With increasingly stringent environmental requirements, China mandates ultra-low emission standards for coal-fired power plants, limiting the nitrogen oxide (NOₓ) emission concentration to below 50 mg/m³. Selective Catalytic Reduction (SCR) is the dominant denitrification process for power plants at present. This technology uses ammonia as the reducing agent to convert nitrogen oxides in flue gas into nitrogen and water under suitable temperature and catalyst conditions. Ammonia sources mainly include liquid ammonia and urea processes. The liquid ammonia process boasts mature technology and low operating cost, yet it is classified as a major hazard source with strict requirements for transportation and storage protection. The urea process consists of hydrolysis and pyrolysis routes. Pyrolysis delivers superior safety but comes with higher capital investment and energy consumption. Green Valley Environmental’s SCR denitrification system adopts the urea route to generate ammonia reducing agent, offering higher safety compared with the liquid ammonia solution. The urea solution is decomposed to produce ammonia, which reacts with nitrogen oxides in flue gas to form nitrogen and water with the action of catalyst at proper temperature.
Field investigations reveal many common operational challenges of SCR denitrification systems in coal-fired power plants. First, under low-load conditions, the flue gas temperature at the reactor inlet fails to fall within the optimal catalyst operating window of 300–420 ℃, which deactivates the catalyst. To meet emission standards, more ammonia is injected, aggravating ammonia slip. This further leads to the formation of ammonium bisulfate, causing blockage and corrosion of the air preheater and endangering unit safety. Second, the high dust content in flue gas leads to catalyst fouling, abrasion and even local deactivation caused by alkali metals and arsenic compounds in fly ash. The ammonia injection system also has obvious drawbacks: the original simple control logic results in a low automatic operation rate. Improper opening of ammonia injection branch valves and clogged nozzles degrade the mixing of ammonia and flue gas. Large flow velocity deviation and uneven gas flow distribution inside the reactor easily cause insufficient NOₓ removal in local areas and excessive ammonia slip. Besides, inaccurate on-line monitoring instruments and improperly arranged measuring points interfere with ammonia injection regulation, which may trigger excessive emissions or higher ammonia consumption.
To tackle the above problems, ammonia injection optimization is carried out from two aspects: branch flow distribution adjustment and automatic total ammonia flow control logic, without modifying major equipment. Precise ammonia control is the core to improve system performance. For branch ammonia injection optimization, the grid sampling test method is adopted. The distributions of NOₓ, ammonia concentration and flue gas flow at the inlet and outlet cross-sections of the SCR reactor are measured and compared with online monitoring data. Branch flow regulation tests are performed under medium and high loads. The opening of each branch valve is adjusted repeatedly to equalize ammonia distribution over the cross-section. Verification tests are then conducted under both high and low loads to ensure uniform distribution of NOₓ and ammonia under various working conditions and eliminate regional differences in denitrification performance.
A cascade PID control scheme with feedforward is developed to overcome the long lag and poor adaptability of the two conventional control strategies, namely fixed molar ratio control and fixed outlet NOₓ concentration control. The main loop takes the target NOₓ concentration at the denitrification outlet as the setpoint and outputs the theoretical ammonia supply rate. The secondary loop uses this theoretical ammonia flow as its setpoint to regulate the ammonia control valve. The inlet NOₓ concentration is introduced as a feedforward signal to compensate for the system’s large lag and nonlinearity. Signal filtering is added to stabilize the regulation process, and PID parameters are retuned to accommodate the large inertial characteristic of the system.
This optimization scheme accelerates the response of ammonia injection regulation during unit load variation and stabilizes the outlet NOₓ concentration. While satisfying ultra-low emission requirements, it reduces ammonia consumption and restrains ammonia slip, balancing the safety, environmental benefits and economic performance of power plant operation.
Post time: Nov-03-2026