Applications – Power Boiler Combustion Control

As mentioned in my previous blogs on the book Control Loop Foundation – Batch and Continuous Processes, the application examples included in Chapter 16 illustrate the various ways single loop and multi-loop control techniques may be used automate plant control. The fourth workshop exercise in this chapter allows the reader to explore the different features of power boiler combustion control.

A power boiler is designed to operate over a wide range of steam demand and to respond to rapid changes in fuel demand. One or more power boilers may be used to meet the changing steam demand of the plant. The fuel demand from the plant master passes to the boiler through a bias/gain station, known as the boiler master. Using the boiler master, the operator may bias the plant master fuel demand to determine the boiler fuel demand. Such operator intervention may be needed if there is a condition that limits the boiler steam production or if it is more economical to generate steam in one boiler versus another boiler. Also, by placing the boiler master in Manual, it is possible for the operator to manually set the boiler fuel demand. A lead-lag (also know as cross limiting) combustion control system that might be found on a power boiler that burns a single fuel is shown below.

To ensure complete combustion, some excess air must be supplied to the combustion process to provide proper mixing. As the fuel BTU content changes, the quantity air required for complete combustion will change. To compensate for BTU content changes and allow the boiler to achieve maximum operating efficiency, the O2 in the gases leaving the boiler may be measured and the measurement used to automatically adjust the air/fuel ratio. It is good practice to limit the range of the O2 controller output to a safe minimum and maximum air/fuel ratio. The control module provided for the power boiler combustion control workshop is shown below.

By accessing the book’s web site, you may complete this power boiler combustion control workshop using your web browser. The viewer below may be used to see the solution to this exercise.