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Silica particle formation and deposition in gas-fired appliances

Visser, P. (2012) Silica particle formation and deposition in gas-fired appliances. Master's Thesis / Essay, Chemistry.

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A growing trend in the Netherlands is the injection of biogases into the natural gas grid for transportation and distribution to end-users (industrial, commercial and residential). Biogases can contain (trace) compounds, like for example siloxanes, which may have adverse effects on the integrity of the gas infrastructure, the safety and performance of gas utilization equipment and even the health of end users. The harmful aspect of siloxanes is that they are converted to silicon dioxide (silica, SiO¬2) particles upon combustion, which deposit as a layer on relatively cold parts of gas utilization equipment. The impact of the presence of siloxanes in biogases on the performance of domestic end-user equipment is discussed in this thesis. In chapter 3, transmission electron microscopy (TEM) measurements and theoretical analysis are combined to construct the physical picture of silica particle formation in premixed laminar methane/air flames. In the reaction zone of the flame siloxanes are quickly converted to a supersaturated SiO2 (g) vapor. Nucleation of silica causes nanoclusters to appear in the early stage of combustion. These nanoclusters will continue to grow into larger clusters via Ostwald ripening, sintering processes (after collision between (nano)clusters) and by taking up free SiO2 (g) molecules. Further downstream, after cooling of the combustion products clusters combine to form fractal aggregates. The measured sizes of clusters and fractal aggregates are in accordance with those calculated with theoretical models. The presented models are divided in two flame regions: one where cluster growth is the dominant process and one where fractal aggregate growth is the dominant process. The position of the dividing line between these regions can be determined experimentally and depends on the temperature of the flame and the concentration of silica particles. An increased temperature favors the cluster growth process and an increased concentration favors the fractal aggregate growth process. In chapter 4, practical tests were performed with domestic appliances (boiler and geyser) operating on siloxane containing natural gas to study silica deposition. In a widely used boiler in the Netherlands clogging of the heat exchanger by silica deposition (>90% yield of deposition) resulted in an increased flow resistance. This flow resistance caused a significant reduction of the thermal output of the appliance. Experiments with different siloxane concentrations yielded that the flow resistance scaled non-linearly with the siloxane concentration in natural gas. This indicates that the density of the silica layer changes with the siloxane concentration. The density influences the layer thickness, which in turn influences the flow resistance. At higher siloxane concentrations the silica layer would be thicker and at lower concentrations the silica layer would be thinner due to the density. This difference may be caused by the morphology or the size of the silica particles in the heat exchanger. Siloxane admixture caused the combined ionization and ignition probe of the boiler appliance to be covered with silica. This silica layer decreased the measured ionization current. After some time the critical value for the ionization current was reached and the boiler automatically turned off. The time till failure did not scale linearly with concentration. This non-linearity may be caused by the density of the silica layer or by an effect of siloxanes (or a combustion product of siloxanes) on the ions in the flame. Silica deposition in the heat exchanger of a domestic gas geyser also resulted in clogging of the heat exchanger by silica deposition. Here, the increased flow resistance caused the CO emissions to increase exponentially. In chapter 5, a simplified model is presented to describe silica deposition in heat exchangers. Several trends observed in experiments were in accordance with the deposition model. For example, the position of silica deposition and the yield of deposition could roughly be determined. In order to make a quantitative analysis of the silica deposition a more detailed model should be developed. With this model, the maximum allowable silicon content in biogases can be determined by extrapolating the results from the practical appliances to low concentrations. This avoids the need to perform time consuming long-term tests at these concentrations.

Item Type: Thesis (Master's Thesis / Essay)
Degree programme: Chemistry
Thesis type: Master's Thesis / Essay
Language: English
Date Deposited: 15 Feb 2018 07:51
Last Modified: 15 Feb 2018 07:51

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