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.
Carbon capture using solar power can now potentially remove excess carbon dioxide from the atmosphere, using a process, STEP, which the scientist Dr. Stuart Licht describes in the Journal of Physical Chemistry Letters. This journal isn’t available to the public but supporting information is available under open access.
Lithium is not the only material which can be used in these cells. According to Green Car Congress, the lithium carbonate cell used in this process provided a more energy efficient alternative to cells which use sodium carbonate or potassium carbonate. The problem is that sodium and potassium are extremely common and cheap, and lithium is much more rare. There is also high demand for lithium because of its other uses, which include batteries, renewable energy systems, and medicine. Lithium deposits in Afghanistan provide a potential source of this metal in the future.
There are really two main questions here. If this process is scaled up enough it can reduce carbon dioxide to preindustrial levels, eliminating the global warming issue. The question is how much lithium would be necessary to accomplish this task. It is also not clear whether the cells using the other materials are effective at removing carbon dioxide and just less effective than lithium, or whether the other cells were not able to remove carbon dioxide from the air at all. Because sodium and potassium are not subject to the same supply constraints, less efficient cells which still remove carbon dioxide from the air may be a cheaper alternative and conserve a metal which is available in limited supply.
As with the other carbon capture methods, this process also extracts carbon which can be recycled for fuel use at a later time. The carbon dioxide will be emitted when this material is burned, although the electrolysis cells can simply capture it again. The electrolysis cells draw their own power from solar cells, so this process does not require carbon sources to operate, although mining the metals and manufacturing the cells themselves will likely use carbon dioxide. The tradeoff is worthwhile if the cells are durable and stable for a long time, and stability of the cells is an issue with this process.
Algae does not require lithium or any other rare metals, and neither do trees. Algae also does not have the other impacts such as mining damage to the environment. The main issue with algae carbon sequestration is the space it requires. A combination of several methods should be an effective method to deal with lithium supply issues. Also, a physical chemist knows how to make many types of electrolysis cells so there is good potential for substituting alternative metals. The cell also uses Gallium and Indium, which are rare and expensive, as well as Arsenic. Lithium is not a rare earth metal, although batteries that use lithium do use it along with rare earth metals.
Algae already have a vital role in the ecosystem since they can produce oxygen from carbon dioxide in the air through photosynthesis. Algae at a power plant provide the flexibility to capture carbon at the source and reduce the carbon emissions before these emissions reach the atmosphere. As with other plant materials, algae can also be dried or converted into methane to store fuel for later usage.
Researchers at Indiana University are using algae to capture carbon emissions from a power plant on the campus. According to the university, the power plant uses coal, and the more expensive natural gas when its budget allows. Using algae reduces the impact of the fossil fuel usage, although completely capturing all of the carbon from a standard coal plant would require more than a hundred acres of algae ponds. Obviously this is not feasible in many locations, although a power plant at a university in a rural location might be able to successfully set up this type of system.
The University of Illinois is also working on demonstration projects to use algae for carbon capture. According to the University of Illinois, the main advantage of algae is their rapid growth, which requires them to absorb carbon much faster than other plants. Trees and bushes can not match an algal reproduction rate that can double the size of a clump of algae in four hours. The flue gas from the coal plant does first have to be scrubbed of pollutants such as sulfur dioxide which can kill algae, and reduced in temperature to a level that the algae can handle.
Projects in Southern California provide a demonstration of carbon sequestration using algae. A joint project from several research institutions in San Diego, including UCSD, SDSU, and associated research labs, are setting up carbon capture projects in the deserts of Imperial Valley. The algae ponds will require a large amount of space, and the coastal region of San Diego is densely populated and most nearby cities have high land values, but some nearby desert areas are nearly empty because of the extreme heat. Some of the desert regions in this area are part of national parks or military facilities, so the approval process for setting up algae ponds may not always move quickly.