CN108709945B - VOCs fixed source online monitoring method - Google Patents
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Abstract
The invention discloses an online monitoring method for VOCs fixed sources, which comprises the following steps: under a normal working condition, carrying out full-scan analysis on the gas discharged by the VOCs fixed source to obtain the ion intensity range of each characteristic ion; calculating to obtain the total equivalent concentration of VOCs discharged by the VOCs fixed source under normal working conditions; performing full-scan analysis on the gas to be detected discharged by the VOCs fixed source in real time by adopting an online mass spectrometer, and calculating to obtain the total equivalent concentration of the VOCs of the gas to be detected; and comparing the full-scanning mass spectrogram and the total equivalent concentration of VOCs of the gas to be detected with the full-scanning mass spectrogram and the total equivalent concentration of VOCs of the gas discharged by the VOCs fixed source under the normal working condition, and judging whether the VOCs fixed source is abnormally discharged at the moment. The online monitoring method does not need to monitor a single VOC index and a total index of non-methane total hydrocarbons, and effectively avoids the restriction of the standard system of the existing VOCs monitoring method.
Description
Technical Field
The invention relates to an online monitoring technology, in particular to an online monitoring method for VOCs fixed sources.
Background
VOCs are acronyms for Volatile Organic Compounds (Volatile Organic Compounds). According to the definition of the World Health Organization (WHO), VOCs refer to organic matters with melting points lower than room temperature and boiling points between 50 ℃ and 260 ℃ and exist in the air in a steam form at normal temperature; the Federal Environmental Protection Agency (EPA) in the United states defines that VOCs are other than CO and CO2、H2CO3Metal carbides, metal carbonates and ammonium carbonate, any carbon compound that participates in atmospheric photochemical reactions. The definition of the united states federal Environmental Protection Agency (EPA) is more reasonable than the atmospheric environmental protection. The harm of VOCs is mainly reflected in three aspects: firstly, some VOCs are directly harmful to human health and ecological environment, such as toxicity, carcinogenesis, teratogenesis, mutagenesis and the like; secondly, the catalyst participates in photochemical smog reaction and is an important precursor for forming ozone and fine particulate matter PM 2.5; third, VOCs are mostly greenhouse gases, which contribute to global warming.
VOCs are from a plurality of sources and almost relate to all aspects of industrial production and life. In terms of movement, there are stationary sources such as fixed discharge ports of industrial enterprises and mobile sources such as mobile discharge ports of automobiles, ships and the like. The types of VOCs are extremely large, hundreds of thousands, and hundreds of them are common, such as hydrocarbons, halogenated hydrocarbons, ketones, esters, alcohols, phenols, aldehydes, amines, nitriles, and the like. The existing environmental monitoring standard method system cannot completely cover all VOCs targets technically or economically, only covers a small part of the targets, does not have pollutant indexes and analysis methods for accurately representing total VOCs, cannot effectively monitor and monitor a waste gas pollution source with complex and variable VOCs components, causes VOCs emission supervision loopholes, and greatly restricts VOCs pollution prevention and control work.
At present, the indexes of VOCs pollutant emission include two types, namely total VOCs indexes and single VOC indexes.
The current environmental standard in China relates to about 300 VOCs, and the monitoring and analyzing methods of single VOC substances include HJ734-2014 solid phase adsorption-thermal desorption/gas chromatography-mass spectrometry for measuring volatile organic compounds in waste gas of fixed pollution sources (measuring method only comprising 24 VOCs), HJ/T33-1999 gas chromatography for measuring methanol in exhaust gas of fixed pollution sources, HJ/T32-1999 spectrophotometry for measuring 4-aminoantipyrine compounds in exhaust gas of fixed pollution sources and the like. Generally, the monitoring of a substance requires corresponding monitoring analysis method standards and standard substances, and the monitoring is not necessary. Limited by monitoring analysis methods and standard gases, less than 300 kinds of VOCs can be monitored at present, and only a few VOCs occupy the family. Common monitoring instruments are gas chromatography-flame ionization detector (GC-FID) and gas chromatography GC-MS. At present, the difficulty in monitoring the unknown VOCs in the fixed source of the VOCs is very high, the repeated verification by using different methods and instruments is needed to possibly obtain a certain ideal VOC substance monitoring result, a large amount of manpower and material resources are consumed, the time of several days or weeks is often consumed, the analysis of the whole components of the fixed source is more difficult, the operation is difficult in reality, and the online monitoring is not mentioned.
VOCs total amount index monitoring can play better management and control effect to VOCs pollution supervision. Different countries or standards have different names and definitions for evaluating total amount of VOCs indexes, and the indexes usually have different forms such as TOC, THC, NMTHC, TVOCs and the like, are usually related to different detection methods and instruments, and need to be selected according to emission characteristics of pollution sources and different application occasions.
(1) The Total Organic Carbon (TOC) refers to the concentration of carbon existing in the form of organic matters in gas, and is adopted by European Union and Japan, and measuring instruments are divided into a catalytic oxidation-nondispersive infrared detector and a hydrogen ion Flame Ionization Detector (FID), wherein the former is limited by an analytical instrument, has higher detection limit and can only detect high-concentration VOCs; the latter adopts FID detector, and the response value to organic matters containing heteroatoms such as oxygen, chlorine, nitrogen, sulfur and the like is very low and different, so that the total concentration of VOCs with unknown components or complex and variable components and containing heteroatoms such as oxygen, chlorine, nitrogen, sulfur and the like cannot be accurately represented.
(2) The Total Hydrocarbon (THC) is defined in the HJ604-2011 standard of China as the total amount of gaseous hydrocarbons and derivatives thereof measured by a hydrogen ion Flame Ionization Detector (FID) in terms of methane; the non-methane total hydrocarbons (NMTHC) HJ/T38-1999 standard is defined as a generic term referring to all volatile hydrocarbons and their derivatives that respond on FID detectors except methane, and as can be seen from the above, the THC and NMTHC cannot accurately characterize the total concentration of VOCs that are not known in composition or are complex and varied in composition and contain heteroatoms such as oxygen, chlorine, nitrogen, sulfur, etc., due to the FID detectors.
(3) Total Volatile Organic Compounds (TVOCs) are defined differently in different standards and methods, one being expressed in terms of the specified VOCs equivalent concentration, e.g., in toluene, propane; the other refers to the sum of the concentrations of single VOC substances in a certain range, as specified in GB21902-2008, which is essentially the monitoring of the complete composition of VOCs. Limited by VOCs component monitoring technology, TVOCs cannot represent the total concentration of VOCs with unknown components or complex and variable components and containing heteroatoms such as oxygen, chlorine and the like.
The VOCs on-line monitoring technology is characterized in that a manual monitoring and analyzing method is used for reference, and a sampling device and an analyzing instrument are subjected to system integration optimization to realize real-time automatic continuous monitoring. The analysis technologies used for on-line monitoring include various sensor technologies, spectrum technologies, chromatography technologies, mass spectrum technologies and the like, and the commonly used VOCs detection instruments include an on-line gas chromatograph (FID flame ionization detector and PID photoionization detector), an on-line gas chromatograph-mass spectrometer, an on-line infrared spectrometer, an on-line laser detector, an on-line differential optical absorption spectrometer and the like.
The total index of the current VOCs fixed source on-line monitoring is non-methane total hydrocarbon, and the index of single VOC substance mainly comprises benzene, toluene and the like. The existing VOCs fixed source online monitoring technology is widely applied mainly to PID and FID online detection technologies, is suitable for occasions with single components or hydrocarbon VOCs emission, can monitor non-methane total hydrocarbon projects, and is successfully applied to industries such as petroleum refining and the like. The online monitoring for single VOC substances mainly comprises a GC-FID or GC-MS online monitoring technology, the system is complex, the operation and maintenance cost is high, the data output is slow, false positive is easy to occur, the data is inaccurate, and the large-scale use is difficult. In practical application, the total discharge indexes of the VOCs are required to be monitored so as to comprehensively reflect the actual discharge condition of the fixed sources of the VOCs, so that cases that the total discharge indexes of the VOCs and the full-component monitoring of the VOCs are not successful are still in an exploration stage under the condition that the main components of the VOCs are unknown or the components are complex and varied and contain heteroatoms such as oxygen and chlorine.
The industries such as chemical industry, pesticide, textile printing and dyeing, spraying and the like are VOCs discharge households, but the discharge conditions of a plurality of fixed source VOCs are extremely complex, and the discharge components of most of the fixed source VOCs are unknown. For example, the waste gas of the setting machine, the VOCs of which are derived from a plurality of components such as polyphenyl organic matters, printing and dyeing auxiliaries, oil and the like which are heated and volatilized by chemical fibers are mixtures, contain heteroatoms such as oxygen, silicon and the like, are not single VOCs components and are not pure hydrocarbons, are monitored according to the existing monitoring project of non-methane total hydrocarbons (NMTHC), the monitoring concentration is very low, the emission reaches the standard, and actually, the total concentration of the VOCs is more than dozens of times of the total concentration of the non-methane total hydrocarbons; if the VOCs waste gas is treated by an acid-base absorption method and a combustion method, the VOCs components in the tail gas are changed due to chemical reactions, according to the standard system of the existing monitoring method, the phenomenon that the actual emission is high due to low monitoring result is often caused no matter the concentration of a single VOC substance or the total concentration is measured, and a plurality of environment-friendly facilities which are abnormally operated cannot be found in time, so that the atmospheric pollution of the VOCs is serious is one of important reasons.
Summarizing, the existing VOCs monitoring method system is restricted, VOCs single substance and total amount monitoring is still to be perfected, and the current VOCs on-line monitoring technology has the following defects:
(1) the monitoring efficiency is low, the data output is slow, the time for one-time monitoring is usually about 1 hour, and one set of on-line monitoring device can only meet the on-line monitoring requirement of one fixed source;
(2) the data quality is poor. False positive is easy to occur when the single VOCs index is monitored on line by adopting GC-FID and GC-MS, so that the data is inaccurate; the total index of non-methane total hydrocarbon can not accurately represent the total discharge amount of VOCs containing heteroatoms such as oxygen, chlorine and sulfur;
(3) the online monitoring system is complex, high in operation and maintenance cost, required to be maintained every day and high in requirement on the technical quality of personnel;
(4) the method is not suitable for the occasions of online monitoring and monitoring of fixed sources of VOCs with unknown or complex and variable components and containing heteroatoms such as oxygen, chlorine and sulfur;
therefore, under the conditions that the current VOCs pollution treatment level is low and the pollution situation of the atmospheric VOCs is very severe, a universally-applicable VOCs fixed source online monitoring and monitoring method is urgently needed.
Disclosure of Invention
Aiming at the defects in the prior art, the invention provides the VOCs fixed source on-line monitoring method, which can comprehensively and accurately master the discharge condition of the VOCs fixed source in real time by comparing and analyzing the fluctuation conditions of various characteristic ions and the total equivalent concentration of VOCs on the detected gas spectrogram, does not need to monitor a single VOC index and a total index of non-methane total hydrocarbons, effectively avoids the restriction of the existing VOCs monitoring method system, and is suitable for various VOCs fixed source on-line monitoring occasions.
The invention provides the following technical scheme:
a VOCs fixed source on-line monitoring method comprises the following steps:
(1) calibrating an online mass spectrometer by adopting VOCs standard gas;
(2) continuously sampling gas discharged by the VOCs fixed source in one discharge period of the VOCs fixed source under normal working conditions, performing full-scan analysis on the sampled gas by adopting an online mass spectrometer, and recording a full-scan mass spectrogram to obtain the intensity range of each ion in the next discharge period under the normal working conditions;
(3) correcting the full-scanning mass spectrogram, screening characteristic ions, calculating the equivalent concentration of each characteristic ion, and adding the equivalent concentrations of all the characteristic ions to obtain the total equivalent concentration of VOCs discharged by the VOCs fixed source under normal working conditions;
on the basis of the steps, obtaining characteristic ions and the intensity fluctuation range thereof, and the total equivalent concentration of VOCs and the fluctuation range thereof in a discharge period under normal working conditions, thereby forming a reference full fingerprint spectrogram of the fixed source of the VOCs;
(4) performing full-scan analysis on the gas to be detected discharged by the VOCs fixed source in real time by using an online mass spectrometer to obtain a full-scan mass spectrogram of the gas to be detected, calculating the equivalent concentration of each characteristic ion of the gas to be detected after correction, and adding the equivalent concentrations of all the characteristic ions to obtain the total equivalent concentration of the VOCs of the gas to be detected;
and (3) forming a full fingerprint spectrogram of one-time scanning of the gas to be detected by using the intensity of each characteristic ion and the total equivalent concentration of the VOCs obtained by full scanning analysis of the gas to be detected, comparing the full fingerprint spectrogram obtained by scanning with a reference full fingerprint spectrogram, and judging whether the VOCs fixed source is abnormally discharged at the moment.
The method comprises the steps of establishing a fixed source reference full fingerprint spectrogram by performing full scanning analysis on VOCs fixed source exhaust gas; when the measured gas component changes, the related characteristic ions of the mass spectrogram on the online mass spectrometer also change, so that the real-time online monitoring and control of the discharge of the VOCs in the fixed source are realized, and the online monitoring and control device has a good monitoring effect on the abnormal discharge condition of the VOCs in the industries such as chemical industry, pesticide industry, textile printing and dyeing, spraying industry and the like.
In the step (1), the calibrating the online mass spectrometer by adopting VOCs standard gas comprises the following steps:
and detecting a group of VOCs standard gases with gradient distribution concentration by adopting an online mass spectrometer, and establishing a standard curve between the concentration of the VOCs standard gases and the ion response intensity of the maximum abundance of the VOCs standard gases.
Preferably, the VOCs standard gas is toluene gas;
the standard curve is as follows: 4.4966 x-1.0319; wherein y is the ionic response strength in 10-11A; x is the concentration of toluene gas in ppm; r2＝0.9961。
In the step (2), the full scanning range of the on-line mass spectrometer for full scanning analysis is 1-200 m/z. The full scan range can effectively cover almost all the VOCs components of the fixed source, and almost all the VOCs components can be expressed on a mass spectrogram, so that the full scan analysis is suitable for all the VOCs fixed sources.
In the step (3), the step of correcting the full-scanning mass spectrogram refers to the following steps: and removing ions of non-VOCs components in the full-scanning mass spectrum.
The ions of the non-VOCs components are ions of gases such as oxygen, nitrogen, carbon dioxide, argon, nitrogen oxide, water vapor and isotopes thereof.
The gas of non-VOCs components in the detected gas is determined through the analysis of the production process and the actual detection result.
In the step (3), the method for screening the characteristic ions comprises the following steps:
sequencing ions on the full-scanning mass spectrogram from high ion intensity to low ion intensity, and taking the first 10-30-bit ions as the characteristic ions; preferably, the ions at the first 20 th position are taken as the characteristic ions;
or, taking a certain VOCs standard gas as a reference for quantitative determination, and taking an ion as the selected characteristic ion when the equivalent concentration of the ion is greater than a certain value; preferably, when the equivalent concentration of an ion is greater than 0.5mg/m3Then, the ion is taken as the selected characteristic ion.
After the characteristic ions of the screened fixed source exhaust gas are subjected to subsequent equivalent concentration calculation, only the equivalent concentration of each characteristic ion needs to be calculated, and the concentrations of other ions except the characteristic ions are low, so that the influence on the equivalent concentration of the total amount of VOCs is small.
In the step (3), the equivalent concentration calculation method of the characteristic ions comprises the following steps:
obtaining the ion intensity A of the characteristic ion from the full-scanning mass spectrogram, calculating the volume equivalent concentration C (ppm) of the corresponding characteristic ion according to the ion intensity A, and calculating the mass-volume ratio equivalent concentration C of the characteristic ion according to the mass number M of the corresponding characteristic ioni(mg/m3) The calculation formula is as follows: ci＝C×M/22.4。
The total normality of VOCs (on the selected standard gas concentration) is the normality sum of all characteristic ions.
In the step (4), the method for correcting the full-scanning mass spectrogram of the gas to be detected is the same as the step (3); in the step (4), the method for calculating the equivalent concentration of the characteristic ions is the same as that in the step (3).
In the step (4), the method for judging whether the discharge of the fixed source of the VOCs is abnormal is as follows:
at some point, the fixed source emissions of VOCs at that point in time are abnormal if one or more of the following conditions occur:
(a) characteristic ions which are not contained in the reference full-fingerprint spectrum appear in the full-scanning mass spectrum at the moment;
(b) in the full-scanning mass spectrogram at the moment, the intensity of a certain characteristic ion is higher than the intensity fluctuation range of the corresponding characteristic ion in the reference full-fingerprint spectrogram;
(c) the total equivalent concentration of the VOCs at the moment is higher than the fluctuation range of the total equivalent concentration of the VOCs in the reference full fingerprint;
otherwise, the discharge of the VOCs fixed source is normal at the moment.
The invention also provides an online monitoring method for VOCs fixed sources under the condition that VOCs components discharged by the VOCs fixed sources are known, which comprises the following steps:
(1) calibrating an online mass spectrometer by adopting standard gas with known components of a VOCs fixed source;
(2) in a discharge period of the VOCs fixed source under normal working conditions, continuously sampling gas discharged by the VOCs fixed source, and performing specified scanning on the sampled gas by adopting an online mass spectrometer to obtain the concentration fluctuation range of each VOCs component;
(3) performing designated scanning on the gas to be detected discharged by the VOCs fixed source in real time by adopting an online mass spectrometer to obtain the concentration of each VOCs component in the gas to be detected;
and comparing the obtained concentration of each VOCs component in the gas to be detected with the concentration fluctuation range of each VOCs component under the corresponding normal working condition or the emission standard of each VOCs component, and judging whether the VOCs fixed source is abnormally emitted at the moment.
In the step (1), the designated scanning means: the maximum abundance ion for each VOCs component was designated for scanning.
The specified scanning in the step (1) can directly and quantitatively detect the VOCs components, but is only suitable for all VOCs fixed sources with known VOCs components. This is because unknown VOCs components, when present, affect the intensity of the relevant ions on the mass spectrum obtained by the given scan, resulting in inaccurate detection results.
The on-line monitoring method of the invention can realize multi-point monitoring, and the reason is that: the full scanning of a fixed source of VOCs only needs 30 seconds, and appointed scanning only needs several seconds, and monitoring efficiency is high, as long as satisfy the requirement of fixed source monitoring frequency, can carry out full scanning or appointed scanning in turn to the fixed source of up to hundreds of VOCs and monitor in real time.
The technical scheme of the invention is suitable for online monitoring of any VOCs fixed source. For fixed sources of known VOCs components, VOCs indexes can be quantitatively monitored and monitored on line by adopting appointed scanning; for VOCs fixed sources which are difficult to monitor in the standard system of the existing method, the VOCs emission condition can be monitored in real time by analyzing the change condition of a full-scanning mass spectrogram without monitoring specific VOCs component indexes.
The technical scheme of the invention is not only suitable for VOCs fixed sources, but also suitable for the occasions of online monitoring and monitoring of environment air with higher concentration, such as factory boundary, unorganized emission and the like, and can be widely applied to online monitoring and monitoring of VOCs pollution in industrial parks in the industries of chemical industry, pesticides, textile printing and dyeing, spraying and the like.
Compared with the prior art, the invention has the beneficial effects that:
(1) the applicability is wide. The device is suitable for any VOCs fixed source on-line monitoring occasions containing heteroatoms such as oxygen, chlorine, sulfur, nitrogen and the like;
(2) the monitoring efficiency is high. The method has the advantages that pretreatment units such as chromatographic columns and the like are not needed, the tested gas directly enters the mass spectrometer through the polar semipermeable membrane, one VOC index only needs several seconds, one fixed source full-scanning only needs 30 seconds, and the data output is extremely fast; the existing online monitoring technology takes about 1 hour to measure one index, and the data output is slow;
(3) the operation and maintenance are simple, and the cost is low. Various carrier gases such as hydrogen, nitrogen, propane and the like are not needed, the chromatographic column is not replaced frequently, a standard curve is not needed to be made frequently, only the air pump is checked periodically, and the polar semipermeable membrane is replaced once a year;
(4) and the data reliability is high. The method has the advantages that the problems of loss or false positive and the like of the VOCs components of the detected gas caused by pretreatment of a chromatographic column and the like are avoided, almost all VOCs components enter an online mass spectrometer for detection, ions of almost all VOCs components (including VOCs components containing heteroatoms such as oxygen, chlorine, sulfur, nitrogen and the like) are reflected by related ions on a mass spectrogram, and the total equivalent concentration of the VOCs is more reliable and representative than the total index of non-methane total hydrocarbon and is not influenced by the heteroatoms such as oxygen, chlorine, sulfur, nitrogen and the like;
(5) the resolution degree is high. The invention not only can monitor and control the change condition of the total equivalent concentration of the VOCs of the fixed source in real time, but also can monitor and control the slight changes of the running condition and the production condition of the environmental protection facility in real time by monitoring and controlling the change condition of each ion intensity on the mass spectrogram in real time;
(6) and monitoring at multiple points. The invention can monitor the discharge condition of the unorganized or organized fixed source of 127 point sites in real time at most, one point site needs 30 seconds (time spent in full scanning) at most, the monitoring period of the 127 point sites only needs 10-64 minutes, and the single multi-point site rapid monitoring can be realized.
Drawings
FIG. 1 is a plot of standard concentration of toluene;
FIG. 2 is a full-scan mass spectrum of the exhaust gas discharged at 10:23 am (under normal conditions);
FIG. 3 is a full scan mass spectrum of exhaust gas discharged at 16:26 PM;
FIG. 4 is a full scan mass spectrum of 23:32 pm exhaust;
FIG. 5 is a full scanning mass spectrum of the exhaust gas emitted during a certain abnormal emission.
Detailed Description
The invention will be described in further detail below with reference to the drawings and examples, which are intended to facilitate the understanding of the invention without limiting it in any way.
The VOCs fixed source on-line monitoring method disclosed by the invention is characterized in that an on-line mass spectrum monitoring technology is applied to carry out full scanning on the measured gas within a certain range, generally within the range of 1-200 m/z, and is specifically determined according to the performance of a mass spectrometer; taking characteristic ions with the ion intensity of the first N positions (such as the first 20 positions) on the full-scanning mass spectrogram,or the quantitative concentration of toluene as standard gas is higher than a certain value (such as higher than 0.5 mg/m)3) Establishing a full fingerprint spectrogram or a characteristic fingerprint spectrogram of the VOCs fixed source under normal working conditions by using a statistical method; when the total equivalent concentration of VOCs of the real-time measured gas exceeds the set limit value of the full fingerprint spectrogram or the characteristic fingerprint spectrogram, abnormal emission of the fixed source of VOCs can be judged. Otherwise, the normal discharge state is achieved. The method mainly comprises the following steps:
step a mass spectrometer calibration
And (4) selecting toluene standard gas to calibrate the online mass spectrometer.
Step b. gas sampling
The mixed gas in the fixed source is sampled by keeping a certain flow rate through a sampling pipe of the gas sampling unit.
Step c. gas full scan analysis
Introducing the mixed gas sample in the sampling pipe into an online mass spectrometer for full scanning analysis, scanning once every 5 minutes at intervals, wherein the scanning range is 1-200 m/z, and recording a full scanning mass spectrogram; determining gases of non-VOCs components in the gas to be detected through analysis of the production process and actual detection results, removing the maximum abundance ions of the gases, and correcting characteristic ions; screening characteristic ions at 20 th position before the corrected ion intensity, or taking toluene as standard gas to make the quantitative concentration be greater than 0.5mg/m3The characteristic ion of (1); and establishing a VOCs fixed source full fingerprint spectrogram or characteristic fingerprint spectrogram.
Step d, VOCs online real-time detection
C, measuring the characteristic ions determined in the step C by using a calibrated online mass spectrometer, and sending the measured related ion intensity A to a data processing unit;
when the peak value of some characteristic ions of the real-time measured gas is found to be out of a certain range (higher than or lower than a limit value, and is respectively judged according to different fixed source conditions) or new ions appear, the abnormal emission of the VOCs fixed source can be judged. Otherwise, the normal discharge state is achieved.
Step e, calculating and outputting the equivalent concentration of real-time emission of VOCs
D, calculating the volume equivalent concentration C (PPM) of the corresponding characteristic ions according to the ion intensity A of the characteristic ions of the measured mixed gas obtained in the step d, and calculating the mass-to-volume equivalent concentration C of the characteristic ions according to the mass number M of the characteristic ions and the following formulai(mg/m3)；
Ci＝C×M/22.4；
The total equivalent concentration of VOCs of the tested mixed gas is the sum of the equivalent concentrations of all characteristic ions.
When the total equivalent concentration of VOCs of the real-time measured gas exceeds the set limit value of the full fingerprint spectrogram or the characteristic fingerprint spectrogram, abnormal emission of the fixed source of VOCs can be judged. Otherwise, the normal discharge state is achieved.
Examples
A certain dye production enterprise mainly produces products such as indigo, sulfur black, dinitroaniline, dinitrochlorobenzene, disperse red dye and the like, and main waste gas pollutants comprise more than 10 types of toluene, DMF (dimethyl formamide), methanol, dichloroethane, chloropropene, acrylonitrile and the like. The organized waste gas sources such as a workshop reaction kettle, a vacuum pump, a drying device, a rectifying tower and the like are collected and then enter an RTO incinerator for incineration treatment, then alkali absorption treatment is carried out through a spray tower, and tail gas is discharged through an exhaust funnel.
In order to effectively monitor the organic waste gas emission situation, the scheme is characterized in that a monitoring point is arranged on an exhaust funnel of the waste gas treatment device, an online mass spectrometer is adopted for online monitoring of VOCs emission, meanwhile, manual VOCs monitoring is carried out at a fixed moment according to the national relevant technical specifications, and relevant monitoring data are compared and analyzed.
The online mass spectrometer has the following specific parameters:
capturing current intensity: 40 muA; quadrupole lens digital-to-analog converter: -2.0V; reflecting plate voltage: 6.04V; focusing voltage: 450.0V; deflection voltage: 1.6V; ion energy: 1000V; a detector: a secondary multiplying electron detector.
The experimental procedure was as follows:
1. instrument calibration procedure
Toluene standard gas with the concentration of 0.5ppm, 2.0ppm, 5.0ppm and 8.0ppm is prepared by using a dynamic mixing dilution gas distribution system, and is introduced into an online mass spectrometer for testing, and a standard curve is drawn by taking the concentration as a horizontal coordinate and the ion response intensity as a vertical coordinate.
The specific values are shown in Table 1, and the standard graph is shown in FIG. 1. .
TABLE 1 toluene Standard concentration values
2. Gas sampling
Sampling the mixed gas in the fixed source at the flow rate of 5L/min by a sampling pipe of the gas sampling unit between 10:23 and 10:26 in the morning.
3. Real-time monitoring and controlling tail gas discharged after treatment of Regenerative Thermal Oxidizer (RTO)
And introducing the collected mixed gas sample into an online mass spectrometer, wherein the VOCs and a small amount of inorganic gas penetrate through a polar semipermeable membrane and enter a detector for full-scanning analysis, the rest inorganic gas is directly discharged, the scanning range is 1-200 m/z, and a full-scanning mass spectrogram is recorded (figure 2). Due to the mixing of a large amount of air in the waste gas collecting process and the generation of CO by combustion2And gases such as water vapor and NO, so the maximum abundance ions of the gases need to be removed when the total equivalent concentration of the VOCs is calculated, specifically: o, 16; h2O，18，；N2And NO, 28; o is2，32；Ar，40；CO2，44)。
And (3) quantifying the characteristic ions at the front 20 positions of the corrected row by using toluene, and converting the quantitative result into mass-volume ratio equivalent concentration by using a formula (1) by using a volume fraction concentration meter, wherein the specific result is shown in a table 2.
Ci＝C×M/22.4； (1)
Wherein, CiIs characterized by the mass equivalent concentration of ions, mg/m3(ii) a C is the characteristic ion volume fraction, ppm; m is the characteristic ion molar mass, g/mol.
TABLE 2 characteristic ion equivalent concentration at the first 20 sites of ion response intensity
4. Calculating the total equivalent concentration of VOCs
And (3) summing the mass-to-volume ratio equivalent concentrations of the characteristic ions in the step (3) to obtain the total VOCs concentration, and the result is shown in Table 2.
5. Mass spectrum full-scanning result monitored in real time at 16:26 pm and 23:32 night
The real-time monitoring results between 16:26 PM and 16:29 PM are shown in FIG. 3.
The monitoring in the afternoon period is close to the time of handing over with the night shift, and the production intensity is reduced to some extent due to the adjustment of the process and the time of handing over, and the pollutant discharge types are changed. Originally, diethylamine is used as a raw material in the sulfonation reaction in the morning, but the sulfonation process is not performed in the reaction process in the afternoon, so that diethylamine is not used, and the peak value of the characteristic ion with the mass number of 58 is obviously reduced on the mass spectrum in the afternoon. The total equivalent concentration of the characteristic ions is 132.2mg/m3。
The real-time monitoring results at night between 23:32 and 23:35 are shown in FIG. 4.
The production intensity was slightly reduced during the night period compared to the afternoon, the total equivalent concentration of the characteristic ions being 120.8mg/m3。
The real-time monitoring result of the abnormality monitored at a time is shown in fig. 5.
According to the inspection, the abnormal condition of the discharge is caused by the operations of cleaning the reaction kettle and emptying residual liquid in the kettle after the reaction is finished, so that a large amount of solvent to be recycled is evaporated and leaked in the form of waste gas in the kettle opening process, the discharge capacity is greatly increased and reaches 313.4mg/m3。
6. Compared with the existing detection method
The existing detection method for the non-methane total hydrocarbons is to carry out detection according to the detection standard HJ 604-2017 direct sample injection-gas chromatography for the determination of the total hydrocarbons, methane and the non-methane total hydrocarbons in the ambient air. Definitions of non-methane total hydrocarbons quoted from this standard: "refers to the sum of the other gaseous organic compounds after subtracting methane from the total hydrocarbons under the measurement conditions specified in this standard".
The sampling method is similar to the method in 10 a.m: and (4) carrying out full-scanning test on the mass spectrometer at 23 hours at the same time, sampling for three times, and taking the average value of the three detection results. The specific data are shown in Table 3.
TABLE 3 comparison of results of the monitoring method of the present invention with the non-methane Total Hydrocarbon detection method
Therefore, the VOCs fixed source on-line monitoring method has the characteristics of quick response, sensitivity and high accuracy, is particularly suitable for organic waste gas monitoring occasions containing a large amount of heteroatoms such as chlorine and oxygen, and can effectively make up the defects of the existing monitoring methods such as non-methane total hydrocarbon and the like.
The method comprises the steps of carrying out full scanning analysis on VOCs fixed source exhaust gas to establish a full fingerprint spectrogram or a characteristic fingerprint spectrogram; when the measured gas component changes, the related characteristic ions of the mass spectrogram on the online mass spectrometer also change, so that the real-time online monitoring and control of the discharge of the VOCs in the fixed source are realized, and the online monitoring and control device has a good monitoring effect on the abnormal discharge condition of the VOCs in the industries such as chemical industry, pesticide industry, textile printing and dyeing, spraying industry and the like.
The above-mentioned embodiments are intended to illustrate the technical solutions and advantages of the present invention, and it should be understood that the above-mentioned embodiments are only specific embodiments of the present invention, and are not intended to limit the present invention, and any modifications, additions, equivalents, etc. made within the scope of the principles of the present invention should be included in the scope of the present invention.
Claims (6)
1. A VOCs fixed source on-line monitoring method is characterized by comprising the following steps:
(1) calibrating an online mass spectrometer by adopting VOCs standard gas;
(2) continuously sampling gas discharged by the VOCs fixed source in one discharge period of the VOCs fixed source under normal working conditions, performing full-scan analysis on the sampled gas by adopting an online mass spectrometer, and recording a full-scan mass spectrogram to obtain the intensity range of each ion in the next discharge period under the normal working conditions;
(3) correcting the full-scanning mass spectrogram, screening characteristic ions, calculating the equivalent concentration of each characteristic ion, and adding the equivalent concentrations of all the characteristic ions to obtain the total equivalent concentration of VOCs discharged by the VOCs fixed source under normal working conditions; the step of correcting the full-scanning mass spectrogram is as follows: removing ions of non-VOCs components in the full-scanning mass spectrogram;
the method for screening the characteristic ions comprises the following steps:
sequencing ions on the full-scanning mass spectrogram from high ion intensity to low ion intensity, and taking the first 10-30-bit ions as the characteristic ions;
or, taking a certain VOCs standard gas as a reference for quantitative determination, and taking an ion as the characteristic ion when the equivalent concentration of the ion is greater than a certain value;
on the basis of the steps, obtaining characteristic ions and the intensity fluctuation range thereof, and the total equivalent concentration of VOCs and the fluctuation range thereof in a discharge period under normal working conditions, thereby forming a reference full fingerprint spectrogram of the fixed source of the VOCs;
(4) performing full-scan analysis on the gas to be detected discharged by the VOCs fixed source in real time by using an online mass spectrometer to obtain a full-scan mass spectrogram of the gas to be detected, calculating the equivalent concentration of each characteristic ion of the gas to be detected after correction, and adding the equivalent concentrations of all the characteristic ions to obtain the total equivalent concentration of the VOCs of the gas to be detected;
the method comprises the steps of obtaining the intensity of each characteristic ion and the total equivalent concentration of VOCs by full scanning analysis of gas to be detected, forming a full fingerprint spectrogram of one scanning of the gas to be detected, comparing the full fingerprint spectrogram obtained by scanning with a reference full fingerprint spectrogram, and judging whether the VOCs fixed source is abnormally discharged at the moment; the method for judging whether the discharge of the VOCs fixed source is abnormal comprises the following steps:
at some point, the fixed source emissions of VOCs at that point in time are abnormal if one or more of the following conditions occur:
(a) characteristic ions which are not contained in the reference full-fingerprint spectrum appear in the full-scanning mass spectrum at the moment;
(b) in the full-scanning mass spectrogram at the moment, the intensity of a certain characteristic ion is higher than the intensity fluctuation range of the corresponding characteristic ion in the reference full-fingerprint spectrogram;
(c) the total equivalent concentration of the VOCs at the moment is higher than the fluctuation range of the total equivalent concentration of the VOCs in the reference full fingerprint;
otherwise, the discharge of the VOCs fixed source is normal at the moment.
2. The method according to claim 1, wherein the calibrating the online mass spectrometer with the standard gas of VOCs in step (1) comprises:
and detecting a group of VOCs standard gases with gradient distribution concentration by adopting an online mass spectrometer, and establishing a standard curve between the concentration of the VOCs standard gases and the ion response intensity of the maximum abundance of the VOCs standard gases.
3. The method according to claim 2, wherein the standard gas of VOCs is toluene gas;
the standard curve is as follows: y = 4.4966 x-1.0319; wherein y is the ionic response strength in 10-11A; x is the concentration of toluene gas in ppm; r = 0.9961.
4. The method for on-line monitoring and controlling fixed sources of VOCs according to claim 1, wherein in step (2), the full scan range of the full scan analysis by the on-line mass spectrometer is 1-200 m/z.
5. The method according to claim 1, wherein the ions of non-VOCs are oxygen, nitrogen, carbon dioxide, argon, nitrogen oxides, water vapor and isotopes thereof.
6. The method for on-line monitoring and controlling fixed sources of VOCs according to claim 1, wherein in step (3), the equivalent concentration of characteristic ions is calculated by:
obtaining the ion intensity A of the characteristic ion from the full-scanning mass spectrogram, calculating the volume equivalent concentration C of the corresponding characteristic ion according to the ion intensity A, and calculating the mass-to-volume equivalent concentration C of the characteristic ion according to the mass number M of the corresponding characteristic ioniThe calculation formula is as follows: ci=C×M/22.4。
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