Solar Fuels: Making Hydrocarbon Fuels Directly from CO2 and Sunlight

by Jim Lane (Biofuels Digest) Drop-in Hydrocarbon fuels made directly by plants? Here. we look at the science and risks of turning an “ability” into an “industrial reality”.

It’s common knowledge that terrestrial plants (and many aquatic species) use carbon dioxide, water and sunlight to make the molecules they need — using a complex series of metabolic pathways established and controlled by their genetic code. And its common knowledge that there’s carbon in CO2 and hydrogen in water.

So, why don’t we teach plants to produce hydrocarbons, directly, usable for fuels and chemicals with which we power our industrial lives?

It’s an “oft-praised, not-so-oft-seen” alternative to using sugars, starches or oils — that is, the materials that plants produce, to make fuels, chemicals and materials. Turns out that cutting out the middle step and producing hydrocarbons out of the box is, as the Australians would say, “hard yakka” (tough sledding).

This is the goal which has animated Joule, a stealthy technology developer in Boston which has attracted legions of supporters and detractors — developing a system which photosynthetically grows a modified cyanobacteria that produces ethanol, diesel or jet-range molecules, or even a large array of renewable chemicals.

You might well ask, if a ton of b. brauniican contain up to 1720 pounds of hydrocarbons, or about 230 gallons of hydrocarbons for fuel, why aren’t we all driving around on it?

Three limitations. First, b. braunii makes a slightly complex form of hydrocarbon that needs to be “cracked down” to the fuel range. Second, no one has yet worked out a commercially-affordable production and harvesting system for algal fuels — companies like Algenol and Cellana say they are close to cracking that problem, but not with our friend b. braunii. Third, our friend here grows sloooooowly.

According to this impressive review which appeared earlier this year in Plant Biotechnology, the challenges are four:

a) improving photosynthesis efficiency
b) fine-tuning the MEP pathway
c) optimizing key terpene enzymes
d) designing proper storage strategies

The sustainability equation. Though research into hydrocarbon production from terrestrial plants is ongoing, the timelines look long. And, we would expect that the Joule / Algenol approach, which uses non-productive land and non-potable water, is going to be the winner on sustainability hands down as long as the economics work.

So, think “photosynthetic organisms” for now, terrestrial plants maybe one day down the line, maybe never.

The CO2 equation. The systems that have been developed to date use direct CO2 fed from point sources. Think “climbing Mt. Everest” where the optimal result is from using a “bottled gas” supplement to make the trip. There’s been a lot of work on sequestering CO2 from the atmosphere — best we’ve seen is a mid-term target of $100 per ton, and with lower-cost CO2 available from point sources and plenty of it, it’ll to go that way, for now.

Who’s in the lead. For now, Joule and Algenol. But a growing cadre of researchers is working on this. We sure hope to see a highly-productive terpene-secreting organism one of these days. That really would be something.

Why make a $2 fuel when you can make a $5 chemical? The answer is simple, you make all the $5 chemicals you can, but one would rapidly run through all the available demand with a couple of scaled commercial faclities, in many cases.

Can it happen? It already does. Will it happen, industrially? A first-generation of the technology is expected in the next few years. READ MORE