Using heats of oxidation to evaluate flammability hazards

The heat of oxidation is the net heat of combustion per mole of oxygen consumed. For complete combustion it is given by “ΔH C /S” (kcal/mol oxygen), where “ΔH C ” is the net heat of combustion (kcal/mol fuel) and “S” is the stoichiometric ratio of oxygen to fuel as written in the stoichiometric equation. Although previously unrecognized, the heat of oxidation “ΔH C /S” is the single most powerful parameter for evaluating the flammability hazards of fuels. This article derives and discusses eight simple rules for estimating flammability parameters of single or mixed organic fuels in air under atmospheric conditions. The rules apply to fuels containing combinations of C+H+O+N atoms that burn to carbon dioxide plus water. Apart from the trivial First Rule, each rule is expressed in terms of the heat of oxidation. Two Rules are presented for lower flammable limit (LFL) estimation. The First Rule is approximate and requires only the stoichiometric ratio of oxygen to fuel “S.” Although it outperforms Lloyd's Rule, it similarly overestimates the LFLs of energetic fuels such as acetylene and ethylene oxide. The Second Rule requires both “S” and the net heat of combustion “ΔH C .” It is shown that the LFL is inversely proportional to the product of heat of combustion and heat of oxidation. LFL predictions are typically within experimental error of reported values, including energetic fuels such as acetylene and ethylene oxide. The Third Rule shows that lower limit flame temperatures decrease linearly with increased heat of oxidation and increase stepwise between hydrocarbons and other organic fuels containing oxygen and nitrogen atoms. The Fourth Rule shows that the Limiting Oxygen Concentrations (LOCs) of non‐decomposable fuels decrease with the inverse square of heat of oxidation. It is proposed that the optimum fuel concentration at the LOC is related via diffusivity ratio to the stoichiometric concentration. A fuel‐oxygen Cartesian flammability diagram is suggested for presenting LOC data. The Fifth Rule shows that maximum flame temperatures increase linearly with increased heat of oxidation and decrease stepwise between hydrocarbons and other organic fuels containing oxygen and nitrogen atoms. The Sixth and Seventh Rules address fundamental burning velocity “S u ” and lowest minimum ignition energy (LMIE), second order polynomial expressions being presented for easily calculating these parameters. It is shown how the calculated S u can be used to estimate normalized rate of pressure rise “K G ” data. It is also demonstrated that many published LMIE and quenching distance values are too high. The Eighth Rule gives a tentative expression for quenching distance, which decreases linearly with increased heat of oxidation. A scheme is presented for ranking fuel hazards in terms of heat of oxidation. Demarcations are made in terms of S u , LMIE and Class I NEC and Zone Groups. It is shown how the approach might be extended to deflagration and detonation arrester selection.