Town gas
Town gas is a generic term referring to manufactured gas produced for sale to consumers and municipalities. Depending on the processes used for its creation the gas was a mixture of caloric gases: hydrogen, carbon monoxide, methane, and volatile hydrocarbons with small amounts of noncaloric gases carbon dioxide and nitrogen as impurities.
Prior to the development of natural gas during 1940s and 1950s, virtually all fuel and lighting gas was manufactured, and the side product coal tars were an important chemical feedstock for the chemical industries. The development of manufactured gas paralleled that of the industrial revolution and urbanization. The terms coal gas and manufactured gas are also common.
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Manufacturing process
Manufactured gas is made by two processes: carbonization or gasification. Carbonization refers to the devolitalization of an organic feedstock to yield gas and char. Gasification is the process of subjecting a feedstock to chemical reactions that produce gas.
The first process used was the carbonization and partial pyrolysis of coal. The off gases liberated in the high temperature carbonization (coking) of coal in coke ovens were collected, scrubbed and used as fuel. Depending on the goal of the plant, the desired product was either a high quality coke for metallurgical use, with the gas being a side product or the production of a high quality gas with coke being the side product. Coke plants are typically associated with metallurgical facilities such as smelters, and blast furnaces, while gas works typically served urban areas.
The goal of a public utility gas works was to produce the greatest amount of highly illuminating gas. The illuminating power of a gas was related to amount of soot forming hydrocarbons (“illuminants”) dissolved in it. These hydrocarbons gave the gas flame its characteristic bright yellow color. Gas works would typically use oily bituminous coals as feedstock. These coals would give off large amounts of volatile hydrocarbons into the coal gas, but would leave behind a crumbly, low quality coke not suitable for metallurgical processes. Coal or Coke oven gas typically had a caloric value (CV) between 1 and 2 MJ/m³ with values around 2 MJ/m³ being typical.
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 CO: this reaction is exothermic and self sustaining. It was discovered that adding steam to the input air of a producer would increase the CV of the fuel gas by enriching it with CO and H2 produced by water gas reactions. Producer gas has a very low CV of 3.7 to 5.6 MJ/m³ because the calorific gases CO/H2 are diluted with lots of inert nitrogen (from air) and CO2 (from combustion)
- 2 C (s) + O2 → 2 CO (Exothermic: producer gas reaction)
- C (s) + H2O (g) → CO + H2 (Endothermic: water gas reaction)
- C + 2 H2O → CO2 + 2 H2O (Endothermic)
- CO + H2O → CO2 + H2 (Exothermic: water gas shift reaction)
The problem of nitrogen dilution was overcome by the blue water gas 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-caloric 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 CV of 11 MJ/m³
Because blue water gas lacked illuminants it would not burn with a luminous flame in a simple fishtail gas jet as existing prior to the discovery of the welsbach mantle in the 1890s. Various attempts were made to enrich BWG with illuminants from gas oil in the 1860s. Gas oil was the flammable waste product from kerosene refining, made from the lightest and most volatile fractions (tops) of crude oil.
In 1875 Thaddeus SC Lowe invented the carburetted water gas process. This process revolutionized the manufactured gas industry and was the standard technology untill 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 firebricks 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
Early history
0. Gas producers invented, water gas process discovered. Mond Gas: 1850s Europeans discover that using coal instead of coke in a producer results in producer gas that contains amonia and coal tar, Mond Gas is processed to recover these valuable compounds.
1 Enrichment of BWG with illuminants from gas oil circa 1860s. Gas Oils, are the volatile fractions that evaporate above kerosene, major problem for kerosene industry.
2. The invention of the Carburetted Water gas process by Prof. TSC Lowe in 1875. The gas oil is fixed into the BWG via thermocracking in the carburettor and superheater of the CWG generating set. CWG is the dominant technology from 1880s until 1950s, replacing coal gasification. CWG has a CV of 2 MJ/m³ i.e slightly more than half that of natural gas. Golden age of gas light develops with welsbach mantle.
Development of Pacific coast oil gas process
1912. /Pintsch Railway oil Gas processes 1880s.
Massive problems with lampblack created from the Pacific coast process. Up to 20 to 30 lb/1000 ft³ (300 to 500 g/m³) of oily soot. Major pollution problem leads to passage of early enviromental legislation at the state level.
Layout of a typical gas plant
- 1880s Coal gasification plant.
- 1910 CWG plant
Issues in gas processing
- Tar aerosols (tar extractors, condensers/scrubbers, Electrostatic precipitators in 1912)
- Light oil vapors (oil washing)
- Naphthalene (oil/tar washing)
- Ammonia gas (scrubbers)
- Hydrogen sulfide gas (purifier boxes)
- Hydrogen cyanide gas (purifier)
WWI-interwar era developments
- Loss of high quality gas oil (used as motor fuel) and feed coke (diverted for steelmaking) leads to massive tar problems. CWG tar is less valuable than coal gasification tar as a feed stock. Tar-water emulsions are uneconomical to process due to unsellable water and lower quality by products.
- CWG tar is full of lighter PAH's, good for making pitch, but poor in chemical precursors.
- Various "back-run" procedures for CWG generation lower fuel consumption and help deal with issues from the use of bitumious coal in CWG sets.
- Development of high pressure pipeline welding encourages the creation of large municipal gas plants and the consolidation of the MG industry. Sets the stage for rise natural gas.
- Electric lighting replaces gaslight. MG industry peak is sometime in mid 1920s
- 1936 or so. Development of Lurgi gasifier. Germans continue work on gasification/synfuels due to oil shortages.
- 1935 Public Utilities Holding Companies Act forces break up of integrated coke and gas companies.
- Fischer-Tropff process for synthesis of liquid fuels from CO/H2 gas.
- Haber-Bosch ammonia process creates a large demand for industrial hydrogen.
Post WWII: the decline of manufactured gas
- Development of natural gas industry. NG is 37 MJ/m³
- Petrochemicals kill much of the value coal tar as a source of chemical feed stocks.(BTX, Phenols, Pitch)
- Decline in creosote use for wood preserving.
- Direct coal/natural gas injection reduces demand for metallurgical coke. 25 to 40% less coke is needed in blast furnaces.
- BOF and EAF processes obsolete cupola furnaces. Reduce need for coke in recycling steel scrap. Less need for fresh steel/iron.
- Steel is replaced with aluminum and plastics.
- Pthalic Anhydride production shifts from catalytic oxidation of naphthalene to o-xylol process.
Post WWII positive developments
- Catalytic upgrading of gas by use of hydrogen to react with tarry vapors in the gas
- Interestingly the decline of coke making in the US leads to a coal tar crisis since coal tar pitch is vital for the production of carbon electrodes for EAF/Aluminum. US now has to import CT from china
- Development of process to make methanol via hydrogenation of CO/H2 mixtures.
- Mobil M-gas process for making gasoline from methanol
- SASOL coal process plant in South Africa.
- Direct hydrogenation of coal into liquid and gaseous fuels
Environmental effects
From its original development until the wide scale adoption of natural gas, more than 50,000 manufactured gas plants were in existence in the United States alone. The process of manufacturing gas usually produced a number of by-products that contaminated the soil and groundwater in and around the manufacturing plant, so many former town gas plants are a serious environmental concern, and cleanup and remediation costs are often high.
Commonly associated with former manufactured gas plants (known as "FMGPs" in environmental remediation) are contaminants including:
- BTEX
- Diffused out from deposits of coal/gas tars
- Leaks of carburetting oil/light oil
- Leaks from drip pots, that collected condensable hydrocarbons from the gas
- Coal tar waste/sludge
- Typically found in sumps of gasometers/decanting ponds.
- Coal tar sludge has no resale value and so was always dumped.
- Volatile Organic Compounds
- Semivolatile Organic Compounds
- Many heavier coal tar compounds are not very volitile, i.e PAH's
- Polyaromatic hydrocarbons
- Found in copious quantities in coal tar, gas tar, and pitch.
- heavy metals
- Leaded solder for gas mains, lead piping, coal ashes.
- cyanide
- Purifer waste has large amounts of complex ferrocyanides in it.
- Lampblack
- Only found were crude oil was used as gasification feedstock.
- Tar emulsions