Anchao Zhang

Structural evolution of maize stalk/char particles during pyrolysis.

The structural evolution characteristics of maize stalk/char particles during pyrolysis were investigated. The char was prepared by pyrolyzing at temperatures ranging from 200 to 900 degrees C. Maize stalk and chars were characterized by thermogravimetric analysis, ultimate analysis (TGA), Fourier transform infrared spectroscopy (FTIR), scanning electron microscopy (SEM), helium density measurement and N(2) adsorption/desorption method. The char yield decreased rapidly with increasing temperature until 400 degrees C. As temperature increased, the char became progressively more aromatic and carbonaceous. The hydroxyl, aliphatic C-H, carbonyl and olefinic C=C groups were lost at high temperatures. Below 500 degrees C, the removal of volatile matter made pore opening. High temperatures led to the occurrence of softening, melting, fusing and carbon structural ordering. The aromatization process started at approximately 350 degrees C and continued to higher temperatures. The shrinkage of carbon structure occurred above 500 degrees C, which was concurrent with the aromatization process.

Mechanism Study of Rice Straw Pyrolysis by Fourier Transform Infrared Technique

Abstract The pyrolysis mechanism of rice straw (RS) was investigated using a tube reactor with Fourier transform infrared (FTIR) spectroscopy and thermogravimetric analyzer. The results show that the maximum pyrolysis rate increases with increasing heating rate and the corresponding temperature also increases. The three-pseudocomponent model could describe the pyrolysis behavior of rice straw accurately. The main pyrolysis gas products are H 2 O, CO 2 , CO, CH 4 , HCHO (formaldehyde), HCOOH (formic acid), CH 3 OH (methanol), C 6 H 5 OH (phenol), etc . The releasing of H 2 O, CO 2 , CO and CH 4 mainly focuses at 220-400°C. The H 2 O formation process is separated into two stages corresponding to the evaporation of free water and the formation of primary volatiles. The release of CO 2 first increases with increasing temperature and gets the maximum at 309°C. The releasing behavior of CO is similar to H 2 O and CO 2 between 200 and 400°C. The production of CH 4 happens, compared to CO 2 and CO, at higher temperatures of 275-400°C with the maximum at 309°C. When the temperature exceeds 200°C, hydroxyl and aliphatic C-H groups decrease significantly, while C O, olefinic C C bonds and ether structures increase first in the chars and then the aromatic structure develops with rising temperature. Above 500°C, the material becomes increasingly more aromatic and the ether groups decreases with an increase of temperature. The aromatization process starts at ≈350°C and continues to higher temperatures.

Removal of elemental mercury from coal combustion flue gas by bentonite-chitosan and their modifier

Abstract Adsorption experiments of vapor-phase elemental mercury were carried out using modified bentonite-chitosan in a laboratory-scale fixed-bed reactor. VM3000 online mercury analyzer was applied to detect the inlet and outlet mercury (Hg 0 ) concentrations. The characterizations of the sorbents were analyzed using the methods such as nitrogen (N 2 ) adsorption/desorption, scanning electron microscopy (SEM), and Fourier transform infra-red spectroscopy (FT-IR). It was observed that the porosity and specific surface area decreased after modification. The FT-IR spectra demonstrated that the iodine was found in the inner layer of bentonite and the chemical reactions of iodine and sulfuric acid with the amide of chitosan occurred. The tests in the fixed-bed reactor showed that the bentonite-supported chitosan exhibited lower mercury capture than raw bentonite, which indicated that the mechanism of gas phase mercury removal was different from that of ionic state mercury in liquid. In general, the iodine-modified sorbents demonstrated higher mercury capture efficiency than raw sorbents and the iodine-modified bentonite showed the best one. Mercury removal efficiency of bentonite supported by chitosan sorbents could be promoted from 85 to 100% when added appropriate amount of H 2 SO 4 , while that of iodine- and sulfuric acid-modified bentonite exhibited opposite tendency because of their absolutely different physicochemical properties.

Facile synthesis of ternary Ag/AgBr-Ag2CO3 hybrids with enhanced photocatalytic removal of elemental mercury driven by visible light

A novel technique for photocatalytic removal of elemental mercury (Hg(0)) using visible-light-driven Ag/AgBr-Ag2CO3 hybrids was proposed. The ternary Ag/AgBr-Ag2CO3 hybrids were synthesized by a simple modified co-precipitation method and characterized by N2 adsorption-desorption, scanning electron microscope (SEM), X-ray diffraction (XRD), UV-vis diffused reflectance spectroscopy (DRS), X-ray photoelectron spectroscopy (XPS) and electron spin resonance (ESR) techniques. The effects of AgBr content, fluorescent lamp (FSL) irradiation, solution temperature, SO2 and NO on Hg(0) removal were investigated in detail. Furthermore, a possible reaction mechanism for higher Hg(0) removal was proposed, and the simultaneous removal of Hg(0), SO2 and NO was studied. The results showed that a high efficiency of Hg(0) removal was obtained by using Ag/AgBr-Ag2CO3 hybrids under fluorescent lamp irradiation. The AgBr content, FSL irradiation, solution temperature, and SO2 all exhibited significant effects on Hg(0) removal, while NO had slight effect on Hg(0) removal. The addition of Ca(OH)2 demonstrated a little impact on Hg(0) removal and could significantly improve the SO2-resistance performance of Ag/AgBr(0.7)-Ag2CO3 hybrid. The characterization results exhibited that hydroxyl radical (OH), superoxide radical (O2(-)), hole (h(+)), and Br(0), were reactive species responsible for removing Hg(0), and the h(+) played a key role in Hg(0) removal.

Fe3 xCuxO4 as highly active heterogeneous Fenton-like catalysts toward elemental mercury removal

A series of novel spinel Fe3-xCuxO4 (0<x<0.71) composites, synthesized by chemical co-precipitation method, are proposed synthesized to use as highly active heterogeneous Fenton-like catalysts to remove elemental mercury (Hg0) from the simulated flue gases. Inductively coupled plasma-Atomic emission spectrometry (ICP-AES), X-ray diffraction patterns (XRD), scanning electron microscopy (SEM), Brunauer-Emmett-Teller (BET) surface area, and X-ray photoelectron spectrometer (XPS) were used to characterize the catalysts. The catalysts were confirmed the presence of the redox pairs Fesurf2+/Fesurf3+ and Cusurf+/Cusurf2+ on the surface of the cubic structure. The performance of heterogeneous Fenton-like reactions for Hg0 removal was evaluated in a lab-scale bubbling reactor at the solution temperature of 50°C. The systematic studies on the effects of different catalysts, H2O2 concentration and solution pH values on Hg0 removal efficiencies were performed. The recycling of the Fe3-xCuxO4 catalysts in Fenton-like solution is stable and Hg0 removal efficiency remain above 90% after 3 cycles. The active hydroxyl radical (OH) generated during heterogeneous Fenton-like reactions was confirmed through electron spin resonance (ESR) spin-trapping technique. The Hg0 removal mechanism has been discussed based on the experimental and analytical results.

Effects of Pyrolysis Temperature on Characteristics of Porosity in Biomass Chars

In this study, the influence of pyrolysis temperature (T) in the range of 200-900oC on the characteristics of porosity in biomass chars was investigated. The samples were characterized by N2 isothermal adsorption/desorption method and scanning electron microscopy (SEM). The results indicated that pyrolysis temperature had a notable impact on the pore structure and morphology of biomass char. Between 200 and 500 oC, the removal of a significant amount of volatile matter produced pore opening. Above 500oC, the loss of only a relatively small fraction of volatile matter caused the development of porosity and the occurrence of the structural shrinkage and pore narrowing. High temperature led to plastic deformation of particles resulting in smooth surfaces and large cavities. The surface area reached a maximum value at 500oC, and at higher temperatures, the specific area dropped significantly, probably due to thermal annealing. Char structural ordering was likely to be a mechanism for thermal annealing and thus for thermal deactivation.

Magnetically separable AgI–BiOI/CoFe2O4 hybrid composites for Hg0 removal: characterization, activity and mechanism

A series of magnetically separable AgI–BiOI/CoFe2O4 hybrid composites were successfully synthesized via a solvothermal and subsequent coprecipitation method. The microstructure and magnetism of the materials were characterized by X-ray diffraction (XRD), N2 adsorption–desorption, scanning electron microscopy (SEM), transmission electron microscopy (TEM), high-resolution transmission electron microscopy (HRTEM), X-ray photoelectron spectroscopy (XPS), UV-vis diffuse reflectance spectroscopy (DRS), photocurrent test, electron spin resonance (ESR) and vibrating sample magnetometer (VSM). The photocatalytic performance of AgI–BiOI/CoFe2O4 composites on Hg0 removal from simulated flue gas was carefully designed and evaluated under fluorescent light (FSL) irradiation. The results showed that AgI–BiOI/CoFe2O4 composites displayed superior photocatalytic activities because of the synergistic effects between AgI, BiOI, and CoFe2O4 under FSL irradiation. The optimal weight ratio between AgI and the total weight of AgI–BiOI/CoFe2O4 photocatalyst was 0.3. The presence of a small amount of SO2 had a dramatic inhibition on Hg0 removal, while the inhibitory effect of NO on Hg0 removal could only be observed at a higher NO concentration. The trapping experiments indicated that photoinduced holes (h+) and superoxide radicals (˙O2−) were the primary active substances in the AgI–BiOI/CoFe2O4 photocatalytic oxidation system. According to the experimental and characterization results, one plausible mechanism for enhanced Hg0 removal performance over AgI–BiOI/CoFe2O4 composites was proposed.

Effect of preparation methods on the performance of MnOx-TiO2 adsorbents for Hg0 removal and SO2 resistance

Abstract Aiming at the difficulty of elemental mercury (Hg0) removal from flue gas due to its indissolubility in water and the problem of lower SO2 resistance performance of manganese-based adsorbent, the MnOx-TiO2 adsorbents prepared with impregnation (IM), sol-gel (SG) and deposition-precipitation method (DP) were employed to remove Hg0 in the absence and presence of SO2. The adsorbents were characterized by N2 adsorption-desorption, TG-DSC, XRD, TEM, H2-TPR, and XPS techniques. The results showed that Hg0 removal performance over MnOx-TiO2 adsorbents was markedly influenced by the preparation methods. The adsorbent prepared by DP method exhibited a superior activity for Hg0 adsorption and the best SO2 resistance performance. The characterization results indicated that the Hg0 removal activity did not correlate with the BET surface area. The preparation method of deposition-precipitation could not only lead to an increase of reducibility and high dispersion of MnOx, but also significantly enhance a migration of well-dispersed active phase from bulk to surface, resulting in a higher Mn4+/Mn ratio and the presence of abundant chemisorbed oxygen, which would play an important role in promoting Hg0 removal.

Modified Random Pore Model Study on Coal Char Reactions under O2/CO2 Atmosphere

Evolution characteristics of particle pore structure not only reflect reactive process, but also intensify the complicated degree of the reaction. Variation of pore structure in coal char particles is too complex to be described only by the invariable structure parameter Â? when the random pore model (RPM) is used to describe reaction process. In this paper, the structural parameter Â? in random pore model was modified by the pore properties and evolution characteristics of structural parameter were clarified and a new model, modified random pore model (MRPM), was developed. Compared with the simulation results of RPM, it was found that modified random pore model was more satisfied to explain the reaction processes of char, especially at the end of stage. The value of Â? changes with specific characteristic which could be explained by the pore structure feature. Using the MRPM, isotherm reactive characteristics of coal chars were analyzed at different temperatures under O 2 /CO 2 atmosphere.

Adsorption of Hg0 from Coal Combustion Flue Gases by Novel Iodine-Modified Bentonite/Chitosan Sorbents

Adsorption experiments of vapor-phase elemental mercury (Hg 0 ) were carried out by using modified bentonite/chitosan in a laboratory-scale fixed-bed reactor. VM3000 online mercury analyzer was applied to detect the inlet and outlet mercury concentrations. The characterizations of the sorbents were analyzed using the method of nitrogen (N 2 ) adsorption-desorption, Thermal gravimetric analysis (TGA) and X-ray diffraction (XRD). It is observed that porosity and specific surface area decreases after modifying. The TGA analysis demonstrates these sorbents will operate stably at flue-gas temperatures below 140°C, which can meet the temperature requirement of mercury removal after the electrostatic precipitator. The XRD analysis indicates that the iodine and chitosan is found in the inlayer of bentonite, and the chemical reactions of iodine and sulfuric acid with the amide of chitosan occurr. Fixed-bed adsorber tests show that iodine-modified bentonite-chitosan sorbents exhibit better mercury capture than that of iodine-modified chitosan. For the iodine-modified chitosan-supported bentonite sorbents, mercury removal capacity could be significantly promoted when an appropriate content of H 2 SO 4 was added. The mercury capacities of modified chitosan sorbents increase with increasing temperature. The increase in mercury removal efficiency with an increase in temperature is a typical of a chemisorption mechanism.