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Energy 36 (2011) 4063e4071

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Thermodynamic analysis of hydrogen rich synthetic gas generation from fluidizedbed gasification of rice husk

Chanchal Loha a, Himadri Chattopadhyay b,*, Pradip K. Chatterjee a

a Thermal Engineering Group, Central Mechanical Engineering Research Institute (CSIR), Durgapur 713209, IndiabDepartment of Mechanical Engineering, Jadavpur University, S C Mallik Road, Jadavpur, Kolkata 700032, India

a r t i c l e i n f o

Article history:Received 3 December 2010Received in revised form25 April 2011Accepted 25 April 2011Available online 26 May 2011

Keywords:EnergyExergyEquilibrium modelGasificationBiomassRice husk

* Corresponding author. Tel.: þ91 9332151376; fax:E-mail addresses: chanchal.loha@gmail.com (C.

(H. Chattopadhyay).

0360-5442/$ e see front matter � 2011 Elsevier Ltd.doi:10.1016/j.energy.2011.04.042

a b s t r a c t

In the present work, the generation of hydrogen rich synthetic gas from fluidized bed steam gasificationof rice husk has been studied. An equilibrium model based on equilibrium constant and materialbalance has been developed to predict the gas compositions. The equilibrium gas compositions arecompared with the experimental data of the present group as well as of available literature. The energyand exergy analysis of the process have been carried out by varying steam to biomass ratio (j) withinthe range between 0.1e1.5 and gasification temperature from 600 �C to 900 �C. It is observed that boththe energy and exergy efficiencies are maximum at the CBP (carbon boundary point) though thehydrogen production increases beyond the CBP. The HHV (higher heating value) and the external energyinput both continuously increase with j. However, the hydrogen production initially increases withincrease in temperature up to 800 �C and then becomes nearly asymptotic. The HHV decreases rapidlywith increase in temperature and energy input increases. Therefore, gasification in lower temperatureregion is observed to be economical in terms of a trade off between external energy input and HHVof the product gas.

� 2011 Elsevier Ltd. All rights reserved.

1. Introduction

The conservation of limited supply of fossil fuel, climate changeand the increasing concern over global warming have prompteda search for new and cleaner methods of power generationparticularly from renewable energy sources. Amongst the differentsources of renewable energies, the most promising future energysource is biomass [1]. India has substantial biomass resources inthe form of agricultural residues, which are currently used fordomestic energy and fuel applications mostly through combustion.This is often, however, inefficient as well as contributing to localpollution from inadequately controlled gaseous emissions. There-fore, the gasification of biomass is possibly a more efficient way ofbiomass utilization. Biomass gasification is the thermo chemicalconversion of solid biomass into the fuel gas which containsmainly hydrogen, carbon monoxide, carbon dioxide, methane andnitrogen. The product gas from the reactor also contains somecontaminants like char particle, ash and some higher hydrocarbonsor tar. A limited supply of oxygen, air, steam or a combination

þ91 3324146890.Loha), chimadri@gmail.com

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of these serves as gasifying agent. Biomass gasification by using airproduces a gas with a lower Calorific Value (4e7MJ/Nm3), whereasgasification with steam and oxygen produces the gas havingmedium to higher Calorific Values (10e18 MJ/Nm3) [2]. The gasi-fication with oxygen is not popular due to the fact that it involveslarge investment for production of oxygen. The gasificationwith airdilutes the gas due to the presence of N2 in the air. In recent years,the steam gasification of biomass has become an area of growinginterest because it produces gaseous product having higher H2content. Besides, the steam gasification process has the followingadditional advantages: it is capable of maximizing the gas productwith higher heating rate involved, advantageous residence timecharacteristics, and the efficient tar and char reduction broughtabout by steam reforming.

In biomass gasification, fluidized bed technology is widely useddue to its various advantages which include high heat transfer,uniform and controllable temperature, favorable gas-solid con-tacting etc [3]. Mansaray et al. [4] have studied the air gasification ofrice husk in a dual distributor type fluidized bed system. The effectof varying fluidizing velocity (0.22e0.33 m/s) and equivalence ratio(0.25e0.35) on the gasifier performance were discussed. Miccioet al. [5] studied the biomass gasification in fluidized bed andachieved a maximum C (carbon) post conversion of 70%. Thegasification performance of sawdust in fluidized bed reactor was

Nomenclature

CBP carbon boundary pointCp specific heat at constant pressure, kJ/kmol Kdaf dry ash freeE exergy, kJDGo

T standard gibbs function of reactionDgof ;T ;i standard gibbs function of formation, kJ/kmolH enthalpy, kJHc heat of combustion, kJ/kmolI Irreversibility, kJhof heat of formation, kJ/kmolHHV higher heating valueK equilibrium constantm mole of input steamn mole of product gases

P partial pressureQIN energy input as electricity which is converted

to heat, kJR universal gas constant, kJ/kmol KT, T0 gasification and ambient temperature, KWIN exergy input, which is basically work input as

electricity, kJx mole fraction

Greek lettersj steam-to-biomass ratiog stoichiometric number3chem standard chemical exergyhI first law (energy) efficiencyhII second law (exergy) efficiencyD difference

C. Loha et al. / Energy 36 (2011) 4063e40714064

studied by many authors [6e8]. The effects of gasificationtemperature, equivalence ratio, O/C ratio and steam-to-biomassratio were studied on the product gas composition and heatingvalue. The hydrogen yield potential was found to be the mostsensitive with equivalence ratio by Turn et al. [6], whereas thetemperature was reported to be the most important factor by Lvet al. [7], over the range of experimental condition studied. It wasalso observed by Li et al. [8] that the gas composition and heatingvalue depend heavily on the O/C ratio. The air gasification of woodchips was also carried out by Lim et al. [9] in a bubbling fluidizedbed gasifier and the performance was studied in terms of thermaloutput. They showed that the gas produced has an energy contentof 4.75 MJ/m3 at a bed temperature of 733 �C and equivalence ratioof 0.23. The resulting thermal efficiency was 61.32% with a thermaloutput of 355.55 kWth.

Fluidized bed steam gasification was studied by Rapagna et al.[10,11] and naturally occurring catalytic substances were employedto enhance the yield of fuel gas and reduce its tar content. Thereactions influencing the biomass steam gasification process werestudied in an atmospheric fluidized bed gasifier by Franco et al.[12]. The effects of gasification temperature and steam/biomassratio were investigated on the gas composition and HHV (higherheating value). Recently, fluidized bed gasification process withpure steam to produce hydrogen rich gas has been reviewed byCorella et al. [13]. It is reported that the biomass gasification withpure steam can generates 60 vol% (dry basis) hydrogen rich gaseswith tar content as low as 0.25 g/Nm3.The present group of authors[14] have also studied the fluidized bed steam gasification ofbiomass experimentally and developed a correlation to predict thehydrogen yield.

The assessment of the gasification process by analyzing thefirst law (energy) efficiency and the second law (exergy) efficiencyis an effective method for design and analysis of the process. A lotof works on energy and exergy analysis of biomass gasification infixed bed reactor based on equilibrium modeling have been re-ported in the literature. Some authors have also used theequilibrium model to describe the biomass gasification in fluid-ized bed reactor. Equilibriummodel provides a useful design aid inevaluating the possible limiting behavior of a complex reactingsystem that is difficult or unsafe to produce experimentally or incommercial operation [15,16] and also it is computationallyinexpensive. Pellegrini et al. [17] used chemical equilibriummodelto present the exergetic and energetic behavior of air gasificationof sugarcane bagasse where a parametric study has been carriedout to investigate the influence of many variables such as: gasi-fication temperature, air temperature and moisture content. The

equilibrium model was also developed by Jarungthammachoteet al. [18] to calculate the product gas composition and second lawanalysis has been done for gasification of municipal solid waste ina downdraft waste gasifier. Prins et al. [19,20] have focused theirstudy on the energy and exergy analysis of biomass gasification byusing chemical equilibrium model. Their Study showed that theCBP (Carbon Boundary Point) was the optimum point of operationwith respect to exergy-based-efficiency for both gasification withair and steam. The term CBP implies the situation where exactlyenough gasifying agent is added to obtain complete carbonconversion. Ptasinski et al. [21] showed that the exergetic effi-ciency of gasification also depends on the chemical composition ofbiofuel used as feedstock. They showed that the exergetic effi-ciencies of vegetable oil, straw, treated wood, untreated wood andgrass were comparable with coal, whereas the efficiencies ofsludge and manure were considerably lower. The efficiency ofbiomass gasification was also analyzed by Ptasinski [22] by usingtriangular CeHeO diagram, considering a biomass fuel repre-sented as CH1.4O0.59N0.0017. It was observed that at the equivalenceratio of 0.26, the chemical and the total exergy of the gas reachedmaximum at CBP for an air-blown gasifier. The Exergy analysis forbiomass-to-SNG (synthetic natural gas) conversion was presentedby Jurascik et al. [23]. The analysis was made for a temperaturerange from 650 �C to 800 �C and the pressure range from 1 to15 bar. The results showed that the largest exergy losses take placein the biomass gasifier, CH4 synthesis part and CO2 capture unit.The exergy analysis of hydrogen production from sawdust woodwas analyzed by Abuadala et al. [24,25]. The analysis has beendone by developing an equilibrium model. The results indicatedthat the hydrogen production from biomass steam gasificationdepends on the operating temperature, amount of steam addedand the quality of the biomass. Kalinci et al. [26] studied thethermodynamic performance of an integrated gasifier-boilerpower system with six different biomass fuels and showed thatthe exergy contents of different biomass fuels varied from 15.89 to22.07 MJ/kg, respectively. A chemical equilibrium model has alsobeen developed by Karamarkovic et al. [27] to study the air gasi-fication of biomass in different gasification temperature byanalyzing the energy and exergy associated with the process. Itwas reported that the gasification process at a given gasificationtemperature can be improved by the use of dry biomass and by thecarbon-boundary temperature approaching the requiredtemperature. A non-stoichiometric equilibrium model based onfree energy minimization was developed by Li et al. [28]. It wasshowed that the gas composition and heating value vary primarilywith temperature and the relative abundance of key elements,

1. Steam generator 2. Superheater 3. Screw Feeder 4. Gasifier

5. Electric Furnace 6. Cyclone Seperator 7. Gas Claning and

Cooling 8. Filter 9. Suction Pump 10. Gas Chromatograph

1

2

3

4

65

7

8

910

Fig. 1. Schematic diagram of bubbling fluidized bed biomass gasifier.

C. Loha et al. / Energy 36 (2011) 4063e4071 4065

especially carbon, hydrogen and oxygen. Pressure only influencesthe results significantly over a limited temperature range. Thesimulation of hydrogen production from biomass gasification inan interconnected fluidized bed was presented by Shen et al. [29].A process simulation was conducted with Aspen plus software.The variation of gas composition, hydrogen yield, carbon conver-sion and available efficiencies were presented.

From the state-of the-art, it is observed that the exergetic andenergetic analysis of steam gasification of biomass in a fluidizedbed reactor is rather limited. The analysis of the gasification processbased on the new concept of CBP is even fewer. Some authors havediscussed the effects of parameters like temperature, steam-to-biomass ratio, moisture content, feedstock quality etc. on thesteam gasification process. But the important issue of the energyinput to the allothermal gasification process has not been discussedin detail. Therefore, the objective of the present work is to study thesteam gasification of biomass in a fluidized bed gasifier with specialemphasis on the thermodynamic analysis involving parameterslike energy and exergy efficiency. The rice husk is used as biomassbecause it cultivated in more than 75 countries in the world andIndia alone generates about 22 million tones of rice husk per year[30]. Thus rice husk would be the most natural choice for countrieslike India and China and little work has been reported on thegasification behavior of rice husk in the open literature. An equi-librium model has been developed to predict the gas composition.The model has been calibrated against the experimental investi-gation described in Section 2 and also with the experimentalinvestigation of Rapagna et al. [11]. After calibration the model isused to study the H2 production, energy and exergy efficiencies,HHV and the energy input to the system for a wide range oftemperature and S/B (steam-to-biomass ratio), particularlyfocusing at CBP.

The authors envisage a twin fluidized bed gasifier where therewill be two interconnected reactor, one will act as gasifier andanother will act as combustor. The heat required for gasification,which is supplied from the electric furnace in the present investi-gation, will be replaced by the continuous transfer of hot bedmaterial from the combustor. Accordingly, the consideration ofelectrical energy in the analysis does not arise. The advantages ofusing two separate sections: gasification and combustion zones arealready reported in literature [31,32].

Table 1Ultimate and proximate analysis of rice husk.

Ultimate analysis Percent Proximate analysis Percent

Carbon 38.43 Volatile matter 55.54Hydrogen 2.97 Fixed carbon 14.99Sulfur 0.07 Moisture 9.95Nitrogen 0.49 Ash 19.52Oxygen 36.36Ash 21.68

2. Process description

The gasification experiment is conducted in a laboratory scalebubbling fluidized bed gasifier described previously [14] and pre-sented in Fig. 1. The inner diameter of the gasifier is 50 mm and thelength is 1200 mm. The superheated steam at 200 �C is introducedat the bottom of the gasifier. The steam is used as both gasifying aswell as fluidizing agent. The rice husk is fed into the gasifierthrough a water-cooled screw feeder with a variable speed drive.The ultimate and the proximate analysis of rice husk is presented inTable 1. Sand of size 0.3 mme0.5 mm is used as inert bed materialdue to the non-granular and flaky nature of the rice husk. Thegasifier is placed inside an electric furnace to provide the heatrequired for gasification. The temperatures are measured with typeR thermocouple (PlatinumeRhodium) with calibrated uncertaintyvalue of 2.4%. Hot gas coming out from the gasifier is cleaned andcooled before entering into a Gas Chromatograph [Make-Chemito,Model-GC 1000]. The Gas Chromatograph is calibrated in therange used before analysis. The experiments are carried out fordifferent temperature by keeping the S/B fixed and then the S/B isvaried by keeping the temperature fixed and the product gascompositions are analyzed.

3. Equilibrium modeling

Biomass essentially contains volatiles, fixed carbon, ash andwater. Upon heating the biomass, initially the removal of moisturetakes place up to about 120 �C, followed by devolatization up to350 �C and the gasification of char occurs above 350 �C [33]. Inequilibrium modeling, it is assumed that biomass, which is dry andash free, contains the elements C, H and O. The element of nitrogenand sulfur are not considered because biomass contains negligibleamount of nitrogen and sulfur in comparison to carbon, hydrogenand oxygen. Therefore, the chemical formula of the biomass isrepresented as CHxOy. The steam is used as the gasifying agent here.In general the global gasification reaction of biomass with steam asthe gasifying agent can be written as follows:

CHxOy þmH2O ¼ nH2H2 þ nCOCOþ nCO2

CO2 þ nH2OH2O

þ nCH4CH4 þ ncC (1)

where, x and y are the numbers of atoms of hydrogen and oxygenper single atom of carbon in the biomass. m is the moles of steamadded per mole of dry ash free biomass. Left hand side all are inputparameters and they are known. On the right hand sidenH2

;nCO;nCO2;nH2O;nCH4

and nC are the unknown number ofmoles of hydrogen, carbon monoxide, carbon dioxide, steam,methane and solid char carbon respectively present in the product.Now to solve these unknowns equal numbers of equations arerequired. Three equations are obtained from thematerial balance ofcarbon, hydrogen and oxygen respectively. Carbon balance

nCO þ nCO2þ nCH4

¼ 1 (2)Hydrogen balance

Table 2Heat of formation and the standard Gibbs function of formation from 800 K to1100 K for all components.

Compound hof (kJ/kmol) Dgof ;T ;i (kJ/kmol)

800 K 900 K 1000 K 1100 K

H2 0 0 0 0 0CO �110525 �182497 �191416 �200275 �209075CO2 �393509 �395586 �395748 �395866 �396001H2O �241818 �203496 �198083 �192590 �187033CH4 �74520 �2115 8616 19492 30472

Table 3Coefficient for specific heat for the empirical equation Cp ¼ Aþ BT þ CT2 þ DT3.

Compound Cp ¼ Aþ BT þ CT2 þ DT3 (kJ/kmol K) Temperature range

A 102 B 105 C 109 D

H2 20.09 �0.1916 0.4000 �0.870 273e1800CO 27.11 0.655 �0.1000 e 273e3800CO2 22.24 5.979 �3.498 7.464 273e1800H2O 32.22 0.1920 1.054 �3.594 273e1800CH4 19.87 5.021 1.268 �11.00 273e1500

C. Loha et al. / Energy 36 (2011) 4063e40714066

2nH2þ 2nH2O þ 4nCH4

¼ xþ 2m (3)

Oxygen balance

nCO þ 2nCO2þ nH2O ¼ yþm (4)

Now up to the CBP, the char carbon is considered in the productand hence six equations are needed to solve six unknowns.Therefore, in addition to the three material balance equationsanother three equations are obtained from the equilibriumconstant expression of three heterogeneous reactions as givenbelow.

Heterogeneouswater�gas reaction : CþH2O ¼ CO þ H2 (5)

Boudouard reaction : C þ CO2 ¼ 2CO (6)

Hydrogenating gasification C þ 2H2 ¼ CH4 (7)

The equilibrium constants are as follows

K1 ¼ PCO PH2

PH2O¼ xCO xH2

xH2O(8)

where,

xj ¼njP

j¼product gasesnj

K2 ¼ ðPCOÞ2PCO2

¼ ðxCOÞ2xCO2

(9)

K3 ¼ PCH4�PH2

�2 ¼ xCH4�xH2

�2 (10)

Beyond CBP, only gaseous products are considered and hencethe number of unknowns reduced to five. In that case the reaction(5) and reaction (6) can be combined to give the homogeneouswateregas shift reaction as given below.

Water� gas shift reaction COþ H2O ¼ CO2 þH2 (11)

K4 ¼ PCO2PH2

PCO PH2O¼ xCO2

xH2

xCO xH2O(12)

So, equations (10) and (12) are taken in addition to equations(2), (3) and (4) to solve five unknowns.

The Equilibrium constant and the Gibbs free enthalpy of reac-tion is calculated according to equations (13) and (14).

lnK ¼ �DGoT

R=T(13)

DGoT ¼

Xi

giDgof ;T ;i (14)

Where R is the universal gas constant, DGoT is the standard Gibbs

function of reaction and Dgof ;T ;i represents the standard Gibbs func-tion of formation at given temperature T and g is the stoichiometricnumber of the gas species i. The data for hof and Gibbs function offormation are given in Table 2 [34] for the temperature range form800 K to 1100 K. An Arrhenius type function is fitted to calculate theequilibrium constant in between the given temperature steps.

K ¼ ko exp�� EactR=T

�(15)

Where ko is the pre exponential function and Eact is the activationenergy.

4. Energy balance

Applying the law of conservation of energy, the energy balancefor the gasification system give rise toXi¼ reactant

Hi þ QIN ¼X

j¼product

Hj (16)

where,

Xi¼ reactant

Hi ¼hof ;biomass þm�hof ;H2O þ DhT ;H2O

�(17)

and

Xj¼product

Hj ¼X

j¼product

nj�hof ;j þ DhT ;j

�(18)

hof in the above equation represents the enthalpy of formationand ΔhT represents the enthalpy difference between any given stateand at reference state (298 K, 1 atm). It can be represented by:

DhT ;j ¼ZTTo

CpðTÞdT (19)

where Cp is specific heat at constant pressure and it is temperaturedependent. The temperature dependency of Cp can be expressed bya polynomial function as given below.

Cp ¼ Aþ BT þ CT2 þ DT3 (20)

The values of A, B, C and D and their range of applicability aregiven in Table 3 [35].

The heat of formation of solid biomass containing only carbon,hydrogen and oxygen can be calculated from the heat of combus-tion of the compound (complete combustion to H2O and CO2assumed) and heat of combustion of carbon and of hydrogen. Thusthe heat of reaction

C þ x2H2 þ

y2O2/CHxOyDHR1 ¼ �hof ;biomassDHR1 ¼ �hof ;biomass

Table 4Chemical exergies for different components.

Component Chemical exergy (kJ/kmol)

H2 236100CO 275100CO2 19870H2O 9500CH4 831650C 410260

C. Loha et al. / Energy 36 (2011) 4063e4071 4067

is obtained by an algebraic summation of the heats for thereactions

C þ O2/CO2DHR2 ¼ �hof ;CO2

x2H2 þ

x4O2/

x2H2ODHR3 ¼ �x

2hof ;H2O

CHxOy þ 12��2þ x

2� y�O2/CO2 þ

x2H2ODHR4 ¼ Hc;biomass

By combining the last three equations to give the first equation,one can write

DHR1 ¼ DHR2 þ DHR3 � DHR4

or,

hof ;biomass ¼ HC;biomass þ�hof ;CO2

þ x2hof ;H2O

�(21)

5. Exergy balance

Applying the second law of thermodynamics, qualitativeexpression of law of conservation of energy to the gasificationprocess gives the exergy balance of the process asXIN

E þWIN ¼XOUT

Ek þ I (22)

where,XIN

Ei ¼ Ebiomass þ Esteam (23)

XOUT

Ej ¼ Edry product gas þ Esteam þ Eunconverted carbon (24)

In the present system, the heat supplied to the system is directlyfrom the electricity so the exergy input WIN (equation (22)) isbasically thework input as electricity which is converted to heat QIN(equation (16)). The difference between energy and exergy balanceis that the exergy is never conserved in any irreversible process butalways decreases. Here the irreversibility term in equation (22)isrepresented by I.

The exergy can be divided into two components as the chemicalexergy and the physical exergy as stated below:

E ¼ Echem þ Ephy (25)

The standard chemical exergy of a pure compound is equal tothe maximum amount of energy obtained when the compound isbrought from environment state, characterized by the environ-mental temperature and pressure, to the dead state, characterizedby the same environment condition of temperature and pressure.The values of standard chemical exergy of pure compounds aregiven in Table 4 [17]. The chemical exergy of ideal gas mixture canbe written as;

Echem;M ¼Xj

xj3chem;j þ R=T0Xj

xjln�xj�

(26)

where xj is the mole fraction and 3, j is the standard chemical exergyof Jth product respectively. It is observed from above equation thatthe chemical exergy of mixture is always lower than the averagechemical exergy of the mixture, because the second term of theabove equation is always negative.

The physical exergy of a material stream can be calculated as[36e38]:

ZT ZTCp

Ephy ¼

To

CpdT � ToTo

TdT ¼ ðh� h0Þ � T0ðs� s0Þ (27)

Therefore, the physical exergy of ideal gas mixture can beevaluated using mixture rule:

Ephy;M ¼ZTTo

0@X

j

xjCp;j

1AdT � To

ZTT0

PjxjCp;j

!

TdT (28)

In the present work, the exergy of solid biomass was calculatedfrom its lower heating value using a multiplication factorb [17,39,40] as follows:

Ebiomass ¼ bLHVbiomass (29)

where the coefficient b is given in terms of oxygen-carbon andhydrogen-carbon ratios according to the following equation:

b ¼ 1:0414 þ 0:0177½H=C� � 0:3328½O=C�f1 þ 0:0537½H=C�g1� 0:4021½O=C�

(30)

and the lower heating value of the biomass is given by:

LHVbiomass ¼ 0:0041868f1 þ 0:15½O�gf7837:667½Cþ 33888:889½H� � ½O�=8�g (31)

6. First law and second law efficiency

For the system under study, the first law efficiency can becalculated in two different ways. The first approach is to define the“hot gas efficiency”, is the ratio of energy of product including thesensible energy, to the HHV of the biomass plus the energy of theinput steam plus the external energy used.

hI; hot gas ¼ HHVproduct þ Hsensible;product

HHVbiomass þ Hsteam;input þ QIN(32)

Under the assumption of adiabatic wall condition, the hot gasefficiency value would be 100% with no loss of energy beingconsidered. Therefore it is more meaningful to use so called “coldgas efficiency”, which excludes the sensible energy of the product,is defined as the ratio of the HHV of the product and the HHV of thebiomass plus energy of the input steam plus the external energyused.

hI; cold gas ¼ HHVproduct

HHVbiomass þ Hsteam; input þ QIN(33)

But one of the major thermodynamical concerns is that the firstlaw efficiency, whether hot gas efficiency or cold gas efficiency,does not consider the loss due to increase in entropy during theconversion of solid fuel into gaseous products. The second law

Table 6Comparison of gas composition with published work [11].

Present Model Rapagna et al. (2000)

Case-I Case-II

(Dolomite as fluidizedbed inventory)

(Olivine as fluidizedbed inventory)

Temperature (oC) 770 770 770S/B 1.0 1.0 1.0Gas composition (Vol%)H2 57.98 55.50 52.20CO 23.79 24.00 23.00CO2 16.70 14.10 16.90CH4 1.53 6.40 7.90

C. Loha et al. / Energy 36 (2011) 4063e40714068

efficiency or exergy efficiency takes care of that loss and providesa more equitable measure of conversion efficiency [41]. However,there are various processes of defining the exergy efficiency. Themost easiest and common definition of exergy efficiency is the sumof the exergy of product gases divided by the total exergy input tothe system.

hII; a ¼ EgasEbiomass þ Esteam; input þWIN

(34)

The unconverted carbon could also be considered as the usefulproduct because this could be re-introduced to the gasifier to utilizeits energy. In that case the exergy efficiency is defined as the ratio ofthe exergy of the product including unconverted carbon and thetotal exergy input to the system.

hII; b ¼ Egas þ EcharEbiomass þ Esteam; input þWIN

(35)

The exergy efficiency can also be defined in an alternativemanner as the sum of the chemical exergy of the product gasesdivided by the total exergy input to the system.

hII; c ¼ Echem;gas

Ebiomass þ Esteam; input þWIN(36)

In all the above definition of the efficiency terms only thenumerator changes whereas the denominator remains same.

7. Validation

Equilibrium model is based on some assumptions like thegasification reaction rate are fast enough and residence time is longenough for the equilibrium state to be reached. Due to theseassumptions, equilibrium model sometimes yield greaterdisagreement under certain circumstances. Therefore, a propercalibration of the modeling results are needed with the experi-mental values before applying the model for further analysis. Herethe computed gas compositions from the model are compared withthe experimental results of the present group of author reportedpreviously [14] as shown in Table 5. The error in this comparison isestimated by the RMS (root-mean-square) values for each set ofdata as given below

RMS ¼

ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiPNj

�Experimentj �Modelj

�2N

vuuut(37)

where N is the number of data point.The comparison has been done for five different sets of

Temperature and S/B values. The RMS errors are calculated for eachset of data point. The maximumvalue of RMS observed is 3.197006.

The gas composition is also compared with the experimentalinvestigation on steam gasification of biomass reported by Rapagnaet al. [11] as shown in Table 6. Here the gas compositions are in

Table 5Comparison of result with experimental investigation [14].

Tempe S/B Experimental

H2 CO CO2 CH4

690 1.32 50.50 14.30 26.60 8.60730 1.32 52.20 16.40 23.50 7.90750 1.00 49.50 23.70 21.20 5.60750 0.60 48.8 27.5 19.5 4.2750 1.32 52.30 17.75 22.25 7.40

good agreement except for CH4. The reason can be attributed to thefact that in our experiment only sand bed was used while Rapagnaet al. used sand bed with dolomite and olivine which are active fortar destruction and increases the production of gas, but suppressesmethane reforming. Though some discrepancies arise due to theequilibrium assumptions, still the comparison shows almost a fairagreement of the predicted data with the experimental values.Therefore, the calibrated accuracy of the model can be consideredto be acceptable for the analysis.

8. Results and discussion

8.1. Effect of steam-to-biomass ratio

The effect of j on the gasification of rice husk at temperature700 �C is explored through Figs. 2 and 3 and 4. The value of j can bevaried either by changing the steam flow rate and keeping thebiomass feed rate fixed or vice versa. The variation of hydrogenproduction and the amount of char carbon present in the product isshown in Fig. 2. It is observed that the amount of char carbonpresent in the product decreases linearly with increase of j andreaches to zero at j equal to 0.5, which is the carbon boundarypoint (CBP) corresponding to 700 �C. On the other hand thehydrogen production increases with increase in j due the steamreforming and gas shift reactions. The variation is linear and rela-tively steepwithin the CBP. Beyond the CBP, the rate of change in H2yield reduces with increase of j while the curve assumes a para-bolic shape with an asymptotic value of about 0.098. It was alsoreported by Prins MJ and Ptasinski KJ [42] that the CBP was reachedat an equivalence ratio (ER) of 0.5 corresponding to the gasificationtemperature of 1220� C.

The variations of hI, cold gas, hII, a, hII, b and hII, c are shown in Fig. 3.It is observed that all efficiencies reach a maximum at CBP excepthII, b It is to be noted that hII, b considers the exergy of char carbonpresent in the product which initially reduces with j and can bedistinctly observed till the CBP. However, it merges with hII, a afterCBP and both hII, a and hII, b becomes equal. The increase of hI, cold gas,hII, a and hII, c below the CBP is very steepwhereas the decrease afterCBP is relatively flat. It is also observed that the difference between

Model RMS

H2 CO CO2 CH4

48.76 12.99 31.21 7.03 2.66742950.71 15.48 28.74 5.06 3.1060350.37 20.59 25.01 4.02 2.61923250.79 29.37 18.3 1.54 1.99778351.43 16.64 27.65 4.28 3.197006

0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.60.03

0.04

0.05

0.06

0.07

0.08

0.09

0.10

CBP

H2Char

H2 (kg

/kg

o

f d

af b

io

mass)

0.0

0.1

0.2

0.3

0.4

Ch

ar (m

ole/m

ole o

f d

af b

io

mass)

Fig. 2. Variation of hydrogen production and unconverted char withJ at a gasificationtemperature of 700 �C.

0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.613.0

13.2

13.4

13.6

13.8

14.0

CBP

HHV

HH

V (M

J/N

m3)

4

5

6

7

8

9

QIN

(M

J/kg

of b

iom

ass)

)

ΩIN

Fig. 4. Variation of HHV of the product gas and QIN with J at a gasification temper-ature of 700 �C.

C. Loha et al. / Energy 36 (2011) 4063e4071 4069

energy efficiency (cold gas efficiency) and the exergy efficienciesare a maximum at CBP. The values of hI, cold gas, hII, a and hII, c at theCBP are 0.94, 0.84 and 0.81 respectively. The highest value of 0.88 isobserved for hII, b at the lowest j value of 0.1. It is to be noted thatthe difference between first law and second law efficiency i.e.energy and exergy efficiency is maximum at the CBP indicatinghighest level of irreversibility.

The variation of HHV of the product gas and the external energyinput to the system are also presented in Fig. 4. Initially, the HHV ofthe product increases very fast with increase of j and after CBP isreached, it increases appreciably only at very high value of j. Theexternal energy input also increases with increase in j and the rateof increase is high just after CBP and then it becomes flat after j

equal to 1.1. The variation of energy input with j is somewhat flattill the CBP and the curve is steeper just after the CBP till about an j

equal to 1.0 and then the energy input is almost at a constant level.From the preceding analysis, it is justified to say that although

the hydrogen production increases beyond CBP, CBP is the optimumpoint of gasification in terms of energy and exergy consideration.Similar proclamation was also made by Fryda et al. [43] based onthe exergetic analysis of biomass allothermal gasification, that the

0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.60

10

20

30

40

50

60

70

80

90

100

CBP

I,cold gasηηη

Efficien

cy (%

)

η

II,a

II,b

II,c

Fig. 3. Variation of efficiencies with J at a gasification temperature of 700 �C.

gasification should be carried out with the minimum steamrequirement to accomplish carbon conversion.

8.2. Effect of steam temperature

In the present investigation the gasification temperature isvaried form 600 �C to 900 �C. The effect of gasification temperatureon the hydrogen production is studied for j in the range of0.6e1.0as shown in Fig. 5. The figure shows that the H2 productionincreased significantly with the increase of temperature from600 �C to 800 �C and above 800 �C it becomes nearly asymptotic.ThemaximumH2 yield achieved are 0.09, 0.085 and 0.078 kg per kgof dry ash free biomass forJ ¼ 1.0, 0.9 and 0.8 respectively. Similartrend is also observed by Shen et al. [29] in their study. The varia-tion of HHV and the external energy input are plotted againsttemperature in Fig. 6. It depicts that at lower temperature the HHVof the gas is more and it decreases with increase in temperature. Itis because, as temperature increases, the H2 production increasesbut CH4 production decreases. The decrease of HHV with temper-ature is also reported by Franco et al. [12] in their experimental

600 650 700 750 800 850 9000.04

0.05

0.06

0.07

0.08

0.09

0.10

H2 (kg

/kg

o

f d

af b

io

mass)

T (0C)

Ψ = 1.0Ψ = 0.8Ψ = 0.6

Fig. 5. Variation of hydrogen production with gasification temperature at J ¼ 1.0, 0.8and 0.6.

600 650 700 750 800 850 90012.0

12.5

13.0

13.5

14.0

14.5

15.0

15.5

16.0

HHVΨ

IN

T (0C)

HH

V (M

J/N

m3)

5

6

7

8

9

QIN

(MJ/

kg o

f daf

bio

mas

s))

Fig. 6. Variation of HHV of the product gas and QIN to the system with gasificationtemperature.

600 650 700 750 800 850 900

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

T (0C)

Ψ

Fig. 7. Plot of J at CBP with respect to the corresponding gasification temperature.

C. Loha et al. / Energy 36 (2011) 4063e40714070

investigation. It is also observed from Fig. 6 that as the temperatureincreases the external energy input increases continuously. Thusfor an economically variable operation, a trade off between HHVand energy input is justified as the yield of H2 is higher at highertemperature

In Fig. 7, the steam-to-biomass ratio at CBP is also plottedagainst the corresponding gasification temperature. It is illustratedthat as the gasification temperature increases, the CBP is reached atlower value of j. For example, while the value of steam to biomassratio is 0.9 at 600 �C, it reduces to around 0.22when the gasificationtemperature is 900 �C.

9. Concluding remarks

The thermodynamic analysis can be considered as an effectivemethod for design and analysis of hydrogen rich synthetic gasproduction form fluidized bed steam gasification of biomass. In thepresent work, an equilibrium model has been developed to predictthe gas composition. The variation of hydrogen production, HHV ofthe product gas, energy and exergy efficiencies and external energy

input to the gasifier are studied by varying the steam-to-biomassratio and gasification temperature. The H2 production increasedcontinuouslywith increase inj and it increasedat thebeginningandthen becomes nearly asymptoticwith temperature. It is also evidentform the result that all the efficiencies (hI, cold gas, hII, a and hII, c)reached their maximum value at CBP except hII, b, which considerstheexergyof char carbon. TheHHVof theproduct gas increasedwithj and decreased with temperature. On the other hand, the externalenergy input increased with increased in both j as well as temper-ature. Therefore, as the energy and exergy efficiencies aremaximumat CBP and H2 production increased beyond CBP, a designer has tochose the range of operating parameters depending on theirrequirement so that the external energy input is reasonably less aswell as theHHVof theproduct gas is in theacceptable level. Itmaybementioned here that presence of tar in the product gas is a bigproblem in fluidized bed gasification of biomass. Therefore, to crackthe tar gasification is generally done at higher temperature. Thus,deploying catalysts for tar cracking for low temperature gasificationis justifiable from thermodynamic point and needs further study.

Acknowledgment

The authors expressed since gratitude to Prof (Dr.) GautamBiswas, Director, Central Mechanical Engineering Research Insti-tute, Durgapur for his continuous support and encouragement. HCacknowledges support of Department of Science and Technologyunder the PURSE scheme.

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