## 4 Discussion  
At the time of publication, there is considerable uncertainty on how the expected push towards transport decarbonisation will be realised. If passenger cars and light-duty mobility seem to be more oriented towards electrification and gaseous fuels (i.e., H\({}_{2}\)) [31], the trend for the heavy-duty sector is less clear. The road and maritime freight sector, as well as the airborne segment, are expected to keep relying on current propulsion technologies due to the longer turnover of the vehicles and the need for energy dense fuels. Conversely to what is expected to happen in the light-duty road sector with electrification, the use of alternative liquid fuels can be considered as a short-term option for hard-to-abate sectors, such as maritime and aviation. In this framework, liquid alternative fuels will play a crucial role at least till 2030.  
It is worth recalling that the methodology used contains some limitations, such as the inhomogeneity and the limited number of literature sources available. However, within these limits, the followed methodology allowed us to derive a consistent set of information. In particular, what emerged from the analysis was that the volumes will be further defined by the legislative initiatives, and this is already under development for the maritime and aviation sectors, which have not yet started to use alternative fuels in large amounts to reduce their environmental impacts. Adding to the layer of uncertainties created by COVID-19, which disrupted previous assessments, the quantification of the potential demand for alternative fuels is a challenging task. However, as of today the estimate ranges between 20.4 and 32.6 Mtoe. This is an important outcome, which illustrates that adequate quantities of sustainable feedstock can be made available to supply the development of the low carbon transport markets.  
Under a very conservative approach, expected feedstock availability appears to be sufficient to cover the projected demands. This allows the drawing of a picture where feedstock quantity may not actually be the limiting factor, with respect to a significantly increasing use of alternative fuels by 2030. The only caveat is that high-ILUC lipid materials and food/feed feedstock-based production chains are planned to be progressively phased out. This implies that a shift towards other productions will be needed, to cover this expected cut in the feedstock pool. However, even when this general approach is focused on specific feedstock/fuel pathways, the overall picture does not change. The expected HEFA use in aviation could, for instance, be entirely supplied by waste streams. In the medium term, the forestry residues are expected to sustain growing demands, progressively reducing pressure and lipid materials, as technological conversion processes will be increasing their technology readiness level (TRL).  
Clearly, factors other than the feedstock availability are related to the actual pace with which these alternative fuels will be taken up. For example, the final production cost is widely recognised as a limiting factor, especially for aviation [32] and maritime sector [4]. Additionally, for specific pathways, such as liquefied biomethane in maritime, the need for creating a dedicated infrastructure may also be a relevant barrier.