Page 79 - Wastewater Solids Incineration Systems

48 Wastewater Solids Incineration Systems

• Fluid bed incinerator design considerations.
• A description of major fluid bed incinerator system components.

• Features of the furnace and subsystems.
• Advantages of the technology.

2.1 Principles of Fluidization
Correlation and theoretical formulas on fluidization technology are well developed
in the literature. Thus, these formulas are not presented in detail in this chapter.
Abstracts that are important to understanding the technology, however, are included.

2.2 Definition of Fluidization
Fluidization is defined by Kunii and Levenspiel (1969) as an operation by which fine
solids are transformed into a fluidlike state through contact with an upflowing gas or
liquid. This method of contacting has a number of unusual characteristics. Fluidiza-
tion engineering attempts to take advantage of this behavior. In applying fluid bed
incineration, air is used instead of a liquid to supply oxygen to the combustion and
fluidize fine solids. Because the focus is on the application of fluid bed to incinera-
tion, the following will deal primarily with gas-fluidized systems.
Depending on airflow rates, various kinds of contacting of a batch of solids by
air are illustrated in Figure 5.1. In this figure, superficial air velocity increases from
left to right. As air passes upward at a low flow rate through a bed of fine particles,
the air merely percolates through the void spaces between stationary particles
because the velocity is not high enough to displace the sand particles. This configura-
tion is considered a fixed bed because there is no particle movement.
With an increase in airflow rate, particles move apart and a few will vibrate
and move in restricted regions. This situation is an expanded bed that is
approaching fluidization and is characterized by a higher pressure drop than
required for fluidization.
At higher air velocity, a point is reached when all particles are suspended in the
upward flowing gas. At this point, the frictional force between particle and air
counterbalances the weight of the particle. As a result, the vertical component of
compressive force between adjacent particles disappears. Pressure drops in sections
of the bed approximately equal the weight of fluid and particles in that section. The
bed is considered to be barely fluidized and is referred to as a bed at minimum or