Page 144 - Wastewater Solids Incineration Systems

Heat Recovery and Reuse 111

Intermittent operation can be of greater importance to secondary heat recovery
and reuse than it is to primary heat recovery. The simple reason for this situation is
that if the incineration system (the market for reuse of the primary energy recov-
ered) is out of service, then the need for the recovered energy is nonexistent. On the
other hand, if the secondary energy recovered is used, for example, to heat
processes or buildings or to generate power, then demand will continue indepen-
dent of the incinerator operation. Hence, an alternate means must be provided to
ensure that energy demands are met when the incineration system is not in service
or when operating at partial load. In the latter case, the recovered energy may not
fully satisfy the load it serves when the incineration system is operating at 50 to
60% of the design feed rate. Finding a creative way to address the issues of turn-
down and cyclic operation in design and operation of heat recovery and reuse sys-
tems can be the key factor in success.
Operation of a high-temperature air preheater at turndown is a significant con-
cern, particularly when design temperatures approach design limits of the materials
of construction or of the equipment manufacturer’s warranty. If a preheater is
designed to produce 650°C (1200°F) combustion air temperature at the 100% design
point, then it will tend to produce higher temperature air when the system is turned
down to 60 to 70% of the design point. The typical recuperator has the same amount
of heat transfer area, regardless of operating throughput. Further, fluid bed systems
may experience prolonged temperature excursions for a variety of reasons, thereby
changing the flue gas inlet temperature to the air preheater to an “off-design” condi-
tion. At the extreme, metal temperature limits can be approached, leading to poten-
tial equipment damage; at the least, equipment life may be reduced.
A second consideration with respect to combustion air preheat relates to
changing thermodynamic characteristics of the feed cake to the incinerator. For
example, the incineration system may have been designed for a 25% solids feed cake,
with 70% volatile solids. During the design phase, a combustion air preheat level of
650°C (1200°F) may have been selected for this application to yield autogenous oper-
ation at slightly less than 870°C (1600°F) furnace exhaust. For one reason or another,
this system may be called on to process 27% solids feed cake, with 75% volatile
solids, which may only require the combustion air to be preheated to 480°C (900°F).
If flexibility was not built into the system, then furnace exhaust and preheated air
temperatures would rise above design levels, which are unacceptable conditions. In
the extreme, the operator may have to turn down the feed cake rate to the system and
operate at an artificially inflated excess air level to limit furnace exhaust temperatures