An efficient calcium-based sorbent for flue gas dry-desulfurization: promotion roles of nitrogen oxide and oxygen †

The desulfurization performance of all the as-synthesized sorbents and raw materials was tested in an isothermal fixed bed reactor as mentioned above. The activity tests were conducted by feeding 1000 mg m −3 SO 2 , 750 mg m −3 NO, 8% O 2 , and N 2 as the balance gas at 150 °C with a gas hour space velocity (GHSV) of 12 000 h −1 . As displays, CaSO 4 and fly ash could hardly capture SO 2 , while CaO only maintained 100% SO 2 removal ratio for 7 min, followed by rapid inactivation. Notably, the mixture of fly ash/CaO/CaSO 4 showed distinctly improved desulfurization performance. Among the series of sorbents with different ratios of fly ash/CaO/CaSO 4 , ACS-4 showed the most outstanding SO 2 elimination efficiency maintaining 100% SO 2 removal for 43 min. Moreover, the deactivation rate of ACS-4 was significantly decreased compared with the other sorbents. At the time on stream of 120 min, SO 2 removal dropped to around 25%. Besides, the dynamic adsorption amount of SO 2 was calculated by using eqn (3) , as shown in . Indeed, the SO 2 adsorption capacities of the ACS with different fly ash/CaO/CaSO 4 ratios followed the order of ACS-4 (44.26 mg g −1 ) > ACS-3 (33.85 mg g −1 ) > ACS-5 (28.21 mg g −1 ) > ACS-2 (26.28 mg g −1 ) > ACS-1 (12.42 mg g −1 ). In summary, the combination of fly ash/CaO/CaSO 4 shows superior SO 2 adsorption capacity compared with each single component and the optimal ratio of fly ash/CaO/CaSO 4 is 1 : 2 : 1. Additionally, the SO 2 adsorption capacity of various previously reported Ca-based sorbents is listed in . The SO 2 adsorption amount of the sorbent prepared in this work (ACS-4) surpasses that of a majority of Ca-based sorbents except for the 50CaO/C Foam material. Nevertheless, the differences in SO 2 adsorption capacity between ACS-4 and 50CaO/CFoam are insignificant. By contrast, the sorbent prepared in this work (ACS-4) presents considerable potential in future practical application due to its outstanding desulfurization performance and low cost.

3.2. Sorbent characterization

The pore structure parameters of the three raw materials and the as-synthesized sorbents are listed in . The specific surface area along with pore volume and average pore size of the active raw materials were extremely small, leading to poor desulfurization efficiency. Particularly, the pore volume of fly ash was only 0.003 cm3 g−1, indicating that fly ash has little pore structure. According to previous literature,34 the main chemical constituents of original fly ash are compounds mainly composed of amorphous SiO2 and Al2O3, which can be dissolved in alkaline solutions and then react with Ca2+. The reaction between fly ash and calcium hydroxide is known as a pozzolanic reaction and yields calcium aluminate silicate hydrate compounds that are fibrous gels that have improved surface areas, pore volumes and hierarchical networks. In contrast, the specific surface area, pore volume and the average pore size of the prepared ACS was markedly increased in comparison to the original materials, which suggested that the hydration reaction process had a positive influence on the surface area of the sorbent. It was speculated that the heat released during the hydration reaction greatly swelled the inner pores, causing the increase of specific surface area. Additionally, the nitrogen adsorption–desorption isotherms and pore diameter distributions of the ACS are illustrated in Fig. S1.† All of the sorbents revealed typical Langmuir IV isotherm curves with a type H3 hysteresis loop, indicating the existence of slit-like mesopores in aggregates.18,19 The pore size distributions of all the ACSs further verified the existence of mesopores (2–50 nm). Among them, the pore diameter distribution of ACS-4 was most concentrated and homogeneous (around 30 nm), which may provide abundant adsorption active sites for desulfurization reaction. In conclusion, the combination reaction of the three raw components increased the specific surface area of the ACS, which facilitated the desulfurization reaction compared with the single components. ACS-4 with the biggest specific surface area, pore volume and average pore size presented the most outstanding desulfurization performance.

Samples S BET, (m2 g−1) V p, (cm3 g−1) d p, (nm)Fly ash1.450.0036.89CaO4.210.0119.94CaSO41.650.04510.83ACS-128.490.11616.34ACS-228.590.15822.08ACS-329.290.13918.99ACS-431.430.17622.34ACS-521.240.11120.93Open in a separate window

displays the XRD patterns of fresh ACS-4, CaSO4, CaO and fly ash. Among them, the fly ash was provided by a domestic coal-fired power plant. The major chemical composition of fly ash is shown in Table S2,† and it was characterized by the X-ray fluorescence (XRF) method. The weak diffraction peak of fly ash at 2θ = 26.5° was indexed to quartz (SiO2). Although fly ash is an essentially vitreous material with numerous amorphous structures, it also contained a small amount of SiO2 crystalline phases. In addition, peaks of CaO and CaSO4 closely matched the corresponding standard PDF database. Notably, the strong diffraction peaks of ACS-4 at 2θ = 28.66°, 34.09°, 47.12°, 50.79°, 54.34°, 56.25°, 62.54° and 64.23° were attributed to Ca(OH)2 (JCPDS PDF No. 36-1248),35 and the other major diffraction peaks at 2θ = 9.93°,18.02° and 19.76° corresponded well with a Ca2Al3Si9O36·8H2O standard (#39-1381).13 Thus, the diffraction patterns changed appreciably after activation of the original fly ash with CaO/CaSO4. The major phase of the ACS-4 material was calcium aluminate silicate hydrate and calcium hydroxide (Ca(OH)2) with no presence of CaO, CaSO4 and SiO2. The new component in ACS-4 was considered to be formed in the fly ash/CaO/CaSO4 hydration process during ACS preparation. It was further demonstrated that the occurrence of chemical reaction after the mixing of fly ash/CaO/CaSO4 resulted in the formation of calcium aluminate silicate hydrate and calcium hydroxide, dramatically increasing the surface area of the sorbents. Moreover, the XRD patterns of ACS-1, ACS-2, ACS-3 and ACS-5 were similar to that of ACS-4 (shown in Fig. S2†). Hence, the ACS material presents superior desulfurization performance compared with the raw individual components.

Furthermore, the SO2 adsorption active site in the ACS sorbents was investigated by XRD analysis. As shown in , the XRD patterns of fresh and used ACS-4 showed that no new crystal phase was generated in the desulfurization process, suggesting that SO2 adsorption species were highly dispersed on the surface of the sorbent with no crystallization. After the desulfurization process, peaks at 2θ = 9.93° and 19.76°, attributed to silicate hydrate, almost completely disappeared. Besides, the diffraction peak intensity of calcium hydroxide was weakened.

It was inferred that the silicate hydrate compound and calcium hydroxide were converted to other chemical substances. Furthermore, the pore diameter distributions of ACS-4 before and after desulfurization are shown in . As shown in , the pore volume dropped from 0.176 to 0.094 cm3 g−1, indicating that SO2 adsorption species accumulated in the pore structure. Consequently, the sorbents were entirely inactive due to the consumption of adsorption active sites.

Samples S BET, (m2 g−1) V p, (cm3 g−1) d p, (nm)ACS-431.430.17622.34ACS-4-used18.130.09420.78Open in a separate window

The top surface morphologies of fresh ACS-4 and used ACS-4 were observed using SEM. As shown in , the porous fiber-like and network structure displayed an ordered microscopic pore structure, which provided a sufficient buffer space for SO2 adsorption reaction and significantly improved SO2 diffusion and transformation. Fly ash is typically composed of spherical particles, as shown in Fig. S3.† In addition, Fig. S4† contains the SEM images of fresh ACS-1, ACS-2, ACS-3 and ACS-5, all of which displayed analogous surface morphologies. As shown in , ACS-4 was crushed into smaller particles when used in the reactor. Further, during the adsorption of SO2 molecules, calcium sulfate and calcium sulfite formed, leading to clogging of the channel. It could be observed that the porous fiber-like and network structure of the sorbent was no longer discernible. Furthermore, the thermal stability of the ACS materials was detected with thermogravimetric analysis from 50 °C to 800 °C in an air atmosphere (Fig. S5†). All the samples presented a prominent weight loss peak at around 700 °C, ascribed to the decomposition of the ACS. From 50 to 700 °C, the weight loss of all samples was constant and slow, due to the release of water in the structure. As a result, the ACS remained stable at the practical desulfurization temperature of 150 °C.