Reference Manual

12−2
Two components commonly employed on older, or
conventional boilers, are direct contact
evaporators and black liquor oxidation systems.
Direct contact evaporators may be the cascade
evaporator or cyclone evaporator type.
Black Liquor Oxidation
Black liquor oxidation is the exposure of black
liquor to air (oxygen) to form more stable sulfide
compounds. This exposure prevents the release
of hydrogen sulfide and mercaptans when the
liquor is exposed to hot flue gas in the direct
contact evaporators. Release of these gases
results in sulfur loss and odor emission. The
oxidation process is commonly performed by a
blower forcing compressed air into a liquor filled
tank via a sparger ring.
Direct Contact Evaporators
Direct contact evaporators are used for final liquor
concentration from 45-50% solids to 60-65%
solids. They are so named because the boiler
induced draft fan pulls the hot flue gas into direct
contact with the black liquor to evaporate water
prior to combustion.
The cyclone evaporator, commonly used with
Babcock and Wilcox recovery boilers, consist of a
cylindrical vessel with an opening which allows
flue gas to enter tangentially. Black liquor is
sprayed into the swirling gas to allow mixing and
evaporation of water.
The most common type of evaporator for the past
two decades has been the falling film or cascade
evaporator. This evaporator consists of a rotating
assembly of hot plates or tubes that are alternately
submerged and then exposed to hot flue gas.
These tend to be more efficient and versatile.
Due to the high flue gas temperatures and the
presence of black liquor fuel, the potential for a fire
exist in both types of evaporators. A common
method employed to combat this potential is the
injection of steam to displace oxygen and smother
a fire.
Low Odor Design
As mentioned earlier, both black liquor oxidation
and direct contact evaporators are employed on
older or conventional type boilers. However, most
modern designs referred to as low odor type use
indirectly heated concentrators (similar to
evaporators) to raise the black liquor solids
concentration to the 65-70% range. Similar to the
falling film evaporator, concentrators eliminate the
need for direct contact evaporators, which, in turn,
eliminates the need for black liquor oxidation.
Lower sulfur and odor emissions are a result of
the flue gas and liquor having no direct contact.
This design also requires additional boiler
economizer section(s) to absorb the flue gas heat
which had been removed in the direct contact
evaporators. The concentrator is more energy
efficient and environmentally friendly than its
predecessors.
Precipitator Ash
Following preparation by concentrators or black
liquor oxidation, liquor is transferred to the
electrostatic precipitator. Ash, consisting primarily
of salt cake (Na
2
SO
4
or sodium sulfate), is
collected from the flue gas and mixed with the
liquor. In some older designs, referred to as wet
bottom, the liquor fills the precipitator bottom and
salt cake falls directly into the pool. Since this
presents a potential fire hazard and a source for
odor emissions, most newer precipitators have the
dry bottom design. In this design, chains or screws
convey the dry ash into a liquor filled sluice tank.
Salt Cake Mix Tank
Before introduction to the recovery boiler furnace,
the liquor is mixed with more ash and salt cake in
the blending tank or salt cake mix tank. Salt cake
addition is required due to small chemical losses
occurring in the recovery cycle. Salt cake is
typically added from two sources. One source is
via fallout hoppers below the steam generating
and economizer sections of the boiler. A second
source is purchased make-up. Direct steam
heating of the tank is typically used to maintain
liquor viscosity at suitable values for pumping.
In order to reduce emissions, some mills have
eliminated salt cake as a soda makeup chemical
because of its high sulfur content. Caustic soda
(NaOH) and soda ash (Na
2
CO
3
) have been
substituted since they are sulfur free chemicals.
Liquor Divert
Black liquor is typically at 65-70% solids before it
is sent to the recovery furnace for combustion.
Density, or percent solids, is usually measured
between the liquor heater and liquor guns via