Reheating furnace

01

Set the capacity, stock and discharge temperature

California Steel Industries lists two eight-zone, natural-gas-fired walking beam reheating furnaces feeding its 2.5 million t/y hot strip mill — one started in 1997 (Chugairo), one in 2009 (Tenova-Core) — each 38 ft 9 in wide and 137 ft 10 in long, for carbon steel slabs 7–9 in thick, 24–75 in wide, 158–441 in long and up to 30 tons. AIST reports that ANDRITZ was ordered to supply two top- and bottom-fired walking beam furnaces of 500 t/h of carbon steel slabs each to replace the pusher furnaces at ArcelorMittal Burns Harbor's 80-inch strip mill, with low-NOx burners, long unfired preheat zones, individual burner control and improved insulation including skid insulation. [1] [2]

Air Products' US6290492B1 states that steel reheat furnaces heat billets, ingots and slabs to 2,000–2,300 °F for further processing, that continuous furnaces with two or more zones are used to distribute heat and control temperature, and that such furnaces fire from 20 to 400 million Btu per hour. [3]

Editorial calculation: 2,000–2,300 °F is about 1,093–1,260 °C; the CSI furnace hearth envelope of 38 ft 9 in × 137 ft 10 in is about 11.8 m × 42.0 m; and a 500 t/h furnace running 24 hours would heat about 12,000 t of slabs per day. [3] [1] [2]

Give the supplier the mill and its rolling schedule, stock type and size range (thickness, width, length, weight), steel grades, charge temperature (cold or hot charge), required capacity in t/h for each product, discharge temperature and allowable temperature difference across and through the piece, and the space available in the bay.

02

Set the zones, burners, fuel rate and emissions

The patent describes a typical three-zone continuous reheat furnace with a soak zone at the exit end and upper and lower heating zones toward the entrance, flue gases flowing from the soak zone back through the heating zones and out at the charge end; it states that steel reheat furnaces account for almost 30% of the NOx generated by the steel industry and proposes staging fuel-rich and fuel-lean zones with flue gases mixed above about 1,200 °F (preferably above 1,600 °F) to burn out CO. [3]

A paper on SSAB's 300 t/h walking beam slab furnace reports a prior oil rate of 440 kWh/t (1.58 GJ/t) and that a flameless oxyfuel retrofit, supplying about 75% of the combustion oxygen by high-velocity injection while keeping the existing air-fuel burners, gave 45% lower NOx, 25% lower fuel consumption, flue gas volume under 45% of air-fuel, 15–20% higher throughput and better slab temperature uniformity with no harm to surface quality. [4]

Editorial calculation: 440 kWh/t × 3.6 MJ/kWh = 1.58 GJ/t, and a 25% reduction would bring that to about 1.19 GJ/t, assuming the reduction applies to the whole furnace at the same throughput. [4]

State the fuel (natural gas, mixed gas, oil, oxygen), burner types and number per zone, top and bottom firing, recuperator or regenerative air preheat, fuel rate guarantee in GJ/t at stated capacity and charge temperature, NOx and CO limits, flue gas route, and the level 1 and level 2 control system.

03

Choose walking beam or pusher handling and plan rebuilds

Walking beam furnaces carry the stock on moving beams and can be fired from above and below; the Burns Harbor order replaced pushers with walking beam furnaces, and the patent notes that flue gases pass through openings in the walking beam support mechanism. [2] [3]

A paper on Gerdau Ameristeel's Midlothian section mill describes rebuilding an existing walking beam furnace that could not meet production: the handling equipment, skids, steel, refractory, combustion system and controls were replaced, only the understructure mechanism retained, a new 45 ft section added at the charge end, the furnace rezoned and new level 1 and level 2 automation installed, for more production with less maintenance and fuel. [5]

Say whether the furnace is new or a rebuild, the handling type (walking beam, walking hearth, pusher), charging and discharging machines, skid and beam insulation, and the outage window for a rebuild.

04

Know the reheating furnace parts

This is a functional reference built from the sources cited; it is not one maker's design.

05

Fill the specification fields with values for your mill

A reheating furnace enquiry needs values from your mill and products. The table explains each field, gives published values where a reviewed source states them, and says what to obtain from the supplier.

06

Send the mill, stock and fuel data together

Paste it into your enquiry below or into an email to a maker, then replace every bracket.

Mill: [type; schedule; capacity]
Stock: [slab, billet or bloom; sizes; weight; grades; charge temperature]
Heating: [discharge temperature; uniformity]
Firing: [fuel; burners; air preheat; oxygen]
Limits: [GJ/t guarantee; NOx; CO]
Project: [new or rebuild; handling type; outage]

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