Coal, a naturally occurring fossil fuel, serves as a fundamental raw material in various industrial applications. One significant transformation it undergoes is conversion into coke, a refined carbonaceous product. This process involves heating specific types of coal in the absence of air, leading to a material with distinct properties vital for heavy industries.
Understanding Coke
Coke is a hard, porous, and grey solid primarily composed of carbon, derived from low-ash and low-sulfur bituminous coal. This refined material possesses a high carbon content, exceeding 85%, and contains minimal impurities like sulfur and ash. Its porous structure and strength allow it to withstand significant weight and maintain gas permeability in industrial furnaces. These properties make coke valuable as both a fuel and a reducing agent, particularly in the production of iron. Its main industrial use is in blast furnaces, where it provides the necessary heat to melt iron ore and chemically reduces the ore by removing oxygen.
The Coking Process
The transformation of coal into coke begins with meticulous raw material preparation. Metallurgical coal, selected for its high carbon content, is first crushed to a precise size. Different types of coal are blended to achieve desired properties in the final coke product, and water or oil may be added to control the mixture’s density. This prepared coal blend is then loaded into specialized coke ovens, which are part of larger coke oven batteries.
The coal is charged into these ovens. The step, known as carbonization or pyrolysis, involves heating the coal in the complete absence of oxygen, preventing it from burning. Temperatures within the ovens range from 900 to 1100 degrees Celsius (approximately 1650 to 2000 degrees Fahrenheit), which drives off volatile components from the coal.
During this high-temperature process, the coal softens, decomposes, and then re-solidifies into a dense, solid carbon residue. This “destructive distillation” process can take 15 to 30 hours, with coking considered complete when the center of the coal mass reaches 950-1000 degrees Celsius. After carbonization, the hot coke is discharged from the oven.
The hot coke is transported to a quench tower, where it is rapidly cooled by dousing it with water. This quenching prevents the coke from igniting upon exposure to air and halts the carbonization process. Finally, the cooled coke is moved to a crushing and screening system, where it is sized for its industrial applications.
Valuable Byproducts
The coking process not only yields solid coke but also generates a variety of valuable gaseous and liquid byproducts. As volatile components are driven off the coal during heating, they are captured and processed. These byproducts are important feedstocks for other industries.
One significant byproduct is coke oven gas, a mixture of gases including hydrogen and methane. This gas is used as a fuel within the steel plants themselves, heating the coke ovens or supporting other production processes. It can also be utilized for power generation or in the production of synthesis gas.
Coal tar is another valuable liquid byproduct, which can be further distilled to produce chemicals such as naphthalene, benzene, toluene, and xylene. These derivatives find applications in manufacturing paints, dyes, and other chemical products. Additionally, ammonia is recovered, and sulfur compounds are also removed and processed.