Which chemicals should we make from biomass?
Depends on your definition of "should"
In my last post, I argued that we shouldn’t be using biomass for energy at all. The obvious next question is, what should we be using biomass for? (Or maybe, what should we use for energy, but that’s easy to answer—solar, wind, hydro, geothermal).
What to use biomass for is a much more difficult question. There are a lot of factors to be considered for each possible use, like:
how long does it store carbon?
how much fossil fuel is it displacing?
is the transportation/conversion process efficient?
are there other negative/positive impacts?
how long will it take to scale?
how strong is the market for it?
It’s a bit much to cover in one newsletter, so I’ll limit the scope for this post to my core competency: chemicals.
As we move toward Net Zero, one big problem of the foundation industries is that the chemical industry cannot be decarbonised.
Really, it can’t.
It’s not that it’s challenging, or the technology hasn’t been invented yet. It’s that most of our chemical products rely on carbon as a building block. No matter how energy-efficient we make the manufacturing processes, or how much renewable energy we adopt, we will still need carbon to put into the chemicals. (And if you think we should just get rid of chemicals altogether, you may want to refresh your understanding of what a chemical is.)
Instead, we need to talk about defossilisation: removing fossil sources of carbon as much as possible, and replacing them with circular or renewable sources.
The Renewable Carbon Initiative, from Nova Institute, is working toward that goal. They’ve published a Carbon Flows Report that looks at the big picture of carbon in the materials and chemicals sectors, and proposes a global 2050 scenario that looks something like this:
Of course these are rough numbers, and the predicted growth rate is low, but that’s a debate for another day. The upshot is that, based on some assumptions about limits to recycling and sustainable biomass availability, carbon for future chemicals and materials needs to come from a mix of sources. It’s likely that a significant chunk—about 20%—will need to come from biomass.
So we’ve got to go from 44 Mt of chemical carbon being bio-based, to 230 Mt, in less than 30 years. What’s the most sensible way to go about this transition?
That depends on who you ask. You’ll get one answer from chemists, another from engineers, and a third from economists. Personally I’m a big fan of the engineer answers, so let’s look at one of those first.
A team from RWTH Aachen and Fraunhofer have applied their LCA expertise to the problem, and come up with a simple ranking of efficiency for conversion of sugar to various chemicals. Well, the outcome looks simple; the process to get there was a bit more complex.
First, you need to define efficiency. The goal here is to move away from petrochemicals in order to reduce global warming. Efficiency is the fastest way towards that goal. In this study, the authors quantified it as a “sugar-to-X” (S2X) ratio:
Simply put, when you replace a petrochemical with a bio-based one, how much global warming is avoided per kilo of sugar used? The sugar in the denominator is sugar consumed in fermentation, detaching the S2X ratio from production of the feedstock. This ratio lets us compare efficiency of different sugar-based fermentation processes, so regardless of where/how the sugar is grown, the relative ranking will stay the same.
The authors calculated this ratio for 46 processes with enough available data in the literature, assuming that all stored carbon was released as CO2 at end of life. For petrochemicals being replaced by a chemically different bio-product, functional substitution was assumed on a mass basis (1 kg PLA replaces 1 kg PET) or energy basis for fuels (1 kg ethanol replaces 0.587 kg gasoline).
I won’t get into all of the assumptions (the original paper is open access if you want to get elbow-deep into LCA), but the outcome is:
Remember, a positive S2X ratio is good—global warming is being avoided. A negative S2X ratio means that switching to the bio-based chemical actually emits more carbon than sticking with the petrochemical.
The results have a wide range, from -3 to +6.1, showing that it’s not always good (purely from a carbon footprint perspective) to switch to bio-based. 34 of the 46 ratios are positive, so 74% of these processes do actually produce a bio-based chemical with a lower carbon footprint than its petrochemical version.
There’s a couple of really interesting bits of analysis in the paper. One is that in general, chemicals containing more oxygen achieve higher S2X efficiencies:
Oil and gas feedstocks have very little oxygen, while sugar (glucose) naturally contains it. So it makes intuitive sense that going from no oxygen → high oxygen requires a lot of energy, while going from high oxygen → high oxygen is much more efficient. So as a rule of thumb, we can aim to replace highly oxygenated petrochemicals with bio-based versions.
However, there are exceptions like acrylic acid, which is very easy to make from fossil fuels. So before investing in scaling a bio-based chemical replacement, an in-depth efficiency analysis is needed.
The other interesting bit is looking at ethanol, which has quite a low S2X ratio, but is produced in high volumes from sugar. What if we stopped making sugar into ethanol, and made it into more efficient chemicals instead? Let’s take the 220 Mt of sugar per year used for ethanol production (as of 2019, anyway) and look at what happens if we use it to make more efficient chemicals, maxing out market demand for each:
We start by making a tiny amount of succinic acid (to replace adipic acid), then some diols, then PLA to replace PET, and so on, until we’ve consumed all of the sugar currently used for ethanol.
Compared to making ethanol, we would save 134 Mt of CO2-eq emissions per year. 30% of that reduction is achieved by replacing only 6.3% of the current ethanol production.
So why are we making so much ethanol, if the goal of bio-based chemistry is to avoid global warming? I’m afraid that’s a question for an economist with agricultural expertise.
I love this approach, and have some more papers on bio-based production efficiency on my to-read list for the near future. Do please send your favourites my way!