Researchers Propose New F-T Process for Synfuels; Less Work Required Could Result in 15% Reduction in CO2 Emissions Compared to Conventional Route
|The proposed CO2 and H2 pathway (bottom) rather than the traditional CO and H2 pathway (top) can improve efficiency and reduce CO2 emissions by 15%. Credit: Adapted by P. Huey/Science. Click to enlarge.|
Researchers from the University of the Witwatersrand (Wits), South Africa and Rutgers University are proposing new Fischer-Tropsch (F-T) reaction chemistry and process designs that they say could increase F-T process efficiency and reduce CO2 emissions by 15% compared to the conventional process.
The new process, which uses a carbon dioxide and hydrogen route rather than the traditional carbon monoxide and hydrogen route, could also open up a pathway for the direct use of CO2 and H2 derived from low-carbon processes (nuclear, wind, solar, bio). A brief description of the proposal, derived via thermodynamic analysis, was published in the 27 March issue of the journal Science.
Lead author Professor Diane Hildebrandt is the Director of the Centre of Materials and Process Synthesis (COMPS) at The University of Witwatersrand, a research center that has developed an optimized F-T technology being applied in the Baodan Liquid Fuels Plant in Baoji, ShaanXi Province China, and that has been licensed by Canada-based Alternative Fuels Corporation (AFC) for further deployment. (Earlier post.)
The Fischer-Tropsch process, which catalytically converts synthesis gas to synthetic fuels and chemicals, is “very inefficient in that a large part of the carbon fed into the process ends up as CO2, either directly or indirectly from fuel consumption for heating the reaction.”
Hildebrandt and her colleagues viewed the coal-to-liquids process as a heat engine:
The first step is a high-temperature endothermic reaction that converts the solid coal into gases. This heat input, by virtue of its temperature, carries work Win that must be equal to or greater than the Gibbs free energy change of this process. The second step, the FT synthesis reaction, is a low-temperature exothermic process that emits heat and carries work Wout out with it. Again, Wout should be equal to the Gibbs free energy of this reaction for this step to be reversible. The net work for the overall process is the difference between Win and Wout and is equal to the change in the Gibbs free energy of the overall reaction.—Hildebrandt et al. (2009)
Conventional F-T processes require too much work to be put in the gasifier and emit too much work from the F-T reactor. Gasification adds more than four times as much as theoretically required and nearly three times as much even with co-generation of electricity. While the theoretical minimum net work for a process that would produce 80,000 barrels of liquid fuel per day is 350 MW, the net work for a conventional F-T process of that scale is 1,000 MW, with co-generation.
More efficient operation requires decreasing both Win and Wout. A way to run both reactions to achieve this goal is for the gasifier not to produce CO and H2 but rather CO2 and H2, which is a less endothermic process. Furthermore, making fuel from CO2 and H2 is less exothermic. The synthesis reaction may not go directly via the new gas mixture, but when combined with the reverse water-gas shift reaction, which converts CO2 and H2 to CO and H2O, the process is feasible.
Water can be recycled in both cases, and in the second process, can pump heat back into the gasification section. The CO2 gasification process requires adding about 20% less work to the gasifier than would be required by the CO route. If work is recovered from the heat rejected from the synthesis reactors, the net work required by the overall process in an 80,000 barrels per day facility is 820 MW—nearer the optimum (350 MW) than the conventional route (1000 MW). This process would produce 0.5 MT less CO2 per year than the conventional route (15% reduction).
Note that the second part of the new process also represents a direct way of using CO2. If H2 is produced via nuclear, wind, or solar energy, this process becomes a method for consuming CO2 and may bypass the difficulties in the direct use of H2 as a fuel. Technological advances developed for CTL readily transfer to processes for converting natural gas to liquids, and eventually could be adapted to biomass sources.—Hildebrandt et al. (2009)
Diane Hildebrandt, David Glasser, Brendon Hausberger, Bilal Patel, Benjamin J. Glasser (2009) Producing Transportation Fuels with Less Work. Science Vol. 323. no. 5922, pp. 1680 - 1681 doi: 10.1126/science.1168455