PHYSICAL CHEMISTRY

Academician of the Academy of Sciences of the Uzbek SSR N. V. LAVROV

FEATURES OF THE MECHANISM OF CARBON COMBUSTION

At the present time it may be considered that the process of combustion and gasification of carbon proceeds through the following stages: diffusion of the reacting gases to the reaction surface of the carbon, physical adsorption, formation of an intermediate physicochemical complex, destruction of the complex, and desorption of the combustion products.

Most investigators have come to the conclusion that carbon–oxygen intermediate compounds exist, through whose formation and destruction the chemical stages of the processes of combustion and gasification take place. The first scheme of such a process was developed by Rideal and Wheeler (¹)

\[ x\mathrm{C} + \frac{1}{2}y\mathrm{O}_2 = \mathrm{C}_x\mathrm{O}_y, \tag{1} \]

\[ \mathrm{C}_x\mathrm{O}_y = n\mathrm{CO}_2 + m\mathrm{CO}. \tag{2} \]

Subsequently L. Meyer (²), carrying out experiments on the combustion of carbon under high-vacuum conditions, established that the main amount of carbon reacts according to the reaction:

\[ 4\mathrm{C} + 3\mathrm{O}_2 = 2\mathrm{CO} + 2\mathrm{CO}_2, \tag{3} \]

while at higher temperatures the process proceeds according to the equation

\[ 3\mathrm{C} + 2\mathrm{O}_2 = 2\mathrm{CO} + \mathrm{CO}_2. \tag{4} \]

From Becker’s experiments (³) on the combustion of carbon in a dry medium it is known that, at ordinary temperatures, it reacts slowly with oxygen, forming only carbon monoxide, while carbon dioxide is the product of subsequent oxidation of carbon monoxide.

Studies by Z. F. Chukhanov and N. A. Karzhavina (⁴) and by N. V. Lavrov (⁵) have shown that at low oxygen concentrations and ordinary pressure carbon burns with the formation of carbon monoxide and carbon dioxide in equimolecular amounts.

As a result of the theoretical generalization carried out, Z. F. Chukhanov put forward a bireaction theory of carbon combustion, according to which carbon and oxygen react by two independent reactions: the “combustion” reaction

\[ 2\mathrm{C} + \mathrm{O}_2 = 2\mathrm{CO} \tag{5} \]

and the “oxidation” reaction

\[ 3\mathrm{C} + 2\mathrm{O}_2 = \mathrm{C}_3\mathrm{O}_4 = 2\mathrm{CO} + \mathrm{CO}_2\,(a), \]

\[ \mathrm{C}_3\mathrm{O}_4 + \mathrm{C} + \mathrm{O}_2 = 2\mathrm{CO} + 2\mathrm{CO}_2\,(b), \tag{6} \]

the explosion of the complex according to scheme (6) being effected by the impact of an oxygen molecule diffusing from the gas space.

Reactions (3) and (6) do not explain the accelerating influence of water vapor on the process of carbon combustion and on the change in the mechanism of interaction of carbon–

with oxygen. A shortcoming of the indicated hypotheses on the mechanism of the carbon-combustion reaction is also the isolated consideration of the course of heterogeneous and homogeneous reactions.

The noted shortcomings of the previous schemes are taken into account in the concept of the mechanism of carbon combustion proposed by the author \((^6)\).

In the process of combustion, molecular oxygen, carbon dioxide, or water vapor dissociate, and atomic oxygen forms an intermediate carbon–oxygen complex of the ketone-group type.

The dissociation of oxygen takes place both in the process of chemisorption and as a result of the occurrence of homogeneous chain reactions discovered by Academician N. N. Semenov and his co-workers \((^7)\).

In the zone of reduction reactions, in the absence of oxygen, carbon–oxygen complexes are thermally destroyed with the liberation of carbon monoxide.

Thus, in this zone the occurrence of the following reactions is most probable:

1. The reaction of reduction of carbon dioxide, \( \mathrm{C} + \mathrm{CO}_2 = 2\mathrm{CO} \), proceeding through intermediate stages:

\[ \begin{aligned} \text{a) }& x\mathrm{C} + \mathrm{CO}_2 = x\mathrm{C} + (\mathrm{CO}_2)\,\text{ads},\\ \text{b) }& (\mathrm{CO}_2)\,\text{ads} + x\mathrm{C} = \mathrm{C}_{x-1}\mathrm{CO} + \mathrm{CO},\\ \text{c) }& \mathrm{C}_{x-1}\mathrm{CO} = (x-1)\mathrm{C} + \mathrm{CO}. \end{aligned} \tag{7} \]

1. The reaction of interaction of carbon with water vapor, \( \mathrm{C} + \mathrm{H}_2\mathrm{O} = \mathrm{CO} + \mathrm{H}_2 \), taking place as follows:

\[ \begin{aligned} \text{a) }& x\mathrm{C} + \mathrm{H}_2\mathrm{O} = x\mathrm{C} + (\mathrm{H}_2\mathrm{O})\,\text{ads},\\ \text{b) }& x\mathrm{C} + (\mathrm{H}_2\mathrm{O})\,\text{ads} = \mathrm{C}_{x-1}\mathrm{CO} + \mathrm{H}_2,\\ \text{c) }& \mathrm{C}_{x-1}\mathrm{CO} = (x-1)\mathrm{C} + \mathrm{CO}. \end{aligned} \tag{8} \]

1. Subsequently, under the action of catalysts, the conversion reaction of carbon monoxide with water vapor takes place,

\[ \mathrm{CO} + \mathrm{H}_2\mathrm{O} + [\mathrm{K}] = \mathrm{H}_2 + \mathrm{CO}_2 + [\mathrm{K}], \]

where \([\mathrm{K}]\) is the catalyst. According to the mechanism established by N. V. Kul’kova and M. I. Temkin \((^8)\):

\[ [\mathrm{K}] + \mathrm{H}_2\mathrm{O} = [\mathrm{K}]\cdot\mathrm{O} + \mathrm{H}_2 \;(\text{a}), \]

\[ [\mathrm{K}]\cdot\mathrm{O} + \mathrm{CO} = [\mathrm{K}] + \mathrm{CO}_2 \;(\text{b}). \tag{9} \]

In the oxygen zone, carbon combustion proceeds according to different schemes, depending on whether this process occurs in a dry or a moist medium.

Dry medium

\[ \begin{aligned} \text{a) }& x\mathrm{C} + \mathrm{O}_2 \rightarrow x\mathrm{C} + (\mathrm{O}_2)\,\text{ads},\\ \text{b) }& 2x\mathrm{C} + (\mathrm{O}_2)\,\text{ads} \rightarrow 2\mathrm{C}_{x-1}\mathrm{CO},\\ \text{c) }& 2\mathrm{C}_{x-1}\mathrm{CO} \rightarrow 2(x-1)\mathrm{C} + 2\mathrm{CO}. \end{aligned} \tag{10} \]

or

\[ \text{[schematic aromatic carbon complex]} + (\mathrm{O}_2)\,\text{ads} \rightarrow \text{complex A} \rightarrow \text{complex B} \rightarrow 2(x-1)\mathrm{C} + 2\mathrm{CO}. \tag{11} \]

complex A    complex B

Moist medium

1. \[ \mathrm{H_2O}+M=\dot{\mathrm{O}}\mathrm{H}+\dot{\mathrm{H}}+M, \]
   \[ \dot{\mathrm{H}}+\mathrm{O_2}=\dot{\mathrm{O}}+\dot{\mathrm{O}}\mathrm{H}. \tag{12} \]

2. Reactions of formation of the carbon–oxygen complex

\[ \left. \begin{aligned} \text{a) }&x\mathrm{C}+\mathrm{O_2}=x\mathrm{C}+(\mathrm{O_2})_{\text{ads}},\\ &2x\mathrm{C}+(\mathrm{O_2})_{\text{ads}}=2\mathrm{C}_{x-1}\mathrm{CO};\\ \text{b) }&x\mathrm{C}+\mathrm{O}=\mathrm{C}_{x-1}\mathrm{CO},\\ \text{c) }&x\mathrm{C}+\mathrm{OH}=\mathrm{C}_{x-1}\mathrm{COH}=\mathrm{C}_{x-1}\mathrm{CO}+\dot{\mathrm{H}} \end{aligned} \right\} \tag{13} \]

\[ \dot{\mathrm{H}}+\mathrm{O_2}=\dot{\mathrm{O}}+\dot{\mathrm{O}}\mathrm{H}. \tag{14} \]

1. Reactions of destruction of the carbon–oxygen complex:

\[ \text{a) }\mathrm{C}_{x-1}\mathrm{CO}=(x-1)\mathrm{C}+\mathrm{CO}; \tag{15} \]

\[ \text{b) }2\mathrm{C}_{x-1}\mathrm{CO}+\mathrm{O}=2(x-1)\mathrm{C}+\mathrm{CO}+\mathrm{CO_2} \]

or

\[ \begin{array}{c} \chemfig{*6(=-(-[:0]\mathrm{C}{=}\mathrm{O})-(-[:0]\mathrm{C}{=}\mathrm{O})=-=)} \;+\;\mathrm{O}\;\to\; \chemfig{*6(=-(-[:0]\mathrm{C}{=}\mathrm{O}>[:0]\mathrm{O})-(-[:0]\mathrm{C}{=}\mathrm{O})=-=)} \;\to\;2(x-1)\mathrm{C}+\mathrm{CO}+\mathrm{CO_2}. \end{array} \tag{16} \]

complex C

In the oxygen zone, destruction of the intermediate carbon–oxygen complex occurs as a result not of the action on it of molecular oxygen, as Z. F. Chukhanov assumed, but of atomic oxygen formed in the course of chain oxidation reactions.

Atomic oxygen with complex B can form complex C, discovered by N. A. Shilov and his co-workers \((^9)\), as a result of the decomposition of which CO and \(\mathrm{CO_2}\) are formed in an equimolecular ratio. As follows from the proposed mechanism, the process of direct formation of carbon monoxide (equation 15a) and oxidation (equation 15b) do not proceed as independent reactions, but are different variants of the formation and decomposition of one and the same carbon–oxygen complex.

Combustion of carbon in a moist medium, with some approximation, according to our concepts, proceeds by the following scheme:

\[ 2x\mathrm{C}+\mathrm{O_2}\to 2\mathrm{C}_{x-1}\mathrm{CO}\to 2(x-1)\mathrm{C}+2\mathrm{CO}, \tag{17a} \]

\[ \begin{array}{cccccccccc} & \mathrm{C}_{x-1}\mathrm{CO} &&&& 2\mathrm{C}_{x-1}\mathrm{CO}+\mathrm{O}=2(x-1)+\mathrm{CO}+\mathrm{CO_2} \\ & \uparrow &&&& \uparrow \\ \mathrm{H}+\mathrm{O_2}\to \mathrm{OH}+x\mathrm{C} & \to & \mathrm{H}+\mathrm{O_2}\to \mathrm{OH}+x\mathrm{C} & \to & \mathrm{H}+\mathrm{O_2} & \to & \mathrm{OH}+x\mathrm{C} & \to & \mathrm{H},\\ \downarrow && \downarrow && \downarrow && \downarrow \\ \dot{\mathrm{O}}+x\mathrm{C}\to \mathrm{C}_{x-1}\mathrm{CO} &&& \mathrm{C}_{x-1}\mathrm{CO} &&& \mathrm{C}_{x-1}\mathrm{CO} \\ && \downarrow \\ 2x\mathrm{C}+\mathrm{O_2}\to 2\mathrm{C}_{x-1}\mathrm{CO}+\dot{\mathrm{O}}\to 2(x-1)\mathrm{C}+\mathrm{CO}+\mathrm{CO_2} \end{array} \tag{17b} \]

In a previous work we established the important role of atomic oxygen in the combustion process. From the scheme presented, based on new data on the mechanism of the reaction \(\mathrm{C}+\mathrm{H_2O}\), it follows that, when hydroxyl reacts with carbon, a complex chain reaction with unbranched chains arises (17b), ensuring the continuity of the combustion process.

The introduction of inhibitors (for example, chlorine) can remove active centers from the combustion process \((\mathrm{Cl_2}+\dot{\mathrm{H}}=\mathrm{HCl}+\dot{\mathrm{Cl}})\) and not only prevent

afterburning of CO, but also suspend the course of reaction (17b). In this case the reaction of oxygen with carbon will proceed according to reaction (17a), i.e., according to the scheme of carbon combustion in a dry medium.

Fuel Utilization Institute
Tashkent

Received
4 February 1964

CITED LITERATURE

¹ Rhead, Wheeler, J. Chem. Soc., 1912, 864; 1913, 461.
² L. Meyer, Zs. phys. Chem., 17, 385 (1932).
³ H. Bacher, Phil. Trans. A., 179, 571 (1888).
⁴ Z. F. Chukhanov, ZhTF, 8, issue 2, 147 (1938).
⁵ N. V. Lavrov, Physicochemical Foundations of the Combustion and Gasification of Fuel, Moscow, 1957.
⁶ N. V. Lavrov, Fundamentals of the Combustion of Gaseous Fuel, Tashkent, 1961.
⁷ N. N. Semenov, On Certain Problems of Chemical Kinetics and Reactivity, 2nd ed., Publishing House of the Academy of Sciences of the USSR, 1958.
⁸ N. V. Kul’kova, M. I. Temkin, ZhFKh, 23, No. 6 (1949).
⁹ N. A. Schilow, E. G. Schatunowskaja, K. W. Tschmutow, Zs. phys. Chem. A., 149, 211 (1931).