Page 121 - Wastewater Solids Incineration Systems

90 Wastewater Solids Incineration Systems

Saturated gas, which contains a relatively small amount of water, has to be raised
from a temperature of 60 to 140°C (140 to 280°F) to approximately 80°C (140°F). The
mass loading of the exhaust gas after the scrubber is approximately 70% of the mass
loading out of the furnace.
The RTO in the RHOX process requires gas with low particulate concentrations
to prevent fouling of the ceramic surfaces of the heat exchangers. Therefore, existing
scrubbers may have to be augmented by installation of a wet ESP or replacement
with a scrubber.

9.5.2 Flue Gas Recirculation
Flue gas recirculation (FGR) in MHFs (Figure 5.17) recirculates flue gas from the
hearth onto which the dewatered cake is fed to a hearth below the burning hearth.
Typically, the dewatered cake is fed below the afterburning hearth(s) (referred to as
AB in Figure 5.17). The gas is recirculated through ducting and fans. Typically, two
sets of ducts and fans are provided. The flow rate and temperature of recirculated gas
is measured by a flow meter in the FGR duct and is controlled either through
dampers or by varying fan speed. Cooling air is provided to the FGR to control tem-
perature. The FGR process provides many benefits, including

• Increased stability of operation.
• Reduction in NOx emissions (elimination of yellow plume).
• Reduced slag formation in the MHF.

• Increased furnace throughput capacity (because of lower downtime from slag
removal).
• Reduced THC emissions (because of more stable furnace operation).
• Complete ash burnout (because of higher temperatures in the lower hearths
because of the recirculated gases).

9.5.3 Oxygen Injection
An oxygen-enriched MHF demonstration was carried out at the Frank E. Van Lare
Sewage Treatment Plant by Praxair, Inc., under a New York State Energy Research
and Development Authority (NYSERDA) grant (NYSERDA, 1998).