| The world energy demand has been incessantly thriving due to the rise in population and modernization.Exploitation of renewable and sustainable energy is the optimal solution to energy crisis.Biomass is one of the most abundant renewable energy sources in the world,more and more attention has been paid to the development of the biomass energy in recent years.However,organic wastes could not meet the huge demands of biomass resource,cultivation of energy crops are required to close the gap between the supply and demand of biomass feedstock.To avoid the conflicts between the production of food and energy,large amounts of marginal land resources in China could be utilized.This study comprehensively investigated the anaerobic digestibility of a typical energy crops,switchgrass,and explored the optimal condition of anaerobic digestion.This study also carried out the research on optimizing the temperature during pretreatment process.Pyrolysis of digestate was then carried out to assess the energy recovery.Firstly,comparison of 7 pretreatment methods were performed by the batch anaerobic digestion of pretreated switchgrass.Results collectively showed that all pretreatments could significantly improve the anaerobic digestibility of switchgrass.Experimental methane yield of switchgrass after pretreatments enhanced from 46.3 mL gvs-1 to 197.2 mL gvs-1,with a maximum improvement of 325.61%at 4%NaOH,which illustrated pretreatments could enhance the AD performance greatly.3%KOH pretreatment also showed good effect and more environmental-friendly.Modified Gompertz model fitting illustrated Ca(OH)2 and steam explosion pretreatment could remarkably reduce the lag phase time.Secondly,according to the results above,response surface methodology experiment was designed to to find out the optimal level of factors in order to enhance methane production from switchgrass.Methane yield raised with the increase of organic loading from 3 to 27.3 gVS/L,and then slightly decreased.It was observed that groups having lower F/I ratio had higher methane yield.And with the increasing of initial pH to 7.67,methane production increased firstly,and then decreased gradually.Results showed that the highest experimental methane yield(EMY)was calculated to be 299.48 mL gvs-1 at F/I ratio of 0.50,pH of 6.96 and organic loading of 27.30 gVS/L respectively.At last,combine the conclusion above,the optimum anaerobic digestion condition of switchgrass after 3%KOH,20,37,55(24 h),and 120?(30 min)were as follow:organic loading 24 gVS/L,F/I ratio 1,and initial pH 6.96.Results comprehensively showed that 120 ? 30 min pretreated switchgrass reached the highest the methane production of 288.4 mL gvs-1,significantly higher(P<0.01)than other groups.No significant difference(P>0.05)was found between the methane production at 20,37,55? groups.Besides,this study recycled the digestate from 20? pretreatment group,and carried out a pyrolysis experiment.The experiment gained 422 mL gvs-1 pyrolysis gas which has energy yield of 4.43 MJ kgvs-1.Considering the anaerobic digestion process obtained methane yield of 235.5 mL gvs-1,these two experiments gained 12.88 MJ kgvs-1 totally,which means a recovery of 85.3%energy in switchgrass.Result from this study indicate that planting switchgrass on marginal land and utilizing it through pretreatments and anaerobic digestion might be a promising method to improve energy production in future. |
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