Coal Gas - Gas For Industrial Use

Gas For Industrial Use

Fuel gas for industrial use was made using producer gas technology. Producer gas is made by blowing air through an incandescent fuel bed (commonly coke or coal) in a gas producer. The reaction of fuel with insufficient air for total combustion produces carbon monoxide (CO); this reaction is exothermic and self-sustaining. It was discovered that adding steam to the input air of a gas producer would increase the calorific value of the fuel gas by enriching it with CO and hydrogen (H₂) produced by water gas reactions. Producer gas has a very low calorific value of 3.7 to 5.6 MJ/m³ (100-150 Btu/ft3 (std)); because the calorific gases CO/H₂ are diluted with lots of inert nitrogen (from air) and carbon dioxide (CO₂) (from combustion)

(Exothermic: Producer gas Reaction)

(Endothermic: Water gas Reaction)

(Endothermic)

(Exothermic: Water gas shift reaction)

The problem of nitrogen dilution was overcome by the blue water gas (BWG) process, developed in the 1850s by Sir William Siemens. The incandescent fuel bed would be alternately blasted with air followed by steam. The air reactions during the blow cycle are exothermic, heating up the bed, while the steam reactions during the make cycle, are endothermic and cool down the bed. The products from the air cycle contain non-calorific nitrogen and are exhausted out the stack while the products of the steam cycle are kept as blue water gas. This gas is composed almost entirely of CO and H₂, and burns with a pale blue flame similar to natural gas. BWG has a calorific value of 11 MJ/m³ (300 Btu/ft3 (std)).

Because blue water gas lacked illuminants, it would not burn with a luminous flame in a simple fishtail gas jet as existed prior to the invention of the Welsbach mantle in the 1890s. Various attempts were made to enrich BWG with illuminants from gas oil in the 1860s. Gas oil (an early form of gasoline) was the flammable waste product from kerosene refining, made from the lightest and most volatile fractions (tops) of crude oil. In 1875 Thaddeus S. C. Lowe invented the carburetted water gas process. This process revolutionized the manufactured gas industry and was the standard technology until the end of manufactured gas era. A CWG generating set consisted of three elements; a producer (generator), carburettor and a super heater connected in series with gas pipes and valves.

During a make run, steam would be passed through the generator to make blue water gas. From the generator the hot water gas would pass into the top of the carburetor where light petroleum oils would be injected into the gas stream. The light oils would be thermocracked as they came in contact with the white hot checkerwork fire bricks inside the carburettor. The hot enriched gas would then flow into the superheater, where the gas would be further cracked by more hot fire bricks.