CN102353634A - 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 to be the gas sensor on basis with the Spectrum Absorption Principle, is main with the infrared absorption type gas analyzer.Because the infrared absorption peak of gas with various is different, come detected gas through 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 refills 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 possibly can influence measuring accuracy not on a plane equally; In addition, the optical axis of existing optical system is regulated inconvenient, has influenced the time of demarcating.

Summary of the invention

Demarcate inconvenient technical matters in order to solve existing optical type gas sensor, the present 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 tested gas characteristic spectrum certain wavelengths spectral line; Modulated laser passes the background gas that comprises tested gas componant; Then modulated laser is divided into two-way, wherein one the tunnel directly sends into the reference edge photo-detector, the measuring junction photo-detector is sent into after passing flue and calibrating tube successively in another road;

2] in calibrating tube, feeding concentration known is the tested gas of Nbd1, and the measuring junction photo-detector is measured the output electric signal Xbd1 of this moment;

3] in calibrating tube, feeding concentration known is the tested gas of Nbd2, and the measuring junction photo-detector is measured the output electric signal Xbd2 of this moment;

The difference Δ X of pairing output electric signal when 4] obtaining feeding two kinds of tested gases 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:

Δ X _Bd=| Xbd2-Xbd1| is the poor of pairing output electric signal when feeding the tested gas of two kinds of variable concentrations in the calibrating tube;

Lbd is the length of calibrating tube;

Lcl is the diameter of flue;

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

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

Wherein: Δ N=|Nbd2-Nbd1|;

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

B＝ΔN-kΔX _cl

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

N＝kX+B

Wherein: N is a gas concentration to be measured, and X is the output electric signal.

The advantage that the present invention had: the present invention has adopted the sensing arrangement of two-way light beam; Detection signal is relevant with the concentration of gas with only with the ratio of reference signal; Need not dismantle, measure and demarcate and adopt same set of device and same light path, but direct-on-line 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 structural representation of primary optic axis governor motion;

Fig. 4 is the structural representation of the second optical axis governor motion;

Fig. 5 is the circuit diagram of signal integration circuit;

Fig. 6 is the output signal schematic representation;

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 inlet, the outlet of 4-background gas, 5-primary optic axis governor motion, 6-first breakout box; 7-measuring junction signal cable, 8-purge wind inlet, 9-gate valve, 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 inlet; 17-measuring junction photo-detector, 18-explosion-proof wiring case, 19-signal-transmitting cable, 20-feed cable, the 21-second optical axis governor motion; 22-central authorities switch board, 23-industrial computer, 24-reference edge photo-detector, 25-quartz glass, 26-view window; The 27-sphere is regulated flange, and 28-adjusts screw, 29-second breakout box, 30-optical cable, 31-spectroscope.

Embodiment

Referring to Fig. 1 and Fig. 2; 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 switch board, industrial computer etc.

The optical drive unit is used to generate modulated laser, comprises function generator and double frequency modulation signal combiner circuit, semiconductor laser, semiconductor laser device driving circuit, semiconductor laser temperature-control circuit; After system powered on, function generator produced the signal of two different frequencies, and these two signals send semiconductor laser device driving circuit respectively to and late-class circuit carries out signal Processing, demodulation, sampling usefulness.Semiconductor laser device driving circuit with the signal of two kinds of frequencies synthetic after, noise spectra of semiconductor lasers is carried out the double frequency modulation, makes the laser that sends both have the length scanning characteristic under the low frequency contribution, has the high frequency characteristics of preparing for the signal demodulation again.The effect of semiconductor laser temperature-control circuit is that optical maser wavelength is fixed on the gas characteristic spectrum certain wavelengths spectral line.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 through 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 governor motion, calibrating tube and measuring junction photo-detector; Background gas sealed tube, primary optic axis governor motion, spectroscope, flue, the second optical axis governor motion, 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 include with flue gas in the identical composition of tested gas, be used to provide the characteristic benchmark of tested gas absorption signal.Such as, needs detect the content of oxygen in the flue gas, then seal other gases that pure oxygen perhaps comprises 20% concentration oxygen in the background gas sealed tube.Laser beam is divided into two-way behind the spectroscope of optical signal detection unit; One road laser beam of reflection is received by the reference edge photo-detector provides a reference signal to be used for comparison; 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.

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

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

The electric signal transmission unit is used for reference signal and measuring-signal are transferred to through the Signal Spacing transmission circuit signal pre-processing circuit of central switch board.

Central authorities' switch board comprises the electric signal processing unit, and said 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 processing of reference edge signal and measuring junction signal.In signal pre-processing circuit; Through two paths of signals relatively, reject the influence that light source fluctuation brings, reject the influence that a part of device drift brings; The signal that is suitable for the lock-in amplifier demodulation is provided; Particularly reference signal and measuring-signal are integrated into timesharing and transmit signal in proper order, can save a lock-in amplifier, practiced thrift cost greatly.

Being used in the signal pre-processing circuit that reference signal and measuring-signal are integrated into the circuit that timesharing transmits in proper order can be referring to Fig. 5; Comprise the reference edge signal emitter follower U2A, the measuring junction signal emitter follower U2B that connect reference edge signal (S signal) and measuring junction signal (R signal) respectively; 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 sent into simulant electronic switch U3 after passing through emitter follower U2A, U2B, and 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 spread out of one road signal in proper order; 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 schematic representation.

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 reaches signals collecting and telecommunication circuit with measurement road signal, and industrial computer is delivered in conversion through A/D.

Industrial computer is handled, calculates, is stored the data that receive, and obtains gas concentration.With the form of the concentration data that obtains, on display, show on the one hand, 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 through curve or chart.

The system works principle:

System comes detected gas concentration through the variation of detected gas transmitted light intensity or reflective light intensity.Every kind of gas molecule all has absorption or the radiation spectrum characteristic 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.The phenomenon that this light wave produce to absorb through medium is summarized as the relation of absorbance and dielectric thickness by Bouguer and Lambert in the period of 1729～1760, Beer has proposed the relation of absorbance and concentration of medium again in 1852, promptly

I＝I ₀?exp(-a _lCL)， (3)

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

Formula (1) is called Beer-Lambert law, and it has shown absorbance log and has absorbed the relation between 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: 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.Influencing spectral absorption formula gas sensor for these mainly is to reduce and eliminate through differential absorption method and modulation-demodulation technique.

The principle of Difference Absorption technology is with originally only adopting 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 that goes up gas, other one the tunnel then by gas absorption, and being used as provides reference path.When this two-way adopted the device of identical parameters, the instability of light source and the drift of photoelectric device were identical to the influence of two paths of signals, and then detection signal is relevant with the concentration of gas with only with the ratio of reference signal.

Because the signal intensity of spectral absorption is very faint, is fallen into oblivion by noise usually.In order to detect the poor absorption change in signal strength, must there be very high signal to noise ratio (S/N ratio) in system.Wavelength modulation spectroscopy technology can greatly improve signal to noise ratio (S/N ratio) exactly with phase lock amplifying technology, 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 Δ X _Cl, but can't directly obtain this variable quantity at the scene.It is that light emitted end 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 Δ X _ClDemarcate, again light source and detector are reinstalled after the completion.The benefit of doing like this is not consider the influence of flue dust; Directly obtain calibration result; Disadvantageous aspect also is conspicuous, demarcates and measures not in same light path, and the error of bringing will be offset and get rid of the benefit that dust disturbs demarcation to bring; And increased disassembly process, complicated operationization.

The present invention is installed calibrating tube additional in order to realize on-line proving on light path.Characteristics according to measuring-signal: under the certain condition of concentration, the length L that change amount signal Δ X ∝ is measured section.Suppose the gas concentration in the flue, in a period of time, change very for a short time, so, calibrating tube is charged into the calibrating gas of variable concentrations, the change amount signal Δ 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 Δ X of pairing output electric signal in the time of then can obtaining in the flue feeding the tested gas of two kinds of variable concentrations _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, Δ X _Bd=| Xbd2-Xbd1| is the poor of pairing output electric signal when feeding the tested gas of two kinds of variable concentrations in the calibrating tube; Through type (2) just can obtain Δ X _Cl

Then according to the computing formula B=Δ N-k Δ X of concentration _ClCan know that 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 Δ X _ClCalculate, have only that concentration changes in the calibrating tube this moment, can obtain calibration coefficient k:

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

Wherein: Δ N=|Nbd2-Nbd1|;

Again by N ₁=kX _Cl1+ B can know that the computing formula of mark stable constant value B is:

B＝ΔN-kΔX _cl

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

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, makes operation simplify greatly.Disadvantage has been introduced measuring error with regard to the influence that is flue dust, but this error can be demarcated the sampling time through prolonging, and increases the sample number and subdues.

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 tested gas characteristic spectrum certain wavelengths spectral line; Modulated laser passes the background gas that comprises tested gas componant; Then modulated laser is divided into two-way, wherein one the tunnel directly sends into the reference edge photo-detector, the measuring junction photo-detector is sent into after passing flue and calibrating tube successively in another road;

2] in calibrating tube, feeding concentration known is the tested gas of Nbd1, and the measuring junction photo-detector is measured the output electric signal Xbd1 of this moment;

3] in calibrating tube, feeding concentration known is the tested gas of Nbd2, and the measuring junction photo-detector is measured the output electric signal Xbd2 of this moment;

The difference Δ X of pairing output electric signal when 4] obtaining feeding two kinds of tested gases of variable concentrations in the flue according to following formula _Cl:

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

Wherein:

Δ X _Bd=| Xbd2-Xbd1| is the poor of pairing output electric signal when feeding the tested gas of two kinds of variable concentrations in the calibrating tube;

Lbd is the length of calibrating tube;

Lcl is the diameter of flue;

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

$k=\frac{\mathrm{\ΔN}}{\mathrm{\Δ}{X}_{\mathrm{cl}}}$

Wherein: Δ N=|Nbd2-Nbd1|;

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

B＝ΔN-kΔX _cl

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

N＝kX+B

Wherein: N is a gas concentration to be measured, and X is the output electric signal.

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