Study on the influence of representative Tar and H2S on SOFC performance

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SCUOLA DI INGEGNERIA

CORSO DI LAUREA MAGISTRALE IN INGEGNERIA ENERGETICA

STUDY ON THE INFLUENCE OF A REPRESENTATIVE TAR

AND H

2

S ON SOFC PERFORMANCE

Relatore Candidato

Prof. Umberto Desideri Roberta Bernardini

Supervisori

Dr. Ir. P.V. Aravind Ir. Alessandro Cavalli

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Abstract

The increasing demand of energy and the increasing concern about global warming has more and more drawn the attention on the use of renewable sources of energy.

An interesting solution can be the integration of biomass gasifiers with Solid oxide fuel cells (SOFCs). Indeed, these systems allow to achieve high conversion efficiency of biomass to electrical energy. In addition, high quality heat is available for further applications, because SOFCs operate at high temperatures and the processes that take place in them are overall exothermic.

However, solid oxide fuel cells are generally very sensitive to impurities contained in syngas. For this reason, a gas cleaning unit is present between the gasifier and the SOFC. In particular, employing a hot gas clean-up, the overall thermal efficiency of system is greater, because cooling and re-heating steps typical of low temperature gas cleaning are avoided.

In this work an integrated system with an updraft gasifier is considered. This type of gasifiers has several advantages in comparison to the other types: a greater thermal efficiency, it can handle biomass with high moisture content, syngas entrains a lower quantity of particulate matter and has a quite good LHV. On the other hand, a greater amount of tar is produced in an updraft gasifier. However, if SOFC internal tar reforming is feasible, a higher content of this contaminant will lead to a higher amount of available fuel in the cell and, thus, this will be an advantage. Moreover, the endothermic tar reforming reaction can contribute to reducing the excess air necessary to cool down the cell, thus increasing further the system efficiency.

The aims of this thesis are:

 studying the feasibility of tar internal reforming,

 studying the effects of both hydrogen sulfide and tar on SOFC,

 investigating the tolerance limits of SOFCs for tar and hydrogen sulfide.

Solid oxide fuel cells have a suitable environment for tar reforming: in the anode chamber there are catalyst, high temperature, steam and/or CO2. Moreover, energy is available to carry out reforming, which is an endothermic reaction for most of compounds. If tar reforming can be done in the cell, tar reformer will be not needed or, at least, there will be

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a smaller one or cheaper materials will be used. Moreover, less energy will be required. In this way, a higher system efficiency can be achieve.

Although, tolerance limit is already known for hydrogen sulfide, this are investigated in this work because they depend widely on the operating conditions, such as fuel composition, temperature, current load, anode materials. Indeed, the found tolerance limit is strictly associated to the GCU operating temperature. The higher the tolerance limit, the higher the operating temperature of the GCU. A higher operating temperature of the GCU might result is a higher efficiency for the whole system.

In order to investigate these points, four sets of experiments were carried out. In the first one the cell was exposed to clean syngas to have a reference for the following tests. In the second set of experiments, the effects of several concentrations of acetic acid (20 – 150 g/Nm3) were studied. During the third one, 0.8 ppm(v) and 1.3 ppm(v) of hydrogen sulfide were added to syngas stream. At the end, synergistic effects of tar and H2S on cell performance and the effect of hydrogen sulfide on acetic acid reforming were investigated. All these sets of tests were carried out in the same operating conditions: at 800°C and under a current density of 68 mA/cm2. The employed SOFCs were electrolyte-supported with Ni-GDC anodes. The cells were exposed to each concentration of contaminants for 24 h. When cells performance were affected by these contaminants, a recovery step was carried out.

The effects of these contaminants on the cell were evaluated by i-V curves, monitoring the cell voltage, tar sampling and micro-GC.

It was found that acetic acid was very reactive and reformed in the cell. Carbon deposition due to homogeneous decomposition of acetic acid seemed to occur because solid carbon was observe in the anode inlet ceramic pipe. Micro-GC data suggested that this tar species may produce CH4 from its decomposition. Hydrogen sulfide affected widely the cell performance, methane steam reforming and water-gas shift reaction. Sulfur poisoning due to both H2S concentrations seemed to be partially reversible. These behaviours were confirmed also in the last set of experiments.

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v 3.1.5. Primary tars ... 51 3.1.5.1. Acetic acid ... 53 3.2. Hydrogen sulfide ... 56 3.2.1. H2S in humidified H2 ... 56 3.2.2. H2S in syngas ... 59 3.3. Synergistic effects ... 63 3.4. Conclusions ... 64 4. Methodology ... 66 4.1. Set-up description ... 70 4.2. Tar sampling ... 72 4.3. H2S sampling ... 77 5. Results ... 80 5.1. Theoretical results ... 80 5.1.1. Water content ... 80 5.1.2. H2S tested concentrations ... 81 5.2. Experimental results ... 84 5.2.1. Reference test ... 84 5.2.2. Tar tests ... 87 5.2.3. H2S tests ... 95 5.2.4. Tar + H2S tests ... 105 6. Conclusions ... 114

6.1. Recommendations and future works ... 116

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List of figures

Figure 1 Biomass integrated gasification – SOFC system ... 3

Figure 2 Classification of solid fuels by H/C and O/C ratio [4] ... 4

Figure 3 Classification of biomass conversion processes ... 5

Figure 4 Main sub-process of biomass gasification [5] ... 8

Figure 5 Simplified mechanism of tars formation [5] ... 11

Figure 6 Molecular structure of cellulose ... 11

Figure 7 Molecular structure of lignin [4] ... 11

Figure 8 Tar types as function of temperature [5] ... 12

Figure 9 Classification of biomass gasifiers [3] ... 13

Figure 10 Scheme of an updraft gasifier [5] ... 13

Figure 11 Scheme of a downdraft gasifier [5] ... 13

Figure 12 Scheme of a bubbling bed gasifier [3] ... 14

Figure 13 Scheme of a circulating bed gasifier [3] ... 14

Figure 14 Bubbling bed gasifier [5] ... 15

Figure 15 Circulating bed gasifier [5] ... 15

Figure 16 Influence of temperature in syngas properties [5]... 17

Figure 17 Scheme of a SOFC [12] ... 18

Figure 18 Simple edge connection between cells [12] ... 19

Figure 19 Connection by bipolar plates [12] ... 19

Figure 20 Tubular design [12] ... 20

Figure 21 Interconnects in tubular cells [12] ... 20

Figure 22 Three-phase boundary regions for different SOFC anode materials [12] ... 22

Figure 23 Typical i-V curve of an high-temperature fuel cell [12] ... 24

Figure 24 Distinguishing different types of losses in i-V curve [12] ... 27

Figure 25 Comparison between high-temperature and low-temperature cell performance (adjusted from [12]) ... 28

Figure 26 Effect of temperature on dry reforming of CH4 [16] ... 29

Figure 27 Effect of pressure on dry reforming of CH4 [16] ... 29

Figure 28 Effect of CO2/CH4 ratio and temperature on dry reforming [16] ... 30

Figure 29 Effect of temperature, pressure and S/C ratio on steam reforming of CH4 [17] ... 30

Figure 30 Scheme of indirect internal reforming in a SOFC (adjusted from [12]) ... 31

Figure 31 Scheme of direct internal reforming in a SOFC (adjusted from [12]) ... 31

Figure 32 Possible mechanisms of sulfur poisoning for low H2S concentrations (adjusted from [18]) ... 34

Figure 33 Possible mechanism of sulfur poisoning for high H2S concentrations (adjusted from [18]) ... 34

Figure 34 Classification of gas cleaning methods ... 37

Figure 35 Barrier filtration devices at high temperatures: (a) ceramic filters, (b) cross-flow filters, (c) moving bed granular filters [8] ... 38

Figure 36 H2S level in biosyngas after cleaning with zinc titanate (Zn2TiO4) [23] ... 40

Figure 37 Catalysts classification [23] ... 42

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Figure 39 MBMS sampling of gasifier effluents run in the updraft gasifier [73] ... 68

Figure 40 Scheme of the set-up ... 70

Figure 41 Tar sampling ... 73

Figure 42 Standard set-up for tar sampling ... 74

Figure 43 Reactants [mol/min] in equilibrium calculation ... 81

Figure 44 H2S concentration at equilibrium based on GCU temperature ... 82

Figure 45 Zn concentration in the outlet stream from H2S cleaning device ... 83

Figure 46 Cell voltage during the first day of reference test ... 84

Figure 47 Cell voltage during day 2, 3 and 4 of reference test... 85

Figure 48 i-V curve at the beginning ("syngas day 1") and at the end ("syngas day 4") of the test86 Figure 49 Total outlet flow rate obtained by Micro-GC analysis ... 87

Figure 50 Compounds flow rates obtained by Micro-GC analysis ... 87

Figure 51 Cell voltage during tar tests ... 88

Figure 52 Comparison between cell performance at the end of every acetic acid concentration... 90

Figure 53 Micro-GC results for tar tests... 90

Figure 54 Micro-GC results: methane flow rate at anode outlet during the all tar tests... 92

Figure 55 Carbon deposition ... 94

Figure 56 Carbon deposition on ceramic pipe at anode inlet ... 95

Figure 57 Cell after tar tests ... 95

Figure 58 Cell voltage during H2S tests ... 96

Figure 59 Comparison between cell performance when the cell was fed with clean syngas (reference) and after 50 min from the beginning of H2S injection ... 98

Figure 60 Comparison between cell performance when the cell was fed with clean syngas (reference) and after 24 h of H2S exposure ... 99

Figure 61 Comparison between i-V curves of cell fed with clean syngas (reference), cell exposed to 1.3 ppm of H2S for 24 h (H2S_day 1 #2) and cell after a recovery step of 24 h ... 100

Figure 62 Flow rates of hydrogen, methane, carbon monoxide and dioxide at anode outlet obtained by Micro-GC analysis during injection of 1.3 ppm of H2S ... 101

Figure 63 Total anode outlet flow rate obtained by Micro-GC analysis during injection of 1.3 ppm of H2S ... 101

Figure 64 Comparison between flow rates of hydrogen, methane, carbon monoxide and dioxide at anode outlet obtained by Micro-GC analysis during the exposure to 1.3 ppm of H2S and “recovery step #1”... 102

Figure 65 Comparison between total anode outlet flow rate obtained by Micro-GC analysis during exposure to 1.3 ppm of H2S and "Recovery step #1" ... 103

Figure 66 Flow rates of hydrogen, methane, carbon monoxide and dioxide at anode outlet obtained by Micro-GC analysis during injection of 0.8 ppm of H2S ... 103

Figure 67 Total anode outlet flow rate obtained by Micro-GC analysis during injection of 0.8 ppm of H2S ... 104

Figure 68 Comparison between flow rates of hydrogen, methane, carbon monoxide and dioxide at anode outlet obtained by Micro-GC analysis during exposure to 0.8 ppm of H2S and "Recovery step #2" ... 104

Figure 69 Comparison between total anode outlet flow rate obtained by Micro-GC analysis during exposure to 0.8 ppm of H2S and "Recovery step #2" ... 105

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viii Figure 70 Cell voltage during "Tar + H2S" set of experiments ... 106

Figure 71 Trends of hydrogen, methane, carbon monoxide and dioxide, adding 42.42 g/Nm3 of acetic acid to syngas stream ... 109 Figure 72 Trend of methane adding 42.42 g/Nm3 of acetic acid to syngas stream ... 109 Figure 73 Trends of hydrogen, methane, carbon monoxide and dioxide, adding 42.42 g/Nm3 of acetic acid and 0.8 ppm of H2S to syngas stream ... 110 Figure 74 Total anode outlet flow rate adding 42.42 g/Nm3 of acetic acid and 0.8 ppm of H2S to syngas stream ... 110 Figure 75 Comparison between outlet flow rates obtained by Micro-GC analysis during test with clean syngas, exposure to acetic acid, exposure to acetic acid and H2S, and just H2S ... 111

Figure 76 Comparison between methane Micro-GC data obtained from experiments adding 42.42 g/Nm3 of tar, 42.42 g/Nm3 of tar + 0.8 ppm of H2S and 0.8 ppm of H2S... 112

Figure 77 Carbon deposition on anode inlet pipe ... 112 Figure 78 Cell after H2S and Tar + H2S sets of tests ... 113

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List of tables

Table 1 Biomass classification [4] ... 4

Table 2 Ultimate and proximate analysis of some biomass, compared with bituminous coal [1] ... 5

Table 3 Summary of gasification sub-processes ... 9

Table 4 Concentrations of several contaminants in syngas obtained from gasification [7] ... 10

Table 5 Tars classification [8] ... 10

Table 6 Advantage and disadvantage of fixed and fluidize bed gasifiers [5] ... 15

Table 7 Comparison between different types of gasifiers [1] ... 16

Table 8 Requirement of gas quality for different downstream application [10]... 36

Table 9 Tolerance limits for two different SOFC anodes (adjusted from [22]) ... 36

Table 10 Summary of main hot gas cleaning devices ... 38

Table 11 Summary of papers that used toluene as model tar and Ni-YSZ anode ... 46

Table 12 Summary of papers that used toluene as model tar and Ni-GDC anode ... 47

Table 13 Summary of papers that used toluene as model tar and other anode materials ... 47

Table 14 Summary of papers that used benzene as model tar ... 48

Table 15 Summary of papers that used naphthalene as model tar ... 49

Table 16 Summary of papers that used real gasification tar ... 51

Table 17 Summary of papers that used primary tars ... 52

Table 18 Summary of catalytic studies about acetic acid ... 53

Table 19 Summary of papers that used acetic acid on SOFC ... 55

Table 20 Summary of articles that used a mixture of H2, H2O and CH4 as fuel ... 61

Table 21 Summary of articles that used a mixture of H2, H2O, (N2), CO and CO2 as fuel ... 62

Table 22 Summary of articles that used syngas as fuel ... 63

Table 23 Summary of articles that studied synergist effects of tar and H2S ... 64

Table 24 Experimental plan ... 66

Table 25 Syngas composition (dry basis) [72] ... 67

Table 26 Tar concentration in syngas [1] ... 68

Table 27 Analyses carried out during the sets of tests ... 69

Table 28 Sampling durations for the tar concentrations ... 73

Table 29 Tar sampling experimental plan ... 75

Table 30 Duration of tar sampling depending on acetic acid concentration ... 76

Table 31 Results of tar sampling ... 76

Table 32 Calculation of maximum duration of sampling ... 78

Table 33 Calculation of minimum duration of sampling ... 79

Table 34 Actual flow rates of main syngas components ... 80

Table 35 Actual dry syngas composition (vol% dry basis) ... 80

Table 36 Actual wet syngas composition ... 80

Table 37 Flow rates, expressed in mol/min, of compounds to insert in FactSage ... 81

Table 38 Water content calculated by equilibrium calculation ... 81

Table 39 Products from equilibrium calculation, based on temperatures ... 83

Table 40 Degradation rate of cell voltage during reference test ... 85

Table 41 Values of OCV and ASR at the beginning and at the end of the reference test ... 85

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x Table 43 Degradation rate of cell voltage during tar tests ... 89 Table 44 Comparison between calculated increase of cell voltage due to complete reforming of injected acetic acid and actual voltage gain obtained during the experiments ... 92 Table 45 Minimum reforming rates of acetic acid obtained from tar sampling ... 93 Table 46 Degradation rate of cell voltage during H2S tests ... 97

Table 47 Actual and expected values of cell voltage after the rising trend during recovery steps . 97 Table 48 Degradation rate of cell voltage during “Tar + H2S” tests ... 107

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1. Introduction

Fossil fuels are nowadays the most used energy sources in the world: approximately 80% of global energy demand is covered by them [1]. The increasing concern about global warming and the increasing demand of energy has more and more drawn the attention on the use of renewable sources of energy. Indeed, world carbon dioxide emission has increased by 44% until 2011 in comparison to level in 1993 and earth temperature is expected to rise by 1.7 – 4.9 °C from 1990 to 2100 [1].

Biomass has received significant attention, because it is a renewable energy source, provided it is used sustainably, it is largely available, it can produce energy continuously and it does not contribute to carbon dioxide emissions.

Biomass can be used efficiently in integrated gasifier – SOFC systems: biomass gasification is more efficient than biomass combustion, solid oxide fuel cells are characterized by high electrical efficiencies (55 – 65%) [1] and heat surplus from these devices can be used for heating services.

However, solid oxide fuel cells are downstream applications, which are pretty sensitive to the impurities contained in the syngas obtained from gasification process. For this reason, a gas cleaning unit has to be placed between the gasifier and the SOFC. The type of GCU that allows to achieve the highest system efficiency is the hot gas cleaning unit, because cooling and re-heating steps of gas stream, typical of lower clean-up methods, are avoided. In this work, the effects of two contaminants, tar and hydrogen sulfide, contained in syngas are investigated.

An integrated updraft gasifier – SOFC system has been considered. Updraft gasifier has some advantage in comparison to other types: a greater thermal efficiency, it can handle biomass with high moisture content, syngas entrains a lower quantity of particulate matter and has a good LHV. On the other hand, a greater amount of tar is produced.

Nonetheless, solid oxide fuel cells have a suitable environment for tar reforming: in the anode chamber there are catalyst, high temperature, steam and/or CO2. Moreover, energy is available to carry out reforming, which is an endothermic reaction for most of compounds. If tar reforming can take place in the cell, a higher content of this contaminant will lead to a higher amount of available fuel in the cell and, thus, this will be an advantage. Moreover, tar reformer will be not needed or, at least, there will be a smaller one or cheaper materials

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will be used. Furthermore, less energy will be required. In this way, a higher system efficiency can be achieve.

Although tolerance limit is already known for hydrogen sulfide, this are investigated in this work because it depends widely on the operating conditions, such as fuel composition, temperature, current load, anode materials. Also, the effect of this contaminant on tar internal reforming is not yet known. Indeed, the found tolerance limit is strictly associated to the GCU operating temperature. The higher the tolerance limit, the higher the operating temperature of the GCU. A higher operating temperature of the GCU might result is a higher efficiency for the whole system.

The aims of this thesis are:

 studying the feasibility of tar direct internal reforming,

 studying the effects of both hydrogen sulfide and tar on cell performance.  investigating the tolerance limits of SOFCs for tar and hydrogen sulfide, This thesis is composed by four main sections:

 Section 2 – Theoretical background: the state of art of integrated gasifier – SOFC systems and of all its main components are described.

 Section 3 – Literature review: previous works about the effects of tar and H2S on SOFCs performance are reported in this section in order to understand what has already been studied and found.

 Section 4 – Experiments: experimental plan is defined, explaining also the reason why some choices have been made. Therefore, some results of theoretical calculations, which were used to define some aspects in the experiments, are reported in order to explain the motivations.

 Section 5 – Results: in this last section the results obtained carrying out experiments are reported and discussed.

The present work has been made in collaboration with “Process and Energy” department at TU Delft (Netherlands) and it was part of “FlexiFuel-SOFC” project (European Union Horizon2020 Program) [2].

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2. Theoretical background

In this section, the state of art of biomass integrated gasifier – SOFC systems and the state of art of all their components are described.

Biomass integrated gasifier – SOFC system is an interesting application, because it represents a sustainable and high efficient energy conversion system: the syngas obtained from biomass gasification feeds solid oxide fuel cell, but before reaching this downstream device, the produced gas has to be cleaned because of the presence of some contaminants that may damage the cell.

As Figure 1shows, this system is composed by a gasifier, a gas cleaning unit and a fuel cell. In addition to the electricity, high quality heat is available for further applications, because SOFCs operate at high temperatures and the processes that take place in them are overall exothermic.

Figure 1 Biomass integrated gasification – SOFC system

2.1. Biomass and its conversion

Biomass is “non-fossilized and biodegradable organic material originating from plants, animals and micro-organisms. This shall also include products, by-products, residues and waste from agriculture, forestry and related industries as well as the non-fossilized and biodegradable organic fractions of industrial and municipal wastes”1.

Basically biomass stores energy through photosynthesis process in the presence of sunlight [3]. There are several classifications of biomass; the one based on the EN 14961 and EN 15234, published by European committee is shown in Table 1.

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Table 1 Biomass classification [4]

A. Virgin biomass A.1 Terrestrial biomass i. Forest biomass ii. Grasses iii. Energy crops iv. Cultivated crops A.2 Aquatic biomass i. Algae

ii. Water plant B. Waste biomass B.1 Municipal waste i. MSW

ii. Biosolids, sewage iii. Landfill gas

B.2 Agricultural solid waste i. Livestock and manures ii. Agricultural crop residue B.3 Forestry residues i. Bark, leaves, floor residues B.4 Industrial waste i. Demolition wood, sawdust

ii. Waste oil/fat

2.1.1. Biomass composition

Terrestrial biomass is composed by biopolymers such as cellulose, hemicellulose and lignin [5].

In particular, the biomass composition can be defined through two analyses: ultimate analysis and proximate analysis. The first one can quantify the amount of basic elements that constitute the biomass. Indeed it contains ash, moisture, C, H, O, N and, in smaller percentage, S and Cl.

The proximate analysis can define the amount of moisture, volatile matter, ash and fixed carbon in the biomass.

As Table 2 shows, biomass composition strongly depends on the type; however, in comparison to coal, generally biomass has less content of ash and sulfur, but a greater amount of moisture and volatile matter, lower energy density, higher O/C and H/C ratios and lower HHV.

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Table 2 Ultimate and proximate analysis of some biomass, compared with bituminous coal [1]

Biomass type

Ultimate analysis (% w/w, dry

basis) Proximate analysis (% w/w)

C H O N S Ash VM FC Moisture Larch wood 44.15 6.38 49.32 0.12 - 0.12 76.86 14.86 8.16 Camphor wood 43.43 4.84 38.53 0.32 0.1 0.49 72.047 14.75 12.29 Wood sawdust 46.2 5.1 35.4 1.5 0.06 1.3 70.4 17.9 10.4 Rice husk 45.8 6.0 47.9 0.3 - 0.8 73.8 13.1 12.3 Rice straw 38.61 4.28 37.16 1.08 0.65 12.64 65.23 16.55 5.58 Wheat straw 46.1 5.6 41.7 0.5 0.08 6.1 75.8 18.1 (db) Switch grass 47 5.3 41.4 0.5 0.1 4.6 58.4 17.1 20 Cotton stem 42.8 5.3 38.5 1.0 0.2 4.3 72.3 15.5 7.9 Bituminous coal 80.9 6.1 9.6 1.55 1.88 9 35 45 11

The moisture content affects not only the conversion process of the biomass, but also the handling, storage and transportation of this fuel. So it can be pre-dried, but this involves cost [1], or it can be used in a process that tolerates its water content.

Some other pre-treatments can be carried out in order to increase its low energy density.

2.1.2. Biomass conversion

Several processes for the biomass conversion exist. These can be classified in three main groups:

1. thermochemical (green blocks in Figure 3): conversion using thermal energy (combustion, pyrolysis, gasification),

2. biological (blue blocks in Figure 3): conversion using microbial or enzymatic activity (aerobic and anaerobic digestion, fermentation),

3. mechanical (purple block in Figure 3): conversion using mechanical energy (oil extraction).

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Figure 3 shows the different conversion processes, focusing on the type of biomass suitable for them (orange blocks) and the main products (grey blocks).

Biological conversion differs from the other two types because these processes are generally slow. On the other hand, multiple and complex products are obtained from thermochemical conversion; instead, specific products are often gotten from the other two kinds of process,.

The suitable type of biomass for a process depends on some properties, such as (i) moisture content, (ii) heating value, (iii) amount of fixed carbon and volatile mater, (iv) ash content, (v) alkali metal content, (vi) cellulose/lignin ratio [6].

For instance, biomass with high moisture content (>20%) is more suitable for biological conversion, such as anaerobic digestion; on the contrary, biomass with less than 20% of moisture is better suited to thermochemical conversion [6].

In combustion, the biomass is burnt in excess of air to produce hot gases at about 800 – 1000°C. Feedstock with quite high heating value and with low moisture containing (biomass is sometimes pre-dried) is more suitable for this application.

If the process is efficient enough, the gases will be mainly composed by H2O and CO2 plus small amount of other products depending on the species of feedstock. The hot gases can be used to produce heat or for electricity generation.

In addition, biomass can be used as co-fuel in coal-fired power plants.

In pyrolysis, the biomass is heated up to 400 – 800 °C almost in absence of oxygen. Three products are obtained: solid fraction (charcoal), liquid fraction (bio-oil) and non-condensable gases. The composition and the amount of these different fractions depend on type of biomass, temperatures and residence time. For example, at about 500°C and for short residence times (<2 seconds), the produced bio-oils can be up to 80%. Raising temperature and residence time, the amount of gaseous products increases as well.

Gasification is a partial oxidation of the feedstock in a restricted atmosphere of oxygen. The gasification agent can be air, pure oxygen or one of these two with added steam. The main product is a mixture of gases, called syngas. It is principally composed by H2, CO, CO2, CH4 and N2, if air is used.

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Aerobic digestion is a biological process where microorganisms work to degrade the biomass in presence of air. So compost, water, carbon dioxide are produced instead of fuel gases. It can be used to obtain heat [4].

In anaerobic digestion several species of bacteria decompose the organic matter in the absence of oxygen. This process is especially suitable for high moisture content biomass (up to 80 – 90%).

The main product is biogas; this is principally composed by methane (55 – 75 %) and carbon dioxide (25 – 45 %). The percentages of CH4 and CO2 depend on the water content. Biogas can be used as a fuel (in a turbo-gas or in internal combustion engines) or it can be upgrade to natural gas.

In addition to the gaseous product, also a liquid fraction is obtained. It is usually used as fertilizer.

Alcoholic fermentation is the biological process where the sugars contained in biomass are converted to ethanol by yeast. This suits especially to sugar and starch crops.

During oil extraction, oilseed crops are physically crushed, the oil is extracted and converted to esters. These can be used to replace diesel.

2.2. Gasification process

As it has been introduced previously, biomass gasification is a thermal conversion of feedstock to a combustible gaseous mixture, called syngas, through incomplete combustion.

Air, pure oxygen or steam can be used as gasification agent.

Syngas mainly consists of hydrogen, carbon monoxide and carbon dioxide, methane, nitrogen and steam. Anyway it generally contains some impurities. The composition of the produced gas and the contaminants level depend on several factors, such as type of gasified feedstock, type of gasifier, gasification agent and operating parameters [1].

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Figure 4 Main sub-process of biomass gasification [5]

Drying, pyrolysis and generally reduction are endothermic reactions; on the contrary, oxidation is the exothermic one that provides energy for the other steps. If this energy is enough to carry out the whole gasification process, this is autothermal; on the contrary, if some heat is supplied from outside the gasifier, the process is allothermal [1].

Drying is the step that occurs at temperatures lower than 200°C. There, the moisture contained in the feedstock evaporates. The amount of heat required depends on how much water the biomass contains: the higher is the moisture content, the higher is the thermal energy required and the worse is the quality of the produced gas.

However, during this step the lighter compounds do not volatise because of too low temperatures.

Pyrolysis is the thermal decomposition of feedstock in absence of oxygen. This occurs at temperatures about 200 – 600 °C. In this step the volatile matter is released. So a gaseous fraction, vaporised tars and solid fraction are produced. The latter one includes inert materials contained in biomass in the form of ashes and carbon content (char).

In particular, when the temperature increases up to 300°C, mainly the amorphous cellulose starts to decompose. Then, beyond this temperature, crystalline cellulose and hemicellulose

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decompose with the formation of volatile gases, tar and char. At the end, lignin, the most stable polymer, decomposes at temperatures between 300°C and 500°C and forms methanol, acetic acid, water and acetone [3].

In the oxidation zone, the oxygen/air reacts with the volatile matter from biomass (homogenous combustion) and with char (heterogeneous combustion), producing H2O, CO, CO2 and especially thermal energy. There, the temperatures achieve 1000 – 1500 °C. Reduction step involves the products of the previous processes. The main reactions that occur are:

Boudouard reaction ( ) [1]

Reforming of char ( ) [2]

Water-gas shift reaction ( ) [3]

Methanation ( ) [4]

Overall, the reduction stage is endothermic.

Table 3 Summary of gasification sub-processes

tep T [°C] Reaction equations Products

Drying 200 Dry biomass, H2O Pyrolysis 200 – 600 H2, CO, CO2, CH4, H2O, tar, solid fraction Reduction 600 - 1000 H2, CO, CO2, CH4, H2O, tar, solid fraction Oxidation 1000 - 1500 CO, CO2, CH4, H2O and heat 2.2.1. Contaminants

As it has been explained before, produced syngas contains some contaminants species besides the main compounds. Basically the impurities are particulate matter, tar, sulfur compounds, especially H2S, chlorides, especially HCl, and alkali compounds.

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Table 4 Concentrations of several contaminants in syngas obtained from gasification [7]

Tar [g/Nm3] Particulate matter _[g/Nm3_] H2S [ppmv] HCl [ppmv] Alkali metals [ppmv] 0.01 – 150 0 – 100 20 – 200 < 500 ~1

Particulate matter generally includes inorganic material derived from mineral constituents in biomass feedstock, such as potassium, sodium, calcium, silica, iron, unconverted char, soot and eventual bed material. Its size can range from less than 1 µm to over 100 µm. Particulate matter is considered a contaminant, because it can damage downstream devices. Common issues are fouling, corrosion and erosion.

Alkali and alkali earth metals are naturally contained in biomass, in amount generally greater than in coal. Some alkali compounds remain in ashes but others melt or even vaporize above 600°C and so they can leave the gasifier as aerosol or vapours. These contaminants have to be removed from gaseous stream because these can cause hot corrosion or bed de-fluidization in gasifiers. Moreover some catalysts are extremely sensitive to these [8].

Chlorides are present in syngas especially in form of HCl, because of the chlorine contained in biomass. Issues due to hydrogen chloride are hot corrosion and eventual poisoning of some catalysts.

Tars are a complex mixture of condensable hydrocarbons [5]. They vary from primary oxygenated compounds to polycyclic aromatic hydrocarbons (PAHs).

Biomass is very susceptible to tar formation because of its high amount of volatile matter. There are two tars classifications: one based on their formation, the other on compounds structure and their molecular weight. The first one divides this type of contaminant in primary, secondary and tertiary tars. The second one classifies tars in five classes, as Table 5 shows.

Table 5 Tars classification [8]

Class Description Properties

1 GC-undetectable Very heavy tars; cannot be detected by GC

2 Heterocyclic aromatic Tars containing hetero atoms; highly water soluble 3 Light aromatic (1 ring) Single ring light hydrocarbons; do not pose problems

of condensation or solubility 4 Light PAH compounds (2-3 rings) 2 and 3 ring compounds; condensate at low T 5 Heavy PAH compounds (4-7 rings) Larger than 3 ring compounds; condensation occurs

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The formation process of tars is very complex. During gasification, these compounds derive in the pyrolysis step and then they decompose and recombine. The mechanism includes a lot of reactions. A simplified formation chain for cellulose-derived tars is shown in Figure 5.

Figure 5 Simplified mechanism of tars formation [5]

Primary tars form during the pyrolysis process and they depend on the type of biomass gasified. Oxygenated organic compounds, such as alcohols, acids, aldehydes, are due to cellulose and hemicellulose. Indeed in their structures there are a lot of oxygen atoms. On the other hand, lignin is responsible for aromatic compounds [5].

Figure 6 Molecular structure of cellulose

[4] Figure 7 Molecular structure of lignin [4]

When primary tars pass through oxidation zone, where the temperature is higher than 500°C and there is an oxidizing agent, they may begin to rearrange, forming gases and secondary tars. Some examples of these are furan, dioxin and, in general, alkylated mono- and di-aromatics, including hetero-aromatics.

If temperature increases above 800°C, tertiary tars may form. These are mostly aromatic and poly-nuclear aromatic hydrocarbons, such as benzene and naphthalene. Tertiary tars

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are not part of biomass structure, but these compounds are due to the decomposition and recombination of secondary tars in the reducing environment of syngas [5].

Tertiary tars appear only when primary tars are converted in secondary tars.

Temperature is a key parameter for the types of tars formed, but also for their amount produced. Indeed high temperatures decrease the quantity of tars, because of the occurrence of tar cracking.

Figure 8 Tar types as function of temperature [5]

Tars are generally considered as contaminants because they can lead to operational problems as blocked filters and lines caused by aerosol and soot formation due to re-polymerisation, their interaction with other impurities on fine particles and condensation of heavy tars on cooler surfaces [9].

Hydrogen sulfide is the main representative of sulfur compounds in syngas obtained from gasification. The presence of H2S is due to sulfur in feedstock. In biomass there is generally a lower percentage of this element than in coal. Indeed woody biomass usually contains 0.1% of sulfur by weight, herbaceous crops 0.3 – 0.4%, while coal contains about 1% [10]. Hydrogen sulfide is a contaminant and, therefore, it has to be removed from syngas, because it can corrode metal surface, it is a forerunner of SO2, a regulated pollutant and it can poison catalysts. Indeed, sulfur may be absorbed on their surface, deactivating these.

2.2.2. Types of gasifiers

Gasification process can take place in several types of reactors. Basically, gasifiers are classified as fixed and fluidized bed reactors.

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Figure 9 Classification of biomass gasifiers [3]

Generally, fixed bed gasifiers consist of a cylindrical vessel (made of concrete or steel), fuel feeding unit, ash collection unit and gas exit. The biomass is fed from the top, while the gasification agent from the bottom; the produced gas goes either up (updraft gasifier) or down (downdraft gasifier).

These types are designed to operate at moderate pressure conditions of 25 – 30 atm, at low gas velocity and long residence time [3].

As Figure 10 and Figure 11 show, the sub-processes of gasification is well identifiable in fixed bed reactors.

Figure 10 Scheme of an updraft gasifier [5] Figure 11 Scheme of a downdraft gasifier [5]

Updraft gasifiers are also called counter-current gasifiers because the feedstock and the produced gases flow in counter-current. This means that syngas leaves the reactor at lower temperatures than in the other types. In the drying zone, the reaction is more effective and, thus, this type of gasifier can handle feedstock with higher moisture content. In addition, produced gas contains a negligible quantity of aerosol/vapour of alkali compounds because its temperature is generally lower than those of melting/vaporization of these impurities and, moreover, the gaseous stream entrains a low amount of ashes and dust.

Finally updraft gasifiers have a high thermal efficiency and the syngas has a quite good heating value.

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On the other hand, the pyrolysis products do not pass through the oxidation step and so and higher tar amount is contained in syngas.

In downdraft gasifier, also known as co-current gasifiers, feedstock and syngas move in the same direction. The advantages of these reactors, in comparison with the previous type, are that the pyrolysis products are forced to pass through the oxidation zone, where thermal cracking occurs and, hence, a lower amount of tar is present in the produced gas. On the contrary, syngas obtained from this gasifier can entrain some ashes and dust.

In fluidizes bed gasifiers, solid particles, such as fuel and bed material, behave like a fluid. The gasification agent is also the fluidization medium. Therefore in this kind of reactors, the air, oxygen or steam must have a quite high velocity.

Fluidized bed enhances the heat transfer between fuel particles and thus the process is nearly isothermal. It means that the four sub-processes are not distinguishable there. The operating temperature depends on the melting point of the bed material, but is generally quite low (800 – 900°C) and the gas residence time is short.

Bed material can be inert, for instance silica, or a catalyst, like olivine or dolomite.

These gasifiers have excellent mixing properties, but are not suitable for biomass that contains a lot of ashes and alkali metals. Indeed these can cause agglomeration of solid particles and so bed de-fluidization.

Figure 12 Scheme of a bubbling bed gasifier

[3]

Figure 13 Scheme of a circulating bed gasifier

[3]

Generally, bubbling bed gasifiers are designed to operate at gas velocities very close to the minimum fluidization one (1 – 3 m/s) [3]. In this way, the particles appear to be in a “boiling” state [11].

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Most of the process occurs in the bubbling bed (Figure 14). Some particles can be entrained by syngas and, hence, at the gasifier outlet there is a cyclone to separate these from the gas flow (Figure 12).

Figure 14 Bubbling bed gasifier [5] Figure 15 Circulating bed gasifier [5]

Table 6 Advantage and disadvantage of fixed and fluidize bed gasifiers [5]

Reactor type Advantage Disadvantage

Updraft gasifier

- high thermal efficiency - difficulty controlling the temperature - simple construction

- need for installation of mobile grates to avoid the formation of preferential paths in the fixed bed - can handle feedstock of different size - low production of H2 and CO

- can handle high humidity feedstock - high content of tar in syngas - good contact between the solid material

and the gasification agent - energy content of tar > 20% - reduced entrainment of dust and ashes

Downdraft gasifier

- simple construction - requires feedstock containing low moisture

- high carbon conversion - low coefficient of heat transfer - low production of tar - difficulty controlling the

temperature - quite limited entrainment of ash and dust

Bubbling bed gasifier

- high mixing and gas-solid contact - entrainment of dust and ashes - high carbon conversion - need for low process temperature to

avoid defluidization of the bed - good thermal control

- low level of tar in syngas

Circulating bed gasifier

- high conversion - complex technology

- low level of tar in syngas - difficulty controlling - requires the reduction of material

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Circulating bed gasifiers operate a high gas velocity ranging from 3 m/s to 10 m/s [3]. This type of reactors operates above the minimum fluidization velocity. In this way, the process takes place almost in the whole reactor, as Figure 15 shows, but a higher amount of particle matter may be entrained by the produced gas. In order to decrease this content, there is a cyclone at the gasifier outlet to separate solid particles from the syngas and to recirculate them to the reactor.

2.2.3. Effects of operating conditions on gasification process

There are several factors that affect the gasification process, such as feedstock, type of gasifier, gasification agent, equivalent ratio, temperature, bed material.

Table 7 Comparison between different types of gasifiers [1]

Downdraft Updraft Bubbling bed Circulating bed

Gasification agent Air Air Air/H2O/O2 Air/H2O/O2 Fuel size

[mm] 20 – 100 5 – 100 ≈ 6 ≈ 6

Allowable fuel moisture

[%] < 25 < 60 < 55 < 55

Syngas temperature

[°C] 700 200 – 400 800 – 1000 -

Syngas LHV

[MJ/Nm3] 4.5 – 5.5 5.5 – 6 3.7 – 8.4 4.5 – 13 Tar in produced gas

[g/Nm3] 0.01 – 5 3 – 150 3.7 – 62 4 – 20

Particles in produced gas

[g/Nm3] 0.02 – 8 0.1 – 3 20 – 100 8 – 100

Hot gas efficiency

[%] 85 – 90 90 – 95 89 89

The effect of feedstock on syngas characteristics depends on which substances it contains. The type of gasifier influences syngas composition as well as its LHV, the amount of contaminants in the produced gas and the gas temperature at the reactor outlet, as explained in paragraph 2.2.1 and summarised in Table 7.

Moreover, syngas composition and its heating value depend on what kind of gasification agent is used as well as the Equivalent Ratio (ER).

Air is the most common and economical gasification agent, but it leads to a quite low heating value, because the produced gas is diluted by nitrogen. Oxygen lead to syngas with higher LHV and H2 and CO yields, but it is the most expensive.

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The amount of employed air/oxygen is very important. Indeed, the ER affects the gasification process in two different ways. Increasing this factor, the gas quality becomes less attractive since the CO2 production increases, because of oxidation of combustible gases. However, the more oxidation reactions occur, the more heat is available for the whole gasification process.

On the other hand, when ER is lower, the gas quality is better, but if it is too small, the energy released from oxidation zone is not enough to carry out the endothermic steps of gasification [11]. So, a suitable value of ER is found to be between 0.2 and 0.4 [1].

Steam is generally added to air or to oxygen. An higher steam/biomass ratio (SB) improves steam concentration and so steam reforming and water-gas shift reaction are favoured. In this way, H2 and CO2 yields increase, but CO and CH4 concentrations decrease. Nevertheless, if too steam is added, a large amount of energy is lost in the gasifier in order to heat up the steam. Therefore, it has been found that the suitable SB is about 1 [11]. Temperature influences mainly gas yield, char conversion and tar concentration. Indeed a higher temperature contributes to higher char conversion and gas yield due to release of more volatiles. Moreover tar cracking is favoured and this leads to lower amount of heavy tar in produced gas and higher concentration of hydrogen.

Figure 16 Influence of temperature in syngas properties [5]

H2 is also a product of endothermic reactions such us steam reforming and water-gas shit. Therefore, a greater hydrogen concentration in syngas can be obtained raising the temperature, because a higher temperature favours products in endothermic reactions, as Le Chatelier’s principle says [11]. On the contrary, CO yield decreases at high temperatures because, according to Le Chatelier’s principle, this gas is consumed in water-gas shift reaction and its production is not favoured in char partial oxidation, that is an exothermic reaction.

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Therefore, generally high temperatures favour hydrogen production and gas yield but not always a high heating value.

Finally, also bed materials affect syngas characteristics. Indeed, catalyst particles can be used instead of inert bed material, in order to improve hydrogen yield and tar decomposition.

2.3. SOFC

Fuel cells are electrochemical devices, where chemical energy is directly converted in electrical energy. Therefore, these have higher efficiency than traditional multi-step thermo-mechanical processes [1].

These are composed by an anode, a cathode and an electrolyte, which separates the electrodes, allowing ions to pass and avoiding electrons transition. The electrolyte may be either liquid-state or solid-state.

Solid oxide fuel cells are complete solid-state devices.

Figure 17 Scheme of a SOFC [12]

As Figure 17 shows, electrons pass through an external circuit, going from the anode to the cathode, where they react with O2 at the cathode-electrolyte interface, forming O2- ions. These pass through the electrolyte and react with fuel at the anode-electrolyte interface, forming H2O/CO2. The zone where the reactions occur is the area of contact between gaseous reagents, electrode and electrolyte and it is called three-phase boundary.

SOFCs operate at high temperature, ranging from 650°C to 1000°C. Indeed, at these temperatures electrolytes achieve a good ion conductivity. Therefore, the suitable operating temperatures depend mainly on the nature of the electrolyte.

This type of fuel cell has several advantage in comparison to the others:  carbon monoxide is a fuel as well as hydrogen

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 it can be fed with hydrocarbons containing fuel, because reforming can happen inside SOFCs;

 because of the high operating temperatures, it is suitable for heat and power application (CHP systems):

 system efficiency can be further enhanced driving a gas turbine with anode exhaust gases;

 it is suitable even for small-scale application, because of the modular nature of SOFC.

On the other hand, the high operating temperatures lead to elevated costs especially for interconnect, manufacturing and sealing materials. Moreover, the required time to heat up and cool down solid oxide fuel cells is quite long and so these cannot be used in those applications that require rapid start-up and cool down [13].

In Figure 17 a single cell is shown. Nevertheless, several cells are usually connected in series to achieve a useful voltage, since that of a single cell is quite small under operating conditions (about 0.7 V). A collection of fuel cells is called “stack”.

The connection between the cells may be in two different ways, shown in Figure 18 and Figure 19.

Figure 18 Simple edge connection between cells

[12]

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The first method (Figure 18) consists in connecting the anode of a cell with the cathode of the following one. This is hardly never used because the electrons have to flow across the surface of the electrode to the current collector at the edge; so, although the electrodes may be good electronic conductors, even a small voltage drop has a great effect, because of the low voltage of the cell.

On the contrary, bipolar plates allow to make connections all over the surface (Figure 19). In addition, these have channels to supply fuel to the anode and air/oxygen to the cathode. So bipolar plates have to have good electronic conductivity, but the two gas flows must be separated. Therefore, they have to be impermeable and gas seals are needed.

The cells explained and shown until now are planar ones. Indeed this is the most used type of solid oxide fuel cells. Nevertheless, another kind of SOFCs exist: tubular cells, shown in Figure 20.

Figure 20 Tubular design [12] Figure 21 Interconnects in tubular cells [12]

This latter type is not usually employed because of its low power density and its high fabrication costs. Moreover the type of electrical connection between the tubular cells causes higher ohmic losses (Figure 21). Its advantage, in comparison to planar design, is that high-temperatures gas-tight seals are not needed.

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2.3.1. SOFC materials

Materials choice is very important for the good operation of SOFCs. Therefore, all components have several required properties: they have to be chemically and physically stable in the appropriate environment (oxidising and/or reducing), chemically compatible with other components, they have to have proper conductivity and similar thermal expansion coefficients to the other parts to avoid cracking [13].

Electrolyte

SOFCs have a crystalline oxide ceramic electrolyte. The electrolyte has to be impermeable to fuel gases, a good ion conductor, but it must not allow electrons to pass through.

The ion transit occurs because of vacant sites in the crystalline structure. These vacant sites are obtained doping a solid oxide with another one.

The most commonly used types of electrolyte are yttria-stabilized zirconia (YSZ) and gadolinia-doped ceria (GDC).

Yttria-stabilized zirconia is created doping zirconia (ZrO2) with a certain percentage (usually 8% mol) of yttria (Y2O3). In this way, when two Zr4+ ions are replaced by two Y3+ ions, one oxide-ion site becomes vacant, because three O2- ions take the place of four O 2-ions [12].

If the number of vacancies increases, ion conductivity will increase as well. Nevertheless, there is an upper limit for the amount of doping species: at about 8% mol of this, the cubic phase is fully stabilized; beyond this amount, the ion conductivity begins to decrease [14]. Yttria-stabilized zirconia is one of the most common types of electrolyte because it is stable in both oxidising and reducing atmosphere, unreactive with other components of SOFCs; moreover, it has good ion conductivity and negligible electronic conductivity. Gadolinia-doped ceria is produced, doping ceria (CeO2) with gadolinia (Gd2O3), another lanthanide metal. The mechanism of ion conduction is the same explained before.

In order to achieve the best properties of GDC material, the optimum percentage of gadolinia is around 10 – 20% [14].

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In comparison to YSZ, GDC has higher ion conductivity, but this advantage is especially important at lower temperatures. Indeed, when this electrolyte operates in a reducing environment (such as the anode) at high temperatures, Ce4+ may be partial reduced to Ce3+ leading to electronic transfer [14]. A solution to this issue can be adding a ultra-thin interfacial electrolyte layer to avoid electronic conduction. This phenomenon is negligible at temperatures below 500°C.

Anode

Beside the common characteristics explained in paragraph 2.3.1, other required properties for anode materials are: high porosity and catalytic activity, high electrical conductivity, fuel flexibility, it must be as tolerant as possible to impurities and coking.

Generally SOFC anodes are made dispersing nickel with the solid electrolyte material to form a cermet, which has high porosity (20 – 40%) [12]. Ni particles provide electronic conductivity and catalytic activity; ceramic material provides a structural framework, prevents nickel sintering, gives a thermal expansion coefficient comparable to that of electrolyte and provides ionic conductivity in order to increase the three-phase boundary. Porosity of electrodes leads to an increase in the electrodes area; this expedient and the use of catalyst allow to make the reactions rate faster.

The most common anode materials are Ni-YSZ and Ni-GDC, according to electrolytes described before.

Ni-GDC has some advantages in comparison to Ni-YSZ, such as it is more impurities tolerant and it provides a larger area for reactions because of GDC mixed electronic/ionic conductivity.

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The cathode, as the anode, must have high porosity and both ionic and electronic conductivity. Moreover, it must have elevated activity for reduction of oxygen and it has to be stable in an oxidising atmosphere.

Generally composite materials and/or mixed ionic-electronic conductors are employed as cathode materials.

Metal conductors are typical instable in oxidising environments at high temperatures, hence, these are not used in SOFC cathodes, that are almost always ceramic. Therefore, the cathode electronic conductivity is lower than that of anode.

The most common cathode material is strontium-doped lanthanum manganite (LSM), because of its good physical and chemical stability, electrical conductivity and catalytic activity. Unfortunately, its ionic conductivity is not so high; therefore LMS-based cathodes are usually mixed with YSZ (LSM-YSZ composite cathodes), in order to increase O2- ion conduction.

Interconnects

The required characteristics for bipolar plates are: impermeability to keep the anode and cathode supplies separated, high electrical conductivity.

Generally ceramic interconnects are used. Indeed, although these are worse electrical conductors than metallic ones, they present a thermal expansion coefficient similar to SOFC materials.

The most employed ceramic material is lanthanum chromite (LaCrO3). In order to increase its quite low electrical conductivity, La3+ may be substituted with strontium (St2+), calcium (Ca2+) or magnesium (Mg2+).

Sealing

In order to avoid mixing between fuel and air, an high-temperatures gas-tight seals is needed. For planar stacks with ceramic interconnects glass is usually used as sealant.

2.3.2. Cell performance

The cell characterization is usually evaluated by i-V curve. In Figure 23 a typical one is shown for a cell that operates at 800°C, 1 atm.

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Figure 23 Typical i-V curve of an high-temperature fuel cell [12]

In Figure 23 the value of “No-loss” voltage is shown. This is the theoretical voltage obtained in a reversible process:

̅̅̅

[5]

The [5]shows how to calculate the reversible cell voltage:

 E is the “electromotive force” (EMF) or “reversible open circuit voltage” (OCV)  ̅̅̅ is the Gibbs free energy of formation per molar unit, that is the energy

available for external work, neglecting that due to any changes in volume/pressure  z is the number of electrons involved in the reaction for each molecule of fuel

reacting

 F is the Faraday constant

Gibbs free energy of formation is not constant, but it depends on temperature and the state. In particular:

̅ ̅ ( ) [6]

where ̅ is the molar Gibbs free energy at standard pressure, ( ) is the activity, which is the pressure or the partial pressure of the species divided by standard pressure; J and K are the reactants (respectively j and k moles) that produce m moles of M (product).

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So the relation between EMF and molar Gibbs free energy of formation, known as Nernst

equation, can be written as:

̅ ( ) ( ) [7]

Another possible expression for Nernst Equation exists: the OCV is calculated using partial pressure of O2 at the anode and at the cathode [15]. This formula is useful especially when syngas is employed as fuel.

(

) [8]

Actually cell voltage is always lower than theoretical value, calculated by[7]. This is due to irreversibility and/or losses, that apply slightly even when no current is drawn.

There are four types of irreversibility that cause voltage drop: activation losses, fuel crossover and internal currents, ohmic losses, mass transport or concentration losses. Activation losses are due to the slowness of the reaction on the electrode's surfaces. Indeed the chemical reaction which transfers the electrons from or to electrodes needs some energy to be carried out. So a part of the generated voltage is lost to drive it. This kind of losses is non-linear.

A value for the activation losses can be calculated by the Tafel equation: ( )

( ) The Tafel equation is valid if .

The constant A is bigger for slow reactions and is higher for fast reactions. The constant A is defined as:

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where z is the number of electrons transferred for mole of fuel and is the charge transfer

coefficient. This latter one depends on the reaction involved and on the electrode material.

It must be in the range to , but for a lot of materials .

Although the coefficient A is proportional to the temperature, if T raises, the overvoltage does not increase, because the temperature affects also the reaction rate and thus .

is called exchange current density. This is the value when the overpotantial is zero. The Tafel equation shown is true only if the overvoltage of one of the electrodes is negligible. This happens in the hydrogen fuel cells. In fact it has been studied that at the cathode is smaller than that at the anode and so the overpotential at the anode is negligible. Otherwise, the value of the activation losses is:

(

) (

)

So increasing the exchange current density can be a good way to reduce the activation losses. It can be done in several ways:

 raising the cell temperature  using more effective catalyst

 increasing the roughness of the electrodes (this increases their real surfaces)  increasing reactant concentration

 increasing the pressure

Fuel crossover and internal currents are due to electrons that pass through the electrolyte or to fuel that diffuses from the anode to the cathode. In fact the electrolyte should prevent these events, but small amounts of electrons and fuel could pass through. Then the voltage drop due to the activation and fuel crossover/internal currents losses can be measured through the following equation:

( )

The internal currents losses are less important in high-temperature fuel cells because of greater exchange current density.

Ohmic losses are caused by electrical resistance of the electrodes and interconnects, and by the resistance to the flow of ions in the electrolyte. The latter causes the main resistance in most fuel cells.

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where is the current density, expressed in mA/cm2 and is the area-specific resistance, expressed in kΩ∙cm2

.

The ohmic losses are considerable in each type of fuel cell, but especially in SOFCs. There are several ways to reduce the cell’s resistance and thus the voltage drop:

 using electrodes with high electrical conductivity,

 using appropriate materials for the interconnects and for bipolar plates,  making the electrolyte as thin as possible.

The voltage loss due to mass transport is caused by the decrease in reactants concentration at the surface of the electrodes. This is the results from an insufficient transport of reactants in certain operating conditions.

The voltage drop can be defined empirically:

where m and n are constants.

The concentration losses are more important at high current density, because a greater amount of reactants is needed.

If the concentration of the reactants increased, the would be positive, thus it would be a voltage gain.

Figure 24 Distinguishing different types of losses in i-V curve [12]

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Figure 25 Comparison between high-temperature and low-temperature cell performance (adjusted from [12])

As Figure 25shows, high-temperature fuel cells have a lower OCV and resistance.

2.4. Effects of contaminants on SOFC performance

The impurities which may be contained in syngas can damage the SOFCs.

The effects of tars and hydrogen sulfide on SOFC performance and/or structure will be explained in the following paragraphs.

2.4.1. Tars

Tars may have two different impacts on SOFCs: one is positive and the other negative. Basically these are hydrocarbons and so they can be both fuel and carbon precursors. Indeed tars can be reformed at the SOFC anode and subsequently oxidised, contributing to electricity production, but they can even cause carbon accumulation, deactivating the catalyst. What happens to tars in a SOFC depends on operating conditions, such as temperature, current density and steam content, anode material and the type of tar.

2.4.1.1. Reforming

Reforming is the process where hydrogen and carbon monoxide are produced from hydrocarbons. If the latter ones react with steam, steam reforming occurs ([9]); on the contrary, if hydrocarbons react with carbon dioxide, the process is called dry reforming ([10]).

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[10]

If a catalyst is not employed, steam reforming of tars occurs appreciably only at temperatures above 900°C, because its activation energy is high. Adding a catalyst, this process has high conversion rate at temperatures between 500 – 900°C.

The most common catalysts used are nickel-based ones, because these have high activities, are easily available and are quite cheap relatively at their characteristics, even if they suffer deactivation.

Regarding both steam and dry reforming, the higher the temperature and the lower the pressure, the more complete the conversion. Indeed, according to Le Chatelier’s principles, the equilibrium is moved to the products, if the pressure is low [12].

On the contrary, about the amount of H2O/CO2, the behaviours are different: in steam reforming, the conversion rate is greater at higher S/C ratio; in dry reforming, there is an optimum value of CO2/C ratio, that depends on operating temperature: above this value, the amount of produced CO increases but that of H2 decrease. These trends are shown in Figure 26, Figure 27, Figure 28 and Figure 29for reforming of methane.

Figure 26 Effect of temperature on dry reforming of

CH4 [16]

Figure 27 Effect of pressure on dry reforming of

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Figure 28 Effect of CO2/CH4 ratio and temperature on dry reforming [16]

Figure 29 Effect of temperature, pressure and S/C

ratio on steam reforming of CH4 [17]

Depending on where the reaction takes place, three types of reforming exist: external reforming, direct internal reforming and indirect internal reforming.

Indeed this process may occur either in a separated reactor downstream the gasifier, (external reforming) or inside the SOFC (internal reforming). Indirect internal reforming occurs when the reformer is placed in close thermal contact with the cell, but reforming reaction is separated from electrochemical reactions. On the contrary, direct internal reforming occurs when the process takes place directly on the anode of the solid oxide fuel cell without a separated reformer, in order to use cell thermal energy and catalyst. In this way thermal and chemical interaction exist.

Internal reforming is an interesting application because it allows to increase the efficiency of the system. Indeed, generally, reforming is an endothermic process, therefore, thermal energy produced in the cell (mainly by internal resistance) is enough to carry out this reaction. In this way, further energy has not to be supplied for the reforming and, in addition, the cell is cooled down.

Moreover, direct internal reforming offers further advantages in comparison to the indirect one: a separated reformer is not needed, hence costs, due, for instance, to materials,