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Anaerobic Digestion Overview

Anaerobic Digestion Overview . David Schmidt University of Minnesota Department of Biosystems and Agricultural Engineering. Methane ~ 60% Carbon Dioxide ~ 40% Hydrogen sulfide ~ trace. What is Anaerobic Digestion?. Conversion of Organic Matter. by Anaerobic Microbes. to Biogas.

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Anaerobic Digestion Overview

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  1. Anaerobic Digestion Overview • David Schmidt • University of Minnesota • Department of Biosystems and Agricultural Engineering

  2. Methane ~ 60% Carbon Dioxide ~ 40% Hydrogen sulfide ~ trace What is Anaerobic Digestion? Conversion of Organic Matter by Anaerobic Microbes to Biogas and Manure Effluent University of Minnesota, Department of Biosystems and Agricultural Engineering

  3. Benefits of Digestion • Odor Control • Energy production • Conversion of organic N to inorganic N • Homogeneous effluent • Other crop benefits? • Pathogen reduction? • Weed seed sterilization? University of Minnesota, Department of Biosystems and Agricultural Engineering

  4. Other Key Facts • There is “no” reduction in manure volume • There is no reduction in manure nutrients • Some organic nitrogen is converted to ammonia nitrogen and could be volatilized in the manure storage • There is no increase in manure nutrients University of Minnesota, Department of Biosystems and Agricultural Engineering

  5. History of Digestion • First plant built in India in 1859 • Fueled street lamps in England in 1985 • In 1998 an estimated 600 farm-based digesters were in use. • Estimated 31 digesters are currently in operation on farms in US University of Minnesota, Department of Biosystems and Agricultural Engineering

  6. Terms to Know • Volatile Solids (VS) - A measure of the weight of solids that is combustable “volatilized” at a temperature of 600°C. It is reported as a percent of the total weight of the manure sample. Methane production is often based on the volatile solids portion of the manure. University of Minnesota, Department of Biosystems and Agricultural Engineering

  7. Volatile Solids Production Dairy = 10 lbs VS per day Swine = 8.5 lbs VS per day Layer = 12 lbs VS per day (per 1000 lbs live weight) Approximately 50-70% of the VS can be converted. This depends on species and digester design. University of Minnesota, Department of Biosystems and Agricultural Engineering

  8. Terms to Know • Total Solids (TS) – The weight of the dry matter of a sample of manure and reported as a percent of the total weight of the manure sample. The type of digester used is based primarily on total solids content of the waste. University of Minnesota, Department of Biosystems and Agricultural Engineering

  9. Terms to Know • Hydraulic Retention Time (HRT) - The amount of time the manure spends in the digester. Reported as the ratio of digester volume to the amount of manure added per day. HRT affects the amount of methane produced. 10 gallons per day to a 100 gallon tank has a 10 day HRT. University of Minnesota, Department of Biosystems and Agricultural Engineering

  10. Terms to Know • Loading Rate - amount of volatile solids per unit of time per volume of digester. A “standard” digester will have loading rates of between 0.1 to 0.5 lbs VS/day/ft3 digester (30 day HRT). University of Minnesota, Department of Biosystems and Agricultural Engineering

  11. More Details on the Digestion Process Odor Acid forming bacteria Volatile Solids (VS) Volatile organic acids Methane forming bacteria Methane, carbon dioxide, water, trace gases University of Minnesota, Department of Biosystems and Agricultural Engineering

  12. Temperature considerations • Psychrophilic <68 ºF • Mesophilic 95-105 ºF • Thermophilic 125-135 ºF University of Minnesota, Department of Biosystems and Agricultural Engineering

  13. pH considerations • Methane forming bacteria require pH of between 6.8 and 7.4 University of Minnesota, Department of Biosystems and Agricultural Engineering

  14. Biogas composition • Methane ~ 60% • Carbon Dioxide ~ 40% • Hydrogen sulfide ~ trace University of Minnesota, Department of Biosystems and Agricultural Engineering

  15. Methane is a Fuel Source • Natural gas is 99% methane • Methane is about 900 BTU/ft3 • Propane is 2284 BTU/ft3 University of Minnesota, Department of Biosystems and Agricultural Engineering

  16. Typical Energy Production per1000 lbs live weight Species VS Biogas Energy lb/day ft3/day* BTU/day Dairy 10.0 39 23,400 Swine 8.5 28 16,800 Layer 12.0 37 22,000 Taken from MWPS-18 *Biogas production is typically much higher than these reported values (often more than twice). University of Minnesota, Department of Biosystems and Agricultural Engineering

  17. Other notes 20-40% efficiency in converting BTU’s of methane to electricity with engine generator set. University of Minnesota, Department of Biosystems and Agricultural Engineering

  18. Maximizing Methane Production Control Temperature Control pH Mixing University of Minnesota, Department of Biosystems and Agricultural Engineering

  19. Mixing and Temperature effect on Digestion University of Minnesota, Department of Biosystems and Agricultural Engineering

  20. Covered Lagoon Flexible cover on lagoon or manure storage Lowest gas production Least “controlled” system Longest HRT University of Minnesota, Department of Biosystems and Agricultural Engineering

  21. Complete Mixed Covered Tank with Mixing Heated Mesophilic or thermophilic range 15-20 day HRT 2-10% solids input University of Minnesota, Department of Biosystems and Agricultural Engineering

  22. Rectangular pit (typically concrete) Manure flows from one end to other Heated Mesophilic or thermophilic 15-30 day HRT Requires high solids (>11%) Plugged Flow University of Minnesota, Department of Biosystems and Agricultural Engineering

  23. Options for Biogas Flare University of Minnesota, Department of Biosystems and Agricultural Engineering

  24. Boiler for Heat Hot water, floor heat, room heat Options for Biogas University of Minnesota, Department of Biosystems and Agricultural Engineering

  25. Options for Biogas • Electricity Generation • Engine, micro-turbine, fuel cell University of Minnesota, Department of Biosystems and Agricultural Engineering

  26. Haubenschild Digester • Biogas production 93ft3/cow/day • (66 ft3/day/1000lb lw) • Electrical production 4 kWh/cow/day University of Minnesota, Department of Biosystems and Agricultural Engineering

  27. 30 minutes per day “feeding” digester checking temperatures monitor biogas production change oil (every 700-1000 hours) Management University of Minnesota, Department of Biosystems and Agricultural Engineering

  28. Solids separate better after digestion Phosphorus is concentrated in solids What about solid separation? University of Minnesota, Department of Biosystems and Agricultural Engineering

  29. Methane is explosive at 5-15% Does not work well with sand bedding but efforts are underway to solve this problem AD is a living system therefore you must be consistent Antibiotics (copper sulfate) can inhibit or kill microbial activity in digester Other thoughts University of Minnesota, Department of Biosystems and Agricultural Engineering

  30. Questions • www.bae.umn.edu/extens/manure/ University of Minnesota, Department of Biosystems and Agricultural Engineering

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