Research On Hydrothermal Reutilization Process And Mechanics Of Biomass Wastes

Under the background of energy and resource shortage and the environment pollution, the utilization and research of biomass wastes as a kind of resource attracting worldwide attention. Hydrothermal reaction provides an effective method for the reutilization of biomass wastes. The disposal and reutilization of biomass wastes were realized by hydrothermal reaction in batch hydrothermal reactor. To obtain high yield of reducing sugar and lactic acid by hydrothermal reaction, the influence factors, decomposition process and reaction network were investigated. The work can provide base for the industrialization of hydrothermal reaction.The product distributions of maize straw, sawdust, rice husk and wheat bran were described on basis of carbon balance. The effects of reaction temperature, time, water volume, grain size and oxidant on the aqueous, residual and gaseous sample of biomass wastes by hydrothermal decomposition were researched. The results showed that the hydrothermal decomposition of biomass wastes was composed by the liquefaction, oily production and gasification according to different temperature region. The increase of reaction time and water volume and the decrease of grain size can promote the liquefaction of biomass wastes. The increased amount of H₂O₂ can realized the complete decomposition of biomass wastes by hydrothermal reaction.The saccharification of micro crystalline cellulose, maize straw, sawdust, rice husk and wheat bran under the hydrothermal conditions was carried out in batch reactors. The effect of reaction temperature, time, water volume, oxidant, acetic acid, ethanol, metal ions (Zn²⁺, Ni²⁺, Co²⁺, Cu²⁺ and Cr³⁺) and sodium carbonate on the yield of reducing sugar were investigated. The optimal conditions for production of reducing sugar were obtained. The results indicated that 300-350℃in favor of the hydrothermal decomposition of biomass wastes to obtain reducing sugar. The yield of reducing sugar increased with increasing Na₂CO₃ for micro crystalline cellulose, maize straw and sawdust. For micro crystalline cellulose and sawdust, H₂O₂ could accelerate the production of reducing sugar. At the same time, the yield of reducing sugar increased much when adding acetic acid for micro crystalline cellulose. Furthermore, adding ethanol could increase the yield of reducing sugar and it reached the maximum 70.30% for maize straw. The yields of reducing sugar changed according to the difference of metal ions.As model compound, the hydrothermal decomposition of micro crystalline cellulose and glucose for the production of lactic acid under different conditions were discussed. To obtain a high yield of lactic acid by hydrothermal reaction, the influences of metal ions(Zn²⁺, Ni²⁺, Co²⁺ and Cr³⁺)on the reaction of biomass-relevant sugars such as maize straw, sawdust, rice husk and wheat bran were investigated. Based on the experimental data, the hydrothermal decomposition process and the catalyst's mode of operation in this reaction network were investigated. In comparison with non-catalytic process, the addition of 400ppm Ni²⁺ catalyst increased the yield of lactic acid to 6.62% for micro crystalline cellulose. The lactic acid yield for glucose was achieved as 9.51% for 400ppm Co²⁺ catalyst. Compared with non-catalytic conditions, the lactic acid yield is 9.24% and 6.71%, respectively starting from maize straw and rice husk at 400ppm for Cr³⁺. Sawdust obtains the highest lactic acid yield 5.47% at Zn²⁺ 400ppm. But as for wheat bran, the highest lactic acid yield 7.46% is obtained under non-catalytic conditions. Hydrothermal reactions makes water a Bronsted base-acid and act as an effective catalyst and the metal ions represent Lewis base character. The conversion process occurs in a very complex network of parallel, consecutive and equilibrium reactions, and the last step being the catalyzed Cannizzaro-type reaction of pyruvaldehyde to lactic acid.The pyrolysis behavior and character of micro crystalline cellulose, maize straw, sawdust, rice husk and wheat bran were studied by thermo gravimetric analysis (TGA).The kinetic parameters and model of pyrolysis biomass wastes were established. The experimental results showed that the decomposition of micro crystallite cellulose mainly occurred at the temperature range of 300-400℃.The weight loss(TG) and the rate of weight loss(DTG) shift to higher temperature region with the increase of heating-up rate. The thermal dynamics for biomass wastes was coincidence to the first order reaction dynamics by Coat-Redfern analysis. The Fourier transform spectrum (FTIR), TGA, element, scanning electron microscopy (SEM), X-ray diffraction (XRD) analysis indicated that the hydrothermal decomposition of micro crystallite cellulose was composed by low temperature hydrolysis and high temperature pyrolysis.