A thermochemical reactor powered entirely by electricity has been unveiled by Jonathan Fan and colleagues at Stanford University. The experimental reactor was used to convert carbon dioxide into carbon monoxide with close to 90% efficiency. This makes it a promising development in the campaign to reduce carbon dioxide emissions from industrial processes that usually rely on fossil fuels.
Industrial processes account for a huge proportion of carbon emissions worldwide – accounting for roughly a third of carbon emissions in the US, for example. In part, this is because many industrial processes require huge amounts of heat, which can only be delivered by burning fossil fuels. To address this problem, a growing number of studies are exploring how combustion could be replaced with electrical sources of heat.
“There are a number of ways to use electricity to generate heat, such as through microwaves or plasma,” Fan explains. “In our research, we focus on induction heating, owing to its potential for supporting volumetric heating at high power levels, its ability to scale to large power levels and reactor volumes, and its strong safety record.”
Induction heating uses alternating magnetic fields to induce electric currents in a conductive material, generating heat via the electrical resistance of the material. It is used in a wide range of applications from domestic cooking to melting scrap metal. However, it has been difficult to use induction heating for complex industrial applications.
In its study, Fan’s team focused on using inductive heating in thermochemical reactors, where gases are transformed into valuable products through reactions with catalysts.
Onerous requirements
The heating requirements for these reactors are especially onerous, as Fan explains. “They need to produce heat in a 3D space; they need to feature exceptionally high heat transfer rates from the heat-absorbing material to the catalyst; and the energy efficiency of the process needs to be nearly 100%.”
To satisfy these requirements, the Stanford researchers created a new design for internal reactor structures called baffles. Conventional baffles are used to enhance heat transfer and mixing within a reactor, improving its reaction rates and yields.
In their design, Fan’s team re-reimagined these structures as integral components of the heating process itself. Their new baffles comprised a 3D lattice made from a conductive ceramic, which can be heated via magnetic induction at megahertz frequencies.
“The lattice structure can be modelled as a medium whose electrical conductivity depends on both the material composition of the ceramic and the geometry of the lattice,” Fan explains. “Therefore, it can be conceptualized as a metamaterial, whose physical properties can be tailored via their geometric structuring.”
Encouraging heat transfer
This innovative design addressed three key requirements of a thermochemical reactor. First, by occupying the entire reactor volume, it ensures uniform 3D heating. Second, the metamaterial’s large surface area encourages heat transfer between the lattice and the catalyst. Finally, the combination of the high induction frequency and low electrical conductivity in the lattice delivers high energy efficiency.
To demonstrate these advantages, Fan says, “we tailored the metamaterial reactor for the ‘reverse water gas shift’ reaction, which converts carbon dioxide into carbon monoxide – a useful chemical for the synthesis of sustainable fuels”.
To boost the efficiency of the conversion, the team used a carbonate-based catalyst to minimize unwanted side reactions. A silicon carbide foam lattice baffle and a novel megahertz-frequency power amplifier were also used.
As Fan explains, initial experiments with the reactor yielded very promising results. “These demonstrations indicate that our reactor operates with electricity to internal heat conversion efficiencies of nearly 90%,” he says.
The team hopes that its design offers a promising step towards electrically powered thermochemical reactors that are suited for a wide range of useful chemical processes.
“Our concept could not only decarbonize the powering of chemical reactors but also make them smaller and simpler,” Fan says. “We have also found that as our reactor concept is scaled up, its energy efficiency increases. These implications are important, as economics and ease of implementation will dictate how quickly decarbonized reactor technologies could translate to real-world practice.”
The research is described in Joule.
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