## 2 Current Cellulosic Ethanol Production Process in Commercial Plants  
Cellulosic ethanol production can be summarized in four main steps: pretreatment, hydrolysis, fermentation, and product recovery (Figure 1). Pretreatment makes the lignocellulosic biomass more amenable to biological conversion ensuring complete substrate utilization. Several pretreatment technologies have been developed and tested at various scales [13; 14], and steam explosion has been recognized as the one most widely used by industrial companies [15]. In addition, hydrolysis (of the cellulosic and hemi-cellulosic structures) can also be enhanced by means of enzymes or dilute acid pretreatments. Following pretreatment, enzymatic hydrolysis is the most commonly applied hydrolysis method, although the cost of enzymes currently takes a major proportion of the production costs [15] (see Section 3 for more details on production costs). Bacteria or yeasts are then used to ferment sugars into ethanol. In terms of production, around 109 billion liters (or 86 million tons) of ethanol were globally produced in 2018, most of which was made by US (56%), followed by Brazil (28%), EU (5%), and China (4%) [16]. For cellulosic ethanol, commercial size plants have been constructed in Europe, US, Brazil, and China, but regular and reliable production is yet to be proven. The actual cellulosic ethanol production to date has been markedly below the installed capacity. For the EU, ePURE reports that almost 200 kt of European ethanol has been produced from ligno-cellulosic, other RED Annex IX-A feedstocks, or other feedstocks, representing 4% of the total ethanol production in 2017 [17]. However, according to the US Department of Agriculture (USDA) Global Agricultural Information Network (GAIN) report, EU28 annual production of cellulosic ethanol was estimated to be around 40 kt in 2017 down to 10 kt in 2018 [18]. In the US, the EPA's 2018 RFS data reports US cellulosic biofuel production levels, from which it's possible to estimate a total of about 30 kt of cellulosic ethanol produced domestically in 2017 [8]. In Brazil, total cellulosic ethanol production is estimated to be 25 million liters (or 20 kt) for 2018 representing an insignificant share of total ethanol production in Brazil [9]; while, for China, 2018 cellulosic ethanol production is forecast to stop at 20 million liters (or 16 kt) as its major cellulosic project appears idle [11]. There are several first-of-a-kind commercial scale cellulosic ethanol plants at a global level, according to the International Energy Agency (IEA) database [19] and other sources [9; 11; 18; 20; 21; 22; 23; 24], although operations of some of the plants are currently idle or on-hold (Table 1). Only a few commercial scale plants are reported as operational in Norway, US, and Brazil. In addition to the plants listed in Table 1, Quad County Corn Processors and six other plants (using the Edeniq technology) adapted their conventional corn ethanol refineries to produce ethanol from corn kernel fiber, known as 1.5 generation technology. As reported in Section 1, it will qualify as cellulosic biofuel in US following the EPA definition. However, corn kernels consist of a significant content of fibers (10-12%) that impact on the conversion of fermentable sugar, but they are mainly comprised of starch [7]; therefore, they should not be strictly considered as second generation ethanol production plants. Other plants are also planning to produce 1.5 generation ethanol: D3 Max and ICM are developing their own technologies [7]. Figure 1: Cellulosic ethanol process. The POET-DSM Advanced Biofuels LLC is reported as operational, but it is not clear how much ethanol it is currently producing. The plant has been inaugurated in 2014 in Iowa with a production capacity of 75 kt per year of cellulosic ethanol from corn stover and cob. In summer 2017, the company installed a new pretreatment technology and announced the construction of an on-site enzyme manufacturing facility that will cut costs associated with the process [25].