Small stockpiles of biomass such as food waste, wood chips, and sewage are usually ignored in visions of future sustainable fuel and chemical production. The dismissal appears because moving these materials to a large-scale, centralized bio-refinery would need more energy than they produce. However, there is sufficient carbon stranded in these materials to probably provide about 25 percent of the U.S. need for transportation fuel.
A new study, directed by scientists at Pacific Northwest National Laboratory (PNNL), proposes a solution to capture these unused materials: mini-refineries located near waste sources that process biomass using electrochemical reduction reactions powered by renewable energy.
In the article, which was recently issued in Chemical Reviews, the scientists gather data from over 100 years of chemistry on theory, reactor design, and materials needed for mini-refineries to be active components of industrial biomass processing.
“This work drafts a conceptual framework required to move centuries of study into real-world applicability,” said PNNL computational expert Roger Rousseau, who manages the laboratory’s Chemical Transformations Initiative. For the past four years, the initiative has been studying catalyst design, reactor design, and fundamental electrochemistry needed for functional mini-refineries.
Electrochemical reduction delivers more reliable products from bio-crude
The challenge with turning sewage, plant waste, and food waste into fuel are the basic molecular transformations. The initial step in this transformation includes decomposing the biomass under high temperature to produce a crude bio-oil. This oil comprises molecules such as esters, acids, aldehydes, phenols, and ketones that carry multiple oxygen atoms.
However, fuel is composed of several hydrocarbon molecules, which carry more hydrogen than oxygen.
Adding hydrogen to oxygen-rich molecules needs chemical conversions called reduction reactions. To execute these reactions on bio-oil, existing industrial processes bombard the bio-crude with hydrogen gas at high pressure and temperature.
At large scales, the heat generated during these reactions is stored and utilized in other refining processes. It enhances the overall productivity of the process. However, at small scales, that heat is not reused as it gets lost in the surrounding. This indicates other methods to reduce reactions are required for the local processing of wastes on small scales.
Well-known electrochemical conversion reactions are one path to moderate conditions required for energy-efficient mini-refineries. In these reactions, rather than hydrogen gas and heat, a metal catalyst and electricity propel the molecular transformations. Other molecules in the mixture can also be simultaneously scavenged to provide hydrogen atoms during the reaction.
Compared to thermochemical reduction with hydrogen gas, electrochemical reductions of specific molecules in bio-oil can occur faster without raising the reaction temperature and generate fewer byproducts. This indicates that fewer purification steps are required later in production, which enhances the energy efficiency of the whole process.
Interdisciplinary fundamental science implicating industrial applications
The basic electrochemistry required for electrochemical transformations has been acknowledged for centuries. However, most of that work has included laboratory studies of model compounds that represent molecules derived from biomass.
In this study, the scientists describe the data available—and still required—to move these reactions out of the lab. That data includes a study developing unique catalysts that can control complex mixtures of molecules found in bio-oil, as well as electrochemical analysis to improve energy-efficient processes.
“This study confirms the capability of electrochemical reduction for bio-oil processing and shows how to optimize the reactions so they can be utilized beyond proof-of-principle demonstrations,” said PNNL computational scientist Vanda Glezakou.
The Chemical Transformations Initiative at PNNL gives a one-of-a-kind possibility to improve this work because it unites scientists with expertise in catalysis with scientists specializing in electrochemistry. Together, these diverse perspectives bring knowledge about the basic principles leading every step of an electrocatalytic reaction. Scientists can then build upon this comprehensive framework to advance existing science toward applications and meet specific reactions to particular production steps.
“We’ve learned that processing biomass on a local scale can contribute to sustainable fuel and chemical production,” Glezakou said. “Interdisciplinary science that we pursue at PNNL gives a holistic insight to understand which chemical conversions are most suitable for specific steps in an industrial scale process.”
“Electrocatalytic Hydrogenation of Biomass-Derived Organics: A Review” by Sneha A. Akhade, Yue Liu, Abhijeet Karkamkar, Robert S. Weber, Johnathan E. Holladay, Jonathan L. Male, Nirala Singh, Asanga B. Padmaperuma, Mal-soon Lee, Greg A. Whyatt, Michael Elliott, Oliver Y. Gutiérrez, Juan Lopez-Ruiz, Huamin Wang, Jamie D. Holladay, Johannes A. Lercher, Roger Rousseau and Vassiliki-Alexandra Glezakou, 17 September 2020, Chemical Reviews.
The Chemical Transformations Initiative at PNNL is supported by the Laboratory Directed Research and Development Program.
Review co-authors include Sneha A. Akhade, PNNL and Lawrence Livermore National Laboratory; Mal-Soon Lee, PNNL; Greg A. Whyatt, PNNL; Michael Elliott, PNNL; Nirala Singh, PNNL and the University of Michigan; Oliver Y. Gutiérrez, PNNL; Jamie D. Holladay, PNNL; Yue Liu, TU München; Abhijeet Karkamkar, PNNL; Juan Asanga B. Padmaperuma, PNNL; Johnathan E. Holladay, PNNL; Jonathan L. Male, PNNL;Lopez-Ruiz, PNNL; Huamin Wang, PNNL; Robert S. Weber, PNNL; and Johannes A. Lercher, PNNL and TU München.