Thermal Mass Meters In Waste Incineration Process Measurement
Incineration is the burning of municipal waste at high temperatures to convert combustible waste into carbon dioxide and water. After incineration, the residual ash is only less than 5% of the original volume of the waste, which greatly reduces the amount of solid waste, and can also eliminate various pathogens. Toxic and harmful substances are converted into harmless substances.
Incineration is popular in some countries with limit resource of landfill process, and though incineration process generates heat energy and electricity.
Different technologies in measure the air flow of waste incinerator process
Flow meters included are used in the incineration process:
-Monitoring primary and secondary air to the furnace
-Measuring combustion air to a boiler
-Monitoring stack flue gas
-Measuring and monitoring emissions
At present, air flow measurement mainly by differential pressure flowmeter (eg. double venturi flowmeter, average velocity tube flowmeter, etc.), thermal mass flowmeter and other measurement methods. Because the grate of the waste incinerator has the characteristics of short straight section of the air pipe and wide air volume range ratio, the air volume measurement error is usually large, so the flowmeter selection has always been a difficult problem.
Features and difficulties of air volume measurement in incineration process
Air flow meter on applications
A, Venturi flowmeter
Venturi flowmeter cooperates with differential pressure transmitter and display instrument (or computer) to realize the measurement of fluid flow in pipeline through the measurement of differential pressure.
Qv: volume flow;
K: Coefficients related to flow coefficient, medium density and cross-sectional area of openings, etc.
Δp: differential pressure;
Features:
– high sensitivity
– low pressure loss
– Easy to install, easy to maintain and overhaul
Disadvantages: because it measures the point velocity, the flow velocity distribution in the pipeline has a greater impact on its measurement accuracy.
B, Annubar flowmeter
A new type of differential pressure flowmeter developed based on the Pitot tube principle. Its basic structure is a hollow metal rod called a detection rod. Pairs of pressure measuring holes are opened on the detection rod in the forward flow direction to measure the average total pressure p0 of the fluid in the pipeline, and the hydrostatic pressure p is measured in the back-flow direction of the detection rod or on the downstream pipeline.
The relationship between the difference △p between the average total pressure p0 and the static pressure p and the flow rate Qv is
α is the flow coefficient;
ε is the expansion coefficient;
ρ is the medium density
The advantages include:
① simpler structure and light weight;
② small pressure loss and low energy consumption;
③ better accuracy and stability;
④ convenient installation and disassembly, and low maintenance;
⑤ the average flow rate of the medium on the pipe section is adopted, which can Reflect the changing law of flow velocity distribution in the pipe.
Meanwhile, the measuring principle determines that it has inevitable shortcomings, like: ① Easy to block: Since the flow must be measured through the detection hole, as long as there are dust, particles, condensate, etc. in the fluid, blockage is unavoidable. However, the primary air of the waste-to-energy plant is relatively clean and has less dust. ②The output differential pressure is small: the average velocity tube flowmeter is based on the Pitot tube principle, and calculates the flow rate by measuring
C, Thermal mass flow meter
Thermal Mass Flow meter works base on Thermal diffusion theory. It has two sensors. One to measure the temperature of fluid (Ta), another one is a heated sensor as a velocity sensor (Tv). ΔT is temperature difference between Ta and Tv. As fluid flow increases will take away much heat from Ta, and value decrease, ΔT become decrease two. The terminal board will transfer the temperature signal into DC voltage or current signal and output.
P/ΔT=a+bQm
P: heating source power;
b, it is a coefficient related to the measured gas properties such as thermal conductivity, specific heat capacity, viscosity, etc. If the gas composition and physical properties are constant, it is regarded as a constant.
a, is a constant related to actual flow
Qm: the mas flow of fluid;
Nowadays, there’s two methods on thermal mass flow meter to get the mass flow. Constant heating current(P fixed), and Constant temperature differential(ΔT fixed).
In order to have quick response to the temperature and keep a constant temperature difference, the platinum resistance of the CTD (constant temperature differential) usually is very thin. Once there’s dust, oil or some parities on the thin resistance and caused heat sensor cannot dissipation well. In the end measurement accuracy greatly reduced. Which call it zero-off.
On the contrary, the CHC (constant heating current) the platinum resistance can be made thicker or the heating source and thermal resistance can work separately. In the waste incineration power generation project, the primary air is drawn from the waste storage pit, and its medium features are high temperature, high humidity, and large dust content, so the CHC method thermal flowmeter will be more suitable.
Features of thermal mass flow meters:
- Measure mass flow directly
- Very low-pressure loss, barely can ignore
- Wide turndown ratio
- Less requirements of straight pipeline
- Can measure very low flow rate
- No moving parts, easy to maintain
The CHC thermal mass flow meter used in waste incineration power electric project can effectively avoid the influence of gas humidity on the sensor. Overall, the air volume measurement is accurate and stable, and it avoids the problem that the Annubar flowmeter cannot accurately measure very low flows.
Air volume control is very important, and air volume measurement is the key factor in air volume control. When the wind speed of the waste incineration plant is low and the length of the straight pipe section is insufficient, the Venturi flowmeter is not effective; the Annubar flow meter is moderately priced. Except for the minimum flow condition, other working conditions are accurate and worthy of popularization; Thermal mass flow meters are accurate and stable in various working conditions, but they are expensive and suitable for the construction of high-standard waste incineration plants.