Currently in Brazil, the production of sugar cane bioethanol is based entirely on the fermentation of sugar juice from sugar cane and/or molasses in autonomous distilleries (39% of the cases) and in plants associated with sugar mills (61%) . This technology has been in commercial use for the past 30 years and can be considered to be mature as the cost of feedstock accounts for a major part of the production cost , around 60-70% [3, 4]. Compared to other crops used for bioethanol production from sugar and starch, i.e. first-generation (1G) bioethanol, bioethanol from sugar cane is claimed to have the lowest production cost worldwide . The low cost of Brazilian 1G bioethanol can be explained by a combination of favourable conditions such as the photosynthetic rate of the sugar cane crop per hectare, the meteo-climatic conditions and a renewable energy ratio close to 10, denoting an efficient life cycle from cultivation to processing included . The 2010/2011 sugar cane harvest in Brazil yielded 625 Mton sugar cane (SC) leading to the production of hydrous and anhydrous ethanol of 19.6 and 8.1 billion litres, respectively . Recently, the high sugar prices, the ethanol export and the high internal demand for hydrous ethanol for flex-fuel cars caused the Brazilian Federal Government to reduce the amount of anhydrous ethanol blended in gasoline from 25% to 20%, and to direct it towards hydrous ethanol . In 2011 Brazil was still a net exporter of ethanol even if since 2008 export has decreased 3.4-fold reaching the value 2 billion litres . Also for this reason, it was recently announced that incentives for ethanol production will be available for new autonomous distilleries . Furthermore, the conventional sucrose-to-bioethanol process yields not only ethanol and crystallized sugar, but also bagasse, a lignocellulosic residue with an interesting higher heating value, some of which can be used in boilers to produce the heat and electricity needed to run the plant.
The view of bagasse has changed throughout the years, due to technological breakthroughs, investment opportunities and revenue margin. At the beginning of the Pro-álcool project in the 1970s, bagasse was considered a fluffy waste which was eliminated in low-efficiency 22- bar boilers. As a result of deregulation of the electricity market and the increase in the price of electricity due to the 2001 energy crisis , bagasse has come to be regarded as the perfect solid fuel for bioelectricity generation, and for the past decade it has been combusted in more efficient and expensive boilers . Nowadays, bagasse is also generally recognized as a very promising feedstock for cellulosic ethanol production, i.e. second-generation (2G) bioethanol, and it is also expected that biofuel produced in this way will have less impact on the environment. However, the production cost of 2G bioethanol is still rather high, irrespective of the lignocellulosic feedstock used, and the development of a commercially competitive process for 2G technology poses a challenge [11, 13, 14], although the integration with 1G production could greatly facilitate this development, as will be discussed in the present study.
Sugar cane leaves and tops, often called trash, constitute the residues of mechanical harvesting, and are suitable as raw material for 2G bioethanol production because of their lignocellulosic nature. The amount of trash that can be removed from the field without affecting the nutritional properties of the soil or pest and weed control is generally considered to be about 50% by weight , although it is strongly dependent on the location and weather. Recent estimates show that up to 66% of the trash can be used for industrial processes, if only that amount is removed as dry leaves . For this reason, the term leaves will be used instead of trash since the material removed from the field is mainly composed of leaves.
Generally, the main process bottlenecks in 2G ethanol production, such as the low yield from the conversion of lignocellulose into fermentable sugars and the downstream processing, account for a significant proportion of the production cost . Several techniques for pretreating and hydrolysing the material, as well as process configurations, have been investigated with the aim of remedying the low ethanol productivity and high costs .
Among the viable processes, steam pretreatment followed by enzymatic hydrolysis is one of the most promising approaches for ethanol production from lignocellulose , and this configuration was adopted in the present study for the 2G ethanol production from sugar cane bagasse and leaves using separate hydrolysis and fermentation (SHF). Other techno-economic studies on ethanol production based on more or less the same process concept have been performed previously for various materials, e.g., switch grass , tall fescue , hardwood , softwood [22, 23], straw , poplar , salix , corn stover  and sugar cane bagasse [26–28].
Process design and optimization greatly affect the production cost. The choice between simultaneous saccharification and fermentation and SHF, which have different advantages and disadvantages regarding their effects on downstream processing ; boiler pressure, which affects the electricity production efficiency ; mechanical recompression in evaporation, which reduces the steam demand [31, 32]; thermally integrated distillation, which also reduces energy demand as discussed by Dias ; as well as other integration options has been investigated in various studies [21, 34, 35].
Apart from technical hurdles and the price of petrol, the question of feedstock competitiveness must be addressed at international and local levels. The global price of sugar and the local bioelectricity price may affect 1G and 2G ethanol production, respectively. Although there is no direct competition with food security and supply, 2G ethanol from sugar cane suffers from a higher production cost due to the rise in opportunity cost caused by the rapidly growing bioelectricity market in Brazil. Today, greater profit may be achieved by burning bagasse and leaves than by producing 2G ethanol due to a lack of cheap fuel to complement hydropower in the dry season. However, future development of the 2G ethanol process using leaves and bagasse is recognized to be profitable, even when a high surplus of electricity (184 kWh/ton SC) is sold . In addition high sugar prices may reduce the gap between the 2G and 1G ethanol production costs. Furthermore the goal of more competitive 2G ethanol production can also be achieved by integrating the 1G and 2G technologies in a plant sharing energy and material streams in unit operations in order to exploit synergies. It is also important to utilize the whole material for the production of by-products, as this has been shown to have considerable impact on the economic feasibility [14, 34].
In the present study, a techno-economic evaluation of integrated 1G and 2G ethanol production from sugar cane has been performed for fifteen scenarios. In each scenario several operating conditions were implemented in the flowsheeting models, based on data obtained experimentally within the CaneBioFuel Project # , and different process layouts were designed to optimize the ethanol production based on these data. The final results of the CaneBioFuel Project regarding the technical design and economic assessment are presented, summarizing the feasibility of the completely integrated production process, considering both overall energy efficiency and the ethanol production cost, for each scenario.