| In recent years,the production of crude steel in China is increasing rapidly,and a large number of by-product gases are produced in the process of iron and steel production,which is the main secondary energy used in iron and steel enterprises,but the users are various and the operating conditions are complex and changeable.If the gases production and consumption are not balanced,the gases will be released,resulting in environmental pollution and energy waste.Iron and steel enterprises are large electricity users,and by-product gases used for power generation is one of the most commonly methods for the recovery and utilization of gases.Power load regulation capacity is limited for the combined gas-steam cycle power generation and the gases-fired boiler unit.Since the gases output fluctuates greatly,it will inevitably lead to a large amount of gases release.As a large adjustable gases consumption user,the boiler unit co-firing coal/gases blends has been paid more attention.The boiler unit co-firing coal/gases blends has the advantages of relatively mature technology,high domestication rate and long annual operation time.The boiler unit co-firing coal/gases blends is an adjustable consumer of by-product gases in iron and steel enterprises,which results in the change of combustion characteristics,heat distribution characteristics and pollutant emission characteristics when the gases supply changes.Therefore,a 300 MW pulverized coal co-fired with blast furnace gas and coke oven gas is used as research objec.The effects of gases ratio and air distribution on combustion characteristics and mechanism were studied.Furthermore,the optimum experimental studies on gases ratio and air distribution were carried out.A boiler combustion optimization model is established finally.It provides a theoretical and practical basis for the safety and economic operation of boiler units co-firing coal/gases blends.Firstly,the combustion characteristics and mechanism of co-firing coal/gas blends and adopting air staging were studied by commercial FLUENT software.Under the conditions of co-firing coal/gas blends and adopting air staging,the models of pulverized coal pyrolysis,coke gasification and gas combustion are established because the reduction atmosphere is formed in the primary combustion zone of the furnace.The reduction of NOx by reductive gases(CH4?CO?H2)produced from co-firing gases,pulverized coal pyrolysis and coke gasification is considered.Since the multiple-surface reaction model of FLUENT software does not take into account the gas diffusion resistance in porous medium during coke reaction,and the direct contact between oxidant and unburned coke leads to burn out completely.In this paper,a modified multiple-surface reaction model,which is compiled by FLUENT User defined function(UDF),is proposed to calculate coke reaction.The results showed that the unburned carbon in fly ash calculated by multiple-surface reaction model was close to zero,and the modified multiple-surface reaction model established in this paper could accurately calculate the unburned carbon in fly ash.On the basis of using the operating conditions to verify the accuracy of the numerical simulation results,the temperature field,velocity field,flue gas components,coke burnout,NOx formation and reduction characteristics of five working conditions(coal-fired condition,co-firing blast furnace gas condition,co-firing coke oven gas condition,co-firing coke oven gas and blast furnace gas condition and low separated overfire air rate condition)were studied numerically.It can be concluded,by comparative analysis of temperature field,1850 K iso-surface,cross section of average temperature,heat flux distribution of 5 conditions,that furnace temperature is more uniform,temperature of local high temperature zone is lower and reducing atmosphere is stronger for the condition of co-firing two gases and high separated overfire air ratio(combustion characteristics condition IV).Therefore,the NOx emission concentration is lowest for this condition.Secondly,in order to reduce the optimal experimental conditions,the numerical simulation optimization researches of gases ratio,separated overfire air rate and secondary air rate were carried out by using unburned carbon in fly ash and NOx concentration as the combustion optimization index.The optimization results of numerical simulation show that the heat adding rate of blast furnace gas is equal to that of coke oven gas(?HBFG/?HCOG?1),the rate of separated overfire air is 26%,the excess air coefficient in A-C pulverized coal zone is about 0.70.Based on numerical simulation optimization results of gases ratio and air distribution,the experimental optimization studies on gases ratio,separated overfire rate and secondary air rate were carried out.The optimization results of experimental show that the optimum heat adding rate of blast furnace gas and coke oven gas is 1,the optimum damper opening of secondary air(AA/AB/BC/CC)is 50/30/50/50,and the optimum damper opening of separated overfire air(SOFA 1/SOFA2)is 100/100.The experimental optimization results are in good agreement with the numerical simulation optimization results,and the results show that the numerical simulation results can be used to guide the optimization experimental study,and the experimental results of gases ratio and air distribution optimization provide a reference for boiler operation.Thirdly,the heat distribution and steam temperature characteristics of boiler are greatly affected by staged air distribution after retrofitting low nitrogen combustion.The heat distribution and steam temperature characteristics of boiler co-firing coal/gases blends and adopting air staging are more complicated.Therefore,the effects of heat adding rate of gases,rate of secondary air,rate of separated overfire air and tilt angle of burners on heat distribution are studied experimentally.In order to study the effect of co-firing blast furnace gas on flue gas velocity deviation of upper furnace,numerical simulation was carried out on different working conditions of heat adding rate of blast furnace gas.It was obtained that the velocity deviation of flue gas increased with the increase of heat adding rate of blast furnace gas.Aiming at solving the problem of reheat steam temperature deviation caused by co-firing blast furnace gas,the optimization experiment of adjusting the yaw angle of separated overfire air was carried out,and the optimal yaw angles were obtained.The results showed that the deviation of reheat steam temperature was reduced from 17.1? to 2.55?,the reheat steam temperature was 536.4?,the main steam temperature deviation was 4.15?,and the main steam temperature was 538.70C,when the separated overfire air yaw angles of#1,#2,#3,#4 were adjusted respectively to-7.5°,+7.5°,+7.5°,-7.5°.Finally,in order to establish the boiler combustion optimization model and integrate with the distributed control system(DCS)of boiler unit,the closed-loop control of boiler operation optimization is formed.The advantages of support vector machine(SVM),least squares support vector machine(LS-SVM)and fixed size least squares support vector machine(FSLS-SVM)algorithm in solving different capacity sample are studied.A double grid search method for optimal size is proposed for FSLS-SVM algorithm.The models of NOx emission concentration,flue gas temperature and unburned carbon in fly ash are established respectively based on SVM,LS-SVM and FSLS-SVM algorithm,and the advantages and disadvantages of the three algorithms are evaluated by using the approximation ability,relative error and calculation time.The evaluation results show that the models of NOx emission concentration and flue gas temperature established by FSLS-SVM algorithm are superior to the other two algorithms,while the model of unburned carbon in fly ash established by SVM algorithm is best.The mixed model of boiler efficiency is established by synthesizing flue gas temperature model,unburned carbon in fly ash model and the theoretical calculation model.Finally,the improved particle swarm optimization(IPSO)algorithm,using NOx emission concentration model and boiler efficiency mixed model as fitness function,is applied to optimize damper opening of secondary air(SA)and separated overfire air.The optimization results show that The NOx emission concentration decreased from 210 mg/Nm3 to 182.8 mg/Nm3,and the boiler efficiency increased from 90.21%to 91.12%. |
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