Study On Catalytic Reforming Of Biomass Tar For Hydrogen Production

Biomass tar, a viscous and dark by-product with a strong pungent odour is generated in biomass pyrolysis/gasification process, which contains numerous complex organic compounds, especially oxygen-containing organic compounds. The tar not only affects the normal operation of equipment, but also causes serious harm to the environment and human health, which limits the industrial development of biomass gasification technology. Catalytic method is a promising conversion technology for tar removal, and the traditional catalytic method can improve the decomposition of tar components. However, it has some disadvantages, such as low removal efficiency, low resistance to carbon deposition and low hydrogen production, which limit its wide application in the field of pyro lysis and gasification. Based on these considerations, a research on catalytic reforming of biomass tar for hydrogen production over nickel-based catalyst was carried out to improve the tar removal efficiency and slow the catalyst deactivation in this study. Thermo gravimetric analysis was used to study the dynamic characteristics of biomass tar in pyrolysis and combustion processes and the products distribution was analyzed by Py-GC/MS. With high-efficiency and stable characteristics of NiO/ceramic foam catalyst was prepared for hydrogen production. Then, the optimum experimental parameters for the maximum H2 yield were obtained. The partial oxidization approach was introduced to perform in situ catalyst regeneration, which ensured a long-term activity of catalyst on tar high cracking. The detailed conclusions are drawn as follows:(1) Thermal process of biomass tar was carried out by using TG analysis under different heating rates. The results indicated that the biomass tar pyrolysis process can be divided into three stages, including volatilization, first pyrolysis and second pyrolysis. Both TG and DTG curves had a similar trend under different heating rates. With the increase of heating rates, they shifted to higher temperatures zone, suggesting a higher tar pyrolysis temperatures. Moreover, the pyrolysis kinetics of tar conformed to the first-order kinetic equation. Therefore, the activation energies of first pyrolysis and second pyrolysis calculated by Arrhenius formulas were 33.48-50.27 kJ/mol and 39.18-44.20 kJ/mol, respectively. The dynamic characteristics of combustion was similar to the pyrolysis, however, there was larger mass loss at higher temperature for the combustion process. The activation energy of 79.77-124.25 kJ/mol in the combustion process was much higher than that of pyrolysis. The Py-GC/MS results showed that the main components of tar pyrolysis products were aromatic compounds and ketone compounds under different pyrolysis temperatures. And the content of ketone compounds distinctly increased while that of aromatic compounds decreased with the pyro lysis temperatures rising.(2) The NiO/ceramic foam catalyst used ceramic foam as catalyst carrier was prepared by impregnation method in this study. Assisted by SEM and XRD analyses, a series of experiments were carried out to study the effects of active nickel-based component, calcination temperature and catalyst loading on catalyst activity. It could be concluded that when Ni(NO3)2-6H2O was used as nickel precursors, with calcination temperature of 700℃ and catalyst loading of 3.50+0.2%, the NiO/ceramic foam catalyst showed higher activity and better stability for catalytic reforming of tar.(3) A lab-scale fixed-bed reactor was designed and established for hydrogen production from catalytic reforming of biomass tar. In this study, the effects of reaction temperature, steam to carbon ratio (S/C), equivalence ratio (ER) and regenerated catalyst on catalytic reforming of tar for H2 yield were systematically investigated. It was found that when the temperature varied from 500 to 900℃, S/C ratio from 0 to 4 and ER of 0, H2 yield was in the range of 28.29-105.28 g H2/kg tar. However, H2 yield obviously reduced when ER value increased. SEM and XRD results showed that the active component of catalyst could be in situ regeneration via carbon deposition combustion in partial oxidization process. Besides, the regenerated catalyst could keep high activity compared to the fresh catalyst. The qualitative analysis results of the tar and reforming tar by GC/MS showed the active nickel promoted the C-C bond and C-O bond cleavage of the hydrocarbon and phenyl ring, as well as ring-opening reactions of heterocyclic compounds.