Background

Tancredi et al. [2] investigated the catalytic effect of ash on char gasification for eucalyptus wood chars. The ash content in char was of the order of 1.45% on mass basis. The reactivity of the char increases monotonically with conversion. At low and intermediate conversion, it can be attributed to the increase in surface area as gasification proceeds. At high conversion levels a steeper increase in reactivity has been observed, which cannot be explained by the development of surface area. This region of the reactivity/conversion curves can be better explained as the result of an increase in catalytic effect of the metallic constituents (mainly Na and K) present as inorganic matter in the chars. Here CO₂ was used as the gasifying agent. Activation energies determined were found to vary within a narrow range of 230–257 kJ/mol. Arrhenius plots showed parallel lines for different degrees of conversion. Parallel line of Arrhenius plot indicates similar activation energies. The increase in reactivity was mainly due to an increase in pre-exponential factor. In a similar study by Montesinos et al. [3], steam gasification and CO₂ gasification of grape fruit skin char were investigated. They also observed an increase in reactivity at high values of conversion. However, a different trend of activation energies values was observed; in the case of CO₂ gasification, as the conversion increased, a decrease in activation energy was observed.

On the other hand an increase in activation energy was observed in case of steam gasification. This increase in activation energy was also, observed by Marsh et al. [4]. The decrease in activation energy values in the case of CO₂ gasification was accompanied by a decrease in pre-exponential factor as well. This behavior is called the compensation effect [5]. Montesinos et al. obtained a value of isokinetic temperature of 1150 K. The isokinetic temperature is the temperature at which all reactivities are equal for different conversions. An isokinetic temperature of 1449 K was obtained by Dhupe et al. [6] for CO₂ gasification using catalyzed sodium lignosulfonate. Feistel et al. [7] found this temperature to be 1425 K, obtained using potassium-catalyzed steam gasification.

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Food wastes, especially which have high percentage of vegetable oil and animal fat, provide a good potential for production of liquid fuels though transesterification. Transesterification is the process of exchanging the organic group R00 of an ester with the organic group R0 of an alcohol. The process is widely used to produce biodiesel fuels from vegetable oils and animal fats. The process is often catalyzed by an acid or a base.

Other than acid or base catalysts, enzyme or heterogeneous catalysts might be used as well. Among the mentioned catalysts, alkali catalysts are more effective. However, if the oil has high free fatty acid (FFA) content, higher than 3% (approximately), acid catalyzed transesterification is used rather than a base catalyst [10,11].

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Дата добавления: 2014-10-31