Reference Manual

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Chapter 12
Kraft Recovery Boiler − Black Liquor System
The kraft recovery boiler is the heart of a complex
series of chemical processes referred to as the
kraft recovery cycle. The two main functions of the
kraft recovery boiler are to:
1. Reclaim digester cooking chemicals, sodium
and sulfur, in a suitable form for regeneration of
cooking liquors.
2. Provide efficient heat recovery and steam
generation from combustion of organics in black
liquor fuel.
Since the kraft recovery cycle is far removed from
the finished product of the paper mill, it sometimes
does not receive the attention it deserves.
However, efficient chemical and heat recovery
have a critical impact in overall mill efficiency and
profitability.
Recovery Process Overview
As mentioned earlier, the recovery boiler is a
major component of the kraft recovery cycle.
However, other components perform important
functions in the cycle. For continuity, a brief
description is given to indicate the role of the
recovery boiler in the overall cycle.
The basic components of the kraft recovery cycle
are:
Digester(s)
Wood chips are mixed with a solution of cooking
chemicals called white liquor (sodium sulfide and
sodium hydroxide). The contents are cooked
under pressure with steam to dissolve the glue-like
lignin, which holds the wood fibers together. After
cooking, the contents are blown into a holding
tank.
Washers
The holding tank contents, known as pulp or
stock, are transferred to the washers where water
is used to wash residual cooking chemicals from
the wood fibers. The wash water is sent to the
evaporators and washed pulp to paper making.
Evaporators
The wash water, containing wood by-products and
cooking chemicals, is transferred to the
evaporators. This solution is commonly known as
weak black liquor. Evaporators employ a series of
effects or bodies using steam which evaporates
much of the weak liquor water content. The final
product is referred to as strong black liquor.
Recovery Boiler
Strong black liquor is transferred to the recovery
boiler for combustion. The combustion process
burns the organics (for steam production) and
transforms the chemicals to a molten liquid known
as smelt. The smelt flows from the boiler into a
tank of water (or weak wash) and produces green
liquor.
Recausticizing Plant
The green liquor is transferred to a series of
components which add lime to regenerate the
cooking chemicals or white liquor. Clarified white
liquor is then pumped to the digesters to begin the
cooking process again.
Black Liquor Preparation
As stated in the overview, black liquor preparation
for the recovery boiler actually begins with the
evaporators; however, other components (see
figure 12-1) presented in this guide also play
important roles.