Abdellatif Barakat

Lignocellulosic materials into biohydrogen and biomethane: impact of structural features and pretreatment.

Production of energy from lignocellulosic biomass or residues is receiving ever-increasing interest. Among the different processes, dark fermentation for producing biohydrogen and anaerobic digestion for producing biomethane present considerable advantages. However, they are limited by the accessibility of holocelluloses that are embedded in the lignin network. The authors propose a review of works on the conversion of biomass into biohydrogen and biomethane with the comprehensive description of (a) biomass composition and features that may impact on its anaerobic conversion and (b) the impact of different kinds of pretreatment on these features and on the performance of biohydrogen and methane production.

Do furanic and phenolic compounds of lignocellulosic and algae biomass hydrolyzate inhibit anaerobic mixed cultures? A comprehensive review

Nowadays there is a growing interest on the use of both lignocellulosic and algae biomass to produce biofuels (i.e. biohydrogen, ethanol and methane), as future alternatives to fossil fuels. In this purpose, thermal and thermo-chemical pretreatments have been widely investigated to overcome the natural physico-chemical barriers of such biomass and to enhance biofuel production from lignocellulosic residues and, more recently, marine biomass (i.e. macro and microalgae). However, the pretreatment technologies lead not only to the conversion of carbohydrate polymers (ie cellulose, hemicelluloses, starch, agar) to soluble monomeric sugar (ie glucose, xylose, arabinose, galactose), but also the generation of various by-products (i.e. furfural and 5-HMF). In the case of lignocellulosic residues, part of the lignin can also be degraded in lignin derived by-products, mainly composed of phenolic compounds. Although the negative impact of such by-products on ethanol production has been widely described in literature, studies on their impact on biohydrogen and methane production operated with mixed cultures are still very limited. This review aims to summarise and discuss literature data on the impact of pre-treatment by-products on H2-producing dark fermentation and anaerobic digestion processes when using mixed cultures as inoculum. As a summary, furanic (5-HMF, furfural) and phenolic compounds were found to be stronger inhibitors of the microbial dark fermentation than the full anaerobic digestion process. Such observations can be explained by differences in process parameters: anaerobic digestion is performed with more complex mixed cultures, lower substrate/inoculum and by-products/inoculum ratios and longer batch incubation times than dark fermentation. Finally, it has been reported that, during dark fermentation process, the presence of by-products could lead to a metabolic shift from H2-producing pathways (i.e. acetate and butyrate) to non-H2-producing pathways (i.e. lactate, ethanol and propionate) and whatever the metabolic route, metabolites can be all further converted into methane, but at different rates.

Dry fractionation process as an important step in current and future lignocellulose biorefineries: a review.

The use of lignocellulosic biomass is promising for biofuels and materials and new technologies for the conversion need to be developed. However, the inherent properties of native lignocellulosic materials make them resistant to enzymatic and chemical degradation. Lignocellulosic biomass requires being pretreated to change the physical and chemical properties of lignocellulosic matrix in order to increase cell wall polymers accessibility and bioavailability. Mechanical size reduction may be chemical free intensive operation thanks to decreasing particles size and cellulose crystallinity, and increasing accessible surface area. Changes in these parameters improve the digestibility and the bioconversion of lignocellulosic biomass. However, mechanical size reduction requires cost-effective approaches from an energy input point of view. Therefore, the energy consumption in relation to physicochemical properties of lignocellulosic biomass was discussed. Even more, chemical treatments combined with physicochemical size reduction approaches are proposed to reduce energy consumption in this review.

Effect of lignin-derived and furan compounds found in lignocellulosic hydrolysates on biomethane production

Hydrolysates resulting from the lignocellulosic biomass pretreatment in bioethanol production may be used to produce biogas. Such hydrolysates are rich in xylose but also contain lignin polymers or oligomers as well as phenolic and furan compounds, such as syringaldehyde, vanillin, HMF, furfural. The aim of this study was to investigate the impact of these byproducts on biomethane production from xylose. The anaerobic digestion of the byproducts alone was also investigated. No inhibition of the anaerobic digestion of xylose was observed and methane was obtained from furans: 430 mL CH(4)/g of furfural and 450 mL CH(4)/g of HMF; from phenolic compounds: 453 mL CH(4)/g of syringaldehyde and 105 mL CH(4)/g of vanillin; and, to a lesser extent, from lignin polymers: from 14 to 46 mL CH(4)/g MV. The use of different natural polymers (lignosulfonates, organosolv and kraft lignins) and synthetic dehydrogenative polymers showed that higher S/G ratios and lower molecular weights in lignin polymers led to greater methane production.

Comparison of seven types of thermo-chemical pretreatments on the structural features and anaerobic digestion of sunflower stalks.

Sunflower stalks can be used for the production of methane, but their recalcitrant structure requires the use of thermo-chemical pretreatments. Two thermal (55 and 170°C) and five thermo-chemical pretreatments (NaOH, H(2)O(2), Ca(OH)(2), HCl and FeCl(3)) were carried out, followed by anaerobic digestion. The highest methane production (259 ± 6 mL CH(4)g(-1) VS) was achieved after pretreatment at 55°C with 4% NaOH for 24h. Acidic pretreatments at 170°C removed more than 90% of hemicelluloses and uronic acids whereas alkaline and oxidative pretreatments were more effective in dissolving lignin. However, no pretreatment was effective in reducing the crystallinity of cellulose. Methane production rate was positively correlated with the amount of solubilized matter whereas methane potential was negatively correlated with the amount of lignin. Considering that the major challenge is obtaining increased methane potential, alkaline pretreatments can be recommended in order to optimize the anaerobic digestion of lignocellulosic substrates.

Predictive models of biohydrogen and biomethane production based on the compositional and structural features of lignocellulosic materials.

In an integrated biorefinery concept, biological hydrogen and methane production from lignocellulosic substrates appears to be one of the most promising alternatives to produce energy from renewable sources. However, lignocellulosic substrates present compositional and structural features that can limit their conversion into biohydrogen and methane. In this study, biohydrogen and methane potentials of 20 lignocellulosic residues were evaluated. Compositional (lignin, cellulose, hemicelluloses, total uronic acids, proteins, and soluble sugars) as well as structural features (crystallinity) were determined for each substrate. Two predictive partial least square (PLS) models were built to determine which compositional and structural parameters affected biohydrogen or methane production from lignocellulosic substrates, among proteins, total uronic acids, soluble sugars, crystalline cellulose, amorphous holocelluloses, and lignin. Only soluble sugars had a significant positive effect on biohydrogen production. Besides, methane potentials correlated negatively to the lignin contents and, to a lower extent, crystalline cellulose showed also a negative impact, whereas soluble sugars, proteins, and amorphous hemicelluloses showed a positive impact. These findings will help to develop further pretreatment strategies for enhancing both biohydrogen and methane production.

Bio-nanocomposite films reinforced with cellulose nanocrystals: Rheology of film-forming solutions, transparency, water vapor barrier and tensile properties of films

This study was aimed to develop bio-nanocomposite films of carboxymethyl cellulose (CMC)/starch (ST) polysaccharide matrix reinforced with cellulose nanocrystals (CNC) using the solution casting method. The CNC were extracted at the nanometric scale from sugarcane bagasse via sulfuric acid hydrolysis and used as reinforcing phase to produce CMC/ST-CNC bio-nanocomposite films at different CNC loading levels (0.5-5.0 wt%). Steady shear viscosity and dynamic viscoelastic measurements of film-forming solution (FFS) of neat CMC, CMC/ST blend and CMC/ST-CNC bio-nanocomposites were evaluated. Viscosity measurements revealed that a transition from Newtonian behavior to shear thinning occurred when CNC were added. The dynamic tests confirmed that all FFS have a viscoelastic behavior with an entanglement network structure, induced by the hydrogen bonding. In regard to the cast film quality, the rheological data showed that all FFS were suitable for casting of films at ambient temperature. The effect of CNC addition on the optical transparency, water vapor permeability (WVP) and tensile properties of bio-nanocomposite films was studied. It was found that bio-nanocomposite films remain transparent due to CNC dispersion at the nanoscale. The WVP was significantly reduced and the elastic modulus and tensile strength were increased gradually with the addition of CNC. Herein, the steps to form new eco-friendly bio-nanocomposite films were described by taking advantage of the combination of CMC, ST and CNC. The as-produced films exhibited good optical transparency, reduced WVP and enhanced tensile properties, which are the main properties required for packaging applications.

New opportunities for agricultural digestate valorization: current situation and perspectives

In the agricultural sector of many European countries, biogas production through anaerobic digestion (AD) is becoming a very fast-growing market. AD is a simple and robust process that biologically converts an organic matrix into biogas and digestate, the latter corresponding to the anaerobically non-degraded fraction. So far, digestate has been mostly used at farm-scales for improving soils. However, its ever-increasing production induces problems related to transport costs, greenhouse-gas emissions during storage and high nitrogen content that constrains its use to land application only. Consequently, research on alternative valorisation routes to reduce its environmental impact and to improve the economical profitability of AD plants should draw increasing interest in the future. This review therefore focuses on the different alternatives of digestate valorisation, apart from land applications: (I) the use of the digestate liquor for replacing freshwater and nutrients in algae cultivation; (II) the use of solid digestate for energy production through biological (i.e. AD, bioethanol) or thermal processes (i.e. combustion, hydrothermal carbonization and pyrolysis); (III) the conversion of solid digestate into added-value products (char or activated carbons) through a pyrolysis process.

Mechanical pretreatments of lignocellulosic biomass: towards facile and environmentally sound technologies for biofuels production

The transformation of lignocellulosic biomass into biofuels represents an interesting and sustainable alternative to fossil fuel for the near future. However, one still faces some major challenges for the technology to be fully realized including feedstock costs, novel pretreatment processes, production, transportation, and environmental impact of the full chain. The development of new technologies focused to increase the efficiency of cellulose conversion to biofuels determines successful implementation. Mechanical fractionation is an essential step in order to increase final carbohydrate output, appropriate particle sizes and densification, enzymatic accessibility, and bioconversion affectivity without the production of toxic side streams. In this review article, we surveyed a substantial amount of previous work in mechanical fractionation or pretreatments of a variety of lignocellulosic biomasses; these include numerous milling schemes and extrusions, and their impacts on the physical and physicochemical properties of the lignocellulosic matrix (crystallinity, surface area, particle size, etc). We have also compared results with other pure chemical and physicochemical pretreatments in order to show the new aspects and advantages/disadvantages of such an approach. Last, but not least, the effect of mechanical treatment and physical properties on enzymatic hydrolysis and bioconversion has been discussed, with potentially interesting dry lignocellulosic biorefinery schemes proposed.

Studies of xylan interactions and cross-linking to synthetic lignins formed by bulk and end-wise polymerization: a model study of lignin carbohydrate complex formation

The mechanism of lignin carbohydrate complex formation by addition of polysaccharides on quinone methide (QM) generated during lignin polymerisation was investigated using a model approach. Dehydrogenation polymers (DHPs, lignin model compounds) were synthesized from coniferyl alcohol in the presence of a glucuronoarabinoxylan (GAX) extracted from oat spelts, by Zutropfverfahren (ZT) and Zulaufverfahren (ZL) methods. The methods ZT and ZL differed in their distribution of QM over the reaction period but generated roughly the same QM amount. Steric exclusion chromatography of the ZT and ZL reaction products showed that only the ZT reaction produced high molar mass compounds. Covalent linkages in the ZT reaction involving ether bonds between GAX moiety and α carbon of the lignin monomer were confirmed by 13C NMR and xylanase-based fractionation. The underlying phenomena were further investigated by examining the interactions between GAX and DHP in sorption experiments. GAX and DHPs were shown to interact to form hydrophobic aggregates. In the ZT process, slow addition permitted polymer reorganisation which led to dehydration around the lignin-like growing chains thereby limiting the addition of water on the quinone methide formed during polymerisation and thus favoured lignin–carbohydrate complex (LCC) formation.