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| Fig. 1: Ammonia Molecular Structure. (Source: Wikimedia Commons) |
Ammonia (NH3) has garnered significant attention across academia and industry as a potential energy storage solution due to its unique physical and chemical properties. It can be easily liquefied either by increasing the pressure to approximately 10 bar at room temperature or by cooling down to -33°C under 1 atm, making it a versatile option for energy storage in various environmental conditions. NH3's safety and ease of storage and transportation are facilitated by its low vapor pressure and high boiling point.
Ammonia has been used as an industrial input for decades, making it possible to draw from robust existing technologies to produce, transport, and use ammonia. Moreover, NH3 contains a higher hydrogen (H2) content (17.65%) compared to alternative energy carriers like methanol (MeOH) and methylcyclohexane (MCH).
We wish to evaluate the conversion efficiency of powering a vehicle using ammonia as a means of energy storage. In other words, we wish to track what percentage of input energy is ultimately put to use to move the vehicle.
We will consider the six potential ammonia conversion processes shown in Table 1. These are not meant to be an exhaustive list of potential paths for ammonia use but rather a set of alternatives that show potential given currently available data.
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| Table 1: Evaluated ammonia converstion processes. |
The following sections explain each of the methods of ammonia production and end uses that are part of each process.
SMR+HB (Steam Methane Reforming and Haber-Bosch) production uses fossil fuels as an input and applies carbon capture and utilization (CCU) technologies to mitigate process greenhouse gas emissions. Sometimes called "Blue" ammonia production, this process is based on CCU-equipped steam methane reforming (SMR) facilities and the Haber-Bosch Process. SMR+HB is a mature technology that has been widely deployed and optimized; though carbon capture and utilization equipment and processes can be costly. This process leverages mature, low-risk technologies but it results in greenhouse emissions and can be costly.
In Hydrolysis ammonia production, hydrogen, an input for ammonia production, is generated from renewable energy sources, such as wind, solar, or hydroelectric power. Electrolysis splits water into hydrogen and oxygen, which can then be combined with nitrogen from the air to form ammonia. This processes is sometimes called "Green" ammonia. The key advantage of this process is its potential to reduce greenhouse gas emissions significantly, as it avoids the use of fossil fuels in the hydrogen generation process.
Engine: Ammonia used as fuel for an internal combustion engine. This is a simple approach, though engines are generally less energy-efficient than fuel cells, and direct ammonia use involves safety risks outlined later in this paper.
Hydrogen Fuel Cell: Ammonia is converted back into hydrogen, which is then used as input for a fuel cell. This additional conversion causes additional losses and requires additional infrastructure near the point of use. Furthermore, any ammonia used as a fuel can permanently damage a fuel cell. Conversion to hydrogen must, therefore, be strictly controlled for purity, which presents logistical and economic challenges.
Ammonia Fuel Cell: While direct ammonia fuel cells (DAFCs) are a relatively immature technology, solid oxide fuel cells (SOFCs) show promise for near-future deployment. In SOFCs, NH3 cracking occurs inside the fuel cell, eliminating the need for hydrogen conversion. However, it's worth noting that their high operating temperatures, ranging from 550°C to 900°C, may limit their suitability for continuous stationary applications with minimal on-and-off cycling, like long-haul freight shipping. [1]
Table 2 outlines benchmark conversion efficiencies for each stage for each of the proposed ammonia conversion processes. Each value indicates (Energy Input / Energy Output) for each stage of the process.
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| Table 2: Conversion efficiencies for proposed processes. Values correspond to Energy Out / Energy In; final column corresponds to the total sequence of all stages in the process. [1-3] (Note: Chatterjee et al. provide ranges for potential conversion efficiencies. [1] The number reported here is the average between the top and bottom values for those ranges.) |
NH3 Production: Blue ammonia production corresponds to SMR followed by the Haber Bosch Process; all powered by methane. Green ammonia production consists of hydrogen production through a Proton Exchange Membrane (PEM) electrolyzer, nitrogen (N2) production by an air-separation unit (ASU), and electric-powered Haber Bosch loop compressors.
While currently available PEM electrolyzers are less energy-efficient than SMR, the "green" electric-powered HB process is more energy efficient than the "blue" process powered by methane, leading to similar overall efficiencies for ammonia production.
Fuel Transport: Accounts for ammonia losses due to evaporation and any energy used to transport the fuel. Expected to be similar across all proposed processes.
NH3 to H2: Only applies to processes where ammonia is converted to hydrogen for end use. Includes ammonia cracking and hydrogen compressing processes.
End Use: Energy conversion from powering a vehicle as indicated in each process (through an engine, hydrogen fuel cell, or ammonia fuel cell). Fuel cells are subject to losses to heat than engines but are less mature technologies with higher costs.
One of the main forces driving interest in ammonia as an energy carrier is its potential to displace fossil fuels. However, different modes of ammonia production and use can result in significantly different levels of greenhouse emissions. Blue ammonia production with modern CCU equipment typically emits 1.5-1.6 kg CO2-eq per kg of NH3. [2] Green ammonia could be produced with near-zero greenhouse gas emissions if all power used is effectively sourced from zero-emission sources. However, if energy is not sourced carefully, the carbon footprint of production could be much higher.
As a brief and simple example: what would the carbon footprint of NH3 produced using power whose carbon footprint corresponds to the average of the California power grid?
Energy use for NH3 production through PEM Electrolysis: 3.35 MJ/kg NH3 (9.86 × 10-3 MWh/kg NH3) [1]
Average California Grid Emissions: 175 kg CO2-eq/MWh [4]
Average emissions for NH3 production: 1.73 kg CO2-eq/kg NH3
As calculated above, the average emissions would be 1.73 kg CO2-eq/kg NH3, which is approximately 13% higher than "blue" ammonia production. This highlights the importance of carefully tracking the processes used across the ammonia supply chain to effectively use it as a low-emission fuel.
There are also safety concerns to using ammonia as an energy carrier. It is immediately dangerous to human health at concentrations of 300 ppm, about half the corresponding number for gasoline. [5] It also has a vapor pressure of 8 bar at 20°C, compared to 0.047 bar for gasoline, making it highly volatile. [5] This high volatility combined with high toxicity raises significant safety concerns that may limit ammonia use to highly controlled industrial settings.
Lastly, several technologies involved in the proposed ammonia conversion processes are still in a nascent stage and their deployment can be costly. PEM electrolyzers and direct ammonia fuel cells are still difficult and expensive to produce, but could prove promising as the technologies mature and reach scale.
Blue and green ammonia production currently have similar conversion efficiencies. Fuel cells are significantly more efficient than engines. Conversion back to hydrogen significantly lowers total process efficiency. The processes that used ammonia fuel cells had the highest total process efficiency, with similar numbers for blue and green ammonia production.
Implementing these processes for ammonia production and use may face challenges including safety concerns regarding ammonia toxicity, effectively sourcing low-emission power as an input energy source, and developing early-stage technologies like PEM electrolyzers and direct ammonia fuel cells.
© Daniel Sandoval. 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] S. Chatterjee, R. K. Parsapur and K.-W. Huang, "Limitations of Ammonia as a Hydrogen Energy Carrier for the Transportation Sector," ACS Energy Lett. 6, 4390 (2021).
[2] C. Smith, A. K. Hill and L. Torrento-Murciano, "Current and Future Role of Haber-Bosch Ammonia in a Carbon-Free Energy Landscape," Energy Environ Sci. 13, 331 (2020).
[3] C. Tornatore et al., "Ammonia as Green Fuel in Internal Combustion Engines: State-of-the-Art and Future Perspectives," Front. Mech. Eng. 8, 944201 (2022).
[4] E. Grubert et al., "Utility-Specific Projections of Electricity Sector Greenhouse Gas Emissions: a Committed Emissions Model-Based Case Study of California Through 2050," Environ. Res. Lett. 15, 1040a4 (2020).
[5] A. Klerke et al., "Ammonia For Hydrogen Storage: Challenges and Opportunities," J. Mater. Chem. 18, 2304 (2008).
