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| Fig. 1: An overview of the Fischer-Tropsch process. Carbon containing feedstock is processed into a mixture of CO and H2 called Syngas, which is then processed by way of Fischer-Tropsch catalysts into hydrocarbon chains. Common feedstock include coal, natural gas, and biomass. |
The Fischer-Tropsch process is a chemical technique developed in the 1920s to convert a mixture of carbon monoxide and hydrogen, called synthesis gas or syngas, into hydrocarbon chains of varying lengths. The process is relatively simple: syngas is fed at high temperatures through catalysts (usually the transition metals cobalt, iron, and ruthenium) which facilitate the hydrocarbon formation. When paired with processes such as steam reforming or gasification, which convert methane or solid carbon sources into syngas respectively, the Fischer-Tropsch process can effectively be viewed as a method to transform any available carbon source into hydrocarbon chains. Note that this process is not creating energy, but simply converting heat energy into chemical energy stored in the hydrocarbon bonds. The hydrocarbon chain length of most interest is usually that of liquid hydrocarbons (C5 - C25), which can used as synthetic fuel. [1]
This process is highly relevant to aviation fuels, where the high energy density requirements of efficient flight restrict the energy storage options available to aircraft. Weight is a major aircraft design consideration. As such, once natural petroleum reserves are exhausted, synthetic fuel will likely become the only viable aviation fuel due to its high energy density, unlike the automobile industry, which can use heavier battery technology as weight and energy density is not a crucial design factor. [2]
The issue of Fischer-Tropsch sustainability comes into play when one envisions a world depleted of its natural petroleum reserves. In this world, electrical energy can still be generated via non-carbon based sources (e.g. nuclear, hydro, wind, solar, etc.) which can be used to power the Fischer-Tropsch plants as well as the gasification or steam reforming processes. The major consumable of the process then becomes the starting carbon sources, also known as feedstock. Therefore, provided that energy is available to power the process, the sustainability of synthetic fuel production through Fischer-Tropsch rests on the availability of feedstock.
Fischer-Tropsch feedstock can essentially consist of any material containing carbon. The most common feedstock materials are coal, natural gas (methane), and more recently biomass. [1,3] Of these three, coal and natural gas are both geological and will eventually be depleted with varying time tables. Biomass is renewable; however when it becomes the only major available feedstock, synthetic fuel production will be limited by the availability of the biomass material.
Once coal and natural gas feedstocks are depleted as well, the major remaining renewable feedstock materials are biomass materials. Different types of biomass feedstock have varying carbon contents: approximately 50% for woody crops or wood waste and 45% for grass crops or agricultural residues. [4] Since biomass must originally be grown before it can be harvested, restriction to this category of feedstock can cause yield limitations in the production Fischer-Tropsch synthetic fuel. As discussed previously, synthetic fuel is vital to the aviation industry in the post-petroleum era so these limitations will be evaluated in the context of aviation.
Consider the question of how many trees must be converted into synthetic fuel to cover the US airline fuel budget. The total fuel used by all US airline carriers in the year 2007 was estimated at 13682 million gallons. [5] This translates into 3.41 x 10^10 kg of pure carbon. Using the approximation that woody crops contain ~50% carbon content of their mass, this is roughly the equivalent in carbon of
Since a logging three has a mass of about 1360 kg (3000 lbs), the wood equivalent of the aviation fuel consumed amounts to [6]
Note that as the gasification and Fischer-Tropsch processes are not 100% efficient, the actual amount will be higher. [7]
The total amount of wood harvested in the United States in 2002 according to government reports was approximately 15000 million cubic feet. [8] Assuming the specific gravity of dry wood to be 0.5, we thus obtain [9]
For the US wood harvest. Therefore, if the entire current US airline fleet switched to Fischer-Tropsch synthetic fuels, it would require as much wood as approximately 35-70% of the current annual national harvest. Remember that this is only considering the necessary carbon feedstock and neglecting the large energy costs to run the gasification and Fischer-Tropsch processes.
This now becomes an issue of sustainable logging. It would seem that Fischer-Tropsch production of synthetic aviation fuels is possible on a commercial scale even when coal and natural gas feedstock are depleted, but it is clear that biomass feedstock will have limitations on how large an aviation industry can be sustained.
© Michael Liu. 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] H. Schulz, "Short History and Present Trends of Fischer-Tropsch Synthesis," Applied Catalysis A 186, 3 (1999).
[2] A. Bisio, "Aircraft Fuels," Encyclopedia of Energy, Technology and the Environment (New York, 1995).
[3] M. Tijmensen et al., "Exploration of the Possibilities for Production of Fischer Tropsch Liquids and Power Via Biomass Gasification," Biomass and Bioenergy 23, 2 (2002).
[4] M. Antal et al., "Attainment of the Theoretical Yield of Carbon from Biomass," Ind. Eng. Chem. Res. 39, 11 (2000).
[5] "Fuel Cost and Consumption," Airline Data and Statistics, U.S. Research and Innovative Technology Administration, 14 Dec 10.
[6] D. Waddell, D. L. Weyerman and M. B. Lambert, et al., "Estimating the Weight of Douglas-Fir Tree Boles and Logs With an Iterative Computer Model," Research Paper PNW-RP-374, US Department of Agriculture, Forest Service, Pacific Northwest Research Station, March 1987.
[7] A. van der Drift et al., "Bio-syngas: Key Intermediate for Large Scale Production of Green Fuels and Chemicals," ECN Energietechnologie ECN-RX-04-48, 10 May 04.
[8] D. M. Adams, R. W. Haynes and A. J. Daigneault, "Estimated Timber Harvest by U.S. Region and Ownership, 1950-2002," General Technical Report PNW-GTR-659, US Department of Agriculture, Forest Service, Pacific Northwest Research Station, January 2006.
[9] H. D. Erickson and A. T. Harrison, "Douglas-Fir Wood Quality Studies - Part I: Effects of Age and Stimulated Growth on Wood Density and Anatomy," Wood Science and Technology 8, 255 (1974).