A Prompt Decarbonization Pathway for Shipping: Green Hydrogen, Ammonia, and Methanol Production and Utilization in Marine Engines

The paper addresses how the maritime sector can decarbonize through green fuels derived from renewable energy. Conventional marine fuels produce NOx, SOx, PM, and CO₂; regulatory pressures (e.g., IMO targets: 50% CO₂ reduction by 2050 vs 2008 and zero emissions within this century) demand deeper cuts than efficiency measures alone can deliver. While many studies evaluate alternative marine fuels (hydrogen, ammonia, methanol), the pathway from renewable energy to marine fuel use is less explored. This review analyzes renewable electricity generation (green power) and its role in producing green hydrogen, ammonia, and methanol, and evaluates their applicability to shipping, highlighting costs, infrastructure, and technical challenges.

The paper synthesizes extensive literature on: (1) Carbon capture and carbon cycling, including CCS/CCU options and costs/efficiencies of shipboard capture (e.g., amine systems, membranes), indicating CC as a transitional measure. (2) Renewable electricity (green power) options—hydropower, wind, solar PV/CSP, and bioenergy—covering technology maturity, intermittency, environmental impacts, and deployment trends. (3) Green hydrogen production technologies: water electrolysis routes (AWE, PEMWE, AEMWE, SOWE/SOEC), advances in catalysts for OER/HER, and emerging seawater electrolysis; biomass-based thermochemical/biochemical/bioelectrochemical hydrogen routes (gasification, fermentation, MECs). (4) Ammonia production pathways: conventional Haber–Bosch with green H₂, and alternatives (electrochemical N₂ reduction, nitrate reduction, photocatalysis, biocatalysis), including catalyst developments and limitations (selectivity, Faradaic efficiency, stability). (5) Methanol production: biomass routes (gasification/biogas) and CO₂ hydrogenation (direct/indirect via RWGS) with catalyst advances (Cu-based, In₂O₃-based, Pt/In₂O₃-ZrO₂), including reaction mechanisms and techno-economic considerations. (6) Maritime applications and assessments for H₂, NH₃, and CH₃OH in engines and fuel cells, infrastructure readiness, safety, and lifecycle emissions.

This is a narrative review. The authors compile and analyze recent research on renewable electricity generation and green-fuel synthesis pathways, focusing on technical principles, catalyst and system advances, techno-economic indicators, life-cycle considerations, and maritime applicability. They structure the synthesis by energy source (green power), fuel (hydrogen, ammonia, methanol), and shipboard application (engines, fuel cells, storage and infrastructure), and distill comparative advantages, challenges, and recommendations.

- Green power is foundational for green fuels; increased renewable electricity directly correlates with reduced CO₂ emissions across energy systems. - Viable near- to mid-term green fuel production routes for shipping are: (1) green hydrogen via electrolysis (including potential seawater electrolysis); (2) green ammonia via green H₂ + Haber–Bosch; (3) green methanol via CO₂ hydrogenation powered by renewables or via biomass routes. - Shipping applications: • Methanol: Highest near-term readiness—liquid at ambient conditions, existing infrastructure compatible with minor modifications; engines commercialized; IMO interim guidelines exist. HyMethShip concept can achieve ~97% CO₂ reduction and >80% NOx reduction by onboard reforming and CO₂ capture/return. • Ammonia: Promising in next decade—carbon-free at point of use, tradable commodity, easier storage than H₂; challenges include toxicity, combustion characteristics, NOx/N₂O and NH₃ slip requiring after-treatment, and nascent safety regulations. • Hydrogen: Long-term potential—zero-carbon use with ICEs or fuel cells; main bottleneck is onboard storage (compressed/liquid/material carriers) and supporting bunkering infrastructure; safety and energy density constraints remain. - Cost and lifecycle indicators from reviewed sources: • Table 1 LCA vs MGO: Liquid H₂ (fossil: 166%; renewable: 0%); NH₃ (fossil: 140%; renewable: 6%); Methanol (fossil: 101%; renewable: 1%). Energy densities (MJ/L): H₂ ~8.5; NH₃ ~12.7; Methanol ~10.6–14.9 (temp-dependent). • Shipboard post-combustion CO₂ capture costs/effects: ~EUR 77.5/t CO₂ at 73% capture; EUR 98–120/t CO₂ at 90% capture, with studies showing up to 94.7% removal on a 3000 kW engine. • Green ammonia costs may become competitive post-2030 at favorable locations, with projected costs ~EUR 345–420/t (2030), ~EUR 300–330/t (2040), ~EUR 260–290/t (2050) when powered by optimal PV-wind hybrids. • Electrolysis: SOEC with waste heat shows best economics but durability is a key challenge; PEMWE offers high performance but needs cost reduction; AEMWE shows cost advantages but durability remains an issue. - Recommendations: Reduce green power costs; advance electrochemical production efficiency (for H₂/NH₃/CH₃OH); develop and scale new production technologies; expand green fuel production capacity and supporting maritime infrastructure.

By mapping renewable-to-fuel pathways and their maritime applicability, the paper shows that decarbonizing shipping hinges on abundant low-cost green electricity and mature conversion technologies. Methanol offers a practical near-term route due to logistics compatibility and engine readiness; ammonia provides a mid-term carbon-free onboard option if combustion/emission control and safety frameworks mature; hydrogen is a compelling long-term zero-emission solution contingent on breakthroughs in storage, materials, and bunkering infrastructure. The review contextualizes shipboard carbon capture as a transitional measure to bridge the gap until green fuels are widely available. Overall, aligning technology development (electrolysis, catalysts, reactors), infrastructure (production, storage, bunkering), and policy/regulatory measures will be crucial to achieve IMO climate goals.

Green power drives green fuels, which are pivotal for maritime decarbonization. The review concludes: (1) Electrolytic hydrogen (potentially from seawater) is central as both a fuel and a precursor for green ammonia and methanol; storage/infrastructure are the main marine barriers for H₂. (2) Green ammonia via green H₂ + Haber–Bosch is globally promising; alternative electrochemical/photocatalytic/biocatalytic routes merit R&D to improve yield, selectivity, and stability. (3) Green methanol is technically feasible with minimal supply-chain barriers and can integrate with carbon capture to approach a closed CO₂ cycle. The authors recommend strengthening R&D in renewable power and electrochemical fuel production, improving efficiency and durability, developing safety and environmental regulations (especially for ammonia), and scaling production capacity and bunkering infrastructure to ensure adequate low/zero-emission marine fuel supply. Future research should target durable, low-cost electrolyzers (especially SOEC/AEM), selective/stable NRR/NO₃RR/photocatalysts, efficient CO₂-to-methanol catalysts, and ship-specific storage and safety solutions.

The authors note space limitations precluded detailed reviews of all topics; the paper provides a macro-level synthesis rather than exhaustive quantitative analysis. Many alternative production pathways (e.g., electrochemical N₂ reduction, photocatalysis, biocatalysis) remain far from industrial viability due to low yields, selectivity challenges, and catalyst stability. Shipboard implementation also depends on unresolved infrastructure, safety regulation (notably for ammonia), and cost uncertainties; these factors may affect generalizability and timelines.