Study On Coupling Technology Of Alkaline Thermal Pretreatment At Mild Temperature In Solid State With Straw And Wet-dry Two-stage Anaerobic Fermentation Process

How to use straw and other lignocellulose resources with high efficiency,green and low cost to obtain renewable energy?such as biogas?has become an important issue in the sustainable energy development strategy of all countries in the world right now,and how to effectively destroy or eliminate the anti-degradation barrier with straw to increase the accessibility of microorganisms or enzymes and fermentation substrates has become one of the key common technologies restricting the development of straw biogas industry.Because of the disadvantages of traditional pretreatment methods,such as high energy consumption,strong chemical corrosion,long treatment cycle and harsh equipment conditions,the purpose of this study is to study the feasibility of mild hydrothermal pretreatment and optimize the parameters.The coupling alkali treatment strategy was adopted to improve the biogas production effect of mild hydrothermal pretreatment of straw,and the best straw pretreatment method was obtained,and the technical parameters of combined pretreatment were applied to the newly developed wet-dry two-stage fermentation process.The practical application effect is studied in order to provide theoretical and technical support for straw biogas engineering design,engineering operation regulation and high efficiency biogas production.In this study,the feasibility of promoting biogas production by anaerobic fermentation of straw with mild hydrothermal pretreatment was studied firstly using crop straw as raw material,and the technological parameters were optimized by orthogonal experiment.At the same time,the selection of solid-state chemical pretreatment agent and the optimization of pretreatment period,as well as the comparative study between hydrothermal and alkaline thermal pretreatment at mild temperature in solid state to promote biogas production of straw were carried out,and the optimum technical parameters of mild alkaline thermal pretreatment were obtained.Secondly,through the comparative study of straw two-stage fermentation and single-stage fermentation,a novel high efficiency biogas fermentation process suitable for straw raw material was proposed,the selection of key equipment was optimized,and the optimum parameters of wet-dry two-stage fermentation were also obtained.Finally,further research on the running effect of wet-dry two-stage fermentation process with straw based on mild alkaline thermal pretreatment was carried out.The main results are as follows:?1?The biogas production effect of mild wet heat pretreatment on biogas production of straw anaerobic fermentation was confirmed by straw batch fermentation test.It was found that mild hydrothermal pretreatment promoted the dissolution of organic matter from rice straw and destroyed the cellulose composition of rice straw.The order of effect on three elements was cellulose>hemicellulose>lignin.Furthermore,it was found that the order of the influence of each factor on the cumulative gas production rate of TS was initial moisture content>presoaking time>heat treatment time through orthogonal test,and the optimum pretreatment conditions were as follows:the initial moisture content was 55%,The pre-soaking time is 2 h and the heat treatment time is 6 h.?2?Considering that mild hydrothermal pretreatment has a small degree of damage to the structure of straw lignocellulose,which makes it limited to improve the biogas yield of straw,this study intends to use chemical pretreatment agent to enhance the effect of mild wet-heat pretreatment.First of all,in view of the long treatment cycle of traditional chemical pretreatment agents,the comparative study on shortening the pretreatment time of different chemical pretreatment agents at room temperature was investigated.Three kinds of chemical pretreatment were helpful to improve the volume gas production rate,raw material gas production rate and methane production rate of rice straw.The best methods among the three treatments were H₂O₂-168h,Ca-?OH?₂-72h and NH₃·H₂O-120h.Therefore,the better pretreatment effect can be obtained by properly shortening the pretreatment period under the condition of solid state pretreatment at room temperature,and the treatment group of Ca?OH?₂-72 h is the best.Then,through the comparative study of solid low temperature hot alkali pretreatment and mild hydrothermal pretreatment,the mild alkali thermal pretreatment in solid state was more favorable to improve biogas production rate of anaerobic fermentation of rice straw compared with mild hydrothermal pretreatment.The cumulative gas production rate of TA3 was the highest?411.1mL/g VS?,which was 24.04%higher than that of CK.The hydrolysis dissolution and acidizing effect of organic matter in rice straw were significantly promoted by the mild alkali thermal pretreatment in solid state,and increased with the increase of alkali content,but decreased gradually.In addition,excessive addition of alkali is not conducive to promoting biogas yield of rice straw,and the optimal addition of Ca?OH?₂ was2%.It can be seen that the technical parameters of temperature and wet hot alkali treatment are optimized by coupling mild wet and hot pretreatment technology with solid alkali treatment technology,which not only reduces the dosage of alkali,but also avoids the problem of wastewater pollution.?3?It was found that the biogas production effect of wet single-stage fermentation and wet-dry two-stage fermentation in batch fermentation test was obviously better than that of dry single-stage fermentation and dry-wet two-stage fermentation,through the optimization study of dry and wet single-stage and two-stage fermentation technology of straw.With the prolongation of the conversion period of wet-dry two-stage fermentation,the cumulative gas production rate increased gradually,but it was similars to that of wet single-stage fermentation group,and the experiment group of 20 days was the best.With the prolongation of the conversion period,the cumulative gas production rate of dry-wet two-stage fermentation decreased gradually,and the experiment group for 5 days was the best.The models of LM,RM and RC were used to fit and compare the cumulative methane production.The results showed that the RC model and GM model were suitable for the study and analysis of the kinetics of straw fermentation in wet-dry and dry-wet stages,respectively.According to the comprehensive analysis,wet-dry two-stage fermentation was more favorable to increase the cumulative gas production rate,among which,15 days,20 days and 25 days were increased by79.33%,94.51%and 84.72%respectively,compared with the dry-wet two-stage fermentation?5 days group?.The results showed that the wet-dry two-stage fermentation process was more suitable for straw fermentation.?4?In this study,the key equipment of wet-dry two-stage dynamic fermentation was designed and developed.The continuous dewatering device suitable for high concentration anaerobic fermentation system has been designed and developed,which has both the functions of fermentation material discharge and dehydration.And it is beneficial to improve the effect of material mass transfer,prevent local acidification,realize zero discharge of biogas liquid in fermentation system,and the fermentation residue after dehydration is easy to be used for follow-up processing.In addition,the continuous wet-dry two-stage dynamic anaerobic fermentation equipment is designed and developed,which is suitable for the treatment of multi-component organic waste,and can maximize the treatment capacity and volumetric gas production rate of the reactor,and reduce the investment cost.And the difficult problem of feeding and discharging was solved in the dry anaerobic reactor.In addition,this study relies on the wet-dry two-stage dynamic fermentation technology and supporting equipment has been licensed six patents of China.?5?In view of the advantages of wet straw fermentation process,such as good start-up and uniform mass transfer,but the straw treatment capacity per unit reactor is limited,while dry fermentation also has many advantages,such as high volume gas production rate and less biogas liquid production,but it has some disadvantages,such as poor start-up performance,difficult in and out of material,and poor heat transfer by mass transfer,and so on.Considering that the anaerobic bioconversion rate of straw feedstock has obvious trend of high and then low,a two-stage anaerobic fermentation process combining wet and dry process to prepare biogas is put forward innovatively.In addition,in order to investigate the practical application effect of mild alkali treatment technology parameters in straw wet-dry two-stage fermentation system,a comparative study of dry-wet or wet-dry two-stage fermentation with wet and dry single-stage fermentation was carried out.It was proved that the wet-dry two-stage fermentation process had obvious advantages in promoting biogas production efficiency of straw and increasing the capacity of straw treatment per unit reactor.The average volume gas production rate of straw wet-dry two-stage fermentation process was 1.12 m³/m³·d,increased by 138.30%compared with semi-continuous single-stage wet fermentation process,the fermentation cycle was reduced by 30%,and the capacity of straw treatment per unit reactor was increased by 2.8 times.The anaerobic reactor volume in engineering was greatly reduced and the investment cost in engineering construction was saved.However,although the feasibility study and comparative study of promoting biogas production from straw were carried out in this study,the feasibility study and comparative study on promoting biogas production from straw were carried out for mild hot and humid pretreatment,mild hot alkali pretreatment and so on.However,there are few studies on the structure,surface wetting characteristics and microcosmic characteristics of straw before and after pretreatment,so it is insufficient to analyze the mechanism of improving biogas production by straw anaerobic fermentation.Further research on the mechanism of action should be carried out in order to provide a theoretical basis for the optimization of pretreatment technical measures.In addition,although the application effect of straw wet-dry two-stage fermentation process based on mild wet-hot alkali treatment was confirmed by comparing different fermentation processes of straw,the application effect of straw wet-dry two-stage fermentation process based on mild wet-hot alkali treatment was confirmed,but the time was limited.The technical parameters of straw wet-dry two-stage fermentation?such as SRT of wet fermentation and dry fermentation,total solid concentration of wet fermentation,fermentation temperature,etc.?have not yet been optimized.It provides technical support for the final formation of straw wet-dry two-stage fermentation technology solution.