Environmental Threat
Site Characterization
Man. Gas Processes
Plant Wastes
Contamination Threat Modes
Residuals - Components
Sources of MGP Liquid Effluent
FMG Plants in the US
Parallel MG Technologies
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   Residuals and Wastes of Gas Manufacturing


 Each of the various processes of gas manufacturing used feedstock materials to create gas, first for purposes of illumination and later (generally after 1890) for heat and fuel. In order to generate and capture the gas released from the feedstock, organic matter of some type was sealed in retorts, ovens or generator set shells (names depending on the process) and heated in the absence (pyrolytically) of oxygen. This roasting drove off the volatile content of the feedstock, as gas, which was captured and cleansed of impurities so that it would serve the consumer for the intended purpose (light, heat or fuel energy).


This chapter of the website identifies the residuals and wastes that were also created in generation of gas. As the feedstock was roasted to release the wanted gas, also released were tiny particles of aerosolized tars. The tars were known as “oils” by the historic gas makers who knew that such compounds must be separated from the gas in order to give the gas the correct properties of illumination (in order of their adoption, “candle power,” British Thermal Units (B. T. U.), then (1930) “Therms.”).  Also as the gas was generated, the feedstock would release various amounts of contaminated waters of condensation, which became known as “ammoniacal liquor” in the case of coal gas or “gas liquor” for all of the other gas generation processes.  By 1860 a clear concept of the hierarchy of product and waste was well in place. Gas generation produced residuals and from residuals valuable secondary by-products could be further captured and sold in local, regional or international commerce, for considerable revenue. At the same time whatever was allowed to be wasted was observed and known to have detrimental characteristics and properties when “discharged to the ground” (environment).



There is a hierarchy of product and residual involved in gas manufacture. In most cases gas was manufactured with itself as the desired product, and the other forms of liquids and solids produced in the process were considered residuals and waste. At the first instant, the two of the residuals were recognized as by-products of value (coke) or potential value (tar) and the ammoniacal liquor considered largely as a waste. By about 1820 forms of tar distillation were underway as water sealant-repellent, and ammonia was being concentrated as a cleansing fluid. In the 1850s initial discoveries were made (Britain) of the dye qualities available from oil (tar) fractions separated to the individual compound level and the world chemical industry was launched on the basis of coal tar as its feedstock.  


Sources of Residual By-products and Waste



Waste Source Location & Potential

Transportation Spur

Delivery point of feedstocks;
exit point of salable residuals

Human labor was a significant cost to gas making. Feedstocks were brought as close as possible to the retorts and generator houses.

Coal Yard

Storage area which kept coal dry for optimal use in firing boilers or as retort feedstock

Kept as close as feasible to the retorts and generators.  Many plants chose to place coal in sheds so as to optimize gasification in the presence of minimal water content.

Coke Yard

By-product coke from coal-gas plants

Used symbiotically as feedstock for various water gas plants, especially as co-located

Retort House

Coal-gas retorts housed internally in benches; groups of benches known as stacks

The central building of the gas-making process; generally located at the corner of the plant with highest elevation and near the gate, from which the processed gas left the plant through the station meter.  Origin of coke quench water = ammoniacal liquor.

Generator House

Location of generator sets  for carburetted water gas and oil-gas processes

Generation capacity such that vastly smaller space required for commensurate production over that required for coal-gas process

Condenser House

Building or addition immediately adjacent to retort house or generator house

After 1910, tended to be out-of-doors. Same configuration used for all gas generating processes; a wet process that concentrated and/or precipitated tars for further management.


Tall (5-10 m) right-circular cylinders with slanted trays holding contaminant-absorbing wood fiber/chips

Usually employed a water shower to remove tar and other process residuals from the gas.  Residuals captured in scrubber sump for further management.


Gas immersed in agitated water bath to cool gas and drop tar particles into its sump.

With carburetted water gas and enhanced oil-gas.  When designed as a water-seal/wash box, placed first in the clarification sequence as a seal against back-flow of gas.

Combined Washer- Scrubber

When employed, generally post-1895

Enhanced the recovery of tar from gas.  Trapped tar held on sorbant and in sump.

Sumps of Clarification Devices

Condensers, scrubbers and washers, and their combinations had bottom sumps to trap and yield tar and tar sludges

Tar generally removed manually for recovery, reuse or dumping.
Spills and leaks assumed in a generic sense.
Tar sludges contained refractory geologic impurities such as quartz and feldspar, entering the system mainly from feedstocks.


Steam-driven gas evacuator employed to reduce gas pressure and promote flow through system

Position of exhauster chosen by the plant gas engineer to achieve optimal flow of gas through the tar-removal clarification process; most plants had a backup exhauster in parallel.

(Purifier Boxes)

Gas was passed through “boxes” containing layers of lime, wood chips, oxide of iron (particles) and/or strips of iron as various forms of sorbants, often in conjunction with each other.

 Generally employed minimally as a pair of “boxes” in series, with at least a spare pair in parallel.

Trapped some tar, but designed to trap sulfur, cyanide, arsenic and other heavy metals all of which originated in or from the organic gas feedstock materials

Relief Holder


1) With coal gas, the oldest of the gas holders, serving as a raw-gas exposure to tar-dropping seal water before clarification/purification.

2) With carburetted or oil-enhanced water gas a usually necessary (*) presence to buffer gas-pressure variations on blow-run cycles.

* Under some circumstances it was possible for small CWG plants to operate without a relief holder.

Relief holders of the first variety can be expected to have subsurface "tanks" (pits = basins) commonly abandoned and virtually full of unrecovered tar.

Second variety holder tanks tend to be less commonly abandoned with large volumes of water-gas tar, unless dumped at time of plant decommissioning.

Gas Holders

As many as needed; ever more and larger as the gas business expanded.

Generally predicated on the largest holder being equivalent to one day’s make

Of prime concern are the subsurface tanks most common to pre-1900 varieties.

Of several basic design variations.
Those pre-1900 have a subsurface water-seal tank likely to have leaked considerable amounts of precipitated and trapped PAHs to the subsurface soil, rock and groundwater,0 through various fractures related to brick, masonry and/or  concrete or composite construction materials. Valve pits commonly exhibit hot-spot concentrations of PAH contamination

Tar Wells and Tar Cisterns

Subsurface tanks, right-circular cylinders and rectangular or square-sided; brick, masonry or concrete or composite.

 Less commonly known as “ammonia wells”.

Commonly designed with a self-functioning gas-liquor (process water) discharge system to carry off lightest-fraction of gas liquor while retaining the gravity-separated tar fraction; all subject to through-fracture flow leakage to the surrounding earth during the operational period.

Tar Extractor

Typically an above-ground mechanical device for separating tar particles from the passing gas.

Most common and best known were the "P & E" devices of French manufacture.

Tar Separator

Both as above-ground devices housed in structures and as   subsurface rectangular-form concrete or wood  “tanks,” the latter often made of wood planks subject to between-plank leakage.

Above-ground devices were machines built to physically separate tar particles from gas liquor; below-ground devices contained flow baffles functioning to slow in-out flow of gas liquor carrying suspended tar, the latter dropped to the sump of the tar separator.

Boiler House

Necessary to power the extractor and a variety of small steam engines and fluid pumps

Generally consumed coal or by-product coke; could be rigged for burning tar, under close supervision of temperatures. Ash not expected to be toxic unless later so exposed.

Oil Storage Tanks
(Above Ground and Underground)

Illuminating or enriching oil for non-coal-gas production

Generally petroleum oils susceptible to biodegradation if leaked or spilled; generally no incentive or rationale to dump

Plant Plumbing

Below-ground piping, often in trenches or pipe chases.

Virtually all process piping was subject to corrosion and release of PAHs, or release through joints and seams.  Well known to the gas industry since 1860s.

Yard Drips
(Drip Pots)

Light-oil (drip oil) collection sumps placed along gas-flow pipes in the gas yard.

Used to collect naphthalene and other light oils; these were of value and were recycled, usually as carburettion oils for water gas, or as industrial solvents.  Sometimes disposed of as herbicide or by dumping.


The fire  box located below gas benches and all boilers.

Source of operational heat; residue was only ash, cinder, clinker or slag; not expected to be hazardous by nature of its formation.

Station Meter

Plant production measuring device housed in a building at the gas-outlet from the plant.

Generally co-located with the plant office and in the up-gradient end of the site, near the plant gate.  Not a source of contamination.


Gas flow control device adjusting distributed gas to main distribution pressure.

Should not be a source of contamination.

Rail-Spur Spills

Operational-era spills of tars and other fluid residuals (light oils and ammonia) being transferred off-site as by-products.

Naturally most prominent at larger plants and those plants engaged in by-product recovery operations.

Purification Box Media Spreading Ground

Wood-chip and some forms of iron oxide media could be revivified on this pad and returned for re-use short of ultimate “spent” condition

Action implies shaking and mass-expansion via pitch forks.  Sulfur and Prussian blue (cyanide) could be raked up and sold as by-products in many instances.

Spent Wood-Chip Box Waste Burning Ground

A corner or side area of the gas yard where dry chips could be torched and destroyed by fire.

Required dry climate or dry season; ashes carried to a plant dump.

Plant Dump

Primary disposal site on the gas yard; broken, fractured, slagged retort bricks; generator lining bricks, all manner of scurf or other carbon-slag wastes, ash, clinker, slag, off-specification tar, tar sludge, lampblack, box wastes, bottles, purifier shelf slats, broken window glass, corroded pipe, scrap iron, wagon and vehicle parts, and broken gas-plant equipment.

Expect  a toxic character in general. Plant dump likely will be found in or at the furthest down-slope corner or extension of the gas yard, along the adjacent creek, stream, or river, or filling any original topographic declivity of the ground at the site.  In almost all cases, the plant dump was filled early and supplemented with multiple dumps around the periphery of the gas plant, to within a several-block wagon haul distance.


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