Combustion of Liquid and Solid Fuels

Fuel characteristics
Fuel & Combustion
2/2006
Dr. Suneerat Pipatmanomai

Fuel
• Fuels are any materials that can be burnt to release thermal
energy
• Most familiar fuels consist primarily of C and H – called
hydrocarbon fuels, denoted as C n H m
• Fuels can be broadly classified as
Form of fuel Primary (natural) Secondary (synthetic)
Solid
coal, oil shale,
biomass
charcoal, coke, MSW
Liquid oil liquid biofuel
Gas natural gas biogas, refinery gas

Fossil Fuels
• Natural gas, oil and coal are the three (fossil) fuels that are
abundantly used.
• This energy is a stored form of solar energy that accumulated over
millions of years, and at the current and projected rates of
consumption, fossil fuels will be used up in a fraction of time
compared to the time it took to collect the energy from the sun.

• Natural gas
• Natural gas like petroleum is generally believed to be derived
from deposits of plant and animal remains from millions of years
ago.
• It may be found along with oil or by itself as in many gas fields
where little or no oil is found.
• As supplied is the cleanest fuel with sulfur removed (except for
small amounts of odorants added)
• Simplest in term of composition and being a gas mixes
immediately in the combustor. Along with methane which is by
far the major combustible constituent of natural gas, other light
hydrocarbons, namely ethane, propane, and butane are present.
• No ash and only molecular nitrogen, and a high H/C ratio which
minimizes the greenhouse gas CO 2 emission.

• Coal
• Is the least clean (fossil) fuel containing sulfur, elemental
nitrogen, low H/C ratio and ash
• Coal has a very complex structure and being a solid is more
difficult to burn.
• Coal combustion undergoes devolatilisation and combustion
of the released gases, char combustion and fly ash formation
which are particles 10 microns in size (the low visibility around
certain coal fired power plants is due to the fly ash).

• Almost all of the coal consumed in the world is for
electric power generation by combusting the coal in
boilers and generating steam to power a turbine.
• Coal is being used to a limited
extent in gasification based plants
to produce gas to fuel gas turbine
based combined cycles (IGCCs)
and in some countries such as
China for chemicals synthesis.
With more advanced gas turbines
under development, coal based
IGCC will have a strong economic
and environmental basis to
compete with boiler based power
plants.

• Coal is classified into the following four types according to
the degree of metamorphism:
• Anthracite which is low in volatile matter (which forms tars,
oils and gasses when coal is heated) and consists of mostly
carbon (fixed carbon)
• Bituminous which contains significant amounts of the volatile
matter and typically exhibit swelling or caking properties
when heated
• Sub-bituminous is a younger coal and contains in addition to
the volatile matter, significant amounts of moisture
• Lignite is the youngest form of coal (when peat is not
included in the broader definition of coal types) and is very
high in moisture content resulting in a much lower heating
value than the other types of coal.

• Oil
• Represents an intermediate fuel in terms of quality.
• Petroleum oil is a mixture of a number of hydrocarbons with
some sulfur, nitrogen and organo-metallic compounds also
present.
• A number of processing steps are involved in producing the
various high value salable fuel streams such as gasoline, diesel
and jet fuel from the petroleum.
• Oil which contains more than 300 molecular species needs to
be atomized (less than 10 microns to provide large surface
area), and within the combustor it has to vaporize and mix
before combustion can occur).

• Oil shale
• The organic solids in oil shale rock are a wax-like material
called kerogen.
• Kerogen is extracted by heating in retorts in the absence of air
where it decomposes forming oil, gas, water and some carbon
residue.
• Production of gasoline or jet fuel from the oil produced from the
oil shale, however requires more extensive processing than
most petroleum feedstocks.
• The shale oil also contains more nitrogen than petroleum does
which if left in the fuels produced from the shale oil would result
in significant NOx emissions.

Non-fossil fuels
• Biomass
• Is all plant and animal matter on the
Earth's surface including trees, crops,
algae and other plants, as well as
agricultural and forest residues plus
other wastes, e.g. MSW, industrial
wastes, wastewater
• Renewable (produced sustainably)
• Considered carbon neutral fuel. When
using biomass to displace fossil fuels,
CO 2 emissions are largely avoided
and the overall system is often carbon
neutral or close to it.
• Multiuse – food, energy, materials
• Distributed nature and can be grown
close to where it is used

Fuel properties
• Fuel contains combustibles, which should be known for
stoichiometric calculations.
• Analyses of various solid fuels are conducted for
• Proximate analysis:
• Moisture, Volatile matter (VM), Mineral matter (or ash),
Fixed carbon, Calorific values
• The value of proximate analysis
• Identifies the fuel value of the as-received material
• Provides an estimate of ash handling requirement
• Describes something of the burning characteristics
• Ultimate analysis: C, H, N, O, S
• Describes something of the burning and product
characteristics

• Calorific values = Heat of combustion of fuel
• Defined as “the total heat produced when a unit mass of fuel
is completely burnt with pure oxygen”
• Two terms of calorific values
• NCV or LHV: when water vapour is present in the flue gas
(the latent heat of vapourisation is lost)
• GCV or HHV: when water vapour is condensed and
therefore this latent heat is added
• NCV = GCV – (% mass of hydrogen) x 9 x λ v
λ v = latent heat of vapourisation at reference temperature
= 2442.5 kJ/kg at 298.15 K (25°C)

• Moisture
• Water expelled from fuel in its various forms (when tested
under specified conditions)
• Normally moisture content is determined by drying sample of
known mass at 110°C until no further weight loss is observed.
• Depends on a combination of its origination and
treatment/storage
• Biomass: harvesting method, climatic conditions, time of
year when harvesting takes place and the length and
method of storage
• Coal: coal rank, method of storage, pre-treatment

• Moisture content has a significant effect on many of the
energy conversion processes. For example,
‣ The percentage of solids present in the digestate when
biogas is obtained from an anaerobic digestion process
affects the gas yields
‣ For dry biomass fuels, such as wood or straw, the amount
of water present has a considerable effect on the
proportion of the total heat content of the material that is
possible to recover as a result of combustion
‣ High moisture fuel makes feeding system difficult, render
agglomeration, incomplete combustion

• Volatile matter (VM) and Fixed carbon
• VM = Total loss in the weight minus the moisture in fuel
when heated under specified conditions
• Fixed C is normally obtained by difference
Weight (%)
100
80
60
40
20
0
(1)
(2)
(3)
(4)
0 10 20 30 40 50 60 70 80 90 100 110
Time (min)
1000
800
600
400
200
0
Temperature (C)
(1) moisture
(2) Volatile
matter
(3) Fixed
carbon
(4) Ash
TGA result for proximate analysis of solid fuel

• Generally biomass fuels are highly volatile (= low fixed
carbon) and need to have specialized combustor designs
to cope with rapid gas evolution when heated
• Fuels with low volatiles, such as coal, need to be burnt on
a grate as they take a long time to burn out unless they are
pulverized to a very small size

• Mineral matter or often referred to as ash content
• Inorganic residue left over when fuel is incinerated
(completely combusted) in air to constant mass under
specified condition
• Characterization of ash by elemental analysis and fusion
temperatures is an important aspect of utilizing biomass fuels
• Ash analysis provides
• Information on how much ash there will be to dispose
• Information on whether special ash treatments are needed
before disposal
• Information on slagging, fouling and clinker formation in
the burner and boiler to be predicted

• Ash management presents both a problem and an
opportunity
‣ Removal of ash from the furnace and disposal in
landfill areas incurs costs for power plants
‣ Ash can be recycled in the forest ecosystem, depletion
of plant nutrients (other than nitrogen) and acidification
associated with intensive biomass removal, is then
radically reduced
• Examples of slag problem
• For pure wood combustion, the combustion
temperatures are likely to be low, ash fusion does not
usually represent a problem; however, when wood is
co-fired with coal, combustion temperatures are
considerably higher and may reach a level where
slagging could occur

• In the case of straw or palm EFB combustion, ash fusion
and the resulting slagging represent a considerable problem
which has to be solved by special boiler designs
• The combination of some mineral matters in coal also
increase slagging potential
Bottom ash Slag

• Ultimate analysis: C, H, N, O, S
• For ultimate analysis, fuel sample is burnt in a current of
oxygen producing water, carbon dioxide, nitrogen oxide and
sulfur dioxide, which are measured to determine the amount
of the original elements
• The results are normally presented on air-dried basis
• Converting to as-received basis by
As-received basis = Air-dried basis x (100 – moisture)
100

• Attempts have been made to correlate the ultimate
analysis of a fuel with its calorific value. One of the most
commonly used relationships is that given by Dülong
GCV (kJ/kg) = 33950 C + 144200 [H – (O/8)] + 9400 S
Where C and S = mass fraction of carbon and sulfur
H – (O/8) = mass fraction of net hydrogen
= total hydrogen – 1/8 (oxygen)
• Calderwood equation is relating total carbon content with
the proximate analysis and the GCV
mass % of carbon = 5.88 + 0.00512 (GCV – 40.5 S)
± 0.0053 [80 – 100 (VM/FC)] 1.55
If 100 (VM/FC) > 80, the sign is (-) and vice versa

Exercise
• Calculate NCV at 298.15 K of crude oil having following
properties:
Ultimate analysis: 87.1% C, 12,5% H and 0.4% S (by mass)
GCV at 298.15 K is 45,071 kJ/kg oil
Latent heat of water vapour at 298.15 K = 2442.5 kJ/kg
• The GHV of gaseous propane is 2,219.71 kJ/mol at 298.15 K,
calculate its NHV

Combustion
• Is a chemical reaction during which a fuel is oxidised and a large
quantity of energy is released
• For any combustion reaction, oxygen is the agent which will
combine with carbon, hydrogen and sulfur
• In normal practice, air is used since it is the cheapest source of
oxygen (about 21 mole% of air)
• One drawback of air utilisation is the presence of nitrogen (79
mole%), which reduces the flame temperature considerably and
also accounts for the high heat loss of stack
• Oxygen has much greater tendency to combine with hydrogen
than it does with carbon, therefore hydrogen is normally burned to
completion forming H 2 O. Some of carbon, however, ends up as
CO or just as plain as C particles (soot) in the products.

• It should also be mentioned that bringing a fuel into intimate
contact with oxygen is not sufficient to start a combustion process.
The fuel must be brought above its ignition temperature to start
combustion
• Minimum ignition temperatures of various substances in air
Gasoline 260°C
Carbon 400°C
Hydrogen 580°C
Carbon monoxide 610°C
methane 630°C
• Moreover, the proportions of the fuel and air must be in proper
range for combustion to begin, e.g. natural will only be burn in air
in concentration between 5-15%

Theoretical/ stoichiometric air
• Combustion equations are balanced on the basis of the
conservation of mass principle: The total mass of each
element is conserved during a chemical reaction
2 kg of hydrogen 16 kg of oxygen 2 kg of hydrogen
16 kg of oxygen
H 2 + ½ O 2 = H 2 O
• Theoretical or stoichiometric amount of air = the minimum air
required to burn fuel completely so that C, H and S are
converted into CO 2 , H 2 O and SO 2 , respectively

• Consider combustion reactions
mole of O 2 needed/ 1 mole of reactant
• C + O 2 = CO 2 1
• H 2 + ½ O 2 = H 2 O ½
• S + O 2 = SO 2 1
• Theoretical air demand (in moles)
= Theoretical oxygen demand (in moles)/ 0.21
• CH 4
• C 6 H 12 O 6
mole of air needed/ 1 mole of reactant

Stoichiometry
• For a hydrocarbon fuel given by C x H y , the stoichiometric
relation can be expressed as
C x H y + a(O 2 + 3.76N 2 ) xCO 2 + (y/2)H 2 O + 3.76aN 2
Where a = x + y/4
Composition of air is 21% O 2 and 79% N 2
Each mole of O 2 in air, there are 3.76 moles of N 2

Equivalence ratio
• The equivalence ratio, F, is commonly used to indicate
quantitatively whether a fuel-oxidizer mixture is rich, lean,
or stoichiometric.
(A/F) stoi
F = =
(A/F)
(F/A)
(F/A) stoi
for fuel-rich mixtures, F > 1
fuel-lean mixtures, F < 1
stoichiometric mixture, F = 1
Where A/F = mass ratio of air to fuel

Excess air
• In actual practice, theoretical air is not sufficient to get complete
combustion. Excess air supply (or, in the other words, excess
oxygen supply) is essential for complete combustion.
• % Excess air
= (actual air supply – theoretical air demand) x 100
theoretical air demand
• The actual percentage excess air depends on the fuel used for
combustion. Normally gaseous fuels require very less excess
air, i.e. 5-15% excess air, than liquid and solid fuels, which
require 10-50% excess air.
• Excess air can reduce the flame temperature and increase the
heat losses through the flue gases

• Theoretical as well as actual air requirements are expressed in
• kg/kg of fuel by multiplying with the average molar mass of air
• m 3 /kg of fuel by multiplying with specific volume of air at that
condition
• Normally, flue gases contain CO 2 , CO, H 2 O, O 2 , SO 2 and N 2 , with
very low concentration of SO 3 .
• Water in flue gases
• Interferes with the gas analysis, it is removed prior to the
analysis of dry gases.
• Comes from three sources: water vapour product, evaporated
moisture in fuel, water vapour accompanying air for
combustion

Exercise
• One kmol of octane is burned with air that contain 20 kmol of O 2 .
Assuming the products contain only CO 2 , H 2 O, O 2 , and N 2 ,
determine the mole number of each gas in the products and the
air-fuel ratio for this combustion process.

Exercise
The ultimate analysis of a residual fuel oil sample is given below:
C: 88.4 %, H: 9.4%, and S: 2.2% (mass)
It is used as a fuel in a power-generating boiler with 25 % excess
air. Calculate
(a) the theoretical dry air requirement
(b) the actual dry air supplied
(c) composition of flue gases