Sustainability 2022 , 14 , 4669
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being responsible for the increment of CO and H 2 concentration in syngas. Conversely, exothermic reactions (methanation, ethene formation, and all the combustion reactions) move in the opposite direction with temperature and are responsible for the reduction of CO 2 , CH 4 , andC 2 H 4 concentration in syngas [37,64]. CGE and CCE increase continuously with temperature whereas syngas density de- creases. The energy flow rate exiting from the gasifier associated with the syngas increases with temperature due to the raise of CO and H 2 concentration in syngas, as shown in Figure 4. As a consequence, the CGE increases as well [23,59]. The raising of carbon frac- tion in the syngas through CO concentration with temperature is higher compared to the cumulative decrement of the other three carbon-containing species (CO 2 , CH 4 , andC 2 H 4 ). Additionally, the syngas flow rate increases with temperature, and consequently CCE raises [23]. The kinetic energy of molecules present in syngas increases with temperature decreasing its density [65]. The gasification temperature similarly affects syngas LHV and . P net . Syngas LHV in- creases continuously with temperature due to the raise of CO and H 2 concentration [22,59,66]. Compared to M1 pellets, considering the same operating conditions, the syngas LHV is slightly lower due to the higher ash content. The primary power of the product stream increases with temperature as syngas LHV raises. However, the thermal power required for air preheating increases simultaneously with temperature. For this reason, the net power available from the gasification products increases continuously with temperature up to 850 ◦ C and afterward, it decreases as the energy required for air preheating (to raise the gasification temperature from 850 to 900 ◦ C and afterward) is higher than the energy gain. Based on the current analysis, 850 ◦ C appears to be the optimum temperature for gasification of WP–DIS pellets (M2). Indeed, increasing the gasification temperature above this value does not appear to be convenient in terms of the net energy that can be recovered from the gasification products. 3.3.2. Effect of ER The variation in composition, LHV , and density of syngas, CGE , CCE and . P net is assessed by varying the ER from 0.1 to 0.4 at the predicted optimum temperature of 850 ◦ C. The results are presented in Figures 6 and 7. Regarding syngas composition, the concentration of CO 2 ,H 2 , and CO increases with ER whereas that of CH 4 andC 2 H 4 decreases due to the movement of oxidation reaction to the forward direction with the raise ofO 2 concentration inside the gasifier, as explained by Le Chatelier’s principle [54]. CGE continuously decreases with ER, whereas CCE and syngas density experience an opposite trend. The concentration of C 2 H 4 andCH 4 decreases by 76.8% and 46.9%, respectively, with the increase of ER within the tested range. Conversely, the concentration of CO increases by 24.2% and that of H 2 by 53.8%, as clearly presented in Figure 6. The con- tribution of CH 4 fraction to the energy content of syngas is almost three times compared to that of CO and H 2 [22,59,66]. Consequently, the CGE decreases continuously with ER. Carbon transformation from input biomass to the gasification product increases due to the raise of oxidation reactions with ER. Additionally, the syngas flow rate increases with ER, and consequently CCE raises continuously [23]. The concentration of N 2 inside the gasifier raises with ER being responsible for the reduction of molecular movement in the reacting medium due to its inert nature. This causes an increase in the syngas density [67]. Syngas LHV decreases with ER due to the increase of N 2 volume inside the reactor, which causes a dilution effect [54,61,62]. . P net obtained from syngas decreases with ER due to the reduction of LHV . At the same time, the incoming air flow rate increases with ER requiring more thermal energy for air preheating, further decreasing the available . P net .
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