5. Wood energy technologies - Nest

5. Wood energy technologies
The possibilities of using wood energy are wide. They go from its use in
households for cooking to its application in great capacity units in order to generate
electricity and produce heat in industries. This chapter shows the main technologies of
biomass conversion, particularly wood energy resources that adopt the previously
discussed processes. It will also comment on biomass preliminary treatment processes
such as drying and densification, which are important in some cases.
Although biomass can be frequently employed in a direct way, as for example
the direct burning of firewood in furnaces and boilers, its conversion into other
energetic products allows the increase of its energetic homogeneity and density. As a
consequence, its storage and transport conditions are improved, making its end use
easier and more widespread. Thus, converting solid biomass into liquid and gaseous
fuel, for example, allows its use in internal combustion engines and gas turbines.
Another example is the transformation of firewood into charcoal producing a fuel with
much less moisture, a higher calorific value and much more homogeneous, besides not
producing smoke while burning.
However, it is important to notice that in all of the processes of converting
biomass into other energy products, there is always energy consumption by the process
itself and a cost that will fall upon the final energy product. Only a carefully analysis of
the different processes and of the impact of the losses associated with the process allows
the determination of the feasibility of a certain technology.
5.1. Pre-processing of wood energy resources
Raw biomass, that is, biomass in the condition it is produced in forestial or
agricultural activities, or even as residues, can be presented in many different ways in
terms of size or moisture, but may not be completely fit to its utilization in the
conversion processes. In the pre-processing stages, which will be dealt further on, the
size reduction, densification or drying can be employed in order to adjust the
characteristics and improve the efficiency in the subsequent conversion processes.
A. Size reduction
Trying to increase reactivity and the specific surface of the solid bio-fuels, it is
necessary to reduce the size of the raw biomass in some occasions. As it is indicated in
Table 5.1, it can be observed that the different systems of biomass utilization require

well-defined particle sizes in order to achieve good operating conditions and high
efficiency. For example, moving bed gasifiers are more efficient when operating with
biomass in pieces of 5-10 cm. This way, it is necessary to cut the wood in pieces within
these dimensions, or in case of a fine-sized biomass residue the pieces should be fitted
to 5 – 10 cm by means of briquetting. For those systems where biomass has a short
residence time within the reaction zone, a fine-sized biomass makes them more
efficient. In this case, different types of mills are used for pulverization. Figure 5.1
shows the most widespread types of wood shredders.
Table 5.1 – Biomass recommended particles/sizes for different applications.
Type of biomass utilization system Recommended size, mm
Moving bed. 50-100
Suspension burning < 6.0-7.0
Bubbling fluidized bed 20-30
Circulating fluidized bed < 6.0-7.0
B. Drying
Some types of biomass present very high moisture making their use as fuel
difficult and, therefore, reducing the amount of available energy to be converted into
heat. Within the combustion processes, the evaporated moisture consumes part of the
released energy, the one that is, technically, hard to recover. Besides, it makes the fuel
ignition difficult and reduces the combustion temperature. Thus, most of the combustion
systems require a fuel with moisture lower than 50-60% (wet basis). Considering the
calorific value increasing with moisture reduction, the less the moisture, the less
biomass is needed.
In relation to gasifiers, moisture remarkably affects the composition and the
calorific value of the attained gas, so the recommended moisture ranges between 15% to
20%. Recently cut firewood as well as sugar cane bagasse at the mill outlet present
approximately 50% of moisture in wet basis, a level considered to be high for an
efficient use. This way, a preliminary drying is necessary in order to fit the biomass
moisture for a certain conversion process.
The two main factors that limit the drying rate are: the transfer of biomass
moisture into the air and the diffusion or migration of the moisture inside the biomass
volume. The biomass drying can be carried out in a natural way or by using dryers.
Through the natural drying process it is possible to achieve a final moisture ranging
between 15% to 20% in wet basis within a period of two or three months when the
biomass is stored under good conditions of air circulation and climate.
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Figure 5.1 – Wood shredders
The dryers allow this time to be significantly reduced, even during rainy and wet
periods. The most common drying agents used in dryers are the biomass combustion
products from the burning in boiler furnaces, which are released into the atmosphere at
temperatures ranging between 150 o to 300 o C. In the last few years, drying with
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superheated steam has been highly widespread. The most commonly employed dryers
and their features will be dealt next.
Rotary dryer: This dryer, shown in Figure 5.2, is known by the sugar industry as
Rader-Thompson and it can be employed for a wide scope of biomass types and sizes.
The dryer itself is a drum with blades throughout its internal perimeter that allow the
residence period of the material inside the dryer to be enhanced. The final moisture that
rotary dryers can achieve operating with sugar cane bagasse is approximately 35%.
There are three of these dryers installed in Hawaii. Their average electric energy
consumption is 13.5 kWh/t of bagasse to dry (KINOSHITA, 1989). According to the
same author, a 0.14% reduction of CO concentration in the combustion products was
observed, and this is equivalent to an increase of 0.7% in the boiler efficiency. The
efficiency of the steam generators increases about 5% when the dryer is in operation.
Figure 5.2 – Rotary dryer for biomass.
Figure 5.3 – Pneumatic dryer for bagasse
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Pneumatic dryer: In this equipment, the biomass drying takes place basically during its
movement through pneumatic transport in the dryer vertical duct as it is shown in Figure
5.3. The COPERSUCAR Technology Center in Brazil developed a pneumatic dryer for
sugar cane bagasse with a capacity of 20 t/h. It can reduce the bagasse moisture from
50% to 23% (CAMPANARI, 1984). As a disadvantage, its design and operation
efficiency greatly depend on the biomass particle size. Studies carried out by NEBRA
and MACEDO (1989) detected that an important fraction of the moisture reduction
takes place in the separating cyclone.
Mixed bed dryer: This type of dryer, presented by FORSS and MUONIOVARA
(1996), is coupled to fluidized bed combustors. It uses the heat directly from itself for
the drying, and that takes place in steam atmosphere. This way it is possible to recover
the latent heat for the process at a reasonable thermal level. The increase in the
efficiency of the plant is 15% and it is determined based on the LCV. Figure 5.4 shows
the operating principle of a mixed bed dryer.
C. Densification
Figure 5.4 – Scheme of a mixed bed dryer.
The low density of some types of biomass, particularly agricultural and agroindustrial
residues such as sawdust and wood shaves, rice husks and sugar cane bagasse,
makes their long distance transport and storage difficult, therefore economically
unfeasible. In order to improve this situation, a lot of work has been done within the last
few years in order to develop different densification technologies.
The elements of densified biomass are named pellets or briquettes, and their
predominant particle size ranges between 10 and 30 mm with a density that can reach
from up to 1,100 to 1,300 kg/m3. Typically, the products attained through selfagglomeration
by means of a combined action of heat and pressure are called pellets,
whereas briquettes are the densification products that require biding agents such as
charcoal. The most common densification systems are shown in Figure 5.5. They can be
presses with mechanical or hydraulic driving or with disc or annular-shaped matrices, or
even, extruding ones (or screw systems).
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Figure 5.5 – Equipment for biomass densification.
The energy demand that is necessary to achieve a suitable density has important
influence regarding the densification systems of some sorts of residues. REED et al.
(1980) pointed out that the amount of energy necessary for the densification could be
reduced in approximately twice if the biomass was previously heated at a temperature
ranging between 100 and 230°C. In addition, such fact leads to a certain increase in the
pellets calorific value because of the release of part of the volatiles with low calorific
value.
The energy consumption of a wheat straw densification installation with annular
matrix press ranges typically from 37 to 64 kWh/t. However, considering all the energy
consumed during the harvest, the size reduction and particle size classification, the
number mentioned above grows from up to 90 to 120 kWh/t. (THOMAS, 1980).
Results of researches about sugar cane bagasse densification can be found in
McARTHUR (1981) and SILVA (1988). GREVER and MISHA (1994) compared the
piston presses with the extruding ones during the production of briquettes out of wood
residues, and the main results are displayed in Table 5.2. Their conclusions indicate that
the extruding presses are more appropriate for developing countries. Matrix presses are
frequently used for sugar cane bagasse for they are more economical regarding this
matter.
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Table 5.2 – Comparison between piston and extrusion presses
Parameter or characteristic Piston press Extruding press
Matter optimum moisture 10-15% 8-9%
Briquette density 1,000-1,200 kg/m3 1,000-1,400 kg/m3
Power consumption 50 kWh/t 60 kWh/t
Maintenance high low
Use in gasifiers not recommendable recommendable
5.2. Biomass direct combustion
The burning of biomass, mainly firewood, is the most widespread wood energy
technology and it presents several systems according to the application context. Initially
the aspects associated with household use will be presented, afterwards the industrial
systems will be considered.
A. Residential systems
Millions of people throughout the world depend on biomass as the only possible
energy source for daily cooking. It is estimated that 75% of the biomass used in
developing countries have this purpose. In general, the systems employed for biomass
burning are extremely simple and they don’t allow the appropriate use of the thermal
energy generated by the combustion, besides they produce a lot of smoke, consequently,
they are hazardous for the users’ health. Figure 5.6 shows a classification of biomass
stoves, specially taking the employed fuel into account. The schemes corresponding to
these types of stove are presented in Figure 5.7.
Figure 5.6 – Biomass stove classification.
(The letters in parentheses indicate the corresponding scheme in Figure 5.7)
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Driven by the possibilities of improving life conditions and reducing bio-fuels
consumption, above all in regions where they are scarce, over the last 20 years several
institutions have been working on the development of efficient models of stoves that use
firewood and other sorts of biomass in order to accomplish a more reasonable use of
this fuel. It is considered that an appropriate stove for biomass must be able to meet the
following conditions:
• High efficiency in combustion and heat transfer to the pans.
• Sufficient thermal power (According to MUKUNDA (1993), at least 4 KW for one
hour and a half).
• Low cost and high durability.
• Low pollutant emission into the environment (CO and tar).
• Acceptance among the users.
• Easy manufacturing and repairing (using local material).
• Safe use (low risk of fires and burns).
• Easy fire setting.
• Not making the saucepans dirty on the outside.
a) Three stone stove.
b) “Heavy” stove with chimney.
c) “Light” stove with chimney
(Nepal).
d) No chimney stove for one pan
(Thailand).
e) No chimney stove for two saucepans
(Indonesia).
f) Charcoal metal stove (“Jiko” type).
g) Charcoal pottery stove
h) Sawdust and rice husk compact stove
Figure 5.7 – Different types of biomass stoves scheme
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The presence of a chimney is relevant to reduce the impacts associated with the
smoke emissions and it can also help to control the air excess, which directly affects
efficiency. Suitable culinary practices such as washing vegetables before cooking and
the use of lids, as well as using dry firewood protecting it from inclemency, are
measures that are as important as the stove design.
Programs introducing improved and low cost stoves, such as the Lorena stove
(built with clay and sand) from Guatemala, were introduced in practically every
continent showing distinct degrees of success. In technical literature there is a great
amount of information regarding the development and diffusion activities of traditional
and improved technologies for cooking with firewood or charcoal. So, it is evident that
it is always important to respect the values and interests of the consumers, giving
special consideration to the role of women.
In order to determine the efficiency of biomass stoves, the method that is usually
accepted is the boiling water test. In this case, the efficiency is calculated as the relation
between the amount of heat absorbed by the water in the cooking pans and the amount
of heat supplied by the fuel. The equation for the calculation is:
Where:
M H O . C.( Te− Ti) + Mevap . L
2
η=
. 100 % (5.1)
t
M . PCI
comb
M - Mass of water in the pan at the beginning of the experiment, kg
HO
2
C - Water specific heat, kJ/kg o C
Te - Boiling temperature, corresponding to the atmospheric pressure at the
place where the stove will be used, o C
Ti - Water temperature at the beginning of the experiment, o C
M - Mass of evaporated water, kg
evap
L - Water vaporization heat, for the local utilization conditions of the stove,
kJ
M comb. – Mass of burned fuel, kg
LCV t - Fuel low calorific value, kJ/kg
Table 5.3 shows the efficiencies, which are mentioned in the literature, for
different types of firewood stoves. They were attained by using the boiling water test.
The efficiencies of gas and electrical commercial stoves are used with comparison
purposes.
The estimative of firewood demand within the household sector is almost always
carried out based in surveys and field evaluations, and it can vary significantly from one
region to another, above all due to the fuel availability. The average value of firewood
annual demand is about 1 m 3 per person. In order to help the survey, it is often
necessary to know the load units of firewood for household consumption, and the
number of people necessary for their transport, as shown in Table 5.4. These data were
evaluated within Brazilian conditions.
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Table 5.3 - Efficiencies recorded in different types of firewood stoves and commercial
kitchens.
Type of stove Efficiency References
“Three stone” stove 10-7
10-15
BHATT (1983)
WEREKO-BROBBY e HAGEN
(1996)
“Heavystove” stove with
chimney
15-23 CLAUS e SULILATU (1983)
No chimney stove for one pan 30 DUNN et. alli (1983)
35 WEREKO-BROBBY e HAGEN
(1996)
No chimney stove for two pans 18-22 CLAUS e SULILATU (1983)
Compact stove for sawdust and 15* MUKUNDA et alli (1993)
rice husks
32-36**
Gas stove 57 WEREKO-BROBBY e HAGEN
(1996)
Electrical stove 50 WEREKO-BROBBY e HAGEN
(1996)
* Traditional model, ** Improved model
Table 5.4 – Firewood typical loads for household use
Person that carries firewood typical load, kg
Boys (8-10 years old)
7
Teenagers – male (12-15 years old)
15
Adult women
23
Teenagers – female
33
Men
35
Another way of using wood energy within the household scenario, which is not
as important as the preparation of food, is in heating kilns. Furnaces are basically used
for that purpose, and over the last few years they have shown remarkable development.
In some industrialized countries, such as Canada and France, firewood has become a
source of heat throughout cold periods, where, most of the time, it is consumed in
automized and highly efficient systems. Some countries in Europe, as for example
Denmark, Sweeden and Finland have been using firewood biomass and agricultural
residues for heating in distrital central heating systems where the heat is produced in a
central boiler and it is distributed to the houses as hot water and low pressure steam.
B. Industrial systems (Process heat generation)
Direct biomass combustion is widely used to generate heat in several industrial
processes using kilns or boilers. In the first case, kilns require high temperatures, in
general over 500 o C, as for example, the manufacturing of pottery products where the
heat produced by the combustion must be transferred directly to the material being
processed. On the other hand, the objective of the boilers is the steam production, which
is used as a thermal energy source in the industrial process or also, used for the
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production of mechanical or electrical power by means of steam turbines. The turbine
exhaust steam can also be used for heat supply, which is known as cogeneration.
The industrial processes need steams at relatively low pressure levels,
approximately ranging from 0.2 to 0.4 MPa. However, considering that the elevation of
the steam parameters (pressure and temperature) allows the efficiency to increase during
electricity generation in a cogeneration cycle, it is common to produce steam at
pressures that range between 7 and 14 Mpa and temperatures between 500 and 550 o in
modern boilers.
There are many examples of industrial systems that depend on biomass direct
thermal energy. In plants that produce cellulose pulp and in sugar mills the main fuels
are by-products and lignocellulosic residues such as bagasse, black liquor and cork.
Biomass is also the essential energy source in several types of small and medium-sized
agro-industries that process and treat agricultural and cattle products. This way, its
thermal applications are commonly used for coffee and rice processing are common
where, besides firewood, their own by-products (coffee pods and rice husks) constitute,
most of the time, the fuel used to obtain hot air for the drying and the generation of
electricity required for the treatment process of the grains.
The brick and tile industry is another example of biomass consuming industry,
mainly firewood, which is used in kilns to burn bricks and tiles. In Brazil, orange juice
industries located in sugar cane areas consume a considerable amount of sugar cane
bagasse for fuel. Another interesting case that can be mentioned is the extensive use of
spent coffee grounds in soluble coffee industries because of the high moisture of this
biomass residue. This topic presents the types of equipment and the basic methodology
employed for their evaluation.
B.1. Grates and combustion systems
In order to make use of an efficient combustion, i. e., a complete burn of
biomass using the least amount of air possible, several systems can be employed. There
are two basic types: biomass bed (or piles) or suspension burning. While the first is
simpler, the suspension systems require small-sized biomass allowing the operation
with less air excess, thus they are more suitable for high capacity equipment. Figure 5.8
presents schemes of the different types of combustion systems used for biomass burning
of which technical characteristics are shown in Table 5.5 according to MITRE (1982),
PERA (1990), BAZZO (1992), SILVA (1995), TOPLEY (1992), HUGOT (1986) and
LEPPA (1982).
The most important parameters to define the furnace maximum capacity during
the burn of a certain fuel are the volumetric thermal load (Qv) and the grate surface
thermal load (Qf). These parameters define the amount of heat that is released by each
furnace volume unit or by the grate surface, respectively. Naturally, the high values of
these parameters indicate more compact systems that, at fist, are less expensive. On the
other hand, in order to make use of a suitable mixture of air and fuel, it is necessary to
use of air circulating systems with forced and induced draught, that in turn requires fans
and control and driving systems, consequently increasing their complexity.
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Table 5.5 – Technical characteristics of the furnaces used for biomass combustion.
Combustion system Qf
kW/m 2
Qv
kW/m 3
Boiler
maximum
capacity
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Biomass
moisture
t/h %
Cellular furnaces. 3.9 - 45 100 30-50
Suspension burning furnaces. 2.6-3.7 < 0.41 180-550 < 15
Figure 5.8 – Combustion systems used for biomass.

The combustion systems with cellular furnaces are being progressively set aside, above
all because of their low efficiency, which is a consequence of the high excess of air that
they need. In medium-capacity applications, inclined grate furnaces with inclinations
between 37 and 55 o C seem to be more interesting, for they allow the use of pre-heated
air at temperature levels that are superior to 300 o C, which helps to improve the
combustion conditions significantly. Suspension combustion systems are adopted at
elevated capacities and whenever the biomass is available in particles whose sizes are
smaller than 10 mm, which is the case of bagasse and sawdust
B.2. Boilers
Boilers or steam generators may have two basic types: of fire or water tubes. In
the first group, which is also known as fire tube boilers, the combustion products
circulate inside the tubes and transfer their thermal energy to the water that surrounds
them. This type of boiler is high employed, although it presents limits in terms of
production capacity that reaches approximately 20 t/h and steam maximum pressure of
2.0 MPa. It is mainly applied in small and medium agro-industries that nearly always
demand saturated steam with a pressure lower than 0.4 MPa for heating purposes.
Figure 5.9 – Fire tube boiler for biomass.
In watertube boilers, the water that will be vaporized circulates in the interior of
the tubes that receive heat from the gases resulting from biomass combustion from the
outside. Through this design it is possible to attain great volumes of steam, even
superheated and high-pressure ones. Watertube boilers are adopted whenever the steam
pressure to be produced surpasses 2 MPa and the steam capacity is greater than 20 t/h.
Owing to the development of the technology of constructing boilers, today, there are
different types of boilers for biomass, which are displayed in Figure 5.10. The two drum
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convective boilers and the radiant boilers correspond to the modern technology of heat
generation with high capacity and efficiency. Table 5.6 indicates the most significant
characteristics of watertube boilers.
Figure 5.10 – Basic constructive types of biomass firetube boilers.
Table 5.6 – Most important data and parameters of the different types of biomass
boilers.
Data and parameters Straight
tubes
Typically adopted
furnace
Efficiency (related to the
LCV), %
cellular
furnace or
inclined
grate
Types of boiler
Curved
tubes with
several
drums
moving
grate
Convective
with two
drums
moving
grate
Radiant
suspension
or fluidized
bed
50-60 50-70 70-80 80-87
Steam temperature, o C 300 320 320-510 400-550
Steam pressure, MPa 1.8 1.8-3.0 1.8-14.0 7.0-13.0
Steam maximum
generation, t/h
35 60 80 até 550
Figure 5.11 shows a scheme of a modern watertube boiler for biomass burning.
According to the numbering used in this figure, its main parts and functions are:
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1. Furnace: it is the space or volume where the total combustion of the gases
resulting from the pyrolysis of the biomass on the grate takes place. In
burning in suspension systems, the burning of the solid fuel occurs in all its
own volume.
2. Grate: it is the element that supports the combustion matter at the same time
that distributes the primary air. This device also guarantees the periodic
removal of the accumulated ash on the grate.
3. Biomass feeders: Their function is to feed the fuel that will be burnt in the
furnace distributing it in homogenous layers on the grate. They may be
mechanical or pneumatic.
4. Water walls: they are evaporative surfaces that cover the walls of the furnace
partially or completely. It is formed by tubes where the water circulates
throughout the evaporating process.
5. Convective bank: It is a group of tubes connected to the superior and inferior
drums. It is also an evaporative surface that receives the heat of the gases due
to the convection.
6. Superior drum: it receives the water-steam mixture that comes from the
evaporative surfaces carrying out the separation of the saturated steam from
the water supply to the evaporative surfaces through downcorner feeding
tubes.
7. Inferior drum: It works as a collector-distributor of water during the
evaporation.
8. Superheater: It allows the conversion of saturated steam into superheated
steam with a reduced loss of pressure. It has devices to control the steam
temperature called atemperators.
9. Air pre-heater: It is a surface where the pre-heating of air, which will be
introduced in the furnace together with the biomass, takes place. It uses
residual thermal energy that is available in the combustion products.
10.Economizer: It is a heat exchanger. It pre-heats the feeding water using the
combustion gases until saturation temperature.
Figure 5.11 – Biomass steam boiler scheme.
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The economizer and the air pre-heater are components that don’t necessarily
exist in every boiler. However they are interesting, for they allow the recovery of part of
the thermal energy available in the gases that will go to the stack, therefore reducing
losses and increasing efficiency.
Figure 5.12 shows the scheme of a modern biomass steam boiler of 210 t/h
steam capacity and steam parameters 6.6 MPa and 520 o C. (APU-70-76I-PSE type
boiler manufactured by Caldema Equipamentos Industriais Ltda). It is a suspended or
hanging boiler with seven section pinhole type grate. It has primary and secondary
superheaters, one pass in the convective bank, primary and secundary air pre-heater, and
finned economizer. Superheated steam temperature control is carried out by using a
ventury attemperator located in the intermediate collector between primary and
secundary superheaters. Water used for temperature control comes from a tube-shield
condenser, that condenses the steam from the drum. The reason to use a condenser is
that the water, obtained by steam condensation, injected in the Venturi, has the same
quality as the superheated steam to be attemperated. The fact that this boiler is being
used for cogeneration makes the occurrence of salts deposition on the steam turbine
blades possible, so a high quality of superheater steam is required
Figure 5.12 – Industrial steam boiler for sugar cane bagasse burning (courtesy
of Caldema Equipamentos Industriais Ltda).
A very important operational aspect regarding steam generation is the quality of
the water. It must have a low content of salts, for after the water evaporation these
products will be deposed in the surfaces of thermal exchange and they cause highly
risky situations. This way the water must be treated before being fed to the boiler
through the addition of chemical products, and periodical extractions of the liquid
volume present in the boiler superior drum must be carried out. The most frequently
used chemical products are phosphates and hidrazine.
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In the last 25 years, fluidized bed boilers appeared in the market. Their
advantage in relation to conventional technologies allowed to enhance significatilly the
utilization of different kind of biomass residues for electricity generation and
cogeneration. A common aspect in all the fluidized bed boilers is the combustion of the
fuel in an inert material particulate volume (sand), that guarantees an intensive heat
transfer, allowing the combustion to happen at a relatively low temperature level (800-
900 o C).
There are two types of fluidized bed boilers:
- With a bubbling fluidized bed - BFB (Figure 5.13). The inert bed (sand) expands, but
keeping a defined height. Fluidization velocity (air velocity in the free furnace cross
section) is 1-3 m/s;
- With a circulating fluidized bed - CFB (Figure 5.14). The inert material and the fuel
circulate through the bed, a separating cyclone and finally returns to lower part of the
bed. Combustion is finished in a few circulating cycles, making the residence time and
combustion efficiency to be high. The fluidization velocity in these systems is higher,
attaining 4-12 m/s. CFB units reach a capacity up to 250 MW.
Figure 5.13 – Bubbling fluidized bed boiler for biomass.
Figure 5.14 – Circulating fluidized bed boiler for biomass.
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Fluidized bed boilers have the following advantages (VTT, 2001):
- Combustion stability, even with considerable variation in size distribution, moisture
content, ash content and fuel calorific value;
- Possibility of using fuels with low volatiles and high ash content;
- Possibility burning different fuels simultaneously;
- Possibility of an efficient control of SOx and NOx emissions, not requiring high cost
control equipment.
B.3. Boiler efficiency
A very important parameter to indicate the perfection degree in an energy
conversion process, such as the one that occurs in a steam generator, is the efficiency,
which is shown in this topic. If one considers that the total energy and mass involved in
the combustion and heat transferring processes, the determination of the efficiency in
the conversion of the chemical energy available in the fuels into steam potential energy
will be essentially based on the evaluation of mass and energy flows that can be found
in steam generators as it is shown afterwards.
Figure 5.15 shows the mass balance of a steam generator where:
mvs - steam production, kg/s
La - air flow for the combustion, kg/s
C - fuel consumption, kg/s
∆L1, ∆L2 e ∆L3 – air infiltration in the boiler, kg/s
Gcen1 - ashes removed through the grate, kg/s
Gcen2 - ashes removed through the convective bank hoppers, kg/s
Gcen3 - ashes dragged with the gases, kg/s
Daa - feeding water flow, kg/s
Dext - drum extraction (purge) flow, kg/s
- exhaust gas flow, kg/s
Lg
By using these variables, the mass balance for the fuel and gases, as well as for
water and steam can be directly written. In the same way, once the fuel ash content is
known, A t , the balance regarding these inert materials can also be implemented in terms
of combustion. In this present analysis, the ashes were considered to leave the boiler
from three points: the grate ashtray, the hoppers of the gas vertical conduct and as
volatile ashes dragged together with the gases.
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Fuel and gas balance.
3
Figure 5.15 – Mass balance in a steam generator.
∑ ∑
L + C+ ∆L
= L + Gcen
a i g
i = 1
i = 1
Water and steam balance.
Ash balance.
Daa = mv+ Dext
.
t
A C = Gcen + Gcen + Gcen
1 2 3
3
i
79
(5.2)
(5.3)
(5.4)
In order to carry out the energy balance in a steam boiler the fuel high or low
calorific value can be considered as the available energy. Actually, both of the
parameters are employed. However, it is important to remember that the water present
in the combustion products remains as steam for low calorific values, which is a typical
condition in the gases of a real boiler. This is the reason why, in the scheme of Figure
5.16 where the thermal energy flows in a boiler and in the subsequent balances are
shown, the LCV t is employed. The variables in this case, expressed in kJ/kg of fuel or
power units are:
t
Q d
Qev
Qca
- energy that enters the boiler control volume, known as “available
heat”, and corresponds to the fuel low calorific value
- energy absorbed in the evaporative surfaces
- energy absorbed in the air pre-heater

Qec
Qsa
Qp2
Qp3
Qp4
Qp5
Qp6
Hge
- Energy absorbed in the economizer
- Energy absorbed in the superheater;
- Stack gas heat losses;
- heat losses caused by incomplete combustion due to combustible
gases (CO, H2 and CH4) presence in the combustion products.
In technical literature, they are named heat losses because of
incomplete chemical combustion;
- heat losses caused by incomplete combustion due to carbon and soot
presence in the ashes. The literature calls them heat losses caused by
incomplete “mechanical” combustion (This is related to the fact that
this losses are a consequence of a “mechanical” or physical process of
fuel particles carry-over);
- heat losses to the surroundings removal of ashes through the boiler
walls;
- heat losses due to high temperature removal of ashes;
- exhaust gases enthalpy.
The fraction of available heat that is used or “useful heat” transferred to the
steam may be defined as the difference between the available heat in the fuel and the
several losses, that is:
6
t
Qutil
= Qec
+ Qev
+ Qsc
= C.
PCI −∑
Qpi
i=
2
Table 5.7 – Heat losses in boilers
Losses Value approximate
range, %
Comments
80
(5.5)
Qp2 8 - 18 For a temperature of exhaust gases
from 180 to 200 o C, Qp2 ≈ 12% of the
LCV t .
Qp3 (as fuel gases) 0.5 – 1.5 At normal conditions, the fraction of
fuel gases represents about 0.5% of the
LCV t
Qp4 (as carbon in the
ashes and soot)
1 - 4 This loss depends directly on the
combustion system and fuel size
Qp5 0.8 – 4.5 In boilers that are correctly designed
and operated, this less is about 2% of
the fuel energy
Qp6 < 0.1 It can be neglected.

Observe that Qca was not included as useful heat, for it constitutes an internal
transference in the control volume, that is, the energy that is recovered from the exhaust
gases in the air preheater is reintroduced in the furnace as hot air. Table 5.7 presents the
indicative values of heat losses in real boilers. Considering extreme situations, the
efficiency of well-designed and well-operated steam generators usually ranges between
65 and 85% for small-sized systems with great generation capacity, respectively.
Naturally, the previous expressions could be more detailed or could be presented related
to other parameters. Figure 5.16 shows a diagram of energy flows entering and exiting
the control volume of a steam generator.
Figure 5.16 – Energy balance in a boiler.
Once the mass and energy balance are known, the relations to determine the
efficiency can be shown, that is, the fraction of biomass heat in the produced steam.
Since the sum of the used heat and the losses always equals the heat supplied by the
burning of the fuel, it is possible to calculate the efficiency through two different ways:
evaluating the useful effect or the heat corresponding to the steam (direct balance) or
measuring
or estimating the losses that subtracted from the heat supplied by the biomass
equals the useful heat (indirect
balance):
Direct balance:
Q
=
CPCI .
util
ηc t
Indirect balance:
η
=
C.
PCI
t
−
6
∑
Qp
i
6
∑
Qp
i
∑
i = 2
i=
2 = 1 − = 1 − specific losses
t
C.
PCI
C
81
(5.6)
c (5.7)

The indirect method is the most used one, for it allows the efficiency to be
determined without necessarily impose fuel and steam flow measurements, which are
specially complicated in the case of small and medium-sized systems. In general, the
most important losses take place because of the residual energy of the exhaust gases,
and they are evaluated by measuring the temperature of the gases and the air excess that
most of the time, is measured by using gas analyzers (determining the gas composition).
5.3.
Applied gasification
This topic presents the basic aspects of the conception and operation of gasifiers
that allow the conversion of solid biomass into gas fuel. The biomass gasifier is an
equipment that present a great technological diversity
and they can be classified in
several
ways according to the following parameters:
Produced gas calorific value:
Gas of high calorific value - from 10 to 40 MJ/Nm 3 • Gas of low calorific value – up to 5 MJ/Nm
.
3 .
• Gas of medium calorific value - from 5 to 10 MJ/Nm
•
It must be observed that Nm
nce conditions (normal conditions), that is, 1 atmosphere of pressure and
0ºC.
3 is understood to be the gas volume measured at
refere
Type of gasification agent:
• Air.
• Water steam.
• Oxygen.
Working pressure:
• Low pressure (atmospheric) .
• Pressurized (up to 3 MPa).
Direction of the biomass relative movement and of the gasification agent:
• Bed moving in the opposite direction of the gas flow (counter current
or up-draft).
• Bed moving in the same direction as the gas flow (down-draft).
• Bed moving perpendicularly
to the gas flow (cross-draft).
• Fluidized bed.
The calorific value of the attained gas greatly depends on the type of gasification
agent and the operation pressure as it is indicated in Table 5.8, which, also shows the
possible applications of the produced gas. The gasification employing air is the most
widespread and allows the production of a lower cost gas. However, in this alternative,
the gas presents a low calorific value. When the gasification gas constitutes the raw
material for the production of liquid derived from biomass, steam or oxygen must be
3 .
82

employed as gasification agents. Another factor that affects the gas calorific value is the
biomass moisture, which is recommended to be lower than 20%.
Among the gasifier classification criteria presented
above, the most adopted is
the
direction of biomass movement and the gasification agent relative movement. Figure
5.17 displays the schemes of basic types of gasifiers.
Figure 5.17 – Types of biomass gasifiers.
Table 5.8 – Dependence among the type of gasification agent employed, the gasifier
pressure, the gas calorific value
and its possible application.
Gasification agent air steam oxygen
Operation pressure Atmospheric Atmospheric Pressurized Gas calorific value
3
(MJ/Nm )
4 – 6 10 - 18 18 - 14
Application Production of Production of power or synthetic liquid fuels
power
(methanol, ammonia and gasoline)
83

A. Gasifier comparison
Normally, moving bed gasifiers (up-draft and down-draft) are more simple and
cheaper units than the fluidized bed ones. In addition, the up-draft bed gasifiers present
high thermal efficiency in spite of the relatively high content of tar in the gas, especially
when the gasified biomass is wet. On the other hand in down-draft gasifiers, the tar
content is low, which is a consequence of the gas tar cracking in the oxidation zone.
This
is the reason why down-draft gasifiers are the most employed ones for “in natura”
biomass, such as wood.
Among the main disadvantages presented by moving bed gasifiers we can
highlight the need of biomass size uniformity and the capacity limitation. This
equipment requires fuel particles that must be relatively homogenous in size and inferior
to 100 mm in order to guarantee the biomass descending movement in the interior of the
reactor at the same time it allows the passage of the air and the gases. The open top
gasifier, shown in Figure 5.18,
is a modification of the down-draft bed gasifiers that,
because
of their feeding system, allow the use of some types of agricultural and
industrial
residues as fuel.
Figure 5.18 – Scheme of an open top gasifier.
The capacity limitations are associated with the difficulty in scaling the gasifiers,
specially the down-draft types. The diameter of the throat in the oxidation area is
limited by the need of reaching, in the whole cross section, homogenous temperatures in
the range of 1.400ºC, guaranteeing a high efficiency for cracking the tar. The up-draft
gasifiers, even though they are characterized by producing a relatively dirty gas, present
easier scaling, once they don’t have any restrictions regarding the diameter of the throat.
Because of these reasons, up-draft gasifiers have been applied to supply thermal energy
for domestic heating, as well as for industrial applications. According to
84

BEENACKERS and MANIATIS (1996), a lot of companies have been offering this
type of gasifier in Europe for capacities up to 10 MWt, with an investment cost of 387
US$/KW and a thermal generation cost of 22.1 US $/MWh. The company Bioneer has
already installed 10 up-draft gasifiers of 6 MWt capacity for wood and peat.
Fluidized bed gasifiers are considered to be the most convenient ones for high
capacity applications, as in BIG/GT systems for example, because of their high
flexibility in relation to the fuel (allowing the use of low density and fine-sized fuels,
such as most of the agro-industrial residues), as well as the facility in scaling. The
pressurized systems can have more compact installations, although their biomass
feeding system is more complex.
According to the company Studsvik the advantages of
high
pressure are evident for more powerful installations ranging between 50-80 MWe
(BLACKADDER
et al., 1993).
Table 5.9 presents
data about the average composition and calorific value of the
poor gas, obtained from biomass, in different types of gasifiers with different
gasification agents.
However, there is not a significant difference between the composition and
calorific
value of the gas obtained in the atmospheric and pressurized gasifiers, that use
air as
gasification agent (Bridgwater, 1995).
Table 5.9 - Average composition and calorific value of the poor gas obtained
in
different types of gasifier, using different gasification
agents
(adapted from KALTSCHMITT and HARTM AN,
2001).
Compounds Units
Atmospheric pressure
gasifier
Air S team
Pressurized Gasifier
(0.5 - 2.0 MPa)
H2 % in volume 12.5 38.1 4.0 - 15.0
CO % in volume 16.3 28.1 10.0 - 19.0
CO2 % in volume 13.5 21.2
14.0 - 19.0
CH4 % in volume 4.4 8.6
5.0 - 9.0
HC % in volume 1.2 3.0 ⎯
N2 % in volume 52.0
0 45.0 - 60.0
Calorific Value
3
MJ/Nm 5.1 13.2 3.5 - 6.5
* This value ranges between 3.0 and 6.5 MJ/Nm 3
B. Gasifier efficiency
Figure 5.19 shows the necessary elements to establish the mass and energy
balance
in a gasifier and to determine its efficiency. In this figure and in the next
equations it was adopted that:
mb
PCIb
PCIg i
mg
PCIg
tg
- biomass flow that enters the gasifier,
kg/s
- biomass calorific value, kJ/kg
3
- “i” gas calorific value, MJ/Nm
- gas flow produced during
the gasification, kg/s
- gas calorific value, MJ/Nm
, ºC
3
- gas temperature
85

ma
mcen
Qma
Ci
- gas flow introduced into the gasifier, kg/s
- ash flow, kg/s
- heat lost to the environment, kW.
- volumetric or molar concentration of the “i” gas.
Figure 5.19 – Mass and energy flow in a down-drought gasifier.
The calorific value of the gas can be calculated through its volumetric
composition
by using the following equation:
n
PCIg = ∑
i=
1
PCI
g i
. C
i
PCI +
g 0.
126CCO
0.
358CCH
0.
108CH
+ 0.
59CC
H 0.
637CC
H
+ = , MJ/Nm 3 (5.8)
4 +
CO CH C
4 C2H
4 2 6 H C C H2 2
Where C , C , , e C refer to the volumetric concentrations of
CO,
CH4, C2H4, C2H6 and H2 in the biomass gas measured in percentages, respectively.
The mass balance is expressed by the following equation:
mb+ ma= mg+ mcen
2
4
2
6
86
(5.9)
The energy balance can be carried out by considering the energy that enters the
gasifier, which must be the same as the one exiting. In this case, the
enthalpy is the
measure
of the thermal energy per unit of mass of air, gas and ashes.
m . PCI + m . h = m . PCI + m . h + m . h + Q
(5.10)
b b a a g g g g cen cen ma
For the produced
gas, the enthalpy is the result of the effect of the several gases
that
compound it.
h C h C h C h
g = CO. CO + H . H + ......... + n.
2 2 n
(5.11)

Two important concepts arise from the energy balance: cold efficiency and hot
efficiency:
Cold efficiency: η f
Hot efficiency:
That is:
η η
c = f + h g
*
Where:
h
*
g
mg. hg
=
m . PCI + m . h
b b a
mg . PCIg
=
mb . PCIb + ma . ha
b
b
a
a
87
(5.12)
mg.
PCIg
+ mg.
hg
η c=
(5.13)
m . PCI + m . h
a
(5.14)
- Gas “specific” enthalpy. (5.15)
Figure 5.20 – Efficiency in gasifiers.

Regarding thermal applications of the gasification, when the gas is directly
burned in a kiln, it is more convenient to refer to hot efficiency, for the gas thermal
energy is used. In power applications, such as internal combustion engines and gas
turbines, when the gas is cooled during its conditioning (the removal of particulates and
tar),
it is important to refer to cold efficiency as it is schematized in Figure 5.20.
A major operation parameter for gasifiers is the air/fuel relation, which is also
expressed as air factor. The gas superficial velocity
is also interesting for fluidized bed
gasifiers.
The meaning of these parameters is:
•
•
Air factor (FA): It’s the relation between the amount of air supplied to the gasifier
o
per kg of fuel and its stoichiometric value m a (see Table 4.2). The cold efficiency,
the bed temperature and the gas calorific value depend on the air factor value as it is
shown in Figure 5.21, for the case of fluidized bed gasifiers. The air factor optimum
value is generally ranging from 0.20 to 0.35, and this is equivalent to saying that in a
gasification process each kg of fuel needs between 20 and 35% of the amount of air
theoretically necessary for the combustion.
Gas superficial velocity (Vsg ): it is the velocity of the gas in the reactor free section.
Vsg ≈ 0,7 m/s in bubbling fluidized bed gasifiers. Vsg = 8 - 9 m/s in circulating
fluidized bed gasifiers.
Figure 5.21 – Typical relation between different parameters
and the air factor for
fluidized bed gasifiers.
The processes that take place during the gasification are so complex that the only
way of forecasting the gas composition, efficiency, temperatures in different regions of
the equipment and other operation parameters is a careful mathematical modeling based
on careful experimental data. However, the significant advances that have been
achieved regarding this technology make us believe that the gasifiers of great capacity
and efficiency will be operative within the next few years allowing a greater use of
biomass and their use in modern electricity generation systems, as it will be presented in
the next
chapter.
One of the professional program packages for the simulation of fluidized bed
gasifiers is the CSFB – Comprehensive Simulator for Fluidized Bed Equipment –
88

developed by Professor Marcio Souza-Santos of Faculdade de Engenharia Mecânica da
Universidade de Campinas (Mechanical Engineering College of the University of
Campinas). The program needs information on biomass characteristic, its flow and the
gasifier dimensions. The program calculates the gasifier performance at a stationary
regime using point to point energy and mass differential balance, chemical reaction
kinetics, fluid dynamics and an auxiliary database to calculate the physic-chemical
properties. The use of this program (CSFB) to design fluidized bed gasifiers allows the
optimization of important operating parameters such as the location of biomass feeding
point, the air factor and the bed dynamic height (that is, with an expanded bed at a
fluidization regime). This task is accomplished based on the influence of these
parameters on the gas calorific value, gasifier efficiency, and tar content in gas (Figure
5.22 - 5.25). These results are referred to the design of a 210 kWth gasifier shown in
Figure 6.20.
Figure 5.22 – Relation between the amount of tar in the gas and the location
of the
biomass feeding point (height above the distributing plate).
Figure 5.23 – Relation between the gas calorific value and the air factor.
89

Figure 5.24 – Relation between the amount of tar in the gas and the bed dynamic
height.
Figure 5.25- Relation between the gasifier efficiency and the air factor.
The significant advances that have been achieved in biomass gasification
technology make us believe that the high-capacity and high-efficiency gasifiers will be
operative within the next few years allowing a greater use of biomass in modern
systems for electricity generation. Some of them will be presented on the next chapter.
C – The gas quality issue
The gas, which is a product of biomass gasification, contains contaminants such
as solid particulates, tar, alkaline metals, hydrogen sulfide and ammonia. If they are not
removed, they may cause serious problems during the operation of the energy
equipment that uses this gas.
According to REED (1997), the tar and the particulates may be considered as the
Achilles’ heels of biomass gasification. As a consequence of the variety of substances
that compose the tar and the variation of their amount, which depend on the gasification
process parameters, it is difficult to establish a criterion to define a tar concept. MILNE
et al. (1998) define tar as “the organics produced in thermal or partial oxidation
(gasification) regimes of any organic matter, and they are assumed to de predominantly
aromatic”. It is evident that the technique used for the sampling and for the analysis may
modify the value attained for the amount of tar considerably.
90

The concentration of contaminants varies depending on the type of gasifier
(Figure 5.26). The moving bed down-draft gasifiers are characterized by producing a
gas with a smaller amount of tar (a product of the cracking of gas at the gasifier throat),
while moving bed up-draft gasifiers are characterized by producing a gas with large
amounts of tar. Fluidized bed gasifiers produce high levels of tar and particulates.
By comparing the amounts of contaminants in the gas at the outlet of different
types of gasifiers with the requirements established by the manufacturers for different
energy equipment (Table 5.10), it is possible to verify the need of using gas cleaning
efficient systems. (Table 5.11).
Small capacity systems normally use several stages of spray towers or Venturi
scrubbers, concluding the process with a sand filter. A lot of work has been done on
developing ceramic filters to separate particulate from the gasification gases at high
temperatures.
Figure 5.26 – Concentration of tar and particulates in the producer gas attained from
different gasifiers (GUIGON and LARGE, 1990).
Table 5.10 – Quality standards/requirements of the biomass gasification gas for
different technological applications (KALTSCHMITT and HARTMANN, 2001).
Concentrations and
characteristics of
controlled compounds
Units Internal
combustion
engine
Gas
turbines
Methanol
synthesis
Fuel cells
Particulate amount mg/Nm 3
< 50 < 30 < 0.02
Particle dimensions µm < 3-10* < 5
Tar amount mg/Nm 3
< 100** < 0.1 < 1
Alkali amount (K , Na) mg/Nm 3
< 0.25
NH3 amount mg/Nm 3
< 55 < 0.1 < 0.1
H2S amount mg/Nm 3
< 1150 < 1 < 1
*HASSLER AND NUSSBAUMER, 1999
** JENBACHER (1998) apud OBERNBERGER and HAMMERSHMID (1999)
recommended values less than 10 mg/Nm 3 .
91

Table 5.11 – Reduction of the amount of particulates and tar using different gas
cleaning systems (HASSLER and NUSSBAUMER, 1999).
Cleaning system Temperature Particulate Tar amount
o
C amount reduction,
%
reduction, %
Sand filter 10-20 70-99 50-97
Spray tower 50-60 60-98 10-25
Venturi scrubber - - 50-90
Wet electrostatic precipitator 40-50 >99 0-60
Baghouse filter 130 70-95 0-50
Tar fixed bed absorber 80 - 50
Tar catalytic cracking reactor 900 - >95
5.4. Charcoal production
The result of the biomass thermal decomposition or pyrolysis can be a gaseous
product, as it was shown in the previous section, or other products of energetic value
such as charcoal whose lower moisture, as well as its greater energetic density and
homogeneity are advantages of particular interest. This topic presents the most relevant
technological aspects of the biomass slow pyrolysis carried out in low temperatures, up
to 650oC for charcoal production.
Table 5.12 shows the typical mass balance during charcoal production out of
wood at 450°C in a laboratory kiln. Tar is a dense viscous liquid with phenolic
character. In certain places, where charcoal is produced in Brazil, some techniques to
recover the pyrolysis liquid products have been adopted, and in some cases, the desired
product may be the tar and not the charcoal, so the pyrolysis at higher temperatures is
recommended.
Table 5.12 – Material balance during the carbonization of wood at 450°C in a
laboratory.
Inlet Outlet
Material Weight
Carbon
content
Material Weight
Carbon
content
kg % kg %
Dry wood 100 47.5 Charcoal 42.0 74.3
Non-condensable
gases
20.0 34.2
Piroligneous acid 33.5 19.4
Tar 4.5 64.5
Considering traditional production systems, an average productivity of 165 kg of
charcoal per cubic meter of wood (solid) SCM is usually adopted, that is, 6 cubic meters
of wood are consumed for every tone of produced charcoal. However, this productivity
is greatly affected by the operation conditions, the kiln design and the wood moisture.
Better-operated systems may forecast values ranging between 200 and 250 kg of
92

charcoal per cubic meter. Table 5.13 displays the impact of the wood moisture and the
carbonization technology upon the specific consumption in carbonization systems.
Table 5.13 – Specific consumption in carbonization systems, as a function of the
firewood moisture
Carbonization Firewood moisture (%, dry basis)
Kiln 15 20 40 60 80 100
Specific firewood consumption (SCM/tcharcoal)
Trench 10 13 16 21 24 27
Metallic
transportable
6 7 9 13 15 16
Brick 6 6 7 10 11 12
Continuous 4.5 4.5 5 7 8 9
The systems for the manufacturing of charcoal can be continuous or
discontinuous. The discontinuous kilns are much more employed, mainly the fixed
models. However there are also transportable metallic kilns. Figure 5.27 shows the main
types of discontinuous kilns for carbonization. In this kind of equipment, the energy for
the pyrolysis is attained through the combustion of the raw material itself and its
gaseous products. The carbonization continuous processes have higher efficiency, but
they are much more sophisticated and expensive. They have not been applied
commercially yet and constitute issues for research and development to be carried out
by, above all, the great companies that consume coal such as some Brazilian iron and
steel making industries.
Figure 5.27 – Brick carbonization kilns
93

The kilns made of clay (pit kilns) have a life expectancy (useful life) between 1
and 5 years depending on the firewood that is used and on the operation cares. These
ovens are operated manually, retaining the fire on the top of it and progressively closing
the holes with clay from the bottom to the top. The indication that the end of the
carbonization process has been achieved is when the smoke that is being released has a
change of color, from white to gray. Thus, the preparation of the charcoal makers or
colliers is very important in order to get charcoal with fair level of quality and,
therefore, good productivity. A kiln 1.80m high with a 5-meter-diameter surface can
produce 150 tons of dry charcoal in one year, however, the “beehive” kiln produces an
average of 55 tons. These values vary according to the quality and moisture of the
wood. Because of the discontinuing nature of these kinds of carbonization processes,
several kilns operate in parallel usually constituting a battery employing from 7 to 9
units. The cycle of load, carbonization, cooling and discharge takes from 4 to 8 days
according to the size of the kiln.
The production of charcoal in Brazil presents significant economic importance
and it is developed in two basic ways: the traditional way, using firewood from native
forests, which is cut down for the attainment of agricultural land, and a modern way
carbonizing wood from energy forests (cultivated forest). In the first case, the systems
are characterized by low efficiency and low quality charcoal. The kilns that are
employed are the trench type, surface kilns or the “beehive kilns”, with a total
productivity close to 1.7 t charcoal/ha.year. In modern production, besides the use of
homogenous firewood, higher capacity surface kilns are adopted allowing the
productivity to be elevated up to values ranging between 4 and 6 t charcoal/ha.year.
The composition, properties and charcoal yield are strongly determined by the
carbonization temperature as shown in Table 5.14 and Figure 5.28.
Carb.
Temp.
( o C)
Table 5.14 – Charcoal properties depending on the carbonization temperature, and
obtained from Acacia bussei (WEREKO-BROBBY and HAGEN, 1996)
Carbon
(%)
ultimate analysis proximate analysis
Hydrogen
(%)
Oxygen
(%)
Ash
(%)
Humidity
(%)
Volatiles
(%)
Carbon
(%)
Calorific
value
(MJ/kg)
Vegetable
coal
production
(% in mass)
94
Energetic
efficiency
(%) *
300 30.2 5.67 63.73 0.4 1.9 70.8 28.8 22.40 56.27 65.92
400 71.5 3.93 22.17 2.4 2.8 30.9 66.7 29.88 28.03 43.80
500 87.0 3.10 8.50 1.4 2.8 17.7 80.9 32.14 22.65 38.07
600 87.5 2.67 6.93 2.9 1.0 7.1 90.0 33.20 21.63 37.56
700 92.4 1.71 3.89 2.0 1.8 3.9 94.1 33.40 20.22 34.22
800 93.4 1.03 3.57 2.0 2.2 2.4 95.6 33.90 19.54 34.64
* Calculated as the charcoal calorific value multiplied by its yield and divided by the
wood calorific value.

Figure 5.28 – Yield and carbon, oxygen and Hydrogen content for charcoal obtained
with different carbonization temperatures (WEREKO-BROBBY and HAGEN, 1996).
5.5. Fast pyrolysis and bio-oil attainment
Considered to be a theme that is still being developed, the biomass thermal
decomposition with high heating velocities and short residence periods has the
production of liquid wood energy derivatives, sometimes called bio-oils, as its main
goal. This is possible due to the fact that the pyrolysis intensification allows the
breaking of macromolecules, essentially cellulose and lignin, maximizing the
production of liquid and gaseous organic compounds. The yield of the liquids in
different fast pyrolysis processes is between 50 and 75% of the original biomass mass in
dry basis. The liquids attained through this process are called primary oils and they are
considerably different from those attained through slow pyrolysis processes (secondary
oils or tars). The primary oils are characterized by low density, higher homogeneity and
more stability in ambient conditions than the secondary oils. The calorific value of these
oils range between 22 and 24 MJ/kg (dry basis).
Two technologies can be used to carry out the fast pyrolysis:
• The ablative pyrolysis (the biomass heating is accomplished by means of contact
with a surface at a high temperature).
• The pyrolysis in fluidized bed or during the pneumatic transportation.
A perspective option for electric energy generation out of biomass is the
utilization of bio-oils in internal combustion engines. This option may be more
economical than the gasification for a medium range of power. The main institutions
that develop demonstrative projects and research about biomass pyrolysis are:
• ENSYN - Canada. It has three operating plants with capacities of 10, 25 and 80 t/h of
biomass (dry basis).
95

• Interchem – United States. There is an ablative “pyrolyser” of 1.36 t/h capacity in
operation.
• Waterloo University - Canada. Small fluidized bed reactors from 100 g/h to 3 kg/h.
• Union FENOSA - Spain. They have a fast fluidized bed pyrolysis reactor of 160 kg/h
(based on the process from the University of California).
• NREL - National Renewable Energy Laboratory – United States. They have two
ablative pyrolysers and they carry out studies about the cracking of bio-oils through
zeolites.
• Aston University – Great Britain. They carry out investigations on ablative pyrolysis
and fluidized bed pyrolysers.
The bio-oils can be converted into hydrocarbonlike fuel through a valorization
process in which their molecular weight is reduced allowing them to be eventually used
in systems such as internal combustion alternative engines. These processes are
schematized in Table 5.14.
Table 5.14 – Bio-oil valorization process
Process basic product Comentaries
Hydrotreatment CH2 a product similar to gasoline
Zeolite cracking CH1.2 aromatic product
The bio-oils and the products of their valorization have not met, however, a great
diffusion, basically because of the following factors:
• The bio-oils are corrosive, a consequence of the low PH that characterizes them.
They must be stored in containers made of polipropilene or stainless steal;
• The bio-oils lose their stability at temperatures above 100 o C due to polimerization
reactions;
• The cost of a GJ of energy in the form of bio-oils is 1.9 times greater than the cost of
the corresponding one GJ of raw petroleum with a cost of 20 US$/barrel. On the
other hand, the bio-oil valorized through hydrotreatment presents a cost 2.7 times
greater (Bridgwater, 1992). Evidently, taking the petroleum present prices into
account, the bio-oils are not competitive.
References
BAZZO, E., Geração de Vapor, Editora da UFSC, Florianópolis, 2 a Edição, 1995.
BHATT, M.S., “The efficiency of firewood devices (open-fire stoves, chulabs and
heaters)”, in Wood Heat for Cooking, Edited by Prasad, K.K.; Verhaart, P.,
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