Absrtact: Monitoring particulate matter from pollution sources is an important work of environmental monitoring. Whether particulate matter sampling is accurate or not directly affects whether environmental monitoring can truly reflect the real situation of pollutants discharged by pollution sources. Therefore, this paper analyzes the reasons for the deviation of source particle monitoring data.

keywords: particulate matter; Monitoring data; Reasons for deviation

Chinese library classification number: X83 document identification code: A

Environmental monitoring quality control is a very important technical work and management work in environmental monitoring, a method to ensure the accuracy and reliability of monitoring data, and an effective measure to scientifically manage the monitoring system, which can make environmental monitoring based on reliability. According to the requirements of standard methods such as Ambient Air Quality Standard, Monitoring and Analysis Methods of Air and Waste Gas, Sampling Methods of Particulate Matter and Gaseous Pollutants from Stationary Pollution Sources, the manual monitoring of particulate matter still adopts the gravimetric method. That is, the particulate sampling tube is inserted into the flue, and a certain amount of dusty gas is extracted according to the principle of constant-speed sampling of particulate matter, and the concentration of particulate matter in the exhaust gas is calculated according to the amount of particulate matter captured on the filter cartridge and the amount of gas extracted at the same time.

1 Monitoring status and problems

1.1 The emission concentration of particulate matter is relatively low

Electrostatic precipitator is generally used for dust control in thermal power industry, and the dust removal efficiency is over 99%. The smoke emission standard is mostly 51mg/m3；; Bag filter is mostly used to treat particulate matter in cement industry, which has higher dust removal efficiency, and its particulate matter emission standard is mostly 31 ~ 51mg/m3. However, in actual monitoring, the emission concentration of particulate matter from most pollution sources is far below its implementation standard.

1.2 manual monitoring error is large

In the process of manual monitoring of particulate matter emission concentration, many factors will increase the error of manual monitoring. For example, the pretreatment method of filter cartridge, the weighing environment of balance room, sampling site, sampling parameters, production and operation conditions, sampling instrument error, personnel error and so on. The large error of manual monitoring has more and more obvious influence on the monitoring results of low concentration particles, which has become a problem that cannot be ignored.

1.3 The quality of the filter cartridge is uneven

When sampling for a long time in the discharge pipeline with high flow rate or high temperature and low concentration of particulate matter, there are often situations such as low collection efficiency and weight loss of the filter cartridge; However, when sampling in the pipeline with high humidity, the filter cartridge is easy to be broken and stuck, and the direct consequence is that the monitoring result is negative and cannot be used, let alone the representativeness of the sample.

The above situation has become a difficult problem in particulate matter monitoring at present, but this problem can't be solved for the time being. We can only further explore the causes of data deviation within the scope of methods, seek solutions, and improve the effectiveness of monitoring data as much as possible.

2 Analysis of the reasons for the deviation of test data

2.1 Glass fiber filter cartridge

As an important medium for sample collection, the quality of filter cartridge is very important. Generally speaking, it is required that the material of the filter cartridge should be uniform, with no slag falling and no peeling when tapped lightly, with no weight loss at high temperature and no breakage at high humidity. At present, there are many brands of glass fiber filter cartridges on the market, and the quality is mixed. It is difficult to find the filter cartridges that meet the basic requirements. It is difficult to avoid the weightlessness of the filter cartridge, which leads to the lack of parallelism of the data, and even the samples are all unreasonably negative, so it is impossible to talk about the representativeness of the samples.

1) weight loss of the filter cartridge at high temperature

According to the requirements of conventional methods, the filter cartridge should be baked at 115℃ ~ 111℃ for 1h before sampling, then cooled to room temperature in a dryer, and weighed to a constant weight with a balance with a sensitivity of 1.1mg. In practical application, the flue gas temperature is often higher than 115℃, and the filter cartridge will lose weight when used for more than 31min at this high temperature and high flue gas velocity. By comparing the weights of two brands of glass fiber filter cartridges after drying at 115℃ and 161℃ for 1h, it is found that the average weight loss of filter cartridges is 2.8mg and 3.8mg only in the laboratory.

Therefore, when the smoke temperature is higher than 115℃, it is not only necessary to bake the filter cartridge at 115℃, but at least raise the temperature to 161℃.

2) Loading and unloading error of filter cartridge

According to the current sampling method, at least 3 samples are collected at each monitoring point. This requires the sampling personnel to replace the filter cartridge at the monitoring site. However, most of the monitoring sites are located outdoors, and the process of installation, removal and sealing of filter cartridges will be disturbed by external factors such as wind and sand. The filter cartridges sampled under high-velocity and high-humidity flue gas are more likely to fall off during loading and unloading, which leads to the weightlessness of the filter cartridges.

The errors produced in the process of sampling, loading and unloading of filter cartridges are illustrated by experiments. Three samples were collected under the simulated dynamic pressure of 121Pa in ambient air, and each sample was collected for 31min. After weighing and comparison, the average weight loss of the filter cartridge reached 2.1mg. If the weight loss factor of 4.1.1 at high temperature is taken into account, the weight loss of the filter cartridge will reach 4.9 ~ 5.9 mg. Based on the average sampling volume of 447L in the above test, the final impact on the monitoring results is 11.1 ~ 13.2 mg/m3. This is absolutely not negligible compared with the emission standard of 31 ~ 51mg/m3.

2.2 On-site sampling

(1) Selection of sampling media

To collect low-concentration particulate matter samples, it is necessary to know the characteristics of sampling site in advance and choose the appropriate media according to different conditions. By comparing experiments and considering the factors such as large surface area and easy chipping of filter cartridge, it is feasible to use glass fiber filter membrane instead of filter cartridge under special conditions such as high temperature, high flow rate and high humidity. This method is still based on the gravimetric method. Compared with the filter cartridge, its advantage lies in that the filter membrane and the sampling head are integrated, and the sampling medium is not changed during the sampling process, thus avoiding human operation errors. Under the condition of stable production conditions, the parallelism of monitoring data can be ensured by using filter membrane sampling.

The following is the monitoring and comparison of two representative points, namely, the dust collector outlet and the desulfurization tower outlet of a power plant in Liaoning Province, which were sampled with filter cartridges and filter membranes respectively.

table 1 shows the monitoring data of the dust collector outlet. At this point, the flue gas temperature is high, 1.47℃, the humidity is low, 6.7%, and the smoke concentration is low. The monitoring results of filter membrane and filter cartridge are 22 ~ 34 mg/m3 and 6 ~ 46 mg/m3 respectively. As far as the sample is concerned, the standard deviation of the sample collected by filter membrane is 4.9 mg/m3; The standard deviation of the samples collected by the filter cartridge is 16.5mg/m3.

table 2 is the monitoring data of the desulfurization tower outlet. The flue gas temperature at this point is low, 56℃; High humidity, 13.5%; The smoke concentration is low. The monitoring results of using filter membrane and filter cartridge are 5 ~ 8 mg/m3 and 13 ~ 52 mg/m3 respectively. Similarly, as far as the whole sample is concerned, the standard deviation of the sample collected by filter membrane is 1.4 mg/m3; The standard deviation of the samples collected by the filter cartridge is 15.9mg/m3. The sample results collected by the filter cartridge at the outlet of the desulfurization tower are obviously too large, which does not rule out the possibility that the salt substances in the desulfurization wet flue gas are enriched in the filter cartridge with a large surface area, which is difficult to remove in the laboratory drying process, thus leading to an increase in smoke concentration.

(2) Guarantee of sampling conditions

Isokinetic sampling is one of the key conditions for obtaining representative samples. The so-called isokinetic sampling means that the velocity Vn of gas entering the sampling nozzle should be equal to the velocity Vs of flue gas at the sampling point, and its relative error should be within 11%. When the sampling speed Vn is greater than the flue gas velocity Vs at the sampling point, the sample concentration is lower than the actual concentration; When Vn is less than Vs, the sample concentration is higher than the actual concentration; Only when Vn is equal to Vs can the sample concentration be equal to the actual concentration.

In the field monitoring process, it is occasionally found that the dynamic pressure of the monitoring instrument is unstable, the sampling flow is too large, the load is too high, the sampling gun is offset, and the constant speed sampling is not really achieved. The above situation will affect the measurement of temperature, pressure and flow rate, thus affecting the calculation of sampling volume and smoke volume, and eventually leading to the deviation of smoke emission concentration and emission data.

Therefore, before sampling, it is necessary to predict the flow velocity for many times, master the situation of each sampling site in the flue, select the sampling nozzle and sampling method with appropriate caliber, and use the bracket to ensure the stability of the direction and angle of the pitot tube to ensure constant speed sampling.

2.3 Quality assurance

(1) Sampling instruments must be certified by the metrological department;

(2) periodically check the monitoring instruments;

(3) calibrate the flowmeter and pressure sensor regularly;

(4) Ensure that the Kp value inside the host is the same as that of the pitot tube used;

(5) Ensure the constant temperature and humidity of the balance room;

(6) Select representative monitoring points for sampling monitoring to avoid eddy current area;

(7) Before sampling, the air tightness of the sampling system should be tested to ensure that the sampling system does not leak;

(8) Before sampling, put the sampling gun in the flue with its back to the airflow, and start sampling when the temperature rises to the flue temperature;

(9) When the sampling is stopped, take out the sampling tube before the anti-backflow pump is turned off, so as to avoid the negative pressure of flue gas from withdrawing the collected samples;

(11) Replace silica gel in time to ensure that dry flue gas enters the instrument and prevent overload.

Conclusion

To sum up, in particulate matter detection, we should pay attention to the selection of filter cartridges, control the quality, and prevent the influence of inferior filter cartridges on monitoring data. Pay attention to the pretreatment of the filter cartridge, get to know sampling site before sampling, and choose the appropriate pretreatment method according to the situation to ensure that the filter cartridge is dried at a high enough temperature, and the weight must be constant when weighing. Use glass fiber filter membrane with better effect when collecting low-concentration particulate matter samples, and use filter cartridge when collecting high-concentration samples, so as to ensure that the monitoring process strictly abides by the monitoring specifications and do a good job in quality assurance.

reference

[1] Lu Xing. Study on monitoring and harm of particulate matter emitted from coal-fired power plants to air pollution [D]. Zhengzhou University, 2115.

[2] Liu Xiaoyu. Study on emission characteristics of particulate matter from typical stationary combustion sources [D]. china environmental science Research Institute, 2117.