SOME ISSUES IN THE DESIGN OF FURNACE DEVICES FOR BOILER UNITS IN THE COMBUSTION OF COALS FROM THE KANSK–ACHINSK BASIN

P. K. LEBEDEV

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

1. Selection of the Furnace-Chamber Design

In a number of our works [1, 2] it was shown that the principal direction in combating the formation of bonded deposits on convective heating surfaces when burning coals from the Kansk–Achinsk Basin is the organization of high-temperature combustion of these coals in furnace devices with liquid slag removal.

The necessity of high-temperature combustion and high-temperature treatment of ash in the course of the furnace process imposes definite requirements on furnace devices. On the basis of the conclusions that were drawn earlier [2], these requirements may be formulated as follows.

In the combustion zone of the main part of the fuel, a high temperature must be maintained that would ensure the complete course of the transformation processes involving the mineral part of the fuel, in which all mineral constituents must be converted either into strong aluminosilicates, aluminoferrites, or into inert glasses. To achieve the necessary intensification of these processes, they must proceed not as solid-phase reactions in the diffusion region, but in the kinetic region; this can be ensured by the passage of the entire mineral mass through the molten (liquid) state. Therefore the furnace devices must have liquid slag removal.

In order to ensure high-temperature combustion of all or most of the fuel supplied to the furnace chamber, in that part of the furnace volume where combustion takes place it is necessary to maintain uniformity of the temperature fields both over the horizontal cross section of the furnace volume and along the length of the burning flame path; and the combustion chamber itself must have high heat-release rates per unit volume.

For the most profound transformations in the mineral part of the fuel to occur after completion of the main stages of combustion, it is necessary, as far as possible, to ensure the maximum residence time of the ash particles under high-temperature conditions. Therefore, in furnace devices all stages of combustion must be intensified, with the calculation that they proceed along the shortest possible flame path. This gives rise to requirements for burner devices, which must provide good initial mixing, good turbulization of the flame in the combustion zone, and the most complete and intensive burnout of the fuel in the shortest possible time.

Such are the basic requirements which, in our opinion, apply to furnace devices for the combustion of coals from the Kansk-Achinsk Basin.

Let us consider various modern designs of furnace devices from the standpoint of the extent to which they meet the formulated requirements. Naturally, we shall consider only furnaces with liquid slag removal.

The simplest in design and the least expensive is the single-chamber open furnace with liquid slag removal. One of the features of open furnaces with liquid slag removal is the nonuniformity of the temperature fields, both over the cross section of the furnace chamber and along the length of the flame path. As a result, the combustion core with the highest temperatures occupies a small fraction of the furnace volume. Therefore a significant (if not the greater) part of the fuel in an open furnace is burned under low-temperature conditions, and the ash does not receive the required high-temperature treatment. Thus, open-type furnace devices do not meet the requirements formulated above and cannot be recommended for the combustion of coals from the Kansk-Achinsk Basin. This is also confirmed by practice. Thus, for example, when the PK-38 boiler units of the Nazarovo GRES were converted from dry to liquid slag removal, not only did the formation of bonded deposits on the convective heating surfaces not cease, but the intensity of this process decreased only slightly.

The requirements formulated are most fully met by furnace devices with separated combustion zones in the form of pre-furnaces of various designs, with the combustion processes of the main part of the fuel possibly completed in them at high temperatures. Such furnace devices include VTI furnace devices with vertical cyclone pre-furnaces, TsKTI furnace devices with horizontal cyclone pre-furnaces, semi-open furnace devices of BKZ-TsKTI with octagonal pre-furnaces, and the VTI gamma furnace.

When coals from the Kansk-Achinsk Basin are burned in all the furnace devices listed above, with the exception of the VTI gamma furnace, the formation of bonded deposits on the convective heating surfaces of boiler units is not observed. As for the VTI gamma furnace, operating experience with it is available at Krasnoyarsk TETs-1, where bonded deposits nevertheless do form. The occurrence of bonded deposits is explained by the fact that the use in this installation of a pulverized-fuel preparation system with semi-direct blowing [3] did not make it possible to obtain the required level of high-temperature ash treatment.

2. On the Selection of the Gas Temperature at the Inlet to the Convective Heating Surfaces of Boiler Units When Burning Coals from the Kansk-Achinsk Basin

At present there are various opinions concerning the selection of the gas temperature at the inlet to the convective heating surfaces when designing boiler units for burning coals from the Kansk-Achinsk Basin. Thus, in the draft standards for the thermal calculation of boiler units [4], it is recommended that the temperature at the furnace outlet be taken as 900°C; at the same time it is provided that the combustion products enter the rarefied part of the convective surfaces (screens, widely spaced rows of superheater tubes), in which the gas temperature is additionally reduced by 50–100°C. In [5] it is recommended that, for these coals, the gas temperature at the inlet to the convective heating surfaces be taken as 650–850°C, and in [6] as 850–900°C.

The data of our studies of coals from three deposits of the Kansk-Achinsk Basin, which agree well with VTI data, show

...that the minimum temperatures at which ash deformation begins are: for coals from the Nazarovo deposit, \(1160^\circ\mathrm{C}\); for the Irsha-Borodino deposit, \(1180^\circ\mathrm{C}\); and for the Berezovo deposit, \(1200^\circ\mathrm{C}\). Thus, the proposals considered above substantially reduce the gas temperature at the inlet to the convective heating surfaces in comparison with temperature \(t_1\). The basis for such recommendations was the operating experience of boiler units of the PK-38, PK-10, and other types in the Krasnoyarskenergo system, equipped with open-type mine-mill furnaces with dry and liquid slag removal, where intensive fouling of the convective heating surfaces by bonded deposits occurs when the gas temperature in them rises above \(850^\circ\mathrm{C}\).

A study of the nature and mechanism of formation of bonded deposits during the combustion of Kansk-Achinsk Basin coals in various installations showed that the fouling of heating surfaces by bonded deposits can be eliminated by applying high-temperature combustion of these coals with high-temperature treatment of the ash during the furnace process in specially designed furnace devices with liquid slag removal [2]. These conclusions are confirmed by practice. Thus, BKZ-320-140 boiler units, in whose furnace devices high-temperature combustion and high-temperature treatment of fly ash are provided, show no signs of formation of bonded deposits on the convective surfaces [7]. No signs of formation of bonded deposits were observed either during experimental combustion of Nazarovo coal in BKZ-220-100 BTs boiler units equipped with furnace devices with vertical VTI pre-furnaces.

The gas temperature at the inlet to the convective superheater surfaces on the BKZ-320-140 boiler unit, when burning Irsha-Borodino coal with boiler loads from 125 to 290 t/h, according to measurements performed by TsKTI, varied within the range \(930\text{--}1070^\circ\mathrm{C}\), and during experimental combustion of Nazarovo coal at boiler loads of 160–320 t/h, within the range \(950\text{--}1100^\circ\mathrm{C}\); the temperatures ahead of the screen surfaces were, respectively: when burning Irsha-Borodino coal, \(1030\text{--}1250^\circ\mathrm{C}\), and when burning Nazarovo coal, \(1050\text{--}1300^\circ\mathrm{C}\). Despite the fact that the convective superheater has fairly tight tube banks

\[ \left(\frac{S_1}{d} = 2.5,\ \frac{S_2}{d} = 2.0\right), \]

slagging, the formation of bonded deposits, and entrainment of the banks by fly ash were not observed either when burning Irsha-Borodino or Nazarovo coals. Blowing of the superheaters was carried out only at boiler loads above 220 t/h, two to three times per day, by retractable soot blowers OPK-7 located between the 1st and 3rd stages of the superheater.

Both when burning Irsha-Borodino coal and when burning Nazarovo coal, on the lower bends of the tubes of the screen surfaces there was observed the formation of small accretions (Fig. 1), consisting of slightly sintered slag particles of a dark-brown color. These deposits are very weakly bonded to one another and to the tube surface and are spontaneously destroyed as they grow to more than 30–40 mm.

Microscopic examination of these deposits showed that they consist of strongly deformed ash particles, that is, they are the result of simple slagging, since the gas temperatures ahead of the screen surfaces exceed the temperature at which ash deformation begins. Microscopic examination of loose deposits from superheater surfaces located at the gas inlet reveals in them a certain amount of deformed ash particles, which indicates that, at the inlet to the superheater surfaces, not all ash particles are in a fully solidified state.

This fact indicates that, with proper high-temperature treatment of the ash in the furnace process, even some increase in the gas temperature above the temperature at which ash deformation begins still does not cause slagging of the convective heating surfaces.

Thus, operating experience with BKZ-320 boiler units when burning coals of the Kansk-Achinsk basin with proper high-temperature treatment of the ash does not make it necessary to limit the gas temperature at the inlet to the convective heating surfaces by the conditions of formation of bound deposits. These limitations should be determined, as a rule, only by the temperature at which ash deformation begins; proceeding from this, the gas temperature at the inlet to the convective heating surfaces may be taken as 1100°C. Taking into account the possibility of certain errors in calculations of heat exchange in furnace devices, as well as the possibility of certain distortions of the temperature fields at the furnace outlet, it can be recommended with complete confidence, when designing boiler units for burning coals of the Kansk-Achinsk basin, that the temperature at the inlet to the convective heating surfaces be taken as 1000°C.

Fig. 1. View of the lower part of the screen surfaces of the BKZ-320-140 boiler unit when burning Kansk-Achinsk coals

In work [8] it was convincingly shown that lowering the gas temperature at the furnace outlet causes a sharp decrease in the heat-release intensity of the furnace chamber, primarily due to a decrease in the average specific density of heat absorption by the screen surfaces, and leads to an increase in the dimensions of the boiler unit and, ultimately, to an increase in its cost. Therefore, a reduction in the gas temperature at the inlet to the convective surfaces, based on operating experience with boiler units having furnace devices that are clearly unsatisfactory for burning Kansk-Achinsk coals, cannot be recommended.

It is sometimes considered—and this also follows from equation (3) of work [8]—that

\[ (q_v)_{\mathrm{cool}}=\left(\frac{q_{\mathrm{ne}}}{q_l}\right)\left(\frac{H_e}{V_t}\right), \]

where \((q_v)_{\mathrm{cool}}\) is the heat output released per \(1\,m^3\) of furnace space, based on the conditions of cooling the flue gases in the furnace, MW/m³;

\(q_{\mathrm{ne}}\) is the average specific heat-absorption power of the screen surfaces, MW/m²;

\[ q_l=\frac{Q_n^p+Q_v-I_t''}{Q_n^p} \]

is the fraction of the fuel heat, taking into account the heat of the air, absorbed by the screen surfaces due to radiant heat exchange in the furnace;

\(H_e\) is the effective radiation-absorbing surface of the furnace chamber, m²;

\(V_t\) is the volume of the furnace space, m³;

that a reduction in the first term of this equation can be compensated by an increase in the second term through the use of double-light screens and semi-radiant (screen) heating surfaces. In this case the heat fluxes and dimensions of the furnace chamber, and consequently the dimensions of the boiler unit as a whole, can remain acceptable. But the cost of the boiler unit will nevertheless increase because of the greater metal consumption of the furnace chamber and the reduced efficiency of the radiant heating surfaces. In addition, the use of double-light screens will complicate the design of the boiler unit and create considerable difficulties in cleaning them during operation, since the means for cleaning screen surfaces, including vibration cleaning, are still far from perfect. In view of the fact that the ash of coals from certain deposits of the Kansk-Achinsk basin (for example, Berezovsky) has a high ash fusion temperature at low ash contents (at $A^c < 5\%$), and the introduction of furnaces with liquid slag removal encounters certain difficulties, there is an opinion that furnace arrangements for burning these coals may be designed as single-chamber, open-type units with dry slag removal, and that, by developing deeply lowered screen surfaces in the furnace chamber, the temperature at the inlet to the convective surfaces can be maintained below 850°C, thereby eliminating or sharply reducing the intensity of the formation of bonded deposits on the convective heating surfaces. Assumptions close to these were used as the basis for the reconstruction of PK-38 boiler units at the Nazarovo State District Power Plant (second stage of reconstruction). In this reconstruction, the second stage of the secondary superheater, which before reconstruction had been the first surface in the gas path at the inlet to the convective shaft and had been most intensively fouled by bonded deposits, was moved into the furnace in the form of additional screens. As a result of this, according to calculations, the gas temperature at the inlet to the convective surfaces was to decrease to 790°C and, as was assumed, to lead to a sharp reduction in the intensity of their fouling by bonded deposits. Practice showed that this reconstruction, without a fundamental change in the furnace process and the design of the furnace arrangement, did not lead to the expected results. The screen surfaces proved to be in the temperature zone of the most intensive formation of bonded deposits; therefore the boiler unit could reliably carry load only provided uninterrupted operation of the vibration cleaning of the screens and shot-cleaning of the convective heating surfaces with forced supply of shot by shot blowers, with all means of comprehensive cleaning started two or three times per shift. A brief outage of the vibration cleaning leads to a sharp increase in the temperature in the turning chamber because of intensive fouling of the screens and to intensive formation of bonded deposits on the convective heating surfaces. Therefore, reducing the gas temperature at the inlet to the convective heating surfaces cannot be regarded as a normal method of combating the formation of bonded deposits without the introduction of high-temperature fuel combustion and high-temperature treatment of the ash in the furnace process. From this point of view, the use of open furnaces with dry slag removal and the reduction of the gas temperature at the inlet to the convective heating surfaces below 850°C when burning Kansk-Achinsk basin coals will not ensure reliable operation of the boiler unit.

Conclusions

1. To ensure high-temperature combustion of coals from the Kansk-Achinsk basin, furnace arrangements must meet the following basic requirements:

a) the bulk of the fuel must be burned at temperatures that ensure the passage of the mineral mass of the coal into a molten (liquid) state; the furnaces must have liquid slag removal;

b) in the combustion zone of the main portion of the fuel, uniformity of the temperature fields must be ensured both over the cross section and along the length of the flame path;

c) to ensure high-temperature combustion of the fuel and high-temperature treatment of the ash, it is necessary to intensify all stages of fuel combustion so as to ensure their most complete completion within the combustion chamber, which has high heat-release rates;

d) to ensure the most complete completion of the processes of high-temperature transformations in the mineral mass, one must strive to make the residence time of ash particles in the high-temperature zone as long as possible.

1. These requirements are most fully met by furnace devices with separated combustion zones in the form of pre-furnaces (cyclone furnaces, vortex furnaces).

2. Among the known modern designs that have proved their ability to ensure high-temperature combustion and high-temperature ash treatment with complete elimination of the indications of the formation of bonded deposits are the VTI furnaces with vertical cyclone pre-furnaces, TsKTI furnaces with horizontal cyclone pre-furnaces, and the semi-open BKZ–TsKTI furnaces with eight-sided pre-furnaces.

3. When coals of the Kansk–Achinsk basin are burned in furnace devices that ensure high-temperature combustion and high-temperature ash treatment in the furnace process, the temperature at the gas inlet to the convective heating surfaces should be determined, as usual, by the temperature at which ash deformation begins, and in the design of boiler units should be taken as equal to 1000°C.

4. Lowering the gas temperature at the inlet to the convective heating surfaces to values below 850°C by introducing screen heating surfaces, without organizing high-temperature fuel combustion, will not ensure elimination of intensive fouling of the convective heating surfaces by bonded deposits, is economically disadvantageous, and cannot be recommended when burning coals of the Kansk–Achinsk basin.

REFERENCES

1. I. K. Lebedev. Combustion of coals of the Kansk–Achinsk basin in the furnaces of high-capacity steam generators. In: Results of Research in Power Engineering and Power-Machine Building over 50 Years. Tomsk University Press, Tomsk, 1968.

2. I. K. Lebedev. Concerning the article by M. P. Smorgunova et al., “Combustion of brown coals in mine-type furnaces with liquid slag removal.” Elekt. stantsii, No. 5, 1966.

3. S. N. Mironov, V. R. Kotler. Start-up and adjustment of a boiler equipped with an experimental furnace with a gamma-shaped flame on Kansk–Achinsk coals. In: Proceedings of the scientific and technical conference on the combustion of Kansk–Achinsk coals.

4. Thermal calculation of boiler units. Normative method. (Draft.) Leningrad, 1966.

5. V. I. Reznik, I. P. Epik, Kh. Kh. Arro, M. Z. Gudkin. Recommendations for the design of boiler units for burning Kansk–Achinsk coals. Abstracts of reports of the conference on the combustion of coals of the Kansk–Achinsk basin. Ministry of Power and Electrification of the USSR, Moscow, 1967.

6. E. P. Dik, R. A. Sirokha. Study of the conditions for the formation of ash deposits during the combustion of Nazarovo coal. Teploenergetika, No. 10, 1969.

7. I. K. Lebedev, A. I. Gnatjuk. Behavior of the mineral part of Irsha-Borodino and Azey coals during their combustion in the BKZ-320-140 PT boiler furnace of the Irkutsk TPP-6. Abstracts of reports of the conference on the combustion of coals of the Kansk–Achinsk basin. Ministry of Power and Electrification of the USSR, Moscow, 1967.

8. V. P. Romadin. Basic furnace characteristics and their significance in world practice. Teploenergetika, No. 10, 1969.