Pyrohydrolysis Of Corn Stover Catalyzed By Dilute Nitric Acid And Biological Acidification Of Hydrolysate

The conversion of corn stover pretreated properly to biogas with the industrialscale can realize the use of corn stover efficiently and cleanly. However, there aresome key difficult problems in the conversion process, for example the pretreatmentcost is high and the catalyst employed in the pretreatment could influence thesubsequent anaerobic digestion negatively. From the viewpoint of whole process, alow cost pretreatment which has no effect on the subsequent anaerobic digestion wasexplored. The regularity of pyrohydrolysis reaction of corn stover catalyzed by nitricacid was analyzed. The effects of corn stover hydrolysis with HNO₃and H₂SO₄werecompared by combination of theoretical analysis and experiment. The biologicalacidification characteristics of hydrolysate gained from nitric acid hydrolyzing cornstover were discussed. Based on the acidification experiments, the feasible methodthat the hydrolysate was used for anaerobic fermentation was proved, which offers anovel way to realize the conversion of corn stover to biogas commercially.The variation of H+concentration during the corn stover hydrolysis wasinvestigated. The results show that the H+was involved in hydrolysis reaction andplayed an important roal during the corn stover hydrolysis. On the basis of the sameH⁺concentration, the hydrolysis effect of H₂SO₄and HNO₃on corn stover wascompared. The result shows that the H+of HNO₃was more active than that of H₂SO₄and it had stronger capability of breaking the glycosidic bonds, indicating that HNO₃is superior to hydrolyze corn stover. The regularity of pyrohydrolysis reaction of cornstover catalyzed by HNO₃was investigated. The results show that the nitric acidconcentration, reaction temperature and time all had promotion effects on corn stoverhydrolysis, and the influence of the concentration of HNO₃was more remarkable. Inthis experimental research scope, the optimum condtion was150℃,0.6%HNO₃and2min. Under this condition,12.24%glucan was coverted into glucose,96.87%xylanwas converted into xylose,98.54%araban was converted into arabinose and thehydrolysate contained acetic acid3.11g·L^-1, furfural0.43g·L^-1.The kinetics of hemicellulose hydrolysis at high temperature were studied. In thisexperimental research scope, the experimental value consist with the theoretical value.Arrhenius equation was adopted to calculate the activation energy of hydrolysisproducts, the order from high to low was furfural>xylose>arabinose>acetic acid. The biological acidification characteristics of hydrolysate gained from HNO₃hydrolyzing corn stover were investigated. The results show that the acidification rateof hydrolysate was faster than that of corn stover. The volatile acid production mainlydepended on the monosaccharide content of the hydrolysate. In this experimentalresearch scope, the hydrolysate gained at the condition of150℃,0.6%HNO₃and2min could produce maximum VFAs,2513.35mgCOD·L^-1. Howerver, the distributionpattern of VFAs was relevant to substrates, for example, the end products of cornstover after anerobic acidification were mainly acetic acid and propionic acid,moreover, the content of propionic acid was high during the acidogenic process. Themain end product of HNO₃-hydrolysate after anerobic acidification, was acetic acid,and the content of propionic acid decreased with prolong of the acidification time.The main end product of H₂SO₄-hydrolysate after anerobic acidification were aceticacid and propionic acid, however, the end product only consisted of acetic acid andpropionic acid, and the content of propionic acid was relatively high in the later stage,this appeared a trend of propionic acid type fermentation. The high content ofpropionic acid in the VFAs has adverse effect on the subsequent biomethane, therefore,compared to the H₂SO₄-hydrolysate, the HNO₃-hydrolysate was suitable forbiomethane.