Photo courtesy of Siemens AG.
Carbon capture and storage (CCS) is a group of technologies that includes several steps. Capture of CO2, the first process, can occur before or after coal is combusted. Pre-combustion capture is associated with coal gasification, while post-combustion capture is associated with traditional coal burning technologies such as pulverized coal, circulating fluidized bed, and emerging technologies such as oxy-fuel combustion.
Gasification
Gasification technology is the cleanest way to make energy from coal. A typical coal plant emits more sulfur dioxide and nitrogen oxide in a few weeks than a state-of-the-art gasification plant will produce in an entire year. There are hundreds of gasification plants worldwide making ammonia, methanol, substitute natural gas, and diesel fuel. A smaller number—about 16 plants—use gasification to produce electricity.
In gasification, steam and oxygen convert coal into synthesis gas, also know as “syngas.” This byproduct is composed of hydrogen and carbon monoxide with relatively small amounts of other impurities that must be removed.
Gasification provides a number of environmental benefits, including:
- Vastly lower levels of sulfur dioxide, nitrogen oxide, and mercury emissions compared to conventional coal-fired power plants.
- Lower solid waste, as little as half the volume of conventional coal plants. Solid waste is vitrified (enclosed in a glass-like substance) and therefore less likely to leach contaminants into the ground.
- Less water use, between 20% and 50% less than conventional coal plants.
Electricity is generated from gasification via two methods:
Surface Gasifiers
Two principal options are used to make electricity with gasification: Integrated Gasification Combined Cycle (IGCC) and Substitute Natural Gas (SNG).
IGCC: In this process, syngas is burned in a turbine to produce electricity. Excess heat is captured and used to power a second turbine that makes more electricity. These plants are known as “combined cycle” plants, and the plants that combine gasification with combined cycle units are called “integrated gasification combined cycle,” or IGCC plants.
SNG: Alternatively, syngas produced from coal gasification can be further processed to make substitute natural gas or SNG, which can then be burned in a natural gas combined cycle plant to produce electricity. These plants often make excess SNG which can then be sold through interstate natural gas pipelines to other gas plants.
Approximately 90% of the CO2 created in the substitute natural gas process can be captured and stored—resulting in a net 50% or more reduction in CO2 emissions compared to a conventional natural gas power plant.
Post-Combustion Capture
More than 95% of the coal-fired plants in the world use coal combustion technology. About one-third of this capacity worldwide is less than 10 years old. Installing Post-Combustion Capture (PCC) technology on these plants is a key strategy to avoiding the most serious effects of climate change.
PCC uses a chemical solvent to remove CO2 from the exhaust gas of a traditional coal power plant. The solvent—typically an amine solution—binds with the CO2. The solvent/ CO2 combination is separated from the rest of the exhaust gas and heated in order to isolate the relatively pure CO2 for compression and sequestration. The solvent is then cooled and reused.
Many variations on this basic approach exist and new technologies using selective membranes and solid materials instead of liquid solvents are under development now, and are explored in a recent CATF report, Coal Without Carbon: An Investment Plan for Federal Action.
The largest PCC system operating today is located in Trona, California, where an amine system built in 1978 now captures 900 tons of CO2 per day—roughly equivalent to the CO2 emissions from a 35 MW coal power plant. Amine-based PCC also has been used successfully on the exhaust from a natural gas-fired combined cycle combustion turbine in Bellingham, Massachusetts. Several small systems are currently testing ammonia-based methods.
Retrofitting PCC to the world’s existing coal power plants would be one of the fastest ways to reduce global CO2 emissions. Without CCS in the mix of its projected new coal generating capacity, China’s CO2 emissions will significantly exceed U.S. emissions.
There are other environmental impacts related to PCC. First, the retrofits require that sulfur dioxide emissions be lowered because the solvents used for PCC can be sensitive to sulfur levels in the plant exhaust. Secondly, water use increases at existing conventional coal plants with today’s PCC technology. One U.S. Department of Energy study estimates water use would more than double with installation of a capture system.
