Scientists have warned that carbon dioxide released during combustion of natural gas, oil and coal may lead to global warming. Concern over climate change stimulated research into methods for carbon capture and sequestration—separating it from flue gas and storing it so it won’t add to the atmospheric burden.
There are three general ways to sequester carbon dioxide: terrestrial, geologic and oceanic. Planting a tree is a small-scale example of terrestrial sequestration. As the tree grows, it absorbs carbon dioxide, metabolizing it to form its biomass. The tree’s wood fiber sequesters the carbon until it is burned or ultimately decays. Unfortunately, even large-scale forestation holds very limited potential for carbon sequestration, relative to the amount of carbon dioxide expected to be released from energy production in the next several decades.
Carbon can also be sequestered by capturing carbon dioxide directly at the combustion source, liquefying it by cooling and compressing it and pumping it deep into the ocean where the pressure above keeps it in a stable, liquid form indefinitely. Research on oceanic sequestration is still in its infancy, and it will probably be quite some time before it can be counted upon as an economically viable process.
Placing liquefied carbon dioxide in underground reservoirs—geologic sequestration—is the leading contender for short-term carbon sequestration. The basic technology is well developed because it has long been used to enhance oil recovery. Many parts of the US have geology suitable for carbon sequestration. In order to work for storing carbon, a site must consist of porous rock at a depth of several thousand feet. At this depth, the pressure overhead is sufficient to keep the carbon dioxide in its liquid state. The amount of carbon which can be stored in a given location is determined by the size and porosity of the receiving formation.
Scientists have identified several types of underground formations which are expected to work best for storing carbon dioxide: depleted oil and gas fields, unminable coal deposits and deep saline aquifers. Of these, we have considerable experience with carbon dioxide injection into oil and gas fields. This process is commonly used in order to force out additional oil and gas. Scientists are still exploring the long-term stability of the injected material—it’s important that the carbon dioxide stay put, if it is to be useful in controlling climate change. While depleted fields may be effective carbon dioxide repositories, their use is limited since they tend not to be where you need them.
As with above-ground real estate, the most important factors for geological sequestration sites are “location, location, location”. To be economically viable, a sequestration site needs to be close to the carbon dioxide source. Unfortunately, there is not an oil or gas field near every large power plant (where most of the country’s carbon dioxide is generated) and the cost of transporting carbon dioxide would greatly add to its sequestration cost. Nevertheless, where depleted fields are located near large combustion sources, they may prove a viable solution.
Unminable coal fields are coal seams located too deep for economical recovery. These seams have permeability properties which are expected to make them act as sponges when carbon dioxide is introduced. As with depleted oil and gas fields, the geographic distribution of unminable coal fields is spotty and may not correspond with power plant locations.
Deep saline aquifers represent a huge potential for sequestration. These underground water reservoirs are not suitable for drinking water due to their high concentration of dissolved solids and great depth. While not located everywhere, deep saline aquifers are found in many regions of North America and have excellent porosity and a vast capacity for carbon sequestration. A number of pilot studies have confirmed the feasibility of pumping liquefied carbon dioxide into the reservoirs under pressure. The weight of the water above the carbon dioxide is sufficient to keep it in liquid form.
The U.S. Department of Energy is funding a number of regional studies exploring all three geological sequestration options. The Midwest effort is called the Plains CO2 Reduction Partnership—a consortium of universities, research institutes, energy companies and government agencies. With the second-largest coal reserves in the nation, Illinois is a focus of attention for the Plains CO2 Reduction Partnership. Its second phase of research is currently being performed, featuring underground injection of carbon dioxide.
Illinois is also home to two of the four finalists contending as host site for “FutureGen”—a planned next-generation, minimal environmental impact coal-fired power plant. Scheduled for completion by 2012, FutureGen will be a 275 megawatt power plant capable of capturing and storing more than a million tons of carbon dioxide per year in a deep saline aquifer underground.
Carbon sequestration promises to be an important tool in the effort to achieve energy sustainability. Due to its technical maturity, carbon capture and sequestration can be bridges to renewable energy concepts which will take us into the 22nd century. IBI