THE EFFECT OF TEMPERATURE ON THE YIELD OF PYROLYSIS TAR FROM SOLID FUELS

G. I. KRAVTSOVA, S. I. SMOLYANINOV

(Presented by the scientific-methodological seminar of the CTF)

One of the most important factors affecting the process of tar formation is the temperature of the process.

The investigations carried out [1–5] have not yet made it possible to establish, to a sufficient degree, the regularities of the action of this factor, since the mechanism of tar formation has not yet been fully elucidated.

Let us represent the process of destruction of solid combustible minerals by the following simplified scheme:

\[ \text{SCM} \xrightarrow{k_1} \text{tar} \begin{array}{c} \xrightarrow{k_2} \text{gas} \\ \xrightarrow{k_3} \text{coke} \end{array} \]

Assuming that the reactions taking place are described by a first-order equation,

\[ \alpha = 1 - e^{-k\tau}, \]

where

\[ \alpha \text{ is the fraction of the reacted substance,} \]
\[ \tau \text{ is the heating time,} \]

we find the ratio of the amount of unreacted SCM to the amount of accumulated tar:

\[ \frac{1-\alpha_1}{1-\alpha_2} = \frac{e^{-k\tau}}{e^{-(k_2+k_3)\tau}} \quad \text{or} \quad \ln \frac{x_1}{x_2}=k_1\tau+k_2\tau+k_3\tau \]

where \(x_1=1-\alpha_1\) is the fraction of unreacted SCM,

\[ x_2=1-\alpha_2 \]

is the fraction of accumulated tar,

then

\[ \ln x_1-\ln x_2=[-k_1+(k_2+k_3)]\tau, \]

\[ \ln x_2=\ln x_1+[k_1-(k_2+k_3)]\tau \tag{1} \]

From equation (1) it is evident that the tar yield in one temperature interval will depend on the ratio of the value of the rate constant of tar formation and tar consumption, and also on the time of occurrence of the destruction process, i.e., on the heating time of the SCM at the given temperature or in the given temperature interval.

The equations presented make it possible to consider the effect of temperature on the yield of pyrolysis tar at a constant heating time \(\tau\), min.

We shall write equation (1) as follows:

\[ \ln x_2=\ln x_1+\left[k_{10}e^{-\frac{E_1}{RT}}-k_{20}e^{-\frac{E_2}{RT}}+k_{30}e^{-\frac{E_3}{RT}}\right]\tau, \]

Assuming the pre-exponential factors \(k_{10}, k_{20}, k_{30}\) in the first approximation to be quantities independent of temperature, we see that the change in tar yield must be determined by the relationship between the absolute values of the activation energies of the reactions taking place, \(E_1, E_2, E_3\).

Let us consider particular cases:

if 1) \(E_1 \ll E_2,\ E_1 \ll E_3\), then, as the temperature increases, the tar yield decreases.

2) \(E_1 \gg E_2,\ E_1 \gg E_3\); when the temperature is lowered, tar accumulation will occur.

3) \(E_1 < E_2\), but \(> E_3\)

\[ E_1 < E_2,\quad \text{but } < E_3. \]

In this case a decrease in tar yield occurs.

Let us consider the influence of temperature on tar yield using the Bodenstein principle [6].

The rate of tar formation may be represented by the following equation:

\[ \frac{dc_c}{d\tau}=k_1 c_{\text{tgi}}-k_2 c_c-k_3 c_c=0, \quad \text{whence} \]

\[ C_c=\frac{k_1 c_{\text{tgi}}}{k_2+k_3}, \]

then the rates of formation of gas and coke, respectively, are equal to:

\[ V_{\text{gas}}=\frac{dc_g}{d\tau}=k_2 c_c=\frac{k_2 k_1 c_{\text{tgi}}}{k_2+k_3}, \tag{2} \]

\[ V_{\text{coke}}=\frac{dc_k}{d\tau}=k_3 c_c=\frac{k_3 k_1 c_{\text{tgi}}}{k_2+k_3}. \]

Let us consider, according to equation (2), the limiting stages of this process and the corresponding activation energies:

1) \(k_1 c_{\text{tgi}} \gg k_2=k_3\), then

\[ V_{\text{r-n}} \simeq \frac{1}{2}k_1 c_{\text{tgi}}, \]

i.e., the tar-formation reaction will mainly proceed; then \(E_{\text{r-n}}=E_1\), and, as the temperature increases, the rate of the tar-formation process will increase.

2) \(k_1 c_{\text{tgi}} \ll k_2 \approx k_3\),

then

\[ V_{\text{r-n}}=\frac{k_2 k_1 c_{\text{tgi}}}{k_2+k_3}. \]

In this case the reaction \(E\) will be equal either to \(E_1\) or to \(E_2\), i.e., with an increase in temperature the yield of either tar or gas will increase. Thus, the yield of the reaction products will be determined by the ratio of the quantities \(E_1\) and \(E_2\).

3) \(k_2 \ll k_3\),

then

\[ V_{\text{r-n}}=\frac{k_2 k_1 c_{\text{tgi}}}{k_3}, \qquad E_{\text{r-n}}=E_1+E_2-E_3. \]

In this complex case, raising the temperature will determine an increase in the yield of one or another product depending on the ratio of the absolute values of all activation energies \(E_1, E_2, E_3\).

4) \(k_2 \gg k_3\), then

\[ V_{\text{r-n}} = -k_1 c_{\text{t-n}}, \qquad E_p = E_1 . \]

The limiting stage will be tar formation, and an increase in temperature will cause an increase in its yield.

Conclusions

Using kinetic equations it has been shown that, for a constant heating time, as the temperature changes, the yield of pyrolysis tar from solid fuels will be determined by the relationship between the absolute values of the activation energies of the reactions taking place, \(E_1\), \(E_2\), \(E_3\).

References

1. Ya. I. Khisin. The influence of temperature conditions on the yield of tar during the semicoking of solid fuels. Trudy VNIGI, vol. 3, 1951.

2. V. A. Vekhov. The influence of changes in the temperature regime on the dynamics of the release of volatile substances. Izv. AN SSSR, OTN, No. 8, 1949.

3. N. A. Nikol’skii. On the question of the influence of coking temperature on the composition and yields of chemical products. Khimiya tverdogo topliva, vol. 7, issue 9—10, 1936.

4. V. E. Rakovskii, F. A. Kaganovich, E. A. Novichkova. Chemistry of Pyrogenic Processes. Publishing House of the Academy of Sciences of the BSSR, 1959.

5. A. P. Kashurichev. Study of the thermal decomposition of various solid fuels at low heating rates. Energotekhnologicheskoe ispol’zovanie topliva, issue 1, 1960.

6. G. M. Panchenkov, V. P. Lebedev. Chemical Kinetics and Catalysis. Moscow State University Publishing House, 1961.