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Steam Temperature Control in a Coal-Fired Power Station

Cockenzie Power Station is situated on the shores of the Firth of Forth, 8 miles (13 km) east of Edinburgh. The station is operated by ScottishPower, a wholly owned subsidiary of the Iberdrola Group. This 1200 MW coal-fired power station, which comprises four separate generating units, has been operational since 1967.

The attemperation project was commissioned by ScottishPower to investigate the potential for improving the operating efficiency of the plant through more precise control over the temperature of superheated steam entering one of the steam turbines, particularly during load changes. These cause large changes in process dynamics, and so present complex control challenges. Control of the final steam temperature is carried out by regulating the amount of boiler feed water spray into the attemperator to keep the temperature at the secondary superheated outlet close to a desired value. The objectives were; maximize turbine efficiency; minimize the risk of turbine damage; and finally, minimize the risk of thermal stress in the pipework due to excessive temperature excursions and oscillations. This has an impact on both plant performance and asset integrity.

The integration of ADEX control was achieved without modifying the existing control architecture, using the existing PLC, and through the application of the ADEX controller module, offering maximum control performance and physical robustness with a minimum of intrusion. The results overall were analyzed independently, and an improvement of at least 50% in temperature excursions for both steady state and during load changes was reported.

Attemperation Process Diagram

The attemperation system diagram is shown in the attached figure. Water from the “Heaters and Feed Pumps” is supplied both to the economiser and to the rest of the boiler via feed regulation valves, and in addition, to the pipe supplying all four attemperation units. The flow rate of water passed to the economizer circuit and to the rest of the boiler to obtain the final superheated steam is controlled using the feed regulation valves.

The flow rate of water sent to the common pipe of the attemperation system is controlled by means of a master valve after which the water splits into two paths labelled A and B, corresponding to each side of the boiler and which are further divided into A1 and A2 for side A and B1 and B2 for side B.

The master valve remains fully open most of the time during operations. There is an attemperation unit for each of the four lines A1, A2, B1 and B2. Each of these lines has two valves, the upper one shown in Figure 1 called the “main valve” and the lower one called “by-pass valve”. These valves control the flow rate of spray water injected into the superheated steam in order to maintain the temperature at the set point. At no time during the operation are both lines, main and by-pass, functioning simultaneously (i.e. never a mixture), and for most of the time, the by-pass lines are used since the valves were more precise for fine control purposes. The means of controlling temperature T2 is through controlling T1 using the attemperators. 

ADEX Control Strategy

The ADEX strategy was aimed at controlling each temperature T2 at the outlet from the A1 and A2 superheaters by regulating the set points of T1, at the points of attemperation, and each temperature T1 was controlled by regulating the valve position of the attemperation unit. Thus, the control strategy is represented in the attached cascade scheme where:

  • The master controller establishes a desired steam temperature T1 based on a measurement of the temperature T2 and the setpoint of T2.
  • The slave controller establishes the necessary valve position of the attemperator based on the desired temperature for T1 calculated above, and the measured value of temperature T1 at that point.
  • The master controller takes into account the drum pressure as a perturbation.
  • The slave controller takes into account the differential pressure across the feed regulation valves to the boiler as a perturbation.
  • With T1 variation used to control T2, compromise had to be reached between a precise T2 and a strongly fluctuating T1 which could cause thermal stress in attemperation lines.

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