With the industrial production, centralized heating of boilers and the popularization of transportation tools, a large number of soot and toxic and harmful gases will be discharged. Hazardous substances accumulate gradually in the atmosphere and reach a certain concentration, which will make the normal composition of air change, thus endangering the health of human beings and various animals and plants. Various problems caused by air pollution have attracted the attention of environmental protection departments. In order to achieve accurate and real-time monitoring of environmental quality, ecological environment and pollution sources, and provide accurate basis for supervision and management of environmental protection departments at all levels and environmental decision-making of the government, a large number of modern environmental monitoring instruments are urgently needed.
At present, there are three main methods for gas detection of portable spectrometers in domestic and foreign markets: differential algorithm, electrochemical analysis and infrared spectroscopy. Differential absorption algorithm can accurately calculate the concentration of most gases, but it will lose the broadband continuous absorption information in the characteristic absorption of gases, leading to some gas concentration measurement can not come out. For example, the absorption spectrum of nitrogen dioxide molecule in the ultraviolet band is mostly gradual continuous absorption, so differential absorption algorithm may think that the absorption information of nitrogen dioxide is filtered out by scattering, resulting in the detection of nitrogen dioxide. If the absorption curves of nitric oxide and nitrogen dioxide have the same absorption peaks, the fitting absorption curves are superimposed at the measuring points, so it is still impossible to distinguish the two gases. The electrochemical analysis method has the advantages of simple structure and easy operation. It mainly depends on gas sensors, a gas sensor can only detect a corresponding gas, and the sensitivity of gas sensors is high, but after a period of time, the sensitivity of sensors to gas will decline, it is necessary to replace gas sensors in time, and gas sensors are expensive, which increases the use cost for users. The main principle of gas sensor is to use the oxidation or reduction reaction of gas to generate current, but if there are both oxidizing gas and reducing gas, the measurement results will be inaccurate. Infrared spectroscopy overcomes the shortcomings of electrochemical analysis, but can only measure the approximate concentration of nitrogen oxides, can not accurately measure the specific concentration of NO and NO2, and infrared spectroscopy for environmental humidity, temperature and other external conditions require higher technology is more complex.
Based on defects and deficiencies of the above gas detection methods, an iterative evolution gas solution algorithm is proposed in this paper. According to the good absorption of ultraviolet light by gas at the wavelength of 190-290 nm, the number of absorbed photons can be obtained by measuring the ultraviolet light absorbed by gas. The actual concentration of gas can be obtained from the number of photons by using the iterative gas calculation algorithm.
Mixed gases have characteristic absorption peaks in the range of ultraviolet wavelength 190-290 nm. Gas absorbance has multiple superposition. Assuming that some elementary gas does not absorb other gases on its best characteristic absorption peak, the corresponding table of absorbance and concentration of this single substance gas is searched to obtain the initial concentration of the gas, and then switch to another characteristic absorption peak. The photon number of the gas is subtracted from the total photon number measured, and the initial concentration of another gas is obtained. By analogy, the initial concentration of each gas is obtained one by one. Then, the characteristic absorption peak of the first gas is returned to, and the absorption photon number of other gases is subtracted from the total photon number absorbed, and the iterative concentration of the first gas is obtained again. By analogy, the initial concentration of each elemental gas is obtained again. By repeating the iteration until the difference of gas concentration between two adjacent times is less than a certain value, it is considered that the concentration of the elemental gas is obtained.
The initial concentration
The initial concentration
the absorbance of the second gas is calculated, and the concentration of the second gas is calculated by querying the absorbance and concentration table again, as the initial concentration
Solve the concentration of other elemental gases in mixed gases. Methods 1 and 2. Selecting the characteristic peak absorption wavelength of other elemental gases and reading the number of absorbed photons at that wavelength. The absorbance was calculated by formula
(
Iterative Recursion of the Concentration of the First Element Gas. The concentration of all elemental gases obtained at present is substituted into the formula
and the corresponding
Repeat 2) and 3) to find the iteration concentration
Calculate the error of the calculation results of the same elemental gas in the adjacent two times. The first-order iteration error of each elemental gas is calculated.
Repeat 4, 5 and 6 until the error of two iterations of the same gas concentration is less than 3%.
The last calculated gas concentration is regarded as the final concentration of various elemental gases.
Fig. 1 is the absorption spectra of
From the observation in Fig. 1, we can see that
SPECTRAL TABLES FOR SO2 AND NO2 AT WAVELENGTH 273.33NM AND WAVELENGTH 231.33NM
|
231.33 | 273.33 |
|
0.03003444 | 0.00456189 |
|
0.00482312 | 0.07984884 |
Fig. 3 is the flow chart of the iterative algorithm for the concentration of
After two iterations, the numerical value of the algorithm tends to be stable, and the precise gas concentrations of
There is no interference between
Ultraviolet flue gas analyzer consists of three parts: flue gas data acquisition module, data processing module and data display module.
The data acquisition module is composed of ultraviolet light source and marine optical Maya2000 Pro ultraviolet spectrometer. Ultraviolet light source outputs stable ultraviolet light. Ultraviolet light passes through the optical fiber through the detected gas. After the gas is fully absorbed, the remaining ultraviolet light is transmitted into the ultraviolet spectrometer by the optical fiber. After the optical processing and photoelectric conversion of the gas by the spectrometer, the gas information becomes an electrical signal, waiting for the data processing module to read. In this system, the ultraviolet spectrometer is actually a flue gas acquisition sensor. Data processing module is composed of embedded development board. The embedded development board reads the gas information from the ultraviolet spectrometer, calculates the actual concentration of the elemental gas through the iterative algorithm, and visualizes it through the data display module. This is the composition and working principle of the experimental system.
The following data are obtained when a mixture of
Table 2 shows the number of absorbed photons and dark noise photons at 271.98 and 225.94 nm measured by ultraviolet spectrometer in a mixture of
200PPM SO2 AND NO GAS SPECTRAL DATA
271.98nm | 225.94nm | dark noise | |
---|---|---|---|
|
56434 | 43973 | 2900 |
|
52851 | 42754 | 2900 |
|
56434 | 43973 | 2900 |
|
56386 | 40749 | 2900 |
Table 3 shows the number of absorbable photons at 271.98 nm and 225.94 nm for zero and mixed gases, as well as the number of dark spectral noise photons measured by spectrometer. In practical calculation, the number of photons measured should be subtracted from the number of photons of dark spectral noise to obtain the actual number of photons of zero gas and mixed gas.
SPECTRAL DATA FOR MIXED GAS
271.98nm | 225.94nm | dark noise | |
---|---|---|---|
|
56434 | 43973 | 2900 |
|
52050 | 39588 | 2900 |
Based on the above data, the photon number absorption curves of elemental gases at their maximum absorption peaks are fitted.
Figure 8-11 is a curve drawn by a single gas at the maximum absorption wavelength of ultraviolet light. Analysis table of experimental results
In Table 4, the standard value is the concentration of the standard elemental gas put in the test, and the measured value is the concentration of the elemental gas calculated from the mixed gas using an iterative algorithm. From the experimental results, it can be seen that the maximum error of the measured value is less than the standard value, and the maximum error of the accuracy is 1.94%, which is much less than the original design standard of 3%, which is within the normal standard.
ANALYSIS OF RESULTS
gas species | standard value (ppm) | measured value (ppm) | difference value | maximum difference | error |
---|---|---|---|---|---|
|
102 | 104 | 2 | -4 | 0.8% |
200 | 196 | -4 | |||
499 | 501 | 2 | |||
|
104 | 108 | 4 | -7 | 1.4% |
200 | 206 | 6 | |||
500 | 493 | -7 | |||
|
116 | 112 | -4 | -9.7 | 1.94% |
201.5 | 195 | -6.5 | |||
501.7 | 493 | -9.7 | |||
|
50 | 53 | 3 | 3 | 1.5% |
200 | 199 | -1 | |||
conclusion | Accuracy error is less than 3%, which meets the design standard. |
The software algorithm is written in JavaScript, including the analysis and implementation process of the iterative algorithm gas. The main code is as follows:
/*
*Iterative calculation of gas concentration
*/
function GetGasC() {
//1. Obtain absorbance from NO2
var NO2A_1 = GetAByWa(gasWavebanc[‘NO2’]);
//2. Find the concentration of this point
var NO2C_1, NO2A_2, SO2A_2, SO2C_2, SO2A_1, ONA_3, NH3A_4, ONC_3, NH3C_4;
for(var i = 0; i < 2; i++) {
NO2C_1=GetCByA_Data(NO2_data_231, NO2A_1);
//3. Calculate the absorbance of NO2 at 273.33. //NO2A_2;
NO2A_2= getAByC_Data(NO2_data_273, NO2C_1);
//4. Obtain the total absorbance of SO2 in the optimum band and subtract the absorbance of NO2 here.
SO2A_2=GetAByWa(gasWavebanc[“SO2”])
-NO2A_2;
//5. Looking up Table to Find DeSO2C_1
SO2C_2 = GetCByA_Data(SO2_data_273, SO2A_2);
//6. The absorbance of SO2 at 231.33 was obtained by //looking up the table.
SO2A_1 = getAByC_Data(SO2_data_231, SO2C_2);
//7. NO2A_1 is the total absorbance minus the
//absorbance of SO2A_1 at 231.33.
NO2A_1 = NO2A_1 - SO2A_1;
}
currentGasC_NO2 = NO2C_1;
currentGasC_SO2 = SO2C_2;
//The absorbance of NO at the optimum band is the
//total absorbance S-SO2 absorbance minus the
//absorbance of NO2.
ONA_3 = GetAByWa(gasWavebanc[“NO”]) - getAByC_Data(NO2_data_225, NO2C_1) - getAByC_Data(SO2_data_225, SO2C_2);
//Concentration of NO obtained
currentGasC_NO = GetCByA_Data(NO_data_225, ONA_3);
NH3A_4 = GetAByWa(gasWavebanc[“NH3”]) - getAByC_Data(NO2_data_208, NO2C_1) - getAByC_Data(SO2_data_208, SO2C_2);
currentGasC_NH3 = GetCByA_Data(NH3_data_208, NH3A_4);
$(“#NO2_C”).html(“No2” + currentGasC_NO2);
$(“#SO2_C”).html(“So2” + currentGasC_SO2);
$(“#NO_C”).html(“NO” + currentGasC_NO);
$(“#NH3_C”).html(“NH3” + currentGasC_NH3);
}
Aiming at the detection requirement of main harmful components in air pollution, a fast iteration algorithm of mixed flue gas is designed by using the continuous frequency division method of ultraviolet grating in the experimental system, and the effectiveness of the algorithm is verified. Ultraviolet spectrometer is used as a sensor. The embedded development board reads and calculates the gas concentration. The analysis and calculation of the algorithm are realized by programming. The results show that the iterative algorithm can accurately measure the concentration of flue gas and keep the error within 3%. It can meet the design requirements and solve many kinds of gases at the same time. It is suitable for practical engineering applications.