Abstract
With the growing global attention to climate change and reducing environmental pollution, it is necessary to look for sustainable technologies that help curtail carbon dioxide emissions and create useful products from waste. The hypothesis of this study was that recycling wood waste reduces the negative impact on the environment, and the biochar obtained during processing could be used to create secondary, environmentally friendly products. This study evaluated the environmental effects of biochar production using two pyrolysis reactors, UMT-3 PLUS EcoTeploOtbor (SC–1) and BIO-KILN-1 (SC–2), across various impact categories, including global warming potential (GWP), human toxicity (HT), freshwater aquatic ecotoxicity (FWE), marine aquatic ecotoxicity (MAE), and terrestrial ecotoxicity (TE). The functional unit for this study was defined as 1 ton of biochar, which allowed for meaningful comparisons between studies and ensures data reproducibility. SC–1 showed higher impacts, with preparation and processing of tree stumps (SBP) contributing 643.02 kg CO₂eq total GWP without carbon sequestration, 134.47 kg 1,4-DBeq to HT, and 347,097.03 kg 1,4-DBeq to MAE − 8.65% higher than SC–2. Carbon sequestration potential resulted in net negative GWP values: -2,036.98 kg CO₂eq for SC–1 and − 1,866.31 kg CO₂eq for SC–2. Chemical analysis confirmed heavy metals in biochar are within permissible limits, and most material accumulated on the 2 mm sieve, indicating suitability for further use, such as granulation.
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Introduction
The current European and global efforts are aimed at preventing climate change and environmental degradation by reducing greenhouse gas emissions and increasing the use of renewable resources1,2. The European Union, under the Paris Agreement, has committed to achieving climate neutrality by 2050 through promoting green technologies, sustainable industries, and reducing pollution3. However4, argue that current emission reduction efforts and future commitments are insufficient to meet the Paris Agreement targets, emphasizing that the pace of emissions reduction is too slow to address the urgency of climate action. They suggest that decarbonization should be integrated with emission reductions and complemented by direct carbon removal efforts, such as carbon sequestration, to effectively contain global temperature increases and meet long-term climate goals4,5. In this context, biochar production from waste wood, including oak stumps, presents a valuable opportunity for both carbon sequestration and waste reduction. By exploring sustainable pyrolysis technologies, this study proposes methods that contribute to achieving carbon neutrality by capturing atmospheric carbon and offering an environmentally friendly waste management alternative, thus supporting broader European and global climate goals.
The climate crisis requires efficient and cost-effective CO₂ removal strategies, which is why the use of biomass residues for energy production, the application of natural solid catalysts for biodiesel synthesis, and the production of biodegradable bioplastics and valuable products such as biochar are increasingly being explored6,7,8,9,10. Biochar, a porous, carbon-rich material with high aromaticity and resistance to decomposition, is formed through the thermal decomposition of plant or animal biomass in the absence of oxygen11. Typically, it is produced through pyrolysis, heating, gasification, hydrothermal liquefaction, or hydrothermal carbonization, with yields ranging from 10 to 44%, depending on feedstock type, physicochemical properties, and production conditions12,13,14,15,16,17,18,19,20,21,22,23. Different production conditions, particularly the available oxygen content, significantly influence biochar yield and quality, leading to variations in properties even from the same biomass9,24,25,26.
Due to its stable carbon structure, biochar is an effective long-term carbon sequestrant and CO₂ reduction agent7. Due to its high porosity, surface area and carbon content, biochar is an effective pollutant sorbent used for soil improvement and biofuel27,28,29 and can persist in the soil for hundreds of years30,31. The gas produced during pyrolysis can be used to generate energy or heat7,32,33. Biochar is also used as animal feed, for heavy metal removal and as a slow-release fertilizer component, providing additional soil improvement and carbon sequestration7,27,28,29,34,35,36,37. Biochar has been widely studied for its ability to improve soil properties (pH, nutrient and water retention, microbial activity, aeration, root development) and increase crop yields24,34,35,38,39,40,41,42,43. It is also effective in sequestering carbon in the soil and reducing greenhouse gas (CH₄, N₂O) emissions44,45,46, making it a valuable tool for climate change mitigation.
Biochar production faces challenges, particularly in maintaining quality due to variability in feedstock and production methods. Potential contaminants in the starting biomass, such as heavy metals, can cause undesirable properties of biochar and may even become a contaminant47. Therefore, a comprehensive cost-benefit analysis and Life Cycle Assessment (LCA) are essential before undertaking large-scale projects to assess biochar’s potential contributions to environmental and societal well-being. LCA is a widely used method for evaluating the impacts of biochar on climate change and the environment, but methodological differences among studies can lead to variability in results48,49,50,51,52,53,54,55,56. In this context, increasing attention is being paid to the potential of specific feedstocks, such as oak stump waste, for biochar production. Biochar production from oak stump waste has beneficial properties such as soil improvement, carbon sequestration, nutrient retention and increased microbial activity57,58,59,60. Studies highlight that pyrolysis-derived oak stump biochar significantly improves soil physicochemical properties, increases moisture retention and plant-available nutrients61,62, and can also mitigate soil degradation and promote plant growth63,64. However, gaps remain in recent literature due to the specific properties of biochar from less studied biomass sources, including oak stump waste6566. emphasizes the need for further investigation of the effects of production parameters on carbon sequestration efficiency and pyrolysis emissions, and67 emphasizes the need for standardized methodologies for assessing the impact of biochar. However, it is also noted that a comprehensive analysis of oak stump waste biochar production technologies and their environmental assessment is still lacking.
Oak wood is known for its high density, which affects the yield of biochar during pyrolysis. Higher density leads to greater biochar production per unit volume due to the higher carbon content. Dense woods tend to produce biochar with a more compact structure, which can affect its porosity and surface area. Studies on various oak species have demonstrated notable calorific values, underscoring oak’s potential as an efficient bioenergy source68.
The composition of oak wood includes significant amounts of lignin, cellulose, and hemicellulose. Lignin, being a complex and thermally stable polymer, contributes to a higher fixed carbon content and greater stability in biochar. In contrast, cellulose decomposes more readily during pyrolysis, leading to increased volatile matter and lower biochar yields69.
The ash content in oak biomass can influence the nutrient profile and pH of the produced biochar. Higher ash content may enhance the biochar’s ability to supply essential nutrients and affect soil alkalinity when used as a soil amendment. Biochars derived from oak biomass at pyrolysis temperatures of 450–500 °C have been found to be rich in macronutrients such as calcium (Ca), iron (Fe), potassium (K), magnesium (Mg), phosphorus (P), sulfur (S), which can enhance soil fertility when used as an amendment70.
The porosity of oak wood affects the porosity of the resulting biochar. High porosity can enhance biochar’s ability to retain water and nutrients in soil applications. Oak wood is durable and resistant to decay, which can contribute to the stability and longevity of biochar when used as a soil amendment. For comparison of other biomass sources like corn stalk, corn cob biochars, biochar from spruce wood had a larger surface area, which are favorable for soil application. On the other hand, the corn stalk, corn cob biochars had evidently high concentrations of inorganic elements (K and P), which is a desirable property for improving soil fertility71.Currently, tree stump waste is not used because its wood structure is very different from tree trunks, which makes it difficult to apply it in traditional biomass processing technologies. However, this study aims to demonstrate that stump-derived biochar can be a valuable feedstock suitable to produce secondary, environmentally friendly products. Life cycle assessment aims to demonstrate that certain pyrolysis technologies are more efficient in terms of both carbon sequestration and reducing global warming potential. It, therefore, makes sense to investigate the envisaged biochar using two different biochar extraction technologies, carrying out an environmental assessment where the boundaries of the assessment range from the collection of the waste from the site to the transport of the biochar production to the operator, with a preference for the sequestration indicator.
Results and discussion
The fraction and chemical composition of biochar
The composition of the biochar fraction was determined after stump burning (Fig. 1). A large amount of biochar fraction up to 2 mm in diameter, 42.1% SC–1 (1st scenario – UMT-3 PLUS EcoTeploOtbor reactor) and 35.8% SC–2 (2nd scenario – BIO-KILN-1 reactor) by weight was accumulated on the sieve with round holes. The lowest biochar content (4.76%) was obtained for both SC cases on the sieve up to 0.1 mm diameter. The fraction composition of biochar was similar in both SC cases. As the sieve up to 2 mm had the highest concentration of material, it can be assumed that the biochar is suitable for further use such as palletizations. A principal use of biochar is to serve as a soil amendment. Due to significant adsorption capacity, it boosts soil fertility and helps in the removal of heavy metal contaminants, thus enhancing the overall quality of the soil72,73. Biochar can be used as a means of increasing the amount of organic carbon in the soil, which can improve the biological and physical functioning of the soil74. Soil carbon storage is essential for regulating atmospheric CO2 levels and mitigating climate change75.
Average fraction composition of biochar.
The results of the chemical composition of biochar (Table 1) showed that the pH of biochar (SC–1) is 7.3, which means that the biochar is slightly alkaline, which indicates that it was optimal, especially in acidic soils. The organic carbon content is 31.29%, nitrogen (N) is 28 mg kg−1, phosphorus (P) is 119 mg kg−1, and potassium (K) is 950 mg kg−1. According to76, fixed carbon was 38.5% in oak wood biochar treated for 80–90 min at 300 °C77. pointed out that higher pyrolysis temperatures increased the fixed carbon content for biochar with low ash content but decreased it for biochar with more than 20% ash content. According to78,79, as the pyrolysis temperature increased, the pH of the biochar also increased. Biochar should be used in combination with other N, P and K-containing materials to improve plant nutrient availability80,81.
Analyzing biochar from wood waste, the following concentrations of heavy metals were determined: cadmium (Cd) – 0.17 mg kg–1, zinc (Zn) – 86 mg kg–1, nickel (Ni) – 3.13 mg kg–1, lead (Pb) – 8.70 mg kg–1, copper (Cu) – 7.23 mg kg–1 and chromium (Cr) – 5.67 mg kg–1. These concentrations can affect the effect of biochar on the soil, so it is important to assess the potential risk of contamination and the significance of these indicators for the possibilities of biochar application. Chemical analysis of the experimental biochar produced by SC–2 technology was not carried out because the pyrolysis units were of similar design, operating under similar conditions at 500 °C and the resulting fraction was not significantly different from the biochar produced by SC–1 technology. Discussing the chemical composition results, according to the European Biochar Certificate EBC-Agro (EBC, 2021)82, the experimental biochar (SC–1) complies with the permissible limits for cadmium (Cd) (1.5 mg kg−1), lead (Pb) (150 mg kg−1), zinc (Zn) (400 mg kg−1), copper (Cu) (100 mg kg−1), chromium (Cr) (90 mg kg−1), and nickel (Ni) (50 mg kg−1) in dry matter.
Biochar is of great interest to scientists due to its extraordinary versatility in various fields83. Additionally, beyond agricultural use, biochar can function as an energy resource, for instance, as biofuel. Its gaseous by-products may be used as sources of hydrogen, its liquid by-products as alternatives to fossil fuels, and its solid by-products can replace coal72.
Life cycle impact assessment
The LCA data for the UMT-3 PLUS EcoTeploOtbor (SC–1) and BIO-KILN-1 (SC–2) reactors indicated that the preparation and processing SBP were the largest contributors to their respective Global Warming Potentials (GWPs), accounting for 324.87 kg CO2eq for SC–1 and 297.23 kg CO2eq for SC–2. The higher (approximately 9%) environmental impacts of SC–1 compared to SC–2 likely result from differences in energy efficiency, design, or operational parameters. SC–1 may have less efficient heat recovery or higher energy consumption during pyrolysis, leading to increased emissions. These results are consistent with other life cycle analysis studies84: highlighted that the total emissions from biochar production strongly depend on the feedstock type and pyrolysis method, while85 showed that different production pathways have different impacts on the overall energy-biochar system86. analyzed straw biochar production and found that emissions vary depending on inputs and conditions throughout the life cycle, confirming that optimizing energy consumption and heat recovery is crucial to reduce GWP. Although accounting for these emissions, the net GWP of biochar production remains highly negative due to its high carbon sequestration potential. The LCA reveals that for SC–1, this sequestration reduces GWP by −2,541.59 kg CO2eq, and for SC–2 – by −2,325.34 kg CO2eq. Consequently, the net GWP for SC–1 is −2,036.98 kg CO2eq, and for SC–2 it is −1,866.31 kg CO2eq, demonstrating a significant net reduction in greenhouse gas emissions.
To evaluate the GWP of the SC–1 and SC–2 reactors without considering carbon sequestration, we focus solely on the emissions from their preparation and processing phases. For SC–1, the GWP from the preparation and SBP is 324.87 kg CO2eq, combined with other life cycle inputs resulting in a total GWP of 643.02 kg CO2eq (Fig. 2). For SC–2, the SBP contributes 297.23 kg CO2eq, with a total GWP of 585.65 kg CO2eq when all life cycle inputs are considered. The research results showed approximately 9% higher environmental impacts of SC–1 compared to SC–2. This evaluation highlights the emissions from biochar production processes without accounting for the significant negative impact of carbon sequestration, demonstrating the importance of biochar’s role in reducing overall GWP through carbon storage.
87,88,89,90 conducted a detailed analysis of LCA studies on biochar production from various agricultural residues and acknowledged that the sequestration potential of biochar systems is very promising. Carbon sequestration by biochar involves the long-term storage of carbon in a stable, non-reactive form, effectively removing CO2 from the atmosphere. Unlike other organic matter, biochar remains stable in soil for centuries, ensuring long-term carbon storage91. However, it is worth critically assessing the assumption of a 100-year sequestration horizon used. Our estimates assume that carbon in the form of biochar remains stably stored throughout this period. However, in practice, the duration of sequestration, as argued by92, depends on many factors – soil microbiome, climatic conditions and land use practices, which can accelerate or slow down the mineralization of biochar and the return of CO₂ to the atmosphere66. emphasize that the assessment of carbon removal potential needs to be more flexible, as the interaction of biochar with soil and plants changes over time61. Therefore, in order to achieve greater transparency in the assessment, it is necessary to constantly update the sequestration assessment methodology based on new experimental data.
Furthermore, when analyzing the trade-offs between fossil fuel use and long-term carbon sequestration benefits, it is important to highlight that even with a negative GWP (SC-1: − 2,036.98 kg CO₂eq; SC-2: − 1,866.31 kg CO₂eq), primary emissions during production can have significant short-term impacts. As pointed out by87,93, energy-intensive biochar production technologies that rely on fossil fuels can reduce the benefits of sequestration94. argue that only the use of renewable energy sources can effectively reduce emissions during both production and life cycle.
Finally, it is necessary to consider the challenges of industrial scale-up. Although current research demonstrates the positive effects of biochar on atmospheric carbon reduction, scaling up faces barriers such as residue availability, logistics costs, and economic sustainability95. According to96, biochar solutions must not only be environmentally friendly but also economically viable to be widely adopted in a long-term carbon reduction strategy.
Dependence of GWP on life cycle inputs to produce 1 ton of biochar. Note: CO2 – CO2 emissions from biochar production; FPP – firewood is intended for the pyrolysis process; BPR – bricks are intended to produce reactors, which accounted for 80% of the total weight, with amortization for 5 years; MPR – metal is intended to produce reactors, with amortization for 5 years; MPT – metal is intended to produce troughs, with amortization for 1.5 years; FT – fuel is for techniques; TS – transportation of stumps to the processing site (15–20 km radius); SBP – stumps are intended for biochar production; MTP – metal for techniques production, with amortization for 20 years; EP – electricity for processes.
Abiotic depletion due to fossil fuels (ADff)
Was another critical impact category. The analysis showed that the combined processes of stump processing, fuel consumption, and electricity use were major contributors to ADff. For both SC–1 and SC–2, the impacts of this category were significant, but SC–2 demonstrated an 8.51% lower impact compared to SC–1. This reduction highlights SC–2’s relative efficiency and reduced reliance on fossil fuels. The components contributing to ADff include the use of diesel fuel for machinery, electricity consumption, and materials required for pyrolysis reactor construction and maintenance (Fig. 3). Overall, the LCA results confirm that SC–2 is a more efficient biochar production alternative that contributes to climate change mitigation. However, abiotic depletion due to fossil fuels in our study was considerably high, with SC–1 at 8,306.48 MJ and SC–2 at 7,574.75 MJ97. reported lower values for palm residues, averaging 57–124 kg oil eq., indicating that fossil fuel depletion is an area where our biochar production process could be further optimized.
Dependence of ADff on life cycle inputs to produce 1 ton of biochar. Note: FPP – firewood is intended for the pyrolysis process; BPR – bricks are intended to produce reactors, which accounted for 80% of the total weight, with amortization for 5 years; MPR – metal is intended to produce reactors, with amortization for 5 years; MPT – metal is intended to produce troughs, with amortization for 1.5 years; FT – fuel is for techniques; TS – transportation of stumps to the processing site (15–20 km radius); SBP – stumps are intended for biochar production; MTP – metal for techniques production, with amortization for 20 years; EP – electricity for processes.
Eutrophication (ET) and acidification (AC)
Were also evaluated in the LCA. ET refers to the enrichment of water bodies with nutrients, leading to excessive growth of algae and other aquatic plants, which can degrade water quality. AC refers to the increase in acidity in the environment, primarily due to emissions of sulfur dioxide (SO2) and nitrogen oxides (NOx). The results indicated that SC–2 had lower impacts in both categories compared to SC–1. Specifically, ET impacts for SC–2 were 8.81% lower, and AC impacts were also significantly reduced, reflecting SC–2’s improved environmental performance (Figs. 4 and 5)97. reported an average freshwater eutrophication impact of 71–103 g P eq and marine eutrophication impacts up to 6.82 g N eq. In our study, the eutrophication levels were lower, with SC–1 and SC–2 showing 0.87 kg PO₄³⁻eq. and 0.79 kg PO₄³⁻eq, respectively. This was comparatively better than most of97 results but higher than98 values for mango pits, which had a eutrophication potential of 0.718 kg PO₄³⁻eq. In the case of biochar production per ton of willow biochar, aquatic AC is 0.91 kg SO2eq t−1 and aquatic ET is 0.58 kg PO4 P-lim t−153. Regarding our research, these findings underscore the importance of optimizing biochar production processes to maximize environmental benefits and support sustainable practices. The detailed analysis of the environmental impacts of biochar production provides a comprehensive understanding of its potential benefits and challenges. Future research should focus on optimizing pyrolysis conditions, exploring alternative feedstocks, and including broader environmental assessments.
Dependence of ET on life cycle inputs to produce 1 ton of biochar. Note: FPP – firewood is intended for the pyrolysis process; BPR – bricks are intended to produce reactors, which accounted for 80% of the total weight, with amortization for 5 years; MPR – metal is intended to produce reactors, with amortization for 5 years; MPT – metal is intended to produce troughs, with amortization for 1.5 years; FT – fuel is for techniques; TS – transportation of stumps to the processing site (15–20 km radius); SBP – stumps are intended for biochar production; MTP – metal for techniques production, with amortization for 20 years; EP – electricity for processes.
Dependence of AC on life cycle inputs to produce 1 ton of biochar. Note: FPP – firewood is intended for the pyrolysis process; BPR – bricks are intended to produce reactors, which accounted for 80% of the total weight, with amortization for 5 years; MPR – metal is intended to produce reactors, with amortization for 5 years; MPT – metal is intended to produce troughs, with amortization for 1.5 years; FT – fuel is for techniques; TS – transportation of stumps to the processing site (15–20 km radius); SBP – stumps are intended for biochar production; MTP – metal for techniques production, with amortization for 20 years; EP – electricity for processes.
Photochemical oxidation (PO) and ozone layer depletion (OLD)
Often associated with the formation of ground-level ozone (smog), was another impact category examined. The results indicated that SC–2 had a lower impact on PO compared to SC–1 (Fig. 6). Additionally, the assessment of OLD showed that SC–2 had a reduced impact, highlighting its environmental benefits over SC–1 (Fig. 7). Regarding photochemical oxidation, its values in the research of97 ranged from 16 to 26 kg NOₓeq, whereas in our study, the SC–1 and SC–2 scenarios showed significantly lower values – 0.16 and 0.15 kg C₂H₄eq, respectively. This reduction in photochemical oxidation potential indicated lower NOₓ and VOC emissions during SC-2, which confirmed the effectiveness of SC-2 in reducing precursor emissions and potentially increasing its environmental viability according to99,100. Ozone depletion potential values in97 ranged from − 1457 to −1982 mg CFC-11 eq, reflecting a positive effect on the ozone layer. In our study, ozone depletion values in SC–1 and SC–2 were minimal (6.08E-07 and 6.10E-05 kg CFC-11 eq) but also indicated a positive effect. These findings echoed the literature suggesting that biochar production systems may indirectly affect ozone-depleting substances by altering the course of atmospheric chemical reactions101,102. These changes may be driven by the biomass composition and pyrolysis conditions, which result in a different spectrum of emissions102,103. The lower PO and OLD rates in SC-2 can be explained by several mechanisms related to chemical and physical processes. First, this scenario results in less generation of reactive precursors, such as NOₓ and OH, responsible for ozone formation mechanisms in the atmosphere104,105. In addition, the biomass used in SC-2 and the pyrolysis conditions may support more favorable oxidative degradation mechanisms106.
Dependence of PO on the life cycle inputs to produce 1 ton of biochar. Note: FPP – firewood is intended for the pyrolysis process; BPR – bricks are intended to produce reactors, which accounted for 80% of the total weight, with amortization for 5 years; MPR – metal is intended to produce reactors, with amortization for 5 years; MPT – metal is intended to produce troughs, with amortization for 1.5 years; FT – fuel is for techniques; TS – transportation of stumps to the processing site (15–20 km radius); SBP – stumps are intended for biochar production; MTP – metal for techniques production, with amortization for 20 years; EP – electricity for processes.
Dependence of OLD on the life cycle inputs to produce 1 ton of biochar. Note: FPP – firewood is intended for the pyrolysis process; BPR – bricks are intended to produce reactors, which accounted for 80% of the total weight, with amortization for 5 years; MPR – metal is intended to produce reactors, with amortization for 5 years; MPT – metal is intended to produce troughs, with amortization for 1.5 years; FT – fuel is for techniques; TS – transportation of stumps to the processing site (15–20 km radius); SBP – stumps are intended for biochar production; MTP – metal for techniques production, with amortization for 20 years; EP – electricity for processes.
Human toxicity (HT) and ecotoxicity potential (EP)
Along with various ecotoxicity potentials (EP) (freshwater aquatic, marine aquatic, and terrestrial) were assessed to understand the impact of biochar production on human health and ecosystems (Table 2). LCA results showed that SC–2 had lower HT impacts, with a reduction of approximately 10% compared to SC–1. This trend is consistent with studies showing that toxicological effects strongly depend on feedstock composition and pyrolysis parameters107,108,109. Lower HT values reflected a reduction in harmful emissions associated with biochar production, consistent with the literature explaining the relationship between specific feedstock choices and toxicological footprint108,109. Similarly, SC–2 exhibited lower freshwater aquatic and MAE impacts, indicating that it was a more environmentally benign process. TE was also lower for SC–2, underscoring its advantages over SC–1 in reducing the potential for environmental harm. These results were significant as they suggested that SC-2 may contribute less to the leaching of toxic substances such as heavy metals and persistent organic pollutants than SC-1110,111. Given that biochar production processes can have unintended consequences in the surrounding ecosystems, understanding these mechanisms becomes particularly important112,113. The reduced impact on aquatic ecotoxicity may be attributed to the inherent properties of biochar, which, despite its ability to adsorb pollutants, could also sequester leachable metals and organic compounds, depending on their origin and preparation methodology. The comparative advantage of SC-2 suggested that optimizing pyrolysis conditions can increase the environmental safety of biochar by reducing the release of pollutants that may be harmful to aquatic life114,115.
Although our results highlight the favorable environmental profile of SC-2, certain limitations must be considered, especially with respect to the assumed 100-year carbon sequestration horizon. Biochar interactions with soil ecosystems can be volatile-environmental conditions can both shorten and lengthen its stability and sequestration efficiency, making it difficult to assess long-term benefits116,117. These aspects, together with the mechanisms underlying photochemical oxidation, ozone depletion, human toxicity, and ecotoxicity potential, reveal the multifaceted impacts of biochar production. Therefore, it is necessary to strive for more sustainable practical solutions that assess not only immediate benefits but also long-term impacts on environmental restoration and carbon reduction strategies. This requires more comprehensive research that integrates emission dynamics, ecological responses and technological improvements.
For both reactors, the preparation and processing SBP emerge as significant contributors to HT and MAE, indicating that this stage is critical in determining the overall environmental footprint of biochar production. First, the SBP stage involves significant mechanical processing, which typically requires substantial energy input, often derived from fossil fuels, leading to high emissions of greenhouse gases and other pollutants. For instance, in SC–1, SBP contributes 134.47 kg 1,4-DBeq to HT and 347,097.03 kg 1,4-DBeq to MAE, indicating substantial toxic emissions. Similarly, for SC–2, SBP contributes 123.03 kg 1,4-DBeq to HT and 317,563.61 kg 1,4-DBeq to MAE, illustrating the significant impact of this stage. However, SC–2 obtained about 9% less HT and MAE compared to SC–1. Secondly, the machinery and equipment used for stump processing can release lubricants, hydraulic fluids, and other chemicals into the environment, further contributing to HT and ecotoxicity.
Regarding human toxicity, the data in97 has its levels ranging from 13 to 20 kg 1,4 DBeq, depending on the feedstock. However, in our study, SC–1 and SC–2 human toxicity values were significantly higher − 204.81 and 184.81 kg 1,4 DBeq, respectively, indicating a higher human toxicity impact. While biochar production is seen as beneficial for climate change mitigation44,118,119,120,121,122. However, similar to our research, studies by others123,124,125,126 still showed that the largest energy consumption and emissions were generated during transport. The impact of transport’s fuel consumption and emissions depends on the mode, speed, and distance126. Variables such as feedstock types and pyrolysis conditions also affect environmental outcomes, making comparisons with other LCA studies challenging127,128. However, as86 argue, biochar emissions are climate-positive and influence decisions on biomass projects. Moreover, assumptions like biochar C sequestration potential and pyrolysis reactor energy costs influence the results128.
129 state that in large-scale operations, maintaining precise temperature control is critical. When the temperature exceeds 650 °C, the biochar exhibits significantly higher pH levels, which can lead to pronounced phytotoxic effects. Using higher temperatures for pyrolysis in biochar production might adversely affect the availability of crucial nutrients for plants. Using large-scale equipment, it is possible to produce biochar suitable for agriculture and environmental protection, with temperature and retention time being the two most crucial factors to control129. Additionally, the production of biochar can create various employment opportunities, particularly in rural areas where biomass is abundantly available. Scientists have noted that biochar enhances plant growth and nutrient cycling, which can boost agricultural productivity and lead to more jobs in agriculture and related industries130. The economic benefits of using biochar as a soil enhancer have also been documented, providing incentives for local farming practices and increasing the demand for labor in both producing and applying biochar131. Furthermore, systems that transform locally sourced biomass into biochar can significantly support rural communities by creating localized job markets in sustainable agriculture and bioenergy132. These initiatives not only generate direct employment but also invigorate local economies through related services and products, such as the sale and maintenance of biochar-related equipment131,132.
Also, a sensitivity analysis was performed to determine the influence of key input variables, such as electricity consumption, transportation distance, and feedstock preparation on the environmental impacts. A ± 20% variation in each parameter was simulated using SimaPro. Results indicated that electricity consumption and mechanical preparation of SBP had the greatest impact on GWP and HT scores. For example, a 20% increase in electricity consumption raised the GWP by 9.4% for SC–1 and 8.1% for SC–2. Transportation showed moderate sensitivity, affecting GWP and Acidification AC when transport distances exceeded 30 km. These findings highlight the importance of optimizing on-site processing and using renewable energy sources to further reduce environmental impacts.
In summary, the life cycle assessment demonstrated that the production of biochar using SC–2 offers significant environmental benefits over SC–1. SC–2’s lower impacts across multiple categories, including GWP, ADff, eutrophication (ET), AC, HT, and ecotoxicity, indicate its superior environmental performance. The carbon sequestration potential of biochar further enhances its value as a climate change mitigation strategy, providing a net reduction in GWP.
The study recommends enhancing sustainability in biochar production by improving energy efficiency, adopting cleaner technologies, and optimizing logistics. Reducing fuel use in stump processing, utilizing renewable energy, and improving feedstock handling can lower emissions. Additionally, optimizing pyrolysis conditions will further improve environmental performance.
Materials and methods
In this study, biochar was produced, and theoretical environmental modeling of biochar production was carried out, using tree stumps as a raw material for other forestry and urban management processes.
The tree stumps were collected locally in Lithuania, Ukmergė district, ensuring that only sustainable forestry waste was used. Stumps were chopped into pieces and mechanically cleaned of soil, stones, and other particles remaining between the smaller roots of the stump. Two pyrolysis reactors were used for biochar production: UMT-3 PLUS EcoTeploOtbor (SC–1) and BIO-KILN-1 (SC–2). Specific operating conditions of pyrolysis reactors are presented in Table 3. SC–1 and SC–2 are both types of pyrolysis reactors, each designed for specific applications, including biochar production, waste management, or biomass processing. Both devices are characterized by automatic protection against critical temperatures, automatic control of thermal processes, total absence of harmful emissions during operation, no environmentally hazardous air emissions, low operating costs, no consumables needed, and do not need routine maintenance throughout the entire life cycle period. Reactors were designed to operate under slow pyrolysis conditions, and the main difference between them is their production yield and design features. Although both SC–1 and SC–2 operated at the same pyrolysis temperature of 500 °C and heating rate of 8 °C min⁻¹, SC–2 demonstrated improved energy efficiency due to its better insulation and integrated heat recovery system. SC–2 recirculated hot exhaust gases to preheat incoming biomass, reducing electricity demand to 320.22 kWh t⁻¹, compared to 350 kWh t⁻¹ in SC–1. This contributed directly to lower GWP and ADff indicators for SC–2. The design of SC–1 resulted in higher energy losses during the batch operation mode, making it less efficient in terms of heat utilization.
In the entire biochar production cycle, such process stages as raw material collection, transportation, raw materials for reactor production (bricks, metals, plastic, etc.), fuel for stump processing machines, and electricity consumption were evaluated. The prepared raw material was placed in the pyrolysis devices SC–1 and SC–2.
Technological process for the preparation of biochar
The stumps were chopped into pieces and mechanically cleaned of soil and stones left between the smaller roots of the stump. The prepared feedstock was placed in two pyrolysis units SC–1 and SC–2 operating at 500 °C. While both systems operated at the same temperature and heating rate (500 °C, 8 °C min⁻¹), they differ in structural design. SC–1 is a batch-type reactor with a more enclosed chamber and slower thermal dissipation, resulting in a higher biochar yield of 42.1%. SC–2, a semi-continuous reactor, allows for smoother biomass feeding and throughput but has slightly lower yield (35.8%) due to less heat retention and more efficient gas evacuation. These differences are attributed not to pyrolysis duration but to internal volume, insulation, and material flow design. The primary differences in outcomes in both systems are due to the processing method. SC–1 operates in batch mode, which can be advantageous if the batch processing time is well-optimized. By recovering heat effectively, it can accelerate the drying and pyrolysis processes, leading to higher productivity. SC–2 operates in a continuous mode, which ensures a stable production rate but can have limitations if maintenance or adjustments are needed during operation. The SC-1 is optimized for heat recovery and industrial use, while the SC-2 emphasizes high-quality charcoal production, consistency and safety.
The produced biochar that did not meet the quality parameters for use as grill charcoal (fraction up to 1 mm) was selected and the fraction composition and the chemical elemental composition were analyzed. The technological process for the preparation of biochar is presented in Fig. 8.
Technological process for the preparation of biochar.
Fraction and chemical composition of biochar
The fraction composition of the biochar was determined using a Retsch AS 200 sieve (Germany) with a 200 mm diameter sieve set. The diameter of the holes in the sieves was 0 mm, 0.1 mm, 0.25 mm, 0.5 mm, 0.63 mm, 1 mm, and 2 mm. A sample of 100 g of biochar was sieved and the set of sieves was placed on a horizontal surface in a semicircle for 1 min. The mass remaining on the sieves was weighed on a Kern ABJ (Germany) balance (accuracy 0.01 g) and the percentage of the biochar fraction of the sample was calculated. Three repetitions were carried out.
The chemical composition of Biochar was determined according to the standards presented in Table 4.
Environmental assessment
Life cycle assessment
To assess the environmental impact of different pyrolysis devices (UMT-3 PLUS EcoTeploOtbor (SC–1) and BIO-KILN-1 (SC–2)), the LCA method was used133,134. It is a widely used tool to assess the potential environmental impact and resource use of a product/service system throughout its life cycle, i.e., from raw material extraction to production and use stages to waste management and transportation135 and is standardized by the International Commission of Standardization ISO 14040136 and ISO 14044137. The phases of the LCA conducted included the goal and scope definition, the life cycle inventory (LCI) analysis, the life cycle impact assessment (LCIA) using the CML method, and the interpretation of results. The final phase involved interpreting the environmental performance of the production systems and discussing the potential for carbon sequestration. In this research, to perform the LCA, SimaPro 9 software was used. Environmental impact assessment in the production of biochar was studied based on their midpoint impacts (CML-IA baseline V3.06/EU25), using 10 impact categories for evaluation (MAE, ADff, GWP, FWE), human toxicity (HM), TE, ET, AC, PO, and OLD). The impact categories were chosen due to their relevance to biochar production, which involves processes like pyrolysis, transportation, and energy use. These categories help assess critical environmental impacts, such as climate change and health risks associated with emissions and pollutants. For example, GWPis essential for assessing biochar’s carbon sequestration capabilities, while HT and ecotoxicity measure the potential harm caused by emissions during the pyrolysis process. Categories like abiotic depletion (fossil fuels) were included to evaluate the resource and energy consumption during production. Acidification, eutrophication, and photochemical oxidation were selected to assess broader ecological impacts, such as soil and water degradation and air quality. By evaluating the GWP, human toxicity, and other environmental impact categories, the LCA provides insights into how different pyrolysis technologies, such as SC–1 and SC–2, perform in terms of sustainability. This information is vital for optimizing production processes, reducing the environmental footprint, and making informed decisions about adopting these technologies in industrial biochar production. In addition, the LCA helps ensure that biochar production not only meets carbon sequestration goals but also aligns with broader environmental sustainability objectives in industrial applications.
The CML method was chosen for its standardized approach in life cycle impact assessment (LCIA), allowing for a detailed analysis of midpoint environmental impacts. This method is advantageous because it provides robust, scientifically validated models to quantify potential environmental impacts across multiple categories, ensuring comprehensive insights into the biochar production processes and their respective contributions to these environmental burdens. Its use in this study helps in comparing the environmental performance of different technologies and processes, particularly within the European context, which aligns with the goals of the study. Transportation, equipment, and raw materials were obtained from the Ecoinvent V3 database138.
Life cycle assessment system and inventory
To achieve the scientific goal, a biochar production line was analyzed. It consisted of production processes (stump collection, transportation, loading and crushing, magnetic separation, pyrolysis process, re-shredding, etc.) and equipment used (various specialized transport vehicles, conveyors, shredders, pyrolysis reactor, shredders, etc.) (Fig. 9). The scope of this LCA system includes everything needed to produce biochar using tree stumps as raw material. The functional unit, which is the reference unit for expressing environmental interventions, was expressed as 1 ton of biochar.
Flowchart and system boundaries of the LCA of biochar production, using tree stumps as raw material.
The impact categories assessed in this study are defined by the CML method and the values of the impacts in each category are determined based on the LCI, which includes data on emissions, resource use, and energy consumption throughout the biochar production process (Table 5).
The impact categories were chosen due to their relevance to biochar production, which involves processes like pyrolysis, transportation, and energy use. These categories help assess critical environmental impacts, such as climate change and health risks associated with emissions and pollutants. The CML method was selected to provide a comprehensive framework for evaluating these impacts. Key LCA parameters, like using 1 ton of biochar as the functional unit, allow for standardized comparison, and the inclusion of carbon sequestration is essential to capture biochar’s long-term environmental benefits.
Carbon sequestration
When performing a life cycle analysis of biochar production, it is also very important to evaluate the potential carbon sequestration of biochar. Biochar stands out as a significant advancement that enables the efficient capture, use, and sequestration of carbon from the atmosphere. In this study, the calculation assumed that biochar decomposes very slowly, with a significant portion remaining stable in the soil for centuries. This assumption is based on existing literature that suggests biochar can retain its carbon content over long periods, depending on factors such as soil conditions, temperature, and biochar properties. Studies like30,139 have shown biochar’s potential for long-term carbon sequestration. However, the rate of biochar decomposition may vary under different environmental conditions, which could impact the accuracy of long-term sequestration estimates. Scientists have determined that for every tone of biochar produced, an estimated 2.68 tons of CO2eq (tCO2eq) are effectively and permanently sequestered from the atmosphere, once the carbon emissions associated with the entire production process are considered140. Assumptions about biochar decomposition rates were based on existing literature and were adjusted for local soil conditions and climate. The carbon sequestration potential was calculated over a 100-year time horizon to account for the long-term stability of biochar carbon in soil. The net carbon sequestration was calculated by subtracting the total carbon emissions associated with the biochar production process (including collection, transportation, and pyrolysis) from the total carbon sequestered as stable biochar in the soil over a defined time horizon. The analysis accounts for the potential of biochar to immobilize carbon in a stable form, reducing the amount of carbon available for atmospheric release. This assessment was integrated into the LCA of biochar production to provide a more comprehensive understanding of the environmental benefits of biochar produced from tree stumps.
Conclusions
Chemical analysis confirmed that the concentrations of heavy metals in the prepared biochar were within permissible limits, ensuring its suitability for further applications creating secondary products that are environmentally friendly. Additionally, the physical properties of the biochar, such as particle size distribution, indicated that it was well-suited for granulation, as most of the material accumulated on the 2 mm sieve.
The research showed that SBP contributed greatly to HT and MAE in both scenarios (SC–1 and SC–2). The primary drivers of these impacts were high energy consumption and emissions from mechanical processing and transportation. Additionally, the release of lubricants, hydraulic fluids, and other chemicals during machinery operation further exacerbated the negative effects on HT and ecotoxicity. The LCA analysis revealed that the use of SBP in biochar production contributed to greenhouse gas emissions, but the overall process benefits from biochar’s significant carbon sequestration potential. Despite emissions associated with feedstock preparation and processing, the ability of biochar to store carbon in a stable form ultimately leads to a net reduction in GWP. This highlights biochar’s effectiveness as a strategy for climate change mitigation by significantly reducing overall CO₂ emissions. SC–2 generally demonstrated lower toxic impacts across all categories compared to SC–1, suggesting that SC–2’s design and operational efficiencies contribute to better environmental performance. However, the substantial impacts in the MAE category for both reactors underscored the need for advancements in feedstock management and cleaner processing technologies.
From a scalability perspective, SC–2’s semi-continuous design, lower energy consumption, and consistent performance suggest it is more appropriate for industrial-scale applications. SC–1, while producing slightly higher biochar yield, is more suited for small-scale or decentralized biochar production where throughput is lower and batch processing is acceptable. These insights support informed decision-making based on production scale and environmental objectives.
One key limitation is the study did not account for potential variations in biochar stability under different soil conditions, which could affect long-term carbon sequestration estimates. Future research should explore how different feedstocks, pyrolysis conditions, and soil environments influence both biochar quality and its environmental benefits. Another area for future investigation is the optimization of energy use in biochar production to further minimize the environmental footprint. Expanding the scope to include economic and social factors in life cycle assessments could also provide a more holistic evaluation of biochar’s sustainability.
Data availability
All data generated or analyzed during this study are included in this published article. Additional information related to the LCA model and input parameters can be made available by the author, Andrius Grigas, upon reasonable request.
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Conceptualization, K.L., E.J., A.G., Al.G., S.P., and R.M.; methodology, E.J., A.G., Al.G., and R.M.; software, A.G., Al.G., and R.M.; validation, E.J., A.G., Al.G., and R.M.; formal analysis, K.L., A.G., and S.P.; investigation, A.G., Al.G., and R.M.; resources, E.J., A.G., Al.G., and R.M.; data curation, K.L., A.G., and R.M.; writing—original draft preparation, K.L., A.G., and S.P.; writing—review and editing, K.L., A.G., and S.P.; visualization, K.L., and A.G.; supervision, E.J.; project administration, K.L.
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Lekavičienė, K., Jotautienė, E., Grigas, A. et al. Evaluating biochar extraction from waste tree stumps in different pyrolysis systems using life cycle analysis. Sci Rep 15, 40135 (2025). https://doi.org/10.1038/s41598-025-23855-6
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DOI: https://doi.org/10.1038/s41598-025-23855-6
