TOOL CONTROL THE CONCENTRATION OF CARBON DIOXIDE IN THE FLUE GAS BOILERS BASED ON THE OPTICAL ABSORPTION METHOD

The methodology for controlling the concentration of carbon dioxide in flue gases from boilers is an optical absorption method with improved metrological characteristics. Studies done in the article, provided the new, scientifically-based theoretical and practical results that are essential for improving the accuracy at the required speed process, control the carbon dioxide concentration in dual gas boilers based on the optical absorption method with compensation effect on the factors optical converter.


Introduction
The common disadvantage of most existing methods, with the exception of optical ones, is their low selectivity, that is, often the definition of the concentration of gas hinders the presence of other components. A low threshold of sensitivity, that is, the ability to determine small concentrations of gases, is characteristic only for electrochemical methods of gas analysis. The conducted analysis shows that the best use is the use of optical methods, among which the optical-absorption method [18] is best for solving the problems. Therefore, an important goal of further work is to improve the optimal method for controlling the concentration of components of gases and its unification of the indicative features of the object of control. To do this, it is necessary to solve a number of theoretical and practical tasks [1,15]:  To develop a mathematical model for determining the concentration of carbon dioxide in flue gases of boiler installations, which takes into account and compensates for the influence of influence factors (temperature, pressure, humidity) on the parameters of the meter, which will allow to assess the metrological characteristics of the means of control for real operating conditions [3,4];  To develop algorithmic organization of adaptation of the control of concentration of carbon dioxide in flue gases of boiler plants to real operating conditions [2,17].

Method
Flue gases of boiler installations with an average concentration of more than 0.1vol% and their main oscillationrotational characteristics are presented in Fig. 1.
Analysis of the absorption spectrum [15] shows that the most active region is within the range of 2200-2500 cm -1 . In Fig. 1. The absorption of the main components of flue gases of boiler installations in the range of 4-4.5 microns (2200-2500 cm -1 ) is presented [5,8,16].
At the same time, virtually all absorption bands are adjacent to each other (in some cases overlap). In addition, water vapours present in gases occupy a wide range of absorption band lengths. Therefore, the necessary condition is not only the choice of the lengths of absorption of gases in such a way that they are not superimposed on each other, but also to use high-precision radiation sources and receivers (whose breadth of spectrum does not allow the "adjacent" gases to enter). After analysing the lengths of absorption of flue gases, it is necessary to select infrared emitters and receivers according to already known input data [6,7].
Thus, it is possible to determine the parameters of photodiodes and photodetectors, taking into account the interconnection of the absorption lines. That is, the control points for each gas are such that there are no other absorption lines of other gases that could affect the measurement results.
Graphic modelling of a fragment of the HITRAN database is presented in Fig. 2. As shown on Fig. 2, the absorption spectrum of carbon dioxide in the given range has a complex structure due to overlap of the spectra of gases that are part of its composition, so the absorption coefficient k(λ) of the band is expressed by the sum of the absorption coefficients of the individual lines [12,13] , where Sintensity of the absorption band; λ i and λ 0length and centre of absorption band; σabsorption band width.
An analytical model of the spectral distribution of gas absorption coefficients calculated on the basis of tables of the intensity of the lines S (λ 0 ) of the radiation of the gases in the indicated spectral range, which corresponds to the calculation of the absorption value in the range 4.1-4.3 microns (2300-2400 cm -1 ), was used the following approximation [10,11,14]:

. Model of attenuation of radiation in the environment of flue gases of boiler installations
On the basis of Bouguer-Lambert-Beer's law a mathematical model is presented that describes the process of attenuation of radiation in an optical measuring converter and takes into account the main factors that are associated with the features of an object of control that affect its correctness (2) where I lthe intensity of the radiation that passed through the gas to be studied; I 0initial intensity of radiation; M Mthe molar mass of dry air (0.029 kg/mol); gacceleration of free fall; hheight, which is the sum of the height of the chimney and the surface of its base above sea level; Cgas concentration (mg/m 3 ); llength of the absorption path; σabsorption band width; Mmolar mass of investigated gas; Ttemperature of flue gas; Runiversal gas became; P 0pressure at sea level; P dpressure of dilution.
Graphically, the intensity dependence on concentration is depicted in Fig. 4.
A mathematical model [1] of the measuring converter of carbon dioxide concentration in flue gases of boiler plants was also developed.
where Sarea of the photosensitive layer of the photodetector, which is illuminated; S I0integral current sensitivity of the photodiode at unmodulated irradiation; R ZZresistance in the feedback loop of the operating amplifier; U Foutput voltage of the photodetector based on a pair of photodiode-operational amplifier.
The simulation was carried out at an input intensity of 3 mW, a feedback resistance of 9.88 MΩ, an integral current sensitivity of the photodiode 5.06 A/m 2 , an area of a photosensitive layer of a photodetector of 26 mm 2 .
The simulation of the dependence of the CO 2 concentration on the voltage in the working and full range is shown in Fig. 5    As the output signal of the optical sensor, we will represent the irradiance transmittance in the cuvette with the gas Using a light emitting diode and a photo diode with spectral characteristics that overlap some band of frequencies λ = λ2-λ1. The shifting of the spectral characteristics of the source and the receiver of radiation due to changes in temperature and pressure is inherent in all light emitting diodes and photo-diode. To evaluate the influence of temperature and pressure on the optical sensor and, consequently, on the gas concentration value, we introduce parameters T and P in the function of the spectral characteristics of the source and the receiver of infrared radiation. The photoconductor measures the integral signal and the transfer function of the optical sensor is described by an integral expression The study of the reference and working wavelength for an optical gas sensor was developed and the method of additivemultiplicative compensation of influential factors was developed. It is determined that absorption of carbon dioxide in the 1900 cm -1 range is almost zero. Therefore, as a source (IR) radiation for a reference optical channel, a laser with a working length λ = 1900 cm -1 was used. It should be noted that in this absorption range CO, NO 2 , SO 2 , CH 4 is from 10-21 to 10-23 cm/mole, which is several orders of magnitude higher than the absorption of CO 2 in this range.
The length of the working and reference waves and the absorption sensitivity are presented in Fig. 7. Simulation of the intensity of radiation from pressure and temperature is presented (at constant concentration) in Fig. 8.
Development of the method of additive-multiplicative compensation of influential factors.

Modelling and experimental results
In work [17], two variants of the algorithm of compensating destabilizing factors for the parameters of the optical converterlinear and nonlinear are considered. Based on research data, algorithms of additive and multiplicative compensation of influencing factors, which are adapted to the object of control, have been developed.
The additive compensation of the influential factors expressed through the voltage has the form The multiplicative compensation of influential factors is expressed due to the intensity of the form [17,18]   The modelling of the Add and Mul parameters from the CO 2 concentration was performed on the basis of experimental data with a 2.5 vol%. Theoretically, the models take into account the intensity at the output of the working and reference channels, but practically in these dependencies, voltage was used, that is Having analysed the characteristic in Fig. 9 it is concluded that at concentration of CO 2 up to 3 vol%. It is optimal to use an additive compensation algorithm, and at more than 3 vol%multiplicative.

Discussion of results
To test the mathematical model (3), experimental design studies have been carried out on a model installation (Fig. 10) for determining the concentration of carbon dioxide in flue gases with known content of CO 2 (10-12-14-16 vol%). At a wavelength of 4.267 micron. As a source of radiation, a laser diode based on 1.5 mW InAsSbP was used as a radiation detectora thin-film thermoelectric receiver (working without cooling).   (18) According to international recommendations, the uncertainty of gas concentration measurements with the help of the developed means of control is estimated. For a sample carbon dioxide concentration of 14448 mg/m 3 , the value of the total standard uncertainty is 121.01 mg/m 3 . And the increased uncertainty of the measurement result with the reliability of the control of 0.968 is 237.18 mg/m 3 .
Substituting given and experimental we obtain the value of α = 0.0312, β = 0.0010. Instrumental probability of control D I = 0.968, it is higher compared with the probability of control of known devices of gas analysis (0.8-0.94).

Conclusions
The article develops a means for controlling the concentration of carbon dioxide in flue gases of boiler plants on the basis of an opto-absorption method with improved metrological characteristics. The conducted researches have allowed to receive new scientifically-based theoretical and practical results, which are essential for increasing the accuracy with the required speed of control of concentration of carbon dioxide in flue gases of boiler installations on the basis of the opto-absorption method with the compensation of the influence factors of the optical converter.