Apr
16
Exploring a New Gasification Process
April 16, 2009 | 5 Comments
Diane Hildebrandt, the Director of the Centre of Materials and Process Synthesis at The University of Witwatersrand and colleagues at Rutgers University are proposing new Fischer-Tropsch (F-T) reaction chemistry and process designs. They are saying in their letter published in Science Vol. 323. no. 5922, pp. 1680 – 1681 doi: 10.1126/science.1168455 that the new reaction chemistry and process designs could increase F-T process efficiency and reduce CO2 emissions by 15% compared to the conventional processes in use today.
The new design uses a carbon dioxide and hydrogen route rather than the traditional carbon monoxide and hydrogen route. It could also open up a process pathway for the direct use of CO2 and H2 derived from low-carbon processes such as nuclear, wind, solar, or biomass. A brief description of the thermodynamic analysis proposal was published in the March 27th issue of the journal Science as well.
The issue with the Fischer-Tropsch process, which catalytically converts synthesis gas to synthetic fuels and chemicals, isn’t particularly efficient because a large part of the carbon fed into the process ends up as CO2, either directly or indirectly from the fuel used for heating the reaction.
Fischer Tropsch Section in China Click image for more info.
Professor Hildebrandt has announced that the research center’s optimized F-T technology is being used in the Baodan Liquid Fuels Plant in Baoji, ShaanXi Province China, and it has been licensed by Canada-based Alternative Fuels Corporation for further deployment.
Hildebrandt and her colleagues’ take on the coal-to-liquids process is to see the process as a whole, a large heat engine. Hildebrandt says in the published letter, “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.”
She’s saying the team is very conscience of the energy inputs, mapping if you will allow, the trail of energy through the process. They are answering the matter that 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. Hildebrandt points out that a conventional F-T process as ran today producing 80,000 barrels of liquid fuel per day is 1,000 MW, even with co-generation to use some process heat. Study and the team’s research show the theoretical minimum net work could be 350 MW per day.
Hildebrandt explains in the letter, “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.”
She adds another positive attribute to their design, “Water can be recycled in both cases (gasification and reforming the fuel), and in the second reforming 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, a 15% reduction.
Now here is the most interesting part, the study and research has a payoff that could coincide with other energy developments, “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 coal to liquid fuels readily transfer to processes for converting natural gas to liquids, and eventually could be adapted to biomass sources.”
The significance of this is quite substantial. Gasification and the reforming techniques are energy intensive and also throw off or waste an abundance of CO2 which is a prime resource as its coming out concentrated, heated and ready for reforming. The needed energy inputs and available energy outputs currently have a gap of major proportions. The result of the scientists work has come a long way to closing the gap and finding ways to amplify the value of gasification and reforming.
The University of the Witwatersrand has an interesting set of pages that explore the research, where it’s going and how they got to this point. What is missing is the proprietary information; obviously, so the internal details about the catalysts, how they are used and other internal information is absent.
Yet the gasification path to fuel production from a wide array of sources isn’t going to go away, rather it will be an important part of the future. The work that the group has done closed the energy use gap by about a quarter already and they seem to see the methods that can close it further. It might be, if the economic conditions with increased taxing and government spending reducing available capital, that gasification might be the most practical way to use coal and biomass to add to or replace fossil fuels.
Those tough economic conditions have occurred before; they drove the early Fischer-Tropsch research and development such that the technology works. Gasification on to Fischer-Tropsch absolutely works, and each step made to economize and optimize offer more new fuel sources for the future.
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Great read. Thanks for the info!
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