Effect of pre-oxidation process on V2O5/AC catalyst for the selective catalytic reduction of NOx with NH3

Activated coke-based catalysts have attracted extensive attention in denitration by selective catalytic reduction by NH₃ (NH₃-SCR), due to their excellent catalytic performance at low temperature. In the paper, the V₂O₅/AC catalyst was prepared by the impregnation method to investigate the effect of pre-oxidation process on its NH₃-SCR activity. Activity test results show that the V₂O₅/AC catalyst with 4-h pre-oxidation exhibits the best NO_x removal efficiency, which reaches the NO_x conversion is over 75% in the range of 200-240 °C and exhibits an excellent resistance to SO₂ and H₂O. Characterization results demonstrate that the V⁴⁺ was oxidized by oxygen molecule during pre-oxidation process, which contributes to the formation of V⁵⁺ ions and surface-active oxygen species. The surface-active oxygen species are conducive to promoting the "fast SCR" reaction; thus, the pre-oxidized process can contribute to the superior NH₃-SCR performance for V₂O₅/AC catalyst at low temperature.

Simultaneous removal of gaseous CO and elemental mercury over Cu-Co modified activated coke at low temperature

Cu-Co multiple-oxides modified on HNO₃-pretreated activated coke (AC_N) were optimized for the simultaneous removal of gaseous CO and elemental mercury (Hg⁰) at low temperature (< 200 °C). It was found that 2%CuOx-10%CoOx/AC_N catalyst calcined at 400°C resulted in the coexistence of complex oxides including CuO, Cu₂O, Co₃O₄, Co₂O₃ and CoO phases, which might be good for the simultaneous catalytic oxidation of CO by Co-species and removal of Hg⁰ by Cu-species, benefiting from the synergistic catalysis during the electro-interaction between Co and Cu cations (CoO ⇌ Co₃O₄ and Cu₂O ⇌ CuO). The catalysis removal of CO oxidation was obviously depended on the reaction temperature obtaining 94.7% at 200 °C, while no obvious promoting effect on the Hg⁰ removal (68.3%-78.7%). These materials were very substitute for the removal of CO and Hg° from the flue gas with the conditions of 8-20 vol.% O₂ and flue-gas temperature below 200 °C. The removal of Hg° followed the combination processes of adsorption and catalytic oxidation reaction via Langmuir-Hinshelwood mechanism, while the catalysis of CO abided by the Mars-van Krevelen mechanism with lattice oxygen species.

Promotional removal of gas-phase Hg0 over activated coke modified by CuCl2

Impregnating CuCl₂ on AC (activated coke) support to synthesize xCuCl₂/AC showed superior activity with higher 90% Hg⁰ removal efficiency at 80-140 °C, as well as a lower oxygen demand of 2% O₂ for Hg⁰ removal. The acceleration on Hg⁰ removal was observed for NO and SO₂. The BET, SEM, XRD, XPS, TPD, and FT-IR characterizations revealed that the larger surface area, sufficient active oxygen species and co-existence of Cu⁺ and Cu²⁺ may account for the efficient Hg⁰ removal. In addition, the low demand of gaseous O₂ was contributed to higher content of active oxygen and formed active Cl. After adsorbing on Cu sites, Cl sites, and surface functional groups, the Hg⁰_(ads) removal on xCuCl₂/AC was proceeded through two ways. Part of Hg⁰_(ads) was oxidized by active O and formed Hg⁰, and the other part of Hg⁰ combined with the active Cl, which was formed by the activation of lattice Cl with the aid of active O, and formed HgCl₂. Besides, the Hg²⁺ detected in outlet gas through mercury speciation conversion and desorption peak of HgCl₂ and Hg⁰ further proved it. As displayed in stability test and simulated industrial application test, CuCl₂/AC has a promising industrial application prospect.

Investigation on mercury removal and recovery based on enhanced adsorption by activated coke

This work is to systematically study the mercury-removal behavior of activated coke (AC), regeneration of spent AC by microwave treatment and subsequent recycling of Hg⁰. The powdery (AC) was obtained under coal-fired hot gas conditions in a drop-tube reactor. The adsorption mechanism and capacity of the AC for Hg⁰ removal in a H₂O + SO₂ + O₂ atmosphere were investigated. The regeneration of the AC by microwave heating and recovery of Hg⁰ were studied. The results showed that this AC preparation method can greatly simplify the process, and the AC's large surface area, developed pore structure, and abundant functional groups played a key role in the adsorption of Hg⁰. The adsorption mechanism and the optimum reaction conditions were determined, with a highest average Hg⁰-adsorption efficiency of 91% obtained at 70 °C in 3 h. Desorption of Hg⁰ was also studied, in which the alkaline-functional-group content and pore structure were enhanced, and S was detected by X-ray photoelectron spectroscopy in microwave-regenerated AC, which could improve the Hg⁰ removal efficiency increased to 96% after five adsorption/desorption cycles. The Hg⁰ could subsequently be recovered from the desorbed gas by condensation with an efficiency of 87.4% using ice-water.

Carbon consumption mechanism of activated coke in the presence of water vapor

To reduce chemical carbon consumption in activated coke technology used for flue gas purification, the carbon consumption mechanism of commercial activated coke in the presence of water vapor was studied. A fixed-bed reactor and a Fourier transform infrared (FTIR) spectrometer were combined to study the amount of carbon consumption. Temperature-programmed desorption (TPD) coupled with in situ diffuse reflectance infrared Fourier transform (in situ DRIFT) spectra were used to investigate functional group changes of activated coke. The sources and factors influencing carbon consumption in various adsorption atmospheres and in the N₂ regeneration atmosphere were compared. Carbon consumption during the adsorption and regeneration process was mainly due to the release of C-O and C=C groups. The addition of H₂O increased the formation of carbonates and carboxylic acids during the adsorption process, which decomposed during the regeneration process, thereby increasing carbon consumption. Carbon consumption was reduced during regeneration in an H₂O-SO₂ adsorption atmosphere, mainly because of the formation of C-S bonds, which reduced the formation of CO₂. The C-N bonds generated in an H₂O-NO adsorption atmosphere were decomposed during the regeneration process, thereby increasing carbon consumption. In a complex atmosphere of SO₂, NO, NH₃, and H₂O, SO₂ was absorbed by NH₃, and the amount of carbon consumption was consistent with that in the NO atmosphere during the regeneration process. The total carbon consumption in various adsorption atmospheres ranged from 85.4 to 125.2 μmol/g. Compared with an anhydrous atmosphere, chemical carbon consumption increased by 6.5-14.3% in the presence of H₂O. Chemical carbon consumption was reduced by decreasing the H₂O concentrations, which provides a reference concept for reducing the operating cost of the activated coke process in industry.

Adsorption and oxidation of elemental mercury from coal-fired flue gas over activated coke loaded with Mn-Ni oxides

A series of Mn-Ni/AC (AC, activated coke) catalysts were synthesized by the impregnation method for the removal of elemental mercury (Hg⁰) from simulated flue gas. The samples were characterized by BET, ICP-OES, SEM, XRD, XPS, H₂-TPR, FT-IR, and TGA. Mn₆Ni_0.75/AC exhibited optimal removal efficiency of 96.6% in the condition of 6% O₂ and balanced in N₂ at 150 °C. The experimental results showed that both O₂ and NO facilitated Hg⁰ removal. SO₂ could restrain the Hg⁰ removal in the absence of O₂, while the inhibitory effect of SO₂ was weakened with the aid of 6% O₂. In addition, H₂O exhibited a slightly negative influence on Hg⁰ removal. The characterization of the samples indicated that Mn₆Ni_0.75/AC possessed larger specific surface area, higher dispersion of metal oxides, and stronger redox ability. In the meantime, the results of XPS and FT-IR demonstrated that the lattice oxygen and chemisorbed oxygen made contributions to Hg⁰ removal and the consumed oxygen could be compensated by the redox cycle of metal oxides and gas-phase O₂. Meanwhile, the mechanisms of Hg⁰ removal were proposed based on the above studies.

Study on the removal of elemental mercury from simulated flue gas by Fe₂O₃-CeO₂/AC at low temperature

Fe2O3 and CeO2 modified activated coke (AC) synthesized by the equivalent-volume impregnation were employed to remove elemental mercury (Hg(0)) from simulated flue gas at a low temperature. Effects of the mass ratio of Fe2O3 and CeO2, reaction temperature, and individual flue gas components including O2, NO, SO2, and H2O (g) on Hg(0) removal efficiency of impregnated AC were investigated. The samples were characterized by Brunauer-Emmett-Teller (BET), X-ray diffraction (XRD), scanning electron microscopy (SEM), and X-ray photoelectron spectroscopy (XPS). Results showed that with optimal mass percentage of 3 % Fe2O3 and 3 % CeO2 on Fe3Ce3/AC, the Hg(0) removal efficiency could reach an average of 88.29 % at 110 °C. Besides, it was observed that O2 and NO exhibited a promotional effect on Hg(0) removal, H2O (g) exerted a suppressive effect, and SO2 showed an insignificant inhibition without O2 to some extent. The analysis of XPS indicated that the main species of mercury on used Fe3Ce3/AC was HgO, which implied that adsorption and catalytic oxidation were both included in Hg(0) removal. Furthermore, the lattice oxygen, chemisorbed oxygen, and/or weakly bonded oxygen species made a contribution to Hg(0) oxidation.