoor storage is also a hot spot for ozone formation, representing 39 % of the positive contributions from the reference scenario. For Scenario G, the co-generation of heat and power is the second hot-spot, representing 32 % of the positive contributions from the reference scenario.

In the case of in-house storage, the main contributor is CH₄, which represents about 94 % of the contribution to ozone formation for this process. The overall emissions of CH₄ from in-house may have been overestimated, as discussed in section 4.2.2. The alternative MCF value for estimating CH₄ from in-house slurry storage (see description section 4.2.2) would lead to a reduction of 0.06807 person*ppm*h, representing a reduction of about 76 % of the ozone formation potential. In spite of this, CH₄ would remain the main contributor to ozone formation for in-house storage.

For co-generation of heat and power, the main contributor is NO_x, representing about 85 % of the contribution to ozone formation for this process. This is emitted during the combustion of biogas in the biogas engine.

In Scenario A, CH₄ emission is the main contributing substance (78 %) to ozone formation impact for the outdoor storage process. As it can be observed in table 5.1, slurry storage is much less significant for Scenario G than Scenario A. This is due, as described in section 4.2.2, to the lower VS content of separated and degassed slurry, thus involving a much lower potential for CH₄ emissions. This again highlights the positive effect of slurry separation and digestion as regarding CH₄ emissions during slurry storage.

The production of biogas itself also contributes to photochemical ozone formation through the small amount of CH₄ leakages that was assumed (CH₄ represents 84 % of the contributions for this process).

Avoiding inorganic fertilisers to be produced (N, but to a smaller extent P and K) also contribute to reduce the contribution to ozone formation, in similar magnitude for both scenarios. However, the avoided production of marginal heat and electricity in Scenario G contribute to “extra” avoidance of contribution to photochemical ozone. So does the avoided wheat production induced by increased yield, but this represents a rather small avoided contribution.

Overall, when the “deductions” from the avoided contributions to photochemical ozone formation are accounted for, the difference between managing the slurry as described in Scenario G as compared to the reference slurry management is 0.01 pers*ppm*h (figure 5.7). However, when taking the uncertainties into account, this difference is not significant (figure 5.10.A).

The major results as regarding photochemical ozone formation potential can be summarised as:

• There are no significant benefits in managing the slurry as in Scenario G for the impact category photochemical ozone formation.
   
• For both scenarios, in-house storage is a hot spot process as regarding ozone formation impact, essentially because of CH₄ emissions.
   
• For Scenario G, the emissions of NO_X during the combustion of biogas for co-generation of heat and power also contribute significantly to ozone formation potential.
   
• Contributions to ozone formation during slurry storage are much lower in Scenario G as compared to Scenario A due to the positive effect of slurry separation and digestion on CH₄ emission potential.

This information is summarised in figure 5.7. This figure presents the contributions to photochemical ozone formation of Scenarios A and G only for the processes that are not equal between A and G (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 5.7. Comparison of Scenario G vs Scenario A for photochemical ozone formation, for processes differing between A and G only. Soil JB3, 10 years values.

Click here to see Figure 5.7

5.2.7 Respiratory inorganics (small particles)

This impact category involves, for both scenarios, 2 main contributing processes. These are:

• In-house storage of slurry, representing, for both scenarios, about 35 % of the total positive contributions to respiratory inorganics from the reference scenario.
   
• Field processes
  • Scenario A: This represents about 49 % of the total positive contributions to respiratory inorganics from the reference scenario.
  • Scenario G: This represents about 45 % of the total positive contributions to respiratory inorganics from the reference scenario (11% liquid fraction; 5% degassed fibre fraction and 29 % degassed liquid fraction).

For Scenario G, another important contributor is the co-generation of heat and power from biogas, representing 19 % of the total positive contributions to respiratory inorganics from the reference scenario.

For in-house storage, the main contributor is NH₃ emissions, representing about 93 % of the contribution to respiratory inorganics for this process. This, as discussed in section 5.2.3, emphasises the importance of reducing NH₃ from animal buildings.

For field processes, in Scenario G, NH₃ is also the main contributor to respiratory inorganics, accounting for 95 % of the contributions for this process. The contribution from the degassed liquid fraction is higher than the other two organic fertilisers for the same reasons as explained in section 4.2.3 (acidification).

Co-generation of heat and power from the biogas has for main contributor to “respiratory inorganics” NO_X, which are emitted during the combustion of the biogas in the biogas engine (NO_x represents 96 % of the total contributions to respiratory inorganics for this process).

Avoiding inorganic fertilisers to be produced (N, but to a smaller extent P and K) also contributes to reduce the contribution to respiratory organics, in similar magnitude for both scenarios. However, the avoided production of marginal heat and electricity in Scenario G contributes to “extra” avoidance of contribution to “respiratory inorganics”. So does the avoided wheat production induced by increased yield, but this represents a rather small avoided contribution.

Overall, when the “deductions” from the avoided contributions to “respiratory inorganics” are accounted for, there are no significant benefits to manage the slurry as described in Scenario G in comparison the the reference slurry management. This is illustrated in figure 5.8 and figure 5.10.A.

The major results as regarding respiratory inorganics potential can be summarised as:

• There are no significant benefits obtained with Scenario G as compared to the reference scenario for the impact category “respiratory inorganics”.
   
• Two main processes contribute to “respiratory inorganics”: in-house storage of slurry and field processes, for both Scenario G and the reference scenario. For these two processes, the main responsible substance to “respiratory inorganics” is NH₃.
   
• Co-generation of heat and power for Scenario G also contributes significantly to “respiratory inorganics”, and in this case it is mainly due to NO_X emissions during biogas combustion in the engine.
   

This information is summarised in figure 5.8. This figure presents the contribution to “respiratory inorganics” of Scenarios A and G only for the processes that are not equal between A and G (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 5.8. Comparison of Scenario G vs Scenario A for respiratory inorganics, for processes differing between A and G only. Soil JB3, 10 years values.

Click here to see Figure 5.8

5.2.8 Non-renewable energy resources

This category involves, for both Scenario G and Scenario A, the following common hot spot:

• Transport process
  • Scenario A: 41 % of the total positive contributions to “use of non-renewable energy resources” from the reference scenario. The main contributing substance for this process is crude oil (58 %).
  • Scenario G: 73 % of the total positive contributions to “use of non-renewable energy resources” from the reference scenario. The main contributing substance for this process is crude oil (69 %).
     
• Use of energy (mostly electricity but also heat). This is translated by:
  • Process “outdoor storage-electricity” for Scenario A;
  • Process “Biogas production” for Scenario G;

This impact category is the only category where transport is significant and reflects the use of diesel to fuel the tractors and to transport the slurry by trucks between the farm and the biogas plant.

In the case of Scenario A, “field process” contributes to the use of non-renewable energy in about the same magnitude as “outdoor storage-electricity”. In the case of field process, this reflects the use of diesel for spreading operations.

The process requiring energy inputs appeared as hot spots for the non-renewable energy resources. For Scenario G, the production of biogas can be highlighted, representing 110 % of the total positive contributions to “use of non-renewable energy resources” from the reference scenario. The main contributors to the “non-renewable energy resources” impact from this process are hard coal (79 %) and natural gas (14 %). This is because of the consumption of electricity and heat for running the anaerobic digestion process (see section G.15.3, Annex G). The electricity consumption for producing biogas was estimated as 5% of the net energy production from the biogas. As discussed in section G.15.3 of Annex G, this may however be higher and represent 10 % of the net energy production. In such a case, this would simply double to amount of energy used, and increase the contribution of the biogas production process to the “non-renewable energy resources” category accordingly. The heat consumption for the process was calculated based on heating the biomass mixture from 8°C to the process temperature of 37°C (a temperature difference of 29°C). Reducing this temperature difference through heat exchangers would contribute to slightly attenuate the contribution from the biogas process to the present impact category, but this is likely to represent a rather small contribution overall, as the negative contributions for this impact category are much larger than the positive contributions.

This means, in the case of Scenario G, that producing biogas allow to avoid the consumption of non-renewable energy much more than it contribute to it. This is particularly due to the consumption and production of marginal electricity that is avoided, but also to avoided marginal heat and fertilisers (particularly N). Avoiding fertilisers to be produced and used also has a positive effect on the non-renewable energy consumption for Scenario A, and such benefits are in the same order of magnitude for both Scenarios, except for P where a little more non-renewable energy is saved in the case of Scenario G, as this scenario involve no excess P.

In the case of Scenario G, the avoided wheat production induced by higher yield also contributes to avoided contributions to “non-renewable energy” impact category, but this is rather small as compared to the contributions from the avoided fertilisers and avoided energy.

For this impact category, all storage process appears rather insignificant contributors, as they all require little or no energy input.

Overall, when the “deductions” from the avoided contributions to non-renewable energy use are accounted for, managing the slurry as described in Scenario G allows a reduction of 946 MJ of (primary) non-renewable energy use as compared to the reference scenario.

The major results as regarding the use of non-renewable energy resources can be summarised as:

• Managing the slurry as described in Scenario G allow significant reductions of the use of non-renewable energy resources, as compared to the management of slurry as in the reference scenario.
   
• For Scenario G, the positive contributions to this impact are more than the double of those from Scenario A; on the other hand, the avoided contributions to this impact are approximately 3 times higher for Scenario G than Scenario A, which makes Scenario G overall more environmentally beneficial as regarding this impact category.
   
• The main contributors to this impact are transport processes as well as processes involving energy input. In both cases, the main contributors are fossil resources use (crude oil, hard coal and natural gas, among others).
   
• Avoiding marginal electricity to be produced (through the use of biogas for electricity) contribute significantly to avoid the consumption of non-renewable energy resources. In an important (but smaller) extent, avoiding N fertilisers to be produced as well as marginal heat also contributes to avoid the use of non-renewable energy resources consumption.

5.2.9 Consumption of phosphorus as a resource

This impact category aims to reflect the importance of phosphorus as a limited and valuable resource. Accordingly, any scenario allowing preventing the use of this limited resource can be highlighted. In the case of this study, this category allows to emphasise the benefits of using slurry as an organic source of P rather than using inorganic P from the limited reserves. As Scenario G allows to save more P (because all the slurry P contributes to avoid inorganic P to be produced, the P not being applied in excess as in Scenario A: section A.6.1 of Annex A and G.28.4 of Annex G), it provides more benefits as compared to Scenario A regarding this impact category. The avoided wheat production induced by higher yields in Scenario G also allows a tiny additional contribution in avoiding P resources to be consumed.

Based on this result, it can be concluded that managing slurry as described in Scenario G offers more advantages over Scenario A as regarding the consumption of P (the difference is 0.13 kg P being saved). This is due to a better management of the P in Scenario G, as the P is concentrated in the solid fraction which is applied in a field where P is deficient. Yet, if this P would be applied in excess as in Scenario A, this benefit would be lost, which highlights the importance of using the P-rich fertiliser produced in Scenario G in a field where it is needed.

5.2.10 Carbon stored in soil

Through Scenario G, 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 6.95 kg C per 1000 kg cow slurry ex-animal is stored in soils, corresponding to 25.5 kg CO₂ not emitted per functional unit. This is when considering a horizon of 10 years for C. With the 100 years values, more CO₂ is emitted and consequently less C is stored per functional unit (1.98 kg), resulting to 7.3 kg CO₂ not emitted per 1000 kg cow slurry ex-animal. These values are presented in table 4.2 of Wesnæs et al. (2009).

For Scenario G, less C is added to field per functional unit but also less CO₂ is emitted in the field (as this CO₂ has been emitted already during the combustion of biogas). As a result, the amount of C sequestrated in the soil per functional unit (6.09 kg C considering 10 years values; 1.69 kg C considering 100 years values) is similar to the amount of C sequestrated for the reference scenario. In terms of CO₂ avoided, this corresponds to 22.3 kg CO₂ (10 years values) and 6.2 kg CO₂ (100 years values) per 1000 kg cow slurry ex-animal.

5.2.11 Polymer

This scenario involves the use of cationic polyacrylamide (PAM) as a polymer during the separation (0.60 kg cationic PAM per 1000 kg slurry input in the separation process). This represents 0.28 kg cationic PAM per functional unit. The impact of the polymer fabrication was considered in the assessment, but the fate of this polymer in the environment could not be assessed due to a lack of data.

As explained in section 4.2.11, evidences from the literature suggest that the use of cationic PAM may represent an important concern in the environment. This could not be reflected in this life cycle assessment. Given the importance of the potential risk associated with the cationic PAM fate in the environment, it is suggested to investigate this aspect more deeply before implementing any large scale slurry management projects involving the use of cationic PAM.

5.3 Uncertainties

The uncertainties on the compared results have been estimated by analysing the most important factors that are changed, when comparing scenario G 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 contribution from the reference scenario A (i.e. the total that is set to 100% in figures 5.9 and 5.10 as “index”).

The values of the uncertainty ranges are shown in table 5.5.

5.4 Synthesis of the results for all impact categories assessed

Table 5.5 compares the overall characterised results of Scenario A versus Scenario G, for all impacts categories (including carbon stored in soils). It also presents the uncertainty ranges for all impact category results.

Figures 5.9.A and 5.9.B illustrate the results presented in table 5.5, 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 5.9.B in order to capture the whole impacts of non-renewable energy consumption. Figures 5.10.A and 5.10.B present only the net differences between Scenario A and G, including the uncertainties.

Table 5.5. Comparison of the impacts from Scenario A (reference) versus Scenario G (biogas from raw cow slurry + fibre fraction from chemical-mechanical separation). The number of digits is not an expression of the uncertainty.

Click here to see Table 5.5.

Note: All numbers are all rounded and accordingly, if the reader calculates the difference, it might vary slightly from the numbers shown in the table.

Figure 5.9.A Comparison of the environmental impacts from Scenario A (reference) versus Scenario G (biogas from raw cow slurry + fibre fraction from mechanical-chemical separation). Axis ranging from -180 to 120.

Click here to see Figure 5.9.A

Figure 5.9.B Comparison of the environmental impacts from Scenario A (reference) versus Scenario G (biogas from raw cow slurry + fibre fraction from mechanical-chemical separation). Axis ranging from -1200 to 200.

Click here to see Figure 5.9.B

Figure 5.10.A Comparison of the environmental impacts from Scenario A (reference) versus Scenario G (biogas from raw cow slurry + fibre fraction from mechanical-chemical separation). Net difference only. Axis ranging from -100 to 100.

Click here to see Figure 5.10.A

Figure 5.10.B Comparison of the environmental impacts from Scenario A (reference) versus Scenario G (biogas from raw cow slurry + fibre fraction from mechanical-chemical separation). Net difference only. Axis ranging from -1000 to 200.

Click here to see Figure 5.10.B

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Version 1.0 August 2010, © Danish Environmental Protection Agency