Solar Fuels: Making hydrocarbon fuels directly from CO2 and sunlight

July 6, 2015 | Jim Lane

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.

And lately, more researchers have been delving into the field.

Can it really be done?

For some time, researchers have known that the green algae known as Botryococcus braunii are capable of accumulating huge quantities of hydrocarbons. In fact, as this “cool read” review of b. braunii states, “the best record was 86% of its dry weight for an algal sample harvested from a natural bloom.”

B. braunii’s magic? Ir has a knack for accumulating hydrocarbons in the “extracellular space” instead of “in the cytoplasm” — which is to say, it puts all the hydrocarbons on the driveway because the hall closet’s too small.

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.

So work on the organism continues, but it inspires the question. Could other aquatic or even terrestrial plants ever be trained to directly produce a fuel?

To which the answer is, they already do — what we need to find is a way to make them produce it faster and in higher concentrations. Which is no simple task, as it turns out. That’s what Joule’s up to, more or less. In their case, they’ve done a lot of work to build an organism from the ground up.

So, how close are we and what are the limitations?

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

Over at Joule, we seen some remarkable results reported on the first three fronts — with photosynthetic efficiency and biocatalysts.

According to a report last year, Joule has successfully engineered “a photosynthetic biocatalyst able to divert 95% of fixed carbon normally converted to biomass directly to fuel.”

Joule noted at the time: “Prior research has generally capped the photon energy conversion efficiency of photosynthetic processes at 2 – 3%. By contrast, Joule has applied a systems approach that spans biocatalyst, reactor and process engineering to negate the effects of these conditions, resulting in many-fold greater energy conversion efficiencies and supporting Joule’s estimated maximum of 14%.”

Storage? In most cases, we’ve seen researchers focus on secretion. That is, the cell won’t contain all the hydrocarbons we want to produce, so it’s secreted outside of the cell, ready for harvest. That’s the “milk the cow” rather than “shoot the cattle” approach to harvesting fuels from living cells. Algenol and Joule ar both working on that technology.

Joule, Algenol and their target molecules

Algenol’s demonstration system in Florida.

We know that Joule and Algenol are both producing ethanol — both have the capability to produce diesel-range molecules and other renewable chemicals, but we are not clear on the timelines and progress to date. Which is to say, they can make them, but we’re not clear on whether they can a) make them at commercial-feasible costs or b) make them at commercial-scale. Stay tuned on this channel.

The Joule system – a scaled process, as of October 2014, in Hobbs, NM.

Commercial-scale

Algenol and Joule are both expected to commence commercial-scale construction this decade — in the case of Algenol, we dont have a specific date; Joule says construction will commence in 2017.

What can be expected?

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.