Pulping Byproducts

Byproduct streams at a glance

Energy

Kraft mills burn dissolved lignin, known as black liquor, in a recovery boiler, which regenerates the cooking chemicals and generates steam and electricity. Bark and other residues are burned in a separate biomass boiler.

• Electricity: a large kraft mill is a net exporter to the grid. A standalone market kraft mill generates 920 to 1 770 kWh per air-dry tonne against a mill consumption of 500 to 670, exporting 420 to about 1 130 kWh/ADt. Integrated pulp and board mills export less, 130 to 670 kWh/ADt.
• District heating: excess heat supplied to nearby towns, common in the Nordic countries.
• Biogas: from anaerobic treatment of effluent and organic waste.
• Bioethanol and other biofuels: from fermentation of wood sugars or further processing of residues.

The carbon numbers. About 96 % of a reference market kraft mill's direct CO2 is biogenic and 4 % fossil, and peer-reviewed mill models and measured Swedish mills land at 94 to 97 %. The lime kiln is the main remaining fossil source: it accounts for roughly 10 % of total mill CO2, of which about 60 % is biogenic calcination CO2 and 40 % fossil combustion CO2, giving 100 to 250 kg fossil CO2 per air-dry tonne. Flue gas runs from 8 to 22 volume per cent CO2 depending on the stack, and the lime kiln is the most concentrated of them at 20 to 22 volume per cent.

Those properties are what make pulp mills candidates for BECCS, bio-energy carbon capture and storage: a capture plant needs a large, concentrated and preferably biogenic stream, and mills often sit at deep-water ports where the captured CO2 can be shipped. Estimated technical capture potential is about 135 Mt CO2 per year at kraft mills globally, with individual large mills in the 1 to 2 Mt per year range.

Tall oil derivatives

Tall oil is a mixture of resin acids and fatty acids that separates as a soap during kraft cooking of resinous softwood and is skimmed off before the liquor reaches the recovery boiler. It exists because those extractives survive the cook rather than dissolving into the pulp, which is why the yield follows the species: about 45 kg per air-dry tonne of pulp from Scots pine, 25 from Norway spruce and under 20 from birch, within a general range of 30 to 50 kg/ADt for softwood kraft. Refining splits it into four products.

• CTO (crude tall oil): the raw soap-derived oil.
• TOFA (tall oil fatty acid): surfactants, alkyd resins, lubricants; also a renewable diesel feedstock.
• TOR (tall oil rosin): adhesives, inks and coatings.
• DTO (distilled tall oil): soaps, alkyd resins and metalworking fluids, increasingly also biofuel feedstock.

Chemical byproducts

• Turpentine: volatile terpenes from softwood cooking, used in solvents, fragrances and increasingly as a bio-based chemical feedstock.
• Lignin: the polymer that binds wood fibres together, dissolved during chemical pulping and normally burned as black liquor. Extracted lignin means lignin precipitated out of the liquor beyond what the recovery boiler needs, typically 20 to 25 % of the lignin the liquor carries, which is roughly the margin available before the boiler loses the fuel it runs on. As a solid it substitutes for phenol in adhesives and resins, and it is under development as a precursor for carbon fibre, battery anodes and bio-binders, because its aromatic structure is hard to obtain from any other renewable source at scale. Extraction is still a young industry: about six commercial or demonstration plants have ever been built and operating capacity is around 55,000 tonnes a year, so a mill listing lignin is usually running a first-of-a-kind unit rather than a standard process.
• Wood-derived specialty chemicals: bio-based carbon black substitutes, functional additives and similar products derived from extractives and process streams.

Some facilities have taken this logic to its end point: wood-consuming biorefineries that produce no pulp at all, only chemicals and fuels. WoodComp includes them in the database because they compete for the same wood. See methodology.