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What problems should be noticed in the boiler flue gas and dust online monitoring system?

             阅读:5554 更新时间:2019-11-20 16:04 Source: New Jersey Instrument Co., Ltd. Shandong reading: 5554 Updated: 2019-11-20 16:04

What problems should be noticed in the boiler flue gas and dust online monitoring system ?

During the combustion process of the boiler, a large amount of flue gas and dust is generated, among which there are atmospheric pollutants such as SO2 and NOx. These pollutants are discharged into the atmosphere and will form acid rain, which seriously disrupts the ecological balance and brings great inconvenience to people's lives. Therefore, reducing the emissions of boiler flue gas and dust and reducing the concentration of exhaust gas are important components of China's environmental protection cause. Monitoring boiler smoke and dust is an effective measure to reduce smoke and dust emissions.
1. Indicators and processes for boiler flue gas and dust monitoring
1.1 Monitoring indicators
Monitoring indicators of flue gas and dust include flue gas pressure, smoke velocity and concentration, excess air coefficient of flue gas, and the effect of air leakage on thermal effects.
Flue gas pressure is divided into flue static pressure and dynamic pressure. During monitoring, there are suction flue and press-in flue according to the position of the monitoring hole. The static and dynamic flue pressure in these two cases are generally equal. Is negative.
The monitoring of soot concentration generally adopts the gravimetric method and constant velocity sampling. The soot flow velocity on the sampling section should not be less than 5m / s. The distribution of flue gas and soot in the flue is not theoretically evenly distributed in practice, but will generate eddy current phenomenon with the change of flue diameter, affecting the analysis of monitoring data. In general, the location of the monitoring hole is an important determinant of the accuracy of the monitoring results.
The ratio of the actual air volume to the theoretical air volume during the fuel combustion process in the boiler is called the excess air coefficient. The excess air coefficient of flue gas is used to measure whether the amount of air used for the combustion of the flue gas is suitable, and then to determine the combustion of the fuel. An appropriate excess air coefficient can ensure the complete combustion of the fuel and minimize various heat losses. The national standard stipulates that the excess air coefficient of coal-fired boilers is 1.8, and that of oil-fired gas boilers is 1.2. The excess air volume is approximately large, indicating that the actual air supply is greater than the theoretical air volume required for fuel combustion, and the more O2 is in the furnace, the more fuel is burned.
However, if the excess air coefficient is too large, because a large amount of cold air enters the furnace, the temperature of the furnace will decrease, which will be detrimental to combustion; the exhaust loss will also increase, which will reduce the thermal efficiency of the boiler; the amount of flue gas will increase, and the amount of smoke and dust carried by the flue gas will also increase. With it increased. Therefore, in actual operation, it is always desirable that the excess air coefficient at the smoke exhaust location is below 1.8. Of course, due to the poor condition of the equipment and the low operating level, the actual excess air coefficient is often greater than 1.8, and some are surprisingly large.
This is because the load is low, the burning surface of the grate is small, and a large amount of cold air flows into the hearth from the grate; the draft is not matched; and it is caused by a large amount of air leakage from the tail flue of the boiler or the dust collector itself. This is abnormal and should be eliminated before testing. The appropriate excess air coefficient indicates that the fuel is burned more completely and less harmful substances are generated in the exhaust gas; the lower coefficient indicates that the fuel combustion is insufficient, and the content of harmful gases such as CO and NOx in the exhaust gas is high.
1.2 Key issues in the monitoring process of smoke and dust
1.2.1 Preparation before sampling
The filter cartridge is a necessary tool for monitoring. It is placed in the flue for dust collection and needs to be carefully prepared before sampling. The requirements of the filter cartridge are that the wall of the cartridge should be uniform, and it should be discarded in time if it is too thick, too thin or has needle-like holes in the wall. Due to the thick wall of the cylinder, the resistance during the sampling process is large, and the smoke and dust particles are not easily inhaled; the cylinder wall is thin and the strength is low, and the cylinder wall is easily broken when the smoke and dust pass through. Glass fiber filter cartridges are commonly used, and at the same time, a blank filter cartridge must always be involved in the monitoring process for reference and correction of errors.
Before sampling, a comprehensive and careful inspection of the test equipment, including sampling pumps, sensors and displays, is required. Inspect the piping system of the instrument for leaks and blockages. If there are problems, they should be resolved in time to make the monitoring work smoothly.
1.2.2 Selection of monitoring points
The sampling position should be selected in the vertical and horizontal pipe sections where the air flow is stable in the pipeline, avoiding the pipe sections and elbows where the shape of the section changes sharply, and the air flow cutting the sampling section should not be less than 5m / s. The determination of the number of sampling points is related to the shape and size of the sampling section and the size of the airflow at the section. The number of measurement points per section should be arranged as much as possible according to the specifications to ensure the accuracy of the measurement data. For example, away from the elbow and the section where the cross section is uniform, the airflow is relatively uniform, so you can take fewer monitoring points. You must only choose as many monitoring points as possible when you are selecting the monitoring points from the elbow or the nearby section where the section changes sharply. To ensure more accurate data.
1.2.3 Temperature and humidity control
The temperature and humidity of the flue gas and dust are also the important data to be obtained during the monitoring process. The temperature is usually measured by a sampling gun at the monitoring point, and the data is transmitted to the instrument for analysis. Humidity measurement is commonly used in the dry-wet bulb method. The gas passes through the dry-wet bulb thermometer at a certain flow rate to obtain the temperature of the gas. Based on the exhaust pressure at the monitoring point, the moisture content in the flue gas can be calculated, that is, humidity.
1.2.4 Flow rate control and sampling
In order to obtain more accurate data, the isokinetic sampling method should be adopted. The so-called constant velocity sampling means that the velocity of the gas entering the sampling nozzle is equal to the flow velocity of the smoke at the sampling point. The method of constant velocity sampling is to use different sampling nozzles corresponding to the different flow velocity of different parts of the flue, so that the flue gas with different velocity flows into the sampling nozzle and out of the flue gas with constant velocity.
Start monitoring when everything is ready. First, zero the monitoring instrument, put a weighed filter cartridge into the sampling tube, install the calculated sampling nozzle, and connect the pipeline. At this time, the sampling hole is closed. When the airflow stabilizes and gradually enters the sampling state, at this time, the sampling hole is opened, and the sampling nozzle is aligned with the airflow direction in turn to complete the sampling. After the sampling is completed, the sampling nozzle should be opposite to the direction of the airflow, take out the sampling tube, weigh the mass of the soot particles sucked by the filter cartridge, and measure again. If the difference between the flue gas flow rate and the previous weighing result is greater than 20%, then The sample is invalid. After all the work is done, check whether the instrument is intact and clean.
If the phenomenon of excessive oxygen content is found during the sampling process, it may be caused by a leak in the boiler system or a large induced air volume. Boiler equipment should be inspected in a timely manner. For air leakage conditions, sealing measures should be taken for the flue system. In the case of large air intake, the system matching the boiler should be replaced in time.
1.2.5 Data recording and processing
During the preparation of the monitoring work, the model, manufacturer, production capacity of the boiler equipment, the temperature, humidity, air pressure and wind speed of the environment, the diameter and height of the flue gas, etc. should be carefully recorded. The data of the points are filled into the corresponding form after checking. The form should be filled in strict compliance with national standards to ensure the integrity, accuracy and fairness of the monitoring data.
2.Factors that may affect the monitoring of smoke and dust
The main influencing factors are the monitoring environment, air volume and the use and maintenance of equipment.
The monitoring of the flue gas and dust must be carried out under the environmental conditions specified in the specification. In many cases, the monitoring site, the blower and the valve are not positioned correctly, and the monitoring data will be distorted. The temperature and humidity of the environment will also affect the flow rate of smoke and dust, making the actual monitoring data deviate from the real data. The amount of air volume affects the static pressure of the airflow in the flue and the dynamic pressure of the flow velocity, and affects the choice of sampling nozzle for the calculation result of constant velocity sampling.
With the same equipment and the same environmental conditions, the monitoring data obtained by different staff members is different. The service life of the equipment, the size of the bearing capacity and the maintenance status have a great impact on the monitoring data. For example, when the boiler load is 60%, the soot emission concentration is 30% of the rated load; when the boiler load is 80%, the soot emission concentration rises to 65%.
3.Some suggestions for the monitoring work of boiler flue gas and dust
a) The fuel of the boiler should be selected from the national standard coal type. The boiler and other combustion equipment should be selected in accordance with the national environmental assessment requirements. Be sure to make preparations before monitoring begins, and carefully check whether the performance of the dust removal equipment is good. Whether the flue and trachea have good airtightness and whether the various monitoring tools are in the best working condition; b) the monitoring department should focus on improving the environment, such as the control of the ambient temperature, humidity and air pressure during the monitoring process; c The monitoring department shall cooperate with the production department to strengthen the professional quality of staff and the cultivation of professional ethics. The boiler staff has a high level of professionalism. They master the rationality of the fuel use and the amount of induced air. The excess air coefficient is close to the national standard. The combustion of the fuel is sufficient. The concentration of the flue gas and dust is close to the national standard. The more accurate the data obtained during the monitoring process, the lower the monitoring workload.
4.Conclusion
Boiler flue gas and dust emissions and their concentrations are key factors related to air quality, and their monitoring is also the most basic monitoring content of the Chinese environmental protection department. The monitoring of boiler flue gas and dust is not easy to implement. The reason is related to the diversification of boiler models, the complexity of on-site environmental conditions, and the comprehensive quality of staff. However, no matter how complicated and changeable the situation is, there is a unified national standard for monitoring, namely GB13271-2011 boiler smoke monitoring method. The staff should strictly abide by the national standards and do every detail work carefully to ensure the completeness, accuracy and fairness of the monitoring results.

Shandong Xinze Instrument Co., Ltd. is mainly committed to the research and development, production, sales and agency of environmental monitoring instruments and industrial process gas analysis systems. Xinze people have been working hard to achieve it. When we choose us, we choose high-quality products and services. We do Not only you can see, we hope that the gas detectors, on-line flue gas monitoring systems, ammonia escape analyzers, and coal mill CO analysis systems we provide will give you a different experience. Welcome to inquire and discuss exchanges. lm

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