Research On The Catalytic Conversion Of Biomass Tar Model Compounds Using Ni-Based Catalysts

For the technology of biomass gasification, the presence of the tar is the major problem limiting its large-scale application. Tar could not only block pipeline and corrode equipment, but also cause energy waste and environment pollution. Therefore, how to efficiently remove the tar is an urgent problem needed to be solved. Among various tar removal methods, catalytic conversion is an ideal choice. It could convert the macromolecular tar into the micromolecular fuel gas, which could both remove the tar and increase the fuel gas yield. However, as there are various ways for biomass gasification and the operation processes of each way are different, the concrete operation methods of tar catalytic conversion during each gasification process will be also different. And the most frequently used biomass gasification technologies are air gasification and retorting gasification. Thus, the catalytic conversion research was carried out according to the tar produced in these two processes. The detailed research contents are as follows:1. Catalytic conversion of biomass air gasification tarFor the catalytic conversion of the tar, the selection of catalysts is crucially important. Among various catalysts, after considering the catalytic performance and economy, Ni-based catalysts are the ideal choice. However, the conventional Ni-based catalysts will be deactivated rapidly due to the coke formation, which will greatly shorten its service life. Based on this reason, on the basis of summarizing and analyzing previous studies, CeO2 was selected to dope into the conventional Ni-based catalysts (Ni/γ-Al2O3) as a promoter. Then experiments were performed on the self-designed conventional catalytic steam reforming device by taking toluene (tar model compound) as the research object. The effects of several factors on the toluene conversion, gas composition and coke formation were investigated, including the reaction temperature, CeO2 loading and steam/carbon ratio. The results showed that, with the presence of CeO2, the catalytic activity and anti-carbon deposit performance of the catalysts were remarkably improved and would be further improved with the increasing of CeO2 loading. The toluene conversion could reach 90.4% at the reaction temperature of 850℃ and the steam/carbon ratio of 3 by using the Ni-CeO2(3wt%)/γ-Al2O3 catalyst, while the carbon content of the catalyst was only 0.42wt%.As the toluene conversion obtained by using the Ni-CeO2/γ-Al2O3 catalysts should be improved, mesoporous SBA-15 with better performances was used as the carrier to prepare a series of Ni-CeO2/SBA-15 catalysts. Afterwards, experiments were carried out under the same conditions with that of using the Ni-CeCh/γ-Al2O3 catalysts. The results indicated that the catalytic activity and anti-carbon deposit performance of the catalysts could be further improved by using SBA-15 as the carrier. The toluene conversion reached as high as 98.9% at the reaction temperature of 850℃ and the steam/carbon ratio of 3 using the Ni-CeO2(3wt%)/SBA-15 catalyst, while the carbon content of the catalyst was only 0.01wt%. Besides, this catalyst exhibited great stability and anti-carbon deposit performance during 29 h stability test.In order to decrease the required temperature of tar steam reforming and further suppress the coke formation, the "electrochemical catalytic" technology was introduced in the process of conventional catalytic steam reforming. With benzene, toluene,1-methylnaphthalene and air gasification tar as the research object, experiments were performed on the self-designed electrochemical catalytic steam reforming device using the Ni-CeO2/γ-Al2O3 catalyst. The results showed that this process could decrease the required temperature of toluene steam reforming, increase the toluene conversion and effectively suppress the coke formation. The toluene conversion reached 99.9% under the electric current of 4 A, the catalytic temperature of 800℃ and the steam/carbon ratio of 3, while the carbon content of the catalyst was only 0.10wt%. In addition, throughout 24 h stability test, the conversions of toluene, benzene and air gasification tar could respectively maintained around 99%, 98%and 95%, while that of 1-methylnaphthalene could only maintained around 83%.2. Catalytic conversion of biomass retorting gasification tarCompared with air gasification, the most important feature of retorting gasification is that it is performed in the inert atmosphere. Due to the absence of the oxidation medium, carbonaceous materials could be used as the carrier. Based on this reason, activated carbon was used as the carrier to prepare an efficient composite catalyst of NiO/activated carbon. Then the tar derived from rice husks retorting gasification was underwent thermal and catalytic cracking on the self-designed two-stage fix-bed reactor to investigate the effects of reaction temperature and NiO loading on the tar conversion. The results indicated that thermal cracking was able to convert part of the light tar, but difficult to reduce the heavy tar. However, the catalytic cracking with the NiO/activated carbon catalysts was effective to reduce both the light and heavy tar, and meanwhile increase the yields of H2 and CO2. Compared with non-cracking, after catalysis by the NiO/activated carbon(4.3wt%) catalyst at 700 ℃, the yields of the light and heavy tar were respectively decreased 95.5% and 94.7%, while the yields of H2 and CO2 were about twice that of non-cracking. Moreover, the catalytic performance of the NiO/activated carbon catalysts would be enhanced with the increasing of NiO loading and reaction temperature.