Application of microwave solid flowmeter pulverized coal detection

1 Introduction Large-scale coal-fired units in thermal power plants generally adopt a direct blowing pulverization system. There are 4 to 8 primary air pulverized coal pipes directly connected to the boiler burner at the outlet of each pulverizer, and the pulverized coal is transported through the pulverized pipeline. to the boiler burner for combustion. Due to the different lengths and number of elbows of each pulverized coal pipeline, the pressure loss of each pipeline is different, resulting in uneven distribution of pulverized coal among the pulverized coal pipelines, and as a result, the burner cannot operate under the condition of the best air-to-coal ratio. Normal operation reduces combustion efficiency, increases NOX emissions and increases boiler failure rate. 2 Analysis of problems in pulverized coal detection When pulverized coal is distributed unevenly among pulverized coal pipelines, the concentration of pulverized coal is too high or too low, and the flow rate is too high or too low. (1) When the pulverized coal concentration is too high, the following situations will occur: the pulverized coal is blocked, and the pulverized coal cannot be transported to the furnace, and at the same time, the pulverized coal in the pipeline is caused to spontaneously ignite and the pulverized coal is burned out; the pulverized coal is not completely burned, and the efficiency is low , CO increases, and aggravates the high-temperature corrosion of the heating surface in the furnace and the heating surface of the superheater; local slagging of the furnace and superheater seriously affects the normal operation of the boiler. (2) When the pulverized coal concentration is too low, the following situations occur: the furnace temperature drops, it is easy to extinguish, the boiler pressure drops, and the load requirement cannot be met; a large amount of NOX is generated, the environment is polluted, the superheater is high temperature, and even accidents such as superheater tube burst are caused. ;In order to increase the air pressure and increase the flow rate of the primary air (powder delivery pipe), the tangential circle of the furnace is offset from the center of the furnace, resulting in local slagging of the furnace wall, excessive deviation of the smoke temperature on the heating surface at the tail, and even tube explosion. (3) When the flow rate of pulverized coal and air mixture is too fast, it will affect the optimal concentration of pulverized coal, and the following situations will occur: the wear of the powder delivery pipe will be aggravated; the flow rate of the mixture at the outlet of the burner will be too fast, and the combustion will lag behind, causing the flame center to deviate And it is easy to cause local coking of the furnace wall and local overheating of the superheater at the end of the furnace to burst the tube; the combustion is not complete, the carbon content in the ash and the temperature of the exhaust gas increase, and the boiler efficiency is reduced. (4) When the flow rate of the mixture is too slow, in addition to affecting the optimal pulverized coal concentration, the following situations will occur: the pulverized coal deposited in the powder delivery pipe will increase, causing pipe blockage; causing the pulverized coal to spontaneously ignite, and even the pulverized coal pipeline explosion; the burner If the flow rate of the outlet mixture is too slow, a large amount of pulverized coal will be separated from the main air flow, and the long-term removal will cause an increase in coal consumption, and it will also cause the furnace to stall and secondary combustion to block the ash outlet at the lower part of the boiler. 3 Pulverized coal measurement solution The method to solve the above problems is to measure the pulverized coal flow velocity and mass flow in the pulverized coal pipe online, and use this as a reference to adjust the secondary air volume of each burner to meet the best combustion conditions. In the direct blowing pulverizing system, the control of pulverized coal is monitored by the primary air volume entering the boiler pulverizer. Therefore, the primary air flow signal is particularly important. For the flow rate measured by the Venturi tube solid flow meter, when the front and back flow fields are stable and uniform, the flow coefficient K is constant, and the ventilation rate can be obtained only by measuring the fluid density and pressure difference. Due to the constraints of the working environment and equipment conditions, the differential pressure signal is distorted, the coefficient K is not constant, and the maximum deviation is more than 34%. Therefore, it is unreliable to control the amount of coal burning into the boiler by adjusting the air volume through the baffle.

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