New Membrane Technology for Lower-Cost CO2 Separation from Power Plant Gases
19 September 2007
Researchers in the membrane research group (MEMFO) at The Norwegian University of Science and Technology (NTNU) in Trondheim have developed and are patenting a new nano-structured plastic membrane for the low-cost separation of CO2 from flue gases from power plants.
The polyvinylamine membranes use a fixed-site-carrier (facilitated transport) mechanism. CO2 is transported through the membrane by both solution-diffusion and fast “hops” between reactive fixed-site-carriers. The effectiveness of the membrane increases proportionally to the concentration of CO2 in feed gas mixture.
The novelty is that instead of using a filter that separates directly between CO2 and other molecules, we use a so-called agent. It is a fixed carrier in the membrane that helps to convert the gas we want to remove.
—Professor May-Britt Hägg, MEMFO
The agent helps the CO2 molecules combine with moisture to form HCO3 (bicarbonate) which is quickly transported through the membrane. Conventional membranes without carrier sites may lose their separation ability when swollen by water vapor in the system. By contrast, this polyvinylamine membrane shows even more enhanced CO2 transport in the presence of water molecules by the reversible formation of bicarbonates.
In recent testing under various conditions (pressure of 2-15 bar and temperature of 25-55°C and relative humidity of 30-95%), the team found that both CO2 permeance and CO2 selectivity over N2 were dependent on humidity change in the system.
The permeance increased continuously with humidity and even exceeded more than 0.3m3(STP)/m2•bar•hr, while the selectivity (CO2/N2) showed convex curves with a maximum of approximately 120-250 depending on temperature and pressure.
The CO2 capture performance (high permeance, excellent selectivity and durability) under the presence of water vapor in the system indicates a potential for omitting water removal units from the CO2 capture process when applying this membrane for real power plant process.
According to the team, the excellent CO2 selectivity over N2 (150–250) even at relatively high pressure (10-20 bar) has strong potential for replacing the conventional CO2 capture process using alkanolamine solutions.
MEMFO recently joined a consortium of 26 European businesses and institutions within a project named NanoGloWa—Nanostructured Membranes against Global Warming. The consortium has received €13 million (US$18 million) to develop such membranes.
Within a five-year period, the plan is to test the membrane technology in four large power plants in Europe. We believe this will result in an international breakthrough for energy-efficient CO2 membranes.
—May-Britt Hägg
The new technology is very suited for coal-powered plants. In gas-fired plants, the concentration of CO2 is so low that the pressure in the waste gas must be increased before the gas can be cleaned with this method. Statoil is currently developing a method for pressurized exhaust that could be combined with this membrane technology, and that would make it suitable for purification in gas-powered plants as well, according to Professor Hägg.
Resources:
Can we please use this captured CO2 to feed algae ponds to make biodeisel. Sequestration is nice but using it to make fuel is better.
Posted by: Joseph | 19 September 2007 at 09:28 AM
In addition to filtering out pollution at its source, is it possible, or has it been tried, to remove the pollutants from air above a city? I'm talking like putting some sort of vacuum or fan with filter on top of a building in Los Angeles and filtering out smog. Has this been tried? Sounds like it'd be tough to do, but it'd be something.
Posted by: Elliot | 19 September 2007 at 09:34 AM
Joseph,
Plants are fairly good at separating our CO2 by themselves so full CO2 separation with these membranes seems unnecessary. Also, reuse of CO2 exhaust gases would reduce the CO2 intensity of a fossil fuel power station a lot less than sequestration as the CO2 wold get released to the atmosphere eventually is used to make biofuel.
Posted by: Thomas Lankester | 19 September 2007 at 09:39 AM
is the HCO3 bicarbonate they form a solid? if so, can it simply be buried? how hard would it be to go from bicarbonate to Sodium bicarbonate?
Posted by: | 19 September 2007 at 10:49 AM
@ Joseph -
algal oil may become an option in many places, but Norway probably isn't one of them. They have little level land to begin with and, their fjords probably receive too little sunlight to support year-round algaculture on floating platforms or even skirt ponds.
Instead, the system will likely be applied simply to stripping CO2 from natural gas and/or biogas that has already been treated to remove the sulfur. The alternative is the expensive wet scrubbing with an amine solution, typically involving toxic and/or corrosive compounds that are recycled in the process. The CO2 can then be sequestered in saline aquifers deep underground.
http://www.statoil.com/statoilcom/svg00990.nsf/web/sleipneren?opendocument
In some cases, CO2 injection can also be used as a tertiary oil recovery technology.
@ Elliot -
smog is caused not by CO2 but by HC and NOx emissions in conjunction with strong sunlight. Filtering these pollutants out of the atmosphere, where they are highly dilute, would require massive air flow volume. The 1992 World Expo in Seville (Spain) featured giant towers that were open at the bottom. Clean water was sprayed into the air inside, cooling it sufficiently to sustain modest natural downward convection. The result was outdoor air conditioning at street level. However, no effort was made to use the towers to scrub out airborne pollutants.
The best place to combat smog is at the source, i.e. the exhaust systems of power stations, industrial furnaces and ICEs. Home heating appliances produce very little NOx and virtually no HC anyhow and, are only used in winter when there is too little sunlight to produce any smog.
Posted by: Rafael Seidl | 19 September 2007 at 11:46 AM
Increasing the partial pressure of CO2 in the exhaust might also be a good idea; anyone know if selective O2 membranes (like the high-temperature SOFC ZrO2 type) have ever been studied together with downstream CO2 filers? A combination might be more cost effective, than either of these systems alone...
(Combustion in an oxygen-enriched environment (over the 21% of air, like 40-60%) increases the partial pressure of CO2 downstream, which seems the main factor in the above mentioned technology.
Posted by: realarms | 19 September 2007 at 12:01 PM
If the reaction is:
CO2 + H2O -> HCO3 + H
Where's the H going?
Posted by: Neil | 19 September 2007 at 12:45 PM
If the reaction is:
CO2 + H2O -> HCO3 + H
Where's the H going? -- Neil
It's CO2 + H2O -> HCO3- + H+
The solution is weakly acidic, which is why CO2 is considered an acid volatile. That's also why pristine rainwater is weakly acidic, pH ~5.6.
Ocean waters are weakly alkaline, pH ~8.4, because of the high level of dissolved salts. Adding CO2 will lower the pH -- that's what "ocean acidification" is about.
Posted by: cidi | 19 September 2007 at 12:58 PM
The bicarbonate presumably only exists while the CO2 is chemisorbed in the membrane, I think. When the carbon leaves the other side, I would presume it's changed back to CO2.
There must also be diffusion of hydrogen ions (or, hydroxyl ions, going the other way) to neutralize the current, I imagine.
Posted by: Paul Dietz | 19 September 2007 at 01:55 PM
Some missing information. What fraction of CO2 capture is aimed at eg 80%? What is the energy penalty for dramatically cooling and compressing the flue gas? Do the membranes become clogged or 'poisoned' by SOx? Is there an offsite energy requirement for the absorbent for HCO3?
Posted by: Aussie | 19 September 2007 at 02:10 PM
A note on necessary membrane area:
A small powerplant of 350-450 MW, depending on thermal efficiency, uses 1 GW of coal, corresponding to roughly 40 kg/s. A typical coal might have 63%(mass) carbon, meaning that it would produce 44/16*40*3600*0.63 = 249480 kg/h of CO2. I assume STP means standard temperature and pressure, i.e. 0°C, 101325 Pa. A this condition, one kmol has a volume of 22.4 m3. So the volume of CO2 produced per hour is: 249480/44*22.4 = 127008 m3. At atmospheric pressure, the membrane is capable of absorbing 0.3 m3/m2 per hour, so the necessary membrane area to remove CO2 from a small coal-fired powerplant is: 127008/0.3 = 423,360 m2.
I hope the membrane material is cheap...
And is capable of tight packing...
Anyway, the biggest problem with CO2 capture at this point is energy consumption/waste irrespective of type of process used, so membranes are highly relevant.
How does the membrane get rid of the CO2 on the other side of the membrane, I wonder?
Posted by: Thomas | 20 September 2007 at 06:43 AM
Thanks Rafael, learn something new every day. I'm kind of embarrassed I didn't know what went into smog, just kind of always assumed it was concentrated CO2. That Spain idea is interesting, I'd love to have seen it. No doubt that scrubbing at the source is best, I just wish there was another option.
Posted by: Elliot | 21 September 2007 at 01:05 AM
Nanotecnologia com membrana para separar CO2
Posted by: Vadson | 21 September 2007 at 04:07 AM