Circulating Fluid-Bed (CFB) Processes
The characteristics of a circulating fluid bed combine many advantages of the stationary fluid bed and the transport reactor. The high-slip velocities ensure good mixing of gas and solids, and thus promote excellent heat and mass transfer. Small particles are converted in one pass, or are entrained, separated from the gas, and returned via an external recycle. Larger particles are consumed more slowly and are recycled internally inside the bed until they are small enough for external recycling. The CFB operates with a much higher circulation rate than a classical stationary bed, thus creating a specific advantage in the higher heating rate experienced by the incoming feed particles. This reduces significantly the tar formation during the heating up process.
More recently the focus of new designs for fluid-bed gasifiers has shifted from the lower-velocity bubbling beds to higher-velocity circulating or transport-type designs, which feature higher char circulation rates with consequent improvements to the overall carbon conversion. Another advantage of the circulating fluid beds is that the size and shape of the particles is less important. This is one of the reasons why this type of gasifier is eminently suitable for the gasification of biomass and wastes of which the size, shape, and hence the fluidizing characteristics are even more difficult to control than of coal.
There are two companies offering CFB gasification systems, Lurgi and Foster Wheeler. The fundamental technologies supplied by these two companies are similar, although there are naturally a number of differences in detail.
Lurgi’s atmospheric CFB technology (Figure 5-10) was originally developed for alumina calcination and later, during the 1980s, it was adapted for the combustion of coal. It has since been applied to the gasification of biomass.
The CFB system comprises the reactor, an integral recycle cyclone, and a seal pot. The high gas velocities (5-8 m/s) ensure that most of the larger particles are entrained and leave the reactor overhead. The solids separated from the gas in the cyclone are returned to the reactor via the seal pot. The gasifying agent, usually air, is fed as primary air through the nozzle grate and as secondary air at a level above the fuel supply point. For biomass applications the fuel must undergo size reduction to 25-50 mm (Greil etal. 2002).
The Foster Wheeler circulating fluid-bed technology (originally Ahlstrom) was developed in the environment of the Scandinavian forestry industry and is discussed further in Section 5.5.