I was asked recently to give a rundown on the viability of carbon sequestration. This is a fancy term for separation and storage of carbon, generally in the form of carbon dioxide. Carbon dioxide is the result of burning nearly any kind of hydrocarbon, and is widely cited as a greenhouse gas which would contribute to global warming when released into the atmosphere. There is considerable uncertainty and disagreement on the issue, but let’s put that aside for now.
This issue is particularly pertinent at this time in history. If you believe, as I do, that peak oil is close, then carbon dioxide related to oil will naturally decline. However, unless we are to take a big cut in our standard of living driven by even higher energy costs, this energy source will need to be replaced. While natural gas, nuclear energy or renewables such as solar or wind are relatively low carbon, the cheapest, easiest and most capable current replacement for oil is coal. Coal could easily play a major role in replacing oil, but it has the highest carbon content of any fuel. For coal to play this part we must either be willing to accept higher carbon dioxide emissions into the atmosphere or store the carbon dioxide created by use of coal. Let’s be clear…while many pollutants from coal can be cleaned up, production of carbon dioxide is inevitable in the use of coal. I frequently hear the term “clean coal technology” thrown around, but if carbon is considered, only sequestration can result in clean use of coal.
Carbon dioxide can be stored in one of three places:
At first glance, the idea that we can store enough carbon dioxide to make a difference seems ridiculous. After all, carbon dioxide is a gas, and humans pump about 25 billion metric tons annually into the atmosphere, and that number increases each year. And, a ton of carbon in the form of carbon dioxide at atmospheric temperature and pressure occupies hundreds of times the volume of a ton of carbon in the form of say, oil or coal.
On review, though, the idea may not be as absurd as it first seems. After all, the carbon entering the atmosphere today is essentially being taken out of storage in one of the three locations listed above and put into storage in the atmosphere. Carbon is an element, so that essentially means that no new carbon is created…it is just moved about and changed in form. All the carbon we now have has always existed, and been stored somewhere. So, let’s talk briefly about storage in each of the three locations listed above and see where it leads.
Storage in vegetation. Plants, as they grow, breathe carbon dioxide and use it, along with other elements and compounds, to build their structure. Essentially, carbon dioxide is removed from the atmosphere and stored in the plant and oxygen and water are returned to the atmosphere. This sequence naturally occurs, and it is one of the processes than nature uses to keep things in balance. Unfortunately, this process is generally short lived. When the plant dies, it immediately begins to decompose, returning the carbon dioxide to the atmosphere. Generally, only trees have a reasonably longer term effect, since they store the carbon in wood, which may last for several hundred years. But, even with trees, a significant portion may be returned by decomposing leaves, or by forest fires and general disease and decomposition of the wood. And generally, arable land is largely balanced between forests and food production of one type or another. So, substantially increasing forest land would detract from land available for food production. As a result, a significant boost to carbon storage by this method is unlikely. Even so, many firms which sell carbon offsets purport to do it by planting trees. A better choice would be to plant trees in your yard, where you not only put carbon in storage, but decrease your cooling load-keeping some carbon in storage in the form of hydrocarbons, and in the process saving some money on utilities.
Storage in/under water. Water in contact with carbon dioxide in the atmosphere naturally absorbs some of the carbon dioxide, maintaining a state of equilibrium. As carbon dioxide increases in the atmosphere, more of it will automatically be absorbed in water to maintain the equilibrium. Again, this is one of the ways nature maintains its balance. Unfortunately, as carbon dioxide increases in the water, it makes the water more acidic, effecting reefs, crustaceans and other aquatic life adversely. Also, as temperature increases, the water has less ability to absorb carbon dioxide, so increasing temperatures will have the effect of forcing carbon dioxide from water into the atmosphere. In fact, this is one of the possible explanations for rises of carbon dioxide in the atmosphere after the earth warms, as is usually observed in past warming trends. Besides these complications, we have little ability to affect this natural process in a way that increases carbon storage in the sea.
However, there is one possibility that had been suggested for dramatically increasing carbon dioxide storage under the sea. When carbon dioxide is compressed to pressures higher than those below 5000 feet of seawater, or about 2000 pounds per square inch (psi), it becomes heavier than seawater. The theory goes that, if carbon dioxide is compressed to over 2000 psi and injected into the ocean below 5000 feet, it will pool at the bottom of the ocean and remain there permanently. Unfortunately, the theory is largely unproven and the cost would be high. With such a scheme, the probability of unknown, unintended negative consequences could be high.
Storage in/under earth. Again, some storage of carbon in the earth is a naturally occurring process. As vegetation dies, it gets worked into the top few inches of soil. This increases fertility of the soil and, in the process, stores carbon in the earth. But, again, this process is relatively short term and difficult to change in a large scale way.
However, storage of carbon dioxide under the earth in reservoirs and traps is possible. Generally the hydrocarbons which are burned to produce the carbon dioxide have come from these traps. In effect, we would be returning the carbon to the location from which it came. And, it turns out, this has been done enough to gain considerable expertise in the process because it has substantial benefits. Commercially produced carbon dioxide has been purchased and injected into oil reservoirs because it very effectively sweeps oil toward producing wells, increasing oil production from the reservoir. It also has been used to replace natural gas from coal beds.
As a result, this method of storing carbon dioxide is relatively well proven. In fact, this is usually the method assumed when carbon sequestration is discussed. But is it economically viable enough to make coal a reasonable low carbon alternative energy source? The bad news is that it is expensive. The exact numbers vary widely depending on the pressure and permeability of the reservoir, distances between the source and sequestration sites, etc. But, my rough look at reasonable averages results in costs of about $25-30 per metric ton for sequestration of carbon dioxide. And, since carbon is only about one quarter of the mass of carbon dioxide (carbon has a molecular weight of 12, while carbon dioxide has a molecular weight of 44), the cost for sequestering the carbon from one ton of coal is on the order of $100, approximately equal to the current cost of coal. That means that carbon sequestration would approximately double the cost of coal. Worse, most of the cost of sequestration is energy cost for compressing and transporting the carbon dioxide, meaning about twice as much coal would need to be used to generate the same net energy with sequestration as would be required without it. Even so, since coal costs only about 25% as much per unit of energy as oil, coal including sequestration would still cost about half as much as oil.