Advancing flue-gas flow monitoring techniques

Efficient operation of today’s power plant largely depends upon accurate and repeatable measurement of primary and secondary airflow to coal mills, flue gas recirculation flow, overfire airflow, airflow to individual burners and other areas. Selecting the right flowmeter for flue gas or stack gas emission flow rate measurement is critical. Such measurements are important for quantifying emissions using continuous emissions monitoring systems (CEMS) for environmental reporting for government authorities for pollution control.

Composition of flue-gas combustion

Flue gases are gases emitted due to the combustion process due to heating of fuel (liquid or solid or gaseous) and air with a stoichiometric ratio in boiler heating and process furnaces.

Flue gas production from combustion mainly consist of:

• Nitrogen (N₂)
• Carbon monoxide (CO)
• Carbon dioxide (CO₂)
• Traces of sulphur dioxide (SO₂)
• Nitrogen oxides (NO, NO₂)
• Suspended particulate matter (SPM)
• Moisture.

Flue gases are gases emitted due to combustion process due to heating of fuel (liquid or solid or gaseous) and air in thermal power plants, steel plants and foundries, cement production plants, chemical and fertilizer production process plant, many other industrial, commercial, and other facilities.

Why is flue gas flow measurement so important?

Most flue gases emission contains air pollutants harmful to human health. CEMS are mandatory for providing reporting to state and central pollution control boards for environmental pollution control. It is important to measure the composition and concentration of polluting gases as well as mass flow rates to arrive at the total emission discharge in the environment.

Flue gas flow rate measurement is imperative for:

• Optimizing electrostatic precipitator (ESP) performance by maintaining design parameters, such as specific collection area, gas velocity, and treatment time in the controlled ESP.
• Indicates early warning for preheater condenser failures
• Help regulate harmful pollutants, dust emission controls
• Useful information on optimizing mass balance
• Simple design to operate; helps in energy conservation
• Predictive and preventive measures for optimizing their process efficiency and reducing harmful emissions in the environment.

Process conditions of flue gas in stack

Process engineers design the lowest possible heat loss into the environment for better power plant thermal efficiency. Flue gas process conditions with optimum design often have below process parameters such as:

• Composition, where flue gas has moderate dust/fly-ash particles, as in a coal-fired power generation or steam generating plant or process.
• Process temperature of 130 to 180⁰C.
• Process velocity: Recommended approximately 12 to 20 meters per second (m/s).

Where should the flue gas flow rate be monitored?

To get optimum efficiency, flue gas can be monitored at the chimney or stack near to the point of sampling for lab analysis; at the inlet of the flue gas desulfurization plant (wet FGD/dry FGD ) in a thermal power plant; in the process stack in chemical production, fertilizer and steel plants; and in process venting systems.

Flue gas flow monitoring technologies, selection advice

Main technologies flue gas flow monitoring are differential-pressure (DP)-based flowmeter (with aerofoil, annubar or pitot-tube designs), non-contact ultrasonic flowmeter and an insertion thermal mass flowmeter.

A process instrumentation engineer’s task is identifying which flowmeter will be the most suitable technology for the application. Below are important factors to be considered and evaluated with comparing technologies for optimum selections:

• Duct or pipe dimension
• Insulation thickness, if any
• Process conditions such as flow rate, pressure, temperature, density, viscosity, dirt and moisture, etc.
• Installation conditions, such as horizontal, vertical and available straight lengths, time and effort, etc.
• Accuracy and repeatability needed
• Process turndown ratio needed
• Budgeted price.

Based on the above factors process engineers must understand the working principles, positive and negative attributes of different flow measurement technologies and their possibilities for selection. Each technology is discussed to help with selecting the right flowmeter for the right application.

Help with selecting a differential pressure-based flowmeter

Working principle: A differential pressure-based flowmeter with aerofoil, annubar or pitot-tube measurement restricts the flow path and measures differential pressure across primary flow elements to derive a volumetric flow rate (Figure 1). With additional continuous pressure and temperature (instantaneous density) compensation, mass flow rate can be derived.

Figure 1: A differential pressure-based flowmeter with aerofoil, annubar or pitot-tube measurement restricts the flow path and measures differential pressure across primary flow elements to derive a volumetric flow rate. With additional continuous pressure and temperature (instantaneous density) compensation, mass flow rate can be derived. Courtesy: Leomi Instruments Pvt. Ltd.

Pros:

• Established standard BS-1042/ISO 5167 for volumetric flow measurement
• Suitable up to 5 m or higher
• Rugged designed for any process conditions of industries
• Any orientation is possible
• Highly repeatable
• Site calibration is easy.

Cons:

• Higher pressure drop
• Needs periodic maintenance
• Lower accuracy 3% full-scale deflection (FSD) and may drift over time
• Lower turn-down ratio 4:1 (range of accurate fluid measurement)
• Lower flow sensitivity
• Susceptible to clogging
• High wear factor
• High installation cost.

Help with selecting a non-contact ultrasonic flowmeter

Working principle: A non-contact ultrasonic flowmeter measures volumetric flow rate consisting of a pair of ultrasonic trans-receiver that transmits and receives ultrasonic pulses across the flue gas path in both directions, resulting in a transit time (time difference) proportional to stack gas velocity. It depends mainly on the sound velocity of the gas.

Pros:

• Derives volumetric flow rate by ultrasonic beam transit-time measurement
• Used for pipe diameter up to 10 m
• Suitable for temperature up to 450⁰C
• Horizontal or vertical orientation is possible
• Turn-down ratio better than 100:1.

Cons:

• Gas must be dry and clean
• Accuracy up to ±1.5 to 3% Rd (Rd or reading is closeness to the value as a percentage of the value) and repeatability ±1%
• High initial cost
• Need good technical knowledge when installing
• Not suitable for pipes with inner lining
• Drift due to change in flue-gas sound velocity.

Help with selecting an insertion thermal mass flowmeter

Working principle: Thermal mass (calorimetric) flowmeters work on the physical principle of thermal dispersion from a heated element to the ambient medium (such as air or gases). This is affected by the velocity, density (temperature and pressure), and by the characteristic of the medium. The amount of needed energy is a function of the temperature difference (∆T) and the mass flow (Figure 2).

Figure 2: Thermal mass (calorimetric) flow meters work on the physical principle of thermal dispersion from a heated element to the ambient medium (air or gases). This is affected by the velocity, density (temperature and pressure), and by the characteristic of the medium. The amount of needed energy is a function of the temperature difference ∆T and the mass flow. Courtesy: Leomi Instruments Pvt. Ltd.

Gas flowing through two resistance temperature detector sensors (RTD) Pt-100, one reference (Tref) and another heater (Th). The temperature difference (over-temperature) ∆t between the reference sensor (medium temperature) and the heater sensor is controlled constantly. As per King’s Law, the higher the mass flow rate, the higher the cooling effect of the heater sensor, thus higher the power required to maintain the differential temperature constant. Therefore, the heater power is proportional to the gas mass flow rate.

Pros:

• Works on constant calorimetric temperature anemometry (Thermal dispersion)
• Pipe sizes suitable 15 mm to 10 m
• Insertion is rugged and works up to 400⁰C and 16 bar (232.06 psi) or more
• Any orientation possible
• Better accuracy < ±2%RD of mass flow rate
• Highest turndown ratio 100:1 or better
• Adjustable and versatile
• Lowest pressure drop
• Low cost of ownership against other flow technology
• Online thermal conductivity compensation

Cons:

• Mechanically vulnerable to damage
• Flow straightener recommended
• Affected by high moisture (>10% volume) and dirt/dust. Needs periodic cleaning or system purging.

Suggested flue-gas (stack gas) measurement technology

The guidance above is derived from practical experience. Check with manufacturers for latest design improvements or different configurations that may eliminate some disadvantages. Considering the advantages and disadvantages and process conditions with respect to different flowmeter technologies for the flue-gas (stack gas) measurement helps process engineers decide on a suitable flowmeter.

Figure 3: Efficient operation of today’s power plant depends largely upon accurate and repeatable measurement of primary and secondary airflow to coal mills, flue gas recirculation flow, overfire airflow, airflow to individual burners, and other areas. Courtesy: Leomi Instruments Pvt. Ltd.

Insertion thermal mass flowmeters are preferable for stack diameter up to 8 m or less with moderate wet or dust load conditions with a suitable purging system. Insertion thermal mass flowmeters can be a good and less expensive alternative to a DP-type flowmeter. Ultrasonic gas flow meters can be a good alternative for large stack diameters above 8 m compared to DP and thermal flowmeters.

Manish Patel is director, Leomi Instruments Pvt. Ltd. Edited by Mark T. Hoske, content manager, Control Engineering, CFE Media and Technology, mhoske@cfemedia.com.

KEYWORDS: Flue-gas measurement flowmeter technologies

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