Boilers

Power Plants

Battelle has provided a well-written report that discusses power plant coal utilization in great detail. It gives

a thermal efficiency of 80-83% for steam generation plants and 37-38% thermal efficiency for power generating plants at base load (about 70%). A base load plant designed for about 400 MW and up will run at steam pressures of 2,400 or 3.600psi and 1,000”F with reheat to 1.000”F and regenerative heating of feedwater by steam extracted from the turbine. A thermal efficiency of 40% can be had from such a plant at full load and 38% at high annual load factor. The 3,600psi case is supercritical and is called a once-through boiler, because it has no steam drum. Plants designed folr about 100-350MW run around 1,800 psi and 1,000”F with reheat to 1,000”F. Below 100 MW a typical condition would be about 1,350psi and 950°F with no reheat. Below 60% load factor, efficiency falls off rapidly. The average efficiency for all steam power plants on an annual basis is about 33%.

Controls

Three basic parts of boiler controls will be discussed:

1. Level control

2. Firing control (also applies to heaters)

3. Master control

For steam drum level control, the modern 3-element system-steam flow, feedwater flow. and drum levelshould be selected. Steam and feedwater flows are compared, with feedwater being requested accordingly and trimmed by the drum level signal. This system is better than having the drum level directly control feedwater, because foaming or changing steam drum conditions can cause a misleading level indication. Also, the 3-element controller responds faster to changes in demand.

The firing controls must be designed to ensure an airrich mixture at all times, especially during load changes

upward or downward. SEeam header pressure signals the firing controls for a boiler. The signal to the firing controls comes from a master controller fed by the steam header pressure signal if multiple boilers are operating in parallel.

The firing controls that best ensure an air-rich mixture are often referred to as metering type controls, because gas flow and air flow are metered, thus the fuel-air ratio is controlled. The fuel-air ratio is the most important factor for safe, economical firing, so it is better to control it directly. Do not settle for low budget controllers that attempt to perform this basic job indirectly (such as controlling fuel-steam or other ratios).

The idea for safe control is to have the air lead the fuel on increases in demand and fuel lead the air on decreases in demand. On load increases, the air is increased ahead of the fuel. On load decreases, the fuel is decreased ahead of the air. This is accomplished with high- and low-signal selectors.

A high-signal selector inputs the air flow controller. which often adjusts forced draft fan inlet vanes. The highsignal selector compares the steam pressure and fuel flow signals, selects the highest, and passes it on to the air flow controller. For a load decrease, the steam line pressure tends to rise and its controller reduces the amount of firing air to the boiler. but not immediately. The high-signal selector picks the fuel flow signal instead of the steam pressure signal, which has decreased. The air flow will therefore wait until the fuel has decreased. On load increases, the steam pressure signal exceeds the fuel signal and the air flow is immediately increased.

The reader can easily determine how a low-signal selector works for the fuel flow controller. It would

compare the signals from the steam pressure and the air flow. A flue gas oxygen analyzer should be installed to continuously monitor or even trim the air flow.

A master controller is necessary to control a single steam header pressure from multiple parallel boilers. The boilers are increased or decreased in load together. There is a bias station at each boiler if uneven response is desired. The “boiler master,” a widely used device, can also be used to control other parallel units of equipment. Therefore, it should not be thought of as only a boiler controller if other applications arise, such as controlling parallel coal gasification units.

Source

Branan, C. R., The Process Engineer's Pocket Handbook,

Vol. 2, Gulf Publishing Co., 1983.