Flue gas desulfurization studies using absorbent prepared from coal fly ash
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Date
2004-09
Authors
Lee, Keat Teong
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Abstract
Flue gas desulfurization (FOD) technology utilizing absorbent synthesized from coal fly
ash, calcium oxide (CaO) and calcium sulfate (CaS04) was studied. It was found that
the absorbent surface area and total pore volume were the only absorbent properties that
have significant influence on the desulfurization capacity of the absorbent. Apart from
that, it was found that the total pore volume of the absorbent was linearly correlated to
the surface area of the absorbent. The comparison between the absorbent preparation
method (water and steam hydration) showed that water hydration method was more
favorable in producing absorbent with higher surface area. A modeling-optimization
approach using neural network-genetic algorithm was used to optimize the absorbent
preparation variables to obtain absorbent with the highest surface area. The results
obtained indicated that a maximum surface area of 62.2 m2/g could be obtained using Sg
of CaO, 13.lg of coal fly ash and S.Sg of CaS04 at a hydration period of 10 hr. The
absorbent prepared under these conditions was coded as the reference absorbent.
The desulfurization capacity of the reference absorbent was found to easily outperform
its base components. The reference absorbent exhibited 100% removal of sulfur dioxide
(S02) for 30 min. X-ray Diffraction showed that calcium aluminum silicate hydrated
compounds were the reactive species in the absorbent that reacted/absorbed S02 and
calcium sulfite/sulfate were the products of the desulfurization reaction. The effect of
various operating conditions such as feed concentration of S02 (SOO to 2000 ppm), feed
concentration of NO (0 to 750 ppm), reaction temperature (60 to 300°C), relative
humidity (0 to 70%), absorbent particle size (200 to 600 ~m) and space velocity (3,500
to 11,300 hr-I) on the desulfurization activity of the absorbent were reported. Smaller
absorbent particle size and lower feed concentration of S02 were found to increase the
desulfurization capacity of the absorbent. Although higher reaction temperature favored
higher desulfurization capacity, but an increase of reaction temperature beyond 200°C
gave no advantage. Contradictory, the relative humidity of the feed gas must be above
40% before it could give a positive effect on the desulfurization capacity. On the other
hand, the presence of oxygen (02) and nitrogen oxide (NO) in the feed gas was found to
be necessary to produce calcium sulfate (CaS04) instead of calcium sulfite (CaS03) as
the final product of the desulfurization reaction.
The kinetics of the desulfurization process was studied using thermogravimetric
analysis. The desulfurization reaction was characterized by a substantial decrease in the
rate of reaction at the initial period of reaction time. A one step global reaction model
based on almost second order (n=2.2) reaction was found to fit the kinetic data. The
activation energy and frequency factor for the desulfurization reaction were found to be
22.9 kJ/mol and 64.4 min-I respectively. A mathematical model based on the material
balance for the gaseous and solid phase using partial differential equations to describe
the absorption of S02 from a moving gas stream to the absorbent-bed of changing
composition was proposed to represent the desulfurization reaction. The rate expression
employed in the model was based on the one step global reaction rate equation. The
kinetic parameters of the mathematical model were obtained by fitting the model to a
series of experimental desulfurization reaction carried out under isothermal conditions
at various operating parameters. The partial differential equations were solved using
finite difference method. The model was found to give a very good description of the
experimental data with error less than 10%.
Description
Keywords
The total pore volume of the absorbent was linearly correlated , to the surface area of the absorbent