Not Your Average Refinery: Sustainable Energy Production Through Electrochemical Reduction

Illustration Credit: Cortland Johnson | PNNL

Small stockpiles of biomass such as sewage, food waste, and wood chips are often overlooked in visions of future sustainable fuel and chemical production. The dismissal arises because transporting these materials to a large-scale, centralized bio-refinery would require more energy than they produce. Yet there is enough carbon stranded in these materials to theoretically provide about 25 percent of the U.S. demand for transportation fuel.

A new review, led by researchers at Pacific Northwest National Laboratory (PNNL), offers 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 paper, which was recently published in Chemical Reviews, the researchers gather information from over 100 years of chemistry on theory, materials, and reactor design needed for mini-refineries to be efficient parts of industrial biomass processing.

“This work outlines a conceptual framework needed to move centuries of research into real-world applications,” said PNNL computational scientist Roger Rousseau, who leads the laboratory’s Chemical Transformations Initiative. For the past four years, the initiative has been researching fundamental electrochemistry, catalyst design, and reactor design needed for functional mini-refineries.

Electrochemical reduction produces cleaner products from bio-crude

The challenge with converting sewage, food waste, and plant waste to fuel is the necessary molecular transformations. The first step in this conversion involves decomposing the biomass under high temperature to produce a crude bio-oil. This oil contains molecules such as aldehydes, ketones, esters, acids, and phenols that contain many oxygen atoms.

Fuel, however, is made of various hydrocarbon molecules, which contain more hydrogen than oxygen.

Adding hydrogen to oxygen-rich molecules requires chemical transformations called reduction reactions. To do these reactions on bio-oil, existing industrial processes bombard the bio-crude with hydrogen gas at high temperature and pressure.

At large scales, heat produced during these reactions is captured and reused in other refining steps. This maximizes the overall energy efficiency of the process. At small scales, however, that heat is lost and not reused. This means other approaches to reduction reactions are needed for local processing of wastes on small scales.

Well-known electrochemical reduction reactions are one path toward milder conditions needed for energy-efficient mini-refineries. In these reactions, electricity and a metal catalyst, rather than hydrogen gas and heat, 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 happen faster without increasing the reaction temperature and produce fewer byproducts. This means that fewer purification steps are needed later in production, which improves the energy efficiency of the whole process.

Multidisciplinary fundamental science informing industrial applications

The fundamental electrochemistry needed for electrochemical transformations has been known for centuries. Most of that work, however, has involved laboratory studies of model compounds that represent molecules derived from biomass.

In this review, the researchers outline the information available—and still needed—to move these reactions out of the lab. That information includes research developing new catalysts that can handle complex mixtures of molecules found in bio-oil, as well as electrochemical analysis to develop energy-efficient processes.

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