Applications of Model Based Controls on 5 Coal Fired Units to Improve Thermal and Emissions Performance

摘要

In the late 80's Virginia Power was experiencing outage problems associated with boiler tube leaks on their large, 550 MW, coal fired units at Mt. Storm. In addition the units were not achieving expected thermal performance levels. Following efforts to apply traditional techniques which did not adequately resolve these problems, Virginia Power embarked on an ambitious program to install Stone & Webster's model based Advanced Steam Temperature Control (ASTC) system on Unit 1. The ASTC system included the Model Reference Feedforward steam temperature control system to address the outage issue and the Soot Blow Advisor (U.S. Patent No. 5,181,482) system to address the performance issue.The system which was installed on a pilot distributed control system became fully functional by early 1990 and began to show payoffs in both areas. The unit experienced its longest run without a boiler tube failure and the rate of tube failures was reduced. The ability to maintain steam temperatures at set point and maintain clean boiler conditions improved thermal performance. Following the upgrade to a full distributed control system in '92, the Soot Blow Advisor was upgraded to a fully automatic system, activating soot blowers to maintain optimum thermal performance. Based on the success of the installation on Unit 1, following the recent controls upgrade to distributed control systems, the ASTC system was installed on the other two units at Mt. Storm. Similar results of improved thermal performance and reduced transient variations have been achieved.In '93 Virginia Power tested Stone & Webster's model based NOx Emissions Advisor (U.S. Patent No. 5,280,756) on the two coal fired units at Possum Point station. The testing demonstrated that NOx reductions of 20 % could be achieved on both the 107 MW Unit 3 and the 234 MW Unit 4 with no hardware retrofit. Based on these results Virginia Power embarked on the installation of the Stone & Webster NOx Emissions Advisor and Automation system in conjunction with the ASTC system for both of these units. The installation which was completed in the Spring of '95 hasattained the NOx reduction objectives with little or no penalty on thermal performance. Through automatic control action the combined system minimizes NOx emissions while maintaining satisfactory furnace conditions and high thermal performance over the entire load range for these cycling units.The Model Reference Feedforward steam temperature control system utilizes process models and all relevant plant signals to calculate feed forward signals. These signals are provided for the superheat and reheat steam temperature control loops every 2 seconds. In this way anticipatory signals are provided for effects such as load changes, excess air changes, fuel disturbances, soot blowing, etc. The system not only reduces steam temperature variations following disturbances, but also mitigates fuel transients, thereby reducing peak fuel swings and the associated impact on boiler tube failures.The Soot Blow Advisor and Automation system utilizes a full cycle performance program and a set of performance criteria in conjunction with all relevant plant sensors to select the optimum soot blower action. Calculated soot blower performance measures and advisory information are provided to the operator through the DCS graphic interface. The selected soot blower is transmitted to the DCS then processed by the discrete soot blower control logic and activated.The NOx Emissions Advisor and Automation System applies process models with input from all relevant plant sensors to determine the optimum' setting for a series of plant control settings which minimize NOx formation while maintaining satisfactory furnace conditions. Advisory information is provided to the operator through a graphic interface and the control settings are automatically transmitted to the DCS for on-line regulation.This paper describes these model based systems and their performance on t