Boiler fouling is obstructing the flow of energy from the fire/flue gas to the steam in the waterwall or in the heat exchanger bundles.

Typically the growth of significant fouling in an electrical power plant takes a couple of hours to reach a stage in which it becomes significant.

Figure:
Look through a port in an electrical power plant. The fouling is visible on the waterwall.

The process of slagging is closely related to that of fouling. Slagging refers to deposition of solid layers on the boiler tube, formed by sintering. Slagging is quite hard to remove and also (although it is sintered material) the slagging material is usually still a good insulator. For these reasons, most modern sootblowing systems are aimed at prevention of slagging.

The deposition of slag and fouling is a self-propelling process. The deposition of the first particles strongly facilitates the process of deposition.

Sensors that detect fouling and slagging are often called "Heat flux sensors" or deposition sensors. In fact these sensors in one way or another will measure heat flux.

The fouling behaviour of a boiler changes depending on fuil characteristings, load and other parameters of boiler operation. This makes it necessary to react on demand and therefore sensors as well as Intelligent Sootblowing systems are necessary for a safe and economical plant operation.