Gasworks
A gasworks is an industrial facility designed for the production of manufactured gas, primarily coal gas (also known as town gas), through the destructive distillation of coal in retorts, yielding a flammable mixture used for illumination, heating, and cooking before the widespread adoption of natural gas.[1] These plants typically included retort houses for gas generation, purification systems to remove impurities like sulfur and ammonia, and gasholders—iconic telescoping structures—for storage and pressure regulation, often located near waterways or rail lines for efficient coal delivery.[2] By-products such as coke (a smokeless fuel), coal tar (used in road surfacing and chemicals), and ammoniacal liquor (for fertilizers) were also significant outputs, contributing to related industries.[1] The origins of gasworks trace back to late 18th-century experiments in Britain, where Scottish engineer William Murdoch demonstrated coal gas lighting in 1792 by illuminating his home in Redruth, Cornwall.[2] Commercial production began in 1812 with the establishment of the Gas Light and Coke Company in London, marking the first public gas supply and sparking rapid expansion across the United Kingdom and beyond during the Industrial Revolution.[3] By the 1820s, gasworks had proliferated in major cities, enabling street lighting and domestic use, with over 1,200 such facilities operating in Britain by the early 20th century; the industry was nationalized in 1949 but began declining after the discovery of North Sea natural gas in 1965, leading to the conversion of supplies and closure of most plants by 1977, with the last in Britain shutting in 1981.[1] Innovations like vertical retorts in the 1880s and oil-gas processes in the 20th century improved efficiency, but environmental concerns over waste products, regulated since the Gasworks Clauses Act of 1847, prompted stricter controls on emissions and effluents.[3] Today, many former gasworks sites serve as cultural or recreational landmarks, such as parks or museums, highlighting their role in urban development and industrial heritage, though remediation of contaminated soils from tar and other residues remains a key challenge in redevelopment.[1] Their architectural legacy, including ornate gasholders, underscores the era's engineering prowess and the transformation of energy infrastructure.[3]History
Origins and early development
The early experiments with coal gas production began in the late 17th century, when Reverend John Clayton of Wigan, England, demonstrated the generation of an inflammable gas through the destructive distillation of coal in 1684, referring to it as the "spirit of the coal."[4] This discovery remained largely theoretical for over a century, as the focus shifted to more practical applications in the late 18th century. Scottish engineer and inventor William Murdoch advanced the technology significantly in 1792 by distilling coal to produce gas and using it to illuminate his home and office in Redruth, Cornwall.[5] Murdoch further demonstrated its viability in 1802 by lighting the exterior of the Boulton & Watt factory in Birmingham, marking one of the first industrial uses of coal gas for illumination.[5] The transition to commercial production gained momentum in the early 19th century, spurred by French inventor Philippe Lebon, who patented a method for distilling combustible gas from wood and coal in 1799, envisioning its use in lamps known as thermolampes.[6] In England, German promoter Frederick Winsor showcased the potential for public application with a demonstration in 1807, lighting the street lamps along Pall Mall in London using coal gas, which drew crowds but highlighted the technology's novelty.[7] This paved the way for the establishment of the world's first commercial gasworks by the Gas Light and Coke Company, chartered in 1812 and operational at Great Peter Street in Westminster, London, where it began supplying gas primarily for street and building lighting.[8] Initial commercialization faced significant hurdles, including the high capital costs of building distillation facilities and laying pipe networks, which limited early installations to affluent urban areas.[9] Safety concerns were paramount, as coal gas was highly flammable and prone to leaks, leading to public denunciations of it as unsafe despite its brighter light compared to oil lamps; distillation processes also posed risks of fire and toxic exposure to workers.[6] Consequently, usage remained confined to lighting factories, theaters, and select streets, with broader heating applications emerging only later. The technology's spread accelerated in the 1820s, reaching major European cities like Paris in 1815 and Berlin in 1826.[10] It extended to British colonies in the 1840s, including Sydney in 1841 and Bombay around 1844, through the establishment of local gasworks modeled on London's success.[11][12] In the United States, the first gasworks opened in Baltimore in 1816, initiated by Rembrandt Peale's Gas Light Company, which lit the city's streets by 1817.[13]Expansion and technological evolution
The gas industry in Britain underwent rapid expansion during the 19th century, spurred by the growing demand for street and public building lighting in rapidly urbanizing areas. By 1830, there were approximately 200 gas companies operating across the country, a figure that surged to around 800 by 1850, with 13 concentrated in London alone.[14] This proliferation was supported by legislative frameworks that facilitated the establishment of municipal gas companies, notably the Gas Works Clauses Act 1847, which consolidated provisions for authorizing gasworks construction, land acquisition, and operational standards to ensure reliable supply. By 1860, the number approached 1,000 companies, reflecting the sector's maturation into a vital infrastructure element for industrial and civic needs.[14] Technological advancements further drove efficiency and scalability during this period. Horizontal retorts, the predominant early design since the 1810s, operated in batch processes but allowed for larger coal charges and more uniform heating than initial experimental setups.[15] These intermittent operations gradually evolved toward continuous methods, with vertical retorts introduced in larger facilities by the late 19th century to enable non-stop production. Additionally, in the 1870s, water gas processes emerged as a viable alternative to coal carbonization, involving the reaction of steam with hot coke to produce a higher-calorific gas more suited for peaking demands, with practical implementation around 1873 marking a key shift in production flexibility.[1] The expansion extended internationally, adapting British innovations to local contexts. In the United States, manufactured gas plants grew from about 30 municipal operations in 1850 to over 200 by 1880, fueled by urban development in cities like Baltimore and New York.[16] In Europe, Germany's Ruhr Valley became a hub for large-scale gasworks integrated with the coal and steel industries, supporting industrial lighting and processes from the mid-19th century onward.[17] Colonial networks also proliferated, exemplified by the Oriental Gas Company in Calcutta, India, established in 1857 to supply coal gas for street lighting and households, one of Asia's earliest such ventures.[18] The sector reached its peak output in the early 20th century amid wartime pressures, despite emerging competition from electrification. During World War I, gas production ramped up to meet heightened industrial and civilian heating needs, with similar surges in World War II as lighting restrictions and fuel shortages underscored gas's reliability. However, from the 1880s, electric lighting began challenging gas for illumination, prompting gasworks to pivot toward heating and cooking applications to sustain demand.[19]Design and Components
Retort house
The retort house served as the primary production facility in a gasworks, where coal underwent destructive distillation in sealed vessels to generate coal gas. These structures were typically multi-story brick buildings designed to accommodate benches of iron retorts, often numbering 10 to 50 per house in smaller to medium-sized operations, arranged in horizontal, inclined, or vertical orientations and heated externally by adjacent coke-fired furnaces. Horizontal retorts, the earliest widespread design pioneered by William Murdoch around 1802, consisted of elongated cast-iron cylinders suspended over open fires, while later innovations in the late 19th and early 20th centuries introduced inclined and vertical configurations for improved efficiency and continuous operation.[1][20] Retorts were constructed from durable cast iron to contain the high-pressure process, with interiors lined in firebrick for thermal resistance against temperatures reaching 1,000–1,200°C. Early designs relied on clay-based linings, but by the early 20th century, advancements incorporated more refractory materials such as silica bricks to enhance durability and reduce maintenance amid prolonged exposure to intense heat. The buildings themselves featured robust masonry construction with iron frameworks to support the weight of multiple retort benches and associated machinery, including ascension pipes that directed evolving gas upward.[1][21] In operation, coal was charged into each retort—typically 10–15 hundredweight (approximately 500–760 kg) for horizontal units—sealed, and heated in cycles lasting 8–12 hours to carbonize the fuel without oxygen, producing gas that was drawn off through hydraulic mains. This process yielded around 4,000–5,000 cubic feet (113–142 cubic meters) of gas per charge, depending on coal quality and retort size, with the residual coke discharged for reuse as fuel. Vertical retorts allowed semi-continuous feeding of smaller charges (around 50–100 kg), enabling steadier output without full shutdowns between cycles.[21][22] Safety considerations were paramount due to the extreme conditions, with retort houses equipped with robust ventilation systems, including louvered roofs and exhaust flues, to dissipate heat, smoke, and potential explosive vapors from the furnaces and ascension pipes. Workers, known as stokers or chargers, operated in shifts to manage the intense environment, often using mechanical aids by the early 20th century to load and unload retorts. A prominent example of scale was the retort houses at Beckton Gas Works in East London, established in 1870 as the world's largest facility, spanning over 300 acres with multiple expansive houses that processed millions of tons of coal annually to supply the capital.[21][23] The hot gas exiting the retorts was subsequently cooled in adjacent condensers.Condenser and exhauster
In gasworks, condensers served to cool the hot raw gas emerging from the retorts, causing the precipitation of tar and water vapors as the primary step in gas processing. Early designs included vertical air-cooled condensers, which featured dedicated outlets for gas and tar separation, while more advanced annular condensers—composed of two concentric cylinders with cooling on both internal and external surfaces—provided improved control over cooling rates to optimize tar deposition without excessive condensation. These systems typically employed water sprays or atmospheric cooling to lower the gas temperature rapidly, removing the bulk of condensable matter before further treatment, though small amounts of residual tar remained for subsequent removal.[1] Exhausters were essential mechanical components that created a partial vacuum to draw gas from the retorts at a controlled rate, preventing back pressure that could disrupt the carbonization process in the retorts. The earliest type, the hydraulic main exhauster developed around 1839, operated on principles similar to a water-sealed piston, but was soon superseded by reciprocating exhausters, which resembled miniature gasholders with a vessel suspended in water that rose and fell as gas filled and emptied it. By the early 1900s, rotary exhausters, such as vane-type or double-gear models driven by horizontal steam engines, became standard, offering smoother operation and integration with non-return valves to avoid gas backflow into the retorts. These later designs, often steam- or electrically powered, maintained low pressures to pull gas through the condensers efficiently.[24][1] The evolution from hydraulic to rotary exhausters marked significant efficiency gains, enabling larger-scale operations in urban gasworks by sustaining consistent gas flow and minimizing tar entrainment beyond the condensers. Reciprocating and rotary types reduced operational disruptions, supporting higher throughput in facilities processing thousands of tons of coal annually, while condensers alone captured most tar, limiting carryover to levels manageable by downstream equipment. This integration enhanced overall system reliability, with rotary blowers facilitating up to continuous operation without frequent maintenance interruptions in mid-20th-century plants.[1][25]Scrubber and purifier
In gasworks, the scrubber served as a critical initial stage in gas purification, employing wet lime or water towers to dissolve soluble impurities such as ammonia and hydrogen sulfide from the raw coal gas emerging from the condenser. These towers, often tall cylindrical structures packed with wooden slats or baffles, allowed the gas to pass upward through a counterflow of water or lime slurry, which absorbed up to 90% of the ammonia and a portion of the H₂S, while also aiding in the separation of residual tar droplets through impingement and coalescence. Designs such as Livesey washers, featuring mushroom-shaped perforated plates, enhanced tar removal by generating fine gas bubbles in the scrubbing liquid, capturing entrained tar mists with efficiencies approaching 95% for fine particles. The resulting ammoniacal liquor was collected at the base and processed further for ammonia recovery, typically as ammonium sulfate via reaction with sulfuric acid, which was then sold as a fertilizer.[26][27] Following scrubbing, the gas entered the purifier boxes, where iron oxide beds—composed of bog iron ore containing 30-35% Fe₂O₃—absorbed remaining hydrogen sulfide, achieving removal rates of approximately 95% to meet safety standards for domestic use. These purifiers consisted of rectangular cast-iron or concrete chambers, about 4-5 feet deep, filled with layered iron oxide supported on hardwood grids; the gas passed through multiple such boxes in series for thorough cleansing. The primary chemical reaction involved the oxidation of H₂S by ferric oxide: $2 \mathrm{Fe_2O_3 + 6 H_2S \to 2 Fe_2S_3 + 6 H_2O} This process also addressed trace hydrogen cyanide, forming inert Prussian blue compounds. Spent oxide, saturated after accumulating about 50% sulfur by weight, underwent daily regeneration by exposure to air in open yards, reversing the reaction via: 2 \mathrm{Fe_2S_3 + 3 O_2 \to 2 \mathrm{Fe_2O_3 + 6 S} allowing reuse for 4-5 cycles before disposal as spent oxide.[28][27][26] Purification methods evolved from wet processes, like early lime slurries patented by Samuel Clegg in 1812, to predominantly dry systems by the mid-19th century, which proved more efficient and reduced wastewater issues associated with "blue billy" (calcium sulfide sludge). Dry purification using hydrated lime or iron oxide avoided excessive moisture, minimizing side reactions like the reversal of sulfide formation with CO₂: \mathrm{CaS + CO_2 + H_2O \to CaCO_3 + H_2S} In the 1920s, activated carbon beds were introduced as a supplementary variation for odor control, adsorbing volatile organic compounds and residual naphthalene to improve gas quality without altering the primary H₂S or ammonia removal. This innovation, driven by advancing petrochemical needs, allowed for finer purification while maintaining the core dry oxide process in most facilities.[27][29]Benzole plant
The benzole plant was a key component of larger gasworks, designed to extract valuable aromatic hydrocarbons known collectively as benzole from the coal gas stream after initial purification. This facility targeted benzene and toluene vapors present in the gas, which were absorbed using heavy petroleum oils in a series of vertical scrubber tanks where the gas was bubbled upward through the descending oil. The process prevented these hydrocarbons from remaining in the gas supply, where they could cause issues like excessive luminosity or deposits in pipes, while recovering them as marketable by-products.[1][15] Following absorption, the benzole-rich oil was transferred to distillation units for recovery. The oil was heated in strippers to release the volatiles, and the resulting crude benzole underwent fractional distillation in rectifying columns at temperatures ranging from 80°C for benzene to 110°C for toluene, yielding a purified fraction with boiling points of 80–150°C. Essential equipment included oil washers for absorption, steam-heated stills for stripping, and fractionating columns for separation, often arranged in an open-air layout resembling a small refinery. Yields varied by coal type but averaged about 0.18 gallons of benzole per 1,000 cubic feet of gas produced, equivalent to roughly 2 gallons per ton of coal carbonized.[30][15][31] Benzole plants emerged in the 1860s and 1870s amid growing demand for aromatics in the emerging synthetic dye industry, following William Perkin's 1856 discovery of mauveine from coal tar derivatives, which spurred benzene extraction for aniline production. By the early 20th century, benzole served as a solvent and petrol additive, with the National Benzole Company formed in 1919 to blend it into motor fuels. Production peaked during World War II, when toluene from benzole was nitrated to produce trinitrotoluene (TNT) for explosives, and benzene was converted to nitrobenzene for further munitions applications; the resulting benzole typically met purity standards of at least 40% benzene, with 70–80% total aromatics including toluene and xylenes.[1][30][32][33]Gasholder
The gasholder, also known as a gasometer, served as the primary storage mechanism for purified coal gas at gasworks, enabling the balancing of production rates with fluctuating consumer demand by holding reserves for peak usage periods.[34] These structures received gas after purification from upstream components like scrubbers and purifiers, storing it temporarily before distribution. The design typically featured an inverted bell-shaped vessel that floated within a large water-filled tank, creating a gas-tight seal via the water interface while allowing vertical movement as gas volume changed.[34] Capacities varied by site and era, ranging from smaller units of around 100,000 cubic feet for early installations to larger ones exceeding 1 million cubic feet to accommodate growing urban needs. For stability during ascent and descent, especially in telescoping models with multiple interlocking lifts, the bells were equipped with wheels that rolled along spiral-guided rails or columns integrated into the tank's framework, preventing lateral sway and ensuring smooth operation.[34] In operation, gasholders underwent daily cycles of filling during off-peak production hours and emptying to meet evening demand spikes, with gas pressure maintained at low levels of approximately 10-25 mbar to minimize structural stress and facilitate safe handling.[35] Flow into and out of the holder was regulated by governor valves, which automatically adjusted based on pressure differentials to prevent overfilling or depletion, thus stabilizing supply across the network.[36] The water seal not only contained the gas but also acted as a safety barrier, suppressing potential leaks or ignition sources by submerging the bell's lower edges.[34] The evolution of gasholders began in the early 19th century, with the first practical designs appearing in the 1810s alongside the initial coal gasworks, such as the 1813 installation at Westminster in London, constructed from wrought iron for the floating bells and basic cast-iron supports.[34] By the mid-1800s, telescoping variants with multiple lifts improved efficiency, but stability challenges led to the adoption of more robust guide frames.[37] The late 19th and early 20th centuries marked a shift to steel construction for greater durability and larger scales, with spiral-guided systems patented around 1890 to eliminate bulky external columns and enhance wind resistance.[34] Safety enhancements, including reinforced water seals and anti-freeze heating for the tanks, became standard by the 1900s to mitigate risks in varying climates.[38] Iconic examples include the Vienna gasometers, built in the 1890s as part of the city's municipal gasworks in Simmering, where four massive telescoping holders—each with a capacity of 90,000 cubic meters—demonstrated advanced engineering for the era's industrial demands.[39]Gas Production Process
Overview of coal gasification
Coal gasification refers to the thermochemical conversion of coal into combustible gases through processes such as carbonization, where coal undergoes thermal decomposition in an oxygen-limited environment at temperatures between 900 and 1,200°C. This reaction breaks down the coal's organic structure, releasing volatile matter as coal gas while leaving behind solid coke. The primary output, coal gas or town gas, consists mainly of hydrogen (H₂, approximately 50%), methane (CH₄, approximately 35%), and carbon monoxide (CO, approximately 10%), along with minor components like carbon dioxide (CO₂), nitrogen (N₂), and hydrocarbons.[40][41] In typical historical gasworks operations, one ton of bituminous coal served as input, yielding around 10,000 cubic feet of coal gas with a calorific value of approximately 500 BTU per cubic foot, 700 kg of coke, and about 40 gallons of tar. These outputs varied based on coal type and process conditions, but they established the scale for urban gas supply, with the gas providing a heating value suitable for lighting and heating applications. Energy balances highlighted the process's recovery of 60-70% of the coal's input energy in usable forms, primarily as gas and coke, though significant losses occurred as heat and unrecovered volatiles.[42][43][44] The high-level process flow encompassed coal charging into heated vessels for pyrolysis, where decomposition generated raw gas; subsequent quenching cooled the products and halted reactions; purification removed tars, sulfur compounds, and ammonia; and storage in holders prepared the gas for distribution. Overall efficiency reached 60-70% through optimized heat recovery, though variants influenced outcomes—coal gas from dry carbonization emphasized methane-rich output for higher calorific value, water gas from steam-coal reaction produced hydrogen- and CO-dominant syngas for industrial use, and producer gas from air-blown partial combustion yielded lower-energy gas (around 150 BTU/ft³) suited for on-site power generation.[45][46]Carbonization and initial processing
The carbonization of coal in gasworks retorts primarily involves pyrolysis, a thermal decomposition process conducted in the absence of oxygen to yield coal gas, coke, and by-products such as tar and liquor. This stage occurs within sealed retorts heated externally to temperatures typically ranging from 900°C to 1100°C, allowing the coal to undergo progressive breakdown as heat penetrates the charge. Bituminous coals, particularly those with high volatile content like Newcastle gas coal, were preferred due to their superior gas yield, often producing up to 10,000 cubic feet of gas per ton of coal carbonized.[47][48] Pyrolysis proceeds through distinct phases driven by increasing temperature within the coal mass. The initial devolatilization phase, occurring between 200°C and 500°C, involves the release of volatiles including moisture, carbon dioxide, light hydrocarbons, and tar precursors as the coal softens and intumesces, expelling approximately 20-30% of its mass in gaseous and condensable forms. This is followed by the cracking phase from 500°C to 900°C, where heavier volatiles undergo secondary thermal decomposition, breaking down into permanent gases such as hydrogen, methane, and carbon monoxide through cracking reactions that enhance the gas volume and calorific value. Finally, the charring phase solidifies the remaining structure into coke, a porous carbonaceous residue comprising about 50-60% of the original coal mass, as fixed carbon dominates and volatiles are fully expelled.[48][49][50] Following pyrolysis, the hot coke is quenched to halt further reactions and recover additional value. In many gasworks operations, steam was injected into the retort or over the discharged red-hot coke, cooling the solid while promoting the water-gas shift reaction:\ce{C + H2O -> CO + H2}
This endothermic process enriched the raw gas with hydrogen, increasing its heating value by up to 20% and yielding a mixture known as blue water gas, which was often blended with the primary coal gas stream.[47][51][52] The raw gas exiting the retorts, laden with condensables, enters the hydraulic mains—sloping pipes partially filled with liquor—for initial separation of liquids. Here, tar and ammoniacal liquor condense and settle, comprising approximately 15% of the coal input by weight, with tar yielding about 10 gallons per ton of coal and liquor around 30 gallons of dilute ammoniacal solution. This crude separation prevented downstream fouling while collecting valuable by-products for further processing.[47][53] Operationally, carbonization in horizontal or inclined retorts was batch-based, with cycles lasting 6-10 hours per charge to allow complete pyrolysis without excessive fuel consumption for heating. Each cycle included charging the retort with crushed coal (typically 10-15 hundredweight), heating via producer gas or coke combustion, gas evolution, discharging the coke, and preparation for the next batch, enabling continuous production across multiple retorts in a house.[47][54]