Synergistic Characteristics of Co-Hydrothermal Carbonization of Sewage Sludge and Waste Biomass

Abstract

This study investigated the synergistic characteristics of co-hydrothermal carbonization (co-HTC) of sewage sludge (SS) and wheat straw (WS). Co-HTC of SS and WS synergistically promoted the conversion of organic matter into combustible hydrochar components. The synergistic coefficients for hydrochar yield (YHC), energy recovery efficiency (REN), carbon recovery efficiency (RC) and hydrogen recovery efficiency (RH) first increased and then decreased as the WS blending ratio or hydrothermal carbonization (HTC) temperature increased. Additionally, blending WS or increasing the HTC temperature shifted the main combustion process of hydrochar to the low-temperature region. The average activation energy of hydrochar combustion first decreased and then increased as the WS blending ratio increased. However, the average combustion activation energies of hydrochars prepared via HTC at 200, 230 and 260 °C were essentially identical (160.61, 161.88, and 161.04 kJ mol^-1, respectively), with a relative difference of < 1%. Maintaining an HTC temperature of 200 °C avoids the higher energy input required for elevated temperatures (230 or 260 °C), which is crucial for reducing the industrial application costs of HTC while preserving hydrochar quality. Consequently, from the perspective of energy consumption, 200 °C is more suitable for the HTC. The minimum average activation energy for hydrochar combustion (160.61 kJ mol^-1) was achieved at a SS:WS mass ratio of 2:1 and an HTC temperature of 200 °C. These research findings provide a theoretical basis for the energy-oriented utilization of SS and waste biomass.

Keywords:
sewage sludge; waste biomass; hydrochar; synergistic effect; combustion; kinetic analysis

Introduction

With the rapid acceleration of global urbanization, the production of sewage sludge (SS) has increased sharply.¹ As an inevitable by-product of wastewater treatment, SS has become a major environmental concern worldwide in terms of its disposal.²,³ Traditional SS treatment methods such as landfilling, incineration, and composting, typically face a series of challenges, including secondary pollution, resource waste, and high treatment costs.⁴ Concurrently, wheat straw (WS) is a typical agricultural waste that is often mishandled, for instance through open burning. This practice exacerbates environmental burdens while wasting the energy potential of WS.⁵

Hydrothermal carbonization (HTC) is an emerging thermochemical conversion technology that has attracted attention for its ability to convert wet biomass into high-value hydrochar under relatively mild conditions (typically 180-260 °C and ٢-10 MPa).⁶ For SS, HTC eliminates the energy-intensive drying pre-treatment and enables energy recovery through hydrochar utilization.⁷ However, SS contains large amounts of ash, moisture, and organic components, resulting in hydrochar with low energy density, which greatly limits its widespread application as a high-quality solid fuel.⁸,⁹ For waste biomass (such as WS, crop straws, garden waste, and forestry residues), HTC produces hydrochar with high calorific value.¹⁰,¹¹ Nevertheless, standalone HTC of waste biomass for fuel lacks obvious energy efficiency advantages and yields relatively single products, which significantly limits improvements in economic benefits.¹²

To address these limitations, co-HTC of SS and waste biomass has been proposed as a “treating waste with waste” strategy, which leverages synergistic effects to enhance hydrochar quality and energy efficiency. Cui et al.¹³ reported that co-HTC using SS and pine sawdust as feedstocks exhibited significant synergistic effects in hydrochar preparation, promoting increases in hydrochar yield, carbon content, organic matter retention, and energy yield. Liu et al.⁷ studied the co-HTC of SS and rice straw, and found that co-HTC improved hydrochar properties, including organic matter content, higher heating value (HHV), fuel ratio, and combustion behavior. Wilk et al.¹⁴ discussed the co-HTC of SS and organic additives (such as charcoal, fir, grass, and an undersieved fraction of municipal solid waste), and observed that adding organic waste to SS during HTC increased the HHV, carbon content, and fixed carbon content of the resulting hydrochar.

Significant progress has been made in SS-waste biomass co-HTC research, and synergistic promotion effects have been confirmed. However, relatively few studies have focused on hydrochar preparation via the co-HTC of SS and WS. Additionally, the influence of blending ratio and HTC temperature on the fuel properties and combustion characteristics of hydrochar remains unclear.

Therefore, this study prepared hydrochar via the co-HTC of SS and WS under different blending ratios and HTC temperatures. First, the effects of SS-WS blending ratios and HTC temperatures on the fuel properties and synergistic effects of hydrochar were analyzed. Second, the combustion characteristics and reaction kinetics of hydrochar were investigated using a thermogravimetric analyzer (TGA). Finally, the optimal SS-WS blending ratio and HTC temperature for the co-HTC process were determined. The results of this study are expected to provide theoretical support for the energy-oriented utilization of SS and waste biomass, and to promote the application and development of HTC technology in the field of organic waste treatment.

Experimental

Materials

The SS used in this experiment was activated sludge collected in April 2024 from the secondary sedimentation tank of a wastewater treatment plant in Pingdingshan, China. It was concentrated to a moisture content of 90% before being used as the experimental raw material. The selected waste biomass was WS, collected in June 2024 from local farmland in Pingdingshan, China. After collection, impurities (mainly soil and dust particles) were removed using deionized water. The WS was then naturally air-dried and crushed into particles smaller than 1 mm to serve as the experimental raw material. To ensure representativeness and account for potential compositional variations in the samples, three parallel samples were prepared for both SS and WS.

To thoroughly analyze the physicochemical properties of SS and WS, both materials were dried in a vacuum drying oven at 95 °C until their weight stabilized. After cooling to room temperature, they were crushed and ground. Powders with a particle size of 0.10 to 0.15 mm were screened for proximate analysis, ultimate analysis, and HHV determination. The results are shown in Table 1.

Table 1
Proximate and ultimate analyses of the materials (tested in the air-dried basis state)

Material	Proximate analysis / %			HHV /(MJ kg-1)	Ultimate analysis / %
	A	V	FC		C	H	O	N	S
SS	44.54	49.12	6.34	11.33	32.45	4.87	18.45	3.84	1.03
WS	4.29	74.05	21.66	15.97	45.12	6.21	30.22	1.65	0.32

• HHV: higher heating value; A: ash content; V: volatile matter; FC: fixed carbon; SS: sewage sludge. WS: wheat straw.

Proximate analysis was carried out using a proximate analyzer (TGA2000, Las Navas, Spain). This analysis determined the ash content (A), volatile matter (V), and fixed carbon (FC) of the samples, and these data were used to evaluate the fuel potential of samples. Ultimate analysis was performed using a CHN/O/S elemental analyzer (C-440, Well Group, USA). It quantified the contents of C, H, O, N, and S, and the resulting data were used to calculate fuel-related parameters. The HHV is a core indicator of fuel quality, and it was determined using an automatic calorimeter (ZDHW-5G, HuaTai, China). To ensure the accuracy and reliability of the results, three parallel tests were conducted for each experiment, and the average value was calculated.

Preparation of hydrochar

Mixtures of SS (with 90% moisture content) and WS particles were prepared at mass ratios (on a dry basis) of 4:1, 3:1, 2:1, and 1:1. Each mixture was placed into the reactor liner, with a filling volume of 70%. The HTC reaction was conducted at 200 °C for 2 h. After cooling to room temperature, the mixture was filtered and dried in a vacuum drying oven at 105 °C for 12 h. It was subsequently crushed and milled, and hydrochar particles with sizes ranging from 0.10 to 0.15 mm were screened out as samples. These samples were named HC-S4W1, HC-S3W1, HC-S2W1, and HC-S1W1, respectively. For comparison, hydrochar samples were also prepared from SS and WS individually under the same conditions, and named HC-SS and HC-WS, respectively.

To analyze the effect of HTC temperature on hydrochar performance, samples HC-170, HC-200, HC-230, and HC-260 were prepared. For these samples, SS and WS were mixed at a 2:1 mass ratio, and the HTC reaction was carried out at temperatures of 170, 200, 230, and 260 °C, respectively, while other conditions remained unchanged. It is worth noting that HC-200 and HC-S2W1 are the same sample, as they were prepared under identical conditions.

To ensure the accuracy and reliability of the results, three parallel samples were prepared for each experimental condition, and the average value was calculated.

Combustion experiments of hydrochar

Combustion experiments of hydrochar were carried out in a TGA (Q50, TA Instruments, USA), which consists of a sample holder placed in a programmable furnace inside an alumina crucible. The sample holder was monitored by a sensitive precision balance, and the heating rate and temperature range were controlled via a control panel. To account for any interference of the carrier gas on the measured weights, a blank experiment was conducted for each experimental treatment under the same conditions, and the measured weights were adjusted by subtracting the blank values accordingly.

For each experiment run, 15 mg of hydrochar were weighed and uniformly dispersed in an alumina crucible. After closing the reaction chamber, ultrapure N₂ (99.999%) was introduced at a flow rate of 60 mL min^-1 for 15 min to purge the TGA apparatus. The hydrochar was then heated from room temperature to 900 °C under a simulated air atmosphere (V(N₂):V(O₂) = 4:1) at a flow rate of 60 mL min^-1 and a heating rate of 10, 20, or 40 °C min^-1. The weight loss characteristics of the hydrochar were recorded as a function of time and temperature. All TGA experiments were repeated 3 times, and the weight loss data were averaged to confirm reproducibility (relative standard deviation < 3%).

Formula and calculation

Hydrochar yield (Y_HC), energy recovery efficiency (R_EN), carbon recovery efficiency (R_C) and hydrogen recovery efficiency (R_H) were calculated in accordance with equations 1, 2, 3, and 4, respectively.

(1)$${\text{Y}}_{\text{HC}}(\%)=\frac{{\text{M}}_{\text{HC}}}{{\text{M}}_{\text{raw}}}\times 100$$

(2)$${\text{R}}_{\text{EN}}=\frac{{\text{HHV}}_{\text{HC}}}{{\text{HHV}}_{\text{raw}}}\times {\text{Y}}_{\text{HC}}$$

(3)$${\text{R}}_{\text{C}}=\frac{{\text{C}}_{\text{HC}}}{{\text{C}}_{\text{raw}}}\times {\text{Y}}_{\text{HC}}$$

(4)$${\text{R}}_{\text{H}}=\frac{{\text{H}}_{\text{HC}}}{{\text{H}}_{\text{raw}}}\times {\text{Y}}_{\text{HC}}$$

where M_raw and M_HC respectively represent the mass of raw feedstock and its hydrochar, in mg; HHV_raw and HHV_HC respectively represent the HHV of raw feedstock and its hydrochar, in MJ kg^-1; C_raw and C_HC respectively represent the carbon content in ultimate analysis of raw feedstock and its hydrochar, in percentage; and H_raw and H_HC respectively represent the hydrogen content in ultimate analysis of raw feedstock and its hydrochar, in percentage.

The combustion performance of hydrochar was evaluated using the comprehensive combustion characteristic index (S) and the flame combustion stability index (F). A larger S value indicates better combustion characteristics, while a larger F value indicates better combustion stability. Equations 5 and 6 were used to calculate S and F, respectively.¹⁵,¹⁶

(5)$$\text{S}=\frac{{\text{(dm/dt)}}_{\text{max}}{\text{(dm/dt)}}_{\text{mean}}}{{\text{T}}_i^2{\text{T}}_{\text{b}}}$$

(6)$$\text{F}=\frac{{\text{(dm/dt)}}_{\text{max}}}{{\text{T}}_{\text{i}}(\Delta \text{T})}$$

where (dm/dt)_max is the maximum mass loss rate of hydrochar, in % min^-1; (dm/dt)_max is the average mass loss rate of hydrochar, in % min^-1; T_i is the ignition temperature, in ºC; T_b is the burnout temperature, ºC; and ∆T is the combustion temperature range, which is calculated by subtracting T_i from T_b.

The synergistic coefficient (SC), which was used to evaluate the synergic effect of co-HTC, was calculated by equation 7.¹⁷

(7)$$\text{SC(\%)}=\left|\frac{{\text{K}}_{\text{ex}}-{\text{K}}_{\text{ca}}}{{\text{K}}_{\text{ca}}}\right|\times 100$$

where K_ex is the experimental value parameters of hydrochar; and K_ca is the theoretical value parameters of hydrochar, which can be calculated by equation 8.

(8)$${\text{K}}_{\text{ca}}={\lambda }_{\text{SS}}{\text{K}}_{\text{HC-SS}}+{\lambda }_{\text{WS}}{\text{K}}_{\text{HC-WS}}$$

where λ_SS and λ_WS are the mass ratios of SS and WS used to prepare hydrochar, respectively; and K_HC-SS and K_HC-WS are the experimental value parameters of HC-SS and HC-WS, respectively.

Combustion reaction kinetics

The combustion conversion rate (α) of the hydrochar was defined by equation 9.

(9)$$\alpha =\frac{{\text{m}}_0-\text{m}}{{\text{m}}_0-{\text{m}}_{\text{a}}}$$

where α is the combustion conversion rate, in percentage; m₀ is the initial mass of hydrochar, mg; m is the hydrochar mass at time t, in mg; and m_a is the mass of hydrochar at the end of combustion reaction, in mg.

The kinetic equation of the combustion reaction follows the general equation of gas-solid reaction, which can be described by equation 10.

(10)$$\frac{\text{d}\alpha }{\text{dt}}=\text{kf(}\alpha )$$

where t is the reaction time, in s; k is the reaction rate constant; f(α) is the combustion reaction mechanism function, which is mainly determined by reaction type or mechanism.

The reaction rate constant (k) follows the Arrhenius law, which can be described by equation 11.

(11)$${\text{k=A}}_{\text{r}}{\text{e}}^{\frac{\text{E}}{\text{RT}}}$$

where T is the reaction temperature, in K; A_r is pre-exponential factor, in s^-1; E is the activation energy, in kJ mol^-1; R is the universal gas constant.

The heating rate (β) was expressed by equation 12.

(12) $$\beta =\frac{\text{dT}}{\text{dt}}$$

Substituting k and β into equation 10, equation 13 can be obtained.

(13) $$\frac{\text{d}\alpha }{\text{dT}}\text{=}\frac{\text{A}}{\beta }{\text{e}}^{\frac{\text{E}}{\text{RT}}}\text{f(}\alpha \text{)}$$

The traditional single heating rate method struggles to ensure the rationality of the chosen reaction mechanism function. Therefore, the Model-free method was used in this study to calculate the activation energy of the combustion reaction. The Model-free method performs combustion kinetic analysis based on the thermal analysis curves obtained at different heating rates, which separates the reaction mechanism function and obtains the reliable value of activation energy without introducing the kinetic model function.¹⁸,¹⁹

The activation energy was calculated using the Ozawa-Flynn-Wall (OFW) method, which can be described by equation 14.¹⁹,²⁰

(14) $$\text{ln(}\beta \text{) = ln}\left(\frac{\text{AE}}{\text{Rf}(\alpha )}\right)-5.331-1.052\frac{\text{E}}{\text{RT}}$$

According to different heating rates and a given α, a linear relationship was observed by plotting ln(β) vs. T^-1, and E was calculated by the slope. During the experiment, different heating rates were controlled by TGA, and then the value of E was subsequently obtained.

Results and Discussion

Effects on fuel properties

Table 2 shows the proximate and ultimate analyses of the hydrochars. As observed, compared with SS, HC-SS exhibited increased A, accompanied by decreases in the proportions of V and FC. This phenomenon can be attributed to the hydrolysis reaction of organic matter in SS during HTC. Zhuang et al.²¹,²² also observed the dissolution of organic matter in their co-HTC studies of SS and penicillin mycelia waste, finding that 69.3% of the organic matter in SS and 56.2% in penicillin mycelia waste dissolved after 30 min of HTC at 240 °C.

Table 2
Proximate and ultimate analyses of the hydrochars (tested in the air-dried basis state)

Hydrochar	Proximate analysis / %			HHV /(MJ kg-1)	Ultimate analysis / %
	A	V	FC		C	H	O	N	S
HC-SS	50.29	44.24	5.47	9.97	30.15	3.54	12.45	2.15	0.64
HC-WS	3.14	65.45	31.41	21.34	55.78	5.97	22.14	1.36	0.24
HC-S4W1	41.09	47.56	11.35	12.54	35.57	4.33	15.68	1.98	0.62
HC-S3W1	37.55	49.75	12.70	13.57	37.15	4.49	16.48	2.03	0.55
HC-S2W1	34.93	50.15	14.92	14.56	40.45	4.98	17.54	1.95	0.57
HC-S1W1	27.22	53.24	19.54	16.52	45.32	5.48	18.32	1.85	0.46
HC-170	29.86	58.75	11.39	12.95	36.29	5.12	27.63	1.83	0.54
HC-230	35.61	48.36	16.03	15.07	43.49	4.33	12.34	2.65	0.59
HC-260	36.03	47.23	16.74	15.36	44.04	4.02	10.65	2.87	0.60

• HHV: higher heating value; A: ash content; V: volatile matter; FC: fixed carbon; SS: sewage sludge; WS: wheat straw.

As a result of hydrolysis reaction, the C and H, which constitute the combustible components of hydrochar, were partially dissolved into the aqueous phase. Consequently, the C and H contents in HC-SS were lower than those in SS, which reduced the HHV of HC-SS to 9.97 MJ kg^-1. As evident, while standalone HTC of SS can reduce the energy consumption required for SS carbonization, it does not facilitate the calorific value of HC-SS.

In contrast, compared with WS, HC-WS showed decreases in A and V, while the proportion of FC increased from 21.66 to 31.41%. This indicates that during HTC, although some volatile matter in WS undergoes hydrolysis, another portion is converted into FC with aromatic structures via pathways such as dehydration, cyclization, and polymerization,²³ and thus the HHV of HC-WS increased to 21.34 MJ kg^-1.

As the WS blending ratio increased during the HTC process, the A gradually decreased, while the V, FC, HHV and fuel ratio gradually increased. When the SS:WS blending ratio reached 1:1, the fuel ratio of HC-S1W1 was 0.37 and its HHV was 16.52 MJ kg^-1, which were 3.08 times of the fuel ratio and 1.66 times of the HHV for HC-SS. This indicates that blending SS with WS can significantly increase the combustible component content of hydrochar, thereby improving the fuel ratio and HHV of hydrochar. Zhang et al.²⁴ also discovered this trend in their study of co-HTC of SS and pinewood sawdust, proposing that co-HTC of SS and waste biomass synergistically promotes the cyclization and aromatization of solid-phase molecules, as well as the polymerization of water-soluble molecules to form hydrochar.

As the HTC temperature increased, the A, FC, C, HHV, and fuel ratio of hydrochar gradually increased, while the V and H gradually decreased. When the HTC temperature increased from 170 to 260 °C, the FC increased from 11.39 to 16.74%, the C increased from 36.29 to 44.04%, the HHV increased from 12.95 to 15.36 MJ kg^-1, and the fuel ratio increased from 0.19 to 0.35. These results confirm that increasing the HTC temperature improves the fuel properties of hydrochar. This aligns with the research findings of Petrović et al.²⁵ on the HTC of corn cob, paulownia leaves, and olive pomace at different temperatures (180, 220, and 260 °C). This improvement occurs primarily because during the HTC process, the hydroxyl groups (-OH) in the volatile matter are removed as H₂O through dehydration. Meanwhile, the carboxyl groups (-COOH) and carbonyl groups (-C=O) in the volatile matter are released as CO₂ and CO, respectively, through decarboxylation.²¹ Finally, volatile matter is converted into aromatic substances through the above-mentioned dehydration and decarboxylation reactions. Higher HTC temperatures are more likely to promote these reactions, which not only increase FC but also gradually raise the HHV of hydrochar. In addition, the H gradually decreased as the HTC temperature increased, and this phenomenon was also caused by the dehydration and decarboxylation of volatile matter.²⁶

When the HTC temperature increased from 170 to 200 °C, the V decreased by 8.60%, the FC increased by 3.53%, and the fuel ratio increased by 0.11. These changes were the most significant, indicating that the carbonation process of the organic blends was most pronounced within the temperature range of 170 to 200 °C. However, Lei et al.²⁷ found that the carbonation process of SS and corn straw blends varied most significantly between 220 and 250 °C. This discrepancy may stem from differences in the composition of the SS and straw used in the respective studies.

Effects on energy recovery

Figure 1 shows the experimental values, theoretical values, and SCs of Y_HC, R_EN, R_C and R_H for hydrochars prepared by SS and WS at different blending ratios. The Y_HC of HC-SS was 68.24%, while that of HC-WS was 55.52%. However, the R_EN of HC-SS was only 60.05%, which was significantly lower than that of HC-WS (74.19%). Additionally, the R_C and R_H of HC-SS were lower than those of HC-WS.

Figure 1
The impact of different blending ratios of raw materials on the energy recovery.

When SS and WS were blended for co-HTC, the Y_HC gradually decreased as the WS blending ratio increased, while the R_EN, R_C, and R_H gradually increased. The R_EN of HC-S2W1 and HC-S1W1 were 74.66 and 76.06%, respectively, which were higher than that of HC-WS (74.19%). This indicates that the co-HTC of SS and WS can promote the conversion of more organic matter into combustible components fixed in hydrochar, thereby synergistically promoting the Y_HC and R_EN of hydrochar. Wang et al.²⁸ concluded that lignin in straw waste can produce phenolic compounds through hydrolysis reactions, which interact with SS during co-HTC processes to improve the yield and fuel properties of hydrochar.

Moreover, the Y_HC, R_EN, R_C and R_H of HC-S4W1, HC-S3W1, HC-S2W1, and HC-S1W1 were all higher than their respective theoretical values. This further confirms that SS and WS exert a synergistic promoting effect on co-HTC. Wang et al.²³ also reached similar conclusions in their study of the co-HTC of SS and cornstalk, noting that biomass produces carbonyl compounds (such as aldehydes and ketones) during HTC, which polymerize with the amino compounds formed by the degradation of SS to generate solid-phase microparticles. Additionally, water-soluble substances formed by degradation undergo bonding reactions with the functional groups in SS and biomass matrix, and are re-fixed.²⁸ For these reasons, the experimental value of Y_HC was higher than the theoretical value.

To objectively evaluate the synergistic effect of WS blending ratio on the fuel properties of hydrochar during co-HTC, the SC was calculated. As the WS blending ratio increased, the SCs of Y_HC, R_EN, R_C and R_H first increased and then decreased, reaching a maximum at a SS:WS mass ratio of 2:1. This may be because when the WS blending ratio was too high, the amount of organic acids, such as acetic acid and valeric acid, produced by degradation, increased. This enhanced the acidity of the HTC environment, promoting the dehydration and decarboxylation of the volatile components in the SS and WS. In turn, this inhibited hydrochar formation, leading to decreased Y_HC. It also inhibited the retention of C and H, resulting in decreased R_EN.

Figure 2 shows the experimental values, theoretical values and SCs of the Y_HC, R_EN, R_C and R_H for hydrochars prepared from SS and WS at a 2:1 mass ratio under different HTC temperatures. As the HTC temperature increased, the Y_HC, R_EN, R_C and R_H of hydrochar all decreased, because higher HTC temperatures enhance the reactivity and solubility of subcritical water, causing a large amount of organic matter to hydrolyze or vaporize. Consequently, the Y_HC of hydrochar decreases. Therefore, although increasing the HTC temperature helps to increase the HHV of hydrochar, it reduces the R_EN. Koprivica et al.²⁹ and Petrović et al.³⁰ also proposed that elevated HTC temperatures reduce the oxygen content of biomass and alter the O/C and H/C atomic ratios, which facilitates the production of hydrochar with improved fuel properties, particularly a higher HHV.

Figure 2
The impact of different HTC temperatures on the energy recovery.

Moreover, the degree of decrease in Y_HC, R_EN, R_C and R_H of hydrochar varied across different HTC temperature ranges. The Y_HC showed the largest decrease (9.31%) between 170 and 200 °C. The R_EN and R_C showed the largest decreases (4.93 and 5.42%, respectively) between 230 to 260 °C. In contrast, the R_H showed the largest decrease (13.28%) between 200 and 230 °C. This is mainly because the SS-WS mixture contains substances such as proteins and cellulose, which undergo intense hydrolysis reactions at different HTC temperature ranges.³¹,³² Petrović et al.³⁰ also found that hydrolysis of the thermally least stable hemicellulose occurs at low HTC temperature (180 °C), while cellulose and lignin degradation require more intense conditions.

Additionally, the Y_HC, R_EN, R_C and R_H of HC-170, HC-200, HC-230, and HC-260 were all higher than the theoretical values. This further confirms the synergistic promoting effect of SS and WS on the co-HTC of hydrochar under different HTC temperatures. However, the SCs of Y_HC, R_EN, R_C and R_H first increased and then decreased as the HTC temperature increased, reaching their maximum at 200 °C.

Effects on combustion characteristics

Figure 3 shows the combustion TGA curves of hydrochar. As observed, the maximum weight loss rate of hydrochar increased with the increase of WS blending during HTC. Hydrochar combustion is a non-isothermal reaction of a non-homogeneous solid, and the combustion TG curves of hydrochar can be divided into 3 stages.

Figure 3
The combustion TGA curves of hydrochar.

The first stage occurs below 200 °C and is primarily characterized by the release of water molecules and weakly bound light volatile compounds. The insignificant weight loss in the first stage is because the hydrochar contains minimal moisture and weakly bound volatile compounds.³³

The second stage occurs between 200 and 500 °C, arising mainly from the pyrolysis of organic matter, such as cellulose and hemicellulose.²⁹ This stage involves the greatest weight loss of the hydrochar, with mass losses exceeding 50% of the total weight loss. This is due to the escape and combustion of large amounts of volatile components in the hydrochar at this stage.²⁹

The third stage occurs between 500 and 700 °C, resulting from the decomposition of difficult-to-pyrolyze organic matter (such as lignin) and the combustion of fixed carbon.³⁴

As shown in Table 3, the combustion characteristic parameters were derived from the combustion TG curves of hydrochar. The T_i of HC-SS was 235.4 °C, lower than the 303.8 °C of HC-WS, indicating that HC-SS is easier to ignite. However, the T_b of HC-SS was 685.5 °C, much higher than the 519.5 °C of HC-WS, indicating that the combustion process of HC-WS mainly occurs in the low-temperature region.¹³ The ∆T of HC-SS was 450.1 °C, higher than the 215.7 °C of HC-WS, indicating that the complete combustion of HC-SS requires a longer time. As the WS blending ratio or the HTC temperature increased during HTC, the T_i of hydrochar gradually increased, while the T_b and ∆T gradually decreased. This is consistent with the phenomenon observed by Koprivica et al.²⁹ in their study of the combustion behavior of paulownia leaf hydrochars. This indicates that adding WS or increasing the HTC temperature during HTC can shift the main combustion process of hydrochar to the low-temperature region and shorten the combustion time, but it deteriorates the ignition performance1.¹³,²⁹

Table 3
Combustion characteristic parameters of hydrochar

Hydrochar	Ti / ºC	Tb / ºC	∆T / ºC	(dm/dt)max / (% min-1)	(dm/dt)mean / (% min-1)	S / (min-2 oC-3)	F / (min-1 oC-2)
HC-SS	235.4	685.5	450.1	3.97	1.03	1.08 × 10-7	3.75 × 10-5
HC-WS	303.8	519.5	215.7	17.59	1.56	5.72 × 10-7	26.84 × 10-5
HC-S4W1	253.1	645.3	392.2	6.89	1.15	1.92 × 10-7	6.94 × 10-5
HC-S3W1	259.4	630.1	370.7	7.63	1.20	2.16 × 10-7	7.93 × 10-5
HC-S2W1	268.2	611.9	343.7	8.83	1.27	2.55 × 10-7	9.58 × 10-5
HC-S1W1	275.9	595.4	319.5	10.96	1.37	3.31 × 10-7	12.43 × 10-5
HC-170	257.3	635.8	378.5	7.43	1.19	2.10 × 10-7	7.63 × 10-5
HC-230	271.6	605.3	333.7	9.26	1.31	2.72 × 10-7	10.22 × 10-5
HC-260	273.4	603.2	329.8	9.35	1.33	2.75 × 10-7	10.37 × 10-5

• (dm/dt)_max: maximum mass loss rate of hydrochar; (dm/dt)_max: average mass loss rate of hydrochar; T_i: ignition temperature; T_b: burnout temperature; ∆T: combustion temperature range; S: combustion characteristic index; F: flame combustion stability index.

The (dm/dt)_max and (dm/dt)_mean of HC-SS were 3.97 and 1.03% min^-1, respectively, which were much lower than the 6.89 and 1.15% min^-1 of HC-WS. This indicates that HC-WS exhibits higher reaction intensity and a faster reaction rate than HC-SS during the volatile decomposition stage of combustion.⁷,¹³ As the WS blending ratio or the HTC temperature increased during HTC, the(dm/dt)_max and (dm/dt)_mean gradually increased. This suggests that adding WS or increasing HTC temperature during HTC can promote the reaction intensity and rate of hydrochar combustion.¹³,³³

The S and F values of the HC-SS were 1.08 × 10^-7 min^-2 °C^-3 and 3.75 × 10^-5 min^-1 °C^-2, respectively, which were much lower than those of HC-WS. As the WS blending ratio or the HTC temperature increased during HTC, the S and F values of hydrochar gradually increased. This indicates that adding WS or increasing the HTC temperature during HTC can improve the combustion characteristics and flame combustion stability of hydrochar.¹⁴,³⁴

Figure 4 shows the SCs of the combustion characteristics of hydrochar. The SC value of (dw/dt)_mean gradually increased as the WS blending ratio or the HTC temperature increased, indicating that a higher WS blending ratio or higher HTC temperature leads to a more obvious synergistic effect on the average weight loss rate of hydrochar. However, the SC values of T_i, T_b, ΔT, (dw/dt)_max, S, and F all first increased and then decreased, reaching their maximum when the SS:WS ratio was 2:1 and the HTC temperature was 200 °C. This indicates that blending at this ratio and temperature exerted the strongest synergistic effect on hydrochar combustion.

Figure 4
The SCs of the combustion characteristics of hydrochar.

Combustion kinetic analysis of hydrochar

Figure 5 shows the fitted lines obtained by the OFW method for HC-S2W1 (identical to HC-200) at different conversion rates (α = 0.1-0.9). Similar fitting calculations were performed for other hydrochars, and the fitted lines are not listed individually. The correlation coefficients of the fitted lines were all not less than 0.95, indicating that the OFW method is suitable for calculate the E of the hydrochar combustion reaction. The slope of the fitted line was used to calculate the E corresponding to different conversion rates. Table 4 shows the E values obtained by the OFW method at different heating rates (10, 20, and 40 °C min^-1) during hydrochar combustion.

Figure 5
The OFW method linear fit of combustion for HC-S2W1 at different heating rates.

Table 4
Activation energy (E) of hydrochar during combustion

α	E / (kJ mol-1)
	HC-SS	HC-WS	HC-S4W1	HC-S3W1	HC-S2W1	HC-S1W1	HC-170	HC-230	HC-260
0.1	104.94	143.86	109.57	122.02	128.98	135.99	108.45	134.07	146.26
0.2	134.19	152.05	138.24	141.85	140.18	155.70	137.84	145.70	150.35
0.3	165.72	155.19	162.47	161.25	160.29	163.00	154.45	162.05	153.34
0.4	188.28	157.53	175.65	171.05	162.82	168.22	164.54	165.22	157.55
0.5	190.27	162.05	179.13	172.79	165.18	170.68	175.36	166.68	161.66
0.6	192.24	166.15	189.26	173.47	166.83	171.88	180.65	168.68	165.02
0.7	194.15	167.09	191.99	184.24	167.63	172.29	187.54	168.78	165.37
0.8	203.20	170.53	195.65	185.27	175.47	172.71	188.48	171.47	172.18
0.9	208.21	176.86	196.57	190.23	178.14	177.45	190.44	174.23	177.55
Average	175.69	161.26	170.95	166.91	160.61	165.32	165.31	161.88	161.038

• α: combustion conversion rate.

As the α increased, the E of hydrochar generally showed a gradual upward trend. The similar relation between E and α of wood sawdust hydrochar combustion was also observed in the study of Kabakcı,³⁵ and are consistent with the trend identified by some scholars that the activation energy of fuel reactions increases as the reactant conversion rate increases.³⁶ This may be because as the reaction proceeds, the proportion of the hydrochar components that are easy to combust gradually decreases, while the proportion of components that are more difficult to combust increases, making the reaction more difficult.³⁷

The E of HC-SS was smaller than that of HC-WS at low conversion rates (α = 0.1-0.2). This indicates that the initial combustion of HC-SS requires less activation energy than HC-WS. This is consistent with Table 3, which shows that the initial ignition temperature (T_i) of HC-WS is lower than that of HC-SS. However, as the combustion reaction progressed, the E of HC-SS increased much more than that of HC-WS. Therefore, the average E of HC-WS was ultimately lower than that of HC-SS. The similar research findings were also observed in the study of Wilk et al.³⁸ who applied Arrhenius method to find out the activation energy of SS hydrochar combustion is higher than that of acacia and pine hydrochars. Probably, the hydrothermal process already degrades the complex and heterogeneous nature of biomass with the highly cross-linked cell wall components of cellulose, hemicellulose and lignin, which should lead to lower activation energies.³⁸

The blending of WS during HTC resulted in a decrease in the average E of each hydrochar compared with that of HC-SS. Cardarelli et al.³⁹ also found that the hydrochar from the co-HTC of vine pruning and exhausted grape marc exhibited reduced activation energy for combustion processes. As the WS blending ratio increased during HTC, the E of hydrochar gradually increased at low conversion rates (α = 0.1-0.2). However, the average E of hydrochar first decreased and then increased, with the average E of HC-S2W1 reaching a minimum of 160.61 kJ mol^-1. This is mainly because co-HTC can reduce unstable biomass compounds (such as hemicellulose and cellulose) while increasing those with higher bond energies (such as lignin).³⁹

As the HTC temperature increased, the E of hydrochar gradually increased at low conversion rates (α = 0.1-0.2). This indicates that a higher carbonization temperature makes the combustion reaction more difficult to proceed. The similar relation between E and HTC temperature of coffee beans hydrochar combustion was also observed in the study of Santana et al.⁴⁰ who believed that the hydrochar structure, formed by transformations occurred during hydrothermal carbonization process, is more resistant to initial stage oxidation in the combustion process. Furthermore, except for HC-170, which has a higher average E, the average E values of HC-200, HC-230, and HC-260 were similar (with a relative difference of < 1%). This is consistent with the phenomenon observed by Yu et al.⁴¹ in their study of the combustion behavior of maize straw hydrochar. However, Wu et al.⁴² found that the average activation energy of agricultural waste hydrochar combustion increases with the HTC temperature, while Lang et al.⁴³ found that the average activation energy of the cattle manure hydrochar combustion first increases and then decreases with the HTC temperature. This difference may reflect significant variations in the composition of hydrochar used in different experiments.

Maintaining an HTC temperature of 200 °C avoids the higher energy input require for elevated temperatures (230 or 260 °C), which is crucial for reducing the industrial application costs of HTC while preserving hydrochar quality. From an energy consumption perspective in the HTC process, a temperature of 200 °C is more suitable.

Conclusions

The synergistic characteristics of co-HTC for SS and WS were investigated. The Y_HC, R_EN, R_C, and R_H of hydrochars derived from co-HTC at different blending ratios and HTC temperatures all exceeded their respective theoretical values. During HTC, increasing the WS blending ratio or HTC temperature shortened hydrochar combustion time while improving its combustion characteristics and flame stability. Notably, the synergistic promotion of hydrochar combustion was strongest at a SS:WS mass ratio of 2:1 and an HTC temperature of 200 °C.

As the WS blending ratio increased during HTC, the activation energy of hydrochar combustion gradually rose at low conversion rates (α = 0.1-0.2), whereas the average activation energy first decreased and then increased. Additionally, the average combustion activation energies of hydrochars prepared via HTC at 200, 230, and 260 °C were essentially identical, with a relative difference of < 1%. Maintaining an HTC temperature of 200 °C avoids the higher energy input required for elevated temperatures (230 or 260 °C), which is crucial for reducing the industrial application costs of HTC while preserving hydrochar quality. Therefore, from an energy consumption perspective, 200 °C is more suitable for the HTC.

The co-HTC of SS and WS converts two types of waste into a clean solid fuel, which helps reduce the environmental burden associated with waste treatment. Moreover, the optimized co-HTC operating conditions provide benchmark data for industrial scale-up. Future work will extend this study to other organic wastes to establish a universal co-HTC strategy.

Supplementary Information

is available free of charge at http://jbcs.sbq.org.br as PDF file.

Data Availability Statement

All data are available in the text.

Acknowledgments

The authors gratefully acknowledge the financial support provided by the Science and Technology Research Program of Chongqing Municipal Education Commission under grant number KJQN202502602 and KJQN202502603, and the Science and Technology Project of Henan Province of China under grant number 252102321080.

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Edited by

• Editor handled this article:
  Josué Carinhanha Caldas Santos (Associate)

Publication Dates

• Publication in this collection
  17 Nov 2025
• Date of issue
  2025

History

• Received
  16 Aug 2025
• Accepted
  01 Oct 2025