The shift to alternative bunker fuels risks raising the GHG emissions of the shipping industry if additional feedstock for the renewable alternatives is not sustainably produced, according to a recently published paper.
The paper was published in the journal Environments, and compares CO2 emissions from two life cycle assessment methodologies for eight marine fuels.
A standard narrative of the emissions generated when burning alternatives such as HVO, bio-LNG or bio-methanol suggests these fuels should be a clear path towards reducing GHG emissions from shipping.
But comparing the alternatives emissions from a marginal and attributional method reveals another story.
The world's largest database for low-carbon assessment now show drastically worse values for marginal compared to average emissions for the renewable alternatives, Gustav Krantz, a lead researcher on the paper, told Ship & Bunker.
"HVO can be produced from a different set of feedstock and the average values can thereby be dominated by the preferable feedstock." Krantz said.
"By gathering all feedstock used the past year in a basket and then normalise it by energy content gives the average value for CO2 per product unit.
"This average value was valid in 2022 but does not answer what the value is in 2023 if demand increases.
"We would then require one additional feedstock unit; so the question would then be which feedstock that is able to scale up?
"Since some of the feedstocks, like used cooking oil, are limited by how much is coming out as a byproduct from the food industry; demand does not affect supply, which is determined by the food industry.
"There are only certain production chains that can actually increase their production, and if those are in the upper range of emission values, then the marginal emissions will differ quite a lot from the average emissions."
Krantz emphasises that the calculations of marginal values have been collected from the Ecoinvent database and that the above example is schematic.
The paper reports marginal emissions values of 169-220 kg CO2/MJ for biofuels, 162 kg CO2/MJ for biomethane and 180 kg CO2/MJ for LNG. That compares with 148-262 kg CO2/MJ for conventional bunkers.
"Since the underlying demand for bio-based fuels is currently higher than the supply, there is a potential risk that GHG emissions of marginal biofuels are currently considerably higher than their average emissions," the paper argues.
"This must be considered before initiating a rapid large-scale fuel switch, e.g., in the shipping industry.
"Biofuels can potentially reduce the GHG emissions from shipping significantly, but that requires that the marginal emissions from biofuel production be reduced, eg via the use of more sustainable feedstocks.
"Additional measures are therefore required to reach the climate goals of the maritime industry, such as more climate-efficient production of biofuels, more energy-efficient ships, reduced shipping volumes, and the use of carbon capture techniques.
"In the long term, the utilisation of climate-neutral electrochemical fuels, such as hydrogen and ammonia, is of paramount importance."