Investigation On Oxy-fuel Co-gasification/Co-combustion Mechanism Of Municipal Solid Waste Blended With Coal And System Performance

With the economic development and growing population,more and more municipal solid waste?MSW?is normally released,causing serious environmental and social problems.Among these MSW treatment technologies,thermochemical conversion technology?gasification and combustion?has gained more and more attention due to its advantages of the large capacity reduction and resource recovery.However,the inherent high moisture content and low calorific value of MSW materials have largely restricted the large-scale development of this technology.If a certain proportion of coal is blended,the co-gasification/co-combustion of MSW and coal can overcome these shortcomings of MSW treatment alone.In addition,the oxy-fuel technology can not only reduce the emission of pollutants,but also realize the storage and capture of CO₂ and reduce the greenhouse effect.If these two technologies are combined,it will surely be able to achieve better harmless treatment of MSW.Based on this background,a comprehensive and in-depth research on MSW and coal co-gasification/co-combustion technology has been conducted,which will provides a feasible technical solution to achieve the environmentally friendly treatment and resource recovery of MSW.Based on the thermogravimetric experiments and complex reaction kinetics theory,the CO₂ co-gasification characteristics of MSW and bituminous coal?BC?are studied in detail.It is found that the gasification process of MSW is mainly divided into four stages,while the gasification process of BC has only three stages.The gasification reaction temperature of coal char is much higher than that of MSW char,a significant synergistic effect is found for the char gasification stage of blends.According to the two methods of Starink and FWO,the kinetic parameters of the co-gasification process are analyzed,and the Malek method is used to determine the most probable mechanism function.Besides thermodynamic parameters of co-gasification process are calculated and analyzed.It is concluded that when the bituminous coal mixing ratio is 40%,the average activation energy of the co-gasification process reaches a minimum value of184.13 k J/mol.Therefore,the 60MSW40BC sample may be a better mixing ratio of MSW and BC in terms of kinetic parameter.For this sample,models F6,A1 and D7 are more suitable to describe the three main stages of light volatile release,heavy volatile decomposition and char gasification.In addition,analysis of thermodynamic parameters shows that the average values of?H,?G and?S for co-gasification of this blends are176.82 k J/mol,257.89 k J/mol,and-89.16 J/mol·K,respectively.According to above research,co-combustion characteristics and kinetic parameters of MSW and coal semi-coke in oxy-fuel atmosphere is studied.It is found that both air and 30O₂/70CO₂ atmospheres,the addition of MSW and higher oxygen concentration is beneficial to improve the co-combustion characteristics of the blends.The decrease of peaks number of DTG curve in the combustion process under O₂/CO₂ atmosphere indicated that the presence of CO₂ is conducive to the burnout of semi-coke.The synergistic effect of the oxy-fuel co-firing process mainly occurs between 260-650?.In addition,the average activation energy E_a showed an upward trend with the increase of oxygen concentration,and the average activation energy of the combustion process in 30O₂/70CO₂ atmosphere is closer to that of air atmosphere.The E_a,?H,?G and?S of50MSW50LCSC sample in 30O₂/70CO₂ atmosphere are 140.86 k J/mol,138.33 k J/mol,161.94 k J/mol and-41.44 J/mol·K,respectively.The zero-dimensional thermodynamic equilibrium model and three-dimensional numerical model of MSW/BC oxy-fuel co-gasification process is focused to investage in detail.First,based on the principle of system Gibbs function minimization,an integrated MSW/BC oxy-fuel co-gasification system is established,and the main operating parameters of the system are analyzed.As the gasification temperature increases,the gasification system exhibits better gasification performance.With the R_cratio increases,it can enhance the water gas shift reaction to proceed in a reverse direction,resulting in a decrease in the H₂ mole fraction.Besides as the MSW blending ratio R_M,the mole fraction of CO,CGE and OEE of co-gasification system all shows a gradual decreasing trend.The relatively optimal operating parameters of this system are T=900?,R_o=0.2,R_c=0.5 and R_M=0.6.And the OEE of this system is 0.57 in this condition.Based on the Euler-Lagrange method a three-dimensional full-loop numerical simulation of the MSW/BC oxy-fuel fluidized bed co-gasification process is carried out,focusing on the gas-solid flow characteristics,distribution characteristics of furnace temperature and component field in different oxygen concentrations.It is found that the temperature field of co-gasification in 21O₂/79CO₂ atmosphere is relatively lower than that of air atmosphere,and 30O₂/70CO₂ atmosphere is easier to achieve the temperature field similar to air.As the increase of oxygen concentration,the temperature field,the concentration distribution of CO,H₂ and CH₄ all show an increasing trend.In addition,the molar concentration of outlet syngas?CO+H₂?of oxy-fuel co-gasification is greater than that of air,indicating that the oxy-fuel condition contributes to improving the quality of MSW/BC co-gasified syngas.According to the operating data of an actual MSW incineration power plant,the whole process simulation of MSW/BC oxy-fuel co-combustion power generation system is designed and established.The sensitivity analysis of the combustion system is carried out,and the influence of combustion temperature and excess oxygen ratio on the main components of flue gas is investigated.In addition,an optimization analysis of system's energy consumption is performed.The results indicate that the amount of NO_xproduced in this system under O₂/CO₂ atmosphere is significantly lower than that in the air atmosphere,the 96%supplied oxygen concentration is a better choice,and the net power generation efficiency of optimized system is improved by 2.69%.In addition,the addition of bituminous coal has further improved the net efficiency of the oxy-fuel combustion system,to attain 11.65%.At the same time,the power production cost of the system also increase to 148.40€/MWh.