Co-Product Treatment For Asphalt Materials And Supplementary Cementitious Materials
The methodology for co-product treatment for asphalt materials and supplementary cementitious materials when conducting a life-cycle assessment (LCA) is discussed in this article.
Asphalt Materials
Asphalt is one of many co-products produced in oil refineries. Because of the importance of refinery products on the environment and economy, many of these products have been studied using life-cycle assessment (LCA). Each study has had to select a co-product treatment approach. Nearly all rely on allocation, although some have combined subdivision methods with allocation by distinguishing processes within the refinery that can be attributed to particular products while relying on allocation by energy or mass to partition oil extraction and transport impacts and other processes that cannot be reasonably partitioned. Since different refinery products have different fuel contents, weights, or economic values, the method of allocation can have a significant effect on the calculated impact. Different allocations can be applied to different steps in the asphalt production. The extraction and transport of the crude to the refinery is treated in the same way for all the products obtained from the crude: actual crude origins and transportation distances and means should be applied. At the refinery level, depending on the refinery setup, some processes may be common to some products while other processes are unique to a single product.
Most LCA studies use mass allocation for each individual processing step that can be attributed to asphalt. To help understand the impact, sensitivity analyses are often performed in LCA using alternative allocation methods. Wang, Lee, and Molburg (2004) suggest that the different approaches for allocation for different refinery products can lead to differences in assigned environmental impacts of up to 25 percent. A more recent life cycle inventory (LCI) considering a typical set of crude sources in Northern European refineries has been prepared by Eurobitume for conventional asphalt binders. A hybrid approach using allocation based on mass for parts of the process and economic value for other parts was used to determine the environmental impacts for the cradle-to-gate inventory (Eurobitume 2012; Bernard, Blomberg, and Southern 2012).
A complication for most LCA practitioners that can influence the allocation choices is the limited availability of specific process data at the desired level of a refinery that is within scope of a particular LCA.
Supplementary Cementitious Materials
There are different approaches on how to allocate environmental impacts for supplementary cementitions materials (SCMs), such as fly ash and slag cement. In the past these materials were considered wastes from industrial processes (coal-powered electricity generating fly ash, and steel production generating blast furnace slag). The current practice in the U.S. is to consider fly ash a waste material diverted from a landfill for beneficial use, meaning that none of the environmental impact associated with electricity generation is typically assigned to the fly ash. As long as the cost of transport and processing of the fly ash is the only source of economic value, a waste classification is appropriate. However, once the fly ash has value beyond this, it should no longer be considered waste, but instead a co-product. Already in some markets fly ashes are in high demand and economically valuable, meaning they are no longer waste flows. In these cases, it is appropriate to allocate some of the environmental burden associated with coal-fired power plants to fly ash. The most common means to accomplish this is through economic worth of the co-products. LCAs in some regions (e.g., Europe) show that the economic worth of fly ash compared to electricity generation is small and hence the assigned environmental impacts are also small. The same practices can be applied to slag cement as a co-product in steel production. It is noted that different allocation methods can lead to differences in assigned environmental impacts. There are also other motivations for industries to seek classification as waste or co-product. For example, in Europe fly ash producers often do not want classification as "waste" because that requires a much more difficult regulatory environment for handling, storing, and transporting the material. Chen et al. (2010) considered different allocation methods for slag cement and fly ash used in Europe, arguing that demand for these products outpaced production and thus their designation as a waste may not be appropriate. Allocation based on economic value as compared to allocation by mass leads to significantly lower environmental flows attributable to both SCMs, and seems to better reflect the purpose of the industries that produce the SCMs - production of steel or electricity.
For more information on asphalt materials and supplementary cementitious materials, see Chapter 3 (.pdf, 5 mb) and for more details on assessing pavement sustainability, refer to Chapter 10 (.pdf, 5 mb) of the Reference Document.
References
Bernard, F., T. Blomberg, and M. Southern. 2012. "Life Cycle Inventory: Bitumen." Proceedings, International Symposium on Life Cycle Assessment and Construction. Nantes, France.
Chen, C., G. Habert, Y. Bouzidi, A. Jullien, and A. Ventura. 2010. "LCA Allocation Procedure Used as an Incitative Method for Waste Recycling: An Application to Mineral Additions in Concrete." Resources, Conservation and Recycling. Vol. 54, Issue 12. Elsevier, Philadelphia, PA.
Eurobitume. 2012. Life Cycle Inventory: Bitumen. 2nd Edition. European Bitumen Association-Eurobitume, Brussels, Belgium.
Wang, T., I. S. Lee, J. Harvey, A. Kendall, E. B. Lee, and C. Kim. 2012. UCPRC Life Cycle Assessment Methodology and Initial Case Studies on Energy Consumption and GHG Emissions for CAPM Treatments with Different Rolling Resistance. UCPRC-RR-2012-02. California Department of Transportation, Sacramento, CA.