Waste Heat Recovery Systems for MIDREX® Plants

The MIDREX® direct reduction process is highly efficient, utilizing direct heat integration of the reformer flue gas to preheat various process gases, including combustion air, process gas, top gas fuel, and natural gas. In addition, valuable waste heat from the reformer flue gas, seal gas, and/or top gas can be recovered and used to generate energy for various purposes.

Heat Recovery from Reformer Flue Gas

Downstream of the standard heat recovery system, the flue gas still has a temperature of about 350°C. This heat can be further utilized by installing additional heat recovery bundles to produce hot water or steam. The outlet temperature can be reduced to about 200°C. This results in some power gains for the hot flue gas fans and an additional 18 MWth of energy, resulting in approximately 22 t/h of steam production (sample of a MIDREX® plant, which produces 2.5 million tons per year).

Heat Recovery from Seal Gas

A portion of the reformer flue gas is used as seal gas in the MIDREX® process for inertization purposes. This seal gas has a temperature of 1,100°C before cooling in a direct contact cooler. The contained heat can also be recovered down to 300°C by installing a heat recovery system, which will recover approximately 11-12 MW of thermal heat upstream of the cooler. This amount of heat is equivalent to approximately 19 tons per hour of steam production (sample of a MIDREX® plant with a capacity of 2.5 million tons per year).

Heat Recovery from Top Gas

The dust-laden gas is routed to the waste heat boiler through a refractory-lined duct from the top of the boiler to the bottom. Typically, the top gas temperature is approximately 350°C to 400°C. Cooled top gas is returned to the top gas scrubber. Coarser dust is removed at the bottom of the waste heat boiler. A soot blower system can be installed to clean the heat exchanger bundles.

This system maintains the overall availability of the direct reduction system as an automatic changeover can bypass the heat exchanger without disruption. Such heat recovery systems have already been installed in smelting reduction plants. Based on the excellent operating results and the similarity of the process conditions, the described waste heat recovery system can be adapted for installation in MIDREX® direct reduction plants.

Utilization of Waste Heat

Captured waste heat from the waste heat boiler can be used as hot water or steam for electrical power generation, e.g., using waste heat in an organic rankine cycle (ORC) module, using steam in existing power plants at the steel mill or with conventional steam turbines.

Alternatively using steam from flue gas or top gas in a carbon capture plant, or driving mechanical equipment directly.

Main Advantages

• Utilization of valuable waste heat from flue gas, seal gas, and/or top gas
• Improved energy efficiency of the MIDREX® process
• Power generation via organic rankine cycle module steam turbine in an (existing) power plant
• High plant availability due to automatic switch-over to the existing wet scrubber in an on/off operating modus
• Successful implementation in smelting reduction plants