| With the rapid development of the world economy and the increasing population, the demand for energy in human life is growing. However, coal, oil and natural gas that supply energy for human being at present are non-renewable resources. Nevertheless, biomass that is from the photosynthesis of green sources is an inexhaustible renewable energy. Therefore, it is necessary to find an efficient and green way to transform biomass into useful chemicals and bio-energy.Lignocellulosic biomass is composed mainly of cellulose, hemicelluloses and lignin, which composition and structure are very different and need different treatments. As we know, the conversion conditions of hemicellulose are much milder than that of cellulose. So we can hydrolyze hemicelluloses under mild conditions; and then hydrolyze cellulose under harsh reaction conditions. This is so called two-step hydrolysis treatment of biomass. Our paper mainly took a two-step treatment of converting corn cob and rice straw, two kinds of popular and easy available biomass, in.H2O-SO2system. At first, SO2was dissolved in hot water, afterwards, SO2and H2O could generate H+by the formation of sulfurous acid. After the reaction, SO2could be recycled by steam stripping.Experimental results indicate that after the first-step treatment of corn cob, with time, temperature and SO2concentration increasing, yields of xylose, arabinose and glucose increase at first and then decrease after reaching a maximum; the yield of acetic acid increases at first and keep a constant value after the hydrolysis reaction was carried out for a period of time; furthermore yield of furfural increases at the same time.26.6%xylose,3.3%glucose and3.2%arabinose could be obtained at140℃for1.5h with0.024g/mL SO2; after the second-step treatment8.6%glucose could be obtained at190℃for0.5h with0.048g/mL SO2. The total yield of products via the two-step treatment of corn cob was48.4%, which is more than that in one-step treatment,32.4%at190℃for0.5h with0.048g/mL SO2. The two-step treatment can not only improve the yields of the products, but also reduce the yields of degradation products. We used infrared spectroscopy, XRD and SEM to characterize the raw materials and residues, the characterization results demonstrate the feasibility of the two-step treatment of corn cob in H2O-SO2system. According to the Saeman model, we completed the kinetic fitting for the first-step treatment of corn cob in H2O-SO2system to determine the kinetic equation of this reaction. Compared with the previous literature, the results show that the activation energy which xylose was degraded into furfural when we used SO2as catalyst is greater than that used sulfuric acid as catalyst. This is because SO2as a gas acid catalyst is weaker than sulfuric acid. This system can reduce the degradation of xylose and the yield of furfural.Then, we took a two-step hydrolysis treatment to transform rice straw in H2O-SO2system. After the first-step hydrolysis treatment,17.1%xylose,3.1%glucose and3.5%arabinose could be obtained at140℃for1.5h with0.024g/mL SO2; after the second-step hydrolysis treatment10.9%glucose and little degradation products could be obtained at190℃for0.5h with0.048g/mL SO2. We compared the rice straw experimental results with the results of the transformation of corn cob. The results show that yields of xylose and the total reducing sugar in rice straw experiment are lower that of corn cob, this is mainly because the mass percentage of hemicelluloses in rice straw is17.25%lower than that in corn cob.The structure and the kinds of ash of corn cob and rice straw are very different, so the hydrolysis reaction rate of rice straw in the first-step treatment is faster than that of corn cob. In summary, the different composition and structure of raw material are the underlying causes that the yields of products and kinetic parameters of different raw materials in H2O-SO2system are quite different. |
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