Application of Microwave Solid Flowmeter in Pulverized Coal Detection
1 Introduction
The large-scale coal-fired generating units of thermal power plants generally use a direct-fired pulverizing system. Each coal mill has 4-8 primary pulverized coal pipes directly connected to the boiler burners, and the pulverized coal is conveyed to the boiler through the powder feeding pipeline. The burner burns. Due to the difference in the length and number of elbows of each pulverized coal pipeline, the pressure loss of each pipe is different, thereby resulting in non-uniform distribution of pulverized coal between pulverized coal pipes, resulting in that the burner cannot be under the optimum ratio of wind to coal. Normal operation reduces combustion efficiency, increases NOX emissions, and increases boiler failure rates.
2 Pulverized coal detection problem analysis
When the distribution of pulverized coal is not uniform between the pulverized coal pipelines, the pulverized coal concentration may be too high, too low, and the flow rate may be too high or too low.
When the pulverized coal concentration is too high, the following conditions occur:
Pulverized coal plugging can't transport pulverized coal into the furnace, causing the pulverized coal to burn spontaneously in the pipe to burn out the pulverized coal pipe; pulverized coal is not completely burned, the efficiency is low, CO increases, and the heating surface in the furnace and the superheater heating surface are increased. The high-temperature corrosion; furnace slagging and superheater local, seriously affect the normal operation of the boiler.
When the pulverized coal concentration is too low, the following conditions occur:
Furnace temperature is reduced, easy to extinguish, boiler pressure decreases, can not meet the load requirements; produce a lot of NOx, pollute the environment, high temperature of the superheater, and even cause superheater explosion and other accidents; in order to increase the pressure, increase the primary air (powder) flow rate Furnace slicing circle offset center of the furnace, resulting in a partial slag on the furnace wall, the exhaust temperature deviation of the tail heating surface is too large, and even cause tube burst.
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 conditions will occur:
Intensify the wear of the powder conveying pipe; the flow rate of the mixture at the outlet of the burner is too fast, and the combustion lags behind, causing the center of the flame to be deflected and easily causing local coking of the furnace wall and partial over-temperature explosion of the furnace tail superheater; combustion is not complete, carbon content in the ash As the temperature of the exhaust gas increases, the efficiency of the boiler is reduced.
When the mixture flow rate is too slow, in addition to affecting the optimal pulverized coal concentration, the following conditions occur:
The increase of pulverized coal deposited by the powder pipe causes blockage of the pulverized coal, causing spontaneous combustion of the pulverized coal, and even explosion of the pulverized coal pipeline. The flow rate of the mixture at the outlet of the combustor is too slow, the pulverized coal is separated from the main air stream, and coal consumption is increased due to long-term removal. Caused flameout of the furnace and secondary combustion blocked the lower ash outlet of the boiler.
3 Pulverized Coal Measurement Solution
The method to solve the above problems is to measure the flow rate and mass flow rate of pulverized coal in the pulverized coal pipe, and use this as a reference to adjust the secondary air volume of each burner to satisfy the best condition of combustion.
In the direct-fired pulverizing system, the amount of pulverized coal is monitored by the primary air volume entering the boiler mill. Therefore, primary air flow signals are particularly important. For the flow rate measured by the venturi flowmeter, when the current flow field is stable and uniform at present, the flow coefficient K is a constant, and only the fluid density and the pressure difference are measured, and the ventilation volume can be obtained. Due to the constraints of operating conditions and equipment conditions, the differential pressure signal is distorted, the coefficient K is not constant, and the maximum deviation is more than 34%. Therefore, controlling the amount of coal entering the boiler through baffle adjustment is not reliable. When the boiler load increases or decreases, the boiler workers can only rely on the field experience and the reference air volume detected to adjust the wind coal. If the powder flow rate and concentration on-line measuring instrument are installed in the powder conveying pipe (ie primary air pipe), the amount of pulverized coal can be controlled optimally and the coal consumption can be reduced. At the same time, the labor intensity of the furnace operator can be reduced and the working environment can be improved. For the direct-fired pulverizing system, the on-line measuring device for the flow rate and velocity of pulverized coal is installed on the primary air duct. In addition to solving the above-mentioned problem of large and unreliable air flow in the two-input and two-outlet pulverizers, it can also be detected in time. The blow-away separator air lock leaks, does not function, etc.
4 Microwave Solid Flowmeter Measurement System
4.1 Measurement principle
Microwave solid-state flowmeters use leading-edge microwave ultra-short pulse technology developed for the detection of material flow in a variety of solid-feed metal tube troughs. The microwave solid flowmeter MF3000 utilizes the microwave energy field and the solid particle's principle of microwave refraction and Doppler. The sensor transmits low-energy microwave signals to the solid materials in the feeding metal pipeline/trunk. The signal is reflected by the material and then received by the sensor. . The density of the material is measured by moving the microwave reflection energy of the material and is equivalent to a microwave counter to determine the solid material flow. It adapts to solid particles (powder) with a diameter of 1 nm to 1 cm and a measurement accuracy of 1-3% (after calibration).
4.2 System Components
A complete set of microwave solids measurement system includes: sensor probe and mounting base; communication unit (optional); transmitter (connecting sensor probe and communication unit). An RS2485 Modbus connection is used between the sensor probe and the communication unit. When the distance exceeds 1.8m, the transmitter must be used. The system is composed as shown in the figure.
4.3 Microwave Solids Flowmeter Installation
According to the conditions of the upstream and downstream areas of the coal pipeline, select suitable installation points. When the pipe diameter is larger than <200mm, 2 to 3 sensor probes should be installed on each coal pipe. The gap between the sensor probes is 150mm, and the angle between the sensor probes is 90° or 120°. For applications where the material falls freely (such as after a screw feeder or rotary valve), the best installation position is 300mm after the material fall point. When installing, first weld the sensor probe base to a fixed point, then drill a <20mm hole from the sensor probe base into the pipe, insert the sensor probe and use the mounting nut to make the sensor probe measurement surface flush with the inner surface of the pipe or lock Back 1 ~ 2mm.
4.4 Calibration of Microwave Solid Flowmeter
The calibration method of the microwave solid-state flowmeter must be calibrated online through the actual measured medium. For the thermal power plant, the pulverized coal mass flow calibration can be calibrated by the Danish P&M's dedicated pulverized coal sampling system.
4.5 Control Improvements after Application of Microwave Solids Flowmeters
The boiler pulverized coal flow measurement and the primary and secondary (three) secondary wind measurement systems are used as separate systems of the DCS to monitor in real time the wind speed, nozzle velocity and air volume of the primary and secondary (three) secondary air ducts, and the primary air ducts. The pulverized coal concentration, pulverized coal quantity and other parameters are used to guide the operators to optimize the boiler combustion.
The pulverized coal flow measurement signal is sent to the DCS, and then the corresponding pulverized coal amount balance according to the coal type is usually simple and economical: only measure the non-uniform amount of pulverized coal in the system, and calculate each burner according to the measurement result. The amount of air, because the adjustment of the air volume is relatively easy to achieve, as long as the damper damper can control the secondary air volume of the burner. Although the imbalance of the amount of pulverized coal between the burners did not improve, it was ensured that each combustor operated within the range of the design pulverized coal ratio.
According to the measurement results, the boiler maker adjusted the proportion of eolian coal in the primary air duct (ie, the powder conveying pipe) to the optimal state and matched and balanced each other, thereby changing the customary operating method of adjusting the damper baffle and the amount of powder according to experience.
For the direct-blast powder boiler, the speed of the coal feeder can be adjusted by the cumulative value of the pulverized coal flow rate of the pulverized coal pipes of the pulverizer to accurately adjust the output of the pulverizer.
The delay of large heat signal detection in the boiler combustion regulation system causes the regulation of the combustion regulation system to be insensitive to restrict the load response speed and its adjustment quality. A closed-loop control link to the powder is added to the heat regulation control link to overcome the powder supply. Intra-disturbance to improve the sensitivity of the combustion regulation system.
The closed-loop control of the amount of powder delivered by each primary air duct of the boiler can truly optimize the combustion conditions of the boiler automatically.
5 Concluding remarks
Through the accurate detection of coal coal flow rate (concentration) and air flow rate (speed) in coal-fired units, the balance of pulverized coal concentration and air flow (speed) of each coal conveying pipe is satisfied, so that the boiler combustion flame is maintained in the hearth of the furnace to ensure a balanced combustion output. The purpose of optimizing combustion and extending the service life of boilers and keeping boilers operating safely and economically for a long period of time can reduce the coal consumption of power plants by 3% and reduce the NOx emissions from thermal power plants.
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