China is the world’s largest coal producer and consumer. Conventional pollutants such as NO
x, SO
x and particulate matter generated by coal combustion, as well as heavy metal pollutants such as As, Se, Pb and Hg, seriously harm human health and natural environment
[1−2]. Arsenic, as a heavy metal pollutant, is now attracting more and more attention due to its strong volatility, bio-accumulation in ecosystems, carcinogenicity and teratogenicity to humans
[3−4]. It can also cause arsenic poisoning in selective catalytic reduction (SCR) catalyst of coal-fired power plants, which is reported to be one of the most common causes of SCR catalysts deactivation
[5]. Therefore, it is urgent to strengthen the control of arsenic pollution from coal combustion process. Especially in recent years, the “dual-carbon” goal requires coal-fired power units with high carbon emissions to operate at medium-to-low loads or perform deep peak-shaving
[6−7], which results in a significant decrease in the tail flue gas temperature. For example, the denitrification device’s inlet temperature will drop below 300 °C. Therefore, it is significantly important to develop arsenic pollution control technology under middle-low-temperature flue gas conditions (<400 °C).
Adsorption is currently a mature method for arsenic removal from flue gas
[8−10]. To reduce arsenic poisoning of SCR catalysts, arsenic removal adsorbents are generally added to the flue before the denitrification unit. In this case, the adsorbent can be easily deactivated or its service life can be reduced due to poisoning by high concentration of SO
2 and other substances in the flue gas. Therefore, developing sulfur resistant adsorbents for arsenic removal from middle-low-temperature flue gas is exceedingly desired.
In previous work, the authors developed Fe-rare earth composite oxide adsorbents, including FeLaO, CeLaO and FeCeLaO
[11], which can efficiently remove arsenic from middle-low-temperature flue gas (<400 °C) and can meet the current needs of low load operation or deep peak shaving of coal-fired power units. Especially for the FeCeLaO, the total adsorbed arsenic reaches 583.7 μg/g at 400 ℃, and the arsenic removal efficiency is as high as 89%. Therefore, this work uses Fe-Ce-La oxides as the active component and further couples it with a sulfur resistant carrier to prepare a composite adsorbent, aiming to achieve both low-temperature arsenic removal and high-efficiency sulfur resistance in coal-fired flue gas.
Up to date, only a few studies on the sulfur resistance of arsenic removal adsorbents have been reported. Hu et al.
[12] used the DFT method to study the effect of the SO
2 on the adsorption of As
2O
3 on γ-Al
2O
3 by calculating the adsorption structure, adsorption energy, electron transfer and other characteristics. They found that the SO
2 mainly changed the electron distribution around O atom of the first layer of γ-Al
2O
3 and caused almost all previous As
2O
3 adsorption structures on γ-Al
2O
3 surface to become or stay in chemisorption. Therefore, the SO
2 generally favors the adsorption of As
2O
3 on the γ-Al
2O
3 surface. Li et al.
[13] concluded that the SO
2 has no significant effect on arsenic adsorption on the CaO. Firstly, the reaction rate constant between CaO and gaseous arsenic was greater than that between CaO and SO
2. These indicate that in the initial stage, the SO
2 cannot compete with As
2O
3 for active sites on the CaO surface. Secondly, they demonstrated that calcium sulfate also can adsorb gaseous arsenic. Therefore, even if the CaO surface was fully sulfated, it still had a certain adsorption capacity for gaseous arsenic. However, Chen et al.
[14] showed that the SO
2 inhibited the adsorption process of arsenic on CaO between 300 and 800 °C. This is attributed to the fact that the generated CaSO
3/CaSO
4 occupies the free surface available for arsenic capture, and reduces the arsenic capture ability of the CaO. Zhao et al.
[15] investigated the effect of the SO
2 on the removal of As
2O
3 by Fe
3Mn
xO
y. Results showed that the SO
2 occupied the active sites for As
2O
3 adsorption, consumed lattice oxygen in the adsorbent, and converted it into
${\mathrm{SO}}_4^{2-} $
, thus inhibiting the adsorbent’s ability to remove As
2O
3. These previous studies have well illustrated sulfur resistance in common metal oxide adsorbents for arsenic removal. However, the sulfur resistance and synergistic mechanisms of multi-metal oxide composite adsorbents require further investigation.
Fly ash is a by-product of coal-fired power plants. After alkali fusion
[16], acid washing
[17] and other modification, the silica-alumina glass with high degree of polymerization, quartz, and mullite are depolymerized or destroyed
[18]. The SiO
2 and Al
2O
3 are dissolved in the amorphous matters
[19]. After drying and calcination, the active Si-Al carrier can be obtained, which has the advantages of porosity, large specific surface area and high thermal stability
[20]. Compared with some common carriers such as activated carbon and TiO
2, it has strong surface acidity and can inhibit the adsorption of acid SO
2 in flue gas. Therefore, it may have good SO
2 poisoning resistance.
In this work, a series of Fe-Ce-La oxides were prepared by co-precipitation method, and active Si-Al carriers were obtained by modifying fly ash. Then the FeCeLaO/SiO2-Al2O3 composite adsorbents were prepared. To investigate the effects of Fe, Ce, La and Si/Al ratio on surface properties, the Fe-Ce-La oxides and carriers were characterized using temperature programmed desorption (TPD), thermogravimetry (TG), X-ray fluorescence spectroscopy (XRF), specific surface area analysis (BET), and X-ray photoelectron spectroscopy (XPS). The effects of temperature, SO2 concentration and Si/Al ratio on the arsenic adsorption performance and sulfur poisoning resistance were also investigated in depth.