Applications of Online Monitoring Technology for Flue Gas Moisture (Humidity) -2
Ion Flow (Zirconia)
Based on in-depth theoretical research and numerous experiments, it is verified that ion flow sensors can achieve accurate humidity measurement. Humidity measurement can be realized by adjusting the voltage applied to the sensor's cathode and anode. This discovery addresses the issue that conventional humidity sensors cannot operate effectively in high-temperature environments (such as those above 100°C).
Platinum electrodes are coated on both sides of stabilized ZrO2. The cathode side is assembled with a cover provided with gas diffusion holes to form a cathode cavity. At a certain temperature, a specific operating voltage is applied across the anode and cathode of ZrO2, then oxygen molecules inside the cavity gain electrons at the cathode to form oxygen ions (O2-). The O2- migrate to the anode through oxygen vacancies in the ZrO2, release electrons, and are converted into oxygen molecules that are discharged outward. This phenomenon is known as an electrochemical pump. In this way, oxygen in the cathode cavity is continuously pumped out of the cavity by the ZrO2 electrolyte, and oxygen ions flow from the cathode through the ZrO2 to the anode, forming an oxygen ion current. When the oxygen concentration in the measured atmosphere is constant, the output current of the zirconia sensor no longer increases with the increase of the applied voltage, but instead reaches a constant value. This constant current value is referred to as the limiting current value for that oxygen concentration, which we call the first limiting current value (as shown in Figure 1-A). Based on this operating principle, when the measured atmosphere contains water vapor, increasing the applied operating voltage causes the water vapor to be ionized into oxygen ions. Similarly, when the water vapor concentration in the measured atmosphere is constant, the zirconia sensor outputs a constant current value, which is referred to as the second limiting current value (as shown in Figure 1-B). The first-stage current value I1 and the second-stage current value I2 are respectively proportional to the oxygen partial pressure and the oxygen partial pressure in the water vapor-containing atmosphere.
The reactions at the sensor's cathode and anode are as follows:
Figure (2) Ion flow principle
In accordance with the Fick's law restricted by gas diffusion holes of the sensor, assuming that the diffusion coefficient of oxygen is equal to that of water vapor, the first limiting current I1 and the second limiting current I2 are expressed by the following formulas respectively:
In the formula:
F is Faraday constant S is the area of diffusion holes
D is the diffusion coefficient of mixed gas molecules P is the total pressure of the gas mixture
PO2 is the partial pressure of oxygen PH2O is the partial pressure of water vapor
R is the gas constant T is the absolute temperature
L is the length of gas diffusion holes 0.21 is the oxygen content in air
The relationship curve between ionic limiting current value and oxygen concentration is shown in Figure (3):
The oxygen content in flue gas can be calculated based on the first limiting current, while the humidity in the flue gas can be calculated according to the difference between the second and first limiting currents. Therefore, humidity meters adopting the limiting- current zirconia principle have a distinct advantage over those using other principles. Since its core function is oxygen detection and oxygen measurement is a prerequisite for humidity testing, users do not need to install a separate oxygen analyzer. A single humidity meter can provide both sets of measurement data simultaneously.
>> Suction-type 3D Ion Flow Humidity Analyzer
Suction-type high-temperature humidity analyzers are manufactured by several companies in China. Here, the products from Chang Ai are introduced as an example.
To address the impact of corrosive atmospheres on sensor electrodes, Chang Ai has improved the zirconia-platinum catalytic electrode materials and adopted a novel electrolyte material via nanochemical synthesis technology. This method addresses electrode corrosion and short service life in high-SO2 flue gas from waste incineration plants, ore calcination, ceramic factories, and power plants. Furthermore, building upon the foundation of limiting current sensors, Chang Ai made a bold innovation by co-firing dual oxygen sensors onto a single chip, completely resolving the challenge that a single oxygen sensor cannot simultaneously measure dynamic oxygen and electrolytic wet oxygen.
The 3D ion flow device adopts a dual-ion-flow sensing unit. One unit measures the content of water vapor and oxygen, while the other measures pure oxygen content. By applying different voltages to ionize oxygen ions and mix them with water vapor, the contents of oxygen ions and water vapor can be obtained through current measurement. Featuring high temperature resistance and anti-pollution performance, this sensor can operate stably in harsh gas environments. Its principle and structure are shown in Figure 4.
Figure (4) Structure of an ion flow humidity sensor
CI-PC196 3D Ion Flow Humidity Analyzer consists of high-temperature sampling probe and the instrument control unit (as shown in Figure 5). The control unit supports automatic backflushing and automatic calibration functions. The probe is equipped with a heat tracing function to prevent condensation from forming inside it, and its end is fitted with a sintered stainless steel or ceramic filter.
Figure (5) CI-PC196 High-temperature humidity meter
The probe includes a primary filter inserted into the flue, a sampling tube, a purge execution unit and sensor located at the normal-temperature end outside the flue. High-temperature flue gas is extracted from the flue by an ejector pump driven by compressed air. The flue gas enters through the sensor inlet and discharges from the air outlet. By controlling the pressure and flow rate of the compressed air, the flow rate of the drawn-in gas can be regulated. The probe is a current-type oxygen sensor with the operating principle different from that of direct-insertion concentration-type zirconia probes. Under high-temperature conditions, the zirconia (ZrO2) material becomes conductive due to the migration of oxygen ions. When the temperature exceeds 650°C, oxygen ions migrate; as the oxygen concentration increases, the current increases proportionally with the rise of ion flow.
Compared to conventional polymer, electrolyte, and ceramic humidity sensors, this instrument differs entirely in terms of structural design, testing methods, and operating principles, thereby offering remarkable advantages: it exhibits excellent temperature resistance and corrosion resistance (the sensor operates at temperature exceeding 600°C), allowing it to be used in high-temperature environments above 200°C. It measures moisture content according to the decomposition amount of water vapor under decomposition voltage, thus delivering superior selectivity. Furthermore, this principle can measure the humidity and output oxygen concentration synchronously. It is widely used in environmental protection, printing and dyeing, wood, building materials, papermaking, chemical, fiber and pharmaceutical industries, as well as the processing and storage fields of food, tobacco, vegetables and grains.
>> Wet-dry Oxygen Method
When oxygen sensors equipped in the CEMS system are used to measure the oxygen content of flue gas before and after dehumidification, and calculate the moisture in the flue gas, the flue gas humidity is calculated using the following formula:
In Formula (1), X´O2 represents the volume percentage of oxygen in the wet flue gas, %, and Xo2 represents the volume percentage of oxygen in the dry flue gas, %.
For example: If the concentration of the wet flue gas is 6.8% O2, and the reading of the dry flue gas after dehumidification is 7.4% O2, let Xsw denote the moisture content of the flue gas, then
The main issue with the dry-wet oxygen method is that it requires two instruments to measure dry oxygen and wet oxygen respectively. The resultant errors include sampling errors caused by inconsistent sampling points, as well as superimposed errors resulting from measurement drift in the two instruments themselves. These errors are difficult to overcome with this method.
Infrared Spectroscopy
In nature, every gas absorbs light of specific wavelengths. When a beam of white light (containing all wavelength components) passes through the gas, the outlet light weakens or lacks those specific wavelength components. In spectroscopy, the components of a substance can be determined according to the composition of gas absorption spectral lines. By analyzing the absorption degree of light at specific wavelengths by a specific absorption spectral lines of the specific gas, we can calculate the concentration of that gas.
There are two main methods for measuring humidity based on near-infrared absorption spectroscopy: Cavity Ring-Down Spectroscopy (CRDS) and Tunable Laser Diode Absorption Spectroscopy (TDLAS). Infrared absorption spectroscopy is based on the principle that the selective absorption of specific infrared wavelengths by water vapor molecules varies with their concentration. However, since Fowle first proposed infrared humidity measurement in 1912, progress in humidity measurement had been slow due to the limitations of traditional infrared absorption techniques (broadband absorption). The rapid development of semiconductor laser spectroscopy technology (TDLAS) in the 1990s facilitated the emergence of current online high-temperature flue gas humidity analyzers. Compared to traditional infrared absorption spectroscopy, TDLAS employs narrow-band absorption, as the spectral width of the semiconductor laser source (less than 0.0001 nm) is far smaller than the broadening of the gas absorption lines.
Every gas molecule has its own inherent absorption spectrum. Absorption occurs when the emission spectrum of the light source matches the absorption spectrum of gas molecules, and the absorption intensity is correlated with the volume fraction of the gas. When a beam of semiconductor laser with an intensity of I0 passes through the gas to be measured, the light is attenuated as it passes through the gas if the spectrum of light source covers the absorption spectrum of the gas molecules. According to the Lambert-Beer law, the relationship among outgoing light intensity I, incident light intensity I0 and gas volume concentration is expressed as follows:
In the formula:
I0: Initial light intensity;
I: Residual light intensity after absorption by water vapor (H2O) in gas sample;
S: Absorption coefficient of water (H2O) for a laser at a specific wavelength;
L: Optical path length;
N: Quantity of water vapor molecules along the optical path, correlated with water vapor content in sample gas.
Therefore, the water content in the gas sample can be determined by measuring the initial light intensity and the light intensity after absorption. Due to the selected laser wavelength is specific, the measurement results are virtually unaffected by other gases. In addition, the calculation using the ratio I/I0 can effectively eliminate the influences caused by variations in light source intensity, mirror reflectivity and electrical parameters.
To achieve higher detection sensitivity or improve it, and to reduce the 1/f noise of the laser, TDLAS technology generally requires the use of modulated spectral detection. This technique significantly reduces the impact of laser noise on measurements through high-frequency modulation. At the same time, by setting a large time constant for the phase-sensitive detector used in phase-sensitive detection (which detects harmonic components), a very narrow bandpass filter can be obtained, thereby effectively compressing the noise bandwidth. Flue gas high-temperature humidity analyzers developed using TDLAS technology perform non-contact measurements when measuring flue gas, eliminating sensor poisoning and interference from background gases. They feature fast response time, high measurement accuracy, a long calibration cycle, and nearly maintenance-free operation, while their main disadvantage is the high cost. However, when using the infrared absorption method for flue gas humidity measurement, it is necessary to avoid interference from wavelengths sensitive to CO2/SO2/NOX, which presents certain challenges. Coupled with the instrument's high cost, this method is currently rarely used for flue gas humidity measurement.
A Comparison of Various Principles
| Comparison Items | Constant Flow Jetting Method | Dual-cell Ion Flow | Zirconia Method | Resistance-capacitance Method | TLDAS |
| TLDAS | |||||
| Measurement Range | 0-100% | 0-100% | 0-100% | 0-100% | 0-100% |
| Response Time | T90＜90S(10～190g/kg) | T90＜30S | T90＜30S | T90＜30S | T90＜10S |
| Display | Dew point temperature: 20～100℃ | Oxygen concentration: 0–100% | Volume ratio (H2O): 0–100% | Relative humidity (RH%) | Relative humidity (RH%) |
| Volume ratio: 2～100% | Volume ratio (H2O): 0–100% | Volume ratio 0–100% | Volume ratio 0–100% | ||
| Absolute humidity: 15～1000 g/kg | |||||
| Water vapor pressure: 10～1000 hPa | |||||
| Displayed Value | Absolute value | Absolute value | Absolute value | Relative value | Relative value |
| Temperature | 0～300℃ | 0～700℃ | 0～700℃ | 0～180℃ | 0～240℃ |
| Precision | ±2%F.S | ±2%F.S | ±3%F.S | ±2%F.S | ±1.0%F.S； |
| Chemical Resistance | Resistant | Resistant | Moderate | Not resistant | Resistant |
| Applicability | Any gas mixture | Flue gas, general gas mixtures | Mixtures of air and water vapor | Flue gas, general gas mixtures | Flue gas, general gas mixtures |
| Measurement Method | Continuous sampling | In-situ/continuous sampling | In-situ | In-situ/continuous sampling | In-situ/continuous sampling |
| Service Life | 10 years | 1–2 years | 1–2 years | 0.6–2 years | ≥2 years |
| Calibration | No calibration required, no drift | Required (oxygen calibration) | Required (oxygen calibration) | On-site calibration unavailable (requires a professional humidity generator) | On-site calibration unavailable (requires a professional humidity generator) |
