Elsevier

Fuel

Volume 89, Issue 9, September 2010, Pages 2549-2555
Fuel

Sulphur dioxide removal using South African limestone/siliceous materials

https://doi.org/10.1016/j.fuel.2010.04.029Get rights and content

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 composition and structural properties of the sorbent with regard to SO2 removal in dry-type flue gas desulfurization (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 Fe2O3 and oxides of other transitional elements which provided catalytic ability during the sorbent sorption process. Generally the sorbents had higher SO2 absorption capacity in terms of mol of SO2 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 structure 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.

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 remains 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 regard 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 order to reduce SO2 from being released into the atmosphere. Consequently, emissions control measures of gas pollutants such as SO2 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 components are usually SiO2, Al2O3, and Fe2O3 [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 parent coal composition [2], [5], [6], [7], [8]. This is also the same case with limestone 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 sorbents and the sorbent surface area changes with respect to the particle size, form of the SiO2 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 SO2 from flue gas.

In this work, sorbents synthesized from calcined limestone conditioned with fly ash both of South African origin have been characterized and sulfated in order to investigate their influence in SO2 absorption capacity.

Section snippets

Materials and methods

The starting materials used for the preparation of sorbents were coal fly ash and limestone which were obtained from local power plants. The limestone was calcined in a laboratory furnace in order to obtain CaO according to the reaction shown in Eq. (1) below.CaCO3(s)+HeatCaO(s)+CO2(g)

The limestone was weighed, put in crucibles and then placed in the furnace where it was heated at 900 °C in the absence of air for 4 h. The cooled sample was weighed in order to determine the weight loss due to

Chemical analysis

Fly ash had the oxides of Na, K, Fe, Ti and Mn which can influence the reaction of the sorbent with SO2: oxides of Na and K would result in the formation of NaOH and KOH which reacts with SO2 to form sulphite salts reducing SO2 from the flue gas as shown in Eq. (2).SO2(g)+2NaOH(aq)Na2SO3(aq)+H2O(l)

Existence of these oxides (Na and K) can however be disadvantageous if in large quantities as it will result in leachable wastes which will further cause environmental harm. The oxides of Fe, Ti and

Conclusion

CaO/fly ash sorbents were prepared and characterized at different hydration periods and temperatures as well as weight ratios. Results showed that the specific surface area of CaO/fly ash sorbents (12.6–108.5 m2/g) was higher than that of the CaO (4.9 m2/g) and fly ash (3.3 m2/g) at all preparation conditions. X-ray diffraction (XRD) analysis showed the presence of complex compound containing calcium silicate hydrate in the synthesized sorbents. This contributed to the high SO2 removal. Hydration

Acknowledgements

The authors acknowledge the financial support provided by Tshwane University of Technology, Universiti Sains Malaysia (Research University Grant), the South African power producer-Eskom and The National Research Foundation.

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