Allocation of process gases generated from integrated steelworks by an improved system expansion method

Abstract

Goal, Scope and Background. Although a large number of life cycle inventory (LCI) analyses for steel-making processes or steel products have been conducted, the allocation of process gases generated from the steelworks has not yet been clearly solved. The most consistent settlement for avoiding the allocation problem has been generally known as a system expansion method. However, the existing subtracted operations for the process gases in that method are inconsistent to a system in which those gases are consumed at their unbalanced consumption ratio. The goal of this study is to suggest a more reasonable substitute for the process gases in the system expansion method and a modified system expansion method resettling the amount of process gases used. Methods. To seek a more suitable one as a substituted operation of the process gas, a kind of by-product gas, in the system expansion method, it is necessary to analyze the composition of whole fuel consumed within a steelworks. Because the steelworks is supplied with a gap of electricity from the national grid electricity other than home power plants, we should also consider various carbon fossil fuels consumed in the external electric-power production. From this procedure, a composite fuel, which is composed of coal, heavy fuel oil and LNG, is derived as the alternative of the process gases such as BFG, COG, CFG, and LDG. In the sequential manufacturing line, IO(gas), which is the ratio of the quantity used to the quantity produced of each process gas, is increased as a functional unit proceeds to a following steel product of the next process. In the LCI system, where IO(gas) is higher than one, the IO(gas) is adjusted to nearly one. The adjustment of IO is conducted in the order of the amount of process gas used in the whole steelworks on the basis of the functional unit. Results and Discussion. LCI analyses were carried out focusing on the alternative of the process gases for five steel products. As a functional unit goes down a lower stage, IO(gas) is increased due to the high consumption of those gases. We found a phenomenon that IO(gas) had a critical influence on the LCI results between no allocation and system expansion for the process gases through sensitivity analysis. To reduce this influence and adjust for the real situation of IO(gas), we applied an improved system expansion method to the process gases. That is, we partly substituted a process gas with LNG and rearranged the ratio between internal and external electricity (RIEE) as close as the values of IO(gas) to one. Conclusion. As the alternative fuel for the process gases, a composite fuel was derived in the system expansion method. In addition to the composite fuel, which consisted of coal, HFO and LNG, an improved system expansion method was revised by adjusting a modified IO(gas) to nearly one, not the high value of IO(gas) for each process gas. Recommendation and Outlook. This improved system expansion method can be applicable in the chemical industry as well as the steel industry, which have multi-function systems. Optimal LCI analysis may be achieved through the redistribution and optimization in the usage of process gases.