นานาคำถามเกี่ยวกับการตรวจวัดการปลดปล่อย (emission testing)

นานาคำถามเกี่ยวกับการตรวจวัดการปลดปล่อย (FAQ Emission testing)


มีคำถามมากมายเกี่ยวกับการตรวจวัดการปลดปล่อย (Emission Test) ที่หลายคนยังสงสัย  หรือยังเข้าใจคลาดเคลื่อนอยู่  วันนี้ทาซาเทคนำความรู้เหล่านี้มาตอบข้อสงสัยของหลายๆท่าน ไม่ว่าจะเป็น 
– ทำไมอุณหภูมิถึงมีผลในการวัด NOx
– ทำไมเราถึงใช้สูตรคำนวนเพื่อชดเชยการผันแปรของแก๊สอื่นๆที่มารบกวนการวัดไม่ได้
– หรือ ในเมื่อแก๊สพิษที่เกิดจากการเผาไหม้นั้นมีไม่เกิน 5000 ppm อยู่แล้ว ทำไมเราถึงใช้เซนเซอร์ตัวเดียววัดไม่ได้  
– ทำไมการวัด CO ถึงไม่ควรวัดนาน
– มาทำความรู้จักกับข้อจำกัดของ SO2 เซนเซอร์
– ทำไมการปรับสภาวะของแก๊สจึงมีความสำคัญมาก ก่อนการวัด

เหล่านี้จะทำให้ท่าน เข้าใจแก๊สมากขึ้น และจะทำให้การวัดของท่านสมบูรณ์ ลดการผิดพลาดที่จะเกิดขึ้นได้

Emission testing

1. ZERO DRIFT OF THE NO SENSOR – WHY CAN’T I USE ELECTRONIC TEMPERATURE COMPENSATION TO ELIMINATE THIS DRIFT? 

(Not to be confused with span drift as a function of temperature.) A number of studies have shown that with an increase in ambient temperature from 70 deg. F to 105 deg. F, the zero of the nitric oxide sensor has drift by as much as 100 PPM. This is unacceptable, particularly when many NOx RACT requirements are between 50 – 150 ppm.

This drift has been observed for all electrochemical sensors, and is due to a combination of filter desorption and additional chemical reactions inside the sensor that take place as the temperature increases. These studies have confirmed that the zero drift depends on the interaction of two equally important factors: ambient temperature and the exposure history (i.e length of exposure and concentration) of the sensor to nitric oxide gas. Its own target gas.

Since it is impossible to know or predict the entire history of exposure of an NO sensor to its own target gas with any precision, temperature compensation can not solve this problem, and may even add an additional factor of data uncertainty.

Fortunately, the only effective remedy to this problem is both simple and elegant; to maintain the filter and sensor’s temperature to below 30 Degree Centigrade. This will insure the effects of these factors on zero drift is negligible.

2. WHY CAN’T I USE A MATHEMETICAL CORRECTION TECHNIQUE (COMPENSATING MATRIX OF INTERFERENCES) TO CORRECT FOR A SENSOR’S RESPONSE TO INTERFERRING GASES? 

Electrochemical sensors do not lend themselves to the use of mathematical correction, a matrix, to correct for cross interfering gases. Although in principle it is possible to carry out a mathematical correction, it is only practical if the sensor’s response to interfering gases is small (typically less than 5%). There are, however, two major technical difficulties in trying to rely on this method to remove the effects of cross sensitivities:

The sensor’s characteristics change continuously with time. This adds a substantial degree of uncertainty. When the interference response of the sensor is large or uncertain, it is impossible to achieve acceptable accuracy by mathematical correction techniques.

A better design consists of using a chemical filter or a scavenging electrode to minimize interference. Only when one is assured of the effectiveness of these primary cross interfering barriers, can a limited back-up mathematical correction be used to remove any residual interference. But the effectiveness on the primary removals systems must be checked and confirmed on a continuous basis to insure sensor selectivity and accuracy.

The analyzer must be equipped with an additional sensors to measures all cross-interfering gas. Of course, the specificity of these sensor’s must also be maintained, or else one can experience a potentially complex and uncertain series of mathematical interactions. Again the simple and elegant solution is to confirm that the effectiveness of the sensors’s primary interference barriers.

Sometimes the interfering gas interferes only in the presence of another gas. (i.e. SO2 in the presence of NO gas on the NO sensor). In this case only a selective filter can correct for this interference.

3. THE ANALYTICAL RANGE OF TOXIC GASES OF COMBUSTION SOURCES IS TYPICALLY 30- 5000 PPM. WHY CAN’T I USE ONE SENSOR TO TAKE MEASUREMENTS OVER THIS RANGE?

Unlike analyzers using optical sensors that respond over many orders of magnitude, electrochemical sensors operate accurately over a much smaller range, typically one order of magnitude. These sensors are designed with a diffusion opening to admit the gas and respond to chemical reactions at the electrodes. If the gas concentration is relatively high for a given diffusion opening, the sensor electrodes saturate and do not respond linearly to the gas concentration. If on the other hand the gas concentration is too low for a given diffusion opening, sensor drift caused by temperature changes and residual interference by other gases become significant and reduce sensor accuracy. The proper optimum solution is to divide the entire analytical range into three ranges covering one order of magnitude each and for each sensor to determine the optimum diffusion opening for its operating range.

4. WHAT IS THE MAXIMUM CARBON MONOXIDE CONCENTRATION THAT I CAN MEASURE?

This depends on how long the measurement period is. The longer the required measurement period the lower the maximum CO concentration will have to be. Required accuracy is also a factor, since the sensor will begin to drift with time as it becomes saturated.

The diffusion opening of the sensor is the governing feature that determines maximum permissible concentration. The smaller the opening the higher the allowable maximum concentration.Unfortunately there is a practical lower limit for the opening, before other factors begin to affect sensor performance. Typically, for the lowest practical diffusion opening (and not using dilution of gas methods) and reasonable accuracy (10%), 40,000 PPM for 10 minutes or 20,000 PPM for 20 minutes are possible. (Such a sensor will not operate with sufficient accuracy below 1000 PPM).

5. WHAT ARE THE LIMITATIONS OF ELECTROCHEMICAL SULFUR DIOXIDE SENSORS?

There are two major drawbacks associated with standard electrochemical sensors. They exhibit a fairly rapid drop in sensitivity when exposed continuously to medium gas concentrations over a period of several hours to three days. They also exhibit very large cross sensitivity (typically up to 160%!!!) to nitrogen dioxide gas. The first problem is associated with the sensor’s internal structure. The second problem cannot be corrected by a selective filter. Filters that remove NO2 will also remove SO2.

The new SEM(tm) sensor uses a novel electrode configuration that minimizes sensitivity loss and incorporates an additional scavenging electrode that minimizes NO2 interference.

6. WHAT IS MEANT BY A CONDITIONING SYSTEM?

Extractive type analyzers draw a small sample of the stack gas, which is then introduced to the analyzer. Extracted stack gas contains a large amount of water vapor and, in the case of liquid and solid fuels, it also contains soot particles. Soot particles are removed by means of porous or fiber filters that are designed not to remove any of the desired gases. In portable analyzers water vapor is removed either by condensing it or by using a permeation type drier. Condensation can be accomplished either by a simple catch pot or by cooling the sample using a chiller. The combination of filters and water removal system is called the analyzer’s conditioning system.

7. WHY IS A PERMEATION DRIER SUPERIOR TO ANY CONDENSING SYSTEM?

Nitrogen dioxide and sulfur dioxide gases react with water, but not with water vapor. Even in the most carefully designed condensing type of water removal system, some of the gas will come into contact with the condensing water and this will cause partial removal of NO2 and SO2 from the sample stream resulting in incorrect measurement of their concentrations. In a permeation drier the water is removed while it is still in its vapor state and before it begins to condense. Thus, there is virtually no loss of NO2 and SO2 from the sample stream.

8. ARE THERE ANY EPA APPROVED METHODS FOR NOX MEASUREMENTS?

There are two EPA approved methods:

Reference Methods 7A through 7E (see 40CFR60 App. A). Method 7E regulating the use of chemiluminescent analyzers is the most commonly used.

EMTIC CTM-022 Conditional test Method for Electrochemical analyzers.

The accuracy requirements for both methods are approximately the same. The procedures for the two methods differ in addressing the special requirements of the two different technologies.

The ENERAC models 3000 & 3000E meet fully the requirements of CTM-022.

9. WHAT IS A CONDITIONAL TEST METHOD ?

According to EPA, a conditional test method is a method that has been technically verified by both OAQPS and ORD, that has not yet gone through the Federal register as a reference method. It can however, be used by states for compliance purposes.

10. WHY CAN’T I USE JUST A HEATED PROBE ?

Using a heated sampling probe only, as some manufacturers do, prevents condensation only for the small section of the probe that is heated. If the sampling line is not heated, condensation will occur in the sampling line. This will cause most of the NO2 and SO2 components of the sample gases to react with the condensing water and disappear from the sample.

11. HOW ACCURATE ARE THE ENERAC’S OXYGEN READINGS ?

The ENERAC’s accuracy for oxygen is plus or minus 0.2 percentage points. However, when comparing the ENERAC’s readings with zirconium oxide in situ analyzers keep in mind that there is a substantial fraction of water vapor in the stack which is removed by the conditioning system of an extractive analyzer, such as the ENERAC, This results in a higher oxygen reading for the ENERAC than for an in situ analyzer. Since the water vapor in the stack can be as high as 20% of the total gas, the difference between the readings can be as high as 1.0 percentage points.

There is also one other point to consider. At the end of its AUTO ZERO period the ENERAC calibrates its oxygen reading to 20.9%. This is the concentration of oxygen in a DRY air (i.e. relative humidity near zero). However, if the ambient temperature is high and the realtive humidity also high the concentration of oxygen in the ambient air may be as low as 20.2%. Under these circumstances the ENERAC calibrating at 20.9% will tend to read proportionally high. To obtain an accurate calibration of the ENERAC, introduce the ambient air through an air purifier that typically consists of a desiccant, such as silica gel, to remove the water vapor and activated charcoal to remove gaseous contaminants.

12. HOW CAN I ARRANGE IN A SPREADSHEET FORMAT DATA SAVED IN THE ENERAC’S INTERNAL MEMORY ?

When you store data in the ENERAC, these data are stored in a form similar to the print out that you get when you press the “TEXT” key.

To arrange the stored data into a form suitable for a spreadsheet program such as Excel or Quattro Pro you must first use the ENERCOM(TM) for Windows program to retrieve the data from the ENERAC. (This program arranges the ENERAC data into the proper form).

To retrieve the data using the ENERCOM program, connect the ENERAC to your computer. Start ENERCOM and establish communication. When the main ENERCOM screen appears enter a suitable file name where you wish the retrieved data to be stored. (You may wish to switch the baud rate at this point to 9600 Baud for faster data retrieval). Switch next to the COMMANDS window. Click the DUMP data box. The ENERAC data will be retrieved and stored in your computer under the chosen file name.


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