## 1 Introduction  
Ethanol production from cellulosic material such as agricultural residues (e.g., wheat straw and corn stover) and energy crops (e.g., switchgrass and miscanthus) is considered a highly promising option for future ethanol production, helping the energy diversification and decarbonization of the transport sector. Compared to conventional (or first generation) ethanol production from food and feed crops (mainly sugar and starch based crops), cellulosic (or second generation, 2G) ethanol is considered to provide better performance in terms of greenhouse gas (GHG) reduction and low risk of direct and indirect land-use change impacts. Those advantages have led to the promotion of cellulosic ethanol in the legislation around the globe. In the EU, the current 2009 Renewable Energy Directive (RED) [1] and Fuel Quality Directive (FQD) [2] (in force until 2020), contain a 10% renewable energy target for the transport sector by 2020 and a 6% GHG reduction target for transport (set in FQD) that are both expected to be met largely with food-based biofuels. In response to the controversial issue of indirect land use change (ILUC), related to the global market-mediated agricultural area expansion, the 2015 ILUC Directive [3], amending RED and FQD, introduced a cap on crop-based biofuels, which can contribute up to 7% of the final consumption of transport energy in 2020. The directive [3] also introduced requirements for reporting ILUC emissions and measures to promote advanced biofuels. It [3] requires Member States to promote the use biofuels produced from feedstocks listed in Part A of Annex IX (which includes waste,residue, non-food cellulosic, and ligno-cellulosic feedstocks) by setting a non-legally binding target of 0.5% in energy content for their use in transport fuel. RED [1] also has a 'double-counting' mechanism, according to which the energy content of those biofuels is double counted towards the overall renewable energy in transport target. The recast of RED, Directive EU 2018/2001 [4], with effect from 1 July 2021, makes advanced biofuel mandate compulsory containing a sub-target of 3.5% for advanced biofuels for 2030 within the 14% target for renewable energy in transport. Advanced biofuels are defined by the directive as 'biofuels that are produced from the feedstock listed in Part A of Anne IX' that includes among others, agriculture and forestry residues as well as energy crops. Those biofuels will continue to count double towards the targets. In addition, biofuels must meet a 65% greenhouse gas reduction threshold (in installation starting operation from 1 January 2021), to try to ensure substantial GHG savings compared to fossil-based fuels. Focusing on the US, a fuel is classified as advanced on the basis of its lifecycle GHG savings (which need to be at least 50% when compared to fossil fuels). Cellulosic biofuel is an advanced biofuel subcategory: it is required to have a greater than or equal to 60% GHG saving compared to a 2005 petroleum baseline [5]. The Renewable Fuel Standard (RFS) program required 36 billion gallons of biofuels to be used by 2022, of which at least 16 billion gallons were supposed to come from cellulosic biofuels [6]. However, every year, the Environmental Protection Agency (EPA) has exercised its 'cellulosic waiver authority' to reduce the cellulosic biofuel target. In 2014, EPA also redefined the term 'cellulosic biofuels' expanding the definition to include certain types of biogas and ethanol from corn kernel fiber [7]. Since then, most of the increase in cellulosic biofuel consumption is from biogas, in either compressed or liquefied form. The current cellulosic biofuel mandate (2019) has been set by EPA at 418 million gallons, corresponding to about 5% of the target envisioned by the Energy Independence and Security Act of 2007 [8]. Moving to Brazil, the recent RenovaBio program aims to decrease transport emissions by 10% in the next 10 years favoring fuels with lower carbon intensities [9].