Potential of free nitrous acid (FNA) for sludge treatment and resource recovery from waste activated sludge: A review

The escalating production of waste activated sludge (WAS) presents significant challenges to wastewater treatment plants (WWTPs). Free nitrous acid (FNA), known for its biocidal effect, has gained a growing focus on sludge dewatering, sludge reduction, and resource recovery from WAS due to its eco-friendly and cost-effective properties. Nevertheless, there have been no attempts made to systematically summarize or critically analyze the application of FNA in enhancing treatment and resource utilization of sludge. In this paper, we provided an overview of the current understanding regarding the application potential and influencing factors of FNA in sludge treatment, with a specific focus on enhancing sludge dewatering efficiency and reducing volume. To foster resource development from sludge, various techniques based on FNA have recently been proposed, which were comprehensively reviewed with the corresponding mechanisms meticulously discussed. The results showed that the chemical oxidation and interaction with microorganisms of FNA played the core role in improving resource utilization. Furthermore, current challenges and future prospects of the FNA-based applications were outlined. It is expected that this review can refine the theoretical framework of FNA-based processes, providing a theoretical foundation and technical guidance for the large-scale demonstration of FNA.

Combustion, Chemistry, and Carbon Neutrality

Combustion is a reactive oxidation process that releases energy bound in chemical compounds used as fuels─energy that is needed for power generation, transportation, heating, and industrial purposes. Because of greenhouse gas and local pollutant emissions associated with fossil fuels, combustion science and applications are challenged to abandon conventional pathways and to adapt toward the demand of future carbon neutrality. For the design of efficient, low-emission processes, understanding the details of the relevant chemical transformations is essential. Comprehensive knowledge gained from decades of fossil-fuel combustion research includes general principles for establishing and validating reaction mechanisms and process models, relying on both theory and experiments with a suite of analytic monitoring and sensing techniques. Such knowledge can be advantageously applied and extended to configure, analyze, and control new systems using different, nonfossil, potentially zero-carbon fuels. Understanding the impact of combustion and its links with chemistry needs some background. The introduction therefore combines information on exemplary cultural and technological achievements using combustion and on nature and effects of combustion emissions. Subsequently, the methodology of combustion chemistry research is described. A major part is devoted to fuels, followed by a discussion of selected combustion applications, illustrating the chemical information needed for the future.

Interactions in Ammonia and Hydrogen Oxidation Examined in a Flow Reactor and a Shock Tube

Ammonia (NH₃) is a promising fuel, because it is carbon-free and easier to store and transport than hydrogen (H₂). However, an ignition enhancer such as H₂ might be needed for technical applications, because of the rather poor ignition properties of NH₃. The combustion of pure NH₃ and H₂ has been explored widely. However, for mixtures of both gases, mostly only global parameters such as ignition delay times or flame speeds were reported. Studies with extensive experimental species profiles are scarce. Therefore, we experimentally investigated the interactions in the oxidation of different NH₃/H₂ mixtures in the temperature range of 750-1173 K at 0.97 bar in a plug-flow reactor (PFR), as well as in the temperature range of 1615-2358 K with an average pressure of 3.16 bar in a shock tube. In the PFR, temperature-dependent mole fraction profiles of the main species were obtained via electron ionization molecular-beam mass spectrometry (EI-MBMS). Additionally, for the first time, tunable diode laser absorption spectroscopy (TDLAS) with a scanned-wavelength method was adapted to the PFR for the quantification of nitric oxide (NO). In the shock tube, time-resolved NO profiles were also measured by TDLAS using a fixed-wavelength approach. The experimental results both in PFR and shock tube reveal the reactivity enhancement by H₂ on ammonia oxidation. The extensive sets of results were compared with predictions by four NH₃-related reaction mechanisms. None of the mechanisms can well predict all experimental results, but the Stagni et al. [React. Chem. Eng. 2020, 5, 696-711] and Zhu et al. [Combust. Flame 2022, 246, 115389] mechanisms perform best for the PFR and shock tube conditions, respectively. Exploratory kinetic analysis was conducted to identify the effect of H₂ addition on ammonia oxidation and NO formation, as well as sensitive reactions in different temperature regimes. The results presented in this study can provide valuable information for further model development and highlight relevant properties of H₂-assisted NH₃ combustion.

Non-premixed combustion and NOX emission characteristics in a micro gas turbine swirl combustor fueled by methane and ammonia at various heat loads

In comparison to methane (CH₄), ammonia (NH₃) is considered a potential carbon-free alternative fuel that can reduce greenhouse gas emissions. But a principal concern is the generation of elevated nitrogen oxide (NO_X) emissions from NH₃ flame. In this study, the detailed reaction mechanisms and thermodynamic data of CH₄ oxidation and NH₃ oxidation were performed using the steady and unsteady flamelet models. After validation of the turbulence model, the combustion and NO_X emission characteristics of CH₄/air and NH₃/air non-premixed flames in a micro gas turbine swirl combustor under a series of identical heat loads were numerically investigated and compared. The present results show that the high-temperature zone of the NH₃/air flame migrates more rapidly toward the outlet of the combustion chamber than that of the CH₄/air flame as the heat load increases. The average NO, N₂O, and NO₂ emission concentrations at all heat loads from NH₃/air flame are respectively 6.12, 161.05 (given the very low N₂O emission concentration from CH₄/air flame), and 2.89 times higher than those from CH₄/air flame. There are correlation trends between some parameters (e.g. characteristic temperature and OH emissions) with the variation of the heat load, and the relevant parameters can be tracked to predict the emission trends after changing the heat load.

Theoretical insights into the gas/heterogeneous phase reactions of hydroxyl radicals with chlorophenols: Mechanism, kinetic and toxicity assessment

Emission of 2-chlorophenols (2-CPs) can cause serious air pollution and health problems. Here, the reaction kinetics and products of key radicals in 2-CPs photo-oxidation are explored in both gaseous and heterogeneous reactions. Quantum chemical calculations show that •OH-addition pathways are more preferable than H-abstraction pathways in gas phase, while that is opposite in heterogeneous phase. At 298 K, the overall rate coefficients of the title reactions in gas and heterogeneous phases are 3.48 × 10^-13 and 2.37 × 10^-13 cm³ molecule^-1 s^-1 with half-lives of 55.3 h and 81.2 h, respectively. The strong H-bonds between linear Si₃O₂(OH)₈ and 2-CPs change the energy barriers of initial •OH-addition and H-abstraction reactions, resulting in the competition between heterogeneous reactions and gas phase reactions. The products in heterogeneous reactions are chloroquinone and HONO, which can cause atmospheric acid deposition and eco-toxicity. In gas phase, self-cyclization of alkoxy radical (RO•) leads to formation of •HO₂ and highly‑oxygenated molecules, which cause formation of secondary organic aerosol. It is emphasized that oxidation of 2-CPs by •OH leads to formation of more toxic products for aquatic organisms. Therefore, more attention should be focused on the products originated from •OH-initiated reactions of (2-)CPs in gaseous and heterogeneous reactions.

Involvement of a Formally Copper(III) Nitrite Complex in Proton-Coupled Electron Transfer and Nitration of Phenols

A unique high-valent copper nitrite species, LCuNO2, was accessed via the reversible one-electron oxidation of [M][LCuNO2] (M = NBu4+ or PPN+). The complex LCuNO2 reacts with 2,4,6-tri-tert-butylphenol via a typical proton-coupled electron transfer (PCET) to yield LCuTHF and the 2,4,6-tri-tert-butylphenoxyl radical. The reaction between LCuNO2 and 2,4-di-tert-butylphenol was more complicated. It yielded two products: the coupled bisphenol product expected from a H-atom abstraction and 2,4-di-tert-butyl-6-nitrophenol, the product of an unusual anaerobic nitration. Various mechanisms for the latter transformation were considered.

The impact of NOx addition on the ignition behaviour of n-pentane

Modern engine concepts present several opportunities for nitrogen combustion chemistry, particularly the interaction of NOx (NO + NO2) with fuel fragments and products of partial combustion.

Threshold photoionization shows no sign of nitryl hydride in methane oxidation with nitric oxide

No nitryl hydride was detected in partial oxidation of nitric oxide doped methane, despite recent theoretical reaction rates suggesting otherwise.

Structural Changes in Cell-Wall and Cell-Membrane Organic Materials Following Exposure to Free Nitrous Acid

Previous studies demonstrate that free nitrous acid (FNA, i.e., HNO₂) is biocidal for a range of microorganisms. The biocidal mechanisms of FNA are largely unknown. In this work, it is hypothesized that FNA will break bonds in molecules found in the cell envelope, thus causing cell lysis. Selected molecules representing components found in the cell envelope were treated with FNA at 6.09 mg N/L (NO₂^- = 250 mg N/L, pH 5.0) for 24 h (conditions typically used in applications) to evaluate the hypothesized chemical interactions. Molecular changes were observed using analytical techniques including proton (¹H) nuclear magnetic resonance spectroscopy (NMR) and electrospray ionization mass spectrometry (ESI-MS). It was found that FNA broke down a range of cell envelope molecules. The spectral data demonstrated that the FNA reactions proceeded via two general pathways. One consisted of electrophilic substitution, whereby the nitrosonium ion (NO⁺) was the reactive electrophile. The other was via oxidative reactions involving nitrogen radicals (e.g., •NO₂ and •NO) formed from the decomposition of FNA. We further revealed that it was HNO₂ that caused the breakdown, rather than the exclusive action of the acid (H⁺) or nitrite (NO₂^-) counterparts. The fragmentation of these representative cell envelope molecules provides insight into the biocidal effects of FNA on microorganisms.

Ab Initio Kinetics of Methylamine Radical Thermal Decomposition and H-Abstraction from Monomethylhydrazine by H-Atom

Methylamine radicals (CH₃NH) and amino radicals (NH₂) are major products in the early pyrolysis/ignition of monomethylhydrazine (CH₃NHNH₂). Ab initio kinetics of thermal decomposition of CH₃NH radicals was analyzed by RRKM master equation simulations. It was found that β-scission of the methyl H-atom from CH₃NH radicals is predominant and fast enough to induce subsequent H-abstraction reactions in CH₃NHNH₂ to trigger ignition. Consequently, the kinetics of H-abstraction reactions from CH₃NHNH₂ by H-atoms was further investigated. It was found that the energy barriers for abstraction of the central amine H-atom, two terminal amine H-atoms, and methyl H-atoms are 4.16, 2.95, 5.98, and 8.50 kcal mol^-1, respectively. In units of cm³ molecule^-1 s^-1, the corresponding rate coefficients were found to be k₈ = 9.63 × 10^-20T^2.596 exp(-154.2/T), k₉ = 2.04 × 10^-18T^2.154 exp(104.1/T), k₁₀ = 1.13 × 10^-20T^2.866 exp(-416.3/T), and k₁₁ = 2.41 × 10^-23T^3.650 exp(-870.5/T), respectively, in the 290-2500 K temperature range. The results reveal that abstraction of the terminal amine H-atom to form trans-CH₃NHNH radicals is the dominant channel among the different abstraction channels. At 298 K, the total theoretical H-abstraction rate coefficient, calculated with no adjustable parameters, is 8.16 × 10^-13 cm³ molecule^-1 s^-1, which is in excellent agreement with Vaghjiani's experimental observation of (7.60 ± 1.14) × 10^-13 cm³ molecule^-1 s^-1 ( J. Phys. Chem. A 1997, 101, 4167-4171, DOI: 10.1021/jp964044z).

An experimental, theoretical and kinetic-modeling study of the gas-phase oxidation of ammonia

A wide-range experimental and theoretical investigation of ammonia gas-phase oxidation is performed, and a predictive, detailed kinetic model is developed.

Shock Tube Laser Schlieren Study of the Pyrolysis of Isopropyl Nitrate

The decomposition of isopropyl nitrate was measured behind incident shock waves using laser schlieren densitometry in a diaphragmless shock tube. Experiments were conducted over the temperature range of 700-1000 K and at pressures of 71, 126, and 240 Torr. Electronic structure theory and RRKM Master Equation methods were used to predict the decomposition kinetics. RRKM/ME parameters were optimized against the experimental data to provide an accurate prediction over a broader range of conditions. The initial decomposition i-C₃H₇ONO₂ ⇌ i-C₃H₇O + NO₂ has a high-pressure limit rate coefficient of 5.70 × 10²²T^-1.80 exp[-21287.5/T] s^-1. A new chemical kinetic mechanism was developed to model the chemistry after the initial dissociation. A new shock tube module was developed for Cantera, which allows for arbitrarily large mechanisms in the simulation of laser schlieren experiments. The present work is in good agreement with previous experimental studies.