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The grand challenge of cellulosic biofuels

Nov 07, 2017   |   by Lee R. Lynd   |   Nature Biotechnology

Why cellulosic biofuels have fallen short of expectations and what we can do about it.

A robust second-generation biofuels industry based on inedible cellulosic biomass available as wood, grass, and various wastes was widely expected to be in place by now. Anticipated benefits include climate change mitigation and rural economic development while avoiding the limitations of first-generation biofuels. Progress has been made but at a much slower pace than expected. It is important to understand why. The experience of the past decade and the need for low-cost technology in a world of low oil prices necessitates a strategic reset for biofuels as part of a 'grand challenge' renewables strategy.

The promise

Two years ago, at the Conference of the Parties to the United Nations Framework Convention on Climate Change (COP21), over 190 nations (including the United States) committed themselves to keeping the increase in global average temperature 2 °C below pre-industrial levels, with an aim of limiting the increase to 1.5 °C. Plant biomass provides 10% of global primary energy today and is widely expected to provide on the order of a quarter of primary energy in prominent low-carbon scenarios for 2050. Biomass provides as much energy as oil, natural gas, and coal combined in Shell's (The Hague, The Netherlands) net zero energy scenario, as well as opportunities for carbon removal that must be deployed at a large scale to have more than a 50% chance of achieving the 2 °C goal.

Among various types of plant biomass, cellulosic feedstocks are thought to have the greatest potential for mitigating climate change and are widely available at a lower cost per unit energy (e.g., per megajoule) than petroleum. Transport is both one of the largest and fastest-growing energy sectors and one of the most difficult to decarbonize. Even if the rest of the global economy were completely decarbonized, a failure to displace the fossil fuels used in aviation, ocean freight, and long-haul trucking with low-carbon alternatives would result in emissions exceeding the 2 °C COP21 target. Biofuels are the leading low-carbon option for these transport modes, which represent about half of global transport energy.

A spate of recent studies recognize the substantial number of jobs created by renewable energy technologies, including biofuels. Bioenergy is responsible directly and indirectly for almost 3 million global jobs globally—about the same as photovoltaics and three times that of wind—with liquid biofuels responsible for a little over half this total, and solid biomass and biogas making up the balance. Estimates for direct liquid biofuel jobs in the United States range from 100,000 to 300,000, which may be compared to about 370,000 direct jobs in the US solar industry and about 70,000 for coal mining. Sugarcane production in Brazil, about half of which is used for ethanol, is the largest agricultural employer in that country. Compared with other agricultural workers, laborers in the cane industry have the greatest representation in the formal economy and achieve higher levels of education. Towns with ethanol plants in Brazil have higher tax revenues than comparable towns that do not.

Yet biofuels in the United States and across the globe have progressed little over the past decade—in sharp contrast to other renewable energy technologies. Expansion of global production of biofuels has leveled off, policy support has weakened, and research and development (R&D) funding has decreased and/or narrowed in many countries. Cellulosic biofuel investment and expectations have decreased markedly, although the rationale for their use is widely accepted and in some ways stronger than a decade ago.

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