Tar fouling during heat recovery in solid fuel gasification

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Examensarbete för masterexamen
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2020
Författare
Calvo Bascuñán, Javier
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Sammanfattning
Solid fuel gasification is the thermal conversion of carbon-based materials into gases. Depending on the gasification conditions the main share of products will be components that are in gas phase at ambient conditions. A minor share of the products will be either solid, derived from inorganics and char, or liquids, usually called tars described as hydrocarbon molecules with a molecular weight higher than benzene. Fouling, the unwanted accumulation of material on surfaces, is greatly enhanced by tars. This issue has been a recurring problem in gasification technologies for many years. Because of a growing interest in a circular economy and sustainable alternatives, gasification is an interesting technology expected to develop in the future. Gasification is a thermally intensive process, so energy recovery in the process should be as high as possible. The main source of recoverable energy is the raw gas at high temperatures, which can be used for heating. However, recovering energy from the raw gas can provoke tars to condense and increase fouling. There is a trade-off between energy recovery, to increase efficiency and maintenance with a decrease in availability because of tar fouling. The condensation of tars, required for fouling, was investigated. A method to analyse the dew point based on tar analysis was implemented. It consisted of raw gas extraction at decreasing temperatures, first high enough to avoid any tar condensation. Condensed tars were capture and the remaining total tar concentration for different temperatures was analysed to determine the amount of tars condensing at different temperatures. The experiments were implemented in a lab reactor using gasification conditions from references to mimic real conditions. The results validated the concept for the method, but further experiments would be required to collect more data, reducing errors and improving the accuracy. Tar fouling can occur by direct condensation over surfaces or condensation as aerosols that stick to walls. Condensation alone does not guarantee to foul, aerosols contained in the raw gas could travel with the flow without sticking to surfaces. A review of aerosols and the influence of different flow conditions was made to investigate the fouling process. Even though heat transfer is enhanced by turbulence, it also might enhance fouling from tar aerosols. An experimental set up to compare fouling in laminar and turbulent flows was presented for future research.
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Gasification, Tars, Heat recovery, Fouling, Condensation, SPA analysis
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