Freeing Hydrogen From Glucose

tarawa writes: "This story at CNN reports that researchers have developed a new and easy way to extract hydrogen from a glucose solution that could provide a clean, environmentally safe fuel for our cars in the future." Stay calm, though -- ""We are not talking about spooning glucose into your car to make it go. That is 'Back to the Future' stuff."

Comments

Re:Questions for the chemists

[GMontag451]

How much heat do you get from C6H12O6 + 6O => 12H + 6CO2? And what is the molecular mass of C6H12O6?

First of all you wouldn't get 12H, you'd get 6H2. I don't know about the heat, but the molecular mass is pretty easy to figure out. 6C = 6*12. 12H = 12*1. 6O = 6*16. That makes 72+12+96=180. What I want to know is how they are keeping the H2 from combining with the O2 at those temperatures.

Two inaccuracies in the story

[GMontag451]

He says the process does not produce extra carbon dioxide, as this would have been released back into the air anyway through biodegradation of the plants.

This statement, which I suppose is technically true, is absolutely ludacris. It would be like saying that burning down the rainforest wouldn't produce any extra carbon dioxide for the same reason! If this process becomes widespread, even though it won't technically produce any more carbon dioxide than would have been produced, it will produce it a hell of a lot faster and there will be higher concentrations of it in the atmosphere.

At present in the laboratory bench process, hydrogen constitutes about half of the product from a catalyzed glucose solution. But with further refining of the sugar solution the percentage rises.

The percentage cannot rise above 50%. The reaction is C6H12O6 (glucose) + 6O2 -> 6H2 + 6CO2. And that's only 50% by volume. By mass its only 4.3% H2 and 95.7% CO2.

Re:Two inaccuracies in the story

[Sgt York]

Got a chance to read the article (the real one, not the CNN article)

The CO2 production/recovery by plants issue is well addressed in the first reply.

As for the 50% figure....

Glucose is C6O6H14, not H12. Sugars are CnOnH(2n+2) in these conditions. The H12 refers to the ring structure. They're saying they get almost, but not quite, 100% of the H2 recovered if they use glucose.

On other stuff in the letter (it's not a full paper)....

The guy even proposes a complete system, with their reactor connected to a fuel cell working at 50% efficiency (Engineers.....Is that realistic?). He states that based on the rate of H2 formation that they got at conditions for optimal efficiency, you could get 1kW of electrical power per liter reactor volume. The reactor would use bio waste (wood, grain chaff, etc).

He says the main drawback is the use of Pt as a catalyst; he says there may be better and/or cheaper ones.

The thing that really got me was the first line:

We consider production of hydrogen by low-temperature reforming (at 500K)

Since when is 227 degrees C a low temperature?

Re:Questions for the chemists

[troemyd]

According to my only readily available source ('Energy: a Guidebook' by Janet Ramage) the combustion of hydrogen releases 37 kWh/kg. That's more than twice the heat released from methane.

Re:More efficient is more CO2

[norton_I]

Except that glucose would be made by photosynthesis, which is a C02 consuming process. So this would not release any "fixed" CO2.

Possibly an answer

[Spamalamadingdong]

What I want to know is how they are keeping the H2 from combining with the O2 at those temperatures.

They don't, because there is no O2; they start with a water solution of glucose. The reaction would appear to go something like this:

C6H12O6 + 6 H2O -> 6 CO2 + 12 H2