Scientists at the US Pacific Northwest National Laboratory (PNNL) have developed an innovative flow cell reactor that can refine sewage into valuable chemicals while simultaneously generating hydrogen that can be used to power vehicles or generate heat. This refinement process is potentially CO2 neutral and could ultimately even be CO2 negative.
Led by Catalyst Research Engineer Juan A. Lopez-Ruiz, the PNNL team's autocatalytic oxidation fuel recovery system starts with bio-oil that can be made from crops, algae and even sewage. This bio-oil is made through a process called hydrothermal liquefaction (HTL), which is admittedly energy intensive because it uses high temperatures and pressure to convert raw organic material into bio-oil. By mimicking the natural processes that created the world's fossil fuels, HTL can “achieve in minutes something that takes Mother Nature millions of years.”
“To make bio-oil today, hydrogen is required under high pressure. This hydrogen is usually made with natural gas,” says Juan A. Lopez-Ruiz
This is where PNNL's patented process comes into play. Research showed that electrocatalytic processes can provide a more sustainable means of refinement compared to thermocatalytic processes such as HTL that use hydrogen under high temperature and pressure.
As Lopez-Ruiz explains, “Our system can generate that hydrogen itself while simultaneously treating the wastewater under near-atmospheric conditions using excess renewable electricity, making it cheap to run and potentially carbon neutral.”
The process starts with a mixture of bio-oil and wastewater entering the flow cell reactor. The reactor is divided by a membrane that is permeable to protons, but not to electrons. As the mixture enters the anode side of the cell, it comes into contact with a thin titanium foil coated with ruthenium oxide nanoparticles. By reacting with this anode, the waste stream undergoes a catalytic conversion that changes its chemistry. This breaks down the main components, including carboxylic acid, and separates the useful oils and paraffins. Soluble compounds containing oxygen and nitrogen are also broken down and converted into these common gases.
The waste stream meanders around to the cathode side of the reactor where it passes through charged carbon felt. Here it undergoes further reactions that can hydrogenate organic molecules or generate hydrogen gas that can be used to fuel part of the process. The fibers of the carbon felt are not only an excellent conductor of electricity, but allow the molecules of the current to mix with a high degree of turbulence, further accelerating the catalytic reactions. The energy requirement of the cell is so relatively low that the remainder of the requirement can potentially be met with electricity generated by solar cells.
The system has now run for almost 200 hours without any problems. The test had to be stopped because the input of biological material had run out. “It's a hungry system,” Lopez-Ruiz said. “It can run indefinitely as long as we keep feeding it with sewage.”
But there are also disadvantages to the system. The device is heavily dependent on rare metals. But this problem is surmountable, Lopez-Ruiz thinks. Thanks to the ruthenium oxide coating, the reactor works with titanium, which is a lot easier to obtain than platinum, which is more often used in these types of reactors.
While it is common to use a concept like this to build large, centralized fuel refineries, it may be more environmentally friendly to locate smaller versions near their source material. Farms, breweries and wastewater treatment plants could be ideal for this, allowing them to be both producers and consumers.
he now patented system from PNNL, which is part of the US Department of Energy, is already available to municipalities and companies. The American energy company CogniTek is one of the first to use the system in their biomass processors.
Source: geekwire.com
