Introduction

The rectangular flue is generally an ideal choice for flue gas sampling points in coal-fired power plants. As for the selection of sampling location, the sampling point should avoid the position with elbow and section change, and try to choose the vertical pipe section, so as to ensure the uniformity and stability of the flue gas flow field on the section. If elbows, valves, etc. cannot be avoided, the sampling point shall be located at a location more than 4 times the equivalent diameter from the downstream direction and 2 times the equivalent diameter from the upstream direction (Figure 1).

In order to make the flue gas parameters collected representative, relevant specifications require that the flue section be divided into small pieces with the same area, and the center point of each piece is the sampling point.

As the flue gas contains dust, CO2, SO2, NOx, etc., and is high temperature, the environment is relatively harsh, so the monitoring equipment needs to meet the requirements of anti-corrosion and wear resistance for a long time.

Fig. 1 Schematic Diagram of Sampling Position

Classification of flow meters

There are many instruments to measure the flow rate on the market, but there are few in such high-temperature, acid, and dust environments as coal-fired power plants. There are mainly three types of flowmeter, namely pitot tube flowmeter, thermal flowmeter, and matrix flowmeter.

(1) Pitot tube flowmeter. Pitot tubes commonly used for flue gas velocity measurement can be divided into standard pitot tubes and S-shaped pitot tubes. The standard pitot tube, also known as an L-shaped pitot tube, is a right-angled bent metal tube with a semicircular cone or oval head. One end of the right angle is the measuring head, and the other end is the supporting rod. Two holes are opened at the probe end, which is the total pressure hole and the static pressure hole. There is a directional hollow rod inside the strut to connect the probe opening with the micro manometer. The S-shaped pitot tube is made of two identical metal hollow tubes welded back to back. During fabrication, it is required that the two metal tube probe parts shall be drilled in two directions, and the section of the holes shall be strictly parallel.

When measuring the gas flow rate, the total pressure hole is the direction of gas flow and the static pressure hole is the direction of gas flow. Connect the end of the pitot tube to the micro manometer, and use the Bernoulli equation to obtain the velocity and differential pressure of a point in the fluid.

(2) Thermal flowmeter. The thermal flowmeter uses the diffusion of heat to work. There are two temperature detection elements in the probe: one is the unheated end and the other is the heating end. The unheated end is used to measure the temperature of the fluid medium itself as a reference end; The heating end is the measuring end, which is close to the heat source and is used to measure the temperature of the constant heat source surface after being taken away by the fluid medium. The gas mass flow rate is proportional to the temperature difference between the reference end and the measuring end (Figure 2).

Fig. 2 Thermal flowmeter

According to the measuring principle, a thermal flowmeter can be divided into the thermostatic differential type and constant power type. The principle of constant temperature difference is mainly to maintain a constant temperature difference by constantly adjusting the heater current and measuring the constant current change. However, this kind of thermal flowmeter requires a harsh environment, which will produce unstable flow measurement signals in humid and dirty gas environments. The principle of constant power is to keep the current of the heater unchanged, and the temperature difference between RTDs (Resistance Temperature Detectors) varies with the flow. The constant power thermal flowmeter has loose environmental requirements and stable output.

(3) Matrix flowmeter. The measurement principle of a matrix flowmeter is the differential pressure method. The matrix flowmeter is placed in the flue. When the flue gas flows in the flue, the kinetic energy of the airflow at the windward side is converted into pressure. The pressure at the windward side is relatively high, and the pressure here is called full pressure; The leeward side is not impacted by smoke, and its pressure is called static pressure. The difference between total pressure and static pressure is called differential pressure, and the wind speed in the pipe is related to the differential pressure, so the wind speed is in direct proportion to the differential pressure. With differential pressure, find the corresponding relationship between differential pressure and wind speed, and then measure the wind speed in the flue.

Summary

From the statistical results of power generation enterprises, the reliability of matrix differential pressure flowmeter is better than that of single point differential pressure flowmeter and thermal flowmeter; Thermal flowmeter is suitable for measuring relatively clean gases such as primary air and secondary air in thermal power plants. It is not suitable for measuring flue gas of denitrification systems containing smoke and dust, nor for measuring flue gas of desulfurization systems with high humidity.

In these power generation enterprises, except for individual power plants, the transformation of matrix differential pressure flowmeter has been basically completed at the outlet of the desulfurization system, which can meet the requirements of environmental protection departments. However, due to such reasons as fund and site, a few power plants still use single point differential pressure flowmeter at the inlet of the desulfurization system, which is not conducive to the operation control of the desulfurization system. 

It is recommended that power plants where conditions permit should install a matrix differential pressure flowmeter at the inlet of the desulfurization system. There are also some power plants that do not install flue gas flow meters at the inlet and outlet of the denitration system at the same time, which will lead to problems in the denitration system, which is not conducive to cause analysis.