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background    feedstocks    processes    products

Wisconsin produces more paper than any other state, producing more than 5 million tons of paper and more than 1 million tons of paperboard annually. This represents $12.4 billion of the $16.8 billion in wood products shipped by Wisconsin companies each year.

The papermaking process begins in the forest, where trees are harvested, leaving forest residues behind. As these trees are converted into logs, wood waste such as small-diameter wood and sawdust are created. The mill removes bark from the logs, creating debarking waste, and the logs are then chipped, producing more sawdust.

In the Kraft and sulfite processes, the wood chips enter a digester, where a solution of caustic chemicals (such as sodium hydroxide) called white liquor dissolves the lignin, leaving cellulose fibers. The resulting slurry is washed with fresh water, and the wastewater from this process, called black liquor, is treated so it becomes white liquor that can be introduced to new wood chips. The major byproduct of this treatment process is high-quality carbon dioxide, although there are also corrosive gases.

The alternative to these processes is mechanical pulping, which expends more energy in the chipping stage to reduce the particle size and furnishes lower-quality pulp, but which is less expensive. Mechanical pulping also produces a greater quantity of wood waste (known as fines) than chemical processes which are better able to extract value from these scraps.

The pulp is then filtered, screened and otherwise mechanically treated for uniformity before being bleached. Excess water is driven from the pulp through pressing. The pulp can be transported wet, or it can be dried—often to less than 10 percent water content—to facilitate transport.

When dry pulp arrives at the paper mill, water is added and it is re-pulped into slurry. Mills can mix pulps of different qualities—for instance, Kraft pulp and mechanical pulp—to provide different characteristics to the final product. The pulp (whether fresh or reconstituted) is then mechanically and chemically cleaned and treated, including thickening and screening processes, before being formed into sheets.

The sheets are pressed and dried, and certain fine-quality products are surface treated using minerals such as clay and calcium carbonate precipitate. The paper—whether it is writing paper, tissue paper or diaper filler—is then cut to size and can be shipped in either rolls or sheets.

The major waste product of both the pulping and papermaking processes is paper mill residue, or sludge, which is essentially "scrap" pulp. Wastewater that emerges during the process is often reintroduced back into the process.

• Black liquor
• Debarking waste
• Forest residues
• Paper mill residue
• Sawdust
• Waste wood chips
• Wastewater (pulp & papermaking)
• Wood chips

• Aerobic digestion/composting
• Anaerobic digestion
• Biomass gasification
• Black liquor gasification
• Combustion
• Fast pyrolysis
• Fermentation of lignocellulosic biomass
• Fiber composites manufacturing
• Lipid extraction
• Thermochemical liquefaction
• Vitrification

• 1,3-Propanediol (PDO)
• 3-Hydroxypropionic acid (3-HP)
• Acetic (ethanoic) acid
• Anaerobic digestion effluent
• Biobased fuel gas & syngas
• Bio-derived liquefaction oil
• Biogas
• Butanol
• Char
• Durable building materials
• Ethanol
• Fertilizer
• Glass aggregate
• Glycerin
• Itaconic acid
• Lactic acid
• PHA (Polyhydroxyalkanoate) polymers
• Pyrolytic bio-oil
• Specialty chemicals
• Succinic acid
• Volatile gases (energy)

Brown, Harry L., et al. 1996. Energy Analysis of 108 Industrial Processes. The Fairmont Press, Lilburn, GA.