Methane
Partial oxidation to methanol, for example, is difficult to achieve; the reaction typically progresses all the way to carbon dioxide and water. In the combustion of methane, several steps are involved: Methane is believed to form a formaldehyde (HCHO or H2CO). It is believed to have been created by abiotic processes, with the possible exception of Mars. .The radiative forcing effect due to this increase in methane abundance is about one-third of that of the CO2 increase. Gas pipelines distribute large amounts of natural gas, of which methane is the principal component. In the chemical industry, methane is the feedstock of choice for the production of hydrogen, methanol, acetic acid, and acetic anhydride.
At room temperature and standard pressure, methane is a colorless, odorless gas; the smell characteristic of natural gas as used in homes is an artificial safety measure caused by the addition of an odorant, often methanethiol or ethanethiol. Methane hydrates/clathrates (icelike combinations of methane and water on the sea floor, found in vast quantities) are a potential future source of methane.
This means that a methane emission will have 25 times the impact on temperature of a carbon dioxide emission of the same mass over the following 100 years. Usually, excess methane from landfills and other natural producers of methane are burned so CO2 is released into the atmosphere instead of methane because methane is such a more effective greenhouse gas.
Methane is also an asphyxiant and may displace oxygen in an enclosed space. Methane can trap about 20 times the heat of CO2.
When structures are built on or near landfills, methane off-gas can penetrate the buildings interiors and expose occupants to significant levels of methane. Recently methane emitted from coal mines has been successfully converted to electricity. Methane has been detected or is believed to exist in several locations of the solar system.
Instead, such engines will most likely propel voyages from our moon or send robotic expeditions to other planets in the solar system. Recently methane emitted from coal mines has been successfully converted to electricity. Methane is used in industrial chemical processes and may be transported as a refrigerated liquid (liquefied natural gas, or LNG). In general, methane reactions are hard to control.
In this process, methane and steam react on a nickel catalyst at high temperatures (700–1100 °C). The ratio of carbon monoxide to hydrogen in synthesis gas can then be adjusted via the water gas shift reaction to the appropriate value for the intended purpose. CO + H2O → CO2 + H2 Less significant methane-derived chemicals include acetylene, prepared by passing methane through an electric arc, and the chloromethanes (chloromethane, dichloromethane, chloroform, and carbon tetrachloride), produced by reacting methane with chlorine gas. Acetylene is replaced by less costly substitutes, and the use of chloromethanes is diminishing due to health and environmental concerns. The major source of methane is extraction from geological deposits known as natural gas fields.
In addition, there is a large, but unknown, amount of methane in methane clathrates in the ocean floors. This mechanism for this process is called free radical halogenation.
Cattle belch methane accounts for 16% of the world s annual methane emissions to the atmosphere. Industrially, methane can be created from common atmospheric gases and hydrogen (produced, for example, by electrolysis) through chemical reactions such as the Sabatier process, Fischer-Tropsch process. In its natural gas form, it is generally transported in bulk by pipeline or LNG carriers; few countries transport it by truck. Methane was discovered and isolated by Alessandro Volta between 1776 and 1778 when studying marsh gas from Lake Maggiore. Methane is a relatively potent greenhouse gas with a high global warming potential of 72 (averaged over 20 years) or 25 (averaged over 100 years).
In many cities, methane is piped into homes for domestic heating and cooking purposes. Methane in the atmosphere is eventually oxidized, producing carbon dioxide and water.
However, the use of these chemicals is declining. Methane is violently reactive with oxidizers, halogens, and some halogen-containing compounds.
When X is Cl, this mechanism has the following form: 1. An example of this type of system is in the Dakin Building, Brisbane, California. Main reactions with methane are: combustion, steam reforming to syngas, and halogenation.
Methane has a large effect for a brief period (a net lifetime of 8.4 years in the atmosphere), whereas carbon dioxide has a small effect for a long period (over 100 years). The Earth s methane concentration has increased by about 150% since 1750, and it accounts for 20% of the total radiative forcing from all of the long-lived and globally mixed greenhouse gases.
At about 891 kJ/mol, methane s heat of combustion is lower than any other hydrocarbon; but a ratio with the molecular mass (16.0 g/mol) divided by the heat of combustion (891 kJ/mol) shows that methane, being the simplest hydrocarbon, produces more heat per mass unit than other complex hydrocarbons. It is the simplest alkane, and the principal component of natural gas.
This occurs very quickly, usually in significantly less than a millisecond. 2H2 + O2 →2H2O Finally, the CO oxidizes, forming CO2 and releasing more heat. In this context it is usually known as natural gas, and is considered to have an energy content of 39 megajoules per cubic meter, or 1,000 BTU per standard cubic foot. Methane in the form of compressed natural gas is used as a vehicle fuel, and is claimed to be more environmentally friendly than other fossil fuels such as gasoline/petrol and diesel. Research is being conducted by NASA on methane s potential as a rocket fuel. Current methane engines in development produce a thrust of 7,500 pounds , which is far from the seven million pounds needed to launch the space shuttle.
Asphyxia may result if the oxygen concentration is reduced to below 19.5% by displacement. Uncontrolled build-up of methane in the atmosphere is naturally checked—although human influence can upset this natural regulation—by methane s reaction with hydroxyl radicals formed from singlet oxygen atoms and with water vapor. Methane in the Earth s atmosphere is an important greenhouse gas with a global warming potential of 25 kg CO2 over a 100-year period.
Liquid methane does not burn unless subjected to high pressure (normally 4–5 atmospheres). Methane is not toxic; however, it is highly flammable and may form explosive mixtures with air. -182.5 °C, 91 K, -297 °F -161.6 °C, 112 K, -259 °F Methane is a chemical compound with the chemical formula CH4.
The concentrations at which flammable or explosive mixtures form are much lower than the concentration at which asphyxiation risk is significant. Other sources include mud volcanoes, which are connected with deep geological faults, and livestock (primarily cows) from enteric fermentation. Methane is the major component of natural gas, about 87% by volume.
Because of this difference in effect and time period, the global warming potential of methane over a 20 year time period is 72. As a gas it is flammable only over a narrow range of concentrations (5–15%) in air.
As a result, methane in the atmosphere has a half life of seven years. The abundance of methane in the Earth s atmosphere in 1998 was 1745 parts per billion, up from 700 ppb in 1750. Radical generation: 2.
Radical exchanges: 3. Despite the high activation barrier for breaking the C–H bond, CH4 is still the principal starting material for manufacture of hydrogen in steam reforming.
While leaks from a refrigerated liquid container are initially heavier than air due to the increased density of the cold gas, the gas at ambient temperature is lighter than air. The process is called oxidative pyrolysis: CH4 + O2 → CO + H2 + H2O Following oxidative pyrolysis, the H2 oxidizes, forming H2O, replenishing the active species, and releasing heat.
The gas at shallow levels (low pressure) is formed by anaerobic decay of organic matter and reworked methane from deep under the Earth s surface. In general, sediments buried deeper and at higher temperatures than those which give oil generate natural gas.
Radical extermination: Methane is important for electrical generation by burning it as a fuel in a gas turbine or steam boiler. The formaldehyde gives a formyl radical (HCO), which then forms carbon monoxide (CO).
However, because it is a gas at normal temperature and pressure, methane is difficult to transport from its source. The relative abundance of methane and its clean burning process makes it an attractive fuel.
When used to produce any of these chemicals, methane is first converted to synthesis gas, a mixture of carbon monoxide and hydrogen, by steam reforming. Burning methane in the presence of oxygen produces carbon dioxide and water.
Coal bed methane extraction is a method for extracting methane from a coal deposit, while enhanced coal bed methane recovery is a method of recovering methane from an unminable coal seam. Scientific experiments have given variable results in determining whether plants are a source of methane emissions. Methane is created near the Earth s surface, and it is carried into the stratosphere by rising air in the tropics. It is associated with other hydrocarbon fuels and sometimes accompanied by helium and nitrogen.
Compared to other hydrocarbon fuels, burning methane produces less carbon dioxide for each unit of heat released. Methane s bond angles are 109.5 degrees.
Some buildings have specially engineered recovery systems below their basements to actively capture such fugitive off-gas and vent it away from the building. Large amounts of methane are produced anaerobically by methanogenesis.
This process is generally slower than the other chemical steps, and typically requires a few to several milliseconds to occur. 2CO + O2 →2CO2 The result of the above is the following total equation: CH4(g) + 2O2(g) → CO2(g) + 2H2O(g) - 890 kJ/mol where bracketed g stands for gaseous form and bracketed l stands for liquid form. The strength of the carbon-hydrogen covalent bond in methane is among the strongest in all hydrocarbons, and thus its use as a chemical feedstock is limited. Methane is also produced in considerable quantities from the decaying organic wastes of solid waste landfills. Apart from gas fields, an alternative method of obtaining methane is via biogas generated by the fermentation of organic matter including manure, wastewater sludge, municipal solid waste (including landfills), or any other biodegradable feedstock, under anaerobic conditions.
Methane has a boiling point of −161 °C at a pressure of one atmosphere. The Earth s crust contains huge amounts of methane.
In the same time period, CO2 increased from 278 to 365 parts per million. The search for catalysts which can facilitate C–H bond activation in methane and other low alkanes is an area of research with considerable industrial significance. Methane reacts with all halogens given appropriate conditions, as follows: CH4 + X2 → CH3X + HX where X is a halogen: fluorine (F), chlorine (Cl), bromine (Br), or iodine (I).
