•Effects of CH4 mixing ratio and O2 concentration on NH3/CH4 combustion are studied.•The mechanisms of NO formation and reduction are clarified.•The microkinetic of NH3/CH4 co-combustion is ...revealed.•Develop and validate the optimized mechanism and skeletal model for the NH3/CH4 co-combustion.
Under the vision of “low carbonization”, ammonia (NH3), as a carbon-free hydrogen-rich fuel, blended with reactive additives, is an effective means to achieve low carbon emissions and reduce the consumption of traditional fossil fuels. In this work, the combustion behaviors of NH3 and CH4, the distribution of main products and the formation and reduction characteristics of NO at different temperatures, CH4 mixing ratios, and O2 equivalence ratios are explored by means of reactive molecular dynamics simulation to reveal the microscopic mechanism of NH3/CH4 co-combustion. The calculated results show that CH4 takes the lead in producing H radicals which react with O2 to generate OH radicals to promote the oxidation of NH3, and it is found that the co-combustion process of NH3/CH4 in the radical chain transfer stage is similar to the respective pyrolysis and oxidation reactions of CH4 and NH3. Accordingly, an optimized kinetic mechanism of NH3/CH4 co-combustion is constructed based on the mechanisms proposed by Glarborg et al. and Okafor et al. by incorporating the NH3 pyrolysis sub-mechanisms related to NH2, HNO, and N2H2 and the skeletal mechanism of CH4 oxidation. The optimized mechanisms of NH3/CH4 co-combustion are found to improve the prediction of the ignition delay time (IDT) and laminar burning velocity (LBV) of NH3/CH4 mixture under wide ranges of conditions compared to the original mechanisms of Glarborg et al. and Okafor et al. To decrease the computational cost of three-dimensional combustion numerical simulations, the skeletal mechanism models are established by simplification of the optimized mechanisms of NH3/CH4 co-combustion using direct relation graph method with error propagation and sensitivity analysis, which can provide nearly the same estimates of IDTs and LBVs as the optimized mechanisms mentioned above.
To achieve comprehensive prediction of ammonia combustion in terms of flame speed and ignition delay time, an improved mechanism of ammonia oxidation was proposed in this work. The present model ...(UT-LCS) was based on a previous work Song et al., 2016 and improved by relevant elementary reactions including NH2, HNO, and N2H2. The model clearly explained reported values of laminar flame speed and ignition delay time in wide ranges of equivalence ratio and pressure. This suggests that NH2, HNO, and N2H2 reactivities play a key role to improve the reaction mechanism of ammonia oxidation in the present model. The model was also applied to demonstrate NH3/H2/air combustion. The present model also appropriately predicted the laminar flame speed of NH3/H2/air combustion as a function of equivalence ratio. Using the model, we discussed the reduction of NO concentration downstream and H2 formation via NH3 decomposition in NH3/H2 fuel-rich combustion. The results provide suggestions for effective combustion of NH3 for future applications.
An improved mechanism of ammonia oxidation was proposed to achieve comprehensive prediction of ammonia combustion in terms of flame speed and ignition delay time. We also discussed the reduction of NO concentration and H2 formation via NH3 decomposition in NH3/H2 combustion. Display omitted
•An improved mechanism of NH3 and NH3/H2 combustion was proposed.•The improved mechanism explained laminar flame speed and ignition delay time of NH3 combustion very well.•This model suggests appropriate conditions to reduce NO concentration in NH3/H2 combustion.•The model also suggests H2 formation via NH3 decomposition in NH3/H2 mixed fuel.•The model can contribute to effective combustion of NH3/H2/air with new combustors.
The present study includes an experimental investigation of the performance, combustion, and emission parameters of a hydrogen port fueled SI engine under wide-open throttle. The compression ratio ...(CR) is varied from 10 to 15, equivalence ratio (φ) from 0.4 to 1.0, and speed from 1400RPM to 1800RPM. The ignition timing is maintained at 20° before the top dead center. The brake thermal efficiency increases by nearly 10% from CR10 to CR15, and it also increased by 13.7% by changing φ from 0.4 to 0.9. Similarly, BP increases in the same fashion. The combustion enhances with an increase in peak pressure by increasing CR from 10 to 15 and φ from 0.4 to 0.9; however, φ 1.0 exhibits a negative trend. However, the NOX emission increases continuously with CR and φ, and so as the exhaust gas temperature. The carbon-based emissions are negligible, and volumetric efficiency decreases with φ and increases with CR.
•High heat content and carbonless nature are key factors of hydrogen as a fuel.•High laminar flame speed and low ignition energy provide suitability in SI engines.•Increasing compression ratio improves lean combustion characteristics of hydrogen.•Equivalence ratio 0.7, exhibit better brake thermal efficiency and brake power.•Carbonless burning of hydrogen produces negligible CO, CO2, and HC emissions.
•Ultra-lean burn characteristics of spark ignition methanol engine was assessed.•A non-uniform spray-line nozzle was adopted to achieve stratified mixture.•Equivalence ratio of methanol engine at ...high compression ratio is as low as 0.20.•Thermal efficiency increases with decrease high compression ratio at ultra-lean burn.
Lean burn is one of the most important characteristics of a stratified-charge direct-injection spark-ignition engine. In this study, a non-uniform 10-hole × 0.30 mm spray-line distribution nozzle was chosen to achieve stratified-charge of the methanol/air mixture. The mixture formation, combustion, and emissions characteristics of a stratified-charge direct-injection spark-ignition methanol engine under five global equivalence ratios and three high-compression ratios were simulated to assess its ultra-lean burn characteristics. The results showed that the equivalence ratio of the stratified-charge direct-injection spark-ignition methanol engine for stable combustion at high compression ratio was as low as 0.20. The ignition delay and combustion duration decreased as the compression ratio increased at high equivalence ratio, and vice versa at low equivalence ratio. The stratified-charge direct-injection spark-ignition methanol engine greatly reduced nitric oxide emissions under ultra-lean combustion. The unburned methanol, carbon monoxide, and soot emissions increased slowly as equivalence ratio decreased at all compression ratios and equivalence ratio >0.25. The indicated thermal efficiency decreased with decreasing compression ratio at high equivalence ratio, and vice versa at low equivalence ratio. For an equivalence ratio of 0.67, the indicated thermal efficiency at compression ratio of 14 was approximately 14.9% lower than at compression ratio of 16. However, for an equivalence ratio of 0.20, the indicated thermal efficiency at compression ratio of 14 was approximately 13.5% higher than at compression ratio of 16. The compression ratio of 14 was more favorable for the stratified-charge direct-injection spark-ignition methanol engine to achieve ultra-lean burn and find practical application.
•Gasification performances of raw and torrefied biomass are thermodynamically analyzed.•A downdraft fixed bed gasifier is tested using Aspen Plus.•The modified equivalence ratio and steam supply ...ratio are considered.•The cold gas efficiency and carbon conversion are examined.•The optimum operating conditions for the gasification are found.
The gasification performances of three biomass materials, including raw bamboo, torrefied bamboo at 250°C (TB250), and torrefied bamboo at 300°C (TB300), in a downdraft fixed bed gasifier are evaluated through thermodynamic analysis. Two parameters of modified equivalence ratio (ERm) and steam supply ratio (SSR) are considered to account for their impacts on biomass gasification. The cold gas efficiency (CGE) and carbon conversion (CC) are adopted as the indicators to examine the gasification performances. The analyses suggest that bamboo undergoing torrefaction is conducive to increasing syngas yield. The higher the torrefaction temperature, the higher the syngas yield, except for TB300 at lower values of ERm. Because the higher heating value of TB300 is much higher than those of raw bamboo and TB250, the former has the lowest CGE among the three fuels. The values of CC of raw bamboo and TB250 are always larger than 90% within the investigated ranges of ERm and SSR, but more CO2 is produced when ERm increases, thereby reducing CGE. The maximum values of syngas yield and CGE of raw bamboo, TB250, and TB300 are located at (ERm, SSR)=(0.2, 0.9), (0.22, 0.9), and (0.28, 0.9), respectively. The predictions suggest that TB250 is a more feasible fuel for gasification after simultaneously considering syngas yield, CGE, and CC.
In order to reveal the influence of incoming flow velocity and mixture equivalence ratio on the characteristics of oblique detonation, Euler equations coupled detailed chemical reaction model were ...used to numerically simulate the flow field of oblique detonation. We investigated the characteristic parameters and the morphology of oblique detonation with different wedge angles, velocities and mixture equivalence ratios of incoming flow. The results show that the variations of parameters for uniform inflow condition have obvious influence on the structure of the wave system and the internal stabilization of the oblique detonation flow field. With the increase of incoming flow velocity, the instability of oblique detonation flow field is suppressed. And the initiation position of the oblique detonation is advanced. Under the condition of high velocity inflow, the performance of oblique detonation propulsion is obviously improved. With the increase of the mixture equivalence ratio of inflow, the change law of oblique detonation initiation distance is U-shaped. The internal flow field of the oblique detonation becomes stabilized with the increase of excess oxygen. For the inflow equivalence ratio inhomogeneity, the deflagration front and oblique detonation front become distorted. In addition, the flow field properties of oblique detonation can be optimized by adjusting the inhomogeneity of equivalence ratio.
•We synthesize information from three new calculation procedures of the emergy baseline.•We propose a unified global emergy baseline (12.0 E+24seJy−1).•We suggest this baseline should be used in all ...subsequent emergy evaluations.
The concept of emergy defined as the available energy (or exergy) of one form used up directly and indirectly to produce an item or action (Odum, Environmental Accounting Emergy and Environmental Decision Making, John Wiley & Sons, Inc., 1996) requires the specification of a uniform solar equivalent exergy reference, or geobiosphere emergy baseline (GEB). Three primary exergy sources of different origins interact to drive processes within the geobiosphere. Each of these sources are expressed in solar equivalent exergy from which, all other forms of energy can be computed, so that they may be expressed as emergy in units of solar emjoules. If emergy practitioners reference their work to a single agreed-upon baseline, then all research products resulting from the application of the emergy approach will be inherently consistent and valid comparisons can then be made easily. In this paper, we synthesize information from three new calculation procedures of the emergy baseline for the geobiosphere and propose a unified solution.
We achieve current-induced switching in collinear insulating antiferromagnetic CoO Pt, with fourfold in-plane magnetic anisotropy. This is measured electrically by spin Hall magnetoresistance and ...confirmed by the magnetic field-induced spin-flop transition of the CoO layer. By applying current pulses and magnetic fields, we quantify the efficiency of the acting current-induced torques and estimate a current-field equivalence ratio of 4 × 10−11 T A−1 m2. The Néel vector final state ( n ⊥ j ) is in line with a thermomagnetoelastic switching mechanism for a negative magnetoelastic constant of the CoO.
•NH3/H2/air laminar burning velocities measured at high temperatures (up to 200 °C).•Comparison with a laminar burning velocity correlation from the literature.•The promoting effect of the ...temperature is well captured by the correlation.•The effect of the mixture composition is not well captured by the correlation.•Correlations to be improved thanks to updated reaction mechanisms and experiments.
The present study introduces new laminar burning velocity data for ammonia/hydrogen/air mixtures measured by means of the outwardly propagating spherical flame method at atmospheric pressure, for previously unseen unburned gas temperatures ranging from 298 to 473 K, hydrogen fractions ranging from 0 vol% to 60 vol% in the fuel and equivalence ratios in the range 0.8–1.4. Results show increasing velocities with increasing hydrogen fraction and temperature, with maximum values obtained for rich mixtures near stoichiometry. The new experimental dataset is compared to dedicated laminar burning velocity correlations from the literature and to simulations using detailed kinetic mechanisms. The ammonia/air correlation presents a good agreement with measurements over the whole range of experimental conditions. The ammonia/hydrogen/air correlation captures the effect of the initial temperature satisfactorily for equivalence ratios below 1.3 and hydrogen fractions below 50 vol% in the fuel, but discrepancies are observed in other conditions. The effect of hydrogen addition is reproduced satisfactorily for hydrogen fractions between 20 and 40 vol% in the fuel, but discrepancies are observed for rich mixtures below 20 vol% hydrogen and for all mixtures containing 50 vol% hydrogen and more. An optimization of both correlations is proposed thanks to the experimental data obtained, but only with partial improvement of the ammonia/hydrogen/air correlation. State-of-the-art detailed kinetic reaction mechanisms yield values in close agreement with the present experiments. They could thus be used along with additional experimental data from different techniques to develop more accurate correlations for time-effective laminar burning velocity estimates of NH3/H2/air mixtures.
Ignition delay times for 1,3-butadiene oxidation were measured in five different shock tubes and in a rapid compression machine (RCM) at thermodynamic conditions relevant to practical combustors. The ...ignition delay times were measured at equivalence ratios of 0.5, 1.0, and 2.0 in ‘air’ at pressures of 10, 20 and 40 atm in both the shock tubes and in the RCM. Additional measurements were made at equivalence ratios of 0.3, 0.5, 1.0 and 2.0 in argon, at pressures of 1, 2 and 4 atm in a number of different shock tubes. Laminar flame speeds were measured at unburnt temperatures of 295 K, 359 K and 399 K at atmospheric pressure in the equivalence ratio range of 0.6–1.7, and at a pressure of 5 atm at equivalence ratios in the range 0.6–1.4. These experimental data were then used as validation targets for a newly developed detailed chemical kinetic mechanism for 1,3-butadiene oxidation.
A detailed chemical kinetic mechanism (AramcoMech 3.0) has been developed to describe the combustion of 1,3-butadiene and is validated by a comparison of simulation results to the new experimental measurements. Important reaction classes highlighted via sensitivity analyses at different temperatures include: (a) ȮH radical addition to the double bonds on 1,3-butadiene and their subsequent reactions. The branching ratio for addition to the terminal and central double bonds is important in determining the reactivity at low-temperatures. The alcohol-alkene radical adducts that are subsequently formed can either react with HȮ2 radicals in the case of the resonantly stabilized radicals or O2 for other radicals. (b) HȮ2 radical addition to the double bonds in 1,3-butadiene and their subsequent reactions. This reaction class is very important in determining the fuel reactivity at low and intermediate temperatures (600–900 K). Four possible addition reactions have been considered. (c) 3Ö atom addition to the double bonds in 1,3-butadiene is very important in determining fuel reactivity at intermediate to high temperatures (> 800 K). In this reaction class, the formation of two stable molecules, namely CH2O + allene, inhibits reactivity whereas the formation of two radicals, namely Ċ2H3 and ĊH2CHO, promotes reactivity. (d) Ḣ atom addition to the double bonds in 1,3-butadiene is very important in the prediction of laminar flame speeds. The formation of ethylene and a vinyl radical promotes reactivity and it is competitive with H-atom abstraction by Ḣ atoms from 1,3-butadiene to form the resonantly stabilized Ċ4H5-i radical and H2 which inhibits reactivity. Ab initio chemical kinetics calculations were carried out to determine the thermochemistry properties and rate constants for some of the important species and reactions involved in the model development. The present model is a decent first model that captures most of the high-temperature IDTs and flame speeds quite well, but there is room for considerable improvement especially for the lower temperature chemistry before a robust model is developed.
PDF
