16/03/2022
Bruton Town Gas Works were on the south side of Shute Lane.
Gas Works and Coke or Coal Tar Plants
Coal consists of carbon mixed with a range of impurities, if it is heated in a sealed container with no air (pyrolysis) the various impurities in the coal are driven off as a thick dirty smoke to leave almost pure carbon in the form of coke. If the smoke from the retort is allowed to cool tars and oils are condensed out, leaving a fairly clear gas which can be used as a fuel. This process was developed by a William Murdock in the 1790's and his 'coal gas' was soon developed for domestic and industrial use.
The tar and oils that were condensed out of the gas were initially a waste problem for the gas works but it was not long before people began looking at these tars and oils to see if they contained anything of value. With the water content reduced the tar was found to be useable as a protective 'paint' on buildings (rammed earth cottages had a black strip of tar around the base of their whitewashed walls where the rain might cause most damage). By the early nineteenth century someone had developed a method of producing 'roofing felt', essentially a woolen fabric coated in tar and then covered by sand. Roofing felt was popular in Germany and the USA throughout the nineteenth century but was less common in Britain other than on flat roofed buildings.
Coal gas tar consists on a range of materials, basically divided into 'Naphtha' (valuable) and 'Pitch' (less valuable). Naphtha is a generic term used for the assorted light distillates of coal tar, wood and petroleum. A ton of coal produced about seven and a half gallons of tar, of which about 60% ends up as 'pitch' and only about 5% constitutes the more valuable elements in the naphtha. One of the first valuable by-products from coal distillation was creosote and by the 1860's the remaining lighter fractions were distilled to obtain benzene, toluene naphthalene & etc. Some of these fractions ended up in the gas being produced and from the 1870's Benzol was being recovered from coal gas at many larger works.
In the larger gas works the gas from the condensers was passed to the Benzol plant where it was re-heated to about 170 degrees centigrade to recover these lighter distillates. A ton of coal only produced a few pounds of Benzol. Benzol is a mixture containing mostly benzene but also toluene, anthracene, xylene and the aniline so important to the dyestuffs industry. Aniline is colourless but turns brown on exposure to air and light, it is one of the most important of the organic bases and as well as dyestuffs it is used herbicides, fungicides, photographic chemicals, in the making of urethane foam and pharmaceuticals. Over half the dyes used today are the nitrogen based 'azo dyes', the basis of which is naphtha. Since the 1930's these chemicals have mainly been obtained from petroleum oil at oil refineries.
The unrefined naphtha, a deep amber to dark red liquid, was often shipped out for further processing in drums from smaller gas works, or in tank wagons for larger establishments. Coal tar naphtha contains the more valuable light distillates usually lumped together under the general term 'Benzol'.
The tar could be processed on site, first being passed to the Naphthalene plant where it is the again heated to a slightly higher temperature (200-250 degrees centigrade). From this you can take the naphthalene direct as a white flaky powder or get heavier oils including carbolic oil. At still higher temperatures (up to 400 degrees centigrade) you get creosote oil, a yellowish to dark green-brown liquid (also known as tar oil and occasionally as liquid pitch oil). The remaining tar, a thick black oily liquid is called 'pitch', was stored on site in a pit and shipped out in barrels or tank wagons to be used for tarring roads and as a wood preservative.
During the Second World War experiments were made with introducing hydrogen into the tar to produce something akin to fuel oil (the process was called 'gasification') but this was expensive and was not followed up at the time. More recently there has been a resurgence of interest in this process, but this is mainly based in the USA.
The residual coke from the gas works was rather soft and could only be used as fuel, although it has the benefit of being smokeless. Most would be sold locally and hauled away by road in sacks or two wheeled carts, although some might be shipped out by rail for delivery to a coal merchant (the coke from the gas works was sold as Firemax after Nationalisation of the gas industry).
Gas Works (see photo)
Prior to the development of natural gas supplies in the late 1960's and early 1970's gas was made from coal in a local gas works. The town gas works is a popular adjunct to model railways set in any period up to the 1970's, they were built in sizes to suit almost any size of layout and generated an interesting and regular traffic for the line. At the gas works the sealed containers used to cook the coal are called retorts, originally made of metal they changed to moulded fire clay in the 1830's. The evolution of coal gas retorts is illustrated in Fig ___, note that the cross sections shown are generalised to show the basic changes, they are not accurately based on specific prototypes.
Fig ___ Gas retort buildings
Sketch showing typical gas works retort buildings
Fig ___ (1) shows the early form of retort house, there were between five and seven retorts (A) arranged rather like a bank of torpedo tubes in a submarine. The coke used to heat the retorts was shovelled into the furnace (B), which gas works people called the 'producer'. Ash from the producer fell through the fire bars and collected underneath (C), the floor of the retort house was made up of removable plates (D) to allow the ash to be periodically shovelled out. The gas was drawn off from the retorts through pipes (E) which fed a single main pipe called the 'foul main' which lead out to the purification plant.
Once all the gas had been removed the hot coke from the retorts was dug out using long handled rakes and tipped into metal wheel barrows (called coke barrows). These were then quickly wheeled to a small brick or stone shed where they were sprayed with water to cool the coke down. An example is shown in Fig ___ (2), in this case there is a small water tank on top of the structure, fed through a small pipe to the right. The coke would then be graded, probably using angled bars, stockpiled and sold.
In the mid nineteenth century, as the size of retorts increased, mechanical stoking was introduced but this was not a great success. In the 1880's someone thought of building angled retorts, filled from the top and emptied from the bottom with the assistance of gravity. Fig ___ (3) shows the general arrangement. The angled retorts (A) are supplied with coal from bunkers (F) in the upper part of the building. The coal is wheeled in from the stock pile in small trucks (B) which are then hoisted up the outside of the building. A separate 'charging stage' (C), also fed from the hoist, allowed the crew to stoke the 'producer' to heat the retorts. The ash from the producer fell through the fire bars to an ash pit (D) in the base of the building. The hot coke was emptied into small trucks (E) or onto a conveyor belt, quenched with cold water, graded for size and stockpiled.
The logical development was to build bigger retorts and Fig ___ (4) shows a cross section of a retort house dating from the turn of the century. The retort in this case was made of metal and on the plans it is called a 'chamber oven' (A), the building was still called a retort house however. The coal is supplied by a conveyor system (B) from the stockpile and fed into a crusher (C). A gravity bucket hoist (D) lifts the coal into the upper part of the building where it is distributed to the storage bunkers (F) using small tipping trucks (E) or a conveyor system. Small tipper trucks feed the vertical producers (G) through the top and the ash is extracted at the bottom (H). When all the gas has been extracted the coke is emptied with the assistance of a powered ram (I) via a power operated door into a quenching trough (J) and from there it is fed into tipper trucks (K) to be hauled away, graded and stockpiled.
Fig ___ (5) shows a highly mechanised retort house from the 1930's. The retorts (A) are vertical and they are heated with carburrated water gas from an on-site plant. Unlike the older designs the retorts operate continuously with the coal passing down through them changing to coke by the time it reaches the bottom. Coal is supplied via lorry or railway wagons to the loading point (B). Here it is crushed by the machinery (C) then hoisted in a gravity bucket conveyor (D) to the bunkers (E) at the top of the building. The hot coke is mechanically extracted at the bottom of the retorts and is carried away on conveyor belts (F) to be quenched, sorted for size and stockpiled. Conveyor belts under the floor of the building (G) move the coal from one side to the other and feed the hoppers at the top.
Fig ___ (6) shows a coke grading plant which might be built onto the end of the retort houses shown in Fig ___ (3), (4) or (5). The coke is first quenched with cold water. A gravity bucket conveyor lifts the coke into a hopper (A) at the top of the building. The coke is then fed through a series of rotating screens (B) which grade it for size, emptying into the hoppers below. From the hoppers it can be bagged in a bagging plant (C) or dropped straight into road or railway vehicles (D). As a rough guide a hundred tons of coal would typically yield seventy tons of coke.
The dirty smoke generated in the retorts is drawn off through pipes, this smoke contains the desired gas as well as a range of impurities, some of which can be recovered and sold for various purposes (gas works were noted for the smell of camphor). What happens to the smoke depends on the age and size of the associated works, in early gas works it was passed to a condenser consisting of a set of tall iron pipes usually in the open air into which a spray of cold water was injected. The cold water condensed the tars and other oils in the gas and washed it into a tar pit from where it was sold as a wood preservative.
The remaining gas was then passed through water into which lime had been mixed to remove more of the impurities (mainly smelly sulphur compounds) and stored in cast-iron (later riveted iron plate) cylinders called gas holders. Gas holders are often called 'gasometers' although there is in fact no such word. Inside there is a water bath and the gas holder sits in this rather like the plastic cap of an aerosol sitting in a cup of water. The gas is trapped above the water and as the holder fills with water it is lifted up, the weight of the metal providing the pressure for the gas (a booster pump was often required to supply mains pressure as demand and the area being served increased).
Up to the mid 1830's the gas holders had to be encased in a brick building by law. These buildings were usually hexagonal in form, often with a prominent roof ventilator, and a surviving example may be seen at Warwick although the tank holding buildings are now offices. This crude secondary containment was actually highly dangerous, in the tank there is no air so the gas cannot explode, but trapped between the tank and the building an explosive gas-air mix could form if the tank leaked. If the mix was ignited the brick building would in effect become a very large hand gr***de. I have found no evidence of any of these encased gas holders lasting later than the 1850's, for one thing they were all rather small and the demand for gas was growing rapidly.
Early gas holding tanks were simple open-bottomed containers floating on water as a sealbut by the 1840's telescopic tanks were in use, with additional sections (called 'lifts') to increase the height of the tank without increasing the depth of the water tank. These early tanks had supporting masts spaced around the sides with rails for small guide wheels on the top of the tank. If the tank proved too small they were sometimes modified by adding more lifts, this meant they extended above the original frame and they were usually fitted with rope guides to keep them steady. The external supporting frame was sometimes painted in a light colour, typically something similar the 'light earth' or even 'sand' in the standard modelling paint ranges but dark green seems to have been the most common colour for the entire apparatus. The supporting frame designs were quite varied, on larger tanks there might be twelve supports, connected together at the top and one or two points on the ides with horizontal trussed frames and with diagonal metal rods forming X shapes between them.
the gas was often pumped into smaller gas holders in the chain of processes so the flow could be controlled, these were mainly of the multi-lift (masted or spiral) type. At a more rural gas works the gas holders would be proportionately smaller, you can get away with a couple of holders about three and a half inches in diameter for such a works.
The change to North Sea Gas in the 1970s did not make these gas holders totally redundant, they were used for storage and pressure control for the new service. They cost a lot to maintain however and by the late 1990s there were only about five hundred still in existence. The privatised gas distribution company Transco has invested in new pipeline technology which eliminates the need for these gas holders and they plan to demolish all of them by 2010. Some have been 'listed' for preservation as monuments but most will have be demolished in the first few years of the twenty first century.
From the tanks the gas was passed through cast iron pipes, coated with a black enamel material made by dissolving coal dust into coal tar, to the homes and factories. Difficulties in sealing lengths of pipe lead to quite frequent explosions in the streets due to gas leaks in the 1820's and early 1830's. The gas from the early works still contained a lot of impurities, it produced an unpleasant sulphurous smell and soot when it burned. By the 1850's the process was much better understood and the filtering had improved but no one has yet found a way to get all the smelly sulphur compounds out of coal gas. By this time people were making considerable use of the by-products extracted from the coal tar and from the gas itself. The hot smoke was first passed through a water or air cooled condenser which removes the oily coal tar. A pump called an exhauster then passes the gas to a filtering bed to remove the smelly hydrogen sulphide (which gave early gas supplies the smell of rotten eggs). The water and lime mix was replaced by a dry lime filter in the 1830's and this in turn was replaced by iron oxide filtering in the 1850's. The iron oxide was actually a mixture of iron oxide (rust) and wood shavings, called Iron Sponge or Iron Mass.
The gas is then bubbled through water to remove trace acids and nitrates, of which ammonia was the most valuable. Water will absorb over seven hundred times its own volume of ammonia and the resulting liquid was called Ammoniacal Liquor. There were still impurities in the gas, some of which were worth recovering. The larger works would often have a Benzol plant to extract a range of oily hydrocarbons. The benzol mixture from town gas works was of poor quality and was mainly used as an additive for petrol. The gas would then be dried and pumped via a meter into the gas holder.
The coal used in gas works was selected for its gas producing qualities, most was supplied as a rough mixture of small coals mixed with coal dust. The coke from the retorts was therefore usually small (two inch lumps down to dust), it was mostly just graded and sold locally. In the early days children with small hand carts hauled it away but by the early twentieth century it was being sold in sacks carted away on lorries resembling loads of coal. Some was also sold in bulk and shipped by the wagon load for use as a fuel in factories. Most gas works feature a large stockpile of coke, the bigger the works the larger this would be.
The coke from a gas works consists of over 90% pure carbon and its porous structure allows it to burn rapidly. As most of the impurities have been removed it is 'smokeless' and much less smelly than coal, so the larger lumps were popular for domestic stoves.
Up to the 1940's most gas works coke was sold to factories as fuel, some was burnt but more was used to make producer gas and water gas which could be piped through the factory wherever heat was required. In smaller works some of the coke was used as fuel to heat the retorts, often this was loaded directly from the retort using a simple wheeled chute. Coke firing remained the norm at existing small gas works but in the 1870's there was a shift to using producer gas, made on site from the coke, to heat the retorts. This requires another small building, with a chimney, to house the gas plant (see also Lineside Industries - Prototype industrial ancillary structures). This kind of works remained in use right to the end of coal gas as the small communities supplied from such works were the last to be changed over to North Sea Gas in the 1970's. The illustration below shows a 'carburrated water gas' (CWG) plant, a developed form of producer gas which uses superheated steam and an oil spray to add calorific value to the gas produced.
From the First World War up to the 1940's a lot of coke from gas works was used to make synthetic ammonia for fertiliser but this practice died out quickly in the 1950's as ammonia from oil refineries came on stream. Gas works coke was also used (with limestone) to make calcium carbide, used in the production of acetylene gas. Unfortunately gas works coke was not suitable for use in a blast furnace for iron or steel making as it is too soft but some was used in refining other metallic ores.
The used 'iron sponge' is called 'spent oxide', it is liable to spontaneous combustion if it comes into contact with air. To get the sulphur out air was admitted which reacted with the mixture to produce sulphur dioxide (the main use for which is the manufacture of sulphuric acid). At smaller works the spent oxide was taken away in sealed container to a separate works to have the sulphur removed but at the larger works the sulphur recovery was done on site. In a typical installation the iron tanks containing the oxide were mounted on a low raised platform. There would be several banks of filters and each in turn would be un-coupled from the gas lines and lowered to ground level. Air pipes and sulphur dioxide extraction pipes were coupled in place of the gas pipes and the air was admitted to start the process.
The ammoniacal liquor was decanted into holding tanks and either shipped out in tank wagons or, at a larger works, it could be further processed on site. Ammoniacal liquor when distilled with lime (delivered in sheeted or 'cottage topped' open wagons) and treated with sulphuric acid produces ammonium sulphate, a valuable fertiliser. The sulphuric acid could be made at the gas works as described above, additional supplies would be delivered in glass carboys or, from the 1930's, in de-mountable iron tanks. Ammonium sulphate is a white powdery substance, shipped out in sacks, the area where this stuff was handled would have a lot of white staining on the ground and on any loading bank or platform.
In smaller works the tar from the condensers was dumped into a pit to be decanted into wooden barrels and sold locally for use as a wood preservative. Larger works would have a raised tank for filling the barrels and might also ship the tar out in tank wagons for further processing. M
In the early days most tar was used for road surfacing, waterproofing felt for flat roofs and as a wood preservative. After the 1880's people started distilling the tar to extract a range of valuable chemicals. The processes involved favoured large installations and many tar distilling firms set up to process tar purchased from smaller gas works (see preceding section for information on coal tar distilleries). By the end of the century some larger works had their own tar distillery on site, invariably known locally as 'the chemical works'. The degree of processing varied from site to site and from a modelling perspective it is better to rely on a separate firm as this generates more interesting traffic.
Fig ___ Small Gas Works
Sketch showing typical flow diagram for a Small Gas Works
(see photo)
Some works were very small indeed, virtually every town had a gas works and not all towns were large. At Seascale in Cumberland in 1939 the entire staff of the local gas works was one man, Mr. Lee, who also had to maintain the local gas meters and appliances. He bagged and carted the coke from the ovens for sale in the area and also sold barrels of unrefined coal tar. These small works continued in use right up to the change over to North Sea or Natural Gas, in fact the smaller establishments in remote towns were the last to go, outliving their larger more modern brethren. The last coal gas works in Britain was at Muirkirk in Ayrshire which closed in 1977 when the town was connected to the new gas grid.
The small town gas works at Fakenham, about thirty kilometers North West of Norwich in northern Norfolk, is now a listed ancient monument. There is a very nice little gas holder dating from the 1880's and a retort house dating from the 1840's. When it closed in 1965 the works was partially preserved as a museum, operated on a volunteer basis and normally only has open days on Thursdays during the summer months (May to early September). For further information on open days for the Fakenham Museum of Gas call: 01328 863 150 (Summer months and daytime only).
Gas works used coal in small sizes, typically 8 inches (400 mm) cube down to dust. This was often carted from the local railway yard in horse drawn carts but even some quite small gas works boasted a siding or two. At my local town gas works a street tramway was used with a gas works locomotive hauling coal wagons up the middle of the road from the station yard to the works.
In smaller gas works the coal was wheel barrowed or shifted in small hand-trammed tubs on a narrow gauge track way to the retort house. A man with a shovel would then throw the coal into the retort pipes. At smaller works the coke was handled using long handled shovels and rakes. It came out of the retorts red hot and was moved in slatted metal 'coke barrows' to the quenching hut. The cooled coke was then wheeled in the coke barrow to a stockpile.
