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

Figure: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 depositionsensors. In fact these sensors in one way or another will measure heat flux.

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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 economicalplant operation.

Heat

Flux

Sensor

s

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Figures:A heat flux sensor is incorporated in the waterwall of a boiler.

By measuring the flux as a function of time, the process of soot accumulation canbe monitored. The heat flux sensor functions as a slagging and fouling sensor.

The key element of the Intelligent Sootblowing system and for the measurement of

fouling is a heat flux sensor. The sensor measures the exchange of energy (inWatts per square meter) from the flame to the steam inside the waterwall tubes. Inaddition this sensor also measures the temperature of the tube wall. In fact thecombination of the heat flux and the temperature signal allows accurate calculation

of the tube skin temperature. These data are again used for calculations of the tubelifetime. (in fact the sensor is specially calibrated for this application).

The heat flux sensor perfroms two functions:

1. as a deposition probe (indication the degree of slagging and fouling)2. as a chordal thermocouple (offering the possibility to measure tube skin

temperature)

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Figures:

A heat flux sensor incorporated in a boiler tube: cross section, normal situation onthe left, with fouling / slagging on the right

Figure:Signal of a heat flux sensor incorporated in a boiler tube: from the clean to the

fouled condition, the output signal level (heat flux) drops, and also the spectralcontent of the signal changes (the fouling dampens the signal).

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Figure:

Complete heat flux sensor, including lead out tube for the wiring and connectionbox for wiring.

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Sootblow

er

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Boiler cleaning systems

Steam generating plants are cleaned on-line by regularly removing ash deposits

that accumulate on heat transfer surfaces. This maintains maximum thermalefficiency of the boiler (optimizes heat rate) and maintains flue gas

temperatures within design conditions at key location. This, in turn, supportsthe optimum operation of air pollution control equipment. On-line cleaning alsoprevents the blockage and plugging of gas passages in the boiler.

Cleaning medium

An effective method of on-line boiler cleaning is to direct a concentrated jet of

cleaning medium against the soot of ash buildup. The cleaning medium may besaturated steam, superheated steam, compressed air or water. Combinations of

water with other media such as steam or air, have also proven effective but cancause boiler tube damage.

Superheated steam is the most widely used medium for several reasons. Boiler-generated steam avoids the cost and maintenance of a compressor. Because itis generally extracted from a high pressure steam outlet and its pressure

reduced by regulating valves, steam provides flexibility in case more aggressive

cleaning is required. Superheated steam is generally recommended oversaturated steam to avoid condensation in the cleaning flow, which canaccelerate boiler tube erosion or result in thermal cracking.

Application of sootblowers

The application of sootblowers is individual for each boiler. In generalsootblowers are use for cleaning Superheater, Reheater and Economizer.

There are several types of sootblowers like full retractable, half retractable,oscillating and rake type of sootblowers.

The design and layout of sootblowers is individual and specific to meet theinstallation requirements like flue gas temperature, cleaning effect, cleaningarea, etc.

Intelligent Sootblowing enables to use the cleaning force of the sootblowers

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selectively and targetly according to the demand of the steam generator.