|
Life Cycle Assessment of Biogas from Separated slurry 7 Biogas production from raw pig slurry and fibre pellets (Scenario I) – results and interpretation
This section presents the results and the interpretation from the life cycle assessment carried out for “Scenario I”. The results from “Scenario I” 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 utilising pig slurry for producing fibre pellets and utilising the fibre pellets for biogas production - compared to the reference scenario for pig slurry?”. As for Scenario H, the present scenario does not relies 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 I. All life cycle inventory data used for the results presented in this section can therefore be found in Annex I. The environmental impacts and conclusions in this section to a great extent build on data and information delivered by the producer of the technology, Samson Bimatech, and on data made from 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 the data. 7.1 System descriptionThe system constituting Scenario I, as described in section 2.2.5, consists to use pig slurry for the production of fibre pellets, and these fibre pellets are used for biogas production in a mixture with raw pig slurry. After excretion, raw slurry is stored in-house; part of it is separated and part of it is kept as raw slurry. The separation process used is the same mechanical separation technology as used for Scenario H. The fibre fraction from this separation is used as an input for producing fibre pellets, while the liquid fraction is used as an organic fertiliser. The production of pellets also involves the production of ashes which are used on-field as an organic fertiliser. The pellets and un-separated raw slurry are used for biogas production. The biogas produced is used for co-production of heat and power. The degassed biomass effluent resulting from the anaerobic digestion process is stored and used on-field as an organic fertiliser. The flow of the pig slurry “from farm to soil” is shown in figure 7.1. The process numbers refer to the heading of the sections in Annex I. Figure 7.1. Process flow diagram for “scenario I” – biogas production from raw pig slurry and fibre pellets. The process numbers follows the numbers of the sections in annex I. 7.2 Results of the Impact Assessment7.2.1 Overall results of the comparisonTable 7.1 presents the overall environmental impacts from “Scenario I” (biogas from raw pig slurry and fibre pellets) and compare them to the environmental impacts from the reference scenario for pig slurry (described in section 3). Figures 7.2 A and 7.2.B illustrate the results presented in table 7.1. Figures 7.2 A and 7.2.B are identical except for the minimum and maximum at the x-axis. In the case of figure 7.2.B, the minimum and maximum were adjusted in order to present the full impacts covered for 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 7.1 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 I. An attempt to discuss these results based on this focus, impact category per impact category, is presented in sections 7.2.2 to 7.2.10. Table 7.1. Scenario I 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] Figure 7.2.a Overall environmental impacts for the selected environmental impacts categories – Scenario I 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 7.2.a Figure 7.2.b Overall environmental impacts for the selected environmental impacts categories – Scenario I 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 -400 to 250. Click here to see Figure 7.2.b In the following sections, the benefits (and shortcomings) of producing biogas as described in Scenario I instead of the reference slurry management are discussed in details for each impact categories. Sensitivity analyses were performed for some processes, but are only presented using the data of Annex F, see section 8. 7.2.2 Global warmingDifferent trends can be observed from figure 7.2.A as regarding global warming. First, when analysing the positive contributions, it can be seen that there are two major hot spots to global warming contribution:
As for the other scenarios assessed, 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 (84 %), N2O (12 %) and CO2 (4 %). The proportions shown in parenthesis are for Scenario I and for the 10-years value as regarding the C horizon considered for field processes. 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 Scenario I, the contribution of field processes to global warming potential is dominated by biogenic CO2 (due to C applied emitted as CO2) (66 % of the contribution to global warming from field processes) and N2O (31 %). The contribution to global warming from field processes caused by fossil CO2 emissions (due to diesel combustion) is 2 %. The balance comes from a multitude of other substances, having small contributions all together. 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, in turn based on IPCC, 2001, table 6.7). 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. This is why the liquid fraction has the highest contribution to N2O, as it has the highest N content per functional unit. The ashes, assumed to contain no N, therefore do not contribute to generate N2O. The contribution of N2O to global warming from field processes is similar for Scenario A than Scenario I (32 kg CO2 equivalent in Scenario I and 35 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. It shall also be emphasised that the contribution to global warming from field processes from Scenario I are below those of scenario A (table 7.2), but the difference is not as big as in the case of the other scenarios. In the present case, the difference between scenario I and A for field processes is mainly due to biogenic CO2 emissions, which are lower for Scenario I (67.6 kg CO2 equivalent in Scenario I as compared to 88.6 kg CO2 equivalent in Scenario A).This reflects the fact that a share of the C is recuperated for energy production in Scenario I, thus not available for CO2 emissions in the field. For Scenario I, the biogenic CO2 emissions from field processes (i.e. the portion of the C from slurry applied that is emitted as CO2) are the highest with the application of the liquid fraction (51.5 kg CO2equivalent) as compared to the degassed biomass mixture (16 kg CO2equivalent). This is because the liquid fraction has the highest C content per functional unit (19.66 kg C per functional unit as compared to 5.74 kg C per functional unit for the degassed biomass mixture). This reflects the potential for improvement as regarding the separation efficiency of C in the first separation. The co-generation of heat and power from biogas also represents, in the case of Scenario I, a positive contribution to global warming, representing 4 % 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. Another interesting observation to highlight from figure 7.2.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 (Scenario A) represents 21.4 % of the total positive contributions to global warming from the reference scenario, the contribution from separated liquid is 14 %, and it is 2 % for the degassed biomass mixture. This is mainly because of lower CH4 emissions due to the separation (and digestion) of the VS, as explained in section 4.2.2. The fact that the contributions are much lower for the degassed biomass mixture reflects that the flow of degassed biomass to store is small in the overall system (116 kg degassed biomass mixture entering the storage per functional unit, figure 7.1). In comparison, there is 843.7 kg of liquid fraction entering the storage per functional unit. In Scenario I, another significant contributor to global warming is the production of the fibre pellets, representing 10.5 % of the total positive contributions to global warming from the reference scenario (29.9 kg CO2 equivalent for this process, including separation). The main contributing substance for this is CO2. This CO2 is 56 % biogenic and 44 % fossil. The biogenic portion represents 16.10 kg CO2 equivalent and is due to the losses of C through CO2 during the fabrication of the pellets (this data is based on in-situ measurements). The fossil portion represents 12.48 kg CO2 equivalent and is due to the energy consumption. Finally, as it can be observed from figure 7.2.A, the contributions to global warming from the transportation and the biogas production itself 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 additional benefits in terms of global warming contribution avoided. This corresponds to an “avoidance” of 4 % 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 1.2 % 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 I, 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. In the present case, differently from previous scenarios, the amount of avoided N differs slightly between Scenario A and Scenario I. This is because of the fact that part of the produced fibre pellets is burned in the energy plant to be used in order to cover the heat need. This burned fraction allow the use of a different substitution rule for the liquid portion associated with it, see Annex I for more details. However, this difference is so small that it does not make any major difference in the overall picture, as it can be seen from figure 7.2.A. The amount of inorganic P and K substituted in Scenarios A and I 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 I 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, the difference between managing the slurry as in Scenario A and I is 15 kg CO2 equivalent. Accounting for the uncertainties (figure 7.9.A), this benefice becomes rather small. Table 7.2 summarises, for selected processes, the contribution of the main contributing substances to global warming, for both Scenario A and I. Table 7.2. Scenario I 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 [d] This includes 16.10 kg CO2 from biogenic origin (based on measurements) and 12.48 kg CO2 from fossil origin (energy consumption) The major results as regarding global warming can be summarised as:
This information is summarised in figure 7.3. This figure presents the contribution to climate change of Scenario A and I only for the processes that are not equal between A and I (i.e. in-house storage). All processes are presented; the category labelled “other processes non equal” represents the aggregation of all processes not presented in the legend. Figure 7.3. Comparison of Scenario I vs Scenario A for global warming including carbon sequestration, for processes differing between A and I only. Soil JB3, 10 years values. 7.2.3 AcidificationAs it can be observed from figure 7.2, 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 I, about 20 % of the total positive contributions to acidification from the reference scenario (accounting for liquid fraction, degassed biomass mixture and ashes). The main contributing substance in this case is also NH3 (96 % of the contributions for this process). The contribution from the liquid fraction represents 75 % of the share for the total contribution from field processes to acidification in Scenario I. This reflects the important flow of liquid fraction as compared to the flow of degassed biomass involved in the system. The overall contribution to acidification from field processes is much lower for Scenario I (15.7 m² unprotected ecosystem) as compared to Scenario A (9.2 m² unprotected ecosystem). This reflects the positive effect of the separation, where the N is most likely to enter the soil quickly due to the lower dry matter content, and thereby it reduces the potential for this N to be emitted as NH3 in the atmosphere (see detailed explanation in Annex F, section F.7.3) The outdoor storage, if the contributions from the liquid fraction and the degassed biomass are aggregated in the case of Scenario I, contributes to acidification in the same magnitude for both scenarios. The main contributing substance to acidification for outdoor storage is also NH3 (77% of the contributions for this process). The fabrication of the pellets in the energy plant also has some significance as regarding acidification in Scenario I (representing 3.5 % of the total positive contributions to acidification from the reference scenario). The main contributing substance responsible for this is NOX (representing 81 % of the contributions to acidification for this process). The co-generation of heat and power has here a minor contribution to acidification (representing 1.3 % of the total positive contributions to acidification from the reference scenario). The main contributing substance for this process is also NOX. For Scenario I, as it can be observed in figure 7.2.A, the contributions to acidification from the biogas production and the transportation are rather small. The amount of fertiliser avoided, though different, has the same magnitude 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, and the production of marginal electricity avoided as well as the wheat production avoided have even smaller contributions in avoiding acidification. Table 7.3 summarises, for selected processes, the contribution of the main contributing substances to acidification, for both Scenario A and I. Table 7.3. Scenario I 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:
This information is summarised in figure 7.4. This figure presents the contribution to acidification of Scenarios A and I only for the processes that are not equal between A and H (i.e. in-house storage). All processes are presented; the category labelled “other processes non equal” represents the aggregation of all processes not presented in the legend. Figure 7.4. Comparison of Scenario I vs Scenario A for acidification, for processes differing between A and I only. Soil JB3, 10 years values. 7.2.4 Aquatic eutrophication (N)Aquatic N eutrophication is, as illustrated in figure 7.2.A, mostly due to field processes:
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 7.1. The contribution to N-eutrophication from field processes is therefore similar for both scenarios. In both cases, it is essentially caused by N leaching through soils. The other important contributor to N eutrophication is, for both scenarios, in-house storage, through NH3 emissions. In both scenarios, avoiding inorganic fertilisers to be used (mostly N) allow to avoid considerable contribution to N-eutrophication, and this is in similar magnitude for both scenarios. The wheat avoided through higher yields in Scenario I is an extra avoided contribution, but it is rather smaller in the overall picture. 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 I becomes more significant. Overall, when the “deductions” from the avoided contributions to N-eutrophication are accounted for, managing the slurry as described in Scenario I allows a net difference of 0.08 kg N reaching aquatic recipients (characterisation unit for N-eutrophication potential) 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 0.09 kg N. However, when accounting for uncertainties, it does not appear so clear whether there is a net benefit or not (figure 7.9.A). This information is summarised in figure 7.5. This figure presents the contribution to N-eutrophication of Scenarios A and I only for the processes that are not equal between A and I (i.e. in-house storage). All processes are presented; the category labelled “other processes non equal” represents the aggregation of all processes not presented in the legend. Figure 7.5. Comparison of Scenario I vs Scenario A for N-eutrophication, for processes differing between A and I only. Soil JB3, 10 years values. 7.2.5 Aquatic eutrophication (P)As it can be observed from figure 7.2.A and table 7.1, the contribution (positive and negative) to P-eutrophication are about the same for both scenarios. 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 I, it can be highlighted that, for the field processes, the liquid fraction contribute to about 81 % of the total positive contributions to eutrophication (P) from the reference scenario, while it is 16 % for the degassed biomass mixture and 3 % for the ashes. This difference reflects the important difference between the mass flow of these 3 fractions in the system; while there is 916.3 kg liquid fraction ex-storage (including water) per 1000 kg slurry ex-animal, there is only 125.9 kg degassed biomass ex-storage (including water) per 1000 kg slurry ex-animal and 1.5 kg of ashes per 1000 kg slurry 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 I.7 and I.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 I does not allow a significant benefit for P-eutrophication over managing the slurry as in the reference scenario. This is illustrated in figure 7.6. Figure 7.6. Comparison of Scenario I vs Scenario A for P-eutrophication, for processes differing between A and I only. Soil JB3, 10 years values. 7.2.6 Photochemical ozone formation (“smog”)In both scenarios, there are 2 main hot spots for photochemical ozone formation:
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 74 % in the case of Scenario I). The fact that the overall contribution from outdoor storage is lower for Scenario I 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 I, the co-generation of heat and power. The main contributing substance in this case is NOX (85 % of the total contributions for this process), which is emitted during the combustion of the biogas in the biogas engine. The fabrication of the pellets also contributes importantly to the photochemical ozone formation (representing about 12 % of the total positive contributions to photochemical ozone from the reference scenario). The main contributing substance in this case is also NOX. Avoiding marginal heat and electricity to be produced only have, for the compared scenarios, 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 I does not allow a benefit as compared to the reference slurry management. This information is summarised in figure 7.7. This figure presents the contribution to photochemical ozone formation of Scenarios A and I only for the processes that are not equal between A and H (i.e. in-house storage 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 7.7. Comparison of Scenario I vs Scenario A for photochemical ozone formation, for processes differing between A and I only. Soil JB3, 10 years values. 7.2.7 Respiratory inorganics (small particles)Respiratory inorganics is caused by 3 main processes:
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 876% in Scenario I for this process). For outdoor storage, the contributions are divided between NH3 and NOX. In the case of Scenario I, the fabrication of the fibre pellets, also contributes to “respiratory inorganics” formation, representing 9 % of the total positive contributions to respiratory inorganics from the reference scenario. The main contributing substance in this case is NOX. 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 I allows a net difference of 0.018 kg PM2.5 equivalent as compared to the reference scenario. This is not significant when uncertainty is accounted for (figure 7.9.A). This information is summarised in figure 7.8. This figure presents the contribution to “respiratory inorganics” of Scenarios A and I only for the processes that are not equal between A and I (i.e. in-house storage is 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 7.8. Comparison of Scenario I vs Scenario A for “respiratory inorganics”, for processes differing between A and I only. Soil JB3, 10 years values. 7.2.8 Non-renewable energy resourcesBoth 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 7.2.B where the contributing processes are those requiring electricity or fuel inputs. The importance of the process “production of fibre pellets” for this impact category is highlighted in figure 7.2.B. Avoiding marginal electricity but also heat to be produced through the use of biogas for scenario I 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 I 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 I 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. Overall, when the “deductions” from the avoided contributions to non-renewable energy use are accounted for, the difference between managing the slurry as described in Scenario I as compared to the reference slurry management is 19 MJ of (primary) non-renewable energy. This however becomes lower when taking the uncertainties into account (figure 7.9.B). 7.2.9 Consumption of phosphorus as a resourceBoth 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 I (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 7.1 and figure 7.2.A. 7.2.10 Carbon stored in soilsThrough Scenario I, 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 for a C horizon in the field 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 I, less C is added to field per functional unit but also less CO2 is overall emitted (table 7.2). As a result, the amount of C sequestrated in the soil per functional unit (2.99 kg C considering 10 years values; 0.51 kg C considering 100 years values) is similar, though lower than the amount of C sequestrated for the reference scenario. In terms of CO2 avoided, this correspond to 10.9 kg CO2 (10 years values) and 1.9 kg CO2 (100 years values) per 1000 kg pig slurry ex-animal. 7.3 UncertaintiesThe uncertainties on the compared results have been estimated by analysing the most important factors that are changed, when comparing scenario I 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 7.9 and 7.10 as “index”). The values of the uncertainty ranges are shown in table 7.4. 7.4 Synthesis of the results for all impact categories assessedTable 7.4 compares the overall characterised results of Scenario A versus Scenario I, for all impacts categories (including carbon stored in soils). It also presents the uncertainty ranges for all impact category results. Figures 7.9.A and 7.9.B illustrate the results presented in table 7.4, and give an impression of the uncertainty. The difference between these two figures is the x axis, which has a greater range in the case of figure 7.9.B in order to capture the whole impacts of non-renewable energy consumption. Figures 7.10.A presents only the net differences between Scenario A and I, including the uncertainties. Table 7.4. Comparison of the impacts from Scenario A (reference) versus Scenario I (biogas from raw pig slurry + fibre pellets). The number of digits is not an expression of the uncertainty. Figure 7.9.A Comparison of the environmental impacts from Scenario A (reference) versus Scenario I (biogas from raw pig slurry + fibre pellets). Axis ranging from -180 to 120. Click here to see Figure 7.9.A Figure 7.9.B Comparison of the environmental impacts from Scenario A (reference) versus Scenario I (biogas from raw pig slurry + fibre pellets). Axis ranging from -1000 to 250. Click here to see Figure 7.9.B Figure 7.10.A Comparison of the environmental impacts from Scenario A (reference) versus Scenario I (biogas from raw pig slurry + fibre pellets). Net difference only. Axis ranging from -100 to 100. Click here to see Figure 7.10.A
|
||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||