Energy decision making in a pulp and paper mill: selection of LCA system boundary

Background, aim, and scopeNorth American pulp and paper mills are facing tremendous challenges, which may necessitate major mill modernizations. An example is process modification to reduce dependency on purchased power, which is an expensive resource. Such modifications may have environmental implications at the mills’ sites, on their product life cycle, and on other interconnected systems, and therefore, systematic tools such as life cycle assessment (LCA) need to be applied. Different LCA system boundary approaches can be used for such process design applications, and these approaches need to be compared to determine their respective benefits and limitations in this context. This study compares setting the system boundary according to a cradle-to-gate approach [attributional LCA (ALCA)] and a system expansion [consequential LCA (CLCA)] approach using a case study, which deals with implementing cogeneration and increased de-inked pulp production at an integrated newsprint mill.MethodologyA case study considering various process options to reduce purchased electricity at an integrated newsprint mill is defined. These options include implementing cogeneration and increasing de-inked pulp production. The environmental impacts related to these process options are analyzed using two LCA methodologies. The first one consists in setting the system boundary according to a cradle-to-gate ALCA approach, while the second one uses a differential CLCA approach (system expansion). Comparisons of the two methods are based on different parameters: inclusion/exclusion of the indirect environmental consequences, selected allocation procedures, and effect of using the average versus the marginal technology for power production.ResultsBoth the ALCA approach and the CLCA approach indicate that the process options are beneficial. However, the results show that indirect environmental consequences, which are assessed only with the CLCA approach, can be opposite to direct effects (e.g., the results can indicate an improvement of the direct impacts and a deterioration of the indirect ones). In addition, environmental impacts obtained by modeling the average or the marginal technology may be very different. Using an ALCA approach often necessitates the use of an arbitrary allocation procedure, which can greatly affect the results. On the other hand, ALCA can be used to perform “hot spots” identification, which, at this time, is not possible with CLCA.DiscussionThe cradle-to-gate boundary approach, which is usually used in the literature for pulp and paper process design applications, may not always be the most appropriate to determine the environmental consequences of implementing different process options. However, at this time, there is no method based on CLCA allowing for “hot spots” identification, and thus, a combined approach may be required for process design.ConclusionsIt is shown that the choice of the system boundary can significantly affect the results. More specifically, the usage of different allocation procedures in ALCA leads to different interpretation of the results, which does not occur when using the CLCA approach. CLCA provides more complete information for decision making for the cases where most of the consequences occur outside the life cycle of the product investigated.Recommendations and perspectivesResearch opportunities include developing methodologies to integrate ALCA and CLCA for improved process design applications and mechanisms for reducing uncertainty in CLCA.