PNNL researchers announce waste-to-hydrogen conversion process

A research team at the Department of Energy’s Pacific Northwest National Laboratory has developed a system capable of converting waste from sewage, food crops, algae and other renewable carbon sources into hydrogen fuel.

PNNL’s electrocatalytic oxidation fuel recovery system turns waste carbon from farms, sewage and other sources into high-grade bio-based fuels, while simultaneously generating hydrogen. Until now, progress had been made converting waste to useful fuel but not completing the cycle using clean energy, researchers said.

PNNL said the key to making it all work is a catalyst that combines billions of immeasurably 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, 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.”

The research team 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 researchers running out of their wastewater sample, PNNL said. The patent-pending system solves several problems that have hindered efforts to make biomass economically viable, 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.”

Waste carbon from farms, sewage and other sources can be processed into high-grade bio-based fuels more easily with a new PNNL-developed flow cell. In this animation, the flow cell receives biocrude and wastewater from a hydrothermal liquefaction process. It then removes carbon from wastewater, allowing the clean water to be reused. The system even generates hydrogen, a valuable fuel that can be captured, reducing the cost of the whole operation. (Animation by Sara Levine | Pacific Northwest National Laboratory)

Lab researchers said one effective process for converting wet waste carbon to fuel is called hydrothermal liquefaction (HTL). This process essentially compresses the natural, fossil fuel-production time, converting wet biomass into an energy-dense biocrude oil in hours instead of millennia. But researchers noted the process is incomplete in that the wastewater produced 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.

As part of PNNL’s new process, unrefined biocrude and wastewater can be fed into the system directly from an HTL output stream or other wet waste. The PNNL process includes a flow cell, where the wastewater and biocrude flows through the cell and encounters 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. From here, the waste stream undergoes a catalytic conversion, with biocrude being converted to useful oils and paraffin. At the same time, oxygen and nitrogen-containing compounds undergo a chemical conversion that converts them into nitrogen and oxygen gasses. The wastewater that emerges from the system 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.

Researchers said the process’ speed was a bonus. They said they 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.

You can read more on PNNL’s electrocatalytic oxidation fuel recovery system here. Portions of the research were conducted as part of an agreement with Southern California Gas Company.