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Milligram-Scale Flame Calorimetry

Fernando Raffan-Montoya and Catherine Hamel

Sponsor: 
BASF

Flame retardants (FRs) reduce the threats of fires through mechanisms that inhibit or prevent flame spread and development. Among these, brominated flame retardants (BFRs) are a widespread option for textiles, electronics, plastics and other materials. Unfortunately, some BFRs have been related to detrimental health and environmental effects. As a result, alternative FRs are currently being considered by industry. Researchers continue to study FRs in order to understand the mechanisms by which they alter the combustion process, in hopes of choosing a suitable replacement to BFRs. Unfortunately, screening replacement candidates can be costly due to the sample size requirements of currently available testing techniques.

To address these issues, our group has developed the Milligram-scale Flame Calorimeter (MFC), a novel apparatus that can measure the heat release rate (HRR) and heat of combustion (HOC) of milligram-sized pyrolyzable solid samples using the principles of oxygen-consumption calorimetry.  In this apparatus, samples of mass 30-35 mg are pyrolyzed under carefully characterized heating conditions in an anaerobic environment.  The gaseous pyrolysis products are subsequently combusted in a near axisymmetric laminar diffusion flame, effectively uncoupling the condensed phase from the gas phase but maintaining combustion conditions similar to those encountered in real fires.

 

Figure 1. MFC Schematic.

 

Figure 2. Pyroprobe Configuration.

 

As shown in Figure 1, the MFC is composed of multiple sub-assemblies: a control panel, a combustor base, a pyrolyzer, a cylindrical combustion chamber and an exhaust/gas analyzer. The control panel allows the user to prescribe flow rates for co-flow gases and purge gases, and even gaseous fuels such as propane. This feature of the MFC allows for flexible combustion zone environments tailored to the requirements of each user. The prescribed flow enters the combustion chamber, while the solid sample is heated by a platinum coil at a prescribed heating rate. The purge gas carries volatiles out of the sample tube to meet a 50 W resistively heated coil, which ignites the pyrolyzed fuel and a laminar flame is established.

Throughout the test, the exhaust is analyzed through the use of a flow meter and oxygen sensor, and data is later processed using oxygen-consumption calorimetry principles.


Figure 3. Heat Release Rate and Sample Temperature Histories Obtained for Polystyrene-based Materials with and without Brominated Flame Retardants.

 


Figure 4. Image Pair of Polystyrene Flame at Wavelengths 650 nm (Left) and 900 nm (Right) used for Non-intrusive Temperature Measurements.

 


Figure 5. Intensity Profiles of a Polystyrene Flame at Two Wavelengths.

Unlike cone calorimeter (a traditional bench-scale oxygen-consumption-based flammability test), the MFC’s design allows the user to essentially isolate the gas phase and analyze gas phase effects of a combustible. The ability to trap all solid combustion particles, in addition to any char produced during pyrolysis, gives the user a better understanding of condensed and gas phase effects.

One significant benefit of the MFC is that it allows for testing multiple BFR-replacement candidates in a cost-effective, qualitative, and quantitative manner. As noted, the traditional setup of the MFC allows for the HRR and HOC to be obtained through oxygen-consumption methods. Because the MFC uses very small samples, it can be used as cost-effective screening tool to compare one FR to another. For instance, the effectiveness of alternative FR additives, such as other halogens, can be compared to better-characterized brominated flame retardants in the MFC. The respective char yields, soot yields, HRR, HOC, and qualitative flame behavior can be compared to quantify the FRs’ effectiveness, before larger samples are created for further testing.

Because the flames produced in the MFC experiment are optically thin, an additional application of the MFC burner allows for the obtainment of flame temperature through non-intrusive methods using photography and ratio pyrometry. Two cameras, each modified to receive light at a particular wavelength, capture two photos of the same flame at an instant in time. Using Planck’s law, a physical law that relates an emitting body’s temperature to the electromagnetic radiation produced by the body, the ratio of the intensities captured by the two cameras can then be used to compute temperature of the flame sheet. This method is preferable to inserting a thermocouple into the flame, as doing so may disrupt flame behavior.

Overall, the MFC is a cost-effective, versatile apparatus that allows users to conduct a wide-range of tests in order to better understand behavior of flames fueled by solid samples. The design of the MFC gives the user the unique ability to isolate the gas phase for analyzing interaction between fuels and flame retardants, as well as the ability to conduct non-intrusive flame measurements using optical methods.


Fernando

Fernando Raffan-Montoya is a Doctorate student in the Department of Aerospace Engineering and the Department of Fire Protection Engineering of the A. James Clark School of Engineering. He obtained his Bachelor of Science degree in Aerospace Engineering from the Florida Institute of Technology in 2005. His current research is in the field of gas-phase flame retardants for polymer systems, focusing on the characterization of brominated flame retardants. Other interests include experimental design and the use of minimally intrusive diagnostics in combustion and heat transfer applications.

For further information about his work, he can be contacted at: fraffan@umd.edu.

Catherine Hamel is a Graduate student in the Fire Protection Engineering Department at the Clark School of Engineering. She received her Bachelor of Science degree from the Department of Fire Protection Engineering at the University of Maryland. Her research is in the area of material flammability with a focus on non-intrusive methods for flame measurements and combustion diagnostics.