RU2767005C1 - High temperature electrochemical cell - Google Patents

Abstract

FIELD: gas analysis means.

SUBSTANCE: invention relates to means of analyzing gases at high temperatures and can be used in boilers and furnaces for various purposes. Planar type high-temperature electrochemical cell comprises an upper ceramic plate made of a zirconium dioxide-based solid electrolyte material, electrodes deposited on the plate, a heating element, wherein the cell comprises a U-shaped middle ceramic plate made from a ceramics based on an magnesium aluminate spinel, lower ceramic plate made from ceramics based on magnesium aluminate spinel, with possibility of tight connection of upper, middle and lower ceramic plates into a single package, wherein one electrode is a reference and is applied to the lower plane of the upper plate, the second electrode is a measuring electrode and is applied to the upper plane of the upper plate, and the heating element is applied on the upper plane of the lower plate.

EFFECT: high accuracy and reliability of determining oxygen concentration.

3 cl, 5 dwg

Description

The invention relates to tools for the study or analysis of gases at high temperatures and can be used in boilers and furnaces for various purposes at enterprises in the energy, oil refining, chemical and metallurgical industries, ceramic and glass production, and the construction industry. A planar-type high-temperature electrochemical cell is proposed for use as part of a universal stationary industrial oxygen gas analyzer in gaseous media, and which is its main structural element.

Modern requirements for reliability, non-failure operation, service life, accuracy, maintainability at high flue gas temperatures are met only by oxygen gas analyzers with a solid electrolyte ceramic sensor based on zirconium dioxide.

Today, there are domestic solid electrolyte stationary oxygen gas analyzers on the market, for example: TDK-ZM (NPF ZIRCON LLC), AKVT-01 (FGUP SPO Analitpribor), EKON (EKON JSC), IKTS-11 (Promanalitpribor JSC), and some other.

However, due to increasing environmental and technical requirements, a significant improvement in the main characteristics of stationary gas analyzers used in industry is required. In particular, it is necessary: to increase the speed, reduce the time to enter the mode when turned on, reduce the power consumption, reduce the mass of the sensor, reduce the overall diameter of the sensor, increase the maximum cable length, reduce the dimensions and weight of the electronic unit of the gas analyzer, reduce production costs. A fundamental improvement in all of the above characteristics of the gas analyzer is possible with a constructive change in the solid electrolyte ceramic sensor, namely, its manufacture using film technologies (planar sensor).

This will make it possible to create a gas analyzer based on such a sensor with fundamentally improved technical characteristics in order to use it in existing and future automated control systems of fuel-burning installations (boilers, furnaces, etc.).

Known from the prior art is a planar-type high-speed flat wide-range oxygen sensor according to patent CN 1029676420 with priority dated 02.11.2012, assembled from several plates of zirconium dioxide, having an electric heating wire and sensitive electrodes.

An oxygen sensor and a method for manufacturing an oxygen sensor according to US patent 2009044598 with priority dated 08/15/2007, including a layer of zirconium dioxide, two electrodes located on opposite sides of the zirconium dioxide layer, and a porous channel for directing air to the first electrode, preventing flow through hydrocarbons and contact with the first electrode.

A device for determining the characteristics of gas according to the international application WO 2006005332 with conventional priority dated 01.10.2004, having a flat design and consisting of a carrier substrate containing thin layers for at least one heating element, a solid electrolyte of aluminum oxide, glass ceramics or oxide zirconium-aluminum oxide and electrodes.

Known Sensing element according to US patent 2005067283 with priority dated 13.08.2004, having several layers of solid electrolyte of different composition based on zirconium oxide stabilized with yttrium oxide, and two electrodes.

The disadvantage of these devices is that, despite their planar design, they are not intended for use in aggressive environments at high temperatures, and are mainly used to determine the oxygen concentration in the exhaust gases of internal combustion engines.

The closest analogue in technical essence is a high-temperature electrochemical cell-sensor (patent RU 2433394 with priority dated 04/05/2010). The cell-sensor contains a substrate plate made of a heat-resistant insulating material. On one side of the plate, a thin 2–20 µm layer of solid electrolyte is formed, on which catalytic and inert electrodes are applied with leads to the cold zone, coated with a porous layer of ceramics to prevent abrasive wear by the gas under study. On the other side of the plate, a flat heater is applied, covered with a non-porous ceramic layer. The sensor cell can be used in gas analyzers designed to control the exhaust gases of boilers and other fuel-burning installations, as well as for lambda probes used in cars with internal combustion engines in the fuel mixture preparation system, for more complete combustion and reduction of harmful emissions into the atmosphere . The disadvantages of the selected analogue include the lack of temperature control, which is not provided for by the design of this sensor. Without temperature control, it is impossible to accurately determine the oxygen concentration. In this regard, the cell-sensor, due to its design, is not a self-sufficient means of monitoring the oxygen concentration.

It is known that the cell EMF with oxygen concentration and temperature is related by the Nernst formula:

where

R is the universal gas constant,

T - temperature, K,

F - Faraday number,

n is the number of electrons involved in the reaction,

c is the concentration of oxygen in the medium under study,

c ₀ - oxygen concentration at the reference electrode.

Thus, the oxygen concentration is a function of two parameters: EMF and cell temperature. Therefore, cell temperature control is very important for accurate measurement of oxygen concentration.

The present invention solves the technical problem of eliminating these disadvantages, namely, it provides energy-efficient, fast and high-precision measurements of the oxygen concentration directly in the flue gas flow using a gas analyzer with a high-temperature planar-type electrochemical cell.

The technical result of the invention is to increase the accuracy and reliability of determining the oxygen concentration, due to the ability to control two parameters at once: the cell EMF and the cell temperature, as well as to increase energy efficiency and speed due to the multilayer planar structure of the high-temperature electrochemical cell, to reduce the thickness of the solid electrolyte layer, to place the heater as close as possible to the electrodes, the ability to control the cell temperature by controlling the parameters of the heater: current and voltage. In addition, the design of the oxygen gas analyzer is simplified, labor intensity is reduced, and the reliability of the device is increased.

The technical result is realized due to the following design features.

High-temperature electrochemical cell (Cell) of planar type consists of

- from the upper ceramic plate made of solid electrolyte material based on zirconium dioxide,

- from a measuring electrode deposited on the upper plane of the upper plate,

- from a reference electrode deposited on the lower plane of the upper plate,

- from a medium U-shaped ceramic plate made of ceramic based on alumina-magnesia spinel,

- from the lower ceramic plate made of ceramic based on alumina-magnesia spinel,

- from a heating element deposited on the upper plane of the lower plate,

- in this case, the electrodes and the heating element are made of platinum Pt, and also possibly of gold or silver, or tungsten, or molybdenum, or nickel,

- all three ceramic plates are tightly connected in a package.

It should be said that the overall dimensions of the Cell are as follows: length from 30 to 40 mm, width from 4 to 6 mm, thickness from 1 to 1.5 mm. It consists of three hermetically connected ceramic plates, the thickness of each plate is about 0.4 mm. The dimensions are minimal and help to reduce the thickness of the solid electrolyte layer. That gives a decrease in inertia and an increase in speed, an increase in thermal stability, and a decrease in power consumption.

The upper ceramic plate is made of a solid electrolyte - a ceramic material based on zirconium dioxide, stabilized with yttrium oxide (ZrO ₂ ⋅Y ₂ O ₃ ). According to its purpose, the upper ceramic plate is an oxygen-conducting membrane that hermetically separates two gas volumes: the volume of the studied gaseous medium (upper plane) and the volume of reference air with a constant oxygen concentration (lower plane). Electrodes are applied to the upper and lower planes.

The electrodes in the structure perform several functions: electrochemical transformation of matter takes place on them, they are current leads. The electrode material must have good electrical conductivity and adhesion to ceramics, chemical inertness, and porosity for gas supply to the electrolyte. According to the totality of properties, platinum Pt is the best suited and chosen as the electrode material.

The middle ceramic plate with its U-shaped form forms a channel for supplying the reference air to the reference electrode, and also serves as an electrical insulator.

The lower ceramic plate is the bearing part of the heating element in the form of a metallized track deposited on its upper plane. Thus, the heating element is located as close as possible to the electrodes, which allows you to quickly heat up to the required operating temperature (500 ... 700 ° C), accurately maintain this temperature, and significantly reduce power consumption.

The possibility of determining the temperature of the heater, hence the entire Cell, is due to the dependence of the electrical resistance of the metal on its temperature.

Some metals have a pronounced positive temperature coefficient of resistance (TCR), which depends on the properties of the metal. The value of the electrical resistance of a metal conductor depends on its TCS a, and changes in its temperature (J. Ash. Sensors of measuring systems: In 2 books. Book 1. Translated from French - M .: Mir, 1992. - 480 s) . Change in resistance due to heating:

R ₀ - initial resistance;

ΔR - change in resistance, allows you to determine the change in temperature, expressed:

In turn, the electrical resistance of the heater is determined by the parameters of the current and voltage of the heater:

Thus, it is possible to control the temperature of the electric heater by measuring the voltage across the heater and the current drawn by it.

This allows combining the functions of "heater" and "temperature meter" in one device, which greatly simplifies the design of the oxygen gas analyzer, reduces labor intensity, and increases reliability.

This solution can be applied to an oxygen gas analyzer in flue gases, as well as used in the design of other sensors and meters that have their own heating element, for example: semiconductor, thermal catalytic, solid electrolyte, etc.

It is proposed to use platinum as the material of the heating element, which is due to technological advantages (in the future, the general process of firing, due to close temperatures), a significant positive thermal resistance coefficient (3.9 * 10 ^-3 C ^-1 ), characterizing the dependence of electrical resistance on temperature ( J. Ash. Sensors of measuring systems: In 2 books. Book 1. Translated from French - M.: Mir, 1992. - 480 p.). In addition, the chemical passivity of platinum and the absence of crystalline changes ensure the stability of electrical properties during operation in aggressive environments at high temperatures.

The middle and bottom plates are made of magnesium oxide-based ceramic material - alumina-magnesia spinel (MgAl ₂ O ₄ ) with an excess of MgO (AMS). This material has high dielectric properties and thermal linear expansion coefficient (CTEC) close to that of zirconium dioxide.

The ceramic plate material has the necessary mechanical strength and chemical resistance when used in flue gas environments in the high operating temperature range.

All three ceramic plates are hermetically connected in a package and form a planar cell.

Thus, the totality of the above features of a high-temperature electrochemical cell achieves the technical result of improving energy efficiency, accuracy and reliability of determining the oxygen concentration, due to the ability to control two parameters at once: EMF and cell temperature. This solves the problem of providing energy-efficient, fast and highly accurate measurement of the oxygen concentration directly in the flue gas stream. In addition, the design of the oxygen gas analyzer is simplified, the complexity of manufacturing is reduced, and the reliability of the device is increased.

On FIG. 1 and FIG. 2 schematically shows a high-temperature electrochemical cell in section, where

1 - upper ceramic plate,

2 - lower ceramic plate,

3 - measuring electrode,

4 - reference electrode,

5 - heating element,

6 - medium ceramic plate,

7 - channel,

8 - the upper plane of the upper plate,

9 - the lower plane of the upper plate,

10 - upper plane of the lower plate.

The high-temperature electrochemical cell consists of upper (1) and lower (2) ceramic plates. The upper ceramic plate (1) made of a solid electrolyte material based on zirconium dioxide with a measuring electrode (3) deposited on its upper plane (8) and a reference electrode (4) on its lower plane (9) is hermetically connected to the lower ceramic plate (2), carrier heating element (5). Between the upper and lower plates there is a middle U-shaped ceramic plate (6), as a result of which, when all the plates are connected together in one package, a channel (7) is formed inside with an open part for supplying reference air to the reference electrode (4). Thus, the cell separates the two gas volumes, and the analyzed gas goes to the measuring electrode (3), and the reference gas goes to the reference electrode (4). As a result of the resulting difference in partial pressures of oxygen and the occurrence of an electrochemical reaction on the electrodes, the EMF generates EMF E, which, at a given oxygen content (air) on the reference electrode and a controlled cell temperature, determines the oxygen content in the medium under study. The cell temperature is determined by the current value of the electrical resistance of the heater, T;, which is controlled by the voltage and current consumption parameters.

T ₀ - room temperature,

ΔT - temperature difference, which determines the resistance of the heater at room temperature R ₀ , the change in resistance ΔR and TCR α:

Knowing the initial resistance of the heater at room temperature R ₀ , and also, by determining the current value of resistance Ri from the voltage U and current I, one can determine the change in resistance ΔR.

Thus, the current cell temperature can be represented by the expression

Example 1

A sample of a high-temperature electrochemical cell was made. To obtain fundamentally improved technical characteristics, the proposed cell has miniature dimensions and a minimum thickness of ceramic layers, as well as a heater integrated into the design. The electrodes and heater are made of platinum.

The overall dimensions of the high-temperature electrochemical cell sample are as follows:

- length 40 mm,

- width 5 mm,

- thickness 1.5 mm.

A practical measurement of the cell temperature and the electrical resistance of the heater was made. The results are presented in Fig. 3

The correlation of these two values shows the fundamental possibility of implementing the proposed method of temperature control by the resistance of the cell heater.

The practical dependence of resistance on temperature for a given sample of a high-temperature electrochemical cell in the coordinates T(°C) / R(Om) is shown in Fig. 4.

This experimental temperature dependence of the cell heater resistance is close to linear. Software approximation of experimental data using Excel allows you to make the dependence:

Ri=0.012Ti+5.798

Such an individual characteristic of the dependence of the electrical resistance on the temperature of a given sample of the heater of a high-temperature electrochemical cell makes it possible to calculate the temperature from the value of the electrical resistance of a given sample of an electrochemical cell during operation, including the stages of heating and cooling:

Ti=(Ri-5.798)/0.012

The oxygen concentration C (% vol.), expressed from the above Nernst formula, for an air reference electrode (C ₀ \u003d 20.9% vol.), can be calculated, knowing E (mV) and T (°C):

C=100 exp(-46418.11(E/Ti)-l.5612),

where

E - EMF generated by a ceramic sensitive element;

Ti is the temperature at the ceramic sensing element.

The value of EMF E (mV) is fixed on the electrodes of the cell. The temperature value Ti (°C) is determined by the empirical dependence Ti=(Ri-5.798)/0.012, where Ri=U/I.

So, for example, at the values of voltage and current consumption, respectively: U=11.9V, I=0.8A, the resistance value of the heater is 13.8 Ohm. From the graphically presented dependence of the Cell heater resistance on temperature at the Cell parameters indicated above, the cell temperature will be equal to Ti=670°C.

On FIG. 5 shows the measured values of the EMF of cell E, and the corresponding calculated values of the oxygen concentration C at a constant temperature Ti=670°C.

Thus, due to the ability to control two parameters of a high-temperature electrochemical cell at once: EMF and cell temperature, the main technical result is achieved: the accuracy and reliability of determining the oxygen concentration. This will make it possible to use the proposed cell as a sensor for a high-precision measuring instrument - a high-temperature oxygen gas analyzer.

In addition, the functional and constructive combination of two different functions: a heater and a means of controlling its own temperature in one product simplifies the design of the sensor as a whole, eliminating the need for additional devices. Consequently, labor intensity is reduced, and the reliability of the gas analyzer as a whole is increased.

When using the proposed high-temperature electrochemical cell as part of an oxygen gas analyzer, it is possible to reduce the time to establish stable readings by a factor of 3 ... In addition, reduce the mass of the gas analyzer and the overall diameter of the gas analyzer by 1.5...3 times. Reducing the power consumption, and therefore the current strength consumed by the heater of the gas analyzer, will increase the cable length between the electronic unit and the sensor by 3-5 times, up to 200 meters.

Claims (3)

1. A high-temperature electrochemical cell of a planar type, containing an upper ceramic plate made of a solid electrolyte material based on zirconium dioxide, electrodes deposited on the plate, a heating element, characterized in that it contains a middle U-shaped ceramic plate made of ceramic based on alumina-magnesium spinels, a lower ceramic plate made of ceramic based on alumina-magnesium spinel, with the possibility of hermetically connecting the upper, middle and lower ceramic plates into a single package, while one electrode is a reference and is deposited on the lower plane of the upper plate, the second electrode is measuring and deposited on the upper plane of the upper plate, and the heating element is applied to the upper plane of the lower plate. 2. Cell according to claim 1, characterized in that the electrodes are made of platinum Pt. 3. Cell according to claim 1, characterized in that the heating element is made of platinum Pt.

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