biomass conversion

=chemistry =renewable energy =polymers



What chemicals can be made from biomass at competitive prices? Are biofuels economically viable?





The chemical formula for glucose is C6H12O6. If you remove 2 H2O and 2 CO2 from it, you get C4H8, which has 31% as much mass.

Without subsidies, sugarcane is the cheapest source of sugar. Prices for it vary, but let's say it's $300/ton. At that price, oil from sugar can't possibly be less than $960/ton, and conversion is never 100% efficient.

At 2.8 kilograms in a gallon for gasoline, that's an absolute minimum of $2.73/gallon of gasoline equivalent from sugar - before transportation and taxes - and considering losses and processing costs, $4.60 would be more realistic. Using unsubsidized corn, that would significantly higher.

By the way, burning $960/ton oil in a 60% efficient combined-cycle gas turbine would cost at least $0.12/kWh, which is about 3x as expensive as wholesale electricity in the USA.

So you see, while sugar is cheap by mass, it's not cheap enough by energy content. So, if you're making chemicals from sugar or starch, you want to make more valuable things. This isn't exactly a problem, unless you're trying to substantially reduce global warming, in which case fuel is the only product with a large enough volume.

Making pesticides, flavorings, and certain polymer precursors such as butanediol by fermentation is very much viable. Yields of anything that's not the main product of metabolism tend to be low, and lots of things are toxic to cells at high concentrations, and it's usually hard to separate dilute chemicals from fermentation broth, but despite all that, production of high-value chemicals by fermentation will continue to increase.





Grass is much cheaper than sugar. The cost of miscanthus or switchgrass has been estimated at $65 per dry ton, but as with anything so cheap, that cost varies significantly with distance.

Most papers on miscanthus as a source of biomass have focused on Miscanthus x giganteus. This produces much more mass per area than switchgrass, but it doesn't have seeds so it's more expensive to plant, and it requires more fertilizer. Energy cane (a sugar cane variant bred for biomass) produces even more, but needs too much fertilizer.

I actually think it's better to focus on breeding seeded miscanthus, maybe lutarioriparius. Depending on your usage, you might want to maximize cellulose, hemicellulose, or lignin content, and those proportions vary widely between miscanthus and switchgrass species.



biorefineries for fuel


Suppose you feed grass to a biorefinery, converting cellulose to levulinic acid and hemicellulose to furfural. You might get 1/4 of the input dry mass out as those products, and they have about 1/2 the energy per mass of oil, which means about $520 per ton oil equivalent just in feedstock. Depending on how processing is done, the net cost per ton oil equivalent could be anywhere from $700 to $1800, not including processing to gasoline.

I think it's possible to profitably produce levulinic acid and furfural from grass at an average selling price of $500/ton. This estimate is much lower than current prices, and is based on a novel biorefinery design, but I have fairly high confidence in it. Production from sugarcane bagasse would be cheaper, but supply is limited by demand for sugar.

It's possible to produce relatively good gasolines from those products by, for example, hydrogenating furfural to 2-methylfuran. The cost of that obviously depends partly on the cost of hydrogen, and some fuel products involve using methanol too. Also, the economics of combined production of fuel and polymers is better.

If producing only fuel, and using syngas from natural gas, then I'm estimating that the cost of fuel using this approach is slightly less than $3 per gallon of gasoline equivalent, which is slightly more than the current cost of producing gasoline from only natural gas. That's also similar to the cost of biodiesel when the seed meal is sold as animal feed, but biodiesel production requires more land - maybe twice as much land, but this depends on the plant species - and the demand for seed meal would be saturated if biodiesel replaced oil.

Obviously, ethanol from corn is only used in the USA because it's subsidized, and biodiesel is only used in Europe because it's subsidized. And that's...actually, you know what, that's fine - as the first step. Having subsidized something enough to get an industry established, all that's left is the second step: ramping down subsidies to zero to force companies to be economically competitive. China has the system down, just follow their example.

Using biogas on a large scale, it would be more, but less than $4. That's far from civilization-ending, but it is expensive, and doesn't make a very good sales pitch to investors. Still, I guess it looks a lot better economically than most of the stuff Breakthrough Energy Ventures is investing in; that whole group is weird and doesn't make sense in several ways, but I can't really talk about that.



Production of furfural and levulinic acid produces hydrochar as a byproduct, which can be buried for carbon sequestration. If you're only making fuel, the required CO2 mitigation cost including that and avoided oil production would be maybe $60/ton. Not terrible; full mitigation of US CO2 emissions at that price would be about 10% of US federal tax revenue, which is politically questionable but economically very possible. But again, this depends on a theoretical biorefinery design with several novel elements.

In theory, you could gasify hydrochar to produce the hydrogen for biofuel production; it would be about as good for that as lignite coal, but it's much better to make syngas from methane than from coal, which is part of why coal is worth approximately nothing these days. It's also, of course, possible to make hydrogen by electrolysis, which is far from viability as long as natural gas is available but could potentially end up being competitive with biogas.



cellulose fermentation


Most US research on fuel from biomass has focused on cellulosic ethanol. Nothing even close to practical has resulted from this extensive reseach. The concept is fundamentally flawed because ethanol disrupts the cellulose-digesting enzymes, and breaking cellulose into sugars before fermentation is too expensive.

But hey, it's a lot less dumb as an idea than the hydrogen fuel proposals or coal power with CO2 sequestration.

We know that producing net exergy by grinding up grass and fermenting it to butyrate is possible, because cows do that. Here is a process I designed using that approach. The resulting dipropyl ketone is almost ideal as a biofuel:

- good energy density
- good octane ratings
- low toxicity
- not too strong a solvent
- somewhat biodegradable
- suitable boiling point
- low enough melting point


That involves fermenting solids, separating out solid product, and processing it. It's always more expensive to handle solids than liquids, so that process might be too expensive in practice; its costs vary significantly with process design details. But it's plausibly viable, and I think it's still the most promising approach for economically competitive biofuel production today.



biorefineries for polymers


If you want to get investment for biorefinery construction, and at least get some things going, it's better to start with something with less marginal economics than fuel production, and expand from there.

Here's an example of a more profitable process:
furfural -> methyl furoate -> dimethyl 2,2’-bifuran-5,5’-dicarboxylate -> dimethyl biphenyl-4,4'-dicarboxylate -> polyester

With 70% ethylene glycol / 30% butanediol the result is a thermoplastic with >150 MPa flexural strength and >260 MPa tensile strength. It can also be used for self-reinforced composites, and as a matrix for thermoplastic prepreg. This polymer could potentially be about as cheap as PET, and in some ways, it's a higher-performance polymer than anything commercially available now, so production is viable on a small scale even if it's 10x that expensive.

Of course, it's always possible that there's some problem, so I designed a couple other routes from furfural to polymers, as one does. Anyway, while renewable plastics are currently expensive, and some questionable approaches to them are being pursued, they shouldn't be a big long-term problem.


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