Relationship between Physicochemical Transformations in the Mineral Matter of Nazarovo Coal and the Processes of Formation of Bonded Deposits

E. A. Mosin, G. N. Zakurtsev

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

The need to maintain constant combustion conditions in studying temperature transformations in the ash of Nazarovo brown coals and their influence on the intensity and properties of bonded deposits forming on convective heating surfaces posed the task of creating an experimental bench installation with constant parameters for burning coal dust. The design of such an installation and some results of experimental combustion of Nazarovo-coal dust have already been reported by us [1].

All deposit samples obtained as a result of the experimental combustions were subjected to spectral analysis on a DFS-10 quantometer. The most representative of them were subjected to chemical analysis by classical methods of silicate analysis. From the results of chemical and spectral analyses, presented in Figs. 1–4, it is evident that the dust-combustion temperature and the temperature conditions at the site of deposit formation—the gas and wall temperatures—also influence the chemical composition of the deposits. The identical behavior of the contents of CaO and $\mathrm{SiO_2}$ as a function of combustion temperature is noteworthy. Both of these elements have a maximum content in the deposits at a temperature of 1250–1300°C. However, if for $\mathrm{SiO_2}$ it can be said that it passes into the bonded deposits from the fine fractions of carryover, while friable deposits, in terms of the trend in their silicic-acid content, differ little from the coarse fractions of carryover, then from the graph of the change in calcium oxide content it follows that CaO passes into the bonded deposits from both the coarse and the fine fractions of carryover. From the fine fractions this transition was observed up to combustion temperatures of 1150°C, and from the coarse fractions at higher temperatures, up to 1250–1300°C. The content of aluminum oxide changes in an analogous manner; however, its maximum occurs at a combustion temperature of 1200°C (Fig. 2).

The content of iron oxides in the bonded deposits obtained on the experimental installation also depends significantly on the combustion temperature of the coal dust. With an increase in combustion temperature, the content of $\mathrm{Fe_2O_3}$ decreases. The content of MgO fluctuates within very narrow limits, reaching a maximum of 6–7% at a temperature of 1250–1300°C. Thus, the experiments showed the presence of intensive separation of $\mathrm{SiO_2}$, CaO, $\mathrm{Fe_2O_3}$, MgO, and $\mathrm{Al_2O_3}$ into the deposits in the combustion-temperature interval $t_{\mathrm{comb}} = 1100$–1300°C.

Apart from silicic acid, we were unable to establish, for any of the compounds considered, differences in separation into bonded deposits from fractions of different particle size. This phenomenon can be explained on the basis of the structure of the mineral matter of Nazarovo coal. Silicic acid is present in the ash in the form of quartz and clay minerals [2]. The dispersity of these ash constituents and their strength differ substantially from one another; moreover, quartz is included predominantly in the coarse-dispersed constituents of the ash, while clay minerals—

Fig. 1. Dependence of the content of SiO₂ and CaO on combustion temperature:
1 — bonded deposits, 2 — loose deposits, 3 — fine fly-ash fractions, 4 — coarse fly-ash fractions

Fig. 2. Dependence of the content of MgO, Fe₂O₃, and Al₂O₃ on combustion temperature (designations as in Fig. 1)

—(illite, kaolinite, montmorillonite) are present mainly in the finely dispersed fractions, whereas the remaining oxides are included in the finely dispersed ash fractions. (An exception is iron, which is included in the coarse-grained fractions in the form of pyrite.)

The works of N. V. Kuznetsov [5], K. Sheff [6], and others have established certain regularities in the process of formation of loose deposits. Like bonded deposits, deposits of this type begin to arise owing to the adhesion of the finely dispersed component of fly ash to tube surfaces; by finely dispersed fly-ash particles are meant particles smaller than 30 μm [5] or even smaller than 20 μm [6]. Particles of such sizes can settle on heating surfaces owing to stress forces caused by the action of molecular forces or induced electrostatic charges. The presence of the latter on boiler heating surfaces and in the fly-ash cloud was shown experimentally by Gabler [7]. On particles larger than 20–30 μm these forces act much more weakly than the kinetic energy of the flow,

therefore, when they strike the surface of the tube, they tear away from it the fine-fraction particles that have already settled.

However, as is known from studies of the process of ash erosion of tubes [5], the destructive action of entrained ash particles depends on the angle of inclination of the impact surface relative to the flow. At an inclination angle close to 90°, erosion is slight. The greatest erosion is obtained with an oblique impact at an angle of 30–45°. At first, near the frontal generatrix of the tube, coarse ash particles strike at an angle close to 90° and only weakly impede the growth of deposits. Subsequently, as the deposits acquire the shape of a triangular prism, the impacts become oblique, the destructive action is intensified, and dynamic equilibrium sets in. Under unchanged conditions, the growth of deposits practically ceases. And if there is no chemical interaction, the composition of the deposits and their strength do not change.

Fig. 3. Dependence of the content of $\mathrm{SO_3}$ on the combustion temperature (notation as in Fig. 1)

Fig. 4. Dependence of the chemical composition of bonded deposits on the temperature of the probe wall

Fine fractions of fly ash from Nazarovo coals contain a number of elements forming the system $\mathrm{CaO—SiO_2—Al_2O_3—Fe_2O_3—MgO}$. During coal combustion, clay minerals decompose with the formation of reactive amorphous silica. This amorphization of silica begins already at $600^\circ\mathrm{C}$ [8]. At $t = 700^\circ\mathrm{C}$ (in a reducing atmosphere), or $t = 800^\circ\mathrm{C}$ in an oxidizing atmosphere, amorphous silica combines intensively with other oxides, primarily with alkalis and $\mathrm{CaO}$, and in a reducing medium also with $\mathrm{FeO}$. The intensity of interaction is proportional to the size of the contacting surface, the duration of interaction, and the temperature in the mixture.

On the other hand, in the ash of Nazarovo coals there is, liberated as a result of combustion of the organic mass, an extremely active finely dispersed calcium oxide that had been part of the humic compounds of the coal. The interaction of $\mathrm{CaO}$ and $\mathrm{SiO_2}$, beginning at $t = 600^\circ\mathrm{C}$, leads to the formation of $\beta\text{—}2\mathrm{CaO}\cdot\mathrm{SiO_2}$, and the reaction rate increases in the presence of iron oxides [8]. $\mathrm{MgO}$ and $\mathrm{Al_2O_3}$ interact—

react with SiO₂ at \(t \ge 800^\circ\mathrm{C}\), forming Mg₂SiO₄ and mullite. At \(t = 600^\circ\mathrm{C}\), reactions of CaO and Al₂O₃ also occur, the consequence of which is the formation of CaAl₂O₄. As a result of the interaction of ternary systems, compounds of the anorthite, åkermanite, and augite type arise.

Iron oxides catalyze the formation of compounds of this type in the range \(600—800^\circ\mathrm{C}\) [8]. At \(t = 800^\circ\mathrm{C}\) in a reducing medium, the appearance of which in the deposit layer is inevitable if there is some incompleteness of fuel combustion, \(2\mathrm{CaO}\cdot\mathrm{Fe}_2\mathrm{O}_3\), \(\mathrm{CaO}\cdot\mathrm{Fe}_2\mathrm{O}_3\), and a solid solution of CaO and MgO in FeO—calciowüstite and magnesiowüstite—are formed. Pure wüstite in this system is formed through decomposition of pyrite and marcasite, which are included in the ash of Nazarovo coal, at \(t = 800^\circ\mathrm{C}\).

The above-mentioned interactions, taking place under the conditions of the most finely dispersed deposit layer, with a large contact surface of the reacting compounds and at a temperature in the layer of \(500—700^\circ\mathrm{C}\), which for the time being is still determined by the temperature of the tube wall, may lead to an increased tendency of ash particles to sinter. The melting temperature of such oxide mixtures lies within \(1000—1200^\circ\mathrm{C}\) (for example: \(\mathrm{Al}_2\mathrm{O}_3\cdot2\mathrm{SiO}_2+\mathrm{Fe}_2\mathrm{O}_3\cdot\mathrm{SiO}_2+\mathrm{SiO}_2\) has \(t_{\mathrm{m}} = 1000—1100^\circ\mathrm{C}\)), and their sintering temperature \(t_{\mathrm{s}}\sim0.8t_{\mathrm{m}}\) [4], i.e., is close to the gas temperature.

As a result of sintering of individual ash particles, a layer of bonded deposits arises. However, this process alone is insufficient to impart to the primary layer of deposits of Nazarovo-coal ash the strength characteristic of industrial samples. The structure and strength properties of the deposits are a consequence of reactions in the solid phase and the appearance of local formations of a liquid phase, which envelops the grains of fly ash. For the appearance of a liquid phase, a reducing medium is necessary first of all; it is formed in the deposit layer as a result of incomplete combustion of the fuel and slow diffusion of the oxidant through the finely dispersed layer.

The sources of the liquid phase may be the eutectics \(\mathrm{CaSO}_4+\mathrm{CaS}\), \(\mathrm{FeS}+\mathrm{FeO}\), formed in the deposit layer; the conditions for their occurrence have already been considered in sufficient detail [9], and the probability of their occurrence in the deposit layer is considerably higher than in the flow. Owing to the very low surface-tension coefficient, these liquid films envelop the grains of fly ash, cementing them into a strong layer. With time the lower oxides pass into higher oxides, and sulfides into sulfates; since the molar volume of the latter is greater than that of sulfides or pure oxides, the deposit layer is strengthened still further (the molecular volume of \(\mathrm{CaSO}_4 = 45.7\ \mathrm{cm}^3/\mathrm{mol}\), while \(\mathrm{CaCO}_3 = 37.3\ \mathrm{cm}^3/\mathrm{mol}\) and \(\mathrm{CaO} = 16.3\ \mathrm{cm}^3/\mathrm{mol}\)) [10]. Simultaneously, the process of corrosive ingrowth of the lower deposit layer into the oxide surface layer of the tube proceeds, which has been shown very well by I. P. Epik and others [3].

An increase in the thickness of the layer of bonded deposits entails a change in the temperature of the outer deposit layer and in the aggregate state of the ash particles within it. Naturally, this also changes the character of the interaction between an ash particle of the dust-gas flow and the deposit layer. The outer deposit layer becomes more plastic and softens the impact of the ash particle. A large particle can no longer rebound from the surface and “gets stuck,” penetrating into the deposits.

Thus, owing to the difference in the chemical composition of the fine and coarse fractions of the ash, a difference also arises in the chemical composition of the dense tube-side and the looser outer layers of bonded deposits. Chemical analyses of industrial loose deposits show—

caused an increase in the concentration of silicic acid in them in comparison with fly ash and dense deposits [2].

The relationship between the content of \(\mathrm{SiO_2}\) in the deposits and the wall temperature (Fig. 4), constructed from the results of our experiments, shows an increase in the concentration of silicic acid at heating-surface temperatures equal or close to the temperature of the flue gases at the place where the deposits form.

An increase in the size of ash particles retained on the surface of the deposits and, consequently, in the size of the pores between them sharply reduces the magnitude of the reacting surface and facilitates diffusion of the oxidizer to the ash particles, which entails a decrease in the content of lower oxides and sulfides in the deposits.

The consequence of the processes named is a looser and less strong structure of the deposits.

Petrographic and X-ray structural analyses showed a difference between bonded ash deposits and entrainment obtained at \(t_{\mathrm{comb}} \leq 1250^\circ\mathrm{C}\) during combustion of Nazarovo coal dust in an experimental installation. If the entrainment consists mainly of oxides and a number of double compounds—\(\mathrm{CaO}\), \(\mathrm{Fe_2O_3}\), \(\mathrm{SiO_2}\), \(\mathrm{CaSO_4}\), \(\mathrm{MgFeO_4}\)—then in the bonded deposits a significant place is occupied by double and triple compounds (belite, augite, akermanite, calcium and magnesium ferrites, mullite, fayalite, anhydrite).

The content of silicon and iron oxides is small, while such oxides as \(\mathrm{CaO}\), \(\mathrm{Al_2O_3}\) are practically absent in the deposits.

Thus, the appearance of two types of deposits on the front side of tubes, differing in external appearance and chemical composition, can be explained as a continuous process of interaction of fly-ash fractions of different sizes with the heating surface at a variable temperature of the outer layer of contamination. To prevent such a phenomenon, it is necessary to combat not only the processes of sulfation of free calcium oxide, but also the removal of reactive amorphous silicic acid from the furnace. This is possible only when all possible reactions proceed in the mineral part of the fuel, leading to the binding of active \(\mathrm{CaO}\) and \(\mathrm{SiO_2}\) within the volume of the furnace chamber, and also when the activity of these compounds is limited through formation of their crystalline structure. Experiments on the bench installation showed a sharp decrease in the intensity of deposit formation at a combustion temperature \(t_{\mathrm{comb}} \geq 1400^\circ\mathrm{C}\). With an increase in combustion temperature, the qualitative composition of the deposits also changed. Petrographic and X-ray structural analyses showed a great similarity between friable deposits and fly ash (obtained at \(t_{\mathrm{comb}} = 1400^\circ\mathrm{C}\)). The deposits consist of globules of glassy material, practically X-ray amorphous, containing a very small number of \(\mathrm{CaSO_4}\) platelets. \(\mathrm{CaO}\), \(\mathrm{SiO_2}\), \(\mathrm{Al_2O_3}\) are bound into aluminosilicates of the anorthite type \(\mathrm{CaAl_2Si_2O_8}\), belite \(2\mathrm{CaO}\cdot\mathrm{SiO_2}\), mullite, etc. Raising the combustion temperature led to the transfer of most chemical interactions into the furnace chamber and to the formation of chemically neutral fly ash.

Conclusions

1. Experimental combustion of Nazarovo coal dust in a bench installation made it possible to reveal increased separation of amorphous \(\mathrm{SiO_2}\) from the fine entrainment fractions into the deposits. The chemical composition of the deposits depends not only on the gas temperature at the place where the deposits form, but also on the combustion temperature of the dust and the temperature of the surface of the contaminated wall.

1. Chemical reactions in the fouling layer and the associated phenomenon of sintering of ash particles form the layer of deposits that has arisen. The development of these reactions, associated with recrystallization of ash particles and the appearance of a local liquid phase, strengthens the deposit layer, binding it to the oxide film on the tube surface.

2. With an increase in the temperature of the fouling surface, the rates of chemical interactions increase, leading to the appearance of a viscous outer layer of deposits and to its capture of larger particles. This process gives the deposits a highly porous, loose structure and an increased diffusion of oxidizer in comparison with the primary layer, which reduces the possibility of formation of sulfides and lower oxides, but does not exclude the process of ash sulfation.

3. With an increase in the combustion temperature to \(t_{\text{comb}}\sim 1400^\circ\mathrm{C}\), the difference in the chemical composition of deposits and carryover is smoothed out; most chemical reactions are transferred to the furnace volume. The ash obtained as a result of such combustion is chemically neutral. The quantity of bonded deposits on convective heating surfaces decreases sharply.

LITERATURE

1. G. N. Zakourtsev, E. A. Mosin. The influence of the combustion temperature of Nazarovo coal on the intensity of formation of bonded deposits (present collection).

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

3. Investigation of the processes of fouling and ash deposits during combustion of coals of the Kansk-Achinsk basin. Technical report. Tallinn Polytechnic Institute — TKZ. 1966.

4. P. P. Budnikov and A. M. Ginstling. Reactions in Mixtures of Solid Substances. Stroiizdat, 1965.

5. N. V. Kuznetsov. Operating processes and questions of improving convective heating surfaces of boiler units. GEI, 1958.

6. K. Shtef. Steam Boilers and Furnaces. Collection: Furnaces. Fouling of Heating Surfaces. BTI. ORGRES, 1958.

7. H. Gable. BWK, 1963, IX, Bd. 15, No. 9.

8. I. Barauskaite, I. Birmantas et al. Collection of Proceedings of VNII — Thermal Insulation. Issue 2, Vilnius, 1967.

9. I. P. Epik. The influence of the mineral matter of shales on the operating conditions of a boiler unit. Tallinn, 1961.

10. M. Kruel, N. Jungten. Chem. Ing. Technik, 9/10, 1967.