Applications of Online Monitoring Technology for Flue Gas Moisture (Humidity)-1
Fuel Cell Sensor Technology
Flue gas emissions from various burners, industrial and commercial boilers have caused severe atmospheric pollution. Monitoring toxic and harmful gases in flue gas is a important aspect of environmental protection efforts. Continuous Emission Monitoring Systems (CEMS) have emerged to address this need，which generally quantifies flue gas pollutants on the basis of dry flue gas conditions. Nevertheless, industrially discharged flue gas is not ideal dry gas and always contains a certain amount of moisture. Therefore, flue gas humidity has become an essential measurement parameter in flue gas pollution source monitoring, and the accuracy of its measurement directly affects the calculation of total pollutant emissions and concentrations, as well as the evaluation of flue gas purification system efficiency.
In addition, humidity calibration is also a significant challenge. This is due to the difficulty in manufacturing high-temperature humidity generators, which affects the traceability of measurement values from online humidity instruments. To verify and calibrate flue gas humidity meters, it is essential to have devices capable of generating standard humidity sources, as well as humidity benchmarks and standards. Humidity measurement methods capable of absolute humidity determination can serve as humidity benchmarks and gases with known humidity levels can also serve as humidity benchmarks. The standard "The determination of particulates and sampling methods of gaseous pollutants emitted from exhaust gas of stationary source" (GB/T 16157-1996) specifies three methods for measuring flue gas humidity: the wet-dry-bulb method, the condensation method, and the gravimetric method. Serving as reference methods for flue gas humidity detection, these three methods can be used to calibrate flue gas humidity meters. Besides, humidity generators can produce constant moist gas under specific temperature and pressure conditions, and can also be used to calibrate flue gas humidity meters. With the advancement of technology and the nation's increasing emphasis on environmental protection, there are currently four primary methods for online measurement of high-temperature flue gas humidity in China: constant flow injection method (wet-dry bulb), resistance-capacitance method, zirconia-based ion flow (limiting current) method, and infrared spectral absorption method.
Introduction to Measurement Methods for Flue Gas Humidity
>> Wet-dry Bulb Method
The wet-and-dry-bulb method measures relative humidity of air based on the difference effect between wet-bulb and dry-bulb temperatures. Water molecules evaporate from the wet bulb surface into water vapor, which need to absorb latent heat of vaporization. Continuous evaporation keeps absorbing heat from the surface and cools the wet bulb. The cooling degree is determined by the relative humidity of the surrounding air, atmospheric pressure, and wind speed. If atmospheric pressure and wind speed remain constant, the higher the relative humidity, the lower the rate of water evaporation from the wet-bulb surface, and the smaller the surface temperature of wet bulb which is the difference between the wet-bulb temperature and the dry-bulb temperature; conversely, the greater the difference between the wet-bulb and dry-bulb temperatures. Accordingly, by measuring the difference between the wet-bulb and dry-bulb temperatures and determining the relationship between relative humidity and this temperature difference, the relative humidity can be calculated [2,3].
>> The Principle of Humidity Measurement by Wet-Dry Bulb Method
According to the principles of heat and moisture transfer, when thermal and moisture equilibrium is reached, the heat transfer quantity Q1 from the air to the wet bulb equals the latent heat Q2 required for the evaporation of moisture from the gauze, that is: Q1 = Q2 (1)
Based on the heat transfer principle: Q1=α（t-tw）F (2)
In the formula: α is the heat exchange coefficient between air and the water surface of wet-bulb, W/m2 ·℃; t is the temperature of dry bulb, °C; tw is the temperature of wet bulb, °C; F is the surface area of wet-bulb, m².
According to the moisture transfer principle and Dalton's evaporation law, the mass of evaporated water is directly proportional to the vapor saturation deficit of surrounding air and the evaporation area, and inversely proportional to the atmospheric pressure at that time. Therefore, the moisture exchange rate[4] can be expressed as:
In the formula: W is the moisture exchange rate of moisture, kg/s; r is the latent heat of vaporization, J/kg; β is the moisture exchange coefficient, kg/(m²·s·Pa); F is the surface area of wet bulb, m²; B is the actual atmospheric pressure, Pa; P´q,b is the partial pressure of saturated water vapor at wet-bulb temperature, Pa; Pq is the partial pressure of water vapor in the air, Pa.
Derived from Formula (1), (2) and (3):
In the formula: the psychrometer coefficient 
Therefore, the relative humidity is:
The wet-dry bulb method used for monitoring flue gas from pollution sources generally employs two identical thermocouples as temperature-sensing elements, one for measuring dry-bulb temperature and the other for wet-bulb temperature. The sensing element for dry-bulb temperature is positioned within the main flue gas flow, while the sensing element for wet-bulb temperature is wrapped with cotton gauze which is connected to a water container. The wet bulb and the surrounding flue gas are treated as a single system, with no consideration given to radiative heat conduction. An automatic moisture content measurement device, based on the dry-wet bulb principle, uses a microprocessor to control sensors that measure and collect parameters such as surface temperatures of wet bulb and dry bulb, as well as the pressure across the wet-bulb surface and exhaust static pressure. It derives the saturated water vapor pressure at the wet-bulb surface temperature and, combined with the input atmospheric pressure, automatically calculates the flue gas moisture content using the formula.
1-Flue;
2-Dry-bulb thermometer;
3-Wet-bulb thermometer;
4-Insulated sampling tube;
5-Vacuum pressure gauge;
6-Rotameter;
7-Air suction pump
>> Constant Flow Jetting Method
Constant flow jetting method (wet-dry bulb). The basic principle of wet-dry bulb humidity measurement: a temperature sensor (5) measures the flue gas temperature as the dry-bulb temperature, and a measuring cell (3) is filled with a certain amount of water. Water in the water tank (1) is continuously supplied to the measuring cell by a hose pump (2). The water level in the measurement cell is monitored by a sensor, and the pressure within the cell is also detected by a sensor to ensure measurement accuracy and monitor the liquid. A temperature sensor (6) is installed inside the measurement cell and needs to be positioned below the water surface. Flue gas is then continuously jetted at a constant flow rate directly above the water surface in the measurement cell, and the detected temperature is taken as the wet-bulb temperature (see Figure 2). Based on the principles of heat transfer and thermodynamics, the following mathematical formula can be deduced:
In the formula
U—Relative humidity %;
etw—Saturated water vapor pressure at wet-bulb temperature;
et—Saturated water vapor pressure at dry-bulb temperature;
P—Atmospheric pressure;
∆t—Temperature difference between dry-bulb and wet-bulb;
A—Constant related to wind speed.
Chang Ai's Constant Flow Jetting High-temperature Hygrometer
Chang Ai's CI-PC39 has been successfully applied to high-temperature and high-humidity measurement processes, as well as in processes involving corrosive and dust-laden gases. In many industrial processes, it is necessary to monitor and control process gas humidity to ensure product quality, optimize energy efficiency or meet emission reduction targets. The CI-PC39 is a process hygrometer designed for the most stringent industrial application requirements, including humidity measurement in harsh mediums with corrosion resistance (such as waste incineration plants, sulfuric acid, hydrofluoric acid, or aqua regia). It features accurate and reliable measurement data, a wide range of temperature adaptability, low maintenance workload, and long service life. Its disadvantages include a high price, large size, and the need for regular water refilling.
Based on the mathematical formula above, we can clearly see that the constant-current jetting method for measuring humidity in flue gas is achieved through indirect measurement of flue gas temperature. Its representative product is shown in Figure (3).
Temperature measurement technology is relatively mature and reliable. Even under extremely harsh working conditions, on-site replacement of temperature sensors can be accomplished conveniently. Although the constant flow jetting method is the working principle of the wet-dry bulb, it is not similar to the hygrometers commonly used in the market by meteorological departments to measure the air relative humidity. This hygrometer has achieved the aforementioned measurement results through a brand-new—even revolutionary—innovative design.
Condensation Method
The principle of the condensation method is as follows: a certain volume of exhaust gas is extracted from the flue and pass through a condenser. The moisture content in the exhaust gas is calculated by summing up the amount of condensed water and the amount of water vapor contained in the saturated gas discharged from the condenser.
● Condenser, see Figure (1)
● Flue dust sampling device: It consists of a flue dust sampler (capable of measuring tr, tv, Pr, Qrw, timing setting n and gas drying function), sampling tube and air pump.
>> Detection Procedures
● Connect the condenser to the cold water pipe or place it directly in ice water, then connect the entire system as shown in Figure 2 and check the entire system for gas leaks;
● The inspection method of gas leaks: block the rubber tube between the sampling tube and condenser, then start the air pump. When the negative pressure indicated by the pressure gauge before the flowmeter reaches 6.6 kPa (50 mmHg), seal the rubber tube between the flowmeter and the air pump and stop pumping. If the indicator of pressure gauge does not drop by more than 0.2 kPa (1.5 mmHg) within 1 minute, the system is airtight.
Figure (2) Humidity Measuring and Sampling System
Calculation
The volume percentage of water vapor content in the flue gas shall be calculated according to the following formula:
In the formula:
Gw-Amount of water condensed in the condenser, g;
Vs- Sampled volume of flue gas (at measurement conditions), L;
Pv- Saturated vapor pressure of the gas after passing through the condenser, Pa;
this value can be obtained from the attached table according to the saturated temperature tv of the gas at the condenser outlet;
461.4 ...Gas constant of water vapor
In the formula, Vs represents the actual flow rate under the measurement conditions. When a rotameter is used to measure sampled gas volume, the rotameter reading Qrw during sampling shall be corrected to the actual flow rate Qr under measurement conditions, and then multiplied by the sampling time to obtain the sampled gas volume Vs under measurement conditions, that is,
In the formula
Qrw-The rotameter reading during sampling, L/min;
Rr- he gas constant of the gas passing through the rotameter, can be taken as 286.7J/kg.K.
Tr-Gas temperature before rotameter, K; [Note: Tr=273+tr]
Pr-Indicated gas pressure before rotameter, Pa;
n-Sampling time, min;
In actual measurements, the error resulting from this formula of calculating moisture content is very small and generally negligible. Therefore, to simplify the calculation, the rotameter reading can be substituted directly into the formula without correction.
Example: Given the mass of condensed water in the condenser is Gw=15.4g, outlet temperature of condenser is tv=15℃, gas temperature before flowmeter is tr=17℃, pressure before flowmeter is Pr.= -3335Pa, atmospheric pressure is Ba=100584Pa, sampled gas volume (at measured conditions) is Vs=950L, calculate the volume percentage Xsw of water vapor content in flue gas.
Solution: It is found from the attached table that the saturated water vapor pressure Pv.=1706Pa at saturated temperature tv=15℃. Substituting this value into the formula, the volume percentage Xsw. of water vapor content in the flue gas is
Attached Table 1:
| Temperature | Saturated vapor pressure | Temperature | Saturated vapor pressure | Temperature | Saturated vapor pressure |
| t/℃ | /(103Pa) | t/℃ | /(103Pa) | t/℃ | /(103Pa) |
| 0 | 0.6129 | 35 | 5.6267 | 70 | 31.176 |
| 1 | 0.65716 | 36 | 5.9453 | 71 | 32.549 |
| 2 | 0.70605 | 37 | 6.2795 | 72 | 3.972 |
| 3 | 0.75813 | 38 | 6.6298 | 73 | 35.48 |
| 4 | 0.81359 | 39 | 6.969 | 74 | 36.978 |
| 5 | 0.8726 | 40 | 7.3814 | 75 | 38.563 |
| 6 | 0.93537 | 41 | 7.784 | 76 | 40.205 |
| 7 | 1.021 | 42 | 8.2054 | 7 | 41.905 |
| 8 | 1.073 | 43 | 8.6463 | 78 | 43.65 |
| 9 | 1.1482 | 4 | 9.1075 | 79 | 45.487 |
| 10 | 1.281 | 45 | 9.5898 | 80 | 47.373 |
| 11 | 1.3129 | 46 | 10.094 | 81 | 49.324 |
| 12 | 1.4027 | 47 | 10.62 | 82 | 51.342 |
| 13 | 1.4979 | 48 | 1.171 | 83 | 53.428 |
| 14 | 1.598 | 49 | 1.745 | 84 | 5.585 |
| 15 | 1.7056 | 50 | 12.34 | 85 | 57.815 |
| 16 | 1.8185 | 51 | 12.97 | 86 | 60.19 |
| 17 | 1.938 | 52 | 13.623 | 87 | 62.49 |
| 18 | 2.064 | 53 | 14.303 | 8 | 64.958 |
| 19 | 2.1978 | 54 | 15.012 | 89 | 67.496 |
| 20 | 2.38 | 5 | 15.752 | 90 | 70.17 |
| 21 | 2.487 | 56 | 16.52 | 91 | 72.823 |
| 22 | 2.647 | 57 | 17.324 | 92 | 75.614 |
| 23 | 2.8104 | 58 | 18.159 | 93 | 78.494 |
| 24 | 2.985 | 59 | 19.028 | 94 | 81.465 |
| 25 | 3.169 | 60 | 19.932 | 95 | 84.529 |
| 26 | 3.3629 | 61 | 20.873 | 96 | 87.68 |
| 27 | 3.567 | 62 | 21.851 | 97 | 90.945 |
| 28 | 3.7818 | 63 | 2.868 | 98 | 94.301 |
| 29 | 4.078 | 64 | 23.925 | 9 | 97.759 |
| 30 | 4.245 | 65 | 25.02 | 10 | 101.32 |
| 31 | 4.4953 | 6 | 26.163 | ||
| 32 | 4.7578 | 67 | 27.347 | ||
| 33 | 5.035 | 68 | 28.576 | ||
| 34 | 5.329 | 69 | 29.852 |
Gravimetric Method
The principle of the gravimetric method is as follows: a specific volume of exhaust gas is extracted from the flue and passed through a moisture absorption tube containing the hygroscopic agent. The moisture in the exhaust gas is absorbed by the hygroscopic agent, and the increase in weight of the moisture absorption tube represents the amount of moisture contained in the known volume of exhaust gas. This method shares similar principles with the condensation method. Both directly determine the mass concentration of flue gas humidity by weighing moisture content and dividing by sampling volume, and then convert it into volume percentage. The wet-dry bulb method features simple operation and strong adaptability, and is currently a commonly used reference method for online measurement of flue gas humidity. The condensation method and the gravimetric method offer high accuracy but involve complex tests, require highly skilled operators, and take a long time to test. Consequently, they are not suitable for online flue gas humidity measurement and can only be used as laboratory methods for comparison with online measurement methods.
Resistance-capacitance Method (RC Method)
>> Measurement principle
In China, the RC method applied for high-temperature humidity measurement of flue gas is essentially the capacitance method. Most sensors based on this method adopts polyimide as the humidity-sensing material. Polymer humidity-sensitive capacitors made of this material generally exhibit excellent electrical properties with very low dielectric constants and medium loss. In a fully dry state, the dielectric constant of polyimide is 2~3. The dielectric constant of water molecules is approximately 80 at 20°C. The complex dielectric constant after adsorption of water molecules is:
In the formula: εu is the complex dielectric constant at the relative humidity of u%RH, εr is the dielectric constant of the polyimide film at 0%RH, a and b are structural constants, and εh is the dielectric constant of the water adsorbed in the polyimide film, Wu is the mass of adsorbed water unit mass of polymer at the humidity of u%RH, and p/p0 is the equilibrium relative pressure of water vapor. When the aforementioned polymer humidity-sensitive capacitor adsorbs gaseous water molecules from the environment, the dielectric constant of the material changes, leading to variation in capacitance value. By measuring this change in capacitance, the corresponding environmental humidity value can be calculated.
At present, the capacitive sensors used for measuring high-temperature humidity in flue gas—polymer humidity-sensitive capacitors—employ a parallel-plate capacitor structure. They primarily consist of several components, including the glass substrate, the lower electrode, the polymer humidity-sensitive film, and the upper electrode. Figure (1)-a shows a parallel-plate capacitor composed of the moisture-containing medium, and its equivalent circuit is shown in Figure (1)-b. R is a resistance that varies with moisture content; the higher the moisture content, the smaller the value of R, and vice versa. C is a capacitance related to moisture content, whose value increases as moisture increases. When ignoring the edge effect of capacitance, the corresponding relationship between capacitance and relative humidity can be expressed according to the calculation formula of parallel-plate capacitor as:
Parallel-plate capacitive moisture sensor and its equivalent circuit
In the formula:
C—Sensor capacitance;
S—Area of a single plate;
d—Distance between two plates;
ε—Dielectric constant of the medium
If the unit of S is cm², the unit of d is cm, and the unit of C is pF, then formula (3) can be rewritten as
As can be seen from Formula (4), after the dimensions of the capacitor are determined, the value of the sensing capacitance C depends on the relative dielectric constant εr of the medium.
Figure (2) Structure diagram of a polymer humidity-sensitive capacitor
Here, C is the capacitance of the humidity-sensitive capacitor, ε0 is the vacuum dielectric constant, S is the electrode area of the humidity-sensitive capacitor, and D is the distance between the electrodes of the humidity-sensitive capacitor as well as the thickness of the humidity-sensitive film. From Formulas (1), (2), and (3), it can be seen that the relationship between the amount of water molecules adsorbed by the humidity-sensitive capacitor and the equilibrium relative pressure of water vapor should follow a Henry-type adsorption isotherm; that is, there is a linear relationship between the capacitance value and relative humidity. Capacitance-resistance moisture meters operate based on this principle.
Resistance-Capacitance Flue Gas Humidity Meter
Based on the practical application of resistance-capacitive flue gas humidity meters, this method offers fast response time, a compact size, and resistance to damage from condensation. The drawback is that the ambient temperature of flue gas must not exceed 170°C. The higher the temperature, the more prone the data is to fluctuations, and the more difficult it becomes to measure humidity with volume ratio below 6%. This is because, When the relative humidity of water vapor (volume ratio) is 0~40% and the temperature exceeds 30°C, as the temperature rises (above 100°C), the corresponding saturated vapor pressure increases, resulting in a decreasing relative humidity. As a result, the change in capacitance also necessarily decreases. However, the acquisition range or resolution of the circuit for capacitance variation is limited. In addition, flue gas from waste incineration, metallurgy and other industries exhibit a certain corrosiveness, which easily causes electrode failure and shortens the service life of the sensor.
Chen Xingzhu and others from Fix Instruments (Shenzhen) Co., Ltd. have proposed a novel capacitive composite dielectric film humidity-sensitive element. Its dielectric film utilizes the polymer thin film technology with new materials, endowing it with the high humidity response advantages of conventional polymer film sensors, while also offering unprecedented response levels and durability. Comparative tests show that, compared to traditional single-material designs, this new composite material exhibits smaller hysteresis, lower nonlinear error, and a smaller temperature coefficient, with significantly improved repeatability and long-term stability. This provides new insights for the functional design of dielectric materials in capacitive humidity-sensitive elements. The outline drawing of its representative product is shown in Figure 3.
Figure (3) Outline of PC338 High-Temperature Humidity Module
Main Performance Indicators and Application Scenarios
Taking Chang Ai's CI-PC338 series capacitive moisture meter as an example, its main performance indicators are as follows:
| Humidity measuring range | Absolute humidity 0～40 vol% H2O |
| Temperature | 0～190℃ |
| Precision | H2O ≤ 5%: Absolute error ≤ ±1.5% |
| H+O > 5% | Relative error ≤ ±20% |
| Temperature | Less than 3°C |
| Response time | At the flow rate of 1 L/min and the pressure of 1 bar, |
| from high humidity to low humidity | T90 < 150 s |
| from low humidity to high humidity | T90 < 10 s |
| Operating ambient temperature of analyzer | -10～55℃, humidity: < 90%RH |
| Operating ambient temperature of probe | 0～+180℃, humidity: 0～100%RH |
Application scenarios
Capacitive moisture meters feature extensive measuring objects and ranges. They can measure moisture content in both gases and liquids, and are applicable for measurement of both macro moisture (max. measurable humidity content at 180℃) and trace moisture. Several application examples are listed below for reference.
● Measurement of high-temperature humidity in exhaust gas emissions from coal-fired and oil-fired boilers (CEMS);
● Humidity measurement in food processing;
● Humidity measurement in the wood, building materials, and papermaking industries;
● Humidity measurement in the chemical and textile industries;
● Measurement of moisture content in natural gas;
● Measurement of moisture content in ethylene cracking gas;
● Measurement of moisture content in air, carbon dioxide, nitrogen, and inert gases;
● Humidity measurement at high temperature (120～180℃) for open-width dyeing process in printing and dyeing industry.
Installation and Maintenance
Installation
It should be noted that the capacitive moisture meter has specific requirements on the length, core cross-sectional area, shielding and insulation property of the connecting cable between the on-site probe and the display unit. Many factors (especially cable length) will affect the distributed capacitance of the cable and further interfere with the measurement results.
In general, it is recommended to use the matching cables supplied by the instrument manufacturer. If you need to purchase cables on your own, they shall strictly comply with the requirements specified in the instrument installation and operation manual.
The following points shall be noted during installation and operation.
● The cable length shall strictly comply with the instrument manufacturer's requirements. The supplied cable must not be lengthened or shortened to meet on-site requirements, as doing so would increase or decrease the cable's distributed capacitance;
● The cable connectors shall be well protected against damage. For self-matched cables, special attention shall be paid to the compatibility, firmness and sealing performance of cable end connectors;
● The connecting cable shall be integral without intermediate joints, and splicing multiple short cables is prohibited.When calibrating the probe, the matched cable must be connected to the probe for calibration.
Common Faults and Solutions (See Table 1)
The main advantages of the capacitance method are high sensitivity, fast response, ease of manufacturing, and the ability to be easily miniaturized and integrated. It is currently the most widely used method for online flue gas humidity meters in China. Nevertheless, it suffers from unsatisfactory long-term stability; most devices exhibit severe drift after long-term operation, which may lead to functional failure and damage. Capacitive humidity-sensitive elements also feature poor corrosion resistance and require high environmental cleanliness. Some products are even prone to failure caused by light irradiation and static electricity. In summary, this method is still in continuous improvement.
Table (1) Common faults and solutions for capacitive moisture meters
| Fault phenomenon | Fault causes | Solution |
| Instrument indication exceeds upper limit | a. Infiltration of liquid water or excessively high moisture content of the sample | a. Turn off the power and feed air or a dry sample gas. |
| b. Short circuit occurs in the probe or other circuit components | b. Replace the probe or other short-circuited components. | |
| c. The probe is contaminated with a conductive solid or liquid; the meter still reads full scale even after the probe has been cleaned | c. Clean the probe with reagent-grade pure ethane or toluene and dry it thoroughly. | |
| d. It indicates the probe short circuit, send it back to the manufacturer for repair. | ||
| Instrument reading shows "0" or below "0" | a. Poor contact in the plug or cable, causing an open circuit | a. Repair open-circuit components |
| b. Open circuit in the probe | b. Remove the probe. If the instrument reading exceeds full scale when the probe chip is shorted to the outside, this indicates that the probe is open-circuited. | |
| The instrument reading is inaccurate | a. Excessively high operating temperature | a. Extend the length of the sampling pipeline or use other heat exchange methods for cooling |
| b. Influence of particulate matter in sample | b. Filter out dust and recalibrate the sensor | |
| c. Drift caused by prolonged use of sensor | c. Recalibrate the sensor | |
| d. Discrepancy between the moisture content at the sampling point and the humidity reading on the instrument | d. Adjust the flow rate and allow sufficient time for the sample system to equilibrate | |
| e. Sensor contamination | e. Check the pipeline for leaks and inspect the inner surfaces of the pipelines for liquid moisture | |
| f. Sensor corrosion | f. Clean and dry the sensor | |
| g. Clean and dry the sensor, then recalibrate it; if corrosion is severe, replace the sensor | ||
| Extremely slow instrument response | a. Too low the sample flow rate | a. Increase the sample flow rate |
| b. Probe contamination | b. Clean the probe |
