Cellulosic Ethanol

Introduction

While corn-based ethanol continues to attract criticism, ethanol produced from cellulosic material such as switchgrass or corn stalks is viewed as a more palatable alternative, in part because it doesn't rely on food crops to be produced.
 
To get to the cellulose requires special enzymes to remove the lignin that holds the cellulose together. This lignin, however, can be used to fuel the process of turning the remaining material to ethanol, meaning the process relies less on fossil fuels than other methods of creating ethanol.
 
One issue with cellulosic ethanol, however, is the cost of the enzymes needed to break the cellulose down. The National Research Council of Canada said that while waste biomass is cheaper than corn, the current enzyme cost for biomass conversion is about 16 cents per gallon (or 3.78 litres) of fuel ethanol. The ideal economic target, they say, would be in the three-cent-per-gallon range.

Not everyone, however, is convinced that cellulosic ethanol is the answer. University of British Columbia ecologist Bill Rees, a longtime critic of ethanol as a fuel alternative, said cellulosic ethanol is no better. He said taking away corn stalks and other material that would normally return to the land would deplete the soil, requiring further fertilization later. Improving the fuel efficiency of cars on the road is a more cost-effective way to improve the environment, he said.

Cellulosic Ethanol Production Methods:


There are two broad ways of producing alcohol from cellulose. Hydrolysis breaks down the cellulose chains into sugar molecules that are then fermented and distilled. Gasification transforms the lignocellulosic raw material into gaseous carbon monoxide and hydrogen that is then fed to a special kind of fermenter or to a catalyst bed.

Hydrolysis processes

The cellulose molecules are composed of long chains of glucose molecules. In the hydrolysis process, these chains are broken down to "free" the sugar, before it is fermented for alcohol production. There are two major hydrolysis processes: a chemical reaction using acids, or an enzymatic reaction.
 

Chemical hydrolysis

In the traditional methods developed in the 19th century and at the beginning of the 20th century, hydrolysis is performed by attacking the cellulose with an acid. Dilute acid may be used under high heat and high pressure, or more concentrated acid can be used at lower temperatures and atmospheric pressure. A decrystalized cellulosic mixture of acid and sugars reacts in the presence of water to complete individual sugar molecules (hydrolysis). The product from this hydrolysis is then neutralized and yeast fermentation is used to produce ethanol. A significant obstacle to the dilute acid process is that the hydrolysis is so harsh that toxic degradation products are produced which is a hurdle for fermentation. Concentrated acid must be separated from the sugar stream for recycle (simulated moving bed (SMB) chromatographic separation
for example) to be commercially attractive.

 
Enzymatic Hydrolysis

Cellulose chains can be broken into glucose molecules by cellulase enzymes. This reaction occurs at body temperature in the stomach of ruminants such as cows and sheep, where the enzymes are produced by bacteria there are actually at least three enzymes, used at various stages of this conversion. If the enzymatic hydrolysis process takes place with previously isolated enzymes, a steady supply of the cellulase enzymes is needed.


Iogen Corporation is a Canadian producer of enzymes. They are promoting an enzymatic hydrolysis process that uses "specially engineered enzymes". The raw material (wood or straw) has to be pre-treated to make it amenable to hydrolysis. Another Canadian company, SunOpta Inc. markets a patented technology known as "Steam Explosion" to pre-treat cellulosic biomass, overcoming its "recalcitance" to make cellulose and hemicellulose accessible to enzymes for conversion into fermenatable sugars. SunOpta designs and engineers cellulosic ethanol biorefineries and its process technologies and equipment are in use in the first 3 commercial demonstration scale plants in the world: Celunol Corporation's facility in Jennings, Louisiana, Abengoa's facility in Salamanca, Spain, and a facility in China owned by China Resources Alcohol Corporation (CRAC). The

CRAC facility is currently producing cellulosic ethanol from local corn stover on a 24-hour a day basis utilizing SunOpta's process and technology.
 
Genencor and Novozymes are two other companies that have received United States government Department of Energy funding for research into reducing the cost of cellulase, a key enzyme in the production of cellulosic ethanol by enzymatic hydrolysis.
 
Other enzyme companies, such as Dyadic International, Inc. (AMEX: DIL),are developing genetically engineered fungi which would produce large volumes of cellulase, xylanase and hemicellulase enzymes which can be utilized to convert agricultural residues such as corn stover, distiller grains, wheat straw and sugar cane bagasse and energy crops such as switch grass into fermentable sugars which may be used to produce cellulosic ethanol.


Gasification process

The gasification process does not rely on chemical decomposition of the cellulose chain. Instead of breaking the cellulose into sugar molecules, the carbon in the raw material is converted into synthesis gas, using what amounts to partial combustion. The carbon monoxide, carbon dioxide and hydrogen may then be fed into a special kind of fermenter. Instead of

yeast, which operates on sugar, this process uses a microorganism named Clostridium ljungdahlii. This microorganism will ingest (eat) carbon monoxide, carbon dioxide and hydrogen and produce ethanol and water. The process can thus be broken into three steps:
 

• Gasification Complex carbon based molecules are broken apart to access the carbon as carbon monoxide, carbon dioxide and hydrogen are produced
• Fermentation Convert the carbon monoxide, carbon dioxide and hydrogen into ethanol using the Clostridium ljungdahlii organism
• Distillation Ethanol is separated from water

Cellulosic Ethanol Production Process

There are two ways of producing alcohol from cellulose:

1. Cellulolysis processes which consist of hydrolysis on pretreated lignocellulosic materials, using enzymes to break complex cellulose into simple sugars such as glucose and followed by fermentation and distillation.

Gasification that transforms the lignocellulosic raw material into gaseous carbon monoxide and hydrogen. These gases can be converted to ethanol by fermentation or chemical catalysis.

2. They both include distillation as the final step to isolate the pure ethanol. Cellulolysis (biological approach)There are four or five stages to produce ethanol using a biological approach:

• A "pretreatment" phase, to make the lignocellulosic material such as wood or straw amenable to hydrolysis,
• Cellulose hydrolysis (cellulolysis), to break down the molecules into sugars;
• Separation of the sugar solution from the residual materials, notably lignin;
• Microbial fermentation of the sugar solution;
• Distillation to produce 99.5% pure alcohol.

Cellulosic Ethanol Feedstock

Switchgrass is a native prairie grass of "the tall grass prairie", in contrast to the short grass of the "high plains". Known for its hardiness and rapid growth, this perennial grows during the warm months to heights of 2–6 feet. Switchgrass can be grown in most parts of the United States, including swamplands, plains, streams, and along the shores & interstate highways. It is self-seeding (no tractor for sowing, only for mowing), resistant to many diseases and pests, & can produce high yields with low applications of fertilizer and other chemicals. It is also tolerant to poor soils, flooding, & drought; improves soil quality and prevents erosion due its type of root system.

Switchgrass is an approved cover crop for land protected under the federal Conservation Reserve Program (CRP). CRP is a government program that pays producers a fee for not growing crops on land on which crops recently grew. This program reduces soil erosion, enhances water quality, and increases wildlife habitat. CRP land serves as a habitat for upland game, such as pheasants and ducks, and a number of insects. Switchgrass for biofuel production has been considered for use on Conservation Reserve Program (CRP) land, which could increase ecological sustainability and lower the cost of the CRP program. However, CRP rules would have to be modified to allow this economic use of the CRP land.

Miscanthus x giganteus is another viable feedstock for cellulosic ethanol production. This species of grass is native to Asia and is the sterile triploid hybrid of miscanthus sinensis and miscanthus sacchariflorus. It can grow up to 12 feet (3.7 m) tall with little water or fertilizer input. Miscanthus is similar to switchgrass with respect to cold and drought tolerance and water use efficiency. Miscanthus is commercially grown in the European Union as a combustible energy source.

Corn cobs and leaves, wood chips and paper pulp are also feedstocks for cellulosic ethanol.


The Advantages of Cellulosic Feedstocks

Cellulosic feedstocks have many advantages over using corn to produce ethanol. Because cellulosic crops are not used for food, there is inherently less price volatility. And because a wide variety of crops can be used, they can be grown in a wide variety of geographic locations--even on marginal lands--and can, therefore, be more abundant. Plus, with certain crops, more ethanol can be produced per acre than can be made with corn.

With so many advantages, it seems only natural that we have dedicated energy crops, rather than using food crops for ethanol production.

Here are some numbers to think about.

Right now, corn yields, on average, about 160 bushels per acre, with industry predictions climbing all the way up to 300. And we get about three gallons of ethanol per bushel. That means for every acre of corn harvested, about 900 gallons of ethanol can be made.

Add in four tons of stover (converted cellulosically) per acre, with which you can produce 100 gallons per ton, and we're looking at additional ethanol production of 400 gallons per acre--for a grand total of 1,300 gallons per acre

And that's using two different feedstocks, with two different harvest times, two different costs and two different conversion processes.

Now consider a dedicated biomass energy crop like switchgrass, miscanthus or sorghum. These crops can be harvested, at the present time, at a rate of 20 tons per acre, with ethanol production of 100 gallons per ton, for a total of 2,000 gallons per acre. You can see why energy crops and the cellulosic process will be huge successes.

And that's with the current numbers. Imagine how big this would be if crop yields and gallons per acre were increased and cost were continually driven down. That's exactly where this industry is heading.


Cellulosic Ethanol Properties

Ethanol is a monohydric primary alcohol. It melts at -117.3°C and boils at 78.5°C. It is miscible (i.e., mixes without separation) with water in all proportions and is separated from water only with difficulty; ethanol that is completely free of water is called absolute ethanol. Ethanol forms a constant-boiling mixture, or azeotrope, with water that contains 95% ethanol and 5% water and that boils at 78.15°C; since the boiling point of this binary azeotrope is below that of pure ethanol, absolute ethanol cannot be obtained by simple distillation. However, if benzene is added to 95% ethanol, a ternary azeotrope of benzene, ethanol, and water, with boiling point 64.9°C, can form; since the proportion of water to ethanol in this azeotrope is greater than that in 95% ethanol, the water can be removed from 95% ethanol by adding benzene and distilling off this azeotrope. Because small amounts of benzene may remain, absolute ethanol prepared by this process is poisonous.
 
Ethanol burns in air with a blue flame, forming carbon dioxide and water. It reacts with active metals to form the metal ethoxide and hydrogen, e.g., with sodium it forms sodium ethoxide. It reacts with certain acids to form esters, e.g., with acetic acid it forms ethyl acetate. It can be oxidized to form acetic acid and acetaldehyde. It can be dehydrated to form diethyl ether or, at higher temperatures, ethylene.