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THERMODYNAMIC ANALYSIS AND COMPARISON OF DOWNDRAFT GASIFIERS INTEGRATED WITH GAS TURBINE, SPARK AND COMPRESSION IGNITION ENGINES FOR DISTRIBUTED POWER GENERATION

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The objective of the present article is to assess and compare the performance of electricity generation systems integrated with downdraft biomass gasifiers for distributed power generation. A model for estimating the electric power generation of internal combustion engines and gas turbines powered by syngas was developed. First, the model determines the syngas composition and the lower heating value; and second, these data are used to evaluate power generation in Otto, Diesel, and Brayton cycles. Four synthesis gas compositions were tested for gasification with: air; pure oxygen; 60% oxygen with 40% steam; and 60% air with 40% steam. The results show a maximum power ratio of 0.567 kWh/Nm3 for the gas turbine system, 0.647 kWh/Nm3 for the compression ignition engine, and 0.775 kWh/Nm3 for the spark ignition engine while running on synthesis gas which was produced using pure oxygen as gasification agent. When these three systems run on synthesis gas produced using atmospheric air as gasification agent, the maximum power ratios were 0.274 kWh/Nm3 for the gas turbine system, 0.302 kWh/Nm3 for CIE, and 0.282 kWh/Nm3 for SIE. The relationship between power output and synthesis gas flow variations is presented as is the dependence of efficiency on compression ratios. Since the maximum attainable power ratio of CIE is higher than that of SIE for gasification with air, more research should be performed on utilization of synthesis gas in CIE.
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  This article appeared in a journal published by Elsevier. The attachedcopy is furnished to the author for internal non-commercial researchand education use, including for instruction at the authors institutionand sharing with colleagues.Other uses, including reproduction and distribution, or selling orlicensing copies, or posting to personal, institutional or third partywebsites are prohibited.In most cases authors are permitted to post their version of thearticle (e.g. in Word or Tex form) to their personal website orinstitutional repository. Authors requiring further informationregarding Elsevier’s archiving and manuscript policies areencouraged to visit:http://www.elsevier.com/authorsrights  Author's personal copy Thermodynamic analysis and comparison of downdraft gasi 󿬁 ersintegrated with gas turbine, spark and compression ignitionengines for distributed power generation Andrés Z. Mendiburu * , Justo J. Roberts, João A. Carvalho Jr., José L. Silveira São Paulo State University  e  UNESP, Campus of Guaratinguetá  e  FEG, Av. Ariberto P. da Cunha, 333, Guaratinguetá, SP CEP 12510410, Brazil h i g h l i g h t s   Engines and gas turbines integrated with gasi 󿬁 cation were compared.   Four different gasi 󿬁 cation agents were considered in the analysis.   It was found that a syngas e air mixture has to be considered in the compression.   Good accuracy with respect to available experimental data.   For air gasi 󿬁 cation higher power ratio was found for compression ignition engines. a r t i c l e i n f o  Article history: Received 4 December 2013Accepted 9 February 2014Available online 18 February 2014 Keywords: Gasi 󿬁 cationEnginesGas turbinesPower generationComparison a b s t r a c t The objective of the present article is to assess and compare the performance of electricity generationsystems integrated with downdraft biomass gasi 󿬁 ers for distributed power generation. A model forestimating the electric power generation of internal combustion engines and gas turbines powered bysyngas was developed. First, the model determines the syngas composition and the lower heating value;and second, these data are used to evaluate power generation in Otto, Diesel, and Brayton cycles. Foursynthesis gas compositions were tested for gasi 󿬁 cation with: air; pure oxygen; 60% oxygen with 40%steam; and 60% air with 40% steam. The results show a maximum power ratio of 0.567 kWh/Nm 3 for thegas turbine system, 0.647 kWh/Nm 3 for the compression ignition engine, and 0.775 kWh/Nm 3 for thespark-ignition engine while running on synthesis gas which was produced using pure oxygen as gasi- 󿬁 cation agent. When these three systems run on synthesis gas produced using atmospheric air asgasi 󿬁 cation agent, the maximum power ratios were 0.274 kWh/Nm 3 for the gas turbine system,0.302 kWh/Nm 3 for CIE, and 0.282 kWh/Nm 3 for SIE. The relationship between power output andsynthesis gas  󿬂 ow variations is presented as is the dependence of ef  󿬁 ciency on compression ratios. Sincethe maximum attainable power ratio of CIE is higher than that of SIE for gasi 󿬁 cation with air, moreresearch should be performed on utilization of synthesis gas in CIE.   2014 Elsevier Ltd. All rights reserved. 1. Introduction Currently, power generation systems based on biomass down-draftgasi 󿬁 ersareinthedevelopmentstage,butsomeexperimentalresearch supports their feasibility. Spark-ignition engines (SIEs) aremostly studied for use with synthesis gas; works by Coronado et al.[1] and Shah et al. [2] are evidence of this. Compression-ignition engines (CIEs) are more complex because auto-ignition of thesynthesis gas must be assured. The auto-ignition potential of syn-gas improves when the syngas contains more hydrogen and lessnitrogen [3]. Theoretical limits based on the second law of ther-modynamics for ICEs operating with syngas and using diesel fuelforignitionwerestudiedbySahooetal.[4],whodemonstratedthatgreater hydrogen concentration improves the combustion process.The major conversion technologies that can be fueled by synthesisgas are [5]: internal combustion engines, micro turbines, gas tur-bines (GTs), fuel cells, steam engines, steam turbines, Stirling en-gines, and organic ranking cycles. CIE systems have been runningon mixtures of two fuels consisting of up to 90% synthesis gas [6]. *  Corresponding author. Tel.:  þ 55 12 31232838. E-mail address:  andresmendiburu@yahoo.es (A.Z. Mendiburu). Contents lists available at ScienceDirect Applied Thermal Engineering journal homepage: www.elsevier.com/locate/apthermeng http://dx.doi.org/10.1016/j.applthermaleng.2014.02.0271359-4311/   2014 Elsevier Ltd. All rights reserved. Applied Thermal Engineering 66 (2014) 290 e 297  Author's personal copy Chacartegui et al. [7] addressed the use of synthesis gas inheavy duty GTs; Fagbenle et al. [8] studied the use of synthesis gasin a 53 MW gas turbine. Delattin et al. studied combustionchamber conditions and performance when using synthesis gas asfuel [9]. According to Coronado [10], 100 kg/h of biomass and around 240 Nm 3 /h of air are required to produce 200 Nm 3 /h of gas.Biomass gasi 󿬁 cation cogeneration was addressed by Ahrenfeldtet al. [11]. They considered systems with gas engines and microGTs. They concluded that biomass gasi 󿬁 cation cogeneration ispromising for small scale plants. Integrated gasi 󿬁 cation combinedcycles with CO 2  capture were studied by Kanniche and Bouallou[12]. 2. Model development  2.1. Downdraft gasi  󿬁 er non-stoichiometric equilibrium model In the present study, the Gibbs free energy minimizationmethod was implemented with Lagrange multipliers consideringthe general chemical reaction shown in Eq. (1).The input parameters are as follows:(a) Equivalence ratio (ER);(b) Biomass moisture content (MC);(c) Nitrogen/oxygen rate present in the air ( a );(d) Steam molar fraction in the gasi 󿬁 cation agent (  f  vap );(e) Carbon conversion ef  󿬁 ciency ( h CC ).When moisture content in biomass materials reaches over 20%,it reduces the LHV of the synthesis gas [13], so the feedstock mayhave to be dried beforehand. A carbon conversion ef  󿬁 ciency valueabove 80% was used by Vera et al. [14].The input data needed to feed the model is:(a) Proximate and ultimate analysis of the feedstock;(b) Thermodynamicpropertiesof theelementsandcompounds;(c) Higher or lower heating values of the feedstock. C  x C H  x H O  x O N  x N S  x S  þ  x A SiO 2 þ  x H 2 O H 2 O  |fflfflfflfflfflfflfflfflfflfflfflfflfflfflfflfflfflfflfflfflfflfflfflfflfflfflfflfflfflfflfflfflfflfflfflffl{zfflfflfflfflfflfflfflfflfflfflfflfflfflfflfflfflfflfflfflfflfflfflfflfflfflfflfflfflfflfflfflfflfflfflfflffl}  Feedstock þ n vap H 2 O þ n ar ð O 2 þ a N 2 Þ  |fflfflfflfflfflfflfflfflfflfflfflfflfflfflfflfflfflfflfflfflfflfflffl{zfflfflfflfflfflfflfflfflfflfflfflfflfflfflfflfflfflfflfflfflfflfflffl}  Gasification  agent / ð 1  h CC Þ  x C C þ n H 2 H 2 þ n CO CO þ n CO 2 CO 2 þ n CH 4 CH 4 þ n H 2 O H 2 O þ  a n ar þ  x N 2  N 2 þ  x A SiO 2 þ  x S H 2 S(1) The proximate and ultimate fuel analysis can be acquired fromgasi 󿬁 cationliterature[15 e 17].Thethermodynamicpropertiesusedto perform the calculations were obtained from Chase et al. [18].The sensible enthalpies and the Gibbs free energies were  󿬁 tted tosixth degree polynomials. HHV was estimated by means of thecorrelation presented by Channiwala and Parikh [19].  2.1.1. Input parameters Equivalence ratio is determined by Eq. (2), in which the oxygensupplied by the water vapor (if there is any) is also considered. ER   ¼ n O 2  ag n O 2  stq ¼ n vap 2  þ  n ar  x C  þ  x H 4  þ  x S    x O 2 (2) Steam molar fraction (  f  vap ) is de 󿬁 ned as follows Nomenclature  A  ashesER equivalence ratio  f  vap  steam molar fractionHHV higher heating value, MJ/kg or MJ/Nm 3  g  0T  speci 󿬁 c Gibbs free energy, kJ/mol _ G  󿬂 ow, Nm 3 /h h  speci 󿬁 c enthalpy, kJ/mol h 0  f   298  speci 󿬁 c enthalpy of formation, kJ/molLHV lower heating value, MJ/Nm 3 M   molecular weight, kg/kmolMC moisture content, % n i  moles of   “ i ”  species N   number of experimental data P   pressure, kPa P  0  standard state pressure, kPa P  ge  electric power generated, kW P  % CH 4  assumed CH 4  rate R  universal gas constant, kJ/mol K r  C  compression rate s  speci 󿬁 c entropy, kJ/mol K T   temperature,   C and K u  speci 󿬁 c internal energy, kJ/mol V   volume, m 3 w  speci 󿬁 c work, kJ/mol  x  j  amount of   “  j ”  species per biomass carbon mole  y i  “ i ”  molar fraction  Abbreviations CIE compression-ignition engineGT gas turbineICE internal combustion engineRMS root-mean-squareSIE spark-ignition engine Greek letters a  nitrogen/oxygen rate present in the air a ex  air excess D G 0T  Gibbs free energy of formations, kJ/mol h  ef  󿬁 ciency l  Lagrange multiplier Subindex ar aircc carbon conversioncomp compressionexp expansionge electricity generatedgs synthesis gas or syngasgc combustion gasmix air e syngas mixturep productsr realre reagentsstq stoichiometrict totalvap steam  A.Z. Mendiburu et al. / Applied Thermal Engineering 66 (2014) 290 e  297   291  Author's personal copy  f  vap  ¼  n vap n vap  þ  n ar (3) Air mole number is determined byconsidering a given ER valueand the stoichiometric O 2  present in the gasi 󿬁 cation agent. n ar  ¼  n O 2  stq ð ER  Þ  1 þ  f  vap 2  2  f  vap !  1 (4) The ratio of N 2  to O 2  is pre-established for the air present in thegasi 󿬁 cation agent, which permits the study of different cases suchas pure O 2  or enriched air gasi 󿬁 cation. a  ¼ n N 2  ga n O 2  ga (5) Carbon conversion ef  󿬁 ciency is an input parameter which isused in Eq. (6) in order to determine the quantity of convertedcarbon. h CC  ¼ n C  gasified  x C * 100% (6)  2.1.2. Determination of synthesis gas composition Mass and energy conservation equations are shown below. n CO  þ  n CO 2  þ  n CH 4    h CC  x C  ¼  0 (7)2 n H 2  þ 2 n H 2 O  þ 4 n CH 4  þ 2  x S    x H   2  x H 2 O   2 n vap  ¼  0 (8) n CO  þ 2 n CO 2  þ  n H 2 O    x O    x H 2 O    n vap   2 n ar  ¼  0 (9) X M i ¼ 1 _ n i h i ! p  0@X N  j ¼ 1 _ n  j h  j 1A re ¼  0 (10) In equilibrium modeling, methane content is generally under-estimated by these models [20 e 24]. In order to solve this problem,the methane content can be adjusted to an approximate expectedvalue; this is supported by several published experimental workson downdraft gasi 󿬁 cation [20,25 e 31]. This statement can becon 󿬁 rmed by analyzing modeling efforts such as Casella andColonna [32], who did not consider the methane content in theirgasi 󿬁 er model and got good results. However, in the present studyan adjustment was applied in the calculations in Eq. (11), in which P  % CH 4  can have values from 1.5 to 2.0% without compromising theaccuracy of the model. n CH 4  ¼  P  % CH 4 100 24 h CC  x C  þ  x H 2  þ  x H 2 O  þ  x N 2  þ  n vap  þ  l n ar 1 þ  P  % CH4 50 35  (11) The total number of moles in the products is needed to performthe calculations. It is shown in Eq. (12). n t  ¼  h CC  x C  þ  x H 2  þ  x H 2 O  þ  x N 2  þ  n vap  þ  l n ar  þ  n O 2   2 n CH 4  (12) Finally, the Gibbs minimization equations are shown in Eqs.(13) e (16). 24 G 0H 2 RT   þ ln n H 2 n t þ ln  P P  0 35 g þ 2 l H  ¼  0 (13) G 0CO RT   þ ln n CO n t þ ln  P P  0 # g þ l C  þ l O  ¼  0 (14) 24 G 0CO 2 RT   þ ln n CO 2 n t þ ln  P P  0 35 g þ l C  þ 2 l O  ¼  0 (15) 24 G 0H 2 O RT   þ ln n H 2 O n t þ ln  P P  0 35 g þ 2 l H  þ  l O  ¼  0 (16)  2.2. Models used to assess power generation using synthesis gas The following power generation systems were studied:(a) Spark-ignition engine system (SIE);(b) Compression-ignition engine system (CIE);(c) Gas turbine system (GT).FortheGTsystem,Veraetal.[14]usedanisentropicef  󿬁 ciencyof 79% for the compressor with a generator ef  󿬁 ciency of 95%, whileKunze and Spliethoff  [33] set the isentropic ef  󿬁 ciency of thecompressor at 87.7% with a generator ef  󿬁 ciency of 98.5%. In thepresent work, the real points in the thermodynamic cycles arecalculated by means of the isentropic ef  󿬁 ciencies of the compres-sionandexpansionprocesses,adjustedto85%forthethreesystemsstudied. The ef  󿬁 ciency of the electric generator is considered to be96%.A set of equations had to be developed for each speci 󿬁 c case tomodel the power generation systems. The equations that are usefulfor solving simple models in which standard air is used as theworking  󿬂 uid can be found in thermodynamics books [34]. In thecase of systems running on synthesis gas, such approximation re-sults are inaccurate. This is mainly because the air e fuel ratios aresigni 󿬁 cantly lower than those obtained for conventional fuels.For these reasons, an appropriate gas mixture was consideredfor every point of the cycle in which there is one mole of synthesisgas at the input of the generation cycle. Relevant works in the areaofinternalcombustionengines[35]andGTcycles[36]wereusedto developthemodels.Itisalsopossibletousesimpli 󿬁 edequationstopredict engine performance, as in Kalina et al. [37]; however, theauthors consider that it is also important to apply the formalthermodynamic method to this case.  2.2.1. The compression and expansion processes The 󿬁 nal ideal temperatures for the compression and expansionprocesses are obtained by solving the non-linear algebraic equa-tionsshownin Eqs. (17) and (18). The entropyof each compound inthe gaseous mixture was adjusted to a sixth degree polynomial,fromwhichthevaluesofthepolynomialconstants d i wereobtainedfor the syngas e air mixture by factorization, while the constants  e i were obtained for the combustion products.Therealinternalenergiesandenthalpiesattheendof thesetwoprocesses are easily determined since enthalpies and internal en-ergies were also adjusted to sixth degree polynomials. The realtemperatures at the end of the compression and expansion pro-cesses are also obtained. s 2    s 1  ¼ X 7 i ¼ 1 d i T  i  12   X 7 i ¼ 1 d i T  i  11  ¼  0 (17)  A.Z. Mendiburu et al. / Applied Thermal Engineering 66 (2014) 290 e  297  292
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