material obtained from rice husks and rice straw were powders with low hardness [28]. Additionally, the rice husks and rice straw still had a substantial total carbon and fixed carbon content. When comparing rice husks to rice straw, the carbon content was 41.92% and 36.07%, with a fixed carbon content of 11.44% and 6.93%, respectively [29]. The rice husks were probably a more suitable feedstock for the production of carbonaceous material.
Many works also refer to the potential of biomass stones and shells for the production of carbonaceous materials [30, 31]. Olive stone was converted into a bio‐based carbon material, with a carbon content of 47.50% and fixed carbon of 13.80% [32]. Another effective feedstock, peanut shell, possessed a high carbon content of 52.73% and fixed carbon of 22.60% [33]. Carbonized peanut shells at 500 °C resulted in a biochar with little ash and volatile matter. Cocopeat presented a high carbon content of 61.57% and fixed carbon of 25.30% [34]. The cocopeat that was pyrolyzed at 500 °C generated a biochar containing up to 84.44% carbon content with up to 67.25% fixed carbon. However, it contained volatile matter of about 14.30%, volatile matter of about 14.30%, moisture 2.55% and ash 15.90%. Only temperatures over 800 °C can produce pure fixed carbon without any volatile matter. In European countries, a great deal of research is done on the utilization of walnut shells (45.10% carbon and 15.90% fixed carbon) and almond shells (50.50% total carbon and 9.10% fixed carbon) for the production of hard biochars with high yield [35]. In Southeast Asia, palm kernel shells (50.49% carbon) and coconut shells (49.20% carbon), which are local plants, have been converted into carbonaceous materials in the industry [28].
The thermochemical conversion of petioles, leaves, branches, leaflets, and stalk from various plants has also been studied. These feedstocks have a moderate carbon content, low lignin, and fixed carbon that affect the hardness and yield of biochar [36]. Although both date palm leaflets and tobacco stalks (the biomass waste after tobacco harvesting) had a carbon content of up to 50%, the produced biochars were rather fragile [37, 38]. These feedstocks may thus not be applicable to produce carbon material for catalysis, but can be used as additives for soil improvement. Camphor leaves and camellia leaves contained a carbon content of over 70% but they produced a brittle biochar owing to the small amount of lignin and fixed carbon. Such materials are usually applied as an adsorbent instead of a catalyst or catalyst support. Nevertheless, the empty fruit bunch of oil palm containing only 47.78% carbon produced a biochar with high hardness [39].
2.3 Thermochemical Conversion Processes
Lignocellulosic biomass can be chemically transformed into carbon materials by diverse thermochemical processes as demonstrated in Figure 2.3. Each process presents different influences on the properties of the obtained carbon materials, both chemically and physically. The selection of the conversion process mainly depends on the application.
2.3.1 Carbonization and Pyrolysis
Carbonization is a thermal process of biomass conversion in the absence of oxygen with charcoal or biochar as the main product. Similarly, the pyrolysis process results in a condensed liquid product from the gas called bio‐oil, as well as biochar as a by‐product. The equipment used in carbonization and pyrolysis of biomass are seemingly identical; both processes essentially require a furnace for heating and creating an oxygen‐free environment [33]. Some researchers carried out these processes in a horizontal tube furnace under nitrogen flow to remove oxygen from their surroundings [37]. Nevertheless, the pyrolysis process needs additional condensation units to compress the produced gas into the bio‐oil [40]. Both carbonization and pyrolysis processes require an appropriate temperature. As discussed in Section 2.2.1, the destruction of the lignocellulosic structure occurs at around 360 °C. However, many researchers usually operate the carbonization process of biomass at temperatures above 500 °C under atmospheric pressure and oxygen‐free conditions in order to achieve the complete conversion of lignocellulosic material [33, 35]. Nitrogen may be supplied into the processes, where the flow rate is typically around 100–300 cm3 min−1 depending on the process scale. From the fixed carbon analysis, highly crystalline biochar without any impurities can be produced at a temperature higher than 800 °C.
Figure 2.3 Thermochemical processes for biomass conversion.
Considering the cellulose conversion steps, cellulose is first transformed into anhydro‐cellulose. Subsequently, the charcoal or biochar is formed. The overall chemical reaction of the cellulose transformation is shown in Eq. (2.1).
From the preceding equation, the number of carbon atoms can vary according to the reaction conditions such as temperature and pressure. For instance, the use of a reaction temperature of 400 °C, which is slightly higher than the theoretically required 360 °C, presents the incomplete decomposition of cellulose through Eq. (2.2).
Previous research showed that with the aim to produce a biochar with high porosity and surface area, the process should be operated at temperatures in the range of 600–800 °C [41]. A type of pinewood, Pinus sylvestris, was pyrolyzed at various reaction temperatures, and increasing temperature enhanced the porosity and surface area of the obtained carbonaceous product. Pyrolysis at 500 °C generated a biochar with a surface area of 19 m2 g−1, while increasing the temperature to 600 and 700 °C greatly improved the surface area to 254 and 470 m2 g−1, respectively. Conversely, the pyrolysis of non‐lignocellulosic material at temperatures above 700 °C resulted in a decrease in surface area due to the collapse of the biochar structure [42]. The change in pore diameter depends on several parameters, e.g. type of raw material and heating rate. An increase in carbonization temperature strongly enhanced the number of pores but it slightly affected the pore diameter. However, with the biomass having low lignin content or low hardness, increasing the carbonization temperature could significantly increase the pore diameter of biochar. This clearly shows that the carbonization and pyrolysis processes benefit the porosity of biochar, which is an interesting characteristic for catalysis application. However, the production of biochar via these thermochemical processes has some disadvantages as it reduces the amount of valuable chemical groups. Biomass commonly consists of oxygen‐containing functional groups with carbon–oxygen bonds (C–O), carbonyl groups (C=O), and hydroxyl groups (–OH), which are useful in many catalytic processes. An increase in carbonization temperature led to a decrease in such chemical functionalities [43]. For this reason, the physical and chemical properties of the biochar should be modified prior to its use in specific applications including catalysis. For example, mixtures of rapeseed oil cake and walnut shells were carbonized at 400 and 750 °C. Only a few functional groups appeared on the surface of the produced biochar, and either chemical (using agents such as ammonium and monoethanolamide) or thermal treatment were required before use
