CN1641357A - Method for on-line monitoring dioxin by tunable laser spectrum with flight time mass spectrum - Google Patents

Abstract

The invention discloses a method for online monitoring of dioxins by combining tunable laser spectrum with time-of-flight mass spectrometry. Install the laser absorption element and detector of the tunable laser spectrometer on the two sides opposite to the pipe diameter of the combustion system under test. The gas is first detected by the laser absorption element of the tunable laser spectrometer, and the working current of the laser is adjusted to 20-30A , the working wavelength of the laser is adjusted to 112-281nm, and the gas to be tested is locked. After the gas passes through the detector of the tunable laser spectrometer, the gas enters the time-of-flight mass spectrometer for detection, and the precursor is detected by a selective ionization method. On-line detection, and then use the established equivalent relationship between dioxin precursors and total dioxins to obtain the online concentration of total dioxins. The invention is practical and feasible, easy to operate, strong in adaptability, free from the interference of dust in the measurement environment, and can quickly, accurately and real-time monitor the concentration of trace pollutant dioxin and its toxic equivalent.

Description

On-line monitoring of dioxins by tunable laser spectroscopy combined with time-of-flight mass spectrometry

technical field

The invention relates to a method for online monitoring of dioxins by combining tunable laser spectrum with time-of-flight mass spectrometry.

Background technique

Dioxin is a common name for polychlorinated dibenzo-p-dioxin (PCDD) and polychlorinated dibenzofuran (PCDF), which are carcinogenic, teratogenic and mutagenic Toxic substances. Especially the isomer substituted with 2,3,7,8-position chlorine is the most toxic (2,3,7,8-TCDD), which is a known first-class carcinogen in the world, and its toxicity is 50 times that of potassium cyanide. ~ 1000 times, and therefore received special attention.

Studies have shown that the combustion process is the main source of dioxins in the environment (combustion described here refers to all strong oxidative exothermic and luminescent reactions, and incineration refers to the combustion reaction of waste, the same below). At present, China's environmental standards for dioxin emissions are limited to atmospheric standards. There are two standards that have been formulated, namely "Standards for Pollution Control of Hazardous Waste Incineration" (GB18484-2001) and "Standards for Pollution Control of Domestic Waste Incineration" (GB18485- 2001), the emission limits for dioxins in the two standards are 0.5TEQng/m ³ and 1.0TEQng/m ³ respectively. For the time being, there is no discharge standard for dioxins in incineration residues and dust, and they are only treated as hazardous waste. In 2001, the State Environmental Protection Administration conducted a survey on the discharge of pollutants from small waste incineration plants and the impact on the surrounding environment. The results of the survey showed that 57.1% of the incineration plants exceeded the standard in the discharge of dioxins.

In order to control the formation and discharge of dioxin-like substances and meet the emission standards of dioxin-like substances, it is necessary to improve the status of waste incineration and increase post-processing devices. The dioxin-like pollutants produced by waste incineration are continuously produced and discharged, and their concentration in the flue gas varies due to the composition of the incineration and the incineration process. In order to organize combustion reasonably and configure pollution control equipment economically, it is required to detect the concentration of dioxin as accurately and conveniently as possible, so as to guide the optimal operation. However, the currently widely used dioxin analysis and detection methods refer to the US EPA1613 method of high-resolution gas chromatography/high-resolution mass spectrometry. Because dioxins are trace substances, the sampling and purification steps are extremely complicated and the degree of automation is low. , the test period is very long, and the analysis cost is extremely expensive (about 10,000 RMB/sample-domestic or 5,000 US dollars/sample-foreign). Generally, the results can be obtained within a few weeks after sampling, which is far from meeting the needs of optimal operation.

At the same time, the detection of dioxin-like substances has become the bottleneck of waste incineration pollution control. This has also to a certain extent caused the low operating efficiency of waste incineration equipment in my country and the potentially serious dioxin pollution that may be produced, which cannot be monitored and controlled.

In addition, due to the serious harm of dioxins to human health, a large number of researches have been carried out at home and abroad, and a lot of research resources have been invested. However, due to the complexity of dioxin analysis and detection methods and the long analysis cycle, the detection cost is expensive. Therefore, the tests and test results that are of great significance to dioxin research are still incomplete. Especially in the distribution characteristics of dioxins under complex working conditions, the formation mechanism of dioxins, and the correlation with other incomplete combustion products, there have been no breakthroughs. The experimental description is still at the stage of hypothesis and conjecture. However, the method provides a method for rapidly and accurately detecting the concentration of dioxin precursors, and further provides a very feasible method for real-time monitoring of dioxin. Through the application of this method, the test data and test results on dioxins can be obtained more easily and conveniently, which greatly promotes the integrity and breakthrough of dioxin research.

In terms of on-line monitoring of trace organic pollutants, US Patent No. 5,252,060 mainly uses laser absorption spectroscopy to detect low-molecular-weight hydrocarbons to control the combustion process and optimize working conditions. US Patent US712132 uses mass spectrometry to detect the concentration of various compounds in smoke. US Patent No. 696005 also mentions the technology of laser absorption spectroscopy. In addition, Chinese patent CN1467487 uses tunable laser spectrum second harmonic intelligent analysis method to monitor roadside motor vehicle exhaust pollutants in real time, which also proves that this technology is feasible for real-time monitoring of organic compounds.

In terms of real-time monitoring of dioxins, Chinese patent CN2460994 provides a rapid determination method for chemical reagents. However, the disadvantage of this method is that there is a chemical reaction between the chemical reagent and the measured compound, the test cycle is longer, and the operability of the specific combustion system test is not strong.

Contents of the invention

The purpose of the present invention is to provide a method for online monitoring of dioxins by combining tunable laser spectrum with time-of-flight mass spectrometry.

It installs the laser absorbing element and the detector of the tunable laser spectrometer on the opposite sides of the pipe diameter of the measured combustion system. The gas is first detected by the laser absorbing element of the tunable laser spectrometer. The upper limit of the working current of the laser is 20~ 30A, the maximum operating wavelength range of the laser is 112-281nm (multi-mode), lock the gas to be measured, and the gas after the gas passes through the detector of the tunable laser spectrometer enters the time-of-flight mass spectrometer for detection, through a selective ionization method , carry out on-line detection of the precursors, and then use the established equivalent relationship between the dioxin precursors and the total amount of dioxins to obtain the online concentration of the total amount of dioxins.

The method of the present invention is feasible, easy to operate, strong adaptability, free from the interference of dust in the measurement environment, and can quickly, accurately, and real-time monitor the concentration and toxicity of trace pollutant dioxins, and propose a method based on theoretical and experimental research results A new dioxin monitoring method and process can also provide experimental data and theoretical basis for the organization and optimization of the combustion process.

Description of drawings

Figure 1 is a flow chart of a method for online monitoring of dioxins with tunable laser spectroscopy combined with time-of-flight mass spectrometry;

Fig. 2 is a schematic diagram of laser spectrum principle;

Fig. 3 is a schematic diagram of the principle of tunable laser spectroscopy.

Detailed ways

Existing studies have shown that there is an inevitable relationship between incomplete combustion products and dioxin formation in the combustion process, and the degree of correlation is very large, that is, incomplete combustion products are precursors of dioxins. There are many types of precursors of dioxins, from olefins to chlorobenzene, chlorophenols, biphenyls, polycyclic aromatic hydrocarbons, etc., and their concentrations are generally more than 20 times the concentration of dioxins, so the detection limit and Sensitivity requirements have dropped a lot, and some of them can already be detected online. Therefore, we propose a method for real-time online detection of the concentration and toxic equivalent of dioxins by detecting some reliable precursors of dioxins. From the degree of correlation and the difficulty of online detection, we select some of the most reliable precursors of dioxins as substitutes for real-time detection of dioxins, mainly including the following categories: chlorobenzene (CB), chlorophenol (CP), polychlorinated biphenyls (PCBs) and polycyclic aromatic hydrocarbons, ethylene, propylene, butadiene, cyclopentadiene, vinyl acetylene, malonic acid, toluene, styrene, etc. By using the single-line spectral characteristics of the tunable laser spectrum, the spectral line locking function and the online real-time characteristics of time-of-flight mass spectrometry, the online concentration of the above substances is detected, so as to achieve the purpose of online monitoring of dioxin concentration. Monitor the concentration and distribution of dioxin precursors online through the above method, and then select some operating points to conduct off-line dioxin concentration analysis on the flue gas at these operating points, and compare the concentrations of dioxins and their precursors and distribution characteristics, establish a mathematical model of the corresponding correlation, and confirm and evaluate the results of this monitoring method, which will further prove the accuracy and reliability of this monitoring method.

According to the present invention, a tunable laser spectrum combined with a time-of-flight mass spectrometer combined with a real-time monitoring system for dioxins includes: a tunable laser spectrum element capable of testing the gas absorption intensity of the gas to be monitored; A time-of-flight mass spectrometry detection element for sexual ionization; a microprocessor control element capable of process control and data exchange between the tunable laser spectrum element and the time-of-flight; and the corresponding interface.

The specific detection method is described in detail as follows:

1) The laser absorbing element and the detector with tunable laser spectrum are respectively installed on the opposite sides of the measured pipe diameter, and the detected gas enters the time-of-flight mass spectrometer, see Figures 1 and 2. When there are measured gas molecules in the optical channel between the laser and the detector, the laser intensity signal detected by the detector will drop. There is a certain quantitative relationship between the attenuation of laser intensity and the average content of the measured gas within the light transmission distance. The concentration of the component to be measured can be obtained analytically by measuring the attenuation of the laser intensity.

2) By adjusting the temperature of the laser, the laser frequency is adjusted to correspond to the central frequency of the absorption line of the measured component. Then periodically modulate the operating current of the semiconductor laser, so that the laser frequency periodically scans the absorption line of the measured component. The scanning range of the laser frequency is set to be larger than the width of the measured gas absorption line, so that the "I" area not attenuated by the gas absorption line and the "II" area attenuated by the gas absorption line are included in a frequency scanning range In the "I" area, the laser intensity is not attenuated by the absorption line of the measured gas, but only by the dust; but in the "II" area, the laser intensity is not only attenuated by the dust, but also absorbed by the measured gas spectral line attenuation. Therefore, the measurement signal obtained from the "I" area can obtain the light transmittance T _d of the dust, and the measurement signal obtained from the "II" area can obtain the total light transmittance T gd of the dust and the measured gas T _gd = T _d * T _g . Since the scanning range of the entire laser frequency is very small (~1cm ^-1 ), the attenuation characteristics of the laser by particles such as dust will not change in such a small frequency range. Therefore, the laser online gas analysis system developed in this project can accurately obtain the light transmittance T _g of the measured gas by simultaneously measuring the "I" and "II" areas within one laser frequency scanning cycle = T _gd /T _d . The laser absorption spectrum gas analysis technology adopted can distinguish the absorption produced by dust and measured gas, so that the gas analysis will not be disturbed by the dust in the measurement environment.

3) The laser ionization method is used to selectively ionize the substance to be detected, and the interference of other molecules with similar molecular weights is eliminated by selecting lasers of different wavelengths, and the concentration of dioxin online detection substitutes is monitored by time-of-flight mass spectrometry. The time-of-flight mass spectrometer is mainly based on the following principle: After the ions are accelerated by the electric field, they fly over the flight tube and reach the detector. The flight time is: the flight time is:

$\mathrm{<span\; class="notranslate">\; t</span>}<span\; class="notranslate">\; =</span>\mathrm{<span\; class="notranslate">\; L</span>}\sqrt{\frac{\mathrm{<span\; class="notranslate">\; m</span>}}{\mathrm{<span\; class="notranslate">\; e</span>}}\frac{\mathrm{<span\; class="notranslate">\; 1</span>}}{\mathrm{<span\; class="notranslate">\; 2</span>}\mathrm{<span\; class="notranslate">\; V</span>}}}$

Where L is the length of the flight tube;

V is the acceleration voltage;

e is the ion charge.

Therefore, the flight time of the ion is proportional to the square root of the ion mass m. Recording the time when the ion reaches the detector can realize the mass resolution of the ion. The laser pulse width is generally below 10ns, which is called the starting point of time-of-flight measurement. The mass resolution of linear time-of-flight mass spectrometers is relatively low, generally below 1000. The mass resolution of the reflective time-of-flight mass spectrometer can reach 5000-20000. In addition to high sensitivity and high mass resolution, another outstanding feature of time-of-flight mass spectrometers is their high speed. In the present case, the repetition rate of both the laser and the sample introduction system can reach 100 Hz, so a time resolution of 10 ms can be achieved.

In order to eliminate the interference of other molecules with similar molecular weight during detection, a selective ionization method is realized by adjusting the working wavelength of the tunable laser. The specific parameters are as follows: When the dioxin precursor is polychlorinated biphenyl, select the working wavelength of the laser The single photon ionization method is 265-275nm. When the dioxin precursor is polycyclic aromatic hydrocarbons, the single-photon ionization method with a laser operating wavelength of 248-266nm is selected. When the dioxin precursor is ethylene, propylene, butane For diene, cyclopentadiene, vinyl acetylene, malonic acid, toluene or styrene, select the single-photon ionization method with the working wavelength of the laser at 118nm.

4) The single-line spectral characteristics and locking function of the tunable laser spectrometer lock the above-mentioned substitutes for key detection (the detection parameters of the spectroscopic instrument and the detection parameters of the time-of-flight mass spectrometer are adjusted up), and the online concentration of these precursors C _precursor ( t).

5) Monitor the concentration and distribution of the precursors of dioxins online through the above method, and then select some operating points to conduct off-line dioxin concentration analysis on the flue gas at these operating points. By comparing dioxins and their precursors The concentration and distribution characteristics of dioxins can be established, and the mathematical model of the corresponding correlation relationship can be established to achieve the purpose of online monitoring of dioxins. The ratio Φ'(t) of the total amount of dioxins to the concentration of dioxin precursors is preliminarily determined as: when the dioxin precursors are PCBs, the ratio is 0.025-0.019; when the dioxin precursors are polycyclic Aromatic hydrocarbons, the ratio is 0.004~0.006; when the dioxin precursor is chlorophenols (CP), the ratio is 0.014~0.008); when the dioxin precursor is ethylene, propylene, butadiene, cyclopentadiene, For vinyl acetylene, malonic acid, toluene or styrene, the ratio is 0.001 to 0.005. The off-line dioxin analysis method is mainly implemented with reference to the US EPA8280 method, which specifically includes four steps: sampling, separation, purification, and detection. Select the start-up, shutdown, rated load of the combustion system equipment to be tested, and the corresponding operating points when the overload is 20%, conduct off-line dioxin concentration analysis on the flue gas at these operating points according to the US EPA8280 method, and compare the dioxin The concentration and distribution characteristics of dioxin and its precursors are compared with the initially determined ratio, and the original ratio is corrected to establish a corresponding real-time equivalent relationship Φ(t) between dioxin precursors and the total amount of dioxin.

6) Using the above-mentioned mathematical model of the monitored precursor and dioxin concentration, by determining the concentration of the dioxin real-time monitoring substitute, the concentration data of the dioxin real-time online monitoring is obtained. The concentration of dioxin online can be calculated by the following formula:

C _dioxin (t) = C _precursor (t) × Φ (t)

Compared with the existing dioxin detection method, the present invention has the following advantages: real-time performance, accuracy, pertinence, strong adaptability, easy operation, low monitoring cost, and can be widely used in power station systems and municipal waste incineration treatment Systematic real-time monitoring of dioxins is in progress. The content of the present invention will be further described in conjunction with the following examples, but the content of the present invention is not limited to the content involved in the examples.

measurement accuracy:

Test 1#

Experimental conditions:

This system has adopted MCB (m/z=188.5) as a substitute for real-time monitoring of dioxins to monitor the concentration of dioxins in the tail flue of a 160T/D waste incinerator in Hangzhou. The laser ionization wavelength of the time-of-flight mass spectrometer is selected 269nm, the ionization mode is single photon ionization mode.

Measurement result:

Table 1 Example data of on-line monitoring of dioxin concentration in the tail flue of a 160T/D waste incinerator in Hangzhou by this method

Comparison of Results PCDD(ng/kg) PCDF(ng/kg) TEQ(TEQng/kg) Working Hours Time Resolution

Traditional method 104 76 0.927 8.3 hours/sample ——

This method 92～123 63～105 1.060～0.854 —— 0.1s

Experimental results show that this method works well for on-line monitoring of dioxins.

Claims (5)

1. A method of tunable laser spectrum combined with time-of-flight mass spectrometry on-line monitoring of dioxins is characterized in that: the laser absorption element and the detector of the tunable laser spectrometer are respectively installed on the opposite sides of the pipe diameter of the measured combustion system, The gas is first detected by the laser absorbing element of the tunable laser spectrometer. The upper limit of the working current of the laser is 20-30A, and the maximum working wavelength range of the laser is 112-281nm. The gas to be measured is locked, and the gas passes through the detector of the tunable laser spectrometer. The gas then enters the time-of-flight mass spectrometer for detection. Through the selective ionization method, the precursor is detected online, and then the dioxin precursor is used to obtain the equivalent relationship between the dioxin precursor and the total dioxin. Total online concentration. 2. A method for online monitoring of dioxins by tunable laser spectroscopy combined with time-of-flight mass spectrometry according to claim 1, characterized in that: the dioxin precursors are: chlorobenzenes, chlorophenols, poly Chlorinated biphenyls, polycyclic aromatic hydrocarbons, ethylene, propylene, butadiene, cyclopentadiene, vinyl acetylene, malonic acid, toluene or styrene. 3. A method for online monitoring of dioxins by tunable laser spectroscopy combined with time-of-flight mass spectrometry according to claim 1, characterized in that: said selective ionization method: when the dioxin precursor is polychlorinated For biphenyl, choose the single-photon ionization method with the working wavelength of the laser at 265-275nm. When the dioxin precursor is polycyclic aromatic hydrocarbons, choose the single-photon ionization method with the working wavelength of the laser at 248-266nm. When the British precursor is ethylene, propylene, butadiene, cyclopentadiene, vinyl acetylene, malonic acid, toluene or styrene, the single-photon ionization mode with the working wavelength of the laser at 118nm is selected. 4. A method for on-line monitoring of dioxins by tunable laser spectroscopy combined with time-of-flight mass spectrometry according to claim 1, characterized in that: the equivalent relationship is: the total amount of dioxins and the concentration of dioxin precursors The ratio: when the dioxin precursor is polychlorinated biphenyl, the ratio is 0.025-0.019; when the dioxin precursor is polycyclic aromatic hydrocarbons, the ratio is 0.004-0.006; when the dioxin precursor is chlorophenol ( CP) ratio is 0.014-0.008; when the dioxin precursor is ethylene, propylene, butadiene, cyclopentadiene, vinyl acetylene, malonic acid, toluene or styrene, the ratio is 0.001-0.005. 5. A kind of tunable laser spectrum according to claim 1 combines the method for on-line monitoring of dioxin with time-of-flight mass spectrometry, it is characterized in that: the calibration method of described equivalence relationship is: select the start-up of the measured combustion system equipment, Shutdown, rated load, and corresponding operating points when the overload is 20%, conduct off-line dioxin concentration analysis on the flue gas at these operating points according to the US EPA8280 method, and compare the concentration and distribution characteristics of dioxin and its precursors , to establish the equivalent relationship between the corresponding dioxin precursors and the total amount of dioxin.

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