Thermal regeneration is a proven technology that is
generally accepted as the most efficient means of heating an industrial
furnace. But regenerative systems, though commonplace, are far from the norm in
the industry. This is largely because regenerative systems are substantially
more expensive than cold-air systems (Fig. 1)
due to the costs of the regenerators themselves and the required specialized,
paired burners.
History of Thermal Regeneration in the Steel Industry
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| Fig. 1. Cold-air
combustion system |
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Thermal regeneration is the capturing of heat from exhaust
gases and using it to preheat combustion air. Given the enormous quantity of
fuel necessary for any type of heat processing, it is likely that some thought
was given to recapturing waste heat beginning with man’s earliest attempts at
heat treatment. In fact, there were some attempts at thermal regeneration in
the ancient world with some marked success by ancient Chinese and Korean
pottery makers. However, the concept of using waste heat to preheat combustion
air remained largely undeveloped until the time of the industrial revolution.
Regenerative Technology Gave Birth to the Open-Hearth
Steelmaking Era
During the 1830s, Carl Wilhelm Siemens, a young German
engineer living in England, began studying the earlier work of Robert Stirling,
a Scottish minister who patented a power system that became known as the
“Stirling Engine” in 1816. The Stirling Engine is a closed engine system
containing a fixed amount of gas that, when subjected to external heat, cycles
back and forth between a hot end and a cold end. The expansion of the gas when
heated, followed by the compression of the gas when cooled, activates pistons attached
to a fly wheel that can perform work. Stirling placed a simple heat-retaining
media in the midpoint of the system that absorbed heat from the hot portion of
the engine as the gas passed to the cold side and released the heat to the
cooled gas as it returned to the hot side.
While the
Stirling Engine itself is still recognized for its simple ingenuity, its impact
has been relatively small due to the success of the later-developed
internal-combustion engine and electric motor. However, Robert Stirling’s idea
of cycling hot and cold air over a heat-retaining media fostered a revolution
in steelmaking.
Siemens became
fascinated with heat regeneration and spent years trying and failing to apply
Stirling’s regeneration idea to improve steam-powered mechanical functions.
Siemens finally found success after he realized that the regeneration concept
was easier applied to large-scale industrial processes. In 1861, Siemens
secured a patent for a regenerative industrial furnace. Shortly thereafter, he
installed regenerators containing brick checker-work media on a glass furnace
near Birmingham, England. The furnace exhausted gases through two
heat-absorbing chambers made of brick that operated cyclically, with one
serving as an exhaust flue and the other as a combustion draw. Cycling exhaust
and combustion functions back and forth between the two chambers provided a
continuous supply of preheated combustion air and vastly increased efficiency.
Siemens licensed the technology to Pierre Emile Martin who built
a regenerative open-hearth furnace sitting atop two regenerator chambers
containing the Siemens-devised brick checker-work media in 1865. While the idea
of an open hearth was not new, melting ore and scrap in an open hearth was not
previously economical because of the enormous amount of fuel needed to maintain
the required temperatures.
The regenerative
open hearth, or “Siemens–Martin process” as it came to be known, was
cost-competitive with the Bessemer steelmaking process of decarburizing pig
iron by forcing high-pressure air through the molten mix. The open hearth
allowed for a more controlled and refined process of steelmaking, resulting in
substantially superior and more consistent grades of steel. Ironically, after
it appeared that the regenerative open hearth would render the Bessemer process
completely obsolete, a variation of the Bessemer process, the basic oxygen
process, appeared. Employing almost pure oxygen, the basic oxygen process
produced higher-quality steel in one-tenth of the time needed in the open
hearth. Not surprisingly, open hearths began to disappear, and by the late
1980s few could be found in the U.S.
Small-Scale Regenerative Systems are Born
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| Fig. 2. Regenerative
combustion system |
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The demise of the open-hearth furnace was not the end of
thermal regeneration in the steel industry. The same concept of thermal regeneration
employed in the gargantuan open-hearth furnaces was scaled down by industrial
burner manufacturers and applied to heat-treating applications. Hotworks Combustion Technology Ltd. of Yorkshire, England,
claims credit for devising the first compact regenerative systems in 1982. The
credit appears deserved, but it also appears true that others in Germany, Japan
and the U.S. were pursuing the idea at the same time. By the mid-1990s,
thermal-regeneration systems became widely available, and since that time more
and more have been included in new furnaces and retrofits (Fig. 2).
The drawback to
these systems is the need for increased burner capacity in order to maintain a
desired furnace temperature. Standard systems include paired regenerative
burners that are directly connected one-to-one with a regenerator with the
furnace waste gases exhausted through the burner itself to the regenerator box
dedicated to that burner. Because burners cannot fire when in the exhaust mode,
each burner fires only half of the time. Thus, maintaining the desired heat
capacity requires additional burner capacity. Moreover, each additional burner
requires its own dedicated regenerator, which can often be problematic due to space
availability.
How Does it Work?
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| Fig. 3. Airflow
diagram for new system |
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The new system allows all burners to fire continuously and
eliminates both specialized regenerative burners and the one-to-one pairing of
burners and regenerator boxes.
Because the
burners are not part of the new system’s exhaust process, they need not cycle
on and off, and with all burners continuously available, less total burner
capacity is required to maintain the desired heat. Also, without need for
exhaust-function capability, burners can be selected based solely upon
performance and suitability for the particular application. The need for
intricate and complicated specialized regenerative burners is completely
eliminated while all the fuel savings are retained.
Just as is done
in standard regeneration systems, the new system directs the hot furnace
exhaust through a media such as tabular alumina spheres located within the
regenerator box. And just as in standard systems, there is a cycling from one
regenerator box to another (Fig. 3). But, because nothing exhausts through the
burners, the burners can fire continuously and need not cycle. The furnace
gases instead pass from a port in the furnace wall to the regenerator box,
where the media becomes charged with heat. A valve in the exhaust line
positioned between the furnace port and the regenerator box controls the flow
of the hot exhaust into the regenerator. The system recovers the hot furnace
gases to such an extent that the temperature of the exhaust is less than 200°F
after being pulled through the regenerative media. There are no furnace flues
used in the system, and no stack is required. All exhaust gases coming out of
the regenerators are handled through a fan, allowing the exhaust to be easily
routed to a safe location.
When the charging of a regenerator is
completed, the furnace-port valve closes, stopping the flow of the furnace
exhaust gases into the regenerator. An ambient-air valve then opens, allowing
the flow of ambient air through the now-heated media. The ambient air absorbs
the heat charge from the media and passes the now-superheated air to the
burners for combustion. At the same time, a separate port connected to a
separate regenerator opens, causing the exhaust gas to begin charging that
regenerator. The number of regenerators used is dependent on the capacity of
the furnace. Whatever the arrangement, there are always some regenerators
charging and some discharging at any given time, enabling the burners to fire
continuously.
A Common Combustion Air Supply
The key aspect of this system is the incorporation of a
common combustion air supply that receives all of the preheated air from all of
the regenerators and feeds it to all of the burners uniformly and
simultaneously. This is accomplished via a system of piping, valves and fans.
Each regenerator box has four separate piped connections controlled by separate
valves as follows:
- A furnace exhaust line
connects the furnace port to the regenerator.
- A regenerator exhaust
line powered by a fan forcibly draws the furnace exhaust through the media with
the small amount of heat not absorbed by the media exhausted to the atmosphere.
- An ambient-air supply
line is powered by a fan, which pushes cold ambient air through the media
during discharge.
- A heat discharge line
transports the superheated ambient air from the regenerator to the common
combustion-air supply which feeds the burners.
Sequence of Valve Movements
1. Regenerator is charging
- The
furnace exhaust line and the regenerator exhaust line are open such that the
furnace exhaust flows in and through the media.
- The
ambient-air line and the heat-discharge line are closed.
2. Regenerator is
discharging (the valves are reversed)
- The
furnace exhaust line and the regenerator exhaust line are closed.
- The
ambient-air supply line and the heat-discharge line are open. Ambient air is
forcibly pushed across and through the media and out to the common combustion
air supply and finally to the burners.
Fully Automated
Throughout operations, the regenerators run in a timed
sequence of charging and discharging, which provides a constant supply of
heated combustion air that is directed from the common combustion air supply to
each burner. The system is fully automated, and the valve sequencing is
pre-programmed to optimize efficiency. The timing will be variable, depending
on the application. This cycle of transferring waste furnace heat to a common
combustion air supply continues until the combustion air reaches a
pre-programmed maximum allowable temperature. Once this occurs, a thermocouple
feedback control mechanism will cause additional unheated ambient air to mix
with the common combustion-air supply and dilute the superheated combustion
air.
Benefits
The new system provides fuel savings comparable with standard
regeneration systems. However, the elimination of redundant burners,
regenerators and specialized burners significantly reduce the up-front costs. A
more uniform heat is also possible because the common combustion-air supply
allows all of the burners to continuously fire or pulse. The steady supply of
preheated combustion air to all burners also provides more precise temperature
control and is thus adaptable for use in applications that require multiple or
rapid temperature changes. The new system works across any temperature ranges
and allows operators the option of using a number of small firing burners to
maximize control over furnace heat uniformity.
The new system also makes lower-temperature regeneration
economical. In the past, regeneration below a 1200°F furnace temperature
required very expensive self-regenerative burners with the heat-storage media
contained within the burner itself. Self-regenerative burners can fire
continuously, just as is the case with the new system. However, the up-front
cost of self-regenerative burners can be daunting. Also, because the burners
themselves contain the media, they are large and intrude into the heating chamber.
With the new system, smaller and substantially less-expensive burners can be
used, causing minimal intrusion.
Perhaps most
important is the spatial flexibility created by the decoupling of the
regenerators and burners. The system decreases the physical area needed for
installation of a regenerative system and makes the efficiency of regeneration available
in furnace locations where it was previously considered to be unworkable. With
the direct connection between the regenerator boxes and the burners eliminated
and with a reduction in the number of burners required, the placement of the
boxes is unrestricted. The boxes can be stacked in one place, grouped on just
one side of the furnace, installed above the roof or located in reasonable
proximity to the furnace. Adding to the system’s spatial flexibility is the
low-temperature exhaust, which eliminates the need for a raised stack.
