Article guide
Flue gas emissions are an important part of environmental monitoring, which is directly related to air quality and ecological health. In order to meet the requirements of environmental protection, the flue gas CEMS online monitoring system is widely used in various industrial production processes to monitor and monitor flue gas emissions in real time. However, in actual operation, the flue gas CEMS online monitoring system may encounter various problems. This article will summarize and analyze these common problems, and propose corresponding specification requirements and verification methods to ensure the normal operation of the system and the accuracy of the data.
Sampling and preprocessing unit
1.1 Sampling points
Frequently Asked Questions 1:
Flow velocity and particulate matter sampling points are located at flue elbows, valves, reducers, curves, or front and rear straight pipe sections.
Effect:
The flow field at these locations is unstable, with irregular and violent fluctuations in flow velocity and particulate matter concentration.
Specification requirements:
1. Priority should be given to the vertical pipe section and the negative pressure area of the flue.
2. Not less than 4 times the diameter of the flue from the downstream direction of the elbow, valve and reducer, and not less than twice the diameter of the flue from the upstream direction of the above components (HJ/T75-2007).
Verification Method:
On-site observation.
Note: The sampling point has little influence on gaseous pollutants, but it should also meet the requirements of HJ/T75-2007 specification "not less than twice the flue diameter from the downstream direction of the elbow, valve and reducer, and not less than 0.5 times the flue diameter from the upstream direction of the above components".
Frequently Asked Questions 2:
The sampling point is set in the net flue, but the flue gas flow and flue gas temperature monitoring device is not installed in the bypass flue.
Effect:
The bypass opening cannot be effectively monitored.
Specification requirements:
1. When the flue gas purification equipment of the fixed pollution source is provided with a bypass flue, a continuous flue gas flow measurement device (HJ/T75-2007) should be installed in the bypass flue.
2. Flue gas temperature and flow sampling devices should be installed in the bypass flue (Huanban [2009] No. 8).
Verification Method:
1. Observe whether the bypass flue is equipped with flow and flue gas temperature measurement devices on site.
2. Turn on the bypass, observe the changes in flow rate and flue gas temperature on DCS and CEMS, the net flue flow rate should decrease, the bypass flow rate should rise, and the bypass flue gas temperature should be close to the original flue gas temperature.
Note: Currently, many coal-fired power plants do not have a bypass or have eliminated the bypass, and this problem does not exist. However, there is still a bypass for sintering machine desulfurization, which needs to be paid attention to.
FAQ 3:
The reference method sampling hole is located upstream of the CEMS sampling well, or far away from the CEMS sampling hole.
Effect:
The results of the assays are not comparable.
Specification requirements:
The number of sampling holes and sampling platforms shall be determined according to the requirements of the "Determination of Particulate Matter in the Exhaust of Fixed Pollution Sources and Sampling Methods for Gaseous Pollutants" in the downstream of the flue gas CEMS monitoring section for the reference method test. Under the premise of not affecting each other's measurements, they should be as close as possible (HJ/T75—2007).
Verification Method:
On-site observation.
Note: The distance between the reference method sampling hole and the CEMS sampling hole is generally controlled within 1 meter.
FAQ 4:
The particulate matter sampling hole is located upstream of the gaseous pollutant sampling hole.
Effect:
Particulate matter monitoring requires continuous purging, and the purge air will dilute the gaseous pollutants, and the monitoring results are low.
Verification Method:
On-site observation.
Note: The correct arrangement sequence of the sampling holes is as follows: along the flue gas flow direction, the gaseous pollutants, temperature and pressure flow rate, and particulate matter sampling holes are arranged in turn. The distance between each other is not less than 0.5 meters.
1.2 Sampling pipeline
Frequently Asked Questions 1:
1. The sampling pipeline is not fully heat-traced.
2. The heating temperature of the sampling probe or the heating temperature of the sampling pipeline is insufficient.
Effect:
As a result, the temperature of the flue gas in the sampling tube is lower than the dew point, the water vapor condenses, and the sulfur dioxide is dissolved in the water, which increases the measurement error and makes the measurement result low
Verification Method:
1. Observe the sampling pipeline and whether the whole process is heat-traced.
2. Touch the sampling line with your hand to feel if there is an abnormally low temperature.
3. Check whether the constant power heat trace pipe at both ends of the sampling tube is reserved for 1 meter heat trace.
4. Check the heating temperature of the probe (the temperature display instrument is next to the sampling probe or in the analyzer cabinet), and the general heating temperature is not lower than 160°C.
5. Check the heating temperature of the heat trace pipe (the temperature display instrument is in the analyzer cabinet), and the general heating temperature is not lower than 120°C.
Remark:
1. Only complete extraction methods (including hot and wet methods and cold dry methods) instruments use heat trace pipes. The dilution extraction method does not require heat tracing, but the probe needs to be heated.
2. The heating temperature of the sampling probe and the heating temperature of the heat trace tube shall be determined according to the dew point temperature of the flue gas, and it must be guaranteed that the flue gas can be heated above the dew point temperature. For flue gases with high dew point temperatures such as garbage incineration tail gas, the heating temperature of the sampling probe and the temperature of the heat trace pipe should be set at a higher temperature, generally not less than 180 °C.
3. According to the actual test of a certain type of heat tracing pipe, when the length of the bare pipe section is 30 cm, the flue gas temperature can be reduced by about 70 °C, and when the length of the bare pipe section is 60 cm, it can reach about 90 °C. In other words, when the length of the exposed pipe section exceeds 60 cm, the flue gas temperature has been reduced to close to room temperature. In this process, a large amount of condensate is generated, which absorbs the sulfur dioxide in the flue gas, making the measurement results low. When the sulfur dioxide concentration is low, the influence on the measurement results is greater (for example, when the concentration of ordinary wet desulfurization flue gas is less than 50 ppm, the sulfur dioxide loss rate can reach 10% or even higher). Therefore, during the installation process, the length of the exposed section of the sampling tube should be kept as short as possible.
Frequently Asked Questions 2:
The sampling tube forms a U-shaped pipe section.
Effect:
The condensate is easy to accumulate in the U-shaped pipe section, which increases the measurement error and makes the measurement results of gaseous pollutants low.
Verification Method:
On-site observation.
1.3 Pretreatment
Frequently Asked Questions 1:
Particulate matter meter lenses, gaseous pollutant sampling probes, and pitot tube probes are not properly flushed.
Effect:
Abnormal backflushing will lead to contamination of the lens of the particulate matter tester and make the concentration too large, and the gaseous pollutant sampling probe and pitot tube probe will be blocked, the data will be abnormal, and the equipment will not be able to operate in severe cases.
Verification Method:
1. Observe whether the blades of the reverse blower of the particulate matter measuring instrument on the platform rotate, listen to whether the fan has the sound of running, feel whether the fan vibrates with your hands, and judge whether the fan is operating normally.
2. Observe whether the gaseous pollutant probe and the pitot tube probe are normally connected to the blowback pipe on the platform, and whether the blowback valve on the platform is open.
3. Observe the back-blowing air source pressure gauge in the monitoring station room or on the platform, and the pressure is generally 0.4~0.7MPa.
Remark:
1. There are 3 parts that need to be backflushed: particulate matter meter lens, gaseous pollutant sampling probe, and pitot tube probe.
2. The lens of the particle matter measuring instrument adopts continuous backblowing.
3. The gaseous pollutant sampling probe and pitot tube probe are pulsed backblowing, the backflushing cycle is generally 4~8 hours, and the backblowing time is 2~5 minutes each time.
4. When the gaseous pollutant probe is backflushed, the concentration of sulfur dioxide and nitrogen oxides decreases, and the oxygen content increases.
5. When the pitot tube is full pressure backblowing, the pressure is displayed as full scale. When the static pressure is blown, the pressure is shown as zero.
6. At present, the data during the backflushing is generally shielded. In the case of shielding, when querying minute data in the historical data of CEMS and DCS, it can be observed that the concentration and flow rate maintain a fixed value (such as the average value of the previous 5 minutes) during the backflush period. If unshielded, periodic troughs of sulfur dioxide and nitrogen oxide concentrations and flow velocity (hydrostatic backflushing) and periodic peaks of oxygen content and flow velocity (full-pressure backflushing) can be observed.
7. The backflush air source is generally provided by the air compressor in the monitoring station room, and the compressed air is transported to the platform through the pipeline and divided into 3 channels, which are respectively supplied to the lens of the particulate matter measuring instrument, the gaseous pollutant sampling probe, and the pitot tube probe for backblowing.
Some enterprises have their own air source and do not need to be equipped with air compressors. Some particulate matter measuring instrument lenses are purged directly by the platform fan. The pressure of the back-blowing air source is 0.4~0.7MPa.
Frequently Asked Questions 2:
The filter element in the gaseous pollutant sampling probe and the filter element in the pretreatment cabinet have not been replaced for a long time, resulting in the failure of the filter element.
Effect:
The filter element is clogged, resulting in a decrease in the sampling flow rate and the equipment is unable to operate in severe cases.
Specification requirements:
Generally, the sampling probe filter element should be replaced no more than once every 3 months (HJ/T76-2007).
Verification Method:
1. Check whether the surface of the filter element of the gaseous pollutant sampling probe is too dusty.
2. Check whether the cabinet filter element is deformed or discolored, and whether there is a large amount of dust on the surface.
Note: Before the measured gas enters the analysis instrument, it needs to be filtered to remove dust and water vapor, in order: ceramic or stainless steel filter in the gaseous pollutant sampling probe, and 1~2 filters in the pretreatment cabinet. Under normal circumstances, the sampling flow rate of the analyzer is generally 1~2L/min.
FAQ 3:
1. The condensation temperature of the condenser is too high or too low.
2. The condensing temperature is unstable.
Effect:
1. The condensation temperature is too high, resulting in the moisture in the flue gas cannot be filled and analyzed, and the analysis instrument is damaged.
2. The condensation temperature is too low, especially when it is lower than 0°C, which may cause the condenser pipe drain to freeze and cannot drain normally.
Verification Method:
1. Check the display temperature on the condenser, generally the condensing temperature should be 3~5°C.
2. Observe the air pump, if the dehumidification is not good, the air pump is easy to corrode.
Note: The measurement of gaseous pollutants by complete extraction method generally includes two types: cold drying method and hot and wet method, and the main application in China is cold drying instrument. Only cold-drying instruments need to use a condenser, the purpose is to make the moisture in the flue gas quickly condense and condense and precipitate. Thermo-wet and dilution instruments do not require a condenser.
FAQ 4:
1. The condenser drain peristaltic pump tube is aging.
2. Peristaltic pump is damaged.
3. Peristaltic pump leakage.
Effect:
The condensate cannot be discharged normally, and in severe cases, the condenser cannot work normally.
Specification requirements:
At least once every 3 months, check the ash formation and condensate of the filter, sampling probe and pipeline of the gaseous pollutant CEMS, and the aging state of the gas cooling parts, converters and pump membranes (HJ/T75-2007).
Verification Method:
1. Check whether the peristaltic pump motor rotates in the marked direction, and observe whether the peristaltic pump pipe has a water column discharged smoothly.
2. Check the operation and maintenance records and check whether the peristaltic pump tube is replaced regularly (usually at least once every 3 months).
3. Disassemble the peristaltic pump tubing and observe whether it has cracks and whether it can be restored to its original state. If it cannot be restored to its original state after disassembly, or there are cracks on the surface of the pump tubing, it needs to be replaced.
Analytical unit
At present, the national standard only stipulates the technical indicators for judging whether the CEMS is qualified during the commissioning and testing, and the technical indicators for determining whether the data is out of control during regular calibration, regular calibration and irregular comparison and monitoring, but does not clarify the methods and technical indicators for determining whether the CEMS system data is accurate in the daily inspection. In the daily inspection, due to the limitation of time and equipment, the reference method is generally not used to compare and monitor the gaseous pollutants, but refers to Article 5.8.2 of HJ/T76-2007 "The main technical indicators of gaseous pollutants CEMS (including O2 or CO2)" as the judgment standard, that is, the relative error does not exceed ±5%, the response time is not more than 200 seconds, and the zero drift and range drift do not exceed ±2.5% of the full scale. The determination of the accuracy of particulate matter and flow rate must be based on a reference method, which is generally not available for comparison and monitoring during daily inspections. Therefore, the focus of the inspection should be on the actual condition of the equipment. For particulate matter, it is important to check whether the optical path is collimated, whether the optical mirror surface is clean, and whether the installation position is vibrating violently, and whether the installation position is reasonable and whether the probe is blocked for flow rate/flow.
When using the reference method for the determination of sulfur dioxide, it is necessary to pay attention to the interference of carbon monoxide on the measuring instrument. The results show that carbon monoxide has a large degree of positive interference with the instrument for the determination of sulfur dioxide by electrochemical principle, and has basically no effect on the CEMS system. Compared with a certain type of domestic electrochemical instrument and a foreign type of optical instrument, 4000ppm carbon monoxide in flue gas will produce 606mg/m3 positive interference of electrochemical sulfur dioxide, and 8000ppm carbon monoxide will produce 1170mg/m3 positive interference of electrochemical sulfur dioxide. The carbon monoxide concentration in the flue gas of steel plants and coking plants is more than 5000ppm, and the carbon monoxide content of waste incineration exhaust gas is about 3000ppm.
Frequently Asked Questions 1:
The instrument is not calibrated or calibrated in a timely manner.
Effect:
The measurement error increases, reduces the accuracy of the instrument, and the accuracy of the instrument cannot meet the standard requirements in serious cases.
Specification requirements:
For existing instruments, it should generally meet:
1. Zero point calibration: gaseous pollutants (sulfur dioxide, nitrogen oxides and oxygen) once every 24 hours, particulate matter and flow rate once every 3 months.
2. Span calibration: gaseous pollutants (sulfur dioxide, nitrogen oxides and oxygen) once every 15 days, particulate matter and flow rate once every 3 months.
3. System-wide calibration: The extractive gaseous pollutant CEMS conducts a system-wide calibration at least once every 3 months, requiring zero gas and standard gas to be consistent with the path through which the sample gas passes (such as sampling probes, filters, scrubbers, regulators) to detect zero point and span, linearity error and response time.
4. Regular calibration: once every 6 months (HJ/T75-2007).
Verification Method:
1. For gaseous pollutants, the zero drift and span drift measured on site should not exceed ±2.5% F.S.
2. If the zero drift and span drift meet the requirements, the whole system inspection will be carried out with a standard gas close to the measured gas concentration, and the error shall not exceed ±5%.
3. View the historical data during calibration and calibration in CEMS or DCS, and if unmasked, you should be able to find the corresponding concentration values. If it is blocked, it should be kept at a fixed value.
Note: Span drift is range drift.
Frequently Asked Questions 2:
The range is set too high or too low.
Effect:
1. If the measuring range is set too high, the actual concentration of the measured flue gas is much lower than the measured range (such as less than 20%), which may lead to excessive measurement error and affect the accuracy of the data.
2. When the range is set too low, and the actual concentration of flue gas exceeds the upper limit of the range, the measurement data is invalid, and the emission situation cannot be effectively monitored.
Verification Method:
1. Check the historical data of the instrument to observe the actual emission concentration range of pollutants.
2. Generally, the actual emission concentration should be in the range of 20~80% of the range.
3. If the actual emission concentration is less than 20% of the range, a standard gas close to the actual emission concentration is introduced for measurement, and the relative error should not exceed ±5%.
4. Observe whether the data that exceeds the range of the instrument often occurs in the historical data.
FAQ 3:
The measurement data were modified by modifying the slope and intercept of the standard curve of the measuring instrument, setting the calibration coefficient incorrectly, and setting the upper and lower limits of the data.
Effect:
Artificial fraud, the data is not true.
Verification Method:
The whole system inspection was carried out with low, medium and high concentration standard gases, and the error did not exceed ±5%.
FAQ 4:
The actual concentration of the standard gas is inconsistent with the standard gas concentration set by the instrument.
Effect:
1. If the actual concentration of the standard gas is lower than the set concentration of the instrument, the actual measured concentration will be increased by nearly equal proportions.
2. If the actual concentration of standard gas is higher than the set concentration of the instrument, the actual measured concentration will be reduced by nearly equal proportions. If the standard gas concentration set by the instrument is 1000ppm, but the actual concentration of the standard gas is 2000ppm and the actual concentration is 500ppm, the measurement result will be displayed as 250ppm.
Verification Method:
1. Use your own standard gas for measurement, and the relative error should not exceed ±5%.
2. Use a rapid tester or bring the on-site standard gas back to the laboratory for measurement, and its concentration should be consistent with the standard gas concentration set by the instrument.
Source: Environmental Knowledge