Natural gas is a fossil fuel
formed when layers of buried plants, gases, and animals are exposed to intense
heat and pressure over thousands of years. The energy that the plants originally
obtained from the sun is stored in the form of chemical bonds in natural gas.
Natural gas is a nonrenewable resource because it cannot be replenished on a
human time frame. Natural gas is a hydrocarbon
gas mixture consisting
primarily of methane,
but commonly includes varying amounts of other higher alkanes and even
a lesser percentage of carbon dioxide, nitrogen, and hydrogen
sulfide. Natural gas is an energy source often used for heating, cooking,
and electricity generation. It is also used as fuel for vehicles and as a
chemical feedstock in the manufacture of plastics and
other commercially important organic
chemicals.
Natural gas is found in deep underground rock formations or
associated with other hydrocarbon reservoirs in coal beds and
as methane clathrates. Petroleum is
another resource and fossil fuel found in close proximity to, and with natural
gas. Most natural gas was created over time by two mechanisms: biogenic and
thermogenic. Biogenic gas is created by methanogenic
organisms in marshes,
bogs, landfills, and
shallow sediments. Deeper in the earth, at greater temperature and pressure,
thermogenic gas is created from buried organic material.
Before natural gas can be used as a fuel, it must be processed to remove impurities, including
water, to meet the specifications of marketable natural gas. The by-products of
this processing include ethane, propane, butanes, pentanes, and higher molecular weight hydrocarbons,
hydrogen
sulfide (which may be converted into pure sulfur), carbon
dioxide, water vapor, and sometimes helium and nitrogen.
Natural gas is often informally referred to simply as
"gas", especially when compared to other energy sources such as oil
or coal. However, it is not to be confused with gasoline,
especially in North America, where the term gasoline is often shortened in
colloquial usage to gas.
Natural gas was used by the Chinese in about 500 BC. They
discovered a way to transport gas seeping from the ground in crude pipelines of
bamboo to where it was used to boil sea water to extract the salt.[6] The
world's first industrial extraction of natural gas started at Fredonia, New York, USA in 1825.[7] By
2009, 66 trillion cubic meters (or 8%) had been used out of the total 850
trillion cubic meters of estimated remaining recoverable reserves of natural
gas. Based on an estimated 2015 world consumption rate of about 3.4 trillion
cubic meters of gas per year, the total estimated remaining economically
recoverable reserves of natural gas would last 250 years.
Natural gas
In the 19th century, natural gas was usually obtained as a
by-product of producing
oil, since the small, light gas carbon chains came out of solution as the
extracted fluids underwent pressure reduction from the reservoir to the surface, similar to uncapping
a bottle of soda where the carbon dioxide effervesces.
Unwanted natural gas was a disposal problem in the active oil fields. If there
was not a market for natural gas near the wellhead it was
virtually valueless since it had to be piped to the end user.
In the 19th century and early 20th century, such unwanted
gas was usually burned off at oil fields. Today, unwanted gas (or stranded gas without a market) associated with
oil extraction often is returned to the reservoir with 'injection' wells while
awaiting a possible future market or to repressurize the formation, which can
enhance extraction rates from other wells. In regions with a high natural gas
demand (such as the US), pipelines are constructed when it is economically
feasible to transport gas from a wellsite to an end
consumer.
In addition to transporting gas via pipelines for use in
power generation, other end uses for natural gas include export as liquefied natural gas
(LNG) or conversion of natural gas into other liquid products via gas-to-liquids
(GTL) technologies. GTL technologies can convert natural gas into liquids
products such as gasoline, diesel or jet fuel. A variety of GTL technologies
have been developed, including Fischer-Tropsch
(F-T), methanol to gasoline (MTG) and STG+. F-T produces a synthetic crude that
can be further refined into finished products, while MTG can produce synthetic
gasoline from natural gas. STG+ can produce drop-in gasoline, diesel, jet fuel
and aromatic chemicals directly from natural gas via a single-loop process. In
2011, Royal Dutch Shell’s 140,000 barrel per day F-T
plant went into operation in Qatar.
Natural gas can be "associated" (found in oil fields),
or "non-associated" (isolated in natural
gas fields), and is also found in coal beds (as
coalbed
methane). It sometimes contains a significant amount of ethane, propane, butane, and pentane—heavier
hydrocarbons removed for commercial use prior to the methane being
sold as a consumer fuel or chemical plant feedstock. Non-hydrocarbons such as carbon
dioxide, nitrogen,
helium (rarely),
and hydrogen sulfide must also be removed before the
natural gas can be transported.
Natural gas extracted from oil wells is called casinghead
gas or associated gas. The natural gas industry is extracting an
increasing quantity of gas from challenging resource
types: sour
gas, tight
gas, shale
gas, and coalbed methane.
There is some disagreement on which country has the largest
proven gas reserves. Sources that consider that Russia has by far the largest
proven reserves include the US CIA (47.6 trillion cubic meters), the US Energy
Information Administration (47.8 tcm), and OPEC (48.7 tcm). However, BP credits
Russia with only 32.9 tcm, which would place it in second place, slightly
behind Iran (33.1 to 33.8 tcm, depending on the source). With Gazprom, Russia
is frequently the world's largest natural gas extractor. Major proven resources
(in billion cubic meters) are world 187,300 (2013), Iran 33,600 (2013), Russia
32,900 (2013), Qatar 25,100 (2013), Turkmenistan 17,500 (2013) and the United
States 8,500 (2013).
It is estimated that there are about 900 trillion cubic
meters of "unconventional" gas such as shale gas,
of which 180 trillion may be recoverable . In turn, many studies from MIT, Black
& Veatch and the DOE predict that natural gas
will account for a larger portion of electricity generation and heat in the
future.
The world's largest gas field is the offshore South Pars / North Dome
Gas-Condensate field, shared between Iran and Qatar. It is estimated to
have 51 trillion cubic meters of natural gas and 50 billion barrels of natural
gas condensates.
Because natural gas is not a pure product, as the reservoir
pressure drops when non-associated gas is extracted from a field under supercritical (pressure/temperature)
conditions, the higher molecular weight components may partially condense upon
isothermic depressurizing—an effect called retrograde condensation. The liquid thus
formed may get trapped as the pores of the gas reservoir get depleted. One
method to deal with this problem is to re-inject dried gas free of condensate
to maintain the underground pressure and to allow re-evaporation and extraction
of condensates. More frequently, the liquid condenses at the surface, and one
of the tasks of the gas plant is to collect this condensate. The
resulting liquid is called natural gas liquid (NGL) and has commercial value.
Shale gas
Shale gas is natural gas produced from shale. Because shale
has matrix permeability too low to allow gas to flow in economical quantities,
shale gas wells depend on fractures to allow the gas to flow. Early shale gas
wells depended on natural fractures through which gas flowed; almost all shale
gas wells today require fractures artificially created by hydraulic fracturing. Since 2000, shale gas
has become a major source of natural gas in the United States and Canada.[18]
Following the success in the United States, shale gas exploration is beginning
in countries such as Poland, China, and South Africa.
Town gas
Town gas is a flammable gaseous fuel made by the
destructive distillation of coal and contains a variety of calorific gases including hydrogen, carbon
monoxide, methane,
and other volatile hydrocarbons, together with small quantities of
non-calorific gases such as carbon
dioxide and nitrogen,
and is used in a similar way to natural gas. This is a historical technology,
not usually economically competitive with other sources of fuel gas today. But
there are still some specific cases where it is the best option and it may be
so into the future.
Most town "gashouses" located in the eastern US in
the late 19th and early 20th centuries were simple by-product coke
ovens that heated bituminous coal in air-tight chambers. The gas driven off
from the coal was collected and distributed through networks of pipes to
residences and other buildings where it was used for cooking and lighting. (Gas
heating did not come into widespread use until the last half of the 20th
century.) The coal
tar (or asphalt)
that collected in the bottoms of the gashouse ovens was often used for roofing
and other waterproofing purposes, and when mixed with sand and gravel was used
for paving streets.
Biogas
Methanogenic archaea are responsible for all biological
sources of methane. Some live in symbiotic relationships with other life forms,
including termites,
ruminants,
and cultivated crops. Other sources of methane, the
principal component of natural gas, include landfill
gas, biogas, and methane hydrate. When methane-rich gases are
produced by the anaerobic decay of non-fossil organic
matter (biomass),
these are referred to as biogas (or natural biogas). Sources of biogas include swamps, marshes, and landfills (see landfill
gas), as well as agricultural waste materials such as sewage sludge and manure[22] by
way of anaerobic digesters, in addition to enteric fermentation, particularly in cattle. Landfill
gas is created by decomposition of waste in landfill sites.
Excluding water
vapor, about half of landfill gas is methane and most of the rest is carbon
dioxide, with small amounts of nitrogen, oxygen, and hydrogen, and
variable trace amounts of hydrogen
sulfide and siloxanes.
If the gas is not removed, the pressure may get so high that it works its way
to the surface, causing damage to the landfill structure, unpleasant odor,
vegetation die-off, and an explosion hazard. The gas can be vented to the atmosphere, flared or
burned to produce electricity or heat. Biogas can also be produced by separating organic
materials from waste that otherwise goes to landfills. This method is more
efficient than just capturing the landfill gas it produces. Anaerobic
lagoons produce biogas from manure, while biogas reactors can be used for
manure or plant parts. Like landfill gas, biogas is mostly methane and carbon
dioxide, with small amounts of nitrogen, oxygen and hydrogen. However, with the
exception of pesticides, there are usually lower levels of contaminants.
Landfill gas cannot be distributed through utility natural
gas pipelines unless it is cleaned up to less than 3 per cent CO
2, and a few parts per million H
2S, because CO
2 and H
2S corrode the pipelines. The presence of CO
2 will lower the energy level of the gas below requirements for the pipeline. Siloxanes in the gas will form deposits in gas burners and need to be removed prior to entry into any gas distribution or transmission system. Consequently it may be more economical to burn the gas on site or within a short distance of the landfill using a dedicated pipeline. Water vapor is often removed, even if the gas is burned on site. If low temperatures condense water out of the gas, siloxanes can be lowered as well because they tend to condense out with the water vapor. Other non-methane components may also be removed to meet emission standards, to prevent fouling of the equipment or for environmental considerations. Co-firing landfill gas with natural gas improves combustion, which lowers emissions.
2, and a few parts per million H
2S, because CO
2 and H
2S corrode the pipelines. The presence of CO
2 will lower the energy level of the gas below requirements for the pipeline. Siloxanes in the gas will form deposits in gas burners and need to be removed prior to entry into any gas distribution or transmission system. Consequently it may be more economical to burn the gas on site or within a short distance of the landfill using a dedicated pipeline. Water vapor is often removed, even if the gas is burned on site. If low temperatures condense water out of the gas, siloxanes can be lowered as well because they tend to condense out with the water vapor. Other non-methane components may also be removed to meet emission standards, to prevent fouling of the equipment or for environmental considerations. Co-firing landfill gas with natural gas improves combustion, which lowers emissions.
Biogas, and especially landfill gas, are already used in
some areas, but their use could be greatly expanded. Experimental systems were
being proposed for use in
parts of Hertfordshire, UK, and Lyon in France. Using
materials that would otherwise generate no income, or even cost money to get
rid of, improves the profitability and energy balance of biogas production. Gas
generated in sewage treatment plants is commonly used to
generate electricity. For example, the Hyperion sewage plant in Los Angeles
burns 8 million cubic feet (230,000 m3) of gas per day to
generate power New York City
utilizes gas to run equipment in the sewage plants, to generate electricity,
and in boilers. Using sewage gas to make electricity is not limited to large
cities. The city of Bakersfield, California, uses cogeneration
at its sewer plants. California has 242 sewage wastewater treatment plants, 74
of which have installed anaerobic digesters. The total biopower generation from
the 74 plants is about 66 MW.
The McMahon natural gas processing plant in Taylor, British Columbia, Canada.
Crystallized natural gas — hydrates
Huge quantities of natural gas (primarily methane) exist in
the form of hydrates under sediment on offshore continental
shelves and on land in arctic regions that experience permafrost,
such as those in Siberia.
Hydrates require a combination of high pressure and low temperature to form.
In 2010, the cost of extracting natural gas from
crystallized natural gas was estimated to 100–200 per cent the cost of
extracting natural gas from conventional sources, and even higher from offshore
deposits.
In 2013, Japan Oil, Gas and Metals National Corporation
(JOGMEC) announced that they had recovered commercially relevant quantities of
natural gas from methane hydrate.
Natural gas processing
The image below is a schematic block flow diagram of a typical natural gas
processing plant. It shows the various unit processes used to convert raw
natural gas into sales gas pipelined to the end user markets.
The block flow diagram also shows how processing of the raw
natural gas yields byproduct sulfur, byproduct ethane, and natural gas liquids
(NGL) propane, butanes and natural gasoline (denoted as pentanes +).
Depletion
Uses
Mid Stream Natural Gas
Natural gas flowing in the distribution lines and at the
natural gas well head are often used to power natural gas powered engines.
These engines rotate compressors to facilitate the natural gas transmission.
These compressors are required in the mid-stream line to pressurize and to
re-pressurize the natural gas in the transmission line as the gas travels. The
natural gas transmission lines extend to the natural gas processing plant or
unit which removes the higher molecular weighted natural gas hydrocarbons to
produce a British thermal unit (BTU) value between 950
and 1050 BTU's. The processed natural gas may then be used for residential,
commercial and industrial uses.
Often mid-stream and well head gases require removal of many
of the various hydrocarbon species contained within the natural gas. Some of
these gases include heptane, pentane, propane and other hydrocarbons with molecular weights above Methane (CH4) to
produce a natural gas fuel which is used to operate the natural gas engines for
further pressurized transmission. Typically, natural gas compressors require
950 to 1050 BTU per cubic foot to operate at the natural gas engines rotational
name plate specifications.
Several methods are used to remove these higher molecular
weighted gases for use at the natural gas engine. A few technologies are as
follows:
- Joule–Thomson Skid
- Cryogenic or Chiller System
- Chemical Enzymology System
Power generation
Natural gas is a major source of electricity generation through the use of cogeneration,
gas
turbines and steam turbines. Natural gas is also well suited for a
combined use in association with renewable
energy sources such as wind or solar
and for alimenting peak-load
power stations functioning in tandem with hydroelectric
plants. Most grid peaking power plants and some off-grid engine-generators
use natural gas. Particularly high efficiencies can be achieved through
combining gas turbines with a steam turbine in combined
cycle mode. Natural gas burns more cleanly than other hydrocarbon
fuels, such as oil and coal, and produces less carbon dioxide per unit of
energy released. For an equivalent amount of heat, burning natural gas produces
about 30 per cent less carbon dioxide than burning petroleum and
about 45 per cent less than burning coal.
Coal-fired electric power generation emits around 2,000
pounds of carbon dioxide for every megawatt hour generated, which is almost
double the carbon dioxide released by a natural gas-fired electric plant per
megawatt hour generated. Because of this higher carbon efficiency of natural
gas generation, as the fuel mix in the United States has changed to reduce coal
and increase natural gas generation, carbon dioxide emissions have unexpectedly
fallen. Those measured in the first quarter of 2012 were the lowest of any
recorded for the first quarter of any year since 1992.
Combined cycle power generation using natural gas is
currently the cleanest available source of power using hydrocarbon fuels, and
this technology is widely and increasingly used as natural gas can be obtained
at increasingly reasonable costs. Fuel cell
technology may eventually provide cleaner options for converting natural gas
into electricity, but as yet it is not price-competitive. Locally produced
electricity and heat using natural gas powered Combined Heat and Power plant
(CHP or Cogeneration
plant) is considered energy efficient and a rapid way to cut carbon emissions.
Domestic use
Natural gas dispensed from a simple stovetop can generate
temperatures in excess of 1100 °C (2000 °F) making it a powerful
domestic cooking and heating fuel. In much of the developed world it is
supplied through pipes to homes, where it is used for many purposes including
ranges and ovens, gas-heated clothes
dryers, heating/cooling,
and central heating. Heaters in homes and other
buildings may include boilers, furnaces, and water
heaters.
Compressed natural gas (CNG) is used in rural homes without
connections to piped-in
public
utility services, or with portable grills.
Natural gas is also supplied by independent natural gas suppliers through Natural Gas Choice programs throughout the
United States. However, as CNG costs more than LPG, LPG (propane) is the dominant source
of rural gas.
Transportation
CNG is a cleaner alternative to other automobile
fuels such as gasoline
(petrol) and diesel. By the end of 2012 there were 17.25 million natural gas vehicles worldwide, led by Iran (3.3 million), Pakistan (3.1
million), Argentina
(2.18 million), Brazil
(1.73 million), India
(1.5 million), and China
(1.5 million). The energy efficiency is generally equal to that of gasoline
engines, but lower compared with modern diesel engines. Gasoline/petrol
vehicles converted to run on natural gas suffer because of the low compression
ratio of their engines, resulting in a cropping of delivered power while
running on natural gas (10%–15%). CNG-specific engines, however, use a higher
compression ratio due to this fuel's higher octane
number of 120–130.
Fertilizers
Natural gas is a major feedstock for the production of ammonia, via the Haber
process, for use in fertilizer production.
Aviation
Russian
aircraft manufacturer Tupolev is currently running a development program to produce
LNG- and hydrogen-powered
aircraft. The program has been running since the mid-1970s, and seeks to
develop LNG and hydrogen variants of the Tu-204
and Tu-334
passenger aircraft, and also the Tu-330
cargo aircraft. It claims that at current market prices, an LNG-powered
aircraft would cost 5,000 roubles (~ $218/ £112) less to operate per ton, roughly
equivalent to 60 per cent, with considerable reductions to carbon
monoxide, hydrocarbon and nitrogen
oxide emissions.
The advantages of liquid methane as a jet engine fuel are that
it has more specific energy than the standard kerosene mixes
do and that its low temperature can help cool the air which the engine
compresses for greater volumetric efficiency, in effect replacing an intercooler.
Alternatively, it can be used to lower the temperature of the exhaust.
Hydrogen
Natural gas can be used to produce hydrogen, with
one common method being the hydrogen
reformer. Hydrogen has many applications: it is a primary feedstock for the
chemical industry, a hydrogenating agent, an important commodity for oil
refineries, and the fuel source in hydrogen
vehicles.
Other
Natural gas is also used in the manufacture of fabrics, glass, steel, plastics, paint, and other
products.
Storage and transport
Because of its low density, it is not easy to store natural
gas or to transport it by vehicle. Natural gas pipelines are impractical across oceans. Many existing pipelines in America are
close to reaching their capacity, prompting some politicians representing
northern states to speak of potential shortages. In Western Europe, the gas
pipeline network is already dense. New pipelines are planned or under
construction in Eastern Europe and between gas fields in Russia, Near East
and Northern Africa and Western Europe. See also List of natural gas pipelines.
LNG carriers transport liquefied natural gas (LNG) across oceans,
while tank
trucks can carry liquefied or compressed natural gas (CNG) over shorter distances.
Sea transport using CNG carrier ships that are now under development may be
competitive with LNG transport in specific conditions.
Gas is turned into liquid at a liquefaction plant, and is returned to gas
form at regasification
plant at the terminal. Shipborne
regasification equipment is also used. LNG is the preferred form for long
distance, high volume transportation of natural gas, whereas pipeline is
preferred for transport for distances up to 4,000 km (2,485 mi) over
land and approximately half that distance offshore.
CNG is transported at high pressure, typically above 200 bars.
Compressors and decompression equipment are less capital intensive and may be
economical in smaller unit sizes than liquefaction/regasification plants.
Natural gas trucks and carriers may transport natural gas directly to end-users,
or to distribution points such as pipelines.
In the past, the natural gas which was recovered in the
course of recovering petroleum could not be profitably sold, and was simply
burned at the oil field in a process known as flaring.
Flaring is now illegal in many countries. Additionally, higher demand in the
last 20–30 years has made production of gas associated with oil economically
viable. A further option is the gas is now sometimes re-injected
into the formation for enhanced oil recovery by pressure maintenance as well as
miscible or immiscible flooding. Conservation, re-injection, or flaring of
natural gas associated with oil is primarily dependent on proximity to markets
(pipelines), and regulatory restrictions.
A "master gas system" was invented in Saudi
Arabia in the late 1970s, ending any necessity for flaring. Satellite
observation, however, shows that flaring and venting are still practiced in some gas-extracting countries.
Natural gas is used to generate electricity and heat for desalination.
Similarly, some landfills that also discharge methane gases have been set up to
capture the methane and generate electricity.
Natural gas is often stored underground inside depleted gas
reservoirs from previous gas wells, salt domes,
or in tanks as liquefied natural gas. The gas is injected in
a time of low demand and extracted when demand picks up. Storage nearby end
users helps to meet volatile demands, but such storage may not always be practicable.
With 15 countries accounting for 84 per cent of the
worldwide extraction, access to natural gas has become an important issue in
international politics, and countries vie for control of pipelines.[51] In
the first decade of the 21st century, Gazprom, the
state-owned energy company in Russia, engaged in disputes with Ukraine and Belarus over the
price of natural gas, which have created concerns that gas deliveries to parts
of Europe could be cut off for political reasons. The United States is
preparing to export natural gas.
Floating
Liquefied Natural Gas (FLNG) is an innovative technology designed to enable
the development of offshore gas resources that would otherwise remain untapped
because due to environmental or economic factors it is nonviable to develop
them via a land-based LNG operation. FLNG technology also provides a number of
environmental and economic advantages:
- Environmental – Because all processing is done at the gas field, there is no requirement for long pipelines to shore, compression units to pump the gas to shore, dredging and jetty construction, and onshore construction of an LNG processing plant, which significantly reduces the environmental footprint. Avoiding construction also helps preserve marine and coastal environments. In addition, environmental disturbance will be minimised during decommissioning because the facility can easily be disconnected and removed before being refurbished and re-deployed elsewhere.
- Economic – Where pumping gas to shore can be prohibitively expensive, FLNG makes development economically viable. As a result, it will open up new business opportunities for countries to develop offshore gas fields that would otherwise remain stranded, such as those offshore East Africa.
Many gas and oil companies are considering the economic and
environmental benefits of Floating Liquefied Natural Gas (FLNG). However, for the time
being, the only FLNG facility now in development is being built by Shell, due
for completion around 2017.
Environmental effects
Effect of natural gas release
Natural gas is mainly composed of methane. After release to
the atmosphere it is removed over about 10 years by gradual oxidation to carbon
dioxide and water by hydroxyl radicals (·OH) formed in the troposphere or
stratosphere, giving the overall chemical reaction CH4 + 2O2→
CO2 + 2H2O. While the lifetime of atmospheric methane is
relatively short when compared to carbon dioxide, it is more efficient at
trapping heat in the atmosphere, so that a given quantity of methane has 84
times the global-warming potential of carbon dioxide over a 20-year period and
28 times over a 100-year period. Natural gas is thus a more potent greenhouse
gas than carbon dioxide due to the greater global-warming potential of methane.
Current estimates by the EPA place global emissions of methane at
85 billion cubic metres (3.0×1012 cu ft) annually, or
3.2 per cent of global production. Direct emissions of methane represented 14.3
per cent of all global anthropogenic greenhouse gas emissions in 2004.
During extraction, storage, transportation, and
distribution, natural gas is known to leak into the atmosphere, particularly
during the extraction process. A study in 2011 demonstrated that the leak rate
of methane was high enough to jeopardize its global warming advantage over
coal. This study was criticized later for its high assumption of methane
leakage values. These values were later shown to be close to the findings of
the Scientists at the National Oceanic and Atmospheric Administration.Natural
gas extraction also releases an isotope of Radon, ranging from 5
to 200,000 Becquerels
per cubic meter.
CO2 emissions
Natural gas is often described as the cleanest fossil fuel. It produces about
29% and 44% less carbon dioxide per joule delivered than oil and coal respectively, and potentially
fewer pollutants than other hydrocarbon fuels. However, in absolute terms, it
comprises a substantial percentage of human carbon
emissions, and this contribution is projected to grow. According to the IPCC Fourth Assessment Report, in
2004, natural gas produced about 5.3 billion tons a year of CO2
emissions, while coal and oil produced 10.6 and 10.2 billion tons respectively.
According to an updated version of the Special Report on Emissions
Scenario by 2030, natural gas would be the source of 11 billion tons a
year, with coal and oil now 8.4 and 17.2 billion respectively because demand is
increasing 1.9 percent a year. Total global emissions
for 2004 were estimated at over 27,200 million tons.
Other pollutants
Natural gas produces far lower amounts of sulfur
dioxide and nitrous oxides than any other hydrocarbon fuels.
The other pollutants due to natural gas combustion are listed below in parts
per million (ppm):
- Carbon monoxide - 40 ppm
- Sulfur dioxide - 1 ppm
- Nitrogen oxide - 92 ppm
- Particulates - 7 ppm
Safety concerns
A pipeline odorant injection station
Production
In mines, where methane seeping from rock formations has no odor,
sensors are
used, and mining apparatus such as the Davy lamp
has been specifically developed to avoid ignition sources.
Some gas fields yield sour gas
containing hydrogen sulfide (H2S). This untreated
gas is toxic. Amine gas treating, an industrial scale process
which removes acidic
gaseous components, is
often used to remove hydrogen sulfide from natural gas.
Extraction of natural gas (or oil) leads to decrease in
pressure in the reservoir. Such decrease in pressure in turn may
result in subsidence,
sinking of the ground above. Subsidence may affect ecosystems, waterways, sewer
and water supply systems, foundations, and so on.
Another ecosystem effect results from the noise of the
process. This can change the composition of animal life in the area, and have
consequences for plants as well in that animals disperse seeds and pollen.
Releasing the gas from low-permeability reservoirs is
accomplished by a process called hydraulic fracturing or
"hydrofracking". To allow the natural gas to flow out of the shale,
oil operators force 1 to 9 million US gallons (34,000 m3)
of water mixed with a variety of chemicals through the wellbore casing into the
shale. The high pressure water breaks up or "fracks" the shale, which
releases the trapped gas. Sand is added to the water as a proppant to keep the
fractures in the shale open, thus enabling the gas to flow into the casing and
then to the surface. The chemicals are added to the frack fluid to reduce
friction and combat corrosion. During the extracting life of a gas well, other
low concentrations of other chemical substances may be used, such as biocides to
eliminate fouling, scale and corrosion inhibitors, oxygen scavengers to remove
a source of corrosion, and acids to clean the perforations in the pipe.
Dealing with fracking fluid can be a challenge. Along with
the gas, 30 per cent to 70 per cent of the chemically laced frack fluid, or
flow back, returns to the surface. Additionally, a significant amount of brine,
containing salt and other minerals, may be produced with the gas.
Use
In order to assist in detecting leaks, a minute amount
of odorant
is added to the otherwise colorless and almost odorless gas used by consumers.
The odor has been compared to the smell of rotten eggs, due to the added tert-Butylthiol
(t-butyl mercaptan). Sometimes a related compound, thiophane, may be used in the mixture. Situations
in which an odorant that is added to natural gas can be detected by analytical
instrumentation, but cannot be properly detected by an observer with a normal
sense of smell, have occurred in the natural gas industry. This is caused by
odor masking, when one odorant overpowers the sensation of another. As of 2011,
the industry is conducting research on the causes of odor masking.
Explosions caused by natural gas leaks
occur a few times each year. Individual homes, small businesses and other
structures are most frequently affected when an internal leak builds up gas
inside the structure. Frequently, the blast is powerful enough to significantly
damage a building but leave it standing. In these cases, the people inside tend
to have minor to moderate injuries. Occasionally, the gas can collect in high
enough quantities to cause a deadly explosion, disintegrating one or more
buildings in the process. The gas usually dissipates readily outdoors, but can
sometimes collect in dangerous quantities if flow rates are high enough.
However, considering the tens of millions of structures that use the fuel, the
individual risk of using natural gas is very low.
Natural gas heating systems are a minor source of carbon
monoxide deaths in the United States. According to the US Consumer Product
Safety Commission (2008), 56 per cent of unintentional deaths from non-fire CO
poisoning were associated with engine-driven tools like gas-powered generators
and lawnmowers. Natural gas heating systems accounted for 4 per cent of these
deaths. Improvements in natural gas furnace designs have greatly reduced CO
poisoning concerns. Detectors are also available that warn of
carbon monoxide and/or explosive gas (methane, propane, etc.).
Energy content, statistics, and pricing
Quantities of natural gas are measured in normal cubic meters (corresponding to 0 °C
at 101.325 kPa) or in standard cubic feet (corresponding to
60 °F (16 °C) and 14.73 psia). The gross heat of combustion of 1 m3 of
commercial quality natural gas is around 39 MJ (≈10.8 kWh), but this can vary
by several percent. This comes to about 49 MJ (≈13.5 kWh) for 1 kg of
natural gas (assuming a density of 0.8 kg m−3, an approximate
value).
The price of natural gas varies greatly depending on
location and type of consumer. In 2007, a price of $7 per 1000 cubic feet
(about 25 cents per m3) was typical in the United States. The
typical caloric value of natural gas is roughly 1,000 British thermal units (BTU) per cubic foot, depending
on gas composition. This corresponds to around $7 per million BTU, or around $7
per gigajoule.
In April 2008, the wholesale price was $10 per 1,000 cubic feet (28 m3)
($10/MMBTU). The residential price varies from 50% to 300% more than the
wholesale price. At the end of 2007, this was $12–$16 per 1000 cubic feet
(about 50 cents per m3).Natural gas in the United States is traded
as a futures contract on the New York Mercantile Exchange. Each
contract is for 10,000 MMBTU (~10,550 gigajoules),
or 10 billion BTU. Thus, if the price of gas is $10 per million BTUs on the
NYMEX, the contract is worth $100,000.
European Union
Gas prices for end users vary greatly across the EU. A
single European energy market, one of the key objectives of the European Union,
should level the prices of gas in all EU
member states. Moreover, it would help to resolve supply and global
warming issues.
United States
In US units, one standard cubic foot 1 cubic foot (28 L) of
natural gas produces around 1,028 British thermal units (1,085 kJ). The
actual heating value when the water formed does not condense is the net heat of combustion and can be as much as
10% less.
In the United States, retail sales are often in units of therms (th); 1 therm
= 100,000 BTU. Gas meters measure the volume of gas used, and this is
converted to therms by multiplying the volume by the energy content of the gas
used during that period, which varies slightly over time. Wholesale
transactions are generally done in decatherms
(Dth), or in thousand decatherms (MDth), or in million decatherms (MMDth). A
million decatherms is roughly a billion cubic feet of natural gas. Gas sales to
domestic consumers may be in units of 100 standard cubic feet (scf). The typical annual consumption of a
single family residence is 1,000 therms or one RCE.
Canada
Canada uses metric
measure for internal trade of petrochemical products. Consequently, natural gas
is sold by the Gigajoule, cubic metre (m3) or thousand cubic
metres (E3m3). Distribution infrastructure and meters almost always meter
volume (cubic foot or cubic meter). Some jurisdictions, such as Saskatchewan,
sell gas by volume only. Other jurisdictions, such as Alberta, gas is sold by
the energy content (GJ). In these areas, almost all meters for residential and
small commercial customers measure volume (m3 or ft3),
and billing statements include a multiplier to convert the volume to energy
content of the local gas supply.
A Gigajoule (GJ) is a measure approximately equal to half a
barrel (250 lbs) of oil, or 1 million BTUs, or 1000 cu ft of gas, or 28 m3
of gas. The energy content of gas supply in Canada can vary from 37 to 43 MJ
per m3 depending on gas supply and processing between the wellhead
and the customer.
Elsewhere
In the rest of the world, natural gas is sold in Gigajoule
retail units. LNG (liquefied natural gas) and LPG (liquefied petroleum gas) are traded in
metric tons or MMBTU as spot deliveries. Long term natural gas distribution
contracts are signed in cubic metres, and LNG contracts are in metric tonnes
(1,000 kg). The LNG and LPG is transported by specialized transport
ships, as the gas is liquified at cryogenic
temperatures. The specification of each LNG/LPG cargo will usually contain the
energy content, but this information is in general not available to the public.
In the Russian Federation, Gazprom sold
approximately 250 billion cubic metres of natural gas in 2008. In 2013 the
Group produced 487.4 billion cubic meters of natural and associated gas.
Gazprom supplied Europe with 161.5 billion cubic meters of gas in 2013.
Natural gas as an asset class for institutional investors
Research conducted by the World Pensions Council (WPC)[when?]
suggests that large US
and Canadian
pension funds and Asian
and MENA area SWF investors have become particularly active
in the fields of natural gas and natural gas infrastructure, a trend started in
2005 by the formation of Scotia Gas Networks in the UK by OMERS and Ontario Teachers' Pension Plan.
Adsorbed Natural Gas (ANG)
Another way to store natural gas is adsorbing it to the
porous solids called sorbents. The best condition for methane storage is at
room temperature and atmospheric pressure. The used pressure can be up to 4 MPa
(about 40 times atmospheric pressure) for having more storage capacity. The
most common sorbent used for ANG is activated carbon (AC). Three main types of
activated carbons for ANG are: Activated Carbon Fiber (ACF), Powdered Activated
Carbon (PAC), activated carbon monolith.
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