5 Fibre Pellets combusted in Energy Plant

Life Cycle Assessment of Slurry Management Technologies

This chapter contains a comparative life cycle assessment for a scenario where pig slurry is used for energy production in a Samson Bimatech Energy Plant compared with the reference scenario from chapter 3. The life cycle assessment is performed in order to answer the question: “What are the environmental benefits and disadvantages of utilising pig slurry for producing fibre pellets in a Samson Bimatech Plant MaNergy 225 and utilising the fibre pellets for heat production - compared to the reference scenario for pig slurry?”.

The environmental impacts and conclusions in this chapter to a great extent build on data and information delivered by the producer of the technology, Samson Bimatech, and on data made for Samson Bimatech (laboratory measurements of the slurry composition). The conclusions rely on this information, and the authors of this study have not had the possibility of verifying the data.

5.1 System description

The Life Cycle Inventory Data for the Samson Bimatech Energy plant and a description of the Energy Plant and fibre pellet production can be found in Appendix C and D.

The fibre pellets are produced in a number of steps, which include mechanical separation of pig (or cattle) slurry, drying of the fibre fraction and pressing the dried fibres into pellets. The pellets can be used for heat production at the farm in a Samson Bimatech Energy Plant. The drying process of the wet fibres requires heat and this consumes approximately 40% of the energy produced by combustion of the fibre pellets. The produced heat replaces heat production by light fuel oil (or by the use of straw or wooden pellets in the sensitivity analysis) and the “replaced heat” is subtracted from the system.

The assessment includes pig slurry only, as measurement for cattle slurry was not available at the time of collecting data. Data on cattle slurry has been collected just before finalizing the project (May 2009) however, it was not possible to include this within the time frames of this study.

The scenario in this chapter contains the Energy Plant producing energy based on fibre pellets are shown in figure 5.1. The process numbers refer to the heading of the section in this Annex D and the numbers of the processes.

Figure 5.1. Flow diagram for the scenario with the Samson Bimatech Energy Plant (Annex D).

Click here to see Figure 5.1

5.2 Results of the Impact Assessment

5.2.1 Overall results of the comparison

In figure 5.2, the environmental impacts from the “Samson Bimatech Energy Plant scenario” have been compared to the environmental impacts from the reference system described in chapter 3. The results are discussed in the following sections.

Figure 5.2. Environmental impacts for the system for the Bimatech Energy Plant compared with the reference system (both based on soil type JB3) – pig slurry.

Click here to see Figure 5.2

5.2.2 Sensitivity analysis

Sensitivity analysis has been carried out for a number of possible variations of the Energy Plant scenario and uncertainties related to the data. Some of the results of the sensitivity analyses are shown in figures in this section, and some are described in the text under each impact category in the following sections.

In this section, the results of sensitivity analysis are shown for the difference between soil type JB3 and JB6. As can be seen from figure 5.3, the difference between soil type JB3 and JB6 has no significance for the overall conclusions.

The significance of applying a 10 years horizon or 100 year horizon is shown in figure 5.2.

As discussed in Annex C it has not been possible to find data for separating the emissions from the pre-tank from the emissions from the subsequent outdoor storage. Accordingly, the emissions are treated together under “outdoor storage”. As the slurry is not separated until after the pre-tank and as the storage of the liquid fraction has lower emissions than storage of untreated slurry, the emissions from the pre-tank will be too low. A sensitivity analysis (assuming that 30% of the emissions occur from the pre-tank) indicates that the emissions from storage (pre-tank plus outdoor storage) might be at maximum 7% too low, which will not change the overall conclusions.

As mentioned in Annex C, there are significant uncertainties related to the separation indexes from the mechanical separation due to uncertainties on measurements combined with the fact that it is difficult to measure on the relatively inhomogeneous matter, slurry is. Moreover, the separation to a great degree depends on the actual slurry composition, the amount of water in the slurry, DM etc. As discussed in Annex A, the Norm Data slurry used as reference in this study, contains too low amounts of water. In real life, the concentration of the slurry has influence on the separation index. In this study, it has been necessary to keep in mind that the water content of the reference slurry is unrealistic low. The separation indexes have been used for separation of DM, N, P, K, Cu and Zn (see Annex C, table C.2) meaning that the percentageof DM transferred to the fibre fraction of the slurry is the same as “in real life”. The fibre fraction is modelled as close to the measurements from real life as possible. It has the consequence that the liquid fraction becomes too concentrated compared to “real life measurements” as liquid fraction takes over “the lack of water” from the Norm Data.

The “lack of water” in the liquid fraction has no influence on the emissions, as the emission factors are based on the actual amount of N and C rather than the concentrations of these. Adding more water to the liquid fraction (without adding more N and C) would not change the results of this study.

The results of the sensitivity analyses are discussed under each impact category in the following sections.

Figure 5.3. Sensitivity Analysis: Difference between soil type JB3 and JB6. Environmental impacts for the Energy Plant Scenario compared to the reference system – pig slurry.

Click here to see Figure 5.3

5.2.3 Global warming

In figure 5.2 it can be seen that the overall contributions to global warming for the system with the Energy Plant is at the same level as the contributions from the reference system (when keeping the high uncertainty on the data in mind).

The CO₂ emissions from the combustion of fibre pellets from the energy plant give a significant contribution to the global warming. This is partly counterbalanced by decreased contributions from the field processes and partly by the subtraction of “replaced heat” (to the left in figure 5.2).

The reductions from the field are attributable to the CO₂ emissions from the liquid fraction being lower than the CO₂ emissions from the untreated reference slurry in Annex A. The liquid fraction contains less C than the untreated reference slurry because a significant part of the dry matter and carbon is transferred to the fibre fraction during the mechanical separation (described in Annex C).

The energy produced by the Energy Plant replaces heat, which should have been produced by other fuels (for heating the farmer’s private house). The results in figure 5.2 are based on the assumption that the heat replaces heat production based on light fuel oil. As can be seen from figure 5.2 the CO₂ emissions from the replaced heat do not counterbalance the CO₂ emissions from the combustion of fibre pellets. This is because a significant part of the heat produced in the Energy Plant is used internally for heating and drying the fibre pellets combined with a relatively high loss of heat from the process. The “traditional oil burner” for a private house will have a higher efficiency than the Energy Plant. Changing the ”replaced type of heat” change the avoided CO₂ emissions slightly but it does not change the overall conclusions.

There are no data on the CH₄ emissions from the storage of the fibre pellets.

It is assumed that the biological activity is relatively low due to the low water content of the fibre pellets (normally a maximum of 10-15% water). Accordingly, it is assumed that the CH₄ emissions are relatively low compared to the CH₄ emissions from the indoor and outdoor storage. If there was significant biological activity in the fibre pellets during storage, it could increase the total contribution to global warming as the fibre pellets contain approximately 30% of the total carbon from the slurry ex animal. In that case, the “Energy Plant scenario” would have a higher contribution to global warming than the reference scenario.

5.2.4 Acidification (environmental impact)

As for the reference scenario described in chapter 3, the total contributions to acidification are mainly dominated by NH₃ emissions. Part of the contributions comes from nitrogen oxides from the Energy plant during the combustion (less than 8% of the total contributions). The scenario with the Energy Plant has reduced contributions to acidification from the field processes. This is caused by reduced NH₃ emissions from the application of the liquid fraction of the slurry. According to Hansen et al. (2008), application of the liquid fraction of slurry has an emission factor (NH₃ per total N in the slurry) that is approximately 50% of the emission factor for untreated slurry. The overall reduction of the contributions to acidification is at the level of 10% compared to the reference system, however, the actual magnitude should be interpreted with care, as the uncertainty on the nitrogen oxides from the Energy Plant is high.

As mentioned in Annex C, it has not been possible to include the NH₃ emissions during the separation process due to lack of data. For the Energy Plant, the mechanical separator is placed within the plant which uses the NH₃ emissions for the combustion process (see the description in Annex D). The plant has a constant vacuum in order to transfer emissions from the separator to the combustion chamber. Accordingly, it is likely that the NH₃ emitted to the surroundings are rather low for the Energy Plant (but not for the mechanical separator alone).

There has been no data on emissions from the storage of the fibre pellets, as described in Annex D. It is assumed that the biological activity is relatively low due to the low water content of the fibre pellets (normally a maximum of 10-15% water). As the fibre pellets contain only 15% NH₄⁺(out of the total N) it is assumed that the NH₃ emissions from the fibre pellets are insignificant. If assuming that the emission from the fibre pellets has the same emission factor as for deep litter i.e. 13% of the total N (which is a very conservative estimate, as the emissions from deep litter is most likely much higher than from fibre pellets), the NH₃ emissions for the whole system would increase with maximum 2% (due to the low amounts of fibre pellets stored per 1000 kg slurry ex animal and the low content of N in the fibre pellets). Accordingly, it is assumed that the lack of data for the storage of fibre pellets has no significance for the overall results.

5.2.5 Aquatic eutrophication (N)

As for the reference scenario, the main contributions to aquatic eutrophication (N) come from nitrate leaching (and smaller amounts from NH₃ emissions). As some of the N is removed to the fibre fraction, converted to fiber pellets and combusted, a smaller amount of N from the slurry ends at the field, which leads to a slight decrease in the contribution to aquatic eutrophication (N).

5.2.6 Aquatic eutrophication (P)

As all the phosphorous from the original reference slurry ex animal end at the field, there is no change in the contributions to aquatic eutrophication (P) (P in the fibre fraction is converted to fiber pellets, combusted and the ash is assumed to be applied to the farmer’s own field, mixed with the liquid fraction).

In the steering group for the study, it has been discussed whether the ash can be used as fertiliser. According to personal communication with K. S Andersen from the Danish Environmental Protection Agency (2008) and S Sommer (2009), measurements of the ash from the Samson Bimatech Energy Plant indicate that the phosphorus in the ash is plant available. If the ash has a lower phosphorus plant availability, the reduction is limited as 9.1% of the phosphorous is transferred to the fibre fraction (see table C.2 in Annex C). Accordingly it will reduce the aquatic eutrophication (P) by maximum 9.1% if the ash is not used for fertilising of if the fertiliser value of the ash is lower. As it works now, the farmer simply mix the relatively small amounts of ash into the liquid fraction before application (4.3 kg per 1000 kg slurry).

5.2.7 Photchemical ozone formation (“smog”)

The scenario for the Energy Plant has a higher contribution to the environmental impact category “photochemical ozone formation” due to emissions of nitrogen oxides during the combustion of the fibre pellets. This is only partly counterbalanced by a slight decrease of CH₄ emissions from the outdoor storage of the liquid fraction compared to the outdoor storage of the reference slurry due to a lower content of C in the liquid fraction. There are significant uncertainties on the emissions of nitrogen oxides from the Energy Plant due to the fact that the technology is undertaking ongoing product development. With the data that have been available for this project, it must be concluded that the contribution to impact category “photochemical ozone formation” is higher for the scenario with the Energy Plant than for the reference scenario. During future product development of the Energy Plant, it could be a good idea to perform measurements for nitrogen oxides from the Energy Plant (related to the amount of slurry treated) in order to reduce the nitrogen oxide emission level.

As mentioned above, it is assumed that storage of the fibre pellets does not contribute significantly to the CH₄ emissions. If the storage of fibre pellets leads to CH₄ emissions in high amounts it could lead to even higher contributions to ozone formation. It is assumed that this is not the case.

5.2.8 Respiratory inorganics (small particles)

The Energy Plant scenario has increased contributions to respiratory inorganics caused by the emissions of nitrogen oxides (and partly by the emissions of particles). The nitrogen oxides are discussed above. However, the contributions from the field processes are lower for the Energy Plant scenario than for the reference scenario due to the reductions of NH₃ as explained above.

If the emission of nitrogen oxides from the plant is reduced significantly during the product development of the plant, there are possibilities for a net reduction of the contributions to respiratory inorganics from the scenario with the Energy Plant compared to the reference scenario.

5.2.9 Non-renewable energy resources

As can be seen in figure 5.2, the electricity consumption for the Energy Plant increases the consumption of non-renewable energy resources for the total system, compared to the reference system. However, this is partly counterbalanced by the heat produced by the Energy Plant, as it is assumed that the heat is used for the farmer’s private house, and that is replaces heat produced on light fuel oil. If the replaced fuel is not light fuel oil, but e.g. straw or wooden pellets, there is no replacement of non-renewable energy resources (as straw and wood is renewable resources), which means that there is no replacement of non-renewable energy resources. In that case, the Energy Plant scenario has a noteworthy increase of non-renewable energy resources compared to the reference scenario.

5.2.10 Consumption of phosphorus as a resource

There is no difference regarding the consumption of phosphorus as a resource between the Energy Plant scenario and the reference scenario. The phosphorus stays in the ash, and as the ash is added to the liquid fraction before application to field all the phosphorus from the slurry “ex animal” is applied to field. The handling of the ash is modelled as it is done by the farmers of the existing Energy Plants today (i.e. keeping the ash and adding it to the liquid fraction).

In the future, the farmers might be able to sell the ash as fertiliser to other farmers, transfering the phosphorus to areas with more need of phosphorus.

As the mechanical separation system by Samson Bimatech separates approximately 9% of the total amount of phosphorus into the fibre fraction (see Annex C, table C.2), a future system could redistribute approximately 9% of the total phosphorus to other areas.

5.3 Conclusion

The CO₂ emissions from the combustion of fibre pellets from the Energy Plant is partly counterbalanced by the reduced CO₂ emissions from applying the liquid fraction to field (compared to applying untreated slurry to field) and the CO₂ emissions from the replaced heat production (the Energy Plant produces heat that can be utilized for heating the farmer’s private house. The energy that would have been used for heating the farmer’s house has been subtracted from the system).

The NH₃ emissions is reduced in the Energy Plant scenario compared to the reference scenario due to reduced NH₃ emissions from the application of the liquid fraction to field compared to untreated slurry in the reference scenario. The reduced NH₃ emissions lead to a small reduction in the overall contributions to the environmental impact “Acidification“.

The contribution to aquatic eutrophication (N) is slightly reduced for the Energy Plant scenario, as a smaller total amount of N from the slurry ends at the field because some of the N is removed to the fibre fraction, converted to fiber pellets and combusted.

The contribution to aquatic eutrophication (P) is unchanged.

The contribution to the impact category “respiratory inorganics” is at the same level for the scenario with the Energy Plant as for the reference scenario (taking the uncertainties into consideration). The Energy Plant scenario has increased contributions to respiratory inorganics caused by the emissions of nitrogen oxides (and partly by the emissions of particles). The nitrogen oxides are discussed above. However, the contributions from the field processes are lower for the Energy Plant scenario than for the reference scenario due to the reductions of NH₃ as explained above.

The Energy Plant technology is undergoing continuous product development and the technology should be seen as being “in its child-hood”. On one hand it means that it has been difficult to obtain “high quality data” on air emissions and separation indexes. The data on air emissions has significant influence on the nitrogen oxide emissions. On the other hand it probably means that there are plenty of possibilities for improving the technology. The NO_X emissions could probably be reduced significantly by implementing known NO_X reducing technology in the Energy Plant. The separation has huge influence on the overall environmental impacts, as the main contributions come from storage and application of the liquid fraction to field – which to a great extent depend on the proportion of N and C transferred to the fibre fraction. For the mechanical separation, 29.6% of the DM and 6.8% of N is transferred to the fibre fraction (see table C.2). Increasing these separations indexes by “product development” would reduce the overall environmental performance of the system.

Table 5.1. Comparison of the impacts from the Energy Plant scenario to the reference scenario for pig slurry. The number of digits is not an expression of the uncertainty. The uncertainty of the net contribution is based on an estimate with regard to the uncertainty on the data the forms the foundation for the LCA.

Environmental impact / resource consumption	Reference scenario	Energy Plant scenario	Net contribution i.e. ”Energy Plant scenario” minus ”Reference scenario”
Global warming (during 10 years) [kg CO₂ eq.]	From slurry: 284 kg From fertiliser: -36 kg Net: 248 kg	From slurry: 289 kg From fertiliser ¹: -48 kg Net: 241 kg	-7 [-21 – +7] kg CO₂ eq. No significant difference
Global warming (during 100 years) [kg CO₂ eq.]	From slurry: 304 kg From fertiliser: -47 kg Net: 257 kg	From slurry: 307 kg From fertiliser ¹: -59 kg Net: 248 kg	-9 [-27 – +9] kg CO₂ eq. No significant difference
Acidification [m² UES, i.e. area of unprotected ecosystem]	From slurry: 50.3 m² From fertiliser: -5.5 m² Net: 44.8 m²	From slurry: 45.7 m² From fertiliser: -5.9 m² Net: 39.7 m²	-5 [-3- -7] m² UES 6-14% reduction of contribution from slurry
N-eutrophication (aquatic) (during 10 years) [kg N - amount reaching aquatic recipients]	From slurry: 1.51 kg From fertiliser: -0.93 kg Net: 0.59 kg	From slurry: 1.44 kg From fertiliser: -0.94 kg Net: 0.51 kg	-0.08 [-0.04 - -0.12] kg N 2.5-7.7% reduction of contribution from slurry
N-eutrophication (aquatic) (during 100 years) [kg N - amount reaching aquatic recipients]	From slurry: 1.63 kg From fertiliser: -1.03 kg Net: 0.61 kg	From slurry: 1.57 kg From fertiliser: -1.04 kg Net: 0.53 kg	-0.08 [-0.04 - -0.13] kg N 2.5-7.4% reduction of contribution from slurry
P-eutrophication (aquatic) [kg P - amount reaching aquatic recipients]	From slurry: 0.0069 kg From fertiliser: -0.0086 kg Difference not significant due to high uncertainties	From slurry: 0.0069 kg From fertiliser: -0.0086 kg Difference not significant due to high uncertainties	No difference
Photochemical ozone formation [person.ppm.hr - see section 3.4.7]	From slurry: 0.18 p.p.h From fertiliser: -0.014 p.p.h Net: 0.17 p.p.h	From slurry: 0.21 p.p.h From fertiliser: -0.016 p.p.h Net: 0.19 p.p.h	0.025 pers.ppm.hr [0.012-0.038] 7-21% increase of contribution from slurry
Respiratory Inorganics [kg PM2.5 eq, i.e. kg equivalents of 2.5 µm size particles]	From slurry: 0.29 kg From fertiliser: - 0.05 kg Net: 0.24 kg	From slurry: 0.30 kg From fertiliser: - 0.06 kg Net: 0.24 kg	No significant difference
Non-renewable energy [MJ primary energy]	From slurry: 151 MJ From fertiliser: - 369 MJ Net: -217 MJ	From slurry: 358 MJ From fertiliser ¹: - 543 MJ Net: -185 MJ	31 [ 0 – 200 ] MJ 0-130% increase of contribution from slurry ²
Phosphorus Resources [kg P]	From slurry: 0 kg From fertiliser: - 1.3 kg Net: - 1.3 kg	From slurry: 0 kg From fertiliser: - 1.3 kg Net: - 1.3 kg	No difference
Carbon stored in soil during 10 years [kg C] (Corresponding to this amount of CO₂-eq.)	From slurry: 7.5 kg C From fertiliser: - 3.9 kg C Net: 3.6 kg C From slurry: 27.6 kg CO₂ From fertiliser -14.3 kg CO₂ Net: 13.2 kg CO₂	From slurry: 6.6 kg C From fertiliser: - 4.0 kg C Net: 2.6 kg C From slurry: 24.2 kg CO₂ From fertil.: -14.5 kg CO₂ Net: 9.7 kg CO₂	-1.0 [-0.7 – -1.4 ] kg C -3.6 [ -2.6 - -5.0] kg CO2 9-18% decrease of contribution from slurry
Carbon stored in soil during 100 years [kg C] (Corresponding to this amount of CO₂-eq.)	From slurry: 2.1 kg C From fertiliser: - 1.1 kg C Net: 1.0 kg C From slurry: 7.8 kg CO₂ From fertiliser: -4.0 kg CO₂ Net: 3.8 kg CO₂	From slurry: 1.7 kg C From fertiliser: - 1.11 kg C Net: 0.6 kg C From slurry: 6.3 kg CO₂ From fertiliser: -4.1 kg CO₂ Net: 2.2 kg CO₂	-0.4[-0.3 – -0.6 ] kg C -1.6 [ -1.1 - -2.2] kg CO2 14-28% decrease of contribution from slurry

¹ Including replaced heat, which is especially important for non-renewable energy.

² The upper limit for the uncertainty includes the situation where the heat produced by the Energy Plant replaces renewable resources as e.g. wooden pellets or straw.

Figure 5.4. Environmental impacts for the Energy Plant scenario compared to the reference system (both based on soil type JB3) – pig slurry.

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