b i o m a s s a n d b i o e n e r g y x x x ( 2 0 1 3 ) 1 e1 1
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Assaı´ e An energy view on an Amazon residue Marcos Alexandre Teixeira c,*,2, Jose´ Carlos Escobar Palacio b, Ce´sar Rodriguez Sotomonte b, Electo Eduardo Silva Lora b, Osvaldo Jose´ Venturini b, Dirk Aßmann a,1 a
GIZ e Deutsche Gesellschaft fu¨r Internationale Zusammenarbeit GmbH, Energy Program, Brazil. c/o Edifı´cio Rodolpho de Paoli, Av. Nilo Pec¸anha, 50 - 30 andar, grupo 3009, Centro - Rio de Janeiro, RJ 20020-906, Brazil b Nu´cleo de Exceleˆncia em Gerac¸a˜o Termele´trica e Distribuida e NEST, Instituto de Engenharia Mecaˆnica e IEM, Universidade Federal de Itajuba´ e UNIFEI, Av. BS 1303, CP 50, Itajuba´, MG 97500-093, Brazil c Federal Fluminense University, Department of Agricultural Engineering and the Environment, Rua Passo da Pa´tria 156, CEP 24.210-240 Nitero´i, RJ, Brazil
article info
abstract
Article history:
This paper analyzed the technical and economic feasibility of electricity generation using
Received 7 May 2012
the residues from the exploitation of Assaı´, an Amazonian agrosilvicultural product
Received in revised form
(Euterpe oleracea, Mart). Assaı´ biomass characteristics as fuel were reviewed based on
8 July 2013
available literature and its availability assessed. The profile of a typical industrial pro-
Accepted 1 August 2013
cessing unit was described. The electricity generation cost for a 1 MW conversion systems,
Available online xxx
considering 5.5 US$ t1 biomass price, were evaluated: conventional steam cycle with backpressure turbine (66.97 US$ MWh1), steam cycle with extraction condensation tur-
Keywords:
bine (92.11 US$ MWh1), organic Rankine cycle (ORC e 122 US$ MWh1) and a gasifier/in-
Assaı´
ternal combustion engine set (102 US$ MWh1). Based on financial performance, back-
Euterpe oleracea
pressure steam turbine was the best option, and gasifier/internal combustion should be
Biomass
further considered due its operation flexibility. For any system, minimal electricity
Electricity production
commercialization price for economical feasibility found was 150 US$ MWh1. ª 2013 Elsevier Ltd. All rights reserved.
Feasibility
1.
Introduction
The Amazon forest is one of the largest biomass reservoirs in the world, but disregarding the wood as a cooking fuel, the use of its biomass energy potential as fuel is almost zero. The forest has been put down for crops (like soybean), and cattle raising; and recently, some of its agrosilvicultural products are gaining local, national and even international market potential. This is the case of the Assaı´ (or Ac¸aı´ in Portuguese), part of the traditional diet of the local population at the river delta of
the Amazon. It is gaining a market share throughout of the southeast of Brazil and also internationally, now being exported as a sustainable product of Amazon. Although this product is the basis of income for most of the population living at the river shore (the so called “ribeirinhos”), thousands of tonnes of Assaı´ seeds are wasted every day, which are disposed in landfills. The proportion between seeds and pulp in this fruit is more than 60%, which means that one tonne of product gives more than one tonne of highly useful residues [1].
* Corresponding author. E-mail addresses:
[email protected],
[email protected] (M.A. Teixeira). 1 www.giz.de. 2 http://www.ter.uff.br/. 0961-9534/$ e see front matter ª 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.biombioe.2013.08.007
Please cite this article in press as: Teixeira MA, et al., Assaı´ e An energy view on an Amazon residue, Biomass and Bioenergy (2013), http://dx.doi.org/10.1016/j.biombioe.2013.08.007
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There have been some initiatives to untap the Assaı´’s energy potential, mainly by small and micro scale electricity generation projects, focused on isolated communities, as the NERAN project, developed for 4 small communities (systems up to 40 kW) [2] and the GASEIBRAS (Nationalization of biomass gasification technologies and human resources formation for the North) and the GASEIFAMAZ (Comparison between biomass gasification technologies existing in Brazil and abroad and human resources formation for the North) [3]. Taking it into consideration, the objective of this paper is to analyze the basic engineering of a power installation using this silvicultural residue, to be installed and operated to provide a part of the energy requirements of the capital city of Para´, Bele´m do Para´, northern Brazil. The capacity of the electric power station was determined considering the Assaı´’s seed availability and was fixed at 1 MW, while the technologies evaluated were the conventional Rankine cycle, the organic Rankine cycle and the gasifier/internal combustion engine. In each case the energy conversion efficiency was determined and the economic-financial feasibility evaluated.
2. Characteristics and fuel properties of Assaı´ residues Assaı´ is the fruit of the Assaiseiro tree (E. oleracea, Mart), of the order of the Arecales gender Euterpe, family Palmae. It is an endemic palm tree from the river delta of the Amazon, with specific occurrence at the lower region of Tocantins. It grows in floodland rich in nutrients (especially N, Ca, P, K, and Mg) and sediments [4]. According to the Brazilian Institute of Geography and Statistics, in 2008 the Assaı´ production in Para´ State was 107,028 tonnes, representing 88.5% of the total production in Brazil. Once the Assaı´ juice has gained importance in the market, the Assaı´ turned from a pure-simple extractive process into a sustainable crop practice in the region, incorporating techniques of irrigation and fertilization. One growing issue is the use of such plantations for carbon capture and storage within the Amazon Region. A trend in the development of the Assaı´ is to spread it as a sustainable culture option, to generate larger incomes for the poor inhabitants of the river margins of the Amazon Delta Region [5]. In Brazil some works have been done on the quantification of the energetic potential of the Assaı´ residues, specifically for Bele´m do Para´. Ref. [6] estimated a total availability of 93,521 tonnes of kernels per year, leading to 489.01 GWh year1 of energy potential. Ref. [7] analyzing the potential for briquette production, in the state of Para´, and considering a production of 112,676 tonne year1 of fruits, concluded that there is a residue production of 93,521 tonne year1, leading to or 40,751 MWh month1. Ref. [1] conducted an extensive work on the characterization of the Assaı´ as an energy biomass and [8], focused on the logistic aspects of collecting, transporting and burning the biomass in one centralized power plant, considering the steam turbine Rankine cycle, with steam parameters of 21 kg cm2, 213 C, and a specific energy consumption of 2.5 kg biomass kWh1. This author concluded that, based on the transport, O&M costs, and the possible revenue from avoided purchased
Table 1 e Elementary composition for Assaı´ kernels [1]. Component (weight %) Source [1]
H
C
N
O
S
Ashes
5.57 48.80 Not detected 46.19 Not detected
0.44
energy, the operation of a power plant using Assaı´ kernels would be economically feasible. Table 1 shows the elementary composition of Assaı´ kernels and Table 2 the immediate analysis and high heating values on dry basis (one can notice the difference between different literature sources). The average moisture content of Assaı´ kernels is 35%. According to its calorific value, low moisture and ash contents it is possible to conclude that Assaı´ kernels have acceptable characteristics as fuel. However, no information was found about the Assaı´ kernel ash composition, which could be an important factor to forecast its agglomeration characteristics during combustion. As pointed out by Seye [1], not all residues can be used as an energy biomass. In the case of Assaı´, 55.40% of the fruit is the kernel (suitable for the gasification and with 31.8% of moisture content), and 15.2% are fines and residues, that could only be useful in a direct combustion process.
2.1.
Energy demand/biomass availability
In terms of energy consumption, the Assaı´ processing is dominated by two main steps: the pulp extraction, which uses mechanical energy to squeeze fruit and extract the fruit peel, and the storage, which requires high quantities of electricity for cooling systems (freeze and storage). The typical industrial unit is a small to medium enterprise with installed processing capacity varying from 800 up to 2400 tonnes of fruits per year, for an average value of 1946 tonnes of fruits/year [9]. The production is dominated by the Assaı´ peak crop season, although it lasts 6 months, it has only 3 months (Luczynski, 2008), when all industries are operating above peak (extended working hours). This fact e peak fruit processing season e leads units to a high installed e yet under utilized e processing capacity; being usual levels of 80% [9]. According to Ref. [5], equipment suppliers and operational data from local industries, a typical unit that process 1000 tonnes of fruits per year can be described as: 1. The equipments, for conventional installation: one squeezing machine (600 tonnes of fruits per hour e 80 kW), one cooling tunnel (200 kW), a storage facility (75 kW), other additional loads (packaging, handling, filter, etc. e 20 kW);
Table 2 e Immediate analysis and high heating value for Assaı´ Kernels found in literature. Source [6] [8] [1]
Fixed carbon [%]
Volatiles [%]
Ashes [%]
High heating value [kcal kg1]
e 16.46 18.50
e 79.12 80.35
e 1.32 1.15
4500.00 4451.33 3907.51
Please cite this article in press as: Teixeira MA, et al., Assaı´ e An energy view on an Amazon residue, Biomass and Bioenergy (2013), http://dx.doi.org/10.1016/j.biombioe.2013.08.007
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2. The number of working hours for this analysis, 3 different seasons along the year must be considered, as follows: a. Peak of Assaı´ crop season: 3 months (SeptembereNovember), working ours: squeezer 11 h day1 (12 working hours minus 2 cleaning), cooling tunnel 20 h day1, storage 24 h day1; b. Low Assaı´ crop season: 4 months (August, and from December to February), working hours: squeezer 6 h day1 (8 working hours minus 2 cleaning), cooling tunnel 5 h day1, storage 24 h day1; c. End of season Assaı´ crop: it’s when other fruits are processed, like Cupuac¸u Theobroma grandiflorum), 5 months (MarcheJuly), working hours: squeezer 5 h day1 (7 working hours minus 2 cleaning), cooling tunnel 0 h day1 (send direct to storage), storage 24 h day1; 3. The amount of processed fruits at peak: 556 tonne fruits year1; at low processing season: 444 tonne fruits year1. 4. The amount of biomass produced at peak: 309 tonne kernel year1 (at 32% moisture e 260 tonne year1 at 16% moisture); out of peak: 247 tonne kernel year1 (at 32% moisture e 208 tonne year1 at 16% moisture). Total 556 tonne year1 at 32% moisture or 469 tonne kernel year1 at 16% moisture. 5. The maximum power demand is 375 kW, and the minimum 95 kW As for any biomass bases system, raw material supplychain is a key aspect to be considered, first concerning the radio from the processing where to secure the supply as indicated by Mun˜iz-Miret [10], other aspect particular for the Assaı´, is the need to consider the multiple use of the floodplain forest (where Assaı´ is cultivated/exploited) and the heterogeneity of the forest [11]. Finally, it is necessary to build up a forest governance where the risks and gains could be fairly negotiated and divided between the multiple stakeholders [12].
3.
Biomass conversion technology
According to Ref. [13], there are four main categories of conversion technologies for exploitation of biomass energy: direct combustion, thermochemical processes (gasification or pyrolysis), biochemical processes and agrochemical processes. Currently, the most commonly used technologies to convert biomass energy into electricity are direct combustion and gasification. For that reason, these technologies will be investigated in this work.
3.1.
low capacities. So, there are two solutions for small scale electricity generation using biomass: a) the utilization of other fluid as the working fluid, let’s say an organic fluid (organic Rankine cycle e ORC) [14]; b) the utilization of other prime mover instead of the conventional axial turbines (radial turbines, screw or scroll expanders, or steam piston engines); c) the utilization of more than one of these alternatives simultaneously. The last case corresponds to the externally fired gas turbine (EFGT) technology, which operates using air heated in a specially designed heat exchanger using high temperature combustion gases. Between all these alternatives only the ORC systems are available commercially, however its costs is still high [14]. The conventional steam Rankine cycle, burning biomass, has been used in several industries such as sugar, rice, palm oil, paper and wood industry for many years to produce electricity with relatively low efficiency. However, the low price of fuel (biomass wastes from the process), the maturity and reliability of this technology and his relatively low investment cost, make this conversion technology an attractive option. According to Ref. [15], in recent years, increasing political and environmental pressures has prompted the development and utilization of renewable energy. This led to increase both demand and cost of biomass resources. As such, the paradigm has shifted towards more efficient and effective utilization of the biomass resources, aiming to acquire higher electrical efficiencies than the one of the conventional Rankine steam cycle. So, nowadays the organic Rankine cycle is a real option to be considered. The ORC cycle consists of an evaporator driven by a hot thermal oil boiler, a water or air cooled condenser, a solution pump and a condensing turbine. According to ORC manufacturers, this technology has many favorable characteristics [16]:
High electric efficiency Excellent partial load behavior Various possibilities for process integration High turbine isentropic efficiency (up to 85%) No erosion of the turbine blades, due to the absence of moisture in the steam nozzles Low maintenance costs Choosing the most suitable organic fluid for a given ORC cycle application is a key issue and it has been treated in numerous studies, most of them focused on low temperature heat sources. Some general relevant features can be drawn from these studies. According to Ref. [17] the most important are:
Biomass combustion
Biomass combustion is a complex exothermic reaction. The heat from this reaction can be transferred to a working fluid that is supplied to a prime-mover. The most common applications are the steam Rankine cycle, with conventional steam turbines or a variety of different steam engines, or the organic Rankine cycle, which uses an organic fluid instead of water as a working fluid. The utilization of conventional axial steam turbines in small scale electricity generating plants is complicated by the fact that they have very low efficiencies at
Water
Fruits Reception
Washing 3x
Squeezing
Kernel
Pulp Storage
Freezing
Fig. 1 e Schematic of the production process on industrial Assaı´ pulp unit.
Please cite this article in press as: Teixeira MA, et al., Assaı´ e An energy view on an Amazon residue, Biomass and Bioenergy (2013), http://dx.doi.org/10.1016/j.biombioe.2013.08.007
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Fig. 2 e Physical structure of the power plant in the Gate-Cycle screen (case 1 and case 2).
Thermodynamic performance: because it is important to achieve the higher electric efficiency and/or power for a given heat source. Chemical stability: The majority of fluids suffer deterioration when exposed to high pressures and temperatures; in addition, there must be compatibility between the materials and construction equipment. Environmental, safety and health impact: issues such as the global warming potential (GWP), ozone depletion potential (ODP), toxicity and flammability. Availability and costs.
3.2.
Biomass gasification
The main feature of ORC cycles using biomass as a fuel is their higher level of temperature; generally around 600 K, that further restricts the selection of the fluid. Some organic fluids are used, mainly siloxanes, which have many favorable characteristics. They have good lubricating properties, are odorless, are non-toxic, are stable at high operating temperatures, are non-flammable and, especially, have a good thermodynamic performance [18,19].
Biomass gasification produces a fuel gas called poor gas or synthesis gas (syngas) through the thermochemical conversion of the biomass, involving partial oxidation of the feedstock in a reducing atmosphere in the presence of air, oxygen and/or steam. Biomass materials differ greatly in their chemical, physical and morphological properties and, therefore, and they require the development of different pretreatment of the materials, gasification methods and consequently different reactor designs or even gasification technologies [20]. There are three main types of gasifiers: fixed or moving bed, fluidized bed, and entrained flow. Among these designs there are variations, such as spouted bed, conventional and internally circulating fluidized bed gasifier, etc. [21]. Gasification produces not only useful fuel gases, but also some byproducts like fly ash, NOx, SO2 and tar. Tar in the produced gases will condense at low temperature, leading to clog or blockage of fuel lines, filters and engines.
Table 3 e Main parameters adopted for the CEST power plant simulation.
Table 4 e Main parameters adopted for the BPST power plant simulation.
Parameter
Parameter
Biomass power plant Atmospheric air temperature Atmospheric air pressure Steam pressure Steam temperature Steam production Condensing pressure Assaı´ e low heating value (LHV) Boiler thermal efficiency Steam turbine isentropic efficiency Installed power Pumps isentropic efficiency Electric generator efficiency Power plant auxiliary equipment power consumption Feed water temperature Biomass consumption a Wet based. b Calculated using the software Gate-Cycle.
Value, units 25 C 101.3 kPa 2 MPa 300 C 6 t h1 20 Pa 9847a kJ kg1 60% 78% 1 MW 82% 96% 26.3b kWh 109 C 2760 kg h1
Biomass power plant Atmospheric air temperature Atmospheric air pressure Steam pressure Steam temperature Steam production Condensing pressure Assaı´ e low heating value (LHV) Boiler thermal efficiency Steam turbine isentropic efficiency Installed power Pumps isentropic efficiency Electric generator efficiency Power plant auxiliary equipment power consumption Feed water temperature Biomass consumption
Value, Units 25 C 101.3 kPa 2 MPa 300 C 9 t h1 102 kPa 9847a kJ kg1 60% 75% 1054 MW 82% 96% 39.76b kWh 109 C 3090 kg h1
a Wet based. b Calculated using the software Gate-Cycle.
Please cite this article in press as: Teixeira MA, et al., Assaı´ e An energy view on an Amazon residue, Biomass and Bioenergy (2013), http://dx.doi.org/10.1016/j.biombioe.2013.08.007
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Fig. 3 e Organic Rankine cycle and thermal scheme [25].
Moreover, too much tar in product gases will reduce the biomass gasification efficiency. Therefore, the reduction or decomposition of tar into biomass derived fuel gases is one of the biggest obstacles in its utilization for power generation [22]. On the basis of their characteristics, the current tar reduction or destruction methods can be broadly divided into five main groups: mechanical, thermal cracking, catalyst cracking and plasma methods [22]. Mechanical methods include scrubbers, filters, cyclones and electrostatic precipitators. The primary use of these devices is to capture particles from the product gases. These methods only remove the tar from product gases, while the energy of the tar is lost. Besides, the systems generate contaminated water, which induces another waste disposal problem. For the mechanical power generation, spark ignition engines normally used with petrol or kerosene, can be run on producer gas alone. Diesel engines can be converted to full producer gas operation by lowering the compression ratio and the installation of a spark ignition system. Another possibility is to run a normal unconverted diesel engine in a “dual fuel” mode, whereby the engine draws anything between 0 and 90% of its power output from producer gas, the 10% corresponding to diesel oil is necessary for ignition of the combustible gas/air mixture. The advantage of the latter system lies in its flexibility. However, not all types of diesel engines can be converted to the above mode of operation. Compression ratios of diesel engines are, considered by some researchers, too high for satisfactory dual fuel operation. The use of producer gas in those engines could lead to knocking because of auto combustion due to high pressures, combined with delayed ignition. Other authors differ on this issue [23]. The gasifier/engine generation technology has been object of some critical considerations relating to its frequent operational problem, which affects the system reliability and available power [24]. Environmental and health related problems are also indicated. However, some success stories could be remarked such as; the CPM, Xyllowatt, Babcock&Wilcox, Enamora and Gussing gasifier. The high cost of the equipment is another serious drawback to be considered.
4.
Biomass power cycles description
4.1.
Steam Rankine cycle
4.1.1.
Case 1. Condensing/extraction steam turbine e CEST
The system generates electricity using a steam turbine, with a steam extraction to preheat the feed water. Basically, the system consists of a fire-tube boiler, one condensing/extraction steam turbine, an electric generator, auxiliary pumps, a deaerator and a closed cooling water system (Fig. 2). The simulations were performed using the GateCycle software and the general parameters adopted for the selected configuration are presented in Table 3. In the power system shown in Fig. 1 the fuel consumption is 2.76 t h1 and the net electricity produced is 1000 kW, with an overall system efficiency of 13.4%. Different factors can affect the plant efficiency such as: Assaı´ residues moisture and the equipment degradation. However, this technological alternative presents a high potential for electricity generation from Assaı´ fibers in isolated communities.
Table 5 e Main parameters adopted for the ORC power plant simulation. Parameter Biomass power plant Turbine inlet vapor pressure Turbine inlet vapor temperature Steam production Condensing pressure Assaı´ e low heating value (LHV) moisture 35% Boiler thermal efficiency Turbine isentropic efficiency Net electrical power Pumps isentropic efficiency Electric generator efficiency Power plant auxiliary equipment power consumption Biomass consumption
Value, Units 990 kPa 269 C 58.7 t h1 10 kPa 9847a kJ kg1 80% 85% 1 MW 75% 98% 30 kW 2710 kg h1
a Wet based.
Please cite this article in press as: Teixeira MA, et al., Assaı´ e An energy view on an Amazon residue, Biomass and Bioenergy (2013), http://dx.doi.org/10.1016/j.biombioe.2013.08.007
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Fig. 4 e Schematic representation of a set of a gasifier coupled with an internal combustion engine for electricity generation [27].
4.1.2.
Case 2. Backpressure steam turbine e BPST
This alternative considers the use of a backpressure steam turbine instead of a condensing/extraction one, being the configuration similar to the one presented in Fig. 1, but without the steam extraction and the condenser. The main parameters adopted in the simulations are presented in Table 4. The fuel consumption is 3.09 t h1 and the net electricity produced is 1000 kW, with an overall system efficiency of 9.34%. This configuration presents a lower efficiency than the one considered in Case 1, the main reason is the high pressure in the exhaust of the turbine (102 kPa).
4.2.
Organic Rankine cycle e ORC
The proposed ORC cycle was simulated to produce 1000 kW of electric power. In this application octamethylcyclotetrasiloxane (C8H24O4Si4) was chosen as a working fluid. The working fluid is pumped to the evaporator, where it is heated and slightly vaporized by thermal oil cycle. The organic fluid vapor powers the turbine, which is directly connected to the electric generator. The vapor is led to the condenser and then the condensate is pumped back to the evaporator, in a closed cycle. Usually, the expansion in the turbine ends for most
Table 6 e Main parameters used for the evaluation of the BGICE power Plant simulation. Parameter
Value, units
Assaı´ moisture Gasifier cold efficiency Assaı´ e low heating value Total thermal output Gas engine installed capacity Gas engine efficiency De-rating Biomass consumption
20% 80% 12,638 kJ kg1 5078 kW ta 1300 kW 32% 23% 1450 kg h1
a Thermal energy.
organic fluids, normally Siloxanes, in the gas phase at a temperature above the condensing one. For that reason an auxiliary heat exchanger is used, to improve the cycle efficiency as shown in Fig. 3. The following general assumptions are made in the thermodynamic analysis: 1) The kinetic and potential energy are neglected; 2) Reference state temperature (T0) and pressure (P0) are 298.15 K and 101.325 kPa respectively; 3) Temperature and pressure of fuel and air at the inlets are 298.15 K and 101.325 kPa; 4) The system is in steady state operation; 5) The main operating parameters are summarized in Table 5. The overall system efficiency is 13.5% and the net electricity produced is 1000 kW. This cycle presents a higher efficiency than the conventional Rankine cycle. Nevertheless, this option has a high initial investment cost, because it is an imported technology. The domestic production of some components could lead to a considerable reduction in investment cost.
4.3. Biomass gasifier/internal combustion engine e BGICE Fig. 4 shows a schematic representation of a gasifiereinternal combustion engine set. The syngas obtained from the Assaı´ seeds gasification is cleaned and cooled before entering the combustion engine, where the transformation of the chemical energy of the fuel into mechanical work happens (shaft
Fig. 5 e Conversion efficiencies of evaluated conversion technologies.
Please cite this article in press as: Teixeira MA, et al., Assaı´ e An energy view on an Amazon residue, Biomass and Bioenergy (2013), http://dx.doi.org/10.1016/j.biombioe.2013.08.007
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Table 7 e Estimated investment cost of each evaluated scenario. Cost (1000 US$)
Total PCE Civil work and Eng. (73% PEC) Total direct cost Indirect costs (12% TDC) Fixed capital investment Annual maintenance (3% TDC)
ORC
CEST
2530 1846 4376 525 4902 131
1867 1363 3231 387 3.619 161
BPST BG/ICE 1219 890 2110 274 2840 105
3400 588 3989 478 4467 119
power) and after this, the energy is converted, by an electric generator, into electricity. In order to ensure reliable, consistent feeding and optimize gasification products, the feedstock must be properly dried and sized. Most gasification systems require the moisture to be in the range of 10e20%. In this case it was considered as an Assaı´ residue moisture of 20% in the gasification feed, this level of moisture is achieved in a rotatory dryer using the engine waste heat with a thermal requirements of 4:0 Gj t1 evap [26]. The mathematical model developed by Centeno [28] to predict steady state performance of a biomass downdraft gasifier/spark ignition engine power system was used. The main parameters considered for the plant simulation are shown in Table 6. Additionally it was considered that the syngas in the only fuel used by the engine. According to the consideration resumed in Table 6, the electrical power output of the engine fueled with syngas is 1 MW, with a biomass consumption of 1.45 t h1. The overall efficiency of the electricity generation system is 19.6%. An alternative to the use of a producer gas with internal combustion engines, is the combination of gasifiers with gas turbines, Stirling engines or fuel cells. However, all these technologies are at an early stage of development, thus they were not considered in this study. Fig. 5 shows a comparison of the conversion efficiencies for all the technologies evaluated.
5.
Economic evaluation
Table 8 e Economic environment indicators. Indicator Electricity sale price Biomass pricea Planning time horizon Interest Rate Minimum attractivity rate TMA Plant operation per year
Value, Unit 67e233 US$ MWh1 0e33 US$ t1 20 years 12% 15% 7425 h
a The price of biomass varies significantly considering the species, the harvesting methods and the treatments needed. In addition, distance of transportation also affects the costs. In this work, the fuel used is a waste from pulp processing and/or sawmills; for that reason, the total cost of these resource is only associated with loading cost to the transportation vehicles the transportation it self and storage.
electricity production costs and the economic appeal of the considered scenarios. The net present value (NPV) and the internal rate of return (IRR) were used as economic feasibility indicators. The economic analysis of each thermodynamic cycle was performed according to the purchased-equipment cost (PEC). Data from equipment manufacturers were used to evaluate the monetary costs of plant equipment and additional cost calculated using correlations proposed by Peters [29]. Table 7 shows the estimated costs for each thermodynamic cycle and the distribution of fixed capital investment costs (FCI) and cost of operation using the methodology proposed by Bejan [30]. The cost of generated electricity for each scenario, obtained using the data from Table 7 is shown in Fig. 6. From the economic point of view, it can be observed in Fig. 5 that ORC cycle has no significant advantages for the electricity cost generation, compared to the results obtained when using the conventional steam cycle or the gasifier/internal combustion engine. The electricity generation cost of the steam cycle, based on condensing turbines is 27.2% higher than the one based on backpressure turbine. The reason is that the cost of the steam turbine is 46.4% higher when using condensing/extraction steam turbine. However, the steam cycle based on condensing turbine presents an electrical
As a complement of the obtained thermodynamics results an economical analyses was performed in order to determine the
Fig. 6 e Electricity generation cost for the different evaluated alternatives for an Assaı´ kernel cost of 5.5 US$ tL1.
Fig. 7 e NPV and IRR for the steam Rankine cycle based on condensation turbine an electricity market prices of 67e233 US$ MWhL1 and a biomass cost of 5.5 US$ tL1.
Please cite this article in press as: Teixeira MA, et al., Assaı´ e An energy view on an Amazon residue, Biomass and Bioenergy (2013), http://dx.doi.org/10.1016/j.biombioe.2013.08.007
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Fig. 8 e NPV and IRR for the steam Rankine cycle based on condensation turbine with biomass market prices of 0e33 US$ tL1 and an electricity commercialization price of 161 US$ MWhL1.
Fig. 10 e NPV and IRR for the steam Rankine cycle based on backpressure turbine with biomass market prices of 0e33 US$ tL1 and an electricity commercialization price of 161 US$ MWhL1.
efficiency 43.4% higher than the one of backpressure turbine. This shows the necessity of a cash-flow analysis to select the alternative that is the most economically attractive. Taxes and discount rates adopted for the cash flow analysis in this work are presented in Table 8.
electricity efficiency, the higher initial investment required affects their economical performance. A huge reduction in the ORC components investment costs can contribute to the economic feasibility of this type of plants for electricity generation in rural areas. From the economic point of view the most attractive technology is the BPST followed by the CEST, meaning that steam cycle is still a good option for biomass generation systems. For an electricity commercialization cost of 161 US$ MW1 the biomass cost lying in the range of 0 and 33 US$ t1 has low impact in the economic feasibility of the evaluated alternatives, being the electricity commercialization cost more important.
6.
Sensitivity analysis
A sensitivity analysis is carried out aiming to determine the economic feasibility of each one of the evaluated alternatives, considering different options of electricity commercialization and biomass costs. Figs. 7e14 show the main NPV and IRR obtained in different evaluated scenarios. Can be observed in Figs. 6, 8, 10 and 12 that for fixed market prices of biomass and electricity, the market prices are in the range of 67e273 US$ MWh1. The attractiveness of the investment increases when the electricity commercialization price goes from 150 up to 273 US$ MWh1. From a thermodynamic point of view the best energy conversion rate is presented by the gasifier/ICE plant, followed by the Rankine cycles. In spite of the ORC having high
Fig. 9 e NPV and IRR for the steam Rankine cycle based on backpressure turbine an electricity commercialization prices of 67e233 US$ MWhL1 and a biomass cost of 5.5 US$ tL1.
7.
Discussion and conclusions
There is no interest in storage biomass, and once Brazilian electrical system is characterized by a dry season (7 months e May to Nov), when the electricity has a higher cost, and a daily 3-h peak at the highest electricity costs (leading to 66 h/months), it might be possible to use the Assaı´ kernel
Fig. 11 e NPV and IRR for the ORC plant with an electricity market prices of 67e233 US$ MWhL1 and a biomass cost of 5.5 US$ tL1.
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b i o m a s s a n d b i o e n e r g y x x x ( 2 0 1 3 ) 1 e1 1
Fig. 12 e NPV and IRR for the ORC plant with biomass market prices of 0e33 US$ tL1 and an electricity commercialization price of 161 US$ MWhL1.
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Fig. 14 e NPV and IRR for the BG/ICE plant with biomass market prices of 0e33 US$ tL1 and an electricity commercialization price of 161 US$ MWhL1.
at this period of peak hours, at the maximum of the installed capacity (1 MW), and just covering the industry energy consumption for the rest of the day throughout the year. This business model (to operate at the maximum capacity when the electricity costs are higher), could make things easier for the Assaı´ industries to find business partners in the electricity sector, where the risks can be minimized, as the processing unit could avoid the biggest energy costs, and the utility could maximize the profits. On the other hand, there is a restricted availability of biomass at peak crop season, where 80% of the biomass is produced in a very concentrated period of the year (3 months). Considering the three technological options, and its specific biomass consumption, only the ORC and the BGICE might cover 100% of the peak hours (66 h month1), due to their lower specific biomass consumption. From an energy generation unit, the BPST could offer the best investment, but to optimize this system, a bigger integration should be sought, as a trigeneration option with the use of chillers to cover the refrigeration demand of the cold storage of the fruit processing unit. However this will also increase the demand for capital and risk perception for the processing unit.
It is clear that these processing units should operate with other biomass sources that could be used to feed the energy cycle, apart from the Assaı´ crop season, that could only respond for the last three months during the dry season. This should be part of the solution, as a more detailed balance between biomass availability and electricity tariffs should be better considered, as for all cases electricity commercialization price higher than 150 US$ MWh1 is necessary. Taking into consideration all points here presented, the authors consider e for further studies e first the adoption of the BGICE, due its potential fuel flexibility and potential operation during the electricity highest cost hours and the full Assaı´ crop season (and lower investment than the ORC). As a second alternative is the BPST due to its low cost of generation, but it can be better integrated with a three generation system e using chillers. For further studies it should be considered the analysis of the business model, considering other possible sources of biomass, complementary to the Assaı´. This could help to extend the use of the power cycle at the best electricity selling conditions, such as Cupuac¸u (T. grandiflorum), which crop season is from January to May [31]. Finally, it is possible to list some conclusions from the technical and economic evaluation of the four selected cases:
Fig. 13 e NPV and IRR for the BGICE plant with an electricity market prices of 67e233 US$ MWhL1 and a biomass cost of 5.5 US$ tL1.
1- On the current technical and economic conditions the economic feasibility of electricity production from Assaı´ kernels in a 1 MW electric power plant is difficult to be attained. A reduction in the investment costs, as well as higher electricity tariffs is necessary. 2- The technology having the highest conversion efficiency is the BGICE. The most attractive generation cost corresponds to the BPST. 3- For all cases, electricity commercialization prices higher than 150 US$ MWh1 are necessary. 4- The present electricity commercialization prices, biomass costs and investment cost do not justify the utilization of higher efficiency technologies. 5- The utilization of these technologies in cogeneration systems with absorption chillers, to support the cold storage capacity to the fruit processing units, could improve its economic feasibility.
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Acknowledgments [9]
The authors would like to express their thanks to Fabiano Sousa, of Cooperativa Agrı´cola Mista de Tome´-Ac¸u (www. camta.com.br), and other equipment manufacturers3 contacted: TGM Turbinas, AGTherm, Guascor, World Latin Business EIRL, Hauber Macanuda and Max Machine for supplying equipment data and costs. We are also grateful to the Coordination for the Improvement of Higher Education Personnel (CAPES), the National Counsil for Scientific and Technological and Development (CNPq), and the Foundation for Research Support of the State of Minas Gerais (FAPEMIG) for their collaboration and financial support in the development of this work.
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