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Life Cycle Assessment of Biogas from Separated slurry

Processes H.2 to H.7: Raw slurry from which the fibre fraction origins: production, separation and fate of the liquid fraction

• H.2 In-house storage of slurry
• H.3 Storage of raw slurry in pre-tank at farm
• H.4 mechanical separation (no polymer) – screw press and arc strainers
• H.5 Outdoor storage of the liquid fraction
  • H.5.1 General description
  • H.5.2 Addition of water
  • H.5.3 Electricity consumption
  • H.5.4 Emissions of CH₄
  • H.5.5 Emissions of CO₂
  • H.5.6 Emissions of NH₃
  • H.5.7 Emissions of N₂O, NO-N and N₂-N
  • H.5.8 Life cycle data and mass balances for storage of liquid fraction
• H.6 Transport of liquid fraction to field
• H.7 Field processes (liquid fraction)
  • H.7.1 General description
  • H.7.2 Emissions of CH₄ and CO₂
  • H.7.3 Emissions of NH₃
  • H.7.4 Emissions of N₂O-N and NO_X-N and N₂-N
  • H.7.5 Calculation of liquid fraction fertilizer value
  • H.7.6 Nitrate leaching
  • H.7.7 Phosphorus leaching
  • H.7.8 Cu and Zn fate
  • H.7.9 Life cycle data for field application of liquid fraction

Click here to see Figure

H.2 In-house storage of slurry

In this scenario, the storage of the slurry in the barn has not changed compared to the reference scenario (Annex A). Accordingly, the life cycle inventory data are identical to table A.9 in Annex A. This also means, that the process is identical to the process in Annex F, section F.2 of this report.

The slurry composition when leaving the barn is identical to the slurry “ex-housing” from table A.1 in Annex A. This is shown in table H.1 below.

Table H.1. Characteristics of slurry “ex-housing” from fattening pigs
Per 1000 kg of slurry ex-housing

H.3 Storage of raw slurry in pre-tank at farm

The storage of raw slurry is identical to the process in Annex F, section F.3.

The storage duration is, for this study, assumed to be rather short and it has been assumed that losses of CH₄, CO₂ and N₂O in the pre-tank are negligible.

Accordingly, the composition of the slurry “ex pre-tank” is assumed to be identical to the “ex-housing” composition from table H.1 above.

H.4 mechanical separation (no polymer) – screw press and arc strainers

The mechanical separation process used in this Annex is the Samson Bimatech mechanical separation described in Annex C, section C.4. The life cycle inventory data are shown in table C.4.

Note, that the Samson Bimatech mechanical separation in Annex C does not include the use of polymer. It is possible to add polymer to the separation (J. Mertz, 2008), however this is not included in this report.

The slurry before the separation has the same composition as in Annex C, accordingly, the life cycle inventory data and the resulting separated fibre fraction and liquid fraction are identical to the fractions from Annex C. The composition of the slurry before and after the separation is shown in table H.2 below (and this is identical to table C.3 from Annex C).

Table H.2. Mass balances for mechanical separation of slurry from fattening pigs.
Per 1000 kg of slurry “ex-housing”.

Click here to see Table H.2.

H.5 Outdoor storage of the liquid fraction

H.5.1 General description

The main principles for the outdoor storage of the liquid fraction in this annex are basically the same as for the outdoor storage of the liquid fraction in Annex F, however, as the separation process is different, the composition of the liquid fraction is different. As the emissions depend on the composition of the liquid fraction, the emissions will be different from Annex F (but still be based on the same calculation methods).

As in Annex F, the liquid fraction is stored in an outdoor concrete tank covered with a floating layer consisting of 2.5 kg of straw per 1000 kg slurry stored. Because straw is regarded as a waste product from cereal production (rather than a co-product), the life cycle data of straw production are not included in this study (as in Annex F).

H.5.2 Addition of water

Water will be added in the liquid fraction during storage through precipitations. The amount of precipitations is the same as in Annex A and F, i.e. a total of 86 kg of water.

H.5.3 Electricity consumption

The electricity for pumping and stirring is assumed to be identical to the electricity consumption for the pumping and stirring in Annex F (see section F.5.3), i.e. 1.45 kWh per 1000 kg liquid fraction. This consumption was calculated from the data for raw slurry presented in Annex A, but adjusted by a factor of 0.5 to take into account that the separated liquid fraction is likely to offer much less resistance when stirring or pumping than does the raw slurry. There might be a difference between the viscosity and resistance for the liquid fraction in this annex as compared to the liquid fraction in Annex F since the content of DM is not the same, however, this 0.5 factor adjustment is a rough estimate anyway that only aims to take into account. Moreover, the results of the life cycle assessment for the reference scenario in Annex A shows that the electricity consumption in this stage is rather insignificant for the overall results.

H.5.4 Emissions of CH₄

For the calculation of the emissions of CH₄ from the outdoor storage of the liquid fraction, the same method has been applied as in Annex F, see the explanation in section F.5.4.

As described in section F.5.4, the CH₄ emissions are calculated based on the IPCC methodology, but by using the VS content of the separated liquid fraction (the VS being calculated with the hypothesis that VS = DM * 80 %). This gives a CH₄ emission of 1.248 kg per 1000 kg of liquid separated fraction^[2]. When comparing with the values of the CH₄ emission from the outdoor storage of the liquid fraction from Annex F it can be seen that the CH₄ emission from the storage in this annex is significantly higher. This is caused by the higher content of DM (and thereby VS) in the liquid fraction. However, it is still a reduction compared to the outdoor storage of the slurry in the reference scenario. The value of 1.248 kg CH₄ per 1000 kg of liquid fraction represents a reduction of 36 % as compared to the emissions occurring during the storage of raw slurry (which was 1.94 kg CH₄ per 1000 kg slurry ex-housing, table A.11, Annex A). This is in the range of values presented by Martinez et al. (2003), where reductions in CH₄ emissions between 7% and 40% were observed from the storage of different mechanically separated liquid fractions, as compared to raw slurry.

H.5.5 Emissions of CO₂

Emissions of CO₂ were estimated as a function of the methane emissions, based on the Buswell equation (Symons and Buswell, 1933) and the composition of pig slurry in terms of organic components constituting the VS. This calculation is detailed in Annex F (section F.5.5) and demonstrates that an amount of 1.42 g of CO₂ is produced per g of CH₄.

As mentioned in section F.2 in Annex F, part of the produced CO₂ from the outdoor storage is emitted to air immediately and part of the CO₂ is dissolved in the slurry. However, in this life cycle assessment, it is calculated as all the CO₂ is emitted to air immediately, which makes the interpretation of the sources easier, as detailed in section F.2.

H.5.6 Emissions of NH₃

In this project, the ammonia emissions are calculated using the same assumption as for the reference scenario: According to Poulsen et al. (2001), the emission of NH₃–N is 2% of the total-N in the slurry “ex-housing” (i.e. “ex-separation” in the present case). This corresponds to NH₃–N emissions of 0.1077 kg per 1000 kg of separated liquid.

H.5.7 Emissions of N₂O, NO-N and N₂-N

As in Annex F, section F.5.7, the N₂O emissions from the outdoor storage is calculated based on the IPCC guidelines (IPCC, 2006), using the total-N content of the slurry “ex-separation”. This gives a N₂O emission of 0.03244 kg N₂O-N per 1000 kg liquid fraction ^[3]. As mentioned in section F.5.7 it is acknowledged that the N₂O emissions may in fact be lower than this estimate due to the lower DM content in the liquid fraction (and thereby a lower potential for easily converted VS content).

The NO-N and N₂-N emissions were calculated in the same way as in Annex A, i.e. based on the study of Dämmgen and Hutchings (2008). In their study, they assumed that the emission of nitrogen monoxide (NO) is the same as the direct emission of nitrous oxide (N₂O) (measured as NO-N and N₂O-N). Furthermore, they assumed that emission of nitrogen (N₂) is three times as high as the direct emissions of nitrous oxide (N₂O) (measured as N₂-N and N₂O-N).

As regarding the total NO_X emissions (NO_X = NO + NO₂), it was assumed, as in Annex A, that NO_X = NO. This is because it has not been possible to find data on NO₂.

Therefore, this means that the NO-N emissions (and thereby the NO_X-N emissions) correspond to 0.03244 kg per 1000 kg liquid fraction and the N₂-N emissions correspond to 0.09732 kg per 1000 kg liquid fraction.

The indirect N₂O-N emissions can be calculated as described by IPCC guidelines (IPCC, 2006), i.e. as 0.01 * (NH₃-N + NO_X-N). This gives indirect N₂O-N emissions of 0.0014 kg per 1000 kg liquid fraction.

H.5.8 Life cycle data and mass balances for storage of liquid fraction

Table H.3 summarizes the life cycle inventory data for the storage of liquid fraction and presents the comparison with the storage emissions in Annex A. It must be emphasized that 1000 kg liquid fraction do not correspond to 1000 kg slurry ex-animal, so the values of Annex A versus Annex H are not directly comparable. Values from Annex A were only included since they were needed for the calculation of many of the emissions.

Table H.4 presents the mass balance of the liquid fraction in order to establish its composition after the storage. In this table, it can be noticed that the change of DM is estimated as the losses of N and C. It is acknowledged that this is a rough estimation, as other elements of greater molecular weight may also be lost (e.g. dissolved O₂). The estimated DM change shall therefore be seen as a minimum change, the actual DM change may in fact be greater than the one taken into account in this study.

Table H.3 Life cycle data for storage of the liquid fraction. All data per 1000 kg of liquid fraction “ex-separation”.

Table H.4. Mass balances for storage of liquid fraction

^a Changes in total N: 0.1077 kg NH₃-N + 0.03244 kg N₂O-N + 0.03244 kg NO-N + 0.0973 kg N₂-N = 0.270 kg N

^b Changes in total C: 1.772 kg CO₂ * 12.011 [g/mol] /44.01 [g/mol] + 1.248 kg CH₄ * 12.011 [g/mol] /16.04 [g/mol] = 1.418 kg C

^c The change in DM is assumed to be identical to the sum of the loss of N and C

H.6 Transport of liquid fraction to field

The transport of the liquid fraction to field is assumed to be identical to the transport of the untreated slurry in Annex A. Accordingly the same assumptions have been applied.

This means that the process “Transport, tractor and trailer” from the ecoinvent database has been used (Nemecek and Kägi, 2007, p.204), for a distance of 10 km. This includes the construction of the tractor and the trailer. As the transport by trucks (instead of by tractor with a trailer) is required by law in Denmark when the slurry is transported for distances greater than 10 km, Wesnæs et al. (2009) carried out a sensitivity analysis with a transportation distance of 32 km (involving transport by truck). Yet, they found that the transport distance of slurry from the storage to the field had no significance on the environmental impacts they assessed. Therefore, the transport distance from storage to field is fixed to 10 km in the present project.

H.7 Field processes (liquid fraction)

H.7.1 General description

The main principles for the field processes for the liquid fraction in this annex are basically the same as for the field processes for the liquid fraction in Annex F. However, as the composition of the liquid fraction is different, the emissions will be different from Annex F, as the emissions depend on the composition of the liquid fraction (but they are still based on the same calculation methods).

H.7.2 Emissions of CH₄ and CO₂

As described in Annex F, section F.7.2, the CH₄ emissions on the field are assumed to be negligible, as the formation of CH₄ requires an anaerobic environment, which is, under normal conditions, not the case in the top soil.

CO₂ emissions and C-binding in the soil are modelled by the dynamic soil organic matter model C-TOOL (Petersen et al., 2002; Gyldenkærne et al., 2007). The development in organic soil N is modelled by assuming a 10:1 ratio in the C to N development.

H.7.3 Emissions of NH₃

The NH₃-N emissions during application were calculated as in the reference scenario, i.e. 0.5 % of the NH₄⁺-N “ex-storage”. This is based on Hansen et al. (2008). Yet, Hansen et al. (2008) calculated NH₄⁺-N “ex-storage” as 79 % of the total N (instead of 75 % as assumed in this study). In this specific case, because the NH₃-N emissions are calculated based on Hansen et al. (2008), the NH₄⁺-N will be evaluated with the figures presented by Hansen et al. (2008), as it was done in Annex A.

The NH₃-N emissions for the period after application are calculated by using the same method as described in Annex F, section F.7.3. Therefore, they are estimated as 50% of 0.138 kg NH₃-N per kg TAN-N in the pig slurry, assuming that the TAN (NH₃+NH₄⁺), at the liquid fraction pH, corresponds to NH₄⁺ only, and evaluating NH₄⁺-N as 79 % of the total N (as these data are also based on Hansen et al., 2008). As this figure includes the NH₃-N emissions during application mentioned above, these are subtracted.

H.7.4 Emissions of N₂O-N and NO_X-N and N₂-N

Direct and indirect N₂O-N emissions as well as emissions of NO_X-N were calculated as in the reference scenario (section A.5.3 and A.5.4 in Annex A). This means that the direct emissions of N₂O-N are evaluated as 0.01 kg N₂O-N per kg N in the ex-storage liquid fraction (table 11.1 in IPCC (2006)). Yet, it is acknowledged that this may overestimate the N₂O emissions occurring from the spreading of the liquid fraction, as the C/N ratio of the liquid fraction is lower than the C/N ratio of the non-separated slurry. In fact, according to Møller et al. (2007c), the centrifugal separation mainly transfers the organic N to the solid fraction, while the dissolved NH₄⁺ goes in the liquid fraction. A higher NH₄-N content involves more N in a form directly available for plants. This means that less N shall be available to microorganisms for nitrification (where NO₃^- is formed), and thus, the potential for denitrification (where NO₃^- is reduced to N₂O, and subsequently to N₂) is reduced. According to Amon et al. (2006), a lower C/N ratio also reduces the potential for N immobilisation in the soil N pool, and thereby the availability of N for denitrification.

The indirect N₂O-N emissions due to ammonia and NO_X are evaluated as 0.01 kg N₂O-N per kg of (NH₃ + NO_X) volatilized. The indirect N₂O-N emissions due to nitrate leaching correspond to 0.0075 kg N₂O-N per kg of N leaching.

The emissions of NO_X-N are calculated as 0.1* direct N₂O-N, based on Nemecek and Kägi (2007).

The N₂-N emissions are based on the estimates from SimDen (Vinther, 2004). For soil type JB3 the N₂-N:N₂O-N ratio is 3:1 and for soil type JB6 the N₂-N:N₂O-N ratio is 6.

H.7.5 Calculation of liquid fraction fertilizer value

The fertilizer value for liquid fraction is calculated and detailed in section H.23.

H.7.6 Nitrate leaching

In order to calculate N leaching values, the same simplifying assumption as in Annex C is used: the liquid fraction, once the respective ammonia losses have been subtracted, can be modeled as: a given proportion of slurry + a given amount of mineral N. The present liquid fraction has a higher content of N relative to C, as compared to the original reference slurry. This is because the mechanical separation transfers relatively more C (i.e. 29.6%) to the fibre fraction than N (i.e. 6.8%). As the amount of organic matter is one of the key properties for its effect on the N partitioning, the amount of C relative to N in the pig slurry from the reference scenario is used. The N values are taken after ammonia volatilization. The C:N proportion is 29.2 [kg C] / (4.80-0.02-0.48) [kg N] = 6.79 for the slurry and 21.7 [kg C] / (4.71-0.02-0.24) [kg N] = 4.876 for the liquid fraction. The “virtual” proportion of N assumed to affect the soil and plants as raw slurry is therefore 4.876/6.79 = 0.72, and the virtual proportion of N assumed to affect the soil and plants as mineral N is accordingly 0.28. The tables A.14 and A.15 of Annex A are therefore the basis for the calculation of N leaching, after correcting for their respective ammonia volatilizations.

H.7.7 Phosphorus leaching

For P leaching, the same assumptions as those used in Annex A were used, i.e., 10% of the P applied to field has the possibility of leaching and 6% of this actually reach the aquatic recipients, based on Hauschild and Potting (2005).

H.7.8 Cu and Zn fate

As in Annex A, it is considered that the entirety of the Cu and Zn applied will leach through the water compartment.

H.7.9 Life cycle data for field application of liquid fraction

Table H.5 presents the life cycle data for the application of ex-storage liquid fraction on the field. The results of the reference case (Annex A) are also presented for comparison purposes. However, in order to be comparable, both results must be related to the functional unit, i.e. 1000 kg slurry ex-animal.

Table H.5. Life cycle data for the field processes related with the application of liquid fraction. All data per 1000 kg of “liquid fraction ex-storage”.

[2] 80 % * 51.76 kg DM per 1000 kg liquid fraction (see table H.2) * 0.45 * 0.67 * 10 % = 1.248 kg

[3] The content of total-N “ex-separation” is 5.387 kg/1000 kg liquid fraction (table H.2). The content of total-N in the reference slurry is 5.48 kg per 1000 kg slurry ex-housing (table A.1, Annex A). The direct N₂O emissions in the reference scenario were 0.033 kg N₂O-N per 1000 kg slurry ex-housing (table A.11, Annex A). Therefore, the direct N₂O-N emissions are calculated as: 0.033 kg N₂O-N * (5.387 kg/ 5.48 kg) = 0.03244 kg N₂O-N per 1000 kg liquid fraction.

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Version 1.0 August 2010, © Danish Environmental Protection Agency