Reference Manual

12−3
magnetic flow meters. If solids drop below 60%,
auxiliary fuel is added due to the potential of a
smelt/water explosion or bed “blackout”. If solids
fall below 57-58%, the liquor is diverted from the
furnace to the mix tank until solids reach an
acceptable level. Each individual liquor feed line
typically has a valve connected to emergency
shutdown interlocks. These valves snap close on
a trip or shutdown signal.
Black Liquor Heating
A final preparation stage for the liquor involves
heating. Liquor is typically heated to 230°F-250°F
to impart the desired viscosity and burning
characteristics before spraying into the furnace.
Many older designs employ direct steam heaters,
but most newer designs are using indirect steam
heating. The indirect heating does not add water
content to the liquor which is a safety and
efficiency consideration. Recirculation systems
and steam desuperheating are often used with
indirect heating.
Liquor Flow/Pressure Control
A common method used to control the flow or
pressure of black liquor to the furnace is
recirculation to the mix tank. This allows the
nozzle pumps to run at a constant speed and keep
the liquor moving to avoid potential plugging of
transport piping.
Auxiliary Fuel
Black liquor is not used as a fuel to start-up a
recovery boiler. Natural gas or fuel oil is typically
used to bring the boiler up to a prescribed
temperature before black liquor fuel is introduced.
This is done primarily as a safety consideration
due to the potential of a smelt-water explosion in
the lower furnace.
Kraft Recovery Boiler
While recovery boiler design considerations vary
among each manufacturer, their basic two-fold
purpose of chemical recovery and steam
production is common to all. Since the primary
function is chemical recovery, black liquor flow to
the furnace is a constant and steam production is
a by-product. This requires the recovery boiler
steam outlet header to be piped to a common
header with a power boiler steam outlet. Swinging
steam demands due to mill processes are
accommodated by manipulating fuel to increase
steam production of the power boiler.
Combustion Air
Air required for combustion in the furnace is
introduced separately from the black liquor.
Ambient air is forced into the boiler through an air
heater via the forced draft fan(s). The air heater
typically uses steam coils to heat the incoming air.
Most modern designs introduce air at three levels:
primary, secondary, and tertiary. These various
levels are used to ensure chemical reduction,
complete combustion of organics, and proper
shape of the smelt bed. The primary air ports are
located a few feet above the hearth and carry the
responsibility to provide as low a velocity as
practical while still supplying between 50-65% of
the total air requirement. Secondary and tertiary
ports establish higher velocities to ensure
complete mixing and combustion of the unburned
gases. Combustion flue gas is pulled from the
furnace section to the convective section of the
boiler via induced draft fan(s). This creates a
slightly negative pressure inside the boiler. This
action prevents hot gases from leaving boiler
openings and is commonly referred to as
“balanced draft”.
Black Liquor Combustion
Black liquor is introduced into the recovery furnace
via nozzles or liquor guns. The guns produce a
spray of coarse droplets exposed to hot flue gas.
Depending on the manufacturer, the guns may be
stationary or oscillating, and spray the liquor on
the walls or into the center of the furnace. The
flow of this black liquor to the guns is controlled by
a valve. When selecting a control valve for this
application, it is crucial to select proper materials
due to corrosion. As the organics burn and release
heat to the flue gas, the remaining char, consisting
of the sodium and sulfur cooking chemicals, falls
to the smelt bed on the furnace floor.
Sootblowers
Application Summary:
The efficiency of a fossil-fuel boiler is highly
dependent on the heat transfer effectiveness of
the boiler tubes. These tubes are fairly delicate,
and hot spots (due to soot buildup cannot) be