Duct Sorbent Injection
DSI for SO2 emission control is intended to
enable the control directly in the flue gas duct
between the air preheater and the particulate
control device. Since no dedicated absorber
vessel is required, the amount of hardware
needed to control SO2 is minimized for DSI.
DSI utilizes the contacting of finely dispersed
sorbent with the flue gas. Sorbent used in
DSI is typically hydrated lime or,
occasionally, sodium bicarbonate.18 In the
DSI shown schematically in Figure 2-4,
dry hydrated lime sorbent is injected into
the flue gas downstream of the boiler's air
preheater. Water may be injected separately
from the sorbent either downstream or
upstream of the dry sorbent injection point to
humidify the flue gas. The relative position
of dry sorbent and water injection is
optimized to maximally promote the so-called
droplet scavenging or impacts between
sorbent particles and water droplets, both
suspended in the flue gas. Fly ash, reaction
products, and any unreacted sorbent are
collected in the particulate control device.
Additionally, recycling solids from the
particulate control device can boost the
utilization of alkaline material.19
A variation of DSI is duct spray drying
process, in which slurry is atomized and,
subsequently, evaporated in the duct.