June 15, 2007

Drum pressure the key to managing boiler stored energy

Roger A. Leimbach, Metso Automation

At the heart of most boiler combustion control systems (and most coordinated boiler/turbine control systems as well) is throttle pressure correction, usually applied by the "master controller." Throttle pressure is considered a key variable to control because it represents the energy balance between the boiler and the turbine.

 

When throttle pressure is constant, the boiler is supplying the optimal amount of energy to the turbine. It is said that "the boiler is balanced with the turbine." Any prolonged deviation from setpoint will reduce efficiency. For example, at a lower throttle pressure, producing the same amount of electrical energy would require increased steam flow and thus an increased boiler firing rate.

However, because of process latencies, nonlinearities, and interaction, a control system that uses throttle pressure as its primary controlled variable will produce instability and oscillation. Stable operation can be achieved only by restricting the rate and magnitude of load changes.

To overcome the instabilities and allow for faster load changes, "feed-forward" signals—typically based on boiler steam flow or turbine first-stage pressure—are commonly added to the system. The feed-forwards may or may not include intelligence to deal with nonlinear response. However, most steam flow feed-forwards are "regenerative" in nature. That is, they tend to produce negative interaction with the pressure control as a result of disturbances in the boiler or, more importantly, when the heating value of the fuel changes. When the fuel's heating value changes, the feed-forward will produce a response to compensate for the change, but with an opposite (incorrect) polarity, thus creating further instability. As a result, feed-forwards tend to be detuned, and their use with solid fuel–fired boilers is quite limited. Figure 1 shows such a system.

 

Time delays and nonlinear responses must be addressed by the control system designer. For example, a boiler's "time constant"—the time it takes its output to reflect two-thirds of the impact of an input change—can be several minutes in duration. Even with the aid of a well-tuned feed-forward, any increase in load will produce a pressure drop that can cause instability and cycling due to excess integral action by the master pressure control. Realize that most generation control systems are designed to modulate the turbine governor valves to provide maximum linear response to the new load target. Accordingly, when load increases, the system opens the valves to extract more stored energy (in the form of increased steam flow) to meet demand. But it may take as long as 2 minutes before pressure returns to the setpoint level, and during this period pressure may overshoot and break into oscillation.

The long delay is due in part to the fuel delivery system and to the boiler's heat-transfer characteristics. Throttle pressure will lag and continue to fall during the load increase, causing severe controller "windup." When pressure begins to increase, the master controller will integrate in the opposite direction, introducing another lag, which again may lead to cycling. To overcome this problem, it is recommended that the integral action be set to a relatively small value (usually much less than 1).

A well-designed pressure correction algorithm typically comprises a compensated feed-forward (proportional control) and largely derivative action. A well-tuned control system should be able to stabilize pressure within two boiler time constants. Unfortunately, too many pressure control systems still rely on integrating master pressure controls and less-than-optimum feed-forwards. Even coordinated control systems that utilize a common unit demand do not do justice to the problem. Unit demand, though not necessarily regenerative, does not usually include the intelligence to deal adequately with the delays and boiler stored-energy issues.
 

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