Petroleum coke is a manufactured carbon product produced at a limited number of oil refineries. Petroleum coke results from the thermal processing of residual oil, which has been cracked or otherwise processed to remove low boiling fractions.
Coke is made by heat-treating the residual oil (more accurately described as tar) to a temperature high enough to result in its polymerization to form a non-melting solid carbon. The process is performed to maximize the yield of lower molecular weight compounds derived from crude oil feedstocks. Coke, as it is removed from the coking process, is referred to as green coke. Green petroleum coke contains approximately 15-20% residual hydrocarbon materials. These hydrocarbons are those compounds that do not polymerize in the coking process and cannot be removed from the coke substrate due to process limitations.
Calcined petroleum coke (CPC) is manufactured by heating green coke to approximately 1300-1400 C in a rotary kiln. This affects the removal of virtually all residual hydrocarbons and moisture. The final calcined product contains only a trace of volatile matter, and 0.3-6% sulfur, depending on the petroleum base used to make the coke. Calcined and green petroleum cokes are inherently low in ash constituents. The fixed carbon content of most calcined petroleum coke products ranges from approximately 97-99.5%.
Petroleum coke is used in applications where high quality non-graphitic carbons are required. Some applications for petroleum coke include foundry products, wear moderators for PTFE compounds, rubber compounds, reducing reagents, ceramic packing media, oxygen exclusion from molten materials, manufactured carbon shapes, alloys, cathodic backfill, drilling additives, case hardening, seals, mechanical carbons, and flooring.
Metallurgical coke, also known as Met coke, is a carbon material manufactured by the destructive distillation of various blends of bituminous coal. Bituminous coal is a soft, medium-grade coal that contains a high percentage of volatile components. Destructive distillation is performed in coke batteries, which are banks of large enclosed kilns.
Once the kilns are loaded, they are heated to approximately 1000 C in the absence of air. During the heat cycle the volatile components of the coal are released and the solid coal goes through a partial melt and subsequent re-solidification to a non-melting carbon. Volatile components include coal tar, ammonia, and literally dozens of other products of decomposition. Most volatile components are reclaimed or recycled. The final solid is a non-melting carbon called Met coke.
As a result of the loss of volatile gasses and of partial melting, Met coke has an open, porous morphology and may appear glassy in some specimens. As a result of the heat treatment process, Met coke has a very low volatile content. However, the ash constituents that were part of the original bituminous coal feedstock remain encapsulated in the resultant coke.
Met coke feedstocks are available in a wide range of sizes from fine powder to basketball-sized lumps. Typical purities range from 88-92% fixed carbon. Commercial grades are available in sizes from 30-micrometer powders up to 20 cm lumps.
Metallurgical coke is used where a high quality, tough, resilient, wearing carbon is required. Applications include but are not limited to conductive flooring, friction materials, foundry coatings, foundry carbon raiser, corrosion materials, drilling applications, reducing agents, heat treatment, ceramic packing media, electrolytic processes, and oxygen exclusion.
Pitch coke is a high purity carbon residue manufactured by the destructive distillation (coking) of coal tar pitch. Coal tar pitch is a by-product resulting from the thermal distillation of bituminous coal. Because it is condensed from the gas phase, coal tar pitch is inherently low in ash and other mineral impurities. Therefore, the resultant coke is low in impurities.
Coal tar pitch is a highly aromatic feedstock and results in the formation of a coke that is highly graphitizable. Pitch coke is also inherently low in sulfur and has relatively high thermal conductivity. Asbury’s pitch coke is a unique solid carbon that has a wide range of potential uses. Applications that call for calcined petroleum coke would benefit from its higher purity, low sulfur, and vanadium.
Calcined Needle Coke
Calcined needle coke is a variety of petroleum coke (see section on petroleum coke for a basic introduction to PC manufacturing). The term “needle” is used to describe the acicular morphology of the coke since it tends to form oriented needle-like structures that are clearly visible to the naked eye. This unique morphology is a reflection of the high degree of liquid crystal alignment that occurs during the coking process. The formation of highly oriented morphology is consistent with a coke that is manufactured from feedstock that has a high aromatic character.
Needle coke is a highly crystalline form of carbon and can be viewed as being similar to certain crystalline polymers. In this case the “polymer” system is the coke “backbone” that consists of layers of sp2 hybridized carbon atoms in hexagonal ring structures. A high level of two-dimensional orientations exists in these structures, but they are not well oriented in the “c” axis direction. If heated to graphitization temperatures, “c” axis orientation is affected resulting in the highest quality synthetic graphite.
Calcined needle coke is typically higher in carbon and lower in ash constituents, such as sulfur and metals, than standard calcined petroleum coke. In some respects it may offer unique friction and wear properties due to its well developed layer structure. Applications where higher purity, “softer” calcined petroleum coke is required may benefit from utilization of this material.