The Coupling Technology Of High Efficiency Combustion And Low-NO_x Emission In Coal-fired Boiler

Due to the increasing stringent environmental standards and the increasing degree of social attention,large quantities of the low-NO_x combustion technologies have been developed in recent years.Even though many of these technologies have been widely promoted and applied,the contradictory relation of the high efficient combustion and NO_x control restricts the property of various kinds of furnaces.How to achieve high combustion efficiency and low-NO_x emission at the same time is very important for the clean and efficient application of coal-fired boilers.Decoupling combustion means divides the combustion process into pyrolysis and combustion two stages,which could solve the contradictory relationship between the combustion efficiency and NO_x control.Based on this,the characteristics of coal staged combustion of pyrolysis and combustion processes were experimentally studied.The high efficiency combustion and low-NO_x emissions coupling technologies for three low combustion efficiency and high emission furnaces,includes the lean coal pulverized coal boiler,industrial grate-fired furnace and civilian small stove were developed based on the experimental results and literature analysis.In this thesis,the experiment and numerical simulation were used comprehensively to analyzed and optimized the three technologies,in order to obtain the industrial application valued data.Firstly,based on the analysis of the low-NO_x mechanism of the decoupling combustion,the reaction characteristics of pyrolysis and combustion staged reaction were studied.The effects of different pyrolysis degree on coal combustion efficiency were investigated.The study showed that although decoupling combustion solves the contradiction between stable combustion and low-NO_x combustion conditions,it reduces the combustion efficiency of coal.However,the results also showed that the reduce degree of the total reaction rate of long pyrolysis time condition was similar with the short condition,in other words,both cases reduced the reaction rate at the same level.Based on this,the high-efficiency combustion and low-NO_x emission coupling technologies were developed,one is lean coal pulverized coal boiler multi-scale decoupling combustion system,including a horizontal rich/lean burner and a multi-angle tangentially fired furnace;the other one is the combinative low-NO_x combustion chain furnace;and the last one is the cook and heating decoupling stove.Secondly,by means of cold model test,cold numerical simulation and combustion numerical simulation,the stable combustion and low-NO_x mechanism of lean coal decoupling burners was analyzed in detail.The numerical simulation results showed that this burner can produce high concentration efficiency and low velocity recirculation zone,which is benefit for pulverized coal ignition.And the pulverized coal with different concentrations showed a ladder-like ignition,which promoted the rapid ignition of lean coal.Terraced flame holders organized the coal formed a long and stable pulverized coal beam,which was advantageous to maintain the volatiles and coal char together to provide heat to unburned coal,thus formed a self-stabilizing combustion state.The pulverized coal beam strengthened the NO_x reduction of char and reduced the NO_x generation.The structure optimization of the burner nozzle indicated that the straight separator effectively prolonged the time interval between the volatile release and the coke combustion and showed good performance in several aspects and the NO_x concentration of the output decreased 59.36% than the original design structure.The optimal operating parameters of the lean coal decoupling burner are obtained by optimization.The NO_x concentration at the outlet decreased 61.88% than the original design.Thirdly,through the cold model pilot test,cold numerical simulation,the four-wall tangential / corner-type secondary air furnace（FWC）structure was analyzed and the influence of different 2^nd secondary air locations and different swing angle on the aerodynamic was studied.At the meanwhile,the FWC and four corner tangential / wall-type secondary air furnace（FCW）were analyzed roundly by combustion simulation.The results showed that the multi-angle tangential furnaces delayed the combustion reaction by separating part of the secondary air to the upstream or downstream of the primary air.The high temperature and high coal concentration formatted in the center of the furnace,which is conducive to decoupling combustion,provided advantage to the low-NO_x combustion.The output NO concentration of the FWC and FCW furnaces reduced by 17% and 36% separately than the four wall and four corner tangentially combustion furnaces.Also,these two furnaces avoided the uncertainty of the actual tangential caused by the deviation of the main jet,and effectively prevent the water wall high temperature corrosion.Furthermore,within the actual circle was strong turbulence region,which extended the residence time of coal in the combustion zone,facilitated the burnout of coke/semi-coke.Ultimately,these two furnaces structure achieved efficient combustion and low-NO_x emissions coupling.The FWC and FCW furnace can be used for efficient combustion and low-NO_x emissions transformation of four-wall tangentially fired furnace and four corner tangentially fired boiler separately.Finally,the combustion and pollutant release characteristics of the decoupling and reburn combinative low-NO_x chain furnace and the cook/heating decoupling stove were studied by numerical simulation and field experiment separately.The results showed this new design grate furnace enhenced the mixing of the main combustion zone gas and the pyrolysis gas,and avoided the incompletely burnout of the pyrolysis gas.Meanwhile,the NO_x reduction rate was promoted by combine the decoupling combustion and gas reburning.This furnace further improved the performance of the pyrolysis combustion grate furnace.The outlet pyrolysis gas concentration of this furnace decreased by 72% and NO concentration decreased by 89%,compared with the traditional grate furnace.The experimental results of the dual-use stove showed that through separating the furnace area and layout the duct between different sections,this stove controlled the staged combustion in different areas.The combustion efficiency was 36% higher than that of the conventional one,and the SO₂,NO_x and CO concentrations were below 200 mg/m³,100 mg/m³ and 300 mg/m³,respectively.This study can provide a strong theoretical basis and technical guidance for the development and application of efficient clean combustion technology for different coal-fired boilers,thus improving the regional air quality greatly.