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Abstract
| The substance of the review | |||||||||||||||||||||||||||||||||||||||||||
| Introduction 1. The relevance of the theme 2. The objective of the work 3. Formulation of the problem research 4. Scientific novelty 5. The object of the research 6. The subject of the research 7. Planned practical results 8. The difficulty of measuring the concentration of oxygen in the Avdeevka’s coke-chemical plant 9. The justification of the possibility of application of electrochemical method with the use of solid electrolytic cell 10. The results of the work 11. Investigation of the mathematical model Findings The list of the literature | |||||||||||||||||||||||||||||||||||||||||||
| Introduction | |||||||||||||||||||||||||||||||||||||||||||
On the territory of the Donetsk region is situated the largest in Europe coke-chemical plant Avdeevka Coke-Chemical Plant, which is one of the largest enterprises of coke and chemical industry. For the volume of coke production plant occupies the first place among coke plants of Ukraine. Average daily production of coke is more than 16 thousand tons. For more than 40 years on АCP was made more than 200 million tons of gross coke, processed more than 12 million tons of coal tar, produced more than 3,7 million tons of phthalic anhydride, almost 2,1 million tons of benzol and 2.5 million tons of ammonium sulfate, etc [1]. | |||||||||||||||||||||||||||||||||||||||||||
| 1. The relevance of the theme | |||||||||||||||||||||||||||||||||||||||||||
To ensure safety at the enterprises of coke-chemical complex of Ukraine it is necessary to control the management of technological process and working conditions. During the operation of gas paths of coke’s quenching installations content of the oxygen in inert gas which is circulated should continuously be monitored by automatic gas analyzing devices, this is provided by the requirements of "Safety Rules for the enterprises in gas sector of the ferrous metallurgy" (SREGFM-86). In case of increase of the oxygen concentration of more than 1%, you should immediately vacate the premises of the personnel and to stop the technological process. | |||||||||||||||||||||||||||||||||||||||||||
| 2. The objective of the work | |||||||||||||||||||||||||||||||||||||||||||
The development of measuring device for the control of oxygen concentration, which provides reduction of the probability of a potentially explosive situation due to perform the continuous monitoring of the concentration of oxygen in the coke gas fot the conditions of high temperatures during the technological process (of the order of 700 — 1200 °С). | |||||||||||||||||||||||||||||||||||||||||||
| 3. Formulation of the problem research | |||||||||||||||||||||||||||||||||||||||||||
To achieve the research objectives was formulated and solved following tasks:
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| 4. Scientific novelty | |||||||||||||||||||||||||||||||||||||||||||
Was developed mathematical model of the measurement channel based on the electrochemical method with the use of solid electrolytes cell that will allow with sufficient accuracy and speed to determine the oxygen content in the coke gas. | |||||||||||||||||||||||||||||||||||||||||||
| 5. The object of the research | |||||||||||||||||||||||||||||||||||||||||||
Measuring device fot the control of oxygen concentration in the coke gas. | |||||||||||||||||||||||||||||||||||||||||||
| 6. The subject of the research | |||||||||||||||||||||||||||||||||||||||||||
To decrease the risk of a potentially explosive situation due to the use of the developed measuring device in the mode of continuous monitoring of the oxygen concentration in the coke gas. | |||||||||||||||||||||||||||||||||||||||||||
| 7. Planned practical results | |||||||||||||||||||||||||||||||||||||||||||
Based on the result of research, based on the results of the mathematical model develop and sustantiate the structure of a measuring device, develop requirements for the prototype model of the device for measurement of oxygen concentration in the coke gas during the technological process.
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| 8. The difficulty of measuring the concentration of oxygen in the Avdeevka’s coke-chemical plant | |||||||||||||||||||||||||||||||||||||||||||
Determination of the oxygen concentration in Avdeevka’s coke-chemical plant (АCP) is complicated by the conditions of coke production:
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| № | Name of components | Соmposition, Vol.% |
|---|---|---|
| 1 | H2 | from 55 to 60 |
| 2 | O2 | 0,4 — 0,8 |
| 3 | CO2 | 2 — 3 |
| 4 | N2 | 4 |
| 5 | unsaturated hydrocarbons | 2 — 3 |
| 6 | CO | 5 — 7 |
| 7 | CH4 | 20 — 30 |
Coke gas is explosive and toxic. This is important to monitor the composition of the coke gas [2]. Component’s MPC of the coke gas shall not exceed the normative values, as a unit of the gas-dust emissions from the conventional technology of preparation, coking coal and coke processing.
Table 2 —Component’s MPC of the coke oven gas
| Name of components | Composition, mg / m 3 |
|---|---|
| CH4 | 300 |
| CmHn | 50 |
| C6H6 | 5 |
| CT | 20 |
| H2S> | 10 |
| HCN | 0,3 |
| C6H6OH | 0,3 |
| C5H5N | 0,5 |
Solid electrolytes – solids, electrical conductivity of which provided due to the transfer of ions. Their use in electrochemical analyzers provides selectivity analysis that allows you to create exemplary analytical devices appropriate fot the levels [3].
Electrochemical sensors are divided into solid electrolytes, polar-graphical and galvanic. In solid electrolytes sensor as the electrolyte used ceramic zirconium dioxide. Electrolyte properties of zirconium dioxide occur at very high temperatures (about 500-800°C) [4]. This feature makes it convenient for you to use zirconium sensor for measuring the oxygen concentration in the flue gases at high temperature and small concentration of О2. Measurement of oxygen in the room temperature is associated with high energy consumption and complexity of sensor design, which is connected with the necessity of gas heating to a high temperature. In addition, when significant concentrations of oxygen and such a high temperature it is very difficult to satisfy the requirements of fire safety.
Figure 2 — Animation of the measurment princip of coke oven gas (number of frames — 7 , volume — 59 КВ, number of repetition cycles — 20 , delay between frames — 1 s)
In figure 2 are shown:
1 — Solid electrolyte ZrO2;
2,3 — external and internal electrodes;
4 — pin terminal;
5 — "signal contact";
6 — exhaust pipeа.
The electrochemical oxygen sensors with liquid electrolyte have the best selectivity. These sensors are divided into polar-graphical and electroplating. The dignity of polar-graphical sensor – is the small size of the working electrode, and therefore, the possibility of creating a sensor with very small dimensions [10]. Be aware, though, that the smaller the size of the sensor, the more often it is necessary to replace the electrolyte in the process of exploitation. Another disadvantage of the polar-graphical oxygen sensor is the need to maintain accurate voltage potentiometric rheostat.
Galvanic oxygen sensors, with high selectivity, do not require external power supply. The output signal of the sensor oxygen galvanic type is directly proportional to the partial pressure of oxygen, so to handle this signal does not require sophisticated converters. In the simplest case, for measuring the oxygen concentration can be directly connect micro-ammeter to the sensor. Record the testimony of this "gas" on atmospheric air, the volume fraction of oxygen which is approximately equal to 21%, it is easy to indications on micro-ammeter calculate the oxygen concentration in any other of the gas mixture. The advantages of galvanic electrochemical oxygen sensors are also in their small size and the independence of the output signal from the position in space, which together with the lack of power consumption makes them indispensable in portable gas analyzers intended for operation in hard-to-reach places (mines, wells), and also in explosive and flammable premises.
Electrochemical cell with a solid electrolytes used in two modes: potentiometric and coulometric [5]. In potentiometric mode voltage occurs directly in the field of the borders of three phases: electrode – solid electrolyte and gas phase. The voltage does not depend on whether any conductive material (solid or powder) is used as an electrode, as well as for education of the capacity of dissolution of the components of the gas in the electrode layer is not necessary [9]. Such a mechanism education voltage corresponds to the recording of an electrochemical cell, shown in table 3.
Table 3 —Record of an electrochemical cell
Electronic conductor gas phase cathode A (p'O2) |
The solid electrolyte | Electronic conductor gas phase B(p''O2) |
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where p'O2 — is the partial pressure of oxygen.
If as an electronic conductor use platinum, and as solid electrolyte – zirconium dioxide, stabilized by monoxide calcium (ZrOaCaO), which forms an electrolyte with oxygen-ionic conductivity, the abbreviated form of the formula of the electrochemical cell can be written in next form (1):
(1)
EDF of an electrochemical cell is expressed the mutated Nernst equation :
(2)
where R — is the universal gas constant;
T — is the temperature solid electrolyte cell on the absolute scale;
F — is the Faraday’s constant.
Electric driving force of solid electrolyte cells in potentiometric mode is defined as the difference between the two electrode potentials: the capacity of the working electrode (electrode is responding to a defined component of the sample) and a reference electrode.
If the reference electrode is washed by the pure oxygen to the pressure 0,9807*10-5 PA, and the working electrode — break of the AGM with the partial pressure of oxygen pO2, the potential difference between the electrodes (in mV) is the results of equation (3):
(3)
The use of SEC in the electrochemical method in 2 modes is shown in figure 3.
Figure 3 —Solid electrolyte cell
In figure 3 are shown:
а — in potentiometric mode:
1 — camera;
2 — membrane;
3 — electrodes.
In solid electrolyte cell camera 1 is divided into two parts by membrane 2 of SE (Fig. 3, a). On the surface of the membrane plotted gas permeable electrodes 3, made of metal, not entering in chemical interaction with a breakdown of the AGM. On the one side of the diaphragm is washed by the comparative gas with a known oxygen concentration, and in the other — the breakdown of AGN. The potential difference between the electrodes is a function of the oxygen concentration in the sample of AGM.
б — in coulometric mode:
1,3 — electrodes;
2 — Solid electrolyte cell;
4 — the source of direct current;
5 — the device for measuring the strength of the current.
Disadvantages of solid electrolyte gas analyzers — the need to have a comparative gas mixture with a high accuracy to maintain the set temperature in the working area. In solid electrolyte cells, working in coulometric mode, these defects are absent[6].
In coulometric mode sample AGM enters the cell 2 (Fig. 3, b), made of SE in the form of pipes, the outer and inner surface of which marked the electrodes 1 and 3. A voltage applied to the electrodes from the source DC 4 and consistently with them is connected device for measurement of electric current 5.
The oxygen molecules of the sample AGM diffuse to the surface of the inner electrode and, sorb on it, dissociate into atoms O2-About+O, which in turn ionizes due to electrons lead On + 2E-O2 – , penetrating to the boundary of gas — electrode — electrolyte.
Under the voltage oxygen ions are transported through the electrolyte to the outer electrode, in which ions, giving the electrons in the external circuit that recombine to molecular oxygen, exhaust into the atmosphere. Thus, in external circuit electrochemical cell arises electric current. In steady state, when it is virtually complete transfer of oxygen from the samples of the PSA, the flow of gas through the solid electrolyte cell is constant. The dependence between the current transfer and oxygen concentration samples of AGM is expressed by the relation deduced based on the Faraday’s law:
I=QCnF/M (4)
where Q — is the flow rate of the sample of AGM;
С — is the concentration of oxygen in the sample of AGM;
М — is the molecular mass of oxygen.
Except to processes of oxidation and reduction of oxygen on the electrodes no reactions associated with the formation of new substances in solid electrolyte cells, does not occur, that is cell is reversible. This is a fundamental difference and one of the significant strengths of the cells in comparison with liquid electrochemical cells. The advantages of these cells are also a wide range of measurements, low inertia, the possibility of calculating strapping characteristics, ease of equipment. Solid electrolytes have a high resistance to mechanical effects, the serviceability in wide temperature range, have a long service life, easy to miniaturization.
Disadvantages of cells: the difficulty of ensuring a good adhesion leads to a solid electrolyte for a long time work at high temperatures and the need for a high working temperature of the solid electrolyte (from 500 to 1200 °С).
Usual gas sensors based on solid electrolytes can be schematically represented in the form of concentration of the element [7].
Figure 5 —Schematic diagram of the sensor based on a solid electrolyte
In figure 5 are shown:
— Me' and Me'' — two electronic Explorer (probably chemically inert and of the same nature), the contacts of which with solid electrolyte (E, s) form the electrodes;
— Electrolyte ES — substance, physically impermeable to gases and is the ionic conductor, containing ions, Xn-;
— X2 — the analyzed gas (which may be in pure form, dissolved in the gas mixture, or in equilibrium with the chemical system, formed by gases, liquids or solids);
— Р and pr — the partial pressure of the gases on both sides of the electrolyte.
On each electrode we have the reaction:
(5)
In the ideal case, this element creates a potential difference, or EDF between conductors Me' and Me" sublect to the Nernst’s law
(6)
or
(7)
— Nernst’s equation, with t:=400.
. 
— sensitivity.