Science Magazine (EUA)

Make Way for Ethanol

Publicado em 26 novembro 2010

In his News story “Is there a road ahead for cellulosic ethanol?” (Special Section on Scaling Up Alternative Energy, 13 August, p. 784), R. F. Service identifies factors contributing to fading enthusiasm for cellulosic ethanol and observes that policy-makers' decisions this year could shape the nascent U.S. biofuels industry for decades. It is critical at this time to distinguish the fundamental from the ephemeral and to base policy on the former rather than the latter.

Biomass is by far the most viable sustainable source of liquid fuels, which will be needed for a long time, if not indefinitely. Batteries are completely impractical for aviation and likely for long-haul trucks as well. In the most aggressive scenarios for electrification of light-duty vehicles, liquid fuels still provide more than 50% of U.S. transportation energy (1). Policy debates are often framed as if large-scale use of biofuels is discretionary, but achieving a sustainable transportation sector is much more likely with bio fuels than without them, and turning away from biofuels entails substantial risks.

The most important next step in the biofuels arena is commercial production of ethanol from cellulosic feedstocks. The alternative—converting sources of readily fermentable sugars (mostly from corn and sugar cane) to fuel molecules other than ethanol—might allow us to retain current infrastructure, but would contribute little to our larger goals: creating a sustainable energy supply, reducing greenhouse gas emissions, assuring energy security, and promoting rural economic development. Ethanol will very likely be the world's first cellulosic biofuel because it is unrealistic to commercialize new technology for converting lignocellulose to sugars at the same time as new technology for converting sugars to fuels. Infrastructural challenges associated with distribution and utilization of ethanol are readily solvable, as the Brazilian experience shows, and decidedly small compared to challenges associated with other petroleum alternatives such as batteries or hydrogen.

Sharply divergent conclusions have been drawn about biofuel-related land-use issues, contributing to uncertainty among policy-makers. Pessimists typically ask: What are the impacts of adding large-scale use of today's version of biofuels to the world that will exist if current trends and practices continue? Optimists typically ask: What role could the advanced biofuels of the future play in a world reconfigured to meet energy-related challenges? Both questions merit consideration, but only the second illuminates a promising way forward. There is increasing evidence that large-scale biofuel production can be gracefully reconciled with feeding humanity and preserving the environment, provided that changes are made pursuant to this goal. Change is necessary, given that humanity cannot hope to achieve a sustainable and secure future by continuing the practices that have led to the unsustainable and insecure present.

Most of the factors mentioned in Service's News story are peculiar to the United States. Brazil, for example, did not experience the recent economic downturn, does not have a “blend-wall” (ethanol production approaching the maximum amount that can be accommodated in 10% gasoline blends) or policy uncertainty relative to bioethanol, and is confident of the sustainability of its land-use practices. Ethanol is produced in volumes exceeding gasoline usage (2) from about 1.5% of Brazil's arable land, and the cane used to produce this fuel is grown on former pastureland while total pasture output has increased (3). The international community should be careful not to draw negative conclusions from the U.S. malaise, particularly in light of the high stakes involved.