6. Wood energy applications - Nest

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 pretreatment 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 2 Heat transfer surface, (m ) 238 Thermal power, (kWt) 1,453 Biomass consumption, (kg/h) 457 o 3 Drying air flow, (at 70 C), (m /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 installed power 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 20 1,200 5 a 1,000 available technology engines 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) =

26.1 efficiency (%)

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.

C energia =

I .(FRC + FO&M ) C comb (I P) .(FRC + FO&M ) C comb + + = 8760.P.FCAP 3,6.η planta 8760.FCAP 3,6.η planta

FCAP =

E anual 8760 P

Where Cenergia P I I/P FRC FO&M FCAP Eanual Ccomb ηplanta

- cost of generated energy, US$/kWh - installed power, kW - total investment within the plant, US$ - capacity unit cost, US$/kW - capital return factor is related to the discount rate and the payment period - part of the investment that corresponds to the operation and maintenance annual costs. Fuel costs are not included - capacity factor, a fraction of time when the plant could operate at installed load to generate Eanual - energy generated in the plant along one year, kWh - fuel cost, US$/GJ - 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$/kW 1,500 800 discount annual rate 12 6 % ⎯ payment period 10 20 years ⎯ 0.05 0.03 operation and maintenance FO&M ⎯ cost factor US$/GJ 4 2 fuel cost Ccomb plant average efficiency 15 30 % ηplanta

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 Chinese commercial 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 Electric power Gas calorific value Biomass particle size Biomass consumption Diesel specific consumption Biomass specific consumption

valor 40 kW 4.19 MJ/Nm3 10 - 100 mm. 32 - 40 kg/h 0.090 kg/kWh 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.

• Well-prepared and motivated operators. • Constant technical support.

• Capacity improper fitting between the gasifier and the engine. • High value of emissions. Financial

• Imported gasifier high cost.

• Well developed technology.

• Old equipment and personnel lack of motivation.

• Replacement parts availability.

• Rise in biomass prices. Institution al

• Insufficient support.

• Intense support.

• Gasifier installation in inappropriate places without commercial interest.

• 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 10 1-2 10 1-2 10 MWe MWe MWe MWe MWe 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$/kW 3415 1452 3063 1693 3496 1865 Generation cost without 0.152 0.078 0.125 0.083 0.136 0.087 cogeneration, US$/kWh Generation cost with 0.118 0.076 0.129 0.073 − − cogenetarion, US$/kWh

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 out in 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

MWt 18

MPa 2.40

Bed temperature o

C 950-1,000

Gas LCV

gasifier efficiency

MJ/Nm3 5.00

% 82-83

Alhstrom/ air Bioflow TPS air 65 0.18 890-920 6.2 Lurgi air 16 0.10 800 5.8 IGT/ air + 20 2.07 830 4.3 – 4.8 RENUGA steam S2 BCL3 steam 40 0.2 15.65 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.

77 -

75-80 -

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 -

Name and location Type of biomass

-

Gasifier

-

Operational parameters (gasifier) Gas turbine

o

C/atm

Projects BIOFLOW, ARBRE, BIOCYCLE, ENERGY Aire Valley, TBD, FARM, Di Varnamo, Great-Britain Denmark Cascina, Italy Sweden Wood

wood and sorgho TPSCarbona OYAtmospherical Pressurized circulating fluidized bed

850-900/1.5

wood and sorgho LurgiAtmospherical circulating fluidized bed

wood residues

800/1.4

950-1.000/22

850-950/22

Alhstrom – Pressurized fluidized bed

EGT/Typhoon EGT/Typhoon EGT/Typhoon EGT/Typhoon

Electric power electricity generation efficiency

MWe

8.0

7.2

11.9

6.3

%

30.6

39.9

33.0

32.0

Table 6.9 - BIG/GT projects in the United States (BEENACKERS and MANIATIS, 1996). Data Name and location Type of biomass Gasifier

Electric power Electricity generation efficiency

Units -

MWe %

Projects BGF, Hawaii Sugar cane bagasse IGT – Renugas, pressurized fluidized bed 5.0 30-35

Vermont, Burlington Wood BCL – indirect heating atmospherical 15.0 -

117 Table 6.10 – Modeling results of BIG/GT plants with different gasification systems and gas turbines. Variants Gasifier

Gas calorific value, MJ/Nm3 Gas turbine Gross power, MW TG TV Net power, MW LCV based efficiency Specific Investment, $/kWe

I Bioflow Pressurized

II TPS Atmospherical

III BCL – atmospheric with indirect heating 6.0 4.8

5.13

4.3

-

13.2

LM2500*

GE MS6101FA**

LM2500

GE MS6101FA

LM2500

GE MS6101FA

30.6 20.0 10.6 28.8 45.2

139.7 93.1 46.6 131.7 47.6

32.2 23.5 8.8 25.9 41.9

137.2 82.1 55.1 122.0 43.3

27.3 18.4 8.9 24.5 41.1

120.5 72.9 47.6 105.4 45.0

-

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 Capacity Efficiency Fuel consumption Specific investment Total investment

value 32 MW 43% 0.75 t/MWh 2,560 US$/kW US$ 110 million

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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 Total cost US$ 29,200.00 42,000.00 131,500.00 176,000.00 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 economic and 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

R$/US$

2,3

%

15

Exchange rate Interest rate

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). Status Company Power Eletric (KWe) Efficiency (Country) (%) STM / 4-120 32 30 Commercial 2002 (USA) WHISPERGEN 0.5 10 Commercial (New Zealand) 3 24 Developing JOANNEUM 30 ? RESEARCH (Austria) DANSTOKER/DTU 36 22 * Commercial 2002 (Denmark) 150 26 * Developing Developing 35 KOCKUMS 8 35 (Sweden) 40 47 118 SOLO 2-9 27 Commercial 2002 (Germany) * 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 temperature characteristics 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). H 2 + 1 2 O 2 = H 2O + cheat + electricity

(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. Temperature Power Type Electrolyte Applications 0 C MWe Ion-exchange in “situ” membrane Proton Exchange 80 generation 0.25 Membrane (PEMFC) hydrated organic (piles) polymer Phosphoric Acid (PAFC)

Molten Carbonate (MCFC)

Solid Oxide (SOFC)

Phosforic acid

Molten Li/Na/K carbonate

Yttria-doped zirconia

200

850

1000

Distributed generation

1-10

Cogeneration

0.2-1

Distributed generation

1-10

Centralized generation

> 100

Cogeneration

0.25-1

Distributed Generation Centralized generation

1-10 > 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 basedcatalyst. 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). CH 4 + H 2 O → 3H 2 + CO

(6.5)

In the case of a molten cabonate cell, there are to reforming alternatives – the indirect internal 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. CO + H 2 O → H 2 + CO 2

(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). Anode Material Cathode material Technology Electrolyte Anode reaction Cathode reaction PEMFC

Ion-exchange membrane hydrated organic polymer

Platinum

Platinum

H 2 → 2H + + 2e−

MCFC

Phosforic acid

Molten Li/Na/K carbonate

+

H 2 → 2H + 2e H 2 + CO3

−

Nickel → H 2O + CO2 + 2e

Yttria-doped zirconia

1

2 O2

+ 2 H + + 2e − → H 2

CO2 + CO3 2 − → 2CO2 + 2e −

CO + O 2− → CO2 + 2e − CH 4 + 4O 2− → 2 H 2O + CO2 +

+ 2 H + + 2e − → H 2 O Nickel oxide

2−

Nickel H 2 + O 2 − → H 2 O + 2e −

SOFC

2 O2

Platinum

Platinum

PAFC

1

1

2 O2

+ CO2 + 2e − → CO3

Sr-doped lanthanum manganite 1

2 O2

+ 2e − → O 2 −

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 Thermal power Cycle pressure Power net production Efficiency Total cost Specific investment

t/day MW MPa MW % 6 10 US$ US$/kW Electricity cost

2000 331.7 0.16 175.75 53 144.45 822

For biomass 1.8 US$/GJ For biomass 3.7 US$/GJ

0.025 US$/kWh 0.037 US$/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 Gas calorific value (LCV) Pressure drop in the fuel cell Degree of fuel use by the SOFC Degree of oxygen use by the SOFC Temperature of the gas at the microturbine inlet Microturbine isentropic efficiency Compressor isentropic efficiency Compressor pressure relation Cell voltage SOFC electric power Microturbine electric power Additional turbine electric power Thermodynamic efficiency

Value 4.1 MJ/kg 0.05 bar 85 % 27 % 885 oC 87 % 78 % 3.25 0.747 V 75.1 kW 27.2 kW 12.7 kW 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 (2401.000 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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