Particle Deposition on KCl Initial Layers

Biomass is a promising replacement for coal in high-temperature processes. However, the high volatile inorganic volatile content of biomass and waste fuels leads to increased ash-related issues such as slagging, fouling, and corrosion. Through various deposition mechanisms such as thermophoresis and condensation, inorganic species form initial layers on heat exchanger tubes. This can increase the surface stickiness, consequently increasing the likelihood of larger particles adhering to heat exchangers upon impact, accelerating deposit formation. Of particular interest for initial layer formation is potassium chloride as it is enriched in many biomasses. Studies investigating the KCl initial layer formation and its sticking enhancement are scarcely found.

This thesis aimed to further investigate the formation of KCl initial layers and their influence on subsequent particle deposition, involving three experimental parts: Initial layer formation by KCl, Sulfation of the KCl initial layers, and Silica deposition. The experiments were conducted in an entrained flow reactor (EFR). The deposition surface was a temperature-controlled steel tube located downstream of the EFR, simulating a heat exchanger tube. For each experiment, the change in deposit mass was measured, and scanning electron microscope (SEM) images of the deposit layers were recorded.

Initial layer formation by KCl: A temperature-controlled deposition probe was exposed to a KCl-laden gas stream (1400 ppmv KCl). The gas temperature was 640 to 250 °C from the middle to the tip of the probe. The influence of the exposure time of the probe to the gas stream (0 – 120 min) and the probe temperature (350 and 500 °C) on the initial layer formation was investigated. The total deposit mass and the mass of the individual deposit types were measured. Additionally, SEM images of the different structures were taken. The initial layer of KCl forms two distinct morphologies: a fine powder and a coarse crystalline layer. The fine layer consists of sphere-like particles formed through homogeneous nucleation and deposited by thermophoresis, while the coarse layer features crystal-like dendrites formed through heterogeneous condensation. Lower probe temperatures increase the mass of the fine deposit but decrease the coarse deposit mass, with these differences becoming more pronounced over time. However, the total deposit does not change with the probe temperature. This indicates that the total amount of KCl available for deposition is limited, and only the formation mechanism changes with temperature. At long exposure times, the growth mechanism of the coarse deposit layer shifts from forming new crystal-like structures to expanding existing ones. Initially, elongated dendrites are generated, which grow in length and size over time. When dendrites are sufficiently close to each other, material condenses on their surfaces, closing the gap. Melt was observed on the outward-facing side of these structures, with higher exposure times causing larger molten areas due to the structures growing further outward into higher temperature zones.

Sulfation of the KCl initial layers: The KCl initial layers were exposed to an SO₂-containing gas stream (650 ppmv SO₂) for 1 h, causing the KCl to sulfate. The mass increase and thus the sulfation degree was measured. SEM images were taken to explore morphological changes and to analyze the elemental distribution in the initial layer. The outer layers are enriched in sulfur, while the inner layers are primarily composed of KCl. This suggests that both SO2 transport to the inner layers or the lower deposit temperature inside the initial layer could limit the sulfation process. The results indicate that a molten phase can enhance KCl sulfation and that sulfation of condensed KCl occurs rapidly, achieving similar conversion degrees as in the gas phase. SEM images show slight changes in the fine section after sulfation, with smooth-surfaced particles in the outermost layers, likely due to low melting eutectic formation. In the coarse section, SEM images reveal significant structural changes, with spherical particles visible alongside crystal-like structures. The inner layer resembles pure  KCl but with sulfur-rich spherical particles at the edges of the crystal-like dendrites. The outer layer's structures are largely covered with sulfur-rich spherical particles.

Silica deposition: Silica is deposited on pure and sulfated KCl initial layers. The feeding rate of solid silica particles is 0.95 g/min, with the feeding process lasting 10 min. The deposition mass was measured to examine the deposition-enhancing effect of the total initial layer and its distinct morphologies. SEM images were taken to investigate how the silica particles are incorporated into the initial layer. It was found that an initial layer on the deposition probe is necessary for silica deposition, as it initiates and enhances particle deposition. The morphology of the initial layer and the probe temperature influence particle deposition, but their effects are complex. The concept of catching potential was introduced to describe the morphology influence. Usually, the macro surface of the initial layer (assumed to be smooth) is used to calculate impact properties like the impaction angle for particles. However, the micro surface can differ significantly from the macro surface. For instance, dendrites roughen the surface, increasing the available area for particle impaction and altering properties such as the impaction angle. In general, rougher surfaces have a higher catching potential. In the coarse section, longer and more numerous dendrites increase catching potential and deposition, but this effect diminishes as dendrites grow in width over time. Higher initial layer formation temperatures lead to thicker dendrites, reducing the catching potential and thus the deposit mass. In the fine section, longer formation times increase the initial layer mass and thus subsequent particle deposition, while higher temperatures create denser layers that reduce deposition. For both fine and coarse initial layer sections, increasing the temperature during solid feeding can soften or melt the initial layer, thus smoothening it and reducing its catching potential. However, higher probe temperatures and thicker initial layers increase the deposit surface temperature, making the layer stickier. For thin layers, sticking potential is more dominant, while for thick layers, catching potential is more dominant. Sulfation lowers the melting point, increasing densification and smoothening, which reduces catching potential. Additionally, sulfation causes dendrites to lose crystallinity and form spherical particles, further reducing the catching potential. For high sulfation degrees, the melting point is increased making the sulfate layer less sticky than a pure KCl layer.

The findings of this thesis offer some careful suggestions for operating industrial facilities. The influence of surface deposit temperature on particle deposition depends on the initial layer structure. For flat, smooth surfaces, particle sticking is primarily determined by the melt fraction, and thus surface temperature, of the initial layer. In contrast, for rough, highly irregular surfaces, surface morphology is the dominant factor, with surface temperature having only a limited effect. This allows for adjusting the surface temperature to address the most critical issue (e.g., deposition, chemical reactions, corrosion) based on the initial layer structure. To optimize removability, initial layers with fast-growing thickness, highly brittle structures, and high thermal energy demand for melting should be considered.