The Problem With Biobutanol

The Essay I Didn’t Want to Finish

Note: I will insert references a bit later. I have to dig them up.

This is an essay that I have been promising for some time. I have had to start it from scratch a couple of times, and I am starting it from scratch again now. I tend to get a lot of e-mail about biobutanol, especially after people read the essay that I wrote on the subject last year:

Biobutanol

However, I did write in that essay:

I need to spend some time going over the patents and linked reports more closely to see if anything suggests a problem that has been glossed over.

When I first started this essay, I was going to review the literature and outline what I felt needed to happen to make biobutanol a reality. Furthermore, since butanol is about 8% soluble in water, I was going to write that a real holy grail would be to find a microorganism that can tolerate 8% butanol, or one that produces a higher alcohol - one that is completely insoluble in water. That would allow the alcohol that was produced to phase from the water, and eliminate the energy intensive distillation that is typically required. That’s what I was going to write. However, reality butted in.

My Initial Thoughts

Just for completeness, here is what I had originally written:

There has been much press lately about the potential of butanol as an alternative fuel. As I have mentioned before, I spent many years as an engineer in butanol units in Texas and in Germany, and I have received a patent for a butanol process that I invented while working in Germany.

Butanol production via petrochemicals is a very straight-forward process. Once the petro-process was invented, it put the bio-process (ABE) out of business. My guess is that due to the nature of biological processes, the product from the ABE process contains copious amounts of water [I was right]. In contrast, the petrochemical process generally contains product with only 5-10% water. Therefore, the energy requirements for purification are much lower for the petrochemical process.

If butanol could be produced without having to purify it via an energy-intensive distillation, the energy return would be much higher and the costs should be lower. If butanol was completely insoluble in water, for instance, it would float to the top as it was produced and it could just be skimmed off. However, butanol is about 8% soluble in water, which means it won’t start phasing until that concentration is reached. And for a biological process, 8% butanol would most likely poison the microorganisms that were producing it.

However, longer-chain alcohols – pentanol, hexanol, hetpanol, etc. - are essentially insoluble in water. These alcohols would phase out and float to the top as they were produced. Separation would be a snap.

The major problem with butanol is that it doesn’t start to phase out of water until the concentration reaches about 8%. If there was a yeast-based process for butanol that could tolerate concentrations higher than this, you would have the makings of a very cost effective process. Do the fermentation, and then just skim off concentrated butanol. This would be much more energy efficient than a distillation.

What I Learned

While butanol is absolutely a superior fuel to ethanol, the production of butanol from microorganisms is a problem. As I was dreaming about pushing concentration to the point that they start to phase out, a little research showed the current status quo. Here was the first reality check:

Significant improvements in acetone-butanol (AB) fermentation by Clostridium acetobutylicum must be achieved before it can become an economically viable industrial process (8, 12). Key factors which contribute to the elevated costs of fermentative production of acetone and butanol are the low product titers and low product selectivity. Butanol inhibits cell growth even at relatively low concentrations, and its final titers are limited to ca. 13 g/liter. This and the low product selectivity (i.e., the production of more than one product) result in increased costs for product separation. In addition, continuous cultures have been of limited applicability because solventogenic clostridia degenerate under continuous-culture conditions; that is, they stop producing solvents.

13 grams per liter is about 1.3% butanol. While we can expect improvements in the technology, the yield needs to go up by almost an order of magnitude to keep the distillation energy (and costs) reasonable. While one may be able to push the conversion to a high level with extreme dilution, such an approach is simply unworkable from an economical or energy return viewpoint. I won’t address the paper in-depth - I leave that up to others for now - but you can see that this was the approach taken in the paper that is basically responsible for the current biobutanol craze in which 2.5 gallons of butanol per bushel of corn was claimed:

Effects of Butyrate Uptake and Long-term Stability of a Fibrous Bed Bioreactor on Continuous ABE Fermentation by Clostridium acetobutylicum

Here is a graphic that shows the extent of the problem:

Note that the concentration of butanol achieved 1.3% for one specific strain. Typical butanol concentrations were just over 1.0%. The total concentration of acetone, butanol, and ethanol amounted to 1.8%. For reference, the petrochemical process produces a product that is somewhere between 85% and 95% butanol. Now you can see why the biochemical process was dropped when the petrochemical process came along. If your objective is merely to produce butanol for some specific application, then the fermentation process can do that - with enormous energy inputs. If your objective is to make fuel with an EROEI of > 1.0, it is nowhere close to being able to achieve that.

Higher Alcohols

How about my desire to find microorganisms that could make higher chain alcohols that could be phase-separated?

As a follow-up to earlier studies on the emission of long-chain alcohols from broth cultures of Gram-negative enteric bacteria, E. coli was examined for the production of 1-octanol, 1-decanol, and 1-dodecanol. Ten strains of E. coli cultured in tryptic soy broth were assayed for volatile metabolites using solid-phase microextraction. Long-chain alcohols were produced by all strains with 1-decanol predominating with production ranging from 23.6 ng mL(-1) to 148 ng mL(-1). The production of long-chain alcohols followed the onset of the exponential growth phase of the broth culture. Doubling the concentration of glucose (5 g L(-1)) in the broth had no effect on the concentration of long-chain alcohols produced.

I would have to check on each one to be sure, but my guess is that even though the higher chain alcohols are essentially insoluble, 148 ng/mL (0.0148%) is not going to phase out. I was hoping to find that something in the range of 0.5-1.0% could be produced. However, 1% would require the process to become about 70 times as productive. So, it looks like we will colonize Mars before higher chain alcohols will be produced commercially from microorganisms. (Gasification may still have a shot at doing it economically).

Conclusion

Sad to say, but I believe biobutanol is dead. While research will (and should) continue, the process is currently at least 10 years from any sort of commercially feasibility. And I would point out that “never” falls under the umbrella of “at least 10 years.”