Biorefinery models - Lignocellulosic biorefinery
Publication
date: 25/04/2016
Last
update: 25/04/2016
ABOUT
THE SERIES OF POSTS “BIOREFINERY MODELS”
This
post belongs to a series called “Biorefinery Models”. This series is devoted to
briefly describe the models or concepts of advanced biorefineries which have
emerged in the last few years and that are rising currently. These models are
simplified representations which enable us to understand in a simple way the
structure and characteristics of a general biorefinery type. Some of these models
refer to the type of feedstock while other focus on the technologies involved.
A biorefinery may resemble these models or be the result of variations and
combinations of them.
It
should be noted that although these paradigms are very useful and instructive,
they show limitations in describing and classifying complex systems with high
level of integration. In order to define and describe a specific complex case,
the blog recommends use the classification proposed by IEA Bioenergy Task 42
(Feedstocks / Products / Platforms / Technologies). You can learn more about
the general notion of biorefinery and the different biorefinery classifications
in this section of the blog: BIOREFINERY CONCEPT.
LIGNOCELLULOSIC
BIOREFINERY MODEL 1,2,3,4,5,6,7
Lignocellulose
is the most abundant kind of biomass produced from photosynthesis and can be
supplied on a large-scale basis from different low-cost raw materials such as wood,
agricultural residues or municipal and industrial wastes. The conversion of those
'nature-dry' raw materials into different goods (biofuels, biobased chemicals
and biomaterials) is becoming more and more important due to their abundance
and variety, their renewable nature and the good position of the products on
the traditional markets. Additionally, there is no direct resource competition
for food and feedstuff production in the utilisation of lignocellulosic crops
and wastes. Among the models of large-scale industrial biorefineries, the
lignocellulosic biorefinery model in its different forms is the most promising
one.
Chemically,
lignocellulose is a natural composite consisting of three primary fractions:
hemicellulose (sugar polymer of predominantly pentoses), cellulose (a glucose
polymer) and lignin (a polymer of phenols). The structures and compositions of
these biopolymers vary greatly depending on plant species and growth conditions.
Cellulose connects with hemicellulose and lignin by hydrogen bonds, while
hemicellulose and lignin connect with each other by covalent bonds. This tight
and complex spatial structure makes it difficult to use it directly, hence
pretreatment is necessary to achieve efficient conversion.
Figure
1. Lignocellulosic biomass composition (extracted
from Reference 3)
Taking
into account the two previous introductory paragraphs, the specific description
of the lignocellulosic biorefinery concept and its characteristics will be
descripted hereafter. Biorefineries processing lignocellulosic biomass can
follow two primary process routes: thermochemical and biochemical. Most of the
references use the term lignocellulosic biorefinery to refer to models related
to the second route and this post is focused only on that route. The
thermochemical approach to transform lignocellulosic and other type of
feedstocks will be discussed in other posts of this series. The main
characteristics of the biochemical route are summarized in the factsheet below.
Lignocellulosic
biorefinery factsheet (biochemical route)
|
|
Feedstocks
|
Lignocellulosic
crops: wood, Miscanthus, short rotation poplar and willow…
Lignocellulosic
residues: agricultural residues (e.g. straw, bagasse, corncobs), saw mill
residues, municipal wastes…
|
Primary refining
|
Effective
pretreatment is essential for optimal successful downstream operations. It
can include physical, biochemical or thermochemical processes that involve the
disruption of the recalcitrant material of the biomass.
There are several
technologies to carry out this “break”:
• Enzymatic fractionating.
Enzyme systems, specifically tailored to the
particular raw material, are used to transform the cellulose and the
hemicelluloses into monomeric fermentable carbohydrates.
• Hot water or acid chemical
hydrolysis.
• Steam or ammonia fiber explosion.
• Alkaline treatment.
• Organosolv process.
|
Main streams
|
After primary
refining, two different scenarios can be found:
(1) The enzymatic
conversion results in one material flow of fermentable sugars (from cellulose
and hemicellulose) and one material flow of lignin.
(2) The
fractionation of the lignocellulosic raw material generates three material
streams: cellulose, hemicellulose and lignin.
|
Valorization
pathways and products of the fermentable sugars (scenario 1)
|
• Fermentation.
Fermentable sugars (C5/C6 platform) can be sent
directly to biotechnological production of ethanol and other alcohols,
biopolymers, organic acids, amino acids or other bioproducts.
|
Valorization
pathways and products of the cellulose (scenario 2)
|
• Enzymatic hydrolysis and
fermentation.
Cellulose can be hydrolysed to produce sugars
which are then used as a fermentation substrate (the same final products as
the previous point). 3 types of enzyme activity are required for cellulose
hydrolysis to glucose: endo-cellulose, exo-cellulose and β-glucosidase.
• Chemical pulping.
To obtain pulp and paper.
|
Valorization pathways
and products of the hemicellulose (scenario 2)
|
• Enzymatic hydrolysis, fermentation
and other chemical methods.
The separated hemicellulose fraction contains
more or less digested carbohydrates. Due to its variable composition per
biomass source, a tailor made combination of enzyme activities is required
per feedstock.
Monomeric carbohydrates (such as xylose) are
separated and then refined by fermentation. Other valuable compounds (acetic
acid, furfural…) can be extracted and chemically processed.
|
Valorization
pathways and products of the lignin (scenario 1 and 2)
|
• Upgrading to fibre products by
material-oriented technologies.
• Selective degradation to monomeric
and dimeric aromatic compounds.
• Production of binders and
adhesives.
Lignin derivatives are being studied as a
potential alternative to formaldehyde-containing resins for applications such
as fibreboard panels.
• Thermochemical routes (combustion,
gasification…)
Production of bioenergy and biofuels.
|
Figure
2. Scheme with the products that can be obtained from the different fractions
of the lignocellulosic biomass (extracted from Reference 1)
BIOREFINERIES
AT COMMERCIAL SCALE
Currently,
the greatest exponent of the lignocellulosic biorefineries models is the
cellulosic ethanol biorefinery. In the last few years, the first facilities at
commercial scale started up in Italy, Brazil and USA. Several projects are
being constructed or planned. More information can be found in the following
links:
Until
now, the emphasis has been on the production of ethanol. In the following
years, great efforts will have to be made to deploy new integrated facilities
that take advantage of the whole potential of the lignocellulosic biorefinery
model. One of the key points will be the valorization of the lignin to obtain
high added-value products.
____________________________________________________________________________
REFERENCES
1 B. Kamm,
M. Kamm: “Principles of biorefineries”.
Appl Microbiol Biotechnol (2004) 64: 137–145.
2 “The European Biorefinery 2030 Vision”. Star-COLI
BRI -Strategic Targets for 2020 – Collaboration Initiative on Biorefineries.
3 T.G.
Rials: “The Lignocellulosic Biorefinery:
Vision and Implementation”. Presented at American Institute of Chemical
Engineers Knoxville-Oak Ridge Section Meeting, 15 February 2007.
4 “Biorefineries Roadmap as part of the German
Federal Government action plans for the material and energetic utilisation of
renewable raw materials”. May 2012.
5 “Biorrefinerías. Situación Actual y
Perspectivas de Futuro”. Genoma España /CIEMAT.
6 A.
Limayema, S.C. Ricke: “Lignocellulosic
biomass for bioethanol production: Current perspectives, potential issues and
future prospects”. Progress in Energy and Combustion Science, 38 (2012),
449-467.
7 J.H. Reith, R. van Ree, R. Capote, R.R.
Bakker, P.J. de Wild, F. Monot, B. Estrine, A.V. Bridgwater, A. Agostini: “Lignocellulosic feedstock biorefinery for
co-production of chemicals, transportation fuels, electricity and heat”. Presented
at International Workshop on Biorefinery, 22 June 2009, Madrid.
