BACKGROUND OF THE INVENTION
The present invention pertains to an oxygen fueled combustion
system. More particularly, the present invention pertains to an oxy-fueled combustion
system in which the production of green-house gases is reduced and in which fossil
fuel consumption is reduced.
Oxygen fueled burner systems are known, however, their
use is quite limited. Oxy-fueled burner systems are generally only used in those
applications in which extremely high flame temperatures are required. For example,
these systems may be used in the glass making industry in order to achieve the temperatures
necessary to melt silica to a fusion temperature. Otherwise, it is commonly accepted
that structural and material limitations dictate the upper temperatures to which
many industrial systems can be subjected. To this end, air fueled or air fired combustion
systems are used in boilers, furnaces and the like throughout most every industrial
application including manufacturing, electric power generating and other process
In particular, air fueled combustion systems or electric
heating systems are used throughout the steel and aluminum making industries, as
well as the power generation industry, and other industries that rely upon carbon
based fuels. In air fueled systems, air which is comprised of about 79% nitrogen
and 21% oxygen, is fed, along with fuel into a furnace. The air fuel mixture is
ignited creating a continuous flame. The flame transfers energy in the form of heat
from the fuel air mixture into the furnace.
In the steel and aluminum industries, air fueled furnaces
and electric furnaces have been used as the primary heat source for creating molten
metals. With respect to air fueled furnaces, it is conventionally accepted that
the energy requirements, balanced against the thermal limitations of the process
equipment, mandate or strongly support the use of these types of combustion systems.
As to the use of electric furnaces in the aluminum industry, again, conventional
wisdom supports this type of energy source to achieve the temperatures necessary
for aluminum processing.
One drawback to the use of air fueled combustion systems,
is that these systems produce NOx and other green-house gases such as carbon dioxide,
sulfur dioxide and the like, as an inherent result of the combustion process. NOx
and other green-house gases are a large contributor to environmental pollution,
including, but not limited to acid rain. As such, the reduction in emission of NOx
and other green-house gases is desirable, and as a result of regulatory restrictions,
emission is greatly limited. To this end, various devices must be installed on these
combustion systems in order to limit and/or reduce the levels of NOx and other green-house
Another drawback with respect to air fueled furnaces is
that much of the energy released from the combustion process is absorbed or used
to heat the gaseous nitrogen present in the air that is fed to the furnace. This
energy is essentially wasted in that the heated nitrogen gas is typically, merely
exhausted from the heat source, e.g., furnace. To this end, much of the energy costs
are directed into the environment, through an off-gas stack or the like. Other drawbacks
of the air fed combustion systems known will be recognized by skilled artisans.
Electric furnaces likewise have their drawbacks. For example,
inherent in these systems as well is the need for a source of electricity that is
available on a continuous basis, essentially without interruption. In that large
amounts of electric power are required to operate electric furnaces, it is typically
necessary to have these electric furnaces located in proximity to electric generating
plants and/or large electrical transmission services. In addition, electric furnaces
require a considerable amount of maintenance to assure that the furnaces are operated
at or near optimum efficiency. Moreover, inherent in the use of electric furnaces
is the inefficiency of converting a fuel into electrical power (most large fossil
fueled power stations that use steam turbines operate at efficiencies of less than
about 40 percent, and generally less than about 30 percent). In addition, these
large fossil fueled stations produce extremely large quantities of NOx and other
For example, in the aluminum processing industry, and more
specifically in the aluminum scrap recovery industry, conventional wisdom is that
flame temperatures in furnaces should be maintained between about 2500°F and
3000°F. This range is thought to achieve a balance between the energy necessary
for providing sufficient heat for melting the scrap aluminum, and maintaining adequate
metal temperatures in the molten bath at about 1450°F. Known furnaces utilize
a design in which flame temperatures typically do not exceed 3000°F to assure
maintaining the structural integrity of these furnaces. That is, it is thought that
exceeding these temperature limits can weaken the support structure of the furnace
thus, possibly resulting in catastrophic accidents. In addition, stack temperatures
for conventional furnaces are generally about 1600°F. Thus, the temperature
differential between the flame and the exhaust is only about 1400°F. This results
in inefficient energy usage for the combustion process.
It is also believed that heat losses and potential damage
to equipment from furnaces in which flame temperatures exceed about 3000°F
far outweigh any operating efficiency that may be achieved by higher flame temperatures.
Thus, again conventional wisdom fully supports the use of air fueled furnaces in
which flame temperatures are at an upper limit of about 3000°F (by flame stoichiometry)
which assures furnace integrity and reduces energy losses.
Accordingly, there exists a need for a combustion system
that provides the advantages of reducing environmental pollution (attributable to
NOx and other green-house gases) while at the same time providing efficient energy
use. Desirably, such a combustion system can be used in a wide variety of industrial
applications, ranging from the power generating/utility industry to chemical processing
industries, metal production and processing and the like. Such a combustion system
can be used in metal, e.g., aluminum, processing applications in which the combustion
system provides increased energy efficiency and pollution reduction. There also
exists a need, specifically in the scrap aluminum processing industry for process
equipment (specifically furnaces) that are designed and configured to withstand
elevated flame temperatures associated with such an efficient combustion system
and to increase energy efficiency and reduce pollution production.
BRIEF SUMMARY OF THE INVENTION
An oxygen fueled combustion system includes a furnace having
a controlled environment, and includes at least one burner. The combustion system
includes an oxygen supply for supplying oxygen having a predetermined purity and
a carbon based fuel supply for supplying a carbon based fuel. The present oxy fuel
combustion system increases the efficiency of fuel consumed (i.e., requires less
fuel), produces zero NOx (other than from fuel-borne sources) and significantly
less other green-house gases.
The oxygen and the carbon based fuel are fed into the furnace
in a stoichiometric proportion to one another to limit an excess of either the oxygen
or the carbon based fuel to less than 5 percent over the stoichiometric proportion.
The combustion of the carbon based fuel provides a flame temperature in excess of
about 4500°F, and an exhaust gas stream from the furnace having a temperature
of not more than about 1100°F.
The combustion system preferably includes a control system
for controlling the supply of carbon based fuel and for controlling the supply of
oxygen to the furnace. In the control system, the supply of fuel follows the supply
of oxygen to the furnace. The supply of oxygen and fuel is controlled by the predetermined
molten aluminum temperature. In this arrangement, a sensor senses the temperature
of the molten aluminum.
The carbon based fuel can be any type of fuel. In one embodiment,
the fuel is a gas, such as natural gas, methane and the like. Alternately, the fuel
is a solid fuel, such as coal or coal dust. Alternately still, the fuel is a liquid
fuel, such a fuel oil, including waste oils.
In one exemplary use, the combustion system is used in
a scrap aluminum recovery system for recovering aluminum from scrap. Such a system
includes a furnace for containing molten aluminum at a predetermined temperature,
that has at least one burner. The recovery system includes an oxygen supply for
supplying oxygen to the furnace through the combustion system. To achieve maximum
efficiency, the oxygen supply has an oxygen purity of at least about 85 percent.
A carbon based fuel supply supplies a carbon based fuel.
The oxygen and the carbon based fuel are fed into the furnace in a stoichiometric
proportion to one another to limit an excess of either the oxygen or the carbon
based fuel to less than 5 percent over the stoichiometric proportion. The combustion
of the carbon based fuel provides a flame temperature in excess of about 4500°F,
and an exhaust gas stream from the furnace having a temperature of not more than
In such a recovery system, the combustion of oxygen and
fuel creates energy that is used for recovering aluminum from the scrap at a rate
of about 1083 BTU per pound of aluminum recovered. The fuel can be a gas, such as
natural gas, or it can be a solid fuel or a liquid fuel.
In the recovery system, heat from the furnace can be recovered
in a waste heat recovery system. The recovered heat can be converted to electrical
In a most preferred system, the combustion system includes
a system for providing oxygen. One such system separates air into oxygen and nitrogen,
such as a cryogenic separation system. Other systems include membrane separation
and the like. Oxygen can also be provided by the separation of water into oxygen
and hydrogen. In such systems, the oxygen can be stored for use as needed. Other
systems are known for oxygen generation/separation.
The oxygen fueled combustion system, generally, can be
used with any furnace that has a controlled environment. That is, with any furnace
that has substantially no in-leakage from an external environment. Such a combustion
system includes an oxygen supply for supplying oxygen having a predetermined purity
and a carbon based fuel supply for supplying a carbon based fuel.
The oxygen in the oxygen supply and the carbon based fuel
are fed into the furnace in a stoichiometric proportion to one another to limit
an excess of either the oxygen or the carbon based fuel to less than 5 percent over
the stoichiometric proportion. In such a furnace, an exhaust gas stream from the
furnace has substantially zero nitrogen-containing combustion produced gaseous compounds.
That is, because there is no nitrogen fed in with the fuel, unless there is fuel-borne
nitrogen, the exhaust gas contains substantially no nitrogen containing combustion
products (i.e., NOx), and significantly lowered levels of other green-house gases.
This combustion system can use any carbon based fuel including
gas, such as natural gas or methane, any solid fuel such as coal or coal dust or
any liquid fuel, such as oil, including waste and refined oils. In such a combustion
system, any nitrogen-containing combustion produced gaseous compounds are formed
from the fuel-borne nitrogen.
A method for recovering aluminum from scrap includes feeding
aluminum scrap into a melting furnace and combusting oxygen and a carbon based fuel
in the furnace. In the combustion of the oxygen and fuel, the oxygen and fuel are
fed into the furnace in a stoichiometric proportion to one another to limit an excess
of either the oxygen or the carbon based fuel to less than 5 percent over the stoichiometric
proportion. The combustion provides a flame temperature in excess of about 4500°F,
and an exhaust gas stream from the furnace having a temperature of not more than
The aluminum is melted in the furnace, contaminant laden
aluminum is removed from the furnace and substantially pure molten aluminum is discharged
from the furnace. The method can include the step of recovering aluminum from the
contaminant laden aluminum, i.e., dross, and charging the recovered aluminum into
The method can include recovering waste heat from the furnace.
The waste heat recovered can be converted to electricity.
A furnace for recovering aluminum from scrap aluminum includes
a bath region for containing molten aluminum at a predetermined temperature, and
at least one burner. An oxygen supply supplies oxygen having a purity of at least
about 85 percent and a carbon based fuel supply supplies fuel, such as natural gas,
coal, oil and the like.
The oxygen in the oxygen supply and the fuel are fed into
the furnace in a stoichiometric proportion to one another to limit an excess of
either the oxygen or the fuel to less than 5 percent over the stoichiometric proportion.
The combustion of the fuel provides a flame temperature in excess of about 4500°F,
and an exhaust gas stream from the furnace has a temperature of not more than about
In one embodiment, the furnace is formed from steel plate,
steel beams and refractory materials. The furnace walls are configured having a
steel beam and plate shell, at least one layer of a crushable insulating material,
at least one layer of a refractory brick, and at least one layer of a castable refractory
material. The furnace floor is configured having a steel beam and plate shell and
at least two layers of refractory material, at least one of the layers being a castable
A salt-less method for separating aluminum from dross-laden
aluminum is also disclosed that includes the steps of introducing the dross-laden
aluminum into a furnace. The furnace has an oxygen fuel combustion system producing
a flame temperature of about 5000°F, and having substantially no excess oxygen.
The dross-laden aluminum melts within the furnace.
An upper portion of the melted dross-laden aluminum is
skimmed to produce a heavily dross-laden product. The heavily dross-laden product
is pressed in a mechanical press to separate the aluminum from the heavily dross-laden
product to produce a concentrated heavily dross-laden product. The method can include
the step of returning the concentrated heavily dross-laden product to the furnace.
Introduction of the dross-laden aluminum into the furnace is carried out in near
direct flame impingement to release the oxides from the dross.
These and other features and advantages of the present
invention will be apparent from the following detailed description, in conjunction
with the appended claims.
BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS
The benefits and advantages of the present invention will
become more readily apparent to those of ordinary skill in the relevant art after
reviewing the following detailed description and accompanying drawings, wherein:
FIG. 1 is an overall flow scheme of an exemplary aluminum
scrap recovery process having a melting furnace with an oxygen fueled combustion
system, in which green-house gas production and fuel consumption are reduced, embodying
the principles of the present invention;
FIG. 2 is an overall flow scheme of a dross processing
operation continued from FIG. 1 having a recovery furnace having an oxygen fueled
combustion system embodying the principles of the present invention;
FIG. 3 is an exemplary natural gas supply train and oxygen
supply train for use with the oxygen fueled combustion system;
FIG. 4 is an overall plant scheme showing the oxygen supply,
from a cryogenic plant, and flow to the furnaces, and further illustrating an exemplary
waste heat recovery plant;
FIG. 5 is a schematic illustration of an aluminum melting
furnace for use with an oxygen fueled combustion system in accordance with the principles
of the present invention;
FIG. 6 is a side view of the furnace of FIG. 5;
FIG. 7 is a front view of the melting furnace of FIG. 6;
FIGS. 8 and 9 are partial cross-sectional illustrations
of a side wall and the floor of the furnace, respectively;
FIG.10 illustrates a burner assembly for use with the oxygen
fueled combustion system;
FIG. 11 is a schematic illustrations of an exemplary control
system for use with an oxygen fueled combustion system of the present invention
FIG. 12 is a schematic view of an exemplary power boiler
or furnace front wall illustrating a burner and an air feed arrangement, and showing
the incorporation of an oxy fuel combustion system therein embodying the principles
of the present invention; and
FIG. 13 is a schematic illustration of a waste incinerator
showing the incorporation therein of an oxy fuel combustion system embodying the
principles of the present invention.
DETAILED DESCRIPTION OF THE INVENTION
While the present invention is susceptible of embodiment
in various forms, there is shown in the drawings and will hereinafter be described
a presently preferred embodiment with the understanding that the present disclosure
is to be considered an exemplification of the invention and is not intended to limit
the invention to the specific embodiment illustrated. It should be further understood
that the title of this section of this specification, namely, "Detailed Description
Of The Invention", relates to a requirement of the United States Patent Office,
and does not imply, nor should be inferred to limit the subject matter disclosed
An oxy-fuel combustion system uses essentially pure oxygen,
in combination with a fuel source to produce heat, by flame production (i.e., combustion),
in an efficient, environmentally non-adverse manner. Oxygen, which is supplied by
an oxidizing agent, in concentrations of about 85 percent to about 99+ percent can
be used, however, it is preferable to have oxygen concentration (i.e., oxygen supply
purity) as high as possible. In such a system, high-purity oxygen is fed, along
with the fuel source in stoichiometric proportions, into a burner in a furnace.
The oxygen and fuel is ignited to release the energy stored in the fuel. For purposes
of the present disclosure, reference to furnace is to be broadly interpreted to
include any industrial or commercial heat generator that combusts fossil (carbon-based)
fuels. In a preferred system, oxygen concentration or purity is as high as practicable
to reduce green-house gas production.
It is contemplated that essentially any fuel source can
be used. For example, in a present application, as will be described in more detail
below, oxygen is fed along with natural gas, for combustion in a furnace. Other
fuel sources contemplated include oils including refined as well as waste oils,
wood, coal, coal dust, refuse (garbage waste) and the like. Those skilled in the
art will recognize the myriad fuel sources that can be used with the present oxy-fuel
The present system departs from conventional processes
in two principal areas. First, conventional combustion processes use air (as an
oxidizing agent to supply oxygen), rather than essentially pure oxygen, for combustion.
The oxygen component of air (about 21 percent) is used in combustion, while the
remaining components (essentially nitrogen) are heated in and exhausted from the
furnace. Second, the present process uses oxygen in a stoichiometric proportion
to the fuel. That is, only enough oxygen is fed in proportion to the fuel to assure
complete combustion of the fuel. Thus, no "excess" oxygen is fed into the combustion
Many advantages and benefits are achieved using the present
combustion system. It has been observed, as will be described below, that fuel consumption,
to produce an equivalent amount of power or heat is reduced, in certain applications,
by as much as 70 percent. Significantly, this can provide for a tremendous reduction
in the amount of pollution that results. Again, in certain applications, the emission
of NOx can be reduced to essentially zero, and the emission of other green-house
gases reduced by as much as about 70 percent over conventional air-fueled combustion
An Exemplary Scrap Aluminum Recovery Process
In one specific use, the oxygen fueled combustion system
(also referred to as oxy-fuel or oxy-fueled) is used in a scrap aluminum recovery
plant 10. A flow process for an exemplary plant is illustrated in FIGS. 1-2. Scrap
aluminum, generally indicated at 12 is fed into a melting furnace 14, and is liquefied.
The plant 10 can include multiple furnaces operated in parallel 14, one of which
is illustrated. The liquefied or molten aluminum is drawn from the melting furnace
14 and is fed into a smaller holding furnace or holder 16. The holding furnace 16
is also an oxy-fueled furnace. The molten aluminum is drawn from the melting furnace
14 as necessary, to maintain a certain, predetermined level in the holding furnace
16. This can result in continuously drawing down from the melting furnace 14 or
drawing down in "batches" as required.
In the holding furnace 16, chlorine and nitrogen (as gas),
as indicated at 18 and 20, respectively, are fed into the holding furnace 16 to
facilitate drawing the impurities from the molten aluminum. The chlorine and nitrogen
function as a gaseous fluxing agent to draw the impurities from the aluminum. This
can also be carried out in the melting furnaces 14 to increase cleaning of oily
and dirty scrap. Other contemplated fluxing agents include gaseous argon hexafluoride.
The holder 16 is actively heated and operates at a molten metal temperature of about
1300°F. The air temperature in the holder 16 is slightly higher.
The molten aluminum is then filtered. Presently, a bag-type
particulate filter 22 is used. However, other types of filters are known and can
be used. The filtered, molten aluminum is then fed through a degasser 24.
In the degasser 24, a fluxing agent, such as an inert gas
(again, nitrogen is used, as indicated at 26) is fed into the molten aluminum. The
molten aluminum is agitated, such as by a mechanical stirrer 28 and the fluxing
agent 26 bubbles up through the molten aluminum to draw impurities (e.g., oxides)
from the aluminum.
The molten aluminum is then fed into an in-line caster
30. In the caster 30, the aluminum is cast into continuous plate. The cast thickness
can be any where from .010 inches up to .750 inches or more. The aluminum can then
be rolled into a coil, as indicated at 32, for use or further processing. In a present
method, the aluminum proceeds from the caster 30 through a pair of hot milling machines
34 where the plate is milled to a final thickness or gauge, presently about 0.082
inches (82 mils) and is then rolled to form the coil 32. Those skilled in the art
will understand and appreciate the various end forming and finishing processes that
can be carried out on the metal. All such forming and finishing processes are within
the scope and spirit of the present invention.
Returning to the melting furnace 14, as stated above, it
is an oxy-fuel furnace. It is fed with a carbon based fuel, such as natural gas,
in stoichiometric proportion with oxygen. This is unlike known furnaces which use
fuel and air mixtures. The fuel/air mixtures feed nitrogen as well as oxygen into
the furnace to support the combustion process. This results in the production of
undesirable NOx off-gases. In addition, the nitrogen also absorbs energy from the
molten aluminum, thus reducing the overall efficiency of the process. That is, because
the percentage of nitrogen in air is so great, a large amount of energy goes into
heating the nitrogen rather than the aluminum.
The oxygen/natural gas proportions in the present melting
and holding furnaces 14, 16 are about 2.36:1. This ratio will vary depending upon
the purity of the oxygen supply and the nature of the fuel. For example, under perfect
conditions of 100 percent pure oxygen, the ratio is theoretically calculated to
be 2.056:1. However, the oxygen supply can have up to about 15 percent non-oxygen
constituents and natural gas is not always 100 percent pure. As such, those skilled
in the art will appreciate and understand that the ratios may vary slightly, but
the basis for calculating the ratios, that is stoichiometric proportions of fuel
and oxygen, remains true.
This proportion of oxygen to fuel provides a number of
advantages. First, this stoichiometry provides complete combustion of the fuel,
thus resulting in less carbon monoxide, NOx and other noxious off gas emissions
(other green-house gases generally). In addition, the controlled oxygen proportions
also reduce the amount of oxides present in the molten aluminum. This, in turn,
provides a higher quality final aluminum product, and less processing to remove
these undesirable oxide contaminants.
It is important to note that accurately controlling the
ratio of oxygen to fuel assures complete burn of the fuel. This is in stark contrast
to, for example, fossil fueled power plants (e.g., utility power plants), that struggle
with LOI (loss on ignition). Essentially, LOI equates to an incomplete burn of the
fuel. In the present method, on the other hand, substantially pure oxygen, in tightly
controlled stoichiometric proportion to the fuel, minimizes and possibly eliminates
these losses. In addition, in the present method, the only theoretical NOx available
is from fuel-borne NOx, rather than that which could otherwise result from combustion
using air. Thus, NOx, if not completely eliminated is reduced to an insignificant
amount compared to conventional combustion systems.
Oxides in aluminum come from two major sources. First,
from the combustion process; second, from oxides that reside in the aluminum. This
is particularly so with poor grade scrap or primary metal. The present process takes
into consideration both of these sources of oxides and reduces or eliminates their
impact on the final aluminum product. First, the present process reduces oxides
that could form as a result of the oxygen fed for the combustion of the fuel. This
is achieved by tightly controlling oxygen feed to only that necessary by stoichiometric
proportion for complete combustion of the fuel.
The present process takes into consideration the second
sources of oxides (that residing in the aluminum), and removes these oxides by virtue
of the degassing and filtering processes. The benefits are two fold. The first is
that less byproduct in the form of dross D is formed; second, the quality of the
finished product is greatly enhanced.
It has also been found that using a fuel/oxygen mixture
(again, rather than a fuel/air mixture) results in higher flame temperatures in
the melting furnace. Using oxy-fuel, flame temperatures in the furnace of about
5000°F are achieved. This is higher, by about 1500°F to 2000°F, than
other, known furnaces. It has also been observed that using oxy-fuel, in conjunction
with these higher flame temperatures, results in an extremely highly efficient process.
In one measure of efficiency, the energy required (in BTU) per pound of processed
aluminum is measured. In a known process, the energy required is about 3620 BTU/lb
of processed product. In the present process and apparatus, the energy requirements
are considerably less, about 1083 BTU/lb of metal processed. It should also be noted
that although the "fuel" discussed above in reference to the present method is natural
gas, any organic based fuel, such as oil (including waste oil), coal, coal dust
and the like can be used.
For purposes of understanding the thermodynamics of the
process, the theoretical energy required to melt a pound of aluminum is 504 BTUs.
However, because specific process inefficiencies are inherent, the actual energy
required was found to be about 3620 BTU/lb when using an air fired combustion system.
These inefficiencies include, for example, actual processing periods being less
than the actual time that the furnace is "fired", and other downstream process changes,
such as caster width increases or decreases. In addition, other "losses" such as
stack (heat) losses, and heat losses through the furnace walls, add to this energy
Moreover, the value of 1083 BTU/1b is an average energy
requirement, even taking into account these "losses". It has been found that when
the process is running at a high efficiency rate, that is when aluminum is processed
almost continuously, rather than keeping the furnace "fired" without processing,
the "average" energy requirement can be reduced to about 750BTU/1b to 900BTU/1b.
The Melting Furnace
A present melting furnace 14 is constructed primarily of
steel and refractory materials. Referring to FIGS. 5-9, the furnace shell 42 has
outside dimensions of about 20 feet in width by 40 feet in length by 12 feet in
height. The steel shell structure 42 is formed from plates and beams. Plates and
beams will be identified through as 44 and 46, respectively, for the furnace shell
42 structure, except as indicated. The floor 48 is fabricated from one-inch thick
plate 44 steel that is welded together. Each weld is above a beam 46 to assure the
integrity of the furnace shell 42.
Additional beams 46 are provided for furnace floor 48 support.
Each beam 46 provides an 8 inch wide flange about every 18 inches on center. All
of the beams 46 (exclusive of the joining beams which are completely seam welded)
are stitched to the bottom plate 50. This permits "growth" in the steel due to thermal
expansion during heating.
The beams 46 provide support and rigidity to the furnace
bottom 52. The beams 46 maintain the furnace 14 rigid to reduce flexing during installation
of the refractory and long-term use. The beams 46 also provide support so that during
operation of the furnace 14 the mechanical loading on the refractory materials is
minimized. The beams 46 also elevate the furnace bottom 52 from the floor on which
the furnace 14 is mounted. This allows heat, which builds up under the furnace 14,
The furnace side walls 54 are likewise made of a steel
plate and beam construction. Two wall regions are recognized, above metal line and
below metal line. This distinction is made for both strength and thermal value considerations.
Below metal line, the plate is % inch thick. Above metal
line, the plate is 5/8 inch thick. In the present furnace, the first eight feet
are considered (for design purposes) below metal line and the upper four feet are
considered (for design purposes) above metal line.
Beams 46 are used to support the side walls 54 of the furnace
14. The beams 46 are set on 18 inch centerlines running vertically along the furnace
14. Horizontal beams 46 are placed at 18 inch centers below metal line and 24 inch
centers above metal line. Although the metal line in the furnace 14 varies, it is,
for design considerations, the highest level of metal that will be in the furnace
14 during normal operation. Additional factors may be considered, in which, for
example, the metal line can be assumed to be nine inches above the maximum fill
line of the furnace 14.
The roof 56 of the furnace 14 is a hanging refractory design.
Beams 46 are on 18 inch centers along the width of the furnace 14. Additional beams
46 are welded to beams extending across the width, which additional beams are oriented
along the length of the furnace 14. Clips are mounted to the beams, to which precast
refractory blocks are mounted.
The furnace 14 has two main doors 58 on the furnace side
54. The doors 58 are used during operation for skimming or cleaning the main furnace
heat chamber or bath area 60 and for main furnace chamber 60 charging. Dross D (the
contaminant slag that forms of the surface of molten aluminum) builds up inside
the furnace 14 and must be cleaned out at least once a day to maintain heat transfer
rates. The dross D is removed by opening the doors 58 and skimming the surface of
the molten metal pool.
Although during typical operation, metal or scrap is placed
in the charge well 62, and is subsequently melted and transferred to the furnace
heat chamber 60, some types of scrap, such as sows or ingot, are better placed directly
in the main heat chamber 60. The doors 58 can be opened to transfer these types
of loads to the heat chamber 60.
The doors 60 are of steel and refractory construction.
The doors 60 are hung on a mechanical pulley system (not shown) and are protected
by safety chains to prevent them from falling to the ground in the event that the
pulley system fails. Powered winches are used to operate the doors. The doors 60
are hung from a common cross member, which is supported from the side 54 of the
The main charge well 62 is located on the front 64 of the
furnace 14. The well 62 is partitioned from the furnace heat chamber 60 and is partitioned
into two areas: a charging area 66; and a circulation pump area 68. A circulation
pump 70 circulates metal from the hot pool of molten aluminum in the main chamber
60 to the scrap charging area 62.
There are three openings, indicated at 72, 74 and 76, between
the chambers 60, 66 and 68. The first opening 72 is in the partition between the
main chamber 60 and the pump well 68. The second opening 74 is in the partition
between the pump well 68 and the scrap charging area 66. The third opening 76 is
in the partition between the charge well 66 and the main heat chamber 60.
All of the openings 72, 74 and 76 are about one foot below
the physical or actual metal line of the furnace 14. The openings 72, 74 and 76
are below metal line to maintain the heat inside the main chamber 60, and to prevent
the flow of oxides between the partitioned areas of the furnace 14 and to maintain
the furnace airtight (i.e., maintain a controlled environment within the furnace
14). The pump 70 is located in an elevated area to prevent excessive furnace garbage,
rocks and dross from accumulating in and around the pump 70.
An exhaust hood 78 is positioned above the charge chamber
66. The hood 78 is fabricated from steel and is mounted on beams 46 similar to those
from which the side walls 54 are fabricated. The beams 46 are positioned on a plate
that covers the side wall of the well, essentially capping it off. The hood 78 vents
the main furnace chamber 60 through a stack 80 (see FIG. 4). The stack 80 exhausts
gases from the furnace 14 and can be closed off to maintain pressure in the furnace
Exhaust gases exit the furnace 14 and flow to a baghouse
82 (FIG. 4). The baghouse 82 is used primarily for collection of unburned carbon
from paints, oils, solvents and the like inherent in scrap aluminum processing.
The furnace 14 includes four oxy-fuel burners 84. The burners
84 are installed on a side wall 54 of the furnace 14, opposite the doors 58. Steel
is constructed surrounding the burners 84 to allow for mounting the burners 84 and
maintaining the surrounding wall rigid.
The furnace 14 is lined with refractory materials. The
floor 48 is fabricated from two different refractory materials. The first material
86 is a poured slab, about six inches thick, of a high strength, castable refractory,
such as AP Green KS-4, that forms a sub hearth. A floor material 88 is poured above
the sub hearth 86 in monolithic fashion having a thickness of about thirteen to
fourteen inches. The floor material 88 is an AP Green 70AR refractory. It is a 70
percent alumina, aluminum resistant castable refractory.
The walls 54, 64 and 65 are fabricated from two layers
of insulation 90 followed by the 70 AR castable or monolithic, phosphate bonded
85 percent alumina (MONO P85) plastic ramming refractory 92. The alumina content
of this material is 85 percent. The backing insulation 90 is insulating board, about
two inches thick in the side walls 54 of the furnace and about three inches thick
on the front and rear walls 64, 65 of the furnace. The difference in insulation
90 thickness is to accommodate thermal expansion of the furnace 14. The furnace
walls 54, 64 and 65 will grow about 1/8 inch per linear foot. Thus, the furnace
14 will grow (along the 40 foot length) a total of about 5 inches. In that there
is six inches of backing insulation 90 (each the front and rear has three inches),
the insulation 90 will crush and allow for growth in the furnace walls 54, 64 and
65 without damaging the furnace shell 42.
Insulating brick 94 is positioned between the crushable
insulation board 90 and the cast refractory 92. The roof 56 is fabricated from 70
percent alumina castable refractory. The material is poured into six roof sections.
Each door 58 frame is fabricated from 70 percent alumina AR refractory.
The furnace 14 has two sets of tap out blocks (not shown).
The first set is positioned on the bottom 52 of the furnace and serve as drain blocks.
A second set of blocks is positioned sixteen inches from the floor of the furnace
and serves as a transfer set of blocks. The transfer blocks are set on the outside
of the furnace for ease of replacement. The inside of the furnace is formed and
the blocks are set on the outside and keyed in with a plastic ram.
There are two ramps (not shown) in the furnace, one at
each of the main charge doors 58. The ramps are used for deslagging or skimming
dross D from the molten metal and for allowing scrap aluminum to slide into the
furnace. The ramps are composed of two materials. The base is a low grade aluminum
resistant brick, stacked to form a ramp. The brick is covered with a castable refractory
(about 18 inches thick), such as the 70AR material. The ramp extends from the edge
of the sill into the furnace.
The wall 96 that separates the main furnace chamber 60
and the charge well 62 is about 22 inches thick and is formed from 70AR material.
The wall 96 is cast as a single monolithic structure.
The furnace 14 can operate in several modes from empty
to holding and maintaining molten aluminum. When the furnace 14 is at peak operation
it is about 80 percent to 90 percent full. The molten metal is at about 1400°F
and the air temperature in the furnace is about 1800°F. The stack (exhaust)
temperature is about 1000°F. Air temperature is measured by a thermocouple
98 in the upper side wall 54 of the furnace 14. Metal temperature is measured at
the base of the circulating pump 70.
Scrap is charged or introduced to the furnace in the charge
well 62 in increments of about 3,000 pounds. It will be understood that the size
or weight of the introduced scrap will vary depending upon the size and capacity
of the furnace 14.
Molten metal from the main chamber 60 is pumped onto the
cool metal charge by the circulation pump 70. The molten metal transfers heat, by
conductivity, to the cold metal charge. The charge metal rapidly heats and melts.
The primary mode of heat transfer to the charged aluminum
is by conduction. The large heat sink provided by the full furnace enhances this
effective method of heat transfer. When the furnace is 80 percent to 90 percent
of capacity there is about 220,000 pounds of molten aluminum at about 1400°F.
When scrap is charged into the furnace 14 the bath acts as a heat sink and provides
the necessary energy for heat transfer to the charged metal. This is true regardless
of the dimensions and capacity of the furnace, as adapted to the present oxy-fuel
combustion system. The circulating pump 70 assists melt of the scrap by providing
hot molten metal to the charge well 62 from the main furnace chamber 60. In addition,
by circulating the molten metal, heat stratification throughout the furnace 14 is
It has been found that by pumping or circulating the molten
metal, the temperature differential between the top and the bottom of the furnace
14 (a height difference of about 42 inches) is only a few degrees Fahrenheit. Thus,
the furnace 14 acts as a stable heat sink to provide a consistent heat source for
conduction heat transfer to the charge metal.
Heat is input to the furnace 14 by the burners 84. It is
believed that the principal mode of heat transfer to the furnace 14 is radiation,
with some convective heat transfer. Because of the high flame temperatures, the
oxy fuel combustion system provides efficient radiative heat transfer. The geometry
of the furnace 14 is further designed to increase the heat transfer rate by maximizing
the metal surface area over which heat transfer from the flame to the metal occurs.
In addition, the refractory materials above the metal line
are made of a high alumina content material. These materials reflect the heat from
the burners back into the molten metal. This is in contrast to conventional furnace
designs which, rather than reflecting heat back into the molten metal pool, permit
much of the heat to escape from the furnace.
For example, traditional furnaces use refractories that
have a lower alumina content and a higher insulation value on the upper side walls.
The present design, on the other hand, uses higher alumina content refractories
in order to reflect more of the radiative heat from the burners 84 to the bath area
60. Again, this is contrary to conventional furnace design. In traditional furnaces
the lower side walls (defined as below metal line) use higher alumina refractories
for strength. In contrast, the present design uses a lower alumina castable refractory,
which is more advanced and has a higher insulating value. In a sense the present
design goes completely against the traditional application of refractories.
Moreover, because there is no nitrogen fed to the furnace
14 (other than fuel-borne nitrogen) the volume of hot gases (e.g., exhaust) going
through the furnace 14 is very low. Advantageously, this increases the residence
time of the gases in the furnace 14 providing additional opportunity for heat transfer
to the molten metal. Convective heat transfer, while relatively low, is more efficient
than in conventional furnaces. In that the hot gases in the present furnace 14 approach
5000°F and have a relatively long residence time, much of the heat is removed
prior to exhaust.
A present furnace 14 operates at an energy input required
to melt of about 1083 BTU per pound. The maximum heat input to the furnace 14 is
about 40 million BTU (40 MMBTU) per hour, and typical heat input is about 10 to
12 MMBTU per hour. The heat input will, of course, depend upon the scrap being melted
and the production requirements. The furnace is capable of melting up to 40,000
pounds per hour.
The Combustion System
The combustion system, indicated in FIG. 3, generally at
100, is a dual combustion train that operates on a fuel, such as natural gas, fuel
oil, waste oil, coal (pulverized, dust and liquefied), and an oxygen source. The
system is designed as two complete combustion systems to facilitate maintenance,
as well as to conserve energy during low use periods. One oxygen train 102 and one
exemplary natural gas fuel train 104 are shown in FIG. 3.
The combustion system 100 is controlled by a control system
(illustrated in FIG. 11, indicated generally at 120) that includes a central processing
unit ("CPU") 106 that monitors all data inputs from metal temperature, air temperature,
fuel and oxygen flow, and provides an operator interface. Each combustion train
can be operated individually or in tandem based on operating conditions and requirements.
The main process input variable used to control the combustion
system 100 is the metal bath temperature as measured by a thermocouple 108. Alternative
process input variables include signals from one of several air temperature sensors
98, 110. The control scheme includes inputs from thermocouples (type K) located
in the furnace upper wall, exhaust stack and furnace roof, indicated generally as
inputs 112. The primary thermocouple 108 is located in the molten metal bath are
60. The air thermocouples 112 are sheathed with alumina or like materials to protect
the measuring element from the atmosphere. The bath thermocouple 108 is protected
from molten metal by a ceramic sheath that is resistant to heat and to the corrosive
conditions found in molten metal. The bath thermocouple 108 is configured to signal
initiation of the burner system only when the metal bath temperature falls below
a preset level.
The stack thermocouple or the roof thermocouple 116 is
designed for over temperature protection. This thermocouple 116 is connected to
an over-temperature circuit that shuts down the combustion trains 102, 104 to protect
the refractory and furnace 14 structure in the event that an over temperature limit
The upper wall thermocouple 98 is primarily used to monitor
the furnace 14 air temperature. It can also be used to operate the furnace 14 in
the absence of the molten bath thermocouple 108. The upper wall thermocouple 112
is also used as the process input variable when metal is first being charged in
the furnace 14 or when the level of molten metal drops below the molten bath thermocouple
An operator has full control over individual temperature
set points. A control panel 118 includes temperature indicators for all of the thermocouples
92, 108, 110, 112, 114, 116. The operator can adjust each thermocouple set point
until operation limits are achieved. The operational set point limits can be internally
set within the CPU so that any desired temperature range can be established.
The combustion system control system 120 is configured
in two parts. The first part 122 includes hard wired safety devices, such as relays,
limit switches and the like, as will be recognized by those skilled in the art.
These include all gas pressure switches, shut off and blocking valves, and flame
detectors. The second part 124 of the control system 120 is monitoring and automatic
control functions carried out by the CPU 106.
The gas trains 104 are configured in pairs so that one
train can be in service while the other is out of service for, for example, maintenance
or low-load/use periods. Each gas train 104 is appropriately sized vis-à-vis
oxygen flow requirements. Each gas train 104 commences at a ball-type shut off valve
130. Piping 132 routes the gas through a strainer 134 to remove any debris present
in the line. A gas pilot line 136 extends from the piping 132 after the strainer
A backpressure regulator 138 is used to lower the header
pressure. Presently, the oxygen pressure is set at about 18 pounds per square inch
(psig). A shut off valve 140 and safety valves 142 follow in line. A differential
pressure flow meter 144 is located downstream of the safety valves 142. The flow
meter 144 measures the temperature and differential pressure of the gas as it flows
through an orifice 146. A present flow meter 144 is a Rosemount model 3095 differential
pressure flow meter.
Through these measurements a flow rate is determined and
a signal is transmitted to the control system 120. A control valve 148 is in line
following the flow meter 144. In a present arrangement, a modulating control valve
is used that receives an output signal from the control system 120. The valve 148
transmits a signal to the control system 120, and specifically, the CPU 106, indicating
the actual valve 148 position.
The gas train 104 then splits into two separate lines 104a,b
each having a valve 150a,b. The valves 150a,b are used to balance each burner 84
so that the gas flow is evenly distributed.
The oxygen train 102 is similar to the gas train 104, except
that the line sizes and components are larger to accommodate the larger flow rate
of oxygen. An exemplary oxygen train 102 is illustrated in FIG. 3, in which those
components corresponding to fuel train 104 components are indicated by 200 series
Referring to FIG. 10, the burners 84 are a fairly straight
forward design. Each of the four burners 84 includes a main inlet nozzle body 152
that extends into the furnace 14. A fuel gas inlet 154 extends to the main inlet
body 152 external of the furnace wall 54. Oxygen is input to the main inlet nozzle
body 152 and mixes with the fuel gas. An igniter (not shown) extends through a central
opening 156 in the main inlet body 152. The igniter provides a spark for ignition
of the fuel/oxygen mixture.
Operation of the combustion system 100 is readily carried
out by a combination of operated initiated action and automatic control by the CPU
106. Power is provided to the system controls which enables the CPU 106 and the
hard-wired safeties portion 122 of the control system120. The CPU 106 initiates
communication with the control valves, thermocouples, and relays that are part of
the hard-wired safeties portion 122. The gas and oxygen pressure switches are of
a dual hi/low switch design. The low-pressure switch is a normally closed signal
while the high-pressure side is a normally open signal. The CPU 106 determines whether
a the proper signal is present and allows the program to continue. If an improper
signal is recognized, audible and visual alarms are actuated. The control scheme
also monitors whether the gas and oxygen control valves 148, 248 are in the "low-fire"
position. If the control valves 148, 248 are in the proper position, a signal is
transmitted that allows the control system 120 to continue the startup procedure.
An over-temperature signal must also be clear to allow the system 120 to continue
through the start up procedure.
When all of the startup conditions are met, a nitrogen
purge cycle is initiated. Nitrogen is used to purge the furnace 14 of any combustible
gases that may be remaining in the furnace 14. The nitrogen purge is timed so that
the volume of nitrogen through the furnace 14 is about 2.5 times the volume of the
After the purge is complete, one or both of the combustion
trains is started. A control switch places either a pair of burners or all of the
burners 84 into operation. A flame controller opens the pilot solenoids. The pilot
solenoids are normally closed, however, upon starting, the solenoids are opened
and gas and oxygen flow through a pilot assembly.
At the tip of the pilot assembly the gases mix and are
ignited by a spark emitted controlled by the flame controller. Upon ignition, a
flame detector 126 detects the presence or absence of flame and transmits a signal
to the control system 120. Once a flame is detected, the control system 120 opens
the main blocking valves for both the gas and oxygen.
The main fuel and oxygen shut off valves 140, 240 operate
independently. The safety valves 142, 242 are configured such that if the gas valve
140 does not open, the safety valves 142, 242 do not open. When the main gas valve
140 opens, the gas and oxygen safety valves open 142, 242. With all of the main
valves open, a control relay is energized as well as an indicator light for each
gas train on the control panel 118. A pilot timer remains energized for a preset
time period, about 30 seconds. Once the preset time duration has elapsed, the pilot
circuit is de-energized and the normally closed solenoid valves are de-energized,
isolating the pilot assemblies and the pilot indicator light for each burner train.
The flame detectors 126 continuously monitor the flame.
Upon loss of flame indication, an alarm signal is transmitted to the CPU 106 and
the control circuit isolates the gas and oxygen shut off valves 140, 240 and blocking
valves 142, 242.
Once the pilots are de-energized, furnace automatic operation
is assumed by the control system 120. While the system 120 is set to "low fire",
the oxygen control valves 248 are maintained in the closed position regardless of
process and set point values. The gas control valves 148 are not limited in their
range since gas flow follows oxygen flow. The control system 120 maintains the gas
at the preset ratio.
When operating in the automatic mode, the control system
120 responds to deviations from the process and set point values. Furnace temperature
is monitored and matched to the temperature set point. When the process temperature
deviates from the set point temperature, an error signal is generated, and the control
system 120 transmits a signal to the oxygen control valve 248. The gas control valve
148 is also controlled by the control system 120; the set point variable follows
the (stoichiometrically correlated) flow rate of oxygen as established by the oxygen
flow meter. The control system 120 is configured to limit the control valves 148,
248 that, in turn, limit the output power of the burners 84.
The combustion system 100, and specifically the control
system 120 can be configured to meet any desired application for and in any industry
that relies on carbon based fuels. For example, in the present scrap aluminum processing
plant 10, there are three applications or uses of the oxygen fueled combustion system
100. The first is for melting aluminum in a high production environment (i.e., in
the melting furnace 14). Second, the system 100 is present in the holding furnace
16 primarily for steady state temperature and alloy mixing of the molten aluminum.
The last application is in a dross-melting furnace 166 in which high temperature
burners are used to release the metal units (aluminum which can be recovered for
production) from the dross D (melt byproduct) by thermal shock. In each use, the
burners are installed for energy conservation and environmental reasons.
Applications of the present combustion system 100 vary
by thermal output (measured in maximum MMBTU per hour), size and orientation of
the burners 84, as well as the temperatures at which the furnaces 14, 16, 166 are
designed to operate. Those skilled in the art will recognize that mechanical differences
(e.g., line sizes and the like) are needed to accommodate these differing needs,
and that the specific programming of the control system 120 and CPU 106 may vary.
The present combustion system 100 provides a number of
advantages over known and presently used combustion systems. For example, it has
been shown through operation that there is considerable energy savings using the
present combustion system 100. The oxy-fuel burners 84 operate at a much higher
temperature than conventional furnaces. Thus, there is an observed increase in the
heat available for melt (in other industrial applications, this increased heat can
be made available for, for example, steam generation , refuse incineration and the
like). This provides a reduction in the amount of fuel required to operate the furnaces
14, 16, 166. In practice of the present invention, it has been observed that the
average (and estimated) thermal input required per pound of aluminum melted is decreased
from about 3620 BTU per pound (in a conventional furnace) to about 1083 BTU per
pound in the melting furnace 14. This is a decrease of about 70 percent. In addition,
the fuel needed to maintain temperature in the holding furnace 16 has been shown
to be about one-half of that of a conventional furnace.
It is believed that the fuel savings is attributed to three
principal factors. First, the increased heat of the combustion system 100 permits
complete burn of all fuel without excess oxygen. Second, being held to theory, it
is believed that the combustion system 100 operates within a radiative (or radiant)
heat transfer zone, with some heat transfer by conduction.
The system 100 is designed to take advantage of the radiant
heat transfer within the furnaces 14, 16, 166 to transfer heat effectively to the
metal baths. Third, because there is no nitrogen in the combustion process, the
amount of gas flowing through the furnaces 14, 16, 166 is low. Thus, an increased
residence time of the hot gases permits the release of a larger proportion of energy
(in the form of heat) prior to exhaust from the furnaces 14, 16, 166.
Typical exhaust gas volume is fractional of that of conventional
furnaces. In that there is about 80 percent less gases (essentially the nitrogen
component of air) in an oxy-fueled furnace, combustion efficiency is greatly increased.
In conventional furnaces, the nitrogen component of air absorbs much of the energy
(again, in the form of heat) from the melt. In the present combustion system 100,
oxygen (rather than air) and fuel are fed to the furnaces 14, 16, 166 and burned
in a stochiometric ratio. This is carried out without excess oxygen. Thus, there
is no energy absorbed by non-combustion related materials e.g., excess oxygen or
The present combustion system 100 also provides for increased
production. When installed as part of a melting furnace, the melting capacity or
throughput of the furnace will be increased. This again is attributed to the rapid
and effective heat transfer in the furnace 14. As new metal is introduced into the
furnace 14, the combustion system 100 responds rapidly to provide heat to melt the
fed metal and to maintain the heat (temperature) of the molten metal in the pool
60 at the set point temperature. It has been found that aluminum accepts heat very
efficiently from a radiative heat source.
Perhaps most importantly, is the reduced environmental
impact of the present combustion system 100, compared to presently known and used
combustion systems. The present system 100 advantageously uses no nitrogen (from
air) in the combustion process. Typically, NOx production occurs in a furnace as
a reaction product of the heated air that is fed by the combustion system. However,
in that the present system 100 uses oxygen, rather than air, any NOx produced by
the present combustion system is due solely to the amount of elemental nitrogen
that is in the fuel (i.e., fuel-borne nitrogen). In that fuel-borne nitrogen levels
are extremely low (compared to that contributed by air in conventional furnaces),
the NOx levels of the present combustion system are well below any industry standards
and governmental limitations. In addition to reducing NOx production, the production
of other green-house gases, such as carbon monoxide, is also greatly reduced.
In addition, to the reduced environmental impact, the present
oxy fuel combustion system conserves energy because significantly more aluminum
can be processed at considerably less fuel input (any carbon based fuel, including
coal, coal dust, natural gas or oil). As a result of processing with less fuel usage,
conservation of fuel resources is achieved. Essentially, less fuel is used in the
aggregate, as well as on a per pound basis to produce aluminum. This reduces processing
(e.g., fuel) costs, as well as the taxing use of fossil fuels.
As will be recognized by those skilled in the art, the
oxygen requirements for the present combustion system 100 can be quite high. To
this end, although oxygen can be purchased and delivered, and stored for use in
the system, it is more desirable to have an oxygen production facility near or as
part of an oxy fuel combustion system, such as the exemplary scrap aluminum processing
Referring now to FIG. 4, there is shown a cryogenic plant
180 for use with the present combustion system 100. The illustrated, exemplary cryogenic
plant 180 produces 105 tons per day of at least 95 percent purity oxygen and 60,000
standard cubic feet per hour of nitrogen having less then 0.1 part per million oxygen.
The plant 180 includes a 1850 horsepower three-stage compressor 182. The compressed
air, at 71 psig enters a purifier/expander 184. The air exits the expander 184 at
a pressure of 6.9 psig and a temperature of -264°F, and enters a cryogenic
distillation column 186. In the column 186, air is separated (distilled) into gaseous
nitrogen, liquid nitrogen, gaseous oxygen and liquid oxygen. The gaseous oxygen,
indicated generally at 188, is fed directly to the combustion system 100 and the
liquid oxygen, indicated generally at 190, is stored for example in tanks 191, for
later use for in the combustion system 100. The oxygen pressure from the cryogenic
plant 180 may be lower than that required for the combustion system 100. As such,
an oxygen blower 192 is positioned between the oxygen discharge from the column
186 and the combustion system 100 feed to raise the pressure to that need for the
combustion system 100.
The gaseous nitrogen, indicated generally at 194, is fed
to a downstream annealing/stress relieving system (not shown) within the plant 10.
These systems, which use nitrogen to treat aluminum to relieve stresses in the metal
and to anneal the metal, will be recognized by those skilled in the art. In addition,
the nitrogen 194 is used in the degassing units 24. The plant 10 also includes a
back up supply of oxygen and nitrogen 191, 196, respectively, in liquid form in
the event of, for example, maintenance or other situations in which the cryogenic
plant 180 cannot supply the plant requirements. The back-up systems 191, 196 are
configured to automatically supply oxygen and/or nitrogen as required, such as when
the cryogenic plant 180 is off-line. Excess nitrogen can be stored, bottled and
sold. Systems such as these are commercially available from various manufacturers,
such as Praxair, Inc. of Danbury, Connecticut.
The aluminum processing system 10 also takes advantage
of waste heat from the various processes. Specifically, the processing plant 10
can include a waste heat recovery system, indicated generally at 200 in FIG. 4.
Exhaust gas, indicated at 202, from the melting furnace 14 and the holding furnace
16 is directed to one side of a waste heat recovery heat exchanger 204. In that
the exhaust gas 202 is at a temperature of about 1000°F, there is a considerable
amount of energy that can be recovered. In addition, energy can be recovered from
the exhaust above the main furnace bath area 60.
The exhaust gas 202 is directed to the waste heat exchanger
204. A working fluid, indicated at 206, such as pentane, flows through the other
side of the heat exchanger 204 under pressure. It is anticipated that a plate-type
heat exchanger or a plate-and-tube type heat exchanger is best suited for this application.
Those skilled in the art will recognize the various types of working fluids that
can be used for the present waste heat recovery system, as well as the heat exchange
systems that are used with these types of working fluids. All such systems are within
the scope and spirit of the present invention.
The heated fluid 206 is then directed to a vaporizer 208
where the fluid 206 is allowed to expand into vapor. The vapor 206 is directed to
a turbine-generator set 210 to produce electricity. The vapor is then condensed,
in a condenser 212, and returned to the heat exchanger 204. It is anticipated that
sufficient energy to produce about 1.5 to 2.0 megawatts of power in the form of
electricity can be recovered from the exhaust gas 202 from the above-described scrap
processing plant 10.
Although a wide variety of working fluids 206 can be employed
for use in such a waste heat or waster energy recovery system 200, in a presently
contemplated system, pentane is used as the working fluid 206. Such an organic based
system provides a number of advantages over, for example, steam-based systems. It
is anticipated that a pentane-based working fluid 206, in a standard Rankine-cycle
arrangement will allow for variations in vapor supply more readily than a steam-based
system. In that the heat output from the furnaces (melting 14 and holding 16) is
dependent upon metal production, rather than electrical needs, the energy input
to the recovery system 200 is likely to vary and will be the controlling characteristic
for power production. As such, a fluid 206 such as pentane provides the greater
flexibility that is required for such a recovery system 200.
As will be recognized by those skilled in the art, the
electrical power generated can be used to provide some of the power necessary for
the scrap processing plant 10, including the cryogenic plant 180. Power for operating
the plant 10 can be provided by an oxy fueled combustion system employed in an electric
power generating plant (using a furnace or boiler), to generate steam for a steam
turbine-generator set. In such an arrangement, when the power generated exceeds
plant 10 requirements, the excess power can be sold to, for example, a local electric
Referring now to FIG. 2, the contaminants or dross D from
the melting furnace 14 is further processed, separate and apart from the in-line
aluminum recovery in a dross recovery process, indicated generally at 164. The dross
D is removed, as by skimming, from the top of the molten aluminum pool 60 in the
melting furnace 14. The dross D is pressed in a sieve-like bowl 168 by mechanical
means. Pressing pushes the aluminum A from the dross D, through openings 170 in
the bowl 168. The aluminum A that is pressed from the dross D is recovered and is
returned to the melting furnace 14.
The oxide laden dross is fed into the recovery furnace
166 for reheating. The recovery furnace 166 is of a similar design to the melting
furnace 14 in that it uses an oxy-fuel combustion system 100 design. In operation,
however, the recovery furnace 166 "shocks" the dross laden material by using near
direct flame impingement of about 5000°F to release the aluminum metal from
the dross D. The molten bath 172 temperature in the recovery furnace 166 is also
considerably higher, about 1450°F-1500°F, with a furnace air temperature
of about 2000°F-2200°F. In addition, the "shocking" process is carried
out in a highly reduced atmosphere with substantially no excess oxygen within the
furnace 166 (in contrast to conventional furnaces that operate at excess oxygen
levels of about 3 to 5 percent).
The recovery furnace 166 is likewise skimmed and the resulting
dross is pressed. The recovered aluminum A is transferred to the melting furnace
14. The remaining dross D2 is then sent for processing off-site, to a dross processor,
for further aluminum recovery. It has been found that the present process, including
the dross recovery process, provides a significant increase in the recovery of metal.
The dross D2 that is ultimately shipped for further processing is only a fraction
of the original quantity of dross D, thus reducing processing costs and increasing
Importantly, the present dross recovery process 164 is
carried out without the use of salts or any other additives. Rather, thermal shocking
is used to release the metal from the oxides. Known recovery processes use salts
to separate the oxides from the metal. In that the salts remain in the oxides, which
are in turn disposed of, ultimately, the salts are likewise sent for disposal. These
salts can be environmental hazards and/or toxic. As such, the present process 164
is environmentally beneficial in that it eliminates the need for these salts and
thus their disposal.
As to the overall processing scheme 164, again, it has
been found that the present recovery steps (e.g., double pressing with intermediate
reheating) result in aluminum recovery rates that are significantly improved over
those of known processes, depending upon the grade of the scrap. Multi-percent increases
in the amount of metal recovered from the dross D have been achieved.
Other Applications for the Combustion System
As discussed above, it is apparent that increased efficiencies
from the use of oxygen in all continuous processes can be achieved. For example,
power generating plants can increase flame temperature or reduce LOI in boilers
by introducing oxygen to the burning formula (rather than air). This can increase
efficiencies in operation. Essentially, burning of any carbon based fuels can be
enhanced by the introduction of oxygen. The benefits are both economical and environmental.
To date no industry other than glass-making has embraced oxy fuel technology. In
the glass making industry this technology is used not for the efficiencies that
result, but because of the high melting temperature required for the glass production
Nevertheless, use of oxy-fueled combustion systems in all
industrial and power generating applications can provide reduced fuel consumption
with equivalent power output or heat generation. Reduced fuel consumption, along
with efficient use of the fuel (i.e., efficient combustion) provides greatly reduced,
and substantially zero, NOx emissions and significant reductions in the emission
of other green-house gases.
Due to the variety of industrial fuels that can be used,
such as coal, natural gas, various oils (heating and waste oil), wood and other
recycled wastes, along with the various methods, current and proposed, to generate
oxygen, those skilled in the art will recognize the enormous potential, vis-à-vis
industrial applicability, of the present combustion system. Fuel selection can be
made based upon availability, economic factors and environmental concerns. Thus,
no one fuel is specified; rather a myriad, and in fact, all carbon based fuels are
compatible with the present system. In addition, there are many acceptable technologies
for producing oxygen at high purity levels. Such technologies includes cryogenics,
membrane systems, absorption units, hydrolysis and the like. All such fuel uses
and oxygen supplies are within the scope of the present invention. Those skilled
in the art will recognize that the other gases produced, such as hydrogen and nitrogen,
can be stored, bottled and sold.
As discussed in detail above, one application for the present
combustion is scrap aluminum processing or recovery. Other exemplary applications,
as will be discussed below, include industrial power generation boilers and incinerators.
These exemplary applications focus on the flexibility and applicability of this
technology for broad industrial uses.
In general, the use of oxygen fuel fired combustion over
current or traditional air fuel systems offers significant advantages in many areas.
First is the ability to run at precise stoichiometric levels without the hindrance
of nitrogen in the combustion envelope. This allows for greater efficiency of the
fuel usage, while greatly reducing the NOx levels in the burn application. Significantly,
less fuel is required to achieve the same levels of energy output, which in turn,
reduces the overall operating costs. In using less fuel to render the same power
output, a natural reduction in emissions results. Fuel savings and less emissions
are but only two of the benefits provided by the present system.
Steam generators for the production of electricity, e.g.,
by industrial power boilers, are varied but are nevertheless fundamentally dependent
upon their combustion systems to produce steam to turn a turbine-generator set.
The fuels used vary based upon the design of the steam generators. However, all
of the boilers require an oxidizing agent. Using the present oxy fuel combustion
system, high purity oxygen is used as the sole oxidizing agent throughout the boiler
or is used as a supplement to air providing the oxygen for combustion.
The benefits that can be enjoyed by other industrial applications
hold true for the power industry. For example, the use of oxygen within the combustion
zone enhances flame temperature while effectively cutting LOI (loss on ignition)
by providing readily available oxygen for combustion. By increasing flame temperatures,
greater rates of steam generation can be accomplished with the same fuel burn rate.
Conversely, equal power generation or output can be recognized with lower fuel burn
rates. Flame temperature will be dependent upon the concentration of the oxygen
provided for combustion. To this end, with no oxygen supplementation or enrichment
(i.e., pure air for combustion), flame temperatures will be about 3000°F. Referring
to the above discussion, with pure oxygen as the oxidizing agent, the flame temperature
will be about 4500°F to about 5000°F. The anticipated flame temperatures
for varying degrees of oxygen supplementation can be interpolated (it is believed
linearly) between these temperatures.
Oxygen can also be used in conjunction with over-fired
air systems or lox NOx burners to reduce NOx and other green-house gases while ensuring
stable flame at stoichiometry. Typical low NOx burners often increase LOI. This
requires operators to burn more fuel. By adding enriched oxygen to the combustion
process complete burn becomes available for fuel while at stoichiometry without
additional nitrogen present (by additional air input) to create NOx.
It is anticipated that boilers will be designed around
oxygen fueled combustion systems to take full advantage of the benefits of these
systems. It is also anticipated that retrofits or modifications to existing equipment
will also provide many of these benefits both to the operator (e.g., utility) and
to the environment.
For example, FIG. 12 illustrates, schematically, a coal
fired boiler or furnace 300. A wind box 302 is formed at a wall 304 of the furnace
300. A burner 306, through which the coal is introduced into the furnace 300, extends
through the wind box 302. The coal is carried to the furnace 300 by a coal conduit
308. Primary air (as indicated at 310) is supplied to carry the coal (from a pulverizer,
not shown) through the conduit 308 and burner 306 into the furnace 300. Tertiary
air (as indicated at 312) is provided to the coal conduit 308 to assure that the
coal is conveyed to the burner 306.
Secondary air (as indicated at 314) is provided from the
wind box 302 directly into the furnace 300 through registers 316 on the furnace
wall 304. The secondary 314 air is the primary source of air for the combustion
process. In one well recognized and known system for controlling NOx, an over-fired
air system (as indicated at 318) injects air (from the wind box 302), into the furnace
300 over the flame F. The underlying purposes for the over-fired air are two-fold.
First is to provide sufficient oxygen to assure complete combustion of the fuel.
Second is to reduce the flame temperature and thereby reduce the production of NOx.
It is anticipated that the present combustion system can
replace existing combustion systems, in total, or, in the alternative, can be used
to provide an oxygen supplement to the air used for combustion. Specifically, it
is anticipated that high purity oxygen can be used in place of any or all of the
primary 310, secondary 314 and tertiary air 312 that is used in these known combustion
systems. Those skilled in the art will recognize the benefits that can be obtained
using the present oxy fuel combustion system (or as in certain applications oxygen
supplementation system) in power boilers or furnaces that use other fossil fuels,
such as oil or gas.
Use of the present combustion system is also contemplated
for use in connection with industrial waste incinerators. Typical waste incinerators
operate on the basis of resonant time, temperature and excess oxygen. An oxy-fuel
system will allow for greater efficiency in the operation.
Resonant time is dependent upon the physical size of the
heated chamber or stack, and the velocity and volume of gases passing through the
chamber or stack. As nitrogen is taken out of the mix the resonant time naturally
increases because the volume of gas used in the combustion process is less (by about
80 percent). When an incinerator is specifically designed with an oxy-fuel combustion
system, the incinerator requires considerably less capital cost because of the reduced
size that is required.
Typical flame temperatures of oxy-fueled combustion systems
are much higher then air fueled systems. Thus, the efficiency of the burn ultimately
requires less thermal input from the fuel, resulting in less operating costs. One
of the benefits of the oxy-fuel combustion system is the control over excess oxygen
levels that is achieved. In the case of conventional incinerators, excess oxygen
is required to burn the volatile organic carbons (VOCs) and unburned carbon. This
excess oxygen is provided by injecting air into the chamber or stack where the oxygen
(from the air) is used to complete the burn of VOCs and unburned carbon. Although
air provides the necessary excess oxygen, it also permits nitrogen into the chamber.
The excess nitrogen that is introduced (to provide the excess oxygen) results in
increased production of NOx. Additionally, the excess air, overall, results in the
generation of other green-house gases, and further acts to cool the chamber. This
undesirable cooling then requires additional heat from the combustion system to
overcome this cooling effect.
FIG. 13 illustrates, schematically, a typical industrial
furnace 400. Waste (as indicated at 402) is introduced into a stack 404. A burner
406 is fed with air (as indicated at 408) and fuel (as indicated at 410) to produce
a flame F to incinerate the waste 402. A carbon monoxide (CO) monitor 412 is located
above the flame F to determine the level of CO in the exhaust gas. When the level
of CO is too high, additional air is fed to the burner 406. Optionally, air can
be fed into the stack from a location 414 apart from the burner 406 to provide the
There are a number of drawbacks to this method of operation.
As discussed above, the two controlling factors in waste incineration are time and
temperature. That is, higher temperatures and greater resonant times increase the
incineration of the waste. However, the addition of air (to reduce CO levels) increases
the flow rate through the stack 404 thus reducing the resonant time. In addition,
although the increased air flow reduces flame temperatures (which in turn reduces
NOx production), it also introduces high levels of nitrogen, which tends to increase
NOx production and offset the cooling (and reduced NOx production) effect. Moreover,
because of the cooling effect of the air, the efficiency of the incineration process
The present oxy-fuel combustion system, on the other hand,
uses high purity oxygen which permits burning the unburned material without the
production of NOx and other green-house gases and without cooling effects. The present
oxy-fuel system thus affords several advantages over conventional or traditional
incinerator systems. In that the primary duty of an incinerator is to burn VOCs
and other contaminants before they reach the atmosphere, the present combustion
system reduces the fuel used and thus results in reduced production of NOx and other
green-house gases, and a reduced volume of flue gases generally.
In addition, the installation (e.g., capital) and operating
costs of incinerators employing oxygen fueled combustion systems will be greatly
reduced. The capital cost of the incinerator will be reduced because the volume
of gases through the system is expected to be much lower. As provided above, because
the throughput of gas is much less, the overall size of the incinerator can be considerably
less than conventional systems while maintaining the same resonant time. Thus, the
incinerator can be physically smaller to handle the same waste load, and the required
support systems and ancillary equipment and systems can likewise be smaller.
In addition, oxy-fueled combustion systems are generally
considerably more efficient than conventional incinerator systems and require a
fractional amount of the required energy input. The system also lends itself quite
well to incinerator applications in which the fuel is unburned carbon or VOCs. Likewise
since there is no nitrogen present in the flame envelope the development of NOx
is kept to a minimum, relegated to NOx formed from fuel-borne nitrogen only.
The industries described above are only a few exemplary
industries that can benefit from the use of the present oxy fuel combustion system.
Those skilled in the art will recognize the applicability of this system in the
chemical and petro-chemical industries, the power generation industry, plastics
industries, the transportation industry and the like.
Oxy Fuel Combustion - The Benefits and Advantages
The benefits and advantages of oxy fuel combustion will
be appreciated by those skilled in the art. Nevertheless, in an exemplary aluminum
scrap processing facility, using an air-fired furnace outfitted for natural gas,
it was found that the energy required to process or melt one pound of scrap aluminum
(as determined by the cubic feet of natural gas used), was 3,620 BTUs (presented
as 3,620 BTUs/lb). That is, about 3.45 standard cubic feet (SCF) of natural gas
was need to melt each pound of aluminum. The energy requirement of 3,620 BTU is
based upon each SCF of natural gas having a heat content of 1,050 BTUs.
In contrast, using the present oxy fueled combustion system,
it was found that only 1.03 SCF of natural gas (or 1083 BTUs) was needed to melt
each pound of aluminum. Thus, the present oxy fuel combustion system used 1083BTU/3620BTU
or 29.9 percent of the fuel required for an air-fired furnace. This is a reduction
of 1.0 less 0.299 or about 70 percent in the fuel consumption.
Similar though not quite as drastic reductions in fuel
consumption have been observed with an oxy fueled combustion system that uses waste
oil as a fuel. It was found that the heat content of the waste oil fuel need to
melt each pound of aluminum was 1218 BTUs. Thus, the reduction observed with waste
oil was 1218/3620 or 33.6 percent, resulting in a reduction in fuel consumed of
about 66 percent. As such, even before considering the reduction in pollutants produced,
the present oxy fuel combustion system exhibited reductions in fuel consumption
of about 70 percent and 66 percent using natural gas and waste oil, respectively,
over an air-fired, natural gas fired furnace.
Table 1, below illustrates a comparison of the pollutants
produced using an air-fired (gas fueled, shown as "AIR-GAS") combustion system,
an oxy fueled (gas, shown as "OXY-GAS") combustion system and an oxy fueled (waste
oil, shown as "OXY-OIL") combustion system. The pollutants shown are carbon monoxide
(CO), gaseous nitrogen compounds (NOx), particulate matter under 10 microns in size
(PM10), total particulate matter (PT), sulfur containing gaseous compounds (SOx)
and volatile organic carbon compounds (VOC).
The data is shown in two forms, namely, tons per year produced
(TPY) and pounds produced per million BTUs used (1bs/MMBTU). The parentheticals
following the OXY-GAS and OXY-OIL data represent pollutant reductions over those
of the air-fired, gas fueled combustion system.
TABLE 1- FLUE GAS ANALYSIS FOR AIR-GAS, OXY-GAS AND OXY-OIL COMBUSTION SYSTEMS
The values for PM10, PT, SOx and VOC for the oxy fueled
waste oil combustion system show increases (as negative reductions). This is due
in part to no "post-burn" treatment processes used in the exemplary combustion system.
It is anticipated that proper "post-burn" processes would include bag houses (for
particulate matter) and scrubbers (for sulfur-containing gases) and would result
in reductions of at least about 98.99 percent and 95 percent, respectively, in emissions
quantities. The values attained in TABLE 1 were based upon the reduction in fuel
consumption observed and were determined in accordance with accepted United States
Environmental Protection Agency (USEPA) criteria, as determined from USEPA tables
AP42 (available from the USEPA website).
It must be noted that the above values are based upon controlling
the environment within the furnace in which the oxy fueled combustion system is
used. That is, the values shown above that indicate reductions in pollutants for
the OXY-GAS and OXY-OIL combustion systems require that the furnace in which the
combustion systems are installed is designed to limit to negligible air in-leakage
(i.e., nitrogen in the combustion atmosphere).
Thus, as will be appreciated by those skilled in the art,
the use of high purity oxygen (or highly oxygen-enriched air) and any carbon based
fuel is highly adaptive to many existing industrial systems. It is anticipated that
the uses for such a system in standard and conventional industrial applications
will provide myriad advantages and benefits over known, presently used air fired
and air over-fired systems. Although many present physical plants may require redesign
and modification to incorporate the present oxy-fueled combustion systems to enhance
performance and production, it is contemplated that the benefits gained by making
these changes in design and structure, such as lowered operating. costs, e.g., reduced
fuel costs, lowered capital costs and reduced emissions, will far outweigh the costs
to make these changes.
In the present disclosure, the words "a" or "an" are to
be taken to include both the singular and the plural. Conversely, any reference
to plural items shall, where appropriate, include the singular.
From the foregoing it will be observed that numerous modifications
and variations can be effectuated without departing from the true spirit and scope
of the novel concepts of the present invention. It is to be understood that no limitation
with respect to the specific embodiments illustrated is intended or should be inferred.
The disclosure is intended to cover by the appended claims all such modifications
as fall within the scope of the claims.