Biochar: Production, Application and the Future

Abstract

Biochar, or carbon obtained from biomass, is a particularly rich source of carbon created by thermal burning of biomass. There is a rise of interest in using biochar made from waste biomass in a variety of disciplines to address the most pressing environmental challenges. This chapter will provide an overview on the methods employed for the production of biochar. Biochar has been considered by a number of analysts as a means of improving their ability to remediate pollutants. Process factors with regards to biochar properties are mostly responsible for determining biomass production which is discussed in this present chapter. Several characterization techniques which have been employed in previous studies have received increasing recognition. These includes the use of the Fourier transform infrared spectroscopy and the Scanning electron microscope which duly presented in this chapter. This chapter also discusses the knowledge gaps and future perspectives in adopting biochar to remediate harmful contaminants, which can inform governmental bodies and law-makers to make informed decisions on adopting this residue.

1. Introduction

The word char, is a common terminology used for the solid product of the combustion of carbonaceous material [1]. Generally, char product is rich in carbon content; an example is charcoal, which is almost the earliest invention of humans from fire or heat creation. Another vivid example of char is biochar. In this case, the study, is made from organic compounds such as forest, agricultural or animal products but in the absence/limited supply of oxygen compared to charcoal. Therefore, biochar is derived from biomass combustion in the presence of a limited oxygen supply and at relatively low temperatures below 700°C. The earliest known purpose for creating biochar was specifically for soil application such as carbon storage or sequestration in soil; improvement of soil performance such as increase in nutrient availability, reduction of compactness in soil, soil pH improvement; soil water filtration. Recent applications involve energy production, biochemical process stability and improvement, climate change mitigation, and construction additive [1, 2, 3]. The raw material determines carbonized organic matter properties and the operational parameters used during it production. Pyrolysis (slow or fast) and gasification are the main methods for the production of biochar. The physical nature of the biochar produced is directly affected by the chemical composition of the biomass feedstock. Most organic matter begin to thermally decompose at temperatures above 120°C. Hemicelluloses degrade between 200 and 260°C, cellulose between 240 and 350°C, and lignin between 280 and 500°C. As a result, the proportions of these components will affect the degree of reactivity and, as a result, the extent to which the physical structure is modified during processing [4]. Biochar is characterized with high porosity with pores ranging in size from micro to macropores. Large holes, which originate from the raw biomass’s vascular bundles, are critical for increasing soil quality because they can serve as habitats for symbiotic microbes. Biochar major components are carbon, volatile matter, mineral matter (ash), and moisture. The percentage composition of each components varies based on the feedstock material and the operating parameters [1]. Biochar from plant-based materials have higher carbon composition which range from as low as 51% to as high. The understanding of the key mechanisms for changes in physicochemical properties of biochar during processing for various feedstock types and operating parameters is required to determine biochar’s potential for application both now and in future. Therefore, this chapter explains biochar production techniques, factors affecting its properties and compositions and its application.

2. Biochar production techniques

An ever-growing appetency for using biochar for various applications has orchestrated an increase in converting it into biochar. Thermochemical conversion is a common technology for making biochar. Thermochemical conversion techniques are pyrolysis, hydrothermal carbonization (HTC), gasification, torrefaction, and hydrothermal liquefaction [5, 6].

2.1 Pyrolysis

Pyrolysis is a thermochemical technique that produces biochar, bio-oil, and syngas derived from biomass [7]. The process involves heating and thermally decomposing biomass under anaerobic conditions or limited oxygen supply (low stoichiometric oxygen atmosphere) with temperatures ranging between 400°C and 1200°C [2]. The absence of oxygen enables biomass heating beyond its thermal stability limit, causing the creation of more robust products, including solid residues. By creating an anaerobic atmosphere, it is also ensured that combustion will not occur when the biomass is heated. It is a highly complex process involving many distinct reactions in the reacting zone [8]. In another study, a low-temperature range for pyrolysis was recorded between 250°C and 900°C. Biomass from Agriculture comprises lignin, cellulose, hemicelluloses, and silica. Typically, cellulose pyrolyzes at 350°C, whereas the melting point of lignin is well above 350°C [6]. Although the product yield depends on various operating variables, char formation is generally favored by low temperatures and long residence times [9]. Therefore, it can be decoded that the effective temperature range for pyrolysis was between 300 and 700°C. The cracking of heavy chemicals happens in secondary pyrolysis and converts biomass into biochar or gases. Figure 1 is a summary of the pyrolysis technique and the operating variables affecting pyrolysis.

Figure 1.

Schematic representation of pyrolysis process [3].

In essence, this is an alternative way to valorize biomass into various products such as bio-oil, syngas and biochar. Depolymerization, fragmentation, and cross-linking are chemical mechanisms that occur during the process at specific temperature points, resulting in a different product state for lignocellulosic components, including cellulose and hemicellulose (solid, liquid and gas). Biochar and bio-oil are the solid and liquid products, whereas CO₂, CO, H₂, (collectively known as syngas) are evolved as the gaseous by-products (C₁-C₂ hydrocarbons) [3]. Biochar is made in a different type of reactors, such as paddle kiln, bubbling fluidized bed, wagon reactor, and agitated sand rotating kiln. The biomass nature and employed type determine the biochar yield during the pyrolysis route. The major operating parameter that impacts product efficiency is the temperature [10, 11]. When the pyrolysis temperature is increased, biochar’s yield decreases and the generation of syngas increases. The gas yield is represented by the initial section of the product side (as shown in Eq. (1)), with various gases created during the process.

$${\left({\mathbf{C}}_{\mathbf{6}}{\mathbf{H}}_{\mathbf{6}}{\mathbf{O}}_{\mathbf{6}}\right)}_{\mathbf{n}}\to \left({\mathbf{H}}_{\mathbf{2}}+\mathbf{C}\mathbf{O}+\mathbf{C}{\mathbf{H}}_{\mathbf{4}}+\dots +{\mathbf{C}}_{\mathbf{5}}{\mathbf{H}}_{\mathbf{12}}\right)+\left({\mathbf{H}}_{\mathbf{2}}\mathbf{O}+\mathbf{C}{\mathbf{H}}_{\mathbf{3}}\mathbf{O}\mathbf{H}+\mathbf{C}{\mathbf{H}}_{\mathbf{3}}\mathbf{COOH}+\dots \right)+\mathbf{C}$$E1

The mixture of multiple sorts of liquid outputs is shown in the second part of the products’ side, and the solid yield is represented in the last component [12]. One of the most significant masteries of this technology is that it may be optimized to achieve the desired outcomes. Slow pyrolysis, for example, can be utilized to produce a considerable amount of biochar, whereas fast pyrolysis is better for dominantly producing bio-oil [13].

2.1.1 Types of pyrolysis

Pyrolysis is strongly dependent on the operating parameters, namely temperature, heating rate, and residence time [14]. These operating conditions further help to categorize pyrolysis into other six subclasses. These subclasses are slow pyrolysis, fast pyrolysis, flash pyrolysis, vacuum pyrolysis, intermediate pyrolysis, and hydropyrolysis [15]. Each classification of pyrolysis has its own documented benefits and drawbacks. The subclasses in question foster an environment for different reaction conditions and mechanisms to have various products. The pyrolysis technology mechanism is shown in Figure 2.

Figure 2.

Representation of a pyrolysis process [6].

2.1.1.1 Slow pyrolysis

As indicated by the name, to complete the process, slow pyrolysis has a long residence time (more than 1 hour), and biochar is produced as a major product [16]. Slow pyrolysis is dubbed conventional pyrolysis, where biomass is heated at temperatures ranging between 300 and 600°C accompanied by a heating rate of 5–7°C/min [12, 17]. A lower heating rate and longer vapor residence time provide a suitable environment and adequate time for the secondary reactions to proceed. Furthermore, a prolonged residence period permits vapors created during the secondary reaction to be evacuated [15, 18]. This leads to the creation of solid carbonaceous biochar in the end. Slow pyrolysis favors char development, but liquid and gaseous products are also created in modest quantities. Biochar is formed as a primary product (35–45%) together with other products such as bio-oil (25–35%) and syngas (20–30%), as indicated in Eq. (1) [6, 19].

2.1.1.2 Fast pyrolysis

Fast pyrolysis is a direct thermochemical process for converting solid biomass into high-energy liquid bio-oil. A high-efficiency thermochemical technique to produce biomass-derived biofuels, with reduced amounts of solids and gases produced [20, 21]. Fast pyrolysis is carried out without oxygen at temperatures above 500°C and a heating rate of over 300°C/min. Fast pyrolysis is a rapid biochar generation technique that takes only a few seconds. Fast pyrolysis produces 60% bio-oil, 20% biochar, and 20% syngas, as reported in other studies [21, 22]. Even higher temperatures in the range of 850–1250°C with a heating rate of 10–200°C for a short residence time ranging from 1 to 10 s have been reported in several experiments. 60%-75% of liquid products, 15%-25% of biochar and 10–20% of non-condensable gaseous products are produced by a typical pyrolysis process [23]. Fast pyrolysis takes biomass to temperatures in which thermal cracking can occur and minimizes the exposure time, which supports biochar production [24].

2.1.1.3 Flash pyrolysis

This is dubbed to be an enhanced and modified version of fast pyrolysis. Biomass decomposes quickly, usually in less than a minute, at 1000°C and even higher temperatures. Heating rates of above 1000°C/sec have been recorded on occasion. Flash pyrolysis is carried out at temperatures between 900 and 1200°C, which can be reached in less than one second (usually between 0.1 and 1 s) [25, 26]. A high bio-oil yield combines a high heating rate with a high temperature and a short vapor residence time. However, the yield of biochar is reduced because of the process [27, 28]. In flash pyrolysis, heat and mass transfer processes, reaction chemical kinetics and biomass phase transition behavior all play a role in product distribution. Although flash pyrolysis is performed in a fluidised bed reactor and a twin-screw mixing reactor, it has limited industrial applicability because of the reactor’s architecture, which requires it to run at a high temperature with a very high heating rate [12].

2.1.1.4 Vacuum pyrolysis

This is the thermal decomposition of biomass under vacuum or relatively low pressure in an isolated oxygen environment [15, 29]. Pressure is usually regulated in the region between 0.5 and 2 bar, and temperature is maintained at 450–600°C [30]. Like slow pyrolysis, vacuum pyrolysis has comparably low heating rates. However, these two techniques, in comparison, yield significantly different products. This owes to the constant and effective discharge of the vapor produced during vacuum pyrolysis through condensation train. The rapid evacuation of organic vapors created during the primary pyrolysis also considerably minimizes the vapor residence time, which in turn minimizes the occurrence of secondary reactions and assures a high liquid product yield during the secondary pyrolysis [31]. As a result, only vacuum or low-pressure extraction is utilized to remove vapor evolved during pyrolysis, which substantially affects product quality and yield by preventing inorganic devolatilisation.

2.1.1.5 Intermediate pyrolysis

As the name suggests, this is a combination of slow and fast pyrolysis processes, and it is crucial when there is a need to balance solid and liquid products. This means that slow pyrolysis is more efficient at producing large amounts of char, but it also results in lower amounts of liquid products, while it is vice versa with fast pyrolysis. Generally, pressure is kept at 1 bar during the process. Intermediate pyrolysis has temperatures ranging between 500 and 650°C, with heating rates between 0.1 and 10°C/min and residence time between 5 and 17 mins [32]. 40–60% liquid, 20–30% non-condensable gases, and 15–25% biochar are typical constituents of finished products [33, 34]. Using intermediate pyrolysis conditions prevents the synthesis of high molecular reactive tars and results in dry biochar, which can be utilized for agricultural purposes or directly in boilers and engines in conjunction with high-quality bio-oil [2].

2.1.1.6 Hydropyrolysis

It relatively a novel technique that is used for the conversion of biomass into high quality products by injection of hydrogen or hydrogen based material into the reactor under high pressure, typically above the atmospheric pressure, stretching from 50 bar to 200 bar [15, 35]. The heating rate (10–300°C/s), residence time (over 15 sec) and temperature (350–600°C) are not highly deviated from fast pyrolysis [36]. In essence, hydropyrolysis can be considered a special type of fast pyrolysis subjected to high pressure in an atmosphere infused with hydrogen or hydrogen-based material. This method is not ideal for the production of biochar as the introduction of hydrogen under high temperature and pressure acts as a reducing agent, hence reducing oxygen content in the bio-oils produced and synchronously inhibiting the production of biochar [37, 38]. The employment of a catalyst to eradicate oxygen, water, and CO_x from the liquid product is typically linked with hydropyrolysis. Catalysts also reduce depolymerisation and coking reactions [39]. However, developing the catalyst for this intention remains a notable example of the difficult aspects of catalytic hydropyrolysis.

2.2 Carbohydrate decomposition

The majority of the material used in biochar production via pyrolysis contain carbohydrates in various forms (cellulose, hemicellulose and lignin), and these react differently based on the operating conditions they are subjected to, thus influencing the product yield of pyrolysis [15]. More specifically, lignin and cellulose are the major parts of biomass, making up its bulk [40]. On pyrolysis, cellulose mostly creates tar, a mixture of discrete ketones, aldehydes, organic liquids, and char, whereas lignin essentially produces char and a minimal amount of water. As the cellulose content grows but the char and tar content decreases, the yield of gaseous content increases. It’s also been discovered that structural differences in biomass cause changes in the pyrolysis product’s composition [41].

2.2.1 Cellulose decomposition

By lowering the extent of polymerization, the process of cellulose degradation is determined, which consists of two principal reactions:

1. Slow pyrolysis involves cellulose degradation over a prolonged period with a lower heating rate.

2. Fast pyrolysis occurs at high heating rates through speedy volatilization and leads to levoglucosan formation.

In addition to producing the solid product biochar, levoglucosan is dehydrated to generate hydroxymethylfurfural, which can break down to produce liquid and gaseous products such as bio-oil and syngas, respectively. Furthermore, the hydroxymethylfurfural can undergo several processes, including aromatization, condensation, and polymerization, to generate solid biochar [42, 43]. At low temperatures, cellulose degrades to a reasonably stable anhydrocellulose that produces a lot of char, but it decomposes into volatiles [25, 44].

2.2.2 Hemicellulose decomposition

The hemicellulose degradation mechanism is like that of cellulose. Depolymerisation of hemicellulose leads to oligosaccharides production [45]. Decarboxylation, intramolecular rearrangement, depolymerisation, and aromatisation reactions can be used to synthesize biochar or the compound can degrade into syngas and bio-oil [46]. The volatile products and lignin are responsible for the char yield of the cellulose and hemicellulose components in biomass [40].

2.2.3 Lignin decomposition

Unlike the degradation of cellulose and hemicellulose, lignin decomposition is more complicated [47]. The creation of a more condensed solid structure and the shattering of relatively weak bonds result in the formation of char from lignin [48]. The β-O-4 lignin bond is broken and causes free radicals to be released. The protons emanating from other particles are captured by these free radicals, causing the production of degraded substances or compounds. Chain propagation is accomplished by free radicals moving to other molecules. Different amounts of lignin related to variable wood types bring about different breakdown rates. Coniferous lignin has been discovered to be more stable than deciduous lignin, and the former creates more char [49, 50].

2.3 Gasification

This is a thermochemical process that decomposes carbon-rich materials into gaseous products, including CO, CO₂, CH₄, H₂, and traces of hydrocarbons; these gases are referred to as syngas [51, 52]. Gasification happens at high temperatures between 700 and 900°C in an environment with restricted oxidizing agents such as oxygen, air, nitrogen, steam, carbon dioxide, or a mixture of these gases. It was discovered that when the temperature rose, carbon monoxide and hydrogen production increased, while other components such as methane, carbon dioxide, and hydrocarbons declined [53]. The main product of this process is syngas (mostly hydrogen), while char is referred to as a by-product (or waste) with a lower yield, along with ash, tar, and some oil [51]. Partial oxidation of biomass, unlike combustion, takes the energy available in the biomass and bundles it into chemical bonds in the form of gaseous products. The intrinsic chemical energy of carbon in biomass is transformed into combustible fuel gases, which are more efficient and convenient to utilize than raw biomass [54]. Commercial use of the gasification technique has also been documented. Because of its lower Levelised emissions and higher volume of syngas, gasification outperforms other traditional techniques including pyrolysis, combustion, and fermentation. The O/C ratio is critical to achieving high gasification efficiency. High gasification efficiency is achieved by using biomass with a low O/C ratio during gasification. Biomass can be reduced in its O/C ratio by the process of torrefaction. Before conventional gasification, torrefaction might be regarded as a pretreatment for better product quality. It is a low-temperature process between 200 and 300°C with a heating rate of roughly 50°C/min depending on the biomass composition and type [55, 56]. Pyrolysis and gasification are closely related processes. When gasification and pyrolysis are combined, there is no apparent separation between the two approaches [57, 58]. The little composition of oxygen used in gasification causes the biomass to undergo partial oxidation, changing the final product’s characteristics. The product type is one of the most significant variations between pyrolysis and gasification. Gasification produces around 85% gaseous products, 10% solid char, and 5% liquid products [15, 58]. The schematic of the gasification process is shown in Figure 3.

Figure 3.

Process diagram for gasification [54].

The gasification mechanism can be sub-divided into many steps as follows [5]:

2.3.1 Drying

Biomass moisture is entirely removed from the material, and no energy is recovered in the process. Different types of biomass have varying moisture contents. When the biomass has a high moisture content, drying is used as a distinct step during gasification.

2.3.2 Pyrolysis

The biomass is heated from 200 to 700°C with restricted oxygen or air during the pyrolysis process. The volatile components of the biomass are evaporated under these circumstances. The volatile vapor contains CO, CO₂, CH₄, H₂, tar (heavier hydrocarbon) gases, and water vapor [59]. Tar and char are also formed [60].

2.3.3 Oxidation/combustion

The oxidation and combustion reactions of the gasification agents are the primary energy sources for the gasification process. These gasification agents react with the gasifier’s combustible species to create CO₂, CO, and water.

2.3.4 Reduction

The CO₂ and H₂O are produced when the oxygen provided to the gasifier combines with the combustible elements. Upon contact with the char formed by pyrolysis, some of this CO₂ and H₂O are converted to CO and H₂ [60, 61]. Furthermore, the hydrogen in the biomass can be oxidized, resulting in the production of water. The reduction reactions that take place inside the gasifier are endothermic, and the energy necessary for them comes from the combustion of char and volatiles. Through a series of reactions, biomass reduction produces combustible gases such as hydrogen, carbon monoxide, and methane [62, 63].

2.3.5 Cracking

Furthermore, during the gasification process, the tar gases formed during the pyrolysis step are cracked, resulting in non-condensable gasses, light hydrocarbons, and unconverted tar [64]. The cracking stage follows more or less Eq. (2).

$$a{C}_n{H}_x\to b{C}_m{H}_y+C{H}_2$$E2

Where C_nH_x is tar and C_mH_y is dehydrogenated hydrocarbons; a, b and c are mole ratios.

3. Factors affecting the properties of biochar

3.1 Feedstock

Biomass is a composite solid substance made up of organic, inorganic and biological material produced from living or non-living creatures/organisms. There are two main categories of biomass, namely Woody and Non-woody biomass. Woody biomass is mainly forestry and tree residue [1]. It is characterized by low moisture and ash content, high calorific and bulk density values, and low voidage; in contrast, Non-woody biomass is made up of agricultural crop residue, animal waste, and municipal and industrial solid waste [1]. Non-woody biomass is characterized by high moisture and ash content, decreased calorific value, low bulk density, and increased voidage compared to woody biomass [1]. The moisture content of the biomass has been shown to have a significant effect on the physicochemical characteristics of the derived biochar [2]. A study conducted by [3] comparing the pyrolytic charcoals produced from hard and softwood bark samples reported a direct correlation between initial sample moisture content and the surface chemistry derived charcoal; the study found that a decrease in the moisture content of maple bark resulted in charcoal surface becoming more graphite-like and polyaromatic attributed to prolonged pyrolysis time. The effect of feedstock lignin and cellulose content on biochar formation is a well-researched area [4]. Lignin is an amorphous, high molecular weight polymer that is hydrophobic in nature and has several aromatic functional groups in comparison; cellulose and hemicelluloses are made up of simple sugar monomers that disintegrate at temperatures below 450 degrees Celsius [5]. Studies conducted by Tripathi et al. 2016 and Yu et al. 2014 [2, 6] showed that the cellulose content of feedstock aided the formation of tar (which comprises aldehydes, organic liquids, ketones, and char); while a high lignin concentration is beneficial to the formation of char during pyrolysis. According to Demirba (2004) [7], high feedstocks lignin content will increase char formation. It has been shown that increased lignin content in plant biomass promotes carbonization and increases biochar carbon and ash content [8, 9].

3.2 Residence time

Residence (pyrolysis time) has been shown to affect the degree of carbonization and biochar yield of feedstock; this effect is particularly pronounced at low temperatures [18]. According to Zornoza et al. (2016), increased residence time during pyrolysis results in a higher degree of carbonization, reducing the liable organic matter mitigation the vulnerability of the biochar to microbial attack [19]. Residence time has also been shown to influence the specific surface area of biochar produced. A study conducted by Wang et al. (2019) found that the surface area of biochar’s derived from the co-pyrolysis of sewage sludge and cotton stalks increased as residence time increased from 30 minutes to 90 minutes [20]. This was attributed to the formation and extension of pore structures of the biochar caused by the increased thermal decomposition of organic matter and volatiles released from etching pores during the increased residence time [21]. The same study noted a decrease in the surface area of the biochar’s as the residence time was increased from 90 minutes to 150 minutes; this reduction was accounted for by the collapse of the pore structure of the biochar during the extended residence time [20]. Residence time has also been shown to affect the calorific value of the biochar produced; a study conducted by Ahmad et al. (2020) on coconut shell derived biochar showed an increase in calorific value from 25.99 MJ/kg to 29.54 MJ/kg as residence time increased for 45 minutes to 75 minutes [22].

3.3 Biomass pretreatment

The pre-treatment of biomass before the pyrolysis has been shown to influence biochar characteristics. Pre-treatment is primarily divided into four categories: physical, physiochemical/thermal, chemical, and biological. Physical pre-treatment describes methods (milling, grinding etc.) that use mechanical energy to alter biomass properties. The most common form of physical pre-treatment is particle size reduction via mechanical comminutions. The effect of particle size reduction and fractionation of ash content is well researched. A study conducted by Liu et al. showed that the ash content of switchgrass and pine bark varied considerably with particle size fractions [22]. The study also reported the potential 20% removal of inorganic constituents from switchgrass and a 30% removal of inorganic constituents from raw pine bark. A similar study conducted by Bridgeman et al. found that the ash content of switchgrass and reed canary greatly increased in fines with particle sizes smaller than 90 micrometers, increasing to 3.62 wt. % to 6.0 wt. % for reed canary grass and 3.12 wt. % to 6.88 wt. % (dry basis) for switchgrass [23]. Besides the ash content, feedstock particle size is also correlated to biochar particle size, with finer feedstocks producing finer biochar particle sizes [18]. Studies have found that biochar’s derived from finer feedstocks exhibit lower nitrogen content as well as increased surface area, electrical conductivity, and pH [24, 25]. A study conducted by Sun et al. (2012) evaluating the properties of fine apple wood and corn stover-derived biochar (feedstock = 0.25 mm) reported a higher surface area when compared to applewood or corn stover-derived biochar stover-derived biochar of feedstock particle size = 1.5 mm [27]. Thermal pre-treatment describes methods that make use of thermal energy to produce changes in biomass properties; the most common forms of thermal pre-treatment are steam explosion, HTC and hot water extraction. Steam explosion involves the subjection of biomass to high temperatures and pressures between (160-260°C) and (0.69–4.83 MPa); the biomass subsequently undergoes sudden decompression scattering the fiber material and breaking the covalent bonds between the hemicellulose and lignin [28, 29]. Steam explosion increases the lignin content of the biomass by facilitating the depolymerisation of lignin into lower molecular weight molecules, which then condense with other degradation products [30]. A study conducted by Chen et al. 2017 [46] evaluating the effect of the steam explosion of crop straws before pyrolysis reported a change in the surface structure of the derived biochar; exhibiting a rougher surface when compared to the smoother, clearer and distinct pore structure of the untreated crop straw [31]. The same study also showed an approximate increase in the specific surface area of oil-rape straw-derived biochar 16 times greater than that on the untreated sample.

4. Biochar characterization

Properties of biochar produced depend on the composition, type of biomass and the conditions under which it is carbonized. Both physical and chemical characterizations are necessary when identifying the basic properties of biochar and predicting the various application uses. Biochar serves as a promising alternative to its surface area, charged surfaces and functional groups. Figure 4 below displays the different physical and chemical methods used for biochar characterization, focusing on BET and FTIR, belonging to the chemical characterization and SEM as physical characterization.

Figure 4.

Overview of a proposed characterization techniques for biochar [65].

The main aim of quantification to distinguish biochar from organic matter and other forms of black carbon produced. Majority of the potential technology is dependent on spectroscopic characteristics rather than physical separation or isolation.

Biochar being produced from a range of biomass that has different chemical and physical properties results in materials of different properties. Properties of each biomass are important during thermal conversion processes, proximate analysis (ash and moisture content); calorific value; fractions of fixed carbon; volatile components; fractions of lignin, cellulose and hemicellulose; inorganic substances; true density; particle size and moisture content.

4.1 Porosity and surface area

Chemical composition of biomass feedstock and biomass is subjected to a range of analyses to achieve the basic physicochemical characteristics of each raw material. Figure 5displays the physiochemical characteristics of biochar. Biochar production is often assessed through changes in the elemental concentrations of C, H, O, S and N and the associated ratios. The fixed carbon is the solid residue that remains after the particle size is carbonized and the volatile matter is expelled. The H/C and O/C ratios are used to determine the degree of aromaticity and maturation. Elemental ratios of O/C, O/H and C/H have been used to provide a reliable measure of the extent of pyrolysis and the level of oxidative adjustment of the biochar. Irrespective of the pyrolytic temperature, the BET areas increased with an increase in carbon burn off, indicating that the carbon burns off had a significant role in increasing pore volume and surface area while the average pore size increased with residence time and pyrolytic temperature. The BET surface area of biochar value of (1057 m2 .g − 1) has been reported, which appears slightly higher than that of activated carbon (970m² .g⁻¹). Biochar micropore volume of (0.24 mL .g⁻¹) also appeared smaller than that of activated carbon, having a value of (0.32 mL .g⁻¹), however having an average pore diameter of (5.2 nm).

Figure 5.

Fourier-transform infrared spectra (FTIR) of the biochar samples [66].

4.2 Scanning electron microscope (SEM)

Scanning electron microscopy is categorized as a physical characterization technique used to determine the samples macroporosity and the physical morphology of solid substance (Figure 6). A study by Amin 2016 [1] approximated that the biochar produced from cellulose plant materials had a pore diameter of 1 𝜇m. This characteristic is highly dependable in the intrinsic architecture of the feedstock use.

Figure 6.

SEM micrograph of biochar with magnification of 500x [67].

SEM micrographs displayed that the biochar produced at different pyrolytic temperatures has a distinguishable and clear honeycomb structural appearance due to the original tubular structures present in plant cell materials (Figure 6). The well-developed pores have a direct impact on the high surface area. According to Cantrell et al. (2012), biochar produced at lower temperatures is appropriate for regulating fertilizer nutrients and absorbing pollutants from the soil. Higher temperatures lead to material analogous to activated carbon and environmental remediation. SEM micrographs of biochar displayed a clean surface as the pyrolysis process had stabilized the volatile hydrocarbons, therefore smoothening the surface of the biochar. Pyrolysis at lower temperatures displays molded structures with small pores and uneven surface structure. In general, it is safe to say that since the biomass wastes contain lignin and high volatile matter content, the pore creation in biochar is directly affected.

4.3 Fourier transform infrared spectroscopy (FTIR)

FTIR spectroscopy serves as a great tool to observe the shift change of chemical compositions. The commonly used technique for biochar characterization using the FTIR is the pellet technique, which mixes 1 mg of dried biochar with 300 mg of pre-dried and pulverized spectroscopic grade KBr. Novak ae al. (2012) used the pellet technique to conclude 3400to 3410 cm⁻¹, H-bonded O–H stretching vibrations of hydroxyl groups from alcohols, phenols, and organic acids, 2850 to 2950 cm⁻¹, C–H stretching of alkyl structures; 1620–1650 cm⁻¹, aromatic and olefinic CDC vibrations, CDO in amide (I), ketone, and quinone groups; 1580 to 1590 cm⁻¹, COO- asymmetric stretching; 1460 cm⁻¹, C–H deformation of CH₃ group; 1280–1270 cm⁻¹, O–H stretching of phenolic compounds; and three bands around 460, 800, and 1000–1100 cm⁻¹, bending of Si–O stretching [68]. Figure 5 illustrates the FTIR spectra of biochar collected during different stages of the production, i.e. (Biochar: Original, −1: pre-incubation, −2: jointing, −3: Heading; −4: Mature).

5. Applications of biochar and future perspective

Biochar is a product (together with bio-oil and gases) resulting from biomass pyrolysis. Biochar usage has increased because it reduces the negative impacts of biomass on the environment [69]. The physicochemical properties of biochar are what govern the applications of this material. Depending on the feedstock type, production technology and process conditions [70]; the quality, yield and toxicity of the resulting biochar differs (as shown in Table 1) [72, 73]. These applications (including potential applications) range from adsorption for water and air pollutants [74], activated carbon [75], anaerobic digestion promoter/catalyst [76], construction material [77], agriculture and horticulture use such as soil conditioning, compost additive [78], carbon sequestration, etc. [73]. Figure 7 demonstrates these applications and how biochar contributes to the circular economy through its uses in agriculture and horticulture. Also, these numerous biochar benefits show a great potential to contribute to the economic sustainability of emerging cellulosic bioenergy production systems [79, 80]. It is worth noting that as the number of applications of biochar increases, so does the number of manufacturers, leading to a need for regulated standards and guidelines for the production of this material (see Table 2) [81, 82].

Type of characterization	Determination method	Results and remarks
Elemental analysis	C, H, O, S and N associated ratios	The H/C, O/C and N ratios are used to determine the aromaticity and maturity of the biochar
BET	Surface area, pore structure, average pore diameter, pore volume and average pores of biochar	1057m^2g^−1; macroporosity and microporosity; 5.2 nm; 0.24 mLg^−1; 3.3 nm. [71]
FTIR	Changes which occur in the biochar preparations as well as its functional groups present from the original biochar.	Changes include dehydration, pyrolysis, graphene; nucleation, and finally carbonization; O–H; (3600–3100 cm − 1), C=C and C=O stretching; (1740–1600 cm − 1),; C–O–C symmetric stretching (1097 cm − 1), –COOMe; (1400–1500 cm − 1), and so on

Table 1.

List of notable chemical characterisations of biochar.

Figure 7.

Biochar uses in agriculture and horticulture and its contribution to the circular economy [78].

Process	Process temperature	Residence time	Solid product yield on a dry wood feedstock basis (mass %)	Carbon content of the solid product (mass %)	Carbon yield (masscarbon, product/masscarbon feedstock
Slow pyrolysis	$~$400	Minute to days	$\approx$30	95	$\approx$0.58
Fast pyrolysis	$~$500	$~$1 s	12–26	74	0.2–0.26
Gasification	$~$800	$~$10–20 s	$\approx$10	—	—
HTC	$~$180–250	1-12 h	$<$66%	$<$70%	$\approx$0.88
Flash carbonization	$~$300–600	$<$30 min	37	$\approx$85	$\approx$0.65
Torrefaction	$~$290	10–60 min	61–84	51–55	0.67–0.85

Table 2.

Comparison of typical operating conditions and product properties of various biochar production processes [81].

5.1 Biochar in agriculture and horticulture

Biochar application in agriculture and horticulture has been explored both on a laboratory scale and in the field. These applications include being used as a component of chemical fertilizer [83], soil microbial activity, soil amendment for crop productivity improvement through nutrient availability [84, 85] as well as water holding capacity [86]. Biochar has also been reported to alleviate heavy metals release in the soil while having a limiting effect that aids in increasing the pH of highly acidic soils [87, 88]. Though biochar is another soil conditioner type, it differs from compost by production pathways. Biochar is produced by thermal decomposition of food, horticultural and municipal solid waste in the absence of oxygen, while natural biodegradation of organic substrates produces compost by the microbial community under aerobic conditions. Another difference is that; compost degrades fast, making its benefits relatively short-lived compared to biochar which persists in the soil for more prolonged periods [78, 89].

5.1.1 Biochar as a compost additive

Low soil organic carbon and fertility are challenges faced by many agricultural farmers around the globe. Biochar offers a solution to this challenge because it gives two options, i.e. returning nutrients and carbon to the soil while producing energy [90]. Also, the compositing rate can be increased by using biochar as an additive. Zhang and Sun [91] have examined spent mushroom compost and biochar co-composting. Their results showed a great increase in nutrients content of the resultant compost product and an improved composed quality while reducing the composting time from 90 to 270 days to only 24 days. Also, the large porosity of biochar enables it to facilitate microbial growth in the compost pile, leading to accelerated nutrient recycling [92]. The addition of biochar to poultry manure has been found to increase the maximum temperature reached and shorten the thermophilic phase [93].

5.1.2 Biochar as an adsorbent

An issue of heavy metals/metalloids (HMS) and polycyclic aromatic hydrocarbons (PAHs) in soil and water poses detrimental environmental problems and poor quality of agriculture, affecting all forms of life [94, 95]. These pollutants are toxic, persistent, non-biodegradable and potentially bioaccumulate [96]. Among other bioremediation technologies used to solve the HMS and PAHs issue, biochar is one of the best solutions due to its advantages [97]. These advantages include sustainability, low costs, sequestration of carbon, etc. [94]. Various physical and chemical characteristics of biochar, such as pore structure, specific surface area and functional groups, have been used to adsorption different pollutants [98]. For instance, Mahmoud, et al. [99] have used modified Switchgrass biochar for efficient decolorization of reactive red 195 A dye from aqueous and wastewater samples. Other biomass materials such as rice husks and dairy manure have also been used for biochar production with varying adsorption capacities according to the biomass used upon other factors [100].

5.2 Biochar in construction

Biochar has been used in road construction and as a concrete admixture. Wang, et al. [77] assessed this where a novel production of fill material and pedestrian/vehicle paving blocks were done. In this study, biochar addition was found to be beneficial to cement hydration even though it was noticed that the studied particle sizes could incur microcracks and strength degradation. Also, biochar’s incorporation resulted in enhanced immobilization of potentially organic contaminants and toxic elements in the sediment product, which is significant for moderately to heavily contaminated products. Therefore, biochar from wood can be used as a green combination for cement-based recycling procedures for highly contaminated waste. The use of biochar in construction material to trap atmospheric carbon dioxide in buildings also offers the potential to reduce greenhouse gasses by 25%. High pH and high water retention rate of biochar enable it to absorb some of the mixing water used in concrete mixing, thereby reducing the amount of free water in the concrete [101].

5.3 Future perspective

Since biochar’s applications depend greatly on its properties, future research must elucidate the production process effects on biochar’s properties. Biochar used in water treatment would differ from the one used in energy/agriculture. Likewise, there are diverse literature findings on the effects of biochar on agriculture, particularly on crop production caused by soils being different. For instance, crop yields may be increased or decreased by adding biochar depending on the soil type and fertilizer management [90, 102]. Also, the chemical behavior of biochar with heavy metal ions has been found to be inconsistent [103]. It is apparent that the interaction mechanisms between biochar, soil and plants are critical and yet not thoroughly known. Therefore, more efforts are still needed concerning biochar properties to soil and crop responses equally in the field and climate-controlled environment.

6. Conclusion

Biochar has been applied to remediate contaminated agricultural soil and improve soil fertility by reducing acidity and increasing the availability of nutrients. Thus, the addition of biochar to soils can be one of the best practices to overcome any biotic stress in soil and increase crop productivity, mainly in the agricultural sector. The properties of biochar have significantly been influenced by processes such as pyrolyscould, which have been discussed in this chapter. Thus, biochar appears as a highly promising option for pollutant removal. Economic impacts and recyclability should be considered in developing recoverable biochar for wide environmental applications. The relationship between various solutions for waste management and energy production differs in parameters and multiple techniques for its production and economic, social and ecological constraints. This review paper detailed the state-of-art information that would be helpful to find new opportunities in scientific innovation in the field of biochar research.

Acknowledgments

The authors are thankful to the Green Engineering and Sustainability research group in the Department of Chemical engineering at the Durban University of Technology, South Africa.

Conflict of interest

The authors declare no conflict of interest.