Study on the influence of representative primary Tar and HCl on SOFC performance

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UNIVERSITÀ DI PISA

SCUOLA DI INGEGNERIA

CORSO DI LAUREA MAGISTRALE IN INGEGNERIA ENERGETICA

Study on the influence of representative primary Tar

and HCl on SOFC performance

Relatore Candidato

Prof. Umberto Desideri Tommaso Del Carlo

Supervisors

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

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II

Abstract

Biomass – integrated gasification fuel cell (B – IGFC) systems represent an interesting alternative to state of the art fossil-fuel based power plants due to their high overall efficiency and low emissions in terms of pollutants and greenhouse-gas (GHG). The aim of this thesis is the improvement of a B – IGFC system, composed by a biomass updraft gasifier, a high temperature gas-cleaning unit (GCU) and a solid oxide fuel cell (SOFC) by acting on overall efficiency, system cost and its complexity. The focus of the study is on the possibility to reform tar internally in the SOFC and on the cross-influence of another contaminant present in syngas, HCl, which may influence SOFC performance and internal reforming. Higher tolerability level might result in less stringent cleaning requirements, thus increasing the system efficiency and reducing its complexity and costs.

The experimental campaign consisted of three sets of tests in which a Ni-GDC cell was fed with syngas and different concentrations of the contaminants, HCl and tar, separately or together. The model tar chosen was acetic acid, because it is the most abundant species in the updraft gasifier produced gasses. The concentrations (dry basis) tested were 17 – 41 – 83 – 128 g/Nm3_{for tar and 3,4 – 20 – 50 ppm for hydrogen chloride.} Furthermore, the synergistic effects of both contaminants together were examined, testing a constant tar content of 41 g/Nm3 and an increase concentration of HCl equal to 3,4 – 20 – 50 ppm.

All the experiments were carried out under a constant current density of 68 mA/cm2, at 800°C and feeding the cell with simulated wet syngas (9,5% H2; 12,6% CO; 9,5% CO2; 1,3% CH4; 30,3% N2; 36,8% H2O).

The results revealed that tar injection led to higher immediate cell performance due to internal tar reforming and it has been demonstrated that the cell exposed to 128 g/Nm3 of acetic acid had stable operation, for overall two days of test.

The HCl set of tests showed that the tolerance limit in the tested conditions was 3,4 ppm. Indeed, the higher concentrations resulted in voltage drop and higher cell degradation rates. As a consequence, a high-temperature HCl removal reactor, where sodium or potassium based sorbents are adopted, can be operated around 600°C and low chlorine feedstock as paper residue sludge or cacao shells can be used without the need of a chlorine removal stage.

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III When the two contaminants were fed together into the SOFC, a positive synergistic effect was observed, which led to lower cell degradation rates in comparison with the set of test with HCl only.

In conclusion, the aim of this thesis has been achieved through a demonstration on the possibility to reform acetic acid internally in the SOFC and a redefinition of the tolerance limits for HCl. This last outcome allows an increase in HCl reactor temperature or the possibility of remove it, when feedstock with low chlorine content are used.

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IV

Sommario

I sistemi integrati di gassificazione della biomassa con celle a combustibile (B – IGFC) rappresentano un'interessante alternativa agli impianti di potenza a base di combustibili fossili, grazie all'elevata efficienza complessiva e alle basse emissioni in termini di inquinanti e gas serra (GHG). Lo scopo di questa tesi è il miglioramento di un sistema B – IGFC, composto da un gassificatore updraft a biomassa, un’unità di pulizia del gas ad alta temperatura (GCU) e una cella a combustibile ad ossido solido (SOFC) agendo sull'efficienza complessiva, sul costo del sistema e la sua complessità. Questo studio è focalizzato sulla possibilità di riformare il tar internamente alla cella e sull'influenza di un altro contaminante presente nel syngas, l’HCl, il quale può influenzare le prestazioni della SOFC ed il reforming interno. Livelli di tollerabilità superiori permetterebbero requisiti di pulizia meno severi, aumentando così l'efficienza del sistema e riducendo la sua complessità e il suo costo.

La campagna sperimentale è stata svolta in tre serie di prove in cui una cella Ni-GDC è stata alimentata con syngas e diverse concentrazioni di contaminanti, HCl e tar, separatamente o insieme. Il modello di tar scelto è stato l'acido acetico, poiché è la specie più abbondante nei gas prodotti dal gassificatore updraft. Le concentrazioni (secche) testate sono state 17 – 41 – 83 – 128 g/Nm3 per il tar e 3,4 – 20 – 50 ppm per cloruro di idrogeno. Inoltre, sono stati esaminati gli effetti sinergici di entrambi i contaminanti, testando un contenuto di tar costante pari a 41 g/Nm3 e una concentrazione di HCl uguale a 3,4 – 20 – 50 ppm.

Tutti gli esperimenti sono stati eseguiti con una densità di corrente costante pari a 68 mA/cm2, a 800°C e alimentando la cella con syngas umido, ottenuto miscelando più flussi gassosi (9,5% H2; 12,6% CO; 9,5% CO2; 1,3% CH4; 30,3% N2; 36,8% H2O).

I risultati hanno rivelato che l'iniezione di tar porta ad una maggiore prestazione della cella a causa del reforming interno del tar ed è stato dimostrato che la cella esposta a 128 g/Nm3 di acido acetico ha avuto un funzionamento stabile per due giorni di prova complessivi. I test con HCl hanno mostrato che il limite di tolleranza nelle condizioni sperimentate è 3,4 ppm. Infatti, le concentrazioni più elevate hanno determinato una caduta di tensione e un aumento dei gradi di degradazione della cella. Di conseguenza, un reattore di rimozione di HCl ad alta temperatura, in cui vengono adottati sorbenti a base di sodio o di potassio, può essere impiegato a circa 600°. Altrimenti, se vengono utilizzate biomasse con basso

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V contenuto di cloro, come i residui di carta o gusci di cacao, può essere tolta del tutto la fase di rimozione del cloro.

Quando i due contaminanti sono stati alimentati insieme nella SOFC, è stato osservato un effetto sinergico positivo, che ha portato a ridurre i gradi di degradazione della cella rispetto al set di test con solo HCl.

In conclusione, l'obiettivo di questa tesi è stato raggiunto attraverso la dimostrazione sulla possibilità di riformare l'acido acetico all'interno della SOFC e una ridefinizione dei limiti di tolleranza per l'HCl. Quest'ultimo risultato consente un aumento della temperatura del reattore HCl o la possibilità di rimuoverlo quando vengono utilizzate biomasse con basso contenuto di cloro.

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VI

List of Figures

Figure 1 - Ultimate composition of several biomass type [2]_{... 3}

Figure 2 - Biomass conversion processes. Readapted from [5] ... 6

Figure 3 - Syngas quality on the basis of temperature. Readapted from [8] ... 9

Figure 4 - Updraft gasifier [9]_{... 10}

Figure 5 - Downdraft gasifier [9]_{... 10}

Figure 6 - Entrained flow [9]_{... 11}

Figure 7 - Bubbling fluidized bed [9]_{... 11}

Figure 8 - Circulating fluidized bed [9]_{... 12}

Figure 9 - Dual fluidized bed [9]_{... 12}

Figure 10 - Plasma [9]_{... 12}

Figure 11 - Rotary kiln reactor [8]_{... 13}

Figure 12 - Six species of contaminant contained in syngas. Readapted from [1]... 13

Figure 13 - Comparison Updraft-downdraft gasifier ... 15

Figure 14 - Biomass Integrated Gasification Fuel Cell system ... 17

Figure 15 - Coal IGFC system ... 18

Figure 16 - Zones and reactions that occur inside an updraft gasifier [11]_{... 20}

Figure 17 - Primary, Secondary and Tertiary tar [8]_{... 23}

Figure 18 - MBMS analysis of updraft and downdraft gasifiers [16]_{... 24}

Figure 19 - Internal reforming, direct or indirect. Readapted from [18] ... 26

Figure 20 - Basic principles of an alkaline electrolyte fuel cell [18]_{... 33}

Figure 21 - Solid oxide fuel cell structure ... 35

Figure 22 - External and internal manifolding ... 35

Figure 23 - Gibbs free energy of formation of water [18]_{... 36}

Figure 24 - Fuel cell equivalent circuit ... 39

Figure 25 – i-V curves at low and high temperature [18]_{... 39}

Figure 26 - Reactions taking place at anode and cathode ... 41

Figure 27 - Fuel cell layer configurations [23]_{... 42}

Figure 28 - Yttria-stabilized zirconia structure ... 43

Figure 29 - Cermet vs MIEC conductor [18]_{... 44}

Figure 30 - SOFC flow configurations. Readapted from [1] ... 47

Figure 31 - Hot gas clean up configuration. Readapted from [1] ... 52

Figure 32 - a) Long-term test with 255 mg/Nm3_{of toluene in syngas; and b) EIS analysis}[27]_{... 54}

Figure 33 - Tests with naphthalene in wet H2 or syngas [28] ... 55

Figure 34 - Test with toluene (up to 14,2 g/Nm3_{) in syngas for 25 hours}[31]_{... 56}

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X

Figure 36 - i-V curves referred to the five tests [33]_{... 57}

Figure 37 - Possible reaction in SOFCs operating on tar-containing syngas, and impact of temperature on 4 reactions [34]_{... 58}

Figure 38 - Carbon deposited on the two type of anode, due to exposure to the whole real tar or two its fractions [35]_{... 59}

Figure 39 - i-V and power density curves for the three tests [37]_{... 60}

Figure 40 - Carbon deposited on two type of anode, due to exposure to toluene or real tar; and based on steam content [38]_{... 61}

Figure 41 - i-V curves referred to tests carried out in different operating condition; and for different time of exposure [39]_{... 62}

Figure 42 - Polarization curves obtained feeding the cell with H2, DME or DME-CO2[41] ... 62

Figure 43 - Carbon deposited using fuel with different steam content [46]_{... 65}

Figure 44 - Cell degradation rates with cell fed with H2 or syngas and HCl [57] ... 68

Figure 45 - i-V curves carried out after the tests with different concentrations of HCl [57]_{... 69}

Figure 46 - Polarization curve of the whole test with syngas and HCl (up to 90 ppm) [57]_{... 69}

Figure 47 - Polarization resistances for different HCl concentrations ... 70

Figure 48 - Polarization curve of the test, with 10 ppm of HCl [58]_{... 71}

Figure 49 - Polarization curves. Tests carried out with 50 ppm of HCl and at different temperatures [59]_{... 71}

Figure 50 - Poisoning mechanism by chlorine compound [62]_{... 73}

Figure 51 - Tests with 0, 20, 160 ppm HCl at two temperatures (800 or 900°C) [64]_{... 74}

Figure 52 - Basic scheme of the fuel cell set-up ... 81

Figure 53 – Photo of the new solid oxide fuel cell used ... 84

Figure 54 - Set-up assembling ... 85

Figure 55 - Impinger train for tar sampling [66]_{... 89}

Figure 56 - FactSage water content calculation ... 94

Figure 57 - HCl cleaning, at different temperature, using Na2CO3 ... 99

Figure 58 - HCl cleaning, at different temperature, using K2CO3 ... 100

Figure 59 - Syngas preliminary test ... 107

Figure 60 - Photo cell, after syngas preliminary test ... 108

Figure 61 - Cell potential during reduction procedure ... 109

Figure 62 - Preliminary i-V curves ... 110

Figure 63 - Cell potential during reference test ... 112

Figure 64 - i-V curves reference test ... 114

Figure 65 - Cell potential during tar test ... 116

Figure 66 - Cell potential during the test with 17 g/Nm3_{of acetic acid ... 118}

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XI

Figure 70 - Cell potential during the first recovery test ... 120

Figure 71 - Cell potential during the second test with 128 g/Nm3_{of acetic acid ... 121}

Figure 72 - Comparison between the i-V curves carried out at the end of each test ... 122

Figure 73 - GC. Total outlet volume flow rate ... 123

Figure 74 - GC. Outlet volume flow rates ... 123

Figure 75 - Carbon deposited at ceramic block inlet ... 125

Figure 76 - Carbon deposited at inlet pipe ... 125

Figure 77 - Carbon deposition and delamination (red circle) at the cell anode... 125

Figure 78 - Tar sampling methodology ... 126

Figure 79 - Cell potential during HCl test ... 130

Figure 80 - Cell potential during the test with 3,4 ppm of HCl ... 131

Figure 81 - Cell potential during the test with 20 ppm of HCl ... 131

Figure 82 - GC. Total outlet volume flow rate ... 132

Figure 83 - GC. Volume flow rates ... 132

Figure 84 - Cell potential during the first recovery step ... 133

Figure 85 - Cell potential during the test with 50 ppm of HCl ... 134

Figure 86 - Cell potential during the second recovery ... 135

Figure 87 - Cell potential during the synergistic effect test ... 138

Figure 88 - Cell potential during the test with 3,4 ppm of HCl and 41 g/Nm3_{of acetic acid ... 139}

Figure 89 - Cell potential during the test with 20 ppm of HCl and 41 g/Nm3_{of acetic acid ... 140}

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XII

List of Tables

Table 1 - Contaminants level based on type of gasifier. Readapted form [11] ... 16

Table 2 - Updraft gasifier characteristics [1]_{... 20}

Table 3 - Tar classification (5 classes). Readapted form [8] ... 22

Table 4 - Tar transformation. Readapted form [16] ... 23

Table 5 - HCl released during dasification of different type of biomass ... 29

Table 6 - Fuel cell type. Readapted from [18] ... 33

Table 7 - SOFC characteristics. Readapted from [1] ... 45

Table 8 - SOFC tolerance limits. Readapted from [11], [24] ... 46

Table 9 - Particulate matter removal methods... 49

Table 10 - Literature review on acetic acid ... 67

Table 11 - Cell characteristics ... 84

Table 12 - Reduction procedure ... 86

Table 13 - Syngas composition [67]_{and volume/moles flow rates ... 92}

Table 14 - Tar concentrations... 95

Table 15 - Real tar composition ... 96

Table 16 – Comparison between hydroxyacetaldehyde and acetic acid ... 97

Table 17 - Voltage increase due to acetic acid injection ... 98

Table 18 - Water content in syngas necessary to avoid carbon deposition, at different temperatures ... 98

Table 19 - Voltage increase due to a lower water content ... 99

Table 20 - HCl cleaning, at different temperature, using Na2CO3 ... 99

Table 21 - HCl cleaning, at different temperature, using K2CO3 ... 100

Table 22 - Preliminary tar sampling results ... 104

Table 23 - Preliminary tar sampling analysis ... 105

Table 24 - OCV and ASR of preliminary i-V curves ... 110

Table 25 – Voltage and degradation rates during reference test ... 112

Table 26 - Expected and actual tar concentrations ... 114

Table 27 - OCV increase due to acetic acid internal reforming ... 115

Table 28 - Voltage and degradation rates during tar test... 116

Table 29 - Comparison between calculated and actual voltage increase values ... 117

Table 30 - Degradation rates during the tar test ... 117

Table 31 - Difference volume flow rate between the absence of tar or its highest concentration.. 124

Table 32 - Tar sampling results ... 127

Table 33 - Comparison between acetic acid, acetone and hydroxyacetone ... 128

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XIII

Table 35 - Voltage and degradation rates during the synergistic effect test... 138 Table 36 – Synergistic effect test. Tar sampling results. ... 141 Table 37 - Comparison between the degradation during the HCl and synergistic effect test ... 141

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XIV

Nomenclature

AcOH Acetic acid (CH3COOH)

ASR Area specific resistance CHP Combined heat and power

db Dry basis

EIS Electrochemical impedance spectroscopy GC Gas chromatography

GC – FID Gas chromatography – Flame ionization detector GCU Gas cleaning unit

i Current density LHV Low heating value

LSM Lanthanum strontium manganite MFC Mass flow controller

Ni-GDC Nickel - gadolinia-doped ceria (or Ni-GCO) Ni-ScSZ Nickel - scandia-stabilized zirconia

Ni-YSZ Nickel - yttria-stabilized zirconia NiO Nickel oxide

OCV Open circuit voltage P Pressure or partial pressure ppm Part per million weight (or ppmw) ppm(v) Part per million (volume)

PAH Polycyclic aromatic hydrocarbon S/C Steam over carbon ratio

S/F Steam over fuel ratio

SEM Scanning electron microscope SOFC Solid oxide fuel cell

T Temperature

t Time

TPB Triple phase boundary

TPO Temperature programmed oxidation

V Cell voltage

wb Wet basis

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1

1. Introduction

Nowadays, climate changes, increase energy demand along with depletion of fossil fuel have gained more attention. At the same time, the use of biomass as renewable energy source has spread widely, since it is considered carbon-neutral, releasing during the combustion the same amount of CO2 previously captured during its growth.

Biomass, which is available nearly worldwide, can be converted through gasification technologies into syngas that can be used into different type of devices to generate electricity.

In this scenario, solid oxide fuel cells can act a key role due to their ability to convert the chemical energy of fuel directly into electricity, with high efficiencies and without any atmospheric greenhouse gases emission.

The combination of an updraft gasifier with SOFC is a system that allows the combined production of heat and electric power using biomass as feedstock. This type of system is called Biomass – Integrated Gasification Fuel Cell (B – IGFC).

Moreover, the updraft scheme assures a high conversion efficiency from biomass feedstock to syngas. Another advantage is that thanks to the fuel cell the conversion from fuel to electricity has higher efficiencies.

The overall aim of this work is the study and optimization of the B – IGFC system. Although the produced syngas contains varying type of contaminants, this work will be focussed on two of these impurities: tar and hydrogen chloride.

Testing different concentrations of tar and hydrogen chloride, the key objectives of the present experimental campaign are determine the feasibility of the internal tar reforming, study the impact of HCl on cell performance; valuate the effect of HCl on internal tar reforming; study the synergistic effect of tar and HCl on SOFC.

A higher tar tolerance limit will lead to the possibility of decrease the size of the external tar reformer or allow the use of a less-performing but cheaper catalyst, or if the cell can handle the whole load of tar produced from the gasifier, to remove it completely from the system, leading to lower costs and complexity.

Generally, higher HCl tolerance limit could lead to higher temperature of the HCl reactor, or even to remove the HCl cleaning step.

An higher temperature of the HCl reactor will lead to higher system efficiency and lower needed of water content in the syngas flow, to avoid carbon deposition, which lead to lower costs and higher cell performance.

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2 The outline of the thesis is the follow:

 Chapter 2

In this section will be given the theoretical information about the topics of this work. Particular attention will be reserved to SOFCs and the poisoning mechanism that can influence their performance.

 Chapter 3

In this chapter, the literature reviews about tar, hydrogen chloride and synergistic effect will be presented and discussed, in order to know what have been done until now on these topics.

 Chapter 4

The methodology, aims and approach used for this study are discussed in this section. The criteria used to plan the actual experimental campaign, the set-up used and the necessary preliminary procedures will be discussed.

 Chapter 5

In this section, the theoretical results are presented. They included the water content necessary in the syngas stream to avoid carbon deposition, the theoretical values of voltage increase due to tar injection, the simulation of HCl clean-up through the software FactSage.

 Chapter 6

The experimental results obtained from the experimental campaign are reported and deeply discussed.

 Chapter 7

The final chapter will contain both conclusion and final remarks of the thesis; included the suggestions for future works and research on these topics.

The present work has been made in collaboration with the department of “Process and Energy” at Delft University of Technology (Netherlands), by means of the European Erasmus Plus Programme.

The research was performed within the project “FlexiFuel – SOFC”, which has received funding from the European Union Horizon2020 Program (H2020/2014-2020) under Grant Agreement n° 641229.

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3

2. Technical Background

2.1 Biomass[1], [2], [3]

Biomass is defined as the biodegradable fraction of agriculture products, residues or waste as well as animal, industrial and municipal waste. It can be classified mainly as woody or non-woody biomass; the first one includes material from plants and trees, but without leaves, bark and roots. The biomass obtained directly from energy crops is the perfect example of this first class. Instead, the non-woody classification includes many kinds of waste and residues, such as grass, straw, manure and sludge. Rice husk is a particular, but significant example of agricultural residue; it is produced worldwide, but underutilized, and could be used for instance as fuel in a gasifier to obtain energy. Municipal Solid Waste and animal manure can be potential biomass feedstock, even if with some issues related to large amounts of ash.

Generally, biomass is composed by lignin, cellulose and hemicellulose polymers interlinked in a heterogeneous matrix. However, its chemical composition is rather heterogeneous as well as moisture content, thus, its energy density may vary largely and a deeper analysis is necessary. Indeed, the characterization of the biomass is fundamental to define the quality of the feedstock and to establish which process it can undergo. The biomass analyses are classified in proximate and ultimate analysis. The first one describes the material in terms of gross components, thus quantifying volatile matter, moisture content, fixed carbon and ash. Through the ultimate analysis is possible to determine the exact elemental composition of the biomass and thus, calculate the theoretical air/fuel ratio needed, depending on the following application, and take into consideration its potential emissions. The following Figure 1 summarize the analysis of some classes of biomass:

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4 Detailed information on the composition of many other different kinds of biomass can be found on an online database at www.ecn.nl/phyllis2. The principal aspects that affect deeply the quality of the biomass material are elemental composition, moisture and ash content, density, particle size.

The elemental composition of the feedstock allow quantifying the higher heating value (HHV). In particular, are important the carbon, hydrogen, oxygen and nitrogen contents, indeed [4]:

 C, H and O are the main constituents of biomass;

 C and H are oxidized during the combustion through exothermic reactions, forming CO2 and H2O;

 The amount of oxygen released during the combustion is a part of the oxygen required for the combustion itself, the lacking part is introduced by injecting air from the outside;

 C is present in various oxidized forms which explains the lower calorific value of biomass compared to coal;

 The concentration of C is higher in woody biomass than those herbaceous, so the firsts have more calorific power;

Moisture may vary from 10 to 60% of the feedstock volume, or even higher. H2O content has an important role in the gasification process, because it influences the gas yield and composition. Indeed, water act as gasification agent, converting the biomass, as well as increasing the hydrogen yield through the water-gas shift reaction. However, the key problem is that many conversion processes cannot handle a biomass with too high H2O content: if the moisture is 50% or higher the biomass usually undergo to biochemical process conversion, instead, if it is lower than 50% the feedstock is processed through thermochemical processes [4].

Moreover, the moisture content affects negatively the feedstock energy value as well as many processes, such as handling, storage and transportation, increasing the costs. In addition, feedstock with too high H2O content needs more energy to be heating up to gasification temperature and this means a lower whole system efficiency.

Ash may generate deposits inside the combustion chamber or the gasifier, this lead to increasing costs of maintenance. For instance, grasses, bark and field corps have an higher ash content than wood.

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5 Feedstock size and density influence the drying and heating processes as well as the conversion process which the biomass can undergo, for instance, an entrained flow gasifier have to be fed with a biomass with small size.

Chlorine content is another key point to analyse; indeed, it is the most important difference between herb biomass and wood biomass or carbons. While chlorine is typically found in negligible amounts in coal or wood (<0.05% by weight db), various types of herbaceous biomass have a chlorine content of 0.1 ÷ 2% or more. Chlorine is absorbed by plants through a wide variety of environmental sources and plays an important role in certain functions of the plant.

“The chlorine in plants is mainly bounded as inorganic salt in the form of potassium chlorine (KCl) and quaternary ammonium chlorine.” Moreover, “the level is influenced by the location of growth and the period of harvesting” indeed, “chlorine content of biomass harvested in the winter is normally lower than in the summer because of the low plant activity during the winter. Furthermore the chlorine content in biomass is higher on locations near the sea due to the increased NaCl concentration in the soil” [2].

Chlorine content leads directly to the formation, during the gasification process, of contaminants such as hydrogen chloride (HCl). Indeed, at high temperatures the halide gases, derived from the alkali metal salts of biomass, reacts with steam generating HCl, HF, HBr, etc. These chloride compounds are in gaseous state at the temperature conditions of the gasifier and thus, they will flow out with the syngas. Hydrogen chloride is the most abundant chloride species in the produced gas; it can react with other gas species to form NaCl or NH4Cl, which may deposits and create fouling in some downstream equipment.

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6

2.2 Conversion processes[3], [5]

Biomass can be processed with many methods, which can be classified in three main classes, as shown in the Figure 2:

1. Thermal -> combustion, torrefaction, pyrolysis, or gasification 2. Biological -> fermentation, aerobic or anaerobic digestion 3. Mechanical -> oil extraction

Figure 2 - Biomass conversion processes. Readapted from [5]

The choice of one process or the other depends on the feedstock characteristics, which in case of biomass is rather heterogeneous. Indeed, if on the one hand, the combustion can handle a wide range of feedstock, on the other hand, others require much more homogeneous feedstock.

Combustion is a process in which the biomass is burned inside a high-pressure boiler to generate steam with high enthalpy, through an exothermic chemical reaction with an

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7 oxidant such as air. The steam is used to drive a steam turbine, in order to generate electricity, and even to feed a heating system. In a power system, the fuel can be for instance biomass, coal or both of them (co-firing).

Torrefaction is a biomass treatment, carried out at 200 ÷ 320°C, transforming the feedstock in a higher energy density product: bio-coal. This process is energetically rather expensive, but once obtained the final product, it can be pelletized in order to be easier transportable and used it such as the traditional coal. Moreover, the gasses obtained from the process can be used to supply part of these thermal needs [6].

Pyrolysis is a subset of gasification systems. This process occurs at lower temperature (300°C to 600°C) and low presence of oxygen. The partial combustion results in the formation of a liquid bio-oil, as well as gaseous and solid products. The pyrolysis process can be classified in three types:

1. slow pyrolysis: obtained with temperatures lower than 600°C and long periods of permanence; the main product obtained is a char-coal that accounts for about 30% of the initial dry substance;

2. fast pyrolysis: obtained between 500 and 650°C and the products are mainly gasses, almost 80% of the initial weight;

3. flash pyrolysis: carried out at temperatures between 650 ÷ 1000°C and very short residence times, less than 1 second; this process allows to get 60% of liquid products.

Gasification of the biomass is the partial combustion of the feedstock at high temperature up to 1000°C and reductive environment; producing mainly gaseous products (syngas), char and bio-oils. The gasification occurs inside a gasifier that can have different configurations: fixed bed, circulating bad or entrained flow. The syngas is a mixture of hydrogen, carbon monoxide, carbon dioxide, nitrogen, methane, steam and many other kind of contaminants such as tar, char, sulphur etc. After the clean-up the resulting gas can be used in many ways to produce energy and heat: gas turbine, fuel cell, combustion engine, etc.

Fermentation is a biochemical process in which the sugar in the biomass are converted in ethanol through the reaction 𝐶6𝐻12𝑂6 → 2𝐶2𝐻5𝑂𝐻 + 2𝐶𝑂2.

Digestion is a process in which the feedstock is transformed through bacteria, in an environment in presence (aerobic digestion) or absence (anaerobic digestion) of oxygen. The obtained biogas is mainly composed of methane (CH4) and carbon monoxide (CO). “The biogas can be used, after clean-up, in internal combustion engines, micro-turbines,

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8 gas turbines, fuel cells and Stirling engines or it can be upgraded to bio-methane for distribution” [3]_.

Oil extraction is a process with which the oil is extract form the feedstock mechanically. The product can be used as bio-fuel in specific vehicles, or being upgraded and used as traditional fuel.

2.2.1 Gasification

Gasification is the thermal conversion of biomass, through an incomplete combustion, into a combustible mixture of gas called syngas, composed mainly of hydrogen, carbon monoxide, carbon dioxide, nitrogen, methane, steam and contaminants. The conversion is obtained heating up the feedstock at high temperature, up to 1500°C, in an environment with low presence of oxygen. Usually the biomass require, for a complete combustion, an air to fuel ratio equal to 6, instead the gasification is carried out with a sub-stoichiometric ratio equal to 1,5 ÷ 1,8 [1], [7].

The process can be divide in some steps [7], [8]: A. Drying

During this first step, the biomass lose its moisture content through the evaporation with the temperature. It occurs inside the gasifier and it can be considered complete when the feedstock reaches 150°C. Due to the low temperature, the biomass does not undergo any thermal decomposition of its volatile matter. The general drying reaction is: 𝑀𝑜𝑖𝑠𝑡 𝑓𝑒𝑒𝑑𝑠𝑡𝑜𝑐𝑘 + 𝐻𝑒𝑎𝑡 → 𝐷𝑟𝑦 𝑓𝑒𝑒𝑑𝑠𝑡𝑜𝑐𝑘 + 𝐻₂𝑂(𝑔)

The heat necessary for this process, as well as for the others endothermic steps, is given by the combustion reactions that occur inside the gasifier.

B. Pyrolysis

During the pyrolysis, the dry biomass is thermal decomposed in absence of oxygen/air. Thus, with temperature between 250 and 700°C the feedstock is transformed into a solid charcoal, a liquid part usually called tars and a gasses mixture. The process can be express briefly as:

𝐷𝑟𝑦 𝑓𝑒𝑒𝑑𝑠𝑡𝑜𝑐𝑘 + ℎ𝑒𝑎𝑡 → 𝐶ℎ𝑎𝑟 + 𝑇𝑎𝑟𝑠 + 𝐻2+ 𝐶𝑂 + 𝐶𝑂2+ 𝐶𝐻4+ 𝑁2+ 𝐻2𝑂(𝑔) More details on the thermal decomposition and consequent tar formation will be given in the [2.3.2 Tar] section.

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9 C. Partial Oxidation

The partial oxidation occurs in the zone where the gasification agent is injected and it is carried out in sub-stoichiometric conditions, in order to oxidize only part of the fuel. The reactions that can occur are:

𝐶 + 𝑂2 → 𝐶𝑂2 𝛥𝐻 = −394 𝐾𝐽/𝑚𝑜𝑙 𝐶ℎ𝑎𝑟 𝑐𝑜𝑚𝑏𝑢𝑠𝑡𝑖𝑜𝑛

𝐶 + 1/2 𝑂₂ → 𝐶𝑂 𝛥𝐻 = −111 𝐾𝐽/𝑚𝑜𝑙 𝐶ℎ𝑎𝑟 𝑝𝑎𝑟𝑡𝑖𝑎𝑙 𝑐𝑜𝑚𝑏𝑢𝑠𝑡𝑖𝑜𝑛 𝐻₂+ 1/2 𝑂₂→ 𝐻₂𝑂 𝛥𝐻 = −242 𝐾𝐽/𝑚𝑜𝑙 𝐻𝑦𝑑𝑟𝑜𝑔𝑒𝑛 𝑐𝑜𝑚𝑏𝑢𝑠𝑡𝑖𝑜𝑛

These are all exothermic reactions and are those that assure the require heat of the whole gasification process. The highest temperature of the gasifier are in this zone, with peaks that reach 1100 ÷ 1500°C.

D. Reduction

In the reduction zone, in absence of oxygen, occurs reactions between all those products formed in the other steps of gasification. The reactions can be either endothermic: 𝐶 + 𝐶𝑂₂ ↔ 2𝐶𝑂 𝛥𝐻 = +172 𝐾𝐽/𝑚𝑜𝑙 𝑅𝑒𝑣𝑒𝑟𝑠𝑒 𝐵𝑜𝑢𝑑𝑜𝑢𝑎𝑟𝑑 𝑟𝑒𝑎𝑐𝑡𝑖𝑜𝑛 𝐶 + 𝐻2𝑂 ↔ 𝐶𝑂 + 𝐻2 𝛥𝐻 = +131 𝐾𝐽/𝑚𝑜𝑙 𝐶ℎ𝑎𝑟 𝑠𝑡𝑒𝑎𝑚 𝑟𝑒𝑓𝑜𝑟𝑚𝑖𝑛𝑔 Or exothermic: 𝐶𝑂 + 𝐻₂𝑂 ↔ 𝐶𝑂₂+ 𝐻₂ 𝛥𝐻 = −41 𝐾𝐽/𝑚𝑜𝑙 𝑊𝑎𝑡𝑒𝑟 − 𝑔𝑎𝑠 𝑠ℎ𝑖𝑓𝑡 𝑟𝑒𝑎𝑐𝑡𝑖𝑜𝑛 𝐶 + 2𝐻₂ ↔ 𝐶𝐻₄ 𝛥𝐻 = −75 𝐾𝐽/𝑚𝑜𝑙 𝑀𝑒𝑡ℎ𝑎𝑛𝑎𝑡𝑖𝑜𝑛

In addition, Le Chatelier’s principle assure that at higher temperature are favoured the exothermic reactions; instead, at lower temperature the opposite is true.

Thus, the temperature of the reduction zone is important in order to determine the quality and composition of the syngas.

Syngas lower heating value, tar content and efficiency of char conversion depend on temperature, as briefly shown in Figure 3:

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10 More particular on how and why some gasification parameter may influence the syngas characteristics are reported in the paragraph [Parameters that affect the syngas production and composition].

Type of Gasifier[7], [8], [9] Updraft fixed bed

The biomass is fed from the top of the gasifier, instead the air, oxygen or steam is injected from the bottom, passing upward through the biomass. The syngas leaves from the upper part of the gasifier, instead the ash falls through the grate for the removal. The main advantages of this gasifier are: high thermal efficiency, it can handle feedstock with high moisture content and with different sizes.

Figure 4 - Updraft gasifier [9]

Downdraft fixed bed

As in the updraft gasifier, the biomass is fed from the top, but the gasification agent in this case is fed from the top as well. Thus, biomass and air stream co-flows to the bottom, where syngas and ash are collected. This gasifier requires feedstock with low moisture content.

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11 Entrained flow (EF)

The biomass, after has been pulverized, is fed from the bottom part of the EF gasifier with pressurized steam and/or oxygen (25 ÷ 30 bar). The partial combustion at high temperature, 1200 ÷ 1500°C, provide the main part of heat to the process. This type of gasifier assure a fast conversion of the feedstock and results in high quality syngas. On the other hand, this process require a feedstock with low moisture content and small size, 0,1 ÷ 1 mm particles; thus, it may need a torrefaction-based pre-treatment. The advantages of the EF are: fuel flexibility, high carbon conversion and short reactor residence time.

Figure 6 - Entrained flow [9]

Bubbling fluidized bed (BFB)

The biomass is fed in the BFB, in which is mixed with an inert granular material (such as sand, silica, olivine or dolomite), and the gasification agent is blown upwards with a velocity of 1 ÷ 3 m/s in order to fluidize both fuel and bed material. This gasifier have a particular layout in order to optimize the heat transfer between the particles and obtain a better gasification process. The feedstock is inserted from the side and when the syngas is produced, comes out form the top. The temperature inside a BFB depends mainly on the melting point of the bed material and can vary from 800 to 900°C.

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12 Circulating fluidized bed (CFB)

The basic functioning of a CFB is rather similar to that of a BFB, but with higher gasification agent velocity, 3 ÷ 10 m/s. This velocity allows the fuel to be suspend in the whole gasifier and thus, the reactions of gasification occur everywhere. In addition, a cyclone is necessary to separate the syngas and the suspended particles.

Figure 8 - Circulating fluidized bed [9]

Dual Fluidized bed (Dual FB)

This system is composed by two chambers: a gasifier and a combustor. The feedstock is fed inside the gasifier, BFB or CFB, and the char generated there is burnt in air in the combustor. The products of the combustion are sent back into the gasifier in order to provide the necessary heat.

Figure 9 - Dual fluidized bed [9]

Plasma

Inside a Plasma reactor, the biomass can be fed without any pre-treatment, here an electrically generated plasma actuate an atomic degradation of the material. The temperature are usually really high, between 1500 and 5000°C. The quality of the resulted syngas is very good and the inorganic matter is vitrified into inert

slag. Figure 10 - Plasma

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13

Contaminants[10], [11]

The contaminants contained in the syngas are classified in six species and they can be in solid, liquid or gaseous state.

Figure 12 - Six species of contaminant contained in syngas. Readapted from [1]

 Particulate matter

The particulate is basically constituted by unconverted carbon in the form of char or soot, and inorganic compounds, generating from the mineral content of the feedstock such as K, Na, Ca, Fe, Mg. In addition, many types of additives or bed materials, used during the gasification, may slip out from the gasifier and these are classified as Rotary kiln reactor

It consist in a cylindrical chamber, in which the feedstock is fed from one side, syngas and slag are extracted from the other. The contact between the all solid matter and the gasses are allowed by the slow rotation of the chamber along its own axis. The gasification agent usually streams counter flow. This technology is not so effectiveness, but is really simple and is often use for the treatment of waste materials.

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14 particulate matter as well. The size of these particles are usually in a range of 1 ÷ 100 µm and its composition depends on both temperature and type of biomass used. The main problems caused by the particulate matter are fouling, erosion and corrosion in the downstream equipment.

 Tar

Tars are a complex mixture of condensable hydrocarbons, which can be represented with the general chemical formula 𝐶𝑥𝐻𝑦 or, in case of oxygenated organic compound, 𝐶_𝑥𝐻_𝑦𝑂_𝑧. They are formed mainly during the pyrolysis step and then, they may be involved in further reactions, recombination and/or condensation.

More details on tar characteristics, classification and formation will be given in the [2.3.2 Tar] section.

 Sulphur compounds

During the gasification, the sulphur contained in the biomass is converted into hydrogen sulphide (H2S), mainly, and carbonyl sulphide (COS). The main problems of sulphur compounds are corrosion of metal surfaces and poisoning of catalyst, especially nickel-based catalyst.

 Halides

Chlorides, formed during the gasification of alkali compounds contained in the biomass, react with steam, forming halides compounds such as HF, HBr and HCl. This last one is present in the highest concentration, because is the most stable at those temperature. Halides may corrode the metal surface and reacting with other species to form NH4Cl or NaCl which may cause deposits and fouling in the downstream equipment.

More details on hydrogen chloride will be given in the [2.3.3 HCl] section.  Alkali compounds

Biomass contains both alkali and alkaline earth metals that during the gasification may react with chlorine generating KCl, NaCl, ZnCl, etc. The melting point of some alkali is around 600°C and thus, they may be found as aerosol or in gaseous state, which leave the gasifier flowing out with syngas. They may generate corrosion in downstream equipment and so they must be removed to reach concentrations under few ppm.

 Nitrogen compounds

The main nitrogen compound is ammonia (NH3) and the limits of it in flue gas from power plants are fixed by civil regulations.

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15

Parameters that affect the syngas production and composition[12]  Gasifier type

In this section will be compared two type of fixed bed gasifier: updraft and downdraft.

Figure 13 - Comparison Updraft-downdraft gasifier

In Figure 13, taken from [www.fengyugroup.com/technician/index/mid/1_185], one can observe that the direction of the biomass flow is the same, from the top to the bottom, for both type of gasifier. Instead, the air and so the syngas extraction is different.

In the case of a downdraft layout, the air is injected from the top and then flows downstream to the bottom, where the syngas is extracted. Thus, the flow of all the gasses inside the gasifier is downwards. This means that the pyrolysis products have to pass through the oxidation zone. In particular, the tars generated during the pyrolysis undergo to reactions of thermal cracking:

𝐶𝑥𝐻𝑦 → 𝐶𝑥𝐻𝑦−𝑚+ 𝑚 2 𝐻2 → 𝐶𝑥−𝑛𝐻𝑦−𝑚+ 𝑚 2 𝐻2+ 𝑛𝐶 → ⋯ → 𝑦 2𝐻2+ 𝑥𝐶 Eq. 1 Thus, syngas from a downdraft gasifier has usually a low tar content, but on the other hand, the particulate matter is quite high, because part of the ash of the bottom part slip out with syngas.

On the other hand, in an updraft layout the syngas is extracted from the top of the gasifier and so there is much less dragging of ash with the gas, this means that the

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16 syngas has a lower particulate content. The Table 1 summarize the comparison between updraft and downdraft layout, according of contaminants production:

Contaminant in the syngas Updraft gasifier Downdraft gasifier

Tar High Low

Particulate matter Low High

Ash Low High

Table 1 - Contaminants level based on type of gasifier. Readapted form [11]

 Temperature

How temperature influences the syngas composition and production is rather complex. Indeed, the temperature effect the equilibrium of all the reaction that occur in a gasification process; for instance higher temperature means lower char and tar concentration in the syngas, higher production of hydrogen, higher overall gas yield. Moreover, according with Le Chatelier’s principle, higher temperature leads to higher generation of products in endothermic reactions and reactants in exothermic reactions. On the contrary, lower temperature favours the products in exothermic reactions and reactants in endothermic reactions. In addition, higher temperature favours the endothermic reaction of tar thermal-cracking Eq. 1. Thus, with a complete thermal decomposition of the tar there is formation of hydrogen and char.

 Biomass particle size

This aspect is generally less relevant in comparison with the others. For instance, a feedstock with larger particle size needs longer time for complete all the reactions, but at high temperature, this behaviour is negligible.

 Gasification agent

The gasification agents are usually air, pure oxygen, steam or a mixture of these. A parameter used to quantify the agent is the equivalence ratio (ER) and it is defined as follow:

𝐸𝑅 = 𝜙 =(𝐴 𝐹⁄ )𝑠𝑡𝑜𝑖𝑐 (𝐴 𝐹⁄ )

Where: 𝐴 = gasification agent 𝐹 = fuel

If the mixture is fuel rich, ER>1; if it is fuel lean, ER<1 and if it is stoichiometric, ER=1.

ER is an important parameter to determine the quality of the syngas, indeed higher ER generally leads to lower LHV, lower concentration of CO, higher concentration of

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17 CO2, lower char and tar yields. However, if ER is too high there could be too low concentration of fuel gasses such as H2 and CO.

 Catalyst or bed material

The presence of bed material, such as calcium oxide CaO, or catalyst in the gasifier may lead to decreasing concentration of tar in the produced gas, increase the total gas yield and hydrogen concentration.

2.3 Biomass – Integrated Gasification Fuel Cell system

A biomass – integrated gasification fuel cell (B – IGFC) system is generally composed by a) a gasifier that transform the biomass in order to obtain a mixture of gasses called syngas, b) a gas cleaning unit (GCU), in which the syngas is cleaned up from the unwanted impurities, and c) a fuel cell that convert the syngas directly into electricity.

A particular type of integrated system considered the use of a biomass updraft gasifier and a solid oxide fuel cell (SOFC), in which the syngas is fed after a cleaning in the GCU.

Figure 14 - Biomass Integrated Gasification Fuel Cell system

“The main technical challenge is the adjustment of the three main system components gasification, gas processing and fuel cell. A B-IGFC concept has been developed based on the findings that producer gas originating from the updraft gasification of wood can be electrochemically converted in a SOFC whereby tars are degraded to hydrogen and carbon monoxide and contribute to the electrochemical reactions and power generation” [13]. Generally, the B – IGFC systems are characterized by the following advantages:

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18  High electrical efficiency, even in small-scale power plant, basically by taking

advantages of intrinsically high efficiency of SOFCs and process integration [13];  Combined heat and power generation;

 “SOFCs in the IGFC cycle can be operated so as to isolate the carbon dioxide-rich anodic exhaust stream, allowing efficient carbon capture to address greenhouse gas emissions” [14]_.

The combined heat and power generation can be obtained with different configurations of the system:

 Burning part of the syngas, instead of feed all the syngas into the cell, using the generated heat to fed an heating system;

 Extract the heat directly from the hot exhaust gases from the cell, using a heat exchanger. Otherwise that heat can be used for pre-heated the inlet streams;

 Using a post-combustor that burn the fuel contained in the outlet anode flow and using that heat to generate steam. This steam can be used into an heating system network and/or to feed a steam turbine, as shown in the Figure 15 for an integrated coal gasification SOFC system:

Figure 15 - Coal IGFC system

[en.wikipedia.org/wiki/Integrated_gasification_fuel_cell_cycle]:

Furthermore, the integrated system has high efficiency by merging the advantages of solid oxide fuel cell with updraft gasification. Indeed, a SOFC can operate with a fuel different from hydrogen and thus, it can be fed with syngas generated from the gasification of biomass. Moreover, fuel cell can handle even the contaminants contained into the produced gases up to certain tolerance levels; a clean-up is anyway always necessary. The main advantages of an updraft gasifier can be briefly summarized in the next points:

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19  Simple construction and possibility to handle feedstock with high water content;

 High thermal efficiency due to high charcoal conversion and good internal heat exchange [15]. In addition, the product gas from an updraft gasifier contains a significant proportion of tars and hydrocarbons, contributing to its high heating value [15]_{, “which makes updraft gasifiers more suitable where heat is needed, for example in} industrial furnaces” [http://biomasspower.gov.in].

Overall, these integrated systems generate electric power and heat as though a common combined heat power (CHP) plant, but with the advantage that it does not require a heat engine and a generator to convert the fuel into electricity. Indeed, inside a heat engine, the chemical energy of the fuel, firstly, is converted in heat through the combustion, generating mechanical energy that is then converted into electrical energy through a generator. Instead, a fuel cell perform the electrochemical transformation directly from fuel to electric power, avoiding intermediate steps and thus, obtaining higher efficiency.

Note that, after the gasifier, a certain quantity of steam has to be added in the syngas stream in order to avoid the risk of carbon formation inside the GCU. Indeed, the cleaning unit is the place with the lowest temperature of the system and so where the risk of carbon deposition is higher. The steam injection is the most common method used to decrease this risk. When the gasification agent is steam, the water content in the produced syngas is higher and so the steam injected after the gasifier can be lower, with lower cost for the water evaporation and, thus, higher system efficiency.

In the following paragraphs, the three major components of the system are analysed in detail. A first paragraph describes the updraft gasifier and those contaminants this work focuses on, tars and HCl. Then a paragraph will be completely reserved to the fuel cell, with an important focus on SOFCs and the effect of the contaminants, contained in the syngas, on cells performance. Finally, the gas cleaning unit will be analysed and, section by section, will be explained how it works and how it reduces the contaminants concentration under certain limits.

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20 2.3.1 Updraft Gasifier [1], [11]

In an updraft layout, the biomass is inserted from the top and then streams downward through the gasification zones. In the first part it is heated up and dried, then it passes through pyrolysis, reduction and oxidation zones, in which it undergo into many reactions and transformations, as shown in Figure 16:

Figure 16 - Zones and reactions that occur inside an updraft gasifier [11]

The gasification agent, usually air, is injected from the bottom of the gasifier and flow upward. The produced mixture of gasses, extracted from the top, contains different types of contaminants, described in a previous paragraph, with concentrations that depends on some factors such as type of gasifier, temperature and equivalence ratio. Generally, syngas produced by an updraft gasifier is characterized by quite high tar content, low particulate matter. In Table 2 are summarized the main features of an updraft:

Updraft gasifier

Fuel size 5 ÷ 100 mm

Allowable fuel moisture < 60% Syngas temperature 200 ÷ 400°C Syngas LHV 5,5 ÷ 6 MJ/Nm3 Tar content 30 ÷ 150 g/Nm3 Particulate content 0,1 ÷ 3 g/Nm3 Reactor size 0,1 ÷ 20 MWth Thermal efficiency 90 ÷ 95%

Table 2 - Updraft gasifier characteristics [1]

One can observe that the updraft gasifier has a really high thermal efficiency, higher than a downdraft gasifier (85 ÷ 90%) [1]_{. The definition of thermal efficiency can be found} at [http://www.fao.org/docrep/t0512e/T0512e09.htm] and it is:

𝜂_{𝑡ℎ𝑒𝑟𝑚𝑎𝑙} = 𝑚̇𝑠𝑦𝑛𝑔𝑎𝑠∗ 𝐿𝐻𝑉𝑠𝑦𝑛𝑔𝑎𝑠 𝑚̇_{𝑏𝑖𝑜𝑚𝑎𝑠𝑠}∗ 𝐿𝐻𝑉_{𝑏𝑖𝑜𝑚𝑎𝑠𝑠}

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21 Where, 𝑚̇ is the mass flow rate

𝐿𝐻𝑉 is the lower heating value

Thus, considering the same feedstock inlet, a high 𝜂_{𝑡ℎ𝑒𝑟𝑚𝑎𝑙} can be due to either a high outlet syngas mass flow rate or an high LHV, which is higher, for instance, in case of high presence of hydrogen, methane or even hydrocarbons.

In the next paragraphs the six type of contaminants will be analyse more in details. In particular, one paragraph will be completely reserved for tars, with two specific focus on tar reforming and carbon deposition; one paragraph for hydrogen chloride and some short observations will be done of the alkali compounds. For the aim of this work the other three contaminants will not further analysed, because:

o it has been done the assumption that particulate matter and hydrogen chloride are both clean-up in the GCU, reduced with concentration lower than one ppm each. With this assumption the downstream equipment and applications, including the SOFC, are not affected by these two classes of compound;

o ammonia, the most abundant of the nitrogen compounds, is considered present with negligible concentration; although NH3 is a potential fuel for a solid oxide fuel cell, since it is dissociated into N2 and H2 at the anode.

2.3.2 Tar [8], [16], [17]

This paragraph will be totally focused on tars: their characteristics, classification, reforming reactions under which can undergo and finally, the problems that can generate, in particular carbon deposition.

Tars are a complex mixture of condensable hydrocarbons, formed during biomass gasification, mainly throughout pyrolysis phase and then, they may be involved in further reactions of recombination or decomposition inside the gasifier.

Due to their heterogeneous nature, tars have multiple definitions:

a) “All components of product gas having a molecular weight larger than that of benzene” (Knoef, 2005, p. 278). In agreement with the International Energy Agency (IEA) and the US Department of Energy (DOE).

b) “All the organic compounds that are present in the syngas, excluded the gaseous hydrocarbons, from C1 to C6” (European Committee for Standardization)

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22 c) “Tar is a complex mixture of condensable hydrocarbons, including, among others, oxygen-containing 1- to 5-ring aromatic, and complex polyaromatic hydrocarbons” (Devi et al., 2003)

Moreover, they can be classified according to two main ways:

1) Classification in five classes, from [16], (Knoef, 2005), (Monteiro et al 2007) Practically, it is based on the molecular weight of tar compounds.

# Type name Peculiarity Characteristic compounds

1 GC-undetectable Very heavy tars, cannot be detected by GC 2 Heterocyclic aromatics Tars containing hetero

atoms, highly water soluble compounds

Pyridine, phenol, cresols, quinoline, isoquinoline, dibenzophenol

3 Light aromatic (1 ring) Usually light

hydrocarbons with single ring; no problems

regarding condensability and solubility

Toluene, ethylbenzene, xylenes, styrene

4 Light PAH compounds (2-3 rings)

2 - 3 rings compounds; condense at low

temperature even at very low concentration Indene, naphthalene, methylnaphthalene, biphenyl, acenaphthalene, fluorene, phenanthrene, anthracene

5 Heavy PAH compounds (4-7 rings)

Larger than 3-ring, these components condense at high-temperature at low concentration

Fluoranthene, pyrene, chrysene, perylene, coronene

Table 3 - Tar classification (5 classes). Readapted form [8]

GC = Gas Chromatography

PAH = Polycyclic Aromatic Hydrocarbon

The GC-undetectable tars are determined by subtracting the GC-detectable tar fraction from total gravimetric tar, which is another tar analysis instead of GC. Gravimetric analysis is carried out evaporating the lighter compounds and then weighing the remained tar matter.

2) Classification in Primary, Secondary and Tertiary tars. (Milne and Evans, 1998)

Tar reactions of formation, decomposition and recombination can be simplified, as shown in Figure 17 below, with a mechanism based on three main phenomena, each one that lead to the formation of primary, secondary or tertiary tars.

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23

Figure 17 - Primary, Secondary and Tertiary tar [8]

Biomass, composed by lignin, cellulose and hemicellulose, is decomposed during the pyrolysis step, at temperatures up to 500°C. Here, cellulose and hemicellulose that are especially made of oxygen atoms, splitting up generate mainly oxygenated organic compounds such as methanol, acetaldehyde, acetic acid, hydroxypropanone, methyl furfural and creosol; and in lesser quantities aromatic compounds such as phenol, benzene and toluene. When primary tars pass through the oxidation zone, the higher temperature (500÷700°C) and the presence of the oxidant, they undergo to a rearrangement that lead to formation of the secondary tars. They go through reactions such as dehydration, decarboxylation or decarbonylation, generating alkylated mono- and di-aromatics including hetero-aromatics like pyridine, furan, dioxin, and thiophene. Tertiary tars, instead, are formed at temperature higher than 800°C as a further rearrangement of secondary tars. They are mainly aromatic and polynuclear aromatic hydrocarbons (PAHs) such as benzene, naphthalene, phenanthrene, pyrene, and benzopyrene. Thus, basically, depending on the different temperatures at which is exposed the tar matter, it will undergo into a certain transformation and so it will change its nature.

Tar transformations as a function of temperature are summarized in Table 4:

Mixed Oxygenates → Phenolic Ethers → Alkyl Phenolic → Heterocyclic Ethers → PAH → Larger PAH 400°C 500°C 600°C 700°C 800°C 900°C

Table 4 - Tar transformation. Readapted form [16]

Thus, as just explained, tar compounds are formed as primary tars in the pyrolysis zone and then, they may undergo to further rearrangement at higher temperatures forming secondary and tertiary tars. One can observe that this is why the type of gasifier used influences so deeply the concentration and nature of tars in the produced syngas.

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24 Indeed, tars in a syngas produced in updraft gasifier are present in large quantity and mainly primary tars, because, since the syngas is extract from the top, tars produced during pyrolysis will not undergo further reaction of neither recombination nor decomposition and thus, they flow out with the gaseous product. A further support for this theory come from [16], which presented a Molecular Beam Mass Spectrometry (MBMS) analysis of products from updraft and downdraft gasifiers. The MBMS results are reported in Figure 18 below:

Figure 18 - MBMS analysis of updraft and downdraft gasifiers [16]

Where, M/Z = mass to charge ratio; in which the M refers to the molecular or atomic mass number and Z to the charge number of the ion. Anyway, it is numerically related to the molecular weight: 𝑀/𝑍 ≃ 𝑚𝑜𝑙𝑒𝑐𝑢𝑙𝑎𝑟 𝑤𝑒𝑖𝑔ℎ𝑡 (𝑀𝑊) [𝑔/𝑚𝑜𝑙]

One can observe that:

- an updraft gasifier generates much more tars than a downdraft;

- tars from an updraft are mainly primary, thus with low values of molecular weight. In particular one can observe that the highest peak is for MW = 60, which correspond to acetic acid;

- tars from a downdraft are especially with an MW = 78, 92 and 128 that correspond to benzene, toluene and naphthalene. As expected, these three compounds belong to secondary or tertiary tars.

Furthermore, one can connect the two type of classification, indeed, primary tars correspond with the first class of the previous classification; secondary tars match with second and third classes and, finally, tertiary with fourth and fifth.

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25

Tar Reforming[18]

Reforming is a chemical process by which the molecular structure of a hydrocarbon is rearranged through an endothermic reaction with H2O, called steam reforming, or CO2, called dry reforming. The products can be hydrogen and carbon monoxide or carbon dioxide, thus, potential fuel for a solid oxide fuel cell. For instance, the steam reforming of methane is:

𝐶𝐻₄+ 𝐻₂𝑂 → 3𝐻₂+ 𝐶𝑂 Eq. 2

Instead, representing the hydrocarbons with the general chemical formula of 𝐶_𝑥𝐻_𝑦, or in case of oxygenated organic compound 𝐶𝑥𝐻𝑦𝑂𝑧, the general reactions of steam and dry reforming are the follow:

𝐶_𝑥𝐻_𝑦+ 𝑥𝐻₂𝑂 → (𝑥 +𝑦 2) 𝐻2+ 𝑥𝐶𝑂 Eq. 3 𝐶𝑥𝐻𝑦+ 𝑥𝐶𝑂2 → 𝑦 2𝐻2+ 2𝑥𝐶𝑂 Eq. 4

Furthermore, here below are reported the two steam reforming reactions, of a generic oxygenated organic compound, which may occur, generating either carbon monoxide or carbon dioxide: 𝐶𝑥𝐻𝑦𝑂𝑧+ (𝑥 − 𝑧)𝐻2𝑂 → (𝑥 − 𝑧 + 𝑦 2) 𝐻2+ 𝑥𝐶𝑂 Eq. 5 𝐶_𝑥𝐻_𝑦𝑂_𝑧+ (2𝑥 − 𝑧)𝐻₂𝑂 → (2𝑥 − 𝑧 +𝑦 2) 𝐻2 + 𝑥𝐶𝑂2 Eq. 6

Then, in particular, applying the reactions above to acetic acid, 𝐶𝐻₃𝐶𝑂𝑂𝐻 = 𝐶₂𝐻₄𝑂₂:

𝐶𝐻3𝐶𝑂𝑂𝐻 + 2𝐻2𝑂 → 4𝐻2+ 2𝐶𝑂2 Eq. 7

One can observe that, since the number of atoms of carbon and oxygen in the acetic acid molecule are the same (𝑥 = 𝑧 = 2), the steam reforming reaction cannot have as product carbon monoxide, because the number of moles cannot be balanced.

Another key fact to remember is that the reforming reactions are endothermic, thus they need to be supplied with heat and an high temperature (above 500°C) to goes on. Thus, SOFCs have the main characteristics to allow reforming reactions inside the cell itself: a) high temperature, b) a nickel catalyst, in order to reduce the activation energy required, c) presence of steam and/or carbon dioxide. The internal reforming can be actuated following two approaches: direct (DIR) or indirect internal reforming (IIR), as shown in Figure 19:

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Figure 19 - Internal reforming, direct or indirect. Readapted from [18]

The IIR approach consider a reforming chamber positioned as close as possible to the cell, but still separated from it; this system assure only the thermal connection between reforming and cell. Instead, with the direct approach, reforming reactions are carried out within the anode chamber with the advantages of good heat transfer and chemical integration between all the reactions. Indeed, the reforming reactions can use the “waste” heat from the exothermic reaction of oxidation, and moreover, contribute to reduce/control the temperature of the cell itself. The advantages of an internal reforming are briefly summarized in the next points:

 lower costs, because is not needed a separate external reformer;  higher system efficiency, typically > 50%;

 the steam produced during cell operation is used for steam reforming, which lead to less steam required;

 a DIR approach assure a more even distribution of hydrogen along the anode surface and consequently to more homogenous temperature.

In conclusion, one of the major advantages of high temperature fuel cells is their ability to use other fuels than hydrogen. Indeed, the internal reforming of hydrocarbons lead to several benefits: availability of additional fuel (H2 and CO), lower request of thermal removal, higher efficiency and lower costs linked with absence of an external reformer.

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Carbon Deposition[19], [20]

The risk of carbon deposition is one of the most critical issues in a SOFC. The presence of carbonaceous compounds in syngas, such as CO, CO2 and CH4, plus the presence of tars, lead to carbon formation, which may nucleate into solid state, deposit and accumulate in any zone of the system where there are the favourable conditions. Carbon formation may take place by different routes:

1) Whisker carbon

These type of carbons are formed by a) cracking decomposition of carbonaceous species, b) carbon monoxide reduction, and c) carbon monoxide decomposition, the last one through Boudouard reaction, as shown below.

𝐶_𝑥𝐻_𝑦 → ⋯ → 𝑥𝐶(𝑠)+ 𝑦

2𝐻2 Eq. 8

𝐶𝑂 + 𝐻₂ → 𝐶(𝑠)+ 𝐻₂𝑂 Eq. 9

2𝐶𝑂 → 𝐶(𝑠)+ 𝐶𝑂₂ Eq. 10

Whisker carbon represent the main issue for Ni-based catalysts, because they grow on nickel, forming graphene tubes that may result in a breakdown of the catalyst material. Indeed, through Boudouard reaction, CO is adsorbed on the Ni catalyst and carbon migrates to the back of the Ni particle forming filaments or whiskers carrying Ni particles on the top. The reverse process, gasification, is also possible. The filament shrinks until Ni reintegrates on the support. Depending on the severity of the carbon deposition, the Ni particle can be completely removed during the accumulation or the gasification process. The whiskers are made of pure carbon.

In case of reforming reaction, the mechanism is similar to what happens during Boudouard reaction, the hydrocarbon is adsorbed on the catalyst. The C-H bonds are broken until only C remains. The C can migrate to the back of the particle resulting in the formation of C whiskers.

With the cracking reaction, the hydrocarbons (including CH4) might undergo cracking which results in the formation of pure C. This carbon is sometimes named graphitic carbon and less frequently coke or pyrolytic carbon. This is pure carbon powder that deposits on the surfaces and on the catalyst as well.

2) Gum formation