Today, 45% of U.S. electricity generation and 36% of energy-related carbon-dioxide emissions come from coal-fired power plants. Coal is a cheap and abundant energy resource in the U.S. (and in two of the countries with the most rapidly expanding power sectors, China and India). However, as we transition to a carbon-constrained world, conventional coal power is becoming a less attractive method for generating electricity. New technologies for generating coal that emit only a fraction of the carbon-dioxide emissions of conventional coal plants are being developed and offer a promising technological path forward to continue utilizing coal as an energy source while significantly reducing greenhouse gas emissions.

Carbon capture and sequestration, or CCS for short, is a class of technological system that is comprised of a source of carbon-rich flue gas (i.e. a stream of gaseous emissions), a technological method for separating and compressing carbon-dioxide from flue gas, a method to transport carbon-dioxide, and a method for long-term storage for compressed carbon-dioxide. While there are several sources of flue gas that CCS technology could be applied to (e.g. cement production, iron/steel facilities, petroleum refining, and biofuel processing), this post will focus on the largest stationary source of carbon-dioxide emissions and one of the most promising applications for CCS, coal-fired power plants. However, many of the technologies relevant for CCS with coal-fired power are also applicable for other types of fossil fuel power generation (e.g. natural gas) and other flue gas sources. This post will describe three of the most promising technologies for carbon capture with coal electric power generation.

Carbon Capture Technologies for Coal-Fired Power Plants

1. Post-combustion capture

Post-combustion capture is the process of removing carbon-dioxide from the flue gas stream of a coal-fired power plant. In a conventional coal power plant, coal is burned in air, turning a turbine, and the remaining gas from burning coal is emitted into the atmosphere. Post-combustion capture adds an additional facility to a coal power plant that separates the carbon-dioxide from the flue gas. In most designs of post-combustion capture coal power plants, an organic chemical solvent, typically monoethanolamine (MEA), is used to separate carbon-dioxide from a coal plant’s flue gas stream. After separation, the carbon-dioxide is compressed and transported for storage and the remaining gas is released to the atmosphere.

2. Oxygen-fired combustion with capture

Oxygen-fired combustion, or oxyfuel combustion, is a method for coal power generation that burns coal in high-purity oxygen rather than in air, as a conventional coal plant does. By burning coal in oxygen, the flue gas of a power plant comes out as primarily water and carbon-dioxide. Therefore, separation of carbon-dioxide from the flue gas stream of an oxygen-fired combustion power plant is much simpler than from the flue gas stream of a conventional coal plant. Similar to a post-combustion capture plant, the carbon-dioxide is separated, compressed and transported and the remaining gas is released to the atmosphere.

3. Pre-combustion capture

Pre-combustion capture systems remove carbon-dioxide from coal before the combustion step. In pre-combustion capture systems, coal is gasified (transformed into synthesis gas, or syngas), becoming carbon-monoxide and hydrogen. The synthesis gas is then reacted with water to form carbon-dioxide and hydrogen. Next, the carbon-dioxide is separated from the hydrogen, compressed and tranported, while the hydrogen is used for power generation (either through combustion or through a fuel cell).

Many of the technologies needed in a carbon capture plant system are already well-established and are currently used for other applications. However, two important questions still remain unanswered. First, how to sequester carbon-dioxide (i.e. storing carbon-dioxide in such a way that it is isolated from the environment for very long time scales) is still unproven at scale. In the next post, I will take a look at promising options for carbon sequestration. Second, it is still unclear whether or not coal plants with CCS will be economically viable as a method for generating electricity at large scale. In a subsequent post, I will take a look at the economics of a coal plant with CCS and how coal with CCS can compete with other methods of generating electricity in a carbon-constrained world. In the meantime, for more information on CCS, I recommend taking a look at the 2005 IPCC Special Report on Carbon Dioxide Capture and Storage, available here: www1.ipcc.ch/ipccreports/srccs.htm