TOWARD THE DETERMINATION OF THE REACTIVE PROPERTIES OF SOLID FUELS IN A PULVERIZED COAL–AIR FLOW AT DIFFERENT STAGES OF BURNOUT

E. A. KONKOV, I. K. LEBEDEV

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

An important indicator of a solid energy fuel is its reactivity, by which in the present case is meant the activity of interaction with atmospheric oxygen at high temperatures, that is, its combustibility. The reactive properties largely determine the kinetics of heterogeneous combustion and therefore inevitably exert a substantial influence on the dynamics of the entire furnace macroprocess.

However, the reactive properties of energy fuel have been very poorly studied. In particular, there is no classification of fuels from the standpoint of these properties, nor are there sufficiently established practical criteria for such an assessment. This situation, to a certain extent, is apparently connected with the fact that in ordinary chamber furnaces the role of the kinetic features of the fuel being burned has not been sufficiently clarified. In conceptions of the operation of such furnaces, primary attention is given to the significance of heat-transfer factors, so that in flame fuel utilization the “bottleneck” of the process is considered to be not so much the rate of fuel burnout as the possibility of sufficiently rapid removal of heat from the furnace gases [1]. This circumstance is clearly expressed in the established practice of engineering calculation of furnace devices, which is reduced, in essence, to calculating the furnace as a radiant heat exchanger, with practically complete disregard for its role as a chemical reactor.

Nor can conceptions of the regime regions of fuel reaction in a real furnace device be regarded as reliable; these, likewise, can be refined only on the basis of trustworthy information about the reactive properties of the fuel being burned. In the relevant works on this question, different views are found, even to the point of being contradictory. Thus, some authors consider the combustion of coal fuel in a chamber furnace as a diffusion process [2], while others proceed from a model of combustion in an intermediate [3] or kinetic [4] region. At the same time, reliable determination of the regime region of combustion in each specific case is one of the determining conditions for the rational selection of effective practical measures for optimizing furnace operation.

Progress in the field of flame processes, proceeding along the path of an all-round increase in the thermal loading of furnace volumes and the introduction into energy practice of furnace schemes with processes separated in time—

the combustion of fuel itself and the removal of heat from the flue gases (furnaces with overheating, cyclone furnaces, etc.), defines a number of primary scientific and engineering problems associated not only with the development of designs for combustion devices, but also with the further, more profound study of the various properties of the fuels used. In this respect, the reactive (kinetic) properties also acquire great importance; they are needed in order to extend the limits of practical applicability of the diffusion-kinetic theory of combustion to furnace-process practice.

The features of natural coal fuel, determined by its substantial thermal instability and the associated variability of its macromolecular and fine-pore structure under thermal action [5], provide grounds for posing the question of reactive properties in a new way: namely, there arises a need to study them at different stages of fuel burnout. The desirability of posing such a problem is justified both by the needs of further deepening our understanding of the mechanism of the furnace process and by the prospects for introducing modern high-speed computing equipment into engineering practice. The latter will inevitably require that the various properties of the fuel be represented not only as quantities averaged over the burnout time, but will pose the problem of finding them in the form of continuous functions of this process. In this formulation we have considered the kinetics of burnout of several brown-coal fuels of the Kansk-Achinsk basin.

The concept of the structure of coal substance and of its thermal instability provides sufficient grounds for the assumption that the reactive properties of natural fuel are not constant during the combustion process [8]. However, the specific laws governing the change in these properties can be found only experimentally. The very complex character of heterogeneous combustion imposes on the corresponding experiment a number of specific requirements, which must ensure the determination of kinetic properties in the form of objective fuel characteristics, not complicated by any random factors from among the features of the macroprocess (thermal, diffusion, hydrodynamic, etc.). The indicated experimental problem, as analysis shows, can be solved by using a scheme of suspended combustion of finely dispersed fuel, while, however, taking special measures for thermal stabilization of the burning stream. In accordance with this, the experimental apparatus used by us (Fig. 1) is a vertical tubular furnace with downward forced motion of the burning pulverized-coal–air stream. A heater (reactor) of special design, made of heat-resistant steel in the form of a small-diameter tube with a bend, is heated by electric current from a single-phase low-voltage transformer. Thermal stabilization of the stream is achieved by strongly ballasting it with an inert finely dispersed heat carrier (alumina or quartz sand), with the air being dust-laden with ballast in an earlier section relative to the point at which the fuel is introduced into the reaction furnace. This makes it possible to feed the fuel into a ballast–air stream that has been thermally stabilized in advance, and thus practically to eliminate the unsteady portion of the temperature curve of fuel heating. A special experimental check shows that the measures considered make it possible to carry out the flare burnout process under practically isothermal conditions. Burnout in different sections of the reaction tube (i.e., at different moments of the process) is monitored by gas analysis using the method of gas-adsorption chromatography.

The difficulties of estimating the actual reactive surface in the process of heterogeneous combustion are well known. In determining the reactive properties at different stages of burnout, and especially in connection with the method of thermal stabilization of the flame adopted by us, which leads to dilution of the fuel with mineral ballast by tens of times, these difficulties become practically insurmountable. Therefore, as the

Fig. 1. Diagram of the experimental setup. 1—heater (reactor); 2—fuel feeder; 3—ballast-material feeder; 4—power transformer; 5—direct-current source; 6—flow-measuring device; 7—thermocouple; 8—multiway gas valve; 9—wet aspirator; 10—KhL-3 chromatograph.

criterion of reactivity we adopted the coefficient*) of the relative weight rate of combustion, analogous in meaning to the constant of internal reaction [6]. In comparison with the so-called true kinetic constants (\(k_0\) and \(E\) in the Arrhenius equation), this indicator characterizes the combustion process in a more general form, since it contains the undisclosed dependence of the combustion rate on the magnitude of the actual reacting surface. However, it is not difficult to convince oneself that this indicator, which determines the “output” (effective) rate of the process, is quite sufficient for kinetic calculations of real furnace processes, and in the limiting region of a purely kinetic regime it is an objective measure of the fuel reactivity.

Thus, determination of the kinetic properties of the fuels considered consisted in the experimental determination of burnout curves along the length of the flame (burning in a purely kinetic regime), determi-

*) We avoid using the term “constant” to designate this criterion, in order to emphasize its variable character as burnout proceeds.

...determination, on this basis, of the combustion rates in each section of the flame and calculation of the reaction-rate coefficients for each section by the method set forth in [8].

Fig. 2. Burnout of Nazarovo brown coal as a function of time. □—at a temperature of 700°C, ●—at a temperature of 720°C, ▽—at a temperature of 747°C; \(y = 1 - \Omega\), where \(\Omega\) is the degree of burnout.

The results of the study are presented in Figs. 2–4. Let us recall that, in accordance with the method we adopted for determining the reaction constants [8], representation of the burnout curve \(\ln(1-\Omega)=f(\tau)\) in semilogarithmic anamorphosis qualitatively determines the character of these constants; namely, if this representation is depicted by a straight line, then the reaction proceeds at a constant rate coefficient. Conversely, if the indicated representation is nonlinear, then this corresponds to a reaction-rate coefficient that varies as burnout proceeds. Figs. 2 and 3 show that in reality the latter case occurs; that is, the combustion-rate coefficients of the fuels considered are monotonic functions of the combustion process, but by no means constant quantities. Such a character of the reaction was found by us for a broad range of bituminous power-station coals. The numerical values of the combustion-rate constants at each instant of time are proportional to the tangent of the angle of inclination of the curve \(\ln(1\Omega)=f(\tau)\) at the corresponding point. Therefore, finding the derivatives of this curve makes it possible to determine the numerical values of the combustion-rate coefficient at different instants of time (Fig. 3) or at different stages of burnout (Fig. 4).

The index of kinetic activity adopted by us does not make it possible to give a rigorously substantiated separation of the role of surface factors and factors of chemical affinity in the manifested reaction properties of the fuel; that is, it is impossible to estimate precisely to what extent the change

Fig. 3. Change in the combustion-rate constant of Nazarovo brown coal with burnout time.
1 — at 700°C; 2 — at 720°C; 3 — at 747°C.

of the combustion-rate constant during burnout is due to a change in the pre-exponential factor \(a_0\) or to a change in the activation energy \(E\) in the equation

\[ a=a_0 \exp\left(-\frac{E}{RT}\right). \]

Such a separation can be made only approximately. If, in accordance with the concepts of the theory of heterogeneous kinetics and catalysis [7] [4], \(a_0\) is interpreted as a function of the number of reaction-capable centers (a surface function), and \(E\) as a function of chemical affinity (an energy factor), then from Fig. 4 one may conclude that the surface function plays the predominant role (\(a_0\) depends strongly on the degree of burnout), while the variability of the activation factors is less pronounced (\(E\), as a function of the degree of burnout, changes only slightly (Fig. 4, b). On this basis it is possible to conclude that,

Fig. 4. Dependence of the combustion-rate constant of Nazarovo brown coal on the degree of burnout (a) and on temperature (b). 1, 2, 3 — the same as in Fig. 3.

that in the course of combustion, apparently, the process of change (consumption) of the quantity of reactive clusters becomes more noticeable, while the activity of these clusters changes less.

The independence of the activation energy from the degree of burnout at late stages of reaction of formed fuels was also found by W. Peters [9]. However, since that work was carried out under conditions substantially different from ours, a direct comparison of the results obtained by us with work [9] cannot be made.

Conclusions

1. The variability of the kinetic constants of natural fuel as burnout proceeds, substantiated by modern concepts of its macromolecular and fine-pore structure, as well as by the mechanism of thermal effects, is confirmed experimentally.

2. The boundaries of the regime regions of reaction of natural fuel in a furnace device with suspended combustion of fuel particles must have an unstable character. The decrease in the kinetic activity of the fuel, together with the reduction in the size of the fuel particles during burnout, should rapidly shift the furnace process toward the kinetic regime.

3. Accordingly, for intensifying the operation of furnaces using coals of the Kansk-Achinsk basin, measures leading to an increase in the temperature levels of the process appear desirable.

REFERENCES

1. S. A. Tager. Problems of modern furnace technology and tasks of scientific research. Teploenergetika, No. 9, 1965.

2. A. B. Reznyakov. Combustion of a pulverized-coal flame as a polydisperse system. Alma-Ata, 1958.

3. V. V. Pomerantsev, S. L. Shagalova, K. M. Aref’ev. An approximate method for calculating the burnout of a pulverized-coal flame. Teploenergetika, No. 11, 1958.

4. I. A. Yavorskii. Questions of the theory of combustion of fossil coals and intensification of their use. Publishing House of the Siberian Branch of the Academy of Sciences of the USSR, Novosibirsk, 1961.

5. Z. F. Chukhanov. Some problems of fuel and power engineering. Publishing House of the Academy of Sciences of the USSR, Moscow, 1961.

6. L. N. Khitrin. Physics of combustion and explosion. Moscow State University Press, 1957.

7. G. M. Panchenko, V. P. Lebedev. Chemical kinetics and catalysis. Moscow State University Press, 1961.

8. E. A. Kon’kov. On the calculation of the kinetic constants of an isothermal pulverized-coal flame at different stages of burnout. Izv. TPI, vol. 139, 1965.

9. W. Peters, I. Langhoff. Brennstoff-Chemie, Bd. 44, No. 2, 1963.