EPA" 600/R-98-090
July 1998
B10MASS DRYING TECHNOLOGIES
by
Eila Salomaa
(U.S. EPA Visiting Scientist)
EPA Project Officer: Robert H. Borgwardt
Air Pollution Prevention and Control Division
National Risk Management Research Laboratory
Research Triangle Park, North Carolina 27711
Prepared for:
U.S. Environmental Protection Agency
Office of Research and Development
Washington, DC 20460
-------
TECHNICAL REPORT DATA
-------
NOTICE'
This document has been reviewed in accordance with
U.S. Environmental Protection Agency policy and
approved for publication. Mention of trade names
or commercial products does not constitute endorse-
ment or recommendation for use.
PROTECTED UNDER INTERNATIONAL COPYRIGHT
ALL RIGHTS RESERVED.
NATIONAL TECHNICAL INFORMATION SERVICE
U.S. DEPARTMENT OF COMMERCE
Reproduced from
best available copy.
~
-------
FOREWORD
The U. S. Environmental Protection Agency is charged by Congress with pro-
tecting the Nation's land, air, and water resources. Under a mandate of national
environmental laws, the Agency strives to formulate and implement actions lead-
ing to a compatible balance between human activities and the ability of natural
systems to support and nurture life. To meet this mandate, EPA's research
program is providing data and technical support for solving environmental pro-
blems today and building a science knowledge base necessary to manage our eco-
logical resources wisely, understand how pollutants affect our health, and pre-
vent or reduce environmental risks in the future.
The National Risk Management Research Laboratory is the Agency's center for
investigation of technological and management approaches for reducing risks
from threats to human health and the environment. The focus of the Laboratory's
research program is on methods for the prevention and control of pollution to air,
land, water, and subsurface resources; protection of water quality in public water
systems; remediation of contaminated sites and groundwater; and prevention and
control of indoor air pollution. The goal of this research effort is to catalyze
development and implementation of innovative, cost-effective environmental
technologies; develop scientific and engineering information needed by EPA to
support regulatory and policy decisions; and provide technical support and infor-
mation transfer to ensure effective implementation of environmental regulations
and strategies.
This publication has been produced as part of the Laboratory's strategic long-
term research plan. It is published and made available by EPA's Office of Re-
search and Development to assist the user community and to link researchers
with their clients.
E, Timothy Oppelt, Director
National Risk Management Research Laboratory
-------
ABSTRACT
The aim of the present study was to examine the available technologies for drying of biomass, and the
energy requirements of biomass dryers. The published studies on drying methods for biomass and various
types of dryers were reviewed, focusing mainly on methods using superheated steam as the drying medium.
The amount of steam required to provide the heat needed for drying biomass was examined at different
pressures and temperatures, calculated based on tabulated values of steam properties, and compared to
the reported values of various steam dryers.
Biomass fuels have usually a high initial moisture content. The traditionally used and most common method
of drying biofuel is in air, by using the sensible heat of the flue gases from a boiler. Another method is drying
in a superheated steam atmosphere. Compared to air or flue gas drying, lower net energy consumption and
reduced risk of fire or explosion are important advantages attained in steam drying. On the other hand, a
steam drying system is more complex and susceptible to leakage.
The most common flue gas or hot air dryer types used for biomass drying are rotary, cascade, and flash
dryers. The energy of the vaporized water cannot usually be recovered from the flue gases. The existing
commercial scale superheated steam dryer types for biomass fuels are fluidized bed and pneumatic
conveying flash dryers. Other types have been tested in pilot scale, or used for various related materials.
Steam dryers include both directly and indirectly heated types.
The heat requirement of indirectly steam heated steam dryers has been reported to be lower than that of
hot air dryers, and not far from the theoretical heat requirement for drying of biomass, as calculated in this
report. In steam drying it is possible to recover most of the energy spent for the drying process, which
reduces the net energy consumed.
The composition and amount of emissions released from biomass during drying depend on the drying
conditions applied, and on the type of biomass. Generally, a gas cleaning process is needed after a flue gas
dryer, and waste water treatment after a steam dryer.
This report was prepared during the period October 1,1997 to July 31,1998, while the author was a visiting
scientist at the Air Pollution Prevention and Control Division of NRMRL.
ii
-------
CONTENTS
ABSTRACT ii
LIST OF TABLES v
LIST OF FIGURES vl
ABBREVIATIONS AND SYMBOLS viii
1. INTRODUCTION 1
2. BIOMASS DRYING 2
2.1. Drying of solid particles 2
2.2. Equilibrium moisture 2
2.3. Methods of biomass drying 3
2.4. Principle of steam drying 5
2.5. Comparison of air and steam as drying medium 5
2.6. Drying rates in air and steam 6
3. DRYING TECHNOLOGY 8
3.1. Types of dryers for biofuel 8
3.2. Flue gas dryers 8
3.2.1. Rotary dryers 10
3.2.2. Cascade dryers 10
3.2.3. Pneumatic conveying or flash dryers 10
3.3. Multiple-effect dryers 10
3.4. Steam dryers 12
3.4.1. Types of steam dryers 12
3.4.2. Fluidized bed steam dryers 13
3.4.3. Flash or pneumatic conveying steam dryers 14
3.4.4. Other steam dryers 16
4. ENERGY ASPECTS OF DRYERS 18
4.1. Utilization of steam 18
4.2. Vapor recompression 19
iii
-------
4.3. Theoretical steam requirement for drying 21
4.3.1. Indirectly steam heated steam drying 22
4.3.2. Directly heated steam drying 25
4.4. Energy consumption of dryers 28
5. ENVIRONMENTAL BURDENS FROM DRYING OF BiOMASS 31
5.1. Environmental burdens from flue gas dryers 31
5.2. Environmental burdens from steam dryers 32
6. CONCLUSIONS 33
REFERENCES 34
APPENDIX A A-1
APPENDIX B B-1
iv
-------
LIST OF TABLES
Table Page
1. Electrical energy consumption of superheated steam drying installations using
mechanical vapor recompression. Compiled after data presented by Aschard
etai. (1992), Lurgi (1994), and Wimmerstedt & Hager (1996) 20
A-1. Summary of commercial and pilot scale steam dryers used for drying of biomass
or biofuel. A-1
B-1, Energy consumption of steam dryers B-1
B-2. Energy consumption of air or flue gas dryers and multiple-effect dryers B-2
v
-------
LIST OF FIGURES
Figure Page
1. Principle of flue gas drying 4
2. Principle of superheated steam drying by (a) directly heated steam dryer and
(b) indirectly steam heated steam dryer 4
3. Examples of direct dryers: a) rotary dryer, b) conveyor dryer, c) cascade dryer,
d) fluidized bed dryer, e) flash dryer 9
4. Examples of indirectly heated dryers; a) fluidized bed dryer, b) rotary dryer,
c) flash dryer 9
5. Example of a multiple-effect superheated steam dryer which produces dry fuel of
three different temperatures 11
6. Niro fluidized bed steam dryer. 1} fan, 2) heat exchanger, 3) feeder, 4) outlet conveyor,
5) fixed vanes creating vortex flow, 6) cylinder, 7) cyclone for dust separation,
8) ejector discharge of dust, 9) vortex flow breaker, 10) high pressure steam inlet,
11) excess steam outlet. (Courtesy of Niro) 13
7. Exergy pneumatic conveying steam dryer. (Courtesy of Stork Engineering) 15
8 Flow diagram of the bed mixing dryer. (Courtesy of Imatran Voima) 17
9. Thermal energy needed for heating 1000 kg of wood containing 50 % moisture, and
vaporizing the water to 10 % final moisture content, as a function of the dryer pressure. 22
10. Volume of heating steam needed at three different pressures and temperatures, as a
function of the dryer pressure, when 1000 kg of wood containing 50 % moisture is dried
to 10 % final moisture content in an indirectly steam heated steam dryer 23
vi
-------
11. Amount of heating steam needed at two different dryer pressures and heating steam
temperatures, as a function of the heating steam pressure, when 1000 kg of wood
containing 50 % moisture is dried to 10 % final moisture content in an indirectly steam
heated steam dryer 23
12. Volume of heating steam needed at two different dryer pressures and heating steam
temperatures, as a function of the heating steam pressure, when 1000 kg of wood
containing 50 % moisture is dried to 10 % final moisture content in an indirectly steam
heated steam dryer 24
13. Volume of vaporized moisture, when 1000 kg of wood containing 50 % moisture is
dried to 10 % final moisture content, as a function of temperature 25
14. Amount of steam needed at four different dryer pressures, as a function of the
temperature, when 1000 kg of wood containing 50 % moisture is dried to 10 % final
moisture content in a direct steam dryer. 27
15. Volume of steam needed at four different dryer pressures, as a function of the
temperature, when 1000 kg of wood containing 50 % moisture is dried to 10 % final
moisture content in a direct steam dryer 27
vii
-------
ABBREVIATIONS AND SYMBOLS
BGCC Biomass gasification combined cycle
BOD Biological oxygen demand
COD Chemical oxygen demand
e.g. For example (exempli gratia)
EPRI Electric Power Research Institute
et al. And others (et alii)
etc. And others (et cetera)
i.e. That is (id est)
IGCC Integrated gasification combined cycle
IVOSDIG BGCC system including a direct steam dryer (Imatran Volma)
pH Negative logarithm of hydrogen ion concentration
SFBD Steam fluidized bed drying
TOC Total organic carbon
WTA Fluidized bed drying process with integral waste heat recovery (Lurgi)
bar Bar (= 100 kPa)
bar g Bar (gauge pressure)
C Carbon
°C Degrees Celsius
cal Calorie (=4.1868 J)
°F Degrees Fahrenheit (=1.8 toC + 32)
GJ Gigajouie (=10a J)
g Gram
h Hour
J Joule
kg Kilogram
kJ Kilojoule
kPa Kilopascal
kW Kilowatt
kWh Kilowatt-hour
I Liter
m Meter
viii
-------
m3
Cubic meter
mg
Milligram
MJ
Megajoule (=10® J)
mm
Millimeter
MW
Megawatt (=10® W)
Pa
Pascal
rpm
Revolutions per minute
s
Second
t
Ton (=1000 kilograms)
w-%
Percentage by weight
-------
1. INTRODUCTION
Utilization of biomass arid peat fuels has been increasing in the industrial sectorand in district heating during
the last few decades. Biomass fuels are mainly waste materials of plant origin like wood waste, bark, straw,
and bagasse. In the future, short rotation woody crops grown for energy production purposes will also
probably be used in increasing amounts. Peat is used in several countries where domestic resources are
available. The initial moisture content of such fuels is usually high, 45-65 % (wet basis).
Advantages of drying biomass include an improvement in thermal efficiency, when low-temperature waste
heat can be used for drying and the energy of the biomass needed for vaporizing the water component can
be reduced. When used as a fuel in a boiler, a reduction in the flue gas volume and an increase in
combustion temperature are possible, which allow the boiler to be designed smaller in size and accordingly
also in cost. For gasification processes a lower wafer content of the biomass is desired in order to reduce
energy required for gasification, producing less vaporized water within the gasification product gases and
ensuring a higher gas quality.
Thermal drying is a highly energy intensive process and, considering energy aspects, is important in dryer
selection and design. In traditional air or flue gas biomass drying methods, the energy used for vaporizing
the moisture has not been utilized. In a potentially more energy-efficient method, as in steam drying, the heat
of evaporation can be recovered and utilized.
The purpose of this study is to review the technologies available for biomass drying and examine the energy
requirements of biomass dryers. Biomass drying methods and different types of dryers are discussed
generally, and more emphasis is given to methods and dryers utilizing superheated steam as drying
medium. The operating conditions, energy requirements, types of biomass utilized, and anticipated
environmental burdens of steam dryers are described. Further, steam requirements for drying of biomass
at different pressures and temperatures are theoretically calculated and compared to the steam consumption
in different types of dryers.
1
-------
2. BIOMASS DRYING
2.1, Drying of solid particles
In thermal drying of solid particles, two processes occur simultaneously: (1) transfer of energy {mostly as
heat) from the surrounding environment, usually brought to the surface by convection or conduction, and
(2) transfer of moisture to the surface of the solid and its subsequent removal to the ambient gas.
Any drying process usually consists of three different drying periods:
1. Initial heat up period, while the particle absorbs heat from its surroundings and is heated to the
temperature at which surface moisture begins to evaporate.
2. Constant drying rate period, when the most easily removed moisture leaves the particle, and the
surface of the particle remains wetted with moisture transferred from the interior.
3. Falling rate period, when the surface of the particle is no longer wetted in air drying, and the rate of
internal movement of moisture decreases.
The moisture has to be removed as vapor from the surface of the particle to the ambient gas. This process
depends mainly on the external conditions. External conditions are of importance especially in the initial
stages of drying when unbound surface moisture is removed. From the inside of the particle to the surface,
the moisture has to be transported as liquid or vapor. This process is a function of the physical nature of the
solid, temperature, and moisture content. Internal movement of moisture is the controlling factor in the falling
rate period, the later stages of drying, when the average depth of moisture level has gradually increased,
especially if dried to low final moisture contents (Mujumdarand Menon 1995).
2.2. Equilibrium moisture
Materials like wood retain some of their moisture, depending on the type of bonding of moisture to the
material and the surrounding atmosphere, and this moisture is called equilibrium moisture content at
specified conditions. The moisture in excess of the equilibrium moisture content corresponding to saturation
humidity is unbound moisture. As a hygroscopic material, wood may also bind water after drying, if exposed
to such conditions.
2
-------
Hygroscopic properties of materials are usually described by a sorption isotherm which is the relation
between equilibrium moisture content of the material arid water activity at the given temperature. In humid
air the water activity is the relative humidity, and in superheated steam it is equal to the relative pressure,
which is the ratio of the actual pressure to the saturation pressure at the superheated temperature
(Wimmerstedt and Hager 1996).
The equilibrium moisture of wood has been studied in superheated steam, mostly because of interests in
drying of lumber (Rosen et a!. 1983, Resch et al, 1988). Recently the equilibrium moisture contents of wood
and bark used as biofuel, and of peat, have also been reported (Wimmerstedt 1995b, Bjoerkand Rasmuson
1995). Bjoerk and Rasmuson (1995) have studied spruce chips, spruce bark, and aspen chips in
superheated steam. In bark the degree of crystallization of cellulose is lower than in wood, and the
equilibrium moisture content in certain conditions is higher. It is concluded that equilibrium moisture fraction
of about 0.05 seems to be reachable with water activities in the range of 0.2 - 0.4 for ail real materials. For
different types of peat the drying curves are similar in shape, and resemble those presented for wood
(Wimmerstedt 1995b).
2.3. Methods of biomass drying
The most common and traditionally used method to dry biofuel is hot air drying, using flue gases as drying
medium. The hot flue gases flow through the moist fuel, and the sensible heat of the flue gases is used to
evaporate the moisture. Another method, possibly gaining wider acceptance in the future, is steam drying.
In this method superheated steam is used as drying medium instead of air or flue gases (Tapanainen 1982,
Turnbull etal. 1996, Wimmerstedt 1995b). Principles of these drying methods are shown in Figures 1 and
2. In indirectly steam heated steam drying, the heat for drying is supplied by heating steam, which is
condensed in a heat exchanger placed in the dryer. The drying steam that acts as drying medium in
indirectly heated steam drying may also be called fluid izing steam or conveying steam. In this report the term
drying steam is used. In direct drying, the steam is superheated before entering the dryer, and it acts both
as heat supplier and as drying medium. In this report the term heating/drying steam is used in this meaning.
3
-------
flue gases
+ vapor
11
flue gases_
dryer
moist fuel
dry fuel ^
Fig. 1, Principle of flue gas drying.
generated
steam
generated
steam
" dry'n9
steam
steam
moist fuel
dry fuel
heating steam
dry fuel
moist fuel
heater
dryer
dryer
condensate
a b
Fig. 2. Principle of superheated steam drying by (a) directly heated steam dryer and (b) indirectly steam
heated steam dryer.
4
-------
2.4. Principle of steam drying
Drying of materials by using steam as drying medium is based on the fact that in superheated steam the
temperature is higher than the saturation temperature of water vapor corresponding to the dryer pressure.
When the temperature of the material to be dried increases, the pressure of water vapor inside the material
increases and the water vaporizes into the surrounding lower-pressure steam. Drying in superheated steam
can be understood as boiling of water contained in the material to the environment During the constant
drying rate period the temperature of the surface of the particle has been found to remain at the boiling point,
and the constant rate period has been found to continue longer than in air drying (Mujumdar 1995, Beeby
and Potter 1985, Potter and Beeby 1994, Shibata and Mujumdar 1994, Wimmerstedt and Hager 1996).
Beeby and Potter (1985) have presented the equilibrium moisture content of various woods and paper pulps
as a function of superheat used for steam drying. This shows that for timber (spruce and beech), only 3 -
4° C superheat is needed to achieve 10 % moisture content, which is less than what is needed for paper
pulp and brown coal.
2.5. Comparison of air and steam as drying medium
Compared to air drying, the advantages of steam drying of biomass, in general, include (Mujumdar 1995,
Beeby and Potter 1985, Frame etal. 1983, Nomura and Hyodo 1985, Fagernaes and Sipilae 1996):
1. Lower net energy consumption
When using superheated steam as drying medium, the only material leaving the dryer (except the
dried biomass), is steam, which can usually be utilized. Thus the energy used for evaporating the
moisture can be largely recovered, giving lower net energy consumption.
2. Reduced risk of fire or explosion
Steam offers an inert (low oxygen content) atmosphere, which minimizes the risk of fire or
explosion. This is of importance particularly for combustible dusty products like small particle size
biomass.
3. Improved product quality
Overdrying of fine material is minimized because the equilibrium moisture content in steam is close
to the desired moisture content.
5
-------
4. Higher drying rates possible
In a steam atmosphere the constant rate period is prolonged, and in the falling rate period the
mobility of water is enhanced because of the absence of airand the resulting diffusions! resistance.
Thus in both the periods there are factors that tend to speed drying in steam atmosphere. Below the
inversion temperature, however, the drying rate in air is faster (see next chapter).
5. Reduced airborne emissions
Airborne emissions are reduced because the vaporized compounds are not emitted to the
atmosphere. Instead, they are in the condensate of the steam, which thus needs biological waste
water treatment before releasing to watercourses.
Disadvantages of steam drying, and points where particular consideration is needed, include (Mujumdar
1995, Potter and Beeby 1994):
1. The drying system is more complex, and leaks of air or steam are not allowed
Start-up and shut-down, as well as feeding and discharge are more difficult, especially if operated
under pressure.
2. Condensation on the particles
If the product to be dried enters at ambient temperature, condensation of water occurs on the
surface of the product before the evaporation begins. This will affect the flow properties of the
particles and may delay the beginning of drying. The water must be vaporized; the energy load,
however, is not increased.
3. Insulation of the walls
High temperatures are used, and the walls of the dryer must be insulated to prevent any
condensation of steam. If condensation occurs in the dryer, it adversely affects the flow properties
of the particles and is especially serious in fluid bed dryers.
2.6. Drying rates in air and steam
The general characteristics of drying processes are similar for both airand superheated steam as the drying
medium. The overall drying rate in steam can be faster or slower than in air depending on the amount of
condensation, steam temperature, and other conditions, including the nature of the material to be dried.
6
-------
Evaporation rates of water in air and steam have been compared in several studies (Yoshida and Hyodo
1970, Nomura and Hyodo 1985, Faberetal. 1988, Sheikholeslami and Watkinson 1992a, 1992b,Tarnawski
et al. 1996, Woods et al. 1994). It has been found fiat at a certain temperature, an inversion point, the
evaporation of water is independent of moisture content in the air. Below the inversion temperature
evaporation is faster in air than in steam, and above the inversion temperature evaporation is faster in steam
than in air.
The existence of the inversion point has been proved by several experimental studies, and various inversion
temperatures have been reported. The shifting of the inversion temperature has first been found to depend
on the mass flow of the drying gas and on a radiant heat supply (Yoshida and Hyodo 1970, Nomura and
Hyodo 1985). Theoretically the existence and the temperature of an inversion point has been explained
through thermodynamics of the evaporating medium. This implies that at high temperatures the specific heat
of the drying medium is the determining factor in the drying rate (Sheikholeslami and Watkinson 1992a).
According to Strumillo et al. (1995) the temperature of the inversion point may vary from 160° C to 300° C
(320° F to 572° F). The locus of the inversion point is expected to shift up when superheated steam at higher
pressures is used, and shift down when indirect heat supply is used (Sheikholeslami and Watkinson 1992a,
M ujumdar 1995). Woods et al. (1994) found that drying of wood waste to 10 % moisture, was not any faster
in superheated steam than in air over the range of temperatures investigated, 150° C to 190° C (302° F to
374° F). According to their graph on drying times the overall drying rate would have been faster in steam
above about 185° C (365° F) for wood waste in the conditions used.
Operating a superheated steam dryer above the inversion temperature is, however, not necessary if
evaporation rate is not a key question. The other advantages of steam drying can be obtained also at lower
temperatures, and the total drying time actually depends on several factors, as mentioned above.
7
-------
3. DRYING TECHNOLOGY
3.1. Types of dryers for biofuel
The dryers used for drying of biofuels and peat can be classified as flue gas dryers, superheated steam
dryers, and multiple-effect dryers (Wimmerstedt 1995b). In the following chapter those types of dryers will
be reviewed, with closer discussion on existing steam dryers. Heat for drying may be supplied by the drying
medium itself (direct or convective dryers), or by a heat exchanger inside the dryer (direct/indirect or
combined convection/conduction dryers). Examples of direct dryers are shown in Figure 3, and of indirectly
heated dryers in Figure 4.
The residence time of biomass varies in different types of dryers, and is related to the size of the particles
to be dried. In flash type dryers the residence time is shortest, typically 0-10 seconds. In such a case the
particle size has to be small, to get proper drying. The most common residence time is several minutes,
which is typical for dryers like fluid bed, rotary, belt, and continuous tray dryers. In batch dryers the residence
time may be several hours (Mujumdar and Menon 1995).
3.2. Flue gas dryers
Where flue gases from a boiler are available, the sensible heat of the flue gases can be used for drying
biofuel. In the dryer, the hot flue gases are led to direct contact with the fuel to be dried. The temperature
of the flue gases range generally from 250° C to 450° C (482° F to 842° F), and after being cooled during
the drying process leave the stack at a temperature of 90-120° C (194° F - 248° F). A dryer operating at a
lower temperature has also been developed (Lundqvist and Oesterman 1995). After leaving the dryer, the
biomass is usually separated from the flue gases and the water vapor in a cyclone. The energy of the
vaporized water cannot normally be utilized because of the low partial pressure of the water vapor in the flue
gases (Fagernaes and Sipilae 1996, Tapanainen 1982).
The most common types of flue gas dryers are rotary, cascade and flash dryers, which are briefly described
below. Other flue gas dryer types that have been commercially used for peat drying are the fluid bed dryer
and the whirly bed dryer, which is a combination of fluid bed and a flash dryer (Wimmerstedt 1995b). One
of the power plants using flue gas drying is the IGCC (Integrated Gasification Combined Cycle) plant of
Sydkraft AB in Vaernamo, Sweden. The fuel is dried in a rotary drum dryer by flue gases, after which the
flue gases are scrubbed with water before being discharged (Fagernaes and Sipilae 1996).
8
-------
air or steam
Fig, 3. Examples of direct dryers: a) rotary dryer, b) conveyor dryer, c) cascade dryer, d) fluidized bed dryer,
e) flash dryer.
=>
\
/
o
b
ft
—fXI-
r
(V
biomass air or steam —heating steam, condensate
Fig. 4. Examples of indirectly heated dryers: a) fluidized bed dryer, b) rotary dryer, c) flash dryer.
-------
3.2.1. Rotary dryers
Typical materials dried in rotary drum dryers are: hog fuels, sawdust, and bagasse. The dryer is a rotating
cylinder, horizontal or slightly inclined in the solids-flow direction. The sizes of the dryers may be up to 4.5
m diameter and up to 10 m length, and the rotating speed is usually 2 - 8 rpm. Flue gases may flow either
co-currently or counter-currently with the fuel. Rotary dryers are very flexible. They may be of the open-
center type or the center-fill type. The open-center type is equipped with flights or lifting vanes on the inside
shell to lift the solids up the sides and disperse in the gas. In the center-fill type Internal structures project
radially from the shaft and help convey the particles. The dryer processes wet fuel according to its density,
and can thus handle particles of different sizes, with different residence time needs, from a few minutes up
to an hour (Wimmerstedt 1995b).
3.2.2. Cascade dryers
In a cascade dryer the fuel is revolved in a cascading bed, the flue gases conveying the fuel to hit a
reflecting board in the chamber. Coarse material is removed from the drying chamber via overflow, and the
fines are removed with the exiting gas and separated in a cyclone. The residence time is about two minutes,
which requires smaller particle size than in the rotary drum drying. Space requirement is also less than that
of the rotary drum dryer (Wimmerstedt 1995b, Tapanainen 1982).
3.2.3. Pneumatic conveying or flash dryers
In flash dryers the fuel is conveyed by flue gas flow upwards through the vertical dryer pipe, and then
separated from the gas in a cyclone. The gas velocity is 15-50 mis. The residence time in the dryer is short,
only a few seconds, and thus the particle size should be small. Usually the fuel is pulverized or milled before
drying (Wimmerstedt 1995b, Tapanainen 1982).
3.3. Multiple-effect dryers
A double-effect dryer, called the Peco dryer, was developed in the 1930s and has been used for drying peat
in Ireland and the former Soviet Union (Wimmerstedt 1995b, Mujumdar 1995, Tapanainen 1982). The dryer
is a 2-stage dryer, consisting of 5 columns altogether, each column containing about 500 pipes of about 70
mm diameter. Milled peat is fed into the lower part of the first column and is carried upward in the pipes by
air flow through the first two columns. The peat passes all the columns in sequence and is separated from
the gas by cyclones after each column. Energy for drying is supplied to the first two columns by circulating
10
-------
water, which is heated to about 75 °C (167 °F) by the evaporated moisture from the last three columns. The
last three columns are heated by condensing steam in the jackets outside the pipes.
Because of utilizing the vaporized moisture in the Peco dryer, the thermal energy consumption per kilogram
of evaporated water is less than that of hot air dryers. Electrical power demand, however, is high for the
compressors used for pneumatic conveying the fuel inside the pipes, and the capital costs are higher than
those of other common dryers due to more complex apparatus. In spite of the high capital costs of the Peco
dryer, it was considered a feasible option, especially for large scale drying, because of low total energy
consumption compared with the more common flue gas dryers (Tapanainen 1982, Wimmerstedt 1995b).
Among the various types of multiple-effect dryers studied in Australia, mainly for drying brown coal, are two-
stage and three-stage dryers operated either at atmospheric pressure or stages at different pressures
(Figure 5). The highest pressure stage is heated by supplying high pressure steam through the heating coils
in the fluid bed, and the steam produced in the dryer is applied to the heating coils of the next effect, and
so on. In contrast to the above described Peco dryer, moist fuel is fed to, and dry product collected from
each stage. It has been declared that in multiple-effect dryers it is, in principle, possible to remove several
kilograms of water per kilogram of steam fed to the first stage, depending on the number of effects.
Mechanical vapor recompression is also possible (Hovmand 1995, Beeby and Potter 1985, Potter et at.
1990, Potter and Beeby 1994).
steam
fuel drying steam
steam, condensate
Fig. 5. Example of a multiple-effect superheated steam dryer which produces dry fuel of three different
temperatures.
11
-------
3,4. Steam dryers
Heat can be provided for steam atmosphere drying either indirectly or directly, and both types of dryers have
been developed. In indirectly heated dryers usually high pressure steam is condensed in a heat exchanger
located in the dryer. In directly heated dryers the steam used as drying medium is heated to a high
temperature, and the sensible heat of the steam is used for drying. In some types of dryers a high
temperature heat transfer medium is circulated through the dryer.
Operating pressure of the dryers may vary from atmospheric to high pressure exceeding 20 bar. Steam
drying atsubatmospheric pressures has also been studied, and used for instance in drying of sewage sludge
(Shibata and Mujumdar 1994). When operating at higher pressures, the needed gas volumes can be
reduced and smaller equipment size is possible. On the other hand, operation and maintenance is more
demanding (Potter and Beeby 1994).
Blomass dryers which can handle large quantities of material are of continuous type. Batchwise operation
of steam dryers in principle is possible, but it is feasible mainly for smaller amounts of valuable material.
3.4.1. Types of steam dryers
Early steam dryers using superheated steam were batch type dryers. For instance, Karrer(1920) has studied
steam drying of peat and some other materials by using an electrically heated batch dryer, and reported
a lower energy consumption compared to hot air drying. A batch process using steam, called Fleissner
process, has been in commercial use for drying of coal in Austria (Potter and Beeby 1994).
In principle, any direct or indirectly heated (combined convection/conduction) dryer can be operated as a
superheated steam dryer by replacing steam for hot air as the drying medium. This conversion, however,
is not simple because of more complex technology involved (Mujumdar 1995).
The existing commercial scale steam dryer types for biomass fuels are fluidized bed and pneumatic
conveying flash dryers. Mainly these types will be discussed below. Other types tested in pilot scale or used
for various special materials are, for instance, tunnel, conveyor, and multishelf dryers. Dryer types are
tabulated in Appendix A.
12
-------
3.4.2. Fluidized bed steam dryers
In fluidized bed drying the gas velocity blown upward through a layer of particles is such that all the particles
are suspended in the upward flowing gas. Steam fluidized bed drying tends to combine the advantages of
superheated steam drying and the excellent heat and mass transfer characteristics of a fluidized bed.
The steam-fluidized bed dryer by Niro, in Denmark, is a compact design and originally developed for sugar
factories to dry sugar beet pulp (Figure 6). The dryer is a pressurized, cellular fluid-bed dryer with a heat
exchanger placed centrally in the dryer. The only moving part in the vessel is a fan in the bottom to circulate
steam up through the 16 cells around the heat exchanger. The particles to be dried are fed to the first cell
and then move through all the cells in sequence. Dried product is collected from the bottom of the cell 16.
For the fines, a side cyclone is placed inside the dryer, above the 16th cell. Residence time in the dryer is
a few minutes and can be controlled by the amount of pulp fed or by changing the pressure of the heating
steam. The circulation steam at 3 bar is reheated to 200° C (392° F) when returning down through the tubes
of the heat exchanger, before being blown through the cells. The heat is provided by heating steam which
is higher pressure steam; e.g., 16 bar. The excess steam produced in the dryer is continuously bled off
through a pipe on the top and can be utilized (Jensen 1995,1996, Niro 1997, Muenter 1996).
Fig. 6. Niro fluidized bed steam dryer. 1) fan, 2) heat exchanger, 3) feeder, 4} outlet conveyor, 5) fixed vanes
creating vortex flow, 6) cylinder, 7) cyclone for dust separation, 8) ejector discharge of dust, 9) vortex flow
breaker, 10) high pressure steam inlet, 11) excess steam outlet. (© A S. Jensen. Drying Technology, 13:
5-7, pp. 1377-93 (1995). Reproduced by permission of Marcel Dekker, Inc., New York.)
13
-------
The Niro dryer is suitable for products that have particle size between 0.5 mm arid 50 mm. According to Niro,
(1997) 11 plants have been built with water evaporation capacities from 5 to 40 t/h. One of the dryers is
drying wood chips and one is drying baric.
Steam-heated steam-fluidized bed dryers for drying of brown coal have been studied intensively and
operated pilot scale in Australia (Beeby and Potter 1985, Potter et al. 1990, Potter and Beeby 1994). The
initial moisture content of brown coal may be 67% (wet basis). The fluidized bed has immersed heat
exchange surfaces of horizontal tubes fitted inside the bed. The bed temperature applied has been relatively
low, 110-127° C (230-261° F). The final moisture content of dried coal has been reported to be lower than
anticipated based on the equilibrium data at the bed temperature (Potter etal. 1990). A steam-fluidized bed
for drying brown coal of 63% moisture content, with a capacity of 24 t/h water evaporation, has been
completed in Victoria State, Australia in 1993 (Lurgi 1994).
Drying studies using an indirectly heated fluid-bed steam dryer on brown coal have been carried out also
in Eastern Europe where this fuel is found as well. The first commercial scale plant based on the research
and development done in Australia and the Eastern part of Germany for drying of brown coal, was
demonstrated in Germany with a capacity of 8 t/h water evaporation, in 1986. In 1993 a drying plant with a
capacity of 44 t/h of brown coal was commissioned in Germany. This factory reduces the coal's moisture
from 60% to 12% and utilizes the recompressed vapor as heating steam (Lurgi 1994, Beckmann 1990).
3.4.3. Flash or pneumatic conveying steam dryers
In pneumatic conveying or flash type dryers, the moist fuel is conveyed with the steam through the dryer
which may be a system of several columns. In case the dryer is indirectly heated, the tubes are heat
exchangers with condensing steam jackets outside the tubes to supply heat for the circulating superheated
steam inside the tubes.
The dryer developed in Sweden by Chalmers University of Technology and Modo Chemetics, consists of
transport pipes, heat exchangers, cyclone, and fans (Figure 7). The superheated steam circulates at a
pressure of 2 to 6 bar and pneumatically transports the particles through the tube side of a series of tubular
heat exchangers (Svensson 1979,1985, Frame etal. 1983, Stork 1997), The heat for drying is supplied by
heating steam which is condensed on the shell side of the heat exchangers, usually at a pressure of 8 to
15 bar. The dried material is separated from the steam in a cyclone, and the steam is recirculated by the fan.
The excess steam evaporated in the dryer is continuously bled off from the system and is available at a
pressure of 2 to 6 bar as process steam. Alternatively, the vapor can be recompressed for internal use, as
14
-------
is done for instance in a peat briquetting factory (Aschard et al. 1992, Svensson 1985).
The flash dryer, now marketed as Exergy Steam Dryer, has first been taken into commercial use at
Rockhammar mill in Sweden for drying of pulp. The first commercial drying unit operating on hog fuel has
been installed in Husum pulp mill in the 1980s (Svensson 1985).
Heating
steam
..J '
Generated
steam
Steam dryer with super
heater for materials with
small particle size
Dried
product
Fig. 7. Exergy pneumatic conveying steam dryer. (Courtesy of Stork Engineering)
15
-------
Imatran Voima, Finland, has developed a pressurized, direct flash dryer for moist fuels, where the fuel Is
dried in direct contact with superheated steam which supplies heat for the drying process (Hutkkonen et al.
1993, 1994). The fuel is conveyed by steam inside the dryer tube and separated from the steam in a
cyclone. The dryer was designed for use as a part of an integrated system consisting of a pressurized steam
dryer, pressurized air-blown gasifier, and gas turbine and is suitable for wet fuels that include peat, biomass,
and brown coal. The dryer produces injection steam for the gas turbine and operates at the gasifier pressure
(e.g., 23 bar), and at temperatures up to 400° C (752° F). Part of the steam in the dryer circuit is, however,
recirculated by a blower to the waste heat boiler superheater and returned as superheated steam back to
the dryer.
A steam drying pilot plant was built in 1991, with a capacity of 1000 kg/h of feedstock having an initial
moisture content of 70% (wet basis). It operates normally at 23 bar superheated steam atmosphere and has
been used for drying experiments on peat and wood biomass as saw dust and crushed wood chips
(Hulkkonen et al. 1994).
Imatran Voima has also developed a fuel dryer called the Bed Mixing Dryer (Figure 8), which operates in
a superheated steam atmosphere at atmospheric pressure and utilizes hot bed material from fluidized bed
combustion as its energy source. The circulating hot bed material is mixed with the steam in the dryer for
efficient heat transfer, and returned to the combustor with the dry fuel. The two first applications, Kuusamo
in Finland and Oerebro in Sweden, are designed as flash dryers (Hulkkonen and Heinonen 1997).
3.4,4. Other steam dryers
A pilot scale, continuous type superheated steam dryer for drying biomass has been built and tested in
Canada (Charron 1990). A piston was used to push the wood through the dryer which has a drying section
two meters long. The steam supplied to the dryer could be directed either co-current or counter-current to
the biomass flow. The steam was heated by a duct heater to a temperature of 200-265° C (392-509° F)
before entering the dryer and was condensed after exiting the dryer.
Other steam dryers designed for materials somewhat I ike wood include agitated multishelf, tray, tunnel, and
conveyor types. They have been operated either in pilot and/or full scale plants (Appendix A).
16
-------
STEAM
CONDENSER
DRYER
STEAM
FLUID1ZED
BED BOILER
FUEL +
BED
MATERIAL
HOT BED MATERIAL
NON-CONDENSING GASES
Fig. 8. Flow diagram of the Bed Mixing Dryer, (Courtesy of Imatran Voima)
17
-------
4. ENERGY ASPECTS OF DRYERS
Thermal drying is a highly energy intensive process. Traditionally flue gases have been considered to
contain economical available energy for drying blomass fuel. If the flue gases cannot be cooled and utilized
in the process, drying of the biomass could be a useful option. The moisture is removed from the biomass,
and the heat used for evaporation removes from the process with the exiting flue gases.
There are, however, also other possible uses for flue gases in the process, and often they are used for
preheating the air for combustion. In this method, much of the thermal energy is retained in the process but
is not available for drying of biomass. If steam drying method is used for biomass drying, it is possible to
recover a considerable part of the energy used for the drying process, usually by condensing the steam
produced in the dryer. The economics often depends on the possibilities to utilize the excess steam
produced in the dryer.
The dryer is always a part of a process or a system, and the economics actually depends on that total
system. If the economics of different drying methods, flue gas drying and steam drying, were to be compared
properly, the whole system with the inputs, outputs, the fuel, and produced energies should be considered.
In this chapter, energy aspects of mainly steam drying and possibilities of steam utilization are considered.
The thermal energy requirement of steam dryers, especially the amount of steam needed in different drying
conditions, is studied. Of flue gas dryers, energy requirements reported in the literature are tabulated in
Table B-2.
4.1. Utilization of steam
In steam drying the utilization of the excess steam produced during drying is often the key factor which can
make the method more economical than conventional methods. The possibilities for utilizing the steam
include (Lurgi 1994, Hulkkonen etal. 1994, Mujumdar 1995):
1. The steam can be condensed to recover the heat of evaporation and thus be used for heating
purposes.
18
-------
2. The steam can be compressed for use as heating steam in the drying process.
3. The steam can be used to boost power production in a gas-fired turbine.
The vaporized steam from steam drying can usually be used for heating purposes, either in the process or
in district heating network. As examples of steam utilizing purposes, Svensson (1985) has mentioned that
in a pulp mill the generated 3 bar steam from drying of pulp is used for drying the boiler fuel (bark) and for
heating process water; and in another mill the steam generated from drying of bark is condensed in a
reboiler where heating steam condensate is vaporized to form 4.5 bar process steam. From a direct steam
dryer, operated at higher pressure, the generated steam can be used as injection steam for a gas turbine
for power production (Hulkkonen et al. 1993).
4.2. Vapor recompression
If the steam produced in the drying process is not needed as such, another method for utilizing the steam
is to recompress it and use in the drying process. In certain conditions the mechanical energy required for
compressing is significantly less than the heat value that can be recovered from the vapor (Mujumdar 1995).
Dibella et al. (1991) have calculated that a reduction of 55 % in the overall net energy consumption for drying
in the U.S.A. can be achieved when steam drying with recompression is used, compared to using the
traditional method with hot air as drying medium.
According to Mujumdar (1995) compressors that can be used for steam may be of several types: screw,
centrifugal, lobe, reciprocating, axial, turboblowers, etc. In steam drying systems, however, one compressor
does not always meet the requirements of the drying system, but more than one compressor may be needed
to boost the steam pressure to the level used.
Aschard et al. (1992) have reviewed the superheated steam drying installations using mechanical vapor
recompression in France and Sweden. The briquetting plant in Sveg, Sweden, drying peat in a pneumatic
conveying steam dryer since 1988 is one example of those using steam recompression. Compressors are
used to compress the steam from 3 to 14 bar. The system pressure is 3.6 bar, but a steam reboiler is
needed to provide the compressor with clean steam. Of the total electricity consumption, the compressors
use 70% and the circulation fans 30% (Wimmerstedt 1995b). Electricity consumption of steam dryers using
mechanical vapor recompression are presented in Table 1.
19
-------
Table 1. Electrical energy consumption of superheated steam drying installations using mechanical vapor
recompression. Compiled after data presented by Aschard et al. (1992), Lurgi (1994), and Wimmerstedt
& Hager(1996).
Type of dryer
Material dried
Capacity
Compressors
Fans
Electricity
consumption of
the dryer
Tunnel dryer
Board of maritime pine
20 000 nf/year
two stage, 250 kW,
from 1 to 5 bar
two fans of 15 kW
each
not available
Conveyor dryer
Sugar beet pulp
20 t/h water evaporation
two stage, 2,5 MW,
rate 4.6
12 fans of 100 kW
each
190 kWh/t water
evaporated
Pneumatic conveying dryer
Peat, 2 dryer lines
30 t/h water evaporation
each dryer line
4 stage, 4.3 MW
each dryer line,
rate 4.7
two fans of
2 MW each
200 kWh/t water
evaporated
Fluidized bed dryer
Brown coal
24 t/h water evaporation
yes
yes
typically around
120 kWh/t water
evaporated
20
-------
4,3. Theoretical steam requirement for drying
The following calculations are theoretical and based on tabulated values of specific enthalpy and specific
volume of steam at different temperatures and pressures and on boiling points of water at different
pressures (Parrish 1977, Shrivastava 1967). They represent the minimum heat requirement. Heat losses
are not included, and the possible extra heat required for removing bound or pore water is not included.
The assumptions in the base case are the following:
amount of biomass feed 1000 kg
initial moisture content 50 w%
final moisture content 10 w%
in drying to final moisture content of 10%, amount of water evaporated is 444 kg
moisture is evaporated at the boiling point of water at the dryer pressure
- feeding temperature 25° C (77° F)
specific heat of water 1.0 cai/g °C (Perry and Chilton 1973)
specific heat of wood 0.570 cal/g °C (Perry and Chilton 1973).
The amount of heat required is determined by the heat needed for heating of water and wood to the boiling
temperature of water at the dryer pressure and for vaporization of water at this temperature. Other factors
(e.g., the thermal conductivity of steam at different temperatures, the thermal conductivity properties of the
heat exchanger in the dryer, and the heat losses) are not considered here.
The heat requirement is presented in Figure 9 for drying of 1000 kg moist biomass. The heat needed
corresponds to 2700 - 3300 MJ/t evaporated water in the studied pressure range 0.5 - 25 bar.
21
-------
1600
1400
11200
m 1000
800
if 600
400
200
20
25
0
5
10
15
Dryer pressure (bar)
Total
Vaporization
Water
—" A—
Wood
Fig. 9. Thermal energy needed for heating 1000 kg of wood containing 50% moisture, and vaporizing the
water to 10% final moisture content, as a function of the dryer pressure.
4.3.1, Indirectly steam heated steam drying
Heating steam in heating side
In indirect steam drying nearly all the thermal energy is needed for generating the heating steam. Properties
of the heating steam (i.e., specific enthalpy and specific volume) depend on the pressure and temperature
of the steam. In the heat exchanger inside the dryer, the heating steam is condensed when it has been
cooled to the boiling temperature of water at the heating steam pressure. The sensible heat above the
condensing temperature and the latent heat of condensation are assumed to be transferred to the biomass
via the heat exchanger and the drying steam. The amount of the heating steam required, both weight and
volume, is determined by the heat required for biomass drying and the heat available from the steam.
The dryer pressure has only a slight effect on the steam requirement (Figure 10). The weight and volume
of the needed heating steam depend mainly on the pressure and the temperature of that steam, the volume
being remarkably larger at lower pressures (Figures 11 and 12).
22
-------
_60Q
CO
t500
c
(1)
e
S400
3
CT
£
lioo
x
2 3 4
Dryer pressure (bar)
2 bar, 130 oC
5 bar, 160 DC
10 bar, 200 oC
Fig. 10. Volume of heating steam needed at three different pressures and temperatures, as a function of
the dryer pressure, when 1000 kg of wood containing 50% moisture is dried to 10% final moisture content
in an indirectly steam heated steam dryer.
660
¥640
o>
| 620
£
0-6OO
£
E 580
! 560
,1* 540
1 520
0 2 4 6 8 10 12 14 16
Heating steam pressure (bar)
m , 160 oC, dryer pressure 1 bar T 210 oC, dryer pressure 2 bar
Fig. 11. Amount of heating steam needed at two different dryer pressures and heating steam temperatures,
as a function of the heating steam pressure, when 1000 kg of wood containing 50% moisture is dried to 10%
final moisture content in an indirectly steam heated steam dryer.
23
-------
c
0)
E
2
800
cr
2
600 --
E
3 400 ...
cn
c
200 -
CO
-------
1200
R-1000.
E
E 800.
(0
¦i 600 .
0
| 400 ¦
1 200 .
0 ¦
100 120 140 160 180 200 220 240
Temperature (oC)
1 bar
2 bar
5 bar
Fig. 13, Volume of vaporized moisture, when 1000 kg of wood containing 50% moisture is dried to 10% final
moisture content, as a function of temperature.
4.3.2. Directly heated steam drying
Heating/drying steam
The amount of heat required is determined by the heat needed for heating of water and wood to the boiling
temperature of water at the dryer pressure and for the vaporization of water at the boiling temperature of
water at the dryer pressure. Other factors, like the thermal conductivity of steam at different temperatures
and the heat losses, are not considered here.
The heating/drying steam both supplies the heat needed for drying and acts as the drying medium.
Properties of the heating/drying steam {i.e., specific enthalpy and specific volume) depend on the pressure
and temperature of the steam. For heating the material to be dried and for vaporizing the water, only the
sensible heat of the heating/drying steam is used. The latent heat is not used forthis purpose in direct steam
dryers. The needed amount of the heating/drying steam, both weight and volume, is determined by the heat
requirement and the amount of sensible heat available for heating from the steam (Figures 14 and 15).
As can be seen by comparing Figures 11,12,14, and 15, the amount of steam required as Input in direct
steam drying, heated entirely with the steam which acts as drying medium, is far more than in indirectly
heated steam drying. The volume decreases, however, at higher temperatures, and above 300° C (572° F)
25
-------
the volume of steam required approaches that needed in indirectly heated dryers, as shown in Figures 11
and 12, These differences also mean considerably higher mechanical energy requirement than in indirectly
heated drying for circulating the steam, especially at lower temperatures.
In contrast to indirect drying, where most of the thermal energy in the heating steam is used for drying, in
direct drying the outlet steam still contains the major proportion of the thermal energy of the steam. In cases
studied in Figures 14 and 15, only 1-13% of the thermal energy of the steam has been used in drying.
26
-------
60000
50000
40000
30000
20000
10000
10 bar
15,7 bar
20 bar
-M-
25 bar
200 220 240 260 280 300 320 340 360 380
Temperature (oC)
Fig. 14. Amount of steam needed at four different dryer pressures, as a function of the temperature, when
1000 kg of wood containing 50% moisture is dried to 10% final moisture content in a direct steam dryer.
8000
7000
6000 --
3000 -
m
10 bar
15,7 bar
20 bar
-H-
25 bar
200 220 240 260 280 300 320 340 360 380
Temperature (oC)
Fig. 15. Volume of steam needed at four different dryer pressures, as a function of the temperature, when
1000 kg of wood containing 50% moisture is dried to 10% final moisture content in a direct steam dryer.
27
-------
Recoverable heat
Because the heating/drying steam is not condensed inside a direct steam dryer, the outlet steam can be
assumed to contain enthalpy equivalent to saturated steam at the dryer pressure. This amount of steam is
available as recirculation steam, if reheated, or for other purposes, at the dryer pressure. In addition to the
input steam, the outlet steam contains also the evaporated steam removed from the biomass. With the
assumption that 75% of the heat transferred from the heating/drying steam to the biomass and moisture,
is as latent energy of this excess steam, nearly all of the input thermal energy can be recovered.
4.4. Energy consumption of dryers
Typical energy consumption in existing common industrial dryers (air dryers), according to Mujumdar and
Menon (1995), is in the range 3000 - 10 000 kJ/kg water evaporated. The dryer types used for biomass
applications, flue gas or hot air dryers, need approximately 3000 - 6000 kJ/kg water removed (Mujumdar
1995, Ihren 1991), and the double-effect dryers need somewhat less, around 1700 - 2700 kJ/kg water
removed. Because in steam drying the steam produced can be utilized, the energy consumption of steam
dryers is often expressed as net energy consumption and is normally lower than the above mentioned
energy consumption ranges. There are several energy consumption ranges given in the literature for steam
drying in general:
200 - 600 kJ/kg water removed; net energy consumption (Wimmerstedt and Hager 1996)
600 - 900 kJ/kg water removed; specific energy consumption (Ihren 1991)
1000 -1500 kJ/kg water removed; net energy consumption (Mujumdar 1995)
More specific information on energy requirements of different types of dryers, compiled from literature and
information from dryer manufacturers, can be found in Appendix B. Except for the type of the dryer and
drying conditions used, according to Wimmerstedt and Hager (1996), the energy usage strongly depends
also on the initial moisture content of the biomass and the inlet temperature, especially below initial moisture
content of 40%.
Woods etal. (1994) have compared air and steam drying and calculated the energy requirements of a flue
gas dryer of rotary type and a steam dryer of fluid bed type. The wood used in these experiments had an
initial moisture content of 55% and was dried to 10% moisture content. The reported steam consumption
of the steam dryer was low compared to the consumption of other dryers listed in Table B-1. The test was
performed by using a batch drying process, and in this respect differs from the other fluidized bed drying
processes presented in Table B-1.
28
-------
Liinanki et al. (1994) have tested the applicability of three different steam dryers for drying of biomass:
manufactured by Modo Chemetics (pneumatic conveying dryer), Stork Friesland (pneumatic conveying
dryer), and Niro (fluidized bed dryer). The Modo Chemetics dryer was tested in a full scale plant, the Stork
Friesland dryer in a pilot plant, and the Niro dryer in a laboratory scale plant. All dryers were tested with the
same type of biomass, logging residues including bark and green parts. The initial moisture content of the
fuel was 50%, and it was dried to 10-15% moisture content. The tests showed that there were only small
differences in the performance of the different dryers, depending on the specific design and test facilities,
and all the dryers were working very well for drying of biomass. Based on the test results, the authors
consider an internal power consumption down to 35-40 kWh/t evaporated moisture to be possible with
proper design of the dryer.
Jensen (1995,1996) presented operating results of Niro dryers used in the sugar industry for drying sugar
beet pulp and in the pulp and paper industry for drying wood chips. The dryer used for drying wood chips
at a capacity of 20 t/h is reported to use 14 bar g steam 10 t/h, but more exact energy consumption is not
given for wood chips. Instead, the energy used for drying of sugar beet pulp is reported in more detail (Table
B-1).
Svensson (1981) presented results of test runs on bark and wood shavings done in both pilot and full scale
and of drying of pulp by pneumatic conveying or flash type dryers. In drying bark from 62% moisture content
to 25% average moisture content, the energy inputs were: heating steam (13 bar) 3900 kJ/kg dry solids and
electrical energy 110 kJ/kg dry solids. The generated steam (4 bar) was condensed and heat recovered
down to 70° C (158° F) before leaving the mill (Table B-1).
The performance and cost of the IVOSDIG biomass-gasification combined cycle system including a direct
flash steam dryer has been evaluated by EPRI (Electric Power Research Institute) in USA (Vivenzio et al.
1995). It has been concluded that the flash steam drying process provides additional mass and energy to
the biogas (from the gasifier), and as a result a notable improvement in conversion efficiency is gained
without any extra internal power consumption. The report describes the performance of the entire system,
including the dryer. The amounts of the steam streams, both inlet and outlet streams, however, can be seen
in the flow diagram as being massive compared to those in indirectly heated dryers (Table B-1). In this case
also the outlet steam is at high pressure and has uses that depend on the system the dryer is part of.
The published values presented in Appendix B, table B-1, on thermal energy requirement of different types
of indirectly heated steam dryers, 2700 - 3700 kJ/kg, are close to the theoretical minimum heat requirement
2700 - 3300 kJ/kg, as calculated for dryer pressures 0.5 - 25 bar (Figure 9), The data listed in Table B-1,
29
-------
in the column "steam requirement", and expressed in megajoules/ton, include both the heat available for
drying and the heat thatwill not be used for drying, except for that of Svensson (1981), which is expressed
as latent (available) heat. The heat that will not be used for drying would be contained in any condensate
or warm water after its heat is not further utilized in the system, and this portion must be contained in the
incoming steam in addition to the minimum heat requirement. Considering this, the steam consumption of
the fluid bed dryer reported by Woods et ai. (1994) seems to be quite low.
Variations in the feeding temperature of biomass, and thus the need for warming the biomass before
evaporation begins, have also some effect on the energy consumption of the dryer. In the cases tabulated
in Table B-1, the inlet temperature of biomass has been 10° C - 50° C (50° F -122° F). This 40° C difference
in inlet temperature can cause a difference of around 10% in the heat consumption.
The closeness of the consumption figures with the theoretical heat requirement values suggests that the
heat losses in these types of dryers are not significant. Examples of losses by radiation and leakage in
indirectly heated dryers are given by Jensen (1996), which is 0.2% of the energy contained in the heating
steam in drying of sugar beet pulp and given by Svensson (1981), which is about 5% of the latent heat
contained in the heating steam in drying of bark. These figures do not contain losses as discarded warm
water.
The heat input requirement of direct dryers is far more than that of indirectly heated dryers, as seen in
Figures 14 and 15. Because, in the former case, the steam is not condensed inside the dryer, only a minor
part of the thermal energy is actually consumed for the drying process. The outlet steam contains
considerably more energy than that of indirectly heated dryers and can be utilized for recirculation and other
purposes.
30
-------
5. ENVIRONMENTAL BURDENS FROM DRYING OF BIOMASS
Wood consists mainly of the polymeric materials cellulose, hemicellulose, and lignin, Except polymers, wood
and other biomass fuels also contain low molecular weight extractable compounds like sugars, salts, fats,
pectin, and resins. Bark contains a larger fraction of extractables than wood. When exposed to high
temperatures, volatilization of low molecular weight components of the biomass and of thermal degradation
products occurs. Among the organic compounds released are both volatiles and condensable compounds
(Fagernaes and Sipilae 1996, Bjoerk and Rasmuson 1996).
The volatiles consist mainly of monoterpenes, the principal components of which are alpha-pinene and beta-
pinene. Non-condensable gases are mainly carbon dioxide, but some hydrogen, carbon monoxide, methane
and C2-C4-hydrocarbons are also present Monoterpenes are strong smelling irritants.
The condensable category consists of components like fatty acids, resin acids, diterpenes, and triterpenes.
They have sufficient vapor pressure to volatilize at high drying temperatures, but condense after leaving the
dryer stack and form aerosols. Condensable hydrofilic compounds consist mainly of thermal degradation
products, and are small molecular weight acids like formic and acetic acids, alcohols like methanol and
ethanol, aldehydes, furfurals, and carbohydrates like anhydro sugars. The amount of thermal degradation
products increases with the increase in the drying temperature, especially above 200° C (392° F) (Fagernaes
and Sipilae 1996, Fagernaes 1992).
The composition of the emissions depends on the type of biomass, its composition, and the drying
conditions applied. Best examined are the emissions from drying of wood and bark, in which the main
classes are monoterpenes, other lipid compounds, and thermal degradation products (Fagernaes 1992).
5.1. Environmental burdens from flue gas dryers
Emissions of organic substances from drying of wood fuel in flue gas dryers, as estimated in Swedish
studies in connection with the planning of new biomass plants, have been 600 - 2700 mg C/kg dried fuel,
when drying the fuel to 30% moisture content. Gaseous emissions may contain malodorous and toxic
substances, and compounds which react with nitrogen oxides to produce ozone (Fagernaes and Sipilae
1996).
31
-------
To reduce the formation of emissions, lower drying temperatures and shorter drying times should be used.
For minimizing the emissions entering the atmosphere it has been advised to use gas cleaning devices, like
scrubbing with water before being discharged (Fagernaes and Sipilae 1996).
5.2. Environmental burdens from steam dryers
In steam drying, the organic compounds volatilized during drying are mixed with the steam produced in the
dryer. When the steam is condensed after the dryer, they mostly remain in the condensate. Fagernaes and
Sipilae (1996) have summarized the amounts of emissions in the condensate of drying steam from several
studies of different dryers and materials. There are great variations depending on the drying conditions and
different materials:
Fagernaes and Sipilae (1996) have found that the samples from atmospheric dryers contained a higher
organic load than those from pressurized drying, while Woods et al. (1994) have reported higher TOC
concentration in condensates obtained from pressurized drying tests. According to Muenter (1996) the
components causing problems with the condensate are the suspended material, the terpenes, the BOD, and
the toxic effect on nitrogen removal at a municipal sewage tratment plant.
The condensate produced in steam drying may first need pH control and after that, biological waste water
treatment (Fagernaes and Sipilae 1996, Muenter 1996, Reinbold and Mallevialle 1975). To minimize the
amount of organic compounds in the exhaust steam, it is recommended to use stored biomass and dry at
'low' temperature with short drying time (Bjoerk and Rasmuson 1996).
pH
BOD
COD
TOC
solids
3.3 - 7.5
phosphorus
total nitrogen
ammonium nitrogen
130-1700 mg/l
300 - 2900 mg/l
70 - 9700 mg/l
5-1500 mg/l
0.1 - 0,4 mg/l
14 -160 mg/l
11-14 mg/l
32
-------
6. CONCLUSIONS
The traditionally used arid most common method of drying biomass is by using the sensible heat of the hot
flue gases from a boiler. Another method, possibly gaining wider acceptance in the future, is drying in
superheated steam. In spite of the more complex system needed for steam drying, it offers advantages like
lower net energy consumption and reduced risk of fire, which is of importance especially for combustible,
small particle-size biomass.
Superheated steam dryers, using steam as drying medium, can be heated directly by the steam that acts
as drying medium, or indirectly, often by higher pressure steam. Operating pressure varies in different steam
dryers from subatmospheric up to over 20 bar. The existing commercial scale steam dryer types for biomass
fuels are fluidized bed and flash or pneumatic conveying dryers. Other types have been tested in pilot scale
or used for various related materials. The reported heat requirements of indirectly heated steam dryers, as
compared with the calculated minimum heat requirement, suggest that heat losses are not significant in
these types of dryers.
In indirectly heated steam dryers the heating steam is condensed in the heat exchanger inside the dryer,
and the latent heat is used for the drying process, while in direct dryers only the sensible heat of the steam
is used for drying. Thus the requirement for heat input of indirectly heated steam dryers is considerably less
than that of direct steam dryers heated entirely with the steam being used as drying medium. On the other
hand, the outlet steam of direct dryers still contains most of the thermal energy content of the steam, and
additionally contains the vapor produced in drying, and is available for recirculation or utilization for other
purposes. From indirectly heated dryers the steam output is less and usually at lower pressure. In either
case, the latent heat of vaporization of the biomass moisture, which is major part of the thermal energy used
for drying, is recoverable.
Utilizing the heat of the outlet excess steam may be the factor which can make the steam drying method
desirable over conventional methods. The method of utilization of the heat depends on the process upon
which the dryer operates. Low-pressure excess steam can be used for heating applications, or where there
is no need for low pressure steam, recompressed to be reused in the drying process. Higher pressure
steam from a direct dryer may be reheated for circulation, while the excess steam is available for other uses.
The composition and amount of emissions released from biomass during drying depend on the drying
conditions and on the type of biomass. Low temperatures are recommended to minimize volatilization of
organic compounds. Generally, after a flue gas dryer, the gases need to be cleaned, and after a steam dryer,
the condensate of the produced steam needs to be treated.
33
-------
REFERENCES
Aschard, J-L., Missirian, C., Costa, P., Causse, M. (1992). S 6chage a la vapeur d'eau surchauffde:
gfenferalites et applications industrielles avec compression me'chanique de vapeur. Electrotech 92:
proceedings of the 12th congress of the International Union for Electroheat, Montreal, Canada, June 14 to
18, 1992, p. 634-642.
Beckmann, G. (1990). Dampf-Wirbelschicht-Trocknung; Entwicklung, Prinzip undAnwendung. Chem.-Ing.-
Tech. 62 (1990) 2,109-111.
Beeby, C., Potter, O.E. (1985). Steam drying. In: Toei, R., Mujumdar, A.S. (Eds), Drying '85, Hemisphere
Publishing Corp., Washington, DC, 1985, p. 41-58.
Bjoerk, H., Rasmuson, A. (1995). Moisture equilibrium of wood and bark chips in superheated steam. Fuel
74(1995)12, 1887-1890.
Bjoerk, H., Rasmuson, A. (1996). Formation of organic compounds in superheated steam drying of bark
chips. Fuel 75(1996)1,81-84.
Charron, J.M (1990). Development of direct contactsuperheated steam drying process for biomass. Energy,
Mines and Resources Canada, Bioenergy Development Program, SSC-34SZ23283-7-6040, Winnipeg,
Canada, 1990, 72 p.
Dibella, F.A., Doyle, E.F., Becker, F.E., Lang, R. (1991). Research and development of industrial drying
concepts using a superheated steam atmosphere with exhaust steam recompression. Phase 1 - Final report.
U.S. Department of Energy, Washington, DC., DOE/ID/128261, Waftham, MA, 1991, 280 p.
Faber, E.F., Heydenrych, M.D., Hicks, R.E. (1988). Techno-economic comparison of air and steam drying.
Chemsa 14 (1988) 9, 266-273.
Fagernaes, L. (1992). Peat and bark extractives and their behaviour in drying processes. Technical
Research Centre of Finland, VTT Publications 121, Espoo, Finland, 1992, 146 p.
34
-------
Fagernaes, L., Sipilae, K. (1996). Emissions from biomass drying. Proc, Conference on Developments in
Thermochemical Biomass Conversion, Banff, Canada, May 20-24,1996, 15 p.
Frame, G.B., Galland, K.V., Svensson, C, (1983). Steam drying of industrial and agricultural products and
wastes. Energy Progress 3 (1983) 1, 36-39.
Hovmand, S, (1995). Fluidized bed drying. In: Mujumdar, A.S. (Ed.), Handbook of Industrial Drying, Vol. 2.
Marcel Dekker, Inc., New York, NY, 1995, p. 195-248,
Hulkkonen, S , Aeijaelae, M., Raiko, M. (1993). Development of an advanced gasification process for moist
fuels. 12th EPRI Conference on Gasification Power Plants, San Francisco, CA, Oct 27-29,1993,16p.
Hulkkonen, S., Heinonen, 0., Tiihonen, J., Impola, R. (1994). Drying of wood biomass at high pressure
steam atmosphere; experimental research and application. Drying Technology 12 (1994) 4, 869-887.
Hulkkonen, S., Heinonen, O. (1997). Bed mixing dryer for high moisture content fuels. Proceedings of the
3rd Biomass Conference of the Americas, Montreal, Canada, Aug 24-29,1997. Vol. 1, p 695-702.
Ihren, N. (1991). Torkning av skogsbraensle - teknisk jaemfoerelse av torkmetoder vid koppling till
foergasning och elproduktion fraan kombicykel. Vattenfall Utveckling AB, SV-UV-91-30, Aelvkarleby,
Sweden, 1991, 56 p.
Jensen, A.S. (1995). Industrial experience in pressurized steam drying of beet pulp, sewage sludge and
wood chips. Drying Technology 13 (1995) No. 5-7,1377-1393.
Jensen, A.S, (1996). Pressurized steam drying of sludge, bark and wood chips. Pulp & Paper Canada 97
(1996) 7, 61-64.
Karrer, J. (1920). Drying with superheated steam. Schweizerische Bauzeitung 76 (1920) pp. 213-216,228-
230. Ref. Eng. Index 1921 p. 164.
Kullendorff, A. (1987). Anordning foer torkning av biobraensle. Swedish Patent 450971 B, 6 p.
35
-------
Liinanki, L., Horvath, A., Lehtovaara, A., Lindgren, G. (1994). The development of a biomass based
simplified IGCC process. 13th EPRI Conference on Gasification Power Plants, San Francisco, CA,
Oct 19-21, 1994, 18 p.
Lundqvist, R., Oesterman, J. (1995). Petikuivuritutkimus (Fluidized bed dryer research). In: Yearbook 1994,
Part III. Bioenergy Res. Progr., Publications 6, VTT Energia, Jyvaeskylae, Finland, 1995, p. 49-55.
Lurgi Energie und Umwelt GmbH (1994). SFBD: Steam fluidized bed drying. WTA: Fluidized bed drying with
integral waste heat recovery. Marketing material.
Muenter, M. (1996). Use of dried biomass in Boraas. Biomass for energy and the environment. Proc. 9th
European Bioenergy Conference, Vol. 2. Pergamon 1996, p.923-928.
Mujumdar, A.S. (1995). Superheated steam drying. In: Mujumdar, A.S. (Ed.), Handbook of Industrial Drying,
Vol. 2. Marcel Dekker, Inc., New York, NY, 1995, p. 1071-1086.
Mujumdar, A.S., Menon, A.S. (1995). Drying of solids: Principles, classification, and selection of dryers. In:
Mujumdar, A.S. (Ed.), Handbook of Industrial Drying, Vol. 1. Marcel Dekker, Inc., New York, NY, 1995,
p. 1-39.
Niro A/S 1997. Fluid bed drying with superheated steam. Pressurized steam fluid bed drying. Marketing
material.
Nomura, T., Hyodo, T. (1985). Behavior of inversion point temperature and new applications of superheated
vapordrying. In: Toei, R., Mujumdar, A.S. (Eds), Drying '85, Hemisphere Publishing Corp., Washington, DC,
1985, p. 517-522.
Parrish, A. (1977) (ed.). Mechanical Engineer's Reference Book. Newnes-Butterworths, London, England,
1977, p. 2.86-2.103.
Perry, R.H., Chilton, C.H. (1973) (eds). Chemical Engineers' Handbook, 5lh ed. McGraw-Hill BookCompany,
New York, NY, 1973, p. 3.126, 3.136.
Potter, O.E., Beeby, C.J., Fernando, W.J.N., Ho, P. (1983). Drying brown coal in steam-heated, steam-
flu id ized beds. Drying Technology 2 (1983) 2, 219-234.
36
-------
Potter, O.E., Guang, L.X., Georgakopoulos. S., Ming, M.Q. (1990). Some design aspects of steam-flu idized
steam heated dryers. Drying Technology 8 (1990) 1, 25-39.
Potter, O.E., Beeby, C. (1994). Scale-up of steam drying. Drying Technology 12 (1994) 1&2,179-215.
Reinbold, M,, Mallevialle, M. (1975). Biodegradation by the activated sludge system of steam process water
in the timber industry. Water Research 9 (1975) 1, 87-93.
Resch H., Hoag, M.L, Rosen, H.N. (1988). Desorption of yeilow-poplar in superheated steam. Forest
Products Journal 38 (1988) 3,13-18.
Rosen, H.N. (1981). Drying of lumber in superheated steam above atmospheric pressure. AlChE
Symposium Series 77 (1981) No. 207, 32-37.
Rosen, H.N., Bodkin, R.E., Gaddis, K.D. (1983). Pressure steam drying of lumber. Forest Products J. 33
(1983) 1, 17-24.
Sheikholeslami, R,, Watkinson, A.P. (1992a). Rate of evaporation of water into superheated steam and
humidified air. Int. J, Heat and Mass Transfer 35 (1992) 7,1743-1751.
Sheikholeslami, R., Watkinson, A.P. (1992b). Convective drying of wood-waste in air and superheated
steam. Canad. J. Chem. Eng. 70 (1992) 470-481.
Shibata, H., Mujumdar, AS. (1994). Steam drying technologies: Japanese R&D. Drying Technology 12
(1994) 6, 1485-1524.
Shrivastava, S.B, (1967). Steam Tables and Mollier Chart. Asia Publishing House, Bombay, India, 1967,
8 p.
Stork Engineering (1997). Exergy steam drying technology, Marketing material.
Strumillo, C., Jones. P.L., Zylla, R. (1995). Energy aspects in drying. In: Mujumdar, A S. (Ed ), Handbook
of Industrial Drying, Vol. 1. Marcel Dekker, Inc., New York, NY, 1995, p. 1241-1275.
Svensson, C. (1979). Back pressure drying, a new system for hogged fuels. Svensk Papperstidning 82
(1979) 10, 281-287.
37
-------
Svensson, C. (1981). Steam drying of hog fuel. Tappi 64 (1981) 3,153-156.
Svensson, C. (1985). Industrial applications for new steam drying process in forest and agricultural industry.
In: Toei, R., Mujumdar, AS. (Eds), Drying '85, Hemisphere Publishing Corp., Washington, DC, 1985,
p. 415-419.
Tapanainen, J. (1982). Turpeen ja puubiomassan kuivaus. Kirjallisuuskatsaus (Drying of peat and wood
biomass. Literature review). Valtion teknillinen tutkimuskeskus, VTT-TIED-107, Espoo, Finland, 1982,
44 p.
Tarnawski, W.Z., Mitera, J., Borowski, P., Klepaczka, A. (1996). Energy analysis on use of air and
superheated steam as drying media. Drying Technology 14 (1996) 7&8,1733-1749.
Turnbull, J.H., Hulkkonen, S., Bruce, D.M., Sinclair, M.S. (1996). Options for biomass drying; What's
feasible? What's practical? Bioenergy '96 - The 7th National Bioenergy Conference, Nashville, TN,
Sep 15-20, 1996. Vol. 1, p.557-564.
Vivenzio, T.A., Schectman, D.L., Wells, P.D. (1995). Performance and Cost of an IVOSDIG Biomass
Gasification Combined Cycle System. Electric Power Research Institute, Palo Alto, CA, TR-105356. Boston,
MA, 1995, 20 p.
Wimmerstedt, R. (1995a). Steam drying - history and future. Drying Technology 13 (1995) No. 5-7,1059-
1076.
Wimmerstedt, R. (1995b). Drying of peat and biofuels. In: Mujumdar, A.S. (Ed.), Handbook of Industrial
Drying, Vol. 2. Marcel Dekker, Inc., New York, NY, 1995, p. 809-824.
Wimmerstedt, R., Hager, J. (1996). Steam drying - Modelling and applications. Drying Technology 14(1996)
5, 1099-1119.
Woods, B.G., Nguyen, Y , Bruce, S. (1994). Assessment of superheated steam drying of wood waste.
Bioenergy '94 - The 6th National Bioenergy Conference, Reno/Sparks, NV, Oct 2-6,1994. Vol. 1, p.177-184.
Yoshida, T., Hyodo, T. (1970). Evaporation of water in air, humid air, and superheated steam. Ind. Eng.
Chem. Process Des. Dev. 9 (1970) 2, 207-214.
38
-------
APPENDIX A
Table A-1, Summary of commercial and pilot scale steam dryers used for drying of biomass or biofuel.
Type of the dryer
Reference
Scale
Designer
Biomass dried
Installations
Comments
Drying conditions
Fluidized bed
Fluid bed dryer
Heating tubes
inside the bed.
- indirectly heated
Beeby & Potter
1985,
Potter et al.
1983,1990,
Potter & Beeby
1994
Pilot
Brown coal from
63-67% moisture
Indirectly steam-heated by
heating element tubes fitted
inside the bed.
Bed temperature 110-127° C
Fluidized bed
dryer
Heat exchanger
tubes immersed
in the fluidized
bed
- indirectly heated
- compression of
evaporated steam
possible
Lurgi 1994
Lurgi
Germany: brown coal
from 60% to 12%
moisture 1986 and
1903.
Australia: brown coal
1993
Heating steam
400-900 kPa in SFBD process,
400-500 kPa in WTA process.
Fluidizing steam 115-125 kPa.
Bed temperature 110-120° C.
Coal feed size 0-6 mm.
Part of the dedusted product
vapor is compressed and
returned to the dryer as fluidizing
steam (SFBD) or heating steam
(WTA)
Fluid bed
cellular (16 cells)
dryer. Heat
exchanger
centrally placed in
the dryer.
- indirectly heated
- pressurized
Jensen 1995,
1998, Niro 1997
Aschard et al.
1992,
Fagernaes &
Sipilae 1996
Niro
Boraas Sweden:
wood chips 1994,
Moensteraas
Sweden: bark 1995.
Mackolsheim France:
sludge 1993.
Sugar beet pulp in
several countries e.g.
Nangis France 1990.
Product is dried in steam blown
through the cells by a fan in the
bottom.
Particle size 0.5-50 mm
Steam pressures
max 4 bar inside the dryer and
max 25 bar in heating side.
Outlet temperature of steam
about 150° C
Fluidized bed
dryer
- atmospheric or
pressurized
Shibata &
Mujumdar 1994
Pilot
Sewage sludge from
73% to 5% moisture
Inlet temperature of steam 230°C,
exit temperature 150° C
Fluidized bed
dryer
Vertical drying
chamber.
Kullendorff
1987 (patent)
Biofuel
Steam is heated by a heater, and
outlet steam is used in a
condenser.
(continued)
A-1
-------
Table A-1. (continued)
Type of the dryer
Reference
Scale
Biomass dried
Comments
Designer
Installations
Drying conditions
Flash
(pneumatic
conveying)
Pneumatic
Svensson
Formerly
Rockhammar mill
Dryer tubes 75-150 mm diameter
conveying dryer.
1979, 1981,
called
Sweden: wood pulp
and 10-20 m length, often
Dryer tubes are
1985, Frame et
Modo or
from 60% to 12%
several columns.
heat exchangers
al. 1983,
Stork
moisture 1979.
Particle size about 5-10 mm.
where material is
Stork Eng.,
Friesland
Husum Sweden: hog
Steam pressure inside 2-6 bar,
conveyed with low
(US Pat. 1977
fuel, bark about 10
heating steam usually 8-15 bar.
pressure steam
4043049)
Markete
years.
Drying time typical 10-30 seconds
through the dryer
d now as
Sveg Sweden: peat
Cyclone for separating fuel and
(several
Exergy
1988.
steam.
columns).
Wimmerstedt
Steam
Other materials:
Fan to circulate the steam.
1995a, 1995b,
Dryer by
sugar beet pulp from
- indirectly heated
Mujumdar
Stork
80% to 10% moisture
- pressurized
1995,
Enginee-
content
- steam
Aschard et al.
ring
recompression
1992,
possible
Fagemaes &
Sipilae 1996
Flash dryer
Hulkkonen et
Pilot
Peat from 50-70% to
Needs relatively small particles.
vertical tube
ai. 1994
10-25% moisture,
Capacity of pilot 1000 kg/h of wet
Imatran
wood biomass
feedstock.
Voima
Operating pressure about 23 bar
Wimmerstedt
and temperatures max 400° C.
1995a,
Residence time a few seconds.
Fagemaes &
Planned to be operated in
- pressurized
Sipilae 1996
combination with pressurized air-
blown gasifier and a gas turbine.
Bed mixing dryer
Hulkkonen &
Finland: peat and
Utilizes hot bed material of
Circulating hot
Heinonen 1997
imatran
wood 1994,
fluidized bed combustion as
bed material is
Voima
Sweden: saw dust
energy source.
mixed with the
steam used as
drying medium
A-2
(continued)
-------
Table A-1. (continued)
Type of the dryer
Reference
Scale
Designer
Biomass dried
Installations
Comments
Drying conditions
Other
continuous
types
Conveyor dryer
- with steam
recompression
Aschard et al,
1992
Promill
France: sugar beet
pulp 1985
Superheated steam 145° C,
1 bar. Dryer is 30 m long and
11m diameter.
Semicontinuous
tunnel dryer
- steam
recompression
Aschard et al.
1992
Nordon
Landes France:
maritime pine 1989
Superheated steam 145° C,
1 bar.
Steam recompression from 1 to 5
bar.
Compressed steam used as
heating steam in heat exhanger.
Agitated multi-
shelf dryer.
Superheated
steam flows
parallel to the
material which is
placed on shelves
and moved
towards next
lower shelf
- subatmospheric
Shibata &
Mujumdar 1994
Full
scale
Japan: sewage
sludge to final
moisture content of
below 10%
Capacities 161 dewatered sludge
/day and 901 dewatered
sludge/day.
Steam inlet temperature 390° C,
outlet temperature 150° C.
Evaporated steam is circulated
and used as heating medium in
the dryer.
Pressure 0.1-1 kPa below
atmospheric pressure
Batch
Batch dryer
Electrically
heated
Karrer 1920
Wimmerstedt
1995a
Peat
Temperature about 120° C
Tray dryer
Electric steam
generator and
heater,
- pressurized
Rosen 1981,
Rosen et al.
1983
Pilot
Timber
Pressure up to 2.4 bar,
temperature to 160° C.
A - 3
-------
APPENDIX B
Table B-1. Energy consumption of steam dryers.
Reference
(Dryer type)
Electricity
consumption
MJ/t water evaporated
Steam requirement
MJ/t water evaporated
Recoverable
steam
MJ/t water evaporated
Total energy consumption
(net energy consumption)
MJ/t water evaporated
Mujumdar 1995
1000-1500 MJ/t
net energy consumption
Ihren 1991
600-900 MJ/t
specific energy consumption
Wimmerstedt &
Hager 1996
200-600 MJ/t
net energy consumption
Lllnanki et al. 1994
130-140 MJ/t
internal power consumption
Beckmann 1990
(fluid bed dryer)
< 3000 MJ/t
Woods et al. 1994s
(fluid bed dryer)
39 kWh/t
140 MJ/t
775 kWh/t
2790 MJ/t
682 kWh/t
2450 MJ/t
Lurgi 1994
(WTA-process)
120 kWh/t
430 MJ/t
no external steam
yes
430 MJ/t
electric power requirement
Wimmerstedt &
Hager 1996 (Niro
dryer)
200 MJ/t
typical energy use
Niro 1997
(Niro dryer)
20 30 kWh/t
80-110 MJ/t
3700 MJ/t
3200 MJ/t
Jensen 1996"
(Niro dryer)
19 kWh/t
70 MJ/t
1040 kWh/t
3800 MJ/t
770 kWh/t
2800 MJ/t
Stork 1997
(Exergy dryer)
yes
most part
75-80 %
2000-2300 MJ/t
2700-2900 MJ/t
specific energy consumption
Svensson 1981°
(Modo dryer)
85 MJ/t
3000 kJ/kg"
3000 MJ/t
2700 kJ/kg
2700 MJ/t
Vivenzio etal. 1995"
(direct dryer)
160 kW/t
570 MJ/t
29000 MJ/t
26000 MJ/t
Rosen 1981'
(batch dryer)
2 kWh/kg
7200 MJ/t
7200 MJ/t
a drying of wood waste from 55% moisture to 10% moisture content, 16 t/h, tested as pilot batch process
b drying of sugar beet pulp from 70.8% moisture to 10% moisture content, 25000 kg/h water evaporation, biomass feeding temp.
50° C
c drying of bark from 62% moisture to 25% moisture content, 500 kg/h water evaporation, biomass feeding temperature 10° C
d latent heat
e drying of biomass from 50% moisture to 20% moisture content, process simulation and evaluation study, biomass feeding temp.
1S°C
f drying of timber (yellow poplar)
B-1
-------
Table B-2. Energy consumption of air or flue gas dryers and multiple-effect dryers.
Reference
(Dryer type)
Electricity consumption
Thermal energy
consumption
Total energy consumption
MJ/t water evaporated
Air and flue gas dryers
Mujumdar 1995
4000-6000 MJ/t
Ihren 1991
2800-4500 MJ/t
Tapanainen 1982
(cascade dryer)
yes
5800 yj/t
Tapanainen 1982
(rotary dryer)
30 kWh/t
110 MJ/t
3600 - 3900 MJ/t
Multiple-effect dryers
Ihren 1991
(2-effect dryer)
1700 - 2700 MJ/t
Tapanainen 1982
(multiple-effect dryer)
1700-1 BOOMJ/t
2900 MJ/t
B -2
------- |