Experimental Study On Biomass Gasification And Its Reaction Mechanism At Lower Temperatures

Biomass, with the advantages of abundant reserve and environmental friendship, is considered as a promising green energy, and its utilization has great significance to ease the energy crisis and reduce the environmental pollution. Biomass steam gasification at lower temperatures is is of broad prospect due to the ability of producing H2-rich gas from the carbon in it. However, the shortcomings of this technology comprise high content of tar, low conversion efficiency and reaction rate of char, etc. For solving the above problems, biomass pyrolysis, char gasification, tar reforming and biomass gasification were studied in this paper, which optimized the overall experimental conditions and clarified part of the reaction mechanism.Firstly, the experiments of biomass pyrolysis and char gasification using wheat straw were performed based on Response Surface Methodology (RSM), in order to convert biomass char into H2-rich gas efficiently at high rates and make clear the impact of pyrolysis conditions on char properties. The response surface was set up with pyrolysis temperature, K2CO3 loading concentration and the content of K-based catalyst in the bed materials for char gasification performances of carbon conversion efficiency (Xc), hydrogen yield (RH2) and reaction rate (Yc). The results show that the suggested conditions of char steam gasification are 650? for pyrolysis temperature,22% for K2CO3 loading concentration and 70% for the content of K-based catalyst in bed materials. In these conditions, the model prediction indicates that Xc is 97.9% while RH2 and Yc reach 186.0mol/kg-char and 1.56%/min, which is in good agreement with the confirmatory experiment. In addition, H2 concentrations are able to attain 70%. Moreover, results show that the influence of K2CO3 loading in biomass pyrolysis is more significant than K-based catalyst during char gasification. X-ray diffractometry (XRD), scanning electron microscopy (SEM) and Fourier-transform infrared (FTIR) spectroscopy were employed to draw the conclusions of char characterizations and account for the optimization results, i) Chars obtained by the pyrolysis around 650? are liable to be gasified due to the cracked structures without carbonization occurring, and ii) properly loaded K2CO3 tends to cause the formation of porous sponge-like textures on char surfaces and contribute to maintaining the variety of functional groups with H and O, resulting in enhanced char reactivity.Secondly, as for the remarkable generation of tar, the reforming experiments were conducted based on RSM, involving a-methylnaphthalene as the model compound of biomass tar. In this part, the reforming mechanism of a-methylnaphthalene was studied and the different catalytic performances of Ni-based catalysts after the modifications by Li and Mg were also compared. The two-stage reactor system (TSR), which comprised a fluidized bed reactor as the first reactor (FR) and a fixed bed reactor as the second one, (SR) was used in the reforming experiments. The response surface was set up with the temperatures of TSR and the molar ratio of steam to carbon (S/C) for a-methylnaphthalene reforming performances of Xc and RH2 to analyze and optimize the experimental conditions. The optimized conditions are 900? for the temperatures of TSR and 9 for S/C. In these conditions, the results of model prediction indicate that Xc is 76.1% while RH2 reaches 120.2mol/kg-tar, good agreement achieved with the corresponding experiment. Under the optimized conditions, the activities of modified catalysts were further tested. The results show that it is more suitable for the modified Ni-based catalysts to be loaded in FR because of their significant promotion to the hydrocracking of tar while it is favorable for the Ni-based catalyst to be placed in SR considering the enhancement of reforming and water gas shift reaction. Furthermore, when the temperatures of TSR were 900? and S/C was 9, stability tests of the modified catalysts were carried out for 10h, and no apparent carbon deposition was found. The proper introduction of H2 tends to improve the reforming effect significantly.Finally, based on RSM, two groups of experiments concerning biomass steam gasification were conducted with the employment of TSR for gasification performances of Xc and Rm. The first group of experiments was about the optimization of catalyst proportions, which intended to find out the proper contents of K-based catalyst and Ni-based catalyst in TSR. The results show that the optimized temperature of FR is 800?, the content of K-based catalyst in FR is 82% and the content of Ni-based catalyst in SR is 74%. The second group of experiments was focused on the catalytic performances of the modified Ni-based catalysts in biomass conversion and the optimization of gasification conditions. Analysis and optimization were made about the temperatures of TSR as well as the proportions of Ni-based catalysts modified by Li and Mg. The results indicate that the optimized temperatures of TSR are 800? and the content of modified Ni-based catalyst (by Li) is 90%. Under the optimized conditions, Xc and RH2 of both groups of experiments are able to attain 97% and 66mol/kg-bio. In addition, the modification of Li promotes the catalytic performance of Ni-based catalyst especially at lower temperatures, which is mainly embodied in the increment of Xc while RH2 is raised with an appropriate amount of K-based catalyst introduced. In this case, when the temperatures of TSR are 700 ?, Xc and RH2 reach 91.7% and 57.6mol/kg-bio separately. Moreover, the employment of SR tends to enhance both Xc and RH2, and this effect is more significant when temperatures are relatively low.