VOCs are an important class of environmental pollutants that can be carcinogenic, teratogenic, and mutagenic when ingested over a long period of time. In addition, VOCs are the main precursors for ozone and secondary organic particulate matter (SOA) generation, which have strong photochemical reactivity and can trigger urban photochemical smog. According to studies, China’s emissions come from three main sectors: transportation, industry and biomass combustion, and the total VOCs emissions from industrial stationary sources in China will show a significant growth trend. In order to control VOCs emissions from stationary sources, protect the environment and safeguard human health, China has gradually incorporated VOCs control into the air pollutant control system, and gradually established and continuously improved emission standards and related management methods.
VOCs monitoring technology for stationary sources
Common analytical methods for VOCs include gas chromatography (GC), gas chromatography-mass spectrometry (GC-MS), high performance liquid chromatography (HPLC), fluorescence spectrophotometry, Fourier transform infrared spectrometry (FTIR), etc. In addition, there are also reflection interference spectrometry, off-line supercritical fluid extraction GC-MS and pulse discharge detector methods, among which The most applied methods are GC and GC-MS.
1. GC-FID hydrogen flame ionization method
1.1.1 Detection principle:
When the monitored VOCs gas is burned in a high temperature hydrogen flame flowing through H2 and air, high temperature ionization occurs and the electrons generated by the reaction are collected under the action of electric field to form a weak ionization current, the current intensity is proportional to the concentration of the measured components, so that the VOCs of the sample to be measured can be judged. This technique responds to all types of VOCs, and is particularly sensitive to hydrocarbon or carbon VOCs, and is therefore used in the monitoring of these VOCs. However, FID is usually large in size and weight, and requires H2 as an auxiliary agent, which is dangerous, and the presence of gaseous water, O2, and N, O, and halogen atoms in VOCs can cause errors in monitoring results.
1.1.2 Working Principle:
The sampling probe is responsible for sampling the flue gas, the built-in ceramic filter element is used to filter the dust in the flue gas, the accompanying pipeline is heated at high temperature to avoid condensation of water vapor in the flue gas, and the temperature-pressure flow is used to measure the temperature, pressure and flow rate of the flue gas in the chimney or flue. The temperature meter is used to measure the humidity of the flue gas in the chimney or flue, and the control cabinet has built-in control unit, industrial control machine, heating box, high temperature pump, etc. The standard gas is used to calibrate the analytical instruments, and the zero gas generator, hydrogen generator, and high purity nitrogen cylinder provide the gas source. Air compressor generates compressed air, which is used for regular back-blowing of the tracing pipeline, sampling probe and temperature and pressure flow.
1.1.3 Detection range:
Monitor gaseous organic pollutants such as methane, total hydrocarbons, non-methane total hydrocarbons, low-carbon enzymes and ketones, benzene (benzene, toluene, xylene, ethylbenzene, isopropylbenzene, etc.) and some halogenated hydrocarbons in the exhaust gas, as well as parameters such as humidity, temperature, pressure and flow rate of the exhaust gas.
1.2 Features and characteristics.
1.2.1 Gas chromatography/flame ionization detection (GC+FID) is the internationally recognized standard method for VOCs detection. With high accuracy, high sensitivity and high stability, stable and reliable operation and easy maintenance, it is suitable for online analysis and monitoring of VOCs in industrial waste gases under complex and demanding environmental conditions1.
1.2.2 The system can monitor a variety of organic waste gases such as total hydrocarbons, non-methane total hydrocarbons, benzene, ketones, olefins, alcohols, etc., which can meet the monitoring needs of different customers.
1.2.4 FID detector with automatic ignition function, automatic ignition after the flame is extinguished, safe and reliable, automatic over-temperature protection function to avoid damage to the device, reliable and stable chromatographic components and gas circuit design. The use of Silcosteel-treated gas pipeline greatly reduces the residual adsorption of gas samples in the wall of the tube.
1.3 Application range.
Petroleum refining and petrochemistry, pharmaceutical manufacturing, paint and ink manufacturing, rubber product manufacturing, plastic product manufacturing, ferrous metal smelting, electronics industry, printing industry, automotive surface coating, furniture surface coating, other surface coating, other VOCs emission industries.
2. Gas chromatography / flame photometric detector method (GC+FPD)
2.1 FPD detection mechanism
The FPD is based on the principle that sulfur and phosphide emit characteristic light with wavelengths of 394nm and 526nm respectively when they are burned in a hydrogen-rich flame.
Hydrogen-rich flame: The presence of a hydrogen-rich flame in the detector provides the basic conditions for combustion and excitation of organic compounds containing sulfur and phosphorus.
Characteristic wavelengths: When a sample is burned in a hydrogen-rich flame, sulfur-containing and phosphorus-containing organic compounds emit their characteristic wavelengths of light.
Photoelectric conversion: The detector is equipped with a filter and a photomultiplier, which converts the light into an electrical signal after the filter is selected by the photomultiplier.
2.2 Basic structure
FPD is mainly composed of flame nozzle, filter and photomultiplier tube and other three parts, the combustion chamber and hydrogen detector combustion chamber structure is very similar, if properly improved and installed above the nozzle collection pole, may also be used for hydrogen flame detector.
2.3 Detection process
The detection process of FPD is as follows: The carrier gas flowing after the GC column is mixed with air and hydrogen and then flows out through the nozzle where it burns. When the sample components flowing out of the column enter this hydrogen-rich flame together with the carrier gas, the sulfur and phosphorus compounds emit their characteristic light. Phosphorus-containing organics emit their characteristic light as HPO fragments, and sulfur-containing organics emit their characteristic light as excited S2 molecules. Phosphides are selected with a 526 nm filter; sulfides are selected with a 394 nm or 384 nm filter. The photomultiplier tube converts the filtered light into an electrical signal, which is sent to a microcurrent amplifier for amplification and then transmitted to a recording device (recorder, chromatography data processor or chromatography workstation, etc.) for data processing, image display, printing of spectra and printing of analysis results, etc.
2.4 Features and characteristics
The FPD is a dedicated differential detector with high sensitivity for compounds containing sulfur and phosphorus. the FPD has a minimum detection of 10-11 g and a linear range: up to 104 for organic phosphorus; and not linear for sulfides, with a linear range of 102 using double logarithmic plotting.
FPD must be hydrogen-rich flame, the ratio of oxygen to hydrogen flow rate in the range of 0.2 to 0.5 can obtain high sensitivity.
The practical flow rate and temperature of various gases have a large impact on the detection sensitivity. For example, when measuring phosphorus measurement flow rate: hydrogen 160 ~ 180 mL / min, air 150 ~ 200 mL / min, nitrogen 40 ~ 80 mL / min; measurement of sulfur flow rate: nitrogen flow rate of 90 ~ 100 mL / min when its sensitivity is higher; detection chamber is too high to reduce the sensitivity of detection when measuring sulfur. The practical flow rate is related to the instrument type, sample type and other operating conditions and analytical requirements, so their flow rates should be determined according to the specific circumstances.
Gas chromatography / flame photometric detector method is widely used in petrochemical, environmental protection, food hygiene, biochemistry and other analytical fields.
3. Gas chromatography photoionization (GC-PID) analysis method
3.1 Working principle.
PID (photoionization detector) consists of the main parts such as UV light source and ionization chamber. There are positive and negative electrodes in the ionization chamber, forming an electric field, and the UV lamp produces high-energy UV light. After the organic gas flows into the measurement cell (ionization chamber), it is ionized under the excitation of the UV light source. The ionized particle “fragments” have a positive and negative charge, which generates a current signal between the two electrodes. PID is a non-destructive detector, ions can be detected and recombined into the original gas molecules, PID detector response is fast, sensitive and does not require H2, air as an auxiliary, and more convenient to carry, so it is very suitable for on-site emergency monitoring, indoor monitoring and dangerous gas warning. However, the energy limitation of UV lamp makes the response of some kinds of VOCs gases (such as short-chain alkanes) slow or even undetectable.
Gas chromatography (GC) generally consists of a gas circuit system, injection system, separation system, temperature control system, and detection and recording system. The chromatographic column is the key to the separation of components; according to the size and length of the inner diameter of the column, it can be divided into packed column and capillary column. The inner diameter of filled column is 2~4mm and the length is about 1~10m, while the inner diameter of capillary column is 0.2~0.5mm and the length is generally 25~100m. Whether the components can be identified after separation lies in the detector, so the separation system and detection recording system are the core of GC instrument.
According to the different components of VOCs, the gas chromatography column can effectively separate the components of VOCs gas, and the separated gas molecules are detected by PID detector to realize the monitoring and analysis of multiple components. The electrical signal generated by the PID of the VOCs gas to be measured is output to the recording instrument, and a chromatogram with peak area proportional to the mass of the organic compound is obtained.
GC-PID technology, which adopts a high temperature of over 120℃ for sample acquisition and transmission, and strong corrosion resistance and inert materials, thus greatly reducing the adsorption phenomenon during the sample transmission, is selective and sensitive for the detection of VOCs, and is convenient to operate, and the measurement results are more stable and reliable, and can realize the simultaneous measurement of multiple components. It is widely used because it is easy to operate, stable and reliable, and can measure multiple components simultaneously.
3.2 Technical parameters
British PID-AH sensor technical parameters
Resolution: 1ppb.
Sensitivity: >20mv/ppb.
Measuring gas: ionization potential <9.6eV,<10.6eV,<11.7eV of VOCs
Monitoring range: PID-AH:1ppb～50ppb(isobutene), high sensitivity
Supply voltage: 3.2~3.6VDC
Output signal: Offset Voltage to Vmax (Vmax = Vsuppply-0.1v)
Power consumption: 27mA typical (inrush current 112mA<0.2s)
Response time: T90<3s (diffusion mode)
Temperature range: -40℃～55℃; Humidity range: 0～95%RH (no condensation) Life time: 5 years (without bulb and gate)
Response time: <3s in the case of free diffusion 2.
Technical parameters of portable monitoring instruments and equipment
Detectable: benzene, toluene, xylene, styrene, etc. (other gases can be extended)
Ionization: standard UV lamp (10.6 eV) Sensitivity: 0.5 ppb (in benzene)
Humidity: 0-90% RH
Data output: Integrated GUI, 4-20mA (analog), RS-232 (optional), RS-458 (optional), USB (optional), LAN/WLAN (optional)
Power: 100V-240V/50-60Hz
Li-ion battery life: more than 8 hours gas load: no gas load
Instrument size (L * W * H: mm): 280 * 100 * 280
Weight (kg) 7
Operating temperature (℃): -10~50
At the same time, industrial procurement network to provide a variety of ranges of PID sensors for customers to choose: PID-AH5, PID-AR5, PID-AY5, PID-A15, PID-AH, PID-A1, as follows.
