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Life Cycle Assessment of Biogas from Separated slurry Processes G.8 to G.10: Handling the fibre fraction from the farm to the biogas plant before biogas is produced
G. 8 Storage of the fibre fraction at the farmAt the farm, the fibre fraction is stored in a covered concrete container (Rosager, 2009). The storage duration varies between 1 and 3 days, the maximum being approximately 7 days (Jensen, 2009). It is here important to highlight that the fibre fraction is stored in a covered structure. In fact, covering has a major mitigation impact on composting activity and on production and emission of greenhouse gases and NH3, as further discussed in section G.21. The emissions occurring during the storage of the fibre fraction are highly variable and are dependent upon various parameters such as the chemical composition and structure of the fibre fraction, the temperature and the storage time (Hansen, 2009; Dinuccio et al., 2008). The porous structure of the fibrous fraction causes a potential for a higher N loss during the storage and handling phases, especially as ammonia, as compared to raw manure (Petersen and Sørensen, 2008). However, Jørgensen and Jensen (2009) found no major differences in the proportion of NH4-N of total N when they compared samples of fibre fractions that underwent 2 to 8 weeks storage to fibre fractions samples that were taken directly from the separator. This was true for 6 of their 7 samples of stored fibre fractions. Their stored fibre fractions samples were issued from both close and open storage. One of the hypotheses formulated by the authors to explain the non-significantly different proportions of NH4-N from stored and fresh fibre fractions is that the storage period was not long enough to influence the apparent composition of N in the solids. Based on these findings as well as on information from biogas producers (Rosager, 2009), the emissions occurring during the temporal storage of the fibre fraction at the farm are considered as negligible. Because it is considered that no losses occurs during this 1 to 3 days storage, the fibre fraction after the storage has the same composition as at the outlet of the separator (as presented in table G.8). It is acknowledged that this is an important assumption impacting the whole mass balances for all subsequent process. As such, it is not a suitable to carry out a sensitivity analysis on this. Instead, the importance of this assumption is raised as a discussion point in the interpretation of the results. The material consumption related to the storage facilities is as described in table G.19. This is based on the process “slurry store and processing” from the Ecoinvent database (Nemecek and Kägi, 2007, p.182), which is for a covered concrete tank with 300 m³ capacity. It is estimated that an annual amount of 15 000 m³ of fibre fraction will be handled per year. Table G.19 Material consumption data related to the infrastructures needed for the storage of the fibre fraction.
a) The density of the fibre fraction was assumed to be 600 kg/m³, based on Brauer (2006). G.9 Transport of fibre fraction to biogas plantThe calculations for the transport of the fibre fraction to the biogas plant will be made for a transportation distance of 10.6 km (based on Laursen, 2009). 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). 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. G.10 Storage of the fibre fraction at the biogas plantOnce at the biogas plant, the fibre fraction is stored for a very short period – from a few days to maximum a week (Rosager, 2009). As for process G.8, this means that the emissions occurring during the temporal storage of the fibre fraction at the biogas plant are considered as negligible. The equipment and materials for this storage are included in the material list for the biogas plant in table G.24.
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