Evolution of the char physicochemical properties during gasification

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Biomass and its composition

Biomass definition

The European Parliament directive 2009/28/EC of 23 April 2009 on the promotion of the use of energy from renewable sources defined biomass as “the biodegradable fraction of products, waste and residues from biological origin from agriculture (including vegetal and animal substances), forestry and related industries including fisheries and aquaculture, as well as the biodegradable fraction of industrial and municipal waste” (European Parliament, 2009). Therefore, biomass is a feedstock that includes a variety of different resources. Vassilev et al. classified biomass into six groups: wood and woody biomass, herbaceous and agricultural biomass, aquatic biomass, animal and human biomass wastes, contaminated biomass and industrial biomass wastes (semi-biomass), and biomass mixtures (Vassilev et al., 2010). In this work, as the focus was on the gasification process, only the first two groups—i.e. dry land-based vegetation—were studied. For simplification purpose, these groups were referred as biomass or lignocellulosic biomass.

Biomass composition

Biomass composition is complex and involves several hundreds of compounds, divided between organic and inorganic fractions detailed below. However, some compounds do not perfectly fit with these two fractions. For example, N- and S-compounds can be in both fractions while oxalates are considered as organic minerals (Vassilev et al., 2012).

Organic fraction

Organic fraction elements

Biomass is typically composed of organic-forming elements, namely C, O, H, N and S (Vassilev et al., 2010). The proportion of these elements in lignocellulosic biomass expressed in dry ash-free basis is reported in Table I.1. The typical content of coal, the main solid fossil fuel, is also presented. Table I.1 shows a higher ratio of C and S and a lower ratio of O for coal compared to lignocellulosic biomass (Vassilev et al., 2010).
In addition to the composition difference, the biomass energy content is lower than solid fossil fuels. The lower heating value (LHV) of biomass is 15 – 20 MJ.kg-1 whereas for solid fossil fuel it is 20 – 40 MJ.kg-1. Differences between biomass and solid fossil fuels in terms of composition and LHV can be correlated with H/C and O/C ratios, as shown in the Van Krevelen diagram (Figure I.1). Such difference can be explained by the fact that C – H and C – O bonds have a low energy content compared to C – C bonds (McKendry, 2002a).

Organic fraction macromolecules

The organic fraction of the lignocellulosic biomass is organized in three types of macromolecules: cellulose, hemicellulose and lignin. The lignocellulosic biomass structure is represented in Figure I.2.
Cellulose is a linear and partly crystalline glucose polymer with an average polymerization degree around 10 000 for wood (Bajpai, 2016; Deglise and Donnot, 2017) and an average molecular weight around 100 000 (McKendry, 2002a). It constitutes 40 – 50 % of the biomass by weight (de Lasa et al., 2011; McKendry, 2002a).
Hemicelluloses are polysaccharides whose units are glucose, xylose, galactose, arabinose and mannose. It is a branched macromolecule with a random and amorphous structure and with an average molecular weight lower than 30 000. It represents 20 – 40 % of the biomass weight (McKendry, 2002a; Vassilev et al., 2012).
Lignin is a highly branched polyaromatic macromolecule. Its building blocks are made up of a three carbon chain attached to an aromatic ring of six carbon atoms, with zero to two methoxyl groups. These building blocks—depicted in Figure I.2—are linked mainly through ether bonds and arranged irregularly forming an amorphous three-dimensional structure that varies among biomass species (McKendry, 2002a; Vassilev et al., 2012).
The exact arrangement of these macromolecules is a subject of ongoing research. The biomass structure seems to be formed of cellulose macromolecules linked by hydrogen and van der Waals bonds in microfibrils, themselves grouped in fibers (Bajpai, 2016). This rigid matrix would then be covered by hemicelluloses and lignin macromolecules.

Inorganic fraction

In addition to the organic fraction, biomass contains inorganic elements, namely Cl, Ca, K, Si, Mg, Al, Fe, P and Na. Some other elements can also be found as trace elements (<1 % of the inorganic content), for instance Mn, Ti, B, Be, Rb, Cr, Ni, Cu, Se, Zn (Vassilev et al., 2013).
Few extensive studies are available on the inorganic fraction. This section is mainly based on the work of Vassilev et al. who gathered data from literature to publish several reviews on the subject that were complemented by their own experimental work (Vassilev et al., 2010, 2012, 2013).

Inorganic elements in biomass

Biomass composition differs from solid fossil fuels. Inorganic elements in biomass are mainly Si, K and Ca, while those in solid fossil fuels are rather Si, Al, Fe and Ca (Vassilev et al., 2010). More specifically, Vassilev et al. (Vassilev et al., 2010) identified the main inorganic elements several biomass subgroups:
Wood and woody biomass: Ca > Si > K > Mg > Al > P Herbaceous and agricultural biomass:
Grass: Si > K > Ca > P > Mg > Al
Straw: Si > K > Ca > Mg > P > Al
Other residues such as shells and husks: K > Si > Ca > P > Mg > Al
These rankings are trends from mean values calculated for each subgroup. However, in each subgroup, individual biomass species can have slightly different compositions that modify the order of occurrence of the main inorganic elements.

Inorganic compounds in biomass

These inorganic elements are present in the form of various compounds. However, as stated by Vassilev et al. (Vassilev et al., 2012), “the direct methods for determination of the structural components [of raw biomass] are very rare”. Nevertheless, they compiled the data of 197 samples from 25 references and of their own characterizations on 8 biomass samples to establish a list of the compounds identified in biomass. Identified inorganic compounds were:
Silicates such as SiO2, Ca-silicates or aluminosilicates;
Oxides and hydroxides such as Mg(OH)2, Ca(OH)2 or Fe2O3;
Sulphates, sulphites and sulphides (not typical in lignocellulosic biomass) such as CaSO4 or K2SO4;
Phosphates such as Ca-phosphates, Ca-Mg-phosphates or K-phosphates; Carbonates such as CaCO3 or CaMg(CO3)2;
Chlorides such as KCl or K-Ca-chloride; Nitrates such as KNO3 or Ca-nitrates;
Other inorganic matter such as metals or glass (not typical in clean lignocellulosic biomass).
The available data did not allow to link these compounds to particular biomass subgroups. Moreover, there was a high variability due to the inorganic content dependence on several factors such as the biomass genetics, its environment or the biomass part considered (Vassilev et al., 2012).
For example, silicates and in particular SiO2 are typically found in soil. The presence of these compounds in biomass can be either because they are formed in the biomass (authigenic fraction) or because they are formed outside and then fixated in the biomass (detrital fraction). The detrital fraction of silicates can come from the fixation of fine particles, brought by water or wind from the soil, on the plant surface. These fine particles can also be introduced into the plant by water suspensions. In the particular case of SiO2, the authigenic fraction is formed by silicic acid absorption from the soil solutions that precipitates in the biomass structure. SiO2 gives rigidness to the plant tissues where it is found—husk, straw, bark and other supportive tissues. Other compounds can also have various origins. Other examples of authigenic compounds (i.e. formed in the biomass) are sulphates, nitrates and chlorides that come from the evaporation and precipitation of water in the biomass (Vassilev et al., 2012).

Inorganic compounds in biomass ashes

Inorganic elements are often referred to as ash, since the inorganic content of the biomass is usually measured through ash formation by combustion in air at 550 °C (European Standards, 2009). The standards for ash yield measurement are slightly different for biomass and for solid fossil fuels. Ashes are formed at 815 °C for the latter (International Organization for Standardization, 2010). In the case of biomass, such a high temperature can volatilize alkali and alkaline earth compounds, in particular KCl, and induce the release of inorganic carbon as CO2 from alkali and alkaline earth carbonates, in particular CaCO3 (Arvelakis et al., 2004; Xiao et al., 2011). The ash content of wood and woody biomass is typically 0.1 – 8 %. It is typically 0.9 – 20 % for herbaceous and agricultural biomass with straws having the highest ash content, grasses the lowest and other residues such as husks and shells ranging in between (Vassilev et al., 2010). In comparison, the ash yield of solid fossil fuels at 815 °C is commonly 4 – 30 %. Solid fossil fuels contain therefore more inorganic elements than wood and woody biomass but can have the same content as some herbaceous and agricultural biomasses.
More data is available regarding the inorganic composition of biomass ashes obtained from combustion rather than that of raw biomass. This is partly due to the higher concentration of inorganic compounds in ashes which makes the characterization easier compared to the case of raw biomass in which detection issues can occur. However, the results obtained on ashes have to be considered carefully as they are obtained after a thermochemical conversion of the biomass. Vassilev et al. (Vassilev et al., 2013) stated that the phases identified in biomass ashes were “mostly secondary”, i.e. formed during combustion, and “occasionally primary”, i.e. formed originally in the biomass. Out of 96 identified mineral phases, 52 were dominantly secondary and 26 were dominantly primary.
Vassilev et al. reviewed the data from more than 600 references and established a list of 229 species or groups of species identified in the characterization of biomass ashes. They identified 188 species for coal ash by conducting the same work. There were not sufficient quantitative data in literature to determine the proportions of these species in the ashes of each biomass subgroup. Nevertheless, from the characterization of ashes from woody biomass, straw and switchgrass, they established that the mineral species contained in biomass ashes were, in decreasing order of concentration:
Forming (> 10 % of ashes) such as glass, sylvite KCl, calcite CaCO3, leucite KAlSi2O6, anorthite CaAl2Si2O8, K–Ca silicate K2CaSiO4 or K4CaSi3O9 or others, and quartz SiO2.
Major (1–10 %), namely albite NaAlSi3O8, anhydrite CaSO4, ankerite Ca(Mg,Fe)(CO3)2, kaolinite Al2Si2O5(OH)4, siderite FeCO3, cristobalite SiO2, arcanite K2SO4, hematite α-Fe2O3, illite (KH2O)Al2(Al,Si)Si3O10(OH)2, lime CaO, Na silicate Na2Si3O7 or Na2SiO3, fairchildite K2Ca(CO3)2, hydroxylapatite Ca(PO4)3(OH), Ca5(PO4)3(OH) or Ca10(PO4)6(OH)2, merwinite Ca3Mg(SiO4)2, periclase MgO, Ca chlorosilicate Ca2SiO3Cl2, diopside CaMgSi2O6 or CaMg(SiO3)2, glaserite K3Na(SO4)2, K feldspar KAlSi3O8, larnite Ca2SiO4, and portlandite Ca(OH)2.
Minor (0.1–1 %) such as K silicate K2Si4O9 or others, K phosphate K3PO4, K5P3O10 or others, and K carbonate K2CO3.
Some accessory phases (< 0.1 % or only traces) according to the elemental composition of the biomass.
Overall, most of the species identified in biomass ashes were also found in coal ashes. The discrepancies could be explained by the difference in elemental composition. In particular, Ca–K–Mn silicates, Ca–Al–Mn oxides, K–Na–Ca chlorides and K–Ca–Mg–Na carbonates, sulphates and phosphates that were identified in biomass were mostly not found in coal ashes. This result is in accordance with the higher occurrence of Ca, Cl, K, Mg, Mn, Na and P in biomass than in coal. On the opposite, many compounds containing Al, Fe and Ti that are typically found in coal ashes were logically not identified in biomass ashes since the latter contain low amounts of these elements (Vassilev et al., 2013).
Inorganic species such as oxalates, silicates, oxides, hydroxides, sulphates, sulphites, sulphides, phosphates, carbonates, chlorides, nitrates, amorphous inorganic matter and others can be primary phases. Various silicates, oxides, hydroxides, carbonates, sulphates, sulphides, sulphosalts, sulphites, thiosulphates, phosphates, chlorides, chlorites, chlorates, nitrates, oxalates, amorphous inorganic matter and glass can be secondary phases. In particular, glass contained in biomass ashes is the result of the fusion and rapid cooling of inorganic compounds from the biomass during ash formation, i.e. combustion. It mainly contains Si, K, Ca and Na.

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Biomass conversion to energy

Lignocellulosic biomass can be processed to obtain:
Energy through heat and electricity generation;
Energy through conversion to liquid or gaseous fuels; Chemical feedstock materials.
In the context of the conversion to a fuel, biofuels generations are identified and are related to the biomass nature. First generation biofuels are obtained from edible biomass such as sugar, starch and oil crops. Second generation biofuels rely on non-edible feedstocks such as wood, agricultural residues and forestry waste. They have a lower negative impact on the environment than the first generation as they have a limited influence on the usage of arable land and food crops (Luque et al., 2008; Sikarwar et al., 2017). Third generation biofuels are produced from algae, in particular microalgae. Finally, a fourth generation of biofuels is appearing. It is based on the genetic modification of microorganisms to make them consume higher quantities of CO2 that what would be released through their use (Alalwan et al., 2019). Dry lignocellulosic biomass studied in the present work belonged to the second generation.
Processes for energy production can be grouped in three categories: mechanical extraction, biochemical processes and thermochemical processes (McKendry, 2002b; Sikarwar et al., 2017). However, only thermochemical processes are applied to dry lignocellulosic biomass.
Thermochemical conversion processes include combustion, pyrolysis, gasification and hydrothermal processes. The energy products obtained from each thermochemical process are presented in Figure I.3. Hydrothermal processes, i.e. hydrothermal carbonization and liquefaction, are rather used in the case of wet biomass and not of dry lignocellulosic biomass as in the present work. Combustion produces hot gases at temperatures between 800 and 1000 °C and its energy can be used in the form of heat or can be converted to mechanical power or to electricity (McKendry, 2002b). Pyrolysis and gasification are discussed in detail in the following section.
Figure I.3 | Thermochemical conversion processes, intermediate products and final energy products. Reprinted from (Patel et al., 2016) with permission from Elsevier.
The focus of the present work was the biomass gasification thermochemical process.

The pyrogasification of biomass

Though it is not widely used in literature, the term of pyrogasification could rather be used to refer to the gasification process. Indeed, it is usually decomposed in three main steps: biomass drying, biomass pyrolysis and residual char gasification (Basu, 2010a; de Lasa et al., 2011; Sikarwar et al., 2017; Van Swaaij, 1981). These steps are overlapping and are themselves groups of various reactions. Figure I.4 shows a more detailed scheme of the pyrogasification steps with the intermediary and final products.
Figure I.4 | Biomass pyrogasification steps. Reprinted from (Baker and Mudge, 1984) with permission from Elsevier.
The following sections present the existing pyrogasification technologies to date and describe the pyrolysis and gasification steps.

Gasifier technologies for biomass

Three main reactor technologies are in use for biomass pyrogasification: fixed bed gasifiers, fluidized bed gasifiers and entrained flow gasifiers (Basu, 2010a; de Lasa et al., 2011; Sansaniwal et al., 2017; Sikarwar et al., 2017). The gasification requires a heat input. In most gasifiers, this heat is brought by the combustion of a part of the biomass in air or O2. Such a configuration is called autothermal, while a configuration where heat is brought from outside the reactor is called allothermal.
The main characteristics of each type of technology are summarized in Table I.2 with their advantages and limitations (Dahmen and Sauer, 2015; de Lasa et al., 2011; McKendry, 2002c; Sikarwar et al., 2017). They are then described in more details in the following subsections.

Table of contents :

Introduction
Chapter I. Context and state of the art
1. Biomass and its composition
1.1. Biomass definition
1.2. Biomass composition
1.2.1. Organic fraction
1.2.2. Inorganic fraction
1.3. Biomass conversion to energy
2. The pyrogasification of biomass
2.1. Gasifier technologies for biomass
2.1.1. Fixed bed gasifiers
2.1.2. Fluidized bed gasifiers
2.1.3. Entrained flow gasifiers
2.2. Pyrogasification reactions
2.2.1. Pyrolysis reactions
2.2.2. Gasification reactions
3. The effects of inorganic elements on the steam gasification process
3.1. Assessment of the inorganic effects on char gasification
3.1.1. Effects of all inorganic compounds
3.1.2. The specific effects of K and Si
3.2. Proposed mechanisms for the K-catalysis
3.2.1. Mechanism with intercalation compounds
3.2.2. Mechanisms with a mechanical action
3.2.3. Mechanisms for a carbon atom
3.2.4. Mechanisms for carbon structures
3.2.5. Explanation of the reactivity changes at high conversion
3.3. Proposed mechanisms for the inhibiting effect of Si
3.4. Proposed models
4. Conclusions and objective of the work
Chapter II. Evolution of the char physicochemical properties during gasification
1. Materials and experimental installations
1.1. List of biomass species
1.2. Experimental set-ups
1.2.1. Pyrolysis furnace MATISSE
1.2.2. Thermogravimetric analyzer
1.2.3. Gasification devices
1.3. Characterization techniques
1.3.1. Raman spectroscopy
1.3.2. N2 and CO2 adsorption
1.3.3. Temperature programmed desorption coupled to mass spectrometry
1.3.4. Inductively coupled plasma with atomic emission spectroscopy
1.3.5. Scanning electron microscopy coupled to energy dispersive X-ray spectroscopy
1.3.6. Powder X-ray diffraction
2. Choice of the methodology
2.1. Choice of the biomass samples
2.2. Choice of the char production method
2.3. Choice of the gasification device
2.4. Thermodynamic equilibrium simulation method
3. Results and discussion
3.1. Gasification kinetic profiles
3.2. Results of the characterization of the char carbon matrix
3.2.1. Structure of the carbon matrix
3.2.2. Porosity of the carbon matrix
3.2.3. Surface chemistry of the carbon matrix
3.2.4. Conclusions on the carbon matrix properties
3.3. Results of the characterization of the inorganic fraction of the chars
3.3.1. Volatilization of the inorganic elements
3.3.2. Determination of the inorganic condensed phases
3.3.3. Conclusions on the inorganic fraction
4. Conclusions
Chapter III. Experimental study on the influence of K and Si on biomass gasification kinetics
1. Materials and methods
1.1. Biomass samples
1.2. Model inorganic compounds
1.2.1. Amorphous silica and quartz
1.2.2. Potassium carbonate
1.3. Experimental installation and procedure
1.4. Thermodynamic equilibrium simulation method
2. Steam gasification of the pure materials
2.1. Steam gasification of pure inorganic compounds
2.1.1. Amorphous silica and quartz
2.1.2. Potassium carbonate
2.2. Steam gasification of pure biomass
3. Results of the interactions between materials
3.1. Influence of K2CO3 addition on pyrolysis and steam gasification
3.1.1. Influence of K2CO3 addition on the pyrolysis reaction
3.1.2. Influence of K2CO3 addition on the gasification reaction
3.1.3. Characterization of the ashes
3.1.4. Conclusions on the influence of K2CO3 addition
3.2. Influence of the addition of SiO2 on steam gasification
3.2.1. Influence of the addition mode
3.2.2. Influence of the crystalline form
3.2.3. Influence of the concentration
4. Conclusions
Conclusions and perspectives
1. Conclusions
2. Perspectives
2.1. Towards a full understanding of the K-catalysis mechanisms
2.2. Towards a phenomenological modeling of the inorganics influence on biomass gasification kinetics
2.3. Towards practical applications to the gasification process
References

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