Energetic and exergetic analysis of biomass gasification

Universidade de Trás-os-Montes e Alto Douro

Energetic and Exergetic analysis of biomass gasification

Dissertação de Mestrado em Engenharia Mecânica

Nome do Candidato: Nuno Tiago Dinis Couto Nome do Orientador: Abel Rouboa Nome do Co-Orientador: Armando Soares

Nome do Co-Orientador: Valter Silva

Universidade de Trás-os-Montes e Alto Douro

Energetic and Exergetic analysis of biomass gasification

Dissertação de Mestrado em Engenharia Mecânica

Nome do Candidato: Nuno Tiago Dinis Couto Nome do Orientador: Abel Rouboa

Nome do Co-Orientador: Armando Soares Nome do Co-Orientador: Valter Silva

Composição do Júri:

Professor Doutor Abel Rouboa Professor Doutor Armando Soares Doutor Valter Silva

Professor Doutor Nuno Dourado Professor Doutor Luís Alexandre

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Agradecimentos

Ao meu orientador, Professor Doutor Abel Rouboa, e aos meus co-orientadores, Doutor Valter Silva e Professor Doutor Armando Soares, agradeço toda a ajuda e oportunidades que me foram dando ao longo dos últimos 2 anos.

Ao Doutor Eliseu Monteiro pela sua ajuda e experiencia ligada à combustão e gasificação.

Ao Professor Doutor Paulo Brito do Instituto Politécnico de Portalegre pelo fornecimento dos dados experimentais usados nesta tese.

A todos os amigos que me apoiaram ou ajudaram de alguma forma nos últimos anos.

Queria agradecer também à minha família, especialmente aos meus pais e aos meus avós maternos, por tudo que me concederam ao longo da minha vida.

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Resumo

A crescente preocupação energética, económica e ambiental obriga a uma utilização eficiente dos recursos energéticos naturais. Recursos energéticos renováveis poderão ser um bom substituto aos combustíveis fósseis que estão a ser consumidos rapidamente.

Atualmente, biomassa e biocombustíveis estão a ser considerados devido às suas características não poluentes e às suas capacidades de fornecimento de energia. Uma das preocupações mais urgentes em relação ao uso da biomassa como energia renovável é o aumento da sua eficiência energética e principalmente a sua eficiência exergética. De forma a melhorar a eficiência exergética é necessário melhorar o processo de gasificação, criando modelos numéricos que otimizem o seu design e as suas condições operatórias. Seguindo este raciocínio, a presente tese realiza uma revisão exergética e de seguida um modelo numérico é apresentado para a otimização das condições operacionais de forma a melhorar a eficiência do processo de gasificação.

O primeiro capítulo desta tese apresenta uma revisão dos artigos existentes na análise de exergia da gasificação da biomassa. Muita atenção foi dada à produção de hidrogénio a partir do processo de gasificação e à tecnologia da gasificação de carvão integrada com o uso de uma célula de combustível (SOFC). Na maioria das análises exergéticas realizadas, a gasificação foi considerada como o processo termodinâmico principal.

Adicionalmente, os modelos exergoeconómicos permitem combinar economia com termodinâmica de forma a atingir uma avaliação conjunta. Uma das mais populares estratégias para desenvolver análises exergoeconomicas é o método SPECO.

O método SPECO apresenta de uma forma geral, sistemática e simples uma aproximação para o desenvolvimento das eficiências energéticas de um sistema térmico e os seus respetivos componentes.

A necessidade deste tipo de investigações é devida à falta de informação nas análises exergéticas da gasificação da biomassa.

De forma a otimizar as condições operatórias do processo de gasificação da biomassa foi desenvolvido um modelo numérico com base na estrutura CFD. Este modelo consiste num modelo computacional de 2 dimensões que descrevia o processo de gasificação da biomassa

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num gasificador de leito fluidizado usando cascas de café com o auxílio do código comercial

FLUENT. Ambas as fases, sólida e gasosa, foram descritas usando uma aproximação

Euleriana-Euleriana havendo troca de massa, energia e de momento. Resultados do modelo numérico foram depois comparados com resultados experimentais. O estudo foi realizado numa central semi-industrial instalada no parque industrial de Portalegre baseada na tecnologia de leito fluidizado, com uma capacidade de processo de 70 Kg/h e operando à volta de 800 °C. Os testes foram realizados continuamente durante vários dias, usando diferentes resíduos de forma a otimizar a composição do syngas produzido. Resultados obtidos apresentam boa concordância com os experimentais.

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Abstract

The growing concern for energy, economy and environment calls for an efficient utilization of natural energy resources in developing useful work. Renewable energy sources can be a good substitute of the fossil fuels which are being terminated fast.

Nowadays biomass and biofuels are considered because of their environment friendly characteristics and their ability of supplying much more energy. One of the most urgent concerns regarding the use of biomass as a renewable energy is getting a superior energetic and most importantly an exergetic efficiency. In order to improve the exergetic efficiency it is necessary to improve the gasification process, by developing numerical models that optimize the design and operation conditions. Accordingly, the present thesis shows an exergy review and later a numerical model is developed for operational condition optimization in order to make the gasification process more efficient.

The first chapter of this thesis has reviewed the existent papers on the exergy analysis of biomass gasification. A much extended report was made on hydrogen production by the process of gasification and coal gasification technology integrated with a solid oxide fuel cell.

In most of the exergy analysis that had been done, gasification was found to be the main thermochemical process.

Additionally, the Exergoeconomical modeling allows combining economics with thermodynamically required energy inputs in order to achieve an integrated evaluation. One of the most popular strategies to develop the exergoeconomical analysis is the SPECO method.

The SPECO method presents a general, systematic, simple and unambiguous approach for developing the exergetic efficiencies of a thermal system and its components.

The need for such investigations is due to the lack of information in the exergy analysis on the biomass gasification.

To optimize the operating conditions of a biomass gasification process it was developed a numerical model based on the CFD framework. This model was based on a two-dimensional computational model that described the biomass gasification process in a fluidized bed reactor using coffee husks within the commercial CFD code FLUENT. Both, gas phase and solid

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phase, were described using and Eulerian-Eulerian approach exchanging mass, energy and momentum. Results from the numerical model were later compared with experimental data. The study was conducted in a pilot thermal gasification plant, installed at Portalegre’s Industrial Park based on the fluidized bed technology, with a processing capacity of 70 kg/h, and operating at around 800 ºC. The gasification tests were performed continuously for several days, using different wastes in order to optimize the composition of produced syngas. Numerical results shows good agreement with experimental data.

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General Index

Agradecimentos iii

Resumo iv

Abstract vi

General Index viii

Figure Index x Table Index xi Nomenclature xii Main Introduction 1 References 6 Chapter I 8 1. Introduction 9

2. Research made on exergy analysis 12

3. Exergoeconomical Modeling 19 3.1. SPECO Approach 20 4 Exergoeconomical Modeling 23 Chapter II 31 1. Introduction 32 2. Experimental Set Up 34 3. Mathematical Model 37

3.1. Mass Balance Model 37

3.2. Momentum Model 38

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3.4. Granular Eulerian Model 39

3.5. Chemical Reactions Model 39

3.5.1. Homogeneous Gas-Phase Reactions 39

3.5.2. Heterogeneous Reactions 40

3.6. Energy Conservation 41

3.7. Numerical Procedure 41

4. Results and Discussion 43

5. Conclusions 47

6. References 48

Main Conclusions and Future Work 49

x

Figure Index

Figure 1 - Main conversion options for biomass to secondary energy carriers 2

Figure 1.1 - Exergy distribution for gasification of various fuels. 12

Figure 1.2 - Comparison of gasification efficiency for various fuels. 13

Figure 1.3 - Relative exergy losses for subprocesses in air-blown oxidation of solid carbon

versus equivalence ratio for an adiabatic case. 15

Figure 1.4 - Exergy efficiency versys gasified wood at a gasifier temperature of 1500 K. 17

Figure 2.1 - Biomass gasification pilot scale plant at Escola Superior de Tecnologia e Gestão,

Instituto Politécnico de Portalegre, Portalegre, Portugal. The main components of the unit

are described in the text. 34

Figure 2.2 – Gasifier’s Mesh 42

Figure 2.3 - Mass Fraction Contours for CO 44

Figure 2.4 – Mass Fraction Contours for CO2 44

Figure 2.5 – Mass Fraction Contours for H2O 45

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Table Index

Table 1.1 - Operational parameters of the tests 14

Table 2.1 - Biomass Properties 35

Table 2.2 - Operational parameters of the tests 40

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Nomenclature

Volume fraction

Density

Instantaneous velocity

Mass source term due to heterogeneous reactions

Molecular weight

stoichiometric coeficiente

Reaction Rate

universal gas constant

Gas Pressure

Temperature

Mass Fraction

Mean velocity

gas-solid interphase drag coefficient

Tensor Stress

generation of turbulence kinetic energy due to the mean velocity gradients

generation of turbulence kinetic energy due to buoyancy

contribution of the fluctuating dilatation in compressible turbulence to the overall dissipation rate

Constant Constant Constant

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user-defined source terms

user-defined source terms viscosity

thermal conductivity

specific heat capacity

_{specific enthalpy}

heat flux

source term due to chemical reactions

heat transfer intensity between phases enthalpy of the interface

1

Main Introduction

Over the last decades, it has been carried out a great effort to develop a set of new solutions to minimize the power generation supply, based on fossil fuels, but also to minimize the pollutants emissions, green house effects and global warming [1]. The economical benefits and environmental risks resulting from growth in energy consumption are well recognized, however, they are challenged by our ability to ensure the availability of endless resources to sustain a global energy future. Energy conversion represents 80% of the total greenhouse gas (GHG) emissions in EU [2]. The main origin of GHG emissions in Portugal is also related with the energy sector, more specifically to the fossil fuel combustion. Based on the Kyoto protocol, replacing the fossil fuel, which leads to global problems and greenhouse gas emission, by the hydrogen energy system and biomass is essential [3]. The obvious action to meet this challenge is to maximize energy productivity at the least cost.

Bioenergy is seen as one of the main options to mitigate the GHG emissions and to substitute fossil fuels. This is evident in Europe, where a set of activities and programs is supported to stimulate the use of the biomass for energy production [1]. Biomass has great potential as a renewable alternative to fossil fuels and it has relatively clean feedstock for producing modern energy carriers, such as electricity and transportation fuels. In order to compete with fossil energy sources an optimal utilization of biomass resources is desired. For biomass-based systems, a key challenge is thus to develop efficient conversion technologies [4].

Biomass production can generate employment and bring environmental benefits, such as reduced leaching of fertilizers and reduced use of pesticides. Also, when produced by sustainable means, biomass emits roughly the same amount of carbon during conversion as is taking up from the atmosphere during plant growth. Therefore, the use of biomass does not contribute to a buildup of CO2 in the atmosphere.

Usually, the biomass conversion processes are divided in two main process technologies as shown in Fig. 1. Firstly, thermochemical decomposition, suitable for low moisture herbaceous materials. Secondly, biochemical conversion suitable for high moisture herbaceous plants marine crops and manure. Within thermochemical conversion three process options are available: combustion, pyrolysis and gasification. Biochemical conversion embraces two process options: digestion (biogas) and fermentation (ethanol). The biomass conversion processes are normally selected based on the desired final form for the energy, the

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environmental standards, the type and quantity of biomass available in each region, the project specific factors and the governmental policies. However, it is the form in which the energy is required that determines the process type, followed by the available types and quantities of biomass [5].

Production of heat, electricity and fuels is nowadays more reachable than never with these technological developments, which in turn allow to the European nations to achieve stipulated targets in order to reduce the petroleum and environmental dependence.

Fig. 1 – Main conversion options for biomass to secondary energy carriers [6].

Pyrolysis process converts biomass into liquid, gaseous and solid fraction at temperatures around 500 ºC in the absence of oxygen or partially combusted in a limited oxygen supply [7]. Pyrolysis now receives most attention as a pre-treatment step for long distances transport of bio-oil that can be used in further conversion (i.e. efficient power generation or oil gasification for syngas production) [8].

The major role of the combustion is to allow the production of work which is largely used by the transportation sector, by electricity generating plants or by process industries. The efficiency of the combustion process is decreased by losses due to unburnt fuel, incomplete combustion and heat transference to the surrounding across the combustor walls [9].

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Gasification is a clean and highly efficient technology able to handle with different bio-feedstocks to a wide variety of applications such as, heat, electricity, chemicals and transports fuel [10]. Indeed, there are several methods to convert the biomass feedstocks to energy but it is expected that the biomass gasification will become the dominant technology in the future. The biomass gasification is very attractive because offers the most economical route for the production of renewable hydrogen and the obtained gases are intermediates in high efficient power production.

Gasification plants can be distinguished concerning the type of reactor, type of heating supply, medium of gasification as well as pressure ratio in the reactor. It has to be differentiated between fixed bed reactors and fluidized bed reactors [11].

The use of fluidized bed technology in biomass gasification units makes it possible to use relatively smaller gasifiers and larger capacities than with fixed bed. The higher efficiency of fluidized bed comes from its higher specific reaction area when compared to fixed bed. Fluidized beds also proved to permit more versatile units in terms of feeding materials of different characteristics and origins, although the relatively high temperatures used to avoid sintering problems of the bed material leads to the formation of tars that complicate operation of equipment due to clogging of equipment and piping.

In Portugal according to SFA Gasification Database, three gasification projects have been deployed, with six gasifiers. However relevant information or scientific work is inexistent to take of conclusions on these national experiences.

Gasification is one of the most promising thermochemical conversion routes to recover energy from biomass [12]. Since the gasification is an endothermic process, heat must be added to the gasifier, which may be provided by an external source which supplies the necessary amount of heat. This feature can be achieved using the heat supplied by the depleted fuel and air streams of a fuel cell. The biomass-fuelled integrated solid oxide fuel cell (SOFC) system has been identified as one of key energy technologies for the future since it combines the merits of renewable energy sources and hydrogen energy systems.

One way to improve the efficiency of gasification processes is to perform exergy analysis. energy-based performance analyses are often misleading, compared to exergy ones, as they fail to identify the deviation from ideality. This is due to practical processes that generate thermodynamic irreversibilities internal to them and result in a loss of exergy even when there

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is no loss of energy external to the system. The exergy of a system (also known as ‘exergy reference environment’) can be defined as the maximum theoretical work obtainable if the system of interest and the prescribed environment interact with each other and reach the equilibrium.

Another way to improve the efficiency of gasification processes is the use of numerical simulation using high advance calculation techniques, the CFD. CFD modeling techniques are becoming widespread in the biomass thermochemical conversion area. Researchers have been using CFD to simulate and analyze the performance of thermochemical conversion equipment such as fluidized beds, fixed beds, combustion furnaces, firing boilers, rotating cones and rotary kilns. CFD programs predict not only fluid flow behavior, but also heat and mass transfer, chemical reactions, phase changes, and mechanical movement. Compared to the experimental data, CFD model results are capable of predicting qualitative information and in many cases accurate quantitative information. CFD modeling has established itself as a powerful tool for the development of new ideas and technologies.

On the reviewed papers there is a lack of knowledge concerning numerical data on biomass gasification compared to semi industrial gasifier units.

Pablo Cornejo and Oscar Farías [13] developed a three-dimensional computational model in order to describe the coal gasification process in fluidized bed reactors, taking into account drying, volatilization, combustion and gasification processes. Validation was performed by using existing experimental data from a colombian benchmarking coal gasification case available in the literature, results were in well agreement with experimental data.

Fu Yang Wang and Suresh K. Bhatia [14] developed a generalized model for the prediction of single char particle gasification dynamics, accounting for multi-component mass transfer with chemical reaction, heat transfer, as well as structure evolution and peripheral fragmentation is developed in this paper.

Jun Xie et al [15] developed a comprehensive three-dimensional numerical model to simulate forestry residues gasification in a fluidized bed reactor using Eulerian–Lagrangian approach.. The model used an Eulerian method for fluid phase and a discrete particle method for solid phase, which takes particle contact force into account. The numerical model was employed to study the gasification performance in a lab-scale pine gasifier. The model predicted product gas composition and carbon conversion efficiency in good agreement with experimental data.

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In this thesis we will start by presenting the state of art knowledge on thermodynamic irreversibility and exergy loss that occur in fundamental physical processes in gasification of various fuels with specially biomass. The different approaches adopted by the researchers to achieve the goal have been reviewed with their relative merits and demerits.

The development of exergoeconimical models was also given emphasizes. These models allows to combine economics (market based prices) and natural science (thermodynamical required energy input) in order to achieve an integrated evaluation. One of the methods used in exergoeconomical modeling is the SPECO method. It is applied to thermal systems to investigate them from an economic point of view.

On the second part of this work a numerical methodology within the framework of the commercial CFD code Fluent is applied to the gasification of biomass residues from the Portuguese agro industry carried out in a bubbling fluidized pilot unity. The numerical simulation results were compared and validated versus a set of runs using coffe husks.

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References

[1] S. Ferreira, N. Moreira, E. Monteiro, Bioenergy overview for Portugal, Biomass Bioenerg, 33 (2009) 1567-1576.

[2] A.L. Cowie, W.D. Garner, Competition for the biomass resource: Greenhouse impacts and implications for renewable energy incentive schemes, Biomass Bioenerg, 31 (2007) 601-607.

[3] European Comission (EC), (2004), Commission Decision 29/01/2004 – Establishing guidelines for the monitoring and reporting of greenhouse gas emissions pursuant to Directive 2003/87/EC of European Parliament and of the Council.

[4] Krzysztof J. Ptasinski, Thermodynamic efficiency of biomass gasification and biofuels conversion, Biofuels, Bioproducts and Biorefining, 2 (2008) 239–253.

[5] McKendry P. Energy production from biomass (part 2): conversion technologies. Bioresource Technology 83 (2002) 47–54.

[6] Commission of the European Union. Working paper – production capacity of the renewable energies in the European Union. Commission staff working document; 2003. Available from: http://www.europarl.europa.eu/stoa/publications/studies/stoa115_en.pdf last access 02-11-2012 15:08.

[7] Demibars A. Biomass resource facilities and biomass conversion processing for fuels and chemicals. Energy Conversion and Management 42 (2001) 1357–78.

[8] Faaij APC. Bioenergy in Europe: changing technology choices, Energy Policy 34 (2006) 322–42.

[9] S.K. Som, A. Datta, Thermodynamic irreversibilities and exergy balance in combustion processes, Prog Energ Combust 34 (2008) 351- 376.

[10] A. Kirkels, G. Verbong, Biomass gasification: Still promising? A 30-year global overview, Renew Sust Energ Rev 15 (2011) 471-481.

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[11] European Commission. Biomass conversion technologies: achievements and prospects for heat and power generation Luxembourg. Office for Official Publications of the European Communities; 1998. X, 178 S. EUR; 18029: Studies.

[12] M.K. Cohce, I. Dincer, M.A. Rosen, Energy and exergy analyses of a biomass-based hydrogen production system, Bioresource Technology 102 (2011) 8466–8474.

[13] P. Cornejo, O. Farías, Mathematical Modeling of Coal Gasification in a Fluidized Bed Reactor Using an Eulerian Granular Description, International Journal of Chemical Reactor Engineering, 9 (2011) 1515-1542.

[14] F. Y. Wang, S. K. Bhatia, A generalised dynamic model for char particle gasi&cation with structure evolution and peripheral fragmentation, Chemical Engineering Science 56 (2001) 3683–369.

[15] J. Xie , W. Zhong, B. Jin, Y. Shao, H. Liu, Simulation on gasification of forestry residues in fluidized beds by Eulerian–Lagrangian approach, Bioresource Technology 121 (2012) 36– 46.

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Chapter I

Exergy analysis applied to biomass

gasification: an overview

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1. Introduction

Fossil fuels, the largest resource of the world’s energy demand, are being diminished fast [1]. The economical benefits and environmental risks resulting from growth in energy consumption are well recognized, however, they are challenged by our ability to ensure the availability of endless resources to sustain a global energy future. Based on the Kyoto protocol, replacing the fossil fuel, which leads to global problems and greenhouse gas emission, by the hydrogen energy system and biomass is essential [2]. The obvious action to meet this challenge is to maximize energy productivity at the least cost [3].

Biomass has great potential as a renewable alternative to fossil fuels and it has relatively clean feedstock for producing modern energy carriers, such as electricity and transportation fuels. In order to compete with fossil energy sources an optimal utilization of biomass resources is desired. For biomass-based systems, a key challenge is thus to develop efficient conversion technologies [4].

Gasification is one of the most promising thermochemical conversion routes to recover energy from biomass [5]; it is a clean and highly efficient technology able to handle with different bio-feedstocks to a wide variety of applications such as, heat, electricity, chemicals and transports fuel. Biomass gasification means incomplete combustion due to the surplus of the solid fuel, resulting in the production of combustible gases as carbon monoxide, hydrogen and traces of methane. This mixture is generally called producer gas or syngas. The gasification technology still remains in the early development stage, being need a set of research and development efforts to mitigate some of the technological challenges, as the scale-up of process and its integration with other conversion methods [6].

One way to improve the efficiency of gasification processes is to perform exergy analysis. Several papers are available in the literature advocating the importance of exergy-based analysis for the performance evaluation of thermodynamic systems [7–9]. According to them, energy-based performance analyses are often misleading, compared to exergy ones, as they fail to identify the deviation from ideality. This is due to practical processes that generate thermodynamic irreversibilities internal to them and result in a loss of exergy even when there is no loss of energy external to the system.

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The exergy of a system (also known as ‘exergy reference environment’) can be defined as the maximum theoretical work obtainable if the system of interest and the prescribed environment interact with each other and reach the equilibrium. The term exergy is sometimes referred by thermodynamically synonymous term ‘availability’ and is a composite property of the system and the reference environment.

Historically, second-law-based analysis was developed to evaluate the process of power generation from heat. Applications of exergy analysis for the performance evaluation of power-producing cycles have increased in the recent years. A lot of works is now available in the literature where the second-law-based analyses have been applied for optimizing performance on coal-based electricity generation using conventional [10–13], fluidized bed and combined cycle technology [14] as well as for gas turbine [15–18], internal combustion engine [19–22] and blast furnace [23]applications.

The losses due to process irreversibilities can be calculated using the second-law analysis. Accordingly, two different approaches have been evolved for second-law-based process performance evaluation and optimization studies, the exergy analysis approach and minimization of entropy generation approach [24].

The concept of minimization of entropy generation is simply to reduce the exergy loss for improving the efficiency in a given process. Consequently, the most efficient performance is achieved when the exergy loss in the process is minimum. On the other hand, a structured exergetic analysis is able to retrace the origin of the thermodynamic losses from subsystems down to single units. Both the approaches find wide acceptability in heat transfer engineering, i.e., in the optimization of heat exchangers, fins, thermal insulation, electronic package cooling, etc. and have been chronologically reviewed in many references [24–27]. Entropy production and exergy destruction phenomena, which are related to the second law of thermodynamics, are well described by Rosen and Scott [28].

The present review will present the state of art knowledge on thermodynamic irreversibility and exergy loss that occur in fundamental physical processes in gasification of various fuels with special attention given to biomass. The different approaches adopted by the researchers to achieve the goal have been reviewed with their relative merits and demerits.

This thesis will also emphasize the development of exergoeconimical models. These models allows to combine economics (market based prices) and natural science (thermodynamical

11

required energy input) in order to achieve an integrated evaluation. One of the methods used in exergoeconomical modeling is the SPECO method. It is applied to thermal systems to investigate them from an economic point of view.

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2. Research made on Exergy Analysis

As previously said, during the past decades exergy analyses have been used to evaluate the performance of different systems and to improve their efficiencies [29–34]. R. Saidur et al. [1] reviewed the existent surveys on the exergy analysis of different kind of biomass. From this paper, it was concluded that in most of the exergy analysis that have been done, gasification was found to be the main thermochemical process. On the other hand, gasification, methanation and CO2 removal were determined as the main sources of exergy losses. Also, in

most of the studies, Aspen Plus was used to simulate the process and analyze the mass and energy balances.

In the work of Ptasinski et al. [35] the gasification of various biofuels was analyzed at the so-called carbon boundary point (CBP). The CBP is obtained when exactly enough gasifying medium is added to avoid carbon formation and achieve complete gasification. Desrosiers [36], Double and Bridgwater [37] proved that the CBP is the optimum point for gasification with respect to energy-based efficiency, and Prins et al. [38] proved that it is also the optimum point with respect to exergy-based efficiency. On the other hand, it was concluded that gasification of sludge and manure is not possible at the optimum point, because these fuels contain large amounts of moisture so that the carbon boundary temperature is lower than 600 ºC.

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Figure 1.2 – Comparison of gasification efficiency for various fuels. [34]

Fig. 1.1 shows the exergy distribution for gasification of various biofuels, and according to it, the largest irreversibility occurs for vegetable oil and coal gasification, which is related to the large amount of oxygen that needs to be added to vegetable oil and coal gasifiers. Fig. 1.2 shows that the efficiency based on chemical exergy is higher for coal and vegetable oil than for the other biomass streams. This may be explained because large molecules are broken up into smaller ones [34].

Following the work of Ptasinski, Karamarkovic et al. [39] focused on biomass gasification with air at different moisture and at different gasification temperatures. The optimal moisture content for air gasification of biomass at a given temperature is the one that corresponds to the moisture content in the biomass at the CBP.

According to the results presented on this paper, the gasification process at a given temperature can be improved by the use of dry biomass and by the temperature at the CBP approaching the required temperature.

The promotion of large-scale electricity generation in sugarcane mills has been object of study of many researchers. Pellegrini and Oliveira Jr. [40] performed an exergy analysis in order to evaluate irreversibilities associated to the process, and the influence of temperature, moisture, charcoal production, and thermal losses on them. Table 1.1 illustrates the operational conditions of the three tests performed.

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Table 1.1 - Operational parameters of the tests

Operational Parameters Test 1 Test 2 Test 3

Chip flow rate (Kg/h) 19.3 17.1 15.9

Moisture content (%) 8.0 8.2 20.8

Air flow rate (kg/h) 33.1 31.8 28.7

Temperature of the gas after

14.6 11.0 10.3

the cleaning system (ºC)

Ashes flow rate (kg/h) 1.24 0.6 0.6

Difference between inflow

3.5 2.5 3.8

and outflow rates (kg/h)

It was adopted the non-stoichiometric approach to find the chemical equilibrium, in which only gaseous species (CO, H2, H2O, CO2 and N2) have been considered in equilibrium.

The volume of CH4 and unconverted carbon have been calculated based on experimental data

and empirical relations [41], The produced gas was modeled as a mixture of ideal gases, and no nitrogen pollutants were presented (NOX, N2O).

From the paper, it can be concluded that moisture is responsible for an increase in the destruction of exergy inside the reactor, as a result of an increase of the energy required to evaporate the moisture. Also pre-heating air and/or bagasse may reduce irreversibilities, although it is not sufficient to compensate losses due to high moisture content and/or thermal losses. The results showed good agreement with results found in literature and real operation.

Prins et al. [42] compared gasification and combustion processes in terms of exergy, and the source of exergy losses in both processes. The aim for this comparison was to find out whether the exergy losses in a gasifier could be minimized by optimization of process parameters.

To compare exergy losses in gasification and combustion of solid carbon, the processes were conceptually divided into several subprocesses: instantaneous chemical reaction, heat transfer from reaction products to reactants (internal thermal energy exchange) and product mixing. For stoichiometric combustion of carbon with air, exergy losses due to internal thermal energy exchange are larger than those due to the chemical reaction. Fig. 1.3 shows the relative

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exergy losses for the sub-processes of carbon oxidation using air, for the adiabatic. The relative exergy loss was defined as the irreversibility for each sub-process divided by the consumed chemical exergy of the fuel.

Figure 1.3 – Relative exergy losses for subprocesses in air-blown oxidation of solid carbon versus equivalence ratio for an adiabatic case. [42]

In gasification, the reaction remains very efficient while the exergy losses related to internal thermal energy exchange are reduced due to the lower temperature. While the exergy losses due to chemical reaction are inherent, those due to internal thermal energy exchange are in principle preventable.

As referred earlier, hydrogen energy systems, alongside biomass, are essential to replace fossil fuels. However hydrogen production by gasification is a complex process and is influenced by a number of factors, such as: feedstock composition, moisture content, gasifier temperature, gasifier pressure, geometry, amount of oxidant present, and more. The hydrogen production from biomass gasification can be improved through optimization of the operating parameters and efficiencies. Walawender et al. [43] considered the gasifier temperature to be the most important parameter. Mahishi et al. [44] predicted hydrogen production from biomass gasification in existing of air–steam agent and in view of the first law of thermodynamics. It was observed that an increasing of the gasifier pressure reduces the hydrogen yield and the highest hydrogen yield occurred at atmospheric pressure. Hanaoka et

al. [45] gasified wood to produce hydrogen in existing of CO2 sorbent. Cohce et al. [5]

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free energy minimization approach and chemical equilibrium considerations were used, in addition, exergy and energy analysis were used throughout the investigation. The plant was simulated and analyzed thermodynamically using the Aspen Plus process simulation code.

Abuadala et al. [46] performed a parametric analysis of biomass gasification to produce hydrogen. The study was focused on the influence of the gasification temperature, biomass feeding and steam injecting on the hydrogen yield and energetic efficiencies. The results showed that the hydrogen produced by following this approach of modeling and from the desired quantity of this type of biomass reaches 80 – 130g H2/kg biomass while at the

examined operating gasifier temperature; the hydrogen yield reaches 80g H2/kg biomass.

Their results deviated from the ones found by Herguido et al. [47] at the same operating temperature. Schuster et al. [48] reported that this deviation could be attributed to the results deviation from the equilibrium state (since they were obtained at equilibrium state).

A mathematical approach was written in order to solve the modeled approach, the code was able to calculate the gas fraction content, the energy, exergy and exergy destruction at different amounts of steam and biomass as well at different gasifier temperatures.

In this study three forms of rational exergetic efficiencies, ηEx1, ηEx2 and ηEx3, were applied as

follows: (1) (2) (3)

Where ExH2 is the exergy flow rate of the product hydrogen, Exlost is the thermal exergy out,

Extar is the exergy flow with tar, Exchar is the exergy flow with char, Exsteam is the exergy flow

17

Figure 1.4 – Exergy efficiency versys gasified wood at a gasifier temperature of 1500 K. [46]

In Figure 1.4 we can see that the exergy efficiency, ηEx3 has the highest value because it

considers all products from the gasification process. It is also clear that there is a point where the exergetic efficiencies ηEx2 and ηEx3 are minimal. From the paper, we can conclude that

higher steam–biomass ratio leads to a weak enhanced hydrogen production especially in bench scale units.

Recently, a considerable number of studies have been made on integrated systems for hydrogen, gasification and/or Solid Oxide Fuel Cells (SOFC). SOFC are energy conversion systems, which convert chemical to electrical energy directly. In essence, fuel cells (SOFC included) are similar to batteries except that where batteries run down and become depleted; fuel cells are continually replenished with fuel and are able to provide a continuous supply of electric power.

Cosmos [49] performed an investigation of the processes of system integration for combined hydrogen and electricity co-production. On this paper, energy integration aspects for hydrogen and electricity co-production scheme based on an IGCC design with carbon capture and storage were investigated. The principal focus was the evaluation of energy integration aspects for overall plant energy efficiency optimization. Ghosh and De [50, 51]performed an energy and exergy analysis for a cogeneration gasification and SOFC system. The effects of the pressure ratio were studied on the performance of the system at different values of the SOFC operating temperature. The exergy analysis showed the exergy destruction in different components of the system and how it was affected by the pressure ratio. Bedringås et al. [52] analysed exergetically two methane fuelled SOFC systems, with limiting fuel utilisation

18

factor of 0.75 due to pre-heating and methane reforming requirements. Monanteras and Frangopoulos [53] studied the performance of a SOFC system using the Pinch method with exergy analysis. Oostenkamp [54] performed a fuel cell system energy and exergy analysis using Aspen Plus, and presented their results in Sankey and Grassmann diagrams. De Groot [55] presented in his thesis an exergy analysis of SOFC. Chan et al. [56] examined pressurised SOFC systems focusing on thermodynamic modelling and exergetic evaluation. It was shown that in a simple SOFC power system with only waste heat recovery used for pre-heating the fuel and air, the system efficiency can be improved by operating the fuel-cell stack at a high fuel utilization rate, but not excessively high to cause problems of concentration overpotential or overheated cells due to high stack temperature associated with high cell polarization.

Panopoulos et al. [57] Investigated the integration of a near atmospheric SOFC with a novel allothermal biomass steam gasification into a combined heat and power system. The energy and exergy evaluation was based on a earlier work on a steady state model built in Aspen Plus in which four major subsections were incorporated: gasification, heat pipes, SOFC, and gas cleaning.

El-Emam et al. [58] did a thermodynamic analysis on an integrated system based on the production of syngas using coal gasification, which was directed to a solid oxide fuel cell. Different values for the reference temperature, which directly affect the exergy performance and their effects on the system performance, were studied. This study performed thermodynamic analyses based on energy and exergy to investigate the performance of the integrated system and determine the extent of the system components to be enhanced for higher efficiency. The effect of variations of pressure ratio of the gas turbine compressor on different devices were also been studied. For the gasifier, SOFC, and the combustion chamber, increasing the pressure ratio enhanced the performance of these devices. Increasing the fuel cell operating temperature caused more exergy destruction in the combustion chamber and the fuel cell.

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3. Exergoenonomical Modeling

As stated throughout the paper, exergy-based analyses are essential tools for providing us important information on the design and operation of a system. However, further insight into the improvement potential of a given system is significant when optimal operation is required.

This necessity led to the developmentof advanced exergy-based analyses, in which the exergy destruction aswell as the associated costs and environmental impacts are splitinto avoidable/unavoidable and endogenous/exogenous parts [59].

Exergoeconomical modeling allows combining economics with thermodynamically required energy inputs in order to achieve an integrated evaluation.

The exergoeconomic analysis enables to assess the results of cost causing technological or technical modifications in the process. It presents guidelines for economical justifiable expenditures to reduce exergetic losses. The use of exergetic costing makes it possible to compare products with different qualities (i.e. energy carrier, heat, cold, electricity) in consideration of the manufacturing process [60].

Nowadays, more and more costs have been included in the models during the development of thermoeconomical analysis and evaluation [61–63]. Thereby, the models now imply local, global and social aspects instead of being predominantly process oriented [64–69]. Calculation of the relative effort for each product is especially interesting in cases of co-production of different materials and energy carriers. Knowledge about the relative costs is important for setting prices and for improving the efficiency of the manufacturing method [60].

The literature regarding this type of studies is still short. Tsatsaronis and Moran [70] carried out exergy-aided cost minimization on a cogeneration system. The aim of the paper was to investigate exergy destructions, investment and operating-maintenance cost rates, exergetic cost flows and exergetic factors for all components. Based on this work, Hebecker et al. [71] developed a thermoeconomic evaluation method. This method provided a clear representation of complex technical systems, a quantification of the losses, which occur in the system, a localization of the causes of losses in the structure and an assessment of the state of the art of simple and complex units by using corresponding evaluation coefficients. The method was demonstrated on a complex system providing electricity, heat and cold based on biomass.

20

Balli et al. [72] conducted an exergoeconomic analysis of a combined heat and power (CHP) system. Turgut et al. [73] performed exergoeconomic analysis of an aircraft turbofan engine. Klimantos et al. [74] performed a system analysis and economic assessment with the aid of computer simulation tools on an integrated gasification combined cycle using air-blown biomass gasification (BGCCs). The outcome of the work suggests that commercial availability of reliable and efficient of gas turbines, within the power range of 10–40MWe modified for hot syngas operation, is the key element for the successful commercial break through of BGCCs.

3.1 SPECO Approach

In order to calculate the costs associated with each material and energy stream in a system, cost balances are written for each system component, and auxiliary costing equations are used.

There are 2 different approaches for formulating efficiencies and auxiliary costing equations, the exergoeconomic accounting methods and the Lagrangian-based approaches. Exergoeconomic accounting methods use principles from business administration. Lagrangian-based approaches, on the other side, employ mathematical techniques to arrive at costs.

Some of the papers on exergoeconomic accounting methods discuss their foundations ([75– 78]). Others [79–81] present a general and comprehensive way for calculating efficiencies and costs in complex energy systems. In all the papers reviewed, only total exergy values were used and the auxiliary costing equations were formulated explicitly by using assumptions derived from experience, postulates, or the purpose of the system being analyzed.

On the second approach for formulating efficiencies and auxiliary costing equations, a Lagrangian method was used to solve an optimization problem and calculate marginal costs as Lagrange multipliers associated with exergy streams. The pioneer work in [82] was followed by [83–86] where a significant effort was committed to the development of a functional structure representing the interactions among components in terms of fuel and product. The fact that marginal costs could be calculated in the optimum as Lagrange multipliers suggested to calculate these costs starting directly from their definition. Accordingly, the cost formation process was described through exergy derivatives in the ‘Structural theory’ [87] to calculate exergetic or monetary costs associated with different exergy flows. This principle was extended to include investment costs in [88]. In these papers,

21

it is shown how to directly obtain the component cost balances and the auxiliary costing equations using derivatives, once the fuel and product have been defined. Further developments of the Structural Theory, proposed as a standard for thermoeconomics, are presented in [89].

The same cost balances and auxiliary equations used in accounting methods can be obtained through partial derivatives in the Lagrangian-based approaches.

Tsatsanonis et al. [90] presented a different approach, based on the LIFO (Last In First Out) accounting principle. On this approach, fuels, products and costs were defined by systematically registering exergy and cost additions and removals from each material and energy stream. The name SPECO was given to this approach because of the need of using specific exergies and costs for registering all additions and removals of exergy and cost.

The basic principles of the SPECO approach were then directly applied to exergy streams instead of material and energy streams in [91–93].

The SPECO method was applied to thermal systems to investigate them from an economic point of view. Tsatsaronis and Moran [70] used the SPECO method to exergoeconomically improve a power plant components and Orhan et al. [94] investigated a thermochemical copperechlorine cycle for hydrogen production using the SPECO method.

For a system operating at steady state there may be a number of entering and exiting material streams as well as both heat and work interactions with the surroundings. Since exergy measures the true thermodynamic value of such effects and cost should only be assigned to commodities of value, it is meaningful to use exergy as a basis for assigning costs in thermal systems. Indeed, thermoeconomics rests on the notion that exergy is the only rational basis for assigning costs to the interactions that a thermal system experiences with its surroundings and to the sources of inefficiencies with it. This approach is referred as exergy costing.

In exergy costing, a cost is associated with each exergy stream. Thus, for entering and exiting streams of matter with associated rates of exergy transfer and , power and the

exergy transfer rate associated with heat transfer , we write respectively,

22

(5)

(6)

(7)

, , and stand for average costs per unit of exergy, , , and for the

cost streams associated with the corresponding exergy streams.

In conclusion, the SPECO method presents a general, systematic, simple and unambiguous approach for developing the exergetic efficiencies of a thermal system and its components. In addition, in the SPECO method there are no exergy losses associated with material streams at the component level since all exergy streams associated with material streams exiting a component are considered either on the fuel or on the product side. All other approaches, with the exception of LIFOA, consider some exergy losses associated with material streams at the component level.

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Chapter II

Experimental and Numerical Analysis

of Coffee Husks biomass Gasification

32

1. Introduction

Agroindustry is one of the most important economic activities in the Portugal’s Alentejo region, generating large amounts of residues, in particular, vine prunings, bagasses, coffe husks, forest residues, among others, which require treatment or an adequate recovery to minimize environment impacts and increase the economic value of these wastes. A large variety of technologies has been developed over the past decades to deal with this problem. Among the proposed technologies, those oriented towards energy recovery, including combustion and gasification of biomasses has attracted much interest [1].

Gasification appears to be a promising process to convert biomass in syngas containing methane and hydrogen to be used directly as energy source or as raw material for the production of liquid fuels and other chemicals [2, 3].

The gasification system includes a set of phenomena, such as, fluid flow, heat transfer and complex chemistry that could only be solved applying several governing mathematical equations mostly based on conservation equations, i.e., mass, heat and momentum. This level of complexity can be achieved using CFD tools. The recent improvement of state-of-the-art computational fluid dynamics (CFD) models allow for the design and optimization of these processes [4].

In the open literature there several papers concerning the use of CFD approaches to model the gasification process. Pablo Cornejo and Oscar Farías [5] developed a three-dimensional computational model in order to describe the coal gasification process in fluidized bed reactors within the commercial multipurpose CFD code FLUENT 6.3. Wang and Bhatia [6] developed a generalized model for the prediction of single char particle gasification dynamics. Xie et al [7] developed a comprehensive three-dimensional numerical model to simulate forestry residues gasification in a fluidized bed reactor using Eulerian–Lagrangian approach. Although, the rising of complexity of these numerical approaches, there is a lack of experimental data where the numerical results could be confirmed with semi-industrial or industrial set-ups..

This paper aims at presenting a numerical methodology within the framework of the commercial CFD code Fluent applied to the gasification of biomass residues from the Portuguese agro industry carried out in a bubbling fluidized semi-industrial pilot unity. The