Life Cycle Assessment of Biogas from Separated slurry
3 Reference scenario for pig and for cow slurry (scenario A)
- 3.1 System Description
- 3.2 Composition of reference slurry
- 3.3 Data for the reference scenarios
- 3.4 Results of the Impact Assessment
3.1 System Description
The reference scenario, for both fattening pig and dairy cows, consists to store the slurry in-house, to transfer it to a pre-tank and then to store it in an outdoor storage until application on-field is possible (suitable) (described in section 2.2.1). The final use of the slurry is then as an organic fertiliser. This scenario is thoroughly described in the first part of this LCA foundation for slurry management (section 3 of Wesnæs et al., 2009). All life cycle inventory data used for this reference scenario can be found in Annex A of Wesnæs et al. (2009). The major elements from this can be found in Annex A of the present study.
A simplified diagram illustrating the main processes involved in the reference scenario is presented in figure 3.1 (after Wesnæs et al., 2009).
Figure 3.1. Simplified flow diagram for the reference scenario (After Wesnæs et al., 2009)
As mentioned in Wesnæs et al. (2009), it has been necessary to define the preconditions concerning the reference scenarios regarding e.g. housing units, type of storage, technology for application to the field and reference cropping scenarios. These pre-conditions also apply for all subsequent alternatives assessed. These pre-conditions are summarised in the present study, but their full description and justification can be found in Wesnæs et al. (2009).
For fattening pigs, these pre-conditions include (after Wesnæs et al., 2009):
- Housing conditions based on a housing system with “Fully slatted floor”.
- Pumping of the slurry from the pre-tank (in connection with the housing units) to the outdoor storage.
- Outdoor storage in concrete slurry tanks covered by a floating layer of straw.
- A transport distance from storage to application to fields of 10 km.
- Slurry application by trail hose tankers.
- Relevant soil types for application of pig slurry: soil type JB3 (representing sandy soil) and soil type JB6 (representing clay soil).
- It is assumed that pig slurry is applied to all crops in the crop rotation pattern, with a farm average of 140 kg N ha-1 y-1. It is also assumed that the slurry is applied during spring.
- A six year crop rotation, with slurry N (kg ha-1 y-1) applied shown in parenthesis: winter barley (133.5) – winter rape (133.5) - winter wheat (133.5) – winter wheat (133.5) – spring barley with catch crop (165) – spring barley (145). As mentioned in section 2, the crops are not included within the system boundaries. They are only defined as the uptake and emissions of N and P in slurry depends on the crop, and in order to model the further fate of the N not removed with harvested products.
For dairy cows, these pre-conditions include (after Wesnæs et al., 2009):
- Housing conditions based on a “Cubicle housing system with slatted floor (1.2 m channel)” [2].
- Pumping of the slurry from the pre-tank (in connection with the housing system) to the outdoor storage.
- Outdoor storage in concrete slurry tanks covered by a natural floating layer.
- A transport distance from storage to application to fields of 10 km.
- Slurry application by trail hose tankers.
- Relevant soil types for application of pig slurry: soil type JB3 (representing sandy soil) and soil type JB6 (representing clay soil).
- It is assumed that cattle slurry is applied to all crops in the crop rotation pattern, with a farm average of 140 kg N ha-1 y-1. It is also assumed that the slurry is applied during spring.
- A five year crop rotation, with slurry N (kg ha-1 y-1) applied shown in parenthesis: spring barley harvested as whole crop silage (156) – grass clover mixture (182) – grass clover mixture (182) – spring barley with catch crop (0) – spring barley (132). Besides this, 15 % of the area is assumed utilised for continuous silage maize (188).
3.2 Composition of reference slurry
As described in Wesnæs et al. (2009), the slurry composition is the very basis for the different process flows involved in this LCA foundation. The “reference slurry ” upon which all flows of this study will be normalized is defined as “ slurry ex-animal”, i.e. the slurry produced right after animal excretion. This is chosen as the reference point as this is where the system boundaries start. The composition of the slurry in the reference scenario is calculated at three points:
- Slurry “ex animal”, i.e. right after excretion;
- Slurry “ex housing”, i.e. in the slurry pit under the animals right before flushing to the pre-tank;
- Slurry “ex storage”, i.e. after months of covered outdoor storage, measured right before application to field.
The chemical composition of pig slurry is given in table 3.1. The composition of dairy cow slurry is given in table 3.2. The explanations for the composition are given in Annex A of Wesnæs et al. (2009). The number of digits in table 3.1 and 3.2 should not be seen as a measure of the precision, but is only included as the values are the foundation for further calculations.
Table 3.1. Characteristics of slurry from fattening pigs in the reference
scenario.
Per 1000 kg of slurry “ex animal”, “ex housing” and “ex storage”. (after Wesnæs
et al., 2009)
| Ex Animal |
Ex housing |
Ex storage |
|
|---|---|---|---|
| Total mass | 1000 kg Slurry ex animal |
1000 kg Slurry ex housing |
1000 kg Slurry ex storage |
| Dry matter (DM) | 77.4 kg | 69.7 kg | 61 kg |
| Ash content | 13.2 kg | 13.2 kg | 12.2 kg |
| Volatile solids (VS) | 64.2 kg | 56.5 kg | 48.8 kg |
| Of total VS: - easily degradable |
41.7 kg | 34.0 kg | 28.1 kg |
| - heavily degradable | 22.5 kg | 22.5 kg | 20.7 kg |
| Total-N (Norm Data, DJF, 2008) | 6.60 kg | 5.54 kg | 5.00 kg |
| Total-N in this study | 6.60 kg | 5.48 kg | 4.80 kg |
| NH4+-N | No data | No data | 3.60 kg |
| Total-P | 1.13 kg | 1.13 kg | 1.04 kg |
| Potassium (K) | 2.85 kg | 2.85 kg | 2.60 kg |
| Carbon (C) | 37.0 kg | 33.3 kg | 29.2 kg |
| Copper (Cu) | 30.0 g | 30.0 g | 27.6 g |
| Zinc (Zn) | 89.4 g | 89.4 g | 82.4 g |
| Density | 1053 kg per m³ | 1053 kg per m³ | 1053 kg per m³ |
| pH | 7.8 | 7.8 | 7.8 |
Table 3.2. Characteristics of slurry from dairy cows in the reference scenario.
Per 1000 kg of slurry “ex animal”, “ex housing” and “ex storage”. (after Wesnæs et al., 2009, with corrections of the units for Cu and Zn)
| Ex Animal |
Ex housing |
Ex storage |
|
|---|---|---|---|
| Total mass | 1000 kg slurry ”ex animal” |
1000 kg slurry ”ex housing” |
1000 kg slurry ”ex storage” |
| Dry matter (DM) | 125.7 kg | 113.2 kg | 103 kg |
| Ash content | 21.5 kg | 21.5 kg | 20.6 kg |
| Volatile solids (VS) | 104.2 kg | 91.7 kg | 82.4 kg |
| Of total VS: - easily degradable |
50.0 kg | 37.5 kg | 30.5 kg |
| - heavily degradable | 54.2 kg | 54.2 kg | 51.9 kg |
| Total-N (Norm Data, DJF, 2008) | 6.87 kg | 6.41 kg | 6.02 kg |
| Total-N in this study | 6.87 kg | 6.34 kg | 5.79 kg |
| NH4+-N | No data | No data | 3.47 kg |
| Total-P | 1.02 kg | 1.03 kg | 0.98 kg |
| Potassium (K) | 5.81 kg | 5.90 kg | 5.65 kg |
| Carbon (C) | 55.2 kg | 49.7 kg | 45.2 kg |
| Copper (Cu) | 12.1 g | 12.1 g | 11.6 g |
| Zinc (Zn) | 23.4 g | 23.4 g | 22.4 g |
| Density | 1053 kg per m³ | 1053 kg per m³ | 1053 kg per m³ |
| pH | 7.8 | 7.8 | 7.8 |
3.3 Data for the reference scenarios
All data for the references scenarios are presented in Annex A of Wesnæs et al. (2009) and summarised in Annex A of the present study.
3.4 Results of the Impact Assessment
The results of the impact assessment for the reference scenario are presented and discussed in detail in Wesnæs et al. (2009). In the present study, the results of all assessed scenarios are compared to the results for the reference scenario, as presented in the next sections.
[2] In Danish: Sengestald med spaltegulv (1.2 m kanal)
Version 1.0 August 2010, © Danish Environmental Protection Agency