Critical Issues of Chemical Kinetics in MILD Combustion

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Easy tuning of nanotexture and N doping of carbonaceous particles produced by spark discharge

A better understanding of the effects of carbonaceous particulates in air pollution on human health and on the transmission of viruses requires studies with artificially produced aerosols that mimic the real ones. To produce such aerosols, methods to precisely tailor the morphology as well as the physical and chemical properties of carbon-based nanomaterials are crucial. Here we describe a facile and flexible approach to produce carbon-based nanoparticles with tailored N content by spark discharge utilizing graphite rods. Carbon-based nanoparticles with different nanotexture and N doping could be obtained by simply changing dilution gas (nitrogen, argon) and dilution gas purity (99 and 99.999%). The effect of the discharge frequency (50, 300 Hz) was also explored. The carbon-based nanoparticles were characterized by Fourier transform infrared and X-ray photoelectron spectroscopy, thermogravimetric analysis, and transmission electron microscopy. We find that the nanotexture is strictly linked to the chemical reactivity and to the surface chemistry. The use of N2 as dilution gas allowed for the incorporation of significant amounts of nitrogen (5–7 wt.%) in the carbonaceous particle network mainly as pyrrolic N, graphitic N and N-oxide functional groups

A kinetic study of the oscillating combustion of hydrogen and syngas in well-stirred reactors

The establishment of permanent oscillations (“limit cycles”) has been often observed in the oxidation of several hydrocarbons in premixed, non-adiabatic systems like well-stirred reactors [1-3]. In such cases, the interaction between mass flow, heat exchange and chemical kinetics results in a periodic extinction and reignition of the system. Several operating parameters have been found to influence the establishment of periodic limit cycles: beyond the fuel type and the dilution level, the oscillatory behavior is affected by the reactor temperature, pressure and residence time. Thus, the high number of parameters makes theoretical analysis a necessary step to understand the causes of such phenomena. The simplest system to be studied is the combustion of hydrogen in a premixed reactor. Such configuration was first studied by Baulch et al. [2, 4, 5]. The oxidation of CO was also separately analyzed [6, 7]. In this work, a kinetic analysis of the hydrogen and syngas oxidation in isothermal, well stirred reactors is carried out. By adopting detailed kinetic mechanisms, the boundaries of the oscillating regions are defined through a parametric study. The Rate of Production (ROP) Analysis is adopted to understand the critical reaction paths

Effects of benzydamine and mouthwashes containing benzydamine on Candida albicans adhesion, biofilm formation, regrowth, and persistence

Objectives To assess the effects of benzydamine and mouthwashes (MoWs) containing benzydamine on different stages of Candida albicans biofilm: adhesion, formation, persistence, and regrowth (if perturbed). Materialsandmethods C.albicansCA1398,carryingthebioluminescenceACT1p-gLUC59fusionproduct,wasemployed. Fungal cells were exposed for 1\u2032, 5\u2032, or 15\u2032 to 4 different benzydamine concentrations (0.075 to 0.6%) to 2 mouthwashes (MoWs) containing benzydamine and to a placebo MoW (without benzydamine). Treated cells were tested for adhesion (90 min) and biofilm formation (24-h assay). Next, 24- and 48-h-old biofilms were exposed to benzydamine and MoWs to assess regrowth and persistence, respectively. The effects of benzydamine, MoWs containing benzydamine, and placebo on different biofilm stages were quantified by bioluminescence assay and by the production of quorum sensing (QS) molecules. Results Benzydamine and MoWs containing benzydamine impaired C. albicans ability to adhere and form biofilm, counter- acted C. albicans persistence and regrowth, and impaired a 48-h-old biofilm. Some of these effects paralleled with alterations in QS molecule secretion. Conclusions Our results show for the first time that benzydamine and MoWs containing benzydamine impair C. albicans capacity to form biofilm and counteract biofilm persistence and regrowth. Clinical relevance Benzydamine and MoWs containing benzydamine capacity to affect C. albicans biofilm provides an interesting tool to prevent and treat oral candidiasis. Likely, restraining C. albicans colonization through daily oral hygiene may counteract colonization and persistence by other critical oral pathogens, such as Streptococcus mutans, whose increased virulence has been linked to the presence of C. albicans biofilm

Evaluation of benzydamine effects on Candida albicans adhesion, biofilm formation and persistence onto abiotic surfaces

Introduction. Candida albicans is the most abundant yeast colonizing the oral cavity. It behaves as an opportunistic pathogen, causing mucosal infections mainly in immunocompromised individuals; in addition, it is often associated to patients suffering from diabetes, oral cancer and terminally ill conditions. Benzydamine hydrochloride is a non-steroidal and anti-inflammatory agent. It has been included in the formulation of several mouthwashes because endowed with analgesic and anesthetic properties. Since benzydamine exerts antibacterial and antifungal activity in vitro, we assessed if this molecule could affect C. albicans virulence traits, such as adhesion, biofilm formation and persistence on abiotic surfaces. Materials and Methods. C. albicans CA1398, carrying the bioluminescence ACT1p-gLUC59 fusion product, was employed. Firstly, fungal cells were exposed for 1\u2019, 5\u2019, or 15\u2019 to 4 different benzydamine concentrations (0.075%, 0.15%, 0.3% and 0.6%) and then tested for their capacity to adhere to plastic (90\u2019 incubation) or to form a biofilm (24h assay). Secondly, 24 and 48h-old biofilms were exposed to the same concentrations of benzydamine and for the same times in order to assess biofilm persistence and regrowth. Benzydamine effects were quantified by measuring, in parallel, metabolically active fungal cells (bioluminescence assay) and viable cells (Colony Forming Units assay). Results. Benzydamine impaired ability to adhere to plastic and to form biofilm, in a dose-dependent fashion; such effects could be ascribed to a direct effect of benzydamine on Candida viability only when using the highest dosage. Moreover, benzydamine caused a dose-dependent decrement in the viability of Candida cells embedded in biofilm, no matter whether a 24h- or a 48h-old sessile community was tested. Discussion and Conclusions. Benzydamine not only impairs C. albicans biofilm formation, profoundly affecting the initial step of fungal cell adhesion to abiotic surfaces, but it is also able to counteract persistence and regrowth of a preformed biofilm. The capacity of benzydamine to affect C. albicans, a fungus responsible of oral diseases in several categories of susceptible subjects, makes this molecule a very interesting tool for both prevention and treatment of oral candidiasis. Studies employing benzydamine-containing mouthwashes will be carried out, in order to assess and compare the anti-Candida effects of different commercial products

Effects of Bath Gas and NO x Addition on n -Pentane Low-Temperature Oxidation in a Jet-Stirred Reactor

International audienceThe oxidation of n-pentane (C5H12) in different bath gases (He, Ar, and CO2) and in Ar with NO2 or NO addition has been studied in a jet-stirred reactor at 107 kPa, temperatures between 500 and 1100 K, with a fixed residence time of 2.0 s, under stoichiometric conditions. Four different quantification diagnostics were used: gas chromatography, a chemiluminescence NOx analyzer, continuous wave cavity ring-down spectroscopy, and Fourier transform infrared spectroscopy. The results showed that the onset temperature of the fuel reactivity was the same (575 K) regardless of the type of bath gases. Although the low-temperature fuel oxidation window was not affected by the type of bath gas, the n-pentane conversion was slightly larger when diluted in Ar through the negative temperature coefficient (NTC) region (625-725 K). Above 800 K, the reactivity according to the diluent was in the order CO2 > Ar > He. In the presence of NO2 or NO, it was found that the consumption rate of n-pentane exhibited a different trend below 700 K. The presence of NO2 did not modify the fuel conversion below 675 K. On the contrary, NO addition increased the onset temperature of the fuel reactivity by 75 K and almost no NTC zone was observed. This clearly indicated that NO addition inhibited n-pentane oxidation below 675 K. Above 700 K, n-pentane conversion was promoted by the presence of both NOx additives. The intermediate species HONO was quantified, and a search for HCN and CH3NO2 species was also attempted. A new detailed kinetic mechanism was developed, which allowed a good prediction of the experimental data. Reaction rate and sensitivity analyses were conducted to illustrate the different kinetic regimes induced by the NOx addition. The inhibition by NO of the n-pentane oxidation below 675 K can be explained by its direct reaction with C5H11O2 radicals disfavoring the classical promoting channels via isomerizations, second O2 addition, and formation of ketohydroperoxides, the well-known branching agents during alkane oxidation. With respect to NO2 addition, the major consumption route is via NO2 + CH3 = NO + CH3O, which is not directly related to the direct fuel consumption. HONO formation mainly derives from NO2 reacting with CHiO (i = 2, 3). The reaction, HONO + M = OH + NO + M, is one of the most sensitive reactions for HONO depletion.

Methane combustion in various regimes: first and second thermodynamic-law comparison between air-firing and oxyfuel condition

MILD oxyfuel combustion has been attracting increasing attention as a promising clean combustion technology. How to design a pathway to reach MILD oxyfuel combustion regime and what can provide a theoretical guide to design such a pathway are two critical questions that need to be answered. So far there has been no open literature on these issues. A type of combustion regime classification map proposed in our previous work, based on the so-called ”Hot Diluted Diffusion Ignition” (HDDI) configuration, is adopted here as a simple but useful tool to solve these problems. Firstly, we analyze comprehensively the influences of various dilution atmosphere and fuel type on combustion regimes. The combustion regime classification maps are made out according to the analyses. In succession, we conduct a comparison between the map in air-firing condition and its oxyfuel counterpart. With the aid of the second thermodynamic-law analysis on the maps, it is easy to identify the major contributors to entropy generation in various combustion regimes in advance, which is crucial for combustion system optimization. Moreover, we find that, for the first time, a combustion regime classification map also may be used as a safety indicator. With the aid of these maps, some conclusions in previous publications can be explained more straightforwardly

Oscillatory Behavior in Methane Combustion: Influence of the Operating Parameters

The influence of the main process parameters on the oscillatory behavior of methane oxidation was analyzed in conditions relevant for low-temperature combustion processes. The investigation was performed by means of direct comparisons between experimental measurements realized in two jet-stirred flow reactors used at atmospheric pressure. With the operating conditions of the two systems coupled, wide ranges of the inlet temperature (790-1225 K), equivalence ratio (0.5 < Φ < 1.5), methane mole fraction (XCH4 from 0.01 to 0.05), bath gases (i.e., He, N2, CO2, or H2O) and different overall mixture dilution levels were exploited in relation to the identification of oscillatory regimes. Although the reference systems mainly differ in thermal conditions (i.e., heat exchange to the surroundings), temperature measurements suggested that the oscillatory phenomena occurred when the system working temperature accessed a well-identifiable temperature range. Experimental results were simulated by means of a detailed kinetic scheme and commercial codes developed for complex chemistry processes. Simulations were also extended considering systems with different heat losses to the surroundings, thus passing from adiabatic to isothermal systems. Results highlighted the kinetic nature of the dynamic behavior. Because predictions were consistent with experimental tests, further numerical analyses were realized to identify the kinetics responsible for the establishment of oscillatory phenomena. Temperature oscillations were predicted for a significant reactor working temperature range, where oxidation and recombination kinetic routes, involving carbon C1-2 species as well as reactions of the H2/O2 sub-scheme, become competitive, thus boosting limit cycle behaviors. Oscillatory phenomena cease when the system working temperatures exceed characteristic threshold values with the promotion of faster oxidation routes that diminish the inhibiting effects of recombination reactions

Review on ammonia as a potential fuel: from synthesis to economics

Ammonia, a molecule that is gaining more interest as a fueling vector, has been considered as a candidate to power transport, produce energy, and support heating applications for decades. However, the particular characteristics of the molecule always made it a chemical with low, if any, benefit once compared to conventional fossil fuels. Still, the current need to decarbonize our economy makes the search of new methods crucial to use chemicals, such as ammonia, that can be produced and employed without incurring in the emission of carbon oxides. Therefore, current efforts in this field are leading scientists, industries, and governments to seriously invest efforts in the development of holistic solutions capable of making ammonia a viable fuel for the transition toward a clean future. On that basis, this review has approached the subject gathering inputs from scientists actively working on the topic. The review starts from the importance of ammonia as an energy vector, moving through all of the steps in the production, distribution, utilization, safety, legal considerations, and economic aspects of the use of such a molecule to support the future energy mix. Fundamentals of combustion and practical cases for the recovery of energy of ammonia are also addressed, thus providing a complete view of what potentially could become a vector of crucial importance to the mitigation of carbon emissions. Different from other works, this review seeks to provide a holistic perspective of ammonia as a chemical that presents benefits and constraints for storing energy from sustainable sources. State-of-the-art knowledge provided by academics actively engaged with the topic at various fronts also enables a clear vision of the progress in each of the branches of ammonia as an energy carrier. Further, the fundamental boundaries of the use of the molecule are expanded to real technical issues for all potential technologies capable of using it for energy purposes, legal barriers that will be faced to achieve its deployment, safety and environmental considerations that impose a critical aspect for acceptance and wellbeing, and economic implications for the use of ammonia across all aspects approached for the production and implementation of this chemical as a fueling source. Herein, this work sets the principles, research, practicalities, and future views of a transition toward a future where ammonia will be a major energy player