Needle Coke - Rising to the Quality Challenge of a Resurgent Market

Dr. Marcio Wagner da Silva and Dr. John Clark

Introduction

 The 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 (EV’s). This demand is likely to show sustained 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 aforementioned applications which is driven by a requirement to derive maximum in-situ performance from these materials. The two main Ultra Premium needle coke precursors 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 technical issues stemming from process origin, environmental concerns (carcinogenetic and emissions), and inconsistent quality. Quality issues relating to FCCDO are generally associated with heavier, higher sulfur 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 the future broaden the scope of potential precursors in line with the trend towards cleaner fuel technology (e.g. natural gas) over the medium-term period at least. Utilizing natural gas a source, Gas to Liquid (GTL) technology 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 (C20+) streams. The lack of inherent contamination (thermally stable nitrogen and sulfur 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 contrast with historically desirable characteristics associated with needle coke precursors.

This article discusses forward looking concerns associated with the quality of needle coke precursors as a function of their origin and or process, especially in light of the drive to enhance quality, purity, and consistency to enhance in-situ performance. Embracing cleaner fuels technology (GTL) and the unlikely potential opportunity it affords the needle coke industry is also discussed. 

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 (sulfur, nitrogen, and metals) and a low Coefficient of Thermal Expansion (CTE). Historically needle cokes have been used to produce graphite electrodes for steel smelting in an Electric Arc Furnace (EAF). The advent of Electric Vehicles (EV’s) has additionally diversified the scope of application for needle coke (inclusion in graphite anodes of lithium-ion batteries). 

Additionally, the needle coke market has utilized Low Sulphur Vacuum Residues (LSVR) and Ethylene Tar Pitches (ETP) precursors, although they have mostly been associated with lower quality needle coke grades. Solvent Refined Coals (SRC) have been trialed at a pilot plant scale but there is no evidence of any sustained commercial production. 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 (dependant on the thermal stability of the feedstock). Unlike other coke types, needle coke microstructural and crystalline order is particularly important. The aromaticity associated with most needle coke feedstocks infers a degree of thermal stability. During the polycondensation of higher molecular weight radicals, they form an intermediate phase (mesophase). The longer the mesophase remains within an optimal viscosity 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 hybridized orbitals) and essentially exists as layers of covalently bonded benzene sheets separated by interplanar spaces. The microstructural 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 quality grades (UltraPremium, Super Premium, and Intermediate Premium), the specifications of which are presented in Table 1. 

The quality ascribed to needle coke grades is generally characterized by differences in microstructural order, crystalline order, CTE, and impurities. Higher quality needle coke grades are usually associated with larger diameter graphite electrodes. Petroleum based needle cokes have historically dominated the market, however, all recent expansion initiatives in China are based on coal-tar needle cokes (which have historically been plagued by the inconstant quality and problematic electrode graphitization). A diagram showing highly ordered needle coke microstructure with parallel aligned porosity is shown in Figure 1.

Figure 1 - Diagram of the microstructure of highly ordered needle coke (Mochida et al ed, 1994).

Being a specialty and derived from a by-product, needle coke exhibits a concentrated supply structure. There are around 10 major producers globally while most refineries do not have delayed cokers (especially being those suitable for needle coke production) or calciners. Following delayed coking (450°C-480°C) the green needle coke is calcined (1350°C) to reduce volatiles and promote microstructural densification.

The science associated with the needle coke value chain is complex and beyond the scope of this discussion which merely serves to introduce the topic. 

Needle Coke - A Resurgent Market

Over the last decade, the appetite for needle coke has diminished given the abundance of scrap steel emanating from China. This demand decline has exerted downwards pressure on needle coke pricing to under $1000 per tonne. However, in the two years to the end of the decade (2018-2020), there has been a resurgence in the demand for needle coke and associated pricing, which although sluggish has rebounded with sustained growth. The demand for needle coke is witnessed by the substantial ramp-up of Chinese EAF capacity.

The demand has further been impacted by market diversification based on the exponential growth of the EV automotive sector (needle coke is utilized for the production of synthetic graphite anodes in lithium-ion batteries). Competition as an EV battery graphite source is provided by natural graphite which while commanding a lower price is also associated with lower comparative quality (higher natural impurities). The sustained requirement of graphite for the EV market is demonstrated by the fact that the Tesla Model S contains up to 85 kg of graphite. While the growth of the EV market has been exponential, it has been off a small base. The growth of the EV market is expected to command a 30% automotive market share by 2030. To demonstrate the expected impact of this sector (on needle coke demand), a six percent EV growth would relate to a 250kilo-tonne per annum (ktpa) demand increase. The comparative demand from the graphite electrode market has historically remained stable at approximately one million tpa.

Competition for the battery market between natural and synthetic (needle coke based) graphite will depend on a variety of factors. Compared to synthetic graphite, natural graphite is:

·        More abundant with a simplified mining process flow

·        The inherent ash content is comparatively high requiring substantial floatation and purification

·        The effect of purification to remove impurities may leave voids affecting both microstructural order and density

·        Natural graphite purity is linked to the price which has historically been comparatively lower

·        Purified natural graphite exhibits a larger BET surface area which may be associated with the removal of impurities. While this may beneficial especially in light of battery applications, both the impurity themselves and their removal introduce crystalline imperfections limiting electronic performance characteristics. 

One of the determining choice factors between natural or synthetic (coal-tar or petroleum based) graphite is the cost of purification steps required to enhance the purity and thus operational performance. The majority of these contaminants (e.g. nitrogen, sulfur, ash, and semi-carbonaceous MIQ) find their origin at the source of the value chain. The majority of contaminants act as physical occlusions leading to crystalline imperfections and microstructural defects during the phase transition between liquid and solid hydrocarbons (during delayed coking). After this, further thermal treatment or purification processes only serve to amplify theses imperfections and translate to restricted electronic properties and an elevated CTE. Finding a graphite precursor which naturally excludes these contaminants would produce a higher purity needle coke, improved performance with lower process costs. 

The Coal-Tar Needle Coke Route

Needle coke produced from the coal-tar pitch finds its origin as a by-product of destructive distillation (during the reduction of iron ore in a blast furnace). The severe thermal environment of the blast furnace (temperatures in excess of 1000°C) produces stable Poly Aromatic Hydrocarbons (PAH) and aromatic heterocycles (mainly nitrogen-based). The concentration of pure aromatics in HT-CTP is enhanced as a function of the associated thermal kinetics which destroys any alkyl or heteroatom (hydroxy, thiol, or amide) side chains to the aromatic. Apart from needle coke production, HT-CTP is used in the aluminum sector (as a binder pitch for the production of carbon anodes and cathodes) and in steel smelting (as a binder and impregnation pitch for the production of graphite electrodes).

The use of HT-CTPas a needle precursor in a Delayed Coker Unit (DCU) is determined by a multitude of factors:

·        The high aromaticity (especially in the two-ring [naphthalene] to seven rings [coronene] range) are ideal molecular (needle coke) precursors as their thermal stability allows for optimal mesophase development resulting in a highly ordered coke microstructure

·        In contrast, the high aromaticity has a distinctive disadvantage given its eco-toxicological carcinogenic potential. Apart from quality factors, this attribute may in the future deter substantial expansion of this route especially when associated with production systems which are not closed

·        The high temperature associated with HT-CTP production forms semi-carbonaceous solids (called primary MIQ) which (if not removed using filtration) presents an inert physical barrier introducing crystalline imperfections in the graphene layers. These crystal imperfections disrupt the microstructural order. Any microstructural disorder limits the electronic properties and increases the CTE of the needle coke

·        The prevalence of stable nitrogen heterocycles form three dimensional structures providing steric hinderance (also known as ONS Obstruction) during mesophase development and disrupt the formation of an ordered microstructure.

·        The thermal stability of nitrogen based aromatic heterocycles (including carbazole, pyridine, and quinoline based derivatives) in HT-CTP result in their incorporation within the coke matrix, even surviving calcination (1350°C). However, within the electrode graphitization cycle (between 1500°C and 1800°C) nitrogen dissociates causing irreversible volumetric expansion ‘puffing’, potentially triggering electrode cracks. While the incorporation of inhibitors (e.g.borates) can reduce nitrogen puffing, this science is comparatively less developed (than sulfur puffing) as the majority of needle cokes are petroleum based. Graphitization of coal-tar (based needle coke) electrodes is energy intensive given the long residence times required to achieve a slow temperature increase without electrode cracking

·        The comparatively higher density of HT-CTPis beneficial in terms of coke yield

·        The geographical production of HT-CTP based needle coke may further be associated with large coal resources (e.g. China)

While theoretically (at least based on aromaticity), HT-CTP should produce the highest quality needle coke, in practice its quality is inconsistent and electrode graphitization is expensive. The high carcinogenic potential of HT-CTP is a further deterrent. Based on published reports (Mackenzie, W 2019), the expansion of Chinese capacity is based on coal-tar needle coke alone.

Future quality-based initiatives relate to purity (semi-carbonaceous or metals contaminants) to ensure consistency. The graphitization of electrodes with high nitrogen content is problematic and expensive although based on the above-mentioned expansion initiatives processes to limit graphitization firing times and reduce puffing would be realistic. In an ironic twist, the desirability of HT-CTP(especially the concentration of pure aromatics creating well established microstructural order), may in contrast act as a deterrent when considering the potential carcinogenic impact.

The Petroleum Needle Coke Route

The production of Ultra Premium needle cokes based on heavy petroleum residuals is limited to FCCDO residuals as a precursor. Fluidized Catalytic Cracking (FCC) is a carbon rejection conversion technology using heavier residuals to produce lower molecular weight distillates. While FCC process conditions are not usually altered to enhance residual (FCCDO) quality it may be utilized as a bunker fuel viscosity cutter stock or as a needle coke precursor.

Fluid Catalytic Cracking (FCC) is one of the main processes which give higher operational flexibility and profitability to refiners. The catalytic cracking process was widely studied over the last decades and became the principal and most employed process dedicated to converting heavy oil fractions in higher economic value streams.

The installation of catalytic cracking units allows the refiners to process heavier crude oils and consequently cheaper, raising the refining margin, mainly in higher crude oil prices scenario or in geopolitics crises that can become difficult the access to light oils. The typical Catalytic Cracking Unit feedstream is gas oils from the vacuum distillation process. However, some variations are found in some refineries, like sending heavy coke naphtha, coke gas oils, and deasphalted oils from deasphalting units to processing in the FCC unit.

The catalyst normally employed in fluid catalytic cracking units is a solid constituted by small particles of alumina (Al2O3) and silica (SiO2) (zeolite). By the catalyst characteristics and the operational conditions in the catalytic cracking process (temperature higher than 500 °C), the process is inefficient to cracking aromatic compounds, therefore, how much more paraffinic is the feedstream, the higher is the unit conversion. Figure 2 presents a process scheme for a typical Fluid Catalytic Cracking Unit (FCCU). 

In a conventional scheme, the catalyst regeneration process consists of the carbon partial burning deposited over the catalyst, according to the chemical reaction below:

C + ? O2 → CO

The carbon monoxide is burned in a boiler capable of generating higher pressure steam that supplies other process units in the refinery. 

Figure 2 – Schematic Process Flow for a Typical Fluid Catalytic Cracking Process Unit (FCCU).

The principal operational variables in a fluid catalytic cracking unit are reaction temperature, normally considered the temperature in the top of the reactor (called riser), feed stream temperature, feed stream quality (mainly carbon residue), feed stream flow rate, and catalyst quality. Feedstock quality is especially relevant, but this variable is a function of the crude oil processed by the refinery, so is difficult can be changed, but for example, aromatic feedstock’s with high metals content are refractory to cracking and conducting to quick catalyst deactivation. 

An important variation of the fluid catalytic cracking technology is the residue fluid catalytic cracking unit (RFCC). In this case, the feedstock to the process is basically residue from the atmospheric distillation column, due to the high carbon residue and contaminants (metals, sulfur, nitrogen, etc.) are necessary some adaptations in the unit like a catalyst with higher resistance to metals and nitrogen and catalyst coolers furthermore, it’s necessary to apply materials with most noble metallurgy due to the higher temperatures reached in the catalyst regeneration step (due to the higher coke quantity deposited on the catalyst), that raises significantly the capital investment to the unit installation. Nitrogen is a strong contaminant to the FCC catalyst because they neutralize the acid sites of the catalyst which are responsible for the cracking reactions.  

When the residue has high contaminant content, is common the feed stream treatment in hydrotreating units to reduce the metals and heteroatoms concentration to protect the FCC catalyst.

Typically, the average yield in fluid catalytic cracking units is 55% in volume in cracked naphtha and 30 % in LPG. Figure 3 presents a scheme for the main fractionator of the FCC unit with the principal product streams.  

Figure 3 – Main Fractionator Scheme for a Typical Fluid Catalytic Cracking Unit.

The decanted oil stream contains heavier products and has high aromatic content, is common that this product is contaminated with catalyst fines and normally this stream is directed to use as fuel oil diluent, but in some refineries, this stream can be used to produce black carbon.

Light Cycle Oil (LCO) has a distillation range close to diesel and normally this stream is directed to treatment in severe hydrotreating units (due to the high aromaticity), after this treatment the LCO is sent to the refinery diesel pool.

Heavy cracked naphtha is normally directed to the refinery gasoline pool, however, in scenarios where the objective is to raise the production of middle distillates, this stream can be sent to hydrotreating units for further diesel production.

Refiners focused to produce high-quality needle coke tend to operate their FCC units optimized to maximize the Decanted Oil (DO) due to the high aromaticity. This is the less common operation mode in FCC units, where the main objective is to maximize the yield of decanted oil and achieve the quality requirements of aromatic residue. The aromatic residue is normally applied to produce black carbon, these derivative presents great demand in some markets.

The main difficulty to comply with the aromatic residue specification is regarding ash content in the decanted oil. This parameter is strictly related to the cyclones efficiency in the catalyst regeneration section, to achieve this objective some refiners apply additives to promote de ash decantation in the final tanks or specific filtration systems that require more capital spending.

Another key quality parameter to meet aromatic residue specification is the BMCI (Bureau of Mines Correlation Index) that is related to the aromaticity of the decanted oil, to achieve the current specifications of black carbon it’s necessary to achieve a minimum BMCI higher than 120. The BMCI is calculated based on the viscosity of the decanted oil at the temperature of 210 °F. The metals content in the decanted oil needs to be also controlled, especially, sodium, aluminum, and silicon.

The operating severity in maximum aromatic residue mode tends to be high with high TRX, high catalyst/Oil ratio, and high catalyst activity. As a side effect is observed the raise of octane number in cracked naphtha due to the incorporation of aromatics compounds in this intermediate.

In maximum aromatic residue operation mode, the main restrictions are the temperature of the bottom section in the main fractionators that can lead to coke formation, metallurgic limitations in the hot sections as well as the capacity of blowers and cold area compressors.

The use of FCCDO as a needle precursor in a Delayed Coker Unit (DCU) is determined by a multitude of factors:

·        The relatively high temperature (approximately 700°C) of the FCC reaction promotes aromaticity (although not to the same degree as HT-CTP). Thus long chain alkyl functions may persist on the aromatic ring structure leading to steric hindrance during polycondensation and a reduction of microstructural order during delayed coking

·        As a carbon rejection conversion process, FCC is dehydrogenative. While this favors aromatization, it may additionally lead to the production of olefins or conjugated dienes which are known to promote crosslinking during delayed coking leading to microstructural defects and a higher coke CTE. However, olefin formation is usually associated with lighter distillates and its effect on heavier residual molecules may not be so profound

·        The reference to Vanadium (V) and Nickel (Ni) impurities in Table 1 would (based on an initial assessment) seem out of place. However, it holds considerable value as both Ni and V are bound as organo-metallics (and not inorganically as with other metallic contaminants). Thus Ni and V pyphorins cannot be extracted using traditional filtration methods. The only unintended (and undesirable) refinery reaction to reduce the Ni / V content is through hydrocracking (when Ni and V deposit on the Nickel-Molybdenum fixed bed catalyst reducing activity)

·        The formation of coke on the fluidized bed FCC catalyst causes significant deactivation and is an undesirable reaction. However, it does serve to initiate polycondensation of thermally reactive species (asphaltenes and unstable hydrocarbons) forming coke on the catalyst. It thus leaves a thermally stable residual (FCCDO) which is highly desirable during the coking reaction resulting in greater microstructural order

·        The prevalence of sulfur heterocycles may form three dimensional structures providing steric hinderance (also known as ONS Obstruction) disrupting the formation of an ordered microstructure

·        The thermal stability of sulfur heterocycles results in their incorporation in the coke matrix, even surviving calcination (1350°C). However, within the electrode graphitization cycle (between 1500°C and 1800°C), sulfur dissociates (CS2) causing irreversible volumetric expansion potentially causing electrode cracks due to the puffing reaction. The market dominance of petroleum-based needle cokes has meant that the study of sulfur puffing inhibitors (e.g. borates, iron oxides, and sodium carbonates) has received greater attention

·        A potential concern with FCCDO is the incorporation of the catalyst (alumina-silicates) in the DCU feed. As with other metal complexes they can disrupt the crystalline structure by forming obstruction to microstructural order and resulting in an increased CTE

Petroleum based needle cokes are typically associated with lighter, sweeter crude oil. Refineries are increasingly having to process cheaper, heavier, and higher sulfur crudes. This substantially increases the concentration of thermally stable sulfur heterocycles presenting both in the FCCDO and ultimately in the coke (eventually contributing to increased electrode puffing). However, perhaps more of an immediate impact is the competition for low sulfur crude oil derivatives from the bunker fuel market. From 1 January 2020, the International Maritime Organization (IMO) requires ships to reduce their SOx emissions. Thus in accordance, the sulfur content of marine fuels has been reduced from a 3.5% max to 0.5% max. This will result in substantial refinery reforms away from Heavy Fuel Oil (HFO) towards lower sulfur lighter fuels (e.g. Marine Gas Oil [MGO] or Very Low Sulphur Fuel Oil [LSSFO]) at least over the medium term. This will lead to increased competition for the downstream refinery products associated with lighter sweeter crude oils, increasing the value of these residuals.

Future quality assurance measures will very much relate to the nature of the crude oil and development of the FCC process to further ensure consistent precursor characteristics.

Due to the necessity to comply with the IMO 2020, refiners with low bottom barrel conversion capacity are looking for middle API crude oils with low sulfur content, capable to produce the new marine fuel oil (Bunker) through the simple blending of bottom barrel streams with dilutants like hydrotreated diesel, as presented in Figure 4 the availability of this kind of crude is limited.

Figure 4 – Availability of Low Sulfur Atmospheric Residue (Source: McKinsey Energy Insights' Global Downstream Model)

The low availability of low sulfur crudes to comply with IMO 2020 tends to create a competition by refiners focused to produce needle coke and refiners looking to comply with the IMO 2020 in a cheaper way than deep bottom barrel conversion units. This competition relies on the price gap between the HSFO (High Sulfur Fuel Oil) and LSFO (Low Sulfur Fuel Oil) as well as the price differential between the needle coke and LSFO. In this business environment, can be expected to raise prices of low sulfur crudes and a competitive advantage to refiners with high complexity refining hardware.

Alternative Options - The GTL Route

One of the most promising and well-developed technologies currently is the conversion of syngas (CO + H2) in longer-chain hydrocarbons such as gasoline and other liquid fuel products, known as Gas to Liquids Technologies (GTL). The liquid hydrocarbons production can be carried out by direct syngas conversion, in Fischer-Tropsch synthesis reactions or through methanol production as an intermediate product (Methanol to Olefins technologies).  

Fischer-Tropsch is a chemical process that can produce liquid hydrocarbons according to the following chemical reactions:

Paraffin Production: n CO + (2n+1)H2 = CnH2n+2 + nH2O

Olefin Production: n CO + 2nH2 = CnH2n + nH2O

These reactions are strongly exothermic and the CO/H2 ratio in the syngas is a key parameter to define the hydrocarbon chain extension that will be produced.

The reactions occur normally under temperatures that vary from 200 to 350 oC and operating pressures in the range of 15 to 30 bar. The catalyst commonly applied to these reactions is based on cobalt or iron as active metals deposited upon alumina as a carrier. 

Figure 5 presents a block diagram for a typical process plant dedicated to producing liquid hydrocarbons from Fischer-Tropsch synthesis.

Figure 5 – Block Diagram to a Typical Fischer-Tropsch GTL Process Plant

The process in Figure 5 is based in the syngas gas generation from steam reforming of natural gas, this is the most common route, however, there are process variations applying syngas production through coal, biomass or petroleum coke gasification route.  

The process starts with syngas generation and, as aforementioned, the produced hydrocarbon chain extension is controlled in the Fischer-Tropsch synthesis step through the CO/H2 ratio in the syngas fed to the FT reactors (beyond temperature and reaction pressure), following the produced hydrocarbons are separated and sent to refining steps as isomerization, hydrotreating, hydrocracking, catalytic reforming, etc. According to the application of the produced derivative (Gasoline, Diesel, Lubricant, etc.). 

Some side reactions can occur during the hydrocarbons production process, leading to coke deposition on the catalyst, causing his deactivation according to the following chemical reactions:

2 CO = C + CO2 (Boudoir Reaction)

CO + H2 = C + H2O (CO Reduction)

The type of reactor applied in the FT synthesis step has a strong influence on the yield and quality of the obtained products, the campaign time of the process units also depends on the type of reactor. Fixed bed reactors are widely employed to FT synthesis, however, show a reduced campaign time due to the low resistance to catalyst deactivation phenomenon. Modern process units apply fluidized bed or slurry phase reactors that present a higher resistance to coke deposition on the catalyst and better heat distribution, leading to higher campaign periods.

The several geopolitics crises over history motivated the development of new technologies and the improvement of the Fischer-Tropsch original process.

The complete transition away from fossil fuel energy (e.g. coal and crude oil) towards renewable energy (e.g. solar, wind, and geothermal) to address climactic phenomena will occur over the medium to long term. Within this transitional phase, the medium-term outlook favors “cleaner” fuels with higher energy efficiency, fewer SOx or NOx emissions, and less greenhouse gas impact (e.g. 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 utilizes 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 process also produces chemical intermediates, it is expensive and associated with a large greenhouse gas component. It is thus not considered a future prospect although the formation of liquid fuels using synthesis gas (from an alternative source like natural gas) may well be attractive.

Gas to Liquids (GTL) technology utilizes 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 component. Natural gas has further attracted a lot of 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 (C20+) are attractive given the absence of either nitrogen or sulfur-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 aromatic precursors (in-situ) at the onset of carbonization.

To examine this potential, research was conducted to thermally treat (400°C) a Waxy Oil fraction followed by carbonization (delayed coking). At the onset of carbonization, the stabilized alkanes slowly form cycloalkanes which rapidly dehydrogenate forming aromatics primarily 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 sulfur heterocycles.

The use of GTL based Waxy Oil as a needle coke precursor in a Delayed Coker Unit (DCU) is interesting:

·        Globally there is an abundance of natural gas and GTL production to produce liquid hydrocarbon fuels would produce heavier 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, the resultant quality is determined entirely by reaction conditions and not determined 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 stabilization (of Waxy Oil at 400°C) does not create aromaticity. The function thereof is to remove oxygen functionalities (e.g. hydroxy, aldehyde, or carboxylic 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 stabilized alkanes.

·        At the onset of carbonization (450°C; delayed coking) the stabilized alkanes slowly form cycloalkanes which are rapidly dehydrogenated to form aromatics. Usually, the thermal production of highly unstable alkane radicals (during carbonization) 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 aromatization takes place during delayed coking within a closed system and thus poses no threat

·        The absence of nitrogen or sulfur in the coke matrix eradicates the detrimental influence of electrode ‘puffing’ and 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 microstructural order, high real density, low CTE, and comparable purity

 A photomicrograph of the highly anisotropic microstructure of Waxy Oil needle coke is shown in Figure 6.

Figure 6 Photomicrograph (Mag. x50) of Waxy Oil needle coke exhibiting a highly ordered flow domain with parallel porosity (Clark, J 2011)

Conclusion

The use of both coal and petroleum derivatives in the energy sector will slowly be replaced by the cleaner or renewable energy over the medium to long term. The science of the needle coke value chain is well understood and the demand for needle coke is expected to experience sustained growth based on increased demand from both the graphite electrode and EV (lithium-ion batteries) sectors. However, coal-tar residuals are known to produce inconsistent needle coke qualities. The future viability of this precursor (specifically for the Ultra Premium needle coke market) is limited by contaminants (metals, semi-carbonaceous MIQ, and nitrogen) as well as environmental concerns (in terms of carcinogens). Petroleum based needle cokes enjoy market dominance, but their future viability may largely be dependent on competition (especially from bunker fuels) for the downstream derivatives of sweeter crude oil slates. The practice of processing heavier, higher sulfur crude oils further serves to deteriorate the outlook. The ability to produce higher purity needle cokes whose characteristics (nitrogen or sulfur) are not limited by their source would drive quality initiatives.

The use of cleaner fuels (e.g. natural gas) is virtually guaranteed during this transitional period. The use of GTL (based on natural gas conversion) to produce liquid hydrocarbon fuels also produces a heavy residual Waxy Oil fraction devoid of either nitrogen or sulfur, however, also lacking inherent aromaticity. The ability to self-assemble (in-situ) desirable aromatic precursors at the onset of carbonization leads to the development of a stable mesophase and long-range microstructural order. The lack of stable nitrogen and sulfur heterocycles further removes a realistic quality impediment. While Waxy Oil may not be the only novel needle coke precursor, it does demonstrate the value of examining unlikely feeds, to meet the future market demand of needle coke which is so dependent on quality. 

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Dr. John Clark is a fossil fuel scientist and industrial development specialist. He holds a BSc (Chemistry), MSc (Eng) in heavy petroleum shipping fuels, and a Ph.D. (Applied Materials Engineering) in sustainable needle coke. He has substantial experience in fossil fuels research, sustainable product, and business development. While he specializes in delayed coking chemistry and coke markets, he has made considerable contributions in aluminum, steel, bunker fuel, heavy petroleum residues, coal to oil, bitumen, and energy value chains. He has lectured as an honorary professor on a pro bono basis at the University of the Witwatersrand (South Africa) and held a seat on the executive council of an international coal industry consortium (USA). He is a seasoned international lecturer and industrial adviser to academic, industry, and government audiences. His specific interests involve the utilization of heavy petroleum and coal-tar residues in sustainable markets. He lives in Brisbane, Australia.

Dr. Marcio Wagner da Silva is Process Engineer and Project Manager focusing on Crude Oil Refining Industry based in S?o José dos Campos, Brazil. Bachelor’s in chemical engineering from the University of Maringa (UEM), Brazil, and PhD. in Chemical Engineering from the University of Campinas (UNICAMP), Brazil. Has extensive experience in research, design, and construction to oil and gas industry including developing and coordinating projects for operational improvements and debottlenecking to bottom barrel units, moreover Dr. Marcio Wagner have MBA in Project Management from the Federal University of Rio de Janeiro (UFRJ) and is certified in Business from Getulio Vargas Foundation (FGV).