What Is Coking? What Happens to the Coal?

Petroleum coke ranked among the top 10 major types of U.S. petroleum products consumed in 2022, with 255,000 barrels consumed per day, according to the U.S. Energy Information Administration. 

However, the process of creating petroleum coke — called “coking” — remains largely unfamiliar to the average American consumer. The guide below includes a basic discussion of the coking process, including what happens to the coal and how coke influences multiple elements of our daily lives. 

What Is Coking?

Coking describes the catalytic process by which metallurgical coal (a feedstock/raw material) becomes petroleum coke, also known as petcoke. 

Petroleum coke is porous and dark in color, similar to charcoal, and contains 90% carbon, 3% to 6% sulfur, and small traces of nitrogen, oxygen, and hydrogen. Depending on the specific steps involved in coking, petcoke can appear in various shapes and sizes. 

The American Petroleum Institute identifies two types of petcoke: green and calcined. Green petcoke acts as fuel, while calcined petcoke can be used for the following applications:

• Aluminum
• Brick and glass
• Fertilizer
• Paint
• Paper
• Steel 

Petcoke is most often associated with power generation and the manufacturing of aluminum or steel using iron ore. It’s transported mainly via ocean freight and barges overseas and by rail and semi-truck over land. 

What Happens to the Coal During the Coking Process? 

The multi-step coking process relies on numerous chemical reactions to achieve coke formation and extract and divert coke-oven gases. Delayed coking, fluid coking, and flexi-coking are all methods used in the production of coke for consumption. 

Below is an overview of what happens to the coal during the coking process. 

1. Metallurgical Coal Sourced and Prepared

Metallurgical coal — as opposed to thermal coal, a type of coal which is used for generating electricity — contains high concentrations of carbon, which is critical to the coking process. 

Manufacturing companies source metallurgical coal (also known as coking coal or bituminous coal) from open-cut and underground mining operations. Different grades of metallurgical coal and the resulting coke exist based on the carbon content and porosity. 

Before metallurgical coal enters a coke-oven battery (more on that in a moment), it must undergo several preparatory procedures. Depending on how the coal arrives, it will be crushed to a specific size to ensure maximum efficiency during the carbonization process. 

Increasing the surface area allows the heat to penetrate the coal and produce coke faster. The coal may also be mixed with water or oil and enter the coke-oven battery wet or dry, depending on individual manufacturing processes. 

2. Coal Carbonization Produces Coke

A coke-oven battery consists of stacked rows of 10 to 100 coke ovens. High-temperature carbonization and the coking process take place within these coke-oven batteries. Temperatures exceed well over 1,000 degrees Celsius (1,832 degrees Fahrenheit) to extract impurities from the metallurgical coal and produce coal-oven gases and coke.

On average, the coking cycle lasts anywhere from 15 to 30 hours. Most coke processing plants have developed a specific protocol designed to optimize both the processes required during coking and the subsequent results. 

Coking produces two distinct elements: the solid, porous coke the process is named for and coke-oven gases. 

3. Coke-Oven Gas Extracted and Diverted

Coke-oven gases are a byproduct of coking but are equally valuable as the coke itself. Indeed, depending on consumer demand, these gases are separated and diverted to various applications. The Environmental Protection Agency (EPA) identifies several chemicals in coke-oven gases, such as coal tar and tar pitch, creosote, naphtha, and polycyclic aromatic hydrocarbons. 

Through various distillation processes, these chemicals can be diverted to produce the following: 

• Dyes
• Insulation 
• Paints 
• Pesticides 
• Plastics
• Roads 
• Sealants 
• Solvents 

Similar to coke, coke-oven gases support steelmaking and steel production. Other coke-oven gas applications include carbon black production, the glass industry, power generation, chemical manufacturing, environmental management, and synthesis gas (syngas) production. 

4. Quenching Process and Distribution 

Once the coke-oven gases have been removed during the coking process, the resulting coke must be quenched. Failure to quench the coke results in the continuation of the carbonization process, where the coke begins to burn once it meets oxygen within the ambient air. 

Coking plants typically transport petcoke via quench carts to the quench tower, where water douses the coke to halt the burning process.

These four steps highlight the fundamental processes involved in coking, which turns metallurgical coal into petroleum coke and coke-oven gas. At this point, coking plants continue processing coke to package it for sale and consumption. Subsequent steps vary depending on the end user. 

What Are the Benefits Associated With Petcoke?

Petcoke and its by-products offer manufacturers, businesses, and consumers multiple benefits. The following list includes perks of petcoke you benefit from without knowing it:

• Product affordability: Petcoke is an efficient fuel source (especially for blast furnaces) that lowers manufacturing overhead costs and allows companies to extend those savings to their customers. 
• Economic support: Each of the businesses involved in coking and any subsequent processes provides numerous employees with financial stability. 
• Sustainability: Manufacturers redirect numerous byproducts of the coking process to maximize sustainability efforts at various points. 

Although you may not immediately think of petcoke as a valuable resource, it’s easy to see its influence over multiple aspects of daily life. 

Are There Health Concerns Surrounding the Coking Process?

Once it’s undergone coking, petcoke remains inert and poses little health risk. However, the emissions associated with coke-oven gases include toxic elements such as arsenic and cadmium, which can increase the risk of lung and kidney cancer in anyone exposed to such toxins. 

Therefore, it’s vital for the safety of all personnel involved that petcoke manufacturers follow the appropriate safety protocols. Failure to do so can expose employees to hazardous substances that could negatively impact their health. 

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Sources: 

Use of oil - U.S. Energy Information Administration (EIA) | U.S. Energy Information Administration
PETROLEUM COKE: ESSENTIAL TO MANUFACTURING | National Association of Manufacturers
Health Effects of Petroleum Coke | Environmental Protection Agency
12.2 Coke Production | epa.gov
COKE PRODUCTION - Chemical Agents and Related Occupations | International Agency for Research on Cancer
Coke Oven Emissions | Environmental Protection AgencyCoke Oven Emissions - Cancer-Causing Substances | National Cancer Institute