Numerical Study on the Ignition and Flame Propagation of Ammonia/ n -Heptane Dual Fuels

Self Cite

Blending with a high-reactivity fuel is recognized as an effective way to overcome poor combustion and heavy emissions of ammonia (NH 3 ). In this study, we investigated the combustion performance and nitrogen-based emissions of an nheptane/ammonia blend in an optical engine operating under reactivity-controlled compression ignition (RCCI) mode, addressing the n-heptane energy ratio and injection timing. Considering the long ignition delay time of pure ammonia, the n-heptane energy ratio was selected as 20−40%, and the injection timings were selected as −70 to −35 °CA aTDC. The results demonstrate the substantial impact of both variables on engine performance. Notably, a critical injection timing (SOI = −50 °CA aTDC) was identified, exhibiting optimal combustion stabilities, while an increased n-heptane energy ratio facilitated the expansion of the stable operating conditions and increased the indicated mean effective pressure by up to 6.4%. Combustion visualization showed a transition in combustion mode from flame propagation to sequential autoignition with the advancement of injection timing, resulting in a more homogeneous combustion. Meanwhile, optimal flame development was observed at the critical injection timing. As the heptane fraction decreased, autoignition timing was obviously retarded and flame development depended on the local temperature field, suggesting the potential for the modulation of combustion mode through controlling the injection timing and energy ratio. Regarding nitrogen-based emissions, an advanced injection timing correlated with more homogeneous combustion, leading to a decrease of NH 3 by up to 47.7% and an increase of NO and NO 2 by approximately up to 65.9 and 22.4%, respectively. As the n-heptane energy ratio decreased, unburned NH 3 emissions increased by up to 40.4%, while NO 2 emissions decreased by up to 11.2%. NO emissions exhibited an initial increase followed by a subsequent decrease, and the maximum value reached 2698 ppm at the n-heptane energy ratio of 30%. N 2 O emissions displayed negative correlations with the autoignition timing and flame propagation speed, consistent with the observed variations in combustion stabilities.

Section: Resultsmentioning

confidence: 99%

Section: Pressure-based Combustion Characteristicsmentioning

confidence: 99%

Effects of Injection Timings and Energy Ratios on the Combustion and Emission Characteristics of n-Heptane/Ammonia RCCI Mode

Zhang,

Chen,

et al. 2024

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“…Although NH 3 has many benefits, several obstacles need to be addressed before NH 3 can be applied in combustion equipment (e.g., spark ignition (SI) and compression ignition (CI) engines). These obstacles include high-autoignition temperature, , minimum ignition energy, low flame propagation speed, , narrow flammability range, and toxicity . Taking the NH 3 application in the CI engine as an example, it requires a higher compression ratio than that of conventional transport fuel to maintain stable operation.…”

Section: Introductionmentioning

confidence: 99%

Investigating Autoignition Characteristics of Ammonia/Heptamethylnonane Mixtures Over Wide Pressure Ranges: Rapid Compression Machine Measurements and Kinetic Modeling Study

Zhang,

Zhou,

et al. 2024

Ammonia (NH 3 ) blending combustion with highreactivity fuel has garnered substantial attention in terms of decarbonization potential in internal combustion engines. 2,2,4,4,6,8,8-Heptamethylnonane, denoted as HMN, an important large-molecular weight component for diesel and jet fuel surrogates, was selected to be blended with NH 3 in this study. The ignition delay times (IDTs) of NH 3 /HMN mixtures were measured using a heated rapid compression machine (RCM) over an extensive range of conditions (temperature of 680−1025 K, pressure of 20−100 bar, equivalence ratios of 0.5−1.0, and NH 3 energy ratio (NER) of 50−90%). Experimental results show that increasing the pressure, equivalence ratio, and oxygen concentration reduces both the total and first-stage IDTs, while an increase in the NH 3 energy ratio prolongs the IDTs. For the mixture with the lowest NH 3 energy ratio of 50%, non-Arrhenius-type behavior was observed at a pressure of 20 bar, while it transfers to a monotonic decrease of IDTs with increasing temperature at a pressure of 40 bar. An NH 3 /HMN blending mechanism was developed by merging individual NH 3 and HMN submechanisms, updating NH 3 submechanism, and adding C−N cross-reaction subset. Simulation results show that under most experimental conditions, the blending mechanism exhibits reasonable prediction on the measured NH 3 /HMN IDTs. Kinetic analysis shows that the discrepancy in the first-stage ignition between experiments and simulations may be associated with the inaccurate OH consumption proportion between HMN and NH 3 , while at the intermediate-temperature region, it may be related to the core C 0 −C 4 mechanism and the NH 3 -related reactions. Further experimental or quantum calculations are needed in the future to refine the NH 3 /HMN blending mechanism on the basis of this work.

“…Ammonia serves as a refrigerant in large industrial refrigerators and is a key component of fertilizers and various industrial goods. − In recent years, ammonia has been considered as a potential alternative fuel and hydrogen storage carrier in power systems and energy storage due to its carbon-free nature and high hydrogen density. − Due to its excellent thermal properties, ammonia is well suited for large-scale storage and transportation. , In addition, ammonia is a highly toxic chemical that can cause environmental pollution and injury if accidentally burned or exploded. − The lower molecular weight of ammonia allows for faster diffusion compared with ambient air. During the storage, transport and bunkering of ammonia as well as hydrogen production during ammonia cracking, ammonia leakage in the high-pressure storage containers and hitting ignition sources can lead to explosions, structural damage to buildings, and destruction of production facilities, endangering personal safety .…”

Section: Introductionmentioning

confidence: 99%

Flammability Limits of Ammonia in Air from 298 to 423 K at Elevated Pressures

Fu,

Zhou,

et al. 2024

The flammability limit (FL) data of ammonia at elevated temperatures and pressures are useful for fire prevention and control of the storage, transportation, and bunkering process of a large amount of ammonia, as well as for the detailed study of the reaction kinetics of ammonia-containing alternative fuels. In this work, the FLs of ammonia were measured in the temperature range from 298 to 423 K and in the pressure range from 0.05 to 0.5 MPa based on the ASTM E918 and EN 1839 standards, and the ammonia concentrations at the FLs were characterized using the mole fraction and volume fraction, respectively. The FLs of ammonia varied linearly with the temperature and logarithmically with the pressure. The combined effect of the temperature and pressure expanded the FLs of ammonia and increased the likelihood of an explosion. By correlating the FLs of ammonia, the correlated results of ammonia FLs varying with the temperature and pressure were obtained. The mean absolute deviation between the fitted values and the experimental values of ammonia at the lower FLs (LFLs) was 0.22%, while at the upper FLs (UFLs) it was 0.37%. The influence of the temperature and pressure on the ammonia FLs was explained by using the reaction kinetics of ammonia combustion. The presence of OH radicals was crucial for the LFLs in the ammonia/air mixture, followed by NH2 radicals. The UFLs in the ammonia/air mixture were highly dependent on the change of NH2 radicals. The formation of NH2 radicals primarily relied on the elementary reaction NH3 + OH⇔NH2 + H2O at the lower and upper FLs. The peak value of the free radical mole fraction gradually decreased with increasing experimental pressure. The increase in pressure resulted in a significant increase in the change rate of radical ROP, while the increase in temperature resulted in a slight decrease in the rate of change.