CN102353634B - Online calibration method of laser online detection system for direction of gas contents in smoke - Google Patents

Abstract

The invention relates to an online calibration method of a laser online detection system for the detection of gas contents in a smoke. The method comprises the following steps: 1, a beam of modulation laser, which is generated and penetrates a background gas, is divided into two beams: one beam is directly sent to a reference terminus light detector, and the other beam which traverses a smoke pipeline and a calibration tube in order is sent to a measure terminus photodetector; and 2, different concentrations of a gas to be detected are respectively accessed into the calibration tube to respectively measure the output electric signal of the measure terminus photodetector, a calibration coefficient k and a calibration constant B are obtained through calculating, and a calibration formula is obtained. According to the invention, the system which adopts a sensing structure of the two beams and allows the ratio of the detection signal to the reference signal to be only related with the concentration of the gas has the advantages of no need of dismounting, realization of the measure and the calibration with a same apparatus and a same light path, direct online calibration, fast calibration speed, and high calibration precision.

Description

The online calibration method of flue gas content laser on-line detecting system

Technical field

The present invention relates to a kind of scaling method of optical type gas sensor, relate in particular to a kind of online calibration method of optical type gas sensor.

Background technology

In the production run of refinery, the control of oxygen, carbon monoxide, carbon dioxide content in the flue gas not only is related to the quality and yield of oil refining, and relates to the consumption of main air blower electric energy.Now the optical type gas sensor system that three-stage cyclone separator is imported and exported that is installed in is mainly adopted in the detection of oxygen, carbon monoxide, carbon dioxide content in the flue gas.

Optical type gas sensor mainly is take the gas sensor of Spectrum Absorption Principle as the basis, take the infrared absorption type gas analyzer as main.Because the infrared absorption peak of gas with various is different, detect gas by measuring and analyze infrared absorption peak.The optical type gas sensor major advantage is that the response time is fast, and is highly sensitive, good reliability.Optical type gas sensor belongs to untouchable measurement, can be inflammable, explosive in industries such as oil, chemical industry, use under the severe environmental conditions of high temperature, high corrosion.

The demarcation of existing optical system is optical system to be taken off from flue demarcate, and is loaded on after demarcation is good, and such scaling method can produce certain error.And along with the dismounting of calibration process, the optical axis of optical system also may not on a plane, can affect measuring accuracy equally; In addition, the optical axis of existing optical system is regulated inconvenient, has affected the time of demarcating.

Summary of the invention

Demarcate inconvenient technical matters in order to solve existing optical type gas sensor, the invention provides a kind of online calibration method of flue gas content laser on-line detecting system.

Technical solution of the present invention:

A kind of online calibration method of flue gas content laser on-line detecting system may further comprise the steps:

1] produces a branch of modulated laser, the modulated laser wavelength is fixed on the specific wavelength spectral line of tested gas characteristic spectrum, modulated laser passes the background gas that comprises tested gas componant, then modulated laser is divided into two-way, wherein one the tunnel directly send into the reference edge photo-detector, the measuring junction photo-detector is sent into after passing successively flue and calibrating tube in another road;

2] pass into the tested gas that concentration known is Nbd1 in calibrating tube, the measuring junction photo-detector is measured the output electrical signals Xbd1 of this moment;

3] pass into the tested gas that concentration known is Nbd2 in calibrating tube, the measuring junction photo-detector is measured the output electrical signals Xbd2 of this moment;

The difference △ X of corresponding output electrical signals when 4] obtaining passing into two kinds of tested gas of variable concentrations in the flue according to following formula _Cl:

${\mathrm{\ΔX}}_{\mathrm{cl}}=\frac{{\mathrm{\ΔX}}_{\mathrm{bd}}}{L_{\mathrm{bd}}}{L}_{\mathrm{cl}}$

Wherein:

△ X _Bd=| Xbd2-Xbd1| is the poor of corresponding output electrical signals when passing into two kinds of tested gas of variable concentrations in the calibrating tube;

Lbd is the length of calibrating tube;

Lcl is the diameter of flue;

5] according to the △ X that obtains in the step 4 _ClCalculate calibration coefficient k:

$k=\frac{\mathrm{\ΔN}}{{\mathrm{\ΔX}}_{\mathrm{cl}}}$

Wherein: △ N=|Nbd2-Nbd1|;

6] the calibration coefficient k that obtains according to step 5 calculates mark stable constant value B:

B=N ₁-kX _cl1

7] according to calibration coefficient k and mark stable constant value B, obtain calibration formula, finish demarcation:

N=kX+B

Wherein: N is gas concentration to be measured, and X is output electrical signals.

The advantage that the present invention has: the present invention has adopted the sensing arrangement of two-way light beam, the ratio of detection signal and reference signal is with only relevant with the concentration of gas, need not dismantle, measure and demarcate the same set of device of employing and same light path, but direct-on-line is demarcated, it is fast to have demarcation speed, and stated accuracy is high.

Description of drawings

Fig. 1 is the structural representation of flue gas content laser on-line detecting system;

Fig. 2 is the schematic block circuit diagram of flue gas content laser on-line detecting system;

Fig. 3 is the block diagram of signal pre-processing circuit

Fig. 4 is the structural representation of primary optic axis governor motion;

Fig. 5 is the structural representation of the second optical axis adjusting mechanism;

Fig. 6 is the circuit diagram of signal integration circuit;

Fig. 7 is the synoptic diagram of two paths of signals timesharing sequential delivery;

Wherein: 1-Optical Transmit Unit, 2-background gas sealed tube, 3-background gas entrance, the outlet of 4-background gas, 5-primary optic axis governor motion, 6-the first breakout box, 7-measuring junction signal cable, 8-purges wind entrance, 9-gate valve, the 10-flue, 11-gate valve flange, 12-optical fiber, 13-reference edge signal cable, 14-calibrating tube, the outlet of 15-marked gas, 16-marked gas entrance, 17-measuring junction photo-detector, 18-explosion-proof wiring box, the 19-signal-transmitting cable, 20-feed cable, 21-the second optical axis adjusting mechanism, the 22-central control cabinet, the 23-industrial computer, 24-reference edge photo-detector, 25-quartz glass, the 26-view window, 27-sphere regulating flange, 28-adjusts screw, 29-the second breakout box, the 30-optical cable, the 31-spectroscope.

Embodiment

Referring to Fig. 1 and Fig. 2, the flue gas content laser on-line detecting system adopts the correlation optical measurement, mainly comprises optical drive unit, Optical Transmit Unit, optical signal detection unit, electric signal transmission unit, instrument wind distribution unit, central control cabinet, industrial computer etc.

The optical drive unit is used for generating modulated laser, comprises function generator and double frequency modulation signal combiner circuit, semiconductor laser, semiconductor laser device driving circuit, Temperature control circuit of semiconductor laser; After system powered on, function generator produced the signal of two different frequencies, and these two signals send respectively semiconductor laser device driving circuit to and late-class circuit carries out signal processing, demodulation, sampling usefulness.After semiconductor laser device driving circuit was synthetic with the signal of two kinds of frequencies, noise spectra of semiconductor lasers was carried out the double frequency modulation, made the laser that sends both have length scanning characteristic under the low frequency contribution, had again the high frequency characteristics of preparing for the signal demodulation.The effect of Temperature control circuit of semiconductor laser is that optical maser wavelength is fixed on the specific wavelength spectral line of gas characteristic spectrum.When measuring oxygen, optical maser wavelength is near 760nm, and when measuring carbon monoxide, optical maser wavelength is at 1570nm, when measuring carbon monoxide, optical maser wavelength near 1580nm,

To Optical Transmit Unit, the optical alignment mirror of Optical Transmit Unit is made into the outgoing of collimation parallel beam with laser to laser after the wavelength-modulated by Optical Fiber Transmission.

The optical signal detection unit can convert laser signal to electric signal, comprises spectroscope, reference edge photo detecting unit and measuring junction photo detecting unit; The reference edge photo detecting unit comprises background gas sealed tube, primary optic axis governor motion, reference edge photo-detector; The measuring junction photo detecting unit comprises the second optical axis adjusting mechanism, calibrating tube and measuring junction photo-detector; Background gas sealed tube, primary optic axis governor motion, spectroscope, flue, the second optical axis adjusting mechanism, calibrating tube and measuring junction photo-detector are successively set on the optical axis of Optical Transmit Unit; The reference edge photo-detector is arranged on spectroscopical reflected light path.Be sealed with background gas in the background gas sealed tube, background gas includes the composition identical with tested gas in the flue gas, is used for providing the feature benchmark of tested gas absorption signal.Such as, need to detect the content of oxygen in the flue gas, then sealing pure oxygen or comprise other gases of 20% concentration oxygen in the background gas sealed tube.Laser beam is divided into two-way behind the spectroscope of optical signal detection unit, the referenced end photo-detector of one road laser beam of reflection receives provides a reference signal to be used for relatively, one road laser beam of transmission is through tested gas sample cell (being flue), and the measured end photo-detector of this road laser beam that is coupled to measured gas concentration information receives and forms measuring-signal.

Primary optic axis governor motion and the second optical axis adjusting mechanism communicate with flue by the gate valve flange respectively.The internal diameter of gate valve flange is 100mm.The primary optic axis governor motion comprises the first converter 6, be arranged on two blocks of quartz glasss 25 of the first converter both ends of the surface, be arranged on the first converter two sides two view windows 26, be arranged on the sphere regulating flange between background gas sealed tube and the first converter, the sphere regulating flange is connected with the background gas sealed tube is coaxial, and is connected with the side of the first converter by four adjustment screws; Spectroscope and reference edge photo-detector just are arranged in the first converter, and the reference edge photo-detector is by reference edge signal cable output signal.The second optical axis adjusting mechanism comprises the second converter 29, be arranged on two blocks of quartz glasss 25 of the second converter both ends of the surface, be arranged on the second converter two sides two view windows 26, be arranged on the sphere regulating flange between the second converter and the calibrating tube, the sphere regulating flange is connected with calibrating tube is coaxial, and is connected with the side of the second converter by four adjustment screws.The effect of sphere regulating flange is to adjust optical axis, guarantees that collimated light beam that light source sends by behind the measured flue, can shine on the measuring junction photo-detector.Adjust screws by adjusting up and down four, optical axis can be adjusted in twice among a small circle around exit direction is positive and negative.

Both ends of the surface at the first converter and the second converter all are equipped with quartz glass, play the effect of isolation flue dust and measurement mechanism.The both sides of converter respectively have a square view window, by these two pollution condition that window can be observed the quartz glass surface.Purge the cleaning that wind keeps glass pane although system and device has, long time running inevitably can be infected with dust, stops passing through of measuring beam, affects measuring accuracy.When not having view window in the past, judge whether not too easily to cause system to work because quartz window is subject to dust pollution.After view window has been arranged, can see through easily view window, see the situation of quartz window.Clean-up performance by regular supervision quartz window glass can normally move by assurance device.

The electric signal transmission unit is used for transferring to by the signal isolation and transmission circuit with reference to signal and measuring-signal the signal pre-processing circuit of central control cabinet.

Central control cabinet comprises the Electric signal processing unit, and described Electric signal processing unit comprises signal pre-processing circuit, lock-in amplifier, signals collecting and telecommunication circuit;

Signal pre-processing circuit is used for filtering, amplification and the integration of reference edge signal and measuring junction signal to be processed.In signal pre-processing circuit, by two paths of signals relatively, reject the impact that light source fluctuation brings, reject the impact that a part of device drift brings, the signal that is suitable for the lock-in amplifier demodulation is provided, particularly be integrated into timesharing with reference to signal and measuring-signal and sequentially transmit signal, can save a lock-in amplifier, saved greatly cost.

Being used for being integrated into the circuit that timesharing sequentially transmits with reference to signal and measuring-signal in the signal pre-processing circuit can be referring to Fig. 5, comprise the reference edge signal emitter follower U2A that connects respectively reference edge signal (S signal) and measuring junction signal (R signal), measuring junction signal emitter follower U2B, the simulant electronic switch U3 that joins with reference edge signal emitter follower U2A and measuring junction signal emitter follower U2B output terminal respectively, the control end of simulant electronic switch U3 is connected on the output terminal of counter U5A, one of them low-frequency signals of the input end input function generator of counter U5A and double frequency modulation signal combiner circuit, the timesharing sequential signal after the output terminal output of simulant electronic switch U3 is integrated.S signal and R signal are by behind emitter follower U2A, the U2B, send into simulant electronic switch U3, simulant electronic switch is under the digital square-wave control that counter U5A provides, R signal and the synthetic timesharing of S signal two-way are sequentially spread out of one road signal, when the counter number signal was high level, the output of R signal was when the counter number signal is low level, the output of S signal, Fig. 6 is the output signal synoptic diagram.

Lock-in amplifier (being modulator-demodular unit) is used for reference edge signal and the measuring junction signal after demodulation is integrated, and the reference arm of lock-in amplifier demodulation output and measurement road signal reach signals collecting and telecommunication circuit, and industrial computer is delivered in conversion through A/D.

Industrial computer is processed, calculates, is stored the data that receive, and obtains gas concentration.On the one hand the concentration data that obtains is passed through the form of curve or chart, show at display, on the other hand the concentration data that obtains is passed through serial ports or 4～20mA, pass to client's production control network, for it provides necessary reference data.

System Working Principle:

System detects gas concentration by the variation that detects gas transmitted light intensity or reflective light intensity.Every kind of gas molecule has absorption or the radiation spectrum feature of oneself, and the emission spectrum of light source is only just producing absorption with the overlapping part of gas absorption spectrum, and the light intensity after the absorption will change.This light wave produces the phenomenon that absorbs was summarized as absorbance and dielectric thickness by Bouguer and Lambert between 1729～1760 years relation by medium, and Beer has proposed again the relation of absorbance and concentration of medium in 1852, namely

I=I ₀exp(-a _lCL),(3)

I in the formula ₀Be that wavelength is the light intensity of monochromatic light before entering absorption of gases of l, C is the concentration of absorbing medium, a _lBe the absorption coefficient of unit length unit's concentration, L is that light is by the length of medium.

Formula (1) is called Beer-Lambert law, and it has shown the relation between absorbance log and absorption path and the concentration of medium.

Circuit system adopts single wavelength bifocal path technique, Wavelength modulation spectroscopy technology and the phase lock amplifying technology of Difference Absorption.

The key factor that has influence on spectral absorption formula gas sensor includes: the stability of light source, light path coupling place lotus root are closed drift of element in variation, driving and the testing circuit of variation, optic path characteristic of variation, the environmental factor (temperature etc.) of state etc.Affecting spectral absorption formula gas sensor for these mainly is to reduce and eliminate by differential absorption method and modulation-demodulation technique.

The principle of Difference Absorption technology is originally only to adopt the sensing arrangement of one road light beam to change two-way into, and wherein one the tunnel is can be by gas absorption and with the spectral absorption information of upper gas, other one the tunnel then by gas absorption, as reference path is provided.When this two-way adopted the device of identical parameters, the drift of the unstable and photoelectric device of light source was identical on the impact of two paths of signals, and then the ratio of detection signal and reference signal is with relevant with the concentration of gas.

Because the signal intensity of spectral absorption is very faint, is usually fallen into oblivion by noise.In order to detect faint absorption signal Strength Changes, must there be very high signal to noise ratio (S/N ratio) in system.Wavelength modulation spectroscopy technology and phase lock amplifying technology can greatly improve signal to noise ratio (S/N ratio) exactly, the technology that useful signal is detected from noise.

The principle of the online calibration method of flue gas content laser on-line detecting system of the present invention:

When demarcating, use measured section change amount signal △ X _Cl, but can't directly obtain this variable quantity at the scene.It is that light source transmitting terminal and optical signal detector receiving end are removed that a kind of way is arranged, and is contained in preprepared calibrating tube two ends, measures △ X _ClDemarcate, again light source and detector are reinstalled after finishing.The benefit of doing like this is not consider the impact of flue dust, directly obtain calibration result, disadvantageous aspect also is apparent, demarcate and measure not in the same light path, the error of bringing will be offset and get rid of the benefit that dust disturbs demarcation to bring, and increased disassembly process, complicated operation.

The present invention is installed calibrating tube in order to realize on-line proving additional in light path.Characteristics according to measuring-signal: under the certain condition of concentration, the length L that change amount signal △ X ∝ is measured section.Suppose the gas concentration in the flue, within a period of time, change very littlely, so, calibrating tube is filled with the calibrating gas of variable concentrations, the change amount signal △ X that obtains _BdThe length L of ∝ calibrating tube _Bd, just have:

$\frac{{\mathrm{\ΔX}}_{\mathrm{bd}}}{L_{\mathrm{bd}}}=\frac{{\mathrm{\ΔX}}_{\mathrm{cl}}}{L_{\mathrm{cl}}}---\left(1\right)$

The difference △ X of corresponding output electrical signals in the time of then can obtaining passing into two kinds of tested gas of variable concentrations in the flue _Cl:

${\mathrm{\ΔX}}_{\mathrm{cl}}=\frac{{\mathrm{\ΔX}}_{\mathrm{bd}}}{L_{\mathrm{bd}}}{L}_{\mathrm{cl}}---\left(2\right)$

In following formula, the diameter L of flue _Cl, calibrating tube length L _BdKnown, △ X _Bd=| Xbd2-Xbd1| is the poor of corresponding output electrical signals when passing into two kinds of tested gas of variable concentrations in the calibrating tube; Through type (2) just can obtain △ X _Cl

Then according to the computing formula of concentration

As can be known, the calibration formula under the variable concentrations is:

N ₁=kX _cl1+B

N ₂=kX _cl2+B

Two formulas are subtracted each other, obtain:

N ₁-N ₂=(kX _cl1+B)-(kX _cl2+B)

That is:

ΔN=kΔX _cl

Because of △ X _ClCalculate, only have that concentration changes in the calibrating tube this moment, can obtain calibration coefficient k:

$k=\frac{\mathrm{\ΔN}}{{\mathrm{\ΔX}}_{\mathrm{cl}}}$

Wherein: △ N=|Nbd2-Nbd1|;

Again by N ₁=kX _Cl1The computing formula that+B marks stable constant value B as can be known is:

B=N ₁-kX _cl1

At last, according to calibration coefficient k and mark stable constant value B, obtain calibration formula, finish demarcation:

N=kX+B

The benefit of doing like this is to have avoided demarcating the measuring error of introducing under the different light paths condition, simultaneously, has reduced the device disassembly process, so that operation is simplified greatly.Disadvantage has been introduced measuring error with regard to the impact that is flue dust, but this error can be demarcated the sampling time by prolonging, and increases the sample number and is subdued.

Claims (1)

1. the online calibration method of a flue gas content laser on-line detecting system is characterized in that: may further comprise the steps:

1] produces a branch of modulated laser, the modulated laser wavelength is fixed on the specific wavelength spectral line of tested gas characteristic spectrum, modulated laser passes the background gas that comprises tested gas componant, then modulated laser is divided into two-way, wherein one the tunnel directly send into the reference edge photo-detector, the measuring junction photo-detector is sent into after passing successively flue and calibrating tube in another road;

2] pass into the tested gas that concentration known is Nbd1 in calibrating tube, the measuring junction photo-detector is measured the output electrical signals Xbd1 of this moment;

3] pass into the tested gas that concentration known is Nbd2 in calibrating tube, the measuring junction photo-detector is measured the output electrical signals Xbd2 of this moment;

The difference △ X of corresponding output electrical signals when 4] obtaining passing into two kinds of tested gas of variable concentrations in the flue according to following formula _Cl:

${\mathrm{\ΔX}}_{\mathrm{cl}}=\frac{{\mathrm{\ΔX}}_{\mathrm{bd}}}{L_{\mathrm{bd}}}{L}_{\mathrm{cl}}$

Wherein:

△ X _Bd=| Xbd2-Xbd1| is the poor of corresponding output electrical signals when passing into two kinds of tested gas of variable concentrations in the calibrating tube;

Lbd is the length of calibrating tube;

Lcl is the diameter of flue;

5] according to the △ X that obtains in the step 4 _ClCalculate calibration coefficient k:

$k=\frac{\mathrm{\ΔN}}{{\mathrm{\ΔX}}_{\mathrm{cl}}}$

Wherein: △ N=|Nbd2-Nbd1|;

6] the calibration coefficient k that obtains according to step 5 calculates mark stable constant value B:

B=N ₁-kX _cl1

7] according to calibration coefficient k and mark stable constant value B, obtain calibration formula, finish demarcation:

N=kX+B

Wherein: N is gas concentration to be measured, and X is output electrical signals.

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