Effect of pre-oxidation on the porosity development in a heavy oil fly ash by CO2 activation
Introduction
In previous works [1], [2], the possibility of converting heavy-oil fly ashes, after acid leaching to reduce the mineral matter content, into porous matrices by CO2 activation has been investigated. The resultant activated products showed structures mainly mesoporous with a maximum in surface area of about 160 m2/g.
A considerable amount of work on the conversion of coals and other carbonaceous materials into activated carbons has shown that the pre-oxidation plays an important role in the development of highly microporous structures [3], [4], [5], [6], [7], [8], [9], [10] by formation of oxygen groups which reduce the thermoplastic properties of the material [4].
In the present note, the pre-oxidation has been introduced in the activation process of a heavy-oil fly ash.
Section snippets
Starting material
A heavy-oil fly ash collected from electrostatic precipitator of an Italian power electric plant has been used. The as-received fly ash has been sieved and the particle size fraction less than 300 μm has been chosen for this study.
The acid leaching was conducted in accordance with the operative conditions used in previous works [11], [12] regarding the recovery of vanadium from heavy-oil fly ashes. The solid residue, after filtration and washing, has been dried in an oven at 105 °C and then
Results and discussion
The chemical analyses and porosimetric characteristics of the raw fly ash are shown in Table 1. The low values of surface area and pore volume are typical for these residues [1], [2], [14] and are the result of their formation path [15].
The SEM morphological analysis shows that the raw fly ash consists of well-distinct spongy and hollow particles (cenospheres) and aggregates. The SEM–EDS microanalysis relieves that the cenospheres contain mainly unburned carbon (85–90 wt%) and organic sulphur
Conclusions
A preliminary oxidation treatment performed in air at 250 °C for 36 h on leached heavy-oil fly ash, followed by pyrolysis at 900 °C for 2 h and CO2 activation at 900 °C, permits to reach higher surface areas trough a noticeable development of mesoporosity during activation, leading to obtain activated samples with a surface area of about 270 m2/g at a 40% burn-off.
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