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Overview
Nitrogen oxides (NOX) pose significant environmental risks. Although most NOX produced by various pollution sources initially exists as NO, which isn’t highly toxic, it transforms into NO2 in the atmosphere, causing harm to human health. Additionally, NOX contributes to the formation of photochemical smog and acid rain.
In China, coal-fired power plants account for a substantial portion of NOX emissions. The power sector is foundational to the national economy, and with the rapid economic growth, electricity demand has surged. High-capacity and high-parameter thermal power units, such as those above 300 MW, dominate the industry. By 2004, total NOX emissions in China reached approximately 16 million tons, with the electricity sector contributing about half of these emissions.
Qingdao Lingfeng Automation Engineering Co., Ltd. specializes in designing desulfurization and denitrification DCS automation control systems. These DCS systems offer numerous advantages, including high reliability, openness, powerful functionality, and ease of maintenance. They enhance the efficiency of SCR denitrification processes, stabilize control operations, and improve denitrification effectiveness, aligning with national standards for gas emissions.
System Principle
The denitrification process employs the selective catalytic reduction method. This reaction occurs in a reactor containing a catalyst. Flue gas interacts with injected ammonia under the catalyst's influence, removing nitrogen oxides. Typically, flue gas contains approximately 95% NO and 5% NO2, which undergo chemical reactions to form water and nitrogen.
Process Introduction
The system primarily consists of two components: an SCR system and a reducing agent supply system, specifically an ammonia supply system. The SCR subsystem’s main equipment is located in the SCR area, which is the boiler's tail-end. Ammonia produced by the ammonia supply system is transported to the SCR inlet flue via an ammonia gas/air mixer located in the SCR zone and mixed with dilution air using the gaseous ammonia pipeline along the site's pipe rack.
SCR System
Flue gas enters the boiler outlet flue and flows horizontally into a vertically arranged SCR reactor. Inside the reactor, the flue gas passes through a flow splitter, the catalyst layer, and then into the rotary air preheater, electrostatic precipitator, induced draft fan, and FGD before finally being discharged into the atmosphere through the chimney.
The SCR system includes a flue gas system and a catalyst purge system. The flue gas system comprises dilution blowers, ammonia/air mixers, ammonia flow control valves, ammonia shut-off valves, and ammonia injection grids. Gaseous ammonia originates from the ammonia preparation area of the utility system, while air is supplied by the dilution blower and mixed with the ammonia/air mixer at a specific volume ratio before being injected into the reactor via the ammonia injection grid to serve as a reducing agent. The catalyst purge system includes the catalyst and steam soot blower. The catalyst used is a honeycomb type, with TiO2 and V2O5 as the primary active ingredients, supplemented by a small amount of WO3.
Ammonia Supply System
Liquid ammonia from the storage tank is sent to the evaporator to produce gaseous ammonia. The ammonia unloading compressor draws ammonia gas from the ammonia storage tank and compresses liquid ammonia from the tanker into the liquid ammonia storage tank. The ammonia tank is designed to prevent solar radiation, with industrial water spray pipes and nozzles installed around it. If the tank temperature becomes too high, the automatic watering device activates to cool it down. Similarly, if there is a trace ammonia leak, the automatic watering device starts to absorb ammonia and control pollution.
Control Strategy
The hardware and software of the denitrification DCS control system and the unit DCS system both utilize the DCS series. The denitrification process system, or SCR system, is integrated as a substation within the unit's DCS system for control purposes. The agent storage, preparation, and supply system is incorporated as a remote I/O station of a public DCS system for centralized control. Public DCS and unit DCS are connected via redundant optical fiber communication. Operators monitor the parameters and equipment of the denitrification system through the DCS operator station in the control room, while the ammonia preparation area is monitored on the public DCS.
The out-of-stock DCS control system primarily handles two functions: controlling the ammonia system and managing the ammonia preparation area. The ammonia preparation area is controlled by a pair of CPs in the public DCS. This part focuses on ammonia unloading, transferring between storage tanks, providing ammonia steam to reactors, monitoring various process parameters of the tank, and handling emergencies. The second function involves controlling the reactor zone. The SCR_DCS manages two reactors per furnace, with a pair of CPs assigned. This part focuses on controlling NOx concentrations, reactor monitoring, and soot control.
Application Effect
Selective catalytic reduction (SCR) implemented with the DCS distributed control system reduces worker workload, boosts denitrification efficiency, and leverages DCS’s distributed control capabilities to coordinate field instrument operations. The unique interlock control of the denitrification DCS system enhances the safety of the denitrification process. The stability of the DCS collection control system ensures smooth production processes and easier management.
In conclusion, Qingdao Lingfeng’s advanced DCS solutions play a crucial role in reducing NOX emissions and ensuring compliance with environmental regulations, marking a significant advancement in modern power plant technology.
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