Petroleum coke

Petroleum coke, often called petcoke, is a carbon-rich material made during oil refining. It comes from a process called cracking, where long chains of molecules in oil are broken into shorter ones. This happens in special units called coker units. Petroleum coke is also made when producing 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 leftover gases, making it ready for further 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. It produces 5% to 10% more carbon dioxide per unit of energy and between 30% and 80% more per unit of weight. This makes petcoke a significant 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. They also have different properties, especially how much ash and volatile matter they contain.

Needle coke, also known as acicular coke, is a special type of petroleum coke that is used to make electrodes for the steel and aluminium industries. It is very important because the electrodes need to be replaced often. 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 process, it usually does not release heavy metals into the air.

The amount of carbon in petcoke can be very high, up to 98-99%, depending on the oil used to make it. It also contains small amounts of hydrogen, nitrogen, and sulfur. When petcoke is heated, some of these parts, like sulfur, are released 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 either sponge coke or shot coke. Scientists still do not fully understand why these two types form, and they cannot predict which will form. Generally, lower temperatures and higher pressures help create sponge coke. The amount of certain materials in the oil and the mix of lighter parts also play a role.

Petroleum coke can be used as fuel in power plants because it gives off a lot of heat and has low ash. However, it has high sulfur and low volatile content, which can cause environmental and technical issues when burned. It provides nearly twice the energy of average coal used for electricity. Special methods like SNOX Flue gas desulfurisation and fluidized bed combustion are often used to burn it safely. Gasification is also becoming more common for processing this type of coke.

heptane ash boilers volatile gross calorific value SNOX Flue gas desulfurisation WSA Process Fluidized bed combustion Gasification

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, which are needed for making aluminium, steel, and titanium. The raw coke, called green coke, must have low metal content to be used for this purpose. If the green coke has too much metal, it is used as fuel instead of being heated.

Desulfurization

Petroleum coke sometimes has a lot of sulfur, which can make it less useful as fuel. This is because having too much sulfur can create harmful gases when the coke is burned, which is bad for the environment.

There are several ways scientists have thought about to remove sulfur from petroleum coke, like using special liquids, heat, or different gases. However, as of 2011, none of these methods were being 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, which is over 20 times more than in 2010. Tests in India near New Delhi showed that the petroleum coke had sulfur levels 17 times 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 as demand for these heavier oils falls. Petroleum coke is also used in special plants to make synthetic natural gas.

Health hazards

Petroleum coke can create fine dust that may get into our lungs and cause health issues. However, studies show that petroleum coke itself is not very poisonous and does not cause cancer.

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

Overall, experts believe 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 unique to petroleum coke.

Environmental hazards

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

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