Friday, November 04, 2005

Chemical Looping Combustion

One of the proposed ways of dealing with global warming is the capture of CO2 at stationary powerplants, with the gas being injected into wells, deep aquifers, or even (after liquifaction) into the deep ocean.

The showstopper here is the cost of separating CO2 from the flue gases. Existing techniques led to estimates of this taking perhaps 1/3 of the energy roduced by the powerplant, raising the cost significantly.

Several groups are now looking at a new technology for this problem, called chemical looping combustion, or CLC. Here, combustion is separated into two phases. In the first, particles containing a transition metal oxide (typically an oxide of Fe, Mn, Cu, Co, or Ni) is reduced by a fuel gas, either to another oxide or to the base metal. In the second phase, the metal is reoxidized with air. The oxide particles are recirculated between the two fluidized bed reactors in which these reactions occur.

Because the air and fuel never mix, no nitrogen is mixed into the combustion products of the fuel. For a hydrocarbon fuel, the exhaust of the combustion reactor is CO2, water, and unburned fuel. This gas is cooled, compressed, and the CO2 liquified; any unburned fuel gas is recirculated to be burned again (or bled off to the other reactor for disposal to the atmosphere).

How efficient is this? Calculations show the system can achieve overall efficiency on natural gas of better than 54%; this is reduced by about 2% if the cost of compressing/liquifying the CO2 is included. The high efficiency is in part due to a reduced irreversibility of combustion, and also due to the lack of a separation stage to remove nitrogen from either air or the flue gases. The cost estimates appear reasonable; with lab measurements of fluid bed erosion rates the cost of separating a ton of CO2 is less than 1 euro.


Blogger FuturePundit said...


Is that Euro per ton in metric tons? Also, is it tons of CO2 or tons of carbon?

The reason I ask is that I was looking at charts of how many tons of CO2 are emitted per country per year and found that in carbon equivalent terms the US is between 5 and 6 tons per person per year. Well, at 1 dollar or 1 euro to prevent the emissions the cost per ton would be piddlingly small.

See from a Japanese web site 1994 data with the US at about 5 and a half tons per person per year. Or see this 2004 chart with Americans at 5.76 tons per person per year.

But even if your cost is in carbon dioxide weight terms the weight would only approximately triple and that would still cost very little.

But either hydrogen or electricity would need to be delivered to cars to make this work.

5:53 PM  
Blogger Chris said...

I'm sorry FuturePundit, but I don't understand your final comment on having to deliver, "either hydrogen or electricity to cars to make this work."

What it sounds like is that you mean a CLC system will be installed in a car. I am one who is doing research into this technology and I don't believe that vehicles will be equipped with such a system. It's too big to be in place for a car.

Besides, the biggest producers of CO2 aren't vehicles, but are energy plants (see Besides, if hydrogen is used as a combustion source no CO2 would be generated any way - it's just not a product of the chemical reaction.

You are correct though in saying that the cost of CO2 sequestration is piddingly small when utilizing a CLC system - especially if you compare it to current costs.

8:14 AM  

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