Optimization Of Straw Pretreatment For Anaerobic Fermentation And Analysis Of Economic Practicability

With the deterioration of rural envirionment pollution and energy shortage, thedevelopment of rural household biogas has attracted the people’s attention. It has been animportant part of new countryside construction and main content of rural energy developmentin China. The development of biomass energy and the utilization of agriculture wastes foranaerobic biogas production are important for solving the energy crisis and environmentpollution in rural China. The present study compared the characteristics of household biogasand large-medium biogas plants in rural China, and also put forward the chemicalpretreatment for straw to improve its biodegradability in order to solve the problem ofhousehold biogas raw material shortage caused by large-scale farming. The effect of differentacid and alkaline chemicals pretreatments on the anaerobic digestion of rice straw, wheatstraw and corn straw was investigated, and the effective acid and alkaline pretreatmentschemicals was optimized to obtain the optimal operation parameters. On the basis ofabovement, the economic-practical application of two chemical pretreatment was analyzed toprovide the theoretical basis and data support for the efficient utilization of straw bigasficationin rual China. The main results are as follows:1. In the present study, the effect of rice straw, corn straw and wheat straw pretreated byhydrogen peroxide （H₂O₂）, hydrochloric acid （HCl）, sulfuric acid （H₂SO₄） and ethylic acid（CH₃COOH） on the biogasfication performance was investigated. The results showed the acidpretreatements with different concentration have a different effect on the the cellulose content,biogas yield and biogas efficiency of straws. The optimal treatment of H₂O₂and CH₃COOHfor bigas production was3%, and the optimal treatment of HCl and H₂SO₄was4%. Amongthe four pretreatments, the optimal treatment biogas production was2%H₂O₂, which yieldedthe biogas of315、320and285mL/g VS for rice straw, wheat straw and corn strawrespectively.2. The effect of rice straw, corn straw and wheat straw pretreated by sodium hydroxide（NaOH）, calcium hydroxide （Ca（OH）2） and aqueous ammonia （NH₃·H₂O） on thebiogasfication performance was investigated. The result showed the the alkalinepretreatements with different concentration have a different effect on the the cellulose content,biogas yield and biogas efficiency of straws. Among the three pretreatments, the optimal treatment for biogas production was8%Ca（OH）2,which yielded the biogas of325,285and320mL/g VS for rice straw, wheat straw and corn straw respectively.3. Pretreatment time has a significant effect on the anaerobic digestion of straws.Degradation of H₂O₂、CH₃COOH、HCl、H₂SO₄and NaOH on the cellulose and hemicelluloseincreased with the increase of pretreatment time. There was no significant difference between1d and4d pretreatment on the cellulose content for Ca（OH）2and NH₃H₂O, while the7dresulted in a great decrease on it. The biogas efficiency of straw pretreated by H₂O₂,CH₃COOH, Ca（OH）2and NH₃·H₂O increased with the increase of pretreatment time whilepretreated by HCL and H₂SO₄have the higest value in the4d pretreatment.4. Pretreatment temperature can affect the anaerobic digestion of straw. amongthe15℃,35℃and55℃pretreatment, no significant difference was found between the15℃and35℃on the cellulose, hemicellulose and biogas efficiency of straw pretreated byH₂O₂, CH₃COOH, Ca（OH）2and NH₃H₂O, while at the55℃, the cellulose, hemicellulosecontents greatly decreased and biogas efficiency significantly enhanced. The cellulose andhemicellulose contents of straw pretreated by HCl, H₂SO₄and NaOH significantly decreasedand its biogas efficiency increased with the rise of the pretreatment temperature.5. The pretreatment time, chemicals concentration and inoculum amount of wereselected as parameters to investigate the effect of H₂O₂and Ca（OH）2on rice strawbiggasfication. The result showed the the optimal conditions of H₂O₂treatment foranaerobic digestion were6.18d treatment time,2.68%H₂O₂and1.08inoculum ratio,which yielded the max theoretical methane of287.90mL/g VS. The optimal conditions ofCa（OH）2treatment for anaerobic digestion were5.89d treatment time,9.81%H₂O₂and45.12%inoculum content, which yielded the max theoretical methane of217.82mL/gVS. The determination coefficient of H₂O₂and Ca（OH）2optimal model are95.2%and96%, this indicated two optimal models to predict the the actual anaerobic fermentationprocess have high credibility. To verify the the incredibility of predicted resutls, H₂O₂andCa（OH）2pretreatment was used for straw anaerobic digestion at the optimal conditions. Theresult showed the methane yield of rice straw pretreated by H₂O₂and Ca（OH）2were90.36mL/gVS and225.3mL/g VS, respectively, which was increased by88%and73.3%,respectively, compard to the untreated rice straw. In conclusion, H₂O₂and Ca（OH）2can usedas pretreatment for rice straw biogasifciaton, and both can effectively improve the energyconversion efficiency of straw.6. The anaerobic digestion of cow manure and rice straw mixture was conducted toinvestigate the effect of pretreatment on the mixture anaerobic digestion. The results showedthat the pretreatment on straw after can significantly improve the biogas yield and biogas yield efficiency. Compared with the straw without pretreatment, the biogas yield and biogasyield efficiency of pretreated straw mixture was increased by29%-43%and24%-31%. Thus,H₂O₂and Ca（OH）2pretreatment is an effective method for single material and mixturematerial to improve the anaerobic digestion.7. The economic practicability analysis of chemical pretreatment for straw anaerobicdigestion indicated that compared with the use of raw coal for fuel, the use of H₂O₂andCa（OH）2as pretreatment for straw anaerobic digestion can save311.4and312.4Yuan peryear per household, respectively. The evaluation of straw biogas potential and the estimationof greenhouse gas emission reduction showed that the direct buring of straw will annuallycause the8.85×10⁸t CO₂in China, if these straws was all used for anaerobic digestion, notonly12.97×10¹¹m³biogas can be produced per year, but the reduction of6.70×108t CO₂emission can be made. Considering the advantage of chemical pretreatments for stawanaerobic digestion in energy conversion efficiency improvement and the greenhouse gasemission reduction, our study recommend that chemical pretreatment technology should bewidely promoted for straw anaerobic digestion in the rural househould biogas development.