| 英文摘要 |
Recently, Over 0.21 million tons per year (TPY) of flyash hazardous wastes were produced from steel industries in Taiwan having high risk for enviroment or human beings. In stainless stell/carbon steel manufacturing plants, the ZnFe2O4/NiFe2O4/MnFe2O4 are major materials and can be easily speparated/purified using magnetic method or ball mill unit. Carbon dioxide is the main greenhouse gas of 50~60% with global warming potential (GWP) and plays a role in the greenhouse effect especially for the CO2 emission from steel companies, powder generation plants, and petrochemical industries. Experimentally, ZnFe2O4/NiFe2O4/MnFe2O4 catalysts were speparated from flyashes with the efficiency of 73% and further purified to obtain nanocatalysts with the efficiency of 15-20% using magnetic separation or ball mill methods. The XRD patterns indicated the ZnFe2O4/NiFe2O4/MnFe2O4 catalysts are spinel structure. Based on FE-SEM and TEM micrographs, the particle size ranged of 35-45 nm of ZnFe2O4/NiFe2O4/MnFe2O4 nanocatalysts were found. Decomposition of CO2 into carbon and oxygen was carried out within few minutes when it comes into contact with oxygen deficient nanocatalysts through incorporation of oxygen into catalysts. The Type IV mesoporous nanomaterials with a specific surface area of 46 m2/g for magnetic selection flyashes using nitrogen adsorption isotherms. Flyashes and ZnFe2O4/NiFe2O4/MnFe2O4 nanocatalysts increase the CO2 decomposition efficiencies and aprticle size with increasing reaction temperatures. The CO2 decomposition effiencies of all the ferrite nanocatalysts synthesized by hydrothermal method are >99% but magnetic selection flyashes are only 88%. CO2 yields of NiFe2O4/ZnFe2O4/MnFe2O4 and flyashes are 99.8, 99.6, 89.6, and 22.3%, respectively. The first-order reaction and activation energy of CO2 decomposition NiFe2O4/ZnFe2O4/MnFe2O4 and flyashes are 117, 139, 172, and 279 kJ/mol, respectively. In addition, the ΔH = -252 kJ/mol and △S = 244, 249, and 252 J/mol K respectively at 300, 350, and 400℃ for CO2 decomposition.
Based on the theoretical calculation, the energy efficiency of the route from CO2 to CH4 is higher compared with the one from C or H2 combustion directly. The in-situ complicated offgases contains 15-20% CO2, it needs to be purified by pressure swing adsorption (PSA) before decomposition reaction in fixed-bed or fluidized-bed catalytic reactors at 300-350℃ and 1 atm. When CO2 contacts with oxygen deficient ferrites, decomposition of CO2 occurs by the incorporation of oxygen anions in the vacancies in the oxygen deficient ferrite thereby restoring the ferrite to stoichiometry at 1 atm and 573-673 K. At the same time, electrons are donated from the oxygen deficient ferrite to produce carbon or carbon monoxide. Next, the deposited carbon on the surface converts into methane (methanation) upon treatment with H2 (hydrogenation) while regenerating used ferrite to oxygen deficient ferrite. Decomposition of CO2, moreover, recovery of valuable methane using heat energy of offgas produced from steel companies, powder generation plants, and petrochemical industries is an appealing alternative for energy recovery. Moreover, the simulated calculation of scale-up 20 TPD flyash recycling and CO2 utility plant were perfomed with benefit of 166,674 NT$/ton flyash and extra 2 million NT$/day of nanocatalysts that shows highly potential for industrial application in the near future.
|