Thermal measuring and testing – Calorimetry – Heat value of combustion
Reexamination Certificate
2000-12-22
2002-10-15
Gutierrez, Diego (Department: 2859)
Thermal measuring and testing
Calorimetry
Heat value of combustion
C374S045000, C374S014000, C422S051000, C436S157000, C436S160000
Reexamination Certificate
active
06464391
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates generally to calorimeters, and more particularly to calorimeters used to measure heat release rates of small samples. Such calorimeters are useful in determining flammability parameters of milligram samples of combustible materials.
BACKGROUND OF THE INVENTION
The rate at which heat is released during the burning of a combustible solid in a fire is the primary indicator of its hazard to life and property. Consequently, a method to study the heat release rate of solids in a fire or under fire conditions is of theoretical and practical importance to fire protection engineers and materials scientists. Fire calorimeters are used to measure the rate of heat released in flaming combustion, often with simultaneous measurement of the fuel generation (mass loss) rate of the solid. Since the combustion reactions in the flame are orders of magnitude faster than the fuel generation rate of the solid and the transit time of the gaseous fuel between the burning surface and the flame where combustion occurs is less than one second, the slow step (that is, the step limiting the burning rate in flaming combustion of solids) is the fuel generation rate. This means that the heat released in flaming combustion is essentially simultaneous with, and proportional to, the mass loss of the solid, and that the heat release rate in a fire is simultaneous with, and proportional to, the mass loss rate at the surface of the material.
Fire calorimeters measure the heat release rate in flaming combustion directly but have drawbacks for materials research or quality control testing. These drawbacks include: 1) the heating rate in the sample varies with location; 2) the temperature of the sample is non-uniform because of thickness and edge effects; 3) the amount of oxygen diffusing into the flame is usually less than is required to effect complete combustion of the fuel gases, so that measured heat release rate depends on the test environment; 4) the measured heat release rate depends on the sample thickness, orientation, and method of holding the sample, (i.e., on the test method); 5) fire calorimeters give qualitative (about 15% error) rather than quantitative (about 5% error, or less) heat release rate data because of the poor repeatability of the test as a result of drawbacks 1) through 4) above; 6) samples must be large (on the order of about 100 grams) to support steady flaming combustion: and 7) replicate samples are needed because of the poor repeatability of fire calorimetry tests. Thermoformed kilogram samples are impractical for research where initial synthesis of new materials typically yields one (1) gram of material. Gram quantities of sample can be used for ease of ignition (flammability) tests utilizing a Bunsen burner but only pass/fail results are obtainable and reproducibility is poor.
Because flaming combustion requires large samples and the thermal history and combustion environment vary from sample to sample, fire calorimetry is not the method of choice for measuring the fire performance properties of materials. Likewise, ignitability tests provide only relative rankings but no properties that can be related to fire performance. A number of thermoanalytical methods have been developed which use thermal decomposition of milligram-sized samples and analysis of the evolved gases to measure the heat released under controlled (laboratory) conditions. Of those known laboratory thermoanalytical methods which have been used to measure the non-flaming heat of combustion of the sample gases under simulated fire conditions, all measure the total heat of combustion of the sample pyrolysis (fuel) gases. However, only the methods that measure the mass loss rate of the sample can determine heat release rate of an individual material particle (specific heat release rate) as it occurs at a burning surface in a fire. The heat release rate in a fire during steady flaming combustion is equal to the specific mass loss rate (rate at which the solid particle decomposes into fuel which can enter the gas phase/flame) multiplied by the thickness of the surface burning layer (number of solid particles involved), the heat of combustion of the particles (heat released per particle by complete combustion), and the efficiency of the combustion process in the flame (fraction of solid particles which enter the gas phase and are completely combusted). Because the rate of mass loss at the burning surface is a relatively slow process in comparison to the gas phase combustion reactions, the heat release in a fire is coincident in time with, and proportional to, the mass loss (fuel generation) rate of the sample. Consequently, unless the evolved gas measurement is synchronized with the sample mass loss in a laboratory test, the heat release rate as it occurs in a fire cannot be measured. One approach to obtain the rate of heat released by the sample under fire conditions is to measure mass loss (fuel generation) rate and heat of combustion of the fuel gases separately and then multiply them together.
Lyon and Walters have invented and patented a microscale combustion calorimeter that measures flammability parameters of milligram samples of combustible materials. U.S. Pat. No. 5,981,290. In order to obtain results consistent with other techniques, the invention requires the simultaneous measurements of the mass loss rate of the sample, and the amount of oxygen consumed by combustion of the fuel gases given off by the sample. The mass loss rate is measured by using a thermogravimetric analyzer (TGA), while the amount of oxygen consumed is measured using a mass flow meter and oxygen analyzer downstream from the combustor.
Errors of more than 50% result when the heat release rate of the sample is determined solely from the oxygen consumption rate without a mass loss rate measurement. These errors arise principally from two sources: (1) distortion of the heat release rate curve and (2) reduced area under the heat release rate curve. Each of these sources of error will be discussed in turn.
DISTORTION OF THE HEAT RELEASE RATE CURVE: The mass loss (fuel generation) event for a rapidly heated small sample of combustible material occurs over a narrow time interval. The mass loss/fuel generation rate versus time curve has the form of a narrow peak or “fuel pulse”. Multiple fuel pulses are observed for multi-component materials. The shape of the fuel pulse is unique to a particular material or component. This fuel pulse becomes spread out, broadened, or “smeared” if it is generated in a large volume (e.g., the pyrolysis chambers of commercial thermogravimetric analyzers) where the fuel gases can be intermingled, mixed, and diluted with the purge gas before exiting to the combustion chamber or furnace. This same intermingling, mixing, and dilution occurs anywhere in the flow calorimeter where a large volume is introduced (e.g., the scrubbers). The large volume of the TGA has the effect of distorting the shape of the fuel pulse prior to its reaction with oxygen in the combustor or furnace. Large volumes further downstream distort the shape of the combustion gases before the oxygen content of the combustion products can be measured. Consequently, the oxygen consumption history measured at the downstream oxygen detector has been distorted by the instrument and is not synchronized with the fuel pulse of the material. In particular, instrumental broadening or smearing reduces the height and increases the width (duration) of the fuel pulse as deduced from oxygen consumption.
Of particular interest to fire scientists is the peak specific heat release rate of the sample (W/g). The peak specific heat release rate of a material is a quantitative measure of its fire hazard and is obtained by multiplying the height of the fuel pulse (maximum fuel generation rate, g/g−s) by the instantaneous heat of combustion of the fuel (J/g) at peak mass loss rate. The average heat of combustion of the fuel is proportional to the area under the oxygen consumption curve and is unaffected by instrumental br
Drew James J.
Gutierrez Diego
The United States of America as represented by the Secretary of
Verbitsky Gail
Wildensteiner Otto M.
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