How to Manage Inputs from Co-production Processes in Emergy Accounting

In life cycle assessments it is a challenge to allocate resource use and environmental impact in processes with multiple outputs. This is especially the case when systems include agricultural products that in their production cannot be separated from each other. For emergy accounting, Bastianoni and Marchettini (2000) suggested how to calculate transformities and other indices for joint production systems. Their proposals however, do not include how to manage inputs from joint production systems. Thus a practical method for making systems with inputs from joint production processes comparable with systems that do not depend on joint production processes is still lacking. As a consequence, a product relying on inputs from joint production processes appears to compete poorly with a similar product that does not have to account for co-products appearing upstream. This is counter to perceived benefits of integrated production systems. Inspired by the ISO 14044 standard for LCA, this paper investigates two approaches to making systems comparable: system expansion and allocation. The methods are exemplified by comparing willow production for combined heat and power (CHP), where pig manure is used for nutrient application, with CHP based on fossil fuel. Implications of applying the different approaches to making the systems comparable are discussed. Using the first approach, the systems are expanded to generate the same services namely electricity, heat, meat, and nutrients. Applying the second approach, the bioenergy system is simplified by allocating the emergy flows of pig production between meat and manure. Allocation was based on energy content and on weight for comparison. Both approaches significantly reduce the unit emergy value (UEV) of the bio-based system compared to the existing emergy algebra procedure of implicitly allocating the entire emergy of pig farming to manure without considering pig meat as an output. In general, it is recommended to expand systems whenever practically possible. Allocation should be considered a second-best alternative since it is not in accordance with emergy theory. If subsystems are insufficiently described, and allocation is used, it should be based on energy content to ensure consistency of emergy analyses.