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Energies 2020, 13, 420 36 of 96 compressing biomass feedstock [468]. The process enables several advantages such as facilitate transport and handling and improve the uniformity, the density and the quality (low moisture content) of biomass feedstock. 4.2. Pyrolysis Biomass pyrolysis is a thermochemical decomposition process in which large complex hydrocarbon compounds are broke down in molecules with a lower molecular weight. The thermal degradation of biomass results in the production of gaseous, liquid (bio oil) and solid (char) products. It is always the first step in gasification. It is carried out either in the total absence of oxidizing agents (air, oxygen or steam) or in an oxygen-limited condition that does not permit gasification or combustion processes. The temperature is in the range from 300 to 1000 ◦C [469]. The heat required can be provided by solar radiation concentrated by solar collectors [470]. Thermodynamic and kinetic conditions, including temperature, heating rate and residence time, determine the nature and composition of products. Pyrolysis generally occurs in two stages: primary reaction involves the cracking of condensable gas bond linkages by dehydration, decarboxylation and dehydrogenation, resulting in the production of condensable gases, a liquid phase and a solid phase (mainly char). The solid fraction ranges between 5–10 wt% for fluidized bed gasifier and 20–25 wt% for fixed bed gasifier. It has a high heating value, includes char (an aromatic polycyclic carbon formed by condensation of benzene rings) and ash. Tar constitutes the liquid phase, and his fraction ranges from 1 wt% for downdraft gasifiers, and 10–20 wt% for updraft gasifiers [46]. In the secondary stage compounds with high molecular weight are cracked into small molecules of non-condensable gases (CO, CO2, H2 and CH4) through steam reforming, water–gas shift, thermal cracking and tars are thermally cracked in the gasification process [471]. 4.2.1. Pyrolysis Process The following overall reaction may summarize the pyrolytic thermochemical process [472]: CnHmOp(Biomass) → CxHyOz + CaHbOc + H2O + C(char) (47) liquid gas The pyrolysis process can be divided into three subclasses depending on the operating conditions: Conventional or slow pyrolysis: the process is characterized by an operating temperature of nearly 500 ◦C, a low heating rate (0.6–6.0 ◦C/min) and a long residence time (5–30 min) [473]. These conditions allow the repolymerization reaction to maximize char production. Products are composed of solid, liquid and gaseous phase nearly in the same portion: approximately 35%, 30% and 35% respectively. The carbon content of the solid product is very high (~95%) [474]. Particles can be relatively large, with a size range from 5 to 50 mm [475]. Fast pyrolysis: the process is characterized by a very high heating rate (10–200 ◦C/s), high temperature (500–650 ◦C) and very short residence time in the reaction zone (1–5 s). The rapid quenching that follows the heating generates intermediate liquid products. Therefore, not only chemical reaction kinetics is important, but also heat and mass transfer process and phase transition phenomena [476]. These conditions promote the liquid (bio oil) production (60–75 wt%), whereas the char fraction constitutes 15–25 wt% of the mass production and incondensable gases are approximately 10–20 wt%, depending on the feedstock quality. The carbon yield is low (0.2–0.26) compared to slow pyrolysis (~0.58) [477,478]. The fast pyrolysis requires a dry pre-treatment to reduce the moisture content up to 10% and a grinding process to decrease the particle size to approximately 2 mm (see Figure 17) [479].PDF Image | Green Synthetic Fuels
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