PHOTOELECTRONIC DEVICE FOR RAPID DETERMINATION OF CARBON CONTENT IN ASH FROM THERMAL POWER PLANTS

K. N. POPOV, V. N. TITOV

(Presented by the scientific seminar of the Faculty of Physics and Technology)

This article considers a device, improved by the authors, for the rapid determination of carbon content in the ash of thermal power plants burning hard coals with a high yield of volatiles.

Fig. 1. Schematic diagram of the device: FS₁, FS₂ — photoresistors; 1 — measuring bridge; 2 — cathode follower; 3 — electronic amplifier; 4 — reversible motor; 5 — disk with a set of standard samples; 6 — sample under investigation; 7 — sample-feed motor; 8 — chamber; 9 — micromanometer; 10 — measuring bridge on FS; 11 — electronic amplifier; 12 — motor with a scale; 13 — electromagnetic valves; 14 — press; 16 — power-supply unit.

The device consists of two circuits: a) a photoelectronic circuit for measuring the carbon content \(C_y\), which is shown in the upper part of Fig. 1; b) a photoelectronic circuit for measuring the granulometric composition of the samples under investigation (the lower part of Fig. 1).

The operating principle of circuit “a” is based on the use of the dependence of the magnitude of the reflected light flux on the carbon content in the sample. Carbon, being the main component in the combustible elements of ash [1], has a high absorptive capacity for light waves in the visible range; therefore, an increase in the content of \(C_y\) in the sample reduces the amount of light reflected from its surface.

The main element of the circuit is disk 5 with a set of standard samples, arranged in order of increasing \(C_y^{e}\) from 2 to 12% at intervals of 1%.

Lamp 15 simultaneously illuminates the surface of the investigated and standard samples. If the reflected light fluxes, measured by photoresistances \(FC_1\) and \(FC_2\), are not equal, then a voltage appears at the output of amplifier 3, which is fed to reversible motor 4. The motor rotates disk 5 in the direction of equalizing the light fluxes; i.e., the circuit carries out an automatic search for that standard sample in which the content \(C_y^{e}\) is equal to \(C_y^{i}\) of the investigated sample.

Since the operating principle of the instrument is based on measuring the intensity of the reflected light flux, a difference in the granulometric composition of the standard and investigated samples introduces an error into the measurement of the content.

Analysis of the granulometric composition of samples taken from the station over one and a half years of its operation showed that in some samples it differs considerably. For such samples, the error in measuring \(C_y\) due to the difference in granulometric composition amounted to 25–30% of the scale.

In connection with the influence of granulometric composition on the readings of the instrument, the authors developed and investigated an additional circuit “b,” intended for introducing a correction into the readings of the main instrument in the case of different granulometric composition of the investigated and standard samples.

The operating principle of circuit “b” is based, as in the surface meter of V. V. Tovarov [2], on measuring the rate of pressure drop in a chamber from which air is passed through the investigated sample 6.

Figure 2 shows the circuit for feeding air through the sample. The accepted designations are: \(P_0\) is the initial pressure in the chamber, \(V_0\) is the specific volume of air at \(P_0\), \(V_k\) is the volume of the chamber, \(P_a\) is atmospheric pressure, \(h\) is the height of the layer of the investigated sample above the measuring tubes, \(\delta\) is the mean effective particle diameter [3], \(F\) is the total cross section of the tubes, \(P_i\) is the pressure measured by the circuit according to the micromanometer, \(l\) is the distance between the photoresistances \(FC\), \(\mu\) is the coefficient of dynamic viscosity of air, \(\gamma_a\) is the specific weight of air at atmospheric pressure, and \(K\) is a coefficient taking into account the porosity and structure of the sample [3].

The law of pressure variation in the chamber with time, when measuring a sample, is described by the equation

\[ P = P_0 e^{-Bt}, \tag{1} \]

where \(P\) is the pressure in the chamber at time \(t\);

\(B\) is a coefficient determined by the measurement conditions and is determined by the formula

\[ B = \frac{P_0 V_0 \gamma_a F \delta^2}{96 V_k \mu h K}. \]

It follows from formula (1) that the pressure change in the chamber proceeds exponentially and depends on the square of the mean effective diameter of the sample. The process is isothermal.

Fig. 2. Schematic of air delivery through the sample under investigation.

Fig. 3. Plot of the dependence of pulse duration $\tau$ on particle diameter $\delta$.

Figure 3 presents curves for different diameters, with \(\delta_1 > \delta_2 > \delta_3\).

The designs of the hoppers and of the sample-feeding mechanism provide identical filling conditions and a constant layer thickness for all samples.

The pressure \(P_o\) is produced by a Sylphon bellows press. The pressure value is chosen so that the column of displaced liquid interrupts the light fluxes incident on the photoresistors. During measurement, the release of air from the chamber through the sample into the atmosphere is opened. As the liquid column passes the upper photoresistor \(FS\), the measuring bridge goes out of equilibrium; at the output of amplifier 11 a pulse appears, switching on motor 12. The duration of the pulse \(\tau\) (see Figs. 2, 3) is determined by the time taken for the liquid column to pass the distance \(l\) from the upper to the lower photoresistor \(FS\). As a result, the angle of rotation of the scale is found to depend on the particle diameter. For a diameter of \(200\,\mu\), the angle of rotation of the scale is \(38^\circ\). For a diameter of \(60\,\mu\), it is correspondingly \(88^\circ\).

Measurement of the amount of carbon with correction for granulometric composition is carried out as follows.

The sample under investigation is delivered by the feeding mechanism under photoresistor \(FS_1\).

The press creates pressure in the chamber; with the “measurement” button, the discharge of air through the sample is opened and voltage is applied to circuit “a.” At this moment circuit “a” searches for the reference sample by rotating the disk with a set of reference samples \(C_y^{э}\), while circuit “b” measures the air permeability of the sample.

Fig. 4. Graph for obtaining the correction \(\Pi\) when \(\alpha_i\) is not equal to \(\alpha_э\).

The reading is taken from the screen, onto which are projected the figures for the carbon content in the reference samples \(C_y^{э}\), the rotation angle \(\alpha_э\)—for each reference sample of the fractional-composition scale, measured before the disk is loaded with reference samples—and the rotation angle \(\alpha_i\) for the given sample under investigation. If the angle \(\alpha_i\) is not equal to \(\alpha_э\), then a correction \(\Pi\) is introduced into the instrument reading.

The magnitude of the correction is determined from the graph shown in Fig. 4.

When \(\alpha_i > \alpha_э\), the instrument readings are too high. To obtain the actual carbon content in the sample, it is necessary to subtract the correction \(\Pi\) from the reading of circuit “a”; the magnitude ...

which is determined from the graph. When \(z_i < z_e\), the instrument reading is underestimated; in this case the correction \(\Pi\) is added. For example, when \(z_i < z_e\) by \(20^\circ\) and the instrument indicated \(C_y^3 = 6\%\), \(\Pi = +1.8\%\). The actual carbon content in the sample will be \(C_d = C_y^3 + \Pi = 7.8\%\).

The graph for obtaining the correction was constructed from experimental data.

The results of measurements on ash samples from the Cheremkhovo open-pit coal mine, with different carbon contents and different particle-size compositions, are shown in Fig. 5.

Fig. 5. Instrument readings with correction for particle-size composition.

With the introduction of the correction, the scatter of readings decreased to \(0.4 \div 0.6\%\). The total processing time for one sample is \(40 \div 50\) sec. The instrument contains a device for automatically setting the particle-size-composition scale to “zero,” an automatic device for fixing the sample on the belt opposite the measuring head of the instrument, and a device for periodic purging of the measuring tubes.

REFERENCES

1. P. P. Elizarov. Operation of High-Pressure Boiler Installations at Power Stations. Gosenergoizdat, Moscow, 1961.

2. V. V. Tovarov. An instrument for measuring the specific surface of powdered materials by the method of air permeability. Factory Laboratory, Metallurgizdat, vol. XIX, No. 5, 1953.

3. L. S. Leibenzon. Collected Works, vol. II, Underground Gas Hydrodynamics. Publishing House of the Academy of Sciences of the USSR, 1953.