An Introduction to the Combustion of Carbon Materials

Abstract

Combustion is arguably as old as homo sapiens ability to observe and use fire. Despite the long tradition of using carbon combustion for energy production, this reaction is still not fully understood. This can be related to several facts that are intertwined and complicate the investigation, such as the large variety of possible carbon structures, the actual surface structure, porosity, the solid-gas nature of this reaction, diffusion limitation and fundamental reaction steps. In this review, a brief history of carbon combustion science is given, followed by a detailed discussion of the most important aspects of carbon combustion. Special attention is given to limitations for example diffusion. In carbon combustion, kinetic control can rarely be observed. The literature of the fundamental reaction steps actually occurring on the carbon framework is reviewed and it becomes apparent that the reaction is occurring primarily on defects on the basal plane. Thus, the reaction between oxygen and carbon may be used as an analytical tool to provide further insights into novel materials, for example synthetic carbon materials, fibres and graphene type materials. Mastering the combustion reaction in all its complexity may prove to be very valuable in the future.

Keywords: carbon nanomaterials; combustion; diffusion; kinetics; surface reactions.

Figure 1

Classification of some carbonaceous materials from reference. Depicted are crystalline forms of carbon such as diamond graphite and nanoforms such as fullerene and carbon nanotubes (CNTs). Moreover, there are also various carbon forms that exhibit an amorphous structure, such as cokes, carbon black and charcoal. Adapted with permission from Ref. [10]. Copyright 2022, Wiley‐VCH.

Figure 2

General structure of carbon black. Elementary particles are first made of randomly oriented graphitic crystallites (containing sp² hybridized carbon) and of amorphous domains (containing sp² and sp³ hybridized carbon) of varying size, domain size, crystallinity, defect density, etc. Covalently linked together, they form indivisible primary aggregates of various sizes. Less cohesive secondary aggregates are finally formed by Van der Waals interactions between primary aggregates. Adapted with permission from Refs. [28–31]. Copyright 2022, Wiley‐VCH.

Figure 3

Van Krevelen diagram for various solid fuels. Data extracted from reference. By plotting the hydrogen:carbon atomic ratio as a function of the oxygen:carbon atomic ratio, the  origin and maturity of different carbon based materials can be assessed. It is a common technique to classify a natural material into a specific group for potential applications e.g their heating value for power plants. Energy density values increase following the same arrow that the increased temperature treatment on the diagram.

Figure 4

Use of combustion reactions over the centuries. Combustion reaction is used in pottery, heat generation, as well as fuel for different types of engines. It is truly one of the central technologies of mankind that is used from ancient times until today.^[55–57] (Credit for ceramics picture: Brigitte Pénicaud).

Figure 5

SEM (a), AFM (b) and STM (c) images of HOPG surfaces after partial oxidation. Value for the scale bar (a and c) is 1 μm. Carbon combustion occurs not randomly on the carbon basal plane but is occurring on specific sites (also called active surface sites). Typically, the reaction occurs in proximity to the initial active sites due to progressive formation of reactive sites in the course of the carbon combustion (see also chapter 6 for a more detailed discussion). Adapted with permission from Ref. [20, 23, 24]. Copyright 2022, Wiley‐VCH.

Figure 6

Catalytic effect of inorganic component on carbon combustion. A large variety of different elements, especially transition metals atoms, are capable of catalyzing combustion reaction with oxygen but also with water or CO₂ which is of crucial importance when discussing kinetics or reaction mechanism. Adapted with permission from Ref. [167]. Copyright 2022, Wiley‐VCH.

Figure 7

Ellingham diagram (standard Gibbs free energy versus the temperature, for the oxidation/reduction) of few species (P=1 atm). The predominance domains of C (orange), CO (green) and CO₂ (blue) are represented. The red legend reports the equilibrium oxygen pressure. It also corresponds to the minimal oxygen pressure needed to oxidize an element at a given temperature (dry corrosion pressure). To read it, a line in between the diagram origin point (O in red) and the point of interest (element / temperature) should be reported on the scale. Taking the black C point on the ΔG⁰ as origin, the same method should be used to read the CO/CO₂ ratio from the corresponding scale. Adapted with permission from Ref. [175]. Copyright 2022, Wiley‐VCH.

Figure 8

Schematic progress from a molecular, over a directed molecular flow to a macroscopic understanding of a bulk gas phase and a boundary layer. a) impenetrable barrier b) penetrable, with a very large chemical resistance c) the barrier is penetrable, out of equilibrium, some random passing of the barrier depending on the relative energy d) equilibrium situation, where the situation can be described by Fick's laws and the flow experiences a chemical resistance by the barrier.

Figure 9

Single film model containing a stationary boundary layer and a bulk gas flow. In the flow the concentration of the educt and the product is constant, whereas in the boundary layer a concentration gradient for both the educt and the products can be found. This is dependent on the flow and the temperature and constitutes a physical limitation (see chapter 6 for a complementary discussion).

Figure 10

Scheme of the carbon combustion reaction (1) diffusion of oxygen to the carbon surface through the boundary layer (including pore diffusion, see chapter above). (2) Adsorption of oxygen atoms onto the surface (see chapter above) (3) Splitting of dioxygen and successive reaction steps forming the primary product CO or CO₂ (discussed in this chapter) (4) Desorption of primary products from the surface (see chapter above) (5) Diffusion through the boundary layer of CO/CO₂ (see chapter above).

Figure 11

Kinetic and diffusive regimes by Walker. In the lower part the oxygen concentration in the bulk gas phase and the boundary layer (external and internal) is depicted. Only in the kinetic regime, the concentration of oxygen is identical in all regions and only there activation energies can be obtained. In all other regions only apparent activation energies can be measured as here physical limitation such as diffusion are rate limiting. Adapted with permission from Ref. [122]. Copyright 2022, Wiley‐VCH.

Figure 12

Schematic representation of the main chemical features in a graphene sheet, with its typical surface functionalities, including free edge sites. The pairing of σ (*) and π (•) electrons at the zigzag sites and the presence of triple bonds at the armchair sites is indicated. The abundance of aromatic sextets and the degree of π electron delocalization depends on size, shape, and connectivity of the graphenes, as well as on their edge termination. Adapted with permission from Ref. [209]. Copyright 2022, Wiley‐VCH.

Figure 13

a),b),c) Optimized geometries of carbon clusters representing the indirect path to CO₂ formation involving different transition states d) mechanism for O‐hopping (surface diffusion) on the graphene basal of the oxygen surface complex e,f) direct versus indirect mechanism and thermochemistry of CO₂ desorption. The grey spheres represent the carbon atoms, red spheres the oxygens and the white, hydrogens. The ▵E values shown are in kcal/mol. The lactone structure is favored. Adapted with permission from Ref. [227]. Copyright 2022, Wiley‐VCH.

Figure 14

SIS program developed for the analysis of different carbon (nano) materials, allowing a kinetic analysis with one TGA experiment. Adapted with permission from Ref. [9]. Copyright 2022, Wiley‐VCH.