PTQ Q4 2022 Issue

aromatic precursors (in-situ) at the onset of carbonisation. To examine this potential, research was con - ducted to thermally treat (400°C) a waxy oil fraction followed by carbonisation (delayed coking). At the onset of carbonisation, the sta- bilised alkanes slowly form cycloalkanes, which rapidly dehydrogenate, forming aromatics pri- marily in the ideal three-to-six ring range. These aromatics result in a highly ordered anisotropic carbon microstructure without the associated naturally acquired limitation of nitrogen or sul - phur heterocycles. The use of GTL-based waxy oil as a needle coke precursor in a DCU is interesting from the following perspective: • Globally, there is an abundance of natural gas and GTL production to produce liquid hydro - carbon fuels, and this could produce heavier

The complete transition away from fossil fuel energy (such as coal and crude oil) towards renewable energy (like solar, wind, and geothermal) to address climactic phe- nomena will occur over the medium to long term. Within this transitional phase, the medium-term outlook favours 'cleaner' fuels with higher energy efficiency, fewer SOx or NOx emissions, and less greenhouse gas impact (such as natural gas). Over this period, the production of liquid hydrocarbon fuels will still be a requirement (although this will slowly diminish over time). Historically, coal-to-liquid (CTL) technology utilises coal gasification (in the presence of oxygen and water) to form synthesis gas (H₂ and CO), which is followed by the Fischer- Tropsch reaction forming largely straight chain alkanes (petrol and diesel) as well as heavier waxes. While this pro - cess also produces chemical intermediates, it is expensive and associated with a large greenhouse gas component. Thus, it is not considered a future prospect, although the formation of liquid fuels using synthesis gas (from an alter - native source like natural gas) may well be attractive. GTL technology utilises steam reforming of natural gas to produce synthesis gas (H₂ and CO). This is followed by the Fischer-Tropsch reaction forming largely straight chain alkanes (petrol and diesel) as well as heavier waxes. Natural gas is abundant and has a far lower greenhouse gas com- ponent. Natural gas has further attracted much attention given its viability as a medium-term hydrogen source. Thus, natural gas will be an important fuel in the transition to renewable energy. From a needle coke standpoint, heavy residual waxy oils (C 20+ ) are attractive given the absence of either nitrogen or sulphur-based organics (and their associated detrimental effects within the needle coke value chain). However, the absence of any notable inherent aromaticity is at first a concern. The inherent aromaticity traditionally associated with needle coke precursors (both petroleum and coal tar) is based on source and/or process variables and cannot be manipulated. The paraffinic nature of waxy oil-type streams is attractive as it offers the potential to create optimal Figure 6 Photomicrograph (Mag. x50) of waxy oil needle coke exhibiting highly ordered flow domains with parallel porosity¹

residual waxy oil streams as a by-product. Natural gas is a cleaner fuel and will be critical within the medium-term energy transition • As waxy oil is a synthetic product, its resulting quality is determined entirely by reaction conditions and not by its source (in contrast to coal tar and petroleum sources). The potential guarantee of a consistent, high purity needle coke precursor would be highly beneficial • The thermal stabilisation (of waxy oil at 400°C) does not create aromaticity. The function thereof is to remove oxygen functionalities (such as hydroxy, aldehyde, or car - boxylic acids). Hydroxy derivatives readily form unstable radicals (forming cross-linked ether bridges), prematurely increasing the viscosity of the incipient mesophase and introducing microstructural disorder. Other reactions involve cracking to produce C1 to C₄ hydrocarbon gasses and stabilised alkanes • At the onset of carbonisation (450°C; delayed coking), the stabilised alkanes slowly form cycloalkanes, which are rapidly dehydrogenated to form aromatics. Usually, the thermal production of highly unstable alkane radicals (dur- ing carbonisation) would promote crosslinking, resulting in disrupted microstructural order. However, the abundance of hydrogen (within cycloalkanes) provides a key source of ‘bay protons,’ which cap these radicals. This induces kinetic stability within the reacting system, allowing the optimal progression of mesophase viscosity. Thus, promoting microstructural order: • The FT catalyst needs to be removed prior to delayed coking to prevent microstructural disorder (as previously discussed) and catalytic polycondensation • A key benefit is the negligible carcinogenic potential of waxy oil. Any aromatisation takes place during delayed coking within a closed system and thus poses no threat • The absence of nitrogen or sulphur in the coke matrix eradicates the detrimental influence of electrode ‘puffing’ and the requirement for puffing inhibitors • Waxy oil-based needle coke produces highly anisotropic carbon devoid of SOx or NOx emissions • Waxy oil coke further offers a comparably high

PTQ Q4 2022