Research On Improving Biodegradability Of Corn Stalk By Ensiling And Hydrothermal Pretreatment Method

Using corn stalk to produce biogas by anaerobic fermentation in large-scale is one of the most effective and promising utilization methods for agricultural wastes. While the lignocelluse content in corn stalk is very high, it will lead low efficieny, low biogas production and low utilization rate if corn stalk was used to produce biogas by anaerobic fermentation without pretreatment. Therefore, suitable pretreatment to corn stalk is very necessary for good utilization and biogas production. In this thesis, the ensiling methods and pretreatment methods were investigated to improve corn stalk and enhance anaerobic digestion process.Corn stalk was cut to pieces with length of 1-3 cm, and some additives of nitric acid, acetic acid and ammonium hydroxide were added to make corn silages. Moreover, some corn stalk was compressed by extractor and acetic acid was added to the residues after compressed to make silage. The quality of the silages were evaluated by determining the varieties of chemical compostion in corn stalk after silage period. The results showed that the content of Klason lignin and crystallinity index for corn stalk silage decreased 12.3-25.8% and 4.4-20.1% compared to air-dried corn stalk.Corn stalk silage and air-dried corn stalk were treated with liquid hot water. The pretreatment results showed that the maximum soluble sugar yield of corn stalk silage was 2.4 times compared with that of air-dried corn stalk.Five inorganic salts, ZnCl₂, FeSO₄, Fe₂（SO₄）₃, FeCl₃ and Fe（NO₃）₃ were chosen as catalysts to determine their effects on hemicellulose hydrolysis in control silage （no silage additive）. The results indicated that Fe（NO₃）₃ was the most efficient catalyst for hemicellulose hydrolysis. The effects of Fe（NO₃）₃ pretreatment conditions on sugar yields were investigated for corn stalk silage. Optimum pretreatment condition was obtained at 150 oC for 10 min with 0.05 M Fe（NO₃）₃, at which the yields of xylose in liquid achieved 91.8% of initial xylose, and the concentration of total sugar achieved 33.48 g/l,while the inhibitor concentration was only 0.03 g/l.In order to improve the hydrolysis efficiency of corn stalk and decrease the dosage of catalyst, the study introduced two-stage method to pretreat corn stalk. First, corn stalk was immersed by different solution at low temperature, and then the residues after immersion were pretreated by Fe（NO₃）₃ at high temperature. The results showed that compared with pretreated by only Fe（NO₃）₃, the total sugar yield improved 17.5% and 12.1%, while the dosage of Fe（NO₃）₃ reduced 31.9% and 28.3%, respectively, when Tween-80 and deionized water used as immersed solution. When ammonia water was used as immersed solution, the total sugar yield was lower than that pretreated by only ferric nitrate. Though the effect of Tween-80 is a little bit better than deionized water, while it can increase the cost of the process, so deionized water was recommended to be used as immersion solution. Moreover, the two-stage method was used to pretreat different types of corn stalk when deionized water was used as immersion solution, and all the experiments got good results.The kinetics of Fe（NO₃）₃ catalyzed hydrolysis for control silage and acid silage （treatment with HNO₃） were investigated at various pretreatment conditions. The results demonstrated that Saeman model was well consistent with Fe（NO₃）₃ catalyzed hydrolysis reaction for corn stalk silage, and kinetic parameters for this model were developed by the Arrhenius equation. The activation energies for hemicellulose hydrolysis in control and acid silage were 44.3 kJ/mol and 3.1 kJ/mol, respectively.