Synthetic Alcohols Could Provide a Fully Renewable Liquid-Fuel Future

By James Turner and Richard Pearson | March 10, 2008
It is difficult to assign much confidence to projections of how long fossil fuel reserves will last, but a shift away from a fossil fuel-based energy economy is inevitable, motivated in the short term by growing concern over the impact of greenhouse gases on global climate change and energy security issues resulting from political events. In response to this and following the failure of the Association of European Automotive Manufacturers' voluntary agreement to reduce fleet average carbon dioxide emissions to 140 grams per kilometer by 2008, it looks increasingly likely that the European Union will mandate a tailpipe average of 130 grams per kilometer by 2015 as part of the process.

The intention is to force change in the near term. Longer term, molecular hydrogen has been suggested as the primary energy vector for transportation, the attraction being that as the energy is released by combination of the hydrogen with oxygen, water is the sole product. Provided the energy used to produce the hydrogen is renewable, no carbon dioxide impact results. However attractive this may be, though, molecular hydrogen has some severe drawbacks in terms of its transportation and distribution, notwithstanding the considerable issue of providing a sufficient quantity of renewable energy.

While hydrogen has a very high energy content per unit mass of 120 Megajoules per kilogram, it is also the least dense of elements. At standard atmospheric temperature and pressure, its density is 0.09 kilograms per cubic meter (or 90 grams per cubic meter), meaning that 1 cubic meter under these conditions contains only 10.8 Megajoules of energy. Compare this with gasoline under these conditions, which, with a density of 740 kilograms per cubic meter and energy per unit mass of 42.7 Megajoules per kilogram, contains 31,600 Megajoules per cubic meter—more than 2,900 times as much.

This illustrates the problem with hydrogen. Whereas gasoline (being a liquid) is easily transported and stored, a higher-energy-density method of handling hydrogen has to be arranged. Those currently showing the most promise are compression (to 700 bar) or liquefaction (requiring minus-253 degrees Celsius or minus-424 degrees Fahrenheit). Under these conditions the energy stored per cubic meter is 6,900 Megajoules and 8,496 Megajoules, or 22 percent and 27 percent that of the cheaper-to-store gasoline, respectively.

Further, in order to process hydrogen to these conditions, pressurization requires the equivalent of approximately 10 percent of the energy stored, and liquefaction between 30 percent and 40 percent.

To compound matters, in order to encourage pressurized hydrogen to flow into a vehicle pressure vessel it must be pressurized to a still higher level, and liquid hydrogen suffers badly from boil-off—any heat input causes a loss of hydrogen from the system—meaning the complete loss of the energy contained and that used to put it into that state.

In any future where availability of energy is limited, minimizing energetic losses will be of paramount importance, and it is the energetic losses associated with storage and distribution which mitigate against molecular hydrogen as an energy carrier. This is before the costs associated with a completely new infrastructure are considered—some estimates for which are in the region of $1 trillion for the United States alone.

The alcohols ethanol and methanol, however, can 1) be made renewably, 2) are liquid at atmospheric conditions, and 3) while their energy contents per unit of mass are not as high as that of gasoline, can store significantly greater energy per unit volume than hydrogen—approximately 67 percent and 50 percent that of gasoline, respectively.

Furthermore, they share similar physico-chemical characteristics and are both miscible with gasoline, meaning that they can be introduced via a single modified infrastructure and into existing vehicle architectures.

Ethanol can readily be produced from sugars or other forms of biomass with the correct enzymes (lignocellulosic ethanol). Both routes permit high levels—up to 90 percent—of carbon dioxide to be trapped in the loop. However, ethanol supply cannot satisfy the total energy requirement for transport.

Methanol, the simplest alcohol, can be made more easily from a much greater range of feedstocks than ethanol. For example, it can be made from wood more easily than the lignocellulosic approach can yield ethanol. It can also be synthesized from carbon monoxide or carbon dioxide and hydrogen. Indeed, the limiting factor over the years as been the availability of feedstocks. The carbon and oxygen chemically liquefy the hydrogen in an energetically efficient manner.

Note that once hydrogen is liquefied into methanol, the significant downstream energetic losses associated with pressurizing or liquefying it in its molecular form are eradicated, and therefore the total energy supply to the system is reduced. As mentioned above, this is of paramount importance when maximizing the proportion of energy supplied renewably.

An interesting concept is the coupling of the electricity and transport markets by using the carbon dioxide output from coal-fired power stations as a feedstock for methanol—the combined methanol and power approach, giving one more use for the carbon dioxide before its eventual atmospheric release. The reduction in power station output to achieve this has been estimated to be 1 percent, and the carbon dioxide mitigation of the order of 56 percent, which is better than ethanol from sugar beet. Similar approaches can be used for factory flues.

Unfortunately, the power and transport sectors have not historically been linked. The setting of a monetary value for carbon dioxide emissions will enable a financial argument for the production of methanol from such flue gases to be made, and at a sufficiently punitive level it could become compelling.

In the longer term, to ensure sufficient feedstock, the solution could be to scrub carbon dioxide from the atmosphere and combine it with renewable hydrogen, which would already have to be created for a hydrogen economy. Removal of carbon dioxide from the atmosphere should be given financial credits by governments. In this case, the atmosphere becomes a resource and (if some of the methanol is used for petrochemicals instead of fuel) a form of "autosequestration" takes place. One hundred percent closure of the carbon dioxide cycle could theoretically be achieved, as shown in Figure 1.

While this perhaps sounds like science fiction, it is important to note that all of the science is in place and being researched. The University of Southern California has already achieved it. Consequently, this is a most exciting long-term opportunity because the whole process can be driven by the existing economics of supplying liquid fuel to the marketplace, since 1) gasoline, ethanol and methanol are all miscible in the same tank (minimizing costs), 2) suitable engines can be created cheaply, and 3) no one has to invest in a completely new fuel infrastructure or vehicle energy storage concept.

Indeed, the on-cost for flexible-fuel gasoline/alcohol operation in a vehicle is trivial. Creation of a suitable fleet to kick-start the process is effectively a stroke of the legislator's pen. The future existence of such a fleet would draw greater investment in lignocellulosic ethanol production, and a later software change would effectively then be all the modification required to introduce renewable methanol into the then-existing fleet.

If this could be arranged, one could foresee a process which both shifted the balance of power with respect to the holders of the world's energy resources and provided a means of influencing carbon dioxide level in the atmosphere within a relatively short period of time, a process which could be encouraged by market forces. The collective nature of this, initiated by the mandating of flexible-fuel capability in all new spark-ignition vehicles from 2012 onwards, is shown in Figure 2.

We believe that this is an entirely workable solution to the problem of global warming associated with transport, and can be used to reduce the effect of carbon dioxide emissions from other areas as well.

James Turner and Richard Pearson are with the powertrain department of Norwich, Norfolk, U.K.-based Lotus Engineering. The company's U.S. office can be reached at (734) 995-2544.