Power Plant Fly Ash: The Link between Immersion Freezing Behavior and Physico-Chemical Particle Properties

Due to the globally increasing electric power demand and the slowly progressing development of renewable energy sources, coal combustion is still a cornerstone of the global energy supply. Despite particulate control devices being a standard in coal-fired power plants, significant amounts of Coal Fly Ash (CFA) particles in atmospherically relevant sizes are emitted because of inefficient filtering of submicron particles [4]. To date, only a few studies have investigated the potential of CFA particles to trigger heterogeneous ice nucleation in cloud droplets [7, 11, 5]. Our measurements aim at expanding the sparse dataset and improving process understanding of how physico-chemical particle properties influence the immersion freezing behavior of CFA.

Immersion freezing measurements were performed with CFA samples from four anonymous German coal-fired power plants. Three instruments were used to achieve good coverage of the temperature range between the first occurrence of heterogeneous ice nucleation and the homogeneous freezing limit. Firstly, the Leipzig Aerosol Cloud Interaction Simulator (LACIS [6]), a laminar flow tube where single, size-selected particles are immersed in airborne droplets, was used. Here, the influence of suspension time of CFA particles in water could be investigated by employing dry particle generation (aerosolization of dry sample) and wet particle generation (atomization of CFA-water suspensions). Secondly, two cold stage setups, one using microliter sized droplets (Leipzig Ice Nucleation Array; LINA [2]) and one using nanoliter sized droplets (WeIzman Supercooled Droplets Observation on Microarray setup; WISDOM [9]) were applied.

We found that CFA particles are comparable to mineral dust in their immersion freezing behavior when being dry-generated. However, a significant decrease in immersion freezing efficiency was observed during experiments with wet-generated particles in LACIS and the cold stage measurements. In order to understand the reason behind the deactivation, a series of chemical composition, morphology, and crystallography analyses (e.g., single particle mass spectrometry [1], scanning electron microscopy coupled with energy dispersive X-ray microanalysis [8], X-ray diffraction analysis) was performed with dry- and wet-generated particles. From these investigations, we conclude that anhydrous CaSO₄, which shows the same qualitative immersion freezing behavior as was observed for dry-generated CFA particles and is known to occur as a coating on submicron CFA [3], triggers heterogeneous ice nucleation at the CFA-water interface. The observed deactivation is likely caused by CaSO₄ being dissolved with time which leads to the adsorption of hydrated ions on the CFA surface and hinders direct interaction with the surrounding water molecules [10].

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