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.
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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.

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