Study On Anaerobic Fermentation Of Corn Straw Based On Ball-milled Pretreatment

The treatment of lignocellulosic biomass by anaerobic fermentation technology is of great significance for the development of low carbon economy and green energy in China.However,the complex structure of lignocellulose seriously hinders its bioenergy conversion.Ball-milled as a green and non-polluting mechanical pretreatment means can destroy the dense structure of lignocellulose,but the mechanism of its effect on the anaerobic fermentation process still needs to be further investigated.In addition,how to balance the high energy input of ball-milled pretreatment process is a hot issue of current research.Based on this,this study characterized the physicochemical properties and microstructure of corn straw powder at different ball-milled times,and clarified the changes of materiality parameters and deconcentration mechanism during the ball-milled process.In addition,the anaerobic fermentation of corn straw was carried out with different ball-milled times and different total solid(TS)as the fermentation conditions,and the changes of physicochemical parameters during the fermentation process were further investigated to reveal the influence mechanism of ball-milled pretreatment on anaerobic fermentation.Finally,energy analysis and energy conversion assessment were performed for all experimental groups to determine the optimal pretreatment and fermentation conditions.This study can provide theoretical guidance for the practical application of ball-milled pretreatment and scientific support for the continuous promotion of the"Carbon peaking and carbon neutralization"strategic policy.The main findings and conclusions of this paper are as follows:(1)Ball-milled pretreatment(10,20,30,40,60,120,240 min)changed the microstructure and physicochemical properties of corn straw:during the ball-milled process,the particle size of straw became smaller and smaller,and the median particle size(D50)of BM60(ball-milled 60 min group)was significantly reduced to 13.8μm,and the homogeneity of the powder gradually deteriorated(the maximum particle size span was 6.76);The cell structure was severely damaged,and the surface morphology changed from flakes to rods and finally to granules;the macropores were gradually torn into mesopores,and the pore volume(87.5%)and specific surface area(5.83 times)of BM60increased significantly compared with the unball-milled group(BM0);the hydrogen bonds between cellulose were broken,and the crystallization order was disrupted.The cellulose grain size and crystallinity were reduced by up to 88.97%and 84.13%.The functional groups and chemical bonds were gradually exposed,the dense mesh structure between the lignocelluloses was broken,the intensity of the IR peaks increased and the peak positions were shifted;the voids between the particles were reduced,the fluidity of the powder became poor,and the packing density(5.42 times)increased significantly;the absorption rate of water by the straw became faster,the adsorption capacity became weaker,and the binding capacity became stronger.Combining the variation pattern of materiality parameters of corn straw and the generation of agglomeration effect(120 min),the optimal ball-milled time was determined to be 60 min.(2)Anaerobic fermentation of corn straw with different ball-milled times(0,20,30,40,60,120 min)and different TS(4%,5%,6%,7%,8%,9%,10%)showed that compared to the control group BM0-6(ball-milled time of 0 and TS of 6),ball-milled pretreatment increased the accessibility of microorganisms and enzymes to the straw,promoted the hydrolysis of anaerobic fermentation acidification process,increased the peak volatile fatty acids(VFAs)(5325.69 mg/L)and chemical oxygen demand(COD)(14524 mg/L)concentrations,increased the substrate available to methanogenic bacteria,and significantly increased the daily methane production of the test group(89.01%);reduced the initial density of the fermentation slurry(6.91%-9.58%),which facilitated the mixing and heat and mass transfer of the fermentation process,but the density increased significantly with the increase of TS.The release rate of total ammonia nitrogen(TAN)was accelerated,which was used to neutralize the VFAs produced by hydrolysis so that the buffering capacity of the fermentation system was increased;the water distribution of the fermentation slurry was changed(the free water content of BM60-6 was increased to 70.4%),and the combined water was continuously converted to free water as the fermentation process proceeded,and the conversion process mainly occurred in the gas production stage;TS threshold(9%)of straw anaerobic fermentation was reduced,but too high TS would cause acid accumulation and ammonia inhibition,which would unbalance the fermentation system.The modified Gompertz model(R~2≥0.98)was well-fitted and predicted the variation of gas production performance in each experimental group,and the results showed that the ball-milled pretreatment significantly reduced the fermentation delay period(37.41%-56.12%)and increased the cumulative methane production(45.63%-104.11%).BM60-6 presented the highest VS removal(78.79%),cellulose degradation(92.07%),and biochemical methane potential(BMP)(309.69 m L/g VS).(3)The relationship between ball-milled pretreatment and rheological characteristics was constructed,and the results showed that the ball-milled pretreatment improved the rheological characteristics of the fermentation slurry,reduced the consistency coefficient(K)and increased the rheological index(n)of the fermentation slurry.Therefore,the theoretical stirring power consumption of the anaerobic fermentation process in the ball-milled pretreatment group was significantly reduced,while the stirring power consumption increased with the increase of TS.The methane production capacity,ball-milled power consumption,and stirring power consumption of all experimental groups were evaluated for energy analysis and energy conversion,in which the net energy benefit(28.2101 KJ/g VS)and the net energy surplus(0.5000)were the largest for the BM60-6 group,so the optimal conditions for ball-milled corn straw anaerobic fermentation were 60 min for ball-milled and 6%TS.