Techno-economic evaluation of polygeneration system for olefins and power by using steel-mill off-gases

Abstract Utilization of industrial off-gases to produce valuable chemicals has the advantages of reducing dependence on fossil fuels. Despite many studies of off-gas utilization, production of secondary chemicals including detailed kinetic models integrated with a power-generation system to fully utilize byproducts has not been considered. The goal of this study was to techno-economically evaluate a polygeneration system to produce olefins and power from steel-mill off-gases. Two process cases were evaluated; they use carbon monoxide-rich Linz-Donawitz converter gas with either hydrogen or coke oven gas as feed. The hydrogen was concentrated from coke oven gas by using membrane-separation technology. These gases reacted with Linz-Donawitz converter gas to yield methanol, which was then converted to olefins. Kinetic models were applied for methanol synthesis and methanol-to-olefin reaction. Heat exchanger networks are designed to minimize utility usage, and a power generation system that uses the Rankine cycle to fully utilize byproduct stream. Calculated minimum selling prices of ethylene by discounted cash flow analysis were US$ 2030∙tonne−1 when hydrogen was the feed gas, and US$ 1064∙tonne−1 when coke oven gas was the feed gas; both were higher than the current minimum selling price (~US$ 720∙tonne−1) of ethylene derived from naphtha. The reduction in minimum selling price by use of coke oven gas is due to the revenue of power generation from coke oven gas retentate. Linz-Donawitz converter gas production with hydrogen is the most sensitive to feed price, whereas Linz-Donawitz converter gas-with- coke oven gas is the most sensitive to capital investment. The analysis demonstrates that neither system is economically viable under the current condition. The findings of this study represent a basic analysis for future improvements of the off-gas to olefins process, especially highly integrated system with power generation.