The effect of coal composition on carbon dioxide adsorption
Date
2010-03-15T07:41:11Z
Authors
Bhebhe, Siboniwe
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Abstract
Greenhouse gas emissions due to fossil fuel combustion and gasification are recognized
as the major contributor to climate changes. South Africa is ranked the 12th highest
emitter of carbon dioxide in the world and accounts for 38 % of carbon dioxide emissions
in Africa. Carbon dioxide sequestration in unminable coal seams is one of the options
available for the reduction of carbon dioxide emissions in South Africa.
The adsorption behavior of carbon dioxide on four parent coal samples from major coal
fields in South Africa was studied. Heavy media separation was used to concentrate coal
macerals and mineral matter. The effect of the coal macerals, mineral matter and coal
rank on the carbon dioxide adsorption of the samples was evaluated. The coal samples
covered a maturity range from 0.69 % to 2.28 %. The maceral compositions of the
fractions varied from 92.8 % to 0.4 % vitrinite and 78.8 % to 0.4 % inertinite. Half of the
samples had 0 % liptinite and the maximum liptinite obtained for the rest of the samples
was 9.2 %. Mineral matter content ranged between 0.4 % and 93.0 %. Adsorption
measurements were performed on 2.0 g samples with a grain size of 300 – 600 μm. The
adsorption isotherms were measured at 303-304 K and a pressure range of 250kPa to
1500 kPa using the volumetric method. The volumetric system was specifically designed
and constructed for this research work.
The adsorption capacity was determined by fitting the data with a Langmuir equation.
The model provided very good fits, the relative error between the measured value and
the predicted value over the entire pressure range was less than 5.0 % for all the
samples.
The carbon dioxide adsorption capacity appears to positively correlate to the vitrinite
content and negatively correlate to the mineral matter content. There appears to be no
relationship between the inertinite content and gas adsorption capacity. The highest
adsorption capacity obtained was for the highest rank vitrinite-rich coal sample, and the mineral matter-rich sample had the lowest adsorption capacity.