Posts Tagged dry ice

Cryogenic Carbon Capture

Posted by on Saturday, 4 December, 2010

Coal plants release a lot of carbon when they burn coal, along with sulfur and other contaminants. The ashes and particles can travel long distances after their release, and can create a breathing hazard, as well as causing other problems such as acid rain and creating a dark layer on ice that melts the ice. One of the main methods of getting rid of this carbon is using a scrubber, but a method known as cryogenic carbon capture provides some advantages.

The idea behind cryogenic carbon capture is that equipment can blast the air leaving the plant with a stream of freezing nitrogen gas. According to Sustainable Energy Solutions, the freezing nitrogen will solidify other contaminants such as sulfur and nitrogen oxides, as well as carbon dioxide and other carbon compounds.

Cryogenic carbon capture has another advantage over traditional filtration methods such as scrubbers. The freezing blast of nitrogen gas cools down the air leaving the plant, making the ventilation system more efficient and allowing the coal plant to burn coal more efficiently. The coal plant will not need to spend as much money on a water based coolant system. Since the air coming out of the plant is cooler, cryogenic carbon capture also reduces the effects of releasing hot air into the nearby environment, which can be harmful.

Cryogenic carbon capture is very effective at removing almost all of the carbon from the air. According to Purdue University, the cooling system reduces the temperature of the air leaving the plant to -135 Celsius, or -211 degrees Fahrenheit. At this temperature all of the carbon dioxide freezes into dry ice and drops out of the air in solid form. Sulfur dioxide, with a -73 Celsius freezing point, and nitrous oxide, with a -88 Celsius freezing point, are even easier to freeze and remove from the air. Carbon dioxide has a much lower freezing point than most airborne pollutants.

The cryogenic filtration system includes several devices which attach to the flue that vents the gas as it comes out of the plant. A Brigham Young University presentation explains how this equipment works. A condensing heat exchanger first removes excess water vapor from the gas. This helps conserve energy because water takes more energy than most compounds to freeze. A compressor compacts the gas and sends it to the heat exchanger, which freezes it. The gas is sent through one separator, uncompressed, and then sent through a second separator, removing the frozen carbon dioxide, or dry ice. The contaminants have different melting points so the machinery can use this property to separate compounds such as sulfur oxides from carbon dioxide.