How conservation evaporates ethanol’s benefits for cars
The sharp increase in crude oil prices during the past year created an urgent need to search for alternative sources of transportation fuels. Ethanol emerged as the primary candidate to replace a part of the gasoline pool used for transportation. Ethanol can be produced from carbohydrates which are abundantly present in nature as key structural and functional constituents of plants. Carbohydrates come in a great variety including simple sugars, most prominently glucose, and in more complex forms such as starch and cellulose, which consist of several thousand interconnected glucose units. Ethanol production from complex carbohydrates is a two-step process: first the carbohydrate structure has to be broken down to its simple sugar units which in turn need to be converted to ethanol by fermentation. Starch can be converted to ethanol with relative ease while other carbohydrates such as cellulose are more resistant to ethanol conversion. Factors influencing commercial scale ethanol production include availability of the renewable source material in sufficient quantities, efficient means of conversions to ethanol, attractive pricing conditions, government subsidies and tax incentives.
In the USA more than 95 percent of the ethanol is produced from corn, while other sources include cheese whey, barley and shorgum. The United States Department of Agriculture (USDA) forecasted 10,550 million bushels of corn crop from 71 million acres of land for 2006. Approximately 20 percent (2,100 million bushels) of the corn crop is slated for ethanol production.
The first question we ask, “How much ethanol can be obtained from the annual corn crop dedicated to ethanol production?” The next logical question is what impact this quantity of ethanol has on the domestic gasoline consumption. Regarding the first question we need to estimate the material yield for ethanol defined as the quantity of ethanol that can be obtained from one mass unit of corn. On the average two-thirds of corn’s dry mass is starch. Assuming that all of the starch can be broken down to glucose which in turn can be converted to ethanol without losses, the ethanol yield is estimated 0.057 gallons of ethanol per pound of dry corn corresponding to 0.125 gallons of ethanol per kilogram of dry corn or 2.71 gallons of ethanol per corn bushel (1 corn bushel equals 56 lbs at 15 percent moisture). Using 149 bushels per acre average corn yield predicted for 2006 and the maximum material yield for ethanol, one acre corn crop would result in 404 gallons or 9.6 Barrels of ethanol. Since corn is harvested once a year this also represents the annual ethanol production limit from the fresh crop for this year.
Using these estimates approximately 5.7 billion gallons or 135 million Barrels of ethanol can be obtained from the 2006 corn crop slated for ethanol. Due to incomplete material conversions, material losses, and limitations in distillation capacity the actual ethanol yields are lower. The current domestic distillation capacity is 4.3 billion gallons per year (280,000bbl per day) with an additional 1.98 billion gallons capacity under construction.
Now we are in the position to estimate the impact of ethanol on gasoline consumption.
Assuming 3.4 billion Barrels (average 9.3 million Barrels per day) of gasoline consumed in 2006 the 5.7 billion gallons (372,000 Barrels per day) of maximum ethanol production represents 4 percent of the gasoline pool. The percentage of gasoline that can be replaced by ethanol drops to a modest 2.9 percent (266,000bbl per day) because it takes 1.4 unit volume of ethanol to replace 1 unit volume of gasoline to account for equivalent heat contents in these fuels.
The Energy Policy Act of 2005 mandated a steady growth in annual ethanol production from renewable sources reaching 7.5 billion gallons of output in 2012. Renewable sources include dedicated energy crops, trees, wood, plants, grasses, fibers, agricultural residues, and waste materials. Assuming 10 percent increase in gasoline consumption in 2012 compared to present levels, the 7.5 billion gallons of ethanol targeted by the Energy Policy Act from any renewable sources would still replace only 3.4 percent of the gasoline pool adjusted for equivalent heating values. To achieve this goal from corn alone would require 32 percent increase in the quantity of corn dedicated to ethanol production compared to the present level. This scenario would likely upset the balance with other important uses of corn such as food for humans and feed for livestock.
The question then arises whether improvements in fuel economy can achieve comparable gasoline savings as predicted by ethanol substitution.
Based on statistical information from the Federal Highway Administration in 2004 approximately 90 million pickups trucks, SUVs and vans were registered in the USA. Assuming 90 percent of this fleet was gasoline powered with 18mpg average fuel economy and 15,000 miles driven per vehicle annually this fleet of vehicles consumed 1.79 billion Barrels of gasoline per year (4.9 million Barrels per day). By raising the fuel economy to 20 miles per gallon corresponding to 11 percent increase would save 7.5 billion gallons of gasoline per year. This gasoline saving is equivalent to using 10.5 billion gallons of ethanol as gasoline substitute which is 40 percent more ethanol than set by the Energy Policy Act for 2012.
In sum, ethanol from corn alone is not a significant source as gasoline substitute at the national level but it can ease the gasoline squeeze in and around areas where the corn industry thrives. The goals by the Energy Policy Act for ethanol production from any renewable source can be met by a relatively modest increase in fuel economy for SUVs, vans and pickup trucks not even taking into account passenger cars. Pending commercial viability, cellulose conversion to ethanol has a better chance to ease the gasoline squeeze at the national level.
The following can be concluded:
- Ethanol from renewable sources at the levels set by Energy Policy Act will have a minor contribution to alleviate gasoline consumption;
- A modest increase in fuel economy for SUVs, pickup trucks and vans (not even including passenger cars) appears to achieve similar if not better gasoline saving compared to ethanol production targets from renewable sources by the Energy Policy Act.
Ethanol from renewable sources can play a significant role to replace MTBE in reformulated gasoline. The Clean Air Act Amendments (CAAA) of 1990 required the use of reformulated gasoline containing at least 2 wt percent oxygen in areas of severe ozone non-attainment. In the ensuing years MTBE became the choice of oxygen bearing compound blended to gasoline to fulfill this mandate. For MTBE the 2wt percent oxygen content in gasoline corresponded to 11 volume percent. During the intervening years however the use of MTBE fell out of favor because of several incidents of groundwater contamination from leaking gasoline storage tanks. While not banned at the federal level, several states including California banned MTBE use in gasoline since 2005.
Ethanol is emerging as the preferred candidate for the replacement of MTBE. For ethanol the 2wt oxygen content in reformulated gasoline corresponds to 5.3 percent by volume that is 50 percent less than by using MTBE. The present corn to ethanol production appears to meet this blending volume requirement. Interestingly the Energy Policy Act removed the oxygen content mandate from the CAAA. The current production level of ethanol from corn seems adequate however for replacing the volume of MTBE as a blending component in gasoline for reducing emissions.
|Number of motor vehicles in U.S. (2004) ||243 million|
|Number of licensed drivers in U.S. (2004) ||198.8 million|
|Total U.S. gasoline consumption (2004) ||141.2 billion gallons|
|Total U.S. highway gasoline consumption (2004) ||136.5 billion gallons (96.6% of total)|
|Total U.S. non-highway gasoline consumption, e.g. aviation, marine (2004) ||4.7 billion gallons 19.5 (3.4% of total)|
|Total annual gasoline consumption per vehicle (2004)||581 gallons|
|Total vehicle miles traveled (2004) ||2.96 trillion|
|Total URBAN vehicle miles traveled (2004) ||1.89 trillion|
|Average urban speed ||20mph|
|U.S. urban travel time (2004)||94.6 million hours|
|Percentage of urban drive time spent idling ||18%|
|Annual urban idling time in U.S. (2004)||17 million hours|
|Idle time fuel consumption ||0.5 gallons/hour|
|Gasoline consumed idling annually||8.5 billion gallons|
|Gasoline consumed idling annually per licensed driver||42.8 gallons|
|U.S gasoline price as of 5/22/06||$2.883|
|Estimated savings per driver per year||$123.39|
The Energy Policy Act set the goal of annually increasing ethanol production from renewable sources reaching 7.5billion gallons in 2012. Assuming present gasoline consumption annually.
Expert No. 28193 is a consultant on energy and environmental issues, experienced in chemical engineering issues with over 30 years in the industry; areas include air pollution control, accidental release prevention / risk management program, storm water pollution prevention, hazardous waste management, chemical and refinery processes, environmental project management, CAL/OSHA, air quality compliance, and more.
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