Effect Of Metal Salt On Hydrolysis And Anaerobic Acidogenesis Of Corn Straw And Its Mechanism

The industrialization of biogas from anaerobic fermentation of corn straw is an effective way to achieve high-value and clean utilization of straw.The dense rigid structure and complex chemical composition of straw are the key factors restricting its conversion.Moreover,the composition and content of volatile fatty acids in anaerobic fermentation directly determine biomethane production.This study focused on the two key processes of straw to biogas,namely hydrolysis and anaerobic fermentation,and explored a new method of co-utilization of nutrient metal ions for catalytic hydrolysis and anaerobic acidogenic fermentation.The influences of nutrient metal salt on the catalytic hydrolysis of corn straw were studied.Moreover,the catalytic mechanism of nutrient metal salt on cellulose and hemicellulose of corn straw is revealed.The further study was about the compatibility of nutrient salt in the two key processes of hydrolysis and anaerobic fermentation.The study aims to provide core basic theory and key technical support for low-cost and efficient conversion of corn straw.The catalytic hydrolysis performances of three typical nutrient salts,such as Fe Cl₂,Ni Cl₂ and Co Cl₂,on corn straw under the conditions of multivariate parameters coupling were analyzed,using the combined severity factor and the response surface optimization.It was found that Fe Cl₂ has excellent catalytic ability,especially on hemicellulose hydrolysis.Temperature is the key factor to affect the hydrolysis effect by nutrient metal salt,while the nutrient salt concentration has a weak influence on the hydrolysis effect.The acidity of the metal salt solution and the special chemical property of the cation play a key role in enhancing the catalytic hydrolysis effect.Saeman model was used to analyze the kinetics of hemicellulose hydrolysis catalyzed by Fe Cl₂,and it was concluded that Saeman model can simulate and predict the hydrolysis process of hemicellulose catalyzed by Fe Cl₂.According to the minimum amount of nutrient metal elements required in the anaerobic fermentation,the effect of corn straw hydrolysis by ultra-low concentration of Fe Cl₂ catalysis was explored.It was found that 97.63%of hemicellulose and30.71%of cellulose can be transformed,and the recovery rate of xylose reached94.46%with 1×10^-5 mol/L Fe Cl₂ at 180?for 10 min.Moreover,the production of5-HMF and furfural were 0.31 g/100 g and 1.53 g/100 g,respectively.They only accounted for 4.36%of the total liquid product.82.68%of the organic matter in the hydrolysate can be used for subsequent anaerobic fermentation.Based on the density functional theory in quantum chemistry,the hydrolysis mechanism of cellulose under the action of iron ions was discussed at the high basis level.According to Gibbs free energy in the microscopic dynamics,combined with activation energy and reaction enthalpy change,two possible paths were found for the combination of iron ion complex and oxygen atom.In the two paths,different transition states are generated during the reaction.O?H bond or C?OH bond is broken to form H₂O,and C3?O22 bond is broken,resulting in the degradation of cellulose.Based on the above analysis about the hydrolysis effect and action mechanism of corn straw catalyzed by nutrient metal salt,the study on the matching of the two key processes of hydrolysis and anaerobic fermentation by the metal salt was carried out.It was concluded that the hydrolysates from the catalytic reaction of Fe Cl₂,Co Cl₂ and Ni Cl₂,promote the production of volatile short-chain fatty acids,and accelerate the process of acidogenic fermentation.The concentration of volatile short-chain fatty acids produced from the hydrolyzate of the single or combined catalytic reaction by Fe Cl₂,Co Cl₂ and Ni Cl₂,were compared in the anaerobic system.Combined with the material analysis,it is shown that the nutrient salt ion co-utilization technology of the ultra-low concentration Fe Cl₂ has a significant advantage in terms of input to output ratio,and also plays an important role in the hydrolysis effect and fermentation efficiency in the key process of converting corn straw into biogas.