Modeling of Wheat Straw Torrefaction as a Preliminary Tool for Process Design2

8/9/2019 Modeling of Wheat Straw Torrefaction as a Preliminary Tool for Process Design2

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ORIGINAL PAPER

Modeling of Wheat Straw Torrefaction as a Preliminary Toolfor Process Design

N. Nikolopoulos R. Isemin K. Atsonios D. Kourkoumpas

S. Kuzmin A. Mikhalev A. Nikolopoulos M. Agraniotis

P. Grammelis Em. Kakaras

Received: 24 July 2012 / Accepted: 15 January 2013 Springer Science+Business Media Dordrecht 2013

Abstract Torrefaction is considered as a kind of mildpyrolysis that is carried out under inert atmosphere (usuallynitrogen) conditions. During this process, the moisture of the initial fuel and a portion of its volatiles are removed fromthe biomass particles towards the inert atmosphere. Theresulted torreed solid biomass has high energy density,durability and less hydrophilic character. The most bene-cial result of torrefaction process is that biomass feedstock logistics cost can be reduced, as less tones of biomass arerequired for a given amount of energy input. The develop-ment of a process model examining basic parameters asreaction temperature and residence time can provide usefulinformation, which can be used for the more efcient designof a torrefaction reactor. This study presents such a processmodel for a straw torrefaction pilot plant. This model isbased on the thermodynamic calculation of a single and/or atwo batch reactor, built on the commercial software ASPEN

Plus. The calculation of required ow rates of inert gas,cooling medium for a specic biomass feedstock value, isbased on relevant results found in literature.

Keywords Wheat straw Torrefaction Thermodynamicmodeling ASPEN Plus

List of symbolsCp Heat capacity coefcient (J/kg K)E Activation energy (J/mol K)M Mass yield (kg)K Kinetic rate (1/s)t Time (s)theat Heating time required for achievement of torrefac-

tion temperature (s)t* Critical time at which the second stage of torrefaction

begins and dominates over the rst one (s)

N. Nikolopoulos K. Atsonios D. Kourkoumpas A. Nikolopoulos P. Grammelis Em. KakarasCentre for Research and Technology Hellas, Chemical Process& Energy Resources Institute (CERTH/CPERI), Athens, Greecee-mail: [email protected]

D. Kourkoumpase-mail: [email protected]

A. Nikolopoulose-mail: [email protected]

P. Grammelise-mail: [email protected]

Em. Kakarase-mail: [email protected]

N. Nikolopoulos ( & )ARKAT Building, 357-359 Mesogeion Ave., Halandri, Athens,Greecee-mail: [email protected]

R. Isemin S. Kuzmin A. MikhalevTambov State Technical University, Tambov, Russiae-mail: [email protected]

S. Kuzmine-mail: [email protected]

A. Mikhalev

e-mail: [email protected]

K. Atsonios A. Nikolopoulos Em. KakarasLaboratory of Steam Boilers and Thermal Plants, NationalTechnical University of Athens, Athens, Greece

M. AgraniotisClean Energy Ltd., 4th km Ptolemais-Mpodosakeiou, Ptolemais,Greecee-mail: [email protected]

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Waste Biomass ValorDOI 10.1007/s12649-013-9198-y


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T Temperature (K or C)W Solid residual mass at each time step (kg)Greek lettersk Thermal conductivity (W/m K)q Density (kg/m 3)

Abbreviations

CV Caloric value (MJ/kg)ER Energy recovery (-)HHV High heating value (MJ/kg)LHV Low heating value (MJ/kg)

Subscripts0 Initial properties of biomass at t = 0 sA Biomass properties during the rst stage of torre-

factionB The intermediate reaction solid product propertiesC The nal torreed biomass propertiesV1 Properties of volatiles as produced during the rst

phaseV2 Properties of volatiles as produced during the second

phaseraw Raw materials propertiestor Torrefaction material properties1 Stage 12 Stage 2

Introduction

Nowadays, the efforts for the achievement of high caloricvalue alternative biogenic fuels, are intensied. Torrefac-tion is a thermal pre-treatment technique applicable forbiomass conversion into energy and heat by combustion,gasication or co-ring with conventional solid fuels. Thetorrefaction treatment not only decomposes the brousstructure and tenacity of biomass, but it as well increasesits caloric value. Additionally, after the torrefaction thebiomass obtains less hydrophilic characteristics, thusallowing the higher time storage of torreed biomass,because of its rotting behavior. Specically, due to thebreakage of OH-groups in torrefaction process, the materiallosses its tendency to absorb water so it remains morestable and hydrophobic [ 1] compared to the raw fuel.Specically for straw, because of its abundance and lowmoisture, in comparison with other types of biomass(wood), this is considered as a very promising fuel in whichtorrefaction can be applied.

In general, the major advantage of using torreedpelletized biomass instead of raw biomass is the consider-able reduction of the CO 2 footprint associated with itstransport and its co-combustion with conventional solidfuels. Moreover, torreed biomass has properties whichresemble more those of conventional solid fuels H/C and

O/C ratios) compared to raw one. These facts have a con-siderable added value for the energy utilization of torreedbiogenic fuels in comparison with their raw form, as thebiomass fuel transportation cost is reducing for the sameenergy gain, its heating value is increasing, while the cost of modications that should be made in small and/or largescale power plants when co-ring scenarios are applied, isdecreasing. From an operational point of view, the moisturecontent of torreed pellets much less than the correspondingof raw one and therefore the amount of heat required to beprovided by other energy sources for their nal energyutilization (i.e. pyrolysis, gasication, combustion) is muchless than that the one required for the raw one.

On the other hand, the installation and operational costof the torrefaction plant increase the overall investmentcost of a power producer. However, for one to make a moreaccurate estimation of the advantages and disadvantagesof this process, a LCA (Life Cycle Assessment) and a CBA(Cost Benet Analysis) analysis should be in depth made.

Torrefaction is a mild pyrolysis process carried out attemperatures 200300 C, in which three basic componentsare produced including, (a) a solid product of a brown/dark color, (b) a condensable liquid including mostly water,acetic acid, and other oxygenates and (c) non condensablegasesmainly CO 2, CO, and small amounts of CH 4 and H 2.Their yield depends upon the (a) reaction temperature,(b) the residence time and (c) the raw material biomassproperties.

The solid phase consists of a chaotic structure of sugarsand other reaction substances. The content of carbon in thesolid product increases, the higher the torrefaction temper-ature and the longer the residence time are. The increase of the torreed biomass caloric value is owed to the decreaseof hydrogen and oxygen [ 2, 3]. The gas phase includes maingases, such as CO, CO 2 and CH 4 , but as well light aromaticcomponents such as benzene andtoluene, butwith very smallconcentration (traces) [ 4]. According to Ferro et al. [ 2] andWang et al. [ 5] the content of CH 4 , H 2 , C xHy and CO in theproduct gases increases as the torrefaction temperatureincreases, while the content of CO 2 decreases. The con-densable part or liquids comprises mainly of water, organicsand lipids. Water content can reach up to a high percent, asthis is released from the biomass during its evaporationphase. The organics subgroup (in liquid form) comprises of compounds mainly produced during devolatilization andcarbonization. Finally, the lipids subgroup contains com-pounds such as waxes and fatty acids that are present in theoriginal biomass. According to Wang et al. [ 5], the yield of liquid products increases as the temperature increasesbecause of the decomposition of hemicellulose.

Torrefaction has a great inuence on the physical/ chemical properties of the solid product, mainly caused bythe removal of oxygen from the original solid biomass

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resource. In this way, the net caloric value is increasingand the product becomes more energy dense. According toShang et al. [ 6], the percent of energy loss increases fasterthan the weight loss, with the torrefaction conditions get-ting more severe. This happens because of the degradationof lignin and cellulose in the temperature range of 270300 C. So in order to achieve the most desired energycontent in the torreed material, special attention should bepaid on the parameters of temperature and residence time,during this complicated process.

In this frame, an important parameter inuencing sig-nicantly the characteristics of the solid yield, is the rawmaterial composition (in this case study: straw pellets), interms of hemicellulose, cellulose and lignin content. Strawpellets consist of around 7.7 % lignin 41.3 % cellulose and30.8 % hemicellulose [ 3]. The composition is given in dryash free basis. Hence, the total dry matter digestibility(DMD) of straw pellets is equal to 79.8 %. The rest of thecomposition includes other materials such as resin. Hemi-cellulose is the most reactive among the three lignocellu-loses components found in biomass and during torrefactionit undergoes the most signicant decomposition reactions.According to Peng et al. [ 7], hemicelluloses isolated fromdignied wheat straw are consisting of arabinoxylan anduronic acids, which have the typical structure of straw he-micelluloses. Such typical monomers are glucose, xylose,galactose, arabinose, glucuronic and galacturonic acids. TheTG and DTG curves from the Peng et al. [ 7] suggest thatdecompositionof hemicellulose mainly happens in the rangefrom 190 to 350 C. The corresponding temperature of cellulose decomposition is in-between 305 C and 375 C,whilst lignin gradually decomposes over a temperaturerange of 250500 C. It is mentioned that studies conductedusing xylan (the prominent hemicellulose found in herba-ceous biomass) have concluded that decomposition of hemicellulose initiates at temperatures above 200 C, whilefull devolatilization occurs by 400 C.

Table 1 presents a short description of all the experi-mental investigations for straw torrefaction, found in theliterature. Some of them are carried out in various types of reactors andother using theTGAequipment. Theparametersinuencing the torrefaction process include the straw andnitrogen mass ow rates, the heating rate, the operatingtorrefaction temperature and nally the residence time. Thestudies summarized in this Table areconductedby Bridgmanet al. [3], Lanzetta and Blasi [ 8], Prins et al. [ 9], Ferro et al.[2], Wang et al. [ 5] and Sadaka et al. [ 10]. The experimentalresults from the aforementioned studies conclude that thehigher the torrefaction temperature is, the lower the qualityyield of charcoal is, while the energy density increases andthe energy yield decreases. The apparent volume of the solidproducts decreases signicantly and their grind abilityimproves greatly with an increase in temperature.

For quite high temperatures (above 247 C), two con-secutive reactions describe the temporal evolution of weight loss. In the rst phase, raw biomass particles break into volatile gases and an intermediate solid compound,which in turn break again into volatiles and a solid residual(second phase), often named as char residual.

The scope of this paper is the denition of a set of design parameters for a pilot scale straw pellets torrefactionreactor, of a capacity equal to around capacity 100 kg/h.For this scope, a thermodynamic tool describing the tor-refaction process is built in the ASPEN Plus commercialpackage. Very few simulation tools for this process can befound in the recent literature, [ 11 , 12]. A parametric studyon the effect of residence time and temperature on the nalproperties of the torreed biomass for a given raw biomassow rate is conducted. Among all the derived results, theconditions reecting the most efcient operation of thepilot plant are selected for its further design, drawing andmanufacturing can be derived.

Set up of the Thermodynamic Model

Products Yield

The prediction of weight loss of the biomass feedstock isbased on the two-stage mechanism, rst introduced by DiBlasi and Lanzetta [ 8], [13]. According to it, for an oper-ating torrefaction temperature above 250 C, torrefaction isconducted in two consecutive stages, as depicted in Fig. 1.

In this Figure, subscript A denotes the raw biomass, Bthe intermediate reaction solid product, C the nal torreedbiomass and V 1, V2 the volatiles, as produced during therst and second phases respectively. In fact, the rst stagerepresents the degradation of hemicellulose, whereas thesecond one the degradation of lignin. The differentialequations that express the mass loss or production for aconstant temperature along with time for stage 1 are:

dM Adt

K 1 M A ) M A M 0e K 1 t 1a

dM V 1dt

K V 1 M A ) M V 1 K V 1

K 1 M 0 1 e K 1 t

1b

dM A! B Bdt

K B M A ) M A! B B K BK 1

M 0 1 e K 1 t 1cwhere M 0 is the initial mass of biomass and M A! B B is themass of B as produced from the mass of A (M A ), at eachtime step. The corresponding kinetic rates for each stage (1and 2) are expressed as:

K 1 K B K V 1 2a

K 2 K C K V 2 2b

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For each rate, an Arrhenius type temperature depend-ency is considered:

K i k 0;i eE i

RT ; i A; B; C ; V 1; V 2 3

For a torrefaction operating temperature below 250 Conly the rst stage takes place. For temperature higher than250 C, there is a specic time, denoted as critical time t*,at which the rst stage ends and the second stage begins.This value is important in order to dene the evaluation of the process for a certain reaction time, as the kinetic ratesrepresenting the torrefaction process change signicantly.t* can be calculated analytically taking into account that atthis time instant the yield of the intermediate solid Bmaximizes, according to the following formula:

dM Bdt

K B M A K 2 M B 0 ) K B M 0 1 e K 1 t K 2

K B M 0K 1 K 2

e K 2 t e K 1 t ) t ln K 1K 2

K 1 K 24a

At this time instant t*, the product yield of solid B isdenoted as M B* . Moreover it stands that:

M B M 0

M V 21

M 0

M C 1 M 0

1 M V 11

M 04b

where M V 11 M 0 ; M V 21

M 0are the maximum dimensionless (with the

initial biomass mass), amount of released volatiles duringstages 1 and 2 correspondingly. M C is the solid residualmass after the end of the devolatilization phase. For strawpellets, based on the literature data from [ 8], the followingexpressions were built so that the mass ratios of products Cand volatiles V 2 respect to the initial mass M o

M C 1 M 0

and M V 21 M 0

are calculated as a function of temperature: M C 1 M 0

0:60 0:0041 T C 200 K C K BK 1K 2

5a

M V 21 M 0

0:17 0:001 T C 260 K V 2K BK 1 K 2

5b

and M V 11

M 0

K V 1K 1

5c

According to [ 1] and [2] the solid residual mass W ateach time step is given by:

Table 1 Summary of experimental studies about straw torrefaction

Studies Experimentalequipment

Input (straw) Nitrogenow(mL/min)

Heating rate( C/min)

Torrefactiontemperatures( C)

Residencetime (min)

Ferro et al. [ 2] Cylindrical reactor 65 g 83.33 N.A 230

250

280

60

120

180Bridgeman et al. [ 4] TGA analyzer 2535 mg 50 10 250

270

290

30

Wang et al. [ 3] Fixed bed reactor 22 g 500 30 200

250

300

30

Lanzetta and Di Blasi [ 7] TGA analyzer 10 mg N.A 2570 200300* 2050

Prins et al. [ 8] Fixed bed reactor 10 g N.A 1020 250 30

Sadaka et al. [ 9] Mufe furnace 50 g N.A N.A 200

260

315

15

30

45

60120

180

* examined range: 127375 C

K C

K V2K V1

A

V 1

K BB C

V 2

raw strawintermediate

solidtorrefied

straw

stage 2stage 1

Fig. 1 Two-stage mechanism of torrefaction

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W M 0

M A M B M c

M 0

M B M 0

M B M 0

M C 1 M 0 1 e K 2 t t ;

t [ t heat

6

In these equations the time t is measured from the time that

torrefaction begins.Mass M A has been determined from the former stage,and M V2 is calculated by:

M V 2 M 0

M B M A W

M 0)

M V 2 M B

1 M A W

M B7a

The reduction of solid M B due to the reactions at stage 2 is:

dM B! C Bdt

K 2 M B! C B ) M B! C B M B 1 e

K 2 t t )

M B! C B M B

1 e K 2 t t ;

t [ t heat 7b

where M B! C B is the mass of M B during the second reaction.Hence, the nal solid char mass M C production is easilyobtained by:

dM C dt

K C M B! C B ) M C M 0

K C 1 1 e K 2 t t ) M C M B

K C K BK 2 K 1 K V 1

1 e K 2 t t ;t [ t heat

7cSolid and Gas Products Composition

The proximate (VM (volatiles matter), FC (xed carbon))and ultimate (C, H, O) composition of the torreed biomasscan be determined by corresponding arithmetic expressionsthat are yielded based on data from experimental cam-paigns of wheat straw torrefaction found in the literature.As far as sulfur content prediction is concerned, there is nodata in literature related to the fate of S after the torre-faction process. This is attributed to both the difculty foraccurate S measurement and the low industrial interestabout it. Hence, it is made the rough assumption thatthe whole sulfur remains at the solid biomass. Regardingthe nitrogen fate included within the biomass particles,according to the recent study of Medic et al. [ 14] this isassumed to mainly remain within the solid biomass. Nev-ertheless, the nitrogen content in the torreed fuel is sosmall, that has little effect on the mass and energy balanceof the process. Moreover, the initial ash content in terms of absolute mass (kg) still remains in the solid char residue,after torrefaction. This is a reasonable consideration since

any mechanism transferring ash from solid to gas phase isnot taking place during torrefaction.

For the solid product composition (C, H, O analysis), thearithmetic expressions built-up and used, are based on theexperimental data, derived by the work performed by Ferro[2]. In this study, several experimental series are conductedin a lab scale unit using straw pellets as a fuel. The tem-perature and residence time for which these equations areregarded as valid are 230280 C and 03 h, respectively.

C solid C raw

kg=kg 0:0014 T C 0:010 t s3600

1:22 ;

t [ t heat

8

H solid H raw

kg=kg 0:0040 T C 0:020 t s3600

1:87 ;

t [ t heat

9

Osolid O raw

kg=kg 0:0050 T C 0:015 t s3600

2:02 ;

t [ t heat

10

Onthe otherhand, xed carbon (FC) and volatiles mater (VM)are dependent on the torrefaction temperature according tothe following equations. These equations have been derivedfrom relevant experimental measurements conducted byBridgeman et al. [ 3]. In this study, the proximate analysisof torreed wheat straw was performed for a temperaturerange of 200300 C.

FC solid FC raw

kg=kg 0:0003 T 2C 0:1762 T C

24:149

11

VM solid VM raw

kg=kg 0:0122 T C 3:88 12

As aforementioned, the volatiles species are divided intotwo categories, (a) liqueed at low temperatures namedas condensable and (b) non-condensable. According tothe literature, the main components of the condensablevolatiles part (liquid), derived by wheat straw torrefactionare acetic acid (CH 3COOH) and water (H 2O) [3, 5, 9].Apart of them, formic acid (HCOOH), methanol (CH 3OH),lactic acid (C 3H6O3), furfural (C 5H4O2) and hydroxyacetone are also detected [ 3]. For the reasons of modelsimplicity, acetic acid and water are assumed as the onlycondensable components in this study. The produced wateroriginates from the moisture of the initial fuel. Theassumption to consider only two condensable species inthe products is not expected to inuence the accuracy of themass and energy balance because of the small percentage

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of liquid yields produced during this mild pyrolysis process[5].

On the other hand, the vast majority of non-condensablecomponents (gas) are carbon dioxide (CO 2) and carbonmonoxide (CO) [ 3, 5, 9]. The quantity of produced meth-ane (CH 4) can be neglected, without a signicant error,since its contribution to the caloric value of the gaseousproduct is very small [ 2, 9]. Ferro et al. [ 2] showed that theCO 2 /CO ratio is not highly affected by the residence time;the opposite stands for temperature. The arithmeticexpression, which the experimental data [ 2] can t ontoand is incorporated in the thermodynamic model is givenby Eq. 13 :

CO 2CO

0:0004 T 2 0:22 T 32:5 kmol

kmol 13The fraction of gas to liquid components is determined

based on the relevant studies of Bridgeman et al. [ 3], Wang

et al. [5] and Prins et al. [ 9]. Fuel origin plays an importantrole on this ratio, as the devolatilization is stronglyaffected by the decomposition mechanism and rate of thehemicellulose structure. For instance,Prins et al. [ 9] detectedthat the composition and quantity of non-condensablevolatile yields, for willow and straw are similar, in contrastto larch. This is attributed to the presenceof xylan.Ferro etal.[2] measured a small amount gas content (volatiles), theresults of whom are in disagreement with the results of Prinset al. [ 9]. Based on the literature data [ 2], we made the roughassumption that the gas to liquid mass ratio is equal to 0.25.

Concerning the heating value, this gets higher as the

torrefaction temperature and the residence time increase.For the calculation of the caloric value of the productsolid yield the following equation, [ 2] is used:

HHV daf 0:34% C 1:40% H 0:16% O MJ =kg 14

Description of the ASPEN Plus Process Model

For the simulation of torrefaction process, very few processtools are available in the literature, [ 11 , 12]. The thermo-dynamic tool, describing the straw pellets torrefaction isbased on the afore-described modeling approach, inte-grated in the commercial accredited commercial softwareASPEN Plus. The ASPEN Plus owsheet is depicted inFig. 2. Torrefaction process is modeled to being performedin two stages as aforementioned, while the equationsdescribed in the previous paragraphs, simulate the gov-erning physical mechanisms taking place during thiscomplicated process. In the thermodynamic setup algo-rithm, reactors type of stoichiometric (RSTOIC) for thetwo stages are applied.

The feedstock biomass enters the rst reactor (RSTOIC)named as STAGE1, where the rst stage reactions taking

place are simulated. Similarly, the reactions taking placeduring the 2nd stage are conducted in the second RSTOICnamed as STAGE2.

Both V 1 and B are dened in the process tool, as non-conventional components, the properties of which (Ulti-mate, Proximate analysis and Caloric Values) are esti-mated by the previously described arithmetic expressionsThe synthesis of composition of outlet streams is done atthe RYIELDS TORSYN for solid biomass and VOL-SYN for volatiles.

Data for the oil thermophysical properties come fromAB&CO [ 15], and the correlation of each property isextracted ( T in C):

C p 4:5426 T 1864 :6 [J/kg K] 15

k 7 10 5 T 0:1356 W=m K 16

q 0:8096 T 902 :24 kg=m3 17

The present model focuses on the calculation of the massand energy balance of the torrefaction system. In this light,a detailed analysis of heat transfer mechanism within themass of each particle is not included, as a more rigorousmodeling of the reactor would be then required. The onlyassumption related to the heat transfer mechanism withinthe particles is related to the temperature difference (25 C)between the temperature of the heat/cooling medium (i.e.hot oil, cooling water, respectively) and the correspondingtemperature of particles at each stage of the reactor. Theparticles are considered to exhibit a uniform temperatureprole, by the adopted approach.

Operation Principles of the Torrefaction Reactor

A schematic view, representing the operation principles of the reactor is shown in Fig. 3. This is a Hearth type reactor.

Operating Conditions

The overall torrefaction process can be sub-divided intotwo main steps, (a) torrefaction and (b) solids cooling. Themedium, which provides the biomass with the requiredheat, is considered to be hot mineral oil, previously heatedup in a furnace. It is assumed that the inlet temperature of the hot oil is 350 C and its corresponding outlet temper-ature 15 C higher than the operating torrefaction tem-perature, in the reactor. The maintenance of the inertatmosphere inside the reactor is accomplished by injectingpure nitrogen, which is preheated at the torrefaction tem-perature before it enters the reactor. The required nitrogenow rate is calculated based on previous studies of torre-faction process at experimental devices [ 2], where for 65 g

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of biomass 5 l/h of nitrogen are used. For pilot plants, suchdata could not be found.

In the second step, the torreed product is further cooleddown below 200 C, until it reaches the desired naltemperature of around 50 C. The cooling medium iswater. The water inlet temperature is set to around 15 C,while its outlet to 28 C. The latter value was chosen to bebelow the value of 50 C, in order to conform with relevantenvironmental legislations for an open waste water dis-charge open circuit.

Pilot Plant Components

The biomass enters the treatment vessel through an upperinlet. Within the vessel, the biomass is subjected totorrefaction and latter is discharged. The biomass is fed

continuously from the upper surface of the torrefactionreactor vessel. In its inlet, a chute is applied. This receivesthe biomass from the inlet and directs it to the trailingsection portion of the upper tray. The chute ensures thatbiomass entering the vessel is retained on the upper tray fornearly a full rotational period of the tray.

The reactors trays (plates) are circular discs having agenerally planar upper support surface. The trays includean opening similar to a pie shaped open section of a cir-cular disc. For each tray there is one circular area, whichallows biomass on the upper surface of the tray, to fallthrough towards the underlying one. The open sections of each tray are not vertically aligned with the correspondingones in the trays immediately above and below it. This isdesigned in such a way, so that the required residence timeof the biomass at each stage is achieved. The trays arestationary and mounted to the sidewall of the vessel, whilean impeller rotates with the shaft and the upper surface of each tray.

The scraper bars may be solid rods, frames with sup-porting ribs or other radially extending rigid or semi-rigidarm. As the scrapper bars rotate with the shaft, the barssweep the biomass over the surface of the stationary trays.As the biomass slides over the trays, the biomass is driedand heated by the hot inert gas circulating inside the vessel.For inert gas injection one or more gas inlets are designed.Finally, there may be one or more scraper bars arranged inthe radial array for each tray.

Results and Discussion

Model Validation

The model is valid up to a temperature equal to 300 C.Above this limit, not a mild pyrolysis process takes place

torrefaction process

cooling

stage 1stage 2

torrefied solid& gas formation

preheating

torrefied biomass

rawbiomass

nitrogen nitrogenpreheating

gases

energy balance

Fig. 2 ASPEN Plus Process owsheet

N

Fig. 3 Schematic representation of the torrefaction pilot plant

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and any rates and products compositions calculations basedon the equations described in this paper cannot be regardedas valid. As it is discussed in the previous section, allcalculations described in this paper are based on experi-mental data found in literature for wheat straw exclusively.Nevertheless, the described modeling approach can bemodied and applied for any solid biogenic fuel based onadequate experimental data for reaction rates and compo-sition yields.

For the validation of the thermodynamic tool, the TGAexperimental data presented by Prins et al. [ 12], are used asa reference. Prins et al. present data for the wheat strawpellets mass reduction rate for two temperatures (248 and267 C), the digitizing of which allows for the extrapola-tion of the kinetic parameters, which describe the masstemporal decrease, during torrefaction. These data aregiven in Table 2.

The numerical results along with the correspondingexperimental data are presented in Fig. 4, depicting a verygood agreement for both temperatures.

It should be noted that the model is not capable of providing valid results for lower than 240 C temperatures,as the experimental data used as an input in the model, donot allow for a torrefaction process to take place for lowerthan this temperature value.

In the following paragraphs, a parametric study onthe implementation of either one single and/or a twobatch pilot plant reactor is presented. The developed andvalidated process tool is capable of giving importantinformation, which can be used for the design and manu-facturing of a torrefaction plant, including the effect of:

HHV (high heating value) of the solid product; liquid, gas and solid composition; ow rates of the heating (oil), inert gas (nitrogen) and

cooling (water) mediums; residence time and torrefaction temperature on the

torrefaction process overall.

The input data used for the calculations are the rawmaterial characteristics and its feeding ow rate, while theoutput is the characteristics both of the produced torreedbiomass and the composition of volatiles V2.

In the following Figures, we dene as heating time (t heat )(0 \ t \ theat ), the time needed for the biomass particles toreach the torrefaction temperature, T tor , and as reactiontime the residence time of the particles beyond it. It isassumed t heat to be equal to around 1,500 s according to thekinetics data, we used for this study [ 16]. This value mayvary from case to case, depending on the heating rate, thesize of the reactor and the thermochemical properties of thebiomass ( q , Cp, e.t.c). Moreover, during the temperatureincrease from ambient temperature up to the torrefactionone, devolatilization of biomass is not considered to takeplace. This assumption is close to reality, since during thisperiod, only internal heating of biomass takes place. Con-sidering biomass evaporation above the temperature of 100 C, drying of biomass takes place in reality, but this isconsidered to be included in the rst step of the process andrepresented by the corresponding arithmetic expressions asa function of time. These data are provided by conductingTGA analysis, while the preheating time required to reachthe operating torrefaction temperature starting from theambient one (20 C) is included.

One Batch Reactor-Solid Product Composition

Table 3 presents the fuel composition of the torreed bio-mass (at time t = 3,500 s), as a function of the operatingtemperature. Since during torrefaction a devolatilization

Table 2 Kinetic parameters

i k 0 (s- 1) E (J/mol)

B 0.00327 3,485.6

V1 5 9 1019 240,657

1 0.1656 20,376

C 0.01921 1.5894 9 104

V2 84.79 5.629 9 104

2 0.5135 28,692

Fig. 4 Model prediction of solid mass yield in comparison with theexperimental results (with dash lines )

Table 3 Fuel analysis dependence on temperature, t = 3,500 s

T ( C) Proximate analysis (%w/w dry-basis)

Ultimate analysis (% w/w dry-ash free basis)

FC VM ASH C H O M

25 (raw) 1 9.3 69.6 11.1 49.04 5.72 44.67 0.56

240 17.66 70.50 11.84 50.66 6.00 42.69 0.64

260 30.68 56.55 12.77 52.92 5.90 40.39 0.78

280 42.59 42.69 14.71 55.47 5.77 37.63 1.13

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step is conducted, the volatiles mass production is increasingwith temperature. In addition, among the three mostimportant substances of the fuel (C, H, O), oxygen isreleased with the highest rate, thus resulting in higher HHV.Finally, the increase in the ash content should be as wellpointed out.

Figure 5 presents the H/C to O/C ratio for the torreedbiomass, reecting the fact that the torreed biomassproperties resemble more the properties of conventionalfuels (coal, lignite) than standard biogenic fuels, thuscontributing signicantly to the more efcient co-ringconcepts of torreed biomass with conventional fuels,when compared to the raw biomass co-ring.

Furthermore, Fig. 5 provides with extra informationabout the beneciary role of torrefaction on the improve-ment of caloric value of the solid biomass (RS: RawStraw, TS: Torreed Straw). Low O/C ratios imply highcaloric value (HHV), whereas low H/C ratios imply lowLHV. As the temperature increases, more Oxygen isreleased through devolatilization. On the other hand theHydrogen content for the raw biomass remains practicallyconstant.

Table 4 gives the molar composition of the releasedvolatiles. The increase of temperature beneciates theincrease of gas heating value (Fig. 6). This is owed to theincrease of its combustible content (CO and acetic acid).

One Batch Reactor-Data Performance of the Process

The basic design parameters including the ow rates of thebasic streams for the torrefaction pilot plant are presentedin Table 5.

From Table 5, it is evident that as the torrefaction tem-perature is increasing, the required ow rate of the heatingmedium (thermal oil) is increasing, especially in the rangeof temperature values more than 250 C. For a temperatureincrease of 10 C, the required ow rate almost doubles. On

the other hand, the corresponding cooling water ow ratedecreases as less torreed biomass needs to be cooled downto a temperature equal to around to 50 C.

In addition, the energy content of the torreed biomassis increasing with temperature, Fig. 6. For temperaturesabove 240 C the HHV of solid upgrades up to 20 %, whencompared to the raw material corresponding one. Similar toFerro et al. [ 2] study, the parametric study on the reactiontime last, for residence times higher than 3,660 s, has notan impact on the achieved caloric value of the solid yield.

The effect of temperature on the temporal evolution of the torreed mass is depicted in Fig. 7. As the temperatureincreases, larger portion of the hemicellulose contentreacts. For a temperature above 250 C, where lignindecomposition happens, the mass yield curves are moreabrupt. According to this graph, torrefaction temperatureshigher than 260 C should be avoided owed to the very lowmass yield earned. This temperature limitation presents abottleneck for the straw torrefaction as a process, at leastfor the case of straw pellets. Nevertheless, if the operatorsintention is to increase the heating value of torreed bio-mass as much as possible, without paying an attention onthe increase of energy recovery (ER), higher temperaturesthan 350 C are preferable.

Figure 8 depicts the temporal evolution of energyrecovery (ER) based on Eq. 18 , in dry-ash free basis ordifferent operating temperatures. Values above 100 % atthe beginning of the process are valid without violation of

Fig. 5 O/C and H/C ratio at solid samples (RS: raw straw, TS:torreed straw), at the time instant of t = 3,500 s

Table 4 Dependence of gas analysis on temperature (% v/v-N 2 free),

t = 3,500 sT ( C) CO 2 (%) CO (%) H 2O (%) CH 3COOH (%)

240 8.13 3.00 85.34 3.71

260 7.78 3.30 85.22 3.69

280 7.75 3.42 85.18 3.65

Fig. 6 Upgrade of biomass Caloric Value (HHV), for differentTorrefaction temperatures at the time instant of 3,500 s

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energy balance, because of the heat input at the system,which is used for the energy upgrade of the fuel and isreected to the increase of the Heating value of the bio-mass. However for residence time higher than 1 h, ERconverges to an almost constant value. This value is alwaysdecreasing with the increase of operating torrefactiontemperature.

ER _mtor LHV tor _mraw LHV raw

; _miraw ;tor ; LHV iraw ;tor in daf 18

To summarize, based on Figs. 6, 7 and 8 results, oneshould design a pilot plant operating at temperaturesaround to 240 C, for a the highest energy yield to beachieved. This is owed to the fact that the decreasing ratioof mass is higher than the corresponding increasing ratio of the Heating value at quite high temperatures (above250 C). This time period allows as well for the operator toget a homogenized torreed biomass product.

Two Batch ReactorSolid Product Composition

Except for the standard case of a single batch reactor, a twobatch one was as well investigated. In this two-batchreactor, the torrefaction process is conducted in two stageswith different operation temperature and residence time. Inthe rst stage, the straw pellets undergo torrefaction up to a

certain point, before they re-enter into the second stage,until the torrefaction is completed. In the end an additionalstage exists for solids cooling. A schematic view of theapparatus is depicted in Fig. 9.

The process is simulated applying the same methodol-ogy. Nevertheless it is made the assumption that thereaction rates at temperatures T 1 and T 2 for torreed bio-mass is calculated from the same kinetic model indepen-dently the state of the fuel at the beginning of the process(i.e. at the 1st batch the initial fuel is raw whereas at thenext batch the initial fuel has already been torreed up to apoint). The corresponding time for the biomass to reach thetemperature dened at the second stage (T 2) theat2 has beencalculated as being proportional to t heat , starting from thecorresponding of the rst stage (T 1).

For all cases examined, with parameters being the twooperating torrefaction temperatures (one for each stage)and the corresponding residence time, the total residencetime is taken equal to around 5,100 s (reaction time of around 1 h). This reaction time lies in between the reportedreaction time of biomass during torrefaction, based onvarious experiments conducted for torrefaction process inthe literature [ 2, 3, 5, 6]. Moreover, this time is consideredas enough for the particles to undergo torrefaction, increasetheir heating value without a signicant loss of mass yield.

In order to investigate the effectiveness of a two-batchreactor, a parametric study regarding the effect of tem-perature and residence time on the operation of the two-batch reactor is conducted. For all cases examined, theresidence time, the biomass particles remain at each of the two steps reactor, is the same and equal to the half of the total one. Table 6 presents the nal process results of the four scenarios examined.

According to the results of the parametric study, adouble-batch reactor with a higher operating temperature atits rst stage (scenario 2) has considerable advantages

Table 5 Stream ow rates, required for the torreed biomass to havethe properties as presented in Table 3 at t = 3,500 s

Mass rate (kg/h) T = 240 C T = 250 C T = 260 C

Thermal oil 288 628 1,200

Cooling water 436.8 410.0 376.6

Nitrogen make-up 7.7 7.7 7.7

Solid yield 86.71 80.18 72.27

Fig. 7 Temporal evolution of mass yield of torreed Biomass fordifferent Torrefaction Temperatures

Fig. 8 Temporal evolution of energy recovery ( ER) of torreedbiomass as a function of Temperature

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(higher mass yield and energy efciency) over the singleone, as presented in the previous paragraphs (scenario 4).Moreover, the application of a higher temperature at therst stage compared to the second one, favors the efciencyincrease, as derived from the comparison of scenario 2 withscenario 1 and scenario 3. Additionally, an equivalent timedistribution (scenario 1) between the two stages has not asignicant effect on the operation efciency of the plant,when the time difference is not very high (less than 10 %scenario 3), in terms of efciency.

From these results, it is deduced that the division of theprocess in two steps has a benecial effect in terms of ER,

only if the second batch is operating at a smaller temper-ature than the rst one. Figure 10 depicts the mass losstemporal evolution for each scenario. To conclude, the

major advantage of following two stages torrefaction pro-cess compared to one stage is that one can achieve higherenergy recovery (92 % compared to 81 %) while on theother hand, one has to build a more complex and thus moreexpensive apparatus for this to be achieved. Specic valuescannot be given as we have not made any Cost BenetAnalysis so far.

Conclusions

In this study, a process model capable of predicting thevalues of temperature and residence times, for the mostefcient operation of a straw pellets pilot torrefaction plantis described. This model is developed in the commercialsoftware ASPEN Plus . The model predicts the gas/solidproducts composition and the rate of biomass mass lossduring the process, while the simulation results agree quitewell with corresponding experimental data. This model isapplied for a straw pellets pilot scale torrefaction reactorand the ow rates of the inert and cooling medium streamsare reported. Several preliminary design parameters(number of stages, torrefaction temperature and residencetime) are parametrically investigated, allowing for theselection of the most optimal one to achieve maximumefciency. Within these parameters, the application of atwo batch reactor seems to be more preferable than a singleone, always under specied conditions and for the case of straw pellets. The composition of the torreed biomassconrms the beneciary effect of torrefaction process, forthe energy upgrade of the fuel. The comparison of simu-lation results between one and a two batch reactor revealsthat a two-batch reactor can be more efcient than a singleone.

make-up N 2

make-up N 2

raw biomass

thermal oil -in

thermal oil -out

cooling water -in

cooling water -out

volatiles-moisture-

N2

torrefiedbiomass

Fig. 9 Schematic representation of two-batch torrefaction reactor

Table 6 Comparison of single-batch and double-batch reactoreffectiveness

Scenario 1 Scenario 2 Scenario 3 Scenario 4

T batch 1( C)

240 260 240 260

T batch2 ( C)

260 240 260

theat (s) 1,500 1,500 1,500 1,500

t1 (s) 1,800 1,800 1,800 3,600

t2 (s) 1,800 1,800 1,600

Mass yield(daf %)

68.11 % 77.62 % 69.15 % 72.09 %

HHV tor / HHV raw

1.19 1.19 1.19 1.13

ER (%) 81.20 % 92.53 % 82.43 % 81.62 %

Fig. 10 Mass yield along the reaction time for each scenario

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References

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12. Hardianto, T.H., Pasek, A. D., Suwono, A., Azhari, R., Ardi-ansyah, W.: The Aspen Simulation of a Peat Torrefaction System.Proceedings of International Symposium on Sustainable Energyand Environmental Protection (ISSEEP) (2009)

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http://www.abco.dk/http://www.abco.dk/

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