In the quest for chemicals and fuels made from biomass, there are a few important black boxes that make it difficult to compare different companies’ business models and likelihood of success. One of them is the process by which a particular facility obtains sugars from its biomass feedstock.
In many cases, the first step is expensive, but low-tech – chopping up the stuff. This is the part that reminds me of Choppin’ Broccoli, the Saturday Night Live song as performed by Dana Carvey. Since cellulosic ethanol is sort of an offshoot of corn ethanol, it’s helpful to imagine how different it is to process a corn cob or stalk or an entire sugar cane, compared to grinding up a starchy corn kernel. Getting sugar from cellulose is difficult enough, getting the cellulose away from the clutches of a plant’s lignin first requires heavy machinery to chop it into little pieces.
So say you have tidy chipped up pieces of biomass. What do you do then? Like the SNL song, it ain’t pretty. Generally it requires some combination of thermochemical assaults to get the sugar out. Steam, alkali-acid washes, and pricey enzymes… In an otherwise green business, the pretreatment steps use energy and possibly chemicals that you wouldn’t want to spill.
Since pretreatment of biomass has a lot to do with both costs and the yield of sugars from feedstock, it is a busy area of research. An article by Chris Hanson in the appropriately named Biomass Magazine delves into some intriguing ideas. To release the useful cellulose from lignin, researchers at University of Illinois at Urbana-Champaign and the U.S. DOE’s Joint BioEnergy Institute are investigating ionic liquids. Instead of using a traditional, two-stage alkali-acid pretreatment, a dose of butadiene sulfone got the job done in one step, according to U. of Illinois scientist Hao Feng. Another major benefit is that the butadiene sulfone can be recovered and recycled.
In California, the JBEI has been experimenting with imidazolium chloride. It has succesfully obtained sugar yields of 95% from mixed feedstocks and recycled 95% of the ionic liquid.
And a company called Leaf Energy has been studying a glycerol pretreatment method. Compared to acid pretreatments, the company says their method gets more sugars faster by dissolving lignin with a relatively inexpensive reagent with low temperature and standard pressure.
The goal with improving pretreatment steps is to bring down the cost of sugar from cellulose so that it is not more expensive than sugar from corn or sugar cane. Maybe if major cellulosic ethanol producers take up these technologies, we’ll have a better window into how they get the sugar out.