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Ammonia is becoming increasingly popular as a carbon-neutral fuel with zero carbon dioxide emissions. However, a significant hurdle lies in its combustion, which leads to substantial emissions of NOX. The current research involves conducting a chemical kinetic investigation to examine the characteristics of Intense Low oxygen Dilution (MILD) or Moderate combustion in ammonia (NH3)/air flames. This study is carried out under specific conditions, such as oxygen concentrations ranging from 11 % to 23 %, premixed reactant temperatures between 1300 and 1700 K, and a pressure of one atmosphere. The study focuses on investigating the combustion characteristics of MILD using dilution with H2O and N2. With the rise in the inlet temperature of the premixed reactant, the peak temperature of the flame also rises. Moreover, flames diluted with H2O exhibit lower peak temperatures compared to flames diluted with N2. Flames diluted with H2O result in lower NOX emissions compared to flames diluted with N2. Additionally, for N2 diluted flames, the exit NOX emissions rise as the oxygen concentration increases. Despite this, NOX emissions from H2O diluted flames demonstrate non-monotonic behaviour. This means that the exit NOX increases initially as the oxygen concentration reaches 21%, but then begins to decrease. In contrast to N2 and H2O diluted flames exhibit a wider regime of no-ignition. Moreover, the rise in peak temperature in H2O diluted flames is less apparent than in N2 diluted flames, corresponding to broader ranges MILD combustion ranges. Furthermore, to attain MILD combustion in H2O diluted flames at a specific O2 concentration, the temperature of reactant needs to be higher than that required for N2 diluted flames. [ABSTRACT FROM AUTHOR]

Numerical simulations for moderate and intense low oxygen dilution (MILD) combustion with essential solvers and detailed mechanisms involve more complications and computational time. Various advanced combustion modeling techniques have recently been developed to study MILD combustion characteristics. However, every combustion model has specific issues predicting the temperature and emissions of the MILD combustion flames. The diffusive nature of the MILD flame is considered, and individual Lewis numbers are investigated on a nonpremixed flame. The current study analyzes the methane/hydrogen flame propagation with different Lewis number combinations in a hot co-flow environment. Individual Lewis numbers for methane and hydrogen are investigated from stochiometric to ultra-rich mixtures in non-premixed flames. Several numerical simulations are performed in the OpenFOAM9 environment using a modified EDC model with tuned turbulence and combustion model constants. The numerical simulation results with hydrogen and methane Lewis numbers of 0.4 and 0.9, respectively, show promising agreement with the experimental findings of Dally1, et al. Various combustion parameters are studied with different CH4 and H2 Lewis number combinations. In addition, the unity Lewis number case is simulated and compared to the situations that are taken into consideration. [ABSTRACT FROM AUTHOR]

In this study, a phase change material (PCM)‐encapsulated packed‐bed thermal energy storage (PB‐TES) system is intended for Day‐round space heating in the winter. Solar concentric collectors act for day‐time heating and TES charging, while the stored energy can be used at night and intermediate time for space heating. A region‐wise techno‐economic analysis of PCM candidates is made to compare the costs and sizes of solar collectors and PB‐TES. A spherical capsule filled with sodium nitrite (NaNO2) and sodium nitrate (NaNO3) is numerically investigated during the PCM charging at constant thermal input. After complete liquefication, the charging time for NaNO2 and NaNO3 capsules is 3280 and 4300 s, respectively, holding the maximum stored energy of 733.4 KJ/kg at 285°C and 687.27 kJ/kg at 310.12°C, respectively. In addition, the stored exergy is obtained 238.78 and 234.74 kJ/kg for NaNO2 and NaNO3, respectively; however, the average exergetic effectiveness for NaNO2 and NaNO3 are 0.54 and 0.46. Despite the lower energy storage capacity (kJ/kg) of NaNO3, the PB‐TES volume is less with NaNO3 than with NaNO2 due to higher density. However, the average cost ($/tonne) of NaNO3 PCM is 50% higher than NaNO2. Subsequently, the thermal efficiency of the proposed system is estimated at 27.68% and 26.94% during day and night‐time operations. [ABSTRACT FROM AUTHOR]

This work investigates both autoignition and combustion characteristics in highly preheated and diluted combustion of a laminar premixed stoichiometric CH4/O2/N2 mixture in a cylindrical combustor operating at elevated pressures. The analysis was carried out for a range of operating parameters, including reactant preheat temperatures of 1100–1500 K, combustor pressures of 1-10 atm, and in a highly diluted mixture, achieved by decreasing the oxygen content in the oxidizer from 21% to 3% on volume basis. Simulations were conducted using the laminar premixed adiabatic PFR (plug flow reactor) model of Ansys Chemkin Pro. Two-dimensional pictorial representation was performed using the finite volume-based CFD code Ansys Fluent 19.2. Finite-rate chemistry with the detailed chemical mechanism GRI Mech 3.0 was used for combustion analysis. Results showed that OH and HCO mole fractions decreased with increasing combustor pressure and N2 dilution (or decreased O2 content), while the mole fractions increased with reactant temperature. It was also found that, by reducing the oxygen content in the mixture, the flame stabilized far away from the combustor inlet. In contrast, an increase in combustor pressure and reactant temperature stabilized the flame toward the combustor inlet. These flame stabilization characteristics at different locations of the combustor are explained in terms of ignition delay time, which were calculated using the closed homogenous reactor (CHR) model available in the Ansys Chemkin Pro package. The flame peak temperature decreased with increased N2 dilution and increased by increasing the reactant temperature. Moreover, the peak temperature varied marginally when the combustor pressure was increased. Finally, a regime diagram was prepared to show the various combustion modes, such as HiTAC, MILD combustion, and the no ignition region as a function of O2 content and reactant temperature for different operating pressures. The CO and NO emission were reduced with an increase in pressure in the MILD combustion region. [ABSTRACT FROM AUTHOR]

Summary: In recent years, ammonia has shown great potential for future green fuel. However, ammonia combustion is a challenging task due to its narrow flammability limit, low flame speed, and slow kinetics compared with conventional hydrocarbon fuels. The blending of ammonia with higher reactive fuel such as hydrogen seems a novel alternative to overcome these difficulties. So, a comprehensive study of NH3/H2/air combustion characteristics is necessary. In the literature, numerous ammonia reaction models can be found, but most of them failed to demonstrate good combustion characteristics with NH3/H2/air mixtures. A new NH3/H2/air reaction model is suggested or proposed in the present work by referring to the previously available literature. The newly proposed reaction model consists of 32 species and 259 reactions. Model validation for laminar flame speed and ignition delay time at different operating conditions are performed in this study using the newly proposed reaction mechanism. Also, an extensive chemical kinetic modeling for heat release rate, flame speed and OH sensitivity analysis, reaction pathway analysis, and NOx emission characteristics of NH3/H2/air mixture is studied. The results show that the proposed reaction model closely follows the experimental trends, and lesser inconsistency is observed than the other reaction models compared in this study. All referenced reaction models used in the present work for comparison shows large discrepancies with experimental results of NH3/H2/air combustion, especially more discrepancies are observed at higher hydrogen enrichment conditions. Hydrogen enrichment positively affects the heat release rate intensity and flame speed of NH3/air mixture. Through the addition of hydrogen, the burning velocity of ammonia can be increased up to the level of hydrocarbon fuels, and the combination of ammonia and hydrogen blend can be used as an effective industrial fuel. [ABSTRACT FROM AUTHOR]

Flameless combustion is an effective method to suppress CO and NOXemissions from combustion systems with improved combustion efficiency. Flameless combustion is achieved through simultaneous preheating and dilution of fresh reactants with internal recirculation of hot combustion products leading to a distributed reaction zone and uniform temperature distribution throughout the volume of the combustor. In the present study, a high-intensity swirl-based combustor operating with liquid fuel in a flameless combustion mode is developed for industrial applications and designed to run at atmospheric pressure conditions. Detailed investigations on the emission characteristics for various operating conditions in flameless combustion mode have been reported. The experiments have been carried out for a range of heat release rates varying from 5 to 10 MW/m3. Swirl flow pattern is achieved through tangential air injection to enhance the internal recirculation of hot combustion products into fresh reactants. Preliminary numerical studies aimed at understanding flow pattern in four different combustor configurations show that recirculation rate increases with an increase in the divergence angle of the combustor. This helps achieve flameless combustion mode. Recirculation rate increases with a decrease in the outlet diameter from 80 mm to 25 mm. Based on the degree of the combustion completeness, extended frustum of the cone-based configuration (60°) has been proposed to obtain low emission combustion mode with liquid fuels. Temperature distribution and emissions are measured and compared for different thermal inputs (heat release intensities) and air-preheat temperatures. Measurements of acoustic, CO, and NOXemissions show that these emissions are reduced by an order of magnitude when the combustor is operated in flameless combustion mode as compared to conventional combustion mode. [ABSTRACT FROM AUTHOR]