This is an excerpt from chapter three of Carbon Capture by Howard Herzog. Part of the MIT Press Essential Knowledge Series, Carbon Capture offers a concise overview of carbon dioxide capture and storage (CCS), a promising but overlooked climate change mitigation pathway.
Chapter 3: Carbon Capture
In the high desert, about 250 km northeast of Los Angeles, is the Searles Valley Minerals plant. This plant produces a number of chemicals such as soda ash from the brines that they mine. The manufacturing process requires significant quantities of CO2 to carbonate the brines. Being in a remote area, it would be very expensive to transport CO2 to the site. It turns out that carbon capture provides a cheaper solution. In 1978, the then owner North American Chemical built a process to capture up to 800 tons per day of CO2 from a coal-fired boiler. This process, based on amine technology, was originally patented in the 1930s. However, this was the first time that amines were adapted for use on a coal-fired boiler exhaust, termed flue gases. In fact, this was the first implementation of carbon capture from any type of boiler. Constructed well before people considered carbon capture for climate change mitigation, this project demonstrated that carbon capture was feasible on flue gases from fossil fuel combustion.
Carbon capture is most effective on large, stationary sources of CO2 because the capture process exhibits significant economies of scale. It is much easier and cheaper to implement CCS on the smokestacks of power plants and factories than on the tailpipe of an automobile or the chimney of a house. The IPCC assessed the most appropriate targets for CCS worldwide as coal-fired power plants (60%), other power plants, primarily natural gas (19%), cement (7%), refineries (6%), iron and steel (5%), and petrochemical (3%). The number in parentheses is the amount of CO2 emissions for each industry divided by the total amount of CO2 emissions for all the industries listed. This breakdown shows why carbon capture has been generally associated with coal, but it also shows that there are other significant targets. The industrial processes outside the power sector are starting to draw more attention because, while CCS must compete with renewables and nuclear in the power sector, CCS is the only practical option for most of the other industrial sector targets.
A simple indicator of the degree of difficulty and cost of capturing carbon from a gas stream is the partial pressure, which is simply the pressure of the gas stream multiplied by its CO2 concentration. The higher the partial pressure, the easier it is to capture the CO2. While most streams of interest are at atmospheric pressure, there are some processes with gas streams at high pressure. These include the cleanup of natural gas, production of ammonia in fertilizer plants, and production of hydrogen at refineries. Overall, these make up a very small percentage of the target CO2, but because they are the least costly options, they have dominated as a source of CO2 in carbon capture projects operating today (see Chapter 5). There are some small sources of high purity, atmospheric pressure CO2. The biggest example is fermentation plants that produce ethanol to use as a gasoline additive. The overwhelming amount of CO2 emissions from large, stationary sources come from dilute, atmospheric pressure flue gases with CO2 concentrations running 3–20%. At the low end of this range are natural gas-fired power plants, while cement plants are at the higher end of the range. Coal-fired power plants are in the middle at about 12%. The amine process has become the standard carbon capture technology for these dilute, atmospheric pressure flue gases.
To learn more about Carbon Capture, click here.
