Buysse, Jeroen

De Dobbelaere, Anke

Meers, Erik

Van der Heyden, Caroline

Vanrolleghem, Wouter

Vergote, Tine

Volcke, Eveline

English

Ghent University Academic Bibliography

Model-based analysis of greenhouse gas emission reduction potential through farm-scale digestion  T. Vergotea,b, W. Vanrolleghema, C. Van der Heydena, A. De Dobbelaerec, J. Buysseb, E. Meersa, E.I.P. Volckea a Department of Green Chemistry and Technology, Faculty of Bioscience Engineering, Ghent University, Coupure Links 653, 9000 Ghent, Belgium, Tine.Vergote@UGent.be b Department of Agricultural Economics, Faculty of Bioscience Engineering, Ghent University, Coupure Links 653, 9000 Ghent, Belgium c Inagro vzw, Research & advice in agriculture & horticulture, Ieperseweg 87, 8800 Rumbeke-Beitem, Belgium, www.inagro.be   Abstract: A fit-for-purpose anaerobic digestion model was set up to estimate desired methane production in the form of biogas as well as unwanted methane emissions associated with farm-scale digestion of manure. The generally accepted Anaerobic Digestion Model No. 1 was simplified assuming hydrolysis as the rate-limiting step. The carbon footprint of several manure management scenarios typical for Flemish dairy farms was assessed based on model simulations and literature data. The scenarios assessed differed in the possible presence of a digester as well as in the manure collection and storage method. Dairy farms with fresh manure collection by a manure scraper, followed by controlled digestion and storage of the digestate in a gas-tight tank located outside achieved the lowest carbon footprint. Controlled digestion must take place in a properly managed and correctly dimensioned reactor as high digester methane losses and low digester retention times increase the carbon footprint significantly.  Keywords: Farm-scale anaerobic digestion; ADM1 reduction; greenhouse gas emissions  Session: Sludge and slurry digestion   Introduction The livestock sector accounts for 14.5% of the global anthropogenic greenhouse gas emissions, 4.3% of which is methane emitted during manure management like storage (Gerber et al., 2013). In most European countries, long-term manure storage takes place because the EU Nitrates Directive (91/676/EEC) restricts the period during which fertiliser application is allowed. Farm-scale anaerobic digestion has become increasingly popular because it allows farmers to be (partly) energy self-sufficient and less dependent of fluctuating and generally increasing energy prices. Besides, it seems a promising strategy to prevent methane emissions from long-term manure storage, as storage facilities can be coupled directly to a digester (Miranda et al., 2015).  Modelling and simulation are useful tools to describe and optimize the anaerobic digestion process, allowing the comparison of different process operations in a time-efficient way. The Anaerobic Digestion Model No. 1 (ADM1) (Batstone et al., 2002) has been widely accepted, mainly for anaerobic digestion of sewage sludge. However, it is not often applied by practitioners due to its complexity and the large number of parameters. In this study, a fit-for-purpose model was set up to describe anaerobic digestion of dairy manure and was subsequently applied to estimate the biogas production and methane emission reduction potential through farm-scale digestion.  Material and Methods Manure mainly consists of non-readily biodegradable compounds (Vavilin et al., 1996). As a result, the rate-limiting step during anaerobic digestion of manure is typically the extracellular hydrolysis process (Pavlostathis & Giraldo-Gomez, 1991). Based on this assumption, ADM1 was simplified to a one-step model, describing the direct conversion of composites into methane and microbial biomass, besides soluble inerts and particulate inerts. Specific kinetic parameter values for dairy manure were included. Liquid phase mass balances over the anaerobic digester and the storage tanks for manure and digestate were set up for all state variables, taking into account the possible variation of stored volumes in time. Moreover, temperature dependency of the liquid-gas transport as well as the hydrolysis and decay rate was accounted for.  The carbon footprint of four manure management scenarios for Flemish dairy farms (Figure 1) was calculated by complementing the simulated amount of annual methane emissions and production with literature values for emissions related to electricity consumption, digester operation and fertiliser spreading.  Results and Conclusions The simulation results obtained with the anaerobic digestion model were in line with fixed values put forward in literature. For example, the simulated amount of methane emitted during storage in the manure pit (97.5 kg CO2,eq. m-3 manure) lies between the maximal amount of emissions from slurry calculated by Marañón et al. (2011) and the average amount defined by Owen and Silver (2015).  Methane losses through (open) storage increased with increasing storage time, increasing temperature and higher organic content of ingoing manure or digestate (Figure 2). Methane emissions from storage were about equally divided over the seasons, given that large stored volumes (autumn and winter) and high temperatures (spring and summer) mostly do not coincide. Both stored manure volume and temperature are thus key variables in methane emission quantification.  From the investigated manure management scenarios, the following strategies for methane emission reduction were identified (Figure 2): to lower the temperature by storing manure externally, to decrease the organic content of manure through digestion and to reduce the open storage time by feeding fresh manure to the digester. The latter will not only result in fewer emissions but also in a higher biogas production. Hence, the largest reduction in methane emissions (about 70%, in agreement with Miranda et al. (2015)) could be realised by fresh manure digestion combined with external digestate storage. However, emissions from digestate storage can still be significant and need to be avoided by implementing gas-tight storages.  Methane emissions from storage take the largest share in the carbon footprint of dairy farms (Figure 3), up to over 80% for open manure storage under relatively warm conditions and without controlled digestion. Fresh manure digestion in a dedicated reactor can reduce the carbon footprint, if it concerns a properly managed and correctly dimensioned reactor. If not, low digester retention times and high digester methane losses (e.g. through overpressure safety devices, leakages and methane slip) could be induced, resulting in an increased carbon footprint. Overall, when following the above recommendations, farm-scale manure digestion could significantly reduce (by up to over 50%) the carbon footprint of dairy farms, which is consistent with results from Miranda et al. (2015).  References Batstone, D. J., Keller, J., Angelidaki, I., Kalyuzhnyi, S. V., Pavlostathis, S. G., Rozzi, A., Sanders, W. T. M., Siegrist, H., & Vavilin, V. A. (2002). Anaerobic digestion model No. 1 (ADM1). Scientific and Technical Report No. 13. IWA Publishing. Gerber, P. J., Steinfeld, H., Hendersond, B., Mottet, A., Opio, C., Dijkman, J., Falcucci, A., & Tempio, G. (2013). Tackling climate change through livestock - A global assessment of emissions and mitigation opportunities. FAO. Rome. Marañón, E., Salter, A. M., Castrillón, L., Heaven, S., & Fernández-Nava, Y. (2011). Reducing the environmental impact of methane emissions from dairy farms by anaerobic digestion of cattle waste. Waste Management, 31, 1745–1751.  Miranda, N. D., Tuomisto, H. L., & McCulloch, M. D. (2015). Meta-analysis of greenhouse gas emissions from anaerobic digestion processes in dairy farms. Environmental Science and Technology, 49(8), 5211–5219.  Owen, J. J., & Silver, W. L. (2015). Greenhouse gas emissions from dairy manure management: a review of field-based studies. Global Change Biology, 21, 550–565. Pavlostathis, S. G., & Giraldo-Gomez, E. (1991). Kinetics of Anaerobic Treatment: A Critical Review. In Critical Reviews in Environmental Control (pp. 411–490). Vavilin, V. A., Rytov, S. V., & Lokshina, L. Y. (1996). A description of hydrolysis kinetics in anaerobic degradation of particulate organic matter. Bioresource Technology, 56(2–3), 229–237.  Figures   Figure 1: Dairy manure management scenarios considered in this study. Scenario 1: stable with a manure pit beneath the slatted floor in which the manure temperature varies between 7.5 and 18.4°C. Scenario 2: stable with a solid floor, manure scraper and external manure storage in which manure temperatures range from 5 to 15°C. Scenario 3: stable with a solid floor and manure scraper which collects fresh manure, farm-scale digester operating at a constant temperature of 37°C and a retention time of 25 days and external digestate storage in which the digestate temperature varies between 5 and 15°C. Scenario 4: stable with a manure pit beneath the slatted floor in which pre-storage of 20 days takes place at manure temperatures ranging from 7.5 to 18.4°C, farm-scale digester operating at a constant temperature of 37°C and a retention time of 25 days and external digestate storage with digestate temperatures varying between 5 and 15°C.  Figure 2: Dynamic simulation results of a stable with a manure pit beneath the slatted floor (-) (scenario 1) compared to a stable with a solid floor, manure scraper and external manure storage (--) (scenario 2), a stable with a solid floor, manure scraper, farm-scale digester and external digestate storage (-) (scenario 3) and a stable with a manure pit, farm-scale digester and external digestate storage (--) (scenario 4) with A: daily emitted methane quantity [kg CH4 d-1], B: stored manure or digestate volume [m³] determined by both the ingoing flow [m³ d-1] which depends on the excretion per animal, and the fertiliser spreading pattern (outgoing flow [m³ d-1]) based on restrictions imposed by Flemish legislation, C: hydrolysis constant [d-1] determined by manure or digestate temperature [°C], D: composite concentration (organic content) of manure or digestate [kg COD m-3].  Figure 3: Carbon footprint for all scenarios [kg CO2,eq. m-3 manure] based on results from model simulations (anaerobic digester retention time of 25 days) and literature, given a global warming potential for methane and nitrous oxide of respectively 34 kg CO2,eq. kg-1 CH4 and 298 kg CO2,eq. kg-1 N2O.  Acknowledgements This research was supported by the LA project Pocket Power, funded by Flanders Innovation and Entrepreneurship (VLAIO, http://www.vlaio.be) and co-financed by Boerenbond, ABS, Biolectric, Continental Energy Systems, Innolab, AB Milieusystemen, Vermeulen Construct, United Experts, Biogas-E, Inverde and VLACO. 

Model-based analysis of greenhouse gas emission reduction potential through farm-scale digestion

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Model-based analysis of greenhouse gas emission reduction potential through farm-scale digestion

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