Cellulosic Ethanol

Quan Bui
November 26, 2010

Submitted as coursework for Physics 240, Stanford University, Fall 2010


Obtaining a sustainable source of energy can stabilize the United States economy, government, and politics. Fortunately, the United States government has began to recognize the importance of energy independence by implementing the Energy Independence and Security Act of 2007, which requires the replacement of 36 billion gallons of annual petroleum with renewable fuel by 2022. [1] The predominant, enticing method of achieving this goal is through the production of ethanol, which can be produced from a variety of processes.

Cellulosic Ethanol vs. Grain Ethanol

Cellulosic and grain ethanol represent the two major types that differ in their synthetic processes. The most common type of grain ethanol is corn, a starch that is easily broken down into energy-rich glucose. [2] On the other hand, cellulosic ethanol is extracted from the cellulose found in the thick, tough cell walls of plants. These rigid cell walls make the process of conversion of cellulose to simple sugars and glucose very difficult and more energy intensive. [3]

Despite its energy drawbacks, cellulosic ethanol still remains a popular source of renewable energy because of its potential environmental benefits, including lowering greenhouse gas emissions. Also, the input energy concern could be combated with evidence that cellulosic may yield ~3 times for energy than grain. [4] In addition, the raw materials needed for cellulosic ethanol production are relatively cheap and abundant since it primarily consists of waste from food crops. Researchers expect that cellulosic ethanol could displace 30% of the United States current petroleum consumption. [3]

Harnessing Cellulosic Ethanol

Cellulosic ethanol can be obtained from either cellulosic biomass or from energy crops, including switchgrass and miscanthus. Cellulosic ethanol is primarily harnessed in two manners: biochemically and thermodynamically. The biochemical process involves pretreatment, biological conversion, fermentation, product recovery, and distillation. [3] In the pretreatment step, the plants are ground into bits and treated with strong acid or base. The strong acid or base serves to disintegrate the cell walls and lignin in order to free hemicellulose, which can be further broken down into simpler sugars, such as xylose, pentose, and hexose. [4] At this point, cellulase enzymes facilitate cellulose hydrolysis to produce glucose. This hydrolysis step is typically the rate-limiting step. Various fermentation methods and microorganisms are implemented for the conversion of glucose and hemicellulose sugars into ethanol. Finally, the ethanol is separated and distilled to meet the standards for fuel. [4]

In contrast, the thermodynamic process first involves the drying of the plants that allows it to be burned into a synthetic gas, consisting of carbon monoxide (CO) and hydrogen gas (H2). [3] Next, the relatively pure synthetic gas is then reacted with a metal catalyst to produce ethanol or other hydrocarbons resembling gasoline. This is commonly known as the Fischer Tropsch Synthesis (FTS) Process. The thermodynamic process has the benefit of extracting the most ethanol from any plant and excludes the hydrolysis rate-limiting step in the biochemical process. A major drawback of FTS is the costs of metal catalysts. [3]


Switchgrass has become the poster child for cellulosic ethanol because of less-stringent agricultural requirements and its ability to be grown on limited land spaces. Switchgrass requires less fertilizer and water than grain ethanol. Vogel and colleagues at the University of Nebraska performed studies on growing switchgrass as a biomass energy crop on ten different farms across the Midwest in order to calculate agricultural energy input costs, biomass yield, estimated ethanol output, greenhouse gas emissions, and net energy results. Their research found that switchgrass, grown as a biomass energy crop produces 540% more renewable energy compared to the nonrenewable energy inputted. [5] In addition cellulosic ethanol derived from switchgrass was found to have 94% less green house gas emissions compared to gasoline and also had soil conservation benefits. [5] Switchgrass can produce significantly greater quantities of energy per unit of land than corn. The net energy outputs of switchgrass are expected to increase further as conversion rates are improved through technological advancements in hybrids and breeding techniques.

Concerns and Moving Forward

One of the concerns for cellulosic ethanol is the finding available land to grow it on large scale. Some fear that the switchgrass may grow uncontrollably and reduce food crop yields through spatial competition. The ethanol byproduct has also been shown to have lower mileage outputs compared to gasoline because of its low energy density. For example, the E85 (85% ethanol and 15% gasoline) fuel blend yields 30% less gas mileage than normal gasoline because it has two-thirds the energy density of gasoline. [3] It is definitely worth weighing out the benefits and drawbacks when considering cellulosic ethanol as a reasonable renewable resource.

© Quan Bui. The author grants permission to copy, distribute and display this work in unaltered from, with attribution to the author, for noncommercial purposes only. All other rights, including commercial rights, are reserved to the author.


[1] J. M. Broder, "Bush Signs Broad Energy Bill," New York Times, 19 Dec 07.

[2] D. Greer, "Creating Cellulosic Ethanol: Spinning Straw into Fuel," BioCycle, April 2005.

[3] J. R. Regalbuto, "Cellulosic Biofuels - Got Gasoline?" Science 325, 822 (2009).

[4] L. R. Lynd, "Overview and Evaluation of Fuel Ethanol from Cellulosic Biomass: Technology, Economics, the Environment, and Policy," Annu. Rev. Energy Environ. 21, 403 (1996).

[5] M. R. Schmer et al., "Energy of Cellulosic Ethanol from Switchgrass," Proc. Natl. Acad. Sci. 105, 465 (2008).