Page 267 - Wastewater Solids Incineration Systems

Incinerator Operations 229

their incinerators. However, U.S. Environmental Protection Agency personnel have
indicated that this method of operation is not acceptable.

2.7 Air Pollution Control Systems
As discussed in Chapter 7, Emission Control and Monitoring, particulate matter,
metals, and other pollutants are removed from the incinerator’s exhaust gases by the
air pollution control system. The vast majority of MHFs are equipped with
Venturi/impingement tray scrubbing systems.
The key to proper operation of the offgas scrubber is optimization of the differen-
tial pressure across the impingers. Typically, if the differential pressure is too low, par-
ticulate removal is minimized. If the differential pressure is too great, excessive nega-
tive pressure may occur in the incinerator, which results in an increase in gas velocity
through the system and excessive fly ash being carried out in the exhaust gas.
Although an increased velocity and resulting increase in differential pressure across
the impingers are methods to enhance particulate removal, an increased exhaust gas
solids load may cause a higher particulate discharge. Scrubber differential optimiza-
tion must be done in a manner to maintain the desired incinerator pressure and stay
within permit operating conditions. For additional details see Chapter 7, Emission
Control and Monitoring.

2.8 Emergency Operations

2.8.1 Power Failure
During complete electrical power failure, the greatest immediate danger is heat
damage to the center shaft and rabble arms. In addition, a temporary unsafe condi-
tion may result because the induced draft fan shuts down and the pressure within the
incinerator becomes positive. The emergency bypass stack damper should automati-
cally open to restore a negative pressure condition.
If the power failure lasts only a short time, the incineration equipment may be
restarted and the combustion process continued. If the power failure lasts for an
extended period of time, the incinerator may rapidly cool down, resulting in severe
damage to the refractory.
To prevent rapid cool down, the emergency bypass stack damper should be mod-
ulated to maintain the natural draft at a level that minimizes offgas heat losses. The
center shaft will stop rotating during a complete power failure. Without rabbling, the
combustion process is reduced to smoldering cake. The smoldering cake produces