Turning sugarcane cellulose into ethanol: Energy for the future?

Linda Benedict  |  7/3/2008 2:28:16 AM

Giovanna A. DeQueiroz is an assistant professor at the LSU AgCenter’s Audubon Sugar Institute. (Photo by Mark Claesgens)

Photo By: Mark Claesgens

Table 1. Ethanol production.

Donal F. Day, Giovanna A. DeQueiroz and Benjamin L. Legendre

An expanding economy requires fuel for transportation. As long as our vehicles are powered by internal combustion engines, this means some source of liquid fuel. There has been much talk about carbon balances, global warming, energy requirements to produce ethanol, hydrogen fuel and hybrids. But the simple fact is the United States has more than 250 million vehicles on the road, which use 560 million gallons of fuel a day. In Louisiana terms, each man, woman and child in the state, on average, is responsible for 0.25 gallons of gasoline and 0.75 gallons of diesel fuel to support their daily activities. The problem is how to replace this large volume of fuel with nonpetroleum sources.

The focus has been on ethanol as a replacement for gasoline. Ethanol is a combustible fuel that has about 80 percent of the Btu value of gasoline. It can be used in existing internal combustion engines and can be made from simple sugars. It takes about 15 pounds of sugar (sucrose) to produce one gallon of ethanol by fermentation. Because of ethanol’s ability to absorb water and because it must remain dehydrated to be used as a fuel, the logistics of transporting large volumes of ethanol long distances are difficult and expensive. So most ethanol is used in the region where it’s produced.

The United States is the world’s largest producer of ethanol, followed closely by Brazil. We produce about 13 million gallons of ethanol a day. To close the large gap between fuel use and available ethanol would require a 50-fold increase in ethanol production. Currently, America’s primary agricultural source of simple sugars used for ethanol production is the starch in corn. But an expansion of this magnitude is well beyond our available corn supplies.

The largest potential source of renewable, fermentable sugars is the glucose tied up as cellulose in plants – generally referred to simply as biomass. Government estimates place renewable U.S. biomass resources at 510 million dry tons per year. Biomass contains about 40 percent sugar as cellulose (poly-glucose) and 20 percent sugar as xylose. It takes about 1 ton of dry biomass to make 80 gallons of alcohol by fermentation – and somewhat less if made by a process called gasification. Sufficient liquid fuel could be made from biomass to fill 20 percent of U.S. demand, fueling the excitement at the potential for converting cellulose to ethanol. Converting all the sugar in the available biomass to ethanol could potentially yield 40 billion gallons of ethanol. As a practical matter, not all of this material could be available.

The price of a gallon of gasoline in Louisiana includes a 16-cent federal tax and a 25-cent state tax. Assuming a 10 percent retail markup – when gasoline is $3 per gallon at the pump – fuel must be produced for about $2.30 per gallon to be profitable. Feedstock represents approximately 65 percent of the cost required to produce ethanol – or at these prices about $1.50 per gallon. That means sugar must cost 10 cents per pound to be competitive with other feedstocks. The current estimated costs of feedstocks, the equivalent sugar value and the amount of ethanol produced from each feedstock are presented in Table 1.

On a tax-free basis, the sugar cost for biomass-based ethanol would be about $0.11 per pound, which is within the range of economic feasibility. With the current tax structure, the biomass price must drop – or the productivity (tons per acre) must rise – for cellulose ethanol to compete. It is possible a rise in the cost of oil will push the selling price to where biomass ethanol is profitable (for example, at a retail price of $4 per gallon, it would be profitable to make ethanol directly from sugar).

Even without incentive programs, cellulosic ethanol can be produced at a profit, but it requires taking advantage of byproducts and using existing industrial systems to reduce capital expenses. This is the basis of the biorefinery concept, where biomass arrives at a facility and several different products are produced from the same feedstock. By producing multiple products, a biorefinery can take advantage of different biomass components and intermediates, maximizing the value derived from biomass. Today, corn wet mills and dry mills, pulp paper mills and sugar factories fit the concept of biorefineries because they produce combinations of food, feed, power and industrial and consumer products.

A commercial industry based on cellulosic biomass conversion to fuels and other byproducts does not yet exist in the United States, but several precommercial facilities are in development. These facilities will produce ethanol from biomass components, cellulose and hemicellulose and burn another component – lignin – for energy. One demonstration plant in Jennings, La., has a capacity to produce 1.4 million gallons of ethanol a year from sugarcane-derived bagasse and other feedstocks, such as energy cane, wood chips and corn stalks.

Some of the challenges in the cellulosic industry include the development of high-performance energy feedstocks that can be harvested in rotations to supply factories throughout the year. A series of potential biomass crops has been proposed ranging from sugarcane to Miscanthus, a perennial grass native to subtropical and tropical regions of Africa and southern Asia.

Problems that must be solved for biomass-based cellulosic ethanol to achieve its full potential include:
  • Harvesting and transporting feedstocks to biorefineries.
  • Storing materials on-site. Improving pretreatment technologies.
  • Reducing the cost of enzymes for converting biomass polymers to fermentable sugars.
  • Developing microorganisms capable of using all fermentable sugars simultaneously.
  • Expanding the biofuel transportation infrastructure.
Ethanol from cellulosic biomass does offer a supplement to the conventional liquid fuels that support national economic growth. It can help provide national energy security and satisfy environmental goals. But increased productivity and efficiency must be achieved through operations that maximize the use of all feedstock components, byproducts and waste streams. The nation can benefit from an expanding, sustainable energy supply and reducing dependence on foreign oil.

Donal F. Day, Professor; Giovanna A. DeQueiroz, Assistant Professor; Benjamin L. Legendre, Professor and Interim Head, Audubon Sugar Institute, St. Gabriel, La.

(This article was published in the spring 2008 issue of Louisiana Agriculture.)
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