Numerical Simulation On Combustion Optimization Based On Soft-measuring Technique

Power plants have their own corresponding designed the types of coals,however,in practice,typically in China;they normally cannot burn a single type of coal for long time as the cost of transportation and limited coal stock will have to be considered.Changing coal type or quality during the operation period of a boiler will likely decrease boiler combustion thermal efficiency and result in a series of problems,such as combustion instability,ash accumulation and slagging,tube explosion,etc.In this paper,a combustion optimization scheme based on soft-measuring technique is put forward to solve the problems above.Explore the influence of industrial analysis indexes on coal combustion characteristics,it is found that the content of volatile on dry ash-free basis and low calorific value of coal are the two crucial parameters,and they are generally used as guides to rearrange boiler air distribution.In this paper,a soft-measuring model of coal quality is developed by finding the corresponding relationship between flue gas composition and coal quality.Solving heat balance equations combined with coal quality elements,real-time data of coal ultimate analysis indexes and low calorific value can be obtained,and then the mathematical model for volatile on ash-free basis can also be gained by fitting the relationship between the content of each element in coal and it.Based on the mathematical models above,a software program for on-line measurement of coal quality is created.The percentage of CO₂?H₂O?O₂?SO₂ in flue gas and other physical parameters such as atmospheric density and air humidity can be input into the interface,and the coal type used in the boiler at this time can be determined.It turns out that the error of coal quality information calculated by soft-measuring model is about 10%compared with the actual data.To simulate the design condition of the boiler,a three-dimensional physical model of the boiler is established and meshed according to the design instructions and appropriate mathematical models and boundary conditions are set to describe the streaming and combusting process of pulverized coal in the furnace.Comparing the simulation results with the field test results,it is found that they are in good agreement,proving that the simulation method used in this paper is of high accuracy.According to the coal type calculated online,the aerodynamic field in the furnace is reorganized pertinently.The thermal efficiency of boiler,the burnout degree of pulverized coal particles and the NOx emission are taken as the comprehensive indicator to evaluate the combustion condition of boiler under different schemes.When boiler using Wula lignite coal,the primary air velocity should be decreased because of its lower volatile content than the designed type of coal,as for its distribution mode of secondary air,the shrinked-middle one is recommended.While for Xingan lignite coal with a high content of volatile,the primary air velocity ought to be enhanced,and its recommended distribution mode of secondary air is the positive-tower one when only thermal efficiency and the burnout rate of coal particles are taken into consideration,or the shrinked–middle one when NOx emission is also included.For Gepu bituminous coal,whose volatile content is much lower than the designed type,the primary air velocity is supposed to be reduced,and the recommended distribution mode of secondary air is reversed-tower one.The content of the key components of coal currently used by the boiler can be figured out through the soft-measuring method,according to which,operators can modify the air distribution mode when the type of coal changes,thus,the operation condition of the boiler is optimized.On-line analysis and targeted air redistribution can achieve efficient,economical and clean combustion of boilers and also provide theoretical support for optimization of the air distribution of power plants when real-time information of coal quality can be obtained.