High Carbon Yield Research Articles

Identifying the importance of functionalization evolution during pre-oxidation treatment in producing economical asphalt-derived hard carbon for Na-ion batteries

Na-ion batteries (NIBs) are emerging as frontrunners for next-generation energy storage systems, boasting superior safety profiles and promising cost-effectiveness. A crucial hurdle in the commercialization of NIBs lies in developing cost-effective hard carbons (HC) with high carbon yields. This work leverages ultra-affordable asphalt as a precursor, coupled with pre-oxidization to specifically tailor the microstructures of HC, achieving an impressive carbon yield of 63%. Both experimental and theoretical calculation results reveal that the successful introduction of oxidation sites during the pre-oxidation process is a prerequisite for producing non-graphitizable HC with large interlayer spacing. The inclusion of carbonyl groups effectively restricts the movement of carbon atoms during subsequent carbonization, thus hindering the graphitization of carbon layer. Furthermore, abundant ether-oxygen bonds enable the solid-phase carbonization mechanism and prevent the shrinkage of carbon layers, facilitating the crispation of carbon layers. This breakthrough significantly boosts the potential of harnessing low-cost, high-performance HC for NIBs.

A Recyclable Polyimide Derived Hard Carbon as a High‐Performance Negative Electrode Material for Sodium‐Ion Batteries

AbstractHard carbon, characterized by high ion storage capacity, low operating voltage, and excellent cycling stability, is considered an ideal negative electrode material for sodium‐ion batteries. However, the high cost and low carbon yield of thermosetting precursors limit their practical application in SIBs, while low‐cost and high‐yield raw materials exhibit highly ordered carbon structures and narrow interlayer spacing under high‐temperature carbonization. Discarded polyimide materials are inexpensive, which offer a high carbon yield and possess good thermal stability and thermoplasticity, with functional groups of imide rings (−CO−N−CO−) on their main chains. This study recycled and converted polyimide materials into hard carbon materials from discarded engineering plastics, investigating the influence of carbonization temperature on the degree of graphitization, interlayer spacing, and pore structure of the materials to achieve high reversible sodium storage capacity. After carbonization at 1300 °C, the polyimide material exhibited 319 mAh g−1 reversible capacity and excellent electrochemical performance (with a capacity retention of 91.92 % after 100 cycles at a 0.5 C current rate) and rate performance (up to 242.5 mAh g−1 at 2 C). This study provided a simple, high‐yield, and effective method for the reuse of discarded organic polymers, promoting the sustainable utilization of resources.

Pre-oxidation modification of bituminous coal-based hard carbon for high-quality sodium ion storage

Bituminous coal, with its moderate anthracene content, high reactivity, and ease of modulation, stands out as a favorable choice as a precursor for hard carbon. However, due to the highly condensed aromatic rings in bituminous coal, it tends to form highly graphitized structures after high-temperature carbonization. Therefore, pretreatment of bituminous coal is necessary to suppress the graphitization process. Here, we combine pre-oxidation techniques with high-temperature carbonization to produce a cost-effective, high carbon yield, and superior performance coal-based hard carbon. When utilized as anode for sodium-ion batteries, the prepared coal-based hard carbon exhibits a high reversible capacity of 313.5 mAh g−1, along with excellent rate capability and long cycling stability.

Coal-derived flaky hard carbon with fast Na+ transport kinetic as advanced anode material for sodium-ion batteries

Hard carbon (HC) has demonstrated a significant potential in anode for sodium-ion batteries (SIBs) due to its superior electrochemical properties. Coal is considered a promising hard carbon precursor due to its high carbon yield. However, the widespread adoption of coal-derived HC faces challenges such as limited storage capacity and decreased rate performance, primarily attributed to the dense structure of coal. In this study, a molten salt-assisted approach was developed to adjust the microstructure of coal-derived HC. The dense coal is etched into a flaky structure by molten salt, which is conducive to the diffusion and transport of sodium ions. The optimized HC materials possess large interlayer spacing and abundant pseudo-graphitic domains, exhibiting a remarkable reversible capacity of 303.6 mA h g−1 at 30 mA g−1 and 82.1 mA h g−1 at 500 mA g−1. Besides, a full cell with the configuration of as-prepared coal derived HC versus Na3V2(PO4)2O2F (NVPOF) is presented with a high initial coulombic efficiency of 76.15 % and capacity of 100.7 mA h g−1. The results indicate that our synthesis strategy is feasible for the application of coal derived carbon materials in SIBs.

Synthesis, Activation, and Characterization of Carbon Fiber Precursor Derived from Jute Fiber.

Activated carbon (AC) fiber is a carbonaceous material with a porous structure that has a tremendous scope of application in different fields. Conventionally, AC is derived from fossil fuel-based raw materials like polyacrylonitrile (PAN) and pitch. In this work, AC was synthesized from eco-friendly, renewable, and ubiquitous jute fiber. Systematically, the jute fiber was washed and pretreated with NaOH. Raw jute and NaOH-treated jute were carbonized/pyrolyzed at 500 °C for 1 h in presence of N2 gas. The carbonized carbon was activated with H3PO4 and KOH and again pyrolyzed at 650 °C for 1.5 h maintaining the inert condition. The different features of activated carbons were characterized with field emission-scanning electron microscope, energy-dispersive X-ray spectroscopy (EDX), transmission electron microscopy (TEM), X-ray diffraction (XRD), and thermogravimetric analysis. The average yield of carbonized and activated carbons was recorded at 19 and 13.8%, respectively. The scanning electron microscopic images confirmed a honeycomb-like porous structure. It was observed that KOH-activated carbon exhibited a more porous structure than the H3PO4-activated carbons. The average pore diameter of activated carbons was noted to be 1.3 μm. The pore density was higher in case of KOH-activated carbons accounting for 2.15 pore/μm. The EDX analysis showed that H3PO4-activated carbons had more than 90% carbon atoms indicating a significant carbon content. The TEM images revealed that AC particles were in the nanoscale range. The average particle sizes of H3PO4-activated carbon and KOH-activated carbon were 36.38 and 32.8 nm, respectively. The XRD study demonstrated the highly disordered and low level of crystallinity of AC. It was detected that the AC showed much higher thermal resistance than the jute fiber. The H3PO4-activated carbon obtained from NaOH-treated jute remained at 84% even after 500 °C. A higher thermal resistance was achieved with H3PO4-activated carbon since it contains 0.56% phosphorus, which was confirmed by EDX investigation. It was found that a higher carbon yield was obtained from NaOH-treated jute. The porous structure of the material showed that it could be used as an adsorbent. Due to its high thermal stability, it is recommended for flame retardants and heat insulation applications as well.

Facile construction of porous carbon fibers from coal pitch for Li-S batteries

Coal pitch, an important by-product in the coal coking industry with a high output, is a low-cost and high-carbon yield precursor for the manufacturing of high-value carbon materials. Herein, N/O co-doped carbon fiber (CFCP), fabricated by electrospinning using pre-oxidized coal pitch as the precursor, was employed as the sulfur host for Li-S batteries. The presence of more pyrrolic N and graphic N in CFCP than carbon fiber made from polyacrylonitrile benefits the adsorption of lithium polysulfide and the battery’s life. Sulphur-CFCP cathode (S@CFCP) exhibited excellent specific capacity and cyclability, with a specific capacity of 701.1 mAh/g and a low capacity decay rate of 0.088% per cycle over 200 cycles at 2.0 C, respectively. The high ion diffusion rate, low charge transfer resistance, and effective conversion of lithium polysulfides enable the high electrochemical performance of S@CFCP.

Oxidative Upcycling of Polyethylene to Long Chain Diacid over Co-MCM-41 Catalyst.

Plastic pollution is an emerging global threat due to lack of effective methods for transforming waste plastics into useful resources. Here, we demonstrate a direct oxidative upcycling of polyethylene into high-value and high-volume saturated dicarboxylic acids in high carbon yield of 85.9 % in which the carbon yield of long chain dicarboxylic (C10-C20) acids can reach 58.9% over cobalt-doped MCM-41 molecular sieves, in the absence of any solvent or precious metal catalyst. The distribution of the dicarboxylic acids can be controllably adjusted from short-chain (C4-C10) to long-chain ones (C10-C20) through changing cobalt loading of MCM-41 under nanoconfinement. Highly and sparsely dispersed cobalt along with confined space of mesoporous structure enables complete degradation of polyethylene and high selectivity of dicarboxylic acid in mild condition. So far, this is the first report on highly selective one-step preparation of long chain dicarboxylic acids. The approach provides an attractive solution to tackle plastic pollution and a promising alternative route to long chain diacids.

Oxidative Upcycling of Polyethylene to Long Chain Diacid over Co‐MCM‐41 Catalyst

AbstractPlastic pollution is an emerging global threat due to lack of effective methods for transforming waste plastics into useful resources. Here, we demonstrate a direct oxidative upcycling of polyethylene into high‐value and high‐volume saturated dicarboxylic acids in high carbon yield of 85.9 % in which the carbon yield of long chain dicarboxylic (C10‐C20) acids can reach 58.9% over cobalt‐doped MCM‐41 molecular sieves, in the absence of any solvent or precious metal catalyst. The distribution of the dicarboxylic acids can be controllably adjusted from short‐chain (C4‐C10) to long‐chain ones (C10‐C20) through changing cobalt loading of MCM‐41 under nanoconfinement. Highly and sparsely dispersed cobalt along with confined space of mesoporous structure enables complete degradation of polyethylene and high selectivity of dicarboxylic acid in mild condition. So far, this is the first report on highly selective one‐step preparation of long chain dicarboxylic acids. The approach provides an attractive solution to tackle plastic pollution and a promising alternative route to long chain diacids.

Demineralization activating highly-disordered lignite-derived hard carbon for fast sodium storage

Lignite, as one of the coal materials, has been considered a promising precursor for hard carbon anodes in sodium-ion batteries (SIBs) owing to its low cost and high carbon yield. Nevertheless, hard carbon directly derived from lignite pyrolysis typically exhibits highly ordered microstructure with narrow interlayer spacing and relatively unreactive interfacial properties, owing to the abundance of polycyclic aromatic hydrocarbons and inert aromatic rings within its molecular composition. Herein, an innovative demineralization activating strategy is established to simultaneously modulate the interfacial properties and the microstructure of lignite-derived carbon for the development of high-performance SIBs. Demineralization process not only creates numerous void spaces in the matrix of lignite precursor to assist aromatic hydrocarbon rearrangement, thereby reducing the ordering and expanding interlayer spacing, but also exposes more interfacial oxygen-containing functional groups to effectively increasing the sodium storage active sites. As a result, the optimal demineralized lignite-derived hard carbon (DLHC 1300) delivers a high reversible capacity of 335.6 mAh g−1 at 30 mA g−1, superior rate performance of 246.3 mAh g−1 at 6 A g−1 and nearly 100 % capacity retention after 1100 cycles at 1A g−1. Furthermore, the optimized DLHC 1300 material functions as an outstanding anode in sodium ion full cells. This work significantly advances the development of low-cost, high-performance commercial hard carbon anodes for SIBs.

A soft carbon materials with engineered composition and microstructure for sodium battery anodes

Sodium-ion batteries (SIBs) have advantages in high sodium resources, providing powerful supplement to the current energy storage system. However, the lack of low-cost and high-performance anode materials still limits its practical application. Herein, a soft carbon anode derived from petroleum coke was successfully synthesized by engineering its composition and microstructure through resin and sodium phosphate compositing with optimal heat treatment process, delivering significant merits over existing composite anode in term of price density, initial Coulombic efficiency (ICE) and carbon yield. Resin helps to increase micropores and inhibit graphitization. Na3PO4 contributes to expand layer spacing and increase reversible groups, meanwhile facilitates the cross-linking of graphite microcrystalline and provides additional sodium supplement with improved ICE and conductivity. Through the synergistic effect by the additives, the optimized sample (P-ONH-1200) exhibits a superior reversible charge specific capacity of 311.9 mAh g−1 with high cycling stability, ICE (90.7 %) in SIBs, and high carbon yield (70 %). It also gains rate performance of 209.7 mAh g−1 at 4 C with 98 % retention after 1000 cycles. The full cell with Na3V2(PO4)3 (NVP) cathode at 1.05 N/P ratio exhibits an excellent stability with a capacity retention of 70 % after 500 cycles at 1 C. It provides a model reference for the microstructure regulation of petroleum coke and a revenue for preparation high-performance soft carbon anode for SIBs.

Kinetics insights into size effects of carbon nanotubes’ growth and their supported platinum catalysts for 4,6-dinitroresorcinol hydrogenation

Size effects are a well-documented phenomenon in heterogeneous catalysis, typically attributed to alterations in geometric and electronic properties. In this study, we investigate the influence of catalyst size in the preparation of carbon nanotube (CNT) and the hydrogenation of 4,6-dinitroresorcinol (DNR) using Fe2O3 and Pt catalysts, respectively. Various Fe2O3/Al2O3 catalysts were synthesized for CNT growth through catalytic chemical vapor deposition. Our findings reveal a significant influence of Fe2O3 nanoparticle size on the structure and yield of CNT. Specifically, CNT produced with Fe2O3/Al2O3 containing 28% (mass) Fe loading exhibits abundant surface defects, an increased area for metal-particle immobilization, and a high carbon yield. This makes it a promising candidate for DNR hydrogenation. Utilizing this catalyst support, we further investigate the size effects of Pt nanoparticles on DNR hydrogenation. Larger Pt catalysts demonstrate a preference for 4,6-diaminoresorcinol generation at (1 0 0) sites, whereas smaller Pt catalysts are more susceptible to electronic properties. The kinetics insights obtained from this study have the potential to pave the way for the development of more efficient catalysts for both CNT synthesis and DNR hydrogenation.

Advanced Structural Engineering Design for Tailored Microporous Structure via Adjustable Graphite Sheet Angle to Enhance Sodium‐Ion Storage in Anthracite‐Based Carbon Anode

AbstractCarbon material has emerged as a highly promising anode for sodium‐ion batteries (SIBs) due to its abundance of resources, cost‐effectiveness, and high carbon yield. This work elaborately designs the precursor structure for the self‐assembly of melamine‐cyanuric acid on the anthracite surface through hydrogen bonding, and successfully constructs N‐doped carbon with tailored microstructure and expanded interlayer spacing. Serving as anode for SIBs, the optimized sample delivers high specific capacity (371.3 mAh g−1 at 0.05 A g−1), superior rate capability (295.8 mAh g−1 at 10.0 A g−1), and excellent ultra‐long cycling performance (the retention of 91.5% after 3000 cycles at 0.5 A g−1). The systematic investigations reveal the enhancement of sodium‐ion storage in the low‐voltage plateau region involving the interlayer intercalation coupled with nanopores filling. It is discovered that the microporous structure formed by the appropriate graphite sheet angle influences the migration and storage of sodium ions. Density functional theory calculations indicate that the adsorption capacity for sodium ion is enhanced and the migration energy barrier perpendicular to the graphite layer is reduced at the appropriate angle of 8°. This study provides novel insights into the sodium‐ion storage mechanism, offering guidance for the better design of anthracite‐based carbon anode with superior performance.

Boosting Molecular Cross-Linking in a Phenolic Resin for Spherical Hard Carbon with Enriched Closed Pores toward Enhanced Sodium Storage Ability.

Phenolic resin (PF) is considered a promising precursor of hard carbon (HC) for advanced-performance anodes in sodium-ion batteries (SIBs) because of its facile designability and high residual carbon yield. However, understanding how the structure of PF precursors influences sodium storage in their derived HC remains a significant challenge. Herein, the microstructure of HC is controlled by the degree of cross-linking of resorcinol-benzaldehyde (RB) resin. We reveal that robust molecular cross-linking in RB resin induced by hydrothermal treatment promotes closed-pore formation in the derived HC. The mechanism is devised for the decomposition of a highly cross-linked RB three-dimensional network into randomly stacked short-range graphitic microcrystals during high-temperature carbonization, contributing to the abundant closed pores in the derived HC. In addition, the high cross-linking degree of RB resin endows its derived HC with a small-sized spherical morphology and large interlayer spacing, which improves the rate performance of HC. Consequently, the optimized hydrothermal treatment HC anode shows a higher specific capacity of 372.7 mAh g-1 and better rate performance than the HC anode without hydrothermal treatment (276.0 mAh g-1). This strategy can provide feasible molecular cross-linking engineering for the development of closed pores in PF-based HC toward enhanced sodium storage.

Co-carbonization of hazelnut shell and lemna minor: its effectiveness in adsorption of crystal violet from an aqueous solution

ABSTRACT Co-carbonization of waste biomass is of great interest to reduce production costs and increase carbon yields. In this study, it was investigated to produce cost-effective and high carbon yield activated carbons by carbonizing lemna minor (LM) and hazelnut shell (HS) wastes together. In this context, LM and HS were co-carbonized at 800°C, 100 mL/min of N2 for 90 min, and their carbonization yields, adsorption capacities, physical and chemical properties were determined. X-ray diffraction (XRD), Fourier-transform infrared spectroscopy (FTIR), ultraviolet-visible spectroscopy (UV), energy-dispersive X-ray spectroscopy (EDX), scanning electron microscopy (SEM), ultimate and proximate analysis were performed. According to XRD analysis, HS is amorphous, LM is semi-crystalline, but the crystalline structure increased after co-carbonization. Based on the FTIR analysis LM, HS, and cLM/HS contain various functional groups, including O-H, C-H, and C-O. The adsorption capacity and CV removal, obtained by co-carbonized LM and HS (cLM/HS), are 87.95 mg/g and 88%, respectively. Its specific surface area is 745 m2/g. This study showed that the cLM/HS is a cost-effective adsorbent for the removal of crystal violet dye.

Increased CO2 fixation enables high carbon-yield production of 3-hydroxypropionic acid in yeast

CO2 fixation plays a key role to make biobased production cost competitive. Here, we use 3-hydroxypropionic acid (3-HP) to showcase how CO2 fixation enables approaching theoretical-yield production. Using genome-scale metabolic models to calculate the production envelope, we demonstrate that the provision of bicarbonate, formed from CO2, restricts previous attempts for high yield production of 3-HP. We thus develop multiple strategies for bicarbonate uptake, including the identification of Sul1 as a potential bicarbonate transporter, domain swapping of malonyl-CoA reductase, identification of Esbp6 as a potential 3-HP exporter, and deletion of Uga1 to prevent 3-HP degradation. The combined rational engineering increases 3-HP production from 0.14 g/L to 11.25 g/L in shake flask using 20 g/L glucose, approaching the maximum theoretical yield with concurrent biomass formation. The engineered yeast forms the basis for commercialization of bio-acrylic acid, while our CO2 fixation strategies pave the way for CO2 being used as the sole carbon source.

Effect of asphalt component distribution characteristics in layered porous carbon on performance of supercapacitors

Petroleum asphalt, characterized by its rich aromatic hydrocarbon structures and high carbonization yield, emerges as an ideal precursor for generating porous carbons intended for supercapacitors (SCs). Despite asphalt's classification into four distinct components, the intricate composition due to its diverse sources still presents an unresolved challenge when it comes to elucidating the predominant structural characteristics underlying porous carbons. In the present investigation, we adopted potassium bicarbonate (KHCO3) as a templating agent, and the potassium-containing compounds were thermally polymerized with carbon-based free radicals to facilitate the synthesis of porous carbon materials through carbonization process. We observed that the fabricated porous carbon with a substantial content of heavy fraction (68.5 wt. %) demonstrated the maximal pore volume (1.02 cm3 g−1) and specific surface area (SSA) of 1229 m2 g−1, yielding in the power density of 625 W kg−1 at the energy density of 17.88 W h kg−1 used 1 M tetraethylammonium tetrafluoroborate in acetonitrile (TEATFB/AN). Notably, we corroborated the reliability of the entire process through a rigorous scaling-up experiment to 10 g, demonstrating entirely consistent performance. Our research introduces a novel avenue for strategically selecting petroleum asphalt precursors, with the overarching objective of producing cost-effective carbon materials endowed with commendable performance attributes.

Poly (vinyl alcohol) based carbon nanofibers as freestanding anodes for lithium‐ion batteries

AbstractIn this study, we investigated the iodination process for the preparation of water‐soluble polyvinyl alcohol (PVA)‐based carbon nanofibers (CNFs) and their electrochemical properties. Iodinated nanofibers were carbonized at different temperatures (600°C, 800°C) under an argon (Ar) atmosphere. Morphology and physical properties of fiber mats were characterized by scanning electron microscopy (SEM), Fourier transform infrared spectroscopy (FTIR), Raman spectroscopy, and thermogravimetric analysis (TGA), while electrical conductivity measurements were done using four probes. As a freestanding anode in half‐cell lithium batteries, the CNFs were prepared by electrospinning of PVA and PVA–CNT blend, which was exposed to iodine vapor at various temperatures (60, 80, and 100°C). Iodine vapor treatment at 80°C and subsequent carbonization at 800°C provided a 29% carbon yield. The resulting CNT‐containing optimized CNFs exhibited superior flexibility, electrical conductivity (2.16 S/cm), and handleability, making them a promising, binder‐free environmentally friendly carbon source for lithium‐ion batteries with high capacity (672 mAh/g at 50 mA/g). The study clearly showed that PVA might be an alternative precursor for the production of carbon micro‐ and nanofibers.Highlights Anodic performance of PVA‐based carbon nanofibers was analyzed. PVA precursor offers economic and environmental option for binder‐free electrodes. PVA–CNT composite nanofibers with optimized iodination and carbonization time identified as a promising high carbon yield carbon source for lithium‐ion batteries. CNT‐containing optimized carbon nanofibers exhibited superior flexibility and electrical conductivity (2.16 S/cm). Freestanding fiber anodes exhibited a higher capacity of 672 mAh/g, at 50 mA/g compared to conventional graphite anode (372 mAh/g).

Preparation and characterization of lignin-derived carbon aerogels.

Lignin is considered a valuable renewable resource for building new chemicals and materials, particularly resins and polymers. The aromatic nature of lignin suggests a synthetic route for synthesizing organic aerogels (AGs) similar to the aqueous polycondensation of resorcinol with formaldehyde (FA). The structure and reactivity of lignin largely depend on the severity of the isolation method used, which challenges the development of new organic and carbon materials. Resorcinol aerogels are considered a source of porous carbon material, while lignin-based aerogels also possess great potential for the development of carbon materials, having a high carbon yield with a high specific surface area and microporosity. In the present study, the birch hydrolysis lignin and organosolv lignin extracted from pine were used to prepare AGs with formaldehyde, with the addition of 5-methylresorcinol in the range of 75%-25%, yielding monolithic mesoporous aerogels with a relatively high specific surface area of up to 343.4m2/g. The obtained lignin-based AGs were further used as raw materials for the preparation of porous carbon aerogels (CAs) under well-controlled pyrolysis conditions with the morphology, especially porosity and the specific surface area, being dependent on the origin of lignin and its content in the starting material.

A Universal Method for Regulating Carbon Microcrystalline Structure for High-Capacity Sodium Storage: Binding Energy As Descriptor.

Sodium-ion batteries (SIBs) are attracting worldwide attention due to their multiple merits including abundant reserve and safety. However, industrialization is challenged by the scarcity of high-performance carbon anodes with high specific capacities. Here, we report the metal-assisted microcrystalline structure regulation of carbon materials to achieve high-capacity sodium storage. Systematic investigations of in situ thermal-treatment X-ray diffraction and multiple spectroscopies uncover the regulation mechanism of constructing steric hindrance (C-O-C bonds) to restrain the aromatic polycondensation reaction. The carbon precursor of polycyclic aromatic hydrocarbon-type pitch contributes to a high carbon yield rate (40%) compared with those of resin and biomass precursors. The as-synthesized carbon materials deliver high capacities of up to 390 mAh g-1, surpassing many reported carbon anodes for SIBs. Through correlating specific capacity with ID/IG values in Raman spectra and theoretical calculation of carbon materials regulated by different metal elements (Mn, Nb, Ce, Cr, and V), we identify and propose the binding energy as the descriptor for characterizing the capability of regulating the carbon microcrystalline structure to promote sodium storage. This work provides a universal method for regulating the carbon structure, which may lead to the controlled design and fabrication of carbon materials for energy storage and conversion and beyond.

Enhancing Fuel Properties of Napier Grass via Carbonization: A Comparison of Vapothermal and Hydrothermal Carbonization Treatments

Napier grass is a herbaceous biomass that can be used as biofuel; however, its high ash, potassium, sulfur and chlorine content may cause problems when combusted. Napier grass was submitted to vapothermal carbonization (VTC) and hydrothermal carbonization (HTC) processes at 190 and 220 °C to compare their ability to enhance its fuel properties. The different water distribution between phases in the two processes was verified: up to 14.5% of the water vaporized to steam in the VTC ran at 220 °C, while over 99% of the water remained in the liquid state and in contact with the solids during all HTC runs. Both processes improved the calorific value of the Napier grass (up to 20.6% for VTC220 and up to 29.8% for HTC220) due to the higher C content in the chars. Both processes reduced the sulfur content, removing up to 15.3% of it with VTC190 and 28.5% of it with HTC190 compared to that of Napier grass. In contrast, the two processes had different effects on the ash and chlorine content. While HTC removed both ash and Cl from the Napier grass, VTC concentrated it in the chars (ash: 5.6%wt. Napier grass, 3.3%wt. HTC chars, 7.1%wt. VTC; chlorine: 1.08%wt. Napier grass, 0.19%wt. HTC chars, 1.24%wt. VTC). Only the HTC process leached high percentages of Cl (up to 80%), S (up to 70%), sodium (Na, up to 80%) and potassium (K, up to 90%) into the process water. This may prevent fouling and slagging problems when burning HTC char. The biofuel qualities of the raw Napier grass, VTC, and HTC chars were evaluated using two standards: the international standard for solid biofuels, EN ISO 17225, and the Korean regulation for biomass solid recovered fuels (Bio-SRF). Napier grass and VTC chars presented problems regarding Cl content thresholds for both EN ISO 17225 and Bio-SRF. Both VTC and HTC chars along with the Napier grass fulfilled the requirements for heavy metals (Pb, Ni, Cr, and Cd) except for copper. The choice of process in practical applications will depend on the goal; HTC improves fuel quality and VTC has higher high solid, carbon and energy yields.