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. Hardwood hemicelluose Softwood hemicellulose

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. 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 Species Cellulose Hemicellulose Lignin Softwoods 43% 28% 29%** Hardwoods 45% 34%** 21% ** These 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 30 minutes after kraft cooking 1 hour 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