The development of a low cost process for removing air pollutants is highly desirable. For this purpose, a bag filter system with Ca(OH)2 absorbent is suitable because of its simple structure, the ease of maintenance, the low running cost, and the absence of slurry treatment. However, there are also some drawbacks, such as low conversion of the absorbent and low removal efficiency of acid gases (Seno et al., 1996; Mariana et al., 2003).
To enhance the efficiency of gas removal and the sorbent utilization, various additives have been examined. Sorbents obtained by mixing hydrated lime with different sources of silica have led to significantly higher conversion of calcium compared to the conversion obtained using pure hydrated lime (Garea et al., 1996). The use of fly ash (FA) as a silica source for absorbing SO2 has been studied and reported in many literatures (Lin et al., 2003a; Lin et al., 2003b; Lin and Shih, 2003; Liu et al., 2002; Ho and Shih, 1992; Ishizuka et al., 2000; Sanders et al., 1995; Al-Degs et al., 2001; Rastogi, 1999). However, the study on the use of diatomaceous earth (DE) as a silica source for absorbing acid gases has not been studied deeply (Jozewicz et al., 1988a; Jozewicz et al., 1988b). DE is a pozzolonic material and its main components are SiO2, Al2O3, and CaO. Many literatures reported that the pozzolonic reaction of silica with Ca(OH)2 in the slurry to form highly hydrated calcium silicates (CSHs) and calcium aluminates (CAHs) has been thought to be responsible for the improvement in solid utilitization (Ho et al., 1992; Ishizuka et al., 2000; Sanders et al., 1995; Rastogi, 1999).
In this work, DE as an additive was slurried with Ca(OH)2 to prepare Ca(OH)2/DE sorbents and then used to enhance the efficiency of HCl removal and the sorbent utilization in a bag filter reactor. The sorbent was prepared for various Ca(OH)2/DE weight ratios under the optimum condition of Ca(OH)2/FA preparation reported by previous works (Lin et al., 2003a; Liu et al., 2002; Ho and Shih, 1992). The results are compared with those for the sorbents of Ca(OH)2/FA and Ca(OH)2 only.
The hydrated lime used in this experiment was from Wako Pure Chemical Industries Ltd. Japan (99.9% - Ca(OH)2 ), had a mean particle size of 10.2 μm. DE was also from Wako Pure Chemical Industries Ltd. Japan (86.8% SiO2, 4.1% Al2O3, 1.6% Fe2O3, 1.7% CaO, 0.4% MgO, 0.8% others, and 4.6% ignition loss), had a specific surface area, SBET, of 7.26 m2/g and a density of 2.0 g/cm3. Fly ash was from Nihon Huntai Kougyou Gijyutu Kyoukai Japan (57.95% SiO2, 27% Al2O3, 5% Fe2O3, 3% CaO, 2.5% K2O, 2.3% Na2O, 2.1% MgO, and 0.15% SO3), had a specific surface area, SBET, of 0.65 m2/g and a density of 2.23 g/cm3. The mean particle size of the DE and the FA are 14.48 and 15.43 μm, respectively and measured by a laser scattering particle size distribution analyzer (LA-910, Horiba, Ltd. Japan).
The hydrated lime and the additives (DE or FA), together with 100 g...