ABSTRACT
Circulating fluidized bed boilers, as the name suggests, circulate solids around an endless loop. The
recycle system moves the solids from the low-pressure cyclone to the high-pressure bottom of the
furnace or riser. The amount of solid to be moved is so large that it is nearly impossible to find an
inexpensive, motorized mechanical device to transfer it from the standpipe to the furnace. For
example, A 165-MWe CFB boiler would need to transfer nearly 3600 tons/hr of hot solids. It is very
difficult for any mechanical feeder to move such a large amount of hot solid from the low-pressure
cyclone to the high-pressure end of the riser of a CFB gasifier or boiler. A simple nonmechanical
device called loop-seal with no moving parts is used for this purpose. It is also sometimes called a
J-valve, fluoseal, siphon seal, or simply an overflow standpipe. Several other types of nonmechanical
devices are also used in other CFB reactors. As the loop-seal transfers the solids across a pressure
barrier which allows the solids to move from the standpipe to the furnace (riser) but does not allow
the gas tomove from furnace to the standpipe it is also called a nonmechanical valve. It may not be an
exaggeration to call these valves the heart of a CFB plant. Even a short interruption of this device will
fill the cyclone with solids and stop the entire circulating fluidized bed. Thus a good understanding of
its operating principle and design is very important. This chapter describes the operating principles
of nonmechanical valve, and the design methods of some commonly used valves.
