(343g) NOx-to-Ammonia for Circular Ammonia-Fueled Container Ships

The shipping industry is highly dependent on fossil-based fuels like heavy fuel oil (HFO), and marine diesel oil (MDO), being responsible for around 2.5% of the global GHG emissions [1]. Notably, decarbonization efforts set by the International Marine Organization anticipate a GHG-reduction of 70% by 2050 [2]. Adopting alternative fuels i.e., low carbon CO₂-based fuels or zero-carbon fuels, is thus imperative. In this context, ammonia, hydrogen, and electricity could be (almost) net zero-carbon fuels that hold a promise to mitigate CO₂ emissions in the shipping industry. However, they face technical challenges, i.e., toxicity, low energy density, and high investment costs [3].

Among the options available, Stolz et al. [4], found that ammonia and methanol are two of the most balanced fuels for the shipping sector in terms of technoeconomic suitability. Recently, MAN Energy Solutions started working in designing a two-stroke ammonia engine, and the auxiliary system that will support the engine. Combusting ammonia requires careful design of emission reduction technologies due to the high expected NO_x emissions, however, there is still no clear view on how NO_x emissions will be handled. Selective catalytic reduction (SCR), has been the main technology that is used to remove NO_x from exhaust gases [5]. Nowadays, ships use SCR to comply with emissions regulations, although the expected large-scale adoption of ammonia-fueled ships might create opportunities for other alternatives to handle NOx emissions.

Motivated by the use of ammonia in two-stroke marine engines, this work applies circular economy principles to reduce NO_x emissions from ammonia-fueled ships and regenerate back ammonia, decreasing the fuel. It was estimated that approximately 34 Mton of ammonia could be produced by using all the NO_x emissions in the exhaust gas from all the power plants in the world. Several studies have looked at the technical feasibility of producing ammonia from NO_x emissions, while still relying (t a lesser extent) on SCR, particularly in power plants [6]. This study will transfer the above idea to the shipping industry. The concept is to regenerate ammonia fuel from NO_x emissions on board an ammonia-fueled ship.

The studied concept involves the modeling of ammonia combustion, the ammonia cracker, NOx-to-ammonia (NTA) reactor, separators, and ammonia tank. The process was modeled in Aspen PLUS v.12. First, a mixture of ammonia/H₂/N₂ generated from ammonia in a cracker is fed to the engine. The ammonia decomposition process is simulated as in Sánchez et al. [7]. The combustion of the mixture has been proposed to reduce the combustion temperature and time, but causes an upsurge of NO_x emissions. Next, the exhaust gases are sent to the NTA reactor where H₂ coming from the cracker is fed to the reactor. The NTA process requires that NO_x reduction with H₂ produces ammonia, although experimental studies have shown that complete selectivity for ammonia is difficult to achieve in stoichiometric conditions. NO_x conversion is enhanced in the presence of CO and H₂O on a Pt/TiO₂ catalyst and at a temperature of 300 ^oC and ambient pressure. The yield of the reaction with CO can be up to 86% [8]. Therefore, we consider that the designed engines operate in dual-fuel mode, and that diesel fuel is mixed at certain operation modes during the ships trip. We further assume that part of the carbon emissions will be used as a source of CO for the NTA reaction [9]. The process simulation of the NTA reactor was based in experimental studies [8]. After the regeneration of ammonia, separation of the gases takes place and ammonia is sent back to the tank. The production of green ammonia was not simulated, but data for its production were taken from D’Angelo et al. [10].

An economic and environmental assessment was performed as well as a technical feasibility analysis of the developed concept. For the technical feasibility, the weight and space taken by the equipment added on-board the ship is considered and compared to the actual cargo weight (DWT – deadweight tonnage) and space (TEU – twenty-foot equivalent units). In order for the circular concept to be feasible and profitable, the added equipment should not take up most of the space, hindering the ships’ purpose. Moreover, the amount of regenerated ammonia dictates the cost of ammonia per tonne·kilometer (tkm).

The environmental assessment was implemented in Brightway2 v.2.4.2, an open-source Python-based LCA software, and its GUI, Activity Browser. The goal of the LCA was to compare the use of ammonia in two-stroke marine engines with and without regeneration of the fuel with the business-as-usual HFO-fueled ships, based on the IPCC 2013 GWP 100a impact assessment method. The scope of the analysis is cradle-to-propulsion, and the functional unit is 1 tkm. The inventories integrate data of the foreground and background systems. For the background system, data available in Ecoinvent v3.8 were used [11]. The mass and energy flows in the foreground system were taken from process simulations, and are complemented with literature data for the production of green ammonia.

Our results show that the circular economy principles could be applied to an ammonia-fueled ship enabling their faster uptake due to, mainly, the possible reductions in investment costs. Our work opens up a new pathway by proposing an alternative concept for tackling the shipping industry's NOx emissions while regenerating the fuel.

Acknowledgments: This work was created as part of the NCCR Catalysis, a National Centre of Competence in Research funded by the Swiss National Science Foundation.

References

1. IMO et al., "Third IMO Greenhouse Gas Study 2014," Int. Marit. Organ., p. 327, (2014)
2. IMO et al., "Initial IMO GHG Strategy." (2018)
3. N. Gray et al., "Decarbonising Ships, Planes and Trucks: An Analysis of Suitable Low-Carbon Fuels for the Maritime, Aviation and Haulage Sectors," Adv. Appl. Energy, vol. 1, p. 100008, (2021)
4. Stolz, B. et al., “Techno-Economic Analysis of Renewable Fuels for Ships Carrying Bulk Cargo in Europe,” Nat. Energy 2022 72, 7 (2), 203–212, (2022)
5. “MAN B&W Two-Stroke Engine Operating on Ammonia” MAN Energy Solutions.
6. Matsumoto, H. et al., “Synthesis and Assessment of NOx to Ammonia Conversion Process in Combined Cycle Power Generation Systems,” Computer Aided Chemical Engineering, Elsevier, Vol. 49, pp 253–258 (2022)
7. Sánchez, A. et al., “Evaluating Ammonia as Green Fuel for Power Generation: A Thermo-Chemical Perspective,” Appl. Energy, 293, 116956 (2021)
8. Kobayashi, K. et al., Effect of the TiO₂ Crystal Structure on the Activity of TiO₂ -Supported Platinum Catalysts for Ammonia Synthesis via the NO–CO– H₂O Reaction. Catal. Sci. Technol., 9 (11), 2898–2905 (2019)
9. Nadimi, E. et al., “Effects of Using Ammonia as a Primary Fuel on Engine Performance and Emissions in an Ammonia/Biodiesel Dual-Fuel CI Engine” Int. J. Energy Res., 46 (11), 15347–15361 (2022)
10. D’Angelo, S. C. et al., “Planetary Boundaries Analysis of Low-Carbon Ammonia Production Routes,” ACS Sustain. Chem. Eng., 9 (29), 9740–9749 (2021)
11. G. Wernet et al., “The ecoinvent database version 3 (part I): overview and methodology, Int. J. Life Cycle Assess., vol. 21, no. 9, pp. 1218–1230 (2016)