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Applied Thermal Engineering 57 (2013) 116e124

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Applied Thermal Engineering

journal homepage: www.elsevier .com/locate/apthermeng

Energetic, ecologic and fluid-dynamic analysis of a fluidized bedgasifier operating with sugar cane bagasse

Paulo Tasso Diniz Filho a,*, Jose Luz Silveira a,*, Celso Eduardo Tuna a,Wendell de Queiroz Lamas a,b,c

a Laboratory of Energy Systems Optimization, Department of Energy, Faculty of Engineering at Guaratingueta, Sao Paulo State University, Brazilb Post-graduate Programme in Mechanical Engineering, Department of Mechanical Engineering, University of Taubate, BrazilcDepartment of Basic and Environmental Sciences, Engineering School at Lorena, University of Sao Paulo, Brazil

h i g h l i g h t s

� we develop a methodology to size a fluidized bed gasifier.� we validate this methodology comparing to a fixed bed gasifier values.� we aggregate ecological efficiency to this methodology.

a r t i c l e i n f o

Article history:Received 23 July 2012Accepted 27 January 2013Available online 15 March 2013

Keywords:Biomass gasificationFluidized bed gasifier modellingSugar and alcohol industry

* Corresponding authors. Laboratory of Energy Syment of Energy, Faculty of Engineering at GuaratingueAv. Dr. Ariberto Pereira da Cunha, 333, Pedregulho,Brazil. Tel.: þ55 12 3123 2240; fax: þ55 12 3123 283

E-mail addresses: paulotd@gmail.com (P.T. Diniz(J.L. Silveira), wendell@feg.unesp.br, lamaswq@aol.com

1359-4311/$ e see front matter � 2013 Elsevier Ltd.http://dx.doi.org/10.1016/j.applthermaleng.2013.01.04

a b s t r a c t

This work aims to study the thermodynamic, ecological and fluid-dynamic aspects of a circulating flu-idized bed gasifier using sugar cane bagasse as biomass, in order to estimate a model of its normaloperation. In the initial stage was analysed the composition of biomass selected (sugar cane bagasse) andits lower heating value (LHV) was calculated. The energy balance of the gasifier was done, being thevolumetric flow of air, synthesis gas and biomass estimated. Also the power produced by this gasifier wastheoretically estimated. Then the circulating fluidized bed gasifier was designed for operation withapproximately 100 kg/h of processed biomass. Cross-sectional area of the reactor, feeder size, diameter ofthe exit zone of the gases and minimum height of the expanded bed were selected. Some bed gasifierhydrodynamic factors were also studied. The minimum fluidization velocity, fluidization terminal ve-locity, and average fluidizing velocity were calculated, in order to understand the fluid-dynamicbehaviour of gasification of this fuel. It was obtained a theoretical model that can support a possibleprototype of circulating fluidized bed gasifier biomass. Finally, there were studied the ecological aspectsof the gasifier, through an overall methodology. Ecological efficiencies were estimated for two scenarios:first considering the carbon cycle and thereafter disregarding the carbon cycle. In both cases, it can beproved the ecological viability of the project.

� 2013 Elsevier Ltd. All rights reserved.

1. Introduction

The energy alternatives to replace or to supplement the sourcesused today have been a constant challenge for scholars and re-searchers. According to Ref. [1], there is untapped potential forenergy generation in the Brazilian sugar and alcohol sector, which

stems Optimization, Depart-ta, Sao Paulo State University,12516-410 Guaratingueta, SP,5.Filho), joseluz@feg.unesp.br(W.deQ. Lamas).

All rights reserved.5

could be offered to the utilities and subsequently to society withoutthe need for large investments in the economic order.

One of the factors that contribute to this aspect stems from thefact that, in general, is employed in the biofuels industry powersystems and low pressure steam to the expanded use of steam inthe process, technology known as traditional back pressure cycle.However, this technology has low exergy efficiency, causing the losspart of exergy that could eventually be transformed into heat,mechanical power or electricity.

The biomass gasification has been identified by experts as one ofthe best alternatives for recovery of energy from biomass. This typeof technology does not require a large investment demand and canbe inserted into the production process of ethanol.

Table 2Chemical composition of the synthesis gas generated considering an incompletegasification [11].

Gas Value (%)

Carbon monoxide (CO) 12.0Hydrogen (H2) 4.0Methane (CH4) 3.0

P.T. Diniz Filho et al. / Applied Thermal Engineering 57 (2013) 116e124 117

Fundamentals of gasification have been explained in detailssuch as technologies, modelling, simulation, comparison to com-bustions in use etc [2e7].

The sizing of the gasifier was developed in accordance withinformation available in the literature by several authors [8e19],detaching its methodology, modelling, and specific features foreach type of gasifier.

Specifically the fluidized bed gasification had been studied byauthors such as [20e28].

This work aims to study the thermodynamic, ecological andfluid-dynamic aspects of a circulating fluidized bed gasifier usingsugar cane bagasse as biomass, in order to estimate a model of itsnormal operation.

2. Methodology

In the initial stage was analysed the composition of selectedbiomass (sugar cane bagasse) and its lower heating value (LHV) wascalculated through the methodology reviewed in the literature[15,29,30]. Fromthe calculated LHV, the energybalance of the gasifieroperating with this type of biomass was performed. The volumetricflow of air, synthesis gas (syngas) and biomass were estimated.

Then the circulating fluidized bed gasifier was designed foroperation with approximately 100 kg/h of processed biomass.Cross-sectional area of the reactor, feeder size, diameter of the exitzone of the gases and minimum height of the expanded bed wereselected.

Some bed gasifier hydrodynamic factors were also studied. Theminimum fluidization velocity, fluidization terminal velocity, andaverage fluidizing velocity were calculated, in order to understandthe fluid-dynamic behaviour of gasification of this fuel.

The calculations were divided separately for each segment asfollows: properties of bagasse; energy balance; mass flow ofbiomass and volume of air and synthesis gas; methodology for thegasifier sizing; fluid-dynamic study of the gasifier; ecologicalefficiency.

3. Results and discussion

3.1. Properties of the bagasse

The chemical composition of sugar cane bagasse on a dry basisfor the development of a bullet mass process is formed by carbon(C), hydrogen (H), nitrogen (N) and oxygen (O) [31]. This value wasobtained after analysis of various types of species. Table 1 showsthese elements and their contribution for the chemical compositionmentioned.

Table 2 shows the composition of synthesis gas generated fromincomplete sugar cane bagasse gasification.

This table shows the expected concentrations of the energeticcompounds in the fuel gas (% volumetric), information given byOlivares-Gomez [11]. It is not the complete composition of thesynthesis gas or the fuel gas. The energy compound on the gas is inmajority given by CH4, H2, and CO.

The feed moisture content is 20%; therefore it is necessary totransform the composition of the biomass on dry basis [11]. Table 3

Table 1Basic composition of sugar cane bagasse in natura [31].

Element Value (%)

Carbon (C) 44.80Hydrogen (H) 5.35Nitrogen (N) e

Oxygen (O) 39.55Sodium (Na) 0.01

shows the transformation of the composition on a dry basis of9.85% to work with biomass of 20% wet basis.

3.2. Energy balance

According to the energy balance, the total energy entering thecontrol volume (the gasifier) is equal to the total energy leavingcontrol volume. In this case, the enthalpy will be used formeasuring thermal energy per unit mass of air, gas and ash.Therefore, an energy balance of the gasifier can be represented asfollows in Eq. (1).

_mbio$hbio þ _mair$hair ¼ _mgas$hgas þ _mash$hash þ Qenv (1)

where:

hair e enthalpy of the air [kJ/kg];hash e enthalpy of the ash [kJ/kg];hbio e enthalpy of the biomass [kJ/kg];hf e enthalpy of formation [kJ/kg];hgas e enthalpy of the gas [kJ/kg];_mair e air flow in the gasifier [kg/s];_mash e ash flow [kg/s];_mbio e biomass flow in the gasifier [kg/s];_mgas e gas flow produced in the gasification [kg/s];Qenv e heat lost to the environment [kW].

3.2.1. Calculation method for synthesis gas LHV (LHVgas)According to Ref. [30], the composition of synthesis gas on dry

basis uses to be CO2 (13%) and C2H4 (0.19%); C2H6 (0.15%) and C2H2(0.01%); H2 (16%), O2 (0.6%) and N2 (48%); C2H4 (2%) and CO (20%).Thus, the lower heating value for biomass synthesis gas producedin the gasifier is calculated through Eq. (2) [15,29].

LHVgas ¼ 0:126$CCO þ 0:358$CCH4þ 0:108$CH2

þ 0:59$CC2H4

þ 0:637$CC2H6

(2)

where CCO2, CCH4

, CH2, CC2H4

, and CC2H6are the volumetric concen-

trations of the gas produced as a percentage. Therefore:

LHVgas ¼ 0:126$ð20Þ þ 0:358$ð2Þ þ 0:108$ð16Þ þ 0:59$ð0:19Þþ 0:637$ð0:15Þ

LHVgas ¼ 5:172�MJ

Nm3

�:

Table 3Transformation of the composition on a dry basis of 9.85%.

Element Value (%)

Carbon (C) 49.70Hydrogen (H) 5.94Nitrogen (N) e

Oxygen (O) 43.87Sulphur (S) 0.01

P.T. Diniz Filho et al. / Applied Thermal Engineering 57 (2013) 116e124118

3.2.2. Calculation of the enthalpy of the synthesis gas (hgas)For the produced synthesis gas, the enthalpy hgas is calculated

according to Eq. (3).

hgas ¼ CCO2$hCO2

þ CCH4$hCH4

þ CH2$hH2

þ CN2$hN2

þ CO2$hO2

þ CCO$hCO þ CH2O$hH2O

(3)where helem is the enthalpy of each component gas. Table 4 showsthe composition of the synthesis gas.

The enthalpy of each component of the exit gas can be deter-mined using the specific heating at constant pressure (cp),expressed as a function of temperature (T) in kJ/kmol K [32].

cpCO2¼ 10:34þ 0:000274$T � 195;500

T2

cpCO ¼ 6:6þ 0:0012$T

cpO2¼ 8:27þ 0:000258$T � 187;700$T2

cpH2¼ 6:62þ 0:00081$T

cpN2¼ 6:5þ 0:00100$T

cpCH4¼ 5:34þ 0:0115$T

cpH2O ¼ 8:22þ 0:00015$T þ 0:00000134$T2

The enthalpy of the gas from each element is given by Eq. (4).

h ¼ hf þZTf298

cp$dT (4)

By calculating the specific heating for an approximate temper-ature Tf of 600 �C, the Eq. (4) can be solved.

hCO2¼ �366:87

kJmol

hCO ¼ �92:93kJmol

hO2¼ 18:326

kJmol

hH2¼ 16:714

kJmol

hN2¼ 17:38

kJmol

hCH4¼ �45:33

kJmol

Table 4Composition of the produced gas volume produced in wet basis.

Element Percentage

CO2 13.4CO 13.4O2 0.9H2 17.9N2 40.2CH4 3.6H2O 10.6

hH2O ¼ �220:752kJ

mol

These values are introduced in Eq. (3).

hgas ¼ 0:134$ð�366:87Þ þ 0:036$ð�45:33Þ þ 0:179$ð16:714Þþ 0:179$ð16:714Þ þ 0:402$ð17:38Þ þ 0:09$ð18:326Þþ 0:134$ð�92:93Þ þ 0:106$ð�220:752Þ

hgas ¼ �76:5012kJmol

:

This amount is divided by the molecular mass of gas(24.034 g/mol).

hgas ¼�76:5012

kJmol

24:034g

mol

$1000g

mol

hgas ¼ �3183:04kJkg

The negative signal is due to the enthalpy of formation of certainelements of the gas composition [32].

3.2.3. Calculation of biomass LHV on a wet basisUsing the equation developed by Makray [33], Eq. (5), the lower

heating value (LHV) on wet basis of the selected biomass can becalculated [31].

LHVwet ¼ HHVbio$

�1� Wu

100

�� 22:11$Hs � 0:442$

��Wu

18

�

��Hs$Wu

2

��(5)

LHVwet ¼ 14;870kJkg

where:

LHVwet e lower heating value of the wet bagasse [kJ/kg];HHVbio e higher heating value of the biomass in the dry basis[kJ/kg];Wu e humidity of the biomass in the wet basis [%];Hs e hydrogen amount in the dry basis [%].

3.2.4. Calculating the amount of energy required for the processAn average power of Ed ¼ 254 kW in the gasifier was selected.

This value had considered the experience provided by Refs. [15,18].According to Ref. [21], due to incomplete gasification of thebiomass, discontinuities in the feed, among others, the actual po-wer required in the gasifier is always approximately 10e20% higherthan the theoretical operation. Thus, the actual operating powerwas calculated by multiplying the estimated theoretical power bythe safety coefficient fsec ¼ 1.1, Eq. (6).

Et ¼ Ed$fsec (6)

where:

Et e corrected power of the gasifier [kW];Ed e theoretical power of the gasifier [kW];fsec e safety coefficient [e].

Fig. 1 shows the power operation of the gasifier according to theamount of biomass burned.

Table 5Calculated parameters estimated during the theoretical power.

Parameter Value

HHVbio 18.850 kJ/kgLHVwet 14.970 kJ/kgEd 254 kWfsec 1.1Et 280 kW_mbp 112.22 kg/h_mash 24.69 kg/hEmg 60%

P.T. Diniz Filho et al. / Applied Thermal Engineering 57 (2013) 116e124 119

3.2.5. Calculating the amount of biomass processed to meet thetheoretical power of operation

Calculating the actual power of operation, it is now necessary toestimate the amount of biomass to be processed in the gasifier togenerate so much energy. It was considered an average efficiency ofbiomass gasification of Emg ¼ 60%, value obtained through the expe-rience of Refs. [15,18]. The amount of processed biomass can be ob-tained by dividing the actual power operation by lower heating value(LHV) and efficiency Emg, both previously calculated, as Eq. (7).

_mbp ¼ Et

LHVwet$Emg

100

$3600 (7)

where:

_mbp e biomass processed flow [kg/h];Emg e efficiency of the gasifier [%].

The Eq. (7) corresponds to the processed biomass flow into thegasifier. The methodology proposed in this work corresponds to asmall prototype of fluidized bed gasifier, operating with approxi-mately 100 kg/h of biomass. However, according to Ref. [21], due toincomplete gasification of the biomass, discontinuities in the feed,among others, it is necessary to adopt a safety coefficient. Also theoperation power of the gasifier is a little higher than that wasinitially expected. Using the Eq. (7), which is a relationship betweenthe efficiency of the gasifier, the lower heating value (LHV), and thecorrected operation power (Et), it is possible to calculate the pro-cessed biomass flow into the gasifier ð _mbpÞ.

3.2.6. Calculation of the ash produced during gasificationAccording to Ref. [34], the ash produced in the process of gasi-

fication of biomass can be estimated at about 22% of biomass, so itcan be determined through Eq. (8).

_mash ¼ _mbp$0:22 (8)

where:

_mash e ash produced [kg/h].

Several authors had discussed this estimative. The values of theash are always between 15 and 30% for the almost types of biomassused in gasification. The values of the ash content for bagasse in afluidized bed gasifier it is not defined with clarity. Some authorshad explained that the major factor of influence is the percent ofcarbon (C) in the biomass. The value adopted in this work is thesame adopted by Ramirez et al. [34] that have experimental results

Fig. 1. Estimated theoretical power operation.

for rice husk as biomass, and used the same value as estimative. Therice husk and the sugar cane bagasse have approximately the samecomposition and approximately the same combustion behaviour.Therefore, it is possible to use this value.

Table 5 shows the parameters obtained to the required energybalance of the gasifier.

3.3. Volumetric flow of biomass, air and synthesis gas

3.3.1. Determination of required flow of fuelThe volumetric rate of required biomass can be calculated by

dividing the actual power of the gasifier Et by the lower heatingvalue (LHV) of syngas generated. This is known from the globalequation of bagasse combustion calculated through the method-ology proposed by Zainal et al. [35], Eq. (9).

_Q fuel ¼ EtLHVgas

$3600sh

(9)

where:

_Q fuel e fuel flow [Nm3/h];LHVgas e lower heating value of the synthesis gas [MJ/Nm3/h].

3.3.2. Determination of flow rate of air consumption for this gasifierThe gasifier is characterized by a device where there is an

incomplete gasification of the biomass. Therefore, from Eq. (10)overall combustion is estimate. From the equilibrium modeldeveloped by Zainal et al. [35], it is possible to calculate therequired air flow.

½C1H1:434O0:66N0S0:00008� þ 0:35O2 þ 1:32N2/0:638H2

þ 0:6408COþ 0:34CO2 þ 0:374H2Oþ 0:019CH4 þ 1:315N2

(10)

From Eq. (10) of overall combustion, it is possible to calculate thetheoretical air/fuel ratio (TAFR) as Eq. (11).

hairhbagasse

¼ 1:67$kmolairkmolbagasse

(11)

TAFR ¼ 1:67$kmolairkmolbagasse

¼ 1:67$28:552

kgairkmolair

30:04kgwet bagasse

kmolwet bagasse

¼ 1:6kgair

kgwet bagasse

As previously calculated by the item Section 3.4.1, it is knownthe mass of pulp used as a fuel, so the volume of air consumptioncan be expressed by Eqs. (12) and (13).

Table 6Parameters relating to the biomass combustion.

P.T. Diniz Filho et al. / Applied Thermal Engineering 57 (2013) 116e124120

_Qair ¼ _Q fuel$TAFR (12)

Parameter Value

mc 112.22 kg_Q fuel 194.91 Nm3/h_Qair 311.86 Nm3/h_Q syngas 506.77 Nm3/h

_Qair ¼ 1:6$ _Q fuel (13)

where:

_Qair e air flow [Nm3/h];_Q fuel e fuel flow [Nm3/h];_Q syngas e synthesis gas flow [Nm3/h];TAFR e air/biomass rate [kg/kg].

3.3.3. Determination of syngas flowThe syngas flow is the sum of the air flow and fuel gas at

considered control volume (the gasifier), such as Eq. (14).

_Q syngas ¼ _Q fuel þ _Qair (14)

Applying Eq. (13) into Eq. (14) is obtained:

_Q syngas ¼ 2:6$ _Q fuel:

Fig. 2 shows the relationship between the synthesis gas, air flowand produced ash.

Table 6 shows the flows of synthesis gas, air and fuel when112.22 kg/h biomass is treated.

3.4. Methodology for gasifier sizing

It was selected for study a circulating fluidized bed gasifier. Thistype of gasifier was chosen because the synthesis gas generated bythis model has a low tar content, as demonstrated by Williams andLarson [36], and this type is the most recommended and used forthe gasification of bagasse, due to higher amount of synthesis gasgenerated by this type of biomass, as stated by Sanchez [12]. Theincrease of ash content in the solid residue is a consequence of theincrease of the gasification. More biomass is gasified, the amountsynthesis gas increases and thus the ash content in the residue alsoincreases.

3.4.1. Determination of cross-sectional area of the gasifierAccording to Ref. [15], the cross-sectional area of a fluidized bed

gasifier may be obtained by Eq. (15).

Ag ¼_Q fuel_Qair

(15)

where:

Ag e sectional area of the gasifier [m2].

Fig. 2. Relationship of flow of synthesis gas, air and ash.

The Eq. (15) calculates the sectional area of the gasifier. Thisequation was developed by Olivares-Gomez [11]. However, therewas a unit inconsistency in this equation and the corrected versionwas done in this one, using a more precisely estimative done byCoronado-Rodriguez [15] through the experience provided by [30].

The gasifier diameter can be calculated using the cross-sectionalarea calculated through Eq. (16).

Dr ¼�4$Ag

p

�0:5

(16)

where:

Ag e sectional area of the gasifier [m2];Dr e diameter of the bed in the gasifier [m].

3.4.2. Determination of the minimum height of the expanded bedAs Ag, _Q fuel and _mbp were previously calculated, then the value

of the minimum height of the gasifier bed can be obtained throughEq. (17) [15].

hlmin ¼_mbp

Ag$ _Q fuel(17)

where:

hlmin e minimum height of the gasifier bed [m].

The Eq. (17) calculates the minimum height of the gasifier,which is given by a relationship between the sectional area (Ag), themaximum volumetric capacity of the gasifier ð _Q fuelÞ, and the pro-cessed biomass flow ð _mbpÞ. This methodology was proposed byexperimental tests and the experience of Ref. [11] and adopted bythe same author during a construction of a real prototype of flu-idized bed gasifier.

3.4.3. Determination of the diameter of the exit zone of the gasesAccording to Refs. [22,37,38], the diameter of the exit zone of the

gas in the gasifier should be 1.5e2 times the bed of the gasifier.Therefore it is obtained a minimum bed for this gasifier throughEq. (18).

Dsg ¼ 1:5$Dr (18)

where:

Dr e diameter of the bed in the gasifier [m];Dsg e diameter of the gas exit zone [m].

Table 7 shows the parameters obtained for the scaling of thegasifier burning about 100 kg/h of biomass.

Table 9Parameters and hydro-dynamic profiles of the gasifier.

Parameter Value

Number of Archimedes 249.98Fluidization velocity 0.148 m/sFluidization minimum velocity 0.052 m/sTerminal fluidization velocity 3.516 m/sReynolds number for minimum fluidization velocity 0.153Reynolds number for the terminal velocity of fluidization 10.357

Table 7Parameters calculated onto the gasifier sizing.

Parameter Value

Cross-sectional area 0.494 m2

Diameter of the body of the gasifier 0.793 mDiameter of the exit zone of the gas 1.19 mMinimum fluidization bed height 0.622 mPorosity of the bed 0.46Diameter of the bed zone 0.417 mNumber of side holes air nozzles 4

P.T. Diniz Filho et al. / Applied Thermal Engineering 57 (2013) 116e124 121

3.5. Fluid-dynamic study of the gasifier bed

Table 8 lists hydro-dynamic operating parameters for a flu-idized bed gasifier that produces synthesis gas at around 600 �C.These values are based on the methodology adopted by Ramirezet al. [34].

Number of Archimedes: is calculated by the Eq. (19) such asdemonstrated by Kunii and Levenspiel [21].

Ar ¼g$dp3$rf$

�rp � rf

�m2

(19)

Minimum fluidization velocity: is the minimum speed in whichgas can flow into the gasifier bed using biomass such as sugar canebagasse. It can be calculated through Eq. (20) [24].

Umf ¼dp2$

�rp � rf

�$g

150$m� ε

3$f2

1� ε

(20)

Fluidization terminal velocity: consists of the maximumspeed in which the gas can flow into the gasifier bed and is

Table 8Hydro-dynamic operating parameters.

Parameter Value

G 9.81 m/s2

dp 0.379 mmrag30 1.165 kg/m3

rp 2650 kg/m3

rf 0.342 kg/m3

Hmf 750 mmεmf 0.52εf 0.4Ai 0.0045 m2

mag30 1.86 � 10�5 Pa smag760 4.4 � 10�5 Pa sefl 0.56

where:Ai e Synthesis gas escape zone [m2];dp e Diameter of the particle [m];g e gravity acceleration [m2/s];H e height of the expanded bed [m];Hmf e minimum height of the bed [m];Re e Reynolds number;_Uf e fluidization velocity during the gasification [m/s];_Ut e terminal particle velocity [m/s];_Umf e minimum fluidization velocity [m/s];rs e density of material in the bed [kg/m3];rf e density of the air at the temperature and pressure of the gasifier (750 �C and101.3 kPa) [kg/m3];rg e produced gas density at normal conditions pressure and temperature(101.3 kPa and 25 �C) [kg/m3];rp e density of the particle [kg/m3];ε e particle porosity;εf e bed porosity;f e sphericity of particles in the bed;me air viscosity to the temperature and pressure operation conditions of the gasifier(approximately 750 �C and 101.3 kPa).

determined from a relationship between the material andthe bed, also depending on the particle’s Reynolds number [39],Eq. (21).

Ut ¼ dp$

"4$

�rp � rf

�2$g2

225$rf$m

#13

(21)

Fluidizing velocity during the gasification: is the superficial gasvelocity to be used during operation of the gasifier. It has beenestablished taking into account the height of the fluidized bed [23].It can be calculated through Eq. (22).

HHmf

¼ 1þ10:978$

�Uf � Umf

�0:738$r0:376p $dp1:006

U0:937mf $r0:126f

(22)

Since the relationship between the height of the expanded bedand the minimum height of the bed [21]:

1:2 <H

Hmf< 1:4: (23)

For calculation purposes was adopted an average ratio of 1.3.Table 9 shows the parameters calculated in the fluid-dynamic

study of the gasifier.

3.6. Ecological efficiency of the gasifier

The ecological efficiency evaluates the pollutant amount of asystem, considering gases emissions per kg of used fuel. This effi-ciency is ranged between 0 and 1; where an ecological efficiencyequal to 0 means 100% of environmental impact, or high polluter,and efficiency equal to 1 means 0% of environmental impact, ornon-polluter.

Cardu and Baica [40,41] had introduced the concept of carbondioxide equivalent [(CO2)e], based on maximum concentrationallowed for CO2, which is 10,000mg/m3. The equivalent coefficientsfor some pollutants, in kg per kg of fuel (kg/kgfuel), called globalwarming potential (GWP), are related according to Eq. (24) [42e46]. These values consider a time horizon of 100 years for thesegases [47,48].

ðCO2Þe ¼ CO2 þ 1:9$ðCOÞ þ 21$ðCH4Þ þ 42:4$ðH2SÞþ 50$ðNOxÞ þ 80$ðSO2Þ þ 310$ðN2OÞ þ 67$ðPMÞ

(24)

Table 10Emissions of SO2, NOx and PM in the combustion of sugar cane bagasse.

Components Sugar cane bagasse combustion [kg/kgfuel]

SO2 0.0NOx 0.0012PM 0.0071

Table 11Amount of CO2, SO2, NOx and PM emitted during the combustion process.

Components Sugar cane bagasse combustion [kg/kgfuel]

Without the carbon cycle With the carbon cycle

CO2 0.498 0.09433SO2 0.0 0.0NOx 0.0012 0.0012PM 0.0071 0.0071

Table 13Thermodynamic efficiency and ecological efficiency of the gasification of bagasse ina fluidized bed gasifier with the studied design.

hsystem [%] ε [%]

Without the carbon cycle With the carbon cycle

60.0 80.99 93.54

P.T. Diniz Filho et al. / Applied Thermal Engineering 57 (2013) 116e124122

An indicator is proposed by Cardu and Baica [40] to quantifyenvironmental impact and it is defined as the difference betweencarbon dioxide equivalent of fuel and its low heat value. This in-dicator is called pollution indicator represented by Pg, Eq. (25).

Pg ¼ ðCO2ÞeLHV

(25)

where:

(CO2)e e carbon dioxide equivalent [kg/kgfuel];LHV e low heat value of fuel [MJ/kgfuel];Pg e pollution indicator [kg/MJ].

Relating carbon dioxide emitted by fuel combustion processwith its lower heating value [40], makes possible a comparisonbetween different fuels. However a fuel can have a high lowerheating value and to emit a wide amount of pollutants into atmo-sphere or has negligible, or null, emissions of noxious gases, butcannot have the energy required to obtain a good efficiency in anindustrial process.

Based on assumption that the best fuel is one that has the lowestpollution indicator [40], propose a more complex and dimension-less index that expresses the ecological component of noxiousgases emitted into atmosphere from the combustion of a fuelcompared to useful energy produced in thermal power plants. Theindicator proposed is called ecological efficiency (ε), such as Eq.(26).

ε ¼"0:204� hsystemhsystem þPg

� ln�135�Pg

#0:5(26)

According to Refs. [48,49], Brazil has the lowest average annualemissions of greenhouse gases, around 659 kgCO2/t, against worldaverage around 800e880 kgCO2/t.

The ecological analysis is done through comparison betweenecological efficiency, pollution indicator and values for CO2 equiv-alent from cosmetic industry rate, before and after adoption ofwater solar pre-heating.

The molecular weight of the biomass (MWet Biomass) with 20% ofmoisture instead of humidity is 30.04 kg/kmol [35].

Based on Eq. (10), the amount of CO2 produced could be esti-mated empirically.

Table 12Amount of carbon dioxide equivalent of the combustion process in the gasifier andthe pollution indicator.

Components Sugar cane bagasse combustion [kg/kgfuel]

Without the carbon cycle With the carbon cycle

(CO2)e 1.0337 0.630Pg 0.1412 0.0861

30:04kg

kmolBiomass/14:96

kgkmol

CO2 (27)

For emissions, regarding the gasification of sugar cane bagasse,it can be used the values suggested by [50] presented in Table 10.

Based on Eq. (23) and the data of Table 10, it can be calculate theamount of equivalent carbon dioxide (CO2) generated in the com-bustion process of biomass. In the calculations of emissions twoscenarios were considered: one without considering the cycle ofCO2 and the other considering this cycle.

The gasifier operates with sugar cane bagasse. The combustionof lignocellulose material on different types of gasifiers can beestimated using the equilibrium model developed by Zainal et al.[35]. In this work was adopted this methodology to estimate thecombustion reactions in the fluidized bed gasifier, as throughoutthe text. Using this combustion results, can be calculated theecological efficiency of the gasifier using the methodology pro-posed by Cardu and Baica [40].

Themethodology considering the carbon cycle was based on thework of [51], where each 1000 L of produced ethanol captures1211 kg of CO2 from the atmosphere. According to Ref. [52], a ton ofsugar cane generates 83.33 L of ethanol and 250 kg of bagasse. Byadopting these values, the amount of CO2, SO2, NOx and PM emittedduring the combustion process can be calculated, as shown inTable 11.

The amount of carbon dioxide equivalent can be determinedusing the values of CO2, SO2, NOx and PM in Eq. (24). Consideringthe LHV of the bagasse as 7.32 MJ/kg, the pollution indicatorassociated with the system can be calculated using the Eq. (25),which results are shown in Table 12.

Considering the thermodynamic efficiency of the gasifierEmg ¼ 60% as demonstrated previously, it can be calculated theecological efficiency of the system for the two cases: with orwithout carbon cycle using the Eq. (26), as show in Table 13.

It is observed that the ecological efficiency for this gasifier isquite good, and this value increases evenmore if the carbon cycle isconsidered. These values are represented in Fig. 3.

Fig. 3. Relationship between the ecological efficiency with or without the carboncycle.

P.T. Diniz Filho et al. / Applied Thermal Engineering 57 (2013) 116e124 123

4. Conclusions

The following conclusions could be obtained from themodellingof fluidized bed gasifier operating with sugar cane bagasse:

� Themain idea is to design the feeder in order to produce higheramount of synthesis gas. The increase of ash content in thesolid residue is a consequence of the increase of the gasifica-tion. More biomass is gasified and thus the ash content in theresidue increases;

� Compared with eucalyptus, bagasse gasification is moreincomplete, with a greater production of elements that preju-dice the combustion, like SO2, and its LHV is about 30% lower;

� In the design of a gasifier operating with sugar cane bagasse, itis necessary a bed material of high porosity, because thefluidization velocity and Reynolds number use to be very low;

� It is feasible to burn bagasse for the generation of syngas.However, using bagasse, some necessary technical re-quirements increase the costs in comparison to other types ofbiomasses with higher LHV;

� In terms of ecological efficiency, the study fluidized bedgasifier operating with bagasse proves to be an environ-mentally friendly way, with an ecological efficiency of 81%regardless of the carbon cycle of bagasse and 93.54%considering the carbon cycle. This proves that this type ofsystem is an excellent alternative. Thus, this technology canbe inserted with energy and environmental gains in theproduction chain of ethanol.

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