10th International Aerosol Conference September 2 - September 7, 2018 America's Center Convention Complex St. Louis, Missouri, USA
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Water Sorption Phenomenon on Aerosols Emitted during a Fire: Determination of the influencing Physico-Chemical Parameters
LAURA LINTIS, Alexis Coppalle, François-Xavier Ouf, Cécile Vallières, Université de Lorraine/IRSN
Abstract Number: 1604 Working Group: Combustion
Abstract Fire constitutes a hazard in a nuclear facility. It can lead to the clogging of HEPA (High Efficiency Particulate Air) filters by soots which are aggregates of carbonaceous primary nanoparticles. The presence of water leads to a higher filter airflow resistance. The water condensation on particles, initiated by an adsorption process, must be taken into account in order to better estimate the clogging process. This study consists in determining the most influencing physico-chemical parameters of soots on the adsorption phenomenon.
Combustion of isolated fuels heptane, PMMA and hydraulic oil has been carried out at analytical scale in a controlled atmosphere calorimeter cone. Fire tests were performed at two dioxygen concentrations: 21% and limit dioxygen concentrations before extinction in order to mimic the vitiated surrounding atmosphere occurring during a fire in a confined local. Online analysis including a SMPS (Scanning Mobility Particle Sizer) and ex situ physico-chemical analysis were carried out to determine the morphology, densities, specific surfaces and chemical surface compositions.
The CMD (Count Median Diameter) measured at 21% of dioxygen concentration presents values between 160 and 200 nm. A CMD diminution of 10% up to 50% depending on the fuel was observed for the minimal dioxygen concentrations. This aggregates CMD obviously influenced by dioxygen concentration is a parameter controlling the soot cake porosity during clogging processes. Transmission electronic microscopy pictures analysis lead to values of primary particles diameters of 30 and 36 nm for heptane and PMMA soots respectively. Unlike the aggregates CMD, these primary particle diameters do not change significantly with the dioxygen concentration. The elemental analysis leads to almost identical carbon amounts of about 90% at 21% of dioxygen concentration and show a diminution of 2% at the minimal dioxygen concentration. Organic carbon amounts are systematically higher for soots produced in a vitiated atmosphere. Heptane shows the most significant increase from 1% to 18%.
Water uptake has been measured at a constant temperature of 25°C for different relative humidity using a Dynamic Vapour Sorption Vacuum microbalance. As a preliminary step, water sorption measurements have been performed for two reference black carbons with primary particle diameters of 25 and 95 nm. Water uptakes of 10% for the lowest diameter and 1% for the highest highlighted a clear influence of this primary particle diameter on water adsorption. Identical water adsorption isotherms were obtained for samples compacted in pellet of 60% apparent porosity and left at looser powder state characterized by porosities close to 98%. It excluded the apparent porosity and indirectly the CMD as influencing parameters on sorption process. Soots present similar water uptakes of 3% at 90% of relative humidity. The isotherm shapes are characteristic of Vth type isotherm (Rouquerol et al., 2014), reflecting a multilayer adsorption mechanism on hydrophobic adsorbents. However, water uptakes obtained at minimal dioxygen concentrations are systematically 0,5% higher than those obtained at ambient concentration. Those discrepancies can be explained by higher organic carbon amounts and lower carbon element amounts with the dioxygen concentration decrease previously indicated. The primary particle diameter was brought out as major influencing parameter on water sorption phenomenon occurring on soots.
For comparison, water isotherms of IIth type were obtained with realistic fire soots. They were collected on a HEPA filter during a large scale fire of a gloves box essentially composed of PMMA walls. The concave shape at low humidities is characteristic of high interactions with water. The water uptake increase up to 6% at a relative humidity of 90% reveals the beginning of the capillary condensation stage. Realistic soots have been found having a hydrophilic behavior compared to the soots produced at analytical scale.