Abstract
Biochar is highly valuable in various applications due to its unique physicochemical properties such as high thermal efficiency, high surface area, surface functional groups, and crystal structure. The goal of this review is to establish a systematic strategy of biochar production for applications in various fields. First, the characteristics of biomass as feedstock for biochar production and their classification are discussed according to the types present in nature. Second, the technology for biochar production and the production yield are examined. In thermochemical conversion for biochar production, five major types of pyrolysis processes are suggested, and the production yield is evaluated according to pyrolysis parameters (feedstock pretreatment, operating temperature, heating rate, residence time, carrier gas). In addition, biochar production from pyrolysis of mixed feedstock has recently been suggested; thus, the evaluation of the production yield from co-pyrolysis is included. Finally, analytical techniques for biochar characterization are investigated and the application of biochar in various fields is considered, such as in adsorbents, energy storage devices, and catalysts.
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Recent Advancements in Biochar Production According to Feedstock Classification, Pyrolysis Conditions, and Applications: A Review
Youngsang Chun,a,1 Soo Kweon Lee,b,1 Hah Young Yoo,c,* and Seung Wook Kim a,b,*
Biochar is highly valuable in various applications due to its unique physicochemical properties such as high thermal efficiency, high surface area, surface functional groups, and crystal structure. The goal of this review is to establish a systematic strategy of biochar production for applications in various fields. First, the characteristics of biomass as feedstock for biochar production and their classification are discussed according to the types present in nature. Second, the technology for biochar production and the production yield are examined. In thermochemical conversion for biochar production, five major types of pyrolysis processes are suggested, and the production yield is evaluated according to pyrolysis parameters (feedstock pretreatment, operating temperature, heating rate, residence time, carrier gas). In addition, biochar production from pyrolysis of mixed feedstock has recently been suggested; thus, the evaluation of the production yield from co-pyrolysis is included. Finally, analytical techniques for biochar characterization are investigated and the application of biochar in various fields is considered, such as in adsorbents, energy storage devices, and catalysts.
Keywords: Biochar; Biomass; Classification; Pyrolysis; Applications
Contact information: a: Department of Interdisciplinary Bio-Micro System Technology, College of Engineering, Korea University, 145 Anam-Ro 5, Seongbuk-Gu, Seoul 02841, Republic of Korea; b: Department of Chemical and Biological Engineering, Korea University, 145, Anam-Ro, Seongbuk-Gu, Seoul 02841, Republic of Korea; c: Department of Biotechnology, Sangmyung University, 20, Hongjimun 2-Gil, Jongno-Gu, Seoul 03016, Republic of Korea; 1These authors contributed equally to this work;
* Corresponding authors: y2h2000@smu.ac.kr (H.Y.Y.) and kimsw@korea.ac.kr (S.W.K.).
GRAPHICAL ABSTRACT
INTRODUCTION
Global warming, accelerated by the reckless use of fossil fuels after the industrial revolution, has led to various social and economic problems such as rising sea level, climate change, species extinction, and food insecurity. According to the International Panel on Climate Change (IPCC), global warming is significantly affected by intensive greenhouse gas emissions (GHGs), including carbon dioxide, into the atmosphere. The problem is expected to increase as the world’s population grows to about 9 billion by 2050 (Antar et al. 2021). Therefore, it is necessary to replace fossil fuels with renewable energy to prevent global warming.
Biomass, which participates in the fast carbon cycle within the biosphere, has been described in many reports as a renewable energy source that can serve as an alternative to fossil fuels (Amiri et al. 2014). The term biomass (Greek bio meaning life + maza meaning mass) is referred to all organic matter derived from plants, animals, and microorganisms (Demirbas 2010). A biorefinery is a process that converts biomass to energy and other beneficial byproducts such as biochar. The International Biochar Initiative (IBI) defines biochar as a solid material produced during oxygen-limited pyrolysis of biomass (Meyer et al. 2017). Biochar is utilized in a variety of industries due to its physicochemical properties such as high thermal efficiency, high surface area, surface functional groups, and crystalline structure. According to market report, the market size of biochar is expected to reach $3.14 billion by 2025 (Hersh et al. 2019).
In order to apply biochar to various fields such as environment, energy, and fuel, it is important to characterize biomass as a feedstock and determine various pyrolysis parameters in the biochar production process (Fig. 1). Conventional biomass contains food resources; thus, this review is focused on non-edible resources as feedstock for biochar production. Non-edible biomass can be classified into second, third, and fourth-generations according to its origin and characteristics (Yang et al. 2015b; Yoo and Kim 2021). The second-generation biomass is a plant composed of lignocellulose, and the third-generation includes energy crops, engineering plants, and algae. The fourth-generation biomass, which has recently begun to be classified, refers to organic wastes such as coffee grounds, fruit peels and sewage sludge. The pretreatment including drying, crushing, and sieving is carried out to prepare feedstock in the specific condition required for each process. Biochar is produced through the pyrolysis of the prepared feedstock that is controlled by various parameters.
Fig. 1. Schematic diagram of biomass into biochar conversion process for various applications
Pyrolysis is a well-established method for obtaining high value-added substances such as biochar, bio-oil, and syngas through the thermochemical conversion of biomass. Pyrolysis can be classified into various types such as torrefaction, hydrothermal carbonization (HTC), slow pyrolysis, fast pyrolysis, and flash carbonization with different reaction conditions depending on the target substance (Pandey et al. 2020). Slow pyrolysis is the most reported thermochemical conversion to obtain high-yield biochar as the main product. However, the disadvantage of slow pyrolysis is a long reaction time. In recent years, reports of torrefaction procedures that react at milder operating temperatures than conventional pyrolysis have been increasing, and the method of producing biochar from HTC using wet feedstock is also drawing attention.
Various methods for using the produced biochar have been proposed in recent years. Physicochemical analysis of biochars identifies biochars that exhibit high and stable performance in their applications. Because specific surface area, surface functional groups, and mineral contents of biochar are various, physicochemical properties could be different according to feedstock and pyrolysis process. As analytical techniques to understand the physicochemical properties of biochar have become more advanced, the application of biochar as an adsorbent, energy storage material, and fuel production catalyst has been under considerable research and development.
In biochar research, most reports focus on the characterization and analysis of pyrolysis products based on optimization of process parameters at the laboratory level under their own conditions. In order to produce consistent yield and quality of the biochar, the whole process should be managed carefully. Figure 1 represents the process of biomass conversion into biochar for various applications. Understanding of feedstock is required to convert it into biochar of consistent quality and yield. This is because the pyrolytic product will be affected by the energy conversion efficiency of pyrolysis depending on the type of feedstock composed of various components having a different physicochemical property (Weldekidan et al. 2019). In addition, it is necessary to establish thermal decomposition conditions and control the entire process of biomass conversion.
In this review, fundamental data (Table 1) are provided by the classification of biomass used as a feedstock for biochar production. Table 2 provides various pyrolysis methods for producing biochar, and the effects of feedstock pretreatment (crushing and drying). Pyrolysis conditions (operating temperature, heating rate, residence time, and carrier gas) on the production yield of biochar are examined in Table 3. Table 4 investigates the conversion of biochar through a recently proposed mixed feedstock. Finally, analytical techniques for identifying physicochemical properties of biochar are investigated, and the applications of biochar such as adsorbent, energy storage device, and catalyst, are summarized in Table 5.
CLASSIFICATION OF BIOMASS AS FEEDSTOCK FOR BIOCHAR PRODUCTION
Biomass is the generic term for all organic matter derived from animals and algae as well as plants such as shrubs, trees, and crops (Jacobsson and Johnson 2000). Since the types of biomass are diverse and their composition depends on their origin, systematic classification of biomass is required to utilize as a beneficial feedstock in thermochemical processes. In general, biomass has been classified into starch-based (grain), cellulosic (agricultural by-products), and protein-based (organic waste) resources according to the type. Depending on their origin, they have been classified as agricultural residues, forestry residues, animal residues, algae, and aquatic crops. Recently, wastes generated by human activities, such as food waste, construction waste, and sewage, also have been included in the biomass category (Yuan et al. 2019).
In order to utilize biomass as a feedstock more efficiently, it has been classified into 1st to 3rd generation based on major biotechnology (Dalena et al. 2019; Ibrahim et al. 2018; Naik et al. 2010; Tursi 2019). Recently, organic wastes derived from industry have attracted attention as fourth-generation biomass, which provides a way to prevent environmental pollution (Yang et al. 2015b; Yoo and Kim 2021). Thus in the present review, based on the classification of previous reports (considering both the major biotechnologies and biomass characteristics), biomass will be classified into first, second, third, and fourth-generation. First-generation biomass will be defined to include in food crops such as corn, potato, wheat, sorghum, sugarcane, and rice. The direct usage of edible biomass for energy purposes is recognized as a threat to food security, including rising grain or feed prices and resulting starvation for the marginalized. Therefore, its use as a feedstock for bioindustry should be avoided (Naik et al. 2010). The second-generation biomass feedstock is lignocellulose, which exists in nature as non-edible biomass. As a potential fuel source, lignocellulosic biomass can be an effective alternative to first-generation biomass because of its low cost and high abundance (Wang et al. 2021). Third-generation biomass contains algae, a completely different generation of feedstock. Algae exists as aquatic biomass in nature (Chun et al. 2018). Macroalgae (blue, green, brown, red) and microalgae are representative forms. Unlike lignocellulosic biomass, algal strains have a different growth environment. Besides, it is proposed as an important feedstock for modifying global warming because it could produce renewable energy through the absorption of CO2 (Dalena et al. 2019; Ibrahim et al. 2018). The fourth-generation biomass is organic wastes, which includes organic resources not included in the first, second, and third generations. Unlike other generations, the fourth-generation biomass has the advantage that it does not require cultivation space to obtain the feedstock (Chun et al. 2019; Yoo and Kim 2021). According to the standards, feedstock for biochar can be classified into first to fourth generations, and their compositions are shown in Table 1.
The second-generation lignocellulosic biomass is a plant resource that is mainly composed of cellulose, hemicellulose, and lignin. Cellulose and hemicellulose are polysaccharides, but each has a unique composition and structure. Cellulose is an important structural component of the primary cell wall in plants, and is derived from D-glucose units, which are composed of long-chain linear via β(1, 4)-glycosidic bonds. Hemicellulose can comprise diverse sugar monomers such as xylose, arabinose, rhamnose, mannose, and galactose, and is a short-chain branched polymer with an amorphous structure. In addition, major forms of hemicellulose such as xylan, glucuronoxylan, arabinoxylan, glucomannan, and xyloglucan are well known. Lignin, an aromatic polymer, is a heterogeneous polymer derived from precursor lignols that crosslink in various ways. Hydrophobic lignin plays an important role in plants by covalently binding to hemicellulose. It interferes with the absorption of water into the cell wall, enabling the plant’s vascular tissue to conduct water efficiently, and its other functions are known as mechanical strength and disease resistance (Gundekari et al. 2020). According to Sarip et al. (2016), lignocellulosic biomass, on average, contains 32 to 54% cellulose, 11 to 37% hemicellulose, and 17 to 32% lignin. Similar results were found in a report analyzing wood, leaves, grass, and straw classified as second-generation biomass. Wood’s (birch, pine, willow, spruce) chemical composition has been reported to be 40 to 46% cellulose, 20 to 23% hemicellulose, and 25 to 29% lignin. Platanus leaf, grasses, and straws were reported to have chemical compositions of 18 to 49% cellulose, 8 to 35% hemicellulose, and 10 to 51% lignin
The third-generation biomass, algae, is considered an ideal raw material for biochar production because it could produce a large amount of biomass per hectare. Algae is comprised of organisms, and unlike second-generation biomass, it has been reported to contain a large amount of protein and lipid (Bardhan et al. 2015). Algae can be largely divided into macroalgae and microalgae. The chemical composition of macroalgae (Saccharina japonica, Sargassum fusiforme, Undaria pinnatifida, Capsosiphon fulvescens, Spirulina, Gelidium amansii) has been reported as 2 to 69% cellulose, 0 to 65% hemicellulose, 0.5 to 30% ash, 1 to 58% protein, and 0.04 to 25% lipid. Microalgae (Chlorella vulgaris, Nannochloropsis gaditana, Chlorella pyrenoidosa, Haematococcus) was reported as 5 to 49% cellulose, 0 to 15% hemicellulose, 0 to 8% ash, 21 to 56% protein, and 5 to 23% lipid.
The fourth-generation biomass is organic wastes derived from the bio-industries (Yoo and Kim 2021). Organic wastes from food processing residue (apple residue, soybean curd residue (SCR), instant noodle residue, potato peel residue) was reported as 5 to 84% cellulose, 0 to 34% hemicellulose, 0 to 24% lignin, 0 to 1% ash, 0 to 4% protein, and 0 to 2% lipid.
Table 1. Composition of Feedstock for Biochar
In addition, fruit peel (banana peel, orange peel, grapefruit peel, lemon peel) was reported as 30 to 48% cellulose, 10 to 30% hemicellulose, 0 to 16% lignin, 0 to 1% ash, 4 to 5% protein, and 6 to 8% lipid. The composition of organic wastes from industry (newspapers, waste papers from chemical pulps, oil palm trunks, empty fruit bunch (EFB), and spent coffee grounds (SCG)) is present at 12 to 65% cellulose, 15 to 39% hemicellulose, 8 to 24% lignin, 0 to 1% ash, 0 to 17% protein, and 0 to 2% lipid. The chemical composition of various shells from food processing (soybean shell, peanut shell, coconut shell, ginkgo nut shell, walnut shell, almond shell, pine nut shell, chestnut shell, acorn shell, pumpkin seed shell, sunflower seed shell) is 21 to 47% cellulose, 0 to 32% hemicellulose, 0 to 1% ash, 0 to 17% protein, and 0 to 2% lipid.
Various biomass can be classified into first to fourth generations according to the characteristics and major biotechnologies. However, it was found that several feedstocks have not been analyzed in detail for their chemical composition. Further studies on biomass composition analysis should be performed on the basis of systematized criteria as feedstock for biochar production.
BIOCHAR PRODUCTION
Types of Pyrolysis Processes for Biochar Production
Pyrolysis has been used to produce biochar for thousands of years through traditional earthen, brick, and steel kilns (Laird et al. 2009). The recent thermochemical technologies with different types of reactors for converting biomass into renewable products include torrefaction, hydrothermal carbonization (HTC), gasification, slow pyrolysis, flash pyrolysis, and fast pyrolysis (Garcia-Nunez et al. 2017). These modern pyrolysers are also designed to capture the volatiles for the production of bio-oil and syngas along with biochar. Achievements of higher yield and quality of target product are significantly dependent on operating parameters and the properties of the feedstock. Various thermochemical techniques operating with different reaction conditions (operating temperature, heating rate, residence time, and etc.) for biochar production are summarized in Table 2.
Table 2. Operating Conditions on Different Types of Pyrolysis Process
Torrefaction
Torrefaction, which is often referred to as roasting, is carried out under relatively mild conditions with an inert environment at atmospheric pressure and a temperature range of 200 to 300 °C. In general, the residence time in the reactor is less than 1 hour and the yield of solid product is about 60 to 85%. Under mild conditions, biomass releases a variety of volatile gases from the partially decomposed hemicellulose fraction, resulting in a large amount of solid product suitable for use of bio-coal (Gan et al. 2018). It is assumed that the earliest method originated from France in the 1930s for producing syngas. The torrefaction can be designed for either batch processing or continuous processing equipment, both of which are often performed in laboratory-scale studies. On the pilot-scale, devices such as tray ovens and screw reactors are used. The study of thermochemical conversion of biomass through torrefaction is mainly aimed at the production of syngas or biochar (bio-coal) (Ciolkosz and Wallace 2011).
Hydrothermal carbonization
Hydrothermal carbonization (HTC) is especially used for treating biomass with high moisture content that is generally carried out in a temperature range of 180 to 250 °C under autogenous pressure for over a day. The solid product via HTC is referred to as hydrochar. It is obtained with a high yield of 40 to 70 wt%, depending on the reaction temperature, pressure, residence time, and water-to-biomass ratio (Yan et al. 2010). Since biomass containing moisture is directly used, HTC, which does not require pretreatment for drying, is attracting attention as an economical process compared to other pyrolysis methods. Water molecules act as acid catalysts without the addition of acid under a specific residence time and pressure in the HTC process. This method was firstly reported by Bergius in 1913 to convert cellulose into a coal-like material and is currently referred to as hot compressed water treatment, wet torrefaction, or hydrothermal treatment (Wang et al. 2018).
Slow pyrolysis
In general, the aim of the slow pyrolysis process with a relatively low heating rate (0.1 to 1 °C/s) is the production of high-yield biochar (10 to 90%) from biomass. The reaction is usually carried out in an operating temperature range of 300 to 700 °C and a wide range of residence time (from several hours to days) (Li et al. 2019). In slow pyrolysis, relatively large sizes (~50 mm) of the feedstock can be used, and the process is not significantly dependent on the size of the feedstock. The feedstock is loaded into the reactor in an inert atmosphere from the beginning of the pyrolysis reaction. Recent studies on thermochemical conversion of biomass for various biochar applications have mainly been carried out with slow pyrolysis in fixed-bed reactors (Meyer et al. 2011).
Fast pyrolysis
The fast pyrolysis process is carried out at a temperature range of 450 to 600 °C with a relatively higher heating rate of 10 to 200 °C/s for a short residence time (<30 min). The main purpose of fast pyrolysis is to maximize the conversion of liquid products such as bio-oil and bio-oil-derived products, and the yield of biochar is relatively low, about 10 to 20%. Fast pyrolysis requires the feedstock to be prepared in a size of 1 mm or less for efficient heat transfer during a short residence time, and the feedstock is loaded into the reactor after reaching the operating temperature. The process has been operated in various types of reactors such as conical spouted bed, bubbling fluidized bed, circulating bed, rotating cone, and ablation reactor (Garcia-Nunez et al. 2017).
Flash pyrolysis
Flash carbonization is carried out with a heating rate of at least 1000 °C/s, which is the highest of the pyrolysis methods in the operating range of 700 to 1000 °C for a residence time of less than 30 min. The advantage of this reaction is a relatively short process time, but the production yield of biochar is about 20 to 30%. A dust-like feedstock is prepared to facilitate fluidization, leading to the effective thermochemical conversion of biomass. In this process, the feedstock is first loaded into a packed bed reactor and then a constant pressure of 1 to 2 bar is applied with air. The bottom of the pressurized reactor is heated by a flame, air flows downstream, and the flame rises up, heating the entire packed bed (Meyer et al. 2011).
Biochar Production from Various Feedstock
Biochar production technology is highly dependent on both the pyrolysis process and the characteristics of biomass. In general, pyrolysis has been performed at a high temperature with limited oxygen, but heating conditions such as operating temperature, heating rate, and residence time are inconstant. In particular, the nature of the feedstock is one of the important factors in biochar production because the components and contents are different according to the classification of biomass. The yields of biochar production under various pyrolysis conditions and feedstock are summarized in Table 3.
Masek et al. (2013) performed the slow pyrolysis of pine wood chips (second-generation) and softwood pellets (second-generation) which consists of cellulose, hemicellulose, and lignin to biochar at an operating temperature of 350, 450, and 550 °C under 0.5 L/min N2 flow rate during 1 h. The yield of biochar production based on pinewood chips was measured to be 42% (at 350 °C) and 31% (at 450 °C), respectively, which indicates a decrease in yield as the pyrolysis temperature was increased. This was attributed to the different properties of thermal decomposition of cellulose and hemicellulose.
The yield of biochar production on pyrolysis at 550 °C is 30%, exhibiting an increase in pyrolysis temperature from 450 to 550 °C, did not significantly affect the biochar production in decrease. This is due to the high lignin portion in the feedstock, which is not sensitively responsive to the operating temperature of 450 and 550 °C. The yield of pinewood chips-derived biochar production according to temperature (350 to 550 °C) is related to feedstock composition and the range of their decomposition temperatures. The thermal decomposition temperature and the identification of biomass composition are highly correlated with the yield of biochar. Also, the yields of softwood pellets derived biochar in the pyrolysis temperatures range of 350 to 550 °C showed a similar trend as those of pine wood chips. Thermochemical conversion of second-generation biomass performed at 350 to 550 °C showed similar characteristics due to their composition of the feedstock.
In order to understand the relationship between the pyrolysis temperature and biochar production, several studies have been conducted for biochar production over a wide range of temperatures (200 to 900 °C). Selvarajoo and Oochit (2020) performed various tests at operating temperature in a range of 300 °C, 500 °C, 700 °C, and 900 °C to produce palm fiber (second-generation) derived biochar with an N2 flow rate of 0.03 L/min during 2 h. Biochar production was 54% at 300 °C of pyrolysis temperature. After a temperature (about 500 °C) of lignin decomposition, biochar yield was measured as 29% (at 500 °C), 28% (at 700 °C), and 26% (at 900 °C), resulting in a significant decrease in production at elevating pyrolysis temperature. This is attributed to the fact that the yield of biochars significantly decreased after the temperature at which lignin degradation occurs. Operating temperature, which is responsive to thermal degradation of lignin, influences the biochar production from thermochemical conversion of second-generation biomass. Jung et al. (2016) produced marine macroalage (third-generation) based biochar pyrolyzed with operating temperatures from 200 °C to 800 °C under an N2 atmosphere. Four different biochar yields were 78%, 63%, 37%, and 27% at pyrolysis temperatures of 200 °C, 400 °C, 600 °C, and 800 °C, respectively. As the temperature was increased, thermal degradation of cellulose and hemicellulose occurred in algal biomass, and the biochar yield was significantly decreased. Especially in the third-generation of biomass, which does not contain lignin, the decrease of biochar yield over 500 °C is caused by the complete removal of volatile substances during the pyrolysis process. Therefore, it was observed that the biochar yield between pyrolysis at 400 °C and pyrolysis at 600 °C was significantly reduced by 25.7%, which is a higher gap than that of a temperature range of 600 to 800 °C.
Table 3. Summary of Feedstock and Pyrolysis Conditions for Biochar Production