Pyrolysis technologies for biochar production in waste management: a review

Abstract

Pyrolysis is a thermal conversion process in the absence of air to derive energy components from the residues. Renewable-energy technologies will play a major role in addressing future challenges related to environmental safety and energy security. One of the many easily available renewable energy sources is biomass—an organic material that is thought to be carbon-neutral. Pyrolysis technology is a thermochemical process that can be used to produce useful products from biomass, such as biochar, bio-oil and combustible pyrolysis gases. The structure and relative product yield are impacted by the pyrolysis method employed. This article evaluates different approaches for biomass pyrolysis. Fast, slow and advanced pyrolysis methods using various pyrolyser reactors have been studied in the literature and are provided to increase the variety and use of these methods in upcoming studies and research. Slow pyrolysis can lead to increased ecological well-being, as it increases the amount of biochar produced using auger and rotary-kiln reactors. Rapid pyrolysis, mainly in fluidized-bed reactors with bubbling and rotating circulation, can be used to obtain bio-oil. Advanced pyrolysis methods offer a good probability of yielding great prosperity for specific applications. The selection of a pyrolysis process is based on the required output in terms of solid, liquid and gaseous fuels, and the parameter plays a crucial role in the pyrolysis performance.

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Introduction

A country’s sustainable, affordable clean energy supply is essential for better economy, ecosystems and society, which will remain pioneering in the twenty-first century [1]. Also, the greenhouse effect, which has resulted in Earth’s atmosphere being warmer, has been brought on by an increase in CO₂ emissions over recent decades as well as several other environmental contaminants [2]. The energy generated by fossil fuels has sustainability challenges in addition to carbon emissions and ecological problems. International organizations have worked hard to find solutions to this problem by adopting renewable and carbon-neutral energy sources to fulfil energy demands without harming the environment [3]. In this situation, the implementation of legislation to reduce carbon emissions and the concentration on green and renewable-energy technology are important [4]. Biomass is mostly abundant and has fewer emission bioenergy materials among the renewable-energy methods. Biomass operates with the effect of a carbon preoccupation process and releases the same quantity of CO₂ as is used during photosynthesis for plant growth [5]. Therefore, by removing the elevated CO₂ concentrations and landfill methane emissions, biomass is a carbon-neutral fuel. As a result, dumping and landfilling will gradually decline. Biomass provides obvious environmental benefits and lessens the threats to natural ecosystems that are posed by diverse carbon emissions [6]. The World Energy Assessment study estimates that biomass accounts for ~65% of all renewable energy sources. This will supply 11–12% of the total energy used in the world. Other benefits of biomass include its consistent supply of renewable energy, extremely low sulphur content, affordability of biomass materials used as a substrate and mass production of syngas as a way for numerous polygenesis uses [7].

A variety of renewable resources make up biomass, including wasted coffee grounds, municipal solid waste, forestry and wood residues, crop residues and other energy crops for the production of biofuels and bio-hydrogen [8]. Using various types of biomass materials for energy generation satisfies major objectives such as fuel security as well as a decrease in CO₂ emissions. When bonds are broken, chemical energy from the Sun is taken and stored in biomass [9]. Thermochemical and biochemical conversion techniques can be used to extract energy from biomass and turn it into biofuels [10, 11]. Biochemical conversion entails anaerobic digestion and fermentation to convert biomass into flammable liquids and gas fuels [12]. In recent times, membrane-integration methods have come to light as a highly possible substitute for recovering and separating biofuels, biogas and bio-hydrogen as well as waste gases and gases produced during the processing of biomass [13, 14]. Biomass materials can be transformed into liquid fuel, gaseous by-products and bio-oil by the well-established thermochemical process known as pyrolysis [10]. There are three types of pyrolysis: slow, rapid and flash. Each method of pyrolysis produces different compounds with unique compositions [15].

By using thermal energy to convert biomass materials into different by-products, such as charcoal, syngas and pyro-oils, pyrolysis takes place in an inert environment [16]. The liquid fuel is a blend of many chemical molecules that have had their oxygen removed. Depending on the various operational parameters, including the heating rate, burning temperature, residence duration and particle size of the biomass, a variety of products are created [17]. The composition of the finished goods also depends on the concentrations of lignin, cellulose and hemicellulose, which are polymers mainly found in biomass [18]. The pyrolysis of biomass occurs at relatively lower temperatures (400–600°C) than thermochemical conversion processes such as combustion and gasification, and is often preferred because the pyrolysis outputs, namely char and pyrofuels, are simple to store and transport [19]. The pyrolysis of various biomass materials—more recently, electronic waste items such as electronic scrap components—has been the subject of extensive investigation. When compared with those other thermochemical conversion methods, pyrolysis has several benefits, including its being a much easier and less expensive conversion process [20]. A larger range of feedstock can be used for pyrolysis. Both the need for landfills and greenhouse gas (GHG) emissions are decreased. It has a small potential to pollute water. Building a pyrolysis reactor is a fairly quick process [21, 22]. The pyrolysis mechanisms of three biopolymers in biomass materials were discussed in detail by Wang et al. [23], who also emphasized the complexity of the structures involved. To identify the technology gaps and consider fresh possibilities for merging models of biomass pyrolysis of various sizes, Surya et al. [24] undertook a critical study of pyrolysis modelling. A thorough analysis of the pyrolysis product characteristics and pyrolysis parameter impacts was reported by Melati [25]. They claimed that the primary determining factors impacting the pyrolysis quality and yield are the temperature and heating rate. A review on comprehending the non-catalytic and catalytic chemistry of the pyrolysis process was written by Dai et al. [26].

They discussed current developments in the process of biomass catalytic pyrolysis using zeolites and metal oxides for the production of high-valued hydrocarbons, phenol compounds and anhydrous sugars along with chemicals containing nitrogen [27]. The main element in the biomass pyrolysis process that transforms biomass into useful products is the pyrolysis reactor. The process of biomass pyrolysis is covered in many review papers; however, the authors identified few pieces of research that covered the full range of biomass pyrolysers [28]. In general, experimental and modelling research was reported in the majority of review papers on biomass pyrolysis. Only a few papers described how the items were characterized (pyro-oil and biochar). Additionally, many review papers on the various catalysts utilized in experimental reactions, the upgrading of products and the pyrolysis process parameters are provided in Fig. 1. Garcia-Nunez [29] presented research on various pyrolysis reactors for biomass materials, but these review papers gave a historical overview of the pyrolysis technology.

Where appropriate, specific subjects that have been adequately represented in prior review papers were abridged or the relevant various review papers were cited. This review paper offers a thorough analysis of pyrolysis technology for biomass materials, covering the choice of various biomass materials, handling them, selection of the necessary biomass pyrolysis procedure and pyrolysis in an appropriate reactor [25]. This review paper thoroughly covers pyrolysis technology for biomass materials, including all the steps from choosing the feedstock to creating the finished product. Particular attention is placed on biomass pyrolysis reactors. The studies on pyrolysis processes for biomass by considering the various pyrolysis reactors and cutting-edge biomass pyrolysers are highlighted in this present review paper. Researchers in this discipline will be very interested in this review article. This present review study will also aid in the investigation and improvement of pyrolysis procedures for biomass. This review study also emphasizes novel pyrolysis conversion technologies and various pyrolysis reactors that improve the capacity of the pyrolysis process to recycle waste to produce energy. The review paper makes a substantial contribution to the research field by examining the fast, slow and advanced pyrolysis procedures. It also suggests the prospects of pyrolysis in terms of insulation, recirculation and indirect heating.

1 Methodology

The analysis of publications was based on a search for documents using specific terms that were entered into Scopus, Google Scholar and other databases. The themes of this paper were chosen after a study of the primary applications investigated for various biomass. Articles were initially looked for that addressed the terms ‘biomass’ and ‘pyrolysis’ in the title, abstract and keywords. The search was conducted from 2015 to 2023.

The results of the initial Scopus and Google searches were analysed to separate duplicate content, reclassify some articles using the most appropriate approach given their content and, perhaps most importantly, omit articles that did not fall under the purview of this paper. A flowchart that depicts the approach of the study is shown in Fig. 2.

2 Conversion mechanisms of biomass by using a pyrolysis process

The matrix of the biomass residue polymers undergoes chemical modifications because of heating. The biomass consists of volatiles, ash, cellulose, hemicellulose and lignin content, which act as an energy source to produce renewable fuels. Based on the degradation of bio-compounds, energy production will occur, and it also depends on the conversion process used. The different biomass classifications are agricultural crop residues, forest biomass, industry, municipal solid waste, etc. Volatile chemicals are released because of heating and rearrangement processes [30]. Some unstable, volatile chemicals are subsequently transformed after these initial reactions. As a result, there are two types of biomass conversion reactions: primary and secondary reactions, which are shown in Fig. 3.

2.1 Primary conversion mechanisms of pyrolysis

The main components of biomass are lignin, cellulose and hemicellulose, which are collectively known as biopolymers. The primary properties of these compounds are displayed by their conversion reaction. The following three paths can be used to describe the different methods for releasing chemical bonds.

2.1.1 Char formation mechanism

Char is the solid by-product of the thermal heating of biomass in the absence of air. The char has an aromatic polycyclic structure. Higher thermal ability and quality of residue are produced as a result of the inter- and intramolecular reconfiguration of the molecular structure, which dominates the approach [31]. The primary mechanism in this route is the synthesizing of the benzene rings and the later clustering of those rings. Non-condensable gases along with moisture are released from the biomass component as a result of these rearrangements.

2.1.2 Depolymerization mechanism

Lignin, cellulose and hemicellulose are examples of biopolymers that break down into smaller molecules known as monomers as part of the depolymerization phenomena in this route [32]. When the molecules generated are volatile, the level of polymerization is lowered. There is a liquid percentage of the molecules that condense at room temperature.

2.1.3 Fragmentation mechanism

Polymer covalent bonds are required for fragmentation. Additionally, the constituent monomers of the polymer have links between them. Many non-condensable gas components and other simple chained organic molecules that condense at room temperature are released by this mechanism [33].

2.2 Secondary conversion mechanisms of pyrolysis

In the secondary conversion mechanism—the method for converting biomass—the range of the reactor temperature is crucial. When volatile, unstable molecules are released from the reactor, they may experience additional secondary reactions such as cracking or recombination [34]. The disintegration of volatile substances into substances with a lower molecular weight is known as cracking. Because the same chemical bonds might be broken inside the polymer or the volatile chemicals, the products generated via cracking and fragmentation share some similarities. In contrast to cracking, recombination or recondensation results in the synthesis of molecules with larger molecular weights. In the reactor settings, the newly produced chemicals are primarily non-volatile [35]. Additionally, this route produces more secondary char.

2.3 Pyrolysis principle and product distribution of different pyrolysis processes

Biomass feedstock that degrades thermally without oxygen is called pyrolysis. This phenomenon is the result of several intricate reactions occurring in the reaction zone. Heating the biomass material releases its volatile biomolecules, which are subsequently condensed into bio-oil. The biomass is heated past its thermal stability limit by the inert environment, producing more steady by-products and solid outputs [36]. The primary benefit of the inert condition is the non-combustion heating of biomass sources [37]. Biomass components are divided into dicarboxylic, dehydrate and dehydrogenate groups during the volatile removal process. The secondary pyrolysis breaks down the heavy chemicals into char and pyro gases, and the primary conversion process takes place (CH₂, CO₂, CO and CH₄). The volatile vapours are then collected to create bio-oil. The circumstances during the overall pyrolysis process, such as temperature, residence time and heating rate, determine the formation of these solid as well as liquid outputs [38]. The general reaction of the pyrolysis process is as follows:

The first component (H₂ + CO + CH₄ + …. + C₅H₁₂) of the product side is a composition of distinct combustible gas known as syngas, followed by a blend of various liquid fuels that produce bio-oil and a solid output (H₂O + CH₃OH + CH₃COOH +...) (char) [39]. The distribution of the product is considerably impacted by pyrolysis processes, depending on their operating conditions. The temperature, heating rate, residence time, size of particle and pressure all play a role in how the product is distributed. These operational factors determine the different pyrolysis processes. The conditions for the pyrolysis process are included in Table 1. Some of the work carried out by the researchers is also discussed.

Slow pyrolysis has a low heating rate of (0.1–1°C/s), the residence period is long (5–30 min) and the temperature is moderate (350–500°C) [40]. The primary by-product of the slow pyrolysis of biomass is charcoal or char. The pyrolysis conversion reaction tends to produce the most solid product char under these circumstances. On the other hand, although in much lower amounts, pyro-oil and syngas are also obtained. The longer residence time and slower heating rate in the slow pyrolysis of biomass encourage the secondary processes to complete the mechanism for a more solid product [41]. A longer vapour residence time enables the removal of vapours generated at the time of secondary reactions, increasing the yield of char.

The fast pyrolysis has a high operating temperature range of 800–1250°C, the heating rate is faster (10–200°C/s) and the residence period is incredibly short (1–5 s) [42]. The biomass pyrolysis reaction process is more likely to produce liquid fuel under these circumstances. There is 10–25% solid biochar, 10–20% non-condensable syngas and 65–75% of bio-oil as the secondary products of the rapid pyrolysis process [19]. The objective is to raise the temperature above which char production is not preferred to decomposition. Before the biomass material can turn into biochar, the extremely rapid heating rate transforms it into condensable vapours. The bio-oil generated has a higher heating value that is half that of crude oil components [43].

Flash pyrolysis of biomass is an advanced process of fast pyrolysis. The exceptionally high heating rate of 1000°C/s is what sets it apart from the quick pyrolysis method. The working temperature is maintained at between 900°C and 1200°C, and the biomass materials only spend comparatively short periods in these settings (0.1–1 s) [44]. When observed with rapid pyrolysis, flash pyrolysis produces much less solid and gaseous waste and a higher yield of bio-oil (>75%). Industrial-scale applications are additionally hampered, which means that progress is still needed for the proper implementation of flash pyrolysis working conditions. Still, research is ongoing to enhance the utilization of flash pyrolysis at the industrial level.

To achieve steadiness between the outputs of liquid, solid and gaseous products, the intermediate pyrolysis method is used so that evenness may be achieved in the ratios within the product output. The pyrolysis operating conditions are kept between the slow and fast pyrolysis processes; 500–650°C, 0.1–10°C/s and 300–1000 s of residence time are the typical intermediate pyrolysis conditions. Fast pyrolysis with 40–60% of bio-oil production, 15–25% of biochar and 20–30% of non-combustible gases is highly prevalent in intermediate pyrolysis [45]. The fact that the pyro-oil created by the intermediate pyrolysis process has less tar content and can be used straightaway for thermal energy generation is a benefit.

The vacuum pyrolysis process is a slow pyrolysis process (low-pressure range: 0.05–0.20 MPa) for the transformation of biomass materials while maintaining all other parameters necessary for the process [46]. The method for removing vapours from the reaction area, however, is what distinguishes the two procedures. As a substitute for using an inert gas, which would be primarily used in other biomass pyrolysis procedures, a vacuum is used to remove the vapour. The biomass components disintegrate at substantially lower temperatures while under vacuum or low pressure. A higher output of bio-oil is possible due to the quick removal of fumes by the primary pyrolysis mechanism. Vacuum pyrolysis increases the bio-oil output and the resulting biochar has a high porosity.

Hydro pyrolysis is an emerging technology that can turn a biomass feedstock into high-quality bio-oil that includes biomass materials and has proceeded at hoisted pressures of 5–20 MPa (H₂ or hydrogen-based material); the temperature range is the same as that for fast pyrolysis [47]. High pressures and temperatures with hydrogen present decrease the amount of oxygen produced in the pyro-oil and prevent the production of solid char.

3 Pyrolysis reactors

3.1 Slow pyrolysis

The primary outcome of this slow pyrolysis process is biochar, commonly referred to as charcoal. The slow pyrolysis of biomass resources uses heating rates that are roughly between 0.1 and 0.8°C/s [48]. The residence period inside the biomass slow pyrolysis operation is maintained for a longer time compared with the quick pyrolysis methods. Most pyrolysis reactors have an approximate residence time of 5–30 min, or occasionally 25–35 h, at between 300°C and 550°C [49]. The yield of biochar and bio-oil depends on the biomass material, operation temperatures, heating rate and pyrolysis circumstances. Whether pyrolysis takes place in the presence of nitrogen (N₂) or carbon-dioxide (CO₂), or even in materials with changed beds, depends on the pyrolysis environment [40]. Fewer minerals are present in woody biomass than in herbaceous biomass, but it also yields more products. The yield of biochar reduces as the pyrolysis temperature rises. This is due to the burning of organic materials at intense heat, which also results in the decomposition of cellulose and hemicellulose components [50]. The several biomass pyrolysis technologies are the fixed-bed reactor, auger reactor, rotary-kiln reactor and catalytic slow pyrolysis of biomass. The recent evaluation of slow pyrolysis is described in Table 2.

3.1.1 Fixed-bed reactor slow pyrolysis

These reactors are useful for biomass pyrolysis due to their various special qualities. As a result, they are often employed by scientists and businesses for the biomass slow pyrolysis that aims to produce a solid product output called biochar [59]. A working model of fixed-bed reactor slow pyrolysis is shown in Fig. 4.

To improve output product yield and adapt or change the process working parameters, fixed reactor beds are constructed from solid impetus. As a result, their use in the slow pyrolysis of biomass is crucial [61]. Fixed-bed reactors have been employed by numerous researchers for the biomass slow pyrolysis process.

3.1.2 Auger reactor slow pyrolysis

A growing number of small and medium-sized businesses are interested in using auger reactors to pyrolyse lignocellulosic biomass sources [62]. It is a popular technology because of its straightforward design and functionality. The biomass material is continuously fed into the entrance of a screw-type feeder, thus providing the materials to the pyrolysis cylinder heating zone along the axis rotation, as part of the working mechanism of this auger reactor [63]. Depending on several variables, including the rate of feeding and the required size of feed particles for the heating chamber, the system may use a single or a dual screw feeder [64]. Following the breakdown of the lignocellulosic materials, solid output called biochar is collected at the lower side of the reactor, as shown in Fig. 5. Other volatilities, such as pyrolysis gases, also exit the reactor [65].

The system has three benefits: (i) easy operation, (ii) no need for an inert atmosphere and (iii) minimal usage of energy. Controlling the residence duration of the biomass in the heating chamber by varying the speed of rotation of the screw feeder is also a significant benefit [66]. The auger reactor typically yields bio-oil at a rate of 40–60% of the biomass materials. The yield was often lower than that attained in fluidized-bed-type pyrolysis reactors and is largely dependent on the operating parameters [67].

3.1.3 Rotary-kiln reactor slow pyrolysis

The preferred reactor design for the thermal processing of particulate particles is a rotary kiln [68]. Significant studies have been undertaken to assess how well they work with lignocellulosic biomass feedstock. Because of their numerous special advantages, these types of pyrolysers are preferred in place of many other types of reactors. The gentle rotation of the inclined rotary-kiln-type pyrolyser can be used to produce specific qualities of goods made from biomass feedstock [69]. A working diagram is shown in Fig. 6.

It is possible to regulate how long the feedstock stays in the kiln chamber and to make the necessary changes for smooth operation. Additionally, these kilns permit the use of biomass that may be fed constantly or in equal batches and come in a variety of sizes, forms and calorific values [71]. Additionally, these pyro reactors are less responsive to the type of fuel source. As a result, these may process a wide variety of biomass without any pretreatment.

3.1.4 Summary of the slow pyrolysis process

By reducing the amount of oxygen in its chemical make-up, a catalyst is used in the fast pyrolysis of biomass to boost pyro-oil yield and change its content [72, 73]. To boost the yield of biochar, which is the major by-product of slow pyrolysis, biomass is subjected to catalytic slow pyrolysis. As a result, it is crucial to use catalysts in slow pyrolysis to regulate the content and quality of the biochar [47]. The process uses catalysts for some benefits, which include (i) a low temperature for pyrolysis, (ii) increased physical and chemical stability and (iii) a high yield of the desired products [74]. The research community is considering additional study of a catalytic biomass slow pyrolysis that has been used in industries to produce biochar. Because of the homogeneous structure of their pores as well as adequate acidity, catalysts are recognized as efficient bio-oil upgraders, as they demonstrate considerable resilience to deactivation [75]. According to numerous pieces of research, catalysts are essential for producing high-quality product recovery. Plastic catalytic pyrolysis is a promising technique that needs particular reactor set-ups and operating environments.

3.2 Fast pyrolysis

In this pyrolysis method, the lignocellulosic material decomposes very quickly, even without air. When the biomass material is allowed to undergo a quick pyrolysis process, the principal by-products are primarily vapours and aerosols, with minor amounts of char and gas. After being cooled along with condensing the vapours, a dark brown mobile liquid is produced [76]. That liquid has a calorific value that is roughly equal to half that of fossil fuel. The rapid pyrolysis process has some of the following crucial traits. The phenomena occur under conditions of intense heat and rapid heat transfer. Consequently, biomass materials must be extremely tiny. In the vapour phase, the regulated range of temperatures is 550–650°C. There are vapour residence times of only 2 s. The bio-oil is obtained from the instantaneous cooling of vapours. The recent advances in fast pyrolysis are discussed in Table 3.

3.2.1 Bubbling fluidized-bed reactor fast pyrolysis

The phenomenon known as fluidization involves fine particulates coming into close proximity with a gas or liquid component and changing into a fluid-like condition [86]. The fluidization is caused by the upward pull of fluid by the gas on the solid matter. The fluidized bed contains particles that are present in a semi-suspended condition [87]. Thus, the pressure decreases as a result of the fluid drag continuing to climb as the gas flow rate through the fixed bed is raised [88]. This behaviour persists until the minimal fluidization velocity, which is a critical value, is maintained. At the point at which the fluid haul equals the weight of the particle, this fixed bed becomes a fluidized bed, as shown in Fig. 7. Every time the fluidizing gas reaches the bed through an aperture, bubbles are created. They develop because the rate of gas input is greater than what can pass through the gaps with a frictional resistance that is smaller than the weight of the bed at the contact point of the bed hole just above.

As a result, the solid layers above the perforations are pushed aside, creating a gap with a porous surface through which the gas can pass at the initial velocity of fluidization. The homogeneous mixing, homogeneous temperature distribution and continuous operation are the benefits of bubbling fluidized-bed reactors.

3.2.2 Circulating fluidized-bed reactor fast pyrolysis

This type of reactor differs from numerous gas–solid reactors in that it has several unique characteristics [90]. It is therefore hopeful for a variety of uses. These reactors do not have a bed and distinct top surface; a real working diagram of a circulating fluidized-bed (CFB) reactor as shown in Fig. 8. However, it is between the phase of dense fluidization and light pneumatic conveyance in terms of density [92]. Over many other technologies, including those employed in the chemical process industry, fixed-bed reactors, entrained-flow reactors (EFRs), dense phase fluidized phase, rotary kilns and CFB reactors have selection superiority [93].

The main key characteristics of CFB reactors that set them apart from other reactor designs are the recycling of internal large bulk particles that reach the upper part of the vessel and then fall to the bottom side, a good void range and the lack of an obvious upper bed surface in the reactor column [94].

3.2.3 Fixed-bed reactor fast pyrolysis

The most popular kind of reactors utilized in the process sector are fixed-bed reactors. Even though they are available in many sizes and several other dimensions, they are most frequently found in circular cylinder shapes [95]. Solid catalysts are most frequently used to fill fixed-bed reactors. The product is taken from one side while the feed enters from the other. In a chosen portion, the catalyst pellets are stationary and they cannot travel in relation to a reactor reference section. The catalyst is where the majority of the chemical reactions take place. Thus, catalyst recycling and recovery are two main factors of the reactor that determine the required economy and have a high impact on the decision to use one.

3.2.4 Ablative reactor fast pyrolysis

A fast form of pyrolysis called ablative pyrolysis is performed in ablative reactors. The process by which the interacting biomass particles receive heat is the essential idea [96, 97]. The following circumstances favour ablative pyrolysis: high contact pressure and relative motion present between the surface and biomass material that will be contacting the heat source [93]. The higher rate of ablation in quick biomass pyrolysis using these ablative-type reactors is caused by these two simultaneous impacts of high pressure at contact and relative velocity. A schematic diagram of the reactor is shown in Fig. 9.

Over traditional rapid pyrolysis, ablative fast pyrolysis has a number of benefits. The highly frequent and better advantages are: (i) reactor volume is lower, (ii) low capital cost due to less use of inert gas, (iii) no gas recycling, (iv) large biomass of ≤50 mm can be used directly, (v) feed preparation is less expensive, (vi) the ease with which it can be modified to produce sustainable output, (vii) throughputs are uniquely high and (viii) operating expenses can be reduced [99].

3.2.5 EFR fast pyrolysis

For many years, coal, its derivatives and biomass have been transformed into energy fuels using EFRs, also referred to as drop tube furnaces [100]. In order to thermally degrade the coal or biomass material, it primarily consists of a vertical tube that is heated from the outside. The gas flowing is heated to the temperature of the wall of the reactor in these reactors, which are typically operated isothermally [101]. The coal or biomass can experience the same heating rate, temperature and residence duration as the tube of the EFR pyrolysis reactor, as described in Fig. 10. For the highest conversion efficiency, these reactors have the following three features: (i) biomass is only present in the reactor for a few seconds, (ii) the feedstock is very small (<100 m) and (iii) the temperature is quite high (>1000°C) [103].

The two primary categories of this type of reactor are slagging and non-slagging flow reactors. The ash melts with the reactor walls in the slagging type, leaving the liquid slag at the bottom. Slag is not an issue in the latter non-slagging type, which makes it appropriate for biomass with lower ash contents. At high pressure and temperatures, a wide variety of biomass can be processed in an EFR.

3.2.6 Catalytic fast pyrolysis of biomass

Lignocellulosic material converted into pyro-oil, a liquid fuel, is highly sought after due to rising fossil fuel prices, national security concerns and impending climatic catastrophes. Numerous processes are being considered, with rapid pyrolysis being one that is usually used to produce pyro-oil. About 70% of the biomass energy stock is found in bio-oil [104]. However, compared with liquid fuels based on crude oil, the commercialization of bio-oil is constrained by some defects in its qualities. The following characteristics are among them: (i) low calorific value, (ii) reduction in volatile compounds, (iii) undesirable level of acidity, (iv) unstable and (v) incompatible with other fossil fuels [105]. These unfavourable characteristics of pyro-oil made from biomass components are brought about by the organic oxygenated molecules that predominate in its chemical make-up. To increase the economic acceptance of pyro-oil and to broaden the adoption, oxygen must be removed [106]. The method utilized in pyrolysis technology to achieve this purpose is catalytic pyrolysis, which produces fuels in the aromatic range utilizing a variety of commercially available catalysts, including zeolites [107].

4 Advanced pyrolysis technology

A third type of pyrolysis is known as advanced biomass pyrolysis. These procedures offer advantages that are occasionally not attainable with one type of pyrolysis process technology because they operate somewhere between the fast and slow pyrolysis operating domains. As a result, these sophisticated biomass pyrolysis processes can be divided into a variety of categories. The following are the most advanced technologies: (i) flash pyrolysis, (i) vacuum pyrolysis, (iii) microwave pyrolysis, (iv) biomass pyrolysis via plasma technology and (v) solar pyrolysis.

4.1 Flash pyrolysis of biomass

The quality, content and yield of the products of biomass pyrolysis are significantly influenced by specific physical circumstances. The operational temperature, heating rate and residence time make up these three parameters [44]. If the operating circumstances fall under the following limitations, biomass pyrolysis will produce more liquid products and less char and gas. (i) a greater heating rate of 104 K/s, (ii) a temperature of 650°C and (iii) quick quenching. Flash pyrolysis is the term used to describe biomass pyrolysis carried out under these circumstances [108]. Slow heating rates with the lowest possible temperature favour the creation of char, while faster heating rates with temperatures of >650°C favour the formation of gaseous products [109]. The residence period at high temperatures, which can be as little as a few seconds or even less, necessitates the use of a pyrolysis reactor set-up with extremely high heating rates. The majority of flash pyrolysis investigations were conducted in a fluidized bed or EFR [50]. Technology for biomass flash pyrolysis is adaptable, less complicated and expensive. The process uses a complex mixture of oxygenated compounds, the composition of which depends on the biomass material utilized and the pyrolysis working conditions, to produce bio-oil with a yield of 60–75 wt% [110]. Klaimy et al. [111] investigated the potential of fuel recovery from plastic waste using pure polypropylene (PP), polyethylene (PE), polystyrene (PS) and polyethylene terephthalate (PET) at temperatures of 550°C and 600°C and obtained product yields of 55 wt% for PP, 31 wt% for PE at 550°C and also a higher quantity of xylene at 39 wt%.

4.2 Vacuum pyrolysis of biomass

Biomass materials are vacuum pyrolysed in moving-bed vacuum reactors. Although it is not a rapid pyrolysis process, the goal is to increase the production of bio-oil. According to the working theory, adding vacuum circumstances shortens the period in residence of the pyrolysis vapours, as shown in Fig. 11. This prevents secondary vapour-phase reactions from happening. Because vacuum pyrolysis requires less heat transmission than other pyrolysis methods, it can handle bigger biomass particles. In this pyrolysis process, the inert carrier gas medium is also not required. The idea is to use both fast and slow pyrolysis conditions at once. As a result, the temperatures at which the coarse biomass particles are heated are higher than those used in slow pyrolysis. When lower pressure is used, the gases from pyrolysis are swiftly evacuated from the heating zone.

Despite this shortcoming, it offers the following benefits: (i) good product quality, (ii) liquid product condensation, (iii) good particle size, (iv) ease of component extraction and (v) little to no char development, which are among the criteria for a good product. Rathore et al. [112] conducted a vacuum pyrolysis for wheat straw with an operational parameter of reduced pressure of between 18 and 25 kPa; 1 kg of raw material was carbonized in a vacuum pyrolysis reactor at 500°C for 30 min. Initially, the reactor was purged with N₂ gas at a flow rate of ~50 cc/min to prevent the introduction of oxygen. It was evident that the wheat (Triticum aestivum) straw (WS) biomass had been converted into wheat straw biochar, which is simply carbonized wheat straw. Pawar and Panwar [113] produced biochar using the following experimental conditions in a vacuum pyrolysis process: pyrolysis temperature of 500°C, residence time of 40–60 min and decreased pressure of 10–12 kPa. To produce liquid oil, the acquired gases were contained in a condenser of the shell-and-tube type.

4.3 Microwave pyrolysis of biomass

Since the heating of biomass material occurs intrinsically rather than extrinsically, microwave pyrolysis technology operates differently from other proven biomass pyrolysis procedures [114]. To break down the biomass material, a heat source of very high temperature is not necessary. For microwave pyrolysis, biomass material with a high dielectric constant or loss factor is preferred. Water is an excellent example of a material that can be microwave pyrolysed [115]. The water is first quickly pushed off when a biomass material with a high moisture content is pyrolysed using microwave heating [116]. The leftover biomass begins to develop char and hold heat. Due to the electrical conductivity of the microwave pyrolysis, eddy currents are created, causing rapid heating. Therefore, the primary goal is to control the microwave operating parameters in order to achieve the desired results [46]. Only 1–2 cm can be penetrated by the microwaves. As a result, scaling up a microwave reactor presents intriguing issues. Due to the homogeneous heating of the biomass material during microwave pyrolysis, a conducive atmosphere is created for learning and studying the basic principles of the pyrolysis mechanism. This aids in our comprehension of the secondary reactions that take place during the pyrolysis of biomass as well as the impact of the thermal gradient in a pyrolysis particle [63, 117]. A working diagram of the process is shown in Fig. 12.

Mong et al. [118] carried out microwave pyrolysis for horse manure. Overall, pyrolysis temperatures of 450–550°C produce greater gaseous and liquid yields, whereas temperatures of <350°C tend to encourage the synthesis of biochar. Additionally, it was discovered that the greater N₂ flow rate boosted the solid yield while decreasing the gas yield, with the effect being most pronounced at temperatures of 450°C and 550°C. Nzediegwu [119] selected WS, canola (Brassica napus) straw (CS), white spruce (Picea glauca) sawdust (SD) and manure pellets as the four feedstocks and pyrolysed at temperatures of 300°C, 400°C and 500°C; they concluded that the temperature and feedstock utilized for MAP (microwave-assisted pyrolysis), together with other process variables, had a big impact on the surface, thermal and fuel properties of the biochar.

4.4 Plasma technology pyrolysis of biomass

Plasma pyrolysis technology has many special advantages over traditional biomass pyrolysis methods. Low temperatures and slow heating rates are necessary to get these benefits [120]. The plasma pyrolysis technique can solve the issues that are related to traditional biomass pyrolysis, such as low gas output and high tar levels [45]. This is made possible by the quick response times, high energy density and temperature of plasma pyrolysis technology. Because plasma pyrolysis requires high-power energy to operate, it has several uses in the destruction of toxic compounds. Due to financial limitations, thermal plasma technology for biomass pyrolysis has received less attention [44]. Thermal biomass plasma pyrolysis produces temperatures of 3000–10 000 K, and a large portion of the energy is transmitted and radiated into the environment. The thermally activated plasma contains a variety of energy species, including electrons, ions, atoms, free radicals and activated molecules. When thermally activated by an electric arc discharge, the temperature exceeds 3000 K. When carbonaceous materials, such as biomass or coal, are treated with plasma, they undergo a quick thermal decomposition and release volatile components. Among them are primarily CH₄, CO, H₂, C₂H₂ and several light hydrocarbons. A schematic representation is shown in Fig. 13.

Titov et al. [122] conducted non-thermal plasma pyrolysis in the liquid phase at low temperatures (≤100°C) without the use of additional reagents or catalysts. Hydrogen (46.5–50.0 mol%), acetylene (28.8–34.3 mol%), ethylene (7.6–8.6 mol%), methane (4.2–6.2 mol%) and C₃–C₅ hydrocarbons made up the majority of the resultant gas along with graphite and multilayer nanotubes as the solid-phase by-products. Fulcheri [123] carried out plasma methane pyrolysis for hydrogen production along with solid carbon black in a lab-scale reactor showing that methane feedstock conversions can reach >99%, hydrogen selectivity can reach >95%, solid recovery can reach >90% and carbon particles of various crystallinities can be produced, all of which have the potential to replace conventional furnace carbon black.

4.5 Solar pyrolysis

The endothermic turning of biomass material using an inert effect appropriate for pyrolysis is called solar energy-assisted biomass pyrolysis [124]. Solar energy is concentrated to provide the required thermal energy. Energy from the Sun is redirected and focused on the biomass pyrolysis reactor with the help of an optical device. As a result, concentrated solar energy is used to reach the necessary biomass pyrolysis temperatures [125]. Three potential methods of transferring solar energy into biomass materials have been identified. Thermosolar devices can also be used for biomass solar pyrolysis. The heating source is provided by this system by deflecting solar radiation from a wide surface to a more focused area. The three main parts of this system are the following: a solar concentrator, a solar collector and a supporting framework, as shown in Fig. 14.

Luo et al. [127] used bifunctional photo–thermo catalysts in a unique photothermal reaction system to study the photothermal catalytic pyrolysis of low-density polyethylene (LDPE) polymers in a solar simulator-heated in situ fixed-bed reactor operating at 500°C. According to the findings, the Ni–Ti–Al catalyst produced 34 mol/kg of hydrogen and had an 80% selectivity for jet fuel compared with the 7 mol/kg and 38% of Ti–Al, respectively. Strong aromatic hydrocarbon selectivity was demonstrated by catalysts that were Ni-loaded, with particular success rates of 49.51% for Ni–Al and 53.46% for Ni–Ti–Al. Acids and phenols in bio-oils were dramatically reduced by pyrolysis in molten salt while aromatics were increased between pyrolysis temperatures of 450°C and 850°C.

5 Conclusion

The pyrolysis process nowadays promises technological invention for converting biomass materials into more valuable renewable energy. To suit home, industrial and commercial needs, the process can produce green and sustainable energy. This review provides an overview of ongoing initiatives, recent advancements, as well as the environmental factor benefits of this conversion technology for energy production. Low-value biomass is converted during pyrolysis into high-value solid fuel biochar, liquid fuel pyro-oil and combustible gas called syngas as a product. The potential for reducing the rise in greenhouse gases from the pyrolysis process depends on a number of variables, including the kind of biomass used as the feedstock, the kind of conversion technology utilized, the size of the pyrolysis unit and the method employed for recycling co-products. Slow pyrolysis can produce better ecological results, as it produces more biochar, which can be added to the soil for the sequestration of carbon. Fast pyrolysis generates a profit by turning biomass into bio-oil—a more valuable product. For particular applications, advanced pyrolysis methods can potentially offer great welfare. The cost of biomass material, product yields, ability to produce high-value by-products and production balance can all show the efficient utilization of the pyrolysis process.

6 Future recommendations

Pyrolysis reactors should be effective and efficient at transferring heat, increase the reactivity of pyrolysis, create bio-oil that is lower in weight, be thermally stable, have less ash agglomeration in reactor beds, produce non-toxic pyrolysis products, and have good temperature and heating rate control. One of the challenges in pyrolysis is to minimize heat loss during the process, as this can reduce the efficiency of the process and increase operating costs. Several heat reduction technologies can be used in pyrolysis to improve the efficiency of the process in the future:

• (i) Insulation: one of the simplest ways to reduce heat loss is to insulate the pyrolysis reactor using materials such as ceramic fibres or refractory bricks. This helps to maintain a more stable temperature inside the reactor, which in turn improves the efficiency of the process.

• (ii) Recirculation: another way to reduce heat loss is to recirculate the hot gases produced during the pyrolysis process back into the reactor. This can be done using a gas recirculation system, which helps to maintain a higher temperature inside the reactor and improve the efficiency of the process.

• (iii) Indirect heating: in some cases, it may be more efficient to heat the pyrolysis reactor indirectly, rather than directly. This can be done using a heat exchanger, which transfers heat from a separate heating source to the reactor. Indirect heating can be more energy-efficient than direct heating, as it reduces heat loss and improves temperature control.

Overall, several heat reduction technologies can be used in slow pyrolysis to improve the efficiency of the process and reduce operating costs. The choice of technology will depend on the specific characteristics of the biomass being processed, as well as the desired product.

Acknowledgements

This work was supported by the Department of Renewable Energy Engineering, College of Technology and Engineering, MPUAT, Udaipur, Rajasthan, India.

Conflict of interest statement

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Funding

This work did not receive any specific grant from funding agencies in the public, commercial or not-for-profit sectors.

Data Availability

Data will made available on request.

References