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Alternative Energy Potentials Of Biogas

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Alternative Energy Potentials of Biogas

In just the past one hundred and fifty years, humans have entered onto a path drastically different than anything we have known in the past. We have developed and embraced modern technologies, and with these a new way of life. This boom in new technologies has been driven and made possible by energy in the form of fossil fuels. Fossil fuels come from plant and animal matter deposits that are millions and millions of years old. They have very high-energy potentials (one pound of coal contains 13000 Btu (Ristinen 1999), and therefore are excellent for our needs. The most utilized of these forms are coal, petroleum and natural gas. All over the world fossil fuels are being extracted at a faster and faster rate as the world's energy demands increase. This presents a large problem because fossil fuels are not renewable within a practical timescale, since they take millions of years to form. According to estimated calculations based on current consumption (not taking into account the escalation of need that the increasing world population would demand) the world's supplies of fossil fuels have already peaked (Ristinen 1999) and will run out sometime in this century (Ristinen 1999).

Another problem with the large-scale use of these fuels are their negative effects on the environment. To access and use the energy held in fossil fuels, we burn them using the heat energy released to do work. A variety of pollutants are part of the byproducts of this process. The pollutants depend on the energy source and way it is used, whether in an electrical plant or an automobile. These emissions include carbon dioxide, ozone, sulfur dioxide, carbon monoxide, particulate matter and more (Ristinen 1999). These substances, in the quantities they are expelled, are damaging the atmosphere. Beyond the poisonous qualities many of these have, many are green house gasses, contributing tremendously to the very real and present global warming that is occurring.

In 2003, the world consumption of fossil fuels was approximately 421 QBtu (EIA 2006). The world population is steadily growing, as is the fuel use of several developing counties; both of these are escalating this demand. With the intensifying and accelerating depletion of these resources and an unwillingness to give up the lifestyle these resources afford us, there is a strong need to find alternative forms of cleaner energy that can feasibly take the place of fossil fuels. There does not appear to be an easy solution to this problem. There are a few alternatives in use today including solar, hydropower, wind, biofuel, and nuclear power. None of these however, have made much of a dent in fossil fuel consumption due to different factors such as economics, environmental impacts and the fact that many require some input of fossil fuels into their initial production. Until existing technologies improve or a new unforeseen solution is found, energy conservation and use of these alternatives is essential. This paper will explore the potentials of an alternative energy source, biogas, a renewable waste product containing methane. I will explore the process of extracting biogas, the overall benefits, economics and the environmental repercussions of biogas generated from livestock wastes. Taking these factors into account may determine whether or not methane gas from waste is an economic, sustainable, alternative energy source, and if so, what beneficial impact it can make in the world.

Biogas is made up primarily of methane (60%), which is the main component of natural gas; it has an energy content of 600 Btu/ft3 (Barker 2001). Methane and a few other gasses are naturally created from wastes by bacteria during the decomposition of organic compounds that make up the waste. This process is known as anaerobic fermentation or digestion. The types of bacteria that perform most of the decomposition are anaerobes, meaning that they can only operate in anaerobic (oxygen-free) environments (Penn State 2006; Hansen 2006). There are two basic steps in the processing of digestion. First acid-forming bacteria break down the complex matter of manure into simple compounds. Then these compounds and the acid are broken down by a different group of bacteria resulting in primarily methane and carbon dioxide waste (Hansen 2006). Animal manure is the primary source of waste that biogas is extracted from. It is also extracted from human waste and landfills, which will not be explored in this paper.

Useable biogas can be produced from manure quite simply as long as the two groups of bacteria maintain balance. This is necessary, so that the methane-formers use up the acids produced by the acid-formers, and they do not accumulate in excess (Hansen 2006). Manure is put in a sealable (oxygen-free) container, which should have a valve or some other way of collecting the gas that accumulates near its top. A fairly constant temperature and pH are needed for maximum methane production. Wide temperature variations increase the chances of acid accumulation, and thus cause disruption to the digestion process (Hansen 2006). The best temperatures for digestion are between 90 and 100o F (Laichena 1997), which can be achieved from heating by passive solar or by burning a fuel. The bacteria do not perform very well in cooler temperatures. Gas production is approximately cut in half (or will take twice as long) for each 20o F decrease (Hansen 2006). The best pH range for this process is 6.6-8 (Paxton n. d.).

Anaerobic digesters that are run on mesophilic or thermophilic conditions (distinguished by operating temperature) follow the basics of the simple system, but are optimized for faster and more efficient results. These processes generate more methane and can be used when large-scale amounts of waste need to be processed. Commercial dairies that are capturing biogas generally follow one of these systems. Mesophilic digesters contain bacteria that are active in temperature ranges of 95 to 105 oF, while thermophilic digesters contain bacteria active between a 130 to 135 oF range (Penn State 2006). There are advantages and disadvantages to mesophilic and thermophilic digesters. Digesters operating under mesophilic conditions are much more stable than ones operating in the thermophilic range; thermophilic bacteria are vulnerable to changing operational and environmental conditions, such as fluctuating temperature (Man-Chang 2006). Mesophilic digesters use a much smaller energy input to run than thermophilic digesters, since they are running at lower temperatures; as a result their net energy productions are higher (Man-Chang 2006). Thermophilic digesters have a much shorter processing time than mesophilic digesters, taking up to half as much time as a mesophilic digester would (Man-Chang 2006). The other huge advantage of the thermophilic process is the fact that

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