CONTINUOUS NOx MONITORING SYSTEM
FIELD OF THE INVENTION
The present invention in general relates to system for continuous monitoring of NOx in
gas emission and to a method for such continuous monitoring and in particular to a
non dispersive infra red system and method for continuous monitoring of NOx in a gas
emission such as flue gas that is emitted out of ducts/stacks/chimneys in industrial
factories, automobiles and other industries/vehicles during their operation.
BACKGROUND OF THE INVENTION
In view of rapid industrialization and consequent release of industrial effluents into the
atmosphere causing immense harm to the society, it has now become mandatory for
industrial units to monitor and control the emissions of waste pollutants from ducts / stacks
/ chimneys with specific requirements from process to process. Therefore, the legal
requirements associated with these emissions are diverse, and to monitor them with a
continuous On-line instrument is an indispensable necessity. To assess the load
accurately it is required that the measurement must be continuous and reliable.
Oxides of Nitrogen (hereinafter and hereinbefore referred to as NOx) is found in the emissions
from industrial factories, automobiles, air craft etc. and contributes to the production of acid
rain, smog, and the depletion of the ozone layer and accordingly are considered a most
important category of air pollutants. Monitoring of oxides of nitrogen in the flue gases is
essential and is one of the recommendations of national metallurgical laboratories for
cleaner production under life cycle assessment study.
Presently most of the industries in India, are using imported Non Dispersive InfraRed
(hereinafter referred to as NDIR and IR wherever referred means Infra Red) systems for
the measurement of NOx. As indicated before, continuous and on line monitoring of gases
such as NOx in emissions are now an indispensable necessity, for running of operations
which involve generation of such emissions, having regard to the immense threat caused by
NOx to the environment. However, NDIR equipment and technology for continuous
monitoring of NOx gas emission could not be hitherto arrived at/applied to the best of the
knowledge of the applicants and inventors.
Hence, there was a long felt need for a system and method for continuous monitoring of
NOx gas emission by applying NDIR technology.
The present invention meets the aforesaid long felt need.
All through out the specification including the claims, the words "NOx", "converter",
"detector", "Computer", "Non-Dispersive", "Lens", "filter", "emission", "flue gas" are
to be interpreted in the broadest sense of the respective terms and includes all
similar items in the field known by other terms, as may be clear to persons skilled in
the art. Restriction/limitation, if any, referred to in the specification, is solely by way
of example and understanding the present invention.
OBJECTS OF THE INVENTION
It is the principal object of the present invention to provide a system for continuous monitoring
of NOx in gas emission emitted from industrial factories, automobiles and other
industries/vehicles involving such generation of said emission, during their operation.
It is another object of the present invention to provide a system for continuous
monitoring of NOxin gas emission to ensure reduction in atmospheric pollution.
It is yet another object of the present invention to provide a system for continuous
monitoring of NOxin gas emission by application of non dispersive Infra Red technology.
It is another object of the present invention to provide a system for continuous monitoring of
NOx in gas emission which is adapted to withstand the harsh environmental conditions
prevailing in industry worldwide.
It is a further object of the present invention to provide a system for continuous monitoring of
NOx in gas emission which involves low maintenance, simple and versatile constructional
features.
It is yet another object of the present invention to provide a system for continuous monitoring
of NOx in gas emission which involves easy and fast installation.
It is a further object of the present invention to provide a system for continuous monitoring of
NOx in gas emission which possesses high reliability and is adapted to display of NOx
concentrations in ppm and Mg/M3,
It is a further object of the present invention to provide a system for continuous monitoring of
NOx in gas emission which is adapted to issue video and audio over emissions alarms.
It is a further object of the present invention to provide a system for continuous monitoring of
NOxin gas emission which is adapted to control of source intensity against aging.
It is yet another object of the present invention to provide a system for continuous monitoring
of NOx in gas emission which is adapted to generate continuous data logging.
It is a further object of the present invention to provide a system for continuous monitoring of
NOx in gas emission which avoids the application of moving parts.
It is a further object of the present invention to provide a system for continuous monitoring of
NOx in flue gas emitted from industrial factories, automobiles and other industries/vehicles
involving such generation of said emission, during their operation.
It is yet another object of the present invention to provide a method for continuous monitoring
of NOx in gas emission emitted from industrial factories, automobiles, aircrafts and other
industries/vehicles involving such generation of said emission, during their operation, by
application of non dispersive Infra red technology.
How the foregoing objects are achieved and the other aspects of the present invention
will be clear from the following description which is purely by way of understanding and
not by way of any sort of limitation.
SUMMARY OF THE INVENTION
Accordingly the present invention provides a system for continuous monitoring of
NOx in gas emission, including an infra red light source adapted to emit light beam
having absorption wave length of NOx gas, said beam being caused to pass through
said gas emission, said light source being operatively connected to a sample detector
for focusing said light beam on said detector, said detector being adapted to measure
and store NOx intensity in the event of both absence(I0) and presence(I) of NOx gas
in said emission , said detector being operatively connected to a processing unit
adapted to identify and reproduce in interpretable form, signal intensity measured by
said detector in the event of both presence and absence of said NOx in said emission,
whereby said system is adapted to carry out continuous on line monitoring of NOx ,
during running of the main operation.
In accordance with preferred embodiments of the system of the present invention:
-said light source is a broad band infra red light source, built in with parabolic
reflector and there exists a beam splitter operatively connected to said light source
and adapted to cause transmitted beam to pass through said gas emission to focus
on said sample detector and to cause the reflected beam to focus on a reference
detector to which it is operatively connected, said reference detector being
operatively connected to said signal processing unit.
-said reference detector is adapted to check the aging of said infra red light source by
means of a knob provided outside said signal processing unit.
-said emission is caused to pass through a sample gas cell and said transmitted beam
is caused to focus on said sample detector by an infra red lens.
-said processing unit is adapted to receive and identify intensity signals from said
sample detector and said reference detector.
-said signal processing unit is equipped with A/D converters to compute signal
intensity ratio(I/I0) out of the intensity values measured and stored by said sample
detector, in the event of presence(I) and absence(I0) of NOx in said gas emission .
-said signal processing unit is operatively connected to a computer system adapted to
plot a calibration curve for a selected path length and to display the same, said
system being further configured to generate an alarm in the event of said emission
exceeding a value pre-set according to a reference value and as accordingly reflected
by said curve.
-a narrow band NOx filter is built one each on said sample detector and said reference
detector.
The present invention also provides a continuous method of monitoring NOx in gas
emission, by applying a continuous NOx monitoring system as described hereinbefore,
said method including:
-passing infra red light beam having absorption wave length of NOx gas through said
gas emission,
-focusing said light beam on said detector, measuring and storing NOx intensity by
said detector both in the event of absence(I0) and presence(I) of NOx gas in said
emission gas,
-processing said I0 and I values by said processing unit,
-plotting a calibration curve out of the values obtained and
-displaying the same and
-optionally, issuing an alarm in the event of said emission exceeding a value, pre-set
according to a reference value and as accordingly reflected by said curve,
the said method being performed continuously and on line, during the running of
main operation.
In accordance with a preferred embodiment of the method of the present invention:
-said gas emission in flue gas, emitted from industrial factories, automobiles and
other industries/vehicles involving such generation of said emission, during their
operation.
BRIEF DESCRIPTION OF THE ACCOMPANYING DRAWINGS
The nature and scope of the present invention will be better understood from the
accompanying drawings, which are by way of illustration of some preferred embodiments
and not by way of any sort of limitation. In the accompanying drawings:
Figure 1 illustrates a preferred embodiment of the system for continuous monitoring of
NOx in gas emission, according to the present invention.
Figure 2 illustrates a calibration curve and NOx trend curve generated by the system in
accordance with the present invention.
DETAILED DESCRIPTION OF THE PRESENT INVENTION
The present invention provides a system and method for continuous monitoring of NOx in gas
emission emitted from industrial factories, automobiles, aircrafts and other
industries/vehicles involving such generation of said emission, during their operation by
application of non dispersive Infra Red technology. The technical advancements and
economic significance achieved by the present invention were hitherto unknown and not
conceived by persons skilled in the art.
In the above context it is hereby clarified, that simply by way of example and for
understanding the present invention and not by way of any limitation, the system for
continuous monitoring of NOx in gas emission, in accordance with the present invention,
has been explained with reference to flue gas. It should be understood to persons skilled in
the art that, the present invention is equally applicable to other gas emissions having NOx and
will be equally effective, in achieving the objects of the present invention.
The present invention provides a indigenous NDIR equipment capable of monitoring continuously
and on-line the NOx gas present in the flue gases. The system is capable to withstand the harsh
environmental conditions prevailing in industry world wide.
Following are the features of the system for continuous monitoring of NOx in gas emission,
* continuous on-line and non-contact method
* high reliability
* display of NOx concentration in ppm and Mg/M3
* Video and audio over emissions alarms
* continuous data logging
* integration with next level
* no moving parts
* control of source intensity against aging
* low maintenance
design for mechanical mounting of the system is
rugged, simple and versatile to make installation easy and faster
The system in accordance with the present invention developed for the measurement of
continuous NOx gas concentration in flue gas, operates on the principle of Infra Red Absorption.
Gas molecules are made up of atoms that are bonded together. These bonds constantly
undergo vibrations and rotations. The frequencies of these vibrational and rotational motions
are the strong functions of the size of the atoms and bond strengths. By nature, these
frequencies match with the frequencies of the middle portion of the infrared spectrum (called
mid IR).
Infrared radiation interacts with all molecules, except homonuclear diatomic like Oxygen (O2),
nitrogen (N2), Hydrogen (H2) and Chlorine (Cl2) by exciting the molecular vibrations and
rotations. The oscillating electric field of the IR interacts with the electric dipole of the
molecule, and when the IR frequency matches the natural frequency of the molecules, some of
the IR power is absorbed. The pattern of wavelengths or frequencies absorbed identifies the
molecules in the sample. The strength of absorption at particular frequencies is a measure of
their concentration.
The absorbance of the gas is directly proportional to its concentration, in accordance with the
Lambert-Beer Law:
I = IoeA.d
Where I0 = Initial light intensity emitted from the IR light source
I = Intensity of IR light when the sample gas present in gas cell
A = Absorbance of the gas
d = Optical path length of gas cell
The absorbance of the gas is directly proportional to its concentration.
So above equation will become
I = I = IoeA.d
Where C = Concentration of sample gas in gas cell.
K = preoperational constant
1 lo
For the given geometry, d is fixed and only two parameter I0 and k remain to be established
before this formula can be used to experimentally determine C. So the light intensity ratio I/Io
gives the concentration of the sample gas.
The term non-dispersive refers to the fact that all the light passes through the gas sample
and is only filtered immediately before the detector. Some times the desired filter will be
built onto the detector itself.
In dispersive technique a prism or grating is used to pre-select the desired
wavelength of light and pass only this through the gas sample to the detector.
Dispersive IR techniques are usually used in bench top analytical instruments for their
ability to scan a broad wavelength range. However they tend to be larger, heavier, more
complicated and more expensive and therefore are less suitable for portable instruments.
In the present investigation, NO, concentration is measured experimentally and NOx
concentration is calculated as given below.
NO to NOx Convertion :
It is generally assumed that nitric oxide NO contained in combustion gases makes up about
95% of the total amount of nitrogen oxides NOx. Some combustion analyzers calculate
the total concentration of nitrogen oxides Nox according to the formula
NOx[ppm] = NO[ppm] / 0.95
Calculation of NOx from NO is possible if there is a reliable and known ration between NO and
NOx in the test gas.
In general a common ratio is fixed in the standards of most countries. This ratio is not
always the same in all countries and NOx measurements must therefore state
clearly what ratio of nitric oxide to nitrogen oxides or nitric oxide in NOx was applied for
the results.
The optical schematic of NDIR set up is shown in the accompanying Fig.1. The broad band IR
light source S, built in parabolic reflector emits near parallel IR light beam containing the
absorption wavelength of NOx gas. The emitted IR beam is then split into two beams by the
beam splitter BS. The transmitted beam from beam splitter is then passed through the
sample gas cell(4) containing flue gas(3) and is focused on to the sample detector DT by
the IR lens LI. This detector Dt measures the intensity I0 when there is no NOx gas , and
stores the value in memory. This detector also continuously measures intensity I when there
is sample gas in the gas cell. The narrow band NOx filter F1 is built onto the detector Dt
which detects only the presence of NOx gas and eliminates the detection of all interfering
gases.
The reflected IR beam from the beam splitter, BS, is adjusted to fall on to the reference
detector Dr. The narrow band NOx Filter F2 is built onto the detector dr. This detector is used to
check the aging of the IR light source by means of a knob provided out side the signal
processor unit(1), which supplies the required power to the IR source S, sample detector DT
and reference detector DR. The signal processing unit(l) also contains all designed electrons to
receive the signal from the sample detector Dt and reference detector dr. The signal
processing unit(1) is equipped with A/D converts and process the signal of DT when there
is zero gas, Io, and when there is NOx gas, I and computes the signals ratio I/Io to give the
NOx gas concentration. A dedicated software is developed in VB and is loaded in the
computer which plots the calibration curve for a selected path length. This selects the
intensity ratio from the measured signals,I/Io for each fixed integration time and displays
on-line results - gas concentration in ppm & mg/m3 and generates the trend graph as
shown in the accompanying figure 2. It is also adapted to generate reports in ppm and
mg/m3 for selected path lengths, as will be clear to persons skilled in the art. The results
can also be displayed on monitor(2) operatively connected to the system.
The various components of the system in accordance with a preferred embodiment of the
present invention and their various working interrelations are explained hereinafter.
Source(S): This is a modulated broad band IR source S with Calcium Fluoride window
(emitting 2 to 9 micron wavelength) containing the absorption frequency of the NOx gas
whose concentration is to be measured.
Filter(Fi and F2): Filters F1 and F2 are same narrow band IR filter of 5.3 microns
(NOx filters) which allow only the absorption wavelength of NOx gas and eliminating
all other wave lengths.
A Beam Shaping Optics(L1) : This is an IR Double convex lens which focuses the
incident IR beam on to the sample detector, DT.
A Beam Splitter(BS) : It is a semi transparent IR mirror which splits the single
incident IR beam into two beams, the transmitted beam and the reflected beam.
Gas Cell(4): The main purpose of the gas cell is to measure the unknown
concentrartion of the flue gas(3). Variable path length gas cell is used in the
present investigation. It is made up of borosilicate glass with gold coated mirrors
inside and a micrometer out side the cell for setting various path length from 0.6
meters to 7.2meters with 0.6meter steps. As the path length increases, the lower
concentration of the NOx gas can be measured. The system has been calibrated for
different path lengths to select optimum sensitivity and concentration of the sample gas
to be detected.
There are two glass windows(5,6) on both sides of the Gas cell. Through the first
window(5), the IR beam is allowed inside and after multiple reflections, goes out
through the other window(6). It has one gas inlet for entering the flue gas(3) and one out
let for leaving the gas(3) as clearly shown in the accompanying figure 1. A vacuum
pump(not shown in figure) is connected at the end of the outlet gas cell so that the
flue gas is sucked from stack through inlet of the gas cell and leaves from the pump.
Detectors: DT and DR are two IR detectors. DT measure the intensity of the IR beam
transmitted through the beam splitter, BS, and after it comes out of the exit window(6),
of the gas cell. DR measures the intensity of the reference beam reflected by the beam
splitter, BS.
Signal Processing Unit(1) : This is the heart of the total system. This unit contains
the total electronic components and circuits required are designed and developed for
the system. This unit supplies required input power to the source, detectors and also
receives the signals from both the detectors.
Computer system with Software:
A computer system with dedicated software takes care of all computations, calibration
of the system with standard gases and the results are displayed on the monitor. The
output from the detectors are fed to the signal processing unit and using the custom
made software, developed in VB. The monitor/ on-line monitors (2), stores and display the
results and various parameters. This computer system with dedicated software selects
the intensity ratio from the measured signals, I/Io for each fixed integration time and
displays on-line results - gas concentration in ppm & mg/m3 and generates the trend
graph as shown in the accompanying figure 2. It is also adapted to generate reports in
ppm and mg/m3 for selected path lengths, as will be clear to persons skilled in the art.
The system developed is calibrated using some known concentrations of the test gas, No
gas and a calibration curved plotted for the system. This calibration curve remains valid
till any component of the system is changed. First, the system is calibrated for zero
concentration / when there is no NO gas in the gas cell. To measure the known
concentration of the flue gas, the system has to be calibrated first using some unknown
concentrations of the NO gas and a calibration curve is plotted with normalized detector
reading of the detector Dt Vs the concentration of the NO gas. For unknown
concentration of the normalized detector reading of DT is measured and the unknown
concentrations are obtained which corresponds to this detector reading for a
particular calibration.
As soon as the software is on it will display" select a mode to run", where in two
parameters calibration and monitoring are displayed. Calibration for calibrating the
system is selected and the value of path length say - 3 meters, integration value -1
minute and gas concentration - zero are entered. Thereafter, the software is run. The
computer stores the value lo for zero concentration and plots the value of Transmission Tx
= Io/Io on the Y-axis and concentration on X-axis of the calibration curve. This is
repeated with known concentrations of the calibration gas NO keeping, the path
length, integration time same. Final calibration curve is plotted automatically. For each
concentration the computer calculates the transmission Tx = I/Io.
Variable path length gas cell is used in the system for different gas sensitivities. For
this purpose different calibration curves were plotted for various path lengths from 0.6
meters to 7.2 meters with 0.6 meters steps.
How the computer system operatively connected to the main system in accordance
with a preferred embodiment of the present invention works, is further elaborated
hereinafter.
Different values are entered before on-line monitoring NOx in flue gas. In this context the
accompanying figure 2 may be referred.
As soon as the software is on, a parameter path length appears with the box on the
screen. It contains all the path lengths for which the calibration curves are plotted. One
path length is selected for monitoring.
Integration Time - Any Integration Time value from 1 to 30 minutes is selected. This
need not be the same as calibration integration time.
Over emission - The value of over emission is preselected. When this value crosses
the set over emission value. Video message appears on the monitor as OVER
EMISSION monitor and at the same time audio alarm rings.
A mode to run has to be selected first, say monitoring is selected for on-line
monitoring of the NOx concentration in flue gas.
The values applied and the abbreviations therefor are as follows:
DTo - Value of the of the sample detector DTo when there is a zero gas /no NO gas in the
gas cell.
DR Fixed- This is fixed before calibrating the system for various NO gas concentrations.
Due to aging of the IR source this value may fall to bring the value to the original value
DR Fixed, a knob is provided on the signal processing unit. This avoids the process of
recalibrating the system.
ppm- Display of the current value of NOx in ppm.
mg/m3- Display of the current value of NOx in mg/m3 .
Tools- Once the tools is clicked the following parameters will display.
Comtest-For testing the communication port.
Reports-To see the reports generated when the system is in continuously monitoring.
The reports generated contains date, time, path length in meters, ppm, mg/m3, and
integration time(minutes) a shown in Table 1, report generated during field trials of the
system.
ppm to mg/m3 calculator -For converting ppm to mg/m3
Backup data- To get the stored data.
Import earlier data- To import all earlier data.
The system in accordance with the present invention has been developed successfully
after exhaustive tests and experimentation and the results thereof were validated using
standard non-continuous imported system such results substantiate the improvements
achieved by the present system as stated hereinbefore which are again reiterated as
follows:
* continuous on-line and non-contact method
* high reliability
* display of NOx concentration in ppm and Mg/M3
* Video and audio over emissions alarms
* continuous data logging
* integration with next level
* no moving parts
* control of source intensity against aging
* low maintenance
* design for mechanical mounting of the system is
rugged, simple, versatile to make installation easy and faster
The continuous method of monitoring NOx in gas emission, by applying a
continuous NOx monitoring system according to the present system comprises, at
the first point passing infra red light beam having absorption wave length of NOx
gas through said gas emission and focusing said light beam on said detector. The
next steps involve measuring and storing NOx intensity by the detector both in the
event of absence(Io) and presence(I) of NOx gas in said emission gas. Then
processing of the Io and I values are done by the processing unit, a calibration
curve is plotted out of the values obtained and the same is displayed. Optionally,
an audio and/or visual alarm is issued in the event of said emission exceeding a
value, pre-set according to a reference value and as accordingly reflected by said
curve. This method is being performed continuously and on line, during the
running of main operation.
The present invention has been described with reference to some drawings and
preferred embodiments, purely for the sake of understanding and not by way of
any limitation and the present invention includes all legitimate developments within
the scope of what has been described hereinbefore and claimed in the appended
claims.
We Claim :
1. A system for continuous monitoring of NOx in gas emission, including an infra
red light source adapted to emit light beam having absorption wave length of NOx
gas, said beam being caused to pass through said gas emission, said light source
being operatively connected to a sample detector for focusing said light beam on
said detector, said detector being adapted to measure and store NOx intensity in the
event of both absence(Io) and presence(I) of NOx gas in said emission , said
detector being operatively connected to a processing unit adapted to identify and
reproduce in interpretable form, signal intensity measured by said detector in the
event of both presence and absence of said NOx in said emission, whereby said
system is adapted to carry out continuous on line monitoring of NOx , during
running of the main operation.
2. The system as claimed in claim 1 wherein said light source is a broad band infra
red light source, built in with parabolic reflector and there exists a beam splitter
operatively connected to said light source and adapted to cause transmitted beam
to pass through said gas emission to focus on said sample detector and to cause the
reflected beam to focus on a reference detector to which it is operatively connected,
said reference detector being operatively connected to said signal processing unit.
3. The system as claimed in claim 2 wherein said reference detector is adapted to
check the aging of said infra red light source by means of a knob provided outside
said signal processing unit.
4. The system as claimed in claim 1 to 3, wherein said emission is caused to pass
through a sample gas cell and said transmitted beam is caused to focus on said
sample detector by an infra red lens.
5.The system as claimed in claim 2 to 4, wherein said processing unit is adapted to
receive and identify intensity signals from said sample detector and said reference
detector.
6. The system as claimed in claim 5 wherein said signal processing unit is equipped
with A/D converters to compute signal intensity ratio(I/Io) out of the intensity values
measured and stored by said sample detector, in the event of presence(I) and
absence(Io) of NOx in said gas emission .
7. The system as claimed in claim 6, wherein said signal processing unit is
operatively connected to a computer system adapted to plot a calibration curve for
a selected path length and to display the same, said system being further
configured to generate an alarm in the event of said emission exceeding a value
pre-set according to a reference value and as accordingly reflected by said curve.
8. The system as claimed in claim 5 wherein a narrow band NOx filter is built one
each on said sample detector and said reference detector.
9. A continuous method of monitoring NOx in gas emission, by applying a
continuous NOx monitoring system, said system including an infra red light source
adapted to emit light beam having an absorption wave length of NOx gas and said
beam being caused to pass through a gas emission, said light source being
operatively connected to a sample detector for focusing said light beam on said
detector, said detector being adapted to measure and store NOx intensity in the
event of absence(Io) and presence(I) of NOx gas in said gas emission , said
detector being operatively connected to a processing unit for identifying signal
intensity measured by said detector in the event of absence and presence of said
NOx
said method including:
-passing infra red light beam having absorption wave length of NOx gas through
said gas emission,
-focusing said light beam on said detector, measuring and storing NOx intensity by
said detector both in the event of absence(Io) and presence(I) of NOx gas in said
emission gas,
-processing said Io and I values by said processing unit,
-plotting a calibration curve and trend curve out of the values obtained and
-displaying the same and
-optionally, issuing an alarm in the event of said emission exceeding a value, pre-
set according to a reference value and as accordingly reflected by said curve,
the said method being performed continuously and on line, during the running of
main operation.
10. The method as claimed in claim 9, wherein said gas emission is flue gas,
emitted from industrial factories, automobiles, aircrafts and other
industries/vehicles involving such generation of said emission, during their
operation.

A system for continuous monitoring of NOx in gas emission, including an infra red
light source(S) adapted to emit light beam having absorption wave length of NOx
gas, said beam being caused to pass through said gas emission(3), said light
source being operatively connected to a sample detector(DT) for focusing said
light beam on said detector, said detector being adapted to measure and store
NOx intensity in the event of both absence(I0) and presence(I) of NOx gas in said
emission , said detector being operatively connected to a processing unit(1)
adapted to identify and reproduce in interpretable form, signal intensity measured
by said detector in the event of both presence and absence of said NOx in said
emission, whereby said system is adapted to carry out continuous on line
monitoring of NOx , during running of the main operation.