Cellulose is a major constituent of plants and soon may become an important component in our mix of energy sources.  Cellulosic ethanol can be made from wood, perennial grasses, agricultural residues, and municipal solid waste.  By producing ethanol from biomass, we can reduce our country’s demand for petroleum, lower fuel transportation costs, reduce greenhouse gas emissions, and provide more economic stability.  The federal Energy Independence and Security Act of 2007 mandates that by 2022, the U.S. produce 36 billion gallons of biofuel per year, of which 16 billion gallons must be cellulosic ethanol.

The production of ethanol from biomass is a complex and expensive process.  There are two basic ways to do this.  In the biochemical method, the feedstock is pretreated to separate the cellulose and lignin from the hemicellulose (Figure 1).  Hydrolysis breaks down the complex carbohydrates into simple sugars.  Fermentation of these sugars by microbes produces ethanol.  An alternative method converts biomass into liquid fuels using pyrolysis, a thermochemical process of partial combustion.  This produces synthesis gas (primarily carbon monoxide and hydrogen), which is then converted into ethanol and other alcohols by fermentation or in a catalytic reactor.

As with all new technologies, turning a potential energy source into a profitable one requires economic analysis of each step.  Recent improvements in enzyme production, chemical treatments, and energy efficiency will make the production process more efficient and profitable.

A market for byproducts also improves the economic equation.  These products include lignin, furfural, plastics and other chemicals, protein, and charcoal, depending on the process used. Lignin, with an energy content similar to coal, may be burned directly for heat or electricity or combined with coal in a coal-fired plant. Highly purified lignin can be used to produce biodegradable plastics, polyurethane, adhesives, and resins.  Protein may be removed from the biomass before the pretreatment process and used as animal feed.  Charcoal, also known as biochar, can be burned like coal or used as a soil additive to improve soil health and sequester carbon in a stable form.

New technology is expected to improve the current ethanol output of 50 gallons per ton of biomass to 80 to 100 gallons per ton.  In the case of switchgrass, the annual production is projected to increase to 15 tons per acre using improved strains, resulting in 1200 to 1500 gallons of ethanol per acre (compared to the current 370 to 450 gallons of ethanol per acre of corn).  Some researchers have projected that production costs of cellulosic ethanol could go as low as $0.60 per gallon, or less than $1.00 per gallon of gasoline on an energy equivalent basis.

Further improvements in plant selection, the production process, and the demand for byproducts will make the production of cellulosic ethanol more feasible and profitable.  With commercial plants coming on line in the next few years, we will soon realize the economic and environmental benefits of ethanol made from cellulose, the most abundant organic compound on Earth.