In a world with dual concerns about climate change and growing energy demand, the development of renewable fossil fuel alternatives has been a long-standing dream. Although considerable progress has been made in converting various biomaterials into usable energy in recent years, refining them in an environmentally sustainable manner remains a challenge.
Now, a team at the Pacific Northwest National Laboratory has developed an innovative mobile battery reactor that can refine waste carbon into valuable chemicals and produce hydrogen to power or generate heat for vehicles. This refining process may be carbon neutral or even carbon negative in the end.
Under the leadership of catalyst research engineer Juan A. Lopez Ruiz, the PNNL team’s automatic catalytic oxidation fuel recovery system starts with bio crude oil that can be produced from crops, algae and even sewage. This bio crude oil is produced through a process called hydrothermal liquefaction (HTL), which is energy intensive because it uses high temperature and high pressure to convert raw organic materials into bio crude oil. Imitating the natural process of creating the world’s fossil fuels, HTL can “accomplish something that takes nature millions of years in a few minutes”
However, like traditional petroleum refining, the refining process of bio crude oil is also energy intensive. Juan A. Lopez Ruiz, PNNL chemical engineer and project leader, said: “the current method of treating bio crude oil requires high-pressure hydrogen, which is usually produced by natural gas.”.
This is the process in which PNNL is applying for a patent. It is found that the electrocatalytic process can provide a more sustainable refining method than the thermal catalytic process using hydrogen at high temperature and high pressure, such as HTL.
As Lopez Ruiz explained, “our system can produce hydrogen itself, use excess renewable power and treat wastewater under near atmospheric conditions, making it low-cost and possibly carbon neutral.”
The process starts with the mixture of biological crude oil and wastewater, and directly enters the flow tank reactor from HTL process or other suitable wet waste source. Cells are separated by a membrane permeable to protons but impermeable to electrons. When the mixture enters the anode side of the battery, it contacts a thin layer of titanium foil covered with nano ruthenium oxide. The reaction with the anode will cause catalytic conversion of the waste stream, thus changing its chemical properties. This breaks down its main components, including carboxylic acids, and separates useful oils and paraffins. Soluble compounds, including oxygen and nitrogen, also break down and convert them into these common gases.
The waste stream winds to the cathode side of the battery, where it passes through a charged carbon felt. Here, it has undergone further reactions, which can not only hydrogenate organic molecules, but also generate hydrogen, which can be used as fuel for some processes. Carbon felt fiber is not only an excellent conductive material, but also can make the molecules in the gas flow mix under the condition of high turbulence to further accelerate the catalytic reaction. The energy demand of the cell is relatively low, so the remaining demand may be provided by the power generated by the solar cell.
Despite nearly 200 hours of operation test, the system still maintains its efficiency without obvious loss. The test could have lasted longer, but the team ran out of bio oil.
“It’s a hunger system,” observed Lopez Ruiz. “If wastewater continues to circulate, it may run indefinitely.”
Once the pollutants are removed, the wastewater can be recycled back to the reactor to further reduce the environmental impact of the process. Post filtration can be used to remove any residual harmful chemicals before water is used for growing crops or even drinking.
A potential problem in reactor design is its dependence on rare earth metals. Like many clean energy technologies, PNNL’s mobile battery design requires these energy intensive and difficult to obtain elements, sometimes referred to as platinum group metals. According to a recent survey by the Department of energy, 14 of the 35 metals in the United States are 100% dependent on imports, while the other 17 are more than 50%. This makes improving domestic supply a top priority.
To solve this problem, PNNL team made anodes by coating ruthenium oxide nanoparticles on titanium films. Compared with the use of metal films such as platinum, this method greatly increases the surface area of catalytic reaction and reduces the amount of materials required.
“If wastewater continues to circulate, it may run indefinitely.”
According to a recent paper published in Applied Catalysis B: environmental, Lopez Ruiz et al. Determined that the optimal particle size of ruthenium oxide was about 12 nm. Therefore, the new method of PNNL requires 1000 times less rare earth metals than those usually required for similar platinum based reactors. At the same time, compared with the thermal system using intermediate hydrogen pressure and temperature, the conversion rate achieved by this process is more than 100 times higher.
Another advantage of PNNL method is that it is adjustable and allows the formation of different molecules and compounds according to the voltage applied to the circuit. By changing the voltage of about 2 to 5 volts within the operating range of the system, the biological crude oil will experience different reactions. All of these can be monitored and driven by software, so that people can operate it without a degree in chemical engineering.
This is consistent with some ideas on how to eventually use these reactors. Although it is natural to consider using such a concept to build large centralized fuel refineries, it may be more environmentally friendly to place small fuel refineries near raw materials. Farms, breweries and wastewater treatment plants may be ideal places to achieve this goal, making them both producers and consumers.
To achieve this, PNNL’s new process is currently applying for a patent for clean and sustainable electrochemical treatment, which can be licensed by companies and municipalities. This technology was recently licensed by cognitek, a global company that brings energy products and technology solutions to market. Cognitek plans to combine PNNL’s technology with other biomass treatment systems developed by them and their strategic partners for commercialization.
Together with Lopez Ruiz, PNNL’s research team includes Yang Qiu, Evan Andrews, Oliver Gutierrez and Jamie horadi. Part of the funding for this work comes from the Ministry of energy.