Carbon dioxide (CO2) is a slightly toxic, odorless, colorless gas with a slightly pungent, acid taste. Carbon dioxide is a small but important constituent of air. It is a necessary raw material for most plants, which remove carbon dioxide from air using the process of photosynthesis.
A typical concentration of CO2 in air is about 0.038% or 380 ppm. The concentration of atmospheric carbon dioxide rises and falls in a seasonal pattern over a range of about 6 ppmv. The concentration of CO2 in air has also been steadily increasing from year to year for over 60 years. The current rate of increase is about 2 ppm per year.
Carbon dioxide is formed by combustion and by biological processes. These include decomposition of organic material, fermentation and digestion. As an example, exhaled air contains as much as 4% carbon dioxide, or about 100 times the amount of carbon dioxide which was breathed in.
Large quantities of CO2 are produced by lime kilns, which burn limestone (primarily calcium carbonate) to produce calcium oxide ( lime, used to make cement); and in the production of magnesium from dolomite (calcium magnesium carbonate). Other industrial activities which produce large amounts of carbon dioxide are ammonia production and hydrogen production from natural gas or other hydrocarbon raw materials.
The concentration of CO2 in air and in stack gases from simple combustion sources (heaters, boilers, furnaces) is not high enough to make carbon dioxide recovery commercially feasible. Producing carbon dioxide as a commercial product requires that it be recovered and purified from a relatively high-volume, CO2-rich gas stream, generally a stream which is created as an unavoidable by product of a large-scale chemical production process or some form of biological process.
In almost all cases, carbon dioxide which is captured and purified for commercial applications would be vented to the atmosphere at the production point if it was not recoved for transport and beneficial use at other locations.
The most common operations from which commercially-produced carbon dioxide is recovered are industrial plants which produce hydrogen or ammonia from natural gas, coal, or other hydrocarbon feedstock, and large-volume fermentation operations in which plant products are made into ethanol for human consumption, automotive fuel or industrial use. Breweries producing beer from various grain products are a traditional source. Corn-to-ethanol plants have been the most rapidly growing source of feed gas for CO2 recovery.
CO2-rich natural gas reservoirs found in underground formations found primarily in the western United States and in Canada are another source of recoverable carbon dioxide. CO2 from both natural and industrial sources is used to enhance production of oil from older wells by injecting the carbon dioxide into appropriate underground formations. Carbon dioxide is used in selectively, primarily in wells which will benefit not only from re-pressurization, but also from a reduction in viscosity of the oil in the reservoir caused by a portion of the CO2 dissolving in the oil. (The extent to which carbon dioxide will dissolve in the oil varies with the type of petroleum present in the reservoir. If the viscosity reduction effect will be minimal, nitrogen, which is usually less expensive, may be used as the pressurant instead.)
Carbon dioxide will not burn or support combustion. Air with a carbon dioxide content of more than 10% will extinguish an open flame, and, if breathed, can be life-threatening. Such concentrations may build up in silos, digestion chambers, wells, sewers and the like. Caution must be exercised when entering these types of confined spaces.
CO2 gas is 1.5 times as heavy as air, thus if released to the air it will concentrate at low elevations. Carbon dioxide will form "dry ice" at -78.5ºC (-109.3º F). One kg of dry ice has the cooling capacity of 2 kg of ordinary ice. Gaseous or liquid carbon dioxide, stored under pressure, will form dry ice through an auto-refrigeration process if rapidly depressured.
Carbon dioxide is commercially available as high pressure cylinder gas, relatively low pressure (about 300 psig or 20 barg) refrigerated liquid, or as dry ice. Large quantities are produced and consumed at industrial sites making fertilizers, plastics and rubber.
Carbon dioxide is a versatile material, being used in many processes and applications - each of which takes advantage of one or more these characteristics: reactivity, inertness and/ or coldness.
Carbon dioxide is commonly used as a raw material for production of various chemicals; as a working material in fire extinguishing systems; for carbonation of soft drinks; for freezing of food products such as poultry, meats, vegetables and fruit; for chilling of meats prior to grinding; for refrigeration and maintenance of ideal atmospheric conditions during transportation of food products to market; for enhancement of oil recovery from oil wells; and for treatment of alkaline water.
Carbon dioxide in air is considered to be a greenhouse gas because of its ability to absorb infrared light.
The concentration of CO2 in the Earth's atmosphere has been increasing at a noticeable rate for much of the past century, There is much interest and concern over the inter-relationship between the levels of carbon dioxide in air and the subject of global warming,
Carbon dioxide plays a major role as a component of the carbon cycle in which carbon is exchanged between the atmosphere, the terrestrial biosphere (which includes freshwater systems and soil), the oceans, and sediments (including fossil fuels). These interactions are complex and widespread. They undoubtedly can be, and are, influenced by many types of human activities, but the extent to which humans have impacted these processes, and will impact them in the future, remains the subject of much research and debate.