nceq

ter263

The Pennsylvania State University, 205 Hosler Building, University Park, PA 16802, USA

bility ofs paramne the

heat exchanger network are analyzed using pinch methodology. A 46.5% exer-

increase the use of biomass-derived fuels, especially in Latin Amer-

Each ton of sugarcane contains 142 kg of juice primarily sucroseand molasses, 140 kg of ber (bagasse) and 140 kg of agriculturalresidues with an average energy content of 15.7 MJ/kg on dry basis[2,3]. The brous fraction of sugarcane (bagasse) on a 50% wet basiscontains at about 2.5 bbl of oil equivalent. Many researchers aredeveloping technologies based on the energy potential of bagasse,

ication [4]. Combustion of bagasse is widespread in the combinedne mills. Most ofbackpressu030 kWhkg/kW) an

bagasse surplus [2,5,6].Pyrolysis has been less developed and is focused on the p

tion of bio-oil, while gasication is mainly used to produce aexible gas stream with both energy and chemical applications[4]. The gases produced in gasication contain mainly H2, CO,CH4, CxHy, CO2 and N2.

Many congurations have been studied to use gasication gasesfor energy production, the most widespread: gas engines, gas tur-bines, combined cycles, and more recently, high efciency fuel

Corresponding author. Tel.: +56 (41) 266 1855.E-mail address: luiseap@gmail.com (L.E. Arteaga-Prez).

Chemical Engineering Journal 258 (2014) 402411

Contents lists availab

n

w.ica, where energy from biomass is as much as 20% of the demand[1]. In this framework sugarcane and its harvesting residues havethe potential to produce rst and second generation biofuelsthrough both: biological and thermochemical conversion route.

production of heat and power (CHP) in sugar cathese systems are based on the Rankine cycle andbines, resulting in low electricity production (2sugar cane), high steam consumption (1025http://dx.doi.org/10.1016/j.cej.2014.07.1041385-8947/ 2014 Elsevier B.V. All rights reserved.re tur-/ton ofd low

roduc-Keywords:Biomass gasicationCombined cycleExergyThermodynamic analysis

gy saving by recovering heat in the steam cycle and drying stage can be achieved. Best results areobtained when the turbine inlet temperature is 1323 K and for a 1:10 cycle compression ratio: underthese conditions the total exergy efciency is 32.3% and 35.4% energy efciency. The atmospheric pres-sure gasier was operated at 72% hot gas efciency and 1073 K. Major exergy destruction occur in thegasier, dryer and heat exchanger network with a combined 94% of total losses.

2014 Elsevier B.V. All rights reserved.

1. Introduction

Both global and national energy polices promote research to

its renewability, CO2 neutrality and the possibility of conversionto higher-value-added products. The most common processes forenergy and chemical upgrading are combustion, pyrolysis and gas-inlet temperature (1000 Klosses and recovery in theh i g h l i g h t s

An investigation on the exergetic feasi A survey about the inuence of proces Use of exergy composite curves to de

a r t i c l e i n f o

Article history:Received 13 May 2014Received in revised form 23 July 2014Accepted 24 July 2014Available online 4 August 2014a BIGCC.eters on system performance.saving potentialities.

a b s t r a c t

The objective of this study was to develop a comprehensive mathematical model of bagasse gasicationintegrated with a gas turbine combined cycle (BIGCC). The model uses a quasi-equilibrium approach toevaluate the thermodynamic performance of the plant, considering both rst and the second law of ther-modynamics. The inuence of pressure ratio in the compressor (1:4 < rp < 1:10) and of the gas turbine

< TiT < 1400 K) on system efciencies is explored. The exergy destruction,Departamento de Ingeniera Qumica, Universidad Central de Las Villas, Carretera a Camajuan km 5.5, Santa Clara, Villa Clara, CubacDepartment of Biosystems Engineering, Ghent University, Coupure Links 653, 9000 Ghent, BelgiumdThermodynamic predictions of performagasication combined cycle under quasi-

Luis E. Arteaga-Prez a,, Yannay Casas-Ledn b, WolaUnidad de Desarrollo Tecnolgico (UDT), Universidad de Concepcin, Av. Cordillera N.b

Chemical Engi

journal homepage: wwe of a bagasse integrateduilibrium conditions

Prins c, Ljubisa Radovic a,d

4, Parque Industrial Coronel, Coronel, Chile

le at ScienceDirect

eering Journal

elsevier .com/locate /ce j

Nomenclature

BG biomass gasierBIGCC biomass integrated gasication combined cycleBPST back pressure steam turbinebbl barrelsCEST compression extraction steam turbineCHP combined heat and powerCpg mass heat capacity, (kJ/K/kg)CPR compressor pressure ratioeb chemical exergy of bagasse (kJ/kg)ch

L.E. Arteaga-Prez et al. / Chemical Enginei molar chemical exergy of species i (kJ/kmol)ei total exergy of each chemical species (kJ/kmol)Ein activation energy (kJ kmol1)Ej exergy of the j streams entering the system (MJ)EK exergy of the K streams leaving the system (MJ)eoi standard chemical exergy of species (kJ/kmol)ephi molar physical exergy of species i (kJ/kmol)Fhot,cold molar ow of cold and hot stream in heat exchangers

(kmol/s)GG gasication gasesGGSR gasication steam reformingh enthalpy (kJ/kmol)HHV higher heating value (kJ/kg)HRSG heat recovery steam generatorIr,tot system exergy destruction (kW)IrCOMP exergy destruction in compressor (kW)IrG exergy destruction in gasier (kW)IrHEx exergy destruction in heat exchangers (kW)IrTURB exergy destruction in the turbine (kW)LHV lower heating value (kJ/kg)cells. One possible conguration has been evaluated by the presentauthors where a bagasse gasier was integrated with a solid oxidefuel cell [7]. Another feasible option is to integrate the gasicationof bagasse with a gas turbine combine cycle for the production ofpower and heat [8]. The basic elements of such a BIGCC powerplant include a biomass dryer, a gasier, a cleanup system, a gasturbine-generator fueled by combustion of the biomass-derivedgas, a heat recovery steam generator (to raise steam from the hotexhaust of the gas turbine), and a steam turbinegenerator (to pro-duce additional electricity) [6].

Several models have been developed to simulate the integrationof biomass gasiers with traditional and novel combined cycles.The main results of such studies are summarized in Table 1. Untilnow there are many works focused on the mathematical descrip-tion of gasier based on equilibrium calculations with a very lim-ited report of experimental data [5,9].

Larson et al. [6] have already presented a general review ofseveral cogenerating technologies in sugar cane mills. They

Table 1Performance indicators of different gasication-energy production systems.

System Biomass gEXE (%) Refs.

BIGCC Sugarcane bagasse 30 [9]BOILER Sugarcane bagasse 2434 [10]GTCC Wood 29 [11]GTCC Biomass mixture 33 [12]BIGCC Sugarcane bagasse 2033 [6]CEST Sugarcane bagasse 1628 [6]BIGCC Cobs 28 [13]BIGCC Paper 34 [14]BG-SOFC Straw slurry No [15]GGSR-SOFC Sugarcane bagasse 31 [16]BG-SOFC Olive residues 36 [17]BG-SOFC Sugarcane bagasse 32 [7]mB mass ow of bagasse (kg/s)mg mass ow of gasication gases (kg/s)QET quasi-equilibrium temperature (K)rp pressure ratio of cycleS entropy (kJ/kmol/K)SOFC solid oxide fuel cellTiT turbine inlet temperature (K)TPR turbine pressure ratiotc tons of sugar cane (ton)WAC power consumed by air compressor (kW)WGGC power consumed by gasication gas compressor (kW)WGT power delivered gas turbine (kW)Wnet,GT net power of gas turbine cycle (kW)Wnet,ST net power of steam turbine cycle (kW)WP power consumed by water pump (kW)WST power delivered by the steam turbine (kW)ytar tars concentration in producer gas (g/Nm3)yD,k exergy destruction ratioyD,k exergy destruction coefcient

Greeks letters

gex exergy efciency of system (%)gen energy efciency of system (%)nc fractional carbon conversiongCG cold gas efciency (%)gHG hot gas efciency (%)gT turbine isentropic efciencygc compressor isentropic efciency

eering Journal 258 (2014) 402411 403demonstrated the economic feasibility of using BIGCC systemswhen they are compared with traditional backpressure steam tur-bines and condensing extraction steam turbines. Moreover, carbonemissions savings for the BIGCC are almost twice CESTs at samemilling capacity. However, authors focused their analysis on gen-eral issues and no details of gasication systems are given.

Pellegrini et al. [9] reported a simplied model for the gasica-tion of bagasse based on chemical equilibrium considerations.Authors developed a parametric study on the inuence of severaloperation parameters and an exergy analysis of the system. Theyrecommend using mathematical correlations of experimentalresults for the CH4 composition and for carbon conversion to studydifferent operational modes and to understand exergy and energybehavior.

De Kam et al. [13] analyzed the BIGCC integration into a sugarethanol factory using a simulation model that represents thegasication reactor as an equilibrium block. They emphasized onthe importance of considering both the economic and environmen-tal criteria in such analysis. Even when this communication wasvery explicit a low exactitude in the gasication unit predictionsassociated to the equilibrium consideration (Gibbs model) can beidentied.

In a more recent paper, Pellegrini et al. [19] treated economicand environmental issues associated to BIGCC in their assessmentof combined production of sugar, ethanol and electricity. Theseauthors demonstrated that the electricity surplus obtained bycogeneration improves the exergo-environmental performance ofthe system and that modern cogeneration systems can produce2.5 times the electricity obtained by traditional BPST, generatingin this way a remarkable economic prot for the factory.

More recently, Dias [5] analyzed three different cogenerationsystems for an integrated sugarethanol factory: a traditional Ran-kine cycle with backpressure and condensing steam turbines, and a

BIGCC comprised by a gas turbine set operating with producer gasfrom a bagasse gasier. The feasibility of the BIGCC was demon-strated, generating 130% more electricity than the traditionalRankine cycle with condensing steam turbines and at low exer-gy-based costs of electricity and ethanol. The gasier was identi-ed as a critical point for system analysis, but a comparison ofmodel predictions with experimental results was not reported.

Another recent paper, Soltani et al. [14] reported an advancedexergy analysis for a novel conguration of an externally redcombined-cycle power plant integrated with biomass gasication.They concluded that the focus in improving cycle performanceshould be on the heat exchanger and not the combustion chamberor gasier, even though these have the highest exergy destructionsamong all the components. The system exergy efciency was 34%and the gasier was described using an equilibrium model.

In summary most theoretical studies of the BIGCC have beenbased on unmodied thermodynamic equilibrium calculations ofthe gasier, and have a general shortcoming that they cannot accu-rately predict temperatures, tars formation, heat losses or carbonconversion. This approach affects the robustness of models aswas demonstrated by Kersten [19] and introduces errors in theapplication of the second law of thermodynamics.

To overcome the above mentioned we have performed such athermodynamic analysis of the gasier considering a quasi-equilib-rium approach [7], here we apply rst and second laws of thermo-

experimental procedure are given in Arteaga et al. [7]. Table 2summarizes some of the most important properties used in thecalculations.

The content of moisture in bagasse as well as the structural oxy-gen, are high enough to produce a reduction in the air needed forpartial combustion.

3. Systems modeling

3.1. Biomass gasication

Fig. 1 shows a schematic biomass gasication model. Accord-

404 L.E. Arteaga-Prez et al. / Chemical Engineering Journal 258 (2014) 402411dynamics to a sugar cane bagasse gasication reactor integratedwith a gas turbine combined cycle (BIGCC). Operation feasibilityis evaluated for different pressure ratios and gas turbine inlet tem-peratures. Irreversibility and exergy destruction ratio are calculatedat all stages. The exergy composite curve is represented for the heatexchanger network, to calculate the minimum requirements ofexergy at specied DTmin in the network. Using these results thepotential of the proposed scheme have been demonstrated.

2. Sugarcane bagasse property data

The biomass characterization was developed for a Cuban sugar-cane bagasse collected from the harvest of 201112. Details of the

Table 2Sugarcane bagasse property data.

Ultimate analysis Proximate analysis

Elements %w/w Property %w/w

Carbon 48 Fixed carbon 14.7Hydrogen 6.2 Volatile matter 47.8Oxygen 43.1 Moisture 35Nitrogen 0.4 Ash 2.5Sulfur 0 HHV (MJ/kg) 10.69Ash 2.3Fig. 1. Simulation diagram for theingly, the model is divided into several sub-units to simulate dry-ing, gasication and producer gas cleaning stages, Table 3summarizes all the Aspen One modules and the related approxi-mations. Steady state calculations are assumed, drying and pyroly-sis are instantaneous and carbon as conventional solid (graphite)with specic thermodynamic properties. In all cases, the RedlichKwongSoave (RKS) equation of state was used for propertyestimation [7,13,18]. Biomass was dened using proximate andultimate analysis data and it was treated as an unconventionalsolid.

3.1.1. DryingThe drying processwas simulated using two blocks, a stoichiom-

etric reactor (D-01) and a ash separator (D-02). The wet bagasseenters at atmospheric conditions (298 K and 101.325 kPa) and itsmoisture content is reduced up to 10%. Although biomass dryingis not normally considered a chemical reaction, this approximationallows converting a fraction of nonconventional component (mois-ture in bagasse) to form conventional water in Aspen One and tocalculate the heat needed to vaporize this water. A Fortran blockis used to solve the mass balance in the process. The heat load inthis stage is included in the system balance as a sink which shouldbe satised by a fraction of exhaust gases from the combined cycle(S27 in Fig. 2). Heat calculated by this procedure was checked usingthe Eq. 4.19 of Basu [4].

3.1.2. GasicationThe gasication reactor is divided into several unit operation

blocks (G-01, G-02, G-03) to introduce the approximations of themodel and to allow calculating the heterogeneous and homoge-neous phases by a quasi-equilibrium Gibbs free energy model:

Quasi-equilibrium temperature model (QET), which integratesthe minimization of Gibbs free energy method and temperatureequilibrium restrictions.

Equilibrium model combined with empirical relations forproduction of tars and carbon conversion [20].gasication section of BIGCC.

ytar 27:22 exp 2:78ER 2ER in Eqs. (1) and (2) is dened as the ratio of actual ow of air

to air needed for stoichiometric combustion (equivalence ratio). XCis the carbon conversion dened as the fraction of carbon con-tained in the biomass which is converted into gaseous species.The models represented in Eqs. (1) and (2) were validated usingthe experimental data obtained by Prez-Bermdez [20] in a bub-bling uidized bed pilot scale gasier fed with sugarcane bagassefor the same temperatures of this paper.

3.1.3. Gas cleaningThis section includes physical and chemical cleaning stages

such as: cyclone (GC-01) and a tar cracking reactor (GC-02). Thecyclone is used to eliminate particulate materials of Dp < 5 lm atan efciency of 80%. Complete conversion of tars in the cracker isassumed by considering the following reaction:

C10H8 12O2!4H2O 10CO2 DH 5100 kJ=mol

The heat involved in the cracking of tars was included in theenergy balance.

3.1.4. Combined cycle

Table 3Description of models.

Stage Aspen blocks/ID Description

Drying RStoic/D-01Flash2/D-02Calculator

A stoichiometric reactormodel and ash separator arecombined to simulate the dry-ing stage as was previouslydescribed

Calculator is used to relate thereactor conversion to themoisture removal (see aspenplus user guide-gettingstarted with solids)

Gasication RYield/G-01CalculatorRGibbs/G-02, G03

As the Gibbs free energy ofbiomass cannot be calculated,a yield model is used to con-vert non-conventional bio-mass constituents into theirconventional homologs priorto gasication. Calculatorblock is used to specify theyield in G-01

Minimization of Gibbs freeenergy is used to model gasi-cation, pyrolysis and partialoxidation taking place duringgasication process

Gas cleaning Cyclone/GC-01 Simulates the cyclone opera-

L.E. Arteaga-Prez et al. / Chemical Engineering Journal 258 (2014) 402411 405RStoic/GC-02 tion to separate solids fromgasication gases

A stoichiometric reactormodel with a 100% of conver-sion for the oxidation ofnaphthaleneA detailed discussion on gasier model validation was reportedin a previous paper [7]. Empirical equations for carbon conversion(Eq. (1)) and tar formation (Eq. (2)) are implemented [20]. The tarconcentration (ytar) calculated by Eq. (2) is used to determine themass ow of tars (naphthalene as model compound) which willbe converted downstream:

XC 0:25 0:75 1 expER=0:23 1

Fig. 2. Simulation diagram for theThe combined cycle is represented in Fig. 2. Gases exiting thecleaning section pass through a gas cooler (CC-03) which generatessome process steam. The gas is then compressed in a two stagecompressor (CC-04) in front of combustion chamber of the gasturbine, at a pressure ratio of 1:10, 85% isentropic efciency and3% mechanical loss. The entering ambient air is compressed in anadiabatic process (CC-05) under the same conditions as the gasi-cation gas compressor. Minimization of the Gibbs free energy isused to model the combustion chamber (CC-06) at adiabatic condi-tions. The exhaust gas from the gas turbine (CC-07) enters the heatrecovery steam generator (CC-08) from where superheated steamat 561 K and 1100 kPa is piped to a backpressure steam turbine(CC-09). The steam turbine has an outlet pressure of 152 kPa andis modeled as an adiabatic expansion process with isentropicefciency 75%. The turbine exhaust is still superheated steamwhich is brought to saturation in the water preheater downstreamcombined cycle in the BIGCC.

nginWnet;ST WST WP 5

Cold gas efficiency : gCG mg HHVgmB HHVB 6

Hot gas efficiency : gHG mg HHVg Cpg Tg To

mB HHVB 7

The HHV of bagasse (kJ/kg) can be calculated by the followingequation [21]:

HHVB 351:70 C 1162:49 H 104:67 S 110:95 O 62:80 N 8

4.2. Second law analysis

Exergy is the maximumwork that can be produced when a heator a material stream is brought to equilibriumwith respect to a ref-erence environment. Here the reference or dead state is To = 298 K,Po = 101.325 kPa and contains 75.67%v/v N2, 20.35 of O2, 0.03 ofCO2, 3.03 of H2O and 0.92 of Ar [22].

The exergy balance of the biomass integrated gasication com-bined cycle can be represented by the following general equation:Xin

Ej Xout

EK Ir;tot 9

here the term Ir,tot accounts for irreversibility which is thedifference between exergy entering and leaving the systems.Irreversibility represents the internal rate of exergy destruction.The contributions of kinetic and potential exergy are neglected.The exergy of material streams is represented by two components.

Physical exergy: this is dened as the maximum amount of workobtained when a material stream is taken reversibly from its initialstate, at pressure (Pi) and temperature (Ti), to the ambient state atPo and To by physical processes (e.g. without a change of chemicalcomposition) and it can be written as follows:

eph Hi Ho To Si So 10(CC-02) making possible using this utility to fulll energy needs inthe sugarcane production process. The water ow to the steamcycle is calculated considering a turbine generation index between8 and 19 kgsteam/kWh, working with superheated steam at1100 kPa and 459 K. All the heat exchanger models were calculatedusing HeatX blocks in Aspen One and the pressure changers (tur-bines/compressors) with Compr block.

4. Thermodynamic analysis

4.1. First law analysis

The rst law of thermodynamics was applied to the BIGCC inorder to obtain the performance indicators. Even when energyanalysis cannot account for the irreversibilities in the process, itgives useful measures of merit for energy use and allows the deter-mination of bottlenecks in the process. The following parameterswere calculated based on the inputoutput principle:

Energy efficiency : gen Wnet;GT Wnet;ST

mB LHVB 3

with:

Wnet;GT WGT WAC WGGC 4

406 L.E. Arteaga-Prez et al. / Chemical EThe exergy of all the gaseous mixtures in the present analysiswas calculated by assuming ideal behavior:eph Cp Ti To To ln T=To RTo ln P=Po

i streams11

The physical exergy for biomass was calculated using the de-nition given in Kotas [22]:

eph Cp Ti To To ln T=To v Pi Po 12

Chemical exergy: This is given by transformation of i compo-nents contained in a stream to reference compounds and diffusionof those compounds into the dead state:

ech X

xi eoi RToX

xi lnxi 13The second term in Eq. (13) quanties the exergy of mixing and

it is always negative. The standard exergy of the components in thegaseous mixtures is reported in Kotas [22].

Exergy of bagasse was calculated from the statistical correla-tions of Szargut and Styrylska [23].

eb b LHVB 14

b1:0440:016 XH=XC 0:349 XO=XC 10:53 XH=XC 0:049 XN=XO 10:412 XO=XC

15

Here Xi is the mass fraction of each component in the ultimate anal-ysis of the bagasse and the LHVB is the lower heating value ofbagasse (kJ/kg) which can be calculated by relating it with the mois-ture content as follows [24]:

LHVB 17;807:5 20;321:5M 16Variable M in Eq. (16) is the mass fraction of water in Table 2.

4.2.1. Irreversibility and lossesExergy destruction by irreversibility and the exergy losses are

calculated for each stage in the BIGCC. The mathematical deni-tions are given bellow:

Gasier: The calculation of the irreversibility in the biomass gas-ier assumes the inuence of physical and chemical exergy of thestreams and the exergy of heat (Carnot parameter):

IrG Xni1

ephch Xmj1

ephch 1 To=TG QG 17

with; i = inlets, j = outlets.Turbines and compressors: For compression and expansion

equipment [7]:

IrCOMP F in R T 1 gC=gC lnCPR 18

IrTURB F in R T 1 gT ln TPR 19Heat exchangers: In the heat exchangers only the change in

physical exergy was considered, as follows:

IrHEx Fhot hin hout To Sin Sout hot Fcold hin hout To Sin Sout cold 20

The opportunities for exergy savings in the heat exchanger networkwere calculated using the PINCH technology approach. The target-ing at a specied minimum DT allows identifying the minimumexergy required by the heat exchanger network (HEN) [25]. Thisprocedure is similar to that of minimum energy targets in a heatexchanger network but it uses exergy content of streams insteadenthalpy. Hence, the minimum exergy targets can be calculatedby using composite curves constructed by plotting Carnots factor

eering Journal 258 (2014) 402411(1 To/T) vs. cumulative exergy of hot and cold streams. Thismethod has been applied in very few cases to study BIGCC schemes,further explanation on this procedure is given in Section 5.2.1.The

streams leaving the system are the external exergy losses:

Steady state calculations.

Naphthalene is used as model compound for tars.

5.1. Base case simulation

Initial biomass ow was calculated considering a low thermalload (25 MWth), common for bubbling uidized bed gasicationunits [4]. Table 4 summarizes the base case conditions.

At base case conditions the cycle produces 286.12 kWh/tc, this is100 kWh higher than the potential estimated by Alonso-Pippo et al.[27] for a similar BIG/GTCC using Cuban bagasse. More similaritiescan be found regarding the work of Larson et al. [6] who reported aBIGCC with a generation capacity of 298 kWh/tc off-season. Theactually installed conventional back pressure steam turbines can

ngineering Journal 258 (2014) 402411 407Combined cycle

Air is simulated as a mixture of 79% nitrogen and 21% oxygen(molar basis).

Pressure drop in heat exchangers is negligible. Turbine combustor operates adiabatically. The intercooler temperature in the two stage compressor

(CC-04) is 400 K. The DTmin for exergy targeting in the heat exchanger net- Dryer operates at atmospheric pressure and 393 K. Exhaust gases from combined cycle are used to fulll energy

needs in the process. Gasier operates isothermally at 1073 K and atmospheric

pressure. Equivalence ratio in the gasier is constant (0.30).System exergy efficiency : gex Wnet;GT Wnet;ST Eloss Ep

mB eb23

here Eloss and Ep are the external exergy losses and the exergy ofproducts (kW).

5. Results and discussion

The model developed in the preceding sections was imple-mented in Aspen One v8.7, licensed by the Unit of TechnologicalDevelopment of Universidad de Concepcin (Chile) and then usedas a forecasting tool to study a small capacity BIGCC. Main assump-tions for simulation are:

Gasication sectionminimum temperature approach for overlapping the exergy curves(hot and cold) was xed at 20 K.

Exergy destruction ratio and coefcient: The exergy destructionratio (Eq. (21)) is dened as the ratio between the irreversibilityof stage Irk and the total exergy of fuel entering the BIGCC EF,tot.Also, the exergy destruction coefcient (Eq. (22)) is the fractionof exergy destroyed at stage k.

yD;k Irk=EF;tot 21

yD;k IrkXk

k1Irk 22

here EF,tot includes the exergy of biomass and air entering the gas-ier and combined cycle.

4.2.2. Exergy efciencyThe denition given by Szargut [26] stating that the exergy

efciency is the ratio of the useful exergy of products in a system(power and heat in BIGCC) to the driving exergy, was used to eval-uate the combined production of heat and power from sugarcanebagasse (Eq. (23)). The net power delivered by the cycle, the exergycontent of saturated steam and a fraction of the exergy in theexhaust gases which is used for drying and to preheat the gasica-tion air were considered as products. The remaining exergy

L.E. Arteaga-Prez et al. / Chemical Ework is 20 K.

The rest of assumptions were included opportunely in Section 3.only produce up to 2030 kWh/tc while a compression extractionsteam turbine (CEST) could cogenerate 130150 kWh/tc whenoperated at 6.3 MPa. The foregoing analysis allows us to state that,if the cogeneration scheme is intended to export electricity to thegrid, then the CEST or BIGCC are the most attractive options. Evenunder these basic conditions the proposed BIGCC can provide steamand power on-season to satisfy electromechanical demands of theprocess while exporting electricity to the grid.

The results of mass balance and exergy ow for the base caseare summarized in Table 5. An estimated of 7956 ton CO2/yearare emitted by the cycle.

Hot and cold gas efciencies under the above dened conditionsare 72% and 58% respectively. The effect of drying process isincluded in the efciency calculations by considering the HHVBfor the wet bagasse in Eqs. (6) and (7); this approximation canderive in a 10% difference to that of dry feedstock [9,28]. Thescheme proposed here has the additional advantage of usingexhaust gases to dry the bagasse. The values in Table 5 were usedalong with the system boundaries dened in Fig. 3 to estimate theexergy performance of the base case. Table 6 summarizes the irrev-ersibilities and the exergy destruction coefcient at the conditionsexplored, whereas Fig. 4 shows the exergy destruction ratios for allprocess stages.

The boundaries in Fig. 3 remained constant for the simulationstudy and only the operational parameters were varied.

The stages with highest irreversibilities are the gasier, bagassedryer and heat exchanger network. The exergy destruction coef-cient in the gasier is similar to that found by Meyer et al. [29]for an allothermal biomass gasier coupled to a solid oxide fuelcell. This is a reasonable result considering that the gasier is thestage involving the major chemical transformations.

Irreversibility in the gasier (1396.8 kW) represents 23.4% ofthe exergy contained in the fuel; this result is closely related tothe succession of several reactions which draw a change in thechemical component of exergy [4,7]. The HEN has an 8.7% of exergydestruction similar to that found by Datta [30]. Manipulation ofoperational parameters in the gasier to reduce irreversibilitywas previously reported [7] and hence here are focuses on theparameters of gas turbine and steam cycles; therefore the fractionof exergy destruction in the gasier remains constant in subse-quent analyses.

Table 4Base case conditions.

Stage Parameter Value U/M

Gasier Gasier capacity 1000 kgB/hGasier temperature 1073 KEquivalence ratio 0.30

Compressor Air compressor isentropic efciency 85 %Air compressor inlet temperature 298.15 K

Gas/steam turbines Gas turbine isentropic efciency 90 %Steam turbine isentropic efciency 75 %

Tar formation 11.82 g/Nm3

Carbon conversion 79.7 %

52

69.90 118.72 2565 620.85

100 0.0 5.05 5.05

nginTable 5Mass balance summary for the base case.

Streams? S6 S13 S14Mass ow (kg/s) 0.3056 0.5233 0.500Temperature (K) 373.15 1072.38 298.1Pressure (kPa) 101.32 101.30 101.3Exergy ow (kW) 17.36 2947.27 0.250

Mass composition (%)H2O 0.0 12.51 100

408 L.E. Arteaga-Prez et al. / Chemical EOur present analysis specially deepens on the heat exchangernetwork associated to the steam cycle by estimating exergy con-sumption and minimum required exergy. As previously reportedby Soltani et al. [14,31], thermal integration of heat exchange pro-cesses using low pressure steam is crucial for improving the entiresystem efciency.

5.2. Model application

A sensitivity study is carried out to evaluate the effect ofturbine inlet temperature (10001400 K) and cycle pressure ratio

N2 76.7 44.78 0.0O2 23.3 0.719 0.0CH4 0.0 1.67 0.0CO 0.0 23.71 0.0CO2 0.0 15.21 0.0H2 0.0 1.401 0.0

Fig. 3. System boundaries f

Table 6Exergy parameters for the base case.

System component Exergy destructionIr (kW)

Exergy dest. coefcientyD,k (%)

Compressor (CC-04) 25.63 0.80Compressor (CC-05) 84.79 2.64Gas Turbine (CC-07) 48.52 1.51Steam Turbine (CC-09) 34.02 1.06Gasier 1396.78 43.53Cyclone (GC-01) 2.97E-05 0.00HEN 518.30 16.15Dryer 1100.40 34.31

Total 3211.01 100S16 S21 S23 S270.500 2.444 2.967 2.967386.36 298.15 1323 580.64151.98 101.32 1000 150.32

eering Journal 258 (2014) 402411(1:41:10) on energy and exergy efciencies, as well as on the heatrecovery capacity. Turbine temperature is studied between 1000 Kand 1400 K to avoid using special materials for heat exchangersand turbine blades which could sharply increase the cost of thecycle [14,30]. Moreover, operation within this range allows con-trolling contaminant concentration in exhaust gases e.g. NOx whichexponentially increase with temperature and are standardized byISO 11042-2 (1996) [32,33]. The lower temperature limit is xed

0.0 76.7 71.07 71.070.0 23.3 13.81 13.810.0 0.0 0.00 0.000.0 0.0 0.00 0.000.0 0.0 10.07 10.070.0 0.0 0.00 0.00

or the exergy analysis.

Fig. 4. Exergy destruction ratios for process stages.

at 1000 K to minimize the unconverted hydrocarbons and CO con-centration in exhaust gases [33]. For pressure ratio we used the1:41:10 range based on previous results of Soltani [14,31] whohave found a weak effect of pressure ratio on thermal efciencywith an optimum around 1:10. Moreover, Najjar and Ismail [34]and Brooks [35] demonstrated that working at higher pressureratios does not involve important improvements of efciency forcombined cycles, instead using modest values with appropriatetemperatures could lead to important increments of efciency.Analysis of the exergy efciency included the exergy content ofbagasse and in minor extent the water as the inlet ows. The frac-tion of this input exergy converted to useful work denes the exer-getic efciency of the cycle; the remaining fraction is divided indestruction at all process stages and losses. Furthermore exergycontent of saturated steam and exhaust gases can be consideredas product only for well integrated systems; otherwise they arelosses and could reduce the exergetic performance as well as par-

at TiT = 1323 K to avoid the use of expensive heat exchangermaterials at reasonable high energy and exergy efciencies.Exhaust gases and superheated steam leaving the turbines aretwo valuable streams for cycle prociency and hence the minimi-zation of their exergy losses is essential to obtain high efciencylevels. This minimum exergy target is identied by applyingpinch-based approaches, specically the exergy composite curvea concept that was rst introduced by Linnhoff [25]. For each linearsegment in the enthalpy-temperature prole, the heat exergydelivered by the stream i delivering heat load Q from Tin to Tout iscomputed by Q * (1 To/Tlm) where Tlm is the log mean tempera-ture difference.

For the system under consideration, the exergy compositecurves were calculated for the HEN including the use of exhaustgases to dry bagasse and to preheat air in the gasier. The resultsare presented in Fig. 7.

The loss of exergy in hot streams, without considering heatrecovery, is as high as 519.5 kW. This waste of useable energycan be minimized to 241.6 kW (46.5%) by considering the integra-

L.E. Arteaga-Prez et al. / Chemical Enginticulate material separated in the cyclone.

5.2.1. Energy and exergy analyses5.2.1.1. Turbine inlet temperature. The turbine inlet temperature isone of the most important parameters in the design of combinedcycles. It inuences heat exchanger design, values above 1400 Kneed special materials, increasing the cycle investment cost. Nev-ertheless, operation in such a condition allows reducing the owof air and hence the cost of operation, thus providing a trade-offwhich only can be resolved by optimization [31]. The analysis pre-sented here is expanded to include stainless steel and advancedmaterials for heat exchangers, and it is based on a xed producergas temperature, 1073 K.

It is seen in Fig. 5 that the heat recovered (Q) by the HRSGincreases sharply with TiT. This 45.45% increment is reverted inhigher steam quality at the HRSG output and in higher power pro-duction in the steam cycle. At 1400 K an extra 400 kW is deliveredby the cycle in relation to that at 1000 K; this fact allows to satisfypower consumption by water pumping and producer gas compres-sion, both at higher levels of efciency, illustrated in Fig. 6.

A variation of 21.5% of thermal efciency is found in theexplored range of temperatures for a total thermal capacity of heatexchanger UA of 2.6 kW/m2K. Considering a xed overall heattransfer coefcient (U), the size of the heat exchanger can bereduced at higher values of the log mean temperature difference,nevertheless the operation at such a condition implies less recoverypotential in the heat recovery steam generator (CC-08) and henceless power production in the steam cycle. When temperatureexceeds 1323 K, Q reaches a plateau and the cost of the heatFig. 5. Effect of turbine inlet temperature on heat recovery and delivered power.rp = 1:10.exchanger and turbine are increased due to the need of specialmaterials (e.g. Nickel-based superalloys and ceramics).

The exergy efciency of the system is affected by severalparameters, the most important among them, being the gasiertemperature, and the turbine inlet temperature.

The rst was discussed previously [7] and it was concluded thatat higher temperatures the HHV of the producer gas is incrementedby the content of H2 and CO. These gases are then combusted in thegas turbine and contribute to a better energetic performance.Exergy efciency of the system reached 35.6% at 1400 K, speci-cally due to the contribution of the power delivered by the gasturbine. It is demonstrated that working over 1250 K the systemoperates at efciencies above 31%, which can be considered a verygood result if it is compared to previous studies (see Table 1). Themain cycle irreversibilities are found in the water-steam heatexchanger network which varied from 333 kW at 1000 K to559 kW at 1400 K. This effect is closely related to the inuence oftemperature on enthalpy and entropy of the streams (Eq. (20)).On the other hand, gas turbine irreversibility (49 kW) is the dualresult of chemical transformation in the combustion chamberand physical changes in the turbine itself. It can be concluded fromthe aforementioned that the more convenient operation conditionscould be 1300 K < T < 1400 K with 35.441.6% thermal efciencyand 32.335.6% exergy efciency.

According to the results presented above, it is advisable to work

Fig. 6. Effect of turbine inlet temperature on energy and exergy efciency.rp = 1:10.

eering Journal 258 (2014) 402411 409tion between streams in the steam cycle and also the recovery ofsensible heat of exhaust gases to dry the bagasse, as demonstratedin Fig. 7.

nginFig. 7. Exergy composite curves. TiT = 1323 K, rp = 1:10.

410 L.E. Arteaga-Prez et al. / Chemical E5.2.1.2. Cycle pressure ratio. Fig. 8 shows the behavior of heat recov-ery and turbine delivered power as a function of pressure ratio. Theanalysis was developed at the adiabatic temperature delivered inthe turbine combustor. This temperature was calculated assuminga Gibbs free energy minimization method for chemical reactions,as described in Section 3.1.

As the pressure ratio of the cycle is increased, the heat releasedby the uid compression suffers a proportional effect and hencethe producer gas and air are combusted at higher temperature.These simple and well-known transformations have a positiveinuence on the cycle performance by increasing the potential ofgases to be expanded in the turbine, and reducing the air owneeded by the cycle (m3/kWh).

For the process being analyzed the power generation can rise upto 8.60% when rp varies from 1:4 to 1:10. On the other hand, thepressure ratio has additional effects that should be carefully takeninto account; material thickness of the equipments casings isincreased at higher pressures. The cost-benet of decreasing theair ow needed can be affected by the design and characteristicsof the heat exchangers.

From an integral analysis of such factors, it is concluded that thebest rp is a complex function of investment costs, power deliveredby the system and operational restrictions. According to the resultspresented here, a 1:10 ratio is advisable, higher values can result insevere penalties to the system economy.

The variation of rp has an inuence on the physical componentof exergy balance and therefore is lower than that found for stages

Fig. 8. Effect of pressure ratio on heat recovery and turbines delivered power.that involves changes in chemical composition of streams such asthe gasier. Fig. 9 shows how both energy and exergy efcienciesare increased with pressure ratio. If this parameter is varied, thethermal efciency could change, as Datta [30] found a 10% ef-ciency reduction when temperature falls from 1350 to 1050 K. Bestresults are found at rp = 1:10 and TiT = 1323 K for a generationindex of 290 kWh/tc, 32.3% exergetic efciency and 260 kWh/tcof export capacity to the grid.

6. Conclusions

A model for a biomass gasier connected with a combined cycleis presented. A sensitivity analysis, including turbine inlet temper-ature and cycle pressure ratio integrated to a PINCH methodology,was used to compute exergy losses in the process and the heatexchanger network. Although turbine inlet temperatures increasesfavor exergy and energy efciencies, operation above 1323 Krequires the use of expensive ceramic materials for heat exchang-ers and turbine blades increasing cycle cost. On the other handoptimization of recoverable heat in the HRSG becomes less impor-tant at TiT > 1400 K. Exergy losses in the heat exchanger networkcan be reduced to 241.6 kW if the heat of exhaust gases and steamis recovered.

Total exergy efciency is 32.3% at 1323 K and 1:10 pressure

Fig. 9. Effect of pressure ratio on system efciencies.

eering Journal 258 (2014) 402411ratio while almost 290 kW/tc can be delivered to the grid, demon-strating the technical feasibility of BIGCC for cogeneration in sugarmills.

Acknowledgment

We acknowledge the contribution of editor and anonymous ref-erees. Preparation of this paper has been nancially supported byProject Basal UDT PFB-27 of the Unidad de Desarrollo Tecnolgico Universidad de Concepcin, Chile.

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