Smoke humidityThe measurement accuracy of flue gas flow rate, pollutant concentration, and standard dry flow rate is directly affected by physical expansion effects, instrument interference, and data conversion requirements, which are manifested as:

1、 Direct impact on smoke flow rate

Measurement deviation caused by volume expansion

When the humidity of the flue gas increases, water vapor occupies the space and causes the total volume to expand. If the flow rate of online monitoring equipment is calculated by volume, humidity changes will directly introduce errors. For example:

When the humidity increases from 6% to 10%, the calculated deviation of SO ₂ dry concentration can reach 0.9mg/m ³ (from 21.3mg/m ³ to 22.2mg/m ³).

In the calculation of standard dry flow rate, for every 2% increase in humidity, the converted flow rate value decreases by about 2% (for example, the standard dry flow rate is 0.92Qsh at 8% humidity and 0.9Qsh at 10% humidity).

Differences in instrument principles exacerbate errors

Differential pressure flowmeter: The increase in humidity causes the expansion of flue gas volume, changes in airflow velocity and pressure distribution, and the measured value deviates from the true value.

Thermal flowmeter: The thermal conductivity of water vapor is different from that of dry flue gas, which interferes with the perception of heat transfer and causes errors in flow rate judgment (similar to the phenomenon where humid air appears colder in winter).

2、 Indirect impact on pollutant concentration measurement

Zero drift of cold dry CEMS

The cold drying method removes most of the moisture through pretreatment (HJ76-2017 requires dew point ≤ 4 ℃ after dehumidification), but if air without humidity treatment is used to calibrate the zero point, the difference in humidity between the calibration gas and the sample gas will cause zero drift and introduce mg/m ³ level errors.

Case: In ultra-low emission scenarios, when the humidity of the zero calibration gas of the NDIR analyzer is not processed, the measured value of SO ₂ may be 3-5mg/m ³ lower.

Conversion error of wet basis concentration

The hot wet method and dilution method CEMS have not completely removed moisture, and the measured SO ₂/NOx concentration is the wet basis value, which needs to be converted to the dry basis concentration through humidity data.

Formula: Dry basis concentration=Wet basis concentration/(1- Humidity percentage)

Impact: When the humidity monitoring error is ± 2%, the deviation of SO ₂ dry basis concentration can reach ± 0.5mg/m ³ (taking 20mg/m ³ wet basis concentration as an example).

SO ₂ dissolution loss

In high humidity flue gas, SO ₂ is easily soluble in water to form sulfuric acid, which increases the resistance of the sampling system or causes the filter cartridge to rupture, resulting in a generally low SO ₂ test data by 10-15%.

3、 Composite impact on standard dry flow calculation

The standard dry flow rate needs to be converted based on temperature, pressure, and humidity, and the formula is:

Humidity error propagation: If the humidity monitoring value is 2% higher and the calculated standard dry flow rate is 2% lower, it will result in a 4% underestimation of the total pollutant emissions (assuming that the concentration and flow rate errors are independent).

Human interference risk: Installing phosphoric acid titration at the inlet of the cold dry CEMS sampling pipeline (increasing the humidity of the sample gas) can underestimate the standard dry flow rate by 5-10%, suspected of evading environmental taxes.

4、 The influence of humidity in typical scenarios

scene	Humidity affects performance
High humidity flue gas (>15%)	The wet and rotten sampling filter cartridge, SO ₂ dissolution loss, and abnormal pressure distribution of the differential pressure flowmeter result in a flow measurement error of>10%.
Cold dry CEMS	When the humidity of the zero calibration gas is not processed, the measurement deviation of low concentration SO ₂ reaches 5mg/m ³; When the dehumidification efficiency is insufficient, the SO ₂ loss rate is greater than 3%.
Hot wet CEMS	When the humidity monitoring value is 2% lower, the dry basis concentration of NOx is underestimated by 0.8mg/m ³, the standard dry flow rate is overestimated by 2%, and the total emission error is offset.
Start stop process	When the slope of humidity change is greater than 5%/min, the sensor response lags behind, causing data fluctuations of more than 20% within 10 minutes, and process interlock adjustment needs to be triggered.