Catalytic decomposition of ammonia and tar for hot gas cleanup in biomass gasification using activated carbon supported catalysts and natural limonite ores.

摘要

As Part-I of this thesis, activated carbons (ACs) were produced from a Canadian peat by chemical activation using either H3PO4 or ZnCl2 as the activation agent, followed by carbonization at a relatively low carbonization temperature (400°C). ZnCl2 was found to be an effective activation agent for developing microporous structures in the ACs, leading to greater surface areas, while H3PO4 is highly active in developing the mesopores, leading to much higher mesopore volumes and average pore sizes. The effects intrinsic minerals in the precursor on the textural properties of the activated carbon products were examined by demineralization of the peat with HCl washing before the activation and carbonization. The demineralization of the precursor greatly promoted the development of micropores during the activation process, leading to significantly higher surface areas of the resulting ACs irrespective as to which activation agent was used, and the AC derived from the demineralised peat activated by ZnCl2 attained the highest BET surface area of 888 m2/g. The demineralization of the precursor could also significantly improve the mesoporous structure of the ZnCl2-activated ACs.In Part-III of this work, the catalytic performance of the peat-derived activated carbon supported Fe/Ni catalysts as well as three natural limonite ores towards hot gas NH3 decomposition in a simulated gas (14.9% CO, 2.9% CH4, 11.2% H2, 11.2% CO2) with and without 5-15% H2O, was investigated at 750°C. The Fe/AC and Ni/AC catalysts and all natural limonite ores were very active for ammonia decomposition in the inert atmosphere. However, both AC-supported catalysts could be severely deactivated by the simulated gas, and the Fe/AC catalyst was also deactivated by the presence of H2O in the gas. In the presence of the simulated gas and H2O, the activities of these two catalysts dropped drastically to as low as 10%. The three limonite ores showed high activities towards ammonia conversion to N2 (>90% at 750°C) in both inert atmosphere or in a simulated gas with 0-15% H 2O.In the Part-IV of this work, three types of natural limonite iron ores were tested as the inexpensive catalysts for tar reforming/cracking experiments at 500-900°C using benzene as the model compound (1000-1400 ppm) in the co-existence of H2O/helium a simulated gas mixture containing H 2/CH4/CO/CO2 with and without H2O. The activities of these limonite catalysts of benzene decomposition follow the order of priority of BL > AL > CL. Canadian Limonite (CL) was inactive for steam reforming of benzene, probably resulting from the chemical deactivation of catalyst by the H2O vapor to prevent formation of the active alpha-Fe species on the catalyst surface. However, in the presence of the simulated gas consisting without H2O, the CL showed improved higher activity, of about 65% at 900°C, while its performance was deactivated slightly by the presence of H2O in the gas. The Brazilian limonite (BL) showed the highest activities in benzene decomposition in the presence of the simulated gas with and without H2O, owing to theist high Fe content with smaller crystalline sizes of active Fe-species in the fresh sample or during the benzene decomposition tests. The use of BL catalyst obtained almost complete conversion of benzene (>95%) at above 650°C in the simulated gas irrespective of whether or not 15 vol% H2O was present in the reactant gas. (Abstract shortened by UMI.)In Part-II of this research, two novel carbon-based Ni/Fe catalysts were developed and tested for catalytic decomposition of ammonia into N2 and H2. These catalysts were prepared using a meso-porous activated carbon (AC) support derived from a Canadian peat by H3PO4 activation. The newly developed catalysts proved to be highly active for ammonia decomposition. The conversion of 2000 ppm NH3 diluted in helium over the Fe catalyst reached as high as 90% at 750°C and at the space velocity of 45000 h-1, compared with only about 15% with the activated carbon alone without metal loading. In addition, the new Fe/Ni catalysts showed superior performance with respect to their resistance to catalyst deactivation. Both catalysts remained active as the reaction time increased up to 10 hours without showing a sign of deactivation. Fresh and spent catalysts were characterized by X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS) and temperature programmed desorption (TPD). A cycle mechanism, involving both metal phosphides and metal nitrides, was proposed for the NH 3 decomposition reactions over these new Fe/Ni catalysts.