In this study, SM and CW were used as feedstocks for the Co-HTC process. SM was collected from a livestock farm in Gyeonggi Province, South Korea, and CW was obtained from a commercial coffee shop in Seoul, South Korea. Prior to HTC experiments, both feedstocks were dried at 105 °C for 24 h until constant weight was achieved, based on preliminary tests to ensure complete moisture removal. The dried samples were then ground to a particle size of ≤2 mm to ensure homogeneity.

The proximate and ultimate compositions of SM and CW are summarized in Table 1. SM exhibited a high ash content (29.8%) and a relatively low fixed carbon content (8.8%), which may limit its direct application as a solid fuel due to its reduced combustion efficiency and increased pollutant emissions (Moura et al., 2025). Additionally, the ratio of volatile matter to combustible carbon (0.87) was significantly high, indicating a lower combustion efficiency and higher emissions when used as a standalone solid fuel.

On the other hand, CW had a significantly lower ash content (1.3%) and higher fixed carbon content (20.6%) compared to SM. Although CW exhibited a higher volatile matter content (78.4%) than SM (61.4%), its volatile-to-combustible carbon ratio (0.79) was also high, posing challenges for its direct use as a solid fuel.

The ultimate analysis revealed that CW contained higher carbon (51.0%) and oxygen (38.5%) contents, whereas SM exhibited lower carbon (40.0%) and oxygen (18.7%) contents. Generally, fuels with lower H/C and O/C ratios tend to experience reduced energy loss, smoke, and water vapor formation during combustion, making them more suitable for solid fuel applications (Yang et al., 2024). However, CW exhibited higher hydrogen and oxygen contents, resulting in H/C and O/C ratios that did not reach those of lignite or fossil fuels. Additionally, SM had relatively higher nitrogen (3.9%) and sulfur (1.7%) contents than CW, suggesting its potential application as an agricultural soil amendment.

In this study, HTC experiments were conducted using a custom-designed high-temperature, high-pressure reaction system to ensure stable operation under extreme conditions. The experimental setup consisted of a reactor, a heating system, a pressure monitoring and control system, and a cooling system, each optimized for efficient control and stable operation of the HTC process (Supplementary Figure S1).

The HTC reactor was made of stainless steel (SUS 316) with a total internal volume of 1,000 mL. To enhance corrosion resistance, an anti-corrosion coating was applied to the internal surface. The reactor was designed to withstand a maximum operating temperature of 400 °C and a maximum pressure of 150 bar, allowing for stable operation even under high-pressure conditions generated during the HTC process.

The internal temperature of the reactor was monitored in real-time using a K-type thermocouple, and a PID (Proportional-Integral-Derivative) control system was employed to maintain temperature fluctuations within ±5 °C. The electric heating system provided a maximum heating rate of 10 °C/min and was automatically regulated to maintain the set temperature for a predefined reaction duration.

During the reaction, internal pressure was continuously monitored using a high-pressure gauge. To prevent overpressure, the system was equipped with a safety valve that was automatically activated when the maximum allowable pressure was exceeded.

An external cooling water circulation system was used for rapid cooling after the reaction. The cooling process was facilitated by a cooling coil attached to the reactor, which allowed for an efficient temperature reduction immediately after the reaction, thereby preventing excessive prolongation of the reaction.

HTC experiments were conducted at 160 °C, 200 °C, 240 °C, 280 °C, and 320 °C for individual feedstocks (SM and CW) to explore the decomposition behavior of components like cellulose and lignin, as suggested in previous studies. For Co-HTC, SM and CW were mixed in 1:1, 3:1, and 5:1 mass ratio and treated at 160 °C, 200 °C, and 240 °C, which represent a practical operating range for evaluating fuel performance. These ratios were selected to reflect not only the compositional differences between SM and CW, but also their realistic generation volumes, as SM is produced in much larger quantities globally than CW. This allowed us to evaluate the synergistic potential and the application feasibility of Co-HTC under SM-dominant conditions. The Co-HTC approach was adopted to determine whether combining the two feedstocks could offset their individual limitations and improve the overall fuel quality of the resulting hydrochars.

In all experiments, the total feedstock loading was maintained at 80 g, and based on previous experimental results, distilled water was added to maintain a fixed moisture content of 80%. The reaction time was set to 1 h after the target temperature was reached. To ensure an oxygen-free environment, N₂ purging was performed at a flow rate of 400 mL/min for 10 min before heating.

To ensure reproducibility and minimize experimental uncertainty, all HTC and Co-HTC experiments were independently performed in triplicate under each condition. The resulting data are presented as mean values with standard deviations to reflect measurement variability and enhance the statistical reliability of the findings.

The biochemical composition of raw feedstocks was characterized through a suite of complementary analyses. Proximate analysis (moisture, volatile matter, fixed carbon, and ash) was performed according to ASTM D7582, and ultimate analysis (C, H, N, S) was conducted following ASTM D5373 and ISO 17247 using a Flash 2000 Elemental Analyzer (Thermo Fisher Scientific, USA). HHV was determined by ASTM D5865 using a Parr Model 1341 Plain Jacket Calorimeter (Parr Instrument Company, USA). Protein content was quantified via the Kjeldahl method (Standard Methods 4500-Norg B) using a Foss KjeltecTM 8400 analyzer (Foss, Denmark), and lipid content was measured according to ASTM D7060 by ether extraction at 80 °C for 8 h using A2000 and XT 15 analyzers (Ankom Technology, USA). Carbohydrate content was estimated by difference as: Carbohydrate (%) = 100 − Moisture (%) − Protein (%) − Fat (%) − Ash (%). Fiber fractions (lignin, cellulose, and hemicellulose) were determined following Standard Methods 973.18 and ASTM D1103 using an Ankom A2000 analyzer, with lignin assessed by 72% H₂SO₄ pretreatment and cellulose and hemicellulose quantified via Acid Detergent Fiber (ADF) and Neutral Detergent Fiber (NDF) methods, respectively. All procedures were conducted in accordance with the Standard Methods for the Examination of Water and Wastewater, 24th edition (2023) (APHA, 2023).

Thermal decomposition behavior of the hydrochar samples was evaluated by thermogravimetric analysis (TGA) and derivative thermogravimetric (DTG) analysis using a TG 209 F3 analyzer (Netzsch, Germany) following ASTM E1131 (Cai et al., 2016). Approximately 5 ± 0.2 mg of each pulverized sample (≤2 mm) was heated from 30 °C to 900 °C at a rate of 20 °C/min under an airflow of 20 mL/min. Prior to analysis, all hydrochars were thoroughly homogenized to ensure sample representativeness.

Hydrochar yield (HY), energy densification ratio (EDR), and energy yield (EY) were calculated using Equations 1–3, respectively, based on mass and HHV measurements obtained after drying and weighing the recovered hydrochar (Poomsawat and Poomsawat, 2021; Zhang et al., 2016). The combustion characteristic index (S), a key parameter for evaluating ignition ease, burning velocity, and burnout temperature, was determined using Equation 4, which incorporates the maximum and mean mass loss rates alongside the ignition and burnout temperatures from TGA results (Poomsawat and Poomsawat, 2021). H Y % = M h / M b × 100 (1) E D R = H H V h / H H V b (2) EY  % = EDR × HY  % (3) S = d w / d t max × d w / d t m e a n / T i 2 × T f (4) where M_h is the mass of dried hydrochar (g), M_b is the initial mass of biomass (g), HHV_h is the higher heating value of hydrochar (MJ/kg), HHV_b ​is the higher heating value of raw biomass (MJ/kg), (dw/dt)_max is the maximum mass loss rate (wt%/min), (dw/dt)_mean is the average mass loss rate (wt%/min), T_i is the ignition temperature (°C), and T_f is the burnout temperature (°C).

HTC is an efficient thermochemical process for converting biomass into high-energy-density solid fuel. The yield and characteristics of hydrochar significantly depend on the reaction temperature. In this study, HY was analyzed at different HTC temperatures for SM and CW.

The reaction temperature had a significant effect on HY (Figure 1). HY values were calculated based on the total mass of fixed carbon, volatile matter, and ash, ensuring consistency with standard biomass yield determination. For SM, HY decreased from 61.8% at 160 °C to 27.6% at 320 °C. Similarly, for CW, the yield decreased from 69.9% at 160 °C to 44.4% at 320 °C. This decline is attributed to the decomposition of volatile organic compounds and thermal degradation of biomass components, which reduces the residual solid mass at higher temperatures. These results align with previous studies, which reported that HY typically decreases with increasing HTC temperature due to mass loss from devolatilization (Mendoza Martinez et al., 2021; Zhang et al., 2024).

The mass recovery of SM and CW decreased with rising HTC temperature. At 160 °C, the mass recovery rates were 61.8% for SM and 69.9% for CW. As the HTC temperature increased, the loss of volatile matter increased, and the residual solid fraction declined. CW exhibited a more pronounced decrease in mass recovery due to its higher volatile matter content.

The decomposition of hemicellulose, cellulose, and lignin during HTC varies across different temperature ranges. Hemicellulose begins to decompose at approximately 180 °C–200 °C and is largely degraded by 260 °C, releasing acetic acid, furfural, and hydroxymethylfurfural, which contribute to reduced oxygen content and enhanced fuel quality in hydrochar (Shahnawazi et al., 2025). Cellulose starts to decompose around 240 °C, with major decomposition occurring between 280 °C and 320 °C. This process generates hydroxymethylfurfural, levoglucosan, and other volatile compounds, which contribute to an increase in fixed carbon content and energy density (Smith and Ross, 2019). Lignin decomposition begins at approximately 280 °C and continues gradually up to temperatures above 500 °C, leading to the formation of aromatic compounds. The thermal stability of lignin contributes to the increase in fixed carbon content and the reduction of oxygen content in hydrochar, thereby improving its long-term fuel stability (Gascó et al., 2018; Khan et al., 2022). Furthermore, Wang et al. (2021) demonstrated that delignification significantly influences the thermal degradation reactivity of hemicellulose and cellulose in wood cell walls, providing useful insights into the thermal behavior of lignocellulosic biomass during thermochemical processing.

The higher mass recovery of the CW compared to SM at increasing HTC temperatures can be attributed to two key factors. First, CW contains a higher proportion of lignin and fixed carbon than SM. As HTC temperature increases, lignin transforms into stable aromatic compounds, thereby increasing the fixed carbon content of hydrochar (Mendoza Martinez et al., 2021). In contrast, SM has a relatively high ash content, leading to an increase in residual ash proportion as temperature rises, which limits fuel conversion efficiency (Gascó et al., 2018). Second, the lignin-cellulose network structure of CW enhances hydrochar formation. However, at temperatures above 280 °C, CW’s mass recovery decreases as cellulose decomposition intensifies. Since lignin decomposes over a broad temperature range (280 °C–500 °C), a portion of carbon is retained, but overall mass continues to decline (Lu and Gu, 2022).

TGA results indicate that SM and CW exhibit slight mass loss below 100 °C due to the removal of free and bound moisture (Figure 2). This initial mass loss caused by the evaporation of residual water from the biomass is commonly observed in HTC preheating stages (Jang et al., 2020; Lee et al., 2022). Between 160 °C and 280 °C, CW exhibited a more rapid mass loss compared to SM, primarily due to the decomposition of hemicellulose (180 °C–260 °C) and cellulose (240 °C–320 °C) (Lee et al., 2022; Apaydın Varol and Mutlu, 2023). CW, which has a higher cellulose and lipid content, released more volatile matter within this temperature range. In contrast, SM exhibited a slower decomposition rate, with a portion of organic components persisting even at 240 °C.

Above 280 °C, CW continued to exhibit gradual mass loss, although the rate of decomposition slowed. This trend is attributed to the thermal stability of lignin, which decomposes gradually between 280 °C and 500 °C, leading to the formation of stable aromatic structures (Cho et al., 2020). Meanwhile, SM demonstrated an increase in ash content above 280 °C due to the presence of mineral components that remain stable at high temperatures. This suggests that SM’s conversion to hydrochar is limited by its higher inorganic content, which increases the proportion of noncombustible material rather than fixed carbon.

CW was evaluated as having superior fuel properties compared to SM due to its higher fixed carbon content and lower ash content. At 320 °C, CW exhibited optimal fuel characteristics, making it a favorable feedstock for high-quality solid fuel production. Conversely, SM, due to its high ash content, showed limited improvement in fuel quality with increasing HTC temperature, however, its potential application as a carbon storage material remains significant.

The elemental composition and fuel characteristics of hydrochar provide critical insights into the thermochemical transformation of biomass during HTC. In this study, C, H, O, N, and S contents, H/C and O/C atomic ratios, and HHV of hydrochar produced from SM and CW at various reaction temperatures were analyzed.

As shown in Figure 3, the carbon content of hydrochar exhibited a consistent increasing trend with rising HTC temperature. The carbon content of SM increased from 45.0% at 160 °C to 54.2% at 320 °C, while that of CW increased from 56.2% at 160 °C to 76.5% at 320 °C, indicating that CW had a higher carbon retention than SM. The greater increase in CW’s carbon content can be attributed to its high lignin content, which facilitates the formation of aromatic structures during HTC, leading to an increase in carbon density (Sun et al., 2023). In contrast, the carbonization of SM was relatively limited due to its high ash content, which increased with rising temperature and restricted fuel quality enhancement (Gascó et al., 2018).

The hydrogen and oxygen contents showed a decreasing trend with increasing HTC temperature. This reduction is mainly attributed to dehydration and decarboxylation reactions during HTC, which facilitate biomass carbonization by removing oxygen and hydrogen from the solid phase (Petrović et al., 2024). The oxygen content of SM decreased significantly from 21.0% at 160 °C to 4.4% at 320 °C, while CW’s oxygen content dropped from 32.8% to 3.5% over the same temperature range. The enhanced oxygen removal with increasing HTC temperature led to an increase in fixed carbon content, resulting in improved fuel quality (Maniscalco et al., 2020).

As oxygen and hydrogen were progressively eliminated during HTC, the fixed carbon content increased, leading to an enhancement in HHV. The HHV of SM increased from 21.2 MJ/kg at 160 °C to 25.1 MJ/kg at 320 °C, while CW exhibited a more substantial increase from 23.7 MJ/kg to 32.9 MJ/kg. The greater improvement in HHV for CW is primarily attributed to its high lignin content, which promotes efficient carbonization at elevated temperatures. Lignin-rich biomass generally retains a higher proportion of fixed carbon during HTC, resulting in the production of high-energy-density fuel, aligning with previous findings (Wang et al., 2025).

The Van Krevelen diagram in Figure 4 illustrates the reduction in the H/C and O/C atomic ratios of SM and CW with increasing reaction temperature. As temperature increased, the H/C ratio of SM declined from 1.65 to 1.39, while CW’s H/C ratio decreased from 1.62 to 1.15. Similarly, the O/C ratio exhibited a significant decrease, with SM declining from 0.35 to 0.06 and CW from 0.44 to 0.03. This trend indicates that HTC effectively facilitates dehydration and deoxygenation reactions, leading to a reduction in atomic ratios (Reza et al., 2014). Notably, CW exhibited a sharper decrease in O/C ratio with increasing HTC temperature, which is characterized by low oxygen content and high carbon stability (Dang et al., 2024). In contrast, SM exhibited a relatively gradual reduction in O/C ratio, which can be attributed to its high ash content, limiting the extent of fuel quality improvement during HTC (Moura et al., 2025). However, while CW-derived hydrochar approached that of anthracite in terms of oxygen depletion, its H/C ratio remained close to that of lignite. This suggests that, although the HTC process promoted aromatic structure formation and improved carbonization, hydrogen removal was less extensive. As a result, hydrochar may not fully exhibit the combustion behavior or thermal maturity of high-rank coals such as anthracite.

In summary, CW underwent more effective carbonization than SM during HTC, resulting in a higher energy density fuel. The increase in HTC temperature led to an enhancement in the carbon content of hydrochar and a reduction in oxygen content, which significantly improved fuel quality. However, the presence of high ash content in SM restricted fuel quality improvement despite increasing HTC temperatures. The contrasting behaviors of SM and CW highlight how lignin-rich feedstocks benefit more from HTC in terms of oxygen elimination and fuel densification. Nonetheless, the residual hydrogen content in hydrochar indicates that further thermal upgrading or co-processing strategies may be needed to fully replicate the combustion properties of fossil fuels. Optimizing HTC conditions can further maximize biomass conversion into high-energy fuel.

The mixing ratio and reaction temperature in Co-HTC play a critical role in determining hydrochar fuel characteristics. Co-HTC is recognized as a promising technique for overcoming the limitations of individual biomass feedstocks and enhancing fuel quality through biomass interactions. This study evaluated the fuel characteristics of hydrochar produced at different mixing ratios (1:1, 3:1, 5:1) and reaction temperatures (160 °C, 200 °C, 240 °C).

The yield of hydrochar in HTC is influenced by the decomposition rate of biomass, the extent of volatile matter loss, and the efficiency of the carbonization reaction. In Co-HTC, biomass interactions further affect pyrolysis behavior and hydrochar formation process (Ighalo et al., 2025). As shown in Figure 5, HY decreased consistently with increasing HTC temperature for all mixing ratios. At a 1:1 mixing ratio, HY declined from 67.4% at 160 °C to 53.4% at 240 °C. For the 3:1 ratio, HY decreased from 73.6% to 55.0%, while the 5:1 ratio showed a smaller decline from 67.6% to 57.9%. This downward trend reflects the typical behavior of biomass under HTC conditions, where rising temperatures accelerate the breakdown of volatile organic matter, resulting in greater mass loss. The sharp HY decrease in mixtures with higher CW proportions (1:1 and 3:1) can be attributed to CW’s higher volatile matter and lipid content, which decomposes and escapes as gas or liquid at elevated temperatures. In contrast, the relatively higher HY retention at the 5:1 ratio indicates that SM’s higher ash and protein content contributes more to solid residue, even though it may not enhance fuel quality (Gascó et al., 2018). Beyond these compositional effects, the alkaline mineral components in SM ash, particularly potassium and calcium, promote the hydrolysis and decomposition of cellulose and hemicellulose derived from CW, accelerating volatile release and contributing to the steeper HY decline observed in CW-rich mixtures (Dang et al., 2023; Ighalo et al., 2025). Simultaneously, Maillard-type reactions between nitrogen-containing compounds released from SM proteins and reducing sugars generated by CW carbohydrate hydrolysis produce nitrogen-heterocyclic aromatic compounds that incorporate into the hydrochar matrix, partially offsetting mass loss through repolymerization (Li et al., 2021a; Li et al., 2021b). These opposing tendencies create a compositional trade-off in Co-HTC: higher CW proportions favor carbon quality through lignin-driven aromatization but reduce HY, while higher SM proportions retain solid mass through ash accumulation but dilute fuel quality.

Furthermore, hydrochar’s carbon content increased while oxygen content decreased with rising HTC temperature and CW proportion. This trend highlights the occurrence of dehydration, decarboxylation, and deoxygenation reactions in HTC, which contribute to fuel quality improvement (Wang et al., 2020). At a 1:1 mixing ratio, the carbon content increased from 56.4% at 160 °C to 64.2% at 240 °C. In contrast, the 5:1 mixing ratio showed a smaller increase, from 47.5% at 160 °C to 58.4% at 240 °C, indicating that higher SM proportions restricted carbonization efficiency. These contrasting trends reflect the fundamentally different carbonization pathways of each feedstock: CW undergoes lignin-driven aromatization, in which phenolic intermediates released during partial lignin depolymerization condense and repolymerize into stable polycyclic aromatic structures that progressively increase carbon density (Dang et al., 2024; Sun et al., 2023). In SM-rich mixtures, this aromatization pathway is partially suppressed by the diluting effect of inorganic ash. Nonetheless, SM-derived mineral ions (particularly K⁺ and Ca²⁺) may facilitate cross-linking reactions between CW-derived aromatic intermediates and SM organic matter, generating a more heterogeneous but thermally stable hydrochar matrix compared to single-feedstock products (Dang et al., 2023).

Biomass subjected to HTC undergoes an increase in carbon density while simultaneously experiencing a reduction in oxygen and hydrogen contents, leading to a decrease in the O/C and H/C atomic ratios (Dang et al., 2023). While Figure 4 tracks the evolution of these atomic ratios with increasing temperature, Figure 6 contextualizes the changes in relation to the HHV across different mixing ratios, providing complementary insight into the Co-HTC process. At a 1:1 mixing ratio, the O/C ratio decreased from 0.27 at 160 °C to 0.18 at 240 °C, while the H/C ratio declined from 1.65 to 1.34 (Figure 6). The more pronounced O/C reduction at higher CW proportions (1:1) reflects lignin-driven aromatization, in which oxygen is expelled through dehydration of hydroxyl groups and decarboxylation of organic acids. In Co-HTC, SM’s alkaline mineral environment raises the local pH of the reaction medium, which favors decarboxylation over dehydration, selectively reducing the O/C ratio while moderating H/C reduction (Li et al., 2021a; Dang et al., 2023). This pH-mediated mechanism explains why SM-dominant mixtures (5:1) show a comparatively smaller O/C decrease despite reaching the same maximum temperature.

This atomic ratio variation plays a critical role in determining fuel properties, as a lower O/C and H/C ratio is directly linked to an increase in energy density (Petrović et al., 2024). At a 1:1 mixing ratio, the HHV increased from 24.4 MJ/kg at 160 °C to 27.9 MJ/kg at 240 °C. The higher CW proportion contributed to a substantial increase in HHV, primarily due to the enhanced formation of aromatic carbon structures. These results indicate that HTC promotes the removal of oxygen while increasing carbon density, thereby enhancing the energy density of the fuel (Dang et al., 2023).

TGA and DTG analyses (Figure 7) demonstrated that increasing HTC temperature significantly affected volatile matter release and thermal decomposition behavior. At a 1:1 mixing ratio, volatile matter loss occurred gradually at 160 °C, whereas at 240 °C, it became more pronounced, indicating substantial mass reduction. CW-rich hydrochars exhibited greater volatile release, attributed to the high lignin content in CW, which undergoes gradual thermal degradation over a broad temperature range (Pahnila et al., 2023).

DTG curves revealed two distinct peaks for CW-rich hydrochars, corresponding to volatile release and fixed carbon formation. The high lignin content in CW contributed to a broader, slower degradation process, which enhanced combustion stability by sustaining char formation over an extended temperature range. Conversely, SM-rich hydrochars decomposed more rapidly, leading to higher volatile losses and reduced carbon retention. The concentrated mineral ash in SM-dominant hydrochars acts as a physical barrier restricting oxygen diffusion to the carbonaceous matrix, broadening the DTG peak and shifting burnout temperature to higher values, a mineral-mediated inhibition mechanism consistent with the S index decline observed at higher SM proportions (Section 3.4) (Gascó et al., 2018; Dang et al., 2023).

In contrast, at a 5:1 mixing ratio, where the SM content was predominant, the thermal decomposition process was more gradual. The increased ash content in SM likely hindered thermal degradation, limiting overall mass loss despite the rise in HTC temperature (Gascó et al., 2018).

Collectively, the SM-CW interactions during Co-HTC are governed by three principal mechanisms: (1) mineral-catalyzed hydrolysis of CW carbohydrates by SM ash components, accelerating volatile release and modulating HY; (2) Maillard-type cross-reactions between SM-derived nitrogenous compounds and CW-derived reducing sugars, contributing nitrogen-containing aromatic structures to the hydrochar matrix; and (3) pH-mediated shift from dehydration to decarboxylation pathways driven by SM’s alkaline mineral environment, selectively reducing O/C ratios beyond simple additive blending. A higher CW proportion led to a marked increase in carbon density and HHV, highlighting the effectiveness of lignin-rich biomass in fuel property enhancement. In contrast, higher SM proportions resulted in increased ash content, which restricted fuel quality improvement but suggested potential applications for agricultural utilization rather than combustion-based energy production. With increasing HTC temperature, the continuous decline in the O/C and H/C ratios indicated that the hydrochar structure progressively transitioned to a composition resembling that of coal.

The fuel performance of hydrochar produced through HTC is a critical factor in determining combustion characteristics and energy efficiency. In HTC-treated hydrochar, as temperature increases, volatile matter decreases while fixed carbon content increases, directly impacting combustion reactivity and energy density. This study evaluated fuel performance based on S, EDR, and EY.

The S index is a crucial indicator of ignition ease, combustion rate, and burnout temperature. As shown in Table 2, the 1:1 mixing ratio exhibited the highest S value at 160 °C (39.0 × 10⁻⁹%²/min²·°C³), while the lowest S value was observed at 240 °C (9.1 × 10⁻⁹%²/min²·°C³). This decline suggests that as HTC progresses, volatile matter is removed while fixed carbon increases, leading to a reduction in combustion reactivity. In contrast, the 3:1 and 5:1 mixing ratios showed the highest S values at 200 °C, which subsequently decreased at 240 °C. In particular, the 5:1 mixing ratio demonstrated a higher ignition temperature (Ti) and lower combustion rate, indicating decreased reactivity. This behavior reflects the dual role of SM-derived ash: at moderate temperatures (200 °C), sufficient volatile matter is retained to sustain ignition while the mineral matrix provides structural support to the carbonaceous framework. At 240 °C, however, extensive volatile removal combined with the physical barrier effect of concentrated inorganic ash restricting oxygen diffusion to the combustible carbon matrix, collectively suppresses combustion reactivity in SM-dominant mixtures (Gascó et al., 2018; Dang et al., 2023). Consequently, the optimal HTC temperature for combustion performance shifts with mixing ratio: 160 °C for the 1:1 ratio and 200 °C for SM-dominant conditions.

HTC reduces volatile matter, leading to mass loss; however, it enhances carbon density, which improves HHV and overall fuel quality. Figure 8 illustrates the EDR variations under different HTC conditions. At 160 °C, the EDR was 1.19 for the 1:1 ratio, increasing to 1.36 at 240 °C. The 3:1 ratio increased from 1.01 to 1.36, and the 5:1 ratio from 1.02 to 1.38. These increases demonstrate effective carbon densification with rising temperature, consistent with oxygen elimination via dehydration and decarboxylation reactions (Dang et al., 2023). The comparably high EDR of the 5:1 ratio (1.38) despite elevated ash content is explained by mineral-catalyzed deoxygenation: alkali and alkaline earth metals (K, Ca) from SM ash promote decarboxylation of organic acids derived from CW carbohydrate hydrolysis, expelling oxygen as CO₂ and enriching the solid phase in carbon. This pathway enables effective energy densification even when absolute carbon content remains lower than in CW-rich mixtures, because HHV improvement is driven by oxygen removal rather than carbon addition alone (Dang et al., 2023; Wang et al., 2020). Interestingly, the 5:1 ratio reached the highest EDR at 240 °C, though its combustion reactivity remained low, indicating that high energy density does not always coincide with better ignition performance.

EY (%) quantifies the proportion of energy retained in hydrochar compared to raw biomass. For the 1:1 ratio, EY decreased from 79.8% at 160 °C to 72.4% at 240 °C, indicating that although energy densification improved, the reduction in hydrochar yield at higher temperature lowered the total retained energy. For the 3:1 ratio, EY increased slightly from 74.7% to 74.9%, showing relatively stable retention despite temperature change. In contrast, the 5:1 ratio exhibited a notable increase in EY from 69.3% at 160 °C to 79.9% at 240 °C, despite its high ash content. This elevated EY at the 5:1 ratio under 240 °C is attributable to the synergistic effect of increased hydrochar yield and moderate HHV improvement. Because HY is calculated based on the total hydrochar mass, including ash, SM-rich mixtures such as the 5:1 ratio retain a larger solid fraction, which further increases EY even though ash does not contribute to fuel quality. Therefore, EY reflects the proportion of energy retained in hydrochar relative to the raw biomass and does not indicate fuel purity, as it inherently accounts for all mass components, including ash. This interpretation aligns with the EY definition in Equation 3, where HY includes inert ash mass, while EDR is derived from HHV, which primarily reflects organic carbon content. Overall, the increase in EY with temperature indicates that HTC enhances energy retention via improved carbonization and fixed carbon formation, especially under higher temperature and SM-rich conditions (Yao et al., 2024).

The inverse relationship between S index and EDR/EY across all conditions reflects the mechanistic trade-off inherent to Co-HTC: the carbonization reactions that enrich fixed carbon and increase HHV simultaneously deplete reactive volatile matter required for ignition. In SM–CW Co-HTC, this trade-off is further modulated by mixing ratio, higher CW proportions favor aromatization-driven energy densification (high EDR, lower S), while higher SM proportions prioritize mass retention through ash accumulation (high EY, lowest S). This distinction has direct practical implications: solid fuels for fixed-bed or fluidized-bed combustion systems may prioritize EDR, whereas fuels intended for co-firing with coal may benefit from higher S index values achievable at lower HTC temperatures or higher CW proportions. These findings confirm that Co-HTC with appropriate CW proportions and temperature optimization enhances carbonization efficiency and fuel energy potential. Therefore, optimizing both HTC temperature and feedstock mixing ratio is essential to achieve a balance between combustion performance and energy retention. Compared to continuous HTC systems reported in recent studies (Ipiales et al., 2024), which primarily focus on throughput and operational scalability, the batch-mode design employed in this study enables a more controlled evaluation of carbonization behavior and feedstock synergy. This enhances the understanding of the mechanistic interactions governing hydrochar fuel quality and complements large-scale system analyses.