Considerations for repurposing a fuel-grade coker into a needle coker
How a delayed coking unit originally designed and operated to produce fuel-grade coke could be repurposed to produce needle coke.
Karen Cais and Srini Srivatsan
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A comprehensive overview of needle coke production, markets, key operating parameters, feedstock characteristics, and processing steps demonstrates needle coke production opportunities for meeting the demand for synthetic graphite used in lithium-ion batteries in the manufacture of anodes, thereby enabling a pathway for a sustainable future.
Needle coke, as shown in Figure 1, is a specialty market held by half a dozen or so manufacturers with a production of ~2.1 MMTPA and is currently valued at ~$4 billion. However, this market is expected to grow significantly in the coming years due to the rising global demand for synthetic graphite made from needle coke and used in the manufacture of lithium-ion batteries for electric vehicles (EVs).
Delayed coking units (commonly known as DCUs or cokers) are designed to maximise refinery yields of transportation fuels by thermally cracking the heavy residue feed into lighter products and making fuel-grade petroleum coke as a by-product.
These units have been underutilised due to reduced global demand for transport fuels since the start of 2020. An attractive option for the refiner is to consider repurposing the unit for needle coking operation. According to one market analysis, the forecasted needle coke market is expected to grow from the current $4 billion to over $6.5 billion by the end of this decade.¹
What is needle coke?
Needle coke is a carbonaceous material of defined, anisotropic structure. When crushed, the coke particles are shaped like needles because of the crystalline structure. Historically, needle coke has been used in the manufacture of graphite electrodes used in the steel industry’s electric arc furnaces (EAF). However, recent uses include the production of synthetic graphite for lithium-ion battery anode material in electric vehicles EVs. Needle coke is also used to manufacture electrodes for electric double layer capacitors (EDLC) as an auxiliary source of power in EVs. Depending on the properties, needle coke is classified into various grades: regular, premium, and super or ultra-premium.
Compared to other grades of coke, such as fuel or anode, the market price of premium needle coke is on average 10 to 20 times higher, with recent price spikes being 40 times higher than fuel-grade coke to satisfy the growing demand for li-ion battery manufacture. The difference between needle and other grades of coke is the stringent quality specifications required for electrode manufacturing and the fact that only a small quantity of needle coke produced meets or exceeds the quality specifications for the ultra-premium grade. A thorough understanding of the relationship between feedstock characteristics, processing conditions, and needle coke properties is required to determine the potential of the feedstock required to make needle coke and define the optimum operating conditions of the coker.
As opposed to fuel-grade or anode-grade coke, needle coke is anisotropic in that the properties of the sample when taken in one direction are different from a sample taken perpendicular to it. The anisotropic, crystalline structure of needle coke imparts a lower electrical resistivity and lower coefficient of thermal expansion (CTE) relative to regular cokes. In addition to CTE, other properties that can affect the performance of a graphite electrode are electrical resistivity, coke hardness, sulphur content, and nitrogen content. Table 1 provides typical needle coke properties.
The base coker unit configuration for a needle coking operation is similar to that of fuel-grade coke. Unlike traditional fuel-grade cokers, whose goal is to maximise liquid product yield (minimise coke product yield), high-quality needle coke and maximum coke yield are the primary goals during needle coke production. The processing capacity for needle coking units is typically much less than fuel-grade coking units due to feedstock availability and operating conditions.
Needle coke quality is achieved through mesophase management controlled by feedstock quality and operating conditions. Feedstocks promoting the production of needle coke are aromatic tars with low sulphur and metals content such as decant oils, thermal tars, or other multi-ring aromatics with short aliphatic side chains.
In general, a highly aromatic feedstock must be used to produce premium-quality, readily graphitisable needle coke. The feedstock must also be low in sulphur and nitrogen to avoid ‘puffing’ during the electrode manufacturing process. Puffing causes internal cracks in the electrode and significantly reduces its strength.
The feedstock must also be low in ash to avoid interference with the growth and coalescence of the mesophase required to achieve the anisotropic structure of needle coke. In some cases, pretreatment processing may be necessary to achieve the ultimate feedstock required. However, having the right feedstock does not guarantee a high-quality needle coke since the quality is also a function of the processing path and proper selection of operating conditions. Typically, pilot plant tests are conducted to confirm the feedstock and operating conditions required for needle coke production. Table 2 shows typical needle coke feedstock properties.
Temperature, pressure, recycle rate, coking time, and post-treatment affect the quality of needle coke. The most important of these conditions is temperature since this ultimately determines the reaction rate and viscosity of the reacting medium. The optimum temperature for needle coke formation depends on the feedstock used and is usually determined by pilot testing. Too high or too low temperature would result in an isotropic coke.
Needle cokers are normally operated at higher coke drum pressures, higher coil outlet temperatures, higher recycle rates, longer coking times, and lower superficial velocities than fuel or sponge/anode cokers. The operating parameters are set to increase the fluidity and hence reaction time of the mesophase while increasing the yield of needle coke.
Higher recycle also leads to higher coke density and improved coke quality (i.e., lower contaminants due to dilution by increased coke make and lower CTE). However, excessively high pressures and recycle rates are not recommended because of diminishing returns, and the additional quantity of light material retained in the drum at these conditions can detrimentally affect the final coke properties. Table 3 compares typical DCU operating conditions for the various coke types.
General processing steps for quality needle coke production
For some feedstocks, especially ones derived from coal tar pitch, pretreatment of the feedstock may be necessary for needle coke production. For example, solids or quinoline insoluble (QI) must be removed by filtration or solvent extraction as these components hinder mesophase coalescing. Distillation of the feedstock may also be required to remove the light-end components and optimise the distillate cut/quality to be used as feedstock. For feedstocks high in sulphur and nitrogen, hydrotreating will also be required to reduce sulphur and nitrogen content. In some cases, the feedstock after hydrotreating may have to undergo a thermal pretreatment step to recover the aromaticity lost by hydrotreating. A preliminary schematic for needle coke production is shown in Figure 2.
There are four distinct steps in the transition of isotropic liquid feedstock to anisotropic graphitisable carbon:
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