Conversion Characteristic And Enhancement Method Of Biomass Hydrothermal Hydrolysis With Anaerobic Fermentation For Hythane Production

Biomass energy is a green and eco-friendly renewable energy.Accelerating the development and utilisation of biomass energy can not only solve the energy shortage caused by the massive consumption of fossil fuels,but also improve the increasingly serious problem of environmental pollution.Biohydrogen and biomethane(named as biohythane)production by anaerobic fermentation is regarded as a low-cost and sustainable approach for the gaseous biofuel production.Biomass wastes such as microalgae and lignocellulosic biomass can be converted into biofuels by anaerobic microorganisms at mild temperatures(35–55°C)and ambient pressures.However,the compact cell wall structure of biomass hinders the accessibility of microorganisms to intracellular organic matters,thereby limiting the transmembrane transfer of small molecular substrates.The lack of easily degradable carbon and nitrogen sources with the low microbial activity is also not benifical to provide an effective biochemical conversion of biomass.Although hydrothermal hydrolysis can enhance the release and depolymerisation of intracellular organic matters,the hydrolytic by-products,such as furan and phenolic compounds,exhibit stronge inhibitory effect on microbial growth.They may alter the anaerobic metabolic pathway,thereby leadding to a low biohythane yield.Based on this,the hydrothermal hydrolysis of biomass coupled with anaerobic fermentation for biohythane production were deeply explored.The objectives of this thesis were to reveal the effect of hydrothermal release and depolymerisation property of biomass intracellular organic matters on microbial biochemical conversion,to clarify the inhibitory effect of hydrolytic by-products on anaerobic fermentation and the principle of microbial detoxification,as well as to achieve the strengthening approach of thermochemical/biochemical transformation that converting biomass wastes to biohythane.Firstly,to solve the problems of limited mass transfer of biomass organic matters and low efficiency of microbial biochemical transformation,the hydrothermal release and depolymerisation property of intracellular lipids,carbohydrates and proteins were analysed in this thesis.The changes of cell morphology,functional groups,and organic components for the hydrolytic residues,as well as the interaction effects on mass transfer and biochemical transformation of bacterial/enzyme,were comparatively studied.To solve the problems of acid corrosion to equipment and the difficulty in the recovery of liquid acids,a new hydrothermal system of carbon-based solid acid catalyst composed of combined and catalytic functional domain was established.This solid acid could achieve the effective absorption of substrate and the directional hydrothermal depolymerisation.The kinetic characteristic of hydrothermal reaction via the solid acid catalysis was also obtained.Thirdly,the microbial metabolic degradation mechanism of the typical hydrolytic by-products such as furan and phenolic compounds was explored,and the inhibitory effect of hydrolytic by-products on anaerobic fermentation was clarified.Based on this,the key rate-limiting step of the biochemical degradation of“aldehyde–alcohol–acid”for furan compounds,as well as the microbial detoxification principle,were identified.The hydrothermal side reaction characteristic of model compound monosaccharide/amino acid was further studied.The degradation property of complex hydrolytic by-products and the microbial structure succession in continuous anaerobic fermentation process were analysed.Fourthly,the impact of hydraulic retention time of hydrogen fermentation on biomass acidification degree and biohydrogen production was assessed.Meanwhile,the principle and method of biomass hydrothermal hydrolysis and the deep acidification during hydrogen fermentation for the enhancement of anaerobic transformation were also proposed.On the basis of experimental data,a life-cycle assessment of biohythane production using biomass was estabolished,and the energy budget and the environmental effect in such a process were obtained.The main conclusions of this thesis were shown as follows:(1)The hydrolysis and depolymerization of biomass intracellular proteins were weaker than that of carbohydrates.Under hydrothermal conditions of 140°C,10 min,and 0.5%–1%(v/v)H₂SO₄,the hydrolysis efficiency of polysaccharide for carbon-rich starchy biomass reached up to 100%,while the soluble protein yield was only 65.8%of the initial content of nitrogen-rich microalgae biomass.Dilute acid catalysis could destroy the biomass cell structure.The hydrolytic residues were composed of a large number of porous particles,which could increase the specific surface area of microorganisms contacting with substrates,and provide an effective mass transfer channel for bacteria/enzymes.The relative intensity of C=N stretching vibration in the microalgae residues was high,indicating a more severe Maillard reaction.When microalgae was hydrolysed by HNO₃(0.5%v/v),the effect of nitrogen coordination would promote the thermochemical conversion of amides,amines,and aliphatic amines.Whereas,formic acid was more beneficial to promote the formation of aliphatic ester and carbonyl compounds,and thus increasing the release of intracellular lipids.Additionally,when adding liquid acid catalysts during hydrothermal hydrolysis,a rapid pyrolysis weight loss process was observed in the low temperature range from 170 to220°C,showing a significant increase in unstable small-molecule compounds.The organic matters were activlly transported across the membrane to microbial cells,thus providing the enssential energy for the microbial growth at fermentation initial stage and achieving an effective anaerobic biochemical conversion of the substrates.(2)The carbon-based solid acid catalyst with the absorption function was prepared by sulfonation of co-carbonized?-cyclodextrin and polyvinyl chloride,which was an amorphous aromatic carbon sheet with a graphitized structure.Carbon precursors had a large number of fatty side chains and bridge structures.The sulfonation could introduce the hydrophilic absorption groups such as hydroxyl and carboxyl groups and the sulfonic acid-based catalytic group,through oxidation and substitution of the alkyl side chains.However,such a process caused a large number of micropores and mesopores to collapse and reduced the specific surface area,which affected the mass transfer of hydrothermal reaction and limited the effective contact between the catalytic site and the reactant molecule.Thus,the introduction of chlorine-based binding site was important to promote adsorption of substrate and catalytic site,and to enhance thermochemical conversion of organic matter.The cellobiose absorption of solid acid was 45.8%.The solid acid catalyst also had a strong bond-breaking effect on?–1,4 and?–1,4 glycosidic bonds,with the directional hydrolysis ability of the targeted products.However,the destruction of?–1,1 glycosidic bond was relatively weak,and the catalytic hydrolysis of?,?–1,2 glycosidic bond was likely to cause excessive depolymerization of fructose.During cellobiose hydrolysis via solid acid catalyst,the reaction rate constant of glucose production and degradation increased with the increase of temperature.The reaction activation energy was 98.9 and 126.2 k J/mol,respectively.Compared with the hydrothermal catalysis by liquid acid and conventional solid acid catalysts without the absorption sites,the reaction activation energy decreased by11.4%–45.3%and 40.3%–44.4%,respectively.(3)The microbial degradation of typical furan hydrolytic by-product(i.e.,furfural)was considered as a‘furfural–furfuralcohol–furoic acid'biochemical conversion process.The hydrogen production inhibition occurred in the‘aldehyde–alcohol'conversion stage,and the lack of related enzyme and microorganism led to the‘alcohol–acid'conversion as a key rate-limiting step.After 96 h of hydrogen fermentation,49%–70%of furfuralcohol remained undegraded.The furoic acid with the low molecular weight and high lipid solubility was more easily transported across the membrane to microbial cells by facilitated diffusion,thus promoting hydrogen fermentation by acetic acid metabolic pathway,and the maximum hydrogen yield increased by 6%–15.3%.The inhibitory effect of typical phenol hydrolytic by-product(i.e.,vanillin)on anaerobic fermentation were stronger than furfural.It would alter the metabolic pathways of microorganisms,and the concentration of the produced acetic acid and butyric acid decreased by 64.5%and 49.7%,respectively.Even so,the inhibition of hydrolytic by-products on subsequent methane fermentation could be effectively alleviated via two-stage biohytane production.Compared with single-stage methane fermentation,the maximum methane yield increased by 105%–263%,and the methane production peak rate enhanced by 78.5%–204%.(4)The thermo-chemical decomposition of target hydrolysis products monosaccharide and amino acid was due to the Maillard reaction between carbonyl group and amino group.While,the effect of direct thermal decomposition caused by caramelization or deamination was not significant.The process of Maillard reaction was closely related to the solution p H value.The dilute acid hydrothermal condition(1%H₂SO₄,v/v)could hinder the conversion of intermediate small-molecule compounds(such as 5-HMF)into polymers(such as melanoids).The complex by-products produced in such a process would adversely affect the stability of hydrogen fermentation,and the changes of microbial structure led to the variable fermentation modes and large fluctuation of hydrogen production.Additionally,the sulphate ions contained in the feedstock treated by dilute sulphuric acid hydrolysis would increase the growth and competition of the sulphate reducing bacteria,thereby leading to the stuck methane fermentation.The biochemical transformation of by-products was accompanied by hydrogen production,which improved the relative abundances of hydrogenotrophic and mixotrophic methanogens,and inhibited the aceticlastic methanogens growth.(5)Increasing biomass hydrolysis intensity and extending hydrogen fermentation time could enhance biomass acidification degree and hydrogen yield.As the hydrolysed substrates were used for 12–144 h of fermentation,the biomass acidification rate increased from 15.5%to 78.5%,and the maximum hydrogen yield increased from 87.2to 223.1 m L/g VS.A deep biomass acidification during hydrogen fermentation could obviously change the degradation transformation pathway of target substrates such as monosaccharides,thereby affecting the composition distribution of soluble metabolites;the increase of acetic acid concentration would promote the subsequent methane production,and the maximum methane production rate increased by 10%.Additionally,with increasing biomass hydrolysis intensity and acidification degree,the delay time of methane fermentation was shortened by 55.8%–67.6%,and the peak gas production time was advanced by 61.4%–71.7%.In this case,the whole fermentation period in such a two-stage process was also reduced by 60%–72.5%,compared with the single-stage methane fermentation.Hydrothermal hydrolysis of biomass with a deep acidification during hydrogen fermentation could achieve maximum energy conversion potential in a relatively short fermentation period.(6)On the basis of experimental data,a system of biohythane production via anaerobic fermentation using microalgae and food waste was established,and the systemic net energy ratio was 0.24.Meanwhile,the total greenhouse gas emissions were173.15 g CO_2-eq/MJ,with a greenhouse gas reduction of 49.01 g CO_2-eq/MJ.The processes of microalgae cultivation,food waste pretreatment,and hythane purification played important roles on systemic energy budget and environmental impact.The sysmetic output parameters have significant sensitivity to the changes of microalgae growth rate,paddlewheel operating efficiency,and total hythane yield.Additionally,energy recovery and nutrient recirculation were considered as the indispensable processes for microalgae-based energy project,and the lack of these processes would result in a 1.75-1.88 fold increase in the net energy ratio.