32

Drying of Peat and Biofuels

Roland Wimmerstedt

CONTENTS

32.1

Introduction ......................................................................................................................................... 743

32.2

Requirements on Moisture Content of Fuels....................................................................................... 744

32.3

Thermodynamics of Fuel Drying ......................................................................................................... 745

32.4

Flue-Gas Dryers................................................................................................................................... 747

32.5

Steam Dryers........................................................................................................................................ 748

32.6

Environmental Aspects ........................................................................................................................ 750

32.7

Operation Experiences ......................................................................................................................... 751

32.8

Equilibria and Kinetics ........................................................................................................................ 751

References ...................................................................................................................................................... 752

32.1 INTRODUCTION

The use of biomass and peat, for both the industrial
sector and the district heating, has greatly increased
since the first oil crisis in 1973. The biomass utilized
so far is wood and agricultural wastes, including
bark, straw, and bagasse. Biomass grown especially
for fuel purposes is still a technique in the experimen-
tal state. The use of peat as a fuel is limited to coun-
tries with domestic resources, such as the former
Soviet Union, Canada, the United Kingdom, Ireland,
Finland, and Sweden.

The moisture content of the fuel is an essential

combustion parameter. Peat produced directly from
the bog contains 80 to 95% water (counted on the
total mass), which is reduced to about 50% through
natural drying on the bog during summer. Biomater-
ials collected in forests typically have a water content
around 50%, which is higher in fall and winter and
lower during spring and summer.

The chemical composition of biomass and peat

differs considerably from those of coal and oil mainly
by the high content of oxygen. The content of volatile
material is high. The combustion of biomass and peat
can accordingly be divided into a drying stage (evap-
oration of water), driving off the volatiles that burn in
the gas phase, and, finally, burning of the residue
(charcoal). The moisture content of the fuel thus
influences the drying period and, if the fuel contains
water, part of the boiler system must be used to dry
the fuel before burning. In Table 32.1, some typical
figures for the combustion of a wood fuel containing

different amounts of water could be found. The table
is based on a constant flue-gas temperature of 1508C
and an entering air temperature of 408C. The calcu-
lations are based on 1 kg of dry substance (DS).

As can be seen from the Table 32.1, the amount

of flue gases drastically increases with increasing

TABLE 32.1
Combustion Parameters for Burning of a Moist
Wood Fuel

Moisture Content (%)

65

50

15

Water amount (kg/kg)

1.9

1.0

0.2

Excess air level

(anticipated)

1.6

1.4

1.2

Higher calorific

value (MJ/kg)

20.6

20.6

20.6

Lower calorific

value (MJ/kg)

14.4

16.5

18.6

Flue-gas volume

(1 bar, 08C) (m

3

/kg)

10.3

8.8

6.2

Flue-gas loss (sensible

heat) (MJ/kg)

2.1

1.8

1.3

Efficiency based on

higher value

0.60

0.71

0.84

Efficiency based on

lower value

0.85

0.89

0.93

Adiabatic combustion

temperature (8C)

900.0

1200.0

1800.0

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2006 by Taylor & Francis Group, LLC.

moisture co ntent. Thi s is due to the evaporat ed wat er
and the need for high er levels of ex cess air at high er
fuel mois ture. The antic ipated value of 60% at 6 5%
moisture content is conserva tive, whereas 20% for
almost dr y fuel can be regarde d as normal in practi ce.
The high er amo unt of flue gases means a lower tem-
peratur e in the boiler , as reflect ed in the adiabat ic
combust ion tempe rature.

In the case of 65% moisture content, the adiabatic

combustion temperature is so low that preheating of
the combustion air would be necessary to ensure
complete combustion. The combined influence of
increased flue-gas volumes and decreased temperature
level means that a larger and more expensive boiler
is required when firing a fuel with higher moisture
content.

The boiler e fficiency can be defined in two differ-

ent ways . In Englis h-speaki ng co untries, the usu al
way is to con sider the latent heat of con densatio n of
the water vapor in the flue gases as boiler losse s. The
boiler efficien cy is thus de fined as the quotient be -
tween the utilized heat an d the gross (highe r) calorific
value. In other c ountries, such as Germ any and the
Nordi c countri es, the vapo r losses are consider ed
when defin ing the heat value of the fuel. The pricing
of a biofuel or peat is thus made on the basis of its net
(lower) calorific value and the boiler efficiency is de-
fined as the quotient between the utilized heat and the
net calorific value.

Finally, it should be borne in mind that the effi-

ciency in

Tabl e 32.1

neithe r includes the increa sed

amount of unburned material in the ash from the
moist fuel nor the increased radiation losses that re-
sult from the larger boiler required for the combus-
tion of moist fuel.

32.2 REQUIREMENTS ON MOISTURE

CONTENT OF FUELS

It is not obvious that a moist fuel must be dried before
firing. On the contrary, this is considered uneconom-
ical in many cases. There are, however, cases where
drying is necessary for different reasons, and demands
on the fuel moisture content must be raised.

One such case is the gasification of fuels. To ensure

a high gas quality, the moisture content of a fuel to
be used in a gasification process should not exceed 10
to 15%. Gasification of biomass and peat has hitherto
been limited to limekilns in the pulp industry, but
commercial-scale gasifiers for district-heating systems
with cogeneration of electricity in combined cycle
processes are gradually introduced on the market.

Another application where fuel drying is necessary

is in the manufacture of fuel as powders, pellets, or

briquettes in ‘‘fuel factories,’’ for which the moisture
content must not exceed 10 to 20%. The need for
drying is partly due to technological reasons in the
manufacturing process and partly due to storing and
transport reasons. Storing high moisture–content
wood chips can support large populations of fungi,
many of which cause allergic reactions in humans [1].
Microbiological processes are also assumed to be the
cause of spontaneous ignition in large wood-fuel
piles. To avoid significant microbiological degrad-
ation during storage, the material should be dried to
a moisture content of 20 to 25% [2].

The degree of drying required in direct combus-

tion varies with the type of boiler used. If the fuel is to
be used in small scale, 1 to 5 MW, it is preferentially
fired as dried material in grate ovens with effective
cooling of the grate. Grate-fired boilers of traditional
type are dominating in the 5- to 20-MW area. If they
are designed for moist fuels, the first part of the grate
is designed as a band dryer. Flue gases are recycled to
the first part of the grate, where they together with the
radiating walls deliver the heat necessary for drying.
If this type of boiler is fired with dry fuels, a cooling of
the grate by the combustion air must be foreseen. At
higher moisture contents the capacity is decreased;
hence, if capacity is a crucial factor, a booster effect
can be achieved by installing a predryer before the
boiler.

In large-scale applications, fluidized-bed boilers

are now dominant on the market. Both bubbling
fluidized beds (BFBs) and circulating fluidized beds
(CFBs) are used for biofuel firing. BFBs were origin-
ally limited to a rather narrow specification of fuels
but now flue-gas recycling is used in both types of
boilers. Especially, CFBs are considered to be toler-
ant to varying fuel qualities and, if this type of boiler
is chosen, there would be no need to combine it with a
dryer [3,4].

Suspension firing requires pulverized fuels con-

taining less than 15% moisture to ensure complete
combustion in the furnace. Dry fuel also means a
high combustion temperature, which in turn means
higher capacity in an existing boiler and a smaller and
less expensive new boiler. This technique may be used
when an oil-fired boiler is used to convert biofuels. It
has also been used as a substitute for oil burning in
limekilns in pulp mills, where this technique competes
with gasification of the fuel. A combination of a dryer
and a suspension-fired boiler may be seen as an alter-
native to installing a fluid-bed boiler.

In conclusion, there are cases where a fuel must be

dried and a choice has to be made between different
drying methods. In many cases, however, the primary
choice is between the firing of wet fuel and the firing
of dried fuel (i.e., between drying and not drying).

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2006 by Taylor & Francis Group, LLC.

32.3 THERMODYNAMICS OF FUEL DRYING

Biofuel s and peat can be dried acco rding to diff erent
thermo dynami c princi ples. The most common
method is the use of flue gases. Anothe r is the use of
superheat ed steam. Finall y, double-ef fect drying will
be discus sed.

Fig ure 32.1 shows the princi ples of flue-gas dr ying

in co mbination with combust ion in a boiler [5]. Fro m
the figure it can be seen that afte r the boi ler the flue
gases are taken through a fuel dryer in whi ch all the
fuel from the boiler is dried. The degree of drying is
determ ined by the flue gases entering the dryer. By far
the most c ommon method is to dry the fuel using the
sensib le heat of the flue gases after the eco nomizer
down to a drye r exha ust tempe ratur e of 100 8 C. If the
fuel has a moisture content of 60% (1.5 kg water/ 1 kg
dry material ) and the flue -gas tempe ratur e of 170 8 C,
the fuel can be dried to abo ut 55%. A co nsider able
part of the avail able en ergy is used to heat the fuel
with all its water to the wet bulb tempe ratur e of the
flue gas.

A complet e pictur e of the en ergy stat us for a plant

as descri bed in Figu re 32.1 can be attained by com-
bining dryer ca lculations with a combust ion an alysis.
Results from such a calculati on a re shown in Fig-
ure 32.2, based on the assum ptions from Figure 32.1
and an origi nal exit flue-gas tempe rature of 170 8 C.
Figure 32.2 displ ays the useful en ergy (i.e., energy
that can be extra cted in the boiler ) coun ted pe r wet
basis for a fuel originall y contai ning 60% mois ture.
The point marks the en ergy statu s when there is no
dryer. To achieve a high dry ness, the flue gas must be
taken from the boiler at tempe ratures seen on the
lower hor izonta l axis. If, for exampl e, a dryness of
0.3 is desired, the flue -gas tempe ratur e into the dr yer
must be abo ut 410 8 C.

As can be seen, the prim ary impr ovement takes

place when the ex haust tempe ratur e is low ered to
100 8 C. Further drying leads to margi nal savings
only, due to the decreas e in the amount of exce ss
air. The figu re also shows the decreas e in flue -gas
volume (1 bar, 08 C) per MWh useful energy in the
boiler. A numb er of papers discus sing flue-gas drying
of biofuel s have been publis hed recent ly [6–9] .

Figure 32.3 illustrates the principle of steam drying.

The steam necessary for the dryer is taken from the
boiler and the evaporated water from the dryer is recov-
ered as low-pressure steam. The economy of the dryer is
dependent on whether or not the low-pressure steam
can be used in the process.

Figure 32.4

corresponds

Flue gases

Dryer

T

wet

+

108C

Boiler

Steam

Air

Fuel

1008C

58C

58C

FIGURE 32.1 Principle of a flue-gas dryer in combination
with a boiler.

7500

kJ/kg

m

3

/MWh

7100

6700

6300

5900

5500

0

0.1

0.2

500

400

Moisture content

Temperature of flue gases into dryer (

°C)

300

200

0.3

0.4

0.5

0.6

0.7

3000

2600

2200

1800

1400

1000

FIGURE 32.2 Useful energy and flue-gas volume when dry-
ing to different moisture contents in a flue-gas dryer (inlet
fuel with 60% moisture content).

Dryer

Fuel

58C

1508C

58C

Air

Boiler

Flue gases

1708C

5 bar

5 bar

40 bar

Steam

FIGURE 32.3 Principle of superheated steam dryer in com-
bination with a boiler.

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2006 by Taylor & Francis Group, LLC.

to

Figure 32.2

. The underlying assumptions are:

pressure in the dryer, 5 bar; the produced steam can
be used down to condensate at 1008C; and the fuel
entering the boiler has a temperature of 1508C. Radi-
ation and leakage losses are estimated to be 5% of the
total drying-energy consumption. It can be seen that
this type of drying leads to considerable energy savings.
The flue-gas volumes are reduced in the same way as
for flue-gas drying.

In situatio ns in which there is no use of the low -

pressur e steam, steam drying can be combined wi th
mechani cal recompr essio n of the evaporat ed steam .
A compara tive study betw een dryers worki ng at over-
pressur e and atmos pheric pressure ha s be en done

by Wimmerst edt and Hall stro¨ m [5]. The e nergy con -
sumpt ion is low er at reduced pressur e, but the capit al
cost for the dryer as well as the compres sor is lower in
the elevat ed-pres sure case. To obtain a low energy
consumpt ion at elevated pre ssure, the conden sate
leaving the plant must be cooled by he at exchange
to the mois t mate rial so that the tempe ratur e of this is
increa sed. This kind of cou pling presupp oses that the
ratio between elect ricity and fuel price is low.

The third possibi lity to be discus sed he re, mainly

on grounds of history, is the double-effect dryer. An
example of such a dryer is the Peco dryer developed in
the 1930s and it has been used extensively for the
drying of milled peat. The principle can be seen in
Figure 32.5.

The moist peat (50 to 60%) together with ambient

air is fed to apparatus II B. The heating medium is
water at about 658C. The air is exhausted from
apparatus II A. The peat is then brought to dryer
stage I consisting of three drying towers. The heating
medium is steam at a pressure of about 4 atm. Air is
recirculated in this effect and the humidity is con-
densed in the heat exchanger, the cooling water of
which is used as the heating medium in dryer stage II.
The specific steam consumption of the Peco dryer is
0.65–0.70 kg steam/kg evaporated water.

Peco dryers are normally combined with a special

peat-fired boiler and a back-pressure turbine provid-
ing steam and electricity to the dryer and also electri-
city to the whole briquetting plant. A Peco dryer was
built in Ireland as late as 1982. The high capital cost
associated with the dryer and the need for a boiler
and electricity-generating plant have, however, led to
the Irish peat industry reassessing their choice in
favor of rotary-type flue-gas dryers. The flue gases
are produced in special flue-gas generators [6].

7500

kJ/kg

7100

6700

6300

5900

5500

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

3000

m

3

/MWh

2600

2200

1800

1400

1000

Moisture content

FIGURE 32.4 Useful energy and flue-gas volume when dry-
ing to different moisture contents in a steam dryer (inlet fuel
with 60% moisture content).

Dryer stage II

Dryer stage I

Wet scrubber

Condenser

Sepa-
rator

Dry
peat

Wet

peat

II B

I C

I B

I A

Air

II A

FIGURE 32.5 Two-stage Peco dryer.

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2006 by Taylor & Francis Group, LLC.

32.4 FLUE-GAS DRYERS

Rotary drum dryers have been used for a long time in
the drying of biofuels. Typical materials that have been
dried are hog fuels, sawdust, and bagasse. Sawdust is
also dried for the particleboard industry. Flue gases
and fuel normally pass along the drum in a cocurrent
flow. This means that the hot flue gases contact the
moist material, which reduces fire hazards and emis-
sion of organic compounds. The drum rotates at a
speed of 2 to 8 rpm. Factory-assembled units have
diameters up to 4.5 m and lengths up to 10 m.

The dryers may be of the open-center type or the

center-fill type. The former are equipped with lifting
vanes or flights on the inside shell to carry fuel up the
sides and disperse in the hot gas stream. In the open
dryers the diameter is limited to about 2.5 m, and
these provide a cheap and reliable system with low
horsepower requirement. In the center-fill system, an
internal structure helps to pneumatically convey the
particles within the dryer. The center-fill dryer pro-
cesses wet fuel according to its density. Particles of a
particular moisture content are automatically main-
tained at a fixed bed temperature within a section of
multizoned rotating cylinder. As moisture is evapor-
ated from the particle it moves further along the
dryer. This means that the dryer can handle particles
of different sizes that need a broad spectrum of resi-
dence times, from a few minutes up to 1 h.

Rotary dryers are thus very flexible and come

to extensive use in the particleboard industries and
fuel factories. Another advantage is that the flue-gas

velocities through the dryer can be optimized to
achieve a low fan horsepower requirement. To obtain
a good thermal efficiency, part of the exiting gas is
usually recycled over the dryer. This ensures a high
moisture content of the exiting gas, which also is
essential when the waste heat is utilized in, for in-
stance, district-heating systems [3]. In Figure 32.6, a
typical biofuel-drying system utilizing a rotary dryer
is shown.

Cascade dryers came to extensive use, especially in

the Nordic countries, during the 1970s and 1980s.
Moist material is fed into the dryer with a flue-gas
stream with high velocity. In the drying chamber, the
flue gases cause the fuel to whirl around in a cascad-
ing bed. To maintain the cascade during partial load,
flue gases are recirculated. The fine particles leave the
dryer with the exiting gas and are separated in a
cyclone. Coarse material is removed from the drying
chamber by an overflow. The residence time in the
dryer can amount from 1 to 2 min.

An advantage of the cascade dryer over the rotary

drum dryer is the reduced space requirement, which
may be critical when installing a dryer in combination
with an existing boiler. Applications have been
mainly as predryers in combination with wood-fuel
boilers in saw and pulp mills.

Pneumatic conveying dryers or flash dryers have

been used for a long time in the drying of milled peat,
for instance, in the former Soviet Union and Finland.
These units are directly fired and the peat is conveyed
by the flue-gas flow through two or more towers, or
open pipes, in total about 30-m long. The gas velocity is

Boiler

Stack

Dryer

Mog

Fuel

Storage

Auxilliary burner

if required

Fuel infeed

FIGURE 32.6 Biomass-fired system utilizing a rotary dryer. (From Rader Companies Inc.)

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2006 by Taylor & Francis Group, LLC.

15 to 35 m/s. Some of these units also include a hammer
mill and a short tower. The flue-gas flow is restricted so
that only small, dry particles can be conveyed.

Pneum atic conveying dr yers have been developed

for simulta neou s pulverizati on and drying of bark or
hog fuel, for inst ance. The resul ting powder with a
moisture content of 10 to 15% can be used in a susp en-
sion-fi red boiler or a limek iln in a pulp mill. In Fig-
ure 32.7, a bark dryer that delivers pulveriz ed fuel to a
limekil n is shown. Wet biomas s is metered into the
system via a rotary valve. It is co nveyed by the flue
gases, which in this case are de livered from a recov ery
boiler, up and down in two drying tow ers. Pred rying of
the material takes place he re. This predryi ng redu ces
the surfa ce moisture and the pul verizing process in
the hamme r mil l is thus facilitated. In the c lassifier,
coarse mate rial is br ought back to the ha mmer mil l
where extens ive drying takes place as ne w, hot flue
gases are add ed here. The material passes the milling
circuit until it attains a particle size that allow s it to
pass the classifier. After this , the mate rial passes
through a final flash dryer and then dry fuel is separ-
ated from the flue g ases in a cyclone.

The power demand for the mill and fan is reported

to be 60 to 120 kWh/t DS [5] and the average residence
time is about 30 s. Advantages of the system are its
compactness, versatility (can be used for different

fuels), and low manpower requirements. A dis-
advantage is the high power requirement. Other flue-
gas dryers that have been commercially used for peat
drying are fluid-bed dryers and a combination of a
fluid-bed and flash dryer (whirly bed dryer).

32.5 STEAM DRYERS

Steam drying of peat and biofuel s reached a break -
through during the 1980 s. The co mmercial -scale ap -
plications rep orted so far are either of the indir ectly
heated flash type or of the direct ly heated fluid-bed
type.

Figure 32.8

shows a flow scheme of a flash dr yer

combined with a suspen sion-fired boiler in a district -
heatin g ap plication. Moist hog fuel is disi ntegrated
and fed into a syst em wi th circul ating low -pressure
steam (2 to 5 ba r). The material is co nveyed with the
steam through the dryer. The drying towers are es-
sentiall y tubular he at exchangers wi th the drying ma-
terial and low-pres sure steam insi de tubes of 75- to
150-mm diame ter. Heat is trans mitted from the out-
side of the tubes through con densing steam or from
the cooling of hot water to the superhea ted transp ort
steam. The temperature driving force is of the order
of 408C or higher. The amount of transport steam
that corresponds to the change in moisture content of

Bark silo
for boiler

Wet Bark silo

Steam
Drying
cyclones

System fan

Steam

Shut-off
valves

Flue gas from
recovery boiler

Finishing mill

Cyclone
bag filter

Steam

Pulverizer

Pulverized
fuel silo

Operated from
Boiler control
room

Operated from
chemical
department

Limekiln

Fan

Blower

Classifier

1

2

FIGURE 32.7 Limekiln fired by pulverized biomass produced in a BIOMASSTER. (From ABB-Fla¨kt.)

ß

2006 by Taylor & Francis Group, LLC.

the fuel is extracted at the system pressure. The steam
is separated from the material in a cyclone and then
transported to a condenser where water for district-
heating purposes is warmed. The product condensate
is further cooled and then led to a sewer. The steam
velocity in the tubes is of the order of 30 m/s. As the
steam has a higher density than the flue gases due to
higher pressure, the power demand for the fan is
normally higher than for a flash dryer of the flue-gas
type. The electricity consumption for the disintegra-
tion and transportation of a hog fuel that is dried
from 60 to 10% moisture content is quoted as being
140 kWh/t DS [10]. Further development of this type
of dryer includes an enlarged presuperheater and de-
creased number of drying towers, which contributes
to a decreased electricity consumption [3].

During the 1990s, two steam dryers of the fluid-

bed type were taken into operation in Sweden. These
were the same type of dryers that have successfully
been developed by NIRO for drying of sugar beet
pulp [11]. The principle of this dryer is shown in
Figure 32.9. The dryer is cylindrical with drying cells
around the periphery. The drying steam is circulated
up through the fluid bed and down again through a
superheater installed in the center of the vessel. The
residence time varies from a few seconds for small
particles that are entrained in the drying steam to
about 10 min for coarse material that is transported
in the bed.

The capacity of steam dryers is determined by the

heat transport, and thus by the driving temperature
force between the condensing heating steam (which is

the normal heating medium) and the circulating
steam. This means that the pressure of the condensing
steam is crucial to the cost of the dryer. For example,
the capacity of a dryer with an operating pressure of
4 to 5 bar is doubled when the pressure of the heating

Fuel

Silo

Disc
refiner

Drying stages

PIC

Cyclone

Steam

boiler

Primary heat
exchanger

Secondary
heat
exchanger

District
heating

Preheater

Fan

Rotary
valve
feeder

Feed
water
tank

Preheating
air

District
heating

708C

1208C

To
sewer

PIC

TRC

Rotary
valve
feeder

Fan

FIGURE 32.8 Steam dryer for biomass installed in a district-heating plant. (From MoDo-Chemetics AB.)

1. Material feed

2. Stationary blades

3. Cylinder

4. Side cyclone

5. Ejector

6. Blading

7. Superheater

8. Fan

9. Steam bleed-off

10. Material exit

6

9

3

7

8

4

2

5

1

10

FIGURE 32.9 Fluid-bed steam dryer of the NIRO-type.

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2006 by Taylor & Francis Group, LLC.

steam is raised from 12 to 25 bar. This is true for both
flash and fluid-bed dryers. The conclusion is that the
price information for a steam dryer must always be
related to both the capacity and the heating steam
pressure.

When steam drying is used in situations where

there is no demand for waste energy in the form of
low-pressure steam, mechanical vapor recompression
(MVR) can be applied as mentioned earlier. Such a
plant has been in operation at Ha¨rjedalen, Sweden,
since 1988, where peat is dried in a flash-steam dryer.
The production of briquettes is based on air-dried
peat with 60% moisture and amounts to 300,000 t
DS per year with 10% moisture. The dryer unit con-
sists of two identical lines with a capacity of 20 t of
DS. The dryer in each line has five heat exchangers in
series with a tube length of 20 m. The total heat-
transfer area is 2700 m

2

. Turbo-type compressors

are used with a compression ratio of 1:4.7 from 3 to
14 bar; the system pressure is 3.6 bar giving a tem-
perature driving force of 608C. The total electricity
consumption is 270 kWh/t DS [12].

32.6 ENVIRONMENTAL ASPECTS

Increased use of biofuels and peat, which can be
facilitated by efficient drying methods, means in itself
a mitigation of the greenhouse effect. A further ad-
vantage is that no emission of sulfur oxides takes
place. The emission of NO

x

is strongly dependent on

the combustion temperature. A dried fuel would
hence mean increased emissions of NO

x

. This may

be partly counterbalanced by the decreased amount
of excess air and better possibilities for control of the
combustion process achievable with a dried fuel. The
low temperature at the combustion of very moist fuels
might lead to increased amounts of unburned hydro-
carbons in the flue gases.

Many sources report on fewer particulate emis-

sions when burning dry fuels. This is especially true
for old grate-fired boilers. This is again achieved by
the higher combustion temperature, which enables
the smaller particles to burn faster and more com-
pletely. Drying of bagasse prior to firing has been
found to reduce particulate emissions from boilers
by roughly one half [7].

Drying of biofuels and peat, however, also leads to

emissions of organic compounds. In principle, all vola-
tile organic compounds in the material to be dried
might vaporize during drying. The amount of volatile
material is 60 to 80% in wood and 50 to 70% in peat. In
most cases, however, drying takes place at such condi-
tions where only the most volatile compounds can
vaporize; but, nevertheless, biofuel drying is contrib-
uting to the release of volatile organic compounds

(VOC). Of most concern is the emission of terpenes.
Terpenes are hydrocarbons present in conifer oleo-
resins. They could also be found in many hardwoods,
especially of tropic origin. Natural emissions take
place everywhere in our forests and the quantity re-
leased naturally is much higher than the anthropo-
genic emissions. The problem with the latter is the
very high local concentrations that can be achieved.
Terpenes have a high reactivity in the atmosphere and
may contribute to the formation of ground-level
ozone.

In the last decade, there has been quite an extensive

research in this area and there is now a good under-
standing of the problem [13–16]. The emitted com-
pounds, at normal drying temperatures, are volatile
extractive compounds, mainly terpenes, carbolic acids,
and light aldehydes and alcohols. The emitted amounts
are principally controlled by the material temperature,
and also by the type of material. Sawdust originating
from pine has, for instance, a much higher terpene
content compared with sawdust from spruce. The stor-
ing time of the material after harvesting is important for
finely divided materials like sawdust and flakes as the
terpenes are easily released from those materials. It is
practically impossible to dry a material without releas-
ing about 80% of the original terpenes. Experiments
with batch fluid-bed drying of sawdust showed a very
typical behavior. The terpene release showed one dom-
inating peak directly after the first air contact, during
the rest of the constant rate drying the release was on a
low, stable level. A second peak, much smaller than the
first, could be seen during the final part of the drying
with increasing material temperature [13].

The nature of the emission is, as mentioned, con-

trolled by the material temperature. This temperature
is normally below 1008C for flue-gas dryers whereas a
typical value for a steam dryer is 1408C. On the other
hand, the gas temperature is much higher in flue-gas
dryers. Small overdried particles might therefore be
exposed to very high temperatures. The occurrence of
blue haze in rotary flue-gas dryers is a result of pyr-
olysis of small particles.

Most of the VOCs released during drying in a

steam dryer could be found in the condensate; the
flow of noncondensibles is rather small and is nor-
mally taken to incineration in the boiler. Also, the
effluent from a traditional flue-gas dryer could be
taken to a condenser for heat recovery and gas clean-
ing. The remaining inert-gas flow is, however, much
higher in this case. If further cleaning is required by
the national legislation, this could be achieved by
using a wet electrostatic precipitator followed by a
regenerative thermal oxidation unit. Also closed-loop
drying systems are being developed [3]. In these sys-
tems, the circulating dryer gas in a rotary dryer is

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2006 by Taylor & Francis Group, LLC.

indirectly heated by hot flue gases from the boiler in a
heat exchanger. The bleed from the circulation is
incinerated in the boiler. The circulating gas stream
has a very high dew point and heat could be recovered
from the bleed via a condenser. This type of dryer is
essentially a steam dryer working at atmospheric
pressure and thereby avoiding problems associated
with feeding the materials into a pressurized system.

32.7 OPERATION EXPERIENCES

In Sweden, a number of plants for drying of peat and
biofuels are in operation. In a recent study [17], oper-
ation experiences from eight plants were collected and
evaluated. Four of the plants had steam dryers and
the other four had flue-gas dryers. The total availabil-
ity of the flue-gas dryers was typically 80% or higher
whereas the corresponding values for steam dryers
were 35 to 90% with an average around 70%.
The highest value was for the peat dryer mentioned
earlier, which was installed already in 1986, whereas
the recently installed dryers had a very low availabil-
ity. Common problems of all dryers, steam dryers as
well as flue-gas dryers, were corrosion and erosion.
The condensate in biofuel dryers is acidic and corro-
sive. All parts of the dryers, where condensation can
take place, must be either lined with or manufactured
in stainless steel materials. In the original design,
carbon steel had been the normal construction mater-
ial. The drying material is also erosive due to sand
particles and other inorganic materials. Especially,
bends and pipes with high flow velocities are easily
eroded and in one case lining with wear-protection
plates is part of the regular maintenance.

Common problems with the steam dryers are

related to the feeding-in and feeding-out devices.
Original cell feeders did not work satisfactorily and
had been substituted by plug-screw feeders. In some
cases, these must also be continuously exchanged due
to wearing.

The dried product is highly inflammable and fires

could occasionally take place, and in some cases
water nozzles have been installed as a fire extin-
guisher. In steam dryers it is well over 1008C during
drying, and opening the dryers with air supply must
be avoided until the material has cooled off.

The general impression from the experiences is

that maintenance must be carefully planned for
these type of dryers. A tight dialogue with the manu-
facturer and taking advantage of experiences from
earlier installations seem highly recommendable.

32.8 EQUILIBRIA AND KINETICS

Equilibrium moisture contents of wood have been
presented in a number of published papers. The inter-
est has been caused by the drying of timber. Wood is a
moderately hygroscopic material at moisture contents
below about 0.3, at which the free water has been
removed and the residue can be regarded as cell-
bound water. During the last few decades, consider-
able interest has been devoted to the drying of timber
in superheated steam and a number of papers on
equilibrium moisture content at high temperatures
have been published [18–20]. In Figure 32.10, a typ-
ical set of data are presented [20]. Figure 32.10a
shows the original way of presentation and Figure
32.10b shows the presentation for which the data
has been recalculated and presented as isobars. As is

100

0.20

0.20

0.10

Moisture content

Moisture content

0.10

1.0

1.3 1.6 2.0

Saturated steam line

2.5 bar

120

140

0.5

0

(a)

(b)

Pressure

Saturation pressure at current temperature

1.0

2.5 bar

1.6

1.0

Temperature (8C)

160

FIGURE 32.10 Equilibrium moisture content of wood at different temperatures and pressures.

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2006 by Taylor & Francis Group, LLC.

evident, the latter way of presentation gives a much
smaller parameter influence.

A number of equilibrium moisture data have been

found in the literature for peat also. Larsson and
Wimmerstedt [21] presented a data for peats of differ-
ent origin and humification. Results from the investi-
gation are presented in Figure 32.11. The sorption and
desorption isotherms at 308C and for different peats
are shown in Figure 32.11a. As can be seen, the simi-
larity between different peats is pronounced. As can be
expected from a biological material, a considerable
hysteresis effect is demonstrated. Figure 32.11b pre-
sents the ‘‘average’’ isotherms valid for desorption of
different peats. A pronounced temperature depend-
ence over the whole relative-humidity range can be
seen. The shapes of the isotherms are similar to those
of wood and imply that most of the water is not
strongly bound. It can be concluded that the hygro-
scopicity is of rather limited importance for the drying
process.

Rather few papers deal with the kinetics of biofuel

drying and most designs seem to be ‘‘experience-
based.’’ Bagasse is reported to be easily dewatered
with exit temperatures for commercial rotary dryers
approaching the wet bulb temperature [7]. A study of
drying rates of milled peats [21] in a fluid-bed bench-
scale dryer showed no influence on the origin of the
peat. On the other hand, different size fractions of the
same peat gave quite different results. The intraparti-
cular resistance is pronounced and, of course, con-
trolled by the particle size.

Similar results are reported from the drying of

bark and peat in superheated steam in a pilot-plant
pneumatic conveying dryer [22]. The results are pre-
sented as a convective apparent heat-transfer coeffi-

cient,

defined

with

the

assumption

that

the

temperature of the particle surface coincides with
the saturation temperature of the transport steam.
This transfer coefficient shows a clear dependence
on the moisture content of the particles and the par-
ticle sizes. Fyhr [23] presented a model for a pneu-
matic conveying steam dryer. The dryer model
consists of two submodels, one for the single particle
and the other for the hydrodynamics of gas and par-
ticles in the dryer.

Hermansson et al. [24] reported the results from

bench-scale drying of bark and wood chips in super-
heated steam in a fixed bed. The results are presented as
a thermal efficiency defined as the ratio between the
time-averaged steam temperature decrease over the
bed and the maximum obtainable temperature de-
crease. This efficiency proved to be almost independent
of pressure and temperature. When the mass load of the
bed exceeded 30 kg/m

2

the thermal efficiency was above

85%, even at mass fluxes as high as 0.6 kg/m

2

/s.

REFERENCES

1. JG Riley, CS Drechsel, Drying and storage of woody

biomass fuels, American Society of Agricultural Engin-
eers 1983 Winter Meeting, Paper 83-3560, pp. 1–19,
1983.

2. O Gisterud, Drying and storing of comminuted wood

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3. R Wimmerstedt, B Linde, Assessment of technique and

economy of biofuel drying, Stockholm: Va¨rmeforsk
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4. R Wimmerstedt, Recent advances in biofuel drying,

Chemical Engineering and Processing 38: 441–447, 1999.

5. R Wimmerstedt, A Hallstro¨m, Drying of peat and

biofuels. Techniques, economy and development needs,

0

0

(a)

(b)

20

40

Relative humidity (%)

60

80

100

0.30

Sphagnum peat H3
Sphagnum peat H5
Sphagnum peat H7
Swamp peat H4

0.25

0.20

0.15

0.10

0.05

0

0

20

40

Relative humidity (%)

Moisture content (kg/kg)

60

80

100

0.30

308C

60

8C

908C

0.25

0.20

0.15

0.10

0.05

Moisture content (kg

/kg)

x

x

x

x

x

x

x

x

x

x

x

x

x

x

x

x

x

x

x

x

x

x

x

x

x

FIGURE 32.11 Equilibrium moisture content of peat.

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2006 by Taylor & Francis Group, LLC.

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2006 by Taylor & Francis Group, LLC.