Posted by MoparNorm on Tuesday, August 01, 2006 at 3:59PM :
In Reply to: Tsk, Tsk Norm.... posted by David on Tuesday, August 01, 2006 at 5:44AM :
We both agree that the US burns approximately 40% of the worlds fossil fuel, a large source of CO2. However that amount of CO2 is equal to only 350 metric tons of the worlds estimated 3,600 metric tons burned per year.
For some more information you may read below about how and why we have CO2. NOTE that at times, the earth's consentration of CO2 was as high as 90%. It is now at less than 3% although rising.
Most CO2 is not man made, it is naturally occuring.
The Basics.
Sunlight plays a much larger role in our sustenance than we may expect: all the food we eat and all the fossil fuel we use is a product of photosynthesis, which is the process that converts energy in sunlight to chemical forms of energy that can be used by biological systems. Photosynthesis is carried out by many different organisms, ranging from plants to bacteria (Figure 1). The best known form of photosynthesis is the one carried out by higher plants and algae, as well as by cyanobacteria and their relatives, which are responsible for a major part of photosynthesis in oceans. All these organisms convert CO2 (carbon dioxide) to organic material by reducing this gas to carbohydrates in a rather complex set of reactions. Electrons for this reduction reaction ultimately come from water, which is then converted to oxygen and protons. Energy for this process is provided by light, which is absorbed by pigments (primarily chlorophylls and carotenoids). Chlorophylls absorb blue and red light and carotenoids absorb blue-green light (Figure 2), but green and yellow light are not effectively absorbed by photosynthetic pigments in plants; therefore, light of these colors is either reflected by leaves or passes through the leaves. This is why plants are green.
Carbon Fixation.
Electron flow from water to NADP requires light and is coupled to generation of a proton gradient across the thylakoid membrane. This proton gradient is used for synthesis of ATP (adenosine triphosphate), a high-energy molecule. ATP and reduced NADP that resulted from the light reactions are used for CO2 fixation in a process that is independent of light. CO2 fixation involves a number of reactions that is referred to as the Calvin-Benson cycle. The initial CO2 fixation reaction involves the enzyme ribulose-1,5-bisphosphate carboxylase/oxygenase (RuBisCO), which can react with either oxygen (leading to a process named photorespiration and not resulting in carbon fixation) or with CO2. The probability with which RuBisCO reacts with oxygen vs. with CO2 depends on the relative concentrations of the two compounds at the site of the reaction. In all organisms CO2 is by far the preferred substrate, but as the CO2 concentration is very much lower than the oxygen concentration, photorespiration does occur at significant levels. To boost the local CO2 concentration and to minimize the oxygen tension, some plants (referred to as C4 plants) have set aside some cells within a leaf (named bundle-sheath cells) to be involved primarily in CO2 fixation, and others (named mesophyll cells) to specialize in the light reactions: ATP, CO2 and reduced NADP in mesophyll cells is used for synthesis of 4-carbon organic acids (such as malate), which are transported to bundle sheath cells. Here the organic acids are converted releasing CO2 and reduced NADP, which are used for carbon fixation. The resulting 3-carbon acid is returned to the mesophyll cells. The bundle sheath cells generally do not have PS II activity, in order to minimize the local oxygen concentration. However, they retain PS I, presumably to aid in ATP synthesis. Even though C4 plants have reduced amounts of photorespiration, the amount of ATP they need per amount of CO2 fixed is a little higher than in other plants, and therefore their total production rate is similar to that of plants with higher rates of photorespiration.
Some plants living in desert climates, such as cacti, keep their stomates closed during the day to minimize evaporation (stomates are openings in the leaf surface to enhance gas exchange). These plants take up CO2 during the night when the stomates are open, and temporarily bind the CO2 to organic acids in the leaf. During the day the CO2 is released from the acids and used for photosynthesis.
Carbon dioxide is one of the earliest “natural” products. Although carbon dioxide currently only makes up about 0.03% of the atmosphere it is possible that, in the early stages of the development of the earth, it could have consisted of as much as 80% of Earth’s atmosphere. These high concentrations of carbon dioxide were produced by the volcanic activity which was ubiquitous across the Earth at that time. Indeed, most “natural” carbon dioxide sources which exist on the Earth today are produced from magma deep underneath the Earth’s surface. Over the course of billions of years the high concentrations of atmospheric carbon dioxide were reduced when sea-dwelling life and, eventually, plankton, plants and trees, evolved the process of photosynthesis. This process released oxygen and sequestered carbon in the form of carbonate minerals, oil shale, coal, and petroleum within Earth’s crust. Since the start of the industrial revolution the consumption of these materials by a range of “human“ activities has re-released carbon dioxide into the atmosphere.
Although the combustion of carbonaceous materials generates a large amount of carbon dioxide this material is usually only present in the gaseous products of such reactions at concentrations of about 10%. This concentration is considered to be too low to allow the economic recovery of carbon dioxide from such streams and these types of emissions are not commonly used as sources of carbon dioxide. Natural gas streams may also contain significant concentrations of carbon dioxide (up to about 20%) and this carbon dioxide is often separated in order to upgrade the quality of the methane rich product gas. However, most of this carbon dioxide is simply vented or, in some cases, re-injected into the well. Hence, at the moment, the carbon dioxide which is used in downstream processing applications is only recovered from sources in which the carbon dioxide concentration within the raw gas is very high. The principal sources of carbon dioxide which are used in commercial processes are listed below. These sources can be categorized as those associated with natural processes, those resulting from fermentation processes and those produced by chemical processing.
Natural Sources
These sources of carbon dioxide are produced by the Earth’s natural volcanic activity and can include both geothermal sources and natural wells. Carbon dioxide can be found in underground wells at concentrations of 90% to almost 100% depending on the location of the well. Large carbon dioxide wells exist in the United States (e.g. in Colorado, Mississippi, New Mexico, Utah and Wyoming) and in Europe (e.g. at Répcelak and Oelboe in Hungary and at Bad Driburg-Herste and Rottenburg in Germany). Geothermal carbon dioxide is found in numerous locations across Spain and Italy (e.g. at Torre Alfina).
Fermentation
The fermentation of sugars and starches (catalyzed by yeast) produces ethanol and gaseous carbon dioxide. This reaction is commonly employed where the use of agricultural-based ethanol as a fuel additive is actively encouraged (e.g. the federal tax system in the U.S. exempts agricultural-based ethanol-gasoline blends from motor fuel excise tax) . The reaction is also the basis for brewing operations around the world. The carbon dioxide produced by fermentation processes can be of high purity but can be tainted by odor.
Carbon dioxide is produced naturally through the process of respiration, the decay of plant and animal matter, and natural forest fires. However, there are many anthropogenic sources of carbon dioxide. For example fossil fuel combustion, land use changes and cement manufacture are all major anthropogenic sources of CO2.
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