The Impact Analysis Of Combined Primary Air Pipe On Boiler Operation

In order to improve the operation flexibility of coal-fired power plants,China need to tap the potential of deep peak shaving of coal-fired units.The lowest stable load of units is mainly restricted by the combustion stability of the boiler and the safety of the environmental protection equipment.We need to start from the boiler side to improve the peak load capacity of the boiler.There is a problem of low concentration of pulverized coal in a primary air flow when the boiler run in a low load.It is easy to lead to unstable combustion and increase the amount of nitrogen oxides generated.Solving the problem properly could greatly improve the combustion stability,reduce the pressure of environmental protection equipment and finally improve the unit peak regulating ability.This paper proposes the design of combined primary air pipe to solve the problem above,that is to say,the single tube burner adopts multiple tubes combined powder feeding pipeline,and the corresponding burner is transformed into multiple nozzles burner matching the pipe diameter.The flow area can be shrunk with load decreasing by controlling the valves on the combined powder feeding pipeline which preventing the pulverized coal deposition and maintaining the concentration of pulverized coal near the optimal value at the same time.The paper simulates the aerodynamic field,temperature field and nitrogen oxide concentration field of a 660MW four angle tangentially fired boiler which compares the traditional primary air duct and combined primary air duct at low load to analyze systematically the influence on boiler operating system with combined primary air duct.The simulation above select pulverized coal combustion model,pollutant emission model and other mathematical models with ANSYS FLUENT.The results show that the aerodynamic field may stay still when upgrading the boiler with combined primary air pipe under the circumstance of proper choice of the number of parallel powder feed pipe and optimizing of multiple nozzle burner.Adapting combined primary air pipe at a low capacity could guarantee the stable ignition timely of pulverized coal,fill the furnace with more flames and balance the temperature distribution,which resolve the problem of unsteadiness of combustion at a low capacity and secure the hydrodynamic performance.The combined primary air pipe can ensure the steady combustion without oil spray at extremely low load.It can reduce the cost of operation at extremely low load,reduce the harm to boiler brought by oil spray and reduce the minimum steady combustion load.The combined primary air pipe can improve the inlet flue gas temperature of the SCR denitration system,which alleviate the problem of low efficiency and formation of ammonium hydrogen sulfate damaged the boiler’s safe at low load.On the other hand,it can control the excess air coefficient at the initial stage of ignition,reduce the production of fuel type NO_X,which reduce the total NO_X emission of the furnace outlet.The effect of two aspects can effectively control the emission of NO_X at low load and reduce the pressure for environmental protection.The combined primary air pipe can alleviate the main problems that limit the potential of coal fired units in deep load shaving,improve the deep peaking potential of large coal-fired units and improve the operation flexibility of thermal power plants.