Researchers Propose New F-T Process for Synfuels; Less Work Required Could Result in 15% Reduction in CO2 Emissions Compared to Conventional Route
28 March 2009
|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
Hopefully the finished fuel would have an EROEI of at least 4 after taking into account the effort needed to get extra hydrogen. If the carbon source was biomass and not coal it would be recycled within the biosphere with no addition from underground sources. The next step would be to scale down the process so it can be done in forestry and farming areas.
Posted by: Aussie | 28 March 2009 at 02:02 PM
This is really good. You have a time tested process that they looked at in a different way and came up with this. I like biomass and solar generated H2 and O2 for added yields. You could also use concentrated solar thermal to preheat the biomass. It seems to be a cross over point with costs and the price of oil.
Posted by: SJC | 28 March 2009 at 02:50 PM
I don't see how this differs from the initial stages of the proposed FutureGen plant, which produces a mixture of CO and H2 and then uses the water shift to create a CO2 and H2 mixture. FutureGen supposedly then separates the CO2 from the H2 (and from the H2S, don't forget) and uses the H2 all by itself - supposedly, because it has never been demonstrated in practice.
Second, I don't see how this deals with the other F-T problem - the synthesis of a very wide range of hydrocarbons, which then must be separated - drawing even more energy.
Third, the process doesn't really work that well when the coal is highly contaminated with sulfur and arsenic, as the catalysts involved get poisoned.
Finally, this is purely theoretical study with no experimental proof-of-concept - and EROEI's are notoriously susceptible to manipulation, depending on what is and is not counted as a cost.
Without a working prototype, all it is is hand waving. Who says that the thermodynamic efficiencies at each stage can really be reduced that much? We can imagine efficient heat transfer and waste heat recycling can improve things quite a bit, as was the case with newer ethanol and biodiesel plants, but that seems unlikely with coal - look at the history of FutureGen - a complete technological flop, so bad they won't even release even basic performance data (even though they are still trying for a couple billion in direct taxpayer financing).
Posted by: Ike Solem | 28 March 2009 at 07:15 PM
I do not think this can be compared with FutureGen, the objectives and methods are different. They are going for CO2 from the start to lower the energy required.
The article states "being applied in the Baodan Liquid Fuels Plant in Baoji, ShaanXi Province China.." so it is not like this is just theoretical. This plant sounds like it is more than a prototype.
Posted by: SJC | 28 March 2009 at 09:53 PM
I think there is a lot of hand waving on the part of some respondents to this comment section. Some folks just can't stand anything having to do with coal no matter what. Preconcieved biases and good science don't mix.
It would help to do some background research before shooting off at the hip. Reading the March 27 issue of the Journal of Science to look at the actual paper might be a good starting point?
Posted by: Mannstein | 29 March 2009 at 11:58 AM
Posted by: Engineer-Poet | 31 March 2009 at 11:10 PM