|
Life Cycle Assessment of Biogas from Separated slurry Processes G.20 to G.23: fate of the degassed fibre fraction
G.20 Transport of the degassed fibre fraction to the farmThe degassed fibre fraction will be transported to a farm where a fertilizer rich in P is needed. The transport distance from the biogas plant to this farm was modelled as 100 km. This distance takes into account the assumption that the degassed fibre fraction is not transported between the eastern and western parts of Denmark, as this would not pays off. This is in conformity with Dalgaard et al. (2006). The fibre fraction 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.21 Storage of the degassed fibre fractionG.21.1 General descriptionIn this study, it is assumed that the degassed fibre fraction is stored in a covered heap, outdoor. The effect of covering has tremendous impacts on the resulting emissions, as this contribute to reduce the degradation of organic matter favoured when the heap is exposed to air, and thereby the resulting gaseous emissions to the environment. In an experiment where air-tight covered heap from solid fraction of swine manure were compared to uncovered heap, Hansen et al. (2006) observed emissions reductions of 12 %, 99 % and 88% for NH3, N2O and CH4 when the heap were covered. In another study carried out by Dinuccio et al. (2008), the authors concluded from their results that, because of the emissions occurring during the (uncovered) storage phase, mechanical separation of cattle and pig slurry has the potential to increase the emissions of CO2 equivalents by up to 30 % as compared to raw slurry. Amon et al. (2006) raise similar concerns applying particularly for the fibre fraction. In this study, the fibre fraction is stored in heap lying on a concrete slab. The heap is covered by a polyethylene plastic sheet. This is considered as the best management practice, as this does not involve any specific energy requirements, and as this limits the C losses occurring when the heap is not covered (i.e. through the natural composting thereby occurring). This also contributes to limit the ammonia volatilization and complies with the Danish law stipulating that the stores of solid manure that do not receive daily input of materials have to be covered (Miljøministeriet, 2006). Fibrous fractions of separated slurry that are not used for biogas production are normally stored temporally for about a week (Hansen, 2009). During that temporal storage phase, new material is regularly added until the storage capacity is full. The fibre fraction is then moved to a static store, where it is, in practice, stored for up to half a year (Hansen, 2009). In this study, it is considered that the truck delivering the degassed fibre fraction from the biogas plant will come to the farm only once, with the amount needed by the farmer. Therefore, only static storage is involved. G.21.2 Material consumptionTable G.29 presents the material consumption for the storage of the degassed fibre fraction. The dimensions of the storage platform used for calculating the amount of concrete needed are based on the data found in Petersen and Sørensen (2008). The annual amount of degassed fibre fraction to store is also based on Petersen and Sørensen (2008). The height of the heap is 2 m (based on Petersen and Sørensen, 2008). Table G.29. Material consumption for the storage of the degassed fibre fraction
a) The density considered for polyethylene is 0.96 g/cm³. G.21.3 Water additionSince the heap is covered, it is considered that there is no water addition during storage. This in fact may not be exactly true since the fibre fraction might absorb some moisture from the air. G.21.4 CH4 emissionsAmon et al. (2006) measured, for the uncovered storage of a fibre fraction from cattle slurry, CH4 emissions of 510.6 g CH4 per m³ slurry. The density of their fibre fraction is not specified, but assuming a density of 600 kg/m³ (as in table G.19) yields CH4 emissions of 0.851 kg per 1000 kg fibre fraction. This was for a non-degassed fibre fraction stored during 80 days under warm conditions (mean slurry temperature of 17°C). In the study of Dinuccio et al. (2008), for 30 days of open storage, the authors measured CH4-C emissions for the fibre fraction of cattle slurry corresponding to 0.77 % and 0.23 % of the VS for fibre fraction stored at 5 and 25 °C, respectively. This is for a non-degassed fibre fraction. In the present study, these figures would correspond to CH4 emissions of 2.20 and 0.656 kg CH4 per1000 kg fibre fraction (assuming the VS content corresponds to 80 % of he DM). In Annex F (section F.21.4), it was assumed that CH4-C emissions corresponded to 0.17 % of the C content of the fibre fraction ex-separation. This estimate was based on a study of Hansen et al. (2006) for covered storage of degassed fibre fraction from pig slurry. In the present case, this would correspond to 0.257 kg of CH4 per 1000 kg fibre fraction. This is the estimate that will be used in the present Annex. Using value that were obtained with degassed fibre fraction from pig slurry may not be exactly representative of the actual emissions occurring with digested fibre fraction from cow slurry, but with no better available data, it is nevertheless judged as the best estimate to reflect the emissions occurring under covered storage. G.21.5 CO2 emissionsIn the study of Dinuccio et al. (2008), for 30 days of open storage, the authors measured CO2-C emissions for the fibre fraction of cattle slurry corresponding to 16.4 % and 25.6 % of the VS for fibre fraction stored at 5 and 25 °C, respectively. This is for a non-degassed fibre fraction. In the present study, these figures would correspond to CO2 emissions of 128.4 and 200.4 kg CH4 per1000 kg fibre fraction (assuming the VS content corresponds to 80 % of the DM). The study of Dinuccio et al. (2008) clearly shows that most of the C losses from the covered storage of cattle slurry fibre fraction occurred through CO2. In fact, for a non-covered storage at 5°C, the ratio between the emissions of CO2-C and CH4-C as measured by Dinuccio et al. (2008) is 21.30, while it is 111.30 for a storage at 30°C. In Annex F, based on a study of covered degassed fibre fraction from pig slurry carried out by Hansen et al. (2006), it was assumed that CO2-C = 1.9 % of the C content of the fibre fraction ex-separation. This estimate will also be used in the present annex, as the fibre fraction is covered (and thereby is likely to emit less than the amount reported in the literature for uncovered storage). For the present study, this corresponds to a CO2 emission of 7.89 kg per 1000 kg degassed fibre fraction. G.21.6 NH3 emissionsAccording to Petersen and Sørensen (2008), a “significant proportion” of the N losses during the storage of degassed fibre fraction shall be attributed to NH3-N losses. This is in line with the results of Amon et al. (2006), who report a net increase in total NH3 emissions from stored cattle fibre fraction as opposed to stored raw cattle slurry. Amon et al. (2006) measured 287.8 g NH3 per m³ fibre fraction, which corresponds to 0.480 kg NH3 per 1000 kg fibre fraction, assuming a density of 600 kg/m³ (as in table G.19). This is, however, for non-covered and non-degassed fibre fraction, stored during 80 days, with a mean slurry temperature of 17 °C. In the present project, this would correspond to approximately 6 % of the initial N content of the fibre fraction. Dinuccio et al. (2008) measured NH3-N losses corresponding to 6.03 % and 5.21 % of the initial N content, for cattle fibre fraction stored at 25 °C and 5 °C, respectively, during 30 days. In this study, this corresponds to emissions of 0.398 and 0.344 kg NH3-N per 1000 kg fibre fraction. This is also for non-covered and non-degassed fibre fraction. In this study, the heap is covered, so NH3-N emissions are expected to be lower than those reported in the literature for non covered (and non-degassed) heaps. Yet, Hansen (2009) recommends to use a value of NH3-N emissions corresponding to 13 % of the initial N (as it was done in Annex F, for pig degassed fibre fraction under covered storage, section F.21.3). This is based on recent experiments showing that cattle fibre fraction composts as much as pig fibre fraction (Hansen, 2009). The results of Dinuccio et al. (2008) in fact tend to acknowledge that, as the authors measured similar NH3-N losses for both cow and pig fibre fractions. However, a value of 13 % of the initial N is higher than the values reported in recent studies for uncovered storage of non-degassed fibre fraction. A value of 5.75 % of the N in the degassed fibre fraction ex-storage will therefore be use for estimating NH3-N emissions. This corresponds of the average of the values above-mentioned from the studies of Amon et al. (2006) and Dinuccio et al. (2008), for temperatures between 5 and 25 °C. This value should be considered as a rough estimate. G.21.7 N2O emissionsCovering allow to restrict the air inflow over the heap and therefore the potential for nitrification (and thereby denitrification) processes. In fact, Hansen et al. (2006) observed emissions reduction of 99 % for covered heap as compared to uncovered heap. Dinuccio et al. (2008) did not succeed to measure significant amount of N2O emissions during open storage of stored fibre fraction from cattle manure. This was true for both 5 and 25°C storage temperature. Amon et al. (2006) measured N2O emissions of 13.2 g per m³ fibre fraction for separated cattle slurry. Assuming a density of 600 kg/m³ (as in table G.19), this corresponds to 0.022 kg N2O per 1000 kg fibre fraction (i.e. 0.007 kg N2O-N per 1000 kg fibre fraction). When applied to this study, it corresponds to N2O-N emissions of 0.106 % of the initial N content In Annex F, N2O-N emissions were estimated as 0.04% of the initial N of the fibre fraction, based on a study carried out by Hansen et al. (2006) with covered heap from degassed fibre fraction from pigs. This estimate is also used in the present annex, as there are no other data for degassed cow fibre fraction stored in covered heap. It should therefore be seen as a rough estimate. The indirect N2O emissions are calculated as in Annex A, i.e. based on IPCC guidelines (IPCC, 2006). Therefore, the indirect N2O emissions are calculated as 0.01 kg N2O-N per kg (NH3-N + NOX-N) volatilized. G.21.8 NO, NOx and N2 emissionsAs it was not possible to find data for NO, NO2 and N2 emissions, the same hypothesis as those detailed in section A.2.3 of Annex A were used, 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. G.21.9 Life cycle data and mass balances for storage of degassed fibre fractionTable G.30 summarizes the LCA data for the storage of degassed slurry, while table G.31 presents the mass balances. The estimation for C in the degassed fibre fraction after storage presented in table G.31 may overestimate the actual amount of C. This is because CH4 and CO2 emissions considered were based on the study of Hansen et al. (2006). Yet, in that study, a significant portion of the C was lost and could not be accounted for as CH4 or CO2 emissions. This non-accounted for portion is 4.9 % of the initial C content, for covered heap (Hansen et al., 2006), as compared to measured 0.17 % for CH4-C and 1.9 % for CO2-C. In the present study, these “unexplainable losses” are not included (this would correspond to 5.55 kg C/1000 kg degassed fibre fraction in the present project). In table G.31, 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.30 Life cycle data for storage of the degassed fibre fraction. All data per 1000 kg of degassed fibre fraction.
Table G.31 Mass balances for storage of degassed fibre fraction
a Changes in total N: 0.3795 kg NH3-N + 0.00264 kg N2O-N + 0.00264 kg NO-N + 0.00792 kg N2-N = 0.393 kg N b Changes in total C: 7.89 kg CO2 * 12.011 [g/mol] /44.01 [g/mol] + 0.257 kg CH4 * 12.011 [g/mol] /16.04 [g/mol] = 2.74 kg C c The change in DM is assumed to be identical to the sum of the loss of N and C G.22 Transport of the degassed fibre fraction to the fieldThe transport of the degassed fibre 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.23 Field processes for the degassed fibre fractionG.23.1 General descriptionFor this process, the data from the Ecoinvent process “solid manure, loading and spreading, by hydraulic loader and spreader” (Nemecek and Kägi, 2007, p.200) has been used for the emissions occurring during spreading. This includes, among other, the diesel consumption and the consumption of spreading equipment. G.23.2 Emissions 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). G.23.3 Emissions of NH3Emissions of NH3 are estimated based on the data for solid slurry presented in a recent publication from Hansen et al. (2008). Assuming the application takes place in the spring and that the applied degassed fibre fraction is ploughed or harrowed within 6 hours after the application, the overall NH3 losses are calculated as 40 % of the NH4-N (based on table 18 from Hansen et al., 2008). Yet, the values presented by Hansen et al. (2008) assumed that NH4-N corresponds to 25 % of the N content of the solid slurry ex-storage. Applied to the data of the present study, this means that NH3 emissions corresponds to 0.621 kg (40% * 6.21 kg N * 25 %). The NH3-N losses therefore correspond to 0.5107 kg. G.23.4 Emissions of N2OThe direct and indirect N2O emissions were based on IPCC guidelines (IPCC, 2006), as in Annex A, section A.5. This considers that the direct N2O emissions correspond to 0.01 kg N2O-N per kg N ex-storage, while the indirect N2O-N emissions are estimated as 0.01 kg N2O-N per kg (NH3-N + NOX-N volatilized). The indirect N2O-N emissions based on nitrate leaching are also considered, based on IPCC guidelines (IPCC, 2006), thereby they are estimated as 0.0075 kg N2O-N per kg N leaching. G.23.5 Emissions of NOX and N2-NAs in previous sections, the emissions of NO and NO2 are combined as NOX-emissions, as separate data on NO and NO2 has not been available. According to Nemecek and Kägi (2007) (page 36) the NOX emissions can be estimated as: NOX = 0.21 * N2O. When taking the molar weights into consideration this corresponds to NOX–N = 0.1 * N2O-N. It is considered to be a “rough expert estimate”, but since the relative contribution has minor significance for the overall results, it is considered to be adequate. 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 6:1. G.23.6 Calculation of degassed fibre fraction fertilizer valueThe calculation of the fertilizer value of the degassed fibre fraction is presented and detailed in section G.28. G.23.7 Nitrate leachingThe content of C of the degassed fibre fraction is rather high, which gives rise to a substantial increase in soil C, after 10 years the C content in the soil is still increased with 26.4 (JB3) and 27.8 (JB6) kg C per 1000 kg fiber fraction, according to C-TOOL. The majority of the C in the degassed fibre fraction is released as CO2 (table G.32). The above increase in soil C gives rise to a modeled increase in soil N of 10% of the C increase, i.e. 2.64 (JB3) and 2.78 (JB6) kg N per 1000 kg degassed fibre fraction. According to this modeling, 3.57 (JB3) and 3.43 (JB6) kg N are left for both plant uptake and all N losses (before gaseous losses). After the gaseous losses (table G.32), there is 2.81 (JB3) and 2.48 (JB6) kg N left for harvest and leaching. For the 100 years values, there is, after the gaseous losses, 5.4798 (JB3) and 5.4392 (JB6) kg N left for harvest and leaching. For simplicity, the distribution of the surplus between harvest and leaching is assumed to be as for cattle slurry (table A.16, Annex A), which gives the leaching values of table G.32. When transforming the above 10-year considerations to 100-year values, the additional mineralisation of N is calculated first, utilising C-TOOL. The mineralized N is assumed to be subject to denitrification, with the same factor as for N amendment. The plant uptake value of mineralized N relative to mineral fertilizer is assumed to be an average of 65.3 % on JB3 and 73.0 % on JB6, in accordance with the calculations in Annex A, section A.5. The remainder after denitrification and harvest removal is assumed to go to N leaching, which results to the 100-year figures in table G.32. G.23.8 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.23.9 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.23.10 Fate of the polymerThe fate of the polyacrylamide polymer (PAM) used in the separation process described in section G.4 is considered here, assuming that no losses occurred and that 100 % of the polymer is transferred to the degassed fibre fraction. This assumption is made for simplification purposes only, but is not of importance, as both fractions end up to be spread in the field. As described in section G.4, an amount of 0.60 kg of polymer was used per 1000 kg of slurry ex pre-tank input. As extensively detailed in Annex F, it is considered that 100 % of the PAM present in the applied degassed fibre fraction is accumulating in the environment. G.23.11 Life cycle data for field application of degassed fibre fraction and field processesTable G.32 presents the life cycle data for the application of degassed ex-storage fibre fraction on the field. Table G.32. Life cycle data for application of degassed fibre fraction and field processes. All data per 1000 kg of “digested fibre fraction ex-outdoor storage”. Dairy cow degassed fibre fraction ex-storage.
|
|||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||