PTQ Q4 2022 Issue

Needle coke – overcoming quality challenges in a resurgent market The quality of needle coke precursors as a function of their origin and/or process is discussed against a backdrop of the drive to enhance quality, purity, and consistency

Marcio Wagner da Silva Petrobras John Clark Coke Consulting Company

T he needle coke market is in the process of a demand resurgence based both on Chinese demand for graphite electrodes and additional diversification into lithium ion battery anodes, driven by the market expansion of electric vehicles (EVs). This demand will likely show sus- tained growth for the foreseeable future with associated pricing support over the period. There is a specific demand for ultra-premium, high purity needle coke from both applications, driven by a require- ment to derive maximum in-situ performance from these materials. The two main ultra-premium needle coke pre- cursors include high-temperature coal tar pitch (HT-CTP) and fluidised catalytic cracker decant oil (FCCDO), the latter enjoying market dominance (80%). Quality issues associated with HT-CTP relate to techni- cal issues stemming from process origin, environmental concerns (carcinogenetic and emissions), and inconsistent quality. Quality issues relating to FCCDO are generally associated with heavier, higher sulphur crude oils and the competition (by the bunker fuel market) for sweeter crude oil downstream derivatives. The search for consistent quality, high purity needle cokes may in future broaden the scope of potential precur- sors in line with the trend towards cleaner fuel technology (such as natural gas) over the medium-term period at least. Utilising natural gas as a source, gas-to-liquid (GTL) tech- nology offers the automotive industry a cleaner source of liquid hydrocarbon fuels. With the benefit of these fuels being synthetic, their quality is less dependent on origin (as is much the case with crude oil). This not only relates to the lighter automotive fuels, but additionally applies to the purity of the heavier residual waxy oil (C 20+ ) streams. The lack of inherent contamination (thermally stable nitrogen and sulphur heterocycles, which have a detrimental effect on needle coke quality) makes this fraction an intriguing prospect. However, the additional lack of any inherent aromaticity would seem to be in con- trast with historically desirable characteristics associated with needle coke precursors. What is needle coke? Needle coke is produced in a delayed coker from aromatic petroleum or coal tar heavy residues. They generally form as highly crystalline graphene-like carbons, exhibiting

long-range microstructural order with minimal impurities (sulphur, nitrogen, and metals) and a low coefficient of thermal expansion (CTE). Historically, needle cokes have been used to produce graphite electrodes for steel smelt- ing in an electric arc furnace (EAF). The advent of EVs has additionally diversified the scope of application for needle coke (inclusion in graphite anodes of lithium ion batteries). Additionally, the needle coke market has utilised low sulphur vacuum residues (LSVR) and ethylene tar pitches (ETP) precursors, although they have mostly been associ- ated with lower quality needle coke grades. Solvent refined coals (SRC) have been trialled at a pilot plant scale, but there is no evidence of any sustained commercial produc- tion. However, the two dominant ultra-premium needle coke precursors stem from the petroleum industry (FCCDO) and the coal blast furnace industry (HT-CTP). These heavy liquid residuals are converted to solid coke and cracked distillates in a delayed coker unit (DCU) in the temperature range of 450-500°C (dependent on the ther- mal stability of the feedstock). Unlike other coke types, nee- dle coke microstructural and crystalline order is particularly important. The aromaticity associated with most needle coke feedstocks infers a degree of thermal stability. During polycondensation of higher molecular weight rad - icals, they form an intermediate phase (mesophase). The longer the mesophase remains within an optimal viscos- ity range, the greater the propensity to develop long-range microstructural order (sometimes referred to as the carbon 'blueprint'). The structural order on a micro-scale (10-6) translates to the crystalline scale (10-10). The crystalline order is that of graphene (sp2 hybridised orbitals) and essentially exists as layers of covalently bonded benzene sheets separated by interplanar spaces. The microstruc- tural and crystalline order determine the electronic and CTE characteristics of both needle coke and consequently graphite electrodes. Needle coke quality is generally classified into three qual - ity grades (ultra premium, super premium and intermedi- ate premium), the specifications of which are presented in Table 1 . The quality ascribed to needle coke grades is generally characterised by differences in microstructural order, crys- talline order, CTE, and impurities. Higher quality needle coke grades are usually associated with larger diameter graphite

PTQ Q4 2022