Boiler Flue Gas Treatment of Cooling Water Circulation Device Control. System Based on PLC. Qiuping Wang, Lei Han, Miao Zhang, Feng Li

Transcription

1 Applied Mechanics and Materials Submitted: ISS: , Vol. 598, pp Accepted: doi: / Online: Trans Tech Publications, Switzerland Boiler Flue Gas Treatment of Cooling Water Circulation Device Control System Based on PLC Qiuping Wang, Lei Han, Miao Zhang, Feng Li School of Automation Engineering, ortheast Dianli University, Jilin , P.R.China Keywords: Boiler flue gas; circulation water; dilute sulfuric acid; PLC control system. Abstract. Boiler flue gas treatment of cooling water circulation device is a device which can produce dilute sulfuric acid by absorbing SO 2 in the flue gas of boiler combustion and add dilute sulfuric acid to the circulating water. Adding dilute sulfuric acid to the circulating water, can reduce ph value of the thermal power plant cooling water, obstacles and reduce the condenser fouling. This paper has introduced the working process of the boiler flue gas treatment of cooling water circulation device and the design method of the control system based on SIEMES S7-300 series PLC. The control system realizes data acquisition of the water level and the ph value, and control the operation of each equipment according to the collected data. Finally realized the purpose of producing dilute sulfuric acid. Introduction In the production process of thermal power plant, a lot of cooling water is needed to condense the worked vapour into water by convection cooling for repeated use. While in the process of convection cooling, part of evaporative circulating water will be taken away by air convection, which makes the circulating water in the cooling tower concentrating continuously and the ph value increases. This will lead to condenser fouling [1]. Condenser fouling, will cause the phenomenon that generating the same electricity but consuming more energy and more coal than usual. This has a direct impact on the economy and security of the whole power plant. Therefore, in the process of operation, sulfuric acid needs to be added into circulation water to adjust the ph value, in order to obstacle and reduce the thermal power plant direct cooling circulating water fouling [2]. At present, most domestic thermal power plant using the method of manual direct casting concentrated sulfuric acid into circulating water, to adjust the ph value. However, casting concentrated sulfuric acid directly, makes a higher capital investment, even the safety is not guaranteed [3]. Boiler flue gas treatment of cooling water circulation device is a designed device which is based on the patent number ZL invention patents and the patent number ZL utility model patent. The device can absorb SO 2 in the boiler combustion flue gas to produce dilute sulfuric acid and add dilute sulfuric acid into circulating water instead of concentrated sulfuric acid, so that can adjust the ph value of the circulating water and reduce the condenser fouling [4,5]. According to the thermal power plant circulating water treatment requirements, this paper designs the control system of the device by using PLC technology. The control system can realize the expected function of the device and can ensure the stable operation. All rights reserved. o part of contents of this paper may be reproduced or transmitted in any form or by any means without the written permission of Trans Tech Publications, (ID: , Pennsylvania State University, University Park, USA-19/02/16,05:58:52)

2 588 Advanced Materials, Mechanics and Industrial Engineering Working Process of Boiler Flue Gas Treatment of Cooling Water Circulation Device The working process of boiler flue gas treatment of cooling water circulation device is shown in Fig. 1, the water injection is an electric regulating and other s are electric switch s. Clean gas Flue gas Flue gas inlet Washing area Clean gas outlet Oxidation air Water injection Oxidation air inhaler Washing water Circulating pump Water Drain Wastes The lower pool Dilute sulfuric acid outlet Dilute sulfuric acid Circulating water pool Fig. 1. The working process of the device The following shows the process of the device: After pressing the start button, the water injection is opened at first, then the water in the water injection pipe flows into the lower pool through the water injection and the injected water is used as washing water. The water injection is closed automatically when the washing water level reached the highest level of the lower pool. The circulating pump is started automatically when the washing water level reached the eighty percents of the highest level of the lower pool, then the washing water is ejected from spray head in washing area. The flue gas inlet and clean gas outlet are opened after starting the circulation pump. Through the flue gas inlet, the flue gas goes into the washing area where flue gas can make contact with washing water. Most of the SO2 in the flue gas is absorbed by the washing water, and the clean gas after washing is exhausted through the clean gas outlet. The washing water which has absorbed SO2 flows into the oxidation air inhaler and mixes with oxidation air. After that, the washing water flows back to the the lower pool and completes the deeper oxidation in the lower pool, then dilute sulfuric acid is made. Washing finishes when the ph value of the washing water is less than 4.5. Then close the flue gas inlet, the clean gas outlet, the water injection and stop the pump operation automatically. Under the action of gravity, dilute sulfuric acid leads to the circulating water pool by dilute sulfuric acid outlet pipe as the dilute sulfuric acid outlet is opened, then close the when the lower pool is empty. Finally, open the drain to discharge wastes in lower pool, then close when emission is complete. The above content is the technological process for preparing dilute sulfuric acid. The Hardware System The main task of this device is to produce quantitative dilute sulfuric acid which ph value is less than 4.5. To achieve this function, the control system needs to gather the data of water level and ph value of the washing water in the device. Then the system can control all the operation of every equipment according to the collected data, in order to achieve the desired purpose. It requires that the system can work stably, have a strong anti-interference ability and a relatively low cost. So that Siemens S7-300PLC 314CPU is chosen as the control core, which can accomplish the data processing and the equipment controlling of the system [6].

3 Applied Mechanics and Materials Vol The input of the system: DI(24V), are the system startup and stop ; AI(4~20mA), are the water level and ph value of washing water in the lower pool, and the ph value of washing water in the circulating water pool. The output of the system: DO(24V), are the control of the electric switch s and the start/stop of the circulating pump; AO(4~20mA) is the position control of the water injection. According to the system requirements, other modules are chosen including the digital quantity input module SM321, the digital output module SM322, analog input / output module SM334 and power supply module PS 307 5A [7]. The hardware structure of the control system is shown in Fig. 2. PS cpu D I SM 321 DO SM 322 A I/A O SM 334 U PS D I Input DO Output 4~ 20mA Current 4~ 20mA Current Relay stop sw itch state Switch Valve Liquid level sensor PH sensor Circulating 环循 pump 泵 Regulating control Fig. 2. The hardware structure diagram of PLC control system The software system 314CPU mainly realizes data acquisition conversion and control. This control system uses analog input/output module SM334 to collect 4 ~ 20mA current from the PH sensor and liquid level sensor,then convert them to 0 ~ integers. The collected data would be processed in PLC. The module can also convert 0~27648 integers to 4~20mA current output, which is used to control the opening degree of the regulating. In order to make the operation more intuitive and convenient, it needs to convert the collected data to real values.so that two function blocks are used in the program, whose name are SCALE(FC105) and USCALE(FC106). The SCALE module can convert 0~27648 integers to the specified number; the USCALE module can convert the specified number to 0~27648 integers. The control process of 314CPU main program and liquid level control subroutine is shown in Fig. 3. The liquid level control subroutine is used to control the liquid level keeping between 85% and 100% of the highest level, in order to achieve the purpose of producing quantitative dilute sulfuric acid. Similar with the level control subroutine, circulating pump control subroutine is a subroutine which can control start and stop states of circulating pump, according to the liquid level data. Figure 6 shows the circulating pump control subroutine ladder diagram.

4 590 Advanced Materials, Mechanics and Industrial Engineering Data acquisition ph of washing water below 4.5? The liquid level acquisition Open the regulating control subroutine Circulating pump control subroutine Close s and stop pump in turn Open dilute sulfuric acid outlet and drain Close dilute sulfuric acid outlet and drain Finish reaches 100%? Close the regulating Stop button is pressed? Finish Open the regulating below 85%? Fig. 3. Flow diagram of main program and liquid level control subroutine Conclusions This control system is a system for the device based on the patent requirements of patent number ZL and ZL In this work, PLC technology was applied to the control system. The control system realizes the current time collection of the liquid level and ph value and control, meanwhile, controls the operation of all equipments according to the collected data. Finally, the function of making quantitative dilute sulfuric acid is realized. The Dilute sulfuric acid made by this device can be casted into the circulating water cooling tower instead of the concentrated sulfuric acid, thereby reducing water treatment costs. Acknowledgements Thanks for the support of 2013 Jilin postdoctoral scientific research project start-up funds. References [1] ingli u, Bingyun Shen, Fengying Wu, etc: Scale prevention experimental study on the circulation cooling water with flue gas in power plants. Industrial Water Treatment 2006; 26 (6) : [2] Weina Wang: Experimental Analysis and Research of Scaling and Scale Prevention in Condenser in a Power Plant. Inner Mongolia university of technology, [3] ingli u: Experimental study on the development and scaling power plant condenser cooling water system simulation test bench. Inner Mongolia university of technology, [4] Aiping Cheng, Qiuping Wang, China, Patent ZL (2013). [5] Aiping Cheng, Qiuping Wang, China, Patent ZL (2013). [6] Chunyuan Bian, Shuangyan Ren, Shuikui Man, in: PLC S7-300/400 practical development guide. Beijing: China Machine Press(2007). [7] Changchu Lao. PLC programming and application. Beijing: Mechanical Industry Press(2005).

5 Advanced Materials, Mechanics and Industrial Engineering / Boiler Flue Gas Treatment of Cooling Water Circulation Device Control System Based on PLC /