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Life Cycle Assessment of Biogas from Separated slurry Processes I.19 to I.22: Fate of the degassed biomass
I.19 Transport of the degassed biomass to the farmThe transport of the degassed biomass to the farm is identical to the process from Annex F, “Transport of the degassed liquid fraction” – except that the amount is different. Accordingly, the transport distance is 5 km. I.20 Outdoor storage of the degassed biomassI.20.1 General descriptionThe outdoor storage of the degassed biomass is assumed to be mostly identical to the outdoor storage of the reference slurry in Annex A, including some adjustments in order to take the “degassed” perspective into account. The degassed biomass is thus stored in an outdoor concrete tank covered with a floating layer consisting of 2.5 kg of straw per 1000 kg slurry stored. As in section I.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). I.20.2 Addition of waterThe degassed biomass will be diluted by precipitation in the same amount as described in I.5.2, i.e. a total of 86 kg of water. I.20.3 Electricity consumptionIt is assumed that the electricity consumption is identical to the electricity consumption for the storage of the reference slurry in Annex A. Accordingly, the electricity for pumping and stirring is taken from table A.10 (Annex A). 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. Note, that for the liquid fraction in section I.5.3, the energy consumption 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. As the degassed fibre fraction has a DM content (105.74 kg DM per kg degassed biomass, see table I.8) that is even higher than the DM content of the reference slurry “ex-housing” (69.7 kg DM per kg slurry “ex-housing”, see table A.1 in Annex A), the energy consumption for pumping and stirring has not been reduced. I.20.4 Emissions of CH4It has not been possible to find high quality data about the CH4 emissions occurring during the storage of degassed biomass. 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[12], coupled with a reduction potential factor of 50 % in the case of pig 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 is estimated as 80% of the DM content. This corresponds to a VS content of 84.592 kg per 1000 kg degassed biomass (= 80% of the 105.74 kg DM per 1000 kg degassed biomass from table I.8). As regarding the reduction potential factor, in this project, the interest is not the reduction, but the emissions occurring, so a factor of (100 – 50 %) will be used instead of 50 % (which in this case does not change anything mathematically). The CH4 emissions are therefore calculated as: 84.592 kg VS/1000 kg degassed biomass * 0.45 m³ CH4/kg VS * 0.67 kg CH4/m³ CH4 * 10% * (100-50) % = 1.275 kg CH4/1000 kg degassed biomass. I.20.5 Emissions of CO2Emissions of CO2 were estimated with the calculated ratio between emissions of CO2 and CH4 in anaerobic conditions, i.e. 1.42 kg CO2 per kg CH4 (see Annex F, section F.5.5). As mentioned in section F.5.5, 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 I.2. This gives a CO2 emission of 1.42 kg CO2 per kg CH4 * 1.275 kg CH4/1000 kg degassed biomass = 1.811 kg CO2. I.20.6 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 biomass 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 5.716 kg N/1000 kg degassed biomass, the NH3-N emissions are 0.114 kg NH3-N per 1000 kg degassed biomass. I.20.7 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 biomass. The fact that the biomass 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 pig slurry, this reduction potential factor is 40 %. In the present project, the direct N2O-N emissions will be estimated as in section I.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 (100-40) % in order to consider the fact that the biomass is degassed. The direct N2O-N emissions are therefore calculated as: 0.033 kg N2O-N/1000 kg slurry ex-housing * (5.716 kg N in 1000 kg of degassed biomass/ 5.48 kg N in 1000 kg slurry ex-housing) * (100-40) % = 0.02065 kg N2O-N/1000 kg degassed biomass. 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 0.02065 kg N2O-N per 1000 kg degassed biomass, and the N2-N emissions correspond to 0.06196 kg per 1000 kg degassed biomass. 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 0.001347 kg per 1000 kg degassed biomass. I.20.8 Life cycle data and mass balances for storage of degassed biomassTable I.5 summarizes the life cycle inventory data for the storage of the degassed biomass and presents the comparison with the storage emissions in Annex A. It must be emphasized that 1000 kg of degassed biomass do not correspond to 1000 kg slurry ex-animal, so the values of Annex A versus Annex I are not directly comparable. Values from Annex A were only included since they were needed for the calculation of some of the emissions. Table I.6 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. 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 I.5 Life cycle data for storage of the degassed biomass. All data per 1000 kg of degassed biomass “ex-biogas plant”.
Table I.6. Mass balances for storage of degassed biomass
a Changes in total N: 0.129 kg NH3-N + 0.02332 kg N2O-N + 0.02332 kg NO-N + 0.06995 kg N2-N = 0.2456 kg N b Changes in total C: 1.8108 kg CO2 * 12.011 [g/mol] /44.01 [g/mol] + 1.2752 kg CH4 * 12.011 [g/mol] /16.04 [g/mol] = 1.4491 kg C c The change in DM is assumed to be identical to the sum of the loss of N and C I.21 Transport of degassed biomass to fieldThe transport of the degassed biomass to the field is identical to the process described in section F.6 (transport of the liquid fraction to the field). I.22 Field processes for degassed biomassI.22.1 General descriptionThe field processes for the degassed biomass is assumed to be mostly identical to the field processes for the reference slurry in Annex A, including some adjustments in order to take the “degassed” perspective into account. As in the process described in section F.7 (field processes for 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). I.22.2 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 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. I.22.3 Emissions of NH3Since the degassed biomass is subjected to both increasing and reducing factors as regarding the ammonia emission potential, the ammonia emissions were calculated as in the reference scenario. This is further detailed in section F.7.3 in Annex F (but without the 50% reduction factor as this reduction factor only applies for liquid fractions, and as the degassed biomass is not separated). Accordingly, the NH3-N emissions for the period after application are calculated by using the same method as used in section F.7.3 (but without the 50% reduction factor). I.22.4 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, the estimate of Sommer et al. (2001) for digested slurry will be used as the best available data. This should be regarded as a rather rough estimate. A more precise value for the magnitude of this value would require either an adequate number of scientific based field measurements or detailed modelling in an appropriate tool, which has been beyond the frame of this project. As in section F.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). I.22.5 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. I.22.6 Calculation of degassed biomass fertilizer valueThe calculation of the fertilizer value is presented on section I.26. I.22.7 Nitrate leachingThe C/N ratio of the degassed biomass is higher than for raw pig slurry. Hence, a simplifying approach is used: the N “remaining” after gaseous losses and incorporation in the soil N pool is assumed to be divided between harvest and leaching in the same proportion as for pig slurry. See Annex F, section F.23.7 for further description. After the gaseous losses (table I.7), there is 3.7746 (JB3) and 3.5462 (JB6) kg N left for harvest and leaching. For the 100 years values, there is, after the gaseous losses, 5.4155 (JB3) and 5.4007 (JB6) kg N left for harvest and leaching. I.22.8 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). I.22.9 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. I.22.10 Life cycle data for field application of degassed biomass “ex-storage”Table I.7 presents the life cycle data for the application of degassed biomass “ex-storage” 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 I.7. Life cycle data for application of degassed biomass and field processes. All data per 1000 kg of “degassed biomass ex-outdoor storage”.
[12] 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 = 0.45 m³ CH4 per kg VS for market swine (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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