Study On The Process And Mechanism Of Hydrothermal Liquefaction Of Crop Straws

Biomass resource is very rich in China and the annual production of crop straws is up to 7 million tons. If these crop straws can be converted to liquid fuels by direct liquefaction, it can not only make a compensation for the shortage of fossil fuels, but also be favorable for the environmental protect. As one of the most promising techlogies for biomass liquefaction, hydrothermal liquefaction is attracted much attention over the world. Therefore, investigations on the process and mechanism of hydrothermal liquefaction of crop straws will not only provide a theoretical basis and fundamental data for the further development of straw biomass liquefaction technology, but also provide references for the effective utilization of other biomasses or organic wastes. The main research contents and results were summarized as follows:(1) Thermal weight loss behavior and kinetics of crop straws and biomass model compounds (BMCs:cellulose, hemicellulose and lignin) under different atmosphere (N2, H2, CO, CO2 and air) were studied using a thermogravimetric analyzer. It was found as follows: among the three kinds of BMCs, the temperature range for the weight loss of cellulose was the narrowest and its weight loss rate was the highest; the onset temperature for the weight loss of hemicellulose was the lowest; the weight loss of lignin was the lowest; under the same atmosphere, the weight loss behaviors of different crop straws were similar to each other. The weight loss order of the same crop straws at different atmosheres was CO2>air>H2>N2> CO. The weight loss order of the same BMC at different atmosheres was air>CO2>H2>N2 >CO. The results on kinetic investigations showed that the apparent activation energy order of BMCs was cellulose(212-338kJ/mol)> lignin (63-97kJ/mol)>hemicellulose(20-65kJ/mol), and the apparent activation energy of crop straws was in the range of 74-121kJ/mol.(2) The effects of atmospheres on noncatalytic hydrothermal liquefaction of crop straws were investigated using a high pressure autoclave. It was found as follows:reducing atmospheres (especially CO) could significantly promote the hydrothermal liquefaction convesion of crop straws and improve the qualities of liquid products; among the three crop straws, the yield and heating value of liquid products from corn straws were the highest and the hydrothermal conversion of cotton straws was the highest. Under N2 atmospheres, the conversion of corn straws, and the yield, O content and heating value of liquid products were separately up to 72.58%,35.40%,14.86% and 33.53MJ/kg; under H2, those were seperately up to 79.40%,38.32%,12.01% and 35.64%; under CO, those were separately up to 82.39%, 39.53%,10.64% and 36.72MJ/kg. The liquid products mainly consisted of alkanes with long chains, phenols, carboxylic acids, etc.(3) The effects of technical conditions on the hydrothernmal liquefaction of crop straws under CO atmosphere were investigated using a micro-vibration device. It was found as follows:with the increasing reaction temperature and reaction time, the conversion of corn straws increased, and the yield of liquid products initially increased and then decreased; As the initial pressure of CO, the equilibrium pressure and the mass ratio of water and biomass increased, the conversion of corn straws and the yield of liquid products increased. The optimal conditions of hydrothermal liquefaction of crop straws under CO atmosphere were as follows:reaction temperature of 350-400?, reaction time of 15-35min, CO initial pressure of 3.0-6.OMPa, water/biomass mass ratio of 4-7. Under the optimal operating conditions, the conversion of corn straws and the yield of liquid products were respectively up to 91.17% and 45.55%.(4) The effects of homogeneous catalysts on the hydrothermal liquefaction of crop straws under CO atmosphere were investigated. It was found as follows:The conversion of crop straws under different catalysts was ordered as Na2CO3>NaOH>K2CO3>KOH, which was opposite to the ordering of the yield of liquid products under different catalysts; the conversion of crop straws, and the yield, O content and heating value of liquid products were separately up to 98.94%,40.71%,8.80% and 39.39MJ/kg. The effects of heterogeneous catalysts on the hydrothermal liquefaction of crop straws under CO atmosphere were investigated. It was found as follows:both the conversion of crop straws and the yield of liquid products under different catalysts were ordered as JT-203> JT-201> JB-1; the conversion of crop straws, and the yield, O content and heating value of liquid products were separately up to 99.99%,48.01%,6.84% and 41.62MJ/kg. The catalysts could simutaineously enhance the water-gas shift reaction, the direct removal of O and the decarboxylation process, which would obviously improve the quality of liquid products. Using, the liquid products at homogeneous catalysts were characterized as smaller molecular weight and higher O content; those at heterogeneous catalysts was characterized as higher molecular weight and lower O content. In comparison with the liquid products obtained from fast pyroylysis of biomasses (the O content and heating value of liquid products were 30-37% and 22-26MJ/kg), the liquid products obtained from the hydrothermal liquefaction of crop straws presented a lower O content and a higher heating value.(5) The effects of atmospheres and catalysts on catalytic and noncatalytic hydrothermal liquefaction of CMCs were studied. It was found as follows:under noncatalytic conditions, reducing atmospheres had quite small influences on the hydrothermal liquefaction of cellulose; under catalytic conditions, reducing atmospheres (especially CO) could indistinctly elevate the conversion of cellulose and the yield and heating value of liquid products. Under CO atmospheres, the catalysts, which showed better catalytic activity for the water-gas shift reaction, could show better catalytic action for the hydrothermal liquefaction of cellulose. The elemental compositions and heating values of liquid products from the hydrothermal liquefaction of cellulose under different alkali metal catalysts were similar to each other. Among the three kinds of CMCs, the conversion of hemicellulose and the yield of gas/water for its hydrothermal liquefaction were the highest and the yields of liquid products and solid organic matters from the hydrothermal of lignin were the highest.(6) Using wood vinegars as the solvent, the hydrothermal liquefaction of cotton straws under CO atmosphere was investigated. It was found that the organic compounds and inorganic metal elements in wood vinegars could promote the hydrothermal liquefaction process. Compared with water solvents, the conversion of cotton straws and the yield and heating value of liquid products at the liquefaction condition of wood vinegars as solvents increased from 91.56% to 97.37%, from 35.75% to 54.23% and from 34.14MJ/kg 37.58%, respectively.