Methanol from Limestone?

Thanks in part to my esteemed friend Craig I've been drinking a shot or two of the Peak Oil Kool-Aid. One of the things he mentions is how great methanol is as a post-petroleum renewable fuel since it's an almost direct substitute for gasoline, with many of the benefits that other synthetic fuels (hydrogen, anhydrous ammonia, etc.) are lacking...save for the minor problem of actually getting our hands on the CO2 necessary to economically make it from hydrogen (preferably the atmosphere). The folks at LANL are working on a CO2 sequestration method that involves tossing a bunch of quicklime (CaCO) in the air, where it absorbs atmospheric CO2 and turns into, more or less, limestone. An endothermic (but not break-the-bank endothermic) reaction turns limestone back into quicklime and liberates the CO2. Meanwhile, back at the Bat Cave, we're taking some sort of renewable electric power (e.g. wind power, which if you've drunk enough of the Kool-Aid is cheaper than coal/nuclear/etc.) and are electrolyzing water. Mixing H2 and CO2 generates methanol and water (a third of what we started with). In the end, you're turning limestone, heat, electricity, and water into quicklime and methanol. Numbers-wise, you'd need 2.5kg of limestone, 900ml of water, and about .62 kilowatt-hours (net) of electricity to make a liter of methanol, plus a fair amount of oxygen. Let's see, the limestone gets recycled, water is cheap, and that much electricity would cost a little over a penny if you don't necessarily need it 24/7. That works out to about 5 cents a gallon...clearly too good to be true. What am I missing here?


At 5:04 PM, Blogger Frank said...

Note that the LANL folks figure a cost of about US$.20 per gallon of gasoline (a bit more energy than a gallon of methanol) as the cost to sequester the carbon that one has so thoughtlessly spewed into the atmosphere.

So clearly the quicklime process is a bit more energy intensive than the delta-H's would suggest.

Likewise with the electrolysis: The practical electrolysis of water is supposed to use about 0.066 kilowatt-hours per mole. We need about 75 moles to make our liter of methanol, which is about 5 kilowatt-hours, or about 38 cents per gallon.

So in the end you're talking under 60 cents a gallon, which is still pretty sweet.

At 5:26 PM, Blogger Frank said...

Although 60¢ per gallon sounds cheap, the efficiency isn't great. You're only getting out about a quarter of the energy you put in on a delta-H basis, and probably a lot less in reality. Plus squirt-and-boom combustion is maybe 35% efficient on a good day.

It's not nearly as good as an efficient electric car, which could also charge up on get-it-when-it's-cheap grid electricity. There you'd be paying about 10¢ for the same amount of travel. On the other hand, that electric car costs a lot and needs a charging break (30-60 minutes at a minimum) after every 100-odd miles of driving.

But a liquid fuel's crappy energy balance comes without all the caveats of hydrogen and it's ilk.

At 2:34 PM, Blogger Frank said...

Just a note: mixing H2 and CO2 generates methanol, water, and a soup of all sorts of other organic molecules, like formaldehyde. So this step certainly isn't a drop-kick.


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