Conditions and Possibilities for the Formation of Sticky Components in Ash During Combustion of Coals from the Kansk-Achinsk Basin

I. K. Lebedev

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

Many researchers, assigning the main role in the formation of bonded deposits on convective heating surfaces to the stickiness of fly ash, explain these properties by the presence of films of alkali-metal oxides either on the heating surfaces or on the surfaces of the particles themselves. Such films on the surfaces of ash particles during combustion of coals from the Kansk-Achinsk basin may be melts of eutectic mixtures of $\mathrm{CaS—CaSO_4}$. Below, the conditions under which sulfide melts may form on the surface of ash particles, causing the formation of bonded compounds, and methods for preventing this undesirable phenomenon are considered.

Experimental studies by I. P. Ivanova and V. I. Babii [1] showed that the entire process of burnout of a particle of Nazarovo coal at various temperatures and in various gaseous media consists of four clearly expressed separate stages. At the first moment, a particle placed in a high-temperature gaseous medium undergoes drying, after which ignition and burnout of volatile combustibles occur. The actual process of onset of combustion of volatiles begins at temperatures close to about $800^\circ\mathrm{C}$; during the entire course of this second stage of combustion, the temperature of the solid particle itself remains relatively low and, in the photographed frames, appears as a dark core surrounded by a bright halo of burning volatiles. After the volatiles have burned out, the coke particle is further heated to its ignition temperature, and then the stage of coke burnout begins. In the experiments of Ivanova and Babii, the temperature at which this last, fourth stage of combustion proceeds, with an oxygen content in the gaseous medium of 5%, which most closely corresponds to the conditions of particle burnout in the furnace chamber, is close to the temperature of the surrounding medium.

Let us analyze the possible course of transformations in the mineral part and the most probable reactions during these transformations. In view of the fact that sticky ash components may be created through the formation of eutectic melts of calcium sulfates and sulfides, the most interesting objects for study will be products containing calcium and sulfur.

* According to preliminary studies.

According to the view of VTI, a considerable part of the calcium in the coals of the Kansk-Achinsk basin is associated with the coal substance in the form of calcium humates. As for calcite and calcium sulfates, their content, judging from the yield of carbonate carbon dioxide and the content of sulfate sulfur in the coal, is insignificant.

When a solid fuel particle is heated to 1000°C (1273°K), which corresponds to the temperature at which the first two stages of combustion proceed (heating, devolatilization, and burnout of volatiles), organic sulfur will evaporate and be oxidized by oxygen according to the reaction

\[ \mathrm{S}_2 + 2\mathrm{O}_2 = 2\mathrm{SO}_2, \tag{1} \]

and in this case the reaction, before ignition of the volatiles, will take place near or on the surface of the particle, while during burnout of the volatiles, in all likelihood, it will take place in the gas volume outside the particle.

Pyritic sulfur will be oxidized according to the equation

\[ 4\mathrm{FeS}_2 + 11\mathrm{O}_2 = 2\mathrm{Fe}_2\mathrm{O}_3 + 8\mathrm{SO}_2, \tag{2} \]

but during the period of burnout of the volatiles, owing to the fact that diffusion of oxygen to the solid particle will be impeded by the flame of the burning volatiles, the reaction may proceed according to the equation

\[ 2\mathrm{FeS}_2 = 2\mathrm{FeS} + \mathrm{S}_2, \tag{3} \]

as a result of which iron sulfide will be formed; its melting point, in the presence of other sulfides as well as oxides, especially FeO, may decrease to 800°C [5].

Calcite will dissociate according to the equation

\[ \mathrm{CaCO}_3 = \mathrm{CaO}^{*} + \mathrm{CO}_2, \tag{4} \]

and Katler [3] believes that immediately after the reaction according to equation (4) has occurred, lime is in an unstable structural state between the state of the initial rhombohedral \(\mathrm{CaCO}_3\) and the final well-crystallized \(\mathrm{CaO}\) of cubic structure. In this intermediate state, which with time passes into the final state according to the equation

\[ \mathrm{CaO}^{*} \to \mathrm{CaO}, \tag{5} \]

lime possesses high chemical activity and can again react with \(\mathrm{CO}_2\), forming carbonates. For this reason Katler regards the reaction of thermal decomposition of carbonates as a reversible reaction and denotes it as

\[ \mathrm{CaCO}_3 \rightleftarrows \mathrm{CaO} + \mathrm{CO}_2. \tag{6} \]

But since the reactions of sulfur oxidation and thermal decomposition of carbonates in the coal particle proceed simultaneously, the active lime released as a result of carbonate decomposition reacts with the more active \(\mathrm{SO}_2\) or \(\mathrm{SO}_3\), forming, as shown by Ketov and Pechkovskii, three types of products of lime sulfuration: \(\mathrm{CaSO}_4\), \(\mathrm{CaSO}_3\), and \(\mathrm{CaS}\) [3]. Thus, already during the first two stages of combustion, which proceed at temperatures up to 800°C (1037°K), formation is possible of both \(\mathrm{CaSO}_4\) and \(\mathrm{CaS}\), as well as iron sulfide with its subsequent transition in the deposits into calcium sulfide.

The presence of a reaction between lime and sulfurous anhydride during the first two stages of combustion is indicated by the fact that, when coals of the Kansk-Achinsk basin are ashed in a muffle furnace at a temperature of 800°C (1037°K), the greater part of the sulfur (about 80%) remains in the ash residue. During subsequent calcination of this ash at var-

At various temperatures it has been established that, up to temperatures of 1000–1200°C (1273–1473°K), the sulfur content in the ash remains constant, and then there is a sharp decrease almost to zero [4]. Comparison of these experiments with the work of Hedvall, Öberg, and Weiberg [5] shows that sulfur in the ash residue is represented mainly by CaSO₄, which at temperatures above 1000°C (1273°K) reacts with oxides and silicates with the evolution of SO₂.

The temperature of the fuel particle in the coke-burnout stage is determined, as the experiments of Ivanova and Babii [1] have shown, by the temperature of the furnace chamber. Here two cases are possible: the temperature is below 1350°C (1623°K) (low-temperature combustion) and above 1350°C (1623°K) (high-temperature combustion). In low-temperature combustion, at temperatures of 1000–1200°C (1273–1473°K), in the absence of oxygen at the surface of the burning particle (diffusion regime [6]), in addition to the sulfidization of lime, processes of formation of various kinds of calcium silicates, aluminates, and ferrites will occur; however, the reactions forming these compounds will proceed in the solid phase, will be governed by diffusion processes, and, consequently, their course will be slow. It is quite noteworthy that, in the low-temperature regions of coke burnout, reactions of calcium sulfate with carbon will occur. P. P. Budnikov and A. M. Ginstling [7], on the basis of a detailed thermodynamic analysis of the CaSO₄—C system carried out by A. D. Volkov [8], indicate that in this system, within the temperature range 800–1100°C (1073–1373°K), the most probable reaction will be

\[ \mathrm{CaSO_4 + 4C = CaS + 4CO}. \tag{7} \]

Thus, in low-temperature fuel combustion, in the coke-burnout stage, a significantly higher CaS content may arise than through the reactions of direct sulfidization of lime according to Ketov and Pechkovskii, since the reactions of CaSO₄ formation and the reactions of interaction of CaSO₄ with carbon will proceed practically simultaneously.

In view of the fact that all the reactions considered above proceed over time, especially substantial formation of sulfide melts may occur under any disturbances of the furnace regime that cause elongation of the flame and its extension into the region of convective heating surfaces. This agrees with the observations of practitioners in the operation of boiler units burning coals from the Kansk-Achinsk basin, which show that, in all cases of deterioration of the furnace regime associated with disruption of mixture formation (reduction of the excess-air coefficients in the furnace chamber and other disturbances), the intensity of fouling of heating surfaces increases sharply.

In high-temperature combustion, the burnout time of the particles is reduced considerably, which will lead to a sharp decrease in the sulfide-sulfate eutectics formed and, consequently, to a reduction in the danger of fouling of heating surfaces. Therefore, in order to eliminate sulfide-sulfate films that have formed on the surface of ash particles, high-temperature treatment of the ash after coke burnout acquires particular importance as a means of combating deposit formation.

From the same thermodynamic studies [8], to which reference was made above, during high-temperature treatment of the ash after burnout of the organic part, reactions of interaction are possible—

interactions between calcium sulfide and sulfate according to the following equation

\[ \mathrm{CaS}+3\mathrm{CaCO}_4=4\mathrm{CaO}+4\mathrm{SO}_2 . \tag{8} \]

But the occurrence of this reaction is possible only at temperatures above \(1150^\circ\mathrm{C}\) (\(1423^\circ\mathrm{K}\)), since at lower temperatures this reaction with the formation of CaO and SO\(_2\) is impossible, as is evident from Fig. 1. It is also evident from this figure that even at temperatures of \(1200\)—\(1300^\circ\mathrm{C}\) (\(1373\)—\(1473^\circ\mathrm{K}\)) the magnitude of the negative thermodynamic potential is small; therefore, in order to intensify these reactions it is desirable to maintain higher temperatures of the gaseous medium in the furnace chamber in the zone of high-temperature ash treatment. Maintaining high temperatures is also desirable because the free lime released in reaction (8) requires high-temperature treatment in order to eliminate its chemical activity with respect to subsequent interaction with SO\(_2\) in the zone of arrangement of the convective heating surfaces. Such a process of lime deactivation at high temperatures is called dead-burning of lime by Doctor of Technical Sciences I. Ya. Zalkind. We believe that the temperature at which fuel combustion and subsequent treatment of fly ash from coals of the Kansk-Achinsk basin should occur must be no less than \(1400\)—\(1450^\circ\mathrm{C}\) (\(1673\)—\(1723^\circ\mathrm{K}\)).

At such high temperatures, other chemical reactions involving calcium sulfides and sulfates may also proceed. Thus, in the presence of high temperatures and excess oxygen, further oxidation of sulfides to sulfates is possible according to the equation

\[ \mathrm{CaS}+2\mathrm{O}_2=\mathrm{CaSO}_4 \tag{9} \]

with subsequent thermal decomposition of the sulfates according to the equation

\[ \mathrm{CaSO}_4=\mathrm{CaO}+\mathrm{SO}_2+0.5\mathrm{O}_2 \tag{10} \]

Therefore, in order to ensure the course of the last two reactions during high-temperature treatment of fly ash, it is necessary to provide the most complete mixing, ensuring intensive diffusion of oxygen to the ash particles.

Fig. 1. Change in thermodynamic potential during reactions in the CaSO\(_4\)—C system according to [8].
1 — reaction \(\mathrm{CaSO}_4+4\mathrm{S}=\mathrm{CaS}+4\mathrm{SO}\);
2 — reaction \(4\mathrm{CaO}+6\mathrm{SO}_2=4\mathrm{CaSO}_4+\mathrm{S}_2\);
3 — reaction \(\mathrm{CaS}+3\mathrm{CaSO}_4=4\mathrm{CaO}+4\mathrm{SO}_2\).

Ensuring the best mixing is required not only at the stage of high-temperature ash treatment, but also in the process of burning out

Fig. 2. Adiabatic combustion temperatures of Nazarovo coal and zones of possible temperatures in the flame core at \(\alpha = 1.2\); \(t_{\mathrm{sv}} = 400^\circ\mathrm{C}\).
1 — \(W^p = 0\%\); 2 — \(W^p = 10\%\); 3 — \(W^p = 20\%\);
4 — \(W^p = 30\%\); 5 — \(W^p = 40\%\); 6 — \(W^p = 50\%\).
7 — minimum temperature for desirable ash treatment;
8 — temperature of the liquid-molten state of the ash \((t_3)\).

fuel particles, because, as the experiments of Ivanova and Babiya [1] showed, at high oxygen concentrations in the combustion zone there is a sharp shortening of all stages of combustion. Hence follows the need for a careful approach to the selection and design of burner devices when designing boiler units for firing coals of the Kansk-Achinsk basin.

E. P. Dik and A. I. Filimonov [10], in order to reduce the formation of bonded deposits, propose ensuring that the particle temperature during burnout is not less than \(t_3\). For Kansk-Achinsk coals this temperature, in their opinion, should lie within the range \(1220—1340^\circ\mathrm{C}\), but, as was shown above (which is also confirmed by practice), it is not sufficient to prevent the formation of bonded deposits.

Figure 2 shows the value of the adiabatic combustion temperature \(\vartheta_a\) (curves 1, 2, 3, 4, 5, 6) at different moisture and ash contents of Nazarovo coal at \(\alpha = 1.2\) and a hot-air temperature of \(400^\circ\mathrm{C}\). The same figure also shows the temperature of the desired high-temperature combustion and high-temperature ash treatment (curve 7). As can be seen, the desired temperature treatment of the ash lies above the temperature of the liquid-molten state of the ash of Nazarovo coal (curve 8). This situation applies not only to Nazarovo coal, but also to coals from other deposits of the Kansk-Achinsk basin. Therefore, the main type of furnaces for firing coals of the Kansk-Achinsk basin should be furnaces with liquid slag removal. It should also be borne in mind that the temperature characteristics of ash have a sharply variable value with changes in ash content [9]. Furnaces with dry slag removal, when firing coals with such variable characteristics, are subject to severe slagging. Therefore, the use of furnaces with liquid slag removal is a reliable means of combating slagging of radiant heating surfaces.

It is known that the maximum temperature of the combustion core in an open pulverized-coal furnace is \((0.8—0.85)\,\vartheta_a\) [12]. In the figure these zones of possible temperatures for different coal moistures are shown by hatched fields. As can be seen from the figure, at moisture contents above 40% it is difficult in an open furnace to attain the level of high-temperature combustion and high-temperature ash treatment that we recommend, even in the combustion core. And since in the combustion core the combustion of only an insignificant part of the fuel is completed, while the greater part of the fuel, owing to the great nonuniformity of the temperature field, is burned at lower temperatures, it is difficult to expect the absence of bonded deposits in boilers with an open furnace even with liquid slag removal. Therefore, open furnaces cannot be recommended for firing coals of the Kansk-Achinsk basin.

The necessity of high-temperature combustion and high-temperature ash treatment imposes special requirements both on the method of preparing the fuel for combustion and on the design of the furnace chamber.

As can be seen from Fig. 2, to ensure high-temperature combustion it is necessary to use open-cycle drying schemes, but, as practice shows, it is possible to use semi-open pulverization schemes with fuel drying by flue gases and with discharge of the drying agent outside the ash of combustion of the main fuel [12].

When designing furnace devices, it is necessary to provide for aerodynamic and thermal conditions for creating high temperatures in the combustion zone with a uniform temperature field.

These conditions are well met by multi-chamber furnaces, or furnaces with pre-furnaces having fully insulated surfaces in the combustion zone and the maximum degree of completion of combustion within these pre-furnaces.

Brief Conclusions

1. In low-temperature combustion of coals from the Kansk-Achinsk basin, the formation in fly ash of sulfide-sulfate calcium compounds is possible; these compounds impart adhesive properties to ash particles. This circumstance causes the occurrence of heavy fouling of convective heating surfaces associated with ridge-like deposits of fly ash.

2. In order to eliminate the adhesive properties of the ash, high-temperature combustion of these coals must be carried out with subsequent high-temperature treatment of the fly ash. To create the necessary conditions under which the ash loses its adhesive properties, the temperature both in the combustion zone and in the zone of high-temperature ash treatment must be maintained at not less than 1400–1450°C (1673–1723°K).

3. The principal type of furnace equipment for boilers intended for the combustion of coals from the Kansk-Achinsk basin should be furnaces with liquid slag removal.

4. In designing furnace equipment, great attention must be paid to the selection and design both of burner devices that provide the best mixing and burnout of the fuel along the shortest flame path, and of the configuration of the furnace chamber, ensuring high-temperature combustion of the main mass of the fuel and subsequent high-temperature treatment of the fly ash. From this point of view, open furnaces are unsuitable.

5. Pulverized-fuel preparation systems should have an open drying circuit. It is also possible to use semi-open pulverized-fuel preparation circuits with drying of the fuel by flue gases and discharge of the drying agent outside the ash of combustion of the main fuel. The latter circuits, apparently, may be recommended for boiler units of medium steam capacity.

REFERENCES

1. I. P. Ivanova and V. I. Babii. Teploenergetika, No. 4, 1966.

2. Katler. In: Kinetics of High-Temperature Processes. Metallurgiya Publishing House, 1965.

3. A. A. Ketov, V. V. Pechkovskii. Journal of Applied Chemistry, Vol. XXXI, Issue 12, 1958.

4. I. K. Lebedev, N. V. Trikashnyi, G. K. Prikalikhin. In: Materials of the Scientific-Technical Conference on the Combustion of Kansk-Achinsk Coals, Krasnoyarsk Regional NTOEP, Krasnoyarsk, 1967.

5. V. Eitel. Physical Chemistry of Silicates. Izdatlit, Moscow, 1962.

6. G. F. Knorre. Furnace Processes. Gosenergoizdat, Moscow–Leningrad, 1959.

7. P. P. Budnikov, A. M. Ginstling. Reactions in Mixtures of Solid Substances. Gosstroiizdat, Moscow, 1961.

8. A. D. Volkov. Transactions of the Leningrad Technological Institute, Issue 7, 1958.

9. I. K. Lebedev, N. V. Trikashnyi, A. A. Torlopov. Teploenergetika, No. 10, 1966.

10. E. P. Dik, A. I. Filimonova. Teploenergetika, No. 2, 1966.

11. S. N. Mironov, V. R. Kotlyar. Teploenergetika, No. 10, 1966.

12. V. D. Zorichev, N. V. Pavlov. Energomashinostroenie, No. 10, 1967.