An Inconvenient Truth, A Surprisingly Convenient Solution

Rachel Meserole | rachel.meserole@yale.edu November 23, 2008

An Inconvenient Truth, A Surprisingly Convenient Solution

Fossil fuels. Carbon dioxide. Greenhouse effects. Global warming. We have all heard the news: the environment is changing. Temperatures have been creeping up at an alarming rate over the last century, due in large part to carbon emissions from industrialization.

Seeking potential methods of reducing carbon emissions, Yale professors in the Geology and Geophysics Department are studying some of the questions raised with the development of carbon sequestration techniques and raising their own.

A Primer on Global Warming

At the most basic level, global warming is a rise in temperatures across the world. The Earth has experienced natural temperature cycles in the past that have been gradual and long lasting.

These changes have been significant (the term ice age is used for a reason), but their slow onset has allowed the flora and fauna of the world to adapt and continue thriving.

A sudden temperature change (sudden being a relative term) could have dramatically adverse effects. The release of large quantities of carbon dioxide and other greenhouse gasses is having a tangible influence on the environment.

As these gasses accumulate in the atmosphere, they trap radiation near the Earth’s surface, acting like the glass panels of a greenhouse, and the higher their concentrations, the more efficiently they trap heat.

During ice ages, glaciers advance and oceans retreat. The climate changes of global warming will have the opposite effect: retreating glaciers and advancing oceans. Low-lying coastal cities, such as New York City, will need to fortify themselves against rising oceans. Animals will need to migrate or adapt.

The increase in temperature will affect plants as well. Farmers may yield poorer crops or may need to change crops altogether; wild plant life may suffer and, along with it, the animals it feeds. This is a frightening picture, and the effects of global warming will certainly not reach this degree for years. However, we need to take steps now to counteract the effects of our developing world in order to prevent the situation from ever reaching such a dire point.

According to the Energy Information Administration, the United States alone was responsible for the release of more than 7 million metric tons of carbon dioxide in 2006.

Yale Power Plant on Ashmun Street, which produces several hundred thousand tons of carbon dioxide each year.

The need for alternative energy sources has become very clear in recent years, but it has also become clear that a clean, efficient, abundant energy source (or a portfolio of clean, efficient, less abundant energy sources) will probably not be available for some time.

Thus, dependence on fossil fuels is unlikely to wane in the near future. As we continue to fulfill our energy needs with fossil fuels, carbon dioxide production will continue to increase. Recognizing the importance of addressing this greenhouse gas output to slowing the effects of global warming, researchers have already begun work on finding feasible methods of disposing of our carbon dioxide waste.

Carbon Sequestration: A Convenient Solution?

A number of promising carbon sequestration techniques have been considered, but few have been put into action. For example, it may be possible to store some excess carbon dioxide in oceans or in ocean sediments.

Before this can be implemented, however, several important questions must be answered: How will increased levels of carbon dioxide affect the pH of the oceans? How well will the oceans store the carbon dioxide? How much carbon dioxide can the oceans realistically hold?

Solid sequestration is another option that has received a fair amount of attention. Carbon dioxide can be induced to react with magnesium-rich rocks to create a stable solid known as magnesium carbonate (MgCO3).

The diagram shows several different methods of underground carbon sequestration.

This process must overcome a number of hurdles, including potential health threats. One problem is that injecting carbon dioxide into basalt formations that are high in magnesium may lead to release of asbestos.

Although the type of asbestos that would be released is not the kind that has been banned for its adverse health effects, it is still considered harmful.

Another logistical hurdles is the slow reaction rate. The transformation is actually energetically favorable, being an exothermic reaction with a negative Gibbs free energy, but it is very slow at room temperature. The reaction rate could be accelerated through an increase in temperature or a decrease in reactant mineral size, but providing large quantities of heat and grinding rock to a fine size both require a large energy input. Researchers are currently exploring alternative reactions that are more efficiently catalyzed.

Disposing of the end-product also presents difficulties, as a great deal of magnesite would be produced from the sequestration of an environmentally significant amount of carbon dioxide.

Professor Jay Ague of the Geology and Geophysics Department calculated that the sequestration of the carbon dioxide emissions of Yale’s power plants in 2004 (roughly 235,000 tons according to the Yale University Environment Report) would produce a block of MgCO3 a little larger than the Kline Biology Tower.

If a use for huge quantities of magnesite is discovered, such as utilizing it as a building material, however, solid sequestration may become an important part of a carbon reduction portfolio.

Not all types of carbon sequestration are theoretical. Injection of carbon dioxide into geological formations has been used for more than a decade as a means of carbon disposal. Since 1996 Sleipner, a natural gas field in the North Sea, has been pumping one million metric tons of carbon dioxide back into the ground each year.

The carbon is injected about 1,000 meters below the sea bed into a sandstone layer known as the Utsira Formation. The injected carbon dioxide displaces the salty water that occupies the pore space between the grains of the rock and becomes trapped by an impermeable rock layer above the Utsira Formation.

The Sleipner oil field in the North Sea, which has been sequestering carbon underground for more than a decade.

Injection of carbon dioxide into rock formations is being used in other locations as well, but on a smaller scale. This type of carbon sequestration on a grand scale could have a dramatic impact on the reduction of carbon emissions in the future.

Lingering Questions

Fortunately, the capacity to store carbon dioxide in the Earth’s crust is essentially limitless. Before it can be exploited to its fullest potential, however, a number of questions about this type of carbon disposal must be answered.

Ague studies deep fluid flow through rocks and has pointed out several important unknowns of this technique. For instance, scientists do not know what effects large carbon dioxide deposits will have on mineral deposition in rock formations.

By dissolving and redepositing minerals, the carbon dioxide might cause a weakening of the rocks that could be disastrous in the event of an earthquake.

The probability that the water and carbon dioxide might escape is also unknown and of significant concern, because the sudden release of one of these reservoirs could have adverse effects on the carbon cycle. Also, these rock formations are hundreds of feet below ground, under very high pressure. Few studies to date shed light on the kinds chemical reactions that may be caused by the unique conditions seen in these reservoirs.

Perhaps most importantly, researchers need to identify the best locations for carbon dioxide reservoirs. Any potential injection site must meet two key criteria: it must be porous and covered with an impermeable layer.

This backscattered electron microscope image of a cut and polished slice of sandstone shows tiny pore spaces (black) between grains. These pores could hold CO2 if the sandstone was used for carbon sequestration. Abbreviations refer to mineral varieties and the rock matrix; Qtz: quartz; Kfs: K-feldspar; Hem: hematite; Kln: kaolinite; Cal: calcite; M: mica; FM: matrix between grains.

A number of sites have been considered, including depleted oil and gas reservoirs, unmineable coal seams, saline aquifers, and basalt formations.

Carbon sequestration in some sites may provide secondary benefits. For instance, the use of depleted oil and gas reservoirs as injection sites could aid in the collection of extra fossil fuel.

When most oil fields are abandoned, they still contain nearly half of their original petroleum supply. The remaining hydrocarbons are held so strongly by surface tension that continued pumping is no longer cost-effective.

As carbon dioxide fills an oil reservoir, it lowers the surface tension of the remaining hydrocarbons and allows workers to retrieve more oil.

In the future we may turn to geoengineering sequestration sites to maximize carbon dioxide holding capacity, minimize undesirable chemical reactions, and prevent leakage of carbon waste.

The field of carbon sequestration is still largely in its formative stages. At present there are a number of ideas for disposing of carbon dioxide waste, but there is very little research to support putting those ideas into action.

The growth of this area of study will be a subject of public attention in future years, as the demand for methods of carbon disposal will become greater with our increasing dependence on fossil fuels.

ABOUT THE AUTHOR
RACHEL MESEROLE is a junior in Trumbull College studying Chemistry.

FURTHER READING
Ague Lab webpage: http://earth.geology.yale.edu/~ja5/
Yale School of Forestry and Environmental Studies http://environment.yale.edu