Anaerobic Digestion Anders Damgaard, Ph.D. Senior Researcher S WOLF go.ncsu.edu/swolf www.easetech.dk 1
Significance of anaerobic digestion Rapidly developing as main technology for wet household organics, manures and slurries. Can potentially conserve nutrients in waste (N,P,K) May have significant effect on eutrophication impacts in solid waste systems. 2
Remanufacturing Solid Waste Systems Comingled Recyclable Collection Commingled MRF Thermal WtE Ash Landfill Mixed Waste/ Residual Collection Mixed Waste MRF Anaerobic Digestion Organics Collection Soil Amendment Composting Mixed Waste Recyclables Combustibles Organics Ash Landfill 3
The Anaerobic Digestion Process Model Incoming Waste Materials (Mg in ) Direct Emissions (kg/mg in ) Equipment Fuel Use (L/Mg in ) User Inputs Waste composition Waste properties Anaerobic technology Equipment cost and performance Pretreatment technologies AD Process Model Electricity Use (kwh/mg in ) Capital Cost ($/Mg-yr -1 ) Operating Cost ($/Mg in ) Residual Contaminants (Mg out / Mg in ) Final Digestate Properties (Mg out / Mg in ) 4
Anaerobic Technology factors Pretreatment Reactors Gas collection tanks Screens Mixers (5%-35%? TS) Heating (mesophilic 36 C, thermophilic 53-55 C), shredders, magnets gas utilization equipment, flares, biofilters, dewaters, etc. Each plant has its own configuration 5
Anaerobic Digestion Models Empirical Estimate outputs based on static inputs Gas and Digest properties set a priori Mechanistic Estimate emissions, electricity/fuel use, mass and substance flows through each sub-process Digest and gas properties calculated from mass and substance flows 6
Wet weight TS Dry matter Sardinia Symposium 2015 - Solid Waste Life-Cycle Modeling Workshop Degradation H2O CO2 CH4 H2O: user defined VS Ash VS: partly degraded Ash: unchanged Raw waste Digestate
Degradation and mass transfer Gas generation is linked to VS degradation: VS all material fractions CH 4 -generation (m 3 CH 4 /tonne VS ) each material fraction CH 4 yield (% C_bio_and) each material fraction Fractions different degradability Hint experience + fit with overall m 3 CH 4 TCs transfer mass to solid out puts after degradation Hint: fit with reject and compost mass C_bio_and(%TS) = CH 4pot m 3 tvs VS %TS 10 6 ( t g) 12 CH4%(% in biogas) 0.0224 m 3 mol g mol C n H a O b N c + n a 4 b 2 + 3c 4 H 2O n 2 + a 8 b 4 3c 8 CH 4 + n 2 a 8 + b 4 + 3c 8 CO 2 + c NH 3
Emissions Air emissions: Leakages (can be problematic) Collected CH 4 unburned % of methane production Process-specific emissions from gas utilization equipment Liquids from wastewater treatment: Estimate treatment as process-specific emissions or as input-specific if data can be obtained
Energy utilization Heating value from the gas calculated (CO2 to CH4 ratio, must remember some CO2 can be in the liquid phase when doing mass balancing) Utilization is often expressed as a % of the heating value, and linked to the specific type of utilization (Engine, upgrade for Natural gas networks, further upgrade for liquid fuels etc.=
End Product Use Soil conditioner: high organic content increases moisture holding capacity of soil Nutrient content depends on the starting material nutrients and/or soil may be added for certain markets Markets (identify before producing compost) landfill cover soil mixed MSW nurseries and landscapers for seedlings yard waste state roads and parks yard waste city residents (give away or sell) yard waste Agriculture IF the material is pure and has a nutrient value Alternate daily cover in landfills Dependent of feedstock and product purity 11
Research and Data Needs How should we model mass and substance flows through AD processes? How do we account for feedstock mix and contamination? Emission allocation Use on land impact and allocation How do we account for pretreatment and it influence on the AD process? Very technology specific 12
Questions and Discussion S WOLF go.ncsu.edu/swolf www.easetech.dk 13
Additional Resources Boldrin, A., Neidel, T. L., Damgaard, A., Bhander, G. S., Møller, J., & Christensen, T. H. (2011). Modelling of environmental impacts from biological treatment of organic municipal waste in EASEWASTE. Waste Management, 31(4), 619 30. Levis, J. W.; Barlaz, M. A. (2011). What is the most environmentally beneficially way to treat commercial food waste? Environ. Sci. Technol, 45 (17), 7438-7444. Møller, J., Boldrin, A., Christensen, T.H., 2009. Anaerobic digestion and digestate use: accounting of greenhouse gases and global warming contribution. Waste Management. Res. 27, 813 24. Hansen, T.L., Sommer, S.G., Gabriel, S., Christensen, T.H., 2006. Methane production during storage of anaerobically digested municipal organic waste. J. Environ. Qual. 35, 830 6. 14