OH Radical Formation Research Articles

Mechanism study on the auto-ignition and reaction kinetic characteristics of hydrogen/n-dodecane mixture under mid and high temperature conditions

To acquire the ignition promotion methods for scramjet engine, the auto-ignition and reaction kinetic characteristics of hydrogen/n-dodecane mixture under mid and high temperatures were revealed by detailed chemical kinetic mechanism and experimental data. The results illuminated that the increases of n-dodecane's content and ϕ significantly declined the ignition delay time (IDT) of hydrogen mixture (by two orders of magnitude at most) at mid-temperature high-pressure (T = 950 K&p = 40 bar). The addition of n-dodecane induced the OH radical formation in initial stage and the advance of fuel's H-abstraction reactions (hydrogen: R3, n-dodecane: R2741/R2742/R2743/R2746). Whereas, the corresponding reduction in hydrogen content inhibited the establishment of active radical pool in dual-fuel reaction system before ignition time point. The ignition promoting effect of advanced initial reaction stage overweighted the ignition inhibiting effect of weakened rapid reaction stage in mid-temperature low-reactivity dual-fuel reaction system. Interestingly, at high-temperature high-pressure (T = 1300 K&p = 40 bar), the IDT firstly decreased (by 18.8%), and then continuously increased (by 105.3% at most) with the elevating n-dodecane's content. The minor addition of n-dodecane (1% of mole fraction) at high-temperature had a similar ignition promoting effect as that at mid-temperature. Whereas, the further reduction in hydrogen content dramatically suppressed the formation of active radical pool (H/OH/HO2/H2O2), which overweighted the ignition promoting effect of slightly advanced initial reaction stage. In other words, the hydrogen played a more critical role than the n-dodecane in high-temperature reaction system due to the originally high reactivity of hydrogen. The auto-ignition properties of hydrogen/n-dodecane mixture relied on the competition relationship of initial and rapid reaction stages under specific thermodynamic conditions and blending ratios. This investigation not only reveals internal interaction mechanism between fuels and active radicals in hydrogen/n-dodecane reaction system, but also provides fundamental insights for the precisely ignition promition methods of hydrogen/aviation-kerosene dual-fuel scramjet engine.

Enhanced Monitoring of Photocatalytic Reactive Oxygen Species: Using Electrochemistry for Rapid Sensing of Hydroxyl Radicals Formed during the Degradation of Coumarin.

Many recent research studies have reported indirect methods for the detection and quantification of OH radicals generated during photocatalysis. The short lifespan and high reactivity of these radicals make indirect detection using probes such as coumarin a more viable quantification method. Hydroxyl radical production is commonly monitored using fluorescence spectroscopy to determine the concentration of the compound 7-hydroxycoumarin, which is formed from hydroxyl radical attack on coumarin. There are, however, a number of additional hydroxylated coumarins generated during this process, which are less amenable to detection by fluorescence spectroscopy. Consequently, limitations and inaccuracies of this method have previously been reported in the literature. As an alternative approach to those previously reported, this work has developed an electrochemical screening method using coumarin as a OH radical trap, that is capable of in situ monitoring of not only 7-hydroxycoumarin, but all the main mono-hydroxylated products formed. As a result, this technique is a more representative and comprehensive method for the quantification of OH radicals produced by photocatalysts using coumarin as a probe molecule. Moreover, the electroanalytical method provides a portable, rapid, sensitive, and accurate in situ method for the monitoring of OH radical formation without the need for sample preparation.

Peroxymonosulfate activation with an α-MnO2/Mn2O3/Mn3O4 hybrid system: parametric optimization and oxidative degradation of organic dye.

The present study proposed the synthesis of low-toxicity and eco-friendly spherically shaped manganese oxides (α-MnO2, Mn2O3, and Mn3O4) by using the chemical precipitation method. The unique variable oxidation states and different structural diversity of manganese-based materials have a strong effect on fast electron transfer reactions. XRD, SEM, and BET analyses were used to confirm the structure morphology, higher surface area, and excellent porosity. The catalytic activity of as-prepared manganese oxides (MnOx) was investigated for the rhodamine B (RhB) organic pollutant with peroxymonosulfate (PMS) activation under the condition of control pH. In acidic conditions (pH = 3), complete RhB degradation and 90% total organic carbon (TOC) reduction were attained in 60 min. The effects of operating parameters such as solution pH, PMS loading, catalyst dosage, and dye concentration on RhB removal reduction were also tested. The different oxidation states of MnOx promote the oxidative-reductive reaction under acidic conditions and enhance the SO4•-/•OH radical formation during the treatment, whereas the higher surface area offers sufficient absorption sites for interaction of the catalyst with pollutants. A scavenger experiment was used to investigate the generation of more reactive species that participate in dye degradation. The effect of inorganic anions on divalent metal ions that genuinely occur in water bodies was also studied. Additionally, separation and mass analysis were used to investigate the RhB dye degradation mechanism at optimum conditions based on the intermediate's identification. Repeatability tests confirmed that MnOx showed superb catalytic performance on its removal trend.

Mechanisms and Thermochemistry of Reactions of SiO and Si2O2 with OH and H2O.

This paper reports on computational studies of gas-phase reactions of SiO and Si2O2. The oxidation of SiO can initiate efficient formation of silica or silicate dust particles in a wide range of environments. Both OH radicals and H2O molecules are often present in these environments, and their reactions with SiO and the smallest SiO cluster, Si2O2, affect the efficiency of eventual dust formation. Density functional theory calculations on these reactions, benchmarked against accurate coupled cluster calculations, indicate that the Si2O2 + OH reaction should be faster than SiO + OH. The reaction SiO + H2O → SiO2 + H2 is both endothermic and has high activation energies to reaction. Instead, the formation of molecular complexes is efficient. The reaction of Si2O2 with H2O, which has been suggested as efficient for producing Si2O3, might not be as efficient as previously thought. If the H2O molecules dissociate to form OH radicals, oxidation of SiO and Si2O2 could be accelerated instead.

Spontaneous dark formation of OH radicals at the interface of aqueous atmospheric droplets.

Hydroxyl radical (OH) is a key oxidant that triggers atmospheric oxidation chemistry in both gas and aqueous phases. The current understanding of its aqueous sources is mainly based on known bulk (photo)chemical processes, uptake from gaseous OH, or related to interfacial O3 and NO3 radical-driven chemistry. Here, we present experimental evidence that OH radicals are spontaneously produced at the air-water interface of aqueous droplets in the dark and the absence of known precursors, possibly due to the strong electric field that forms at such interfaces. The measured OH production rates in atmospherically relevant droplets are comparable to or significantly higher than those from known aqueous bulk sources, especially in the dark. As aqueous droplets are ubiquitous in the troposphere, this interfacial source of OH radicals should significantly impact atmospheric multiphase oxidation chemistry, with substantial implications on air quality, climate, and health.

Insight into the Mechanism and Kinetics of the Heterogeneous Reaction between SO2 and NO2 on Diesel Black Carbon under Light Irradiation.

The heterogeneous oxidation of SO2 by NO2 has been extensively proposed as an important pathway of sulfate production during haze events in China. However, the kinetics and mechanism of oxidation of SO2 by NO2 on the surface of complex particles remain poorly understood. Here, we systematically explore the mechanism and kinetics of the reaction between SO2 and NO2 on diesel black carbon (DBC) under light irradiation. The experimental results prove that DBC photochemistry can not only significantly promote the heterogeneous reduction of NO2 to produce HONO via transferring photoinduced electrons but also indirectly promote OH radical formation. These reduction products of NO2 as well as NO2 itself greatly promote the heterogeneous oxidation of SO2 on DBC. NO2 oxidation, HONO oxidation, and the surface photo-oxidation process are proven to be three major surface oxidation pathways of SO2. The kinetics results indicate that the surface photooxidation pathway accounts for the majority of the total SO2 uptake (∼63%), followed by the HONO oxidation pathway (∼27%) and direct oxidation by NO2 (∼10%). This work highlights the significant synergistic roles of DBC, NO2, and light irradiation in enhancing the atmospheric oxidation capacity and promoting the heterogeneous formation of sulfate.

Mechanisms of SiO oxidation: Implications for dust formation

Reactions of SiO molecules have been postulated to initiate efficient formation of silicate dust particles in outflows around dying (AGB) stars. Both OH radicals and H2O molecules can be present in these environments and their reactions with SiO and the smallest SiO cluster, Si2O2, affect the efficiency of eventual dust formation. Rate coefficients of gas-phase oxidation and clustering reactions of SiO, Si2O2 and Si2O3 have been calculated using master equation calculations based on density functional theory calculations. The calculations show that the reactions involving OH are fast. Reactions involving H2O are not efficient routes to oxidation but may under the right conditions lead to hydroxylated species. The reaction of Si2O2 with H2O, which has been suggested as efficient producing Si2O3, is therefore not as efficient as previously thought. If H2O molecules dissociate to form OH radicals, oxidation of SiO and dust formation could be accelerated. Kinetics simulations of oxygen-rich circumstellar environments using our proposed reaction scheme suggest that under typical conditions only small amounts of SiO2 and Si2O2 are formed and that most of the silicon remains as molecular SiO.

Nitration of Phenols by Reaction with Aqueous Nitrite: A Pathway for the Formation of Atmospheric Brown Carbon

Nitrophenols are a major component of light-absorbing atmospheric organic aerosols, commonly referred to as brown carbon (BrC). Most nitrophenol formation pathways involve reactions of phenolic compounds with OH, NO3, and NO2 in the gas phase. In this study, an aqueous nitrophenol production pathway is investigated that can proceed in the dark without apparent OH radical formation. Using high-performance liquid chromatography–mass spectrometry, we demonstrate that catechol reacts in acidic solutions with dissolved nitrite to form nitrocatechol. The rate of nitration increases significantly from pH 4.4 to pH 3.4 such that nitrocatechol is susceptible to second-generation reactions under the most acidic conditions producing chromophores that absorb in the visible region (peak at 425 nm). Increases in the N:C ratio of the reaction solution, as detected by aerosol mass spectrometry, and enhanced absorption from 300 to 500 nm of wood and peat smoke aqueous extracts exposed to nitrite suggest that this is a general nitration pathway. The atmospheric conditions under which this BrC formation process may occur are discussed.

Construction of SnO2/CuO/rGO nanocomposites for photocatalytic degradation of organic pollutants and antibacterial applications

Water contamination by reactive dyes is a serious concern for human health and the environment. In this study, we prepared high efficient SnO2/CuO/rGO nanocomposites for reactive dye degradation. For structural analysis of SnO2/CuO/rGO nanocomposites, XRD, UV–Vis DRS, SEM, TEM-EDAX, and XPS analysis were used to characterize the physicochemical properties of the material. The characterization results confirmed great crystallinity, purity, and optical characteristics features. For both Rhodamine B (RhB) and Reactive Red 120 (RR120) degradation processes, SnO2/CuO/rGO nanocomposites were tested for their photocatalytic degradation performance. The SnO2/CuO/rGO nanocomposites have expressed the degradation rate exposed to 99.6% of both RhB and RR120 dyes. The main reason behind the photocatalytic degradation was due to the formation of OH radical's generation by the composite materials. Moreover, the antibacterial properties of synthesized SnO2/CuO/rGO nanocomposites were studied against E. coli, S. aureus, B. subtilis and P. aeroginosa and exhibited good antibacterial activity against the tested bacterial strains. Thus, the synthesized SnO2/CuO/rGO nanocomposites are a promising photocatalyst and antibacterial agent. Furthermore, mechanisms behind the antibacterial effects will be ruled out in near future.

Interactions of H2O and O2 with char during gasification in mixed atmosphere analyzed by isotope tracer method and in-situ DRIFTS

The interactions of char with H2O and O2 were important for describing the synergistic mechanism of char gasification in H2O/O2 atmosphere. The gases produced during gasification in specific atmospheres were tracked by isotope-labeled oxygen tracing experiment. The interactions of H2O with char and O2 with char during gasification in H2O and O2 atmospheres were separately studied. This helped to explore the interactions between H2O and char and O2 and char during the gasification in mixed-gas atmosphere. It was found that H2O inhibited oxidation of char during gasification, whereas O2 promoted the char-H2O reaction in mixed atmosphere. The overall gasification reactivity in mixed-gas atmosphere was determined by superimposing the effects of O2 on char-H2O reactions and the effects of H2O on O2-char reactions. This could also explain the reason for the influence of H2O on the overall changes in reactivity, based on O2 concentration. The interactive mechanisms during char gasification with H2O and O2 atmospheres were investigated by in-situ DRIFTS experiment and molecular dynamics simulation. In the H2O/O2 atmosphere, H2O was more likely to promote the curling of char structure. The free H radicals obtained in the decomposition of H2O molecules consumed the active sites of reaction between char and O2 and decreased its reactivity. Oxygen could inhibit the curling of the char structure caused due to water and promote the formation of OH radicals. This in turn, promoted the cracking of aromatic rings, increased the number of sites for reaction between char and H2O, and improved the gasification of char in H2O. In case of mixed-gas atmosphere, the interactions between isotope decoupled single reactions could provide new understanding.

Influence of Butanol Isomerization on Photothermal Hydrogen Production over Ti@TiO2 Core-Shell Nanoparticles

In this work, we reported for the first time the effect of butanol isomerization on the photothermal production of hydrogen in the presence of a noble, metal-free Ti@TiO2 core-shell photocatalyst. The experiments were performed in aqueous solutions of 1-BuOH, 2-BuOH, and t-BuOH under Xe lamp irradiation (vis/NIR: 8.4 W, UV: 0.6 W) at 35–69 °C. The increase in temperature significantly enhanced H2 formation, indicating a strong photothermal effect in the studied systems. However, in dark conditions, H2 emission was not observed even at elevated temperatures, which clearly points out the photonic origin of H2 photothermal formation. The rate of H2 production followed the order of 1-BuOH >> 2-BuOH > t-BuOH in the entire range of studied temperatures. In the systems with 1-BuOH and 2-BuOH, hydrogen was the only gaseous product measured online in the outlet carrier argon using mass spectrometry. By contrast, a mixture of H2, CH4, and C2H6 was detected for t-BuOH, indicating a C–C bond scission with this isomer during photocatalytic degradation. The apparent activation energies, Ea, with 1-BuOH/2-BuOH isomers (20–21 kJ·mol−1) was found to be larger than for t-BuOH (13 kJ·mol−1). The significant difference in thermal response for 1-BuOH/2-BuOH and t-BuOH isomers was ascribed to the difference in the photocatalytic mechanisms of these species. The photothermal effect with 1-BuOH/2-BuOH isomers can be explained by the thermally induced transfer of photogenerated, shallowly trapped electron holes to highly reactive free holes at the surface of TiO2 and the further hole-mediated cleavage of the O-H bond. In the system with t-BuOH, another mechanism could also contribute to the overall process through hydrogen abstraction from the C–H bond by an intermediate •OH radical, leading to CH3• group ejection. Formation of •OH radicals during light irradiation of Ti@TiO2 nanoparticle suspension in water has been confirmed using terephthalate dosimetry. This analysis also revealed a positive temperature response of •OH radical formation.

Влияние ультразвука на кинетику электроосаждения диоксида свинца на стальную основу и свойства покрытия

The influence of the ultrasonic field on lead dioxide electrodeposition from nitrate electrolyte was studied. The adsorption of electroactive particles and the amount of nucleation on the electrode surface were found to have increased as well as the formation of OH-radicals accelerated under the influence of the ultrasonic field. Fine-crystalline deposits of lead dioxide were formed, which contributed to obtaining high current yield of lead dioxide, coefficient increasing of the electrode active mass and increasing of the discharge capacity of the emergency current generator cell layout. It was shown that the application of ultrasound in producing a lead dioxide electrode made it possible to reduce the time of the technological procedure.

Effects of Nitrogen Oxides on the Production of Reactive Oxygen Species and Environmentally Persistent Free Radicals from α-Pinene and Naphthalene Secondary Organic Aerosols.

Reactive oxygen species (ROS) and environmentally persistent free radicals (EPFR) play an important role in chemical transformation of atmospheric aerosols and adverse aerosol health effects. This study investigated the effects of nitrogen oxides (NOx) during photooxidation of α-pinene and naphthalene on the EPFR content and ROS formation from secondary organic aerosols (SOA). Electron paramagnetic resonance (EPR) spectroscopy was applied to quantify EPFR content and ROS formation. While no EPFR were detected in α-pinene SOA, we found that naphthalene SOA contained about 0.7 pmol μg–1 of EPFR, and NOx has little influence on EPFR concentrations and oxidative potential. α-Pinene and naphthalene SOA generated under low NOx conditions form OH radicals and superoxide in the aqueous phase, which was lowered substantially by 50–80% for SOA generated under high NOx conditions. High-resolution mass spectrometry analysis showed the substantial formation of nitroaromatics and organic nitrates in a high NOx environment. The modeling results using the GECKO-A model that simulates explicit gas-phase chemistry and the radical 2D-VBS model that treats autoxidation predicted reduced formation of hydroperoxides and enhanced formation of organic nitrates under high NOx due to the reactions of peroxy radicals with NOx instead of their reactions with HO2. Consistently, the presence of NOx resulted in the decrease of peroxide contents and oxidative potential of α-pinene SOA.

High photoactivity of ZrxOy-Bi2O2(CO3) composite materials prepared by one-step synthesis for efficient photodegradation of 4-chlorophenol in water

The ZrBi composite materials were prepared in a single step by the chemical co-precipitation method. Bi2O2(CO3) joined ZrxOy modifying the mole percentage from 5 to 11 mol %. ZrBi composites dried at 80 °C were characterized by: XRD, FTIR, DRS, SEM, N2 adsorption, HR-TEM, PL, XPS and EDS-SEM. Later they were evaluated in the photodegradation of 4-Chlorophenol under UV irradiation. The degradation and mineralization percentages were determined after 150 min of reaction by UV–vis spectroscopy and Total Organic Carbon (TOC), respectively. The composite containing 9 mol % of Bi2O2(CO3) (ZrBi-9) with a specific volume of 0.03 m3/kg in solution with specific catalyst density of 1*10−3 kg/L, showed the highest photoactivity with 96 % photodegradation, a higher value compared to that obtained with the reference material TiO2-P25 (72 %). Finally, a possible reaction mechanism was proposed based on recent studies, which allow to follow the formation of active species OH, O2− and h+. The formation of OH radicals was measured by fluorescence spectroscopy while the inhibition of O2− radicals as well as the scavenge of h+ were determined by UV–vis spectroscopy. The results showed that ZrBi composite materials do not promote OH radical formation. The scavenge of h+ showed a total loss of photoactivity and the inhibition of O2− in the reaction medium results in a partial decrease of photoactivity. The formation of heterojunctions, the presence of localized states and oxygen vacancies in the synthesized materials promise an excellent alternative for the rapid photodegradation of 4-Chlorophenol under UV radiation.

Mactanamide and lariciresinol as radical scavengers and Fe2+ ion chelators – A DFT study

A DFT based kinetic study of OOH radical scavenging potency of mactanamide (MA) and lariciresinol (LA), two natural polyphenols, indicates their nearly equal potential via the proton coupled electron transfer (PCET) mechanism in lipid media. Contribution of C–H bond breaking to this potency is negligible compared to O–H bond breaking, contrary to recent claims. The predicted potency of both compounds is not sufficient to protect biological molecules from oxidative damage in lipid media. In aqueous media, the scavenging potency of MA and LA phenoxide anions via the single electron transfer (SET) mechanism is much higher and may contribute to the protection of lipids, proteins, and DNA from OOH radical damage. Also, MA and LA have the potential to chelate catalytic Fe2+ ions, thus suppressing the formation of dangerous OH radicals via Fenton-type reactions. The monoanionic species of MA and LA show stronger monodentate chelating ability with Fe2+ ion compared to its neutral form. The dianionic specie LA2− exhibited the highest chelation ability with Fe2+ ion via bidentate 1:2 coordination. However, direct radical scavenging and metal chelation could be rarely operative in vivo because MA and LA presumably achieve very low concentrations in systemic circulation.

Experimental and Theoretical Kinetic Studies of the Ozonolysis of Selected Allyl Sulfides (H2C═CHCH2SR, R = CH3, CH3CH2): The Effect of Nascent OH Radicals.

Rate coefficients of the O3-initiated oxidation of allyl methyl sulfide (H2C═CHCH2SCH3, AMS) and allyl ethyl sulfide (H2C═CHCH2SCH2CH3, AES) were determined at atmospheric conditions by "in situ" FTIR. The relative kinetic experiments were performed using methylcyclohexane (McH) and carbon monoxide (CO) as nascent OH radical scavengers and in the absence of any scavenger, to determine the impact that the formation of OH radicals has on the rate coefficients. In the absence of scavengers, values of kAMS+O3 = (5.23 ± 3.57) × 10-18 and kAES+O3 = (5.76 ± 1.80) × 10-18 cm3 molecule-1 s-1 were obtained. In the presence of the scavengers, however, the rates decreased to kAMS+O3+McH = (3.92 ± 1.92) × 10-18 and kAMS+O3+CO = (2.63 ± 0.47) × 10-18 cm3 molecule-1 s-1 for AMS, and kAES+O3+McH = (4.78 ± 1.38) × 10-18 and kAES+O3+CO = (3.50 ± 0.27) × 10-18 cm3 molecule-1 s-1 for AES. On the basis of these results, we have decided to recommend the values obtained using CO as scavenger as those best representing the rate coefficient for the reactions of O3 with AMS and AES. The reaction mechanism was explored using DFT and post-Hartree-Fock computational methods. It is shown that the barrier for the common cyclization to primary ozonide (-3.7 ± 0.1 kcal mol-1) followed by other reactions, as well as that for the reaction with the sulfur atom (-5.1 ± 0.1 kcal mol-1), is small and quite close, meaning that both reaction paths should contribute significantly to the global reaction rate.

Fast and Efficient Atmospheric NO2 Collection for Isotopic Analysis by a 3D-Printed Denuder System.

Being major species of atmospheric reactive nitrogen, nitrogen oxides (NOx = NO + NO2) have important implications for ozone and OH radical formation in addition to nitrogen cycles. Stable nitrogen isotopes (δ15N) of NOx have been sought to track NOx emissions and NOx chemical reactivities in the atmosphere. The current atmospheric NOx collection methods for isotopic analysis, however, largely suffer from unverified collection efficiency and/or low collection speed (<10 L/min). The latter makes it difficult to study δ15N(NOx) in pristine regions with low NOx concentrations. Here, we present a three-dimensional (3D)-printed honeycomb denuder (3DP-HCD) system, which can effectively collect atmospheric NO2 (a major part of NOx) under a variety of laboratory and field conditions. With a coating solution consisting of 10% potassium hydroxide (KOH) and 25% guaiacol in methanol, the denuder system can collect NO2 with nearly 100% efficiency at flow rates of up to 70 L/min, which is 7 times higher than that of the existing method and allows high-resolution (e.g., diurnal or finer resolution) NO2 collection even in pristine sites. Besides, the δ15N of NO2 collected by the 3DP-HCD system shows good reproducibility and consistency with the previously tested method. Preliminary results of online NO oxidation by a chrome trioxide (CrO3) oxidizer for simultaneous NO and NO2 collection are also presented and discussed.

Diesel soot photooxidation enhances the heterogeneous formation of H2SO4

Both field observation and experimental simulation have implied that black carbon or soot plays a remarkable role in the catalytic oxidation of SO2 for the formation of atmospheric sulfate. However, the catalytic mechanism remains ambiguous, especially that under light irradiation. Here we systematically investigate the heterogeneous conversion of SO2 on diesel soot or black carbon (DBC) under light irradiation. The experimental results show that the presence of DBC under light irradiation can significantly promote the heterogeneous conversion of SO2 to H2SO4, mainly through the heterogeneous reaction between SO2 and photo-induced OH radicals. The detected photo-chemical behaviors on DBC suggest that OH radical formation is closely related to the abstraction and transfer of electrons in DBC and the formation of reactive superoxide radical (•O2−) as an intermediate. Our results extend the known sources of atmospheric H2SO4 and provide insight into the internal photochemical oxidation mechanism of SO2 on DBC.

Fuel-NO formation mechanism in MILD-oxy combustion of CH4/NH3 fuel blend

MILD-oxy combustion operates conventional oxy-combustion processes under moderate or intense low-oxygen dilution (MILD) condition, which offers a practical approach towards the synergetic control of CO2 and NOx emissions. This numerical work investigates the MILD combustion characteristics and NO emission of CH4/NH3 fuel blend under N2, CO2 and H2O dilutions, aiming at revealing the fuel-NO formation mechanism under MILD-oxy condition. It is found that relative to MILD-N2 combustion, the flame temperature is reduced under MILD-CO2 and MILD-H2O combustion, while the reaction zone is expanded and reduced, respectively, due to the altered co-flow properties. CO2 dilution lowers the radical pool level, while the formation of OH radical is boosted under H2O dilution. Overall NO emission (EINO) under MILD-N2 combustion is the largest while that under MILD-H2O generates the lowest value, showing its low-NOx emissions superiority. NH3 is converted into NO via routes NH3 → NH2 → HNO → NO and NH3 → NH2 → NH → HNO → NO under MILD-CO2 combustion. By contrast, the importance of route NH3 → NH2 → NH → N → NO in producing NO is largely strengthened under MILD-H2O combustion, wherein the direct formation of NO via HNO is inhibited with the enhanced availability of reactive N-containing species, contributing to lower NO emission under MILD-H2O combustion over MILD-CO2 combustion.

The effect of ozone addition on the combustion characteristics of hydrogen-jet fuel

Ozone (O3) addition combustion is a trending technique to enhance the ignition characteristics of hydrocarbon fuel. In this study, the spray ignition characteristics of jet fuel along with hydrogen (H2) and O3 seeding were experimentally investigated and numerically simulated with an updated chemical kinetic model. The ignition delay (ID) period of jet propellant 5 (JP-5) with H2/O3 addition was measured using a constant volume combustion chamber (CVCC) at 600 K to 818 k, the pressure of 15 bar, equivalence ratio of 0.5. The intake manifold was added with air, H2 (10%, and 20% volume fraction), and 2000 ppm ozone (O3). In general, pure JP-5 has a shorter ID than those obtained with H2 addition. At 600 K, with 10% and 20% H2 to JP-5, the ID was increased by 18.49%, and 35.12% respectively. When O3 was added, the ID of JP-5 was shortened by 10%, while for 10% and 20% H2 addition, the ID was shortened by 19.25% and 22.5%, respectively. An existing jet fuel mechanism was chosen and validated with the previous and the present study experimental ID. The model was in agreement with the practical ID. The H2-mechanism rate constants were updated with the recently optimized values available in the literature to reflect the newly obtained experimental data at low temperatures. An existing sub-mechanism of O3 was adopted and combined with the JP-5 model. The updated model was validated with the present study ID data. Good predictions were noticed between the model and JP-5 experimental ID. However, a small deviation of 5.01% and 0.71% was found for 10%, and 20% H2 ID at 600 K. The reaction H2 + OH = H2O + H and HO2 + HO2 = H2O2 + O2 were found to be the critical reactions responsible to reduce the systems reactivity and makes the fuels ID to become longer. The O-atom from O3 was decomposed through the reaction O3 + N2 = O2 + O + N2, and O3 + O2 = O2 + O + O2 at an early stage of combustion. Due to H2 addition to O3 and air, the early formation of OH radicals is due to O3 + H = O2 + OH, which further accelerates the oxidation of the fuel, resulting in a shorter ID.