1857 oil gas fuel fired furnaces steam boilers water treatment

7/31/2019 1857 Oil Gas Fuel Fired Furnaces Steam Boilers Water Treatment

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FUEL FIRED FURNACES

Written by Norrie

Wednesday, 17 February 2010 20:49 - Last Updated Wednesday, 24 February 2010 19:37

INTRODUCTION

Furnaces are constructed as a rectangular 'Box' or a cylindrical 'Bottle' shape. A Furnace

consists of a combustion chamber or 'Firebox', in which a fuel is burned to produce largeamounts of heat energy for addition to process fluids. (The firebox is often called the 'Radiant

Section' of the furnace where the heat transfer principle is that of Radiation). The side walls of

the firebox, and, in some cases the floor and roof, are lined with the tubes carrying the process

fluid.

The radiant heat is transferred into the process fluid by conduction through the tubing metal.Between the tubes and the outer shell of the furnace - walls, roof and floor, a 'Refractory Lining'

of special brickwork is placed which acts as an insulator, preventing loss of heat to the outside

by reflecting it back into the chamber.

The furnace will generally also contain another chamber, separated from the firebox by a

'Bridgewall'. This is called the 'Convection Section' which also contains tubes carrying the

process fluid. The process feed would pass through these tubes before entering the main

firebox section. On entering the radiant section, the first set of tubes is called the 'Shock Bank'

where they receive the initial shock of radiant heat.

The combustion gases from the radiant section flow over the bridge-wall, through the

convection section giving up heat to the process fluid. The waste combustion gases, now

called 'Flue Gases', then pass out of the furnace by way of the 'Breeching' or 'Ducting' to the

'Stack' which carries them to the atmosphere. (In some installations, these hot flue gases are

used as heating medium in a Waste-heat Boiler' or other heating process).

(See Figure: 34)

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{/artsexylightbox}Figure: 34

THE COMBUSTION PROCESS

The combustion process requires a fuel, oxygen (air), and ignition in order to take place. The

combustion reaction produces flue gases which contain mainly, Carbon dioxide (CO2), Watervapour (H2O), Nitrogen & Oxygen (N2 & O2) - from the unused air.

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If the oxygen (air) supply is low, the reaction will not be complete and some Carbon Monoxide

(CO), would be present. This is a poisonous gas and its formation should be avoided. This is

done by ensuring an adequate supply of air to the furnace.

Combustion Air:i. Primary Air.

This is air which mixes with the fuel to give the initial combustion reaction.

ii. Secondary air.

Mixes with the initial combustion products and ensures complete combustion of the fuel.

Secondary air also gives the correct flame profile of the burning mixture. Too much air will give

a ragged, very bright flame and decrease the heat of combustion while too little air will give a

long smoky flame and incomplete combustion - a long, lazy, smoky flame can also cause

'Flame Impingement' on the furnace tubes. This in turn will cause 'Hot-spots' on the tubes andcoke formation inside them. (The secondary air also ensures thorough mixing of the flue gases

for even distribution of heat).

The flue gases leaving the furnace are sampled periodically and tested for their composition.

This is done to allow the operator to adjust the air supply and the furnace 'Draught' - (flow of

gases through the furnace -sometimes spelt 'Draft') in order to give maximum efficiency of the

system.

The draught through the furnace is controlled by air doors or louvres around the fuel burners in

conjunction with a stack damper to control the flow of gases to the stack.

Furnaces (and boilers) generally, are also fitted with:

1. Observation glasses for the operator to view the flame profile and other internal features.

2. Snuffing steam for fighting fires in the chambers in the event of tube rupture.

3. Explosion doors that will relieve the pressure of an internal explosion.

4. Access doors for maintenance purposes.

THE BURNERS

The burners in a furnace, as their name implies, are used to mix the fuel and air before ignition.

Burners are designed in varying ways in order to deal with the combustion of gases and/or

liquids. Where gas is the fuel, the burner is of a fairly simple construction as gas needs no

atomisation before combustion.

Liquid burners however, are more complex in that they need special parts to impart a 'swirl' to

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the liquid and then to convert the liquid into a fine mist. This is called atomisation. In any

burning process, only the vapour of a substance will burn. The atomised liquid will therefore

vaporise much more quickly and burn more readily.

Gaseous fuel to a furnace must be free of solids, liquid hydrocarbons and water, before goingto the burners. This is achieved by filtering the gas and passing it through knock out drums to

separate it from any of these undesirable substances.

FURNACE DRAUGHT (or Draft)

Draught through a furnace is a slight pressure difference that causes the flow of gases through

the furnace. There are four methods of producing draught:

1. Natural Draught:This is the natural flow of gases due to the decrease in density of the inlet air caused by the

heat in the furnace. The hot gases passing up the stack by natural convection, will give a slight

pressure drop (negative pressure), across the inside of the furnace and cause fresh air to flow

in at the air doors (louvres) around the burners. The draught is controlled by the air doors and

stack damper and is generally measured in water-gauge units.

2. Forced Draught: (F.D. Fan)

In this system, a fan is used to supply air to the burners. The air doors and stack damper are

again used for control. The draught in this case is at a slight positive pressure.

3. Induced Draught: (I.D. Fan)

This is produced by a fan at the stack inlet. The fan is used to pull the gases out of the furnace

and discharge them to the stack. The draught here is a slight negative pressure.

4. Balanced Draught:

In this case, two fans are used, one to supply forced draught (FD fan) to the burners, and one

to give induced draught (ID fan), to pull the gases out of the furnace.

With the other three types of draught, weather conditions, (wind direction and/or high winds

etc.) can affect the draught. With balanced draught, it is not affected.

(See Figure: 35)



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Figure: 35

STEAM GENERATION (BOILERS)

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In essence, a boiler is virtually the same as a furnace in most aspects.

The differences between a furnace and a boiler are:

The raw feed to the boiler tubes, is water that has been treated to remove dissolved salts and

gases and other extraneous impurities.

These impurities cause corrosion, scale build up and other unwanted effects to the piping and

equipment and machinery associated with the boiler system and the steam produced.



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Figure: 36

The hot flue gases before being put to atmosphere, can be utilised in a 'Waste Heat' Exchanger

or boiler to economise on fuel usage by pre-heating the boiler feed water or for heating other

process fluids.

The exchanger tubes carrying the fluid to be heated will be placed in the trunking that takes the

hot flue gases to the stack.

pH of SOLUTIONS, WATER CONDITIONING & CHEMICALS

USED IN WATER TREATMENT pH of a Solution

There is a constant need in industry, to monitor the presence of acids and bases in process

streams.

For many purposes, it is more important to know whether a solution is acidic or alkaline, rather

than knowing its precise composition.

The 'pH' of a solution, is the measure of the concentration of hydrogen ions ( H+) which is an

indication of the acidity or alkalinity of a solution.

The pH of solutions follows a 'Scale' of 0 to 14, where 0 is the most acidic, 7 is neutral and 14 is

most alkaline. Consider the following scale:-


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The control of the pH in many operations is very important. The formation of salts and corrosionof plant equipment can lead to serious damage and failure of the equipment.

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The pH scale is only useful for weak concentrations of acids and alkalis, (up to about 5% by

weight. Stronger solutions will only read at the ends of the scale.

Pure water is a neutral substance and has a pH of 7.0. Following is a list of some common

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WATER TREATMENT TECHNIQUES 1. REMOVAL OF DISSOLVED GASES

Dissolved gases, such as oxygen and carbon dioxide, should be removed from water which is

to be used for boiler feed. These gases, in water, are corrosive. A number of methods for their

removal are available. Following are two such methods: -

i. Chemical Treatment :

Sodium sulphite (Na2SO3), when added to the water, will combine with oxygen to form sodium

sulphate (Na2SO4) which effectively removes free oxygen from the water as in the following

equation: -

2Na2SO2 + O2 = 2Na 2SO4Sulphite Free oxygen Sulphate

This does not remove the CO2 , but, it is probable that the oxygen is the more harmful as a

corrosion agent.

ii. De-aeration :

With this method, the water is sprayed down a tower fitted with baffles and a heater. The

baffles break up the falling water into a fine spray and the heater 'boils out' the dissolved

gases. The water which is thus de-aerated is taken from the tower bottom while the CO2 and

O2 gases are vented from the top. The process may be operated under vacuum which will

help the de-aeration by decreasing the solubility of the gases. The vacuum production system

will also 'suck out' the gases and prevent them re- dissolving in the water.

2. REMOVAL OF DISSOLVED SALTS

As water passes through the ground, it will dissolve some soluble materials out of the soil and

rocks. Two main compounds which dissolve are 'Limestone' - calcium carbonate - CaCO3 and

a mixture of the carbonates of calcium and magnesium - 'Dolomite' - CaCO3

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& MgCO

3

- both of which are insoluble in pure water. However, when rain water, which contains

dissolved CO2 contacts

limestone, a slow reaction occurs which produces calcium bi-carbonate. This is shown in the

following reaction: -

2CaCO3 + 2H2O + 2CO2 = 2Ca(HCO3)2

2CaCO3=Calcium Carbonate

2CO2=Carbon Dioxide

2Ca(HCO3)2=Calcium Bi-Carbonate

A similar reaction occurs with the magnesium carbonate. The Bi-carbonates formed are more

soluble in the water.

Also present in the ground, are other salts of calcium and magnesium - chlorides, sulphates

and nitrates and salts of sodium and iron. (Calcium sulphate is only very slightly soluble).

The term 'Hard Water', refers to water containing calcium and magnesium and, hard water

does not easily form a lather with soaps.

The soaps will react with these salts but their ions will form a 'Scum' of precipitates. However,

the reaction of the continued addition of soap will eventually use up all the ions and the soapwill begin to form a lather.

When these ions of 'Ca' and 'Mg' are removed, the water is termed 'Soft Water'. Water which

does contain some of these ions has some advantages in that it has a more pleasant taste and

calcium is good for healthy teeth and bones. Pure water is tasteless and not pleasant to drink.

Water used in industry for the production of steam etc.., does cause some major problems

when heated. For example:

- Bi-carbonates, when heated, will break down to form insoluble carbonates which give rise

to deposits of scale or ' fur ' inside kettles and piping. This build up of layers of scale will

cause poor heat transfer and the heating element can overheat and burn out or, in the case of

a steam generation boiler, can cause pipe blockages. The kind of hardness caused by the

bi-carbonates of calcium and Magnesium, is called ' Temporary Hardness ', as boiling removes

the chemicals concerned.

- Calcium sulphate has 'Inverted Solubility' -as water temperature increases, the solubility

of the sulphate decreases causing crystals to form that build up into scale deposits. Scale build

up can also cause blockage of piping which will result in plant shut down for cleaning of theequipment.

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- The chlorides, sulphates and nitrates of 'Ca' and 'Mg' cause what is termed 'Permanent

Hardness' in water. For the above reasons, when water is being used for steam

generation, it must be treated to remove these compounds. Methods of water treatment are

outlined below:

3. REMOVAL OF HARDNESS BY 'ION EXCHANGE RESINS'

Some materials that are insoluble in water, called 'Zeolites', have the property of combining with

certain harmful ions in a solution and, at the same time producing other harmless ions.

Zeolites are referred to as 'Ion Exchange Resins' and are complex compounds of sodium,

aluminium, silicon and oxygen.

When water containing ' Ca2+ ' and ' Mg2+ ' ions, is passed through Zeolite beds, these ionsare picked up by the Zeolite which then replaces them with harmless sodium ions ' Na+ '.

If we represent the Zeolite as a letter 'Z', the equation can be shown as follows:

Ca2+ + Na2Z --> CaZ + 2Na+

This indicates that the calcium ions have come out of solution and are replaced by sodium

ions in the solution.

This process is called 'Water Softening by Ion Exchange' as follows: -{artsexylightbox path="images/stories/norrie/4" autoGenerateThumbs="true"


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When the Zeolite is 'saturated', (all Na ions used up), it is regenerated by passing a

concentrated salt (NaCl) solution through the Zeolite bed.

This forces the Ca and Mg ions out of the Zeolite back into the water and replaces them with

sodium. The solution containing the Ca and Mg is disposed of.

The sodium salts remaining in the treated water are harmless and will not form deposits ofscale.

4. DE-IONIZATION

Taking the above process a stage further, by using synthetic Zeolites, (insoluble synthetic

resins), ALL ions of ALL compounds in the hard water can be replaced.

One type of resin will remove all the positive metallic ions and replace them with Hydrogen

ions, while other resins will replace non-metallic ions of sulphates and chlorides with hydroxide

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ions. The treated water will then be free of all harmful substances.

The resins which remove the positive elements are called 'Cation Exchange Resins' and those

that replace the negative elements are referred to as 'Anion Exchange Resins'.

The following illustrates the treatment process: -

This process is called 'Water Softening by Ion Exchange' as follows: -



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Despite the water treatment methods outlined above, the feed water to a boiler for high volume,

high pressure and temperature steam generation, must be processed further in order to

remove trace elements and dissolved gases.

The main water treatment here consists of the following:

i. Phosphate Removal:

The addition of sodium phosphate (or a similar phosphate) to boiler feed water; will convert

soluble calcium hardness salts into insoluble calcium phosphate. This is precipitated as a light

sludge which will not adhere to surfaces and is easily removed by 'Blowdown' of the boiler

'Mud-drum'. This process is carried out periodically to drain the sludge from the blowdown mud

drum.

ii. Oxygen Scavenging:

Oxygen in the boiler feed water will cause corrosion by oxidation of the metal. The oxygen is'Scavenged' from the system by the addition of sodium sulphite or hydrazine which will actually

remove the oxygen. Other substances like 'Tannin', will form a protective film over the metal

surfaces to exclude the corrosion effects.

iii. Boiler Feed Water 'pH' Factor:

The feed water needs to be maintained at a pH above the neutral value of 7.0. (Generally

about pH 8.5). This is to prevent corrosion due to acid gases like CO2 and SO2. The pH is

maintained by the careful addition of caustic soda (NaOH). If the pH is allowed to get too high,

embrittlement of the metal can result in damage due to vibration.

About the Author

Norrie is a retired professional who has been working in Oil and Gas and LNG production in

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FUEL FIRED FURNACES

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Marsa-el-Brega, Libya for 30 years.

Norrie used to be in the Training Dept. and prepared Programmes for Libyan Traine

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1857 oil gas fuel fired furnaces steam boilers water treatment

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