Search This Blog

Tuesday, October 28, 2025

Writing system

From Wikipedia, the free encyclopedia
https://en.wikipedia.org/wiki/Writing_system

A writing system comprises a set of symbols, called a script, as well as the rules by which the script represents a particular language. The earliest writing appeared during the late 4th millennium BC. Throughout history, each independently invented writing system gradually emerged from a system of proto-writing, where a small number of ideographs were used in a manner incapable of fully encoding language, and thus lacking the ability to express a broad range of ideas.

Writing systems are generally classified according to how their symbols, called graphemes, relate to units of language. Phonetic writing systems – which include alphabets and syllabaries – use graphemes that correspond to sounds in the corresponding spoken language. Alphabets use graphemes called letters that generally correspond to spoken phonemes. They are typically divided into three sub-types: Pure alphabets use letters to represent both consonant and vowel sounds, abjads generally only use letters representing consonant sounds, and abugidas use letters representing consonant–vowel pairs. Syllabaries use graphemes called syllabograms that represent entire syllables or moras. By contrast, logographic (or morphographic) writing systems use graphemes that represent the units of meaning in a language, such as its words or morphemes. Alphabets typically use fewer than 100 distinct symbols, while syllabaries and logographies may use hundreds or thousands respectively.

Background: relationship with language

The relationship between spoken, written, and signed modes of language, as modelled by Beatrice Primus et al. While many spoken or signed languages are not written, there are no written languages without a spoken counterpart that they originally emerged to record.

According to most contemporary definitions, writing is a visual and tactile notation representing language. As such, the use of writing by a community presupposes an analysis of the structure of language at some level. The symbols used in writing correspond systematically to functional units of either a spoken or signed language. This definition excludes a broader class of symbolic markings, such as drawings and maps. A text is any instance of written material, including transcriptions of spoken material. The act of composing and recording a text is referred to as writing, and the act of viewing and interpreting the text as reading.

The relationship between writing and language more broadly has been the subject of philosophical analysis as early as Aristotle (384–322 BC). While the use of language is universal across human societies, writing is not; writing emerged much more recently, and was independently invented in only a handful of locations throughout history. While most spoken languages have not been written, all written languages have been predicated on an existing spoken language. When those with signed languages as their first language read writing associated with a spoken language, this functions as literacy in a second, acquired language. A single language (e.g. Hindustani) can be written using multiple writing systems, and a writing system can also represent multiple languages. For example, Chinese characters have been used to write multiple languages throughout the Sinosphere – including the Vietnamese language from at least the 13th century, until their replacement with the Latin-based Vietnamese alphabet in the 20th century.

In the first several decades of modern linguistics as a scientific discipline, linguists often characterized writing as merely the technology used to record speech – which was treated as being of paramount importance, for what was seen as the unique potential for its study to further the understanding of human cognition.

General terminology

Comparison between double-storey |a| (left) and single-storey |ɑ| (right) lowercase forms of the Latin letter A

While researchers of writing systems generally use some of the same core terminology, precise definitions and interpretations can vary by author, often depending on their theoretical approach.

A grapheme is the basic functional unit of a writing system. Graphemes are generally defined as minimally significant elements that, when taken together, comprise the set of symbols from which texts may be constructed. All writing systems require a set of defined graphemes, collectively called a script. The concept of the grapheme is similar to that of the phoneme in the study of spoken languages. Likewise, as many sonically distinct phones may function as the same phoneme depending on the speaker, dialect, and context, many visually distinct glyphs (or graphs) may be identified as the same grapheme. These variant glyphs are known as the allographs of a grapheme: For example, the lowercase letter ⟨a⟩ may be represented by the double-storey |a| and single-storey |ɑ| shapes, or others written in cursive, block, or printed styles. The choice of a particular allograph may be influenced by the medium used, the writing instrument used, the stylistic choice of the writer, the preceding and succeeding graphemes in the text, the time available for writing, the intended audience, and the largely unconscious features of an individual's handwriting.

Orthography (lit.'correct writing') refers to the rules and conventions for writing shared by a community, including the ordering of and relationship between graphemes. Particularly for alphabets, orthography includes the concept of spelling. For example, English orthography includes uppercase and lowercase forms for 26 letters of the Latin alphabet (with these graphemes corresponding to various phonemes), punctuation marks (mostly non-phonemic), and other symbols, such as numerals. Writing systems may be regarded as complete if they are able to represent all that may be expressed in the spoken language, while a partial writing system cannot represent the spoken language in its entirety.

History

Diagram comparing the abstraction of pictographs in cuneiform, Egyptian hieroglyphs, and Chinese characters – from an 1870 publication by French Egyptologist Gaston Maspero

In each instance, writing emerged from systems of proto-writing, though historically most proto-writing systems did not produce writing systems. Proto-writing uses ideographic and mnemonic symbols to communicate, but lacks the capability to fully encode language. Examples include:

Writing has been invented independently multiple times in human history – first emerging as cuneiform, a system initially used to write the Sumerian language in southern Mesopotamia; it was later adapted to write Akkadian as its speakers spread throughout the region, with Akkadian writing appearing in significant quantities c. 2350 BC. Cuneiform was closely followed by Egyptian hieroglyphs. It is generally agreed that the two systems were invented independently from one another; both evolved from proto-writing systems between 3400 and 3100 BC, with the earliest coherent texts dated c. 2600 BCChinese characters emerged independently in the Yellow River valley c. 1200 BC. There is no evidence of contact between China and the literate peoples of the Near East, and the Mesopotamian and Chinese approaches for representing sound and meaning are distinct. The Mesoamerican writing systems, including Olmec and the Maya script, are likewise associated with an independent invention.

With each independent invention of writing, the ideographs used in proto-writing were decoupled from the direct representation of ideas, and gradually came to represent words instead. This occurred via application of the rebus principle, where a symbol was appropriated to represent an additional word that happened to be similar in pronunciation to the word for the idea originally represented by the symbol. This allowed words without concrete visualizations to be represented by symbols for the first time; the gradual shift from ideographic symbols to those wholly representing language took place over centuries, and required the conscious analysis of a given language by those attempting to write it.

The Indus script (c. 2600 – c. 2000 BC), found on different types of artefacts produced by the Indus Valley Civilization on the Indian subcontinent, remains undeciphered, and whether it functioned as true writing is not agreed upon. While its origins are not visually obvious, the opportunity for Mesopotamian cultural diffusion to have introduced the concept of writing to the Indus peoples is clear.

Alphabetic writing descends from previous morphographic writing, and first appeared c. 1800 BC to write a Semitic language spoken in the Sinai Peninsula. Most of the world's alphabets either descend directly from this Proto-Sinaitic script, or were directly inspired by its design. Descendants include the Phoenician alphabet (c. 1050 BC), and its child in the Greek alphabet (c. 800 BC). The Latin alphabet, which descended from the Greek alphabet, is by far the most common script used by writing systems.

Classification by basic linguistic unit

Table of scripts in the introduction to the Sanskrit–English Dictionary by Monier Monier-Williams

Writing systems are most often classified according to what units of language a system's graphemes correspond to. At the most basic level, writing systems can be either phonographic (lit.'sound writing') when graphemes represent units of sound in a language, or morphographic ('form writing') when graphemes represent units of meaning (such as words or morphemes). Depending on the author, the older term logographic ('word writing') is often used, either with the same meaning as morphographic, or specifically in reference to systems where the basic unit being written is the word. Recent scholarship generally prefers morphographic over logographic, with the latter seen as potentially vague or misleading – in part because systems usually operate on the level of morphemes, not words. Some authors make a distinct primary division – between pleremic (from Greek plḗrēs 'full') systems with graphemes that have semantic value in isolation (like logographs), and cenemic (from Greek kenós 'empty') systems with graphemes that lack any such separable meaning (like letters).

Many classifications define three primary categories, where phonographic systems are subdivided into syllabic and alphabetic (or segmental) systems. Syllabaries use symbols called syllabograms to represent syllables or moras. Alphabets use symbols called letters that correspond to spoken phonemes (or more technically, to diaphonemes). Alphabets are generally classified into three subtypes, with abjads having letters for consonants, pure alphabets having letters for both consonants and vowels, and abugidas having characters that correspond to consonant–vowel pairs. David Diringer proposed a five-fold classification of writing systems, comprising pictographic scripts, ideographic scripts, analytic transitional scripts, phonetic scripts, and alphabetic scripts.

In practice, writing systems are classified according to the primary type of symbols used, and typically include exceptional cases where symbols function differently. For example, logographs found within phonetic systems like English include the ampersand ⟨&⟩ and the numerals ⟨0⟩, ⟨1⟩, etc. – which correspond to specific words (and, zero, one, etc.) and not to the underlying sounds. Most writing systems can be described as mixed systems that feature elements of both phonography and morphography.

Logographic systems

A logogram is a character that represents a morpheme within a language. Chinese characters represent the only major logographic writing systems still in use: they have historically been used to write the varieties of Chinese, as well as Japanese, Korean, Vietnamese, and other languages of the Sinosphere. As each character represents a single unit of meaning, thousands are required to write all the words of a language. If the logograms do not adequately represent all meanings and words of a language, written language can be confusing or ambiguous to the reader.

Logograms are sometimes conflated with ideograms, symbols which graphically represent abstract ideas; most linguists now reject this characterization. Chinese characters are often semantic–phonetic compounds, which include a component related to the character's meaning, and a component that gives a hint for its pronunciation.

Syllabaries

A stop sign in Tahlequah, Oklahoma written in Cherokee using both the Cherokee syllabary (top) and Latin alphabet (middle), alongside English (bottom)

A syllabary is a set of written symbols (called syllabograms) that represent either syllables or moras – a unit of prosody that is often but not always a syllable in length. Syllabaries are best suited to languages with relatively simple syllable structure, since a different symbol is needed for every syllable. For example, the Japanese writing system has two kana syllabaries (hiragana and katakana) intended for use in distinct circumstances; both have syllabograms for each of the roughly 100 moras found in Japanese. By contrast, English features complex syllable structures, with a relatively large inventory of vowels and complex consonant clusters – for a total of 15–16 thousand distinct syllables. Some syllabaries have larger inventories: the Yi script contains 756 different symbols.

Alphabets

An alphabet uses symbols (called letters) that correspond to the phonemes of a language, e.g. its vowels and consonants. However, these correspondences are rarely uncomplicated, and spelling is often mediated by other factors than just which sounds are used by a speaker. The word alphabet is derived from alpha and beta, the names for the first two letters in the Greek alphabet. An abjad is an alphabet whose letters only represent the consonantal sounds of a language. They were the first alphabets to develop historically, with most used to write Semitic languages, and originally deriving from the Proto-Sinaitic script. The morphology of Semitic languages is particularly suited to this approach, as the denotation of vowels is generally redundant. Optional markings for vowels may be used for some abjads, but are generally limited to applications like education. Many pure alphabets were derived from abjads through the addition of dedicated vowel letters, as with the derivation of the Greek alphabet from the Phoenician alphabet c. 800 BC. Abjad is the word for "alphabet" in Arabic, and analogously derives from the traditional order of letters in the Arabic alphabet ('alif, bā', jīm, dāl).

A passage from the biblical Gospel of Luke printed using Balinese script

An abugida is a type of alphabet with symbols corresponding to consonant–vowel pairs, where basic symbols for each consonant are associated with an inherent vowel by default, and other possible vowels for each consonant are indicated via predictable modifications made to the basic symbols. In an abugida, there may be a sign for k with no vowel, but also one for ka (if a is the inherent vowel), and ke is written by modifying the ka sign in a way consistent with how la would be modified to get le. In many abugidas, modification consists of the addition of a vowel sign; other possibilities include rotation of the basic sign, or addition of diacritics.

While true syllabaries have one symbol per syllable and no systematic visual similarity, the graphic similarity in most abugidas stems from their origins as abjads – with added symbols comprising markings for different vowels added onto a pre-existing base symbol. The largest single group of abugidas is the Brahmic family of scripts, however, which includes nearly all the scripts used in India and Southeast Asia. The name abugida was derived by linguist Peter T. Daniels (b. 1951) from the first four characters of an order of the Geʽez script, which is used for certain Nilo-Saharan and Afro-Asiatic languages of Ethiopia and Eritrea.

Featural systems

Originally proposed as a category by Geoffrey Sampson, a featural system uses symbols representing sub-phonetic elements – e.g. those traits that can be used to distinguish between and analyse a language's phonemes, such as their voicing or place of articulation. The only prominent example of a featural system is the hangul script used to write Korean, where featural symbols are combined into letters, which are in turn joined into syllabic blocks. Many scholars, including John DeFrancis, reject a characterization of hangul as a featural system – with arguments including that Korean writers do not themselves think in these terms when writing – or question the viability of Sampson's category altogether.

As hangul was consciously created by literate experts, Daniels characterizes it as a "sophisticated grammatogeny" – a writing system intentionally designed for a specific purpose, as opposed to having evolved gradually over time. Other featural grammatogenies include shorthands developed by professionals and constructed scripts created by hobbyists and creatives, like the Tengwar script designed by J. R. R. Tolkien to write the Elven languages he also constructed. Many of these feature advanced graphic designs corresponding to phonological properties. The basic unit of writing in these systems can map to anything from phonemes to words. It has been claimed that even the Latin script may have sub-character features.

Classification by graphical properties

Linearity

All writing is linear in the broadest sense – i.e., the spatial arrangement of symbols indicates the order in which they should be read. On a more granular level, systems with discontinuous marks like diacritics can be characterized as less linear than those without. In the initial historical distinction, linear writing systems (e.g. the Phoenician alphabet) generally form glyphs as a series of connected lines or strokes, while systems that generally use discrete, more pictorial marks (e.g. cuneiform) are sometimes termed non-linear. The historical abstraction of logographs into phonographs is often associated with a linearization of the script.

In Braille, raised bumps on the writing substrate are used to encode non-linear symbols. The original system – which Louis Braille (1809–1852) invented in order to allow people with visual impairments to read and write – used characters that corresponded to the letters of the Latin alphabet. Moreover, that Braille is equivalent to visual writing systems in function demonstrates that the phenomenon of writing is fundamentally spatial in nature, not merely visual.

Directionality and orientation

Writing systems may be characterized by how text is graphically divided into lines, which are to be read in sequence:

Axis
Whether lines of text are laid out as horizontal rows or vertical columns
Lining
How each line is positioned relative to the one previous on the medium – in practice only vertical scripts vary whether columns are read in a left- or rightward order, as all horizontal scripts sequence rows from top to bottom
Directionality
How individual lines are read – whether starting from the left or right on a horizontal axis, or from the top or bottom on a vertical axis

In left-to-right scripts (LTR), horizontal rows are sequenced from top to bottom on a page, with each row read from left to right. Right-to-left scripts (RTL), which use the opposite directionality, include the Arabic alphabet.

Egyptian hieroglyphs were written either left-to-right or right-to-left, with the animal and human glyphs turned to face the beginning of the line. The early alphabet did not have a fixed direction, and was written both vertically and horizontally; it was most commonly written boustrophedonically: starting in one horizontal direction, then turning at the end of the line and reversing direction.

The right-to-left direction of the Phoenician alphabet initially stabilized after c. 800 BC. Left-to-right writing has an advantage that, since most people are right-handed, the hand does not interfere with what is being written (which, when inked, may not have dried yet) as the hand is to the right side of the pen. The Greek alphabet and its successors settled on a left-to-right pattern, from the top to the bottom of the page. Other scripts, such as Arabic and Hebrew, came to be written right to left. Scripts that historically incorporate Chinese characters have traditionally been written vertically in columns arranged from right to left, while a horizontal direction from left to right was only widely adopted in the 20th century due to Western influence.

Several scripts used in the Philippines and Indonesia, such as Hanunoo, are traditionally written with lines moving away from the writer, from bottom to top, but are read left to right; ogham is written from bottom to top, commonly on the corner of a stone. The ancient Libyco-Berber alphabet was also written from bottom to top.

Orthographic regularity and depth

Polygraphy in English
Phoneme Grapheme Example
/eɪ/ ⟨e⟩ saute
⟨ai⟩ hail
⟨ay⟩ bay
⟨ea⟩ steak
⟨ei⟩ veil
 
Polyphony in English
Phoneme Grapheme Example
/eɪ/ ⟨e⟩ saute
/ɛ/ red
/i/ area
/ə/ taken
smile

Writing systems, especially alphabets, often include characters that can represent multiple sound values, or conversely sound values that can be represented by multiple characters – this phenomenon is referred to as polyvalence. Orthographies with lower or higher polyvalence are referred to as shallow or deep respectively. While polyvalent graphemes are often perceived as defects, they can serve to distinguish homophonic words, and to indicate etymological or semantic connections between words not clear from pronunciation alone – e.g. between English sign and signal or child and children. Specifically, an orthographic relationship where one grapheme may represent multiple sound values can be termed polyphony, while a relationship where one sound value may be represented by multiple graphemes can be termed polygraphy.

Scholars have increasingly analysed different patterns of phonological spellings versus morphological spellings in a writing system as being better suited depending on the characteristics of the spoken language, with neither principle being ideal in all circumstances. While not adhering strictly to phonological rules, morphological spellings often follow other patterns that allow for transparent identification and parsing by readers and writers.

Methane

From Wikipedia, the free encyclopedia
Methane
Stereo, skeletal formula of methane with some measurements added
Ball and stick model of methane
Spacefill model of methane
  Carbon, C
  Hydrogen, H
Structure
Td
Tetrahedral at carbon atom
0 D
Thermochemistry
35.7 J/(K·mol)
186.3 J/(K·mol)
−74.6 kJ/mol
−50.5 kJ/mol
−891 kJ/mol
Hazards
GHS labelling:
GHS02: Flammable
Danger
H220
P210
NFPA 704 (fire diamond)
Flash point −188 °C (−306.4 °F; 85.1 K)
537 °C (999 °F; 810 K)
Explosive limits 4.4–17%
Related compounds
Related alkanes
Related compounds
Supplementary data page
Methane (data page)
Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa).
☒ verify (what is check☒ ?)

Methane (US: /ˈmɛθn/ METH-ayn, UK: /ˈmθn/ MEE-thayn) is a chemical compound with the chemical formula CH4 (one carbon atom bonded to four hydrogen atoms). It is a group-14 hydride, the simplest alkane, and the main constituent of natural gas. The abundance of methane on Earth makes it an economically attractive fuel, although capturing and storing it is difficult because it is a gas at standard temperature and pressure. In the Earth's atmosphere methane is transparent to visible light but absorbs infrared radiation, acting as a greenhouse gas. Methane is an organic compound, and among the simplest of organic compounds. Methane is also a hydrocarbon.

Naturally occurring methane is found both below ground and under the seafloor and is formed by both geological and biological processes. The largest reservoir of methane is under the seafloor in the form of methane clathrates. When methane reaches the surface and the atmosphere, it is known as atmospheric methane.

The Earth's atmospheric methane concentration has increased by about 160% since 1750, with the overwhelming percentage caused by human activity. It accounted for 20% of the total radiative forcing from all of the long-lived and globally mixed greenhouse gases, according to the 2021 Intergovernmental Panel on Climate Change report. Strong, rapid and sustained reductions in methane emissions could limit near-term warming and improve air quality by reducing global surface ozone.

Methane has also been detected on other planets, including Mars, which has implications for astrobiology research.

Properties and bonding

Covalently bonded hydrogen and carbon in a molecule of methane.

Methane is a tetrahedral molecule with four equivalent C–H bonds. Its electronic structure is described by four bonding molecular orbitals (MOs) resulting from the overlap of the valence orbitals on C and H. The lowest-energy MO is the result of the overlap of the 2s orbital on carbon with the in-phase combination of the 1s orbitals on the four hydrogen atoms. Above this energy level is a triply degenerate set of MOs that involve overlap of the 2p orbitals on carbon with various linear combinations of the 1s orbitals on hydrogen. The resulting "three-over-one" bonding scheme is consistent with photoelectron spectroscopic measurements.

Methane is an odorless, colourless and transparent gas at standard temperature and pressure. It does absorb visible light, especially at the red end of the spectrum, due to overtone bands, but the effect is only noticeable if the light path is very long. This is what gives Uranus and Neptune their blue or bluish-green colors, as light passes through their atmospheres containing methane and is then scattered back out.

The familiar smell of natural gas as used in homes is achieved by the addition of an odorant, usually blends containing tert-butylthiol, as a safety measure. Methane has a boiling point of −161.5 °C at a pressure of one atmosphere. As a gas, it is flammable over a range of concentrations (5.4%–17%) in air at standard pressure.

Solid methane exists in several modifications, of which nine are known. Cooling methane at normal pressure results in the formation of methane I. This substance crystallizes in the cubic system (space group Fm3m). The positions of the hydrogen atoms are not fixed in methane I, i.e. methane molecules may rotate freely. Therefore, it is a plastic crystal.

Chemical reactions

The primary chemical reactions of methane are combustion, steam reforming to syngas, and halogenation. In general, methane reactions are difficult to control.

Selective oxidation

Partial oxidation of methane to methanol (CH3OH), a more convenient, liquid fuel, is challenging because the reaction typically progresses all the way to carbon dioxide and water even with an insufficient supply of oxygen. The enzyme methane monooxygenase produces methanol from methane, but cannot be used for industrial-scale reactions. Some homogeneously catalyzed systems and heterogeneous systems have been developed, but all have significant drawbacks. These generally operate by generating protected products which are shielded from overoxidation. Examples include the Catalytica system, copper zeolites, and iron zeolites stabilizing the alpha-oxygen active site.

One group of bacteria catalyze methane oxidation with nitrite as the oxidant in the absence of oxygen, giving rise to the so-called anaerobic oxidation of methane.

Acid–base reactions

Like other hydrocarbons, methane is an extremely weak acid. Its pKa in DMSO is estimated to be 56. It cannot be deprotonated in solution, but the conjugate base is known in forms such as methyllithium.

A variety of positive ions derived from methane have been observed, mostly as unstable species in low-pressure gas mixtures. These include methenium or methyl cation CH+3, methane cation CH+4, and methanium or protonated methane CH+5. Some of these have been detected in outer space. Methanium can also be produced as diluted solutions from methane with superacids. Cations with higher charge, such as CH2+6 and CH3+7, have been studied theoretically and conjectured to be stable.

Despite the strength of its C–H bonds, there is intense interest in catalysts that facilitate C–H bond activation in methane (and other lower numbered alkanes).

Combustion

A young woman holding a flame in her hands
Methane bubbles can be burned on a wet hand without injury.

Methane's heat of combustion is 55.5 MJ/kg. Combustion of methane is a multiple step reaction summarized as follows:

CH4 + 2 O2 → CO2 + 2 H2O
ΔH = −802 kJ/mol, at standard conditions (for water vapor, ΔH = −891 kJ/mol for liquid water)

Peters four-step chemistry is a systematically reduced four-step chemistry that explains the burning of methane.

Methane radical reactions

Given appropriate conditions, methane reacts with halogen radicals as follows:

•X + CH4 → HX + •CH3
•CH3 + X2 → CH3X + •X

where X is a halogen: fluorine (F), chlorine (Cl), bromine (Br), or iodine (I). This mechanism for this process is called free radical halogenation. It is initiated when UV light or some other radical initiator (like peroxides) produces a halogen atom. A two-step chain reaction ensues in which the halogen atom abstracts a hydrogen atom from a methane molecule, resulting in the formation of a hydrogen halide molecule and a methyl radical (•CH3). The methyl radical then reacts with a molecule of the halogen to form a molecule of the halomethane, with a new halogen atom as byproduct. Similar reactions can occur on the halogenated product, leading to replacement of additional hydrogen atoms by halogen atoms with dihalomethane, trihalomethane, and ultimately, tetrahalomethane structures, depending upon reaction conditions and the halogen-to-methane ratio.

This reaction is commonly used with chlorine to produce dichloromethane and chloroform via chloromethane. Carbon tetrachloride can be made with excess chlorine.

Uses

Methane may be transported as a refrigerated liquid (liquefied natural gas, or LNG). 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. Gas pipelines distribute large amounts of natural gas, of which methane is the principal component.

Fuel

Methane is used as a fuel for ovens, homes, water heaters, kilns, automobiles, rockets, turbines, etc.

As the major constituent of natural gas, methane is important for electricity generation by burning it as a fuel in a gas turbine or steam generator. Compared to other hydrocarbon fuels, methane produces less carbon dioxide for each unit of heat released. At about 891 kJ/mol, methane's heat of combustion is lower than that of any other hydrocarbon, but the ratio of the heat of combustion (891 kJ/mol) to the molecular mass (16.0 g/mol, of which 12.0 g/mol is carbon) shows that methane, being the simplest hydrocarbon, produces more heat per mass unit (55.7 kJ/g) than other complex hydrocarbons. In many areas with a dense enough population, methane is piped into homes and businesses for heating, cooking, and industrial uses. In this context it is usually known as natural gas, which is considered to have an energy content of 39 megajoules per cubic meter, or 1,000 BTU per standard cubic foot. Liquefied natural gas (LNG) is predominantly methane converted into liquid form for ease of storage or transport.

Rocket propellant

Refined liquid methane as well as LNG is used as a rocket fuel, when combined with liquid oxygen, as in the TQ-12, BE-4, Raptor, YF-215, and Aeon engines. Due to the similarities between methane and LNG such engines are commonly grouped together under the term methalox.

As a liquid rocket propellant, a methane/liquid oxygen combination offers the advantage over kerosene/liquid oxygen combination, or kerolox, of producing small exhaust molecules, reducing coking or deposition of soot on engine components. Methane is easier to store than hydrogen due to its higher boiling point and density, as well as its lack of hydrogen embrittlement. The lower molecular weight of the exhaust also increases the fraction of the heat energy which is in the form of kinetic energy available for propulsion, increasing the specific impulse of the rocket. Compared to liquid hydrogen, the specific energy of methane is lower but this disadvantage is offset by methane's greater density and temperature range, allowing for smaller and lighter tankage for a given fuel mass. Liquid methane has a temperature range (91–112 K) nearly compatible with liquid oxygen (54–90 K). The fuel currently sees use in operational launch vehicles such as Zhuque-2, Vulcan and New Glenn as well as in-development launchers such as Starship, Neutron, Terran R, Nova, and Long March 9.

Chemical feedstock

Natural gas, which is mostly composed of methane, is used to produce hydrogen gas on an industrial scale. Steam methane reforming (SMR), or simply known as steam reforming, is the standard industrial method of producing commercial bulk hydrogen gas. More than 50 million metric tons are produced annually worldwide (2013), principally from the SMR of natural gas. Much of this hydrogen is used in petroleum refineries, in the production of chemicals and in food processing. Very large quantities of hydrogen are used in the industrial synthesis of ammonia.

At high temperatures (700–1100 °C) and in the presence of a metal-based catalyst (nickel), steam reacts with methane to yield a mixture of CO and H2, known as "water gas" or "syngas":

CH4 + H2O ⇌ CO + 3 H2

This reaction is strongly endothermic (consumes heat, ΔHr = 206 kJ/mol). Additional hydrogen is obtained by the reaction of CO with water via the water-gas shift reaction:

CO + H2O ⇌ CO2 + H2

This reaction is mildly exothermic (produces heat, ΔHr = −41 kJ/mol).

Methane is also subjected to free-radical chlorination in the production of chloromethanes, although methanol is a more typical precursor.[35]

Hydrogen can also be produced via the direct decomposition of methane, also known as methane pyrolysis, which, unlike steam reforming, produces no greenhouse gases (GHG). The heat needed for the reaction can also be GHG emission free, e.g. from concentrated sunlight, renewable electricity, or burning some of the produced hydrogen. If the methane is from biogas then the process can be a carbon sink. Temperatures in excess of 1200 °C are required to break the bonds of methane to produce hydrogen gas and solid carbon. Through the use of a suitable catalyst the reaction temperature can be reduced to between 550 and 900 °C depending on the chosen catalyst. Dozens of catalysts have been tested, including unsupported and supported metal catalysts, carbonaceous and metal-carbon catalysts.

The reaction is moderately endothermic as shown in the reaction equation below.

CH4(g) → C(s) + 2 H2(g)
H° = 74.8 kJ/mol)

Refrigerant

As a refrigerant, methane has the ASHRAE designation R-50.

Generation

Global methane budget (2017). Shows natural sources and sinks (green), anthropogenic sources (orange), and mixed natural and anthropogenic sources (hatched orange-green for 'biomass and biofuel burning').

Methane can be generated through geological, biological or industrial routes.

Geological routes

Abiotic sources of methane have been found in more than 20 countries and in several deep ocean regions so far.

The two main routes for geological methane generation are (i) organic (thermally generated, or thermogenic) and (ii) inorganic (abiotic). Thermogenic methane occurs due to the breakup of organic matter at elevated temperatures and pressures in deep sedimentary strata. Most methane in sedimentary basins is thermogenic; therefore, thermogenic methane is the most important source of natural gas. Thermogenic methane components are typically considered to be relic (from an earlier time). Generally, formation of thermogenic methane (at depth) can occur through organic matter breakup, or organic synthesis. Both ways can involve microorganisms (methanogenesis), but may also occur inorganically. The processes involved can also consume methane, with and without microorganisms.

The more important source of methane at depth (crystalline bedrock) is abiotic. Abiotic means that methane is created from inorganic compounds, without biological activity, either through magmatic processes or via water-rock reactions that occur at low temperatures and pressures, like serpentinization.

Biological routes

Most of Earth's methane is biogenic and is produced by methanogenesis, a form of anaerobic respiration only known to be conducted by some members of the domain Archaea. Methanogens occur in landfills and soilsruminants (for example, cattle), the guts of termites, and the anoxic sediments below the seafloor and the bottom of lakes.

This multistep process is used by these microorganisms for energy. The net reaction of methanogenesis is:

CO2 + 4 H2 → CH4 + 2 H2O

The final step in the process is catalyzed by the enzyme methyl coenzyme M reductase (MCR).

Testing Australian sheep for exhaled methane production (2001), CSIRO
This image represents a ruminant, specifically a sheep, producing methane in the four stages of hydrolysis, acidogenesis, acetogenesis, and methanogenesis.

Wetlands

Wetlands are the largest natural sources of methane to the atmosphere, accounting for approximately 20–30% of atmospheric methane. Climate change is increasing the amount of methane released from wetlands due to increased temperatures and altered rainfall patterns. This phenomenon is called wetland methane feedback.

Rice cultivation generates as much as 12% of total global methane emissions due to the long-term flooding of rice fields.

Ruminants

Ruminants such as cattle belch out methane, accounting for about 22% of the U.S. annual methane emissions to the atmosphere. One study reported that the livestock sector in general (primarily cattle, chickens, and pigs) produces 37% of all human-induced methane. A 2013 study estimated that livestock accounted for 44% of human-induced methane and about 15% of human-induced greenhouse gas emissions. Many efforts are underway to reduce livestock methane production, such as medical treatments and dietary adjustments, and to trap the gas to use its combustion energy.

Seafloor sediments

Most of the subseafloor is anoxic because oxygen is removed by aerobic microorganisms within the first few centimeters of the sediment. Below the oxygen-replete seafloor, methanogens produce methane that is either used by other organisms or becomes trapped in gas hydrates. These other organisms that utilize methane for energy are known as methanotrophs ('methane-eating'), and are the main reason why little methane generated at depth reaches the sea surface. Consortia of Archaea and Bacteria have been found to oxidize methane via anaerobic oxidation of methane (AOM); the organisms responsible for this are anaerobic methanotrophic Archaea (ANME) and sulfate-reducing bacteria (SRB).

Industrial routes

This diagram shows a method for producing methane sustainably. See: electrolysis, Sabatier reaction

Given its cheap abundance in natural gas, there is little incentive to produce methane industrially. Methane can be produced by hydrogenating carbon dioxide through the Sabatier process. Methane is also a side product of the hydrogenation of carbon monoxide in the Fischer–Tropsch process, which is practiced on a large scale to produce longer-chain molecules than methane.

An example of large-scale coal-to-methane gasification is the Great Plains Synfuels plant, started in 1984 in Beulah, North Dakota as a way to develop abundant local resources of low-grade lignite, a resource that is otherwise difficult to transport for its weight, ash content, low calorific value and propensity to spontaneous combustion during storage and transport. A number of similar plants exist around the world, although mostly these plants are targeted towards the production of long chain alkanes for use as gasoline, diesel, or feedstock to other processes.

Power to methane is a technology that uses electrical power to produce hydrogen from water by electrolysis and uses the Sabatier reaction to combine hydrogen with carbon dioxide to produce methane.

Laboratory synthesis

Methane can be produced by protonation of methyl lithium or a methyl Grignard reagent such as methylmagnesium chloride. It can also be made from anhydrous sodium acetate and dry sodium hydroxide, mixed and heated above 300 °C (with sodium carbonate as byproduct). In practice, a requirement for pure methane can easily be fulfilled by steel gas bottle from standard gas suppliers.

Occurrence

Methane is the major component of natural gas, about 87% by volume. The major source of methane is extraction from geological deposits known as natural gas fields, with coal seam gas extraction becoming a major source (see coal bed methane extraction, a method for extracting methane from a coal deposit, while enhanced coal bed methane recovery is a method of recovering methane from non-mineable coal seams). It is associated with other hydrocarbon fuels, and sometimes accompanied by helium and nitrogen. Methane is produced at shallow levels (low pressure) by anaerobic decay of organic matter and reworked methane from deep under the Earth's surface. In general, the sediments that generate natural gas are buried deeper and at higher temperatures than those that contain oil.

Methane is generally transported in bulk by pipeline in its natural gas form, or by LNG carriers in its liquefied form; few countries transport it by truck.

Atmospheric methane and climate change

Methane (CH4) measured by the Advanced Global Atmospheric Gases Experiment (AGAGE) in the lower atmosphere (troposphere) at stations around the world. Abundances are given as pollution free monthly mean mole fractions in parts-per-billion.

Methane is an important greenhouse gas, responsible for around 30% of the rise in global temperatures since the industrial revolution.

Methane has a global warming potential (GWP) of 29.8 ± 11 compared to CO2 (potential of 1) over a 100-year period, and 82.5 ± 25.8 over a 20-year period. This means that, for example, a leak of one tonne of methane is equivalent to emitting 82.5 tonnes of carbon dioxide. Burning methane and producing carbon dioxide also reduces the greenhouse gas impact compared to simply venting methane to the atmosphere.

Sources of global methane emissions

As methane is gradually converted into carbon dioxide (and water) in the atmosphere, these values include the climate forcing from the carbon dioxide produced from methane over these timescales.

Annual global methane emissions are currently approximately 580 Mt, 40% of which is from natural sources and the remaining 60% originating from human activity, known as anthropogenic emissions. The largest anthropogenic source is agriculture, responsible for around one quarter of emissions, closely followed by the energy sector, which includes emissions from coal, oil, natural gas and biofuels.

Historic methane concentrations in the world's atmosphere have ranged between 300 and 400 nmol/mol during glacial periods commonly known as ice ages, and between 600 and 700 nmol/mol during the warm interglacial periods. A 2012 NASA website said the oceans were a potential important source of Arctic methane, but more recent studies associate increasing methane levels as caused by human activity.

Global monitoring of atmospheric methane concentrations began in the 1980s. The Earth's atmospheric methane concentration has increased 160% since preindustrial levels in the mid-18th century. In 2013, atmospheric methane accounted for 20% of the total radiative forcing from all of the long-lived and globally mixed greenhouse gases. Between 2011 and 2019 the annual average increase of methane in the atmosphere was 1866 ppb. From 2015 to 2019 sharp rises in levels of atmospheric methane were recorded.

In 2019, the atmospheric methane concentration was higher than at any time in the last 800,000 years. As stated in the AR6 of the IPCC, "Since 1750, increases in CO2 (47%) and CH4 (156%) concentrations far exceed, and increases in N2O (23%) are similar to, the natural multi-millennial changes between glacial and interglacial periods over at least the past 800,000 years (very high confidence)".

In February 2020, it was reported that fugitive emissions and gas venting from the fossil fuel industry may have been significantly underestimated. The largest annual increase occurred in 2021 with the overwhelming percentage caused by human activity.

Climate change can increase atmospheric methane levels by increasing methane production in natural ecosystems, forming a climate change feedback. Another explanation for the rise in methane emissions could be a slowdown of the chemical reaction that removes methane from the atmosphere.

Over 100 countries have signed the Global Methane Pledge, launched in 2021, promising to cut their methane emissions by 30% by 2030. This could avoid 0.2 °C of warming globally by 2050, although there have been calls for higher commitments in order to reach this target. The International Energy Agency's 2022 report states "the most cost-effective opportunities for methane abatement are in the energy sector, especially in oil and gas operations".

Clathrates

Methane clathrates (also known as methane hydrates) are solid cages of water molecules that trap single molecules of methane. Significant reservoirs of methane clathrates have been found in arctic permafrost and along continental margins beneath the ocean floor within the gas clathrate stability zone, located at high pressures (1 to 100 MPa; lower end requires lower temperature) and low temperatures (< 15 °C; upper end requires higher pressure). Methane clathrates can form from biogenic methane, thermogenic methane, or a mix of the two. These deposits are both a potential source of methane fuel as well as a potential contributor to global warming. The global mass of carbon stored in gas clathrates is still uncertain and has been estimated as high as 12,500 Gt carbon and as low as 500 Gt carbon. The estimate has declined over time with a most recent estimate of ≈1800 Gt carbon. A large part of this uncertainty is due to our knowledge gap in sources and sinks of methane and the distribution of methane clathrates at the global scale. For example, a source of methane was discovered relatively recently in an ultraslow spreading ridge in the Arctic. Some climate models suggest that today's methane emission regime from the ocean floor is potentially similar to that during the period of the Paleocene–Eocene Thermal Maximum (PETM) around 55.5 million years ago, although there are no data indicating that methane from clathrate dissociation currently reaches the atmosphere. Arctic methane release from permafrost and seafloor methane clathrates is a potential consequence and further cause of global warming; this is known as the clathrate gun hypothesis. Data from 2016 indicate that Arctic permafrost thaws faster than predicted.

Public safety and the environment

An International Energy Agency graphic showing the potential of various emission reduction policies for addressing global methane emissions.

Methane "degrades air quality and adversely impacts human health, agricultural yields, and ecosystem productivity".

The 2015–2016 methane gas leak in Aliso Canyon, California was considered to be the worst in terms of its environmental effect in American history. It was also described as more damaging to the environment than Deepwater Horizon's leak in the Gulf of Mexico.

In May 2023 The Guardian published a report blaming Turkmenistan as the worst in the world for methane super emitting. The data collected by Kayrros researchers indicate that two large Turkmen fossil fuel fields leaked 2.6 million and 1.8 million metric tonnes of methane in 2022 alone, pumping the CO2 equivalent of 366 million tonnes into the atmosphere, surpassing the annual CO2 emissions of the United Kingdom.

Extraterrestrial methane

Interstellar medium

Methane is abundant in many parts of the Solar System and potentially could be harvested on the surface of another Solar System body (in particular, using methane production from local materials found on Mars or Titan), providing fuel for a return journey.

Negative methane, the negative ion of methane, is also known to exist in interstellar space. Its mechanism of formation is not fully understood.

Mars

Methane has been detected on all planets of the Solar System and most of the larger moons. With the possible exception of Mars, it is believed to have come from abiotic processes.

Methane (CH4) on Mars – potential sources and sinks

The Curiosity rover has documented seasonal fluctuations of atmospheric methane levels on Mars. These fluctuations peaked at the end of the Martian summer at 0.6 parts per billion.

Methane has been proposed as a possible rocket propellant on future Mars missions due in part to the possibility of synthesizing it on the planet by in situ resource utilization. An adaptation of the Sabatier methanation reaction may be used with a mixed catalyst bed and a reverse water-gas shift in a single reactor to produce methane and oxygen from the raw materials available on Mars, utilizing water from the Martian subsoil and carbon dioxide in the Martian atmosphere.

Methane could be produced by a non-biological process called serpentinization involving water, carbon dioxide, and the mineral olivine, which is known to be common on Mars.

Titan

Titan lakes (September 11, 2017)

Methane has been detected in vast abundance on Titan, the largest moon of Saturn. It comprises a significant portion of its atmosphere and also exists in a liquid form on its surface, where it comprises the majority of the liquid in Titan's vast lakes of hydrocarbons, the second largest of which is believed to be almost pure methane in composition.

The presence of stable lakes of liquid methane on Titan, as well as the surface of Titan being highly chemically active and rich in organic compounds, has led scientists to consider the possibility of life existing within Titan's lakes, using methane as a solvent in the place of water for Earth-based life and using hydrogen in the atmosphere to derive energy with acetylene.

History

Alessandro Volta

The discovery of methane is credited to Italian physicist Alessandro Volta, who characterized numerous properties including its flammability limit and origin from decaying organic matter.

Volta was initially motivated by reports of inflammable air present in marshes by his friend Father Carlo Giuseppe Campi. While on a fishing trip to Lake Maggiore straddling Italy and Switzerland in November 1776, he noticed the presence of bubbles in the nearby marshes and decided to investigate. Volta collected the gas rising from the marsh and demonstrated that the gas was inflammable.

Volta notes similar observations of inflammable air were present previously in scientific literature, including a letter written by Benjamin Franklin.

Following the Felling mine disaster of 1812 in which 92 men perished, Sir Humphry Davy established that the feared firedamp was in fact largely methane.

The name "methane" was coined in 1866 by the German chemist August Wilhelm von Hofmann. The name was derived from methanol.

Etymology

Etymologically, the word methane is coined from the chemical suffix "-ane", which denotes substances belonging to the alkane family; and the word methyl, which is derived from the German Methyl (1840) or directly from the French méthyle, which is a back-formation from the French méthylène (corresponding to English "methylene"), the root of which was coined by Jean-Baptiste Dumas and Eugène Péligot in 1834 from the Greek μέθυ méthy (wine) (related to English "mead") and ὕλη hýlē (meaning "wood"). The radical is named after this because it was first detected in methanol, an alcohol first isolated by distillation of wood. The chemical suffix -ane is from the coordinating chemical suffix -ine which is from Latin feminine suffix -ina which is applied to represent abstracts. The coordination of "-ane", "-ene", "-one", etc. was proposed in 1866 by German chemist August Wilhelm von Hofmann.

Abbreviations

The abbreviation CH4-C can mean the mass of carbon contained in a mass of methane, and the mass of methane is always 1.33 times the mass of CH4-C. CH4-C can also mean the methane-carbon ratio, which is 1.33 by mass. Methane at scales of the atmosphere is commonly measured in teragrams (Tg CH4) or millions of metric tons (MMT CH4), which mean the same thing. Other standard units are also used, such as nanomole (nmol, one billionth of a mole), mole (mol), kilogram, and gram.

Safety

Methane is an asphyxiant gas, meaning that it is non-toxic and the primary health hazard is displacement of oxygen in high enough concentrations, potentially causing death by asphyxiation. No systemic toxicity has been detected at 5% concentration in air.

Methane is an extremely flammable gas at normal ambient temperature. It may form explosive mixtures with air. Methane gas explosions are responsible for many deadly mining disasters. A methane gas explosion was the cause of the Upper Big Branch coal mine disaster in West Virginia on April 5, 2010, killing 29. Natural gas accidental release has also been a major focus in the field of safety engineering, due to past accidental releases that concluded in the formation of jet fire disasters.

Patient advocacy

From Wikipedia, the free encyclopedia https://en.wikipedia.org/wiki/Pat...