Combined production is a production process in which the relative amounts of co-products can be varied independently. This characteristic distinguishes it from joint production, where the proportions of product outputs are technically or physically fixed by the nature of the production process itself.
In combined production, the producer has operational flexibility to adjust the output ratios between different products according to market demand, technical preferences, or economic considerations. This flexibility means that the production volumes of individual co-products can respond independently to changes in their respective demands. For example, a refinery might adjust its operations to produce more diesel and less petrol, or vice versa, depending on market conditions, even though both products come from the same crude oil input.
This distinction between combined and joint production is particularly significant for Life Cycle Assessment because it influences how the system should be modelled. According to ISO 14040 clause 3.10, co-products are "any of two or more products coming from the same unit process or product system". When dealing with combined production, the ability to vary output proportions independently suggests that changes in demand for one product may not necessarily affect the production of the other co-products in fixed ratios.
The flexibility inherent in combined production has important implications for consequential LCA modelling and system expansion. When a producer can independently adjust output levels, the marginal effects of increased demand for one product may be different from situations where all co-products must be produced in fixed proportions. This can affect decisions about which activities are included in the product system and how substitution effects are modelled.
Understanding whether a production process involves combined or joint production helps practitioners make more accurate modelling choices regarding co-product allocation and system expansion approaches.
