Estimation of Rate of Heat Release by Means of Oxygen Consumption Measurements

Intuitively, the rate of heat release from an unwanted fire is a major indication of the threat of the fire to life and property. This is indeed true, and a reliable measurement of a fire’s heat release rate was a goal of fire researchers at NBS and other fire laboratories at least as early as the 1960s. Historically, heat release measurements of burning materials were based on the temperature rise of ambient air as it passed over the burning object. Because the fraction of heat released by radiant emission varies with the type of material being burned, and because not all the radiant energy contributes to temperature rise of the air, there were large errors in the measurements. Attempts to account for the heat that was not captured by the air required siting numerous thermal sensors about the fire to intercept and detect the additional heat. This approach proved to be tedious, expensive, and susceptible to large errors, particularly when the burning “object” was large, such as a full-sized room filled with flammable furnishings and surface finishes. A novel alternative technique for determining heat release rate was developed at NBS during the 1970s. It had distinct advantages over the customary approach, but its widespread acceptance was hampered by uneasiness in the fire science community concerning potential errors if the technique were used in less-than-ideal circumstances. In 1980 Clayton Huggett, a fire scientist at NBS, published the seminal paper [1] that convinced the fire science community that the new technique was scientifically sound and sufficiently accurate for fire research and testing. The technique is now used worldwide and forms the basis for several national and international standards. The underlying principle of the new heat release rate technique was “discovered” in the early 1970s. Faced with the challenge of measuring the heat release of combustible wall linings during full-scale room fire tests, William Parker, Huggett’s colleague at NBS, investigated an alternative approach based on a simple fact of physics: in addition to the release of heat, the combustion process consumes oxygen. As part of his work on the ASTM E 84 tunnel test, Parker [2] explored the possibility of using a measurement of the reduction of oxygen in fire exhaust gases as an indicator of the amount of heat released by the burning test specimens. Indeed, for well-defined materials with known chemical composition, heat release and oxygen consumption can both be calculated from thermodynamic data. The problem with applying this approach to fires is that in most cases the chemical compositions of modern materials/ composites/mixes that are likely to be involved in real fires are not known. In the process of examining data for complete combustion (combustion under stoichiometric or excess air conditions) of the polymeric materials with which he was working, Parker found that, although the heat released per unit mass of material consumed (i.e., the specific heat of combustion), varied greatly, the amount of heat released per unit volume of oxygen consumed was fairly constant, i.e., within 15 % of the value for methane, 16.4 MJ/m of oxygen consumed. This fortunate circumstance—that the heat release rate per unit volume of oxygen consumed is approximately the same for a range of materials used to construct buildings and furnishings—meant that the heat release rate of materials commonly found in fires could be estimated by capturing all of the products of combustion in an exhaust hood and measuring the flow rate of oxygen in that exhaust flow. The technique was dubbed oxygen consumption calorimetry, notwithstanding the absence of any actual calorimetric (heat) measurements. Later in the decade, Huggett [1] performed a detailed analysis of the critical assumption of constant proportionality of oxygen consumption to heat release. Parker’s assumption was based on enthalpy calculations for the complete combustion of chemical compounds to carbon dioxide, water, and other fully oxidized compounds. Indeed, a literature review by Huggett revealed that Parker’s findings were actually a rediscovery and extension of the work of W. M. Thornton [3], published in 1917, which found that the heat released per unit amount of oxygen consumed during the complete combustion of a large number of organic gases and liquids was fairly constant. Nevertheless, since in real fires and fire experiments the oxygen supply is sometimes limited, incomplete combustion and partially oxidized products can be produced. Huggett’s paper examined in detail the assumption of constant heat release per amount of oxygen consumed under real fire conditions and assessed its effect on the accuracy of heat release rate determinations for fires. Instead of expressing results on a unit volume basis, as Parker did, Huggett expressed results in the more convenient and less ambiguous unit mass of oxygen