1. Introduction
The increasing energy demand and the associated dependency of fossil fuels is one of the major challenges of the XXI century, as renewable sources are the key to a sustainable model of energy supply. However, developing attractive economically renewable fuels is a difficult task due to the relatively low cost of the energy produced by fossil fuels, and despite some innovative technologies are technically feasible, they present themselves more expensive than the non-renewable sources and, therefore, their industrial application becomes difficult (Goldemberg, 2007