Biochar Production from Biomass by Thermochemical Conversion Technologies
Thermochemical conversion technologies for biomass into biochar include carbonization, pyrolysis, and gasification processes (Masek et al. 2011; Shackley et al. 2011; Yoder et al. 2011; Mahinpey et al. 2009). The difference between carbonization and pyrolysis processes is subtle, as both processes have similar principle of operation; however, carbonization is aimed at biochar as primary product, whereas pyrolysis produces biochar as a by-product. Different biomass conversion processes for biochar production are presented in Fig. 3.2. The biochar produced from
Fig. 3.2 Different thermochemical processes for biochar production
these processes varies greatly in terms of C, H, N, S, O, and ash content depending not only upon feedstock source and pretreatments such as particle sizing and drying but also upon the operating conditions of production (e.g., temperature, heating rate, residence time, and pressure). Likewise, the characteristics of biochar such as density, particle size distribution, moisture content, and pH depend on the feedstock type as well as pyrolysis reaction conditions. For example, biochar produced from wood is reported to be coarse and highly resilient with carbon content up to 80 %. These properties of biochar were reported to be due to the high lignin content (e.g., olive husks) which also yields high biochar content in the pyrolysis products as a result of the stability of lignin to thermal degradation. On the other hand, biochar produced from crop residues (e.g., maize, wheat, rye) and animal manures is generally finer and less robust with low mechanical strength due to lower lignin content (Demirbas 2004a, b). Similarly, particle size distribution of biochar produced from sawdust and woodchips under different slow pyrolysis conditions showed that particle size generally decreased with increase in temperature in the range of 450– 700 °C (Abdullah and Wu 2009). In general, the process conditions in the slow pyrolysis are long vapor residence times (≥10 s), reaction temperatures between 450 and 650 °C at atmospheric pressure, and heating rates in the range of 0.01– 2.0 °C s−1. At these conditions, the rate of increased cracking reactions increases that shifts the liquid organic (bio-oil) yield towards the biochar yield (Sohi et al. 2009). From the existing literature, it can be concluded that slow pyrolysis and intermediate pyrolysis both are suitable for higher biochar yields, whereas fast pyrolysis provides higher liquid yields, which can be suitable for biofuel production for energy. Therefore, the production of biochar can be optimized via slow pyrolysis and intermediate pyrolysis processes. The operating conditions for high biochar yields are (a) high lignin, ash, and nitrogen contents in the biomass, (b) low pyrolysis temperature (<400 °C), (c) reaction at high pressure, (d) higher residence time, (e) extended vapor/solid interaction, (f) low heating rate, (g) large biomass particle size, and (h) optimized heat transfer. These processes for the production of biochar can be carried out in batch or continuous modes depending upon production scale.