Valorization of biomass waste to engineered activated biochar by microwave pyrolysis: Progress, challenges, and future directions

Journal Pre-proofs Valorization of biomass waste to engineered activated biochar by microwave pyrolysis: Progress, challenges, and future directions Shin Ying Foong, Rock Keey Liew, Yafeng Yang, Yoke Wang Cheng, Peter Nai Yuh Yek, Wan Adibah Wan Mahari, Xie Yi Lee, Chai Sean Han, Dai-Viet N. Vo, Quyet Van Le, Mortaza Aghbashlo, Meisam Tabatabaei, Christian Sonne, Wanxi Peng, Su Shiung Lam PII: DOI: Reference:

S1385-8947(20)30392-2 https://doi.org/10.1016/j.cej.2020.124401 CEJ 124401

To appear in:

Chemical Engineering Journal

Please cite this article as: S.Y. Foong, R.K. Liew, Y. Yang, Y.W. Cheng, P.N.Y. Yek, W.A.W. Mahari, X.Y. Lee, C.S. Han, D.N. Vo, Q. Van Le, M. Aghbashlo, M. Tabatabaei, C. Sonne, W. Peng, S.S. Lam, Valorization of biomass waste to engineered activated biochar by microwave pyrolysis: Progress, challenges, and future directions, Chemical Engineering Journal (2020), doi: https://doi.org/10.1016/j.cej.2020.124401

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Highlights: 

Key concepts, mode, operating parameters, and products of current pyrolysis applications are reviewed.



Heat transfer is more efficient in microwave pyrolysis of biomass waste.



Lack of top-tier reactor design blocks the commercialization of microwave pyrolysis.



Microwave pyrolysis produces engineered activated biochar with desirable properties.



Engineered activated biochar has wide application in pollution control, catalysis and energy storage.

Valorization of biomass waste to engineered activated biochar by microwave pyrolysis: Progress, challenges, and future directions Shin Ying Foonga,b,#, Rock Keey Liewc,#, Yafeng Yanga,#, Yoke Wang Chengd,#, Peter Nai Yuh Yeke, Wan Adibah Wan Maharib, Xie Yi Leeb, Chai Sean Hanb , Dai-Viet N. Vof, Quyet Van Leg, Mortaza Aghbashloh, Meisam Tabatabaeii,j, Christian Sonnek, Wanxi Penga,*, Su Shiung Lamb,* Henan Province Engineering Research Center For Biomass Value-Added Products, School of Forestry, Henan Agricultural University, Zhengzhou, 450002, China. a

Pyrolysis Technology Research Group, Institute of Tropical Aquaculture and Fisheries (AKUATROP) & Institute of Tropical Biodiversity and Sustainable Development (Bio-D Tropika), Universiti Malaysia Terengganu, 21030 Kuala Nerus, Terengganu, Malaysia. b

c

NV WESTERN PLT, No. 208B, Jalan Macalister, Georgetown, 10400, Pulau Pinang.

Centre for Biofuel and Biochemical Research, Institute of Self-Sustainable Building, Universiti Teknologi PETRONAS, 32610, Seri Iskandar, Perak, Malaysia d

e

University College of Technology Sarawak, Department of Engineering, 96000, Sibu, Sarawak, Malaysia.

Center of Excellence for Green Energy and Environmental Nanomaterials (CE@GrEEN), Nguyen Tat Thanh University, 300A Nguyen Tat Thanh, District 4, Ho Chi Minh City 755414, Vietnam. f

g

Institute of Research and Development, Duy Tan University, Da Nang 550000, Vietnam.

hDepartment

of Mechanical Engineering of Agricultural Machinery, Faculty of Agricultural Engineering and Technology, College of Agriculture and Natural Resources, University of Tehran, Karaj, Iran Faculty of Plantation and Agrotechnology, Universiti Teknologi MARA (UiTM), 40450 Shah Alam, Selangor, Malaysia. i

Department of Microbial Biotechnology, Agricultural Biotechnology Research Institute of Iran (ABRII), AREEO, Karaj, Iran. j

Aarhus University, Department of Bioscience, Arctic Research Centre (ARC), Frederiksborgvej 399, PO Box 358, DK-4000 Roskilde, Denmark k

#

Co-first authors.

* Corresponding authors: [email protected] (S.S. Lam); [email protected] (W. Peng)

Abstract Biomass waste represents the promising surrogate of fossil fuels for energy recovery and valorization into value-added products. Among thermochemical conversion techniques of biomass, pyrolysis appears to be most alluring owing to its low pollutant emission and diverse products formation. The current pyrolysis applications for valorization of biomass waste is reviewed, covering the key concepts, pyrolysis mode, operating parameters and products. To date, existing types of pyrolysis include conventional pyrolysis (poor heat transfer due to nonselective heating), vacuum pyrolysis (lower process temperature because of vacuum), solar pyrolysis (entirely “green” with solar-powered), and a newly touted microwave pyrolysis. In microwave pyrolysis of biomass, the heat transfer is more efficient as the heat is generated within the core of material by the interaction of microwave with biomass. The plausible mechanisms of microwave heating are dipole polarization, ionic conduction and interfacial polarization. The lack of top-tier reactor design is identified as the main obstacle that impedes the commercialization of microwave pyrolysis in biomass recycling. Based on the existing works, it is surmised that microwave pyrolysis of biomass produces solid biochar as a main product. To confront the great market demand of activated biochar, it is proposed that the solid

char could be upgraded into engineered activated biochar with desirable properties for wide application in pollution control, catalysis and energy storage. Hence, the production of engineered activated biochar from microwave pyrolysis process and its applications are reviewed and explicitly discussed to fill the research gap, and the key implications for future development are highlighted.

Keywords: biomass pyrolysis; microwave heating; waste valorization/recycling; energy recovery; biochar; sustainable production.

Table of content: Abstract 1.0 Introduction 1.1 Biomass waste 1.2 Biomass recycling and energy recovery via thermal methods 2. Pyrolysis 2.1 General reaction of biomass pyrolysis 2.2 Modes of pyrolysis and targeted yield 2.2.1 Fast pyrolysis – bio-oil 2.2.2 Slow pyrolysis – solid char 2.2.3 Flash pyrolysis – syngas production 2.3 Types of pyrolysis 2.3.1 Solar pyrolysis 2.3.2 Vacuum pyrolysis 2.3.3 Conventional pyrolysis 2.3.3.1 Conventional heating mechanism 2.3.3.2 Microwave heating mechanism 2.3.4 Microwave pyrolysis 2.4 Recent progress, challenges and future direction of microwave pyrolysis 3. Engineered activated biochar 3.1 General application of engineered activated biochar 3.1.1 Wastewater treatment 3.1.2 Catalysis 3.1.3 Energy storage 3.1.4 Gas adsorption 3.2 Production of biomass-derived engineered activated biochar 3.2.1 One-step and two-step thermal treatments 3.2.2 Types of activation 3.2.2.1 Chemical activation

3.2.2.2 Physical activation 3.2.3 Influence of activating agent on the porous characteristics of engineered activated biochar 4. Conclusion and recommendation Acknowledgement References

1. Introduction 1.1 Biomass waste Biomass represents an inexhaustible source of carbon as its organic constituents are originated from animal and plants, which can further classify into virgin biomass and biomass waste. Particularly, biomass waste includes forestry and agricultural residues (e.g. wood, leaves, trunk, frond, peels, husks, and nuts), carcass, and excreta from human and animals. The plant biomass mainly consists of three lignocellulosic components, viz. cellulose (40–60 wt%), hemicellulose (15–30 wt%), and lignin (10–25 wt%) in which the compositions are dependent on the source of biomass. Cellulose is a crystalline or amorphous homopolymer composing of multiple linear chains of glucose units that interlinked by β-1,4-glycosidic bonds whereas hemicellulose is an amorphous heteropolymer that made up of various polysaccharides (such as xylan, xyloglucan, arabinoxylan, etc.). For instance, the seed-producing biomass contains mainly the glucomannans-based hemicellulose while the flower-bearing biomass has more xylan-based hemicellulose. Meanwhile, lignin is a complex, amorphous heteropolymer comprised of three propylbenzene units (i.e. p-propylphenol, p-propylguaiacol, and ppropylsyringol) that connected by different ether and carbon to carbon linkages (e.g. β-O-4, 4-

O-5, α-O-4, β-β, β-1, β-5, 5-5) [1]. Figure 1 depicts the monomer structures of cellulose, hemicellulose, and lignin. Upon pyrolysis, each lignocellulosic components will undergo different reaction mechanisms (i.e. decarboxylation, dehydration, and demethylation) to yield pyrolysis products like biochar, bio-oil, and syngas [2].

Figure 1 Monomer structures of lignocellulosic components of biomass. In the rural areas, biomass waste is a renewable and sustainable fuel source for energy generation by virtue of its high accessibility (i.e. easy to obtain) and lower cost as compared with non-replenishable sources such as coal and natural gas. Due to the rapid urbanization, intensified anthropogenic activities (e.g. deforestation, over-consumption of food, industrialization) generate huge biomass wastes in the form of agricultural residues, municipal solid waste, excreta from animals, and industrial wastewater. Prevalently, there are two common disposal methods to dispose solid biomass waste, namely landfilling and open burning. In spite of their simplicity, these methods ultimately contribute to global warming and groundwater pollution [3-5]. During landfilling, microbial degradation of biomass waste emanates greenhouse gases (e.g. methane and carbon dioxide), and the seeping of resulting pollutant-laden leachates could engender groundwater contamination by heavy metals and toxic organics [4-7]. Through open burning of crop residues, weed control and nutrient recycling in soil could be co-accomplished; nonetheless, this approach emits enormous undesirable gaseous pollutants (e.g. CO2, CO, NOx and SO2) that provoke air pollution and acid rain formation [8]. Thus, it was envisaged that the biomass waste should be systematically disposed via a more environmental-benign method.

1.2 Biomass recycling and energy recovery via thermal methods The world energy consumption is increasing due to the growth of human population and technology advancement. On top of that, energy is mainly sourced from the combustion of non-renewable fossil fuels (e.g. natural gas and coal) which will be depleted within the next 40 years [9]. With its renewability, biomass waste represents a promising surrogate of fossil fuels for energy recovery. In the past few years, countless research had been performed to explore and optimize the potential of biomass waste as feedstock for energy recovery and production [10-13]. In general, the energy recovery of biomass can be done by oxidative and non-oxidative thermochemical approaches such as combustion, gasification, torrefaction, and pyrolysis. Combustion is an oxidative thermal process that has been widely used in many energyrelated applications. For example, waste-to-energy plant incinerates biomass in the presence of oxygen for electricity production with the aid of a steam turbine. During the winter season, the biomass can serve as an alternative fuel in wood-fired stoves to produce heat energy for warming. In some restaurants, biomass is burned as a fuel for food processing process like smoking and barbequing. However, uncontrolled combustion of biomass could inflict air pollution by releasing flue gases containing ash, CO, and NOx into the atmosphere [14]. Similar to combustion, gasification is a thermochemical pathway whereby the organic constituents of biomass are oxidized by gasifying agents such as air, limited oxygen, steam, or carbon dioxide at elevated temperature (up to 1600 oC) [15]. Biomass gasification chiefly produces gaseous fuels called syngas (H2 and CO) with a minor proportion of carbon dioxide and methane [9]. Dependent on the syngas ratio (H2:CO), the syngas is either purified to hydrogen (if H2:CO ratio > 2) or upgraded to liquid fuel via Fischer-Tropsch synthesis (if H2:CO ratio ≈ 2) [16, 17]. Without further processing, the raw syngas could be utilized directly to fuel in some modified internal combustion engines for power generation. Gasification is

ostensibly to be an ideal solution for biomass energy recovery; withal, it is discouraged by its endothermicity (i.e. high temperature requirement) and catalyst deactivations (for the case of catalytic approach) [18-21]. Biomass torrefaction is frequently executed in the non-oxidative environment at mild temperature (200–300 oC) and moderate residence time (30–60 min) to give torrefied biomass as the main product [22, 23]. In relative to the raw biomass, the torrefied biomass possesses an improved energy value [24]; thus, it can serve better as a low-grade solid fuel in a steam boiler or a more durable barbeque fuel. Nevertheless, the mild process temperature is incapable of promoting degradation of recalcitrant lignocellulosic component (particularly lignin) through volatilization. Upon the burning of torrefied biomass in a boiler, the lignin likely to form sticky tar that severely affects the thermal efficiency of the boiler following the fouling issue in the combustion chamber [25, 26]. To mitigate the tar formation, biomass pyrolysis that involves thermal degradation of lignocellulosic components in inert condition at high temperature (300–1000 oC) is strongly recommended. Briefly, pyrolysis of biomass produces three value-added products with distinct physical forms, viz. solid biochar, liquid bio-oil, and incondensable syngas [26-29]. Based on previous literature, the proportion of each pyrolysis product is significantly influenced by biomass composition and process conditions such as temperature, heating rate, residence time, and types of catalyst [15]. Pyrolysis process has also been receiving great attention from researchers for its potential convert waste materials into fuel products. Recently, biomass wastes like sewage sludge, waste lubricating oil, waste cooking oil, pine wood, and coconut fibre were pyrolyzed into tar-free fuel gas [30], polyaromatic hydrocarbons [31], and biochar [32-35]. Kwon et al. [36] applied the CO2 pyrolysis to recover energy from biomass and waste material. Pyrolysis with CO2 as purging gas could produce more combustible gases with less tar observed. This type of pyrolysis also enhances the thermal cracking of volatile matter to

suppress the formation of harmful benzene derivatives and polycyclic aromatic hydrocarbons [36]. In overall, the pyrolysis is an alluring option for biomass recycling and energy recovery since it is associated with lower NOx and SOx emissions, more value-added product generation, and lesser tar formation.

2. Pyrolysis The phrase “pyrolysis” was derived from Greek wherein ‘pyro’ and “lysis” refer to fire and decomposition, respectively [37]. The use of pyrolysis was commenced about 38,000 years ago when charcoal was produced by Cro-Magnon man for wall drawing purpose [38]. Starting from the Bronze Age, the charcoal obtained from pyrolysis was used as a reducing agent in metals smelting as well as fuel in cooking. During Victorian times, coal pyrolysis was performed to extract the liquid and gases products as fuel [39]. Besides, the tars produced by pyrolysis were employed as boat caulking agent or embalming agent to prevent the bacterial attack in the ancient Egyptian times. The biomass pyrolysis was reported for the first time in the 19th century [40], while Gruner [41] studied the influences of process parameters on the pyrolysis products yield in 1875. Then, the first patent of pyrolysis prototype for solid wastes was filed by Garrett and Mallan [42] on 17 May 1976. In the 1980s, the scientists discovered that fast pyrolysis of biomass with greater heating rate contribute to higher liquid product yield following a rapid condensation of the pyrolysis vapour [43]. Henceforth, tremendous biomass pyrolysis researches were performed to establish the underlying concept of the pyrolysis process. Later, Bridgwater, Gerhauser and Effendi [44] patented a more advanced biomass pyrolysis prototype on 11 February 2009. Until present, the pyrolysis process still remains as a popular research topic due to its capability of producing multi-products with wide applications [25]. Distribution of these pyrolysis products is deemed to be significantly varied by the composition of lignocellulosic components in the biomass [45].

2.1 General reaction of biomass pyrolysis Based on the literature, the reaction mechanisms of biomass pyrolysis were proposed. Basically, biomass pyrolysis comprised three stages reactions namely: i) evaporation of water/moisture, ii) de-volatilization emerged from the decomposition of lignocellulosic components via complex reaction mechanisms such as depolymerization and fragmentation [46, 47] and the emission of chemically bonded water and CO2 [25], iii) charring that involves cracking [48, 49] and repolymerization/recondensation to form a stable and carbon-dense solid product [48, 50]. After the removal of moisture (first stage) from the biomass, each of the lignocellulosic components will be decomposed at a wide range of temperatures. It was reported that the hemicellulose would be decomposed first at around 180–285 °C probably due to its amorphous structure that possesses the lowest thermal stability. When the temperature increases further to 365 °C [26], the heat energy is sufficient to fully decompose the cellulose and depolymerize it into its monomer unit (i.e. β-glucopyranose). For lignin, decomposition happens at a higher temperature (circa from end temperature of cellulose degradation to 1000 oC)

probably attributed to the stable, complex polyaromatic structure [51]. During de-

volatilization (second stage), numerous heavy organic compounds such as cresol, phenol, guaiacol, levoglucosan, furfural, levomannosan, and 5-hydroxymethylfurfural were released, which could be recovered as bio-oil via a subsequent condensation [46, 52, 53]. When charring (final stage), some volatiles liberated from second stage will either repolymerized/recondensed to become part of the biochar or further cracked into non-condensable syngas (mainly H2 and CO, with trace CO2 and CH4) and lighter organic molecules (e.g. formaldehyde, methanol, acetic acid, formic acid, hydroxyacetone, and acetaldehyde) that can also be recovered as biooil [2]. Hence, the biomass pyrolysis renders the formation of solid biochar, liquid bio-oil, and syngas via the three stages reaction aforementioned.

2.2 Modes of pyrolysis and targeted yield The yields of each pyrolysis products (solid biochar, liquid bio-oil, and syngas) are greatly influenced by the types of feedstock used and the process parameters of pyrolysis (refer Section 2.3). The product yields of biomass pyrolysis can be manipulated by tuning the pyrolysis parameters such as process time, temperature, heating rate, and also the vapour residence time. Frequently, the pyrolysis is classified into different modes (fast pyrolysis, slow pyrolysis, and flash pyrolysis) by the combination of these parameters as shown in Table 1. Apart from that, Table 1 also summarizes the product distribution obtained from different pyrolysis modes. A disparity in product distribution between each pyrolysis mode is indicated in Table 1, suggesting a prevailing effect of process parameters on the product distribution. Generally, pyrolysis has a wide temperature range (400–1200 oC) since most of the volatiles are formed at a temperature of 250–500 oC during biomass pyrolysis [54]. At lowest pyrolysis temperature (400 oC), limited volatiles could be recovered from biomass in the forms of biooil and syngas, resulting in high biochar yield.

Table 1 Summary of the different pyrolysis modes with their process conditions and product distribution. Process conditions Temperature (oC) Heating rate (oC/s) Process time (min) Vapor residence time (s) Biochar yield (wt%) Bio-oil yield (wt%) Syngas yield (wt%)

Slow pyrolysis 400–900 0.1–10 > 5 (can go up to several hours) < 550 25–50 20–40 10–25

Fast pyrolysis 450–850 10–200 10–25

Flash pyrolysis 600–1200 > 1000 <1

0.5–10 15–25 60–75 10–20

< 0.5 5–15 25–40 50–60

Howbeit, different biomass feedstock has a non-identical optimum temperature for recovery of volatiles from biomass pyrolysis. For instance, the optimum pyrolysis temperature to recover most of the volatiles contents from oil palm wastes and fruit wastes are 410–520 oC [26] and 400–550 oC [25], respectively. With a longer vapour residence time, higher biochar yield could be predicted as prolonged time permits the volatiles to react with the biochar surface via secondary recombination reaction. Table 1 could serve as a guideline to foresee the product distribution of biomass pyrolysis from the reference ranges of process conditions. As the combination of process parameters is manifold, it is unlikely to purpose a reliable correlation between the process parameters and the product distribution. Besides, the product distribution can also be affected by other factors such as the types of pyrolysis (Section 2.3) with different reactor configuration (e.g. auxiliary equipment and heating source). In the work of Wang, Chen, Luo, Shao and Yang [55] pyrolysis of pinewood sawdust, peanut shell, and maize stalk under identical process conditions give rise to dissimilar product distributions [53]. Owing to the compositional complexity of biomass, it is virtually impossible to predict the product distribution of biomass pyrolysis. To the best of authors’ knowledge, no one has yet established a reliable correlation between the biomass type and the product distribution of pyrolysis products since its validity have to be verified by huge experimental results. Despite the lack of a consensus on the main product of biomass pyrolysis, each pyrolysis product possesses multi-functionalities. In brief, the solid biochar has wide applications such as solid fuel, adsorbent for pollution control, soil additive, supercapacitor, gasification feedstock, and activated biochar precursor. Meanwhile, liquid bio-oil and syngas are usually ameliorated into valuable chemicals and liquid fuel for energy purposes.

2.2.1

Fast pyrolysis – bio-oil

Fast pyrolysis is renowned with bio-oil as the main product. On account of the feedstock types, fast pyrolysis is usually carried out over a wide range of temperature (450–850 oC) with a short vapour residence time (0.5–10 s) and a high heating rate (10–200 oC/s). Since the volatiles released would leave the reactor within 10 s, the contact time of the volatiles released with the biochar surface is appreciably reduced. As a result, re-combination of volatiles on the biochar surface can be minimized, which eventually lead to the recovery of more volatiles as bio-oil [2]. Furthermore, the employment of a high heating rate favours the occurrence of depolymerization and fragmentation. Hence, the chemical bond cleavage between the lignocellulosic components takes place at a higher rate and simultaneously expedites the formation of light volatiles before rearrangement reaction is likely to occur. Fast pyrolysis of biomass yields approximately up to 75% of bio-oil [56]. Bio-oil is a dark-brown liquid product obtained from pyrolysis of biomass, whereby bio-oil yield obtained by biomass pyrolysis is around 20–75 wt% [43]. Typically, the bio-oil consists of 15–35 wt% water content, organic compounds (e.g. acids, alcohols, phenols, ketones and sugars), nitrogen compounds, and miscellaneous oxygenates [57]. Bio-oil is commonly used as combustion fuel in the furnaces, burners and boilers for heat generation owing to its considerable high heating value (HHV) of 15–46 MJ/kg [29, 35, 58]. However, the bio-oil obtained by fast pyrolysis displays high corrosiveness with its low pH value (pH < 3.6) [59] and chemically unstable [60]. The high oxygen content of raw bio-oil causes the biooil more prone to chemical reactions (e.g. esterification and polymerization), which results in instability of bio-oil during storage and deteriorates the fuel quality of bio-oil (more viscous) [60]. Thus, the raw bio-oil usually required to be upgraded by hydrotreating (i.e. hydrogenation and hydrodeoxygenation) before its fuel application [61, 62]. Recently, extensive researches were conducted to investigate the feasibility of bio-oil as a substitute for petroleum products (e.g. diesel) [29, 33-35, 63, 64]. The successful catalytic upgrading of bio-oil into petroleum-

like biodiesel over zeolite and metallic char evinced the sustainability of bio-oil as a surrogate fuel [15, 65, 66]. Recently, catalytic pyrolysis in different reaction conditions and catalyst selection for production of highly proportion of aromatic microalgal bio-oil have been reviewed. The research shows that zeolites are the most widely used catalyst for production of bio-oil from catalytic pyrolysis of microalgae [67]. Furthermore, bio-oil can also be utilized as chemical feedstocks to produce edible chemicals for food products (e.g. food flavour). For example, the extracted aldehydes and phenolic compounds from bio-oil could be used as meat browning agents and smoky flavouring agents, individually [58]. Table 2 shows the application of bio-oil obtained from various biomass. Table 2 Application of bio-oil obtained from various biomass. Feedstock American hardwoods Birch wood

Application Blending of acetone-pretreated bio-oil with epoxy resin for wood bonding Phenolic extracts of bio-oil as an antioxidant in soybean oil bio-lubricants Corn stalk Bio-oil/diesel emulsions with Ce0.7Zr0.3O2 nanoadditive as diesel fuel Japanese pine Antibacterial agent for foodborne pathogen wood sawdust (Staphylococcus aureus) control Lard wood Partial replacement of resorcinol formaldehyde resin with bio-oil in wood adhesive Lignin Catalytic upgrading of bio-oil into jet and diesel fuel range hydrocarbons over sulfided CoMo/γ-Al2O3 Lignocellulosic Raw bio-oil as fuel for anion exchange membrane fuel biomass cells to generate electricity Pine wood sawdust Antibacterial agent for foodborne pathogen (Bacillus cereus and Listeria monocytogenes) control Residual wood Antifungal (Gloeophyllum trabeum and Trametes fines versicolor) and hydrophobic agent for wood protection

2.2.2

Reference [68] [69] [70] [71] [72] [73] [74] [75] [76]

Slow pyrolysis – solid char Slow pyrolysis is also known as carbonization as it produces solid biochar as the main

product from biomass pyrolysis [26, 28]. In contrast to fast pyrolysis, slow pyrolysis is featured

by a slower heating rate (0.1–10 oC/s) and a longer vapour residence time (up to 550 s). For slow pyrolysis, the processing time can go up to several hours to ensure the volatiles released have sufficient time for recombination with the biochar to maximize the biochar yield [2]. Withal, a slow heating rate facilitates the breakdown of weaker chemical bonds but inoffensive to remaining stronger chemical bond. In this sense, rearrangement reaction is promoted to generate a more structurally stable solid biochar as well as reduces the formation of volatile compounds [54, 77]. Slow pyrolysis of biomass produces up to 50% of solid biochar with remaining yield accounted by the bio-oil and syngas. Despite this, the yield of solid biochar can be varied by using different pyrolysis temperatures. It was found that the yield of biochar decreased with increasing temperature, attributed to the formation and release of volatiles from the biochar at elevated temperature (> 700 oC) through secondary cracking reaction [56]. To date, the optimum temperature of slow pyrolysis to maximize biochar yield remains ambiguous due to the unavailability of any supportive scientific theory. Biochar is a solid carbonaceous material with aromatic polycyclic structure, and the biochar yield obtained by biomass pyrolysis is usually around 5–50 wt% [78, 79]. When biomass is pyrolyzed, biochar possibly formed by the intra- and intermolecular rearrangement reactions, whereby the benzene rings of lignin component combined into a polycyclic structure [49]. The biochar exhibits highly porous morphology with a wide range of surface area (10– 300 m2/g). Moreover, the surface chemistry of biochar can be altered by introducing several chemical functional groups (e.g. hydroxyl and carbonyl) for specific adsorption purpose [80]. In the past few years, biochar has been widely applied in wastewater treatment to remove heavy metals (e.g. copper, cadmium, lead, mercury) [81-85]. The adsorption of heavy metals on biochar occurred through ion exchange mechanism that facilitated by its surface carboxyl and hydroxyl groups [86]. The porous biochar with high surface area (< 300 m2/g) can also be used as an adsorbent to capture carbon dioxide (CO2) and sulphur dioxide (SO2) [87, 88]. Besides,

the biochar could also improve the physicochemical properties of the soil by neutralizing its acidity [89], improving its cation exchange capacity (CEC) [90], and retaining the water and nutrients contents in the soil for plant uptake [91, 92]. Consequently, the addition of biochar into the soil can enhance the growth rate of the plant and thereby increase the crops yield. On the other hand, the carbon-dense biochar can be utilized for energy generation where it can be burned as solid fuel [93] or served as gasification feedstock to produce syngas [94]. Biochar is also a promising material for the synthesis of supercapacitor electrodes due to its low production cost and high performance [95, 96]. Table 3 shows other applications of biochar prepared from various biomass.

Table 3 Application of biochar prepared from various biomass. Feedstock Beech sawdust and spruce sawdust Brich wood

Application Biosorption of Cd (II), Cu (II) and methylene blue from simulated wastewater Soil amendments and growth of lettuce, radish, ryegrass and barley Camphor tree, rice Adsorption of H2S from simulated gas hull, bamboo Crab shell Phosphorus removal from phosphate solution and biogas effluent Fruit peel Treatment of palm oil mill effluent (POME) Olive husk Adsorption of Hg (II) from simulated wastewater Palm kernel shell Bio-fertilizer for mushroom cultivation Peanut shell Adsorption of carbamazepine (CBZ) and bisphenol A (BPA) from simulated wastewater Pine nut shells Preparation of activated biochar from biochar Pruning residues Soil amendments on tomato, watercress and lettuce Sugarcane bagasse Water retention on calcaric dark red soil

2.2.3

Reference [97] [98] [99] [100] [101] [102] [103] [104] [105] [106] [107]

Flash pyrolysis – syngas production Flash pyrolysis is characterized by its extremely high heating rates (> 1000 oC/s) and

short vapour residence time (< 0.5 s). Flash pyrolysis allows the high temperature to be achieved in a short time to promote the production of volatiles, which can be recovered as bio-

oil or incondensable syngas. As the feedstock is heated, most of its chemical bonds will be broken to release more volatiles; additionally, some chemically unstable volatiles will crack into incondensable syngas because of secondary cracking [108]. With a comparatively shorter process time (< 1 min), flash pyrolysis eliminates the re-condensation and recombination possibilities of volatiles with the biochar [2]. The process temperature of flash pyrolysis can quickly rise to 1200 oC, thus producing bio-oil and incondensable syngas as the main products with a small amount of biochar. Theoretically, more syngas will be obtained than bio-oil due to the secondary cracking of volatiles that took place at ≥ 600 oC [2]. Nonetheless, the actual yields of bio-oil and syngas are affected by the types of feedstock and the design of the pyrolysis reactor. Biomass pyrolysis also harnesses syngas as the least product, wherein the syngas yield usually falls in the range of 10–60 wt%. Syngas is a gaseous mixture made up of hydrogen (H2), carbon monoxide (CO), carbon dioxide (CO2), and light hydrocarbons (e.g. methane (CH4), ethane (C2H6) ethylene (C2H4) and propane (C3H8)), with trace amounts of ammonia (NH3), nitrogen oxides (NOx), and sulphur oxides (SOx) [57]. The compositions of these gaseous products are significantly influenced by the feedstocks and the process parameters used for the pyrolysis (See Section 2.3). The syngas obtained could be potentially used as a chemical feedstock or gaseous fuel. Specifically, the syngas (CO and H2) could be upgraded into diesel-like fuel via Fischer-Tropsch synthesis or burned as fuel in the gas [109, 110]. Alternatively, the H2 can be purified through water-gas-shift and CO2 adsorption before its utilization as a secondary fuel while the CH4 can be further steam reformed with H2O to enrich the H2 content. Also, molten carbonate fuel cells (MCFC) could utilise the syngas containing CO, CO2, H2, CH4, and some light hydrocarbons (C2–C6) for electricity generation [111]. The hot syngas could also be used to preheat inert gas (e.g. N2) that employed in the biomass pyrolysis [57]. Syngas could be recycled in the fluidized bed pyrolysis reactor to support

fluidization or simply burnt to supply process heat for relevant heating application (e.g. drying). Table 4 shows the application of syngas produced from biomass pyrolysis.

Table 4 Application of syngas produced from biomass pyrolysis. Feedstock Apple pomace Marine biomass Not stated Sewage sludge Various agricultural residues

Application Feedstock for H2 production Fuel gas Reducing agent of zinc and lead from Waelz slag Fuel gas to sustain the pyrolysis process Fuel gas

Reference [112] [113] [114] [115] [116]

2.3 Types of pyrolysis 2.3.1 Solar pyrolysis By using concentrated solar energy as the heating source, solar pyrolysis is a type of “green” pyrolysis that usually performed over a wide ranges of temperature (150–2000 oC) [117-119], heating rate (5–450 oC/s) [120], and heat flux intensity of 0.01–12 MW/m2 [119, 121]. In relative to other pyrolysis, solar pyrolysis has a comparatively complicated set-up to capture, reflect, and concentrate the solar energy to drive the pyrolysis reaction. Figure 2 shows the schematic diagram of solar pyrolysis set-up, which consists of a reflector mirror (heliostat), a shutter, a ‘solar-blind’ pyrometer, a parabolic mirror (parabola), and a transparent reactor [119]. The heliostat reflects the solar energy received from the Sun to the parabola, which concentrates the solar energy to the sample resided on the transparent reactor [119]. In solar pyrolysis, the sample temperature is measured by the pyrometer while the heating rate is controlled by the shutter that regulates the amount of reflected solar energy [119].

Figure 2 Schematic diagram of solar pyrolysis set-up [119]. Depending on the pyrolysis temperature, solar pyrolysis of biomass mainly generates bio-oil (25–78 wt%) and syngas (1.4–63 wt%) with biochar (8–29 wt%) as the minor product [119]. The solar energy from the Sun is a renewable, zero-cost, and sustainable heating source that can be used for biomass pyrolysis. Without the external heating source, solar pyrolysis eventually provides an eco-friendlier yet cost-effective approach for waste valorization as neither fossil fuels nor electricity is required for heat generation. Howbeit, several hindrances impede scaled-up the commercialization of solar pyrolysis. These challenges include costly solar reactor and heliostat, lack of understanding on the heat and mass transfer phenomena, underdeveloped reaction kinetics, difficulties of continuous operation, and the choices of reactor material that allow the best penetration of solar radiation.

2.3.2 Vacuum pyrolysis

Vacuum pyrolysis is a modified version of conventional pyrolysis, whereby the pyrolysis is conducted under vacuum to imitate an inert environment without the sparging of unreactive gases (e.g. nitrogen, argon). The phrase “vacuum” implies that the pressure inside the reactor is lower than the atmospheric pressure. Typically, vacuum pyrolysis is operated at low vacuum pressure (0.5–50 kPa) and moderate temperature (400–600 oC) [103, 122, 123]. Before pyrolysis, a vacuum pump is employed to evacuate the air inside the reactor to create an inert environment. During pyrolysis, the volatiles released will diffuse immediately towards the direction of the vacuum pump owing to the pressure gradient between the reactor and the vacuum pump. As compared to conventional pyrolysis, the energy consumption of vacuum pyrolysis is relatively lower without the heating of inert gas. Aside from that, the reduced vapour pressure of the biomass in the reactor possibly decreases the decomposition temperature of the biomass material, thereby permitting pyrolysis to occur at a lower temperature [124, 125]. The production cost of vacuum pyrolysis is comparatively low since it is liberated from the requirement of continuous inert gas purging. Furthermore, vacuum pyrolysis also eliminates the possibility of combustion (free of oxygen) and re-condensation of volatiles on the biochar surface [26]; thus, quality biochar with higher surface area and cleaner pores can be obtained. Figure 3 illustrates the effects of different pyrolysis environments on the development of biochar. Vacuum pyrolysis is capable of converting biomass waste into bio-oil and biochar with considerable high heating value (22.4–40 MJ/kg) for fuel application [123].

Figure 3 The effects of different pyrolysis environments on the production of biochar [26].

Vacuum pyrolysis possesses a few limitations as well. From vacuum pyrolysis of biomass, the bio-oil obtained is primarily composed of polycyclic macromolecular compounds (e.g. 2,5-dimethyl benzophenone and 1,1-biphenyl-3-ethoxy) [124]. The existence of polycyclic macromolecular compounds in the bio-oil is probably due to the immediate removal of volatiles formed, which impedes the further decomposition of the volatiles into smaller molecular compounds [126]. Therefore, the raw bio-oil should be further treated by a cracking reaction to form smaller molecular compounds (e.g. C6–C12 hydrocarbon chain) before fuel application. The necessity of this pre-treatment is justified by high viscosity of the raw bio-oil because of concentrated polycyclic macromolecular compounds, which will be troublesome for the fuel injector of the engine. Moreover, combustion of the biomass feedstock might occur in the reactor during vacuum pyrolysis if an improper design is used.

2.3.3 Conventional pyrolysis

Conventional pyrolysis is the most common and widely used pyrolysis technique in waste transformation and recovery. Conventional pyrolysis is conducted with several reactor design such as fixed bed reactor, melting vessel, tubular or blast furnace [127]. Conventional pyrolysis also has been recognized as a waste to wealth approach until countless of pyrolysis plants existed worldwide (e.g. United Kingdom, Australia, China, India, Singapore). Table 5 details several existing pyrolysis plants in the world. For the conventional pyrolysis plants, the common feedstock used is usually biomass wastes such as agricultural/forestry wastes, scrap tires, and waste plastics. By pyrolysis, these wastes are thermally decomposed for valorization into bio-oil and biochar with multifunctional applications (refer Tables 1–2). Both bio-oil and biochar from biomass pyrolysis contribute to the major source of profit for the pyrolysis industries. Specifically, the bio-oil can be sold as fuel for energy application while the biochar can be refined into carbon black or activated biochar that has wider demand. Aside from that, incondensable syngas produced can be channeled to the combustion engine for energy production to actualize a self-sustainable pyrolysis process with lower operation cost. Through this approach, the overall energy consumption could be minimized while more profit could be generated from biomass pyrolysis. The field-scale pyrolysis plants listed in Table 5 have proven the practicality of pyrolysis in sustainable utilization/recycling of biomass wastes. In short, biomass pyrolysis plant offers several alluring advantages: (i) amelioration of waste into renewable fuel for sustainable energy production, (ii) better waste treatment alternative to divert the waste from landfilling and composting, and (iii) alleviation of global warming via lessened greenhouse gases emission.

Table 5 Example of pyrolysis plants in the world. Application of pyrolysis plant Company Website Biomass to biofuel Melbourne Pyrotech https://pyrotechenergy.com/ Energy Pty Ltd.

Biomass to biofuel

BTG Bioliquids

https://www.btgbtl.com/en/technology Plastic to fuel PK Clean Technologies http://www.pkclean.com/ Inc. of Salt Lake City Plastic to oil Agile Process Chemicals http://pyrolysisplant.com/ LLP Scrap tires to oil, carbon black, China Doublestar http://www.doublestar.com. steel wire, and gas cn/ Waste tire to carbon black char, Metso Corporation https://www.metso.com/pro steel radials, oil and gas ducts/pyro-process/tirepyrolysis-systems/ Wastes to energy Klean Industries Inc. http://www.kleanindustries. com/s/Home.asp Wood-derived feedstock to Ensyn Technologies Inc. http://www.ensyn.com/over biofuel and chemical feedstock view.html 2.3.3.1 Conventional heating mechanism Presently, the concept and heating mechanism of conventional pyrolysis is wellestablished and understood. The heating mechanism of these equipment are non-selective to target, wherein the thermal energy is externally supplied from the heating coil to the reactor [26, 28]. The heat transfer from the heating coil to the sample surface is governed by conduction and convection, which is slow and energy inefficient due to non-selective heating that longer time and more energy are required. Moreover, higher temperature gradient is required to transfer heat among the sample to reach the targeted temperature for pyrolysis to occur. Figure 4 shows the heating occurs inside an electrical furnace.

Figure 4 Conventional pyrolysis of the sample using an electrical furnace.

2.3.3.2 Microwave heating mechanism As electromagnetic radiation, microwave consists of electric and magnetic fields that are perpendicular to each other. In the electromagnetic spectrum, microwave is the electromagnetic wave that lies in between the infrared and the radio-wave regions, with a wavelength range of 0.001–1 m (corresponds to a frequency range of 300 MHz–300 GHz). For microwave heating purpose, the Federal Communications Commission (FCC) only permits the utilization of microwave from two applicable frequencies (i.e. 915 MHz and 2.45 GHz) to prevent interferences with telecommunication equipment and devices [128-130]. In general, microwave irradiation interact differently with three types of materials: i) the insulator that allows the direct penetration of microwave without causing any losses of the microwave, ii) the conductor that blocks and reflects the microwave, iii) the absorber (also known as “dielectrics”) that absorbs microwave and converts it into heat energy [131]. Figure 5 shows the interaction of the microwave with different types of materials.

Figure 5 The interaction of microwave with different types of materials.

The interactions between microwave radiation and the absorbent materials is governed by the dielectric loss tangent of the material (tan δ) as in the Equation (1).

tan δ =

dielectric loss factor (ε′′)

(1)

dielectric constant (ε′)

The tan δ in equation (1) refers to the efficiency of a microwave heating process, particularly the final temperature and heating rate achieved by selected microwave-absorbing material [132]. The dielectric loss factor (ε′′) determine the efficiency of heat energy converted from microwave energy, whereas the dielectric constant (ε′) refers to the number of microwave irradiation being absorbed and reflected by the selected material. Carbon materials such as carbon and activated biochar are microwave absorbers with high capacity to absorb and convert microwave energy into heat energy [133]. On the grounds of this, some researchers attempted to improve the microwave pyrolysis of biomass by adding microwave absorbents (e.g. carbon materials) to biomass since few studies reported a low susceptibility of biomass towards microwave radiation [29, 134, 135]. In this approach, the microwave absorbent is preheated by microwave radiation to the desired pyrolysis temperature; then, the biomass waste is pyrolyzed

via thermal contact with the microwave absorbent. Likewise, microwave pyrolysis of biomass gives a different product distribution from other types of biomass pyrolysis. The conversion of microwave energy into heat energy is usually facilitated by three mechanisms, viz. dipole polarization, ionic conduction, and interfacial (Maxwell-Wagner) polarization [134, 136]. Figure 6 shows the dipole polarization (a) and ionic conduction (b) occur between microwave radiation and feedstock. Dipole polarization happens when the heat energy is generated in polar molecules with net dipole moment such as water. For water, a net dipole moment of 1.85 D is pointing from the hydrogen atoms that have lower electronegativity (2.2) towards oxygen atom with higher electronegativity (3.44). When the microwave is radiated to the water molecules, the dipoles of the water molecules will align and rotate with the electric field of the microwave. The alignment and rotation of the dipoles will occur up to a million times per second, so the resulting molecular friction and collisions of water molecules create the heating effect [137]. For ionic conduction, the heating effect is created by the charged ions in the sample. Upon microwave exposure, the ions (cations or anions) will oscillate back and forth with kinetic movements (i.e. vibration, translation, and rotation) to generate an electric current [138]. The kinetic movements of these ions induce their random collisions with each other and neighbouring atoms, in which the electrical resistance of the collisions produces heat energy [139]. The interfacial polarization takes place on the material with free surface charges, which possesses an equivalent amount of positive and negative charges. When the microwave is applied, the external electric field triggers the mobilization of free surface charges to separate opposing charges, which leads to the accumulation of surface charges at the material interface [140].

Without the microwave irradiation, the relaxation and

displacement of these surface charges back to their original location releases the heat energy [141].

Figure 6 (a) Dipolar polarization mechanism. (b) Ionic conduction mechanism [137].

2.3.4 Microwave pyrolysis Microwave pyrolysis is a newly touted pyrolysis which applies microwave as the heat source to pyrolyze the biomass wastes. This microwave system shows a distinct advantage such as rapid, targeted and energy-efficient heating process compared to conventional oven and furnace heating technologies, thus decreasing the production cost and increasing the production rate. In addition, microwave radiation can be selectively absorbed by carbon-based material with good microwave absorbency. Thus, certain chemical reaction could be promoted by the selective heating of carbon-based material with some reactants, leading to improved product’s yield and more uniform temperature profile [142]. Microwave pyrolysis is superior over both conventional and vacuum pyrolysis owing to the adoption of different heat transfer mechanism. For conventional or vacuum pyrolysis, the energy from electricity or fuel combustion is converted to heat, which is transferred to the surface and finally the inner parts of target material through convection and conduction. Thus, the heating efficiency is often limited by the surface temperature and the thermophysical properties of the target material such as its density, heat capacity, and thermal conductivity. On the other hand, microwave pyrolysis exploits the interaction of microwave with the dipoles of

target material for heat generation. Instead of an external heating source, heat is generated within the core of target material, which is more efficient than conventional surface heating by virtue of even heat distribution and more precisely controlled heating. Since microwave heating can achieve high temperatures and heating rates, microwave pyrolysis is applauded as a fast, energy-efficient, and time-saving process as compared to both conventional and vacuum pyrolysis [28, 66, 143, 144]. From the perspective of eco-friendliness, solar pyrolysis that powered by the Sun outweighs microwave pyrolysis that still requires electricity for microwave generation. Nonetheless, microwave pyrolysis offers several advantageous features over solar pyrolysis, such as lower installation cost (only magnetron is required), irresponsive to everchanging weather (stormy/shady days and night), and less spatial-occupying (without solar photovoltaic cells).

2.4 Recent progress, challenges and future direction of microwave pyrolysis Domίnguez et al. [145] took the first initiative to investigate biofuel (syngas, bio-oil, biochar) produced from microwave-assisted pyrolysis of sewage sludge in 2006. Their successful valorization of sewage sludge into valuable syngas had inspired countless researchers to exert strenuous efforts on microwave pyrolysis of biomass wastes. Microwave pyrolysis of biomass is attractive since it concurrently facilitates biomass recycling and energy recovery without expending energy as much as that of conventional pyrolysis. The higher heating efficiency of microwave pyrolysis than conventional pyrolysis represents the main momentum to drive its advancement via further research and development. Energy saving valorization pathway like microwave pyrolysis is sought owing to the gradual depletion of nonreplenishable fossil fuels (e.g. petroleum, coal and natural gas) and environmental pollution inflicted by the coal-fired power plant. To date, microwave pyrolysis of biomass has been

extended to recycle various biomass wastes such as rice straw [146], corn stover [147], oil palm wastes [103, 117], rice husk [148], fruit peel [28, 143], and nutshell [149]. Principally, recent studies on the microwave pyrolysis of biomass highlight the influence of process parameters on product distributions. Table 6 presents the recent microwave pyrolysis of biomass and its products yield while Figure 7 presents the product yield obtained from microwave pyrolysis of biomass from different kinds of feedstock. As shown in Table 6, several common process parameters are usually investigated, such as microwave power, temperature, biomass loading, microwave absorber to biomass ratio, microwave absorber used, process time, and operation mode (batch or continuous). Since the heat energy is converted from microwave irradiation, microwave power has a direct influence on the process temperature such that a proportional relationship exists between process temperature and microwave power. From Table 6, it is perceptible that most researchers opted to manipulate microwave power rather than the temperature during microwave pyrolysis of biomass. Salema et al. [150] highlighted the underlying reason by claiming the lack of efficient and robust temperature sensor for precise temperature measurement. To overcome this issue, they highlighted the need to install costly infrared temperature sensors which is indirect contact with the sample [150]. Moreover, the biomass loading plays an important role during microwave pyrolysis of biomass because of heat transfer efficiency, particularly when the microwave power is constantly fixed. Small biomass loading is usually being heated faster and more uniformly as compared to large biomass loading. For a fair comparison with previous literature, Song et al. [151] suggested the use of specific microwave power (microwave power per biomass loading). Numerous studies claimed that biomass was less susceptible to microwave radiation [29, 134, 135]. Consequently, several microwave absorbers such as silicon carbide (SiC), activated biochar, biochar, iron (Fe) powder, cobalt (Co) powder have been employed to overcome low

susceptibility of biomass wastes toward microwave irradiation. The microwave absorber to biomass ratio also represents one of the critical process parameters as the microwave absorber represents the heat medium that enhances the biomass pyrolysis via direct thermal contact with biomass. Unlike other types of pyrolysis, the processing time for microwave pyrolysis of biomass is rarely specified in published works. Only a few researchers fixed the pyrolysis time while others performed the microwave pyrolysis of biomass until the cessation of syngas flow. While most of the reported works executed in batch mode, Wang et al. [152] and Liu et al. [153] carried out microwave pyrolysis of biomass in continuous mode with a higher rate of biomass recycling. Similarly, conventional pyrolysis is virtually impractical to predict the product distribution without any experiments being conducted. In the present, any generalization attempts for the prediction of product distribution will be meaningless due to the scarcity of related experiments and the non-standardized experimental design.

Figure 7 Product yield obtained from microwave pyrolysis of biomass from different kinds of feedstock [148-157].

Table 6 Recent studies on microwave pyrolysis of biomass and the products yield. Feedstock

Process parameters* P (W) T (oC) WB (g)

Product yield (wt%) Biochar Bio-oil

N/A

400–600

100

MA/B ratio N/A

900– 1500

N/A

500– 1000

0.075– 0.15

Activated 120 biochar

30.9–41.1

13.4–19.6

41.6–54.0

[150]

300– 1800

N/A

75

7.5

15.0–25.0

39.0–58.0

20.0–44.0

[154]

Printed circuit 480 board Rice straw# N/A

N/A

10–100

0.1–10

Activated 20 biochar/ biochar Fe/Co <10

48.2–66.9

10.2–27.0

6.5–41.4

[155]

400–600

100

N/A

SiC bed

N/A

31.3–42.0

16.2–31.9

35.7–52.5

[152]

Shoot of Vitis 3000 vinifera Shrub willow 1000

N/A

100

0.5

Fe/SiC/C

0.5

28.3–71.4

15.3–34.9

13.3–45.7

[156]

N/A

100

0.1

19.3

40.1

21.1

[157]

Sugarcane 600 bagasse Textile dyeing 450–850 sludge# Tire powder 270–720

500

10–20

0–33.33

Activated N/A biochar Fe/Co N/A

21.5–47.4

22.5–31.0

27.6–54.5

[158]

N/A

500–700

0

N/A

20–40

79.8–88.9

6.9–8.0

1.6–6.9

[153]

N/A

30

0

N/A

N/A

43.0–48.9

37.0–45.0

12.0–18.5

[151]

Xylan

N/A

0.1–0.7

0

N/A

10

40.0–82.0

2.0–21.0

16.0–40.0

[159]

Camellia oleifera shell# Corn stalk biomass briquette Mediterranean forest biomass

* P:

200

MA

t (min)

SiC bed

N/A

24.9–35.5

14.6–27.5

39.0–60.6

[152]

microwave power, T: temperature, WB: biomass loading, MA/B ratio: microwave absorber/biomass ratio, t: time Continuous mode N/A: Not available/not specified. #

Reference Syngas

Frequently, selected or all pyrolysis products are purposed for their potential application based on extensive characterization or real application. Gas chromatography-mass spectrometry (GC-MS) and Fourier transform infrared spectroscopy (FTIR) can be used to scrutinize chemical composition and functional group of bio-oil, singly. To verify the potential of fuel application, thermogravimetric analysis (TGA) and bomb calorimeter can be applied for bio-oil and biochar to ascertain their thermal stability and caloric value, respectively. By knowing the gas composition, the caloric value of the syngas could be calculated as higher heating value (HHV) using numerous existing correlations that compiled in the work of Channiwala and Parikh [160]. Furthermore, biochar is usually subjected to scanning electron microscopy (SEM), X-ray fluorescence (XRF), CHNS elemental analysis, and nitrogen physisorption analysis to examine its morphology, chemical composition, organic elemental composition, and surface area. Table 7 summarizes the product of interest from microwave pyrolysis of biomass and its specific application. From the microwave pyrolysis of biomass (algae, agricultural wastes, and industrial wastes), the biochar obtained could be used as activated biochar precursor, adsorbent in wastewater treatment, fuel, growing media amendment, and soil amendment agent [161]. Often, the biochar is upgraded into engineered activated biochar that possesses better quality in terms of chemical compositions and porous characteristics by chemical or physical activation (refer Section 4.2.2). This kind of ‘upgraded’ biochar serves as a better adsorbent than common biochar in wastewater treatment. Comparatively, the bio-oil and syngas from microwave pyrolysis of biomass have narrower applications than the biochar. The bio-oil is recommended either for fuel application or the production of extractive components (i.e. nitrogen-containing compounds and limonene). Meanwhile, the syngas produced is solely suggested for fuel application by most researchers, which eventually neglects the possibility of syngas upgrading into liquid fuel via Fischer-Tropsch synthesis. The in-situ syngas upgrading

attempt is usually omitted since the installation of auxiliary instrumentation and secondary reactor incurs additional project cost.

Table 7 Product of interest from microwave pyrolysis of biomass and its application. Feedstock Algae

Product of interest Application Biochar Activated biochar production by CO2 gasification Banana peel Biochar Activated biochar for dye adsorption Biosolids Biochar Growing media amendments Chromolaena Biochar and bio-oil Fuel, soil repair agent (biochar odorata and soybean only) soapstock Distillers dried grain Bio-oil Fuel with solubles Microalgae Bio-oil Nitrogen-containing chemical as a precursor of pharmaceuticals Moso bamboo Syngas Fuel Oil palm wastes Biochar Dye adsorption, fuel, mushroom cultivation Empty fruit bunch Biochar Solid fuel and waste oil Palm kernel shell Biochar Dye adsorption Palm kernel shell Biochar Landfill leachate treatment Palm kernel shell Biochar Mushroom cultivation Palm kernel shell Biochar Activated biochar for treatment of palm oil mill effluent Pecan nutshell Biochar Wastewater treatment Scrap tires Bio-oil Limonene as chemicals, food additives, medicines Sludge Bio-oil and syngas Fuel Soapstock Bio-oil and biochar Fuel, soil amendment (biochar only)

Reference [162] [143] [163] [164] [165] [166] [167] [26] [168] [169, 170] [171] [103] [172] [149] [173] [174] [175]

As tabulated in Table 8, myriad researches have emphasized on the bio-oil production from microwave pyrolysis of biomass wastes. The results in Table 8 evince that the pyrolysis of single feedstock and co-pyrolysis of two feedstocks can produce valuable bio-oil. A discernible finding could be speculated from Table 8, in which the microwave pyrolysis of waste oils (frying oil, palm oil, etc.) give rise to a high bio-oil yield. This observation can be anticipated as the liquid oil typically has low ash contents that probably contribute to biochar

formation upon pyrolyzed [176]. Several researchers proposed that the merging of microwave pyrolysis and catalytic upgrading into a novel process called catalytic microwave pyrolysis, which strives to lower the production cost and increase the caloric value of bio-oil. The reduction of production costs most likely related to the catalytic approach, which furnishes an alternative pathway with lower activation energy (thereby process temperature) for microwave pyrolysis to occur. Moreover, the catalytic upgrading of bio-oil grants the formation of less deoxygenated bio-oil, which contributes to its higher caloric value. Regrettably, the employment of catalyst often renders a declined bio-oil yield as it promotes secondary cracking of heavy organic compounds into non-condensable syngas. The reduction of bio-oil is more conspicuous especially in the case of supported transition metal catalyst like iron and cobalt.

Table 8 Recent studies on microwave pyrolysis production of bio-oil from biomass. Feedstock

Type of catalyst

Bagasse, groundnut shell, mixed wood sawdust, Prosopis juliflora, rice husk and polypropylene (PP) or polystyrene (PS) Corn stover Distillers dried grains with solubles and waste agricultural plastic mulching films Douglas fir pellet

N.C.

N.C. Hierarchical ZSM-5/MCM-41 Ferrum-modified activated biochar Lignin Co/ZSM-5 Microalgae and polyvinyl chloride (PVC) N.C. Microalgae and scum HZSM-5 Rice straw and low-density polyethylene ZSM-5 (LDPE) Soybean straw and soapstock SiC ceramic foam Used frying oil N.C. Used frying oil and plastic wastes N.C. Waste palm oil N.C. Waste polyolefins and waste cooking oil N.C. Waste shipping oil N.C. N.C. – No catalyst.

Bio-oil yield Reference (wt%) 25.0–60.0 [177] 58.1 10.0–15.0

[178] [165]

23.3–45.2

[179]

20.0 37.7 22.0 24.5

[180] [181] [182] [183]

41.3 73.0 81.0 70.0 62.0 66.0

[184] [29] [185] [186] [35] [33]

The main driving force for biofuel production in the industry is due to its expanding global market at a compound annual growth rate (CAGR) of 4.5% that is expected to achieve about 220 billion dollars in 2022 [187]. The market statistics of Grand View Research [188] predicted that porous carbon materials such as engineered activated biochar has a global market size of USD 10.15 billion by 2024, which mainly shared by Asia Pacific (40%) and United State (35%). The global demand for activated biochar will approach 5.1 million tons by 2024, computed from its compound annual growth rate (CAGR) of 13.3% (2016–2024). The rising demand for activated biochar is believed to be affected by the stringent legislation on environmental protection and pollution prevention [189]. For instance, Environmental Protection Agency (EPA) in the United States launched the Mercury and Air Toxics Standard (MATS) to regulate the mercury and acid gas emissions from the coal-fired power plant and cement industry [190]. In China, the rapid industrialization and urbanization provoked severe air and water pollutions. The increasing demand for activated biochar is galvanized by its application in pollution control. Based on Table 6, the primary product from microwave pyrolysis of biomass wastes is speculated to be biochar since both bio-oil and syngas yields show meagre value (2.0 wt% bio-oil and 1.6 wt% syngas). In light of the high biochar yield, it is envisaged that the considerable amounts of biochar could be upgraded into multifunctional engineered activated biochar to meet the huge market demand. Biofuel production from microwave pyrolysis of biomass has recently gained industrial attention to initiate the commercialization because of the unique advantageous features of microwave heating (i.e. fast heating and energy efficient). Today, the commercialization of microwave pyrolysis technology for biochar production is still restrained by several technical issues despite extensive research at laboratory scale. For successful commercialization, this relatively new technology has to overcome several impediments such as the construction of a high-power magnetron, the design of effective evacuation system to handle enormous

microwave leakage, the discovery of durable and sturdy material to construct microwavetransparent reactor, and the design of an effective condensation unit. Nevertheless, continuous microwave pyrolysis system should be a main research focus due to its potential as a promising biomass waste valorization technology with a constant supply of feedstock. However, operation parameters and reactor design for continuous pilot scale have yet to be fully investigated owing to the limited understanding on microwave propagation pattern, feedstock feeding and simultaneous products (biochar, bio-oil and syngas) discharge methods. The biomass waste preloaded inside the reactor prior to pyrolysis is restrained by the reactor size and this could lead to low production rate. The advancement of microwave pyrolysis of biomass waste from batch to continuous mode would offer a faster processing rate at a lower energy consumption. Hence, continuous reactor should be applied and tested in microwave pyrolysis of AW as a newly touted technology to overcome the shortcoming of batch feeding mode.

3. Engineered activated biochar Activated biochar (also termed as ‘activated carbon’) is a highly porous carbon substance with hard texture, many surface adsorptions sites, and high chemical stability. In 3750 BC, the Egyptians and Sumerians applied activated biochar for bronze production (removal of copper, zinc and tin), fuel combustion, and medicinal purpose (detoxification agent for food poisoning) [191]. By 1500 BC, activated biochar was used to adsorb unpleasant odours and intestinal toxins [192]. In 400 BC, the activated biochar filter was developed to purify drinking water. From the 18th century onwards, activated biochar production from biomass (e.g. animal and plant remains) began, thereby inspired their application in liquid purification such as a decolouring agent for raw sugar syrups [193]. In 1909, the first commercial powdered activated biochar was produced from wood by a production plant called “Chemische Werke”

in Europe [194]. At the early 20th century, the activated biochar was synthesized in both powdered and granular form, in which the former for decolourization in chemical and food industries and the latter for gas mask manufacture in arms industry [193]. Currently, activated biochar plays a significant role in wastewater treatment, catalysis, energy storage, and gas adsorption.

3.1 Production of biomass-derived engineered activated biochar Indeed, Section 3.1 explicitly discusses the broad application of engineered activated biochar in wastewater treatment, catalysis, energy storage, and gas adsorption regardless of the biomass origin. Section 2.4 highlights the capability of microwave pyrolysis to valorize vast biomass wastes into the biochar as primary product. Herein, the production methods of engineered activated biochar from biochar will be meticulously briefed under this section to facilitate the upgrading of biochar. The preparation methods have a predominant effect on the quality and target application of engineered activated biochar.

3.1.1 One-step and two-step thermal treatments Generally, the transformation of biomass into engineered activated biochar (AB) involves thermal treatments, viz. carbonization (slow pyrolysis to produce carbon-dense biochar) and activation (gasification using a gasifying agent to intensify the porosity). The amelioration of biomass into AB can proceed in one-step or two-step thermal treatments, in which the carbonization and activation are conducted simultaneously in one-step method but sequentially in two-step method. The one-step method involves simpler production steps and shorter processing time thereby reduces the AB production cost with lesser energy consumption. In contrast, the two-step method gives higher AB yield and produces more porous AB (higher surface area and pore volume), probably by dint of two stages of biomass

heating [195-197]. During the carbonization (first step), the release of most volatiles from biomass renders the pore creation of fixed carbon fraction to form the biomass-derived biochar [101]. While activation (second step), the gasifying agent further enhances the biochar porosity by existing pore enlargement and new pore creation via gasification and rearrangement [144, 198]. On the other hand, one step method could probably stimulate pore creation without any pore enlargement. Regardless of the thermal treatment methods, it is constantly reported that the synthesized AB have high surface area and pore volume that renders its high adsorption capacity towards targeted adsorbate (e.g. textile dye, heavy metal, CO2) [199-203]. Up to date, there is no any concluding statement about which method is preferable to produce engineered AB. From the standpoint of commercialization, one-step method is more economically feasible for massive AB production that targets low-cost adsorbent market. Meanwhile, two-step method is more recommended for the generation of more refined AB as advanced material precursors (e.g. supercapacitor, medicine, high-grade pharmaceutical mask) to compensate for its higher production cost. The overall production cost of engineered AB is also governed by the types of the activating agent (physical or chemical) used in the biochar activation.

3.1.2 Types of activation Table 9 summarizes the differences between chemical activation and physical activation. The chemical activation involves one-step or two-step thermal treatment whereas the physical activation consists of two-step thermal treatment. The activating agent in chemical and physical activations are chemical compounds and gasifying agent, respectively. The porosity (e.g. microporous, mesoporous) of activated biochar is affected by the types of activating agent used. Since the chemical activation occurs at a lower temperature, the glass reactor made from borosilicate is used. In contrast, quartz reactor with a higher melting point

is utilized in the physical activation that often requires higher process temperature. For the preparation of biomass-derived engineered activated biochar, the microwave heating is superior over conventional heating with its more efficient and uniform heating, which allows the valorization of biomass into activated biochar with lower activation temperature and reduced activation time.

Table 9 Summary of chemical and physical activations. Chemical activation Number of steps for thermal One or two treatments Types of activating agent H3PO4, ZnCl2, NaOH, KOH, Na2CO3, K2CO3, or their mixtures Types of glass reactor Borosilicate Conventional heating Activation temperature (oC) 400–700 Activation time (min) 30–120

Physical activation Two Steam, CO2, mixtures

or

their

Quartz 750–1000 30–300

Microwave heating Activation temperature (oC)

300–650

600–900

Activation time (min)

5–20

15–210

Microwave power (W)

350–1200

900–3000

3.1.2.1 Chemical activation Chemical activation can be performed via one step [204] or two steps [196] thermal treatments, such that the second thermal treatment by gasification is optional. In chemical activation, the activating agent could be various types of chemicals, which inclusive of acidic (e.g. H3PO4, ZnCl2) [205, 206], alkali (e.g. NaOH, KOH, NaOH and KOH mixture) [143, 207, 208], or neutral (e.g. Na2CO3, K2CO3) [209, 210] compounds. Basically, chemical activation made up by three essential steps: i) impregnation of AB precursor (raw biomass or biochar) with a chemical agent, ii) thermal treatment of the impregnated precursor, and iii) washing.

During the impregnation, the AB precursor is dissolved in the aqueous solution of chemical agent (3.5–15 mL) of specified impregnation ratio (mass ratio of the chemical agent to AB precursor). For a predetermined impregnation time (5–24 h), the colloidal mixture is constantly stirred to promote the diffusion of the chemical agent into the interior part of AB precursor [211-213]. It was reported that longer impregnation time would enhance the BET surface area and pore volume of the engineered AB [214]. Before thermal treatment, the impregnated AB precursor will be filtered and oven-dried to remove moisture, which eliminates unnecessary energy consumption for de-moisturization. If microwave heating is used instead of conventional heating, the oven-drying procedure could be neglected as water is a good microwave absorber that hastens microwave heating. The dried impregnated AB precursor is then pyrolyzed at 400–700 oC [215], whereby the heat energy triggers the chemical reaction between the impregnated chemical and AB precursor for new pore creation and existing pore widening. Afterwards, the resulted AB is washed with dilute HCl or NaOH (e.g. 0.1 M) followed by warm water to neutralize and leach out the residual chemical used for impregnation. For instance, the AB is washed with dilute acid if the AB precursor is impregnated with alkali (such as metal hydroxide), and vice versa. The completion of washing step is indicated by the pH value (6–7) of the solution. The washing step is crucial to prevent the pore occlusion of AB by residual chemicals, thereby maximize the adsorption sites of AB.

3.1.2.2 Physical activation Physical activation is conducted via two steps thermal treatments [216]. The biomass will be first carbonized into carbon-dense biochar, and subsequently activated with various gasifying agents (e.g. steam, CO2) to give highly porous engineered AB [217]. Unlike chemical activation, physical activation is a simpler process without impregnation and washing steps,

hence shorter processing time. Comparatively, physical activation is more environmentally benign as no chemicals are required. Nonetheless, physical activation often performed at higher activation temperature (750–1000 oC) with longer activation time (30–300 min), which correspond to greater energy consumption and potentially higher operation cost [218]. Moreover, physical activation is often associated with lower AB yield than that of chemical activation due to the carbon burn-off via side reactions like char gasification or reverse Boudouard reaction [219, 220].

3.1.3 Influence of activating agent on the porous characteristics of engineered activated biochar Porous characteristics of engineered activated biochar (AB) is affected by the types of activating agents (acid, alkali, metal carbonates, steam, and CO2) due to the different interaction between the activating agent and AB precursor (raw biomass or biochar). For instance, KOH commonly produces microporous AB [221] while NaOH and H3PO4 favour the preparation of mesoporous AB [205, 222, 223]. For other activating agents, ZnCl2 [204, 224, 225], Na2CO3 [226, 227], K2CO3 [228, 229], steam [230, 231], and CO2 [232, 233] are capable to produce both microporous and mesoporous AB. The acid agent such as ZnCl2 and H3PO4 act as a dehydrating agent that improves the carbon content of the AB [224, 234]. The interaction between the acid agent and the carbon structure for pore formation is relatively complicated, which involves several reactions like degradation of carbon structure, dehydration, condensation, and emission of volatiles [235]. For the case of H3PO4 activation, several phosphorus functional groups (e.g. phosphate and polyphosphate esters) remained on the surface of AB after activation, which could be beneficial for adsorption [234]. The chemical reaction between the alkali agent (e.g. NaOH and KOH) and the carbon structure have been proposed and well-explained in the literature [222, 236, 237]. For alkali

agent, the pore formation of the resulted AB could be explained by the intercalation of the alkali metal atoms into the carbon structure [238]. During the thermal treatment, the alkali metal atoms of impregnated precursor are thermally energized and migrated into the carbon structure to create new pores and widen the existing pores on the AB surface [143, 237]. For chemical activation using metal carbonates, the mechanism of pore formation is similar to that of alkali agent. Upon thermal treatment, it is conjectured that the metal carbonates are thermally decomposed into the metal atoms, metal oxides, and carbon oxides (CO2 and CO) [229, 239]. Then, the metal oxides are reduced by the carbon structure to form metal atoms with the release of CO [214]. Finally, the metal atoms will diffuse into the carbon structure of biochar via intercalation to widen the existing pores and form new pores [239]. In the case of activation by steam and CO2, the mechanism of pores formation is simpler. Both steam and CO2 act as an oxidizing agent [232] to react with the biochar via steam reforming [240] and dry reforming [241], respectively.

3.2 General application of engineered activated biochar 3.2.1 Wastewater treatment Enormous volume of wastewater has been produced inevitably from the residence areas, commercial buildings, agricultural sectors, and all sorts of processing industries. Since a few decades ago, engineered activated biochar is commonly used in domestic water and industrial wastewater treatment facilities to remove the water and wastewater-borne pollutants through adsorption process that occurs on its adsorption sites [242-244]. However, the activated biochar can only last for several months (maximum up to 1 year) of a continuous function and subsequently discarded as waste, causing secondary environmental pollution. Despite the regeneration of spent activated biochar (i.e. regeneration by steam, thermal, chemical, and biological) for reuse is feasible, the cost of the regeneration process is often higher than the

procurement of new activated biochar. Thus, cost-effective and efficient regeneration of activated biochar is sought. Meanwhile, the production of engineered activated biochar from a renewable source like biomass wastes is alluring. In academia, treatment of wastewater containing textile dyes, heavy metals, and other chemical residues using biomass-derived activated biochar are commonly reported. On top of that, the engineered activated biochar could be chemically-modified to enhance its performance in wastewater treatment. For instance, the activated biochar modified by polymer matrix has enhanced adsorption capacity towards heavy metal [245]. It was also reported that the modified activated biochar demonstrated higher removal efficiency of cationic dyes than pristine activated biochar [246]. The hydroxyl-iron-lanthanum modified activated biochar fibre exhibited a higher adsorption capacity for phosphate [247]. Despite the high effectiveness of modified activated biochar in wastewater treatment, the production cost should be taken into consideration before commercialization; otherwise, such product will remain only at the research stage. Table 10 illustrates the recent application of engineered activated biochar derived from different biomass in wastewater treatment.

Table 10 Recent progress in wastewater treatment by biomass-derived engineered activated biochar (AB). Source of AB Banana peel Coconut shell Coffee grounds Date press cake Laundry sewage sludge Lignocellulosic wastes Oil palm mesocarp fibre Palm kernel shell Palm kernel shell Palm shell Pecan nutshell Pistachio wood waste Plum stones Salvadora persica

Applications Adsorption of Cu2+, Ni2+, Pb2+ Removal of COD and polyphenol Adsorption of methyl orange Adsorption of Cr3+ Adsorption of dye (Remazol Brilliant Blue R) Adsorption of Cd2+ and Ni2+ Treatment of palm oil mill effluent Treatment of palm oil mill effluent Removal of herbicides Removal of dye (Procion Red MX-5B) Adsorption of Zn2+, Cd2+, Ni2+, Cu2+ Removal of Hg2+ Adsorption of Cu2+ and Pb2+ Adsorption of Cu2+, Pb2+, Ni2+

Reference [248] [249] [250] [207] [251] [252] [253] [172] [254] [255] [256] [257] [258] [259]

Sunflower piths Waste tires

Adsorption of dye (methylene blue) Adsorption of Pb2+, Cr3+, Cd2+

[260] [261]

3.2.2 Catalysis Diverse catalysis is an inspiring field with the constant development of novel catalysts to discover effective and economical catalysts. The growing interest in the employment of biomass-derived engineered activated biochar in catalysis is stimulated by the expensiveness of commercial catalyst supports [262]. Hence, the activated biochar is widely used as catalyst support in various reactions such as methane decomposition, hydrogenation, and transesterification [263-265]. As catalyst support, the activated biochar with large surface area provides numerous attachment sites for active metal atoms (e.g. Fe, Ni, Cu), thereby forming the active sites that responsible for the catalysis [266]. Besides, the activated biochar possesses high chemical stability that prevents any unnecessary chemical reactions with the reactant. The surface of engineered activated biochar can also be modified by chemical (e.g. sulfuric acid, phosphoric acid) to introduce certain functional groups for specific catalytic reaction. For example, Matos, Silva, Ruiz-Rosas, Vital, Rodríguez-Mirasol, Cordero, Castanheiro and Fonseca [267] utilized the olive stone-derived activated biochar, commercial activated biochar, and xerogel mesoporous carbon to catalyze the methoxylation of α-pinene into α-terpinyl methyl ether. In their findings, the phosphoric acid treated-olive stone activated biochar showed higher catalytic activity than commercial activated biochar and xerogel mesoporous carbon due to the addition of phosphate groups. Besides that, sulfonated palm kernel shell-derived activated biochar was utilized as an acid catalyst for the hydrolysis of cellobiose (glucose dimer) [268]. The activated biochar was treated with concentrated sulphuric acids to introduce reactive sulfonate groups (-SO3H) on its surface that responsible for catalyzing cellobiose hydrolysis [269]. In short, the use of biomass-derived engineered activated biochar in catalysis provides a great research direction to be explored further since

the activated biochar exhibits comparable or even better catalytic activity than commercial catalysts. Table 11 shows the applications of biomass-derived engineered activated biochar in catalysis.

Table 11 Application of biomass-derived engineered activated biochar in catalysis. Source of AB Biomass Bulgarian peach stone Castanea mollissima shell Coconut shell Coconut shell Groundnut Olive stone Olive stone Palm kernel shell Wood sawdust Bamboo

Applications Photocatalytic degradation of Orange G dye Methanol decomposition Propane dehydrogenation Guaiacol hydrodeoxygenation Ozone decomposition Reduction of organic dye Bio-oil deoxygenation Methanol dehydration Methane dry reforming Isobutene dimerization Oleic acid esterification

Reference [270] [271] [272] [273] [220] [274] [275] [276] [144] [277] [278]

3.2.3 Energy storage The supercapacitor (or electric double-layer capacitor) is an indispensable energy storage material in this modern era that is widely applied in automobiles, air-craft, electronic devices, and locomotive systems. The supercapacitor is characterized by its rapid chargingdischarging, longer cycle life, higher energy and power densities than the conventional capacitors such as a ceramic capacitor and electrolytic capacitor [279]. The performance of supercapacitor is significantly affected by the material of the electrode used. Activated biochar represents one of the common electrode materials used in the supercapacitor due to its highly porous nature and large surface area. The porous activated biochar with a large surface area can provide many electroactive sites for charges accumulation (Faradaic reaction) and subsequently achieve high capacitance [280, 281]. It was forecasted that the global market of supercapacitor would be expanded from USD 910 million (2018) to USD 5530 million (2028) at a CAGR of 19.8% [282]. The application of activated biochar as electrodes of supercapacitor

has been arising as a hot topic in the last two decades ago probably due to its impactful utilization. Table 12 summarizes the recent application of biomass-derived engineered activated biochar as electrodes of the supercapacitor.

Table 12 Application of biomass-derived engineered activated biochar (AB) as supercapacitor. Source of AB Argy warmwood Bamboo Corn silks Cotonier strobili fibers Dead ginkgo leaves Fructose corn syrup Kapok shell Loofah sponge Wild rice stem

Capacitance (F/g) 344 293 260.8 346.1 374 168 169 309.6 301

Energy density (Wh/kg) 175 10.9 17.8 33 9.2 4.2 12.5 16.1 13

Power density (W/kg) 850 63 360 160 4.8 1500 1900 160 250

Reference [283] [284] [285] [286] [287] [288] [289] [290] [291]

3.2.4 Gas adsorption Almost every country in the world has executed its own legislation of air quality to control the emission of air pollutants. The rapid industrial development has engendered the continuous discharge of airborne pollutants such as nitrogen oxides (NOx), sulphur oxides (SOx), volatile organic compounds (VOC), and particulate matters (PM). These pollutants will deteriorate air quality, induce human respiratory problem, and trigger formation of acid rain [292]. Furthermore, poor air quality would also shorten the average lifespan of human for up to two years [293]. Despite this deadly consequence, uncontrol air pollution still happens in certain country. For example, the unsupervised burning of forest and oil palm plantation in Indonesia have caused the formation of haze every year since 2005 [294], which deprived the right of citizens from neighbouring countries (e.g. Malaysia and Singapore) to enjoy the clean air.

Concerted efforts have been made to counter the detrimental impacts on the environment and health caused by the excessive emission of air pollutants. To name a few, these mitigations include adsorption, catalytic reduction, post-combustion amine scrubbing, desulfurization, and storage reduction [292, 295]. Among the aforementioned techniques, adsorption using engineered activated biochar represents a promising method for gas pollution control by dint of its less energy usage, simple operation, and high effectiveness. Engineered activated biochar had been widely adopted in the industry (e.g. petrochemical plant) to minimize the discharge of air pollutants into the atmosphere. For gas adsorption by activated biochar, the emphasis of researches is propagating towards the surface modification of activated biochar surface to bolster specific adsorption application. In comparison with regular activated biochar, the modified activated biochar possesses better gas adsorption performance. For instance, the amine-modified activated biochar was capable of adsorbing more SO2 [296] while the hydroxide-modified activated biochar favoured the adsorption of NOx [297]. Moreover, the modification of activated biochar with metal nitrates (e.g. barium nitrates, cerium nitrates, copper nitrates, zinc nitrates) effectively promoted the simultaneous adsorption of CO2, NOx and SO2 [298, 299]. Table 13 summarizes the recent applications of biomassderived engineered activated biochar in gas adsorption.

Table 13 Application of biomass-derived engineered activated biochar (AB) in gas adsorption. Source of AB Coffee wastes Date seed Lignin Lyocell fibre Microalgae-sodium alginate Palm kernel shell coconut shell

Type of AB Normal AB Normal AB Normal AB CuO-AB Nitrogen-doped AB and Metal oxide-AB

Adsorbate NO2 CO2 SO2 SO2 CO2 and CH4

Reference [300] [301] [302] [303] [304]

CO2

[305]

Pigskin collagen Pistachio nutshells Rice husk Rice straw Urea-formaldehyde resin Walnut Waste lime mud and sawdust Wood pellets

Nitrogen-doped AB Normal AB Cu-AB SiC-AB Nitrogen-doped AB Fe-AB Calcium-rich AB

CO2 and H2 NO2 and H2S Trimethylamine and H2S H2 CO2 H2S SO2

[306] [307] [308] [309] [310] [311] [312]

Normal AB

CO2

[313]

4. Conclusion and recommendation Biomass waste is a renewable and sustainable resource that should be reused and explored further for any value-added uses. Depending on the treatment approach, the biomass waste represents a double-edged sword where it could either inflict negative environmental impact or generate useful feedstocks without harming the environment. Among the utilization using thermochemical conversion techniques, pyrolysis represents a most promising approach that can valorize biomass wastes to form three distinct multifunctional products (solid biochar, liquid bio-oil, and syngas). Nonetheless, the conventional pyrolysis of biomass performed and fueled by fossil fuel and electricity is associated with several drawbacks, such as non-selective heating, poor heat transfer, and high energy consumption. Thus, it is envisaged that the newly touted microwave pyrolysis could be a potential solution by virtue of its fast heating rate, short processing time, and efficient heat transfer. As a concluding remark of this review, several key messages are presented as follows:



Solid biochar is a primary product obtained from the microwave pyrolysis of biomass waste. Syngas produced from microwave pyrolysis can be upgraded into liquid fuel via Fischer-Tropsch synthesis.



In conjunction with the huge market of engineered activated biochar, it is inspired that conventional biochar could be upgraded to form value-added engineered activated biochar that possesses better quality in terms of chemical compositions and porous characteristics by chemical or physical activation. It can be used in diverse applications, such as wastewater treatment, catalysis, energy storage, and gas adsorption.



Engineered activated biochar can be applied to improve the quality of bio-oil with formation of less deoxygenated bio-oil, which contributes to its higher caloric value.



The global demand for activated biochar is estimated to approach 5.1 million tons by 2024, computed from its compound annual growth rate (CAGR) of 13.3% (2016–2024).



Extensive researches that emphasize on the reactor design and biomass pyrolysis are needed to expedite the growth of microwave pyrolysis application at the industrial level over worldwide.

Acknowledgement The authors acknowledge the financial support of FRGS research grant (Vot 59512) by the Ministry of Education Malaysia, Golden Goose Research Grant (GGRG) by Universiti Malaysia Terengganu, Henan Agricultural University for conducting this review. The first author also acknowledges the comments and ideas contributed by all the co-authors throughout this review.

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 Table 9 Summary of the different pyrolysis modes with their process conditions and product distribution. Process conditions Temperature (oC) Heating rate (oC/s) Process time (min) Vapor residence time (s) Biochar yield (wt%) Bio-oil yield (wt%) Syngas yield (wt%)

Slow pyrolysis 400–900 0.1–10 > 5 (can go up to several hours) < 550 25–50 20–40 10–25

Fast pyrolysis 450–850 10–200 10–25

Flash pyrolysis 600–1200 > 1000 <1

0.5–10 15–25 60–75 10–20

< 0.5 5–15 25–40 50–60

Table 10 Application of bio-oil obtained from various biomass. Feedstock American hardwoods Birch wood

Application Blending of acetone-pretreated bio-oil with epoxy resin for wood bonding Phenolic extracts of bio-oil as an antioxidant in soybean oil bio-lubricants Corn stalk Bio-oil/diesel emulsions with Ce0.7Zr0.3O2 nanoadditive as diesel fuel Japanese pine Antibacterial agent for foodborne pathogen wood sawdust (Staphylococcus aureus) control

Reference [68] [69] [70] [71]

Lard wood

Partial replacement of resorcinol formaldehyde resin with bio-oil in wood adhesive Lignin Catalytic upgrading of bio-oil into jet and diesel fuel range hydrocarbons over sulfided CoMo/γ-Al2O3 Lignocellulosic Raw bio-oil as fuel for anion exchange membrane fuel biomass cells to generate electricity Pine wood sawdust Antibacterial agent for foodborne pathogen (Bacillus cereus and Listeria monocytogenes) control Residual wood Antifungal (Gloeophyllum trabeum and Trametes fines versicolor) and hydrophobic agent for wood protection

[72] [73] [74] [75] [76]

Table 11 Application of biochar prepared from various biomass. Feedstock Beech sawdust and spruce sawdust Brich wood

Application Biosorption of Cd (II), Cu (II) and methylene blue from simulated wastewater Soil amendments and growth of lettuce, radish, ryegrass and barley Camphor tree, rice Adsorption of H2S from simulated gas hull, bamboo Crab shell Phosphorus removal from phosphate solution and biogas effluent Fruit peel Treatment of palm oil mill effluent (POME) Olive husk Adsorption of Hg (II) from simulated wastewater Palm kernel shell Bio-fertilizer for mushroom cultivation Peanut shell Adsorption of carbamazepine (CBZ) and bisphenol A (BPA) from simulated wastewater Pine nut shells Preparation of activated biochar from biochar Pruning residues Soil amendments on tomato, watercress and lettuce Sugarcane bagasse Water retention on calcaric dark red soil

Reference [97] [98] [99] [100] [101] [102] [103] [104] [105] [106] [107]

Table 12 Application of syngas produced from biomass pyrolysis. Feedstock Apple pomace

Application Feedstock for H2 production

Reference [112]

Marine biomass Not stated Sewage sludge Various agricultural residues

Fuel gas Reducing agent of zinc and lead from Waelz slag Fuel gas to sustain the pyrolysis process Fuel gas

[113] [114] [115] [116]

Table 13 Example of pyrolysis plants in the world. Application of pyrolysis plant Company Website Biomass to biofuel Melbourne Pyrotech https://pyrotechenergy.com/ Energy Pty Ltd. Biomass to biofuel BTG Bioliquids https://www.btgbtl.com/en/technology Plastic to fuel PK Clean Technologies http://www.pkclean.com/ Inc. of Salt Lake City Plastic to oil Agile Process Chemicals http://pyrolysisplant.com/ LLP Scrap tires to oil, carbon black, China Doublestar http://www.doublestar.com. steel wire, and gas cn/ Waste tire to carbon black char, Metso Corporation https://www.metso.com/pro steel radials, oil and gas ducts/pyro-process/tirepyrolysis-systems/ Wastes to energy Klean Industries Inc. http://www.kleanindustries. com/s/Home.asp Wood-derived feedstock to Ensyn Technologies Inc. http://www.ensyn.com/over biofuel and chemical feedstock view.html

Table 14 Recent studies on microwave pyrolysis of biomass and the products yield. Feedstock

Process parameters* P (W) T (oC) WB (g)

Product yield (wt%) Biochar Bio-oil

N/A

400–600

100

MA/B ratio N/A

900– 1500

N/A

500– 1000

0.075– 0.15

Activated 120 biochar

30.9–41.1

13.4–19.6

41.6–54.0

[150]

300– 1800

N/A

75

7.5

15.0–25.0

39.0–58.0

20.0–44.0

[154]

Printed circuit 480 board Rice straw# N/A

N/A

10–100

0.1–10

Activated 20 biochar/ biochar Fe/Co <10

48.2–66.9

10.2–27.0

6.5–41.4

[155]

400–600

100

N/A

SiC bed

N/A

31.3–42.0

16.2–31.9

35.7–52.5

[152]

Shoot of Vitis 3000 vinifera Shrub willow 1000

N/A

100

0.5

Fe/SiC/C

0.5

28.3–71.4

15.3–34.9

13.3–45.7

[156]

N/A

100

0.1

19.3

40.1

21.1

[157]

Sugarcane 600 bagasse Textile dyeing 450–850 sludge# Tire powder 270–720

500

10–20

0–33.33

Activated N/A biochar Fe/Co N/A

21.5–47.4

22.5–31.0

27.6–54.5

[158]

N/A

500–700

0

N/A

20–40

79.8–88.9

6.9–8.0

1.6–6.9

[153]

N/A

30

0

N/A

N/A

43.0–48.9

37.0–45.0

12.0–18.5

[151]

Xylan

N/A

0.1–0.7

0

N/A

10

40.0–82.0

2.0–21.0

16.0–40.0

[159]

Camellia oleifera shell# Corn stalk biomass briquette Mediterranean forest biomass

* P:

200

MA

t (min)

SiC bed

N/A

24.9–35.5

14.6–27.5

39.0–60.6

[152]

microwave power, T: temperature, WB: biomass loading, MA/B ratio: microwave absorber/biomass ratio, t: time Continuous mode N/A: Not available/not specified. #

Reference Syngas

Table 15 Product of interest from microwave pyrolysis of biomass and its application. Feedstock Algae

Product of interest Application Biochar Activated biochar production by CO2 gasification Banana peel Biochar Activated biochar for dye adsorption Biosolids Biochar Growing media amendments Chromolaena Biochar and bio-oil Fuel, soil repair agent (biochar odorata and soybean only) soapstock Distillers dried grain Bio-oil Fuel with solubles Microalgae Bio-oil Nitrogen-containing chemical as a precursor of pharmaceuticals Moso bamboo Syngas Fuel Oil palm wastes Biochar Dye adsorption, fuel, mushroom cultivation Empty fruit bunch Biochar Solid fuel and waste oil Palm kernel shell Biochar Dye adsorption Palm kernel shell Biochar Landfill leachate treatment Palm kernel shell Biochar Mushroom cultivation Palm kernel shell Biochar Activated biochar for treatment of palm oil mill effluent Pecan nutshell Biochar Wastewater treatment Scrap tires Bio-oil Limonene as chemicals, food additives, medicines Sludge Bio-oil and syngas Fuel Soapstock Bio-oil and biochar Fuel, soil amendment (biochar only)

Reference [162] [143] [163] [164] [165] [166] [167] [26] [168] [169, 170] [171] [103] [172] [149] [173] [174] [175]

Table 16 Recent studies on microwave pyrolysis production of bio-oil from biomass. Feedstock

Type of catalyst

Bagasse, groundnut shell, mixed wood sawdust, Prosopis juliflora, rice husk and polypropylene (PP) or polystyrene (PS) Corn stover Distillers dried grains with solubles and waste agricultural plastic mulching films Douglas fir pellet

N.C.

N.C. Hierarchical ZSM-5/MCM-41 Ferrum-modified activated biochar Lignin Co/ZSM-5 Microalgae and polyvinyl chloride (PVC) N.C. Microalgae and scum HZSM-5 Rice straw and low-density polyethylene ZSM-5 (LDPE) Soybean straw and soapstock SiC ceramic foam Used frying oil N.C. Used frying oil and plastic wastes N.C. Waste palm oil N.C. Waste polyolefins and waste cooking oil N.C. Waste shipping oil N.C. N.C. – No catalyst.

Bio-oil yield Reference (wt%) 25.0–60.0 [177] 58.1 10.0–15.0

[178] [165]

23.3–45.2

[179]

20.0 37.7 22.0 24.5

[180] [181] [182] [183]

41.3 73.0 81.0 70.0 62.0 66.0

[184] [29] [185] [186] [35] [33]

Table 9 Summary of chemical and physical activations. Chemical activation Number of steps for thermal One or two treatments Types of activating agent H3PO4, ZnCl2, NaOH, KOH, Na2CO3, K2CO3, or their mixtures Types of glass reactor Borosilicate Conventional heating Activation temperature (oC) 400–700 Activation time (min) 30–120

Physical activation Two Steam, CO2, mixtures Quartz 750–1000 30–300

Microwave heating Activation temperature (oC)

300–650

600–900

Activation time (min)

5–20

15–210

Microwave power (W)

350–1200

900–3000

or

their

Table 10 Recent progress in wastewater treatment by biomass-derived engineered activated biochar (AB). Source of AB Banana peel Coconut shell Coffee grounds Date press cake Laundry sewage sludge Lignocellulosic wastes Oil palm mesocarp fibre Palm kernel shell Palm kernel shell Palm shell Pecan nutshell Pistachio wood waste Plum stones Salvadora persica Sunflower piths Waste tires

Applications Adsorption of Cu2+, Ni2+, Pb2+ Removal of COD and polyphenol Adsorption of methyl orange Adsorption of Cr3+ Adsorption of dye (Remazol Brilliant Blue R) Adsorption of Cd2+ and Ni2+ Treatment of palm oil mill effluent Treatment of palm oil mill effluent Removal of herbicides Removal of dye (Procion Red MX-5B) Adsorption of Zn2+, Cd2+, Ni2+, Cu2+ Removal of Hg2+ Adsorption of Cu2+ and Pb2+ Adsorption of Cu2+, Pb2+, Ni2+ Adsorption of dye (methylene blue) Adsorption of Pb2+, Cr3+, Cd2+

Reference [248] [249] [250] [207] [251] [252] [253] [172] [254] [255] [256] [257] [258] [259] [260] [261]

Table 11 Application of biomass-derived engineered activated biochar in catalysis. Source of AB Biomass Bulgarian peach stone Castanea mollissima shell Coconut shell Coconut shell Groundnut Olive stone Olive stone Palm kernel shell Wood sawdust Bamboo

Applications Photocatalytic degradation of Orange G dye Methanol decomposition Propane dehydrogenation Guaiacol hydrodeoxygenation Ozone decomposition Reduction of organic dye Bio-oil deoxygenation Methanol dehydration Methane dry reforming Isobutene dimerization Oleic acid esterification

Reference [270] [271] [272] [273] [220] [274] [275] [276] [144] [277] [278]

Table 12 Application of biomass-derived engineered activated biochar (AB) as supercapacitor. Source of AB Argy warmwood Bamboo Corn silks Cotonier strobili fibers Dead ginkgo leaves Fructose corn syrup Kapok shell Loofah sponge Wild rice stem

Capacitance (F/g) 344 293 260.8 346.1 374 168 169 309.6 301

Energy density (Wh/kg) 175 10.9 17.8 33 9.2 4.2 12.5 16.1 13

Power density (W/kg) 850 63 360 160 4.8 1500 1900 160 250

Reference [283] [284] [285] [286] [287] [288] [289] [290] [291]

Table 13 Application of biomass-derived engineered activated biochar (AB) in gas adsorption. Source of AB Coffee wastes Date seed Lignin Lyocell fibre Microalgae-sodium alginate Palm kernel shell and coconut shell Pigskin collagen Pistachio nutshells Rice husk Rice straw Urea-formaldehyde resin Walnut Waste lime mud and sawdust Wood pellets 

Type of AB Normal AB Normal AB Normal AB CuO-AB Nitrogen-doped AB

Adsorbate NO2 CO2 SO2 SO2 CO2 and CH4

Reference [300] [301] [302] [303] [304]

Metal oxide-AB

CO2

[305]

Nitrogen-doped AB Normal AB Cu-AB SiC-AB Nitrogen-doped AB Fe-AB Calcium-rich AB

CO2 and H2 NO2 and H2S Trimethylamine and H2S H2 CO2 H2S SO2

[306] [307] [308] [309] [310] [311] [312]

Normal AB

CO2

[313]