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Energies 2020, 13, 420 40 of 96 to 0.25 and then decreases, whereas the CO2 has an inverse trend. If the oxidizing agent is air, the N2 content increases due to the higher amount of air injected into the gasification reactor [515]. Syngas quality mainly depends on operating temperature since gasification is a thermochemical process. Temperature affected a variety of parameters of gasification process influencing gas yield, heating value, cold gas efficiency, tar production and composition [516]. Higher operational temperature enhances endothermic reactions (steam reforming, Boudouard and water–gas reaction) raising the production of hydrogen and carbon monoxide at the expense of char and methane [517]. The amount of tar decline with temperature increase as a consequence of a higher degree of thermal cracking and steam reforming reactions [518]. 4.3.2. Design of Gasifiers A variety of gasifiers have been developed, and they can be classified in: • fixed-bed (or packed-bed) gasifiers; • fluidized-bed gasifiers; • entrained-flow gasifiers. Fixed-bed gasifiers are realized with a grate that supports the fuel into the reactor. These gasifiers are inexpensive and are suitable for small-medium scale reactors. The efficiency is not maximized because of the non-uniform distribution of fuel, temperature and gas composition. Updraft fixed-bed reactors (see Figure 18a) are configured for the counterflow of the solid phase that moves downward and the gaseous phase that moves upward. This type of gasification reactor is appropriate for high moisture and high ash biomass content. The syngas near the outlet removes the moisture from the feedstock falling, and the ash drops through the grate simplifying the ash discharge. The cold gas efficiency and the heating value is high, but also, the tar generation is high [519]. Downdraft gasifiers are co-current reactors (see Figure 18b). The biomass feedstock is introduced from the top with the gasification agent and products are released from the bottom. The product gas passes through the reduction and the combustion zone in which tar is cracked due to high temperature. Downdraft gasifiers are simple and reliable, but they require low moisture and low ash content [520,521]. In the cross-draft gasifier, biomass is fed from the top and the oxidizing medium form the side of the reactor. Ash is removed from the bottom, and the tar content is higher [44,522,523]. Fluidized-bed gasifiers meet a more uniform temperature and mass transfer distribution achieved by a bed of granular materials (such as sand) that is kept semi-suspended by the gasification medium, ensuring a deep mixing of the hot bed material and the feedstock (see Figure 18c). Two main fluidized-bed gasifiers are commercialized: the bubbling and the circulating fluidized-bed gasifier. The former consists of a vessel with a fine-grained material on the moving bed with a fluidization velocity of 0.5–1.0 m/s [454,524]. The latter is characterized by a circulation of the hot gas between the vessel and the cyclone, which separates solid particles [525]. The size of particles is a critical issue for the corrosion of internal walls, but the gaseous product has a low tar content, and the main difference between circulating and bubbling fluidized-bed gasifiers is the much higher fluidization velocity (3.5–5.5 m/s) [454,521,526]. Dual fluidized-bed reactors have two separate beds in which combustion and gasification are separated. The feedstock is fed to the gasifier bed fluidized with the stream. The second bed behave as char combustor [527]. Entrained-flow gasifiers are not suitable for biomass because the feedstock requires to be very fine and for fibrous biomass is difficult, and ash require to be molten [46]. Entrained-flow gasifiers operate at high temperature and pressure and water slurry, or dry feeds can be used [528,529].PDF Image | Green Synthetic Fuels
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