Effects Of Liquid Hot Water Pretreatment And Modified Biochar On The Anaerobic Digestion Of Wheat Straw

The production of wheat straw in China reached 157,177,000 tons,accounting for 23%of the straw resources,in 2017.Wheat straw is a kind of abundant agricultural waste that is rich in lignocellulose.However,its waxy surface,highly crystallized stable structure,and limited available surface area make it difficult to hydrolyze and be used efficiently by microorganisms.So the efficiency of AD and biogas yield are low,and even cause stagnation of the AD process.The purpose of this paper is to explore the material characteristics of the pretreated wheat straw by liquid hot water?LHW?and the characteristics of biogas production,and further improve the methane yield through the addition of modified biochar.The main research contents and results are as follows:?1?The skeletal structure of wheat straw was gradually broken with LHW pretreatment.Some pores and cracks appeared on the surface of the pretreated wheat straw,and the microbial attachment sites increased.Under different hydrothermal temperatures?150-225??and retention times?5-60 min?,the degradation rate of hemicellulose was 27.69%-99.07%,and the degradation rate of cellulose reached 80.24%.With the increase of temperature and pretreatment time,the kinds of organic acids showed an increasing trend.Acetic acid accounted for the largest proportion amongst all acidic substance,reaching the highest yield at 1250.71 mg/L.The content of reducing sugar in liquid phase after pretreatment was positively correlated with pretreatment time and temperature,respectively.When the temperature was higher than 175?,the reducing sugar content in the liquid phase was at a lower level for a longer pretreatment time.At the same time,the relationship between the content of raw sugar and the degradation rate of lignocellulose was not positive.It showed that lignocellulose not only decomposed into oligosaccharides,but also formed other compounds in the process of hydrothermal pretreatment.The inhibitor appeared at 175?for 30 min,and the content increased with pretreatment conditions becoming harsh.At the temperature of 200?,5-hydroxymethylfurfural was detected,and the highest content reached 35.6%.?2?The maximum methane yield was achieved by LHW at 175?for 30 min,which was a 62.9%increase in methane productivity compared to non-treated straw?from 124.51to 201.81 mL CH₄/g volatile solids?.By calculating the biodegradation rate of pretreated wheat straw,it was found that under 30 min pretreated conditions wheat straw had better biodegradability;LHW at high temperatures such as 225?was not suitable for the AD of wheat straw.Methane yield results were fitted with the first order and modified Gompertz equations to evaluate substrate availability,reaction rate,and inhibitory effects of the pretreated materials in AD.?3?To use hydrothermal temperature A,retention time B and biochar modifier concentration C as factors,the methane yield as response value in the experiment of central composite design?CCD?,the result shows the theoretical model for methane yield is Y=832.47-43.21 A+5.00 B+9.14 C-65.84 AB+32.76 AC-35.46 BC-187.23 A²-80.74 B²-91.36 C².Through the significance test of the model,the correlation coefficient,R²=0.9831,p<0.0001,showed that the model fitting effect was good.The model can be used to describe the change of methane yield with hydrothermal temperature,hydrothermal retention time and the concentration?NaOH?of biological carbon modifier of pretreated wheat straw in AD.Through optimization of model parameters,the optimal process parameters of modified biochar-mediated hydrothermal pretreatment of wheat straw in AD were as follows:pretreated temperature was 173?,retained for 31.0 min,and added modified biochar which was soaked in 2.01 mol/L sodium hydroxide,the maximum yield of methane is 835.5mL.