A Critical Look at Cellulosic Ethanol and Other Advanced Biofuels
A Critical Look at Cellulosic Ethanol and Other Advanced Biofuels
by Alexander A. Koukoulas, Ph.D.
Over the last 10 years, the United States has made enormous investments in the pursuit of bio-based fuels the so-called “Second Generation” or 2nd Gen biofuels. Unlike 1st Gen biofuels—ethanol from corn starch—2nd Gen biofuels are derived from lignocellulosic sugars, those that come from woody biomass and agricultural sources, such as corn and wheat stover, and purpose-grown energy crops, like miscanthus and fast-growing poplar.
The pursuit of 2nd Gen fuels has been motivated by several factors including: their inherently low-cost, at least from a theoretical standpoint; their potential for drastically reducing carbon emissions relative to 1st Gen Fuels; national security (less dependence on foreign oil); and job creation, especially in our rural communities.
Unfortunately, while significant technical progress has been made in the last decade, 2nd Gen fuels, especially those produced using biochemical platforms, have yet to achieve commercial viability due to their inability to compete with conventional fuels on price. In fact, several years ago, researchers and developers alike recognized these technical challenges and began shifting their focus from lignocellulosic ethanol to “drop-in” fuels, such bio-butanol, in the hope that enhanced compatibility with the existing liquid fuels infrastructure would make these fuels more cost competitive.
It should come as no surprise that these technical and commercial challenges have not gone unnoticed from a policy perspective. Delays in achieving widespread commercial success has had significant impact on the entire renewable fuels industry. An extensive analysis from the Government Accountability Office (GAO) issued last month stated that “the investments required to make these fuels more cost-competitive with petroleum-based fuels, even in the longer run, are unlikely in the current investment climate.”
To better understand the current state of biochemical-based production pathways, we reviewed recent developments in the production of 2nd Gen biofuels produced under a number of biochemical routes. Our intent was to provide a state-of-the-art assessment of progress of advanced biofuels within three market applications: gasoline, middle distillates and aviation fuels (see table below).
Particular attention was paid to the success rate of genetic engineering and their commercialization prospects. What we found was that in spite of the significant progress made in the genetic engineering of microbes designed for advance biofuel production, titer and yield of these biomolecules are currently too low to allow these products to compete with their petroleum-derived equivalents.
What surprised us was how far off we are relative to corn-based ethanol. For example, the highest reported yield for the best available alternative to starch-derived ethanol (iso-butanol derived from engineered E. Coli, is still one-eighth the level of achieved in the production of fuel ethanol from corn using that industrial workhorse, S. cerevisiae (see figure below). Moreover, high titers and yields from alternative routes are always reported using model sugar substrates, like glucose, not real-world hydrolysates derived from cellulosic biomass. Neither wild-type nor genetically-engineered microorganisms have been isolated or developed with all the necessary traits for the bulk production of advanced biofuels. And, unlike yeast and conventional ethanol fermentation, recycling of microbial cells are difficult in a lignocellulosic system and genetically engineered strains seem highly susceptible to contamination, which further increases operating costs.
Of course, upstream challenges with cellulosic ethanol are not completely solved either. Recovery of sugars at high concentration from a highly water-holding (hydrophilic) substrate is still challenging. And, the downstream challenges with fermentation are multi-fold including: the toxicity of the inhibitors to both microbes and enzymes; conversion of multiple sugars; and, enzyme and microbe recycling of the enzymes.
As a result, attention was refocused from cellulosic ethanol to other fuel types, such as cellulose-based butanol, fatty acids, and isoprenoids. Based on current progress, it is evident that these also will not be commercially viable as the inherent complexity of micro-organism development for these pathways continues to present formidable technical barriers. Moreover, development is expected to be painstaking as the challenges with microbes used in these systems are multi-fold higher compared to even cellulosic ethanol.
The question of inhibitors still presents an on-going challenge. As is well known, along with the presence of multiple sugars, lignocellulosic hydrolysates contain a spectrum of compounds, which are potentially toxic to the enzymes and/or microbes used in the bioconversion process. These toxic compounds or inhibitors are both naturally present in the lignocellulosic substrates and also process derived including the toxicity imparted by the final products. Techniques for in situ removal of inhibitors and strategies that can enhancing titer, such as gas stripping and solvent extraction, while marginally effective were found to add significant cost to production.
It has been suggested that the cell membrane is the primary target of toxicity as most of these molecules has been shown to fluidize the cell membrane. Increased membrane fluidity also results in uncontrolled transport of solutes that can decrease the proton flux across the membrane and cause leakage of amino acids and enzymes. Over-expressing products that are inherently toxic to the cell membrane appears to be the greatest limitation in achieving high yields. Despite extensive research efforts, there has been limited success in developing a commercial microbial strain for producing advanced biofuels that is both multiple-sugar consuming and inhibitor tolerant while obtaining an industrially acceptable titer and yield. As a result, substantial R&D in metabolic engineering and optimization will be needed to develop a suitable microbial strain capable of producing advanced biofuels from lignocellulose.
We concur with the GAO that lignocellulosic ethanol is far from being commercially viable given the present state of the technology, the unfavorable economic conditions and the policy uncertainty. We also seriously question the commercial viability of certain “drop-in” fuels. Imparting microbial cells with numerous and often competing functions without interfering with their basic physiological characteristics remains a formidable challenge. Key success criteria, like product yield, are still far from commercially relevant levels. Adding the complexity of the cellulosic substrate just raises the technical hurdle for commercialization.
The potential for expanding 1st Gen ethanol as a fuel and as a feedstock for chemicals production is enormous. In most cases, commercialization hurdles are a question of policy rather than technical readiness. However, with respect to 2nd Gen biofuels, only a renewed commitment to basic R&D can provide the tools needed to bridge the many technical gaps that stand between the current state and commercial success. Clearly, the need to fund R&D programs will be a difficult argument to make given the current economic and political climate. But, it is possible if a broad, strategic view is taken. The alternative—widespread defunding of programs—will be a huge set-back. As for commercial opportunities, given this analysis, only value-added specialty chemicals—those that are differentiated from their petroleum analogues—have the potential to be commercially viable at least in the short run.
References
GAO-17-108. Nov 28, 2016. DOI: Nov 28, 2016.
This article first appeared in the Thought Leadership section of Biofuels Digest on December 12, 2016.
© A2K Consultants LLC 2017