Danfoss The soot and scale problems Compendium

Dr. Albrecht Kaupp

Page 1

The soot and scale problems

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

Learning		Understanding the implications of soot at
Objectives		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

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.

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 NOTES

The soot and scale problems

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.

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 NOTES

The soot and scale problems

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 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