Steam Explosion Pretreatment of Lignocellulosic Biomass for Advanced Biofuels
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2019
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Doctoral Thesis
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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.
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Examiner : Rudolf von Rohr, Philipp
Examiner : Panke, Sven
Examiner : Studer, Michael
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ETH Zurich
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03348 - Rudolf von Rohr, Philipp (emeritus) / Rudolf von Rohr, Philipp (emeritus)
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136709 - Use of scavengers in the pretreatment of lignocellulosic biomass for improved chemicals production (SNF)