26 Bagasse handling. >tornge and drying
606
100
26.2.5 Baling
I
!
/
~0~~--~--~--7
8--~17
0--712~-1~~--~16
P>le heoghtln m
Figure 26.4: 01) bulk dcnslly of bogas:.< .., • fun<:ll~n of
heoghl ofoosa-'-"' piI< ((rom Mofll<m et aJ. 1974)
i1 is preferable 10 con,ider dry bulk densities. A u..,.
ful relationship becwcen dry bulk dcn>ily and pile
heigh• was escublished by Morgtm ec ul. ( 1974) nnd
is shown in Figure 26.4. Allhough chis dacn was ob·
tnioed for wee bulk Mornge (di;.cu,sed in Section
31.2.1 on By-Product;). il appears 10 have more
general applicability for baga>>e >tomge and ac the
lower end conforms 10 other dal3 on bagasse packing den>ily. The Figure shows the dry bulk density
increasing from 100 kglm' :11 a 2 m heigh• to 180
kglm' in a 14 m high pile. At 40 % moisturo. this im·
plies a bulk density for moist bagasse varying from
166 to 300 kglm!.
Because loose bagasse has such a low bulk den·
sity. numerous in\'~Ligations have been made into
baling baga'se 10 reduce the volume of stored ba·
gnsse. Baling hns generally been proven to be better
chan pelleti1jng. which re-quires costly equipment that
is expcn<h·e to maimain. Baled bagasse tempemcurc<
are also found to increa.sc in stomge as n result of mi·
crobial activity. Temperatures rise co about60 •c after
a few days and remuin there for about 30 days. They
arc oflen M3ckcd with ajr pa.~g~ bclwccn them.
to provide a means of dis;ipming che heat produced.
Dawson et al. ( 1990) show that dried bag"""' of lcs;
than 20 g water/I 00 g bagasse is obtained alter scor·
age between 20 und 30 days. A' a re>uh the material
is microbiologically and chemically inactive and mosc
suitable for use in by-products or as a boiler fuel.
Bales have a densicy. depending on the press
used. of nboul 600 kglm' . i.e. roughly three time<
that of loose bagasse (Section 32.2.2). However if
bales are often scored with air spaces bee ween them.
the ud,antage Of greater densities in >COring bagasse
in bale> is lnrgely eliminated.
Dtm·stm et ul. ( 1994) studied the populations of
microorganisms in bales and in baga..e stored loose.
They found that drying the baga>se prior to baling
reduced the numbct'l> of >pore> de\'eiOped in stornge.
buc concluded tbot baling of fresh and partially dried
bagasse had liule advanwge over loose scoragc.
26.3
Baga.<sosis. There i' a potential heallh hazard
associated with the scorage and handling of bagasse.
namely bagasso;is. an allergic reaccion of lung ti,.
!<IUC
to I he ptC-i:>Cncc of T!Jemmactinomyc~s .wcclwri
spores. These develop only in stored baga>se and are
not u;u:clly present in sufficient quantity 10 cause
problems in a normal mill environmem. Funher de·
tails are given by Dawso11 et nl. ( 1995).
Trnns port in bulk. In some cases baga>se is
mnsponed from one mill 10 another '" road vehi·
cles. It is imponant 10 design loading systems which
contain the dust nod ensure thac the mocks are sealed
before venturing ou1 on the road,.
In general bulk storage of bagasse requires addi·
tiona] environmental monitoring and management.
Bagasse d rying
Final bagasse ha.~ a high water content (48-52
g/1 00 g bagasse). which both reduces iiS heating value and i\ the cause of the largest energy loss in th•
boiler>- because the fuel moisture carries the lacem
heat of vaporization up the 'tack:
The e-quation for the gross calorific \'alue of bagasse (equacion 27.1 ) shows a decren.e or 196
kJ/kg for C\"Cry increase or I 'k in the mobture
comem.
It i• shown in Section 27.3.2 thac ubout 95 % of
the losses from a bagasse fired boi ler are st:tck
losses and. for bagasse. the moisture loss is the
most significant: the extent of the loss is propor·
tionnlt<> the final gas tempernture.
26.3.1 Effect on boiler effi cie ncy and capacity
Drying the bagasse is therefore beneficial to boiler
efficiency for several reaso ns: (a) th e lower amount
of water reduces the excess air required. at least
down to a level of 40 '7c moi sture. below which there
is no red uction in excess air required (Magasin er et
al. 2002) : (b) it is possible to reach a higher adiabatic
flam e temperature: (c) a smaller flow of gases passes
to the stack. The ways in which bagasse moi sture
and gas exit temperature affect boiler efficiency are
illustrated in Figure 27.4.
However. drying the bagasse with the flu e gas
goes well beyond that as it effective ly reduces th e
final gas temperature passing up the stack. usin g residual energy to effect the drying. This equates to
returning some of the gas enthalpy to the boiler an d
reducing losses accordingl y. Driers. mainl y rotary
drum and fla sh types. have been insta ll ed in Am erica. Brazil, Philippines, Australia, China. Cuba and
Indi a. bur consistent resul ts and concl usions have
not always been obtained and the degree of success
varies widely.
26.3.1 Effect on boiler efficiency and
capacity
The use of boiler exhaust gas to dry bagasse redu ces the stack temperature and hence from an effici ency point of view is eq uivalent to adding more
boiler heat recovery surface . An economizer does
not give an effic iency gain as high as a drier. but has
been usuall y the prefened option because of lower
initial costs. better reli ability and lower maintenance
requirements. However bagas se driers can reduce
stack temperature to a lower figure than either an
air heater or economizer. The use of an economi zer
enables a reliable and relatively cheap reduction in
gas temperature to be obtained with simple heat exchange equipm ent with no additional fa ns. gas ducts
or conveyors. With no moving parts. operational
and maintenance costs are lower. The limit on outlet
temperatures must be high enough to ensure that the
conosion will be unimportant unless very abnormal
conditions occur (Magasiner 1996).
Drying of bagasse not only redu ces the effective boiler gas ex it temperature, but also reduces the
amount of boiler gas produced, thu s improving the
efficiency significantl y more than additi on al heat re-
607
covery eq uipment on a boiler can achieve. However
it never mak es economic se nse to pay for the energy
required to dry bagasse . Thi s only makes sense if
waste heat in boiler flue gas is used for drying.
An economic compari son of the two options is
necessary to dete rmine which is the most cost effective alternative between economizers an d bagasse
driers in each case. Maranhao (1986) reported that
the cost of install ation of a bagasse drier represented
around 12 'k of the value of the boilers. with a stack
temperature of 135-140 oe, while Magasiner ( 1996)
suggests that the cost of an eco nomi zer represe nts 6
to 8 '7c of the cos r of the boi ler with a stack temperature of 150 oe, Thus it appears that an economi ze r
costs be tween 50- 65 'k of a bagasse dri er.
Using info rm ation obtained in a prototype install ed at the Nun orco sugar mill, Argentina, Cardenas et al. (1994 ) pe rformed an evaluat ion concluding th at divertin g 50 'k of the stack gases to the bagasse drier increases th e boiler effi ciency from 7 1.4
to 84. 9 'k. large ly by redu cing stack losses. and with
a smaller reductio n in furn ace loss.
Dixon et al. ( 1998) show that reducing the moisture co ntent of bagasse has a substantial effect on the
capacity of the boiler. However they estimate that a
limit of 30 'k moisture is necessary for safe conditi o ns in conventi onal spreader firing. At this moi sture
content an increase in steam output ofjusr over 10 '7c
is expected. Narendranath and Rao (2002) report an
increase in boiler capacity between 5 and 8 '7c following the installation of bagasse dri ers achieving
bagasse moistu res in the 42-44 '7c ran ge .
26.3.2 Types of drier
Initially. most driers were of the rotary type.
Furines (1976) described some earl y installations
in Florida. These were not always satisfactory and
often gave considerable trouble. Subseq uentl y fla sh
driers were developed. since it was established that
very quick drying of the order of seconds. could be
achi eved, particularly with good cane preparati o n.
Thi s type has now been widely adopted. A re view
of some of the bagasse dry ing plants used in several
countries showing the rotary drum and the pneumatic suspension drier as the most popular alternati ves
is give n by w 111 der Poe/ et al. ( 1998:458). Most of
them have ach ieved a drop in moisture content of
References p. 6 16
608
26 B agas se handling. storage a nd drying
bagas se from 50 ck or just over to somewhere in the
range of 30 to 40 'k .
The swirl burner deve loped by SRI in Australia
dri es bagasse being fed to th e boilers usi ng fu rn ace
gases. This falls into a so mewha t differe nt category
to the driers considered here. in that it is an integral
part of the boiler and does not benefit ti·o m using
resid ua l flue gas energy which would otherwi se be
lost.
Rotary drum driers. This type of equ ipmen t
has bee n widely used fo r various drying duties. co nsisting of a rotating cylindrical drum. prov ided w ith
internal deflectors and cocurrent fl ow of bagasse a nd
hot gases . Airl ock sys tems must be us ed on the ends
of the cylinder feeding and disc harging bagasse .
to avoid air ingress which may cause a fi re. These
driers have often proved troubleso me. they are mechanicall y cumbe rsome. a nd the ri sk of fires is hi gh.
A number of driers installed in the past have bee n
decommissioned and re moved.
Pneumatic flash driers. Direc t contac t with
ho t flu e gases gi\·es hi gh heat tran sfer rate s. short
residence times and uniform dryi ng. Although differe nt des igns are availabl e. in general a fu el feeder
is used at the bagasse inle t. which simultaneously
ac ts as a seal preve nting the entrance of air. Bagasse
is transported in the hot gases in long heating tubes
and direct contact heat transfer take s pl ace. The bagass e is subseque ntly separated fro m the gas in a
cyclone. The wet gases go to the stack. while th e
bagasse separated in the cyclo ne is discharged via a
rotary air-lock v-alve onto a conv-eyor or directly into
the combustion chamber.
~a u st
Cyclones
Sid e view
Moisture contents down to 35-40 'k can be
achieved. in the process reducing the gas stack temperature to 80 cc_ just above the dew poi nt. The installation at Zai o mill in Morocco is reported to have
inc reased the bo il er efficiency fro m 72 to 78 '7c and
the steam production from 1.6 to 2.7 kg steam/kg
bagasse (mn der Poe/ et al. 1998:459).
gas 137 'C
Gas return
figure 26.5: Pneumatic su spension countercurrent bagasse drier at Sa nto Antonio (Maranha,·
1986)
26.3.3 Operatio nal issues
The plant described by Maranhao (1986) involves the use of an auxiliary fan to direct part of the
gases before or after th e ai r preheater to the front of
the boiler through an underground pipe, to the drier
located ju st before the bagasse feeding system (Figure 26.5). Bagasse is separated from the hot gases
in cyclones, from which it discharges into th e boiler
furn ace. The advantage of this system is the absence
of additional conveyors. However wear on most of
the components in the system is significant. requiring ongoing maintenance .
A limit is imposed on the temperature of the
gas leavi ng the drier. whic h is in contact with metal and therefore must be kept below the de w point
of the co mbustion gases to avoid the form ation of
acids that cause conosion. As bagasse has a lov..sulfur content. the temperature is lower than in the
case of boi lers which use foss il fuels. For conventional bagasse boilers the dew point is below 90 oc
(Magas iner 1996).
After 7 years of experience with three pneumatic
bagasse driers installed in the Santo Antonio sugar
mill, M ara nhao ( 1986) described briefiy the units
and results observed. summarized in Table 26.1.
The initial boiler effic ie ncy was low and so signifi cant gai ns were possible with only an air hea rer.
The comparison shows how much more the efficiency can be improved with a drier. with the best
perform ance obtained with a co mbination of air preheater + bagasse drier. The reduction in excess of
air from 93 'K to 35 'k has a considerable inftuence
on the efficiency. apart from the reduction in stack
temperature.
The main concerns with bagasse driers are the
maintenance, reliability and susceptibility to blockages. Careful design can minimize these problems.
This req uires that all parti cle impacts occur at a shallow angle to minimi ze erosion . Erosion is not an issue on bends on vertical up-ftow. but all other bends
609
need to be very long radius or have easily replaceable wear plates at the bends. It is also important to
have oversi1ed rotary va lves. It is reported that the
valves have a working life of four seasons.
A potential environmental advantage of bagasse
driers in a well-designed system is the retention
in the moist bagasse of small particles entrained
wi th the flue gas. reducing the particulate emission
through the stack (1·an de r Poe/ et a!. 1998) . In India
it is reponed that the gas exiting the cyclones has a
part iculate loading of< 100 mg/m 3 (Na rendranath
and Rao 2002). Thi s also means a smaller quantity
of sa nd in the gases , which together with lower velocities (because excess air is reduced) has resulted
in a longer service life of exhaust fa ns and stacks
(Maranhao 1986).
26.3.3 Operational issues
The main operational issue is the mai ntenance of
equipme nt. particularly related to wear. Even though
the concept has been around for man y yea rs, so far
the tec hnol ogy has not proved reliabl e and drier
projec ts have not bee n consistentl y successful. Reports of successful installations of bagasse driers exist. but none of the se riously failed experie nces have
bee n published. In Colombia for example. a bagasse
drier was installed in the late 1980s. whi ch led to a
smooth and efficient operation of the boiler, eliminati ng suppleme ntary fuel consumpti on. However.
serious \Vear of some elements and problems wi th
the rotary seal valves were so severe that the drier
became a mainten ance problem and jeopardized the
reliab ility of the plant. Finally it was dismantled.
Popular opinion in places like Brazil and South
Afric a suggests that an economi ze r is better than
a bagasse drier beca use of few er problems. even
though the efficiency is inferior. Whilst bagasse driers can reduce stack temperature
Table 21i.l: Results v. ith bagasse driers in San Anton io sugar mill.
to a lower figure than either an air heater or
Brazil (Maranhao 1986)
economi zer. they are expensive. mechanicalBo il er
+Air
+Dri er
+Air
ly complicated, use additi onal powe r and are
alone
preheater
preheater
difficult to integrate successfully into a con+Drier
ventional boiler station. Indepe ndent of the
Stack temperature in '' C
drying technology selected , conveyors to and
217
260
140
330
Bagasse moisture in '1c
52.7
52. 7
40
from the bagasse driers are usuall y required,
35
Excess air in '7c
78
41
which may represent a signi fica nt additional
93
35
46.5
Boiler efficiency in
cost.
55.6
64.3
69.3
Re(em1cesp. 616
610
26.3.4 Other alternati\es ror bagasse
drying
Or)ing using superheated ~t ea m. A lluidozed
hed dner fur heet pulp ha' hecn de,crihed CJ•nsm
2003). whoch is in use on n number nf liuropean beet
'u~ur fnctorie,. Es'ientially. 'uperlteuted 'team (e.g.
2.g \1Pa. 250 Cl " u\ed to e\aporate moi,ture.
)teldon~ a \tream of \apor l<a,in~ the drier with
ell<>Ugh energ) to he u>ed "' the heaung media in
the toN cfln'l of the C\3)10011<"' ce.g. 370 I..Pa. 150
C). The \apor generated on the drier " fe\.'0\ered
on the fuctO<). In tho; \\3). nlm<"t all the energ) and
wntcr " recO\ered. but the energy lc,cl ol the Steam
i, lov.cred.
The main complicatoon with hosh pre»ures is
the feeding and o.ealing of the dncr. "hich in the
ca\C of beet pulp has been wl'ed by employing
I"N3<) \ ahe' u\ed in the paper pulp tndU>Il'). The
~Njor change required "ithon the IO<.'I<") " the 'Piit
of the lir\t effecJ e>-apon~tor '"'" '"" h<llhts. or the
addouon of a Mt:un uan,forrntr or anolher effa:t. to
3\oicJ ctmt.unuwlloo of the extuu't "-ondcn\Mc v.lth
the 'ap<>r comong from the drier.
Other dc"gn> lor heet pulp lxl\ed on the pnnco·
pie <>I 'uperhcaung drying ure pre\Cnted by •wo der
f'o<'i e t ul. ( 1998). The>C rely on lluid11ution of the
beet pulp. but for sugureane mill,, lluidituuon of bn·
1!3!11\C " 11tlf10"ible. and initial trial' ha\C rnud~ u~
Of \Upcrbeated >I~ flO\\ through <t4110na<) poleked
bed\, at atlll<hphenc pres<ure t.lfon:nonl/11 ond Bat·
rmnt 10051. A further problem \\til he corong ""h
\\Car due to ""'din the OOg3'"'· Boiler\ "<!Old aho
1\a'e l<l he modohed to operate \\ith
IS<; mol\turc.
Ng"'"' at tO to
Solnr dr)ing. Solar energ) hn' hecn u.ed for
the drying of beet pulp in Cnhfomoa nnd other ngricullurol products (e.g. coffee). and ;, an opuon
for ba~.l\\C. A pil<ll plant '""in the Domonocan Repul>hc 'ho\\ed a reduction on m<ll,ture I rom SO'<
to 36-1-1 <;, but 11 ·~ 1101ed that on a larger .,.;ale the
tC\Uih can \at) coru.idernbl) Iran Jcr l'r'<'l ct al.
19981. It could he an interNinJ! option '" the ri~ht
can.:um,tJn\..~'
26.4
:I
,\ hne fmcllon of OO£a>\C t<ml(J b~acollo 1>
u\ed ._, a hlter :ud to the filter <t311t>n It " gcncr·
311) con..-eded that ltne " better than '"tl.lr\C lxlga.:illo: a e<>mm<lnl) 3CCqlled quaht) cntenon " that
at lea'l K5 '• of the material ,houJd P"" through a
20 mc'h '<recn (0.83 mm open•ngl. Cullm ( 19671
recommended that 80-90 '.f <houhJ P"" through a
14 mc'h ,crcen ( 1.19 mml nnd nm more tlmn S <;
'hould P"" through a 36 me'h .ereen Uom~<t
!1984! ha\ '"'"'n that the fincr lxlgacolln ornpro'·"
filtrate ljUaht) but ~ not impro'c the rate of ltltrauon The t>agacillo '>hould he lrec Ill l;>ng tiber\ .
"hoch do 0<>1 help achi.-e good filtrauoo
Baga.ollo p.utocle\ foliO\\ a log·oormal \IIC dov
tribuuon The t)pocal range of p.utoclc
encoon-
701
60
~
50
!
20
t
J
.,.
40
30
101
:!I
0 I
'"e'
02
03 0405
Partldt loaM tn mm
ll~urt
Bagacillo collection
26.6: s"~ dosmbuoon ol "''""'lin
I0
zo
tcred " 'h<)Wil 111 Figure ::!6.6.
B••sncillo i> ..epuroted from the b.'l'"'"' produced by performed screens in the b<Jttoon deck of a
bagn'"' carrier. or b) pneumatic ..epnrauon throu11~
lou\er or perforated \CI"eeO,, or by \UCkutg hoe par·
licit, out uf a bal;3'\e \tream.