On a Method for Determining the Kinetic Properties of Pulverized Power Fuel

E. A. Konkov

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

The dynamics of processes in pulverized-coal furnaces are determined to a considerable extent by the kinetic properties of the fuels being burned, but because these properties have been poorly studied, they are practically not taken into account in the modern method of engineering calculation of boiler units.

For the further improvement of designs and the rationalization of operating conditions of furnace devices, the need to study the indicated properties becomes highly urgent; this explains the appearance of a significant number of works in this area. However, most of these works amount to an evaluation of combustibility under arbitrary conditions (usually in a bed), whereas flame combustion is characterized by a specific nature of the heat- and mass-transfer processes of reaction of the fuel particles.

Studies of the last 10–15 years have established that coal substance is a complex of complicated polymer compounds [1] [2]. According to modern concepts, the principal structural unit of coal substance is a flat atomic network of carbon of aromatic polymerization, with lateral linearly polymerized chains. Questions concerning the structure of the structural units, their interconnection, orientation, interaction forces, etc., have not yet been fully studied. However, there are sufficient grounds to believe that these factors determine to a considerable extent all the basic qualities of the fuel, including its reactivity.

The elements of linear polymerization are believed to be the least chemically stable. During chemical transformations they are readily broken off, exposing free valence bonds that cause reaction with other substances. The nuclear part of the carbon-polymerization network is the most stable; it is destroyed last, with the exposure of valence bonds, and therefore is the least reactive. It has also been shown (Z. F. Chukhanov, V. I. Kasatochkin) that, depending on the intensity and level of thermal action, destruction of the fuel macromolecule may proceed along different bonds, giving rise to a different chemical composition of the decomposition products obtained and to a different number of opened reactive sites.

The indicated factors of the molecular structure of coal substance and its variability under thermal action must lead

to the constancy of the quantities characterizing the rates of the corresponding chemical acts of the fuel-combustion process.

Not only the chemical structure of the substance, but also its fine-pore structure undergoes considerable changes under thermal action. In Fig. 1, borrowed from [4], an example is presented of the change in the surface of fuel pores and their sizes. It follows from this that,

Fig. 1. Dependence of the surface of fuel pores on the degree of burnout.
●—surface formed by pores with size \(d>1000\,\text{\AA}\); △—surface formed by pores with size \(d<100\,\text{\AA}\); ○—total surface of the fuel pores.

in the process of fuel burnout the factors of diffusional supply of oxidizer to the reacting surface will also not remain unchanged.

Thus, the nonconstancy of both the molecular and the fine-pore structure of the fuel substance during burnout permits one to consider that in the flame there reacts by no means some unchanging physicochemical system, but, on the contrary, products of the thermal decomposition of the fuel that are continuously changing in composition and structure. The properties of these products depend substantially on the character of the thermal transformations, i.e., in essence, are connected with the corresponding method of fuel utilization. This determines the practical advisability of studying fuel activity at different stages of burnout, and the experimental method corresponding to this study must be closely akin to the type of furnace process most widespread in power engineering, which at present is the flame process. A flame scheme for organizing kinetic studies is also very desirable because of the possibility of maintaining a kinetic reaction regime up to very high temperatures [3].

To choose a criterion of reactivity it is necessary to settle on two concepts that occur in kinetic studies. The first concept consists of attempts to find the so-called true kinetic constants, i.e., the parameters of the Arrhenius equation—the pre-exponential factor \(k_0\) and the activation energy \(E\). These attempts cannot yet be regarded as entirely successful because of the difficulty of completely separating the physical factors of combustion from the chemical ones, and because of the impossibility, by the means of modern experimental technique, of accurately estimating the magnitude of the actual reaction surface.

Therefore, from the standpoint of the needs of thermal power engineering, for assessing the kinetic properties of solid fuel the concept proposed by L. N. Khitrin [5] of the coefficient of reactive gas exchange, which regards combustion as a complex physicochemical process, appears justified.

The formal form of the equation for expressing the coefficient of reactive gas exchange remains analogous to the Arrhenius equation:

\[ \alpha = \alpha_0 \cdot \exp\left(-\frac{E_{\mathrm{eff}}}{RT}\right), \tag{1} \]

however, the quantities \(\alpha_0\) and \(E_{\mathrm{eff}}\), in comparison with the corresponding quantities \(k_0\) and \(E\) of the Arrhenius equation, have a broader effective meaning, since, in addition to the chemical nature of the fuel substance, these quantities also reflect nonchemical factors of combustion. Since the number of possible combinations of the latter is practically inexhaustible, it is expedient to determine the coefficients of reactive gas exchange by calibrating the corresponding experimental conditions. In particular, it is possible to show the expediency of carrying out experimental burnout in one of the limiting combustion regimes—the purely kinetic one.

The most important condition in kinetic investigations is to ensure definiteness of the temperature regime of the processes taking place, since, with the Arrhenius dependence of the combustion rate on temperature, an insufficiently accurate estimate of the latter leads to very large errors in estimating the kinetic properties. However, a pulverized-coal flame has substantial nonisothermicity.

An effective means of combating this drawback is ballasting the flame with a solid heat carrier [3], as well as a justified choice of the diameter of the reaction vessel and of the method for heating the latter.

Fig. 2. Diagram of the experimental setup.
1 — reaction tube; 2 — fuel feeder; 3 — ballast-material feeder; 4 — power transformer; 5 — VTI-2 gas analyzer; 6 — KhL-3 chromatograph.

We are carrying out an experimental investigation of the reactive properties of certain power fuels of the Magnitogorsk Metallurgical Combine in an elementary isothermal flame. A diagram of the experimental setup is given in Fig. 2. The principal element of the setup is a small-diameter reaction tube made of heat-resistant—

steel, heated by an electric current by the short-circuit method. Fuel is supplied by means of a drum feeder with smooth regulation of the feed rate. Isothermal conditions in the reaction tube are maintained by strong ballasting of the flame with a solid inert heat carrier (calcined quartz sand). In the apparatus, continuous measurement is made of the fuel flow rate and of the temperature of the reaction space. The air flow rate is measured with a rotameter. The burnout process is monitored by analyzing the gaseous combustion products sampled at four points along the length of the tube. On the basis of the gas-analysis data, the coefficients of reaction gas exchange are calculated for different sections of the reaction tube at different temperatures, and the apparent kinetic constants are also calculated—the pre-exponential factors \(a_0\) and the activation energies \(E_{\mathrm{eff}}\).

Figure labels: \(\ln \alpha\); I section; II section; III section; \(1000/T\).

Fig. 3. Dependence of the coefficients of reaction gas exchange on temperature in different burnout sections.
▼ — Karaganda coal, grade K; ◐ — Kuznetsk coal, grade K; □ — Kuznetsk coal, grade Zh.

Figures 3 and 4 present the results of the investigation of the kinetic properties of three types of hard-coal fuels:

\[ \begin{gathered} \text{Karaganda, grade K,}\\ \text{Kuznetsk, grade K,}\\ \text{Kuznetsk, grade Zh.} \end{gathered} \]

Figure labels: \(\ln \alpha\); \(\Omega\).

Fig. 4. Dependence of the coefficient of reaction gas exchange on the degree of burnout (for the symbols see Fig. 3).

The chemical composition of the fuels investigated is given in Table 1. Analysis of the research results shows a quite clear confirmation of the assumption that the reaction properties of fuels vary as combustion proceeds.

Table 1

Composition of the fuels investigated

Although, on the basis of the available experimental material, the specific form of the dependence of the reaction gas-exchange coefficient on the degree of burnoff still cannot be considered reliably established, the fact of a regular decrease of this coefficient with burnoff appears indisputable (Fig. 3). The apparent activation energy $E_{\mathrm{eff}}$, determined from the experimentally found dependence of the reaction gas-exchange coefficients on temperature, changes with increasing degree of burnoff, and does not remain constant, as is presently assumed in calculations of the combustion rate (in Fig. 3, $E_{\mathrm{eff}}$ is proportional to the tangent of the angle of inclination of the curves $\ln \alpha = f(T)$).

Conclusions

1. A method has been developed and applied for estimating the reaction properties of solid power fuels in a dust–air isothermal stream.

2. On the basis of modern ideas concerning the structure of natural coals, the assumption has been made that their kinetic properties are variable. This assumption has been confirmed experimentally.

3. Data are presented on the magnitude of the apparent kinetic constants for the three fuels investigated at different stages of combustion.

References

1. Z. F. Chukhanov. Some Problems of Fuel and Power Engineering. Publishing House of the Academy of Sciences of the USSR, Moscow, 1961.

2. Van Krevelen, J. Schuyer. Coal Science. Gostoptekhizdat, Moscow, 1960.

3. L. N. Khitrin, M. B. Ravich, L. L. Kotova. Method and results of investigation of the kinetic characteristics of combustion of pulverized fuel. IFZh, No. 1, 1962.

4. W. Peters, J. Langhoff. Brennstoff—Chemie, Bd. 44, No. 2, 1963.

5. L. N. Khitrin. Physics of Combustion and Explosion. Moscow State University Press, 1957.