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The use of pure oxygen for fuel combustion in a billet reheating furnace delivers benefits

including better product quality, higher productivity and increased plant flexibility by reducing

production bottlenecks. While competitive methods also can provide many of these features,oxycombustion also lowers NOx emissions and requires lower investment and downtime for

furnace upgrading.

There are a number of conventional solutions to increasing output and product flow rate in astandard air/fuel combustion furnace. Solutions include increasing the length of the furnace and

the number of burners, improving flue gas heat recovery to improve preheating, installation of

alternating regenerative burners to preheat combustion air and charging hot billets directlyfrom a previous production operation. Major features of all of these are high investment and

length of downtime required to refurbish or refit the furnace. In addition, preheating air can

result in unwanted high NOx emissions and an increase in scale (oxidation) on the heatedbillets.

The Pure-Oxygen Solution

The use of pure oxygen instead of air for combustion has both efficiency and product quality

advantages including:

Improved thermal homogeneity of heated billets (including thick section billets)

resulting in less deformation and a more uniform temperature throughout the billetReduced surface oxidation (scaling) despite the oxidizing nature of the process

Improved efficiency of an existing heat recovery system (by not heating nitrogen in the

air)

At least as good as energy efficiency as the highly preheated air solution, together with

minimized NOx emissions

Decreased use of natural gas per ton (although outweighed by an increased use of

oxygen)
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Low investment due to unchanged furnace configuration, limited adaptation of work to

the burners and minimal downtime

An oxyfuel flame has an enhanced capability to transfer heat energy by radiation so directenergy transfer to load and cells is much improved. With the smaller volume of flue gases,

induced losses are automatically smaller. Table 1 shows the consumption of fuel andcombustion constituent and the generation of fumes in a system at 900?C (1650?F), whichtransfers an effective 18 MJ (5 kWh) of power to the load.

A properly located oxyfuel flame contributes to more uniform heating of a furnace resulting in

fewer hot and cold spots (e.g. high vault temperatures and lows in charging zones). This speeds

up heating of large section steel product, reduces delays, decreases temperature differentialsalong or through a product, and, thus, reduces distortion and scaling. Product quality also can

be better maintained due to the more homogenous condition of the product when undergoing

rolling.

While the efficiency of using high-temperature preheated air for combustion is similar, high-temperature combustion in the presence of nitrogen leads to the risk of high NOx levels and

requires an investment in flue-gas scrubbers to meet emission regulations. By comparison, the

use of pure oxygen minimizes nitrogen in the combustion process.

Modeling Predicts Quality and Efficiency

Air Liquide's advanced modeling techniques enable the evaluation of an existing plantconfiguration to determine how it will improve through the addition of oxyfuel burners. Figure

Fig 1 Modeling an existing furnace heatingconfiguration predicts the improvement in furnace and

product temperature uniformity through the additionof oxyfuel burners. This model demonstrates the

closeness of fit between the predicted and actualtemperatures of the furnace wall and the surface

temperature of a billet in a trial run.


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1 demonstrates the closeness of fit between the predicted and actual temperatures of the

furnace wall and the surface temperature of a billet in a trial run.

Three-dimensional modeling techniques enable fine tuning of the process, such as influencing

burner location and control of the flame size and temperature. Modeling also assists predictingfurnace temperature throughout the furnace, thus allowing optimization of temperature to

improve product heating control.

The difference in temperature homogeneity of a billet achieved in air combustion and in

oxyfuel-assisted combustion of the same overall power is demonstrated in Fig. 2 and 3. The

overall temperature differential is halved in the oxyfuel process.

Improved Efficiency: Case Study

Fig 2 Air combustion furnace at a power of 29 MW

produces a temperature differential across the billet(from top surface to bottom surface) of 50C (90F);

ideal billet temperature is indicated in the middle

(green) range.


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LME Trith Saint L?r (Valenciennes, France), Air Liquide and the French Agency for Energy

and Environment Management (ADEME) collaborated in a project to demonstrate the

improvement in a walking-beam billet reheating furnace by switching to oxyfuel-assisted

combustion. The 31 m (102 ft) long natural gas-fired furnace comprises a charging zone, a longnon-fired recovery zone, a preheating zone (10.5 MW), a main firing zone (15.5 MW) and a

soaking zone (4 MW). A typical exhaust zone temperature of 700?C (1290?F) enables

preheating combustion air to 300?C (570?F), while the target reheat temperature is 1100?C(2010?F). Temperature regulation of each heating zone provides the major furnace process

control.

Two options preselected during preliminary studies were global enrichment of combustion air

to a maximum of 24% oxygen through existing burners and the addition of a new oxyfiringzone, using oxyfuel burners.

A model of the furnace (global energy balances) is shown in Table 2, which shows the

expected output gains using oxygen solutions. With oxyfuel burners, the long recovery zone in

the LME furnace means that exit temperature and the small increase in fumes volume will notmaterially affect the energy recovery performance. Because an increase in productivity was

one of LME's primary objectives, four oxyfuel burners were installed on the reheating furnace

in the heat recovery zone immediately after the existing air combustion zone.

The oxycombustion zone can be regulated by firing two or four burners, as well as throughregulating the gas flow to the burners. This allows the oxycombustion to be used when

required to help the throughput of thick products, which otherwise would cause bottlenecks

and restrict output.

The efficiency of recovery changed very little as indicated by an increase in exhaust

Fig 3 Air combustion (26 MW) plus oxyfuel

combustion (3 MW) produces a more homogenous

temperature through billet than air combustion alonewith a temperature differential of 25C (45F); ideal

billet temperature is indicated in the middle (green)range.


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temperature of only 20?C (35?F); the energy from oxycombustion is transferred to the product.

Visual observation in the discharge zone shows less scale on the billets, despite the oxidizing

nature of the process. Reasons for this are the low temperature (900?C, or 1650?F) in theoxyfiring zone and a shorter time for the product in the high temperature zones. A side benefit

is that scale formed under primary oxidizing conditions is more porous and easily removed.

NOx emissions are reduced by up to 15 %. This is achieved both through empirical

optimization of the oxygen/fuel gas mixture and by matching the number of oxyfuel burnersunder combustion to the total power output of the furnace.

Productivity Increases

LME increased output by 20% (92 to 110 t/hr) at the same average cost per metric ton, which

allowed the company to pay off the capital cost through these savings. Operating cost savings

of the reheated, rolled and finished product are shown in Table 3 with the previous air

combustion process costs given as 100. A saving of 12% per metric ton of finished steel wasachieved, which is a significant cost advantage for a steel producer.

For more information: Daniel Levert is corporate foundry market manager, Air Liquide, 75Quai d'Orsay, Paris 75007; tel: ++ 33 (0) 1 40 62 53 05; fax: ++ 33 (0) 1 40 62 54 36; e-mail:

[email protected]. Mike Grant is senior metals applications engineer, Air Liquide


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America Corp., 5230 South East Ave., Countryside, IL 60525; tel: 708-579-7859; fax: 708-

579-7858: e-mail: [email protected]

This article is derived from a paper presented at AISE September 2000 by O. Delabroy and G.

Le Gouefflec (Air Liquide) and C. Lebrun, A. Barbotin and R. Cervi (LME). Proceedingsavailable from AISE.

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