Mini Review of Ammonia for Power and Propulsion: Advances and Perspectives

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: Introductionmentioning

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

“…Recently, ammonia has been considered as a promising carbon-free fuel, which has a hydrogen content of approximately 17.8% . The volumetric energy density is about three times higher than that of hydrogen. − Because ammonia is easily liquefied at 25 °C and 0.8 MPa, it is convenient to store and transport. Furthermore, the fundamental facilities for production, storage, and transportation have been well established, resulting in lower cost.…”

Section: Introductionmentioning

confidence: 99%

Blow-Off Limits, Flame Structure, and Emission Characteristics of Lean Partially Premixed Swirl-Stabilized Flames with NH₃/CH₄

Lai,

Chen,

Zhang

et al. 2024

Recently, ammonia/methane mixtures have been considered as potential alternative fuels. The present study experimentally investigates the blow-off limits, flame structure, and pollutant emission of ammonia/methane mixtures in a partially premixed swirl-stabilized burner. The effects of fuel composition and the equivalence ratio are also studied. The results show that the NH 3 /CH 4 flame can be stabilized at an equivalence ratio of 0.55. The addition of methane has a positive effect on the blow-off limits. For the chemiluminescence profile, the OH* distribution and CH* distribution of NH 3 /CH 4 flames show a similar tendency under various conditions. The peak OH* and CH* radiances monotonically decrease with increasing E NHd 3 , while the maximum NH 2 * shows an opposite trend. The influence of the equivalence ratio on OH* and CH* in the NH 3 /CH 4 flames is negligible. However, when E NHd 3 is 60%, the equivalence ratio has a significant effect on NH 2 *. In terms of pollutant emissions, the peaks of NO and NO 2 emissions are all found around an E NHd 3 of 20%. When the equivalence ratio reaches 1.0, NO and NO 2 emissions are as low as 15 and 10 mg/m 3 , respectively. At the same time, both CO and unburned NH 3 emissions increase. Therefore, further studies on the application of secondary air or improvement of the fuel injection method to improve the combustion efficiency and reduce the pollutant emissions for ammonia combustion are still needed.

“…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.