Application of CO2 infrared analyzer in calciner gas

【Abstract】: Calciner is a kind of equipment used for high temperature treatment, using natural gas, oil and electricity as energy, iron making, recycling rare metals, catalyst production, improving the environment and producing special chemical products.

In the production process of soda ash, the CO2 concentration of the calciner furnace gas is an important indicator. After cooling, washing, and compressing, the calciner gas reacts with the lower part of the carbonization tower (called the lower section gas) and the cleaning ammonia mother II that enters the upper part of the carbonization tower. Generate NaHCO3. The increase of CO2 concentration in the lower gas can not only save the power consumption of compression, but also benefit the balance and speed of the carbonization reaction, and have a decisive role in improving the carbonization output, the carbonization conversion rate, and improving the heavy alkali crystallization.

The CO2 concentration of the furnace gas after washing is "85%. The operator can avoid the excessive vacuum of the calciner gas system through the remote control of the disc valve of the calciner gas to reduce the amount of air leakage. It can also be sealed by keeping the pipeline equipment and its interface well sealed. To increase the concentration, how to detect the CO2 concentration of the furnace gas? There are two commonly used methods: one is to use an austenitic gas analyzer to measure the CO2 concentration with the sodium hydroxide absorption CO2 method, but the disadvantage of this method is that it cannot be continuously and intuitively It shows the CO2 concentration, and the analysis operation is cumbersome, and it is not suitable for the continuous production of chemical industry; the second is to continuously detect the CO2 concentration of the furnace gas with an infrared CO2 concentration analyzer, and then use a recorder to realize the indication and continuous recording of the CO2 concentration , To facilitate the smooth operation of the chemical industry.

1. The working principle of infrared analyzer The working principle of infrared concentration analysis is based on the property that different gases selectively absorb infrared rays of a certain wavelength. The absorption of infrared rays by gas molecules is due to the specific frequency of the molecules themselves. Infrared radiant energy. The light source of infrared gas analyzer produced in China adopts nickel chromium wire, supplying a constant current to raise the temperature to 700-900. C, emit infrared rays within the wavelength of 2 ~ 7μm, symmetric diatomic gas and monoatomic gas (such as O2, H2, Ar) do not absorb infrared radiation energy in the range of 2 ~ 7μm; CO2, CO, NH3, H2O, etc. There is a strong absorption band in this wavelength range (such as CO2 4.25μm, CO 4.25μm, NH3, 6.15μm, H2O, 2.67μm). CO2 infrared concentration analyzer is to use CO2 2 ~ 7μm band with strong absorption band properties to work.

When infrared ray passes through CO2 gas, the outgoing light intensity and the incident light intensity obey Lambert-Bell's law, and its formula is: from equation (1), it can be seen that K is a constant for a certain component to be measured. When I0 and L are fixed, I is a single-valued function of C. In this way, the intensity of the light after passing through the gas medium is detected and the concentration is detected. From the formula, I decreases exponentially with the increase of the concentration of the components, which is a nonlinear relationship and brings a certain error to the scale of the instrument. There are two methods for the infrared gas analyzer to achieve nonlinear processing; one is to select the length L of the gas chamber according to the range of the gas concentration to be measured. When the C · L value is very small, the transmitted light intensity is between the concentration C Enter the approximate linear region of Figure 1 to achieve linearity. Another method uses a linear correction network in the circuit portion of the analyzer to make the output and concentration linear.

2. Application of infrared analyzer The instrument used by our company is the XLZ-1090 infrared analyzer produced by Beijing Xilinzi. The measuring range is 0 ~ 100% CO2. Its selection is determined by the composition of the sample gas. The factory produces the analyzer based on the information provided by the user.

The components of our company's furnace gas include CO2, NH3, Na2CO3, dust and water vapor. Under the action of the compressor, the furnace gas first enters the separator to recover alkali dust (separation efficiency reaches 95%), and then enters the condensing washing tower to wash part of NH3 and CO2 with cold mother liquor to form condensate, and then the furnace gas enters the furnace gas washing tower Wash the furnace gas with water spray to remove ammonia. After the above treatment, the temperature of the furnace gas dropped to 32. Around C, can meet the requirements of the instrument. The main components of the furnace gas are high-concentration CO2 and trace NH3 (200-300Mg / m3), water gas, and Na2CO3 dust. This is what we want to measure. Because the infrared analyzer is a precision instrument, the requirements for the sample gas are very strict. The long-term effect of trace NH3, H20, and Na2CO3 will cause corrosion of the inner wall of the gas chamber and wafer scaling, resulting in inaccurate measurement and damage to the instrument. Remove harmful ingredients as much as possible.

The sample gas enters the bottom of the water collection bottle through the sampling tube and then flows out from the upper part. The purpose is to play a buffering role and prevent the water in the furnace gas line from directly entering the analyzer. The role of the drying tank is to absorb the remaining moisture in the sample gas. Because it is measuring CO2 gas, silica gel cannot be used as a desiccant in the drying tank. This is because silica gel has a certain adsorption effect on CO2. The amount of CO2 adsorbed decreases with time, resulting in a gradually higher value of the instrument, causing an increase in error , So we choose calcium chloride as the desiccant. Put a lot of defatted cotton in the dust filter tank, the purpose is to filter the trace Na2CO3 dust in the sample gas with cotton fibers. After these few steps, only CO2 and trace NH3 remain in the sample gas, because the characteristic absorption wavelength of NH3 is The characteristic absorption filter length of 6.15μm and CO2 is 4.25μm long distance, trace NH3 has little effect on CO2 measurement, and the filter room can be omitted, but NH3 is somewhat corrosive, and the NH3 corrosion-resistant material should be used in the measurement room. The control valve is to control the flow rate in the sampling tube to ensure 0.2-0.5L / min, and the valve opening can be controlled by the rotameter. This allows the gas source to continuously enter the analyzer for uninterrupted measurement.

3. Existing problems and solutions in application In practical applications, due to the following problems, the concentration analyzer cannot be used normally. Therefore, we propose corresponding solutions to various problems to ensure the accurate measurement and analysis of the analyzer. To facilitate the smooth operation of chemical industry.

1) The on-site process conditions are not good, so that the liquid in the main gas pipe of the furnace is serious. After the liquid passes through the sampling pipe, it enters the analysis gas chamber of the analysis instrument through the pretreatment device. Because the liquid contains NaCO3 and other substances, when it enters the gas chamber and acts on the wafer, it affects the infrared light path and the analyzer cannot be used.

In order to avoid the influence of the liquid in the furnace gas on the measurement, we opened the sampling point directly above the furnace gas pipeline, so that the condensed liquid in the pipeline and the pipeline slag are not easy to enter the sampling tube, and then connect a water collection bottle after coming out of the sampling tube ( A 500mL lower-mouth bottle can be used). The sample gas enters from the lower part of the lower-mouth bottle and exits from the upper mouth. Even a small amount of liquid will be collected at this time, which prevents liquid from entering the instrument. If the liquid enters the analyzer and becomes unusable, it can be handled as follows: open the analyzer to expose the analysis components, do not disassemble the screws of the analysis components, pull the rubber hoses of the sample gas inlet and outlet into the pressurized water gun, and replace it with distilled water The liquid and crystals entering the measuring gas chamber are replaced with high-pressure N2 to dry the moisture in the measuring gas chamber after replacement, and can be used after adjustment.

2) Because the calciner gas is under negative pressure (generally fluctuating around negative several kilopascals), if there is a leak at any link between the sampling port and the gas chamber inlet of the analyzer, it will cause air to enter, After the air enters, it will change the concentration of the gas, resulting in inaccurate measurement.

The causes of air leakage are: 1 the aging of the rubber hose; 2 the drying tank and dust filter tank are not sealed properly after changing the desiccant and cotton; 3 the water collection bottle is not tightly covered due to each flashing water; 4 the analyzer's filter device is due to each The replacement of the filter paper at this time causes the gasket to age and seal poorly. How to check for leaks? The method is as follows: jam the sampling rubber tube and observe the rotor flowmeter. If the rotor flow gradually returns to zero, it means that the pretreatment device has no air leakage; if the rotor flowmeter never returns to zero, it means there is a leak. Gas point, use this method to check the leak points of each can and bottle one by one. The joints and sealing points can be sealed with petroleum jelly. If the hose is aging, it should be replaced with a new pretreatment in a timely manner. No leakage is the guarantee.

3) The analyzer stipulates that the gas sample flow rate is between 0.2-0.5 l / min. If the size of the pretreatment device is not suitable, it will cause measurement lag and affect the operation.

In order to ensure the accurate measurement of the instrument, the gas sample flow band should not be too fast. If the gas flow rate is too fast, the infrared absorption is insufficient, it will cause inaccurate measurement; slow flow rate will increase the volume of the pretreatment device and cause a long lag time Therefore, the volume of pretreatment should be reduced as much as possible. First, the length and inner diameter of the sampling tube should be reduced as much as possible. It is best to place the infrared analyzer near the sampling port of the furnace gas pipeline in order to reduce the length of the sampling tube. In order to prevent the corrosion of the instrument on site, build a small house and put the instrument inside; second, try to reduce the volume of the water collection bottle, drying tank and filter tank.

4. Calibration of the instrument 1 First preheat the infrared instrument for 4h. 2 After passing in high-purity nitrogen, adjust the zero-point potentiometer to make the output current 4mA.

3 After passing 100% CO2 sample gas, adjust the sensitivity potentiometer to make the output current 20mA.

4 Connect the calibrated infrared analyzer to the measurement gas circuit as shown in Figure 2, and you will find that the measured value is lower than the actual value. The reason is that the furnace gas pipeline is under negative pressure, and the calibration meter is at normal pressure. According to Lambert-Beer law, the variable concentration C is a molar concentration. When the pressure is reduced, the molar concentration also decreases, so the indication is low.

5 The standard gas must be analyzed with an on-site austenitic gas analyzer, and then auxiliary calibration of the infrared instrument can be accurate.

Because the instrument has its own drift, in order to ensure accurate measurement, the device should be checked once a day.

Over the past few years, our company has used infrared analyzers with good results, which guarantees the production efficiency of the carbonization tower, and its value is relatively large.

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