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 W_in 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 W_out out with it. Again, W_out 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 W_in and W_out 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 W_in and W_out. 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)

Resources

• 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