| Hydrogen is considered as one of the most ideal clean fuels because of very high heating value and no pollutant produced. Traditional methods of hydrogen production are subjected to energy reserves and environmental pollution, so biomass is recognized as one of the most promising and attractive options, because it is rich and friendly environmental renewable resource. Presently, the main methods to produce hydrogen with biomass are gasification and flash pyrolysis followed by steam reforming of the bio-oil produced in the flash pyrolysis process. The latter was known as one of the more economically viable methods for hydrogen production with fairly mature pyrolysis technology (liquid yield as high as 70%-80%), the convenient storage and transportability. However, the steam reforming of bio-oil still is confronted with these key problems:poor hydrogen quality, catalyst deactivation and not long-time running, etc. In view of these problems, a novel approach for hydrogen production from bio-oil was proposed in our research. The main content of the present thesis was focused on the items as follows:(1) The novel approach of continuously sorption-enhanced bio-oil steam reforming for hydrogen production was proposed. The approach coupled bio-oil reforming reaction and CO2 adsorption reaction in the catalytic reforming reactor, resulting in decreasing CO2 concentration and then improving H2 concentration and yield. The desorption reaction of CO2 sorbent occurred in the desorber reactor, realizing the recycle of CO2 sorbent.(2) The characteristic and kinetics of the pyrolysis processes of three kinds of bio masses were studied through temperature-programmed thermogravimetry, and the obtained results were as followed. With the increase of the temperature, the reaction rate gradually increased, although the initial pyrolysis temperature and the temperature of the maximum reaction rate moved to higher temperature; the biomass pyrolysis mainly resulted from the pyrolysis of hemi-cellulose and cellulose, and there were synergistic effects between the three compounds (hemi-cellulose, cellulose and lignin) for biomass pyrolysis. With the combination of Coat-Redern method and Malek method, the mechanism functions of corn cob, peanut shell and pine cone were selected out, and the fittest ones were R2, A1 and A1, respectively; the values obtained by the kinetics model agreed well with the experimental values.(3) Biomass fast pyrolysis to produce bio-oil was studied by small experiment system, and the obtained results were as followed. With the increase of carrier gas velocity, the bio-oil yields increased, but the increase rates gradually decreased, and 0.35m/h was been confirmed as the best; as the temperature increased, the bio-oil yields increased, but decreased after 500℃ mainly due to the secondary cracking of bio-oil steam at higher temperature; for corn cob, the smaller the granularity was, the higher is the bio-oil yield, but the tendencies of the two other biomasses were reverse; bio-oil was a very complex mixture, mainly consisting of oxygen functional groups, such as phenols, esters, ketones, alcohols and methoxys, causing lower caloric value.(4) Based on the principle of the Gibb’s energy minimization, the thermodynamic analysis of the steam reforming processes for four typical model compounds (ethanol, acetic acid, acetone, phenol) of bio-oil was systematically performed, and the CO2 sorbent used into the novel continuously sorption-enhanced bio-oil steam reforming process was also selected out, and the obtained results were as followed. A CO2 sorbent applied into the novel process has to meet the following requirements:solid state; high CO2 adsorption capacity, especially in the condition of the co-existence of CO2 and H2O; CO2 desorption easily happens under the temperature zone just slightly higher than the temperature of bio-oil steam reforming; good absorbing-desorbing cycle stability, i.e. still maintaining high CO2 adsorption performance after multiple cycles. Through selecting from 9 kinds of common metal-oxide sorbent, CaO was the most fitting one to meet all the above requirements. Through the comparison with the case of no sorbent addition, after the addition of CaO, the H2 concentrations and yields were improved, and the temperatures of the maximum H2 yields moved forward to low temperature, and the temperature ranges with higher H2 yield were widened. In the range of 477℃～677℃, the H2 yields kept over 93%, and the H2 concentrations were improved from about 70% at no sorbent addition to 86%-98%. With thermogravimetric analysis, CaO from calcinated calcium acetate shows the highest CO2 adsorption capacity and the best cycle stability, compared to the other two kinds of CaO with analytical pure and from calcinated calcium hydroxide.(5) Eleven kinds of Ni-based catalysts supported on Al2O3 modified by Mg, Ce or Co were prepared and the catalyst with the best catalytic performance was selected out through investigating the effect of them on the steam reforming processes of model compounds, and the obtained results were as followed. Among these catalysts, Ce-Ni/Co catalyst showed the best activity for the steam reforming reactions of all the four model compounds, with the hydrogen yields of over 85%. For the steam reforming of the simulated bio-oil (mixed by the four compounds with equal masses) over Ce-Ni/Co catalyst, the Hb yield and carbon selectivity increased with the temperature increase, but fattened out after 700℃, when the H2 yield and carbon selectivity kept over 84% and 94%; the H2 yield increased as the S/C ratio increased, but almost flattened out when the S/C ratio was over 9, when the main restriction factor for steam reforming to produce hydrogen was not the steam amount, but the number of the active sites of Ce-Ni/Co catalyst; the less the LSHSV was, the more adequately the steam reforming reacted, and then the more the H2 yield was, but changed not much when the LsHSV was lower than 0.23h-1; after long-term test at 700℃, the S/C ratio of 9 and the LsHSV of 0.23h-1, the Ce-Ni/Co catalyst still maintained excellent stability for the steam reforming of the simulated bio-oil.(6) Sorption-enhanced bio-oil (and simulated bio-oil) steam reforming for hydrogen production was investigated with the small experimental system, and the obtained results were as followed. The addition of CaO obviously improved the H2 concentration and yield. For the simulated bio-oil, the change rules of H2 concentration and yield as a function of temperature were similar with that of the CO2 adsorption performance of CaO, i.e. first increasing and then decreasing, and H2 concentration and yield reached the maximum at 700℃～750℃; H2 concentration and yield gradually increased with the increase of CaO/C ratio, but H2 yield decreased at the CaO/C ratio of over 3, resulting from the excessive CaO restraining the contact between the reactants and the catalyst; adequate steam also inhibited the steam reforming with in continuous situ CO2 capture, and the optimal hydrogen yield and concentration of simulated bio-oil were obtained at the S/C ratio of 9. For the real bio-oil, its steam reforming reaction and sorption-enhanced steam reforming reaction showed the similar change rules with those of the simulated bio-oil. However, due to the various constituents and macro molecular structure, the decomposition and steam reforming of bio-oil were more difficult than those of the simulated bio-oil, so for the sorption-enhanced real bio-oil steam reforming, the optimum temperature was improved to 750℃～800℃, the S/C ratio improved to 12 and the LsHSV decreased to 0.15h-1, and under the optimum condition, the H2 concentration and yield reached over 90% and 85%, respectively. |
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