Abstract
Routine maintenance of fruit trees generates a substantial quantity of pruning waste each year. This waste is potential feedstock for producing energy, materials, and other products. The feasibility of making biochar from the waste via pyrolysis was evaluated. The effects of seven tree species, different pruning sites, and temperature on the pyrolysis process, and the physicochemical properties of the biochar were studied. Pyrolysis of different tree species at 500 °C yielded 27.5 to 33.3% biochar, with a high calorific value (approximately 30 MJ/kg), low ash content (approximately 4%), and capturing up to 60% of the carbon element present. Simultaneously, when the temperature was increased from 400 to 700 °C, the yield of biochar decreased from 35.8% to 24.3%, but the properties improved with the higher heating value rising from 29.2 to 31.3 MJ/kg and the iodine value from 234 to 252 mg/g. The biochar has a good pore structure with a specific surface area of 237 m2/g, total pore volume of 0.175 cm3/g, and average pore size of 2.96 nm. In general, biochar from the pyrolysis of fruitwood pruning waste generated here could be an ideal feedstock to produce high-value-added products, such as solid fuels, activated carbon, and electrode materials.
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Preparation and Physicochemical Properties of Biochar from the Pyrolysis of Pruning Waste of Typical Fruit Tree in North China
Xuelei Liu,a Xueyong Ren,a,* Jiangchuan Dong,a Bowei Wang,a Jianli Gao,b Ruijiang Wang,b Jingjing Yao,c and Wenbo Cao a,c
Routine maintenance of fruit trees generates a substantial quantity of pruning waste each year. This waste is potential feedstock for producing energy, materials, and other products. The feasibility of making biochar from the waste via pyrolysis was evaluated. The effects of seven tree species, different pruning sites, and temperature on the pyrolysis process, and the physicochemical properties of the biochar were studied. Pyrolysis of different tree species at 500 °C yielded 27.5 to 33.3% biochar, with a high calorific value (approximately 30 MJ/kg), low ash content (approximately 4%), and capturing up to 60% of the carbon element present. Simultaneously, when the temperature was increased from 400 to 700 °C, the yield of biochar decreased from 35.8% to 24.3%, but the properties improved with the higher heating value rising from 29.2 to 31.3 MJ/kg and the iodine value from 234 to 252 mg/g. The biochar has a good pore structure with a specific surface area of 237 m2/g, total pore volume of 0.175 cm3/g, and average pore size of 2.96 nm. In general, biochar from the pyrolysis of fruitwood pruning waste generated here could be an ideal feedstock to produce high-value-added products, such as solid fuels, activated carbon, and electrode materials.
DOI: 10.15376/biores.18.4.8536-8556
Keywords: Fruit tree pruning waste; Pyrolysis; Biochar; Pore structure; Physical and chemical properties
Contact information: a: National Forestry Grassland Wood Material Recycling Engineering Technology Research Center, College of Materials Science and Technology, Beijing Forestry University, Beijing 100083, China; b: Chengde Forestry and Grassland Technology Extension Station, Chengde, 067000, China; c: Institute of Resources and Environment, Beijing Academy of Science and Technology, Beijing, 100095, China; *Corresponding author: rxueyong@bjfu.edu.cn
INTRODUCTION
Fruit trees, as a commercially important forest type in China, not only generate immense economic value by supplying fruits every year but also absorb carbon dioxide and help to achieve carbon reduction targets. In the daily management of orchards, fruit trees must be pruned to ensure the quality and yield of fruit, which generates a considerable amount of pruning waste worldwide (Brand and Jacinto 2020). If orchards produce an average of 0.5 tons of fruit tree pruning wastes per 667 m2 per year, then the aggregate number of pruning wastes in China’s orchards could potentially reach 97 million tons in 2022. The European Union yields an estimated 25 million tons of wood waste annually (Aliaño-González et al. 2022). Pruning waste from fruit trees needs to be treated and utilized properly. Otherwise, the material is susceptible to fire, pests, and diseases, which affect fruit production and reduce the economic benefits of the orchard. Further, such wastage is also not conducive to reducing carbon emissions. Seibold et al. (2021) reported that dead wood worldwide releases 1.09 billion tons of carbon per year, which is 1.15 times the amount of fossil fuel emissions. Therefore, there is an urgent demand for the effective utilization of fruit tree pruning waste.
In fact, pruning waste of fruit trees are a type of lignocellulosic biomass, which is a clean renewable resource rich in organic matter including cellulose, hemicellulose, and lignin. Conventional disposal methods typically involve incineration or landfill, which not only pollute the environment but also result in wasting precious resources. Numerous studies have reported on the potential uses of pruning, such as pyrolysis for the preparation of biochar, cultivation of edible fungi, fermentation for the production of organic fertilizers, manufacture of wood-based panels (Lee et al. 2022), and the making of solid biofuel for heating (Soltero et al. 2020). Among these, pyrolysis technology deserves attention because of its benefits of strong adaptability to feedstock, high processing capacity, continuous production, and clean and renewable products.
Pyrolysis is a promising technology that enables biomass to be thermally broken down to obtain products such as biochar, bio-oil, and syngas in an oxygen-free, high-temperature environment (300 to 900 °C) (Kan et al. 2016). The pruning waste of fruit wood have a high lignification degree and low ash content, which means that it is a good raw material for pyrolytic carbonization. Typically, a higher yield of biochar is obtained with a heating rate of no more than 30 °C/min, a heating temperature between 400 and 800 °C, and a residence time of more than 600 s (Yu et al. 2022b). Biochar is one of the main products of pyrolysis and has a high calorific value, high carbon content, well-developed pore structure, abundant surface functional groups, good stability, and durability. It can be used to produce a wide range of high-value-added char materials, such as fuels (Thakkar et al. 2016), activated carbon (Januszewicz et al. 2020; Cheng et al. 2021), carbon-based fertilizers, soil amendments (Li et al. 2020), and electrode materials (Cheng et al. 2022), which are widely used in the energy, environmental, and agricultural sectors. Therefore, the pyrolysis of fruitwood pruning wastes not only creates additional economic value for the agroforestry industry, but also effectively reduces the amount of carbon dioxide emitted from the natural decomposition of those waste.
Generally, the yield and quality of biochar are affected by pyrolysis factors such as temperature (Lin et al. 2022; Agweh et al. 2023), feedstock type (Bednik 2022; Burachevskaya et al. 2023), heating rate, and residence time. Higher temperatures, lower heating rates, and longer residence times generally help to improve biochar quality (Yang et al. 2020). Liu et al. (2023) investigated the effects of temperature and type of pecan feedstock (branches, leaves, and nut shells) on biochar. It was found that with the temperature increase, the specific surface area, total pore volume, pH value, and content of certain elements (calcium, magnesium, iron, manganese, and copper) of the three types of biochar showed an upward trend, but the yield apparently decreased. Among them, leaf biochar had the highest yield. Pecan branch and leaf biochar contained more nutrient minerals than nutshell biochar and could be used to improve soil fertility. Chen et al. (2016) studied the impact of heating rate and temperature on the pyrolysis products of poplar wood, and it was found that the bio-oil yield was higher at 500 °C with larger heating rate, while higher mass and energy yield of biochar was achieved at lower temperature and heating rate. Increasing the heating rate favored the production of CO and CH4, which in turn increased the carbon content of the biochar. The residence time also has a significant influence on the structure of biochar (Shaaban et al. 2014). The degree of carbonization, pH, and aliphatic functional groups of the biochar increased with the extension of the residence time.
The potential of fruitwood waste as a valuable resource for the production of biochar remains largely unexploited. In this study, based on the concept of minimizing energy consumption and CO2 emissions with precisely optimizing the pyrolysis process of typical feedstock, the pyrolysis conditions were studied. The index of energy consumption (electric power) data and carbon yields were used as the evaluation target. The goal here was to correlate the data with the feedstock composition of fruitwood, as well as the properties of final biochar products. Starting from the composition and basic characteristics of fruit tree pruning waste, this study investigated the effects of tree species, temperature, and pruning site on the physicochemical properties of biochar produced by pyrolysis, and studied the effects of different experimental conditions on the energy consumption, product yield, and carbon sequestration. The aim was to provide a viable method of using fruit tree pruning waste and broaden the source of feedstock for biochar production, and then supply a theoretical basis for the optimization of the charring process. Preparation of biochar from fruit tree pruning wastes not only minimizes the environmental burden caused by the feedstock, but it also provides a means of using the fruit wood waste in a resource-efficient manner.
EXPERIMENTAL
Materials
Pruning waste from Hebei and Henan provinces in northern China was collected from the seven different types of fruit trees, namely apricot (Armeniaca vulgaris Lam) tree (AT), apricot plum (Armeniaca mume var. bungo Makino) tree (APT), walnut (Juglans regia Linn) tree (WT), chestnut (Castanea mollissima Blume) tree (WT), peach (Prunus persica (L.) tree (PT), mountain apricot (Armeniaca sibirica (Linn.) Lam) tree (MAT), and mountain peach (Amygdalus davidiana (Carrière) de Vos ex Henry) tree (MPT). Among them, the main branches (MB) and lateral branches (LB) of mountain apricot and mountain peach trees were clipped from different parts. These raw materials were sourced from fruit trees that were growing vigorously, except for chestnut tree pruning wastes, which were cut from old trees and had a relatively high bark content. This batch of fruit wood was cut to a size of 5 to 6 cm and dried in a ventilated and dry place for several days to reach an air-dried state.
Composition Analysis Methods of Feedstocks
The carbon, hydrogen, and nitrogen content in fruit tree branches were determined using a SDCHN 435 elemental analyzer (SDCHN435 Carbon & Hydrogen & Nitrogen analyzer, Changsha, China). The oxygen content was calculated by subtracting the mass fraction of carbon, hydrogen, nitrogen, and ash from the total mass fraction of the sample (100%). The proximate analysis was performed using a SDTGA industrial analyzer (SDTGA5000a proximate analyzer, Changsha, China). The Extract, Holocellulose, and Klason lignin contents in fruit trees were determined in accordance with Chinese national standards GB/T 2677.6(1994), GB/T 2677.10(1995), and GB/T 2677.8(1994), respectively. The higher heating value (HHV) of different fruit tree branches was determined using an IKA C2000 calorimeter (IKA C2000 Calorimeter, Staufen, Germany).
Pyrolysis Equipment and Procedures
Pyrolysis of fruit tree pruning wastes took place in a carbonization furnace, which was independently self-designed and manufactured. Figure 1 is a schematic diagram of the pyrolysis equipment. The weighed fruit wood pruning was placed in the material frame of the equipment, and then the material frame was placed in the carbonization furnace for pyrolysis. Approximately 4 kg of sample was pyrolyzed at a time under the experimental conditions of 15 ℃/min and 500 ℃. While the peach wood pruning was pyrolyzed at temperatures of 400, 500, 600, and 700 ℃ to analyze the influence of temperature on power consumption, pyrolysis product distribution, and properties of biochar.
Fig. 1. Schematic diagram of the pyrolysis equipment
Equations
The normal operation of the equipment requires electricity to drive, therefore the amount of electricity consumed during heating and insulation was recorded by the meter.
The yield of biochar (α), the product of pruning pyrolysis of fruit wood, was calculated with Eq. 1,
(1)
where and respectively represent the mass (g) of biochar and feeding material (fruit tree pruning wastes).
The carbon sequestration of biochar was calculated using Eq. 2,
(2)
where 
respectively indicate the yield of biochar, the carbon content (wt%) of biochar, and carbon content (wt%) in the feedstock.
Properties Analysis Methods of Biochar
Biochar was divided into three categories according to the different pyrolysis tree species, pruning sites, and pyrolysis temperatures. The first type of biochar varied according to the tree species, while other conditions remained the same, including apricot tree biochar (ATB), apricot plum tree biochar (APTB), walnut tree biochar (WTB), and chestnut tree biochar (CTB). The second category was biochar obtained by pyrolysis of different pruning sites (main branches, lateral branches) of the same tree species (mountain apricot tree, mountain peach tree), abbreviated as MATB-MB, MATB-LB, MPTB-MB, and MATB-LB, respectively. The third type was the biochar obtained from the same tree species (peach tree) at different pyrolysis temperatures (400, 500, 600, and 700 ℃), named PTB-400 ℃, PTB-500 ℃, PTB-600 ℃, PTB-700 ℃, respectively. The basic composition, micromorphology, crystalline structure, and pore structure of the biochar were characterized as follows.
Composition analysis methods and calorific value of biochar
The elemental content of carbon, hydrogen, nitrogen, and oxygen in biochar was determined by using the SDCHN 435 elemental analyzer (Changsha, China). The contents of moisture, ash, volatile, and fixed carbon in biochar were determined using a SDTGA 5000a industrial analyzer (Changsha, China). The working temperature of each index of the industrial analyzer was 105 ℃ for moisture, 815 ℃ for ash content, and 910 ℃ for volatile matter, respectively. The amount of fixed carbon was calculated by subtracting the amount of water, ash, and volatile matter. The HHV of biochar obtained under different experimental conditions was measured using an IKA C2000 calorimeter (IKA C2000 Calorimeter, Staufen, Germany).
Characterization of microscopic morphology and crystal structure of biochar
The micromorphology of biochar was studied via JSM-7610F Field Emission Scanning Electron Microscope (JSM-7610F Schottky Field Emission Scanning Electron Microscope, Tokyo, Japan). The acceleration voltage was 5 KV. The scanning electron microscopy (SEM) images were obtained using an excitation of secondary electrons and backscattered electrons in high vacuum mode. Prior to the observation, biochar samples were sprayed with gold by the JEC-3000FC gold spraying equipment (Auto Fine Coater – JEC-3000FC, Tokyo, Japan).
X-ray diffractometry (XRD) was used to measure the graphitization degree of biochar samples via a D2 Phaser diffractometer (BRUKER X-RAY Diffraction D2 Phaser, Karlsruhe, Germany). The experimental conditions of X-ray diffraction were as follows: initial angle was 10°, end angle was 90°, and the time of each step was 0.1 s.
The functional groups present on the surface of the biochar were detected by FTIR using a spectrophotometer (PerkinElmer SPECTRUM 100, Waltham, America) employing the KBr pressing method. The infrared spectrometer was used to measure the range of 4000 to 500 cm-1.
Analysis of adsorption characteristics of biochar
The iodine adsorption value (IAV) of biochar was tested to study its adsorption characteristics and analyzed the degree of pore development according to Chinese National Standard GB/T 12496.8 (2015).
The N2 isothermal curves, total pore volume, specific surface area, and pore size distribution of the biochar were determined by a fully automated specific surface area porosity analyzer (Autosorb IQ, Boynton Beach, America). The specific surface area of the samples was calculated using the Multi-Point BET method, and the pore size distribution of the biochar was obtained using the DFT method and the BJH method.
RESULTS AND DISCUSSION
Feedstock Properties and Composition
To analyze the properties of pruning wastes as the feedstock for biochar production, ultimate, proximate, chemical composition analysis, and the HHV were conducted and discussed as follows.
The ultimate analysis of feedstock is recorded in Table 1. The elemental content of the wastes of fruit tree pruning varied considerably among tree species. Most of the feedstock is composed of approximately 50% carbon, 6% hydrogen, 0.3 to 0.7% nitrogen, and 40 to 44% oxygen, which is similar to the elemental composition of the pruned fruit tree branch in existing reports (Yu et al. 2020). Fruitwood has a higher carbon content than crop straws (around 40%) (Zhao et al. 2023), which means that it has a higher calorific value and the potential to produce solid pellet fuels. The low nitrogen of the feedstocks means that virtually no environmentally harmful nitrogen oxides are released during combustion or decomposition.
Table 1. Ultimate Analysis (wt%, Air Dry Basis) of Feedstocks
The results of the proximate analysis and the HHV of fruit tree pruning wastes are recorded in Table 2. The proximate analysis of feedstocks revealed that the volatile content was the highest at 70 to 80%, followed by fixed carbon at around 20%, with ash being the lowest. The calorific value of fruit tree branches was found to range from 17.9 to 19.9 MJ/kg, which is similar to the value of 18.9 MJ/kg reported in previous literature (Bilandzija et al. 2012). Compared to crop straws, fruit wood contains lower ash content (Zhai et al. 2022), but has higher fixed carbon, making it an ideal material for carbonization. It is worth mentioning that the ash content of chestnut wood pruning wastes was as high as 6.8%, which might be related to the age and high bark content of the tree. In both MAT and MPT tree species, the main branch contained a higher fixed carbon and lower ash content compared to the respective lateral branches, which might be associated with the longer development and greater lignification of the main branches. Fixed carbon, composed chiefly of carbon elements, is the primary substance responsible for heat production during biomass combustion, while ash does not contribute to the heat generation process. Therefore, the calorific value is positively correlated with the fixed carbon content and negatively correlated with ash content, just as APT has the highest fixed carbon content and calorific value of 19.9 MJ/kg. It is worth noticing that the content of each component in the proximate analysis and the HHV is strongly impacted by the plant species and sampling parts (Ślipek et al. 2022).
Table 2. Proximate Analysis (wt%, Air Dry Basis) and the Higher Heating Value (MJ/kg) of Feedstocks
The chemical composition of the raw materials analyzed is recorded in Table 3. Pruning wastes of fruit trees are principally composed of carbohydrates (cellulose and hemicellulose, accounting for 70 to 80%), lignin (about 20 to 30%), and extractives (2 to 8%). For the identical tree species, the holocellulose content of lateral branches was higher than that of the main branches, while the lignin content showed the opposite tendency. This may be because of the shorter growth cycle of lateral branches and the lower degree of lignification, which in turn leads to lower lignin content in lateral branches.
Table 3. Chemical Composition Analysis (wt%) of Feedstocks
Yield and Energy Consumption of Pyrolysis Products
The distribution of pyrolysis products from fruit tree pruning wastes, the amount of carbon sequestration of biochar, and the electricity consumption during the pyrolysis process are recorded in Fig. 2.
The biomass type and pyrolysis temperature had an important effect on the yield of biochar, wood vinegar, and non-condensable gas (Ortiz et al. 2020). When diverse types of feedstocks were pyrolyzed at 500 °C (Fig. 2a), the highest yields of wood vinegar were in the range of 40 to 50%, succeeded by biochar, which accounted for approximately 1/3 of the entire product. The lowest yield was non-condensable gas, approximately 20%.
Fig. 2. Distribution of pyrolysis products of fruit wood, carbon sequestration of biochar, and electricity consumption: (a) Yield of pyrolysis products from different tree species; (b) Yield of pyrolysis products from different pruning sites of peach wood; (c) Yield of peach tree pyrolysis products at different temperatures; (d) Carbon sequestered amount of biochar obtained from different tree species; (e) Carbon sequestration amount of biochar obtained from different pruning sites; (f) Carbon sequestration amount biochar obtained from peach wood at different temperatures; (g) Pyrolysis energy consumption of different tree species; (h) Pyrolysis energy consumption of different pruning sites; (i) Pyrolysis energy consumption of peach wood at different temperatures
It is worth mentioning that for different feedstocks under the same experimental conditions, a higher lignin content resulted in a higher biochar yield, which was the case of walnut trees with a maximum lignin content of 25.6% and the highest biochar yield of 36.5%. This was consistent with previous studies that found that lignin contributed most to the formation of biochar (Babinszki et al. 2021). As shown in Fig. 2 (b), the distribution of pyrolysis products of the main and lateral branches of mountain peach (mountain apricot) was not apparent, indicating that the effect of biomass species may be more relevant than that of biomass pruning sites in the pyrolysis. When pyrolysis temperature increased (Fig. 2c), biochar yield noticeably decreased, the wood vinegar yield showed a trend of first increasing and then decreasing, reaching a maximum at 500 °C, and the gas yield increased overall. The biochar yield decreased from 35.8% at 400 °C to 24.3% at 700 °C, and the lower biochar yield at higher temperatures was induced by more complete pyrolysis of the feedstocks. Higher temperatures supply more energy for the biomass to break bonds between chemical components, thereby facilitating secondary cracking of hydrocarbons, generating and releasing volatiles, resulting in less biochar but improving its quality and economic value (Zhang et al. 2020). The wood vinegar yield was highest at 500 °C, reaching 46.25%, which was presumably attributable to the primary reaction of the feedstock. When the temperature was elevated from 500 to 700 °C, the wood vinegar yield decreased 7.5%, while the gas yield increased 17%, which may be due to a secondary reaction of the molecules in the wood vinegar (Kaur et al. 2023).
When considering the carbon content analysis of feedstock and biochar in Table 1 and Table 3, the biochar captured approximately 44 to 55% of carbon elements present in feedstocks, as illustrated in Fig. 2 (d) through (f). The data presented in Fig. 2 (f) indicates that as the temperature increased, the amount of carbon sequestered by the biochar decreased, which could be because high temperatures cause unstable carbon-containing substances to undergo continuous reactions and be released as gases. Therefore, biochar remains a highly stable substance, which persisted in nature for hundreds of years or even longer, effectively reducing greenhouse gas emissions compared to feedstocks. Studies have demonstrated that if wolfberry branches in Qinghai Province were returned to the soil via biochar, 34.42 × 103 tons of carbon element could be sequestered and 68.56 × 103 tons of carbon emissions could be decreased annually from combustion in the atmosphere (Yu et al. 2022a). Therefore, the pyrolysis of biomass to manufacture biochar is becoming one of the sustainable solutions for carbon sequestration, lowering greenhouse gas emissions, and combating climate change (Kumar et al. 2023).
Electricity was used as the power source for the pyrolysis equipment in this study. In each experiment, 4 kg of feedstocks were pyrolyzed, and electricity consumption was recorded to investigate the effect of species, pyrolysis temperature, and pruning sites on the pyrolysis process, to investigate the economics of this method of biochar production and to optimize the charring process. It has been observed that at a pyrolysis temperature of 500 °C (Fig. 2 (g) through (h)), the energy consumption varies depending on the tree species. Energy consumption was within the range of 20 to 28 kW•h, which may be associated with chemical fraction content of the feedstock. A positive correlation between energy consumption and cellulose content of the feedstock was observed when considering the results in Table 3. Previous studies have shown that the pyrolysis of cellulose exhibited differently from hemicellulose and lignin from the point of energy consumption. The former was endothermic, while that of the latter was exothermic (Yang et al. 2007). Furthermore, it should be noted that the activation energy required for the pyrolysis stage of cellulose is higher than that for hemicellulose and lignin (Yeo et al. 2019). When pyrolysis of peach wood occurred, biochar obtained at a higher temperature had greater economic value, although the power consumption climbed from 21 kW•h at 400 °C to 34 kW•h at 700 °C, as shown in Fig. 2 (i). This might be because more additional heat was required when the charring furnace reached the targeted temperatures, and the corresponding current intensity would be higher. A forthcoming study will consider the full utilization of the pyrolysis gas to provide power for the pyrolysis of the feedstock, thereby reducing electricity consumption. Furthermore, when different sites of identical tree species were pyrolyzed, the energy consumption of the lateral branches was slightly higher compared to that of the main branches. This was probably because for the identical mass of material, there were more side branches and more gaps between them, so the heat transfer route was relatively longer, which tended to cause more heat loss in the process and thus more power consumption. Overall, these results suggest that the energy consumption during pyrolysis is influenced by various factors, including species, temperature, and pruning location.
Ultimate and Proximate Composition of Fruitwood Biochar
To analyze the properties of pruning waste as the feedstock for biochar production, ultimate, proximate, chemical composition analysis, and the higher heating value were conducted and discussed as follows.
The ultimate analysis of biochar is presented in Table 4. When compared to the elemental content of feedstocks, biochar exhibited a substantial increase in carbon and nitrogen content, while hydrogen and oxygen content demonstrated a decreasing tendency, notably in high-temperature peach wood biochar. The increase in carbon content may be ascribed to the higher degree of carbonization of biomass at elevated temperatures. Additionally, there is a positive correlation between the nitrogen element in biochar and that in the raw material, which is predominantly present in protein and inorganic forms. During the pyrolysis process, these forms are transformed into more stable structures, such as pyrrole-N and graphite-N, thus enhancing the stability of the subsequent biochar (Xu et al. 2021). Meanwhile, the decrease in oxygen and hydrogen content could be explained by the cracking of chemical bonds with weak bond energy in biochar at higher temperatures, leading to dehydration and depolymerization reactions. Furthermore, the O/C and H/C ratio in peach wood biochar decreases as the temperature rises (Novak et al. 2009), which is an indicator of biochar stability. A lower ratio value indicates that biochar remains to aromatize and carbonize, resulting in higher stable carbon content (Wei et al. 2022). Therefore, biochar manufactured at higher temperatures may be relatively higher quality and be more suitable for material applications. These findings have profound implications for the production and utilization of biochar, especially in the context of soil amendment, heavy metal adsorption, and carbon sequestration strategies (Liao et al. 2023).
Table 4. The Ultimate Analysis (wt%, Air Dry Basis) of Biochar
Proximate analysis and calorific value of biochar are shown in Table 5, the most abundant component in biochar was fixed carbon, making up around 70 to 80% of its composition. In contrast, volatile content accounted for roughly 20%. Moreover, the HHV of biochar produced at 500 °C was observed to increase almost 50% compared to the feedstocks, reaching 28 to 32 MJ/kg. This value is parallel to, or slightly higher than, the calorific value of standard coal. This rise in HHV was ascribed to the considerable increase in carbon element and fixed carbon content. As the temperature gradually increased from 400 to 700 °C, the volatile content of peach wood biochar underwent a remarkable reduction, dropping from 30.4% to 5.6%. In contrast, the fixed carbon and ash content increased from 64.1% and 3.2% to 87.1% and 5.2%, respectively. In addition, the HHV of biochar also rose from 29.2 to 31.6 MJ/kg. A possible explanation for this phenomenon is that the unpredictable and low-boiling components in biochar continue to react and discharge volatile substances, leading to the formation of biochar with higher carbon content. This conclusion is consistent with previous experimental results (Kazimierski et al. 2021).
Table 5. Proximate Analysis (wt%, Air Dry Basis) and the HHV (MJ/kg) of Biochar
Micromorphology and Crystalline Structure Analysis of Fruitwood Biochar
The biochar samples obtained from various tree species at 500 °C are depicted in the SEM images in Fig. 3. The biochar exhibited a well-defined porous structure with a distinct regularity. At smaller magnifications (100 μm), the biochar pores derived from walnut branches were observed to be arranged radially in the direction of the wood rays. However, the pore structure of different biochar types exhibited some flocculation when viewed at higher magnifications, which is believed to be caused by incomplete release of volatile components during pyrolysis and subsequent re-condensation of volatiles on the biochar surface during cooling. The good pore structure could promote heat transfer within the feedstock, thereby expediting the pyrolysis process. Simultaneously, the release of water vapor, condensable volatiles, and gases in biomass also had positive effects on the formation of pore structures.
Figure 4 displays the SEM images of peach wood biochar obtained at different pyrolysis temperatures (400, 500, 600, and 700 °C). With an increase in pyrolysis temperature, the number of pores in the biochar increased, along with an expansion in pore size and a thinning of the pore walls of the tubular structure. The changes in the pore structure of the biochar could be ascribed to the formation of air bubbles, while the release of volatile gases at higher temperatures may have acted as pore openers (Mu et al. 2023). In particular, at 700 °C the pore structure was smoother and there was nearly no matter produced by condensation of volatile fractions, indicating that volatile gases were released thoroughly. Thus, the biochar prepared at high temperatures had a rich pore structure compared to low temperatures, which might be attributed to the distinct decomposition temperatures of the various components (cellulose, hemicellulose, lignin, and inorganic minerals) during the pyrolysis procedure. The SEM images of biochar from different parts of the MAT (MPT) were broadly analogous and were therefore not described in detail.
Fig. 3. SEM images of biochar from different tree species; First column (100×), second column (500×), and third column (3000×)
X-ray diffraction is one of the most important means to analyze the crystal structure changes of biochar. The XRD spectra of different types of biochar are shown in Fig. 5. For biochar from different species (Fig. 5a) and pruning sites (Fig. 5b), the positions of the diffraction peaks were consistent, while the intensities varied. The characteristic diffraction peaks around 29.2° and 40° were induced by the presence of mineral crystals in biochar, predominantly including potassium salts, calcium salts, and SiO2.