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Petroleum coke, abbreviated pet coke or petcoke, is a final carbon-rich solid material that derives from oil refining, and is one type of the group of fuels referred to as cokes. Petcoke is the coke that, in particular, derives from a final cracking process–a catalytic chemical engineering process that splits long chain hydrocarbons of petroleum into shorter chains—that takes place in units termed coker units. (Other types of coke are derived from from coal.) Stated succinctly, coke is the "carbonization product of high-boiling hydrocarbon fractions obtained in petroleum processing (heavy residues)."
Contents
- Types of petroleum coke
- Fuel grade coke
- Calcined petroleum coke
- Desulfurization of petcoke
- Storage disposal and sale
- Health Hazards
- References
In petroleum coker units, residual oils from other distillation processes used in petroleum refining are treated catalytically 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 employ chemical engineering plant operations for the specific process of delayed coking.
This coke can either be fuel grade (high in sulfur and metals) or anode grade (low in sulfur and metals). The raw coke directly out of the coker is often referred to as green coke. In this context, "green" means unprocessed. The further processing of green coke by calcining in a rotary kiln removes residual volatile hydrocarbons from the coke. The calcined petroleum coke can be further processed in an anode baking oven in order to produce anode coke of the desired shape and physical properties. The anodes are mainly used in the aluminium and steel industry.
Petcoke is over 90 percent carbon and emits 5 to 10 percent more carbon 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 percent more CO2 than coal per unit of weight. The difference between coal and coke in CO2 production per unit energy produced depends upon the moisture in the coal (increases the CO2 per unit energy – heat of combustion) and volatile hydrocarbon in coal and coke (decrease the CO2 per unit energy).
Types of petroleum coke
There are at least four basic types of petroleum coke, namely, needle coke, honeycomb 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.
Needle coke, also called acicular coke, is a highly crystalline petroleum coke used in the production of electrodes for the steel and aluminium industries and is particularly valuable because the electrodes must be replaced regularly. Needle coke is produced exclusively from either FCC decant oil or coal tar pitch.
Honeycomb coke is an intermediate coke, with ellipsoidal pores that are uniformly distributed. Compared to needle coke, honeycomb coke has a lower coefficient of thermal expansion and a lower electrical conductivity.
Fuel grade coke
Fuel grade coke is classified as either sponge coke or shot coke morphology. While oil refiners have been producing coke for well 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 of heptane insolubles present, and fraction of light components in the coker feed contribute.
While its high heat and low ash content make it a decent fuel for power generation in coal fired boilers, petroleum coke is high in sulfur and low in volatile content, and this poses environmental (and technical) problems with its combustion. To meet current North American emissions standards, some form of sulfur capture is required, a common choice of sulfur recovering unit for burning petroleum coke is the SNOX Flue gas desulfurisation technology, which is based on the well-known WSA 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
Calcined petroleum coke (CPC) is the product from calcining petroleum coke. This coke is the product of the coker unit in a crude oil refinery. The calcined petroleum coke is used to make anodes for the aluminium, steel and titanium smelting industry. The green coke must have sufficiently low metals content in order to be used as anode material. Green coke with this low metals content is referred to as anode grade coke. The green coke with too high metals content will not be calcined and is used for burning. This green coke is called fuel grade coke.
Desulfurization of petcoke
A high sulfur content in petcoke reduces its market value and may prevent its use as fuel due to restrictions on sulfur oxides emissions for environmental reasons. Methods have thus been proposed to reduce or eliminate the sulfur content of petcoke. Most of them involve the desorption of the inorganic sulfur present in the pores or surface of the coke, and the partition and removal of the organic sulfur attached to the aromatic carbon skeleton.
Potential petcoke desulfurization techniques can be classified as follows:
- Solvent extraction
- Chemical treatment
- Thermal desulfurization
- Desulfurization in an oxidizing atmosphere
- Desulfurization in an atmosphere of sulfur-bearing gas
- Desulfurization in an atmosphere of hydrocarbon gases
- Hydrodesulfurization
As of 2011 there was no commercial process available to desulfurize petcoke.
Storage, disposal, and sale
Nearly pure carbon, petcoke is a potent source of carbon dioxide if burned.
Petroleum coke may be stored in a pile near an oil refinery pending sale. One example, as of 2013, was the large stockpile owned by Koch Carbon near the Detroit River which was produced by a Marathon Petroleum refinery in Detroit which began refining bitumen from the oil sands of Alberta in November 2012. Large stockpiles of petcoke also existed in Canada as of 2013. China and Mexico were markets for petcoke exported from California to be used as fuel. As of 2013 Oxbow Corporation, owned by William I. Koch, was a major dealer in petcoke, selling 11 million tons annually.
Health Hazards
Petroleum coke is sometimes a source of fine dust, which can get through the filtering process of the human airway and lodge in the lungs. Once these small dust particles lodge in the lungs they can cause serious health problems. However, studies have shown that petroleum coke itself has a low level of toxicity and there is no evidence of carcinogenicity.
Petroleum coke can contain vanadium, 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 the EPA.
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 respiratory inflammation due to dust particles, but not specific to petroleum coke.