The present invention relates to an On-line LED opacity monitor for
detecting and monitoring of dust (i e, dust density) released out of a duct,
chimney or stack of an industrial unit.
In view of rapid industrialization and consequent release of industrial
effluents into the atmosphere causing immense harm to the environment it has
now become mandatory for industrial units to monitor and control the emissions
of waste pollutants from ducts, stacks and chimneys with specific requirements
differing from one process to another process To assess the load accurately it
is required that the measurement must be continuous and have reliability.
Presently most of the industries in India are using Iso Kinetic Sampling
technique for quantifying the dust emissions This method is tedious, not
continuous, does not give accurate results due to the procedure adopted for the
measurement and is time consuming due to the nature of the measurement
Therefore the legal requirements associated with these emissions are diverse,
and it is therefore necessary to monitor them with a continuous On-line
instrument
Optical Transmission method provides the only accepted technique for
continuous measurement of these emissions. These instruments operate on the
principle that the intensity of a light beam (LED source) transmitted through the
duct will be reduced by particulate matter The amount by which it is reduced is
a function of the density of the particulate in the flue gases The measured
variation of the light intensity is used to calculate the density of the particulate
matter.

The aim of the present invention is to develop an equipment capable of
measuring continuously and on-line, the opacity and density of the particulate
matter present in the flue gases, using a low cost high performance Light
Emitting Diode (LED) as a source. The system should be capable to withstand
the harsh environmental conditions prevailing in industry world wide.
It has so far been recognised that optical method can give a continuous
on-line and non-contact measurement There may be two types of optical
methods, namely, Single pass and Double pass In single pass method the
optical beam is allowed to pass only once across the duct, stack or chimney
through which the flue gas is allowed to release in the atmosphere. The
detector, which measures the intensity of this attenuated beam, is placed on the
side of the duct, stack or chimney opposite to that of the incident beam The
only problem of single pass method is that, when the particulate density in the
flue gas is very small, that is, change in intensity of the incident beam is very
small, the detector may not respond to the difference in intensity.
To avoid the above problem, the present invention provides a double
pass method In this double pass method the beam of light is allowed to pass
across the duct, stack or chimney twice by putting a mirror/retroreflector on the
stack on the side opposite to that of the incident beam
The object of the present invention is to provide an equipment capable of
measuring continuously and on-line the opacity and density of particulate
material present in the flue gases coming out of a duct, chimney or stack of an

industrial unit, using a low-cost, high-performance Light-Emitting Diode (LED) as
a source of light The system is capable of withstanding the harsh environment
conditions prevailing in the industry at the place of installation
The present invention provides an on-line LED opacity monitor for
detecting and monitoring of dust released out of a duct, chimney or stack of an
industrial unit, said monitor comprising
(a) a LED source provided on one side of the duct, chimney or stack at the
appropriate height,
(b) a first lens, an aperture and a second lens provided on the same side of
the duct, chimney or stack as the LED source for the light beam from the LED
source to pass there through, said light beam coming from the second lens being
parallel,
(c) a beam splitter onto which the light beam from the second lens is allowed
to fall, so that the light beam is split into a reflected component and a transmitted
component,
(d) a first focusing lens for focusing the reflected component on to a
reference detector, said detector measuring the intensity of said reflected
component,
(e) a processing unit having a first input for receiving the measured intensity
of the reflected component from the reference detector, and an output connected
to a computer,
(f) a retroreflector provided on the other side of the duct, chimney or stack
remote from the LED source, to receive and reflect the transmitted component

through windows on the opposite sides of the duct, chimney or stack, so that the
transmitted beam from the beam splitter passes through the duct, chimney or
stack, falls on the retroreflector and again travels back through the said windows
in the same path as the transmitted component to fall on the beam splitter to
form a test beam ;
(g) a second focusing lens for receiving the test beam, reflected by the beam
splitter to focus the reflected test beam on to a test detector which measures the
intensity of the reflected test beam ;
(h) a second input of said processing unit being connected to said test
detector for receiving the measured intensity of the test beam ,
(i) based on said measured intensities of the reflected component and the
test beam, said processing unit calculating opacity and dust density with the help
of software in the processing unit and displaying the opacity and dust density
along with time on the monitor of said computer
Thus, in the monitor, the test beam passes twice through the
duct/stack/chimney and gets attenuated due to presence of the dust in the flue
gas passing through duct/stack/chimney
The invention will now be described with reference to an embodiment
shown in the accompanying drawings in which Fig 1 shows an on-line LED
opacity monitor according to the present invention.
In Fig. 1, the monitor according to the present invention is mounted on a
duct, chimney or stack (p) (hereinafter referred to as "stack" for brevity) at a
location where the flow of the particulate matter in the stack is linear and smooth.

At this location, the stack is provided, at diametrically opposite sides, with two
windows (m, n) preferably covered by glass covers, so that the gases and
particulate matter are prevented from coming out and dusting the
components of the monitor The windows are preferably provided with means
for cleaning the glass of the windows (not shown)
A LED source of light (a) is provided on one side of the stack (p) so that
a light beam from the LED source is in alignment with the windows (m, n) A first
lens (b) passes the light beam from the LED source (a) through an aperture (c)
onto a second lens (d). The out put from the lens (d) is parallel, and is allowed
to fall on a beam splitter (e).
The beam splitter (e) is a semi-transparent mirror which splits the light
beam from the second lens (d) into two components, namely, .a reflected
component and a transmitted component which are _complementary to each
other. The reflected component forms a reference beam which is not
attenuated The transmitted component forms a test beam The intensity of the
two components depends on the thickness of the reflective coating provided on
the beam splitter.
The reference beam from the beam splitter (e) is focused by a first
focusing lens (f) onto a reference detector (g) which measures the intensity of
the reference beam The measured intensity of the reference beam is fed to a
first input 01) of a processing unit (j).
'
The test beam from the beam splitter (e) passes through the windows
(m, n) of the stack (p) and falls on a retroreflector (o) The retroreflector (o)

reflects the test beam back through the windows (m, n) along the same path as
the incident beam, to fall on the beam splitter (e). Here the purpose of the
retroreflector is to reflect back the incident beam into the stack So, the final out
put test beam has traversed from beam splitter (e) to retroreflector (o) and
again from retroreflector (o) to beam splitter (e). To increase the sensitivity of
the system, the test beam is forced to pass twice through the duct/chimney/stack
by the retroreflector so that the test beam is attenuated two times due to the
presence of dust particles in the flue gas For this reason this type of system is
called the double pass system.
The reflected test beam falling on the beam splitter (e) is focused by a
second focusing lens (h) on to a test detector (i) The test detector (i) measures
the intensity of the test beam The output of the test detector (i) is connected to
a second input (j2) of the processing unit 0) so that the processing unit receives
the measured intensity of the test beam from the test detector (i)
The reference detector (g) and the test detector (i) must be sensitive to
the frequency region of the light beam from the LED source (a) as well as the
intensity of falling light. They should also be capable of fast response. These
can be identified by a person skilled in the art to which the present invention
relates. It is preferable to provide detectors of PIN photo diodes
The processing unit (j) is the heart of the system and it contains the
required electronic components and circuits It does all the calculations based
on the measured intensities received at inputs (j1, j2) The output from the
processing unit (j) is fed to a computer consisting of a CPU (r), a monitor (k) and

a keyboard (I) Based on the measured intensities of the reference beam and of
the test beam, the processing unit (j) feeds the computer for display of opacity
and dust density with the help of necessary software installed in the computer.
The processing unit (j) and computer are installed in a control room and
connected to the reference detector (g) and test detector (i) through inputs (j1)
and (j2) The computer is connected to an output (j3) of the processing unit (j)
The computer-monitor (k) displays an over-emission video alarm whenever the
pre-set emission limits are crossed An audio alarm (not shown) may also be
connected to the computer to give an audio alarm in the above case.
It is advisable to provide a cover (s) to encase the LED source (a), the
focusing lenses (f, h), lenses (b, d) and aperture (c), so that these parts are not
affected by the surrounding environment. For the same reason, the
retroreflector (o) may be provided with a cover (t).
Measurement of opacity and density from the measured intensities of the
reference beam and test beam will now be explained
The opacity and dust density are derived from the measurement of
intensities by the following calculation
Iref = Reference Beam Intensity (Response of the Reference Detector)
Itest = Test Beam Intensity (Response of the test Detector)


Opacity in % = OP x 100 (2)
Extinction E = l„ (1/T) (3)
where ln is a natural logarithm
The following standard relationship between Extinction E and Dust
Density is used to calibrate the system and to convert the measured Extinction
value into Dust Density.
Dust Conversion Factor, D = fine Dust Density / Extinction
Fine Dust Density S is measured by the standard accepted Iso- Kinetic
Sampling Method in mg/m3, for a particular period
At the same time, the average Extinction value E, over the same period
as the above Iso-kinetic Sampling Method, is measured by the Opacity Monitor
So, for the particular stack,
S
Dust Conversion Factor D = — . (4)
E
This Dust conversion factor D mg/m3 per Extinction is used to convert
any measured Extinction value Ei by the Opacity Monitor, to Dust Density in
mg/m3, as
Dust Density = Ei x D mg/m3 (5)
The processing unit 0) will calculate the above Opacity and Dust density
as per equations (2) and (5) and the computer will display the same as per the
software command

Dust conversion factor D is constant for a particular Stack/Duct/chimney,
and is stored in the computer, till there is a change in the Stack/Duct/Chimney or
the industrial process.
Following are the main features of the present invention
continuous on-line and non-contact method ,
high reliability,
proven technology,
display of opacity and dust density ,
central logging and reporting capability ,
continuous data logging ,
integration with next level,
simple installation ,
low maintenance ,
design for mechanical mounting of the system will be rugged, simple and
versatile to make installation easy and fast,
Modular concept of design is envisaged for easy maintainability
The light from the LED source (a) is allowed to pass through the lens (b),
aperture (c) and lens (d) to shape the beam It is then allowed to fall on the
beam splitter (e) The reflected component (reference beam) from the beam
splitter passes through the focusing lens (f) on to the reference detector (g)
which measures the intensity of the reference beam The other beam transmitted
through the beam splitter (e), known as test beam, is allowed to pass across the
stack along the diameter of the stack. It is allowed to retrace the path after

getting reflected by the retroreflector (o) fitted at the opposite side of the stack
This beam, after passing twice across the stack, is again reflected at the beam
splitter (e) and focused on the detector (i) by the focusing lens (h) This detector
(i) measures the intensity of the test beam after double pass across the stack
The output from the detectors (g) and (i) are fed to the processing unit 0)
On the basis of these two measurements and using a custom made software the -"— ! — ■ ■■——• >— .>..«.-..—--^-.---^^.^^^^^^^ __
monitor (kLdisplays the required parameters eg 'Opacity' and 'Dust density'. '
The processing unit will continuously store the data in the memory which
can be retrieved as and when required.
The software developed for the purpose has several features
incorporated in it, such as, 'Calibration', 'Setting', 'Processing'.
By clicking the 'Calibration' a dialogue box will open where P
(calibration factor) and S (isokinetic method) values are to be entered
By clicking the 'Setting' a dialogue box will open where 'Integration
time' (any value from 1 to 10 minutes) and 'Alarm setting' (any desired opacity
value) are to be entered
By clicking the 'Processing' the measurement of the opacity and dust
concentration will start and the monitor (k) will display the average values of the
integration taken over the period of 'Integration Time'.
Whenever the average of the integration value of the 'Opacity' crosses
the above 'Alarm Setting' value, the monitor (k) will start blinking with a red
coloured font message to convey that 'Alarm Setting' value has been crossed.
An audio alarm (not shown) can also be installed with the monitor to send

any audio alarm. However, till the operator attends to the monitor and
conveys through the monitor that he has observed this, the video and the audio
message will continue. During this period also the measurement will continue to
be recorded
The on-line opacity monitor according to the present invention has the
following advantages
i) Low cost high output LED source is used.
n) The temperature stability of LED is much higher than Laser and other
conventional sources, so that the system can be operated at elevated stack
temperatures
In) By using high frequency modulated LED source, the effects of stray light
and ambient light conditions are eliminated.
iv) Since no laser source is used in the system, no high voltage generation
is required, ensuring no chance of any accident.
v) The measurements are made at the rate of approximately 2500 samples
per second. At these high sampling rates, the effect due to variation of light
intensity due to fluctuation in supply voltage is eliminated This also eliminates
the effect due to vibration of the stack
vi) By using the double pass method along with a highly sensitive sensor,
the system is capable of measuring very low particulate density in the stack
vii) By using much bigger diameter receiving optics than the diameter of the
beam, the alignment of the system has been made much simpler.

vin) Maintaining a small positive pressure inside the system minimizes dust
accumulation on the optics
The on-line opacity monitor can be used in the following industrial units •
Steel plants ,
Fertilizer industry ,
Sugar industry,
Cement plants,
Chemical & Petro Chemical industry ,
Waste incineration plants ,
Furnace plants with hard coal, brown coal, fuel oil and mixed
combustions,
The preferred operation parameters are as given below


Although the invention has been described with reference to a particular
embodiment, the invention is to be construed as including all mechanical and
electronic equivalents of the units described hereinbefore.

WE CLAIM :
1 An on-line LED opacity monitor for detecting and monitoring of dust
coming out of a duct, chimney or stack (p) of an industrial unit, said monitor
comprising .
(a) a LED source (a) provided on one side of the duct, chimney or stack (p)
at the appropriate height,
(b) a first lens (b), an aperture (c) and a second lens (d) provided on the
same side of the duct, chimney or stack as the LED source for the light beam
from the LED source to pass there through, said light beam coming from the
second lens being parallel,
(c) a beam splitter (e) onto which the light beam from the second lens is
allowed to fall, so that the light beam is split into a reflected component and a
transmitted component,
(d) a first focusing lens (f) for focusing the reflected component on to a
reference detector (g), said detector measuring the intensity of said reflected
component,
(e) a processing unit 0) having a first input (j1) for receiving the measured
intensity of the reflected component from the reference detector, and an output
(j3) connected to a computer,
(f) a retroreflector (o) provided on the other side of the duct, chimney or
stack remote from the LED source, to receive and reflect the transmitted
component to pass through windows (m, n) on the opposite sides of the duct,

chimney or stack, so that the transmitted beam from the beam splitter passes
through the duct, chimney or stack, falls on the retroreflector (o) and again
travels back through the said windows, in the same path as the transmitted
component, to fall on the beam splitter (e) to form a test beam ,
(g) a second focusing lens (h) for receiving the test beam reflected by the
beam splitter (e), to focus the reflected test beam on to a test detector (i) which
measures the intensity of the reflected test beam ,
(h) a second input (j2) of said processing unit (j) being connected to said test
detector (i) for receiving the measured intensity of the test beam ;
(i) based on said measured intensities of the reflected component (j1) and

the test beam (j2), said processing unit (j) calculating opacity and dust density
with the help of software in the processing unit and displaying the opacity and
dust density along with time on the monitor (k) of said computer.
2. On-line LED opacity monitor as claimed in claim 1, wherein the test
detector (i) and reference detector (g) comprise PIN photo diodes
3 On-line LED opacity monitor as claimed in claim 1 or 2, wherein said
windows (m, n) are provided with glass covers to prevent dusting of the
components of the monitor by the gases and particulate material passing
through the duct, chimney or stack

4. On-line LED opacity monitor as claimed in any of claims 1 to 3, wherein
an over-emission alarm device is connected to said computer for providing an
alarm when the pre-set emission limits are crossed, as determined by the
computer.
5 On-line LED opacity monitor as claimed in any preceding claim wherein,
said LED source (a) and optical elements are encased in a cover (s), and the
retroreflector (o) is encased in another cover (t).
6. On-line LED opacity monitor substantially as herein described,
particularly with reference to the accompanying drawing

Abstract

ON-LINE LED OPACITY MONITOR FOR A DUCT, CHIMNEY OR STACK
An on-line opacity monitor for detecting and monitoring of dust coming
out of a duct, chimney or stack (p) comprises
a LED source (a), a first lens (b), an aperture (c) and a second lens (d) provided
on one side of the duct (p) to form the light from the LED source into a parallel
light beam ;
a beam splitter (e) for splitting the light beam into a reference beam focused by a
first focusing lens (f) onto a reference detector (g), and a transmitted beam ;
a retroreflector (o) provided on the side of the stack remote from the LED source
(a) to receive the transmitted beam from the beam splitter (e) through windows
(m, n) provided on the stack (p), so that the transmitted beam is reflected back
by the retroreflector (o) onto the beam splitter (e),
a second focusing lens (h) for focusing the reflected transmitted beam from the
beam splitter (e) to a test detector (i);
a processing unit (j) having inputs (j1, j2) connected to the reference detector (g)
and test detector (i) and an output (j3) connected to a computer