The present industrial production of olive oil in Spain generates a relevant amount of sub-products. The fact that some of those products are underused may lead to a decrease in the supplying costs. In particular, the two- phase continuous centrifugation system of extraction is characterized by the presence of a semi-solid product, known as sludge, which is composed by the olive husk (OH), by retained oil – between 2.5% and 3.5% weight – and by the olive mill wastewater (OMW). As stated by INE [1], between 1997 and 2002, Spain produced more than 106 t of virgin olive oil and around 4 · 106 t of sludge annually.
The strong seasonal character of olive oil production (mainly during a period of four months per year – fromNovember to February), the important amount of sludge generated in the production process, its high moisture con- tent (over 65% wet basis in most cases), and the presence of polyphenols and other organic compounds, make the treat- ment of this residue a hard and interesting problem.
During the last years, there has been a marked tendency to use energy recovery techniques to eliminate the sludge produced in olive oil extraction industrial processes. It must be noted that one of the main limitations of the use of biomass residues as energy source lies in the fact that their availability and moderate heating power lead to small scales and high costs if compared with fossil fuel. However, the small emissions of SOx and greenhouse gases caused by the use of biomass for power generation meets the Euro- pean Community policy on pollution control [2]. Besides, according to the state-of-the-art in energy conversion tech- niques, biomass-based systems are the only renewable source of electricity – apart from hydroelectricity – that can be used for base-load electricity generation. This fact