Annex H. Fibre fraction from mechanical separation for biogas production – Life Cycle Inventory data – Environmental Project No. 1329 2010 – Life Cycle Assessment of Biogas from Separated slurry

Life Cycle Assessment of Biogas from Separated slurry

Annex H. Fibre fraction from mechanical separation for biogas production – Life Cycle Inventory data

H.1 System description

H.1 System description

This annex contains Life Cycle Inventory data for biogas production from a mixture of fibre fraction (from mechanically separated slurry from the Samson Bimatech separation process) and raw untreated pig slurry. The biogas is used for co-production of heat and power.

This scenario is set up in order to answer the question: “What are the environmental benefits and disadvantages of using the fibre fraction from Samson Bimatechs mechanical separation of pig slurry (see Annex C) for biogas production compared to the reference scenario for pig slurry?”.

Accordingly, it is different than the scenario in Annex F, where the aim was to analyse an optimised system using “Best Available Technology” for biogas production as far as possible.

The main differences compared to Annex F are:

The separation technology in this Annex H is based on the Samson Bimatech separation technology.
Polymer is not added to the separation (polymer is added for the separation in Annex F). It is possible to add this for the Samson Bimatech technology, but data has not been available for this. Adding polymer would give very different results for the separation than the data used in this report.
The methane conversion rate for the fibre fraction from the Samson Bimatech separation is set to 187 Nm³ CH₄/ton VS compared to the 319 Nm³ CH₄/ton VS for the fibre fraction from the mechanical-chemical separation used in Annex F, based on information from Møller (2007).
Separation after the biogas plant is not included, as this scenario is not set up to be a modelling of “best available technology” and as separation after the biogas plant is not commonly used today. Furthermore, the aim of separating the degassed biomass after the biogas plant is to recover phosphorous, and with that in mind, it would not be profitable to separate a degassed mixture partly originating from a mechanical separation (the one before the biogas plant) that only separates 9.1% of the phosphorus to the fibre fraction (see table H.2). In this scenario, less than 22% of the phosphorous in the original pig slurry “ex-animal” actually reach the biogas plant ^[1] and accordingly, it is not the optimal system for phosphorous recovering.

A flow diagram for the scenario for biogas production based on the fibre fraction from mechanically separated slurry and untreated slurry is shown in figure H.1. The process numbers in figure H.1 follows the numbers of the sections in this annex.

The present annex describes a total of 23 main processes, which were divided into 6 main sections:

Section 1: Processes H.2 to H.7
This section focus on 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 pig barn and stored in the barn (H.2). The slurry is then stored in the pre-tank (H.3) and separated (H.4). This section then continues with the fate of the liquid fraction only. The liquid fraction is stored outdoor (H.5), until it is transported to the field (H.6) and used as a fertilizer (H.7).
Section 2: Processes H.8 to H.10
This section is a continuation of the previous, and starts with the fibre fraction output from the separation process (H.4). The fibre fraction is stored on-farm (H.8), transported to the biogas plant (H.9) and temporarily stored at the biogas plant (H.10).
Section 3: Processes H.11 to H.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 pig barn and stored in the barn (H.11). The slurry is then stored in pre-tank at the farm (H.12), and transported to the biogas plant (H.13). Once at the biogas plant, the raw slurry is stored temporarily (H.14).
Section 4: Processes H.15 to H.18
This section focuses on the biogas production (H.15) and the resulting heat and power co-generation (H.16). This co-generation avoids marginal electricity to be produced (H.17) as well as marginal heat (H.18).
Section 5: Processes H.19 to H.22
This section focuses on the fate of the degassed biomass. After the biogas plant, it is transported back to the farm (H.19), stored (H.20) until it is transported to the field (H.21) to be used as a fertilizer (H.22).
Section 6: Process H.23
Throughout this annex, two organic fertilizers were used: the liquid fraction from the separation (H.7) and the degassed biomass from the biogas plant (H.22). The use of the slurry and degassed biomass as organic fertilizers results in a reduced use and production of inorganic fertilizers (H.23), which is the main focus of this section.

Figure H.1. Flow diagram for the scenario for biogas production based on fibre fraction from mechanically separated slurry by the Samson Bimatech separation.

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[1] The initial phosphorus content is 1.13 kg P per 1000 kg pig slurry “ex-animal”. From figure H.1, it can be seen that an input of 863 kg pig slurry is separated (H.4), and from this, 818.11 kg ends as liquid fraction. The liquid fraction contains 1.0833 kg P per 1000 kg, i.e. 818.11 kg * 1.0833 kg P per 1000 kg = 0.8863 kg P ends in the liquid fraction. This corresponds to 0.8863 kg P/1.13 kg P*100% = 78.4%, which means that only 21.6% of the P ends at the biogas plant in this system. Therefore only 21.6% of the initial P has the potential to be found in the degassed biomass from the biogas plant, which is judged not enough to justify a separation post biogas.