Coal fly ash based carbons for SO₂ removal from flue gases

Abstract

Two different coal fly ashes coming from the burning of two coals of different rank have been used as a precursor for the preparation of steam activated carbons. The performance of these activated carbons in the SO₂ removal was evaluated at flue gas conditions (100 °C, 1000 ppmv SO₂, 5% O₂, 6% H₂O). Different techniques were used to determine the physical and chemical characteristics of the samples in order to explain the differences found in their behaviour. A superior SO₂ removal capacity was shown by the activated carbon obtained using the fly ash coming from a subbituminous–lignite blend. Experimental results indicated that the presence of higher amount of certain metallic oxides (Ca, Fe) in the carbon-rich fraction of this fly ash probably has promoted a deeper gasification in the activation with steam. A more suitable surface chemistry and textural properties have been obtained in this case which explains the higher efficiency shown by this sample in the SO₂ removal.

Introduction

Fly ash (FA) is the main by-product of coal combustion. When pulverized coal is combusted in a dry-ash, dry-bottom boiler, about 80% of all the ash leaves the furnace as fly ash. FAs consist mainly of oxides of silica, aluminum, iron and calcium. Some unburned or partially burned carbon residue is collected with the FA in the precipitators (Lindon, 2001).

Approximately 45 million tons were produced in 2005 in the EU15 (ECOBA, 2005). About 50% of this annual production is used in the cement industry, as FAs show pozzolanic properties after reacting with lime and water and contains SiO₂ and Al₂O₃ as major components. However, NO_x emissions reduction, through the installation of “low-air” burner retrofits and low NO_x burners, often results in higher unburned carbon contents, and therefore in a decreased marketability of the ashes. According to ASTM C618, a FA having a loss-on-ignition (LOI) greater than 6% becomes unusable for cement or concrete manufacture, this value decreasing in many US departments to 3–4%. The European Standard (UNI-EN 206) accepts a LOI of 5%. Another 42% of the annual production of FAs in the EU is used as a replacement for naturally occurring resources and the rest is disposed.

The fate of the unburned carbon present in FAs is mainly disposal, due to the present lack of routes for their effective use. In last years, unburned carbon has been explored as low cost adsorbent replacing activated carbon as an adsorbent for the treatment of wastewater (Graham et al., 1996, Wang et al., 2005, Wang and Li, 2007) mercury adsorption (Maroto-Valer et al., 2005, Lopez-Alonso et al., 2007), CO₂ capture (Arenillas et al., 2005, Maroto-Valer et al., 2008) and also as catalyst in low temperature NO reduction with ammonia (Rubio et al., 2007). Indeed, 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 (Baltrus et al., 2001, Lu et al., 2008).

An application few explored until the present of the activated carbons obtained from carbon fly ash is the SO₂ removal from stack gas (Davini, 2002, Izquierdo and Rubio, 2008). Sulphur dioxide is an important air pollutant that causes photochemical smog and acid rain. This knowledge has led to increasing regulation of exhaust emissions from stationary combustion sources. 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 sulphuric 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 sulphuric acid and the storage of the generated acid in the pores (Richter, 1990). So, the adsorptive capacity of these carbons will be determined not only by their porous structure but also by their surface chemical composition.

The aim of the present work is to evaluate the SO₂ abatement capacity of steam activated carbons prepared from two coal fly ashes of different origin and characteristics.