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What's the future of coal?

March 15, 2007 By Emma Marris This article courtesy of Nature News.

A US study calls for more investment and focus on carbon capture and storage to make for cleaner coal. finds out what's taking so long.

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After two years of work, an interdisciplinary group of Massachusetts Institute of Technology (MIT) professors released their report on The Future of Coal yesterday in Washington DC, making recommendations about how the United States should use coal for energy.

Isn't coal the dirtiest fuel there is?

It makes the most carbon dioxide, yes. Coal is mostly carbon, after all. In the United States in 2005, 1 kilowatt-hour of electricity made from coal produced about 1 kilogram of carbon dioxide emissions. The same amount of electricity made from natural gas emits half as much, whereas nuclear, wind and solar emit no CO2.

But if some of those coal emissions could be captured and pumped underground, then coal could clean up its act. As challenging as this may be, it may be easier than replacing coal altogether; coal currently provides a quarter of world energy and half of electrical power in the United States. China is building an average of two 500-megawatt coal plants each week, the report says.

How hard is it to get the emissions underground?

Hard. Here is the process as it would work with the most common type of power plant: carbon on its way out of the plant passes upwards through a tower in which a solution of amines is flowing downwards. The CO2 binds to the solvent, which flows out the bottom. The CO2 is then stripped from the solvent so that the liquid can be re-used. This is done with heat, usually some of the steam that would otherwise be turning turbines and making power. The upshot is that the efficiency of the power plant goes down. The CO2 then has to be compressed into a supercritical fluid and piped deep into the earth, into porous rocks or saltwater aquifers where it will stay — one hopes — for millennia.

How much does all this cost?

The report estimates that capture, pressurization, transport and storage would cost about $30 per tonne of CO2. Plants that have all these capabilities are more complex and so also cost more to build.

Could existing plants be modified to do this?

Yes, but retrofitting plants is more complicated than strapping on additional equipment. One of the co-chairs of the report's authoring panel, Ernest Moniz, likened the process to "major surgery". The report's authors believe that retrofits are unlikely to happen for the most common type of plants.

If you're going to build an entirely new plant, better cost returns can be found - at the moment — by turning coal into a gas before burning it, and then capturing the carbon. However, any plant with carbon capture will always be less efficient than one without it.

Why would a power company build a more expensive plant that will produce less energy?

They won't, until it makes business sense for them to do so. If it costs $30 a tonne to sequester the carbon, they won't do it until it costs that much to emit a tonne of carbon. That is why the report calls for carbon regulation in the United States as soon as possible.

And they won't do it until the technology is proven, which most people say it isn't. There are three small-scale demonstration sequestration plants around the world. But they aren't big enough, and aren't collecting enough data, according to the report, which calls for the United States to build three to five more demonstration plants, each pumping on the order of 1 million tones a year into the ground.

This sounds like it will take a while.

Yes, and conventional coal plants are likely to be built in the meantime. Interestingly, NASA policy guru Jim Hansen and some US politicians are talking about moratoriums on building new plants while the new technology is developed. The report does not go this far, merely suggesting they be built as efficiently as possible.

What about all the coal plants in China? Are they going to change?

The short answer is 'not soon'. Economies in China and India are rapidly growing and, particularly in China, that growth is being fuelled with coal. The two will probably account for 70% of new coal demand through 2030. But central governmental control of all this growth is weak.

The MIT group conducted several case studies in China and concluded that the government probably can't force a technology on its populace if it isn't economically feasible. But they end the section with a caution against being too critical: "We would be unwise to expect of the Chinese what we do not expect of ourselves."


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