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Photograph 1. The Mond gas plant at the former South Staffordshire Mond Gas Company plant,Tipton. Image courtesy of the National Grid Gas Archive.

Producer Gas PlantsA profile of Producer Gas Plants, their design,development, application and type of contaminantspresent.

Prepared by Dr Russell Thomas, Technical DirectorParsons Brinckerhoff, Redland, Bristol, UK, 0117-933-9262, thomasru@pbworld.com orgasworkshistory@gmail.com. The author is gratefulto fellow members of the Institution of GasEngineers and Managers Panel for the History ofthe Industry and the staff of the National Grid GasArchive for their kind assistance.

Introduction

When William Murdock used coal gas to light hishouse and office in Redruth in 1792, it was the firstpractical demonstration of how coal gas could beused commercially. Different combustible gaseshave been used ever since for commercial,industrial and domestic applications. Gas was firstmanufactured from coal and later from oil until itsreplacement in Britain by natural gas in the mid1970s. The conventional production of gas from coalis well documented; however, there was alsoanother simpler method of gas production which isless well known, called “producer gas”. Althoughproducer gas was manufactured at gasworks, it wasnot generally used to provide a public supply. Itsmain application was supplying a cheap low calorificvalue gas for industrial heating purposes.

Producer gas plants started to become popular inthe early 1880s and were in extensive use by 1910.As producer gas plants developed from the firstplant built by Bischof (Figure 1) until their demise inBritain from competing technologies in the mid-20th

century, many varied types evolved.

The German Bischof undertook the early pioneeringwork on the development of the gas producer.Bischof, from Magdeburg in the Saxony-Anhaltregion of Eastern Germany, constructed the first gasproducer in 1839. This was built simply from bricksas shown in Figure 1. It worked under suction

conditions with air drawn through the producer fromthe top of the fuel bed. Bischof was closely followedby Ebelman in France in 1840. Ebelman’s designwas based on a blast furnace and operated quitedifferently to Bischof’s. Ebelman’s producer was of aslagging type, using a mixture of coke and charcoal

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as fuel which was admixed with lime or furnace slagto produce a fusible ash. The producer wasoperated at a high temperature to ensure the slagwas removed in a molten form.

The next major development was that of FredrickSiemens who developed a combined gas producerand regenerative furnace in 1857. This system wasgradually improved and introduced to the UK throughWilliam Siemens. Producer gas plants provided aconsiderable benefit to those industries requiringhigh and uniform temperatures. This greatly aidedthose industrial processes which were unable orfound it very difficult to use directly fired solid fuelfurnaces. It also saved fuel as the gas could be burntat the exact point required.

A simple drawing of a gas producer using just air orair and steam is shown in Figure 2. A represents thefire bars or grate, B is the air inlet, C is the column offuel, D is a hopper with a close-fitting valve throughwhich the fuel is introduced, and E is the gas outlet.

The next major advance in the application of gasproducers came in 1878, when Dowson developedthe Dowson complete gas plant. This plant could beused both industrially and domestically. Dowsonwent on to demonstrate the effectiveness of gasengines (developed by Otto circa 1876) when in1881 he combined one of his producer gas plantswith a 3 horsepower (HP) Otto gas engine. Theseearly gas engines had a maximum of 20 HP,equivalent to 14.9 kilowatts. But by 1910, gasengines had reached 2,000 HP, equivalent to1,491 kilowatts.

Circa 1900, suction gas plants and engines wereintroduced; these plants were able to make moreeffective use of the lower quality producer gas andbecame a popular system in their own right.

Principles of Producer Gas

Producer gas manufacture differed from traditionalgas production in the way and conditions in whichthe gas was made. A traditional gasworks wouldmanufacture gas by indirectly heating coalcontained within a retort through a separate furnacelocated beneath the retort. The retort was anoxygen-free environment, meaning that as the coalwas heated, it would not combust but instead wouldthermally decompose, releasing gas and other by-products such as tar. This gas has a complexcomposition.

By comparison, and in simplistic terms, a producergas plant would manufacture gas by partiallycombusting coke in an oxygen-limited atmosphere.The gas produced primarily consisted of carbonmonoxide, carbon dioxide and nitrogen.

In slightly more detail, the producer gas plant madegas by forcing or drawing air, with or without theaddition of steam, through an incandescent deepbed of fuel in a closed producer vessel. The fuelwas gradually consumed during the process andthe gas was simply piped to where it was required.

An important characteristic of the producer gasprocess was that no external heat was applied tothe producer: it was heated by the combustion ofthe fuel within the producer itself. The skill ineffectively operating a gas producer was to ensurethat the fuel bed was of sufficient depth and the airsupply was not too great, limiting the amount ofcombustion.

Figure 2. Gas producer working with air or airand steam. Image courtesy of Russell Thomas.

E

D

BA

Air or air and steam

C

Figure 1. Bischof Gas Producer. Air was drawninto the producer (A) through the fire bars (B)and fuel, exiting via the vent (D). Fuel wasloaded via door C. Image courtesy of RussellThomas.

BA

D

C

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Once the fuel inside the producer had started toburn, the air supply was carefully controlled to allowcontinuous combustion in the lower regions of thefuel bed. This provided the high temperaturerequired to produce the necessary reactions higherup the fuel bed and, if steam was added, todecompose the steam.

The producer gas process focussed on theincomplete combustion of carbon to maximise thecarbon monoxide produced and minimise theamount of carbon dioxide (which has no calorificvalue). This was achieved through the reactionsshown below.

Within a conventional fire, the carbon in coal wouldreact with oxygen forming carbon dioxide, anexothermic reaction where each kilogram (kg) ofcarbon would produce 33 megajoules (MJ) ofenergy.

(i) 1 kg C + O2 = CO2 + 33 MJ/kg

This reaction also occurred within the fuel pile at thebase of the producer. Due to the limited oxygensupply, carbon monoxide was also formed in the fuelbed, in the reaction below. This was alsoexothermic, producing 10 megajoules for each kg ofcarbon.

(ii) 1 kg 2C + O2 = 2CO + 10 MJ/kg

As the carbon dioxide formed passed up through thebed of coke, it was reduced by further hot carbonhigher up the fuel bed. This formed carbon monoxidethrough an endothermic reaction where13 megajoules of energy would be consumed foreach kg of carbon:

(iii) 1 kg CO2 + C = 2CO - 13 MJ/kg

This reaction was reversible and the amount ofcarbon dioxide converted to carbon monoxide washighly dependent on temperature. At 850°C, thereaction forming carbon dioxide was found toproceed 166 times more rapidly than the reversereaction.

Where moisture was present in the fuel, or wheresteam was injected into the producer, additionalreactions between the carbon and carboncompounds and water would occur. When steaminteracts with carbon at a high temperature, itdecomposes and the oxygen is transferred to thecarbon, producing hydrogen. The oxygen releasedfrom the reaction of the steam could, depending onthe conditions, combine with carbon to form carbonmonoxide or carbon dioxide. These reactions are thebasis of water gas production, which is the subject ofa separate profile called Water Gas Plant. It is alsodiscussed later in the section on Mond gas.

When coal gas was produced in a retort, complexorganic compounds within coal would thermallydecompose, forming gaseous and vapour phaseorganic compounds within the gas. If soft orbituminous coal was used in the producer, similarby-products would form in the gas (Table 1). In GreatBritain, coke and anthracite were primarily used asthe fuel in a gas producer. These fuels wereprimarily composed of carbon and produced feworganic by-products within the gas (Table 1).

Theoretically, producer gas would consist of 34.2%carbon monoxide and 65.2% nitrogen, but theseconditions would never actually occur. A compositionof 25% carbon monoxide would have been thetarget.

Figure 3. An advert for a suction gas producerplant. Image courtesy of Russell Thomas.

Considering the composition in more detail, producergas was a mixture of carbon monoxide, hydrogen,carbon dioxide and nitrogen, in varying proportions,and a very small quantity of gaseous hydrocarbons(predominantly methane).

The carbon monoxide, hydrogen, gaseoushydrocarbons were combustible (30-45% of the gascomposition), and the calorific value of the gas wasdependent on the relative proportions in which theywere present. The carbon dioxide and nitrogen werediluents which lowered the calorific value andsubsequent flame temperature of the combustiblegases when burnt.

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Photograph 2. Two gas engines installed at the former Wandsworth power house. Image courtesyof the National Grid Gas Archive.

The nitrogen concentration in producer gas wasmuch higher than in coal gas. This was because theproducer was aerated by a restricted supply of air(nitrogen forms 78% of air) and coal gas was anenclosed process and not aerated.

Component ofthe gas

Coke (%composition)

Soft coal (%composition)

Carbon monoxide 25 27Carbon dioxide 5 4Hydrogen 6 10Methane 1 3Nitrogen 63 55Oxygen - 0.5

Table 1. Composition of producer gas from cokeand American soft coal.

Gas from producers can be split into two differenttypes: “hot unpurified gas” and “cooled and purifiedgas”. For most industrial heating purposes, the gaswas used in a hot and unpurified state, allowing theentrained heat in the gas to be used in addition tothe heat generated from burning the gas and any tarwhich may be present in the gas.

This avoided the cost of cooling the gas andminimised the use of regenerators to heat incomingair. There were problems using producer gas in thisway; in particular, any precipitated tar and dust couldblock pipes, allowing only short pipe runs to be used.Using coke would minimise tar deposition andbituminous coal would greatly exacerbate theproblem.

If the item being heated was sensitive, such as kilnsfired for glass or ceramic ware, then the dust and tarcould damage the finished product. In thesesituations, and when used for heating retort/cokeovens or powering gas engines, the gas would bepurified, removing any dust, ammonia and tarry

residues. The gas was cleaned with a scrubber,which is described on page 5.

Producer gas could be obtained from almost anycarbonaceous fuel. The type of fuel used dependednot only on the purpose for which the gas was to beused, but on its cost and the ease with which eachfuel could be purchased locally.

Producer gas was predominantly made fromanthracite or coke, especially where the gas use wassensitive. Where the end use of the gas was notsensitive, bituminous or semi-bituminous coal couldbe used (Photograph 8), and in some circumstancesit was also possible to use brown coal, lignite, peator charcoal. The composition of the gas and by-product was largely influenced by the nature of thefuel used as a feedstock.

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Suction Gas

Early gas producers operated using the suction ofgas through the fuel; this was later disregarded inpreference to pressurised gas injection.Developments in the 1860s gradually led to theconstruction of efficient suction gas plants based onDowson’s design (Figure 4).

Suction gas plants were very effectively employed incombination with gas engines optimised for suctiongas producers. The operation of the system can beexplained by referring to Figure 4, where A was thegrate on which the fuel was placed; B was thecontainer holding the store of fuel, which enteredthrough the hopper and valve at the top; C was acircular chamber filled with broken firebrick; D was acircular pipe which sprayed water into the system; E

was the air inlet and F the gas outlet; G was thechimney; H was the scrubber with a water seal at thebottom; and I was the gas outlet leading to theexpansion box (J) and gas engine (K).

To ignite the fuel in the producer some oily wasteand wood were placed on the grate and the producerwas filled with small pieces of anthracite or coke.The feeding hopper was closed and the fire then lit.The fan (not shown in Fig. 5) was set in motion, andthe exiting gases from the producer were initiallyallowed to escape through the chimney. Oncecombustion was effective, the water supply would beturned on; as soon the gas produced was burningeffectively it was connected to the gas engine. Theengine would be started and the fan stopped. Fromthis time, the engine itself would suck the airrequired into the producer. Before entering theengine, the gases passed upwards through thecoke-filled scrubber, ascending through a column ofcoke continually sprayed by water. The role of thescrubber was to purify the gas, removing fine dust,ammonia and tarry residues in particular. The gasesthen passed along the pipe main and into anexpansion box, which was in direct communicationwith the engine cylinder.

Mond Gas

Mond gas was a variant of producer gas and was inessence a form of complete gasification wherebycoal would be fully converted to ash, rather than tocoke as would happen in a retort. The Mond gasprocess was designed to enable the simultaneousconversion of bituminous small coal (slack) intoflammable gas, largely composed of hydrogen, andat the same time recover ammonium sulphate.

Sir George Bielby and William Young (of oil shalefame) did much of the early work on both thecomplete gasification process and the steaming ofthe char subsequently produced. Despite this,

recognition for the Mond gas process goes to itsnamesake, Dr Ludwig Mond, who commerciallydeveloped the process. Mond realised that by greatlyrestricting the air supply and saturating that air withsteam, the fuel bed could be kept dark red in colour,providing a low working temperature. There weretwo key reasons for the low temperature. Firstly, itwas below the temperature of dissociation forammonia, which prevented its destruction andmaximised the amount of ammonia which could beobtained from the nitrogen entrained in thebituminous coal. Secondly, the low temperatureprevented the formation of clinker which wouldhamper the operation of the process, the ash beingeasily removed from the water seal around the baseof the cone of the producer.

The first Mond gas plant was put into operation atthe Brunner, Mond & Co's Works at Northwich,Cheshire. These plants required a massive capitaloutlay in order for them to be profitable, as only verylarge plants were economically viable. They had touse over 182 tonnes of coal per week for theammonia recovery to be profitable. The efficiency ofthe Mond plant was as high as 80 per cent. In orderto achieve this, however, a large excess of steamwas required so that the small proportion of steamwhich was decomposed (about one third) wassufficient to absorb the heat evolved in the formationof carbon dioxide and carbon monoxide from air andcarbon. For each tonne of coal, two tonnes of steamwould be required for the process. This amount wasreduced to one tonne of steam if ammonia was notbeing recovered by the plant.

Coal would be fed by coal elevators, as can be seenon the left side of the building in Photograph 1, up tohoppers which would feed the small pieces ofbituminous coal down into the Mond producers. TheMond producer operated at about 600oC and wasfed with hot moist air (250oC) from the superheater.

Figure 4. A suction gas plantof the Dowson design. Imagecourtesy of Russell Thomas.

A

BCD

L

E

G

H

I

F

J

KB

H

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Following the mechanical washer, the gas wassubjected to treatment in the acid tower (labelled as4 on Figure 5), which was designed to removeammonia. The gas passed upwards through thetower against a counter-current flow of weaksulphuric acid sprayed down the brick- and tile-filledtower, forming ammonium sulphate. The weaksulphuric acid solution would be recirculated until aconcentration of between 36 and 38% ammoniumsulphate was reached. At this point, the solutionwould be removed and replaced by fresh weaksulphuric acid. The ammonium sulphate solutionwould be removed and evaporated, yielding theammonium sulphate. The acid tower was lead lined(steel would have been corroded by the acid), aslead was resistant to corrosion and had beencommonly used in processes involving acids (e.g.lead chamber process). The acid tower wastherefore a source of potential lead contamination onthese former Mond gas plants.

With the ammonium removed, the gas was thenpassed through the gas cooling tower (labelled as 7on Figure 5), where the upflow of gas was met with adownward spray of cold water, cooling the gas.Following this treatment, the gas could be used forits intended purpose. The water from the gas coolingtower emerged hot, and any suspended tar withinwater was removed in the settling tank (labelled as 8on Figure 5). This hot water was then pumped up tothe top of the air saturation tower where it was usedto heat (to 85°C) the hot moist incoming blast airbeing blown into the Mond producer.

The Mond gas process would produce between19 kg and 40 kg of ammonium sulphate andbetween 3,960m3 and 4,530m3 of gas per tonne ofcoal. The amount of ammonia produced wasdependent on the nitrogen content of the coal, thelatter having a preferred nitrogen content higher than1.5%. The predominant reaction in the Mond gasprocess is between carbon and water forming

carbon dioxide and hydrogen. The water gasprocess which predominates at higher temperaturesforms carbon monoxide and hydrogen. Bothreactions are shown below.

Predominant reaction in Mond gas process:C + 2H2O = CO2 + 2H2

Predominant reaction in water gas process:C + H2O = CO + H2

The gas manufactured was hydrogen rich andcarbon monoxide poor (water gas has a much highercarbon monoxide content). It was of limited use forheating or lighting, but it could be used for someindustrial purposes and power generation. The tarproduced would have been brown in colour andtypical of a low temperature coal tar, being high inparaffinoid components and tar acids. It would havebeen removed and processed elsewhere.

The Mond gas process was further developed bythe Power Gas Corporation as the Lymn system.

This process was found on some larger gasworksand was more popular than the earlier Mond gassystem. It was similar to the Mond gas system butused much weaker sulphuric acid and a differentconfiguration of washers. Lymn washers can oftenbe found recorded on plans of large formergasworks. The gas leaves the Mond producer via apiece of plant referred to as either a superheater ora regenerator (labelled as 2 on Figure 5). Thepurpose of this plant was twofold.

The heat of the gas and steam leaving the produceris transferred to the incoming blast of air and steamfrom the air saturation tower (heated to 250°C). Thereverse of this is that the gas and steam leaving theproducer is cooled by this process equally. From thesuperheater, the gas enters a mechanical washer(labelled as 3 on Figure 5), a rectangular ironchamber where the gas was thoroughly washedwith a fine spray of water generated by rotatingdashers. This further cooled the gas (to 100oC),whilst removing dust or heavy tarry residues.

Gas AirAcid Water

1. Mond producer 8. Settling tank2. Superheater 9. Water pump3. Mechanical washer 10. Air saturation tower4. Acid tower 11. Blower5. Settling tank 12. Settling tank6. Acid pump 13. Water pump7. Gas cooling tower

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2

3 6 5

10

8 11

Air

Acid

Gas

Gas toworks

Water

9 12131

Figure 5. The Mond system of gas production and ammonia recovery. Based on historicalprocess drawings, image courtesy of Russell Thomas.

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Gas Producers, Gasworks and Coking Works

Gas and coking works were major users of gasproducers, not for producing gas to distribute(although it was sometimes used to dilute town gas)but to produce a cheap low-grade carbon monoxidegas for the heating of the retorts.

Early gasworks used horizontal retorts which wereheated directly by a shallow fuel bed of coke lit

beneath the bench of retorts. The direct radiant heatfrom the fuel bed in the furnace and the hot wastegases heated the retort. This approach was not veryefficient and was only able to heat the retort totemperatures of approximately 600°C. As a result,the amount of gas produced was relatively low incomparison with later methods and thedecomposition of the organic compounds in the gasand resulting tar was limited.

The heating of the retorts developed from theseearly directly fired settings, through semi-gaseousfired settings (allowing some secondary combustionof gases), to gaseous producer fired settings, asshown in Figure 6.

The gaseous-fired setting used a gas producer toprovide gas to heat the retorts. This system wasused on all the different retort designs fromhorizontal to vertical. The gas producer did not needto be adjacent to the retorts (as shown in Figure 6),although if it was the heat loss was minimised. Theproducer could be located remotely on the gasworkssupplying multiple benches of retorts. The fuel bedin a producer would be approximately 1.5m to 1.8mdeep and the primary air supply was very carefullycontrolled to enable the correct composition of theproducer gas. The producer gas was channelled toa combustion chamber directly adjacent to theretorts, where it was mixed with a secondary supplyof air and burned. The subsequent hot exhaust gas

Secondary Air

Secondary AirSecondary Air

Primary Air

CombustionChamber

Retorts

ProducerGas

Producer

Figure 6. Cross section of a horizontal retort,showing the gas producer. Based onhistorical drawings, image courtesy ofRussell Thomas.

Photograph 3. The Trefois producer house, built by Drakes at the Partington Gasworks,Manchester, which supplied producer gas to the retorts. The ancillary washers and scrubbers canbe seen outside the building. Image courtesy of the National Grid Gas Archive.

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was routed through flues around the retort, heatingthe coal in the retort.

The gas producer was the most efficient method ofheating retorts. Fuel consumption was improvedfurther in gaseous-fired settings if advantage wastaken of the waste heat in the gas after heating theretorts. If the hot waste gas was used to heatincoming air via a heat exchanger then this wascalled a recuperative or regenerative gaseous-firedsetting. If the hot waste gas just passed out of thechimney directly or via a waste heat boiler then itwas termed a non-recuperative gaseous-firedsetting. These developments helped make the gas-making process more cost effective and much moreefficient.

For large gasworks such as those at Partington andGarston, the producers were housed in externalbuildings (Photograph 3) and the gas was purifiedthrough washers and scrubbers before being pipedto the retorts. Like most other producers, this plantwas generally located above ground; therefore littleevidence is found on former gasworks sites wherethe plant had previously existed.

Later gasworks, for example the one at EastGreenwich in South London, used larger moreadvanced gas producers such as the Marishka typegas producer shown in Figure 7. This type of gasproducer was separate from the gas-making plantwhich at the East Greenwich works included bothretorts and coking works. The producer gas wasused for heating coke ovens as well as retorts. Itwas common practice at coke works to useproducer gas to heat the ovens. As the value ofcoke oven gas has dropped (it cannot easily besold for domestic or industrial use) and the value ofthe coke increased, most coking works use cokeoven gas to heat the coke ovens, rather thanproducer gas.

The more advanced gas producers, such as theMarishka producer, used steam injection into the airblast. The purpose of the steam was to control theendothermic water gas reaction, the temperature ofthe zone of combustion, the degree of fusion of theash, and the temperature of both the grate andexiting producer gas. The formation of water gasraised the calorific value of the gas above that ofproducer gas.

Producer gas production was a highly efficientprocess. It had low capital costs and became one ofthe most widely used industrial gas productionmethods in Britain, as it did not require cooling orgas treatment. As natural gas, liquid petroleum gas

and oil-based town gases became available andcoke became costly and scarce, the popularity ofgas producers diminished; they are now largelyobsolete.

Contaminants Associated with Producer GasPlants

In general terms, producer gas plants were not ascontaminating as traditional coal gas productionmethods which used retorts to produce gas. Thiswas primarily because the feedstock fuel usedwithin a producer was predominantly either coke oranthracite (a high-rank coal with a low concentrationof volatile hydrocarbons). In some circumstances,however, other feedstocks such as coal were used;these would produce much greater concentrationsof oily and tarry components when heated. TheMond gas producer and other later developments,such as the Power Gas Corporation’s Lymn System,did produce tar, typically of a low temperature (500-600ºC). The Mond gas process used an acid-washing process to produce ammonium sulphatewhich required a lead-lined acid tower.

Ash/Coal Dust

Ash was the waste material remaining after theburning of the coal or coke in the producer; itcontained heavy metals (e.g. As, Pb, Cu, Cd, Ni,Zn) though generally only at low concentrations.Ashes were often used for raising ground levelsor for use on cinder paths.

Ammoniacal Liquor and AmmoniumSulphate

Ammonia-rich liquors were formed in thescrubber of a conventional producer by sprayingthe gas with water. In the Mond gas process,ammonia-rich liquors were formed by spraying

Figure 7. Cross section of a Marishka typegas producer. Based on a historicaldrawing, image courtesy of Russell Thomas..

Air

Gas outlet

Steamout

Water

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the gas with a weak sulphuric acid solutionwithin the acid tower. The action of the water orweak acid dissolved the soluble ammonia and ifphenolic compounds were present they wouldalso be dissolved. In conventional producer gasplants, the ammoniacal liquor would consist ofup to 1% ammonium and a much lowerconcentration of phenol. Ferrocyanide andthiocyanate may also be present. Within theMond gas process (and similar subsequentprocesses) the concentration of ammoniumcould reach 38% and then solid ammoniumsulphate would be produced from theconcentrated liquor by evaporation.

High concentrations of ammonium may befound in the ground around scrubbers, washersand settling tanks and the connecting pipes.

Coal Tars

Significant concentrations of coal tars weregenerally not produced by producer gas plants,however those plants designed to be operatedusing bituminous coal (e.g. Mond gas) didproduce coal tars. The exact composition of thecoal tar produced depended on many factors,the most important being the type of gasproducer operated (e.g. conventional or Mondtype) and the type of coal or other fuel used.

In terms of elemental composition, coal tar isapproximately 86% carbon, 6.2% hydrogen,1.8% nitrogen and 1% sulphur, with theremaining 5% composed of oxygen and ash. Interms of the types of organic compoundspresent, a composition of a typical crude coal tarcarbonised in retort is given below.

o Saturates 15%

o Aromatics 37%

o Resins 42%

o Asphaltenes 6%

The exact proportions are likely to be different inproducer gas tars. Producer gas tar wasrecorded by Young in 1922 as being veryviscous and containing large amounts of waterwhich would prove difficult to separate. Ifdistilled, producer gas tar would contain no lightoils, paraffins or high boiling tar acids, but wouldcontain a large percentage of pitch. Thissuggests it was a highly degraded tar, similar tocoke oven tar.

Mond gas tar, which was produced by arelatively low temperature process, wouldproduce a low-temperature tar which would bebrown, oily and contain unsaturatedhydrocarbons (olefins), naphthenes, paraffins,phenols and pyridines; benzene and itshomologues and aromatic compoundsnaphthalene and anthracene would be absent.

The main contaminants of concern within coaltar would be:

o Polycyclic aromatic hydrocarbons (PAH), inparticular carcinogenic PAH such asBenzo(a)pyrene.

o Phenolic compounds (e.g. phenol, cresols,xylenols).

o Benzene, toluene, ethylbenzene andxylenes (BTEX).

o Aromatic and aliphatic petroleumhydrocarbons.

o Ammonia, styrene, carbazole anddibenzofuran.

Lead

Lead was used to line the acid towers of theMond gas plant. Lead may therefore be foundassociated with the site of the former acidtowers on Mond gas plants.

Sulphuric AcidWeak sulphuric acid was used within the acidtowers in the Mond gas process to removeammonia from the gas as ammonium sulphate.

Scenarios Where Producer Gas Plants WereUsed

Gas producers were employed in Britain in manyand varied industrial, commercial and domesticsettings from 1880s to the mid-20th Century. Theyare still used in some other countries.

Gas producers were used in the following settings:

o Gasworks, to heat the retorts and occasionally toproduce gas at times of high demand.

o Coking works, to heat the coke ovens.

o Steel works.

o Ore roasting plants.

o Power stations.

o Factories and mills.

o Railway works.

o Glass works.

o Potteries and kilns.

o Muffle furnaces.

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o Chemical works (e.g. those using the Mondprocess).

o Country estates to power gas engines forelectricity generation and to directly drive plantsuch as saw mills.

o Large schools, hospitals or other publicinstitutions to power gas engines for electricitygeneration and to directly drive plant.

Unlike conventional coal gasworks which are oftenvisible on Ordnance survey maps, producer gasplants are not always clearly marked. They did notalways use large gasholders which would bemarked on maps (labelled gasometer). Often, if theplant was small, it would be housed within a buildingand therefore not visible to the map surveyors. Theymay, however, be marked on site plans.

Case Studies

Small-Scale Gas Producer Plants - Canwell Estate

Canwell was typical of many country estates; itconsisted of a substantial house, containing 43rooms. The estate also included stables, garagesand farms with associated tenanted cottages. Aswith many such estates, lighting would be verydesirable, as would a readily available source ofpower.

The estate was powered by a conventional coalgasworks until 1905, providing light and power tothe whole estate. Power came from two gas enginespowered by the gasworks and was used for bothpumping and powering the farm machinery. Wherethe tenants used gas, they were charged at the costof production.

In 1905, an electric plant was installed to replacethe gasworks. The plant consisted of two 30 HP gas

engines (equivalent to 22.3 kW), each with suction-gas producers and two generators. The generatorspowered an accumulator (battery) capable ofmaintaining all the lights that were required for ninehours (overnight). The plant powered a maximum of720 lights plus two additional 15 HP motors

(equivalent to 11.1 kW) running various pieces ofplant such as a saw mill and laundry. Theconversion to the producer gas system wasapproximately 10 to 15% cheaper than the previousenergy provided by the gasworks. This conversionto gas producers and electric power generation wascommon place circa 1900, when many countryestates ceased coal gas production.

Medium-Scale Gas Producer Plants – ElectricalGenerating Stations and Gasworks

During the gradual switch to electrical powergeneration, some power plant used gas producersto power gas engines which in turn poweredgenerators producing electricity.

Towns such as Chelmsford and Walthamstowswitched to producer gas powered electricitygeneration. The electricity generating station of theUrban District Council of Walthamstow providedelectric power for the electric lighting of the townand also for powering the electric tramway service.In this particular plant, the gas engines were built byWestinghouse and the producer gas plant used wasa Dowson steam-jet type.

Photograph 4. Suction Gas Producers atCanwell. From Country House and ItsEquipment, L. Weaver, Country Life 1912.

These works had an aggregate power of 3,000 HP(equivalent to 2.2 megawatts) in 1905.

As mentioned previously, gasworks were majoruses of producer gas plants. They provided a cheapsource of low calorific value gas which could beused to heat retorts and utilise the ready supply ofsurplus coke generated by the coal gasificationprocess. Photograph 6 shows a gas producer at thethe Garston gasworks located near Liverpool.

Photograph 5. Gas engine powering anelectrical generator at a colliery powerhouse,1914. Image courtesy of the National GridGas Archive.

Photograph 4. Suction Gas Producers atCanwell. From Country House and ItsEquipment, L. Weaver, Country Life 1912.

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This plant operated producers for heating retorts,however it is also known that the producers wereused to dilute the town gas supply at times of peakdemand. Given that producer gas contained highquantities of nitrogen and carbon monoxide, thencare would have had to be used not to dilute the gastoo significantly.

The gas from the producers was cleaned using gasscrubbers, shown on the right of Photograph 6.These towers would be filled with material with ahigh surface area such as coke, ceramic or woodand would be continually sprayed with water toremove dust, any residual tar and ammonium.

Large-Scale Gas Producer Plants – SouthStaffordshire Mond Gas Company

The largest example of a producer gas plant in theUK was that built at Dudley Port, Tipton. This Mondgas plant was built by South Staffordshire MondGas Company circa 1902 after it had obtained theparliamentary powers to distribute producer gas inSouth Staffordshire via a gas distribution network.The plant was designed to house 32 producers,capable of gasifying over 600 tonnes of coal perday. To ensure a supply of gas could be maintained,the plant was designed in duplicate, including theproducers, ammonia recovery, gas washing andcooling apparatus.

Photograph 7. The former South StaffordshireMond Gas Company works. Image courtesy ofthe National Grid Gas Archive.

The gas was distributed from the plant through theuse of compressors at a pressure of 10 PSIequivalent to 68.9 kilopascals. The mains weremanufactured as specialised asphalt-covered steelmains. The works provided gas to industrialcustomers via a specialised high-pressure gasnetwork which covered a large area of SouthStaffordshire, competing against other gascompanies. This was the first example of such ashigh-pressure gas network in the UK.

When the Mond gas plant switched to coke as afeedstock, the resulting gas was of a lower calorificvalue, as volatile and semi-volatile hydrocarbon andorganic compounds were not present in coke. Gasfrom the plant therefore had to be mixed withconventional coal gas from a nearby gasworks toenrich its calorific value to make it suitable for use.

Known Producer Gas Plants

The sites listed below are examples of known sitesor companies in the UK where producer gas plantswere previously installed. This is not an exhaustivelist and many other sites were also known to haveexisted, especially small producer gas plants suchas that described at Canwell. It should also be notedthat most medium- and large-scale gasmanufacturing plants and many coke ovens alsoused gas producers to heat the retorts and cokeovens. These gas producers could be integrated orseparate from the retort house or coke ovens.

Medium to large sized gasworks By-product coking works The Castner-Kellner Alkali Co Ltd, Runcorn Albright & Wilson Ltd, Oldbury Ashmore, Benson, Pease & Co Ltd, Stockton-

on-Tees Gloucester Asylum, Coney Hill The Railway and General Engineering Co Ltd,

Nottingham Birmingham Small Arms Factory, Smallheath The Salt Union Ltd, Liverpool The South Staffordshire Mond Gas Co Brunner, Mond & Co Ltd, Northwich Cadbury Bros Ltd, Birmingham D&W Henderson & Co Ltd, Glasgow The Premier Gas Engine Co Ltd, Nottingham J&E Wright of Millwall The Trafford Power and Light Co Ltd,

Manchester

Photograph 6. Gas producer (left) and Scrubber(right) at the former Garston gasworks, 1947.Image courtesy of the National Grid GasArchive.

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Walthamstow District Isolation Hospital The Farnley Iron Co Ltd, Leeds

Selected Bibliography

Below is a selected bibliography of books whichmay be of interest to the reader:Clegg Jnr S., A Treatise on Gas Works and thePractice of Manufacturing and Distributing CoalGas, 1841 (other later editions), John Weale,London.

Newbigging, T., and Fewtrell, Wm., three volumespublished between 1878-1913 King’s Treatise onthe Science & Practice of the Manufacture &Distribution of Gas, Walter King, London.

Wyer, S.S. A treatise on producer-gas and gas-producers, 1906, New York, McGraw-Hill.Hunt, C., A History of the Introduction of GasLighting, 1907, Walter King, London.Dowson, J.E, Larter, A.T., Producer Gas,Longmans, Green and Co. 1907.Smith, C.A.M, Suction gas plants, 1909, London, C.Griffin.Latta, M.N. American Producer Gas Practice andIndustrial Gas Engineering, 1910, New York, D. VanNostrand company.Meade, A., Modern Gas Works Practice, 1916,1921, 1934, Benn Brothers, London.

Lowry, H.H. - Chemistry of Coal utilisation, Vol. 2,Chapter 37, Water Gas, 1945, John Wiley And SonsInc.

King C. Ed - Kings Manual of Gas Manufacture,1948, Walter King, London.

Terrace, J., Terrace’s Notebook for Gas Engineers& Students, 1948, Ernest Benn, Ltd., London.

Chandler, D. and Lacey, A.D. The rise of the gasindustry in Britain, 1949, British Gas Council.British Petroleum - Gasmaking, 1959 and 1965, TheBritish Petroleum Company Ltd, London.

Disclaimer. The purpose of this document is to act as a pointer tothe activities carried out on former producer gas plants. The authorwill not be responsible for any loss, however arising, from the use of,or reliance on, this information. This document (‘this publication’) isprovided ‘as is’, without warranty of any kind, either expressed orimplied. You should not assume that this publication is error-free orthat it will be suitable for the particular purpose which you have inmind. We assume no responsibility or liability for errors or omissionsin this publication. Readers are advised to use the informationcontained herein purely as a guide and to take appropriateprofessional advice where necessary.

Photograph 8. A producer gas plant with cooling and purifying plantfor gasification of bituminous coal. Image courtesy of the NationalGrid Gas Archive.