| In this study,three types of lignocellulosic biomass(switchgrass,wheat straw,and corn straw)were used as raw materials.After the subcritical water pretreatment of materials,two processes of ethanol-methane coupling production by closed-circulation(BMS-CC)based on the recycling of methane fermentation liquid and ethanol fermentation waste under biomass materials shiftting(BMS)were established.With the purpose of providing reference for future practical production applications,the influences of biomass materials shitting on the two processes and fuel production were studied.The main results were as follows:1.Subcritical water pretreatment:The total reducing sugar content and residue yield of 30 cycles in the pretreatment liquids of SW and SC were divided into two different stages by the BMS point(Cycle 12),and they were relatively stable within the same material stage.When switchgrass was changed to wheat straw and corn straw respectively,the residue yield of the two processes increased overall,and the average residue yield(%)increased from 55.85 and 56.54 to 70.62 and 72.56,respectively;the total reducing sugar content(mg/L)of the pretreatment liquid decreased as a whole from 2067 and 1805 to 700 and 633 on average,respectively.The relative content of main components in residues after the BMS were significantly increased.Before and after BMS,the average content(%)of cellulose,hemicellulose and lignin in the SW were 32.55 and 43.03,8.14 and 14.27,12.11 and 13.33,respectively;while the average content(%)of cellulose,hemicellulose and lignin in the SC were 32.60 and 42.43,7.4 and 13.82,12.56 and 14.45,respectively.Under the experimental conditions,the average removal rates(%)of cellulose,hemicellulose,and lignin were 6.84,13.60 and 8.77 for switchgrass,3.62,8.00,and 7.32 for wheat straw,3.72,9.34,and 7.27 for corn straw,respectively.It is obvious that more lignin was removed from biomass,and the dissolution of hemicelluloses was promoted,and most of the cellulose remained after SWP.Compared with wheat straw and corn straw,SWP has a more significant effect on switchgrass.2.Ethanol fermentation:The changes of glucose concentration after enzymatic hydrolysis of residues in two process were different.By the BMS point(Cycle 12),both SW and SC were divided into two different stages.When switchgrass was changed to wheat straw,the average glucose concentration in two stages(g/L)was 11.43 and 11.18,and the enzymolysis rates(%)were 32.16 and 23.63,respectively;when converted to corn straw,the average glucose concentration in two stages(g/L)was 13.73 and 10.05,respectively,and the enzymolysis rates(%)were 38.17 and 18.26,respectively.The average concentration of glucose in 30 cycles of SW and SC(g/L)was 11.27 and 10.43,respectively.After digestion,the glucose content in the SW was higher than that in the SC.Changes of ethanol production were different before and after the BMS.The average concentrations of ethanol(g/L)in the SW and SC were 6.14 and 5.12,respectively.Before BMS(Cycle 1-11),the two processes were switchgrass,and the average concentrations(g/L)were 6.51 and 6.84,respectively;the ethanol conversion rate(%)was 108.20 and 102.49,respectively;And the difference was not significant.After BMS(Cycle 12-30),the average concentration(g/L)of SW and SC was 5.94 and 4.22,respectively,and the ethanol conversion rate(%)was 104.34 and 97.40,respectively.There was a more significant decreasing of ethanol production in SC.3.Methane fermentation:The anaerobic fermentation process can be basically divided into three periods,Rapid growth period(0～2 d),Slow growth period(2～8 d),and the Stable period(after 8 d).The production process of different cycles was similar in the SW and three stages were basically observed with no significant difference under BMS.However,it can be divided into two categories in SC.One was a three-stage process with high gas production,and the other was a two-stage process including only a rapid growth period(0 to 2 days)and a stable period(2 days later)with low gas production generally.The total gas production(mL)of SW and SC before and after BMS was 1027 and 1172,704 and 775 respectively;the methane concentration(%)was 44.18 and 51.06,44.55 and 48.32,respectively;the methane production(mL)was 490.20 and 599.23,315.24 and 373.33,respectively.Although the total reducing sugar content after BMS was significantly reduced,all three methane production indicators increased,indicating that the bacterial population was gradually acclimated and the ability of methanogenesis was increased.Meanwhile,the remaining reducing sugars after fermentation in the whole BMS-CC were all around 200 mg/L,having nothing to do with the initial reducing sugar level,which was related to the existence of a threshold for methanogens in the synthesis of methane using substrates.In addition,the promotion to methane fermentation by reusing the ethanol fermentation waste slag was confirmed.4.Community analysis of methane fermentation system:16S rDNA Sequencing analysis showed that the species abundance and community diversity of the two process fermentation systems were high with the dominant species succession.According to the PCA analysis at the OUT level and analysis of the dominant species in the fermentation system,the community succession rules were different at different stages.Compared with the Initial community(Y),the diversity and changes in the SW were significant with the increase of Cycle,but there was little difference at the process end(SW.27～30),indicating that after long-term fermentation and acclimation,the microbial structure of the fermentation system was similar and tended to be stable.However,there were quite difference among the Initial community,the BMS stage(SC.11～14),and the process end(SC.26～30)in the SC,with the community structure of the adjacent circulation being similar,which showed that the BMS-CC process gradually evolved and eventually formed a more stable community structure.There were significant differences of methanogenic bacterias between the two processes.In the SW the methanogens were mainly hydrogen-utilizing methanogens(Methanobacterium spp.),while both the methanogenic bacteria Methanobacterium(H2/CO2 utilization type)and Methanosaeta(acetic acid utilization type)were dominant bacterias in the SC,indicating that the main pathways for substrate utilization and methanogenesis were different,resulting in different changes of methane production.In addition,the functional analysis of the dominant species at the genus level showed that methane fermentation involves three types of bacteria:fermenting bacteria,such as cellulose,glucose,aromatic compounds degrading bacteria;acidified bacteria,such as Anaerolinea,Bacteroides,Pseudobutyrivibrio,Syntrophobacter,etc.;Methanogens such as Methanobacterium,Methanosaeta and Methanolinea.And three Proteobacteria strains,namely S yntrophorhabdu s,Syntrophus,and Dechloromonas,of degrading aromatic compounds were also identified.5.GCMS analysis of pretreatment and methane fermentation liquids:There were kinds of substances in each sample(over 150 kinds on average),and it all can be classified into furans,ketones,phenols,aromatics,aldehydes,esters,etc.according to the main functional groups in the two processes indicating that there were no significant differences in the types of liquid components.Under the experimental conditions,the main degradation products of SWP were phenols,esters,aldehydes and ketones.The higher content of furan substances in the SC was the main difference between the pretreatment liquids of SW and SC.In addition,decomposition of lignin in SWP produced a large number of aromatic poisons such as phenols,aldehydes,etc..However,the amount and relative content of the substances in most of the categories(except esters and hydrocarbons)reduced greatly after the methane fermentation;And after each methane fermentation,the main components types changed into esters,hydrocarbons and phenolic compounds basically.With the circulation increasing,other types of substances were consumed by the metabolic activities or reactions,and inhibitors such as 2,3-dihydro-Benzofuran,2-Methoxy-4-vinylphenol,Vanillin,2,4-Di-tert-butylphenol,Apocynin,2,4-dimethoxy-Phenol,2,6-dimethoxy-Phenol etc.were significantly reduced after fermentation bringing about non-mass accumulation in the BMS-CC,which indicated that though inhibitors(such as aromatics,ammonia nitrogen,etc.)still accumulated in the fermentation system,the domesticated fermenting bacteria groups had a certain ability to adapt to toxic substances. |
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