How was coal used to make gas?

Answer 1

The earliest methods of producing gas from coal relied on simply heating it in the absence of air until it began decomposing and released "coal gas" which was composed of various combustible gases such as hydrogen, methane, carbon monoxide and various other volatile organic components. The coal gas was mostly a by-product of "coking" coal; besides the coal gas, coking yielded a superior fuel for iron (and steel) production as well as coal tar that was useful in the nascent chemical industry as a raw material for things like dyes.

Later processes improved on this process to make "Producer Gas" via the reaction: 2C + O2 + 3.73 N2 → 2CO+ 3.73 N2
(the nitrogen is present because they use air)

"Water Gas" is made by first converting the carbon in the coal to synthesis gas and then further reacting it:
H2O + C → H2 + CO (both H2 and CO are flammable)
CO + H2O ⇌ CO2 + H2 (thus producing additional H2 which is more energetic than CO and not toxic)

One very clean gassification method is the Integrated Gassification Combined Cycle (IGCC). In this process the coal is injected as a slurry of coal in water into closed pressurized reactor with a shortage of oxygen. The shortage of oxygen ensures that coal is broken down by the heat and pressure as opposed to burning completely. The chemical reaction between coal and oxygen produces a product that is a mixture of carbon and hydrogen, or syngas where the overall process is:
CxHy + (x/2)O2 → (x)CO2 + (y/2)H2 where CxHy represents the carbon/hydrogen ratio of the coal and not all of the carbon is necessarily converted.

The first step is the production of syngas via:
H2O + C → H2 + CO
The heat from the production of syngas is used to produce steam from cooling water which is then used for steam turbine electricity production. The raw syngas is then subjected to the water-shift reaction
CO + H2O ⇌ CO2 + H2
to get hydrogen, then fed to scrubbers where carbon dioxide is dissolved into water (carbon capture) and piped off as essentially carbonated water and problem components like sulfur compounds (which can cause acid rain) and heavy metals are removed. The result is a fairly pure stream of hydrogen gas which is either then fed to another set of electricity generating turbines or captured and sold off-site as fuel.