As the world continues to get on the green bandwagon, biofuels are driving the new fossil fuel market. The goal of biofuels is to create an alternative source of energy that benefits the environment, as well as society. Researchers around the world are in a constant struggle to develop self-sustaining processes that will convert sewage, food crops, and other renewable carbon sources into fuel. Although much progress has been made in converting waste to useful fuel, finishing the cycle using clean energy has always been a tough nut to crack.
Recently, a research team at the Department of Energy’s Pacific Northwest National Laboratory developed a system that simply focuses on using clean energy from wastewater. PNNL’s electrocatalytic oxidation fuel recovery system not only simultaneously turns what has been considered unrecoverable, diluted “waste” carbon into valuable chemicals, but also generates hydrogen that can be used in various ways. The entire process being powered by renewables makes it carbon-neutral or even potentially carbon-negative. The secret to making the idea work was designing a catalyst that would combine billions of small metal particles and an electric current to speed up the conversion of energy at room temperature and pressure. Juan A. Lopez-Ruiz, a PNNL chemical engineer and project lead stated that their current method of treating biocrude required high-pressure hydrogen, usually generated from natural gas. But this new system that they have introduced generates the hydrogen by itself, while also simultaneously treating the wastewater at near atmospheric conditions using excess renewable electricity. This, therefore, makes the overall process inexpensive and carbon neutral.
During the laboratory experimentation, the research team tested the system using a sample of wastewater from an industrial-scale biomass conversion process. This went on for almost 200 hours of continuous operation, that too without losing any efficiency in the process. The limitation was evident – the research team ran out of their wastewater sample. “It is a hungry system,” Lopez-Ruiz said. The reaction rate of the experiment during the process was proportional to how much waste carbon one is trying to convert. Simply speaking, it could run indefinitely if wastewater keeps cycling through it. According to Lopez-Ruiz, this patent-pending system will aid in solving several problems, which have constantly plagued the efforts to make biomass an economically viable source of renewable energy.
“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.” He reiterated that the system runs on electricity that can come from renewable sources. It also generates its own heat and fuel to keep it running. The system thus has the potential to complete the energy recovery cycle. Because the electric grid starts to shift its energy sources toward integrating more renewables, it only makes more and more sense to rely on electricity for 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 sulphur compounds,” he adds.
One of the processes for the conversion of waste carbon to fuel is called hydrothermal liquefaction (HTL). This process compresses the natural, fossil fuel-production time. It aids in the conversion of wet biomass into an energy-dense bio-crude oil in hours instead of millennia. However, truthfully speaking, the process is incomplete. The wastewater that is produced as part of the process should be further treated to obtain added value from what would otherwise be a liability. Lopez-Ruiz said, “we realised that the 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.” The new research feeds biocrude and wastewater into the system directly from an HTL output stream or other wet waste. The process consists of 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 waste carbon from sewage and other sources can easily be processed into high-grade bio-based fuels using a developed flow cell. The flow cell simply receives biocrude and wastewater from a hydrothermal liquefaction process. It then removes carbon from wastewater, allowing the clean water to be reused. The system event generates hydrogen, a valuable fuel that can be captured, reducing the cost of the whole operation. The anode, the positively charged half, contains a thin titanium foil coated with nanoparticles of ruthenium oxide. The waste undergoes a catalytic conversion here with crude being converted to oils and paraffin that can later be used. Simultaneously, water-soluble contaminants like oxygen and nitrogen-containing compounds undergo a chemical conversion that turns them into nitrogen and oxygen gases—normal components of the atmosphere. The wastewater emerging from the system, with contaminants removed, can then be fed back into the HTL process. On the other hand, on the cathode, the negatively charged half generates an energy source that can be used as a potential fuel. This can either be hydrogenated organic molecules or hydrogen gas, per se. Research explicitly points out that the hydrogen by-product generated by the process is a net plus. When collected and fed into the system as a fuel, it could keep the system running with fewer energy inputs, thereby potentially making it more economical and carbon-neutral than current biomass conversion operations. The speed of chemical conversion also proves to be an added benefit to the system. Lopez-Ruiz said that they did a comparison of rates – how fast they could remove the oxygen from organic molecules as opposed to the energy-intensive thermal removal. He said that 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.
What makes this entire process unique is the fact that the system investigates the deposition of nanoparticles of the metals responsible for this particular chemical conversion. These particles because of having a large surface area require less metal to do their work. “We found that using metal nanoparticles as opposed to metal thin films and foils reduced the metal content and improved the electrochemical performance”, said Lopez-Ruiz. The novel catalyst requires 1,000 times less precious metal, in this case, ruthenium, than is commonly needed for similar processes. Specifically, the laboratory-scale flow reactor uses an electrode with about 5 to 15 milligrams of ruthenium, compared with about 10 grams of platinum for a comparable reactor. Researchers also demonstrated that the PNNL process can handle the processing of small water-soluble carbon compounds – by-products found in the water waste stream of current HTL processes, along with many other industrial processes. Although there are several small, carbon compound processes in the wastewater streams available at low concentrations, there has been no cost-effective technology to handle them until now. These short-chain carbon compounds undergo transformation to fuels during the newly developed process, which can be very much useful.
The expected increase in world population to 10.5 billion by 2050, combined with significant economic growth in emerging economies will result in substantially increasing energy consumption. To be able to respond to this growing demand, we need to use natural resources more efficiently and increase the use of renewable energy, such as biofuels. Biofuels help to enhance and safeguard energy security by reducing the world’s dependence on fossil energy sources. Using waste and residue as raw materials for biofuels is an excellent example of answering the needs of a circular economy. Reducing the amount of waste and making the most of our valuable natural resources is crucial for our future survival.