Aki Valkeapää / Martti Veuro
Jarmo Tuunanen
HEATING SYSTEMS
Building services
Part 12: Fuels and burning
2014
MIKKELIN AMMATTIKORKEAKOULU
Mikkeli University of Applied Sciences
MAMK
Fuels and burning
TABLE OF CONTENT
1
2
3
4
5
6
Fuels ............................................................................................................................................. 3
Combustion reactions ................................................................................................................... 7
2.1 Oxygen and air demand in combustion ................................................................................. 9
2.2 The amount of flue gases..................................................................................................... 11
2.3 Combustion with excess amount of air ............................................................................... 12
2.4 The determination of n excess air factor from the flue gases ............................................. 13
2.4.1 By measuring the CO2 –content of flue gases.............................................................. 13
2.4.2 The determination of n by measuring the oxygen (O2 ) content in the flue gases ....... 14
2.5 The mass flow and volume flow of gases ........................................................................... 17
Net calorific value of fuels ......................................................................................................... 18
Carbon dioxide emission of fuels............................................................................................... 19
Efficiency and consumption of fuel ........................................................................................... 20
5.1 Combustion efficiency ηp .................................................................................................... 20
5.2 Boiler efficiency ηk.............................................................................................................. 20
5.3 Efficiency of the boiler plant ηkl.......................................................................................... 20
Real (actual) combustion air flow rate qvi and flue gas flow rate qvs ....................................... 21
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1
Fuels and burning
Fuels
Liquid fuels
-light oil
-heavy oil
-bio oils
Gaseous fuels
-natural gas
-liquid gas
-bio gas (methane, CO2, water vapour: main components in bio gas)
Solid fuels
-coal, coke, anthracite
-peat; sod peat, milled peat, peat briquette, peat pellet
-wood in different forms
wood chips
log wood
briquette
pellet
others
-REF (recovered fuel, sorted waste etc.) = Recycled fuels (REF)are manufactured from sorted, nonreusable, burnable materials, which include wood, paper, cardboard, or burnable plastic.
Black liquor (residue of cellulose production used as fuel).
Fuels contain as a burning substance / element carbon C and hydrogen H and sometimes small
amounts of sulphur S.
Other elements of fuels hardly take part into the burning process.
Fig. The classification of fuels
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Fuels and burning
Fig. Storage of wood chips in a 1 MW boiler plant
The principles of analysing fuels
(Source / referred: Vapo Oy; Local Fuels, Properties, Classifications and Environmental impacts)
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Fuels and burning
Fig. Local renewable fuel, chopped wood
Chemical content of different fuels (Vapo Oy: Local Fuels, Properties, Classifications and Environmental impacts)
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Fuels and burning
Calorific value of fuels (heat / energy released by burning process) per mass unit or per volume unit
depends on the proportional shares of different elements in fuel and also on the content of oxygen O
and water H2O.
Calorific value is given as MJ/kg,pa (pa=fuel) or kWh/kg,pa for solid fuels and for gaseous fuels
MJ/Nm3 or kWh/Nm3 .
In fossil fuels there is only a small part or not at all oxygen and only a little bit water. “Younger”
fuels contain remarkable amounts of oxygen and water.
Oxygen reduces the need of combustion air. Water reduces the net calorific value of the fuel because the heat / energy of evaporation of water is taken from the energy released in combustion
process.
The non-burning compounds of fuel comprise / make up the ash of the fuel. Ash increases the solid
particles in flue gases. Ash can also weaken the heat transfer in the furnace and convection part of
the boiler. Ash must be removed regularly by sooting the boiler.
Part of the nitrogen of the fuel forms / generates harmful nitrogen dioxides (NOx) in the combustion
process especially when the temperature of burning is high.
Chemical composition of different fuels (ref. Vapo Oy: Local Fuels, Properties, Classifications and
Environmental impacts)
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Fuels and burning
Natural gas and biogas properties
fuel
net calorific value
moisture
net calorific value
as received
bulk density
3
4 – 8 MWh/1000 m
10 MWh/1000 m3 (0 °C)
2,77 – 3,6 kWh/kg
biogas
natural gas
black liquor
0,732 t/1000 m3
0
energy density
ash
content
-
0
The combustion process needs air, burning uses the oxygen of air to the combustion reactions. The
combustion process is an exothermic reaction.
Burning needs more air than theoretically (excess air) . This is because part of the air do not participate in the combustion process. Without excess air the combustion is not complete. Ratio of excess
air is so more than 1 (one).
The combustion process produces flue gases. The flue gases contain gaseous substances (chemical
compounds) produced in the combustion. The flue gases also contain excess air (mostly nitrogen
and oxygen, small amounts of other gases and also water vapour from the combustion and from the
water content of the fuel. When hydrogen burns the reaction produces water vapour.
NTP = gas in normal temp. and pressure (normal temperature and pressure, t=273,14 K and
p=101,325 kPa)
2
Combustion reactions
The elements in the fuel burn according to the following combustion (chemical) reaction formulas:
C (carbon)
C
O2
CO2
1 kmol of carbon = 12.01 kg + 1 kmol oxygen = 22,39 Nm3 → 1 kmol carbon dioxide = 22,26 Nm3
H (hydrogen)
2H 2
O2
2 H 2O
2 kmol of hydrogen = 2 x 2,015 kg + 1 kmol oxygen = 22,39 Nm3 → 2 kmol water vapour = 44,80
Nm3
( 4H
O2
H 2O hydrogen in fuel in not actually in form of molecule of two atoms)
S (sulphur)
S
O2
SO2
1 kmol of sulphur = 32,00 kg + 1 kmol oxygen = 22,39 Nm3 → 1 kmol sulphur dioxide = 21,89
Nm3
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Fuels and burning
The oxygen, nitrogen and water of the fuel change into gaseous form (state).
Oxygen
O2
O2 1 kmol oxygen 32,00 kg = 1 kmol oxygen 22,39 Nm3
Nitrogen
N2
N 2 1 kmol nitrogen 28,06 kg = 1 kmol nitrogen 22,40 Nm3
Water
H 2O
H 2O 1 kmol water (liquid) 18,02 kg = 1 kmol water vapour 22,40 Nm3
The oxygen in the fuel reduces the need of air in the combustion process. The moisture (water) content affects to the net calorific value of the fuel. The more water in the fuel the less is the calorific
value of the fuel.
Fig. A burner (atmospheric) for burning natural gas in a boiler. Air for burning is got by the draught
caused by chimney effect (density difference). (Maakaasukäsikirja, p..72)
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2.1
Fuels and burning
Oxygen and air demand in combustion
When 1 kg of fuel containing carbon, hydrogen and sulphur burns the theoretical demand of oxygen
can be calculated according to the following.
Theoretical demand of oxygen:
The shares (amounts) of different elements are marked as follows ( the proportional mass share /
contribution per 1 kg of fuel, i.e. the contents of carbon, hydrogen, sulphur and oxygen in the fuel)
C carbon kg/kg , H hydrogen kg/kg, S sulphur kg/kg, O oxygen kg/kg
Moisture in wood
Moisture content, moisture ratio
If the fuel contains water it must be taken into account. The mass shares are then calculated with the
following way:
e.g. Creal kg/kg (%)
Creal
1 H 2O% / 100
Cd
C d = share of carbon in dry fuel (mass share %)
Creal = real / actual share of carbon in moist fuel (mass share %)
H 2O% = mass share of water in moist fuel (%)
The following formula can be written (oxygen demand per 1 kg of the fuel)
O0 = 22,39/12,01 C + 22,39/4,03 H + 22,39/32,06 S – 22,39/32,00 O
O0 = 1,86 C + 5,56 H + 0,70 S – 0,70 O2 ( Nm3/kg )
(1)
Composition of air
Element/compound
Nitrogen, N2
Oxygen, O2
Argon, Ar
Carbon dioxide, CO2
Hydrogen, H2
volume %
78,03
20,99
0,94
0,03
0,01
Inert (noble): Argon, Neon, Helium, Krypton, Xenon, Radon
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Fuels and burning
The oxygen for combustion is got from atmosphere / air.
Theoretical demand of dry air I0
O0 = 20,99/100 I0 → I0 = 100/20,99 O0 → I0 = 4,764 O0
(2)
substitution of (2) to the formula (1) and then
I0 = 8,88 C + 26,49 H + 3,33 S - 3,33 O2
(3)
I0 = demand of dry combustion air Nm3/kg
Demand of moist air
Moist combustion air contains always also water vapour (air is a mixture of dry air and water vapour). Therefore the amount of dry air must be multiplied with a correction factor f to get the
amount of moist air.
If0 = f I0 = (f-1) I0 + I0
(4)
substitution of (3) to the formula (4) and then
If0 = f I0 = f (8,88 C + 26,44 H2 + 3,33 S – 3,33 O2)
(5)
Term f in formula ( 5 ) is
f
1
pk
p pk
= relative humidity of air
pk = the saturation pressure of water vapour in combustion air
p = the total pressure of combustion air
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Fuels and burning
Fig. Combustion air fans in a wood chip boiler.
2.2
The amount of flue gases
Dry flue gas flow
The amount of dry flue gases per 1 kg of fuel
V0 = 1,85 C + 0,0003 I0 + 0,68 S + 0,8 N2 + 0,78 I0 + 0,009 I0
(6)
V0 = amount of dry flue gases Nm3/kg
Moist flue gas flow
Moist (actual) flue gases contains in addition to dry flue gases
water vapour from combustion of hydrogen
water vapour from the water (liquid) in fuel
water vapour of the combustion air
hydrogen in the combustion air (very small amount and can be ignored)
Taking into account the previous factors the moist flue gases per 1 kg of fuel
Theoretical moist flue gases V0f
V0f = V0 + 11,12 H + 1,24 H2O + (f-1) I0
(7)
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Fuels and burning
V0f = moist flue gases Nm3/kg (Nm3 per one kg of fuel)
Fig. Moist fuel and burning with non excess air, fuel: wood residue chips
2.3
Combustion with excess amount of air
Excess combustion air factor / ratio is the ratio between theoretical and real amount of air needed
for complete combustion. The excess air do not take part the combustion process. It only ensures
complete combustion in the furnace.
Excess air factor n = real amount of air / theoretical amount of air
Typical excess air factors for different fuels
logwood n = 1,5...2,0
wood chips n = 1,2...1,5
light oil, natural gas n=1,1…1,2
Real amount of dry air I
I = n I0 = (n-1) I0 + I0 ( 8 )
n = excess air factor
I0 = theoretical amount of dry air, Nm3/kg
(n-1) I0 = dry amount of combustion air, Nm3/kg
Real amount of moist air
If = n I0f ( 9 a )
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Fuels and burning
....
Real amount of moist air
If = f n I0 = n I0f (9 b)
Real amount of dry flue gases
V = V0 + (n-1) I 0
( 10 )
V0 = theoretical dry flue gases, Nm3/kg
(n-1) I0 = dry amount of dry air
Real moist flue gases, Vf
Vf = V0f + (n-1)(f-1) I0 + (n-1) I0
V0f = theoretical moist flue gases
(n-1)(f-1) I0 the water vapour in the excess air
(n-1) I0 = dry excess air
2.4
The determination of n excess air factor from the flue gases
The excess air factor is determined from the flue gases by measuring CO2- or O2-content.
2.4.1
By measuring the CO2 –content of flue gases
The amount of carbon dioxide which is composed from the fuel (CO2-amount, Nm3 per one kg of
fuel) does not depend on the excess air factor.
Marked as follows
CO2 V = (CO2)max V0 ( 19 )
CO2 = the carbon dioxide content measured from the flue gases
(CO2)max = the maximum content of carbon dioxide in the flues gases (theoretical combustion) can
be got by calculating the theoretical amount of CO2 (Nm3/kg,pa) and the it is divided with the total
amount of the dry flue gases
substitution formula (10) to formula (19)
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Fuels and burning
CO2 (V0 + (n-1) I0 ) = (CO2)max V0 ( 20 )
By solving the value of n (excess air factor)
n = 1 + (CO2 max /CO2 – 1) (V0 / I 0 )
( 21 )
Generally for fuels V0 = ca. I0
2.4.2
The determination of n by measuring the oxygen (O2 ) content in the flue gases
Oxygen in the flue gases is due to the excess air (combustion air). Determination of the formula for
oxygen content in the flue gases by the measured O2-content (from the flue gases) (the amount of
oxygen of the dry excess air).
O2 (V0 + (n-1) I0 ) = 20,99/100 (n-1) I0 ( 22 )
Solving the n
n = 1 + O2/(21-O2) (V0/I0) ( 23 )
The O2-content does not depend on the fuel used, only on the excess air factor.
If the CO2-measurements are used for analysing combustion and excess air factor the excess air
factor calculated in that way depends on the fuel used.
n = CO2 max /CO2 ,
because CO2 max depends on the carbon content of the fuel used.
The goal in combustion is to achieve as small as possible value of n. Anyway there must be no unburnt compounds in flue gases.
The principal diagram of CO, CO2,max and O2 contents in flue gases.
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Fuels and burning
oxygen, carbon dioxide and carbon monoxide content in flue gases according to excess air ratio
CO2-content decreases because of
excess air ratio and oxygen in flue
gases
CO2-content decreases because of
incomplete combustion
CO –composition because of
poor mixing of gases
stoikiometric point
Figure. CO2 , CO and O2 contents and stoikiometric point (Maakaasukäsikirja, p. 16)
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Fuels and burning
Figure. Oxygen measuring device in a boiler plant, boiler power 15 MW (Siekkilä, ESE Oy).
Figure. Part of a flue gas horizontal chimney, damper with an actuator, oxygen sensor, temperature
sensor and thermometer, boiler power 20 MW (Rokkala, ESE Oy).
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2.5
Fuels and burning
The mass flow and volume flow of gases
The mass flow of the flue gases is the same in every point of the system but the volume flow changes according to the temperature.
The affect of the temp. to the volume flow, absolute temp. (thermodynamic temp.) T (K)
The equation of state (gases), pV/T = constant
V2
T2
V1
T1
V2
T2
V
273
Fig. Flue gas fan.
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3
Fuels and burning
Net calorific value of fuels
Dry fuel, no water content, no attenuation in net calorific value of fuel
Moist fuel, water content reduces the net calorific value of fuel and must be noticed
The affect of the moisture (water) to the net calorific value of fuel
-
moisture in fuel reduces the net calorific value because part of the heat generated in the
combustion goes into the heat of evaporation of water
-
the mass flow of fuel increases because of the mass of water in fuel
-
the combustion process is more difficult to handle
-
with solid fuels (like wood in different forms) moisture affects to the structure of the furnace
and boiler, it is impossible to burn both very dry (like shavings / chips from wood plane,
grinding dust of dry wood tc.) and moist wood (like moist bark of logs, debarking of logs
e.g. in a saw mill intake) in the same furnace / boiler
Calculation of net calorific value of fuels (wood based)
The net calorific value (at constant pressure) as received (CEN/TS 15234) (Vapo Oy: Local Fuels,
Properties, Classifications and Environmental impacts)
The net calorific value of the flue as received (net calorific value of the moist bio fuel) is calculated
according to the following formula:
q p , net, ar
q p , net, d
100 M ar
100
0,02443 M ar
qp,net,ar = the net calorific value (at constant pressure) as received, MJ/kg
qp,net,d = the net calorific value (at constant pressure) in dry matter, MJ/kg
Mar = the moisture content as received, wet-%, wet basis
0,02443 = the correction factor of the enthalpy of vaporization (heat of evaporation)(constant pressure) for water (moisture) at +25 °C temperature, MJ/kg per 1 w-%of moisture)
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Fuels and burning
(Source / referred: Vapo Oy; Local Fuels, Properties, Classifications and Environmental impacts)
For wood: Approximate net calorific value for wood: Formula depending on carbon, hydrogen and
oxygen content in the fuel ….
More precise method calculating energy content
Melting of water in wood based fuels (or peat) two terms more…
FSP, fibre saturation point, heat of evaporation, “free water” and water in lumen cell walls…
Fuel measurements at boiler plant
4
Carbon dioxide emission of fuels
Burning of fuel containing carbon produces carbon dioxide in flue gases and therefore carbon dioxide emissions.
CO2 emissions per mass unit of fuel, specific carbon emission factor per mass unit:
fm
m CO2
mp
M CO2
M C
yC
M CO2
M C
1 yv
yk C
where mp is mass of fuel, m(CO2) is mass of carbon dioxide formed in complete combustion,
M(CO2) (44,0095 kg/kmol) and M(C) (12,0107 kg/kmol) are the molar mass of carbon dioxide and
carbon, y(C) mass share of carbon in moist fuel, yv is mass share of water in moist fuel and yk(C)
mass share of carbon in dry fuel.
Unit of fm can be considered as kg(CO2)/kgfuel.
CO2 emissions per energy unit of fuel, specific carbon emission factor per energy unit:
fq
m(CO2 )
Qp
fm
qp
where Qp is the amount of released energy in burning per mass of fuel, qp is net calorific value of
fuel per unit of mass.
Unit of fq is e.g. g(CO2)/MJ or g(CO2)/kWh.
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5
Fuels and burning
Efficiency and consumption of fuel
For a boiler system can be defined several different efficiencies depending on the calculation limits
(boundaries).
Combustion efficiency ηp
5.1
kv
p
k
kv
= heat power moved to the boiler water, kW
k
= the power feeded to the boiler as fuel, kW
qmp
k
Ha
qmp = the mass flow of the fuel to the boiler, kg/s
H a = net calorific value of the fuel, MJ/kg
- calculation limit is the wall between (furnace + convection parts) and water in the boiler
5.2
Boiler efficiency ηk
h
k
k
h
=the net heat output (power) given by the boiler, kW
- calculation limit is the envelope of the boiler (pipe connections)
- difference between Φk - Φh is due to the combustion losses and boiler losses
5.3
Efficiency of the boiler plant ηkl
hl
kl
k
hl =
the net power given by the boiler plant = the net power feeded to the heating system and the
hot domestic water system
- the calculation limit is the walls of the boiler plant
- difference between Φhl - Φk is due to the combustion losses and boiler losses and from the heat
demand of the boiler plant itself and e.g. preheaters of oil or combustion air preheaters
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6
Fuels and burning
Real (actual) combustion air flow rate qvi and flue gas flow rate qvs
From the combustion formulas / equations is got the amount of flue gases and combustion air demand for 1 kg of fuel as Nm3/kg,pa.
The mass flow of the fuel per net boiler power
k
q mp
k
H a 1000
qmp = the mass flow of the fuel to the boiler, kg/s
h
=the net heat output (power) given by the boiler, kW
H a = net calorific value of the fuel, MJ/kg
= boiler efficiency
k
The real / actual combustion air flow rate qvi (m3/s) corresponding the real / actual mass flow of the
fuel (qmp)
qvi
273 ti
273
If
qmp
ti = temp. of combustion air, °C
I f = the amount of moist combustion air, Nm3/kg,pa
qmp = the mass flow of the fuel, kg/s
The real / actual flow rate of the flue gases qvs (m3/s) corresponding the real / actual mass flow of
the fuel (qmp)
qvs
273 t sk
V
273
f
qmp
t sk = temp. of flue gases, °C
V
f
= the amount of moist flue gases, Nm3/kg,pa
qmp = the mass flow of the fuel, kg/s
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Fuels and burning
Figure. Dimensioning conditions, outdoor temperature -36 °C!!!
22