BENCH-SCALE LABORATORY SETUP FOR STUDYING DEPOSIT FORMATION DURING COMBUSTION OF BROWN COALS FROM THE KANSK-ACHINSK BASIN

E. A. MOSIN, G. N. ZAKOURTSEV

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

As a result of numerous studies, extensive information is now available on the conditions and nature of ash deposits during the combustion of pulverized fuel in boiler installations. The composition and properties of deposits are studied by both domestic and foreign researchers [1, 2, 3, 5, 6]. Most authors [1, 3, 6] attribute the principal causes of deposit formation, apart from the composition of the fuel, to the influence of the temperature conditions of its combustion. Indeed, different thermal treatment of a fuel particle causes transformations in the mineral matter, after which the particle, under certain conditions, acquires the ability to adhere to the heating surfaces of the boiler.

In large industrial installations, monitoring the combustion conditions during the formation of ash deposits presents considerable difficulties. Above all, this involves monitoring the uniformity of combustion, intensive mixing of coal dust and air, uniform mixture formation in the flame, etc.

In this connection, it became necessary to create an experimental laboratory-scale setup with stabilized fuel-combustion parameters.

A setup of this type was designed and manufactured in the laboratory of firing test stands of the Department of Steam-Generator Engineering, TPI.

A general view of the setup is shown in Fig. 1.

1. Description of the Setup

DUST-FEEDING SYSTEM

1. Dust hopper. Made in the form of a cylinder with a conical transition into the housing of the dust feeder; mounted on a common frame. To prevent coal dust from hanging up in the hopper, its inner surface is covered with polished sheet iron. The cone opening angle is 50°. In addition, the sealed design makes it possible to keep the hopper volume under positive pressure of dry air. The temperature and pressure are regulated by the addition of hot air. The dust capacity of the hopper is 6 kg.

2. Dust feeder. The design is described in the article by V. M. Tretyakov [4]. The feeder drum is set in rotation through a reducer.

Fig. 1. General schematic of the setup: 1—supporting frame, 2—dust hopper, 3—dust-feeder motor, 4—selenium rectifier, 5—reducer, 6—drum-type dust feeder, 7—dust line, 8—burner, 9—furnace chamber, 10—layer of fireclay concrete, 11—asbestos chips, 12—outer casing, 13—transition to the convective shaft, 14—cylinder with a propane–butane mixture, 15—frame of the convective shaft, 16—fireclay brickwork, 17—probes, 18—damper, 19—dust collector, 20—glass-fiber filter, 21—smoke exhauster, 22—forced-draft fan, 23—compressed-air supply line, 24—shutoff valve, 25—manometer, 26—flowmeter, 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—observation windows, 39—gas analyzer.

by a direct-current motor, which makes it possible to smoothly regulate the fuel feed within the range of 0.5–8 kg/h. The dependence of the fuel feed rate on the voltage at the motors is shown in Fig. 2.

Fig. 2. Change in fuel feed rate as a function of motor voltage.

AIR HEATING AND DISTRIBUTION UNIT

The air for combustion is supplied from the compressed-air line through a flow-metering device, an electric air heater, and an air-distribution head. Hot air is fed in five regulated streams to the combined gas–pulverized-coal burner. To measure the air flow rate, a standard Prandtl tube connected to a micromanometer is used.

The air-heating system consists of a heater, a power transformer, and lead-in wires. The main part of the heating system is a tube made of heat-resistant steel, 36 mm in diameter. The tube heater is connected to the output winding of the power transformer. To ensure smooth regulation of the input voltage, the power transformer is supplied from the alternating-current mains through two parallel-connected autotransformers of the RNO-220/40 type, having a rigid mechanical linkage of the regulating screws. By smoothly varying the input voltage to the power transformer, the temperature of the hot air is regulated up to 450°C.

The lining of the heater tube is made of fireclay cylinders 200 mm in diameter and 250 mm long. The entire heating system is enclosed in a metal casing, which also serves as the frame.

BURNER DEVICE

The combined gas–pulverized-coal burner (Fig. 3), mounted on the upper end of the combustion chamber, consists of three concentric tubes.

Primary air is supplied by two branch pipes axially into the middle tube of the burner; secondary air is supplied by two branch pipes tangentially into the outer tube. Tertiary air is supplied axially into the central tube. The helical surface of the secondary-air tube and the conical diffuser installed at the inlet from the tertiary-air tube

air, ensure maximum turbulence of the dust–air mixture directly under the burner.

For preliminary heating of the furnace to the operating temperature, two nozzles are introduced tangentially into the primary-air duct of the burner; through them a propane–butane mixture is supplied.

Labels in the figure: Coal dust; Primary air; air mixture; gas; Secondary air; Cooling-water inlet; Cooling-water outlet; diffuser; furnace.

Fig. 3. Combined gas–pulverized-coal burner

The burner body has a water cooling jacket.

FURNACE CHAMBER AND CONVECTIVE SHAFT

1. Furnace. Of vertical design, cylindrical, assembled from corundum cylinders 125 mm in diameter and 1400 mm long. The insulation is two-layer: refractory concrete 150 mm thick and asbestos chips.

The furnace structure is enclosed in a steel casing mounted on a rigid steel frame carrying all the remaining units of the installation. The metal sheathing prevents air inleakage. The outside diameter of the furnace with insulation is 800 mm.

2. Convective shaft. The furnace chamber is connected to the convective gas duct by a corundum tube 75 mm in diameter and 400 mm long. The transition tube is insulated and enclosed in metal sheathing.

The shaft with the convective bundle is made of fireclay brick. The gas-duct size is \(0.125 \times 0.065\) m, height \(0.55\) mm.

The convective bundle consists of tubes 12/8 mm in diameter, material—steel 1Kh18N9T. The number of rows of probes and the number of tubes in a row can be varied arbitrarily. The probes are cooled with compressed air. The air flow rate makes it possible to regulate the temperature of the probe wall within the range \(300^\circ \div t_{\text{gas}}\ ^\circ\mathrm{C}\).

CONTROL AND MEASUREMENT OF THE OPERATING CHARACTERISTICS OF THE INSTALLATION

The flame temperature along the entire gas path is measured by means of PR 30/6 and TXA thermocouples installed in ceramic sleeves. Measurement of the temperatures of the probe walls was carried out by means of

thermocouples embedded in the metal walls. All thermocouples are connected to ÉPP-09 electronic self-recording potentiometers.

To observe the pulverized-coal flame and to monitor its temperature by means of an optical pyrometer, sight glasses are installed along the length of the furnace. Sight glasses are also installed opposite each row of probes. Gas samples are taken and draft is measured by means of gas-sampling tubes installed along the gas path and connected to draft gauges.

II. Combustion Procedure

PREPARATION OF THE FUEL FOR COMBUSTION

Brown coal from the Nazarovo deposit, crushed in a crusher and dried to an air-dry state, was passed through a laboratory disk attritor. The fractional composition of the dust is shown in Fig. 4. To avoid smearing of the drum dust feeder and in order to achieve maximum temperatures, before being fed into the dust bunker the dust was dried to an absolutely dry state \((W^b = 0 \div 0.5\%)\).

Fig. 4. Fractional composition of coal dust

HEATING OF THE FURNACE VOLUME AND IGNITION OF THE PULVERIZED-COAL FLAME

After preliminary ventilation of the setup and the supply of air, the propane–butane mixture is smoothly fed to the burner and the gas–air flame is ignited. By simultaneously regulating the gas–air ratio, the required length and color of the flame are established. Completeness of combustion of the mixture components is monitored by gas analysis. Electric air preheating is then switched on. Depending on the specified fuel-combustion mode, heating of the furnace volume is carried out for two to three hours.

When the specified temperature of the furnace-chamber volume is reached, the supply of coal dust is switched on with a simultaneous reduction in gas flow. With ignition of the pulverized-coal flame, the gas is shut off completely, and the flow rate of hot air is adjusted according to the fuel flow rate. The required dust-combustion temperature is achieved by changing the flow rate of the dust–air mixture.

SAMPLING AND ANALYSIS OF COMBUSTION PRODUCTS

A cassette with a glass-fiber filter for sampling fly ash is installed in the gas path upstream of the induced-draft fan. The cassette design makes it possible to replace the filter rapidly while the unit is operating. The gas temperature at the filter installation point does not exceed 160°.

Samples for gas analysis are taken throughout the entire experiment every 15 min under steady-state conditions. In the start-up mode—

sampling is performed at substantially shorter time intervals.

Analysis of the flue gases for the content of CO, CO₂, O₂, and N₂ is carried out on an KhL-3 gas chromatograph, with preliminary absorption of CO₂, on a VTI-2 gas analyzer.

III. Results of Commissioning Tests

CHARACTERISTICS OF FURNACE OPERATION

The commissioning-test program provided for the combustion of coal dust under 5 temperature conditions:

\[ t_{\text{comb}} = 1000,\ 1100,\ 1200,\ 1300,\ 1400^\circ\mathrm{C}. \]

When coal was burned in an amount of 1–6 kg/h, the heat release rate of the furnace volume was

\[ (0.3 \div 1.92)\,10^6\ \frac{\text{kcal}}{\text{m}^3\cdot\text{h}}, \]

which is characteristic of modern pulverized-coal cyclone pre-furnaces and furnaces with liquid slag removal.

Thanks to the burner device, which is capable of regulating the content of the components of the dust–air mixture over wide ranges and of intensively mixing it at the burner outlet, a stable flame was obtained.

Completely filling the furnace cross section, the flame length varied within the range 0.5–0.8 m, not exceeding 2/3 of the furnace length.

The change in gas temperature over the furnace cross section under various conditions is presented in Table 1.

Table 1

Change in gas temperature over the cross section of the furnace chamber
in the zone of maximum temperatures

The temperature field across the section is uniform. The deviation of the flow temperature from the mean over the entire interval does not exceed ±0.5%.

Table 2

The change in gas temperature along the length of the furnace chamber is presented in Fig. 5. As can be seen from the graph, not at all combustion temperatures

acceptable from the standpoint of deposit formation [1, 5] were obtained at the furnace outlet. Therefore, at the transition from the furnace to the transitional gas duct, an additionally cooled probe for collecting deposits was installed.

Fig. 5. Change in gas temperature along the length of the furnace chamber

Operating conditions at all temperatures were easily stabilized, and at a CO₂ content in the flue gases of about 17% no difficulties were encountered in stabilizing combustion.

DEPOSIT FORMATION

Ash deposits, both loose and bonded, arose at combustion temperatures of 1100, 1200, and 1300°C.

By “loose” deposits were meant those deposits that were blown off by a jet of compressed air issuing from a nozzle at a velocity of 30 m/s. Deposits

Fig. 6. Bonded deposits on the cover of the turning gas duct

formed in all sections of the gas duct where the gas temperature exceeded 850°, irrespective of the wall material—Fig. 6. Comparison of the chemical

the composition of the deposits obtained with samples of deposits from industrial installations, recalculated to a sulfate-free mass, shows their considerable similarity—Table 2.

The color of the deposits ranges from light brown to dark brown.

Conclusion

A laboratory bench-scale setup for the combustion of brown coals from the Kansk-Achinsk Basin has been described. Fuel combustion can be carried out with coal feed rates from 0.5 to 8.0 kg/h. The setup permits combustion at temperatures up to 1400° under stable combustion conditions.

As a result of experimental combustions, ash deposits were obtained that are similar to industrial deposits in chemical composition and color.

REFERENCES

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

2. I. P. Epik. The Influence of the Mineral Part of Shales on the Operating Conditions of Boiler Units, 1961.

3. Corrosion and deposits in coal and oil-fired boilers and gas turbines. Report ASME. Pergaman Press, New-York, 1959.

4. V. M. Tretyakov. “Teploenergetika,” No. 4, 1956.

5. Experimental Investigation of the Formation of Ash Deposits during the Combustion of Nazarovo Coal, 1966, technical report of VTI, KTO. Authors: E. P. Dik, R. A. Sirokha.

6. The Mechanism of Corrosion by Fuel Impurities. Proceeding of the International Conference of the Marchwood 20 th, † 24 th, May, 1963, London, Butterworth, 1963.