Petroleum coke

Petroleum coke, often called petcoke, is a material rich in carbon. It is made when oil is refined. This happens in a process called cracking, where long chains of molecules in oil are broken into shorter ones. This occurs in special units called coker units. Petroleum coke is also made when creating synthetic crude oil from bitumen in places like Canada’s oil sands and Venezuela’s Orinoco oil sands.

Petcoke can be fuel grade, which has a lot of sulfur and metals, or anode grade, which has less. The raw coke from the coker is called green coke. It is then heated in a rotary kiln to remove gases, making it ready for use. This processed coke is important for making aluminium and steel because it is used to create anodes.

When burned, petcoke releases more carbon dioxide than coal. This makes petcoke a notable source of greenhouse gases.

Types

There are three main types of petroleum coke: needle coke, sponge coke, and shot coke. These types look different under a microscope because they are made in different ways and from different materials.

Needle coke, also called acicular coke, is a special type of petroleum coke. It is used to make electrodes for the steel and aluminium industries. Needle coke is made only from fluid catalytic cracking decant oil or coal tar pitch.

Composition

When petcoke is heated in a process called calcining, it loses many of its parts and becomes more pure. After this, it usually does not release heavy metals into the air.

Petcoke has a lot of carbon, up to 98-99%, depending on the oil used to make it. It also has small amounts of hydrogen, nitrogen, and sulfur. When petcoke is heated, some of these parts, like sulfur, go into the air. The final product is a solid made mostly of carbon with small holes, like a honeycomb.

Fuel-grade

Fuel-grade coke can be sponge coke or shot coke. Scientists do not fully know why these two types form. Lower temperatures and higher pressures help make sponge coke. The amount of certain materials in the oil also matters.

Petroleum coke can be used in power plants because it gives off a lot of heat and has low ash. But it has high sulfur and low volatile content, which can cause problems. It gives almost twice the energy of average coal. Special methods are used to burn it safely. Gasification is also becoming more common.

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Calcined

Calcined petroleum coke (CPC) is made by heating petroleum coke. This type of coke comes from a special part of an oil refinery called the coker unit.

Calcined petroleum coke is important because it is used to make anodes. Anodes are needed for making aluminium, steel, and titanium. The raw coke, called green coke, must have low metal content to be used for this. If the green coke has too much metal, it is used as fuel instead.

Desulfurization

Petroleum coke can have a lot of sulfur. This can make it less useful as fuel because burning sulfur can create harmful gases.

Scientists have ideas for removing sulfur, such as using special liquids, heat, or gases. But as of 2011, none of these methods were used in real factories to clean the coke.

Storage, disposal, and sale

Petroleum coke, which is almost pure carbon, releases a lot of carbon dioxide when it is burned.

Petroleum coke is often stored in large piles near oil refineries until it is sold. For example, in 2013, there was a big pile owned by Koch Carbon near the Detroit River made by a Marathon Petroleum refinery in Detroit. This refinery started processing bitumen from the oil sands of Alberta in November 2012. Large amounts of petroleum coke were also stored in Canada, and it was sold to places like China and Mexico from California. At that time, the Oxbow Corporation, owned by William I. Koch, sold about 11 million tons of petroleum coke each year.

In 2017, a quarter of the United States' exports of this fuel went to India. In 2016, this was more than eight million metric tons. Tests in India near New Delhi showed that the petroleum coke had sulfur levels higher than allowed by law.

Starting in 2020, international rules set by the International Maritime Organization (IMO) say that ships cannot use fuels with sulfur content higher than 0.5%. Because of this, many oils are being changed into lighter oils, which creates more petroleum coke as a byproduct. The amount of petroleum coke is expected to grow in the future. Petroleum coke is also used in special plants to make synthetic natural gas.

Health hazards

Petroleum coke can make fine dust that may get into our lungs and cause health problems. But studies show that petroleum coke itself is not very poisonous and does not cause cancer.

Petroleum coke sometimes has a metal called vanadium, which can be harmful even in tiny amounts. This has been found in dust near places where petroleum coke is stored.

Overall, experts think petroleum coke does not pose big health risks to people. Tests on animals showed that breathing in the dust could irritate their lungs, but this was not special to petroleum coke.

Environmental hazards

Storing and burning petroleum coke can harm the environment. When processed, it makes waste that needs careful handling. The dust from petroleum coke can spread easily, especially when it is windy, which can be a problem in cities like Chicago, Detroit, and Green Bay.

Burning petroleum coke releases carbon dioxide and sulfur into the air. It can also cause water pollution from metals like nickel and vanadium running off from storage areas.