Nowaday, under the impact of emissions from the production process, the environment is seriously affected, leading to a serious decrease in the quality of human life. The monitoring of emissions of factories into the environment is extremely important in state management.

In Vietnam, according to the regulations promulgated in Decree 40/2019/ND-CP, which stipulates that businesses and industries with the same capacity must install (For details, see Appendix I, Decree 40/2019). /ND-CP). Already operating businesses need to install automatic and continuous emission monitoring stations before December 31, 2020. At the same time, the Government’s Resolution 129 now extends the installation of automatic emission monitoring before December 31, 2021. For newly established enterprises, automatic and continuous emission monitoring stations (CEMS) need to be installed immediately during trial operation. The main parameters that need to be continuously monitored include: Temperature, pressure, flow, dust, pressure, CO, SO2, NOx and residual O2. In addition, some specific industries will have to install more: HF, HCL, NH3…

Currently, the market providing Online Emission Monitoring System (CEMS) has many different products (according to different technologies) of different brands and prices are also different. Therefore, businesses need to have reviews and comments to choose the correct equipment to bring the best effect.


Since all smoke chimneys are quite tall, the house where the analyzer is located is also often far from the base of the smoke column. Therefore, in order for the sample gas to reach the analyzer and remain original compared to the exhaust gas from the chimney, it is very important to take the exhaust gas and conduct the exhaust gas.

Based on the method of sampling and taking the sample to analizer, We divided into two basic types and the analysis system also follows the corresponding technology:


  1. Methods of measurement and sampling.

  • Hot wet

With this method, the gas stream will be sampled directly on the chimney and heated to 160-180 oC, then the sample is led to the analyzer. The temperature entering the analyzer also reaches the hot threshold of 160-180 oC. Why use this method? Because if the temperature is reduced to acidic gases, alkalis will be condensed, causing errors for measurement methods such as HCL, HF, NH3 … The current Hot and humid method mainly uses FTIR, NDIR technology. This allows to measure CO, NO, NO2, SO2, Oxygen, HCL, HF, NH3 gases…

  • Cold dry

With this method, the gas stream will be sampled directly on the chimney and heated to 120-130 oC, then through the air filters, condensing water vapor and cooling the gas stream down to 4-40 oC. before entering the analyzer. For this method mainly currently using NDIR, NDUV technology. This technology allows measurement of CO, NO, NO2, SO2, O2.


    1/ Gas sampling probe with heated filter
    2/ Transport the sample through the heated sampling tube
    3./ Dry the sample usingHigh efficiency air cooler.
    4./ Analysis in dry sample state
    1/ Heated gas sampling probe filter.
    2/ Transport the sample through the heated sampling tube
    3. …….

    4. Sample analysis in hot state

    1/Many gases
    2/ Measure by module
    3/ Can auto-calibrate
    4/ Only suitable for some exhaust gas
    5/ Low investment cost
    1/ Lots of gas
    2/ Measure by module
    3/ Can auto-calibrate
    4/ Suitable for all types of exhaust gas
    5/ Low maintenance cost
    1/ Not suitable for water soluble gases such as
    NH3, HCl, HF, etc.
    2/ Loss of SO2 during drying
    3/ Acid gas is absorbed with water to create liquid acid, which is easy to damage and corrode system components.
    4/ High maintenance cost

    High investment cost


    By the above Pros/Cons analysis, we can easily see that Cold Dry has more disadvantages than Hot-wet.
    When cooling the sample gas and separating water, firstly, it can lose the exhaust gas elements with water absorption properties, so it affects the final analysis results.
    Second, the absorption of acid gas into water creates strong and liquid acids that can corrode metal parts. This leads to very high maintenance costs in the future.
    Since the Hot-wet is suitable for all types of exhaust gases, it is easy to choose the system.
    The maintenance cost of the Hot-Wet system is also much lower than that of the Cold-Dry system.
    On the contrary, it is necessary to evaluate the exhaust gas composition carefully before choosing a cold-dry, otherwise the investor may khápay a heavy price for choosing the wrong solution/system.
  1. Epilogue

    Wili Co Ltd hopes that we will join hands to protect the environment, starting with the investment in continuous emission monitoring systems (CEMS).
    We hope this article will help investors choose the right analysis system and technology.
    However, whether you choose the Hot-wet or Cold-Dry solution, the gas from the sampler to the analyzer still needs to be heated up to a temperature of 110C – 180C.
    With many years of supplying Sampling Tubes, Wili is very willing to advise, support and design for customers to choose the right type of sampling tubes for their particularities.

    CESM sample line list