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• Coal ash and AAEMs have a significant promoting effect on the NH 3 oxidation. • AAEMs promote the decomposition of N 2 O at 800 °C. • Alkali metals (K and Na) only promote the reduction of NO at 600–800 °C. • AAEMs enhance the adsorption capacity of NH 3 on the active site of the coal ash. • A appropriate O 2 concentration should be selected for the power plant. Ammonia as an eco-friendly and carbon-free energy has been greatly concerned at home and abroad. It has been utilized in the ammonia and coal co-firing process, demonstrating robust market viability. Nonetheless, due to the element N within ammonia causing the production of NO x , the catalytic effect and reaction mechanics of AAEMs in coal ash on the NH 3 oxidation remain elusive. In this study, the catalytic effects of AAEMs (K, Ca, Na and Mg) on NH 3 oxidation were investigated through experiments, and the reaction mechanism was elucidated from a microscopic perspective by density functional theory (DFT). The experimental results show that the addition of AAEMs does enhance the oxidation potential of NH 3 during coal and ammonia co-firing process, and promote the transformation of NH 3 to NO, N 2 O and N 2. However, the promotion effect of different AAEMs is different, consequently influencing the selectivity of NH 3 oxidation pathway. At 600–800 °C, alkaline earth metals (Ca and Mg) facilitate the oxidation of NH 3 while alkali metals (K and Na) have the opposite effect. Mg has the highest promoting effect on the conversion of NH 3 to NO at 600–800 °C, but the catalytic effect of K and Na is more dominant from 800 °C to 900 °C. The transformation from NH 3 to N 2 O remains the same, but the catalytic impact of Ca is mostly pronounced. AAEMs can reduce thermal decomposition activation energy of N 2 O, and the decomposition of N 2 O can be promoted at 800 °C. The DFT results indicate that AAEMs can enhance the adsorption capacity of NH 3 on the active site of the coal ash surface, and reduce the activation energy of NH 3 oxidation, thereby accelerating the transformation of NH 3 to other nitrogen-containing species. This study provides new insight into the evidence for the enhanced catalytic effect of AAEMs on NH 3 oxidation during ammonia and coal co-firing process. [ABSTRACT FROM AUTHOR]

Abstract: The surface chemistry of NH3 decomposition and oxidation was examined over M n+/Ce n+/Na+/γ-Al2O3 commercial catalysts incorporating Pt, Pd, and Rh in order to clarify the role of these materials in converting NH3 into N2 and NO x within the dense phase of FCC regenerators. All catalysts were active in promoting the decomposition of NH3 into N2 in the 500–700°C temperature range, with the Rh containing catalyst being the most active at 700°C. Oxygen species activated on the surfaces of these materials significantly enhanced the selective conversion of NH3 into N2. When O2 was present in the feed, complete conversion of NH3 – with approximately 80% selectivity to N2 – was observed over the reduced Pt-, Pd-, and Rh-containing materials at temperatures as low as 200, 300, and 350°C, respectively. At temperatures below 350°C, the NH3 oxidation reaction follows an iSCR-type pathway, in which a fraction of NH3 is converted into NO x and subsequently into surface nitrite/nitrate species capable of reacting with the remaining portion of NH3 to form N2. Although the oxidation of NH3 to NO x prevails at 700°C under excess O2 conditions, optimal O2 concentrations can be found under which both complete NH3 conversion and high N2 selectivity are achieved. Under these conditions, the noble metal component of the M n+/Ce n+/Na+/γ-Al2O3 materials promotes the reaction of NH3 with the NO x formed, yielding N2. Excess O2 effectively hinders this reaction pathway at high temperatures. Overall, the results presented herein indicate that the significant increase in NO x emissions, typically observed over conventional Pt-based FCC CO combustion promoters, could be directly attributed to the oxidation of NH3 formed in the reducing zone of the FCC regenerators. [Copyright &y& Elsevier]

The oxidation of NH3 over three different types of partially sulphated limestone is investigated experimentally and theoretically. The experiments were made in a laboratory fixed bed reactor at atmospheric pressure at 850°C to simulate conditions of fluidized bed combustion. The catalytic activity of calcined limestone decreases strongly when it is sulphated. The decrease in the rate of NH3 oxidation is mainly caused by the conversion of CaO into less active CaSO4 and by the formation of an impervious layer of CaSO4 around the CaO core.A model based on the grain model has been developed to describe the decrease of activity for NH3 oxidation with increasing conversion of CaO to CaSO4. The changes in structure and in composition of the limestone particle are incorporated in the model. The simulation results are in good agreement with the experimental data.Besides the decrease in activity, the selectivity for the formation of NO decreases during sulphation of the limestone. In experiments, it was shown that the decrease in selectivity is independent of the NO concentration and therefore is not the result of NO reduction by NH3 as proposed in the literature. Further, it was shown that while the formation of NO decreases with increasing CaO conversion the formation of N2 is almost not influenced. From these observations and from simulation results it was concluded that the NH3 oxidation to N2 over CaSO4 is the major cause of the decrease in selectivity. To our knowledge, this is the first validated explanation for the decrease in selectivity for NO during sulphation of calcined limestone. [Copyright &y& Elsevier]

Natural manganese ore (NMO) catalysts were characterized and tested in the selective catalytic oxidation of ammonia to nitrogen oxides under dilute conditions. Also, the oxidation of ammonia (NH3) was carried out using pure MnO2, Mn2O3 for comparing with the activity. It is found that the activity of NMO was similar to that of MnO2 at low temperature below 150 °C but above this temperature, the activity of these catalysts showed the difference. In the course of NH3 oxidation, N2, NO, N2O and H2O were produced. But the quantity of NO2 produced in this experiment was negligible. At temperature below 250 °C, selectivity into N2 from NH3 oxidation was in the order, NMO > MnO2 > Mn2O3. This is the reverse of activity of these manganese oxides. Also the characterization of NH3 oxidation was proposed and supported by the effect of space velocity, inlet O2 and NH3 concentration. The increase of space velocity remarkably influenced not only the conversion but also selectivity into N2. The higher the reaction temperature was, the higher the effect of inlet O2 and NH3 concentration on the reaction rate was. By introducing NO during NH3 oxidation reaction, the possibility of NMO as selective catalytic reduction catalyst at low temperature was studied and showed positive results. [Copyright &y& Elsevier]

The influence of ammonia and nitric oxide oxidation on the selective catalytic reduction (SCR) of NO by ammonia with copper/nickel and vanadium oxide catalysts, supported on titania or alumina have been investigated, paying special attention to N2O formation. In the SCR reaction, the VTi catalyst had a higher activity than VAl at low temperatures, while the CuNiAl catalyst had a higher activity than CuNiTi. A linear relationship between the reaction rate of ammonia oxidation and the initial reduction temperature of the catalysts obtained by H2-TPR showed that the formation rate of NH species in copper/nickel catalysts would be higher than in vanadia catalysts. In situ diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) showed that copper/nickel catalysts presented ammonia coordinated on Lewis acid sites, whereas ammonium ion adsorbed on Brønsted acid sites dominated on vanadia catalysts. The NO oxidation experiments revealed that copper/nickel catalysts had an increase of the NO2 and N2O concentrations with the temperature. NO could be adsorbed on copper/nickel catalysts and the NO2 intermediate species could play an important role in the reaction mechanism. It was suggested that the presence of adsorbed NO2 species could be related to the N2O formation. [Copyright &y& Elsevier]