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Capturing Carbon: Expert Q&A

On July 8, 2008, geophysicist Klaus Lackner answered selected questions about carbon capture and related issues.


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Q: Would it be possible to put a smaller version of your CO2 filter in the exhaust pipe of a car? That would stop the pollution at its source, right? Jake Moore, Colorado Springs, Colorado

Klaus Lackner: A car produces about one pound of CO2 per mile. There is no problem with collecting the CO2 in the tailpipe, but one might easily end up with a trailer hitched to the car for carrying all this CO2 back to the filling station. The gas burned from a 15-gallon tank would fill up five 60-inch-tall gas bottles (the kind you see at a fair for filling helium balloons).

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Q: Could your synthetic tree technology be applied to the underbelly of commercial jets if a special compartment was developed to house it? If so, this could offset some of the CO2 that jet engines expire. Farley Tokley, Madoc, Ontario, Canada

Lackner: The high wind speed would be very hard on the filter, and there is no need to push the air through the filter this fast. Luckily, air mixes very fast, so it is not necessary to catch the CO2 near the plane or near the car. The emissions are not causing harm locally but contribute to the steady accumulation of CO2 in the atmosphere. Capture can happen anywhere and is best done where the CO2 can be put away safely.

Q: It seems to me that large quantities of CO2 from sources like power plants might be sequestered in ponds of green algae. The CO2 would make it grow much faster, and the algae could be harvested. There might be problems such as pH control, but such problems could be addressed through research. Do you think that this is a viable idea? Willard Beattie, Las Cruces, New Mexico

Lackner: This is indeed a viable idea. There are several groups working on it. For example, GreenFuel in Massachusetts is actively developing this concept.

Q: I've been pretty intensely interested in this issue for about two years now, since I first saw you on PBS, and I applaud your continued efforts. I'm curious to know:

  1. Why have you apparently changed from convection towers, with liquid sorbent to filter the CO2 to these hanging sheets?

  2. Also, if these were produced, how many towers of hanging sheets would be needed to significantly reduce CO2in our atmosphere?

  3. Thirdly, the big hang-up seems to be what to do with the sequestered carbon, where to store it, etc. We have a similar problem, it seems, with spent nuclear fuel rods, yet the technology goes on, and the French seem to have made great progress with storage and recycling the fuel rods. Assuming that carbon leakage or temporary storage of sequestered carbon would not pose as significant a threat as nuclear fuel rod storage, don't we need to move rapidly to put these CO2 collectors in place?

  4. For weeks now, the air for millions of Californians has been smoky and hazardous to our health. California drought and Midwest flooding are no doubt results of climate change. It would seem that we need to act quickly, even if we haven't fully answered the CO2 storage question.

Ed Schilling, Magalia, California


  1. The solid sorbent works faster than the liquid sorbent we used before, and it requires less energy in the regeneration. These two advantages have led us to a different design. The design is still changing and improving, even since NOVA took pictures.

  2. The current plan calls for a unit that can collect one ton of CO2 per day. There is nothing special about this size, except that such a unit can be factory-built and shipped in a standard shipping container. The world emits 70 million tons of CO2, so 10 million units would make a difference. For comparison, in 2006 more than 73 million motor vehicles were produced worldwide.

  3. For CO2 storage to become accepted, it is important that carbon storage is carefully regulated, that the process is transparent to the public, and that there is a clear accounting of what happened to the CO2. This is particularly true of underground storage, where there is always a small chance that pressurized CO2 could escape. Nevertheless it is possible to try things out, because in a well-monitored storage site it is always possible to release CO2 in a controlled manner in the unlikely event that it threatens to escape. Such a release is certainly no worse than ignoring the emission in the first place.

  4. Quick action is important. Even if we act fast, CO2 levels will still keep rising for many years. Most of the uncertainties about CO2 storage relate to public acceptance. Public acceptance will only come with experience. Therefore, we should start now and make sure that public institutions like the Environmental Protection Agency develop the right procedures and regulations for oversight, safety, and transparency. By starting with a large number of small projects, one can experiment and create a level of familiarity, which is necessary to step up to the full scale.

Q: Capturing CO2 is an interesting idea, but plants and trees leave behind freshwater and oxygen instead of some undesirable waste product. Is there any research being done on mimicking the process of photosynthesis? Or perhaps in the genetic engineering of plants/trees, which have an accelerated photosynthesis cycle? C. Tam, Vancouver, British Columbia, Canada

Lackner: There is research in such fields, but it is worth pointing out that all the plants and trees in the world are not enough to absorb all the carbon that people have released and are expected to release over the next 100 years. The world has consumed 300 billion tons of carbon since the beginning of the industrial revolution and is consuming seven billion tons per year today. The entire biomass on Earth amounts to 600 billion tons. Forty years from now we would have to have doubled the standing biomass in order to absorb all the CO2that will have been emitted by then. I doubt that we can double the total biomass without severe ecological consequences. This is why carbon capture and storage is one important tool in our toolbox.

Q: Is there a biological way to store the carbon? I've heard that there are bacteria that take CO2 and turn it into biological matter. Could we use the by-product of that as a liquid fertilizer in hydroponics?

Also, have you looked into using the updraft from tall buildings as a power source? If you use these updrafts you could set up the "leaves" in a ring. The ring could slowly turn so that part of it is exposed to the updraft while the other side is being washed of the carbon. This would get more air to pass the surface of the material, it wouldn't use electricity other than wind, and it would only run when an updraft was occurring, thus making it very efficient. And it would work in the areas where carbon is produced in excess. Anonymous

Lackner: It takes energy to convert CO2 into biological matter. This energy has to come from somewhere. Plants get this energy from sunshine. Some bacteria can use chemical sources, like hydrogen sulfide or ammonia to convert CO2 into biological matter. I don't think we have enough of those energy sources to cancel out fossil fuel use.

Regarding your second question, we have looked at funneled, wind-like updrafts. In some cases, they can help in providing a more steady flow of air. Unfortunately, the energy collected is far too little to pry the CO2 off the sorbent material. Even though this energy is only a small fraction of what was released when the CO2 was formed, it is huge compared to the wind energy that passes through the collector. The size of a windmill that offsets the CO2 emissions collected by a single air collector unit is many times larger that the air collector.

Q: Professor Lackner,
I read that some scientists in Italy were working on a way to convert carbon dioxide into hydrocarbons. If they succeed, do you think that this would prove to be a practical method of capturing carbon dioxide? Thank you for your attention. Duane, Pomona, California

Lackner: There are many people working on making hydrocarbon fuels from CO2 and water, myself included. It is certainly possible to make such fuels. They provide an extremely convenient way of carrying around large amounts of energy. However, for this technology to succeed, one needs very cheap electricity that is made from non-carbon energy sources, like solar energy. The process also requires CO2 which our air collector could provide. Once solar energy is truly affordable, it would be possible to build a solar field in the desert, collect CO2at the site, and deliver liquid hydrocarbon fuels for cars and airplanes without using fossil fuels. When the fuel is burned, CO2 and water are released back into the environment.

Q: I am a retired Chemistry Professor at UMass, Amherst and a member of the NAS and NAE who listened with interest to your presentation "Capturing Carbon" on NOVA scienceNOW on July 2, 2008. I have just returned from a meeting with Dr. Alan Page, a forester from Belchertown, Mass., where I am helping him and his group develop an alternative method on which I'd like your opinion. This involves preparing biochar from biosources (mostly waste wood) and using it, along with fertilizer, to nourish plant growth. This appears to help growth as well as sequester the carbon in a quite stable form in the soil. Thus, it is "carbon negative" in that CO2 is captured from the air by the growing plants and then gets transformed into a stable form, and at the same time, contributes to plant growth. This seems like a good plan. Richard Stein, Amherst, Massachusetts

Lackner: There recently has been quite a bit of discussion about Terra Preta as an example of a soil that is holding on to a lot of organic carbon in the form of char and its potential for carbon sequestration. Apparently these soils can last for centuries, and it is well known that certain chars are very stable and thus could sequester carbon for a long time. Soil sequestration is a valid method for carbon management. More data on the longevity of these chars on the millennium timescale would be useful. However, in the end, it is a matter of scale, of cost, and of the environmental impact of running millions of charcoal kilns. Given the net efficiency of converting sunlight into char, there seems to be not enough arable land to solve the carbon problem with this method alone. On the other hand, it could well help in creating credible carbon offsets.

Q: Scientists have found living organisms deep underground. Would capturing and then storing carbon in rocks destroy those organisms' environment? Richard Hendricks, Austin, Texas

Lackner: Injecting CO2 into an underground reservoir would certainly change the local environment and thus affect the organisms that live there. Some will thrive and others will suffer. While we should minimize such impacts, they cannot be avoided completely. The same happens when one plows a field, builds a house or a road, or waters a lawn.

Q: Is it true that as long-term temperatures fluctuate, temperature increases precede increased greenhouse-gas levels, and the increased GHG levels then enhance and/or accelerate the temperature increase? I've read about feedback. Is this feedback? John D., Asheville, North Carolina

Lackner: There are complex feedbacks between CO2and the climate—this I know even though I am not a climate scientist. At the end of the most recent ice age, the atmospheric CO2concentration rose from 180 ppm to 280 ppm, without injection of additional carbon into the system. The transition between ice ages and warm times correlates with changes in the distribution of the sunlight on the planet due to changes in the Earth's orbit, and thus those temperature changes were not driven by CO2 changes. Warming led to increased CO2 levels. Since CO2 is a greenhouse gas, this in turn causes further warming.

Today, humans interfere with the natural system by injecting vast quantities of additional CO2into the atmosphere. The rise of CO2 in the air over the last 200 years from 280 ppm to 380 ppm has been driven mainly by fossil fuel consumption. Indeed, the real question is where more than a third of all our emissions ended up, because they are not in the atmosphere. It seems most of it has been dissolved into the top layers of the ocean.

Q: If we are setting free so much carbon from coal and oil, is there any way to reproduce and reverse the coal formation process? I mean, take the CO2, make it back into coal, and bury it in open mines or in huge holes, cover it, and then restore the environment to its previous state? I know economic factors are important, but to have an economy we will need a stable planet first. Vladimir Sanchez, San Francisco, California

Lackner: It is possible to return the carbon to the ground, but it would require us to give back the energy we harnessed by making carbon dioxide from carbon. Since none of these chemical processes is perfectly efficient, we actually would need far more energy than we got out of the fossil fuels in the first place.

Q: CO2 is less than .04 percent of the air, and water vapor is 95 percent of the greenhouse effect. Man-made CO2 is a very small effect on greenhouse, if at all. We are coming out of an ice age that we cannot control, and CO2 is coming out from warmer water. Why the focus on man-made CO2? Why not water vapor? It makes no sense. Ken Oh, San Jose, California

Lackner: CO2 is about 0.04 percent of the air, and it is a controlling greenhouse gas. Water vapor is a powerful and abundant greenhouse gas, but the amount of water vapor is not fixed. It will change with temperature and CO2 content of the air. Water vapor indeed accounts for a large fraction of the greenhouse warming, but by most measures far less than 95 percent. Nevertheless, if the planet were bone dry, it would indeed be miserably cold. However, the water vapor content of the air responds rapidly to temperature and prevailing climate. Water evaporates and rains back out on fast timescales. A better way of looking at the system is that the addition or removal of CO2 will change both temperature and water vapor simultaneously.

Doubling CO2 in the air will result in a climate with a higher temperature and a higher water vapor level. Indeed, most of the warming that results is driven by the water-vapor-induced greenhouse effect, not by the CO2-induced greenhouse effect. The doubling will have many other effects like cloud cover changes and snow cover changes, which all will feed back into the temperature change, and some of them are difficult to predict.

For an analogy of such a system, consider a small rowboat with a flat bottom with an inch of water on the floor. As you shift your weight from the middle of the boat to its side, the boat will list, first because your weight is shifting and then because all the water is sloshing to your side as well. In the end, the boat's list may be explained mainly by the sloshing water, but it is the shifting of your weight that is responsible for the entire change. (By the way, moving back to the middle or even to the other side may not right the boat anymore.) Similarly, in climate, the question is not how much of the greenhouse effect is caused by water vapor, but how much change will be caused by doubling CO2in the air.

Q: If you were to integrate your artificial leaf with the exhaust outlet of a factory or a car, you might be able to get more out of it.

  1. You would have greater concentration of CO2; and

  2. You could harness the kinetic energy of the gas leaving the exhaust system.

One more thing to consider, the material binding to the CO2, what if you could have it flowing in pipes coplanar to your "leaves" and moving countercurrent to the exhaust fumes? Arald Jean-Charles, North Arlington, New Jersey

Lackner: The leaves are designed for capturing CO2from ambient air. They would benefit some from the higher concentrations, but I would build a different system for that case. The advantage of working with ambient air is that we can collect the CO2anywhere in the world. Most emissions occur in places far from CO2 storage sites. This saves the expense of transporting CO2 which can get very large if CO2 has to be shipped out of the middle of a city. Finally, taking advantage of the kinetic energy in the gas would yield very little energy. It would even be counterproductive if some pump or some fan would have to work harder in return.

Q: Instead of sequestering the CO2 why can't we strip off the carbon, let the oxygen go free into the atmosphere, and use the carbon to make useful things like composite auto bodies or tires? Essentially turn the carbon black industry into a carbon-recycling industry? Anonymous

Lackner: There really is no market for six tons of carbon black per person per year. We don't use that many tires, ink cartridges, etc. It is straightforward to recycle copper or aluminum. Recycling carbon is difficult because we stripped the fossil carbon of its energy. Unlike matter, quality energy is truly consumed, so one would need another energy source to recycle carbon, but then we might as well use this energy source rather than fossil carbon.

Q: Why not create a substance that steals the carbon in CO2, scrape it off, turn it into diamonds, and sell them for a net gain? Matt, Allentown, Pennsylvania

Lackner: Six tons of diamonds for everybody every year? Seriously, making diamonds is an expensive, energy-intensive business.