Catalytic Pyrolysis Characteristics of Potassium Chloride on Ash Branch Wood and Its Kinetic Study

Abstract

Branch wood, as a renewable biomass resource, presents certain challenges due to its high volume, complex physical properties, difficulty in handling, and relatively high production costs. Potassium chloride (KCl) treatments were applied to ash branch wood (ABW) using solutions with concentrations of 5%, 10%, and 15% via immersion. Pyrolysis tests were performed at different pyrolysis temperatures (450 °C, 600 °C, 750 °C) and different pyrolysis times (2 h, 3 h, 4 h). The thermal degradation behavior was meticulously examined through Thermogravimetric Analysis (TGA). Furthermore, the pyrolysis kinetics were assessed using the Flynn–Wall–Ozawa (FWO) model, which allowed for the determination of the kinetic parameters and the exploration of the catalytic influence of KCl on the pyrolysis process. The morphology and adsorption properties of the biochar were evaluated employing SEM-EDS and BET characterization methods, respectively. The results show that the higher the impregnation concentration of ABW, the greater the shift in the TG and DTG curves, and the lower the initial temperature and maximum weight loss temperature in the devolatilization stage. The calculation of pyrolysis kinetic parameters indicates that adding a higher concentration of KCl to ABW results in a lower initial temperature and activation energy for the volatile phase of ABW. At the same time, a higher KCl concentration leads to an increased biochar yield; under single-factor conditions, a biochar yield of up to 35.81% can be achieved with an impregnation concentration of 15%. A lower KCl is more conducive to the pyrolysis reaction, with a lower activation energy throughout the devolatilization stage compared to raw ABW. Additionally, ABW treated with a low concentration of KCl results in a higher specific surface area and pore volume of the biochar. The maximum values are achieved when the KCl solution concentration is 5%, with a specific surface area of 4.2 m²/g and a pore volume of 0.00914 cm³/g. Based on these results, this paper explores the catalytic pyrolysis patterns of KCl on branch waste, providing theoretical guidance for the effective utilization of branch wood and the preparation process of biochar.

1. Introduction

According to the data published in China Statistical Yearbook 2023, China’s urban greening area has reached 3.58 million hectares, based on which it is estimated that China’s annual production of garden waste exceeds 53.7 million tons [1].The stacking, burning, and landfilling of a large amount of garden waste have caused serious ecological environmental pollution and waste of wood resources [2]. The elimination and disposal of these wastes still need to be resolved. In contrast to common biomass wastes, branch and lumber wastes are often unfavorable for processing and unsuitable for industrial use due to their high bark content, non-uniform shape and size, irregular shrinkage, and high moisture content [3,4], so there is an urgency to find a way to utilize this type of renewable energy source. Pyrolysis is one of the thermochemical technologies for converting biomass into valuable energy and chemical products consisting of solid biochar, liquid bio-oil, and pyrolysis gas [5,6]. Moreover, pyrolysis can be ecologically beneficial by avoiding the negative environmental impacts of disposal methods such as incineration, landfills, and composting, as well as excessive emissions of greenhouse gases (GHGs) CO₂ and CH₄ [7].

Extensive research has delved into the pyrolytic conversion process of biomass waste, which can be elucidated through thermogravimetric analysis (TGA) and kinetic methods. Zeynep et al. conducted a TGA study on tobacco waste, varying the heating rates from 5 to 40 °C/min, and identified the main decomposition occurring within a temperature range of approximately 150 to 390 °C [8]. Kinetic parameters were determined using the Distributed Activation Energy Model (DAEM) method, with the Ea and k₀ ranging from 280.38 to 201.96 kJ/mol and from $2.45\times {10}^{14}$ to $8.19\times {10}^{25}$ s⁻¹, respectively [8]. In addition to the DAEM method, other kinetic methods include the Flynn–Wall–Ozawa (FWO), Kissinger–Akahira–Sunose (KAS), and Friedman methods, each potentially influencing the magnitude of the kinetic parameters of the materials. Cerda-Barrera et al. performed kinetic analysis on the TGA and pyrolysis of representative species of biomass from southern Chile, using model-free methods to calculate the activation energy parameters [9]. The average Ea varied between KAS (117 and 171 kJ/mol), FWO (120–170 kJ/mol), and Friedman (115–194 kJ/mol) for the five biomasses used [9]. The pyrolysis kinetics of individual materials have been extensively studied; however, co-pyrolysis studies have also gained momentum. Memon et al. co-pyrolyzed peanut shells (PS) and tea plant branches (TPB), determining the average activation energy (Ea) of the composite mixture using Friedman, KAS, and Starink methods, which were 181.65, 166.87, and 167.14 kJ/mol, respectively [10]. However, the aforementioned studies only discussed the pyrolysis behavior of different experimental materials without considering the impact of pretreatment factors on the kinetic parameters of biomass.

Alkali and alkaline earth metals (AAEMs) are catalysts for the pyrolysis process, and the addition of AAEMs to biomass is an ideal pretreatment [11]. Research has considered the impact of various parameters on this catalytic effect, including type of feedstock and catalyst, impregnation mode, and pyrolysis process [12]. While different catalysts exhibit comparable roles in biomass pyrolysis, they present variations in carbon residual degrees and thermal cracking capabilities. Wang et al. explored the TGA of raw beech wood and samples treated with NaCl, KCl, and MgCl₂, uncovering that all AAEM compounds could facilitate the decomposition of biomass by reducing both the initial and peak pyrolysis temperatures [13]. Wang et al. utilized various catalysts, including KCl, CaCl₂, and FeCl₃, to catalyze the pyrolysis of alkaline lignin. By employing the integrated Coats–Redfern kinetic method, they revealed a reduction in activation energy during the primary pyrolysis phase of alkali lignin [14]. Concurrently, the order of the three additives in terms of carbon residue was K⁺ > Fe³⁺ > Ca²⁺; the catalytic pyrolysis capability for alkali lignin was ranked as Fe³⁺ > Ca²⁺ > K⁺, and the ability to produce small molecule gases was in the order of K⁺ > Fe³⁺ > Ca²⁺ [14]. Therefore, potassium salts are ideal catalysts to enhance the char yield of biomass. Inorganic salts have a significant impact on pyrolysis temperature and product distribution, which also depends on the selection of anions. Müller-Hagedorn et al. have investigated the catalytic effects of pH–neutral inorganic salts on pyrolysis temperature and product distribution, finding that alkali metal chlorides significantly reduce pyrolysis temperatures [15]. Potassium salts (KCl, KHCO₃, and K₂SO₄) decreased the amount of levoglucosan, showing the order of effectiveness as chloride > sulfate > bicarbonate. The above indicates that treating branch wood with KCl to affect its pyrolysis behavior and product distribution is a feasible approach. Moreover, KCl also has highly catalytic and migration properties, and the addition of potassium to the raw material could enhance the carbon sequestration potential of biochar, promoting carbon sequestration and soil amelioration, thus opening up new potential applications and prospects for biochar [16,17].

A substantial amount of research has been conducted to explore the effects of AAEMs on the pyrolysis of biomass, including factors influencing the process, reaction mechanisms, species migration and transformation, the enhancement of product yield, and the improvement of product performance [18,19,20]. However, there is relatively little research on the catalytic effects of AAEMs on the pyrolysis of branch wood, especially in terms of kinetics and product characteristic analysis, due to the complex physicochemical properties of branch materials [21]. Therefore, this study focuses on common garden waste, ash branch wood, as research material. The wood is impregnated with KCl solutions of different concentrations, and further investigation into the pyrolysis behavior and kinetic characteristics of ash branch wood is conducted through TGA. This aims to better utilize the catalytic role of AAEMs in the thermal conversion process of biomass. Additionally, this paper examines the characteristics of biochar under various pyrolysis conditions (impregnation concentration, pyrolysis temperature, and pyrolysis time) using SEM and BET analysis. This research aims to enhance the utilization efficiency of garden branch waste and the value of pyrolysis products, providing technical support for the industrial application of branch materials.

2. Materials and Methods

2.1. Experiment Material

The experimental material was selected from the common garden species ash wood (Fraxinus chinensis), sourced from a park in Tongzhou (Beijing, China). Prior to the experiment, it is essential to ensure that the particle size of the experimental material is uniform. The branch wood was crushed once using a garden branch chipper, followed by sieving to obtain particles with a diameter of approximately 10 mm. These particles should then be dried in the atmosphere to reach an air-dry state with a moisture content of approximately 11%.

2.2. Experiment Method

2.2.1. Pretreatment

The ash branch wood (ABW) was immersed in 5%, 10%, and 15% KCl solutions, named ABW-5%, ABW-10% and ABW-15%, respectively. They then were placed in a vacuum-drying oven and pumped into a vacuum state, and then kept under pressure for half an hour, which was repeated three times. After the impregnation was completed, we dried the material in an oven at 60 °C until the moisture content was approximately 11%.

2.2.2. Thermogravimetric Analysis

The treated and untreated ABW samples were crushed and sieved to obtain an 80-mesh powder for thermogravimetric analysis testing. A thermogravimetric analyzer (TG 209 F3, NETZSCH, Germany) was used to test the ABW under a nitrogen atmosphere, with a flow rate set at 40 mL/min. Before the experiment, the samples were put into a constant temperature drying oven at (103 ± 2) °C to eliminate the impact of moisture content on the analysis results. Four milligrams of the sample powder were placed into an alumina crucible and heated from ambient temperature to a final temperature of 750 °C at different heating rates (10, 20, 30 °C/min).

2.2.3. Kinetic Analysis

Model-free methods are reliable approaches for determining the apparent activation energy when the mass conversion of a sample is fixed. By determining the apparent activation energy, model fitting can provide information about the pyrolysis reaction mechanism, mainly including the Flynn–Wall–Ozawa (FWO) method, the Kissinger–Akahira–Sunose (KAS) method, and the Friedman method, along with several other approaches [22]. The pyrolysis kinetic characterization of ABW was investigated before and after treatment using the Arrhenius equations in conjunction with non-isothermal kinetic equations.

$$lg\beta =lg{\displaystyle \frac{AE}{RF\left(\alpha \right)}}-2.315-0.4567{\displaystyle \frac{E}{RT}}$$

(1)

Based on Equation (1), the $lg\beta$ ∼ ${\displaystyle \frac{1}{T}}$ relationship was plotted and linear fitting was performed. The activation energy E can be determined from the slope of the fitted line. Assuming that the pyrolysis reaction of garden branch wood is a first-order chemical reaction, $F\left(\alpha \right)=-ln(1-\alpha )$. The pre-exponential factor A can be determined from the intercept of the fitted line [23,24].

2.2.4. Pyrolysis Experiment

The pyrolysis process of ABW was carried out using homemade pyrolysis equipment, which consists of three parts: a reactor, a condensation device, and a collection device, as shown in Figure 1. The pyrolysis equipment was equipped with a temperature control system and two temperature sensors, which can monitor and control the changes in pyrolysis temperature in real time. The wet gas flowmeter was connected to the end of the collection device, and the non-condensable gas volume data could be obtained by its display value difference.

During the experiment, the material was placed inside the reactor, and the target temperatures (450 °C, 600 °C, 750 °C) and target time (2 h, 3 h, 4 h) were set through the temperature control system. After the pyrolysis was completed, the solid-phase product biochar and the liquid-phase product bio-oil were collected. The yields of these products were calculated, and the gas product yield was calculated by the difference method.

2.2.5. Characterization of Biochar

KCl impregnation affects the elemental composition and properties of biochar. To elucidate the effects of impregnation concentration, pyrolysis temperature, and pyrolysis time on the pore structure and elemental changes of the biochar, the lateral surface structure and K element distribution of the biochar samples were analyzed using a Scanning Electron Microscope-Energy Dispersive X-ray Spectrometer (Regulus 8100, Hitachi, Tokyo, Japan), operated at an acceleration voltage of 3.0 kV). Before scanning electron microscopy observation, a layer of conductive gold film was applied to the sample surface using an ion sputter coater.

The surface area was determined on dry biochar samples via N₂ adsorption at 77 K on a Surface Area Analyzer (Tristar II 3030, Micromeritics, Norcross, GA, USA). The calculations were performed using the BET equation for specific surface area, single-point adsorption total pore volume, and the BJH model for pore size distribution analysis.

3. Results and Discussion

3.1. Effect of KCl on the Thermal Degradation

The TG and DTG curves presented similar characteristics, indicating that the pyrolysis of the ABW occurs in three primary degradation stages: drying (up to 150 °C), devolatilization (200–500 °C), and char formation (above 500 °C) [25]. In the drying stage, the mass loss of the test material is minimal, with a small peak in weight loss observed in the DTG curve around 100 °C. This peak primarily results from the evaporation of water and light volatiles, along with the dissociation of weak chemical bonds [26,27]. As the temperature increases uniformly in the thermogravimetric analyzer, the moisture in the sample transitions from free water to bound water. Upon reaching 150 °C, pyrolysis progresses to the second stage, during which the hemicellulose and cellulose in the sample begin to decompose [28]. The sharply declining TG curve indicates that the sample experiences the most significant decomposition during this phase, commonly referred to as the active pyrolysis zone. The DTG curve reveals a shoulder peak on the left side of this active pyrolysis zone, which corresponds to the decomposition of a substantial amount of hemicellulose in the ABW. This is followed by a peak of the maximum rate of weight loss, attributed to the pyrolysis of cellulose in the ash material [29]. The decomposition of lignin continues from the stage of devolatilization to the char formation, under which the TG and DTG curves of the sample gradually flatten out, indicating that the mass of the sample remains almost unchanged, with the primary formation of char, ash, and a small amount of small molecular gases [30].

By analyzing Figure 2a,c,e, it is evident that the pyrolysis process of ABW showed significant differences after the KCl impregnation treatment. Specifically, the initial temperatures of the devolatilization stage shifted towards lower temperatures to varying degrees. At a heating rate of 10 °C/min, the initial temperatures of the second stage for ABW-5%, ABW-10%, and ABW-15% were reduced by 30 °C, 32 °C, and 49 °C, respectively, when compared to the untreated material. That is, the higher the impregnation concentration, the greater the degree of shift, and the lower the initial temperatures of the active pyrolysis zone. Consequently, the addition of KCl appears to facilitate a more active release of volatiles, suggesting that KCl treatment can effectively promote the pyrolysis reaction in biomass [31]. This phenomenon might be attributed to the ability of K to modify the strength and stability of chemical bonds in ABW, making them more prone to breakage, thereby promoting the progress of pyrolysis reactions [32]. Furthermore, Figure 2b,d,f reveal that, at a heating rate of 10 °C/min, the maximum weight loss temperature during pyrolysis for untreated material is 324 °C. In contrast, ABW-5%, ABW-10%, and ABW-15% experiences temperature decreases of 6 °C, 5 °C, and 4 °C, respectively, relative to the untreated material. This trend continues at heating rates of 20 °C/min and 30 °C/min, where the treated samples also demonstrate a reduction in degradation temperature. These findings highlight the significant catalytic pyrolysis effect of KCl impregnation.

3.2. Kinetic Analysis of Pyrolysis of KCl Impregnated Materials

Based on the TG and DTG curves of the ABW, the pyrolysis kinetic characteristics were analyzed under different concentrations of KCl solutions. The main pyrolysis stage of ABW is the devolatilization stage, with a conversion degree of 0.2–0.7. As shown in Figure 3, at lower and higher conversion degrees, the degree of linear fitting is low, hence 0.1 and 0.8–0.9 are not considered here [33]. As we can see in

Table 1. The kinetic parameters of the FWO model-free method of ABW, ABW-5%, ABW-10%, and ABW-15%.

$\mathit{\alpha }$	ABW			ABW-5%			ABW-10%			ABW-15%
	E (kJ·mol−1)	A (s−1)	R2	E (kJ·mol−1)	A (s−1)	R2	E (kJ·mol−1)	A (s−1)	R2	E (kJ·mol−1)	A (s−1)	R2
0.2	150.17	4.11 $\times {10}^{13}$	0.9997	143.90	1.64 $\times {10}^{13}$	0.9963	102.99	3.18 $\times {10}^{13}$	0.9618	123.24	2.58 $\times {10}^{11}$	0.9890
0.3	155.56	6.99 $\times {10}^{13}$	0.9998	151.88	4.34 $\times {10}^{13}$	0.9918	144.81	1.05 $\times {10}^{15}$	0.9984	149.02	3.55 $\times {10}^{13}$	0.9755
0.4	159.34	8.53 $\times {10}^{13}$	0.9996	158.82	1.06 $\times {10}^{14}$	0.9916	164.91	4.41 $\times {10}^{16}$	0.9939	168.83	1.21 $\times {10}^{15}$	0.9778
0.5	163.33	1.19 $\times {10}^{14}$	0.9997	162.88	1.62 $\times {10}^{14}$	0.9960	186.08	1.02 $\times {10}^{17}$	0.9995	179.49	6.22 $\times {10}^{15}$	0.9953
0.6	167.93	2.14 $\times {10}^{14}$	0.9999	160.51	9.74 $\times {10}^{13}$	0.9957	193.29	6.26 $\times {10}^{16}$	0.9994	187.12	1.96 $\times {10}^{16}$	1.0000
0.7	186.02	5.06 $\times {10}^{15}$	0.9924	162.82	3.26 $\times {10}^{15}$	0.9949	193.19	2.07 $\times {10}^{17}$	0.9955	211.95	1.89 $\times {10}^{18}$	0.9984

, it can be observed that the activation energy of the ash sample generally increases with the deepening of the reaction extent [13,25]. A low conversion rate corresponds to a lower degree of reaction, at which point a significant amount of hemicellulose in the sample decomposes. The pyrolysis activation energy of biomass hemicellulose is relatively low, at around 100 kJ/mol, which is reflected in the lower apparent activation energy of the sample. A high conversion rate corresponds to a higher degree of reaction, at which point a significant amount of cellulose in the sample decomposes. The pyrolysis activation energy of biomass cellulose is relatively high, at around 200 kJ/mol, which is reflected in the higher apparent activation energy of the sample [34].

The pyrolysis reaction primarily involves the formation of volatiles. As indicated in Table 1, the apparent activation energies of ABW-5%, ABW-10%, and ABW-15% all decreased compared to ABW at low conversion rates. The apparent activation energies of ABW-5%, ABW-10%, and ABW-15% decreased at a 0.2 conversion by 6.27 kJ/mol, 47.18 kJ/mol, and 26.93 kJ/mol, respectively, and at a 0.3 conversion by 3.68 kJ/mol, 10.75 kJ/mol, and 6.54 kJ/mol, respectively. This is because the cleavage of active cellulose requires less energy [35]. When the conversion rate is between 0.5 and 0.7, the activation energy for ABW-10% and ABW-15% increases with the concentration of KCl solution, which is consistent with the research findings of Fan, possibly attributed to the extensive decomposition of carbohydrates that requires energy consumption [36]. In contrast, for ABW-5% with a lower concentration of potassium chloride, its activation energy is lower than that of raw ABW throughout the entire devolatilization stage. These findings suggest that the addition of potassium to ABW is not favorable for pyrolysis reactions at higher temperatures; a higher potassium content leads to higher activation energy, while a lower concentration of potassium is more conducive to the progress of pyrolysis reactions.

3.3. Pyrolysis Product Distribution

The distribution of the three-phase product yields of ABW under different treatment conditions is shown in Figure 4. It can be observed that the biochar yield of ABW after KCl impregnation exceeds 30%, the liquid product yield is relatively stable, with a value between 35% and 40%, and the gaseous product yield varies widely, approximately between 20% and 35%. Figure 4a illustrates the yield of three-phase products of pyrolysis of ABW, which were impregnated by different concentrations of KCl solution, under the conditions of a pyrolysis temperature of 600 °C and a pyrolysis time of 3 h. The figure shows that the yield of solid-phase biochar increases with the increase in KCl impregnation concentration, while the yield of gaseous products gradually decreases. The biochar yield of ABW-15% is the highest at 35.81%. These results indicate that impregnating ABW with KCl can promote the formation of solid-phase products from pyrolysis, and the char-forming effect increases with the increase in potassium content. During the slow pyrolysis process, KCl can be retained in biochar up to approximately 800 °C, which may be one of the reasons for the increased yield of solid-phase products from pyrolysis [36].

To further investigate the effects of pyrolysis temperature and time on branch wood, a uniform impregnation concentration of a 10% KCl solution was adopted. The study examined the distribution of pyrolysis products of ABW under the conditions of the same pyrolysis time (3 h) but different pyrolysis temperatures (Figure 4b), and the same pyrolysis temperature (600 °C) but different pyrolysis times (Figure 4c). Figure 4b indicates that the higher the pyrolysis temperature, the more pronounced the decrease in solid product yield, while the yield of gaseous products increases significantly. At a pyrolysis temperature of 450 °C, the biochar yield is the highest, at 46.38%. Figure 4c shows that the biochar yield decreases with increasing pyrolysis time, while the yield of gas-phase products shows the opposite trend. The biochar yield was highest at 35.61% when the pyrolysis time was 2 h.

3.4. Morphology and Potassium Element Surface Distribution on Biochar

SEM-EDS was employed to investigate the surface morphological structure and the atomic distribution of C, O, K, and Cl in the biochar of both raw materials and impregnated samples. Figure 5a–h represent the SEM–EDS images of biochar under various pyrolysis conditions, showing a relatively intact structure with residual wood cell morphology.

Figure 5a is the SEM-EDS image of the biochar produced from untreated material under pyrolysis conditions of 600 °C and 3 h. From the figure, it can be observed that, compared to the untreated material, the biochar derived from ABW after KCl impregnation has noticeably thinner cell walls, enlarged cell cavities, and a more open structure. The scanning electron microscopy images of the impregnated samples show that the biochar surface is covered with many aggregates of different shapes. EDS analysis reveals that the surface of the biochar is covered with potassium elements, confirming that potassium has been successfully loaded onto the biochar surface and uniformly deposited on its skeleton [37]. The biochar surface is smooth and possesses a unique porous structure, which is conducive to the adhesion of KCl crystals [38].

Figure 5b–d represent the SEM-EDS images of biochar derived from ABW impregnated with different concentrations of KCl solution under the same pyrolysis conditions. The atomic mass and normalized mass indicate that the concentration of potassium on the biochar surface is positively correlated with the concentration of the impregnation solution. When the initial potassium salt impregnation concentration of the ABW is increased, the potassium content attached to the biochar cell walls also increases, and its crystal form undergoes significant changes, gradually transforming from needle-like crystals to spherical and irregular blocky structures. The phenomenon of residual KCl ash content in the matrix increasing with concentration may be one of the reasons for the increased yield of biochar from ABW [39]. Figure 5c,e,f demonstrate that, under the same impregnation concentration and pyrolysis time, the relative content of KCl is related to the degree of carbonization. As the pyrolysis temperature increases, the degree of carbonization of the impregnated samples deepens, leading to significant changes in the surface morphology of the biochar. The degree of graphitization of the biochar and the content of KCl correspondingly increase. KCl dissolves into smaller molecules and is adsorbed onto the cell walls of the biochar [39].

At 600 °C, it was observed that needle-like or blocky crystals formed on the surface of the biochar. This phenomenon is likely due to the direct outward condensation and growth of salt in the form of vapor. At higher temperatures (as shown in Figure 5), the crystals transform into spherical and fused appearance, which is primarily attributed to the process of sintering [40]. The observation that chlorine content increases and then decreases with rising temperature during the pyrolysis of biomass is attributed to the release pattern of chlorine in the pyrolysis process. Chlorine is released in two stages: approximately 60% of the chlorine is released when the temperature rises from 200 °C to 400 °C, and the majority of the remaining chlorine is released between 700 °C and 900 °C [41]. Based on the SEM-EDS results shown in Figure 5c,g,h, it can be observed that, under the same impregnation concentration and pyrolysis temperature, the normalized mass of potassium increase correspondingly with the increase of pyrolysis time. The KCl crystals also exhibit a consistent trend of growth, and their distribution on the biochar skeleton becomes denser.

3.5. Pore Structure Analysis of Biochar

The pore structure of biochar derived from ABW under different pyrolysis conditions was analyzed using the BET method. The $N_2$ adsorption/desorption isotherms and pore size distribution diagrams are shown in Figure 6. It can be observed that the nitrogen adsorption–desorption isotherm in Figure 6a belongs to Type I; the nitrogen adsorption–desorption isotherms in Figure 6b–i belong to Type IV. There is no distinct plateau of saturation adsorption in the isotherms, and H3-type hysteresis loops are observed, indicating the heterogeneity of the material’s pores, all of which are mesopores. At low temperatures, the majority of the biochar pore sizes are within 50 nm, showing a broader mesoporous distribution. When the temperature rises to 750 °C, some larger pores between 50 and 105 nm appear in the biochar pore sizes, possibly due to the high temperature merging adjacent small pores into larger ones, leading to an increase in pore size.

Table 2. Specific surface area and pore size analysis of ABW and its biochar.

Name of Sample	SBET (m2/g)	VBJH (cm3/g)	Dap (nm)
ABW	0.8	0.00258	1.5
Biochar 600−3	2.4	0.00677	12.6
Biochar 600−3−5	4.2	0.00914	10.5
Biochar 600−3−10	2.5	0.00693	29.5
Biochar 600−3−15	2.3	0.00508	12.0
Biochar 450−3−10	1.9	0.00569	23.4
Biochar 750−3−10	5.5	0.00961	29.7
Biochar 600−2−10	2.8	0.00626	15.6
Biochar 600−4−10	2.7	0.00617	12.3

presents the specific surface area, pore volume, and average pore diameter of ABW and biochar under various pyrolysis conditions. The pore volume was calculated from the desorption isotherm profile. It clearly indicates that, under single-factor conditions, the treatment of ABW with KCl can increase the specific surface area and pore size of biochar. The specific surface area and pore volume of the biochar decrease with the increase in the impregnation concentration, reaching a maximum when the KCl solution concentration is 5%, with a specific surface area of 4.2 m²/g and a pore volume of 0.00914 cm³/g. The average pore size increases and then decreases with increasing impregnation concentration. This may be due to the addition of an appropriate amount of KCl promoting the pyrolysis process of the biochar, leading to larger pore structures and a reduction in surface area. When the impregnation concentration is increased to 15%, the pore structure of the biochar may be chemically impregnated and blocked, continuing to cause a decrease in specific surface area and pore volume [42].

At the same concentration and pyrolysis time, it is evident in Table 2 that the specific surface area, pore volume, and average pore diameter of the biochar are significantly enhanced with increasing pyrolysis temperature. The increase in surface area is attributed to the breaking of hydrogen bonds and phase transformations [43]. Biochar 750−3−10 exhibits the highest specific surface area of 5.5 m²/g, which is approximately three times that of Biochar 450−3−10. Concurrently, the higher the reaction temperature, the more pronounced the thermal cracking effect, which aids in the release of more gases, thereby generating internal pores and increasing the pore volume and surface area within the structure [43]. The specific surface area and pore volume of biochar do not show a significant trend with pyrolysis time.

4. Conclusions

This study delves into the influence of potassium chloride (KCl) on the pyrolysis of ash branch wood (ABW). The incorporation of KCl was observed to diminish the activation energy of ABW within conversion rates of 0.2–0.3. TG and DTG curves revealed that higher impregnation concentrations led to lower initial temperatures and peak temperatures of the devolatilization stage. Concurrently, the biochar yield of ABW increased with the rise in KCl impregnation concentration. The research further evaluated the effects of impregnation concentration, pyrolysis temperature, and pyrolysis time on the characteristics of biochar. To achieve a higher biochar yield, it is recommended to select pyrolysis parameters of 15% KCl impregnation concentration, a pyrolysis temperature of 450 °C, and a pyrolysis time of 2 h. For obtaining a higher specific surface area and pore size, it is suggested that parameters of 5% KCl impregnation concentration, a pyrolysis temperature of 750 °C, and a pyrolysis time of 3 h are chosen. SEM-EDS analysis indicated that, with the increase in KCl solution concentration, there were significant changes in the content and crystalline form of potassium in the biochar. This could affect the physicochemical properties of the biochar and its performance in various application scenarios, which requires further exploration. In summary, investigating the role of AAEMs in pyrolysis kinetics is necessary for promoting the industrial deployment of branch wood waste pyrolysis, which has a positive impact on the disposal and utilization of branch wood waste, the application of biochar products, and environmental protection.