Tikalon Header

Oxygen-Enriched Combustion

May 28, 2012

My first experience with gas-permeable membranes was as a young child. In those days, before mylar was used for this purpose, helium balloons were made from rubber. I found that my balloon became deflated after just a day. Anlayzing the problem, I decided that the neck hadn't been tied properly, and I decided that the next time I would tie my own, second knot to prevent escape of the gas.

Well, that didn't work, since my hypothesis was wrong. Of course, at age five I knew nothing about the element, helium, or the properties of rubber. Later, I was using helium for leak testing of vacuum systems. Helium is inert, it's a monoatomic gas, and it's also the smallest gas molecule. For these reasons, helium leaks through almost everything.

Polymers other than rubber have been engineered to be specifically permeable to gas molecules smaller than a particular size. These are used for gas separation, one application for which is fuel tank inerting. These membranes, used in nitrogen generators, separate nitrogen and oxygen from air so that only nitrogen exists in the empty space (ullage) of fuel tanks, for obvious safety reasons.

Although polymer membranes are useful at low temperatures, they will decompose at high temperatures. Fortunately, polymers are not the only materials that act as separators. Some ceramics will work as well.[1] A team of engineers at MIT, led by Ahmed F. Ghoniem, the Ronald C. Crane Professor of Mechanical Engineering, has been investigating ceramic membranes as a way to separate oxygen from the air as a means of doing cleaner combustion of fossil fuels.[2-4]

Figure captionDanger!
Hot Technology.

Members of
Ahmed F. Ghoniem's
lab at MIT.

Left to right, Anton Hunt, Ahmed Ghoniem, Patrick Kirchen and James Hong.

(MIT Photo).[2]

When combustion of fuels is done with air, a gas that's about three-quarters nitrogen, small quantities of nitrogen dioxide (NO2) are produced, along with the expected carbon dioxide and water vapor. One global greenhouse mitigation method is carbon sequestration, in which the CO2 is captured and "buried" underground. It would be nice if the effluent gas going into the carbon sequestration process didn't contain all that nitrogen.

Burning fuels in pure oxygen, and not air, is called oxyfuel combustion. The MIT engineering team has done previous work in pressurized oxyfuel combustion, a process that improved fuel efficiency. Their current research is in the development of a ceramic membrane process so this technology can be used in future and existing powerplants as a way to reduce greenhouse gases.[2] Says team leader, Ahmed Ghoniem,
"The whole objective behind this technology is to continue to use cheap and available fossil fuels, produce electricity at low price and in a convenient way, but without emitting as much CO2 as we have been."[2]
The MIT membrane process is pictured, below.[4]
MIT oxygen membrane combustion process
The MIT oxygen membrane combustion process. Oxygen molecules (red) and nitrogen molecules (blue) from air pass over the ceramic membrane, and only oxygen passes through to react with carbon (black) and hydrogen (green). The resultant carbon dioxide product can be captured and stored. (Screen capture from a YouTube video).[4]

Readers might remember Le Chatelier's principle from their high school or college freshman chemistry courses. One expression of this principle, which is actually a qualitative statement of the affect of reactant and product concentrations on the rate constant, is that removal of products leads to a faster reaction. In the MIT combustion membrane process, the combustion removes oxygen from one side of the membrane, allowing more oxygen to pass through. Says Ghoniem,
"It turns out to be a clever way of doing things... The system is more compact, because at the same place where we do separation, we also burn. So we’re integrating everything, and we’re reducing the complexity, the energy penalty, and the economic penalty of burning in pure oxygen and producing a carbon dioxide stream."[2]

Aside from experiments on various temperatures, pressures and fuels, the MIT team is developing a computational model for their process. One interesting result is that gas flow protects the material when the gas temperature exceeds material limits.[2-3]

References:

  1. See, for example, Renate M. de Vos and Henk Verweij, "High-Selectivity, High-Flux Silica Membranes for Gas Separation," Science, vol. 279, no. 5357 (March 13, 1998) pp. 1710-1711.
  2. Jennifer Chu, "Oxygen-separation membranes could aid in CO2 reduction," MIT Press Release, May 21, 2012.
  3. Jongsup Hong, Patrick Kirchen, and Ahmed F. Ghoniem, "Numerical simulation of ion transport membrane reactors: Oxygen permeation and transport and fuel conversion," Journal of Membrane Science, vol. 407-408, July 15, 2012, pp. 71-85.
  4. Oxygen-separation membranes could aid in CO2 reduction, YouTube video, May 11, 2012.