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 320°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 220°C, allowing power gains for the hot flue gas fans and an additional 18 megawatts of thermal heat, resulting in approximately 22 tons per hour of steam production (sample of a MIDREX® plant, producing 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 1,150°C prior to cooling in a direct contact cooler. The contained heat can also be recovered down to 220°C by installing a heat recovery system, which will recover approximately 16 megawatts of thermal heat upstream of the seal gas cooler. This amount of heat is equivalent to about 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 top gas, with a temperature of approximately 350°C to 400°C, is routed through a waste heat boiler. By heat exchange, the top gas is cooled and 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 using steam from flue gas, seal gas, or top gas in a carbon capture plant, or mechanical equipment can be driven 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 an Organic Rankine Cycle (ORC) 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 mode
• Referenced by successful implementation in smelting reduction plants

*MIDREX® is a registered trademark of MIDREX Technologies, Inc.