A Review of Pyrolysis Technologies and the Effect of Process Parameters on Biocarbon Properties

Abstract

Biomass-based solutions have been discussed as having the potential to replace fossil-based solutions in the iron and steel industry. To produce the biocarbon required in these processes, thermochemical treatment, pyrolysis, typically takes place. There are various ways to produce biocarbon, alongside other products, which are called pyrolysis oil and pyrolysis gas. These conversion methods can be divided into conventional and non-conventional methods. In this paper, those techniques and technologies to produce biocarbon are summarized and reviewed. Additionally, the effect of different process parameters and their effect on biocarbon yield and properties are summarized. The process parameters considered were final pyrolysis temperature, heating rate, reaction atmosphere, pressure, catalyst, use of binders, and particle size. Finally, the effect of different reactor configurations is discussed. Understanding the combination of these methods, technology parameters, and reactor configurations will help to produce biocarbon with the desired quality and highest yield possible.

1. Introduction

Energy consumption is growing worldwide. According to the International Energy Agency report “Net Zero by 2050”, energy demand is expected to rise by 20% compared to 2020 by the end of 2050 [1]. At the same time as the growing energy demand, there are targets to reduce greenhouse gas (GHG) emissions by 40% compared to the levels in 2000 [2]. One way to achieve this is the use of bioenergy. Bioenergy is expected to cover almost 20% of the world’s energy demand in 2050. This use of bioenergy includes biofuels, biogas, and biocarbon [1]. At the same time as the rising energy demand, the world’s GHG emissions have constantly been rising over the last 30 years and reached an all-time high in 2021, with 36.6 gigatons of carbon dioxide (CO₂) being released [3]. It is estimated that the industrial sector contributed 24% of global GHG emissions in 2019. The steel industry’s share was about 8% of global CO₂ emissions in 2018 [4].

The United Nations set a goal in the Paris Agreement to maintain climate change at a 1.5 °C higher global average temperature than in the pre-industry era [5]. According to Shukla et al. [6], GHG emissions need to be cut by 23% by 2030 and 75% by 2050. Cutting CO₂ emissions in the steel industry is part of this plan. It is estimated that in Europe, it is possible to cut CO₂ emissions in the steel industry by 14–21% by 2030 compared to the CO₂ levels in 2010. This reduction potential can be achieved without losing the steel industry’s competitiveness [7]. Suopajärvi et al. [8] reported that the CO₂-cutting potential in the steel industry is over 50% when biobased solutions are used. Because of this potential, the use of biocarbon has gained attention in recent years. Biomass-based solutions can be used in metallurgical coke production or injected into a blast furnace using tuyeres injection [8]. However, there are some problems related to the use of biocarbon in metallurgical applications. These are related to the properties of biocarbon; for example, its low density and high porosity affect how it can be used in metallurgical applications [9]. Other important properties of biocarbon are carbon content and calorific value [10]. These properties are affected by pyrolysis conditions [9]. For example, increasing the pyrolysis temperature from 300 °C to 600 °C increases porosity when the wood chips are pyrolyzed [11].

There are two ways to convert biomass into biofuels: biochemical or thermochemical routes. The difference between these two routes is that the biochemical process takes days to complete, whereas, in the case of a thermochemical process, it is finished within seconds to hours [12]. Typically, in the thermochemical process, pyrolysis is used when biocarbon is produced [13]. Depending on the process parameters such as the final pyrolysis temperature, heating rate, and residence time, pyrolysis can be divided into three main categories: slow (often referred to as conventional pyrolysis), fast, and flash pyrolysis [14,15]. The other way to categorize pyrolysis is based on the technology used—for example, solar- or microwave-based technologies [16].

The objective of this study was to summarize and review biocarbon production technologies and gather information on how they affect the biocarbon yield and its properties. The focus is to evaluate different production methods and how the biocarbon yield differs from different pyrolysis methods and technologies.

Finally, the effect of process parameters such as the heating rate, pyrolysis temperature, and atmosphere used are summarized. Understanding the combined effect of these parameters will help to produce biocarbon with suitable properties.

2. Pyrolysis Methods and Technologies

Pyrolysis is a thermochemical decomposition of biomass that takes place at medium (300–800 °C) to high temperatures (800–1300 °C) in the absence of oxygen [17]. At these temperatures, the main components of wood-based biomass, namely cellulose, hemicellulose, and lignin, decompose into other compounds. The decomposition temperature of these compounds differs. Hemicellulose has the lowest decomposition temperature range of those compounds, from 220 °C to 315 °C [18,19]. Pyrolysis should not be confused with torrefaction, which takes place at lower temperatures, from 200 °C to 300 °C in the absence of oxygen [20]. Because of the torrefaction temperature range, only hemicellulose decomposes into other compounds, and pyrolysis gases and torrefied wood are formed [21].

During pyrolysis, three main products are formed: solid products, referred to as biocarbon, liquid (i.e., pyrolysis oil), and non-condensable gases (pyrolysis gas). The relative amount of these products is affected by the feedstock properties and operating parameters such as temperature and heating rate [22]. The two main process parameters affecting biocarbon yield are temperature and heating rate [23]. Altamer et al. [24], showed that when wild Brassica juncea L. seeds are pyrolyzed in a temperature range from 350 °C to 600 °C, the biocarbon yield decreases at a heating rate of 10 °C/min. The pyrolysis oil yield increases until the pyrolysis temperature reaches 475 °C. After that, the pyrolysis oil yield decreases in the studied temperature range. At the same time, the pyrolysis gas yield increases. The heating rate used also affects the product distribution of pyrolysis.

Higher heating rates lead to a decreased formation of biocarbon and pyrolysis oil and an increased pyrolysis gas formation. This has been reported when the pyrolysis takes place at a temperature of 475 °C, and the heating rates used varied from 10 °C/min to 30 °C/min [24]. In addition to the pyrolysis temperature and heating rate, other parameters such as residence time and pressure also affect the product distribution of pyrolysis. These parameters affect the formation of the biocarbon physicochemical properties and structure [25,26].

One of the main reasons pyrolysis is used is that the technology is adaptable, and the process can be optimized based on the desired product. For example, for high-volume biocarbon production, slow pyrolysis is the best method, while flash pyrolysis is preferred when the aim is to maximize the amount of pyrolysis gas. Vacuum pyrolysis produces more pyrolysis oil than slow pyrolysis, while biocarbon production is decreased [27,28].

Based on the heating rate and temperature, pyrolysis can be generally classified into three groups: slow pyrolysis (often referred to as conventional pyrolysis) [15], fast pyrolysis, and flash pyrolysis [17,29]. Typically used process parameters and product distributions are presented in

Table 1. Pyrolysis types and their main process parameters.

	Slow Pyrolysis	Fast Pyrolysis	Flash Pyrolysis	Reference
Temperature (°C)	300–700	400–800	800–1000	[14]
Heating rate (°C/s)	0.1–1	10–200	>1000	[14,30,31]
Vapor residence time	5–30 min 10–100 min	<5 s	<0.5 s	[14,30,31,32]
Particle size (mm)	5–50	<1	<0.2	[33]
Products (%)				[14]
Biocarbon	35	20	12
Pyrolysis oil	30	50	75
Pyrolysis gas	35	30	13

.

The boundary between slow and fast pyrolysis is somewhat questionable. Generally, sources draw the line between slow and fast pyrolysis at a heating rate of 10 °C/s where lower than 10 °C/s heating rates are referred to as slow pyrolysis and faster than 10 °C/s as fast pyrolysis. The temperature ranges between different pyrolysis types also overlap. Depending on the sources, the temperature range could be narrower (400–600 °C in the case of slow and fast pyrolysis and 600–900 °C for flash pyrolysis) [34] or wider (550–1000 °C for fast pyrolysis and 800–1100 °C for flash pyrolysis) [30,35] than that presented in Table 1. Generally, it seems that the temperature range is not the defining factor; instead, the heating rate defines the type of pyrolysis.

2.1. Pyrolysis Stages and Biocarbon Formation

Biocarbon formation consists of three temperature-dependent stages. These are called pre-pyrolysis, main pyrolysis, and the production of carbonaceous products [13]. When wood-based material is turned into biocarbon, the first stage takes place at temperatures under 200 °C. At these temperatures, moisture and light volatiles are evaporated [36]. In the second stage, the organic compounds such as hemicellulose and cellulose are devolatilized. This stage takes place in the temperature range of 200–500 °C [36,37]. The last stage occurs at temperatures over 500 °C. In this stage, the chemical compounds with strong chemical bonds, such as lignin, decompose into other compounds [36]. It is also noteworthy that lignin decomposes at lower temperatures, starting at 160 °C [19].

Biomass typically contains three main compounds: cellulose, hemicellulose, and lignin. In addition, there are three minor compounds: proteins, sugars/aliphatic acid, and fats. The combustion and degradation behavior of these compounds differs depending on the biomass type and composition [18]. For example, studies have shown that a high lignin content in the feedstock increases biocarbon yield [23,38]. According to Yang et al., cellulose, hemicellulose, and lignin decomposition occur following temperature-dependent stages [19].

• Hemicellulose decomposes into acetic acid, carbon dioxide, and aromatic compounds in the temperature range of 220–315 °C. Biocarbon is also formed during the decomposition phase [19,39,40].
• Cellulose decomposes into pyrolysis oil, gaseous products, and biocarbon at 315–400 °C [19,41]. The decomposition products depend on the feedstock heating rate. At slow heating rates, the process favors biocarbon formation. Rapid volatilization occurs at high heating rates, leading to the formation of levoglucosan, which breaks down further into liquid and gas products [40].
• Lignin decomposes at a much wider temperature range of 160–900 °C, with studies suggesting that the main reaction takes place at a much broader range of 200–500 °C [19,42].

According to Nachenius et al. [43] in the temperature range of 180–270 °C decomposition reactions are endothermic, and when the temperature reaches 280 °C decomposition reactions for hemicellulose and lignin become exothermic [19,43]. However, the existence of exothermic reactions is contradictory, and their existence varies depending on the source [44]. At temperatures over 400 °C, the remaining volatile compounds still present in the solid material are volatilized. The remaining solid material has a high fixed carbon content and low volatile content. When the temperature exceeds 600 °C, the condensable components in the gas phase undergo cracking and polymerization reactions. This leads to a decrease in the pyrolysis oil formation [43].

2.2. Pyrolysis Methods

Depending on the process parameters, such as heating rate and final pyrolysis temperature, pyrolysis can be divided into four subclasses. Each of these classes has its advantages and limitations. In the following sequence, the main features of these classes are discussed.

2.2.1. Slow Pyrolysis

Slow pyrolysis has been used for thousands of years for biocarbon production. A typical process feature is a slow heating rate (0.1–1 °C/s) combined with a low pyrolysis temperature (300–700 °C) and a long vapor residence time, between 10 and 100 min. Compared to fast pyrolysis, the vapors do not escape as fast. Thus, different components in the vapor phase continue to react with each other as the biocarbon and pyrolysis oil is formed. It is also possible to remove vapors continuously through pyrolysis and thus decrease vapor residence time [14,15].

2.2.2. Fast Pyrolysis

In fast pyrolysis, the feedstock is heated rapidly in the absence of oxygen with a very short vapor residence time. Because of these process features, the biomass decomposes and forms pyrolysis oil, biocarbon, and pyrolysis gas [15]. Depending on the source, the temperature range of fast pyrolysis varies from 400–800 °C [14] to 850–1250 °C [45]. Typically, the process involves high heating rates of 10–200 °C/s and is combined with a very short vapor residence time, under 2 s. These parameters favor pyrolysis oil formation, and a typical product yield distribution is 30% pyrolysis gas, 20% biocarbon, and 50% pyrolysis oil [14]. According to some reports, a 70% pyrolysis oil yield is even possible [26,46]. A short residence time is the key to producing pyrolysis oil with a high yield. This is because it prevents secondary cracking reactions from occurring and thus improves pyrolysis oil yield [14].

2.2.3. Flash Pyrolysis

Flash pyrolysis uses a very high heating rate, up to 2500 °C/s. The vapor residence time is very short, below 0.5 s. The temperatures used are typically around 1000 °C. The primary product is pyrolysis oil. The main difference between flash and fast pyrolysis is that the heating rate is considerably higher than in fast pyrolysis [46]. Given the product yields, flash pyrolysis leads to more pyrolysis oil generated during pyrolysis than fast pyrolysis [14].

2.2.4. Intermediate Pyrolysis

In some publications, intermediate pyrolysis is separated by its own pyrolysis type because the heating rate, 1–10 °C/s, settles between slow and fast pyrolysis [47,48]. Other operation conditions are also between slow and fast pyrolysis: temperature range is typically between 400 °C and 650 °C with several suggested vapor residence times starting from 0.5–20 s [47] to 300–1000 s [45]. Operating pressure is typically 0.1 MPa [45,47]. When intermediate pyrolysis is used, it is possible to obtain a higher biocarbon and pyrolysis oil yield and a lower pyrolysis gas yield compared to conventional pyrolysis, according to Kazawadi et al. [47]. These pyrolysis gases contain little or no dust or tar, and the pyrolysis gas can be used directly for generating electricity and heat [45,48].

2.2.5. Segmented Heating

Segmented heating is not its own pyrolysis type. Rather, it can be described as an alternative way to heat feedstock to the final pyrolysis temperature [49]. Overall, pyrolysis is an endothermic process, but in the early stages, around 280 °C, pyrolysis is exothermic [43,50]. Because of the exothermicity, it has been shown that it is possible to utilize heat released from these reactions to meet the energy requirements in the stage where the reactions are endothermic. Based on the exothermicity of pyrolysis in the early stages, Lam et al. [49] introduced the two-stage pyrolysis concept in 2010. This approach was later developed further by Cheung et al. [50]. In both concepts, the pyrolysis process is divided into several stages, for example, a heating stage, followed by an adiabatic stage, a second heating stage, and a second adiabatic stage. This type of segmented pyrolysis could reduce pyrolysis energy consumption by 22.5% compared to conventional pyrolysis. [50]. According to Oyedun et al. [51] if the temperature in the first adiabatic stage is too low, below 270 °C, exothermic reactions may not occur. If the temperature is too high, over 400 °C, the exothermic reactions have already occurred. Because of this, the suitable temperature for the first adiabatic stage is between 270 °C and 400 °C.

In another study, Oyedun et al. [52] showed that when two-staged pyrolysis is carried out with a final temperature of 400 °C, it leads to a higher biocarbon and fixed-carbon yield, with a higher volatile matter content compared to conventional pyrolysis. However, in conventional pyrolysis, the fixed carbon content was higher. The difference in the biocarbon yield was 5.65% at the highest, which was achieved when the first stage occurred at 250 °C. The overall time of segmented pyrolysis was slightly longer than conventional pyrolysis when the final pyrolysis occurred at 400 °C.

Cai et al. [53] continued studying and optimizing two-staged pyrolysis. They found that when three biomasses, namely rice straw, cedar wood, and Dalbergia wood, were pyrolyzed using a segmented method, the optimal temperature of the first stage was between 270 °C and 360 °C. This was based on the release of volatile matter. More recently, Babinszki et al. [54] found that using two-stage pyrolysis leads to an increased biocarbon yield compared to conventional pyrolysis when the final pyrolysis takes place in the temperature range of 500–700 °C.

Han et al. [55] introduced the concept of three-stage pyrolysis. This was further studied by Qi et al. [56]. In three-stage pyrolysis, the idea of the first stage is to reduce the feedstock’s moisture content. In the second stage, the temperature is brought close to the decomposition temperature of the feedstock, and the third stage is used to finalize the carbonization process. This process concept leads to higher mass and energy yields compared to conventional methods [56].

Table 2. Comparison of different thermochemical conversion methods [14,30].

Pyrolysis Type	Advantages	Challenges
Slow pyrolysis	Leads to a high yield of biocarbon	Energy consumption when the temperature is greater than 600 °C
	Easy to scale up
	Diverse range of possible feedstock materials
Fast pyrolysis	Leads to a high yield of pyrolysis gas
	Easy to scale up
	Diverse range of possible feedstock materials
Flash pyrolysis	Leads to high yield of pyrolysis oil
	Easy to scale up
	Diverse range of possible feedstock materials

shows a comparison of the three main thermochemical conversion methods. As the table shows, the advantages and challenges are quite congruent between methods. The primary challenge of these three is related to energy consumption when pyrolysis temperature reaches over 600 °C [14,30].

When considering the suitability of these production methods in view of producing biocarbon suitable for metallurgical applications, slow pyrolysis seems to have the most potential. This is mainly because of the high yield of biocarbon, which results from low heating rates and pyrolysis temperatures.

2.3. Pyrolysis Technologies

Besides the pyrolysis method used, pyrolysis technology also plays a crucial role in biocarbon production. Generally, these technologies are divided into four subclasses. Each of these technologies has its advantages and limitations. In this section, the main features of these technologies are discussed.

2.3.1. Microwave Pyrolysis

Microwave-assisted pyrolysis is an alternative pyrolysis method compared to the traditionally used pyrolysis methods. The main difference is that it does not require any external temperature field. Instead of an external field, it is based on the use of electromagnetic radiation. The electromagnetic radiation wavelength varies from 1 mm to 1 m. In the case of frequency, it varies between 300 MHz and 30 GHz. Industrial and domestic microwave applications typically operate at 2.45 GHz, corresponding to a wavelength of 12.2 cm [16,57]. The penetration depth of these waves varies due to several things: material type, microstructure, and temperature. For example, at a temperature of 25 °C, the penetration depth of these waves is 8.5 cm in the case of biocarbon when the frequency used is 5.8 GHz. The penetration depth rises to 55.6 cm when the frequency is dropped to 0.915 GHz.

Another example is water: at a frequency of 2.54 GHz, the penetration depth is 1 cm–4 cm, but it increases to 5 cm–7 cm when the temperature rises from 25 °C to 95 °C. Because of this, the biomass’s density and water content should also be considered when pyrolyzing biomass using microwaves [58]. In order to improve the absorption of microwaves, an absorbent material is used. Usually, this is mixed into the feedstock before pyrolysis [59]. Graphite and silicon carbide are typically used as absorbent materials [60,61]. Calcium oxide (CaO) utilization as an absorbent material in microwave-assisted pyrolysis has also generated good results as it has been found to increase the yield of solid biocarbon in pyrolysis, according to Yu et al. [62].

Compared to conventional pyrolysis, the heating mechanism of microwave-assisted pyrolysis is different. This is because, in conventional pyrolysis, the heat is transferred from the surface to the core, and in microwave pyrolysis, the electromagnetic energy is converted into heat from the core to the surface [57,63,64]. Therefore, the heating mechanism of microwave-assisted pyrolysis can be described as an energy conversion method rather than heat transfer. The difference between these two mechanisms is illustrated in Figure 1.

Based on the heating rates used, microwave-assisted pyrolysis can be categorized as either slow [63] or fast pyrolysis [65]. Typically, microwave-assisted pyrolysis is performed in the temperature range of 450–600 °C [63]. The pyrolysis of the feedstock consists of two stages: drying and pyrolysis. In the drying stage, the temperature of the feedstock is between 100 °C and 200 °C. At these temperatures, the feedstock’s moisture is being released. Additionally, some volatile compounds are released. At higher temperatures, starting at 200 °C, the feedstock decomposes into biocarbon, pyrolysis gas, and pyrolysis oil. The reaction rate also decreases when the temperature rises because of the moisture evaporation [17].

Because of the unique features of microwave pyrolysis, it offers advantages compared to traditionally used pyrolysis methods. When pyrolysis is performed at temperatures from 150 °C to 300 °C, the process consumes less energy than conventional pyrolysis [66]. It can be optimized to produce biocarbon, pyrolysis gas, or pyrolysis oil with good quality, depending on process parameters. This is because the heating rates are flexible, and a heating rate of up to 200 °C/s can be achieved [30]. Additionally, there is no need to dry the biomass before pyrolysis because water is a good absorption material and is thus needed to carry out pyrolysis efficiently [66]. The disadvantages of microwave-assisted pyrolysis are that it is hard to know the exact temperature of the biomass in the reactor and scaling from laboratory to industrial scale is difficult [30].

Microwave pyrolysis produces a relatively low pyrolysis oil yield. Reports show that it could be less than 30%. This is much less than fast pyrolysis, which usually generates a 60–70% liquid yield [67]. Generally, it can be said that microwave pyrolysis produces more biocarbon with the same pyrolysis oil yield compared to conventional pyrolysis. Because of this, the pyrolysis gas content decreases compared to conventional pyrolysis [67]. There are also reports that the effect of increasing microwave power is similar to the effect of increasing pyrolysis temperature in conventional pyrolysis. Using a higher microwave power leads to a decreasing biocarbon yield [67,68,69].

Several studies have been conducted on the use of microwave-assisted pyrolysis with different feedstocks, and studies have shown that microwave pyrolysis affects the physical properties of biocarbon. Maŝek et al. [70] showed that at lower temperatures of ~200 °C, the fixed carbon content of biocarbon was higher compared to biocarbon that was pyrolyzed with conventional methods. To obtain a similar carbon content with slow pyrolysis, higher pyrolysis temperatures would be required [70]. Microwave pyrolysis also increases the surface area of the biocarbon [71]. According to Abas and Ani, microwave pyrolysis increases biocarbon’s calorific value [72]. Mohd [73] reported that when an empty oil palm fruit bunch was pyrolyzed at 500 °C, the biocarbon’s gross calorific value increased compared to conventional pyrolysis. However, when changing the feedstock material to rice husks, microwave pyrolysis led to a decreased gross calorific value. Additionally, when the pyrolysis temperature was increased to 800 °C, conventional pyrolysis produced biocarbon with a higher gross calorific value; these results indicate that the evolution of the gross calorific value is linked to the feedstock material.

Ge et al. [74] combined microwave pyrolysis with a vacuum when palm kern shells were pyrolyzed. The experiments were performed by using three different heating powers: 500 W, 600 W, and 700 W. The effect of different radiation times (5 min, 10 min, and 15 min) was also considered. Increasing the power from 500 W to 700 W decreased the biocarbon yield from 86% to 79%, respectively. When considering the effect of the radiation times, the results show that increasing the radiation time from 5 min to 15 min reduces the biocarbon yield from 87% to 70%, respectively. These results mean that the effect of microwave power is a less significant factor for biocarbon formation, and radiation time plays a vital role in biocarbon yield when microwave pyrolysis is used.

Md Said et al. [75] studied the pyrolysis of empty oil palm fruit bunches with and without pelletizing the feedstock. The pyrolyses were carried out in the 200–400 °C temperature range. They found that the highest biocarbon yield of 47.37% was achieved when pyrolysis took place at 300 °C. When the temperature rose, the biocarbon yield decreased. Comparing biocarbon properties with and without pelletizing, they found that if the material was not pelletized before pyrolysis, it led to a higher biocarbon yield. Additionally, the power consumption was 60% less without pelletizing. This is because when the material was not pelletized, its density was lower; therefore, the microwaves had a deeper penetration depth.

Zhang et al. [76] studied the microwave pyrolysis of pine sawdust in the presence of a metal wire needle. The purpose of the metal wire needle was that it increased the electromagnetic distribution density during pyrolysis. The pyrolysis experiments were performed with a heating power of 400 W, which corresponded to a pyrolysis temperature of 450 °C. They found that at this temperature, the biocarbon yield was 23.77% without the needle but increased to 29.60% when the metal wire needle was added to the reactor.

In view of biocarbon properties and the suitability of biocarbon in pyrometallurgical applications, it can be said that this is quite a promising technology to produce biocarbon. Biocarbon’s properties, such as carbon content and higher calorific value, are increased compared to conventional methods.

2.3.2. Solar Pyrolysis

In solar pyrolysis, feedstock with a low energy density is converted into energy-denser products [77,78]. This pyrolysis type represents fast pyrolysis, and it is an endothermic process based on the utilization of solar energy. The basic principle of solar pyrolysis is quite simple. Solar radiation is used to heat a reactor, and pyrolysis occurs in an inert environment [79]. This can be performed by using different types of concentrators and reactor configurations; for example, using parabolic dishes or parabolic troughs [79,80].

Because of the unique features of solar pyrolysis, it offers some advantages compared to traditionally used technologies. The concept used for performing pyrolysis allows fast shutdowns and startups of reactors [81]. The process maximizes the amount of products formed during pyrolysis. This means that in conventional pyrolysis, a small amount of feedstock is burned for heat to maintain the pyrolysis process. This does not occur in solar pyrolysis because the heat for the pyrolysis is maintained by solar radiation [81,82].

Additionally, less environmental pollution is generated because no feedstock is used to produce the heat needed for pyrolysis [81]. According to Rahman et al. [79] this means that as much as 32.4% less CO₂ is formed during pyrolysis compared to fast pyrolysis. The CO₂ reduction could be as much as 62% compared to conventional pyrolysis, according to Giwa et al. [83]. The reactor temperature and heating rate can be adjusted within a broader range, and the adjustment is more flexible than in conventional pyrolysis [80].

The highest possible operating temperature and the heating rate depend on the technology used. When parabolic mirrors are used, the operating temperature is from 400 °C to 700 °C, whereas in the case of parabolic dishes, the operating temperature rises over 1200 °C [84]. However, the limited size of parabolic dishes sets challenges for large-scale applications. Therefore, it seems that on a larger scale, another option, such as solar tower systems, would be better [85]. In terms of heating rates, Li et al. [80] reported that it is possible to achieve a heating rate range from 10 °C/s to 150 °C/s when a vertical axis solar furnace is used. Parthasarathy et al. [84] reported that it is possible to achieve heating as high as 450 °C/s when heliostat parabolic mirrors are used.

However, solar-assisted pyrolysis faces some challenges compared to traditionally used technologies. For example, achieving a uniform heat flux distribution through the reactor is difficult. This problem may be solved by using a rotating reactor. The heat losses of reactors are also challenging, especially when the wind is strong. Additionally, the heat generated by solar energy is not constant because it depends highly on the time of day and the season [86].

Generally, the technology used to carry out solar pyrolysis is divided into three main groups: directly heated reactors, indirectly heated reactors, and separated reactor systems. The difference between the first two is that solar radiation is directly concentrated onto the feedstock in the directly heated reactor. In contrast, in indirectly heated reactors, solar energy is first concentrated onto the reactor surface and then transferred to feedstock. The third main type is a separated reactor system, where solar radiation is first used to heat a transfer fluid, which is then used to heat the reactor directly [79].

Because the effect of different reactor configurations and feedstock is not well known, several studies have been conducted to understand how these affect biocarbon yield during solar pyrolysis. Zeng et al. [87] studied the fast pyrolysis of beech wood pellets in a vertical solar furnace with several temperatures, starting from 600 °C and ending at 2000 °C. Additionally, the effect of the heating rate on the final product distribution was evaluated using two heating rates of 10 °C/s and 50 °C/s. They found that increasing the final pyrolysis temperature and heating rate increased pyrolysis gas formation during pyrolysis. At the same time, biocarbon and pyrolysis oil yield was reduced. The results mean that, like conventional pyrolysis, the distribution of the final products is linked to the pyrolysis temperature and heating rate.

Bashir et al. [78] focused on modeling fast solar pyrolysis using parabolic concentrators. They found that when the pyrolysis temperature in solar pyrolysis was 465 °C it was possible to obtain a product yield that consisted mainly of pyrolysis oil (51.5%) and biocarbon (43.7%) with a small amount of non-condensable gases (4.8%). After comparing the results to those obtained in the literature, they noticed that the modeled results were quite close to those found in the literature [78].

Weldekidan et al. [88] focused on studying the pyrolysis yield of rice husks when pyrolyses were carried out using concentrated solar radiation. The temperature varied from 500 °C to 800 °C and the heating rate was 160 °C/min. They found that increasing the pyrolysis temperature from 500 °C to 800 °C decreased biocarbon and pyrolysis gas yield. At the same time, the pyrolysis oil yield increased.

Chen et al. [89] studied solar pyrolysis using three different biomasses: peanut shells, soybean straw, and pine wood. They found that when pyrolysis took place at 550 °C, more pyrolysis gas and pyrolysis oil were formed than biocarbon. More recently, Chen et al. [90] studied the effect of biomass torrefaction before the main solar pyrolysis phase. These experiments were also carried out at a temperature of 550 °C. They reported that when pyrolysis took place at 550 °C, the biocarbon yield was 26.8%. However, when the samples were torrefied in a temperature range of 200–300 °C before actual pyrolysis, the biocarbon yield increased from 27.9% to 56.2%. This is because the feedstock material had increased lignin content along with the decomposition of cellulose and hemicellulose. Because lignin is one of the main components behind biocarbon formation, the overall biocarbon yield increased when the actual pyrolysis occurred. Finally, Singh et al. [91] studied the solar pyrolysis of ground Jojoba seeds. The pyrolysis reactor’s average temperature varied from 240 °C to 310 °C. They achieved a 48% yield of biocarbon and 22% pyrolysis oil yield. The low pyrolysis oil yield was related to the solar radiation variation, which meant the reactor temperature was not constant.

It is hard to say anything about how suitable solar-assisted pyrolysis is for producing biocarbon suitable for pyrometallurgical applications because of the relatively limited number of studies. Because most of the reactor configurations utilized high heating rates, it is hard to envision this technology becoming popular to produce primarily biocarbon.

2.3.3. Plasma Pyrolysis

Plasma pyrolysis represents a form of flash pyrolysis and is used to pyrolyze a carbon-based material [92,93,94,95]. Typically, the operating temperature varies from 4273 °C to 5273 °C when using microwave-powered plasma and up to 10,273–12,273 °C if plasma is inductively coupled or arc discharge plasma [96].

Compared to conventional pyrolysis methods, plasma pyrolysis offers a high heating rate and short residence time [94]. Because of the high heating rates, organic compounds are decomposed, and inorganic materials are melted thus forming slag [94]. Plasma pyrolysis produces a relatively high pyrolysis gas yield, and the produced gas has a good heating value. Because of this, the produced gas is suitable for energy generation [97,98]. However, there are some downsides related to the use of plasma pyrolysis. The reactor has high energy consumption, and because of heat loss during pyrolysis, the overall efficiency of the process is reduced [99].

Due to the high gas yield and high energy consumption, it is hard to see that this technology will be implemented further to produce biocarbon in metallurgical applications. However, this technology definitely needs further studies.

2.3.4. Vacuum Pyrolysis

Vacuum pyrolysis is based on using vacuum conditions during pyrolysis. Based on the heating rate used, it represents either slow or fast pyrolysis [100,101,102]. Because of the vacuum conditions, there are no molecules or just a small number of molecules that disturb the reactions. This makes the decomposition products uniform and accelerates the reaction rate [100,101]. Because the reaction rate is fast, the process consumes less energy than traditional pyrolysis methods [101]. A second characteristic feature of this process is that the vapor residence time is short. This leads to fewer side reactions and, therefore, pyrolysis oil with higher quality and yields than other pyrolysis methods [99,103]. The downside of using vacuum pyrolysis is that it is complicated, so the investment cost and maintenance are high [104].

This technology is not the most suitable to produce biocarbon in metallurgical applications because the main product of this process is pyrolysis gas. Additionally, a limited number of studies are focused on the properties of biocarbon produced in this way; thus, this technology needs further study.

Table 3. Comparison of different thermochemical conversion technologies.

Pyrolysis Technology	Advantages	Challenges	Reference
Microwave pyrolysis	Lower energy consumption than traditionally used technologies	Exact temperature of the reactor is hard to determinate. Scaling problems from laboratory to industrial scale.	[30,66,67]
	Flexible heating rates
	Process can be optimized to produce biocarbon, pyrolysis gas, or pyrolysis oil
	No need to dry the biomass before pyrolysis
Solar pyrolysis	Less environmental pollution compared to conventional pyrolysis	Uniform heat flux throughout the reactor is difficult to achieve. Heat losses on the reactor surface, especially when the wind is strong. Generated heat flux depends highly on the time of day and the season.	[77,78,79,80]
	More flexible heating rates and pyrolysis temperatures than in conventional pyrolysis
Plasma pyrolysis	High heating rate	High energy consumption and because of this decreased process efficiency.	[93,99]
	Short residence time
	Because of short residence time, fewer side reactions occur
Vacuum pyrolysis	Short residence time	High maintenance and investment costs.	[101,103]
	Reduced energy consumption compared to traditionally used pyrolysis technologies

shows a comparison of different thermochemical conversion technologies presented in this study. As the table shows, each of these technologies has unique features that make them suitable for producing various products.

2.4. Reactor Types

The reactor type determines the final product yield and its distribution during pyrolysis alongside the operating conditions. The desired product yield depends on the reactor configuration used and the residence time of the volatiles [105]. Pyrolysis reactors can be divided into two main groups: fixed bed reactors, where the feedstock does not move, and moving bed reactors, where movement could be caused by a mechanical force or fluid flow [106]. Several other reactor types are under these two main groups, such as a fluidized bed, vacuum pyrolizer, and ablative pyrolizer [107]. In addition, there are other reactor types, such as auger, rotating cone reactors [108], and batch-type reactors [109].

2.4.1. Fluidized Bed Reactor

Based on operating parameters such as vapor residence time and pyrolysis temperature, a fluidized bed reactor conducts fast pyrolysis [110]. In this reactor type, gasifying agents maintain the biomass in a fluidized state. The biomass is mixed with an inert bed material, improving heat transfer. This bed enables a uniform temperature in the conversion zone. In the conversion zone, the devolatilization, drying, oxidation, and gasification co-occur [111]. The typical operating temperature varies from 700 °C to 900 °C [112]. According to some publications, the operating temperature could also be higher, between 1000 °C and 1050 °C [113,114].

2.4.2. Ablative Plate Reactor

An ablative plate reactor type is also used for fast pyrolysis [115]. In an ablative reactor, the feedstock is pressed onto a hot surface, and the heating is conducted by using hot flue gas. The gas is produced by pyrolysis combustion gases [116]. The main product is pyrolysis oil [117]. The reaction rate is affected by the reactor surface temperature and pressure. The disadvantage of this type of reactor is related to heat transfer efficiency from the surface of the reactor to the feedstock. Additionally, feedstock properties such as particle size impose restrictions because the feedstock must be pressed onto the reactor’s surface [12,108].

2.4.3. Auger Reactor

Based on the pyrolysis temperature and residence time, an auger reactor represents a slow pyrolysis [118]. In an auger-type reactor, the feedstock is fed into the reactor with a screw. The screw mixes the particles, moves them to the reactor, and at the same time controls the residence time in the reactor by its speed. The reaction heat is carried out by heating the wall around the screw. The advantage of auger-type reactors is that they control the mass flow well. The disadvantages are the risk of plugging, mechanical wear to the moving parts at high temperatures, and the possibility of heat transfer problems [119].

2.4.4. Rotating Cone Reactor

Rotating cone reactors represent a fast pyrolysis [120]. In a rotating cone reactor, the feedstock and sand are fed into the bottom of the reactor. Because of the reactor’s constant motion, the feedstock is forced against the wall [108]. The wall is heated, and in this area, pyrolysis occurs. After pyrolysis, the biocarbon and sand are collected, and the biocarbon is burned to heat the sand. Typically, when this reactor type is used, a liquid yield of 60%–70% is achieved [120].

2.4.5. Cyclone/Vortex Reactor

Cyclone/vortex-type reactors represent fast pyrolysis [121]. The feedstock is fed into the reactor alongside hot steam or nitrogen gas. After entering the reactor, the feedstock is forced into contact with the reactor wall at high speed caused by centrifugal forces. This reactor type leads to pyrolysis oil yields of up to 65% [109].

The reactor types sorted by their biocarbon yield are shown in

Table 4. The effect of reactor type on biocarbon yield.

Reactor Type	Pyrolysis Type	Feedstock Material	Biocarbon Yield (%)	Temperature (°C)	Reference
Batch	Fast	Rice straw	47.7	500	[122]
Auger	Slow	Rice straw	44.9	500	[122]
Rotating cone	Fast	Wood waste pellets	29.1	450	[123]
Fluidized bed	Fast	Rice straw	26.8	500	[122]
Ablative plate	Fast	Corncob	24.5	500	[115]
Cyclone/vortex	Fast	Pine	13.9	500	[121]

.

As seen in Table 2, the reactor type strongly affects the biocarbon yield. The best biocarbon yield was achieved using batch and auger-type reactors, whereas cyclone/vortex-type reactors resulted in a decreased biocarbon yield. However, the results are not directly proportional because the feedstock material used differs from one reactor type to another.

The biocarbon yield varies from one reactor type to another, and the product distribution changes depending on the reactor type used. Nam et al. [122] pyrolyzed wheat starch in three different reactor configurations at a temperature of 500 °C. The results of these experiments are shown in Figure 2. In a batch-type reactor, the whole feedstock is fed into the reactor once, then pyrolysis is performed [109].

As Figure 2 shows, the product distribution influences the reactor type used. Using auger and batch-type reactors increases biocarbon yield, while pyrolysis gas yield decreases. Additionally, the reactor type used affects the proportion of pyrolysis oil. According to Nam et al. [122] the higher biocarbon yields in the case of auger and batch reactors are related to the fact that the overall process is slower in these reactors compared to fluidized bed reactors. The faster heating rate in the case of fluidized bed reactors favors pyrolysis oil formation. In contrast, slow reactions in the auger and batch reactors favor the biocarbon formation.

Figure 3 shows what thermochemical conversion methods each conversion technology used represents. Additionally, suitable reactor types for each pyrolysis technology are presented. Microwave and vacuum pyrolysis are separated as pyrolysis reactor types because they do not represent any of the typically used reactor types [57,124].

As seen in Figure 3, most of the new, development-stage pyrolysis technologies are unsuitable for a wide selection of reactor types. In fact, there is mainly only one reactor type that can be used for the respective technology. The only exception is solar pyrolysis, where two reactor types are suitable. Based on this figure, it can be summarized that if the aim is to produce biocarbon with suitable properties for metallurgical applications, the best combination is to use microwave pyrolysis.

3. The Effect of Reaction Conditions and Process Parameters

Biocarbon yield during pyrolysis is influenced by reaction conditions such as temperature, heating rate, pressure, and reaction atmosphere. These parameters are responsible for biocarbon yield during pyrolysis and affect product quality, pyrolysis gas, and oil yield. Discussing the effect of those parameters is essential because, in most cases, the purpose is to increase the yield of one of the three main products.

3.1. Effect of Final Temperature and Heating Rate

The pyrolysis temperature and heating rate are two main features that affect biocarbon yield during pyrolysis [23]. For example, biocarbon produced at higher temperatures has a higher carbon content and greater pore volume. The heating rate, conversely, defines the type of pyrolysis, i.e., if it is slow, fast, or flash. It also affects the product distribution during pyrolysis. Higher heating rates lead to increased pyrolysis gas production [23,128]. These parameters also affect the biocarbon yield, porosity, and surface area [23,128]. Higher heating rates tend to favor high pyrolysis oil yields [128] whereas slow heating rates tend to favor biocarbon formation [129].

According to Solar et al. [130] the biocarbon yield decreases when pyrolysis temperature rises in conventional pyrolysis. This is because secondary reactions affect biocarbon yield at higher temperatures. The effect of the final pyrolysis temperature on the yield of sugarcane bagasse with a heating rate between 45 °C/min and 50 °C/min with a 60 min holding time at the final temperature is presented in Figure 4. The data is based on ref. [131]. These results show that increasing the pyrolysis temperature causes the biocarbon yield to decrease as the temperature increases. The pyrolysis gas yield increases, and the pyrolysis oil yield decreases when the pyrolysis temperature rises. In this case, losses during pyrolysis are mainly due to volatiles condensing inside the pyrolysis equipment.

Solar et al. [130] found that biocarbon quality improves when pyrolysis occurs at higher temperatures. According to Tomczyk et al. [13], biocarbon produced at a higher temperature has an increased surface area and more carbonized fraction. Mlonka-Mędrala et al. [132] found that when the pyrolysis temperature increases, the oxygen to carbon (O/C) ratio and hydrogen to carbon (H/C) ratio both decreased when oat straw was pyrolyzed. Penzik et al. made similar observations when pinewood was pyrolyzed [133]. Because the O/C ratio was reduced, the biocarbon properties of the final products were improved, and they could be used as raw material for further processes; for example, for activated carbon production [132].

The heating rate also has an impact on the biocarbon properties. Angin [129] found that an increased heating rate from 10 °C min⁻¹ to 50 °C min⁻¹ affected the fixed carbon content of the final product. The fixed carbon content tended to be higher as the heating rate rose. With a heating rate of 10 °C min⁻¹ and final temperatures of pyrolysis of 400 °C and 600 °C, the fixed carbon content was 67.30% and 79.20%, respectively. With a heating rate of 50 °C/min⁻¹, the fixed carbon yield was 71.70% and 80.70%, respectively. They also reported that the biocarbon Brunauer–Emmett–Teller surface area and pore volume decreased as the heating rate increased. This is because increasing the heating rate decreases the time the volatiles are discharged [129].

Figure 5 shows the temperature and heating rate effects on biocarbon yield for different organic-based biomasses. These results only include data obtained at atmospheric pressures. In summary, it can be said that higher heating rates lead to decreased biocarbon yields, and higher temperatures also reduce the biocarbon yield. The effect of different biomasses is also visible, especially at lower temperatures, but the difference is not as significant when higher temperatures are being used. Biocarbon yield tends to decline when the heating rate rises, especially when the final pyrolysis temperature is over 500 °C. At lower temperatures, the effect of the heating rate is contradictory. It seems that increasing the heating rate raises the biocarbon yield when the final pyrolysis takes place at temperatures lower than 400 °C. This phenomenon may be linked to the limited size of the reference material.

3.2. Vapor and Biomass Residence Time

Generally, longer vapor residence times favor biocarbon formation, whereas shorter times tend to favor pyrolysis oil and pyrolysis gas formation [15]. Organic vapors are quickly removed when shorter vapor residence times are used, preventing secondary reactions [137]. On the contrary, when the vapor residence time is long, it causes secondary cracking reactions to take place and thus leads to an increased biocarbon yield [138].

The biomass residence time in the reactor also affects biocarbon yield during pyrolysis. Solar et al. [130] reported a decreased biocarbon yield when a woody biomass waste was pyrolyzed in a laboratory-scale continuous pyrolysis plant. Liu et al. [139] reported similar results when peanut shells were pyrolyzed. The reason behind this is likely to be related to the increased decomposition of biocarbon as the residence time is increased. This causes more pyrolysis gas to be formed during pyrolysis [139].

3.3. Feedstock Particle Size

Feedstock particle size and particle size distribution are important factors that affect biocarbon yield during pyrolysis [43,140]. Kirubakaran et al. [141] suggested that in view of heat transfer, a smaller particle size below 0.2 cm would be ideal. This is because, at this particle size, achieving a uniform temperature is possible throughout the particles, thus allowing chemical reactions to occur through the particle.

Şensöz et al. [142] found that the biocarbon yield was independent of the particle size when fast pyrolysis was carried out at a temperature of 500 °C. Liu et al. [139] reported that smaller particle sizes generated less biocarbon when pyrolysis took place at 600 °C. According to Liu et al. [139], this is possible because a greater temperature gradient is achieved when the particle size is larger. It leads to more biocarbon and less pyrolysis oil and pyrolysis gas produced during pyrolysis. Yu et al. [143] reported that a larger particle size generated more biocarbon when pyrolysis took place at 300–900 °C. This was especially visible at temperatures lower than 400 °C. When the temperature was increased, the impact of the particle sizes was reduced. Additionally, the O/C and H/C ratios were higher in the case of larger particles when the pyrolysis temperature was below 400 °C. At the same time, the effect of particle size on the O/C and H/C was insignificant when the pyrolysis temperature was above 400 °C.

Somerville and Deev [144] came to the same conclusion as Şensöz et al. that biocarbon yield is independent of particle size when Radiata pine (Pinus radiata) was slowly pyrolyzed at 650 °C. However, when a purge gas flow was added to the pyrolysis process, the biocarbon yield became dependent on particle size, and smaller particles produced less biocarbon compared to larger particles. More recently, Altmear et al. [24] also showed that a feedstock with smaller particles generated less biocarbon than larger particles when pyrolysis occurred at 475 °C under a sweeping gas flow. According to Altamer et al. [24], this is related to faster heat transfers for smaller particle sizes.

3.4. Reaction Atmosphere

Usually, the pyrolysis of biomass is carried out in an inert, non-oxidizing gas atmosphere [137]. Other gases, such as water, nitrogen, hydrogen, carbon dioxide, and carbon monoxide, can also be used to modify the pyrolysis process [145,146]. The effect of mixing carbon dioxide with other gases has also been evaluated [147].

Minkova et al. showed that when a water vapor flow is present during slow pyrolysis, it increases pyrolysis oil yield but decreases the biocarbon and pyrolysis gas yield. The produced biocarbon had a higher surface area together with a good adsorption capacity [146]. Özbal et al. [148] confirmed Minkova’s observation of an increased oil yield when steam was used. However, the use of steam has a negative impact on biocarbon yield. The reason behind the increased pyrolysis oil content and decreased biocarbon yield is that the presence of steam prevents secondary cracking reactions [148]. According to Önal et al. [149], using steam increases biocarbon’s surface area. The same study also reported that increasing gas velocity affected the pyrolysis product distribution. When the gas velocity was increased, it decreased the biocarbon yield and increased the pyrolysis gas yield. Using a higher amount of nitrogen during pyrolysis had the same effect. The pyrolysis oil yield also decreased when the nitrogen flow rate was doubled from 200 cm³ min⁻¹ to 400 cm³ min⁻¹. Aladin et al. [150] reported that when nitrogen was used during pyrolysis, it increased the biocarbon calorific and fixed carbon value compared to pyrolysis without using nitrogen.

Lee et al. [151] pyrolyzed peat in the presence of CO₂. They found that using CO₂ enhanced the pyrolysis gas production at temperatures over 440 °C. At the same time, tar formation was reduced. The produced biocarbon had a larger surface area than the biocarbon produced in an N₂ atmosphere. This finding suggests that CO₂ had a role in the formation of the pores on the biocarbon’s surface. Manyà et al. [152] also studied the effect of using a CO₂ atmosphere instead of an N₂ atmosphere in the case of slow pyrolysis. They found that using a CO₂ atmosphere did not significantly change the pyrolysis product distribution or potential stability of the biocarbon. Their findings indicate that when pyrolysis takes place in a CO₂ atmosphere, it is possible to obtain two high-value products: biocarbon and pyrolysis gas. However, Pilon and Lavoie [153] reported that using CO₂ gas led to decreased biocarbon production compared to a nitrogen atmosphere at the temperature of 500 °C.

3.5. Pressure

Typically, pyrolysis is performed at an atmospheric pressure [23]. Only a few studies focused on using different pressures during pyrolysis and their effect on the biocarbon yield during pyrolysis. Basile et al. [154] studied the effect of pressure on solid biocarbon yield in the pyrolysis of three different raw materials; namely, corn, poplar, and switchgrass. The pressure range in the experiments was between 0.1 MPa and 4 MPa, while the pyrolysis temperature remained at 500 °C in all the experiments. They found that increasing the pressure from 0.1 MPa to 4 MPa increased the biocarbon yield in all cases. They also found that the reactions may shift from endothermic to exothermic when the pressure is increased.

Qin et al. [155] reported an increased biocarbon yield at a temperature of 550 °C when the pyrolysis pressure was changed from 0.1 MPa to 2 MPa. The most significant change in biocarbon yield occurred when the pressure was raised from 0.1 MPa to 1.0 MPa. Above this pressure range, biocarbon yield did not notably change. Gouws et al. [156] performed the pyrolysis of pine and eucalyptus mixtures at temperatures from 400 °C to 600 °C at three different pressures: 1 bar, 15 bar, and 30 bar. The study concluded that increased pressure led to increased biocarbon yields, which aligns with Basile et al. [154].

3.6. Catalyst

Generally, a catalyst is used in pyrolytic processes to modify the composition, chemical, and physical properties of pyrolysis products [157]. According to Tripathi et al. [45], it is hard to state any general rule on how different catalysts affect product yield. This is because each biomass is different in composition, ash, and water content; thus, each catalyst’s reactions may differ. Still, it is possible to say that using acidic catalysts leads to increased biocarbon yields, and using basic catalysts leads to lower biocarbon yields [45]. In recent years, catalyst-related studies have focused on increasing pyrolysis oil yield and its quality rather than biocarbon yield and quality [158,159,160,161]. Despite this, several studies still show that the biocarbon yield was increased by using different catalysts.

Aho et al. [162] found that when using a zeolite-based catalyst at a temperature of 450 °C, the biocarbon yield was increased compared to noncatalytic pyrolysis. This was possible due to the coking of the catalyst. Wan et al. [159] studied how several different catalysts (Na₂HPO₄, H₃BO₃, MgCl₂, Al₂O₃, KaC, and K₂Cr₂O₇) affected the product distribution when corn stover was pyrolyzed using microwaves. They found that only one of the studied catalysts increased biocarbon yield (Al₂O₃), whereas several others increased pyrolysis oil yield (H₃BO₃, MgCl₂, Al₂O₃, KaC, and K₂Cr₂O₇). The increase in pyrolysis oil occurred at the cost of both the biocarbon and pyrolysis gas yields. The results may indicate that these catalysts work as a microwave absorbent, thus speeding up heating.

More recently, Mohamed et al. [163] studied the effect of several catalysts and catalyst mixtures in microwave pyrolysis. The catalysts used were bentonite (Al₂O₃₄SiO₂H₂O), Potassium Phosphate Tribasic (K₃PO₄), and clinoptilolite ((K,Ca,Na)₂O-Al₂O₃-10SiO₂-6H₂O). They found that depending on the catalyst and mixtures, it either reduced (10% for clinoptilolite) or increased (10% for K₃PO₄ and 10% for Bentonite) the microwave heating rate. They also observed that combining different catalysts significantly improved catalyst performance; thus, less catalyst was needed for pyrolysis. Chai et al. [164] found that using a zeolite-based catalyst (ZSM-5) improved biocarbon yield in pyrolysis.

3.7. Binders

Binders are used when the material undergoes briquetting [165]. This technique is used for compacting loose material into a more energy-dense material [166]. The binder can be added before [167] or after pyrolysis [168]. When binders are added before pyrolysis, they are mixed with feedstock and pelletized. This aims to improve the pellet properties, such as their tensile strength, and achieve a higher heating value [169]. When binders are added after pyrolysis, the feedstock is first pyrolyzed, and then the binders are mixed with biocarbon and pelletized. This affects pellet properties, such as energy and bulk density [168].

Binders can be divided into three groups based on differences in their material composition. These groups are organic binders, inorganic binders, and composite binders. Each group has their own advantages. For example, organic binders have a low ash content, but their use is limited because they easily decompose with heat. Inorganic binders are cheap, but their ash content is relatively high. Composite binders are a mixture of two or more binders, and they combine the advantages of different binders [165]. These binders offer high mechanical strength and thermal stability, but the downside is their high price and increased ash content [170]. Different binders, categorized into organic, inorganic, and composite categories, are presented in

Table 5. Binder types and different binders.

Organic Binders	Inorganic Binders	Composite Mixtures
Peach starch [171] Sodium carboxymethylcellulose [171] Cassava starch [172] Asphalt [173] Pyrolysis oil [174,175,176,177] Molasses [178] Distillers dry grain [178] Crude glycerol [169] Lignosulfonate [169,179] Lignin [168] Lignin powder [167] Pinecones [167] Pyrolytic Lignin [180] Protein, starch, lignin, and molasses [181] Waste cooking oil [182] Deionized water [183]	Bentonite [184,185,186,187] Ash [188] Limestone [165] Clay [165] Magnesium oxide [165] Calcium oxide [165] Calcium hydroxide [168] Sodium hydroxide [168] Attapulgite [189] Sodium silicate [165] Magnesium chloride [165] Waste Plastic [190]	Kaolin-bentonite-sodium humate [191] Corn straw-sodium hydroxide-MgCl/MgO [191] Coal tar pitch phenolic resins [191] Coal tar pitch and molasses [192]

.

Kang et al. [176] reported that when pyrolysis oils are used as a binder, they improve surface morphology and the pellets’ physical stability. More precisely, these oils improve pellet drop resistance and compression strength. However, there is an increased risk of self-ignition when pyrolysis oil is used as a binder [175].

Bentonite possesses a good ability to form bonds [165]. This is why, in recent years, several studies have been made focusing on using bentonite [185,187]. Russell et al. [187] found that in the temperature range of 400–700 °C, adding bentonite increases biocarbon and pyrolysis gas yields and decreases the pyrolysis oil yield. This is in line with the results obtained by Dou and Goldfarb [187]. Dou and Goldfarb also reported that adding bentonite to a biomass increases biocarbon surface area when pyrolysis takes place at the maximum temperature of 350 °C. Lu et al. [169] reported that bentonite increases the ash content and density of pellets while higher heating value is decreased.

4. Discussion

Process parameters such as heating rate and final pyrolysis temperature must be optimized for selected biomass to achieve better biocarbon yield during pyrolysis. Biomass typically contains three main compounds: cellulose, hemicellulose, and lignin. The amount of these varies from feedstock to feedstock, and each has its own effect on biocarbon yield during pyrolysis. For example, it is suggested that the high lignin content of biomass-based feedstock leads to increased biocarbon yield [14,19,22,26,38].

Pyrolysis temperature and heating rate play key roles in biocarbon formation during pyrolysis. Using low temperatures below 400 °C favors biocarbon formation and thus improves biocarbon yield during pyrolysis. This increased biocarbon yield during low-temperature pyrolysis is because biocarbon yield at higher temperatures is affected by the occurrence of secondary reactions [14,130]. Additionally, using lower heating rates instead of high heating rates increases biocarbon yield during pyrolysis, especially when final pyrolysis takes place at temperatures over 500 °C [67,136]. Using a longer vapor residence also increased biocarbon yield during pyrolysis. This is because a longer vapor residence time guarantees that secondary cracking reactions will occur [14,137].

Feedstock particle size also has its own impact on biocarbon yield during pyrolysis. It is suggested that using a small particle size below 0.2 cm increases biocarbon yield during pyrolysis. This is because, in this particle size, it is easier to achieve uniform temperature throughout the particle, thus allowing a reaction to take place through the particle [141]. Furthermore, the atmosphere used has its own effect on biocarbon yield during pyrolysis. Typically, pyrolysis is carried out in an inert, non-oxidizing atmosphere. Nitrogen and carbon dioxide are typically used as atmosphere gases. The effect of using steam has also been evaluated. Steam negatively affects biocarbon yield by preventing secondary reactions, increasing pyrolysis oil and pyrolysis gas yield at the expense of biocarbon yield [137,145,146,148].

In recent years, there has been quite significant process development related to pyrolysis technologies. These technologies are microwave-, solar-, plasma-, and vacuum pyrolysis. Each has advantages and disadvantages, and their biocarbon yield varies [30,86,91,97,98,99,103].

Microwave-assisted pyrolysis has shown great potential to increase biocarbon yield during pyrolysis, and other properties such as carbon content and calorific value are increased compared to conventional pyrolysis. However, microwave pyrolysis has faced problems scaling from laboratory to industrial scale [30,70,71]. Solar-based pyrolysis is a promising technology for producing biocarbon with good yield at relatively low temperatures. However, this technology is not applicable globally because the efficiency of the process is highly dependent on the day and current season [86,91]. Other technologies such as plasma and vacuum pyrolysis do not produce biocarbon with a high yield. These technologies focus on producing pyrolysis oil and pyrolysis gases rather than biocarbon. Moreover, they also have some disadvantages. For example, plasma pyrolysis consumes high energy, and vacuum pyrolysis has high maintenance costs due to reactor configuration [97,98,99,103].

Finally, reactor type strongly affects biocarbon yield. Only a few combinations are suitable for producing biocarbon with a high yield for those reactor types. This is because of the heating rates used and the structures of these reactors. For the reactor types presented in this review, only the auger reactor represents the slow pyrolysis reactor type [105,109,118]. On the other hand, an auger reactor also produces biocarbon with a high yield. The following recommendations for production conditions can be made based on combining these factors, leading to the best biocarbon yield and highest quality in metallurgical applications.

• Slow heating rates, below 1 °C/min.
• Low pyrolysis temperatures, below 500 °C.
• Batch or auger reactors.
• Nitrogen as a purge gas.
• Use of small particle size, below 0.2 cm.
• Use of atmospheric pressure.

5. Conclusions

The presented work reviewed different ways to produce biocarbon and evaluated the efficiency of those processes to produce biocarbon suitable for metallurgical applications. This included a discussion of different pyrolysis techniques and technologies, reactor types, and process parameters that could be used to increase biocarbon yield.

Based on this review, some remarkable points are concluded. (1) For traditionally used technologies, slow pyrolysis leads to the best biocarbon yield during pyrolysis. It is mainly due to the combined effect of slow heating rates and longer residence time in the reactor compared to fast and flash pyrolysis. Additionally, slow pyrolysis leads to biocarbon that has suitable properties for metallurgical use. (2) For new, emerging technologies, microwave and solar pyrolysis are the most preferred technologies for producing biocarbon for industrial use. However, these two technologies have their limitations. Microwave pyrolysis has faced problems scaling from laboratory to industrial scale, and solar pyrolysis is limited to only a certain time of day and a specific part of the world because radiation varies depending on location and time of day. Vacuum and plasma pyrolysis are more suitable for producing pyrolysis oil and pyrolysis gas than biocarbon, and they also have problems related to high costs and high energy consumption. In view of other ways to increase biocarbon yield, segmented pyrolysis seems to have great potential for increasing biocarbon yield and enhancing its properties. Because of the limited number of studies, more research is needed to gain information about biocarbon properties produced using segmented pyrolysis.