FLUIDIZED COMBUSTION OF LIQUID FUELS: PIONEERING WORKS, PAST APPLICATIONS, TODAY’S KNOWLEDGE AND OPPORTUNITIES M. Miccio 1 , F. Miccio 2 1 Dipartimento di Ingegneria Chimica ed Alimentare - Università di Salerno – Fisciano SA - ITALY 2 Istituto di Ricerche sulla Combustione - C.N.R., Napoli - ITALY Abstract: The use of liquid fuels for fluidized bed (FB) combustion is gaining importance for co-firing, waste incineration, switch to renewable energy sources or during plant start-up. The design of a suitable liquid injection system is a key requirement to ensure that all of heat is released within the bed of a bubbling fluidized combustor or the riser of a circulating system. The scientific literature has revealed around 100 papers dealing with FB combustion of liquid fuels since the pioneering work in 1975. This paper conveys an effort to review the relevant aspects of FB combustion of liquid fuels, with the exclusion of mixtures of solid fuels in liquids, which were left out of this work. Following a logical path that goes from early investigations toward a more sound knowledge, it is organized in four sections: Fuels, Fuel Feeding, Combustion Results and Emissions. The Conclusions summarize the main aspects and draw prospective for future research and application of liquid fuel FB combustion. Keywords: bubbling fluidized bed, circulating fluidized bed, liquid fuel, fuel injection, combustion efficiency, sulfur oxides , NO x , carbon monoxide INTRODUCTION The use of liquid fuels for fluidized bed combustion (FBC) is gaining importance for application during co-firing and waste incineration or for plant start-up. In parallel, increasing interest in biomass- and waste-derived fuels, as a consequence of the Kyoto protocol and the need of optimal energy-source share, calls for innovative combustion solutions. Since the fluidized bed technology is effective for burning high-volatile fuels like biomass and solid opportunity fuels, the extension to liquid fuels is rather straightforward, in particular if benefits are obtained, for instance burning liquid wastes with medium heating value and raw vegetable oils without any preventive chemical treatment. These fuels can be reliably processed using fluidized bed combustion thanks to FBC robustness, flexibility and effectiveness. The literature on FB combustion of liquid fuels is still sparse. A part of studies have a fundamental character for understanding the mechanism of liquid fuel FB combustion; a number of papers are available on different aspects as well as on different technical options. So far, both options of bubbling fluidized bed (BFB) and circulating fluidized bed (CFB) have been used for liquid fuel combustion. The search of the scientific literature revealed around 100 papers dealing with FB combustion of liquid fuels, after the exclusion of mixtures or suspensions of solid fuels in liquids, which were left out of this work. Nevertheless, no dedicated reviews are available on this matter. Therefore, this paper represents a first effort in this direction by addressing relevant practical aspects and scientific issues in FB combustion of liquid fuels. FUELS Table 1 reports the list of the liquid fuels used in the research activities reviewed by the present paper. Concerning nature and origin, the majority of the fuels is originated upon petroleum refining processes as valuable products (diesel fuel, kerosene, gasoline, fuel oil), by-products (pitch, heavy oil, mazut) and liquid wastes to be disposed (acid tar waste, waste oil). Orimulsion is a bitumen-based water suspension that is very similar to a heavy fuel oil in its appearance. It is produced from Venezuelan bitumen deposits in the Orinoco region by direct extraction with steam from the underground reservoirs and a further addition of surfactants for its stabilization (Zlatar, 1989). Concerning renewable fuels, pyrolysis oils are generated by dedicated processes of thermo-chemical conversion of biomass/wastes into liquid fractions. Sunflower oil, commonly used for cooking, is produced by milling, pressing and separating the oil fraction of sunflower seeds. Biodiesel is obtained via trans-esterification of rape oil for generating a fuel with rheological properties similar to those of a common diesel fuel.