Effects of Fluidization Velocity and Equivalence Ratio on the Energy and Exergy of the Syngas Produced from Wheat Straw in A Dual-distributor Type Fluidized Bed Gasifier

Abstract

Aim: Exergy is a measurement of how far a certain material deviates from a state of equilibrium with the environment. The aim of the study was to evaluate the energy and exergy of a syngas obtained from the gasification of wheat straw. Methodology: A pilot scale dual distributor fluidized bed gasifier was used to gasify wheat straw at various fluidization velocities (0.28, 0.33 and 0.37m/s) and equivalence velocities (0.17, 0.20, 0.25 and 0.35). Results: The energy values of CO, H2, N2, CO2, CH4, C2H4 and C2H6 varied within the ranges of 3466.27-5790.62, 570.34-2025.86, 774.42-1973.99, 343.22-726.34, 1970.30-3694.93, 1225.03-3453.06 and 0.00-456.65 kJ/kg fuel, respectively. The overall energy distribution was CO>CH4>C2H4>(H2& N2)>(CO2 & C2H6). The results showed that increasing the FV from 0.28 to 0.37 (32.14%), initially decreased the total energy of syngas and then increased it, resulting in a net increase of 7.90-19.80% depending on the ER used. However, when the ER was increased from 0.17 to 0.35 (105.88%), the total energy of syngas increased by 61.82-77.70% depending on the FV used. The effect of the ER on the total energy of syngas was much greater than that of the FV. The exergy values of CO, H2, N2, CO2, CH4, C2H4 and C2H6 varied within the ranges of 3123.99-5200.75, 433.63-1548.33, 224.62-751.35, 262.97-751.35, 1780.38-3327.70, 1151.65-3237.09 and 0.00-425.11 kJ/kg fuel, respectively. The overall exergy distribution was CO>CH4>C2H4>H2>(C2H6& CO2 & N2). Increasing the FV from 0.28 to 0.37 (32.14%) increased the total exergy of the syngas by 8.62-21.35% depending on the ER used. On the other hand, increasing the ER 0.17 to 0.35 (105.88%) increased the total exergy of the syngas by 63.86-79.83% depending on the FV used. The effect of the ER on the total exergy of syngas was also much greater than that of the FV. Conclusions: The exergy values of the syngas were lower than their energy values because the gas components contributed differently to the energy and exergy (the physical exergy of gas components are lower than the corresponding physical energy and the chemical exergy of combustible gases are lower than the corresponding chemical energy). The highest energy (16861.89kJ/kg fuel) and exergy (14532.49kJ/kg fuel) of syngas were obtained at the FV of 0.28 m/s and the ER of 0.35.