S

Ma

b

a

ARRAA

KBSEFFB

1

aitmcvchcccaficpie3

mct

0d

Chemical Engineering and Processing 50 (2011) 944– 951

Contents lists available at ScienceDirect

Chemical Engineering and Processing:Process Intensification

j ourna l h o me pa ge: www.elsev ier .com/ locate /cep

elf-heat recuperative fluidized bed drying of brown coal

uhammad Aziza, Yasuki Kanshab, Atsushi Tsutsumib,∗

Solution Research Lab., Tokyo Institute of Technology, 2-12-1 Ookayama, Meguro-ku, Tokyo 152-8550, JapanInstitute of Industrial Science, The University of Tokyo, 4-6-1 Komaba, Meguro-ku, Tokyo 153-8505, Japan

r t i c l e i n f o

rticle history:eceived 14 April 2011eceived in revised form 5 July 2011ccepted 6 July 2011vailable online 18 July 2011

a b s t r a c t

Brown coal drying based on self-heat recuperation (SHR) technology which recovers effectively bothlatent and sensible heat was developed to reduce energy consumption which is required during drying.A fluidized bed dryer (SHR–FBD) with heat exchanger immersed inside the bed was adopted as the evap-orator. To evaluate the energy efficiency of the proposed SHR–FBD system, a comparison to the availablemechanical vapor recompression (MVR) based drying system concerning the effect of the fluidizationvelocity and bed aspect ratio to the required energy input for brown coal drying was conducted. From

eywords:rown coalelf-heat recuperationnergy efficiencyluidized bedluidization velocity

the results, the proposed SHR–FBD system was found to be able to drastically reduce the drying energyconsumption at all evaluated fluidization velocities and bed aspect ratios. Numerically, the proposedsystem reduced the energy consumption to about 15% and 75% of that required in hot air and MVR dryingsystems.

© 2011 Elsevier B.V. All rights reserved.

ed aspect ratio

. Introduction

Long term future of coal is predicated on its utilization as fuelnd raw material for some chemical synthesis processes which isncreasing. As a low rank coal (LRC), brown coal has some advan-ageous characteristics of low mining cost (possibility for open-cut

ining), high reactivity, and low sulfur content. Moreover, brownoal has a high content of volatile matter causing it easier to be con-erted into gas and liquid products compared to other higher rankoals. Unfortunately, brown coal also has some disadvantages onigh risk of spontaneous combustion and high inherent moistureontent ranging from 45 to 66 wt% (wet basis, wb). High moistureontent results in lower calorific value, higher transportation cost,omplex handling including loading and unloading, reduction in fri-bility, etc. Therefore, brown coal is mostly required to be dried intonal moisture content below 15% depending on its utilization. Driedoals could increase its calorific value as well as efficiency of powerlants, decrease transportation cost, simplify loading and unload-

ng, decrease power plant emission (GHG), etc. In addition, thefficiency of coke-oven in pre-heating and drying increases about0–50% and 10–15% respectively when the dried coal is fed [1–4].

Brown coal (and other LRCs) drying is quite different to bitu-

inous coal drying. In brown coal, the existence of inherent and

hemically bound moistures besides free moisture inside the par-icle leads to higher energy consumption to evaporate the water

∗ Corresponding author. Tel.: +81 3 5452 6727, fax: +81 3 5452 6728.E-mail address: a-tsu2mi@iis.u-tokyo.ac.jp (A. Tsutsumi).

255-2701/$ – see front matter © 2011 Elsevier B.V. All rights reserved.oi:10.1016/j.cep.2011.07.005

[5–7]. Water removal requires a large amount of energy inputbecause of large latent heat required for water evaporation. The-oretically, assuming the ambient temperature of 15 ◦C, the energyrequired for water evaporation ranges from 2.48 to 2.57 MJ kg-evaporated water−1 depending on the wet bulb temperature [8].Furthermore, in hot air fluidized bed drying, heat consumptionfor water evaporation is typically in the range of 3.1–4.0 MJ kg-evaporated-water−1 [3,4]. Moreover, brown coal drying in largepower plant should be able to handle high mass flow rate (upto 200 ton h−1) of raw brown coal and it must have high energyefficiency to avoid a huge amount of energy input. Hence, the devel-opment of drying technology with high energy efficiency has beenemphasized in last few decades.

To reduce the energy consumption for drying, recently, mechan-ical vapor recompression (MVR, or usually also called as vaporrecompression/VRC) based brown coal drying has been developed.Moreover, Potter and Keogh [9] have proposed a fluidized bed dry-ing system utilizing superheated steam as the fluidizing/dryingmedium in a cascading multistage fluidized bed dryer with heatrecovery between successive beds. Furthermore, this system is fur-ther refined by RWE into WTA (German abbreviation standing forfluidized bed drying with internal waste heat utilization) [10,11].Unfortunately, this system could not utilize effectively the energyrecovery of all the heat brought by the drying medium, the evapo-rated water and the dried products.

To enhance more energy saving, recently, a self-heat recuper-ation (SHR) technology which recovers both latent and sensibleheat effectively has been developed [12–14]. Fushimi et al. [14]has developed a drying system based on SHR technology with a

M. Aziz et al. / Chemical Engineering an

Nomenclature

AR bed aspect ratio, dimensionlessAr Archimedes number, dimensionlessC heat capacity (kJ kg−1 K−1)c conversion factor (1 kg m N−1 s−2)d particle diameter (m)F fixed carbon amount (wt% (dry basis, db))g gravity acceleration (m s−2)H bed height (m)l length (m)M moisture content (wt% (wb))p pressure (kPa)po saturated vapor pressure (kPa)T temperature (K)U fluidization velocity (m s−1)V volatile matter amount (wt% (db))W work (W)

Subscriptsb bedbl blowercp compressord distributorex expanderf fluidizationg gasht heatermf minimum fluidizations brown coal sample particler ratiotot totalv vapor

Greek lettersε voidage, dimensionless

sphericity, dimensionless� density (kg m−3)

daacp

tbpm

iderpMio

2

i

� dynamic viscosity (Pa s)

ryer applying countercurrent type of heat exchanger utilizing airs drying medium and they presented a great possibility of SHRpplication to the drying process. However, they provided no con-rete calculation for specific dryer including its applicability anderformance in brown coal drying.

Fluidized bed was selected as the evaporator in this work withhe consideration of its characteristics on large contact surface areaetween solids and gas, good degree of solid mixing, uniform tem-erature distribution across the bed and rapid transfer of heat andoisture between solids and gas that shortens drying time [15,16].In this research, a self-heat recuperation based continuous flu-

dized bed dryer (SHR–FBD) which is designed for brown coal waseveloped with the objective of improvement in overall energyfficiency. Furthermore, with the purpose of comparative studyegarding the energy efficiency, the developed system was com-ared to the hot air drying with conventional heat recovery andVR drying systems in some different fluidization conditions

ncluding fluidization velocity and bed aspect ratio (i.e. the ratiof bed height to its side length).

. Brown coal drying characteristics

The kinetics of coal drying depends on some important factorsncluding coal origin and composition, drying temperature, particle

d Processing 50 (2011) 944– 951 945

size, residence time distribution (RTD) and permeability. Allardiceand Evans [17] have measured the vapor pressure isotherms forVictorian brown coal and concluded that the equilibrium moisturecontent depends strongly on the relative vapor pressure (i.e. theratio of partial vapor pressure to saturated vapor pressure). Fur-thermore, the heat of desorption which is required for brown coaldrying can be approximated simply as the latent heat of watervaporization for its initial 80% of water amount and it is increasinggradually for remaining 20%. Numerically, considering the initialmoisture content of the as-mined brown coal is 65 wt% (wb), theheat of desorption is equal to the latent heat of water evaporationuntil drying to moisture content of about 37 wt% (wb). However,it increases gradually as drying is continued to a lower moisturecontent due to strong hydrogen bonding and capillary movement(diffusion) inside the particles. In case of drying with superheatedsteam (SHS), drying to moisture content of 15 wt% (wb) requiresa bed temperature of about 108 ◦C under atmospheric pressure. Ifdrying is continued to moisture content of 5 wt% (wb), the requiredrelative vapor pressure is about 0.2 corresponding to bed tempera-ture of about 150 ◦C. Different to drying with SHS, the bed temper-ature in drying with air is not constant and it changes following themixing ratio of air–steam mixture. In case of fluidized bed drying,considering that the amount of water to be evaporated is constant,the bed temperature depends strongly on the fluidization velocity.Regarding drying of low rank coals with fluidized bed, Calban [18]has studied the effect of the bed height to the equilibrium moisturecontent and concluded that there is no effect of bed height to theequilibrium moisture content of the low rank coals during drying.

On the other hand, one thing that must be paid attentionis concerning the spontaneous combustion characteristic of thebrown coal during drying. In drying, this spontaneous combustiondepends strongly on the moisture content, particle size, chemicalstructure of the brown coal, oxygen concentration, drying time andtemperature [19–21]. Regarding the relation of moisture contentto critical ignition temperature, Kadioglu and Varamaz [20] foundthat drying to moisture content of about 5 wt% (wb) has a criticalignition temperature ranging from 200 to 240 ◦C for particle size upto 0.85 mm and it decreases to about 130 ◦C as drying is continuedto near complete dry condition. Hence, in brown coal fluidized beddrying employing air as fluidizing/drying medium, the bed temper-ature must be below 130 ◦C as well as the inlet air temperature forsafer operation.

Drying employing SHS as drying medium is advantageous onlower risk of fire hazard or explosion due to oxidation. Unfortu-nately, SHS drying usually requires more complex drying systemand maintenance. Air infiltration due to sample feeding, prod-uct discharging and piping leak must be avoided. Condensation atinitial drying process before evaporation is inevitable in SHS dry-ing. Moreover, superheated steam requires drying system whichis resistant to high temperature and corrosion which is usuallyleading to higher equipment cost. In addition, large energy inputis required to produce steam at every start up which means higheroperating cost.

On the other hand, drying with air as drying medium leads tolower initial equipment cost but usually it has higher risk of firehazard and explosion than drying with SHS. Also, in the conven-tional hot air drying, poor heat recovery method results in higheroperating cost in the respect of energy which is required for drying.Hence, it is necessary to establish a drying method with lower riskof fire hazard or explosion but higher energy efficiency.

3. Self-heat recuperative brown coal drying–drying concept

A state-of-the-art SHR technology is developed based on theconcept of exergy recuperation [12–14]. The characteristic of SHR

946 M. Aziz et al. / Chemical Engineering and Processing 50 (2011) 944– 951

Drain

Raw WetBrown Coal

Dried Brown Coal

Compressor

Fluidized bed

Blower

Flash

Dryer 3(Superheating)

Dryer 1a(Pre-heating)

Dryer 1b(Pre-heating)

Separator

Expander

CondenserDryer 2 (Evaporation)

Drain

Fine grinder

Fig. 1. SHR–FBD system designed for brown coal utilizing air as fluidizing/drying medium.

sestsacawre

ditctwnoFmhcattt1et

pwttat

ystem is on the exergy recuperation through compression, heatxchange and heat pairing for each sensible and latent heat. The hottream is heated by compression to provide the minimum tempera-ure difference required for heat exchange and pairing with the coldtream. As the result, all of the energy involved in drying is recuper-ted and utilized as the heat source for the subsequent drying pro-ess. This includes recuperation of sensible heat of the gas servings drying medium, both sensible and latent heat of the evaporatedater and sensible heat of the dried products. In addition, SHR also

ecovers the energy utilized in increasing the exergy rate of thevaporated water and drying medium exhausted from the dryer.

Fig. 1 shows the basic schematic layout of SHR–FBD systemeveloped for brown coal drying which utilizes air as fluidiz-

ng/drying medium. Drying is divided into three continuous stages,hey are pre-heating, evaporation and superheating. Raw brownoal is heated initially in a pre-heating stage (dryer 1a) to a cer-ain temperature. Subsequently, the main drying process which isater evaporation is performed inside the FBD (dryer 2). The inter-al heat exchangers which are fulfilled by the compressed mixturef fluidizing/drying medium and steam are immersed inside theBD providing the heat for water evaporation. Then, the exhaustedixture of the fluidizing/drying medium and steam are super-

eated (dryer 3) and continuously it flows to the compressor. Theompressed vapor having higher exergy rate is circulated backnd utilized as heat source for superheating (dryer 3), evapora-ion (dryer 2) and pre-heating (dryer 1), in this order. In addition,he sensible heat of the hot dried brown coal is also recovered byhe drying medium to improve the overall energy efficiency (dryerb). By this method, all enthalpy-rich streams are utilized and thexergy is recovered and recuperated leading to minimization ofotal energy input.

Superheating stage is introduced in this system to keep a com-lete vapor quality (vapor quality x equals to 1.0) avoiding anyater condensation during compression. Low vapor quality leads

o difficult design and maintenance of the compressor, etc. Fromhe isosteric heat of desorption which was reported by Allardicend Evans [17], the required evaporation heat is proportional tohe amount of evaporated water until drying to about 37 wt% (wb).

As the result, the exhausted vapor will be same or near to the satu-rated condition of drying until moisture content of 37 wt% (wb).Hence, in this case, the superheating process is proposed to berequired to avoid any condensation during compression. As dryingis continued to lower moisture content, the relative vapor pressuredecreases, hence the exhausted steam–air mixture is in completevapor condition and the superheating process could be omitted.

Initially, raw brown coal having particle size of up to 8 mm isground in the grinder to fine size of 1–2 mm and subsequentlyit enters the pre-heater for pre-heating to certain temperature.The preheated fine brown coal enters the second stage of evap-oration inside FBD where the moisture is evaporated by the heatsupplied by both fluidizing/drying medium (direct heat transfer)and in-bed immersed heat exchangers (indirect heat transfer). Theimmersed heat exchangers increase the temperatures of both flu-idizing/drying medium (air) and brown coal particle. As the result, itleads to increase the moisture-carrying capacity of the air accelerat-ing the water removal from the brown coal. Here, the required heatfor water removal inside FBD is provided mainly by the compressedair–steam mixture. This indirect heat transfer could decrease therisk of fire hazard and explosion which becomes a key issue in coaldrying [4].

4. Process calculation

To evaluate the performance of the proposed SHR–FBD systemin terms of energy efficiency, mass and energy balance is calcu-lated using a commercial dynamic process simulator Pro/II ver. 8.1(Invensys Corp.). The energy consumption required for drying in theproposed drying system was compared to that in the hot air dryingwith conventional heat recovery and MVR drying system employ-ing superheated steam (SHS) with the following assumptions: (1)FBD with internal heat exchanger is approximated consisting of amixer, a heat exchanger and a separator, (2) complete mixing inside

FBD, (3) the minimum temperature approach in FBD and other heatexchangers are 10 and 20 K respectively, (4) raw brown coal has aninitial moisture content of 50 wt% (wb), (5) drying is conducted tomoisture content of 15 wt% (wb), (6) the adiabatic efficiency of com-

M. Aziz et al. / Chemical Engineering and Processing 50 (2011) 944– 951 947

Table 1Properties of material and fluidized bed used in process calculation.

Properties Value Notes and sources

Brown coalInitial moisture content [wt% (wb)] 50Average diameter ds [mm] 1.5Molecular weight [–] 384 Kumagai et al. [22]Coal composition [wt% (db)] C: 66.7, H: 4.7, S: 0.3, N: 0.6, O: 26, Ash: 1.7 Li [23]Volatile matter V [wt% (db)] 51.1 Secondary volatile matter equals to 10% of total volatile matterBulk density �s,bulk [kg m−3] 900Particle density �s [kg m−3] 1470 Neavel et al. [24] dry conditionSphericity ˚s [–] 0.6

Fluidized bedShape SquareSide length l [m] 4

p(d(pemwdsbtbsflv

(msmf

icps

U

e

TC

b

Bed aspect ratio AR [–] 1, 2, 3

ressor, blower and expander are 80%, 80% and 90%, respectively,7) drying is performed under atmospheric condition, (8) thermo-ynamic calculation is performed based on Soave–Redlich–KwongSRK) method, (9) no heat loss from the system, (10) no mass trans-ort resistance in all phases and no pressure drop during heatxchange in each heat exchanger. Table 1 shows the properties ofaterial and fluidized bed used in process calculation. In this study,ith the purpose of evaluating the effect of the bed height to therying energy efficiency, three different bed aspect ratios AR areet, they are aspect ratio of 1, 2 and 3, respectively. In this study,ed aspect ratio AR is defined as the ratio of the bed height Hb tohe bed side length lb. One thing must be noted here is that theed side length lb is fixed and the change of the bed aspect ratio ARimply means the change of the bed height Hb. Coal amount anduidization gas volume in each bed aspect ratio AR and fluidizationelocity U are shown in Table 2.

In this study, as the final moisture content was fixed on 15 wt%wb), the relative vapor pressure of the exhausted air–steam

ixture was quite below the saturated condition. Hence, theuperheating stage could be omitted and the exhausted air–steamixture was directly compressed by the compressor before per-

orming heat recuperation in all drying stages.The minimum fluidization velocity Umf for brown coal is approx-

mated with the equation recommended by Chitester et al. [25] foroarse particle with involving the correction factor for wet coalarticles as it was proposed by Pata et al. [26] and finally it can beummarized as follows:

�g{[(28.7)2 + 0.0494Ar]1/2 − 28.7}

mf ={�g[1.182ds − 5.65 × 10−4 + (7.413 × 10−2)/(M1.58)]}

(1)

On the other hand, the heat capacity Cs is calculated by thequation proposed by Eisermann et al. [27] which is correlated by

able 2oal amount and fluidization gas volume in each bed aspect ratio and fluidization velocit

Bed aspect ratio AR Fluid velocity U Wet brown coa[–] [×Umf] [ton h−1]

1 1 57.6

2

3

4

2 1 115.2

2

3

4

3 1 172.8

2

34

Bed height Hb 4, 8, 12 m

Eq. (2).

Cs = F(−0.218 + 3.807 × 10−3Ts − 1.758 × 10−6Ts2)

+ V(0.883 + 3.307 × 10−3Ts) (2)

Furthermore, the blower work Wbl is defined as the work toprovide a pressure increase �Pf which is required for fluidizationand is calculated according to the equation proposed by Kunii andLevenspiel [28] as follows:

�Pf = �Pb + �Pd (3)

where �Pb and �Pd are pressure drops across the bed and distrib-utor respectively and they could be calculated as follows [28]:

�Pb = (1 − εmf )(�s − �g)Hbg

c(4)

�Pd = 0.4�Pb (5)

The sorption isotherm for brown coal which is required for cal-culating the relative vapor pressure as driving force in drying iscalculated using an equation proposed by Chen et al. [29] which wasformulated based on the experimental work conducted by Allardiceand Evans [1,17] as following equation.

p

po= 1 − exp

[−2.53(Tb − 273)0.47

(M

(100 − M)

)1.58]

(6)

4.1. Hot air drying with conventional heat recovery

Fig. 2 shows the schematic process diagram of hot air dry-ing with conventional heat recovery system. There is no heatexchange inside the fluidized bed dryer. The heat required for waterevaporation is provided mainly by the sensible heat of the air as

y.

l mass Evaporated water mass Gas volume[ton h−1] [×103 m3 h−1]

23.7 28.857.789.9

113.747.4 29.6

57.586.8

115.471.2 30.2

57.486.4

120.9

948 M. Aziz et al. / Chemical Engineering and Processing 50 (2011) 944– 951

HX2

HTBL

MIX SEP

Fluidized bed

MC 50 wt% (wb)MC 15 wt% (wb)

Wbl

P2

P 3

101.33 kP aHX1

AirTg,1

Tg,2

Tg,3 T

g,4

Tg,6

Tg,5

Tg,7

P1

Ts,1

Ts,2

Ts,3

Ts,4

Fig. 2. Schematic process of hot air drying w

Re-circulated SHS

HX1

HX2BL

CP

MIX SEP

Fluidized bed

MC 50 wt% WB MC 15 wt% WB

Wbl

T v,1Tv,2

T v,3

T v,4

T s,1

T s,2

T s,4T s,3

T s,5

Tv,5P2

P 3

101.33 kPa

Wcp

Purged S HS

Fi

fllbIr

tot,MVR

ig. 3. Schematic process of MVR–FBD system for brown coal with steam as fluidiz-ng/drying medium.

uidizing/drying medium. The exhausted air–steam mixture is uti-ized to pre-heat both drying medium and raw brown coal. Dried

rown coal is abandoned and could not be recovered in the system.n this case, there is no addition of energy for the purpose of heatecovery. However, additional energy in the form of heat Wht is

air-water mixtu

water

HX1

H

BL

MIX

FL

Fluidized b

HX5

MC 50 wt% WB

Wbl

Tv,4

Tv,5

Ts,1 Ts,2

Ts,4

Ts,3

Tg,3Tg,4P2

Tv,6

101.33 kP

HX3

Fig. 4. Schematic process of SHR–FBD system for bro

ith conventional heat recovery system.

required to evaporate the water. The total energy consumption inthe case of hot air drying with conventional heat recovery systemWtot,HA is calculated as follows:

Wtot,HA = Wht + Wbl (7)

4.2. MVR–FBD employing SHS as fluidizing/drying medium

Fig. 3 shows the schematic process diagram of MVR–FBD sys-tem with SHS as fluidizing/drying medium as it has been suggestedby Fitzpatrick and Lynch [30] which also referred to work doneby Potter et al. [31]. The exhausted steam from the dryer is splitinto re-circulated and purged SHS which is equivalent to the waterevaporated from the wet brown coal. The purged SHS is compressedby compressor CP and hence its temperature is elevated from Tv,1to Tv,2. Then, it flows to HX2 (FBD) for latent heat exchange (con-densation of compressed air–steam mixture and evaporation ofwater possessed by brown coal) and finally to HX1 for sensible heatexchange (brown coal pre-heating).

The total energy input/duty in the optimized MVR–FBD systemW is calculated as follows:

Wtot,MVR = Wcp + Wbl (8)

re

EX

HX4

X2

CP

SEP

ed

MC 15 wt% WB

Wcp

Wex

Tv,1

Tv,2

Tv,3 Ts,6

Ts,7

T=20 °C

Tg,2

P1=101.33 kPa

P3

water

CD

Tg,1=20 °C

a

Ts,5

wn coal with air as fluidizing/drying medium.

M. Aziz et al. / Chemical Engineering and Processing 50 (2011) 944– 951 949

0

10

20

30

40

50

60

543210

Tota

l inp

ut e

nerg

y Wtot

[ MW

]

Fluidization velocit y U/Umf [ - ]

HA, AR=1 HA, AR=2 HA, AR=3

Fs

4

atH

0

5

10

15

543210

Tot

al in

put e

nerg

y Wtot

[ MW

]

Fluidi zatio n veloci ty U/Umf [ - ]

SHR, AR=1 MVR , AR=1SHR, AR =2 MVR , AR=2SHR, AR=3 MVR , AR=3

Fe

ig. 5. Total energy input in case of hot air drying with conventional heat recoveryystem.

.3. SHR–FBD utilizing air as fluidizing/drying medium

The schematic layout of the developed SHR–FBD system with

ir as fluidizing/drying medium is shown in Fig. 4. Compared tohe MVR–FBD system, some additional heat exchangers HX (HX3,X4 and HX5), an expander EX, a flash FL and a condenser CD are

0

20

40

60

80

100

0 1 2 3 4 5

Inle

t air

tem

pera

utre

Tg,

4[ º

C ]

Fluidi zatio n veloci ty U/Umf [ - ]

AR=1 AR=2 AR=3

a

c

e

0

5

10

15

20

0 1 2 3 4 5

Com

pres

sor w

ork Wcp

[ MW

]

Fluidization velocity U/Umf [ - ]

AR=1 AR=2 AR=3

0

2

4

6

8

0 1 2

Pres

sure

ratio

pr

[ -]

Fluidization

AR=1

ig. 7. Detail of process calculation results of the proposed SHR–FBD system: (a) inlet axpander work Wex , (e) compression pressure ratio pr .

Fig. 6. Total energy input comparison Wtot of both SHR–FBD and MVR–FBD systemsin relation with fluidization velocity and bed aspect ratio.

installed in respect of heat/energy recovery. In SHR–FBD system,

the compression work Wcp conducted by compressor CP is partlyrecovered by the expander EX as expander-work Wex. Moreover,the sensible heat of the dried brown coal is recuperated by the

b

d

0

20

40

60

80

100

0 1 2 3 4 5

Bed

tem

pera

ture

Ts,5

[ ºC

]

Fluidization velocity U/Umf [ - ]

AR=1 AR=2 AR=3

0

5

10

15

20

0 1 2 3 4 5

Expa

nder

wor

k Wex

[ MW

]

Fluidization velocity Uo/Umf [ - ]

AR=1 AR=2 AR=3

3 4 5 velocity U/Umf [ - ]

AR=2 AR=3

ir temperature Tg,4, (b) bed temperature Ts ,5, (c) compressor work Wcp , (d) gained

9 ing an

flc

W

5

iWriO(SUdr3ttbfcwteht

swdiphHscet

titaTlw

Saattalthha

Wtt

50 M. Aziz et al. / Chemical Engineer

uidizing/drying medium in HX3 before it is pre-heated by theompressed vapor.

The total energy input for SHR–FBD, Wtot,SHR, is defined as:

tot,SHR = Wcp + Wbl − Wex (9)

. Results and discussion

Fig. 5 shows the total amount of energy input which is requiredn hot air drying with conventional heat recovery system (Wtot,HA).

tot,HA increases gradually following the increase of bed aspectatio AR. It can be observed that a large amount of energy inputs still required for drying although heat recovery is conducted.n the other hand, Fig. 6 shows the comparison of energy input

total duty) required for drying in MVR (Wtot,MVR) and proposedHR (Wtot,SHR) drying systems in relation to fluidization velocity

and bed aspect ratio AR. As comparison, Pikon and Mujum-ar [4] reported that in conventional hot air drying without heatecovery, the heat consumption in FBD is typically in the range of100–4000 kJ kg-evaporated-water−1. From Figs. 5 and 6, it is clearhat both drying systems (MVR–FBD and SHR–FBD) decreased theotal energy input significantly in all fluidization velocities U anded aspect ratios AR. Numerically, the heat consumption requiredor drying to moisture content of 15 wt% (wb) in hot air drying withonventional heat recovery, MVR and proposed SHR drying systemsere about 2970, 730 and 550 kJ kg-evaporated-water−1, respec-

ively. The proposed SHR–FBD system could reduce the requirednergy input to about 15% and 75% of those required in conventionalot air FBD with heat recovery and MVR–FBD systems, respec-ively.

It is very important to note that in MVR–FBD system, the sen-ible heat brought by the dried brown coal is abandoned anywayithout recovery because the temperature and exergy rate of theried brown coal are same or lower compared to that of the flu-

dizing/drying medium (which is SHS). On the other hand, in theroposed SHR–FBD system, all of the heat including the sensibleeat of the dried brown coal and the condensate can be recovered.ence, higher energy efficiency could be earned from the SHR–FBD

ystem employing air as fluidizing/drying medium. In addition, theompression work Wcp of the compressor CP is recovered as gainedxpander work Wex in SHR–FBD system but not in MVR–FBD sys-em.

In MVR–FBD system, almost no significant difference regardingotal energy input Wtot,MVR in relation to the fluidization veloc-ty U and bed aspect ratio AR. It is considered mainly because ofhe constant bed temperature Ts,4, that was 108 ◦C, resulting inlmost constant temperature of the discharged dried-brown coals,5 (108 ◦C) and condensate Tv,4 (40 ◦C). The amount of SHS uti-ized as fluidizing/drying medium only affects the blower work Wbl

hich is required for fluidization.On the other hand, the total energy input of the proposed

HR–FBD system Wtot,SHR depends on the fluidization velocity Und bed aspect ratio AR. The inlet air and bed temperatures, Tg,4nd Ts,5, compressor work Wcp and gained expander work Wex inhe proposed SHR–FBD system are shown in Fig. 7. As the fluidiza-ion velocity U increases, both inlet air and bed temperatures, Tg,4nd Ts,5, decrease gradually. Higher fluidization velocity leads toower mixing ratio of the air–steam mixture, resulting in lower bedemperature at the same relative vapor pressure. On the other hand,igher bed aspect ratio causes higher mixing ratio and, as the result,igher bed temperature is required following the increase of bedspect ratio.

Furthermore, compressor work Wcp and gained expander workex are increasing following the fluidization velocity U. However,

he increase gradient of the compressor work Wcp is larger thanhe expander work Wex resulting in increase of the total energy

[

[

d Processing 50 (2011) 944– 951

input Wtot,SHR in all evaluated fluidization velocity U. Regard-ing the compression pressure ratio pr, as shown in Fig. 7(e), itdecreases as the fluidization velocity U increases. It is consideredthat this phenomenon is mainly influenced by the decrease of bedtemperature Ts,5 resulting in lower required temperature of thecompressed air–steam mixture Tv,2. Moreover, although the com-pression pressure ratio pr is lower at higher fluidization velocity U,the compressor work Wcp and the gained expander work Wex areincreasing following the rise of fluidization velocity U. It is supposedthat this result is brought mainly by the increase of air amountfollowing the increase of fluidization velocity U.

Regarding the bed aspect ratio AR, it affects the amount of theevaporated water. As the bed aspect ratio AR increases, the amountof evaporated water increases. As the result, the bed temperatureTs,5, compressor work Wcp, pressure ratio pr and total energy inputWtot,SHR increase following the increase of bed aspect ratio AR.

6. Conclusions

A novel brown coal drying based of SHR technology employingfluidized bed as the main dryer has been proposed and its perfor-mance regarding energy efficiency has been evaluated. From theresult, it can be concluded that the proposed SHR–FBD system uti-lizing air as fluidizing/drying medium can reduce the drying energyconsumption compared to hot air drying with conventional heatrecovery or MVR–FBD system utilizing SHS as fluidizing/dryingmedium. Numerically, the developed SHR–FBD system can reducethe energy consumption to about 15% and 75% of that requiredin hot air drying with conventional heat recovery and MVR dry-ing technologies. Hence, SHR–FBD system can be considered as theleading-edge of technology for coal drying concerning the energyefficiency. Furthermore, drying in lower fluidization velocity seemsto be preferable because it shows a higher energy efficiency com-pared to in higher fluidization velocity.

Different to conventional hot air drying, in SHR based drying,the inlet air temperature is considerably low because the heatwhich is required for drying is provided indirectly through the in-bed immersed heat exchanger. Hence, it improves the feasibility ofthe SHR drying system to be applied in brown coal drying as thistechnology could decrease significantly the risk of fire hazard orexplosion due to oxidation.

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