| The production of bioethanol from renewable lignocellulose biomass is significant for the present increasingly serious energy crisis and environment pollution problems.High solids enzymatic saccharification and fermentation with the advantages of improved final products concentration,lower capital costs due to fewer reactors,less energy consumption and reduced disposal costs is essential for cellulosic ethanol production.However,in reality,the problems caused by the increased solids loading led to a decline in the substrate conversion efficiency,partially offset the benefits of high solids system.In order to strengthen the important technology in cellulosic ethanol production process to reduce its economic costs,this paper expanded the related research based on the key issues of the low conversion efficiency appeared in high solids system.Sugarcane bagasse(SCB)collected from the sugar industry was used as the substrate to study the high solids process.The pretreatment technology,enzymatic hydrolysis process and the probable inhibition factors,high solids pre-hydrolysis simultaneous saccharification and fermentation(DSSF)technology were systemically studied to improve the substrate conversion efficiencies.Meanwhile,the unfermented stillage mainly containing C5 sugars was sequentially subjected to biogas production to improve the yield of the utilized biomass.Based on the above study,the technology with low-cost cellulosic ethanol production,the utilization of the multicomponent of SCB was developed.Four different pretreatment approaches including alkali,ionic liquid(IL),Microwave irradiation(MWI)-alkali and IL-alkali pretreatments were compared based on the alkali methods.MWI and IL assisted methods were chosen to reinforce the alkali pretreatment process to improve the saccharification efficiency of SCB.The results suggested that IL assisted treatment brought the highest lignin removal of 78.51% and the glucan content in the substrate was increased to 69.24%.After enzymatic hydrolysis of 72 h,the glucan and xylan conversion efficiency reached about 99.51% and 92.76%,respectively.The field emission scanning electron microscope(SEM),X-ray diffraction(XRD)and Energy dispersive spectrometer(EDS)analyses showed that the morphology,crystal and photon energy performance of the substrates after pretreatment and enzymatic hydrolysis were all differently changed.The characteristics diffraction peaks of the regenerated cellulose after [Amim]Cl treatment was coincided with the diffraction peaks of the cellulose II,and there were significant enhancement in photon energy for the carbon and oxygen element in these samples.The analysis of the fractal-like kinetic showed that for glucan hydrolysis,cellulase was more likely to combine with the IL assisted treated samples.For xylan degradation,xylanase had the strongest capacity to combine with the alkali-treated residuals.Fractal dimension changes suggested that the effects of different pretreatment methods on xylan hydrolysis were not apparent.High solids enzymatic hydrolysis was enhanced by fed-batch and mixture enzyme feeding modes.The results indicated that fed-batch system could overcome the challenges such as increased viscosity resulting in mass transfer limitations,stirring difficulties emerged in high solids enzymatic hydrolysis process.(1)The batch enzymatic hydrolysis was carried out with solids loading of 9%,12%,15% and 18% dry mass(DM)(w/v)to investigate the substrate conversion efficiencies.The liquefaction time was similar for the slurries with 12% and 15%(w/v)initial solids,12% was 9 h,and for 15%(w/v)solids was 10 h.After enzymatic hydrolysis of 96 h,the glucan and xylan conversion efficiency of 80.4% and 81.0% were achieved respectively in the slurry with 15% initial solids system.To facilitate the subsequent fed-batch hydrolysis,15%(w/v)solid loading was selected based on its high conversion efficiencies.(2)High solids fed-batch enzymatic hydrolysis was initiated with 15%(w/v)solids loading,during the hydrolysis,the system viscosity was measured at specific time points.8%,7% and 6%(w/v)new substrate were fed consecutively when the viscosity was reduced to the minimum value,respectively.The whole cellulase(8.5 FPU/g substrate)added at the beginning of the reaction produced the highest sugar yields,with approximately 231.7 g/L total sugars and 134.9 g/L glucose being obtained after enzymatic hydrolysis of 96 h.(3)The mixture enzyme could achieve higher conversions of lignocellulosic biomass.The hydrolysates produced from 25%(w/v)solids system with cellulase loading of 20 FPU/g solids,hemicellulase loading of 1200 IU/g solids were able to obtain 241.91 g/L total sugars after 96 h enzymatic hydrolysis,and the glucose and xylose concentration reached 155.73 g/L and 63.16 g/L with conversion efficiencies of 98.58% and 94.87%,respectively.The polysaccharides were substantially degraded.Supplementing cellulase with hemicellulase played a key role in the efficient conversion process of high solids substrate.The analysis of the probable mechanism behind the decreasing conversion showed that,(1)Hemicellulose directly affected the glucan degradation.(2)Subsequent lignin analysis found that a little amount 0.05 g/g solids of lignosulfonate could enhance the SCB enzymatic process in the present experiment conditions.(3)Oleyl alcohol was used to replace water in order to investigate the water-solids ratio while keeping the system viscosity similar.By substituting part of the water,the water-solids ratio could be altered but with a constant viscosity of slurry.The results showed that glycerol was more seriously inhibiting the glucose production than sorbitol,while glycerol has apparent effect in improving hemicellulose hydrolysis,its mechanism was unclear in the present study.(4)To investigate the role of the product inhibition in high solids enzymatic system,various amounts of glucose were added to the hydrolysis of SCB.Study of the effects of different substrate,cellulase and glucose loading on the substrate conversions found that the produced glucose and xylose concentration decreased as the glucose loading increased.The main hydrolysis product of glucose inhibited the substrate conversions.Dayed inoculation of simultaneous saccharification and fermentation i.e.DSSF technology,was developed to alleviate the different optimum working temperature of enzymes and yeast in high solids fed-batch SSF process,and the pre-hydrolysis time of DSSF was determined.The experiments found that 24 h of pre-hydrolysis time had better effect on the subsequent fermentation than other parallel groups.Increasing solids loading from 18% to 36%(w/v)enhanced glucose production,while ethanol conversion efficiency was decreased.When the substrate loading was higher than 24%(w/v),the ethanol conversion efficiency decreased obviously.DSSF,with batch feeding mode of 36%(w/v)hydrolyzed medium to 24%(w/v)fermentation system,achieved as high as 68.05 g/L(74.13% theoretical yield)ethanol concentration with 30%(w/v)solids loading at fermentation of 96 h.After evaporation,the residual mainly containing C5 sugars obtained 306.97 mL/g volatile solids(VS)methane through anaerobic digestion after inoculating the medium temperature anaerobic methanogenesis for 6 days fermentation.A total heat values obtained in the ethanol-gas process was 340.08 KJ,with an energy conversion efficiency of 59.51% in DSSF process with 30%(w/v)substrate loading(heat value 571.50 KJ).Sequential bioethanol and biogas production enhanced the yield of utilized biomass and augmented the products diversity.The research of this paper relates to the pretreatment of cellulosic biomass,high solids enzymatic saccharification and its probable inhibition mechanism,the improvement of DSSF technology and followed by C5 sugars to produce biogas.These works strengthened the substrates conversion process to a certain extent,and achieved high conversion efficiency in high solids enzymatic saccharification and simultaneously enhanced the final ethanol production yield.The research of this thesis is significant for improving the economics of cellulosic ethanol production and then promotes its industrialization process. |
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