how to save energy and money
TRANSCRIPT
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BOILERS &
FURNACES
G u i d e B o o k 2
3E STRATEGY
STRATEGY
EFFICIENCY
ENERGY
EARNINGS
EUROPEAN COMMISSION
N e t h e r l a n ds M i n i s t e r y o f E c o n o m i c A f f a i r s
TSITechnical Services International
MY
IN GRE ER
A NL ES DAN
H
ow
to
save
energy
and
m
oney
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HOW TO SAVE
ENERGY AND MONEY
IN BOILERS AND FURNACE SYSTEMS
This booklet is part of the 3E strategy series. It provides advice on
practical ways of improving energy efficiency in boilers and furnace
systems.
Prepared for the European Commission DG TREN by:
The Energy Research Institute
Department of Mechanical Engineering
University of Cape Town
Rondebosch 7700
Cape Town
South Africa
www.eri.uct.ac.za
This project is funded by the European Commission and co-funded by
the Dutch Ministry of Economics, the South African Department of
Minerals and Energy and Technology Services International , with the
Chief contractor being ETSU.
Neither the European Commission, nor any person acting on behalf of
the commission, nor NOVEM, ETSU, ERI, nor any of the information
sources is responsible for the use of the information contained in this
publication
The views and judgements given in this publication do not necessarily
represent the views of the European Commission
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HOW TO SAVEENERGY AND MONEY
IN BOILERS AND FURNACESYSTEMS
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HOW TO SAVE
ENERGY AND MONEY
IN BOILERS AND FURNACE SYSTEMS
Other titles in the 3E strategy series:
HOW TO SAVE ENERGY AND MONEY:THE 3E STRATEGY
HOW TO SAVE ENERGY AND MONEY IN ELECTRICITY USE
HOW TO SAVE ENERGY AND MONEY IN STEAM SYSTEAMS
HOW TO SAVE ENERGY AND MONEY IN COMPRESSED AIR SYSTEMS
HOW TO SAVE ENERGY AND MONEY IN REFRIGERATION
HOW TO SAVE ENERGY AND MONEY IN INSULATION
Copies of these guides may be obtained from:
The Energy Research Institute
Department of Mechanical Engineering
University of Cape Town
Rondebosch 7700
Cape Town
South Africa
Tel No: (+27 21) 650 3892
Fax No: (+27 21) 686 4838
Email: [email protected]
Website: http://www.3e.uct.ac.za
ACKNOWLEDGEMENTS
The Energy Research Institute would like to acknowledge the following for their contribution in the production of
this guide:
Energy Technology Support Unit (ETSU), UK, for permission to use information from the Energy
Efficiency Best Practice series of handbooks.
Wilma Walden of Studio.com for graphic design work ([email protected]).
Doug Geddes of South African Breweries for the cover colour photography.
Canadian gov. See other guides.
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G u i d e B o o k E s s e n t i a l s :QUICK CHECK-LIST FOR SAVING ENERGY
and MONEY IN BOILERS AND FURNACE
SYSTEMS
This list is a selected summary of energy and cost savings opportunities outline in the text. Many
more are detailed in the body of the booklet.These are intended to be a quickchecklist.
BOILERS (CHAPTER 9)
Maintain efficient combustion.
Maintain good water treatment. Repair water and steam leaks.
Recover heat from flue gas and boiler blowdown whenever possible (see Steamguidebook).
Ensure good operational control and consider sequence control for multi-plantinstallations).
Attempt to match boilers to heat demand. Valve off idle boilers to reduce radiation
losses. Use flue dampers where appropriate to minimize flue losses when the plant is not firing. Ensure that boilers and heat distribution systems are adequately insulated.
Blowdown steam boilers only when necessary (see Steam guidebook). Ensure as much condensate as practicable is recovered from steam systems.
Insulate oil tanks and keep steam or electric heating to the minimum required.
FURNACES (CHAPTER 12)
Minimise heat losses from openings on sealed units such as doors. Use high efficiency insulating materials to reduce losses from the plant fabric.
Attempt to recover as much heat as possible from flue gases. The pre-heating of
combustion air or stock or its use in other services such as space heating is well worthconsidering.
Reduce stock residence time to a minimum to eliminate unnecessary holding periods. Ensure efficient combustion of fuels where applicable.
Avoid excessive pressure in controlled atmosphere units. If maintaining stock at high temperature for long periods, consider the use of specialized
holding furnaces. Make sure excessive cooling of furnace equipment
is not occurring. Ensure the minimum amount of stock supporting
equipment is used.
Ensure there is effective control over furnaceoperating parameters computerized controlshould be considered for larger units.
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Ta b l e o f C o n t e n t s1. INTRODUCTION ............................................................................................................................................................................................1
2. COMBUSTION ..................................................................................................................................................................................................1
2.1 Combustion air .........................................................................................................................................................................................1
2.1.1 Excess Air.....................................................................................................................................................................................4
2.1.2 Glue Gas Analysis....................................................................................................................................................................42.1.3 Determination of Excess Air ............................................................................................................................................5
2.2 Heat losses ..................................................................................................................................................................................................7
2.2.1 Heat loss due to incomplete combustion................................................................................................................8
3. HEAT TRANSFER ...........................................................................................................................................................................................10
3.1 Conduction ...............................................................................................................................................................................................10
3.2 Convection................................................................................................................................................................................................11
3.3 Radiation.....................................................................................................................................................................................................12
4.THE FUELS...................................................................................................................................................................13
4.1 Pipeline gas................................................................................................................................................................................................13
4.2 Liquid Petroleum Gas ........................................................................................................................................................................14
4.3 Fuel Oil ........................................................................................................................................................................................................14
4.4 Coal .........................................................................................................................................................................................................15
4.5 Choice of Fuel ........................................................................................................................................................................................16
5. COMBUSTION EQUIPMENT: OIL AND GAS BURNERS..............................................................................18
5.1 Gas Burners .............................................................................................................................................................................................18
5.2 Oil Burners ...............................................................................................................................................................................................18
5.2.1 Pressure Jet ..............................................................................................................................................................................18
5.2.2 Air or Steam Blast Atomiser............................................................................................................... 19
5.2.3 Rotary Cup ..............................................................................................................................................................................19
5.2.4 Low Excess Air Burners ...................................................................................................................................................19
5.3 Burner Controls ....................................................................................................................................................................................19
6. COMBUSTION EQUIPMENT: SOLID FUEL COMBUSTION....................... .......................... ......................21
6.1 Stokers .........................................................................................................................................................................................................21
6.2 Chain Grate Stoker .............................................................................................................................................................................21
6.3 Sprinkler Stoker.....................................................................................................................................................................................22
6.4 Fluidised Bed Combustion..............................................................................................................................................................22
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7. ENERGY SAVING EQUIPMENT ........................................................................................................................................................23
7.1 Flue gas heat exchangers ................................................................................................................................................................23
7.1.1 Economiser (Feedwater heater)..................................................................................................................................26
7.1.2 Recuperator (Air heater) ................................................................................................................................................26
7.2 Accumulators ..........................................................................................................................................................................................26
7.3 Insulation ....................................................................................................................................................................................................26
7.4 O2 Analysers ............................................................................................................................................................................................27
7.5 Variable speed fan drives ................................................................................................................................................................28
7.6 Flue gas dampers ..................................................................................................................................................................................28
7.7 Waste heat boilers ..............................................................................................................................................................................28
8. POLLUTION ....................................................................................................................................................................................................29
8.1 Environmental Equipment ..............................................................................................................................................................30
8.1.1 Ash Handling Equipment ................................................................................................................................................30
8.1.2 Air Pollution Control Equipment................................................................................................................................30
9. BOILERS ........................................................................................................................................................................................................31
9.1 Types of boilers......................................................................................................................................................................................31
9.1.1 Water Tube Boilers..............................................................................................................................................................32
9.1.2 Multi-Tubular Shell Boilers ..............................................................................................................................................34
9.1.3 Reverse Flame or Thimble Boilers..............................................................................................................................36
9.1.4 Steam generators ................................................................................................................................................................37
9.1.5 Sectional Boilers ....................................................................................................................................................................38
9.1.6 Condensing Boilers..............................................................................................................................................................39
9.1.7 Modular Boilers ....................................................................................................................................................................409.1.8 Composite Boilers ..............................................................................................................................................................41
9.2 Boiler system selection ....................................................................................................................................................................42
10. ENERGY AND COST SAVING FOR BOILERS ..............................................................................................43
10.1 Potential Losses ..............................................................................................................................................................................43
10.2 Boiler Energy Balance ................................................................................................................................................................43
10.3 Minimizing Boiler Losses ..........................................................................................................................................................44
10.3.1 Maintenance saving opportunities ..............................................................................................................................44
10.3.2 Blowdown Heat Loss ........................................................................................................................................................45
10.3.3 Heat Transfer ..........................................................................................................................................................................46
10.3.4 Excess Air Reduction..........................................................................................................................................................48
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10.3.5 Flue gas heat recovery ......................................................................................................................................................49
10.3.6 Combustion air pre-heat ................................................................................................................................................53
10.3.7 Load Scheduling ....................................................................................................................................................................54
10.3.8 On-Line Cleaning ................................................................................................................................................................56
10.3.9 Flue Shut-Off Dampers ....................................................................................................................................................56
10.3.10 Variable speed fan drives ................................................................................................................................................56
10.3.11 Integrated control ................................................................................................................................................................57
10.4 What to do first a quick checklist ................................................................................................................................58
10.4.1 Check list ..................................................................................................................................................................................58
11.TYPES OF FURNACES ............................................................................................................................................................................59
11.1 Batch Furnaces ................................................................................................................................................................................59
11.2 Continuous Furnaces ..................................................................................................................................................................59
11.3 Direct Fired Furnaces ................................................................................................................................................................60
11.4 Indirect Heated Furnaces ........................................................................................................................................................61
12. ENERGY AND COST SAVINGS FOR FURNACES ............................................................................................................62
12.1 Potential Losses ..............................................................................................................................................................................62
12.1.1 Furnace Energy Balance....................................................................................................................................................62
12.2 Minimizing Furnace Losses ......................................................................................................................................................63
12.2.1 Flue gas heat loss..................................................................................................................................................................63
12.2.2 Heat Loss to incomplete combustion......................................................................................................................66
12.2.3 Radiation Heat Loss............................................................................................................................................................66
12.2.4 Furnace pressure control ................................................................................................................................................67
12.2.5 Furnace efficiencies and Monitoring and targeting ..........................................................................................68
12.3 What to do firsta quick checklist ................................................................................................................................69
APPENDIX ........................................................................................................................................................................................................70
Conversion Tables ................................................................................................................................................................................................70
Boiler Efficiency Test ............................................................................................................................................................................................71
Furnace Efficiency Test ........................................................................................................................................................................................83
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1
This guide examines the energy savings potentials
for boilers and selected furnaces. The boiler
section starts with a description of different
boilers plant, combustion equipment used and
fuels available. Environmental impacts are
described, boilers selection processes outlined and
finally a list of measures and a strategy outline for
saving energy in boiler operation.
2. COMBUSTION
In all aspects of boilers and furnaces (including
dryers and kilns) heat is produced from
combustion or by the use of electrical energy.The
heat is transferred to the product or water toproduce stream in the case of a boiler.
The fuel (with the exception of electricity which
heats an element) burns in the combustion
chamber, which varies in shape and size
depending on the application. Common fuels
include pipeline gas, liquid petroleum gas, heavy
fuel oil, lighter oils and solid fuels such as biomass
or coal. If gas is produced on sitethis can also be
used.
The in the case of a furnace the product is then
exposed directly to the heat generated in the
combustion chamber, flue gas heat or a gas/fluid
that has been heated by the combustion process.
2.1 COMBUSTION AIR
Stoichiometric air represents the amount of air
required for complete combustion with the
perfect mixing of the fuel and air Stoichiometric air
is sometimes called theoretical air. If perfect mixing
is achieved, every molecule of fuel and air takes
part in the combustion process. Excess air must be
supplied to ensure complete combustion of thefuel because perfect mixing of fuel and air does
not occur. Percentage excess air is defined as the
The guide then moves on to savings in furnaces.
Various types of furnaces and energy saving
measures are described.The emphasis here is on
savings from excess air reduction, combustion air
preheat, correct insulation and furnace pressure
control.
1. INTRODUCTION
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total amount of combustion air supplied in excess
of the stoichiometric air, expressed as a
percentage of the stoichiometric air.
Total air = Stoichiometric air x (1 % Excess Airr)Total air = Stoichiometric air(x (1 +( 100 )The minimum amount of excess air required
varies with the fuel used and the efficiency of
mixing the air and fuel. If less than the minimum
quantity of air is supplied, some of the fuel will not
burn completely and there is a waste of fuel
energy. Evidence of incomplete combustion usually
shows up as carbon monoxide (CO) in the
products of combustion (flue gas). A continuous
gas analyser, or a manually operated Orsat, can be
used to check for CO in the flue gas.
Too much air also wastes energy. The gases leaving
the furnace are hot and contain heat energy. If
excessive amounts of air are supplied to the
furnace, the excess will also be heated.The effect
on heat losses by varying the amount of air
supplied to the furnace is shown in Figure 1.The
minimum losses occur when the amount of air
supplied is slightly greater than the
stoichiometric amount.
The weight or volume of each element or
compound in the fuel is required to determine the
stoichiometric air. It is often inconvenient to
determine stoichiometric air in this manner, as in
many instances the precise fuel analysis is
unknown or varies. A more convenient method
is to determine the quantity of air per unit of heat
in the fuel, i.e. kilograms of air per gigajoule of heat
in the fuel as fired (kg/GJ). Expressed in this
manner, the stoichiometric air required forcommon types of fuel is almost constant. Table 1
provides values for several different types of fuel,
which may be used in boilers or furnaces.
It may be suspected that a supply air fan, air inlet
louvers, ducting or the air flow control method is
inadequate. Knowledge of the required amount of
furnace combustion air enables checking the
adequacy of the air supply system.The combustion
air requirements can be calculated and compared
Figure 1: Zone of maximum combustion efficiency (Source:
Canadian Gov.) (Energy Management Series 7. Page 4. Figure 2)
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to the capacity of the components in the air
supply system.
Combustion air can be supplied to the equipment
by natural or forced draft systems. Natural draft
uses the negative pressure (draft) produced by the
furnace stack to draw combustion air into the
furnace and the resulting flue gases out of thefurnace.The most common example of this is the
ordinary domestic gas furnace. Natural draft is
usually applied only to small furnaces with less
than about one GJ/h heat input.
There are several disadvantages related to natural
draft firing. The amount of combustion air drawn
into the furnace cannot be controlled accurately
and the fuel and air mixing is inefficient.This means
that higher levels of excess air must be maintainedto ensure that complete combustion is achieved
under all conditions.The furnace pressure is always
negative which allows air to leak into the furnace,
and create additional flue gas volume and heat
losses.
Forced draft firing uses a fan to supply combustion
air to the equipment. Airflow is regulated by
means of dampers so that accurate control of the
proportion of air to fuel for various firing rates ispossible. A common method used to achieve this
is to operate the fuel valve and the damper with a
common mechanical linkage. Some form of
adjustable cam is used to vary the relative
positions of the fuel valve and damper to provide
proper fuel/air ratios at all firing rates.
The combustion air fan also provides bettermixing of the fuel and the air. The air is introduced
into the furnace around the burner(s) and vanes,
which produce a swirling motion in the air as it
enters the furnace, can create turbulence. A high-
pressure drop between the air supply and the
furnace is required to produce turbulence, and this
can only be achieved with a forced draft system.
These advantages mean that the excess air for a
forced draft system can be lower than for natural
draft firing, with resulting lower heat losses to theflue gas.
Forced draft firing permits a slightly positive
furnace pressure at all times. Leaks will then be
from the furnace outwards, which may lead to a
dangerous situation when a furnace door is
opened. Therefore, it is desirable to control
furnace pressure at a slight positive value of not
more than about 10 Pa.This is normally achieved
by regulating a damper in the breeching betweenthe furnace flue gas exit and the base of the stack.
Example: Combustion air requirements for a furnace using 700 l/h of Number 6 fuel oil, at 15 per centexcess air can be calculated. From Table 1, theoretical combustion air is 327 kg/GJ.The heating value of fuel
oil with 2.5 per cent sulphur is about 42.3 MJ/L (sulphur content can usually be obtained from the fuel
supplier).
Combustion air requirement = 700L/h x 42.3 MJ / L x 327 kg / GJ x 1.15Combustion air requirement =
Combustion air requirement = 1000 MJ / GJ
= 11135 kg/h
11135 kg/hor
1.204 kg / m3
= 9248 m3
/h at standard conditions.
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It may not be possible to maintain furnace
pressure as low as desired if heat recovery
equipment is installed in the flue gas system or if
the stack provides insufficient draft.
2.1.1 EXCESS AIR
The actual percentage of excess air supplied to
the furnace is one of the most informative items
of information to the furnace operator.The most
accurate way of determining this is to analyse the
flue gas leaving the furnace.
2.1.2 FLUE GAS ANALYSIS
A furnace in which heat is produced by the
combustion of fuel can be considered to have fuel
and combustion air as inputs, and flue gas as the
output (Figure 2). Practically all fuels used in
furnaces are hydrocarbons, which contain the
elements hydrogen and carbon. Although some
fuels contain other constituents they are not
usually important to the combustion process.The
hydrogen in the fuel burns to form water vapour,
and the carbon burns to form carbon dioxide
(CO2), or a mixture of carbon dioxide and carbon
monoxide (CO).Air contains nitrogen (N2) as well
as oxygen (O2).The N2 does not take part in the
combustion process, except for the formation of
small quantities of nitrogen oxides (NOx).
The major constituents of the products of
combustion are water vapour, CO2, CO, N2, and
any excess O2 left over from the combustion
process. Not all of the constituents will be present
in all instances. The presence of CO indicates
incomplete combustion.
Flue gas analysis can be determined by the use of
a continuous analyser or by periodic sampling.The
sample should be taken as close to the furnace
exit as possible to reduce air infiltration errors.
Some continuous analysers measure O2 content
and record or indicate the results. Other
continuous analysers measure the combustibles
content of the flue gas, which is mostly CO but
may also include some unburned fuel in gaseous
form. If a continuous flue gas analyser is not
available, a sample of the flue gas can be taken and
analysed with the use of an Orsat. The Orsat
determines the percentage by volume of O2, CO2,
and CO in the flue gas. The remaining gas is
assumed to be N2, plus a small quantity of water
Figure 2: Combustion process. (Source: Canadian Gov.) (Energy Management Series 7.
Page 6. Figure 3)
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vapour, which did not condense out of the sample.
There are other manually operated analysers
available, which measure either CO2 or O2 in the
flue gas.These are simpler to use and can be useful
as a cross check against an Orsat.
2.1.3 DETERMINATION OF EXCESSAIR
Flue gas analysis provides sufficient data to
calculate the excess air to the furnace. In most
furnaces, CO is absent or very low because of high
levels of excess air. For natural gas or fuel oil firing
with no CO in the flue gas, the per cent excess air
can be determined from Figure 3. If other fuels are
used or if CO is present, the following equation
can be used:
% Excess air = O2 0.5CO% Excess air = x 100% Excess air = 0.2682N2 (O2 0.5CO)
Where O2 = oxygen by volume in flue gas (%)
CO = carbon monoxide by volume (%)
N2 = nitrogen by volume (%)
Examples: The flue gas analysis by volume on a
furnace burning natural gas gives the following
results:
O2 = 9.8%
CO2 = 6.2%
CO = 0%
From Figure 3, excess air is approximately 79 per
cent. This number can be compared to the
following calculation.
%N2 = 100% - (9.8% + 6.2% + 0%)
= 84%
Figure 3:Excess air versus flue gas analysis. (Source: Canadian Gov.) (Energy Management Series 7.
Page 7. Figure 4)
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% Excess Air = 9.8(0.5 x 0)% Excess Air = x 100% Excess Air = (0.2682 x 84)[9.8(0.5 x 0)]
= 77%
This value is very high for a furnace burning
natural gas, and the possibility of reducing the
excess air level should be investigated.
Another example will provide greater familiarity
with the calculation procedures. A furnace is
burning coke-oven gas with the following flue gas
analysis.
O2 = 2.1%
CO2 = 10%
CO = 0%
N2 = 87.9% (by difference)
The equation should be used to calculate the
excess air since Figure 3 is not applicable for coke-
oven gas.
% Excess Air = 2.1(0.5 x 0)% Excess Air = x 100% Excess Air = (0.2682 x 87.9)[2.1(0.5 x 0)]
= 9.8%
This excess air is quite acceptable for a furnace
burning coke-oven gas.
In a furnace burning natural gas with a deficiency
of air, the flue gas analysis is as follows.
O2 = 0%
CO2 = 11%
CO = 2%
N2 = 87% (by difference)
Figure 3 cannot be used because of the presence
of CO.
% Excess Air = 01(0.5 x 2)% Excess Air = x 100% Excess Air = (0.2682 x 87)[0(0.5 x 2)]
= 4.1%
Table 1: Combustion Air Requirements
Fuel Stoichiometric Air Typical Excess Air Total Air kg/GJ As
kg/GJ As Fired (minimum as a %) Fired
Natural Gas 318 5 334
#2 Fuel Oil 323 10 355
#6 Fuel Oil 327 10 360
Coke-oven Gas 1 295 15 340
Refinery Gas 2 312 10 343
Propane 314 5 330
CO 12%
H2 42%
CH4 37%
C2H4 and higher 5%
CO2 Remainder
1
Analysis by volume CH4 31%
C2H6 20%
C3H8 38%
H2 5.6%
C4H10 and higher 1.0%
Inert Gases Remainder
2
Analysis by volume
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This means that approximately 4 per cent less
than the theoretical air required for complete
combustion is being supplied to the burners. If the
type of process permits it, increasing the
combustion air supply should reduce the carbon
monoxide.
Occasionally, CO occurs with high O2. This is
usually an indication of poor mixing of the fuel and
combustion air. Sometimes improvements can be
made by adjusting the burner air dampers to
create more turbulence where the fuel and air
mix. In other instances it may be necessary to
replace the burner assembly.
2.2 HEAT LOSSES
The heat discharged from the stack, is usually the
largest loss in a fuel fired boiler or furnace. Flue gas
analysis and flue gas temperature can be used to
calculate the loss. If there is no heat recoveryequipment on the furnace or boiler, these
measurements should be taken at the outlet to
minimize the possibility of the readings being
affected by air infiltration. With heat recovery
equipment the readings should be taken
immediately downstream of the equipment.
The flue gas heat loss has four components, which
can be calculated separately.
Dry gas heat loss.
Heat loss from the water vapour
contained in the combustion air1
.
Heat loss from the water vapour
produced by the combustion of the
hydrogen in the fuel2.
Heat loss from the water vapour
produced by the evaporation of moisture
in the fuel3
.
For natural gas and oil, the moisture in the fuel is
minimal, and the evaporation of the moisture heat
loss can be ignored.The values for flue gas losses
can be calculated using figures from the appendix,
which gives a boiler efficiency test. Figure 4 below
shows this graphically for fuel oil.
1 This is often very small and is a function of atmospheric humidity.2 This quantity is a function of the fuel and therefore cannot be changed by
operation. It is therefore not included in this discussion.3 As above this quantity is primarily a function of the fuel and therefore cannot
be changed by operation. It is therefore not included in this discussion.
Figure 4: Flue-gas loss for fuel oil. (Source: Canadian Gov.) (Energy Management Series 6.
Page 12. Figure 10)
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In practice this loss can vary from 8% to 50%
depending on the fuel. The major influencing
factors are the exit flue gas temperature and the
degree of excess air present.To minimize losses in
coal-fired plant, correct combustion is essential
including better fuel preparation, better stoking
practices and improved control of combustion air
both the undergrate and the overgrate draughts.
The same factors apply to oil-fired boilers. Fuel
preparation should be correct (uncontaminated
and at the right temperature),burners undamaged
and properly maintained, and combustion air
(both primary and secondary) should be
introduced at the right rate and with adequate
turbulence.
For fuels such as coal, biomass, and industrial waste
or municipal refuse, the heat loss from the
moisture in the fuel can be considerable. Wood,
for instance, could have a moisture content of up
to 60 per cent, depending on the source and
capability of the wood burning equipment. Figure
5 shows the variations in the moisture heat loss
for a typical biomass fuel having different moisture
contents at a flue gas temperature of 200 C. At
30 per cent moisture, this fuel heat loss is 5.5 per
cent of the fuel heat content. At 60 per cent
moisture, the loss increases to 21 per cent.
2.2.1 HEAT LOSS DUE TO
INCOMPLETE COMBUSTION
Heat can also be lost by the incomplete
combustion of fuel, this is indicated by the
presence of CO and, in the case of coal,
combustible material left in the ash.
2.2.1.1 HEAT LOSS TO CO
By controlling the amount of dark smoke
produced, the level of CO can be kept to a
practical minimum. The three influencing factors
are insufficient combustion air, inadequate fuel/air
mixing, or the ingress of cold air freezing the
combustion reaction. The heat loss, which is
measured in terms of the non-conversion of
Figure 5:Flue-gas loss with moisture content for biomass fuel. (Source: Canadian Gov.)
(Energy Management Series 6. Page 13. Figure 11)
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carbon into carbon dioxide, is relatively small, but
the rapid fouling of heat transfer surfaces under
these conditions adversely influences the boilers
performance.
2.2.1.2 HEAT LOSS TO
COMBUSTIBLES IN THE ASH
(COAL APPLIANCES)
This loss generally varies from 2% to 5%. It is a
clear indication of combustion air starvation for
which there are three possible causes: poor air
distribution under the grate: too thick a fire bed: or
uneven bed thickness resulting from poor stoking
practices.
The unburned combustibles heat loss is not
significant for properly operating oil and gas fired
installations, but it can be for solid fuel units. Figure
1 demonstrates that there could be a minor
unburned fuel loss at the maximum efficiency
point, but the real significance of this figure is that
the losses increase very rapidly as the total air is
decreased. The measure of this condition is
reflected by the presence of significant
combustibles in the flue gas.
In coal, biomass and other solid fuels, unburned
combustible material will be found in the refuse
collected in the ash pit and the fly ash hopper.The
loss should be determined when the boiler is
tested for efficiency.To do so requires a method of
collecting and weighing the refuse under
controlled conditions and laboratory testing the
refuse for its HHV. The loss can be calculated as
shown.
Unburned combustible heat loss = Dry refuse
quantity x Refuse heat content
Where units are:
Heat loss (MJ/kg fuel as-fired)
Dry refuse (kg of refuse/kg of as-fired fuel)
Refuse heat content (MJ/kg of refuse)
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The transfer of heat from the burner flame to the
product can be by conduction, convection, or
radiation, and in most instances a combination of
all three.
3.1 CONDUCTION
Heat transfer to the product by conduction is only
significant in indirect heated equipment, where the
product is isolated from the flame by a heat
exchange surface. Muffle furnaces and furnaces
using radiant tube heaters (Figure 6) are examples
of indirect heating arrangements. Heat conducted
through a solid can be calculated.
Q = k x A x T x 3.6Q =Q = t
Where, Q = Heat conducted (kJ/h)
k = Thermal conductivity of solid
[W/(mC)]
A = Surface area (m2
)
T = Mean temperature differen-
tial across solid (C)
T = Thickness of solid (m)
3.6 = Conversion factor from watts
to kilojoules per hour.
The foregoing equation shows that rate of heat
transfer increases in proportion to surface area,
and to temperature differential across the solid,
and is inversely proportional to material thickness.
3. HEAT TRANSFER
Figure 6: Radiant Tube Gas-Fired Rotary Furnace. (Source: Canadian Gov.)
(Energy Management Series 7. Page 13. Figure 7)
10
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Example: A muffle furnace has a 10 mm thick, high nickel steel enclosure with a surface area of 55 m2
.
Useful heat to the product, all of which is transmitted through the wall, is 1.9 GJ/h.The thermal conductivity
of high nickel steel is 31 W/(mC).The temperature drop through the muffle wall can be determined asfollows:
Heat Conducted = 31W / (mC) x 55m2
x DT x 3.6Heat Conducted =Heat Conducted = 0.01 m
Heat conducted is 1.9 GJ/h, or 1.9 x 106
kJ/h
Rearranging the equation,
T = 1.9 X 106
X 0.01T =T = 31 X 55 X 3.6
= 3.1C
The temperature drop across the enclosure is 3.1C at the specified rate of heat transfer.
surface increases, but not proportionally. The
following equation can be used for gases:
Q = 23.46 x A x T x V0.78
x d
Where, Q = Rate of convection heat transfer
(KJ/h)
A = Area of heat transfer (m2
)
T = Temperature differential between
solid and fluid (C)
V = Fluid velocity (m/s)
d = Gas density (kg/m3
)
3.2 CONVECTION
Heat transfer by convection takes place at the
boundary between a solid wall and a gas or liquid.
Intermingling takes place between the stagnant
layer of fluid at the wall and the moving fluid
stream next to the stagnant layer.Tests on rate of
heat transfer by convection show that the rate is
proportional to surface area and temperature
differential between the solid and the fluid. It also
increases as the velocity of the fluid over the wall
Example: A furnace is 3 metres long and has a 1 metre by 1 metre cross-section. Flue gas flows through
the furnace at an average velocity of 0.5 m/s with a gas temperature of 500C.The temperature differential
between the furnace walls and the flue gas averages 150C. For most practical purposes, the density of air
can be used for flue gas. From standard references, the density of air at 500C is 0.458 kg/m3
.The average
rate of heat transfer by convection to the walls, floor and roof can be determined as follows.
Furnace area swept by flue gas = (1 + 1 + 1 + 1) m x 3m
= 12 m2
Q = 23.46 x 12m2
x 150C x (0.5m/s)0.78
x 0.458kg/m3
= 11 263 kJ/h
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3.3 RADIATION
Heat transfer by radiation becomes significant for temperatures above 600C. A hot body emits
radiation in the form of heat, which can be
received by another solid body in the path of heat
radiation. In an electric furnace or boiler, the walls
or tank, which are heated by the electrodes, emit
heat radiation to the furnace contents.
The amount of heat radiated from a solid body is
proportional to the fourth power of its absolute
temperature, and directly proportional to itsemissivity. Absolute temperature is the number of
degrees above absolute zero and is measured in
Kelvin (K), which is equivalent to degrees Celsius
plus 273.
Emissivity is a measure of the heat radiated from
an object compared to that radiated from a similar
sized black body at the same temperature.The
maximum value of emissivity is that of the black
body; which is 1. Typical emissivity values forfurnace walls and oxidized steel are 0.8 to 0.9.
Because both the hot body, (the furnace wall) and
the cooler body, (the furnace contents) are
emitting radiation, the net total heat received by
the contents is the difference between the heat
emissions of the two bodies. The equation for a
furnace is:
Q = K x F x [( T14
( T24
]Q = K x F xQ = K x F x [(100) (100) ]
Where, Q = Rate of radiation heat transfer
(kJ/h)
K = Black body coefficient (20.6)
F = Overall radiation factor
depending on emissivity and
surface areas of the furnace
walls and contents
T1,T2 = Absolute temperature of hot
and colder bodies respec-
tively (K)
F = A1F =F = 1 + ( A1 ) ( 11)F = 1 + ( A1 ) ( 1 1F = e1 + ( A2)( e21)
Where,A1 = Surface area of furnace
contents exposed to walls(m
2
)
A2 = Surface area of furnace walls
(m2
)
e1
= Emissivity of furnace contents
e2
= Emissivity of furnace walls
Example: A furnace with a square cross section of 1 metre by 1 metre is heating carbon steel billets
100mm by 100mm.The furnace wall temperature is 1000C.The furnace floor does not radiate heat. From
Table 3, the emissivity of a fireclay brick furnace wall is 0.75, and the emissivity of oxidized carbon steel is
0.80.The heat input to the billet per metre of length when the steel is heated to 650C can be calculated.
A1 = (0.1 + 0.1 + 0.1) x 1
= 0.3m2
A2 = (1 + 1+1) x 1
= 3m2
F = 0.3F =F = 1 + ( 0.3 ) ( 11)F = 1 + ( A1 ) ( 1 1F = 0.8 + ( 3 )( 0.751)
= 0.234
T1 = 1000C + 273
= 1273K
T2 = 650C + 273= 923K
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Heat radiated/metre length
Q = K x F x [( 12734
(9234
]= 20.6 x 0.234 x
Q = K x F x
[(100
) (100
) ]= 91 604 kJ/h
Radiation also takes place from hot gases to the
furnace contents. This method of heat transfer
does not follow the same laws as the radiation
from solid bodies.Radiation from a luminous flame
is higher than from a clear flame of hot gases.
4. the fuels
Each conventional fuel differs from the others in
its combustion characteristics, and this influences
heat transfer. Fuels may be solid, liquid or gaseous,
and either commercial or waste. Commercial
fuels are fossil fuels, which are extracted,
treated/refined to varying degree and sold
nationwide by organizations such as oil companies.
Waste fuels are by-products or adjuncts of
processing or domestic activities and are,
obviously, only economically available locally.
Factors other than simple conversion to heat must
also be considered, including those relating to: the
storage and handling of the fuels, maintenance,
environmental impact etc. All of these influence
the overall efficiency and true cost of burning a
fuel.
4.1 PIPELINE GAS
Because gas mixes so readily with air and burns
without producing smoke and soot, boiler and
furnace maintenance costs are low. Natural gas
burners tend to be simpler with fewer mechanical
parts and are also therefore cheaper to maintain.
Natural gas would normally be the preferred fuel
for burning in boiler plant if convenience alone is
considered. It does not have to be stored; in
common with all the gaseous hydrocarbons it
mixes readily with combustion air to burn clearly;
and, ideally, the products of combustion are just
water and carbon dioxide. These basic arguments
would seem to carry a great deal of weight because
globally the majority of new boiler and furnace
installations in recent years have been gas tired.
The availability of an adequate gas supply at
individual sites needs to be checked in advance as
local constraints in the distribution system can
sometimes lead to delays in providing a
connection. A second factor is safety. Complying
with legislation regarding the supply and use of gas
involves some specialised equipment that has to
be maintained.
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Thirdly, burning gas does cause pollution. While
the pollutants do not include smoke or noxious
substances, they do include gases that contribute
to the so-called greenhouse effect. Gas, being
composed predominantly of methane, is in itself
one such gas. Carbon dioxide, which is produced
by the combustion of all fuels, is another: its
production is not only unavoidable but also
desirable as its presence indicates complete
combustion of the gas. However, pipeline gas also
produces oxides of nitrogen (NOx). This is
because the gas burns at high temperatures and
this provides the additional energy necessary to
make the oxygen and nitrogen in the air combine.
As regards the pricing of gas, the actual price that
a customer will pay, as for any fuel, depends on the
amount used and the type of supply, and can vary
over a wide range. Prices are generally competitive
with oil products, for example with gas oil for firm
gas supplies and with heavy fuel oil for
interruptible supplies. Continued plant operation
during interruptions of an interruptible supply
requires a boiler to be dual-fuel fired usually with
oil as an alternative. In firing these two fuels the
burner would normally be set to achieve the most
effective results on gas, because gas is used for
most of the year, with oil firing only on the few
days of interruption sometimes experienced.
4.2 LIQUID PETROLEUM GAS
Liquid Petroleum Gas (LPG) is used to describe
two fuels: propane and butane. In practice the vast
majority of installations use propane. All the
general comments about natural gas apply equally
to LPG.
One major difference between the two fuels is
that LPG requires both storage facilities and the
special precautions needed in relation to leakages.
The first can be very significant in terms of both
the capital cost of a project and its overall
operational and maintenance costs. The storage
tanks involved are pressure vessels and therefore
subject to both annual and long-term inspection
and testing. If a customer owns his own tanks he is
responsible for carrying out all inspections and
tests at his own expense. In practice, many
customers lease or rent the tanks from the fuel
suppliers, eliminating both this responsibility and
also that of general maintenance.
The second major difference is that LPG is heavier
than air. If natural gas, which is lighter than air,
escapes, all sources of ignition should be removed
and windows opened: it will then disperse
naturally. LPG, on the other hand, may find its way
down into pipe ducts, cable tunnels, drains, cellars
etc., and will not disperse unless forced to using a
fan. This characteristic influences the location of
storage tanks in relation to buildings, hollows,
drains, cellars etc. and plant location may be
affected.
4.3 FUEL OIL
Crude oil is a complex mixture of hydrocarbons.
The other fuel users mainly require the lighter
fuelspetrol, kerosene, diesel, oil, gas oil etc.This
end of the barrel also provides the main
feedstock requirement for the petrochemicals and
plastics industries. However, the primary
separation of oil provides mainly the heavier more
viscous fuel oils, which potentially cause problemsin storage , handling, combustion and
environmental pollution.The main advance of fuel
oil, on the other hand, derives from the fact that
these heavier fractions tend to be cheaper.
Problems relating to fuel oil storage include both
the capital cost of the storage tanks and the
problem of handling the oil. Fuel oils are viscous
liquids, which become thicker and more
intransigent the colder they become. Gas oil, the
lightest and least viscous of the fuels, will usually
remain in liquid form no matter how cold the
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winter. This either allows it to flow under gravity
from the tank to the burner or enables it to be
easily pumped. This holds true unless prolonged
periods of cold weather occur where the
temperature remains below freezing for a week or
more. Under these conditions, some of the waxes
contained in the oil begin to alter into sticky solids.
Typically, these solids build up on the filters in the
burner supply line, eventually blocking them.
Although this is an infrequent occurrence, some
exposed sites have installed electric trace heating
on the filters and/or the external distribution
pipework as a precaution.
The heavier grades of oil require heating in order
to remove them from the tank at all.To reduce the
amount of energy required for pumping the oil to
the burners, an appropriate pumping temperature
should be maintained.
Table 2 shows the recommended minimum
storage temperatures for the different grades of
oil and also the minimum temperatures for
optimising pumping costs.The temperatures given
in this table, especially for the heaviest oils are only
meant as an indication.With the exception of gas
oil, the general trend is for the heavier and more
viscous oil grades to require higher storage and
pumping temperatures.
The oil is heated either electrically or by taking
steam from the boiler, thereby reducing its overall
efficiency. The uncontrolled overheating of oil can
be very expensive, and uninsulated or poorly
insulated tanks or pipes are also a major waster of
energy.
Considerable energy is wasted if all the oil in a
tank is heated to the required pumping
temperature, and it is also bad practice to have
too much hot oil circulating and not being used by
the burners. A well designed hot oil ring main
circulates sufficient oil plus about 10% in order to
meet the maximum demand for all the burners it
serves. Fresh oil is drawn from the storage tank as
required, but the storage tank never forms part of
the basic circulation system thereby allowing all
the oil to heat up to the pumping temperature.
This ensures that both the size and the capital and
running costs of the oil heaters are kept to a
practical minimum.
The penalty of this oil heating requirement is that
it is uneconomic to use these heavier grades of
fuel oil on small boiler plant. Below 3 MW heavy
oil would be inefficient and, for bunker oil, 20 MW
is probably the lower limit. However, the market
price for the heavier fuel oils over recent years has
encouraged their greater use.
Provided that a grade of fuel oil is delivered to the
burner in good condition and at the correct
temperature for the burner, the production of
smoke or carbon monoxide should be minimal.
Table 2: Recommended Minimum Storage Temperatures for Different Grades of Oil
Fuel Oil Grade Viscosity Minimum Storage Typical Pumping
Type * *Cst @ 100C Temperature C Temperature C
Gas/Oil D 1.0 None stated None stated
Light E 8.2 10 10-12
Medium F 20.0 25 30-35
Heavy G 40.0 40 55-60
Bunker H 56.0 45 70
* Refers to BS 2869 - 1986.
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The fact that all fuel oils contain some sulphur
means that sulphur oxides (SOx) are produced
during combustion. Such gases are now
considered to contribute to the global pollution
problem. Oil , however, burns at a lower
temperature than the gaseous fuels and therefore
produces less NOx gases.
4.4 COAL
The clean burning of solid fuels presents a
problem because the air required for combustionis less readily available to the mass of fuel,
compared with atomised liquid fuels and gas.As a
result, coal burning has been responsible for most
of the traditional forms of air pollution smoke,
soot, grit and dust. Modern coal plant using
microprocessor control, on boilers with improved
stoker design, has eliminated this problem.
Stringent control of SOx and particulates can be
achieved through the use of limestone injection,
cyclones and bag filters.
Throughout the sub-tropical and temperate
regions of the world coal deposits are generally
significantly larger than crude oil or natural gas
deposits. As crude oil prices have risen, many oil-
importing countries with significant coal deposits
have undertaken considerable research into coal
burning and, in some cases, have implemented
policy decisions promoting the use of coal for
boiler firing.
Coal is the cheapest of the available conventional
fuels. Furthermore, coal prices tend to be more
stable than prices for other fuels, and long-term
price contracts with only moderate built-in
increases are available.
A coal-fired plant does, however, incur higher
capital and operating costs.As well as the boiler or
furnace plant itself, the capital cost incurred
includes bunkerage, coal handling equipment, and
facilities for ash removal, handling and storage.
Operational costs are high because, despite
considerable development efforts by plant
manufacturers to reduce the labour component, it
is rare that coal fired plants are ever fully
automated and unmanned.
Maintenance costs are also significantly higher than
for the other fossil fuel.The difficulty of achieving
clean combustion means that the boilers require
more frequent cleaning. Both the fuel and the ash
are very hard and abrasive so levels of wear and
tear on coal and ash handling equipment are high.
The disposal of ash in a manner that avoids
pollution is a significant operational component
and, in some regions of the country,can be a costly
business.
Low combustion temperatures limit pollution
from NOx, but the SOx released by coal
combustion must be considered. Both the calorific
value and the sulphur content of coal vary from
source to source.The average South African coal
sold into the industrial market has low sulphur
content and is less polluting than the heavier fuel
oils.
4.5 CHOICE OF FUEL
The choice of fuel is not a simple matter. It involves
balancing a number of factors including the capital
cost of the plant, the price of the fuel, and
operating and maintenance costs. Some
consideration should also be given to likely future
changes in fuel and pricing policies and to
pollution control legislation.
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Table 5 summarises those advantages and
disadvantages that can be estimated and quantified
for each fuel.
Table 4: Calorific values of Some Fuels
Fuel Calorific ValueMJ/Unit
GasNatural Gas 38.0/cu m
LPG Propane 50.0/kg
LPG Butane 49.3/kg
Fuel Oil
Gas Oil 38.0/liter
Heavy Oil 41.0/litre
Coal 29.0/kg
Table 5:The pros and cons of various fuels.
COAL FUEL OIL NATURAL GAS LPGDisadvantages Disadvantages Disadvantages Disadvantages
Capital Capital Cost For: Capital Cost For:Cost For:
Tanks Storage Tank (or Bunkerage leased)
InsulationFuel Handling
Heavy Fuel OilAsh Handling
Running Cost For: Running Cost For: Running Cost For:
Tank Heating Fuel (Especially for Small Fuel Cost Installations)
Heavy Fuel Oil Interrupt TariffHeavy Oil as Second Fuel
Maintenance Maintenance Costs Maintanance Costs For: Maintenance CostsCosts For: For: For:
Safety Equipment Safety Equipment Wear from Abrasive Boiler/FurnaceFuel & Ash Cleaning
Boiler Cleaning Burners
Environmental Costs: Environmental Environmental Costs: EnvironmentalCosts: Cost:
Smoke Emission High NOxSmoke Emission High NOx
Grit & Dust EmissionSulphur Emission
Sulphur Emission
Clean up Heavy Fuel Oil
Ash Disposal Cost Higher NO x
Advantages
Advantages
Advantages
Advantages
LowC
ost
CheaperThanGas
NoStorage
NoSulphur
NoSulphur
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In order to ensure the proper mixing of fuels with
combustion air and the correct flame shape, for
maximum heat transfer from the flame to the
water/steam or heated product, specialized
equipment is used. The type of equipment is
dependent on the furnace/boiler conditions and
the fuel or fuels of choice. (Boilers and furnacescan be set up to fire more than one fuel.)
5.1 GAS BURNERS
Apart from the safety requirements in their
design, gas burners are essentially simple. Very
small boilers use a simple atmospheric burner,which entrains its combustion air from its
surroundings. However, as the air and gas are not
forced to mix, surplus air is required to ensure
complete combustion. This surplus is heated and
then passes out via the flue, thereby reducing
boiler efficiency.
A larger boiler with a fully enclosed combustion
chamber needs a burner that will force the air and
gas to mix thereby controlling the length andshape of the flame.The quantity of combustion air
can be precisely controlled to maximise
combustion efficiency.
Natural gas mixes readily with air.The ring-type gas
burner consists of a circular barrel ringed with
multiple outlet ports. The spud type burner
consists of a ring of 4 to 8 single barrels, each with
a widened end containing multiple outlet ports. In
either case the register surrounds the barrels with
air.
Many boilers are equipped with combination
natural gas and oil burners with the second fuel
used as back up for the prime fuel.
5.2 OIL BURNERS
Oil burners are more complicated because the
fuel has to be in the right condition for clean and
rapid combustion. This entails atomising the oil
into small droplets of the correct size, which can
only be done if the oil is at the right temperature
and therefore the right viscosity. At too low a
temperature the droplets are too big: combustion
is poor and produces soot and smoke.At too high
a temperature the droplets can be too small,passing through the flame too rapidly to burn. In
neither case is the full energy content of the fuel
being used: furthermore, the heat transfer surfaces
become fouled.
Oil burners are of three basic types.The simplest
and most widely used is the pressure jet where
the oil is pumped at pressure through a nozzle.
The air or steam blast type uses gas pressure to
shatter the oil into droplets, while the Rotary Cupuses centrifugal force to break the oil up. Each
type of burner has its benefits and disadvantages.
5.2.1 PRESSURE JET
Advantages:
Very simple in construction and cheap to
replace.
Comes in many sizes to suit most
applications.
5. combustion equipment: oil and
gas burners
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Can produce all flame shapes from long
and thin to short and fat so can fit all
types of boiler or furnace combustion
chamber.
Disadvantages:
Prone to clogging by dirty oil so needs
fine filtration.
Limited turndown ratio of only 2:1.
Easily damaged during cleaning.
Highest oil pre-heat temperature requi-
red for atomisation.
5.2.2 AIR OR STEAM BLAST
ATOMISER
Advantages:
Very robust in construction.
Good turndown ratio of 4:1.
Good control of the combustion air/fuel
over the whole firing range.
Good combustion of the heavier fuel oils.
Disadvantages:
Energy used either as compressed air or
as steam for atomisation.
5.2.3 ROTARY CUP
Advantages:
Good turndown ratio of better than 4:1.
Good atomisation of heavy fuel oils.
Lowest oil pre-heat temperature required
for atomisation.
Disadvantages:
Most complex and costly to maintain.
Electrical consumption required for the
cup drive.
Oil and gas burners produced or sold in this
country have to meet statutory safety and
emission standards.
5.2.4 LOW EXCESS AIR BURNERS
Standard natural gas and oil burners operate at 10
to 15 per cent excess air at full capacity and higher
excess values at lower firing rates.The increasing
excess air with decreasing firing rate phenomenon
results from burner registers, which are fixed at
settings that provide best results at full capacity.
Low excess air burners permit operation at 2 to 5per cent excess air. A reduction of excess air from
15 to 5 per cent would reduce fuel costs by
almost 1 per cent.These savings result from higher
cost features as follows:
Better design of the air diffusers, air
register, and burner, which achieve better
mixing and combustion.
Burner registers which are modulated
with the tiring rate to provide bettercombustion at firing rates below 100 per
cent.
5.3 BURNER CONTROLS
In conjunction with the choice of burner type,
consideration must be given to the control system
required. The simplest ON/OFF control means
either that the burner is firing at full rate or that it
is off.The major disadvantage with this method of
control is that the boiler is subject to large and
often frequent thermal shocks every time the
boiler tires. Its use is therefore limited to small
boilers with an output up to 300 kW.
Slightly more complex is the HIGH/LOW/OFF
system where the burner has two firing rates.The
burner operates first at the lower tiring rate and
then switches to full firing as needed, thereby
overcoming the worst of the thermal shock.The
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burner can also revert to the low-fire position at
reduced loads, again limiting thermal stresses
within the boiler. Typically this type of system is
fitted to boilers with an output of up to 3.5 MW.
A modulating burner control will alter the firing rate
to match the boiler load over the whole turndown
ratio. Every time a burner shuts down and restarts,
the system must be purged by blowing cold air
through the boiler passages: this wastes energy and
reduces efficiency. Full modulation, however, means
that the boiler keeps firing, and fuel and air are
carefully matched over the whole firing range to
maximise thermal efficiency and minimise thermal
stresses.Typically this type of control can be fitted to
boilers above 1 MW.
In matching a burner and a control system to a
boiler three factors must be taken into
consideration.
The maximum output of the plant:
Whether the load is steady or fluctuating:
The fuel being used.
An ON/OFF control, for instance, is not suitable
for heavy fuel oil
The basic choices as they relate to oil burners are
summarised in Figure 7. There is always some
overlap between burner types and control system
types but the preferred combinations are outlined.
Figure 7:Type of fuel oil with recommended burners and controls. (Source: ETSU)
(Good Practice Guide 30.Page 67. Figure 38.)
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Because carbon burns fairly slowly and coal needs
to be in the combustion chamber for a relatively
long period for the air to reach it and cause
complete combustion, many forms of stoker (for
transferring coal to the grate) have been
developed. Some have experienced periods of
popularity and have now declined, while othershave stood the test of time.
Coals from different pits or washeries can have
very different combustion properties.
Furthermore, coals from the same pit that have
been stocked for long periods are very different
from newly mined coal. As a result a boiler
combustion system must be regularly adjusted to
maximise energy conversion. In the following
section only those types of stoker that would befitted to a boiler with an output of 1.5 MW and
above are considered. Below this level there is
limited choice: each boiler comes with its own
proprietary form of stoker, screw feeding the coal
either onto the top of the fire or pushing it up
from below.
Three basic types of stoking system are commonly
used with the larger boilers - two of them
traditional designs and one a relatively modern
development.
6.1 STOKERS
Stokers are mechanical devices that burn solid fuel
in a bed at the bottom of a combustion chamber.
They are designed to permit continuous or
intermittent fuel feed, fuel ignition, adequate
supply of combustion air, release of gaseous
products, and disposal of ash.
Stokers are classified according to the manner in
which the fuel reaches the fuel bed. In an underfed
stoker, the fuel and air enter the burning zone
from beneath the bed. Overfed stokers have the
fuel entering the combustion zone from above, in
the opposite direction to the airflow. The
spreader-type overfeed stoker delivers fuel so thata portion burns in suspension while the remainder
falls and burns on the moving grate.
6.2 CHAIN GRATE STOKER
The chain grate stoker has for many years been
the most widely used method for firing coal on
medium sized industrial and commercial boilers,
even though it is relatively expensive to buy,
operate and maintain. To reduce operating costs
equipment manufacturers are working to develop
a fully automatic system requiring little or no
intervention from trained operators.
The coal is fed onto one end of a moving steel
belt. As the belt moves along the length of the
furnace, the coal burns before dropping off the
end as ash. Some degree of skill is required,
particularly when setting up the grate, air dampers
and baffles, to ensure clean combustion leaving the
minimum of unburnt carbon in the ash and to
achieve maximum heat transfer in the furnace
chamber.
This type of stoker will only operate effectively
using certain types and qualities of coal. Coal must
be uniform in size, as large lumps will not burn out
completely by the time they reach the cod of the
grate. Furthermore, small pieces or fines may
block the air passages in the grate and make it
6. combustion equipment:
solid fuel combustion
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more difficult for combustion air to reach the coal.
The grate also relies on having a layer of ash on
top of it to protect it from the highest
temperatures of the burning coal, so using coals
with a very low ash content will result in rapid
grate damage.
6.3 SPRINKLER STOKER
The sprinkler stoker is an original mechanical
stoker system,which has been brought up to date.
The principle is to spread fresh coal on top of an
already, burning firebed.Once the system has been
set up to spread this coal evenly it is simple to
operate and has many fewer mechanical parts to
maintain than the chain grate stoker.
Many units of this type have been manufactured
with control systems very similar to those for gas
or oil-fired boilers. Fuel feed rate and combustion
air are adjusted in parallel to give a turndown ratio
of 3:1.The chain crate stoker can also achieve this
but the sprinkler can be regulated much more
quickly.
This type of stoker was popular initially because it
was very much cheaper than the chain grate
equivalent. Its main drawback was that it had to be
de-ashed by hand. Effort has been put into
developing an automatic de-ashing system but,
obviously, this has considerably eroded the
sprinkler stokers price advantage.
Like the chain grate stoker, this type of stoker is
selective with regard to fuel size. Fines in the coal
are picked up by the combustion air and flue gases
and carried through the boiler. This can cause
considerable erosion within the boiler and result
in high grit emissions from the stack.
6.4 FLUIDISED BED
COMBUSTION
Fluidised bed combustion is the most recent coal-
burning technology, the fuel being fed onto a hot,
air-agitated bed of refractory sand.This system has
two main advantages:
1. It is much less selective in terms of fuel quality
and can burn not only very poor coal with ahigh ash content but even industrial or
commercial waste.
2. The lower combustion temperature involved
allows cheaper materials and refractories to be
used in its construction.
However, this technology is still new and is in the
experimental stage in South Africa.
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A short description of common equipment used
for saving energy in boilers and furnaces follow. In
some cases these are discussed further under the
energy savings sections of either boilers or
furnaces.
7.1 FLUE GAS HEAT
EXCHANGERS
Since most of the heat losses from a fuel fired
furnace appear as heat in the flue gas, the recovery
of this heat can result in substantial energy savings.
A common method is to install a heat exchanger
at the furnace exit.
A heat exchanger can be used to transfer heat
from the hot flue gas to the incoming combustion
air, or to the heat water used elsewhere in the
plant.The rate of heat transfer is proportional to
the surface area of the heat exchanger, and to the
mean temperature differential between the flue
gas and the combustion air.
Q = U x A x LMTD x 3.6
Where, Q = Rate of heat transfer (kJ/h)
U = Heat transfer coefficient of
heat exchanger [W/(m2
C)]
A = Surface area of heat ex-
changer (m2
)
LMTD = Logarithmic mean tempe-
rature difference (C)
3.6 = Conversion factor from
watts to kilojoules per hour
LMTD = T1T2LMTD =LMTD = T1
LMTD = LnLMTD =
(T2
) Where, LMTD = Log mean temperature dif-ference (C)
T1 = Greater temperature differ-
ence between the flue gas
and the heated air or water
(C)
T2 = Lesser temperature differ-
ence between the flue gas
and the air or water (C)
Ln is the natural logarithm
A heat exchanger may be used to heat water with
the heat from flue gases. An important design
consideration is how close the heated water
temperature should be to the temperature of the
hot gas entering the exchanger. It is not possible to
heat the fluid to a temperature above the
temperature of the hot gas entering, regardless of
the relative fluid and hot gas flows. Small
temperature differentials imply large heat
exchanger surfaces. This is illustrated by the
following example.
7. energy saving equipment
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Figure 8:Tempering Air Heat Exchanger. (Source: Canadian Gov.)
(Energy Management Series 7.Page 18. Figure 11.)
Example of savings
A heat exchanger is to be added to a dryer which is exhausting 450 000 m3
/h of moist air at 100C.The
exhausted air is used to heat up 350 000 m3
/h of incoming air from an ambient temperature of 10C to
85C, which is within 15C of the hot exhausted air (Figure 8). The heat exchanger design has a heat
transfer coefficient quoted by the manufacturer of 28 W/(m2
C). Heat given up by the exhausted air is
equal to the heat gained by the incoming air, since there are no significant heat losses in a heat exchanger
of this type. Density of air at standard conditions is 1.204 kg/m3
, and specific heat is 1.006 kJ/(kgC).The
surface area of the heat exchanger required can be calculated as follows:
Cold air heat gain (Q) = Volumetric flow x Density x Specific heat x Temperature rise
= 350 000 m3
/h x 1.204 kg/m3
x 1.006 kJ/(kgC) x (85-10)C.
= 31.79 x 106
kJ/h
Exhaust air heat loss = Volumetric flow x Density x Specific heat x Temperature drop
= 450 000 x 1.204 x 1.006 x (100CTout) kJ/h
Cold air heat gain = Exhaust air heat loss
This can be expressed as:
31.79 x 106
= 450 000 x 1.204 x 1.006 x (100CTout) kJ/h
Rearranging the equation:
(100C - Tout) = 31.79 x 106
(100C - Tout) =
(100C - Tout) = 450 000 x 1.204 x 1.006= 58.3C
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Heat exchanger exhaust temperature,Tout = 100C58.3C = 41.7C
Maximum temperature differential, T1 = 41.7C10C
= 31.7C
Minimum temperature differential, T2 = 100C85C = 15C
The logarithmic temperature difference (LMTD) is:
LMTD = 31.7 C15 CLMTD =LMTD = 31.7 CLMTD = InLMTD = ( 15 C )
= 22.3C
Cold air heat gain (Q) = 31.79 x 106
kJ/h = 28 W/(m2
C) x A x 22.3C x 3.6 kJ/Wh
Surface area, A = 31.79 x 106
Surface area, A =Surface area, A = 28 x 22.3 x 3.6
= 14 142m2
If the cold air is heated to within 5C of the exhausted moist air instead of 15C, the size of the heat
exchanger required in increased considerably.The calculations are as follows:
Temperature of heated air = 100C5C
= 95C
Cold air heat gain = 350 000 m3
/h x 1.204 kg.m3
x 1.006 kJ/(kgC) x (9510)C
= 36.03 X 106
kJ/h
(100CTout) = 36.03 x 106
(100CTout) =(100CTout) = 450 000 x 1.204 x 1.006
= 66.1C
Tout = 100C66.1C
= 33.9C
T1 = 33.9C10C
= 23.9CT2 = 100C95C
= 5C
LMTD = 23.9 C5 CLMTD =LMTD = 23.9 CLMTD = InLMTD = ( 5 C )
= 12.1C
Surface Area (A) = 36.03 x 106
Surface Area (A) =Surface Area (A) = 28 x 12.1 x 3.6
= 29 541 m2
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7.1.1 ECONOMISER (FEEDWATERHEATER)
This is applicable mostly to boilers, and is an
option used for heating incoming boiler water by
cooling the flue gases. The equipment is a gas-
liquid heat exchanger. Care must be taken not to
allow the flue gases to cool below the sulphur
dew point. Economizers can be considered where
hot water is required. For furnaces, if the use ofhot water and the operation of the furnace do not
always occur simultaneously, it may be practical to
install an insulated hot water storage tank. This
would level out the effect of variations in the hot
water supply and demand.
7.1.2 RECUPERATOR (AIR HEATER)
In a recuperator air entering the combustion
chamber is preheated using the heat of the hot
exhaust flue. This is an important measure for
furnaces where preheating the feed with flue gases
is more difficult that for boilers.The hot gas passes
inside tubes arranged in bundles.The combustion
air is directed over the outside of the tubes by
means of a series of baffle plates. Combustion air
pre-heat has always been regarded as the poor
cousin of the economizer for boilers because air
pre-heaters are large and less efficient than a gas-
liquid heat exchanger - or economizer - used toheat boiler feed water.
7.2 ACCUMULATORS
Boilers produce steam to meet demand. When
spikes in this demand occur, or the load is uneven,
it is often the case that an extra boiler would have
to be used intermittently, or output of severalboilers would rise to meet this demand. In the first
case this can be inefficient due to losses associated
with the heating and cooling of the boiler shell. In
both cases, some of the required boiler capacity
(and running and capital outlay) could have been
avoided by using an accumulator.
An accumulator effectivelystores oraccumulates
steam from boilers during times of low demand
and then can release it during short high demandintervals.
7.3 INSULATION
Insulation is used to retain heat within the furnace
or boiler enclosure. Common insulation materials
include calcium silicate, mineral fibre, ceramic fibre,
cements, cellular glass and glass fibre.An indication
of the heat loss from the hot walls of a furnace or
boiler is given in figure 9.
It should be noted that the reduction in the temperature differential to 5C would require the heat
exchanger area to be slightly more than doubled. An increase in design temperature rise of the incoming
air from (85C10 0C) = 750C to (95C10C) = 85C results in an increase in heat recovery of
(85 C75 C)(85 C75 C) x 100 = 13%
75 C
A careful analysis of capital costs and savings in fuel costs for different possible heat exchanger sizes is
important.
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A significant development in this field, for furnaces,
has been the use of ceramic fibre insulation, which
is a better insulator than solid refractory material
and also requires less heat to reach the operating
temperature. The disadvantages are higher initial
cost and low resistance to physical damage.A layer
of refractory on the bottom of the furnace and
other areas subject to damage is normally used to
protect the ceramic fibre. Further layers ofceramic fibre insulation can be installed on the
outside of the refractory as required.
7.4 O2 ANALYSERS
Systems for checking the O2 or CO2 content of a
boiler flue gas have been available for a long time
but, historically, none have been sufficiently reliable
to be incorporated in an automatic control
strategy. Portable or permanently installed O2 or
CO2 monitoring equipment used by a well trained
and intelligent boiler operator is still the best
method of limiting excess air and hence increasing
efficiency.
The production of the zirconium cell for O2
detection has made available a reliable measuring
system, and this has resulted in the development
of various systems,which automatically control theamount of excess air, thereby overcoming
variations in the fuel and air parameters. Using
these oxygen detection feedback controllers,
usually termed oxygen trim control, allows much
lower excess air levels to be achieved throughout
the operating range.
The simplest systems use the feedback signal