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A global kinetic model for the pyrolysis of pretreated second generation biofuels

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Index

1. INTRODUCTION ... 8

1.1 Biomasses as energy sources and the thermochemical conversion ... 8

1.2 Thermochemical processes ... 9

1.2.1 Biomass Combustion ... 9

1.2.2 Co-firing ... 11

1.2.3 Gasification ... 12

1.2.4 Pyrolysis ... 15

1.3 Thermochemical conversion reactors ... 18

1.3.1 Fixed bed reactors ... 18

1.3.2 Fluidized bed reactors ... 18

1.3.3 Entrained flow reactors ... 18

1.4 Technical Issues ... 19

1.5 Pretreatments ... 19

1.6 The need for modeling and the BRISK project ... 20

2. METHODOLOGIES FOR BIOFUEL CHARACTERIZATION ... 21

2.1 Preliminary analyses ... 21

2.2 Advanced analyses ... 22

2.2.1 Low and medium heating rates facilities ... 23

2.2.2 High heating rate facilities... 25

3. DEVOLATILIZATION KINETIC MODELS FOR BIOMASSES ... 33

3.1 Biomass devolatilization ... 33

3.2 Classification of devolatilization models ... 34

3.3 Classification of kinetic models ... 35

3.3.1 Single First Order Reaction (SFOR) Model ... 36

3.3.2 The ‘Isoconversional’ Methods ... 37

3.3.3 The Kobayashi Model ... 38

3.4 Application of a two-steps model to non-isothermal runs ... 40

4. EXPERIMENTAL RESULTS AND KINETIC PARAMETERS ... 43

4.1 Tested biofuels ... 43

4.2 Low Heating Rate Kinetics ... 44

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4.2.2 Determination of Conversion ... 45

4.2.3 Application of Single First Reaction Order model ... 45

4.2.4 Application of the Friedman Isoconversional method ... 49

4.2.5 Two-steps model for low heating rate non-isothermal runs ... 50

4.3 High Heating Rate Kinetics ... 52

4.3.1 Determination of conversion and Pyrolysis tests with the IPFR ... 53

4.3.2 Application of SFOR model to HHR isothermal runs ... 54

4.3.3 Two-steps model for HHR and isothermal runs ... 58

4.4 ‘Global’ Kinetic Models ... 59

4.4.1 Global SFOR Model ... 59

4.4.2 Global Two-Steps Kinetic Model ... 61

4.5 Remarks on the model results ... 62

5. UNCERTAINTY QUANTIFICATION AND RELIABILITY OF THE MODELS ... 64

5.1 Uncertainty deriving from the determination of the ash content and the ash loss ... 64

5.2 Errors on the particles’ residence time ... 69

5.3 The effective thermal history ... 69

5.4 Effect of the uncertainty on the kinetic parameters ... 71

6. REMARKS ON THE RESULTS ... 77

6.1 Preliminary fingerprinting ... 77

6.2 The volatile matter content ... 78

6.3 The effect of the pretreatments and the torrefaction degree ... 82

6.4 Gaseous species in volatiles ... 90

7. CONCLUSIONS ... 95

7.1 The need for qualified dataset ... 95

7.2 Further developments ... 96

REFERENCES ... 97

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Index of Tables

Table 1.1 Gasification reactions ... 12

Table 1.2 Volatiles species dependency on gasification agent ... 14

Table 4.1Analysis of the considered biomasses ... 43

Table 4.2 SFOR kinetic parameters optimized with a single heating rate dataset ... 46

Table 4.3 SFOR kinetic parameters optimized with three heating rate dataset ... 47

Table 4.4Intrinsic kinetic parameters resulting from the Friedman method applied to biomasses .... 50

Table 4.5 Kobayashi model kinetic parameters resulting with α2 = HTVM approximation ... 52

Table 4.6 Kobayashi model kinetic parameters resulting with α2 = 1 approximation ... 52

Table 4.7 Considered biomasses size fraction ... 54

Tables 4.8 Grids for the devolatilization tests performed on the considered biomasses in the IPFR .. 55

Table 4.9 SFOR kinetic parameters applied to high heating rate dataset ... 56

Table 4.10 SFOR kinetic parameters applied to high heating rate dataset with particle heating approximation ... 57

Table 4.11Kobayashi two step model kinetic parameters with particle heating approximation ... 58

Table 4.12’Global’ SFOR kinetic parameters ... 61

Table 4.13’Global’two-step kinetic model kinetic parameters ... 62

Table 5.1Initial ash content of the considered biomasses and uncertainty ... 65

Table 5.2 Nominal activation energy of the SFOR model and uncertainty deriving from uncertainty on conversion ... 73

Table 5.3 Nominal activation energy of the SFOR model and uncertainty deriving from uncertainty on residence time ... 74

Table 6.1Comparison between standard temperature and high temperature volatile matter yields . 79 Table 6.2 Peak temperature of the considered biomasses at different heating rate ... 85

Table 6.3Intrinsic kinetic parameters obtained for the considered pretreated biofuels ... 86

Table 6.4 Kobayashi model kinetic parameters of the PKS and the torrefied biomasses ... 89

Table 6.5Analysis of solid residues of devolatilization tests on the considered biomasses at different residence time and temperature ... 90

Table 6.6 Gas species content (daf volumetric fraction) resulting from the Gibbs energy equilibrium for the devolatilization of the considered biomasses ... 93

Table 6.7 Gas species content (daf volumetric fraction) experimentally detected during torrefied biomasses devolatilization test ... 94

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Index of Figures

Figure 1.1 Combustion phases ... 9

Figure 1.2 Product gas composition dependency on temperature ... 13

Figure 1.3 Dependency of the cold gas efficiency to steam-fuel ratio ... 15

Figure 1.4 Dependency of the pyrolysis products mass fraction on temperature ... 16

Figure 1.5 Dependency of char yield mass fraction on temperature ... 17

Figure 2.1(a) Thermogravimetric balance, (b) pressurized thermogravimetric balance ... 23

Figure 2.2 Pressurized wire mesh reactor ... 24

Figure 2.3(a) Lab-scale fixed bed reactor,(b) Curie point pyrolyzer ... 25

Figure 2.4(a) Fluidized bed reactor, (b) schematic drop tube ... 26

Figure 2.5 Isothermal Plug Flow Reactor: layout (left) and characteristics (right) ... 27

Figure 2.6 Pressurized Isothermal Entrained Flow Reactor: layout (left) and characteristics (right) ... 30

Figure 2.7 Baby Isothermal Entrained Flow Reactor ... 31

Figure 3.1Multi-component decomposition scheme ... 35

Figure 3.2 Kobayashi two-step devolatilization model ... 39

Figure 4.1Weight loss, derivative weight loss and temperature as time functions ... 44

Figure 4.2HTC conversion runs at different heating rates ... 45

Figure 4.3Comparison of experimental results and SFOR model predictions with a single HR dataset47 Figure 4.4 Comparison of experimental results and SFOR model predictions with three HR dataset . 48 Figure 4.5 SFOR model predictions at 10K /min with three HR dataset approximation ... 48

Figure 4.6 SFOR model predictions at 40 K/min with three HR dataset approximation ... 48

Figure 4.7Activation energy as a function of conversion resulting from the Friedman method applied to biomasses ... 49

Figure 4.8 Logarithm of the frequency factor as a function of conversion resulting from the Friedman method applied to biomasses ... 49

Figure 4.9 Comparison of experimental results at 20 K/min and Kobayashi two-step model predictions with α2 = HTVM ... 51

Figure 4.10 Comparison of experimental results at 20 K/min and Kobayashi two-step model predictions with α2 = 1 ... 51

Figure 4.11Comparison of experimental results and SFOR model prediction for Lignin conversion at high heating rate and two temperature levels ... 56

Figure 4.12 Comparison of experimental results and SFOR model prediction for Lignin conversion at high heating rate and two temperature levels with particle heating approximation ... 57

Figure 4.13Comparison of experimental results and Kobayashi model prediction for Lignin conversion at high heating rate and two temperature levels with particle heating approximation ... 59

Figure 4.14 Comparison among experimental run at 10 K/min, the SFOR and the’global’ SFOR predictions... 60

Figure 4.15 Comparison among experimental run at 20 K/min, the SFOR and the’global’ SFOR predictions... 60

Figure 4.16 Comparison among experimental run at 40 K/min, the SFOR and the’global’ SFOR predictions... 61

Figure 4.17Arrhenius plot for decomposition rate based on mean intrinsic parameters at low heating rate ... 63

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5 Figure 4.18 Arrhenius plot for decomposition rate based on SFOR kinetic parameters at high heating

rate ... 63

Figure 5.1Uncertainty on conversion due to the uncertainty on the ash content ... 66

Figure 5.2Ash loss phenomena ... 66

Figure 5.3 Uncertainty on conversion due to the uncertainty on ash content and to the ash loss ... 68

Figure 5.4Comparison of the effective themal history of two particles in a drop tube with the nominal temperature ... 71

Figure 5.5 Comparison among the conversion error bars and the range of reliability of the SFOR model ... 72

Figure 5.6 Comparison among the residence time error bars and the range of reliability of the SFOR model ... 73

Figure 5.7 Range of reliability of the SFOR model deriving from the maximum uncertainty ... 74

Figure 5.8 Arrhenius plot for the global decomposition rate of wood/biomass based on one-component mechanism reported in the work of Di Blasi (left) and obtained in this work (right) ... 75

Figure 6.1 Van Krevelen diagram ... 77

Figure 6.2 Van Krevelen diagram for the considered biofuels ... 78

Figure 6.3 Weigh loss rate as a temperature function at different heating rate ... 78

Figure 6.4 HTVM yields of different biofuels with varying volatile matter content ... 79

Figure 6.5 Weight loss trends during devolatilization of three biomasses at 1173 K ... 80

Figure 6.6 Weight loss trends during devolatilization of three biomasses at 1473 K ... 80

Figure 6.7 Weight loss trends during devolatilization of HTC at different pressure and at 1473 K... 81

Figure 6.8 Weight loss trends during oxygen gasification of HTC at different pressure and at 1473 K 81 Figure 6.9 Influence of pressure on the volatile yield of different biofuels in a pressurized wire mesh reactor (Tremel et al.2012) ... 82

Figure 6.10 Weight loss runs of the PKS and the torrefied materials at 20 K/min as temperature function ... 83

Figure 6.11Derivative weight loss of the PKS and the torrefied materials at 10 K/min as temperature function and the peak temperature ... 84

Figure 6.12 Derivative weight loss of the PKS and the torrefied materials at 20 K/min as temperature function and the peak temperature ... 84

Figure 6.13 Derivative weight loss of the PKS and the torrefied materials at 40 K/min as temperature function and the peak temperature ... 85

Figure 6.14Arrhenius plot for the decomposition rate of the PKS and the torrefied material based on mean intrinsic kinetic parameters ... 86

Figure 6.15 Weight loss trends during devolatilization of torrefied biomasses at 773 K ... 87

Figure 6.16 Weight loss trends during devolatilization of torrefied biomasses at 973 K ... 87

Figure 6.17 Weight loss trends during devolatilization of PKS and the torrefied biomasses at 1173 K 87 Figure 6.18 Comparison among the STVM and the HTVM yields of the PKS and the torrefied biomasses ... 88

Figure 6.19 Arrhenius plot for the decomposition rate at high heating rate of the PKS and the torrefied biomasses based on a SFOR model... 88

Figure 6.20 Decomposition rate of the torrefied biomasses trends as functions of HTVM ... 89

Figure 6.21 Lignin devolatilization products elemental composition (daf mass fraction) evolution in time at 1173K (left) and at 1473K (right) ... 91

Figure 6.22 PKS and torrefied biomasses devolatilization products elemental composition (daf mass fraction) at 1173K ... 92

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6 Figure 6.23 Gas species content (daf volumetric fraction) runs as a function of temperature resulting from the Gibbs energy equilibrium for the devolatilization of lignin at 1173 K and 30 ms ... 92 Figure 7.1Comparison among the mean value of the kinetic parameters obtained at three different heating rate ... 95 Figure 7.2 Decomposition rate trends as function of the HTVM for all the biomasses at 1173 K (left) and for the torrefied biomasses at three temperatures (right) ... 96

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1. INTRODUCTION

In the most recent energetic framework, characterized by the increasing power demand and the possible fossil fuels depletion, the attention paid to the biomasses as energy sources can be explained by mean of the benefits offered. E.g., the opportunity to re-enter in a production cycle an agricultural or food-industry 'residual' allows obtaining lower economic impact in the energy production cycle. Moreover the life-cycle assessments show an almost closed CO2 balance and, consequently, a lower environmental impact.

There are many ways to better express biomasses energetic potential and mainly the thermochemical conversion processes allow integrating the use of biomasses in high efficiency energetic systems (e.g., IGCC). Nevertheless, technical enhancements in the contribution of biomass to commercial energy needs are focused on improving both the efficiency and environmental impacts of conversion processes and to this aim mathematical modeling is fundamental and requires studying biomasses characteristics and phenomena occurring during thermochemical conversion processes.

1.1 Biomasses as energy sources and the thermochemical conversion

In the meaning of organic material deriving from plants, biomass can be regarded as a mixture of polymers combined with a small fraction of minerals. The numerous different polymers present in the organic fraction of the fuel are generally divided in cellulose, hemicellulose and lignin whose mass fraction may varies determining different biomass categories.

By considering the characteristic elemental composition biomasses show a very low energetic density so that several processes have been developed aiming to increase this value. Processes can be classified on the basis of the conversion mechanisms and are:

- Biological (mainly fermentation and anaerobic digestion) consist in bio-chemical transformation of the biomasses structures;

- Physical consist in mechanical transformation aiming to extract the oils useful as fuels; - Thermochemical consist in physical and chemical reactions occurring by providing heat

allowing to obtain different forms of energy from matter.

By considering that the technical solutions available to convert biofuels into electrical or thermal energy are similar to those using fossil fuels, from which they have been developed by taking into account the characteristic differences among the material, thermochemical conversion represents the most reliable technology in terms of energy efficiency.

Biomasses to be thermally converted are woody and cellulosic products or residues with a low content of humidity (less than 30%) and values for the C/N ratio over 30. Wood and derivatives (sawdust, chips, etc…), lingo-cellulosic cultivation by-products (straw, pruning, etc…) and some processing waste (husks, shells, kernels, etc…) are generally used in thermal plants after several pretreatments aiming to obtain the most suitable characteristics to be processed.

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9 Hence, the need for studying the biofuel characteristics and behavior during thermochemical processes is fundamental to understand the phenomena involved and overcome the technical issues by modeling new solutions.

1.2 Thermochemical processes

By considering the operating conditions and the obtained products different thermochemical processes are available to be integrated in several energy production systems.

1.2.1 Biomass Combustion

Combustion is the most important and mature technology available nowadays for direct biomass utilization although improvements with respect to efficiency, emissions and cost are needed for further exploitation.

Figure 1.1 Combustion phases

Biomass combustion is a complex process that consists of consecutive heterogeneous and homogeneous reactions. The main process steps, represented in Fig. 1.1, are the following:

- Heating and drying. The first phase in the combustion process is the heat up phase. During this phase the particle temperature rises to 100 ° C and moisture evaporates from the particle. At this temperature the particle is assumed to be chemically inert and no chemical source terms are present.

- Devolatilization. When the moisture is evaporated the particle temperature rises further up to 200 – 300 ° C and the devolatilization process starts. During the devolatilization or pyrolysis process gas and tar species leave the solid matrix of the particle. At the end of this stage a very porous char structure is left of the particle. Volatile fractions of the fuel can be in the order of 90 mass percent on dry bases. During this phase the particle is heated further by transport of external heat. In literature a heat of reaction ranging from slightly exothermic to endothermic, depending on experimental setup and biomass source, can be found. Although,

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10 the pyrolysis process itself is usually regarded as endothermic, often a value of -418 kJ/kg is used in literature [3].

- Volatile combustion. The volatiles released during the pyrolysis process leave through the pores of the particle and mix with the surrounding air where they, in case of an oxidizing environment, further react to CO2 and H2O.

- Char burnout. Char oxidation starts when oxygen from the surrounding atmosphere reaches the char formed in the particle. In a heterogeneous surface reaction the fixed carbon is consumed by oxygen, steam or carbon dioxide when all char is converted, the solid residue completely consists out of ash.

The time used for each reaction depends on the fuel size and properties, on temperature, and on combustion conditions. Batch combustion of a small particle shows a distinct separation between a volatile and a char combustion phase with time.

For the design of combustion appliances, the high content of volatiles (80% to 85%) needs to be respected. For large particles, the phases overlap to a certain extent. Nevertheless, even for log wood furnaces, a certain separation of distinct combustion regimes with time can be found.

Since automatic combustion systems are operated continuously, the consecutive reactions occur simultaneously at different places in the furnace (e.g., in different sections on and above a grate). Hence the zones for different process steps during combustion can be optimized by furnace design. A distinct separation of different process steps can be advantageous with respect to pollutant formation.

The main combustion parameter is the excess air ratio (λ) that describes the ratio between the locally available and the stoichiometric amount of combustion air. For typical biomass, the combustion reaction can then be described by the following equation if fuel constituents such as N, K, Cl, etc., are neglected:

where describes the average composition of typical biomass used for combustion.

Indeed, combustion can be applied for biomass feedstocks with water contents up to a maximum 60% [Nussbaumer, 2003]. Fuel constituents beside C, H and O are undesired since they are related to pollutant and deposit formation, corrosion and ash. The most relevant constituents in native biomass are nitrogen as a source of NOx and ash components (e.g., K and Cl as a source of KCl) that lead to particulate emissions. Native wood is usually the most favorable biofuel for combustion due to its low content of ash and nitrogen. Herbaceous biomass such straw, mischantus, switch grass, etc., have higher contents of N, S, K, Cl, etc. that lead to higher emissions of NOx and particulates, increased ash, corrosion and deposits. While wood is as well suited for household heating as for

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11 larger plants, herbaceous biomass is dedicated for larger plants. The same is true for urban waste wood and demolition wood. The combustion of such contaminated biomass should be strictly limited to combustion plants with efficient flue gas cleaning for the abatement of toxic pollutants such as heavy metals and chlorine compounds.

1.2.2 Co-firing

A co-utilization of biomass with other fuels can be advantageous with regard to cost, efficiency and emissions. Lower specific cost and higher efficiencies of large plants can be utilized for biomass and emissions of SOx and NOx can be reduced by co-firing. However, attention must be paid to increased deposit formation in the boiler and limitations in ash utilization due to constituents in biomass, especially alkali metals that may disable the use of ash in buildings materials. Due to undesired changes of ash compositions, the share of biomass is usually up to a limit of 15% of the fuel input.

Different options can be adopted for co-utilization of biomass with coal with different plant solutions:

- direct co-firing. The biomass is directly fed to the boiler furnace (fluidized bed, grate or pulverized combustion), if needed after physical preprocessing of the biomass such as drying, grinding or metal removal, by mixing with the coal or by using separated feeding systems. - indirect co-firing or parallel combustion. The biomass is gasified and the product gas is fed to

a boiler furnace or is burnt in a separate boiler for steam generation and the steam is used in a power plant together with the main fuel.

The former system can have several effects on the emissions and the plant operation: positive effects are that SOx and NOx emissions usually decrease due to the lower sulfur and nitrogen content in biomass than in coal. Furthermore, alkali components in biomass ash can have an effect of SOx removal. Since biomass has a high volatile content, it can also be used as reburnt fuel for NOx reduction from the coal combustion, which gives a further potential for significant decrease of the NOx emissions. Negative effects of co-firing are additional investment cost for biomass pretreatment and boiler retrofitting, higher operation cost due to increased fouling and corrosion, and a possible decrease of the electric efficiency (if the superheater temperature has to be decreased due to high temperature corrosion). Besides potential poisoning of SCR catalyst also the efficiency of electrostatic precipitators may be reduced. Furthermore, the utilization of the ash and the residues from the flue gas cleaning system (especially the De-SOx installation) has to be considered when co-firing biomass.

Parallel combustion enables a complete separation of the ashes and flue gases from different fuels such as biomass and coal. Hence, no disadvantages or limitations result from undesired alkali metals or contaminants in the ash. Further, the flue gas cleaning equipment can be optimized for each fuel. Indirect co-firing of producer gas from biomass gasification also enables the separation of the ashes to a certain extent, while the flue gases cannot be separated. In comparison to parallel combustion, investment cost can be reduced because only one boiler and flue gas cleaning are needed.

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1.2.3 Gasification

For the energy and industrial applications, thermal gasification is a promising technology. Solid feedstock is converted to a synthesis gas that can be used as the feed material for both chemical synthesis and power production.

Gasification can be defined as a controlled thermal decomposition of an organic material producing a fuel gas and an inert residue. The process consists in the rupture of the macromolecular bonds yielding to a widely lower molecular weight product.

Gasification generally occurs after a drying and a pyrolysis phases that consists in an endothermic demolition of bonds producing permanent gases, pyrolytic liquid (bio-oil/tar) and solid residue constituted by porous carbon often polluted with mineral fractions, hydrogen and oxygen (char). The main gasification reactions are reported in the Table 1.1.

Primary tar ( ) Carbon Secondary tar -242 H2 - Combustion (oxidation) -283 CO - Combustion (oxidation) -110 CH4 - Combustion (oxidation)

+247 Dry reforming reaction

+206 Steam reforming methanisation

-40,9 Water-gas-shift reaction

-123,1 Partial oxidation

-393,5 Oxidation of carbon

+118,5 Water gas reaction (steam reforming)

+159,9 Boudoard equilibrium

-87,5 Methane production reaction

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13 During the process several consecutive and/or parallel, endothermic or exothermic chemical reactions occur being influenced by:

- temperature and pressure, - gasification reactor type, - fluid-dynamic regime, - gasification agent, - fuel.

The gasification temperature is a promoting factor (except for the conversion reaction of carbon oxide in hydrogen) allowing a complete conversion to a synthesis gas and the tar thermal cracking around 1200 °C. However, even at a lower temperature of 1200 °C, partial melting of the biomass ash components cannot be avoided leading to stickiness and the blockage of the gasifier with a decreasing in the energy efficiency of the process. Generally the process is controlled in the temperature range of 800-900 °C.

As known from literature (TU Wien – Institute of Chemical Engineering. Test Rig and Pilot Plant. 2011), the gas composition depends mainly on the used fuel, on the temperature and on the gasification agent. The diagrams in Fig. 1.2 show the dependency of the gas composition on temperature:

Figure 1.2 Product gas composition dependency on temperature

It can be seen that with increasing temperature the hydrogen and carbon monoxide concentrations are increasing and the carbon dioxide and methane concentrations are decreasing with increasing temperature. The reasons for these dependencies are that the reactions at higher temperatures are faster and the gas composition is nearer to equilibrium.

As for the gas composition as a function of the gasifying agent, typical values are listed in the Table 1.2 (Burberi et al., 2008).

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14 CO [%] CO2 [%] H2 [%] CH4 [%] N2 [%]

Air 12-15 14-17 9-10 2-4 56-59

Oxygen 30-37 25-25 30-34 4-6 2-5

Steam 32-41 17-19 24-26 12-14 2-5

Table 1.2 Volatiles species dependency on gasification agent

Hence the produced fuel gas that can be classified into three categories depending on the gasifying agent that is used in the process, which may be air, pure oxygen or a mixture of oxygen and steam (Jil et al. 1999):

- low heating value gas : gasification occurs with combustion air and no enrichment with oxygen so that the produced gas is diluted, especially for the presence of inert nitrogen since the heating value is around 11MJ/Nm3;

- medium heating value gas: obtained recurring to oxygen and the heating value is in the range of 12-26 MJ/Nm3;

- high heating value gas: obtained recurring to steam and oxygen from the previous medium heating rate gas recurring to catalytic conversion process of CO in CO2 (shift reaction) and subsequent carbon dioxide removal by mean of a lavage. A further catalytic methanisation allows obtaining a gas with a heating value in the range of 35-41 MJ/ Nm3 .

Syngas can be also characterized by contaminants that could damage or influence the subsequent treatments and are to be reduced in concentration to respect the regulatory limits. The contaminants can be gaseous (NOx, HCl, etc.), carboneous condensable compounds (TAR), heavy metals, alkali and dusts. Tar condenses at reduced temperature, thus blocking and fouling process equipments such as engines and turbines so that considerable efforts have been directed on tar removal from fuel gas.

The level of depuration requested depends on the final use of the fuel gas and it can be stated that:

- dust removal is sufficient using the gas as a reducing in industrial processes or in furnaces for the production of cement or bricks;

- a higher level of depuration is requested using the syngas in internal combustion volumetric engine or in gas turbine for electric power generation. Tar removal technologies can broadly be divided into two approaches; hot gas cleaning after the gasifier (secondary methods), and treatments inside the gasifier (primary methods). Although secondary methods are proven to be effective, treatments inside the gasifier are gaining much attention as these may eliminate the need for downstream cleanup. In primary treatment, the gasifier is optimized to produce a fuel gas with minimum tar concentration. The different approaches of primary treatment are a proper selection of operating parameters, the use of bed additive/catalyst, and gasifier modifications (Devi et al. 2003)

By considering the external energy providing the gasification can be:

- direct: the chemical oxidation reactions provide the energy necessary to keep the process temperature;

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15 - indirect: the gasification process using steam and oxygen needs external energy but the

heating value of the output product is higher since the hydrogen concentration is higher.

Figure 1.3 Dependency of the cold gas efficiency to steam-fuel ratio

Hence, the gasification process efficiency can be evaluated by mean of the ratio between the chemical energy content in the product gas compared to the chemical energy in the fuel (based on the lower heating value) and it can be stated that it depends strongly on the temperature and the gasification agent-fuel ratio. In Fig. 1.3 it is shown the cold gas-efficiency depending on the steam-fuel ratio (TU Wien – Institute of Chemical Engineering. Test Rig and Pilot Plant. 2011)

1.2.4 Pyrolysis

Pyrolysis, consisting of solid thermal degradation occurring in the temperature range of 400 °C and 800 °C in the absence of oxidizing agents, is a possible thermochemical conversion route, resulting in the production of a huge number of chemical compounds. However, for engineering applications, reaction products are often lumped into three groups: permanent gases (syngas), a pyrolytic liquid (bio-oil/tar) and solid residue (char). The components fractions depend on the process and above all on the reaction parameters.

The pyrolysis process is complex but the most accepted theory considers that first forming syngas, char and primary vapors, the characteristics of which are mainly influenced by the heating rate. Primary vapors then degrade the syngas and secondary tar if kept at high temperatures for a long enough time to do the secondary reactions occur.

The chemical reactions are similar to those preceding the gasification and the main chemical process can be outlined according to the following reaction:

The steam reforming of methane with the other produced hydrocarbons and the water-gas shift reactions allow maximizing the hydrogen content obtaining a higher heating value. The other

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16 products can be treated to increase the heating value of the flue by separating the water soluble fraction from the insoluble and by treating the former furtherly whereas the latter is used in various commercial applications. As a process output there can be inorganic salts, such as chlorides and carbonates, which can produce positive effects on the kinetics of pyrolysis.

Figure 1.4 Dependency of the pyrolysis products mass fraction on temperature

The pyrolysis process can be classified in respect of the temperature and the reaction time in:

- slow pyrolysis is used to maximize the production of the char that comes to constitute up to 50% on the weight of residues since the process is characterized by low temperature (< 500 °C) and long reaction times;

- flash pyrolysis is used to maximize the production of the liquid fraction that comes to constitute up to 80% on weight since the process temperature is relatively low (500-650°C) with high reaction rates and reaction times shorter than 1 s;

- fast pyrolysis at relatively high temperature (> 650 °C) and short residence times, gas production is maximized up to 80% on the weight;

- conventional pyrolysis at moderate temperature (below 600 °C) and residence times produces comparable yields of the three lumped classes of products.

- carbonization process at low reaction rate char production is maximized whereas gas and liquid are sub-products used to thermal energy source to the process.

An example of the dependency of the pyrolysis products mass fraction on the operating parameters is given in Fig. 1.4 that shows the output mass as a function of temperature.

It can be noted that more severe conditions take to a higher gas production making the pyrolysis similar to gasification.

Beyond the temperature, also particle dimensions can influence the resulting char content as it is possible to see in Fig. 1.5 (Overend, 2004).

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Figure 1.5 Dependency of char yield mass fraction on temperature

The applications of the products are listed below:

- the syngas is similar to that deriving from the gasification process since it is a mixture of hydrogen, methane, ethane, nitrogen oxides and other light hydrocarbons. It is generally used in energy recovery cycles depending on the heating value that varies with the input material and the operating conditions;

- the bio-oil has as much energetic applications as chemical applications since it can be used in co-firing with coal, in co-gasification, in thermo-electric plants combustion or it can be used for the production of chemicals (Overend, 2004),(Zhang et al., 2013), and it can be easily stored and transported. Fast pyrolysis has now achieved a commercial success for production of chemicals and is being actively developed for producing liquid fuels. Bio-oils have been successfully tested in engines, turbines, and boilers, and have been upgraded to high-quality hydrocarbon fuels, although at a presently unacceptable energetic and financial cost

- the char product is useful as a renewable fuel since it has many attractive features:  it contains virtually no sulfur or mercury and is low in nitrogen and ash; it is highly reactive yet easy to store and handle. It can be used for other applications such as metal reductant, soil amender and the production of activated carbon with a large surface area and biocarbon electrodes since it can be a semimetal with an electrical resistivity comparable to that of graphite (Antal et al., 2003).

The enhancements in the comprehension of the pyrolysis are focused on the study of products composition to a better application for energy needs or chemical uses.

Among the mentioned thermal conversion processes, for the energy and industrial application thermal gasification is a promising technology since the obtainable products can be used also in a poly-generation plant, e.g. IGCC, where the CO2 abatement is simpler and the electricity output is adaptable to the present power demand.

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1.3 Thermochemical conversion reactors

Beyond the traditional combustion systems, reactor designs for pyrolysis and gasification have been researched for more than a century, which has resulted in the availability of several designs at the small and large scales. They can be classified in several ways and by considering the reactor design there are: fixed-bed (updraft, downdraft, cross-draft and open-core), fluidized-bed (bubbling, circulating and twin-bed), entrained-flow, stage gasification with physical separation of pyrolysis, oxidation and/or reduction zones.

1.3.1 Fixed bed reactors

The fixed-bed gasifier has a bed of solid fuel particles through which the gasifying media and gas either move up (updraft), move down (downdraft) or are introduced from one side of the reactor and are released from the other side on the same horizontal level (cross-draft). It is the simplest type of gasifier usually consisting of a cylindrical space for fuel and gasifying media with a fuel-feeding unit, an ash-removal unit and a gas exit. In the fixed-bed gasifier, the fuel bed moves slowly down the reactor as the gasification occurs. Fixed-bed gasifiers are simple to construct and generally operate with high carbon conversion, long solid residence time, low gas velocity and low ash carry-over.

1.3.2 Fluidized bed reactors

The gasifying agent is blown through a bed of solid particles at a sufficient velocity to keep the particles in a state of suspension. Fuel particles are introduced at the bottom of the reactor, are very quickly mixed with the bed material, and almost instantaneously are heated up to the bed temperature. As a result of this treatment, the fuel is pyrolized very fast, resulting in a component mix with a relatively large amount of gaseous materials. Further gasification and tar-conversion reactions occur in the gas phase. Twin-bed gasification uses two fluidized-bed reactors. The biomass enters the first reactor, where it is gasified with steam, and the remaining char is transported to the second reactor, where it is burnt with air to produce heat. The heat is transported to the gasification reactor by the bed material, normally sand. The flue gas and the product gas have two separate exits.

1.3.3 Entrained flow reactors

These gasifiers are commonly used for coal because they can be slurry-fed in direct gasification mode, which makes solid fuel feeding at high pressures inexpensive. These gasifiers are characterized by short residence time, high temperatures, high pressures and large capacities.

Fixed and fluidized bed reactors are based on simple and proven technology because of the high fuel flexibility in terms of both size and type but Entrained flow gasification is arguably the most suitable of the gasification technologies, since, despite the relatively complex construction and operation and the fuel specificity in term of size, its low residence time allows compact gasifiers and the high operating temperature provides a high quality, tar-free syngas with high conversion efficiency, high capacities (> 1 MW) and very good scale-up potential (Puig-Arnavat et al. 2010).

According to Tremel et al. (2012) the US Department of Energy has databased all existing and projected larger scale gasification facilities: from 2005 until today, 94% of all registered gasifiers worldwide, in operation or planned, are based on entrained flow gasification. However, almost all

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19 gasifiers are designed for coal or pet coke as feedstock. Despite its high development stage, ash and conversion related problems in entrained flow gasifiers still exist and are being investigated.

1.4 Technical Issues

Compared to coal, biomass gasification and pyrolysis systems have technical issues that, related to the biofuel nature, mainly are:

- slagging and fouling, - reactor surface corrosion, - ash handling,

- fuel preparation.

High amounts of inorganic constituents, especially potassium and calcium, in some residues and herbaceous biomass often contribute to adverse impact on the different elements of the conversion systems trough fouling, slagging and in the case of fluidized-bed reactors, bed agglomeration. Moreover chlorine and sulfur traces are important contributor to corrosion, metal wastage and pollution.

Entrained flow gasifiers are typically operated above the ash melting temperature. Liquid slag is formed protecting the gasifier wall from corrosion and enables a stable and steady drain of ash. It is not clear, whether a stable slag layer forms with woody biomass that has low ash content and a high ash melting temperature. Despite of the presence of low-melting alkali compounds, an ash with a high melting temperature is formed due to the volatilization of these compounds into the gas phase, favored by the presence of chlorine.

In coal gasification, flux materials (minerals) are used to enhance the rheological properties of slag. The addition and possibly recycling of flux material or the co-gasification with coal may be a viable option for the entrained flow gasification of biomass.

Alternatively, a non-slagging reactor operates below the ash melting temperature. But even at a lower temperature of 1200 °C, partial melting of the biomass ash components cannot be avoided leading to the blockage of the gasifier. Water washing (leaching) is a mean to eliminate alkali metals from herbaceous biomass allowing operating at low temperature so that the thermal conversion efficiency is increased.

Aiming to evaluate the best operation mode (slagging or non-slagging), reactivity data for biofuels at high temperature are required, but so far not available. E.g., a higher reactivity should mean that the operating temperature could be lower managing to operate in non-slagging mode and increasing the efficiency whereas a lower reactivity allows operating at the typical operating temperatures of the coal gasification systems.

1.5 Pretreatments

Biomass pretreatment processes have the potential to improve fluidization behavior, reduce the energy demand for pulverization and increase the energy density and homogeneity of the feedstock so that biofuels may be useful in co-gasification and co-firing applications.

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20 Wood torrefaction is a mild pyrolysis process since it is performed at temperatures between 230 and 300 °C. The hemicellulose fraction of the wood decomposes, so that torrefied wood and volatiles are formed. The severity of the process is expressed by mean of the torrefaction degree that is the ratio between the released volatile matter during torrefaction and the initial content of volatile matter of the parent material. Hence higher torrefaction temperature and residence times correspond to a higher torrefaction degree since the release of volatile matter is increased by increasing temperature.

A similar pretreatment process is the hydrothermal carbonization (HTC): it takes place in pressurized water at 200 – 250 °C at or above saturation pressure (steam explosion) and is slightly exothermic. As the process takes place in water, HTC is well suited for wet biomass. During HTC, oxygen is removed from the feedstock by decarboxylation and dehydratisation reactions, thus decreasing the O/C ratio and the higher the degree of carbonization, the higher the calorific value of the biocoal.

According to Tremel et al. (2012) during torrefaction a destabilization of fibers is induced by a shortening of individual cellulose chains and that this is due to the depolymerization rather than the decomposition of cellulose. Therefore, good grindability and more spherical particles can be expected for HTC derived biocoal, but at lower process temperatures than for torrefaction.

Beyond the technical advantages, pretreatments allow maximizing the potential of the extraction sector and enhancing the biomasses stocking management for the overcoming of seasonality.

1.6 The need for modeling and the BRISK project

Studying the phenomena occurring during thermochemical conversion processes, with a special attention paid to gasification and pyrolysis in entrained flow reactors, is fundamental to technical enhancements in the contribution of biofuels and pretreated biomasses to commercial energy needs.

Improving both the efficiency and environmental impacts of conversion processes are necessary to large-scale development and to this aim mathematical modeling is required allowing quantitative representation for process design, prediction of reactor performances, understanding of pollutants evolution, analysis of process transients and examination of strategies for effective control.

Thermo-chemical conversion of biomass in practical systems results from a strong interaction between chemical and physical processes at the levels of both the single particle and the reaction environment. Hence, biofuels nature and operating conditions are directly involved in the modeling phase and qualified experimental data are required to validate models.

To this aim the Brisk (Biofuels Research Infrastructure for Sharing Knowledge) European Union co-funded project has been developed. Several European and international university and research foundations are involved to share their results out of tests performed on their facilities trying to create a database and outline standard experimental procedures and parameters, especially for advanced analysis.

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21

2. METHODOLOGIES FOR BIOFUEL CHARACTERIZATION

The optimization of a thermochemical conversion process begins studying the phenomena occurring that are linked to operating conditions as well as to the basic characteristics of the considered fuels so that, before starting advanced analyses on industrial scale facilities, preliminary analyses are fundamental to identify a material, make comparisons and anticipate fuels behavior.

2.1 Preliminary analyses

The preliminary analyses are performed in laboratory facilities in mild operative conditions (heating rate on the order of 1 °C/s and temperatures under 1000 °C) simple to be reproduced on small amounts of samples. It is worth repeating each test a sufficient number of times (at least three times) to overcome the heterogeneity of the fuel which depends on the nature, pre-treatments, sampling method. Indeed providing an average value with the determined experimental error is important to elaborate parameters from the raw data.

Composition, content of moisture, volatile matter, fixed carbon and thermal characteristics are detected during traditional preliminary analyses such as ultimate analysis, proximate analysis and the relief of the heating value.

Ultimate analysis, that gives the content of C, H, N, S of the fuel is carried out by completely oxidizing the fuel and measuring CO2, H2O, N2, SO2 respectively, in the gaseous form. The oxygen content is obtained by difference (after ash and moisture).

Proximate analysis that is carried out by a progressive heating of the sample and measuring the weight at standard temperatures gives the content of:

- moisture, evaluated after an isotherm at 110 °C; it depends on the fuel origin and conservation conditions and thus may be not representative of the fuel in itself.

- volatile matter (VM) after a low heating rate ramp to 900 °C in nitrogen atmosphere. This parameter can be defined Standard Volatile Matter (STVM) released and generally differs significantly from the High Temperature Volatile Matter (HTVM) released.

- ash after changing the atmosphere with air. - fixed carbon (FC) by difference.

Parameters given in ultimate and proximate analyses are generally normalized (to 1 or 100%) and may be expressed on different basis: as received, dry (db) or dry and ash free (daf).

The heating value of the fuel is measured after the combustion of the sample in standard conditions and is generally expressed on a dry sample as MJ/kg. The Higher Heating Value (HHV) is experimentally determined by bringing the combustion products to the room temperature and thus the water is considered as liquid while the Lower Heating Value (LHV) considers the water as a vapor.

Additional analyses give important parameters for specific investigations (e.g., the ash analysis for deposition and corrosion studies) and are less common or not standardized at all (like the chemical analysis). They are not strictly necessary for the advanced analysis hence they rarely are all

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22 performed for a fuel. Obviously the more detailed is the information available, the more complete will the characterization of a new fuel be. They are:

- the ash analysis that gives the main elements present in the ash of the fuel after combustion; therefore the elements are often expressed as oxides (SiO2 for silicon, Al2O3 for aluminum and so on). Ash in solid fuels may be classified as minerals, organically associate or water soluble; generally the mineral part is preponderant in coals while biomass fuels contain almost completely organically associated ash. This analysis is important for evaluating the size of fly ash, quantifying some species and possible tracers (e.g. titanium) and defining indexes to predict fouling and slagging phenomena;

- the Nuclear Magnetic Resonance analysis that provides the chemical structure and structural parameters of a fuel (the quantification of aromaticity, aliphatic bridges, coordination number of carbon, for instance), which are useful for estimating the reactivity of a fuel and providing input for structural models (e.g. ABCD model);

- the chemical analysis that gives the amount of cellulose and lignin of biomass fuels, useful for those models that simulate the biomass behavior on the basis of the chemical compounds. It can be derived from a series of analytical procedures: dissolution of the fuel with different solvents and conditions to remove selectively the extractives, the hemicellulose, the lignin and finally obtained the cellulose. There are various methods but none of them are standardized and are reported to be hardly reproducible so that many tests should be repeated to obtain reliable data.

The parameters of fundamental and additional analyses give a preliminary characterization of the fuel, but the same analyses can be used also for any solid residue (e.g. chars) after an experimental test.

2.2 Advanced analyses

The advanced analyses are carried out under conditions similar to those of large scale applications (high temperature and heating rate) and give significant fuel specific parameters. These analyses focus on devolatilization and char oxidation.

Low heating rate facilities are very common in laboratories and for this reason it is simple to collect data, share and compare them; as for other analysis it is difficult to find out the same apparatuses in different laboratories so that it becomes difficult to compare data on the base of the same operative conditions and to validate models.

Drop tubes, entrained flow and moving bed reactors, fluidized beds and other advanced systems on a laboratory or pilot scale are used to study solid fuel combustion and other thermochemical mechanisms: all these systems have high heating rate and temperature and work as dynamic reactors (the fuel particle inside the reactor is moving).

Despite the lack of direct application of models validated with low heating rate runs on high heating rate datasets, performing tests in different operating conditions is important to understand the influence on the process results.

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23

2.2.1 Low and medium heating rates facilities

The facilities available to perform low and medium heating rate analyses allow studying thermochemical conversion on static samples in mild operating conditions. There are several systems and reactors based on different techniques for heat transfer.

The thermogravimetric (TG) balance (Fig. 2.1(a)) is a fundamental tool for the characterization of a material and the comparison by mean of parameters such as the temperature at which the derivative weight loss shows a maximum. It can be used also for studying reactions (oxidation or gasification) in different gaseous environments.

Basically, it measures the amount and rate of change in the weight of a sampled material as functions of temperature and time in a controlled atmosphere. Common and standard procedures can be found for this instrument. However the conditions are far from those of large scale furnaces: the maximum temperature is generally around 1000 °C, the maximum heating rate can be 100 °C/min. The sample is static, is inserted in appropriate crucibles (of different materials depending on the final temperature) and the maximum amount can be on the order of some tenths of mg, typically ranging from 1 mg to 50-100 mg. The residence time depends on the thermal program and can be on the order of some minutes or more. The sample weight and temperature are continuously monitored and the accuracy is generally high.

Small amounts of sample (5 mg or less) should be studied for obtaining the devolatilization kinetics and avoiding the heat transfer limitation during the test. At least three runs (under different heating rates) should be programmed to elaborate the intrinsic kinetics by using iso-conversional methods. Larger amounts of sample can be used for studying the gas products by coupling the TG balance with a gas analyzer, e.g. a Fourier Transform Infrared Spectroscopy (FTIR) that allows identifying the gaseous species by mean of the obtained absorption spectra of compounds that are a unique reflection of their molecular structure.

A thermogravimetric balance can work also at different pressure levels (Fig. 2.1(b)) revealing the influence on the weight loss of the sample.

(a)

(b)

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24 Advanced reactors on a lab-scale can be used to overcome the mild thermal conditions of the TG balance. The high heating rates and temperatures can be achieved with a particular configuration of the reactor, small amount of sample and a strong heat transfer by radiation or induction, for instance.

A wire mesh reactor (Fig. 2.2) enables the adjustment of high temperature and pressure in a very small scale and has become an ideal instrument for the experimental analysis of pyrolysis, gasification and combustion. A very small solid sample lies on a fine mesh of noble metal, which is directly heated through electric flow (resistance heating principle) and the temperature is measured through two fine wire thermocouples. Because of the very small masses and the high heat flow density, heating rates of more than 1000 K/s are possible. The mesh is constantly swept by an inert gas (e.g. argon) to remove any pyrolysis products and to prevent the re-condensation of tar. The reaction room can be evacuated and can currently be operated at different pressure levels (up to 5 MPa). After the heating period the mesh is cooled down only by thermal radiation and the inert gas sweep stream. Pressure has an influence also on the temperature profile during cooling. If the reactor is optically accessible various measurement operations, such as laser measurement technology or FTIR for the analysis of gas composition are possible. An optional gas discharge above the sample also allows for an external gas analysis.

Figure 2.2 Pressurized wire mesh reactor

In fixed-bed reactors the heat transfer is allowed by a thermal flywheel effect while filament pyrolyzers work with an induction mechanism.

Generally a lab-scale fixed bed batch reactor (Fig. 2.3 (a)) allows heating samples uniformly and slowly up to 400 – 500 °C. To compensate for heat losses through the external wall and to assist in uniform sample heating, a tube furnace is placed around the reactor. The pyrolysis reactor consists of a stainless steel vertical tube containing a grid in the middle to support the sample (in a glass cup) and 2 flanges at the top and bottom to easily fill and empty the reactor. The gas, heated by an electrical heater which is controlled via a PID controller, enters the reactor at the bottom. The connecting tube between heater and reactor inlet is covered by an electrically heated tape to compensate for heat losses to the surroundings.

To clean the outlet gas consisting of nitrogen, organics, tars and possibly metals, a series of impingers is added. To prevent clogging of the pressure reduction valve placed at the outlet of the reactor a first impinger is placed before the pressure reduction valve. This impinger was designed to capture all tar components and to operate at elevated pressure (up to 5 bar gage pressure). After the impinger, an expansion valve is placed to reduce the pressure. After the expansion valve two

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25 impingers are placed to capture all metal emissions (if there are any). Thermocouples have been inserted indifferent position in respect of the grid.

In Fig. 2.3(b) an example of a Curie Point pyrolyzer is represented: the sample is heated by induction in rapid heating mode. Electrical current is induced onto a wire made of ferromagnetic metal by use of magnet or high frequency coil while the pyrolysis chamber which is surrounded by coil, is opened and sample wire is dropped or place inside. Pyrolysis temperature is determined by the composition of the ferromagnetic material and this is a drawback because the heating is not linear and the temperature values are limited in a range of 350 - 1000°C so that it is harder to optimize the pyrolysis temperature. The advantages are that the test is quite automatic because of the self-limiting temperature and that the composition of pyrolysis products is demonstrated to be constant even with sample weight increased.

(a)

(b) Figure 2.3(a) Lab-scale fixed bed reactor,(b) Curie point pyrolyzer

Hence, conditions more similar to those of large scale furnaces can be studied. However, the sample cannot be continuously weighted. The static configuration allows one to weight the sample before and after the test, so that the evaluation of the devolatilization conversion is accurate. However, different tests should be programmed by programming a set of final temperatures and residence times.

Although the heating time could be programmed to study also short residence times, the subsequent cooling time is often very long (if quench devices do not operate) and thus the reactions can be hardly stopped in the desired time. Also the effective thermal history of the sample differs from the nominal conditions, as internal gradients in the sample are expected to be not negligible. Therefore, a thermal model should be developed for estimating the thermal history of the sample in all studied conditions.

2.2.2 High heating rate facilities

The most common types of laboratory scale facilities for the study of gasification and pyrolysis in condition similar to those of industrial reactors are the fluidized bed reactors and the drop tubes. The sample is dynamic so that the determination of the conversion of the fuel is indirect.

Fluidized bed reactors are dynamic reactors where the gaseous stream trough the solid fuel sample allows the mass and the energy transport. Laboratory-scale facilities are diffused especially for

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26 studying the waste thermochemical conversion and despite beyond the intent of this work, a schematic example is given in Fig. 2.4(a).

Drop tube reactors (Fig. 2.4(b)) are generally formed by a vertical tube inserted in a furnace; the fuel is fed at the top section generally with a pneumatic transport with preheated gas and a secondary gas can be introduced too. Electrically heated elements along the internal wall of the furnace provide the heat to the gas/solid mixture along the tube: the appropriate set up of these resistances can give an isothermal profile inside the tube. A water cooled collector probe is generally moved along the axis of the reactor to vary the residence time of the particle and stop the reactions. The gas is monitored during the test and the solid residue is collected at the bottom to follow several off-line analyses. However, determining the conversion has several problems to be considered such as the balance on macro-products that is hardly closed or the effective thermal history of solid particle that can be only estimated.

(a) (b)

Figure 2.4(a) Fluidized bed reactor, (b) schematic drop tube

The Isothermal Plug Flow Reactor (IPFR) is an example of drop tube and has to be mentioned since the data available for this work have been collected from this system during several experimental campaigns performed by the International Flame Research Foundation (IFRF).

The IPFR was built in 1985 in Ijmuiden (The Netherlands) and, after the realization of the current version in 1994, has been located inside the Enel Ricerca experimental area in Livorno (Italy) where the IFRF has the headquarters.

The IPFR is an advanced facility for solid fuels characterization. There, the combustion behavior of solid fuels, such as coal, biomasses and their blends, under conditions similar to those typical of industrial boilers, can be studied. Moreover, devolatilization and char burnout can be studied separately, under several well-controlled environments, with reference to flue gas chemical composition, heating rate and temperature, particles residence time.

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27 The reactor can be used to reproduce combustion processes with heating rates up to 104–105 K/s, whereas residence time can vary between 10 and 1500 ms. The collected particles undergo several laboratory analyses, from whose results it is possible to determine the combustion reactions kinetics, that are useful tools for predictive mathematical modeling.

As shown in Fig. 2.5, the IPFR is mainly composed of three parts: a gas pre-heating section, a vertical isothermal reactor, and an exhaust line. In addition to these sections, a fuel feeder and a water-cooling system exist.

The gas pre-heating section consists of a pre-combusting chamber and the pre-combustor is fired by a 60 kW aerodynamically air staged natural gas burner. The amount of natural gas which has to be fed to the pre-combustor is determined by the desired operating temperature, which almost fixes the IPFR thermal load. Once the natural gas flow is fixed, the air flows (for the conventional combustion) have to be fixed to achieve the desired oxygen concentration in the flue gases. The temperature and the chemical composition can be adjusted by mixing with some ambient temperature additional gases like nitrogen and carbon dioxide.

Characteristic Value Reactor tube total

length 4.5 m

Reactor tube

operating length 4.0 m

Reactor tube inner

diameter 150 mm Number of operating modules 8 Number of feeding ports 19 Maximum thermal input - natural gas - electrical 60 kW 54 kW Operating temperature 1400°C 700-Residence time 10-1500 ms Maximum gas flow

rate Nm3/h 75

Quenching gas flow rate

20-25 Nm3/h Carrier gas flow rate Nm3/h 1.8-2.0

Minimum fuel

feeding rate 0.08 kg/h

Figure 2.5 Isothermal Plug Flow Reactor: layout (left) and characteristics (right)

The reactor tube is composed of eight modules. Each one can be independently controlled and electrically heated. With these heating elements, isothermal conditions within a margin of 10-20°C

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28 along the reactor tube can be obtained when working within the operating range between 700 and 1400°C. A fluxing system with air or nitrogen is used to guarantee the heat exchange for the electrical heaters and the temperature is monitored with several thermocouples.

The reactor is provided with 18 ports, which are used to inject the solid fuel, to measure temperature with a suction pyrometer, or to determine the chemical composition of the atmosphere using dedicated probes. Changing the solid fuel injection port is the main way to collect particles with different residence times within the reactor.

As preliminary operations the solid fuels have to be milled into smaller particles and sieved in a narrow range of particle size distribution in order to determine the combustion behavior of particles having almost the same characteristics since the material as received is not suitable to be characterized in the IPFR because of the dimensions.

The pulverized fuel mass flow is guaranteed by a mass flow controller, which consists of a reservoir with an electrically driven screw on a balance. A control unit, which can be commanded manually or automatically, varies the rotational speed of the screw according to the mass decrease measured by the balance. At the outlet of the mass flow controller, solid fuel is mixed with gases in order to transport the particles to the reactor. The carrier gas has the same oxygen concentration of the flue gases in the reactor, and its mass flow can be varied in order to get the same velocity as the assumed mean reactor velocity (iso-kinetical injection).To introduce the pulverized fuel into the reactor, a radial water-cooled injection probe is used. The probe is inserted through one of the above described ports, and the injection nozzle is placed exactly on the axis of the reactor tube.

The sampling probe is inserted in the bottom of the reactor in order to collect the particles after the desired residence time within the reactor. As seen above, the main contribution to the residence time value is given by the choice of the injection port, but also varying the height of the probe inside the reactor, residence time can be significantly modified. The position of the probe inside the reactor is fixed through a manual transmission system.

The most suitable injection port and the height of the sampling probe inside the reactor that ensures the desired residence time are determined considering the adiabatic combustion temperature that is calculated with an iterative method which considers the variation of each gas heat capacity with the temperature, according to the following Perry’s formula.

The flue gas mean velocity is calculated with the following relation:

The volumetric gas flow in standard conditions is converted to the actual operating temperature value with the following correlation:

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29 Once the flue gas mean velocity is calculated, the residence time can be determined with the following formula:

Thus, the sampled particles are quenched with an ambient temperature nitrogen flow to a temperature below 200°C in order to interrupt all the reactions. Then, particles are separated from the flue gas through a cyclone separator, and collected in two cylinders and a bag filter. The non-sampled particles and mixture are quenched with air to get a temperature below 200°C in order not to damage the cyclones and the bag filter. This quenching can be done with air because the particles are not further analyzed and so it is not important to avoid their interaction with oxygen.

After sampling, the solid residues undergo several analyses: proximate analysis is fundamental to determine biomass and char conversion whereas ultimate analysis are performed on some selected samples, e.g. those collected after the lowest and the highest residence time for each kind of test, as well as on the parent fuel and the produced char.

Particle size distribution is determined, to identify the morphological evolution of particles during devolatilization and char combustion. Finally, ash composition is determined on other selected samples such as the parent biofuel, the produced char, and a few partially burnt chars, to validate conversion calculations, which are based on the assumption that ash remains inert through all processes occurring in the IPFR.

The procedure described above is inevitably a source of error for the determined conversion values since the actual residence time and thermal history are dependent on the particles dimensions and the collector probe acts as a selective sampler. Furthermore there are experimental errors on the residence time deriving from the measurements of the position of the sampling probe or of the mass flow whereas the error on the ash content and the assumption that an ash loss can happen during the test are at the basis of the conversion uncertainty that has to be determined to validate the mathematical models.

Some of the mentioned sources of error can be easily identified and overcome, e.g. sieving particle in a narrow range or installing a positioning system with a higher accuracy, while the ash loss can be due to chemical and physical phenomena so that only a closed mass balance system could allow evaluating the uncertainties.

The PiTER, Pressurized High Temperature Entrained Flow Reactor,(Fig. 2.6) is another example of drop tube and has to be mentioned to make a comparison with the IPFR.

The facility is located at Technische Universität München (TUM) and enables measurements over a wide range of operating conditions. The PiTER is designed for experiments at temperatures up to 1800 °C and pressure up to 5.0 MPa but actually it works up to 1400 °C and pressures up to 4.0 MPa.

The structure is similar to the IPFR since there are a preheating zone, a reaction zone, an exhaust line and seven operating modules with several ports to bring in depositions probes and fuel gas analysis but the first three modules are to the gas-preheating whereas the fuel injection probe is located right below where the reaction zone begins.

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