Paper and board LCA: pulping route, recovered fibre and the energy balance

The paper sector has an unusual property in environmental assessment: an integrated kraft mill can produce more energy than it needs. Lignin separated during pulping is recovered as black liquor and burned in a recovery boiler to raise steam and generate electricity. That makes the sector's footprint resistant to generalisation.

Three pulping routes, three profiles

Chemical (kraft) pulping dissolves lignin to give long, strong fibres; yield is low (around 45-50 per cent) but the separated lignin becomes energy. Mechanical pulping breaks wood into fibres physically; yield is very high (90 per cent plus) but the process is electricity-intensive and the fibre weaker. Recovered fibre pulping has the lowest energy intensity of the three.

So the claim that recycled paper is always better is incomplete. Recovered fibre is advantaged on energy, but it cannot draw on the renewable energy an integrated kraft mill generates for itself and stays dependent on grid electricity. Where the grid is carbon-intensive the gap narrows.

Fibre has a finite life

Cellulose fibre shortens and weakens with each recycling cycle. A fibre can typically be recycled five to seven times before it is no longer usable for papermaking. That means fresh fibre must keep entering the system; a hundred per cent recycled paper economy is not physically possible.

• Recovered fibre share is limited by the product's strength requirement
• Deinking sludge creates an additional waste stream
• Corrugated board is the product group that best tolerates a high recovered fibre share
• Brightness requirements in graphic papers raise the fresh fibre share
• Without documented fresh fibre origin, land use risk cannot enter the calculation

Water: withdrawn versus consumed

Papermaking withdraws large volumes of water, but most of it is treated and returned. The difference between water withdrawn and water consumed is decisive in water footprint indicators; a report looking only at withdrawal volume badly overstates the impact. ISO 14046 defines that distinction and the regional water scarcity weighting.

Discharge quality is a separate matter and shows up in the eutrophication indicator. Chemical oxygen demand and nitrogen-phosphorus load depend directly on treatment plant performance, and that is site-specific data; using a sector average here produces large errors.