Biomethane Production From Lignocellulosic Biomass By Two-phase Anaerobic Digestion And Energy Recovery From Solid Residue

Crops straw is an important type of lignocellulosic biomass,which is biodegradable in abundance and wide range.Its application has been proven to be suitable for biogas projects.The cellulose,hemicellulose and lignin are the main components in the lignocellulose,the interaction of which forms an anti-biodegradation barrier in the spatial structure and make it difficult to degrade.The existing single-phase anaerobic digestion(AD)process generally has many problems,such as low solid concentration,scum crusting,large amount of biogas slurry,and low value-added products from biogas residue.Therefore,aiming to solve these issues,the metabolic regulation and pretreatment methods of AD of straw silage and dry straw were explored,and the bio-methane potential of lignocellulosic hydrolysate was investigated,which would provide theoretical and technical support for subsequent applications.In addition,the hydrothermal carbonization(HTC)was used to prepare carbon-based materials that can be used as fuel,functional adsorbent,fertilizers,etc.The energy recovery from biogas residue was realized.This study is of great significance for the efficient and economic operation of large scale biogas plants.The main researches and conclusions are as follows.(1)Corn straw silage(CSS)is one of the lignocellulosic biomass available for biogas production.The possibility and optimal controlling strategy for AD of CSS were investigated.Leaching bed reactors(LBR)were operated at different pH(6,7,8).The results showed that alkaline pH was beneficial for the degradation of lignocellulose with TS and VS removal rates of 35.3%and 64.8%in 7 days.Maximum volatile fatty acids(VFAs)concentration of 19.34 g/L,corresponding to VFAs yield of 0.31 g/g VS,was reached at pH 8.0 with acetic(HAc),propionic(HPr),and butyric(HBu)acids as dominant VFAs.The subsequent microbial analysis by high-throughput sequencing indicated that abundant bacteria,which were functional in the organics degradation and VFAs production,were Bacteroidetes(34.23%),Firmicutes(26.70%),Actinobacteria(11.53%),and Proteobacteria(9.52%).Methanogenic performance and microbial communities including both bacteria and archaea in up-flow anaerobic sludge bed(UASB)reactor treating acidogenic metabolites from acidification of CSS were investigated.Gradient organic loading rates(OLRs)were adopted to investigate the conversion of volatile fatty acids(VFAs)characterized by high concentration of HAc and HPr.The removal rate of chemical oxygen demand(COD)reached above 80.0%at OLR of 8 g/L·d and volumetric methane production rate improved significantly as the OLR increased.The abundances of some representative acetogenic bacteria such as Proteobacteria(14.18%)and Bacteroidetes(11.16%)increased obviously,which contributed to the oxidation and decomposition of organic acids.Acetotrophic methanogen(Methanosaeta)and hydrogenotrophic methanogen(Methanobacterium)played synergistic roles with acetogens to efficiently complete the VFAs conversion.Syntrophobacter occurred in the bacteria to deal with the degradation of HPr.When OLR was 10 g COD/L·d,HAc and HPr in effluent were accumulated and failure of methane production occurred.This study will be of great importance to guide the application of two-phase AD of agricultural waste in full-scale plant.In the whole process,the results showed that methane yield of 143.4 mL CH4/g VS was obtained.pH and OLR controls in two-phase AD were feasible for methane production from CSS.(2)Dry corn straw is an another kind of crucial lignocellulosic materials(LM).Pretreatment using organic acids including HAc and HBu at 2,4,and 6 g/L was applied to reduce the recalcitrance of LM in methane production by two-phase AD.The structural and compositional changes of solids and aqueous phase were analyzed.It showed that higher severity factor was obtained at higer acid concentration.However,the VFAs yield exhibited the opposite trend.Pretreatment with 2 g/L HAc/HBu at 120℃ was optimum for the acidification,under which the net VFAs production could reach the maximum yield of 0.172 g/g VS and 0.189 g/g VS,respectively.The decrease of VFAs yield was due to the feedback inhibition on the microbial metabolism.The species of organic acids also influenced the metabolic pathway of acidogenesis.The HAc was predominant in the HAc-pretreated group while the difference of HAc and HBu concentrations was small.The acidogenic metabolites can be efficiently transformed in UASB with OLR up to 12 g COD/L·d.The methane yields of 94.7 mL CH4/g VS and 98.9 mL CH4/g VS were obtained for HAc/HBu-pretreated groups,respectively,which were approximately 67.1%and 70.0%of BMP of dry corn straw.The hydrothermally pretreatment with organic acid could provide important information for subsequent application.(3)Biogas production from lignocellulosic hydrolysate is of great potential for LM.The performance of AD of lignocellulosic hydrolysate was studied.Using anaerobic sludge(AnS),different inoculum to substrate ratios(ISRs)of 0.5:1,1:1 and 2:1 were investigated.The distribution of gaseous and aqueous products were analyzed.The results showed that acidification occurred at ISR of 1:1 and even 2:1 due to the fast degradation of hydrolysate.The methane production could not proceed normally.The highest VFAs yield was obtained at ISR of 0.5:1 with negligible amount of methane.VFAs composition analysis showed that HBu was the predominant acid,followed by HAc and HPr.Based on the fermentative properties of hydrolysate,it was proposed that two-phase AD was appropriate for the methane production of lignocellulosic hydrolysate in this study.The effects of AnS and aerobic sludge(AeS)on the acidogenic performance of hydrolysate were compared.The optimum VFAs yields were 0.38 g/g COD for AnS and 0.32 g/g COD for AeS with HRT of 5 d,respectively.The equivalent COD value of VFAs accounted for 89.3%of the soluble COD in digestate.The bacterial diversities of inocula and digestates were analyzed by high-throughput sequencing.Two origins of inocula had distinct bacterial structures,but they did share core communities that included Firmicutes,Chloroflexi,Proteobacteria and Bacteroidetes at phylum level.The bacterial communities of both digestates changed significantly as compared with those in inoculum.Firmicutes was absolutely dominant in all the bacterial species.Therefore,the AeS could be an option as the acidogenic inoculum.In methanogenic process,VFAs obtained in acidogenic stage could be efficiently converted into methane.The ultimate methane yield at OLR ≤ 12 g/L·d could reach 279.5 mL CH4/g COD and 270.1 mLCH4/g COD for AnS and AeS acidogenic digestate.(4)The hydrothermal carbonization(HTC)technology was used as a mild thermochemical method to treat biogas residues with high moisture content and to prepare carbon-based materials.The temperature range of 180-240℃ was selected for hydrothermal treatment of solid residues derived from AD of CSS.The yield and basic physicochemical properties of hydrochar were analyzed.The carbon resources in aqueous phase was utilized by AD for energy recovery.The results showed that the hydrochar yield was the highest at 180℃.As compared with the biogas residue,the calorific value of the hydrochar prepared at 210℃ and 240℃ increased by 14.2%and 16.6%,respectively.The specific surface areas,which are closely related to the adsorption capacity,were 34.8 m2/g and 27.1 m2/g,respectively.The pore structure was mainly mesoporous.It was further demonstrated by FTIR that hydrochars rich in oxygen-containing functional groups and aromatic ring were obtained through a series of reactions such as dehydration,decarboxylation,polycondensation,and aromatization.Meanwhile,the hydrothermal carbonization wastewater(HTCWW)produced mainly included short chain organic acids,phenolics and heterocyclic compounds.Through batch experiments at gradiant organic loading,it was found that due to the increase in the concentration of VFAs,sugars and other organic substances that were easily utilized by microorganisms,the methane production showed an upward trend when the initial organic loading was ≤8 g COD/L,reaching 275.9 mL CH4/g CODremovat and 277.6 mL CH4/g CODremoval,respectively.However,as the organic loading continued to increase,the concentration of toxic substances was elevated accordingly.The organic acids could not be consumed and began to accumulate,which resulted in the decrease of methane production.From the perspective of microbial community,the abundance of phenol-degrading bacteria such as Advenella and Pseudomonas increased significantly,but the diversity of bacteria and archaea declined.Analyzing the inhibitory factors such as VFAs and NH4+/N concentrations,the distribution and fluorescence intensity of extracellular polymer substances(EPS),as well as the structure and diversity of microbial communities,it could be concluded that the toxicity to microorganisms or inhibition of metabolic activity existed due to the presence of representative pollutants in HTCWW.Therefore,the initial organic loading should be controlled at ≤ 8 g COD/L.In summary,the energy recovery can be achieved by coupling HTC with AD,which provides an alternative for the utilization of biogas residues.