THE INFLUENCE OF THE COMBUSTION TEMPERATURE OF NAZAROVO COAL DUST IN AN EXPERIMENTAL INSTALLATION ON THE INTENSITY OF THE FORMATION OF BONDED DEPOSITS

E. A. MOSIN, G. N. ZAKOURTSEV

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

The combustion of coals from the Kansk-Achinsk brown-coal basin and, in particular, from the Nazarovo deposit in the furnaces of steam boilers is associated with operational difficulties revealed during the commissioning of 150-thousand-kW power units at the Nazarovo State District Power Station. In PK-38 boilers, at design loads, intensive fouling of convective heating surfaces is observed, along with the formation on them of bonded ash deposits that sharply limit the output of boiler units in their original design (to 60% of nominal).

On the basis of a number of adjustment tests and experimental combustions of Nazarovo coal in furnace devices of various types (furnaces with dry and liquid slag removal, cyclone furnaces, etc.), the assumption was put forward that combustion of Nazarovo coal dust in highly intensified furnace devices, with high-temperature treatment of the ash particle and intensive supply of oxidizer, helps prevent the formation of bonded deposits [1].

At present, sufficiently detailed data are available on the structure of bonded deposits and on the development of the process of their formation over time, on the dependence of the intensity of deposit formation and of some of their properties on the gas temperature at the place of deposit formation and on gas velocity [1]. The influence, however, of such important factors as the combustion temperature of the dust, the excess-air coefficient in the furnace or the degree of mixing of the aerosol mixture, moisture content, and other factors determining the conditions of fuel combustion cannot be established on the basis of data from industrial tests even with a rough approximation, since there is no possibility of changing, or even at least maintaining constant over the cross section, the above-listed parameters, and it is not possible to reveal regularities on the basis of comparing experiments on different industrial installations. This is not allowed by the low accuracy of the results of industrial tests, as well as by the absence of a reliable criterion for evaluating the magnitude of the deviation of the obtained data from the general mean of a given factor in each experiment. One can come closer to meeting these requirements by using an experimental bench installation. In such an installation, having

a small transverse cross section of the combustion chamber, it is possible to maintain the dust-combustion parameters within certain limits and to relate them to the parameters of formation of bound deposits.

The general layout of the experimental test-bed installation is shown in Fig. 1. Its main purpose is the study of the process of formation of bound deposits on metallic probes with a controlled wall temperature.

Fig. 1. General layout of the installation.
1 — supporting frame, 2 — dust bunker, 3 — motor of the dust feeder, 4 — selenium rectifier, 5 — reducer, 6 — drum dust feeder, 7 — dust conduit, 8 — burner, 9 — combustion chamber, 10 — layer of chamotte concrete, 11 — asbestos chips, 12 — outer casing, 13 — transition to the convective shaft, 14 — cylinder with propane–butane mixture, 15 — frame of the convective shaft, 16 — fireclay lining, 17 — probes, 18 — damper, 19 — dust collector, 20 — glass-fabric filter, 21 — smoke exhauster, 22 — forced-draft fan, 23 — compressed-air supply line, 24 — shutoff valve, 25 — pressure gauge, 26 — flow meter, 27 — power transformer, 28 — electric air heater, 29 — distribution manifold for hot air, 30 — thermocouples, 31 — sampling of flue gases for analysis, 32–33 — electronic potentiometer, 34–35 — micromanometers, 36 — flow-metering device, 37–38 — sight glasses, 39 — gas analyzer.

temperature of the wall. The installation is designed for the combustion of 1–8 kg of coal dust per hour and consists of two parts connected in series along the gas flow—the combustion chamber and the convective shaft—equipped with auxiliary systems for supplying dust, hot air,

control and measurement of operating parameters, cleaning, and removal of flue gases.

For preliminary heating of the furnace-chamber volume to the operating temperature, two nozzles are installed tangentially in the primary-air duct of the burner; through these nozzles a propane–butane mixture is fed into the furnace. Upon reaching operating temperatures, the installation is gradually switched over to combustion of the pulverized-air mixture, and the supply of propane–butane to the furnace is stopped. Further maintenance of the flame temperature is achieved only by burning the dust. To avoid overheating the burner body, the latter has a water-cooling jacket.

Sampling of fly ash is carried out using a fiberglass filter installed in a special cassette upstream of the smoke exhauster. The cassette design permits rapid replacement of the filter while the installation is operating.

Commissioning tests showed the possibility of uniform heat treatment of ash in the dust-combustion temperature range from \(900^\circ\mathrm{C}\) to \(1500^\circ\mathrm{C}\). When burning dust of Nazarovo coal \((A^{p}=8\text{--}10\%;\ W^{p}=0\div0.5\%)\) in an amount of \(1\text{--}6\ \mathrm{kg/hour}\), the heat release rate of the furnace volume was

\[ \frac{B\cdot Q^{p}_{n}}{V_{t}}=(0.3\text{--}1.92)\times 10^{6}\ \frac{\mathrm{kcal}}{\mathrm{m}^{2}\cdot\mathrm{hour}}, \]

which corresponds to data for cyclone furnaces and furnaces with liquid slag removal. Owing to the burner device, which is capable of regulating the content of the components of the pulverized-air mixture over wide limits, and to its intensive mixing at the burner outlet, a stable flame was obtained, making it possible to achieve practically complete burnout of the dust \((A^{p}_{un}=0\div0.5\%)\). The gas-temperature field over the cross section of the furnace chamber is uniform. The deviation of the flow temperature from the average over the entire temperature interval \((800\text{--}1500^\circ\mathrm{C})\) does not exceed \(\pm 1.0\%\). The combustion core and the maximum dust-combustion temperature usually lay at a distance of \(200\text{--}600\ \mathrm{mm}\) from the burner support plate. The flame length under the most severe operating conditions did not exceed \(850\ \mathrm{mm}\), i.e., dust combustion was completed entirely within the furnace chamber.

For combustion, dust of Nazarovo brown coal was used, ground to \(R_{90}=35\%\) and dried to an absolutely dry state, \(W^{p}=0.5\%\).

Combustion of coal dust was carried out under various conditions (see Table 1). The dust-combustion temperature, i.e., the maximum temperature in the furnace chamber, was selected as the operating factor. As follows from the table, the change in dust-combustion temperature covered the range from \(900^\circ\mathrm{C}\) to \(1450^\circ\mathrm{C}\); the excess-air coefficient at the furnace outlet varied within \(1.05\text{--}1.48\), and the temperature of the walls of the probes for collecting deposits was regulated within the range from \(450^\circ\mathrm{C}\) to a temperature equal to the gas temperature at the place of deposit formation. To avoid difficult-to-account-for local air inleakage, throughout the entire experiment the installation operated under a slight positive pressure. The duration of each combustion run, excluding the time spent on preparatory operations, was 4–5 hours. Analysis of the flue gases for their content of CO, \(O_{2}\), and \(N_{2}\) was carried out on an KhL-3 gas chromatograph, with preliminary determination of \(CO_{2}\) on a VTI-2 gas analyzer.

As a result of burning Nazarovo-coal dust in the bench-scale experimental installation, ash deposits were obtained. Deposits arose on various sections of the gas duct and surfaces regardless of the wall material. In appearance, strength, and chemical-

composition they were subdivided into two types—bonded and loose. Loose deposits were understood to mean those that could be removed from the probe surface by a jet of compressed air issuing from a nozzle at a velocity of 40 m/sec. Deposits remaining on the probe wall after such blowing were taken to be bonded. The color varied from light brown to dark brown depending on the combustion temperature and the local gas temperature. The bonded deposits always had a darker shade than the loose ones.

Table 1

The experiments confirmed the existence of the previously assumed dependence of the intensity of formation of bonded deposits on the combustion temperature of Nazarovo coal dust [1]. This dependence, constructed from the results of experimental combustion of dust on a test stand, is shown in Fig. 2. The amount of loose deposits, as Fig. 2 shows, changes only slightly, decreasing somewhat with increasing temperature, whereas the bonded deposits have a clearly expressed maximum in the region of combustion temperatures equal to 1250–1300°C. At combustion temperatures above 1300°C the amount of bonded deposits dropped sharply, and at temperatures above 1400°C only loose deposits appeared. When the combustion temperature was reduced from 1100°C to 900°C, the intensity of formation of bonded deposits gradually decreased; however, even at this combustion temperature the process of formation of bonded deposits proceeded quite intensively.

Fig. 2. Dependence of the intensity of formation of bonded deposits on the combustion temperature.
1 — loose deposits;
2 — bonded deposits

With a decrease in the combustion temperature, the region of formation of bonded deposits also shifted. Deposits usually arose on those elements of the test stand where the gas temperature was not less than 850°C.

The temperature of the surface on which the deposits form has a somewhat different effect. This temperature has little influence on the intensity of formation of bonded deposits. Deposits arise at any wall temperature in the interval we studied (450–490°C). However, our studies confirm the previously obtained data [1, 2], indicating that the wall temperature can determine the microstructure or fractional composition of deposits. Thus, on probes having a wall temperature of 450–550°C, a fine-grained first layer of bonded deposits with a grain size of less than 0.02 mm formed, whereas on probes with a wall temperature of 800–900°C the greater part of the deposits was coarse-grained (0.1–0.2 mm). The wall temperature also affects the growth rate of bonded deposits. On probes with a higher wall temperature the deposits grow considerably faster than on “cold” ones, \(t_{\text{st}} = 50^\circ\text{C}\).

The occurrence of bonded deposits did not depend on the wall material. Bonded deposits were observed on probes made of stainless steel, on ceramic sheaths for thermocouples, on gas-sampling tubes, and on the walls of the corundum tube connecting the furnace chamber and the convective shaft. At combustion temperatures of 900–1000°C, ridges of deposits arose on the burner support plate and the walls of the furnace chamber.

The chemical composition of the bonded deposits was not constant [4]. The content of the components is determined by the combustion conditions, temperature, and composition of the gases along the gas duct. However, a comparison of the chemical composition of bonded deposits obtained in the experimental unit with samples of deposits from industrial boiler units, recalculated to a sulfate-free mass, shows their great similarity (Table 2).

Table 2

Thus, the experimental combustion of Nazarovo coal dust made it possible to obtain bonded deposits on the surfaces of the convective gas duct of the bench-scale unit. The study of the properties of these deposits and of the influence exerted on the intensity of their formation by the combustion temperature made it possible to confirm the conclusion made earlier [1] concerning the need to burn Nazarovo coal dust at temperatures \(t_{\text{comb}} \geq 1400^\circ\text{C}\).

Conclusions

1. The study of bonded deposits formed on convective heating surfaces during the combustion of Nazarovo brown coal in furnace devices of various types has made it possible to conclude that there is an interrelation between the intensity of deposit formation and the conditions of dust combustion. Establishing this interrelation using the results of field tests of boiler units of various types is very difficult because of the low accuracy and cumbersome nature of industrial experiments.

2. The experimental installation, by modeling the process of dust combustion and the conditions of deposit formation, makes it possible to maintain the experimental conditions within definite limits and to evaluate their deviation from the prescribed mean value with sufficient accuracy.

3. The installation is designed for burning Nazarovo-coal dust within the range of 1–8 kg/h. The burner device is capable of regulating the content of the components of the pulverized-coal–air mixture over wide limits and of mixing them intensively. A pulverized-coal flame stable within the range of variation of the dust-combustion temperature \(t_{\text{comb}} = 800—1500^\circ\mathrm{C}\) was obtained.

4. As a result of the experimental combustion of Nazarovo brown-coal dust in the bench installation, deposits, both loose and bonded, were obtained on probes simulating heating surfaces and on the walls of the gas duct. In chemical and mineralogical composition the deposits are similar to industrial samples. The strength of the deposits and their disperse composition are determined by their residence time in the convective gas duct, by the temperatures of the gases and of the walls of the gas-duct section on which the deposits form.

5. Experiments carried out on the bench installation confirmed the existence of an interrelation between the combustion temperature of Nazarovo-coal dust and the intensity of formation of bonded deposits. The maximum formation of the latter corresponds to the combustion-temperature interval \(t_{\text{comb}} = 1250—1300^\circ\mathrm{C}\). The intensity of deposit formation increases twofold from \(t_{\text{comb}} = 900^\circ\mathrm{C}\) to \(t_{\text{comb}} = 1300^\circ\mathrm{C}\), and then decreases sharply. Thus, from the point of view of contamination of convective heating surfaces when burning Nazarovo-coal dust, combustion temperatures \(t_{\text{comb}} > 1400^\circ\mathrm{C}\) may be considered the safest.

References

1. Materials of the Scientific and Technical Conference on the Combustion of Kansk-Achinsk Coals. Krasnoyarsk, 1967.

2. I. P. Épik. Influence of the Mineral Part of Oil Shales on the Operating Conditions of Boiler Units. Tallinn, 1961.

3. Experimental Study of the Formation of Ash Deposits during the Combustion of Nazarovo Coal. Technical Report of VTI-KTO. Authors: E. P. Dik, R. A. Sirokha, 1966.

4. E. A. Mosin, G. N. Zakourdev. “Relation of Physicochemical Transformations in the Mineral Matter of Nazarovo Coal to the Process of Formation of Bonded Deposits” (present collection).