As you have probably learned from high school chemistry, combustion is the process of burning a fuel source in the presence of oxygen. You may also recall that combustion is an irreversible chemical reaction , meaning that you can’t take the ashes and smoke from a burning fire and turn them back into fuel. However, what if I were to tell you that what you learned in high school is technically false? In other words, what if there is such a thing as “reverse combustion”?!
Well, reverse combustion does exist, and no, it doesn’t involve time travel. In fact, this rather straightforward but rather challenging method of carbon recycling was discovered in the early 90s.1 Even though this shocking phenomenon was discovered earlier , interest in this field hasn’t picked up until recently. Just this year, a research team from the University of Texas at Arlington consisting of co-principal investigators Brian Dennis and Frederick MacDonnell published a paper outlining the reverse combustion of carbon dioxide and water to make a variety of liquid hydrocarbon fuels including diesel, petroleum, and kerosene.2 What’s most significant about the team’s findings is that the team members are apparently the first to use just concentrated heat and light to synthesize liquid hydrocarbons in a single stage reactor. The reactor operates at 180-200 oC with a pressure of 1-6 atmospheres while a catalyst consisting of cobalt and titanium dioxide is used to help absorb light and initiate the necessary photochemical reactions. Unfortunately, even with these conditions, the team’s best run still produced a lot of waste by-products including oxygen gas, with hydrocarbon yield topping at only 13%.
Despite these shortcomings, the process is still a significant step forward in the development of renewable fuels. Because this process uses mostly heat and light, the reaction, in principle, can be coupled with solar energy so that the principal driving force for the reaction is sunlight rather than artificial light and heat coming from the reactor. The paper even suggests the use of parabolic mirrors as a possible engineering consideration to achieve this: the parabolic mirror would focus and concentrate sunlight conto a catalyst bed that would contain the reaction. In terms of the big picture, this process gives us yet another perspective on the ideal of a carbon-neutral environment that puts a twist on the traditional idea of reducing atmospheric greenhouse gas levels. Instead of doing away with the fuels we currently use and replacing them with more environmentally-friendly alternatives like biodiesel or hydrogen gas, reverse combustion focuses on the current fuel economy and looks to provide sustainable recycling of the fuels’ waste-products.3
Now that I’ve introduced two seemingly simple yet cutting-edge developments in the reconversion of carbon dioxide to liquid fuels, you can take your pick - air capture or reverse combustion? Whatever you choose, hopefully you will agree that among the methods of producing renewable energy that’s out there, taking the carbon dioxide junk that’s literally everywhere around us and converting it into something that multimillion-dollar industries are founded upon is truly an amazing feat.
James Siriwongsup is a sophomore from McMurtry College at Rice University.
1. Biello, D. Reverse combustion: Can CO2 be turned back into fuel? http://www.scientificamerican.com/article/turning-carbon-dioxide-back-into-fuel/ (accessed 3/6/2016), part of Scientific American
2. Chanmanee, W. et. al. Solar photothermochemical alkane reverse combustion. Proceedings of the National Academy of Sciences 2016, DOI: 10.1073/pnas.1516945113.
3. O’Callaghan, J. Reverse combustion breakthrough could lead to carbon-neutral fuel cycle. http://www.iflscience.com/chemistry/reverse-combustion-breakthrough-could-lead-carbon-neutral-fuel-cycle (accessed 3/6/2016), part of IFLScience