Page 88 - Wastewater Solids Incineration Systems

Combustion Technology 57

Changes in water content will affect operating temperature and capacity of the
incineration system. When feed material is wetter than design condition more fuel
will be required to maintain bed temperature and, therefore, decrease feed capacity.
To maintain design capacity and optimize operation, the percent total solids should
be monitored twice daily.

3.2 Temperature
Combustible materials must be introduced into the fluid bed incinerator when the
bed operating temperature is greater than the ignition point of those combustibles. If
the bed operating temperature is not high enough to instantaneously ignite com-
bustible material in the bed, then overbed combustion would be excessive and result
in incomplete combustion. Municipal WWTP solids can generally be completely
combusted with a bed temperature range of 650 to 760°C (1200°F to 1400°F) and a
freeboard temperature range of approximately 815°C to 870°C (1500°F to 1600°F).

3.3 Gas Residence Time
Combustible materials must have sufficient time to react. The fluid bed incinerator is
designed to allow sufficient time for the feed material and any auxiliary fuels to react
with oxygen in the combustion air. The bed section is designed to completely disinte-
grate the feed and combust some of its volatiles to keep temperatures greater than
650°C (1200°F). The freeboard is designed to completely combust any volatiles that
escape from the bed. Typically, the gas residence times are two to three seconds in the
bed and six to seven seconds in the freeboard. Although lower freeboard residence
times would achieve high combustion efficiencies, sufficient disengagement height is
provided in the freeboard to reduce sand carry-over in the exhaust gases. This limita-
tion results in the long freeboard residence times.

3.4 Sufficient Air
Oxygen is supplied to the fluid bed incinerator system in the form of fluidizing and
combustion air. This air must be supplied in an amount slightly greater than that the-
oretically required for complete combustion. The normal indicator for excess air is
percent free oxygen released to the atmosphere. Dependent upon the feed material
and combustion temperature, the oxygen content in the exhaust gases should be a
minimum of 4% by volume on a dry basis, or approximately 2% on a wet basis as
measured in the exhaust gas before the scrubber.