Sparks

MIT process produces fuel from exhaust

 By RP Siegel

Is it possible that a fossil fuel power plant could be zero carbon, while also recycling some portion of its exhaust as fuel, thereby reducing consumption?

Recent research by Mechanical Engineering professor Ahmed Ghoniem and post-doctoral associate Xiao-Yu Wu at MIT suggests that it could be. The key is a new membrane reactor technology that can pull pure oxygen from an exhaust stream. Use of this membrane can reduce carbon dioxide into carbon monoxide which can then be used either as a fuel directly or combined with hydrogen to produce any number of hydrocarbon fuels.
Use of this technology in conjunction with a natural gas-fired power plant would allow some portion of the exhaust stream to be converted back into fuel. This fuel could then be recycled back through the process, reducing some portion of the fuel consumption required.

The heat source

According to Wu, the process will require a heat source. And while waste heat is available in a power plant environment, the required temperature of 900° C will necessitate the use of an external energy source, which, says Wu, could be renewables. This temperature, however, represents an improvement over previous systems that required higher values.
Commenting on this achievement, Xuefeng Zhu, a professor of chemical physics at the Chinese Academy of Sciences, in Dalian, China, said, “Using an oxygen-permeable membrane can significantly reduce the reaction temperature, from 1,500° C to less than 1,000° C, indicating a great energy saving compared to the traditional carbon dioxide decomposition process.” Zhu acknowledged the importance of this work for its ability to convert sustainable thermal energy to chemical energy, where it can be stored indefinitely.

In order to maintain a flow of oxygen from the exhaust stream, across the membrane, some force needs to be applied. The application of a vacuum could work, but that uses considerable energy. Instead the researchers propose a flow of other chemicals such as hydrogen or methane, which are both chemically attractive to oxygen, to draw the oxygen across.
Using this model, the researchers envision a natural gas (methane) power plant in which the incoming gas is split into two streams. One stream goes directly into combustion, while the remaining uncombusted methane flows around the combustion chamber and along the sweep side of the membrane where it draws oxygen from the exhaust, which is on the feed side. This process effectively converts the exhaust stream into some combination of syngas, that is hydrogen and carbon monoxide, and water vapor.

The discovery

The team came across this idea as they were investigating oxygen sources to be used in oxy-fuel combustion. Oxy-fuel combustion is a subject of considerable interest to utilities, since, by replacing air with pure oxygen, they can eliminate all forms of NOx criteria pollutants, along with all the expensive emission controls required to remove them from the exhaust, besides being a very effective carbon (dioxide) capture technology.
The method they discovered for extracting oxygen from CO2, turned out to have important synergies with a power plant environment, which suggested these other useful applications, well outside of what they had originally sought. While carbon monoxide (CO) was originally a byproduct of the extraction of oxygen from CO2, it immediately became clear that this compound, while dangerous to work with, is an important industrial chemical, and an effective precursor for many types of hydrocarbon compounds, ranging from fuels to plastics.
The ceramic material, perovskite, that is used in the membrane, has been the subject of intensive investigation due to its unusual combination of properties. It has garnered considerable interest for solar photovoltaic applications. In this application, it’s the oxygen vacancies or ‘defects’ that allows it to filter oxygen so cleanly. The fact that it’s ceramic also means that it can withstand high temperatures, which is also quite helpful.

The actual use

Now that the team has shown that the process works, what remains is to improve its efficiency, lower its cost, and identify how to best utilize it in the context of an overall system.
How much of the energy coming into the plant can potentially be recovered? Every MJ of natural gas emits 0.05 kilograms of CO2. To recover that today, at full efficiency, requires almost 1.3 times the amount of energy in the gas. This could be supplied by renewables and used as a form of energy storage. The team is working to improve this, in order to increase the syngas production rate and CO2 conversion ratio. Areas of exploration include “changing the material used to build the membrane, changing the geometry of the surfaces, or adding catalyst materials on the surfaces.”
Another scenario, involves converting the exhaust into liquid fuels for cars and trucks and aviation, thereby reducing the amount of petroleum reserves required in the years ahead.
Wu explained yet another scenario involving the production of hydrogen, which could be an improvement over the current steam reforming process. When water is used as the oxygen source in the presence of the membrane reactor, hydrogen can be extracted as a byproduct .
This technology is an important step towards an emission-free future that preserves the benefits of chemical storage which are not found with most renewables. The fact that it facilitates the conversion of thermal energy into a form of chemical energy, gives it that much greater potential.

READ MORE: Turning CO2 into fuel by Rob Davies

about the author
RP Siegel
Skilled writer. Technology, sustainability, engineering, energy, renewables, solar, wind, poverty, water, food. Studied both English Lit.and Engineering at university level. Inventor.