1 Introduction

Forestry waste, which includes residues from logging, wood processing, and forest management activities, represents a significant and underutilized biomass resource. The increasing volume of forestry waste poses environmental challenges, but it also offers a substantial opportunity for energy production. Utilizing forestry waste for energy can reduce reliance on fossil fuels, mitigate greenhouse gas emissions, and contribute to sustainable forest management practices. The global production of agricultural and forestry wastes (AFWs) is immense, and their conversion into biofuels and chemicals can play a crucial role in the circular economy by transforming waste into valuable resources (Song et al., 2020).

Thermochemical conversion methods, such as pyrolysis, gasification, and liquefaction, are pivotal in transforming forestry waste into energy-rich products like bio-oil, syngas, and biochar. These methods leverage high temperatures and chemical reactions to break down complex organic materials into simpler, energy-dense compounds. Thermochemical processes are advantageous due to their ability to handle diverse biomass feedstocks and produce a range of valuable outputs. For instance, thermocatalytic reforming (TCR) has been shown to convert biomass waste into syngas, bio-oil, and biochar, with significant environmental and economic benefits (Moreno et al., 2019). Additionally, the integration of advanced analytical techniques, such as TG-FTIR, enhances the efficiency and optimization of these conversion processes (Ong et al., 2020).

This study summarizes the most advanced thermochemical conversion technologies and their applicability to forestry waste, evaluates the environmental and economic sustainability of these conversion processes, identifies key challenges and future research directions to optimize thermochemical conversion methods to maximize energy recovery and minimize environmental impact. By synthesizing recent research findings, this study will highlight the potential of thermochemical conversion methods to transform forestry waste into valuable energy resources, thereby contributing to sustainable energy solutions and forest management practices.

2 Types of Forestry Waste

2.1 Classification of forestry waste

Forestry waste can be broadly classified into several categories based on their origin and composition. The main types include logging residues, sawdust, bark, and wood chips. Logging residues are the residues left after logging, including branches, stumps, and leaves, which are usually left on the forest floor and contribute to the accumulation of biomass. Sawdust is produced during the process of sawing logs into lumber and is a fine wood particle that can be used for a variety of purposes, including energy production. Bark is the outer protective layer of trees, which is usually stripped away during wood processing and can be used as fuel or in the production of mulch and other products. Wood chips are small pieces of wood produced by cutting larger pieces of wood and are usually used in pulp mills, as fuel, or for landscaping.

2.2 Quantitative data on forestry waste availability

Forestry waste is generated in substantial quantities globally. For instance, agricultural and forestry wastes (AFWs) are produced in huge amounts and are considered an important resource for reducing dependence on fossil fuels (Lin et al., 2021). The availability of forestry waste varies by region and type of forestry activity. For example, sawdust and wood chips are commonly produced in sawmills, while logging residues are more prevalent in areas with active logging operations.

2.3 Environmental impact of forestry waste accumulation

The accumulation of forestry waste can have significant environmental impacts. If not managed properly, it can lead to increased fire hazards, pest infestations, and the release of greenhouse gases as the biomass decomposes. Utilizing forestry waste for energy production through thermochemical conversion methods can mitigate these environmental risks. The smart use of AFWs, including forestry waste, requires a combination of available waste streams and local technical solutions to meet sustainability criteria (Song et al., 2020; Bin et al., 2022). This approach not only reduces the environmental impact but also contributes to the circular economy by transforming waste into valuable resources.

3 Overview of Thermochemical Conversion Methods

3.1 Definition and principles of thermochemical conversion

Thermochemical conversion refers to the process of converting biomass into energy and valuable chemicals through the application of heat and chemical reactions. This method leverages high temperatures to break down complex organic materials into simpler molecules, which can then be utilized as fuels or chemical feedstocks. The primary principles involve the decomposition of biomass in the absence or presence of limited oxygen, leading to the production of gases, liquids, and solid residues (Uzoejinwa et al., 2018).

3.2 Main thermochemical processes

Pyrolysis is a thermochemical process that involves the thermal decomposition of biomass in the absence of oxygen. It produces a mixture of solid (biochar), liquid (bio-oil), and gaseous (syngas) products (Figure 1). Pyrolysis can be further categorized into slow, fast, and flash pyrolysis, depending on the heating rate and residence time (Uzoejinwa et al., 2018; Foong et al., 2020; Jesus et al., 2020). Recent advancements include microwave pyrolysis, which enhances heat transfer efficiency and product yield.

The figure illustrates a process for converting lignocellulosic biomass into sustainable energy using pyrolysis technology. The characteristics of the pyrolysis products significantly vary based on process temperature, heating rate, biomass composition, and residence time. Introducing microwave heating into the pyrolysis process is considered an effective solution. Compared to traditional pyrolysis, microwave-assisted pyrolysis offers several advantages, such as faster heating rates, quicker and more uniform heating of large feed quantities, higher energy efficiency, bio-oil with lower water content, and rapid response in pyrolyzer startup and shutdown.

Gasification is a process that converts biomass into syngas (a mixture of carbon monoxide, hydrogen, and methane) by reacting the material at high temperatures with a controlled amount of oxygen and/or steam. This process is highly efficient for producing energy-dense gases that can be used for power generation or as chemical feedstocks (Figure 2) (Labaki and Jeguirim, 2017; Ong et al., 2020; Jha et al., 2022). Gasification operates at higher temperatures than pyrolysis and can handle a variety of feedstocks, including forestry waste (Vega et al., 2019).

The figure illustrates a process for converting biomass into syngas using Hydrothermal Gasification (HTG) technology. HTG employs subcritical or supercritical water as the reaction medium to produce syngas. Traditional methods for converting wet biomass, such as pyrolysis, liquefaction, and gasification, are usually inefficient because the heat required to evaporate water often exceeds the biomass's combustion heat. In contrast, the products of HTG technology have multiple applications, including chemical synthesis, power generation, and fuel cells related to hydrogen production.

Combustion is the process of burning biomass in the presence of excess oxygen to produce heat, which can be used directly for heating or to generate electricity. This process is the most straightforward thermochemical conversion method and is widely used due to its simplicity and efficiency in energy recovery (Labaki and Jeguirim, 2017; Jha et al., 2022). Combustion of biomass results in the complete oxidation of the material, producing carbon dioxide, water, and ash (Vega et al., 2019).

3.3 Comparative analysis

When comparing pyrolysis, gasification, and combustion, several factors such as efficiency, product yield, and applicability to forestry waste must be considered. Gasification generally offers higher efficiency in converting biomass to energy compared to pyrolysis and combustion. This is due to its ability to produce a high-energy syngas that can be used in various applications (Labaki and Jeguirim, 2017; Ong et al., 2020). Pyrolysis, particularly fast and microwave pyrolysis, can also be efficient but is more complex and requires precise control of operating conditions (Foong et al., 2020; Jesus et al., 2020). Combustion, while straightforward, is less efficient in terms of energy conversion but is highly effective for direct heat and power generation. Pyrolysis is notable for its ability to produce a diverse range of products, including biochar, bio-oil, and syngas, making it versatile for different applications (Uzoejinwa et al., 2018; Foong et al., 2020). Gasification primarily produces syngas, which is valuable for its high energy content and versatility in chemical synthesis. Combustion yields heat and ash, with no intermediate products, making it less versatile but highly effective for immediate energy needs (Labaki and Jeguirim, 2017; Vega et al., 2019).

All three methods are applicable to forestry waste, but their suitability depends on the desired end products and specific application requirements. Pyrolysis is advantageous for producing biochar and bio-oil, which can be used for soil amendment and as a renewable fuel, respectively. Gasification is suitable for producing syngas from forestry waste, which can be used for power generation or as a feedstock for chemical production. Combustion is ideal for direct energy recovery from forestry waste, providing a straightforward solution for heat and power generation (Labaki and Jeguirim, 2017; Vega et al., 2019).

4 Pyrolysis of Forestry Waste

4.1 Mechanism and process conditions of pyrolysis

Pyrolysis is a thermochemical conversion process that involves the thermal decomposition of organic materials in the absence of oxygen. The process conditions, such as temperature, heating rate, and residence time, significantly influence the yield and quality of the pyrolysis products. Typically, pyrolysis occurs at temperatures ranging from 300 ℃ to 700 ℃. The heating rate can vary from slow to fast, affecting the distribution of bio-oil, syngas, and biochar produced. For instance, slow pyrolysis, which operates at lower heating rates and longer residence times, tends to favor biochar production, while fast pyrolysis, with higher heating rates and shorter residence times, is optimized for bio-oil production (Lee et al., 2020; Jha et al., 2022).

4.2 Types of pyrolysis

There are three main types of pyrolysis: slow, fast, and flash pyrolysis. Slow Pyrolysis operates at low heating rates (0.1 ℃/s~1 ℃/s) and long residence times (hours to days), producing a higher yield of biochar (Lee et al., 2020). Fast pyrolysis features a rapid heating rate (10 ℃/s~200 ℃/s) and short residence times (seconds), maximizing the production of bio-oil (Jha et al., 2022). Flash Pyrolysis is an extreme form of fast pyrolysis with very high heating rates (>1 000 ℃/s) and very short residence times (milliseconds), primarily producing bio-oil and syngas.

4.3 Products of pyrolysis and their energy potential

The primary products of pyrolysis are bio-oil, syngas, and biochar, each with distinct energy potentials. Bio-oil is a liquid fuel that can be directly used for heating or further refined into transportation fuels. It has a high energy density and can be upgraded to improve its performance (Jha et al., 2022). Syngas, a mixture of hydrogen, carbon monoxide, and other gases, can be used for power generation or as raw materials for producing chemicals and fuels. Biochar is a carbon-rich solid product that can be used as a soil amendment to improve soil health and sequester carbon, or as a raw material for combustion and gasification due to its high fixed carbon content and energy density (Lee et al., 2020).

4.4 Optimization strategies for maximizing bio-oil yield and quality

To maximize the yield and quality of bio-oil, several optimization strategies can be employed. The type of biomass used can significantly affect the yield and quality of bio-oil. Lignocellulosic feedstocks, for example, are known to produce higher quality bio-oil. Optimizing the pyrolysis temperature, heating rate, and residence time is crucial. For instance, fast pyrolysis at moderate temperatures (around 500 ℃) and high heating rates can enhance bio-oil yield (Lee et al., 2020; Jha et al., 2022). The use of catalysts can improve the quality of bio-oil by reducing oxygen content and increasing the proportion of desirable hydrocarbons. Combining biomass with fossil fuels or other waste materials can enhance the synergistic effects, improving the overall quality and yield of bio-oil (Yang et al., 2019).

5 Gasification of Forestry Waste

5.1 Mechanism and process conditions of gasification

Gasification is a thermochemical process that converts forestry waste into syngas (a mixture of carbon monoxide, hydrogen, and other hydrocarbons) through partial oxidation at high temperatures. The process involves several stages: drying, pyrolysis, oxidation, and reduction. The efficiency of gasification depends on various factors such as temperature, pressure, and the presence of catalysts. Optimal conditions typically involve temperatures ranging from 700 ℃ to 1 000 ℃ and controlled oxygen supply to ensure partial rather than complete combustion (Ong et al., 2020; Song et al., 2020).

5.2 Types of gasifiers and their suitability for forestry waste

Several types of gasifiers are used for the gasification of forestry waste, including fixed-bed, fluidized-bed, and entrained-flow gasifiers. Fixed-bed gasifiers, such as updraft and downdraft gasifiers, are suitable for small-scale applications and can handle a variety of feedstocks, including forestry waste. Fluidized-bed gasifiers offer better mixing and heat transfer, making them suitable for larger-scale operations. Entrained-flow gasifiers operate at higher temperatures and pressures, providing higher syngas quality but requiring more stringent feedstock preparation (Ong et al., 2020; Song et al., 2020).

5.3 Syngas composition and applications

The composition of syngas produced from forestry waste typically includes hydrogen (H₂), carbon monoxide (CO), carbon dioxide (CO₂), methane (CH₄), and other trace gases. The exact composition depends on the feedstock and gasification conditions. Syngas can be used for various applications, including electricity generation, production of synthetic natural gas, and as a feedstock for chemical synthesis, such as methanol and Fischer-Tropsch fuels (Stasiek and Szkodo, 2020).

5.4 Optimization techniques for enhancing gasification efficiency and syngas quality

Optimization of gasification processes involves several strategies to enhance efficiency and syngas quality. These include the use of catalysts to lower reaction temperatures and increase reaction rates, co-gasification with other biomass types to improve synergetic effects, and advanced gas cleaning technologies to remove impurities (Perera et al., 2021). Additionally, the integration of gasification with other thermochemical processes, such as pyrolysis, can further enhance overall energy efficiency and product yield.

6 Combustion of Forestry Waste

6.1 Basic principles and process conditions of combustion

Combustion is a thermochemical process that involves the oxidation of biomass to produce heat, which can be used for power generation. The basic principle of combustion is the exothermic reaction between the carbon and hydrogen in the biomass with oxygen, resulting in the release of energy, carbon dioxide, and water. The process conditions for efficient combustion include maintaining an adequate supply of oxygen, controlling the temperature to ensure complete combustion, and managing the moisture content of the biomass to optimize energy output (Pang, 2019; Solarte-Toro et al., 2021).

6.2 Direct combustion and co-firing with other fuels

Direct combustion of forestry waste involves burning the biomass alone to generate heat and power. This method is straightforward but can be less efficient due to the high moisture content and low energy density of raw biomass. Co-firing, on the other hand, involves burning biomass along with other fuels such as coal. This approach can improve the overall efficiency and reduce greenhouse gas emissions by partially replacing fossil fuels with renewable biomass. Co-firing also helps in utilizing existing coal-fired power plants with minimal modifications, making it a cost-effective solution for integrating biomass into the energy mix (Patel et al., 2016; Chen et al., 2021).

6.3 Emission control and environmental impact

Combustion of forestry waste can lead to the emission of pollutants such as particulate matter, nitrogen oxides (NOx), sulfur oxides (SOx), and volatile organic compounds (VOCs). Effective emission control strategies are essential to minimize the environmental impact. Technologies such as electrostatic precipitators, fabric filters, and scrubbers can be employed to capture particulate emissions. Additionally, optimizing combustion conditions and using advanced combustion technologies can reduce the formation of NOx and SOx. The environmental impact of biomass combustion is generally lower than that of fossil fuels, as biomass is considered carbon-neutral due to the CO2 absorbed during the growth of the plants (Solarte-Toro et al., 2021; Jha et al., 2022).

6.4 Technological advancements and optimization of combustion systems

Recent advancements in combustion technology have focused on improving the efficiency and environmental performance of biomass combustion systems. Innovations such as fluidized bed combustion and staged combustion have been developed to enhance the combustion process. Fluidized bed combustion allows for better mixing of the biomass with air, leading to more complete combustion and lower emissions. Staged combustion involves dividing the combustion process into multiple stages to control the temperature and reduce the formation of pollutants. Additionally, the integration of biomass combustion with other processes, such as gasification and pyrolysis, can further optimize energy recovery and reduce emissions (Patel et al., 2016; Pang, 2019; Yang et al., 2019).

7 Comparative Analysis of Thermochemical Methods

7.1 Energy efficiency and yield comparison

Thermochemical conversion methods such as pyrolysis, gasification, torrefaction, and combustion are widely recognized for their efficiency in converting forestry waste into energy. Pyrolysis and gasification, in particular, have shown high energy yields and efficiency. For instance, gasification of forestry residues can generate net power in the range of 100 to 200 kW/ton, with some systems achieving up to 363 kW/ton. Pyrolysis, especially when combined with waste plastics, can enhance biofuel production and energy security. Torrefaction, while producing lower energy yields compared to gasification, still offers significant improvements in energy density and fuel properties (Vega et al., 2019; Ong et al., 2020).

7.2 Economic feasibility and cost analysis

Economic feasibility is a critical factor in the adoption of thermochemical conversion technologies. The techno-economic analysis of various thermochemical processes, including incineration, pyrolysis, gasification, and integrated gasification combined cycle (IGCC), indicates that these methods can be economically viable at an industrial scale. Key performance indicators such as capital and operational costs, electricity generation per tonne, and net revenue per tonne of feedstock are essential metrics for evaluating economic feasibility (Gabbar and Aboughaly, 2021). The circular economy perspective further supports the economic use of forestry waste by integrating waste streams and local technical solutions to meet sustainability criteria (Song et al., 2020).

7.3 Environmental impact assessment

The environmental impact of thermochemical conversion methods is a significant consideration. Gasification, for example, has been shown to be environmentally friendly for most systems, with low global warming potential (GWP), acidification potential (AP), and eutrophication potential (EP) (Safarian et al., 2021). Co-pyrolysis of biomass with waste plastics not only improves energy yields but also addresses waste management and reduces dependency on fossil fuels, thereby mitigating environmental pollution (Uzoejinwa et al., 2018). The use of advanced analytical techniques like TG-FTIR can further optimize the environmental performance of these processes by accurately characterizing biomass and improving conversion efficiency (Ong et al., 2020).

7.4 Suitability for different types of forestry waste

Different types of forestry waste exhibit varying suitability for thermochemical conversion processes. For instance, Tectona Grandis has shown high conversion efficiency and superior synthesis gas quality in gasification due to its favorable physicochemical characteristics (Vega et al, 2019). The suitability of various forestry residues for gasification has been demonstrated through performance analysis and simulation models, which highlight the potential of specific feedstocks like tamarack bark and birch bark for high energy output and low environmental impact (Safarian et al., 2021). The choice of thermochemical method and feedstock is crucial for optimizing energy production and environmental benefits.

8 Case Studies

8.1 Thermochemical conversion projects using forestry waste

Thermochemical conversion methods have been successfully implemented in various projects to utilize forestry waste for energy production. For instance, the gasification of woody biomass and forestry residues has been demonstrated to produce significant amounts of electrical and thermal energy. A study conducted in Iceland assessed the power generation potential from different types of forestry residues, revealing that tamarack bark could generate up to 363 kW/ton of input feedstock, significantly outperforming other types of biomass (Safarian et al., 2021).

8.2 Detailed analysis of specific case studies

In Iceland, an equilibrium simulation model using ASPEN Plus was developed to evaluate the performance of gasification systems using 28 different types of woody biomass and forestry residues. The study found that tamarack bark was the most efficient, producing 363 kW/ton of input feedstock (Figure 3). The environmental impact assessment indicated that electricity generation from these systems was environmentally friendly for 75% of the studied systems, with tamarack bark and birch bark showing the lowest normalized environmental impact (Safarian et al., 2021).

This flowchart represents a simulation model for the integration of wood chips and crop residues (WB&FR) gasification and power generation units. The model adopts the equilibrium method and applies the PR-BM equation of state (Peng Robinson Boston Mathias alpha) to calculate the physical properties of components during the gasification process. For modeling the enthalpy and density of unconventional materials such as biomass and ash, HCOALGEN and DCOALIGT models were used.

8.3 Scalability and replicability of the projects

The scalability and replicability of thermochemical conversion projects using forestry waste depend on several factors, including the availability of feedstock, technological advancements, and economic feasibility. The gasification project in Iceland demonstrates that with the right technology and feedstock, significant energy production can be achieved. The use of ASPEN Plus for simulation and performance analysis can be replicated in other regions with similar biomass resources, making it a scalable solution (Safarian et al., 2021).

9 Challenges and Opportunities

9.1 Technical challenges in the thermochemical conversion of forestry waste

The thermochemical conversion of forestry waste into energy and valuable products faces several technical challenges. One significant issue is the complex physical structure and chemical composition of biomass, which hinders its efficient conversion to gaseous and liquid fuels. The high temperatures required for processes such as pyrolysis, gasification, and liquefaction can lead to operational difficulties and increased energy consumption (Yang et al., 2022). Additionally, the presence of contaminants in the feedstock can affect the quality and yield of the final products, necessitating advanced pretreatment and purification steps (Pang et al., 2019). The development of robust and efficient catalysts is also crucial to enhance the selectivity and efficiency of these conversion processes.

9.2 Economic and regulatory barriers

Economic and regulatory barriers significantly impact the adoption and optimization of thermochemical conversion technologies. The high initial capital investment and operational costs associated with these technologies can be prohibitive, especially for small-scale operations. Furthermore, the economic viability of these processes is often dependent on the fluctuating prices of fossil fuels and the availability of subsidies or incentives for renewable energy projects (Gabbar and Aboughaly, 2021; Song et al, 2022). Regulatory frameworks and policies also play a critical role, as stringent environmental regulations can both drive the adoption of cleaner technologies and impose additional compliance costs (Korai et al., 2016). The lack of standardized regulations and incentives across different regions can create uncertainty and hinder the widespread implementation of these technologies.

9.3 Future research directions and technological innovations

Future research should focus on optimizing the efficiency and sustainability of thermochemical conversion processes. This includes the development of advanced catalysts and pretreatment methods to improve the quality and yield of biofuels and chemicals (Pang, 2019; Yang et al., 2022). Innovations in reactor design and process integration can also enhance the overall efficiency and reduce the environmental footprint of these technologies (Stasiek and Szkodo, 2020). Additionally, research into the co-processing of biomass with other waste materials, such as plastics, can offer synergistic benefits and improve the economic feasibility of these processes. Exploring the potential of integrating thermochemical conversion with other renewable energy systems, such as solar or wind power, can further enhance the sustainability and resilience of energy production from forestry waste (Jha et al., 2022).

9.4 Potential for integration with other renewable energy systems

The integration of thermochemical conversion technologies with other renewable energy systems presents a promising opportunity to enhance energy security and sustainability. For instance, combining biomass gasification with solar thermal systems can provide a continuous and stable energy supply, leveraging the strengths of both technologies (Jha et al., 2022). Similarly, integrating these processes with wind or hydroelectric power can help balance the intermittency of renewable energy sources and optimize the overall energy output. The co-location of biorefineries with other renewable energy facilities can also facilitate the efficient use of resources and reduce transportation costs (Song et al, 2022). Such integrated systems can contribute to a more resilient and diversified energy infrastructure, supporting the transition to a low-carbon economy (Jha et al., 2022).

10 Concluding Remarks

The review of thermochemical conversion methods for the energy utilization of forestry waste has highlighted several key findings. Thermochemical processes such as pyrolysis, gasification, and liquefaction have been identified as effective methods for converting forestry waste into valuable energy products like bio-oil, syngas, and biochar. The physicochemical properties of the biomass significantly influence the efficiency and yield of these processes. Additionally, the integration of catalysts and appropriate solvents can enhance the quality and yield of the conversion products. The environmental benefits of these processes, including reduced carbon footprints and the potential for carbon-neutral energy production, have also been emphasized.

To optimize thermochemical conversion processes for forestry waste, several recommendations can be made. The use of robust and sustainable catalysts can improve the efficiency and selectivity of thermochemical conversions. Implementing pretreatment methods such as torrefaction can enhance the energy density and conversion efficiency of the biomass. Combining different thermochemical processes, such as pyrolysis and gasification, can maximize energy recovery and improve overall system efficiency. Utilizing advanced reactor designs like fluidized bed reactors can facilitate scale-up and improve the economic viability of the processes. Applying pinch analysis to integrate thermal streams can reduce external energy demands and increase process sustainability.

The application of thermochemical conversion methods for forestry waste has significant implications for waste management and energy policy. By converting forestry waste into valuable energy products, these methods provide a sustainable solution for waste disposal and energy generation. This can reduce the reliance on fossil fuels, lower greenhouse gas emissions, and promote the use of renewable energy sources. Policymakers should consider supporting research and development in this field, providing incentives for the adoption of these technologies, and implementing regulations that encourage the sustainable management of forestry waste.

The future prospects of thermochemical conversion methods for forestry waste are promising. Continued advancements in catalyst development, reactor design, and process integration are expected to enhance the efficiency and economic viability of these technologies. The growing emphasis on sustainability and renewable energy sources will likely drive further research and investment in this area. As these technologies mature, they have the potential to play a crucial role in the global transition to a more sustainable and circular economy, providing a valuable pathway for the utilization of forestry waste.

Acknowledgments

I appreciate the feedback from two anonymous peer reviewers on the manuscript of this study, whose careful evaluation and constructive suggestions have contributed to the improvement of the manuscript.

Conflict of Interest Disclosure

The author affirms that this research was conducted without any commercial or financial relationships that could be construed as a potential conflict of interest.

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