1. IntroductionWorld faces the prog

1. Introduction
World faces the progressive depletion of its energetic resources mainly based on non-renewable fuels. At the same time, energy consumption grows at rising rates. The USA is the first oil consumer, but China’s spectacular economic growth has imposed serious pressure on the oil market. Global panorama in that market is dark. Permanent crises in the Middle East and the speculation in the stock exchange, among other factors, have caused the oil price to reach such elevated values of 100 dollars per barrel. World economy could experience stagnation if the oil maintains these high prices. In addition, the intensive utilization of fossil fuels has led to the increase in the generation of polluting gases released into the atmosphere, which have caused changes in the global climate. The solution to this problematic depends on how the development and implementation of technologies based on alternative sources of energy will be undertaken. Through the use of renewable energetic resources, humankind can find part of the solution to their energy requirements in an environmentally friendly way.
One renewable solution is the use of solar energy in form of biomass (bioenergy). Global potential of bioenergy is represented in energy crops and lignocellulosic residues. Conversion of these feedstocks into biofuels is an important choice for the exploitation of alternative energy sources and reduction of polluting gases. In addition, the utilization of biofuels has important economic and social effects. For instance,Sheehan and Himmel (1999) point out that the diversification of fuel portfolio would bring money and jobs back into the USA economy. Moreover, the development of energy crops dedicated to the biofuels production would imply a boost to agricultural sector. This analysis is also valid for developing countries, especially in Latin America, considering the perspective of drastic reduction of proven oil reserves in the mid term. In addition, agricultural products from developing countries have to face a fierce competition from rich countries that grant huge subsidies for their agricultural production.

Ethanol (ethyl alcohol, bioethanol) is the most employed liquid biofuel either as a fuel or as a gasoline enhancer. Ethanol has some advantages when it is used as an oxygenate. Firstly, it has a higher oxygen content that implies a less amount of required additive. The increased percentage of oxygen allows a better oxidation of the gasoline hydrocarbons with the consequent reduction in the emission of CO and aromatic compounds. Related to MTBE, ethanol has greater octane booster properties, it is not toxic, and does not contaminate water sources. Nevertheless, ethanol production costs are higher than those of MTBE, gasoline mixed with alcohol conduces the electricity, and Reid vapor pressure is higher that entails a greater volatilization, which can contribute to ozone and smog formation (Thomas and Kwong, 2001). Many countries have implemented or are implementing programs for addition of ethanol to gasoline (see Table 1). Fuel ethanol production has increased remarkably because many countries look for reducing oil imports, boosting rural economies and improving air quality. The world ethyl alcohol production has reached about 51,000 mill liters (Renewable Fuels Association, 2007), being the USA and Brazil the first producers (seeTable 2). In average, 73% of produced ethanol worldwide corresponds to fuel ethanol, 17% to beverage ethanol and 10% to industrial ethanol.
The fuel ethanol can be obtained from energy crops and lignocellulosic biomass. The complexity of the production process depends on the feedstock. In this way, the spectrum of designed and implemented technologies goes from the simple conversion of sugars by fermentation, to the multi-stage conversion of lignocellulosic biomass into ethanol. The big diversity of technological alternatives requires the analysis of the global process along with the design and development of each one of the involved operations. Among the new research trends in this field, process integration has the key for reducing costs in ethanol industry and increasing bioethanol competitiveness related to gasoline. This issue is the main topic analyzed in a previous paper (Cardona and Sánchez, 2007). Several reviews have been published on the theme of fuel ethanol production especially from lignocellulosic biomass (Chandrakant and Bisaria, 1998, Lee, 1997, Lin and Tanaka, 2006 and Lynd, 1996). The amount of reviews covering ethanol production form other types of feedstocks like sucrose-based or starchy materials is more reduced (e.g., Kosaric and Velikonja, 1995 and Bothast and Schlicher, 2005). Nevertheless, an analysis of this process from the viewpoint of the three major types of feedstock has not been the main objective of those works. In addition, some issues concerning the feedstocks features on a comparative basis have not always been sufficiently emphasized. This paper attempts to achieve this aim considering the literature reviewed in the last one decade. Therefore, the purpose of this work is to analyze the different trends in fuel ethanol production taking into account both mature and developing technologies and making emphasis on the different types of raw materials from which fuel ethanol is obtained, and the possibilities for using alternative feedstocks leading to an improvement of the global process.