Numerical Simulation Study On The Factors Influencing Co-firing Of Municipal Solid Waste In Large-scale Grate Furnace

With the development of society,the production of urban solid waste is increasing.Grate combustion technology has become the main method for treating urban solid waste due to its adaptability and reliability.However,the high moisture content and low calorific value of municipal solid waste result in low combustion heat quality.Meanwhile,organic solid waste such as sludge and industrial waste have a large production volume,high pollution and are difficult to handle.Therefore,co-firing organic solid waste with municipal solid waste can not only compensate for the low calorific value but also provide coordinated treatment for organic solid waste,which has great development potential.Numerical simulation methods have become a common technology for large-scale incineration research due to their efficiency,economy,and safety advantages.In this study,FLIC and Fluent software were coupled to simulate the process of co-firing municipal solid waste with sludge and the optimization of the grate furnace structure.The influence of factors such as sludge moisture content,mixing ratio,air ratio,total air volume,and feed rate on the incineration characteristics was analyzed.Results show that at an appropriate mixing ratio,incineration performance is improved.Furthermore,optimizing the grate furnace structure can also improve combustion efficiency.Using a 500 t/d waste incineration plant as the research object,the actual operating conditions were first simulated to ensure that the simulation results accurately reflect the incineration process in the boiler through comparison with experimental results.On this basis,the significant effects of sludge mixing ratio,sludge moisture content,air ratio,total air volume,and feed rate on the incineration characteristics during the co-firing process of municipal solid waste and sludge were analyzed.Results showed that the increase in sludge mixing ratio and high sludge moisture content could cause a maximum decrease of 51 K in furnace temperature.Air ratios higher than 72:28 would reduce the combustion temperature in the furnace,and higher ratios would result in insufficient solid-phase combustion and high wall temperatures.Total air volume below 69900 m~3 would result in a low flame height,while exceeding this range would significantly reduce furnace temperature.A feed rate below 500 t/d is difficult to maintain a high-temperature environment inside the furnace,while exceeding 550 t/d would result in incomplete combustion.Secondly,for the simulation of co-firing municipal solid waste with industrial waste,it was found that the calorific value and volatiles content of industrial waste were much higher than those of municipal solid waste.As the mixing ratio increased,the weight loss rate of the mixed fuel accelerated,and the solid-phase combustion process advanced.The highest furnace temperature increased by 87 K and 165 K respectively,but too large a mixing ratio caused high temperatures on the back wall.An air ratio below 28%would result in insufficient solid-phase combustion and high wall temperatures,while too large an air volume(too small secondary air volume)would lower the furnace temperature.Inadequate or excessive total air volume would have an adverse effect on combustion.A feed rate below 500 t/d resulted in an average furnace temperature of less than 1400 K,while exceeding 550 t/d would result in incomplete combustion.Compared with sludge,mixed leather and plastic had higher furnace temperatures.Finally,based on the simulation of sludge co-firing,the grate furnace structure was optimized for uneven furnace temperature distribution and poor gas filling degree.Multi-level working conditions were simulated for the width of the flue gas inlet,combustion chamber structure,addition of baffles,and secondary air injection angle.The results showed that reducing the width of the flue gas inlet could effectively improve the position of the combustion zone and reduce the temperature on the back wall.The optimization effect was best when the ratio of the inlet width to the flue width was BB/AA=0.6.Moving the secondary air nozzles of the front and rear walls to the same height and horizontally opposite spraying could achieve better combustion performance.Adding baffles could change the gas flow direction,causing combustion to occur in the middle of the furnace,but excessively long baffles would reduce the gas passage and cause high gas velocity to directly rush toward the front wall.Adding a disturbance plate could change the flow field of the combustion zone and form a vortex to enhance the mixing of flue gas and secondary air,resulting in the combustion zone shifting towards the center of the furnace.An upward shift of the secondary air injection angle could increase the gas flow rate and reduce the residence time of flue gas in the combustion zone,while a downward shift could enhance the turbulence intensity of the combustion zone and improve temperature distribution.