A number of technological challenges need to be overcome if algae are to be utilized for commercial fuel production. Current economic assessment is largely based on laboratory scale up or commercial systems geared to the production of high value products, since no industrial scale plant exits that are dedicated to algal biofuel. For macroalgae (‘seaweeds’), the most promising processes are anaerobic digestion for biomethane production and fermentation for bioethanol, the latter with levels exceeding those from sugar cane. Currently, both processes could be enhanced by increasing the rate of degradation of the complex polysaccharide cell walls to generate fermentable sugars using specifically tailored hydrolytic enzymes. For microalgal biofuel production, open raceway ponds are more cost-effective than photobioreactors, with CO₂ and harvesting/dewatering costs estimated to be ~50% and up to 15% of total costs, respectively. These costs need to be reduced by an order of magnitude if algal biodiesel is to compete with petroleum. Improved economics could be achieved by using a low-cost water supply supplemented with high glucose and nutrients from food grade industrial wastewater and using more efficient flocculation methods and CO₂ from power plants. Solar radiation of not <3000 h·yr⁻¹ favours production sites 30° north or south of the equator and should use marginal land with flat topography near oceans. Possible geographical sites are discussed. In terms of biomass conversion, advances in wet technologies such as hydrothermal liquefaction, anaerobic digestion, and transesterification for algal biodiesel are presented and how these can be integrated into a biorefinery are discussed.

Ammonia (NH₃) is gaining recognition as a viable “green” transportation fuel due to its zero-carbon characteristic, its high energy density and its widespread availability. However, NH₃ has a high auto-ignition temperature, resulting in potential emissions of NO_x and unburned NH₃. Addressing combustion challenges requires innovative solutions, such as the application of combustion promoters to enhance NH₃ combustibility. This review article focuses on the compatibility of NH₃ as a fuel for spark-ignition (SI) engines, examining its combustion under various modes including pure NH₃ combustion, gasoline blends, NH₃/hydrogen (H₂) blends, and NH₃/natural gas blends in single or dual-fuel configurations. The formation of nitrogen oxides (NO_x) and slip-NH₃ is explored to understand emissions species such as NO and N₂O. Additionally, the article highlights the limitations of NH₃ as a fuel for SI combustion. The comprehensive discussion provided in this review aims to fill a critical gap in the literature regarding NH₃’s feasibility as a zero-carbon fuel for SI engines, particularly in the maritime sector. By offering insights into NH₃ combustion characteristics and emissions profiles, the article seeks to provide a roadmap for leveraging NH₃ as a suitable non-carbon fuel to decarbonize the marine sector and advance global sustainability goals.

The fuel qualities of several biodiesels containing highly saturated, mono, and poly unsaturated fatty acids, as well as their combustion and exhaust emission characteristics, were studied. Six biodiesel samples were divided into two groups based on their fatty acid composition, including group 1 (coconut, castor, and jatropha) and group II (palm, karanja, and waste cooking oil biodiesel). All fuels (in both groups) were tested in a single-cylinder off-road diesel engine. Castor and karanja biodiesel, both rich in mono-unsaturation level, have a high viscosity of about 14.5 and 5.04 mm²/s, respectively. The coconut and palm biodiesels are rich in saturation level with cetane numbers of 62 and 60, respectively. In both groups, highly saturated and poly-unsaturated methyl esters presented better combustion efficiency and less formation of polluted emissions than mono-unsaturation. At full load, coconut and palm biodiesel displayed 38% and 10% advanced start of combustion, respectively, which reduced ignition delay by approximately 10% and 3%, respectively. Mono-unsaturated methyl esters exhibited a higher cylinder pressure and heat release rate, which results in higher NOx gas emissions. The group II biodiesels showed about 10–15% lower exhaust emissions owing to an optimum level of fatty acid composition. Our study concluded that highly saturated and poly-unsaturated fatty acid performed better than mono-unsaturated biodiesels for off-road engine application.