Ind. Eng. Chem. Res. 2008, 47, 7897–7902
7897
Characterization of Blast Furnace Slag-Ca(OH)2 Sorbents for Flue Gas Desulfurization Guozhuo Gong,†,‡ Shufeng Ye,*,† Yajun Tian,† Yanbin Cui,†,‡ and Yunfa Chen† State Key Laboratory of Multi-phase Complex System, Institute of Process Engineering, Chinese Academy of Sciences, Beijing 100080, China, and Graduate UniVersity of Chinese Academy of Sciences, Beijing 100049, China
A series of sorbents for flue gas SO2 scrubbing have been prepared from hydrated lime (HL) and blast furnace slag (BFS) with the factorial experiment design method. The sorbents were characterized and tested for SO2 reactivity in a differential fixed-bed reactor. It was found that, due to the formation of calcium silicate hydrates (CSHs), the reactivities of the sorbents prepared were higher than that of Ca(OH)2 alone. The present sorbent gave better utilization of the Ca originated from the Ca(OH)2 precursor and provided additional utilization capacity of the Ca derived from BFS. The present sorbents were mesoporous, and their specific surface areas were linearly correlated with their mesopore volumes. When the weight ratio of BFS/HL varied between 1/9 and 3/7, the SO2 capture of the sorbents prepared was independent of specific surface area. When the weight ratio of BFS/HL varied between 7/3 and 9/1, the SO2 capture was seen to increase linearly with specific surface area. Among the four process variables studied for the sorbent prepared in this work, the weight ratio of BFS/HL was found to be the most significant for the specific surface area of the sorbents. The optimum BSF/HL ratio looked to be around 9/1. The second and the third process variables studied were hydration time and the ratio of water/solid, respectively. The optimum hydration time was about 10 h, while the optimum water/solid ratio was 25/1. The last process variable studied was slurrying temperature. Slurrying temperature (T) had little effect on the specific surface area of the BFS/HL sorbent in the range 50-80 °C. This is useful for preparing high-performance silica-enhanced desulfurization sorbents for dry and semidry flue gas desulfurization processes, taking advantage of the waste BFS. Introduction The control of SO2 emission in large coal-fired plants is usually performed with three kinds of postcombustion flue gas desulfurization systems: dry, semidry, and wet. More and more attention has been paid to circulating fluidized-bed flue gas desulfurization (CFB-FGD), which is a superior semidry process. Compared to the other processes, the CFB-FGD process needs less space, is easier to retrofit, and produces dry solid products. Many coal-fired power plants employ this process to remove SO2.1 However, the sorbent, which is typically hydrated lime (HL), is not highly converted in the operation of this process. Increasing the utilization and reactivity of the sorbent has been an important technology area for further improvement of this process.2 One way to improve the sorbent utilization and reactivity is addition of certain inorganic and/or organic additives as potential performance additives into the conventional calcium hydrate.3 It seems that inorganic hygroscopic salts, such as barium, potassium, sodium and calcium chlorides, and cobalt, sodium, and calcium nitrates, would be the most effective ones. Some researchers4,5 also consider sodium hydroxide an effective additive due to its alkaline and hygroscopic properties. The other way is to find more active sorbents. Many investigators6-31 have shown that sorbents obtained by mixing lime or Ca(OH)2 with silica of different sources, such as diatomaceous earth, montmorillonitic/bentonitic clays, silica fume, and fly ash, could lead to significantly higher utilization of calcium than that obtained by using hydrated lime alone. It has been confirmed that the pozzolanic reaction between silica * To whom correspondence should be addressed. Tel.: +86-1062588029. Fax: +86-10-62542803. E-mail:
[email protected]. † Institute of Process Engineering, Chinese Academy of Sciences. ‡ Graduate University of Chinese Academy of Sciences.
and hydrated lime through a hydration process, which results in sorbents of large surface areas, is responsible for the enhancement of sorbent reactivities. Recently, several researchers24,29-31 have used blast furnace slag (BFS) to prepare SO2 reactive sorbents. The BFS is one of the wastes generated in steel factories. It is mainly composed of SiO2, Al2O3, and CaO, and is similar in chemical composition to fly ash but with somewhat higher CaO content. Brodnax and Rochelle30 first used BFS and hydrated lime to prepare SO2 sorbent. Liu and Shih24 found that sorbent prepared from BFS and hydrated lime had a greater specific surface area and higher reactivity than the hydrated lime. Lin and Shih29,31 further discussed the effects of the preparation conditions on SO2 capture of sorbent. Nevertheless, the importance degree of preparation variables which is important for industrial application and the effects of preparation variables on specific surface area of the sorbents are still scarcely reported in the literature. In this work, a series of sorbents for SO2 from flue gas were prepared from Ca(OH)2 and BFS using a factorial experiment method. The sorbents prepared were characterized, and their activities toward SO2 were measured in a differential fixed-bed reactor. Furthermore, the effect of specific surface area on SO2 capture of the sorbents is discussed. Besides, through a statistical method, the importance degree of preparation variables was studied, and the effects of preparation variables on the specific surface areas of the sorbents were fully evaluated. Experimental Section Sorbents Preparation and Design of Experiments. The sorbents were prepared using a commercial Ca(OH)2 supplied by the Beijing Chemical Reagents Company and a BFS supplied by the Jinan Iron and Steel Company, Ltd. The chemical compositions and physical properties of these materials are listed in Table 1.
10.1021/ie800873b CCC: $40.75 2008 American Chemical Society Published on Web 09/11/2008
7898 Ind. Eng. Chem. Res., Vol. 47, No. 20, 2008 Table 1. Chemical Compositions and Physical Properties of Ca(OH)2 and BFS BFS chemical composition (wt %) SiO2 Al2O3 Fe2O3 MgO CaO SO3 Ca(OH)2 CaCO3 insoluble ignition loss physical properties particle mean diameter (µm) true density (g/cm3)
33.26 15.63 0.83 9.41 38.69 0.35
Ca(OH)2
(20 °C min-1)
