Ultrasonic flowmeters are not a new technology, but they are used for flue gas measurement and require products with high technical content. So first, we need to solve the problem of low-frequency high-power ultrasonic transducer that can adapt to large pipe diameter and low-pressure gas, followed by the key for noise interference digital filtering technology, to consider the sensor temperature and corrosion resistance, and finally to solve the sensor alignment, blowing The final solution is the alignment of the transducer, the configuration of the blowing air and other engineering issues.
1. Difficulties in flue gas flow measurement
1.1 Unstable flow pattern
We usually refer to the flue before entering the chimney as the flue. For small boilers or incinerators, the chimney and flue are generally circular and usually in the range of 0.5 to 2m in diameter. Due to the high vents, for large coal-fired boilers, CEMS systems are usually installed on horizontal rectangular flue pipes, which are very large: up to 5.3 x 6m for 300MW units and up to 5.7 x 10m for 600MW teams. They are often irregular in shape as they are welded on site with steel plates. Due to the lack of straight pipe sections, the flow pattern in the flue is usually unstable. The flow pattern in a circular vent is shown in the diagram above. The rectangular chimney is more extensive, and the flow field is irregular, so its internal flow pattern is more difficult to describe.
1.2 Contains dust, water vapor, sticky impurities
The Flue gas has corrosive gases, dust, water vapor, etc. When the outlet flue gas of a wet desulphurization plant contains sticky gypsum slurry droplets, these can cause a range of problems for the flow measurement instrument, such as corrosion, staining, and blockage.
1.3 Low pressure, high temperature, and low gas flow rate
Flue gas is usually slightly positive or negative pressure, its temperature is generally between 50 and 150°C, and the flow rate is relatively low, which sometimes puts differential pressure flow meters with high lower limits to the test
2. Comparison of commonly used flue gas flow meters
2.1 Differential pressure flow meters such as Pitot tube
Domestic flue gas flow measurement is based on differential pressure flow meters, mainly S-type Pitot tubes. It is based on the principle of differential pressure. Throttle element placed in the flue, the flue fluid through the throttle, before and after the throttle will produce a distinct pressure difference. For the throttle element’s specific shape and size, a particular pressure measurement position and straight pipe section before and after, specific fluid parameters, the relationship between the differential pressure ΔP before and after the throttle element and the flow rate Q is following Bernoulli’s equation. The flow rate is obtained by measuring the differential pressure.
Advantages: the point flow rate measurement has imperfections when measuring the average differential pressure at multiple points in the same cross-section, despite the availability of numerous throttling devices, and is rarely used in practical applications. The throttling element is in contact with the measuring medium, and impurities such as dust and liquid droplets in the medium will adhere to the throttling element. It can cause flow meter pressure measuring hole clogging and high maintenance.
There are a small number of flue gas measurements using this flowmeter, generally used in boilers where the fuel is relatively pure, such as gas boilers.
Principle: Using two platinum metal wire wound RTD sensors, one heated, the other feeling its thermal diffusion, divided into constant power and constant temperature difference in two forms, direct measurement of the mass flow of gas. If the gas components are relatively large, the obtained flow values may not correspond to the actual thermal flow. The limitations of thermal flowmeters are broadly based on the following four points:
(1) Heating the sensor does not tell whether the flow rate is reduced or whether particles are adhering to the outer surface of the sensor, as both will reduce. A practical recommendation is to add a back-blowing system for compressed air, but we are not sure when the air-cleaning method will; however, we are unsure when the air-cleaning procedure will be completed, and therefore thermal flow meters are not suitable for measuring in media with adhering particles.
(2) The heating sensor does not distinguish between increased flow velocity, droplet impact, or evaporation of surface water vapor. Both of which would increase the heat loss and therefore cannot be used in liquid droplet streams.
(3) Even if there is no flow in the axial direction, the non-axial gas flow will still cool the sensor, while in the non-axial order, there is a very high gas flow. The thermal flowmeter is also unsuitable in cases where there is a very high gas flow in the non-axial direction.
(4) Point flow measurement is available with multiple sensors for the same line or for the same cross-sectional average differential pressure. However, pressure is rarely used in practical applications.
2.3 Optical Scintillation Flue Fas Flowmeters
This is a relatively new technique used to measure flue gas flow velocity before it was used to measure aircraft runway wind speed, atmospheric wind speed by starlight flashes, etc. The principle is schematically illustrated as follows.
The LED light is transmitted through the flue and is received by two sensors, A and B, close to each other. Due to the scattering and refraction of eddies and dust in the flue gas, the signals received by the sensors form a specific characteristic spectrum, and by measuring this time difference, the wind speed can be obtained. There is also a lightless method that uses the infrared signature of the hot flue gas itself to detect the relevant data.
Advantages: line average flow velocity measurement across the flue; can measure particulate matter simultaneously.
Disadvantages: eddy dust disturbance is not present in all flue pipes and cannot be used in some sites. As with optical dust meters, the optical window will adhere to impurities such as dust and liquid droplets in the medium, even with blowing air, resulting in measurement failure and high maintenance; the blowing air can cause a flicker effect and cause measurement errors.
3. Ultrasonic gas flow meters
The ultrasonic flow meter is developed for high humidity, low pressure, low flow rate, large pipe diameter industrial or municipal site conditions. A flow meter to meet municipal and industrial measurement needs. It is versatile and can work alone or be integrated into a CEMS system.
3.1 Working principle
The speed of ultrasound propagation in a fluid is related to the fluid flow rate, and therefore, it enables flow measurement. Ultrasound is transmitted and received in a dual probe mode. They can be used interchangeably or can be shared and received in both directions. As shown in the diagram below, two ultrasonic sensors are installed in opposite directions upstream and downstream of the gas pipe.
3.2 Unique advantages
Let the ultrasonic signal be S(n), and the noise interference signal be U(n). That is, the actual sampled signal X(n) = S(n) + U(n). Assuming S(n) is assumed to be periodic. The period is M, the sample length of X(n) is N and N” M. Then the autocorrelation function of S(n) is:
Ultrasonic gas flowmeter using self-correlation detection technology effectively overcome the pipeline vibration and valve opening and closing and other reasons caused by noise interference, greatly improving the anti-interference ability of the instrument. The instrument can be adapted to the harsh conditions of the industrial site.
3.3 Fully digital electronic unit
The electronic unit uses the latest microelectronic technology and components, significantly reducing the number of electronic components in the secondary instrument. The use of The digital algorithm program makes the device’s signal processing more accurate and the calculation speed faster. It also increases the anti-interference capability of the secondary device.
3.4 Advanced sensors
The ultrasonic sensor uses a corrosion-resistant stainless steel housing to measure the flow of various gases in industrial and municipal processes. The Teflon surface is not easily adhered to and is adaptable to wet, dirty, and mixed gas media. Selected frequency, beam angle, and transmitting power can solve the measurement problems of large pipe diameter, low pressure, and low flow rate. The sensor surface is in contact with the process gas during measurement and for flue gas with plug-in installation.
3.5 High accuracy
The line average flow rate through the flue can be measured with an accuracy of ±1 to ±2% of the reading and a wide range ratio of up to 150:1. The sensor can be arranged in a dual-channel X arrangement for places where the straight section is insufficient. The two channels can be summed and averaged to improve measurement stability and accuracy. The instrument can measure in both directions. Additional temperature and pressure measurements can be added according to customer requirements to calculate mass. The flow rate can be calculated by adding temperature and pressure measurements according to customer requirements.
4. Flue Gas Flow Measurement Systems
4.1 System Components
4.2 Installation specifications
4.2.1 Selecting the right measuring point
Ultrasonic flowmeter sensor installation requirements: in the flue/chimney on the opposite sides of the hole, the sensor is strictly in the center to be integrated. Consider the flue diameter/span, straight pipe section, temperature, particle concentration, and the site platform to build a suitable measuring point must be selected, taking into account the flue diameter/span, straight pipe section, temperature, particle concentration, and the construction of the site platform.
4.2.2 Blowing air configuration
Blowing air has two purposes, one is to protect the sensor from the high-temperature impact of the flue gas; the second is to avoid the sensor being stained with particles, sticky droplets, and other impurities, of course, a small amount of staining on the ultrasonic sensor does not have a significant impact. Blowing air can be used as instrument air, and pressure requirements are not high but higher than the flue pressure.
4.2.3 Limitations of ultrasonic flue gas flow meters
The installation of the sensor requires holes to be punched on two opposite sides of the flue/chimney and the sensor to be strictly centered, so in practice adds to the difficulty, with the flue often requiring platforms to be built on both sides. Since the sensors must be installed upstream and downstream at an angle of up to 60°, more space is required, making installation in the already short straight pipe section even more confined. This makes installation in an already inadequate linear area even more confined.
Flue gas is the final exhaust gas released from the combustion process, and it is mainly produced in China by various coal-fired boilers and incinerators. Still, there are also a few gas and oil-fired boilers, blast furnaces, coke ovens, industrial kilns, etc. However, because flue pipes are usually large in diameter (many rectangular flue pipes) when it has low pressure, short straight pipe sections, contains dust, water vapor, and other impurities, these flow measurement techniques also have some specific application problems, which can cause problems for the measurement. This can cause problems in terms of stability and routine maintenance.
As an essential parameter in the continuous flue gas emission monitoring system CEMS for stationary sources, flow measurement systems must be enhanced with automatic calibration and diagnostic techniques to improve accuracy and stability. Flow measurement systems must be improved with automatic calibration and diagnostic procedures to improve measurement accuracy and tranquility by establishing an emissions trading rights system. As the emissions trading rights system is established, the demands on the measurement system will increase.