Recent restrictions on marine fuel sulfur content and a heightened regulatory focus on maritime decarbonization are driving the deployment of low-carbon and low-sulfur alternative fuels for maritime transport. In this study, we quantified the life-cycle greenhouse gas and sulfur oxide emissions of several novel marine biofuel candidates and benchmarked the results against the emissions reduction targets set by the International Maritime Organization. A total of 11 biofuel pathways via four conversion processes are considered, including (1) biocrudes derived from hydrothermal liquefaction of wastewater sludge and manure, (2) bio-oils from catalytic fast pyrolysis of woody biomass, (3) diesel via Fischer-Tropsch synthesis of landfill gas, and (4) lignin ethanol oil from reductive catalytic fractionation of poplar. Our analysis reveals that marine biofuels' life-cycle greenhouse gas emissions range from -60 to 56 gCO₂e MJ^-1, representing a 41-163% reduction compared with conventional low-sulfur fuel oil, thus demonstrating a considerable potential for decarbonizing the maritime sector. Due to the net-negative carbon emissions from their life cycles, all waste-based pathways showed over 100% greenhouse gas reduction potential with respect to low-sulfur fuel oil. However, while most biofuel feedstocks have a naturally occurring low-sulfur content, the waste feedstocks considered here have higher sulfur content, requiring hydrotreating prior to use as a marine fuel. Combining the break-even price estimates from a published techno-economic analysis, which was performed concurrently with this study, the marginal greenhouse gas abatement cost was estimated to range from -$120 to $370 tCO₂e^-1 across the pathways considered. Lower marginal greenhouse gas abatement costs were associated with waste-based pathways, while higher marginal greenhouse gas abatement costs were associated with the other biomass-based pathways. Except for lignin ethanol oil, all candidates show the potential to be competitive with a carbon credit of $200 tCO₂e^-1 in 2016 dollars, which is within the range of prices recently received in connection with California's low-carbon fuel standard.

The production of biomass‐based sustainable aviation fuel (SAF) is gaining traction to reduce the carbon footprint of the aviation sector. We performed a techno‐economic analysis to estimate the break‐even price and life cycle carbon emissions of the SAF derived from carinata (Brassica carinata) in the Southeastern United States. Carinata has the potential as a feedstock for SAF production in the selected region due to higher yield, low fertilizer use, co‐product generation (animal feed, propane, and naphtha), and compatibility with current farming practices. The system boundary started at the farm and ended when the SAF is delivered to an airport. Without co‐product credit or other subsidies such as Renewable Identification Number (RIN) credit, carinata‐based SAF was more expensive ($0.85 L⁻¹ to $1.28 L⁻¹) than conventional aviation fuel ($0.50 L⁻¹). With co‐product credit only, the break‐even price ranged from $0.34 L⁻¹ to $0.89 L⁻¹. With both co‐product and RIN credits, the price ranged from ‐$0.12 to ‐$0.66 L⁻¹. The total carbon emission was 918.67 g CO₂e L⁻¹ of carinata‐based SAF. This estimate provides 65% relative carbon savings compared with conventional aviation fuel (2618 g CO₂e L⁻¹). Sensitivity analysis suggested a 95% probability that relative carbon savings can range from 61% to 68%. Our study indicates that carinata‐based aviation fuel could significantly reduce carbon emissions of the aviation sector. However, current policy support mechanisms should be continued to support manufacturing and distribution in the Southeastern United States.

Methanol is gaining interest in the marine sector from energy security and reducing emissions perspective. This study provides a comparative life cycle assessment of methanol as a marine fuel, across GHG and criteria air pollutant emission metrics, when it is used in a dual-fuel engine. Twelve methanol pathways from four different feedstock categories were considered, including (1) cellulosic biomass─forest residues and clean pine mix, corn stover, switchgrass, and miscanthus; (2) organic wastes─renewable natural gas from wastewater sludge, swine manure, food waste, and landfill gas; (3) fossil resources─coal and natural gas (NG); and (4) e-methanol using captured carbon dioxide. When used in a dual-fuel engine with pilot fuel, life cycle GHG emissions for woody biomass-based methanol were approximately 19 gCO₂e MJ^-1, while emissions from waste-based sources ranged between -154 and 31 gCO₂e MJ^-1. Methanol from renewable sources showed a GHG reduction potential between 58 and 226% compared to conventional NG-based methanol (122 gCO₂e MJ^-1), primarily due to the avoided emissions from conventional waste management. When carbon from process emissions were captured, the reduction could be up to 327%. All pathways exhibited lower NO_X, and particulate matter emissions compared to the baseline marine fuel (MGO 0.1% sulfur), while woody biomass and coal pathways had higher SO_X emissions.