# Coal into Gas

Laplace’s Mini World · Plate F60 · Energy, Flight & Sensing · Energy

> A gasifier is kept short of oxygen on purpose: ground coal and water only partly burn at over a thousand degrees, and out comes mostly carbon monoxide and hydrogen. The ash melts and runs down the wall.

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- Updated: 2026-11-01

## 01 What Goes In, What Comes Out

A gasifier takes in coal, water and oxygen, and gives out a gas and a slag. In a design case by the US National Energy Technology Laboratory (NETL), every kilogram of coal goes in with 0.41 kg of water and 0.81 kg of oxygen-⁠rich gas; out come 2.11 kg of raw syngas and 0.11 kg of slag (computed here from its stream table).

Raw syngas is mostly carbon monoxide and hydrogen. Both still burn, and both can be built into other things: hydrogen, ammonia, methanol, and from those fuels and plastics.

At the front are lumps of bituminous coal and a dish of slag. The slag is the coal’s ash: its minerals melt in the vessel, run down and set in water as black glass.

*per kg of coal: 0.41 kg of water, 0.81 kg of oxygen-⁠rich gas → 2.11 kg of raw syngas, 0.11 kg of slag*

## 02 One Gives Heat, One Takes It

Carbon joining oxygen as carbon monoxide gives out heat, about 110 kJ a mole. Carbon and steam making carbon monoxide and hydrogen take heat in, about 131 kJ a mole (computed here from the NIST-⁠JANAF tables).

Both run in the same chamber. The first keeps the vessel above a thousand degrees; the second frees the hydrogen in the water. Burnt all the way to carbon dioxide, carbon gives out about 394 kJ a mole: the vessel runs hotter, but less of the gas is any use.

The injector in the top head sprays the slurry and the oxygen down into the chamber together. In NETL’s case the chamber runs at 5.6 MPa and 1,316 °C.

*C + ½O₂ → CO: −110.5 kJ · C + H₂O → CO + H₂: +131.3 kJ · C + O₂ → CO₂: −393.5 kJ*

## 03 Less than Two-⁠Fifths of the Oxygen

Burning a kilogram of this coal completely takes about 2.01 kg of oxygen; NETL’s case gives it 0.76 kg, under two-⁠fifths (computed here), within the “25 to 40 percent of the theoretical oxidant” NETL gives. Short of oxygen, the carbon stops at carbon monoxide and the hydrogen stays hydrogen, instead of going on to carbon dioxide and water.

The oxygen comes from an air separation unit next door: the gas sent to the vessel is 95 % oxygen, the rest argon and nitrogen. With air instead, the same oxygen would bring nearly four times as much nitrogen: in the same case about 46 % of the raw syngas would be nitrogen, heated for nothing and diluting the product (computed here).

The coal is ground and mixed with water into a slurry that a pump pushes into the injector. In NETL’s case dry coal is about 63 % of the slurry (computed here from the stream table); the slurry’s water is the steam the reactions need.

*to burn it all: 2.01 kg of O₂ · given: 0.76 kg, about 38 % · oxygen 95 % pure · with air: about 46 % nitrogen*

## 04 A Chamber Lined with Brick

NETL describes this gasifier as a refractory-⁠lined, high-⁠pressure combustion chamber. The slurry and the oxygen are sprayed down from the injector at the top and react at about 1,200 to just under 1,500 °C; they cross the chamber in a few seconds, and over 98 % of the carbon becomes gas.

The layer facing the fire is high-⁠chrome brick: chromium oxide stands the heat and dissolves only slowly in the molten slag. Behind it are backup brick and insulation, keeping the heat in so the shell stays at a temperature steel can stand. Slag soaking into the brick makes it flake away. Where the wear is worst a lining may last only 90 days; typically it lasts 10⁠–⁠18 months. To make it last two years, the Polk station in Florida ran its gasifier cooler, and converted less of its carbon.

The model’s vessel is drawn about twice as wide as a real one, its brick in fewer, thicker layers. The orange in the chamber stands for the heat, brightest in the jet from the injector.

*a few seconds inside · over 98 % of the carbon converted · a high-⁠chrome lining: replaced every 90 days to two years*

## 05 Ash Turned to Glass

The heat in the vessel melts the coal’s ash. The molten slag runs down the wall, through the throat in the floor, and falls with the gas into the quench chamber below.

There a dip tube takes the gas and the slag down into a pool of water. The gas bubbles up through it, cooled in NETL’s quench design to about 289 °C, and leaves at the side; the slag sets as glassy grains and sinks. NETL says these solids are essentially all of the ash plus a little unconverted carbon, “in the form of glass, which fully encapsulates any metals”.

The slag leaves through a lock hopper, a vessel between two valves: with the upper valve open and the lower one shut, slag falls in; the other way round, it falls out, and the gasifier keeps its pressure. By NETL’s stream table about 98 % of the coal’s carbon goes into the gas; the rest stays in the slag (computed here). Leaching tests show this slag barely leaches and can be classed as non-⁠hazardous; volatile metals such as mercury do not stay in it and are taken out of the gas instead.

*about 0.11 kg of slag per kg of coal: 97 g of ash, about 13 g of unconverted carbon · quenched to about 289 °C*

## 06 Carbon Monoxide and Hydrogen

In NETL’s case the raw syngas leaving the gasifier at 1,316 °C is by moles about 36 % carbon monoxide, 34 % hydrogen, 14 % carbon dioxide and 14 % steam; the rest is hydrogen sulfide, argon, nitrogen, ammonia and a little methane. Hydrogen to carbon monoxide is about 0.95 (computed here).

Syngas is not natural gas: natural gas is mostly methane, while syngas is mostly carbon monoxide and hydrogen, with about 0.1 % methane. NETL puts syngas at 250 to 400 Btu per cubic foot against about 1,000 for natural gas. Syngas is mainly a feed for making other things.

The gases keep turning into one another: the cooler it gets, the more carbon monoxide and steam become carbon dioxide and hydrogen. NETL’s gas sits where that balance would be at about 840 °C, not at 1,316 °C (computed here from JANAF).

*CO 35.8 % · H₂ 34.1 % · CO₂ 13.8 % · H₂O 13.7 % · H₂S 0.7 % · CH₄ 0.1 % (NETL, leaving the gasifier, wet)*

## 07 Washed, and the Sulfur Taken Out

From the quench the gas goes into a scrubber, where water washes out the remaining fine ash, chlorides and ammonia. In the gasifier most of the coal’s sulfur becomes hydrogen sulfide and 3⁠–⁠10 % carbonyl sulfide; the carbonyl sulfide is turned into hydrogen sulfide, the gas cooled and passed up an absorber where a solvent such as Selexol takes the hydrogen sulfide out.

The hydrogen sulfide goes to a Claus unit: part of it is burnt to sulfur dioxide, which reacts with the rest to give elemental sulfur. NETL puts the Claus unit at about 98 % recovery, and about 99.8 % with tail-⁠gas treatment.

This coal is 2.51 % sulfur, about 25 g in every kilogram (computed here). A coal power station takes sulfur out of its flue gas; a gasification plant takes it out before the gas is burnt, when there is far less gas to treat.

*scrubber → COS hydrolysis → H₂S absorbed → Claus unit makes sulfur · about 99.8 % overall*

## 08 Trading Carbon Monoxide for Hydrogen

The raw syngas has about as much hydrogen as carbon monoxide: about 0.95 to 1 in NETL’s case (computed here). Methanol takes two parts of hydrogen to one of carbon monoxide by its equation; ammonia takes hydrogen only.

The fix is the water-⁠gas shift: carbon monoxide and steam react on a catalyst to give carbon dioxide and hydrogen, giving out about 41 kJ a mole, and the cooler the more hydrogen. Coal gasification plants commonly use a sulfur-⁠tolerant cobalt-⁠molybdenum catalyst after the scrubber, with the gas going in at about 230⁠–⁠260 °C. NETL notes that several plants needing a hydrogen-⁠rich gas quench the raw syngas in water precisely to add the water for the shift.

How far to shift depends on the product: for ammonia nearly all the carbon monoxide is shifted, the carbon dioxide removed and nitrogen from the air separation unit added (how the ammonia is made is on the Haber⁠–⁠Bosch page). The model’s shift converter, cut open, holds a bed of catalyst between two layers of ceramic balls.

*CO + H₂O → CO₂ + H₂: −41.2 kJ · raw H₂ : CO about 0.95 · methanol needs about 2 · ammonia needs hydrogen only*

## 09 Gas for the Street Lamps

Most of the gas in 19th-⁠century towns was not made this way. Coal was heated from outside in closed retorts, with no air and no steam, and the gas given off went through the hydraulic main, the condensers and the lime purifiers into a holder floating in water. That is carbonisation: about 70 % of a ton of coal came out as coke, and the gas was only about 17 % by weight (Encyclopædia Britannica, 1911).

London’s Gas Light and Coke Company received its charter in 1812; in 1813 Westminster Bridge was lit by gas. By the 1820s London had about 50,000 gas lamps. That coal gas was about half hydrogen and over a quarter methane.

The gasifier’s own reaction was in water gas: steam blown through red-⁠hot coke, giving carbon monoxide and hydrogen. The reaction takes heat in and the bed cools, so air was blown in turns to heat it again before more steam. In 1875 Thaddeus Lowe patented water gas enriched with oil in the United States; in Britain such gas was often mixed into town gas, 25⁠–⁠50 % of it.

*charter 1812 · Westminster Bridge 1813 · coal gas: H₂ 51.8 %, CH₄ 27.0 % · water gas: H₂ 50.0 %, CO 41.0 %*

## 10 Under Pressure, on Oxygen

In 1926 the first Winkler producer started at Leuna in Germany: fine coal held up and tumbling in the gas blown into it, as if boiling. In the 1930s Lurgi gasified brown coal with oxygen and steam at 20⁠–⁠30 atmospheres; in 1937 Nature reported that the town of Hirschfelde was supplied with its gas.

Texaco’s gasifier made ammonia feed gas from coal in a trial of 1956⁠–⁠1958; its first commercial coal gasification plant started in Tennessee in 1983. South Africa’s Sasol began making synthetic fuels from coal in 1955; in 1984 the Great Plains plant in North Dakota began turning lignite into pipeline gas, and it still runs.

Meanwhile town gas went away: from 1967 to 1977 Britain moved about 13 million customers from coal gas to North Sea natural gas.

*Winkler 1926 · Lurgi under pressure, 1930s · Sasol 1955 · Texaco’s commercial plant 1983 · Great Plains 1984*

## 11 Today, Mostly in China

The International Energy Agency estimates that the world made about 97 Mt of hydrogen in 2023, about 20 % of it from coal gasification without carbon capture, mostly in China; China made about 28 Mt, nearly 60 % of it from coal.

About 35 % of methanol comes from coal gasification, most of it in China (IRENA, 2021). The IEA wrote in 2021 that about 85 % of China’s ammonia is made by coal gasification. In 2019 there were about 130 coal gasification plants in the world, over 80 % of them in China.

In China, the hydrogen that ammonia needs mostly comes from coal gasification.

*2023: about 20 % of the world’s hydrogen from coal gasification · China nearly 60 % · methanol about 35 % · China’s ammonia about 85 %*

## 12 The Coal’s Carbon

When the product is hydrogen, all the carbon monoxide is shifted, and every bit of the coal’s gasified carbon ends up as carbon dioxide. In NETL’s case a kilogram of coal gives about 0.15 kg of hydrogen and about 2.29 kg of carbon dioxide, about 15.5 kg per kilogram of hydrogen (computed here, leaving out the energy for the oxygen, the steam and the power). The IEA puts coal-⁠based hydrogen at about 19 tonnes of carbon dioxide per tonne, against about 10 from natural gas.

Methanol keeps some of the carbon. By the equations, even with nothing wasted only about 47 % of the gasified carbon ends up in the methanol (computed here); the rest becomes carbon dioxide. The figures IRENA cites are 2.6⁠–⁠3.8 kg of CO₂-equivalent per kilogram of methanol from coal, against about 0.5 from natural gas.

NETL’s case is one with carbon capture: after the shift, a second Selexol stage takes the carbon dioxide out as a stream of its own, and by its carbon balance about 90 % of the gasified carbon goes into it. Since 2000 the Great Plains plant in North Dakota has piped carbon dioxide to Canada for oil recovery; the company says it captures about two-⁠thirds of the readily available carbon dioxide at full rates. What carbon dioxide does in the air is on the greenhouse effect page.

*all to hydrogen: about 2.29 kg of CO₂ and 0.15 kg of H₂ per kg of coal · IEA: about 19 t of CO₂ per t of coal-⁠based hydrogen*

## 13 The Gasifier Taken Apart

The injector comes out of the top head’s neck, then the head and the reaction chamber lift apart, each still cut open so the brick shows; the quench chamber stays on its lugs. Beside it: the lock hopper, the scrubber, the shift converter and the slurry tank.

Rest the pointer on a part and it lifts alone and is named; otherwise the sections part together. A real shell is welded in one piece; the model’s flanges are there for taking it apart.

*injector · top head · reaction chamber · quench chamber · lock hopper · scrubber · shift converter · slurry tank*

## 14 Oxygen and Water

Computed here: a kilogram of the coal NETL uses, with set amounts of oxygen and water, reacting at 56 bar to chemical equilibrium; the temperature is the one where the heat in equals the heat out, with no heat lost through the walls. The gas holds carbon monoxide, carbon dioxide, hydrogen, steam and methane; when the gas is saturated with carbon, the rest stays as a solid.

With NETL’s feed the model gives about 1,230 °C, where NETL has 1,316 °C: it leaves out the 2 % of carbon that does not react, and it is not NETL’s process simulation. With only 0.5 kg of oxygen per kilogram of coal the vessel drops to about 910 °C and about 13 % of the carbon is left; with more oxygen it runs hotter, but the share of carbon monoxide and hydrogen falls again.

Readouts: the temperature (to the nearest 10 °C), the share of carbon monoxide and hydrogen in the dry gas, the carbon turned to gas. In the chart, across: the oxygen per kilogram of coal; above, the temperature; below, the dry gas.

*adiabatic equilibrium model · NETL’s feed: about 1,230 °C (NETL: 1,316 °C) · 0.5 kg of oxygen: about 910 °C, 13 % of the carbon left*

**Try this**: Take the oxygen down to 0.5 kg per kilogram of coal: how cold does the vessel get, and how much of the carbon is left? Then take it up to 0.9 kg: does the share of carbon monoxide and hydrogen in the dry gas rise or fall?

## What the model leaves out

This is a model of the principle: the gasifier is drawn about twice as wide as a real one, its brick in fewer, thicker layers, and the flanges on its shell are there for taking it apart; the scrubber, the shift converter and the slurry tank are drawn small and close by. The air separation unit, the coal mills, the black-⁠water treatment, the acid gas removal and the Claus unit are not shown. The orange in the chamber stands for the heat; the bubbles and grains in the quench chamber are schematic too. The lab is an adiabatic equilibrium model: no heat loss, no unconverted carbon, only the five main gases, and it stops at the top of its tables above 1,900 °C.

## Easy to get wrong

Gasification is not combustion: the oxygen is kept to under two-⁠fifths of what burning the coal would take, and what comes out still burns. Syngas is not natural gas: it is mostly carbon monoxide and hydrogen, with very little methane. Most 19th-⁠century town gas was coal gas made by carbonisation, not gasification; only the water gas mixed into it used the same reaction. Gasification takes the sulfur out before anything is burnt, but most of the coal’s carbon still ends up as carbon dioxide, and nearly all of it does when the product is hydrogen.

## Common questions

### Why does a gasifier get only part of the oxygen?

Short of oxygen, the carbon stops at carbon monoxide and the hydrogen stays hydrogen, instead of going on to carbon dioxide and water.

See: [03 Less than Two-⁠Fifths of the Oxygen](https://world.lapla.si/en/coalgas/#oxygen)

### Where does a gasifier’s heat come from?

Carbon joining oxygen as carbon monoxide gives out heat, about 110 kJ a mole.

See: [02 One Gives Heat, One Takes It](https://world.lapla.si/en/coalgas/#heat)

### How is syngas different from natural gas?

Syngas is not natural gas: natural gas is mostly methane, while syngas is mostly carbon monoxide and hydrogen, with about 0.1 % methane.

See: [06 Carbon Monoxide and Hydrogen](https://world.lapla.si/en/coalgas/#gas)

### What does the water-gas shift do?

The fix is the water-⁠gas shift: carbon monoxide and steam react on a catalyst to give carbon dioxide and hydrogen, giving out about 41 kJ a mole, and the cooler the more hydrogen.

See: [08 Trading Carbon Monoxide for Hydrogen](https://world.lapla.si/en/coalgas/#shift)

### How was 19th-century town gas made?

Coal was heated from outside in closed retorts, with no air and no steam, and the gas given off went through the hydraulic main, the condensers and the lime purifiers into a holder floating in water.

See: [09 Gas for the Street Lamps](https://world.lapla.si/en/coalgas/#town)

## Sources

1. [US DOE National Energy Technology Laboratory (2013): Cost and Performance Baseline for Fossil Energy Plants, Volume 1, Revision 2a (the GE gasifier cases: feeds, temperature and pressure, stream tables, quench, slag, cleaning and sulfur recovery; Illinois No. 6 coal)](https://www.netl.doe.gov/projects/files/CostandPerformanceBaselineforFossilEnergyPlantsVolume1BituminousCoalandNaturalGastoElectricityRevision2a_092013.pdf)
2. [M. W. Chase (1998): NIST-⁠JANAF Thermochemical Tables, 4th ed. (reaction heats, equilibrium constants, the lab’s thermochemistry)](https://janaf.nist.gov/)
3. [Encyclopædia Britannica, 11th ed. (1911), “Gas” (the gasworks, what a ton of coal gives, water gas, carburetted water gas)](https://en.wikisource.org/wiki/1911_Encyclop%C3%A6dia_Britannica/Gas)
4. [London Metropolitan Archives: the Gas Light and Coke Company’s records (founded 1810, charter 1812, gas from 1813)](https://atom.aim25.com/index.php/chartered-gas-light-and-coke-company)
5. [London Museum: How gas lamps illuminated London (Westminster Bridge, the lamps of the 1820s)](https://www.londonmuseum.org.uk/collections/london-stories/gas-lamps-illuminated-london/)
6. [Wales & West Utilities, WSP, Northern Gas Networks (2024): Lessons Learnt: Past Energy Transitions in the Gas Industry (coal gas and water gas, after Ward 1960; the change to natural gas)](https://wwutilities.co.uk/media/5331/lessons-learnt-from-the-past.pdf)
7. [US Circuit Court (1889): Guarantee Trust & Safe-⁠Deposit Co. v. New Haven Gas-⁠Light Co., 39 F. 268 (Lowe’s patent of 1875, making water gas in turns)](https://law.resource.org/pub/us/case/reporter/F/0039/0039.f.0268.html)
8. [F. Winkler: German patent DRP 437 970 (filed 1922)](https://patents.google.com/patent/DE437970C/de)
9. [British Intelligence Objectives Sub-⁠Committee, report 333: Winkler Generators (the first Winkler producer, Leuna, 1926)](https://fischer-tropsch.org/primary_documents/gvt_reports/BIOS/bios_333.htm)
10. [US Bureau of Mines, Abstracts of Literature, no. 453 (Lurgi’s pressure gasification, after a 1945 report)](https://fischer-tropsch.org/Bureau_of_Mines/abs_of_lit/lit-abs/0400/lit0453.htm)
11. [H. J. Hodsman (1937), Nature 140, 903 (the Lurgi pressure producer at Hirschfelde)](https://www.nature.com/articles/140903a0)
12. [Sasol: Historical milestones](https://www.sasol.com/historical-milestones)
13. [Dakota Gasification Company: History 1980⁠–⁠1986 and the CO₂ pages (the Great Plains Synfuels Plant)](https://www.dakotagas.com/about-us/history/1980-1986.html)
14. [International Energy Agency (2019): The Future of Hydrogen (hydrogen from coal, the gasification plants, CO₂ per tonne of hydrogen, ammonia and methanol)](https://iea.blob.core.windows.net/assets/9e3a3493-b9a6-4b7d-b499-7ca48e357561/The_Future_of_Hydrogen.pdf)
15. [International Energy Agency (2024): Global Hydrogen Review 2024 (hydrogen in 2023, coal’s share)](https://www.iea.org/files/89c1e382-dc59-46ca-aa47-9f7d41531ab5/GlobalHydrogenReview2024.pdf)
16. [International Energy Agency (2021): Ammonia Technology Roadmap (the gasification step, China’s ammonia)](https://www.iea.org/files/6ee41bb9-8e81-4b64-8701-2acc064ff6e4/AmmoniaTechnologyRoadmap.pdf)
17. [IRENA and the Methanol Institute (2021): Innovation Outlook: Renewable Methanol (coal’s share of methanol)](https://www.irena.org/-/media/Files/IRENA/Agency/Publication/2021/Jan/IRENA_Innovation_Renewable_Methanol_2021.pdf)

## Data

The gasifier’s feeds, conditions, streams and gas cleaning are from the US DOE National Energy Technology Laboratory’s Cost and Performance Baseline for Fossil Energy Plants, Volume 1 (Revision 2a, 2013), its GE Energy gasifier cases and Illinois No. 6 coal. Thermochemistry from the NIST-⁠JANAF Thermochemical Tables (Chase, 1998). The lab’s adiabatic equilibrium model, the mass and carbon balances and the reaction heats are computed here (docs/plates/coalgas/computed.py). Town gas and water gas from the Encyclopædia Britannica (11th ed., 1911, “Gas”), the London Metropolitan Archives, the London Museum and Wales & West Utilities’ Lessons Learnt (2024, after Ward 1960); the 20th century’s gasifiers from the German patent DRP 437 970, BIOS report 333, the US Bureau of Mines, Nature (1937), Sasol and Dakota Gasification; today’s uses and emissions from the IEA (The Future of Hydrogen, 2019; Global Hydrogen Review 2024; Ammonia Technology Roadmap, 2021) and IRENA (Innovation Outlook: Renewable Methanol, 2021). The gasifier, the scrubber, the shift converter and the slurry tank are a model of the principle, not any one plant. Material presets, parts and the softbox studio of the site’s model kit are ported from AirsupHQ/machine-⁠skill (MIT, © 2026 Air Supply Technologies Inc.).

## Licence CC BY-NC 4.0

The text, pictures and models on this page are licensed CC BY-NC 4.0: free to use and adapt for teaching and other non-commercial purposes, with credit to world.lapla.si. Third-party material listed above keeps its own licence.

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## Cite this plate

Laplace’s Mini World, Plate F60, “Coal into Gas”, https://world.lapla.si/en/coalgas/, CC BY-NC 4.0, updated 2026-11-01.