The path to renewable fuel just got easier

The holy grail of bio-fuel researchers is to develop a self-sustaining process that converts waste from sewage, food crops, algae, and other renewable carbon sources into fuels while keeping waste carbon out of our atmosphere and water. Much progress has been made in converting such waste to useful fuel but completing the cycle using clean energy has proved a tough nut to crack.

Now, a research team at the Department of Energy’s Pacific Northwest National Laboratory has developed a system that does just that. PNNL’s electrocatalytic oxidation fuel recovery system simultaneously turns what has been considered unrecoverable, diluted “waste” carbon into valuable chemicals, while simultaneously generating useful hydrogen. Being powered by renewables makes the process carbon-neutral or even potentially carbon-negative.

The key to making it all work is an elegantly designed catalyst that combines billions of infinitesimally small metal particles and an electric current to speed up the energy conversion at room temperature and pressure.

“The currently used methods of treating biocrude requires high-pressure hydrogen, which is usually generated from natural gas,” said Juan A. Lopez-Ruiz, a PNNL chemical engineer and project lead. “Our system can generate that hydrogen itself while simultaneously treating the wastewater at near atmospheric conditions using excess renewable electricity, making it inexpensive to operate and potentially carbon neutral.”

A hungry system

In laboratory experiments, the research team has tested the system using a sample of wastewater from an industrial-scale biomass conversion process for almost 200 hours of continuous operation without losing any efficiency in the process. The only limitation was that the research team ran out of their wastewater sample.

“It’s a hungry system,” Lopez-Ruiz said. “The reaction rate of the process is proportional to how much waste carbon you are trying to convert. It could run indefinitely if you had wastewater to keep cycling through it.”

The patent-pending system solves several problems that have plagued efforts to make biomass an economically viable source of renewable energy, according to Lopez-Ruiz.

“We know how to turn biomass into fuel,” Lopez-Ruiz said. “But we still struggle to make the process energy-efficient, economical, and environmentally sustainable—especially for small, distributed scales. This system runs on electricity, which can come from renewable sources. And it generates its own heat and fuel to keep it running. It has the potential to complete the energy recovery cycle.”

“As the electric grid starts to shift its energy sources toward integrating more renewables,” he added, “it makes more and more sense to rely on electricity for our energy needs. We developed a process that uses electricity to power conversion of carbon compounds in wastewater into useful products while removing impurities like nitrogen and sulfur compounds.”

Closing the energy gap

One very effective process for the conversion of wet waste carbon to fuel is called hydrothermal liquefaction (HTL). This process, in essence, compresses the natural, fossil fuel-production time, converting wet biomass into an energy-dense biocrude oil in hours instead of millennia. But the process is incomplete in the sense that the wastewater that is produced as part of the process needs further treatment to obtain added value from what would otherwise be a liability.

“We realized that same (electro)chemical reaction that removed the organic molecules from wastewater could be also used to directly upgrade the biocrude at room temperature and atmospheric pressure as well,” Lopez-Ruiz said.

This is where the new PNNL process comes into play. Unrefined biocrude and wastewater can be fed into the system directly from an HTL output stream or other wet waste. The PNNL process consists of what’s called a flow cell where the wastewater and biocrude flow through the cell and encounter a charged environment created by an electric current. The cell itself is divided in half by a membrane.

The positively charged half, called an anode, contains a thin titanium foil coated with nanoparticles of ruthenium oxide. Here, the waste stream undergoes a catalytic conversion, with biocrude being converted to useful oils and paraffin. Simultaneously, water-soluble contaminants, such as oxygen and nitrogen-containing compounds, undergo a chemical conversion that turns them into nitrogen and oxygen gasses—normal components of the atmosphere. The wastewater that emerges from the system, with contaminants removed, can then be fed back into the HTL process.

On the negatively charged half of the flow cell, called a cathode, a different reaction takes place that can either hydrogenate organic molecules (such as the ones in treated biocrude) or generate hydrogen gas—an emerging energy source that the flow cell developers see as a potential source of fuel.

“We see the hydrogen byproduct generated by the process as a net plus. When collected and fed into the system as a fuel, it could keep the system running with fewer energy inputs, potentially making it more economical and carbon-neutral than current biomass conversion operations,” said Lopez-Ruiz.

The speed of chemical conversion provides an added benefit to the system.

“We did a comparison of rates—that is how fast we can remove oxygen from organic molecules with our system as opposed to the energy-intensive thermal removal,” Lopez-Ruiz said. “We obtained more than 100 times higher conversion rates with the electrochemical system at atmospheric conditions than with the thermal system at intermediate hydrogen pressures and temperatures.” These findings were published in the Journal of Applied Catalysis B: Environmental in November 2020.

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