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| Fig. 1:Example Methanol Use Cycle as a Transportation Fuel. |
The transportation sector accounts for roughly 30% of total U.S. energy consumption, of which over 90% of this energy comes from crude oil and its derivatives (predominantly gasoline and diesel). [1] The dependence on crude oil can be largely explained by two fundamental properties: energy density and a stable liquid phase at Earth's surface conditions. For most modes of transportation that carry their energy source onboard, high volume and mass energy densities are important to minimize the size and weight of the vehicle, which have associated energy costs (kinetic, potential, drag) during travel. The high energy density of liquid hydrocarbon fuels (~40 MJ/kg) facilitates long range travel and lift (in the case of air transport) that simply are not feasible with other energy carriers such as batteries (~0.4 MJ/kg). [2] Additionally, in the liquid phase, crude oil and its products can be easily distributed and stored, with a much higher volumetric energy density than gaseous hydrogen or methane (natural gas). Correspondingly, a vast and efficient infrastructure for liquid hydrocarbons exists. Unfortunately, crude oil is a diminishing resource, and projections of consumption relative to proven reserves suggest that natural gas and coal will outlive crude oil in the global energy portfolio. [3] Methanol, an oxygenated hydrocarbon, has been suggested as an alternative transportation fuel that may bridge the gap between utilization of current infrastructure and production from non-crude resources such as coal, natural gas, and biomass. [4]
Methanol (CH3OH) has approximately half the energy density (~20 MJ/kg) as gasoline, and a vapor pressure of 4.6 psi, making it a stable liquid at ambient conditions. Due to the lower energy density, higher injection flow rates are needed to achieve similar engine power as with gasoline, and a larger tank is necessary to have similar fuel range (miles/tank). [5] The decrease in miles per gallon from lower energy density is partially offset by improved engine efficiency. A 25-30% efficiency improvement over gasoline may be realized when engines are optimized for methanol. [6] Moreover, methanol may be blended with gasoline and ethanol such that little or no change is required to flex-fuel vehicles (FFV) that are in wide production. [7] Similarly, existing infrastructure technology for gasoline and diesel (including pipelines, tanks, and filling stations) can accommodate methanol with modest technical adjustments. Government programs, namely in California, have funded conversions of vehicles and some infrastructure to support fleets of methanol automobiles at a relatively small scale. Various M85 (85% methanol, 15% additives) fuel programs led to a peak of over 21,000 M85 FFV automobiles and more than 100 M85 fueling stations in the U.S. during 1997. The subsequent decline in methanol as a transportation fuel was attributed to falling gasoline prices (from the 1979 oil crisis peak), and the eventual displacement by ethanol. [5]
Methanol production is a mature and readily scalable industrial process. As of 2009, annual methanol production capacity topped 22 billion gallons worldwide, with over $12 billion in economic activity. [5] Current uses for methanol are predominantly in the chemical production of formaldehyde and acetic acid, as well as other basic industrial chemicals. The well-established process for methanol production happens in two steps. First, a carbonaceous feedstock is converted to a synthetic gas (syn-gas) of CO2, CO, H2O, and H2 via gasification (coal feed) or steam reforming (natural gas feed). In both gasification and reforming, water and/or oxygen are added along with substantial thermal energy to attain an equilibrium composition (CO/H2 ratio) that is desirable for the second step, catalytic synthesis of methanol from syn-gas. Biomass can also be gasified similar to coal and CO2 can be reduced catalytically with hydrogen to yield methanol in a more sustainable manner. At present, natural gas represents the most practical and economical feedstock for methanol production, due to its similar C:H ratio and the corresponding low energy cost of conversion. [4]
With the adoption of hydraulic fracturing in the last decade, proven natural gas reserves have increased dramatically (especially in North America) and natural gas market prices have declined sharply in relation to crude oil (on a $/joule basis). Unfortunately, natural gas is not readily substituted for oil in its ambient state due to the difference in phase (gas vs. liquid). Methanol, which can be efficiently and economically produced from natural gas (and coal), provides a technically viable alternative to gasoline. Methanol can integrate into the existing infrastructure for liquid hydrocarbons and be compatible with much of current internal-combustion engine technology (given modest retrofitting). While shifting to coal and natural gas as feedstocks for domestic liquid fuels provides a reasonable medium-term solution to diminishing crude oil reserves, methanol can also be produced in a sustainable manner (from biomass or CO2). In this regard, methanol has potential as a long-term solution for our consumption of liquid fuels in the transportation sector.
© Mitchell Spearrin. The author grants permission to copy, distribute and display this work in unaltered form, with attribution to the author, for noncommercial purposes only. All other rights, including commercial rights, are reserved to the author.
[1] "Annual Energy Review 2011," U.S. Energy Information Administration, DOE/EIA-0384(2011), September 2012.
[2] K. E. Aifantis, S. A. Hackney and R. V. Kumar, High Energy Density Lithium Batteries (Wiley-VCH, 2010).
[3] "BP Statistical Review of World Energy," British Petroleum, June 2012.
[4] G. A. Olah, A. Goeppert and G. K. S. Prakash, Beyond Oil and Gas: The Methanol Economy (Wiley-VCH, 2009).
[5] L. Bromberg, and W. K. Cheng. "Methanol as an Alternative Transportation Fuel in the U.S.: Options for Sustainable and/or Energy-Secure Transportation," Massachusetts Institute of Technology, November 2010.
[6] M. J. Brusstar and C.L. Gray Jr., "High Efficiency with Future Alcohol Fuels in a Stoichiometric Medium Duty Spark Ignition Engine," Socieety of Automotive Engineers, SAE paper 2007-01-3993, 29 Oct 07.
[7] J. Turner, R. Pearson et al., "GEM Ternary Blends: Removing the Biomass Limit by using Iso-Stoichiometric Mixtures of Gasoline, Ethanol and Methanol," Society of Automotive Engineers, SAE paper 2011-24-0113, 11 Sep 11.