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Index:
Introduction
Combustion
Atomisation
Droplet combustion
model
Catalysis
Metal additives
Soot and cenosphere
NOx
Bycosin combustion additives
References
Case study
Keywords: Combustion, catalysis, cenospheres,
soot, additives, Bacharach, atomisation, NOx
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COMBUSTION OF FUEL
OILS
AND HEAVY FUELS IN BOILERS
Introduction
The increased refinery conversion of crude oil, including thermal
cracking and visbreaking has resulted in residual
fuels with increased levels of asphaltenes, carbon residue and ash. Fuel
oils with carbon residue levels up to, and over, 20% are not uncommon. In both
boilers and diesel engines, these fuels have resulted in poor combustion,
increased fouling, reduced power and increased maintenance and repair.
Combustion
Combustion is the process whereby oxygen reacts with fuel,
resulting in the release of heat and light. The effectiveness of combustion can be measured by
analyzing the flue gas
and the amount of soot. Perfect combustion is obtained when the flue gas
analysis shows no carbon monoxide, hydrogen or oxygen and when the
percentage of carbon dioxide is at a maximum. (Ref 1)
There are several factors or parameters that
will affect combustion of heavy fuels in a boiler:
-
Design of the combustion chamber
-
Design of the burner(s)
-
Condition of the burner(s)
-
Air and fuel ratio. Excess air of a few percent is
normal
-
How well the air and fuel are mixed
-
Temperature and air speed in the combustion chamber
-
Air and fuel preheating
-
Physical properties of the oil (viscosity, density,
surface tension)
-
Chemical properties of the oil (asphaltenes, waxes, metal
content)
-
Fuel droplet size
Atomisation
The size of the fuel droplets is an important parameter in
combustion. The smaller the size of the droplet, the better the
combustion. The mean size
of the droplets is 0.1 mm. A 0.1 mm droplet burns in 0.01 seconds while a 0.2 mm burns in 0.04
seconds. The combustion time is thus more or less square the value of the size of the droplet. As a consequence, a larger droplet may
not burn internally when it starts to reach the cooler end of the flame.
This causes the formation of soot and coke.
During atomisation, the size of the droplets
can be decreased by:
-
Injecting pressure
-
Lower viscosity
-
Oil preheating (this will cause a lowering of the
viscosity)
-
Lower surface tension
-
The right type of burner, or a well trimmed burner
Droplet combustion model (Ref 2)
Experiments on suspended droplets show that three stages of droplet combustion
can be identified.
-
Evaporation stage. The droplet is heated up and volatiles
evaporate from the droplet surface and mix with air.
-
Volatile combustion stage. Ignition and combustion of the
volatiles. The flame remains at a distance from the surface of the droplet
keeping it out of reach of oxygen. The flame supplies additional energy
for heating the droplet. This causes cracking reactions and further
evaporation. The remaining heavy components accumulate at the droplet
surface and eventually form a solid hollow particle called a cenosphere.
-
Coke combustion stage. The cenospheres are heated to even
higher temperatures. It ignites and burns in a hot, oxygen-poor
environment by heterogeneous surface reaction.
Top
Catalysis (Ref 3)
A catalyst is a substance that increases the rate of a chemical reaction,
without itself entering into the reaction products.
In general, the rate of a reaction in a solution increases two-fold when
the temperature is increased by ten degrees. An increase in temperature
causes an incremental increase in the number of collision between molecules, as well
as an increase in the velocity of the molecules that may, in the end, react
with each other. The catalyst will increase the rate of a reaction without
the need to increase the reaction temperature.
A catalyst will thus either increase the rate of a reaction at the same
temperature or, keep the same rate of reaction at a much lower temperature
compared to the same reaction without catalyst. The temperature of
ignition is an example whereby the reaction starts at a lower temperature
with the help of a catalyst. Thus, the energy barrier needed for a
chemical process to occur is lowered, and less energy or temperature is
therefore needed.
For a molecule to react catalytically at a
solid surface, it must be chemisorbed (chemical adsorption). Physical
adsorption (van der Waals forces, electrostatic attraction, etc) has
little relevance to catalysis.
Molecules cannot be chemically adsorbed on a catalyst without undergoing a
radical disruption of the bonds within them. Hydrogen, for instance, must
dissociate into hydrogen atoms. The same applies for hydrocarbons.
Molecules possessing double bonds will not dissociate; instead, their
double bond is converted into a single bond while bonding to the catalyst
with the orbitals from the second bond.
Top
Metal additives (Ref 4)
There are three stages where metal additives are active as a catalyst (in
different degrees and effectiveness, and as a homogeneous and/or a
heterogeneous catalyst).
In the liquid phase. Metal additives will act as catalysts in cracking and
recombination. This will produce lighter components, which will evaporate
more easily.
In the gas phase. Ionised and particulate metal additives will catalyse
the combustion of the volatilized fuel.
In the solid phase. Metal additives get trapped in the coke and soot
particles. The additives will then lower the ignition temperature and catalyze
combustion. An early combustion is favorable so that the
particles have the time to burn before reaching the colder zone of the
flame.
How do metal additives work chemically? (Ref
5)
It is believed that alkali metal compounds positively charge any soot
platelets that from during combustion and reduce the ignition temperature
of the soot. Positively charged platelets would not agglomerate as quickly as
uncharged platelets, and with a greater surface area exposed, would oxidize
more quickly. Group 2a metals (Mg, Ca, Ba) are believed to generate
hydroxyl radicals that are reactive in oxidizing the soot nuclei that
would otherwise grow to the platelets. Transition metals (Fe, Va, etc) are
believed to catalyze soot oxidation. Very many metal oxides are not
selective enough to be used industrially in the petroleum and
petrochemical industry. In a flame, this is not a problem. High molecular
weight hydrocarbons are broken into smaller fragments and rearranged
(branched) into new molecules. (Ref 3)
The pictures below show the effect of metal
additives on the combustion of cenosphere (without and with additives).
The shell of the cenosphere to the right is partially gone. (Ref 6)
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Top
Soot and cenosphere
Soot is made of very small particles (less than 1 mm) which appear
in smoke as a result of incomplete combustion. Soot is produced in the gas
phase (distilled and pyrolysed components from the fuel droplets).
Cenosphere and coke are the remnant of the distillation of a droplet of
fuel oil. There is first a distillation of the lighter fractions, while
the heavy fractions tend to transform itself into coke, forming a shell
round the droplet. Large droplets give large coke spheres or cenospheres.
Incineration of soot particles is
accomplished by providing temperatures above the ignition temperature of
the soot particles and sufficient combustion chamber space (residence
time) for the slow soot combustion to go to completion. Soot is formed in
quenched flames where the temperature of the flame is below the soot's
ignition temperature or when the flame is too long and travels into the cooler parts
of the boiler. Unsaturated fuels are often prone to soot
formation because of their molecular structure, with a higher ratio of
carbon to hydrogen. This is the case with heavy fuel oils which contain
large amounts of unsaturated molecules. (Ref 7)
The amount of soot particles in the flue gas
gives a measurement of how efficient the combustion is. There is a
correlation between excess air and the amount of soot in the flue gas. To
reduce soot, excess air has to be increased.
Top
NOx (Ref 7)
The formation of thermal NOx is dependent on temperature and the
concentration of the oxygen present. The slow reaction kinetics of the
thermal NOx mechanism shows that the growth of NOx is much slower than the
completion of the other hydrocarbon reactions. This allows the problem of
NOx generation to be decoupled from the other chemical reactions which go
on in the flame. The total gas residence time in an industrial furnace is
only a few seconds, after which gases are released to the ambient
atmosphere. The NOx production never reaches equilibrium during such short
time interval.
The formation of chemical NOx (or fuel NOx) is created when nitrogen atoms
chemically bonded in a molecule is oxidized. The first step in the
nitrogen reaction is the formation of HCN and NH3. These two species will
mostly end up as N2, but depending of
the temperature and the availability of oxygen NOx is also produced. (Ref
8)
Most NOx reduction techniques currently applied to boilers are effective
as a result of lower peak flame temperatures in the furnace, reduced
availability of oxygen in the flame or a combination of both. These
techniques include low excess air operation, fuel rich (staged) firing and
flue gas re-circulation. (Ref. 9).
Combustion equipment manufacturers take advantage of the slow rate of
reaction in the formation of NOx when designing burners (including limit
peak flame temperatures, limit localized in-flame oxygen concentrations,
and reduce residence time at peak temperature).
Top
Bycosin combustion additives
-
Catalyzed combustion
-
In the heated droplet, Bycosin's additives act as a
catalyst in cracking and recombination. This will produce lighter
components, which will evaporate more easily.
-
In the gas phase, ionized and particulate iron will
catalyze the combustion of the volatilized fuel.
-
In the volatile combustion stage, when the coke shells are
forming, Bycosin's additives have a catalytic effect during the combustion
process in such a way that the structure of the forming coke is modified.
This will affect the subsequent combustion of the cenosphere.
-
In the coke combustion stage, the ignition temperature is
lowered, resulting in faster and prolonged combustion.
-
Atomisation.
Bycosin´s additives contain dispersant agents and surface active
ingredients that will affect the physical properties of the fuel in such a
way as to improve its atomisation.
-
Reduction of soot and cenosphere. Bycosin's metal additives and asphaltene dispersants will radically reduce
the amount of soot and cenospheres. The picture below shows the
difference. The Bacharach diagram below shows that the amount of soot
can be reduced at a constant excess air or the excess air can be reduced
at a constant soot amount (SSN). A combined reduction of SSN and
excess air could be the best solution. Bycosin's asphaltene dispersants
will contribute very positively in the combustion of the asphaltenes.
Dispersed asphaltene particles will burn better than agglomerated ones.
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References
Ref 1 North American combustion handbook; vol. I, page 53.
Ref 2 Taylor and Burgess, Fuel Sci Technol Int, 6, 43, 1988
Ref 3 Heterogeneous catalysis, priciples and applications. G.C.
Bond, Oxford University press, 1974
Ref 4 L. Witzel et al. Fuel vol.74, no 12 pp 1881-1886, 1995
Ref 5 UK patent (Exxon) GB 2248068A 1992
Ref 6 Rev. Gen. Them Fr. no 262, Octoble, pp 687-692, 1983
Ref 7 North American combustion handbook; vol. II
Ref 8 Zeldovich, Acta Physiochimica, 21(4), page 577, 1946.
Ref 9 Ed L.H. Yaverbaum, Noyes Data corporation, 1979
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