Process G.19: Separation of the degassed biomass mixture

Life Cycle Assessment of Biogas from Separated slurry

G.19 Separation of digested biomass AFTER the Biogas plant
- G.19.1 Separation indexes
- G.19.2 Mass balances

G.19 Separation of digested biomass AFTER the Biogas plant

G.19.1 Separation indexes

The separation technology considered after the biogas plant is the same as in Annex F, section F.19. Therefore, a GEA Westfalia decanter centrifuge (model UCD 305) was used, without polymer addition. Material consumption data and electricity consumption data are therefore identical to those described in Annex F (section F.4.5 and F.4.4, respectively). As in Annex F, data for the separation efficiencies comes from Frandsen (2009) and from Møller et al. (2007b) in the case of Cu and Zn (see table F.25, Annex F). However, the separation efficiencies for Cu and Zn used in this Annex are different from those presented in Annex F since Møller et al. (2007b) present distinct data for cattle and pig slurry.

Moreover, the separation efficiency for the total mass is different from the data presented in table F.25 as it is adjusted based on the slurry type, as described in section G.4.2. Accordingly, as for the separation before the biogas plant, it was assumed that the DM of the solid fraction coming out of the separator would remain approximately constant independently of the water content of the degassed slurry. Based on this, the total mass of fibre fraction can be evaluated, and thereby the separation index for the total mass. Since the amount of DM in the resulting fibre fraction was measured (26.71 %, which means that there is 267.1 kg DM per 1000 kg of fibre fraction according to table 4 in Frandsen (2009)), and since the DM content of the input degassed biomass is known (107.73 kg DM per kg degassed biomass, table G.23), the mass of fibre fraction produced can be calculated. This amounts to 245.638 kg fibre fraction per 1000 kg degassed biomass^[20], which means that 24.56 % of the initial mass is found in the solid fraction. The remaining mass is then going in the liquid fraction, corresponding to 75.44 % (i.e. 100 % - 24.56 %).

The separation indexes considered for the post-biogas separation are summarized in table G.26.

Table G.26. Separation indexes for separation of cow slurry AFTER the biogas plant. (polymer is not added). Data from Frandsen (2009), unless otherwise specified.

	Fibre fraction	Liquid fraction
Total mass^a)	24.56%	75.44%
Dry matter (DM)	60.9%	39.1%
Total-N	21.2%	78.8%
Ammonium-N	14.6%	85.4%
Phosphorous (P)	66.2%	33.8%
Potassium (K)	9.7%	90.3%
Carbon (C)^b)	60.9%	39.1%
Cooper (Cu)^c)	6.7%	93.3%
Zinc (Zn)^c)	25.3%	74.7%

a) This is a calculated value, see text.

b) No data. Assumed to be the same as DM.

c) From Møller et al. (2007b). Data for centrifuge, cattle slurry no.5 used (table 3).

G.19.2 Mass balances

The mass balances of the degassed biomass mixture before and after the separation are presented in table G.27. It should be highlighted that no data as regarding the emissions occurring during the separation process has been found, as it was also the case with the separation before the biogas is produced (i.e. process G.4). This lack of data is particularly critical as regarding ammonia emissions, which are likely to occur given the volatile nature of ammonia. Yet, it appears reasonable to assume that all the emissions likely to occur during the separation are occurring in later stages anyway, so considering them at this stage or at later stages does not change the overall results.

The life cycle data for the separation post biogas production are presented in table G.28.

Table G.27. Mass balances for separation of the degassed biomass.
Per 1000 kg of degassed biomass mixture “ex-digester”.

Click here to see Table G.27.

Table G.28. Life cycle data for separation (decanter centrifuge) after the anaerobic digestion. Data per 1000 kg slurry (ex-digester). Dairy cows digested slurry.

	Dairy cows degassed slurry	Comments
Input
Slurry (ex-digester)	1000 kg	Degassed biomass ex-digester.
Output
Fibre fraction	245.6 kg
Liquid fraction	754.4 kg
Energy consumption
Electricity	2.184 kWh	See table F.9 (Annex F)
Material consumption
Separation equipment	included	See table F.10 (Annex F)
Emissions to air
Carbon dioxide (CO₂)		No data
Methane (CH₄)		No data
Non-methane volatile organic compounds (NMVOC)		No data
Ammonia (NH₃-N)		No data.
Nitrous oxide (N₂O-N)		No data
Nitrogen oxides (NO_X)		No data
Nitrogen monoxide(NO)		No data
Nitrogen(N₂)		No data
Particulates		No data
Hydrogen sulphide (H₂S)		No data
Sulphur dioxide (SO₂)		No data
Odour		No data
Emissions to water
		No emissions to water

[20] The input degassed slurry to separate contains 107.73 kg DM/1000 kg degassed biomass (table G.23). Yet, 60.9% of the DM ends up in the fibre fraction with the separation indexes considered (see table G.26) i.e. 107.73 kg * 60.9% = 65.61 kg DM per 1000 kg degassed biomass. As the fibre fraction contains 267.1 kg DM per 1000 kg fibre fraction (due to measurements, see table 4 in Frandsen (2009)), the total amount of fibre fraction is: 65.61 kg DM / 1000 kg degassed biomass * 1000 kg fibre fraction/ 267.1 kg DM = 245.638 kg fibre fraction per 1000 kg degassed biomass.