A research team at the U.S. Department of Energy’s Pacific Northwest National Laboratory (PNNL), Richland, Wash., has developed a patent-pending system that uses a flow cell bioreactor to turn carbon previously considered unrecoverable from so-called biocrude (aqueous waste streams) into valuable chemicals at room temperature and pressure, while simultaneously generating useful hydrogen.
“The currently used methods of treating biocrude requires high-pressure hydrogen, which is usually generated from natural gas,” says 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.”
Waste first moves through a hydrothermal liquefaction (HTL) process. It then undergoes an electrocatalytic conversion in PNNL’s flow cell bioreactor, dubbed Clean Sustainable Electrochemical Treatment (CleanSET); there, a catalyst composed of minute metal particles converts the biocrude into oils and paraffin. The treatment simultaneously removes carbon from wastewater, allowing the clean water to be fed back into the HTL process.
“…Traditionally, wastewater treatment requires a way to remove ammonia via high temperature oxidation (if present), then a second step involves anaerobic digestion to convert the organic compounds into methane (and CO2). Once those steps are complete, a steam reformer converts methane into hydrogen. The CleanSET system does all of those steps in one unit, as it oxidizes ammonia and organic compounds at the anode while simultaneously generating hydrogen at the cathode.”
The researchers shared in The Journal of Applied Catalysis B: Environmental how they tested the system for almost 200 hours of continuous operation without losing any efficiency in the process. The trial only stopped because the wastewater sample was exhausted.
“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,” notes Lopez-Ruiz.
“We have already tested the performance of our technology with real biomass-derived wastewater and bio-crude containing sulfur and nitrogen containing compounds (e.g., pyrroles). We didn’t see any deactivation for the duration of the experiment, 50 to 160 h. That being said, the system needs to be farther evaluated for thousands of hours to make sure there is not long-term poisoning,” explains Lopez-Ruiz.
In addition, evaluation of the electrodes post experiment showed no changes in their micro- and nanostructure. However, Lopez-Ruiz admits further testing under more aggressive conditions and for longer durations will better assess their structural robustness and address potential points of failure.
The team used the same standard catalyst preparation techniques the catalysis and fuel cell industries already use, so, making the electrodes on a large scale wouldn’t pose a challenge.
HTL technology is already demonstrated at pre-pilot scale; PNNL has process development units (PDU) available for demonstration of biofuel production.
“The CleanSET technology needs to be scaled-up now to match the processing scales of the PDUs, so we can demonstrate the whole process in series at pre-pilot scale… At this point in the development process, we are seeking a commercial partner or partners to scale up the process to industrial pilot-scale,” adds Lopez-Ruiz.
“The next steps are demonstration with different feedstocks, as that will determine the conditions and electrode compositions we need to use in the electrochemical systems. Once we evaluate that at the laboratory scale, we can move forward with the process scale-up,” he concludes.