Page 181 - Wastewater Solids Incineration Systems

148 Wastewater Solids Incineration Systems

then passes through a heat recovery bed in outlet mode. As the hot gas passes
through the bed, it heats up the ceramic bed, thereby regenerating the bed. Every one
to two minutes, the dampers in the inlet and outlet manifolds switch positions and
the incoming exhaust gas now goes through the hot preheated bed (now in inlet
mode) and out the former inlet bed now in outlet mode. By utilizing regenerative
heat recovery, these systems can achieve very high thermal efficiencies of 95%, mini-
mizing fuel use. Some RTO systems will have a third heat recovery chamber which is
in transition mode while the chambers are switching. Volatile organic compound
destruction efficiencies of 98 to 99% are possible with these systems.
The main drawback with the use of RTOs as afterburners is that the inlet gas
must be free of particulate matter because the ceramic media will capture the partic-
ulate matter and eventually plug up the heat recovery beds. To solve this problem,
an RTO on an MHF is preceded by effective particulate control device, typically a wet
ESP. The addition of an RTO and upstream particulate control device is a large cap-
ital expenditure. However, the savings in fuel cost over a direct fired afterburner can
justify the high capital cost.

2.2 Wet And Dry Systems
Emissions control equipment may be a combination of devices that may or may not
use water for their operation. Cyclones, scrubbers, Venturi scrubbers, tray scrubbers,
dry ESPs, wet ESPs, and fabric filters are common units.

2.3 Cyclones
Cyclones are now infrequently used because they have been replaced by more effec-
tive devices. However, they have been used on fluid bed incinerators to reduce the
particulate loading on downstream equipment. As previously stated, 100% of the ash
(and some of the bed sand) exits a fluid bed incinerator in the flue gas which results
in a high particulate loading. A dry cyclone, preferably a multiple cyclone, can
remove the bulk of this high particulate loading. Typical application in a system
would be the use of a cyclone before a waste heat boiler to reduce the dust loading
on the boiler. Another fluid bed incinerator application might be a dry cyclone before
a wet scrubbing system if dry collection of the bulk of the fly ash is desired.
A dry cyclone is a vertical cylindrical vessel that takes advantage of the differ-
ences in densities of the gas stream and particulate matter. The particulate-laden
exhaust gases enter the cyclone tangentially and swirl at high velocity. This high