Decarbonisation of steel mill gases in an energy-neutral chemical looping process

Abstract

The iron and steel industry is the largest industrial carbon dioxide emitter. More than 70% of their emissions originate from conventional combustion of steel mill gases. In this work, a strategy for carbon capture from the steel mill gases is laid out, using a combination of emerging technologies employing oxygen carrying nickel oxide, carbon dioxide accepting calcium oxide, and dioxygen uncoupling manganese(III) oxide. The process combination was simulated and analysed using Aspen Plus. By applying the proposed strategy, up to 85% of carbon dioxide emitted from the combustion of blast furnace gas and coke oven gas can be captured and compressed for storage or utilisation without external heat and electricity supply. The chemical energy in the steel mill gases is used for the reduction of nickel oxide to nickel and the exothermic capture of carbon dioxide by calcium oxide to form calcium carbonate. Auto-thermal conditions for the energy-intensive carbon dioxide release by the endothermic decomposition of calcium carbonate at 1173 K are achieved by combining it with the exothermic oxidation of nickel by dioxygen, thereby regenerating nickel oxide and calcium oxide for another cycle while efficiently using the chemical energy stored in the steel mill gases. The dioxygen necessary for nickel oxidation to nickel oxide is produced by using a fraction of the product carbon dioxide stream at 1173 K for the endothermic manganese(III) oxide reduction to manganese(II,III) oxide, operated between 1100 and 1140 K, making use of the sensible heat of the carbon dioxide product stream and shifting the equilibrium towards dioxygen uncoupling of manganese(III) oxide. Air is used to re-oxidise manganese(II,III) oxide to manganese(III) oxide and complete the process cycle. Self-sufficiency of electrical power for carbon dioxide compression is obtained by recovering high temperature heat and converting it into electricity via high pressure steam in a steam turbine assembly. A proof of concept is obtained in a laboratory scale fixed bed reactor and the effect of operating conditions is experimentally explored.