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UCLA team uses engineered microbe and electricity to convert CO2 to higher alcohols

Liao
An integrated electromicrobial process to convert CO2 to higher alcohols. Electricity powered the electrochemical CO2 reduction on the cathode to produce formate, which is converted to isobutanol and 3MB by the engineered R. eutropha. Li et al. Click to enlarge.

Researchers at UCLA have demonstrated a method for converting carbon dioxide into higher alcohols using electricity. In a study published in the journal Science, James Liao, UCLA’s Ralph M. Parsons Foundation Chair in Chemical Engineering, and his team report a method for storing electrical energy as chemical energy in higher alcohols, which can be used as liquid transportation fuels.

Liao and his team genetically engineered a lithoautotrophic microorganism, Ralstonia eutropha H16, to produce isobutanol and 3-methyl-1-butanol (3MB) in an electro-bioreactor using CO2 as the sole carbon source and electricity as the sole energy input.

The process integrates electrochemical formate production and biological CO2 fixation and higher alcohol synthesis, opening the possibility of electricity-driven bioconversion of CO2 to commercial chemicals. The study was funded by a grant from the US Department of Energy’s Advanced Research Projects Agency–Energy (ARPA–E).

A man-made photovoltaic device is relatively efficient in converting sunlight to electricity, but the electrical energy generated is difficult to store. The biological photosystems, on the other hand, are limited by the intrinsic design and biomaterials available, for which no near-term improvements are in sight. One way to circumvent both problems is to link man-made solar cells to biological CO2 fixation and fuel production. Theoretically, H2 generated by solar electricity can drive CO2 fixation in lithoautotrophic microorganisms engineered to synthesize high energy-density liquid fuels. However, the low solubility, low mass transfer rate, and safety issues of H2 in microbial cultures limit the efficiency and scalability of such processes.

Compared with H2, formic acid is a favorable energy carrier. Electrochemical production of formic acid from CO2 and H2O can achieve relatively high efficiency. Formate is highly soluble and is readily converted to CO2 and NADH in the cells, providing a safe replacement for H2. However, the high solubility of formate increases the cost of separation. Accumulated formate will decompose at the anode, decreasing the yield of the process. Therefore, simultaneous electrochemical formate production and biological formate conversion to higher alcohols is desirable. Unfortunately, introduction of electricity to microbial cultures may impede cell growth.

—Li et al.

According to the team, an integrated process for reduction of CO2 to liquid fuel powered by electricity requires:

  1. metabolic engineering of a lithoautotrophic organism to produce liquid fuels,

  2. electrochemical generation of formate from CO2 in fermentation medium, and

  3. enabling microbes to withstand electricity.

Liao and his colleagues introduced a set of genes previously identified for the production of isobutanol and 3MB into R. eutropha, and altered the organism to use the heterologous isobutanol and 3MB production pathway as the new metabolic sink for carbon and reducing equivalents. In a pH-coupled formic acid feeding fermentor, the engineered strain produced fuels with the final titer of more than 1.4 g/l (~846 mg/l isobutanol and ~570 mg/l 3MB).

When they placed a cathode and anode in the culture medium to produce formate electrochemically, however, they found the growth the bacteria was inhibited upon introduction of the electric current. When the current stopped, cell growth resumed.

The researchers determined that O2 and NO in the medium triggered a stress response in Ralstonia, inhibiting growth. They then used a porous ceramic cup to shield the anode. The inexpensive shield provides “a tortuous diffusion path” for chemicals, enabling the reactive compounds produced by the anode to be quenched before reaching the cells growing outside the cup.

With this approach, the engineered Ralstonia produced more than 140 mg/l biofuels were achieved with the electricity and CO2 as the sole source of energy and carbon, respectively.

The current way to store electricity is with lithium-ion batteries, in which the density is low, but when you store it in liquid fuel, the density could actually be very high. In addition, we have the potential to use electricity as transportation fuel without needing to change current infrastructure. We’ve demonstrated the principle, and now we think we can scale up.

—James Liao

Resources

  • Han Li, Paul H. Opgenorth, David G. Wernick, Steve Rogers, Tung-Yun Wu, Wendy Higashide, Peter Malati, Yi-Xin Huo, Kwang Myung Cho, and James C. Liao (2012) Integrated Electromicrobial Conversion of CO2 to Higher Alcohols. Science 335 (6076), 1596. doi: 10.1126/science.1217643

Comments

HarveyD

Eventually, the CO2 produced at every coal fired power plants could be stored and used at night time with surplus electricity to produce liquid fuel? Interesting potential.

kelly

If a $1000 of 'engineered microbe and electricity' convert CO2 to a dollar of higher alcohols - there's not much gain.

Numbers and bottom lines are determinate and missing.

Herm

Like other "waste" products, CO2 may turn out to be too valuable to discard.

Butanol is too expensive and useful to be burned up as a fuel.

Engineer-Poet

Depends how much of it you have, Herm.  Gasoline used to be a waste product of the production of lamp oil.

Butanol doesn't self-separate until its concentration reaches several percent, so one has to wonder how much energy and equipment it will take to extract pure product from this scheme.  Ralstonia eutropha is a bacterium, not an archaebacterium, so it probably cannot be directly spliced with the electro-autotrophic archaea which have been demonstrated to make methane from electricity and CO2.

Arne

"In addition, we have the potential to use electricity as transportation fuel without needing to change current infrastructure."

That is highly doubtful.

This process probably has an overall efficiency of 15 or 20% (electricity-to-wheel). We simply can not afford to be that wasteful.

JMartin

Why would we want to keep our current infrastructure of gas stations and pipelines? Go electric.

Engineer-Poet

There are some uses where that makes sense, Anne.  Backup systems, extended-range (costly but seldom used), high-value applications.

If the electro-bacterial system is cheap enough to make, it can be used as a dump load.  Cheap dump loads making high-value products allows the generators to be overbuilt without having to throw power away; they set a floor under power prices.

HealthyBreeze

@Anne,

You're quite on point. If this process were incredibly efficient, and the CO2 from flu gas could be stored (to very big "Ifs"), I suppose an additional value of pollution abatement could be garnered for carbon credits or somesuch. The algae guys were looking at getting paid to take the CO2 away from the power plant, and getting paid for the resulting biodiesel, ethanaol, protein animal feed and fertilizer they imagined they could get.

HealthyBreeze

One factor I forgot to mention on my big list of "IFs"...If it is a coal-fired power plant with a lot of wasted nighttime base-load power that could be captured by this process...

...probably and IF too far.

Engineer-Poet

Think nuclear or wind, not coal.

joe.vanni

Wind and sun are a better thought.

The emissions of coal plants are not only CO2, but there are large amounts of other dangerous poisons. Should be closed as soon as possible.
There are other sources from which to derive CO2

Arne

@Engineer-poet

I agree that in niche applications or where no practical alternatives are available (think aviation fuel), this can make sense. However, when they are talking about 'transportation fuels' that suggests widespread use as automotive fuels. But that is an interpretation from my part.

@Healthybreeze,

One thing to bear in mind is that you have to make a choice. Either you consider this process as carbon sequestration from a coal plant, but then the resulting fuel is fossil (not biodiesel). Or you consider the fuel carbon-neutral, but then it doesn't count it as carbon sequestration for the coal plant.

I have noticed in past discussions that people tend to count it double, saying, it gets rid of the CO2 emissions of the coal plant AND it is a carbon neutral fuel. But in essence, you are still burning coal, so it can never be carbon neutral.

And your suggestion to let a coal plant run just for generating electricity for this process is a really bad idea.

The only value I can see for this process is if you can have small reactors situated in areas with high concentrations of renewable energy. In case of overproduction of renewable energy, instead of curtailing you can use the electricity to feed this process. In that scenario, the electricity can be considered 'free'. But I can not see a business case for it. In reality curtailment occurs rarely (which is a good thing) and such reactors would only run for a few days per year.

As soon as you start running this process off the grid, it carries the embedded CO2 from the electricity.

HarveyD

It could become a way to:

1) recycle damaging CO2
2) use excess electricity during low load periods.
3) increase power plants efficiency with steady load.
4) store excess electricity and CO2 into compact useful liquid fuels/chemicals.

Recycling CO2 is considered green and it could get compensation at the rate of $50 to $100/ton to make the process a better business.

HealthyBreeze

@Anne,

I take your point that using coal-plant flu gas as feedstock for bioreactors is really just using the carbon twice before letting it into the atmosphere, but it still increases atmospheric CO2. Pumping water uphill for later Hydropower usage would or somesuch would be better ways to avoid wasting excess base-load energy.

Engineer-Poet

Anne, it might have a place even in segments of major applications.  For instance, if e.g. 75% of energy for vehicles comes from electricity, such fuels might be economical for the other 25% in PHEVs.

As for carbon sources, there are non-fossil options that are much cheaper than atmospheric capture.  Consider separation of CO2 from landfill gas; conversion of the CO2 to alcohols using excess wind or nuclear electricity would both store that energy for later use and be carbon-neutral over the medium and long term (negative over the short term).

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