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Engineered Biochar

Yuqing Sun1 and Daniel C.W. Tsang2

1 School of Agriculture, Sun Yat-Sen University, Guangzhou, Guangdong, China
2 State Key Laboratory of Clean Energy Utilization, Zhejiang University, China

As a product of the thermochemical decomposition of biomass, biochar is produced under the condition of a limited-oxygen or oxygen-deprived environment. Many organic raw materials can be employed for biochar production, including urban organic solid wastes, manures, algae, sewage sludge, agricultural and forestry byproducts, and wood chips. The techniques for the thermochemical decomposition of the organic materials are various, including gasification, torrefaction, microwave heating, hydrothermal carbonization, and pyrolysis, which differ in production duration and thermal condition. The reasons that biochar has drawn much attention are twofold. First, biochar production can mitigate climate change by storing carbon in a relatively stable form, thereby reducing the emission of greenhouse gases into the Earth's atmosphere. Second, due to its porous texture, biochar has plentiful surface functional groups (SFG) and a large surface area, which makes it an environmentally friendly, low-cost, and effective adsorbent/catalyst. Many studies have reported that the production temperature and duration, decomposition techniques, and feedstock type could considerably affect the final product's physical and chemical properties. In addition, pre-treatment of the feedstock and post-treatments can also influence biochar's properties.

1.1 Overview of Biochar Production

Both living and recently living organic materials can be used to produce biochar. They include lignocellulosic materials, such as forestry agricultural byproducts, plants, and wood. Nonlignocellulosic materials are also suitable, including organic municipal solid waste (MSW) and livestock's manure. On the other hand, due to the benefits of quick reaction time and high conversion ratio, thermochemical methods are the most preferred amongst a variety of biomass treatment techniques.

The International Biochar Initiative (IBI) states that "Biochar is a solid material obtained from the thermochemical conversion of biomass in an oxygen-limited environment" [1]. In other words, biomass is processed using the dry carbonization method (e.g., pyrolysis), yielding several products including biochar. Although the term hydrochar is analogous to biochar, it cannot be considered as biochar. Hydrochar is made employing hydrothermal carbonization, through which the organic raw material is heated with water at a temperature range of 200-300 °C. The material undergoes solid and liquid phases (i.e., slurry). Because biochar and hydrochar are essentially distinct, the hydrochar produced using hydrothermal carbonization is not equivalent to biochar.

Among the various thermochemical methods, gasification and dry torrefaction are unsuitable for making biochar. During dry torrefaction, biomass is heated to 200-300 °C and within an inert gas from half an hour to several hours. After the process, the organic raw materials loses energy in gaseous forms, accounting for merely 10% of the total energy, hence the final product's energy density is increased. Because of this, dry torrefaction has been used to pretreat the biomass before combustion. By contrast, gasification is essentially a process of partially combusting biomass at a higher temperature range (600-1200 °C) than pyrolysis, and with a duration of 10-20 seconds. The main reason for using gasification is to transform carbon-based material into gas mixtures (CO2, CO, and H2), such as producer gas and synthesis gas (syngas). However, IBI stresses that the production of biochar must avoid accumulation of toxicants in the final product and emission of unsafe air-pollutant.

Before processing for biochar, the wet organic material must be dried. However, the drying process is usually energy intensive, reducing the production's economic efficiency. That's why some forestry and agricultural wastes that possess low water content (less than 30%) after harvest are preferred sources of biochar. In addition, organic waste, such as livestock manure, sewage sludge, agroforestry waste, food waste, and organic fraction of MSW, are also commonly used as feedstock. Utilizing a variety of organic wastes to produce biochar is undoubtedly cost-efficient because the organic raw materials used in the production do not require arable land that could have been better used for growing energy and food crops. The consensus is that utilizing organic wastes for biochar production is eco-friendly and an ideal way to treat numerous wastes efficiently.

The main technology used to produce biochar is biomass pyrolysis. During the process, biomass is decomposed at a relatively high temperature and in an inert gas atmosphere (i.e., the inert gas N2 with limited O2). The organic constituents in the feedstock undergo thermochemical decomposition, releasing the gas phase and leaving solid phase residual (i.e., biochar). Next, the high-molecular-weight and polar compounds of the gas phase are cooled, producing a liquid phase (i.e., bio-oil), whereas the compounds that have lower molecular weight and primarily consist of noncondensable gases, such as H2, CH4, C2H2, CO, and CO2, are left in the gas phase [2]. Slow pyrolysis can offer the highest yield of solid product (35 to 50 wt.%) and is most commonly adopted for producing biochar. The production takes place at temperatures ranging from 300 °C to 800 °C with heating rates of 5-10 °C min-1, and the gas residence time varies from several minutes to a few hours.

During the pyrolysis process, cellulose starts to decompose between 240 °C and 350 °C, while the decomposition of hemicellulose starts between 200 °C and 260 °C, and lignin decomposes between 280 °C and 500 °C. In cellulose pyrolysis, cellulose first depolymerizes into oligosaccharides. The process of the cleavage of glycosidic bonds to form D-glucopyranose is as follows. It starts from the D-glucopyranose undergoing an intramolecular reorganization, producing levoglucosan. Levoglucosenone is then formed with dehydration of levoglucosan and subsequently proceeds through intermolecular condensation, dehydration, decarboxylation, and aromatization to create a complex solid porous structure (i.e., biochar). The pyrolyzing process of hemicellulose starts with the depolymerizing of hemicellulose into oligosaccharides, succeeded by the reorganization of oligosaccharides and cleavage of glycosidic linkages in xylan chains, forming 1, 4-anhydro-D-xylopyranose. This intermediate step is required for the formation of biochar, which undergoes intramolecular condensation, dehydration, decarboxylation, and aromatization. It is comparable to the pyrolyzing process of cellulose. In the pyrolyzing process of lignin, free radical reaction, among others, is the most crucial pathway. The ß-O-4 bonds are cleaved in lignin to generate free radicals, which is the first stage of the free radical reaction. The generated free radicals capture protons of substances with weak O-H or C-H bonds (such as phenyl groups), yielding decomposition products, including vanillin and cresol [2]. They are passed on to other substances, causing chain propagation [2]. When the reaction progresses further and the two radicals collide, stable compounds are formed, ending the chain reactions of free radicals [2]. However, it is difficult to observe the free radicals being generated during pyrolysis [2]. The exact mechanism of lignin pyrolysis is still unclear, and fully understanding it remains a daunting task [2].

During the first stage (200-300 °C), the organic raw material starts to decompose, and during the second stage (300-600 °C), a large number of aliphatic functional groups originating from hemicellulose and cellulose start to form [3]. It is worth noting that, at 350 °C, hemicellulose and cellulose begin to degrade and create O-H bonding of phenol (1375 cm-1), OH bending of alcohol (3300-3400 cm-1), C-O stretching of conjugated ketones (1600 cm-1), CH3 and CH2 alkanes bend (1375 and 1465 cm-1), C-H stretching (855 cm-1), aromatic C-C stretching (1475-1600 cm-1), and symmetric C-O stretching (1110 cm-1) [3].

During the final stage (600-900 °C), polycyclic structures and benzene derivatives (700 cm-1) start to form by transformation [3]. As the pyrolysis temperature increases, the oxygen-containing functional groups gradually decrease. However, vast majority of the prominent spectral features that could be observed at a lower temperature disappear when the temperature reaches above 600-800 °C, and the FTIR spectrum is analogous to an ordered graphitic structure. Aliphatic structures, including carboxyl, C-O, ester, and ketone, are completely degraded at such a high temperature. As a result, biochar produced at a low temperature is predominantly hydrophilic because of a broad variety of oxygen-containing functional groups [3]. In contrast, biochars produced at a higher temperature were more likely to contain aromatic structures and skeletons. The aromatic rings of biochar lead to hydrophobic functional groups [3]. Therefore, higher temperatures mainly generate aromatic structures, while lower temperatures prominently generate oxygen-containing...