Cellulosic Ethanol

Biomass can be used to make a zero-carbon transportation fuel, like ethanol, which is now used as a gasoline blend. Today, the major biofuel is ethanol made from corn, which yields only about 25% more energy than was consumed to grow the corn and make the ethanol, according to some estimates. Considerable R&D is going on into producing ethanol made from sources other than corn. This so-called cellulosic ethanol can be made from agricultural and forest waste as well as dedicated energy crops, such as switchgrass or fast-growing hybrid poplar trees, which can be grown and harvested with minimal energy consumption, so overall net emissions are near zero.

All cars today can use a mixture of 10% ethanol and 90% gasoline, E10. Some 4 million flexible-fuel vehicles, which can run on either gasoline or a blend with 85% ethanol, E85, are on the road today, but few use E85 because of its high price. This suggests that we cannot solve the chicken-and-egg problem for an alternative fuel merely by delivering a cost-effective vehicle capable of running on that fuel.

The big advantage ethanol has over alternative fuels like hydrogen (and natural gas) is that it is a liquid fuel and thus much more compatible with our existing fueling system. Existing oil pipelines, however, are not compatible with ethanol, so significant infrastructure spending would still be required if ethanol were to become the major transportation fuel.63 Ethanol production will require major technological advances before matching the price of gasoline on an equivalent energy basis. Lester Lave and two other Carnegie Mellon University researchers present the following calculation:

Producing cellulosic ethanol costs about $1.20 per gallon (1.80 per gallon, gasoline equivalent, since ethanol has two-thirds of the energy of a gallon of gasoline). Assuming that the per-gallon distribution costs are the same for ethanol and holding total tax revenue constant, ethanol would sell for $1.80 per gallon at the pump. However, this is equivalent to $2.70 per gallon in order to get as much energy as in a gallon of gasoline.64 16

This calculation should viewed as a projection given that there are no commercial cellulosic ethanol plants anywhere in the world as of 2004. Nonetheless, it suggests two things. First, if oil prices in, say, 2020 are higher than they are today, then cellulosic ethanol will represent a potentially quite competitive alternative fuel. This is particularly true since a price for carbon is virtually inevitable by 2020, further improving the relative cost competitiveness of cellulosic ethanol to gasoline. The average price of gasoline United States has already hit $2.00 a gallon with oil at $40 a barrel.

Given that our first strategy for reducing greenhouse gas emissions must be fuel efficiency, particularly hybrids, we will be unlikely to need substantial amounts of cellulosic ethanol until post-2020. At that time we will have a far clearer picture of future trends in climate change and in the production of both conventional and carbon-intensive unconventional oil. The key will be to ensure that we have taken aggressive measures long before then to bring down the cost of cellulosic ethanol through R&D as well as efforts to subsidize the first initial plants. The Commission, through its support of "The Role for Biomass in America's Future" project, is developing a variety of valuable recommendations in this area. It is possible that would still need technological progress and economies of scale in production plants, cellulosic ethanol could drop to under $2.00 per gallon of gasoline equivalent.

The second conclusion we might draw from cost projections for cellulosic ethanol is that if we can develop a substantial biomass resource for the purpose of creating a low-carbon fuel, it will almost certainly be more cost-effectively used to make cellulosic ethanol than hydrogen. As the National Academy of Sciences panel on the hydrogen economy concluded in February 2004, "hydrogen production from biomass is a thermodynamically inefficient and expensive process, in which approximately 0.2% to 0.4% of the total solar energy is converted to hydrogen at a price of currently about $7.05/kg H2 by gasification in a midsize plant. Even with major technology breakthroughs, "the committee estimates the possible future technology price for hydrogen from gasification of biomass to be $3.60/kg H2, which is noncompetitive relative to other hydrogen production technologies."

For hydrogen production from biomass, perhaps the biggest problem is how expensive and energy-intensive it is to transport hydrogen over long distances. Unfortunately, large biomass resources tend to be quite distant from population centers where vehicle fuel is needed, and transporting solid biomass is also very expensive and energy intensive. Converting that biomass to a liquid fuel like cellulosic ethanol and then transporting that fuel is likely to be the most cost effective and least energy-intensive way of delivering a low-carbon bio-based fuel. A particularly significant benefit of using biomass to make cellulosic ethanol rather than hydrogen is that the switchover to ethanol can be done gradually, as more and more ethanol is blended with gasoline, whereas any switchover to hydrogen almost certainly requires a massive government subsidy for the infrastructure to attempt to solve the chicken-and-egg problem.


Probably the biggest barrier to biofuels, and to biomass energy in general, is that biomass is not very efficient at converting and storing solar energy, so large land areas are needed to provide enough energy crops if biofuels are to provide a significant share of transportation energy. One 2001 analysis by ethanol advocates concluded that to provide enough ethanol to replace the gasoline used in the light-duty fleet, "it would be necessary to process the biomass growing on 300 million to 500 million acres, which is in the neighborhood of one-fourth of the 1.8 billion acre land area of the lower 48 states" and is roughly equal to the amount of all U.S. cropland in production today.67 That amount of displaced gasoline represents about 60% of all U.S. transportation-related carbon dioxide emissions today, but under 40% of what is projected for 2025 under a business-as-usual scenario. Given the acreage needed, using so much land for these purposes would obviously have dramatic environmental, political, and economic implications.

Thus, if ethanol is to represent a major transportation fuel in the coming decades, then U.S. vehicles will need to become much more fuel-efficient. Doubling the efficiency of the fleet by 2030 with hybrid engines and other advanced technology would substantially reduce the biomass acreage requirements. And putting cellulosic ethanol blends into plug-in hybrids with further reduce acreage requirements, especially since there are plausible strategy for cogeneration of biofuels and biomass electricity.

In the long-term, biomass-to-energy production could be exceedingly efficient with "bio-refineries" that produce multiple products. Lee Lynd, professor of engineering at Dartmouth, described one such future bio refinery where cellulosic ethanol undergoes a chemical pretreatment, then fermentation converts the carbohydrate content into ethanol, as carbon dioxide bubbles off.68 The residue is mostly lignin (a polymer found in the cell walls of plants). Water is removed, and the biomass residue is then gasified to generate electricity or to produce a stream of hydrogen and carbon dioxide.

The overall efficiency of converting the energy content of the original biomass into useful fuel and electricity would be 70%, even after accounting for the energy needed to grow and harvest the biomass. The carbon dioxide can be sequestered. Also, this process could be used to generate biodiesel. This is admittedly a futuristic scenario, but is the subject of intense research, and could make ethanol directly competitive with a gasoline, and biomass electricity competitive with other zero-carbon alternatives, especially when there is a price for avoiding carbon dioxide emissions. The syngas could also be used to make synthetic diesel fuel.