Note: Descriptions are shown in the official language in which they were submitted.
:12~3~
Method of image analysis in pulverized fuel combustion
The present invention relates to an image analysis method in
accordance with the preamble of claim 1 for controlling the
combustion of pulverized fuel.
Pulverized fuel combustion implies a method in which the
fuel, i.e., coal in conventional combustion but also peat to
an increasing extent, is milled into a very fine-grained
dust, which is then blown to the boiler via a nozzle using
stack flue gas or air as the carrier. In coal- and peat-fired
power plants, pulverized fuel combustion is a common method
of con~ustion which inherently m~rits an extremely high value
to improvements in the ignition and combustion of pulverized
fuel.
Monitoring of the combustion process is availed to reduce
the proportion of expensive auxiliary fuels. The monitoring
operation is implemented in several ways, of which optical
flame detectors are gaining ground thanks to the large
information avai~able from them.
A conventional method of monitoring combustion in a burner
is to use a video camera, often called a fire-box camera.
The video camera that produces a black-and-white or colour
video signal is located in a heat-resistant and cooled
protective tube. In addition to air cooling, some cameras
are provided with water cooling. The camera installati~ns
are generally provided with an automatic protection that
ejects the camera out from the fire-box when a system
malfunction is encountered.
Furthermore, flame monitoring is implemented with pyrometers
sensitive to radiation intensity as well as with other types
of detectors tuned to a narrow band of wavelengths. The
t
.......
, ~
/
3~7
quality of the combustion process is evaluated on the basis
of flame instability ~from the "DC" and "AC" components of
flame intensity). ~ more advanced version of the afore-
mentioned method is the cross-correlation method, also called
the incremental volume method.
Use of a camera in the conventional methods is restricted to
the monitoring of the averaged combustion process. The
operation of a single burner can be monitored only at the
ignition of the first flames and the extinction of the last
f lames. Detectors of the pyrometer category are hampe~ed by
such factors as placement and ali~nment of the detector, low
temperature o~ ~he flame, etc. Some types of detectors are
prone to erroneous response to nearby flames and background
radiation from the walls of the fire-box. A disadvantage of
tha cross-correlation method is, ~or instance, its high
sensitivity to changes in burning rate.
The aim o~ the present invention is to overcome the disadvant-
ages of the prior art technology and to provide a totally
new kind of monitaring system for the ignition and combustion
of pulverized fuel including a flame monitoring system which
is integxal with the boiler's protective system and conforms
to regulations issued by authorities.
The invention is based on monitoring the ignition and
combustion process over a large area by means of a video
camera and on the localization of the ignition area by the
identi~ication of the average intensity level corresponding
to the maximum intensity changes on selected lines o~ the
video signal, ater which t~e space coordinates corresponding
to this intensity level in the complete video frame signal
are determined.
., .
More specifically, the method in accordance with the in`vention
is characterized by what is stated in the characterizing
part of clalm 1.
.
,~ . , , "" . ~ . .
~373~7
The invention provides outstanding benefits.
The method in accordance with the invention provides a high
reliability because the combustion process is analyzed over
a large area. Furthermore, the method can be adapted to
accept a prede~ined permissible ignition area. Moreover, the
method is compliant with different ignition and combustion
conditions. Thanks to the compliancy of the method, the
number of false alarms can be appreciably reduced. In
accordance with the invention, a common analyzing apparatus
can be adapted to serve for several cameras, thereby reducing
equipment costs per burner. The method can be complemented
with fault diagnostics, which allows for a higher reliability
to be embedded into the system construction. Because
information is readily available on the quality of combustion
and ignition, the quantity of expensive auxiliary fuels can
be reduced and the quality of combustion improved. The
additional information obtained from combustion allows a
hi~her efficiency of the boiler to be achieved.
Next, the invention is examined in detail with he~p of the
~ollowing exemplifying embodiment according to the attached
drawings.
.
Figures la...lc show different types of fire-box cameras in
cross-sectioned side views.
Figure 2 shows schematically an image analysis system in
accordance with the invention.
Fiyure 3 shows a screen display layout in accordance with the
invention.
Figure 4 shows the structure of a computer program executing
the method in accordance with invention in a flow diagr~m form.
" ' ' ' ' . .
~Z8739~7
A fire-box camera, e.g., such a camera illustrated in Figures
la...lc, can be used for investigating the ignition process
of pulverized-fuel combustion. In its typical configuration,
the camera comprises an optics system 1, a protective tube 3,
and a photosensitive element, such as a solid-state matrix
sensor 2 shown in this embodiment. The photosensitive
component could also be a camera tube, but particularly in
conjunction with pulverized fuel combustion, a solid state
matrix camera is more applicable because the photosensitive
area of this kind of a sensor is fully erased during the
frame scan thus allowing an uncorrupted difference between
successive frames to be extracted. Recently, a remarkable
reduction in the size of solid-state cameras has occurred. In
principle, ~his facilitates the placement of the camera to the
tip of the protective tube 3 provided that the problems
associated with cooling can be solved. Furthermore, the camera
could conceivably be located in a tilted position thus
providing a more appropriate view into a greater number o
fire-box types than is possible with the currently used
perpendicular alignment. The tests were performed using a
solid-state camera with bandpass filters for appropriate
wawelength areas mounted in front of it.
Figure 2 illustrates the image analysis equipment used in
the performed tests. Con~entional technology is used in the
equipment. A standard video signal of the fire-box camera
(solid-state camera~ is routed via a selector to analog/-
digital converters. By way of the selector, the equipment
can serve several cameras. The A/D conversion used in the
equipment results in a 6-bit digital signal corresponding to
64 gray scale steps in the video picture. ~he video frame is
stored in an image memory, which in the described equipment
has a size of 256x256 pixels (picture elements). Hence,
each rame consists of 256 lines, and each line comprises
256 pixels, whose numerically ~uantized intensity values may
vary in the range of 0...63, according to the pixel intensity
value. The equipment has two identical image memories; the
image can be stored in either memory, but this application
~2~3~73~
uses image memory 1 for image input and image memory 2 fox
output of processed information. The image stored in the
image memory is printed via colour translation ta~les, which
assign a desired colour from a preset palette of colours to
each of the 64 gray levels. The image is shown in the standard
video signal format on a colour monitor, conventionally
through the R (red), G ~green), and B (blue) video outputs.
On the other hand, the image memories are configured to form
a part of the processing equipment memory space so that the
C~U can read and write pixels in the image memory. The depth
of image memories is 8 bits making 256 hues to be available
at the output although the input siynal is only in a 6--bit
format. The benefit of using 8 bits is that four frames from
the camera can be summed (under program control) into the
image memory without overflow.
The mass memories ~f the equipment comprise Winchester and
floppy-disk type drives serving as mass memories, a real~
time operating system, Pascal and PL/M compilers, which
combination permits concurrent digital image processing with
the development and testing of different klnds of algorithms.
In the following, the outline of program functions is given.
It must be understood that the version illustrated is simply
one possible embodiment of the solutions offered by the
inve~tion, In Figure 4, the ac~ual image analysis program is
~hown in flow diagram form.
Image analysis proceeds principally line-~y-line either
starting Erom le~t to right or vice versa, depending on the
location of the burner nozzle in the image, i.e., if the
nozzle is closer to right margin, the lines are read Erom
right to left.
~,
When the program execution is started, the program requests
the user for the following basic information:
~'73~7
- Line numbers of top and bottom lines outlining
the image area to be processed. The aim is not to
process the whole video frame besause the flame to
be analyzed does not fill the entire image.
Naturally, this procedure speeds image processing.
- A value for coefficient ~k), which controls the
image jitter at the ignition area boundaries, and
thereby variations in the averaged ignition area
shown on the trend display.
- A value for coefficient (b), which is related to
the smoothing of minimum and maximum values o:E
ignition area boundaries.
- Furthermore, the trend display update interval
can be defined in either terms of time or given
number o~ processed images after which the disp~ay
is updated.
- In addition, information on the sidedness of the
nozzle, or the side from which picture processing
is to be commenced, can be given to the program.
Among other things, the aforementioned ~ariables and tables
are loadPd with preset values at the initialization stage.
The tables used in the program are as follows:
L~able, HTable, HMean, LMin, and HMax, each with a size o~
256*2 bytes. The size of trend tables TrMean, TrMin, and TrMax
is selected sufficiently large to make it possible to store
also such historical information into them that does not fit
onko the display. When required, this information is th~n
readily available The memory contents of all tables are
cleared, except ~or tables L~ean, HMean, LMin, and HMax, which
are used for computation of averaged values over a longer
, '
- ~ ,
,
.
.
'7397
period. The aim is to initially load these ta~les with initial
values that are as close as possible to the boundaries of the
expected ignition area. This procedure reduces the time
required for the trend display to settle to its actual value.
In order to find the ignition area, an image is analyzed for
four scan lin~s on which the gradient of pixel intensities is
highest. This is implemented by counting from the start (or
end) of the line the intensity value sum of three successive
pixels which is then subtracted from the intensity value sum
of next stri.ng of three pixels. The difference obtained is
proportional to the intensity gradient. The line is subjected
pixel by pixel to the routine described above. The sums
obtained from two pixel strings rendering the highest
differences are stored. The average of these pixel intensities
is the desired boundary threshold for the processed line.
When each of the four lines is processed for the highest
pixel intensity gradient, the average value of these intensity
levels is computed. ~he front and rear boundaries of the
ignition area are then obtained by subtracting or adding a
preset constan~ from or to the aforementioned a~erage value,
respectively.
Next, an image is stored for computation of ignition area
boundaries. Starting from the beginning of a line, sums of
intensity values of four successive pixels are computed.
When the average computed from the sum exceeds the intensity
threshold of the front boundary computed by way of the routine
described in the foregoing, the front boundary is considered
~ound. The (vertical) video matrix column at which the
boundary was ~ound is stored in the table LTable. T~e same
line is further processed until the rear boundary is found.
Equally, this boundary position is stored in its appropriate
table HTable. To increase the speed of front boundary search,
search is not commenced on the next line from the beginning
but instead close to the position where the boundary was
found on the preceding line.
.
.
', ~ '-
, ~ .
,
.
:
3~
The tables LTable and HTable mentioned above are used for
the update of the tables LMean and HMean, into which the
temporally averaged spatial coordinates of the front and
rear boundaries are computed according to the formula:
LMean=k*LMean+(1-k)*LTable,
where k is the coefficient entered in the initialization
routine with a range of O ~ k <1. Thus, the table LMean is
updated line by line with new values which take into account
ignition area inormation from the last recorded image,
weighed in a desired manner. ~y increasing the value of the
constant k, this procedure helps smoothing the random
variations of intensity values and results in a realistic
indication of actual changes on the trend display. (An
equivalent procedure is applied to the tables HMean and
HTable associated with the intensity values o ignitio~ area
rear boundary).
Further, the variations of front boundary minimum~values and
rear ~oundary maximum values are monitored by gathering these
values to their respective tables LMin and HMax. These tables
are updated by the procedure described in the following. If
the front boundary of a certain line in the latest stored
image has been found spatially earlier than the value given
by the table L~in for the corresponding line, the values on
that row of the table are replaced by the values obtained
from line of the image, or expressed in a formula:
if hTable ~ LMin, then
LMin = LTable.
Next, the value in the table LMin is gradually corrected so
as to make it 810wly approach the temporally averaged value
of the ignition area front boundary. This i9 accomplished by
the formula:
LMin = ~Min~b*(LMean-LMin),
...
.
.
.
:. .
.
~Z~373~
where ~ ls the coefficient (with a ~alue O ~ b ~ 1) described
in the foregoing. The greater the ~alue of coefficient b, the
faster the minimum value in the table LMin approaches the
value given in the table LMean.
Correspondingly, for the computation of the maximum value,
the following formulas are applied:
if HTable > HMax, then
HMax = HTable and HMax = HMax-b*(HMax-HMean).
Information described above is gathered and updated into the
tables at about 5 s intervals, ater which the combined
averaged intensity value of all scan lines from the ignition
area front and rear boundary tables is computed into a table
TrMean. In addition, the average of all lines from the minimum
value table is computed into a table TrMin, and the average
of all lines fr~m the maximum value table is c~mputed into a
table TrMax, respectively. Information obtained in this
manner is then shown toge~her with the average, minimum, and
maximum values on the trend display. The variation range
between the minimum and maximum values is indicative of the
in6tability of the flame, while their mutual distance
characterizes the width of the ignition area.
After a four-fold update of the trend display, the current
image of the flame is shown on the display in a modified
colour picture. The modi~ied colour display is accomplished
by assigning d.ifferent hues of blue varying from dark blue to
light blue to the dark areas outside the ignition area
boundary up to the boundary. ~t the boundary the colour is
changed to red, which changes towards the brighter areas of
flame from dark red to light red, and finally, to white.~
.
.~L2~3~7
A single screen can be used for simultaneous presentation of
information from two different cameras as shown in, e.g.,
Figure 3.
The method illustrated in the foregoing represents only one
possible embodiment within the scope of the invention. The
described methods can be applied to equipment different from
those described above. It is also possible to solve the
problem by using dedicated electronics for the identification
of ignition area values. This approach disposes of image
storage for the input image signal. The dedicated electronics
integrates the video signal by lines and stores the adclresses
(or locations) where the video signal change exceeds the
preset thresholds assigned to intensity values of the ignition
area boundaries. The boundary locations (addresses)
appropriately found on each line are sent by the electronics
to the processor. An appreciable saving in time is obtained
by way of this method.
Moreover, it is possible to construct a preprocessing unit
that logs the intensity values from the entire image to the
tables, a~ter which tables are submitted to analysis. Extended
electronics integration could provide the preprocessing
electronics with a facility to compute in real time ~i.e., by
processing each frame of the video signal) the tables for
the averaged ignition area values as well as for the
fluctuations of the ignition area. Thereby, the system could
also provide for an extremely fast flame monitor. Then, the
flame monitoring ~unctians could be con~igured more reliable
than those offered by ~ conventiorlal flame monitor.
The image display can function well without an image memary
and D/A converters. Due to the synthetic nature of the
displayed picture, the camputatianal resul~s may be output to,
e.g., a graphic terminal.
- . ,
