On the Causes of Formation of Bonded Deposits in Boilers during Combustion of Nazarovo Coals

I. K. Lebedev, E. A. Mosin, G. N. Zakourtsev

The Nazarovo deposit of the Kansk-Achinsk brown-coal basin possesses large reserves of the cheapest fuel in the Soviet Union [1]. However, experience in burning Nazarovo coals has revealed a number of serious difficulties in ensuring economical and reliable operation of boiler units, caused by the formation of intensive bonded deposits on convective heating surfaces. Operating experience with the first units of the Nazarovo State District Power Station showed that, before reconstruction of a number of PK-38 boilers, their output in the design version, because of intensive fouling of the heating surfaces, amounted to 70% of nominal. The use of modern cleaning methods did not yield effective results [1].

Although the study of deposits arising on convective heating surfaces during combustion of Nazarovo brown coals began relatively recently, a large body of factual material has already been accumulated. Observations showed that in the gas-temperature range of 400–1200°C, deposits of mainly three types are encountered [2].

Deposits of the first type form over a wide range of gas and wall temperatures; their thickness reaches 5–7 mm. They grow very slowly and are distinguished by high strength. When an attempt is made to separate them from the tube surface, they are removed together with the scale layer. The deposits have a distinctly expressed layered structure. The color of the layers varies from dark red to brown [3]. Deposits of this type are characterized by a high content of sulfates in the form of CaSO₄ and a low content of silica—SiO₂.

Deposits of the second type form on the frontal surface of tubes and have the form of ridges directed against the flow. As a rule, they have low strength, but grow extremely rapidly and have a reduced sulfate content. Intensive formation of deposits of this type leads to limitation of boiler capacity.

Deposits of the third type form on the rear side of tubes over a wide range of gas and wall temperatures. The strength of these deposits varies—from very loose, crumbling at a light touch, to very strong. Their sulfate content varies, while the content of the remaining components changes little. Forming in the vortex zone behind the tubes, they are not as dangerous as deposits of type I and especially type II.

Deposit formation is due primarily to the characteristics of the ash of coals from the Nazarovo deposit. The ash composition is given in Table 1 [1].

Table 1

There is an extensive literature on the formation of bonded deposits; however, up to the present there has been no generally accepted explanation of the causes of their appearance and of the mechanism of their formation. The presence in the ash of these coals of large amounts of calcium oxide and iron leads, under certain conditions, to the formation of low-melting eutectics in the range 750–1100°C, consisting of mixtures CaSO₄+CaS, \((t_{\text{m}}=850^\circ\mathrm{C})\), FeS+FeO \((t_{\text{m}}=940)\), CaO+FeO+SiO₂ \((t_{\text{m}}=1095^\circ\mathrm{C})\), CaO+FeO \((t_{\text{m}}=1075^\circ\mathrm{C})\).

These eutectics can form the first sticky layer of deposits on convective heating surfaces, on which fly ash settles. According to the studies of E. A. Galileeva and I. P. Epik, the binding substance that ensures the strengthening of the settled ash layer is calcium sulfate CaSO₄, which, at the moment of formation, has the ability to bind ash particles to one another and to the tube surface.

The free lime CaO included in the fuel ash reacts with the sulfurous anhydride of the flue gases according to the equation [4]:

\[ 2\mathrm{CaO}+2\mathrm{SO}_2+\mathrm{O}_2 \rightleftarrows 2\mathrm{CaSO}_4. \]

Therefore, especially strong deposits are formed in those cases where, after combustion of the fuel, the ash contains a large amount of free lime. These conclusions are confirmed by a number of works (2, 5, 6, 9), and the deposits have been called sulfate-bonded.

It is evident that the conditions affecting the presence in the fly ash of free calcium oxide and ferrous oxide, and the sulfation of these compounds by sulfur oxides, determine the occurrence and growth of deposits. Indeed, the intensity of deposit formation is greatly influenced by the factors determining the fuel-combustion regime and the conditions at the place where deposits are formed. Among these, the first to be singled out are the combustion temperature of the dust–air mixture—the maximum gas temperature in the furnace chamber—and the excess-air coefficient, a quantity which, for a constant type of furnace and burner devices, determines the presence in the furnace of an oxidizing, semi-reducing, or reducing medium.

In low-temperature combustion, when the temperature of the burning particle \(t_{\text{comb}} < t_l\) and the liquid phase is practically absent, the free calcium oxide that forms reacts very slowly with the aluminosilicate constituents of the ash and, conversely, very vigorously with sulfur compounds. Wickert [6] investigated the temperature dependence of the binding of SO₂, CaO, and MgO·CaO in the range 400–1300°C in the presence of oxygen. This dependence is shown in Fig. 1. Both compounds have a maximum of desulfurization of the gas stream, i.e., binding of CaO and MgO into sulfates, between 800 and 1000°C. The process ceases only after 1200°C. A similar picture was observed by Kruel and Jungt in experiments on desulfurization of flue gases [7]. Pechkovskii and Maltseva [8] showed thermographically the presence of CaS in the products of the system CaO+SO₂+C(H) at

\(t = 650 \div 1070^\circ\mathrm{C}\). The occurrence of sulfation of free calcium oxide by sulfur and its compounds in the solid products of coal combustion has been confirmed by I. P. Epik [1, 3, 4]. The presence of the eutectic \( \mathrm{CaSO_4 + CaS} \) causes the appearance of a liquid phase already at a temperature of \(850^\circ\mathrm{C}\). A significant influence

Table 2

Melting temperature of eutectic mixtures of the system \( \mathrm{SiO_2 + Al_2O_3 + CaO} \)

on the temperature characteristics of ash is exerted by the presence of iron compounds. During fuel combustion, \(\mathrm{Fe_2O_3}\) and \(\mathrm{FeO}\), depending on the gaseous medium in the furnace, are modified with a change in their fusibility. Thus, oxidic iron compounds are refractory, \(t_{\mathrm{m}} = 1450^\circ\mathrm{C}\); however, when reduced to the ferrous form in a eutectic with other ash components, they

Fig. 1. Dependence of the degree of desulfurization of flue gases on temperature

have a lower melting temperature (see Table 2 on p. 97). With further reduction, ferrous iron compounds are converted into metallic iron, with a sharp increase in temperature

melting. The most favorable conditions for the formation of ferrous iron compounds arise in a semi-reducing gaseous medium, i.e., a medium

\[ \frac{\mathrm{CO}}{\mathrm{CO}_2}=10\text{--}80\%. \]

During high-temperature combustion \((t_{\text{comb}} > t'_{\text{ash}})\), a liquid phase develops intensively, which promotes the rapid occurrence of reactions between the ash components and their binding into various stable chemical compounds. This thereby prevents the sulfation of CaO and the formation of ferrous iron compounds. In addition, according to E. Dick [9], melting of an ash particle disrupts the diffusion of \(\mathrm{SO}_2\) and \(\mathrm{SO}_3\) to calcium oxides, while an excess supply of oxidizer hinders the formation of CaS and FeO, thereby preventing the formation of low-melting eutectics in the range \(700\text{--}1000^\circ\mathrm{C}\).

Such, in general terms, are the causes of deposit formation. As is evident from the foregoing, the formation of bonded deposits depends to a significant extent on the conditions of fuel combustion. Revealing this dependence would make it easier to identify the causes of deposit formation. A theoretical consideration of reactions in 5–6-component systems under combustion conditions presents great difficulties because such systems have been little studied, which makes it hard to obtain any definite solution. It is therefore necessary to determine experimentally the transformations in the mineral portion of the fuel at various combustion temperatures, their influence on the intensity of deposit formation and on deposit properties, to confirm the validity of one or another hypothesis of deposit formation, and, on this basis, to find measures for combating them. But carrying out such an experiment on industrial power installations involves a number of difficulties, the overcoming of which is usually not possible. Under operating conditions it is difficult to maintain the combustion regime specified by the conditions of the experiment, or to assess the deviation of the combustion parameters from those specified. To conduct such a study, it is necessary to create an experimental test-bench installation simulating the formation of bonded deposits on heating surfaces. Only on a test-bench installation is it possible to obtain a uniform flame temperature with a uniform supply of oxidizer across the cross section. In this case, all other factors affecting deposit formation \((W^p, W_g,\) the composition of the fuel ash) can be maintained at one level without particular difficulty, and thus quite definite combustion conditions can be obtained. A test-bench installation will make it possible to clarify the causes of the formation of bonded deposits and to outline measures for combating them.

REFERENCES

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

2. Experimental study of the formation of ash deposits during combustion of Nazarovo coal. 1966. Technical report of VTI, KTO. Authors: E. P. Dick, R. A. Sirokha.

3. Kh. Kh. Arro, V. E. Valdkiwi et al. Study of the fly ash of Nazarovo coal and its deposits on boiler-unit heating surfaces. Proceedings of the Tallinn Polytechnic Institute, Series A, No. 226, 1965.

4. I. P. Epik. Influence of the mineral portion of oil shales on the operating conditions of boiler units. Tallinn, 1961.

5. A. N. Lebedev, G. A. Sheinin. Slagging of pulverized-coal furnaces and combating it. “Energiya,” 1966.

6. K. Wickert. Mitt. VGB, 1963, No. 83.

7. M. Kruel, H. Jungten. Chemie-Ingenieur-Technik, 1967, 9/10.

8. Pechkovskii and Malysheva. ZhPKh, XXXVII, issue 2. 1964.

9. E. Dick, A. Filimonov. Teploenergetika, No. 2, 1966.