Water gas is a synthesis gas, containing carbon monoxide and
hydrogen. It is a useful product but requires careful handling due to its
flammability and the risk of carbon monoxide poisoning. The gas is made by
passing steam over a red-hot carbon fuel such as coke:
H2O + C → H2 + CO
(ΔH = +131 kJ/mol)
The reaction is endothermic so the fuel must be continually
re-heated to keep the reaction going. In order to do this, an air stream, which
alternates with the vapor stream, is introduced for the combustion of carbon to
take place.
O2 + C → CO2 (ΔH =
−393.5 kJ/mol)
Theoretically to make 6 L of water gas, 5 L of air is
required.
Or alternatively to prevent contamination with nitrogen,
energy can be provided by using pure oxygen to burn carbon into carbon
monoxide.
O2 + 2 C → 2 CO
(ΔH = −221 kJ/mol)
In this case 1 L of oxygen will create 5.3 L of pure water
gas.
History
The water gas shift reaction was discovered by Italian
physicist Felice Fontana in 1780.
Water gas was made in England from 1828 by blowing steam
through white-hot coke.
Lowe's gas process
In 1873, Thaddeus S. C. Lowe developed and patented the
water gas process by which large amounts of hydrogen gas could be generated for
residential and commercial use in heating and lighting. This gas provided a
more efficient heating fuel than the common coal gas, or coke gas, which was
used in municipal service. The process used the water gas shift reaction:
CO + H2O → CO2 +
H2
The process was discovered by passing high-pressure steam
over hot coal, the major source of coke gas. Lowe's process improved upon the
chimney systems by which the coal could remain superheated thereby maintaining
a consistently high supply of the gas. The reaction produced carbon dioxide and
hydrogen which after a process of cooling and "scrubbing" produced
hydrogen gas.
The process spurred on the industry of gas manufacturing,
and gasification plants were established quickly along the Eastern seaboard of
the United States. Similar processes, like the Haber Process, led to the
manufacture of ammonia (NH3) by the combining of nitrogen, found in air, with
hydrogen. This spurred on the refrigeration industry which long used ammonia as
its refrigerant. Prof. Lowe also held several patents on artificial ice making
machines, and was able to run successful businesses in cold storage as well as
products which operated on hydrogen gas.
Variations
Carburetted water gas
Water gas had a lower calorific value than coal gas so the
calorific value was often boosted by passing the gas through a heated retort
into which oil was sprayed. The resulting mixed gas was called carburetted
water gas.
Semi-water gas
Semi-water gas is a mixture of water gas and producer gas
made by passing a mixture of air and steam through heated coke. The heat
generated when producer gas is formed keeps the temperature of the coke high
enough to allow water gas to be formed.
Water gas shift reaction
Pure hydrogen can be obtained from water gas by using the
water gas shift reaction, after subsequent removal of the carbon dioxide formed
when carbon monoxide reacts with water.
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