Improvement of pH value and density measurement of slurry in the tower of lime gypsum flue gas desulfurization system
Time:2022-12-14 Read:626
Reading guide: The sampling probes of the pH meter and densimeter used in lime gypsum flue gas desulfurization systems in thermal power plants are easy to scale and wear, and the measuring pipeline is frequently blocked, which leads to deviation and even data distortion in the measurement of pH value and density of desulfurization tower slurry. For this reason, an improved pH meter sampling device is designed, and the diaphragm pressure transmitter is used to replace the traditional densimeter.

Introduction

At present, the pH meter and densimeter commonly used in the lime gypsum desulfurization system in thermal power plants are mostly probed pipe-mounted structures. The viscosity of gypsum slurry is large, which causes serious erosion and wear of the pH electrode and densimeter probe. In addition, the probe is prone to scale and cause pipe blockage, leading to linear distortion in the measurement of slurry pH value and density, resulting in abnormal operation of the desulfurization system. PH meter and densitometer play an important role in the operation of the desulfurization system. Therefore, the installation method of traditional pH meter and densitometer has been improved to achieve stable measurement and prolong the service life of the probe, which has become a research hotspot in recent years.

Project overview and process flow

Shanxi Datuhe Coking Co., Ltd. No. 1 Thermal Power Branch 3 × The traditional mature and reliable lime gypsum wet desulfurization process is selected for flue gas desulfurization of 75 t/h circulating fluidized bed boiler. The desulfurization system adopts one boiler and one tower, and the flue gas treatment capacity (standard state) is 3 × 110000 m3/h, the slurry in the tower is forced oxidized locally by the air sprayed from the oxidation lance in the tower to form gypsum crystals. Three desulfurization systems share one lime slurry system and one gypsum dehydration system.

The density of the slurry made of lime powder is generally 1050 ~ 1100 kg/m3, and the density of lime slurry is measured with a portable densitometer and manually sampled on site. The desulfurization tower specification is D3.9 m × 28 m, during normal operation, the liquid level in the tower is controlled at about 6 m, and the pH value of the slurry in the tower is controlled at 5.8~6.5. When the slurry density in the tower reaches 1200 ~ 1300 kg/m3, start the gypsum slurry discharge pump and the subsequent dehydration system for gypsum dehydration. The pH meter installed on the tower body is used for online monitoring of the pH of the slurry in the tower, and the diaphragm pressure transmitter is used for indirect online monitoring of the slurry density in the tower. The monitoring data are processed and then entered into the DCS system and displayed on the operation screen.

Sampling with a traditional pH meter

At present, most of the pH meters used in the wet desulfurization system of thermal power plants are probe-direct reading types. Most of the probes are pipe mounted, and the installation positions are mainly in the desulfurization tower body, slurry circulating pump pipeline, and gypsum discharge pump pipeline. The common pH meter probe is installed on the desulfurization tower body.

The sampling valve of the desulfurization tower is often opened too small, and the slurry flow rate in the pipeline is slow. The probe, the inner wall of the pipeline, especially the elbow, is prone to scaling. If it is put into use for a long time, it will be blocked, causing distortion of the pH meter reading and even failure to be put into use normally. This flushing method can not fully flush the glass composite electrode at the probe, and even if the flushing frequency is increased, it is difficult to achieve ideal results. If the sampling valve of the desulfurization tower is opened too much, on the one hand, it will cause an excessive slurry loss in the tower, which will affect the liquid level in the tower, and then cause a series of problems such as the reduction of desulfurization efficiency and the rise of slurry temperature; On the other hand, due to the high content of solid substances in the slurry and the fast flow rate, the probe is subject to long-term erosion, easy to wear and tear, and its service life is seriously affected.

The pH meter probe installed on the circulating pump or gypsum discharge pump pipeline has solved the problems of probe scaling and pipe blockage. However, in the process of use, the probe will be quickly damaged due to abrasion due to high-speed scouring of high-concentration slurry.

Improved pH meter sampling device

Structure

Aiming at the problems in the process of traditional pH meter sampling, an improved pH meter sampling device is designed. The sampling device is mainly a tank with a cone structure at the lower part, which is connected to the desulfurization tower. Open the valve connected to the desulfurization tower, and the slurry enters from the bottom of the tank. When the slurry rises in the tank, most of the large particles of solid matter can be well deposited in the cone bucket at the bottom of the tank. When the thinner slurry reaches the top of the tank, it contacts the probe installed on the tank top to measure the pH value of the slurry, and then the slurry enters the trench through the overflow. When there are many sediments, open the drain valve, and discharge the sediments to the trench through the gravity of the slurry in the tank and the differential pressure between the high liquid level in the desulfurization tower and the tank cone. When the improved pH meter sampling device is running, it is generally not necessary to flush the probe in a short time. 

However, in case of abnormal operation of the desulfurization system, such as poor slurry crystallization, large viscosity of solids in the slurry, and difficulty in sedimentation, the tank can be emptied, and then the flushing water valve on the upper part of the tank can be opened for flushing. A spiral nozzle is installed at the outlet of the flushing pipe, which is directly below the probe of the pH meter, to completely clean the probe. The wastewater after flushing falls into the tank, which also plays a certain role in flushing the tank.

The improved pH meter sampling device not only solves the problem of probe erosion and wear caused by high-velocity slurry but also effectively prevents probe scaling.


Design principle and error analysis

During the operation of the desulfurization system, the main reason why the pH value of slurry in the desulfurization tower can be maintained at 5.8~6.5 is that there are a lot of HSO3 - and HSO4 - in the slurry, both of which have a buffer effect. Therefore, the removal of solids in the slurry hardly affects the pH value of the slurry. The addition of lime slurry or the dissolution of SO2 will not change the pH value of slurry in the desulfurization tower too much in a short time. The retention time of slurry in the desulfurization tower is generally 4~6 min. If the slurry in the tower wants to be mixed uniformly again after adding lime slurry, it will take at least one cycle (4~6 min). Therefore, the pH value measured by the improved pH meter sampling device in a short time can basically represent the instantaneous pH value of the slurry in the tower, and the deviation is small. 

According to the above characteristics of the desulfurization system, the retention time of slurry in the tank with a pH meter probe is generally designed to be 4-6 minutes. If the retention time is too long, the measured data will seriously lag behind the instantaneous pH value of the existing slurry in the desulfurization tower, causing large deviation; If the retention time is too short, the slurry cannot reach the effect of sedimentation. The height diameter ratio of the tank body should be appropriately controlled at 2~4 to ensure a lower slurry flow rate and a longer residence time. The cone angle of the lower cone bucket is designed to be 30 °~45 ° to ensure that the solid substances in the slurry can be well precipitated and discharged.

Conventional densitometer

At present, nuclear radiation densitometers, Coriolis force densitometers, and pipe differential pressure densitometers are the most commonly used densitometers for online measurement of slurry density in desulfurization towers.

Online monitoring of slurry density and liquid level by diaphragm pressure transmitter

Error analysis

The diaphragm pressure transmitter is used to measure the slurry density in the desulfurization tower, which solves the problem that Coriolis force densitometer and pipe differential pressure densitometer are very easy to block and wear during use. Compared with nuclear radiation densitometers, their installation and use are unrestricted. During the 2-month system commissioning of the desulfurization project, the slurry density value measured by the diaphragm pressure transmitter was compared with the slurry density value measured by the on-site sampling and weighing method many times. The error range was only ± 20 kg/m3, that is, the relative error of the measurement results of the two methods was small. Generally, when the density of gypsum slurry in the desulfurization tower is 1200 ~ 1300 kg/m3, it will be discharged outwards. Measuring the density of gypsum slurry in the desulfurization tower with a diaphragm pressure transmitter will not affect the judgment of whether the desulfurization system needs to discharge gypsum slurry. At the same time, the measuring device can also monitor the slurry liquid level in the desulfurization tower online, which provides a strong guarantee for the reliable operation of the desulfurization system.

The agitator, slurry circulating pump, and oxidation fan in the desulfurization absorption tower may disturb the measurement results of the diaphragm pressure transmitter during operation, resulting in a large deviation of measurement data. Therefore, the installation position of the diaphragm differential pressure transmitter is crucial. In order to prevent uneven slurry mixing or thick sediment deposition at the tower bottom due to the dead angle of mixing, which may lead to large deviation in the measurement results of the diaphragm pressure transmitter, it is better to install the diaphragm pressure transmitter above the agitator with a certain spacing and staggered in the vertical direction with the agitator. At the same time, the distance between two diaphragm pressure transmitters should not be too small, 0.5~1 m is more appropriate; The distance between the diaphragm pressure transmitter and the highest liquid level of the slurry should not be too small, which should be at least 4~6 times of the distance between the two diaphragm pressure transmitters, and the h ∶ (H-L) should be 1:4~1:6), otherwise the measurement deviation will be large.

Precautions

(1) When the desulfurization system is overhauled or shut down, the slurry in the tank of the improved pH meter sampling device shall be drained, and the probe and tank shall be thoroughly flushed.

(2) The normal operation of the oxidation device in the desulfurization system is critical. If the slurry in the tower is not oxidized sufficiently, the slurry is mostly calcium sulfite crystals with small particles, large viscosity, and extremely slow sedimentation rate, which is easy to cause scaling and blocking of the connecting pipeline and pH meter probe.

(3) The change in fluid flow pattern will interfere with the measurement results of the diaphragm pressure transmitter. When the lime slurry pump is started, the slurry density value on the DCS screen decreases significantly; When starting the gypsum discharge pump, the slurry density value on the DCS screen increases significantly, which means that the short-term data distortion occurs. However, when the slurry supply pump and gypsum discharge pump are shut down, the slurry density data measured by the diaphragm pressure transmitter will soon return to normal, so the data should be read when the two pumps are shut down. In order to avoid the occurrence of data distortion, a choke plate can be installed at the pressure measurement sampling port inside the desulfurization tower.

Conclusion

The practice results show that the improved pH meter sampling device can effectively protect the sampling probe, and has good practicability and economy. The diaphragm pressure transmitter has the dual function of monitoring the slurry density and liquid level in the desulfurization tower, and the maintenance workload is small, which is more in line with the needs of engineering applications. The two technical improvement schemes described in this paper can be used for reference by similar desulfurization projects.