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

1538 J. Barroso et al. / Fuel Processing Technology 91 (2010) 15371550


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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.

1539J. Barroso et al. / F uel Processing Technology 91 (2010) 15371550


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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.

http://localhost/var/www/apps/conversion/tmp/scratch_7/image%20of%20Fig.%E0%B2%80


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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



6/14

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


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


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


Fig. 17.Normalized SO2 vs. O2, burning gasoil and bioethanol at similar input energy


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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


Fig. 19. Dew point vs. O2, burning gasoil and bioethanol at similar input energy


Fig. 20. Dew point vs. O2, burning gasoil and bioethanol at similar input energy


Fig. 21.Normalized NOxvs. O2, burning gasoil and bioethanol at similar input energy


Fig. 22.NOxvs. O2, burning gasoil and bioethanol at similar input energy (1.25 GPH for


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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


Fig.24. Unburntheat losses vs. O2, burning gasoil and bioethanol at similar input energy


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


Fig. 30. Efciency vs. O2, burning gasoil and bioethanol at similar input energy


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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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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Some considerations about bioethanol combustion in oil-fired boilers

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