CPI Website: SEP Ushers in Wide-load De-NOx Technology

Pollutants produced by coal-fired power plants have attracted increasing public attention. NOx, in particular, has become a major emission reduction target in Yangtze River Delta region. China’s NDRC, NEA and MEP jointly published the Energy Conservation and Emission Reduction Upgrading & Transformation Action Plan in Coal-Fired Power Generation (2014-2020), and meanwhile the Shanghai municipal government put forward the plan to achieve NOx emission reduction by 12% in 2014-2015, two percentage points above the national average level.

In order to fulfill the requests of the above three ministries that “coal-fired generation units shall at all times meet stable emission standard with full-loaded minimum technical output” as well as the target of Shanghai government, SEP, as a major power generation company in Shanghai, has already installed SCR de-NOx facilities in all of its owned and affiliated units. However, there exist temperature range limits between 315℃~420℃ in catalytic reaction for SCR de-NOx facilities. Along with the unit load reduction, the flue gas temperature will drop accordingly; when the temperature is below 315℃, the ammonia injection system will automatically drop out, and then the NOx emission will exceed the standard. Therefore, in order to ensure the coal-fired boiler reach the NOx emission standard at full load and all times, it is particularly necessary to carry out research on wide-load de-NOx technology.

For this purpose, SEP established technology research and development team on the platform of Shanghai Shangdian Electric Power Engineering Co., Ltd (SEP Engineering) and Shanghai Minghua Electric Power Technology Engineering Co., Ltd (Minghua Electric Power), and conducted wide-load de-NOx technology development for various types of boiler, actively encouraging research staff to explore innovation, and finally ushered in the wide-load de-NOx technology for coal-fired power plants.
After establishment, the R&D team of SEP Engineering started the studies of related documents, analysis and comparison of operating data. Through large quantities of theoretical calculation and analysis, combined with comprehensive consideration of economic evaluation, rehabilitation costs, construction difficulties and automatic control, the R&D team developed a brand new rehabilitation scheme, that is, transform the current boiler water system, convey part of high temperature and saturated water to the water inlet header of economizer and let it mix with and warm up the feed water, then increase the temperature of flue gas at the economizer outlet to meet the applicable temperature for SCR de-NOx. This rehabilitation system is called Gas Temperature Rising System (GRS), which was first introduced in the renovation project at the boiler of Waigaoqiao Power Station Unit 3 in 2013. However, subject to the lift of boiler water pump, they could not deliver more boiler water to the inlet of economizer, thus unable to further increase the water temperature at economizer inlet. Although the rehabilitation could satisfy the applicable temperature for units with capacity above 140MW, the maximum temperature increase of flue gas was only 10℃, which means the system was still unable to meet the de-NOx requirement for units below 140MW. Hence, based on the former GRS system, the team put forward another new idea of conveying part of feed water to the downcomer through bypass when mixing part of boiler water with feed water, in order to raise the water temperature at the economizer inlet to a larger extent without increasing the flow of economizer, and thus acquire a better effect on flue gas temperature increase. Such upgraded Super Gas Temperature Raising System (SGRS) is able to satisfy the needs for further raising boiler gas temperature, also providing approaches for flue gas in DC and supercritical boiler. SGRS has advantages of small changes on system, low rehabilitation costs and wide range of temperature rise.

After commissioning, SGRS system can satisfy the de-NOx system flue gas temperature conditions for units with minimum load at 125MW, with maximum gas temperature rise of 23℃. The system scheme has effectively solved the problem that the low temperature at the inlet may lead to the suspension of SCR de-NOx system. Furthermore, compared with other methods, SGRS system has the characteristics of wide rising range of gas temperature, simple system, low rehabilitation costs and small impact on other working conditions, etc. The two technologies developed by the system, i.e. Natural Circulating Drum Boiler with GRS and Forced Circulating Drum Boiler with GRS, have acquired utility model patent, while De-NOx Temperature Rising System at Low Load won the first prize of SEP Technology Advancement Award in 2013, the third prize of National Electricity Employee Technology Achievement Award and the first prize of CPI Outstanding Worker Technology Innovation Award in 2014.
The R&D team of Minghua Electric Power put forward the technology scheme of Internally Installed Separated Flue to raise de-NOx temperature, which came from the daily observation of their core member, senior engineer Chen Zenghong. When he noticed that in hot summer we can pull the rope to reduce the gaps between window blinds so as to prevent blazing sunlight from entering the window. When visiting Dujiangyan Irrigation Project, he also noticed that the strong river turned into gentle waves after passing through the V-shaped dike. Inspired by these two details, he came up with the idea of separated flue on economizer. “How clever are our ancestors! Confronted with the wide-load de-NOx problem, my inspiration soon came from Dujiangyan project and window blinds. Later I looked up the existing documents and found that no one has proposed such idea around the world.”

Based on the five advantages of Skillful Method, Simple Construction, Convenient Maintenance, User-friendly Control and Easy Popularization, Minghua R&D team developed the separated flue internally installed on the economizer, which realized bypass system controlled by gas flow division. The technology separates the economizer segments into several flues and install baffles on separated flues to control gas flow in each flue and keep the heat exchange area and flow infinitely variable between economizer and gas, so as to keep the temperature at the economizer outlet above 315℃, ensuring SCR system could normally inject ammonia under low load of boiler and reduce NOx emission, which meets the national environmental requirement.

On December 12, 2014, the wide-load de-NOx project was put into operation at No. 11 Boiler of Shanghai Wujing Power Generation Co., Ltd., fulfilling wide-load de-NOx above load of 120MW in 300MW coal-fired unit. This is a whole new attempt, with all crew’s solidarity and hard work, and it finally achieved an encouraging and remarkable success. The Wide-load De-NOx Technology at Economizer Separated Flue has been highly regarded in the industry, because of its advantages of simple construction, wide application for unit types, low rehabilitation costs, small impact on equipment, wide rising range of gas temperature, convenient operation and maintenance, etc. The two technologies, i.e. Gas Flow Separated Control Facility at Economizer Internally Installed Flue and Gas Flow Separated Control System at Economizer Internally Installed Flue, have applied for utility model patent in 2014.

At present, Water Side Bypass Technology by Shangdian Caojing Power Generation Co., Ltd. and Wide-load De-NOx Rehabilitation Project by Tianji Power Plant have come under operation, while Kanshan Power Plant has also included this technology scheme into the intention list on its ultra-low emission rehabilitation project. At CPI’s training class on thermal power enterprise environmental protection technology and application, senior engineer Chen made a speech on wide-load de-NOx rehabilitation technology, which received enthusiastic responses from the participants, and several companies even directly expressed cooperation intention at the meeting.

Insistence on scientific and technological innovation has already become the consensus of SEP in sustainable development strategy. SEP will continue to explore along this road and eventually achieve the vision of “Providing Green Energy and Serving the Public”.

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