To continue using coal, we have to capture the emitted CO2 and bury it. This is called carbon capture and storage (CCS), but we will continue to avoid acronyms when possible. There are three steps: first, CO2 has to be separated from the flue gas out of a coal burner; second, the CO2 has to be transported to a burial site; and third, it has to be injected into a geological formation that can hold it forever. The last part is of course highly debatable; but it is the first part, capture, that is the most expensive. There are three basic ways to do this.9 In the first method, the flue gas is mixed with a liquid solvent called MEA into which the CO2 dissolves. The MEA’s chemical name is not always spelled the same way, but it is a corrosive liquid found in household products such as paint strippers and all-purpose cleaners. When the MEA is heated to 150°C, pure CO2 is released, and the MEA is cleaned up with steam to be reused. This method can be retrofitted to existing plants, but there is a huge penalty. The heating and steam production takes up to 30% of all the energy produced by the power plant! The cost of this step can be as much as four times higher than that of the other two steps. At the moment, other absorbers are being tried to lower this cost.10

In the second method, the flue gas mixture is controlled by burning the coal in a specific way. When it is burned in air, which is 80% nitrogen and 20% oxygen, there is a lot of nitrogen in the mix, and N2O is a greenhouse gas. A better way is to remove the nitrogen from air at the outset and burn the coal in pure oxygen. What comes out is water and pure CO2, ready to be sequestered. However, separating the nitrogen from the air to get pure oxygen requires 28% of the power plant’s energy, still a steep penalty. This method is being tested by Vattenfall, Sweden’s energy company, in the town Schwarze Pumpe in Germany. The experiment is fairly large — 30 MW — but not of electric utility size. A novel feature was added to this “oxyfuel” process: the flue gas is recirculated into the burner with the oxygen. This keeps the temperature low enough to prevent melting the boiler walls, as would happen with pure oxygen. In effect, the CO2 in the flue gas replaces the nitrogen in air, diluting the oxygen without using nitrogen.

The third method is coal gasification: the coal is heated to a high temperature with steam and oxygen, turning the coal into a gas, called syngas, which is a mixture of carbon monoxide (CO) and hydrogen (H2), plus some nasty contaminants. After the syngas is purified, it is the fuel for generating electricity in an “integrated gasifi­cation combined cycle,” or IGCC, an acronym that seems unavoidable in this case. Coal gasification has been tested in fairly large power plants, but the IGCC sounds like a Rube Goldberg type contraption that has yet to be verified on a large scale. An air separation unit to get pure oxygen is still required both for syngas generation and for burning the syngas later. After the pollutants are taken out, the gas goes into a chamber where the CO combines with steam (H2O) to form CO2 and H2. Pure hydrogen is separated out through a membrane, giving carbon-free fuel. The rest of the gas, containing CO2, CO, and H2, is burned with oxygen in successive turbines, a gas turbine and a steam turbine, to generate electricity. The pure hydrogen sepa­rated by the membrane can be sold or burned to generate more electricity cleanly. The IGCC can be 45% efficient, compared with 35% in ordinary coal plants limited by the Carnot theorem that we described earlier. Meanwhile, the CO2 generation is lower, and it comes out in pure form to be stored. This separation system adds only 25% to the cost of electricity. An even more efficient method called chemical looping is under development.9 New chemical structures for capturing CO2 are described in Chap. 3 under Hydrogen Cars.

In 2003, the FutureGen Alliance had proposed a plan to test IGCC on a large scale by building a $1 billion plant in Illinois, finishing in 2013. That project was canceled by President G. W. Bush in 2008 because the projected cost had almost doubled. Unbelievably, this figure was an accounting error; the actual increase was to only $1.5 billion. Under President Obama, Energy Secretary Steve Chu has pledged $1.1 billion of economic stimulus money to restart the project, with the other funds to be raised by FutureGen. There is $2.4 billion of stimulus money slated for CCS research. This is to be compared with $3 billion spent by the Department of Energy for this purpose since 2001.

Now that we have separated out the CO2, the problem is where to put it. There are three main places: old wells, underground, and undersea. The oil and gas that we mine have been trapped in the earth for millennia, so it is possible that porous rock or underground caverns can hold liquids and gases stably. To carry CO2 to these sites, the gas has to be highly compressed to a small volume and transported by truck or rail. This step entails a certain amount of danger, should there be an accident causing the container to explode and release tons of CO2 into the atmo­sphere. The gas is then injected under pressure into depleted oil or gas wells, where it could stay for millennia if it were not for the leaks made in drilling the wells in the first place. These old wells have to be sealed tightly. The trouble is that carbon dioxide and water combine to form carbonic acid, and the seal has to withstand this acid attack. This storage solution is well tested because it is used to store excess gas and oil mined in the summer for use in the winter. The difference here is that the storage has to be stable essentially forever. The possibility of leaks has to be care­fully monitored. Injection of CO2 into oil wells is actually beneficial, for it helps to push the oil up. Toward the end of life for an oil well, the oil gets quite thick; and gas, which might as well be CO2, is injected to lower the viscosity. This is what happening in those nodding pumps seen along the California coast.

There are many large subterranean formations that can hold carbon dioxide. These are porous sandstone deposits covered with a cap of hard, impervious rock. For instance, such a depository has been found below a little town called Thornton somewhere south of California’s capital of Sacramento. It is estimated that it can hold billions of tons of CO2 in its pores, enough to store away hundreds of years of California’s emissions.11 Of course, no one knows whether it will leak.

There are plans to drill into this formation and test it, to the dismay of local residents. The reaction, NUMBY (Not Under My Back Yard!), is a switch from NIMBY (Not In My Back Yard!), an epithet used against wind and solar power.

Large geologic formations under the sea have also been found for CO2 storage. These are layers of porous sandstone called saline aquifers lying deep below the seabed and capped by impermeable slate. Storage in these aquifers is the only seques­tration method that has been tested on a large scale, and this is a story in itself.9, 11-13 The Sleipner Platform, shown in Fig. 2.17, is a huge oil drilling and carbon sequestra­tion plant located in the middle of the North Sea, halfway between Norway and England. It was built in 1996 by Statoil, Norway’s largest petroleum company to produce oil while testing sequestration. Built to withstand the frigid conditions and storms with 130-mile winds and 70-foot waves, it houses a crew of 240 whose jobs are considered the most dangerous in the world. Below Sleipner lies not only a rich field of natural gas but also a saline aquifer called the Utsira Formation lying a kilo­meter below the seabed (Fig. 2.18). The aquifer is very large: 500 x 50 km in area and 200 m thick. It can hold 100 times Europe’s annual CO2 emissions.

There was a special reason to build sequestration into the plant ab initio. The gas from the Sleipner field contains about 9% CO2, too high to burn properly unless reduced to 2.5%. The gas has to be scrubbed using the MEA solvent described above, thus releasing a million tons of CO2 a year that has to be stored. The way the CO2 is injected involves a little physics. It is compressed to 80 atmospheres because at this pressure it turns into a liquid about 70% as dense as water. So it is stored as a liquid. When it is mixed with the salt water in the aquifer, it tends to rise, since it is less dense. One worries how fast it moves and whether the 200-m thick layer of shale above the storage volume can spring a leak. Such leaks can arise from drilling through the cap to inject the gas, and these holes have to be carefully sealed with acid-proof material. Statoil has spent millions of dollars to develop a way to measure the spreading and leaking of the CO2 using sound waves. Since the system has 25-m resolution and the area is measured in kilometers, the amount of data is many megabytes. These data clearly show that the CO2 is spreading sideways as well as upwards, and that there are no leaks so far. In the best scenario, the CO2 will eventually dissolve into the brine (in 1,000 years or so) and thus become a liquid heavier than water. This then moves safely downwards, and on a geologic timescale will turn into a mineral, thus locking the carbon away permanently. All fossil fuels will be but a distant memory by that time.

The Utsira formation is unusual in that it is located at the same place as the gas deposit, so that no transportation of the CO2 is necessary; but it is not unique as a large burial site. It is estimated that the USA has subterranean reservoirs capable of storing 4 trillion tonnes of CO2, enough to take care of its emissions until coal runs out. Statoil would not have built the Sleipner plant if it did not have to pay an annual $53M carbon tax imposed by the Norwegian government. Global warming cannot be halted without strong legislation by enlightened political leaders. The cost of separating the CO2 and burying it is estimated to be about $25-$50 per tonne. Though this may come down as new techniques are developed, it is still a huge expense. Three tonnes of CO2 is produced for each tonne of coal burned, and a fairly large (1 GW) coal plant gives off 6 million tonnes of carbon dioxide per year. The cost of up to $300M would be passed on to the consumer. That is not even the main problem. It is simply not possible to make a fundamental change in all coal plants or to build enough new-technology plants in a short time. Up to now, except for Sleipner, only small, scattered projects for cleaning up coal have been funded,

with no integrated plan for replacing all dirty coal power with clean coal power. This is in stark contrast to the ITER project for developing fusion power; there, even the political problems of a large international collaboration have been tackled and solved. It may take two or three decades to clean up all coal power, and this is no shorter than the time needed to commercialize carbon-free renewable sources.