Danfoss The soot and scale problems Compendium

Dr. Albrecht Kaupp Page 1

The soot and scale
problems

Issue

Learning
Objectives

Soot and scale do not only increase energy
consumption but are as well a major cause of
tube failure.

 Understanding the implications of soot at

the fire side

 Understanding the implications of scale at

the water side

 Estimating the energy losses through soot

and scale

 Knowing additional negative side effects

of scale and soot build up

 Relating boiler performance parameters to

soot and scale build up

The soot and scale problems Page 2

 NOTES

1. Introduction

Boilers are delivered clean with no soot, slag and scale.
Consequently a soot and scale problem is a classic management
and operational problem that has very little to do with boiler
design.

Soot and slag is a mixture of solid carbon, ash, and molten ash that
sticks to the fire side of the tube and prevents heat transfer. Slag
will also cause corrosion.

Scale is a hard coating or layer of chemical materials on internal
surfaces of the boiler exposed to the water side. Scale mitigates
heat transfer and may lead to corrosion as well.

Scale and soot prevention is one of the most important task of a
boiler operator besides reducing stack gas losses.

The cause of scale will be extensively discussed in lecture 15. Soot
and slag deposits at boiler tube surfaces are mainly a firing
problem and/or mismatch of the fuel and burners.

2. Soot and slag generation

In any combustion process of a fuel there will be always some
unburned carbon (soot) generated and some ash carried with the
stack gas stream. Soot, ash, and molten ash (slag) will accumulate
at the tube banks of the heat exchangers. Some ash will even melt
down at the tube surface. The final result is a layer insulating the
tubes against the hot combustion gases.

Soot generation has various causes such as

a) Freezing the chemical reaction

The flame comes too close to the boiler walls, “freezing” the

kinetic reaction. This effect is best demonstrated with a knife
one passes through a candle flame. The flame will blacken the
knife.

Major causes are overfiring of the boiler, too much excess air,

and worn out burner nozzles.

The soot and scale problems Page 3

 NOTES

b) Insufficient atomization of fuel oil

We talk about the three “T’s” in combustion, turbulence,

temperature and time. Low oil line pressure and lack of
turbulence will result in poor atomization of fuel oil, resulting
in larger droplets leaving the flame envelope partially
unburned.

Major causes are low oil line pressure, lack of primary

combustion air, or low fuel oil preheating temperature.

c) High moisture content in the fuel

Too much water in a fuel leads to a “cold” fire causing

excessive smoke generation because there is not enough fuel
energy available to generate a sufficiently high flame
temperature. However some water helps to speed up kinetic
reaction. It also shortens the flame length.

Major cause of this type of soot generation is a combination of

high moisture content and too much excess air.

d) Erratic feeding of solid fuels

Solid fuel firing is not as “smooth” as liquid or gaseous fuel

firing. Excess air is changing constantly, causing cold pockets
in the furnace, where too much fuel and not enough “fire” and
air are present. The result is smoke generation.

Major causes are bad combustion air distribution and

malfunctioning or badly designed feeding mechanism.

e) Dripping burner

Occasionally liquid oil drips directly from the burner down

into the fire tube and forms a pile of soot. This soot burns up
and generates smoke.

In particular in boilers with superheaters, soot and slag
accumulates at the superheater tube banks first because the
superheater is the first heat exchanger passed by the products of
combustion. All superheaters have “soot blowers” that are
activated periodically to blow off the soot with steam.

The soot and scale problems Page 4

 NOTES

3. Fire tube and water tube differences

There is a basic difference between fire tube and water tube
arrangements, with respect to the location of the soot and scale.

Figure 1 shows a water or superheater tube with soot at the outside
of the tube and scale at the inside. In a water tube boiler the hot
combustion gases pass a bank of tubes at the outside and release
their energy to the water or steam flowing inside the tube.

Figure 1: Water tube with soot outside and scale inside