Steam Explosion Pretreatment of Lignocellulosic Biomass for Advanced Biofuels

Abstract

Biofuels as a sustainable replacement for fuels derived from fossil resources have gathered increasing interest in the past years due to environmental concerns about fossil fuels and unstable oil supply. Lignocellulosic biomass like wood, energy crops or agricultural waste is the most abundant biological material on Earth and is considered as a potential source for sustainable biofuels. Intensive research is carried out on the biochemical conversion process, which consists of the enzyme-catalyzed hydrolysis of cellulose and hemicellulose into sugars and the fermentation of these sugars into the target product. Residual lignin is usually separated and burned for steam and power production but has also a great potential for the production of aromatic monomers. The complex entanglement of cellulose, hemicellulose and lignin makes a pretreatment necessary in order to obtain high yields of fermentable sugars in the subsequent enzymatic hydrolysis. Steam explosion will become most likely the dominating pretreatment technology due to its feedstock flexibility, its cost-savings potential and its comparatively low impact on the further bioprocessing. However, for very recalcitrant biomass, like softwood, steam explosion is not very effective and even after a pretreatment under severe conditions insufficient sugar yields are obtained in the subsequent hydrolysis. Preventing lignin repolymerization by adding a carbocation scavenger like 2-naphthol can significantly increase the enzymatic cellulose conversion in the subsequent hydrolysis. However, this process is still in its infancy and far from its industrial application. The effect of the explosive decompression and the presence of 2-naphthol during the pretreatment on different biomass feedstocks were investigated in order to gain further insight into the process and its application potential. The explosive decompression could significantly enhance the enzymatic cellulose digestibility of woody biomass by particle size reduction. Due to a less pronounced effect of particle size reduction in the pretreatment of herbaceous biomass (corn stover), only minor influences on the enzymatic hydrolysis could be observed. Hardwood (beech) and herbaceous biomass (corn stover) could not benefit from the presence of 2-naphthol during pretreatment as opposed to softwood (spruce). After a steam-pretreatment of beech wood with 2-naphthol even negative influences on the enzymatic digestibility could be observed. The investigation of substituted 2-naphthol molecules in the biomass structure by infraredspectroscopy suggests that 2-naphthol is not integrated into the lignin structure of hardwood. Nevertheless, fundamental investigations on lignin scavenger interactions remain necessary to find out why 2-naphthol only works with softwood. To obtain a homogeneous distribution of the almost non-water soluble 2-naphthol on the biomass, a dip-impregnation of the biomass is necessary. The requirement of large amounts of organic solvents has tremendous ecological and economical impacts on the process. In order to reduce or avoid organic solvents, different 2-naphthol addition methods were developed. Their impact on the enzymatic cellulose conversion and on the economy of the pretreatment process was evaluated. Sprayimpregnation could reduce solvent consumption by almost 90%, while obtaining identical glucose yields compared to a dip-impregnation procedure. Full cellulose conversion could be reached after a pretreatment at a severity of log R0=4.8 and an enzyme dosage of 30 FPU/g cellulose. With a suspension of 2-naphthol in water a cellulose conversion of 82% at a severity of log R0=4.8 and an enzyme dosage of 30 FPU/g cellulose could be reached and at the same time the use of organic solvents completely avoided. This is from an economic point of view the most feasible method, since no expensive solvent recycling is necessary. In order to prevent the degradation of hemicellulose, a two-stage 2- naphthol steam explosion pretreatment was investigated. In the first stage, the process conditions allow for optimal autocatalytic hydrolysis of hemicellulose. The hydrolysate containing the solubilized sugars was withdrawn from the reactor and the remaining solids were pretreated with 2-naphthol under process conditions which allow for high enzymatic cellulose conversion. The two-stage pretreatment could greatly enhance the recovery of hemicellulosic sugars from spruce wood and a maximal yield of 47.5% of hemicellulosic sugars could be reached at a pretreatment severity of log R0=3.75. However, the enzymatic cellulose conversion is reduced and the yield enhancing effect of 2-naphthol is less pronounced compared to a one-stage pretreatment. Therefore, the most likely reason is the high water content of the biomass after the first pretreatment stage, which could have reduced the pretreatment severity of the second pretreatment stage. In addition to influences of 2-naphthol on enzymatic hydrolysis, influences on fermentation were investigated. Therefore, the two-stage pretreated spruce wood was subjected to simultaneous saccharification and fermentation (SSF). SSF of the whole slurry from the second pretreatment stage and a cellulose concentration of 1% w/w, 90% ethanol yield could be reached with a 2-naphthol dosage of 0.205 mol/mol lignin C9-unit compared to a yield of 74% for the control without 2-naphthol. Experiments with higher solid loadings of 5% w/w revealed the inhibitory effect of residual 2-naphthol alone and snyergistically with 5- hydroxymethylfurfural (HMF), furfural, acetic acid. The higher the concentration of HMF, furfural and acetic acid, the lower were the concentrations of residual lignin that were tolerated by the yeast. Lastly, investigations of the molecular weight and the oxidation of the accruing softwood lignin were carried out in order to gain information on its structure and its valorization potential. The studies revealed that spruce which was pretreated with 2-naphthol has a higher extractability with organic solvents and a lower and more defined molecular weight. However, the acidic oxidation of lignin residues pretreated with 2-naphthol could not increase the yield of aromatic monomers. This thesis shows the limitations of the presence of 2-naphthol in the pretreatment of lignocellulosic biomass. It underlines the importance of using not only enzymatic hydrolysis but also fermentation for the evaluation of the pretreatment efficiency so as not to overlook inhibitory effects. The results achieved demonstrate that 2-naphthol can increase the enzymatic cellulose digestibility, but residual 2-naphthol can inhibit the subsequent fermenting yeast cells. Furthermore, it proves that a twostage pretreatment can significantly increase the overall sugar yield from spruce wood. It shows furthermore that the combination of a two-stage pretreatment with subsequent SSF can lead to remarkably high ethanol yields from softwood, even without the addition of 2-naphthol.