Study On Closed-circuit Circulation Process For Ethanol-methane-organic Fertilizer Production With Straw On The Pilot Scale

Converting abundant lignocellulose into renewable energy is significant to alleviate the energy crisis caused by the over-exploitation of fossil fuels,such as petroleum and coal.Based on the closed-circuit circulation process,the wheat straw was converted into ethanol,methane and organic fertilizer on a large scale in this study,achieving the clean and high-value conversion of agricultural waste.The main study contents and results are as follows:(1)The subcritical water was used for wheat straw pretreatment,and the heating,heat preservation and cooling processes of pretreatment were stable.After pretreatment,the hemicellulose in wheat straw was degraded,and its content was reduced from 17.65% to 1.05-3.06%.Meanwhile,the cellulose and lignin were retained,and cellulose increased from 26.87%to 30.01-30.67%,lignin increased from 42.91% to 58.39-67.23% after pretreatment.Pretreatment destroyed the microscopic morphology of wheat straw,after which the surface of the structure changed from flat and dense to disorderly.Besides,the pretreatment solid residue yield(67.10%)after biogas slurry recycling was higher than that of the unrecycled groups(55.42-57.32%),and the reducing sugar concentration(1.88 mg/m L)in the pretreatment liquor was lower than that of the unrecycled groups(1.96-1.99 mg/m L).The above results indicated that subcritical water is a clean and efficient pretreatment method,but biogas slurry recycling reduced the pretreatment efficiency.(2)The cellulase of CEL-01 with the highest enzymatic efficiency was selected from the five cellulase enzymes,and the addition amount was 10 FPU/g.Two different methods,SSF and SHF,were used to convert the pretreated wheat straw into ethanol.The final ethanol concentrations obtained in the SSF-1 and SHF-1 groups were 7.54 mg/m L and 7.64 mg/m L,respectively.There was no obvious difference between the two groups,and the SSF with lower production cost was selected for subsequent recycling fermentation.Due to the low enzymatic hydrolysis efficiency,the final ethanol concentration was 6.97 mg/m L in SSF-2 group,which was 7.56% and 8.77% lower than those of the SSF-1 group and SHF-1 group,respectively,indicating that biogas slurry recycling was suppressive for ethanol conversion.Compared with fermentation,enzymatic hydrolysis had a more profound impact on conversion efficiency of ethanol.(3)The pretreatment waste liquor and fermentation residue were converted into methane and organic fertilizer by anaerobic digestion.The methane volumes of pretreatment waste liquor in SSF-1,fermentation residue in SSF-1,pretreatment waste liquor in SSF-2,fermentation residue in SSF-2 were 233.72,523.18,248.65 and 87.77 L,respectively.The p H is significantly negative with the concentration of acetic acid in anaerobic digestion.Compared with the fermentation residue,the p H of the anaerobic digestion system was more stable when the pretreatment waste liquor is regarded as the substrate.In addition,the solid residue after anaerobic digestion met the technical requirements for organic fertilizer in the agricultural industry standard NY 525-2012.(4)The mass balance of subcritical water pretreatment,ethanol conversion and anaerobic digestion was conducted for SSF-1 and SSF-2,and the mass loss rates were-0.68-4.41%.The yields of ethanol,organic fertilizer and biogas were 0.08-0.09 g/g(wheat straw),0.55-0.79 g/g(mixture)and 0.05-0.10 L/g(mixture),respectively.Similarly,the mass balance of subcritical water pretreatment,enzymatic hydrolysis and ethanol fermentation was conducted for SHF-1,and the mass loss rates were 1.40-5.90%.The ethanol yield is 0.09 g/g(wheat straw).Based on the mass balance,the recycling process consisted of SSF-1 and SSF-2 was considered as a whole to carry out technical-economic balance.It was found that the total cost of recycling process is 697.85 yuan,but the price of ethanol,methane and organic fertilizer is only 18.62 yuan,which showed that the economic efficiency of the recycling process is poor under the conditions of this study,and some measures should be taken to improve economic efficiency in the future.