Pilot-scale steam explosion pretreatment with 2-naphthol to overcome high softwood recalcitrance

Abstract

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.

Homolytic and Heterolytic Cleavage of β-Ether Linkages in Hardwood Lignin by Steam Explosion

“…7. In accordance with previous works [8,13] it is observed that the addition of a carbonium scavenger such as 2naphthol (1) allowed a better lignin extractability during the washing step with ethanol, (2) strongly decreased the formation of Hibbert's ketones in SEL3N and SEL4N (S/S' = 13.4 and 16.6 respectively, Table 1). In acid conditions (SEL6-9) the lower S/S' values observed (S/S' = 6.2-2.7) are in accordance with an acid-catalyzed hydrolytic route producing Hibbert's ketones.…”

“…As seen in Table 1 and Fig. 2 and in accordance with previous reports [3,8,13], the presence of naphthol significantly increased the delignification yields.…”

“…This mechanism can be supported by an improvement in lignin extraction in presence of 2-naphthol. Naphthol acts as a carbonium ion scavenger limiting the lignin recondensation though electrophilic substitution reactions with aromatic moieties of lignin [12,13]. (2) An homolytic mechanism with a -O-4 radical cleavage and subsequent coupling reactions has been also proposed [14].…”

Biomass pyrolysis: A review of the process development and challenges from initial researches up to the commercialisation stage

“…Steam explosion pretreatment does not involve the use of chemicals, which is an advantage compared to other pre-treatments. Therefore, steam explosion pre-treatment of biomass prior to pelletisation can increase heating value, bonding properties and hydrophobicity of the wood [2,[156][157][158][159][160][161][162][163][164][165][166][167][168][169][170][171][172][173][174][175] .…”

Comparative analysis of key technologies for cellulosic ethanol production from Brazilian sugarcane bagasse at a commercial scale

“…In fact, there are number of pretreatment technologies that exist; however, only a few processes match industry norms. Table presents some pilot‐scale studies on LB pretreatment that conform to industrially relevant parameters . Technology scouting enabled the best technology suited to particular LB and the conditions of production facility to be located.…”