Search

The mechanisms of medium- and low-temperature denitrations using selective catalytic reduction (SCR) by combined CO + NH3 on Mn–Ce/activated carbon (AC) catalysts and their poisoning by SO2 were investigated. Several Mn–Ce/AC catalysts prepared and then poisoned by SO2 were used in the study. The physicochemical properties of the catalysts were systematically characterized before and after poisoning, and the mechanism of SO2 poisoning was proposed. SO2 poisoning considerably affects the Mn–Ce/AC catalyst, decreasing its denitration rate from 98% to 47.9% and N2 selectivity from 96 to 85%. This can be attributed to several phenomena. SO2 poisoning increases the surface roughness of the catalyst and results in the formation of Mn(SO4)2, other sulfates, and other substances, which block the catalyst pores and reduce the content of the C and O elements on the catalyst surface as well as its specific surface area, consequently decreasing the adsorption area available for the reaction gas. Moreover, the crystallinity of MnOx on the surface of the catalyst increases, decreasing the contents of Mn4+, Ce4+ and chemosorbed oxygen. In addition, the surface functional groups, such as –COOH, –OH, and C=O, are destroyed, reducing the adsorption sites of the reaction gas and the adsorption capacity of the catalyst, consequently inhibiting the SCR reaction. Ce can reduce the effects of SO2 poisoning on the catalyst because Ce4+ can improve its oxygen storage capacity, increase the adsorption sites for the reaction gas, and react with SO2, preventing sulfate formation and pore blockage. However, excessive Ce loading causes metal agglomeration on the catalyst surface, reducing its denitration efficiency. After SO2 poisoning, the contents of Mn4+, Mn3+, and Ce4+ decreased in the Mn -Ce/AC catalysts, inhibiting the synergistic reaction between Mn and Ce, which decreased the number of oxygen vacancies and Oβ, consequently inhibit the reaction's conversion to fast selective catalytic reduction. Thus, SO2 destroyed the functional groups; occupied the oxygen vacancies and active site; and destroyed the –COOH, O–H, and C=O functional groups on the surface of the Mn–Ce/AC catalyst, reducing the acidity and number of active sites on the catalyst surface. [ABSTRACT FROM AUTHOR]

This study investigates the influences of the evaporation temperature and Mg2+ concentration on the crystallization of an ammonium sulfate mother liquor. Specifically, their effects on the solubility, metastable zone width, crystallization amount, average particle size, and coefficient of variation of ammonium sulfate are examined through the laser and evaporation crystallization methods. Results show that solubility increases and the metastable zone width narrows with an increase in the evaporation temperature. At an evaporation temperature of 338.15 K, the controllability of the crystallization process improves and explosive nucleation does not easily occur. In this case, crystals with large average particle sizes, regular morphologies, and high crystallinity are obtained. With an increase in the Mg2+ concentration in the solvent, solubility decreases. The added Mg2+ covers the active nucleation sites, thus hindering the nucleation of ammonium sulfate and widening the metastable zone width. At a Mg2+ concentration of 0.9 g L−1 or higher, Mg2+ covers the active surfaces of the grains. This inhibits normal crystal growth and hinders the nucleation and growth of ammonium sulfate crystals, so the crystallization amount of ammonium sulfate significantly reduces. [ABSTRACT FROM AUTHOR]

This study aims to explore the influence mechanism of ammonium nitrate produced by ozone denitrification on the crystallisation of ammonium sulfate, a by-product of ammonia desulfurisation. The laser method was used to study the influence of ammonium nitrate on the solubility and metastable zone width of ammonium sulfate. An experiment on the influence of ammonium nitrate on the particle size of ammonium sulfate was designed, and the influence mechanism was explored through scanning electron microscopy and X-ray diffraction. The findings showed that the addition of ammonium nitrate increased the size and aspect ratio of ammonium sulfate crystals. The addition of ammonium nitrate inhibited the dissolution of ammonium sulfate and widened its metastable zone. The addition of ammonium nitrate covered the active sites of crystal nucleus growth, which inhibited the formation of crystal nuclei to a certain extent, and crystal growth dominated the crystallisation process. Moreover, the addition of ammonium nitrate induced the preferred orientation of the specific crystal plane of ammonium sulfate, and the addition of a small concentration of ammonium nitrate decreased the crystallinity of ammonium sulfate. The research results can provide a reference for crystallisation optimisation and quality improvement of ammonium sulfate in the ammonia desulfurisation process. [ABSTRACT FROM AUTHOR]

To investigate the effect of gas activation on the low-temperature ammonia removal performance of activated carbon during nitric oxide (NO) removal, nitrogen (N2), carbon dioxide (CO2), and air were used as activated gases. Orthogonal experiments were designed to examine the activation conditions of each factor. The influence of various activation conditions on the physicochemical properties of activated carbon has been investigated. The interaction between various factors and their effect on the NH3 removal performance of NO were thoroughly investigated. Results show smooth pore walls and large average pore size after N2 activation. Moreover, some nitrogen-containing functional groups are introduced to the surface and help improve the NO conversion rate. CO2 activation can readily disrupt pore structures, causing surface pores to become disordered and some lignin and other functional groups to decompose; thus, the NO conversion rate by the activated carbon following CO2 activation is low. The strong oxidation of air activation causes the pore wall of activated carbon to collapse, the formation of new pores on the surface, and the introduction of oxygen-containing functional groups, so that the denitration rate increases with increasing activation temperature. The kind of activated gas has the most significant effect on the NO removal performance of activated carbon, while the activation temperature has minimal effect on the NO conversion rate of activated carbon. N2 activation at 225°C for 60 min is the best physical activation condition. Herein, we focused on three factors: activation gas; activation temperature; and activation time, which influence the nitric oxide conversion rate of activated carbon. Further, the effect of these factors and their interactions on nitric oxide removal performance were studied. Relevant research findings can be utilized as a reference for the physical activation of activated carbon and optimization of the low-temperature NH3 removal performance of activated carbon during nitric oxide removal. [ABSTRACT FROM AUTHOR]

This study aims to explore the effect of the activator type, activation time, and activator concentration on the denitration performance of activated carbon (AC). Combined with denitration experiments, scanning electron microscopy, pore size analysis, and Fourier transform infrared spectroscopy characterization methods, an orthogonal experiment was conducted using zinc chloride (ZnCl2), phosphoric acid (H3PO4), and potassium hydroxide (KOH) as activators to explore the interaction between various factors and their effect on the denitration performance of AC. The results show that the denitration rate of ZnCl2‐activated AC is generally low because a high concentration of ZnCl2 will block the micropores of AC and destroy the original structure. The denitration rate of H3PO4‐activated AC decreased with the increase in H3PO4 concentration, especially severe blockage occurred on the AC‐H3 surface. KOH activation can optimize the pore structure of AC and generate some oxygen‐containing functional groups, which is beneficial to improve the adsorption performance of AC. Orthogonal experiments showed that the activator type had the most obvious effect on the denitration rate, followed by the activation time, whereas reagent concentration had little effect on the denitration rate. The optimum activation condition was 3 mol L−1 KOH at 80°C for 3.5 h. [ABSTRACT FROM AUTHOR]

The response surface method is used to study the influence mechanism of three factors (seed crystal concentration, pH value, and stirring rate) on ammonium sulfate crystallization (average particle size and coefficient of variation [C.V.] value) under ammonium nitrate to prove the effect of ammonium nitrate doping on ammonium sulfate crystallization. Regression equations between each response value and the influencing factors are established. Results show that the interaction between the seed crystal concentration and the stirring rate is stronger than that between the seed crystal concentration and the pH and that between the pH and the stirring rate. The error between the verified model values and the experimental values is minimized when the seed crystal concentration is 1.5%, the pH is 4.5, and the stirring rate is 250 r min−1, at which the average particle size is 1.238 mm, and the C.V. value is 73.53. After optimization, the crystal morphology changes from needle‐ and flake‐like crystals to cubes, resulting in crystals with large average particle size, uniform distribution, regular morphology, and diffraction peak intensity that is close to that in the analysis of pure ammonium sulfate. [ABSTRACT FROM AUTHOR]

To explore the mechanism of low-temperature carbon monoxide and ammonia (CO + NH 3 ) coupling denitration of manganese/activated carbon (Mn/AC) catalysts, Mn/AC series catalysts were prepared using the impregnation method with AC activated by nitric acid as a precursor and manganese nitrate as a precursor. We characterized the surface morphology, pore structure, active component phase, functional group, and active component valence change law of the Mn/AC catalyst. The denitration rate order with different Mn loadings is 7Mn/AC > 9Mn/AC > 5Mn/AC. When the Mn loading was 7%, the catalyst's surface was smooth, with a good pore structure and uniform surface distribution of metal particles. These features increased the reacting gas's contact area, improving the denitration rate. The reason for this was oxygen chemisorption on the catalyst's surface. The Mn 4+ and the number of oxygen-containing functional groups on the catalyst surface increase after Mn loading increases; this provides more active sites for denitration and promotes the reaction's conversion to fast selective catalytic reduction. The low-temperature CO + NH 3 coupling denitration of Mn/AC catalysts conforms to the Langmuir-Hinshelwood mechanism when the temperature is lower than 230 °C and the Eley-Rideal mechanism when the temperature is higher than 230 °C. The research results can provide new ideas for low-temperature flue gas denitration., Competing Interests: There are no conflicts to declare., (This journal is © The Royal Society of Chemistry.)