Cellulosic ethanol – The basics: Conversion pathway - Biochemical
Section: ADVANCED BIOFUELS
Series: Cellulosic ethanol
- The basics: Conversion pathway – Biochemical
Posts: CELLULOSIC ETHANOL
1. Introduction
The biochemical conversion process has
attracted the most attention because of its similarities to the process used to
produce ethanol from corn grain. As it was
highlighted in the first post of this series, the production process for
ethanol made from cellulosic feedstocks requires breaking down the feedstock
into fermentable sugars through innovative technologies in the form of
pretreatment and hydrolysis. The remaining steps are similar to those used in
conventional ethanol plants.
2. Pretreatment
The pretreatment is the first step in all of
the different processes available for the biochemical conversion pathway but the
definition of this unit operation is elusive. Pretreatment makes the lignocellulosic biomass
more prone to biological conversion ensuring a complete substrate use. The chemical
and biological breakdown resistance of lignocellulosic biomass is known as
recalcitrance. It depends on several factors: crystalline structure of
cellulose, degree of lignification and structural heterogeneity and complexity
of cell-wall constituents. The pretreatment process deranges the recalcitrant
structure of lignocellulose: break the lignin sheath, degrades hemicellulose
and reduces crystallinity and degree of polymerization of cellulose.
At commercial level, a robust pretreatment
section is needed to process feedstocks varying in quality and composition. It
is not possible to define the best pretreatment for all the range of feedstocks,
but two key general features can be described:
- It should maximise the amount of sugar
generated while keeping the overall process as simple as possible.
- It should minimise the formation of toxic or
inhibiting compounds to decrease the risk for negative effects in subsequent steps.
Several pretreatment methods have been developed
and tested at various scales. They can be classified in the following
categories:
(1) Physical: milling, grinding, extrusion, microwaves and ultrasonication.
(2) Chemical: alkali, acid, oxidising agents, organic solvents
(organosolv), ionic liquids and deep eutectic solvents.
(3) Physico-chemical: steam explosion (SE) with and without
catalyst, ammonia fiber explosion (AFEX), CO2 explosion, liquid hot
water (LHW) or hydrothermolysis, wet-oxidation.
(4) Biological: whole cell, enzymatic.
Many variations may be made up of combinations of
two or more methods. In general, physical pretreatment is a prerequisite prior
to any other pretreatment methods.
Case study: PROESA pretreatment
The PROESA® technology is the
basis of the world’s first commercial scale cellulosic ethanol plant located
in Crescentino (Italy). Originally developed by Biochemtex, now it is
commercialised by Versalis (Eni chemical company).
The pretreatment process consists
of a “smart” cooking step. Depending from the biomass and the cleanness
guaranteed by the supplier, the feed handling may include a soaking section
to remove debris and impurities. The process then utilises saturated steam to
disrupt the bonds between lignin, cellulose and hemicellulose. The technology
maintains the principal advantages of standard steam and water-based
processes (no chemical addition and high efficiency separation of cellulose
and hemicellulose) while reducing the formation of inhibitors to the
downstream processes.
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3. Hydrolysis and fermentation
After pretreatment, hydrolysis uses enzymes or acids to break down
the complex chains of sugar molecules from the cellulose and hemicelluloses
into simple sugars in preparation for fermentation. This step is also
termed saccharification.
Hydrolysis will result in both glucose (C6 sugars) and xylose (C5 sugars).
Currently, enzymatic hydrolysis may be the better economic choice. Compared to
acid hydrolysis, it is faster, results in better yields and uses less chemical
input.
The efficiency of hydrolysis is highly
dependent on the effectiveness of pretreatment. If pretreatment leaves behind a large amount of
lignin, the yield of simple sugars from hydrolysis may be reduced. The presence
of lignin prevents enzymes from hydrolysing. It is possible to change some
lignin characteristics in order to make it more compatible with the hydrolysis
process.
The process bears a resemblance to the starch
hydrolysis in corn ethanol production, but its complexity is greater. Unlike
amylases and glucoamylases that are available at low prices for starch
hydrolysis, cellulases for cellulose hydrolysis are much more expensive. Starch
hydrolysis only requires one family of enzymes whereas cellulose hydrolysis
requires several families to break cellulose and hemicellulose polymers.
After pretreatment and hydrolysis have released
simple sugars, fermentation
is used to turn as much of that sugar as possible into ethanol. C6 sugars
can be fermented using traditional yeast strains. C5 are not fermented as
easily. “Co-fermenting” means that the organism can simultaneously ferment
glucose and xylose to ethanol.
Three strategies to address the hydrolysis and
fermentation can be distinguished:
(1) Separate (or sequential) hydrolysis and fermentation (SHF)
Enzymatic hydrolysis is initiated while the
slurry is still at an elevated temperature after pretreatment and conditioning.
At this temperature, the enzyme activity is higher. Once the conversion of
cellulose to glucose is complete, the slurry is cooled to fermentation
temperature and inoculated with the fermenting microorganism.
(2) Simultaneous saccharification and co-fermentation (SSCF)
The temperature of the slurry is reduced and
fermentation is initiated before enzymatic hydrolysis is complete. In SSCF, the
enzyme continues to hydrolyse cellulose even after fermentation is started.
(3) Consolidated bioprocessing (CBP)
Cellulases are produced and supplemented to hydrolyse
the cellulose for ethanol production in SHF and SSCF. In nature, some microbes,
can synthesize and excrete cellulases to hydrolyse cellulose as carbon and
energy sources to support their growth and metabolism. This inspires scientists
to engineer mimic systems which combines the production of cellulases, enzymatic
hydrolysis of cellulose and ethanol fermentation of the resulting sugars.
Figure 1. Diagram of cellulosic ethanol
production for the three strategies (a) SHF, (b) SSCF and (c) CBP (extracted
from the Reference [6])
Case study: Sunliquid® hydrolysis and fermentation
Sunliquid it a technology developed by Clariant
to clear the way for cellulosic biofuels. Its process converts
lignocellulosic agricultural residues, such as cereal straw, into cellulosic
ethanol or other biobased chemicals.
The process uses a SHF strategy:
- An enzyme mixture hydrolyses cellulose and
hemicellulose chains to obtain sugar monomers. The enzymes are highly optimized
based on feedstock and process parameters, resulting in maximum yields and short
reaction times.
- Using optimized microorganisms, a one-pot
system simultaneously converts both C5 and C6 sugars to ethanol, delivering
up to 50% more ethanol than conventional processes which convert only C6
sugars.
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4. Separation and lignin recovery
Distillation is a well-established technology as
it is used in conventional ethanol production. Two energy-demanding steps are necessary to obtain purified
ethanol (95.63% by mass) from binary azeotrope ethanol-water. The first step is
a standard distillation that concentrates ethanol up to the level of 92.4–94%
by mass. The second step involves ethanol dehydration to obtain an anhydrous
ethanol.
In order to reduce energy consumption of
conventional distillation, membrane
techniques have gained attention as an alternative because of a
number of advantages that make them attractive for the separation of liquid
mixtures. They have high separation efficiency, energy and operating costs are
relatively low, they produce no waste streams.
Lignin is traditionally viewed as waste
material or low value by-product to fire. In cellulosic ethanol plants, the
residual lignin is often used just for power generation to drive the
fermentation. Usually, a lignin water slurry is recovered at the bottom of
stripping unit and sent to the lignin recovery and valorisation section.
However, to improve the economic viability of
the cellulosic ethanol production from biomass, it is important to add value to
the lignin produced. The research of novel methods for the efficient and cost-effective
extraction of lignin is active. This topic will be studied in future posts.
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References
[1] Randy Schnepf (Coordinator): “Cellulosic
Ethanol: Feedstocks, Conversion Technologies, Economics, and Policy Options”.
Congressional Research Service, 7-5700, R41460, October 22, 2010.
[2] D. Humbird et al.: “Process Design and
Economics for Biochemical Conversion of Lignocellulosic Biomass to Ethanol”.
Technical Report, NREL/TP-5100-47764, May 2011.
[3] F. Cotana et al.: “Lignin as co-product
of second generation bioethanol production from ligno-cellulosic biomass”. Energy
Procedia 45 (2014) 52 – 60.
[4] A. Bušić et al.: “Bioethanol Production
from Renewable Raw Materials and Its Separation and Purification: A Review”.
FTB, July-September 2018, Vol. 56, No. 3.
[5] J. Baruah et al.: “Recent Trends in the
Pretreatment of Lignocellulosic Biomass for Value-Added Products”. Frontiers
in Energy Research, December 2018, Volume 6, Article 141.
[6] Chen-Guang Liu et al.: “Cellulosic
ethanol production: Progress, challenges and strategies for solutions”. Biotechnology
Advances, Volume 37, Issue 3, May–June 2019, Pages 491-504.
[7] Galbe and Wallberg: “Pretreatment for
biorefineries: a review of common methods for efficient utilisation of
lignocellulosic materials”. Biotechnol Biofuels (2019), 12:294.
[8] Monica Padella, Adrian O’Connell and Matteo
Prussi: “What is still Limiting the Deployment of Cellulosic Ethanol?
Analysis of the Current Status of the Sector”. Appl. Sci. 2019, 9, 4523;
doi:10.3390/app9214523, 24 October 2019.
[9] M.A. Mitchell: “The sunliquid® Process:
Bringing New Proven Solutions for Commercial Cellulosic Ethanol Production”.
Presentation, Bio World Congress 2017, Montreal, (Canada, July 25, 2017.
[10] Minna Yamamoto: “St1 Cellunolix®
process – Lignocellulosic bioethanol production and value chain upgrading”.
Presentation, Bio4Fuels Days, October 12th 2018, Oslo.
[11] “PROESA proprietary process technology”.
Versalis technical information.
