What Shall We Do with Steel Mill Off-Gas: Polygeneration Systems Minimizing Greenhouse Gas Emissions

Abstract

Both the global steel and chemical industries contribute largely to industrial greenhouse gas (GHG) emissions. For both industries, GHG emissions are strongly related to the consumption of fossil resources. While the chemical industry often releases GHGs as direct process emissions, steel mills globally produce 1.78 Gt of off-gases each year, which are currently combusted for subsequent heat and electricity generation. However, these steel mill off-gases consist of high value compounds, which also can be utilized as feedstock for chemical production and thereby reduce fossil resource consumption and thus GHG emissions. In the present work, we determine climate-optimal utilization pathways for steel mill off-gases. We combine a nonlinear, disjunctive model of the steel mill off-gas separation system with a large-scale linear model of the chemical industry to perform environmental optimization. The results show that the climate-optimal utilization of steel mill off-gases depends on electricity’s carbon footprint: For the current electricity grid mix, methane, hydrogen, and synthesis gas are recovered as feedstocks for conventional chemical production and enable a methanol-based chemical industry. For low electricity footprints in the future, the separation of steel mill off-gases supports CO2-based production processes in the chemical industry, supplying up to 30% of the required CO2. By coupling the global steel and chemical industry, industrial GHG emissions can be reduced by up to 79 Mt CO2-equivalents per year. These reductions provide up to 4.5% additional GHG savings compared to a stand-alone optimization of the two industries, showing a limited potential for this industrial symbiosis.

Syngas from What? Comparative Life-Cycle Assessment for Syngas Production from Biomass, CO2, and Steel Mill Off-Gases

“…Several promising alternative pathways for syngas have been developed in recent years: bio-based production, CO 2 -based production via carbon capture and utilization (CCU), and steel mill off-gas (mill gas) separation. These three alternative pathways can reduce syngas’ GHG emissions compared to their fossil-based counterparts. − However, these GHG reductions depend on several factors, such as the carbon intensity of electricity and heat, and each syngas technology shows its own characteristics.…”

“…Instead, mill gases can be separated and used as feedstock for chemicals. Thereby, mill gases can reduce life-cycle GHG emissions of products since valuable chemical intermediates are recovered. , Some mill gases are particularly suited for chemical production since they contain large amounts of H 2 or CO . However, the environmental benefits of mill gas use depend on the composition of the mill gases and the efforts for separation.…”

“…However, the environmental benefits of mill gas use depend on the composition of the mill gases and the efforts for separation. Furthermore, the internal heat and electricity supply currently covered by mill gas treatment must then be covered by other energy sources …”

“…The maximum amount of mill gas per 1 kg syngas is 1.1 kg of COG with a heating value of 38.5 MJ/kg and 1.9 kg of BOFG with a heating value of 5.4 MJ/kg. We adopt the power plants’ electric efficiency of 34% and the thermal efficiency of 15% from Ecoinvent 3.5 (details are given in the Supporting Information), resulting in reference flows of 7.5 MJ heat and 17.7 MJ electricity. , …”

“…Furthermore, the internal heat and electricity supply currently covered by mill gas treatment must then be covered by other energy sources. 9 Due to the multitude of options, industry and policymakers can choose between alternative syngas pathways. Current decision-making primarily relies on economic aspects and regional feedstock availability.…”