Carbon intensity of green ammonia as a fuel for maritime transport

Main Presenter:    William Stafford 

Co-Authors:   Valentina Russo     Koloba Chaba      Taahira Goga                                    

Countries with abundant solar and wind resources are well positioned for a just energy transition with a switch away from fossil fuels. Power-to-X pathways offer an opportunity to convert excess renewable electricity into energy carriers or fuels, such as hydrogen and ammonia. Although these fuels contain zero-carbon, there are carbon emissions associated with the lifecycle of production that need to be assessed. This study is a lifecycle assessment of the carbon emissions of green ammonia production from cradle-to-gate; based on project designs of a coastal production facility that uses seawater for hydrogen production and renewable energy from wind and solar.
The carbon emissions were estimated at 2.69 kg CO2-eq/kg green hydrogen and 0.71 CO2-eq/kg green ammonia. This is a 70% reduction in carbon emissions compared to grey ammonia from natural gas (2.33 CO2-eq/kg ammonia) and a 87% reduction in carbon emissions compared to black ammonia from coal (5.56 kg CO2-eq/kg ammonia). Carbon emissions were also expressed in energy intensity as 38.17 kg CO2-eq/MJ. Most of carbon emissions in the production of green ammonia are attributed to electrolysis (70%) and the electrical energy demands of the electrolyser. The electricity was provided from onsite installation of wind power and solar PV with a combined carbon intensity of 0.0516 kg CO2-eq/KWh. If the embodied carbon emissions from manufacturing renewable energy are assumed zero, as proposed by several developing hydrogen certification bodies, then the carbon emissions are 0.069 CO2-eq/kg hydrogen. Therefore, the production of green hydrogen and ammonia will meet the low-carbon certification
thresholds and support long-term climate mitigation targets for 2050.

The study explores how the carbon emissions of green ammonia production is influenced by the life-cycle scope definition and boundary of the system, particularly the assumptions concerning the sale of co-products (such as oxygen, argon and excess electricity) and whether the indirect carbon emission upstream of production (embodied emissions from the manufacturing renewable energy infrastructure) are included. These are essential aspects to be clarified in the development of standards for the certification of green hydrogen and ammonia as low-carbon fuels

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