Sulphur dioxide removal using South African limestone/siliceous materials D.O. Ogenga a , M.M. Mbarawa a, * , K.T. Lee b , A.R. Mohamed b , I. Dahlan c a Department of Mechanical Engineering, Pretoria Campus, Tshwane University of Technology, Private Bag X680, 0001 Pretoria, South Africa b School of Chemical Engineering, Universiti Sains Malaysia, Engineering Campus, Seri Ampangan, 14300 Nibong Tebal, Pulau Pinang, Malaysia c School of Civil Engineering, Universiti Sains Malaysia, Engineering Campus, Seri Ampangan, 14300 Nibong Tebal, Pulau Pinang, Malaysia article info Article history: Received 4 July 2009 Received in revised form 19 April 2010 Accepted 22 April 2010 Available online 6 May 2010 Keywords: Fly ash Desulphurization Hydration variables Calcined limestone Sorbents abstract This study presents an investigation into the desulfurization effect of sorbent derived from South African calcined limestone conditioned with fly ash. The main aim was to examine the effect of chemical com- position and structural properties of the sorbent with regard to SO 2 removal in dry-type flue gas desul- furization (FGD) process. South African fly ash and CaO obtained from calcination of limestone in a laboratory kiln at a temperature of 900 °C were used to synthesize CaO/ash sorbent by atmospheric hydration process. The sorbent was prepared under different hydration conditions: CaO/fly ash weight ratio, hydration temperature (55–75 °C) and hydration period (4–10 h). Desulfurization experiments were done in the fixed bed reactor at 87 °C and relative humidity of 50%. The chemical composition of both the fly ash and calcined limestone had relatively high Fe 2 O 3 and oxides of other transitional ele- ments which provided catalytic ability during the sorbent sorption process. Generally the sorbents had higher SO 2 absorption capacity in terms of mol of SO 2 per mol of sorbent (0.1403–0.3336) compared to hydrated lime alone (maximum 0.1823). The sorbents were also found to consist of mesoporous struc- ture with larger pore volume and BET specific surface area than both CaO and fly ash. X-ray diffraction (XRD) analysis showed the presence of complex compounds containing calcium silicate hydrate in the sorbents. Ó 2010 Elsevier Ltd. All rights reserved. 1. Introduction There is no doubt that fossil fuels have been and will still play an important role in the global energy mix especially with regard to electric power generation. This is so particularly for developing countries like South Africa, China, India and other fast growing economies. In spite of the environmental concerns posed by coal utilization and the perception of being a dirty energy, its position as the most important source of energy presently and in future re- mains inevitable. Fluctuations in crude oil prices and stability of coal prices coupled with its abundance and availability in almost all parts of the world will certainly increase its (coal’s) usage [1]. Even though technologies such coal gasification, fluidized bed combustion, selective non catalytic reduction and others would sufficiently address the issue of environmental concern with re- gard to coal utilization, it is obviously not easy to switch to these technologies at once. This means pulverized coal combustion (PCC) will be for sometime widely used. The use of PCC requires application of post combustion flue gas control technologies in or- der to reduce SO 2 from being released into the atmosphere. Conse- quently, emissions control measures of gas pollutants such as SO 2 is of utmost necessity. This means development of an efficient dry FGD technology using ash will be very beneficial to these nations. Although dry flue gas desulfurization (FGD) technology has been argued to be more advantageous than the wet FGD processes in terms of capital cost and waste handling, the conversion of the sorbent is low. Increasing the utilization of lime in semi-dry/dry FGD systems is one of the current challenges researchers have to deal with [2,3]. Fly ash, a pozzolanic material whose main compo- nents are usually SiO 2 , Al 2 O 3 , and Fe 2 O 3 [4], has shown good results when used to condition quicklime in a hydration process. The chemical composition of both the major and minor components in fly ash varies from one report to another depending on the par- ent coal composition [2,5–8]. This is also the same case with lime- stone from which quicklime when produced for hydration with fly ash during sorbent synthesis. Besides since fly ash contains both quartz and amorphous silica and the amorphous silica being more active than the crystalline quartz during pozzolanic reaction [9], the amount of the amorphous silica in the fly ash can thus have an effect on the resultant sorbent. For instance, Karatepe et al. [10] reported that the relationship between Ca(OH) 2 /fly ash sor- bents and the sorbent surface area changes with respect to the par- ticle size, form of the SiO 2 present in the fly ash (quartz or silicon oxide) and CaO content of the fly ashes. It is therefore possible that South African calcined limestone/siliceous materials could poses unique properties for the removal of SO 2 from flue gas. 0016-2361/$ - see front matter Ó 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.fuel.2010.04.029 * Corresponding author. Tel.: +27 12 382 5171; fax: +27 12 382 5602. E-mail address: mbarawamm@tut.ac.za (M.M. Mbarawa). Fuel 89 (2010) 2549–2555 Contents lists available at ScienceDirect Fuel journal homepage: www.elsevier.com/locate/fuel