Synergism of trimethylphosphate and carbon dioxide in extinguishing premixed flames

Abstract

This paper presents a method for determining the optimal (in terms of fire-suppression effectiveness) composition of synergistic binary fire suppressants consisting of a chemically active inhibitor (e.g., trimethylphosphate) and an inert diluent (CO₂). The method is based on the numerical simulation of the speed and structure of one-dimensional premixed CH₄/air and methyl methacrylate/air flames with the addition of a binary fire suppressant using reduced multistep chemical kinetic mechanisms. A decrease in the normal flame speed to 4.7–5.5 cm/s by the addition of a binary fire suppressant was used as a flame extinguishment criterion, and the minimum index of interaction of the components of the binary fire suppressant as a criterion for the maximum fire-suppression effectiveness. The proposed simulation method can help reduce the need for laborious experiments to determine effective mixtures of fire suppressants, thereby saving time and resources.

The kinetic analysis performed in this study provides an insight into the mechanism of the synergism between trimethylphosphate and carbon dioxide in flame extinguishment. The results show that at the optimal ratio of fire-suppressant agents (for a specific flame and initial conditions), the rate of H atom production in methane combustion reactions is minimal and, at the same time, the H consumption rate in inhibition reactions is maximal.

Introduction

The search for mixtures of fire retardants/suppressants that are superior in effectiveness to the overall fire-suppression effectiveness of individual components (so-called synergistic fire retardants/suppressants) is of both practical and theoretical interest. An attempt to find a synergistic effect of two flame inhibitors — heptafluoropropane (C₃F₇H) and sodium bicarbonate (NaHCO₃)—was reported in a short paper [1]. Heptafluoropropane is an alternative to Halon 1301 (CF₃Br) for rapid (<0.25 s) flame extinguishment in a confined space. The addition of sodium bicarbonate served not only to improve the flame-extinguishing capability of heptafluoropropane, but also to absorb hydrogen fluoride released during the decomposition of C₃F₇H. Calculations of the propagation speed of a methane/air flame as a function of the concentration of the added inhibitors and their mixtures in different proportions have shown that mixtures of these inhibitors cannot exceed the effectiveness of pure sodium bicarbonate. Babushok et al. [2] developed a kinetic mechanism of flame inhibition by antimony compounds. Considering that antimony trioxide mixed with bromine compounds is a synergistic fire suppressant for polymers, they suggested that this mixture would be a synergistic inhibitor for a methane/air flame. However, calculations of the flame speed using the proposed model of inhibition by antimony compounds and a model of flame inhibition by bromine compounds did not confirm this assumption, since both inhibitors acted on the flame speed independently of each other.

The fire-extinguishing concentration of binary mixtures of halogenated compounds (two bromfluoro alkenes, two fluoro ethers, and one fluoro alkane) in nitrogen were determined using the cup-burner technique [3]. The authors have proposed a method for determination of synergy in studied blends based on adiabatic flame temperature calculations and the dependence of the rate of fire suppression of chemical agents on temperature. The agreement between the experimental data and predicted values of synergism factor is good.

Although the synergism of binary mixtures of chemically active inhibitors and inert diluents in the extinguishment of cup-burner flames has been experimentally proven [[3], [4], [5], [6]], this effect should be studied from the point of view of chemical kinetics at the level of elementary reactions. This is necessary because the effect of some diluents, such as CO₂, is not limited to the physical dilution of the reaction mixture, but they also influence the process chemistry. An example of this effect is given in Ref. [7], where the structure of CH₄/O₂/N₂ and C₃H₈/O₂/N₂ flames was studied by flame-sampling molecular-beam mass spectrometry and numerical modeling. It has been found that replacing part of nitrogen by an equivalent amount of carbon dioxide leads to a change in the concentration of almost all intermediate combustion products, including atoms and radicals. In addition, the modeling of the structure of flames doped with CO₂ and hypothetical inactive f-CO₂ (which is not involved into the reactions, but which has the same thermochemical and transport properties) has shown that the effect on the concentration of intermediates is not purely thermal, but is due, in particular, to the influence of CO₂ on the rate of the reaction CO + OH = CO₂ + H. The addition of nitrogen, another common inert diluent, also leads to changes in the combustion chemistry, although the mechanism of its effect is, apparently, thermal. According to Ref. [8], the flame speed sensitivity to the rate constant of the reaction H + O₂+ M = HO₂ + M changes sign as the dilution of a methane–oxygen flame with nitrogen increases. This is due to the fact that in more dilute flames, the further conversion of the relatively inactive HO₂ radical to hydroxyls by the reaction HO₂ + H = OH + OH has a positive effect on the flame speed. In a CH₄/air flame, this reaction occurs in the preheating flame zone and has little effect on the flame propagation speed.

It is known [9] that the effectiveness of flame inhibition by some chemically active inhibitors depends on the flame temperature. Since flame inhibition by inert diluents (physical inhibitors) mainly involves a decrease in the flame temperature, the observed synergism between a chemical inhibitor and an inert diluent is caused by an increase in the effectiveness of chemical inhibition as the flame temperature decreases due to the inert component of the binary fire suppressant [9].

To numerically describe the interaction between the components of a binary mixture, it is common to use the index of interaction F, which is calculated by the formula [6]:

F = C₁/C₁⁰ + C₂/C₂⁰,

where C_i is the extinguishing concentration of the additive of the binary mixture for the i-th component (i = 1,2) and C_i⁰ is the extinguishing concentration of each individual component in flames of mixtures of the same initial composition. For the first time, the concept of interaction index was introduced by R.S. Sheinson and coworkers [10], attempting to separate the chemical and thermophysical effects of a fire suppressant additive on a flame. Realizing that with an increase in the concentration of the additive, the chemical component will decrease, and the thermophysical component will increase, the authors presented the overall effectiveness of the additive as the sum of the physical and chemical effectiveness. The dependence of this sum on concentration has a minimum. Later, Lott [6], studying the interaction between a physical and chemical inhibitors, expressed the effectiveness of each as the ratio of the concentration of a component in the blend to the minimum extinguishing concentration of this component as the only fire suppressant. Thus, the formula for F has been obtained.

The ratio C_i/C_i⁰ is the ratio of the mole fractions of the considered component of the binary additive. If the action of each component of the fire-extinguishing mixture does not enhance the action of the other (i.e., there is no synergy), then F = 1. This was established experimentally for a binary mixture containing inert fire suppressants — CO₂ and N₂. However, if there is a positive synergistic effect, then F < 1, as was previously obtained for binary mixtures containing organophosphorus and halogenated compounds and CO₂ in extinguishing a diffusion cup-burner flame [4,5]. In some cases, the components of a binary fire suppressant interact with each other in such a way that their total efficiency is lower than the sum of the efficiencies of both components (negative synergy). Examples of such systems are following blends of CHF₃/N₂ and CHF₃/H₂O [9]. In these cases the interaction index has been shown to be greater than one (F > 1).

The effectiveness of binary flame suppressors consisting of an inert diluent and a reactive additive has already been actively studied [[3], [4], [5], [6]]. The main problem with inert gases is the need to store large volumes of gases in cylinders. The addition of a chemically active inhibitor dramatically reduces storage volume. The concentration of active flame arresters in such mixtures is rather low, which improves such characteristics of the blends as toxicity, reactivity, resistance to air and water, compared to individual flame suppressants. Organophosphate compounds are of considerable interest as effective and environmentally friendly halon replacements. Their properties as flame inhibitors and promoters have been well studied [[10], [11], [12], [13], [14], [15], [16], [17], [18]]. Most OPCs have been shown to be soluble in liquid carbon dioxide (CO₂) [19], which facilitate the development of binary fire suppressants consisting of OPCs and CO₂ for practical use. Unlike nitrogen, CO₂ can also function as a combustion agent that inhibits the forward reaction of the chain-branching, making the extinguishing effect of CO₂ stronger than that of N₂ [20].

The goal of this study was to determine the optimal component ratio in a binary fire-suppressant mixture consisting of CO₂ and TMP as OPCs representative with a minimum index of interaction F for extinguishing atmospheric-pressure methane/air and methyl methacrylate/air flames using numerical modeling based on detailed kinetic mechanisms. In addition, in this work, we carried out a detailed kinetic analysis to identify the nature of this interaction. The optimality of the composition can be determined based on various criteria including the cost of the fire suppressant, the emission of hazardous pollutants, the vapor pressure of the components, or other practical criteria. In this work, we consider the composition of a binary fire suppressant to be optimal, based on a single criterion: the maximum efficiency of mutual enhancement of the action of the components, which is determined by the minimum value of the index of interaction.