Other Definitions
town gas (dict)
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Town GasTown gas is a generic term referring to manufactured gas produced for sale to consumers and municipalities. Prior to the development of natural gas during 1940s and 1950s, virtually all fuel and lighting gas was manufactured. The development of manufactured gas paralleled that of the industrial revolution and urbanization. Manufactured gas is made by two processes; title="carbonization or carbonization" title="[gasification">carbonization">[gasification">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 pyrolysis of coal. The off gases liberated in the high temperature carbonization (coking) of coal were collected, and scrubbed and used as fuel. Coal was heated in retorts which were stacked together to form "benches". Heat was provided by burning the coke formed during devolitalzation. The goal of a 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 gas typically had a caloric value (CV) of 250-550 Btu per standard cubic foot (scf). Fuel gas for industrial use was made using producer gas technology. Producer gas is made by blowing air through an incandescent fuel bed 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 100-150Btu/scf because the calorific gases CO/H2 are diluted with lots of inert Nitrogen (from air) and CO2 (from combustion) 2C(s) + O2 ---> 2CO (Exothermic: Producer gas Reaction) C(s) + H2O(g) -----> CO + H2 (Endothermic: Water Gas Reaction) C+2H2O -----> CO2 + 2H2 (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 300Btu/scf 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 Prof TSC 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), carburetor 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
Outline of further work 1 Enrichment of BWG with illuminants from gas oil circa 1860s. Gas Oils, are the volatile fractions that evaporate above kerosene. Problems of gas oil condensing out of the gas. 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 550Btu/scf i.e slightly more than half that of natural gas. Golden age of gas light develops with welsbach mantle. 3. Development of Pacific coast oil gas process/Pintsch Railway oil Gas processes. Massive problems with Lampblack created from the Pacific coast process. Up to 20-30lb of oily soot/1000scf. Major pollution problem leads to passage of early enviromental legislation at the state level. 4 Layout of a typical gas plant. - 1880s Coal gasification plant.
4.5 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 )
5. 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 useless to process.
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- CWG tar is full of lighter PAH's, good for making pitch, but poor in chemical precursors. (Get a good analysis of CWG Tar vs Coal Tar)
- 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.
- Public Utilities Holding Companies Act forces break up of gas companies.
6. Environmental Impacts. FMGP issues. - Coal tar waste/sludges,
- Gas tar emulsions.
- Purifier waste
- Contaminated refuse
- Lampblack (Pacific Coast)
7. Post WWII: The big squeeze: Fast death by 1955. - Development of natural gas industry. NG is 1000 Btu/scf
- Petrochemicals kill much of the value coal tar as a source of chemical feed stocks.
- Decline in creosote use for wood preserving.
- Direct coal/natural gas injection reduces demand for metallurgical coke. 25-40% 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. 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 !!!
* Catalytic upgrading of gas by use of native hydrogen to react with tarry vapors
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