It’s been years since I looked at this article I wrote on LanzaTech in 2007, but today I was made aware that it’s been linked to from an article in Biofuels Digest: Junk or treasure? Looking at carbon monoxide and LanzaTech. LanzaTech CEO Jennifer Holmgren had some comments referencing my previous article that are worth addressing. So let me summarize.
LanzaTech proposes to take waste carbon monoxide from sources like steel manufacturers and ferment that to produce ethanol. Holmgren says that the bacterium they use for their fermentation, Clostridium autoethanogenum, is highly ethanol tolerant. The scientific literature mentions tolerance in the 2% to 4% range, and says that the ethanol production rate slows down beyond 4%. I did see one patent application where they mentioned ethanol via this process in the 5.5% to 6% range.
To my knowledge LanzaTech hasn’t publicly stated the ethanol concentrations they achieve, and this prevents really rigorous calculations. Holmgren states that we needn’t make assumptions since “distillation energy requirements are textbook calculations and easy to calculate.” This only true if we know the ethanol concentration in the solution being distilled. As Holmgren’s own link showed in her response, it takes nearly twice as much steam to distill a 5% ethanol solution as it does a 10% ethanol solution. But without knowing for sure what their ethanol concentration is, we can’t know the energy requirement. So, I gave an example in my previous article to illustrate my point, which is this.
If you have a gaseous feedstock and then ferment it, the resulting solution is mostly water. The presence of a large percentage of water creates a relatively large distillation energy requirement. At a certain point, you expend as much energy removing the water as is contained in the product you are extracting. This situation could be avoided if instead you just did a gas phase reaction to the products instead of fermenting the CO. Why would you do a fermentation instead of a gas phase reaction? In my opinion it’s because one approach qualifies for subsidies and/or mandates and one does not.
Regarding my point that the distillation energy could be prohibitive, Holmgren waves the magic wand of “low value waste heat provides the energy source.” I have seen this waste heat solution invoked plenty of times in a situation with prohibitive energy requirements. It’s not much different than the magic wand of “there’s plenty of waste biomass so our feedstock will be free.” These assumptions are easy to invoke, but challenging to execute.
There are many things that can be said about this “free energy” solution, but one is that you have to reach the boiling point of water to separate ethanol from water in a distillation system. For this, you need either steam to provide the heat or a vacuum distillation column (which isn’t used for basic ethanol/water separations because it is too costly). The reason low value waste heat sources exist is because their temperature is too low to create steam. That’s what makes them “low value.” If they could produce steam, there are more logical places to use it.
There is an academic question of energy efficiency regarding the heat value of the carbon monoxide. Do you have to expend more energy converting and utilizing it than is contained in the carbon monoxide? The answer in the best case will be that the net energy captured from the carbon monoxide is marginal. This is quite a different case from the refinery example Holmgren used, where only about 10% of the contained energy in petroleum is required to refine it into fuel.
Then there is the economic question: If you do have to utilize more energy than is contained in the CO, what was the cost of that energy? And if subsidies weren’t driving the process in that direction, doesn’t it make more sense to do a gas phase reaction and avoid water completely? If you have waste heat, it could be utilized more efficiently in such a process.
So if you want to know LanzaTech’s vulnerability, that’s it. It’s a pretty good bet that the process essentially requires free energy for the economics to work. If I was evaluating LanzaTech as an investment opportunity, I would pay particular attention to the carbon monoxide conversion and selectivity to ethanol, the concentration of ethanol achieved, and the energy requirements for the distillation.
By Robert Rapier. You can find me on Twitter, LinkedIn, or Facebook.