Anaerobic Digestion for Power Generation
Image courtesy of Sustainable Youlgrave (http://www.sustainableyoulgrave.org/moxie/casestudies/from-cow-dung-to-electric.shtml)
Introduction
Anaerobic digestion (AD) is a biological process in which biodegradable organic matters are decomposed by bacteria creating solid and gaseous byproducts [1]. The biogas byproduct consists of methane (CH4), carbon dioxide (CO2), and other trace amount of gases [1]. There are three main sources of organic waste all of which are suitable for anaerobic digestion. These three sources are food waste, industrial waste, and animal waste. Food waste like scraps left on the dinner plate, may come from households, restaurants, and breweries. Pharmaceuticals and paper manufactures are some examples of industrial waste. Lastly animal waste is in abundant supply at commercial farms and sewage plants. Icreasingly finding renewable fuel sources has become an issue as fossil fuels are a limited resource. One solution to producing renewable fuel sources is methane production via the anaerobic digestion process.
Anaerobic Digestion Process
Image courtesy of Anaerobic Digestion (http://www.anaerobic-digestion.com/html/anaerobic_flow_diagram.html)
Inside the Digester
The beginning of the process takes places inside of the digester, which is basically a tank sealed to prevent gaseous oxygen from entering. An anaerobic digester in concept is very similar to the human digestive system [2]. There are two kinds of bacteria that are required for anaerobic digesters to function properly. The first type, fermenting bacteria, feed off organic materials and release organic acids. From these organic acids the second type of bacteria called methanogenic are produced [2]. The methanogenic bacteria also feed on the organic matter and create methane as a by product.
Optimizing the rate of decomposition will in turn optimize how much methane by product is created. Heat, pH balance, and input rate into the digester are all factors that affect the rate of decomposition. Mesophilic and thermophilic are the two types of anaerobic bacteria most commonly found in the digesters. The Mesophillic’s optimum temperature is at 98°F and the thermophilic’s survives best at 130°F. Decomposition is fastest in the thermophilic range however it is also very sensitive to disturbances like waste input, and temperature fluctuations [3]. Also since the thermophilic bacterium requires more energy to operate since it requires a higher operating temperature it is used less than the mesophillic bacterium. The pH level of the digestive system should be kept between the range of 5.5-8.5 [4].
Byproduct - Biogas
The common method of collecting the biogas from the digester is with a floating cover. This device is a weighted pontoon that floats on the liquid surface of a collection/storage basin [9]. The most abundant gas in the gaseous byproduct is methane, but the biogas also includes trace amounts of carbon dioxide, nitrogen, hydrogen, oxygen and hydrogen sulfide. The methane component of the biogas may be used as a fuel to create electricity, vehicle fuel or to generate heat in a boiler. The biogas needs treatment or scrubbing for before it is used to create energy [9]. Hydrogen sulfide, one of the trace gases present in biogas is a toxic gas that most environmental agencies put strict limit to how much may be present in the biogas [4]. Also, volatile siloxanes must be removed from the biogas if it is to be used in any combustion process. This is due to the fact that during the combustion process volatile siloxanes will leave deposits of silicon dioxide on the machine parts leading to a decrease in machine performance. There are several options for transporting the biogas to its final destination. Due to the fact that it is very hard to liquefy methane it is most commonly stored and transported as a gas. Small to medium size farmers find it impractical to store the gas in large quantities due to the limited compressibility of the gas [5]. The farmers will normally burn the gas to fulfill energy requirements on the farm. In larger scale operations the gas may be transported through a piping network. Certain precautions are needed due to the highly explosive nature of the gas. Precautions such as explosion proof motors, wiring, and lights along with alarms and detection devices should be employed [9].
Byproduct -Solids (Seperator)
After material has been processed in the digester process it is loaded into the separator. A separator is similar to a centrifuge where the liquid and solids are separated from one another. Before the waste is loaded into the separator it is called whole digestate. The whole digestate is defined as a material suitable for pumping with less than 12 percent dry matter [7]. Separated liquor and separated fiber are the materials that are removed from the separator. Separated liquor is defined as having less than six percent dry matter and separated fiber may contain no less than twenty percent of dry matter [7]. Both of these materials may be re-used in the fertilization process. Separated liquor helps to reduce nitrogen loss through volatilisation after spreading and through other paths of nitrogen loss [7]. The separated fiber contains ammonia, phosphorus and potassium making in rich in nutrients which is why it is used as a soil fertilizer [3]. The fiber should be tested before used as fertilizer as it may contain concentrated toxins such as pesticides from the input waste [7].
Feasibility of Starting and Maintaining System
The main obstacle for implementing an anaerobic digestion system is that it is a major capital investment. For example a digester system may cost up to two hundred thousand dollars for a farm that uses one-hundred dairy cattle as the main waste source [9]. The cost for commercial farming where seven thousand dairy cattle reside may cost up to 1.8 million dollars [9]. The investor of the system must determine if they will receive a return on their investment within an appropriate time given the associated risks. The first figure shown below gives the typical amount of Btu production that is available from certain animals. The second displays some typical farm heat requirements and the number of animals needed to meet these requirements [5].
Image curtsey of University of Missouri (http://extension.missouri.edu/explore/agguides/agengin/g01881.htm)
Conclusion
Anaerobic digestion for biogas production alone will not solve the world’s energy shortage. It can however, if managed properly result in a renewable energy source that contributes little to greenhouse gases. The high cost of investment may turn away some farmers since the system may not supply all of their energy needs. However if the energy crisis becomes more severe it is an option that governments may utilize with food waste collections from households as is done currently in some parts of Europe [1].
Created by:
Michael Palmer
Clifton Sanders
References
[1] http://www.energy.ca.gov/research/renewable/biomass/anaerobic_digestion/index.html
[2] http://www.michigan.gov/documents/anaerobic_digester_FAQs_2005_137431_7.pdf
[3] http://apps1.eere.energy.gov/consumer/your_workplace/farms_ranches/index.cfm/mytopic=30003
[4] http://en.wikipedia.org/wiki/Anaerobic_digestion#Biogas
[5] http://extension.missouri.edu/explore/agguides/agengin/g01881.htm
[6] http://www.foe.co.uk/resource/briefings/anaerobic_digestion.pdf
[7] http://www.r-e-a.net/biofuels/biogas/digestate-from-ad
[8] http://www.waste.nl/page/248
[9] http://attra.ncat.org/new_pubs/attra-pub/anaerobic.html?id=Pennsylvania
