A certain cement limited liability company is constructing two new dry process cement clinker production lines, equipped with SCR denitrification and powder dry desulfurization. The online process is the Sike Mahak SMC-9012D cold dry CEMS. The problem at the tail of the second line kiln is that the online standard gas data does not meet the standard, SO2 data does not respond when the process changes, and the main influencing factors are ammonia escape and condensate. After installing a dehumidification pre-treatment system for ammonia removal, the problem of condensate and ammonia escape is solved, achieving the system response time and indication error required in HJ75-2017.

On site issues and analysis

1. During the on-site investigation, the following issues were discovered:

1) The online CEMS pre-treatment of Sike Mahak SMC-9012D is a cold dry type, with the core component being the condenser. During the condensation process, liquid water will inevitably be produced due to low temperature and high humidity, and SO2 dissolved in water will inevitably affect the measured values of the analyzer and the test data of the standard gas throughout the process;

2) Due to very low emission standards, such as SO2: 50mg/m3, NOx: 100mg/m3, and particulate matter: 10mg/m3, low concentration control of nitrogen oxides is required, resulting in ammonia escape during the SCR denitrification process. And the ammonium salt crystals generated by the escape of these ammonia will cause corrosion of subsequent process equipment, greatly shorten the service life of online analytical instruments, affect online monitoring data, and correspondingly increase the maintenance workload of operation and maintenance;

3) During the start stop grinding period, there is no significant change in SO2 data, which cannot truly reflect the fluctuation trend of on-site flue gas and accurate flue gas emission data, thus unable to provide effective assistance and reference for adjusting production processes and desulfurization control processes;

4) On site SO2 standard gas was used for full flow testing, but it failed to meet the requirements of HJ75-2017 standard for T90 time ≤ 200 seconds and standard indication error ≤ ± 2.5% F.S;

5) At the same time, ammonia escape test was conducted on the tail of the 2nd line kiln (when starting the raw material mill), and the test data is as follows:

The device used is PreGASS-3500, and the principle is TDLAS

Regarding the issues discovered on site, Shanghai Alwaysbrong technicians analyzed the reasons as follows:

1) The precipitation and corrosion of condensate is one of the main problems in all cold dry direct extraction CEMS systems, which is determined by the condensation method used in the cold dry direct extraction method. The place where the maximum amount of condensate precipitates is the two-stage condenser. The mechanism of condensation dehumidification is to quickly cool the high-temperature and high humidity flue gas, causing the flue gas to rapidly cool down. The water vapor in the flue gas undergoes phase change and condenses, and the condensate is continuously discharged by a peristaltic pump to remove moisture from the flue gas. Therefore, no matter how short the gas/liquid contact time is during condensation, there will inevitably be a problem of dissolution loss of the target gas component (SO2) that is easily soluble in water;

2) Due to the presence of ammonia, acidic gas, and moisture in the heat tracing tube, ammonia is prone to generate ammonium salts, resulting in the production of aerosols and condensed ammonium salt crystals in the CEMS pipeline. This secondary reaction between SO2 and ammonia can lead to deviation in SO2 monitoring results, specifically manifested as low SO2 data in the CEMS test. This phenomenon can be mistaken for an increase in process efficiency, without realizing it is a secondary reaction between aerosols and ammonia. At the same time, the blockage of the sampling pipeline by ammonium salt crystallization also reduces the condensation efficiency of the CEMS condenser. Therefore, the generated aerosols and crystalline ammonium salts are also important reasons for the instability of the calibration data throughout the process.

Solution ideas

1) Through comparison and summarization, it is found that the main cause of CEMS problems is still the problem of condensate and ammonia escape. It is recommended to renovate and upgrade the original flue gas pretreatment system, using the PreGASS-2500 ammonia removal pretreatment system+PreGASS-9500 dehumidification flue gas pretreatment system with high-temperature solid polymer ammonia removal and permeation drying tube gas dehumidification technology as the core, to completely solve the problem of condensation water, completely remove ammonia in the flue gas, retain the tested components in the analyzed flue gas, and ensure the accuracy of analysis, stability of SO2 testing, and stability of CEMS operation.

2) After installing the PreGASS system to solve the problems of ammonia escape and water vapor entering the online system, the entire process of standard gas testing is shown in the following figure.

The response time and indication error of SO2 with standard gas throughout the process (standard gas concentration 91.0mg/m3) reached 50mg/m3 in 200s before the renovation, which does not meet the requirements of T90 time ≤ 200 seconds and indication error ≤ ± 2.5% F.S. in the standard;

After renovation, it can reach 89.8mg/m3 within 200s, fully meeting the requirements of T90 response time and standard indication error of ≤ ± 2.5% F.S.

3) After stable operation of the PreGAS system, the treated flue gas dew point is -8.3 ℃ and the relative humidity is 9.2% RH, which is much higher than the dew point temperature requirements in Appendix D5.6 of HJ76-2017 Dehumidification System.

4) The recent full process test data is as follows: