| dc.description.abstract | This study investigates the kinetic behavior, enhanced hydrogen yield efficiency, trace pollutants removal, trace metals partitioning characteristics, and speciation formation prediction in rice straw gasification combined with hot gas cleaning (HGC). A bubbling fluidized bed gasifier was used by controlling the temperature at 800 °C and equivalence ratio (ER) ranging from 0.2 to 0.4. The hot gas cleaning system consists of zeolite, calcined dolomite, activated carbon, and operated at 250 °C. To further determine the HGC system performances, the prepared Ni-based catalyst was replaced for zeolite as an adsorbent in the HGC system. The tested adsorbents capacity and regeneration performances were also studied in this research.
The kinetic behaviors result shows the activation energy of rice straw is 75.40 kJ/mol and 89.56 kJ/mol under pyrolysis and gasification conditions, respectively. The main gas phases that occur during rice straw pyrolysis/gasification include CO, CO2, CH4, HCl, SO2, aliphatic hydrocarbons, CH3COOH, and some trace gas compounds containing phenol, methanol, etc. The hydrogen concentration of produced gas was also increased from 6.82% to 9.83% with the HGC system used. The maximum syngas LHV was approximately 6.09 MJ/Nm3 as ER controlled at 0.2. Meanwhile, the experimental results indicated that the HGC system for removing trace pollutants has successfully developed. The overall tar removal efficiency was nearly 70% by the HGC system. The HCl and H2S removal efficiencies were approximately 95% and 80%, respectively. Moreover, the enhancing H2 generation and minimizing tar yield using the HGC system combined with prepared Ni-based catalyst was investigated. When zeolite catalyst and hot gas temperature were adjusted from 250 ℃ to 400 ℃, H2 and CO increased slightly from 7.31% and 14.57% to 8.03% and 17.34%, respectively. The tar removal efficiency varies in the 70% to 90% range. When the zeolite was replaced with prepared Ni-based catalysts and HGC operated at 250 ℃, H2 contents were significantly increased from 6.63% to 12.24% resulting in decreasing the hydrocarbon (tar), and methane content.
The trace metals Zn, Cr, Cd, and Pb partitioning in the gas phase were increased with equivalence ratio increased. The crystalline phases of some elements in the particulates were found as SiO2, KClO3, K2SO4, K2Si4O9, CdSiO3, PbCl2, and Pb8Zn(SiO7)3 by XRD identification. The thermodynamic equilibrium simulation results confirmed the dominant gaseous species produced from rice straw gasification, such as KCl(g), NaCl(g), KO(g), K2O(g), ZnCl2(g), CrO2Cl2(g), CuCl2(g), PbCl2(g), PbO(g), and Cd(g). Besides, the metals removal by the hot gas cleaning system was found in decreasing order as: K > Cr > Ca > Pb > Mg > Cd > Na > Zn > Cu. Activated carbon showed a good performance for adsorbing Pb, Cd, Cr, Ca, K, and Mg.
The H2S adsorption capacity order decreased as the activated carbon > calcined dolomite > zeolite. The adsorption capacity of zeolite, calcined dolomite, and activated carbon are 2.22, 3.16, 5.88 mg S/g adsorbents, respectively. In addition, the tested zeolite and activated carbon could be fully regenerated at 350 °C with a stable sulfur adsorption capacity during four adsorption regeneration cycles. In this study, breakthrough curves obtained with several adsorbents for H2S and HCl were shown. Very different breakthrough and saturation times were observed. About HCl, the best adsorption performance was obtained with calcined dolomite and zeolite, in the case of H2S, the best adsorption performance was obtained with activated carbon. In conclusion, the HGC system is proposed as an effective way for improving the syngas quality and reducing trace contaminants emission in rice straw gasification. The results gained from this study could significantly support researchers obtaining more information about the control of syngas contaminants. | en_US |