Carbon Emissions

In 2010, the world emitted 33.2 billion tons of carbon dioxide into the atmosphere. Of this, 25% was emitted by China and 18.5% was emitted by the United States. These two countries are the largest CO₂ emitters by far, accounting for 43.5% of the world’s CO₂ generation. China overtook the United States as the top CO₂ emitter in 2006. Table 14.1 shows the top 15 CO₂ emitting countries in the world.

The vast majority of these emissions come from burning oil, coal and natural gas (see Figure 4.6). Not all of these emissions end up in the atmosphere, however, and contribute to the 400 ppm of CO₂ content already in existence. The carbon cycle is a complicated process. Some of the carbon dioxide emitted into the atmosphere is: utilized by plants to grow cell walls in the photosynthesis process; dissolved in the ocean water; removed from the atmosphere by the world’s food crops (although the human beings and other animals that eat those crops breathe out more carbon dioxide than is removed by crops); and, removed by the new growth forests. Nevertheless, the rising CO₂ levels in the atmosphere are likely to be due to the rising CO₂ emissions over the past 150 years.

Though the world has been trying to find ways to reduce these emissions, Figure 14.1 shows that CO₂ emissions in the world continue to increase. The United States and Europe have begun to reduce their emissions, but China and other developing countries continue to increase their emissions, spurred by their rapid development. The reader may be confused by an apparent discrepancy in the numbers shown in Figure 14.1 compared to those shown in Figure 4.6. Careful comparison of the two figures reveals that Figure 4.6 shows the emissions of carbon (in million tons per year), whereas Figure 14.1 shows the emissions of carbon dioxide (in million tons per year). Each ton of carbon dioxide contains 0.273 tons of carbon.

While carbon dioxide is not the only greenhouse gas emitted by the industrial world, it is by far the most important. Even though methane, nitrous oxide and chlorofluorocarbons have a stronger greenhouse effect, much smaller volumes of these are emitted each year. Nevertheless, there are programs to reduce these emissions as well.

Phase Behavior of Carbon Dioxide

It is useful to understand a little about the phase behavior of CO₂ because it is not only interesting; it is also relevant to how it is handled for transport and storage. The phase diagram for any pure substance will show the pressures and temperatures where the substance is solid, liquid or gas. There is also a fourth physical state above the critical temperature and critical pressure where the substance is called “supercritical,” which is also relevant to its handling.

Figure 14.2 shows the CO₂ phase diagram. Two important points are shown on that diagram: the critical point and the triple point. At the critical point, gas and liquid CO₂ have the same properties (density, viscosity, etc.). It is impossible to distinguish between the liquid and gas phases at this point. At the triple point, the solid, liquid and gas phases are all in equilibrium with one another. Below the triple point, the liquid phase does not exist. It is in this region that the solid CO₂ ice sublimates into a gas phase when it is heated. The triple point of CO₂ occurs at a pressure of 5.11 atmospheres (76.4 pounds-force per square inch absolute or 76.4 psia) and a temperature of − 56.4°C (− 69.6°F). At sea level, the atmospheric pressure is one atmosphere (14.7 psia). This means that CO₂ cannot be a liquid at atmospheric pressure. It must either be a very cold solid (known as dry ice) or a gas, which mixes with the atmosphere. Solid CO₂, or dry ice, is widely used to keep things cold because it has those two properties that make it very useful: it is very cold (less than − 70°F) and, when it is heated, it does not melt to form a liquid, but rather sublimates into the atmosphere. By contrast, the triple point of water occurs at 0.006 atm (0.09 psia); consequently, at atmospheric pressure, ice melts into liquid water when it is heated.

The critical point for CO₂ is 31.1°C (88°F) and 7.37 MPa (1,073 psia). A system pressure of more than 7.37 MPa (which is about 73 atmospheres) and a system temperature of more than 31.1°C will result in a supercritical condition. In the supercritical region, the properties of the substance are somewhere between a liquid and a compressed gas. More importantly, the CO₂ does not undergo a phase change when the temperature and pressure change. If the temperature is decreased, the CO₂ smoothly transforms into a liquid. If the pressure is decreased, the CO₂ smoothly transforms into a gas. Both of these changes occur without undergoing any sudden or rapid phase change.

Between the critical point and the triple point, the CO₂ exists in either a liquid or gaseous state, depending on the temperature and pressure. When in the liquid state, if the temperature is increased or the pressure is decreased, the liquid will reach the boiling point line and be transformed into a gas through the boiling process. Conversely, from the gaseous state, if the temperature is decreased or the pressure is increased, the gas will reach the same line and be transformed into a liquid through the condensation process.

Technically speaking, CO₂ can be transported through pipelines in the form of a gas, a supercritical fluid or in the sub-cooled liquid state. Gas phase transport is disadvantageous because of the lower density and consequently larger pipe diameter required. It is not used for pipelines of any significant capacity and length. Furthermore, in storage, the CO₂ must be injected into deep wells at its destination. It must also be supplied at the appropriate pressure, which is usually around 3,000 psia (204 atmospheres) or more. To be transported in a pipeline, CO₂ should be compressed to ensure that single phase flow is achieved for the entire length of the pipeline. A transmission pipeline can experience a wide range of ambient temperatures, so maintaining stability of this single phase is important in order to avoid considerations of two-phase flow that could result in pressure surges which damage the pipeline.

Most CO₂ pipelines are operated in the liquid region (high pressures and low temperatures). In very hot climates, however, the liquid flowing in the pipeline can be heated above the critical temperature (31.1°C). The liquid thus becomes a supercritical fluid, which oddly enough is easier to handle. Most CO₂ pipelines are buried in the earth a few feet, which keeps the operating temperature below the critical temperature. In that case, care must be taken to ensure the pressure in the pipeline remains higher than the boiling point pressure so that a phase change from liquid to gas does not occur. If this happens, it results in pressure surges that can possibly damage the pipeline. On the other hand, once the CO₂ is injected into the reservoirs under the ground, the earth temperature is usually above the critical temperature (31.1°C (88°F,)) and the CO₂ becomes a supercritical fluid.

In Figure 14.3 in the storage concepts section, some people have proposed that CO₂ be sequestered at the bottom of the deep ocean where the temperatures can be very cold (35°F to 40°F) and the pressures are very high [4,450 psia (303 atm)] at a depth of 10,000 feet. At these conditions, CO₂ is a liquid, slightly less dense than seawater; however, CO₂ is soluble in seawater. When sequestered on the ocean floor in this manner, it will gradually dissolve in the ocean rather than rise to the surface. Another method of sequestering CO₂ in the ocean would be to convert it to dry ice. The density of dry ice is heavier than seawater so it could be dumped into the deepest part of the ocean. It would then sink to the ocean floor where the pressure would be above the triple point pressure. The ocean would then transfer heat to the dry ice, melting it into a liquid form which would then begin rising and dissolving in the ocean water. There are certain disadvantages to this method of storage which will be explained later.

Carbon Capture Methods

Most of the CO₂ that is emitted into the atmosphere comes from burning hydrocarbons in air. Air contains about 80% nitrogen and about 20% oxygen, with many other components of much smaller values. The CO₂ component in the air is currently about 400 ppm, or 0.04%. The flue gas from a coal-burning power plant, however, contains 10-12% CO₂. This can be captured by various methods. The most widely used commercial technology for doing this is to absorb it in Mono-Ethanolamine (MEA) using a contacting column. This process is illustrated in Figure 4.7. After absorbing the CO₂ from the flue gas, the MEA is pumped to a stripping column where it is heated and the CO₂ exits at the top of the column as a mostly pure CO₂ stream that is ready for sale or storage. Using this technology, CO₂ can be stripped out of flue gas from a power plant for about $50 per ton of CO₂ recovered. It costs another $25 per ton to compress the CO₂ and sequester it underground. This increases the cost of coal-fired or gas-fired electricity by about 30%.

Another method of capturing CO₂ is to use solid absorbents, instead of MEA, in the process described above. There are many substances that will absorb and capture CO₂, ranging from activated carbon to poly-ionic liquids. This is the focus of intensive research programs around the world.

A final method of capturing the CO₂ is oxy-combustion, which is illustrated in Figures 13.13 and 13.15. In this process, hydrocarbons are burned in an oxygen environment with recycled CO₂ (instead of nitrogen) as the carrier gas. This results in a flue gas that is mostly pure CO₂, ready for storage.

Carbon Storage Concepts

Figure 14.3 shows the concept of how carbon dioxide is sequestered in the earth or in the ocean. The CO₂ is pumped into depleted oil or gas reservoirs, deep saline formations, coal beds or into the ocean itself.

CO₂ is currently used to increase the recovery of oil from oil reservoirs, a process illustrated in Figure 14.4. The CO₂ is used to sweep oil towards the producing wells, mobilizing oil that would otherwise remain in the reservoir, held in place by capillary forces. At the end of the life of the Enhanced Oil Recovery (EOR) project, the CO₂ remains sequestered in the reservoir. CO₂ is more soluble in oil than it is in water. When it is pumped into the oil reservoir, it dissolves in the oil. If the pressure is high enough, it becomes miscible with the oil. This swells up the volume of the oil and makes it more mobile and able to be swept out of the corners of the porous rock and flow towards the producing well.

Though there are many depleted oil and gas fields, there is not enough space in such reservoirs to sequester all the CO₂ that will need to be sequestered. Consequently, coal beds and saline aquifers will also have to be used. Coal beds use a different physical principle to sequester the CO₂. Coal has a chemical attraction for CO₂ and, when it is injected into the coal bed, it adsorbs onto the surface of the coal (releasing methane in the process). It therefore does not require much additional space in which to store the CO₂. Large volumes of CO₂ can be stored in coal beds, but it must be unminable coal to ensure that the CO₂ remains sequestered. Chapter 13 discussed the large mass of coal reserves in the world; however, there is much more coal in the earth that is too deep to be mined. This coal can also store a large quantity of CO_2.

When sequestered underground, most of the formations capable of holding CO₂ are deep enough that the formation temperature is greater than the critical temperature of CO₂ (31.1°C). The pressure in the formation is also likely to be greater than the critical pressure (73 atm). This means that most of the CO₂ is sequestered in a dense-phase supercritical state. There are some shallow formations where the temperature would be cooler than the critical temperature. In such formations, however, it is unlikely that the pressure could be maintained high enough to keep the CO₂ in a liquid state. Consequently, porous shallow formations are not suitable for storage.

The largest spaces available for carbon storage are in deep saline aquifers. When CO₂ is injected into these formations, there are a number of different mechanisms that contribute to the storage process. Two of the principal mechanisms are the dissolution of CO₂ in the water and the chemical reaction between the CO₂ and the rock minerals and mineral salts present in the water.

When CO₂ reacts with the mineral salts in the formation, it produces carbonate minerals that frequently precipitate into the formation, trapping the CO₂ in-situ. This can be a relatively slow process, but once completed, it keeps the CO₂ trapped and immobile in the formation. If the CO₂ is not mineralized in this manner, some of the CO₂ will dissolve in the water and the rest will remain as a supercritical CO₂ phase that displaces some of the formation water. In this case, it can be beneficial to produce some of the formation water to the surface to create room for the supercritical CO₂ phase in the formation. The produced water can be: desalinated at the surface and used; disposed of in the ocean; or re-injected back into the same aquifer trapping the CO₂ by capillary action. As long as the supercritical CO₂ saturation is kept low (less than about 25%), the CO₂ remains immobile and trapped and cannot migrate or leak out of the containing formation. Over time, the CO₂ saturation gradually dissolves and mineralizes in the aquifer water.

On the bottom of the deep oceans, high enough pressures and low enough temperatures exist that would be sufficient to maintain the CO₂ in a liquid state. Liquid CO₂, however, is lighter than seawater. It would rise and become dissolved in the ocean water. The problem with this method of storage is that the CO₂ is more soluble in cold water. As the ocean currents carry the CO₂-laden water to warmer regions, the CO₂ could be released back into the atmosphere. Solid CO₂ (dry ice) is heavier than seawater. If it was dropped into the ocean, it would sink to the bottom. It would then rapidly dissolve in the ocean water and suffer the same fate.

Another problem with ocean storage is that increasing the CO₂ levels in the ocean will make the ocean more acidic. When CO₂ dissolves in water, it makes carbonic acid which lowers the pH of the ocean water. This will adversely affect the corals and many of the marine species, killing them off. For these reasons, ocean storage of CO₂ is not a viable option on a large scale or on a long-term basis.

Carbon Storage Projects

The Global CCS Institute, based in Australia’s capital city of Canberra, has identified 74 large-scale integrated CCS projects around the world. In their annual survey, undertaken in May–August 2011, they published a detailed assessment of project status, including an analysis of project dynamics, challenges and opportunities. They define large-scale and integrated projects as those that involve the capture, transport and storage of CO₂ at a scale of more than 800,000 tons of CO₂ annually for a coal-based power plant and more than 400,000 tons of CO₂ annually for other emission-intensive industrial facilities (including natural gas–based power generation). There are many more projects around the world which are of a smaller scale or only focus on part of the CCS chain. Twenty-five of these projects are located in the United States and 21 are located in Europe. Of these 74 projects, 8 are currently in operation and another 6 are currently being constructed. These 14 projects are shown in Table 14.2. Eight of the projects are located in the United States.

The Future of Carbon Capture and Storage

Currently, the world is emitting more than 33 billion tons of CO₂ annually. If this were to be sequestered in deep saline aquifers at an average pressure of 3,500 psia and an average temperature of 180°F, the average specific gravity of the supercritical fluid would be 0.65. This correlates to a density of 650 kg/m³, or 40.5 lb/ft³. At this density, the 33 billion tons of CO₂ per year would have a volumetric rate of injection into the formation of 875 million barrels of supercritical CO₂ per day. This is 10 times the volume of oil that is being produced per day in the world. At the surface conditions of 2,600 psia and 75°F, the specific gravity of the liquid phase would be 0.9, giving a density of 56 lb/ft³. At these conditions, the total volume of fluid being injected in liquid form at the surface would be 632 million barrels of liquid CO₂ per day. Consequently, the CO₂ storage business could be larger than today’s oil business. Even if only 10% of the CO₂ produced is sequestered underground, it is still a very large business that today is a relatively small business. If the world chooses to invest more time and energy into carbon capturing and storage, the business potential could be huge.

The Kyoto Accords failed to make much headway in reducing the CO₂ emissions. Any such future agreements would have to begin with a bilateral agreement between United States and China, the two largest CO₂ emitters. Europe, Russia and India would also have to be eventual signatories to the agreement. With this critical mass, most of the other countries in the world would likely follow suit.