Hemicelluloses are the second sugar-based biomacromolecules by importance that can also be found in biomass. It is the second most abundant biopolymer on earth. Hemicelluloses are amorphous heteropolymers composed of pentoses (xylose and arabinose), hexoses (glucose, galactose, mannose and rhamnose) and carboxylic acids (aceric acid, glucuronic acid and galacturonic acid). There are different types of hemicelluloses, such as xylans, that are characteristic of hardwoods, galactoglucomannans, that are predominantly found in softwoods, among others (e.g. galactans, arabinogalactans, arabinoglycuronoxylans and glucomannans).In comparison with cellulose long and linear polymer chains, hemicelluloses exhibit shorter and branched chains. The repeating units of these polymers are shown in Figure 6. While the cellulose is crystalline, solid, and resistant to hydrolysis, hemicellulose has a random amorphous structure with low strength. It is easily hydrolyzed by a dilute acid or base, as well as multiple hemicellulase enzymes.
The biorefinery: a deconstruction game
For several decades, the society relies on petroleum, coal and gas for the production of fuels, energy and chemicals. These are fossil resources and they are called “non-renewable” due to the long term they take to regenerate themselves during hundreds, even millions of years.
Hydrocarbons found in petroleum are fractionated and upgraded in facilities called “refineries” to produce a wide variety of chemicals that made part of current industrial market.
In an analogous sense, a “biorefinery” is a facility that combines several processes allowing to convert biomass feedstocks into fuels, energy and chemicals.
However, in this case the raw material constitutes a complex polymer matrix that needs to be deconstructed to generate carbon-based compounds that can be purified or upgraded into value-added chemicals.
Biomass pretreatment and fractionation
As presented in section I, lignocellulosic biomass constitutes a three-dimensional matrix making it very resistant to chemical and biological conversions. In order to produce biofuels and chemicals from biomass through biological processes, cellulose and hemicelluloses need to be converted into monomeric sugars.
Lignin is not only embedded around the holocellulose (cellulose plus hemicelluloses) protecting it, but also is a source of strong inhibitors for enzymes and microorganism used in the subsequent stages of the biorefinery.61 In order to reduce the recalcitrance of lignocellulosic biomass some pretreatments and fractionation processes have to be used.
Different pretreatments of biomass can be implemented according to the final purpose and process. Commonly, biomass is initially submitted to a mechanical pretreatment (e.g. grinding, ball milling,62 extrusion,63 acoustic cavitation,64 hydrodynamic cavitation,64 etc.). The main objectives of these pretreatments are to reduce the particle size, to decrease the crystallinity of 45 cellulose and to “unravel” the fibers, increasing their accessibility, and thus, the reactivity during the subsequent processes.
Some pretreatments are called pulping or fractionation methods. These technologies ensure an efficient valorization of each one of the constituents of the lignocellulosic matrix, which is a critical point for an economically viable biorefinery.
Different pulping processes such as: kraft,39,68 organosolv,69,70 ammonia fiber expansion (AFEX),71 alkaline,27 chlorite/acid,72,73 ozonolysis,74 ammonia, 75 ionic liquids,76 acid-steam explosion,77 among others.
The key factors to take into account when choosing an efficient pretreatment method for cellulose valorization are a high cellulose recovery yield regardless of the feedstock, high digestibility of the recovered cellulose (if subsequent enzymatic hydrolysis), no significant sugar degradation, no toxic compounds generated, low energy demand for downstream operations and no need of drying and efficient lignin recovery.
The main advantages and drawbacks of different pretreatment and fractionation methods of biomass are indicated in Table 6.
Thermochemical depolymerization of cellulose
The conversion of cellulose into fermentable sugars has been extensively studied for many years because it is crucial for the development of a cost-effective biorefinery.
One of the most common methods used in industrial scale biorefineries to depolymerize cellulose is enzymatic hydrolysis.27 This method has been frequently criticized for compromising the economic viability of biorefineries56 (this will be discussed in more details in the next sub-section) and the requirement of new processes for this purpose are now being studied.
Concerning this objective, thermochemical processes are being studied as an alternative to enzymatic hydrolysis for the production of sugars from lignocellulosic feedstocks,58,83 but also to convert biomass in other streams than sugars.
There are four types of thermochemical processes each one with different reaction mechanisms:
1. Combustion: excess of oxygen.
2. Gasification: partial use of oxygen.
3. Pyrolysis: absence of oxygen.
4. Liquefaction: use of a solvent, commonly water, ethanol, etc.
Among these four thermochemical processes, combustion and gasification are not designed to produce liquid products. Combustion is used mostly for the recovery of energy and gasification, evidently, for the production of a syngas. Only pyrolysis and liquefaction allow recovering soluble sugars which are the targeted product for the present work.
The next sub-sections will be focused on the liquefaction of (ligno)cellulose that is the most interesting method in this work. Pyrolysis will be shortly recapitulated, since it is considered as another potential alternative for sugar production.
Pyrolysis was also studied for this work, but the results are not presented in the main text for the sake of clarity and brevity. These results are briefly reported in the appendices section 2.
Hydrothermal liquefaction of cellulosic materials
Hydrothermal liquefaction is by principle the best thermochemical process to convert cellulose into fermentable sugars. The main characteristic of this process is the presence of water that guarantees a common hydrolytic mechanism and has been widely studied for the depolymerization of cellulose into sugars.85–91 For this reason, this process has attracted significant interest for the purpose of this work.
In this section, the concept and reaction mechanisms of biomass liquefaction are explained. Then, the effect of operational conditions on the liquefaction products is explained. Finally, the reactors used for this process are briefly summarized.
Effect of hydrolysis conditions on product distribution
The most important operating condition in hydrothermal processes is the temperature. As mentioned before, this will strongly affect the properties of water and will provide the energy required to break the linkages in lignocellulosic biomass.
Yun et al.96 found that 150ᵒC are required in order to break a glycosidic bond in the amorphous zone of cellulose and 180ᵒC in the crystalline one. Higher temperature will provoke the degradation of sugars. Since the conversion kinetics of the soluble compounds are much faster than that of the solid cellulose (with sub-critical water), increasing the temperature will drastically increase the secondary reactions of the soluble sugars, transforming them into lighter compounds (e.g. aldehydes, carboxylic acids, carbon dioxide).85,97,98,87
There is no direct effect of the pressure on the reaction mechanisms. However, if water is partially evaporated, the depolymerization will follow similar reaction mechanisms but with different composition and the energetic requirements of the process will increase significantly.88 It has been reported that a pyrolytic mechanism may be promoted if there is not enough water accessibility.86 The residence time plays a fundamental role for the selective conversion of cellulosic materials into sugars or chemicals. Long residence times will provoke secondary reactions and therefore the degradation of the sugars.89,96,99 A good combination of temperature and residence time are needed in order to optimize the sugar yield.
Table of contents :
A. LITERATURE REVIEW
I. Lignocellulosic biomass
2. Global composition
3. Multi-scale organization of lignocellulosic biomass
3.1. Macroscopic structure of biomass
3.2. Microscopic scale: Porous structure
3.3. Biomacromolecules properties
II. The biorefinery: a deconstruction game
1. Biorefinery classification
2. Biomass pretreatment and fractionation
2.1. Mechanical pretreatment
2.2. Pulping pretreatments
3. Thermochemical depolymerization of cellulose
3.1. Hydrothermal liquefaction of cellulosic materials
3.1.2. Effect of hydrolysis conditions on product distribution
3.1.3. Reactors and processes
3.2. Pyrolysis of cellulose
4. Biological conversion of cellulosic materials
4.1. Brief history of fermentation
4.2. Current definition of fermentation
4.3. ABE Fermentation process
4.4. Clostridial species
4.5. Clostridium acetobutylicum
4.5.2. Growth requirements
III. Combining thermochemical and biological conversion of biomass
1. Existing concepts
2. Proposed concept in this PhD
I. Hydrothermal conversion of cellulosic materials
1. Article 1: Decomposition of cellulose in hot-compressed water: detailed analysis of the products and effect of operating conditions
1.2. Materials and Methods
1.2.2. Liquefaction in hot-compressed water
1.2.3. Analysis of the crystalline structure of solid residues by XRD
1.2.4. Soluble products analysis
1.2.5. Permanent gas analysis
1.3. Results and Discussion
1.3.1. Mass balance
1.3.2. XRD of the solid residues
1.3.3. Characterization of liquids
1.3.4. Permanent gas composition
1.3.5. Mechanism of cellulose conversion in HCW
2. Article 2: Production of soluble sugars: Coupling of fractionation and hydrothermal depolymerization of woody biomass
2.2. Materials and Methods
2.2.1. Biomass and reactants
2.2.2. Biomass delignification
2.2.3. Liquefaction in HCW
2.2.4. Chemical analysis
2.3. Results and discussion
2.3.1. Biomass fractionation
2.3.2. Liquefaction of the beech-extracted pulp in HCW
II. Biological conversion of cellulose-derived products
1. Article 3: Diauxic growth of Clostridium acetobutylicum ATCC 824 when grown on mixtures of glucose and cellobiose
1.2. Materials and Methods
1.2.1. Microorganism and media
1.3.1. General growth and metabolic features of Clostridium acetobutylicum ATCC 824 cultivated with glucose or cellobiose
1.3.2. Fermentation of glucose and cellobiose mixtures by C. acetobutylicum:
2. Article 4: From wood to building blocks: ABE fermentation of carbohydrates produced by hydrothermal depolymerization of wood pulps
2.2. Materials and Methods
2.2.1. Substrate preparation
2.3. Results and discussion
2.3.1. Fractionation and hydrolysis of beech wood
2.3.2. Fermentation of a cellulose-derived mixture of sugars
2.3.3. Fermentation of cellulose hydrolysates
2.3.4. Fermentation of hydrolysates from delignified beech
III. Integration of thermochemical and biochemical processes
1. Article 5: Process integration modeling for a wood biorefinery: Pulping, Liquefaction and Fermentation to produce Building Blocks
1.2. Presentation of the Aspen Plus® model
1.2.1. Overview of the process
1.2.2. Definition of compounds
1.2.3. Process modeling
1.3. Results and discussion
1.3.1. Mass balances
1.3.2. Energy balances
C. CONCLUSIONS AND PERSPECTIVES
I. Appendice 1: Article 6 (in development): Hydrothermal conversion of cellulose-rich pulps in
a fixed-bed reactor. Performed at Curtin University in Perth, Australia
II. Appendice 2: Article 7 (in development): Fast pyrolysis of cellulose with a stage condensation system for the recovery of bio-oil fractions.
III. Appendice 3: Fast pyrolysis of cellulosic materias performed in a micro-fluidized bed reactor coupled to SPI-TOF MS
IV. Appendice 4: Main metabolic pathways of cellobiose utilization by C. acetobutylicum.
V. Appendices 5: Effect of the pH on the acidogenic phase of C. acetobutylicum when grown on glucose or cellobiose in a batch bioreactor
E. RESUMÉ DE LA THESE EN FRANÇAIS