Methane (CH4), or natural gas, is commonly used all over the world as a source of energy and fuel. Many homes in the United States combust methane to generate heat for hot water heaters, central air heaters, and is used extensively in gas powered stoves and ovens. Currently methane is mass-produced in a variety of different ways, many of which are not sustainable or environmentally friendly. Many people (see www.builditsolar.com for additional reading if you are interested) have started producing their own methane in a variety of different ways. Even though most of these methods are not very efficient, they can be somewhat sustainable if reclaimed or waste materials are used from local sources. One way to “home-brew” methane in a decently sustainable way is to collect waste vegetable oil from various local restaurants and “pyrolyze” it with high heat (typically ~700 K) in the presence of oxygen. Pyrolysis is a very similar chemical reaction to combustion; however, it differs in the types of products it makes.
Corn oil, a commonly used vegetable oil at fast-food restaurants, is composed of approximately 65% by mass linoleic acid (HC18H31O2) and 35% by mass oleic acid (HC18H33O2). In reality there are many other components of corn oil, but these are the main ingredients. The un-balanced reactions below show the pyrolysis of linoleic and oleic acids, respectively, in the presence of oxygen.
HC18H31O2+ O2(g) àCH4(g) + CO2(g) + H2O(g) + H2(g)
HC18H33O2 + O2(g) àCH4(g) + CO2(g) + H2O(g) + H2(g)
In the pyrolysis of re-claimed, or waste corn oil, both reactions are equally important in the production of methane. Note, many other by-products are also generated in these reactions; however, we can ignore them in this exercise.
In the 2011-2012 academic year, Dr. Harmon used approximately 515 kg of methane (that’s a real “ish” number! I got it from my PG&E power bill) in his home for heating, hot water, and stove/oven use. How many 75-gallon barrels of waste corn oil should Dr. Harmon collect to produce his own methane for 1 entire year assuming he uses the exact same amount of methane as in the 2011-2012 academic year? The density of oleic and linoleic acids are very similar, you may assume both are 0.90 g/cm3. Additionally, you may assume the percent yield of methane production from corn oil pyrolysis is 100% (in really it is much smaller).
HINT: To make this problem easier, you may assume 65% of the methane produced comes from linoleic acid and 35% of the methane produced comes from oleic acid.