Annex G. Biogas production with cow slurry and fibre fraction from a mechanical-chemical separation– Life Cycle Inventory data

Life Cycle Assessment of Biogas from Separated slurry

G.1 System description

G.1 System description

This annex contains Life Cycle Inventory data for biogas production from a mixture of fibre fraction (from mechanically separated slurry, flocculated with polymer) and raw slurry, both from dairy cows. The separation process used in this annex is considered as a “best available technology” for biogas production. The resulting fibre fraction has a degradation potential similar to the one of raw slurry. The biogas 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. The processes described and used for this scenario were built in collaboration with Xergi A/S, as Xergi A/S has experience and data for more than 30 biogas plants established throughout the country. The degassed slurry resulting from the anaerobic digestion is mechanically separated again, but without polymer addition. The liquid fraction is then used on field as a fertilizer, and so is the fibre fraction.

Although biogas exclusively from slurry input (i.e. without supplementary addition of easily degradable carbon) 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 limited availability of the organic waste or the other C-source material that are actually co-digested with the slurry. Co-digesting the raw slurry is practiced in order to ensure the economic feasibility of biogas production. Moreover, the Danish government has set the objective to use more than 50 % of the slurry produced in Denmark for biogas production, in which case the possibility to make biogas from 100 % slurry input represents an interesting option. Several plants running on inputs from slurry only (i.e. raw slurry and separated fibre fraction) are currently under development throughout the country, and the first one has just been put into operation (Morsø Bioenergi was inaugurated 15 of June 2009).

The present annex describes the process flow for a biogas scenario comprising a total of 28 main processes, which were divided into 8 main sections:

Section 1 : Processes G.2 to G.7
This section considers the slurry from which the fibre fraction input in the biomass mixture (for biogas) origins. It starts with the raw slurry being produced in the dairy cow barn and stored in the barn (G.2). The slurry is then stored in the pre-tank (G.3) and separated (G.4). This section then continues with the fate of the liquid fraction only. The liquid fraction is stored outdoor (G.5), until it is transported to the field (G.6) and used as a fertilizer (G.7).
Section 2 : Processes G.8 to G.10
This section is a continuation of the previous, and starts with the fibre fraction output from the separation process (G.4). The fibre fraction is stored on-farm (G.8), transported to the biogas plant (G.9) and temporarily stored at the biogas plant (G.10).
>Section 3 : Processes G.11 to G.14
This section focus on the raw slurry input in the biomass mixture (for biogas). It begins with the raw slurry being produced in the dairy cow barn and stored in the barn (G.11). The slurry is then stored in pre-tank at the farm (G.12), and transported to the biogas plant (G.13). Once at the biogas plant, the raw slurry is stored temporarily (G.14).
Section 4 : Processes G.15 to G.18
This section focuses on the biogas production (G.15) and the resulting heat and power co-generation (G.16). This co-generation avoids marginal electricity to be produced (G.17) and well as marginal heat (G.18).
Section 5 : Process G.19
This section describes the separation of the degassed biomass mixture output from the anaerobic digestion (G.19).
Section 6 : Processes G.20 to G.23
This section focuses on the fate of the degassed fibre fraction. After the separation, it is transported to a farm where a fertilizer rich in P is needed (G.20), stored (G.21) until it is transported to the field (G.22) to be used as a fertilizer (G.23).
Section 7 : Processes G.24 to G.27
This section focuses on the fate of the degassed liquid fraction. After the separation, it is transported back to the farm (G.24), stored (G.25) until it is transported to the field (G.26) to be used as a fertilizer (G.27).
Section 8: Process G.28
Throughout this annex, three organic fertilizers were used: a liquid fraction (G.7), a degassed liquid fraction (G.27) and a degassed solid fraction (G.23). The use of these organic fertilizers allowed to avoid inorganic fertilizer to be produced and used (G.28), which is the main focus of this section.

The scenario described in this annex has been modelled in order to include “Best Available Technology” as much as possible. The conditions considered throughout the scenario were chosen in the light of the best feasible possibilities. This applies for the technologies used as well as for the management practices. However, conservative assumptions were used in the calculation of the emissions, in order to ensure the life cycle assessment reflects the correct picture as regarding the environmental consequences of this manure management option.

It shall also be mentioned that many possibilities could have been included as regarding the different variants in the biogas production, which could be worth another life cycle assessment themselves. For example, instead of being used for co-generation of heat and power through a biogas engine, the biogas could have been cleaned and injected directly to the natural gas grid (this possibility is however considered as a sensitivity analysis). Else, it could also have been upgraded and used as a transportation fuel. Also, the slurry is sometimes treated in a pre-treatment plant before entering the biogas plant, which is another variant not included here.

The overall flow diagram for this scenario is presented in figure G.1.

Figure G.1. Flow diagram for biogas production based on raw slurry + fibre fraction from mechanically separated cow slurry with a decanter centrifuge and polymer addition

Click here to see Figure