Page 99 - Energy from Toxic Organic Waste for Heat and Power Generation

84 Energy from Toxic Organic Waste for Heat and Power Generation

7.2 PAPER MAKING
Paper making process involves number of steps from wood carving to
bleaching and the initial step in paper making is pulping process. The key
steps followed in this part are
• Debarking,
• Wood chipping,
• Chip washing,
• Chip digestion,
• Pulp screening,
• Thickening, and
• Washing.
Various mechanical and chemical processes are followed worldwide in
pulping. The mechanical process includes
• Mechanical pressure,
• Disc refining,
• Heating.
And light chemical processes are performed to increase the yield of
pulping.
In chemical pulping processes, wood chips are cooked under pressure in
pulping liquors at high temperature [1]. In Alkaline media or Krajt process,
sodium hydroxide (NaOH) and sodium sulfide (NaS 2 ) containing liquor
are used to cook the woodchips. On the other hand, the sulfide process uses
−
a mixture of sulfurous acid (H 2 SO 3 ) and bisulfide ions (HSO ) as acidic
3
media. In some of the applications, mechanical and chemical processes are
combined together.
While comparing mechanical and chemical pulping processes, the for-
mer gives higher yield (90%–95%) than later (40%–50%). Nevertheless, the
pulp obtained from the mechanical process is lower in quality which is
highly colored and comprises of short fibers [2]. Hence, the chemical pulp-
ing methodology carried in alkaline or acidic media is mostly preferred.
The statistics about the pulp production by chemical and mechanical
method in North America and Europe from 2004 to 2009 released in Food
and Agriculture Organization (FAO) database (2011) are given in the Table 7.1.
Approximately 5%–10% of the lignin that come out from the initial or
raw materials cannot be removed during the pulp processing and it is respon-
sible for the end product’s dark color. The production of white paper (pulp
bleaching) includes five or optional six treatment steps and the sequence is
• Elemental chlorine (C1),
• Alkali (E1),