Kraft pulping
Wood chips are cooked in a hot solution of alkali and sodium sulfide. Lignin breaks apart and dissolves, while the cellulose fibres stay. The result is strong brown pulp, used for example for kraft paper and board.
The cooking (white) liquor contains NaOH and Na2S; cooking runs at around 160-170 °C. Hydrosulfide ions speed up cleavage of β-O-4 bonds in phenolic lignin units, so delignification is faster and carbohydrates suffer less than in alkali alone. Chemical pulp yield is roughly half the wood mass; the losses are lignin, much of the hemicelluloses, extractives and a little cellulose.
Chemical recovery
A mill does not throw its chemicals away, it sends them round in a loop. Spent black liquor is concentrated and burned. The dissolved lignin burns to give steam and electricity, and the mineral part becomes cooking liquor again.
Black liquor is evaporated and burned in a recovery boiler: the organic part supplies energy and the sodium salts leave as a smelt of Na2CO3 and Na2S (sodium sulfate is reduced to sulfide in the process). The smelt is dissolved into green liquor and causticised with lime: Na2CO3 + Ca(OH)2 → 2 NaOH + CaCO3. The resulting lime mud is reburned in a lime kiln back to CaO.
Sulfite pulping and bleaching
There is also sulfite pulping, usually acidic. Its pulp is brighter but weaker. To make paper white, the remaining lignin is removed by bleaching. Chlorine used to be the bleach; today it is chlorine dioxide, oxygen, peroxide and ozone.
Sulfite pulping uses solutions of sulfites and bisulfites (of calcium, magnesium, sodium or ammonium); lignin is sulfonated and dissolves as lignosulfonates. Bleaching runs in several stages: the industry has moved from elemental chlorine to ECF bleaching (chlorine dioxide) and to TCF bleaching without chlorine compounds (oxygen, hydrogen peroxide, ozone).
Hydrolysis and biofuels
Cut cellulose and hemicelluloses into sugars and they can be fermented. That gives fodder yeast and ethanol. It is harder than with starch, because lignin and the tight packing of cellulose get in the way.
Acid hydrolysis of wood with dilute or concentrated acids was the basis of the wood hydrolysis industry (fodder yeast, ethanol, furfural, xylitol). Enzymatic hydrolysis with cellulases needs a pretreatment that opens up the cell wall: steam explosion, acid, alkaline or organosolv pretreatment.
Pyrolysis and wood panels
Heat wood without air and it breaks down into charcoal, liquid and gas. Birch bark is distilled this way into birch tar. Wood panels, particleboard, plywood and OSB, are glued with synthetic resins, the subject of the polymers block.
In pyrolysis, hemicelluloses decompose at lower temperatures than cellulose, and lignin over the widest range. Charcoal making yields charcoal and liquid products (pyroligneous liquor with acetic acid and methanol, tars); dry distillation of birch bark gives birch tar, and fast pyrolysis gives bio-oil. Plywood and panels are bonded with urea-formaldehyde, melamine-formaldehyde and phenol-formaldehyde resins and isocyanate (pMDI) binders.
Key facts
- white liquor
- NaOH + Na2S
- cooking temperature
- around 160-170 °C
- chemical pulp yield
- about half
- causticising
- Na2CO3 + Ca(OH)2
- bleaching
- ECF and TCF
- birch tar
- dry distillation of bark
Sources for this chapter
- Sjöström E. Wood Chemistry: Fundamentals and Applications. 2nd ed. San Diego: Academic Press, 1993.
- Ek M., Gellerstedt G., Henriksson G. (eds.) Wood Chemistry and Wood Biotechnology. Pulp and Paper Chemistry and Technology, vol. 1. Berlin: De Gruyter, 2009.
- Азаров В.И., Буров А.В., Оболенская А.В. Химия древесины и синтетических полимеров: учебник. 2-е изд. СПб.: Лань, 2010.
- Лукаш А.А. Химия древесины: учебное пособие для вузов. СПб.: Лань, 2025. 129 с.