Research On Numerical Simulation Of Power Plant Boiler Based On The Second Law Of Thermodynamics

Thermal power generation still accounts for a large proportion in China's power industry,and the analysis of power plant boilers cannot be limited to the analysis of thermal efficiency.With the development of society and serious energy waste,the second law analysis method of thermodynamics is gradually applied to many projects.The exergy analysis method is widely used because this analysis method not only considers the "quantity" of energy,but also considers the "quality" of energy.It makes analytical research more comprehensive and scientific.In this paper,an exergy analysis study based on the second law of thermodynamics is carried out for a 350 MW power plant boiler to analyze the combustion of the boiler.The three-dimensional simulation of the combustion of power plant boiler is performed.The distribution of flue gas exergy is observed and a one-dimensional simulation calculation model of the boiler is established to calculate the boiler combustion exergy loss,heat transfer exergy loss,and boiler exergy efficiency.Then put forward an optimization plan for the boiler,verify the feasibility of the optimization plan,and achieve the purpose of saving energy and improving energy efficiency.According to the boiler design specification,a three-dimensional simulation of power plant boiler is performed.A three-dimensional physical model of the boiler is established by CATIA software and meshed by ICEM CFD software.Select appropriate mathematical models and boundary conditions to represent the combustion and flow of pulverized coal particles in the boiler furnace,establish a calculation model for flue gas exergy,and apply it to the three-dimensional simulation of furnace fires,which is convenient to observe the distribution of flue gas exergy in the furnace.The numerical simulation analysis of the combustion conditions under the design conditions was conducted,and the furnace exit temperature was found to be very similar to the furnace combustion exit temperature given by the power plant,which proved the accuracy of the three-dimensional simulation model.Numerical simulations of the combustion conditions of the boiler under different load and coal types were conducted to observe the temperature and the distribution of flue gas exergy.First,the conclusion is drawn by comparing the distribution of boilers under 100% BMCR,75% BMCR and 50% BMCR loads.Under different load conditions,it can be seen that as the load is reduced,the amount of coal is reduced,the temperature is correspondingly reduced,and the flue gas exergy is also reduced.In the main burner section,the exergy of flue gas reaches its peak.At this stage,the exergy loss can be reduced by increasing the air inlet temperature.At the entrance of the blaze angle area,the exergy loss can be reduced by changing the arrangement of the superheater.Then compare the combustion of the designed coal type with lignite,bituminous coal,and anthracite under the same load.It can be seen that the distribution of flue gas exergy is related to the combustion temperature of coal and the composition content of coal type.For different coal types,the overall trend of combustion in the furnace is basically the same,which proves that the improvement of the design coal type is also applicable to other coal types.The Aspen Plus software is used to build a one-dimensional simulation calculation model of the boiler according to the requirements of the boiler design specification.The main parameters of the boiler are used as input parameters.The model is divided into a combustion system module and a heat transfer system module,which mainly simulate the heat transfer of the superheater and reheater in the tail flue of the boiler.The entire process is simulated and calculated,and the calculated value is compared with the power plant value.The error of the working fluid value is within 5% and the flue gas value is within 9%,which proves the accuracy of the model.Then calculate the external exergy loss,combustion exergy loss,heat transfer exergy loss,and exergy efficiency according to the exergy analysis calculation formula.Use Matlab software to write the heat exergy loss calculation programs for each superheater and reheater.The exergy efficiency of the boiler under different load and coal conditions is calculated and analyzed in detail.It is found that the exergy loss of combustion accounts for the largest proportion,accounting for about 30%.The exergy loss of heat transfer is the second,accounting for about 20%,and the proportion of external exergy loss is the smallest.Therefore,to improve the boiler,we must start with reducing the exergy loss of combustion and reducing the exergy loss of heat transfer.Finally,the boiler was transformed using the principle of high-temperature air combustion technology.Comparing the exergy efficiency and exergy loss distribution of the improved scheme with the original scheme,it shows that increasing the preheated air temperature and steam parameters of the boiler can indeed improve the exergy efficiency of the boiler,and the exergy efficiency is increased to 48.35%.Combustion exergy losses have been significantly reduced,and heat transfer exergy losses have also been reduced,proving that this improvement plan has achieved the goals of improving exergy efficiency and saving energy,and put forward a theoretical basis for the improvement plan of power plants.