Elsevier

Fuel

Volume 84, Issues 14–15, October 2005, Pages 1854-1857
Fuel

Effect of pre-oxidation on the porosity development in a heavy oil fly ash by CO2 activation

https://doi.org/10.1016/j.fuel.2005.02.023Get rights and content

Abstract

The effect of pre-oxidation on the porosity evolution in heavy-oil fly ash subjected to activation with CO2 has been investigated. After preliminary acid leaching, used to reduce the mineral matter content, the leached fly ash has been oxidised in air at 250 °C for 36 h. Pyrolysis was conducted on the unoxidised and oxidised leached fly ash at 900 °C for 2 h and the resultant chars were activated with CO2 at 900 °C for different times. The activated samples have been characterised as regards the surface area and the pore volume.

The pre-oxidation enhances the porosity development mainly in terms of mesoporosity leading to obtain activated products with higher surface area (about 270 m2/g at a 40% burn-off).

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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