Study On Coal Combustion Detection And Optimization Of Arch Fired Furnace

The development of coal combustion technology to optimize the coal combustion to improve the efficiency and reduce NOx emission of arch fired boilers, is an important and urgent issue in the combustion utilization of low-volatile coal. Detailed tests and accurate numerical simulation of combustion process is the key point to accurate grasp coal combustion process in furnaces, reveale the mechanism of NOx formation, and comprehensively understand the effects of various factors on the combustion characteristics. They are the the guidance and basis to optimize the coal combustion and develop new high efficiency and low NOx emission combustion technology. This work, based on the analysis of combustion process in the arch fired furnaces by detailed flame detection technology employing image processing technology and numerical simulation, studied a pratical technology to improve the combustion in the arch fired furnace. The detailed description is as below:Combustion detection technology by image processing technique has been applied to study the combustion process in two typical Foster Wheeler (FW) technology arch fired furnaces, each of which is representive for 300MW and 600MW grade boilers. The measurements of the 300MW furnace, show general rules of temperature distribution and flame emissivity along the height of the furnace. Three-dimensional temperature field reconstruction study on a 600MW grade boiler has been performed. The three-dimensional temperature distribution measurements have been verified by a portable image temperature detection system and an infrared pyrometer test results. The comparison shows the temperature measurement differences between the three instruments are within 10%, which indicates the reliability of the mesurements. Numerical simulation method reliability and results are also confirmed by the temperature measurements. The analysis based on above results, shows that the high unburnt carbon and NOx emissions are mainly caused by the actual high flame center in the furnace, which reduce the residence time of pulverized coal and decrease the staged air combustion.A numerical simulation is applied to a FW technology arch fired boiler combustion process, and a method to improve the prediction of NOx emission is presented as well. By the establishment of body-fitted grid of a 310MW FW arch fired furnace, the CFD method is performed to simulate the combustion characteristics in the furnace, and the typical furnace temperature distribution, velocity field and particle trajectory have been presented. The comparison of simulated flow field and temperature distribution along the furnace height with the experimental results, shows the reliability of the simulation methods and results, and the prediction reveal the reasons account for the bad combustion in the furnace. By the CFD combustion results, the heating progress of coal particles in the furnace can be account to predict the nitrogen release by Chemical Percolation model for Devolatilization (CPD) model. Then, the volatile nitrogen and char nitrogen ratio and the proportion of volatile NOx precursors in form of HCN and NH3 are able to be calculated for the actual combustion process in the furnace. Using these computed parameters instead of the traditional experiential parameters, the NOx emission prediction accuracy is improved.On the basis of the above results, four combustion optimization methods for FW arch fired furnace are researched by numerical modeling and field tests. The effects of the proportion adjustments of vent air, C air (oil gun air) and the method to burn blended coal in the furnace on coal combustion process and NOx emission are studied, by comparasions of the momentum ratio, coal igniton, flame depth, coal burnout and NOx emission variations, and optimal operation are suggested. As the conventional combustion optimization methods are not able to fundamentally solve the shortage of the high NOx emission as a reason of lack air-staged combustion. So deep air combustion method by the independent over fire air (OFA) set above the arch of furnace is presented. The numerical simulation is employed to study the effects of the independent OFA strategy on the combustion characteristic, and introduce high-speed OFA above the arch to guarantee the coal burnout with effective reduction of NOx emission.As the conventional optimization methods not only play a limited role to improve combustion efficiency and reduce NOx emission for the arch fired furnace, but also hard to maintain stable combustion while achieve high efficiency and low NOx emission. So a technology to improve the combustion is studied by set guide vanes in the F layer secondary air box to decline the 60-70% secondary air with a certain angle into the furnace. The method is able to increase the downward/horizontal momentum ratio without any change of the flow on the arch. The declination of the secondary air also plays an injection effect on the down-flow primary air, this ensures that the main secondary air gradually offer oxygen for the coal flame along the flame path, not directly impacts the down-flowing primary air. As a result, the flame path is extended and the reduction zone in the down furnace is enlarged, which improve the coal burnout and reduce the NOx emission. Numerical simulation of combustion optimization of different F layer secondary declined angles shows the best angle is 25°. The industrial application of this technology on a FW furnace shows that this method effectively reduces unburnt carbon in fly ash, and increases combustion efficiency more than 1% with more than 20% NOx emission reduction on high load.