Development of an active sorbent from fly ash for dry desulphurization of simulated flue gas in a fluidized-bed reactor

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Abstract

In this work, dry desulphurization of simulated flue gas was investigated in a batchwise operated laboratory-scale stainless steel fluidized-bed reactor (46 mm × 500 mm) by using an active sorbent composed of a mixture of fly ash, FGD-gypsum and calcium-containing local Turkish limestone (Mengen/Bolu) which was calcined at 900 °C with 5% H2O vapour. The sulphation reaction was carried out in a range of SO2 feedstock concentration (1000 ppm SO2  C  5000 ppm SO2) and at a temperature range of 300 °C  T  500 °C. The experimental sulphation conversion data of active sorbent were compared with those of literature. Results obtained with the sorbent of this study indicated an increased sulphation conversion according to the conventional calcium-containing sorbent.

Introduction

Dry flue gas desulphurization by direct injection of calcium-based absorbents into the flue gas duct offers an attractive alternative to semidry or wet methods for controlling SO2 emission at low temperature with a simple technology as a retrofit option for existing coal-fired power plants. Since the residence time of the solids in the duct injection of absorbents is short, a highly active absorbent must be used to achieve acceptable levels of SO2 removal [1]. One method of obtaining that absorbent under medium temperatures is by using a mixture prepared from coal fly ash, calcium oxide and gypsum [2], [3], [4], [5]. It was considered that the high activity of the mixture resulted according to the literature on one hand from the presence of calcium silicate hydrate material formed by the hydration reaction between calcium and alumina silicate in the fly ash, and the difference in reactivity was caused by the structure of calcium silicate hydrate material formed [6]. In this case, the surface area from the formation of hydrated calcium silicates is effective in the desulphurization process. On the other hand, the significant increase of calcium utilization rate was mainly attributed to the Ca(OH)2 covering the surface of fly ash particles, and the hydration reactions did not play an important role under the absorbent preparation condition at ambient temperature [7]. On the contrary of these findings in literature, the desulphurization activity of the absorbent depends on which step calcium sulphate is added in the preparation of absorbents. Recently Ishizuka et al. [8] found that the addition of calcium sulphate in the step of slaking calcium oxide with fly ash brings about a negative effect while the addition of that in the step of hydrothermal treatment following slaking brings about a positive effect.

With this state of knowledge given above, the purpose of this work is to develop highly active desulphurization sorbent by exploring the reaction of this sorbent composed of a mixture of lignite fired power plant waste (a disposed mix of FGD-gypsum and fly ash of Çayırhan/Ankara power plant) and calcined Turkish limestone (Mengen/Bolu) at the simulated conditions of real flue gases of that power plants for regenerative calcium utilization.

Section snippets

Apparatus

The experimental setup consisted of mainly a fluidized-bed reactor (FBR) surrounded by an electrically heated tubular ceramic furnace, and auxiliary equipment for fluidizing gas preparation, gas analysis and on-line data logging system. The FBR is a stainless steel tube of 46 mm inside diameter and 500 mm length. It is fitted with a stainless steel sieve (325 mesh) gas distributor. The reactor is heated by Kanthal-A1 heating wire and the bed temperature is measured by a NiCr/Ni thermocouple, and

Comparison of experimental sulphation conversion values of CS and AS

Experimental sulphation conversion values (Xs,exp) obtained in 15 min reaction time are given in Fig. 2, Fig. 3 as a function of SO2-feedstock concentration at 500 °C according to the sulphite, sulphate and total sulphate conversion of the CS and AS at 500 °C, respectively. As can be discerned from Fig. 2, Fig. 3, the sulphation conversion of the AS was greater than that of the CS at the operating conditions of this study given in Table 3. Therefore, it was concluded that the AS prepared as given

Conclusions

The highly active sorbent for dry desulphurization of simulated flue gas was developed from the LFA, CS and FGDG. For the preparation of this highly active sorbent (AS), FGDG was added in the hydrothermal treatment step after the slaking step of CS and fly ash giving a positive effect reported also in literature. The sulphation conversion of the AS obtained was found to be greater than that of the CS of this study and the previous studies. The lower activation energy values of the sulphation

Acknowledgments

The authors gratefully acknowledge the financial support from Ankara University Scientific Research Projects Directorate (BAP Project No. 98-05-04-012), Gesellschaft für Technische Zusammenarbeit (GTZ Project No. 9120/Germany) as well as the Scientific and Technical Research Council of Turkey (TÜBİTAK) and Cement Producers Association of Turkey (TÇMB) under Grant No. MİSAG/KTÇAG-116.

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