Conversion of liquid and gaseous fuels in fluidized bed combustion and gasification

Fluidized bed combustion (FBC) is a valuable conceptual option for gaseous or liquid fuels by virtue of the advantages of this technology with respect to fuel flexibility and emissions. However, the plant design experience and the practical application of FBC for gaseous or liquid fuels is quite limited. Liquid fuels have been considered for start-up or during co-firing in solid-fuelled FBC plant of various size; some liquid wastes have been proposed or are actually treated in FBC for incineration. More recently, new openings to liquid fuel FBC have been provided by the increasing interest in biomass- and waste-derived fuels, as a consequence of the Kyoto protocol, the need of energy source diversification and the on-going development of innovative combustion solutions with inherent CO2 capture possibilities (Hoteit et al., 2009), like Chemical Looping Combustion (CLC). This subject will be dealt with more details in Chapter 20. Further, since the combustion at low temperature is presently receiving a great deal of interest in view of depressing emission of micro- and nano-pollutants, the option of carrying out liquid fuelled FBC at a temperature lower than the classical value for solid fuels (i.e., 850°C) may offer a number of advantages. Since the fluidized bed technology is effective for burning high-volatile 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. The search of the scientific literature revealed around 120 papers dealing with FB combustion of liquid fuels, after the exclusion of mixtures or suspensions of solid fuels in liquids, which are left out of this book, but were covered by a review work in a past presentation by Greco et al. (1999). Nevertheless, no dedicated reviews are available on this matter. So far, both options of bubbling fluidized bed (BFB) and circulating fluidized bed (CFB) have been considered in liquid fuel combustion studies. In an application perspective, industrial fluidized bed furnaces operating with natural gas can be considered in specific applications such as foundry sand reconditioning, incineration of sludge with a high moisture content, cleaning of metallic parts and calcination of solid particles (Dounit et al., 2001). Thus, FBC of natural gas has received some investigation interest since the early developments of FBC, particularly in the former Soviet Union. The FBC of gaseous fuels is presently restricted to a scientific curiosity and fundamental investigations. However, also for gaseous fuels the new CO2–free combustion technologies may revalue this option: actually, CLC was originally conceived for gas combustion. The survey of the scientific literature revealed around 30 research papers dealing with FB combustion of gaseous fuels