What is Petroleum Coke?

Petroleum coke is a combustible solid byproduct obtained from the coking of residual oil or petroleum pitch during petroleum refining. It belongs to the category of easily graphitizable carbons. Characterized by its black color, porous structure, and low impurity content, petroleum coke is widely used in metallurgy, chemical industries, and serves as a key raw material for the production of graphite products.

Classification of Petroleum Coke

Petroleum coke can be classified in several ways:

(1) By coking process

Petroleum coke can be divided into chamber coke, pitch coke, delayed coke, and fluid coke. The first two have been largely phased out, while delayed coke and a smaller amount of fluid coke dominate current production worldwide.

(2) By heat treatment

It can be classified into green coke and calcined coke. Green coke, produced through delayed coking (or other coking processes), contains high levels of volatile matter and has relatively low mechanical strength. Calcined coke, obtained by calcining green coke at high temperatures, possesses enhanced properties. In China, most refineries produce only green coke, while calcination is typically carried out in carbon plants.

(3) By sulfur content

Petroleum coke is commonly divided into high-sulfur, medium-sulfur, and low-sulfur grades.

(4) By microstructure and performance

Petroleum coke exhibits three primary structural types: sponge coke, honeycomb coke, and needle coke.

• Sponge coke is produced from residuum rich in asphaltenes and resins. Its structure resembles a sponge, with high impurity content and numerous fine pores separated by thin coke walls. After graphitization, the resulting artificial graphite shows a relatively high coefficient of thermal expansion (CTE) and resistivity, making it unsuitable for graphite product manufacturing.
• Honeycomb coke is derived from feedstocks with moderate levels of asphaltenes and resins. It features a more uniform pore distribution and a distinct honeycomb-like texture, giving it superior physical and mechanical properties compared to sponge coke. Honeycomb coke is widely used as a raw material for producing regular power graphite electrodes, carbon products for aluminum electrolysis, and various electrochemical carbon materials.
• Needle coke is obtained from low-sulfur, low-ash aromatic-rich feedstocks with fewer asphaltenes and resins. It is easily recognized by its visible fibrous texture and elongated, elliptical pores aligned in a preferred orientation. When broken, needle coke forms slender particles. Upon graphitization, it yields artificial graphite with low CTE and resistivity, making it the preferred raw material for manufacturing HP graphite electrodes and UHP graphite electrodes.

How to Select High-Quality Petroleum Coke

The quality of petroleum coke largely depends on the nature of its residual oil and the processing conditions under which it is produced. Key quality indicators include:

1. Purity

This refers to the levels of sulfur and ash in petroleum coke. High sulfur content can cause gas expansion during graphitization, leading to cracks in carbon products. Excessive ash inhibits structural crystallization, thereby reducing the performance of carbon materials.

2. Crystallinity

Crystallinity reflects the structural order of coke and the size of the mesophase spheres it contains. Smaller spheres tend to form sponge-like porous structures, while larger spheres form denser fibrous or needle-like structures with superior quality compared to sponge coke. In practice, true density serves as an approximate measure of crystallinity: a higher true density generally indicates better crystallinity.

3. Thermal Shock Resistance

This is the ability of coke-based products to withstand sudden heating to high temperatures or rapid cooling without cracking. Needle coke–based products exhibit excellent thermal shock resistance, which significantly enhances their value. The coefficient of thermal expansion (CTE) is commonly used to evaluate this property: the lower the CTE, the better the resistance to thermal shock.

4. Particle Size Distribution

This refers to the relative proportions of usable coke particles versus fine powders. Most fine powders are generated during coke removal, handling, and storage due to mechanical stress and friction, thus reflecting the mechanical strength of the material. Green coke, after being calcined into mature coke, becomes less prone to breakage. A higher proportion of larger particles and fewer fines indicates superior usability and performance.