The LCA allocation problem: mass, economic, and system expansion compared

Allocation is one of LCA's classical methodological debates. When a single process produces multiple valuable outputs — e.g. a refinery yielding petrol, diesel, kerosene, fuel oil simultaneously — how do you split the environmental burden between products? Wrong allocation hides or amplifies one product's burden at another's expense.

The ISO 14044 hierarchy

ISO 14044 treats allocation in a strict hierarchy:

• 1. If possible, avoid allocation — decompose into sub-processes or use system expansion
• 2. If unavoidable: use allocation reflecting a physical relationship (typically mass or energy)
• 3. If physical relations are insufficient: fall back to economic or other non-physical allocation
• 4. Document your justification regardless of which method you choose

Mass allocation

Splits burden by product mass. Milk processing example: 100 kg of milk yields 90 kg of buttermilk + 10 kg of fat, so the process burden splits 90% / 10%. Simple and intuitive — but not always fair, since some by-products are low-mass and high-value (e.g. concentrated calorie content in milk fat). In steel production, mass allocation puts artificial weight on the slag co-product.

Economic allocation

Splits burden by product market value. From 100 kg of crude oil yielding 30 kg petrol (€200) + 40 kg diesel (€180) + 30 kg fuel oil (€60) = €440 total, allocation runs 45% petrol + 41% diesel + 14% fuel oil. Market values fluctuate so the method is volatile, but it fairly distributes between co-products of different caloric or commercial weight. In refineries and biodiesel co-product analysis (glycerol, fertiliser by-streams) economic allocation is common.

System expansion (avoided burden)

A way to avoid allocation: identify what the co-product substitutes and subtract the substituted product's burden from the main product's burden. Example: if the cement-clinker by-product is used as fertiliser, the fertiliser-production emissions are subtracted from the cement burden. This is methodologically the most natural approach, but data-intensive — you need the actual market structure for the substituted product. Common in consequential LCA, limited use in attributional LCA.

What does EN 15804+A2 prefer?

EN 15804+A2 accepts the ISO 14044 hierarchy but tightens practical rules for construction. The worldsteel methodology for steel is mass-based; EN 16908 for cement permits economic allocation in specific co-product situations. End-of-life "cut-off" allocation is the default for construction products — recycled by-products carry no A1 burden and arrive as a Module D credit.