Open access peer-reviewed chapter - ONLINE FIRST
Abstract
This chapter provides a comprehensive exploration of the various technologies used to convert biomass into valuable products, such as biofuels, biochemicals, and bioenergy. It thoroughly examines the three main categories of current biomass conversion technologies: thermochemical, biochemical, and physicochemical processes. Thermochemical conversion includes processes such as combustion, gasification, and pyrolysis, which utilize heat to transform biomass. Biochemical conversion involves biological processes such as anaerobic digestion and fermentation to produce energy and chemicals. Physicochemical conversion, such as transesterification, chemically alters biomass to create bio-based products. The chapter meticulously examines the principles, mechanisms, and applications of each technology, highlighting their role in creating sustainable, renewable energy solutions and contributing to waste management and environmental protection.
Keywords
- thermochemical conversion
- biochemical conversion
- physicochemical conversion
- biofuel
- bioproducts
1. Introduction
The growing interest in renewable energy stems from two main factors: (1) heightened concern over global climate change and the environmental effects of fossil fuels, and (2) rising worries about the security and sustainability of fossil fuel supplies [1]. Interest in renewable energy arises from two sources: (1) increasing concern about global climate change and the environmental impact of fossil energy sources and (2) increasing anxiety regarding the security and longevity of fossil fuel [1]. Approximately 14% of the world’s primary energy comes from biomass [2]. In developing countries, the percentage is notably impacted, as biomass is mainly burned for household heating and cooking. In contrast, biomass accounts for less than 3% of primary energy in industrialized nations. Globally, only around 40% of the biofuel potential is utilized, with Asia being the only region where consumption surpasses the available potential [2]. This highlights the significant potential for expanding biomass usage as a sustainable alternative or complement to coal. A strategy proposed by the US Department of Agriculture and Energy suggests that biomass sourced from forest and agricultural lands could replace up to 30% of the current petroleum consumption in the US by 2030 [1, 2].
As the world increasingly shifts toward sustainable and renewable energy sources, biomass has emerged as a key player in the production of biofuels, biochemicals, and bioenergy [3, 4]. In order to meet the Net Zero Emissions by 2050 Scenario, biomass, as a zero-carbon fuel, is essential to advancing sustainable energy on a global scale [5]. Biomass, which includes organic material from plants, animals, and waste, can be converted into a variety of valuable products using different conversion technologies [6]. These technologies not only decrease our dependence on fossil fuels but also offer solutions for waste management and the mitigation of greenhouse gas emissions [7].
Biomass conversion technologies are broadly categorized into three types: thermochemical, biochemical, and physicochemical processes [8]. Each of these methods leverages different scientific principles and mechanisms to unlock the energy and material potential stored in biomass. Thermochemical processes, for instance, use heat to break down biomass into energy-rich gases, liquids, or solids, while biochemical processes employ microorganisms or enzymes to convert biomass into biofuels and other high-value chemicals. The physicochemical process involves both physical and chemical reactions that modify biomass into products, such as biodiesel. The thermochemical process happens relatively rapidly at high temperatures that are a few 100°C and sometimes up to over 1000°C. In contrast, the biochemical process proceeds at a few tens of degrees Celsius above ambient temperature, thus it can be quite slow even in the presence of catalysts. The physiochemical process often combines physical actions, such as changes in phase (solid, liquid, gas) or mixing, with chemical reactions that alter the molecular structure or composition of a substance. Therefore, the operating conditions required for physicochemical processes can vary widely depending on the specific process and materials [9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20].
This chapter will delve into these technologies in depth, examining the underlying principles, major processes, and their respective applications. By understanding these conversion pathways, we gain insight into how biomass can be effectively transformed into clean, sustainable energy and materials, helping address critical environmental and energy challenges. As we move forward, these technologies play a pivotal role in the global transition to a circular bioeconomy, where waste is reduced, and renewable resources are maximized.
2. Biomass conversion technologies
Biomass represents the organic material that comes from plants and animals. Plants produce biomass through photosynthesis, which stores the chemical energy. Biomass may contribute little or no net atmospheric CO2 when harvested sustainably and used as a fuel. Biomass sources for energy include: (1) wood and wood processing wastes, such as firewood, wood pellets, and wood chips, and black liquor from pulp and paper mills; (2) agricultural crops and waste materials, such as corn, soybeans, sugar cane, switchgrass, woody plants, and algae, and crop and food processing residues; (3) biogenic materials in municipal solid waste, such as paper, cotton, and wool products, and food, yard, and wood wastes; (4) animal manure and human sewage for producing biogas/renewable natural gas (Figure 1) [21, 22, 23, 24].
Figure 1.
Scheme of the photosynthesis process.
2.1 Thermochemical conversion
Thermochemical conversion is a technique that occurs at high temperatures to decompose biomass into energy and valuable products such as biofuels, syngas, and biochar. Syngas is a mixture of primarily CO and H2 gas components with varying amounts of CO2 and H2O. The syngas converts to other products in one of a variety of catalytic, pressurized processes to produce base chemicals, transportation fuels, or finished chemicals. Several typical applications for syngas in the industry are water-gas shift, catalytic synthesis, fermentation, etc. By utilizing high temperatures, thermochemical conversion can transform organic materials into various energy-rich forms, which makes thermochemical conversion one of the most efficient pathways for extracting energy from biomass. This approach is particularly effective for large-scale applications, including power generation, heat production, and the creation of industrial feedstocks. The three main thermochemical conversion processes are categorized as combustion, gasification, and pyrolysis, which are determined by the temperature range for each stage. Figure 2 indicates that at 0–250°C, moisture in biomass will evaporate. As the temperature goes up to 250–500°C (sometimes can be up to 700°C), pyrolysis (or devolatilization) becomes the dominant stage, which consists of involves many complex reactions and product gases. Combustion and gasification require similar reacting temperatures (typically 800–1100°C), while the key differences are the degree of oxidation and heating rate [25].
Figure 2.
Temperature zones of drying, pyrolysis, combustion, and gasification (reduction) stages [25].
Direct combustion is the most common and simplest foundation approach, where biomass is burned in the presence of oxygen to generate heat, which can then be used directly, or converted into electricity. This process is widely used in biomass power plants and combined heat and power (CHP) systems to produce renewable energy while mitigating carbon emissions. The operation procedure is achieved by rapid oxidation reactions of fuel (biomass raw materials) and oxygen to obtain thermal energy and flue gas, consisting mainly of CO2 and H2O. Several disadvantages hinder the promising application of direction combustion for biomass energy utilization. For instance, high-moisture amounts in the biomass raw materials, agglomeration, and fly-ash deposition due to alkali compounds in biomass, along with the high cost of transporting bulky biomass are currently the most challenging limitations for direction combustion technology [9, 12].
Gasification is the conversion of biomass at elevated temperatures in a limited-oxygen environment to produce syngas, a mixture of carbon monoxide, hydrogen, methane, nitrogen, carbon dioxide, and some hydrocarbons. Syngas can be used directly in gas turbines for electric power generation or be further processed to create synthetic fuels using catalysts or microorganisms (syngas fermentation). The carbonaceous biomass solids or liquids can be gasified into smaller and lower molecular-weight gas mixtures, similar to coal gasification. Coal gasification has been under development for about 200 years, beginning with the aim of providing heating and lighting. Coal gasification has a long and rich history of research in the modern Western world, but its track record of successful operation is much shorter and less impressive. However, countries such as Nazi Germany and Apartheid-era South Africa, which were cut off from petroleum supplies, as well as nations with limited access to oil like China and India, have effectively used coal gasifiers to make significant contributions to their fuel and chemical supplies. These operational processes, along with a few successful government-led gasification demonstrations, make up the current fleet of gasifiers in current operation. Due to the high volatile content in biomass raw materials, biomass gasification can occur more readily than that of coal. This technology possesses higher energy efficiency and flexibility than direct combustion, and produces fewer emissions, making it an attractive option for sustainable energy production, hydrogen production, and synthesis of fuels and high-value chemicals. As petroleum and natural gas become more expensive with reduced reservations in the world, biomass gasification represents promising applications in the near future [1, 9]. The gasification stage in these applications accounts for more than 75% of the total capital cost and poses an even higher proportion of the technical risk. Gasifiers pose significant operational and design difficulties, and even when fully implemented, they tend to be the most problematic and inelegant portions of the overall process [26, 27]. In addition, sizeable energy input, tar production, and significant energy loss especially in smaller-scale operations and plants are the prominent factors that make biomass gasification less ideal in some cases, though ongoing technological advancements aim to address these issues.
Pyrolysis takes place in the absence or very limited presence of oxygen and breaks down biomass into mainly bio-oil (liquid), biochar (carbo-rich solid), and syngas (gas) at moderately high temperatures (250–700°C).
Pyrolysis is a highly versatile process, with bio-oil serving as a precursor for biofuels, biochar used to enhance soil, and syngas providing an energy source. This decomposition process enables the production of a wide variety of products, making it increasingly popular in the development of biorefineries. There are three main types of biomass pyrolysis technologies: slow, fast, and flash pyrolysis. Slow pyrolysis involves gradually heating biomass over several hours, resulting in a higher yield of biochar, which is valuable for soil improvement and carbon sequestration, but it produces less bio-oil and syngas compared to other methods. Fast pyrolysis, on the other hand, rapidly heats biomass in under a minute, yielding up to 75% bio-oil, which can be refined into liquid fuels or chemicals, with smaller amounts of syngas and biochar. Flash pyrolysis heats biomass extremely quickly (within seconds) and is designed to maximize bio-oil production, producing very little biochar [28, 29]. Pyrolysis is applicable to a wide variety of biomass feedstocks, including wood, agricultural waste, and organic residues, leading to a high-value production process. Additionally, biochar from pyrolysis holds great potential for use in carbon sequestration [30].
According to previous studies, the pyrolysis process involves numerous complex reactions and various gas products. Gases such as CH4, C2H2, CO, NO, CO2, C6H6, and C7H8 are monitored. Analyzing CO is challenging with GC-MS because CO and N2 share the same molecular weight and mass-to-charge ratio (m/z). To address this, argon is used as the purge gas to create an environment nearly free of N2. Figure 3 presents sample data, showing gas species during pyrolysis of a 3/8-inch poplar particle in 50% CO2 and balanced argon at 1150°C. The data indicates that H2O is released first, followed quickly by benzene and toluene, then C2H2, CH4, and NO. The sharp decline at the end of the process reflects limitations in sampling heavy gases, not a sudden end to pyrolysis. As a matter of fact, pyrolysis gradually slows, with heavier compounds contributing more to mass loss in the form of tar [1].
Figure 3.
Compositions and concentrations of pyrolysis gases for 3/8-inch poplar wood in 50% CO2 and balanced argon at 1150°C [1].
Biomass pyrolysis is still facing quite several technical challenges. Sizeable energy input, high initial capital cost, process scaling-up limitations, and tar formation issues are associated with biomass pyrolysis. Moreover, the market for pyrolysis byproducts such as biochar, syngas, and bio-oil is still developing, and demand can be limited. This can affect the economic viability of the process, especially if the market for these products does not grow as anticipated [31, 32].
Despite all the technical challenges and issues, biomass pyrolysis is employed in producing biofuels, managing waste, improving soil quality through biochar, and generating renewable energy. It plays a crucial role in advancing sustainable, circular bio-economies by providing alternatives to fossil fuels and lowering carbon emissions.
Thermochemical conversion is a vital component of the renewable energy sector, offering scalable and efficient ways to transform biomass into valuable energy and materials. As technology continues to evolve, this conversion process has the potential to significantly reduce reliance on fossil fuels and contribute to sustainable energy systems.
2.2 Biochemical conversion
Biochemical conversion of biomass involves using biological agents, such as microorganisms and enzymes, to break down organic materials into biofuels or other valuable products. This process typically employs pathways such as anaerobic digestion and fermentation [8, 33, 34]. Both biochemical and thermochemical methods are effective for energy recovery from biomass, as they utilize the entire biomass without needing to isolate specific macromolecules. However, biochemical conversion offers distinct advantages, particularly its ability to handle wet biomass, operate under ambient temperature and pressure, and exhibit greater selectivity for desired products, making it a more sustainable and environmentally friendly option compared to thermochemical processes. Unlike thermochemical conversion, which uses heat, biochemical conversion relies on biological agents. Consequently, the operating condition of biochemical conversion is much more moderate. Common processes of biochemical conversion include anaerobic digestion and fermentation [35]. These processes can convert biomass into various useful products such as bioethanol, biogas, hydrogen, and other chemicals. For instance, anaerobic digestion is commonly used to generate methane-rich biogas, a renewable energy source derived from organic waste [36]. On the other hand, fermentation is primarily applied to convert sugars into bioethanol, a widely used biofuel [37, 38]. Both methods play a critical role in producing sustainable energy from organic materials, helping reduce dependency on fossil fuels and contributing to a more environmentally friendly energy system (Figure 4).
Figure 4.
Scheme of hydrothermal and biochemical routes in biomass utilization from a circular economy perspective [39].
2.3 Anaerobic digestion
Anaerobic digestion is a process in which microorganisms break down organic material, such as agricultural waste, food waste, or animal manure, in the absence of oxygen. The main product of this process is biogas, primarily composed of methane (CH4) and carbon dioxide (CO2). This biogas can be used as a renewable energy source for electricity, heating, or even as vehicle fuel [40, 41].
Anaerobic digestion consists of four key stages, catalyzed by different microorganisms, whereas hydrolysis is considered as the rate-determining step [39, 42]:
Hydrolysis: Complex organic compounds, such as carbohydrates, fats, and proteins, are broken down into simpler molecules such as sugars, amino acids, and fatty acids. This step is often the slowest and determines the overall rate of digestion [42, 43].
Acidogenesis: Fermentative bacteria convert the products of hydrolysis into intermediate compounds, such as volatile fatty acids, alcohols, and other organic molecules [44].
Acetogenesis: Homoacetogenic bacteria convert volatile fatty acids and alcohols into acetate, hydrogen, and carbon dioxide [45, 46, 47].
Methanogenesis: Methanogenic bacteria convert acetate, hydrogen, and carbon dioxide into methane and additional carbon dioxide, to generate biogas [48, 49].
Anaerobic digestion is extensively used in waste management and renewable energy production because it reduces waste volume and produces useful byproducts such as biogas and nutrient-rich digestate that can be used as fertilizer.
2.4 Fermentation
Fermentation is a process where microorganisms, such as bacteria or yeast, break down organic materials, typically sugars, into simpler compounds, usually in the absence of oxygen. This process provides energy to the microorganisms and produces byproducts such as alcohol, gases (such as carbon dioxide), or acids, depending on the type of fermentation involved [50, 51]. Fermentation is generally categorized into several types, including alcoholic, lactic acid, and acetic acid fermentation, based on the microorganisms involved, the end products, and the conditions under which the process occurs [39, 52].
Alcoholic fermentation is the technique for yeasts convert sugars (such as glucose) into ethanol (alcohol) and carbon dioxide. This is the process used in brewing beer, making wine, and producing bioethanol from biomass [53, 54]. Reaction is:
Lactic acid fermentation utilizes certain bacteria (e.g., Lactobacillus) to convert sugars into lactic acid. This process occurs in yogurt production and in human muscles during intense exercise [55, 56]. Reaction is:
Acetic acid fermentation is the process in which acetic acid bacteria (e.g., Acetobacter) convert ethanol into acetic acid (vinegar production) in the existence of oxygen [57]. Reaction is:
Fermentation is a key process in the production of bioethanol, which serves as an important renewable fuel. In addition, fermentation technique has played a vital role in food and beverage industry, and pharmaceutical production. This technology provides sustainable and versatile solutions to food production, energy crisis and environmental waste pollution. Despite the importance of the biochemical conversion offers in the production of biofuels, food and pharmaceutical production, renewable energy, waste reduction, and many other fields, there are still a lot of difficulties and challenges, which are preventing it from being commercialized in a large-scale. One major limitation of biochemical conversion is its inability to efficiently process biomass with high hydraulic retention times, as well as the emission of greenhouse gases during the process. Additionally, biomass requires pre-treatment before undergoing biochemical conversion, adding complexity to the system. During fermentation, the biomass must also undergo enzymatic hydrolysis to break down polysaccharides into monosaccharides. Furthermore, a significant gap exists between proven fermentation technologies and their implementation in large-scale biorefineries, with technical and economic barriers, such as selecting the right conversion process and cost considerations, posing obstacles [34, 58, 59, 60].
2.5 Physiochemical conversion
Physicochemical conversion of biomass refers to a set of integrated processes that utilize both physical and chemical methods to transform organic materials (such as agricultural residues, wood chips, or food waste) into valuable products such as biofuels, chemicals, or energy. This conversion approach blends mechanical, thermal, and chemical techniques, optimizing the breakdown of biomass into simpler components that can be more efficiently used for energy production or further refined into other materials. The methods are distinct from purely biochemical or thermochemical approaches because they integrate multiple stages of treatment to enhance the breakdown of biomass, optimize the efficiency of subsequent energy extraction processes, and maximize product yields [61, 62, 63].
Physicochemical characterization of the biomass properties are essential for this technique. In the physicochemical conversion of biomass, physical methods such as mechanical size reduction and mixing/agitation play a crucial role in preparing the feedstock for subsequent chemical reactions. These physical methods aim to enhance the efficiency of the overall conversion process by increasing the surface area of the biomass and ensuring uniform exposure to chemicals, heat, or biological agents. Mechanical size reduction involves breaking down biomass into smaller, more manageable particles. This can be achieved through techniques such as cutting, grinding, milling, or shredding, depending on the type and hardness of the biomass material. Mixing and agitation are essential steps in physicochemical conversion, particularly in liquid or slurry-based processes. These steps ensure the homogeneous distribution of the biomass particles, solvents, or reactants, which improves the overall reaction efficiency. The physical methods can facilitate handling, decrease the energy cost, and support uniform heat and mass transfer in the chemical reaction process [64].
Chemical techniques, including catalysis, solvent extraction, and super-fluid technology are employed to convert biomass to the desired products. Chemical catalysts are often used to accelerate reactions during physicochemical conversion, such as in biodiesel production where a catalyst is added to oils or fats to enhance the transesterification process. Besides, biomass is treated with solvents to dissolve specific components, such as lignin or cellulose, and extract valuable chemicals. The solvents reduce the energy required for further breakdown of the biomass. In some processes, supercritical fluids (e.g., supercritical CO2) are used to dissolve and extract materials from biomass. These fluids exhibit both liquid and gas properties under specific conditions, allowing for more efficient separation of biomass components [65, 66].
The major processes applied in physicochemical conversion technologies are:
Liquefaction: Biomass is subjected to moderate temperatures (250–350°C) and high pressure in the presence of solvents such as water or alcohols. The goal is to convert solid biomass into a liquid bio-oil, which can be refined into fuels or chemicals. Liquefaction operates at milder conditions than pyrolysis and results in higher liquid yields. This technique can process a wide range of biomass types, including wet feedstocks, and produces bio-oil that is easier to upgrade into fuel than raw biomass [9, 67, 68].
Supercritical fluid extraction: This method employs supercritical fluids to extract valuable substances from biomass. For instance, supercritical CO2 can be used to extract oils from biomass, which can then be processed into biodiesel. Supercritical fluid extraction is highly selective and operates at moderate temperatures, reducing the degradation of sensitive compounds. It is also environmentally friendly because it often uses non-toxic solvents like CO2 [6, 69, 70].
Transesterification: In the production of biodiesel, oils or fats from biomass (such as vegetable oils) are chemically reacted with alcohols (usually methanol or ethanol) in the presence of a catalyst (such as sodium hydroxide). This technique results in biodiesel and glycerol as byproducts. Transesterification is a relatively simple and well-established process, scalable for industrial biodiesel production. It also yields high-quality fuel that can replace conventional diesel in many applications [71, 72].
Solvolysis: Biomass is dissolved in solvents, often organic, to break down its complex structure (like lignocellulose) into simpler chemical building blocks. This process helps separate lignin from cellulose and hemicellulose, allowing for the production of biofuels, chemicals, or bio-based materials. This method can be tailored to specific biomass types and the desired end-products and can work well with complex feedstocks [73, 74].
By integrating physical and chemical techniques, these processes can achieve higher yields and greater versatility than many traditional conversion methods, although their economic viability and energy efficiency continue to present challenges for large-scale deployment. However, every coin has two sides. The physicochemical processes, particularly those utilizing supercritical fluids or operating at high-pressure/temperature conditions, require substantial energy input, which can ultimately offset the environmental benefits of biomass conversion. Furthermore, the need for pre-treatment (physical and chemical methods) in most physicochemical processes to enhance the behavior of chemical agents, and increase the overall efficiency is adding complexity to the operational design, and thus leading to a high cost for the process [3, 39, 75].
3. Conclusions
Biomass conversion technologies are a vital component of the global transition to renewable energy and a more sustainable future. By harnessing the energy and material potential of organic matter, these technologies offer pathways to produce biofuels, bioenergy, and bioproducts that can significantly reduce reliance on fossil fuels, mitigate greenhouse gas emissions, and promote a circular economy.
Thermochemical, biochemical, and physicochemical conversion processes each provide unique advantages and challenges, with the choice of technology depending on factors such as feedstock type, desired end-products, and scalability. Thermochemical methods like combustion, gasification, and pyrolysis are highly efficient for large-scale energy production, while biochemical processes such as anaerobic digestion and fermentation are more suited to producing biofuels like biogas and ethanol from biological sources. Physicochemical approaches like transesterification are valuable for converting oils and fats into biodiesel, offering a cleaner alternative to traditional fossil fuels.
Despite their potential, biomass conversion technologies face technical, economic, and environmental challenges that must be addressed to achieve widespread adoption. Feedstock variability, process inefficiencies, and competition with conventional energy sources remain key hurdles. However, continued research and development, along with supportive policies and investments, are paving the way for more efficient, cost-effective, and environmentally sustainable solutions. To overcome the barriers for the development of biomass conversion technology, governments worldwide are implementing a variety of regulations, incentives, and frameworks to promote the use of biomass as a renewable resource. These include subsidies for biomass power plants, tax credits for biofuel producers, and grants for research and development in advanced biomass conversion technologies. Many countries and regions have established renewable energy mandates and targets that deem biomass as a key contributor, which is to push the adoption of biomass-based energy generation and biofuels. All of these policies focus on encouraging investment, ensuring environmental sustainability, and integrating biomass into the broader energy mix to meet climate and energy goals [76, 77, 78, 79, 80].
In the future, advancements in biomass conversion technologies hold immense promise. Next-generation processes, integrated biorefineries, and innovative approaches to bio-based products can unlock even greater opportunities for reducing waste, enhancing energy security, and driving economic growth. As these technologies mature, they will play a crucial role in building a cleaner, greener, and more resilient energy system that supports both human well-being and environmental preservation.
Acknowledgments
The authors acknowledges the use of ChatGPT for language polishing of the manuscript.
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