PULSED DETONATION COMBUSTOR CLEANING DEVICE AND METHOD
OF OPERATION
RELATED CASES
This application claims priority under 35 U.S.C. §119(e) from Provisional
Application No. 60/763,563 filed on 31 January 2006.
BACKGROUND
The systems and techniques described herein relate generally to a cyclic pulsed
detonation combustion cleaner. More specifically, they relate to removal of buildup
on surfaces within various sections of an industrial boiler system using impulses
generated from pulsed detonations.
Industrial boilers operate by using a heat source to create steam from water or another
working fluid, which can then be used to drive a turbine in order to supply power.
The heat source may be a combustor that burns a fuel in order to generate heat, which
is then transferred into the working fluid via a heat exchanger. Burning the fuel may
generate residues that can be left behind on the surface of the combustor or heat
exchanger. Such buildups of soot, ash, slag, or dust on heat exchanger surfaces can
inhibit the transfer of heat and therefore decrease the efficiency of the system.
Periodic removal of such built-up deposits maintains the efficiency of such boiler
systems.
In the past, pressurized steam, water jets, acoustic waves, and mechanical hammering
have been used to remove this buildup. These systems can be costly to maintain, and
effectiveness of these devices varies depending on location and use. More recently,
the use of detonative combustion devices has been attempted to remove buildup.
These systems tend to require a large footprint, operate infrequently, and in some
cases require oxygen enrichment in order to create the detonations.
Therefore, there is a continued need for development of effective detonative
combustion cleaning systems.
BRIEF DESCRIPTION
Briefly, in accordance with one aspect of the systems described herein, a system for
removing accumulated debris from a surface within a vessel is provided. The system
includes a vessel that has a surface to be cleaned, a fuel source to provide a
combustible fuel, an air source to provide a flow of air and a pulse detonation
combustor. The combustor includes a combustion chamber that has a wall that
defines an airflow path from an upstream end toward a downstream end, an air inlet
disposed upon the combustion chamber and connected to the air source and in flow
communication with the combustion chamber, a fuel inlet in flow communication
with the combustion chamber and connected to the fuel source, an ignition device
disposed downstream of the fuel inlet that is configured to periodically ignite the fuel
within the airflow and produce a flame, and a plurality of obstacles disposed along the •
airflow path and configured to promote the acceleration of the flame into a detonation
as it passes through the combustion chamber. The downstream end of the pulse
detonation combustor is disposed on the vessel such that the shock wave associated
with the detonation from the pulse detonation combustor passes over the surface to be
cleaned within the vessel.
In accordance with another aspect of the systems described herein, a cleaner for
removing accumulated debris from a surface of a vessel is provided. The cleaner
includes a pulse detonation combustor as described above, and the downstream end of
the pulse detonation combustor is configured to direct the shock wave associated with
the detonation in the pulse detonation combustor to pass pver the surface of a vessel
to be cleaned.
In accordance with an aspect of the techniques described herein, a method for
removing accumulated debris from a surface within a vessel is described. The method
includes the steps of receiving a flow of air into a combustion chamber through an air
inlet, the flow of air defining a downstream direction of flow. Another step includes
receiving a flow of fuel into the combustion chamber through a fuel inlet into the flow
of air. Other steps include mixing the fuel and air within the combustion chamber and
periodically igniting the fuel and air mixture using an ignition device. Another step
includes accelerating the flame into a detonation as it passes downstream through the
combustion chamber by passing the flow over a plurality of obstacles disposed along
the path of the flow of air through the combustion chamber. Other steps include
directing the detonation into a vessel having a surface to be cleaned and passing the
Shockwave associated with the detonation over a surface within a vessel to loosen
debris from the surface. The method also includes blowing the loosened debris from
the surface.
DRAWINGS
These and other features, aspects, and advantages of the present invention will
become better understood when the following detailed description is read with
reference to the accompanying drawings in which like characters represent like parts
throughout the drawings, wherein:
Figure 1 is a schematic representation of a pulse detonation combustor system in
accordance with one aspect of the systems described herein;
Figure 2 is a schematic representation of an alternate head end for a pulse detonation
combustor in accordance with another embodiment of the systems described herein;
Figure 3 is a schematic representation of an embodiment of a diverging chamber for
use with the pulse detonation combustors described herein;
Figure 4 is a schematic axial view of an annular ring obstacle for use within a pulse
detonation combustor as described herein;
Figure 5 is a schematic axial view of a circular segment obstacle for use within a
pulse detonation combustor as described herein;
Figure 6 is a schematic axial view of a crescent-shaped obstacle for use within a pulse
detonation combustor as described herein;
Figure 7 is a schematic representation of an alternate embodiment of a combustion
chamber for use in a PDC-based cleaner as described herein;
Figure 8 is a schematic cross-sectional axial view of a bolt used as an obstacle within
a pulse detonation combustor as described herein;
Figure 9 is a schematic representation of a bent combustion chamber for use in a pulse
detonation combustor as described herein;
Figure 10 is a schematic representation of a PDC-based cleaner extending into the
interior of an exemplary boiler as described herein;
Figure 11 is a multiple exit chamber for use in one embodiment of a PDC-based
cleaner as described herein; and
Figure 12 is an alternate arrangement of a multiple exit cleaner disposed within an
exemplary boiler vessel in accordance with the systems described herein.
DETAILED DESCRIPTION
As discussed above, soot or other buildup on heat exchanger surfaces in industrial
boilers can cause losses in the overall system efficiency due to a reduction in the
amount of heat that is actually transferred into the working fluid. This is often
reflected by an increase in the exhaust gas temperature from the backend of the
process, as well as an increase in the fuel-burn rate required to maintain steam
production and energy output. Traditionally, the complete removal of buildup from
such fouled surfaces requires the boiler to be shut down while the cleaning process is
performed. Online cleaning techniques generally lead to higji maintenance costs or
incomplete cleaning results.
In the systems and techniques described herein, a pulsed detonation combustor
external to the boiler is used to generate a series of detonations or quasi-detonaiions
that are directed into the fouled portion of the boiler. The high speed shock waves
travel through the fouled portion of the boiler and loosen buildup from the surface,
which is then allowed to exit the boiler through hoppers. As will be discussed below,
the use of repeated detonations has advantages over traditional cleaning techniques,
such as steam blowers or purely acoustic soot removal devices.
It is also desirable that a cleaning system for a boiler be able to operate to quickly
remove buildups in order to minimize the down-time for the boiler. In addition, it is
desirable that the system be conveniently operable within the boiler environment, i.e.
that it is able to physically fit within the space restrictions necessary, able to reach the
portions of the boiler that require de-fouling, and that it does not interfere with the
operation of the boiler when the cleaning system is not in use. It is also desirable that
the installation of such cleaner not take up excessive flow space outside the boiler or
require major modifications to the boiler for access. A pulse detonation combustor
based cleaning system that can provide these and other features will be described in
more detail below.
As used herein, the term "pulse detonation combustor" (PDC) will refer to a device or
system that produces both a pressure rise and velocity increase from the detonation or
quasi-detonation of a fuel and oxidizer, and that can be operated in a repeating mode
to produce multiple detonations or quasi-detonations within the device. A
"detonation" is a supersonic combustion in which a shock wave is coupled to a
combustion zone, and the shock is sustained by the energy release from the
combustion zone, resulting in combustion products at a higher pressure than tb.e
combustion reactants. For simplicity, the term "detonation" as used herein will be
meant to include both detonations and quasi-detonations. A "quasi-detonation" is a
supersonic turbulent combustion process that produces a pressure rise and velocity
increase higher than a pressure rise and velocity increase produced by a sub-sonic
deflagration wave.
Exemplary PDCs, some of which will be discussed in further detail below, include an
ignition device for igniting combustion of a fuel/oxidizer mixture, and a detonation
chamber in which pressure wave fronts initiated by the combustion coalesce to
produce a detonation wave. Each detonation or quasi-detonation is initiated either by
an external ignition source, such as a spark discharge, laser pulse, heat source, or
plasma igniter, or by gas dynamic processes such as shock focusing, auto ignition or
an existing detonation wave from another source (cross-fire ignition). The detonation
chamber geometry allows the pressure increase behind the detonation wave to drive
the detonation wave and also to blow the combustion products themselves out an
exhaust of the PDC.
Various chamber geometries can support detonation formation, including round
chambers, tubes, resonatmg cavities and annular chambers. Such chambers may be of
constant or varying cross-section, both in area and shape. Exemplary chambers
include cylindrical tubes and tubes having polygonal cross-sections, such as, for
example, hexagonal tubes. As used herein, "downstream" refers to a direction of flow
of at least one of fuel or oxidizer.
One embodiment of an exemplary PDC-based cleaning device suitable for use with an
industrial boiler is illustrated schematically in Figure 1. The PDC cleaner 100 extends
along the illustrated x-axis from an upstream head end that includes inlets for air and
fuel (102 and 104, respectively) located on the left side of the Figure, to an exit
aperture 116 at the downstream end shown on the right side of the Figure. A tube 114
extends from the head end to the aperture 116, defining a combustion chamber 101
within it. In the illustrated embodiment, the aperture 1 Id of the PDC is attached to
the wall 149 of the boiler to be cleaned or another downstream component that can be
used to enhance the cleaning operation, as will be discussed in greater detail below.
As noted above, the head end of the illustrated PDC includes an air inlet 102. The air
inlet 102 may be connected to a source of air that can be provided to the PDC under
pressure. This air source is used to fill and purge the combustion chamber 101, and
also provides air to serve as an oxidizer for the combustion of the fuel. In particular
embodiments, the supply to air inlet 102 may be connected to a facility air source
such as an air compressor. As will be discussed below with respect to the operation
of the PDC, the flow through the air inlet will generally enter the tube 114 and flow
the length of the combustion chamber 114 and exit downstream through the aperture
The air inlet 102 of the illustrated embodiment is connected to a centerbody 112 that
extends along the axis of the tube 114 and into the combustion chamber 101. The
centerbody of the illustrated embodiment is a generally cylindrical tube that extends
from the air inlet 102 and tapers to a downstream opening 109. In addition to the
downstream opening 109, the centerbody 112 also includes one or more holes 108
along its length that allow the air flowing through the centerbody 112 to enter the
upstream end of the chamber 101. These holes connect the interior of the centerbody
with the annular space formed between the centerbody and the upstream portion of
the tube 114.
The opening 109 and the holes 108 of the centerbody 112 allow for directional
velocity to be imparted to the air that is fed into the tube 114 through the air inlet 102.
Such directional flow can be used to enhance the turbulence in the injected air and to
improve the mixing of the air with fuel present within the flow in the head end of the
PDC. In order to enhance these effects, the holes 108 may be disposed at multiple
angular and axial locations about the axis of the centerbody. In some embodiments,
the angle of the holes may be purely radial to the axis of the centerbody. In other
embodiments, the holes may be angled in the axial and circumferential directions so
as to impart a downstream or rotational velocity to the flow from the centerbody. The
flow through the centerbody also serves to provide cooling to the centerbody in order
to prevent excessive heat buildup that could result in degradation of the centerbody.
In addition to the air inlet 102, a fuel inlet 104 is disposed on the head end of the PDC
cleaner 100 illustrated in Figure 1. The fuel inlet 104 is connected to a supply of fuel
that will be burned within the combustion chamber 101. A plenum 106 is connected
to the fuel inlet 104. In the illustrated embodiment, the plenum 106 is a cavity that
extends around the circumference of the head end of the PDC. A plurality of holes
110 connect the interior of the plenum 106 with the interior of the tube 114. Fuel is
supplied to the plenum 106 via the fuel inlet 104, and is then distributed around the
circumference of the PDC where it enters the tube 114 through the holes 110. The
holes 110 extend generally radially from the plenum 106 into the annular space
between the wall of the tube 114 and the centerbody 112.
The fuel is injected into (he chamber 101 and mixes with the air flow coming through
the holes 108 of the centeibody 112. The mixing of the fuel and air may be enhanced
by the relative arrangement of the air holes 108 and the fuel holes 110. For instance,
by placing the fuel holes 110 at a location such that fuel is injected into regions of
high turbulence generated by the flow through the air holes 108, the fuel and air may
be more rapidly mixed, producing a more readily detonable fuel/air mixture. As with
the air holes 108, the fuel holes 110 may be disposed at a variety of axial and
circumferential positions. In addition, the holes 110 may be aligned to extend in a
purely radial direction, or may be canted axially or circumferentially with respect to
the radial direction.
Fuel may be supplied to the fuel plenum 106 through the inlet 104 through a valve
that allows for the active control of when fuel is allowed into the PDC, Such a valve
may be disposed within the inlet 104, or may be disposed upstream in a supply line
that is connected to the fuel inlet. In one embodiment of the system, the valve may be
a solenoid valve, and may be controlled electronically to open and close in order to
regulate the fuel flow.
As seen in Figure 1, an ignition device 130 is disposed near the head end of the PDC.
In the illustrated embodiment, the ignition device is located along the wall of the tube
114 at a similar axial position to the end of the centerbody 112. This position allows
for the fuel and air coming through holes 110 and 108 respectively to mix prior to
flowing past the ignition device. As noted above, the ignition device may take
various forms as known in the art. In particular, the device need not produce
immediate detonation in the fuel/air mixture in every embodiment. However, the
ignition device 130 desirably provides a robust enough combustion ignition that
allows the combustion of the fuel/air mixture can transition to a detonation within the
chamber 101, as will be discussed further below. The ignition device 130 may be
connected to a controller in order to operate the ignition device at desired times.
Although not illustrated, such a controller may be used as is generally known in the
art to control the timing and operation of various systems, such as the fuel valve and
ignition source. As used herein, the term controller is not limited to just those
integrated circuits generally referred to in the art as a controller, but broadly refers to
a processor, a microprocessor, a microcontroller, a programmable logic controller, an
application specific integrated circuit, and other programmable circuits suitable for
such purposes.
The embodiment of a PDC illustrated in Figure 1 includes a tube 114 that generally
extends along the x-axis from the head end described above to an aperture 116 at the
downstream end of the tube. The combustion chamber 101 is defined by the walls of
the tube, and the combustion of the fuel/air mixture takes place within the chamber
101. In general, the combustion will progress from the ignition device 130 through
the mixture that is within the combustion chamber 101. Figure 1 illustrates a crosssection
of tube in the shape of a substantially round cylinder having a constant crosssectional
area. Those of skill in the art will recognize that other configurations are
also possible, as noted above.
The tube 114 contains a number of obstacles 120 disposed at various locations along
the length of the tube. The obstacles 120 are used to enhance the combustion as it
progresses along the length of the tube 114, and to accelerate the combustion front
into a detonation or quasi-detonatiou before the combustion front reaches the aperture
116 at the downstream end of the tube. The obstacles 120 in the embodiment
illustrated in Figure 1 are thermally integrated with the wall of the tube 114. Such
thermally integrated obstacles may be created in various ways. For example,
obstacles may include features that are machined into the wall, formed integrally with
the wall (by casting or forging, for example), or attached to the wall, for example by
welding. In general, a thermally integrated obstacle or other thermally integrated
feature of the wall is in sufficient contact with the wall of the tube that the features or
obstacles 120 exchange heat effectively with the wall of the tube 114 to which they
are integrated.
The tube 114, obstacles 120 and centerbody 112 may be fabricated using a variety of
materials suitable for withstanding the temperatures and pressures associated with the
repeated detonations. Such materials include but are not limited to: Inconel, stainless
steel, aluminum and carbon steel.
Figure 2 illustrates an alternative head end that could be used with a PDC in another
embodiment of a PDC-based cleaner. The head end 200 includes a fuel inlet 104 and
plenum 106 having holes 110 that are each structured and operate in substantially the
same manner as that described with respect to the embodiment shown in Figure 1.
However, rather than air being introduced through an air inlet that is directly
connected to the centerbody, the head end 200 shown in Figure 2 has air and fuel both
being directly introduced into a mixing chamber 215 located upstream of a perforated
plate 224.
Air inlets 210, 212 are used to introduce airflow into the mixing chamber 215 shown
in Figure 2. Each air inlet can be connected to a source of air, as with air inlet 102 in
Figure 1. Fuel is expelled from the holes 110 from the plenum 106 into the mixing
chamber 215 as well. The fuel and air begin mixing in the mixing chamber 215
before they flow through holes 225 in the perforated plate 224 that separates the
mixing chamber 215 from the combustion chamber 101. As the fuel/air mixture is
expressed through the holes 225 of the plate 224, additional turbulence is created in
the flow, further enhancing the fuel / air mixing.
The fuel/air mixture enters the upstream portion of the combustion chamber 101 after
passing through the perforated plate 224, and flows around a centerbody 230 that can
be mounted upon the plate 224. This pre-mixed flow can them be ignited by an
ignition device 130, much as described above with respect to Figure 1. The head end
200 illustrated in Figure 2 can be used in place of the head end features described
above, or with the variations of the PDC that are described below.
Figure 3 shows an embodiment of a diverging chamber that can be connected
downstream of a PDC system, such as that shown in Figures 1 and 2, and that would
receive the flow from the aperture 116 of the combustion chamber 101 of the PDC.
In the illustrated embodiment, the diverging chamber 300 is connected directly to the
exit aperture 116 of the PDC, and the wall 149 of the downstream device shown in
Figure 1 is the upstream wall 149 of the diverging chamber 300. Those of skill in the
art will recognize that although the diverging chamber need not be in direct contact
with the PDC, it is desirable that the chamber 101 of the PDC is in flow
communication with the diverging chamber 300.
As shown in Figure 3, the exemplary diverging chamber 300 has walls 302 that
enclose a flow path 310. The illustrated walls form a horn or bell shape that produces
an increase in the cross-sectional area of the flow path 310 from the upstream end
(connected to the aperture 116) to the downstream exit 320 from the chamber 300.
The increased cross-section as the flow travels downstream serves to increase the
volume of fuel and air that can be combusted within the PDC cleaner during each
combustion cycle. This can be used to increase the penetration and effectiveness of
the shock waves produced.
The illustrated diverging chamber 300 provides a gradually diverging flow path 310,
as opposed to an abrupt change in volume that the flow path would experience if
vented directly into a larger chamber. This gradual divergence allows for the
detonation produced by the PDC to be sustained as it travels through the diverging
flow path 310 of the chamber without causing a failure of the detonation.
The inner surface of the walls 302 of the illustrated diverging chamber 300 are
smooth and substantially circular in cross-section normal to the axis of the chamber.
Those of skill in the art will appreciate that other cross sectional shapes are also
possible, as well as other axial profiles for the diverging chamber. In alternative
embodiments of the diverging chamber, obstacles similar to those described herein for
use in DDT within the PDC chamber 101 can be disposed within the flow path 310 of
the diverging chamber 300. Such obstacles (not shown) can be used to promote flame
acceleration and DDT as the detonation propagates through the expanding profile of
the chamber 300.
In one particular embodiment of a diverging chamber, the chamber was formed from
a 60 inch (approximately 1.52 meters) long chamber 300 of circular cross section in
which the diameter increased from 2 inches (approximately 50.8 millimeters) at the
upstream side to a diameter of 19 inches (approximately 482.6 millimeters) at the exit
320. With detonations produced using an ethylene/air mixture in an upstream PDC,
detonations could be maintained at frequencies up to 20 Hz.
As noted above, the PDC-based cleaning system uses the detonations produced by a
PDC to loosen debris and coatings that can accumulate on the walls of a boiler or
other device, and then the high pressure flow that follows the detonation to help blow
the loosened material away from the surface. In operation, the PDC creates a
detonation and its associated high-pressure flow via a combustion cycle, which is
repeated at high frequency. PDCs can often be operated at frequencies of 1-100 Hz.
Each combustion cycle generally includes a fill phase, an ignition event, a flame
acceleration into detonation phase, and a blowdown phase. The general operation of
the PDC and cleaner will be discussed with reference to the Figures in greater detail
below.
In the discussion that follows, a single occurrence of a fuel fill phase, a combustion
ignition, an acceleration of the flame front to a detonation, and the blow down and
purge of the combustion products will be referred to as "a combustion cycle" or "a
detonation cycle". The portion of time that the cleaner system is active is referred to
as "cleaner operation". Time when the vessel to be cleaned is being actively used for
its purpose will be referred to as "boiler operation". As noted above, the vessel to be
cleaned need not be part of a boiler; however, for simplicity of reference, the term
"boiler operation" will be used to refer to the operation of any device being cleaned
by the cleaner device.
In particular, as will be discussed below, one advantage of the system described
herein is that, unlike other detonation cleaner systems, there is no need to shut down
the boiler or other device whose vessel is being cleaned in order to operate the
cleaner. Specifically, it is possible for the cleaner operation to take place during the
boiler operation. The cleaner need not be run continuously during the boiler
operation; however, by providing the flexibility to operate the cleaner on a regular
cycle during boiler operation, an overall higher level of cleanliness can be maintained
without significant down-time in boiler operation.
In the fill phase of the detonation cycle, air and fuel are fed into the PDC. As shown
hi Figure 1 and discussed above, air can be introduced via the air inlet 102, and fuel
through the fuel inlet 104, after which the fuel and air will mix as described to form a
fuel/air mixture suitable for combustion within the PDC. As more fuel and air are
introduced and mixed, the chamber will tend to fill with the fuel/air mixture, starting
near the head end in the illustrated embodiment, and proceeding along its length as
more fuel and air are introduced. As discussed above, air can be fed continuously to
the PDC through the air inlet 102 during cleaner operation, but it may be desirable to
use a valve to control the introduction of fuel into the PDC by means of a controller in
some embodiments. In addition, the ability to control the flow of air for times when
the cleaner is not operating may also be desirable. In one exemplary embodiment, a
controller can track the amount of time that has passed since the opening of a fuel
valve and, based upon the rate of air input to the PDC, can close the fuel valve one a
sufficient amount of fuel has been added that the fueyair mixture has filled the desh-ed
portion of the combustion chamber 101.
After the combustion event, ah- continues to be introduced into the chamber 101
during combustor or cleaner operation to assist in purging any remaining combustion
products from the previous combustion cycle. In varying embodiments, the valve
may be used to provide a greater or lesser amount of fuel that would be required to fill
the chamber in order to tune the operation of the PDC. Once the valve is closed and
the chamber is no longer being fueled, the ignition device 130 is activated.
The ignition device 130 may be triggered by a controller in order to initiate the
combustion of the fuel/air mixture within the chamber 101. If, for example, a spark
initiator is used as the ignition device, the controller can send electrical current to the
initiator in order to create a spark at the appropriate time. In general, the ignition
device introduces sufficient energy into the mixture near the ignition device to form a
flame within the fuel/air mixture near the device 130. As this flame consumes the
fuel by burning it with the oxidizer present in the mixture, the flame will propagate
through the mixture within the chamber 101.
As the flame propagates through the chamber 101 of the PDC, the flame front will
reach the walls of the tube 114 and the obstacles 120 that are disposed within the tube.
The interaction of the flame front with the walls of the tube and the obstacles will tend
to generate an increase in pressure and temperature within the chamber. Such
increased pressure and temperature tend to increase the speed at which the flame
propagates through the chamber and the rate at which energy is released from the
fuel/air mixture by the combustion at the flame front. This acceleration continues
until the combustion speed raises above that expected from an ordinary deflagration
process to a speed that characterizes a quasi-detonation or detonation. This DDT
process desirably takes place rapidly (in order to sustain a high cyclic rate of
operation), and so the obstacles 120 are used to decrease the run-up time and distance
that is required for each initiated flame to transition into a detonation.
The detonation wave travels down the length of the tube 114 and out of the exit
aperture 116 of the tube. From the aperture 116, the detonation wave may be directed
into the body of an object to be cleaned, or may be sent through a diverging section
300 such as that illustrated in Figure 3 prior to being directed into the object to be
cleaned. High pressure combustion products follow the detonation wave and blow
through the exit aperture 116 along with the detonation wave itself.
As the high-pressure products blow out of the PDC, the continued supply of air
through the air inlet 102 will tend to push the products downstream and out of the
aperture 116, even as the pressure within the combustion products drops. Such
continued supply of air is used to purge the combustion products from the tube 114.
Once the combustion products are purged, the valve for the fuel inlet 104 may be
opened, and a new fill phase may be started to begin the next combustion cycle.
The detonation wave that exits from the tube 114 or exit of the diverging chamber 320
includes an abrupt pressure increase, or shock, that will propagate through the body of
the object to be cleaned. This shock can have several beneficial effects in removing
debris and slag from surfaces such as boiler walls. In one aspect, the shock wave can
produce pressure waves that travel through the accumulated slag and debris. Such
internal pressure waves can produce flexing and compression within the
accumulations that can enhance crack formation within the debris and break portions
of the debris away from the rest of the accumulation, or from the boiler walls. This is
often seen as "dust" that is liberated from the surface of the accumulated slag. In
addition, the pressure change associated with the passage of the shock can produce
flexion in the walls of the boiler itself, which can also assist in separating the slag
from the walls. In addition, the repeated impacts from the subsequent shocks of
repeating combustion cycles may excite resonances within the slag that can further
enhance the internal stresses experienced and promote the mechanical breakdown of
the debris. Behind each shock, the flow of pressurized combustion products provides
a sweeping effect that can blow loosened debris and particles downstream. The
repeated action of shock and purge is used to erode build-up that accumulates upon
the boiler walls.
In order to optimize the cleaning effect, the strength of each wave existing from the
PDC can be increased or decreased, as can the operational frequency at which the
PDC is operated. The strength and frequency can be adjusted by alterations in both
design and operational parameters. For instance, changes in the length of the chamber
101 can be used to alter the amount of £un-up time needed for DDT, or the use of
various lengths or shapes of diverging chamber 300 can be used in order to achieve
different levels of pressure in the shock. Operationally, variations can be made in the
amount of fuel-fill by controlling the duration for which the fuel valve remains open,
or the rate or pressure at which air or fuel is introduced into the PDC through the air
and fuel inlets 102, 104.
By altering the choice of fuel or operational frequency, the overall operational
reliability and cleaning effectiveness can be further tuned for the particular geometry
or debris accumulations experienced. In one embodiment of a cleaner as described
herein, the fuel used is a gaseous fuel, such as ethylene. In particular embodiments, it
should be noted that the fuel need not be stored in a gaseous form, but may be in a
gaseous form at the time of introduction into the combustion chamber 101 through the
fuel inlet. Other possible fuels include but are not limited to: other gaseous fuels
including hydrogen gas, natural gas, methane, and propane; and liquid fuels including
gasoline, kerosene and aviation fuels.
For example, experiments were conducted at up to 20 Hz using an embodiment with a
head end 200 as shown in Figure 2 firing into a rube 114 as shown in Figure 1.
Ethylene was used for fuel and air was used as oxidizer. Test results showed that the
downstream measured pressure (as would be experienced inside a boiler being
cleaned) was strongly dependent upon the duration of the filling phase, and the
effective volume of the chamber 101 that was filled. Varying the effective fuel-filled
length of the chamber between 20 inches (approximately 508 millimeters) from the
inlet to 90 inches (approximately 2.29 meters) from the inlet created a significant
variation in the peak pressure measured downstream during operation of the PDC.
In addition to variations in fuel, variations may also be made to the oxidizer used.
Although the term "air" is used throughout, those of skill in the art will understand
that an appropriate combustible mixture may be formed through the use of oxidizers
other than air. In a particular embodiment, air is used as the oxidizer because it is
generally conveniently available and avoids the expense and complication of
providing a separate oxidizer supply. In addition, the use of air allows for continuous
purging of the PDC cleaner to more effectively cool the system between combustion
cycles.
In addition, the systems described are capable of operating such that detonations can
be produced with the use of the same oxidizer, such as air, for the initial ignition of
the combustion within the chamber, as well as the run-up of the combustion into a
detonation, and the support of the detonation itself. This allows for a simpler system
that does not require separate sources of oxidizer, or the injection of oxidizer at
different pressures or concentrations into the combustion chamber at various points.
Similarly, the use of a single fuel system for both the initial combustion, the run-up,
and the detonation, allows for a simpler system than one that uses separate fueling of
the various portions of the system (for instance, one fueling system for the initial
combustion and run-up, and a second fueling system for a main detonation chamber).
In addition to using the same fueling system, the systems described herein make use
of the same fuel for initiation, run-up and detonation.
It will be understood that other alterations may be made to aspects of the systems and
operational methods described while retaining the benefits shown. For instance, in
one alternative aspect, multiple air inlets 102 may be used in order to allow for a more
rapid introduction of air into the PDC. In other alternative aspects, multiple fuel inlets
104 may be used, either feeding a single fuel plenum 106, or feeding separate
plenums that independently inject fuel into the combustion chamber 101 or mixing
chamber 215. Further possible variations include the use of multiple ignition devices
130, spaced radially or axially along the head end or the combustion chamber 101.
Another example of variation can be found in the configuration of the obstacles 120
discussed above with respect to Figure 1. Obstacles may be in various forms suitable
for improving the DDT process and reliability operating within the PDC environment.
In one aspect, the obstacles 120 may take the form of annular rings 410 that extend
from the walls 114 of the tube, as shown in Figure 4. Such rings 410 provide a
restriction in the cross-sectional area of the tube, and a surface for the flame front to
partially reflect off of. Other forms may include partial obstructions, such as circular
segments 420, for example a half-moon as shown in Figure 5, or crescent shaped
plates 430 as shown in Figure 6. Such forms may be plates that extend from the
surface of the tube 114.
The spacing and placement of obstacles 120 may also be varied in order to produce
more effective cleaning detonations from the PDC. For instance, rather than being
spaced equally as shown in Figure 1, obstacles 120 may be placed with varying
distances between successive obstacles 120 along the length of the tube 114. In
addition, for obstacles 120 such as the circular segment 420, crescent 430, or other
obstacles that are not rotationally symmetrical about the axis ofthe tube 114, varying
circumferential placements are possible. For example, obstacles 120 with a circular
segment shape 420 may be placed on alternating sides of the tube 114 along the
length, such that successive obstacles 120 are disposed opposite one another as shown
in Figure 7. In addition, placement of multiple obstacles at the same axial position
along the tube 114 is also possible for obstacles that do not span the entire area ofthe
tube. One example of such a placement of multiple circular segments is shown on the
right side of Figure 7.
In another embodiment, the obstacles take the form of a cylindrical protrusion that
extends from the wall of the tube into the combustion chamber. As shown in the
cross-sectional axial view of Figure 8. a hole 440 is created in the tube 114 of the
PDC. A cylinder 450 is then placed through the hole 440 and extends through the
wall of the tube 114 and into the combustion chamber 101. In one embodiment, the
cylinder 450 is threaded, as is the hole 440, and the cylinder is held in place by the
threading between the cylindrical bolt and hole. In other embodiments the protrusion
is secured in position by welding or other mechanical restriction. It will be
understood that the cylindrical protrusion can also be formed via casting or being
integrally formed with the wall of the tube. Such an arrangement can be xxsed to
thermally integrated the bolts with the walls of the tube 114 as discussed above.
As shown in Figure 8, the cylinder 450 extends into the combustion chamber 101.
The length which the cylinder extends into the chamber can vary in different
embodiments of the systems described herein. For instance, in one embodiment, the
length may be greater than or equal to about one-half of the inner diameter of the
combustion chamber. In another embodiment, the length may be equal to the inner
diameter of the combustion chamber, in which case the cylinder will extend to the
opposite side of the chamber from the side from which it extends. In varying
embodiments, the ratio of the length which the cylinder extends from the wall of the
chamber to the inner diameter of the combustion chamber at the location of the
cylinder may be: from about .5 to about .625; from about .625 to about .70; from
about .70 to about .80; from about .80 to about .875; from about .875 to about .95; or
from .95 to about 1. In a particular embodiment, the cylinder may have an extending
length of about 1.5 inches (about 38.1 millimeters), and the inner diameter may be
about 2.0 inches (about 50.8 millimeters), for a ratio between the length and the inner
diameter of about .75. Other embodiments will be described below.
As can also be seen with reference to Figure 8, the cylinder 450 also has a width, or
diameter, which may vary in different embodiments of the systems described herein.
For instance, in one embodiment, the width of the cylinder 450 may be greater than or
equal to about one-quarter of the inner diameter of the combustion chamber 101. In
another embodiment, the width may be less than or equal to about one-half of the
inner diameter. In varying embodiments, the ratio of the width of the cylinder to the
inner diameter of the combustion chamber at the location of the cylinder may be: from
about .25 to about .30; from about .30 to about .40; from about .40 to about .45; and
from about .45 to about .5. In a particular embodiment, the cylinder may have a width
of about .625 inches (approximately 15.9 millimeters), and the combustion chamber
may have an inner diameter of about 2.0 inches (approximately 50.8 millimeters), for
a ratio between the width of the cylinder and the inner diameter of about .3125. Other
embodiments will be described below.
Here and throughout the specification and claims, range limitations such as those
recited above may be combined and/or interchanged and such ranges identified can
include all the sub-ranges contained therein unless context or language indicates
otherwise.
In another particular embodiment, the DDT portion of the tube 114 is made up from a
steel tube with a 2 inch (approximately 50.8 millimeters) outer diameter with a length
of 40 inches (1.02 meters) between the head end ignition device 130 and the exit
aperture 116. Obstacles 120 were placed every 2 inches (approximately 50.8
millimeters) along the length of the DDT section, and each obstacle 120 was a Vi inch
diameter (about 12.7 millimeters) threaded bolt 450 driven through a hole 440 in the
wall of the tube 114 and protruding 1.25 inches (about 31.75 millimeters) into the
combustion chamber 101. Each bolt 450 was located circumferentially at a position
approximately 90 degrees from the bolt disposed immediately upstream, creating a
spiral configuration of bolts that extended along the length of the tube 114.
In testing, it was found that the use of cylindrical protrusions, such as bolts, provided
a high degree of robustness of operational parameters that could be used to support
detonation. For example, the use of bolts allowed for variation in the overall air/fiiel
ratio that was present within the combustion chamber at the time of ignition, while
still allowing the combustion to transition to detonation. Such variations in the
fuel/air ratio can be achieved by varying the duration of the fuel fill used prior to each
ignition, thereby varying the fraction of the overall chamber that is filled with fuel.
Such variations may also be achieved by changing the rate at which air or fuel is
introduced into the system.
During operation of a PDC, the heat and pressure produced inside the combustion
chamber can have a damaging effect on the surface of the combustion chamber 101.
In particular, the obstacles 120 that extend into the flow may be heated significantly
during combustion. Having thermally integrated obstacles assists in the transfer of
heat form the obstacles into the tube 114 itself. Because the tube is only heated from
one side, and can also be externally cooled, the tube 114 can be used as a heat sink to
dissipate heav that is transferred to thermally integrated obstacles 120. Such thermally
integrated obstacles will remain cooler during operation and will therefore remain
stronger and less liable to failure than non-thermally integrated obstacles.
In addition to assisting in transition from deflagration to detonation within the
combustion chamber, obstacles 120 in the form of bolts 450 as shown in Figure 8 may
also be removed from the tube 114 and replaced if they become damaged from
extended operation. Because such a removable obstacle can be replaced prior to
failure, degradation of performance of the PDC can be avoided without the need to
replace entire sections of the PDC tube 114.
In addition to the configurations discussed above, other configurations and
arrangement of the components illustrated can be used in creating appropriate PDCbased
cleaning systems. For instance, although tube 114 is illustrated as extending
substantially linearly along the x-axis in Figure 1, in an alternative embodiment, such
as that shown in Figure 9, the tube could contain a bend 510 along its length that
separates a first section 520 of the tube from a second section 530 of the tube. In such
an arrangement, the second section 530 is not coaxial with the first section 520. Such
an arrangement may include obstacles 120 disposed in one or more of the first section
520, second section 530 and bend 530. This configuration creates a combustion
chamber 101 that extends along the curved path of the tube from the head end to the
exit aperture 116. As with straight tubes, PDC embodiments with bends 530 may
optionally be connected to diverging chambers 300 or other downstream components,
or may exit directly into the device to be cleaned.
In another embodiment, a bend may be located in a diverging chamber, such that the
diverging chamber is divided into a first section and a second section which are not
co-axial. As discussed above with respect to Figure 9, such arrangements may
include obstacles in one or more of the first section of the diverging chamber, the
second section of the diverging chamber, or the bend of the diverging chamber. In
addition, the bend itself may be of a diverging cross-sectional area.
In yet other embodiments, a bend may be placed between the PDC and one or more
downstream devices. For example, in a particular embodiment, a bend may be
disposed between the aperture 116 of a combustion chamber and a diverging chamber
300. In addition to providing for more flexibility in the packaging of the components
of PDC-cleaner systems, bends along the length of the flow path may provide gas
dynamic benefits in maintaining the strength and development of a detonation wave
as it passes through such a curved flow path.
In another alternate configuration, a portion of the PDC cleaner may be disposed
within the vessel to be cleaned. For instance, Figure 10 illustrates a schematic view
of a PDC-cleaner having a straight tube 114 that is connected to a diverging chamber
300. However, rather than the exit 320 of the diverging chamber being disposed flush
with the wall 610 of an exemplary boiler 600, a portion of the diverging chamber is
disposed within the boiler 600 such that the diverging chamber 300 extends away
from the wall 610.
Another alternate configuration for a downstream device for use with the PDC
cleaners described herein is shown in Figure 11. A multiple exit chamber 650 is
illustrated schematically. Such a chamber 650 is formed from walls 660 that extend
into the vessel to be cleaned 600 away from the wall 610 of the vessel itself. The
flow from the PDC is directed into the multiple exit chamber 650 through a hole in
the wall 610 of the vessel. A plurality of exit holes 670 are disposed in the walls 660
of the chamber 650 through which the detonation wave and pressurized flow from the
PDC may be directed into the vessel 600. Such an arrangement can be used to more
particularly direct and localize the output from the PDC for more effective cleaning of
specific surfaces within the vessel.
In the illustrated embodiment, the multiple exit chamber 650 extends into the vessel
600 from a wall 610 on the side of the vessel. However, in other embodiments, the
multiple exit tube could be disposed along a wall 610 of the vessel such that the holes
670 are used to direct the detonations from the PDC at multiple locations along the
wall, as shown in Figure 12.
It will also be appreciated that such cleaning systems are not limited to industrial
boilers, but may be used to provide cleaning on a variety of different surfaces which
may experience fouling. Examples of vessels having surfaces which may be cleaned
using the systems and techniques described herein include but are not limited to:
vessels used in cement production, waste-to-energy plants, and coal-fired energy
facilities, as well as reactors in coal gasification plants.
Other features that may be used in varying embodiments of the systems described
herein include area reduction devices that may be disposed within the combustion
chamber 101 or downstream devices such as the diverging chamber 300 or multiple
exit chamber 650. Such area reduction devices may include but are not limited to
nozzles and Venturis, and may be used to increase the pressure within the various
chambers or to reflect shocks ifi order to enhance detonation transition and
propagation. Such devices may be integrally formed with the chamber walls, for
instance by machining, or may be attached to the chambers via techniques such as
frictional fitting, bolting or welding.
In addition to varying the configuration of the cleaner, as described above, the
duration and frequency of the combustion cycles and the cleaner operation can also be
varied. For instance, in a particular embodiment, the cleaner may be activated for
about 2 seconds during each minute of boiler operation. During these two seconds of
operation, the cleaner may operate at a detonation cycle frequency of about 2 Hz. In
such a system, a small number of detonations are used over a short period of time
each minute to shake loose accumulated debris.
In another embodiment, cleaner operation is used for about one minute, followed by a
minute of non-operation in order to allow the cleaner to cool down. Such a one-
minute-on, one-minute-off cycle of cleaner operation is repeated for a period of time,
such as 30 minutes. This operation may be executed once per day, or as needed
during continuous boiler operation. The frequency of the detonation cycle may be
fixed at 2 Hz, as in the previous example, or may be raised or lowered as desired.
Those of skill in the art will recognize that a variety of configurations of cleaner
operation duty cycles are possible, making use of a variety of detonation cycle
frequencies, without deviating from the teachings herein.
In a particular embodiment, the combustor of the cleaner is operated at a frequency
greater than or equal to about 1 Hz. In another embodiment, the detonation cycle
frequency is less than or equal to about 100 Hz. In varying embodiments, the
detonation cycle frequency may be: from about 1 Hz to about 1.5 Hz; from about 1.5
Hz to about 2.5 Hz; from about 2.5 Hz to about 4 Hz; from about 4 Hz to about 8 Hz;
from about 8 Hz to about 12 Hz; from about 12 Hz to about 18 Hz; from about 18 Hz
to about 25 Hz; from about 25 Hz to about 40 Hz; and from about 40 Hz to about 100
Hz. In particular embodiments, the detonation frequency is: about 2 Hz; about 3 Hz;
about 10 Hz; and about 20 Hz.
The various embodiments of cleaning systems described above thus provide a way to
achieve soot or ash removal from a boiler or other vessel. These techniques and
systems also allow for periodic operation without the need to shut down the device
being cleaned for extended periods of time.
Of course, it is to be understood that not necessarily all such objects or advantages
described above may be achieved in accordance with any particular embodiment.
Thus, for example, those skilled in the art will recognize that the systems and
techniques described herein may be embodied or carried out in a manner that achieves
or optimizes one advantage or group of advantages as taught herein without
necessarily achieving other objects or advantages as may be taught or suggested
herein.
Furthermore, the skilled artisan will recognize the interchangeability of various
features from different embodiments. For example, the use of bolts as obstacles
described with respect to one embodiment can be adapted for use with diverging
chambers described with respect to another. Similarly, the various features described,
as well as other known equivalents for each feature, can be mixed and matched by one
of ordinary skill in this art to construct additional systems and techniques in
accordance with principles of this disclosure.
Although the systems herein have been disclosed in the context of certain preferred
embodiments and examples, it will be understood by those skilled in the art that the
invention extends beyond the specifically disclosed embodiments to other alternative
embodiments and/or uses of the systems and techniques herein and obvious
modifications and equivalents thereof. Thus, it is intended that the scope of the
invention disclosed should not be limited by the particular disclosed embodiments
described above, but should be determined only by a fair reading of the claims that
follow.

CLAIMS:
1. A system for removing accumulated debris from a surface within a vessel, the
system comprising:
a vessel having a surface to be cleaned;
a fuel source providing a combustible fuel;
an air source providing a flow of air;
a pulse detonation combustor (100), comprising:
a combustion chamber (101) having a wall defining an airflow path from an upstream
end toward a downstream end;
an air inlet (102) disposed, upon the combustion chamber (101) and connected to the
air source and in flow communication with the combustion chamber (101);
a fuel inlet (104) in flow communication with the combustion chamber (101) and
connected to the fuel source;
an ignition device (130) disposed downstream of the fuel inlet (104) that is configured
to periodically ignite the fuel within the airflow and produce a flame; and
a plurality of obstacles (120) disposed along the airflow path and configured to
promote the acceleration of the flame into a detonation as it passes through the
combustion chamber (101);
wherein the downstream end (109) of the pulse detonation combustor (100) is
disposed on the vessel such that the shock wave associated with the detonation from
the pulse detonation combustor passes over the surface to be cleaned within the
vessel.
2. A system as in Claim 1 wherein the vessel is part of a device that remains in
operation during the operation of the combustor (100).
. A system as in Claim 1 wherein a fuel plenum (106) is disposed in flow
communication with the fuel inlet (104), the fuel plenum (106) having a plurality of
holes (110) that allow the fuel to be injected into the pulse detonation combustor
(100) through the plurality of holes.
4. A system as in Claim 1 wherein the air inlet (102) is in flow communication
with an interior of a hollow centerbody (112) extending along an axis of the
combustion chamber (101), the centerbody (112) having a plurality of holes (108)
providing flow communication between the interior of the centerbody (112) and the
combustion chamber (101).
5. A system as in Claim 1 wherein the air source provides a continuous supply of
air to the combustion chamber (101) through the air inlet (102) during the operation of
the combustor (100).
6. A system as in Claim 1 wherein the fuel passed through the fuel inlet (104) is
in gaseous form.
7. A cleaner for removing accumulated debris from a surface of a vessel, the
cleaner comprising:
a pulse detonation combustor (100), comprising:
a combustion chamber (101) having a wall defining an airflow path from an upstream
end toward a downstream end;
an air inlet (102) in flow communication with the combustion chamber (101) and
configured to be connected to an air source;
a fuel inlet (104) in flow communication with the combustion chamber (101) and
configured to be connected to a fuel source;
an ignition device (130) disposed downstream of the fuel inlet (104) that is configured
to periodically ignite the fuel within the airflow and produce a flame; and
a plurality of obstacles (120) disposed along the airflow path and configured to
promote the acceleration of the flame into a detonation as it passes through the
combustion chamber (101);
wherein the downstream end of the pulse detonation combustor (100) is configured to
direct the shock wave associated with the detonation in the pulse detonation
combustor to pass over the surface of a vessel to be cleaned.
8. A method for removing accumulated debris from a surface within a vessel, the
method comprising:
receiving a flow of air into a combustion chamber (101) through an air inlet (102), the
flow of air defining a downstream direction of flow;
receiving a flow of fuel into the combustion chamber (101) through a fuel inlet (104)
into the flow of air,
mixing the fuel and air within the combustion chamber (101);
periodically igniting the fuel and air mixture using an ignition device (130);
accelerating the flame into a detonation as it passes downstream through the
combustion chamber (101) by passing the flow over a plurality of obstacles (102)
disposed along the path of the flow of air through the combustion chamber (101);
directing the detonation into a vessel having a surface to be cleaned;
passing the shockwave associated with the detonation over a surface within a vessel to
loosen debris from the surface; and
blowing the loosened debris from the surface.
9. A method as in Claim 8 wherein the vessel is part of a device and the device is
in operation during the execution of the method.
10. A method as in Claim 8 wherein the steps of the method are repeated at a
frequency greater than about 1 Hz.