Packed bed combustors burn fairly large solid fuel particles within confining walls, with air supplied from below the grate. As combustion occurs and particles are consumed, fresh particles are fed onto the bed so the level is kept roughly constant. Packed bed combustion is used for wood and biomass combustion in small-scale power plants, wood waste combustion in pulp and paper plants, and trash incineration. The structure of a packed bed is very important to the combustion process and can be defined by particle shape and size, sphericity, particle overlap (decreasing area availability) and chiefly by void fraction. Void fraction has already been proven of great influence in packed beds – it is raised to the third power in the pressure loss equation, and it can also affect heat and mass transfer and surface reaction rates.
This thesis presents results of several experimental combustion tests that were performed in a packed bed combustor, using commercial spruce lumber particles of parallelepipedal geometry as fuel. At the end of each test the bed contents were removed, taking care to preserve their structure, and fixed with liquefied wax. The solidified bed was then cut into circular cross sections at different heights of the bed, and photographs of the cross sections were taken so the local void fraction could be estimated using image analysis. The bed sampling led to the discovery that, surprisingly, the actual bulk void fraction in the combustor, which is the average of local void fraction measurements, is less than that of the unburnt particles, varying from 19% to 30% in decrease in void fraction depending on the particle type used. Local measurements allowed the development of an empirical linear equation model to represent the variation of void fraction with height above the grate. Each combustion test had measurements of gas volume fractions and temperatures at different heights above the bed grate to be compared with the results of a numerical model simulation.
The numerical model used in this work is an existing numerical model of all the relevant processes in packed bed combustion. Previously, the numerical model had assumed the void fraction to be constant and equal to that of the unburnt fuel, since no information on local variation was available, and the packing geometry remained self-similar as particles are consumed. Three models for void fraction were then compared in the combustion model: a constant void equal to that of the unburnt particles, the empirical linear fit of void fraction with height, and a constant void equal to the measured bulk void fraction. Maximum temperatures were higher using the unburnt fuel void fraction because of a thicker oxidation zone, whereas the void fraction model
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based on experiments generated a thicker reduction zone and therefore higher CO concentrations. CO concentrations were experimentally measured and agreed quite well with the CO concentration from the model. Local void fraction differences had the most impact in the diffusion-controlled zone, as shown by comparing the empirical void model and the measured bulk void fraction. How lowering the void fraction can increase gas velocities, heat and mass transfer coefficients, and
burning rates is also discussed in this work.
| Identifer | oai:union.ndltd.org:uottawa.ca/oai:ruor.uottawa.ca:10393/43550 |
| Date | 03 May 2022 |
| Creators | Lovatti Costalonga, Pedro |
| Contributors | Hallett, William |
| Publisher | Université d'Ottawa / University of Ottawa |
| Source Sets | Université d’Ottawa |
| Language | English |
| Detected Language | English |
| Type | Thesis |
| Format | application/pdf |
| Rights | Attribution 4.0 International, http://creativecommons.org/licenses/by/4.0/ |
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