Petroleum coke ( Petroleum coke ) is a product formed by the thermal cracking of heavy oil after separating light and heavy oils from crude oil by distillation. In appearance, it is an irregular, black lump (or granule) of varying sizes with a metallic luster. The coke particles have a porous structure, and their main component is carbon, accounting for 80% or more, 1.5%-8% , the rest being oxygen, nitrogen, sulfur, and metallic elements.

Petroleum coke has unique physical, chemical, and mechanical properties. The non-volatile carbon in the heat-generating portion, volatiles, and mineral impurities (sulfur, metal compounds, water, ash, etc.) determine the chemical properties of the coke. Porosity and density, in terms of physical properties, determine the reactivity and thermophysical properties of the coke. The particle composition, processing method, hardness, abrasion resistance, strength, and other mechanical characteristics determine its mechanical properties.

Physicochemical Properties : The indicators of the physicochemical properties of petroleum coke include ash content, sulfur content, volatile matter, density, porosity, resistivity, thermal expansion coefficient, and mechanical properties.

Ash Content : The main elements in petroleum coke ash are iron, silicon, calcium, aluminum, sodium, and magnesium, as well as small amounts of vanadium, titanium, chromium, nickel, and manganese. , Except for aluminum, magnesium, and calcium, which are elements required for electrolysis, the rest are harmful impurities.

Most of the elements in petroleum coke ash originate from crude oil, while some are added during coking and transportation. Silicon and aluminum exist in crude oil in the form of sand and enter the petroleum coke during the coking process. In addition, if the produced petroleum coke is stored outdoors, the mud and sand on the ground or brought by the wind will also increase the ash content of the petroleum coke. Sodium, calcium, and magnesium are chlorides of these elements from minerals near oil fields that seep into crude oil in the form of aqueous solutions. These elements are related to the geographical location of the well field and the production process of the refinery.

Iron is mainly introduced due to the corrosion of refinery equipment. Sulfur, vanadium, nickel, and titanium are inherent in the high-molecular petroleum heterocyclic chains and exist in petroleum molecules in the form of compounds, so separating these elements is a challenge.

Sulfur Content : Sulfur is one of the impurities that affect the quality of petroleum coke. The sulfur content of petroleum coke depends on the sulfur content of the residual oil. The sulfur content in residual oil is 30%-40% and remains in the petroleum coke. If the high-sulfur residual oil undergoes hydrodesulfurization beforehand to reduce the sulfur content, the resulting petroleum coke will have a correspondingly lower sulfur content.

The sulfur in petroleum coke can be divided into two categories: organic sulfur compounds (sulfides, thiols, sulfonic acids, etc.) and inorganic sulfur compounds (iron sulfide, sulfates). Generally, calcination to 1300 ℃ has little effect on desulfurization. Only by increasing the calcination temperature to 1450 ℃ can a more significant desulfurization effect be achieved. Some sulfides need to be removed at the high temperature of graphitization.

Volatile Matter : The amount of volatile matter in petroleum coke indicates the coking temperature. The coking temperature of kettle coke is higher, reaching 700 ℃, so the volatile matter of kettle coke is lower ( 3%-7% ) (Petroleum coke with low volatile matter is used as fuel, and the combustion speed is particularly slow. Light diesel oil is used for ignition at low temperatures). The coking temperature of delayed coking petroleum coke is only 500 ℃, so the volatile matter is as high as 8%-15% . The volatile matter of petroleum coke produced by delayed coking depends not only on the coking temperature but also on the loading time of the residual oil into the coking tower and the conditions of steam blowing into the coke layer. The volatile matter of coke unloaded from the same tower also varies greatly. For example, the coke at the bottom of the tower has a denser structure, a higher bulk density, and lower volatile matter, while the coke at the top of the tower has a loose structure and much higher volatile matter. The amount of volatile matter in petroleum coke has little effect on the quality of carbon products but does affect the calcination operation. Petroleum coke with high volatile matter is difficult to calcine using general rotary kilns or tank furnaces.

True Density : The true density of petroleum coke after calcination at 1300 ℃ is a major indicator of petroleum coke quality. Generally speaking, the higher the true density after calcination, the easier it is for the coke to graphitize, and the lower the resistivity and thermal expansion coefficient after graphitization. The bulk density of petroleum coke indicates the compactness of the coke structure and is proportional to the mechanical strength. The true density is related to the bulk density of the coke and the particle size of the coke. Resistivity : The resistivity of uncalcined green coke is very high, close to that of an insulator. After calcination, the resistivity drops sharply. The resistivity of petroleum coke is inversely proportional to the calcination temperature. After calcination at 1300 ℃, the resistivity of petroleum coke is reduced to 500 μΩ· m or so.

Thermal Expansion Coefficient : The thermal expansion coefficient of petroleum coke mainly depends on the properties of the residual oil, namely the content of aromatics and asphalt in the residual oil. Residual oil with high aromatics content and low asphalt and resin content produces petroleum coke with a lower thermal expansion coefficient. Needle coke is such petroleum coke. Even for needle coke, there are differences in the thermal expansion coefficient. Needle coke with a lower thermal expansion coefficient should be used to produce large-sized ultra-high-power graphite electrodes and joint blanks.

Mechanical Properties : The mechanical properties of petroleum coke include indicators such as "crushability", brittleness, and wear rate. The "crushability" and brittleness of petroleum coke have certain practical significance in the electrode manufacturing process. Crushability can be evaluated by the size ratio of coke before and after crushing, while brittleness indicates the possibility of coke breaking during transportation and conveying. The rotating drum test method is used to characterize the wear rate of petroleum coke. The wear rate of raw coke is proportional to its volatile matter content and inversely proportional to its bulk density. The wear rate of calcined petroleum coke decreases significantly.

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