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Life Cycle Assessment of Biogas from Separated slurry Processes G.24 to G.27: fate of the degassed liquid fraction
G.24 Transport of the degassed liquid fraction to the farmThe transport of the degassed liquid fraction back to the farm is identical to the process described in section G.13 (transport of raw slurry to biogas plant). This means that a distance of 5 km is taken into account between the farm and the biogas plant. As transport distance is not anticipated to have a considerable influence on the environmental impacts in the overall scenario (based on the results obtained by Wesnæs et al., 2009), no sensitivity analysis was carried out for a greater transport distance. The degassed liquid is transported by trucks. The transport is modelled by use of the Ecoinvent process “Transport, lorry >32t, EURO3” (Spielmann et al., 2007; table 5-124, p.96). G.25 Outdoor storage of the degassed liquid fractionG.25.1 General descriptionThe outdoor storage of the degassed liquid fraction is assumed to be stored in an outdoor concrete tank covered with a floating layer consisting of 2.5 kg of straw per 1000 kg slurry stored (as for process G.5). As in section G.5.1, the life cycle data of straw production are not included in this study, as straw is regarded as a waste product from cereal production (rather than a co-product). G.25.2 Addition of waterThe degassed liquid fraction will be diluted by precipitation in the same amount as described in G.5.2, i.e. a total of 44 kg of water. G.25.3 Electricity consumptionAs with the non degassed liquid fraction in section G.5, the electricity for pumping and stirring is taken from table A.10 (Annex A) and is adjusted by a reduction of 50 %, in order to account for the fact that the liquid fraction will offer less resistance during the pumping and stirring than does the raw slurry. This is further detailed in section G.5. The electricity consumption thus involves: the consumption for stirring when straw is added (1.2 kWh per 1000 kg slurry), the consumption for stirring (1.2 kWh per 1000 kg slurry) and pumping (0.5 kWh per 1000 kg slurry), before application to the field. This gives an electricity consumption of 2.9 kWh per 1000 kg slurry, on which a factor of 50 % is applied, which results in an electricity consumption of 1.45 kWh per 1000 kg degassed liquid fraction. G.25.4 Emissions of CH4It has not been possible to find high quality data about the CH4 emissions occurring during the storage of degassed liquid fraction. Yet, in the latest Danish national inventory report for greenhouse gases, Nielsen et al. (2009) calculated the absolute CH4 reduction of biogas-treated slurry by using the IPCC methodology[21], coupled with a reduction potential of 30 % in the case of cow slurry. When applying this equation, Nielsen et al. (2009) considered the VS content of the treated slurry instead of the VS content ex-animal. This is the methodology that will be applied in this project. The VS of the liquid fraction is estimated as 80% of the DM content. This corresponds to a VS content of 44.66 kg per 1000 kg liquid fraction. The CH4 emissions are therefore calculated as: 44.66 kg VS/1000 kg degassed liquid fraction * 0.24 m³ CH4/kg VS * 0.67 kg CH4/m³ CH4 * 10% * (100-30) % = 0.503 kg CH4/1000 kg degassed liquid fraction. G.25.5 Emissions of CO2Emissions of CO2 were estimated with the calculated ratio between emissions of CO2 and CH4 in anaerobic conditions, i.e. 1.67 kg CO2 per kg CH4 (see section G.5.5). As mentioned in section G.2, 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 G.2. G.25.6 Emissions of NH3Hansen et al. (2008) states that there are no clear differences between the ammonia (NH3) emissions from degassed slurry and untreated slurry. On one hand, the lower content of dry matter might reduce the emission of ammonia, on the other hand, TAN concentration and pH of degassed slurry are higher, which both increase the potential for ammonia emissions. Yet, Sommer (1997), who measured the NH3 volatilization from both covered (one tank covered by straw and one tank covered by clay granules) and uncovered storage tank containing digested slurry, concluded that ammonia volatilization from the covered slurry was insignificant. The ammonia emissions occurring during the storage of the degassed liquid fraction are therefore calculated using the same assumptions as for the reference scenario, i.e. the emission of NH3–N are 2% of the total-N, based on Poulsen et al. (2001). The total N being 7.98 kg N/1000 kg degassed liquid fraction, the NH3-N emissions are 0.160 kg NH3-N per 1000 kg degassed liquid fraction. G.25.7 Emissions of N2O, NO-N and N2-NIn the reference scenario, the direct N2O emissions for storage were based on IPCC guidelines (IPCC, 2006). However, the IPCC methodology does not provide any emission factor for storage of degassed liquid fraction. The fact that the liquid fraction is degassed involves a reduction in the N2Oemissions, as part of the most easily converted dry matter was removed during the biogas production (Mikkelsen et al., 2006). Yet, as for the CH4 emissions, the latest Danish national inventory report for greenhouse gases (Nielsen et al., 2009) considered a reduction potential factor for estimating the reductions in N2O-N emissions obtained when the slurry is biogas-treated. In the case of cow slurry, this reduction potential is 36% (Nielsen et al., 2009). In the present section, the direct N2O-N emissions will be estimated as in section G.5.7 (i.e. relatively to the emissions in the reference scenario but adjusted with the different N content), and this result will be multiplied by (100-36) % in order to consider the fact that the liquid fraction is degassed. The direct N2O-N emissions are therefore calculated as: 0.034 kg N2O-N/1000 kg slurry ex-housing * (7.98 kg N in 1000 kg of degassed liquid fraction/ 6.34 kg N in 1000 kg slurry ex-housing) * (100-36) % = 0.0274 kg N2O-N/1000 kg degassed liquid fraction. 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 NOX emissions (NOX = NO + NO2), it was assumed, as in Annex A, that NOX = 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 NOX-N emissions) correspond to 0.0274 kg N2O-N per 1000 kg degassed liquid fraction, and the N2-N emissions correspond to 0.0822 kg per 1000 kg degassed liquid fraction. The indirect N2O-N emissions can be calculated as described by IPCC guidelines (IPCC, 2006), i.e. as 0.01 * (NH3-N + NOX-N). This gives indirect N2O-N emissions of 0.0019 kg per 1000 kg degassed liquid fraction. G.25.8 Life cycle data and mass balances for storage of liquid fractionTable G.33 summarizes the LCA data for the storage of the degassed liquid fraction and presents the comparison with the storage emissions in Annex A. It must be emphasized that 1000 kg of degassed liquid fraction do not correspond to 1000 kg slurry ex-animal, so the values of Annex A versus Annex G are not directly comparable. Values from Annex A were only included since they were needed for the calculation of some of the emissions. Table G.34 presents the mass balances of the degassed slurry 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. As explained in section G.5.8, 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 G.33 Life cycle data for storage of the degassed liquid fraction. All data per 1000 kg of degassed liquid fraction “ex-separation”.
Table G.34. Mass balances for storage of degassed liquid fraction
a Changes in total N: 0.160 kg NH3-N + 0.0274 kg N2O-N + 0.0274 kg NO-N + 0.0822 kg N2-N = 0.297 kg N b Changes in total C: 0.840 kg CO2 * 12.011 [g/mol] /44.01 [g/mol] + 0.503 kg CH4 * 12.011 [g/mol] /16.04 [g/mol] = 0.606 kg C c The change in DM is assumed to be identical to the sum of the loss of N and C G.26 Transport of degassed liquid fraction to fieldThe transport of the degassed liquid fraction to the field is identical to the process described in section G.6 (transport of the liquid fraction to the field). 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. G.27 Field processes for degassed liquid fractionG.27.1 General descriptionAs in the process described in section G.7 (field processes for [non-degassed] liquid fraction), the data from the Ecoinvent process “Slurry spreading, by vacuum tanker” (Nemecek and Kägi, 2007, p. 198) were used for the emissions related to spreading equipment “consumption”. This includes the construction of the tractor and the slurry tanker, as well as the diesel consumption. The diesel consumption due to the use of the “tanker” in the Ecoinvent process was adjusted to 0.4 litres of diesel per 1000 kg of slurry, based on Kjelddal (2009) (the same as in Annex A). G.27.2 Emission of CH4 and CO2The 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. G.27.3 Emissions of NH3For the ammonia emissions occurring as a result of the fertilisation operations, no data were found in the literature for the specific case of the degassed liquid fraction. Yet, some data are available for degassed slurry (as compared to raw slurry) and for the (non-degassed) liquid fraction (as compared to raw slurry). According to Hansen et al. (2008), there are no clear difference between the emissions from degassed slurry and untreated slurry since degassed slurry presents both factor promoting and inhibiting NH3 volatilization. However, one of the main conclusion in a recent study by Möller and Stinner (2009) is that factors promoting NH3 volatilization (higher amounts of NH4-N and higher pH) predominate over the factors reducing the propensity for volatilization (lower viscosity, lower dry matter content). Different studies also report measurements showing that digested manure is more likely to lose ammonia than untreated manure after surface application (Bernal and Kirchmann, 1992; Sommer et al., 2006; Amon et al., 2006). Bernal and Kirchmann (1992) measured NH3-N losses of 14 % of the total applied N over a 9 days period from anaerobically treated pig manure mixed with soil. In Sommer et al. (2006), accumulated NH3 volatilization after 96 h were increased of about 27.3 % on a sandy loam soil and of approximately 21.6 % on a sandy soil (for digested manure as compare to undigested manure). During the field application of digested cow slurry, Amon et al. (2006) measured NH3 emissions of 220.0 g NH3 per m³ digested slurry. Assuming a density of 1000 kg/m³ for the digested slurry, this corresponds to 0.220 kg NH3 per 1000 kg digested slurry. Börjesson and Berglund (2007) assumed an average increase of 24 % of the NH3 emissions when digested manure is applied as compared to undigested manure (i.e. from 250 to 310 g NH3 per tonne of manure). As regarding the effect of the separation, a reduction of 50 % of the ammonia volatilization can be expected from a liquid fraction, as compared to raw slurry (see section G.7). Since the liquid degassed fraction is subjected to both increasing and reducing factors as regarding the ammonia emission potential, and since no data were found specifically for this, the ammonia emissions were calculated as in the reference scenario. This is exactly as described in section G.7, but without the 50 % reduction factor in the case of the emissions occurring after application. G.27.4 Emissions of N2O and NOX-NThe direct N2O emissions are generally assumed to be smaller for degassed slurry than for untreated slurry (Sommer et al. 2001). This is because digested manure contains less easily decomposed organic matter than undigested manure (Börjesson and Berglund, 2007) and because more N is in a form already available to the plants (NH4+). 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 also reduced. This is also in accordance with Marcato et al. (2009), who concluded from their results that there are fewer risks for oxygen competition between the crops and soil bacteria (and therefore of N2O emissions) with digested slurry as compared to undigested slurry. According to Sommer et al. (2001, table 2) N2O emissions with degassed slurry are in the magnitude of 0.4 % of the applied N. Based on Sommer et al. (2001), Nielsen (2002) used, for field emissions with digested slurry, a reduction corresponding to 41 % of the emissions with raw slurry (i.e. from 34 to 20 g N2O/ton manure) and Börjesson and Berglund (2007) assumed a reduction of 37.5 % (i.e. from 40 to 25 g N2O per tonne of manure). In this project, no specific data as regarding the direct N2O emissions related to the use of the degassed liquid fraction were found. Therefore, the estimate of Sommer et al. (2001) for digested (but non-separated) slurry will be used as the best available data (i.e. 0.4 % of the applied N). This should be regarded as a rather rough estimate. It may also overestimate the N2O emissions, as the slurry is both degassed and separated, which reduced significantly its content in organic N. In fact, according to Møller et al. (2007b), the centrifugal separation mainly transfers the organic N to the solid fraction, while the dissolved NH4+ goes in the liquid fraction. As in section G.7, indirect N2O emissions due to ammonia and NOX are evaluated as 0.01 kg N2O-N per kg of (NH3 + NOX) 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 NOX-N are calculated as 0.1* direct N2O-N, based on Nemecek and Kägi (2007). G.27.5 Emissions of N2-NThe 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 6:1. G.27.6 Calculation of degassed liquid fraction fertilizer valueThe fertilizer value for degassed liquid fraction is calculated and detailed in section G.28. G.27.7 Nitrate leachingThe approach from section G.7.6 is utilized, where the liquid fraction is equaled by a proportion of cattle slurry, and an additional amount of mineral N. The C:N proportion is 45.2 [kg C] / (5.79-0.02-0.73) [kg N] = 8.97 for the cow slurry and 22.11 [kg C] / (7.36-0.02-0.905) [kg N] = 3.44 for the liquid fraction. The “virtual” proportion of N assumed to affect the soil and plants as cattle slurry is therefore 3.44/8.97 = 0.383, and the virtual proportion of N assumed to affect the soil and plants as mineral N is accordingly 0.616. G.27.8 Phosphorus leachingFor 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). G.27.9 Cu and Zn fateAs in Annex A, it is considered that the entirety of the Cu and Zn applied will leach through the water compartment. G.27.10 Life cycle data for field application of degassed ex-storage liquid fractionTable G.35 presents the life cycle data for the application of degassed 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 G.35. Life cycle data for application of degassed liquid fraction and field processes. All data per 1000 kg of “degassed liquid fraction ex-outdoor storage”.
[21] According to IPCC (2006), the methane emission can be calculated as: CH4 [kg] = VS [kg] * B0 * 0.67 [kg CH4 per m³ CH4] * MCF B0 = 0.24 m³ CH4 per kg VS for dairy cows (IPCC, 2006, Table 10A-7). The MCF value used is 10 % (for liquid slurry with natural crust cover, cool climate, in table 10-17 of IPCC (2006)). This is also the MCF recommended under Danish conditions by Nielsen et al. (2009).
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