Ethanol has more than its share of critics, most often due to issues with using food crops for fuel or negative effects on the performance of small engines. At the center of the argument for or against the use of ethanol as a fuel is the environment, with recent reports of more harm than help being done for the environment when considering the use of conservation land for corn planting and total carbon dioxide emissions throughout the entire process from planting to production of ethanol.
Even more core to the issue of ethanol as a fuel is the chemistry that explains and quantifies how much energy is generated by the fuel. I ask my students to evaluate ethanol based solely on its merits as a combustible fuel and the amount of carbon dioxide (CO2) generated from its combustion, leaving arguments over process efficiencies to Archer Daniels Midland (NYSE: ADM ) and the arguments over engine performance to Briggs & Stratton and the American Petroleum Institute, which lobbies heavily on behalf of ExxonMobil (NYSE: XOM ) , Chevron (NYSE: CVX ) , and other American oil companies. Instead of listening to proponents and critics citing contrasting studies, let the chemistry of gasoline and ethanol tell the story.
To compare the amounts of energy generated by gasoline and ethanol using just the knowledge gained in a General Chemistry course, a few significant assumptions need to be made. For ease in the calculations, gasoline is assumed to be 100% octane (C8H18), thus not taking into consideration longer, shorter, and unsaturated hydrocarbons or the 10% ethanol that would typically be found in a gallon of non-premium unleaded gasoline.
The two components used to evaluate the two fuels are energy generation per gallon, measured in kilojoules (kJ), and the environmental impact as determined by the amount of CO2 produced, in pounds, per gallon of fuel.
Energy per gallon
The reaction enthalpy (amount of heat produced by the reaction) for the combustion of octane is calculated by applying Hess' Law to well-established enthalpies of formation for the reaction products (CO2 and water) and reactants (the fuel and oxygen). The initial calculation gives a value in units of kilojoules per mole, which is then converted using molar masses, densities, and standard measurement conversions to units of kilojoules per gallon. When all of the conversions are complete, octane is shown to generate approximately 118,000 kJ per gallon, and ethanol is shown to generate approximately 80,000 kJ per gallon.
These chemistry calculations provide the fundamental reason why the fuel efficiency of a flex-fuel vehicle drops considerably with the use of E85 (a fuel blend of 85% ethanol and 15% gasoline) versus premium gasoline fuel. Across the board independent of the specific vehicle or manufacturer, EPA estimates for flex-fuel vehicles show a 25%-35% decline in realized miles per gallon when E85 is used in place of premium gasoline, which matches very closely with the 32% decline in energy generated between the two fuels in the chemistry calculations above.
As a consumer, the often bigger consideration is the cost of filling up one's vehicle. Given an expected 30% decline in fuel efficiency when using E85 over gasoline, the cost of E85 would need to be over 30% lower than the cost of regular gasoline to be the better financial choice. In other words, with the average price of premium fuel at $4.01 per gallon, the price of E85 would need to be under $2.80 per gallon to provide a financial justification for its use (the current national average is $3.21 per gallon).
Carbon dioxide per gallon
Consumers using E85 in favor of gasoline often cite environmental reasons for their choice, having been sold on the idea that ethanol is a cleaner fuel that produces less greenhouse gases than gasoline. A look at the reaction stoichiometry (remember balancing reactions in chemistry class?) can quantify just how much CO2 is produced. Per gallon, the combustion of ethanol produces about 12.6 pounds of carbon dioxide. The combustion of one gallon of octane produces about 18.1 pounds of carbon dioxide.
When comparing gallons to gallons, ethanol comes out on top as the clear victor in the carbon dioxide emissions test, which is just one component in the overall greenhouse gas emissions consideration. The grander view of the situation, however, should take into account how the amount of CO2 produced relates with the amount of energy produced because emitting less carbon dioxide per gallon does little help for the environment if more gallons need to be used to travel the same distance. When all of the calculations are combined, the amount of carbon dioxide generated by ethanol per kilojoule of energy generated is within 5% of that for gasoline.
The more important consideration than the chemistry behind the fuels is how the general population chooses to interpret the chemistry. A portion would undoubtedly cite the chemistry as a reason why gasoline remains superior to ethanol while a different portion would argue that the chemistry shows ethanol is a reasonable substitute for gasoline that supports the goal of American energy independence. In either case, the nation's largest ethanol producers have little to worry about in terms of a mass revolt against their fuel.
Regardless of the energetics, America has accepted ethanol as a viable blending component for gasoline. Even the American Petroleum Institute has accepted ethanol as a part of America's energy future, and they have gone so far as to conduct initial research suggesting the safety and effectiveness of increasing the ethanol blend limit from 10% to 12%. As for my students and the rest of the American public, very few will actively seek out higher ethanol blends while the vast majority remains content filling their gas tanks with whatever is cheapest and most readily available at the pump.
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