Chemical Composition and Conversion of wood
Cellulose
Cellulose is a long linear molecule with over 7,000 glucoses linked end to end. Bundles of
cellulose molecules fasciculate laterally by hydrogen bonds to form microfibrils. Microfibrils
are the framework of the cell wall and are responsible for the strength of wood.
Chemical Composition of Wood
Hemicelluloses
Hemicelluloses are a group of compounds made up of 60 to 200 units
of different sugars including glucose. Softwood hemicelluloses are
different from hardwood hemicelluloses. Hemicellulose and lignin are
ingredients of natural glue in plant materials and together they make up
the matrix of the cell wall microfibrils are embedded in this matrix.
H
H
H
HO
HO
O
O
HO
H
RO
HO
O
H
OH
H
HO
O
H
RO
O
HO
HO
H
H

H
H
H
HO
HO
OH
H
O
OH OH
Softwood hemicellulose
HO
H
H
H
H O
H
H
O
H
O
OH
H
H
OH
HOH2C
H OH
H
O
H O
O
H
H
OH
H
O
H
R=H, CH3CO
H
O
H OH
H
H
H O
HO
OH O
H
H
H
OH
OH
H
H
H
O
O
H
H
H
H
H
RO
H
H
H
H
H
HO
H
O
OH H
HO
H
OH
H
OH
H
H
(
H
Hardwood hemicelluose
H O
Chemical Composition of Wood
Lignin
Lignin is a three dimensional molecule, formed by linking 15 to 18
phenolic units called phenylpropanes. Lignin contributes to the
rigidity of wood. Lignin together with hemicelluloses form the matrix
system in the cell wall; they also are the natural glue to hold cells
together.

CH2OH

A
CH2OH

61 2
543
B
OCH3
OH
CH2OH
C
H3CO
OCH3
OH
OH
Lignin monomers: A=coniferyl alcohol；B=sinapyl alcohol；C=p-coumaryl alcohol
Chemical Composition of Wood
Extractives
Extractives are defined as compounds in plant materials and wood that
can be extracted with various solvents, such as water and organic
solvents. Extractives include turpentine, fatty acids, tannins, etc. that
give wood colors and odors. Extractive content of wood is extremely
variable depending upon species. Some extractives are responsible
for the durability of wood against fungi decay and insect injuries; other
extractives have medicinal values.
Chemical Composition of Wood
• Chemical Composition
Species
Cellulose
Softwoods
43%
Hardwoods
45%
** These
Hemicellulose
28%
34%**
Lignin
29%**
21%
differences are significant
Chemical composition of crop residues is similar to hardwoods
Mechanical Pulping
•
Groundwood: Drum-debarked bolts are loaded into the chutes and pressed against revolving
grinding stone to convert logs into wood pulp. A water shower is applied to cool the
stone, wash the pulp off the stone into a vat and adjust the consistency of the slurry.
•
Pulp Quality: Over 90% pulp yield; little discoloration of the pulp; suffer certain degree of fiber
breakage; presence of fiber bundles.
•
Species Selection: Light-colored softwoods preferred because the pulp uses less chemicals for
bleaching and long fibers compensates fiber breakage.
•
Major Uses: Add about 15% long-fiber chemical pulp to increase tensile strength for newsprint.
Thermal Mechanical Pulping (TMP)
•
•
Basic Principle: Wood chips are first softened with steam to over 180 oC,
followed by disk refining softened chips into pulp.
Major Use: TMP is mainly used to manufacture medium density fiberboard and
hardboard (high density fiberboard)
Chemical Thermal Mechanical process (CTMP)
• In this process southern pine wood chips (brightness about 55%
to 57%) are impregnated with 2% sodium bisulfite (NaHSO3)
based on dry weight of chips, followed by processing with the
thermomechanical pulping. Sodium bisulfite is able to dissolve
some lignin and softens the wood chips.
• Because it also is a bleaching agent southern pine pulp
produced in this manner has a brightness about 65%, making
the species an important raw material in U.S. for newsprint
production.
Masonite (Explosion) Process
Explosion process is become an important process for pre-treating biomass
for ethanol production.
Semichemical pulping
•
Neutral Sulfite Semichemical Process (NSSC)
– The NSSC process is the most important in the production of corrugating
medium, which is used in the construction of cardboard boxes. Cardboard is
made by sandwiching corrugating medium with linerboards (thick kraft paper).
– The process starts by impregnating wood chips with 12% pulping liquor
containing sodium sulfite (Na2SO3) and sodium carbonate (Na2CO3), cooking the
chips in a screw-digester at 165 oC for 1 hr, after which the softened chips are
refined into pulp with a disk refiner.
--Pulp for corrugating medium typically has a
yield about 80-85%. High lignin content in the pulp
provides the medium with necessary rigidity to
sustain compression strength, while linerboards
provide the tensile strength.
--NSSC pulping also can be used to produce
paper pulp by using more pulping chemicals (~20%)
and cooking at higher temperatures (~180 oC) for
3 hours.
NSSC Pulping
NSSC Pulping
• Spent Liquor of NSSC Process
--The spent liquor contains pulping chemicals and dissolved organic
matters from wood chips. The spent liquor is concentrated to about 50%
solids by evaporating water, followed by spraying the concentrated spent
liquor into a boiler. Combustion of organic matters in the furnace
generates energy, and the pulping chemicals melt in the smelt as sodium
sulfate (Na2SO4) and sodium carbonate (Na2CO3).
--Unfortunately, the two chemicals can not be separated from each other
and reused in the NSSC pulping. But the mixture can be shipped to kraft
mills to be converted into pulping chemicals used by the kraft mills.
Full Chemical Pulping—The Kraft Process
• Kraft Process (Sulfate Process): This is the main full chemical process to
produce paper pulps for the following reasons:
-- All materials including all woody and non-woody species can be pulped
-- Produces strong paper pulp
-- Tolerate about 10% bark in wood chips, allowing the use of whole-tree chips
-- 98% pulping chemicals can be recovered
• The process uses Sodium hydroxide (3 parts, NaOH) and sodium sulfide (1 part,
Na2S) to cook chips. Wood chips are charged into a continuous digester,
impregnated with the pulping liquor and heated 170 oC (1.5 hours), followed by
cooking the chips at that temperature for additional 1.5 hr.
• During washing stages dissolved hemicelluloses precipitate and deposit onto
fiber surfaces, which acting as an adhesive to bond fibers resulting in strong
paper products.
Full Chemical Pulping—The Kraft Process
Fiber Morphology and Paper Strength
• Fiber length, cell wall thickness and cell diameter
•
--Long, thin-walled and large-diameter fibers are flexible and able to
interwoven with each other to form tight hydrogen bonds
--Thin-walled cell is less resistant to tear force across the fiber
Tensile, burst and tear are three important paper strength properties
--Flexible fibers promote fiber-to-fiber bonding and produce paper good
in tensile and burst strength
--Thick-walled fibers make paper good in tear strength
For example,
--Earlywood tracheds of pines are ribon-like fibers that are important for
tensile and burst strength
--Latewood tracheids are rod-like, which is important for tear strength
Before pulping
1 hour after kraft cooking
30 minutes after kraft cooking
Fully pulped (1.5 hours)
Chemical Recovery
• Evaporation: 15% solids spent liquor concentrated to 50%
• Incineration: Concentrated spent liquor sprayed into furnace;
Organic matters burned producing energy:
Inorganic = Na2CO3 + Na2S
Dissolved into water = green liquor
• Causticizing: CaO + H2O
Ca(OH)2
Ca(OH)2 + Na2CO3 + Na2S
2NaOH + Na2S + CaCO3
2NaOH + Na2S = White liquor recovered
• Lime Recovery: CaCO3 heat 100oC
CaO + CO2
Dissolving pulp
• Dissolving pulp is almost pure cellulose, it does not contain
lignin and hemicellulose, which a row material for producing
cellulose derivatives such as rayon, cellulose acetates and
cellulose nitrates
• In the past, dissolving is produced by the acid sulfite process
which removes all hemicelloused during pulping, followed by
removing lignin by exhaustive bleaching.
• Nowadays dissolving pulp can be produced by kraft (sulfate)
process by a slight modification (prehydrolysis)
Pulp Bleaching
• Brightness and Whiteness
--Brightness is physical measurement of how much light is
reflected from an object (pulp or paper)
--Whiteness is physiological perception of how bright an object is
• Importance of initial brightness of pulp
--The brighter the initial brightness the less is the effort to bleach to
certain brightness level
--Protect pulp logs from blue stain
--Pretreatment of water to remove metallic ions
Methods of Bleaching
• Lignin preserving methods
for groundwood (newsprints) and other high-yield pulps such
as low-yield NSSC pulp for printing purposes
• Ligin removing methods for chemical pulps (kraft)
By removing lignin paper is stable in brightness
Lignin Preserving Bleaching
• Lignin and extractives in wood are responsible for color of
mechanical pulps
• Peroxide bleaching has a low impact on the environment
• Oxidation reactions by hydrogen peroxide (H 2O2) remove color
• Procedures
--Treat water with EDTA (0.2-0.5%) to remove heavy-metal ions
--Adjust pH to 10.5-11.0 with NaOH
--Add 1.0 to 3% hydrogen peroxide to 15% consistency pulp
--Bleach 1 to 3 hours at 30 to 50 oC
--Brightness increased from 5 to 12% depending upon conditions
Lignin Removing Bleaching
• Most lignin in chemical pulps must be removed so that brightness
of paper can be maintained for a long time
• Bleach is done in multi-stages
--Chlorination (C): pulp is treated with chlorine (Cl) to breakup
lignin molecules
--Extraction (E): Use NaOH to extract lignin fragments
--Oxidative bleaching
Hypochlorite (NaOCl) bleaching (H)
Chlorine dioxide bleaching (D)
Peroxide bleaching (P)
--
Results of Multi-stage Bleaching
• CEH bleaching increases brightness of kraft pulp from 30%
brightness to 75%
• CEHEH increase brightness from 30% to 80%
• CEHEDP increase brightness from 30% to 90%
• Because chlorine has a great impact on the environment
research is underway to find chlorine-free bleaching methods