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Gas composition and temperature within a spouted bed gasifier

University of British Columbia

Internal gas composition and temperature profiles have been measured in a spouted bed coal gasifier. These, data are essential to develop a mathematical model that can be used for scale-up of the system. As a first step'towards developing a kinetic model, the treatment of Yoshida and Kunii for the fluidized bed gasifier has been adapted to the spouted- bed using the one-dimensional and the streamtube gas flow models. Two Western Canadian bituminous and sub-bituminous coals having particle size -3.36 + 1.19 mm were gasified to produce low and medium calorific value gas. The effect of different operating variables including total bed height, average bed temperature and char recycling on the internal gas composition profiles were investigated. In a typical run eighteen gas samples were collected from different levels and radial positions in the bed. In addition the average axial temperature profiles along different sections were also recorded. Both radial and axial gas composition profiles exist in a spouted bed coal gasifier. The combustion reaction takes place in a very narrow zone close to the gas inlet in the annulus and in the lower section of the spout. The temperature variation in the spout was large, and the upper section of the spout was found to be the hottest region in the bed. A hydrogen/carbon monoxide ratio of about unity has been observed throughout the bed for the system producing low calorific value gas. In this case the contribution from pyrolysis and volatile reforming reactions on the gas composition profiles appears to be significant, especially in the upper section of the bed. Gasifying char instead of coal altered the carbon monoxide and hydrogen distribution in the bed. Less hydrogen was produced. This pattern was however reversed when oxygen rather than air was used to gasify coal and was partly because of the high steam/oxygen ratio. Both the one-dimensional and the streamtube models gave relatively similar predictions of the average gas composition profiles and the overall gas composition based on kinetic expressions from the literature. However the predictions could not be matched with the experimental results. In modelling, the particle movement in the bed should be described. Pyrolysis should not be assumed instantaneous but its kinetics should be included. The streamtube model was superior to the one-dimensional model since it was able to predict radial composition profiles throughout the annulus. These profiles however were much steeper than those found in the experiments.

Text

2010-05-22

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http://hdl.handle.net/2429/24925

Chemical and Biological Engineering

University of British Columbia

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