Search Results (10)

Mn-based oxides are promising catalysts for the selective catalytic reduction (SCR) of NO_x by NH₃ at low temperatures. However, fundamental NH₃-SCR mechanisms and resistance mechanisms against SO_x remain controversial. This study employed density functional theory (DFT) calculations to explore the intrinsic mechanisms of NH₃-SCR and SO_x poisoning on Mn₃O₄(001). Both NH₃ and NO adsorb atop the surface Mn site (the Lewis acid site). In contrast to the traditional Langmuir–Hinshelwood (L-H) mechanism in which gaseous NO is first oxidized to form adsorbed nitrites or nitrates and then react with adsorbed NH_x species to produce H₂O and N₂, a new potential L-H pathway is proposed that involves gaseous NO first adsorbing and then reacting with NH* to generate the key intermediate NHNO*, followed by the formation of H₂O and N₂. This L-H pathway is more efficient as it bypasses the NO oxidation step and is more selective for N₂ formation by avoiding N₂O production. In addition, the L-H mechanism is more favorable than the Eley–Rideal (E-R) mechanism because of the lower free energy profile. SO₂ exhibits limited poisoning effects, whereas SO₃ strongly poisons the Mn₃O₄(001) surface by occupying adsorption sites, hindering intermediate formation and producing ammonium bisulfate. Full article

The ash blockage of the rotary air preheater is a serious problem of the coal-fired boiler that urgently needs to be solved, which is caused by the adhesive deposition of ammonium bisulfate (ABS) and the fly ash. A comprehensive experimental study was performed to investigate the adhesive ash deposition characteristics based on an experimental platform established. The influences of the gas temperature, the gas velocity, the mass ratio of the ABS to the fly ash (R), and the ash particle size on the ash deposition characteristics were mainly analyzed and discussed under different conditions. The experimental results indicate that the liquid ABS is the root cause of the ash particles adhering to the heat transfer elements of the air preheater. The experimental results indicate that when the gas temperature is in the range of 420–493 K, the ABS ash deposition intensity and the ABS adhesion rate both increase with the increase in the gas temperature. When it is 493 K, the ABS adhesion rates of the corrugated plate and the positioning plate both reach maximum values, which are 31.7% and 27.9%, respectively. With the decrease in gas velocity, the total ash deposition intensity, the ABS ash deposition intensity, the ABS adhesion rate, and the growth rate of the ABS adhesion all increase. The content of ABS in the fly ash is also an important factor. When R rises, the ash deposition intensity and the ABS adhesion rate increase significantly. The particle size of the fly ash has little influence on the total ash deposition intensity, but has a great influence on the ABS ash deposition intensity and the ABS adhesion rate. With the increase in the particle size in the range of 30.8–100 μm, the ABS ash deposition intensity decreases by nearly 50%, and the ABS adhesion rates of plates A and B decrease by about 43.9% and 49.6%, respectively. According to the study results, some effective measures can be taken to solve the ash blocking problem of the rotary air preheater, including using the steam air heater, optimizing the operation parameters of the soot blower, and inhibiting ABS formation. Full article

CeNbTi catalyst was poisoned in different sulfur poisoning atmospheres at 300 °C for 6 h and then was evaluated for selective catalytic reduction (SCR) of NO_x with NH₃. The catalyst deactivation upon SO₂ exposure was effectively inhibited in the presence of NH₃. Temperature-programmed decomposition (TPD) analyses were applied to identify deposit species on the poisoned catalysts by comparison with several groups of reference samples. Diffuses reflectance infrared Fourier transform spectroscopy (DRIFTS) over CeNbTi catalysts with different poisoning pretreatments and gas purging sequences were designed to investigate the roles of NH₃ in the removal of surface sulfites and sulfates. More ammonium sulfates including ammonium bisulfate and ammonium cerium sulfate were generated instead of inert cerium sulfate in these conditions. The mechanisms about the formation and transformation of surface deposits upon sulfur poisoning w/wo NH₃ were explored, which provided a basis for developing Ce-based mixed oxides as SCR catalysts for stationary sources. Full article

The application of iron oxide to NH₃-SCR is attractive but largely hindered by its poor acid properties, and surface sulfation is proven to be a prominent way of enhancing the acidity. As such, the method of enriching the sulfate species on iron oxide is crucial for improving the NH₃-SCR performance. In the present study, by employing ammonium bisulfate (ABS) as the source of gaseous SO₂ for the purpose of trapping, we reported an effective strategy for enhancing the SO₄²⁻ immobilization on α-Fe₂O₃ catalyst via spatial confinement in a mesoporous SBA-15 framework. Interestingly, although the presence of the mesopore channel had an adverse effect on the ABS decomposition, which was expected to produce less available SO₂, the measured SO₄²⁻ immobilized on α-Fe₂O₃ in the mesoporous SBA-15 system was significantly greater than that of the regular SiO₂, demonstrating the promoting effect of the spatial confinement on the SO₄²⁻ enrichment. Further characterizations of the NH₃-TPD, NO oxidation, and NH₃-SCR performance tests proved that, as a result of the enhanced acidity, the enrichment of SO₄²⁻ on α-Fe₂O₃ displayed a clear correlation with the SCR activity. The results of the present study provide an effective strategy for boosting the catalytic performance of iron oxide in NH₃-SCR via SO₄²⁻ enrichment. Full article

The food industry is not the only sphere of human activity where inorganic food additives are globally used. In certain concentrations, they are safe for people and agricultural animals. Nonetheless, they impose a negative impact on other classes of living organisms. Therefore, our objective was to determinine the influence of some inorganic food additives (alkalis, acids, salts) on the vitality of nematode larvae that parasitize agricultural animals: Strongyloides papillosus, Haemonchus contortus and Muellerius capillaris. We studied the effects of sodium hydroxide, potassium hydroxide, boric acid, phosphoric acid, potassium chloride, calcium chloride, sodium nitrite, potassium nitrite, sodium nitrate, potassium nitrate, ammonium bicarbonate, sodium bisulfite, sodium bisulfate, sodium sulfate, potassium sulfate, calcium sulfate, sodium thiosulfate, sodium metabisulfite, potassium metabisulfite, copper sulfate pentahydrate, tetrasodium pyrophosphate, sodium triphosphate, sodium borate decahydrate and talc. In in vitro experiments, the strongest effects were produced by alkalis sodium hydroxide and potassium hydroxide. In 24 h, 1% solutions of those substances killed 69% of larvae of S. papillosus, H. contortus and M. capillaris of various development stages. Sodium sulfate was effective against all stages of larvae of S. papillosus, and also against first-age M. capillaris. Nematocidal properties only against all stages of S. papillosus were exerted by copper sulfate pentahydrate. Non-invasive stages of S. papillosus nematodes were affected only by phosphoric acid, ammonium bicarbonate, calcium chloride, sodium nitrite, calcium sulfate, potassium metabisulfite, tetrasodium pyrophosphate, sodium triphosphate and the same stages of M. capillaris—by phosphoric acid, sodium bisulfite and potassium nitrite. Full article

In this review, we elucidate the central role played by fossil fuel combustion in the health-related effects that have been associated with inhalation of ambient fine particulate matter (PM_2.5). We especially focus on individual properties and concentrations of metals commonly found in PM air pollution, as well as their sources and their adverse health effects, based on both epidemiologic and toxicological evidence. It is known that transition metals, such as Ni, V, Fe, and Cu, are highly capable of participating in redox reactions that produce oxidative stress. Therefore, particles that are enriched, per unit mass, in these metals, such as those from fossil fuel combustion, can have greater potential to produce health effects than other ambient particulate matter. Moreover, fossil fuel combustion particles also contain varying amounts of sulfur, and the acidic nature of the resulting sulfur compounds in particulate matter (e.g., as ammonium sulfate, ammonium bisulfate, or sulfuric acid) makes transition metals in particles more bioavailable, greatly enhancing the potential of fossil fuel combustion PM_2.5 to cause oxidative stress and systemic health effects in the human body. In general, there is a need to further recognize particulate matter air pollution mass as a complex source-driven mixture, in order to more effectively quantify and regulate particle air pollution exposure health risks. Full article

Nitrogen dioxide is one of the most dangerous air pollutants, because its high concentration in air can be directly harmful to human health. It is also responsible for photochemical smog and acid rains. One of the most commonly used techniques to tackle this problem in large combustion plants is selective catalytic reduction (SCR). Commercial SCR installations are often equipped with a V₂O₅−WO₃/TiO₂ catalyst. In power plants which utilize a solid fuel boiler, catalysts are exposed to unfavorable conditions. In the paper, factors responsible for deactivation of such a catalyst are comprehensively reviewed where different types of deactivation mechanism, like mechanical, chemical or thermal mechanisms, are separately described. The paper presents the impact of sulfur trioxide and ammonia slip on the catalyst deactivation as well as the problem of ammonium bisulfate formation. The latter is one of the crucial factors influencing the loss of catalytic activity. The majority of issues with fast catalyst deactivation occur when the catalyst work in off-design conditions, in particular in too high or too low temperatures. Full article

Mineral carbonation routes have been extensively studied for almost two decades at Åbo Akademi University, focusing on the extraction of magnesium from magnesium silicates using ammonium sulfate (AS) and/or ammonium bisulfate (ABS) flux salt followed by carbonation. There is, however, a need for proper recovery and recirculation of chemicals involved. This study focused on the separation of AS, ABS and aqueous ammonia using different setups of bipolar membrane electrodialysis using both synthetic and rock-derived solutions. Bipolar membranes offer the possibility to split water, which in turn makes it possible to regenerate chemicals like acids and bases needed in mineral carbonation without excess gas formation. Tests were run in batch, continuous, and recirculating mode, and exergy (electricity) input during the tests was calculated. The results show that separation of ions was achieved, even if the solutions obtained were still too weak for use in the downstream process to control pH. Energy demand for separating 1 kg of NH₄⁺ varied in the range 1.7, 3.4, 302 and 340 MJ/kg NH₄⁺, depending on setup chosen. More work must hence be done in order to make the separation more efficient, such as narrowing the cell width. Full article

Due to the low temperature and complex composition of the exhaust gas of the marine diesel engine, the working requirements of the selective catalytic reduction (SCR) catalyst cannot be met directly. Moreover, ammonium sulfate, ammonium nitrate, and other ammonium deposits are formed at low temperatures, which block the surface or the pore channels of the SCR catalyst, thereby resulting in its reduction or even its loss of activity. Considering the difficulty of the marine diesel engine bench test and the limitation of the catalyst sample test, a one-dimensional simulation model of the SCR system was built in this paper. In addition, the deactivation reaction process of the ammonium salt in the SCR system and its influencing factors were studied. Based on the gas phase and the surface reaction kinetics, the models of the urea decomposition, the surface denitrification, the nitrate deactivation, and the sulfate deactivation were both constructed and verified in terms of accuracy. Moreover, the formation/decomposition reaction pathway and the catalytic deactivation of ammonium nitrate and ammonium bisulfate, as well as the composition concentration and the exhaust gas temperature range were correspondingly clarified. The results showed that within a certain range, the increase of the NO₂/NO_x ratio was conducive to the fast SCR reaction and the NH₄NO₃ formation’s reaction. Increasing the exhaust gas temperature also raised the NO₂/NO_x ratio, which was beneficial to both the fast SCR reaction and the NH₄NO₃ decomposition reaction, respectively. Furthermore, the influence of the SO₂ concentration on the denitrification efficiency decreased with the increase of the exhaust gas temperature because of increasing SCR reaction rate and reversibility of ammonia sulfate formation, and when the temperature of the exhaust gas was higher than 350 °C, the activity of the catalyst was almost unaffected by ammonia sulfate. Full article

The separation of ammonium bisulfate (ABS) from ammonium sulfate (AS) in aqueous solutions by monovalent ion selective membranes was studied. Optimised usage of these chemicals is both an important and challenging step towards a more efficient CO₂ mineralisation process route developed at Åbo Akademi University (ÅA). The membranes were placed in a three or five-compartment electrodialysis stack. Silver, stainless steel and platinum electrodes were tested, of which a combination of Pt (anode) and stainless steel (cathode) electrodes were found to be most suitable. Separation efficiencies close to 100% were reached based on ABS concentrations in the feed solution. The tests were performed with an initial voltage of either 10 V–20 V, but limitations in the electrical power supply equipment eventually resulted in a voltage drop as separation proceeded. Exergy calculations for energy efficiency assessment show that the input exergy (electrical power) is many times higher than the reversible mixing exergy, which indicates that design modifications must be made. Further work will focus on the possibilities to make the separation even more efficient and to develop the analysis methods, besides the use of another anode material. Full article