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Researchers Propose Hydrogen-Augmented Fischer-Tropsch Processes; More Product, No CO2

Agrawal1
One of the possible configurations of the proposed H2CAR process. Click to enlarge.

Purdue University chemical engineers have proposed a modification to the conventional biomass- or coal-to-liquids Fischer-Tropsch process that could, by their calculations, produce sufficient fuel for the entire US transportation sector.

Their process, however, relies on an abundant supply of hydrogen—between 239 and 276 billion kilograms of hydrogen per year—to produce 13.8 million barrels of synthetics per day. The new approach—which the researchers call a “hybrid hydrogen-carbon process,” (H2CAR)—proposes co-feeding a gasifier with hydrogen from a carbon-free energy source, such as solar or nuclear power, and CO2 recycled from the syngas (hydrogen and carbon monoxide) conversion reactor.

The H2CAR process offers a number of advantages compared to conventional biomass- or coal-to-liquids technologies according to models the team developed:

  • No CO2 is released to the atmosphere or is required to be sequestered as a result of the chemical processing system. When conventional methods are used to convert biomass or coal to liquid fuels, 60% to 70% of the carbon atoms in the starting materials are lost in the process as carbon dioxide whereas no carbon atoms would be lost using H2CAR.

  • Approximately 40% of the amount of coal or biomass is needed to deliver the same quantity of liquid fuel. This is advantageous for prolonging the life of the known coal reserves as well as in reducing the land area needed for the bioenergy crop. The land area required to grow the biomass for H2CAR is accordingly less than 40% of that needed by other pathways that solely use biomass to support the entire transportation sector.

  • Current estimates suggest that an annual US biomass supply of 1.366 billion tons could produce approximately 30% of the United States transportation fuel with conventional processes. The H2CAR process shows the potential to supply the entire United States transportation sector from that quantity of biomass.

  • The synthesized liquid provides H2 storage in an open loop system. The addition of hydrogen atoms to carbon atoms from coal or biomass provides a high-density method for storage of massive quantities of hydrogen. On a carbon atom basis, the energy content of the liquid fuel is higher than that of coal or biomass.

The process is detailed in an open access research paper appearing online this week in the Proceedings of the National Academy of Sciences.

The researchers explain the rationale of using H2 in the H2CAR process by the significantly higher annualized average solar energy conversion efficiency for hydrogen generation versus that for biomass growth.

Production of 13.8 Mbpd of synthetic oil by using biomass
CaseGasifier
efficiency
%
Biomass land area
million km2
Req. H2
billion kg/yr km2
H2 land area
thousand km2
Carbon
efficiency %
Energy
efficiency %
Conventional-I 50 5.30 0 0 26.2 29
Conventional-II 70 2.51 0 0 36.7 40.6
H2CAR-I 50 1.41 276 62 ~100 52.7
H2CAR-II 70 0.92 239 54 ~100 58

I’m saying, treat biomass predominantly as a supplier of carbon atoms, not as an energy source.

—Rakesh Agrawal, Purdue University
Argawal2
Land-area and H2 requirement for conventional and process using PHEVs as a function of drivable distance traveled per single full charge H2CAR of batteries. Click to enlarge.

Agrawal argues that the new process also would be more practical than all-electric or hydrogen-powered cars, in part because of the limited storage capacity of batteries and hydrogen storage tanks.

The tremendous convenience provided by the existing infrastructure for delivering and storing today’s fuels is a huge deterrent to introducing technologies that use only batteries or hydrogen alone. A major advantage of our process is that it would enable us to use the current infrastructure and internal combustion engine technology. It is quite attractive for hybrid electric vehicles and plug-in hybrid electric vehicles.

—Rakesh Agrawal

Realizing this requires significant research in two areas: finding ways to produce cheap hydrogen from carbon-free sources and developing a new type of gasifier needed for the process.

The proposed H2CAR-based processes also have a strong impact on the future areas of research. The primary research emphasis needs to be on cost-effective H2 production from a carbon-free energy source such as solar or nuclear. In addition, efficient, low-cost, and easy-to-operate methods are needed for the conversion of biomass through reaction with H2 to a suitable hydrocarbon liquid fuel. In the short term, the same is true for the conversion of coal to liquid. The current conversion route of gasification followed by a H2-CO liquid conversion reaction is quite inefficient, and an alternative efficient hydrogenation process is highly desirable.

In the mean time, until such alternate processes are discovered, the preservation of carbon atoms in the current gasification and H2-CO liquid conversion reaction is essential. A proposed solution in this work is to co-feed H2 and recycle CO2 from the H2-CO liquid conversion reactor to the gasifier. Feasibility and development of such gasifiers especially for biomass will require extensive research.

Purdue has filed a patent for the concept. The approach is in the conceptual stages, and a plan for experimental research is in progress.

Resources:

Comments

Karkus

Maybe I'm missing something here, but here's my problem with this story:

This method relies on an abundant supply of hydrogen! Last time I checked, we didn't have that. If we had it, a lot of our problems would be solved - we wouldn't need to make biofuel, we could just burn the hydrogen, which would be more efficient.
Unless you make hydrogen from renewable sources, it will produce lots of CO2 (or nuclear waste). In addition, making hydrogen from renewable sources is less efficient than making electricity. It would be more efficient to just run things on electicity.
Bottom line, I don't see why this story is getting such good press. Saying "we invented this great fuel process! (assuming there's lots of hydrogen available)" is kind of like saying " I just invented a cure for poverty ! (assuming everybody had a job that made a million bucks a year)

Karkus

OK, sorry for that premature post - I guess I do see the point now, after a more careful reading of the story. They're trying to use this more efficient fuel process because of the limited hydrogen/electricity storage and infrastructure capacity, which limits things like how far cars can travel.

Still, I figure it will take time to demonstrate/scale this up, and by that time battery technology might be far enough along to the majority of cars electric .

mahonj

You could put a nuke plant near one of these plants assuming it was not in NY or wherever.
However, you might have problems with windmills.

On the other hand, if you found a place that was windy and had a lot of coal / biomass, you might have a solution.

Also, if you developed a good way of large scale H2 generation and fuel cells got cheap, you would be in business. On the other hand, if you could generate carbon based liquid fuels, they would be easier to transport than H2.

Butanol anyone ?

Rafael Seidl

Technologically, you can produce hydrogen using renewables (e.g. PV solar, wind, hydro) but using nuclear looks a lot cheaper. I say "looks" because it's only true if you sharply discount the future cost of permanent waste disposal and decomissioning and you assume there will be no major accidents.

Technologically, you can also produce the syngas required for FT using biomass but it's definitely cheaper to do using coal (as South Africa has done for decades). Provided you add enough hydrogen, you will indeed not be emitting any CO2 at the FT plant. However, you absolutely will do so at the tailpipe of the cars and trucks using this clean-burning but very expensive designer fuel. CTL can never lead to lower emissions of CO2 than transportation fuels based on oil or gas.

Ergo: FT based on biomass + H2 from renewables could prove ecologically sustainable but it would be extremely expensive. FT based on coal + H2 from nuclear would be cheaper in the short term and make various lobbyists and politicians rich and happy. However, in ecological terms it's arguably worse - certainly no better - than the status quo.

That said, neither EV technology nor cellulosic ethanol nor biodiesel from algal oil are ready for prime time, so funding basic research into optimizing FT process yields is a good way to hedge your bets. Just remember to ask "cui bono?", i.e. who stands to gain?

Andrey

Great idea. Off peak electricity from already operational nuclear power plants costs nothing, conventional electrolysis will produce not only hydrogen (no matter the low efficiency, uranium is still dirt cheap), but oxygen too for more efficient gasification.
It could be built today. Bonus of biomass versus coal is that biomass is very low in sulfur.

Oulisee

I would agree with this approach once they develop thorium reactors:
http://www.cosmosmagazine.com/node/348

Neil

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