Operating Experience of High-Capacity Shaft-Mill Furnaces Fired with Moscow Region Coal

Eng. V. N. Bereznegovskaya

At one of the Mosenergo power stations, domestic high-pressure boilers with high-capacity shaft-mill furnaces fired with Moscow Region coal have been in operation for a number of years.

Shaft-mill furnaces in the USSR, as is known, have become widely used, especially for burning low-grade coals. High-capacity shaft-mill furnaces of domestic manufacture have been put into operation comparatively recently, and therefore a discussion of operating experience with them is of considerable interest.

Four ShMA 730 (1660) 2004 shaft mills are installed along the front of the boiler. Dust is fed into the furnace through ports in the front wall of the furnace; secondary air enters through slots above and below the port. The volume of the furnace chamber is \(1210\ \mathrm{m^3}\); the flame path length from the center of the port to the middle of the festoon is \(10.5\ \mathrm{m}\). Table 1 gives some design parameters for the boiler and the shaft-mill furnace.

Table 1

Design parameters of boiler TP-230-1 with a shaft-mill furnace

The fuel used is Moscow Region coal of somewhat degraded characteristics:

\[ W^p = 32\text{–}34\%, \qquad A^p = 25\%, \qquad Q_p^n = 2400\ \text{kcal/kg}. \]

Adjustment and testing of the first of the boilers put into operation were carried out by TsKTI,¹) and of the subsequent units by the adjustment and testing service for thermomechanical equipment of Mosenergo.²)

1. Mills

The design arrangement of the fuel-feed unit to the mill is such that the fuel is delivered to the shaft mills from the raw-coal bunker by means of an open belt feeder. As a result, the dust system is not made tight, and cold air enters the shaft together with the fuel.

This “organized” inleakage of air into the shaft amounts to as much as 25% of the quantity of hot air supplied to the mill. Since the cold air enters the shaft above the grinding part of the mill, it does not participate in the process of ventilating the mill, but only increases the velocity in the shaft, thereby coarsening the grinding of the dust without increasing the mill output. On the other hand, the inleakage of cold air worsens the drying conditions in the mill and in the shaft. This defect in the design of the fuel-feed unit was one of the main reasons for the deterioration of the technical and economic performance of the shaft-mill furnaces. Figures 1–5 show the results of mill tests.

The fineness of dust grinding, which in shaft mills depends on the air velocity in the shaft, ranges from \(R_{70} = 50\%\) to \(R_{70} = 70\%\). At the same time, the residue on sieve No. 30 is from 25 to 35%.

In practice, the mills operate with a grinding fineness of \(R_{70} = 60\text{–}65\%\). It should be noted that in the graph of Fig. 1 the apparent air velocity in the shaft, calculated from the hot-air flow rate, is plotted along the abscissa. Taking into account what was said above regarding the inleakage of cold air into the shaft, the actual velocity in the shaft is approximately 25% higher, i.e., of the order of 2.0–2.3 m/sec.

Fig. 1. Dependence of dust-grinding fineness on velocities in the shaft.

The relationship between the dust residues on sieves No. 30 and 70, shown in Fig. 2, gives a uniformity constant of the granulometric composition of the dust equal to \(n \simeq 1.0\). This figure is somewhat lower than the value usual for shaft mills, \(n = 1.3\text{–}1.6\), and characterizes a somewhat worsened granulometric composition of the dust.

¹) Brigade chief: Engineer G. A. Burgvits.
²) Brigade chiefs: Engineers I. P. Gerzhoy and M. F. Dovgaya.

At a fineness of grinding \(R_{70}=60\%\) and an electric-motor load of 230 kW, the mill capacity is 24–28 t/h, or 7.3–8.5 t per \(1\ \text{m}^2\) of the active cross section of the mill rotor (see Fig. 3). At idle operation of the mill, i.e., at an electric-motor load of 110–115 kW, the mill capacity is about 12 t/h.

Fig. 2. Relationship between dust residues on sieves Nos. 30 and 70.

Fig. 3. Mill capacity at different electric-motor loads and different fineness of grinding.

The possibility of obtaining a certain capacity at the power of the mill idling operation, i.e., without additional expenditure of energy, is a characteristic feature of shaft mills. This should be explained by the fact that, at ra-

operation of the mill without coal, the efficiency of the mill as a fan is very low. Feeding fuel into the mill reduces parasitic eddies and increases the efficiency of the mill as a fan. This makes it possible to perform useful work in grinding a certain amount of fuel without additional expenditure of energy.

Figure 4 shows the variation in the specific consumption of electric power for grinding as a function of the fineness of pulverization of the dust and the mill capacity. These curves for shaft mills have a characteristic form, different, for example, from that of drum-ball mills: for each fineness of

Fig. 4. Variation in electric-power consumption for grinding as a function of pulverization fineness and mill capacity.

pulverization the curve has an optimum at a certain mill capacity. The coarser the pulverization, the higher the mill capacity corresponding to this optimum.

When the mill capacity is reduced below the optimum (which corresponds to the left branch of the curves in Fig. 4), the consumption of electric power per ton of pulverized product increases, since the influence of the mill’s idle running increases.

When the mill capacity is increased above the optimum (the right branch of the curves), the consumption of electric power for grinding increases, since the load on the electric motor grows faster than the mill capacity.

Thus, one must always strive to operate not at the maximum but at the optimum capacity of the mills, which in the present case, at a pulverization fineness of \(R_{70} = 60\text{–}65\%\), amounts to 18 to 22 \(m/\text{h}\), or

6–6.5 m per 1 m² of the clear cross section of the mill rotor. Accordingly, and depending on the boiler load, it is necessary to keep one or another number of mills in operation. In operation this condition cannot always be observed; moreover, as the beaters wear, the operating indices of the mills also deteriorate. Fig. 6 shows the operating indices of shaft mills for 1950. As can be seen from the graph, the average operating capacity of a mill ranges from 17 to 20 t/hr; the power consumption is 9–10 kWh/t. The fineness of grinding of the dust is not checked in operation.

Drying of the fuel in the mills is carried out with hot air. According to the calculation, with a hot-air consumption of 1.8 kg per 1 kg of fuel and an air temperature of 381°C, the fuel in the mill should be dried by an amount \(W_p - W_n = 19\%\), where \(W_p\) is the moisture content of the run-of-mine coal and \(W_n\) is the moisture content of the dust.

Thus, with a raw-coal moisture content \(W_p = 33\text{–}35\%\), the dust moisture content should be 14–16%.

In reality, the drying conditions in the mill are considerably worse. Because of the insufficient heating surface of the air heater, the temperature of the hot air is about 40–50°C lower than the calculated value and remains

In the figure: vertical axis—“Dust moisture content,” %; horizontal axis—“Hot-air temperature,” °C; curve field—“Hot-air consumption per 1 kg of fuel,” with values 1.3, 1.6, 1.9 kg/kg.

Fig. 5. Change in dust moisture content as a function of air temperature and air consumption.

within 300–335°C, depending on the boiler load. Further, as already indicated above, cold air is drawn into the mill through the raw-coal feeders in an amount of about 25% of the quantity of hot air entering the mill. As a result, with the same mill ventilation, the consumption of hot air is reduced by approximately 25% as compared with the calculated value, which leads to an increase in the dust moisture content. Fig. 5 shows the change in the moisture content of the dust delivered by the shaft mills at different temperatures and different rates of hot-air consumption per 1 kg of fuel.

When cold air is drawn into the mill in an amount of 25% (which corresponds to a reduction in the consumption of hot air supplied to the mill,

from the calculated \(1.8\ \mathrm{kg/kg}\) to \(1.35\ \mathrm{kg/kg}\), and at a hot-air temperature of \(330^\circ\mathrm{C}\) the moisture content of the dust rises to 20%. Such a dust moisture content is clearly excessive for Moscow-region coal. Ordinarily, pulverized-coal furnaces

Within the figure: “Moisture content,” \(t/\mathrm{h}\); “Power consumption,” \(\mathrm{kW\cdot h}/\mathrm{t}\); “Operating indices of shaft mills”; 1950, 1951.

Fig. 6. Operating indices of shaft-mill operation.

with drum-ball mills operate with a dust moisture content not exceeding 16–17%.

Furnace

As a result of adjustment of the thermal operating conditions, the boiler furnaces are currently operating without chemical incomplete combustion.

The combustible content in the carryover amounts under test conditions to 1–1.5%; in operation it is maintained at a level of 2–2.5%, which corresponds to a heat loss with mechanical incomplete combustion of \(q_4 = 1.5\text{–}1.75\%\). This figure should be considered somewhat high for Moscow-region coal; boilers with drum-ball mills usually have a loss due to mechanical underburning of less than one percent. The loss \(q_4\) depends to a considerable extent on the excess-air coefficient in the furnace.

As can be seen from Fig. 7, \(q_4\) assumes a minimum value at \(\alpha\) downstream of the superheater of 1.36–1.35, i.e., at an excess-air coefficient in the furnace of \(\alpha_t = 1.25\text{–}1.30\).

When the excess-air coefficient in the furnace is reduced below \(\alpha_t = 1.20\), chemical incomplete combustion appears. At normal boiler load the heat release rate of the furnace volume is \(128\) thousand kcal/\(\mathrm{m^3\cdot h}\). The furnace operates stably over a wide range of loads and with any number of operating mills from 2 to 4.

The minimum stable load is about 50% of nominal, which corresponds to a heat release rate of the furnace volume of \(65\) thousand kcal/\(\mathrm{m^3\cdot h}\).

Although combustion proceeds steadily, continuous pulsation of the flame in the furnace is observed. The cause of the pulsation has not yet been clarified in detail, but it is obvious that it may result from the following factors:

1) pulsation of the aeroflow issuing from the burner quarl. This pulsation is clearly visible on the model of the shaft mill.¹ In the gravity shaft of the mill, in the absence of separating and other inserts, some circulation of the air stream takes place, which promotes the creation of pulsation at the outlet from the burner quarl;

2) the high and time-varying moisture content of the dust.

This circumstance, together with the suction through the shaft of a fairly considerable amount of cold air, creates an insufficiently stable ignition balance at the root of the flame, which leads to pulsation.

Fig. 7. Dependence of heat loss with mechanical unburned fuel on the excess-air coefficient downstream of the superheater.

By more careful regulation of the furnace operating conditions it appears possible at times to reduce the intensity of the pulsation, but it cannot be completely eliminated.

The individual components of the furnace air balance are the following quantities: 1) primary air entering with the dust through the burner quarls—45%,

including a) hot air—35%,
b) suction through the coal feeder—8%,

2) secondary air through the upper and lower slots—50%;
3) suction through leaks in the furnace—7%.

Figure 8 shows the isotherms of the furnace space at a heat input close to the design value and with three mills in operation. As can be seen from this graph, even at the design boiler load the temperature level in the furnace remains low. The flame is elongated, and the combustion core is weakly expressed. The zone of maximum temperatures is located at the rear wall, somewhat above the mill burner quarls. The bottom of the furnace is only weakly filled by the flame; ignition of the dust does not occur directly at the outlet from the burner quarl, but at some distance.

The temperature distribution across the width of the furnace is nonuniform and depends on the number and combination of mills in operation. With a considerable furnace width—10 m—the supply of aeromixture through embrasures with a cross section of \(1.56 \times 1.56\) m, i.e., in several powerful streams, naturally cannot promote the formation of a uniform temperature field. This becomes especially noticeable in TP-230-1 boilers, where the superheaters have no mixing headers and the steam temperatures in the individual outlet coils reflect the character of the temperature distribution across the width of the gas stream.

It has been observed that, when one of the mills opposite an idle embrasure is shut down, a zone of reduced pressure is formed, toward which the flame from neighboring operating mills is directed. As a result, a certain temperature peak is created opposite the idle mill, causing a local

Fig. 8. Isotherms of the furnace space at \(Q/V_T = 128\) thousand kcal/m\(^3\) h.

increase in steam temperatures in individual coils of the superheater. Therefore, when operating with an incomplete number of mills, in order to press back the flame it is necessary to admit air through the slots of the idle embrasure.

In order to improve the aerodynamics of the furnace, the angle of inclination of the slots, originally \(15^\circ\) to the horizontal, was increased to \(52^\circ\).

The upper arch of the embrasure was also given a certain inclination. This somewhat improved the filling of the lower part of the furnace by the flame and reduced pul-

tion, but it did not fully resolve the problem of organizing the aerodynamics of the furnace. (The isotherms in Fig. 8 were taken after these modifications had been carried out.)

An attempt was also made to push the flame away from the rear wall by means of additional air supply through the rear wall, but this measure proved of little value.

The temperature of the gases at the furnace outlet is not high: 1000–1050°C. It is somewhat lower than the temperature at which deformation of the ash of Moscow-region coal begins, which is usually 1100°C; nevertheless, slagging of the screen tubes and of the first rows of the superheater is observed.

The slag is friable and, when the boiler is shut down, is easily removed; however, during operation, in the absence of stationary blowing and with the large width of the furnace, it causes a number of difficulties. It should be noted that in the practice of burning Moscow-region coal, which has a comparatively refractory ash, cases of slagging of the screen tubes and the superheater—that is, of the rows of tubes located in the path of the gas flow—are often observed at gas temperatures at the furnace outlet below the temperature at which ash deformation begins.

In all cases this appears to be connected with unsatisfactory aerodynamics, when the flame, without turning through the depth of the furnace, is directed from the mouths of the burners upward directly toward the screen tubes. In this case the ash particles do not have time to cool along the way and are carried by the gas stream to the screen tubes and the superheater, where they adhere to the surfaces of the tubes.

Thus, the conclusion suggests itself that a shaft-mill furnace with simplified ports and a frontal arrangement of the mills, which has proved itself well on small boilers, becomes insufficiently perfected when applied to large boiler units. As the furnace width increases, nonuniformities across the furnace increase as a result of supplying the pulverized-air mixture in separate, powerful streams through a small number of ports.

A powerful pulverized-air stream of considerable dimensions, issuing from a port, in the absence of effective mixing with the secondary air, forms in the furnace an elongated flame without a sharply defined combustion core; and in order to ensure complete carbon burnout it is necessary to operate with increased excess air in the furnace.

Thus, the apparent simplicity of the structural design of shaft-mill furnaces does not fully justify itself when passing to powerful furnaces. This question was raised by some investigators several years ago,^1 but at present this situation has become quite clear.

In order to improve the aerodynamics of large shaft-mill furnaces, design and commissioning organizations are at present developing and testing a number of measures: the installation of wedge-shaped and other inserts in the ports to split the pulverized-air stream and facilitate suction of gases to the root of the flame; increasing the outlet velocities of the pulverized-air mixture to a level corresponding to normal burners; tangential admission of secondary air above the ports in order to swirl the gas flow in the upper part of the furnace, etc. All these measures are at the investigation stage, and no final conclusions can yet be drawn about them.

Likewise, apparently, it should be recognized that the frontal arrangement of ports in wide furnaces is inadvisable. An angular arrangement of the ports should provide better filling of the furnace volume by the flame, just as angular burners in this respect have a number of advantages over frontal ones. Preliminary experience at one of the Mosenergo power stations, where boilers with an angular arrangement of shaft mills—

^1 Nadzhárov M. A. Experience in starting up and adjusting a high-capacity boiler unit with shaft mills. Elektricheskie stantsii, 1947, No. 12.

...units introduced into operation a year ago and now in the adjustment stage confirms this proposition.

Conclusions

The experience of adjusting and operating large shaft-mill furnaces of domestic manufacture when firing Moscow Region coal has shown that the furnace unit as a whole operates quite economically: chemical incomplete combustion is absent; the loss due to mechanical unburned carbon is, under test conditions, 1.0% and, in operation, 1.5–1.75% at a fineness of pulverized fuel of \(R_{70}=60\text{–}65\%\). Thus a furnace efficiency of about 98% is ensured.

The expenditure of electric power for pulverized-fuel preparation is 8–9 kWh/t, which is considerably lower than for drum-ball mills—16 kWh/t. Subsequently, in the course of final adjustment and the implementation of a number of measures, these indicators undoubtedly should improve further. All this opens broad prospects for the further development and introduction of large shaft-mill furnaces.

At the same time, the first operating experience with such furnaces gives grounds to conclude that the simplified design solution for introducing aeropulverized fuel in the form of simple embrasures cannot, under these conditions, ensure good furnace aerodynamics, and that work must be done to create a more advanced device, approaching in type a normal burner, with higher inlet velocities. The angular arrangement of the embrasures should provide better filling of the furnace volume than the frontal arrangement.

It is also necessary to improve the drying conditions in the mills and to work on improving the gravity shaft separator in order further to improve the particle-size distribution of the pulverized fuel and reduce the number of coarse particles in the dust, which chiefly determine the magnitude of mechanical unburned carbon.

1. M. A. Nadzharov. Study of Grinding and Separation on a Model of a Shaft Mill. Report of MOKTI, 1949. ↩