An autohydrolysis pretreatment that suppresses lignin repolymerisation helps overcoming the recalcitrance of softwood for enzymatic hydrolysis of its cellulose.

BackgroundSteam explosion pretreatment has been examined in many studies for enhancing the enzymatic digestibility of lignocellulosic biomass and is currently the most common pretreatment method in commercial biorefineries. The information available about the effect of the explosive decompression on the biochemical conversion is, however, very limited, and no studies prove that the latter is actually enhanced by the explosion. Hence, it is of great value to discern between the effect of the explosion on the one hand and the steaming on the other hand, to identify their particular influences on enzymatic digestibility.ResultsThe effect of the explosive decompression in the steam explosion pretreatment of spruce wood chips on their enzymatic cellulose digestibility was studied systematically. The explosion had a high influence on digestibility, improving it by up to 90 % compared to a steam pretreatment without explosion. Two factors were identified to be essentially responsible for the effect of the explosion on enzymatic digestibility: pretreatment severity and pressure difference of the explosion. A higher pretreatment severity can soften up and weaken the lignocellulose structure more, so that the explosion can better break up the biomass and decrease its particle size, which enhances its digestibility. In particular, increasing the pressure difference of the explosion leads to more defibration, a smaller particle size and a better digestibility. Though differences were found in the micro- and nanostructure of exploded and non-exploded biomass, the only influence of the explosion on digestibility was found to be the macroscopic particle size reduction. Steam explosion treatments with a high severity and a high pressure difference of the explosion lead to a comparatively high cellulose digestibility of the—typically very recalcitrant—softwood biomass.ConclusionsThis is the first study to show that explosion can enhance the enzymatic digestibility of lignocellulosic biomass. If the enhancing effect of the explosion is thoroughly exploited, even very recalcitrant biomass like softwood can be made enzymatically digestible.Electronic supplementary materialThe online version of this article (doi:10.1186/s13068-016-0567-1) contains supplementary material, which is available to authorized users.

The effects of different additives on lignin repolymerisation in the autohydrolysis pretreatment of softwood and the consequences for enzymatic cellulose digestibility have been studied.

BackgroundSteam explosion pretreatment has been examined in many studies for enhancing the enzymatic digestibility of lignocellulosic biomass and is currently the most common pretreatment method in commercial biorefineries. It is however not effective for overcoming the extremely high recalcitrance of softwood to biochemical conversion. Recent fundamental research in small-scale liquid hot water pretreatment has shown, though, that the addition of a carbocation scavenger like 2-naphthol can prevent lignin repolymerization and thus enhance the enzymatic digestibility of softwood cellulose. This work studies the technical application potential of this approach in a larger steam explosion pilot plant for surmounting softwood recalcitrance.ResultsThe addition of 35.36 g 2-naphthol to the steam explosion pretreatment of 1.5 kg spruce wood chips allowed to considerably enhance the enzymatic cellulose digestibility. Different ways of adding the solid 2-naphthol to steam pretreatment were tested. Mixing with the biomass before pretreatment could enhance digestibility by up to 55% compared to control experiments. Impregnation of the biomass with 2-naphthol was yet more effective. Acetone and ethanol were tested to dissolve 2-naphthol and impregnate the biomass. The solvents were then removed again by evaporation before the pretreatment. The impregnation allowed to enhance digestibility by up to 179 and 192%, respectively. A comparison to prevalent acid-catalyzed steam explosion pretreatments for softwood revealed that the scavenger approach allows for obtaining exceptionally high yields in enzymatic hydrolysis. The biomass impregnation with 2-naphthol even renders a complete enzymatic cellulose conversion possible, which is remarkable for a softwood pretreatment not removing lignin. Steam pretreatment experiments without explosive decompression revealed that the enhancing effects of the explosion and the scavenger complement each other well. The explosion enhances the accessibility of the cellulose while the use of the scavenger reduces particularly the deactivation of enzymes.ConclusionsThis is the first study to show that a carbocation scavenger in steam pretreatment can enhance the enzymatic digestibility of lignocellulosic biomass. The approach opens up a novel possibility for overcoming the high softwood recalcitrance in a process that does not require an acid catalyst or the removal of lignin from the biomass.Electronic supplementary materialThe online version of this article (doi:10.1186/s13068-017-0816-y) contains supplementary material, which is available to authorized users.

The utilization of abandoned lignocellulosic residues for chemical production has a strong potential to partially substitute chemicals, which are traditionally produced from non-renewable resources. Softwood especially, with its high availability, presents a sustainable resource for the conversion to higher value-added products such as biofuels and bioplastics. In this study, we investigate mature and innovative technologies for the conversion of softwood to the platform chemical sugar from an economic and environmental perspective. We show that the conventional enzymatic hydrolysis has high economic as well as environmental burdens and that the increase of enzyme availability via a carbocation scavenger process is the key solution to overcome them. Furthermore, we present a process design based on concentrated acid hydrolysis, which is both environmentally and economically competitive compared to conventional production from sugarbeet. The low energy and raw material requirements combined with heat integration and moderate capital costs makes this technology attractive for utilization of softwood residues. This proves that lignocellulosic residues have the potential to become an important raw material in the future bioeconomy.

No abstract

For the production of second generation biofuels from lignocellulosic biomass, pretreatment of the biomass feedstock is necessary to overcome its recalcitrance in order to gain fermentable sugars. Due to many reasons, steam-explosion pretreatment is currently the most commonly used pretreatment method for lignocellulosic biomass on a commercial scale [S. Brethauer and M. H. Studer, CHIMIA, 2015, 69, 572-581]. In contrast to others, we showed that the explosive decompression at the end of this pretreatment step can have a positive influence on the enzymatic digestibility of softwood, especially in combination with high enzyme dosages [T. Pielhop, et al., Biotechnology for Biofuels, 2016, 9, 152]. In this study, the influence of the explosive decompression on the enzymatic digestibility of hardwood and herbaceous plants was systematically studied. Beech and corn stover were pretreated under different pretreatment conditions and enzymatically hydrolysed with different enzyme dosages. The maximum enhancement of the digestibility of corn stover was 16.53% after a 2.5 min pretreatment step at 15 barg steam pressure. For beech, a maximum relative enhancement of 58.29% after a 10 min pretreatment step at 15 barg steam pressure could be reached. With this, we show that the explosive decompression can also enhance the enzymatic cellulose digestibility of hardwood and herbaceous plants.

The recent discovery that the prevention of lignin repolymerisation/condensation in lignocellulosic biomass pretreatment can both enhance the bioconversion of cellulose and the quality of the obtained lignin, has brought a lignocellulose biorefinery closer to reality. In this work, the development of this approach and the last advancements are reviewed. The review reveals the successful implementation for a wide range of lignocellulosic substrates including softwood, hardwood, and agricultural residues. As well, it is shown that the approach can enhance various pretreatment technologies, including steam, acid and organosolv processes. Recent developments involve the discovery of new and greener additives which prevent lignin repolymerisation, the implementation of cellulose saccharification at industrially realistic conditions and high-yield fermentation. In addition, first applications of the lignin obtained in these processes are reviewed, showcasing its enhanced quality for functionalisation and use in polymers, as well as for its depolymerisation to aromatic monomers. The recent progresses bring closer the prospect of a biorefinery that can valorise all fractions of lignocellulosic biomass.