Atmospheric concentrations are determined by the balance of the input rate and the removal rate. Input rates have increased due to human activity removal rates are determined by the effectiveness of sinks, or systems that absorb or neutralize a GHG. The primary methane sinks are oxidation by chemical reaction with tropospheric hydroxyl, stratospheric oxidation, and microbial uptake by trees and soils. The strength and effectiveness of these sinks determine methane's atmospheric lifetime. The lion's share of anthropogenic - human induced - methane emissions comes from landfills, natural gas and oil systems, agriculture and coal mining. Other methane emissions are from natural sources - principally wetlands, gas hydrates, permafrost, and termites.
Methane emissions reducing technologies and practices have already developed to the point where cost-effective solutions now exist for capturing methane gas and converting it into clean energy.
Methane is produced from underground and surface mines, and as a result of post-mining activities including coal processing, storage, and transportation. Underground mines are the single largest source of coal mine methane (CMM) emissions in most countries. Globally, CMM accounts for 8% of total methane emissions resulting from human activities.
At active underground mines, methane must be removed from underground operations for safety reasons. This is done with large-scale ventilation systems that move massive quantities of air through the mines. These ventilation systems keep mines safe, but also release large amounts of methane at very low concentrations. At some active and abandoned mines, methane is also produced from degasification systems (also commonly referred to as gas drainage systems) that employ vertical and/or horizontal wells to recover methane.
The production, processing, transmission, and distribution of oil and natural gas is the second largest anthropogenic (human-influenced) methane source worldwide, releasing as much as 88 billion cubic meters (BCM) or 343 million metric tons of carbon equivalent (MMTCE) of methane to the atmosphere annually. Although natural gas is a clean source of energy, methane losses from natural gas systems account for 18% of total worldwide methane emissions. Emissions primarily result from normal operations, routine maintenance, and system disruptions.
In oil and natural gas systems, one can reduce methane emissions by upgrading technologies or equipment and by improving management practices, inspections, and operational procedures. In countries with large oil and gas infrastructures, such as Russia and the United States, wide application of such programs, from recognizing leak prevention and mitigation as a core business opportunity, and thus directing available capital toward loss reduction projects could yield substantial methane emissions reduction and gas savings.
Globally, landfills are the third largest anthropogenic (human influenced) emission source, accounting for about 13 percent of global methane emissions or over 223 million metric tons of carbon equivalent (MMTCE). LFG is created as a natural byproduct of decomposing organic matter, such as food and paper, disposed of in these landfills. LFG consists of about 50 percent methane, the primary component of natural gas, about 50 percent carbon dioxide, and a trace amount of non-methane organic compounds.
LFG is extracted from landfills using a series of wells and a vacuum system, which directs the collected gas to a point to be processed. From there, the LFG can be used for a variety of purposes. Direct use of LFG is reliable and requires minimal processing and minor modifications to existing combustion equipment.
Methane is produced and emitted during the anaerobic decomposition of organic material in livestock manure. Globally, livestock manure management contributes approximately 72 million metric tons carbon equivalent (MMTCE) of methane emissions, roughly 4 percent of total anthropogenic (human related) methane emissions. Three groups of animals account for more than 80 percent of total emissions: swine (40 percent); non-dairy cattle (20 percent); and dairy cattle (20 percent). In certain countries, poultry is also a significant source of methane emissions.
Techniques for recovery include covered anaerobic lagoons, plug flow digesters, complete mix digesters, and small scale digesters. The waste handled is in the form of liquid, slurry, or semi-solid, depending on the system design requirements. Covered anaerobic lagoons are constant volume reactors that can be operated at ambient temperatures. Manure is treated under anaerobic conditions producing methane, which is recovered by using impermeable floating lagoon covers and applying negative pressure. Complete mix digesters are heated digesters constructed of concrete or steel designed to enhance anaerobic decomposition and maximize methane recovery.Plug flow digesters are heated systems that operate at a constant temperature year round, producing stable gas flows that support gas-to-energy applications in all climates. Small-scale digesters are small-scale versions of the above three types and are well suited for smaller farms in regions with technical, capital, and material resource constraints.
There is an enormous amount of methane on earth frozen into a type of ice called methane hydrate, a type of clathrate. Clathrates were discovered in 1810 by Sir Humphrey Davy, and were considered to be a laboratory curiosity. These gas hydrates are crystalline solids which look like ice, and which occur when water molecules form a cage-like structure around smaller 'guest molecules'. There are CO2 hydrates on Mars, while on Earth most of the hydrates are filled with methane. Most of these are in sediments of the ocean, but some are associated with permafrost soils. In the 1930s clathrate formation turned out to be a major problem, clogging pipelines during transportation of gas under cold conditions.
These structures are estimated to contain 3,000 times as much methane as is in the atmosphere. Some climate models predict that a temperature increase of merely a few degrees would cause these gases to volatilize into the atmosphere, which would further raise temperatures, which would release yet more methane, heating the Earth and seas further, and so on. There are 400 gigatons of methane locked in the frozen arctic tundra, more than enough to start this chain reaction.
Once triggered, this cycle could result in runaway global warming worse than the most pessimistic scenario.
There is an event documented in sediments from 55 million years ago called the Paleocene Eocene Thermal Maximum, during which (allegedly) several thousand gigatons of methane was released to the atmosphere and ocean, driving 5° C warming of the intermediate depth ocean. The theory has been popularized as the 'clathrate gun hypothesis'. But recent analysis of methane trapped in bubbles of Greenland ice cores seems to disprove the idea.
Anthropogenic methane sources have already more than doubled the methane concentration in the atmosphere from pre-industrial levels. Currently methane levels appear stable, but the reasons for this relatively recent phenomena are not yet clear. The amount of permafrost hydrate methane is not known very well, but it would not take too much methane, say 60 gigatons released over 100 years, to double atmospheric methane yet again.
How the world's methane hydrates will respond to future global warming and other disturbances is uncertain. Seafloor reservoirs currently contain twice as much methane as all known conventional fossil-fuel reserves. This makes them a target for the energy industry, but mining the gas could have far reaching environmental impacts.
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