Summary and conclusions

Life Cycle Assessment of Biogas from Separated slurry

Background and objectives
Scope
Main conclusions
Project results

The environmental aspects of biogas production based on pre-treated slurry from fattening pigs and dairy cows have been investigated in a life cycle perspective. The pre-treatment consists of concentrating the slurry using a separation technology. Significant environmental benefits, compared to the status quo slurry management, can be obtained for both pig and cow slurry, especially regarding reductions of the contributions to global warming, but the results depend to a large extent on the efficiency of the separation technology. Adding separation after the biogas plant can contribute to a more efficient management of the phosphorus, and this has also been investigated.

Background and objectives

The objective of the study “Life Cycle Assessment of Biogas from Separated slurry” has been to foster the on-going work on a foundation for Life Cycle Assessment of slurry management in Denmark by biogas production scenarios.

The outcomes of the study are:

A database containing Life Cycle Inventory data for 4 selected biogas scenarios;
A report, containing the Life Cycle Assessment results and the interpretation of these for the four biogas scenarios;
Four detailed Annexes describing all data used, calculations and mass balances.

The goal of the study has been to provide an answer to the question:

“What are the environmental benefits and disadvantages of using fattening pig slurry or dairy cow slurry for biogas production instead of using the raw slurry as an organic fertiliser and spread it on land without any prior treatment?”

This project assesses four biogas production alternatives where slurry is the only input in the process (i.e. without supplementary addition of easily degradable carbon). Although biogas produced exclusively from slurry input is not yet the most common practice in Denmark, it is likely to become an important alternative for the Danish panorama. This is due to the target to use more slurry for biogas production, but also to the limited availability of the carbon-source materials that are actually co-digested with the slurry.

The study is a continuation of the project “Life Cycle Assessment of Slurry Management Technologies”, initiated by “Partnership for Industrial Biotechnology”. Both projects are commissioned by the Environmental Protection Agency of Denmark.

Scope

The study includes 4 biogas scenarios:

Scenario F: Biogas production based on a mixture of raw pig slurry and fibre fraction from chemical-mechanical separation technology (decanter centrifuge combined with the addition of cationic polyacrylamide polymer for increasing the separation efficiency). The biogas is used for co-generation of heat and electricity. After the biogas plant, the degassed biomass is separated by a decanter centrifuge in order to facilitate an optimised utilisation of the phosphorous content of the degassed biomass (i.e. to fields with phosphorous deficiency).
Scenario G: As above, but with dairy cow slurry.
Scenario H: Biogas production based on a mixture of raw pig slurry and fibre fraction from mechanical separation technology (screw press). The biogas is used for co-generation of heat and electricity. No separation is performed after the biogas plant.
Scenario I: As above, but the biogas production is based on raw slurry and processed fibre pellets.

Main conclusions

Based on the results of the study it can be concluded that:

The environmental benefits of biogas from separated slurry are very dependent upon the separation efficiency (for carbon, nitrogen and phosphorous). This particularly applies for carbon, as the separation efficiency defines the extent to which the degradable carbon contained in the slurry is transferred to the biogas plant. Efficient separation can be obtained by using polymer, but also by using a suitable separation technology. It could be mentioned that the decanter centrifuge used has a rather high efficiency of transferring volatile solids (VS) to the fibre fraction also without the use of polymer.
Biogas production from separated slurry can lead to significant reductions in the contributions to global warming, provided that the “best available technologies” described in the report are used. That includes, among others:
- a covered and short time storage of the fibre fraction before entering the biogas plant,
- a 2-step biogas production where the post-digestion tank is covered with air-tight cover,
- a covered storage of the degassed fibre fraction

The benefits are also highly dependent upon the source of energy substituted by the biogas.

Based on evidences from reviewed studies, the cationic polyacrylamide polymer added during separation is probably not degraded in the biogas plant, but spread to land through the degassed biomass fractions. These evidences also suggest that this polymer is rather recalcitrant to degradation, at least under the conditions found in an agricultural field. Therefore, it is suggested that the polymer is likely to accumulate and persist in the environment. More investigation is needed before a final prove or disproval of the potential toxicity of this aspect.

Project results

The overall environmental benefits are expressed per ton of slurry ex animal in order to ensure a common ground for comparisons between technologies. This is the basis on which the results of this study rely.

The environmental assessment of biogas production scenarios based on a mixture of raw and separated slurry has been performed by comparing the selected biogas scenarios with a reference scenario. This reference scenario is defined as the “conventional” way of managing slurry, i.e. storing it and applying it to the field as an organic fertiliser. The Life Cycle Assessment methodology forms the basis for the comparison.

For the biogas scenarios involving an efficient separation technology before the biogas production, the following results have been found:

The overall contribution to the impact “global warming” is significantly reduced when using the slurry for biogas production compared to using the slurry the conventional way. The reduction is mainly caused by two factors:
- Reduction of methane emissions from storage of the slurry fractions. The reductions are caused by:
  - The separation before the biogas plant provides a transfer the easily degradable carbon to the fibre fraction which leaves the liquid fraction with a reduced potential for methane emissions during storage.
  - The conversion of the easily degradable carbon to biogas in the biogas plant, resulting in a lower potential for methane emissions in subsequent storage.
- The produced biogas is used for production of electricity and heat, and this replaces electricity and heat based on fossil fuels which is accordingly subtracted from the system.
The contribution to the environmental impact “acidification” is not significantly lower for the biogas scenarios compared to the reference scenario.
For the impact category “eutrophication with nitrogen” (also known as nitrogen leaching to aquatic recipients) the small reductions obtained with the biogas scenarios can hardly be claimed as significant when taking the uncertainties into consideration.
For the impact “photochemical ozone formation” (or “smog”) there is no significant difference between the biogas scenario as compared to the conventional slurry management. This is mainly caused by a significantly lower methane emission from the storage of the slurry fractions, which is, however, counterbalanced by contributions from the emissions of NO_X from the combustion of biogas during the co-production of and heat and power.
The consumption of phosphorus resources are reduced in the two biogas scenarios, including a separation after the biogas plant (Scenarios F and G). Furthermore, this reduces the eutrophication of aquatic recipients by phosphorus when extracting these resources. However, these benefits depend upon the following pre-condition:
- The recovered phosphorous must be used in fields with phosphorous deficiency (i.e. not applied in excess).
No significant environmental benefits are obtained for the category “respiratory inorganics”, which reflects the emissions of particulate matters. This is mostly because of the emission of nitrous oxides generated during the combustion of the biogas in the biogas engine, but also because of the higher NH₃ emissions involved during the storage of the degassed fibre fraction.
For the impact “non-renewable energy”, transport and consumption of electricity is significant. The biogas scenarios involves slightly more transport, however, this is by far counterbalanced by the fossil fuels that is replaces by the produced electricity and heat. Overall, the biogas scenarios allow significant reductions of non-renewable energy.
The flow of biogenic carbon was included in this study. As a result, the following conclusions can be drawn:
- The emissions of biogenic CO₂ from the slurry represent about 50 % of the positive contributions to global warming for the biogas scenarios. This is slightly lower for the reference scenario.
- For the impact “global warming”, reductions of biogenic CO₂ emissions from field are obtained with the biogas scenarios. However, this is mainly due to the fact that the easily degradable carbon is converted in the biogas plant, and hence, the conversion occurs there instead of in the field.
- The amount of carbon sequestrated in the soil (and accordingly not emitted in the atmosphere) could be determined as the difference between the carbon applied with the slurry and the biogenic CO₂ emitted. The sequestrated carbon is lower with the biogas scenarios, since there is less carbon available for sequestration in degassed slurry as compared to raw slurry. However, these represent rather small differences.

No significant environmental benefits were obtained for the biogas scenario involving a separation technology with a low efficiency and the use of processed fibre pellets for the biogas production. For the biogas scenario involving a separation technology with a low efficiency and the used of fibre fraction for the biogas production, the only significant environmental benefits apply to the impact “global warming” and the impact “non-renewable energy”. The magnitude of these benefits is however much smaller as compared to the scenarios involving a separation technology with high separation efficiency.

For both separation technology types (high and low efficiency), the biogas scenarios resulted in a slurry with higher nitrogen availability and thereby a potential for yield increase. This was taken into account and translated into the production of Danish wheat that is avoided through this increased yield. However, this represented a rather small difference and did not contribute significantly to reduce the overall net impact of the different environmental impact categories considered.

For all scenarios, there are two major hot spots contributing to the impact categories assessed: in-house slurry storage (mostly through NH₃ and CH₄) and field processes. These represent opportunities for potential improvement of the overall environmental performance of the biogas scenarios assessed.

The storage of the fibre fraction before and after the biogas plant is crucial for the overall contributions to especially global warming and acidification. Uncovered storage of the fibre fraction may reduce the global warming benefits of the scenarios assessed.

These results only apply for Danish conditions and the results cannot be transferred to other countries with different climate, different production systems and different laws and rules regarding regulation of nutrients.