Cellulosic Ethanol – Is it an Alternative Fuel?

According to the U.S. Energy Information Association (EIA) the average daily use of petroleum in the U.S. is 20.7 million barrels per day. More than half of this daily intake is imported, leaving the U.S. dependent on foreign petroleum to meet its energy needs. For that reason, the Energy Independence and Security Act of 2007 was passed to require that 36 billion gallons of its annual use of petroleum be replaced by renewable fuels by 2022.

Ethanol will likely be part of this solution, of which corn (i.e. starch) serves as a primary source for the refineries. There is an alternative under development that originates from plant stems, leaves, and trunks rather than supple starch and it is known as cellulosic ethanol. As a replacement for corn ethanol or gasoline, additional research and development is required to make it commercially attractive because it is:

• More expensive than either corn ethanol or gasoline,
• Still being produced on a demonstration-level scale.

In 2009 the U.S. Department of Energy (DOE) allotted over $1.3 billion to seed the start of commercial plants which will likely open as early as the end of 2010. Besides offering significant environmental benefits, there are expectations that with additional research and the uncertainty regarding the price of corn and oil cellulosic ethanol will be a viable economic and technical alternative.

Differences Between Cellulosic and Starch Ethanol

As the name implies, cellulosic ethanol originates from cellulose, one of the most abundant of all carbon-containing material available. In fact, plants produce over 100 billion tons of cellulose each year. In contrast, starch ethanol comes from corn kernels:

• Starch is the means by which plants store energy and is relatively easy to break down. The refineries grind the corn kernels, which combined with other enzymes, transforms into glucose, which is, in turn, converted into ethanol.
• Cellulose, on the other hand, involves a slower and far more complicated process. The plants which need to be ground are woody which makes things more challenging. After the grinding, which is more difficult than with corn kernels, the real complication is the formation of five sugars, four of which are not easy to break down. With the addition of engineered microbes to ferment these sugars, there are toxin buildups, the conversions are incomplete, and the enzymes react slowly – all of these factors resulting a lower ethanol yield.
• Where starch is more predictable regarding the amount of starch, and therefore ethanol, that will be produced, cellulose produces an environmentally “cleaner” product.

Thus, despite the inherent challenges with Cellulosic Ethanol, particularly in comparison with Starch Ethanol, the virtually unlimited supply of environmentally cleaner plant material warrants further investment in developing this fuel source.

Cellulosic Ethanol

Cellulosic biomass is contained within every portion of a tree, grass clippings, farm wastes, and even recycled paper. The first step in producing cellulosic ethanol is to transport plants to the refinery. There are currently two methods for transforming these plants to ethanol:

• Biochemically: After undergoing pretreatment with sulfuric acid, four sugars (xylose, mannose, arabinose, and galactose) are created and combined with glucose, forming somewhat of a “soup of sugars.” This conglomeration is placed in a tank and allowed to ferment, after which the alcohol is separated, distilled, and purified to fuel grade.
• Thermo-chemically: Though this method has the drawbacks of being comparatively expensive and producing tar buildup, it does offer the most efficient transformation from cellulose to ethanol. The process starts with the drying of the plants and then burning them to form a synthesis gas made of carbon monoxide and hydrogen (referred to as “syngas”). This gasification forms the tar and sulfur which is chemically converted to additional syngas. As the gases are cleaned, compressed and run through a metal catalyst, they transform to molecules of ethanol and hydrocarbons that are similar to those found in gasoline. The ethanol is then separated.

Cellulosic Ethanol – Potential Role in Reducing Greenhouse Gases

Assuming that ethanol refineries operate on wood chips and oil refineries operate on coal, cellulosic ethanol releases:

• 86 percent less greenhouse gas (i.e. carbon dioxide, methane and nitrous oxide) than gasoline, and
• 52 percent less greenhouse gas than corn ethanol.

Studies have also compared the emissions of cars using a mixture of ethanol and gasoline, since in the U.S. no car drives on 100 percent ethanol. In fact, the highest percentage of ethanol is 85 percent, as engines do not start well in cold weather as the percentages near 95 percent. The 85 percent mixture releases toxins and pollutants; but, in permeation emissions, those tied to cars’ pipes and hoses, there is a noticeable reduction in emissions as compared to pure gasoline or low-ethanol mixtures of 6 to 20 percent. However, because of the counterbalancing effect of reducing emissions of benzene and butadiene yet increasing the emissions of acetaldehyde and formaldehyde, the total toxicity for tailpipe emissions is not appreciably different.

Benefits of Cellulosic Ethanol

Besides the environmentally-related benefits summarized above, the main benefits of using cellulose to produce ethanol are:

• The virtually endless supply and proximity of cellulosic biomass. Its availability exceeds that of corn or any other existing source for fuel.
• In producing cellulosic ethanol, the waste generated can be made into fuel.
• The volatility in the price of oil or corn does not apply to this alternative.

Challenges Regarding Cellulosic Ethanol

There are a number of challenges or disadvantages in the short term related to making cellulosic ethanol a part of the overall solution in reducing dependence on foreign oil. They include:

• Cost: Cellulosic ethanol currently costs more to produce than starch ethanol, which in turn, is more expensive than gasoline. Though the DOE has established the goal for cellulosic ethanol to be price competitive with gasoline by 2012, there is still a fairly significant gap.
• Production Capacity: Refineries do not have the production capacity to meet the demand in large scale.
• Resource Limitations: There are serious concerns about the availability of land to grow the biomass, or alternatively how to balance the supply of energy and food crops.
• Fuel Mileage: Currently fuel with 85 percent ethanol is 25 percent less efficient in miles per gallon than gasoline.