Working principle and application of flue gas analyzer

No matter how to control the combustion efficiency, how to measure O in flue gas quickly and accurately? Both content and co content are prerequisites to achieve optimal combustion. Therefore, this paper introduces the working principle and usage of some typical smoke analysis instruments.

A, flue gas analyzer (or combustion efficiency tester)

Flue gas analyzer is an instrument for extracting and sampling furnace flue gas and automatically analyzing components, which is divided into on-line monitoring type and portable type. Generally speaking, CO and O in flue gas can be measured and analyzed. Nitrogen oxides, so? Gas content, and flue gas temperature, pressure, etc. And get co? Thermal parameters such as content, excess air coefficient, flue gas dew point, combustion efficiency, exhaust heat loss, flue gas flow rate, etc.

A flue gas analyzer is generally equipped with multiple sensors, including electrochemical sensors and infrared sensors. The measurement principle of electrochemical sensor is that the gas to be measured enters the sensor room after dust removal and dehumidification, and then enters the electrolytic cell through the permeable membrane, so that the gas diffused and absorbed in the electrolyte is electrolyzed at a specified oxidation potential, and the gas concentration is calculated according to the consumed electrolytic current.

The infrared sensor is mainly composed of an infrared light source, an infrared absorption cell, an infrared receiver, a gas pipeline and a temperature sensor. It is based on the absorption principle of various elements to specific wavelengths. When the measured gas enters the infrared absorption cell, it will absorb infrared light to varying degrees, thus calculating the gas content. Infrared sensor has the characteristics of good anti-toxicity, wide measuring range and sensitive response.

Second, the oxygen analyzer

The instrument for measuring oxygen content in flue gas is called oxygen analyzer (oxygen analyzer). Commonly used oxygen analyzers mainly include thermomagnetic type and zirconia type.

(1) Thermomagnetic Oxygen Analyzer

Its principle is to determine the oxygen content in flue gas by using the physical characteristics of high magnetic susceptibility of oxygen in flue gas components. Oxygen is a paramagnetic gas (the gas that can be attracted by magnetic field is called paramagnetic gas), which is attracted by uneven magnetic field and flows to the place with strong magnetic field. There are electric heating wires in this place, which make the temperature of oxygen increase and the magnetic susceptibility decrease, so the magnetic field attraction decreases, and it is pushed by unheated oxygen molecules with higher magnetic susceptibility behind to release the magnetic field, thus causing the phenomenon of "thermomagnetic convection" or "magnetic wind". Under a certain gas sample pressure, temperature and flow rate, the oxygen content in the gas sample can be measured by measuring the magnetic wind. Because the thermistor (platinum wire) is used as both the resistance of the two branches of the unbalanced bridge and the heating resistance wire, a temperature gradient appears under the action of magnetic wind, that is, the temperature of the inlet branch is lower than that of the outlet branch. The unbalanced bridge will output the corresponding voltage value with the different oxygen content in the gas sample.

Thermomagnetic oxygen analyzer has the advantages of simple structure, easy manufacture and adjustment, but it has been gradually replaced by zirconia oxygen analyzer because of its shortcomings such as slow reaction speed, large measurement error, easy blockage of measuring ring chamber and serious corrosion of thermal sensitive elements.

(2) zirconia sensor oxygen analyzer

Zirconia (ZrO? ) is a ceramic, a solid with ionic conductivity. It is monoclinic crystal at room temperature. When the temperature rises to 1 150℃, the crystal form becomes cubic, and the volume shrinkage is about 7%. When the temperature drops, it becomes monoclinic again. If heating and cooling are repeated, ZrO? It will break. Such pure ZrO? Can't be used as a measuring element. If a certain amount of calcium oxide (CaO) or yttrium oxide (Y? o？ ) as a stabilizer, and then calcined at high temperature, it became a stable zirconia material. At this time, tetravalent zirconium is replaced by bivalent calcium or trivalent yttrium, and oxygen ion holes are generated at the same time, so ZrO? Belonging to an anionic solid electrolyte. ZrO？ Conductivity is mainly through the movement of holes. When the temperature reaches above 600℃, ZrO? It becomes a good oxygen ion conductor.

Platinum electrodes are sintered on both sides of zirconia electrolyte. When the oxygen partial pressures on both sides of zirconia are different, the oxygen on the side with high oxygen partial pressure migrates to the side with low oxygen partial pressure in the form of ions. As a result, the platinum electrode on the side with high oxygen partial pressure loses electron positive charge, while the platinum electrode on the side with low oxygen partial pressure gains electron negative charge, thus generating oxygen concentration potential between the two platinum electrodes. This potential is only related to the difference of oxygen content (oxygen concentration difference) in the gas on both sides when the temperature is constant. If the oxygen content on one side is known (for example, the oxygen content in air is unchanged), the oxygen content on the other side (for example, the oxygen content in flue gas) can be expressed by the oxygen concentration potential, and the oxygen content in flue gas can be known by measuring the oxygen concentration potential.

Zirconia oxygen analyzer has the advantages of simple structure and sampling pretreatment system, high sensitivity and resolution, wide measurement range and fast response.

Three. Products and applications

? Flue gas analyzer has a wide range of applications, such as:

(1) Circulating fluidized bed boiler in thermal power plant is used for flue gas monitoring in combustion control room;

(2) The steel rolling heating furnace in iron and steel plant is used to detect the furnace gas atmosphere when reducing the oxidation burning loss or the thickness of decarburization layer;

(3) All-hydrogen heat treatment furnace is used to detect whether the radiant tube burns through and leaks gas.

(4) When developing new burners (regenerative burner, low nitrogen oxide burner, radiant tube burner), it is used for the design and research of burner structure size;

(5) automobile exhaust emission detection;

(6) Other environmental protection monitoring projects.