Factores De Emision

10.2 Chemical Wood Pulping
10.2.1 General
Chemical wood pulping involves the extraction of cellulose from wood by dissolving the
lignin that binds the cellulose fibers together. The 4 processes principally used in chemical pulping
are kraft, sulfite, neutral sulfite semichemical (NSSC), and soda. The first 3 display the greatest
potential for causing air pollution. The kraft process aloneaccounts for over 80 percent of the
chemical pulp produced in the United States. The choice of pulping process is determined by the
desired product, by the wood species available, and by economic considerations.
10.2.2 Kraft Pulping
10.2.2.1 Process Description1 The kraft pulping process (see Figure 10.2-1) involves the digesting of wood chips at elevated
temperature and pressure in "whiteliquor", which is a water solution of sodium sulfide and sodium
hydroxide. The white liquor chemically dissolves the lignin that binds the cellulose fibers together.
There are 2 types of digester systems, batch and continuous. Most kraft pulping is done in
batch digesters, although the more recent installations are of continuous digesters. In a batch digester,
when cooking is complete, thecontents of the digester are transferred to an atmospheric tank usually
referred to as a blow tank. The entire contents of the blow tank are sent to pulp washers, where the
spent cooking liquor is separated from the pulp. The pulp then proceeds through various stages of
washing, and possibly bleaching, after which it is pressed and dried into the finished product. The
"blow" of the digester does notapply to continuous digester systems.
The balance of the kraft process is designed to recover the cooking chemicals and heat. Spent
cooking liquor and the pulp wash water are combined to form a weak black liquor which is
concentrated in a multiple-effect evaporator system to about 55 percent solids. The black liquor is
then further concentrated to 65 percent solids in a direct-contactevaporator, by bringing the liquor into
contact with the flue gases from the recovery furnace, or in an indirect-contact concentrator. The
strong black liquor is then fired in a recovery furnace. Combustion of the organics dissolved in the
black liquor provides heat for generating process steam and for converting sodium sulfate to sodium
sulfide. Inorganic chemicals present in the black liquor collectas a molten smelt at the bottom of the
furnace.
The smelt is dissolved in water to form green liquor, which is transferred to a causticizing tank
where quicklime (calcium oxide) is added to convert the solution back to white liquor for return to the
digester system. A lime mud precipitates from the causticizing tank, after which it is calcined in a
lime kiln to regenerate quicklime.
Forprocess heating, for driving equipment, for providing electric power, etc., many mills need
more steam than can be provided by the recovery furnace alone. Thus, conventional industrial boilers
that burn coal, oil, natural gas, or bark and wood are commonly used.

9/90 (Reformatted 1/95)

Wood Products Industry

10.2-1

10.2-2
EMISSION FACTORS
(Reformatted 1/95) 9/90

Figure 10.2-1.Typical kraft sulfate pulping and recovery process.

10.2.2.2 Emissions And Controls1-7 Particulate emissions from the kraft process occur largely from the recovery furnace, the lime
kiln and the smelt dissolving tank. These emissions are mainly sodium salts, with some calcium salts
from the lime kiln. They are caused mostly by carryover of solids and sublimation and condensation
of theinorganic chemicals.
Particulate control is provided on recovery furnaces in a variety of ways. In mills with either
cyclonic scrubber or cascade evaporator as the direct-contact evaporator, further control is necessary,
as these devices are generally only 20 to 50 percent efficient for particulates. Most often in these
cases, an electrostatic precipitator (ESP) is employed after the direct-contact...