Research On Pollutant Emissions Of Biomass Gas Re-burning In A 600 MW Coal-fired Boiler

The coal coupled biomass gas re-burning system was transformed based on the principle of fuel classification,which can realize the efficient use of biomass resources without affecting the safe and stable operation of the original boiler system.Based on a 600 MW tangential coal-fired boiler,the combustion process of coal coupled with biomass gas re-burning was studied by Fluent,and the combustion characteristics in the furnace during the reburning process and the change of flue gas composition at the outlet of the furnace were analyzed.The main research contents are as follows:（1）A biomass gasifier was built according to the principles of the biomass gasification process and the factors affecting the gasification process.The basic principles of coal coupled biomass gas re-burning was explored,and appropriate mathematical models were selected by analyzing the combustion characteristics in the furnace according to the fuel combustion process,gas flow mode,and heat exchange type.Finally,a 600 MW tangential coal-fired boiler was selected as the research object,a coal coupled biomass gas re-burning model was built based on the principle of fuel grading,and the basic factors affecting biomass gas re-burning were analyzed.The method of thermal calculation was used to determine the modification plan of the re-burning system under different conditions,and prepare for the simulation of the re-burning process of pulverized coal coupled with biomass gas.（2）The effects of biomass gas types and excess air coefficient in the re-burning zone on the reburning system were studied.Straw gas,food waste gas and biogas were selected to simulate the re-burning process under the conditions of excess air coefficient of 0.7,0.8 and 0.9in the re-burning zone respectively to be compared with the pure coal combustion case.It found that:biomass gas re-burning reduces the temperature of the main combustion zone of the boiler,but the establishment of the re-burning nozzle moves the center of the combustion flame in the furnace upwards and increases the temperature of the flue gas at the furnace outlet.When the content of hydrocarbon compounds of CH₄ in the re-burning fuel gas is higher,the effect of reducing nitrogen oxides emissions is better,and the nitrogen reduction effect of biogas re-burning is the best,the nitrogen reduction efficiency can reach to 41.20%.When the excess air coefficient in the re-burning zone is 0.7-0.8,the biomass gas re-burning boiler has the best nitrogen reduction effect.As the excess air coefficient in the re-burning zone continues to increase,the nitrogen reduction effect gradually becomes worse.（3）The effects of SOFA rates on the NO_x and CO emission characteristics of the biomass gas re-burning system were studied.Biomass gas re-burning can reduce the nitrogen oxide content in the furnace effectively,but it will also increase the CO content in the flue gas.Based on the straw gas re-burning system,adjusting the SOFA rate of the reburning system to 15%,20%and 25%respectively,the emission characteristics of NO_x and CO in the flue gas at the furnace outlet have been studied.It found that the NO_x and CO emission characteristics in the flue gas at the furnace outlet are negatively correlated.When the SOFA rate is 20%,the concentration of pollutants in the flue gas at the outlet of the straw gas re-burning boiler is the smallest,which the CO volume fraction in the flue gas at the furnace outlet is 0.76%,and the NO_x concentration is 239.82 mg/m³.Biomass gas re-burning can not only reduce the consumption of pulverized coal in thermal power plants,but also effectively reduce boiler CO₂emissions.The CO₂emission reduction of the straw gas re-burning boiler is 91.47 t/h,and the CO₂ emission reduction of the food waste gas re-burning reaches 109.70 t/h.（4）The effects of the vertical swing angle of the re-burning nozzles and the boiler load change on the re-burning system were studied.Five conditions were the re-burning nozzles swing angle of 0°,±15°and±25°.It found that:When the re-burning nozzle swings down by15°,it does not affect the stable combustion of the flame in the furnace,but also reduces the thermal deviation of the flue gas at the outlet of the furnace,which is helpful for the safe operation of the re-burning boiler.Under the condition of staying the intake volume of biomass gas,the operating conditions of the biomass gas re-burning boiler at 100%,90%,70%,and 50%loads were studied,It found that:as the boiler load decreases,the tangential circle diameter of the flue gas increases significantly,which may cause slagging on the furnace wall.As the load decreases,the emission concentration of pollutants at the outlet of the furnace decreases accordingly.