Biofuel Production for Light-Duty Vehicles
In terms of energy use, light-duty vehicles account for over 58 percent of the 27,600 trillion BTU used for transportation in the U.S. each year. This is a huge market, representing roughly 5 times the annual output from U.S. Nuclear energy plants; the global market is even larger. It will be difficult for any single technology to capture a substantial fraction of this market, so we will require a toolkit of biofuels technologies and political support for these technologies if carbon reduction and energy security goals are taken seriously.
A light-duty vehicle (LDV) is any car or truck that weighs less than 8,500 lbs. For most of us, that represents the vehicles we drive and interact with on a day-to-day basis. You often hear of electric or hydrogen-powered vehicles, and lusting after the Tesla Roadster is all too common. Most people, however, are unaware of what constitutes a biofuel vehicle. Unlike hydrogen or electric vehicles that utilize electric motors and require new fueling stations and driving behaviors, ‘drop-in’ biofuels can use existing infrastructure without substantial modification. As such, most of the interest in biofuel technology innovation and development is associated with drop-in biofuel production rather than with vehicle and infrastructure deployment and adoption. This is a fundamental difference from other transportation technologies.
Without a full transition of the vehicles in use today, which is a long, arduous process, we are left with the ability to design drop-in fuels. Ethanol is at the forefront of drop-in biofuel options for LDVs, and there are several emerging technologies that aim to produce ethanol and other biofuels more efficiently and at reduced cost and lower carbon intensity while not relying on food crops.
Biochemical is just a fancy way of saying that biological organisms are used to create or convert a starting material (e.g. corn) into the desired chemical product (ethanol). Traditional ethanol production is a biochemical process. Biochemical processes are usually appealing because they have relatively high selectivity (you get precisely the product that you want) and can often be cheaper and easier to carry out at a small scale compared to their thermochemical counter part.
While traditional biochemical processes are simplistic enough to carry out in your home (think brewing beer, or baking bread), a vast array of new biotechnology is being designed to improve yields and reduce costs. These come in two primary forms.
- Feedstock yield improvements: like the agricultural industry, genetically engineered crops and feedstock (be they corn or poplar trees) that can be grown using fewer inputs, at higher yields, and on less productive soil may have far reaching implications for biofuels. The cheaper and more abundant your starting material is (biomass), the lower the cost is for your final product (ethanol).
- Feedstock conversion improvements: while traditional fermentation uses yeast to convert starches and sugars to ethanol, there is a lot of plant material that is not made up of sugar such as the cellulosic, hemicellulosic, and lignin parts (think fiber) of the plant. Traditional yeast cannot convert these plant components into ethanol, and so areas of research are being carried out to figure out how to convert this material into ethanol – either directly, or through an initial conversion to starch and sugar.
Furthermore, the yeast typically used in fermentation tends to die when the alcohol content is over 20 percent (this is why most wines and beers are substantially under 20 percent alcohol by volume). Yeast can be engineered, however, to tolerate higher volume alcohol solutions. This ultimately leads to a reduction in the energy needed for distillation, which could lead to lower carbon intensity and potentially cheaper fuels.
Although these process improvements sound appealing and offer great end results, the technology is not yet mature. For any chemical process, there is a tradeoff between reaction rate, reaction yield, and product selectivity. Yeast that can live in 50 percent alcohol solutions might sound great, but in actuality it could take months or years to achieve this conversion, further increasing costs and reducing output. Breaking down cellulose into sugars similarly may be possible, but the level of sugar obtained compared to other products from the breakdown could be low, leading to expensive separation processes. Ultimately, these difficulties translate to higher process costs. Without incentives in place, traditional ethanol fermentation will continue to outcompete any of these next-generation biotechnologies that are still in their infancy.
A thermochemical process, unlike a biochemical process, does not rely on the use of carefully selected and engineered biological organisms for conversion. This typically allows for higher rates of reaction, and larger scale operations. The selectivity, however, may suffer.
Any hydrocarbon material can be used for thermochemical conversion processes (agricultural waste, animal waste, trees, garbage, etc). The hydrocarbon material is split apart into its components, carbon monoxide and hydrogen, and then recombined to form any desired chemical (ethanol, methanol, etc). This process is typically referred to as Fischer-Tropsch synthesis or Mixed-Alcohol synthesis.
Breaking apart the hydrocarbon material requires high temperatures and controlled environments (to prevent combustion). This energy intensive process is known as gasification. Recombining the hydrocarbon components back to form the desired liquid chemical requires fine-tuned catalysts and precise processes engineering
Although Fischer-Tropsch synthesis has been around since the 1920s and has been continually developed and improved since then, the process is far from perfect for the production of ethanol. The process suffers from low selectivity for ethanol on top of low conversion efficiencies. This means that a large quantity of feedstock is needed to generate significant volume, and that energy intensive separation is needed to create ethanol at a purity that is marketable. As the technology matures and becomes increasingly used, process innovations that reduce feedstock use and energy requirements will gradually happen.
On top of these challenges, the cost of creating a chemical plant at scale cannot be understated. Without significant government support, these facilities are likely to remain more expensive and more risky than conventional ethanol production facilities, even if they are able to convert a multitude of hydrocarbons to ethanol while avoiding food-crop concerns.
Other technology options
While ethanol is one option for drop-in fuels, there are others. Biobutanol, a more direct drop-in gasoline replacement, may be produced through analogous biochemical or thermochemical processes. Butanol could more directly act as a substitute for gasoline, and thus avoid ethanol blend wall concerns (getting beyond 10 percent). Methanol or bio-synthesized gasoline may similarly be produced. Unlike ethanol, however, these technologies have not received decades of direct political support and incentives for development. Therefore, these other technologies must not only compete with fossil fuel production, but also with the existing ethanol market.
As we move into the future, advanced biochemical and thermochemical conversion processes will be increasingly relied upon for the production of biofuels to replace gasoline. These technologies will ultimately be necessary to mitigate competition with food crops, and to produce high volumes of biofuel. Furthermore, the conversion processes are likely to increase in efficiency, creating less carbon intensive fuels and contributing to greenhouse gas emission reduction goals. While many of the advanced technologies exist today, the cost of production is often the true barrier for viability. Without extensive political support, it will be difficult for any nascent technology to enter the market so that it may undergo further refinement to emerge as a competitive, viable option for biofuel production.
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