Application of Markov Chain Model to Calculate the Average Number of Times of Use of a Material in Society. An Allocation Methodology for Open-Loop Recycling. Part 2: Case Study for Steel (6 pp)

Abstract

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Preamble. In this series of two papers, a methodology to calculate the average number of times a material is used in a society from cradle to grave is presented and applied to allocation of environmental impact of virgin material. Part 1 focused on methodology development and showed how the methodology works with hypothetical examples of material flows. Part 2 presents case studies for steel recycling in Japan, in which the methodology is applied and allocation of environmental impact of virgin steel is conducted.

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Abstract Goal, Scope and Background. The life cycle of steel begins with the mining of iron ore from the earth. Steel is produced in steel works and used in various products. Some of the steels are recycled at the products' end of life and used as a resource for the production of new steel in electric furnaces, while the remaining steel is used just once in products before being discarded (landfilled). In this paper, case studies were conducted to analyze the average number of times the element of iron is used and its residence time in society, in which the methodology developed in Part 1 of the paper was applied. CO2 emissions caused by steel productions and recycling were allocated by the number of times the element of steel is used in a society.

Results and Discussion

On the basis of the material flows of steel in Japan in 2000, it was calculated that at least 70% of the BF crude iron produced in Japan in 2000 was ultimately exported. On the assumption that steel is used in other countries in the same way as it is in Japan, the average number of times of use and the residence time of elemental iron in society are 2.67 and 62.9 years, respectively. Both of these values depend significantly on the recycling ratios of steel from construction and automobiles. Our model indicated that if the recycling ratio of steel from civil engineering and construction increased from 50% to 60%, the average number of times used would increase to 3.17 and the residence time of elemental iron in society would increase to 75.8 years. If CO2 emissions caused by steel productions and recycling are allocated by the number of times the element of steel is used in a society, it was calculated that steel use of one time generates in average an environmental burden of 1.03 t-CO2/t.

Conclusion

A method was developed to calculate the average number of times a material is used in a society from cradle to grave. Our methodology is based on Markov chain model using matrix-based numerical analysis, and has been successfully applied to steel. The results obtained by this methodology, i.e. the average number of times the element of iron is used in society, could be used for allocation of environmental burdens of virgin material as well as an indicator for assessing the state of material use in a certain year, based on material flow of material in that year.

Recommendation and Perspective

It is recognized that further researches must be conducted to gather data on steel production, use, and recovery in other countries and incorporate them into the transition probability matrix to obtain more precise results. Although this paper deals only with steel, this method can also be applied to other materials.