Comparison of the Characteristics of Hydrochar and Torrefied-Char of Traditional Chinese Medicine Residues

Abstract

With the continuous reduction in fossil energy reserves and the increasingly prominent negative impacts on the environment, the search for sustainable alternative materials has become an urgent task. Biomass-based char has attracted much attention in the field of environmental protection, due to its wide-ranging and renewable raw materials. Hydrothermal carbonization and torrefaction carbonization, as two important biomass carbonization processes, each have their own advantages. This study focuses on the millions of tons of Chinese medicine residue waste generated in China every year. Four common Chinese medicine residues, Shanyao (Chinese yam), Suoyang (Cynomorium songaricum), Yujin (Curcuma aromatica), and Xueteng (Spatholobus suberectus), were selected and treated by hydrothermal carbonization and torrefaction carbonization processes at temperatures of 240 °C, 260 °C, and 280 °C. Through analysis techniques such as Fourier-Transform Infrared Spectroscopy, X-Ray Diffraction, and Scanning Electron Microscopy, the changes in the crystal structure, chemical functional groups, and microscopic morphology of the carbonized products were deeply studied, and the carbon yield was measured. The research aims to reveal the carbonization laws of Chinese medicine residues, provide a scientific basis for their efficient resource utilization, and help promote the development of biomass-based carbon materials in the field of environmentally friendly materials, alleviating energy and environmental pressures.

1. Introduction

In the global energy landscape, fossil fuels have long dominated energy supply systems. In the context of accelerating global energy transition and mounting ecological pressures, the energy sector is witnessing disruptive transitions across its value chain [1]. The International Energy Agency (IEA) reports that at the current rate of consumption, traditional fossil fuels such as oil and coal may face severe depletion within this century. Over-reliance on fossil fuels not only triggers energy crises, but also leads to substantial emissions of greenhouse gases (e.g., CO₂, SO₂), contributing significantly to global ecological degradation, including climate change and air pollution [2]. Against this backdrop, nations worldwide are accelerating renewable energy development. In 2023, renewables accounted for 35% of global electricity generation, with solar- and wind-power capacities continuing their upward trajectory. However, the green transition of energy systems still faces multiple challenges and remains heavily contingent on the advancement of renewable energy technologies [3]. Consequently, there is an urgent need to explore diversified pathways for renewable energy utilization.

The global energy production landscape is progressively transitioning toward circular economy principles and resource-efficient utilization, with the extraction of renewable resources from materials, feedstocks, and residues emerging as a pivotal trend [4,5]. Various biomass resources—including agricultural waste (e.g., crop straw, rice husks) and forestry residues (e.g., sawdust, bark)—as well as organic industrial byproducts and waste liquids exhibit significant potential for energy conversion. Through thermochemical processes (e.g., pyrolysis, gasification) and biological fermentation technologies, these waste streams can be transformed into value-added products such as biofuels and biochar. This dual approach not only mitigates environmental pollution, but also alleviates energy supply pressures. Consequently, the resource recovery and utilization of waste materials represents a critical pathway for achieving sustainable industrial development [6].

China’s traditional Chinese medicine (TCM) industry, with its long history and large-scale production, generates substantial medicinal residues. Under national policy incentives, the TCM sector has flourished, producing over 30 million tons of herb residues annually [7,8]. TCM residues (TCMRs) represent a significant stream of organic solid waste generated throughout pharmaceutical manufacturing and clinical applications. Three primary sources contribute to TCMRs: (1) industrial extraction processes (water decoction, alcohol precipitation, ultrasound-assisted extraction) yielding 40–60% dry basis residues post multi-stage extraction; (2) decoction byproducts from healthcare facilities and households, averaging 30–50% wet residue (60–70% moisture content) per dosage; and (3) concentrate wastes from formula granule production. Current data indicate China’s annual TCMRs output exceeds 35 million metric tons, growing 8% yearly. These residues contain substantial organic components (cellulose 30–50%, hemicellulose 15–25%, lignin 10–20%), residual bioactives (polysaccharides, flavonoids), and notable calorific value (12–18 MJ/kg). Presently, 75% of TCMRs undergo landfilling, 20% uncontrolled incineration, and merely 5% valorization. Improper disposal causes multidimensional impacts: land occupation (2.5 acres/10,000 tons), methane emissions (25 × GWP of CO₂), and leachate pollution (COD 5000–8000 mg/L). Significant regional disparities exist in the treatment and utilization of TCMRs: developed regions are pioneering resource-recovery approaches such as bio-fermentation and active compound extraction, though large-scale industrialization remains unrealized. In contrast, less-developed areas, constrained by technological and financial limitations, still rely on conventional disposal methods (e.g., incineration, open dumping, and landfill). Jia et al. [9] prepared activated carbon from TCMRs through carbonization and activation processes, utilizing its abundant pore structure to fabricate supercapacitors. Li et al. [10] produced biochar from TCMRs via carbonization, achieving an adsorption capacity of 252.0 mg/g for norfloxacin in aqueous solutions. In another study, Li et al. [11] obtained biochar by carbonizing TCMRs at different temperatures and subsequently ball-milling it to produce ball-milled biochar, which demonstrated an adsorption capacity of 293.3 mg/g for quercetin at 318 K. Zhang et al. [12] selected three common TCMRs (Forsythia suspensa, Viola yedoensis, and Lonicera japonica) for pyrolysis at various temperatures to produce biochar as a carrier for immobilizing Bacillus cereus with high chlortetracycline degradation efficiency. Yang et al. [13] prepared biochar through hydrothermal carbonization of Glycyrrhiza uralensis residues for chromium adsorption and recovery. The development of sustainable TCMR valorization strategies—particularly for industrial and environmental applications—to achieve high-value utilization has emerged as a critical bottleneck for the sustainable development of the traditional Chinese medicine industry [14].

Biochar has emerged as a sustainable alternative material with promising environmental applications [15,16,17,18]. The production primarily employs pyrolysis and hydrothermal carbonization (HTC) processes to convert biomass into carbon-rich solid materials. In practical applications, biochar exhibits remarkable versatility. In agriculture, it serves as a soil amendment, enhancing fertility, water retention, and crop productivity. In environmental remediation, it effectively adsorbs heavy metals and organic pollutants from river and soil systems. In the energy sector, high-quality biochar can function as a solid fuel, partially replacing fossil fuels and reducing carbon emissions [19].

HTC and torrefaction are two key thermochemical conversion processes, each with distinct advantages [20,21]. HTC under mild hydrothermal conditions (180–300 °C) does not require pre-drying, making it suitable for high-moisture biomass. This process conserves energy and retains essential nutrients and functional groups, enabling the production of tailored carbon materials [22]. Torrefaction, a mild pyrolysis process (<300 °C), enhances biomass energy density and stability through dehydration and devolatilization, yielding a more suitable solid fuel [8]. At present, there are many applications of biochar, but the formation mechanism of biochar needs to be at depth. It is noteworthy that biochar production from TCMRs carries potential safety risks, due to residual bioactive compounds (e.g., alkaloids) and heavy metals (e.g., lead, cadmium). When applied for soil amendment, these contaminants may inhibit beneficial microorganisms and hinder plant nutrient uptake [23]. In water treatment applications, they could generate harmful byproducts, pose health risks, and cause agricultural product contamination through heavy metal accumulation and secondary pollution [24]. Risk mitigation strategies include raw material pretreatment through physical separation and chemical washing and optimization of process parameters with modification technologies [25], as well as establishment of monitoring systems and quality standards to ensure safe biochar application.

Compared to conventional biomass feedstocks such as wood, straw and sludge, biochar derived from TCMRs exhibits unique advantages including abundant surface functional groups, high specific-surface area, well-developed porous structure and superior adsorption capacity [26]. The enormous and continuously growing production volume of these residues, combined with their geographic concentration near traditional Chinese medicine industrial clusters, significantly reduces raw material collection and transportation costs. With organic composition (cellulose, hemicellulose and lignin) similar to conventional biomass, this feedstock ensures stable biochar production yield. The resulting biochar serves as an optimal low-cost adsorbent for reducing agricultural chemical pollution and represents an effective approach for resource recovery [27].

This study investigates the carbonization characteristics of four TCMRs (Shanyao, Suoyang, Yujin, and Xueteng) under hydrothermal and torrefaction treatments at 240 °C, 260 °C, and 280 °C. The selection of Shanyao, Suoyang, Yujin, and Xueteng was based on three key considerations: (1) chemical representativeness—these herbs, respectively, contain high concentrations of starch (Shanyao), flavonoids (Suoyang), volatile oils (Yujin), and fibers (Xueteng), covering major biochemical components in TCMRs; (2) structural diversity—representing distinct morphological categories including tuberous roots (Shanyao), succulent stems (Suoyang), rhizomes (Yujin), and lianas (Xueteng); (3) resource availability—all are widely used clinically, with substantial annual production, generating significant processing waste. This systematic selection enables comprehensive investigation of thermochemical conversion behaviors across different biochemical compositions and tissue structures, providing universal insights for TCMR valorization. Analytical techniques, including Fourier-transform infrared spectroscopy (FT-IR), X-ray diffraction (XRD), and scanning electron microscopy (SEM), were employed to examine the crystallinity, functional groups, and microstructural evolution of the derived biochars. Additionally, biochar yield was quantified. The findings aim to elucidate the carbonization mechanisms of TCMRs, providing scientific and technical support for their sustainable utilization and advancing biochar applications in eco-friendly materials.

2. Materials and Methods

The four kinds of TCMRs were purchased from Tianjiang Pharmaceutical Co., Ltd., Jiangsu, China. Before the experiment, The TCMRs were first dried in an oven at 105 °C for 12 h. The oven was from Shanghai Jinghong Laboratory Equipment Co., Ltd. (Shanghai, China). Subsequently, the dried material was crushed into a fine powder using a mechanical grinder, achieving a particle size of 0.25 mm (60-mesh sieve). The resulting powder was then stored in a desiccator, under ambient conditions, for subsequent experimental use.

The preparation process of biochar is shown in Figure 1 above. The hydrothermal carbonization process was performed using a 50 mL sealed autoclave reactor. In a typical procedure, 4 g of TCMRs was uniformly blended with 20 mL of deionized water at a solid-to-liquid ratio of 1:5, and the resulting mixture was then transferred into the high-pressure autoclave. A magnetic stir bar was added to ensure homogeneous mixing during the reaction. Then, the reactor was sealed. The high-pressure autoclave used is manufactured by Beijing Century SenLong Co., Ltd. (Beijing, China). The temperature was raised to the target values (240 °C, 260 °C, and 280 °C for comparative studies) under the constant heating rate of 10 °C/min and at autogenous pressure [13,28,29].

Upon reaching the designated temperature, the system was maintained at isothermal conditions for 60 min to complete the carbonization process. After reaction completion, the reactor underwent natural cooling, until reaching room temperature. The resultant biphasic mixture was subsequently separated through vacuum-assisted filtration using a 0.45 μm microporous membrane filter. The solid product (hydrochar) was collected and dried in a convection oven (105 °C, 12 h) to ensure complete moisture removal, preceding material characterization.

The torrefaction experiments were conducted in a muffle furnace. Specifically, 4 g of TCMRs feedstock was placed in a crucible and loaded into the muffle furnace. The reactor temperature was raised to increase at a constant rate of 10 °C/min until attaining the predetermined temperature (240 °C, 260 °C, and 280 °C for comparative studies). After achieving thermal equilibrium, the carbonization reaction proceeded under temperature-controlled conditions for 1 h. After reaction completion, the apparatus cooled spontaneously to room temperature. The torrefied-char was recovered and subjected to oven-drying at 105 °C for 12 h to prevent moisture absorption during subsequent analytical procedures.

Characterization of Hydrochar and Torrefied-Char

FT-IR spectroscopic analysis was performed using a Thermo Scientific Summit X spectrometer (Thermo Fisher Scientific, Waltham, MA, USA) employing the KBr pellet technique. Spectral acquisition covered the 4000–400 cm⁻¹ range with 4 cm⁻¹ resolution to identify functional group signatures. Crystalline phase analysis was conducted via XRD using a PANalytical X’pert 3 Powder diffractometer (Malvern Panalytical, Almelo, The Netherlands) equipped with a Cu-Kα radiation source (λ = 1.5406 Å) operating at 40 kV and 40 mA. Diffraction patterns were recorded over the 2θ range of 10–90°. Microstructural characterization was achieved through field-emission SEM using a Hitachi Regulus 8600 microscope (Hitachi High-Tech, Tokyo, Japan). Prior to examination, specimens were platinum-sputtered (5 nm coating thickness) and imaged under standardized conditions (5 kV acceleration, 8.5 mm working distance) utilizing secondary electron detection. For elemental analysis, an Elementar Unicube system was employed to determine C/H/N/S content through high-temperature combustion (1180 °C) of 2 mg aliquots, with subsequent thermal conductivity detection.

A systematic comparative assessment of hydrochar and torrefied-char was conducted, encompassing six key parameters, including (1) mass yield, (2) carbon recovery rate, (3) energy characteristics, (4) elemental characteristics, (5) surface morphology, and (6) surface functional groups. FT-IR spectroscopy was performed using a Fourier-ransform spectrometer to detect surface functional groups and chemical structures of hydrochars and torrefied-chars. Crystalline phase and structural changes in hydrochars and torrefied-chars were analyzed using an X-ray diffractometer. The microstructure of hydrochars and torrefied-chars was characterized by SEM. The elemental composition of hydrochars and torrefied-chars was determined using an elemental analyzer, with particular focus on the contents of carbon (C), hydrogen (H), nitrogen (N), and sulfur (S). The relative oxygen (O) content was calculated using the equation

$$\mathrm{O} \left(\%\right)=100-\mathrm{C}-\mathrm{H}-\mathrm{N}-\mathrm{S}-\mathrm{A}.$$

(1)

where A is ash, calculated on a dry basis.

The hydrochar yield (HCY) and torrefied-char yield (TCY) were calculated by the followed equation:

$$\mathrm{Yield} \left(\%\right)={\mathrm{m}}_{\mathrm{biochar}}/{\mathrm{m}}_{\mathrm{TCMRs}} \times 100.$$

(2)

where m_biochar and m_TCMRs are the mass of biochar and TCMRs.

The energy recovery efficiency (ERE) was calculated by the following equation:

$$\mathrm{ERE} \left(\%\right)=\mathrm{HCY} \times {\mathrm{HHV}}_{\mathrm{hydrochar}}/{\mathrm{HHV}}_{\mathrm{raw}} \times 100.$$

(3)

The highest heating value was calculated from Mendeleev’s equation, on the basis of elemental composition [30].

$$\mathrm{HHV}=0.339 \times [\mathrm{C}] + 1.256 \times [\mathrm{H}] + 0.109 \times [\mathrm{S}]-0.109 \times [\mathrm{O}].$$

(4)

where C, H, S, O are carbon, hydrogen, combustible sulfur, and oxygen, on a dry basis.

3. Results and Discussion

3.1. Biochar Yield Analysis

The biochar yield was determined, to evaluate the conversion efficiency of the feedstock. Figure 2 reveals a strong correlation between hydrothermal temperature and the extent of TCMR carbonization conversion.

Experimental results revealed that as the hydrothermal temperature increased from 240 °C to 280 °C, the yield of all kinds of TCMRs decreased, under both carbonization methods. Compared with HTC, the yield of torrefaction drops more significantly. HCY exhibited a decreasing trend across all tested materials: Shanyao decreased from 53.37% to 36.61%, Suoyang from 52.77% to 40.40%, Yujin from 42.03% to 37.87%, and Xueteng from 54.22% to 38.11%. The underlying mechanism stems from the differential thermal stability of biomass structural components during pyrolysis. Hemicellulose decomposition initiates at 180 °C, while cellulose and lignin degrade at temperatures exceeding 210 °C. Elevated temperatures promote more intensive decomposition reactions including hydrolysis, dehydration, and decarboxylation, consequently reducing HCY. Furthermore, prolonged exposure to high temperatures may induce secondary degradation of hydrochars, further decreasing the yield [31]. Similarly, torrefaction temperature significantly affected the solid product yield. For TCY, increasing temperature also led to TCY reduction: Shanyao decreased from 83.50% to 46.69%, Suoyang from 61.56% to 48.94%, Yujin from 62.16% to 27.43%, and Xueteng from 65.56% to 42.30%. Torrefaction induces multiple concurrent chemical transformations, principally categorized into (1) oxygen-eliminating reactions (decarboxylation, decarbonylation, and demethoxylation), (2) dehydration processes, and (3) molecular-restructuring phenomena (intermolecular rearrangement, condensation, and aromatization) [32]. Notably, not all these reactions contribute to mass loss. Dehydration and decarboxylation represent the predominant mass-reducing pathways, which particularly affect cellulose and hemicellulose decomposition [33,34].

3.2. Biochar HHV Analysis

HHV is a key criterion for evaluating biochar quality. The HHV was calculated using Equation (4), with the results of the element analysis and ash content testing in

Table 1. Properties of the raw Shanyao and Shanyao hydrochar and Shanyao torrefied-char.

Shanyao	C	H	N	S	O	A	HHV	Yield	ERE
raw	40.81	7.16	1.42	0.17	47.27	3.17	/	/	/
240-H	70.63	5.5	2.39	0.12	16.54	4.82	29.06	43.37%	71.23%
260-H	74.37	5.35	2.7	0.41	8.86	8.31	31.01	39.24%	68.77%
280-H	74.24	5.68	2.88	0.41	3.91	13.15	31.89	36.61%	65.98%
240-T	48.185	6.17	1.68	0.185	40.07	3.71	19.74	83.50%	/
260-T	65.14	4.83	2.65	0.12	21.60	5.66	25.81	56.78%	/
280-T	66.005	4.79	3.125	0.06	19.57	6.45	26.27	46.69%	/

,

Table 2. Properties of the raw Suoyang and Suoyang hydrochar and Suoyang torrefied-char.

Suoyang	C	H	N	S	O	A	HHV	Yield	ERE
raw	44.89	5.37	2.07	0.14	39.08	8.45	/	/	/
240-H	58.69	5.16	3.1	0.10	25.47	7.48	23.61	52.77%	70.32%
260-H	66.44	5.64	2.91	0.20	15.88	8.93	27.90	47.43%	66.81%
280-H	69.48	5.05	2.59	0.18	10.06	12.64	28.82	40.40%	65.71%
240-T	56.76	4.16	2.63	0.19	33.24	3.04	20.86	63.56%	/
260-T	57.16	4.22	2.67	0.15	32.25	3.57	21.17	61.06%	/
280-T	59.65	4.52	2.79	0.16	29.26	3.63	22.73	48.94%	/

,

Table 3. Properties of the raw Yujin and Yujin hydrochar and Yujin torrefied-char.

Yujin	C	H	N	S	O	A	HHV	Yield	ERE
raw	46.21	6.53	0.86	0.12	38.69	7.59	/	/	/
240-H	68.19	4.83	1.19	0.15	15.29	10.35	27.53	42.03%	58.85%
260-H	69.61	5.1	1.41	0.12	12.12	11.64	28.70	41.78%	57.61%
280-H	70.42	4.92	1.57	0.09	10.48	12.52	28.92	37.87%	55.69%
240-T	46.28	6.09	0.70	0.02	46.19	0.73	18.30	62.16%	/
260-T	67.99	2.79	2.65	0.12	25.40	1.14	23.47	42.06%	/
280-T	66.01	4.79	3.13	0.06	29.21	1.31	30.14	27.43%	/

and

Table 4. Properties of the raw Xueteng and Xueteng hydrochar and Xueteng torrefied-char.

Xueteng	C	H	N	S	O	A	HHV	Yield	ERE
raw	51.69	5.97	0.79	0.05	28.38	28.38	/	/	/
240-H	64.17	5.02	1.02	0.04	19.3	10.45	25.96	54.22%	64.18%
260-H	69.35	4.76	1.34	0.04	11.74	12.77	28.21	49.38%	63.52%
280-H	72.51	4.59	1.26	0.04	8.36	13.24	29.44	38.11%	51.15%
240-T	55.37	3.19	1.71	0.02	34.59	5.13	19.01	65.55%	/
260-T	63.64	2.60	1.85	0.06	22.05	6.25	22.44	52.77%	/
280-T	67.94	2.66	1.76	0.06	24.39	9.39	23.71	42.30%	/

.

As shown in Figure 3, the HHV of Shanyao rose from 29.06 MJ/kg to 31.89 MJ/kg, Suoyang increased from 23.61 MJ/kg to 28.82 MJ/kg, Yujin improved from 27.53 MJ/kg to 28.92 MJ/kg, and Xueteng was enhanced from 25.96 MJ/kg to 29.44 MJ/kg, with the hydrothermal temperature increased. Compared to the raw TCMRs, all hydrochars showed significant improvement in HHV. Similarly, with increasing muffle furnace temperature, the HHV of Shanyao increased from 19.74 MJ/kg to 26.27 MJ/kg, Suoyang rose from 20.86 MJ/kg to 22.73 MJ/kg, Yujin improved from 18.30 MJ/kg to 30.14 MJ/kg, and Xueteng was enhanced from 19.01 MJ/kg to 24.39 MJ/kg. The HHV measurements revealed consistent enhancement across all torrefied products, relative to raw TCMRs. With the obtained biochar showing an HHV of ~30 MJ/kg (at 280 °C pyrolysis temperature), theoretical calculations indicate that 1 ton of this TCMR-biochar could yield about 5000 kWh of electricity at 60% gasification efficiency. Temperature-dependent analysis demonstrated intensified aromatization and carbonization at elevated processing temperatures, resulting in energy-dense aromatic clusters.

3.3. Biochar ERE Analysis

The ERE quantifies the energy conversion efficiency during biomass pyrolysis, representing the proportion of feedstock chemical energy transformed into utilizable energy in the biochar. Biochars with high ERE typically exhibit elevated fixed carbon content and reduced volatile matter, rendering them particularly suitable for applications as high-stability fuels or soil amendments.

As illustrated in Figure 4, the ERE of hydrochars demonstrates an inverse correlation with processing temperature, showing progressive decline at elevated temperatures. This observed behavior likely stems from the interphase energy transfer from biomass solids to aqueous media during hydrothermal conversion.

3.4. Biochar FT-IR Analysis

The FT-IR spectra (Figure 5) analysis showed similar peak positions for all samples. The “-H” and “-T” in the figure represent hydrochar and torrefied-char, respectively, and this notation is consistent throughout the following text.

The absorption peaks in the 3500–3000 cm⁻¹ range correspond to O-H functional groups from hydroxyl and carboxyl groups. For torrefied-chars, these groups gradually disappeared with increasing temperature, likely due to the decomposition of hemicellulose at higher torrefaction temperatures. The peaks at 1598 cm⁻¹ and 1033 cm⁻¹ were attributed to aromatic C-C bonds and methoxy groups in lignin, respectively. These peaks were not prominent in torrefied-chars but became more intense and broader in hydrochars. The characteristic absorption at 1033 cm⁻¹ is diagnostic of ether linkages (C-O-C) in cellulose, hemicellulose or lignin structures. The characteristic absorption bands within the 1800–1000 cm⁻¹ spectral region, corresponding to carbonyl (C = O) and alkoxy (C-O) functionalities, exhibited marked intensity reduction, evidencing substantial oxygen depletion through torrefaction-induced dehydration and decarboxylation pathways [35,36]. The FT-IR spectroscopic analysis demonstrated significant structural modifications in all four types of TCMRs during carbonization. With increasing temperature, characteristic functional groups including hydroxyl, carbonyl, and ether linkages underwent successive dehydration, decomposition, and reorganization reactions. This progressive transformation converted the original complex organic structures into more condensed aromatic configurations, indicating an enhanced carbonization degree. Notably, the extent of these structural alterations varied among different TCMRs, due to their distinct biochemical compositions. These structural evolutions directly correlated with critical carbonization characteristics, particularly where the developed aromatic structures contributed to improved thermal stability and elevated energy density of the resulting carbonaceous materials.

3.5. Biochar XRD Analysis

All samples exhibited broad XRD diffraction peaks, consistent with the characteristic patterns of carbon-based materials.

The XRD spectra (Figure 6) consistently displayed a broad peak at approximately 25°, corresponding to the (002) crystallographic plane, indicative of an amorphous carbon structure, along with a weak peak near 50°, attributable to the (100) plane. Distinct diffraction peaks were observed at 2θ values of 20.3° (101), 26.8° (002), and 35.7° (040) in all XRD patterns, representing different crystalline planes of cellulose [37,38]. The intensity of these characteristic cellulose diffraction peaks progressively decreased with increasing hydrothermal or torrefaction temperature, demonstrating the gradual decomposition of crystalline cellulose components under elevated thermal-treatment conditions. With increasing temperature, the chemical constituents in TCMRs undergo a series of physical and chemical transformations, resulting in a gradual transition of crystal structures from disordered to ordered states, along with alterations in crystalline phase composition [39,40]. These differential phase transitions are intrinsically related to the inherent chemical composition and structure of each TCMR. Shanyao residues, rich in polysaccharides and cellulose components, likely form distinct crystalline structures through dehydration and condensation reactions during carbonization. In contrast, Suoyang residues containing various bioactive compounds (e.g., flavonoids and terpenoids) exhibit unique crystalline phase transitions due to thermal decomposition and molecular reorganization. Similarly, the crystalline transformations in Yujin and Xueteng residues are governed by their respective chemical compositions. Elucidating the temperature-dependent mechanisms of crystalline phase transitions in TCMRs provides fundamental insights into the carbonization process.

3.6. Biochar SEM Analysis

SEM image (Figure 7, Figure 8, Figure 9 and Figure 10) observations demonstrated that both hydrochars and torrefied-chars developed extensive porous structures with increasing treatment temperature, accompanied by pore expansion and subsequent structural collapse. The pore formation mechanism can be attributed to the volatilization of thermally labile components during hydrothermal and torrefaction processes [41]. Elevated temperatures induced partial decomposition of cellulose and hemicellulose, initially generating surface cracks, while further temperature increases enhanced degradation, leading to the agglomeration of monosaccharides and oligosaccharides into particulate clusters [29,42]. Comparative analysis revealed distinct morphological differences between the two carbonization methods: torrefied-chars exhibited more densely packed and regularly arranged pore structures, whereas hydrochars displayed significantly more fragmented porosity with greater structural deterioration. The observed microstructural evolution exhibits strong correlations with the chemical decomposition, volatilization, and molecular reorganization occurring during the carbonization process. Distinct microstructural transformation patterns emerge among different TCMRs, primarily governed by their unique biochemical compositions and native structural configurations. These microstructural modifications exert direct influences on the resultant char properties: the developed porous architectures substantially enhance the specific-surface area, thereby improving adsorption capacity, while the formation of fibrous or lamellar structures significantly modulates mechanical properties and electrical conductivity characteristics.

4. Conclusions

This study systematically compares two conversion approaches (HTC vs. torrefaction) for four TCMRs within the 240–280 °C temperature range, revealing their intrinsic trade-off mechanisms. The torrefaction process demonstrated superior mass retention (15–30% higher yield) due to limited devolatilization, while HTC produced biochar with higher energy density (10–20% increased HHV) through enhanced carbonization reactions. Microstructurally, hydrochar exhibited hierarchical porosity with fragmented morphology, contrasting sharply with the uniform pore networks, characteristic of torrefied-char. FT-IR spectroscopic analysis confirmed significant structural transformations across all four TCMRs during carbonization. Temperature elevation induced sequential dehydration, decomposition, and reorganization reactions involving the characteristic functional groups (hydroxyl, carbonyl, and ether bonds). This progressive transformation converted the original complex organic structures into more condensed aromatic configurations, indicating advanced carbonization degrees. XRD patterns revealed gradual amorphization, with increasing temperature. These findings establish a definitive process-selection framework: torrefaction is preferable for soil amendment applications requiring mass yield preservation, while hydrothermal carbonization shows greater advantages for energy recovery and pollutant immobilization. For practical applications of these biomass-derived carbons in porous carbon materials, the optimal carbonization process and conditions can be selected, based on specific requirements.