SOME ISSUES OF BURNING COALS FROM THE KANSK–ACHINSK BASIN IN LARGE BOILER INSTALLATIONS

I. K. Lebedev, N. V. Trikashnyi

(Presented by the scientific seminar of the Department of Boiler Construction and Boiler Installations)

Coals of the Kansk–Achinsk basin possess two distinctive features: a high content of calcium oxide in the ash and sharply variable temperature and viscosity characteristics of the ash depending on its ash content. When these coals are burned in open-type furnace devices with dry slag removal in large boiler units of power stations, a number of difficulties arise, the chief of which are the formation of bonded deposits on convective heating surfaces located in the gas-temperature zone of 800–1000°C, and the slagging of radiant heating surfaces located in the furnace chamber.

On the basis of studying the structure and character of the formation of bonded deposits on the convective heating surfaces of boiler units burning coals of the Kansk–Achinsk basin, a number of hypotheses have arisen concerning the mechanism by which this phenomenon proceeds.

Most researchers believe that this mechanism is based on the appearance in the ash of low-melting components that impart stickiness to ash particles at ash temperatures of 850–1000°C. Such components may be either sulfide-sulfate melts of calcium (Tomsk Polytechnic Institute, All-Union Thermal Engineering Institute), or low-temperature silicates (Tallinn Polytechnic Institute). Other researchers (ORGRES) believe that the mechanism of formation of bonded deposits is based on agglomeration and separation of fine ash fractions on the heating surface, followed by their sintering during sulfatization in the flue-gas stream.

The practice of burning Kansk–Achinsk basin coals in furnace devices of various designs has shown that the process of formation of bonded deposits can be completely eliminated, or the intensity of this process significantly reduced, when the coals are burned in highly forced furnace devices with liquid slag removal, provided that a high-temperature combustion process is organized and the entire mass of ash is subjected to high-temperature treatment during combustion. Theoretical and experimental studies conducted in laboratories also show that high-temperature treatment of ash during the combustion of Kansk–Achinsk coals is useful from the standpoint of all currently existing hypotheses on the formation of bonded deposits and may lead to the cessation of this highly undesirable phenomenon.

On the basis of data from the practice of burning coals of the Kansk-Achinsk basin in furnace devices of various designs, and of theoretical and experimental studies of deposits and fly ash, the following conclusions may now be drawn.

1. The burning of coals of the Kansk-Achinsk basin should be carried out at high temperatures that ensure high-temperature treatment of all or most of the ash during the combustion process.

2. The level of high-temperature combustion and high-temperature treatment of the ash should be no lower than 1450–1500°C.

3. The design of furnace devices for burning coals of the Kansk-Achinsk basin must ensure uniform, high-potential temperature fields in the combustion chamber, excluding low-temperature combustion zones.

4. In order to intensify the physicochemical processes occurring during high-temperature treatment, all the ash in the furnace process must pass through the melting stage, and the furnaces must be equipped with liquid slag removal.

5. In the design of furnace devices, combustion zones with insulated heating surfaces must be provided in the pre-furnace, in which, insofar as possible, all combustion processes should be fully completed.

All the conclusions we have set forth have been discussed and approved at scientific and technical conferences, as well as in periodical publications, and at present are unlikely to be doubted by anyone. Therefore, in the present article we would like to consider the question of how and under what conditions the formulated requirements can be fulfilled in practice.

At present there is no unified opinion regarding the assessment of the suitability of solid fuels for their combustion in furnaces with liquid slag removal. Various firms and scientific organizations, on the basis of experience in burning various fuels with liquid slag removal, give different recommendations, set forth in detail in the literature [1, 2, 3, 4]. On the basis of an analysis of the operating experience of furnaces with liquid slag removal in the FRG, the USA, and Soviet power engineering, Doctor of Technical Sciences Yu. L. Marshak gives the following relationships determining the suitability of solid fuels for burning them in furnaces with liquid slag removal:

\[ \vartheta_a = a_x t_{\mu=250} + b_x, \tag{1} \]

\[ t_0 < t_{\mu=250}. \tag{2} \]

Here
\(\vartheta_a\) — theoretical combustion temperature, °C;
\(t_0\) — critical temperature of the viscosity characteristic, °C;
\(t_{\mu=250}\) — slag temperature at a viscosity of 250 P, °C;
\(a_x, b_x\) — coefficients depending on the type of furnace, the kind of fuel, the temperature of the hot air, and the fuel-drying scheme.

As is evident from these equations, the slag must be removed from the furnace at a viscosity of 250 P, and the slag crystallization temperature \(t_0\) must be lower than the temperature at which slag removal takes place.

These recommendations are not applicable to the coals of the Kansk-Achinsk basin, since in most cases—except for the high-ash coals of the Nazarovo deposit—the slags at critical turning points have a viscosity below 100 P.

Therefore, to assess the possibility of burning various coals of the Kansk-Achinsk basin, we have carried out the following calculations.

We express the heat balance of the furnace by the following equation:

\[ \psi_{\text{k.g}} B_p (Vc)_a \vartheta_a = B_p (Vc)'_{\text{t}} \vartheta''_{\text{t}} + qH, \tag{3} \]

where \(q\) is the magnitude of the heat flux through the slag film on the surfaces of the combustion chamber, kcal/m\(^2\)·h;

\(H\) is the surface area of the combustion chamber, m\(^2\);

\(\psi_{\text{c.c}}\) is the degree of fuel burnout in the combustion chamber.

The remaining quantities are designated in accordance with the standard method for thermal calculation of boiler units [5].

Solving equation (3) with respect to \(\vartheta_m''\) and introducing simple transformations, we obtain

\[ \vartheta_m''=\psi_{\text{c.c}}\vartheta_a''\frac{(Vc)_a}{(Vc)_m''} \left( 1-\frac{q}{\psi_{\text{c.c}}}\frac{H}{V_{\text{c.c}}}\frac{1}{\left(\dfrac{Q}{V}\right)_{\text{c.c}}}\frac{Q_p^p}{Q_T} \right)^\circ\text{C}, \tag{4} \]

where \(\dfrac{H}{V_{\text{c.c}}}\) is the magnitude of the surface area of the combustion chamber referred to its volume, m\(^2\)/m\(^3\);

\[ \left(\frac{Q}{V}\right)_{\text{c.c}} \]

is the apparent heat release rate of the combustion chamber, kcal/m\(^3\)·h;

\(Q_p^p\) is the available heat of the fuel, kcal/kg;

\(Q_m\) is the total heat release of the combustion chamber, kcal/kg.

The absolute temperature of the combustion chamber will be

\[ T_m''=\vartheta_m''+273^\circ\text{K}, \tag{5} \]

and we take

\[ \frac{(Vc)_a}{(Vc)_m''}\simeq 1, \]

then, after simple transformations, equation (5) can be represented in the form:

\[ T_m''=\psi_{\text{c.c}}T_a'' \left[ (1-K)\left(1-\frac{273}{T_a}\right)+\frac{273}{T_a} \right]^\circ\text{K}, \tag{6} \]

where

\[ K=\frac{q}{\psi_{\text{c.c}}}\frac{H}{V_{\text{c.c}}} \frac{1}{\left(\dfrac{Q}{V}\right)_{\text{c.c}}} \frac{Q_p^p}{Q_m}. \]

In calculations of radiant heat exchange in the furnace chamber, the effective temperature of the radiating flame is taken as [6]

\[ T_{\phi}=0.91\sqrt{T_a''T_m''}\ ^\circ\text{K}. \tag{7} \]

Substituting the value of \(T_m''\) from equation (6) into equation (7), we obtain

\[ T_{\phi}=0.91T_a\sqrt{\psi_{\text{c.c}}} \sqrt{(1-K)\left(1-\frac{273}{T_a}\right)+\frac{273}{T_a}}\ ^\circ\text{K}. \tag{8} \]

To estimate the heat flux through the slag wall, we shall use the method developed by Doctor of Technical Sciences Yu. L. Marshak. This method is based on considering the equality of heat fluxes from the flame to the flowing slag film and through a slag film of specified thickness and thermal conductivity. In this case, the thickness of the slag film is determined on the basis of considering the motion of liquid slag as a viscous liquid [3].

The magnitude of the heat flux from the flame to the slag film was determined by the simultaneous solution of the equations

\[ q = 4.9 \cdot 10^{-8} a_k \left[T_{\mathrm{f}}^{4} - T_{\mathrm{p}}^{4}\right] + \alpha_k \left[T_{\mathrm{f}} - T_{\mathrm{p}}\right]\ \mathrm{kcal}/\mathrm{m}^{2}\!\cdot\!\mathrm{h} \]

and

\[ q = \frac{ \lambda_{\mathrm{sl}} \sqrt[3]{\,2\left(e^{\varphi_{\mathrm{sl}}\Delta t}-1\right) -2\varphi_{\mathrm{sl}}\Delta t_{\mathrm{sl}} -\left(\varphi_{\mathrm{sl}}\Delta t\right)^2\,} }{ \varphi_{\mathrm{sl}} \sqrt{ \dfrac{\mu_0 G_0 \eta_{\mathrm{sl}}} {\gamma_{\mathrm{sl}}^{2} U \sin \alpha} } } \tag{9} \]

Here \(a_k\) is the blackness factor of the furnace, taken as equal to 0.63 [3];
\(T_{\mathrm{f}}, T_{\mathrm{p}}\) are the effective temperature of the flame and of the slag film, respectively, K;

\[ \varphi_{\mathrm{sl}} = \frac{\ln \dfrac{\mu}{\mu_0}}{\Delta t} \]

is the viscosity characteristic of the slag according to [3];
\(T_0\) is the critical temperature of the slag (temperature at the turning point), K;
\(\mu_0\) is the viscosity of the slag at the turning point, kg/m\(^2\) sec;
\(G_0\) is the amount of ash introduced into the furnace, kg/sec;
\(\lambda_{\mathrm{sl}}\) is the mean thermal conductivity of the slag within the temperature range from \(T_{\mathrm{p}}\) to \(T_0\), kcal/m h deg;
\(\Delta t = T_{\mathrm{p}} - T_0\);
\(\gamma_{\mathrm{sl}}\) is the specific weight of the slag, kg/m\(^3\);
\(U\) is the perimeter of the walls along which the slag film flows, m;
\(\alpha\) is the angle of inclination of the wall to the horizontal, degrees;
\(\eta_{\mathrm{sl}}\) is the slag-capture coefficient.

The calculations were carried out as applied to the design of the pre-furnaces of the BKZ-320-140 boiler; in this case \(\eta_{\mathrm{sl}} = 0.4\),
\(U = 33.2\) m for the vertical walls and
\(U = 5.6\) m for the perimeter of the burner embrasure; \(\alpha = 15^\circ\).

The coefficient of convective heat transfer from the gases to the slag film, \(\alpha_k\), was determined from the equation [3]

\[ \mathrm{Nu} = 0.0036\,\mathrm{Re}. \tag{10} \]

Calculations were performed for three types of slags: for slags of Nazarovo coal with average ash content, for which \(A^{c} = 10.62\%\), \(\mu_0 = 38\) Pz, \(\varphi_{\mathrm{sl}} = 0.00955\), \(t_0 = 1227^\circ\mathrm{C}\); for slags of Nazarovo coal with increased ash content, \(A^{c} = 30.06\%\), \(\mu_0 = 220\) Pz, \(\varphi_{\mathrm{sl}} = 0.00717\), \(t_0 = 1427^\circ\mathrm{C}\); and for low-ash Berezovo coal, for which \(A^{c} = 4\%\), \(\mu_0 = 10\) Pz, \(\varphi_{\mathrm{sl}} = 0.01\), \(t_0 = 1627^\circ\mathrm{C}\). The results of calculating the temperatures on the surface of the slag film and the heat fluxes through the slag film for one of the coals considered are given in Fig. 1*.

Next, the normal slag-outflow temperature was taken to be a temperature exceeding by \(50\)–\(100^\circ\mathrm{C}\) the temperature at the turning point of the viscosity characteristic; from the curves of Fig. 1 the effective flame temperature and the magnitude of the heat fluxes through the slag film were found, and then the required combustion temperature was determined from equation (8). As applied to the design of the pre-furnaces of the BKZ-320-140 boiler, it was assumed that

\[ \frac{H}{V_{\mathrm{kg}}} = 0.7,\qquad \frac{Q_{\mathrm{r}}^{p}}{Q_{\mathrm{t}}} = 0.9,\qquad \Psi_{\mathrm{kg}} = 0.9. \]

The calculated value of \(q\) for substitution into equation (8) was taken according to the formula

* Similar curves were constructed for calculations with other fuels.

where

\[ q=\frac{q_{\mathrm{v}}H_{\mathrm{v}}+q_{\mathrm{pod}}H_{\mathrm{pod}}}{H_{\mathrm{v}}+H_{\mathrm{pod}}}, \tag{11} \]

\(q_{\mathrm{v}}\) and \(q_{\mathrm{pod}}\) are the heat fluxes through the slag film on the vertical walls and on the hearth, respectively, kcal/m\(^2\) h;

\(H_{\mathrm{v}}, H_{\mathrm{pod}}\) are the surface areas of the vertical walls and the hearth, respectively, m\(^2\).

The results of the calculations are given in Table 1.

It is evident from the table presented that, in order to ensure normal discharge of liquid slag from the furnace when burning Nazarovo coals with an average ash content, the flame temperature near the burner should be maintained within the range 1300–1400°C, and the theoretical combustion temperature within the range 1550–1750°C. Such temperatures can readily be maintained both with direct injection of dust by the drying agent and in furnace units equipped with pulverized-fuel preparation systems using a closed or semi-closed fuel-drying scheme. But at these temperatures the required high-temperature treatment of the ash will not be ensured. Therefore, the temperatures in the furnace chamber must be maintained above 1300–1400°C, according to the greater superheating of the slag, and the pulverized-fuel preparation systems must be either with a semi-closed or with an open drying scheme.

Fig. 1. Dependence of the temperature at the surface of the slag film and of the heat flux through the film on the flame temperature. Berezovo coal, \(A^{c}=4\%\), \(\mu_{0}=10\), \(t_{\mathrm{g}}=1900^\circ\mathrm{C}\), \(\psi_{\mathrm{mp}}=0.01\):
1 — temperature of the surface of the slag film on the hearth; 2 — temperature of the surface of the slag film on the vertical walls; 3 — heat flux through the slag film of the vertical walls; 4 — heat flux through the slag wall.

When burning Nazarovo coal with elevated ash content, in order to maintain liquid slag, the temperature near the burner, as is evident from Table 1, must be within the range 1500–1600°C, and the theoretical combustion temperature must accordingly be 1730–1900°C. In this case, in an open furnace with direct injection, it will no longer be possible to obtain normal discharge of liquid slag, and only the use of furnaces with pre-furnaces and an open or semi-closed drying scheme will be required.

These provisions will be valid for coals of the Irsha-Borodino deposit and for coals of the Berezovo deposit with ash content not lower than \(A^{c}=5.5\%\), since the viscosity characteristics of the ash of these coals are close to the viscosity characteristics of Nazarovo coals.

As for Berezovo coals with ash content below \(A^{c}=5.5\%\), as is evident from Table 1, for normal discharge of liquid slag it is necessary to have a flame temperature near the burner of 1800–1950°C, and the theoretical combustion temperature must be within the range 2350–2800°C. In modern designs of furnaces with liquid slag removal, such temperatures cannot be obtained even with an open drying scheme, since the moisture content of the finished dust, according to explosion-safety conditions, must be within the range

\[ W_{\mathrm{pl}}=W_{\mathrm{g}}+8=22.5\% \]

Table 1

Results of calculation of the theoretical combustion temperature and the temperature near the slag-screen tubes for liquid slag removal during combustion of coals from the Kansk-Achinsk basin.

On the basis of the results of the calculations carried out above, three possible directions may be chosen.

1. Limiting the ash content of Berezovo coal issued from the open pit to an ash content of $A^c = 6\%$, as was recommended by the meeting at VTI in March 1967. In this case, the open-pit design should provide for a technological scheme that would envisage either mixing high-ash coals with low-ash coals, or admixing, to low-ash coals with an ash content below $A_c = 6\%$, siliceous rocks from the floor or roof of the seam, with mandatory reliable control of the ash content of the coal issued from the open pit.

A second possible solution may be to maintain the ash content of the coal burned in boiler units above $A^c = 6\%$ by admixing high-ash coals from the Nazarovo deposit with low-ash Berezovo coals in the power-plant fuel-handling system. This approach, while simplifying the technological scheme of coal mining, causes some complications in the station’s fuel-supply scheme, but is entirely practicable.

A third possible direction is the creation of special boiler units in whose furnace devices it is possible to obtain liquid slag for a broad range of coals of the Kansk-Achinsk Basin. But it should be pointed out that the creation of such units will be associated with the solution of a number of problems that have not yet been encountered either in domestic or in foreign thermal power engineering. Such problems include: the problem of high air preheating, or the use of oxygen-enriched blast to raise the theoretical combustion temperature; the creation of designs for insulated heating surfaces that ensure reliable operation under high-temperature conditions; and reliable insulation of the furnaces in order to reduce the heat flux.

In some works [7] it is also indicated that there is the possibility of increased sublimation of the mineral part of the fuel, which may affect an increase in fouling of the heating surface. But this problem, apparently, will be significant at higher temperatures than those considered by us.

Solving the problems associated with the creation of new types of boiler units will require considerable time, while the development of the coal resources of the unique Kansk-Achinsk coal basin is a matter for the present day; therefore, in developing power engineering on the basis of the coals of the Kansk-Achinsk Basin, the first two directions remain at our disposal.

REFERENCES

1. A. N. Lebedev, S. N. Mironov, Yu. L. Marshak. Field of application of furnaces with liquid slag removal and their thermal economic efficiency. In: Increasing Steam Parameters and Unit Capacities in Power Engineering, Gosenergoizdat, Moscow—Leningrad, 1961.

2. U. H. Rowand, D. U. Kessler. Technical progress in the designs of steam boilers of the Babcock & Wilcox Company (USA). Proceedings of the Fifth Congress of the World Power Conference, vol. 1, TsBTI Mashinostroeniya Publishing House, Moscow, 1958.

3. Yu. L. Marshak. Furnace Devices with Vertical Cyclone Pre-furnaces. Energiya Publishing House, Moscow—Leningrad, 1966.

4. A. N. Lebedev, G. A. Sheinin. Slagging of Pulverized-Coal Furnaces and Combating It. Energiya Publishing House, Moscow—Leningrad, 1966.

5. Thermal Calculation of Boiler Units. Normative Method. GEI, Moscow—Leningrad, 1957.

6. Draft of new standards for thermal calculation of boiler units.

7. S. A. Tager. Problems of modern furnace engineering and tasks of scientific research. Teploenergetika, No. 9, 1955.