Introduction

Pyrolysis is a processing and conversion technology, where a thermal decomposition process occurs when a material is subjected to heat. Many biomass-derived materials are produced through it, such as charcoal, activated carbon, solid char, and liquid fuels1,2. Ahmed et al.3 used furniture sawdust to produce charcoal and bio-oil with a spiral pyrolysis reactor, assessed the energy potential of products through Thermogravimetric Analysis (TGA) and Derivative Thermogravimetry (DTG), and evaluated the application potential of products in adsorption, environmental, and catalyst industries using surface characteristics and Brunauer–Emmett–Teller (BET) technology. Xue et al.4 found that pyrolysis is more effective than torrefaction in enhancing biomass quality for co-combustion with coal. The pyrolysis technology is proven to be crucial for the treatment and utilization of biomass.

Pyrolysis technology includes slow pyrolysis (heating rate < 0.8℃/s), conventional pyrolysis, and fast pyrolysis (heating rate 1000–10,000℃/s)5, where the fast pyrolysis and slow pyrolysis are more common. Pyrolysis under different conditions yields varying product proportions. Fast pyrolysis mainly produces liquid bio-oil, while slow pyrolysis yields solid charcoal. Conventional pyrolysis provides a balanced distribution of products. Different types of pyrolysis equipment include rotary drums, kilns, fluidized beds, vacuum mobile beds, and tube furnaces, resulting in products with different properties and structures in products6. Wang et al.7 treated palm kernel shells at 600°C for 60 min in a tubular furnace to produce charcoal with large content of fixed carbon, low volatile matter, and low sulfur. The produced charcoal was then subjected to co-combustion with anthracite coal to prove its potential as a valuable energy source. Yoo et al.8 studied the devolatilization of torch pine in a tubular furnace (300–1000 °C), proposed a charcoal structural development model, and four carbon crystallite models with X-ray diffraction (XRD), Scanning Transmission Electron Microscopy (STEM), BET, and TA, enhancing the understanding of the complex pyrolysis process. Researchers have found that pyrolyzed biomass shares coal-like qualities such as high calorific value, fixed carbon content and energy density8,9. However, no suitable biomass material has been identified as a direct replacement for coal, emphasizing the crucial importance of finding a suitable biomass alternative.

Robinia pseudoacacia (RP) is a leguminous plant known for its strong adaptability, drought resistance, rapid growth, and significance as a globally important fast-growing tree species. In 2008, Olympia et al.10 used clone selection to rapidly grow RP in large quantities, enhancing its quality and positioning it as a potential future energy source11. This study employed a horizontal pyrolysis furnace to produce RPC and evaluated its potential applications in co-combustion with anthracite through industrial and elemental analyses. The research also examined microscopic structure and combustion behavior and investigated the co-combustion characteristics of RPC and anthracite coal to determine the optimal pyrolysis process.

Experiments

Equipment and materials

RP branches: Collecting 4 × 15 cm RP branches, air-drying naturally and then oven-drying at 105 °C for 24 h before experimentation.

Anthracite coal (AN): Sourced from Shanxi Jincheng Anthracite Coal Mining Co., Ltd.

Equipment: Muffle furnace (SX-2-4-10NP, Shanghai Shenxian Thermostatic Equipment Factory), electric blast drying oven (101-ES, Beijing Yongguangming Medical Instrument Factory), electric constant temperature drying oven (HX-101-1, Shaoxing Shangyu Hongxing Machinery Instrument Co., Ltd.), thermogravimetric analyzer (TGA-Q500, TA Instruments, USA), microcomputer fully automatic calorimeter (ZDHW-5000, Hebi Huayuan Instrument Instrument Co., Ltd.), elemental analyzer (vario EL III, Germany), scanning electron microscope (SEM) (Zeiss EVO/MA/10, Germany), electronic balance (FA2104N, Shanghai Precision Scientific Instrument Co., Ltd.)

Experiment details

Pyrolysis experiments of biomass

The pyrolysis experiments for 4 × 15 cm RP branches were conducted using the setup in Fig. 1. The pyrolysis conditions included a temperature range of 300–900 °C, a heating rate of 10°C/min, and a constant temperature duration of 60 min, followed by natural cooling. The resulting RPC was ground to a particle size smaller than 0.15mm, labelled as RPC300, RPC400, RPC500, RPC600, RPC700, RPC800 and RPC900, and mass yields were calculated12.

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Horizontal pyrolysis furnace.

Analytical testing of RPC

Proximate analysis: following GB/T212-2008 coal standards, RPC at various pyrolysis temperatures was analyzed for fixed carbon (FC), ash content (A), volatile matter (V), and moisture content (M), with samples having a particle size below 0.15 mm and being air-dried.

Ultimate analysis: analyzing RPC with a particle size below 0.15 mm using the German vario EL III elemental analyzer. The content of C, H, N, and S were determined, and the oxygen (O) content was calculated.

Calorific value analysis: using the oxygen bomb method, the calorific value of samples with a particle size below 0.15 mm was measured on the ZDHW-5000 fully automatic calorimeter.

SEM analysis: powdered RPC samples below 0.15 mm were analyzed for microscopic morphology using the German Zeiss EVO/MA/10 electron scanning instrument at 5.0 kV.

Raman spectroscopy analysis: the RPC powder with a particle size of less than 0.15 mm was analyzed using the XploRA PLUS Raman spectrometer in France. The test conditions were set as follows: Raman wavelength λ = 532 nm, with a Raman shift range from 500 to 2500 cm⁻1.

X-ray diffraction analysis (XRD): the RPC powder, with all particles smaller than 0.15 mm, was further ground to 0.045 mm using an agate mortar for testing. The XRD analysis was performed using the Panalytical Axios X-ray diffractometer from the Netherlands. The test conditions were set as follows: Cu-Kα target (λ = 0.15406 nm), scanning range from 10° to 90°, scanning speed of 2°/min, and voltage of 40 kV.

Combustion experiment of charcoal

The finely ground AN particle (less than 0.15 mm) and RPC particles from different pyrolysis conditions were fully mixed at a ratio of 8/2 (total mass of 1 g), respectively, and were subjected to the combustion tests conducted in a thermogravimetric analyzer TGA-Q500. Before the experiment, 3–5 mg of RPC500 particles (< 0.15mm particle size) were weighed and tested three times under consistent conditions to remove equipment errors. The experimental temperature was raised from room temperature to 750 °C at a heating rate of 3 °C/min within pure O2 at 60 mL/min. Note that to purge residual gases from the furnace during the initial stage, a flow of 40 mL/min of N2 gas was used until 40 °C, then switched to pure O2 gas until the end of the experiment. The response indicator was the ignition temperature.

Definition and calculation method of ignition temperature and combustion reactivity

The combustion of solid fuel is complex, typically characterized by ignition temperature and combustion reactivity7,13.

Ignition temperature: the ignition temperature refers to the minimum temperature required for fuel to sustain combustion in an oxygen-containing environment. It indicates the ease of igniting the fuel, with lower values indicating easier ignition and vice versa. Following the standard GB/T 33304-2016, this process entails marking a vertical line at the peak value of DTG, which crosses the TG curve. From this point, a tangent line is drawn on the TG curve, intersecting the initial horizontal line of the TG curve at a precise spot. The temperature corresponding to this intersection point is identified as the ignition temperature. Using RPC300 as an example, the combustion TG-DTG data are shown in Fig. 2.

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Determination of ignition temperature (RPC300).

Combustion reactivity: A combustion reactivity index (R0.5) from Takarada et al.13, was adopted to quantitatively evaluate the combustion reactivity of RPCs, as expressed in Eq. 1.

$${R}_{0.5}=\frac{0.5}{{t}_{0.5}}$$
(1)

where, t0.5 represents the time (min) corresponding to when the reaction conversion rate reaches 0.5. The larger R0.5, the better the combustion reactivity of the fuel and the easier it is to proceed.

Ignition characteristic index (Di): the ignition characteristic index is an important indicator for evaluating the ignition performance of fuel combustion. It is defined based on the maximum rate of thermogravimetric loss and combustion time14, as expressed in Eq. 2.

$${D}_{i}=\frac{{U}_{max}}{{t}_{i}{t}_{max}}$$
(2)

where, Di represents the ignition characteristic index; Umax represents the maximum thermogravimetric loss rate, in wt%/min; ti and tmax represent the time at the ignition temperature and the time at the maximum loss rate, respectively, in minutes.

Comprehensive combustion characteristic index (S): the comprehensive combustion characteristic index is often used as an indicator to reflect the overall combustion performance of a fuel. A higher value indicates superior overall combustion performance15. The calculation method is referenced in Eq. 3.

$$S=\frac{{U}_{max}{U}_{mean}}{{T}_{i}^{2}{T}_{b}}$$
(3)

where, S represents the comprehensive combustion characteristic index; Umax and Umean represent the maximum and average thermogravimetric loss rates, in wt%/min; Ti and Tb represent the ignition temperature and burnout temperature, respectively, in °C.

Flame stability combustion index (Fith): after fuel ignition, the flame stability combustion index is often used to reflect the stability of the combustion flame16. A larger value indicates a more stable flame during the combustion process. The calculation method is referenced in Eq. 4.

$${F}_{ith}=\frac{{U}_{max}}{{T}_{i}\left({T}_{b}-{T}_{i}\right)}$$
(4)

where, Fith represents the flame stability combustion index; Umax represents the maximum thermogravimetric loss rate, in wt%/min; Ti and Tb represent the ignition temperature and burnout temperature, respectively, in °C.

Proximate analysis (PA): based on the results of industrial analysis, Huang et al.17 proposed the PA, which can be used to preliminarily assess the volatility of the fuel. The specific calculation formula is shown in Eq. 5.

$$PA=\frac{{V}_{ad}{FC}_{ad}}{{M}_{ad}{A}_{ad}}$$
(5)

where, PA represents the Industrial Analysis Index; Vad, FCad, Mad, and Aad represent the mass percentages of volatile content, fixed carbon content, moisture content, and ash content in the RPC and anthracite, respectively. The subscript “ad” refers to the air-dried basis of the sample.

Combustion kinetics analysis

The combustion of biochar provides information for the design and optimization of combustion systems. Combustion kinetics were analyzed using the integration method with non-isothermal TG data. Previously, researchers often simplified such combustion reactions into first-order reactions to simplify the kinetic calculation process. However, combustion is a complex process, including a homogeneous combustion reaction of volatile matter and a heterogeneous combustion reaction of fixed carbon18. The overall reaction process is dominated by heterogeneous combustion. Therefore, through different mechanism functions (G(α))19, the Coats-Redfern (CR) method20 was used to calculate the activation energy and pre-exponential factor of RPC produced at different pyrolysis temperatures. The reaction rate is expressed as follows:

$$\text{ln}\left(\frac{G\left(\alpha \right)}{{T}^{2}}\right)=\text{ln}\left(\frac{{k}_{0}R}{E\beta }\left(1-\frac{2RT}{E}\right)\right)-\frac{E}{RT}$$
(6)

where, k0 is the pre-exponential factor, min-1; E is the activation energy, kJ/mol; R is the gas constant, J/(mol·K); T is the temperature, K; α is the conversion rate; G(α) is the mechanism function; β is the heating rate, K/min.

For the general range of temperatures and activation energies, the intercept can be expressed as ln(k0R/Eβ) when (1-2RT/E) « 1. Different functions of G(α) are substituted into Eq. 6, and the resulting plot of the left-hand side against 1/T is fitted with a straight line to select the best-fitting G(α). The intercept and slope obtained are used to determine the pre-exponential factor and activation energy of RPC combustion process, respectively, in order to contribute to the selection of the optimal pyrolysis process. The CR method has been used to determine the combustion mechanism of bio-waste, biochar, coal, municipal solid waste and their mixtures from 15 mechanism functions, and the results are reliable19.

Result and analysis

Effect of pyrolysis temperature on the properties of RPC

The proximate analysis, calorific value and ultimate analysis of RPCs obtained at different pyrolysis temperatures are listed in Table 1. The contents of moisture and volatile matter of RPC decrease with increasing pyrolysis temperature, while the ash content initially increases and then decreases, accompanied by a gradual increase in fixed carbon content. The calorific value initially increases, then stabilizes, and slightly decreases. In terms of the ultimate analysis, the nitrogen (N) and sulphur (S) contents are much lower than those of coal, indicating that burning RPC could reduce emissions of nitrogen oxides and sulphur oxides compared to coal, making it more environmentally friendly. With increasing pyrolysis temperature, the hydrogen (H) and oxygen (O) contents decrease, while the carbon (C) content gradually increases. This is mainly due to the decomposition and volatilization release of cellulose, hemicellulose, lignin and other oxygen-containing small molecule polymers in RPC21. The results indicate that pyrolysis is a thermal treatment method for upgrading biomass to charcoal with high calorific value, high fixed carbon content, low moisture content, low nitrogen content, and high carbon content, thus having the potential for energy substitution.

Table 1 Proximate and ultimate analysis of RPCs.
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The molar ratios of H/C, O/C, and N/C calculated based on ultimate analysis results are shown in Fig. 3. With the increase in pyrolysis temperature, the molar ratios of H/C and O/C decrease gradually, while (N + S)/C reaches its highest value at 500°C and then decreases gradually, indicating a decrease in reactivity, which is attributed to the release of a large amount of volatile matter, and (N + S) is remaining at a low concentration. It was also found that 500°C is a critical point, before which these three ratios change slowly, and after which they decrease faster. Azni et al.22 have reported that higher H/C and O/C ratios lead to a better fuel combustion performance. From the perspective of energy yield and mass yield12, it is found that there is a significant fluctuation between pyrolysis temperatures of 400°C and 500°C, with an energy yield increase of 21.34% compared to RPC400, a mass yield of 35.86% at 500°C, and the comprehensive benefits (mass yield * energy yield) is maximized to 24.20%. With a pyrolysis temperature of 600°C, the maximum energy yield is 71.41%, but the mass yield is low, and the comprehensive yield is lower than that with a pyrolysis temperature of 500°C. When the pyrolysis temperature exceeds 600°C, both the mass yield and energy yield decrease. Compared with AN coal, RPC pyrolyzed at 500°C has similar H/C and (N + S)/C as AN coal, and RPC pyrolyzed at 600°C has similar O/C and net calorific value (Q,net) as AN coal. Overall, RPC pyrolyzed at 500°C exhibits a better combustion performance, suggesting it as the optimal pyrolysis temperature under the conditions and scope of this study, note that this may differ for other industrial uses.

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Analysis on the basic characteristics of RPC at different pyrolysis temperatures: (a) molar ratio; (b) mass yield and energy yield and comprehensive benefits.

Effect of pyrolysis temperature on the microstructure of RPC

Figure 4 illustrates the microstructure evolution of RPC produced at the temperature from 300 to 900 ℃ (with an interval of 200 ℃). At 300 ℃ (Fig. 4a), RPC maintains its carbon skeleton structure typical of lignocellulosic biomass, with some fine particles, likely un-pyrolyzed twig residues, adhering to its surface. At 500 ℃ (Fig. 4b), the inner pore wall becomes smooth, forming honeycomb-like structures with a diameter of about 20 μm. This is because devolatilization provides more active sites, resulting in strong activity at this stage. At 700 ℃ (Fig. 4c), pore blockage occurs due to the presence of fine particles. Gao et al.23 has reported that these fine particles may be those macromolecules formed by self-polycondensation of cellulose, hemicellulose, lignin, and intermediates, and scattered in the pores, increasing the difficulty of pyrolysis. At 900 ℃ (Fig. 4d), particles including these macromolecules are unevenly distributed on the carbon surface, forming a rough surface without obvious pore structure. High temperatures lead to the destruction of the original pore structure, resulting in a denser solid carbon material, complicating grinding and powdering. Compared with AN coal (Fig. 4e), RPC has a better pore structure, which is conducive to the diffusion of combustion gases.

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Micro morphology of RPC at different pyrolysis temperatures: (a) RPC300; (b) RPC500; (c) RPC700; (d) RPC900; (e) AN coal.

The surface structure of materials is characterized by SEM, and the order of carbonaceous is characterized by Raman spectroscopy. Based on Ref.24, the D peak and G peak are segmented into five bands, including one Gaussian band and four Lorentzian bands. The ratio of (AD1 + AG)/Aall and (AD3 + AD4)/Aall respectively signify the presence of crystalline and amorphous carbon in the pyrolyzed sample24. Additionally, the ratios of AD1/AG and AG/Aall indicate the level of defects and orderliness, respectively, of the pyrolyzed sample, with higher values pointing to either increased defects or orderliness7. Figure 5 provides an overview of Raman spectroscopy analysis of RPCs: (a) presents the original Raman data for all pyrolyzed samples, (b) illustrates the Raman peak fitting diagram (RPC600), delineating the G, D1, D2, D3, and D4 bands in orange, grey, purple, green and light brown area, respectively, (c) shows the orderliness and defectiveness in the pyrolyzed samples, and (d) shows the distribution of crystalline and amorphous carbon, indicating that as the pyrolysis temperature rises, there’s a proportional increase in the content of crystalline carbon and decrease in amorphous carbon content within the sample. At higher temperatures, this change slows down, mainly due to the formation of denser carbonaceous materials, complicating the pyrolysis process. Moreover, in Fig. 5c, distinct two peaks and two valleys emerge at 500 ℃ and 900 ℃. It is most obvious at 500 °C, where AD1/AG (4.975) is the largest, AG/Aall (0.1098) is the smallest, and (AD1/AG)/(AG/Aall) is 49.75, indicating that this pyrolyzed sample has a high degree of defects and a more disordered carbonaceous structure, meaning the defective carbon is 49.75 times that of the intact crystalline carbon, that is, this charcoal exhibits higher activity. Consequently, pyrolysis at 500℃ creates numerous active sites for reaction with modifiers, intercalating agents, and swelling agents24, consistent with observations from SEM. Furthermore, increasing the pyrolysis temperature, the charcoal obtained at 900 ℃ displays increased defectiveness compared to those at temperatures beyond 500 ℃. Although the level of orderliness diminishes compared to charcoals at 700 and 800 ℃, there’s an overall trend towards increased orderliness. Additionally, RPC demonstrates lower mass yields at higher pyrolysis temperatures. Therefore, pyrolyzed samples produced at 500 ℃ exhibit the highest activity, albeit with weaker graphitization and orderliness.

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Raman spectroscopy of RPC at different pyrolysis temperatures: (a) all samples; (b) fitting of RPC600; (c) relationship between AD1/AG and AG/Aall and temperature; (d) relationship between (AD1 + AG)/Aall and (AD3 + AD4)/Aall and temperature.

Similarly, to understand the ordering information of pyrolyzed samples, XRD diffraction spectra were utilized to further capture information related to the graphitization trend during the pyrolysis process, such as carbon microcrystal size and interlayer spacing7,9,24. Figure 6 shows XRD analysis spectra of different pyrolyzed samples. Below 600 °C, there is no obvious peak 100 at a diffraction angle (2θ) of 43°, but it becomes increasingly distinct at higher temperatures. The XRD diffraction analysis results are listed in Table 2. The findings reveal that as the pyrolysis temperature increases, both peaks 002 and 100 change from broadened peaks to sharper ones, indicating that pyrolysis is an effective method for inducing graphitization in carbon materials. The diffraction angles of peaks 002 and 100 peaks shift towards larger angles, and the values of La, Lc, and N increase with the pyrolysis temperature, while the values of d002 and d100 decrease. At room temperature, the d002 of natural graphite is 0.33678. Hence, smaller values of d002 and d100 correspond to more ordered carbon structures, suggesting that higher pyrolysis temperatures yield more ordered carbon structures, which aligns with the results obtained from Raman analysis.

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X-ray diffraction spectrum of RPC at different pyrolysis temperatures.

Table 2 XRD results of RPC at different pyrolysis temperatures.
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Based on the above physical and chemical analysis, RPC500 has the highest activity, more disordered carbon structure, and more economical mass effect and energy effect, indicating better combustion characteristics.

Effect of pyrolysis temperature on the combustion characteristics of RPC

TG-DTG analysis

Figure 7 displays the TG-DTG combustion curves of seven RPCs of various pyrolysis temperatures, revealing consistent patterns across them. Typically, the combustion curves of charcoal exhibit three distinct stages: preheating and drying (T < 230 °C, stage 1), combustible burning (combustion of volatiles released, 230 °C < T < 473 °C, stage 2), and oxidative burnout (T > 473 °C, stage 3). In stage 1, various degrees of fuzzy mass loss were observed in these 7 samples at around 105°C. In Fig. 7b, stage 2 can be divided into two regions, corresponding to S1 (230 °C < T < 350 °C) and S2 (350 °C < T < 473 °C). Volatile matter released during the combustion process of RPC typically includes methane, carbon dioxide, carbon monoxide, water vapour, and other volatile small molecules, which facilitates the ignition of the combustion process. Moreover, it is observed that peaks in region S2 are sharper than those in region S1, and with the increase in the pyrolysis temperature of the charcoal samples, the peak in region S1 gradually disappears, while the peak in region S2 becomes stronger, sharper, and shifts towards higher temperatures. Such changes lead to significant delays in ignition and prolonged burning times. In stage 3, the mass of the pyrolyzed sample remains constant at approximately 3wt%, consistent with ash analysis. The main weight loss of different RPCs occurs in stage 2, which is the focal point of attention, thus the combustion reactivity (R0.5) characteristic index is applied in this study to measure the combustion performance of fuels.

Fig. 7
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Combustion TG-DTG curves of RPC at different pyrolysis temperatures: (a) TG curves; (b) DTG curves.

Combustion characteristics analysis of RPCs

According to GB/T 33304-2016 (Fig. 2), the combustion characteristic parameters of RPCs (PRC300—PRC900) obtained at different pyrolysis temperatures were determined with the TG-DTG results shown in Fig. 7. These characteristics include the ignition temperature (Ti), burnout temperature (Tb), average combustion rate (Umean), and maximum combustion rate (Umax), with the data visualised in Fig. 8. The combustion time was calculated as \({t}_{c}=\left({T}_{b}-{T}_{i}\right)/3\). The value of t0.5, representing the time (in minutes) corresponding to when the reaction conversion rate reached 0.5, was obtained from Fig. 7a. The corresponding combustion reactivity (R0.5) was calculated according to Eq. 1.

Fig. 8
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Combustion characteristic analysis of RPCs at different pyrolysis temperatures: (a) relationship between characteristic parameters and temperature; (b) relationship between temperature and reaction time and combustion reactivity.

It could be found from Fig. 8a that the ignition temperature (Ti) and burnout temperature (Tb) gradually rise with higher pyrolysis temperature, consistent with the aforementioned physicochemical property results. As the pyrolysis temperature increases, the average combustion rate (Umean) increases slowly, while the maximum combustion rate (Umax) increases rapidly, attributed to the accelerated heat transfer in high-temperature environments. Furthermore, it could be found from Fig. 8b that with increasing pyrolysis temperature, the combustion time (tc) first decreases and then increases, while the corresponding combustion reactivity (R0.5) gradually decreases. This is because the released volatile matter reduces the combustion time and reactivity of the reduces. Overall, increasing the pyrolysis temperature enhances the ignition characteristics and overall combustion characteristic indices of RPC, and shortens the combustion time overall, while reducing the combustion reactivity of RPC. Considering the two (tc*R0.5), the optimal pyrolysis temperature is 700°C. Based on the physicochemical property results of RPC, RPC500 in Fig. 8 does not exhibit optimal combustion characteristics.

Effect of pyrolysis temperature on combustion kinetics of RPC

The CR method20 was applied to calculate the mechanism function (G(α))19, and then to determine the combustion kinetic results of RPCs obtained at different pyrolysis temperatures, as listed in Table 3. The calculated R2 values are all greater than 0.94, indicating that the mechanisms selected by CR method are reliable and reasonable. Meanwhile, the activation energy of RPC increases with the increase in pyrolysis temperature. The main combustion process changes from D mechanism to F1 and R to D mechanism function. This may be due to the presence of high-volatility substances of RPC obtained at low pyrolysis temperatures, leading to the combustion mechanism being dominated by diffusion (D). RPC500 has large pores and many active sites, making the first-order reaction mechanism (F1) its combustion mechanism. As the pyrolysis temperature continues to rise, these active sites of RPC are destroyed, and the combustion mechanism is mainly power exponents (R) mechanism. When the pyrolysis temperature beyond 900 °C, the pore skeleton is destroyed and the carbon becomes dense, it will return to be diffusion-dominated. Furthermore, Fig. 9 shows the variation of activation energy and its dependence on combustion reactivity. The change in activation energy increases in groups in Fig. 9a. The temperature is 300°C-500°C for the first group, and 600°C-800°C in the second group, with a difference in activation energy of about 15.12 kJ/mol. 900°C is the third group, and the difference in activation energy from the second group is about 20.71 kJ/mol. It can be seen from Fig. 9b that the greater the combustion reactivity of the material, the smaller the energy required for the combustion process, and the stronger thermal decomposition energy of combustion. This is consistent with the analysis of the structural properties of RPC.

Table 3 Kinetic results of RPC at different pyrolysis temperatures.
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Fig. 9
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Kinetic behavior analysis of RPC at different pyrolysis temperatures: (a) relationship between E and temperature; (b) relationship between E and R0.5.

Effect of pyrolysis process on coal-char mixed combustion properties

The co-combustion test of RPC and AN coal (20% charcoal + 80% coal) was completed on a thermogravimetric analyzer (TGA-Q500), with the ignition temperature as the inspection index. The ignition temperature of the co-combustion process is determined according to the standard GB/T 33304-2016. From Fig. 10, the ignition temperature of the co-combustion of RPC and AN coal is generally higher, no matter under conditions of rapid or slow heating rates, high or low pyrolysis temperatures, and long or short isothermal times. Taking the ignition temperature as an index, the optimal pyrolysis conditions of RP biomass are pyrolysis temperature 500 °C, heating rate of 4 °C/min, and a constant temperature time of 60 min, which is consistent with the results of the investigation of pyrolysis temperature. For co-combustion with AN coal, RPC prepared at 500 °C shows better performance than at other temperatures.

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Results of single factor experiment: (a) pyrolysis temperature; (b) heating rate; (c) residence time.

Relationship between the microstructure and combustion properties of RPC

Table 4 presents the molar ratios of nH/nC, nO/nC, and n(N+S)/nC, along with the proximate analysis index (PA), calculated from elemental analysis and proximate analysis for RPC prepared at various temperatures. It also includes the crystallite height (Lc), interlayer spacing (d002), and number of layers (N), calculated from XRD, as well as the crystalline carbon content (A(D1+G)/Aall), amorphous carbon content (A(D3+D4)/Aall), and defective carbon content (AD1/AG)/(AG/Aall), calculated from Raman analysis. Additionally, the table provides combustion performance parameters obtained from combustion curves: ignition characteristic index (Di), combustion reactivity (R0.5), comprehensive combustibility index (S), combustion duration (tc), and flame stability combustion index (Fith).

Table 4 Microscopic structural parameters and combustion characteristic parameters of RPC.
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The data in Table 4 reveal that higher amorphous carbon content correlates with enhanced combustion reactivity and longer combustion time, while greater crystalline carbon content and stacking thickness (Lc) are associated with poorer combustion performance. Combustion reactivity (R0.5) increases with larger interlayer spacing (d002), indicating that greater disorder in the carbon structure promotes reactivity. Although some indices (e.g. S, Di, Fith) exhibit non-linear or unclear trends, the analysis highlights how specific microstructural features—such as amorphous carbon proportion, crystal thickness, and interlayer spacing—significantly influence combustion behaviour. These relationships underpin the conclusion that structural disorder enhances the combustibility of RPC. Based on Table 4, the correlation between the combustion characteristic parameters and the microstructural parameters of the char was established as Fig. 11.

Fig. 11
Fig. 11The alternative text for this image may have been generated using AI.Fig. 11The alternative text for this image may have been generated using AI.Fig. 11The alternative text for this image may have been generated using AI.
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Relationship between the microstructure and combustion properties of RPC.

The combustion reactivity (R0.5) and combustion time (tc) show a linear increase with the increase in amorphous carbon content (A(D3+D4)/Aall) (Fig. 11(2)/(3)) and a linear decrease with the increase in crystalline carbon content (A(D1+G)/Aall) (Fig. 11(5)/(4)). However, there is no clear trend observed between the comprehensive combustion index (S), flame stability combustion index (Fith), ignition characteristic index (Di), and (A(D3+D4)/Aall) or (A(D1+G)/Aall) (Fig. 11(2)–(5)). This suggests that amorphous carbon has better combustion reactivity than crystalline carbon, making it easier to combust and resulting in a longer combustion time. This might be because amorphous carbon mainly consists of transitional carbon with low-energy sp2 hybridized orbitals of carbon valence electrons. Therefore, when the amorphous carbon content is higher, more low-energy carbon atoms are formed, which is more conducive to the combustion reaction.

The combustion reactivity (R0.5) shows a linear decrease with the increase in the stacking thickness (Lc) of crystals (Fig. 11-(6)). The ignition characteristic index (Di) changes parabolically with Lc, reaching its lowest value at 0.94 (Fig. 11-(6)), indicating that the greater the stacking thickness of the carbon crystal flakes, the poorer the combustion performance and the more difficult it is to ignite. However, the ignition performance is optimal when Lc is 0.94. The comprehensive combustion index (S) and the flame stability combustion index (Fith) also exhibit parabolic changes with Lc, with the lowest value occurring at 0.91 (Fig. 11-(7)), suggesting that at Lc = 0.91, the overall combustion performance and combustion stability are at their worst. There is no clear pattern observed between combustion time (tc) and Lc(Fig. 11-(7)).

The combustion time (tc) and the flame stability combustion index (Fith) exhibit a parabolic change with the increase in the interlayer spacing (d002) of carbon crystals, with the lowest values occurring at 0.365 and 0.398, respectively (Fig. 11-(9)). This indicates that as the interlayer spacing of the crystal increases, the flame stability during combustion worsens, while the combustion time extends. The combustion reactivity (R0.5) shows a linear increase with the increase in d002 (Fig. 11-(8)), suggesting that the larger the carbon microcrystal interlayer spacing, the better the combustion performance and the easier it is to ignite. This is mainly because the increase in d002 leads to a larger interlayer spacing, resulting in a more disordered state, an increase in lattice defects, and more active sites. The ignition characteristic index (Di) and the comprehensive combustion index (S) do not show a clear pattern with d002.

The ignition characteristic index (Di), comprehensive combustion index (S), and flame stability combustion index (Fith) all decrease linearly with the increase in the heteroatom nitrogen and sulphur content (n(N+S)/nC) (Fig. 11-(10)/(11)), indicating that the presence of nitrogen and sulphur heteroatoms is detrimental disadvantage factor to combustion. On the other hand, the combustion reactivity (R0.5) increases linearly with the increase in n(N+S)/nC (Fig. 11-(10)), which is related to the fact that the presence of nitrogen and sulphur heteroatoms increases the degree of crystal defects and disorder25,26.The combustion time (tc) changes parabolically with the carbon content, reaching its lowest value at (nC = 6.6) (Fig. 11-(13)). The comprehensive combustion index (S) and flame stability combustion index (Fith) both increase linearly with the increase in carbon content(Fig. 11-(13)).

The combustion time (tc) changes parabolically with the oxygen content, reaching its lowest value at nO/nC = 0.022 (Fig. 11-(15)). The comprehensive combustion index (S) and flame stability combustion index (Fith) both decrease linearly with the increase in oxygen content (Fig. 11-(15)).

The comprehensive combustion index (S) and flame stability combustion index (Fith) decrease linearly with the increase in PA (Fig. 11-(17)).

Conclusion

Using RP biomass, the pyrolysis experiments were conducted in a horizontal furnace to investigate the effects of pyrolysis temperature on its characteristics, physicochemical properties, and combustion performance. Co-combustion experiments of RPC and AN coal were carried out on TGA to determine the ignition temperature with different blends.

  1. (1)

    The preparation process significantly influences the structure and physicochemical properties of RPC. As the pyrolysis temperature increases, both the energy gain and mass gain show that the optimal pyrolysis temperature is 500°C. The charcoal obtained at 500°C has many active sites, high disorder and large carbon microcrystalline size, which is predicted to exhibit good combustion performance.

  2. (2)

    As the pyrolysis temperature increases, RP charcoal has a gradually decreased combustion reactivity, and increased activation energy. 500°C is the point of maximum gradient, where RPC shows an overall good performance in terms of thermal decomposition ability, physical and chemical properties and calorific value.

  3. (3)

    Based on the influence of pyrolysis temperature on the energy utilization of RPC, the pyrolysis process parameters are temperature of 500°C, isothermal residence time of 60 min, and heating rate of 4°C/min.

  4. (4)

    The combustion reactivity (R0.5) of black locust charcoal increases linearly with crystalline carbon content (A(D1+G)/Aall), heteroatomic nitrogen and sulphur content (n(N+S)/nC), and carbon microcrystal interlayer spacing (d002), while decreasing with amorphous carbon content (A(D3+D4)/Aall) and crystalline layer thickness (Lc). The ignition index (Di) follows a parabolic trend with Lc and decreases linearly with n(N+S)/nC. The comprehensive combustion index (S) and flame stability index (Fith) decrease with n(O+N+S)/nC but increase with carbon content, showing a parabolic trend with Lc and d002.