Biomass Modification By Alkali And Biological Conversion Of Methane To Methanol For Anaerobic Digestion Of Lignocellulosic Biomass

Lignocellulosic biomass is an abundant and renewable feedstock for bioenergy production,which could improve energy and environment security by decreasing dependence on fossil fuels and reducing greenhouse gases emission.Biogas production from lignocellulosic biomass through anaerobic digestion（AD）has recently been suggested because of the simple process,reliable performance,and low greenhouse gas emissions.One issue that impedes commercialization of this technology is the recalcitrance of lignocellulosic bioimass to digestion.Although various pretreatment methods have been studies,there have been limited publications in comparison of these methods using the same source of promising energy crops,such as giant reed.Another issue is that biogas has a low energy density,and is difficult to store,transport,and distribute.A plausible approach is to convert biogas to methanol with methanotrophs.However,the methanotrophs could be inhibited by H₂S present in biogas.In this study,liquid hot water（LHW）,NaOH pretreatment with or without leachate reuse,and Ca（OH）2 pretreatment of giant reed were compared for improved enzymatic digestibility and biogas production.Furthermore,simultaneous calcium hydroxide（lime）treatment and solid-state anaerobic digestion（SS-AD）of corn stover was further investigated for improving biogas production with lower additional cost.In order to address the H₂S inhibition issue in biological conversion of raw biogas to methanol,a methanotrophic consortium,HTMC,was isolated from AD systems in presence of 4000 ppm hydrogen sulfide（H2S）.Microbial community of the HTMC was characterized,and effects of H₂S and gas/medium composition on this consortium was evaluated with respect to cell growth and methanol production.Experimental results are summarized as follows:（1）Compared to LHW pretreatment,alkaline pretreatment retained more of the dry matter in giant reed biomass solids due to less severe conditions.Under their optimal conditions,LHW pretreatment（190 oC,15 min）and alkaline pretreatment（20 g/L of NaOH,24h）improved glucose yield from giant reed by more than 2-fold,while only the alkaline pretreatment significantly（p < 0.05）increased cumulative methane yield（by 63%）over that of untreated biomass（217 L/kg VS）.LHW pretreatment obtained negative net electrical energy production due to high energy input.Alkaline pretreatment achieved 27% higher net electrical energy production than that of non-pretreatment（3,859 k J/kg initial total solids）,but alkaline liquor reuse is needed for improved net benefit.（2）The NaOH pretreatment with leachate reuse increased glucose yields during enzymatic hydrolysis by 2.6-fold,and methane yields during anaerobic digestion by 1.4-to1.6-fold,respectively.However,NaOH pretreatment with leachate reuse still had a negative net benefit（i.e.,revenue from increased energy production minus chemical cost）.Pretreatment with 7-20%Ca（OH）2 not only improved glucose yield and methane yield by up to 2.3-fold and1.4-fold,respectively,but also obtained a net benefit of$1.1-5.8/公吨dry biomass.Thus,Ca（OH）2 pretreatment was shown to be more feasible than NaOH pretreatment for biogas production from giant reed.（3）SS-AD of corn stover with different lime addition（0,2,3.5,and 5%）at different feedstock to inoculum（F/I）ratios（6,8,and 10）was conducted.The addition of 2-3.5%lime improved methane yield from 118 L/kg VS to 173-182 L/kg VS（46-54%increase）during SS-AD of corn stover at an F/I ratio of 6.SS-AD of corn stover was upset at F/I ratios of 8 and 10due to accumulation of volatile fatty acids,which was not mitigated by the lime addition（2-5%）.Microbial community analysis indicated that 3.5%lime treatment substantially increased relative abundance of microbes（Thermotogae and Euryarchaeota at phylum level,and Alkaliphilus,Bacillus,S1,and Methanobacterium at genus level）that play important roles in biomass degradation and methane generation.Compared to SS-AD of corn stover without lime addition,simultaneous lime treatment（2-3.5%lime addition）and SS-AD can obtain net benefits of$8.3-12.0/公吨TS from corn stover via SS-AD and CHP.（4）A methanolthrophs consortium,named HTMC,Cyanobacteria（represented by 32.04% of the total bacterial sequences）being the most predominant one,followed by Proteobacteria（31.55%）,Bacteroidetes（29.94%）,Chlorobi（5.87%）.HTMC grew stably on methane/air mixtures containing 4000 ppm H₂S,and cell yields from methane and methanol production showed no significant（p > 0.5）difference at 0 ppm,1000 ppm,2000 ppm,and 4000 ppm percentages of H₂S.The optimal cultivation conditions for cell yield from methane were methane to air ratio 1:4,p H 6.8,and temperature 37℃,and 0.333 g cells/g methane was obtained.（5）The optimal cultivation conditions for methanol production were phosphate concentration 50 m M,format concentration 100 m M,methane to air ratio 1:1,and 0.28 mg/ml of methanol production,0.22 mol /mol of methane yield were obtained.This new isolate may be used in microbial conversion of methane to methanol without scrubbing of H₂S.