THERMOECONOMIC ANALYSIS OF ELECTRICITY GENERATION THROUGH THE BLACK LIQUOR GASIFICATION / GAS TURBINES SYSTEMS

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1 THERMOECONOMIC ANALYSIS OF ELECTRICITY GENERATION THROUGH THE BLACK LIQUOR GASIFICATION / GAS TURBINES SYSTEMS Sílvia Maria Stortini González Velázquez Suani Teixeira Coelho Eletrotechnics and Energy Institute of the University of São Paulo (IEE/USP) Av. Prof. Luciano Gualberto, São Paulo Brasil. ABSTRACT The pulp/paper segment is one of the higher mass/energy consumers of the Brazilian industrial sector. Its industries generate most of the consumed energy from by-products of the process (firewood, barks, residues and black liquor), but still are not self-sufficient and no efficient technology is being stimulated until now. Also, other problems exist, mainly the lack of adequate policies, which must be solved in this segment. Cogeneration is an interesting option to increase electricity offers, contributing for the reduction of the electric system risks of supply together with the diversification of the Brazilian Energy Matrix. In this paper we evaluate the electricity generation costs in a real pulp/paper plant, using the thermoeconomic analysis. Different cogeneration systems are considered, including black liquor gasification/gas turbines systems, and their corresponding electricity costs are calculated and compared with the electricity tariffs, defined by PROINFA. From the obtained results, mechanisms are proposed to stimulate decentralized generation in pulp/paper plants. KEYWORDS: Biomass, Black Liquor, Cogeneration, Gasification, Thermoeconomic Analysis. INTRODUCTION The pulp/paper segment, a very energy-intensive one, is among the main responsibles for mass use in the industrial sector, together with the sugar-cane industry, with the corresponding environmental advantages, generating most of its energy consumption from process by-products (firewood, barks, residues and black liquor), but still not self-sufficient.. From the point of view of these industries, the incentive to the self-sufficiency is at the risk of supply interruptions - therefore the involved costs in these situations (loss of raw material, interruption in the production, loss of final product quality, etc.) are very superior to the costs of the electricity autoproduction. These difficulties are quite well known, as well as the difficulties faced by the Brazilian electric sector. Previous studies (VELÁZQUEZ, 2000) indicate that this segment could reach the selfsufficiency in electricity generation if adequate policies are implemented. However, price policies developed by utilities are the same (tariffs reduced for the large consumers), and the price of the electricity surplus generated, today defined by the PROINFA, is below the value considered attractive by generators. More recent studies (GALLEGO, 2004; LARSON, CONSONNI, KATOFSKY, 2003; VELÁZQUEZ, 2006) search alternatives for electricity generation technologies used in the segment, from commercially available technologies, such as higher pressure boilers, extraction-condensing steam turbines up to gas turbines combined cycles. However, it is observed the need for more efficient technologies (under development) as, for example, the gasification. 1

2 According to Walter (1998), the efficiency and the economic potential of the electricity generation systems from mass gasifiers mainly depend of the performance on the gas turbine for gas on mass. The gas turbines technological advances tend to be significant in the next years, for the integrated action of the gas turbines manufacturers and research centers. To contribute with the discussion of mechanisms to make possible the electric surplus generation, this paper evaluates these costs in a real pulp/paper plant, the Klabin Papel e Celulose Industry - Paraná Business Unit. These results are compared with the price defined by the PROINFA (44,07 US$/MWh 1 ) 2, for wooden residues. METHODOLOGY In the economic analysis for a given plant where the only product is electricity (as, for example, thermoelectric power plants), the capital, fuel and O&M costs are, in general, amortized by the generated electricity, by means of conventional economic analyses. In the case where the process to be analyzed is a cogeneration one, where two different products are generated (steam and mechanical/electric energy), generation costs must be calculated trough thermoeconomics analysis (MARTINS; NOGUEIRA, 1997). These two types of energy produced (heat and work) are different from the point of view of the Second Law of Thermodynamics and, thus, they are quantified based on the exergy concept, which takes in consideration the steam s thermodynamic conditions (COELHO, 1999). Since the cogeneration is a process that involves more than one product, partition methods must be used to determine the specific costs (in an exergetic base) of each one of the products (steam and mechanical/electric energy) (PELLEGRINI; COSTA; OLIVEIRA JÚNIOR, 2005). Among these methods, it has to be considered, in case of this study, the one that the plant allows to keep its energetic process costs, without including in the generated electricity costs all the generation costs, as occurs when the conventional economic analysis is applied (VELÁZQUEZ, 2000), where the costs are all amortized by the generated electricity, admitting cost of the null steam. The partition methods adopted to evaluate these costs are the equality method and the work as by-product method, with the following considerations: - Initially, process steam costs (12.5 bar - medium pressure and 4 bar - low pressure steam) and electricity costs are evaluated for the current situation, with new Tomlinson boiler, that is the technology in use in the segment s industries. The specific cost of process steam (in the exergetic base) is calculated by the equality method, or either, admitting they are equal to the electricity costs (Equation 1); Assuming this value for energy costs in the plant and using the work as by-product method, it is calculated the cost of the electricity generated from different Black Liquor Gasification Combined Cycle (BLGCC) configurations (which are more efficient). There they allow the production of electricity surplus(equation 2); Considering that the generation costs of the electricity consumed in the process are the same of the current situation, the generation costs of the electricity surplus in each BLGCC configuration are calculated (Equation 3). Szargut, Morris and Steward (1988) defines exergy as the obtained amount of work when a mass is taken of the initial state until the state of thermodynamic balance, through reversible processes, having only interaction with the environment. 1 Available in Access in November, Dollar quotation in R$ 2,3. 2

3 C On all the configurations, the thermoeconomic analysis is based on following cost rate balances (BEJAN; TSATSARONIS; MORAN, 1996): econs.. Wecons. Cvb. mvb. EXvb + Cvm. mvm. EXvm = Ccap + Co & m + PCIlix. mlix. Clix + C eger. liq = + PCI. m. C (1) ( C + C + PCI. m. C + PCI. m. C + PCI. m. C ) ( C. m. EX + C. m. EX ) cap o& m lix lix lix econs W eger. liq C = W. C (3) eexc. Weexc Weger. liq. Ceger. liq. econs gn gn gn vb vb vb vm vm vm The Table 1, below, presents the nomenclature of the variable used in equations. Table 1 Nomenclature of the variable used in equations. Weger.liq. = Generated electricity (líquid) (a) MW Ceger.liq. = Generated electricity specific cost (líquid) (a) US$/MWh Cvb = Low pressure steam specific cost US$/kg mvb = Low pressure steam Mass kg/s Exvb = Low pressure steam specific exergy kj/kg Cvm = Average pressure steam specific cost US$/kg mvm = Average pressure steam mass kg/s Exvm = Average pressure steam specific exergy kj/kg PCIlix = Black liquor calorific power kj/kg mlix = Black liquor mass kg/s Clix = Black liquor specific cost US$/kg PCI = Biomass calorific power kj/kg - m = Biomass mass kg/s C = Biomass specific cost US$/kg Ccap = Capital cost US$/s Co&m= Operation and Maintenance cost US$/s PCIgn= Natural gas calorific power kj/kg mgn= Natural gas mass kg/s Cgn= Natural gas specific cost US$/kg Ceexc= Exceeding electricity specific cost US$/MWh Weexc= Exceeding electricity MW Cecons= Specific cost of the electricity consumed in the process US$/MWh Wecons= Consumed electricity in the process MW Cecomp= Specific cost of the electricity bought in the retail US$/MWh Source: Author s denomination. Note: (a) The liquid generated electricity is that one generated in the most efficient configurations. 3

4 In this analysis the same methodology of thermoeconomic analysis applied to the sugar-cane industry sector for Coelho (1999) is adapted to allow the electricity costs real evaluation (in an exergetic base), of the medium and low pressure steam, as well as the surplus electricity cost. It was selected work as by-product method, as a form to keep constants, for the plant, the electricity and the process steam costs. Beyond the system with Tomlinson s recovery boiler (technology in use) three configurations of Black Liquor Gasification in Combined Cycle (BLGCC) are considered, to know, Low Temperature BLGCC with a Mill-Scale Gas Turbine, High Temperature BLGCC with a Mill-Scale Gas Turbine, High Temperature BLGCC with Utility-Scale Gas Turbine, that are evaluated by means of three sceneries where if they present financial conditions next to the used ones in the national market (Discount rate i of 15%, 17.5% and 20% a.a. amortized in 20 years, that is the time of the systems lifetime) beyond specific costs to capital (for acquisition of new equipment and, therefore, not amortized) and to O&M determined for Larson, Consonni and Katofsky (2003), for the fact that the budgets have been carried through by two experienced American companies in gasifiers construction authors consider that these are consistent prices. Initially, the acquisition of a new Tomlinson 3 bolier is considered, identical to that proposed by Larson, Consonni and Katofsky (2003), and calculating the cost of the industry s steam generation, for the equality method (admitting equal costs of the electricity, medium and low pressure steam) (Equation 1) and, then, this value is fixed to calculate the generated electricity costs. In this configuration, the plant still does not reach the self-sufficiency, however it needs to buy a minor amount of electricity (10,9 MW). It is initiated, then, the substitution of these Tomlinson systems for Low Temperature BLGCC systems with a Mill-Scale Gas Turbine and High Temperature with a Mill-Scale Gas Turbine, adequate configurations to the reality of the industry selected for the study. The High Temperature configuration with a Mill-Scale Gas Turbine, after the thermodynamic analysis (Table 2), showed not taken care in the process steam necessities and, therefore, it is not considered in this study. Table 2 - Generated electricity in each configuration that attend the process needs. Liquid generated electricity Consumed electricity in the process Low-Temp Mill Scale BLGCC Hight-Temp Utility Scale BLGCC Source: Author s calculations. Surplus electricity generated For both BLGCC configurations that attend to the plant s needs, are used the steam s generation cost of the industry, having considered the new Tomlinson system acquisition, calculated from the equality method (admitting equal the electricity costs and the medium and the low pressure steam costs) that it is, then, fixed to calculate the generated electricity cost. Pre-defined this value, from the work as by-product method, is calculated the cost of the electricity generated from both BLGCC configurations (which is more efficient), that they attend to the process and that they allow the production of electricity surplus (Equation 2). In this calculation s stage, due to the fact that the Klabin industry consume 31.1 MW of the thermal cogeneration central office (to the cost calculated for Equation 1), to buy 38 MW of the 3 The industry purchase of the local concessionaire, today, 38 MW. 4

5 COPEL (to the price of 35 USS/MWh), besides using 23 MW generated for the hydroelectric plant (Klabin s property), a weighed mean of consumption is calculated, assuming itself that they will continue to use the 23 MW of the President Vargas Plant which was, also, constructed to take care of the plant. Fixed the steam s generation cost and the cost of electricity generation consumed in the process, after that the generation of electricity surplus costs in each BLGCC configuration are calculated (Equation 3). OBTAINED RESULTS Electricity and steam generation costs of the Tomlinson case configuration are obtained considering that ce = clps = cmps (equality method). The obtained result for the electricity costs and the steam of low and medium pressure, calculated for the equality method is presented in Table 3. Table 3 Specific costs of electricity, of low and medium steam pressure from the Tomlinson configuration. SYSTEM (a) GENERATED ELECTRICIT Y COST (US$/MWh) CONSUME D STEAM COST (US$/MWh) PROCESS STEAM COST (b) MEDIUM PRESSURE STEAM (US$/t) HIGH PRESSURE STEAM (US$/t) i = 15% a.a. TOMLINSON 16,14 16,40 4,01 3,20 20 YEARS i = 17,5% a.a. TOMLINSON 18,70 18,70 4,57 3,65 20 YEARS i = 20% a.a. TOMLINSON 21,06 21,06 5,15 4,11 20 YEARS Source: Author s calculations. Notes: (a) Considering equal the costs of the medium pressure steam, the low pressure steam and the generated electricity, for the new Tomlinson system acquired. (b) The costs (specific) of the low and medium steam pressure, calculated into an exergetic base had been transformed into costs (specific) in mass base, aiming better sensitivity in the evaluation of the results. For the two BLGCC configurations, that attends to the industry s steam demand, are calculated the electricity generation costs that kept equal and constants and the specific costs of the necessary steam of low and medium pressure to the process, from the work as by-product method. And, then, calculated the specific costs of the generated surplus electricity, aiming to keep constants the specific costs of the electricity consumed in the process (in an exergetic base). Figure 1 shows the costs of the generated surplus electricity in each configuration. 5

6 SPECIFIC COSTS OF EXCEEDING ELECTRICITY US$/MWh ,33 32,30 94,50 36,52 105,99 40,86 i = 15 % i = 17,5 % i = 20 % BLGCC LOW TEMP. MILL SCALE BLGCC HIGH TEMP. UTILITY SCALE TOMLINSON Figure 1. Specific costs of the generated surplus electricity. Source: Author s calculations. In accordance with Figure 1, the Tomlinson system, that is not self-sufficient, obviously, does not generate excesses. The most interesting configuration is the High Temperature BLGCC with Utility-Scale Gas Turbine, therefore it presents minor generation costs. For better agreement, Table 3, below, summarizes all the calculations results of the specific generation costs obtained in this analysis. Table 4 - Specific costs of electricity generation. SCENERIES i = 15% a.a. 20 years i = 17,50% a.a 20 years i = 20% a.a. 20 years SYSTEMS GENERATED ELECTRICITY COST US$/MWh SURPLUS ELECTRICITY COST US$/MWh TOMLINSON 20, BLGCC LOW-TEMP MILL-SCALE 30,78 83,33 BLGCC HIGH-TEMP UTILITY-SCALE 20,76 32,30 TOMLINSON 22, BLGCC LOW-TEMP MILL-SCALE 34,88 94,50 BLGCC HIGH-TEMP UTILITY-SCALE 23,35 36,52 TOMLINSON 24, BLGCC LOW-TEMP MILL-SCALE 39,10 105,99 BLGCC HIGH-TEMP UTILITY-SCALE 26,12 40,86 Source: Author S calculations. Notes: (a) Considering equal the medium and low pressure steam costs and the generated electricity cost, for the new Tomlinson system acquired. The comparison factors for the results evaluation are the generation costs and the electricity price defined for the PROINFA in Brazil (44,07 USS/MWh). In this analysis is only considered the 6

7 BLGCC configuration of High Temperature with a Mill-Scale Gas Turbine, by the fact to have presented compatible costs of surplus generation with the electricity tariff offered for the concessionaires in Brazil, beyond the amount of generated surplus electricity (120,32 MW). As it can be observed in the results presented in Table 4, the costs of the total generated electricity, in all the configurations, are less than the price defined by PROINFA, what is presented positive. However, the generation costs of the surplus electricity do not reveal competitive with the electric tariffs offered by PROINFA, in all the configurations. The High Temperature BLGCC configuration with a Mill-Scale Gas Turbine was the only one to present a competitive value for the surplus electricity generation cost in relation to the electric tariffs offered by the concessionaires in Brazil, independent of the considered scenario. FINAL CONSIDERATIONS With the results of this study, it is evident the necessity of other mechanisms that could make possible the implementation of new cogeneration technologies. Further studies are necessary to compare environmental aspects of mass-origin electricity to those from conventional generation systems (fossil fuel s origin). The difference between the electricity generation cost and the electricity tariff offered will be better observed when from the generation costs the carbon credits will be deducted, inside Kyoto Protocol chances (MARTINS, 2004) and the increased externalities tariffs (COELHO, 1999). Thus, would be obtained a more precise estimate of the invested capital recovery. Taking in consideration the fact that the pulp/paper segment demonstrate great electricity consumption, associated to the current situation of the Brazilian electric sector, to the lack of guarantee in electricity offers and to the risks of supply interruption, the proposal presented here can be considered viable, supported also in the some inherent advantages to the cogeneration process, that collaborate indirectly in the electricity offer, brightening up the concessionaires overload. 7

8 REFERENCES BEJAN, A., TSATSARONIS, G., MORAN, M. Thermal Design and Optimization. John Wiley & Sons, Inc. New York, COELHO, S.T. Barreiras e Mecanismos para Implementação de um Programa de Larga Escala de Cogeração a Partir de Biomassa. Uma Proposta para o Estado de São Paulo Tesis (Energy Doctorate) Interunits Pos Graduation on Energy Programme, University of São Paulo, São Paulo, GALLEGO, A.G. Modelagem Computacional e Análise Termodinâmica de Sistemas de Geração de Potência Utilizando Gaseificação de Licor Negro. (Doctorate Tesis) State University of Campinas (UNICAMP). Campinas, LARSON, E.D., CONSONNI, S., KATOFSKY, R.E. A Cost-Benefit Assessment of Biomass Gasification Power Generation in the Pulp and Paper Industry. Priceton: Priceton University, (Final Report). Available in: Pdf. Acesso em: 13 jan MARTINS, O.S. Determinação do potencial de seqüestro de carbono na recuperação de matas ciliares na região de São Carlos/SP. (Doctorate Tesis) Pos Graduation Programme in Ecology and Natural Resources of the Federal University of São Carlos, MARTINS, A.R.S., NOGUEIRA, L.A.H. Desenvolvimento Metodológico Para Análise de Sistemas de Cogeração. In: 14º COBEM Mechanical Engineering Brazilian Congress, Anais. Bauru. v. 1. p PELLEGRINI, L.F., COSTA, R.P., OLIVEIRA JUNIOR, S. A Atribuição de Custos em Sistemas Energéticos: A Termoeconomia como base de cálculo. In: XXV ENEGEP. Porto Alegre, RS, SZARGUT, J.; MORRIS, D.R.; STEWARD, F.R. Exergy analysis of thermal chemical and metallurgical processes. Hemisphere Publishing Corporation, New York, VELÁZQUEZ, S.G. Perspectivas para a Geração de Energia Elétrica no Segmento de Papel e Celulose com a Utilização de Sistemas de Gaseificação/turbina a gás. Tesis (Energy Doctorate) Interunits Pos Graduation on Energy Programme, University of São Paulo, São Paulo, VELÁZQUEZ, S.G. A Cogeração de Energia no Segmento de Papel e Celulose: Contribuição à Matriz Energética Brasileira. (Masters Degree Tesis) Interunits Pos Graduation on Energy Programme, University of São Paulo, São Paulo, WALTER, A.C.S. Incentivos Econômicos e Ambientais para a Difusão de Tecnologias Avançadas de Conversão de Biomassa. Semestral Report - FAPESP (Pos doctorate), National Renewable Energy Laboratory. Colorado, In: BIOMASSA - Guia de Investimentos em Energias Renováveis no Brasil ed. ANEEL/CENBIO, 1998 (cdrom). 8