Capitulo 6F (1)

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6. Wood energy applications 
 
 The purpose of the present chapter is to comment on the application of wood 
energy technologies and the processes analyzed in the previous chapters in real 
contexts, specially those cases where the energy coming directly or indirectly from 
forests means a contribution towards the rationalization of energy systems, for the 
enhancement of the economic and social scenario and for the suitable use of natural 
resources. This way it will go on presenting the relevant aspects and a discussion about 
the use of wood energy for useful heat generation and electricity production, including 
cogeneration. 
 
 6.1. The use of by-products to generate heat in ovens and boilers. 
 
 Almost all of the industries require process heat, either as steam or hot air, 
typically at temperatures, which are not very high. In order to attend this need of 
thermal generation, they burn fuels, which, above all, must be easily employed, 
available and must present attractive prices. In this sense, in the most diverse industrial 
sectors, biomass is often the desired fuel, and it is available as a by-product of 
agricultural or industrial activities, a “neighbor” of the consuming industry or even 
produced by the industry itself such as husks, sawdust and other kinds of wood energy 
fuels. It is important to remember that it is sometimes necessary to carry out a pre-
treatment or make the wood energy fuel suitable to be used, for when it is employed, 
besides contributing to the energy supply, it allows the pollution to be reduced. The 
following cases refer to real systems that are being used. 
 
A. Coffee husk ovens 
 
 The low moisture is a basic factor for the suitable conservation of grains, coffee 
among them, and when an appropriate drying is accomplished right after the harvest, the 
final quality of the product will be considerably elevated. As coffee produces 
lignocellulosic by-products with energetic value during its after harvest processing 
process, their application in drying ovens is interesting. One of these by-products or 
residues is coffee pods, which are available at a rate of 4 kg of pods per 100 kg coffee 
grains with a calorific value of 15.91 MJ/kg. Coffee presents another by-product that is 
used as fuel: the pulp that is attained at a rate of 40 kg (wet) per 100 kg of coffee grains. 
Once it is dry, this amount is reduced to 9 kg with a calorific value of 12.56 MJ/kg. This 
way, out of the residues of 100 kg of processed coffee about 177 MJ are available, 36% 
as pods and the rest as pulp. The pods and the pulp may provide 86% of the energy that 
is necessary for the drying of the coffee grains. (Tiraboschi and Coto, 1994). 
 99
 In order to demonstrate a situation in which the pods are used for the heating of 
the drying air, the data from one of the units of Ideal Industries, in El Salvador, were 
considered (Tiraboschi and Coto, 1994). Figure 6.1 shows the scheme of the oven and 
Table 6.1 presents the main operation parameters. 
 
 
 
Figure 6.1 – Scheme of an oven for the production of hot air using coffee pods. 
 
Table 6.1 – Operation parameters of the coffee pod oven from Figure 6.1. 
Parameter Value 
Combustion chamber volume, (m3) 11 
Heat transfer surface, (m2) 238 
Thermal power, (kWt) 1,453 
Biomass consumption, (kg/h) 457 
Drying air flow, (at 70oC), (m3/h) 100,000 
Efficiency, (%) 85 
Cost, (US$/kWt) 17 
 
 
B. Rice husk gasification systems for air heating in rice dryers 
 
 The industry of rice processing, which is usually situated in isolated areas, has 
already got tradition in using rice husks as fuel in order to satisfy its demands of heat to 
dry the product because, as it was mentioned before, it is good for its conservation. In 
this kind of application, the new aspect that will be presented next is the utilization of 
rice husk gasifiers, that is, a purely thermal application of the gasification. 
 
 Figure 6.2 shows the scheme of a rice husk drying system coupled to a gasifier, a 
system that was designed by PRM Energy Systems (PRM Energy Systems, 1996). The 
gas attained from the rice husk gasification and burned in the thermal oxidation 
chamber reaches a combustion product temperature of 815oC. With these gases the air 
in an intermediate heat exchanger can be heated up to a temperature of 480oC and used 
as a drying agent in the dryer. The gasifier is the up-draft type with partial combustion 
in the combustion duct in order to reduce the concentration of tar inside of it. 
 
 
 100
 
 
Figure 6.2 – Rice rusk gasifier coupled to a drying system 
(PRM Energy Systems, 1996). 
 
 
C. Brick kilns that use sugar cane bagasse as fuel 
 
 Most of the ceramic kilns, which are used to produce tiles and bricks, present 
low thermal efficiency because they do not recover the available energy of the exhaust 
gases. Actually, as the load of clay, which is being processed, needs high temperatures 
and because there are no heat recovery systems, the combustion gases are released into 
the atmosphere with a high temperature, so useful heat is wasted. This way, it is 
interesting to consider the situation shown in Figure 6.3, which presents the scheme of a 
brick producing oven that uses sugar cane bagasse as fuel allowing, then, heat recovery. 
In this installation, located in Cerâmica Fazenda do Pinhal, in Boituva, Brazil, the 
exhaust gases of the kiln, where the burning is taking place, are used to pre-heat the next 
kiln to be operated beginning the drying of the material. 
 This scheme is sometimes called Hoffmann or cellular oven, and in this case, it 
is possible to reach an efficiency of 68% and an energy consumption of 1.66 GJ/t of 
bricks (CORRIA et alli., 1998). An oven that was evaluated in another company, 
without gas heat recovery and operating with firewood, presented a specific energy 
consumption of 3.99 GJ/t of bricks. The temperature variation of the gases in different 
points of the installation for an operation cycle over 13 hours is shown in Figure 6.4, 
and it shows that the pre-heating allows a significantly recover of the heat that is still 
available in the exhaust gases of the first oven. 
 
 
 
 101
 
 
Figure 6.3 – Scheme of a brick producing kiln with heat recovery. 
 
 
 
Figure 6.4 – Temperature of the combustion gases during the burning in an operation 
cycle of the oven schematized in Figure 6.4 [Tgs – temperature of the gases in kiln 1 
outlet; Tgs2 – temperature of the gases in the kiln 2 outlet; Tge – temperature of the 
gases in kiln 1 inlet (combustion chamber outlet)]. 
 
 
 
 
 102
6.2. Wood energy and electricity generation 
 
Considering wood energy modern and efficient applications, electricity 
generation is one of the most important ones. Electricity is a noble form of energy that 
can attend, with efficiency and practically with no pollution, a wide scope of end-uses, 
from illumination to “in situ” production of mechanical power. The production of 
electric energy out of fuels can be done by using thermal cycles that convert thermal 
energy into mechanical power that is soon transformed into electricity. The most 
suitable fuels for electricity production must have certain characteristics such as use 
facility, low price for energetic unit and acceptable environmental impacts. In many 
situations the forestial fuels may present more competing advantages against the fossil 
primary energetic fuels in thermal power, especially in isolated systems and in 
cogeneration systems. 
 
The contribution of biomass towards electricity production has always been 
important in some countries. In Brazil, for example, biomass was the first fuel used in 
thermal power plants at the beginning of the century. In 1995, the generation of 
electricity out of bio-energetic resources reached 6.5 TWh with an installedpower 
above 2 GW, representing 30% of the generation with thermal origin and 2.5 % of the 
electricity total generation (NOGUEIRA and MOREIRA, 1997). In the United States 
the installed capacity of electric generation out of biomass in the early 90s was 8.4 GW 
(WILLIAMS and LARSON, 1993), and at the same time the DOE, Department of 
Energy of the US Government, was planning an installed capacity of 12 GW for 2000, 
in addition, it was forecast that this figure might reach 100 GW in 2030 (MUTANEN, 
1993). In fact, biomass is recognized by many researchers within the energy field as one 
of the most relevant “new” energy sources for electricity production and, with the 
development of modern bio-thermoelectric technologies, it shows the tendency of a 
continuing growth within the energy market. (MOREIRA et alli, 1997). 
 
Table 6.2 – Electric generation technologies with biomass 
Technology Efficiency Cost Capacity Technology state of 
the art 
 % US$/kW kW 
Stirling engines > 30 − < 40 being developed 
Locomotives 12 800 40 a 500 available technology 
Gasifiers and alternative 
engines 20 1,200 5 a 1,000 available technology 
Steam boilers and turbines 20 1,000 > 1,000 available technology 
Gas gasifiers and turbines > 30 1,500 > 5,000 being developed 
Fuel cells 80 − − being developed 
 
Table 6.2 shows a general scenario of the electricity producing technologies out 
of biomass with reference values for their basic characteristics, their typical range of 
application and their current development conditions. It is quite interesting to observe 
that for a given efficiency value, the corresponding specific consumption of firewood 
can be calculated directly by employing the following expression, where this fuel is 
assumed to have a calorific value of 13.8 MJ/kg. 
 
 103
firewood specific consumption (kg/kWh) = 
(%) efficiency
1.26 
 
 Thus, for locomotives for example, a specific consumption around 2.2 kg/kWh 
may be expected, but this value can be significantly reduced in more efficient systems. 
Naturally, these values must be regarded as preliminary references, for the efficiency of 
a thermal plant may vary sensitively according to the operation conditions and the load 
factor. 
 It is very important to acknowledge that the viability of an electric generating 
technology is determined, not only by its efficiency, but through a wide set of factors, 
among which the price of the fuel, the investment in the plant and the intensity the plant 
is used are rather important. Typically, the more expensive the fuel is and the more 
hours of energy throughout the year the plant uses, the more important efficiency 
becomes. The following expression indicates the way such variables are related to the 
cost of generated energy. 
 
 
planta
comb
CAP
M&O
planta
comb
CAP
M&O
energia .6,3
C
F.8760
)FFRC.()PI(
.6,3
C
F.P.8760
)FFRC(.I
C η+
+=η+
+=
 
 
 F
E
PCAP
anual=
8760
 
 
 
 Where 
 
 Cenergia - cost of generated energy, US$/kWh 
 P - installed power, kW 
 I - total investment within the plant, US$ 
 I/P - capacity unit cost, US$/kW 
FRC - capital return factor is related to the discount rate and the payment 
period 
FO&M - part of the investment that corresponds to the operation and 
maintenance annual costs. Fuel costs are not included 
FCAP - capacity factor, a fraction of time when the plant could operate at 
installed load to generate Eanual
 Eanual - energy generated in the plant along one year, kWh 
 Ccomb - fuel cost, US$/GJ 
 ηplanta - average efficiency of the plant 
 
 Figure 6.5 shows the electric energy generation costs, featured in Table 6.5, to 
exemplify the application of the previous expressions at distinct contexts allowing the 
demonstration of the great influence of the intensity that the plant is used. Such 
situations require consideration about which should be the limits for the typical values 
in terms of favorable or unfavorable conditions for the use of biomass according to the 
technologies currently available at present. In this analysis, the main simplification to be 
 104
considered facing a real case is the choice of a unique value for the efficiency that, as it 
was mentioned previously, is strongly dependent on the load condition of the plant. It is 
also necessary to consider the load curve to be attended in a more detailed study. 
 
Table 6.3 – Limit situations for the electricity generation with biomass. 
situations parameter unit unfavorable favorable 
capacity unit cost I/P US$/kW 1,500 800 
discount annual rate ⎯ % 12 6 
payment period ⎯ years 10 20 
operation and maintenance 
cost factor 
FO&M ⎯ 0.05 0.03 
fuel cost Ccomb US$/GJ 4 2 
plant average efficiency ηplanta % 15 30 
 
 
 
Figure 6.5 – Load factor effect on the cost of the generated electricity 
 
 
 A. Small and medium capacity systems 
 
 It is usually considered that in the power capacity range from 100 kWe to 2 
MWe it is more feasible to produce electric energy out of biomass by using moving bed 
gasifiers and internal combustion engines. For a power capacity above 5 MWe fluidized 
bed gasifiers are usually predominant. However, according to BRIDGWATER (1995) it 
is possible to find motogenerators with capacities up to 50 MWe and gas turbines have 
been applied for a low calorific value gas and 3MWe powers. There has been, 
nowadays, great efforts towards the technological development of low capacity systems 
using gasifiers and gas turbines. A problem, which was found, is the cleaning of the gas 
to the permissible limits of particulate and other compound concentrations during the 
operation of internal combustion engines. 
 
 105
 
 
Figure 6.6 – Recommendations on the use of biomass gasification systems for 
electricity generation in different power ranges (VTT Energy, 2001). 
 
 The systems that gasify biomass in order to produce gases with low calorific 
value, allowing their use in internal combustion engines, have been known and used 
since the middle of this century. They adopted charcoal as their raw material for its low 
content of volatiles, which allows a considerably reduction of the problems caused by 
the tar during the gasification. This equipment was known as gazogene and it was 
widely used during the World War II. It was adapted to vehicles, this way, allowing 
them to face the limitations imposed by the petroleum derivative supply. Throughout 
the 70s, several Brazilian companies produced charcoal gasifiers, basically using 
conceptions of that time, attending mainly to isolated systems. Nevertheless, this market 
practically vanished over the following years due to the reduction of the Diesel prices. 
With this new context where bio-energy has been revalued, it is possible to observe, 
nowadays, a clear revival of the interest in gasifiers, specially the ones that employ 
biomass directly and are well-reliable. Among the successful applications of this second 
generation of small-scale biomass gasification, we can point out the open top gasifier of 
the Indian Institute of Science and the Chinese gasifier for rice husks. Both of them will 
be discussed further. 
 
 The open top gasifier, from the Indian Institute of Science, shown in picture 6.7, 
is part of an operating pilot plant of 100 kWe. The gasifier efficiency is approximately 
80% (BULHER, 1994; MUKUNDA et alli, 1993). A recent conjoined evaluation 
carried out with the Swiss company Dasag reached the following results: a gas calorific 
value of 4.7 MJ/Nm3, particulate and tar content at the gas cleaning system outlet 
smaller than 50 and 80 mg/Nm3, respectively (DASSAPPA et al., 1996). 
 
 106
 
 
Figure 6.7 – Scheme of the open top gasifier from the Indian Institute of Science. 
 
 
 
Figure 6.8 – Scheme of the Chinesecommercial gasifier for rice husks. 
 
 The Chinese commercial gasifier for rice husks, Figure 6.8, is already being sold 
in commercial scale and estimates show that about 100 units have been installed in 
China. The fuel specific consumption evaluated in Mali in a gasifier of this kind was 
3.75 – 4.0 kg husk/kWh, even though data ranging from 2.0 – 2.5 kg/kWh have been 
reported for gasifiers in China (MAHIN, 1990). The rice husk ashes are greatly valuable 
as raw material in white pottery industries, for they have a very high content of silica. 
However, these ashes represent an important technological problem for the gasification 
systems, because they melt at relatively low temperatures tending to block the gas 
flows. 
 107
 
 As an interesting example in a range of typical capacities for isolated systems 
the 40 kW Boroda plant in Gujarat, India, can be mentioned. It generates electricity 
operating with a Diesel engine of 48 kW. The firewood that is used is produced locally, 
coming from eucalyptus and acacia plantations. It must be previously dried allowing the 
replacement of 70 to 80% of the consumption of petroleum derivatives. The gasifier is 
the down-draftt one and its basic operating features are presented in Table 6.4. 
According to the manufacturer the specific investment cost is 425 US$/kW, and for a 
biomass cost of 30 US$/t, electricity can be generated for 90 US$/MWh (ANKUR 
ENERGY & DEVELOPMENT ALTERNATIVE, 1994). 
 
Table 6.4 - Operation parameters of a gasifier/MCI of 40 kW set. 
parameter valor 
Electric power 40 kW 
Gas calorific value 4.19 MJ/Nm3
Biomass particle size 10 - 100 mm. 
Biomass consumption 32 - 40 kg/h 
Diesel specific consumption 0.090 kg/kWh 
Biomass specific consumption 0.9 kg/kWh 
 
 However, it is important to point out that most of the small-sized electricity 
generating programs based on gasification technology that were developed during the 
period of high petroleum prices failed, this way, operating gasifiers are rarely found 
today. In 1983 the World Bank started the “Small-sized biomass gasifier monitoring 
program”. The data and conclusions achieved by this program in 1993 are (STASSEN 
& KNOEF, 1995): 
 
• The operating gasifiers average biomass specific consumption is 1.1 – 1.4 
kg/kWh for those using wood; 0.9 kg/kWh for those using charcoal; and 2.0 – 
3.5 kg/kWh when the fuel is rice husks. 
• The average efficiency of the gasifier internal combustion engine system is 
13%, a value that is smaller than what was promised by the manufacturers 
• The fraction of Diesel replaced by the gas ranges from 40 to 70%. 
• The specific investment in locally made gasifiers in developing countries 
ranges between 400 and 1.550 US$/ kWe, and in imported gasifiers from 850 
to 4.200 US$/kWe. 
• Biomass gasifiers for power generation are not, in general, an economically 
attractive option considering the petroleum prices ranging from 15 to 20 
US$/barrel. There are some conditions where low cost gasifiers that use wood 
as fuel can be profitable, as well as the ones that use rice husks. 
 
 It is also opportune to verify the basic reasons why these small-sized biomass 
gasification programs failed or succeeded. The reasons, according to STASSEN & 
KNOEF (1995) are presented in Table 6.5. 
 
 
 
 
 108
Table 6.5 - Causes of failure and success in small-sized gasification programs. 
Reasons Failure Success 
Technical • Operational difficulties 
because of design 
technical problems. 
• Inexperienced operators. 
• Capacity improper 
fitting between the 
gasifier and the engine. 
• High value of emissions.
• Well-prepared and 
motivated operators. 
• Constant technical 
support. 
Financial • Imported gasifier high 
cost. 
• Old equipment and 
personnel lack of 
motivation. 
• Rise in biomass prices. 
• Well developed 
technology. 
• Replacement parts 
availability. 
Institution
al 
• Insufficient support. 
• Gasifier installation in 
inappropriate places 
without commercial 
interest. 
• Intense support. 
• Presence of an 
experienced team in 
gasification for personnel 
training and maintenance. 
 
 
 
RABOU and JANSEN (2001) presented the results of an techno-economic 
evaluation of electricity generating systems out of biomass in two capacity ranges: 1-2 
MWe and 10 MWe for electricity generation with or without cogeneration (table 6.6). 
 The biomass chosen for the simulations was willow wood with 20% 
moisture a cost of US$ 38.5/ton (US$ 2.05/GJ). The following technologies were 
considered in the study: steam turbines, gas engines and gas turbines. The results 
displayed in Table 6.6 were calculated by considering a 5000 h/year operation. The 
main conclusions are: 
• A small-scale generation using steam turbines is too expensive; 
• Generation using gas turbines is slightly more expensive than using gas engines; 
• Within a power range of 10 MWe the economic feasibility of the three options are 
very close (from 6000 h/year and over for steam turbine operation and 7000 h/year 
and over for gas engines and turbines); 
• The results change significantly when the rejected heat is used for cogeneration. 
 
 
 
 
 
 
 109
Table 6.6 – Technical and investment data for biomass electricity generating systems 
(RABOU and JANSEN, 2001) 
Steam turbine Gas engine Gas turbine 
1-2 
MWe 
10 
MWe 
1-2 
MWe 
10 
MWe 
1-2 
MWe 
10 
MWe 
Thermal power, MWt 6.30 45.40 6.80 40.10 6.80 39.30 
Net eletrical power, MWe 1.01 10.00 1.73 10.40 1.49 8.90 
Net efficiency,% 16.0 22.00 25.00 25.90 21.90 19.50 
Total investment, 106 US$ 3.45 14.52 5.30 17.61 5.21 16.66 
Specific investment, US$/kW 3415 1452 3063 1693 3496 1865 
Generation cost without 
cogeneration, US$/kWh 0.152 0.078 0.125 0.083 0.136 0.087 
Generation cost with 
cogenetarion, US$/kWh − − 0.118 0.076 0.129 0.073 
 
 
 
B. Biomass gasification for large scale electricity generation 
 
In a context where the environmental advantages of bioenergy are acclaimed, 
firewood may play a role of growing importance in the large-scale electricity production 
and in the interconnected systems. However, in this case, the conversion efficiency is 
determinative in relation to the feasibility, for the transportation costs tend to be each 
time more important. 
 
 There are two essential technological routes to obtain electric energy out of 
biomass within this scale: 
 
(1) Steam cycles based on biomass combustion in conventional boilers, of which 
efficiency would be limited to values around 25%. Unfortunately, however, 
higher values imply superior installed capacities, which are practically 
meaningless regarding biomass use due to the fact of the biomass fuel high 
transportation costs. 
 
(2) Cycles with gas turbines, combined cycles included, which are coupled with 
gasifiers. This technology is still undergoing a demonstrative phase, but it is 
possible to reach efficiencies close to 40-45%. In the combined cycles, the fuel 
is burned inside a gas turbine and the combustion products that are released from 
this turbine pass through a recuperative boiler where the steam is produced and 
employed in the steam turbine. 
 
 There are some variants for the practical accomplishment of a thermal cycle with 
gas turbines using biomass as fuel. The basic differences fall on the adopted turbines as 
it will be presented next: 
 
• BIG/GT systems (Biomass Integrated Gasification - Gas Turbine) – These 
systems, which are the most promising ones, gasify the biomass and the 
produced gas fuel, once it is cleaned from tar, ashes alkaline metals, etc, is 
injected into the gas turbine combustion chamber, as it is shown in Figure 6.9 
 110
(BEENACKERS and MANIATIS, 1996). Cycles derived from modifications 
that were carried outin the gas turbine aiming at the increase of its efficiency 
are: BIG/STIG (Biomass Integrated Gasification – Steam Injected Gas 
Turbine) – with steam injection in the turbine; and BIG/ISTIG (Biomass 
Integrated Gasification – Intercooled Steam Injected Gas Turbine) – with 
intermediate cooling and steam injection in the turbine. Other authors called 
these cycles IGCC - Integrated Gasification Combined Cycles. 
 
• Hot air cycles - HAC. In this case the producer gas is burned and the 
combustion products at high temperature are used to heat the air in the heat 
exchanger - Figure 6.10. This way, once the turbine operates with a clean air, 
there is no need to clean the hot gas. At present, two demonstration plants 
using this cycle are being tested: The BINAGAS project from Free 
University of Brussels, with 500 kWe of power, and the TINA project, 
developed in Austria with 2 MWe of power. 
 
• Biomass direct burning cycles. This installation, as the turbine combustion 
chamber, uses a fluidized bed combustor. ARCATE (1997) proposed a cycle 
of this type operating with charcoal - Figure 6.11. Its net efficiency is 33%. It 
is assumed a carbonization efficiency of 45% for the calculations. 
 
 
 
Figure 6.9 - BIG/GT system (Varnamo plant in Sweden – BIOFLOW process). 
 
 111
 
 
Figure 6.10 – Hot air cycle. 
 
 
 
Figure 6.11 – Cycle with gas turbine and biomass direct burning (ARCATE, 1997). 
 
 The BIG/GT technology is still not being commercialized. The main problems 
yet to be solved are: 
 
• The gas obtained in the gasifier needs to be cleaned, so that the particulates, 
tar, alkaline metals and other compounds that may affect the gas turbine 
operation can be removed; 
• The gas turbines are designed to operate with natural gas, of which calorific 
value is much higher than the calorific value of the gas produced by biomass 
gasification. This way, the gas turbine needs constructive modifications in the 
compressor and in the combustion chamber in order to operate with a greater 
gas volume. 
• In pressurized gasifiers, biomass feeding may present some difficulties. 
 
 At present, several fluidized bed gasifiers for large scale applications are already 
undergoing a demonstrative stage. They are schematized in Figures 6.12 to 6.15 and 
they are briefly described afterwards. Table 6.7 shows a summary of the operation and 
 112
efficiency parameters of these and other demonstrative projects related to biomass 
gasification in fluidized bed. 
 
• TPS atmospheric gasifier. This system was selected by SIGAME Project. It is 
a 30 MW power combined cycle forecast to be built in the state of Bahia, 
Brazil, using wood from eucalyptus plantations as fuel. The distinctive aspect 
of this system is the cracking of the tar present in the gases with dolomite in a 
separate reactor. Lurgi has developed a similar system. 
• Alhstrom pressurized circulating fluidized bed gasifier (Bioflow). This 
system was used in Varnamo plant in Sweden. 
• Institute of Gas Technology - IGT pressurized bubbling bed gasifier. 
Commercially called RENUGAS, this type of gasifier was evaluated at a 
project in the Hawaiian Islands using sugar cane bagasse as fuel. The 
company Enviropower has been purchasing this technology. 
• Battelle Columbus Laboratories (BCL) indirect heating atmospherical 
gasifier. It is being used at Vermont project in Burlington. Its advantage is the 
attainment of a gas with higher calorific value. Steam is used as the 
gasification agent, this way, avoiding the dilution effect of the nitrogen of the 
air. This particularity allows a conventional gas turbine to operate without 
great constructive modifications. 
 
 
 
Figure 6.12 – TPS biomass gasifier. 
 
 113
 
Figure 6.13 – Biomass gasifier developed by Alhstrom/Bioflow 
(SYDKRAFT, 2001). 
 
 
 
Figure 6.14 – IGT biomass gasifier – RENUGAS. 
 
 
 
Figure 6.15 - Battelle Columbus Laboratories biomass gasifier. 
 114
Table 6.7 – Operational and efficiency parameters at circulating fluidized bed biomass 
gasification demonstrative projects. 
Firm Gasification 
agent 
Capacity Operation 
pressure 
Bed 
temperature 
Gas LCV gasifier 
efficiency 
 MWt MPa oC MJ/Nm3 % 
Alhstrom/ 
Bioflow 
air 18 2.40 950-1,000 5.00 82-83 
TPS air 65 0.18 890-920 6.2 77 
Lurgi air 16 0.10 800 5.8 - 
IGT/ 
RENUGA
S2
air + 
steam 
20 2.07 830 4.3 – 4.8 - 
BCL3 steam 40 0.2 - 15.65 75-80 
Omnifuel4 air 23 0.1 760 4.99 - 
1- Garbage pellets were gasified. 
2- Steam/biomass relation – 0.32. 
3- Steam/biomass relation- 0.45. 
4- The Omnifuel gasifier is the conventional fluidized bed type. 
5- Dry matter 
6- Cold efficiency. 
 
 In the 90’s, in Europe and in the United States, several demonstrative plants 
were projected aiming at solving the indicated problems during their operation. The 
main parameters, used equipment, costs and realization stage of these projects are 
displayed in Tables 6.8 and 6.9 (BENACKERS and MANIATIS, 1996). 
ARBRE project is at an advanced stage (Figure 6.16). The plant’s thermal 
scheme is showed in Figure 6.17. At the beginning of 2002, the turbine operated for the 
first time with producer gas for some hours during the commissioning process (TPS, 
2002). In the future a series of similar thermal plants with 35 Mwe of power and 
efficiency close to 50% are intended to be built in Great Britain. 
 
 
Figure 6.16 – ARBRE Plant – photograph taken in June 2001. 
 115
 
 
 
Figure 6.17 – Plant’s thermal scheme. 
 
The BIOFLOW project was concluded in December 2000. A final report was 
elaborate and published (SYDKRAFT, 2001). As its main result, we can highlight the 
possibility of operating pressurized gasification systems and gas turbines with high 
availability. The gas turbine operated with producer gas for 3600 hours and the gasifiers 
for 8500 hours. The plant showed good flexibility in relation to different types of fuel 
and low emissions (except for nitrogen oxides - NOx). 
 
 
Once there are no real operational parameters for these BIG/GT plants, a lot of 
work has been spent in modeling these systems using the already available technologies 
of gasification, gas cleaning and gas turbines. Table 6.10 presents the results of two 
studies, which are quite complete. (CONSONI AND LARSON, 1996 and CRAIG AND 
MANN, 1996). 
 
 
 
 
 
 
 
 
 
 
 
 
 116
Table 6.8 – European community BIG / GT projects (BEENACKERS and MANIATIS, 
1996). 
Data Units Projects 
Name and 
location 
- ARBRE, 
Aire Valley, 
Great-Britain 
BIOCYCLE, 
TBD, 
 Denmark 
ENERGY 
FARM, Di 
Cascina, Italy 
BIOFLOW,
Varnamo, 
Sweden 
Type of biomass - Wood wood and 
sorgho 
wood and 
sorgho 
wood residues
Gasifier - TPS- 
Atmospherical 
circulating 
Carbona OY- 
Pressurized 
fluidized bed 
Lurgi- 
Atmospherical 
circulating 
fluidized bed 
Alhstrom – 
Pressurized 
fluidized bed 
Operational 
parameters 
(gasifier) 
 
oC/atm 
 
850-900/1.5 
 
850-950/22 
 
800/1.4 
 
950-1.000/22
Gas turbine EGT/Typhoon EGT/Typhoon EGT/Typhoon EGT/Typhoon
Electric power MWe 8.0 7.2 11.9 6.3 
electricity 
generation 
efficiency 
 
% 
 
30.6 
 
39.9 
 
33.0 
 
32.0 
 
 
Table 6.9 - BIG/GT projects in the United States (BEENACKERS and MANIATIS, 
1996). 
Data Units Projects 
Name and location - BGF, Hawaii Vermont, Burlington 
Type of biomass - Sugar cane bagasse Wood 
Gasifier - IGT – Renugas, 
pressurized fluidized 
bed 
BCL – indirect 
heating atmospherical 
Electric power MWe 5.0 15.0 
Electricity generation 
efficiency 
% 30-35 - 
 
 
 
 
 
 
 
 
 
 
 
 117
Table 6.10 – Modeling results of BIG/GT plants with different gasificationsystems and 
gas turbines. 
Variants I II III 
Gasifier Bioflow Pressurized TPS Atmospherical BCL – atmospheric 
with indirect 
heating 
Gas calorific 
value, MJ/Nm3 
5.13 4.3 - 13.2 6.0 4.8 
Gas turbine LM2500* GE MS- 
6101FA** 
LM2500 GE MS- 
6101FA 
LM2500 GE MS- 
6101FA 
Gross power, 
MW 
TG 
TV 
 
30.6 
20.0 
10.6 
 
139.7 
93.1 
46.6 
 
32.2 
23.5 
8.8 
 
137.2 
82.1 
55.1 
 
27.3 
18.4 
8.9 
 
120.5 
72.9 
47.6 
Net power, MW 28.8 131.7 25.9 122.0 24.5 105.4 
LCV based 
efficiency 
45.2 47.6 41.9 43.3 41.1 45.0 
Specific 
Investment, $/kWe
- 1,371.0 - 1,108.0 - 1,350.0 
* CONSONI and LARSON (1996) calculations for current technologies. 
** CRAIGH and MANN (1996) calculations carried out for a completely ready technology 
(plant number “n”) using the ASPEN simulator. Advanced energy gas turbine: gas inlet 
temperature - 1.288oC, steam parameters - 100 kg/cm2 and 538oC. 
 
 In Brazil, July 1991, it was given the first pace towards the project Wood 
Biomass Project/Sistema Integrado de Gaseificação de Madeira para a Produção de 
Eletricidade (wood gasification integrated system for electricity production) -WBP / 
SIGAME – aiming at demonstrating the feasibility of generating electricity 
commercially out of wood (eucalyptus) using the BIG/GT technology, a GE (LM 2500) 
gas turbine and a TPS fluidized bed gasifier previously mentioned. This project is 
financed by the World Bank Global Environmental Fund (GEF), and the plant’s 
forecast capacity is 30 MW, with 43% efficiency. At present, such unit is in the stage of 
executive project final discussion and its main features are presented in Table 6.11 
(CARPENTIERI, 1997). 
 Adopting the same conception the COPERSUCAR technology Center-CTC and 
Companhia Paulista de Força e Luz-CPFL (Eletricity Company from the State of São 
Paulo) are proponing a project to be implemented in Brazil using sugar cane bagasse as 
fuel. 
 
Table 6.11 - WBP-SIGAME project main data 
Data value 
Capacity 32 MW 
Efficiency 43% 
Fuel consumption 0.75 t/MWh 
Specific investment 2,560 US$/kW 
Total investment US$ 110 million 
 
 118
6.3. Advanced Technologies: Gas Microturbines, Stirling 
Engines, Fuel Cells and Hybrid Systems 
 
Gas microturbines 
 
The microturbine technology comes from four different technologies: small 
capacity gas turbines, auxiliary power units, gas turbines for automobiles and 
turbocompressors (TANNER, 2000). There is no exact definition of the microturbine 
concept, however the term is usually use when one wants to refer to a high velocity gas 
turbine with a power ranging between 15-300 kW. 
Now-a-days, approximately twelve companies are working on the development 
and commercialization of microturbines. These thermal machines are expected to 
compete directly with alternative engines and fuel cells in relation to their initial cost, 
maintenance requirements and emission levels. In order to achieve the goals mentioned 
above at a low equipment cost, its configuration is kept as simple as possible. The 
following project solutions were included in most of the models: a simple stage radial 
compressor, a radial inlet simple stage turbine, a high velocity and direct drive generator 
cooled by air, a multi-fuel combustor, a high efficiency compact recuperator and a 
simple control system (MASSARDO et al., 2000). 
 
The microturbines have their own characteristics. Among them we can 
highlight: 
• An axis: the generator is placed at the same axis as the turbine (there is not 
transmission box) representing a relatively simple manufacture and maintenance; 
• Air cooled bearings: they avoid the contamination of lubricants caused by 
combustion products, assure the equipment a longer useful life and reduce 
maintenance costs; 
• High velocity: the nominal rotation of the microturbines ranges between 30,000 
and 120,000 rpm, depending on its nominal power and on the manufacturer, so the 
use of a continuous current generator or an induction generator is necessary. The 
frequency conversion is assured by the use of DC/AC convertors; 
• Heat recuperator: it is necessary to achieve efficiency levels of about 30 %. By 
using this equipment it is possible to increase efficiency by 30 and 50%; 
• Start engine: The generator itself is the start engine. 
 
Figure 6.18 shows a cross section view of a gas microturbine manufactured by 
Capstone. The arrows indicate to route of the air and of the combustion gases. Table 
6.12 presents the average values of the most important characteristics of the main 
models of gas microturbines available in the market today. 
 
 119
 
 
Figure 6.18 - Cross section view of a gas microtubine 
(Capstone Turbine Corporation). 
 
Table 6.12 – Technical characteristics of some models of gas microturbines. WILLIS 
and SCOTT, 2000. 
Nominal powerl kW 44 50 175 250 
Fuels* - GN, K, O GN, K, O GN, K, O GN, O 
Regenerator - No Yes Yes Yes 
Efficiency % 27 30 33 32 
Nominal Rotation r.p.m 110,000 110,000 80,000 90,000 
Starting time Minutes 2.0 2.0 2.5 3.25 
US$ 29,200.00 42,000.00 131,500.00 176,000.00Total cost 
Specific investment US$/kW 663.0 840.00 740.00 700.00 
* GN – Natural gas, K – Kerosene, O – Diesel oil. 
 
 The perspective of using biomass, converted in a gas of low calorific value by 
means of gasification of bio-digestion, in gas microtubines is very interesting. Figure 
6.19 shows the scheme of a biomass gasifier and a gas microturbine coupled together. 
The proposal presents two technological difficulties: the first one is the microturbine 
remodeling, so that it can operate with a gas of low calorific value; the second difficulty 
consists of cleaning of the gas after the gasifier. Now-a-days, in the United States, the 
company Reflective Energies has been developing a project called Flex-Microturbine, 
which focus on the modification of a turbine manufactured by Capstone Turbine 
Corporation. The turbine will burn producer gas, which is a product of biomass 
gasification, landfills and animal waste bio-digestion. A potential market of more than 8 
million units is forecast. This project is receiving monetary support from United States 
Department of Energy (DOE) and from the National Renewable Energy Laboratory 
(NREL). The tests are being carried out in the Combustion Laboratory of California 
University. 
Thermal Systems Study Group - NEST, of Federal University of Itajubá, in a 
partnership with CEMIG (Energy Company of the state of Minas Gerais) and 
 120
COPERSUCAR Technology Center is developing the program “Experimental 
Evaluation of a Gas Microturbine System for the Generation of Electricity Using 
Different Fuels”. The goal is to study the performance of microturbines with a nominal 
electric power of 30 kW operating with various fuels, among them the producer gas 
form biomass gasification. A photograph of the gasifier built with resources from this 
project is shown in Figure 6.20. 
 
 
 
 
Figure 6.19 – Main scheme of a installation with biomass gasifier and gas microturbine 
 
 
 
Figure 6.20 – Fluidized bed gasifier for biomass 
 
Presently, microturbines specific investments range from 650 to 1100 US$/kW. 
In a five year period, according to different forecasts these values are expected to be 
reduced down to 400 US$/kW (DUNN, 2000). As a consequence of technological 
 121
development, the efficiency is also expected to increase up to a level of 50% (Figure 
6.21). 
Next, the results of an economic and technical evaluation of a gasifier/gas 
microturbine set using eucalyptus from energy forests as fuel at Brazilian conditions 
will be presented. The current market prices of the eucalyptus and of the 
gasifier/microturbine system were considered, as well as future estimates. Table 6.13 
shows economicand technical data used during the calculations (SILVA et al., 2001). 
 
 
 
 122
 
Figure 6.21 – Technology trends and expected evolution in relation to the thermal efficiency 
of microturbines (MASSARDO et al., 2.000) 
 
Table 6.13 – Parameters and data considered during the economic evaluation of 
gasification/gas microturbine systems (SILVA et al., 2001). 
 
Parameter or data Unit Assumed value 
Biomass LCV kJ/kg 13000 
Biomass price R$/GJ 4.0 
Power kW 45 
Fuel consumption kg/h 52.427 
System efficiency % 0.238 
Gasifier cost R$ 39853 
Cleaning system cost R$ 10000 
Compressor cost R$ 9060 
Microturbine cost R$ 87975 
Equipment toal cost R$ 147888 
Installation and other costs R$ 10147 
Total cost R$ 221832 
Exchange rate R$/US$ 2,3 
Interest rate % 15 
 
 
 123
The analysis considers generation cost current values and the variations 
projected for a near future. The calculation of the different elements that are part of the 
generation cost (investment, fuel and O&M) was carried out, and their specific impact 
on the generation total cost was analyzed as well (Figure 6.22). 
 
 
 
Figure 6.22 – Generation cost elements in a gasifier/gas microturbine system. 
 
A sensitivity analysis was carried out in order to evaluate the influence of the 
biomass price on the generation cost in different scenarios (Figure 6.23). The upper 
curve shows the results that were attained considering the current state of the art of the 
gasifier/microturbine systems. The curve in the middle corresponds to advanced 
systems with an efficiency of 35% and a specific investment of. The lower curve also 
corresponds to advanced systems, however with a 10 % interest rate. Just as a reference, 
the present and the expected cost of electricity in the Brazilian energy system is also 
presented. 
Considering current costs and efficiencies of microturbines, the generation cost 
in gasifier/microturbines systems is higher than in conventional thermal plants (current 
and expected cost). The reduction of the microturbine cost to US$ 400 kWe and the 
efficiency increased to 35 % may lead to generation costs economically feasible for 
low-price biomass (less than 2.5 R$/GJ). The same system financed with a 10 % interest 
rate could be economically attractive for biomass whose price is lower than 4.0 R$/GJ. 
 
 
 
Figure 6.23 – Sensitivity analysis of the generation cost in gasifier/microturbine 
systems in relation to the biomass price in different scenarios. 
 124
 
Another technological trend that implies the use of microturbines is the 
operation in hybrid systems with fuell cells, forming a combined cycle. This new 
technology is one of the most efficient and clean technological options for electricity 
generation. This is a consequence of the fact that the fuel cell operation is based on 
electrochemical reactions and not on combustion ones. Also, microturbines are 
considered to be low emission thermal engines. It is expected an electric efficiency of 
60% for this cycle. 
 
Stirling engines 
 
The Stirling engine was patented by Rev. Robert Stirling, a Scotish minister in 
1816. In the late 19th and early 20th Century thousands of these engines, which had a 
maximum capacity of 4 kW, were operating in the USA and Europe (LIZARRAGA, 
1994). The Stirling engine was replaced with internal combustion engines, for they 
presented more advantages. There was still some interest in those engines because of the 
fact they were quiet, and that led Philips Company to take over the projects in 1930. 
Since its invention, The Stirling prototype has been developed for automobilist purposes 
and it has been tested in trucks, buses and small boats as well. It has also been thought 
to drive higher loads such as yachts, passenger boats, submarines, etc. On the other 
hand, NASA has also been developing research on the field (STINE and DIVER, 1994 
apud WEST, 1986; MEIJER, 1992). Table 6.14 presents a list of companies that are 
working on the development of Stirling engines. 
 
Table 6.14 – Companies involved in the development of Stirling engines 
(CARVALHO, 2001). 
Company 
(Country) 
Power 
(KWe) 
Eletric 
Efficiency 
(%) 
Status 
STM / 4-120 
(USA) 
32 30 Commercial 2002 
WHISPERGEN 
(New Zealand) 
0.5 10 Commercial 
JOANNEUM 
RESEARCH 
(Austria) 
3 
30 
24 
? 
Developing 
DANSTOKER/DTU 
(Denmark) 
36 
150 
22 * 
26 * 
Commercial 2002 
Developing 
KOCKUMS 
(Sweden) 
8 
40 
118 
35 
35 
47 
Developing 
SOLO 
(Germany) 
2 - 9 27 Commercial 2002 
* Global efficiency (Stirling engine and biomass combustion furnace) 
 
Operating principle. The Stirling engine is a device that converts heat into 
mechanical power, so its operation demands high temperatures (STINE AND DIVER, 
1994). It consists of a piston alternative engine driven by an external source of heat, 
which is different from internal combustion engines that operate with an internal source. 
 125
Similarly to the steam machines, the Stirling cycle uses a gas expansion closed system 
in order to attain mechanical power. The Stirling cycle is similar to two stoke gas engine 
cycle with two forced stages (Figure 6.24). The efficiency of this cycle depends on the 
temperature of the heating gas, and it will be limited by the material resistance which 
the heat is supplied through. (WILLIS, 2000). 
 
 
 
Figure 6.24 – Stirling engine operating cycle stages (CARVALHO, 2001). 
 
In stage 1, Figure 6.24, heat is supplied from an external source to the working 
fluid reservoir. Most of the cases use hydrogen, nitrogen or helium because of their high 
heat transfer capacity. When the gas is heated, it expands forcing the piston to move; 
this piston is called displacing piston. This gas is rapidly cooled in the cylinder 
(generally using water or air as a cooling agent) while the piston pushes the cold air, 
which is in the lower part, to the secondary cylinder (stage 2). In stages 3 and 4, the 
secondary piston is forced to move backwards due to mechanical inertia. The gas that is 
in the upper part of this piston goes back to the lower part of the main piston making it 
move upwards. This way, the gas above the main cylinder is once more taken to the hot 
chamber. The heat accumulated in the chamber is transferred into the gas making the 
cycle repeat. 
Stirling engines are, in general, divided into three groups known as Alfa, Beta and 
Gama. In Alfa configuration (Fig.6.25a), the engine has two pistons that are connected 
through the heating heat exchanger, the regenerator and the cooling heat exchanger. On 
the other hand, Beta and Gama engines (Figures 6.25b and 6.25c) use a displacing 
piston and a compression one arranged in just one cylinder. 
 
 126
 
 
Figure 6.25 - Alfa (a), Beta (b) and Gama (c) engine configuration 
(CARVALHO, 2001). 
 
Figure 6.26 shows a Stirling engine installation operating with biomass, whereas 
Figure 6.27 presents the efficiency/heating gas temperature dependency curve for a 40 
kW Stirling engine also operating with biomass. The tests were carried out at Technical 
University of Denmark (CARLSEN et al., 2000). 
 
 
 
Figure 6.26 – Scheme of a 28.5 kWe Stirling engine installation operating with biomass 
(CARLSEN, 2000) 
 
 127
 
 
Figure 6.27 – Efficiency/temperature curve for a 40 kW Stirling engine operating with 
biomass combustion (CARLSEN et al., 2000). 
 
In order to maximize the power of this engine, it usually operates at high 
pressures. For example, in solar installations, the pressure ranges between 5 and 20 MPa 
- these values are far lower for biomass engines (STINE AND DIVER, 1994; 
CARLSEN, 2000; PODESSER, 2000). Figure 6.28 displays the power-heating gas 
temperaturecharacteristics of a Stirling engine having the working fluid operating at 
different pressures. It can be observed that a rise in pressure inceases the power 
(CARLSEN et al., 2000). 
 
 
 
Figure 6.28 – Power vs work fluid pressure and heating gas temperature characterisitics 
of a Stirling engine operating with biomass combustion (CARLSEN et al., 2000). 
 
The operation at high gas pressures creates problem with the machine sealing, 
mainly at the high temperature region. Another important condition is the relation that 
must exist between the air reservoir dimensions and the cylinder volume. The first must 
have approximately the same capacity as the volume of the cylinder with the piston at 
the lower point. This way, the gas can suffer the required changes for the engine 
operation. 
Stirling motors can be run by any type of source that is able to supply enough 
heat for its driving, including fossil and renewable fuels and solar energy. 
 128
This type of engines do not need the fuel gas cooling and, besides, the cleaning 
of the gas is less important mainly because of the technical solutions applied to the tube, 
heater and combustion chamber project, which allow the reduction of the amount of 
contaminants in the gas and the removal of residual dirt with steam jets. Small power 
systems, between 100 and 250 kW, may reach efficiencies of nearly 40 % in more 
advanced production stages (CARLSEN, 2000; HISLOP, 2000). 
Once a small number of these engines have been produced, the current market 
prices are high (2000 to 5000 US$/kW) limiting its competitiveness in relation to other 
technologies. Figure 6.29 shows the results of an economic feasibility evaluation for 
Stirling engines working at cogeneration conditions through the direct use of the gases 
in a district heating furnace of 1 MWt. The simulation was carried out for engines with 
capacities ranging between 20 and 120 kWe using the heating plant real operation data 
and a engine cost of 1418 US$/kWe. Different interest rates and economic subsidy 
percentages were also considered. It can be observed that engines between 40 and 50 
kW have the lowest cost of produced kWh, but they need a subsidy of almost 50% and 
the lowest interest rate for them to be competitive (PODESSER, 2000). 
 
 
 
Figure 6.29 – Electricity production cost simulation of a Stirling engine operating at 
cogeneration conditions in Austria (PODESSER, 2000). 
 
 
Reliability is another difficult aspect to be analyzed because limited hours of 
testing. Other aspects indicate a good technical potential such as primary drivers. Some 
of the Stirling engine advantages are (JACOBSEN et al., 1998): 
 
 129
• Global efficiency of about 30 % making them competitive with other technologies; 
• Good efficiency with partial loads; 
• Low noise level and safe operation; 
• Low maintanance cost; 
• They can use a great variety of fuels; 
• Useful life of about 25000 h; 
• Possibility of operating with cogeneration. 
 
The following disadvantages can be mentioned: 
 
• A few fuels were tested. Other problems may appear when residual fuels are used. 
Among them we can highlight: rust, tar and particles. They can reduce the heat 
exchanger efficiency (CARLSEN et al., 2000); 
• Only small-sized engines were tested; 
• Data regarding reliability and useful life are scarce. 
 
Fuel Cells (FC) 
 
A completely different concept for electricity generation is offered by fuel cells 
(FC). The basic operating principle has been known since 1839, when it was formulated 
by the well-known scientist and judge Sir William R. Grove. It is based on a process 
inverse to electrolysis, that is, attaining electric current, a product of the reaction of the 
hydrogen and oxygen within an appropriate electrolytic environment. In the past 50 
years, a remarkable variety of combustion cells have been developed for space 
exploration, urban transport and electricity stationary generation. 
In general, fuel cells are electro-chemical devices that convert the chemical 
energy of the fuel/oxidatant mixture directly into electricity, allowing the operation at 
high efficiencies (~50-65 % based on the natural gas LCV). This is different from the 
conventional electricity generating systems that use the chemical energy of the fuel 
through its combustion to convert it into mechanical power and then, into electricity. 
In the fuel cells, the chemical energy is directly converted into electric energy 
through a process that is basically the same as a battery process, which is constantly 
recharged involving two reagents (hydrogen and air). This process produces continuous 
currents at low voltages (lower than 2 voltS) (WILLIS; SCOTT, 2000). 
A fuel cell consists of two electrodes (an anode and a cathode one) separated by 
an electrolyte that can have different chemical composition and physical state. The 
hydrogen passes through the anode electrode and the oxygen through the cathode. When 
the hydrogen passes through the anode, it is ionized, so it loses its electrons and then, 
both (hydrogen and the electrons) take different ways to the cathode. The hydrogen goes 
through the electrolyte and the electron goes there through a condutive material. This 
process produces water, electric current and heat (Equation 6.4 and Figure 6.30). In 
order to satisfy power requirements, the area of the cells are enhanced and several 
simple cells connected serially by means of bipolar separating plates and combined. The 
combination of these bateries forms the generating plant (VAN DIJKUM, 1998). 
 
yelectricitcheatOHOH ++=+ 22212 (6.4) 
 
 130
Besides efficiency, fuel cells offer other advantages when compared with 
conventional systems. In fuel cells systems, the hydrogen is consumed at the anode and 
the water is produced at the cathode. Consequently, at first, water is the only by-product 
in fuel cells. These systems high efficiency is translated into a better fuel utilization, and 
therefore, CO2 emissions are smaller. Also, the electricity generating systems with fuel 
cells are completely able to fulfill current and future patterns regarding the emission of 
particulates, NOx and SOx. 
Now-a-days, there are four types of fuel cells, and basically, all of them can use 
biomass gasification gases. Fuel cells are characterized based on the electrolyte they 
use: proton exchange membrane (PEMFC), phosphoric acid (PAFC), molten carbonate 
(MCFC) and solid oxide (SOFC). Table 6.15 presents the main characteristics of these 
types of fuel cells. 
 
 
Figure 6.30 – Fuel cell operation principle. 
 
Table 6.15 – Types of fuel cells, used electrolyte and operation temperature. 
Adapted from VAN DIJKUM, 1998 and SCHMIDT and GUNDERSON, 2000. 
Type Electrolyte Temperature 0C Applications 
Power 
MWe 
Proton Exchange 
Membrane (PEMFC) 
Ion-exchange 
membrane 
hydrated organic 
polymer 
80 
in “situ” 
generation 
(piles) 
0.25 
Distributed 
generation 1-10 Phosphoric Acid 
(PAFC) Phosforic acid 200 
Cogeneration 0.2-1 
Distributed 
generation 
1-10 
 
Centralized 
generation > 100 
Molten Carbonate 
(MCFC) 
Molten Li/Na/K 
carbonate 850 
Cogeneration 0.25-1 
Distributed 
Generation 
1-10 
 Solid Oxide (SOFC) Yttria-doped zirconia 1000 Centralized 
generation > 50 
 
 131
 MCFC and SOFC cells, due to their high operation temperature, present a 
possibility of internal fuel reforming with steam in the presence of a nickel based-
catalyst. This reforming, expressed by the reaction shown in Equation 6.5, allows the 
attainment of hydrogen for the cell out of the methane present in the fuel (natural gas, 
biogas, producer gas, etc). 
 
COH3OHCH 224 +→+ (6.5) 
 
 In the case of a molten cabonate cell, there are to reforming alternatives – the 
indirectinternal reforming – IIR (the reformer is closed to the anode, but physically 
apart) and the direct internal reforming - DIR. 
 Figure 6.31 shows a MCFC cell that combines these two options. The solid 
oxide cells can also use the fuel’s CO attaining hydrogen out of the reaction given by 
Equation 6.6, as it is shown in Figure 6.32. 
 
222 COHOHCO +→+ (6.6) 
 
 
 
Figure 6.31 – Operation principle of a molten carbonate fuel cell with fuel 
internal reforming operating with methane 
(HIRSCHENHOFER et al., 1998). 
 
 132
 
 
Figure 6.32 – Opereation principle of a solid oxide fuel cell 
(HIRSCHENHOFER, et al., 1998). 
Advantages and disadvantages of fuel cells 
Fuel cells offer several advantages over electricity generation based on fossil 
fuels themomechanical energy conversion. These include: 
• High efficiencies, ranging from 15 to 30 % above other technologies that use fossil 
fuels. Fuel cells operate with an efficiency that is practically constant at partial 
loads, whose value is not limited by the Carnot cycle, for it is an electro-chemical 
process, not a thermal conversion one. 
• Wide environmental acceptability: because of its high efficiency, specific emissions 
of CO2 expressed in kg CO2/kg comb. are reduced. They have a low level of noise, 
60 dB at 30 m and specific emissions of SOx and NOx are 0.0013 and 0.00018 
Kg/MWh, respectively (NETL, 2000); 
• Modulability and fast installation: They can be manufactured in “standard” size 
alowing availability within a wide range of powers, form 0.025 to 50 MW for 
natural gas and over 100 MW for gasified coal (NETL, 2000); 
• Flexibility in relation to fuel use: Every kind of fuel having a considerable amount 
of H2 can be used; 
• Cogeneration possibility: Because of the high quality of the residual thermal energy 
attained at the FC outlet, this energy can be used for heating and cooling in 
residential, commercial and industrial sectors. 
 
Disadvantages. 
 
• High initial cost: the cost of fuel cells is twice or three times higher than the cost of 
other technologies that use fossil fuels. It is important to mention that this is still an 
experimental technology, so prices may suffer a reduction after some time. 
According to FETC DOE, (1999) FC price evolution must behave according to what 
is shown in the chart of Figure 6.33; 
• Shortage of people who are specialized in this technology making its maintenance 
difficult; 
 133
• Most of the fuel cells are highly sensitive to fuel impurities, either particulates or 
chemical agents. Filters and cleaning systems are used and they must be replaced 
frequently; 
• Little operation experience. Constant modifications and improvements may give a 
false impression of immaturity. The continuous operating record for this technology 
was registered in Japan with 9478 h of continuous operation and 40000 h of 
cummulative operation in Japan and in the USA, with molten carbonate cells (VAN 
DIJKUM, 1998). 
 
 
 
Figure 6.33 – Fuel cell price evolution perspective 
 (FETC/DOE, 1999). 
 
In gas microturbine/fuel cell hybrid systems, the efficiency is expected to reach 
values of about 70% (LCV basis). When residual heat is used, total efficiency may be 
even higher, over 85 % (CAMPARINI, 2000). Two of the main goals of the operation 
with fuel cells and microturbines in combined cycles are: assuring the pressurized 
operation of the fuel cell, and therefore increasing its efficiency and power (ALI & 
MORITZ, 1997) and, on the other hand, reducing the generating unit global cost. 
Technically, Molten Carbonate Fuel Cells (MCFC) offer the best potential to be 
coupled with large coal and biomass gasifying installations. MCFC cells can be made of 
stainless steel and less exotic materials. Besides, this kind of cells tolerates carbon 
monixide (CO), and the carbon dioxide attained at the cathode is used in the anode 
reaction. This technology operates at temperatures ranging about 650°C, and the offer 
high efficiencies – between 45 and 55 %. Molten carbonate cells have been being 
installed in several demonstrative projects throughout the world, but they have not 
achieved a commercial status yet. The company Fuel Cell Energy has developed a 250 
kW demonstrative model. The current cost of theis technology ranged about 8000 
US$/kW (SCHMIDT; GUNDERSON, 2000). 
Solid Oxide Fuel Cells (SOFC) operate at temperatures of 1000°C, and they use a 
zirconium electrolyte. This allows the use of a resistent ceramic cylinder-shaped 
structure rather than a plain-shaped one, which makes the sealing of the cell more 
difficult. SOFCs present as good flexibility regarding the use of fuels as the MCFCs, 
but it is likely to be used in higher capacity generating applications. This allows the 
attainment of a reasonable economic attractiveness considering the effect of the scale 
 134
factor in the costs related to high temperature materials. Table 6.16 shows a summary of 
the different chemical reactions that take place in FCs and the material used in each one 
of them. 
 
 
Table 6.16 – Main chemical reactions and chemical composition of the materials used 
in the most common fuel cells (SCHMIDT and GUNDERSON, 2000 and 
VAN DIJKUM, 1998). 
Technology Electrolyte Anode Material Anode reaction 
Cathode material 
Cathode reaction 
PEMFC 
Ion-exchange 
membrane hydrated 
organic polymer 
Platinum 
−+ +→ eHH 222
Platinum 
HeHO 2221 22 →++ −+
 
PAFC Phosforic acid 
Platinum 
−+ +→ eHH 222 
Platinum 
OHeHO 2221 22 →++ −+
 
MCFC Molten Li/Na/K carbonate 
Nickel 
−−
−
+→+
++→+
eCOCOCO
eCOOHCOH
22
2
2
2
32
22
2
32
 
Nickel oxide 
− →++ 32221 2 COeCOO
 
SOFC Yttria-doped zirconia 
Nickel 
−
−−
−−
++→+
+→+
+→+
COOHOCH
eCOOCO
eOHOH
24
2
2
22
2
4
2
2
2
2
2
 
Sr-doped lanthanum 
manganite 
 
−− →+ 2221 2 OeO 
 
The most appropriate fuel cells to be coupled with biomass gasifiers are MCFC 
and SOFC cells because of their relatively high tolerance of impurities, their capacity 
for fuel internal reforming, and their favorable thermal integration (temperature levels 
and exhaust gases composition, which are appropriate for the coupling with gas 
microturbines). So far, fuel cells have not been tested with coal gasification gases. The 
first tests of this system are expected to take place in 2003. 
As it was mentioned before, the gasification process produces certain amounts of 
tar and particulate material. Tar may cause the formation of coke in the reformer 
catalyst or in at the FC electrodes. The requirements to remove the tar for gas turbines 
and fuel cells are essentially the same. In both cases, the tar must be removed from the 
fuel gas flow before entering to prevent obstructions. The alkalis, which at high 
temperatures cause corrosion on the surfaces made of steel alloy, do not seem to be 
considerably harmful for the fuel cells as they are for gas turbines. The tolerance for 
alkali metals in the gas turbines is 0.1-0.2 ppm, whereas in the molten carbonate fuel 
cells it is 1-10 ppm (LOBACHYOV; RICHTER, 1998; DAYTON 2000). Alkalis, as 
well as tar, can be removed through catalytic cracking at high temperatures. It is also 
possible to reduce the content of tar by working on the gasifier itself by adding 
catalysts, such as dolomite, which have the property of breaking tar molecules. 
The particulate can be removed from hot gases by using cyclones or filters. The 
ashes of the coal are formed of minerals, basically Fe, Ca and Al. These inorganic 
materials may deactivate the reformer or affect the catalytic removal of sulphur of the 
MCFC or of the SOFC cells because of the electrolyte poisoning. Biomass ashes 
 135
generally have less Fe and Al than coal/charcoal, but,on the other hand they have Si, K, 
Na, and they sometimes present a higher content of Cl than coal/charcoal ashes. Silica is 
harmful to the SOFC electrolytes. The MCFC cell electrolyte is affected by Cl and 
potassium. 
A particulate efficient removal may hold back a considerable fraction of the alkali 
metals present in biomass or coal. The alkali metals in biomass tend to be more volatile 
than the ones present in coal because a significant fraction of these metals in coal are in 
form of refractary mineral compounds. Consequently, alkali metal vapors may be 
formed during biomass gasification and deteriorate the catalyst used in the reformer, the 
electrolyte and the electrodes of the FC. The cooling of the gasification gas contributes 
towards the reduction of it amount of alkalis. 
 
Examples of biomass gasification and fuel cells integrated cycles 
 
The integration of biomass gasifiers and fuel cells can be carried out in two different 
ways: 
1. Gasification-FC-steam turbine system – The gasifier is used as a source of 
hydrogen for the fuel cell. The gas residual energy is used for steam production 
in a recuperative boiler, which is used to run the microturbine. 
2. Gasification-FC-gas microturbine system. The gasifier is used as a source of 
hydrogen for the fuel cell using the exhaust gases of the cell in a gas 
microturbine. 
 
Gasification-FC-steam turbine system (LOBACHYOV and RICHTER, 1998). 
 
The main components of this cycle are the Batelle Columbus indirect heating 
gasifier, a MCFC, a recuperative boiler and a steam boiler. The flow diagram related to 
this cycle is shown in Figure 6.34. The fuel used in sawdust. The modeling results of 
this system are shown in Table 6.17 
 
 
 
Figure 6.34 – Thermal scheme of the gasifier - FC – steam turbine system 
(LOBACHYOV and RICHTER, 1998). 
 136
 
Table 6.17 – Thermal data and operating parameters of the 
gasifier - FC – steam turbine system 
 
Biomass flow t/day 2000 
Thermal power MW 331.7 
Cycle pressure MPa 0.16 
Power net production MW 175.75 
Efficiency % 53 
Total cost 106 US$ 144.45 
Specific investment US$/kW 822 
Electricity cost 
For biomass 1.8 US$/GJ 0.025 US$/kWh 
For biomass 3.7 US$/GJ 0.037 US$/kWh 
 
 
Gasifier-FC-gas microturbine system(BUHRE and ANDRIES, 2000). 
 
BUHRE and ANDRIES (2000) present the results of the performance modeling 
of a hybrid system of fuel cell/gas microturbine formed by a Capstone 28 kW 
microturbine and a SOFC (Figure 6.35). The fuel used is producer gas from biomass 
gasification with a calorific value of 4.1 MJ/kg. The calculation results (Table 6.18) 
show that almost two thirds of the total electric power are produced by the fuel cell and 
the system total thermodynamic efficiency is 54.4 %. The results show that these 
systems are very promising for small-scale decentralized generation of electricity and 
heat using biomass as fuel. 
 
Table 6.18 – Results of the performance simulation of a Gas microturbine/fuel cell 
hybrid system (BUHRE and ANDRIES, 2000). 
 
Parameters Value 
Gas calorific value (LCV) 4.1 MJ/kg 
Pressure drop in the fuel cell 0.05 bar 
Degree of fuel use by the SOFC 85 % 
Degree of oxygen use by the 
SOFC 
27 % 
Temperature of the gas at the 
microturbine inlet 
885 oC 
Microturbine isentropic efficiency 87 % 
Compressor isentropic efficiency 78 % 
Compressor pressure relation 3.25 
Cell voltage 0.747 V 
SOFC electric power 75.1 kW 
Microturbine electric power 27.2 kW 
Additional turbine electric power 12.7 kW 
Thermodynamic efficiency 54.4 % 
 
 137
 
 
Figure 6.35 – Hybrid system formed by a Capstone 28 kW microturbine and a SOFC 
(adapted from BUHRE and ANDRIES, 2000). 
 
 
6.4. Wood energy and iron and steel production 
 
Charcoal was the energy basis for iron and steel production during the Industrial 
Revolution, and nowadays, it is still employed for such purpose in some countries. 
Within the Brazilian context, particularly, this fuel has been considerably applied in the 
production of metals, allowing the attainment of high quality products because of its 
extremely low sulfur content. Nearly 40% of the cast iron and the alloys produced in 
Brazil are based on the use of charcoal as fuel. (REZENDE et al., 1993). 
 
According to Associação Brasileira de Carvão Vegetal - ABRACAVE (Charcoal 
Brazilian Association) (1996), more than 25.103 cubic meters of charcoal are produced 
every year, and the greatest part of it is produced using firewood from planted forests as 
it can be observed in Figure 6.36. This scenario indicates favorable prospects of fuel and 
reducing agent sustainable supply for these industries on the basis of forest resources. 
Figure 6.37 shows the scheme of charcoal supply in Brazil (MEDEIROS, 1996). 
 
Blast furnaces that use charcoal as reducing agents are, in general, small (240-
1.000 tons of cast iron per day). However, large iron and steel industries that produce 
special and stainless steel use more charcoal than coke. The charcoal gets to the iron and 
steel making industries in trucks, and right after a sieving process (removal of fines), it 
is introduced into the blast furnace through its top. Inside the furnace, the charcoal and 
the iron mineral with the melting materials are disposed in alternate layers. 
 
 138
 
 
Figure 6.36 – Forest surface devoted to charcoal production in Brazil 
 
 The so called charcoal fines (particles which are smaller than 9-12 mm) 
represent 15-20% in weight (REZENDE et alli,1995) and they could constitute a loss if 
they were not used for other purposes with or without agglomeration. The injection of 
fines into the furnaces with the iron mineral makes the movement of gases and the 
process itself difficult in the furnace. It is possible to inject the fines into the blast 
furnace through nozzles, consequently reducing the fuel total consumption. Figure 6.38 
shows the evolution of the consumption of charcoal per ton of cast iron within the last 
few years. 
 
 
 
Figure 6.37 – Charcoal blast furnaces production flow chart 
 (MEDEIROS, 1995) 
 
 139
 
 
Figure 6.38 – Evolution of charcoal consumption in Brazilian iron and steel making 
industries (REZENDE et alli., 1993). 
 
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	Coffee husk ovens
	Technical
	B. Biomass gasification for large scale electricity generati
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