October 2024

Refining Technologies

Needle coke and synthetic graphite: Advance performance through technology application

This article looks at the global graphite market and future demand. In turn, it details the author's company's proven delayed coking technology through conventional and two-step processes to produce carbon precursors for synthetic graphite and needle coke for graphite electrodes.

IP: 3.144.106.207

The last two decades of the 21st century have seen an increased focus on the energy transition. Pathways to achieve the transition include switching to sources of energy with zero or low emissions to produce electricity, electrification, and its penetration into all aspects of human activities and, finally, increasing the energy efficiency of existing sources of energy production and consumption. 

Market outlook. The traditional method of steel production from iron ore using blast and basic oxygen furnaces contributes significantly to greenhouse gas (GHG) emissions. Increasing steel demand will exacerbate the industry’s carbon footprint and thus compel steelmakers to reduce their environmental impact by adopting production through electric arc furnaces (EAF) that generate significantly lower GHG. Steel scrap is melted inside EAFs using graphite electrodes to generate a high-current electrical arc. Approximately 30% of world steel production is through EAF, and its share is expected to rise in the long run (FIG. 1). Needle coke, as a specialty coke, is a key ingredient in the manufacture of graphite electrodes. 

FIG. 1. Total world steel production and share of production by EAF. (Source: World Steel Association) 

The electrification of transport systems in the form of electric vehicles (EVs) and hybrid electrical vehicles (HEVs) is already underway. Regulatory policies, combined with a reduction in the cost of batteries and growing consumer support, are boosting sustained growth for further electrification (FIG. 2). This growth is driving increased demand for both natural and artificial graphite with similar qualities as needle coke since both are essential components of EV batteries. 

FIG. 2. World electrical and hybrid car sales.¹ 

The market demand for needle coke—specifically for artificial graphite—is on the rise. Future demand is dependent on many factors, including but not limited to: 

• The health of the world economy and, particularly, China’s economic growth 

• The geopolitical situation and the risk associated with supply chains 

• The development of new materials for power storage applications 

• Additional environmental challenges that will become important with further penetration of electrification in transport and other systems. 

Quality requirements. The properties of the finished product, whether graphite electrode (GE) or anode for power storage applications, are strongly influenced by the characteristics of coke as the starting raw material. To understand the coke quality requirements, it is important to review the process through which the finished products are prepared and their performance requirements. 

The starting raw material for the manufacture of GE is needle coke. FIG. 3 shows the general steps of the manufacturing process. 

The processing steps that are shown in FIG. 3 take a long time (approximately 6 mos) to develop from raw material (needle coke) to the final product (the machined electrode). The only metric for evaluating the needle coke quality is how it will perform as an electrode inside the EAF—this is why needle coke is not just a specification product, but a performance product. 

FIG. 3. GE production steps. 

A similar process, with some differences, applies to the manufacturing of anode material from coke or carbon precursor with similar characteristics as needle coke, as shown in FIG. 4. 

The performance criteria for GE mainly focus on its durability in the harsh environment of the EAF and minimizing its consumption due to side reactions. Generally, GEs are categorized as super-premium, premium, intermediate and regular. Important requirements include: 

FIG. 4. Steps of synthetic graphite production. 

• Low coefficient of thermal expansion (CTE): The electrodes are exposed to severe thermal stresses when charged with high electrical current to generate the arc for melting scraps (FIG. 5). This results in a high-temperature differential of > 2,000°C (> 3,632°F) inside the furnace during the production phase. The induced axial and transverse stresses can compromise the mechanical integrity of the electrode and lead to cracks and failure. The CTE for super-premium coke is less than 0.25 x 10-6 m/m/°C.         FIG. 5. GEs during EAF steel production. 
• Low sulfur and nitrogen content: An irreversible volume expansion occurs during graphitization or the high-temperature baking of electrodes. This phenomenon, known as "puffing," is related to the sulfur and nitrogen contaminants in the coke. When heated during graphitization, these contaminants devolatilize, producing gases such as carbon disulfide (CS2), hydrogen sulfide (H2S) and nitrogen. Puffing can lead to a reduction in the bulk density of GEs and excessive mechanical stress inside the electrode, resulting in both internal and external cracks (FIG. 6). The sulfur content of super-premium coke is < 0.4 wt%. 

FIG. 6. Post graphitization cracks. 

• Low electrical resistivity: GEs must have low electrical resistivity due to the high current required to generate the arc for meting scraps inside the EAFs. High resistivity leads to internal overheating of the electrode, rising thermal stress that is transformed into mechanical stress. Furthermore, high resistivity leads to an increased rate of oxidation reaction between the lateral surface of the electrode and tip loss due to exposure to the furnace atmosphere. 

• Apparent density: The mechanical strength of GE is generally greater when its density is higher. The density is influenced by the basic structural unit formed during the initial stages of the carbonization reaction from the feedstock. The order of the crystalline structure and the porosity of electrodes are important factors affecting the density of the GE. 

The desired characteristics of a typical rechargeable battery consist of high energy density, fast charge capability and adequate cycle life. All three components of a battery—i.e., the cathode, graphite anode and electrolyte—each influence these parameters. The synthetic graphite must possess a suitable crystalline structure, proper particle size and shape, appropriate surface area and chemistry, and extremely high purity. Some of these factors are described below: 

• Soft carbon: The basic structural unit of the carbon precursor for producing synthetic graphite must consist of graphitizable carbon, also known as “soft carbon." The molecular structure of the carbon precursor rearranges, transforming from an amorphous or semi-amorphous structure into a layered crystalline form by prolonged exposure to extremely high temperatures during graphitization. Consequently, the spacing and disorder between adjacent layers decrease while the crystallite size (stacking height of the layers) increases. The larger crystal size and reduced disorder lead to longer battery cycle time and higher specific capacity. The degree of graphitization (DoG) strongly influences specific capacity and is obtained by X-ray diffraction analysis. A higher DoG is directly correlated with specific capacity. Generally, synthetic graphite products with a specific capacity of > 340 mAh/g are suitable for lithium-ion battery anode applications. 

• Morphology/shaping: The particle size and shape influence the final characteristics of the battery cell performance. Optimizing milling and shaping adjusts the average particle size, distribution and morphology to smooth the surface and increase tap density. Optimizing the shape must allow achieving a tapped density of > 1 g/cm3 to achieve high volumetric energy density. 

• First-cycle efficiency: The quantity of lithium ions initially present within the battery is partially consumed to form a protective film on the surface of the anode particles known as the solid-liquid interface (SEI). The formation of the SEI contributes to the first-cycle capacity loss. The target first-cycle efficiency of 95% is preferred. Therefore, it is important to reduce the surface-to-volume ratio of the synthetic graphite particles used in the battery.  

QUALITY FACTORS 

Feedstock quality, carbonization technology, and GE and battery manufacturing technology affect the quality requirements of the final product. The scope of this article is limited to the first two factors. 

It is important to review the process of feedstock carbonization to understand the relationship between feed quality and the thermal conversion (coking) operating parameters. 

Conversion of feedstock into solid coke involves an intermediate phase known as the mesophase, which is neither a liquid nor a solid. The reaction begins with the appearance of small, spherical droplets derived from the basic structural units of the feedstock. These droplets coalesce and grow larger in sizer, eventually forming bulk mesophase with the increase in temperature and time. This is followed by gas evolution, which gives the uniaxial arrangement to the bulk mesophase at the start of solidification to needle coke or carbon precursor. 

Feedstocks. The type and molecular composition of feedstocks influence the quality of coke (TABLE 1). Feedstocks can be divided into two categories: petroleum-derived feedstocks and non-petroleum-derived feedstocks. The first category primarily consists of fluid catalytic cracking (FCC) clarified slurry oil (CSO), thermal tars and pyrolysis heavy oil (tar). The non-petroleum-derived feedstocks are coal tar pitch and solvent-refined coal (SRC). 

 

The molecular structure of feedstocks to produce needle coke and/or synthetic graphite must consist primarily of polynuclear aromatics as a precursor to the formation of graphitizable and crystalline coke. An indirect measure of aromaticity is a high Bureau of Mines Correlation Index (BMCI) or a high specific gravity. 

Suitable feedstocks must contain low quantities of contaminants such as sulfur, nitrogen and transition metals—in particular, nickel and vanadium—to satisfy the performance requirements of high-quality electrodes and synthetic graphite. 

The boiling point range of feedstocks is also an important parameter influencing the quality of coke. Significant quantities of high-boiling components upon the combination reaction produce larger-sized molecules of high molecular weight that precipitate prematurely from the reaction mixture, interrupting the gradual development and coalescence of the mesophase. Moreover, a rapid increase in the molecular weight leads to an increase in the viscosity of the reacting mixture, inhibiting the movement of mesophase spheres to form a highly ordered bulk mesophase. This becomes even more important if asphaltenes make up a significant percentage of the high-boiling components. Light-boiling components, conversely, do not contribute significantly to the carbonization process. 

Feedstock pretreatment to reduce contaminants may be necessary to produce high-quality coke and for flexibility in processing feedstocks from various sources. The author’s company provides a variety of pretreatment technologies: 

• The author’s company’s hydrotreating technology^a to reduce sulfur and nitrogen content 

• Quinoline insoluble (QI) removal for coal tar pitch  

• Mild thermal cracking to increase aromaticity and remove high-boiling components of the feed 

• Solvent de-asphalting to remove heavy components in the feed. 

Coking technology. Delayed coking is used to produce needle coke or carbon precursors. The operating conditions for needle coking units are different from those for fuel-grade delayed cokers. The conditions for the latter include a low coke drum pressure and a low unit recycle rate to minimize the production of coke, which is considered a less valuable product. In contrast, the operation for producing needle coke or carbon precursor is selected under a higher coke drum operating pressure and unit recycle rate. 

Coke drum pressure/recycle rate: High coke drum pressure and the unit recycle rate maximize the coke yield. They also reduce the viscosity of the reacting mixture, facilitating the development of the mesophase that is necessary for the production of a highly ordered coke structure. Higher pressure is also known to affect the quality of needle coke, resulting in GEs with lower CTE. High-pressure operation tends to spread the period for gas evolution, which is responsible for the uniaxial texture during the transition to the solid phase (FIG. 7).  

 

FIG. 7. Solid coke uniaxial orientation. 

Coking temperature: The temperature of the reaction mixture plays a key role in the quality of needle coke. Previous work by the author’s company has shown there is an optimum temperature specific to each feedstock for which the CTE of the corresponding graphite electrode is minimized. FIG. 8 illustrates this observation for a blend of the desulfurized CSO, lube extract and pyrolysis tar that was evaluated at the company’s pilot plant. This observation has been confirmed later by others.²  

 

FIG. 8. The CTE of GE from needle coke produces at different temperatures. 

Residence time: The formation of a highly ordered needle structure depends on adequate time for the growth of the mesophase, its conversion to the bulk mesophase, and the subsequent transition to the solid phase. Therefore, adequate unit cycle time will be required based on the feedstock quality. 

COKING TECHNOLOGY 

The author’s company offers proven delayed coking technology through conventional and two-step processes to produce carbon precursors for synthetic graphite and needle coke for graphite electrodes. Simplified process flows for each are shown in FIG. 9. 

FIG. 9. The author’s company’s delayed coking processes. 

In the conventional process, the feedstock with the recycle stream is heated in the coking heater, and then the heater effluent is introduced to the coke drum. This method of coking imparts the entire energy requirement to the feedstock in a single step. The coking reaction and mesophase development are practically left uncontrolled. In contrast, the two-step coking heats the combined feedstock and recycle stream to a low coking temperature (incipient coking temperature) in the first step. The heater effluent is accepted into the coking drum with adequate residence time for the initiation, formation of the mesophase, and subsequent development of the bulk mesophase. Additional heat is supplied to the content of the coke drum to move the reaction forward toward the formation of a solid phase. This staging of the reaction provides the flexibility to adjust the coking temperature and duration of mesophase development. Furthermore, the rate of transition to the solid phase is controlled. The two-step process allows the achievement of the most optimal operating conditions to produce high-quality premium and super-premium needle coke. The company has licensed units operating in the U.S. and China using the two-step process to produce high-quality needle coke and carbon precursor.  

Pilot capabilities. Many of the performance criteria for electrodes or synthetic graphite cannot be determined theoretically due to the complex nature of the process for their production and the product itself. For example, the puffing tendency can only be gauged when the electrode is graphitized. The same goes for the coefficient of thermal expansion, the degree of graphitization, first cycle efficiency and many other performance requirements. Any project intended to either revamp an existing unit or build a grassroots complex must confirm the feedstock capability for producing the required performance of the end product. Therefore, the first step to be considered must involve pilot testing of the feedstock that allows the manufacture of test electrodes from needle coke and/or the preparation of synthetic graphite from carbon precursor for the assembly of a coin or pouch cell for electrochemical battery testing. 

The author’s company owns and operates pilot testing facilities (FIG. 10) in its research and development centers in Pasadena, Texas, and Richmond, California, with the following pilot plant capabilities: 

• Filtration to remove solid particles 

• Main coker pilot plant with 3-in. and 6-in. coke drums and recycle capability operating round the clock, simulating the actual commercial unit residence time 

• Mini-coker with 1-in. and 1.5-in. coke drums 

• Hydrotreatment to reduce sulfur and nitrogen content 

• Solvent de-asphalting to remove non-crystalline material from the feed 

• Quinoline insoluble removal 

• Carbon test facilities for the manufacture of test electrodes and the preparation of coin cells. 

 

FIG. 10. Main coker pilot plant facility. 

Takeaway. Future challenges that will take center stage include the availability of existing feedstocks and uncovering new sources for the production of specialty coke, improvement in the performance of each component of the batteries, and the reduction of emissions while maintaining increased demand for steel. The journey toward reducing the carbon footprint and enhancing energy efficiency is complex. As the industry advances, the synergy between technology and innovation will drive the progress required to better achieve these goals. 

NOTES 

^a ISOTREATING 

LITERATURE CITED 

1. International Energy Agency (IEA), “Global EV Data Explorer,” 2024, online:  www.iea.org/data-and-statistics/data-tools/global-ev-data-explorer
2. 2 Mochida, I., Y. Korai, Q. F. You and T. Oyama, “Optimum carbonization conditions needed to form needle coke,” Oil & Gas Journal, Vol. 86, No. 18, May, 1988. 

The Author

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