Process and apparatus for furnace operation with gas seal

The invention relates to a method of operating a furnace with gas turbine exhaust as preheated combustion air, in which a gas seal is used. The gas seal housing may for example be a J-shaped tubular device depending from the duct through which the exhaust air flows to the burners, and is open at one end to the duct and open at the other end to the atmosphere. The variation permissible of duct pressure atop the seal in order to keep the seal intact is a function of the vertical range of the interface between the gases in the device and the difference in density between the two gases and can be controlled by adjustment of the damper setting. In normal operation the hot exhaust air flows to the furnace burners under the suction controlled by the damper. The seal is disrupted when the flow of exhaust air is reduced, the colder atmospheric air then being admitted and flowing to the burners so that the furnace continues in operation.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
This invention relates to a method of furnace operation and apparatus in 
which a gas seal is used, in particular where gas turbine exhaust (GTE) is 
employed as preheated combustion air for burning the fuel in the furnace 
burners. 
The furnaces contemplated may be used for various hydrocarbon conversions 
such as the steam reforming of natural gas or light hydrocarbons by steam 
to hydrogen and carbon oxides, which is especially useful in ammonia 
plants; and in steam cracking to convert petroleum fractions such as 
ethane up to naphtha and gas oil to lighter products, especially C.sub.2 
-C.sub.4 olefins and heavier. In steam cracking the feed is generally 
mixed with from about 20 to 92 mol % steam and heated to temperatures in 
the range of about 1200.degree. to 1800.degree. F. 
The operation of such furnaces requires the expenditure of large amounts of 
energy and increasingly is becoming the object of study in order to reduce 
fuel usage. 
One approach to this is to use preheated combustion air in burning the 
fuel. For example, the exhaust gas from gas turbines may be used. Turbines 
of this type find use in many plants for supplying power, for example in 
steam cracking plants for driving olefin gas compressors and/or 
refrigerant compressors. The gas turbine exhaust is generally available at 
temperatures in the range of about 400.degree. to about 1000.degree. F. 
and contains oxygen in the range of about 15 to about 20 weight %. 
One problem that arises in reusing the air in the exhaust from a gas 
turbine--which is a by-product of its operation--is that the turbine is 
run in accordance with its own schedule, viz., its own duty, thus may be 
shut off at times with consequent interruption of the flow of GTE to 
furnaces utilizing it as combustion air. Conversely, before start-up or on 
shutting down a furnace or one or several of a multi-furnace plant, the 
flow of GTE from the turbine is not needed. 
According to the present invention an apparatus and method wherein a gas 
seal is used, are provided in which the furnaces are uncoupled from the 
gas turbine in the sense that the furnaces may be operated independently 
of the turbine and the turbine may be operated independently of the 
furnaces. Additionally, any one of a group of furnaces may be operated 
independently of the others in the group. 
In the event that a gas turbine trips, one concern is that the furnace may 
become starved for oxygen temporarily. The resulting build-up of a high 
concentration of hydrogen or hydrocarbons in the furnace atmosphere 
followed by a sudden surge of oxygen, has the potential for causing an 
explosion and fire. An advantage of the gas seal of the invention is its 
rapid response time. When a gas turbine trips, the seal operates rapidly 
to admit cold air from the atmosphere, which it has up to then blocked 
off, and allow it to flow to the furnace burners as combustion air. This 
is an excellent safety feature. 
2. Description of the Prior Art 
The use of GTE as preheated combustion air in steam reforming furnaces is 
known from the following publications 
U.S. Pat. No. 3,424,695 issued Jan. 28, 1969 to von Wiesenthal. 
U.S. Pat. No. 3,795,485 issued Mar. 5, 1974 to Bogart (Flour Corp.) 
"Hydrocarbon Processing," Apr. 1978, p. 145-151, by Bogart. 
U.K. Pat. No. 1,200,227 published on July 27, 1970, is also of some 
interest. 
However, these workers have not addressed the problem of uncoupling the gas 
turbine from the furnace. Neither the gas seal of this invention nor any 
other means is shown for permitting them to be operated independently of 
each other. 
In U.S. Pat. No. 4,085,708 issued on Apr. 25, 1978, to Ashdown (Foster 
Wheeler Energy Corp.) this problem is considered in connection with 
running a waste heat steam boiler but it may be noted that at all times a 
primary source of air supplied by a forced draft fan is used for at least 
some of the combustion air needs. As regards the secondary source of air, 
it is supplied either by gas turbine exhaust or by air supplied by a 
second forced draft fan, the switching being controlled by the opening or 
closing of mechanical obstructing means. See also U.S. Pat. Nos. 3,301,223 
and 3,118,429. 
U.S. Pat. No. 3,789,804 discloses a method of connecting the exhaust of a 
second gas turbine to the burners of a steam generator, if a first one 
should fail. It may be noted that two turbines have to be available and 
that a gas turbine has to be in operation at all times so that the system 
is not really uncoupled. Furthermore, the switchover is accomplished by 
means of valves. 
SUMMARY OF THE INVENTION 
During the normal operation of a furnace it may be necessary to vary the 
setting of the furnace damper to vary combustion air flow (draft changes 
also), for example, if one wants to change the furnace heat transfer duty 
by changing the flow rate of fuel to the furnace and correspondingly the 
flow rate of combustion air to it. On the other hand, one may wish simply 
to change the air combustion flow rate, i.e., to vary the draft in order 
to strike a desired balance between making the combustion reaction go to 
completion and achieving maximum furnace efficiency. Also, minor 
variations in the draft may occur because of wind at the exit of the stack 
and the like. The present invention teaches using a hot leg of gas to 
balance a cold leg of gas and thereby provide or permit a range of 
pressures at the seal over which the gas seal will remain intact and 
which, when exceeded, automatically allows some air addition to the GTE or 
excess GTE to be safely released to the atmosphere depending on furnace 
requirements. Thus the seal has the flexibility needed for furnace 
performance through the operation of the stack damper. This is achieved by 
means of apparatus comprising a novel gas seal tubular or duct-like device 
of a manometer type, inserted in the duct through which GTE is flowing to 
the burners, one end of this device being open to the atmosphere. 
Seal range is defined as the vertical range or height between the maximum 
(low) and minimum (high) level over which the gas interface can move. The 
following equation applies (see "Flow Measurement with Orifice Meters" R. 
F. Stearns et al, D. Van Nostrand Co., New York, pp. 4-10): 
EQU P=H(.rho.air-.rho.GTE)c Equation 1 
where P is the range of duct pressure atop the seal in inches of water, H 
is height in feet, .rho. is density in pounds per cubic foot, and c is the 
factor to convert from pounds per square foot to inches of water. 
According to the invention an apparatus is employed which comprises at 
least one gas turbine and one or several furnaces each having evacuating 
means for flue gases comprising a stack and/or induced draft fan, with 
associated damper controls, and provided with burners for burning a 
mixture of fuel and air; a duct connecting the gas turbine exhaust outlet 
to the burners for GTE to flow thereto, and means opening into the GTE 
duct encompassing a gas seal wherein a leg of hot GTE coming down from the 
duct is held in by a leg of colder atmospheric air open to the atmosphere. 
The atmospheric air is at a lower temperature giving it a higher density 
than the GTE at the conditions of use and most conveniently is at ambient 
temperature. The form of the means housing the gas seal is not 
particularly limited but uses the principles of a manometer. For example, 
it may be a U-shaped tube or loop, opening at one end into the GTE duct 
and open at the other end to the atmosphere. Alternatively, it may be a 
tube within a tube. However, other structures which can function in the 
same manner may be employed. A hood may be provided at the opening to the 
atmosphere. When there are several furnaces a manifold may be used which 
connects a GTE main duct to individual ducts to the individual furnaces, 
each duct having flow restricting means. 
The factors which determine the range of pressure over which the seal can 
operate are the seal height and the difference in density between the two 
gases (see Equation 1). If the latter is fixed, the seal pressure range 
can be selected by selecting the desired height and is thus a matter of 
choice. However, practical considerations of convenience as regards the 
height of the unit may be limiting. For practicability the seal pressure 
range will generally be about 0.2 inches water but may be as high as 0.4 
inches water. 
The open stack exerts suction to cause flow of the GTE in the duct to the 
burners. The duct pressure atop the seal is controlled by adjusting the 
setting of the stack damper in the furnace concerned. It may be noted that 
the damper need not be located in the stack. In a case where several 
furnaces share a single stack, the damper of each furnace will be in the 
breeching and each will function independently of the others. In this 
application the term damper with reference to furnaces is intended to 
include inlet guide vanes which may be used with induced draft fans, which 
perform the same function. 
In normal operation, GTE flows to the furnace burners under the draft of 
the stack pulling the flue gases out of the furnace. The gas seal is 
maintained over all interfacial levels of the two gases within the 
selected operating pressure range of the seal. The colder air acts as a 
plug, sealing in the hot GTE. Since the colder, heavier air is not above 
the hot GTE, there are essentially no convective currents to disturb the 
seal. The position of the gas interface may be monitored and/or controlled 
by temperature indicating, controlling, and/or alarming thermocouples. In 
controlling mode, these and/or a suitable pressure sensing device will 
actuate the stack damper. Thermocouples may also be used to monitor air 
introduced into the GTE as well as GTE vented to the atmosphere. 
The position of a given furnace stack damper (for a given burner setting) 
controls the amount of suction exerted on the GTE atop the corresponding 
seal. When the gas turbine trips, GTE flow to each furnace slows down 
while increasing quantities of cold air are pulled into the furnaces. The 
furnaces continue to operate safely and satisfactorily. As GTE flow 
gradually diminishes, for a given setting of the stack damper an 
increasingly stronger suction is exerted in the GTE duct atop the seal at 
each furnace. This suction accelerates the intake of cold air into the 
furnace via the seal. In this sequence, it follows that there is no GTE 
exerting pressure on the GTE leg of the manometer. Thus, the cold air in 
the other leg is sucked into the furnaces. The supply of oxygen needed for 
combustion of fuel is thus continued so that the furnaces continue 
smoothly in operation. 
When a particular furnace is shut down, i.e., the vacuum exerted by the 
damper position and the cooling stack gases is substantially reduced, flue 
gases build up in it until the pressure thereof blocks flow of GTE thereto 
and forces it to flow out to the atmosphere through the opening for 
atmospheric air and the hood. In the particular case illustrated, the hood 
contains cold air which mixes with and cools the hot GTE before it emerges 
when that is desirable. 
Thus, the gas seal can be disrupted in either of these two ways, making it 
possible for the furnaces to continue in operation independently of the 
gas turbine or for the gas turbine to continue in operation independently 
of the furnaces and for any furnace in a group of them to operate 
independently of the others. 
Furthermore, it is within the scope of the invention to operate the 
furnaces so as to achieve a combination of effects, viz., by adding some 
cold air continuously to the GTE; or, while supplying GTE to the burners, 
continuously rejecting some GTE to the atmosphere. These effects can be 
achieved by reducing/increasing the supply of GTE from the turbines or 
alternatively by varying the setting of the furnace damper. Such 
conditions represent states in which the seal range of duct pressure is 
exceeded with, however, the furnaces still functioning as well as the gas 
turbines still supplying exhaust as combustion air.

DETAILED DESCRIPTION 
The invention will be described with reference to the drawings. In FIG. 1, 
which is to be considered illustrated but not limiting, a group of steam 
cracking furnaces is schematically represented, furnace 1 being the 
prototype. 
Pyrolysis coils (not shown) through which a steam cracking feed of the type 
described above is passed, are located within the furnace 1. The furnace 
comprises a convection section 2, a radiant section 3 and a stack 4. 
Burners and a fuel supply means (not shown) are provided at the bottom. 
One or several gas turbines indicated at 5, supply hot GTE, for example, 
at a temperature of about 680.degree. F. and with an oxygen content of 
about 18.6 weight %, via line 6 and GTE duct 7 to the burners via two (or 
more) parallel manifolds 8 and 8a. A gas seal housing 9 comes down from 
the GTE duct 7, in open communication therewith, for example in the form 
of a loop or U-tube in this illustration. Suitably, it is in a plane at 
right angles to the GTE duct 7, as shown in FIG. 3, to allow the other end 
to be open and have access to the atmosphere, viz., colder air at about 
60.degree. F. A hood 10 surrounds the open end of the U-tube in this 
illustration. 
The gas turbine exhaust may be at about 955.degree. F., but a heat 
exchanger preheats compressed air from about 250.degree. to 500.degree. F. 
for the gas turbine while extracting heat from the gas turbine exhaust, 
cooling it from about 955.degree. to 680.degree. F. In another 
illustration, this heat exchanger may be omitted. 
FIG. 2 shows a steam cracking unit having six furnaces. The gas turbines 
indicated at 11 send their exhaust through the main GTE duct 12, in which 
a vent stack 13 is provided for use in running the gas turbines 
independently of the furnaces such as during start-up or on shutting down. 
The main duct 12 is connected to the manifold 14 used to distribute GTE to 
each of six furnaces. GTE flows to each furnace by duct 15 via a 
guillotine damper 16 which can only be either open or shut (required to 
isolate a furnace for maintenance, etc.), over the inlet end of the seal 
17 provided for each furnace, then through the forked ducting 18 to the 
floor burners of each furnace 19. A fixed partial butterfly damper 20 may 
be provided in the GTE duct 15 to each furnace for use in balancing the 
design flow among the six cracking furnaces, i.e. to tune the system. In 
the event that a furnace is to remain out of service for some time and one 
wants to avoid continued release of GTE to the atmosphere, the guillotine 
damper 16 is closed. The GTE flow may be redistributed among the furnaces 
remaining in operation by the dampers of the individual furnaces. 
FIG. 3 is a further aid to understanding the invention. A typical profile 
of total pressure in inches of water (relative to atmospheric pressure at 
grade or ground level, i.e., P.sub.A at grade=0 inches water) plotted 
against height or elevation at various points in the GTE and cracking 
furnace system is shown in FIG. 3 by line 21. Absolute pressure in psia is 
also indicated. Also, a profile of atmospheric pressure (taken at 
60.degree. F.) is shown by line 22. At a given elevation, the difference 
between line 21 and line 22 represents the draft or suction or partial 
vacuum, i.e., less than atmospheric pressure, normally in inches water at 
that specific point or elevation in the system. The difference in pressure 
at Point A of curve 21 and Point B of curve 22 represents the draft at the 
furnace bridgewall, i.e., the junction of the convection and radiation 
sections of the furnace. It is not particularly critical so long as some 
vacuum is maintained for safety. A suitable draft for furnace operation is 
about 0.05 to 0.50 inches water as measured at the furnace bridgewall. The 
bridgewall draft, and also the GTE pressure atop the seal of a furnace, 
can be automatically controlled by adjusting the setting of the stack 
damper, i.e. the damper .DELTA.P of FIG. 3. 
GTE flow is shown by the arrows, starting at the right at the top of the 
seal 23 which is shown in side view. GTE flow is to the left and downward 
to the floor burners in the apparatus and downward slightly to the right 
then upward to the left in the pressure profile. A pressure drop, .DELTA.P 
burners, is taken across the floor burners. In the radiant 24 and 
convection 25 sections, flow is upward while total pressure decreases 
slowly first and then faster across the convection section. The profile 21 
reflects the pressure drop incurred in the breeching 26 and at the 
entrance to the stack 27 (shown as furnace supported in this 
illustration). The pressure drop across the stack damper as well as the 
head provided by the centrifugal induced draft fan .DELTA.PID (included in 
this illustration) are also incorporated in the pressure profile. The 
furnace system pressure and the atmospheric pressure profiles gradually 
approach each other with increasing elevation and finally converge at the 
outlet of the stack. 
Another form of gas seal housing is shown in FIG. 4 and comprises a GTE 
duct made up of cooperating tubular ducts 30 and 31. Duct 30 terminates in 
a vertical duct member 32 which enters an outer or sleeve duct member 33 
of duct 31. Sleeve 33 surrounds preferably the entire length of vertical 
duct member 32 preferably coaxially and in spaced relation thereto leaving 
an annular channel 34. The seal range H is indicated in FIG. 4. It 
corresponds to the length of the channel 34 viz., the length of duct 
member 32 to the extent it is covered by sleeve 33. Sleeve 33 may be 
shorter than the vertical duct member 32 but in that case the seal range H 
will be correspondingly shorter. Sleeve 33 may be slanted at the bottom, 
as shown, to protect personnel on the ground. 
In operation, GTE flows from the distribution manifold and flows upward in 
the inner duct 32 of the tube-in-tube seal. GTE exits the inner duct at 
the top of seal 35 and flows to the burners via duct 31. The annular space 
34 between the inner and outer vertical duct members serves as the channel 
for introducing cold air or venting GTE to the atmosphere. 
The following illustrates the method of calculating the range of duct 
pressure atop the seal for a particular seal range or height H, or vice 
versa, using Equation 1 given above. 
For GTE having an O.sub.2 content of 19.01 weight %, the following data can 
be obtained: 
TABLE 1 
______________________________________ 
Fluid GTE Air 
______________________________________ 
.degree.F. 680 60 
Mol. Wt. 28.59 28.84 
.rho. lb/cu/ft. 
0.0344 0.0761 
at conditions 
______________________________________ 
Projecting a height of 23 feet, one can find the duct pressure range by 
substituting in Equation 1 as follows: 
______________________________________ 
P = 23 (0.0761- 0.0344) 
= 0.959 lb./sq.ft. 
= 0.00666 lb./sq.inch 
= 0.1846 inches H.sub.2 O 
______________________________________ 
FIGS. 5a and 5b depict a U-shaped or loop manometer device. In practice, 
the right member of the loop seal, which is open to the atmosphere, could 
end at any desired elevation. The minimum (high) interfacial level and the 
maximum (low) interfacial level are indicated respectively. At those 
levels and at all levels in between, or intermediate levels, there will 
neither be GTE escaping to the atmosphere nor cold air being pulled in to 
the furnace burners. The height H indicated pertains to the left or GTE 
leg; the height of the right leg is not critical and may be equal to, 
greater or preferably smaller than that of the left leg, viz., the unit 
could be a J-shaped loop device. In fact, by analogy to the tube-in-tube 
seal, the device may consist solely of the left leg, open to the 
atmosphere at the bottom, with the rest of the U-tube removed; and this 
constitutes another form of gas seal housing. The process, in this aspect, 
is distinguishable from measurement of a pressure differential with a 
U-tube in that the object is not measurement (in fact a part of the U-tube 
ordinarily used for measuring can be eliminated) but is utilization of the 
sealing-in effect of the seal fluid which in this case is air, not 
mercury. When the atmospheric air passes beyond the high level shown in 
FIG. 5a, and enters the GTE duct, it becomes subject to the furnace system 
pressure at that point, i.e., the draft or suction exerted on the GTE 
flowing in the duct, and the amount of outside air drawn to the burners 
depends on the degree of vacuum. Conversely, when the GTE passes beyond 
the low level, shown in FIG. 5b, and flows out to the atmosphere, it does 
so as a consequence of the GTE supply being excessive for the draft or 
pressure in the GTE duct. 
For illustrative purposes, FIG. 5a shows the high interfacial level for GTE 
flow where the velocity head, P.sub.V, is 0.15 inches water and the total 
pressure is the same, measured at the GTE duct longitudinal center line; 
and FIG. 5b shows the low interfacial level where P.sub.V is 0.15 inches 
water, the static pressure P.sub.S is 0.20 inches water and the total 
pressure is 0.35 inches water. The range of duct pressure atop the seal is 
thus 0.20 inches water and the seal height H is 24.9 feet. 
The total pressure atop the seal can be controlled by adjusting the setting 
of the furnace damper. 
When the gas turbine trips and GTE is no longer flowing to the furnace 
burners, a vacuum is created in the GTE leg. That is to say, the pressure 
drops below the minimum level of the seal. The cold air is then sucked in 
and flows to the burners so that furnace operation continues. Conversely, 
when a furnace is shut down and the damper closed, pressure of the GTE and 
of flue gases builds up in the GTE leg. That is to say, the pressure 
increases above the maximum level of the seal. The GTE is then forced out 
to the atmosphere. 
GTE plus make-up air flow to a typical furnace following a gas turbine trip 
is shown in FIG. 6. Flow to the furnace in lb./sec. is plotted against 
time in seconds following a gas turbine trip. GTE flow decreases slowly 
initially, then faster as the gas turbine inlet guide vanes close, and 
finally slowly again as the gas turbine winds down. Total flow to the 
furnace drops initially and then increases gradually. Air make-up is the 
difference between the top and bottom curves at any time. A line 
representing stoichiometric air flow for the design fuel rate is included 
in the plot for reference. The minimum total flow to the furnace remains 
above the stoichiometric requirements, viz., the air line. 
Thus, the invention provides a gas seal to be operated in conjunction with 
furnaces, whereby GTE can be used as combustion air in a manner such that 
the gas turbine and the furnaces are uncoupled, that is, can operate 
independently of each other. It responds automatically to changes in 
conditions to supply air/GTE as various circumstances require. In one 
condition it admits GTE as preheated combustion air; in another condition 
it admits atmospheric air as combustion air; and in another it causes GTE 
to flow out to the atmosphere. However, it can also operate in other ways, 
if desired, to send any chosen mix of GTE and cold outside air to the 
burners continuously as combustion air or to send GTE to the burners while 
continuously rejecting excess GTE to the atmosphere. The seal per se 
functions without the intervention of any moving or mechanical parts in 
the GTE duct physically admitting or obstructing gas flow. The absence of 
such mechanical parts ensures durability and low maintenance costs. The 
safety factor is high owing to the gas seal's rapid response time. The 
seal can have a range which is practicable for furnace operation. 
Consequently, the present invention has a number of advantages as compared 
with systems which either have no means for accomplishing these ends or 
have means which do not respond as rapidly. 
Thus, the short response time is a fail-safe feature. The furnace can 
always be kept in operation if the gas turbine trips. For those plants 
that generate steam, using furnace effluent, through heat exchangers, this 
will permit continued operation of machinery driven by that steam even in 
the event that the gas turbine exhaust is lost. Consequently, steam 
supplies for refrigeration systems (included in a steam cracking plant to 
recover the valuable ethylene and lighter gases such as H.sub.2) are 
maintained via continued passage of hot cracked effluent to heat 
exchangers for producing steam. With such heat exchangers a simplified 
system for producing this steam can be used as compared with more complex 
fuel fired systems. The equipment layout is thus improved. A pressure 
profile such as illustrated in FIG. 3 can be maintained or at least 
variations therefrom could be moderate as contrasted with a gas turbine 
trip with no other source of air supply where deviations from the normal 
pressure profile could be greater. From the fact that the present 
invention ensures continuous operation of the furnace with little or no 
upset in case of a gas turbine trip, it follows that coil outlet 
temperature of the pyrolysis tubes is controlled within the normal range 
as the furnace is kept onstream with the pyrolysis reaction kept going. 
Wide swings of temperature for pyrolysis tube alloys that are near the 
limit of their temperature tolerance, which would be harmful to these 
metals, are thus avoided. Therefore, since all conditions can be 
continuously maintained, furnace operability is rated high.