Flue Gas Torrefaction of Corn Stover-Densified Fuel: Impacts on Physicochemical Properties and Combustion Performance

Abstract

Corn stover pellets were torrefied under a simulated flue gas atmosphere (220 °C–300 °C, 15–25 min) to evaluate physicochemical and combustion modifications. Analyses revealed that increasing torrefaction severity significantly reduced the volatile matter, hydrogen, and oxygen contents, while enriching the fixed carbon and ash fractions. Consequently, the higher heating value increased by up to 33.41%, peaking at an energy density of 1.33, albeit at the expense of a sharp decline in mass yield. Thermogravimetric analysis revealed that increasing torrefaction severity elevated the ignition temperatures while reducing burnout temperatures. Mild torrefaction (220–240 °C, 15–20 min) maintained optimal combustion stability; conversely, severe conditions (>260 °C, >20 min) significantly deteriorated the combustion kinetics, as evidenced by a marked reduction in both the maximum combustion rate and the comprehensive combustion characteristic index.

1. Introduction

Driven by the dual imperatives of low–carbon transition of the global energy structure and ecological environmental protection, the efficient development and high-value utilization of biomass energya renewable resource with prominent carbon neutrality attributeshave emerged as a core research priority in the energy sector. Corn stover, as one of the most abundantly produced agricultural residues worldwide, can be converted into solid fuel via densification technology, representing a critical pathway for the resourceful utilization of agricultural residuals. However, raw corn stover-densified fuel suffers from inherent drawbacks, including low calorific value, strong hygroscopicity, low energy density, and poor combustion stability, which severely restrict its competitiveness in large-scale industrial applications.

Torrefaction pretreatment, a key technology for modifying the thermochemical properties of biomass, can effectively remove moisture and oxygen-containing volatile matter from biomass and achieve significant carbon enrichment through mild thermal degradation in an oxygen-deficient environment. This process not only substantially enhances the fuel’s calorific value and hydrophobicity but also improves grindability by disrupting the lignocellulosic fiber structure. Consequently, it serves as an effective strategy for optimizing biomass fuel quality, achieving both energy densification and process efficiency.

At present, multidimensional investigations have been conducted in academia on the torrefaction characteristics of biomass under different medium environments. Early studies primarily focused on the regulatory effects of thermochemical process parameters on the biomass upgrading performance. With the deepening of research, innovation in the torrefaction medium has gradually become a core topic in this field. In terms of atmosphere regulation and waste heat utilization, the technical feasibility of using flue gas as the torrefaction atmosphere is verified by previous studies, which provides a new pathway for the synergistic coupling of industrial waste heat recovery and biomass modification. Relevant pilot-scale tests have further validated the significant advantages of industrial flue gas in promoting biomass deoxygenation and upgrading, as well as increasing its energy density. In terms of reaction mechanism and component evolution, existing investigations have focused on the directional regulatory effects of different atmospheres (e.g., nitrogen, carbon dioxide, and composite fuel gas) on the torrefaction behavior of biomass. Through an in-depth analysis of the structural evolution mechanism of core components such as cellulose and hemicellulose during torrefaction, the thermodynamic characteristics of biomass conversion from the raw state to high-quality solid fuel have been revealed. The torrefaction mechanism is illustrated in Figure .

1.

Schematic diagram of the torrefaction mechanism. Note: original figure created by the authors.

Although relevant technical routes have been preliminarily verified, the intrinsic correlation between the physicochemical property evolution and the combustion performance response of densified fuel under the complex component environment of simulated flue gas requires systematic investigation. To date, most existing studies have been confined to laboratory-scale inert atmospheres. In contrast, systematic investigations into the torrefaction characteristics of densified fuels under complex simulated flue gas atmospheres remain notably lacking. In particular, there is a lack of in-depth analysis on the intrinsic correlation between the physicochemical property evolution and the combustion kinetic response of corn stover-densified fuel under flue gas torrefaction conditions. Furthermore, a unified and clear quantitative definition of the optimal process window balancing the energy gain and combustion stability has not yet been formed.

Building on this foundation, torrefaction experiments were conducted across a broad range of temperatures and residence time gradients under a simulated flue gas atmosphere utilizing corn stover densified pellets as the feedstock. The carbon enrichment law, deoxygenation pathway, and energy conversion efficiency of the torrefied products were quantified via ultimate analysis and proximate analysis, and the torrefaction-induced structural reorganization mechanism was characterized by the microscale combined with field-emission scanning electron microscopy (FE-SEM). At the data analysis level, the energy-mass cobenefit index (EMCI) was introduced to evaluate the energy gain performance of the torrefaction process. Employing thermogravimetric analysis (TGA) and derivative characterization methods, this study deciphered the regulatory effects of torrefaction pretreatment on the biomass component thermal degradation kinetics. Furthermore, a multidimensional evaluation system was established, incorporating key metrics such as ignition temperature, maximum combustion rate, and comprehensive combustion characteristic index. By correlating physicochemical evolution and combustion response characteristics, the modification mechanism of flue gas torrefaction on densified fuel was revealed, providing theoretical support for defining the optimal process window that takes both the energy density and combustion stability into account. It should be noted that this work focuses on the overall upgrading performance of corn stover densified fuel under ternary simulated flue gas, and no separate pure N₂ inert torrefaction control group is included in the current experimental design, which limits direct intrastudy quantification of atmospheric component effects. To compensate for this deficiency and clarify the unique combustion-modification effects induced by O₂ and CO₂ in flue gas, a cross-literature comparative analysis based on published corn stover torrefaction data under consistent temperature and residence time gradients is presented in the following section to decouple the independent roles of different atmospheric components. ,

2. Materials and Methods

2.1. Materials

Corn stalk pellets (CS), harvested from Heilongjiang Province, China, were used as the initial experimental feedstock, and torrefied products were prepared under flue gas atmosphere with a set range of torrefaction temperatures and residence times. All samples used in the experiments were processed on an air-dried basis. The corn stalk pellets were dried in a forced-air drying oven at 105 °C for 24 h to reduce the moisture content to less than 1%, ensuring consistent control of initial variables across all feedstock batches. Subsequently, the dried pellets were placed in airtight sealed bags and stored in a dry environment for subsequent biomass torrefaction experiments.

2.2. Experimental Protocol

Torrefaction performance is governed by multiple factors, among which torrefaction temperature and residence time act as the primary determinants, directly dictating the reaction mechanism and reaction rates throughout the torrefaction process. Accordingly, the effects of temperature and residence time on torrefaction performance were taken as the core research focus. Five temperature gradients, namely 220, 240, 260, 280, and 300 °C, were selected as the torrefaction temperatures, and three different torrefaction residence times (15, 20, and 25 min) were set for comparative experiments. Given that mild thermal degradation and partial oxidation of lignocellulosic substrates within the torrefaction temperature range of 220 °C–300 °C are predominantly driven by O₂ and CO₂, dilute SO₂ and NO _x barely engage in solid-state biomass conversion and merely modulate postcombustion emission profiles rather than mediating fuel quality improvement during torrefaction pretreatment. Accordingly, this work adopts a ternary mixed atmosphere composed of the dominant bulk constituents of industrial boiler flue gas (8 vol·% O₂, 12 vol·% CO₂, 80 vol·% N₂). The raw untorrefied corn stalk pellets are denoted as CS, and the torrefied corn stalk pellets are denoted as TCS throughout the study.

Torrefaction experiments were carried out in a horizontal tubular furnace, with the schematic diagram of the apparatus structure shown in Figure . Identical heating and holding processes were controlled to minimize the interference of other factors on torrefaction performance. Prior to each experiment, the constant-temperature zone was identified, and the temperature deviation in this zone was maintained within ±5 °C. Subsequently, 5 ± 0.1 g of corn stalk pellets were weighed and loaded into the inlet section of the reactor. The target temperature of the horizontal tubular furnace was set, and the flue gas valve was opened. When the temperature reached the preset value, the gas flow rate was adjusted to 1000 mL/min via a mass flow controller, and the reactor was purged for 2 min to completely remove the internal air. Subsequently, the gas flow rate was adjusted to 300 mL/min to maintain a stable reaction atmosphere. The left connection port was opened, the sample was pushed into the heating zone via a furnace hook, the left apparatus was quickly reconnected, and the corresponding torrefaction residence times (15, 20, and 25 min) were maintained. Considering the time loss during sample loading and unloading in the operation process, the torrefaction residence time set during the constant temperature process was 2 min longer than required torrefaction duration.

2.

Schematic diagram of the baking system. Note:1. Gas intake device 2. Cooling area 3. Quartz tube 4. Insulation area 5. Temperature control system 6. Power meter 7. Tube furnace body 8. Reaction area 9. Preparation area 10. Push rod 11. Mass flow controller.

After the completion of torrefaction, the right connection port was opened, the crucible was smoothly retrieved via the furnace hook, covered with a lid, and placed at room temperature for slow cooling. Subsequently, the heating program of the tubular furnace was terminated and the flue gas valve was closed. After being cooled to room temperature, the torrefied samples were taken out and weighed, the mass change before and after torrefaction was recorded, and the appearance of the products was documented via photography. The samples were then numbered, placed in airtight sealed bags, and stored in a low-temperature, light-proof environment.

2.3. Characterization of Raw Feedstock and Torrefied Products

Ultimate analysis was performed using a vario MICRO cube elemental analyzer. The carbon (C), hydrogen (H), nitrogen (N), and sulfur (S) contents were directly quantified by the instrument, while the oxygen (O) content was calculated via the difference method, as shown in eq .

$$\mathrm{O}\%=100\%-(\mathrm{C}+\mathrm{H}+\mathrm{N}+\mathrm{S})\%$$ 1

The mass yield (Y _M) is defined as the percentage ratio of the dry basis mass of the solid torrefied product to the dry basis mass of the initial raw feedstock.

$$Y_M=\frac{M}{M_0}\times 100\%$$ 2

where Y _M is the mass yield, %; M is the mass of torrefied corn stalk pellets, g; and M ₀ is the mass of raw untorrefied corn stalk pellets, g.

The energy yield (Y _E ) is defined as the proportion of the total energy contained in the raw feedstock that is retained in the final torrefied product during the torrefaction process.

$$Y_E=(Y_M\times \frac{R}{R_0})\times 100\%$$ 3

where Y _E is the energy yield, %; R is the higher heating value (HHV) of torrefied corn stalk pellets, MJ/kg; and R ₀ is the HHV of raw untorrefied corn stalk pellets, MJ/kg.

The energy density (D) is defined as the energy content per unit mass of the fuel sample.

$$D=\frac{Y_E}{Y_M}$$ 4

The EMCI is defined as a quantitative indicator to evaluate the torrefaction performance under different torrefaction process conditions.

$$\mathrm{E}\mathrm{M}\mathrm{C}\mathrm{I}=Y_E-Y_M$$ 5

In addition, the micromorphology of the samples was characterized by using a Hitachi SU8010 field emission scanning electron microscope (FE-SEM). Ultimate analysis was performed by using an elementar UNICUBE elemental analyzer. The HHV was determined by using an SDACM3000 automatic calorimeter. Thermogravimetric analysis (TGA) was carried out using a STA 449 C simultaneous thermal analyzer.

2.4. Combustion Performance

Thermogravimetric analysis (TGA) was performed using a simultaneous thermal analyzer (STA 449 C) to investigate the combustion characteristics of the densified fuel. Consistent conditions including heating rate and air flow rate were maintained throughout the experiment, and the effects of torrefaction conditions on the combustion performance of the densified fuel were systematically investigated. The experimental parameters were preset as follows: the air flow rate of 50 mL/min, heating rate of 10 °C/min, and final temperature of 800 °C. For data processing of the thermogravimetric experiment, several characteristic parameters were introduced. The ignition temperature (Ti) and burnout temperature (Th) of the densified fuel during combustion were determined via the TG-DTG joint definition method, as shown in Figure .

3.

Schematic diagram of ignition temperature and burnout temperature.

Ignition temperature (T _i ): A perpendicular line (line A) is drawn through the peak point of the DTG curve, which intersects the TG curve at a single point. A tangent line (line B) to the TG curve is generated through this intersection point. The temperature corresponding to the intersection of line B and the horizontal baseline (upper horizontal line) of the TG curve prior to the onset of volatile matter devolatilization is defined as the ignition temperature. This temperature was used to characterize the ignition difficulty of the sample, with a lower value indicating easier ignition.

Burnout temperature (T _h ): The temperature corresponding to the intersection of the aforementioned line B and the horizontal baseline (lower horizontal line) of the TG curve after the completion of weight loss was defined as the burnout temperature. This temperature was used to characterize the termination temperature of the combustion reaction and was defined as the temperature at which 98% of the combustible components in the sample were completely combusted. As no further weight loss of the sample was observed and the DTG curve approached zero with further increasing temperature, the temperature at which the weight loss rate stably decreased to 0.01 mg/min in the late reaction stage was regarded as the burnout temperature.

Maximum combustion rate (dm/dt)_max: The maximum combustion rate was defined as the weight loss rate corresponding to the highest peak of the DTG curve. A higher maximum combustion rate indicated that the fuel could release a large amount of energy in a short period, corresponding to stronger combustion reactivity. Conversely, an excessively low peak value corresponds to a gentle energy release process, which may fail to meet the requirements of high-power applications. This parameter was used to characterize the combustion intensity of the samples. Two peaks were generally observed in the DTG curve during biomass combustion, which corresponded to the volatile matter combustion stage and the fixed carbon combustion stage, respectively.

Average combustion rate (dm/dt)_mean: This parameter was defined as the average weight loss rate of the sample from ignition to burnout, and used to characterize the combustion intensity throughout the entire combustion process. The corresponding calculation equation is expressed as follows

$${(\mathrm{d}m/\mathrm{d}t)}_{mean}=\beta \frac{{\alpha }_i‐{\alpha }_h}{T_h‐T_i}$$ 6

where (dm/dt)_mean denotes the mean mass loss rate, in %/min; β denotes the heating rate, in °C/min; α _i denotes the mass retention at ignition time, in %; α _h denotes the mass retention at burnout time, in %; T _i denotes the ignition temperature, in °C; T _h denotes the burnout temperature, in °C.

Comprehensive combustion characteristic index (S _N ): This parameter was used to characterize both the ignition propensity and comprehensive combustion performance of the sample. A higher value indicated that the sample was more easily ignited and exhibited more intense combustion. The corresponding calculation equation is expressed as follows.

$$S_N=\frac{{(\mathrm{d}m/\mathrm{d}t)}_{\max }\times {(\mathrm{d}m/\mathrm{d}t)}_{\mathrm{m}\mathrm{e}\mathrm{a}\mathrm{n}}}{T_i\times T_i\times T_h}$$ 7

where S _N denotes the comprehensive combustion index, in °C^3–·min^–2.

3. Results and Analysis

3.1. Effects of Torrefaction Conditions on the Basic Properties of Densified Fuel

The results presented in Figure indicated that, under the same torrefaction residence time, the torrefaction temperature was positively correlated with the color, morphology, and torrefaction degree of corn stalk pellets, the torrefaction degree of the pellets was continuously deepened with increasing temperature.

4.

Schematic diagram of torrefied products.

Under the low-temperature condition of 220–240 °C, the corn stalk pellets presented a light brown color with intact morphology, and only slight cracks were observed on the surface. Only mild dehydration and low-level thermal degradation occurred at this stage, and the structures of cellulose and hemicellulose remained relatively stable. Under the medium-temperature condition of 260 °C, the color of the pellets was significantly deepened, the number of cracks and pores on the surface was increased, and fragmentation was observed for partial pellets. At temperatures of 280–300 °C, the pellets exhibited a dark brown to black coloration, accompanied by severe fragmentation and densely distributed surface pores. This morphological evolution arises from two synergistic driving mechanisms. Chemically, extensive hemicellulose decomposition occurs at 260 °C, which disrupts the cellulose framework and markedly intensifies volatile release. At 280–300 °C, severe pyrolytic carbonization of cellulose and lignin proceeds, decomposing a large proportion of organic constituents into char and gaseous volatile products. Mechanically, thermomechanical cracking is triggered by nonuniform heating inside the compacted pellets. Restricted by the low radial thermal conductivity of densified corn stover pellets, a pronounced temperature gradient develops between the outer shell and inner core during rapid heating in a tubular furnace, inducing mismatched thermal expansion strains across microscale domains. The accumulated heterogeneous thermal stress propagates along native fiber boundaries and inherent microdefects, nucleating microcracks before substantial volatile escape. As torrefaction severity increases, these thermally induced cracks widen and interconnect with voids formed by cellulose and lignin thermal degradation, further exacerbating pellet fragmentation.

At the same torrefaction temperature, the progress of the thermal degradation reaction was promoted by extending the torrefaction residence time. A low reaction degree was observed at a torrefaction time of 15 min, the pellet color was deepened and the thermal degradation reaction was enhanced at 20 min, and the thermal degradation reaction was further intensified at 25 min. When the torrefaction temperature was not lower than 280 °C, the effect of extending the residence time on the appearance change of the pellets was weakened, indicating that the torrefaction reaction can reach a high degree within a short time under this temperature condition.

To reveal the micromorphology reconstruction mechanism of torrefaction on densified fuel, 260 °C was selected as the representative temperature for electron microscopic characterization in this study. Previous results demonstrated that 260 °C was determined as the critical thermodynamic threshold for intensive thermal degradation of hemicellulose and the rapid increase in fuel energy density, which constituted the optimal window for observing the fracture of dense fibers and the evolution of porous structures. Meanwhile, a residence time sequence of 15–25 min was introduced at this temperature to reveal the driving effect of thermodynamic accumulation on pore network expansion in the temporal dimension, thereby providing direct microscopic physical evidence for the determination of optimal process parameters.

To mitigate the sampling bias caused by the intrinsic microscale heterogeneity of corn stover pellets, multiple independent observation regions were selected for each sample in SEM testing, and the 3000× micrographs displayed in Figure reflect the universal structural characteristics summarized from dozens of local fields of view rather than single random spots. For each torrefaction condition, at least 5 different pellet fragments were fractured and gold-sputtered for SEM imaging; the crack propagation, pore distribution, and fiber damage features observed in the provided images represent the dominant micromorphological trends after statistical screening of all captured micrographs, eliminating the contingency of partial extreme local regions. The natural dense lignocellulose structure of the untorrefied corn stalk pellets remained intact, with a fibrous texture and relatively dense tissue morphology exhibited, and no obvious pores and cracks induced by torrefaction were observed. At a constant temperature of 260 °C, a progressive evolution of the pellet microstructure occurred with the extension of holding time from 15 to 25 min. Initial torrefaction traces were observed on the pellet surface, and the local structure begins to loosen after 15 min of treatment. Obvious deformation of the fibrous structure occurred, and the number and size of surface pores were significantly increased after 20 min of treatment. The pellets presented an overall porous and loose state after 25 min of treatment, the original fibrous structure was severely damaged, and the degree of structural damage caused by torrefaction was continuously intensified with the extension of residence time.

5.

SEM micrograph of particle morphology at 3000× magnification.

At a constant torrefaction residence time of 20 min, the modification effect of torrefaction temperature on the microstructure was significant. After low-temperature treatment at 220 °C, the pellet structure remained relatively intact, with only a limited number of micropores observed. As the temperature increased to 240 °C, the number of pores proliferated; however, the overall structural compactness was largely preserved. Many pores were formed on the pellet surface after medium-temperature treatment at 260 °C, obvious melting and reorganization of the fibrous structure occurred, and the structure began to loosen significantly. A continuous fibrous loose structure appeared on the pellet surface and the pore network was further expanded after treatment at 280 °C. The original microstructure of the pellets was completely reconstructed after high-temperature treatment at 300 °C, intense thermal degradation of lignocellulose occurred, and dense cracks and porous structures were formed on the surface. The degree of structural damage was continuously enhanced with increasing temperature, which fully reflected the directional modification effect of the torrefaction reaction on the microstructure of corn stalk pellets during flue gas torrefaction2.

Compared with inert N₂ torrefaction, CO₂ in flue gas torrefaction exerts dual effects of physical swelling and mild gasification: CO₂ molecules diffuse into dense pellets to induce swelling and expand the internal pore network, while a weak Boudouard reaction on char surfaces at 260 °C–300 °C further facilitates pore development. This is in good agreement with the well-developed porous structure observed in SEM micrographs, an effect unattainable under pure O₂ or N₂ atmosphere alone. ,

As illustrated in Table , following flue gas torrefaction, the samples exhibited significant and systematic variations in their physicochemical composition and calorific value, as reflected in the proximate and ultimate analyses. In terms of proximate analysis, compared with the untorrefied raw feedstock, the volatile matter content of the torrefied samples exhibited an overall decreasing trend with increasing torrefaction temperature and extended holding time, which was reduced from 62.73% of the raw feedstock to 30.67% under the condition of 300 °C for 25 min. The ash content presented a continuous increasing trend, increasing from 19.50% to 35.57%. The fixed carbon content was significantly increased with the enhancement of torrefaction intensity (temperature and time), rising from 10.31% to 32.76%. Under identical conditions, the influence of torrefaction residence time on fixed carbon content was considerably weaker than that of torrefaction temperature. ,

1. Proximate and Ultimate Analyses as Well as HHV.

sample	proximate analysis			ultimate analysis					heat
	volatile matter	ash	value	C/%	H/%	O/%	N/%	S/%
		content
CS	62.73	19.50	10.31	27.67	3.96	68.01	0.34	0.03	13.53
TCS220 °C–15 min	62.87	20.16	15.40	37.23	5.16	57.07	0.52	0.03	13.57
TCS220 °C–20 min	62.88	20.45	15.19	37.60	5.14	56.83	0.41	0.03	13.58
TCS220 °C–25 min	62.07	21.44	15.34	39.68	5.27	54.60	0.43	0.03	13.62
TCS240 °C–15 min	62.12	20.74	15.98	39.00	5.12	55.41	0.45	0.02	13.64
TCS240 °C–20 min	62.13	21.37	15.57	38.25	5.05	56.22	0.46	0.01	13.79
TCS240 °C–25 min	60.66	22.39	16.06	38.27	4.90	56.33	0.49	0.01	14.15
TCS260 °C–15 min	59.82	22.52	16.39	38.61	4.88	56.00	0.50	0.02	14.21
TCS260 °C–20 min	55.35	24.44	18.65	41.98	4.43	53.03	0.55	0.01	16.34
TCS260 °C–25 min	48.62	27.48	22.29	41.25	4.20	53.99	0.55	0.01	16.51
TCS280 °C–15 min	56.16	23.21	19.11	40.03	4.62	54.81	0.52	0.02	16.27
TCS280 °C–20 min	48.25	26.59	23.95	43.22	4.39	51.86	0.51	0.01	16.68
TCS280 °C–25 min	36.89	32.40	28.99	45.26	3.83	50.26	0.63	0.02	16.83
TCS300 °C–15 min	48.51	27.59	22.89	40.24	4.11	55.04	0.60	0.01	16.56
TCS300 °C–20 min	37.09	31.60	29.84	44.98	3.90	50.51	0.60	0.01	17.62
TCS300 °C–25 min	30.67	35.57	32.76	42.96	3.53	50.80	0.70	0.01	18.05

Ultimate analysis reveals that with increasing torrefaction temperature, the carbon mass fraction of corn stover pellets exhibits an initial rise followed by stabilization, increasing from 27.67% in the raw feedstock to 45.26% under the 280 °C/25 min condition and fluctuating slightly within the 280–300 °C high-temperature range. In contrast, the mass fractions of hydrogen and oxygen decline continuously with escalating torrefaction severity, dropping from 5.27% to 3.53% and from 68.01% to 50.26%, respectively. Correspondingly, the atomic H/C and O/C ratios both decrease monotonically with temperature, falling from 0.14 to 0.08 and from 2.46 to 1.18, respectively. Consistent with this compositional evolution, the HHV increases monotonically with rising torrefaction intensity, growing gradually from 13.53 MJ/kg for raw feedstock to 18.05 MJ/kg; notably, 260 °C serves as the critical inflection point for rapid HHV growth, beyond which the increment amplitude stabilizes with further temperature elevation.

These observed trends are governed by the coupled effects of physicochemical transformation within corn stover-densified pellets and the synergistic action of the simulated flue gas atmosphere during torrefaction. Over the 220 °C–300 °C torrefaction range, carbohydrate fractions (cellulose and hemicellulose) undergo progressive thermal degradation: adsorbed and bound water are first released, followed by continuous evolution of light volatiles including low-molecular-weight organic acids, aldehydes and ketones, which drives a gradual reduction in volatile matter content. As an inorganic mineral constituent, ash does not participate in devolatilization and instead undergoes relative enrichment as organic components are consumed, such that ash content rises with escalating torrefaction severity. The increase in fixed carbon is attributed to the progressive concentration and aromatization of the residual organic carbon skeleton after volatile removal; elevated temperatures yield a higher degree of carbon skeleton polymerization, resulting in a more pronounced fixed carbon enrichment effect.

The variation of elemental composition was essentially dominated by elimination reactions during the torrefaction process. Oxygen was removed via the formation of oxides such as CO, CO₂, and H₂O, while hydrogen was mainly released in the form of H₂O and small-molecule hydrocarbons. As illustrated in Figure , the removal rate of oxygen was higher than that of hydrogen, leading to a relative increase in carbon content and decreases in the H/C and O/C atomic ratios. The reduction of H/C and O/C ratios directly reflected the improvement of the aromatization degree and energy density of the stalk pellets.

6.

H/C and O/C atomic ratios of corn stover.

Compared with a single inert atmosphere, a unique synergistic micro-oxidation and torrefaction environment was constructed by simulated flue gas. Intensive oxidation was suppressed by high nitrogen concentration. Meanwhile, trace O₂ selectively targeted hemicellulose side chains, accelerating deoxygenation and depolymerization. These effects, when synergistically combined with CO₂-mediated physical swelling and mild gasification, collectively drove the profound aromatization and reorganization of carbon skeleton. Consequently, targeted expansion of pore networks and significant enhancement of energy density in briquetted fuels were achieved at a lower thermodynamic threshold. The timely discharge of volatile products was guaranteed by a gas flow rate of 300 mL/min, which further strengthened aforementioned transformation process. The increase in heating value was directly correlated with the rise of carbon content and the reduction of oxygen content. Carbon is the core component for heat release during stalk combustion, while the presence of oxygen reduced the energy density of the fuel per unit mass. Therefore, the heating value of the fuel was gradually increased with carbon enrichment and oxygen deoxygenation during the torrefaction process. The torrefaction reaction tended to be stable after 260 °C, and the increase in the amplitude of heating value slowed down accordingly. Compared with pure N₂ torrefaction conducted by our team under identical thermal conditions, the simulated flue gas atmosphere achieves higher carbon enrichment efficiency and energy density at the same temperature. Specifically, at 260 °C for 20 min, the HHV of torrefied pellets in this work increases by 20.8% relative to the raw feedstock; this relative increment is 4.2–5.7 percentage points greater than the HHV improvement obtained from pure N₂ torrefaction under the same temperature and residence time settings, which confirms the additional fuel-upgrading effect induced by O₂ and CO₂ components in flue gas. Trace O₂ exerts selective oxidative modification on hemicellulose side chains, promoting the cleavage of oxygen-containing functional groups and accelerating deoxygenation and depolymerization at lower temperature thresholds. Coupled with the physical swelling and mild gasification effects of CO₂, the synergistic action drives deeper aromatization and structural reorganization of the carbon skeleton. This mechanism accounts for the faster decline of O/C atomic ratio and higher carbon enrichment rate observed in this study compared with inert torrefaction, and is consistent with the widely recognized oxidative torrefaction mechanism. ,

3.2. Response Law of Core Fuel Property Indicators to Torrefaction Conditions

It is observed in Figure that the mass yield of torrefied products from corn stalk-densified pellets under a simulated flue gas atmosphere presented a significant decreasing trend with rising experimental temperature, and a slow decreasing trend with extended torrefaction residence time. When the temperature was below 240 °C, the mass yield was maintained above 95% with a gentle decreasing amplitude. After the temperature exceeded 240 °C, the mass yield was sharply decreased, and the mass yield of all residence time groups was reduced to below 75% at 300 °C. Under the same temperature condition, the highest mass yield was obtained for the 15 min torrefaction group, while the lowest mass yield was achieved for the 25 min group. The effect of torrefaction residence time on mass yield was weaker than that of torrefaction temperature, and the difference in mass yield caused by residence time was further narrowed in the high-temperature range above 280 °C.

7.

Mass yield, energy yield, energy density, EMCI.

Mass yield evolution during torrefaction is primarily governed by the thermal degradation behavior and compositional transformation of corn stover pellets. At temperatures below 240 °C, only moisture evaporation and minor light volatile release proceed with mild reactivity, sustaining mass yield at a relatively high level. Above 240 °C, pronounced thermal degradation of cellulose and hemicellulose is triggered, releasing abundant volatile species alongside slight carbonization, which induces a substantial decline in solid product mass; further temperature elevation accelerates degradation kinetics and deepens conversion degree, amplifying the reduction in mass yield. Extended residence time affords prolonged reaction duration to promote continuous removal of light fractions and thus moderately lowers mass yield, yet this time-dependent effect diminishes at higher temperatures as thermal degradation rapidly approaches completion, attenuating the incremental effect of prolonged holding on reaction progress. The energy yield was observed to exhibit a stepwise decreasing trend with rising temperature, with an insignificant decreasing amplitude with extended torrefaction residence time. In the low-temperature range of 220 °C–240 °C, the energy yield was maintained above 99.4%, with minimal data fluctuation among all residence time groups and an unobvious decreasing trend. In the temperature range of 240 °C–280 °C, the energy yield began to decrease gently, which was reduced to 97.73% for the 15 min group and 87.12% for the 25 min group at 280 °C. A significant decrease in energy yield was observed at 300 °C, with the minimum value dropping to 86.14%. Under the same temperature condition, the influence of torrefaction residence time on energy yield was much weaker than that of torrefaction temperature, and the difference in energy yield among different residence time groups was only slightly expanded in the high-temperature range of 280 °C–300 °C.

Energy yield variation during torrefaction is governed by the coupled effects of compositional conversion and energy retention within corn stover pellets. Within the low-temperature range of 220 °C–240 °C, only surface moisture evaporation and trace low-boiling fraction removal occur; given the inherently low energy density of these constituents, their loss exerts negligible influence on overall energy retention, thereby sustaining energy yield at a high level. Above 240 °C, progressive thermal degradation of hemicellulose and cellulose liberates small amounts of volatile organic compounds, and escalating reaction intensity with rising temperature releases more energy-bearing volatiles, causing a steady decline in energy yield; extensive lignin decomposition initiated at 300 °C is accompanied by pronounced energy release and loss, triggering a sharp drop in energy yield. Extended residence time only marginally advances the degradation process, resulting in limited conversion and energy loss at low temperatures and thus rendering the effect of holding time statistically insignificant, whereas the cumulative effect of thermal degradation is amplified at elevated temperatures, widening energy loss disparities across residence time groups and producing modest differences in final energy yield. Energy density exhibited a significantly increasing trend with rising temperature, whereas only a marginal increase was observed with extended torrefaction residence time. The effect of torrefaction temperature was found to dominate over that of residence time. In the low-temperature range of 220 °C–240 °C, the energy density was increased gently and maintained between 1.00 and 1.05. A significant increase amplitude of energy density was observed in the temperature range of 240 °C–260 °C, which exceeded 1.05 for all residence time groups at 260 °C, and reached 1.22 for the 25 min group. A continuous increase was found in the temperature range of 260 °C–300 °C, with the peak value of 1.33 achieved under the condition of 300 °C for 25 min. Under the same temperature condition, the energy density was slightly increased with the extension of torrefaction residence time, and the enhancement effect of residence time on energy density in the high-temperature range was slightly higher than that in the low-temperature range.

The core driver of energy density evolution during torrefaction lies in compositional upgrading and moisture removal of corn stover pellets. In the low-temperature regime of 220 °C–240 °C, pellets primarily lose free moisture and minor low-energy light volatiles, causing a marginal increase in the mass fraction of combustible components and hence a gentle rise in energy density. Above 240 °C, moderate thermal degradation of hemicellulose and cellulose commences, further eliminating oxygen-containing functional groups and light fractions and inducing relative carbon enrichment in the solid products; since carbon serves as the dominant contributor to energy density, deepening degradation severity with rising temperature amplifies this enrichment effect, driving a substantial increase in energy density. Extended torrefaction residence time affords adequate reaction duration for component removal and carbon enrichment, facilitating the full escape of low-energy fractions and thus moderately boosting energy density, while higher thermal degradation reactivity at elevated temperatures intensifies the cumulative time effect, rendering the promoting influence of residence time on energy density more pronounced in the high-temperature range. The EMCI exhibited a significant increasing trend with rising experimental temperature and an overall upward trajectory with extended torrefaction residence time. When the temperature was below 240 °C, the EMCI values remained extremely low, registering only 0.8%, 1.88%, and 4.35% for the 15, 20, and 25 min groups, respectively, with a gentle growth rate. After the temperature exceeds 240 °C, the EMCI was sharply increased, reaching 16.46%, 18.12%, and 17.08% for the 15, 20, and 25 min groups at 280 °C, respectively, and the EMCI values of the three groups were further increased to 16.72%, 20.24%, and 21.57% at 300 °C, respectively. Under the same temperature condition, the highest overall EMCI was obtained for the 25 min torrefaction group, while the relatively lowest EMCI was achieved for the 15 min group. The enhancement effect of torrefaction residence time on EMCI was more significant in the high-temperature range, and the increase amplitude of EMCI caused by extended residence time was further expanded in the temperature range of 280 °C–300 °C.

The evolution of the EMCI is governed by the synergistic mechanism between energy density enhancement and mass yield reduction during torrefaction. Below 240 °C, only moisture evaporation and minor light fraction removal occur in corn stover pellets, yielding limited energy density improvement while mass yield remains above 95%; their combined effect results in extremely low EMCI values with gentle growth. Above 240 °C, severe thermal degradation of cellulose and hemicellulose releases abundant volatiles and triggers a sharp decline in mass yield, whereas fixed carbon and energy enrichment in solid products substantially elevates energy density; as the positive effect of energy density enhancement gradually outweighs the negative impact of mass yield loss, EMCI increases dramatically. Extended residence time affords longer reaction duration to promote continuous energy enrichment in solids and steady energy density growth, driving an overall rise in EMCI; this time-dependent effect is more prominent in the 280 °C–300 °C range, so the promoting influence of residence time on EMCI becomes more pronounced with rising temperature.

Figure depicts the mechanistic schematic of flue gas torrefaction for corn stover pellets, constructed based on this study’s experimental characterizations and well-recognized torrefaction mechanisms in existing literature, with all aspects supported by solid experimental evidence. Raw pellets undergo thermochemical conversion in the reactor at 220 °C–300 °C for 15–25 min under a controlled ternary atmosphere (80 vol·% N₂, 12 vol·% CO₂, 8 vol·% O₂), where preferential hemicellulose decomposition acts as the core reaction accompanied by ∼30% mass loss, volume shrinkage, cross-linking reactions, and release of torrefaction gases including CH₄, H₂, tars and acetic acid; microstructurally, the native dense fibrous structure evolves into a rough, porous and fissured amorphous matrix, which disrupts biomass fibrous toughness, significantly improves grindability, and enhances fuel hydrophobicity and energy density via deoxygenation. The preferential hemicellulose decomposition is validated by proximate analysis and DTG profiles showing pronounced volatile reduction and a diminishing hemicellulose peak across 220–260 °C, consistent with canonical torrefaction patterns; , the observed mass loss, shrinkage, and cross-linking align with the measured mass yield at 300 °C for 25 min, corroborated by increased fixed carbon and reduced O/C atomic ratio; the structural transition is directly confirmed by SEM micrographs (Figure ), with improved grindability and hydrophobicity as well-documented functional outcomes of this morphological evolution; , the gaseous product composition matches this study’s volatile release characteristics and agrees with reported torrefaction gas-phase measurements for corn stover; , and the ternary atmosphere formulation exactly corresponds to the simulated flue gas ratio adopted in the experiment.

8.

Schematic diagram of the torrefaction process note: original figure created by the authors.

3.3. Analysis of Combustion Test Results for Torrefied Fuels

As illustrated in Figure , the TG and DTG profiles from combustion tests delineate three distinct stages of thermal decomposition for all samples: moisture evaporation, main devolatilization, and residual char stabilization. The slight weight loss below 100 °C is attributed to the volatilization of free and bound water, corresponding to a faint, narrow peak in the DTG curve. The 200 °C–500 °C temperature range serves as the core decomposition stage, where the TG curve exhibits sharp mass loss and the DTG profile presents characteristic decomposition peaks corresponding to hemicellulose, cellulose, and lignin, respectively.

9.

TG-DTG curves of torrefied products.

Torrefaction pretreatment substantially alters the thermal decomposition behavior of the pellets. With rising torrefaction temperature and extended residence time, the final residual mass of the samples decreases progressively, and the main weight loss peak shifts toward higher temperatures with reduced peak intensity. These phenomena indicate that labile components such as hemicellulose are partially degraded in advance during torrefaction, which enhances the thermal stability of residual fractions and moderates the subsequent decomposition process; this modification effect is amplified under more severe torrefaction conditions (elevated temperature and prolonged holding time), rendering the thermal behavior of treated samples increasingly robust. Combined with results from ultimate analysis and calorific value measurements, the 300 °C/25 min torrefaction condition achieves the most significant upgrading effect.

To quantitatively elucidate the impact of simulated flue gas torrefaction on the combustion and pyrolysis behavior of corn stover-densified pellets across varying temperatures and residence times, the deviations between the TG and DTG curves of torrefied samples and those of the raw feedstock are defined as △TG and △DTG (△TG/DTG = TG/DTG_(TCS)–TG/DTG_(CS)).

From the perspective of pyrolysis mechanism, the interference from the intrinsic pyrolysis behavior of the raw feedstock can be effectively eliminated by this difference characterization method, and the modification effect of torrefaction pretreatment on the thermal weight loss process of the samples can be directly quantified. The positive value interval of △TG corresponded to the temperature range where the mass retention rate of the torrefied sample was higher than that of the raw feedstock, which intuitively reflected the degree of advance removal of volatile components and the improvement of thermal stability during torrefaction. △DTG further accurately captured the shifting and weakening effects of torrefaction on the pyrolysis peaks of different components from the dimension of weight loss rate. A direct and visual quantitative basis for the optimization of torrefaction process parameters was provided by this difference characterization method. The regulation law of torrefaction on the pyrolysis characteristics of corn stalk under different process conditions was clearly revealed, and a theoretical foundation for the subsequent comprehensive evaluation of combustion performance was established accordingly.

As illustrated by the ΔTG and ΔDTG difference curves in Figure , the modification effect of torrefaction on the thermal decomposition behavior of corn stover can be intuitively quantified via mass loss deviation relative to the raw feedstock. Below 200 °C, ΔTG shows a slight upward trend under all torrefaction conditions, corresponding to a weak positive peak in ΔDTG; this arises because preremoval of moisture and partial light volatiles during torrefaction reduces mass loss in this stage and elevates the relative mass retention rate. The 200 °C–400 °C range serves as the core interval for difference evolution, where ΔTG rises sharply to a peak that intensifies with increasing torrefaction temperature and residence time: ΔTG peaks at 15.5%, 23.0% and 30.0% for the 15 min, 20 and 25 min groups at 300 °C, respectively, accompanied by a pronounced positive ΔDTG peak, indicating that near-complete depletion of readily decomposable components such as hemicellulose during torrefaction markedly attenuates mass loss in this range and generates a distinct mass retention advantage. Above 400 °C, ΔTG stabilizes at a high level and ΔDTG exhibits a slight negative peak before returning to baseline, reflecting that torrefaction residuals are dominated by thermally stable lignin and fixed carbon, narrowing the decomposition gap with raw feedstock and yielding a higher final solid residue.

10.

TG-DTG difference curves of torrefied products.

Overall, the influence of torrefaction temperature on the △TG and △DTG curves was much stronger than that of torrefaction residence time. The most significant mass loss difference compared with the raw feedstock was observed under the condition of 300 °C for 25 min, which further verified the optimal modification effect of this pretreatment condition on improving the thermal stability of the samples.

As illustrated in Table , analysis of the TG-DTG profiles and combustion characteristic parameters revealed that the combustion behavior of corn stover densified pellets was significantly governed by torrefaction pretreatment. In terms of ignition temperature, the ignition temperature of the untorrefied raw feedstock was 251.16 °C, while an overall upward trend of the ignition temperature of the torrefied samples was observed after torrefaction treatment. Specifically, the ignition temperature exceeded 270 °C under the conditions of 260 °C for 20 min, 280 °C for 20 min, and 300 °C for 15 min, which is consistent with the reduction of mass loss rate in the low-temperature section of the TG curves. Light volatile matter in the raw feedstock was removed during torrefaction, and the thermal stability of the residual components was improved, thus the initial temperature required for ignition was increased. Meanwhile, the variation of burnout temperature presented a characteristic of “stabilization first followed by reduction.” When the torrefaction temperature was below 260 °C, the burnout temperature was basically maintained between 447 and 450 °C, with little difference from that of the untorrefied raw feedstock. When the temperature exceeded 280 °C and the residence time was extended to 20–25 min, the burnout temperature was reduced to 434 °C–442 °C. This is attributed to the extensive degradation of hemicellulose caused by deep torrefaction, the residual components were dominated by cellulose and polycondensed lignin, and the pyrolysis products of these components exhibited higher oxidation reaction activity, which enabled complete burnout at a lower temperature.

2. Combustion Characteristic Parameters of Torrefied Products.

process parameters	T i /°C	T h /°C	(dm/dt)max	(dm/dt)mean	t h /min	SN/10–7 (°C–3·min–2)
			/(%·min–1)	/(%·min–1)
CS	251.16	447.80	7.24	3.41	42.47	8.74
220 °C-15 min	256.71	449.32	6.97	3.45	42.42	8.32
220 °C-20 min	256.62	447.84	7.11	3.55	42.44	8.56
220 °C-25 min	258.42	448.20	7.02	3.58	42.50	8.40
240 °C-15 min	257.67	450.71	7.10	3.51	43.74	8.33
240 °C-20 min	259.67	449.99	7.04	3.71	42.66	8.61
240 °C-25 min	261.47	448.02	7.37	3.66	42.47	8.81
260 °C-15 min	260.75	447.30	7.17	3.63	42.41	8.56
260 °C-20 min	270.43	452.41	5.45	3.57	42.92	5.88
260 °C-25 min	270.57	447.20	5.85	3.63	42.20	6.49
280 °C-15 min	265.85	446.32	7.18	3.68	42.31	8.38
280 °C-20 min	270.62	442.51	6.33	3.71	41.94	7.25
280 °C-25 min	266.45	439.92	5.09	3.41	41.70	5.56
300 °C-15 min	270.12	441.96	6.29	3.67	41.91	7.16
300 °C-20 min	263.26	434.55	5.43	3.48	41.19	6.27
300 °C-25 min	266.16	443.53	5.40	3.21	42.05	5.52

Note: T _i is defined as the ignition temperature of the fuel; t _h is defined as the burnout time of the fuel; T _h is defined as the burnout temperature of the fuel; (dm/dt)_max is defined as the maximum combustion rate of the fuel; (dm/dt) _mean is defined as the average combustion rate of the fuel; and S _N is defined as the comprehensive combustion characteristic index of the fuel.

The variations of maximum combustion rate and average combustion rate reflect the bidirectional effects of torrefaction conditions. Under low-temperature and short-time torrefaction conditions (220 °C–240 °C, 15–20 min), the maximum combustion rate was maintained between 6.97%·min^–1 and 7.37%·min^–1, which was slightly lower than 7.24%·min^–1 of the untorrefied raw feedstock, while a slight increase was observed for the average combustion rate. This is because the mild degradation of hemicellulose reduced the reaction substrate in the medium-temperature section, while the mild polycondensation of lignin did not significantly hinder the pyrolysis of cellulose, thus the average combustion rate was slightly increased. When the torrefaction temperature rose above 260 °C and the residence time extended to 20–25 min, the maximum combustion rate decreased markedly to 5.09%–5.85%·min^–1, and a more prominent high-temperature DTG peak (i.e., the second peak prevailing over the first in magnitude) was observed for several samples. This observation is consistent with the broadening of the high-temperature DTG band reported in previous torrefaction investigations. A plausible mechanistic hypothesis for this peak-splitting behavior is that polycondensed lignin forms a dense, low-permeability barrier layer that partially encapsulates the residual cellulose skeleton. It should be explicitly clarified that this interpretation is currently derived solely from indirect bulk compositional and thermal evidence, and direct spatial characterization to verify such a core–shell microstructure has not been conducted in the present study. Proximate and ultimate analyses provide supporting circumstantial evidence: escalating torrefaction severity drives progressive lignin enrichment and partial cellulose decrystallization. Within the 260 °C–300 °C range, lignin undergoes aromatic polycondensation via consumption of phenolic hydroxyl groups, yielding cross-linked macromolecular aggregates. At torrefaction temperatures above the glass transition point of lignin, softened lignin may migrate and deposit onto the surfaces of residual cellulose fibers, potentially forming a continuous compact coating. The steady reductions in H/C and O/C atomic ratios reflect the evolution of functional groups, where extensive cleavage of cellulose glycosidic linkages and pronounced accumulation of aromatic CC structures in lignin during deep torrefaction collectively satisfy the chemical prerequisites for the formation of such a lignin-rich surface layer. ,,

From the perspective of thermal conversion mechanisms, the presence of a low-permeability lignin-derived layer would impede internal heat and mass transfer during cellulose pyrolysis, delay the release of cellulose-derived volatiles, and fractionate the combustion process into multistage reactions, thereby reducing the instantaneous combustion intensity. This proposed mechanism aligns with the well-documented interaction patterns among lignocellulosic components. Previous experimental studies focusing on corn stover torrefaction and pyrolysis have confirmed that lignin enrichment and cross-linking can exert significant mass transfer resistance to cellulose devolatilization, which consequently gives rise to shifted and split DTG peaks. ,,

The overall variation of burnout time was gentle. The burnout time of the untorrefied raw feedstock was 42.47 min, and the burnout time of torrefied samples was concentrated in the range of 41.19–43.74 min, with only a slight extension observed under the condition of 240 °C for 15 min. This indicated that the total duration of the combustion process was not significantly altered by torrefaction pretreatment, and the underlying reasons were as follows: although the removal of light volatile matter reduced the reaction time in the low-temperature section, the slowed reaction rate in the high-temperature section caused by lignin polycondensation offset this effect to a certain extent, thus the total burnout time was maintained at a similar level. The variation of the comprehensive combustion characteristic index directly reflected the effect of torrefaction on combustion performance. The index of the untorrefied raw feedstock was 8.74 × 10^–7 °C^3–·min^–2. When the torrefaction temperature was below 260 °C and the residence time was shorter than 20 min, the index was only slightly decreased to 8.32–8.81 × 10^–7 °C^3–·min^–2, indicating that no significant deterioration of combustion performance occurred at this stage. However, when the temperature exceeded 260 °C and the residence time was extended to 20–25 min, the index was reduced to 5.52–6.49 × 10^–7 °C^3–·min^–2. This is because the comprehensive combustion characteristic index integrates parameters such as ignition temperature, combustion rate, and burnout time, and the increased ignition temperature and reduced maximum combustion rate caused by deep torrefaction jointly lead to the decline of comprehensive combustion performance.

Correlating the TG-DTG profiles with the combustion parameters further validates “component-selective regulation” during torrefaction as the core underlying mechanism. The degradation degree of hemicellulose was deepened with the increase of torrefaction temperature and residence time. This process not only removed light components that were easy to burn at low temperature and led to the increase of ignition temperature, but also altered the structural state of residual components and promoted the formation of a barrier layer via lignin polycondensation. In contrast, the crystal structure of cellulose was not significantly damaged when the torrefaction temperature was below 300 °C, thus it remained the main exothermic component during combustion, which prevented drastic changes in average combustion rate and burnout time. Regarding combustion performance, flue gas-torrefied samples exhibit higher ignition temperatures and more pronounced DTG peak splitting than N₂-torrefied counterparts at comparable mass yields, indicating that trace O₂ preoxidation and CO₂-induced structural reconstruction jointly shape fuel combustion kineticsan effect unreproducible by pure thermal decomposition under an inert N₂ atmosphere. Furthermore, O₂ in the simulated flue gas participates in oxidative reactions during torrefaction, and the reduced volatile content of torrefied fuels weakens the homogeneous combustion stage while enhancing the heterogeneous combustion stage, which also accounts for the higher second DTG peak observed in severely torrefied samples. ,

It is worth noting that torrefaction conditions exerted a pivotal regulatory effect on the properties of the resultant products. When the torrefaction temperature was below 260 °C and the residence time was shorter than 20 min, the hygroscopicity of the raw feedstock was reduced via predevolatilization, while good combustion performance was maintained. When the temperature exceeded 260 °C and the residence time was extended to more than 20 min, the carbon content was increased, but the combustion performance was significantly deteriorated. Therefore, appropriate torrefaction conditions should be selected according to application requirements in practical engineering. In summary, directional regulation of the combustion behavior of corn stalk-densified pellets was achieved by torrefaction pretreatment via altering the component composition and structural characteristics of the pellets. This law provides data support and a theoretical basis for the optimization of pretreatment processes for biomass fuels.

4. Conclusions

Severe flue gas torrefaction is found to maximize the energy density and HHV of corn stover briquettes. This enhancement is achieved by inducing structural amorphization and deep deoxygenation, driven primarily by the degradation of hemicellulose. Such intense reactions are found to increase ignition temperature and reduce the maximum combustion rate, thereby deteriorating combustion kinetic performance. Accordingly, mild conditions of 220 °C–240 °C with a holding time of 15–20 min are recommended as the optimal thermochemical window. Under such conditions, the comprehensive benefit index, hydrophobicity, and grindability of fuels are remarkably improved, while combustion stability in practical large-scale application is well maintained.

The authors declare no competing financial interest.