As the old saying goes, all roads lead to Rome. And when it comes to converting biomass into liquid fuels, all roads start with deciding whether the raw plant material should be broken down by exposing it to water or to high temperatures.
It takes chemical engineering expertise to thoroughly compare and evaluate these two basic processes and the many details of implementing either of them in a biorefinery that may—years down the road—be cost-competitive with today’s petroleum refinery.
In a June 2017 interview with Advanced Science News, Christos Maravelias, the Vilas Distinguished Achievement Professor and Paul A. Elfers Professor in chemical and biological engineering at the University of Wisconsin-Madison, and his former postdoctoral researcher Jeffrey Herron, who now works for The Dow Chemical Company, shared their thoughts on designing the kind of biorefinery that may eventually power a next-generation bioeconomy.
The interview was based on a study Maravalias, Herron and their colleagues at the University of Oklahoma recently published in the journal Energy Technology. The study focused on one particular chemical process called torrefaction, or thermal fractionation. In this process, pre-treated biomass is exposed to a series of increasing temperatures (300 to 850 degrees Celsius) to decompose it into a wide range of chemicals of smaller molecular weight.
The next stage upgrades these chemicals to the end products of interest using catalysts, which are compounds designed to speed up a reaction of interest while remaining relatively stable themselves. Since each torrefaction stage yields a different set of chemical species, referred to as a “fraction”, several independent upgrading strategies are needed. The last stage in the process combusts the leftover biomass char to recover heat energy.
“The goal of our study was to provide previously lacking guidelines on how to best integrate the thermal decomposition stages with the subsequent upgrading of the biomass fractions,” Maravelias says. “By identifying the main drivers of this multistage process, future research efforts can focus on improving those stages that are especially complex and expensive.”
One of the key questions in any biomass conversion process is when and how to separate its three main components: cellulose, hemicellulose and lignin. Plant cell walls contain a complex mixture of these three chemicals that needs to be disentangled to generate biofuels and other chemicals of interest.
But deciding whether or not to separate each biomass component in its own fraction is a tradeoff between cost and efficiency.
“On the one hand, you tend to have better chemical efficiency if you separate everything, but on the other hand, it adds complexity and increases the cost of scaling up the process,” Maravelias explains. “That’s because building one large reactor is generally cheaper than building two reactors that are half the size of the large one.”
A closer look at this tradeoff produced the study’s most surprising result: The authors found that the upgrading steps may actually be more efficient when components from multiple fractions are combined.
The main implication for future work is that the thermal decomposition process should be optimized jointly with, rather than separately from, the catalytic upgrading steps. Doing so, Maravelias says, will require a more detailed techno-economic analysis of the conversion process that goes beyond the high-level roadmap the current study provided.
As part of that follow-up work, researchers will also need to evaluate the ability of thermal decomposition-based strategies to generate more than one end product: biofuel and commodity chemicals, such as precursors of plastics, that can be sold at a higher per-unit price.
This is necessary because of some fundamental differences between a biorefinery and a petroleum refinery: The latter facility is often located right next to an oil field, or is supplied by tankers loaded with large quantities of oil, while biomass may need to be collected from a larger surrounding area, which increases costs. Chemically processing fresh plant material also tends to be more complex.
“Producing commodity chemicals is one way to improve the economics of the process,” Maravelias says. “And analyses like ours—whether at the big-picture or more detailed level—help pinpoint exactly where in the process those improvements are needed the most.”
Author: Silke Schmidt