Thermogravimetric analysis and pyrolysis kinetics of typical crop straws

2.1. Biomass sample

Corn stalk samples collected in Zhengzhou, Henan Province, China; rapeseed stalk samples collected in Zhengzhou, Henan Province, China; and cotton stalk samples collected in Lianyungang, Jiangsu Province, China were all pre-processed using a shredder. After grinding, the material was passed through a 50-mesh standard sieve (<0.25 mm) to obtain the initial sample.

2.2. Instrument and experimental conditions

Thermogravimetric analysis was conducted on a Shimadzu DTG-60H analyzer in Japan. Approximately 3–5 mg of straw samples were heated from room temperature to 800°C at heating rates of 10, 20, 30, and 40°C min^-1. High-purity N₂ (99.999%) was used as the purge gas at a flow rate of 30 mL/min to maintain an inert atmosphere. To ensure the authenticity of the curves, each experimental group was designed with at least three replicates.

According to GB/T 28731-2012 Industrial Analysis Methods for Solid Biomass Fuels, the moisture, volatile matter, ash, and fixed carbon content in corn stalks, rapeseed stalks, and cotton stalks were estimated. The fixed carbon content was calculated using the mathematical formula: [fixed carbon% = 100% - (moisture content + volatile matter + ash content)%]. Elemental analysis was performed using an organic elemental analyzer (VARIO EL III Organic Elemental Analyzer) in CHNS mode and O mode.

2.3. Kinetic model

3.1. Industrial analysis and elemental analysis

The industrial analysis results for straw are shown in Table 2. As indicated in Table 2, the moisture, ash, volatile matter, and fixed carbon content of corn straw are 7.72%, 7.71%, 69.38%, and 15.19%, respectively. Rapeseed straw exhibits moisture, ash, volatile matter, and fixed carbon content of 9.08%, 9.75%, 67.02%, and 14.15%, respectively. Cotton straw contains 6.52%, 15.67%, 63.56%, and 14.25% moisture, ash, volatile matter, and fixed carbon, respectively. Significant differences exist among these three typical crop straws. Corn and rapeseed straw exhibit low ash content, classifying them as low-ash fuels. Cotton straw contains markedly higher ash content than corn and rapeseed straw, attributed to its higher silica content in the outer husk. All three crop residues exhibit high volatile matter content, indicating they ignite more readily and burn more completely than coal. This demonstrates the significant potential of crop residues as a biofuel source [21]. The higher fixed carbon content in corn stalks compared to rapeseed and cotton stalks stems from corn stalks’ greater lignification, resulting in relatively higher fixed carbon levels [22].

The elemental analysis results for straw are shown in Table 3. As indicated in Table 3, corn straw contains 49.72% C, 7.16% H, 41.47% O, 1.49% N, and 0.16% S. Rapeseed straw contains 44.24% C, 6.20% H, 48.26% O, 0.92% N, and 0.38% S. Cotton stalks contain 44.13% C, 6.05% H, 48.77% O, 0.70% N, and 0.35% S. All three crop stalks exhibit relatively high C and O content. The H/C molar ratio reflects the content of light hydrocarbons in the straw. A higher H/C ratio indicates a higher proportion of volatile components. The low sulfur content suggests that crop straw contributes minimally to atmospheric SO₂ emissions during energy utilization. Compared to coal, straw qualifies as a clean energy source.

3.2. TG analysis

The thermal decomposition behavior of corn stover, rapeseed straw, and cotton straw was investigated using thermogravimetric analysis (TGA) over a temperature range from room temperature to 800°C. Figures 1-3 illustrate the decomposition behavior of corn stover, rapeseed straw, and cotton stalks, respectively, as a function of temperature. The TG and DTG curves reveal that: At all heating rates, the TG and DTG curves exhibit similar trends. As the heating rate increases, the TG and DTG curves shift toward higher temperatures [23,24].

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Figure 1.

Thermogravimetric curves of corn straw at different heating rates: (a) TG curve; (b) DTG curve.

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Figure 2.

Thermogravimetric curves of rapeseed straw at different heating rates: (a) TG curve; (b) DTG curve.

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Figure 3.

Thermogravimetric curves of cotton straw at different heating rates: (a) TG curve; (b) DTG curve.

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Simultaneously, the pyrolytic weight loss process of straw can be divided into three stages: drying and dehydration, pyrolysis of organic compounds such as cellulose/hemicellulose/lignin, and carbonization [25]. During the first stage, the feedstock does not decompose with increasing temperature but undergoes drying and dehydration. This stage spans a temperature range of approximately room temperature to 149.92°C (corn stover), room temperature to 165.89°C (rapeseed straw), and room temperature to 162.8°C (cotton straw). The weight loss percentages for corn stalks, rapeseed stalks, and cotton stalks were approximately 10.9%, 12.4%, and 9.37%, respectively. The rate of weight loss gradually decreased toward the end of this stage. According to related research reports, other crop straws exhibit similar trends. In the second stage, cellulose, hemicellulose, and lignin decompose. The pyrolysis rate of the raw material accelerates from the start to a maximum value before slowing down. The TG curve declines rapidly, and the DTG curve exhibits a peak. During this process, mass decreases rapidly with a relatively fast pyrolysis rate. The primary pyrolysis temperature range for this stage is approximately 149.92°C to 708.68°C (corn stover), 165.89°C to 646.20°C (rapeseed straw), and 162.8°C to 765.16°C (cotton stalk). During the third stage, residual material undergoes slow decomposition and carbonization until mass changes cease. The temperature range for this stage is approximately 708.68°C to 800°C (corn stover), 646.20°C to 800°C (rapeseed straw), and 765.16°C to 800°C (cotton stalk). The residual components after combustion are primarily ash. Industrial analysis indicates that cotton stalks exhibit the highest ash content at 15.67%, resulting in the greatest residue after thermogravimetric analysis. Corn stalks and rapeseed stalks show similar ash content levels, implying comparable thermogravimetric residues.

Taking corn stover as an example, we also observe that when the heating rate increases from 10 to 40°C min^-1, the temperature of maximum weight loss rate rises from 300.24°C to 352.70°C, while the maximum mass loss rate increases from 39.057% to 44.177%. The results indicate that the reaction rate is closely related to the heating rate. This phenomenon, known as thermal lag, has been reported in numerous studies on biomass pyrolysis [26]. As the heating rate increased from 10 to 40°C/min, the residual mass range expanded from 4.439% to 10.199%. The highest residual mass was observed at a heating rate of 40°C min^-1, while the lowest residual mass occurred at 10°C min^-1. This aligns with the volatiles observed in Table 2. The variation in residual mass primarily stems from slower heating rates favoring solid product formation. Results indicate that heating rate has no significant effect on residual mass. Similar conclusions apply to rapeseed straw and cotton stalks.

3.3. Model-free method

Evaluating kinetic factors of biomass, such as activation energy, aids in understanding the reactivity of specific feedstocks. Figures 4-6 show the T-α curves for the pyrolysis of corn stover, rapeseed straw, and cotton stalks, respectively. We observe consistent patterns among the three straw types concerning heating rate, pyrolysis temperature, and α. At a constant heating rate, α increases with rising pyrolysis temperature; at a constant pyrolysis temperature, α decreases with increasing heating rate.

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Figure 4.

T-α curve of corn stalk pyrolysis.

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Figure 5.

T-α Curve of rapeseed stalk pyrolysis.

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Figure 6.

T-α curve of cotton stalk pyrolysis.

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The apparent activation energies of the three types of straw measured in this study are consistent with previous studies and reflect the differences in raw material characteristics. The FWO, KAS, and Starink methods were employed to predict the thermodynamics of corn stover, rapeseed straw, and cotton straw. Figure 7 shows the linear fitting curve for corn stover at conversion rates ranging from 0.1 to 0.9. Figure 8 shows the linear fitting plot for rapeseed straw at conversion rates ranging from 0.1 to 0.9. Figure 9 presents the linear fitting plot for cotton straw at conversion rates ranging from 0.1 to 0.9. The fitting plots indicate that E changes as the straw pyrolysis conversion rate α increases from 0.1 to 0.9. For corn stover, the FWO method yielded an E range of 56.15–102.31 kJ mol^-1 with an average of 85.16 kJ mol^-1; The KAS method yielded E values ranging from 43.55 to 93.08 kJ mol^-1, with an average of 75 kJ mol^-1; The Starink method yielded E values ranging from 44.06 to 93.45 kJ mol^-1, with an average of 75.41 kJ mol^-1. This aligns with the literature-reported activation energy of corn stalks. The slightly higher value stems from our study’s cross-validation using four heating rates, yielding more precise data [27,28]. For rapeseed straw, the FWO method yielded an E range of 69.45–110.59 kJ mol^-1 with an average of 99.7 kJ mol^-1; the KAS method yielded an E range of 57.09–102.07 kJ mol^-1 with an average of 89.71 kJ mol^-1; Calculations using the Starink method yielded an E range of 57.59–102.39 kJ mol^-1, with an average value of 90 kJ mol^-1. The activation energy of rapeseed straw is higher than that reported in the literature, which is related to the higher lignin content in this study. The stable structure of lignin leads to an increased demand for bond-breaking energy [29-31]. For cotton straw, the FWO method yielded an E range of 47.17–110.77 kJ mol^-1, with an average value of 90.17 kJ mol^-1; Calculations using the KAS method yielded activation energy values ranging from 33.87 to 100.80 kJ mol^-1, with an average of 79.62 kJ mol^-1; calculations using the Starink method yielded activation energy values ranging from 34.40 to 101.196 kJ mol^-1, with an average of 80.04 kJ mol^-1. The activation energy of cotton stalks is highly consistent with the literature, which confirms the reliability of the combined application of the model method in this study [32]. Compared to two other typical crops, rapeseed straw exhibited slightly higher activation energy estimates, while corn stover showed slightly lower estimates. The study revealed that corn stover requires less energy for thermal decomposition than rapeseed straw and cotton stover.

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Figure 7.

Linear fitting curves of corn stover at different conversion rates ranging from 0.1 to 0.9 (a) FWO method; (b) KAS method; (c) Starink method.

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Figure 8.

Linear fitting curves of rapeseed stover at different conversion rates ranging from 0.1 to 0.9 (a) FWO method; (b) KAS method; (c) Starink method.

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Figure 9.

Linear fitting curves of cotton stover at different conversion rates ranging from 0.1 to 0.9 (a) FWO method; (b) KAS method; (c) Starink method.

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At a heating rate of 10°C min^-1, the values of E, A, and R² calculated from the FWO method, KAS method, and Starink method at different conversion rates are shown in Tables 4-6. The activation energy was calculated from the slope and intercept of the fitted lines. The pre-exponential factor was predicted based on the activation energy, and R² was used to assess the accuracy of the kinetic parameters. As shown in Table 4, the R² values for all corn stover curves fell within the narrow range of 0.9593–0.9999, indicating high precision and good data point fitting. At all heating rates, all R² values exceeded 0.95, confirming the reliability of the pyrolysis kinetic parameters for straw. Furthermore, the activation energy and frequency factor were not constant but varied with increasing conversion rate, indicating the presence of multiple single-reaction mechanisms in straw pyrolysis. As shown in Table 5, the R² values for all rapeseed straw curves fall within the narrow range of 0.9831–0.9999. As shown in Table 6, the R² values for all cotton straw curves fall within the narrow range of 0.9695–0.9999.

Low pre-exponential factor estimates indicate that thermal degradation processes exhibit lower reactivity compared to surface reactions. High pre-exponential factor estimates suggest high reactivity associated with complex polyhedral reaction processes. The estimated value of A increases with decomposition progression, attributed to heightened collision frequency between molecules at elevated conversion levels and heating rates.

Figure 10 shows the relationship between E and A of corn stover and α at different conversion rates. Figure 11 shows the relationship between E and A of rapeseed stover and α at different conversion rates. Figure 12 shows the relationship between E and A of cotton stover and α at different conversion rates. Regarding the relationship between E and α, the trends in activation energy under different methods were largely consistent for biomass of the same type. For corn stover, activation energy increased as α rose from 0.10 to 0.30: from 91.20 to 102.31 kJ mol^-1 (FWO), from 82.55 to 93.08 kJ mol^-1 (KAS), and from 82.90 to 93.45 kJ mol^-1 (Starink). This indicates the endothermic nature of pyrolysis during the pyrolysis stage. When α increased from 0.30 to 0.90, the activation energy showed a decreasing trend. It decreased from 102.31 to 56.15 kJ mol^-1 (FWO), from 93.08 to 43.55 kJ mol^-1 (KAS), and from 93.45 to 44.06 kJ mol^-1 (Starink). It peaks at α = 0.30. For rapeseed straw, the activation energy decreases as α increases from 0.10 to 0.40. From 110.59 to 102.8 kJ mol^-1 (FWO), 102.07 to 93.58 kJ mol^-1 (KAS), 102.39 to 93.91 kJ mol^-1 (Starink). Activation energy increased as α increased from 0.40 to 0.50. From 102.80 to 107.17 kJ mol^-1 (FWO), 93.58 to 97.73 kJ mol^-1 (KAS), 93.91 to 98.08 kJ mol^-1 (Starink). When α increases from 0.50 to 0.90, the activation energy shows a decreasing trend. From 107.17 to 69.45 kJ mol^-1 (FWO), 97.73 to 57.09 kJ mol^-1 (KAS), and 98.08 to 57.59 kJ mol^-1 (Starink). This fluctuation suggests a complex reaction mechanism during rapeseed straw pyrolysis. For cotton stalks, the activation energy increases as α rises from 0.10 to 0.50. Values range from 99.93 to 110.77 kJ mol^-1 (FWO), 91.029 to 100.798 kJ mol^-1 (KAS), and 91.38 to 101.20 kJ mol^-1 (Starink). When α increased from 0.50 to 0.90, the activation energy showed a decreasing trend. It decreased from 110.77 to 47.17 kJ mol^-1 (FWO), from 100.798 to 33.865 kJ mol^-1 (KAS), and from 101.20 to 34.40 kJ mol^-1 (Starink). It reached its peak at α = 0.50.

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Figure 10.

Corn stalks. (a) Relationship between E and α; (b) Relationship between A and α.

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Figure 11.

Rapeseed stalks. (a) Relationship between E and α; (b) Relationship between A and α.

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Figure 12.

Cotton stalks. (a) Relationship between E and α; (b) Relationship between A and α.

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For the relationship between A and α, the trends in the indexing factor under the KAS and Starink methods are very similar for biomass of the same type, whereas the FWO method differs from them. For corn stover, the indexing factor shows an upward trend when the conversion rate ranges from 0.10 to 0.30. From 1.13E+06 to 1.31E+07 kJ mol^-1 (FWO), 1.67E+05 to 1.71E+06 kJ mol^-1 (KAS), and 1.80E+05 to 1.86E+06 kJ mol^-1 (Starink). This is primarily attributed to the thermal decomposition of cellulose and non-cellulose polysaccharides. However, when the conversion rate is between 0.30 and 0.90, the prefactor shows a decreasing trend. From 1.31E+07 to 4.47E+02 kJ mol^-1 (FWO), 1.71E+06 to 2.46E+01 kJ mol^-1 (KAS), 1.86E+06 to 2.77E+01 kJ mol^-1 (Starink). It peaks at 0.30. For rapeseed straw, the prefactor exhibits a decreasing trend when the conversion rate ranges from 0.10 to 0.40. From 1.28E+08 to 2.25E+07 kJ mol^-1 (FWO), 1.91E+07 to 2.85E+06 kJ mol^-1 (KAS), 2.05E+07 to 3.07E+06 kJ mol^-1 (Starink). When the conversion rate is between 0.40 and 0.50, the prefactor shows an increasing trend. From 2.25E+07 to 5.96E+07 kJ mol^-1 (FWO), 2.85E+06 to 7.23E+06 kJ mol^-1 (KAS), 3.07E+06 to 7.82E+06 kJ mol^-1 (Starink). When the conversion rate is between 0.50 and 0.90, the prefactor shows a decreasing trend. From 5.96E+07 to 1.22E+04 kJ mol^-1 (FWO), 7.23E+06 to 7.16E+02 kJ mol^-1 (KAS), 7.82E+06 to 8.04E+02 kJ mol^-1 (Starink). Peak reached at 0.10. For cotton stalks, the pre-exponential factor shows an increasing trend when the conversion rate is between 0.10 and 0.50. From 7.18E+06 to 7.68E+07 kJ mol^-1 (FWO), 1.02E+06 to 8.70E+06 kJ mol^-1 (KAS), 1.10E+06 to 9.48E+06 kJ mol^-1 (Starink). However, when the conversion rate is between 0.50 and 0.90, the prefactor shows a decreasing trend. From 7.68E+07 to 5.49E+01 kJ mol^-1 (FWO), 8.70E+06 to 2.44E+00 kJ mol^-1 (KAS), and 9.48E+06 to 2.77E+00 kJ mol^-1 (Starink). It peaks at 0.50.

3.4. Thermodynamic parameter analysis

Thermodynamic parameters are crucial for understanding pyrolysis processes. Knowledge of these parameters provides valuable insights into energetics and kinetics, laying the foundation for subsequent industrial design. We employed equations (6), (7), and (8) to determine thermodynamic parameters for α values ranging from 0.1 to 0.9. The results for corn stover, rapeseed straw, and cotton straw are presented in Tables 7-9, respectively. Notably, ΔH values remain positive across all conversion rates. Rapeseed straw exhibits higher ΔH values compared to other straws. Average ΔH values for corn straw are 81.36 kJ mol^-1 (FWO), 71.21 kJ mol^-1 (KAS), and 71.61 kJ mol^-1 (Starink). The average ΔH values for rapeseed straw were 95.89 kJ mol^-1 (FWO), 85.89 kJ mol^-1 (KAS), and 86.19 kJ mol ^-1 (Starink). The average ΔH values for cotton stalks were 86.13 kJ mol^-1 (FWO), 75.58 kJ mol^-1 (KAS), and 76 kJ mol^-1 (Starink). As α increased from 0.1 to 0.3, ΔH gradually rose for all three biomasses, indicating the endothermic nature of the pyrolysis process. After this conversion level, ΔH values progressively decreased. At each conversion level, the difference between ΔH and E for the three biomasses ranged from approximately 3 to 10 kJ mol^-1. Consequently, an additional 3 to 10 kJ mol^-1 of energy was required for product formation. This minor discrepancy also indicates the feasibility of the pyrolysis process.

The change in Gibbs free energy indicates the spontaneity of a reaction. A lower ΔG value signifies thermodynamically favorable conditions, indicating the feasibility of converting biomass into desired products. A higher ΔG value indicates a reaction prone to extreme heat flows and randomness, suggesting less favorable conditions. Under FWO conditions, the average ΔG values for corn stover, rapeseed stover, and cotton stover were estimated at 168.74 kJ mol^-1, 164.6 kJ mol^-1, and 169.16 kJ mol^-1, respectively. Under KAS conditions, the average ΔG values for corn stover, rapeseed straw, and cotton stalk were estimated at 169.4 kJ mol^-1, 165.12 kJ mol^-1, and 169.86 kJ mol^-1, respectively. Under Starink conditions, the average ΔG values for corn stover, rapeseed straw, and cotton stalk were estimated at 169.37 kJ mol^-1, 165.1 kJ mol^-1, and 169.83 kJ mol^-1, respectively. It can be observed that the ΔG values for the three straws are nearly identical, independent of the model. The positive ΔG values indicate the non-spontaneous nature of pyrolysis [33].

The entropy change represents the degree of disorder within a system. During pyrolysis, ΔS denotes the entropy difference between the products and reactants involved in the thermal decomposition of organic materials. All three types of straw exhibited negative ΔS values under different models [33]. This confirms that the disorder level of pyrolysis products is lower than that of the initial reactants. Simultaneously, these negative values indicate that pyrolysis products in the activated state possess a more ordered structure than pre-pyrolysis materials. This also suggests a low incidence of randomness during the dissociation of complex chemical bonds in actual reactants. The average ΔS values for corn stover were -0.15 kJ K^-1 (FWO), -0.17 kJ K^-1 (KAS), and -0.17 kJ K^-1 (Starink). For rapeseed straw, the average ΔS values were -0.12 kJ K^-1 (FWO), -0.14 kJ K^-1 (KAS), and -0.14 kJ/K (Starink). The average ΔS values for cotton stalks were -0.14 kJ K^-1 (FWO), -0.16 kJ K^-1 (KAS), and -0.16 kJ K^-1 (Starink).

The changes in ΔH, ΔG, and ΔS during pyrolysis of the three straw types are closely related to the reaction mechanisms and provide key theoretical basis for process optimization in bioenergy production. From the perspective of reaction mechanism correlation, ΔH, as the enthalpy change of reaction, directly reflects the energy demand of the pyrolysis reaction. The ΔH of rapeseed straw is significantly higher than that of corn and cotton straw, consistent with the higher apparent activation energy mentioned earlier. This stems fundamentally from its higher lignin content. The stable benzene ring structure of lignin requires the breaking of more chemical bonds during pyrolysis, increasing the energy input required for the reaction. This aligns with the F3 three-dimensional diffusion mechanism. A high enthalpy change indicates that the reaction system must overcome stronger intermolecular forces to achieve three-dimensional diffusion and conversion of the material. ΔG represents Gibbs free energy; its positive values indicate that pyrolysis of all three straw types is a non-spontaneous reaction requiring external heat input [34]. The lower ΔG value for rapeseed straw further confirms its lower energy barrier and greater spontaneity. ΔS, as entropy change, reflects alterations in system disorder. The larger ΔS value for rapeseed straw indicates more severe molecular structural disruption during pyrolysis, with rapid breakdown of the cross-linked structures of cellulose, hemicellulose, and lignin, yielding more small-molecule products. This aligns with the mechanism in the F3 model where “product diffusion dominates the reaction process.” Greater entropy increase implies stronger diffusion driving force, facilitating deeper conversion of the reaction.

From a bioenergy production perspective, ΔH provides guidance for heating process optimization: Corn and cotton straw exhibit lower ΔH values, allowing for energy savings through moderate-to-low heating intensities. Rapeseed straw requires moderately increased heating intensity to ensure complete reaction. ΔG guides reaction condition control. Optimizing pyrolysis temperature or introducing catalysts can lower the reaction free energy barrier, enhance spontaneity, and improve conversion efficiency. ΔS reflects product distribution tendencies. Rapeseed straw exhibits higher ΔS, favoring the formation of gaseous hydrocarbons (e.g., methane, ethylene), making it suitable for targeted biogas production. Corn and cotton straw show relatively moderate ΔS, yielding primarily biochar and bio-oil. Processes can be specifically optimized to enhance target product yields. The synergistic analysis of these three factors provides precise thermodynamic guidance for large-scale bioenergy production from crop residues, facilitating the minimization of energy consumption and maximization of product value.

3.5. Model method

The pyrolysis kinetics of three straw samples was analyzed using the Coats-Redfern model method to elucidate the pyrolysis reaction mechanism. Figures 13-15 show the linear fitting curves of the C-R model for corn stalks, rapeseed stalks, and cotton stems, respectively, under a heating rate of 10°C/min. Seventeen kinetic models were selected for each temperature range. The optimal model for each sample was determined by comparing the correlation coefficients of different reaction models. A correlation coefficient closer to 1 indicates a more ideal fit.

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Figure 13.

Linear fitting plot of corn stalk C-R method.

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Figure 14.

Linear fitting plot of rapeseed stalk C-R method.

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Figure 15.

Linear fitting plot of cotton stalk C-R method.

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The results indicate that for all three straw types, the optimal ideal kinetic model under a heating rate of 10°C/min is identical: the F3 model. For the main pyrolysis stage, the F3 model was selected as the optimal reaction mechanism. The linear correlation coefficients for corn stover, rapeseed straw, and cotton stalk were 0.98471, 0.94482, and 0.97261, respectively.

The F3 model determined by C-R method is consistent with most of the research conclusions of biomass pyrolysis mechanism. In the pyrolysis of apple residue, the chemical reaction of the constituent elements is the dominant one, which follows the F3 model [35]. In addition, the pyrolysis mechanism of biomass is regulated by the proportion of raw material components, and the higher the proportion of lignin, the more complex diffusion behavior is likely to be presented [36]. The higher values of ΔH, ΔS and E of rapeseed straw in this study are consistent with the conclusion of the literature, which further confirms the influence of component differences on the reaction mechanism.

The core principle of the F3 model is that during the primary reaction phase of straw pyrolysis (the volatilization and decomposition stage), heat and product molecules must diffuse through the three-dimensional pore structure within the straw. The reaction rate is thus controlled by the diffusion process rather than the chemical reaction itself. Experimental results show that all three straw types conform to the F3 model at a heating rate of 10°C/min. This indicates that during pyrolysis, volatile components generated by the decomposition of internal fiber structures (cellulose, hemicellulose) must escape through the three-dimensional porous medium channels. This diffusion process becomes the key rate-limiting step, highly consistent with the diffusion-controlled mechanism described by the F3 model. Further validation indicates that the F3 mechanism serves as the optimal kinetic model for biomass microwave pyrolysis at all heating rates ranging from 10 to 40°C/min.

3.6. Characterization of solid product

A scanning electron microscope (JSM-6490LV) with 15 kV acceleration voltage was used to analyze the surface morphology of biomass straw before and after pyrolysis, as shown in Figure 16. The analysis revealed that corn straw exhibited a loose fiber bundle stacking structure with a rough surface and irregular gaps between fibers Figure 16(a). The high cellulose and hemicellulose content resulted in a loose fiber arrangement with minimal pores. Rapeseed straw showed the highest lignification degree, featuring a dense and smooth surface where lignin acted as a binder filling fiber gaps, leaving almost no visible voids Figure 16(b). Cotton straw formed a fine, elongated cellulose fiber network with a smooth surface, where narrow and regular gaps formed at fiber intersections Figure 16(c). The high cellulose purity contributed to strong fiber toughness and a well-structured morphology. Under mild pyrolysis conditions, the pore size remained small, likely due to persistent blockage by volatile matter and ash.

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Figure 16.

SEM images of (a) corn straw, (b) rapeseed straw, (c) cotton straw, (d) corn straw bio-char, (e) rapeseed straw bio-char, (f) cotton straw bio-char.

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Pyrolysis-induced decomposition and volatilization of organic components lead to morphological transformations in surface structures. Corn stalk biochar develops a moderately developed uniform mesoporous structure with thick, structurally stable pore walls, reflecting the pyrolysis characteristics of moderate cellulose/hemicellulose decomposition Figure 16(d). Rapeseed stalk biochar exhibits a honeycomb-like interconnected porous structure with dense and highly interconnected pores, where volatilization occurs during lignin pyrolysis while aromatic structures prevent pore collapse Figure 16(e). Cotton stalk biochar retains elongated interconnected channels and traces of residual fibrous frameworks, with its fibrous structure remaining partially intact during pyrolysis, allowing volatilization to occur directionally through fiber gaps to form characteristic pores Figure 16(f). These morphological differences directly correlate with their pyrolysis kinetics and thermodynamic properties, providing microscopic structural evidence for targeted biochar applications.