Needle-grade coke is produced from highly aromatic feedstocks with low asphaltenes, sulphur, and ash contents. This coke, with high strength and a low coefficient of thermal expansion, is used for graphite electrodes and lithium batteries (anode material). Needle coke is a scarce product in the refining and by-product coke industry.
Owing to an increase in demand, there is increased focus on maximisation of needle coke production with the right quality from existing processes, expanding the process raw material base and towards the treatment of raw material in the coal carbonising process.
Production of high-quality needle coke requires quality control of the feed stream, targeting high aromaticity, low ash content, low sulphur and nitrogen content, and low CCR content. Indeed, these impurities are detrimental to the growth of coke in needle form. Feedstocks such as FCC unit decant or slurry oil and coke derivate cuts are being considered for maximising needle coke production using existing fuel-grade cokers.
Slurry oil was previously blended and sold as bunker fuel but is no longer feasible with the new marine fuel sulphur specification of 0.5 wt%. Axens’ slurry oil hydrotreatment solution enables maximisation of coker feedstock, which could produce high-performing needle coke. This process boosts refinery economics by converting a low-cost feedstock into a high-value product with minimum hydrogen consumption.
Depending on economics, other opportunities could include using coal tar or thermal tar from VGO thermal cracking. Aromatic extracts from lube oil solvent refining are another possible feedstock, even though they are of inferior quality compared to slurry oil.
Coker feed pretreatment objectives include minimising ash, sulphur, CCR, and other coker feed contaminants while maximising aromatics retention. These targets emphasise the experiential importance of designing the feed pretreatment section. For instance, reducing the ash content in FCC decant oil requires a clarifier or similar equipment (filter, centrifuge). Additionally, optimisation of operating conditions (reactor temperature and operating pressure) and catalyst selection for this feed pretreatment section are equally important in maximising hydrodesulphurisation (HDS) while minimising aromatic saturation (HDA).
Converting fuel-grade cokers towards needle coke production requires extensive experience with regard to optimising coker feed pretreatment.
Against this backdrop, the slurry hydrotreating process meets the targets with regards to HDS while minimising HDA (at minimum hydrogen consumption). The proprietary Hyvahl process with the Permutable Reactor System (PRS) maximises the main hydrotreating catalyst’s cycle length while managing the pressure drop using PRS.
All these solutions are already in operation for targeting high-quality needle coke.