| Thermal power generation is the main source of electricity in China.Reducing the coal consumption of thermal power generation is benificial to alleviating the shortage of coal resources,reducing greenhouse gas emissions,and promoting energy conservation and emissions reduction.Using flue gas waste heat recovery system to reduce the exhaust gas temperature,reduce the heat loss of the exhaust gas,and improve the boiler efficiency is conducive to reducing the coal consumption of power generation,and improving the efficiency of the unit.In this paper,using theoretical analysis,laboratory experiments and on-site test methods,the synergism mechanism of a flue gas waste heat cascade utilization system(FWCUS)for boilers based on energy replacement was studied.The factors affecting the economy of the system were analyzed,and the system's operating performance under variable working conditions was explored.The operation mode was optimized,and the energy utilization efficiency of the system was improved,which provides a theoretical basis for the economic,safe and reliable operation of the system and provides a reference for the operation adjustment of the system.In this paper,based on the thermodynamic analysis method,the energy replacement and utilization mechanism of FWCUS were studied.Firstly,the system was set as four component modules,namely the low energy grade energy recovery module,low energy grade energy transfer module,high energy grade energy replacement module and the high energy grade energy utilization module,and the corresponding thermal processes were defined.The exergy analysis method was used to analyze the FWCUS,the thermodynamic evaluation methods based on exergy efficiency of the four modules of FWCUS were established,and the main factors affecting the efficiency of the system were determined.Based on the equivalent enthalpy drop theory,the thermal economy models of the additional economizer(AE)and the front located air preheater(FAH)in the system were established,and the calculation method of economy indicators such as the thermal efficiency improvement value of the power plant was determined.Secondly,to obtain the highest exergy efficiency of the system,according to the energy grade balance theory and the heat balance theory,the energy grade balance coefficient of the FWCUS was given,the optimal energy grade balance node temperature was put farward and proposed to be an adjustment parameter for efficient operation of the FWCUS under off-design conditions.Thirdly,On the basis of theoretical analysis,the characteristics of the system under variable operating conditions were deeply studied.Through experiments,the variation laws of the important thermodynamic parameters such as node temperature and heat of heat exchangers of the FWCUS were studied when the parameters such as flue gas temperature and air temperature changed.Taking the exhaust gas temperature as the control goal,the operation regulation strategy was studied experimentally as the parameters such as flue gas flow,flue gas temperature,and air temperature changed.The low temperature economizer-front located air preheater(LTE-FAH)system was optimized,and the operation adjustment strategy of closed loop circulation flow and the crossover pipe flow was studied experimentally taking the exhaust gas temperature and the inlet liquid temperature of LTE as the control goal.Finally,the theoretical research results and the pilot experimental research results were applied to the FWCUS of a 1000MW ultra-supercritical double reheat unit,and the performance test of the FWCUS was carried out.The obtained working performance of the system under variable working conditions was consistent with the experimental test results.This paper mainly completed the following research contents:(1)Based on energy grade theory,the FWCUS was proposed to be divided into four modules,such as low energy grade energy recovery,low energy grade energy transfer,high energy grade energy replacement and high energy grade energy utilization,and the corresponding four processes were defined.Thermodynamic analysis of the four processes was carried out using exergy theory.The recovery efficiency,transfer efficiency,displacement efficiency and utilization efficiency were used to evaluate the heat utilization effect of each process.The flue gas temperature,flue gas flow,initial air temperature,bypass flue gas flow,AE circulating water flow and increased fan and water pump power consumption due to FWCUS installation were determined as the main factors affecting waste heat recovery.The energy grade replacement coefficient was proposed to measure the energy grade replacement effect of the system.Thermodynamic evaluation method for FWCUS based on exergy efficiency was established.(2)Based on the equivalent enthalpy drop theory,the thermal economy models of the AE and FAH were established,and the calculation methods of the relative increase value of the power plant efficiency,the equivalent enthalpy drop increase value,and the reduction value of the power generation standard coal consumption after the installation of AE and FAH were determined.Thermodynamic evaluation method for FWCUS based on equivalent enthalpy drop was established.(3)Using the energy grade balance theory and heat balance theory,the input and output heat and exergy of the FWCUS were analyzed,the energy grade balance coefficient of the system was determined.To ensure the highest system exergy efficiency,the optimal energy grade balance node temperature(ENT)of the system was proposed,and the calculation method of the optimal ENT was determined.The analysis of an example of a supercritical unit shows that there was an optimal ENT for the FWCUS corresponding to a certain initial temperature of flue gas and initial air temperature.In order to ensure the utilization effect of waste heat,when the initial flue gas temperature and the initial air temperature rise,the ENT should be continuously decreased.The ENT can be used as an important control parameter for the system's off-design operating(4)A low energy grade waste heat recovery,energy displacement and high energy grade waste heat depth utilization pilot test bench was designed and built.Using the pilot test bench,through experiments,when the total flue gas flow,flue gas temperature,air temperature,bypass flue gas flow,and AE water flow changed,the change laws of the outlet air temperature of air preheater(AH),inlet and outlet temperature of LTE,outlet air temperature of FAH,the heat,and input and output exergy of AP,LTE,FAH and AE,as well as the recovery efficiency,transfer efficiency,replacement efficiency,utilization efficiency,and energy grade replacement coefficient of the FWCUS were studied.The experimental results show that:1)when the flue gas temperature and flue gas flow increased,the outlet air temperature of AP,the inlet flue gas temperature of LTE,exhaust gas temperature and outlet air temperature of FAH rise,the exchanging heat and obtained exergy on cold side of heat exchangers increased,which was beneficial to improve the recovery efficiency,transfer efficiency,utilization efficiency and system exergy efficiency.The energy grade replacement coefficient decreased.2)when the bypass flue gas flow increased,the outlet air temperature of AP,the inlet flue gas temperature of LTE,exhaust gas temperature and outlet air temperature of FAH decreased,the exchanging heat and obtained exergy on water side of AE increased,the exchanging heat of AP and other heat exchangers increased,the recovery efficiency and replacement efficiency increased,and the transfer efficiency,utilization efficiency and system exergy efficiency decreased,the energy grade replacement coefficient rise.3)when the air temperature increased,the outlet air temperature of AP,the inlet flue gas temperature of LTE,exhaust gas temperature and outlet air temperature of FAH increased,the exchanging heat of AE and AP decreased,the exchanging heat of LTE and FAH slightly increased,the recovery efficiency,replacement efficiency,utilization efficiency and system exergy efficiency decreased,the transfer efficiency increased,the energy grade replacement coefficient decreased.4)when the circulating water flow of AE increased,the outlet air temperature of AP,the inlet flue gas temperature of LTE,exhaust gas temperature and outlet air temperature of FAH decreased,the exchanging heat of AE slightly increased,the exchanging heat of AP,LTE and FAH slightly decreased,the recovery efficiency and replacement efficiency decreased,the transfer efficiency,utilization efficiency and system exergy efficiency decreased,the energy grade replacement coefficient rise.(5)Taking the exhaust gas temperature as the control goal,the operation adjustment strategy of FWCUS was determined through experiments when the flue gas flow,flue gas temperature and air temperature changed:when the total flue gas flow,the initial flue gas temperature and initial air temperature gradually increased,the proportion of the bypass flue gas flow was gradually increased as the water flow of AE was maintained unchanged,or the water flow of AE was gradually increased as the proportion of the bypass flue gas flow was maintained unchanged.The exhaust gas temperature was maintained at a certain stable value,and the FWCUS has higher system efficiency.(6)The adjustment strategy of ensuring the minimum exhaust gas temperature and the minimum liquid inlet temperature of LTE was studied.The closed loop liquid medium circulation system of the LTE-FAH was improved by setting the variable frequency pump and the crossover pipe valve.Using water as the circulating medium,the adjustment method to ensure the minimum exhaust gas temperature and the lowest liquid inlet temperature of the LTE was determined through experiments:taking he exhaust gas temperature and the LTE inlet water temperature as the adjustment target value,as the crossover pipe flow was continuously increased,the exhaust gas temperature and the inlet water temperature of the LTE were continuously increased;as the proportion of the closed circuit circulation flow was gradually decreased,the exhaust gas temperature and the inlet temperature of the LTE were also continuously increased.During the operation of the LTE-FAH,the crossover pipe flow and the closed-loop circulation flow can be adjusted together to ensure that the exhaust gas temperature and the minimum inlet temperature of LTE simultaneously satisfy the safe and reliable operation of the system.(7)The research results of FWCUS and variable working conditions analysis were applied to a 1000MW ultra-supercritical double reheat unit.The working characteristics of the unit under different loads when the system worked and stop running were tested.The results show:as the FWCUS was shut down or put into operation,the outlet temperature of AP,exhaust gas temperature,outlet air and flue gas temperature of FAH under 100%THA were higher than the corresponding working conditions of 75%THA.The exchanging heat of AP,LTE,FAH and AE under 100%THA was higher than the corresponding heat under 75%THA.When the FWCUS was put into operation,the recovery efficiency,transfer efficiency,replacement efficiency,utilization efficiency,and system exergy efficiency under 100%THA were higher than the corresponding values under 75%THA.The working performance of the FWCUS obtained in on-site test was consistent with the laboratory experiment results.After the FWCUS was put into operation,the unit obtained better coal-saving effect under different loads.the power supply standard coal consumption reduced by 3.21g/(kW·h)under 100%THA,and reduced by 1.52g/(kW h)under 75%THA.Through the above research,the thermodynamic evaluation methods based on exergy efficiency and equivalent enthalpy drop of FWCUS were established.The ENT of the system was defined,and the system operating law under variable working conditions was obtained.The operation adjustment strategy of FWCUS taking the exhaust gas temperature as the control target was determined,which provide a theoretical basis for the efficient operation of the FWCUS,and can guide the engineering application practice of the system. |
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