1 Chemical Kinetics and Fire
12
Gregory T. Linteris and John F. Griffiths
Introduction
2 The purpose of this chapter is to set out the principles of
3 chemical kinetics as they apply to combustion in flames and
4 fires. Chemical equilibrium, which was discussed in a previ-
5 ous chapter, deals with the final preferred state of a given set
6 of reactants after an infinite time has passed. In contrast,
7 chemical kinetics deals with the rate at which the system
8 proceeds to the equilibrium state, i.e., the specific
9 participating chemical reactions and their rates. Chemical
10 equilibrium and chemical kinetics are related in that the
11 thermodynamic, equilibrium state provides the driving force
12 for chemical reaction. The material in this chapter is covered
13 briefly; more detailed descriptions can be found in chemistry
14 [1] and combustion [2–4] text books, from which much of the
15 material was drawn.
16 The foundations of chemical kinetics have validity in gas,
17 liquid or solid phases, but for fires, the gas phase has the
18 greatest relevance because the main heat release normally
19 occurs during flaming combustion. The role of solid- and
20 liquid-phase chemical kinetics in fires is discussed in
21 Chap. 27. Similarly, smoldering combustion is a surface
22 combustion process, and the chemical kinetic description is
23 closely related to that of pyrolyzing materials. The
24 specialized fields of propellants, explosives, and material
25 synthesis also require solid-phase chemical kinetic
26 descriptions, but these are beyond the scope of the present
27 chapter. Nonetheless, many of the fundamental principles of
28 chemical kinetics discussed here are relevant regardless of
29 the phase of the reacting system.
30 Gas-phase chemical kinetics is of interest in fires for many
31 reasons. The heat release in a fire typically occurs in the gas
32phase, and is responsible for the gas-phase temperature field,
33and hence the heat flux to the burning materials (a feedback
34loop which controls the fuel supply rate in the fire, and hence
35the geometric growth in fire size with time). Some fundamen-
36tal fire properties, such as ignition and extinction, are clearly
37controlled by the gas-phase chemical kinetics. Fire suppres-
38sion is controlled by the rates of chemical reaction, both for
39the relatively inert agents (e.g., CO
2
, water) which reduce the
40temperature (and hence overall react ion rate) to the point of
41extinction, and for chemically acting agents (e.g., CF
3
Br and
42hydrofluorocarbons) which interfere with the normal chemis-
43try of the fuels with air. Similarly, the action of the most
44commonly used fire retardants in polymers is controlled by
45their gas-phase chemical behavior. In general, the chemical
46reaction rate must be fast enough to match the local residence
47time for transport (either convective or diffusive); if it is not,
48the flame will extinguish.
49The formation of soot, the major radiating species from
50fire plumes, is controlled by gas-phase chemical kinetics, as
51is the formation of CO, which is the major toxic compound
52responsible for fire deaths. In fires, the formation of other
53toxic compounds, for example of HCN, as well as environ-
54mental pollutants (polycyclic aromatic hydrocarbons,
55dioxins, etc.) is controlled by the chemical kinetics of
56reactions occurring in the gas-phase. Clearly, understanding
57chemical kinetics is central to controlling unwanted fires and
58their deleterious effects. It is of great value if the Fire Scien-
59tist can answer the question: “Is the process at hand con-
60trolled by the rate of chemical reactions or by some other
61physical process?” The goal of the present chapter is to
62provide some fundamental materials for approaching such a
63question.
64The reaction of a fuel (for example methane) with air to
65products can be represented by an expression such as:
CH
4
þ 2O
2
→ CO
2
þ H
2
O ð12:1Þ
G. T. Linteris (✉)
National Institute of Standards and Technology, Gaithersburg, MD,
USA
J. F. Griffiths
University of Leeds, School of Chemistry, Leeds, UK
#
The Society of Fire Protection Engineers 2025
Society of Fire Protection Engineers (ed.), SFPE Handbook of Fire Protection Engineering, The Society of Fire Protection
Engineers Series, https://doi.org/10.1007/978-3-031-59212-6_12
269
