some considerations about bioethanol combustion in oil-fired boilers
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Some considerations about bioethanol combustion in oil-red boilers
Jorge Barroso , Javier Ballester, Antonio Pina
LITEC, University of Zaragoza - CSIC, Mara de Luna 3, Zaragoza, 50018, Spain
a b s t r a c ta r t i c l e i n f o
Article history:
Received 16 April 2010
Accepted 4 May 2010
Keywords:
Gasoil
Bioethanol
Alcohols
Combustion
Boilers
The combustion of bioethanol in boilers has been analyzed and compared with conventional liquid fuels. The
study includes an experimental evaluation of combustion performance as well as the estimation of the
impact of replacing gasoil by ethanol on the thermal efciency of an industrial boiler.
Several works have been dedicated to the study of fuel substitution in internal combustion engines, being theuse of gasoilbioethanol blends in engines a common practice. However, very few studies have addressed
the characterization of switching of conventional liquid fuels by bioethanol in boilers.
Combustion tests demonstrate signicant differences between bioethanol and gasoil ames. Soot, NOx and
SO2emissions are signicantly lower with ethanol, whereas this fuel can produce higher amounts of CO than
gasoil if the burner is not properly adapted. The experimental tests have demonstrated that both the burner
and boiler operation should be readjusted or modied as a result of the change of fuel in industrial boilers. If
thermal input is to be kept constant, nozzles of larger capacities must be used and the air feeding rate needs
to be signicantly modied. Also, the ame detector may have to be replaced and the fuel feeding system
should be revised due to the enhanced tendency of ethanol to cavitation. Using the same thermal input may
not guarantee keeping the same steam production, but some parameters of boiler operation should be
modied in order to avoid reductions in the capacity of the boiler when switching from gasoil to bioethanol,
such as gas recirculation fraction, steam cooling systems and percentage of oxygen in the exhaust gases.
The feasibility of burning bioethanol in gasoil boilers has been analyzed, and the results conrm that fuel
switching is technically possible and offers some advantages in terms of pollutants reduction.
2010 Elsevier B.V. All rights reserved.
1. Introduction
The fossil fuels resources are depleting as quickly as the energy
consumption is increasing and the humanity have the challenge to
nd out environment-friendly alternatives to fulll the energy
demand of the world. In this context, many countries are more and
more concerned of their vulnerability to oil embargoes and shortages,
which would affect not only the development of industrial, transpor-
tation, and agricultural sectors, and many other basic human needs,
but also their political decisions. Hence, the scientists are looking for
alternative energy sources. Considerable attention has been focused
on the development of alternative renewable fuel sources, with
particular reference to the alcohols, as it is pointed in reviews [13].
Hansen et al.[1]recognized the opportunities of bioethanol blended
with gasoline and diesel in internal combustion engines, with the
benecial effects of reducing country dependence on imported fuel,
substituting fossil fuels by a renewable resource, and accomplishing
the more stringent emissions regulations. Agarwal[2]explained that
using an ethanol-unleaded gasoline blend leads to a signicant
reduction in exhaust emissions of CO and HC and using ethanoldiesel
blends up to 20% signicant reduction in CO and NOxemission was
observed.
The main economic, environmental, social implications of biofuels
are discussed by Petrou and Pappis [4], concluding it is necessary to
make a whole Life Cycle Inventory (LCI) analysis to determine
biofuels' performance with respect to all the impact categories, where
the supply cost (in relation to a certain fossil fuel price to be
substituted) is an important consideration. Farrell et al.[5]concluded
that the range of assumptions and data found could play a key role
when comparing fuel resources and that further research into
environmental metrics is needed for obtaining valid comparisons.
Bioethanol is used as alternative fuel in the gasoline and diesel
internal combustion engines, because it improves performance and
reduces pollutant emissions. Parag and Raghavan [6] developed a
fundamental experimental study to determine the burning rates of
ethanol and ethanol-blended fossil fuels such as gasoline or diesel.
They found that the mass burning rate does not vary signicantly for
ethanol blended with diesel, but transition velocity decreases and
ame stand-off distances and ame luminosity increase with the
content of diesel in the blend. On the contrary, for ethanol blended
with gasoline, the mass burning rate increases with the gasoline
content due to the higher volatility of gasoline. Transition velocityrst
decreases when 10% gasoline is added to ethanol, but with further
addition of gasoline, transition velocity gradually increases. Flame
Fuel Processing Technology 91 (2010) 15371550
Corresponding author. Tel.: +34 976 506520x206; fax: +34 976 761882.
E-mail address:[email protected](J. Barroso).
0378-3820/$ see front matter 2010 Elsevier B.V. All rights reserved.
doi:10.1016/j.fuproc.2010.05.036
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stand-off distances and ame luminosity also increase with gasoline
percentage.
There are several works on different proportions of ethanol
gasoline blend[710]. The effect of ethanol blended gasoline fuels on
emissions and catalyst conversion efciencies was investigated in a
spark ignition engine by He et al. [7], concluding that total
hydrocarbon (THC), CO and NOx emissions at operating conditions
can be reduced by adding 30% ethanol by volume to the fuel, but
unburned ethanol and acetaldehyde emissions increase. Bahattin[8]studied different gasolineethanol blends, determining that the most
suitable fuel in terms of performance and emissions was E50 (50%
gasoline +50% ethanol), in which the specic fuel consumption as
wellas CO, CO2, HC andNOx emissions were reduced by about 3%,53%,
10%, 12% and 19%, respectively. Najaet al.[9]obtained good results
studying experimentally the performance and pollutant emissions of a
four-stroke spark ignition engine operating on ethanolgasoline
blends of 0%, 5%, 10%, 15% and 20% with the aid of articial neural
network (ANN). A decrease of CO andHC concentrations wasobserved
when the ethanol level was increased in the blend; on the contrary,
NOx concentration increased with ethanol proportion. Ameria et al.
[10] studied theperformance of a combinedheat andpower plant with
an internal combustion engine fueled with a bioethanolgasoline
blend, demonstrating that the maximum cylinderpressure, the output
temperature, the availability of the ue gas for heat recovery and the
efciency of the CHP system increase and carbon monoxide volume
percentage is reduced when bioethanol is increased in the blend.
Several works have been dedicated to study the behavior of
ethanoldiesel blends. The works [1115] reported a reduction in
heating value, aromatics fractions and kinematicviscosity of the blend
and in the emissions of particulate matter and total hydrocarbons
from diesel engines, but there is a small penalty on CO and unburned
ethanol emissions compared to diesel fuel and the behavior of NOxemissions depends on load, fuel additive, catalytic treatment and
other engine parameters. Rakopoulos et al. [16] analyzed a diesel
engine fueled with ethanoldiesel fuel blends, proving that the
reduction in smoke density, CO and in NOxemissions increased with
the percentage of ethanol in the blend with respect to that of the neat
diesel fuel. On the contrary, the emissions of unburned hydrocarbons(HC) increased in proportion with the percentage of ethanol in the
blend. Kim and Choi[17]tested the effect of ethanoldiesel blend on
the emissions in a diesel engine with warm-up catalytic converter,
concluding that THC and CO emissions were slightly increased
whereas smoke and the total mass of the PM were decreased when
ethanoldiesel blends were burnt. Sahin and Durgun [18] developed a
numerical investigation about the effects of ethanoldiesel fuel blends
on turbocharged direct-injection diesel engines performance and
veried that, at varied equivalence ratios, brake specic fuel
consumption (BSFC) and equivalence ratio reduce and brake effective
efciency and power and ignition delay increase with the percentage
of ethanol in the mixture.
Chen et al. [19] studied the combustion characteristics burning
different esterethanoldiesel blended fuels in a diesel engine,showing that with increasing ethanol in the blended fuel, both
smoke and particulate matter (PM) can be reduced. The reduction of
CO and NOxemissions in diesel engines by introducing bioethanol in
multicomponent diesel fuel mixtures containing fossil diesel fuel (D),
rapeseed oil methyl esters (RME), and ethanol (E) was also tested by
Lebedevas et al.[20]. The experimental and numerical analysis of the
spray characteristics of biodiesel, dimethyl ether (DME), and biodie-
selethanol blended fuels (BDE) in the common-rail injection system
were investigated by Kim et al. [21], concluding that theoverall Sauter
mean diameter has a stable value of30m for biodiesel and BDE20
sprays and 20m for DME spray. The local Sauter mean diameter
distribution as a function of distance from nozzle tip for diesel,
biodiesel and biodiesel 20% ethanol blend is analyzed by Park et al.
[22], concluding that the mean droplet diameter is very similar for the
three fuels in the rst 30 mm from the nozzle tip, but the reduction of
the droplet sizefrom 39to 32m inthe regionbetween 30and 40 mm
from the nozzle tip is only observed for diesel atomization, while
biodiesel and ethanol blended biodiesel have a similar tendency of
atomization with a graduallydecreasing of droplet size over theentire
range of measurement, down to 41m for biodiesel and to 35m
for ethanolbiodiesel blend.
Only a few references about the behavior of ethanoldiesel blendsin
boilers have been found. One of them is the experimental investigationabout the combustion of various kerosenediesel and ethanoldiesel
fuel blends in a continuous ow combustor presented by Asfar and
Hamed[23], where remarkable improvement in combustion quality
and reduction in pollutants and soot mass concentration in the exhaust
are reported, as well as an unavoidable slight raise in NOxemissions.
Increasing the percentage of alcohol in the blend beyond 10% does not
seem to improve combustion or reduce pollutants and soot any further.
The effects of the mixing of alcohol with liquid fuels on the combustion
in furnaces are briey presented by Prieto-Fernandez et al. [24]. They
found that the addition of methanol or ethanol to light oil reduces the
amount of unburnt gas hydrocarbons and solid particulates in the
exhaust gases. On the contrary, the addition of up to 15% of ethanol to
light oil results in a slight decrease in the formation of nitrogen oxides,
butfor ethanolpercentages above 15%the emission of nitrogen oxidesis
greater than that of pure light oil.
Despite the lot of work developed on the combustion of alcohol
light oil blends in engines, the use of this kind of blends in burners has
not been deeply investigated. The present work is an attempt to
contribute to the knowledge in this eld, comparing the combustion
of bioethanol and a conventional heating oil (named as gasoil C in
Spain) in boilers. The work has been subdivided in two parts; the
experimental study of combustion and the simulation of fuel switch-
ing in the operation of large industrial boilers.
2. Experimental facilities and fuel characteristics
Thetests were carried out in a vertical 100 kW experimental boiler
(seeFig. 1) designed and manufactured in the LITEC. An oil burner is
installed in the roof of the combustion chamber, formed by acylindrical water-cooled chamber. The combustion gases are
extracted at the bottom by a chimney.
The burner is a commercial device (SIME MACK 5), with a thermal
power of 33.3 to 46.2 kW, normally used for domestic boilers. Different
Danfoss nozzles with a 60 solid cone spray were used in the tests.
Auxiliary facilities, as fuel and air supplies, gas extraction subsystem,
cooling facility and safety controls, allowed a safe and reliable operation
of the boiler.
Twofuel pipes areconnected to theburner, onefor feeding and the
other one for returning the excess of fuel to the tank. The fuel is
thoroughly ltered (three lter stages) before it passes through the
massow meter, regulation valve, fuel heater and atomizer. The lines
have several bypasses in order to allow the cleaning of the lters and
the start-up operations.The burner was modied so as to achieve a closer control of some
important operating conditions. It is a compact model, incorporating
an air fan, so that the air is admitted directly from the atmosphere
through a manual damper. In order to control and measure the air
ow rate, the air inlet was connected to a compressed air line. The air
ow rate is measured and controlled automatically by means of a
thermal mass ow meter and a control valve.
Special attention was devoted to controlling the atomization
temperature, as it can have a signicant inuence on the drop size
and, hence, on the combustion process. The fuel temperature is usually
measured at the burner inlet, but the injection line and the nozzle are
cooled and heated, respectively, by the combustion air stream and the
radiation from the ame. A thermocouple was installed immediately
upstream of the nozzle in order to determine the actual atomization
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temperature. With air and fuel at room temperature and the fuel
heater off, this sensor revealed that the fuel temperature at the
atomizer reached 40 C for the lower fuel ow rate and ambient air
temperatures, as a result of radiative heating from the ame. In order
to control the atomization temperature, it was automatically con-
trolled by an in-line electrical heater connected to a closed-loop
regulator. The fuel temperature set-point was 45 C in all tests, since
this is about the lowest temperature that could be achieved with fuel
and air at ambient temperature (i.e., fuel heater off).
The pressure in the combustion chamber was controlled by a
manual damper in the exhaust duct and was kept slightly aboveatmospheric in order to avoid errors due to air leaks into the chamber.
An independent water supply is used for the refrigeration of the
boiler, which begins in a 15 m3 water tank. In addition to the feeding
pump, the circuit includes another pump for partial recirculation of
the water at the inlet of the cooling circuit. A control valve and the
recirculation pump allow governing independently the water ow
rate and the temperature in the walls of thecombustion chamber. The
system is adjusted so as to obtain a wall temperature around 60 C,
sufcient to avoid condensations on the inner wall surfaces.
Several lock-in controls automatically turn-off the burner in the
following situations: whenamedetector lossesthe signal, if wall and
exit gas temperatures exceed certain limits, in the event of too low
ow rate of cooling water or if the pressure in the combustion
chamber is outside of the xed range.
Gas analysers, thermocouples, pressure gauges and ow meters
record the main boiler parameters along the test. O2, CO2, SO2, CO and
NOx in the exit gases are measured by two on-line gas analysers. In
each test, conditions were kept stable at least for 192 s, which is the
time needed to determine the Bacharach index. Gas composition,
ow, temperature and pressure of fuel, mass ow rate of combustion
air, temperatures of exhaust gases and inlet and outlet cooling water
were recorded every second along the test. The ow rate of cooling
water was manually recorded at the start and end of the tests.
Gasoil and bioethanol were the fuels used in the tests. Their
characteristics are displayed inTable 1.
3. Use of bioethanol in gasoil burners: some practical issues
3.1. Flame detection
The ame detection system is a very important safety control in
boilers. This system cuts thefuelsupply when theame islost and itdoes
not allow starting until a ame is detected in the combustion chamber.
Flame detection in SIME MACK 5 burners for light oil is based on a
photo-resistance sensor. This type of detector responds to the heat
received from the ame by radiation and is adequate for highly
radiatingames, as that of gasoil. However, it is not suitable for blue
ames (e.g., natural gas) due to their much lower radiating power.
Bioethanolames are less radiating than those of gasoil, mainly in its
Fig. 1.Experimental boiler with thermal capacity up to 100 kW.
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root, where thedetector is directed (see Fig.2). This frequently caused
false diagnostics of ame failure and, hence, burner shut-downs. Adifferent kind of detector is, therefore, needed for bioethanol ames.
In this work, the problem wassolved by installing an ultraviolet cell of
the type used in gas burners.
3.2. Cavitation
Pump cavitation is another undesirable problem detected on
switching from gasoil to bioethanol. The low boiling temperature
(vaporization pressure) of bioethanol at atmospheric conditions can
lead to cavitation inside the pump, reducing its useful life and
inducing harmful pressure uctuations in the system.
It was observed in the test that bubbles appeared in the pump
dischargeeven when fuel deposit andpump were situatedat thesame
height. Xing et al. [25] measured bubble-point vapor pressures for
ethanol in the 324352 K temperature range, proving that Antoine's
equation describes with satisfactory precision the correlation be-
tween vapor pressures and equilibrium temperatures,
lnp= AB
TC 1
where,p is the vapor pressure in kPa, Tis the equilibrium temperature
in K, and A,B, Care constants with the following values for ethanol:
A =17.141,B =3906.2 andC=39.56.
Extrapolating this equation to 45 C=318.15 K (bioethanol tem-
perature in the tests), the bubble-point vapor pressure is 23 kPa
(0.7 gauge bar). Despite the vaporization pressure suggest the
possibility of situating fuel deposit at identical level that pump,
actually, owing to the pressure losses in the network, it is necessary to
place the fuel deposit at a higher level in order to avoid cavitations
problems. The problems disappeared when the fuel tank was placed
5 m above the pump.
3.3. Changes in viscosity
Kinematic viscosity is another important parameter for fuel
atomization. The burner used in the experiments allows atomizing
fuels with a maximum viscosity of 6 mm2/s at temperatures of 20 C,
therefore the nozzle should achieve a suitable atomization burning
bioethanol with viscosities around 1.54 mm2/s.
On the other hand, the viscosity could also affect fuel pump
reducing their efciency and useful life, but a set of tests developed
provedthat pumps with bioethanol have a similar behavior to theone
measured with gasoil in the range of work tested and that the
characteristic curve of the pump remained unchanged after operating
with bioethanol for
300 h.
3.4. Changes in fuelow rate and thermal input
As shown in Table 1, the properties of bioethanol are notably
different from those of gasoil. Particularly relevant are the changes in
density, heating value and oxygen content. Direct fuel substitution is
not possible, but the ow rates of fuel and air must be readjusted in
order to achieve a good performance. Among others, two basic
options can be considered:
Operation with the same atomization pressure used with gasoil
Operation with the same thermal input
Both alternatives have been specically addressed in this work.
The followingsection describes the experimental program as well as a
study of the practical implications for both alternatives.
4. Experimental study
4.1. Tests program
4.1.1. Tests with constant fuel owThe objective of this test series is to compare the combustion
behaviour of both fuels for a similar mass ow rate of fuel. These tests
were performed using the same nozzle and atomization pressure.
The mass ow of fuel discharged by a pressure nozzle can be
determined by the well-known equation[26],
Bf =CDAo ffiffiffiffiffiffiffiffiffiffiffi ffiffiffiffiffi2fpfq kg=s 2
Table 1
Main characteristics of fuels used in the tests.
Parameter Unit Gasoil Bioethanol
Chemical name From C10H20to C15H28 C2H5OH
Carbon (C) %m 85.08 52.14
Hydrogen (H) %m 13.31 13.13
Oxygen (O) %m 1.38 34.73
Nitrogen (N) %m b0.1 0.00
Sulphur (S) %m 0.13 0.00
Ash (A) %m 0.00 0.00Humidity (W) %m 0.00 0.00
Stoichiometric air Nm3/kg 10.67 6.95
High Heating Value MJ/kg 45.54 29.80
Low Heating Value MJ/kg 42.51 27.43
Density at 20 C kg/m3 863 788
Flash temperature C 64 13
Ignition temperature C 230 366
Boiling temperature
at atmospheric pressure
C 160385 78.5
Vaporization pressure at 20 C bar 0.003 0.059
Vaporization pressure at 45 C bar 0.25
Kinematic viscosity at 40 C mm2/s 4.35.2 1.04
Kinematic viscosity at 20 C mm2/s 1.54
Fig. 2.Pictures of gasoil and bioethanol ames for 0.50 GPH nozzles with atomization
pressure of 9 bar and O2=0.6%. a) gasoil ame, b) bioethanol ame.
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whereCDis the nozzle discharge coefcient,Aois the cross section of
the nozzle discharge orice, f is the fuel density andpfis the gauge
atomization pressure.
A rst series of tests was performed with the same atomization
pressure and nozzle used for gasoil. According to Eq. (2), the
difference in mass ow rates with bioethanol and gasoil can be
related to their density ratio as,
Bb
Bg=
Qb
Qgbg
=ffiffiffiffiffibg
s 3
beingQthe volumetric ow. The subscript bis for bioethanol and g
for gasoil.
Therefore, the mass and volumetric ow of bioethanol will be 0.96
times lower and 1.05 times higher, respectively, than the
corresponding values for gasoil.
The ratio of thermal inputs () introduced to the furnace for both
fuels is determined by,
Wb
Wg=
LHVbLHVg
BbBg
4
Having into account the lower heating values (LHV) of both fuels,the thermal power introduced to the furnace is 1.48 times higher
burning gasoil than bioethanol for the same atomization pressure.
On the other hand, the proportion of stoichiometric air in Nm3/
kgfuel is 1.54 times higher for gasoil than for bioethanol. So, for the
same nozzle and atomization pressure, the air mass ow needed for
the stoichiometric combustion of gasoil is 1.47 times greater than for
bioethanol. If the fuel pressure and the air regulation are kept
constant, the excess air would be too high and the oxygen excess in
the furnace would increase in 7.7% when bioethanol is burned,
which would lead to a poor performance and even ame blow-off.
Therefore, it is necessary to reduce the air ow in 1.47 times, in order
to get the same excess of air if the injection pressure is kept constant
when replacing gasoil by bioethanol.
4.1.2. Tests with constant thermal inputA direct fuel switching, with the same nozzle and atomization
pressure leads to a signicant reduction of thermal input. In general,
this would notbe acceptable, butthe burnershould be readjusted so as
to satisfy the energy demanded by the process in which it is installed.
In order to obtain the same quantity of energy with the two fuels
tested, it is necessary to increasethe consumption of bioethanol in 1.55
times with respect to that of gasoil (proportion between their caloric
values). There are two ways for rising fuel ow: using the same nozzle
butsetting an atomization pressure forbioethanol higherthan theone
used forgasoil, or using thesameatomizationpressure, butinstalling a
nozzle with a higher capacity than that used with gasoil.
The atomization pressure for bioethanol should be 2.24 times
higher than the one for gasoil in order to get the
ow rate required.However, such a wide pressure range is not normally possible with
commercial burners (like the one used in these tests).
If atomization pressure is kept constant, a nozzle with a larger
discharge area needs to be installed. The relative change in the size of
the nozzle can be estimated from the ratio of mass ow rates by
means of the Eq. (2). According to the usual standardization, the ratio
between the ow areas of the nozzles can be expressed in terms of
their GPH (gallons per hour),
AobAog
= BbBg
ffiffiffiffiffigb
s = 1:662 =
GPHbGPHg
5
For example, a nozzleof 0.5 GPH working with gasoil produces the
same energy as a nozzle of 0.811 GPH working with bioethanol; and
one of 0.75 GPH of gasoil is identical to one 1.216 GPH of bioethanol.
Since the size of the nozzles must be selected among those
commercially available, the results with gasoil using nozzles of 0.5
and 0.75 GPH were compared, respectively, with data for bioethanol
with nozzles of 0.75 and 1.25 GPH. In both cases, the combustion of
gasoil and bioethanol can be compared for a very similar thermal
power (differences b8%).
The comparison of the thermal inputs to the furnace for both fuels
is shown inTable 2.
4.1.3. Thermal balance in the boiler
Measured parameters were processed to estimate the efciency
and heat losses in the boiler for the different cases. This section
describes the variables involved and the calculation procedure.
The useful heat is calculated by subtracting from the total power
released the losses as sensible heat in the exhaust gases ( q2), due to
chemical and mechanical unburnt emissions (q3andq4, respectively),
and due to convection and radiation to the surroundings (q5).
The heat lost with the exhaust gases is determined as a function of
gas (Hgas) and air (Hair) enthalpies,
q2
= 100HgasHair
Qd% 6
Where Qd is the energy introduced by the fuel in kJ/kg, determined
as the sum ofthe lower caloric value and the sensibleheatof the fuel.
The chemical unburnt loss can be determined from the carbon and
sulphur contents in the fuel (Ctand St) andthe composition of exhaust
gases (CO2, SO2and CO), by the following equation,
q3 = 237 Ct+ 0:375St CO
CO2 + SO2 + CO
100
Qd% 7
The mechanical unburnt loss is determined, whenever the
concentration of unburnt particles in the exhaust gases (Cnq) is
known, by the following equation:
q4 = 10032700
QdCnq % 8
The heat loss by convection (Qcon) and radiation (Qrad) to the
ambient is calculated by,
q5 = 100QconQrad
BQd% 9
whereB is the mass ow rate of fuel in kg/s.
The heat transfer coefcients required to calculate Qcon and Qraddepend on the specic boiler and on the type of fuel.
Table 2
Comparison of tests with constant thermal input.
Atomization
pressure G/B
Ratio of mass ow
rate for fuels G/B
Ratio of thermal power
for fuels G/B
bar/bar kgs1/kgs1 kW/kW
For 0.50 GPH-G/0.75 GPH-B nozzles
8.7/8.8 0.69 1.08
9.3/9.3 0.70 1.08
15/14.8 0.70 1.09
For 0.75 GPH-G/1.25 GPH-B nozzles
7.6/7.5 0.63 0.98
9.3/9.7 0.61 0.95
14.8/14.9 0.63 0.97
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Finally,the boilerefciencycan be easilydetermined from theheat
losses as
= 100 q2 + q3 + q4 + q5 % 10
4.2. Experimental results
4.2.1. Results of tests with constant fuel ow
Four series of tests were performed, each of them including
measurements for a wide range of oxygen concentrations in the ue
gases. The base case is named 0.50 GPH 60S 100% G: a 0.50 GPH
nozzle with 60 solid angle, with 100% swirl and using gasoil (G).
Bioethanol wasburned in theotherthree series. Thedifference among
them is the level of swirl of the air ow. The strongest swirl (100%)
was achieved with the original conguration of the burner; i.e., the six
inclined slots in the air-stabilisers were maintained open. In the other
two cases, the open section of those slots was reduced to 50%. In one
case (50%L), three of the slots were fully closed with an adhesive tape
(seeFig. 3a). In the other case (50%C), the innermost half of the six
slots was sealed (seeFig. 3b). These modications were an attempt to
improve the stability of the ame, since some oscillations in the
attachment of the bioethanol ame to the burner exit were observed
in the preliminary trials with the same nozzle used for gasoil
(constant fuel ow rate), specially for low injection pressures. The
partial sealing of the swirl slots proved an effective means to improve
ame stability. Results for the different congurations of the ame
stabiliser are compared for the test series with constant fuelow rate.
In order to provide a complete description of combustion
performance with both fuels, the results with gasoil and bioethanol
were compared for different airfuel ratios.
The variation of CO emissions with respect to oxygen is shown for
two atomization pressures in Fig. 4. The emissions are consistently
smaller with gasoil. At high excess air, both fuels display a plateau,
with CO levels 100 ppm lower with gasoil. The steep increase in CO
emissions as airfuel ratio decreases is observed at [O2]b1.5% for
bioethanol and is delayed until [O2]b0.7% for gasoil. No signicant
inuence of atomization pressure is observed for any of the fuels.
With bioethanol, the congurations with the swirl slots partially
closed display similar results, withCOlevels slightly lower than those
obtained with maximum swirl.
It should be noted that the inner wall temperature of the
combustion chamber was probably lower for the bioethanol ames,
due to the reduced thermal input. This might contribute to some
extent to the increased CO emissions for bioethanol, but with the data
availableit was not possible to assessthe importance of this effect. For
this reason, tests with constant thermal input are considered more
representative of the inuence of the fuel type on CO emissions.
The hypothesis of an excessive cooling of the combustion chamberwhen burning bioethanol is supported by the comparison of gas
temperature at the boiler exit, which, as it can be observed in Fig. 5, is
signicantly lower than with gasoil.
NOx emissions, corrected to an oxygen concentration in ue gases of
3%, are represented in Fig. 6 forgasoil and bioethanol. A smooth raise of
NOxemissions is observed as the level of oxygen increases until 1.5%,
for allthe fuels and burner congurations tested.A difference of around
60 ppm exits between the emission of NOx for gasoil ames and the
values obtained with bioethanol. This difference is attributed primarily
Fig. 3.Modication of air swirler, a) closing the half of air slots; b) closing the central
half of all air slots.
Fig. 4.CO emissions vs. O2in ue gases, burning gasoil and bioethanol with 100% and
50% swirling, closing 3 of the 6 air slots (L) or the central half of all air slots (C).
Fig. 5.Gas temperature vs. O2, burning gasoil and bioethanol.
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to the presence of certain amount of nitrogen in the gasoil, which does
not exist for bioethanol (seeTable 1). Also, the lower thermal power of
the bioethanol ame is expected to result in reduced ame tempera-
tures and, hence, in smaller amounts of thermal NO.
The normalized SO2 emission oscillates between 26 and 37 ppm
with gasoil (seeFig. 7). Bioethanol does not contain any sulphur and,
therefore, no SO2emissions were detected.
A strong inuence of the excess of air on the Bacharach Index in
smoke (BI) is observed inFig. 8. In all cases, the levels of opacity in
smoke for gasoil combustion are far beyond the values obtained with
the different alcohol variants. The opacity quickly grows as the excess
air is reduced, reaching the value of 9 for oxygen concentration
around 1% and 0.4% with gasoil and bioethanol, respectively.
The difference in the sooting behavior of both fuels is apparent in
Fig. 2. Gasoil produces a much brighter ame, where large amounts of
soot are already present at theame root. On the contrary, the base of
the bioethanol ame is blue with only a few weak yellowish streaks;signicant amounts of soot are only formed beyond 12 burner
diameters downstream of the ame stabilizer.
4.2.2. Results of test with constant thermal input
The combustion of bioethanol and gasoil was compared for two
different levels of thermal input, which required using nozzles of
different capacity. For the same injection pressure, nozzles of 0.5 and
0.75 GPH with gasoil yield similar thermalinputsas those with 0.75and
1.25 GPH nozzles for bioethanol, respectively (see Table 2). Injection
pressures were varied in the range 7.615 bar, corresponding to the
lower limit for which a stable ame could be sustained and the
maximum operating pressure of the burner. The tests were named as
xGPH60S-ybar-Z, where x is the nozzle capacity, y is the atomization
pressure in bar (gauge) and Z is the initialletter of the fuel tested (G/B),
for example, 0.50GPH60S-8.8bar-G is a gasoil test with a 0.50 GPH
nozzle at atomization pressure of 8.8 gbar.
The emission of carbon monoxide with respect to oxygen is shown
inFigs. 9 and 10, for the nozzle/fuels 0.75/0.50 GPH60S bioethanol/
gasoiland 1.25/0.75 GPH60S bioethanol/gasoil, respectively.
CO emission with bioethanol is higher than with diesel oil for
injection pressures lower than 14.8 bar. This difference diminishes as
the atomization pressure and the oxygen percentage increase, dis-
appearing for pressures around 14.8 bar and being much smaller for
larger nozzles (Fig. 10) at all the pressures. The higher CO emissions
measured for bioethanol is an indication of less complete combustion
than forthe equivalent cases with gasoil. This canbe a resultof a numberof effects: differences in spray characteristics, in the physico-chemical
properties ofthe fuels or inthe airfuel mixing pattern. Sincebioethanol
is less viscous than gasoil, and for the same nozzle design and injection
pressure, drop size is expected to be similar (or even smaller) with this
fuel thanwith bioethanol; therefore, differences in the propertiesof the
spray are not thought to explain the results The lower boiling
temperature of the bioethanol favours a faster evaporation and
combustion than for gasoil drops of the same size and, therefore,
Fig. 6. NOx emissions (corrected to 3% O2) vs. O2 in ue gases, burning gasoil and
bioethanol with 100% and 50% swirling, closing 3 of the 6 air slots (L) or the central half
of all air slots (C).
Fig. 7. SO2vs. O2, burning gasoiland bioethanol with 100% and 50%swirling, closing 3 of
the 6 air slots (L) or the central half of all air slots (C).
Fig. 8. Bacharach index vs. O2 in ue gases, burning gasoil andbioethanol with100% and
50% swirling, closing 3 of the 6 air slots (L) or the central half of all air slots (C).
Fig. 9.CO vs. O2, burning gasoil and bioethanol at similar input energy (0.75 GPH for
bioethanol and 0.50 GPH for gasoil).
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would be expected to lead to a more efcient combustion. However,
evaporation rates might be enhanced in the much more radiating gasoil
amethan in thebioethanol ame, which is blue in its root and, visually,
much less bright in global terms; therefore, this may explain, at least in
part, the increased CO emissions for bioethanol. Changes in the airfuel
mixing pattern are also anticipated due to the lower ratio of air-to-fuel
mass ow rates with bioethanol (by 1.47 times with respect to gasoil).
Therefore, the capacity of the air stream to drag and disperse the drops
of fuel is smaller and can lead to a slower mixing process in the
bioethanol ame. Botheffects (delayed evaporationand slowermixing)
may lead to an enhanced axialpenetration of the spray and, therefore, a
slowerdispersion of the fuel intothe oxidizer stream. This interpretation
is conrmed by the increased visible ame length for bioethanol with
respect to the equivalent cases with gasoil (seeFigs. 11 and 12). Since
the amount of air per kg of fuel cannot be adjusted arbitrarily, some
modicationson thegeometry of thethroatand thestabilizer of burners
designed for gasoil might be necessary to improve the aerodynamics of
theame and minimize CO emissions when operated with bioethanol.
The comparison of photographs of the ame obtained burning
gasoil and bioethanol, with similar thermal power inputs (nozzle of0.75 GPH for bioethanol and 0.50 GPH for the gasoil and 9 bar of
atomization pressure), for a low and high Bacharach index are shown
inFigs. 11 and 12, respectively. In these gures, it is clearly observed
that the gasoil ame contains a much higher amount of soot than the
bioethanolame. This can to a certain extent be advantageous by the
higher ame emissivity, but it also has the disadvantage of an
enhanced tendency to the generation of black smoke. Also it can be
appraised, more clearly in theFig. 11, that the gasoil ame is shorter
than the one of bioethanol. This observation seems coincident with
the reasoning pointed previously about which the mixture process is
slower for bioethanol, because this fuel needs a small airow by mass
unity of fuel and thereforethe airow hasa smaller capacity to reduce
the axial speed of the bioethanol drops, which completed its
combustion to a greater distance of the burner.
CO emissions are similar at high pressures, possibly because the
drops aresufciently small to be easilydragged and entrained into the
airstream, which together with theshorter evaporation times of small
drops leads to a fast release and combustion of fuel vapour in the high
temperatureame zone, without appreciable emissions of CO.
Thevariation of theBacharach index with respect to oxygenfor the
two sets of nozzles is represented in Figs. 13 and 14. For the same
excess of oxygen, a cleaner combustion is achieved as the injection
Fig. 10.CO vs. O2, burning gasoil and bioethanol at similar input energy (1.25 GPH for
bioethanol and 0.75 GPH for gasoil).
Fig. 11. Photographs forgasoil a) and forbioethanol b), at 9 bar of atomization pressure
and cero Bacharach index. a) Gasoil 0.50 GPH, O2=5.8%; CO =13 ppm, b) Bioethanol
0.75 GPH, O2=0.7%; CO=380 ppm.
Fig. 12. Photographs forgasoil a) and forbioethanol b), at 9 bar of atomization pressure
and high Bacharach index. a) Gasoil 0.50 GPH, O2=0.7%; CO=88 ppm; IB=9, b)
Bioethanol 0.75 GPH, O2=0.1%; CON
5000 ppm; IB=6.
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pressure and thermal input increase for both fuels. In all cases, soot
emission is much smaller for bioethanol than for the corresponding
gasoil ames. The difference observed between both fuels is
attributed to its chemical composition. The ethanol is an oxygenated
molecule, with simple CC bonds and a higher H/C ratio than gasoil,
which results in a much lower tendency to form soot particles in the
ame. The different behaviour of both fuels is still greater than
suggested by Figs. 13 and 14 if it is taken into account that lower
opacities and higher CO emissions (indicative of a less efcient air-
fuel mixing) are simultaneously obtained with bioethanol.
This is an important difference between both fuels, with a clearly
advantageousbehaviour of bioethanol with respect to the gasoil, since
it is possible to burn bioethanol with a reduced excess of air without
appreciable soot emissions. On the one hand, a reduction in the
amount of combustion air results in an increased efciency, since the
mass ow rate of combustion products is smaller and, for the same
exhaust gas temperature, a reduction in the losses by sensible heat in
ue gases is expected. On the other hand, the ame of bioethanol cantolerate better eventual deviations from the optimum value of the air/
fuel ratio and, in general, drifts in burner performance even at such
low level of oxygen in exit gases.
Figs. 15 and 16show the exhaust gas temperature for gasoil and
bioethanol as a function of the oxygen concentration in the ue gases.
The values and their variation with excess air are similar for both
fuels. This conrms that the difference observed inFig. 5was related
to the lower thermal input of the bioethanol ames; when thermal
input is kept approximately the same, as in Figs. 15 and 16, exit
temperatures are very similar for both fuels.
Normalized sulphur dioxide emissions, shown inFigs. 17 and 18,
are very similar to those shown inFig. 7, with negligible values for
bioethanol (sulphur-free fuel).
Fig. 13. BI vs . O2, burning gasoil and bioethanol at similar thermal input (0.75 GPH
nozzle for bioethanol and 0.50 GPH for gasoil).
Fig. 14.BI vs. O2, burning gasoil and bioethanol at similar input energy (1.25 GPH for
bioethanol and 0.75 GPH for gasoil).
Fig. 15.Gas temperaturevs. O2, burning gasoil and bioethanol at similar input energy
(0.75 GPH for bioethanol and 0.50 GPH for gasoil).
Fig. 16.Gas temperaturevs. O2, burning gasoil and bioethanol at similar input energy
(1.25 GPH for bioethanol and 0.75 GPH for gasoil).
Fig. 17.Normalized SO2 vs. O2, burning gasoil and bioethanol at similar input energy
(0.75 GPH for bioethanol and 0.50 GPH for gasoil).
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A practical consequence of the difference in SO2 emissions is a
reduction of the dew point when gasoil is replaced by bioethanol. As
shown in Figs. 19 and 20, the dew point is about 60 C lower for
bioethanol, leading to a reduced risk of condensation and corrosion of
the boiler.
NOx emissions are consistently lower for bioethanol, with differ-
ences inthe range3060 ppmwith respect to theequivalent gasoiltests
(seeFigs. 21 and 22). As previously noted, this is ascribed to a lower
amount of both fuel-NO (negligible for bioethanol) and thermal-NO.
Even though the thermal input wasabout the samefor both fuels,lower
peak temperatures are expected for bioethanol because its adiabatic
ame temperature is100 C than for gasoil, for the same stoichiomet-
ric ratio.
The unburnt heat losses (q3+ q4) are greater in the gasoil than in
bioethanol combustion (see Figs. 23 and 24), but this difference
diminishes when the atomization pressure and fuelow increase. The
heat losses with exhaust gases (q2) are similar for both fuels (see
Figs. 25 and 26).
The heat losses by radiation/convection to the surroundings (q5)
also display a similar behaviour for both fuels (to see Figs. 27 and 28).
The results indicate that the losses by radiation and convection to the
ambient increase with thepressureand fuel ow (greater power). The
small differences observed in this heat loss for the gasoil andbioethanol inFig. 28must be ascribed to the higher power released
in the case of the gasoil, since the nozzle that offers the same power
for bioethanol is of 0.81 GPH, but this size of nozzle is not
commercially available and one with a slightly lower capacity had
to be used in tests (0.75 GPH).
The efciency is determined with these heat losses and the values
obtained are plotted in Figs. 29 and 30. For similar thermal inputs
Fig. 18.Normalized SO2 vs. O2, burning gasoil and bioethanol at similar input energy
(1.25 GPH for bioethanol and 0.75 GPH for gasoil).
Fig. 19. Dew point vs. O2, burning gasoil and bioethanol at similar input energy
(0.75 GPH for bioethanol and 0.50 GPH for gasoil).
Fig. 20. Dew point vs. O2, burning gasoil and bioethanol at similar input energy
(1.25 GPH for bioethanol and 0.75 GPH for gasoil).
Fig. 21.Normalized NOxvs. O2, burning gasoil and bioethanol at similar input energy
(0.75 GPH for bioethanol and 0.50 GPH for gasoil).
Fig. 22.NOxvs. O2, burning gasoil and bioethanol at similar input energy (1.25 GPH for
bioethanol and 0.75 GPH for gasoil).
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(Fig. 30), the efciencies are similar for both fuels. A slightly higher
efciency was obtained with bioethanol when the thermal input for
this fuel was lower than for gasoil (Fig. 29).
5. Assessment of changes in heat transfer due to fuel switching
The experimental results discussed in previous sections reveal
some differences in ame and emissions with gasoil and bioethanol
when both are burnt in the same burner/boiler. The potential impact
on the thermal performance of a boiler is a relevant issue that should
be considered when planning to replace the conventional fuel (gasoil,
in this case) by bioethanol. This section analyses theexpectedchanges
in the heat transfer characteristics of a generic industrial boiler due to
the substitution of gasoil by gasoil/bioethanol blends.
5.1. Methodology
The simulation involves the evaluation of heat transfer in the
different heat-exchange components of a generic, large-capacity
industrial boiler, including: furnace, steam reheaters, economizer
and air heater. The calculation procedure follows the recommenda-
tions of Ref. [27]. The results on unburnt losses determined in the
previous section for gasoil and bioethanol were used as input data for
the calculations.
The gas temperature at the furnace exit is determined by the
following equation,
tHe = tadiab+ 273:15
CTE aF tadiab + 273:15
3
Bb
0:6+ 1
273:15 C 11
Wheretadiabis the adiabatic ame temperature,CTEis a constant
parameter that includes the Boltzmann number, and the area,
efciency and height of the peak temperature zone in the furnace,
aFis the ame emissivity andBbis the mass ow rate of fuel.
The calculation of every heat exchange surface is solved iteratively,
using the balance of energy and heat transfer equation for each
surface.
The energy delivered by the hot uid (the combustion gases),
Qi = Bb Hg in iHg out i kW 12is equal to the one absorbed for the cooler uid (steam in reheaters,
water in economizer and air in air heater),
Qi = Di hout ihin i kW 13
Where Hg and h are the enthalpies of gas and cold uid,
respectively and D is the mass ow of cold uid. Subscript i takes
the name ofshfor superheater, rhfor reheater, ecofor economizer,
Fig.23. Unburntheat losses vs. O2, burning gasoil and bioethanol at similar input energy
(0.75 GPH for bioethanol and 0.50 GPH for gasoil).
Fig.24. Unburntheat losses vs. O2, burning gasoil and bioethanol at similar input energy
(1.25 GPH for bioethanol and 0.75 GPH for gasoil).
Fig. 25.Heat losses with exhaust gases vs. O2, burning gasoil and bioethanol at similar
input energy (0.75 GPH for bioethanol and 0.50 GPH for gasoil).
Fig. 26.Heat losses with exhaust gases vs. O2, burning gasoil and bioethanol at similar
input energy (1.25 GPH for bioethanol and 0.75 GPH for gasoil).
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and ahfor air heater. The conditions are denoted with subscript in
for inlet and outfor the outlet of each heat exchanger.
The closure is accomplished by the heat transfer equation,
Qi = UiAi TiP
kW 14
BeingUthe heat transfer coefcient, A the heat transfer area, and
Ti the logarithmic mean temperature.
The gas speed across every heat exchanger is required for
determining heat transfer coefcients and its value depends on the
consumption of fuel, which in his turn depends on the gas temperature
at the exit of the boiler (outlet of air heater).
At the beginningof the calculation process the consumption of fuel
is assumed, then the heat exchanges are solved in every heat-transfer
surface and the gas temperature at the boiler and the associated heat
losses are determined. Once the heat lost with the exhaust gases is
known, the boiler efciency is determined by losses method
(Eq. (11)), andfuel consumption (B) and burnt fuel(Bb) arecalculated
by the following equations,
B= QuLHV
100 kg=s 15
Bb = B100q4
100
kg=s 16
WhereQuis the useful heat power of the boiler, determined as the
sum of superheated, reheated and saturated steam powers produced
in the boiler,
Qu = Dsh hout shhin w + Drh hout rhhin rh + Dsat hout sathin w kW
17
Where hin_w is the feed water enthalpy, and Dsatis the massowof
saturated steam removed from the drum for plant necessities.
This process is repeated until the guess and calculated values of
fuel consumption are identical.
5.2. Characteristics of the simulated boiler
The simulations are carried out in a power boiler that produces
superheated and reheated steam at 540 C. The nominal boiler
capacity is 974 t/h of superheated steam at a pressure of 16.7 MPaand 876.4 t/h of reheated steam at 3.6 MPa. The fuel consumption in
the original boilerat nominal conditions is 69.5 t/h. A block diagram of
the heat transfer surfaces of the simulated boiler is shown in Fig. 31.
5.3. Performance of boiler for different gasoil and bioethanol blends
The fact that all heat transfer coefcients are readjusted when the
fuel is changed in an industrial boiler make difcult to correctly
Fig. 29. Efciency vs. O2, burning gasoil and bioethanol at similar input energy
(0.75 GPH for bioethanol and 0.50 GPH for gasoil).
Fig. 30. Efciency vs. O2, burning gasoil and bioethanol at similar input energy
(1.25 GPH for bioethanol and 0.75 GPH for gasoil).
Fig. 28.Heat losses by convection/radiation to the surroundvs. O2, burning gasoil and
bioethanol at similar input energy (1.25 GPH for bioethanol and 0.75 GPH for gasoil).
Fig. 27.Heat losses by convection/radiation to the surroundvs. O2, burning gasoil and
bioethanol at similar input energy (0.75 GPH for bioethanol and 0.50 GPH for gasoil).
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evaluate the fuel consumption needed to produce a given amount of
steam. In order to evaluate the inuence of the change of fuel, the
boiler performance is simulated by the calculation procedure
described inSection 5.1.
The main results of simulations for gasoil/bioethanol blends in the
boiler are shown inTable 3, keeping the design values for the oxygen
excess in exhaust gases at 1.23%, for the fraction of gas recirculation
(r) to the furnace at 0.175, and for the cooling water in the
superheater (Dcool) at 17 t/h.
As shown inTable 3, despite the efciency slightly increases when
the fraction of bioethanol in the blend is raised, an appreciable
penalization in the amount of useful heat (reduction in the steam
ows) is veried, maintaining the gas fraction recirculated to the
furnace, the cooling water ow in the superheater and the air excess
at their design values.
With the purpose of achieving the desired values of steam
production for high fractions of alcohol in the blend, different
modications to the design conditions were tested. A reduction in
the gas recirculation fraction to the furnace led to a marginal
improvement in boiler performance, not enough to achieve that
objective. Thus, this change had to be combined with lower values of
the oxygen excess in exhaust gases and of the cooling water ow in
superheater in order to increase performance when reducing the
amount of gasoil in the blend. The results obtained by modifying
simultaneously the oxygen excess, the cooling water ow and the gas
recirculation fraction are illustrated inTable 4.
The results show that it is practically impossible to reach thenominal capacity of the boiler studied for proportions of bioethanol in
the blend higher than 40%, even by adjusting the three parameters
mentioned at their minimum values; further reductions are not
feasible since the recirculation gas fraction was set at 0%, the cooling
waterow cannot exceed its maximum design value, and lastly, it is
not convenient to reduce the oxygen excess under the value in which
unburnt losses grow abruptly. The inuence of blend composition on
efciency and useful heat in the boiler is easier to observe in the
Fig. 32for the case in which the recirculation fraction, the cooling
waterow and the air excess are simultaneously changed.
Fig. 31.Block diagram of the heat transfer surfaces of the simulated industrial boiler.
Table 3
Main results from simulations burning a blend of gasoil and bioethanol.
x B Dsh Drh Qu tge tadp
% kg/s t/h t/h kW C C
0.0 95.27 15.927 572.8 393.8 424,226 128.3 60.20.1 95.21 16.257 610.4 436.0 454,281 130.9 112.0
0.2 95.14 16.567 647.8 479.1 484,292 133.5 115.6
0.3 95.08 16.891 686.5 524.8 515,502 136.0 117.5
0.4 95.01 17.227 726.4 573.1 547,869 138.5 118.7
0.5 94.94 17.575 767.7 624.0 581,474 141.0 119.6
0.6 94.86 17.927 809.9 677.2 615,964 143.4 120.2
0.7 94.79 18.310 854.6 734.4 652,565 146.0 120.6
0.8 94.71 18.699 900.3 794.2 690,213 148.6 120.9
0.9 94.63 19.094 947.2 856.6 728,989 151.2 121.1
1.0 94.54 19.507 996.0 922.6 769,498 153.8 121.3
Nomenclature: x is the fraction of gasoil in the blend, is the boiler efciency,B is the
fuel consumption,DshandDrhare superheated and reheated steam ows, respectively,
Qu is the usefulheat, tge and tadp arethe temperatures ofthe exhaust gasesand acid dew
point, respectively.
Table 4Main results burning a blend of gasoil and bioethanol modifyingr, O2andDcool(cooling
water in superheater).
x B Dsh Drh Qu tge r O2 Dcool
% kg/s t/h t/h kW C % t/h
0.0 94.7 21.1 745.1 564.1 558801 146.6 0.000 0.4 35.0
0.1 94.7 21.7 799.5 628.5 602663 148.0 0.000 0.4 35.0
0.2 94.7 22.3 854.5 695.5 647325 149.5 0.000 0.4 35.0
0.3 94.7 22.8 911.7 766.7 693926 151.1 0.000 0.4 35.0
0.4 94.6 23.5 971.1 842.4 742612 152.9 0.000 0.4 35.0
0.5 94.6 22.9 981.9 873.5 753781 153.7 0.040 0.5 35.0
0.6 94.6 22.0 982.6 873.7 754253 153.4 0.050 0.9 30.0
0.7 94.6 21.1 977.6 872.6 750858 153.3 0.090 0.8 25.0
0.8 94.6 20.4 979.7 870.7 751959 153.0 0.100 1.0 19.0
0.9 94.6 19.7 975.7 880.1 750627 153.4 0.150 1.0 17.0
1.0 94.5 19.1 976.2 899.4 753496 154.1 0.195 1.2 17.0 Fig. 32. Efciency and useful heat vs. proportion of gasoil in the blend, changing
recirculation fraction, cooling water ow and air excess.
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8/14/2019 Some considerations about bioethanol combustion in oil-fired boilers
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The simulation results indicate that a fuel change from gasoil to
bioethanol/gasoil blends with gasoil proportions lower than 0.5
should not be accomplished in an industrial boiler designed for gasoil
or fuel oil, unless considerablemodications in thedesign of theboiler
are implemented.
6. Conclusions
A series of combustion tests and simulations have been performedin order to evaluate some of the potential issues that may arise when
gasoil is replaced by bioethanol or its blends with gasoil in heating or
industrial boilers.
In the rst place, the operation of the burner (designed for gasoil)
must be modied in order to achieve a similar thermal input with
bioethanol, due to the great differences in the heating value and
composition (especially, oxygen content) of both fuels. The capacity of
the atomization nozzle and the air mass ow rate should be modied
in order to work with higher fuel rates and relative lower air ow
rates. The smaller airfuel mass ratio in bioethanol ames results in a
higher axial penetration of the spray and, hence, can hinder ame
stabilization; this may require some redesign of the ame stabiliser.
The much lower luminosity of the alcohol ame may prevent the use
of some of the ame detectors normally used for gasoil, which should
be substituted by other devices (e.g., UV detectors). Also, special
attention should be given to cavitation problems in the fuel feeding
system.
Bioethanol displayed a much smaller tendency to soot formation
than gasoil. This causes the ame to be much less luminous and
enables keeping the opacity index at very low values down to very
low excesses of oxygen. The alcohol does not contain nitrogen and
sulphur, which results in no SO2emissions and NOxvalues about half
of those measured forgasoil. In many cases, CO emissions were higher
for bioethanol. This is ascribed to the fact that the burner was
originally designed for gasoil and some modications or adjustments
may be necessary to optimise bioethanol combustion; for example,
differences between CO emissions with both fuels disappeared for
high injection pressures.
The various heat lossesand the boilerefciency were evaluated for
both bioethanol, gasoil and their blends in a generic industrial boilers.
This analysis revealed that fuel switching can modify heat transfer in
theboiler and, hence,affect the steam production capacity. Theresults
indicate that steam production is reduced as the fraction of bioethanol
in the fuel blend increases and that the fraction of gasoil in the blend
should not be lower than 50% to keep at acceptable levels the penalty
in the useful heat production, owing to the low heating value of
bioethanol. In order to counteract this effect, a reduction of gasoil in
the blend should be accompanied by certain modications in the
boiler operation to avoid the reduction in steam production.
Acknowledgements
This work was funded by E&M Combustion and the CDTI throughthe IDEA+2 project. The help of Luis Ojeda with the experimental
tasks is gratefully acknowledged.
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