| Bioconversion of resource-rich and renewable lignocellulosic materials into liquid fuels and bulk chemicals is one of the effective ways to alleviate the resource and energy crisis,and reduce carbon emissions for achieving the national goal of "carbon peak and carbon neutrality".The bioconversion contains three main steps,pretreatment,enzymatic hydrolysis,and fermentation.Firstly,lignocellulose materials must to be pretreated by physical or chemical methods due to its complex chemical compositions and structure for improving its enzymatic digestibility,then hydrolyzed into fermentable sugar through enzymatic hydrolysis with cellulase,and converted into liquid fuels and different chemicals through subsequent fermentation.However,during the pretreatment process,substances that inhibit cellulase and microbial fermentation will be produced,thus decreasing the process efficiency of subsequent enzymatic hydrolysis and fermentation.On the other hand,to increase the concentrations of the final products such as ethanol for reducing the cost of subsequent separation and purification,it is necessary to produce the hydrolysis liquid with high sugar concentration in the enzymatic hydrolysis step.Therefore,the enzymatic hydrolysis needs to be conducted under at the high solid loading,which result in the problems of mass transfer and product inhibition,reducing the process efficiency of enzymatic hydrolysis of lignocellulose.So far,the cost of enzymatic hydrolysis is still high,although lots of researches were conducted in the pretreatment and enzymatic hydrolysis of lignocellulose,and further research must to be done for improving the efficiency of enzymatic hydrolysis of lignocellulose at high solid loading and reducing enzyme costs.Based on the above backgrounds,in this thesis,by screening suitable cellulase systems,optimizing enzymatic hydrolysis and saccharification processes,adding surfactants to enzymatic hydrolysis system for reducing the non-productive adsorption of enzyme onto lignin,domesticating the inhibitors-tolerance fermentation strain,etc.,the enzymatic saccharification and fermentation of different pretreated non-wood lignocellulosic materials at high solids loading were investigated for producing high-concentration sugar solution and high content ethanol.This research provides valuable reference for the establishment of sugar platform and production of cellulosic ethanol by using lignocellulose as raw material.The main contents and results of this thesis were as follows:1.Acid catalyzed steam explosion pretreated corn stover(ACSE-CS)contains many components that inhibit enzymatic hydrolysis and fermentation such as formic acid,acetic acid,furfural,etc.But their adverse effects on enzymatic hydrolysis and fermentation can be eliminated by using a simple washing process with water,thus improving the enzymatic hydrolysis efficiency and increasing ethanol yield.The effect of washing with hot water was better than that of washing at room temperature.There were difference in enzymatic hydrolysis efficiency of ACSE-CS when using different sources cellulase systems,and it was shown that the cellulase system produced from Trichoderma reesei SCB-18 was more suitable for enzymatic hydrolysis of ACSE-CS than the commercial cellulase preparations and the cellulase system from P.oxilicum.During the enzymatic hydrolysis of lignocellulose at high solids loading,the addition of surfactant Tween 80 to hydrolysis system improved the enzymatic hydrolysis efficiency.Using the fermentation broth with medium instead of the crude enzyme solution separated by centrifugation to hydrolyze the ACSE-CS,a higher efficiency of enzymatic hydrolysis was obtained,and the conversion of glucan in ACSE-CS was comparable to that by adding Tween 80.This indicated that,if the cellulase is produced on-site in the bioconversion of lignocellulose,the fermentation broth can be directly fed into the enzymatic hydrolysis system without separation,which is beneficial to reduce the devices and operating costs.2.By optimizing the feeding strategy and hydrolysis process,the high-efficient enzymatic saccharifications of different non-wood lignocellulosic materials at high solids concentration were realized by using fed-batch enzymatic hydrolysis process.For the ACSECS,80.9%of glucan conversion rate and 92.8%of xylose yield were obtained after enzymatic hydrolysis of 72 h at a solid loading of 30%through a fed-batch enzymatic hydrolysis process,and total sugar concentration of 161.2 g/L was reached.For the corn cob pretreated with dilute acid and ammonium sulfite and the bamboo chips pretreated by ammonium sulfite,the glucan conversion reached 87.6%at a solid loading of 27%and 78.5%at a solid loading of 20%,respectively,after enzymatic hydrolysis of 72 h,and the conversion of xylan reached 16.5%and 47.5%,respectively.And the concentrations of fermentable sugars obtained by the fedbatch enzymatic saccharification process were 205 g/L and 108.7 g/L,respectively.All the results showed it is potential for industrial production of high-concentration fermentable sugars from lignocellulose by using the fed-batch enzymatic saccharification process.3.As Saccharomyces cerevisiae strain LFB-1 capable of metabolizing cellobiose,glucose and xylose is difficult to normally grow in the saccharification solution obtained by enzymatic hydrolysis of the ACSE-CS because of its high content of inhibitors produced from the acidic pretreatment process.thus the domestication of strain A.LFB-1 was conducted by gradually increasing the concentration of inhibitors in the culture medium and the inhibitorstolerance strain was obtained,which can be directly used in the enzymatic hydrolysis saccharification and co-fermentation process of different pretreated lignocelluloses at high solid loading without detoxification treatment.Using this inhibitors-tolerance strain,ethanol of 46.1 g/L was produced by a fed-batch semi-simultaneous saccharification and co-fermentation(SSCF)process without detoxification at 35%of solid loading using ACSE-CS as substrate.and it was 72.8 g/L for the corn cob pretreated by dilute acid and ammonium sulfite at 29%of solid concentration,and 45.4 g/L for the ammonium sulfite pretreated bamboo chips. |
