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| Fig. 1: BMW Hydrogen 7 Engine. (Source: Wikimedia Commons) |
The consideration of hydrogen as an alternative fuel for several energy applications in place of other hydrocarbon fuels has been the topic of research and development for over a decade. In some of these energy applications, such as transportation, the technology to use hydrogen as fuel has already been developed, while in other aspects such as power generation, the field is still at the earlier stages. Fig.1 shows an example of a hydrogen engine. The potential of hydrogen as a fuel for internal combustion engines is a well-established subject within the global hydrogen initiative aiming at mitigating climate change. [1] Naturally, hydrogen as a combustion fuel has its benefits and limitations. The most circulated benefit is the absence of carbon dioxide emissions when burning hydrogen-air mixtures. However, other harmful emissions (NOx) are indeed a significant challenge to the adaptation of hydrogen in combustion engines. [1] Therefore, understanding the limits of hydrogen chemistry and combustion is crucial to its incorporation into the energy transition.
An important characteristic of hydrogen is its small, light molecules which is the reason for the low density of hydrogen at ambient conditions. [2] At ambient temperature and pressure, hydrogen has a density of 0.08 kg/m3, eight times lighter than methane. [2] There are several implications of hydrogens low density in the context of combustion engines. While the low density means a high energy content per mass, it also means a low energy content per volume (unless compressed), which causes poor volumetric efficiency. This is especially relevant to internal combustion engines since the gaseous fuel-air mixture that would occupy the engine cylinder would contain less energy in the case of hydrogen fuel, hence the low volumetric efficiency. Additionally, hydrogen-air mixtures have lower ignition energies (by an order of a magnitude) than other hydrocarbon-air mixtures as seen in Fig. 2. [1] This property of hydrogen-air mixtures renders them more prone to the effects of preignition. [1] Preignition is undesirable since it causes the mixture to burn early and less efficiently due to the increased heat losses. [1] Low ignition energy is also a safety concern since hydrogen-air mixtures, even as lean as 4%, can be flammable at certain conditions. [2]
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| Fig. 2: Comparison of the minimum ignition energy for hydrogen and other conventional fuels. [1] (Source: G. Alkhamis) |
Another characteristic of hydrogen-air mixture is the relatively high stoichiometric air-to-fuel ratio. The stoichiometric combustion reaction of a hydrogen-air mixture is:
We can use the above reaction to calculate the air-to-fuel ratio by mass using the following equation
Where ma refers to the mass of air and mf is that of the fuel. We can calculate the respective masses knowing the number of moles of each (2 moles of hydrogen and 4.76 of air) and the molecular weights (29 g/mole of air and 2g/mole for hydrogen). Therefore, the stoichiometric air-to-fuel ratio for hydrogen-air mixture is:
| A / F | = | 4.76 mol × 29 g mol-1 2 mol × 2 g mol-1 |
= | 34.5 |
It is important to note that, compared to air-to-fuel ratios for methane (17) and iso-octane (15), hydrogen mixtures have a relatively higher ratio, which can limit the power output. [2,3] Given the hydrogen's stoichiometric air-to-fuel ratio by volume, the maximum air flow intake into the engine cylinder is around 29% less in the case external mixing of hydrogen and air (compared to fuel injection into the cylinder); that reduces the maximum engine power by around the same amount (29%). [3]
Alongside the high air-to-fuel ratio, hydrogen combustion is also characterized by a high combustion temperature. The main consequence of that is the potentially high NOx emissions. [1] Hydrogen combustion engines usually operate with twice the stoichiometric amount of air (which compromises the power output) to minimize the amount of NOx emissions. [4] The high hydrogen combustion temperature facilitates the production of NOx in the combustion byproducts, which is mitigated through burning lean fuel-air mixtures. [1,2]
All the above-mentioned limitations of hydrogen combustion and chemistry are well-known to the hydrogen combustion community, and there are multiple solutions introduced to address those challenges that includes the optimization of engine design or operating conditions. For example, operating the engines with lean mixtures addresses several issues including the NOx emissions and the backfiring. [4]
© Ghufran Alkhamis. The author warrants that the work is the author's own and that Stanford University provided no input other than typesetting and referencing guidelines. The author grants permission to copy, distribute and display this work in unaltered form, with attribution to the author, for noncommercial purposes only. All other rights, including commercial rights, are reserved to the author.
[1] C. M. White, R. R. STeeper, and A. E Lutz, "The Hydrogen-Fueled Internal Combustion Engine: A Technical Review," Int. J. Hydrog. Energy. 31, 1292 (2006).
[2] S. Verhelst and R. Wallner, "Hydrogen-Fueled Internal Combustion Engines," Prog, Energy Combust. Sci. 35, 490 (2009).
[3] L. Dodge and D. Naegeli, "Hydrogen-Air Mixing Evaluation in Reciprocating Engines," U.S. National Renewable Energy Laboratory, NREL/TP-425-6346, June 1994.
[4] S. Verhelst and R. Sierens, "Hydrogen Engine-Specific Properties," Int. J. Hydrog. Energy 26, 987 (2001).
