dc.description.abstract | This research investigated the characteristics of prepared nickel-based MCM-41 (Mobile Composite Matter No. 41) and evaluated the performance of enhanced energy conversion efficiency in rice straw gasification amended with different ratio of nickel-based MCM-41 catalyst. The prepared MCM-41 was synthesized using low-power microwave digestion. Silica content was extracted from rice straw by using 0.05 - 0.30 M citric acid and 500 - 800oC operation temperature. Prepared 5 - 15 wt.% nickel-based catalyst was loaded on the MCM-41 surface using wet impregnation method. The synthesized MCM-41 and nickel-based MCM-41 catalysts were all characterized by X-ray diffraction (XRD), Fourier-transform infrared spectroscopy (FTIR), high-resolution transmission electron microscope (HRTEM), Brunauer-Emmett-Teller (BET) and N2 adsorption/desorption methods, respectively. The catalytic gasification was performed in a fluidized bed gasifier controlled at equivalence ratio (ER) 0.2 and 800oC operation temperature. For each run, 5-10 wt.% tested catalyst was mixed with silica sand as bed material. The resulted syngas was analyzed using gas chromatography equipped with flame ionization detector and thermal conductivity detector (GC-FID/TCD). After gasification reaction, the catalyst was separated from the bed material and characterized by using XRD and HRTEM analysis.
The experimental results indicated that extracted silica was obtained from 0.3 M citric acid extraction and approximately higher than 95 wt.%. The amorphous silica form obtained at 600oC was suitable for MCM-41 preparation. It was also found that the BET area was improved with decreasing the synthesis temperature and prolonging the synthesis time. The highest BET surface area and optimal pore volume of MCM-41 were 1304.71 m2/g and 2.24 nm, respectively. To compare with commercial type, prepared MCM-41 has higher BET surface area and pore volume than that of commercial MCM-41. According to XRD results, all the prepared samples demonstrated the typical peaks of mesoporous structure. The FTIR analysis confirmed that the synthesized MCM-41 exhibited similar vibration signals at 1065 cm-1, 804 cm-1, 1640 cm-1, and a broad-band from 2981-3652 cm-1, comparing with the commercial MCM-41. The representative type IV isotherm and hexagonal structure were observed in all samples based on the results of N2 adsorption/desorption and HRTEM analysis.
The rice straw gasification performance was evaluated under different catalyst condition. Without catalyst addition, the average hydrogen produced and syngas heating value were 2.86 vol.% and 1.56 kJ/Nm3, respectively. It was observed that the hydrogen composition enhanced with either nickel content or catalyst addition. The maximum hydrogen content and syngas calorific value were approximately 7.78 vol.% and 2.60 kJ/Nm3, respectively. In the case of catalyst addition increased from 5 wt.% to 10 wt.%, the average hydrogen yield increased from 3.43 vol.% to 7.35 vol.%. The nickel content was also shown a similar trend, the hydrogen composition increased from 6.00 vol.% to 7.77 vol.% with an increase in Ni content increasing from 5 wt.% to 10 wt.%. It could be explained that the tested catalyst provided more acidic active sites for converting more tar content into syngas. Meanwhile the nickel could facilitate the water-gas shift (WGS) reaction, thus, more hydrogen and carbon dioxide were produced in gasification. However, hydrogen composition was a slight decreased to 7.35 vol.% as 15 wt.% Ni-based and 10 wt.% catalyst addition. This is due to the biomass containing potassium species could deposit on the catalyst surface resulted in the catalyst activity reduction. The results were confirmed by the XRD and HR-TEM analysis.
In summary, the prepared MCM-41 has a higher BET surface area and pore diameter characteristics and also exhibited the similar hexagonal structure with commercial MCM-41. The synthesized Ni-based catalyst has been successfully developed for improving quality of syngas in rice straw gasification.
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