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Life Cycle Assessment of Biogas from Separated slurry

Processes F.20 to F.23: fate of the degassed fibre fraction

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F.20 Transport of the degassed fibre fraction to the farm

The 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).

F.21 Storage of the degassed fibre fraction

F.21.1 General description

In 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.

F.21.2 Material consumption

Table F.28 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 F.28. Material consumption for the storage of the degassed fibre fraction

^a The density considered for polyethylene is 0.96 g/cm³.

F.21.3 Water addition

Since 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.

F.21.4 CH₄ emissions

Only two studies have been found in the literature where the emissions from stored degassed fibre fraction from pig slurry were assessed, and both were performed in a Danish context with a decanting centrifuge separation. The study of Hansen et al. (2006) focus specifically on the effect of covering the heap on specific gaseous emissions, while Petersen and Sørensen (2008) assessed the overall C and N losses from uncovered heaps obtained in farm-scale conditions.

Hansen et al. (2006) measured C losses corresponding to 27.9 % and 7.0 % of the initial C content (the content just before storage) for uncovered and covered heaps, respectively. This was for a 4 months storage period. Petersen and Sørensen (2008) observed total average C losses of 44 % (farm 1) and 43 % (farm 2) in the heap surfaces, for a storage duration between 5 and 9 months. These heaps were not covered.

The C losses measured by Petersen and Sørensen (2008) for uncovered heaps are much higher than those measured by Hansen et al. (2006). To explain this difference, Petersen and Sørensen (2008) highlight the fact that the ratio of aerobic to anaerobic decomposition in heaps depends on the heap size (greater in the study of Petersen and Sørensen, 2008) and on the oxygen supply (the heaps of Petersen and Sørensen, 2008, were loaded weekly while the study of Hansen et al., 2006 assessed static heaps). These differences nevertheless testify of the high degree of variability in the results induced by the different management practices as well as ambient conditions found on-site.

Hansen et al. (2006) also measured that the losses of CH₄-C corresponded to 1.3 % and 0.17 % of the initial C for uncovered and covered heaps, respectively.

In their assessment of slurry management scenarios, Sommer et al. (2009) set the CH₄ (and not C-CH₄) emissions occurring during the storage of the fibre fraction to 1.7 % of the total C, based on the study performed by Hansen et al. (2006). This, however, was for a non-degassed (and most probably uncovered) fibre fraction.

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 pig slurry corresponding to 0.68 % and 0.60 % 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 1.94 and 1.71 kg CH₄ per1000 kg fibre fraction (assuming the VS content corresponds to 80 % of the DM).

In this project, it was decided to consider the figures presented by Hansen et al. (2006) for covered storage, since these are the only data available for degassed fibre fraction stored in covered storage facilities. The data of Hansen et al. (2006) for covered storage are judged to be better data than any data for non-covered storage, since the ratio of aerobic to anaerobic decomposition are rather different for heap stored without cover. Moreover, the data of Hansen et al. (2006) were obtained with degassed fibre fraction, as in this study.

The CH₄-C emissions in this study therefore correspond to 0.17 % of the C content of the fibre fraction ex-separation. The choice of this value has been discussed with one of the leading experts in the area in Denmark, Martin Nørregaard Hansen, who validated the value as the best representative value under current data availability (Hansen, 2009). In the context of this study, it means that 0.2943 kg CH₄ per 1000 kg degassed fibre fraction is emitted.

F.21.5 CO₂ emissions

As for CH₄, data for CO₂ will mostly focus on the study of Hansen et al. (2006), for the same reasons as in the case of CH₄. In their study, Hansen et al. (2006) measured emissions of CO₂-C corresponding to 25.1 % and 1.9 % of the initial C for uncovered and covered heaps, respectively. This was for 4 months of storage.

For both covered and uncovered heap, some C losses were not accounted for in the study of Hansen et al. (2006). This corresponds to 1.5 % of the initial C for uncovered heaps, and to 4.9 % of the initial C for covered heaps.

It can be noticed that most of the C losses in the experiment of Hansen et al. (2006) occurred through CO₂ emissions, for the uncovered storage. This is also the case in the study performed by Dinuccio et al. (2008). This illustrates the importance of the aerobic degradation occurring under uncovered storage.

In this study, CO₂ losses are estimated based on the values given by Hansen et al. (2006) for covered storage, i.e. CO₂-C emissions correspond to 1.9 % of the initial C content of the degassed fibre fraction. In the present study, this amount to 9.02 kg CO₂ per 1000 kg degassed fibre fraction.

F.21.6 NH₃ emissions

The emissions of ammonia are expected to occur mostly during the first week of storage (Hansen et al., 2006; Petersen and Sørensen, 2008). Losses of N through emissions of NH₃-N measured by Hansen et al. (2006) were of 0.3 % of the initial N content, for both covered and uncovered heaps during a 4 months storage period. The authors acknowledge this is rather low. In contrast, Dinuccio et al. (2008) measured NH₃-N losses corresponding to 5.57 % and 7.12 % of the initial N content, for static fibre fraction stored at 25 °C and 5°C, respectively, during 30 days. The fibre fraction of Dinuccio et al. (2008) is not degassed and not covered, however. According 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.

Based on the scarce data availability as well as on a personal communication with an active Danish expert in the area (Hansen, 2009), it was decided that the best estimation for NH₃ emissions of stored degassed fibre fraction would consist to assume that these emissions are in the same order of magnitude as those from stores of pig farmyard manure. In a recent study, Hansen et al. (2008) evaluated, from a compilation of selected emissions factors in the literature, that the NH₃-N emissions from covered stores of pig farmyard manure is 13 % of the N content in the farmyard manure before storage (table 3 in Hansen et al., 2008). This is for a storage period of more than 100 days. In the present study, this would correspond to an emission of 0.9945 kg NH₃-N per 1000 kg of fibre fraction.

Emission of NH₃ from separated fibre fraction of animal slurries is recognized as a “hot spot” from slurry management involving separation (e.g. Amon et al.,2006; Petersen and Sørensen, 2008), so the value of 13 % of the initial N content for NH₃-N emissions used in this study appears to be rather representative. The possibility of lower emissions and the influence of this to the overall system is however raised as a discussion point in the interpretation of the results.

F.21.7 N₂O emissions

Covering 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. The emissions measured by Hansen et al. (2006) for N₂O-N correspond to 0.04 % of the initial N content for covered heap and correspond to 4.8 % of the initial N content for uncovered heap.

In this study, the value of Hansen et al. (2006) for covered storage will be used, i.e. emissions of N₂O-N corresponding to 0.04 % of the initial N content. This choice is validated by Hansen (2009).

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 + NO_X-N) volatilized.

F.21.8 NO, NO_x and N₂ emissions

As 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 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.

F.21.9 Life cycle data and mass balances for storage of degassed fibre fraction

Table F.29 summarizes the LCA data for the storage of degassed slurry, while table F.30 presents the mass balances. The estimation for C in the degassed fibre fraction after storage presented in table F.30 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 the 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 6.25 kg C/1000 kg degassed fibre fraction in the present project).

In table F.30, it can be noticed that the change of DM is estimated as the losses of N and C. As explained in section F.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 F.29 Life cycle data for storage of the degassed fibre fraction. All data per 1000 kg of degassed fibre fraction.

Table F.30 Mass balances for storage of degassed fibre fraction

^a Changes in total N: 0.9945 kg NH₃-N + 0.00306 kg N₂O-N + 0.00306 kg NO-N + 0.00918 kg N₂-N = 1.01 kg N

^b Changes in total C: 9.02 kg CO₂ * 12.011 [g/mol] /44.01 [g/mol] + 0.294 kg CH₄ * 12.011 [g/mol] /16.04 [g/mol] = 2.68 kg C

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

F.22 Transport of the degassed fibre fraction to the field

The transport of the degassed fibre fraction to the field is identical to the process described in section F.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.

F.23 Field processes for the degassed fibre fraction

F.23.1 General description

For 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.

F.23.2 Emissions of CH₄ and CO₂

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).

F.23.3 Emissions of NH₃

Emissions 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 0.664 kg (40% * 6.64 kg N * 25 %). The NH₃-N losses therefore correspond to 0.546 kg.

F.23.4 Emissions of N₂O

The 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 + NO_X-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.

F.23.5 Emissions of NO_X and N₂-N

As in previous sections, the emissions of NO and NO₂ are combined as NO_X-emissions, as separate data on NO and NO₂ has not been available. According to Nemecek and Kägi (2007) (page 36) the NO_X emissions can be estimated as: NO_X = 0.21 * N₂O. When taking the molar weights into consideration this corresponds to NO_X–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.

F.23.6 Calculation of degassed fibre fraction fertilizer value

The calculation of the fertilizer value of the degassed fibre fraction is presented and detailed in section F.28.

F.23.7 Nitrate leaching

The 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 increased with 30.1 (JB3) and 31.8 (JB6) kg C per 1000 kg degassed fibre fraction, according to C-TOOL. The majority of the C in the degassed fibre fraction is released as CO₂ (table F.31). The above increase in soil C gives rise to a modelled increase in soil N of 10% of the C increase, corresponding to 3.01 (JB3) and 3.18 (JB6) kg N per 1000 kg degassed fibre fraction. According to this modelling, 3.63 (JB3) and 3.46 (JB6) kg N are left for the two following fates: plant uptake and all N losses (before gaseous losses).

After the gaseous losses (table F.31), there is 2.81 (JB3) and 2.44 (JB6) kg N left for harvest and leaching. For the 100 years values, there is, after the gaseous losses, 5.800 (JB3) and 5.7535 (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 pig slurry (table A.15), which gives the leaching values of table F.31.

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, obtaining the 100-year figures in table F.31.

F.23.8 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).

F.23.9 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.

F.23.10 Fate of the polymer

The fate of the polymer used in the separation process described in section F.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 F.4, an amount of 0.90 kg of polymer was used per 1000 kg of slurry ex pre-tank input.

Kay-Shoemake et al. (1998) investigated the effect of PAM applied to agricultural soils on soil bacterial communities and nutrient cycling. They found, among others, that the bacterial numbers on soils with and without PAM application were not significantly different. They also found that PAM-treated soils planted to potatoes contained significantly higher concentrations of NO₃^- and NH₃ as compared to untreated soils. For NO₃^-, they found 36.7 mg/kg for PAM-soil as compared to 10.7 mg/kg for control soil. For NH₃, they found 1.30 mg/kg for PAM-soil as compared to 0.50 mg/kg for control soil. This suggests that some biological degradation may take place. In an extensive review on polyacrylamide (PAM) degradation (more than 150 articles were reviewed), Caulfield et al. (2002) also acknowledged this possibility (which they explained as the hydrolysis of the amide group), but they demonstrate that this degradation has to be rather limited, due to the high molecular weight of PAM that cannot pass through the biological membranes of the bacterium. This is in line with El-Mamouni et al. (2002) who suggest that PAM may simply accumulate and persist in the environment. In their review, Caulfield et al. (2002) also concluded that no evidence is existing to suggest that PAM may form free acrylamide monomer units (which are highly toxic) under biodegradation processes.

If PAM appears to be rather recalcitrant to biological degradation, it is more susceptible to undergo thermal degradation (temperatures above 200 °C), photodegradation, chemical degradation (under very acidic or very basic conditions) as well as mechanical degradation (if submitted to high shear). These degradation processes are extensively documented in Caulfield et al. (2002). In the case of application to field, photodegradation may be the most likely degradation mechanism to occur. El-Mamouni et al. (2002) actually studied the degradation of PAM submitted to UV photolysis as a pre-treatment to anaerobic and biological processes. Their results indicate that this UV irradiation pre-treatment did contribute to increase the biological degradation of PAM, under both aerobic and anaerobic conditions. However, El-Mamouni et al. (2002) highlight that the irradiation conditions used in their experiment are unlikely to occur in natural environment, as they used light intensity as low as 254 nm (the lower the wavelength, the higher the energy; visible wavelength are between 400 to 700 nm) and exposition duration ranging between 12 to 72 consecutive hours.

Based on these findings, it was considered reasonable to assume, in the framework of this study, that no degradation of the PAM occur after the application of the degassed fibre fraction to the field. As linear PAM is water-soluble (Wu and Shank, 2004; Sojka et al., 2007), it may dissolve in water during precipitation events and leak through the water compartment. Sojka et al. (2007) in fact report that very few studies have assessed the fate of PAM, as PAM cannot be easily extracted for analysis once it has been adsorbed on solid surfaces.

In this project, as the exact fate of PAM between the soil and water compartment is not known, the impact of PAM was simply considered as a discussion point as “PAM accumulation in the environment”.

F.23.11 Life cycle data for field application of degassed fibre fraction and field processes

Table F.31 presents the life cycle data for the application of degassed ex-storage fibre fraction on the field.

Table F.31 Life cycle data for application of degassed fibre fraction and field processes. All data per 1000 kg of “digested fibre fraction ex-outdoor storage”.

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Version 1.0 August 2010, © Danish Environmental Protection Agency