in

Juay of

Received in revised form31 March 2009Accepted 9 June 2009Available online 17 July 2009

installed capacity [1]).Back in 1970s and 1980s, cogeneration plants in sugarcane mills

were primarily designed to consume all bagasse, and produce steamand electricity to the process. The plants used medium pressuresteam boilers (21 bar and 300 C) and backpressure steam turbines.

bar) is used to fulll the thermal requirements of the process, andits condensate is returned to the boiler. Normally, the electrome-chanical energy produced is for internal use only. However, somemills already use steam with higher parameters (4266 bar),generating an excess of electricity that is sold to the grid. Also, thereis a tendency in the sector to replace old boilers by new ones withgreater capacity (100 bar, for instance).

In the future, some authors suggest that biomass integratedgasication combined cycles (BIGCC) are the best alternative to

* Corresponding author. Tel.: 55 11 3091 9668; fax: 55 11 3091 9681.

Contents lists availab

Ener

els

Energy 35 (2010) 11721180E-mail address: luiz.pellegrini@usp.br (L.F. Pellegrini).1. Introduction

Almost 45% of the Brazilian energy supply comes from renew-able energy sources, while the electricity matrix is composed bynearly 80% of renewables resources. Sugarcane products play animportant role in the Brazilian energy matrix (15%), mainly intransportation due to the use of ethanol as fuel. However, theparticipation of sugarcane and other biomasses in the electricitymatrix is still marginal, less than 4% [1]. According to EPE [2], in thelast energy public sale, a supply of 2.379 MW was contracted frombiomass, mainly from sugarcane bagasse (2% of the Brazilian

Some plants needed also an additional fuel, as the boilers were veryinefcient. In those times, sugarcane bagasse did not have aneconomic value, and it was considered a problem by most mills.During the 1990s and the beginning of the 2000s, sugarcaneindustry faced an open market perspective, thus, there was a greatnecessity to reduce costs in the production processes. In addition,the economic value of by-products (bagasse, molasses, etc.)increased, and therewas a possibility of selling electricity to the grid.

Today, most bagasse-red boilers still raise steam to 300 C and21 bar that is used in backpressure turbines, responsible for theelectromechanical demands of the mill. Backpressure steam (2.5Keywords:CogenerationSugarcane millsGasicationSupercritical cyclesExergy analysis0360-5442/$ see front matter 2009 Elsevier Ltd.doi:10.1016/j.energy.2009.06.011boilers (21 bar and 300 C) and backpressure steam turbines. Some plants needed also an additional fuel,as the boilers were very inefcient. In those times, sugarcane bagasse did not have an economic value,and it was considered a problem by most mills. During the 1990s and the beginning of the 2000s,sugarcane industry faced an open market perspective, thus, there was a great necessity to reduce costs inthe production processes. In addition, the economic value of by-products (bagasse, molasses, etc.)increased, and there was a possibility of selling electricity to the grid. This new scenario led to a searchfor more advanced cogeneration systems, based mainly on higher steam parameters (4080 bar and400500 C). In the future, some authors suggest that biomass integrated gasication combined cyclesare the best alternative to cogeneration plants in sugarcane mills. These systems might attain 3540%efciency for the power conversion. However, supercritical steam cycles might also attain these ef-ciency values, what makes them an alternative to gasication-based systems. This paper presentsa comparative thermoeconomic study of these systems for sugarcane mills. The congurations studiedare based on real systems that could be adapted to biomass use. Different steam consumptions in theprocess are considered, in order to better integrate these congurations in the mill.

2009 Elsevier Ltd. All rights reserved.Article history:Received 8 December 2008Back in 1970s and 1980s, cogeneration plants in sugarcane mills were primarily designed to consume allbagasse, and produce steam and electricity to the process. The plants used medium pressure steama r t i c l e i n f o a b s t r a c tSupercritical steam cycles and biomassfor sugarcane mills

Luiz Felipe Pellegrini a,*, Silvio de Oliveira Junior b,a Laboratory of Environmental and Thermal Engineering, Polytechnic School UniversitSao Paulo, SP, BrazilbMechanical Engineering Faculty, Technological University of Pereira, Pereira, Colombia

journal homepage: www.All rights reserved.tegrated gasication combined cycles

n Carlos Burbano a

Sao Paulo, Av. Prof. Luciano Gualberto, 1289 Cidade Universitaria, CEP: 05508-900,

le at ScienceDirect

gy

evier .com/locate/energy

cogeneration plants in sugarcane mills. These systems might attain3540% efciency for the conversion of power [39].

Supercritical steam cycles (SuSC) might also attain these ef-ciency values, what makes them an alternative to gasication-based systems [10]. Different works discuss the use of supercriticalpower plants as an alternative to integrated gasication combinedcycles [11].

Using the methodology developed in a previous work [12], thispaper presents an exergetic comparative study of these systems forsugarcane mills, using exergy-based costs to evaluate the impact ofthese technologies on the production of sugar, ethanol and elec-tricity. The congurations studied are based on real systems thatcould be adapted to biomass use.

2. Technological aspects

2.1. Supercritical steam cycles (SuSC)

According to [10], supercritical plants have been in use since the1930s, mainly in Europe. In the US, supercritical plants were rstdeveloped in the 1950s and 1960s. The rst units, however, expe-rienced problems related to reliability and operational exibilityand subcritical pressure units became the US norm. Despite that,development continued overseas and there is now little differences

coal-red power plant [10]. The technical realization of a 700 Csteam power plant depends on a successful development andqualication of advanced ferritic, austenitic and Ni-based alloys[15].

Besides materials which admit higher temperatures and pres-sures for the steam, another important feature related to super-critical plants is the use of regenerative heat exchangers andre-heat schemes in the boiler. These characteristics allow higherthermal efciencies of the cycle when compared to conventionalsteam cycles [16].

Regarding the use of biomass as fuel for supercritical plants, theapplicability should be evaluated based on the availability of thebiomass, its composition and combustion characteristics. As far ascombustion of biomass is concerned, the use of CFB boilers seemsto be the best possibility due to its exibility in relation to lowgradefuels. Also, considering the use of such plants for cogeneration, thenumber of regenerative heat exchangers must be set in a way thatthe steam needed in the process is not jeopardized.

This paper presents a supercritical steam cycle compromised bya single re-heat boiler and six regenerative heat exchangers plusa deaerator (Fig.1). The pressure at each regenerative heat exchangerwas chosen based on [14]. Fig. 2 presents the Ts diagramof the SuSC.

L.F. Pellegrini et al. / Energy 35 (2010) 11721180 1173in reliability between subcritical versus supercritical units [13].Improvements in materials, and the increasing demand for high

efciency power generation units are making supercritical thechoice for new coal-red utility plant world wide [10].

In the early 1990s, the Japanese were the rst to put a super-critical plant operating with temperatures near 600 C [14,15]. Bythe late 1990s, the ultrasupercritical (USC) concept was introduced,and steam parameters raised to 290 bar range and temperaturesaround 600 C. Bugge et al. [15] present data for different ultra-supercritical steam plants in service or under construction, calledby the authors 600 C power plants. The efciencies reported varyfrom 40% to almost 50% (LHV) for a single re-heat (580 C/600 C)300 bar plant, located in Denmark.

A 700 C steam power plant will be constructed during thenext 710 years constituting a benchmark for a 50% efciency (LHV)Fig. 1. Schematic represe2.2. Biomass integrated gasication combined cycles

The biomass integrated gasication combined cycle (BIGCC)technology was rst identied over a decade ago as an advancedtechnology with the potential to be cost-competitive withconventional condensingextraction steam turbine (CEST) tech-nology using biomass by-products of sugarcane processing as fuel,while dramatically increasing the electricity generated per unit ofsugarcane processed [7].

Over the last 15 years, many different works arise that dealtwith the different aspects regarding the applicability of BIGCCsystems to sugarcane mills [39]. Other works discussed thecombined use of biomass derived gas and natural-gas in cogene-ration plants in order to overcome some difculties related toBIGCC plants [1719].ntation of the SuSC.

vacuum pans. Backpressure steam used in the multiple-effect

ergyConsonni and Larson [20] described and analyzed the maindesigns available for gasiers to be applied in BIGCC plants: lowpressure air-blown, low pressure indirect heated and high pressureair-blown. Some of the key tradeoffs involved in designing andcommercializing BIGCC systems relate to Atmospheric versusPressurized gasication, hot versus cold Atmospheric cleanup, theadaptability of commercial gas turbines, and the thermal integra-tion between the dryer, the gas production equipment, and theturbomachinery.

Pressurized gasication provides a thermodynamic advantage(it avoids thermodynamic losses associated with compressing thefuel gas), but presents greater technological development chal-lenges. With directly heated (air-blown) gasication, some minormodications are required to use existing aeroderivative turbines,including de-rating the ring temperature. Indirectly heated gasi-cation produces a fuel gas with higher energy content so thatnatural-gas ring temperatures can be readily achieved withexisting machines [20].

This paper presents three congurations for BIGCC technology:two low pressure air-blown and one high pressure air-blown. Forlow pressure air-blown congurations (Fig. 3), the off-designoperation strategy adopted was de-rating the ring temperature inthe turbine. For the high pressure air-blown gasier (Fig. 4), thecompressor blast-off was used, with the air extracted being sent tothe gasier.

2.3. Steam consumption for sugar and ethanol production

Different authors have shown that the integration of BIGCCsystems to sugarcane mills is only possible if there is a considerable

0 1 2 3 4 5 6 7 8 9 10 110

100

200

300

400

500

600

700

Entropy (kJ/kg-K)

Tem

peratu

re (C

)

300 bar 90 bar 2,5 bar 0,1 bar

0.2 0.4 0.6 0.8

Fig. 2. Ts diagram for the SuSC system.

L.F. Pellegrini et al. / En1174reduction in the steam consumption in the processes [3,6,7,9]. Also,in order to avoid an excessive use of biomass in supercritical boilers,it is very important to set up a regenerative heat exchangernetwork to increase the inlet water temperature in the boiler.Hence, a reduction in the steam consumption in the process willmake it possible to extract steam from turbine to these heatexchangers.

Current sugarcane mills do not show a high level of thermalintegration among its heating and cooling demands, presentinghigher steam consumptions when compared to beet sugar mills orcorn ethanol distilleries [2123]. Furthermore, the internal steamconsumption and the level of thermal integration are inuenced bythe production strategy adopted (amount of cane crushed for sugarand ethanol).

It is well-known that reducing the process steam requirementslead to higher surplus of electricity inside sugarcane mills [12,23].

Avram and Stark [24], and Rein [25] present some opportunitiesto reduce the steam consumption in sugarcane mills:and distillery;U Cogeneration plant with bagasse-red boilers (82%, LHV ef-

ciency) with steam generated at 21 bar and 300 C, and lowefciency (5565%) backpressure steam turbines.

The modications cited were implemented to reduce the steamconsumption and integrate the SuSC and BIGCC systems into themill.

3. Modeling approach and simulation

3.1. Sugar and ethanol production

The model developed in [12] for the evaluation of the jointproduction of sugar, ethanol and electricity was used to simulatethe congurations. The model is composed of mass, energy, exergyand cost balances for each component of the system, as well as byrelations to determine the liquidvapour equilibrium in themultiple-effect evaporator and vacuum pans. The model assess allmajor processes involved in the production of sugar and ethanol(extraction system, juice treatment plant pH correction andheating, evaporation, cooking, fermentation and distillation), aswell as the re-use of part of the condensate generated by the use ofvegetable steam as imbibition in the extraction system and dilutionwater in the ethanol process.

The production of sugar is calculated using data for solid andsucrose concentration on the syrup leaving the multiple-effectevaporator and the different ows of the cooking process (vacuumpans). The steam consumption is determined by energy balances ineach equipment.

The amount of ethanol produced is based on the stoichiometricconversion of sugars into ethanol (0.511 L of ethanol/kg of sugars,considering an 89% efciency for the fermentation process and 99%for the distillation process. Furthermore, steam consumption forthe distillation process is calculated based on 3.5 kg of low pressuresteam (2.5 bar)/L of ethanol for the conventional mill, and 1.6 kg ofU Maximum evaporation in multiple-effect evaporators,increasing the solid content of the syrup;

U Increase the number of evaporator effect;U Juice heating using vapours from all effects of the evaporator,

avoiding losses in the condenser;U Use of liquid/liquid heaters as a rst stage in juice heating;U Increase the pressure in each effect of the evaporator, as tomake

better use of extracted vapours.

As for the distillery, Seemann [26] discusses the use of multi-pressure distillation in order to decrease the steam consumption.A new proposal for multi-pressure systems is the Split Feeddeveloped by Dedini-Agro with support of Siemens andChemtech. This system uses three columns operating withdifferent pressure levels, and promotes a thermal integrationbetween columns. This paper considers the use of two columnsoperating under different pressure levels, leading to differenttemperatures in re-boilers and condensers, that allows integratingthe re-boilers and condensers of columns and decreasing thethermal steam demand to distillation system. Such integrationleads to a steam consumption of 1.6 kg/L [27].

In this paper, a base case for a conventional sugarcane mills hasbeen set with following characteristics:

U Production strategy: 50% for sugar, 50% for ethanol;U Low level of thermal integration: extraction from the rst effect

of the multiple-effect for heating juice from the milling and

35 (2010) 11721180low pressure steam/L of ethanol for multi-pressure distillation [26].

ergyL.F. Pellegrini et al. / En3.2. Bagasse drying

The model is based on [28], which considers an energy balancein the dryer and that the outlet air is saturated. It is assumed thatthe thermal loss is equivalent to 1% of the enthalpy variation of thehot gases used for drying.

For the SuSC system, the exhaust gases from the boiler are usedto dry bagasse up to 40% moisture content. As for the BIGCCsystems, it is possible to dry bagasse to 10% moisture content, sincethe amount of exhaust gases from the heat recovery steam gener-ator (HRSG) is higher than that of the SuSC boiler.

3.3. Gasication

A chemical equilibrium model was considered as developed in[9]. The model allows the evaluation of the composition of theproduced gas, under different pressures and temperatures, as wellas for different compositions of the biomass. Table 1 presents thecomposition of the produced gas generated in both low pressureand high pressure air-blown gasiers evaluated in this paper. Forboth cases, it was considered an equivalence ratio of 0.3 and thatall bagasse is converted into produced gas (no charcoalproduction).

Comparison between the values in Table 1 and those obtained inreal systems might be found in [9].

Fig. 3. Schematic representation35 (2010) 11721180 11753.4. Gas turbine

For the gas turbine simulation red with produced gas, twoapproaches were considered [19,29]:

U De-rating for the low pressure air-blown gasication (turbineinlet temperature (TIT) decrease, with the air mass owrateconstant in the compressor);

U Compressor blast-off for the high pressure air-blown gasica-tion (same TIT as the design point, extracting the excess air fromthe compressor before the combustion chamber).

The design conditions were dened based on ALSTOM GT-11operating under ISO conditions, and these parameters were used inthe off-design simulation.

3.5. Other equipment

Table 2 shows other assumptions made in the simulation.It was also considered the total electrication of mechanical

drives with motors with 95% efciency.

3.6. Simulation

The models presented above were implemented in the Engi-neering Equation Solver (EES) [30], and simulated considering

of the Atmospheric BIGCC.

L.F. Pellegrini et al. / Energy1176a steady-state operation, with approximately 50% of the canecrushed to sugar production and 50% to hydrated alcohol produc-tion. EES has been previously used to evaluate the performance ofdifferent bagasse boiler congurations [28], to simulate the processsteam demand reduction in sugarcane mills [23]. Furthermore, it ispossible to nd in the literature the use of EES to evaluate differentenergy conversion systems.

As for the criterion used to distribute costs among differentproducts in a given control volume, it was considered that eachproduct has the same importance. Thus, their exergy-based costswere set equal. This criterion is similar to the equality methoddiscussed in [31]. This means that the cost associated to the irre-versibilities in the control volume is distributed equally among the

Fig. 4. Schematic representation

Table 1Produced gas composition for the two congurations of gasier (low and pressureair-blown).

Component Molar fraction (%)

Low pressure air-blown High pressure air-blown

CH4 0.2 0.8CO 23.2 23.0H2 22.9 20.2CO2 10.3 9.5H2O 5.6 9.2N2 37.4 37.0Ar 0.5 0.4LHV (kJ/kg) 5137 593935 (2010) 11721180exergy content of the outlet product ows. Furthermore, it wasattributed zero cost to all ows leaving the mill control volumewithout further use (excess bagasse, stack gases, contaminatedcondensate, for instance).

The idea of using cost balances is to show the cost formationprocess inside the mill and to evaluate the impact of improving thecogeneration exergy efciency on the cost formation process. Thesame methodology was used to assess different congurations forcogeneration systems in sugarcane mills based on conventionalsteam turbines [12].

3.7. Error analysis

The model developed is based on the real operation of sugar-cane mills installed in Brazil. The following variables weremeasured during a eld research performed during harvests of2005 and 2006 (a table with the values of these data is presented inAppendix I):

U Sugarcane compositionU Fiber compositionU Bagasse moistureU Pressure in each effect of the multiple-effect evaporatorU Pressure in the vacuum pansU Solid and sucrose contents in the ltered juiceU Solid and sucrose contents in the syrup before vacuum pansU Solid and sucrose contents in the mass after vacuum pans

of the Pressurized BIGCC.

Table 2General parameters used for the simulated cogeneration systems.

Parameter Value

Conventional millBoiler efciency (%, LHV basis) 82Power turbine isentropic efciency (%) 65Electric generator efciency (%) 95Mechanical drive turbine isentropic efciency (%) 55Pump isentropic efciency (%) 70

L.F. Pellegrini et al. / EnergyU Sugar and molasses PurityU Steam consumption in the distillery

Except for pressure measurements and the steam consumptionin the distillery, all other data were obtained through laboratorialanalysis with high levels of accuracy. Pressure datawere taken fromgauge manometers installed in each equipment, and the steamconsumption in the distillery was obtained through the use ofowmeters for steam and ethanol.

These data were used to develop mass and energy balancesresponsible for the calculation of steam and electromechanicalenergy demands in process, which are needed to determine theelectricity output in the cogeneration plant. Data for the cogene-

SuSCSteam pressure (bar) 292Steam temperature (C) 590Re-heat steam temperature (C) 590Boiler efciency (%, LHV basis) 87Turbine stages isentropic efciency (%) 6585Electric generator efciency (%) 95Pump isentropic efciency (%) 70

BIGCCSteam pressure (bar) 4080Steam temperature (C) 400510Produced gas compressor isentropic efciency (%) 80HRSG pinch point (C) 10HRSG approach point (C) 5Turbine (condensingextraction) isentropic efciency (%) 7580Electric generator efciency (%) 95Pump isentropic efciency (%) 70ration systems studied were imposed by the authors.All simulations were performed using the EES software with

a convergence criterion of 109.The error analysis was developed using the EES software, and

uncertainties for steam and electromechanical energy demands,exergy-based costs for sugar and ethanol are given in Table 3.

As can be seen in Table 3, the most sensitive variable to errors inmeasured data is the specic steam consumption, which is verydependent on the steam consumption in the distillery, and also inthe sugar production process, specially in the vacuum pans. On theother hand, the sensitiveness of the exergy-based cost of theproducts is more dependent on the composition of the sugarcanethat determines the amount of sugar and ethanol to be produced.

Further analysis were developed considering modications inboth production processes and in the cogeneration plant, based onconcepts to minimize the exergy destruction in energy conversionprocesses.

Table 3Uncertainty evaluation.

Variable Calculatedvalue

Uncertainty Relativeerror

Specic steam consumption (kg/tc) 490.4 9.7 2.0%Electromechanical energy consumption

(kWh/tc)30.00 0.12 0.4%

Sugar exergy-based cost (kJ/kJ) 1.923 0.017 0.9%Ethanol exergy-based cost (kJ/kJ) 3.651 0.021 0.6%4. Results

4.1. Steam reduction

The simulation of the traditional mill resulted in a process steamconsumption of 492 kg/tc (tc tons of cane crushed). Analysing theresults it is possible to come upon ways of decrease thisconsumption, as discussed in Section 2.3.

U 80 kg/tc of vapour are condensed in the barometric condenser.U 175 kg/tc of condensate at 90 C are cooled in spray coolers.U 60% of solid content in the syrup (maximum solubility is 75%).U Only vapour from the rst effect is used to heat the juice.

In order to reduce the process steam consumption the followingmeasures were adopted:

U Increase the pressure in each effect of the multiple-effect;U Vapour extraction from all effects, with no losses to the

condenser, achieving 72% solid content;U 3rd effect vapour is used in the vacuum pans;U Use of liquid/liquid heat exchangers for juice heating;U Use of a multi-pressure distillation process, consuming 1.6 kg of

steam/L of hydrated alcohol [26].U Stillage is used to heat fermented liquor before the distillation

process.

These modications resulted in a decrease of 43% in the processsteam consumption (280 kg/tc).

Considering the exergy destruction in the sugar and ethanolprocesses, including the extraction system, there has beena decrease of approximately 15% (from 259 kWh/tc to 220 kWh/tc).It is important to notice that these processes account for nearly 30%of the total exergy destruction inside the mill [12].

The cogeneration plant accounts for 613 kWh/tc of exergydestruction in traditional mills, including the amount of excessbagasse not used to generate power.

4.2. SuSC

Table 4 presents values for specic mass owrate, temperatureand pressure for each ow indicated in Fig. 1. The supercriticalsteam cycle is capable of generating an excess electricity of142 kWh/tc, reducing the exergy destruction inside the mill bynearly 12% (see Fig. 5). The exergy-based cost of the electricity isreduced by 49%, while the sugar and ethanol exergy-based costs aredecreased by 28% and 30%, respectively (see Fig. 6).

4.3. BIGCC

Three congurations were simulated for BIGCC systems, andvalues for specic mass owrate, temperature and pressure foreach ow are shown in Tables 57:

i. Atmospheric BIGCC, with steam generation at 40 bar and400 C (Table 5);

ii. Atmospheric BIGCC, with steam generation at 80 bar and510 C (Table 6);

iii. Pressurized BIGCC, with steam generation at 80 bar and510 C (Table 7).

These congurations are based on the ones proposed in [20],with heat recovery from the cleaning station to steam productionas well as pre-heating air before the gasier in the case of Atmo-

35 (2010) 11721180 1177spheric gasication. As for the Atmospheric gasicationwith steam

the heat released during cleaning of the produced gas is better usedto superheat steam rather than evaporate other, since it allows theuse of higher steam pressures and temperatures in the steam cycle.The net effect is 3 kWh/tc of additional electricity and a reduction of11 kWh/tc in the irreversibilities (Fig. 5).

The last conguration for BIGCC systems is the most efcientone, generating almost 202 kWh/tc of excess electricity, whilereducing the exergy destruction by nearly 20% (Fig. 5). Also, the

Table 4Calculated values for specic mass owrate, temperature and pressure for the SuSCconguration.

Tag Mass owrate (kg/tc) Temperature (C) Pressure (bar)

1 668 600.0 300.02 45 402.5 87.13 551 356.5 62.24 72 356.5 62.25 551 600.0 62.26 17 495.4 30.67 52 422.7 17.68 477 422.7 17.69 16 333.1 8.010 9 227.7 2.511 260 227.7 2.512 17 159.9 1.013 176 45.8 0.114 217 45.8 0.1

0.000.501.001.502.002.503.003.504.004.505.005.506.006.507.007.50

Electricity Process Steam Sugar Ethanol

Ex

erg

y-b

as

ed

C

os

t (k

J/k

J)

Traditional MillAtmospheric BIGCC (40 bar)Atmospheric BIGCC (80 bar)Pressurized BIGCC (80 bar)SuSC

Fig. 6. Exergy-based costs of electricity, process steam, sugar and ethanol for thedifferent systems.

L.F. Pellegrini et al. / Energy 35 (2010) 11721180117815 198 46.0 17.616 18 46.0 17.617 198 97.6 17.618 198 125.4 17.619 198 169.4 17.620 668 206.0 17.621 668 213.2 300.022 668 235.0 300.023 668 278.9 300.024 668 303.0 300.025 45 301.0 87.126 116 277.9 62.227 133 235.0 30.628 16 170.4 8.029 24 127.4 2.530 41 99.6 1.031 278 140.0 2.5generation at 80 bar and 510 C, the HRSG produces saturatedsteam at 80 bar which is superheated by cooling produced gas fromthe gasier.

For the rst case, the amount of excess electricity generated is152 kWh/tc (Fig. 5), with a decrease in the exergy destructioninside the mill of 15%, in comparison to the traditional mill. Theexergy-based cost of the electricity is reduced by 55%, while thesugar and ethanol exergy-based costs are decreased by 28% and31%, respectively (Fig. 6).

As for the second case, there has been an increase in the excesselectricity generation to 155 kWh/tc, decreasing the total exergydestruction by 16% (Fig. 5). The exergy-based cost of the products isslightly lower than those for the rst case. These results show that

32 264 120.0 2.533 264 120.3 17.634 7 422.7 17.635 21 70.0 17.6

0

200

400

600

800

1000

1200

Excess Electricity Generation Exergy Destruction

kW

h/tc

Traditional MillAtmospheric BIGCC (40 bar)Atmospheric BIGCC (80 bar)Pressurized BIGCC (80 bar)SuSC

Fig. 5. Excess electricity generation and exergy destruction for the different systems.exergy-based cost of sugar and ethanol are 32% and 34% lower thanthat of the traditional conguration (Fig. 6). The absence of theproduced gas compressor is the main reason for the increase in theelectricity generation and the consequent better thermodynamicperformance of the mill. This compressor accounts for 54 kWh/tc(almost 20% of total generation) of electricity in AtmosphericBIGCCs, increasing the cost of the produced gas to the combustionchamber, and, hence, of all downstream ows. Such increase ofcosts leads to higher costs for sugar and ethanol as presented

Table 5Calculated values for specic mass owrate, temperature and pressure for theAtmospheric BIGCC, with steam generation at 40 bar and 400 C.

Tag Mass owrate (kg/tc) Temperature (C) Pressure (bar)

1 212 25.0 1.0

2 212 103.6 2.03 212 300.0 2.04 355 761.4 2.05 355 350.0 2.06 355 258.0 2.07 348 35.0 1.08 7 35.0 1.09 348 447.8 15.710 2519 25.0 1.011 2519 451.9 15.712 2868 995.0 15.213 2868 457.8 1.114 2868 188.8 1.115 372 420.0 40.016 11 146.4 2.517 2 121.2 41.118 277 140.0 2.519 263 125.0 2.520 79 45.8 0.121 79 45.8 0.122 79 45.8 2.523 374 120.4 2.524 372 121.2 41.125 8 420.0 40.026 22 70.0 2.5

and also the negative impact of the use of a produced gas

Table 6Calculated values for specic mass owrate, temperature and pressure for theAtmospheric BIGCC, with steam generation at 80 bar and 510 C.

Tag Mass owrate (kg/tc) Temperature (C) Pressure (bar)

1 212 25.0 1.02 212 103.6 2.03 212 261.2 2.04 355 751.0 2.05 355 311.2 2.06 355 237.2 2.07 349 35.0 1.08 6 35.0 1.09 349 447.2 15.710 2516 25.0 1.011 2516 451.9 15.712 2864 994.6 15.213 2864 457.6 1.114 2864 218.3 1.115 326 510.0 80.016 7 149.3 2.517 2 121.9 81.118 277 140.0 2.519 263 120.0 2.520 38 45.8 0.121 38 45.8 0.122 38 45.8 2.5

L.F. Pellegrini et al. / Energy4.4. Comparative analysis

There is an increase of 7% and 9% in the electricity generation forpreviously. Furthermore, for Pressurized BIGCC, less exergy isdestroyed in the cleaning process due to the hot cleanup.

23 326 120.4 2.524 326 121.9 81.125 6 510.0 80.026 20 70.0 2.5Atmospheric BIGCC (40 bar) and Atmospheric BIGCC (80 bar),respectively, in relation to SuSC systems. However, it is interestingto notice that the impact of SuSC on the exergy-based cost of sugarand ethanol is almost the same as that of the Atmospheric BIGCCs.This is a consequence of the higher exergy-based cost of processsteam for the later, which is connected to higher exergy destruction

systems in their cogeneration facilities then the total amount of

Table 7Calculated values for specic mass owrate, temperature and pressure for thePressurized BIGCC, with steam generation at 80 bar and 510 C.

Tag Mass owrate (kg/tc) Temperature (C) Pressure (bar)

1 212 451.9 15.72 355 857.6 15.73 355 550.0 15.74 2251 25.0 1.05 2251 451.9 15.76 61 451.9 15.77 2333 1119.0 15.28 2333 541.6 1.19 2333 185.7 1.110 362 510.0 80.011 12 149.3 2.512 2 120.0 81.113 277 140.0 2.514 263 120.0 2.515 70 45.8 0.116 70 45.8 0.117 70 45.8 2.518 362 120.4 2.519 362 120.0 81.120 6 510.0 80.021 20 70.0 2.5electricity generated (115 TWh/year) would represent 25% of theBrazilian total generation.

Acknowledgement

The authors wish to acknowledge Fundaao de Amparo a` Pes-quisa do Estado de Sao Paulo (FAPESP) and Conselho Nacional dePesquisa (CNPq) for the nancial support received (grants 03/12094-8 and 301115/2006-0, respectively), and Usina Iracema forcompressor, since Pressurized BIGCC has the lowest exergy-basedcost for every exergy ow in the mill.

5. Conclusions

The congurations presented in this paper, which might becalled advanced cogeneration systems, allow the generation ofroughly three times more excess electricity when compared tocurrent available condensingextraction steam turbines schemes[12]. Pressurized gasication allows even higher excess electricityoutputs, achieving 202 kWh/tc. However, there is a need todecrease the process steam consumption to values lower than350 kg/tc.

The thermal integration proposed in this paper reduced thesteam consumption by 43%, to 280 kg/tc, representing 15% ofexergy destruction savings inside the mill.

As a consequence of the high exergy destruction in equipmentupstream to the steam generation in the Atmospheric BIGCCsystems, when compared to SuSC, those systems present higherexergy-based costs of process steam. However, as for PressurizedBIGCC, the exergy destruction is lower than for SuSC, thus theprocess steam exergy-based cost for this conguration is the lowestone. Also, it is important to comment on the low efciency of thesteam cycles of BIGCC when compared to SuSC conguration.

Although, SuSC are not competitive with Pressurized BIGCC asfar as electricity generation is concerned, the technology toimplement SuSC plants in sugarcane mills seems to be closer tocommercial scale than BIGCC one. However, SuSC systems are notsuitable for small installed capacities, due to problems related tothe operation of the rst stages of the turbine with small massows (reduced volumetric ow) requiring very small blades, withinefcient design related to leakage between stages. The smallerplants for SuSC would have a generating capacity of 280 MW,leading to mills crushing at least 6.5 million tons per year (in Brazil,in the last harvest, only twomills crushedmore 6million tons [32]).

The realization of SuSC plants might be feasible for centralizedplants to be construct near a pool of mills, instead of inside a mill.For instance, to run 280 MW plant based on bagasse during 6000 h(70% of availability), it would be needed something like 2.5 millionsof tons of bagasse. On the one hand, this leads to a problem: whereto nd this excess bagasse in economic range from the thermo-electric plant (50 km). On the other hand, the use of sugarcane trashmight turn it feasible due to the high amount of its availability ifmechanized harvest is used.

Furthermore, if all mills could install Pressurized BIGCCsin equipment upstream to the steam generation in AtmosphericBIGCCs, while the electricity exergy-based cost of SuSC is greaterthan those of Atmospheric BIGCCs. Hence, the net effect is thatthese systems have the same impact on the exergy-based cost ofsugar and ethanol. This indicates the low efciency of the steamcycle of the Atmospheric BIGCCs when compared to SuSC system,

35 (2010) 11721180 1179their technical support and data.

Appendix I

Table A1 presents data measured in a sugarcanemill and used todevelop the mass and energy balances for sugar and ethanolproduction processes.

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Fourth effect pressure 0.56 0.01 1.0%Fifth effect pressure 0.20 0.00 1.0%Vacuum pans pressure 0.20 0.00 1.0%Mass A sucrose content 0.799 0.001 0.1%Sugar sucrose content 0.998 0.001 0.1%Honey sucrose content 0.537 0.001 0.2%Mass B sucrose content 0.749 0.001 0.1%Molasses sucrose content 0.489 0.001 0.2%Magma sucrose content 0.829 0.001 0.1%Sugarcane sucrose content 0.137 0.001 0.7%Specic steam consumption

in the distillery (kg/L)3.5 0.2 5.7%

L.F. Pellegrini et al. / Energy 35 (2010) 117211801180Measured data.

Variable Measure(kg/kg)

Uncertainty(kg/kg)

Relativeerror

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Supercritical steam cycles and biomass integrated gasification combined cycles for sugarcane millsIntroductionTechnological aspectsSupercritical steam cycles (SuSC)Biomass integrated gasification combined cyclesSteam consumption for sugar and ethanol production

Modeling approach and simulationSugar and ethanol productionBagasse dryingGasificationGas turbineOther equipmentSimulationError analysis

ResultsSteam reductionSuSCBIGCCComparative analysis

ConclusionsAcknowledgementAppendixReferences