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Coking
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Coking is the process of heating coal in the absence of oxygen to a temperature above 600 °C (1,112 °F) to drive off the volatile components of the raw coal, leaving behind a hard, strong, porous material with a high carbon content called coke. Coke is predominantly carbon. Its porous structure provides a high surface area, allowing it to burn more rapidly, much like how a bundle of tinder burns faster than a solid wooden log. As such, when a kilogram of coke is burned, it releases more heat than a kilogram of the original coal.
Application to smelting iron
[edit]Coke is used as fuel in a blast furnace. In a continuous process, coke, iron ore, and limestone are mixed together and placed in the top of the blast furnace, and at the bottom liquid iron and waste slag are removed. The raw materials continuously move down the blast furnace. During this continuous process more raw materials are placed at the top, and as the coke moves down, it must withstand the ever-increasing weight of the materials above it. It is the ability to withstand this crushing force, in addition to its high energy content and rapid combustion, that makes coke ideal for use in blast furnaces.
Petroleum coking
[edit]"Coking is a refinery unit operation that upgrades material called bottoms from the atmospheric or vacuum distillation column into higher-value products and produces petroleum coke—a coal-like material".[1] In heterogeneous catalysis, the process is undesirable because the clinker blocks the catalytic sites. Coking is characteristic of high temperature reactions involving hydrocarbon feedstocks. Typically coking is reversed by combustion, provided that the catalyst will tolerate such.[2][better source needed]
A simplified equation for coking is shown in the case of ethylene:
- 3 C2H4 → 2 C ("coke") + 2 C2H6
A more realistic but complex view involves the alkylation of an aromatic ring of a coke nucleus. Acidic catalysts are thus especially prone to coking because they are effective at generating carbocations (i.e., alkylating agents).[3]
Coking is one of several mechanisms for the deactivation of a heterogeneous catalyst. Other mechanisms include sintering, poisoning, and solid-state transformation of the catalyst.[4][5]
See also
[edit]References
[edit]- ^ "Coking is a Refinery Process that Produces 19% of Finished Petroleum Product Exports". U.S. Energy Information Administration.
- ^ H. Schultz (2010). ""Coking" of zeolites during methanol conversion: Basic reactions of the MTO-, MTP- and MTG processes". Catalysis Today. 154 (3–4): 183–194. doi:10.1016/j.cattod.2010.05.012.
- ^ Guisnet, M.; Magnoux, P. (2001). "Organic chemistry of coke formation". Applied Catalysis A: General. 212 (1–2): 83–96. Bibcode:2001AppCA.212...83G. doi:10.1016/S0926-860X(00)00845-0.
- ^ Forzatti, P.; Lietti, L. (1999). "Catalyst Deactivation". Catalysis Today. 52 (2–3): 165–181. doi:10.1016/S0920-5861(99)00074-7.
- ^ Bartholomew, Calvin H (2001). "Mechanisms of Catalyst Deactivation". Applied Catalysis A: General. 212 (1–2): 17–60. Bibcode:2001AppCA.212...17B. doi:10.1016/S0926-860X(00)00843-7.
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Coking
View on Grokipediafrom Grokipedia
Coking is the thermal process of heating bituminous coal, known as coking coal, in the absence of oxygen at temperatures around 1000–1100°C to remove volatile matter through pyrolysis, yielding coke—a porous, carbon-rich solid used primarily as a fuel and reducing agent in blast furnaces for iron smelting and steel production.[1][2] This destructive distillation transforms the coal's organic components into a stable, high-strength material essential for modern metallurgy, with global production exceeding hundreds of millions of tons annually to support the steel industry's demand.[3] Historically, the adoption of coke in 1709 by Abraham Darby revolutionized ironmaking by replacing scarce charcoal, enabling the scale-up of production during the Industrial Revolution and laying the foundation for widespread steel manufacturing.[4][5] Despite its indispensability, coking generates significant byproducts like coal tar and gases, which have raised environmental concerns over emissions and waste management in production facilities.[6]
These factors derive from stack testing and material balance methods across operational coke plants, highlighting quenching and pushing as dominant PM sources and charging as key for VOCs and CO.[86] Coke oven emissions, particularly the PAH-rich PM fraction, are classified as carcinogenic by agencies including the National Toxicology Program, based on elevated lung cancer risks in exposed workers.[85][87]
Definition and Fundamentals
Coal Coking Process
The coal coking process entails the high-temperature pyrolysis of bituminous coal in oxygen-deficient environments to produce metallurgical coke, a carbon-rich, porous solid fuel with minimal volatiles, primarily utilized as a reducing agent and heat source in blast furnaces for iron smelting.[1] This destructive distillation drives off 20–30% of the coal's mass as volatile matter, yielding approximately 70–80% coke by weight, alongside coal gas, tar, and aqueous liquor.[6] Coking coals must possess inherent caking properties, typically medium- to high-volatile bituminous types with free-swelling indices of 3–9 and low ash (under 10%) and sulfur (under 1%) content to ensure the formation of strong, coherent coke with adequate porosity for gas permeability in furnaces.[7] Coal preparation begins with blending multiple seams to achieve consistent quality, followed by crushing to a top size of 25–50 mm and screening to remove fines, with moisture adjusted to 4–8% for optimal charging.[6] The prepared blend, weighing 20–40 metric tons per oven, is charged into slot-type byproduct recovery ovens—narrow, silica-refractory chambers (about 12–15 m long, 0.4–0.5 m wide, 4–6 m high) arranged in batteries of 50–100 units—via mechanical larry cars through top charging holes.[8] Doors are sealed, and levelers distribute the charge evenly to promote uniform heating.[9] Carbonization initiates through indirect heating from oven walls and flues, where producer gas or coke oven gas combustion sustains wall temperatures of 1100–1200 °C.[8] Devolatilization commences at 350–500 °C, softening the coal into a thermoplastic phase via depolymerization of vitrinite macerals, releasing primary volatiles (e.g., CO, H₂, CH₄, tars) that peak between 500–900 °C; secondary reactions then crack heavier hydrocarbons and resolidify the residue into a mosaic of carbon domains.[10] The process duration spans 14–24 hours, with heat transfer progressing from walls to the oven center at rates of 10–20 mm/h, ensuring complete carbonization without oxygen ingress to prevent combustion.[11] Volatiles exit via ascension pipes to collectors, where primary cooling condenses tars and phenols, followed by secondary cooling and electrostatic precipitation for fine tar recovery.[12] Raw coke oven gas, comprising 50–60% hydrogen and methane, undergoes desulfurization (e.g., via ammonia liquor scrubbing), ammonia stripping, and tar/light oil separation to recover chemicals like ammonium sulfate (up to 15 kg/ton coke), benzene-toluene-xylene (20–50 L/ton), and naphthalene.[13] The cleaned gas, providing 70–80% of plant energy needs, fuels oven heating.[9] Coking completion is signaled by thermal profiles or gas evolution cessation; the doors are opened, and a hydraulic pusher rams the glowing coke (at ~1000 °C) onto a quench car for water immersion quenching, reducing temperature to under 200 °C and arresting oxidation while generating steam emissions.[14] The quenched coke is screened, sorted for size (lump >40 mm for furnaces), and stockpiled, yielding products with >85% fixed carbon, porosities of 40–50%, and reactivities tuned for low CRI (coke reactivity index <25%) via precise blend control.[8] Emissions from charging, coking, and pushing include particulates, VOCs, and PAHs, mitigated by engineering controls like enclosed pushes and gas cleaning.[6]Petroleum Coking Process
Petroleum coking is a thermal cracking process applied to heavy residual fractions from crude oil distillation, such as vacuum residuum, to convert them into lighter hydrocarbon distillates and solid petroleum coke by rejecting excess carbon.[15] The process operates at temperatures of 450–550°C and near-atmospheric pressures, promoting endothermic pyrolysis reactions that break down large hydrocarbon molecules into gases, naphtha, gas oils, and coke.[16] It enables refineries to upgrade low-value bottoms streams that would otherwise be limited to fuel oil markets, increasing overall distillate yields by up to 70-80% from the feed while producing coke as a byproduct for fuel or metallurgical use.[17] The predominant method, delayed coking, functions as a semi-batch operation using pairs of large insulated coke drums, typically 6-9 meters in diameter and 30-40 meters tall, to allow continuous feed processing.[18] The heavy feed, often comprising 5-20% asphaltenes and high Conradson carbon residue (10-25%), is preheated and then rapidly heated in a fired tubular furnace to 480-510°C, initiating cracking without full coking in the furnace to avoid fouling.[19] The hot effluent, a mix of vapors and liquid, is routed to one of the coke drums where residence time of 12-24 hours at 400-450°C completes the thermal decomposition: lighter vapors are generated and sent to fractionation towers for separation into coker naphtha (boiling <220°C), coker gas oil (220-540°C), and off-gases, while heavier components polymerize and dehydrogenate into solid coke deposits filling 70-80% of the drum volume.[17] [20] Once the on-stream drum fills with coke, the feed switches to the idle drum, and the filled unit undergoes quenching with steam and water to cool and fracture the coke, followed by mechanical or hydraulic decoking using high-pressure water jets to cut out the solid mass, which is then screened and stored.[18] Cycle times per drum are typically 24-48 hours, with coke yields ranging from 20-35% of feed depending on residuum quality—heavier feeds like those from Venezuelan or Canadian heavy crudes produce more needle coke suitable for electrodes, while lighter residues yield fuel-grade coke with higher sulfur (2-6%) and metals content.[21] The distillates require hydrotreating due to high olefin and sulfur content from the cracking severity.[22] Alternative variants include fluid coking, a continuous process using fluidized bed reactors at 500-560°C with recycled coke particles as heat carrier, achieving lower coke yields (15-25%) and higher distillate conversion through greater severity but producing more olefinic products.[23] Flexicoking extends fluid coking by integrating a gasification step to convert excess coke into syngas, reducing solid waste and enabling hydrogen production, though it requires higher capital investment and is less common.[24] Delayed coking dominates globally, accounting for over 90% of coking capacity as of 2023, due to its lower cost and reliability for processing diverse heavy feeds.[25]Historical Development
Early Coal Coking and Metallurgical Applications
The production of coke from coal for metallurgical use originated in England amid efforts to overcome the fuel shortages plaguing charcoal-dependent iron smelting, which consumed vast woodlands and drove up costs by the late 17th century. Bituminous coal was heated in enclosed structures or pits without air to expel volatile gases and tars, yielding a porous, high-carbon residue suitable as a reducing agent. Early patents for such coking appeared as early as 1627, with sporadic trials yielding inconsistent results due to sulfur impurities contaminating the iron.[26] These impurities caused brittle outputs, limiting viability until process refinements addressed coal variability and oven design.[4] Abraham Darby I achieved the first sustained success in 1709 at his Coalbrookdale furnace in Shropshire, where he smelted iron ore using coke derived from local bituminous coal. Drawing from his prior experience in brass smelting and casting, Darby optimized coking to minimize sulfur, enabling coke to sustain the high temperatures (around 1,200–1,500°C) needed for blast furnace operation. This replaced charcoal's inconsistent heat and volume limitations, producing cast iron pigs that could be molded into goods like pots, with initial outputs demonstrating commercial feasibility through lower fuel needs—coke required roughly one-third the weight of charcoal for equivalent reduction.[27][28] Metallurgically, coke's uniform structure and energy density revolutionized iron production by enhancing carbon transfer to the ore, yielding pig iron with 3–4.5% carbon content ideal for casting rather than the wrought iron favored under charcoal regimes. This shift reduced smelting costs by up to 50% in early adopters and scaled output from tens to hundreds of tons annually per furnace, as coke's abundance from coalfields bypassed timber deforestation constraints. By 1750, coke-fired furnaces outnumbered charcoal ones in Britain, underpinning expansions in machinery and infrastructure, though initial resistance stemmed from perceived quality differences in forge conversion to wrought iron.[29][30]Industrial Expansion in the 19th and 20th Centuries
The expansion of coal coking in the 19th century was propelled by the growing iron and steel industries, which required a reliable, high-carbon fuel to replace charcoal and inefficient bituminous coal in blast furnaces. In the United States, the Connellsville district of southwestern Pennsylvania emerged as the epicenter, leveraging high-quality bituminous coal from the Pittsburgh seam to produce metallurgical coke via beehive ovens, which heated coal in low-oxygen conditions to drive off volatiles. By 1880, U.S. coke output reached 3.3 million short tons, with Connellsville contributing 2.2 million short tons from over 4,200 ovens.[31][32] This growth reflected broader industrialization, as railroads and capital investment enabled scaling; Pennsylvania's coke ovens supplied distant steel mills, with production in the Connellsville area climbing to 6.5 million short tons by 1890.[32] Technological refinements, such as brick beehive ovens introduced in the late 19th century, improved process control and yield compared to earlier mound methods, facilitating further proliferation.[4] By 1900, national coke consumption surpassed 20 million tons, underscoring coke's causal role in enabling efficient smelting and the Bessemer converter's adoption for mass steel production.[31] Regions like western Pennsylvania and northern West Virginia expanded rapidly, with coke output integral to the U.S. becoming a steel powerhouse; for instance, the Rochester and Pittsburgh Coal and Iron Company began oven operations in 1884 using Lower Freeport coal.[32][33] Into the 20th century, coking capacity intensified with the transition to byproduct recovery ovens, which captured gases like ammonia and coal tar for chemical uses, enhancing economic viability over waste-emitting beehive designs. The Semet-Solvay process, implemented in a 1896 Pennsylvania plant, marked this shift, with byproduct ovens displacing beehives by the 1910s amid resource pressures and efficiency demands.[32][34] U.S. production peaked in the World War I era, reaching 56.5 million short tons in 1918, driven by steel demands for munitions and infrastructure; Connellsville hit 13.1 million short tons in 1907, while the adjacent Klondike district peaked at 9.6 million short tons in 1916.[35][32] Appalachian dominance persisted, with expansions in West Virginia's southern counties and Alabama supporting exports and domestic metallurgy, though exhaustion of prime seams and electric arc furnace alternatives began curbing growth post-1920s.[32][33]Rise of Petroleum Coking in Refining
The earliest instances of petroleum coking occurred in the 1860s, when refiners distilled kerosene from crude oil in simple iron stills, leaving a carbon-rich residue of coke and tar that was manually excavated after each batch.[36] This rudimentary process treated coke as a low-value byproduct, with production limited by the batch nature of horizontal still operations and the focus on kerosene for lighting.[36] The development of thermal cracking processes in the early 20th century marked a pivotal shift, as rising demand for gasoline—driven by the internal combustion engine—necessitated upgrading heavier fractions of crude oil.[37] Standard Oil's Burton thermal cracking process, introduced around 1912 at its Whiting, Indiana refinery, cracked gas oils under high pressure and temperature to yield more gasoline, but it generated significant coke deposits that required frequent shutdowns for cleaning.[36] This led to the innovation of delayed coking in 1929, when Standard Oil of Indiana commissioned the first commercial delayed coker unit at the same Whiting facility; the process separated cracking in a heater from soaking and coking in dedicated drums, enabling semi-continuous operation and higher throughput of residual feeds into lighter distillates plus solid petroleum coke.[36][37] Adoption accelerated post-World War II amid surging demand for transportation fuels, as refineries increasingly processed heavier crude residues to maximize light product yields—coking converted "bottom-of-the-barrel" materials that distillation alone could not upgrade.[38] By the 1950s, variants like fluid coking (developed by Exxon) further improved efficiency for fluidizable feeds, allowing gasification of excess coke for additional hydrogen and energy.[39] U.S. petroleum coke production capacity expanded dramatically thereafter, from approximately 53,000 barrels per day in the 1980s to over 900,000 barrels per day by the 2020s, reflecting the processing of heavier global crudes and economic incentives to reject carbon as coke rather than burn low-value residuals.[40] Output rose 64% between 1980 and 1990 alone, underscoring coking's role in refinery profitability.[41] This expansion was causally tied to shifts in crude quality and market economics: lighter sweet crudes yielded less residuum, but access to heavier sours (e.g., from Venezuela and later Canadian oil sands) increased bottoms upgrading needs, with coking providing a cost-effective thermal rejection of impurities like sulfur and metals into the solid coke byproduct.[25] By 2011, U.S. refineries supplied 132 million barrels of petroleum coke annually, much of it exported or used in power generation and aluminum smelting, though production dipped to 39.3 million metric tons in 2024 amid fluctuating heavy oil imports.[25][42] Delayed coking units proliferated globally, comprising a key residue conversion step in over 100 refineries by the early 21st century, prioritizing yield of naphtha, gas oils, and diesel over residual fuel oil.[38]Technical Processes and Chemistry
Key Reactions in Coal Coking
The coal coking process entails the anaerobic pyrolysis of bituminous coal at temperatures typically ranging from 900 to 1100 °C, where thermal decomposition drives the removal of volatile matter to yield metallurgical coke.[43] This transformation proceeds through sequential stages of bond cleavage, devolatilization, and structural reorganization, with the coal's macromolecular network—comprising aromatic clusters linked by aliphatic bridges and heteroatoms—undergoing fragmentation and condensation.[43][44] In the pre-decomposition phase below the onset temperature (Td ≈ 620–670 K), minor reactions include condensation of functional groups and desorption of physically bound species, such as water vapor and gases like CH₄ and CO₂; decarboxylation of carboxyl groups (e.g., R-COOH → R-H + CO₂) predominates between 373–473 K, releasing CO₂ and stabilizing the structure slightly before major breakdown.[43] The active decomposition stage, from Td to ≈820 K, marks the core of primary pyrolysis, characterized by homolytic scission of weak C-C and C-O bonds in aliphatic side chains and bridges, generating free radicals that propagate via hydrogen abstraction and β-scission.[43] Devolatilization intensifies here, evolving tars, light hydrocarbons, and aliphatics between 473–643 K, shifting to methane, polynuclear aromatics, phenols, and nitrogen heterocycles above 643 K; initial dehydrogenation of aromatic moieties forms radical intermediates (e.g., loss of H from C₆H₅-R → C₆H₄•-R + H•), facilitating metaplast formation during the plastic phase around 400–500 °C.[43][45][44] Devolatilization commences as low as 150–200 °C, with volatile content dropping to 3–4% of initial levels by 480–700 °C under slow heating rates typical of coking ovens.[45] Secondary reactions dominate above 820 K, peaking at 973–1173 K for bituminous coals, involving cracking of primary volatiles (e.g., tar decomposition to H₂, CO, and lighter gases via heterocyclic ring opening) and aromatization of the residue.[43] Coke matrix consolidation arises from radical recombination, polymerization of aromatics, and hydrogen elimination, yielding a porous, anisotropic carbon structure; hydrogen gas evolution persists beyond 760 °C, while overall yields reflect competition between primary fragmentation and secondary coking of vapors, with slower heating (as in industrial batteries) favoring higher coke output over tar.[43][45] Mechanisms emphasize free-radical chain processes over ionic pathways, with coal rank influencing activation energies—lower for higher-volatile bituminous coals prone to swelling.[44]Variations in Petroleum Coking Techniques
Delayed coking represents the most prevalent technique in petroleum refining, accounting for the majority of global coker capacity due to its established reliability and adaptability to heavy residues. In this semi-batch process, vacuum distillation residues or other heavy feeds are preheated in a furnace to approximately 480-505°C before being directed into paired, insulated coke drums where thermal cracking proceeds over a residence time of 12-24 hours at temperatures around 400-450°C and near-atmospheric pressure.[24][25] The drums alternate operations—one filling while the other cools—with solid coke buildup hydraulically cut and removed post-cycle, yielding petroleum coke alongside lighter products such as naphtha, gas oils, and gases.[25] This method favors production of sponge coke suitable for aluminum anodes or fuel, though shot coke forms with certain feeds, and its flexibility accommodates varying crude qualities while rejecting metals into the coke fraction.[15] Fluid coking, a continuous alternative, utilizes a fluidized-bed reactor system to achieve higher cracking severity than delayed coking, processing even heavier feeds with reduced coke yields and elevated distillate outputs. Feedstock is atomized onto a circulating bed of seed coke particles in the reactor maintained at 510-570°C, where vapor-phase cracking occurs rapidly; heat is generated by partial combustion of coke in an adjacent burner operating at 595-675°C, with fluidized particles circulating between vessels.[23][46] The process operates above atmospheric pressure and produces finer fluid coke particles, often more olefinic products due to shorter residence times, enabling up to 10-15% higher liquid yields relative to delayed coking for comparable feeds.[23][25] Flexicoking extends fluid coking by integrating a gasification reactor, converting surplus coke into low-BTU synthesis gas via reaction with steam and air, thereby minimizing net solid coke production—often to near-zero—and supplying additional refinery fuel gas.[24] In this configuration, the fluid coker reactor feeds coke to a heater and gasifier operating at elevated temperatures, where partial oxidation and steam reforming produce "flexigas" while recirculating heated coke particles for process continuity.[23] This variation enhances overall carbon utilization, reduces byproduct disposal needs, and lowers emissions like SO2 and NOx compared to standard fluid coking, though it requires more complex infrastructure and is less common in deployment.[24]Quality Control and Coke Properties
Quality control in coal coking processes focuses on ensuring metallurgical coke meets blast furnace requirements through rigorous testing of physical, mechanical, and chemical properties. Key parameters include the Coke Reactivity Index (CRI), which quantifies coke's gasification rate with CO₂ at 1,100°C for 2 hours, and the Coke Strength after Reaction (CSR), measuring the percentage of coke particles larger than 10 mm post-reaction and tumbling; desirable values are CRI below 25–30% and CSR above 60% to minimize degradation in the furnace reducing zone.[47][48][49] Mechanical strength is evaluated via indices like M40 (percentage of particles >40 mm after drum tumbling) and M10 (>10 mm), with targets of M40 >80% and M10 >85% for structural integrity during burden descent.[50] Chemical composition control targets low ash (<12%), sulfur (<0.7%), and moisture (<5%), as higher levels impair furnace efficiency and increase slag volume; these are monitored via proximate and ultimate analyses during coal blend selection and post-carbonization sampling.[51][52] Process variations, such as coal blend optimization and coking temperature (typically 1,000–1,100°C), directly influence coke microstructure—lower porosity and anisotropic domains correlate with superior strength and reactivity resistance, verified through optical microscopy and X-ray diffraction.[53] Quality assurance involves automated sampling from ovens, laboratory assays per ISO or ASTM standards, and feedback loops to adjust blending ratios, with deviations triggering production halts to prevent off-spec output that could elevate blast furnace coke rates by 5–10 kg/t hot metal.[54][7] For petroleum coking, quality control emphasizes feedstock properties and operational parameters in delayed or fluid coking units to yield green or calcined coke suited for anodes or fuel. Critical properties include volatile combustible matter (VCM, <10% for calcined anode grade), sulfur (<3% for low-sulfur variants), and trace metals like vanadium (<300 ppm) and nickel (<200 ppm), which are controlled by residuum quality and heater outlet temperatures (450–500°C); excessive VCM leads to puffing and electrode defects during electrolysis.[55][56] Density (bulk >0.8 g/cm³) and real density (1.9–2.1 g/cm³ post-calcining at 1,200–1,400°C) are assessed via helium pycnometry, ensuring thermal and electrical conductivity for aluminum smelting applications.[57][58] Petroleum coke grading distinguishes anode-grade (high carbon purity, low impurities) from fuel-grade (higher sulfur up to 6–8%, viable for power but limited by emissions), with rapid analytical methods like laser-induced breakdown spectroscopy enabling on-site verification of bulk properties such as calorific value (>8,000 kcal/kg).[59][56] Storage and handling protocols, including water suppression for dust control, maintain particle size uniformity (typically 1–50 mm) and prevent contamination, as verified by API guidelines.[60] Overall, discrepancies in these properties can reduce anode life by 10–20% or fuel efficiency, underscoring the need for empirical testing over predictive models alone.[58]Industrial Applications
Role in Iron and Steel Production
Metallurgical coke, derived from the high-temperature carbonization of coking coals, serves as the primary solid fuel and chemical reductant in blast furnaces for pig iron production, which supplies the basic oxygen furnace stage of steelmaking. In the blast furnace, coke undergoes combustion with preheated blast air to generate the heat required to melt the charge (typically 1,500–2,200°C), while its carbon content reacts with oxygen from iron ore (Fe₂O₃ or Fe₃O₄) to produce carbon monoxide (CO), the key reducing gas that converts iron oxides to metallic iron via indirect reduction: Fe₂O₃ + 3CO → 2Fe + 3CO₂. Coke also permeates the furnace burden to maintain a stable structure, preventing collapse under the weight of descending materials, and contributes carbon to the molten iron, yielding pig iron with 3.5–4.5% carbon content.[61][62] Coke consumption, or "coke rate," in modern blast furnaces typically ranges from 350–400 kg per metric ton of hot metal produced, though advanced operations with pulverized coal injection or hydrogen enrichment can reduce this to around 250–300 kg/t through partial substitution of coke's roles. This BF-BOF pathway accounts for roughly 70% of global crude steel output, underscoring coke's centrality to primary steelmaking, as alternatives like direct reduced iron require significant energy inputs that often rely on reformed natural gas rather than solid reductants.[63][64][65] In the electric arc furnace (EAF) route, comprising about 30% of global steel production and focused on scrap recycling, petroleum-derived needle coke plays a critical indirect role via graphite electrodes. Needle coke, a premium anisotropic petroleum coke with low impurity levels and needle-like microstructure, is calcined and graphitized to form electrodes that conduct electricity and sustain arcs up to 3,000–6,000°C for melting and refining. Its superior thermal conductivity, low thermal expansion (under 1.0 × 10⁻⁶/°C), and resistance to oxidation enable ultra-high-power electrodes essential for efficient EAF operation, where electrode consumption can reach 3–6 kg per metric ton of steel. Standard petroleum coke, however, lacks the structural integrity and purity for blast furnace use or electrode production, limiting its application to minor blending (up to 5–10% in coal cokes) or as a recarburizer in ladle metallurgy due to elevated sulfur (2–6%) and volatile matter.[66][67][68][69]Uses of Petroleum Coke
Petroleum coke, a carbon-rich byproduct of high-boiling hydrocarbon thermal cracking in oil refineries, finds primary application as fuel-grade material in energy-intensive industries and as calcined-grade feedstock for electrode production. Fuel-grade petroleum coke, typically containing higher sulfur levels (up to 6-8%), is burned for its high heating value of about 14,000-15,000 British thermal units per pound, serving as an economical alternative to coal in cement kilns and power plants.[70][71] In cement production, it provides up to 50-80% of kiln energy needs in facilities equipped for high-sulfur fuels, while in power generation, it supports baseload electricity in regions like India and China where imports reached significant volumes in 2023.[70][72] Calcined petroleum coke (CPC), produced by heating green coke at 1,200-1,400°C to remove volatiles and enhance graphitizability, constitutes the bulk of anode-grade material for primary aluminum smelting, where it comprises 65-80% of prebake anodes used in the Hall-Héroult electrolytic process.[59][73] Global aluminum production, exceeding 70 million metric tons annually as of 2023, drives demand for low-impurity CPC (sulfur <3%, metals <300 ppm) sourced from specific refineries processing sweet crudes.[74] In steelmaking, CPC serves as a key ingredient in graphite electrodes for electric arc furnaces (EAF), enabling submerged arc melting of scrap and direct reduced iron; needle-grade CPC, with low ash and high carbon purity, is preferred for this, supporting over 30% of global steel output via EAF routes in 2024.[59][73] Secondary uses include gasification for syngas in chemical production and as a reducing agent in titanium smelting, though these account for less than 5% of total consumption.[75] In 2023, the United States exported approximately 41 million short tons of petroleum coke, primarily fuel-grade to Asia for cement and power applications, underscoring its role in global energy trade amid domestic refining expansions.[70] While versatile, applications are constrained by sulfur and heavy metal content, necessitating specialized handling or blending to meet emission standards in regulated markets.[22]Other Specialized Applications
Foundry coke, a larger and more robust variant of metallurgical coke with lump sizes typically exceeding 100 millimeters, is employed in cupola furnaces for the smelting of cast iron and other metals in foundry operations. It functions both as a high-calorific fuel providing consistent heat for melting scrap steel, iron, and non-ferrous alloys like copper, and as a reducing agent to facilitate carbon addition and oxide removal during the process.[76][77] In the United States, foundry coke constitutes 5 to 7 percent of total metallurgical coke output, underscoring its niche but essential role in producing ductile and gray iron castings for applications in automotive components and machinery.[78] Metallurgical coke also supports non-ferrous metal production, serving as a reducing agent in the smelting of ores for metals such as copper, lead, aluminum, titanium, and silicon. In these processes, coke's high carbon content and low reactivity with slag enable efficient conversion of metal oxides to elemental forms at elevated temperatures, often in electric arc or reverberatory furnaces.[79][80] This application extends coke's utility beyond ferrous metallurgy, contributing to the production of alloys used in electronics, aerospace, and construction materials. Coke breeze, the fine byproduct fraction under 25 millimeters from coking operations, finds specialized use as a carbonaceous backfill in impressed current cathodic protection systems for underground pipelines, storage tanks, and structures. Its low resistivity and high conductivity enhance anode performance by distributing current uniformly and minimizing polarization effects, with calcined petroleum coke breeze preferred for its superior purity and stability in corrosive soils.[81][82] Additionally, fuel-grade petroleum coke, derived from delayed coking, is utilized in cement kilns as a substitute for coal or natural gas, comprising up to 40 percent of fossil fuel inputs globally due to its high heating value of approximately 8,000 kcal/kg and ability to sustain clinkering temperatures above 1,450°C.[83][84]Environmental and Health Considerations
Emissions from Coal Coking Operations
Coal coking operations, conducted in coke oven batteries, emit a complex mixture of pollutants during the destructive distillation of bituminous coal at temperatures of 1,000°C to 1,400°C in the absence of oxygen.[85] These emissions originate from multiple process stages, including coal preparation and crushing, charging into ovens, carbonization, pushing of incandescent coke, and quenching with water.[86] Particulate matter (PM), primarily filterable PM consisting of coal dust, soot, and tar droplets, arises from mechanical disturbances and incomplete combustion, while gaseous and volatile releases stem from thermal decomposition of coal volatiles.[86] Approximately 20% to 35% of the initial coal charge is volatilized as gases and vapors during carbonization, with much captured as coke-oven gas in by-product recovery systems, though fugitive emissions persist via oven doors, lids, and offtake systems.[85] The composition includes PM with a benzene-soluble fraction containing polycyclic aromatic hydrocarbons (PAHs) such as benzo(a)pyrene, benzanthracene, chrysene, and phenanthrene (>40 PAHs identified), alongside trace metals like arsenic, cadmium, and mercury.[87][85] Volatile organic compounds (VOCs) feature benzene, toluene, and xylenes, often as vapors from charging and leaks.[87] Gaseous components encompass carbon monoxide (CO), hydrogen sulfide (H2S), ammonia (NH3), nitrogen oxides (NOx), sulfur oxides (SOx), formaldehyde, acrolein, and aliphatic aldehydes; coke-oven gas itself comprises hydrogen (primary), methane, ethane, CO, CO2, ethylene, and minor H2S and NH3.[86][85] Uncontrolled emission factors, expressed in kg per Mg of coal charged, quantify typical releases absent mitigation, as documented by the U.S. Environmental Protection Agency's AP-42 compilation (updated periodically, with data reflecting empirical measurements from U.S. facilities).[86]| Process Step | PM (filterable, kg/Mg) | VOCs (as CH4, kg/Mg) | CO (kg/Mg) | SOx (kg/Mg) | NOx (kg/Mg) |
|---|---|---|---|---|---|
| Coal crushing (with cyclone) | 0.055 | - | - | - | - |
| Oven charging | 0.24 | 1.25 | 0.3 | - | 0.015 |
| Oven door leaks | 0.27 | 0.75 | - | - | - |
| Oven pushing | 0.58 | - | 0.035 | - | - |
| Quenching (dirty water) | 2.62 | - | - | - | - |
| Underfire stack (raw COG) | 0.234 | - | - | 2.0 | - |