The Grand Challenge of Cellulosic Biofuels

by Lee R. Lynd (Energy Today/Nature Biotechnology/Dartmouth College) 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.

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 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.

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 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.

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.

Many advanced biofuel startups have failed. Those that have survived are trading well below their initial public offering price; most are focusing primarily on higher-value products other than fuels: ...

Although widely expected circa 2008, a price on carbon did not materialize in most of the world. The nascent cellulosic biofuels industry was rocked by the global financial crisis. The collapse in oil prices in 2014 was the final knockout punch to many efforts in the cellulosic biofuels space.

So what has been different about cellulosic biofuels? Overestimation of technological readiness is part of the answer. There has been a marked tendency, encouraged by both government and private sector investors, to focus on large, expensive, stand-alone facilities rather than niche applications. Particularly in the United States, funding agencies prematurely turned away from cellulosic ethanol, although it is now clear that further development is needed to achieve cost-competitive fuel production even with oil prices at $100 per barrel. Amidst frequent claims that economically viable technology was in hand and investment was needed only in scale-up and commercialization, investment in new, potentially low-cost processing paradigms was generally modest. As a result, technological advancement was slower than it might have been, and policies were designed assuming that deployment, rather than technology, was the limiting factor. The impacts of a tendency to try to vault 100-foot cliffs with 10-foot poles were compounded by the very large size of investments of $250–$500 million and relatively long duration of the design-build-operate-learn cycle in the cellulosic biofuels field. In sharp contrast, other renewable energy technologies proceeded in a stepwise fashion, recognized the need for technological advancement and invested accordingly, and benefitted from projects with lower costs and more rapid learning cycles.

There is more to it, however. Biofuels require land. As a result, their production inevitably has strong linkages to food security, rural economic development, and land-based ecological services. Biofuel advocates see these linkages as opportunities to achieve value above and beyond low-carbon energy supply, pointing to the soil fertility and water quality benefits of incorporating perennials into agricultural landscapes, the social benefits resulting from the Brazilian biofuel industry, and the potential role of biofuels in African transformation and enhanced food security. Critics see these linkages as posing risks that arise to a smaller extent with other renewables, and point out that although cellulosic biofuels avoid direct competition with food markets, they do not avoid competition for land. There is a basis for both perspectives, but the critical voices have spoken more loudly over the past decade, and this has contributed to weaker and less-consistent policy support for biofuels compared with other renewables.

Three key measures should be part of any effort to revitalize cellulosic biofuels. First, pursue commercial deployment in achievable, successively enabling steps, proceeding from where the industry is today. Second, maximize social and environmental benefits based on examples and learning from experience. Third, invest in alternative processing paradigms.

The key challenge to cost-effective production of cellulosic biofuels is the difficulty of converting cellulosic biomass into reactive intermediates, termed recalcitrance. The recalcitrance barrier is manifested in the cost of thermochemical pretreatment and added enzymes for biological processing. For thermochemical processing, it is manifested in the cost of gasification or pyrolysis, including clean-up before fuel synthesis. To maximize the probability of developing a robust cellulosic biofuels industry at a scale large enough to meaningfully contribute to climate and other goals, we need an aggressive effort aimed at new processing paradigms.

This article is adapted with permission from an article of the same name published in Nature Biotechnology. Read the full article (with references). READ MORE