Petroleum coke, abbreviatedcoke,pet coke orpetcoke, is a finalcarbon-rich solid material that derives fromoil refining, and is one type of the group of fuels referred to ascokes. Petcoke is the coke that, in particular, derives from a finalcracking process—a thermo-based chemical engineering process that splits long chain hydrocarbons of petroleum into shorter chains—that takes place in units termedcoker units.^[1] (Other types ofcoke are derived fromcoal.) Stated succinctly, coke is the "carbonization product of high-boiling hydrocarbon fractions obtained in petroleum processing (heavy residues)".^[1] Petcoke is also produced in the production ofsynthetic crude oil (syncrude) frombitumen extracted from Canada'soil sands and from Venezuela'sOrinoco oil sands.^[2][3]In petroleum coker units, residualoils from otherdistillation processes used inpetroleum refining are treated at a high temperature and pressure leaving the petcoke after driving off gases and volatiles, and separating off remaining light and heavy oils. These processes are termed "coking processes", and most typically employchemical engineering plant operations for the specific process ofdelayed coking.

This coke can either be fuel grade (high insulfur and metals) oranode grade (low in sulfur and metals). The raw coke directly out of the coker is often referred to asgreen coke.^[1] In this context, "green" means unprocessed. The further processing of green coke bycalcining in arotary kiln removes residual volatile hydrocarbons from the coke. The calcined petroleum coke can be further processed in an anode baking oven to produce anode coke of the desired shape and physical properties. Theanodes are mainly used in thealuminium andsteel industry.

Petcoke is over 80% carbon and emits 5% to 10% morecarbon dioxide (CO₂) than coal on a per-unit-of-energy basis when it is burned. As petcoke has a higher energy content, petcoke emits between 30% and 80% more CO₂ than coal per unit of weight.^[3] The difference between coal and coke in CO₂ production per unit of energy produced depends upon the moisture in the coal, which increases the CO₂ per unit of energy –heat of combustion – and on the volatile hydrocarbons in coal and coke, which decrease the CO₂ per unit of energy.

There are at least three basic types of petroleum coke: needle coke, sponge coke, and shot coke. Different types of petroleum coke have different microstructures due to differences in operating variables and nature of feedstock. Significant differences are also to be observed in the properties of the different types of coke, particularly ash and volatile matter contents.^[4]

Needle coke, also called acicular coke, is a highlycrystalline petroleum coke used in the production of electrodes for thesteel andaluminium industries and is particularly valuable because the electrodes must be replaced regularly. Needle coke is produced exclusively from eitherfluid catalytic cracking (FCC) decant oil or coal tar pitch.

Petcoke, altered through the process of calcining which it is heated or refined raw coke eliminates much of the component of the resource. Usually petcoke when refined does not release the heavy metals as volatiles or emissions.^[5]

Depending on the petroleum feed stock used, the percentage of carbon in petcoke can be as high as 98-99%. This creates a carbon-based compound containing hydrogen in concentrations between 3.0 – 4.0%. Raw (or green) coke contains between 0.1 – 0.5% nitrogen and 0.2 – 6.0% sulfur which become emissions when coke is calcined.^[5]

Through thermal processing the composition in weight is reduced with the volatile matter andsulfur being emitted.^[6] This process ends in the honeycomb petcoke which according to the name giving is a solid carbon structure with holes in it.^[6]

Fuel-grade coke is classified as either sponge coke or shot coke morphology. While oil refiners have been producing coke for over 100 years, the mechanisms that cause sponge coke or shot coke to form are not well understood and cannot be accurately predicted. In general, lower temperatures and higher pressures promote sponge coke formation. Additionally, the amount ofheptane insolubles present and the fraction of light components in the coker feed contribute.

While its high heat and lowash content make it a decent fuel for power generation in coal-firedboilers, petroleum coke is high insulfur and low involatile content, and this poses environmental (and technical) problems with its combustion. Itsgross calorific value (HHV) is nearly 8000 Kcal/kg which is twice the value of average coal used in electricity generation.^[5] A common choice of sulfur recovering unit for burning petroleum coke is theSNOX Flue gas desulfurisation technology,^[7] which is based on the well-knownWSA Process.Fluidized bed combustion is commonly used to burn petroleum coke.Gasification is increasingly used with this feedstock (often using gasifiers placed in the refineries themselves).

Calcined petroleum coke (CPC) is the product fromcalcining petroleum coke. This coke is the product of thecoker unit in a crudeoil refinery. The calcined petroleum coke is used to makeanodes for thealuminium,steel andtitanium smelting industry and as the feed stock for the production of synthetic graphite. Thegreen coke must have sufficiently low metal content to be used as anode material. Green coke with this low metal content is called anode-grade coke. When green coke has excessive metal content, it is not calcined and is used asfuel-grade coke in furnaces.

A highsulfur content in petcoke reduces its market value, and may preclude its use as fuel due to restrictions onsulfur oxides emissions for environmental reasons. Methods have thus been proposed to reduce or eliminate the sulfur content of petcoke. Most of them involve thedesorption of the inorganic sulfur present in the pores or surface of the coke, and the partition and removal of organic sulfur compounds, such as sulfurous aromatic heterocycles.

Potential petroleumdesulfurization techniques can be classified as follows:^[8]

1. Solvent extraction
2. Chemical treatment
3. Thermal desulfurization
4. Desulfurization in an oxidizing atmosphere
5. Desulfurization in an atmosphere of sulfur-bearing gas
6. Desulfurization in an atmosphere of hydrocarbon gases
7. Hydrodesulfurization

As of 2011 there was no commercial process available to desulfurize petcoke.^[9]

Nearly pure carbon, petcoke is a potent source ofcarbon dioxide if burned.^[10]

Petroleum coke may be stored in a pile near an oil refinery pending sale. For example, in 2013 a large stockpile owned byKoch Carbon near theDetroit River was produced by aMarathon Petroleum refinery inDetroit which had begun refiningbitumen from theoil sands ofAlberta in November 2012. Large stockpiles of petcoke also existed in Canada as of 2013, and China and Mexico were markets for petcoke exported from California to be used as fuel. As of 2013 Oxbow Corporation, owned byWilliam I. Koch, was a major dealer in petcoke, selling 11 million tons annually.^[11]

In 2017, a quarter of US exports of the fuel went to India, anAssociated Press investigation found. In 2016, this amounted to more than eight million metric tons, more than 20 times as much as in 2010.^[12] India's Environmental Pollution Control Authority tested imported petcoke in use nearNew Delhi, and foundsulfur levels 17 times the legal limit.^[12]

The International Convention for Prevention of Pollution from Ships (MARPOL 73/78), adopted by theInternational Maritime Organization (IMO), has mandated that marine vessels shall not consume residual fuel oils (bunker fuel, etc) with a sulfur content greater than 0.5% from the year 2020.^[13] Nearly 38% of residual fuel oils are consumed in the shipping sector. In the process of converting excess residual oils into lighter oils by coking processes, pet coke is generated as a byproduct. Pet coke availability is expected to increase in the future due to falling demand for residual oil. Pet coke is also used inmethanation plants to producesynthetic natural gas, etc. in order to avoid a pet coke disposal problem.^[14]

Petroleum coke is sometimes a source offine dust, which can penetrate the filtering process of the human airway, lodge in the lungs and cause serious health problems. Studies have shown that petroleum coke itself has a low level oftoxicity and there is no evidence ofcarcinogenicity.^[15][16]

Petroleum coke can containvanadium, a toxic metal. Vanadium was found in the dust collected in occupied dwellings near the petroleum coke stored next to the Detroit River. Vanadium is toxic in tiny quantities, 0.8 micrograms per cubic meter of air, according to theEPA.^[17]

According to multiple EPA studies and analyses, petroleum coke has a low health hazard potential in humans. It does not have any observable carcinogenic, developmental, or reproductive effects. During animal case studies repeated-dose chronic inhalation did show respiratoryinflammation due to dust particles, but not specific to petroleum coke.^[18]

Environmental concerns stem from the storage and combustion of petcoke. By-waste accumulates as petcoke is processed, making waste management an issue. Petcoke's high silt content of 21.2% increases the risk of fugitive dust drifting away from petcoke mounds under heavy wind. An estimated 100 tons of petcokefugitive dust including PM10 and PM2.5 are released into the atmosphere per year in the United States.^[19] Waste management and release of fugitive dust is especially an issue in the cities ofChicago,Detroit andGreen Bay.^[18]

Externalities stem from petcoke that cause potential environmental impacts. Petcoke is composed of 90% elemental carbon by weight which is converted to CO₂ during combustion. Use of petcoke also produces emissions of sulfur, and the potential for water pollution through nickel and vanadiumrunoff from refining and storage.^[17]

• Air pollution in India
• Coke (fuel)
• Cooler for calcined petroleum coke
• Delayed coker
• Greenhouse gas emissions
• Orimulsion
• Tar

  Media related toPetroleum coke at Wikimedia Commons

• IUPACdefinition of various forms of solidcarbon.
• BP info on how calcined petroleum coke is produced