Carbon-enriched coal fly ash as a precursor of activated carbons for SO2 removal
Introduction
The fly ash (FA) produced from the burning of pulverized coal in a coal-fired boiler is a fine-grained, powdery particulate material that is carried off in the flue gas and usually collected from the flue gas by means of electrostatic precipitators, baghouses, or mechanical collection devices such as cyclones.
When pulverized coal is combusted in a dry-ash, dry-bottom boiler, about 80% of all the ash leaves the furnace as fly ash. Approximately 45 million tonnes were produced in 2005 in the EU15 [1]. Within the EU, the utilization for FA in the construction industry is currently around 47%, about 42% are used as a replacement for naturally occurring resources and the rest is disposed.
FAs consist mainly of oxides of silica, aluminum, iron and calcium. Some unburnt or partially unburnt carbon residue is collected with the FA in the precipitators [2]. Reduction of NOx emissions through the installation of low NOx burners often results in higher unburned carbon contents in the FA, decreasing the marketability of some ashes. Unburned carbon tends to adsorb the air-entrainment reagents that are added to the cement to prevent crack formation and propagation.
The fate of this unburned carbon is mainly disposal due to the present lack of routes for their effective use. In recent years, unburned carbon has been explored as low cost adsorbents replacing activated carbon for flue gas treatment. The carbon present in FAs can be a precursor of activated carbons since it has gone through a devolatilization during the combustion in the furnace of the power station and, therefore, it only requires a process of activation [3], [4].
Unburned carbon has been tested as an adsorbent for the treatment of wastewater from dyeing [5], [6], mercury adsorption [7], [8] and also as catalyst in low temperature NO reduction with ammonia [9].
An application few explored until the present of the activated carbons obtained from carbon-enriched fly ash is the SO2 removal from stack gas.
The removal of SO2 over a carbon in the presence of oxygen and water vapor at low temperature involves a series of reactions that leads to the formation of sulfuric acid as the final product. The role of a carbon material is to allow the adsorption of SO2, H2O and O2 at the internal surface of the carbon, the catalytic oxidation of adsorbed SO2 to sulfuric acid and storage of the generated acid in the pores [10].
The aim of this investigation was to evaluate the SO2 abatement capacity of activated carbons obtained from fly ash carbon-rich fractions activated with steam.
Section snippets
Samples preparation
Coal fly ash (FA) from a pulverized coal power plant in Spain (Escucha, Teruel) was chosen as a starting material (named E2) to obtain an enriched carbon fraction. Approximately 50 kg of FA were sieved into different mesh fractions in order to select the fraction richer in carbon. The characterization of the sieved fractions [11] leaded to select a cut of particle size bigger than 200 μm to obtain carbon-enriched sample E2-EN.
This sample enriched in unburned carbon was agglomerated with vegetable
Characterization
Elemental analysis and ash content of the samples studied in this paper are given in Table 1. Hydrogen content is very low, indicating a high degree of condensation. Sample E2-EN-AG exhibit higher hydrogen content than expected and this fact can be attributed to remaining oil from agglomeration procedure in the sample.
The composition of ash of each sample, expressed as main oxides of metals, is shown in Table 2. It can be seen a high Fe content because the coal burned was a blend containing
Conclusions
This work has shown that the carbon-enriched fraction present in FAs can be a precursor of activated carbons. After an agglomeration process these carbons only requires a process of activation since it has gone through a devolatilization during the combustion in the furnace of the power station. After activation, an activated carbon with medium surface area can be obtained for environmental application. In the present case, SO2 removal capacity has been tested onto these samples, obtained
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