Principle and characteristics of flue gas analyzer
At present, the gas sensors of flue gas analyzer in the market are mostly electrochemical gas sensors. The working principle of the electrochemical sensor is that it reacts with the gas to be measured to generate an electrical signal proportional to the gas concentration: the gas to be measured enters the sensor room through a multi-stage flue gas filter and enters the electrolytic cell through a permeable membrane, so that the gas diffused and absorbed in the electrolyte is decomposed at a specified oxidation potential, and the gas concentration is calculated according to the consumed electrolytic current.
Electrochemical gas sensor has the advantages of stable performance, long service life, low power consumption, fast response to gas, wide working environment temperature, small volume, simple operation and convenient carrying, and currently occupies an important position in flue gas analyzer.
Application of flue gas analyzer in boiler thermal efficiency test
Generally, industrial boilers have low thermal efficiency, high pollution index, large quantity and wide range, and are recognized as large energy consumers. The parameter that affects the thermal efficiency of boiler is exhaust gas. Content, CO content, exhaust temperature, furnace surface temperature, excess air coefficient, carbon content of slag, coal leakage and fly ash, etc. Smoke analyzer can accurately and intuitively detect O? , CO and other gases, through detection, the problems existing in boiler operation can be found in time, and scientific data can be provided. Through equipment rectification and operation adjustment, the boiler operation quality can be effectively improved, fuel consumption can be saved, harmful gas emissions can be reduced, and the purpose of energy saving, consumption reduction and emission reduction can be achieved.
The factors affecting the accuracy of flue gas analyzer in boiler energy efficiency test are analyzed.
In order to ensure the accuracy and reliability of flue gas analyzer data, it is necessary to comprehensively evaluate the detection ability of instruments and equipment, such as strengthening the traceability management of standardized instruments, evaluating the measurement uncertainty, and operating instruments correctly.
Traceability of instruments and equipment is the premise to ensure the accuracy of test results.
Most electrochemical gas sensors use aqueous solution as electrolyte, and the evaporation or pollution of electrolyte will weaken the sensor signal. After long-term use, the sensitivity and repeatability of the sensor will decrease with time, and a large amount of smoke and water vapor will affect the accuracy of the smoke analyzer. The life of general electrochemical sensors is about 2 years. Therefore, according to the verification regulation of JJG968-2002 "Flue Gas Analyzer", the parameters such as indication error, repeatability, response time and stability of the flue gas analyzer are checked regularly.
The basis of ensuring accurate test results when operating the instrument correctly.
In the process of boiler energy efficiency test, it is necessary to avoid the error brought by the instrument itself and ensure the correct operability of the instrument. If we are extremely unfamiliar with the operation and principle of the instrument, such as putting the sampling probe in the flue to start the machine, then the zero position of the instrument will be improved. If the sampling tube and sampling hole are not tightly sealed, the oxygen content in the measured flue gas is high; If the data is read before the data is stable or the selected parameters do not conform to the actual working conditions, the read values will be inaccurate. Correct operation: the instrument should be turned on in clean air, and the self-inspection and zero calibration are completed. Select the correct parameters, insert the probe into the flue test hole with stable working conditions, record readings every 15 minutes after the instrument is stable, and test for five times continuously, and take the average value as the measurement result. Do not turn off the instrument immediately after measurement. Let the instrument run in clean air for (5- 10) minutes, and keep the inside of the instrument in a fresh air environment before turning off the instrument. Otherwise, the sensor will be "poisoned" by high-concentration toxic gas for a long time, which will accelerate the loss of the sensor.
Shorten the normal service life of the instrument. In addition, for instruments equipped with dust filters, the filter element should be replaced in time to avoid dust entering the sensor and polluting the sensor. For portable instruments, no matter whether the instrument is frequently used or not, it should be charged at least once every (2-3) weeks, and the battery charge should not be less than 30% when sampling. When the ambient temperature is too low, it will affect the ionic activity of the electrochemical sensor, so if the ambient temperature is too low, please preheat the instrument in a warm room for a period of time before using it.
Correct evaluation of measurement uncertainty is the key to obtain accurate measurement results.
The accuracy of the measurement results of flue gas analyzer is one of the key factors of boiler energy efficiency test, and the uncertainty is the quantitative characterization of the measurement results. The definition of measurement uncertainty is a parameter related to the measurement result, which indicates the dispersion of the measurement value. It is a part of the measurement result, a part of the magnitude range, and an estimation and inference of the existence range of the truth value, which falls within this range with a given probability. In the process of uncertainty evaluation, all the steps of the whole experiment should be discussed, the source of uncertainty should be found and calculated, and the main factors in the uncertainty component should be clearly obtained from the data.
Referring to JJF 1059- 1999 Evaluation and Expression of Measurement Uncertainty, taking German flue gas analyzer as an example, various factors affecting the uncertainty of flue gas analyzer are analyzed, and the evaluation methods of oxygen and carbon monoxide uncertainty are given.
Analyze the source of uncertainty
Usually, the factors that affect the measurement uncertainty should be considered: the uncertainty of standard instruments, the uncertainty of measurement methods, the uncertainty of measurement environmental conditions, the influence of personnel operation, the fluctuation of measured gas, etc. Among them, the uncertainty of measurement methods and environmental conditions can be ignored when the requirements of evaluation rules are met. The influence of personnel operation and the fluctuation of measured gas are reflected in the repeatability of detection. Therefore, the measurement uncertainty is mainly composed of repeated measurement uncertainty and uncertainty introduced by standard instruments.
Uncertainty evaluation
When the test result is at the specified critical value, the tested boiler can be judged to be unqualified, and the uncertainty of the test result should be given. Through uncertainty analysis, the test error is controlled within the allowable range to ensure the accuracy of the test results and the analysis data is within the given confidence level. When it is found that the quality control data will exceed the predetermined data, measures are taken to correct the problem in a planned way, and measures are taken to prevent erroneous results from being reported, which not only ensures the accuracy of the measurement results, but also reduces the risks borne by testers to some extent.
"China has more coal, less gas and less oil, and the resource utilization rate is low." As a recognized "coal tiger", coal-fired industrial boiler is one of the most energy-consuming equipment in China, and its operation efficiency is low. Therefore, in order to meet the new requirements of energy-saving reconstruction of industrial boilers or the elimination of some old coal-fired boilers with extremely low efficiency and serious environmental pollution, it is an important foundation for the flue gas analyzer to give accurate and intuitive scientific data, and ensuring the accuracy of the flue gas analyzer is the premise of this foundation. In addition, improving the operation and maintenance level of the tester is also a necessary condition to ensure the accuracy of the measurement results.