Scientific American

coal burned since the dawn of the Industrial Revolution.

Coal's projected popularity is disturbing not only for those concerned about climate change but also for those worried about other aspects of the environment and about human health and safety. Coal's market price may be low, but the true costs of its extraction, processing and consumption are high. Coal use can lead to a range of harmful consequences, including decapitated mountains, air pollution from acidic and toxic emissions, and water fouled with coal wastes. Extraction also endangers and can kill miners. Together such effects make coal production and conversion to useful energy one of the most destructive activities on the planet [see box on page 73].

In keeping with Scientific American's focus on climate concerns in this issue, we will concentrate below on methods that can help prevent CO2 generated during coal conversion from reaching the atmosphere. It goes without saying that the environmental, safety and health effects of coal production and use must be reduced as well. Fortunately, affordable techniques for addressing CO2 emissions and these other problems already exist, although the will to implement them quickly still lags significantly.

Geologic Storage Strategy the techniques that power providers could apply to keep most of the carbon dioxide they produce from entering the air are collectively called CO2 capture and storage (CCS) or geologic carbon sequestration. These procedures involve separating out much of the CO2 that is created when coal is converted to useful energy and transporting it to sites where it can be stored deep underground in porous media—mainly in depleted oil or gas fields or in saline formations (permeable geologic strata filled with salty water) [see "Can We Bury Global Warming?" by Robert H. Socolow; Scientific American, July 2005].

All the technological components needed for CCS at coal conversion plants are commercially ready—having been proved in applications unrelated to cli mate change mitigation, although integrated systems have not yet been constructed at the necessary scales. Capture technologies have been deployed extensively throughout the world both in the manufacture of chemicals (such as fertilizer) and in the purification of natural gas supplies contaminated with carbon dioxide and hydrogen sulfide ("sour gas"). Industry has gained considerable experience with CO2 storage in operations that purify natural gas (mainly in Canada) as well as with CO2 injection to boost oil production (primarily in the U.S.). Enhanced oil recovery processes account for most of the CO2 that has been sent into

Affordable methods that prevent CO2 from reaching the atmosphere exist; the will to implement them quickly lags.

underground reservoirs. Currently about

35 million metric tons are injected annually to coax more petroleum out of mature fields, accounting for about 4 percent of U.S. crude oil output.

Implementing CCS at coal-consuming plants is imperative if the carbon dioxide concentration in the atmosphere is to be kept at an acceptable level. The 1992 United Nations Framework Convention on Climate Change calls for stabilizing the atmospheric CO2 concentration at a "safe" level, but it does not specify what the maximum value should be. The current view of many scientists is that atmospheric CO2 levels must be kept below 450 parts per million by volume (ppmv) to avoid unacceptable climate changes. Realization of this aggressive goal requires that the power industry start commercial-scale CCS projects within the next few years and expand them rapidly thereafter. This stabilization benchmark cannot be realized by CCS alone but can plausibly be achieved if it is combined with other eco-friendly measures, such as wide improvements in energy efficiency and much expanded use of renewable energy sources.

The Intergovernmental Panel on Climate Change (IPCC) estimated in 2005 that it is highly probable that geologic media worldwide are capable of sequestering at least two trillion metric tons of CO2—more than is likely to be produced by fossil-fuel-consuming plants during the 21st century. Society will want to be sure, however, that potential sequestration sites are evaluated carefully for their ability to retain CO2 before they are allowed to operate. Two classes of risks are of concern: sudden escape and gradual leakage.

Rapid outflow of large amounts of CO2 could be lethal to those in the vicinity. Dangerous sudden releases—such as that which occurred in 1986 at Lake Nyos in Cameroon, when CO2 of volcanic origin asphyxiated 1,700 nearby villagers and thousands of cattle—are improbable for engineered CO2 storage projects in carefully selected, deep porous geologic formations, according to the IPCC.

Gradual seepage of carbon dioxide into the air is also an issue, because over time it could defeat the goal of CCS. The 2005 IPCC report estimated that the fraction retained in appropriately selected and managed geologic reservoirs is very likely to exceed 99 percent over 100 years and likely to exceed 99 percent over 1,000 years. What remains to be demonstrated is whether in practice operators can routinely keep CO2 leaks to levels that avoid unacceptable environmental and public health risks.

Technology Choices design studies indicate that existing power generation technologies could cap -ture from 85 to 95 percent of the carbon in coal as CO2, with the rest released to the atmosphere.

The coal conversion technologies that come to dominate will be those that

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