Study On Co-gasification Reaction Process And Mechanism Of Digestate And Coal Based On Resource Utilization

With the energy crisis and environmental pollution caused by the increasing consumption of fossil energy,biomass energy has attracted much attention from worldwide scholars due to its advantages of carbon dioxide"zero"emission and renewability.Anaerobic digestion（AD）is an important technology to reutilize biomass waste.However,as a large amount of by-product,the residue is one of the resistances for large-scale industrial application of this technology.When using as an organic fertilizer,new problems will be brought to the environment because of high content of pathogens,heavy metals and residual pesticides,etc.in AD residue.The co-gasification of residue and coal can not only realize the full utilization of residue,but also eliminate the pollutant discharge from coal in the traditional combustion process.However,the co-gasification of residue and coal was seldom reported.This thesis investigated the influence of the mixing process and AD time on the interaction between residue and coal.Meanwhile,the effect of experimental conditions on the co-gasification of residue and coal was systematically analyzed.In addition,the evolutionary mechanism of char physicochemical structure was revealed.Finally,the specific roles of biomass components in the co-gasification process were investigated.This research can provide the theoretical basis for the feasibility of co-gasification of residue and coal,and was of great significance for the energy utilization of AD residue.Firstly,the influence of mixing processes of coal and residue on co-gasification was explored.Dry mixing process can promote the co-pyrolysis of residue and coal,while wet mixing process can promote the co-gasification reaction process because of the improvement in specific surface area,pore volume and pore size of pyrolysis char.The synergistic effect between the residue and coal was the most significant when AD time as0 day and 40 days.It provided theoretical guidance for the coupling of AD system and co-gasification system.The effect of experimental conditions on the co-gasification of residue and coal was investigated in a lab-scale downdraft fixed bed reactor.The lower heating value（LHV）of produce gas can be increased obviously with the increase of gasification temperature and the addition of coal.Compared with the gasification temperature 650℃,the LHV of the product gas increased nearly one time at 950℃.When 30 wt.%coal was added,the synergistic effect between residue and coal during the co-gasification process was the most significant.Compared with air,when using CO₂ as gasification agent,the carbon monoxide content and the LHV of the produce gas increased by 247.55%and 174.69%,respectively.The evolution mechanism of the char structure was investigated by using nitrogen physical desorption-adsorption analyzer（BET）,Fourier transform infrared spectrometer（FT-IR）,laser Raman analyzer and thermogravimetric analyzer（TG）.The effect of final reaction temperature on pore structure of char was complex.When the final reaction temperature increased from 650℃to 850℃,the micropore content increased.The further increase of temperature led to the rapid transformation of micropore to macropore and the sharply decrease of specific surface area and pore volume.In addition,the increase of the temperature promoted the condensation of aromatic rings and the content of large aromatic rings gradually,decreasing the gasification activity of char.The increase of coal content in the mixture obviously improved the pore structure characteristics of char.When the 30 wt.%coal was added,the small aromatic compounds content in the char were relatively the most,and the gasification reaction activity was the highest.What’s more,the increase of the temperature and coal content in the mixture contributed to the decrease of fractal dimension.The presence of O₂ in the gasification agent eroded the internal pores of the char,forming long and fine pores.At the same time,it reduced the condensation degree of aromatic rings and improved the gasification reaction activity of char.Using CO₂ as gasification agent can promote the immobilization of pollution elements P and S in char,which was of great significance for the prevention and control of environmental pollution.Finally,the gasification characteristics of cellulose,hemicellulose and lignin were studied using TG analysis.The co-gasification process mechanism of biomass components and coal,product gas characteristics and development of char structure were studied in a lab-scale downdraft fixed bed.The results showed that the pyrolysis temperatures of the three components followed the sequence of hemicellulose<lignin<cellulose.Hemicellulose and cellulose did not show obvious gasification behavior.Due to the difference of three components molecular structure,the interaction between hemicellulose and coal was the most significant,followed by cellulose.Especially,when the addition of cellulose and hemicellulose reached 50 wt.%and 70 wt.%respectively,the interaction was the strongest.The inhibitory effect on the co-gasification reaction increased gradually with the increase of addition of lignin.When the addition of lignin reached 70 wt.%,the inhibitory effect was the most significant.The research was of great significance for the coupling of AD system and gasification system and revealed the mechanism of biomass components in the co-gasification process.It laid a theoretical foundation for the design and optimization of co-gasification technical scheme of biomass waste and coal.