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The combustion
process
Energy
and combustion Energy's defined as the
ability of a material or system to
perform labor. Energy exists in
different modifications which can be
classified into six categories as
follows:
Nuclear energy (nuclear fission)
The different energy modifications can
be converted into each other, within an
ideally closed system, with the sum
remaining constant (conservation of
energy). In practice, however, energy
losses occur during the conversion
process thereby reducing the efficiency.
The natural energy carriers (coal,
natural gas, crude oil, sun radiation,
water power etc.) are described as
primary energies, while the term
secondary energies stands for what is
received from energy conversions
(electricity, heat, etc.). Energy
carriers differ in energy content. For
comparison reasons the energy content is
described as amount of energy which
could be released from a certain
quantity of an energy carrier in case of
its total combustion. The energy scale
unit is 1 Joule [J]. Some energy content
values are given in table 1.
Combustion is the conversion of
primary chemical energy contained in
fuels such as coal, oil or wood into
heat (secondary energy) through the
process of oxidation. Combustion
therefore is the technical term for the
chemical reaction of oxygen with the
combustible components of fuels
including the release of energy.
Combustion processes proceed with high
temperatures (up to 1000
°C
and above). The oxygen required for the
combustion is supplied as part of the
combustion air fed to the process. From
that a considerable volume of exhaust
gas (flue gas, off gas) is produced
together with, depending on the kind of
fuel, a certain amount of residues
(slag, ash).
Oxidation
Term for all chemical reactions of
oxygen with other substances. Oxidation
processes proceed with the release of
energy and are of great importance in
many technical (combustion) and
biological (breathing) areas.
Combustion plants
Combustion plants are facilities that
generate heat by burning solid, liquid
or gaseous fuels. They are required for
many tasks, e.g.
heating (heating plants, building
heating)generation of electrical energy generation of steam and hot water for
use in process industries
manufacturing certain materials (cement,
glass, ceramics)
thermal surface treatment of metallic
parts
incineration of waste materials and
residues.
See detailed application examples
Combustion occurs in a combustion
chamber; other control units are
required for fuel supply and fuel
distribution, combustion air supply,
heat transfer, exhaust gas cleaning and
for discharge of exhaust gases and
combustion residues (ash, slag).
Solid fuels are fired on a fixed or
fluidized bed or in a flue dust/air
mixture. Liquid fuels are fed to the
burning chamber together with the
combustion air as mist. Gaseous fuels
are mixed with combustion air already in
the burner. The exhaust gases of
combustion plants contain the reaction
products of fuel and combustion air and
residual substances such as particulate
matter (dust), sulfur oxides, nitrogen
oxides and carbon monoxide. When burning
coal, HCl and HF may be present in the
flue gas as well as hydrocarbons and
heavy metals in case of incineration of
waste materials. In many countries, as
part of a national environmental
protection program, exhaust gases must
comply with strict governmental
regulations regarding the limit values
of pollutants such as dust, sulfur and
nitrogen oxides and carbon monoxide. To
meet these limit values combustion
plants are equipped with flue gas
cleaning systems such as gas scrubbers
and dust filters.
Fuels
are available in different forms and
composition:Solid fuels (hard coal, bituminous coal,
peat, wood, straw) contain carbon (C),
hydrogen (H2), oxygen (O2), and smaller
quantities of sulfur (S), nitrogen (N2),
and water (H2O). A major problem when
handling such fuels is the formation of
large quantities of ash, particulate
matter and soot.
Liquid fuels derive mainly from crude
oil and can be classified into light,
medium and heavy fuel oils. Light fuel
oil (i.e. diesel fuel) is
widely used in small combustion plants.
Gaseous fuels are a mixture of
combustible (CO, H2 and hydrocarbons)
and non-combustible gases. Today very
often natural gas is used, which
contains methane (CH4) as the main
component.
The knowledge of fuel composition is
important for an optimum and economical
combustion process. Increasing
percentage of incombustible (inert) fuel
components reduces the gross and net
calorific value of the fuel and
increases contamination of the furnace
walls. Increasing water content raises
the water dew point and consumes energy
to evaporate water in the flue gas. The
sulfur contained in
the fuel is burnt (oxidized) to SO2 and
SO3, which, at temperatures below the
dew point, may lead to the formation of
aggressive sulfureous and sulfuric
acids.
Combustion air; excess air value
The oxygen required for a combustion
process is supplied as part of the
combustion air which consists of (see
table 3) nitrogen (N2), oxygen (O2), a
small amount of carbon dioxide and rare
gases, and a variable content of water
vapor. In some processes pure oxygen or
a air/oxygen mixture is used for the
combustion. The combustion air
components, except oxygen, are all
contained in the resulting raw flue gas.
Stoichiometric and
excess-air combustion; material balance
The minimal amount of oxygen required to
burn all combustible components
completely depends on the fuel
composition. 1 kg of carbon e.g.
requires 2,67 kg oxygen to be burnt
completely, 1 kg of hydrogen requires 8
kg oxygen, 1 kg of sulfur however only 1
kg oxygen! Combustion occurring at these
exact gas quantity ratios is called
ideal combustion or stoichiometric
combustion.
The relevant equations are
Carbon: C + O2
2H2O
Sulfur: S + O2
SO2
The ideal combustion procedure is shown
schematically in figure 1. The amount of
oxygen supplied to the combustion is
just sufficient to burn all fuel
combustibles completely. No oxygen nor
combustibles are left. The excess air
value (ex.air) is 1 in this case.
In a real process this ideal oxygen
volume is not sufficient for complete
burning because of insufficient mixing
of fuel and oxygen. The combustion
process, therefore, must be supplied
with more than the stoichiometric volume
of oxygen. This additional amount of
combustion air is called excess air and
the ratio of the total air volume to the
stoichiometric air volume is the excess
air value ex.air;
another expression for that is (lambda).
Consequently the highest combustion
efficiency is achieved with a (limited)
excess
volume of oxygen, i.e. ex.air >1
(oxidizing atmosphere). The excess air
value is of great importance for an
optimum combustion process
and economic plant operation:
Unnecessary high excess air volumes
reduce combustion temperatures and
increases the loss of energy released
unused into the atmosphere via the hot
flue gas stream.
With too little excess air some
combustible components of the fuel
remain
unburned. This means reduced combustion
efficiency and increased air pollution
by emitting the unburned components to
the atmosphere.
Table 4 shows various excess air value
ranges of specific types of combustion
processes. In general: The smaller the
reactive surface/mass volume ratio of
the fuel particles is the more excess
air volume is required for optimum
combustion. This is also correct
conversely and therefore solid fuels are
ground and liquid fuels are sprayed to
get a larger reactive surface. Only a
few processes exist
which are operated definitely at
deficient air (ex.air<1) conditions
because of their special process
requirements (e.g. heat treatment
processes).
Oxidizing
atmosphere.
In an oxidizing atmosphere more oxygen
is available as necessary for complete
combustion of all combustible components
existing in the gas volume. The
combustion (oxidation) will be complete.
Simplified: Oxidation = Addition of
oxygen (e.g. CO is oxidized to CO2)
Reducing
atmosphere.
In a reducing atmosphere less oxygen is
available as necessary to burn (oxidize)
all combustibles. Simplified: Reduction
= Removal of oxygen (e.g. SO2 is reduced
to S)
Determination of
the excess air value
The excess air value can be determined
from the concentrations of CO, CO2 and
O2 in the flue gas. These relations are
shown in the combustion diagram, fig. 3.
At ideal fuel/air mixing conditions each
CO2 content value is related to a
certain CO value (area with ex.air<1) or
to a certain O2 value (area with
ex.air>1). The CO2 value for itself is
not definite because of the curve
showing a maximum. Therefore it must
additionally be checked whether, besides
the CO2, CO or O2 is present in the gas.
When operating the combustion with
excess air (i.e the normal case)
nowadays the definite determination of
only O2 is preferred. The curves are
fuel-specific which means a separate
diagram for each fuel with a
fuel-specific value of CO2 max is used
for efficiency calculation,
For calculation of the excess air value
from the measured values of CO2 or O2
the
following two formulas may be used:
CO2 max : fuel-specific maximum CO2
value (see table 7, page 21)
CO and O2: measured or calculated
concentration values in the flue gas
Required combustion air volume
The actually required volume of
combustion air can be calculated
from the air volume needed for the ideal
combustion
(depending on the kind of fuel),the desired excess oxygen value and
the relative oxygen content of the used
air or the air/oxygen mixture
The oxygen content of dry air at 1 bar
pressure is 20,95%. In practice, how
ever, ambient air is not dry, therefore
the water content must also be
considered for correct air volume
calculation.
Gas volume; dilution effect;
reference values
Both combustion air and humidity (water
vapor) increase the absolute gas volume.
Fig. 4 shows this effect for the
combustion of 1 kg of fuel. At
stoichiometric conditions (i.e. without
excess air) appr. 10 m3 exhaust gas are
generated from that at dry resp. 11,2 m3
at humid conditions whilst from the same
amount of fuel, burnt with 25% excess
air at humid conditions, 13,9 m3 exhaust
gas are generated.
This is a dilution effect that reduces
the relative concentration of the
particular flue gas components. For
instance the concentration of SO2 is
relatively reduced from 0,2% (stoich.,
dry) to 0,18% (stoich., humid) resp. to
0,14% (25% excess air, humid) and the
content of oxygen is reduced from 4,4%
to 4%. See table 5 and fig. 4.
Reference values
Concentration values are typically
reported in relation to a known resp.
specified reference value. Only then can
the measured value be comparable with
other values, e.g. specified pollution
limits. In practice three definitions of
setting a reference are used:
Reference to a certain value of dilution
through excess air. For that the
oxygen concentration is used as
reference measure by the expression e.g.
Reference value 8% oxygen".This reference is generally applied for
regulatory reporting. Sometimes it is
also used for a specific application so
that the oxygen value is characteristic
for the standard operational conditions
of the plant.
Reference to a certain value of dilution
through the humidity content of
the gas. Typically the gas temperature
is expressed e.g. by "at dew point of
but expressions as "related to dry gas"
are also used.
Reference to the standard conditions of
a gas. This refers to the influence
of pressure and temperature on the gas
volume.
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