Characterization of Pyrolysis Products from Second Generation Biofuels

Sommario

1. INTRODUCTION ... 4

1.1 European Scenario ... 4

1.2 Biomass Generalities ... 5

1.3 Characteristics as Energy Source ... 7

1.4 Second Generation Biomass ... 8

1.5 Thermochemical Processes ... 8 1.5.1 Combustion ... 10 1.5.2 Co-firing ... 12 1.5.3 Gasification ... 13 1.5.4 Pyrolysis ... 16 1.5.5 Pretreatments ... 19 1.6 Reactors... 21

1.6.1 Fixed bed reactors ... 21

1.6.2 Bubbling and circulating fluidized bed reactors ... 22

1.6.3 Entrained flow reactors ... 22

1.7 Technical Issues ... 23

1.8 Purpose of the Thesis ... 24

2. BIOMASS CHARACTERIZATION METHODOLOGIES ... 25

2.1 Fundamental analysis ... 25 2.1.1 Proximate Analysis ... 26 2.1.2 Ultimate Analysis ... 32 2.1.3 Heating Value ... 35 2.1.4 Biochemical Analysis ... 37 2.1.5 Ash Analysis ... 38 2.2 Advanced Analysis ... 38

2.2.1 Low Heating Rate Analysis ... 38

2.2.1.1 Thermogravimetric Analysis ... 39

2.2.1.2 TG-FTIR Analysis ... 40

2.2.2 High Heating Rate Analysis ... 50

2.2.3 Isothermal Plug Flow Reactor (IPFR) ... 52

3. BIOMASS DEVOLATILIZATION MODELS ... 56

3.1 Devolatilization ... 56

3.3 Kinetics models overview ... 58

3.4 Isoconversional Methods ... 62

3.5 Development of a Structural Model based on Chemical Components ... 65

4. EXPERIMENTAL RESULTS ... 68

4.1 Biomass samples ... 68

4. 2 TG and FTIR results ... 72

4.2.1 Thermogravimetry: Devolatilization tests ... 72

4.2.2 Thermogravimetry: Direct Oxidation test ... 82

4.2.3 TG-FTIR Analysis: devolatilization products ... 85

5. MODEL RESULTS ... 106

5.1 Results of the isoconversional techniques... 106

5.1.1 Kissinger-Akahira-Sunose method ... 107

5.1.2 Friedman method ... 109

5.1.3 Coats & Redfern method ... 112

5.1.4 Flynn-Wall-Ozawa method ... 114

5.2 Results of the CHEL structural method ... 119

5.2.1 Devolatilization kinetics ... 119

5.2.2 Devolatilization products prediction ... 127

6. HIGH TEMPERATURE and HIGH HEATING RATE VOLATILE GAS SPECIATION ... 131

6.1 Literature analysis for Drop Tube Reactor ... 132

6.1.1 Experimental overview ... 132

6.1.2 Influencing parameters ... 138

6.2 IPFR Devolatilization experiments ... 148

6.3 Product distribution models ... 155

6.3.1 Thermodynamic Equilibrium: Restricted Equilibrium method... 156

6.3.2 Structural Models ... 157

6.3.2.1 Additive model ... 157

6.3.2.2 Multi-step model ... 158

6.3.2.3 Empirical Model... 162

6.4 Theoretical model applied to IPFR experimental data ... 169

6.4.1 Restricted Equilibrium model ... 169

6.4.2 Application of the empirical model from Neves et al. (2011) ... 176

7. CONCLUSIONS ... 183

REFERENCES ... 184

APPENDIX 2 : MATLAB CODES ... 193 APPENDIX 3 : MODEL RESULTS FOR THE OTHER BIOFUELS ... 225 APPENDIX 4 : The DATABASE ... 268

4

1. INTRODUCTION

With increasing global population and rising living standards, there has been a significant growth in energy demand worldwide over the last several decades. This leads to diminishing fossil fuel reserves, serious environmental pollution and high greenhouse gas (GHG) emissions. To address these challenges, many efforts in developing renewable energy and alternative fuels have been carried out and substantial progress has been made. Although lately the applications of renewable energy grow rapidly, their applications are still limited due to the high cost, poor technology reliability, and limited resource availability. Among the renewable energy and alternative fuels under development, biomass energy (or bioenergy) is one of the promising resources to match the requirements of substituted fossil fuels for reducing GHG emissions. Biomass can be considered as one of the solar Energy resources. Plants grow by absorbing carbon dioxide from the atmosphere as well as water and nutrients from soils followed by converting them into hydrocarbons through photosynthesis. All carbon contained in biomass is gained from carbon dioxide; in other words, carbon is recycled in the atmosphere when biomass is consumed as a fuel. Therefore, biomass is referred to as a carbon-neutral fuel when it is burned.

Bioenergy has also the advantage of being widely distributed on the Earth’s surface so has a great potential as a low-carbon source of large-scale energy production; biomasses can be utilized directly as solid fuels but are also susceptible of being transformed into different fuel forms, such as liquid (bio-oil, biodiesel) or gas (biogas, syngas).

1.1 European Scenario

Studies on biomass characterization and development of new or more efficient technologies of fuel energy conversion fall within the main target of European Decarbonization project (Rapporto Energia e Ambiente, Scenari e Strategie, ENEA 2013). Since 2011 two major Communication from European Commission (COM/2011/112 Roadmap for moving to a competitive low-carbon economy in

2050 and COM/2011/885 Energy Roadmap 2050) set different pathways to reach the aim to diminish

of 80% carbon level compared to 1990, guaranteeing energy security and stability of European economy at the same time.

This goal is consistent with the effort, required from more developed country, to contain global warming avoiding the increase of mean global temperature beyond 2°C until the end of the century.

5 Table 1.1 Greenhouse gas reduction per sector, necessary to achieve the goals of Roadmap 2050

Of many scenarios studied, five main options can be identified: 1) energy efficiency, 2)technological diversification, 3) higher quota of renewable sources, 4) Carbon Capture and Storage technologies and 5) limited recourse of nuclear energy.

In the meanwhile, global GHG emission are in rapid growth (around 400 ppm of CO2 atmospheric concentration in May 2013, first time in millennia). According the majority of studies in this matter, Climate Changing has already begun.

1.2 Biomass Generalities

The term “Biomass” embraces a great variety of organic-inorganic solid products generated by both natural and anthropogenic processes, such as: 1) constituents originated from land-based and water-based vegetation or from animal food digestion , 2) technogenic products derived from processes of the previous natural constituents.

Vassilev et al. (2010) make distinction about the general classification of biomass varieties, dividing them in groups and sub-groups according to their biological diversity and similar source and origin table 1.2.

6 Vegetative biomass (Phytomass) is formed primarily of Cellulose, Hemicellulose and Lignin, along with lesser amounts of Extractives (e.g. terpenes, tannins, fatty acids, oils and resins), moisture and mineral matter. (Legendre et al. 2011).

Cellulose is the most abundant compound in nature (up to 50 wt% of dry biomass); it’s a linear polysaccharide and, except from is degree of polymerization (DP 500÷10.000) it’s undistinguishable from biomass to biomass. Strong hydrogen bonding between the straight chains imparts a crystalline structure to the cellulose, making it highly resilient to dissolution and hydrolization.

On the contrary, Hemicellulose and Lignin composition is rather heterogeneous and can vary greatly within a given biomass species. The first one has an amorphous structure and displays branching in his polymer chain; several sugar monomers are contained, including mostly xylose but also manose, galactose and arabinose.

Lignin accounts for almost 30% of terrestrial organic carbon and provides rigidity and structural framework in plants; the lignin biopolymer consists of a complex network of cross-linked aromatic molecules, which serves to inhibit the absorption of water through cell walls. The structure and chemical composition are determined by the type and age of the plant.

Extractives are organic compounds of low molecular weight which name derives from the possibility to extract those substances with water or organic solvents (they are named after the type of solvent used in the process).

Studies addressing the transformation parameters of biomass must account for the intrinsically heterogeneous nature of the substrate.

7 Figure 1.1 Schematic representation of the three macro components and their chemical schematization

1.3 Characteristics as Energy Source

Nominally, we call Biofuels (or Biomass Fuels) solid, liquid or gaseous fuels derived from processing natural biomass resources, and Bioenergy the energy produced from biofuels. The previous authors also write up the major pro and con offered by using biomass, reassumed in table 3.

Table 1.3 Main advantages and disadvantages of biomass usage

So we can say that, from only an engineering point of view, biomass offer several undoubted advantages that set them on the firsts steps of future energy system, but present also a lot of different problems that remains to be solved:

1) Optimization of the entire life cycle, from producers to users, in a multidisciplinary point of view (engineering, agronomy, economy etc.);

2) Intrinsic randomness of the source (annual products yield may vary a lot); 3) Pretreatments are often required (densification, drying, milling etc.); 4) Insufficient uniformity of products properties;

5) A possible energy plant may need to be versatile to process different fuels, cause the heterogeneity of biomass fuels.

8 6) Low LHV (10÷20 MJ/Kg) that prevent high exergetic efficiency;

7)

Reasonably, two fundamental aspects related to biomass use as fuel are: 1) extend and improve the basic knowledge on composition and properties, 2) apply this knowledge for the most advanced utilization; Which are also the goals of this work.

1.4 Second Generation Biomass

As mentioned before, feasibility of biomass uses must take into account numerous inconvenient linked with its harvesting.

Two of the most important problems are: 1) Energy Crops vs. Food Crops

Some types of plants that are suitable for energy conversion are very important to feed population and/or animals in farms;

2) Deforestation

Big forestry areas have been cut in order to gain space in which crops can be cultivated.

Second Generation Biofuels are the attempt to try not to commit the same errors as before, 1) Abandoning the use of food-dedicated cultivations;

2) Encourage of the use of short production chain;

3) Usage of residual lands valorizing plants as sorghum, miscanthus,

common reed ecc.

When a biomass respect those requisite, we can speak of second generation biofuels.

1.5 Thermochemical Processes

Considering all that has been said above, comes easy to think that biomass cannot be used directly in processes of energy conversion, but have to be treated in order to increase their energetic density; different approaches can be followed, here described on the basis of conversion mechanism:

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

9 2) Physical : consist in mechanical transformation aiming to extract the oils useful as fuels; 3) Thermochemical : consist in physical and chemical reactions occurring by providing heat

allowing to obtain different forms of energy from matter.

It is well known that all the methodologies and logic acquired from coal experiments can be applied to biomass so, considering all the technical solution available to convert fossil fuels into “heat and power”, thermochemical mechanism seem the most reliable 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…), lignocellulosic 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.

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

Below we briefly describe the principals thermochemical methods.

10

1.5.1 Combustion

It’s the best known method of conversion, direct oxidation of the fuel. In total, about 95-97% of global bioenergy is currently produced by direct combustion of biomass (Vassilev at al. 2013).

a) b)

Figure 1.3 Example of combustion weight loss: a) sub-steps highlighted b) rate of weight loss with characteristic temperatures

Vassilev et al. (2013) also report an exhaustive description of the phenomena of transformation organic and inorganic matters occurring during combustion.

Biomass combustion is a complex process that consists of consecutive heterogeneous and homogeneous reactions of degradation and subsequent oxidation. The main process steps, represented in figure 1.3, are the following:

1) 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.

2) 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, 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.

4) 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.

11 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% (T. Nussbaumer, 2003). Gas released from thermal decomposition include dominantly CO2, CO,

H2O, H2 and CH4 and, to lesser extent N2, NOx, SOx, ethane C2H2, ethane C2H6 and others. It was found

that hemicellulose has higher CO2 yield, cellulose generates higher CO yield and lignin higher H2 and

CH4 yield.

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 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.

It should be noted that, due to the high oxygen concentration in natural biomass structures (21÷49 %daf, in contrast to coal 4÷20 %daf), the process curves observed in oxidizing and inert atmosphere have similar decomposition steps and mechanism.

12 It is generally accepted that biomass combustion takes place in two steps: 1) combustion of cellulose and hemicellulose at 250÷350 °C, which is greater for annual plant species and 2) combustion of lignin at 350÷500 °C, greater for perennial plant species. However precise characteristic transformation temperatures are impossible to find because they are strongly dependent on the biomass varieties.

Proportions among structural components though seem to have limited significance during combustion process and the two stage previously described are basically related to: 1) devolatilization and burning of combustible volatile matter and subsequently 2) char formation and combustion.

1.5.2 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:

1) 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. 2) 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.

13 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.

1.5.3 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 1.4.

Table 1.4 Principal gasification reactions 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

14

-393,5 Oxidation of carbon

+118,5 Water gas reaction (steam reforming)

+159,9 Boudoard equilibrium

-87,5 Methane production reaction

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

3) temperature and pressure; 4) gasification reactor type; 5) fluid-dynamic regime; 6) gasification agent; 7) 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 figure 1.4 show the dependency of the gas composition on temperature:

Figure 1.4 Temperature dependency of gasification main products composition

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.

15 As for the gas composition as a function of the gasifying agent, typical values are listed in the table 1.5 (Bocci at al. 2014).

Table 1.5 Typical gas composition for gasification products with different oxidants

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:

1) 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;

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

3) 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:

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

2) 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.

16 1) direct: the chemical oxidation reactions provide the energy necessary to keep the process

temperature;

2) 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.

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.

1.5.4 Pyrolysis

Pyrolysis is the fundamental thermochemical conversion process that can be used to transform biomass directly into gaseous and liquid fuels. Deep understanding of its behaviors is a very hard task but important, for pyrolysis is the very first step in gasification and combustion processes. Knowing its kinetics is therefore vital to the assessment of items including the feasibility, design and scaling of industrial biomass conversion applications.

Pyrolysis of solid materials is classified as an heterogeneous chemical reaction, which dynamics and chemical kinetics can be affected by three key elements: 1)the breakage and redistribution of chemical bonds, 2) changing reaction’s geometry and 3) the interfacial diffusion of reactants and products.

Because of its heterogeneous nature, concentration is an inconsequential parameter that cannot be used to monitor the progress of the process, for it can vary spatially.

Figure 1.5 Pyrolysis process schematization

Pyrolysis consists of solid thermal degradation occurring in the temperature range of 400 °C and 800 °C in the absence of oxidizing agents; this conversion route results 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.

17 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 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 1.6 Temperature dependency of main pyrolysis products (Bridgwater, 2012)

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

1) 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;

2) 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;

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

18 4) conventional pyrolysis at moderate temperature (below 600 °C) and residence times

produces comparable yields of the three lumped classes of products.

5) 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.

Table 1.6 Pyrolysis processes characteristic parameters

Table1.7 Pyrolysis processes main products

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 figure 1.7.

Figure 1.7 Dependency of char yield mass fraction on temperature (Overend, 2004)

The applications of the products are listed below:

6) 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;

7) 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, and it can be easily stored and transported. Fast pyrolysis

19 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

8) the char product is useful as a renewable fuel since it has many attractive features: 1) it contains virtually no sulfur or mercury and is low in nitrogen and ash and 2) 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 areaand 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.

1.5.5 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.

Pretreatments include different techniques. The most known are mechanical processes, such as chipping and densification (also called pelletization) is a process applying a mechanical force to compact biomass residues or wastes into uniformly sized solid particles, such as pellets, briquettes and logs. The objectives of biomass densification are to increase the volumetric energy density to reduce the transportation cost, and to lower the moisture content (Wei-Hsin Chen et al. 2014).

Wood torrefaction is instead a thermal pretreatment, consisting in a mild pyrolysis process, since it is performed at temperatures between 200 and 300 °C in inert atmosphere (nitrogen is commonly used). 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.

When biomass is torrefied, the pretreatment can be further classified into light, mild and severe torrefaction processes, corresponding to the temperatures of approximately 200–235, 235–275 and 275–300 °C, respectively. With light torrefaction, the moisture and low molecular weight volatiles contained in biomass will be released. Hemicellulose in biomass is the most active constituent among hemicellulose, cellulose and lignin; it is thermally degraded to a certain extent from light torrefaction,

20 whereas cellulose and lignin are only slightly or hardly affected. Therefore, the weight loss of biomass is slight and its energy density or calorific value increases only slightly. When biomass undergoes mild torrefaction, hemicellulose decomposition and volatile liberation are intensified. Hemicellulose is substantially depleted and cellulose is also consumed to a certain extent. With regard to severe torrefaction, hemicellulose is almost depleted completely and cellulose is oxidized to a great extent. Lignin is the most difficult constitute to be thermally degraded; its consumption within the temperature range of torrefaction is thus very low. Hemicellulose and cellulose are the main constituents of biomass. By virtue of substantial removal of hemicellulose and cellulose from biomass by severe torrefaction, the weight and energy yield of biomass are usually lowered significantly although the energy density of the fuel is intensified to a great extent. (W.H. Chen at al. 2014)

Main advantages of torrefaction are:

1) Higher heating rate or energy density;

2) Lower atomic O/C and H/C ratios and moisture content; 3) Higher hydrophobicity or water resistivity;

4) Improved grindability and reactivity; 5) More uniform properties.

Figure 1.8 Effects of torrefaction process

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.

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,

21 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.

In this work, we will pay particular attention to torrefaction and its consequences, because samples of torrefied biomass (specifically of palm kernel shells) are a subject matter of this study.

1.6 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.

Table 1.8 Main advantages and disadvantages of different reactor types

REACTOR TYPE PRO CON

Fixed Bed

-Simple Technology; -Low maintenance costs; -Little/medium/big scale; -Tolerates high moisture contents;

-no C in ashes.

-Unsuited for big scale; -Low efficiency.

Fluidized Bed

-Big scale applicability;

-High heat transfer coefficients; -High reaction rate;

-Uniform Temperature.

-High particulate formation; -Medium tar production; -Require fine fuel particle; -Complex technology;

-Possible C presence in ashes.

Entrained Flow

-Big scale applicability; -Great versatility; -Very low tar formation; -Complete C conversion.

-High soot production; -Require fine fuel particle; -Complex technology; -High maintenance costs.

1.6.1 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

22 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.

1.6.2 Bubbling and circulating 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. Bubbling reactors keep the gas stream at relatively low speed and so they can tolerate a great range of fuel particle size; they have good temperature control and vaery efficient heat transfer.

In Circulating reactors the gas stream is at high speed and comprehend fresh and part of recirculated gas; heat transfer is more efficient and temperatures are more uniform, but it’s not a cheap technology.

1.6.3 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 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.

Other kind of reactors, such as Vacuum furnace, Ablative, Rotating cone and Auger, when ready for large scale commercial use, are employed for bio-oil production and are not considered in this work.

23

1.7 Technical Issues

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

1) slagging and fouling; 2) reactor surface corrosion; 3) ash handling;

4) 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.

24

1.8 Purpose of the Thesis

This work is aimed to give a detailed characterization of biomass pyrolysis, specifically devolatilization phenomenon. Experimental study was performed over a selected set of biomass samples of different origins: energy crops (miscanthus), agricultural residues (corn cobs, rice husks and vine prunings) and torrefied biomass (from palm kernel shells, with different degrees of torrefaction).

The study was conducted with laboratory scale facilities, which are important because allow to easily obtain preliminary characterization of biofuels. Thermogravimetric (TG) analysis was used for the study of devolatilization and Thermogravimetry coupled with Fourier Transformed InfraRed spectroscopy (TG-FTIR) was used to study the composition of evolved gases.

Simple models were applied to the acquired data in order to evaluate the kinetics of the process. The simplicity of the modelization, although unable to schematize the complexity of the phenomena, is useful to obtain specific fundamental parameters. They can be applied to more complete systems (like CFD studies) for the feasibility, design, modeling, optimization and scaling of industrial conversion systems of biomass.

The operative conditions reached in the case of industrial scale reactors greatly differ from those of laboratory scale facilities. Therefore further modelization is necessary, in order to predict the gas speciation, when direct measurements are not possible.

This thesis is structured following these ideas. Chapter 2 is dedicated to the detailed description of the methods for biofuels characterization and in Chapter 3 are described the adopted models, used to get the kinetic parameters for devolatilization. In the fourth and fifth Chapters are reported and commented respectively the results of the experimental tests and the application of the kinetic models. In the end, in Chapter 6 is described the speciation of the pyrolysis gaseous products occurring in large scale applications and two predicting models are reported.

25

2. BIOMASS CHARACTERIZATION METHODOLOGIES

All the phenomena that have been seen so far must be described in order to successfully proceed to the design and put into operation a conversion plant. To do so, biomass characterization is an essential step, describing its properties and components through different levels of detail.

Analysis is commonly divided into two major groups:

1) Fundamental analysis: common tests performed by following well known standardized methodologies and procedures. They are of primary importance because they give the “identity card” of a biomass (also called “fingerprinting”), specifying its composition and fundamental properties;

2) Advanced analysis: these tests give a further insight on specific biomass conversion processes. Although they make use of well known technologies there is a general lack of uniform and standardized methodologies, both for laboratory and pilot scale facilities.

2.1 Fundamental analysis

The fundamental analyses are performed in traditional laboratory-scale apparatuses and are based on standardized and commonly accepted procedures and methodologies, so the collected information can be compared and shared. Tests are simple to be reproduced on small amounts of samples. Correct sample preparation methods (e.g., mass-reduction and size-reduction methods) have to be used since the laboratory sample should be representative for the original sample. Furthermore providing an average value with the determined experimental error is important to elaborate parameters from the raw data. Fundamental analysis includes proximate analysis (moisture, volatile matter, fixed carbon and ash content of a biomass), ultimate analysis (carbon, hydrogen, nitrogen, sulfur and oxygen content), heating value determination, biochemical analysis (cellulose, hemicellulose and lignin content) and ash analysis (ash composition). Example of results of these analyses and consequent biomass comparison are reported in figure 2.1.

26 Figure 2.9 Example of resulting triangular diagrams from Vassilev et al. 2013: a) Proximate analysis b) Ultimate analysis…continue…

Figure 2.10 …Example of resulting triangular diagrams from Vassilev et al. 2013: c) Biochemical analysis d) Ash analysis. Abbreviations: B, bituminous coal; BC, beech wood chips; CC, corn cobs; lignite; MM, marine macroalgae; P, peat; PP, plum pits; RH, rice husks; S, sub-bituminous coal; SG, switchgrass; SS, sunflower shells;WS, walnut shells.

2.1.1 Proximate Analysis

Proximate analysis consists of determining moisture (M), volatile matter (VM), fixed carbon (FC) and ashes (Ash) content in biomass sample (figure 2.2).

27 Figure 2.11 Schematic representation of biomass combustible

Volatile matter and fixed carbon constitute the combustible part of the fuel. Moisture represents physically bound water.

The simplified process for the proximate analysis includes three distinct stages: 1. Drying;

2. Devolatilization in N2 atmosphere;

3. Residue (char) Oxidation in oxidizing atmosphere.

Moisture content usually is determined by constantly heating the sample at approximately 110 °C, while volatiles by heating in inert atmosphere until a temperature of 800÷950 °C. The residue is commonly named Char and include both fixed carbon and ash content. They are separated by oxidation in air.

The proximate analysis is fundamental as a preliminary characterization, for the fingerprinting of the fuel and the comparison with similar fuels. The moisture content can be directly related to the heat balance during combustion, the volatile matter content (or VM/FC ratio) is useful for evaluating the ignitability and flame stability in burners, the ash content can be related to deposition mechanism and the use of downstream particulate removal units.

Biomass parameters may be expressed on different bases: “as received” (ar) basis, “dry” (d) basis and “dry and ash free” (daf) basis. The “dry” basis and “dry and ash free” basis consider the parameters without the moisture content and the moisture and ash content of the fuel, respectively. Table 2.1 shows the conversion between different bases.

28 Table 2.9 Conversion from one basis to another

Proximate analysis is usually performed by means of specific ovens. The procedure applied at the Department of Civil and Industrial Engineering (DICI) laboratories uses the thermogravimetric analysis through the thermogravimetric balance.

Equipment

The thermogravimetric (TG) analyzer is a fundamental tool for the characterization of a material and preliminary comparison. Basically, such an instrument 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. Balance resolution is 0,1 μg.

The sample weight and temperature are continuously monitored and the accuracy is generally high.

The small amount of sample required (5 mg or less) is one of the primary advantages of thermogravimetry because allows to save materials and avoid all heat transfer limitations during the test. Usually at least three runs should be programmed in order to guarantee tests reproducibility.

29 Figure 2.12 a) Thermogravimetric balance TA-Q500 b) furnace section

The device used for thermogravimetric analysis is Thermobalance TA-Q500 showed in figure 2.3a. The analyzer is formed by a central body, an external heat exchanger air-water for furnace cooling and a personal computer for device control, data recording and analysis.

The central body comprehends the furnace, gas and water flow pipes, and the micro-balance. Temperature control is operated by setting the desired thermal profile. Information acquired are time, temperature, sample’s weight and rate of weight loss.

Figure 2.3b shows the furnace’s scheme: fuel sample is positioned in an alumina crucible on a platinum plate, that is taken by one of the balance’s arms, right in the furnace’s centre. Microbalance is just atop the furnace.

The furnace is continuously fluxed with a “furnace purge gas” that can vary between 0 and 400 ml/min. The furnace purge gas controls the atmosphere around the sample and removes the volatile products generated in the experimental run. A secondary gas flux (“balance purge gas”) is sent to the balance and then passes through the furnace and has the task of prevent product gas to move toward the microbalance. A thermocouple sited near the sample measures the temperature.

The maximum temperature reachable by the system is 1000°C, while the heating rate may vary from 0 to 100°C/min. The microbalance resolution is 0,1μg. To obtain accurate experimental results is necessary to periodically calibrate the instrument. Two types of calibration are needed for the TGA: temperature and weight calibration.

Weight calibration is almost an automatic procedure performed by the TG dedicated software. It consist of multiple weighing of different, known mass samples (0, 100 and 1000 mg). The program will stop when the measures reach rather stable values.

Temperature calibration is useful for TGA experiments in which accurate temperature measurements are essential. To temperature calibrate the TGA a high-purity magnetic standard is analyzed for its Curie temperature. Those standards are put into the balance's crucible and are posed under a

30 magnetic field generated by a permanent magnet bar. Then a constant temperature ramp until 900 °C is applied.

Figure 2.13 TGA temperature calibration test diagram: Weight change curve (green) with indicated the transition temperatures and the thermal program (red)

As temperature increases, the magnetic properties of the standard materials change so that the effect can be measured by the microbalance as a weight reduction, as can be seen in figure 2.4. Observed transition temperatures of the standards can be determined from the apparent weight-loss curve as the onset temperature of the weight change step. It is useful to repeat the calibration on the same standard sample several times, as shown in figure 2.5, and calculate average observed transition temperatures.

The calibration is repeated for different heating rates.

Figure 2.5 Multi-cycle TGA temperature calibration for a heating rate of 40 °C/min Table 2.10 TGA temperature calibration data: observed and theoretical values

T observed (°C)

T correct (°C)

5 °C/min 10 °C/min 20 °C/min 40 °C/min

23.45 23.45 23.45 23.45 24.4

154.67 158.47 164.88 177.97 145.4

31

747.95 750.73 759.17 775.31 747.5

The observed and correct temperatures correspond to the experimental and theoretical transition temperatures of the calibration material. From one to five temperature calibration points can be entered in the calibration table (Table 2.2). A multiple-point calibration is more accurate than a one-point calibration.

Figure 2.6 TGA temperature calibration points and curves (tendency lines with indicated the correlation factors) for different values of the heating rate

Procedure

Volatile matter and fixed carbon contents are strongly influenced by the operating conditions of the specific method, hence they have to be declared. Every fuel sample is previously milled. A sample of 5 mg is tested. The total gas flow is 100 ml/min. These conditions assure limited thermal gradients inside the sample and negligible secondary reactions.

The thermal program imposed to the fuel sample is the following, for a heating rate of 20 °C/min:

1. Ramp of 20 °C/min from 30 to 105°C in N2 gas flow;

2. Isotherm for 10 minutes; 3. 20 °C/min ramp until 900 °C;

32 4. Isotherm for 10 minutes;

5. Equilibrate at 800 °C; 6. Switch gas from N2 to air;

7. Isotherm of 10 minutes.

The first temperature ramp and isotherm at 105 °C assure the complete moisture loss within the sample. The following ramp and isotherm serve to complete the devolatilization process, in which volatile matter is released. Then, the last isotherm in oxidizing atmosphere allows to evaluate the ash content from the residual weight. The fixed carbon is obtained by difference. The entire process is shown in figure 2.7.

The values immediately obtained by this thermogravimetric analysis are considered “as determined” and then are reported on a “dry” basis.

Figure 2.14 Weight loss curve (green) and thermal program curve (red) during the proximate analysis (M moisture, VM volatile matter, FC fixed carbon, Ash)

2.1.2 Ultimate Analysis

33 The Ultimate analysis is fundamental to give the composition of the fuel. It gives the content of Carbon, Hydrogen and Nitrogen of the fuel and can be integrated with the determination of Sulfur and Chlorine content. Oxygen content is usually determined by difference in a daf basis. This is basic for material balances of all processes and its parameters can also be used in many correlations, i.e. to evaluate the energetic quality of a specific fuel by calculating its heating value (high contents of carbon and hydrogen increase it, while oxygen and nitrogen make it to decrease). Nitrogen and sulfur also have consequences in pollutants production and their relative issues.

The ultimate analysis is carried out according to the Dumas method, that is to measure gaseous oxidation products after complete oxidation of the fuel with pure oxygen. The fuel sample is previously dried at 105°C in a ventilated furnace, then is dropped into a hot furnace (950°C) and flushed with oxygen for rapid and complete combustion. The products of the combustion are passed through a secondary furnace (850°C) for further oxidation and particulate removal. A series of detectors finally measure the different gaseous compounds.

Equipment

This analysis is performed with the CHN analyzer, the “TruSpec CHN” by Leco shown in figure 2.8. The device usually returns the results of the analysis within few minutes for all elements.

34 Figure 2.16 "U" combustion tube of the TruSpec CHN analyzer

The “U” tube showed in figure 2.9 is where combustion takes place. It begins in the primary part of the tube (the right side one) while in the secondary there is a post-combustion that guarantees a complete oxidation. Porous crucibles collect ashes formed during the analysis.

The combustion gases are collected in a vessel (ballast). The combustion gases in the ballast are then purged through two infrared detectors (a CO2 and a H2O detector) and an aliquot loop. Carbon is

measured as carbon dioxide by the CO2 detector and hydrogen is measured as water vapor in the

H2O detector. The gases in the aliquot loop are swept through hot copper to remove oxygen and

reduce NOx to N2, and flowed through adsorbents to remove carbon dioxide and water. A thermal

conductivity cell is used to determine the nitrogen content. The final result is displayed as weight percentage of C, H and N in the sample. A detailed scheme of the analyzer is shown in figure 2.10.

The device requires three gas streams: Oxygen (99,99% purity) that is the combustion agent, Helium (99,99% purity) that transports the combustion products and Dry Air that has a pneumatic function.

Procedure

The standard procedure requires a previous calibration with standard materials.

Approximately 0,10÷0,15 g of previously milled and dried fuel sample have to be placed inside a Tin Foil Cup that is then put inside the CHN device. Thereafter the analysis can be started. At least three replicates have to be made, in order to obtain reliable results.

35 Results are obtained in a dry basis and can be transformed according to table 2.1. Ash is provided by the proximate analysis and oxygen content can be finally obtained by difference after changing results in a daf basis. This oxygen amount excludes the one within the oxides in ashes.

(1)

Figure 2.17 CHN analyzer flow sheet

2.1.3 Heating Value

Heating Value or Calorific Value is the most important biomass property for energetic uses, as it quantify the energy feed during complete combustion (in a normalized calorimeter and under oxygen atmosphere) for mass unity of a certain fuel. It is distinguished between High and Low Heating Value

36 (HHV or LHV) depending on whether the water formed during combustion is in liquid or vapour phase, respectively. HHV contains the latent heat of vaporization of fuel water, while LHV does not. Usually heating values are obtained referring to dry materials, with moisture content equal to zero. If moisture is present, HV must be decreased of an amount proportional to biomass moisture content. It is generally expressed on a dry sample as MJ/kg.

HV determination takes advantage of proved techniques. The sample, previously dried at 105°C in a ventilated furnace, undergoes complete combustion in a controlled environment. The measure is based on the method of Berthelot-Mahler: the heat released by a known quantity of fuel sample combustion, within a specific “bomb” (Mahler bomb), is evaluated as the temperature increase of a known amount of water contained into the calorimeter. Conditions inside the bomb assure that the water formed during combustion is in liquid phase.

The calorimeter itself is composed of a tank containing water (jacket) and a combustion vessel (bomb) immersed in another water vessel (bucket). The combustion vessel is a high-pressure stainless steel vessel where 0.5÷1 g of the sample is inserted. It is filled with pure oxygen up to 25-30 atm and placed into the tank filled with 2 liters of distilled water. The sample is ignited by an ignition wire. The combustion starts, consequently the vessel releases heat to the water, increasing its temperature, which is measured by an electronic thermometer. Once the combustion is completed the temperature increase is recorded. Calibration tests with a compound of known heating value, such as benzoic acid, are previously performed.

Equipment

The calorimeter used in the laboratory test is the “Isoperibol calorimeter” by LECO (LECO AC-500) shown in figure 2.12a).

a) b)

Figure 2.18 a) Isoperibol calorimeter AC-500 b) Calorimeter section: 1.Stirrer; 2.jacket cover; 3.Ignition conductors; 4.Thermometer; 5.Calorimetric vessel; 6.Jacket; 7.Calorimetric Bomb.

The jacket is a double wall container, thermally insulated and filled with water. Inside the calorimeter there is a stirrer, activated by a constant speed motor, which driveshaft has an insulated section to prevent heat loss through it. Section of the calorimeter can be seen in figure 2.12b).

The calorimetric bomb is in a controlled environment and heat is transmitted to the bucket in which the bomb is placed. Water temperature increase is measured with an electronic thermometer, that

37 has a resolution of 0,0001 °C. Measurements are performed every 6 seconds and are instantly corrected with the necessary parameters.

Main characteristic of isoperibolic calorimeter is the automatic management of jacket and bucket temperatures, which allows to effectuate continuous analysis.

Procedure

Usually fuel samples are milled and dried before analysis is started. Standard procedure requires to correctly prepare the bomb with the crucible and the ignition wire connected to the electrodes and fill it with oxygen gas until 25÷30 atm are reached inside.

The vessel is then filled with a known amount (2 liters) of distilled water and the bomb have to be inserted within. The tests start after the bomb is connected with the ignition conductors.

The calorimeter automatically procures the high heating value of the sample, counting the relative correction factors as shown in the following equation.

(2)

where:

water equivalent of the calorimeter; corrected temperature increase;

correction for formation heat of Nitric Acid; correction for formation heat of Sulfuric Acid; correction for combustion heat of ignition wires; correction for ignition heat of wires;

sample mass.

The Low Heating Value LHV is calculated on the basis of the hydrogen content of the sample (provided by the ultimate analysis):

(3)

At least two replicates are generally performed for each sample.

Alternatively to the experimental determination, the heating value can be calculated from ultimate analysis using the following empirical equation.

(4) Where HHV and LHV are expressed in kJ/kg, C,H,O,S,N are the weight percentage on dry basis and A is the ash content in dry basis.