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CHALMERS
Mixture of fibers
Thermal treatment
Syntheticfibers
Other naturalfibers
Residualfibers
Reuse
Recycling
Physico-chemical processing
Different products
Different products
Fuels, chemicals, energy
Waste textiles
Separation of fibers
Cellulosicfibers
Bioprocessing
Biofuels
Chemicals
Waste Textiles Bioprocessing to Ethanol and Biogas AZAM JEIHANIPOUR Department of Chemical and Biological Engineering
CHALMERS UNIVERSITY OF TECHNOLOGY
Gothenburg, Sweden 2011
THESIS FOR THE DEGREE OF DOCTOR OF PHILOSOPHY
Waste Textiles Bioprocessing
to Ethanol and Biogas
AZAM JEIHANIPOUR
Department of Chemical and Biological Engineering
CHALMERS UNIVERSITY OF TECHNOLOGY
Gothenburg, Sweden 2011
Borås, Sweden 2011
ii
Waste textiles bioprocessing to ethanol and biogas Azam Jeihanipour ISBN 978-91-7385-536-5 Copyright © Azam Jeihanipour, 2011 Doktorsavhandling vid Chalmers tekniska högskola Ny serie nr 3217 ISSN 0346-718X Department of Chemical and Biological Engineering Chalmers University of Technology 412 96 Göteborg Sweden Telephone: +46 (0)31–7721000 Skrifter från Högskolan i Borås, nr. 32 ISSN 0280-381X School of Engineering University of Borås 501 90 Borås Sweden Telephone: +46 (0)33–4354000 Cover: Photo from landfill in Damascus, Syria (by permission from Mohammad Taherzadeh) and a waste textiles refinery with highlighted part, subjected to this thesis. Printed in Sweden Repro-service, Chalmers University of Technology Göteborg, Sweden, 2011
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Abstract
In the world today, the need for sustainable processes is increasing. The work of the present thesis has been focused on conversion of the cellulosic part of waste textiles into biogas and ethanol, and its challenges. In 2009, the global annual fiber consumption exceeded 70 Mt, of which around 40% consisted of cellulosic material. This huge amount of fibers is processed into apparel, home textiles, and industrial products, ending up as waste after a certain time delay. Regretfully, current management of waste textiles mainly comprises incineration and landfilling, in spite of the potential of cellulosic material being used in the production of different biofuels. The volume of cellulose mentioned above would be sufficient for producing around 20 billion liters of ethanol or 11.6 billion Nm3 of methane per year. Nevertheless, waste textiles are not yet accepted as a suitable substrate for biofuel production, since their processing to biofuel presents certain difficulties and challenges, e.g. high crystallinity of cotton cellulose, presence of dyes, reagents and other materials, and being textiles as a mixture of natural and synthetic fibers. High crystallinity of cotton cellulose curbs high efficient conversion by enzymatic or bacterial hydrolysis, and the presence of non-cellulosic fibers may create several processing problems. The work of the present thesis centered on these challenges.
Cotton linter and blue jeans waste textiles, all practically pure cellulose, were converted to ethanol by SSSF, using S. cerevisiae, with a yield of about 0.14 g ethanol/g textile, only 25% of the theoretical yield. To improve the yield, a pretreatment process was required and thus, several methods were examined. Alkaline pretreatments significantly improved the yield of hydrolysis and subsequent ethanol production, the most effective condition being treatment with a 12% NaOH-solution at 0 °C, increasing the yield to 0.48 g ethanol/g textile (85% of the theoretical yield).
Waste textile streams, however, are mixtures of different fibers, and a separation of the cellulosic fibers from synthetic fibers is thus necessary. The separation was not achieved using an alkaline pretreatment, and hence another approach was investigated; pretreatment with N-methyl-morpholine-N-oxide (NMMO), an industrially available and environment friendly cellulose solvent. The dissolution process was performed under different conditions in terms of solvent concentration, temperature, and duration. Pretreatment with 85% NMMO at 120 °C under atmospheric pressure for 2.5 hours, improved the ethanol yield by 150%, compared to the yield of untreated cellulose. This pretreatment proved to be of major advantage, as it provided a method for dissolving and then recovering the cellulose. Using this method as a foundation, a novel process was developed, refined and verified, by testing polyester/cellulose-blended textiles, which predominate waste textiles. The polyesters were purified as fibers after the NMMO treatments, and up to 95% of the cellulose content was regenerated. The solvent was then recovered, recycled, and reused. Furthermore, investigating the effect of this treatment on anaerobic digestion of cellulose disclosed a remarkable enhancement of the microbial solubilization; the rate in pretreated textiles was twice the rate in untreated material. The process developed in the present thesis is promising for transformation of waste textiles into a suitable substrate, to subsequently be used for biological conversion to ethanol and biogas.
Keywords: bioethanol, biogas, biorefinery, cellulose recalcitrance, cotton-based cellulose,
enzymatic hydrolysis, pretreatment, recycling, waste textile.
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List of publications
The thesis is mainly based on the results presented in the following articles:
I. Azam Jeihanipour and Mohammad J. Taherzadeh, “Ethanol production from cotton-
based waste textiles”, Bioresource Technology (2009), 100 (2), 1007-1010.
II. Azam Jeihanipour, Keikhosro Karimi, and Mohammad J. Taherzadeh,
“Enhancement of ethanol and biogas production from high-crystalline cellulose by
different modes of NMMO pretreatment”, Biotechnology and Bioengineering (2010),
105 (3), 469-476.
III. Azam Jeihanipour, Keikhosro Karimi, Claes Niklasson, and Mohammad J.
Taherzadeh, “A novel process for ethanol or biogas production from cellulose in
blended fibers in waste textiles”, Waste Management (2010), 30 (12), 2504-2509.
IV. Azam Jeihanipour, Claes Niklasson, and Mohammad J. Taherzadeh, “Enhancement
of solubilization rate of cellulose in anaerobic digestion and its drawbacks”, Process
Biochemistry, in press.
V. Mahdi Khodaverdi, Azam Jeihanipour, and Mohammad J. Taherzadeh, “Kinetic
modelling of high rated enzymatic hydrolysis of crystalline cellulose pretreated by
NMMO”, manuscript.
Statement of contribution
Azam Jeihanipour is responsible for the main part of writing and laboratory work in all
papers, with some aid from supervisors and co-authors. However, the experimental work and
some parts of modelling of paper V was performed by Mahdi Khodaverdi (MSc-student),
under supervision of Azam Jeihanipour.
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List of related publications not included in this thesis
I. Kambiz Mirahmadi, Maryam Mohseni Kabir, Azam Jeihanipour, Keikhosro
Karimi, and Mohammad J. Taherzadeh, “Alkaline pretreatment of spruce and birch
to improve bioethanol and biogas production”, Bioresources (2010), 5(2), 928-
938.
II. Sara Rahim Labafzadeh, Vahid Jafari, Azam Jeihanipour, Keikhosro Karimi, and
Mohammad J. Taherzadeh, “Construction and demolition lignocelluloses wastes to
ethanol”, Renewable Energy, in press.
III. Abas Mohsenzadeh Syouki, Azam Jeihanipour, Keikhosro Karimi, and
Mohammad J. Taherzadeh, “Enhanced ethanol and biogas production from birch
and spruce by alkaline pretreatments”, submitted.
IV. Azam Jeihanipour, Solmaz Aslanzadeh, Karthik Rajendran, Gopinath
Balasubramanian, and Mohammad J. Taherzadeh, “High rate biogas production
from waste textiles using two-stage process”, manuscript.
V. Azam Jeihanipour, Sarah Carrierea, Sung-Woo Cho, Mikael Skrifvars, and
Mohammad J. Taherzadeh, “Recycling polyester fibers from cellulose/PET blend
waste textiles”, manuscript.
VI. Hossein Bidgoli, Akram Zamani, Azam Jeihanipour, and Mohammad J.
Taherzadeh, “Production of superabsorbents from cellulose part of waste textiles”,
manuscript.
VII. Hojatallah Seyedy Niasar, Keikhosro Karimi, Hamid Zilouei, Vida Rahmani,
Peyman Salehian, and Azam Jeihanipour, “Improvement of biogas production
from dairy cattle manure by lime pretreatment”, manuscript.
vi
Table of contents
Abstract .................................................................................................................................... iii
List of publications .................................................................................................................. iv
List of publications not included in this thesis ....................................................................... v
Table of contents ...................................................................................................................... vi
Chapter 1: Introduction ........................................................................................................... 1
Chapter 2: Textiles: Production, consumption, and disposal .............................................. 3
2.1. Fiber classification ..................................................................................................... 3
2.1.1. Cellulosic fibers ............................................................................................. 5
2.2. Fiber production statistics .......................................................................................... 7
2.3. Environmental impact of textile production and its disposal .................................... 7
2.4. Waste management strategies for textile waste ......................................................... 9
2.4.1 Reuse ............................................................................................................ 10
2.4.2. Recycling ..................................................................................................... 11
Chapter 3: Cellulose: Structure, pretreatment, and bioconversion .................................. 15
3.1. Cellulose structure ................................................................................................... 15
3.1.1. Cellulose polymorphs .................................................................................. 18
3.1.2. Cotton fiber structure ................................................................................... 18
3.2. Hydrolysis of cellulose ............................................................................................ 20
3.2.1. Concentrated acid hydrolysis ...................................................................... 20
3.2.2. Dilute acid hydrolysis .................................................................................. 20
3.2.3. Enzymatic hydrolysis .................................................................................. 22
3.2.4. Enzymatic hydrolysis of waste textiles ....................................................... 24
3.3. Bacterial hydrolysis of cellulose .............................................................................. 24
3.3.1 Uptake, phosphorylization, and fermentative catabolism of cellulose degradation products ............................................................................................. 26
3.4. Pretreatment of cellulose to improve hydrolysis and digestion ............................... 27
3.4.1. Pretreatment with sodium hydroxide .......................................................... 28
3.4.2. Pretreatment with phosphoric acid .............................................................. 30
3.4.3. Pretreatment with NMMO ........................................................................... 31
3.4.4. Effect of NMMO treatment on enzymatic hydrolysis of cellulose ............. 32
3.4.5. Effect of NMMO treatment on anaerobic digestion of cellulose ................ 34
vii
Chapter 4: Biofuels ................................................................................................................. 37
4.1. Why biofuels? .......................................................................................................... 37
4.2. Bioethanol ................................................................................................................ 38
4.2.1 Food versus energy – a conflict .................................................................... 39
4.2.2 A possible solution ....................................................................................... 40
4.2.3. Production of ethanol from waste textiles ................................................... 40
4.3. Biogas ...................................................................................................................... 41
4.3.1. Methane fermentation process ..................................................................... 43
4.3.2. Application of granular sludge to increase the degradation rate of anaerobic digestion ................................................................................................................ 44
Chapter 5: Waste textiles refinery ........................................................................................ 47
Concluding remarks ............................................................................................................... 53
Future work ............................................................................................................................ 55
Nomenclature .......................................................................................................................... 57
Acknowledgments ................................................................................................................... 59
References ............................................................................................................................... 61
1
Chapter 1
INTRODUCTION
According to the Waste Framework Directive (European Directive (WFD) 2006/12/EC)1,
waste has been defined as “any substance or object that the holder discards, intends to discard
or is required to discard”. The amount of waste generated by human activities is steadily
increasing. There is a variety of waste and waste streams. The present thesis deals with the
textile fraction of solid waste.
The annual global fiber consumption exceeded 70 Mt in 2009, and almost all these fibers
are converted into various products. Most of these products are replacements of worn-out or
outdated things; thus the volume of the annual production of waste textiles is presumably
comparable with the yearly volume of fiber consumption. As shown in Figure 1.1, the main
methods of disposal of end-of-life waste textiles are currently landfilling or incineration. In
many countries, there is an increasing concern regarding the environmental impact of disposal
of this enormous amount of waste. This waste is in fact a potentially rich source of energy and
materials.
Fear of depletion of fossil fuel reserves, along with rising concerns about climate changes
caused by greenhouse gas (GHG) emissions, have led to a worldwide interest in replacing
fossil fuels with other sources of energy. Within the transportation sector, biofuels such as
bioethanol and biogas are promising alternatives to fossil fuels. An economically and
environmentally sustainable approach when producing these fuels can only be obtained by
using a low-cost feedstock that will not affect global food supplies and that exerts a minimal
negative environmental impact. Thus, the biodegradable fraction of solid waste may be
considered as an alternative sustainable and cost-effective source for biofuel production in a
biorefinery (Figure 1.1). The objective of the present thesis was to investigate the possibilities
for production of biogas and bioethanol from textile waste.
The cellulosic fraction of the world’s total fiber consumption is around 40%, the same
proportion as the average cellulose content of lignocellulosic materials. Waste textiles hold
1 http://ec.europa.eu/environment/waste/framework/index.htm
2
only negligible amounts of lignin or hemicelluloses. Therefore, using waste textiles for the
process of biofuel production appears less complicated than using lignocelluloses. On the
other hand, waste textiles are composed of different materials, including natural as well as
synthetic non-cellulosic fibers, making bioprocessing difficult. In addition, around 90% of the
global cellulosic fiber consumption is cotton, a material with a recalcitrant structure that
makes it difficult to obtain a high yield in an enzymatic hydrolysis. The primary objectives of
the present thesis were to develop methods for separation of cellulose from the mixture of
fibers and to reduce the cellulose recalcitrance. The thesis furthermore aimed at obtaining an
efficient conversion of waste textiles to valuable products, i.e. biogas and bioethanol.
Figure 1.1: Waste textiles management today and in the future.
The thesis constitutes 5 chapters. Chapter 2 provides a background about textiles,
classifying fibers, and describing different possible waste management strategies that can be
applied for waste textiles. Chapter 3 presents an overview of the recalcitrance structure of
cotton-based cellulose. This chapter describes the mechanisms of hydrolysis of cellulose by
acid and enzymes as well as its digestion by anaerobic bacteria, highlighting the main
challenges of these methods. It furthermore, provides possible solutions to overcome the
difficulties of cotton-based cellulose hydrolysis. Chapter 4 gives the background about using
ethanol and biogas as fuels and also reviews the environmental benefits of using biowastes for
production of ethanol and biogas. In chapter 5, the possibility of using waste textiles as raw
material for biorefinery is discussed, as is the potential solution for industrial production of
value added materials from waste textiles, provided by the present thesis. Finally, the thesis
presents conclusions and suggestions for future studies.
Biorefinery
End-of-lifewaste textiles
Incineration
Landfilling
Materials
Energy
Biofuels
Negative environmental
impact
Waste of resources
Currently
Possible solution for future
3
Chapter 2
TEXTILES: PRODUCTION, CONSUMPTION, AND DISPOSAL
Recent scientific research estimates the habit of making and wearing clothing to be about
190,000 years old [1]. Since the beginning, methods of textile production have been
continually developed. Different types of fibers are employed for production of textiles with a
wide range of purposes and applications. Generally, applications of fibers are categorized into
three broad groups, i.e. apparel, home furnishing, and industrial use [2-3].
2.1. Fiber classification
There is a large variety of fibers. However, all the fibers used in textile manufacturing can
be classified into two main groups, natural and man-made fibers [4]. Figure 2.1 shows the
classification of the most important fibers. Natural fibers, like cotton, wool, silk, and flax are
produced by plants in a fibrous form [4], and three major classes according to their sources
can be distinguished (Figure 2.1) [4-5]:
1- Vegetable fibers, such as cotton and flax, are obtained from different parts of plants,
like seeds and stems. They mainly consist of cellulose, which is the structural material
of the plant [4].
2- Animal fibers, including wool, other hair-like fibers, and silk, are protein based. The
principal protein component of hair-like fibers is keratin, while the predominant
protein in silk is fibroin [4].
3- Mineral fibers are less important in the textile sector. Asbestos is the only mineral
fiber available in the nature. Because of the health problems this fiber is causing, its
extraction, manufacture, processing, and use are banned in many countries [5].
4
Figure 2.1: Fiber classification. Adapted from [4-5].
Man-made fibers are not originally in a suitable fibrous form. They can also be subdivided
into three groups, in this case according to the substrate for fiber formation (Figure 2.1) [5]:
1- Natural polymer fibers consist of material from nature. For instance, a large amount of
cellulose is available in plants, but only a small fraction of it is in the form of pure
fiber, like in cotton. Typically, it is available as a structural material mixed with other
substances, like lignin and hemicelluloses. This cellulose may be extracted chemically,
in a process referred to as pulping, and then converted to a suitable fiber form. Fibers
made from maize and soy beans are examples of these natural polymers [4]. Similarly,
animal proteins may also be manipulated into fibers.
2- Inorganic fibers are for example ceramic and glass fibers [5].
3- Synthetic fibers are produced entirely from petroleum-based chemicals. Nylon and
polyester are examples of synthetic fibers [4].
Since the objective of the present thesis was to investigate the potentiality of production of
biogas and bioethanol from the cellulosic part of waste textiles, another classification based
on chemical composition of the fibers was used, in which all fibers fall into three classes as
follows:
Textile fibers
Natural Man-made
Mineral Natural polymer SyntheticInorganicAnimal (protein)Vegetable (cellulose)
Leaf fibers
Hairfibers
Bast fibers
Seedfibers
Woolfibers
Silk fibers
FlaxJute
HempRamie
bamboo
SisalManillaRafiaAbaca
CottonKapok
Coir
Cellulose rayon
Cellulose esters
Others
ViscoseModalLyocell
Acetate RubberCollagen
Starch
GlassCarbonMetal
Polyvinyl Polytetrafluoro ethylene (PTFE)
Polyamide(PA)
Polyester (PET)
Polyolefin(PP/PE)
Polyurethane(PU)
Specialityfibers
Polyvinyl alcohol(PVA)
Polyvinyl chloride(PVC)
Polyacrylonitrile(PAN)
Teflon Acrylic
Nylon 6Nylon 66Aramids
Polyester Polypropylene Polyethylene
Elastane
5
1- Cellulosic fibers: All fibers containing cellulose in its original form (i.e. without any
substitution reactions).
2- Synthetic fibers: All fibers produced from non-renewable sources.
3- Other fibers, from natural sources: All fibers obtained from natural sources, except
those containing cellulose.
2.1.1. Cellulosic fibers
Cellulosic fibers are mixed with other fibers in textile waste, and the main focus of the
current thesis is bioconversion of such materials. The most important cellulosic fibers and
their composition are listed in Table 2.1 and a brief introduction to each of them follows
below:
- Cotton - a fiber obtained from the seed of a plant species of the genus Gyssypium. This
fiber has been used to produce textiles for many centuries and is by far the
commercially most important seed fiber. The dominant component of cotton is
cellulose; the exact proportion varies with the source of cotton and the growth
conditions. Cotton consists of approximately 90% cellulose, and the remaining 10% of
impurities must be removed by processes such as desizing, scouring, and bleaching in
order to acquire a high quality fiber for textile manufacturing. After removing the
impurities, the cotton fibers in textile are almost pure cellulose (Table 2.1) [5].
- Flax fibers - obtained from the stem of the plant and called “linen” when it is spun into
yarn. About 60% of the flax fibers consist of cellulose (Table 2.1). The remaining part
comprises other substances such as hemicelluloses, lignin, pectin, waxes, and proteins.
The ability of flax to absorb water makes linen a useful material for tea-towels and
cloths for glass cleaning, as it removes water effectively without leaving any stray
behind [5].
- Hemp - similar to flax, also obtained from the stem of the plant. The cellulose content
of hemp is about 80% (Table 2.1). This fiber also absorbs water well and shows high
resistance to UV radiation. Hemp fiber is often blended with cotton in clothes
manufacturing [5].
- Jute - Although its use has declined in Europe during the last 30 years, it is still an
important fiber worldwide. Like flax and hemp, it is obtained from the stem of the
6
plant. Jute fibers contain 61-71% of cellulose (Table 2.1) and is mainly used for carpet
manufacturing, furnishing fabrics and ropes, and producing sacks [5].
- Ramie - one of the oldest textile fibers. The fibers are extracted from the stem of the
plant. The extraction process is rather difficult and time consuming. The resulting
ramie fibers have a cellulose content of 92±1% (Table 2.1). The main applications of
ramie fibers are for apparel and interior textiles [5].
- Bamboo - the fastest growing of all woody plants. Bamboo fibers contain about 61%
of cellulose and 32% of lignin. The bamboo fibers are produced, either by a process
similar to that for manufacturing viscose (see next paragraph), or by a process like the
one used for producing flax fibers. Bamboo fibers have a high absorption capacity,
and material produced with these fibers feel cool and soft, making it suitable for
garments used in hot and humid climates [5].
- Viscose - the raw material for production of viscose is cellulose from either cotton
linters (the short and useless fibers from the cotton industry) or wood pulp, extracted
from for example northern spruce or western hemlock. The procedure of producing
viscose requires the cellulose to be dissolved by making sodium cellulose xanthate,
thus involves chemical and physical treatments under special conditions and then
dissolution in a sodium hydroxide solution. The final steps include regeneration of the
dissolved cellulose, spinning of the fibers, and a finishing process [6]. Viscose fibers
thus consist of pure cellulose (Table 2.1). The main features of viscose fibers are their
silk-like feel and shiny-lustrous appearance. Viscose is used extensively for both
apparel and non-apparel applications [5].
- Lyocell - cellulosic fibers obtained by an organic solvent spinning process. Similar to
viscose, lyocell is produced from wood pulp, although the process is much less
complex. The solvent used for manufacturing lyocell is N-Methylmorpholine-N-oxide
(NMMO) which is an environmentally friendly cellulose solvent. The water content
and the temperature of the NMMO are important for the dissolution of cellulose. The
cellulose solution is filtered to remove any remaining impurities and is subsequently
pumped through spinnerets in order to produce long fiber strands. The fibers are then
immersed in a diluted NMMO solution, which regenerates the cellulose and sets the
fiber strands. After that, they are washed with de-mineralized water [7]. Lyocell fibers
are used for instance for shirts, blouses, and sportswear [5].
7
Table 2.1: Composition of cellulosic fibers that are present in textiles. Adapted from
[5].
Cellulosic fiber Cellulose (%) Hemicellulose (%) Lignin (%) Other (%) Cotton 100 - - - Viscose 100 - - - Lyocell 100 - - - Flax 71.3 18.5 2.2 8 Hemp 77.9 6.1 1.7 14.3 Jute 61-71 14-20 12-13 1-13 Ramie 91-93 2.5 0.6-0.7 4-6 Bamboo 61 N.A 32 N.A
2.2. Fiber production statistics
Global fiber consumption in 2009 was 10.4 kg per capita, with a world population of 6.8
billion [8-9]. The consumption has steadily grown over the past six decades, from 9.4 Million
tons (MT) in 1950 to 70.5 MT in 2009 (Figure 2.2) [8], due to the population growth and a
general increase of the living standards [3]. The most important markets for fiber
consumption are apparel, home furnishing, and industrial applications. Since all goods have a
limited lifespan, it is reasonable to assume that the amount of discarded fibers reflect
consumption volumes, taking some delay into account.
Figure 2.3 shows the share of synthetic, cellulosic and other natural fibers in the
worldwide fiber consumption in 2009 [8]. Cellulosic fibers comprised 40% of the total fiber
consumption, i.e. 28.2 MT per year. Moreover, among synthetic fibers, the annual production
of polyester fibers is 31.9 MT, 45.2% of the total fiber production. Consequently, more than
85% of the global fiber consumption is a mixture of cellulose and polyester [8].
2.3. Environmental impact of textile production and its disposal
The most widely accepted method for evaluation of the energy consumption of a product,
process or service, is a life cycle assessment (LCA)1. The LCA method was standardized by
the International Standardization Organization (ISO) in the ISO 14040 series. The most
commonly used system in an LCA study of textile production is cradle-to-factory gate. In this
system, the LCA study includes all steps from the extraction of raw materials, followed by all
conversion steps until the product is delivered to the factory gate. Another approach, which is
1 http://www.epa.gov/nrmrl/lcaccess/index.html
8
called cradle-to-grave, covers the usage and disposal of the product in addition to all the steps
of the cradle-to-factory gate system.
Figure 2.2: The annual global fiber consumption and fiber consumption per capita from 1950
to 2009. Adapted from [8].
Figure 2.3: Amount of synthetic, cellulosic, and other natural fibers, worldwide consumption
in 2009. Adapted from [8].
0
2
4
6
8
10
12
0
10
20
30
40
50
60
70
8019
50
1960
1970
1980
1990
1995
2000
2002
2004
2006
2008
2009
kg/c
apita
Tot
al c
onsu
mpt
ion
(MT
/yea
r)
Total consumption (MT/year) kg/capita
0
5
10
15
20
25
30
35
40
45
Synthetic fibers Cellulosic fibers Other natural fibers
Glo
bal f
iber
con
sum
ptio
n (M
T)
Acetate
Silk
Wool
Non-cotton cellulosic
Cotton
Other synthetic fibers
Acrylics
Polyamide
Polyester
9
Numerous studies have been conducted to quantify the energy used in textile production,
reuse/recycling operations, and disposal [10-16] as well as the environmental impact. Shen
and Patel [15] reviewed the available LCA studies for production of the most common fibers.
Table 2.2 shows the non-renewable energy use (NREU) for production of different fibers and
the energy recovery by waste incineration. Man-made cellulosic fibers have the lowest NREU
requirements among all the reviewed fibers; for example, viscose fibers require 10-30% less
NREU than cotton fibers, and 50-80% less NREU than synthetic fibers. Considering energy
recovery, using incineration as a disposal strategy for waste textiles, the "net NREU" is
calculated as the NREU of production (energy spent) minus the energy recovered from
incineration (energy gained). The polysaccharides fibers hold lower calorific values than the
petrochemical fibers (Table 2.2), but require less net NREU. The exception is cotton, whose
cultivation requires a large amount of water and chemicals [15].
Table 2.2: None-renewable energy use (NREU) for the production of different fibers and the
energy recovery by waste incineration. Adapted from [15].
Fibers NREU (MJ/kg)
Energy recovery from combustion with 100%
recovery (MJ/kg) Cotton 49 16.3 Viscose 35 16.3 Wool 8 20.5 Polyester 109 22.0 Acrylic 158 N.A
2.4. Waste management strategies for waste textiles
Most of the fiber products, applied for e.g. apparels, furniture, carpets, and automotive
interiors, have a short or medium term use and end up as waste after a few years of service
[2]. Fashion changes result in outdating of clothes very quickly. This encourages disposal of
out-of-fashion, yet good quality garments, and purchase of new clothes to replace them [17-
18]. Manufacturers respond to this throw-away-practice of the society by increasing the
development of large quantities of low quality clothing [17]. Increasing wealth enhances this
trend since the production of textiles increases with increased consumer spending. As a
consequence, waste production by manufactures as well as households is also increased.
Textile waste can be classified as either pre-consumer or post-consumer waste. Pre-consumer
textile waste consists of by-product materials from the textile, fiber, and cotton industries.
This category of waste textiles may vary from single polymers to complex multi/material
10
systems, depending on which step of the process that generated the waste. Waste consisting of
single polymers is more valuable, because it is easier to recycle. Post-consumer waste is
defined as any type of garment or household article, made from manufactured textiles, that the
owner no longer needs and decides to discard [3, 19]. These articles are discarded because
they are, e.g., worn-out or out of fashion [19]. Since almost all fibers are produced for
manufacturing new products as replacement of worn-out or outdated products, the volume of
post-consumer waste is comparable with the rate of fiber consumption [3].
The management of waste is an issue of importance. Today, the approach in waste
management, known as the 3 Rs: "Reduce, Reuse, and Recycling" [18], is the most common
strategy. The aim is to protect the environment by minimizing the amount of waste and
extracting the maximum benefits from the products by extending their life span. Two of the
Rs, “Reuse” and “Recycling” of waste textiles are described here in more details:
2.4.1 Reuse
Reuse of textile products “as they are” or after repairing and reconditioning, saves
resources compared with manufacturing new items. Collecting, sorting, and selling the
second-hand garments requires between 10 and 20 times less energy than making new ones
[16, 18]. Charity institutions usually play an important role in the recycling and reuse of
textile waste. These organizations collect clothes that are worn-out, out of fashion, or
outgrown. Once collected, the clothes are sorted and graded by experienced workers in
several steps, on the basis of quality and application. A small portion of the second-hand
clothing is reused in Europe and US, but most of it is shipped abroad to be sold in developing
countries [19]. Some organizations such as TRAIDremade1, run by UK-based fashion
recycling charity TRAID (Textile Recycling for Aid and International Development),
recycles, refashions and remakes the second-hand garments, giving them new life, and are
subsequently selling them at a profitable price. Some other organizations, however, such as
Myrorna2 and Röda Korset3 in Sweden, are charity organizations whose main objective is to
help vulnerable people, but also to conserve the environment. A certain portion of textile
waste cannot be used as second-hand clothes; thus the recycling strategy should in those cases
be considered.
1 http://www.traid.org.uk/remade.html 2 http://www.myrorna.se/ 3 http://www.redcross.se/stod-oss/skanka-till-second-hand/
11
2.4.2. Recycling
Recycling textile waste, which entails a process to convert the waste into products, is a
sustainable waste management strategy to conserve resources. The fibrous wastes should be
sorted and separated as early as possible in the recycle processing steps [2-3]. The different
categories obtained after sorting may undergo different types of recycling processes.
Generally, recycling technologies can be divided into four approaches [2].
1- Primary recycling. Methods recycling a product into its original form.
2- Secondary recycling. Recycling involving conversion of the waste into a new product,
with a lower level of physical, mechanical, and/or chemical properties. The recycled
product may have a completely different application than the original product.
3- Tertiary recycling. Processes, such as pyrolysis, gasification, and hydrolysis,
converting the waste into basic chemicals or fuels.
4- Quaternary recycling. Incineration of solid wastes, utilizing the generated heat.
All four approaches are found in the textile sector and are described below.
Primary recycling or chemical recycling, also called "closed-loop" recycling, is
suitable for synthetic fibers like polyester and nylon. This method includes two steps. The
first step is to break down the polymer chains into monomers, recovering the raw materials
used to produce the polymer. The second step is to repolymerize the monomers, thereby
acquiring a product with a quality equivalent to that of the virgin polymer [3, 20]. Several
depolymerization processes have been proposed to convert nylon, the main component of
carpet fibers, into monomers [21-22]. For instance, aminolysis, which involves heating of the
polymer in the presence of ammonia at high temperature and high pressure, acid- and base-
catalyzed depolymerization, and depolymerization with water, can all break down the nylon
polymer into monomers. Depolymerization of polyester into monomers has also been
intensively investigated, but mainly for recycling of bottle grade polyester [23-26].
Commercial recycling processes, based on depolymerization and then repolymerization, are
currently available for nylon 6 [3] and PET1. Depolymerization processes usually require the
fibers to be free of any other materials. Hence, a separation process is necessary before the
depolymerization [2, 14, 22]. Therefore, primary recycling approaches are more practical for
pre-consumer wastes, mainly substandard yarns, rejected as part of the manufacturing process
[3, 27]. Although chemical recycling is more energy consuming than other methods of
1 http://www.ecocircle.jp/en/
12
recycling, the resulting fibers have more predictable quality. However, for most of the post-
consumer polymeric wastes this "closed-loop" approach is not currently available.
Secondary recycling includes different methods to convert the waste into a new
product. Textiles finished as clothing may for example be converted into a wiping or
polishing cloth for industrial use [3, 18-19], but extraction of fibers from waste textiles,
converting them to new products is the most common method for recycling the end-of-life
textiles. During this process, fibers are cut, shredded, and carded into a middle product called
shoddy by using carding machines [19, 28]. Shoddy can then be converted into value-added
products such as stuffing, automotive components, carpet underlays, and building materials,
e.g. insulation and roofing felt [19]. After introducing synthetic fibers to the market in the 20th
century, textile recycling using this method became more complex for two reasons: (1)
improved fiber strength resulted in the processes of shredding and opening the fibers giving
problems, and (2) the use of fiber blend increased the complexity of the sorting and
purification processes [19].
Another method for recycling thermoplastic polymers is melt processing by extrusion.
Of the waste textiles, carpets or other fibrous waste must first go through a size reduction
step, after which the shredded materials are fed into the extruder to be melted and extruded
through a die, thereby forming polymer strands. The polymer strands are cooled down and
chopped into pellets. These pellets are subsequently used in the production of injection
molded polymers and thermoplastics [3, 21]. This process is relatively cheap. However,
because of the immiscibility of the waste components, the products are usually of low quality
and have limited use [20-21].
Recycled fibers from used carpets have also been utilized for applications such as soil
reinforcement to improve the strength and stability of the soil, and as concrete reinforcement
for enhanced shrinkage and toughness properties [22].
Tertiary recycling, including processes such as pyrolysis, gasification, and hydrolysis,
has been less investigated for recycling waste textiles. Pyrolysis is a method in which textile
fibers are heated and converted to smaller molecules at high temperature, e.g. 400-600 °C, in
the absence of oxygen [29]. Some of these pyrolysis products can be used as fuels. The type
and quantity of the generated fuels depend on the composition of the textiles. Miranda et al.
[30] investigated the pyrolysis of textile waste by thermogravimetry at different heating rates.
They used cotton fabrics, obtained from a T-shirt industry, and studied its pyrolysis kinetics
as well as the resulting products [30].
13
To our knowledge, the present thesis is the first attempt to investigate hydrolysis of
waste textiles using biological processes, in order to produce biofuels (Paper I, II, and III).
The cellulose content of the waste textiles may be assessed as a valuable substance for
production of biogas and bioethanol. The most important challenges lying ahead of this
assumption and possible solutions are further discussed in the forthcoming chapters.
Quaternary recycling, i.e. direct incineration, is a common practice to use the energy
content of solid waste in most European countries. However, it is difficult to incinerate waste
textiles; the pieces of fabrics are too long and may cause fire outside the incinerator. They
may also cause trouble by slowing down the rotating equipments [31]. Further issues also
need to be addressed concerning incineration, e.g. improving the efficiency and reducing the
harmful end products, ash, and noxious gases. Incineration may nevertheless be an option for
those parts of the wastes, difficult to convert to other value added materials, using viable
recycling approaches [3].
14
15
Chapter 3
CELLULOSE: STRUCTURE, PRETREATMENT, AND
BIOCONVERSION
Although the most abundant biopolymer on the earth, cellulose is never found in pure
form in nature. Cotton is the fiber with the highest known content of cellulose, around 90%.
Cellulose is more commonly associated with hemicelluloses and lignin in the cell wall of
plants. Lignocellulosic materials typically contain 40-50% of cellulose, 20-30% of
hemicelluloses, and 15-20% of lignin [32]. The present study focused on cellulose found in
textile. Thus, the forthcoming section mainly concentrates on the structural characteristics,
hydrolysis, and digestion of cellulose.
3.1. Cellulose structure
Cellulose consists of chains, containing 500-15000 D-anhydroglucopyranose residues
joined by β-1,4-glycosidic bonds [33]. Each glucose molecule in the cellulose chain is rotated
180° to its neighbor molecules. Consequently, the repeating unit of cellulose is cellobiose.
Because of the monomer rotation, each side of the cellulose chain has equal numbers of
hydroxyl groups, leaving the cellulose chain highly symmetric (Figure 3.1). The extensive
amount of hydrogen bonds in cellulose, including 2 inter- and 2-3 intra-chain bonds per
glucose residue (Figure 3.1), result in extended and highly stable chains [34-35]. The half-
life1 of crystalline cellulose in water at neutral pH has been estimated to 100 million years
[36].
Cellulose microfibrils are typically composed of about 36 hydrogen-bonded cellulose
chains. The cellulose chains in microfibrils are parallel, in line with the finding of
simultaneous formation of chains in a microfibril [37]. Cellulose synthase, a complex enzyme
in higher plants, (called rosettes), is in most plant cell wall biosynthesis models, believed to
catalyze the synthesis of elementary fibrils [38]. Rosettes are composed of cellulose synthase
1 Half-life is the period of time it takes for a substance undergoing decay to decrease by 50%.
16
(CesA) enzymes (Figure 3.2). These enzymes contain many active sites that coordinately
catalyze glucan polymerization. The resulting glucan chains aggregate to form cellulose
microfibrils. It is not yet known whether the enzymes facilitate the hydrogen bonding of the
glucan chains, or if the proximity between the glucan chains, when emerging from the
enzymes, is the main reason for the hydrogen bond formation in the cellulose structure [39].
Figure 3.1: The cellulose structure and its (●) intra- and (*) inter-chain hydrogen bonds
Figure 3.2: Plant cell wall synthesis. Adapted from [40].
..
*
*
17
The helical twist of cellulose microfibrils in higher plants exhibits a long period in their
native state (Figure 3.3) [41]. The period of the helical structure appears to be dependent on
lateral dimensions1. Larger lateral dimensions leads to longer periods of helical twist (Figure
3.3). Furthermore, increasing the lateral dimension results in higher resistance of the
microfibrils towards helical orientation and subsequently, increases the risk of sheer stress
developing within a microfibril. This may be the reason for small lateral dimensions in the
cellulose structure of load-bearing plants, and large lateral dimensions in the structure of
cotton cellulose [41].
The aggregation type of nanofibrils with helical structure, forming the microfibrils, is
different in different types of lignocelluloses. When cellulose is deposited alone in the cell
wall, e.g. in fibrils of cotton or ramie, the fibrils are collectively subjected to the twist. In
contrast, when the cellulose is deposited in the presence of other cell wall ingredients, the
nanofibrils are twisted individually as well as with the assembly. This pattern gives the most
efficient load-bearing structure and is believed to be the most likely pattern in load-bearing
tissues of higher plants [41].
Figure 3.3: Geometric representation of cellulose nanofibrils with 2 by 2, 4 by 4, 6 by 6, and 20 by 20
nm in cross section and a long period of 1200 nm. Adapted from [41].
When isolating cellulose from plant cell walls using different processes, heating and
drying the cellulose are two treatments that influence the final pattern of the cellulose fibril
aggregation [41].
1 Lateral dimensions of bacterial cellulose microfibrils are approximately 6-7 nm, while only 3-5 nm in higher plants.
18
Cellulose oligomers containing less than ten monomers are water-soluble. Hence, it can be
expected that cellulose chains in their native state are hydrated at the level of elementary
nanofibrils, and that processing the cellulose at high temperatures may reduce its hydration.
Therefore, increasing the temperature changes the state of aggregation of native celluloses
[41]. Water acts as a lubricant, facilitating the relative motion of the nanofibrils, while
removal of water from the native cellulose structure towards dryness, leads to formation of
hydrogen bonds between nanofibrils. Thus, cellulose dehydration by increasing the
temperature or drying generates exceptionally tight aggregates, more recalcitrant to hydrolysis
than native cellulose [41].
3.1.1. Cellulose polymorphs
Cellulose is a polymorphic material, able to adopt various crystalline forms. The most
important cellulose polymorphs are cellulose I and II. The cellulose produced by nature is
cellulose I, composed of cellulose Iα and Iβ. Bacterial cellulose is mainly composed of
cellulose Iα while the dominant form of cellulose in wood and cotton is cellulose Iβ. Cellulose
Iα and Iβ differ in their hydrogen bonding, but their long atom skeletons are identical. The
cellulose molecules are organized in a parallel manner in both forms. However, while Iα has a
triclinic structure with one cellulose chain per unit cell, Iβ comprises a monoclinic unit cell
structure, with two chains per unit cell. Cellulose II can be obtained from cellulose I in either
of two ways: 1) by dissolution of cellulose I in a cellulose solvent, followed by precipitation
by adding an anti-solvent, i.e. regeneration treatment, or 2) by swelling fibers of cellulose I in
concentrated sodium hydroxide, and subsequent removing of the swelling agent. The lattice
crystal structure of cellulose II consists of a monoclinic unit cell, with a nonparallel
arrangement of the chains, in which paired cellulose molecules are rotated 180° around their
axes [35, 42-43].
3.1.2. Cotton fiber structure
Each cotton fiber comprises a single, complete, and elongated cell, developed at the
surface cell layer of the cottonseeds. The chemistry of cotton is often regarded as the
chemistry of cellulose [43]. A cotton fiber consists of a cuticle, a primary wall, a secondary
wall and a lumen [43]. Figure 3.4 shows a schematic representation of the various layers of a
cotton fiber. The cuticle is a waxy hydrophobic layer covering the primary wall, providing
protection in a humid environment as well as a lubricated surface for processing. Most of the
19
cuticle is removed by scouring during the industrial processing. Scouring makes the interior
of the cotton fiber accessible for molecules such as dyes. The primary wall is a thin layer
consisting of numerous microfibrils. The structure of the primary wall is not well understood,
but its function is possibly to provide much of the strength of the cotton. The secondary wall
makes up the main body of the fiber, and is composed of layers of almost parallel fibrils,
oriented spirally. In the secondary wall, the fibrils close to the primary wall are at an angle of
approximately 45° to the fiber axis. Approaching the fiber core, the secondary wall fibrils are
more closely aligned with the fibrilar axis. The secondary wall is considered to be of pure
cellulose, providing the rigidity of the fiber. The molecular weight of cellulose in the
secondary wall is higher than the molecular weight of cellulose in the primary cell wall. The
lumen is on the inside of the secondary wall at the core of the fiber. The lumen is a hallow
canal, where the nutrients of the cotton are transported when the cell is alive. After opening
the cotton boll, the cellular fluid is dried, and the lumen will remain as a cavity, containing
biological material. Drying the cotton fibers causes the lumen to collapse, and the cross-
section of the fiber deforms, resembling the shape of a kidney-bean (Figure 3.4) [43-44].
Figure 3.4: A) Structure of a cotton fiber, B) Cross section of a cotton fiber. Adapted from
[45].
Lumen
Primary layer Second layer
Cuticle LumenWinding
Lumen
Second layer
Primary layer
Layer of envelope
A)
B)
20
3.2. Hydrolysis of cellulose
Extensive research has been carried out on the use of cellulose as a platform for industrial
production of monomeric sugars to be used in the production of different bioproducts, such as
bio-ethanol [46-47]. Nevertheless, a commercial breakthrough of this technology has so far
remained elusive, mainly due to the difficulty executing a cost-efficient hydrolysis of the
cellulose molecules [48-49].
Various hydrolysis strategies are in use, but some of the most commonly applied ones
involve treatment with either concentrated or dilute acid, where the latter requires a high
temperature. Mild acidic treatment may also be combined with enzymes, a method that lately
has received a lot of attention [32, 50-51].
In this thesis, different hydrolysis methods for cotton-based waste textiles, including
concentrated and dilute acid hydrolysis as well as enzymatic hydrolysis, were examined using
either cotton linter or used cellulosic textiles, without pretreatment (Paper I & II).
3.2.1. Concentrated acid hydrolysis
Sulfuric acid, because of its low price, is the acid most used to investigate hydrolysis
of cellulosic materials. The concentrated acid hydrolysis may be performed at a concentration
of 30-70%, at low temperature (e.g. 40 °C) [32, 52-53]. In a preliminary experiment of the
present study, cotton linter was hydrolyzed using 80% w/w sulfuric acid at room temperature
and solid content of 25 g/l. After 6 hours of hydrolysis, not more than 38±1% of the
theoretical glucose yield was obtained [54]. Higher yields may be obtainable under optimized
conditions. However, using concentrated acid gives rise to serious obstacles, such as high
corrosive conditions, requiring expensive corrosive resistant constructions and large amounts
of energy for acid recovery [32, 53]. These drawbacks reduced the potential commercial
interest of this process. However, despite these disadvantages, some research groups, e.g.
Masada1, are still interested in developing this process to make it economically feasible [53].
3.2.2. Dilute acid hydrolysis
Dilute acid hydrolysis has been thoroughly examined for saccharification as well as
pretreatment (prior to enzymatic hydrolysis) of lignocellulosic materials. Sulfuric acid,
H2SO4, is a commonly used acid because of its lower price and less corrosive nature, 1 www.masada.com
21
compared to e.g. hydrochloric acid, HCl (aq). In order to obtain a high glucose yield, with
diminutive inhibitor formation, high temperatures (e.g. 80-230 °C) along with short residence
times (e.g. 2-15 min) should be applied [55-57].
A preliminary study in the present thesis examined the process of dilute acid
hydrolysis of viscose and cotton (i.e. jeans) textiles. Hydrolyses were performed at different
lengths of time (8 and 15 min), temperatures (180 and 200 °C), and acid concentrations (0.5,
1.5, and 3% w/w). The effect of duration and temperature on yield was described by means of
a severity factor Log (R0) as
0100( ) .exp
14.75rTLog R Log t⎛ ⎞−⎡ ⎤= ⎜ ⎟⎢ ⎥⎣ ⎦⎝ ⎠
where t is the reaction time (min) and Tr is the temperature (°C) [32, 51, 58]. The results are
illustrated in Figure 3.5.
Figure 3.5: Dilute acid hydrolysis of viscose and cotton (jeans) textiles.
Hydrolysis of viscose and jeans under identical conditions resulted in significantly
different yields of glucose. By increasing the severity factor from 3.2 to 4.7 at an acid
0
10
20
30
40
50
60
Glu
cose
yie
ld (%
)
Viscose
Acid Conc.=0.5% Acid Conc.=1.5% Acid Conc.=3.0%
0
5
10
15
20
25
30
3 3.5 4 4.5 5
Glu
cose
yie
ld (%
)
Severity factor
Jeans
22
concentration of 3%, the yield of viscose hydrolysis steadily decreased from 54.9% to 5.6%.
However, when using an acid concentration of 1.5% and 0.5%, maximum yields of glucose,
i.e. 53.1% and 46.5% of the theoretical yield, were obtained at severity factors of 3.5 and 3.8,
respectively (Figure 3.5). In contrast, the glucose yield never exceeded 24.4% when
performing dilute acid hydrolysis of cotton, although the hydrolysis was performed under
identical conditions. This may be due to differences in the structure, i.e. high crystalline
cellulose in jeans and low crystalline cellulose in viscose (data not published).
The suggested kinetic models predict that glucose yields higher than 65-70% are not
attainable when performing a dilute acid hydrolysis of cellulose [59], because the hydrolysis
liquor is not able to penetrate the crystalline region in cellulose. Moreover, once glucose is
formed it may be, at a significant rate, degraded to unwanted materials such as hydroxymethyl
furfural. Furthermore, due to optimization problems, up to 30% of the cellulose produces
oligomers, that cannot be assimilated by fermentative microorganisms [57, 59-61].
Performing the process in two to three steps, using more advanced reactor designs,
may improve the sugar yield when performing dilute acid hydrolysis of lignocellulosic
materials [60, 62]. Some of the sugars are, however, inevitably degraded into inhibitors and
thus, the resulting yield tends to be low [32, 53].
3.2.3. Enzymatic hydrolysis
Natural degradation of cellulose is an important step in the global carbon cycle.
Cellulosic microorganisms, responsible for cellulose degradation, produce cellulases, which
hydrolyze the β-1,4 linkages present in cellulose [36]. In contrast to the dilute acid hydrolysis,
enzymatic hydrolysis does not generate inhibiting by-products and is carried out under milder
conditions [32, 34, 63].
Today, cellulases are widely used in different industries, such as cotton processing,
paper recycling as well as juice extraction. Currently, industrial cellulases are derived mainly
from aerobic cellulolytic fungi, such as Trichoderma reesei, Humicola insolens, and
Aspergillus niger [36].
The concept of cellulose degradation by cellulase is based on activities of three types
of enzymes: endogluconases, exogluconases, and β-glucosidases [34, 64]. The action of
cellulase enzymes on cellulose is described in Figure 3.6. The endogluconases randomly
hydrolyze internal β-1,4 glucosidic bonds in the cellulose chain, thereby creating
polysaccharides of varying length and also new chain ends. In contrast, the exogluconases
23
further cleave cellobiose units from reducing as well as nonreducing ends of the cellulose
polysaccharide chains. The β-glucosidases hydrolyze cellobiose into glucose and also cleave
glucose from soluble cellodextrins. Most endogluconases and exogluconases consist of a
catalytic core and a carbohydrate binding module (CBM) (Figure 3.6) [34]. The function of
the CBM is to bind to the cellulose, bringing the catalytic core in close proximity to the
substrate [64]. The active site of endogluconases is located in an open cleft, while being
located in a tunnel in exogluconases [34, 64]. A cellulase system, a mixture of enzymes with
different functions, exerts higher activity than the sum of the activities of individual enzymes,
a phenomenon known as synergism [34].
Figure 3.6: Free cellulase enzyme system. Adapted from [64].
The degree of synergism (DS)1 varies for different types of celluloses [34]. The highest
DS value is reported for bacterial cellulose (5-10) and cotton (3.9-7.6). The DS value for
Avicel is reported in the range of 1.4-4.9, while the smallest synergism of cellulose systems
has been reported for amorphous cellulose (0.7-1.8) [34]. Taking into consideration the
crystallinity and degree of polymerization of the cellulosic substrates mentioned above, it may
be concluded that the synergism action of the cellulase components increases with increasing
difficulty to hydrolyze the substrate. Furthermore, a comparison between the hydrolysis
processes of cellulose and starch (another polymer of glucose, linked by α-glucosidic bonds)
suggests that the differences in substrate characteristics is of greater significance than the
differences in the hydrolysis process of β- and α-linked glucosidic bonds in glucan chains [34,
65].
1 Degree of synergism is expressed as the activity exhibited by the mixture of enzymes, divided by the sum of activities of the individual enzymes.
CBMCatalytic core
Cellobiose
Endogluconase
Exocogluconase β-glucosidase
Glucose
Cellulose
24
3.2.4. Enzymatic hydrolysis of waste textile
Cotton linter, 50/50 cotton/polyester, and 60/40 viscose/polyester textile pieces, with
no pretreatment, were enzymatically hydrolyzed using 20 FPU of cellulase and 30 IU of β-
glucosidase per gram cellulose. The resulting glucose yields from the hydrolysis vs. the
duration of the process, are illustrated in Figure 3.7. Performed under identical conditions,
hydrolysis of viscose was more efficient than hydrolysis of cotton-based celluloses. After 72
hours, 37.8% and 24.7% of the theoretical yield was obtained from hydrolysis of cotton linter
and cotton textile respectively, compared to the hydrolysis of viscose textile, where up to
96.2% of the theoretical yield was attained (Paper III).
Figure 3.7: Enzymatic hydrolysis of cellulosic textiles (Paper III)
3.3. Bacterial hydrolysis of cellulose
Aerobic and anaerobic microorganisms use different strategies for degradation of
cellulosic substrates. Aerobic microbes commonly produce cellulases in high concentrations
that act synergistically to hydrolyze cellulose, whereas the anaerobes have developed a more
energy conserving mechanism for extracellular degradation of polymeric substrates, such as
recalcitrant cellulose in plant cell walls. These microorganisms produce a highly efficient and
unique multi-enzyme complex, called cellulosome, specialized in cellulose degradation
(Figure 3.8) [65-68]. The first, and most studied, cellulosome was that of Clostridium
thermocellum, an anaerobic thermophilic bacterium [64, 69]. This cellulosome consists of
various enzymatic subunits as well as a non-catalytic subunit, referred to as a cellulosome
0102030405060708090
100
0 12 24 36 48 60 72
Glu
cose
yie
ld (%
)
Time (h)
Viscose/Polyester (60/40) Cotton linter Cotton/Polyester (50/50)
25
scaffold. The enzymes are aligned on the scaffold via the cohesin-dockerin1 interaction. The
scaffold also contains a cellulose specific carbohydrate module (CBM), for the attachment of
the cellulosome to the substrate (Figure 3.8) [64-65].
Anchoring proteins mediate the attachment of the scaffolds and their enzymes to the
peptidoglycan of the cell surface. Thus, the cellulosome is involved in the adhesion of the cell
to the insoluble substrate. During the cellulosome-cellulose interaction, an aromatic strip on
the CBM creates a strong binding with the glucose chain of the hydrophobic face of the
cellulose surface [70]. The CBM is able to bind to very high crystalline cellulose as well as to
amorphous cellulose. However, amorphous cellulose has a higher binding capacity than
crystalline cellulose, due to an increased accessibility to binding sites [71].
Figure 3.8: The cellulosome complex. Adapted from [64].
Cellulosomes are packed inside protuberances on the cell surface of the hydrolytic
organism. By their interaction with the cellulose surface, a conformational change occurs,
forming "contact corridors". The corridors are loaded with fibrous materials, used to connect
the cell surface with the cellulose bond-cellulosome. The cellulose degradation products are
subsequently channeled to the cell through the fibrous structure of these corridors [65, 71].
Assimilation of the cellulose degradation products by the host cell of cellulosome and also
by other saccharolytic bacteria, with the same ecosystem as the cell, remove the inhibition
effect of the products, and promote the substrate degradation [65].
The major advantages of the cell-bonded cellulosome to cellulose hydrolysis are: (1)
bringing the various complementary enzymes (e.g. exo- and endocellulases) to the proximity
of the ratio that optimizes their synergism, and (2) allowing efficient uptake of the produced
1 Dockerin is a protein domain and part of the enzyme, and cohesin is its binding partner on the scaffold. Their interaction attaches the enzyme to the scaffold.
Cellulose
CellulosomeProtein Scaffold
Biomass-degradingenzymes
Bacterial Cell
26
oligosacharides by the host cell, by minimizing the diffusion distance for the products [64-
67].
3.3.1 Uptake, phosphorylization, and fermentative catabolism of cellulose
degradation products
The driving force behind the uptake of glucose and its oligomers varies between the
different cellulolytic microorganisms. In C. thermocellum, uptake of cellobiose and
cellodextrins take place via an ATP-binding cassette protein. Cellulolytic anaerobes generally
use either of two mechanisms to assimilate cellodextrins: (1) hydrolysis of cellodextrins,
catalyzed by extracytoplasmic cellodextrinase, or (2) catalyzation of a Pi-mediated (ATP-
independent) phosphorolysis, using intracellular cellobiose and cellodextrin phosphorylases
(CbP and CdP) [64].
Figure 3.9: General strategies of cellulose hydrolysis and utilization by anaerobic bacteria.
Adapted from [64].
Hydrolysis of cellulose by cellulosome
Cleavage via intracellular phosphorylasis
Glc-1-P
Cellulolytic Bacterium
Fermentation via Embden-Meyerhof pathway
Exteracellular hydrolysis of longer cellodextrins
PEP-independentuptake of G1-G4
... ......... ......... .. . ...
Non-adherent cellulolyticbacterium
...Cross-feeding of fermentation products
Loss of cellodextrin by diffusion?
Glc(n-1)
Pi
Cellulose
Non-cellulolyticbacterium
27
It has been suggested that in cellulolytic species, the metabolism of cellodextrins and
cellobiose may occur by several processes: (1) extracytoplasmic hydrolysis with subsequent
uptake and catabolism, (2) direct uptake followed by intracellular phosphorolytic cleavage
and catabolism, or (3) direct uptake followed by intracellular hydrolysis and catabolism [64].
In strictly anaerobic cellulolytic bacteria, the fermentative catabolism is initiated by
glucose-1-phosphate (G-1-P) production through the action of either CbP or CdP. The
glucose-1-phosphate is subsequently metabolized to glucose-6-phosphate. This is the entry
point to the Embden-Meyerhof pathway of sugar catabolism. Acetic acid and CO2 are the
reduced end products in all of these species (Figure 3.9) [64].
3.4. Pretreatment of cellulose to improve hydrolysis and digestion
A number of different pretreatment methods, including physical, chemical, and biological
approaches have been investigated in order to decrease the recalcitrance of lignocellulosic
materials. The overall purpose of the pretreatment is to make the cellulosic part of the
feedstock amenable for digestion by cellulolytic enzymes [49-50, 72-73].
In lignocellulosic biomass, covering the cellulose with lignin and hemicelluloses creates
an additional barrier against enzymatic hydrolysis. Therefore, to increase the exposure of
cellulose to enzymes, it is preferable to fragment the tissue or to chemically remove the lignin
and hemicelluloses [74-75]. In waste textiles, there is no chemical interaction between the
cellulose and other fiber types. Thus, the objective of a pretreatment step when hydrolyzing
waste textiles is to reduce crystallinity, and to enhance the accessible surface area of the
cellulose, rather than to break cellulosic complexes with other materials. Furthermore, waste
textiles are mixtures of different fibers, and it is difficult to pass the mixture through the
whole process of biogas or bioethanol production. Therefore, the current leading methods for
pretreatment of lignocelluloses, such as dilute acid, steam explosion, ammonia fiber
expansion, and lime pretreatments, are not desirable choices for pretreatment of recalcitrance
cellulose in waste textiles. Furthermore, the interaction between fibers and dyes in colored
textiles may affect the enzymatic hydrolysis of cellulose [76].
In principle, the most suitable pretreatment method for cellulosic fibers of waste textiles
are those able to efficiently disrupt the highly ordered hydrogen bonds among glucan chains
in the crystalline cellulose, and possibly simultaneously separate the cellulose part from other
fibers. Pretreatments using sodium hydroxide, phosphoric acid, and NMMO were thus chosen
28
to investigate possible improvement of the hydrolysis of cellulosic waste textiles (Papers I, II,
IV, and V).
3.4.1. Pretreatment with sodium hydroxide
Alkaline pretreatment of lignocelluloses is one of the chemical pretreatments that have
been extensively studied. The method is basically a process of delignification and reduction of
cellulose crystallinity [75, 77]. Among different alkaline reagents, NaOH is the only one that
is able to completely transform cellulose I to cellulose II [78-79], thus facilitating biological
hydrolysis, although its effectiveness is depending on the conditions under which
pretreatment occurs [80-81]. Consequently, to optimize the process of enzymatic hydrolysis
of cotton-based waste textiles, pretreatment with NaOH solution under different conditions
was performed. Cotton linter was chosen as substrate and treated with 0-20% NaOH for 3
hours, at 0, 23, and 100 °C. The treated materials were subsequently hydrolyzed with 20 FPU
cellulase and 30 IU β-glucosidase per gram cellulose. The most important results are
summarized in Figure 3.10 (Paper I).
Figure 3.10: Digestibility (expressed as percent) of alkali treated cotton linter converted to
glucose, by enzymatic hydrolysis (Paper I).
0102030405060708090
100
Yie
ld o
f glu
cose
(%)
23 C, 24 h 100 C, 24 h
0102030405060708090
100
Yie
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f glu
cose
(%)
0 C, 48 h 23 C, 48 h 100 C, 48 h
0102030405060708090
100
0 4 8 12 16 20
Yie
ld o
f glu
cose
(%)
NaOH Concentration (% w/v)
0 C, 96 h
0 4 8 12 16 20NaOH Concentration (% w/v)
23 C, 96 h
0 4 8 12 16 20NaOH Concentration (% w/v)
100 C, 96 h
0 C, 24 ho
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29
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3]. Upon
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Different
30
mechanisms have been suggested for the conversion of parallel chains into an antiparallel
conformation in a solid-state process [35, 84-85]. One possible mechanism, reported by
Okano and Sarko (1985) [84], suggests that once Na-cellulose I is formed, it cannot be
converted back to cellulose I, but acquires an antiparallel arrangement with the same chain
polarity as cellulose II.
Decreasing the temperature from room temperature to 4 °C allows at least two more
water molecules per Na+ to be inserted into the cellulose crystalline lattice. The ability of
NaOH to change the cellulose structure at lower temperatures may be explained by stronger
binding of Na+ and OH- to water at lower temperatures, enabling the breakage of the
hydrogen bonds within the cellulosic structure [82, 86].
Although succeeding in improving the process of enzymatic hydrolysis, the problem
with waste textiles being a mixture of fibers still remained. Hence, other pretreatments were
tested.
3.4.2. Pretreatment with phosphoric acid
Depending on the acid concentration and type of biomass, phosphoric acid is able to
dissolve cellulose or make it swollen [87], and to disrupt the linkage between cellulose,
hemicelluloses, and lignin [88]. Using concentrated phosphoric acid, Zhang et al. [89]
developed a cellulose-solvent-based process for fractionation of lignocelluloses, to produce
regenerated amorphous cellulose.
In the present work, pretreatment with phosphoric acid was tested on cotton-based
waste textiles (cotton linter and jeans textile) prior to enzymatic hydrolysis (Paper I),
according to Zhang et al. [89]. After grinding, the materials were mixed with 85% phosphoric
acid at 50 °C at a concentration of 10% w/v. As one hour was insufficient for a complete
dissolvement of the materials, the retention time was increased to 5 hours. Finally, acetone
was added to the mixture to precipitate the dissolved cellulose.
This kind of pretreatment improved the yield; 63.4% of the cotton linter and 60.7% of
the jeans were digested after 2 days of enzymatic hydrolysis (Paper I). However, using
concentrated acid as a cellulose solvent presents a problem, since metallic things such as zips
and buttons are present in the waste textiles, and these items react with phosphoric acid, and
apart from consuming the acid, also produce undesirable substances.
31
3.4.3. Pretreatment with NMMO
N-Methylmorpholine N-oxide (NMMO) is a cyclic organic amine oxide, recognized as
a non-derivatizing solvent for cellulose. It easily dissolves cellulose of fairly high
concentrations, generating a solution with excellent rheological properties for fiber spinning.
This solvent has therefore, been brought into the textile industry, and is used commercially for
production of regenerated cellulosic fibers under different trade names, such as Tencel,
Lyocell, and Newcell. A nearly complete recovery, along with a recycling potential, non-
toxicity as well as biodegradability, are among the advantages introducing NMMO as an
environment friendly solvent to be used in the fiber formation process. This is potentially
more cost effective than the viscose process when producing fibers [7, 90-91].
The degree of solubilization of cellulose in NMMO-water mixtures depends on the
NMMO concentration [92]. Cuissinat and Navard [93] performed optical microscopy of free
floating fibers in an NMMO solution using a wide range of NMMO concentrations. They
identified four different modes for dissolution of cotton fibers, related to the concentration of
NMMO [93]: (1) fast dissolution by fragmentation without significant swelling (> 83%
NMMO), (2) swelling by ballooning and dissolution (76-82% NMMO), (3) swelling by
ballooning and incomplete dissolution (70-75% NMMO), and (4) low homogenous swelling
and no dissolution (below 65% NMMO).
A previous study, by Chanzey et al. [94], investigated the structural changes of
cellulose after different modes of action by NMMO, i.e. either regeneration of cellulose, or
removing the swelling agent. Depending on the concentration of NMMO, they observed: (1)
dissolution without swelling, (2) an irreversible swelling zone, in which the initial cellulose
fibers were converted into a nonoriented crystalline cellulose II structure, (3) a reversible
swelling zone, in which cellulose I crystals were conserved after a swelling experiment, after
being washed and dried, and (4) no activity, when no visible changes occurred.
Water holds a very strong hydrogen bonding capacity. Cellulose is nevertheless
insoluble in water, due to intra- and inter-molecular hydrogen bonds formed by cellulosic
chains (Figure 3.1). Dissolution of cellulose in a solvent like NMMO, therefore, involves
breaking these hydrogen bonds. The crystallinity of cellulose is also an important factor
concerning the solubility; the crystalline form has a lower energy level than the amorphous
form. Since the degree of solubility corresponds to the difference in the energy level between
the solid and solution state, the crystalline form of cellulose is more difficult to dissolve than
the amorphous [95].
32
3.4.4. Effect of NMMO treatment on enzymatic hydrolysis of cellulose
In paper II, the effects of three modes of action of NMMO, i.e. dissolution (85%
NMMO), ballooning (79% NMMO), and swelling (73% NMMO) of high crystalline cellulose
(cotton linter), on enhancement of subsequent enzymatic hydrolysis were investigated at two
temperatures (90 and 120 °C) during 0.5-15 hours (Figure 3.12) and the results of the 5 hour-
treatments are presented in Figure 3.13.
Figure 3.12: Partial phase diagram of a NMMO-water solution, showing the various regions
of (A) dissolution without swelling, (B) irreversible swelling, (C) reversible swelling, and (D)
no activity on native cellulose fibers. The black points show the selected conditions for
investigating the effect of NMMO-treatment on cellulose digestion in the present thesis
(Paper II). Adapted from [94].
The dissolution mode had the highest effect on fast and nearly complete hydrolysis of
crystalline cellulose (Figure 3.13). The ballooning mode of action was stronger at 120 °C than
at 90 °C, enhancing the rate of hydrolysis, while the efficiency of dissolution and swelling
mode of action were not affected by the temperature (Figure 3.13). Pretreatment duration had
no significant impact on the enzymatic hydrolysis of cellulose. A Fourier transform infrared
spectroscopy (FTIR) analysis, used for characterization of the crystalline structure of
cellulose, confirmed that the dissolution mode of action by NMMO converted cellulose I to
cellulose II, in agreement with the report by Chanzey et al [94].
33
Figure 3.13: Yield from enzymatic hydrolysis of water treated and NMMO treated cellulose.
The pretreatments were carried out using three different modes of NMMO–water solution,
i.e., dissolution (85%), ballooning (79%), and swelling (73%), for 5 hours, at (A) 90 and (B)
120 °C. The enzymatic hydrolyses were performed at 45 °C, using 30 g cellulose/l, 20 FPU/g
cellulase, and 30 IU/g β-glucosidase (Paper II).
The effect of NMMO treatment on the capacity of cellulase to adsorb onto the
insoluble cellulose, and also the reactivity of cellulose during enzymatic hydrolysis, were
investigated in paper V. The data of adsorption of cellulase onto cellulose was examined
using the Langmuir isotherm model. The maximum adsorption capacity was obtained at 212,
35, and 5 mg protein/g substrate for NMMO-treated cellulose, Avicel, and cotton,
respectively. The substrate reactivity (SR) was measured, while the effect of end product
inhibition and enzyme inactivation during enzymatic hydrolysis, was eliminated. The SR of
the regenerated cellulose declined from 1 to 0.43 when 30% of the substrate had been
conversed after 15 min. After that, there was no significant change in the SR until 92% of the
0102030405060708090
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Yie
ld o
f glu
cose
(%)
Dissolution Ballooning Swelling Water treated
0102030405060708090
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0 12 24 36 48 60 72
Yie
ld o
f glu
cose
(%)
Time (h)
A)
B)
34
cellulose was conversed, which was achieved after 6 hours of hydrolysis. The present study
demonstrated that regeneration of cellulose dramatically increased its capacity of cellulase
adsorption. However, the effect of cellulose reactivity on the rate of hydrolysis was
negligible, and hence the SR should perhaps not be included in the kinetic model of the
hydrolysis process (Paper V).
3.4.5. Effect of NMMO treatment on anaerobic digestion of cellulose
Effect of NMMO treatment on the rate of cellulose solubilization in batch anaerobic
digestion, and its conversion to methane at different initial concentrations of cellulose
(between 5 and 40 g/l) was investigated (Paper IV). The yield of methane (% of theoretical),
the total volatile fatty acid production (VFA; g/g cellulose), and the pH-values during 25 days
of digestion, at an initial concentration of 20 g/l of treated and untreated cellulose, are plotted
in Figure 3.14. The yield of methane did not differ significantly between the digestion of
treated and untreated cellulose. The only difference observed was a longer lag phase in the
culture of untreated cellulose, before methane production started (Figure 3.14), in agreement
with the study of Weimer et al. [96]. However, the total VFA-values showed that pretreatment
with NMMO enhanced the rate of hydrolysis during anaerobic degradation. Hydrolysis has
been considered as rate limiting step of cellulose digestion [97]. The enhancement of
hydrolysis by NMMO-pretreatment was furthermore confirmed when comparing the pH-
changes in the cultures containing treated and untreated cellulose. It may thus be concluded
that facilitating the hydrolysis of cellulose may not be the solution for speeding up the
cellulose digestion (Paper IV).
One possible solution to be able to benefit from the high solubilization rate of
pretreated cellulose, along with an increased loading rate, is to use a two-stage anaerobic
digestion process, where hydrolysis and acidogenesis of the cellulose can proceed in the first
reactor, while the volatile fatty acids produced, subsequently can be converted into biogas in
the second reactor.
35
Figure 3.14: The methane yield (A), volatile fatty acid (VFA) production (B), and pH-profile
(C) during 25 days of anaerobic digestion of NMMO-treated (♦) and untreated (■) cotton, at
an initial concentration of 20 g/l of cellulose.
0
20
40
60
80
100
Met
hane
yie
ld (%
)
Treated Untreated
0
0.03
0.06
0.09
0.12
0.15
0.18
t VFA
(g/
g ce
llulo
se)
6.00
6.50
7.00
7.50
8.00
8.50
9.00
0 5 10 15 20 25
pH
Time (day)
A)
B)
C)
36
37
Chapter 4
BIOFUELS
4.1. Why biofuels?
An increasing global population, diminishing oil supplies, and growing concerns about
GHG emissions, caused by the use of fossil fuels, have resulted in an increased global interest
in gradually shifting the energy supply from fossil to alternative fuels [47-48]. About 80% of
the current energy supply in the world originates from oil, natural gas, and coal [98].
According to the BP Statistical Review of World Energy of 2010 [99], the total proven
resources of oil, natural gas, and coal in the world, were estimated to comprise 181.7 billion
tons, 187.5 trillion cubic meters, and 826.0 billion tons respectively, by the end of 2009. With
the present consumption rate, the reserve to production (R/P)1 ratios of the reserves are 46, 63,
and 119 years for oil, natural gas, and coal, respectively [99]. Consequently, fossil fuels are
limited sources and will not last forever, and “peak oil”2, which is the moment at which oil
production reaches its maximum output, may occur in the near future. In brief, this suggests
that when oil production decreases, products generated from oil will become scarce and
expensive, unless alternatives are forthcoming [48, 100]. In 2008, about 61.4 % of the oil
consumption occurred within the transportation sector [98], suggesting that the most efficient
way to reduce the consumption of petroleum, may be to expand the energy options for
transportation [101].
Beside the impact of transportation on the petroleum consumption, the impact on the
environment is severe. Emission of GHG into the atmosphere increases the trapping of the
heat usually radiating from the earth, thus causing an increase in the average global surface
temperature [102]. The probable consequences of this are redistribution of rainfalls, rising sea
levels, more intense floods and tornadoes, and an increasing rate of extinction of species,
leading to a greatly reduced biological diversity [48]. To address these critical environmental
1Reserve to production (R/P) is defined as the length of the time that the reserves would last if the production were to continue at the present rate. 2 http://peakoil.com/what-is-peak-oil/
38
issues, the pattern of energy use in transportation must change within the coming years.
Biofuels, especially biodiesel, bioethanol, and biogas, at present the ones most widely
available on the market, are potential alternatives to petroleum-based fuels for transport on the
road [103].
4.2. Bioethanol
Bioethanol is at present the most common biofuel for transportation, derivable from
biologically renewable resources. Ethanol is a flexible transportation fuel and is easily mixed
with gasoline up to a concentration of 30%, without any engine adaptation being required.
Acting as an octane booster, adding ethanol reduces the need for toxic additives such as
benzene. Ethanol can also be used in high-level gasoline blends such as E85, E95, or even
E100, in dedicated or flexible fuel vehicles [101]. Ethanol has a high octane number, a high
heat of vaporization, and a higher hydrogen/carbon ratio than gasoline, thus producing a
greater volume of gas per energy unit burned. These advantages grant ethanol the same
overall transportation effectivity as diesel in CI engines, and around 15% more effectivity
than gasoline when used in optimized spark ignition (SI) engines [104]. However, there are
some drawbacks when using ethanol as a high-octane additive. Compared to gasoline, the
energy density of ethanol is lower; it may be only partially oxidized, resulting in emission of
acetaldehyde, and it absorbs water and dirt very easily. Furthermore, the blend with gasoline
tends to separate in the presence of traces of water and ethanol can be corrosive. Fortunately,
various technologies and strategies have eliminated these problems [105].
From an environmental point of view, the main advantage of bioethanol is its
sustainability as a renewable raw material, providing a relatively acceptable carbon balance.
In principle, the plants used as a feedstock for bioethanol production, originally assimilated
the CO2 gas emitted by combustion of bioethanol. However, the amount of gas produced
during the combustion of anhydrous ethanol, ethanol-gasoline blends, and gasoline, does not
provide a sufficient indication of the overall consequences of replacing gasoline with ethanol
[103]. It is vital that all steps of the production process are taken into account, since the
environmental effect of ethanol varies greatly, depending on the production process.
39
4.2.1 Food versus energy – a conflict
In 2010, more than 80 billion liters1 of bioethanol was produced globally. Ethanol is
today produced from sugar and starch containing materials [106] such as sugar canes and
corn. Table 4.1 displays the global production of principal grain and sugar crops in 2009. To
estimate the amount of dry crops acquired from the harvest, the production must be multiplied
by 0.85 [107]. Converting the total global agricultural production (counting in the average
yield of anhydrous ethanol from each type of crop) into ethanol would generate around 812
billion liters of ethanol. Taking into account that ethanol contains less energy than gasoline
(21 versus 32 MJ/l), this amount of ethanol would be sufficient to replace 544 billion liters of
gasoline (Table 4.1). By “emptying the stomach” [108], 43% of the global gasoline
consumption (1260 billion liters2 in 2007) can be replaced by this ethanol. On the other hand,
as a result of the growing world population, by 2050, the agriculture will need to provide food
for 9 billion people, an enormous challenge from an agronomic perspective. The
consideration of using crops for food or for ethanol may be referred to as the "food versus
energy" conflict. Furthermore, recent studies disclosed that an increase in the production of
biofuels from sugar and starchy materials might cause a substantial "carbon debt", since the
reduction of GHG emission by replacing fossil fuels is less than the CO2 released from direct
or indirect changes in land use [109].
Table 4.1: Global potential for ethanol from principal grain and sugar crops.
Crop Global production in 2009 (Mt)1
Conversion efficiency (liters/tonne)2
Maximum ethanol (billion liters)
Gasoline equivalent (billion liters)
Wheat 684 340 198 132 Rice 440 430 161 108 Corn 812 400 276 185 Sorghum 59 380 19 13 Sugar cane 1683 70 100 67 Cassava 241 180 37 25 Sugar beet 229 110 21 14 Total - - 812 544
1Data from FAO3 and USDA4 online statistic databases 2Data from [108]
1 http://www.ethanolrfa.org/pages/statistics#E 2 http://www.eia.doe.gov 3 http://faostat.fao.org/ 4 http://www.fas.usda.gov/psdonline/psdHome.aspx
40
4.2.2 A possible solution
One of the best studied options, offering a large reduction in GHG emission [110] with
no direct effect on global food supply [111], is conversion of cellulosic biomass into ethanol.
Ethanol produced this way is referred to as the 2nd generation ethanol. Cellulosic biomass
incorporates materials such as wood materials harvested for this purpose as well as
agricultural residues, forestry waste, waste from the wood product industry (e.g. paper-mills,
furniture manufacturing), and municipal solid waste (yard waste, paper products, and
cellulosic textiles) [109, 111-112]. For example, the cellulose content of annual waste textiles,
(mainly composed of cotton fibers), holds the potential to supply raw material for production
of 20 billion liters of ethanol. At present, these wastes are commonly sent to landfill sites. In
order to minimize landfilling and its consequences, the landfill directive 1999/31/EC obliges
members of the European Union to reduce the amount of biodegradable wastes going to
landfill by 35% of the levels of 1995, by 2016 [113]. The cellulosic fraction of municipal
solid waste may thus be regarded as a sustainable source of bioethanol. Nevertheless,
converting cellulose to bioethanol is problematic when it is mixed with other materials, as is
the case in municipal solid waste. Notwithstanding the technical challenges, developing an
economically feasible process of cellulose hydrolysis provides a major obstacle for the
production of the second-generation bioethanol. Another challenge is to provide the
infrastructure required for supplying raw material in a scale that makes the production
economically feasible [101].
4.2.3. Production of ethanol from waste textiles
Ethanol production from blue jeans textile waste, composed of cotton fibers, was
investigated by means of 24 hours of enzymatic hydrolysis, followed by semi-simultaneous
saccharification and fermentation (SSSF) by Saccharomyces cerevisiae. After 4 days
fermentation, the amount of ethanol produced was 25±1% of the theoretical yield,
corresponding to 0.140±0.005 g ethanol/g textiles. A pretreatment with 12% (w/w) NaOH
solution at 0 °C enhanced the digestibility of the jeans textiles, improving the ethanol yield to
0.48±0.02 g ethanol/g textiles (Paper I). Furthermore, ethanol production from cotton treated
with NMMO at various conditions was compared with production using untreated cotton. The
materials were hydrolyzed by cellulase at 45 °C for 72 hours, followed by 30 hours of
fermentation (separate hydrolysis and fermentation, SHF), using S. cerevisiae. Figure 4.1
illustrates the yield of ethanol from cellulose treated for 5 hours under different conditions
41
and the yield obtained from untreated cellulose. The SHF-results of untreated and pretreated
cellulose disclosed that the concentration of NMMO had a significant impact on the ethanol
yield and that pretreatment had a larger effect at 120 °C than 90 °C. The optimal conditions in
the present study proved to be an 85% NMMO solution at 120 °C, where the ethanol yield
improved over 150% (Paper II).
Figure 4.1: Yield of ethanol of NMMO-treated and water-treated celluloses after 72 h
hydrolysis and 30 h fermentation, as percentage of the theoretical yield (0.56 g/g cellulose).
The pretreatments were carried out at three different modes of NMMO–water solution,
dissolution (85%), ballooning (79%), and swelling (73%) at 90 and 120 °C for 5 hour (Paper
II).
4.3. Biogas
Biogas produced through anaerobic digestion of biological waste is a renewable
bioenergy. Apart from the methane produced, biogas production holds the potential to
minimize the waste pollution, consequently protecting the environment [114]. The digestate, a
byproduct of the biogas production process, is a value added material that can substitute
mineral fertilizers [115]. LCA studies confirm that the environmental impact is minimized by
recycling and anaerobic fermentation, if the manure is used for agricultural purposes [116].
From the perspective of resource effectivity, biogas production has a higher output-to-input
energy ratio compared to current ethanol production systems [117]. Moreover, biogas
production systems can be integrated in existing bioethanol and biodiesel production plants,
thereby increasing the resource effectivity [117]. Furthermore, in terms of air emission, biogas
0
10
20
30
40
50
60
70
80
90
Dissolution Ballooning Swelling Water treated
Yie
ld o
f eth
anol
(% o
f the
oret
ical
)
120 °C
90 °C
42
production may be better for the environment than incineration of waste [118]. Taking all
these aspects into account, as a well established technology for generating bioenergy, biogas
production is one of the most environmentally beneficial processes, offering a drastic
reduction of GHG emissions [114]. From a greenhouse gas perspective, production of biogas
from manure has a dual effect, a reduction of methane emission and a reduction of carbon
dioxide release from using fossil fuels [117].
The EU policies have set a goal that by the year 2020, 20% of the European energy
demands must be supplied from renewable energy sources. Calculations by Holm-Nilsen et
al. [119] show that at least 25% of this bioenergy can be obtained by producing biogas from
animal manure, whole crop silage, and MSW [119-120]. Furthermore, in a report by the
European Biomass Association1, the realistic potential of methane production in EU-27 from
animal manure, energy crops, and waste, was estimated to 78 billion m3 or 40 Mtoe2, in 2020,
compared to the production of 5.9 Mtoe biogas in 2007 [121].
Methane-rich biogas may serve as a source for heat, steam, and/or electricity, and can be
further upgraded to vehicle fuel or for production of chemicals and/or proteins. It may also be
used as a household fuel or in fuel cells [114, 120]. The market for biogas vehicles has in
recent years grown rapidly in Sweden, where 17,000 vehicles, run on upgraded biogas/natural
gas, were in use in 2008 [121].
Anaerobic digestion entails degradation of organic materials by a microbial consortium in
the absence of oxygen [122]. All types of biomass containing carbohydrates, proteins, fats,
cellulose, and hemicellulosic components can potentially be used as feedstock in this process
[114]. Indeed, a wide variety of waste materials, e.g. organic waste from agriculture-related
industries, animal manure, waste from food industries, and biodegradable fractions of MSW
as well as municipality sewage water, can be converted to significant quantities of biogas,
rather than being used as landfills [50, 114, 122].
The composition of the biogas generated from different substrates depends on their
content of organic substances and their respective compositions. Assuming complete
degradation, the formation of methane from biomass undergoes the following general reaction
[123]:
CcHhOoNnSs + yH2O xCH4 + nNH3 + sH2S + (c-x)CO2
x= (4c+h-2o-3n-2s) 1 http://www.aebiom.org 2 Million tons of oil equivalent
43
y= (4c-h-2o+3n+2s)
For example, the overall reaction for biogas production from carbohydrates is [123]:
C6H12O6 3CH4 + 3CO2
4.3.1. Methane fermentation process
Methane fermentation is a complex process, involving the action of different consortia
of microorganisms degrading the organic materials to intermediary products and finally
converting them into methane [124]. This digestion process can be divided into four phases
(Figure 4.2): hydrolysis, acidogenesis, acetogenesis/dehydrogenation, and methanogenesis.
Hydrolysis is the first step of the anaerobic degradation and refers to the
depolymerization of complex insoluble polymers, including carbohydrates, fats, and proteins,
performed by hydrolysis bacteria. The products of the hydrolysis step are simple soluble
compounds, such as sugars, fatty acids, and amino acids, which can easily be assimilated by
bacterial cells [124]. This step is usually considered as the rate limiting step in the anaerobic
digestion of water insoluble organic matters, such as cellulose [97]. The second step of
anaerobic digestion is acidogenesis. In this step, the soluble compounds produced in the
hydrolysis step, or discharged directly to the digester, are absorbed by fermentative bacteria
and converted into volatile fatty acids (VFA), alcohols, hydrogen, and carbon dioxide.
Acidogenesis is usually the fastest step in the anaerobic digestion of organic compounds
[124]. Subsequently, during the acetogenesis phase, acetate-forming bacteria, which are
obligate hydrogen producers, convert the products from the acidogenic phase to acetate,
hydrogen, and carbon dioxide. Acetogenic bacteria can survive only at very low
concentrations of hydrogen. Reproduction of these bacteria is also very slow, with a
generation time longer than 3 days. In the last step, the methanogenic phase, methane is
produced, mostly from acetate, carbon dioxide and hydrogen gas. Methane can also be
produced from some other intermediates, such as formate and methanol. Methanogenic
bacteria are strict anaerobes with a slow growth rate, and methanogenesis is usually
considered as the rate limiting step in the anaerobic digestion of soluble compounds [97, 124].
The four groups of bacteria involved in the anaerobic digestion work in sequence; the
product from a group is used as substrate for the group in the following step [124]. However,
a closer link exists between hydrolytic and acidogenic bacteria as well as between acetate-
and methane-forming bacteria, and dividing the process into two main stages makes it more
44
efficient. As long as the degradation rate is roughly equal in both stages, the process is
balanced [114].
Accomplishment of the anaerobic digestion process depends on environmental factors
such as micro- and macro-nutrient availability, pH, temperature, mixing rate, organic loading
rate, and retention time. These parameters should thus be controlled and kept within the
optimum range to maintain a high efficiency [122]. The structure of the substrate also has an
important impact on the efficiency of the digestion and the recalcitrant structure of cellulosic
material makes it in general very difficult to degrade. It is therefore, advisable to apply a
physical, chemical, or biological pretreatment step to increase the degradation rate [49-50].
Figure 4.2: Degradation steps of the anaerobic digestion process: Hydrolysis, acidogenesis,
acetogenesis, and methanogenesis. Adapted from [124].
4.3.2. Application of granular sludge to increase the degradation rate of anaerobic
digestion
The slow growth rate of acetogenic and methanogenic bacteria is one of the main
reasons for requiring a long residence time in continuous processes of anaerobic digestion.
Keeping a high cell concentration inside the reactor as well as an increasing organic loading
rate may help to reduce the digestion time. Fortunately, anaerobic microorganisms are capable
Intermediary productsAlcohols and Volatile Fatty Acids
Hydrolysis
Soluble Organic MoleculesSugars, Amino acids, Fatty Acids
Compelex SubstratesCarbohydrates, Lipids, Proteins
Acetate Hydrogen, Carbon dioxide
Methane, Carbon dioxide
Acidogenesis
Acetogenesis
Acetoclasticmethanogenesis
Hydrogenotrophicmethanogenesis
45
of biological aggregation and forming of granules. The granules in granular sludge are
conglomerates with dense structures, formed through self-immobilization of bacterial cells
[125-126]. The excellent settling ability of the granules, allows a high biomass and a rich
microbial diversity to be kept in the reactor. This greatly improves the digestion capacity and
efficiency [127]. The upflow anaerobic sludge blanket (UASB) reactor is one of the most
popular biogas reactors, developed by Letinga in 1980 based on the advantages of granular
sludge [128]. It is a high-rate reactor, mostly used for anaerobic treatment of wastewater. The
UASB reactor (Figure 4.3) is typically divided into four sections (from bottom to top): (1) the
granular sludge bed, (2) the fluidized bed, (3) the gas-liquid separator, and (4) the settling
zone [125, 129]. The liquid is pumped in at the bottom of the reactor and flows to the top
through the granular sludge bed. When passing through the sludge, the organic matter in the
liquid is converted to CH4 and CO2 by the action of anaerobic microorganisms present in the
granules. Due to the production of biogas, a fluidized bed is developed above the granular
sludge bed, where further biological degradation can take place. The biogas and the liquid are
subsequently separated in the gas liquid separator [125, 129].
The UASB process offers the following advantages over the other types of biogas
reactor systems [130]:
- More compact structure
- Higher organic loading capacity and lower retention time
- Lower operational costs
- Higher methane yield
One of the limitations of this type of reactor is that feeding the reactor with an influent
containing a high concentration of solid particles causes operational problems. Because of the
dense structure of the granular sludge and a low level of mixing, the suspended solids may
accumulate in the sludge bed, resulting in a decreased methanogenic activity and a lower
efficiency [131]. Therefore, a two-step anaerobic bioreactor system is proposed for anaerobic
digestion of organic waste with high solid contents. The first step of this process can be
performed in a continuous stirred-tank reactor (CSTR), in which partial hydrolysis and
acidification of suspended solids takes place. The effluent of the first step will then be further
processed in the second step, which is a UASB reactor with a high methanogenic activity
[132].
46
Figure 4.3: A lab-scale UASB reactor for biogas production.
Sludge blanket
Influent
Efluent
Gas outlet
Fluidized bed
47
Chapter 5
WASTE TEXTILES REFINERY
Development of biorefineries provides an excellent opportunity to curb the rapidly
increasing consumption of fossil resources enabling establishment of a sustainable bio-based
economy [133]. There are various definitions for biorefinery. According to the US department
of Energy (DOE), the term "biorefinery", is parallel to petrochemical refinery, and is referred
to as "an overall concept of a processing plant where biomass feedstocks are converted and
extracted into a spectrum of valuable products" [134]. The products of a biorefinery may
include chemicals, fuels, and energy [133].
In a report published in 2006, the European Environmental Agency (EEA) stated that by
2030, potentially 9.7 Mtoe can be obtained from biowaste [135]. Biowaste is a residue of
biological origin including municipal, farm, agriculture, and other biodegradable solid waste
streams [135].
First of all, a considerable amount of biowaste still ends up in landfills and consequently,
landfilling increasingly occupies land areas, especially in megacities with a population of
several millions. Furthermore, the release of carbon dioxide and methane into the atmosphere
and the leakage from landfills into the groundwater, are among the most important
environmental impacts associated with landfilling. Thus, the landfill directive [113], issued by
the European Union in 1999, was set up to prevent or to maximally reduce the negative
impact of waste landfilling exerts on the environment.
Using biowaste in the biorefineries as feedstock for generating biofuels, power, and other
value added products, will contribute to long-term sustainability, in terms of CO2 emissions
and supply of raw material. Consequently, this will have a great impact on waste management
issues, reducing the pollutions generated by the waste streams. Moreover, using biofuels
produced from wastes reduces the consumption of fossil fuels and thus, the GHG emissions as
well.
One of the cellulose containing wastes is waste textile, making up 2-5% of the municipal
solid waste in different countries. For instance, in 2008, the influx of clothing and textiles to
48
Sweden was 131,800 tons, equivalent to 14.7 kg per person. After use, approximately 3 kg
textile per person was donated to charities, while 8 kg per person was thrown in the trash
[136].
Disposal of such vast quantities of waste along with pre-consumer waste from fiber and
textile industries has in recent years raised the environmental concerns. Landfilling and
incineration are today the main methods for disposal of waste textiles, in spite of their high
potential as a source of valuable materials and energy. Figure 5.1 shows an overview of the
processes holding the promise of being integrated into a waste refinery concept for production
of materials, fuels, and energy from waste textiles.
Since reuse of waste is at the top of the waste management hierarchy, processing of waste
textiles to make them reusable, as discussed in section 2.4.1, may also be included in a textile
refinery (Figure 5.1). However, when the textiles are not reusable in form of fabrics, one may
instead convert the fiber mixture, without any separation, into heat, electricity, fuels, and
other products by thermochemical processes, i.e. incineration, pyrolysis, and gasification
(Figure 5.1). Furthermore, in an integrated textile refinery, the residual fibers of any step of
the waste textiles processing can be used as feedstock for thermochemical processes.
End-of-life waste textiles (see Chapter 2) comprise natural fibers, e.g. cellulose as well as
synthetic fibers, mainly including polyester, polyamide, and polyacrylonitrile. Developing a
process to separate those polymers, e.g. by solvent extraction, creates a possibility to recycle
them, e.g. by melt processing or depolymerization-repolymerization. Other, previously
investigated, applications of recycled synthetic fibers are as reinforcement of polymer
composites [137], concrete [138-139], and soil [140]. Separation of the acrylic part of waste
textiles to be applied in the production of superabsorbents is under investigation in a parallel
project.
Statistical data on the global fiber consumption indicate that the cellulose content of waste
textile corresponds to 40% of the total global fiber production, which is comparable to the
data on lignocellulosic materials, i.e. 40±10%. Hence, like lignocellulosic waste, waste
textiles may well be perceived as raw materials for a biorefinery. The cellulose may either be
used for production of cellulose-based materials (e.g. regenerated fibers, paper board, and
superabsorbents), or processed biologically into glucose for production of sugar-based
products (Figure 5.1.). A variety of sugar-based products may be produced by biological
conversion of either cellulose or cellulose hydrolyzate (see Figure 5.1), with biogas and bio-
ethanol being in the focus of the present study (Paper I, III). The process of bio-fuel
production from cellulosic waste textiles faces however some challenges.
49
Figure 5.1: A suggested waste textile refinery and the part covered by the present thesis (gray).
Waste textiles
Mixture of fibers
Separation of fibers
Incineration Pyrolysis Gasification
Syntheticfibers
Separation
Syngas
Chemicals
Heat Bio-oil
Other naturalfibers
Polyester Polyamide Polyacrylilonitrile
Melt processing Depolymerization Other applications
Othersynthetic fibers
Cellulosicfibers
Reuse
HeatElectricity
Fischer-Tropschprocess
Syntheticfuels
Gaseous fuels
Recycling
Physico-chemicalprocessing
Fiber
cellophane
Adhesives
Paper
Super-absorbent
Bioprocessing
Methane
Butanol
Acetone
Ethanol
Lactic acid
Malic acid
Citric acid
Acetic acid
50
In a biorefinery of waste textiles, it is possible to pass the fiber mixture as it is through the
whole process in which the cellulosic fibers are converted to bio-fuels (e.g. biogas or
bioethanol) in the subsequent hydrolysis and fermentation steps. However, waste textiles
cannot as yet be considered as a suitable raw material for efficient biofuel production, due to
certain drawbacks expected to arise during the fermentation process, such as:
- High energy consumption
- High equipment costs
- Difficulty in handling and mixing
- High equipment and handling costs
- High hydrolytic enzyme consumption
- Low yield of enzymatic hydrolysis
The main problem is the presence of synthetic fibers in addition to the recalcitrant
structure of cotton. Passing this synthetic fraction through the whole process, creates several
processing problems such as a higher energy demand for heating and cooling, pumping
problems, and a need for larger bioreactors for hydrolysis and fermentation. Moreover,
because the cellulosic fibers are woven with other fibers, e.g. polyester, the available surface
area for cellulolytic enzymes to attack is reduced, hampering the rate of hydrolysis. In
addition, around 90% of the global cellulosic fiber consumption is cotton which is difficult to
hydrolyze because of its recalcitrance structure.
Most of these problems may be overcome by separating the cotton fibers from the textile
and using only the cotton part for the production of biofuel. This is probably an essential step
in the biorefinery of waste textiles. Paper III describes such a separation process, developed in
the present thesis. The basic concept of the process is to dissolve the cellulosic fibers in waste
textiles made up of blended fibers, using a solvent with no harmful effect on the environment,
such as NMMO (Figure 5.2). The dissolved cellulose is separated from the rest of the fibers
by filtration and subsequently regenerated by adding water to the solution. As section 3.4
pointed out, pretreatment with NMMO facilitates the subsequent enzymatic or bacterial
hydrolysis of cellulose. Using NMMO on blended waste textiles effectuates simultaneous
cellulose pretreatment and separation from the other fibers. One of the main economical
advantages of this method is a nearly complete recycling of the solvent. Furthermore, it is
possible to integrate this pretreatment step into the present bio-ethanol and biogas plants,
using other cellulosic materials, e.g. lignocelluloses [141].
51
Figure 5.2: A biorefinery for waste textiles with simultaneous pretreatment of cellulose and
its separation from other fibers (Paper III).
Assuming that the total amount of waste textile in Sweden is nearly equal to the amount of
imported clothing and textiles, and that 40% of the textile fibers consist of cellulose.
Therefore, approximately 53,000 tons of cellulose is wasted every year. Assuming a 90%
yield when producing ethanol from cellulose (0.5 g ethanol per g cellulose), the amount of
Swedish waste textiles is sufficient to supply the substrate for producing around 26,000 tons
of ethanol per year. The amount of ethanol required for running a car 20,000 km/year,
consuming 7 l/100 km, is 1400 l. Consequently, the Swedish waste textiles hold the potential
of providing fuel for 23,000 cars, on a yearly basis.
Dissolution of cellulose Filtration Cellulose
solution
Residuals(Mainlysynthetic polymers)
Precipitationof cellulose
PretreatedceluloseWater-NMMO solutionNMMO (85%) Evaporation
Water
Filtration
Biogas plant
Bioethanol plant
Waste textiles
Water
Recycling
Thermochemicalprocessing
52
53
Concluding remarks
The fate of waste textiles is at present either landfilling or incineration. It holds, however,
a high potential for utilization in a biorefinery to produce biofuel, energy, and a variety of
chemicals. In terms of having cellulose and non- or hardly biodegradable materials, their
composition is comparable to lignocellulosic materials, although the cellulose of waste
textiles is highly crystalline and more recalcitrant than the typical lignocelluloses. The high
crystallinity of cotton used in textile production is one of the challenges when using waste
textiles for biological conversion. Another problem is noncellulosic, mainly synthetic
polymers, being present in waste textiles, causing different problems when processing the
wastes. Thus, two pre-processing steps are necessary to procure a raw material from the
wastes, suitable for biological conversion: (1) a pretreatment for improving the enzymatic
hydrolysis of the cellulose and (2) separation of the cellulose from the noncellulosic parts.
The crystallinity problem can be overcome by different pretreatments similar to the ones
developed for lignocelluloses. Of the processes used in the present work, pretreatment with
phosphoric acid greatly improved the hydrolysis, but treatments with NaOH and NMMO
showed to be even better, with a yield of over 99% in the subsequent enzymatic hydrolysis.
Mixed fibers in waste textiles require a separating process, either of cellulose from the waste
or of other fibers from the cellulose. Since different types of non-cellulosic fibers are present
in the waste, it is logical to separate the cellulose from the waste, which is almost the same in
all the textile wastes.
It would, however, be an interesting idea to consolidate the two processes, i.e. perform
pretreatment and separation in one process. The present work thus developed a process for
simultaneous pretreatment of cellulose and its separation from the waste textile, using
NMMO, an environmental friendly chemical, industrially available, and easy to recycle. The
pretreatment may be performed in three different modes, dissolution, ballooning, and
swelling. The dissolution mode successfully modified the structure of cellulose, decreasing its
crystallinity and making it accessible to hydrolytic enzymes, resulting in a significantly
increased rate and yield of the cellulose hydrolysis. The pretreatment furthermore efficiently
separated the cellulose from the waste textiles. This process may thus provide a promising
option for a better use of waste textiles, converting them into biofuel and chemicals, than the
current alternatives, landfilling and incineration.
54
55
Future Work
The suggested waste textile refinery described in the present study is in its preliminary
stage and much research and development work remains to investigate its different parts. The
possible future work may be classified into the following categories for scientific research and
industrial applications.
A. Suggested experimental work: Since not much work has been performed on different
aspects of waste textiles processing, the following issues may require more experiments and
analyses:
- Effects of different dyes and reagents in the waste textiles on bioprocessing.
- Separation of different types of fibers from waste textiles, other than polyester/cotton.
- Characterization and possible applications of the separated synthetic polymers.
- Characterization of separated cellulose and a possibility of its application as a
cellulosic raw material.
- Conversion of separated cellulose to biological products other than bioethanol and
biogas, such as lactic acid and citric acid.
- Regeneration of cellulose from waste textiles, combined with use of acid hydrolysis
for fermentable sugar production.
- Effects of noncellulosic natural fibers present in waste textiles, in the processing
of the wastes.
These data will give further information on how to proceed.
B. Techno-economical studies: Simulation and economical evaluation of the following
topics are necessary for improving the implementation design for future processes:
- Simulating the suggested process in order to find the bottlenecks, and evaluating
different possibilities for further development.
- Optimization and integration of the suggested process.
- Feasibility study of biofuel production from waste textiles.
- Comparison of bioethanol and biogas production from waste textiles with other energy
production processes, i.e. gasification, pyrolysis, and incineration.
- Feasibility of integrating the process developed in the present work in an existing
ethanol or biogas plant.
This means that there are a numbers of issues and questions to be addressed and answered.
56
57
Nomenclature
CbP Cellobiose phosphorylases
CdP Cellodextrin phosphorylases
CesA Cellulose synthase enzyme
Ds Degree of synergism
GHG Greenhouse gas
G-1-P Glucose-1-phosphate
ISO International standardization organization
LCA Life cycle assessment
Mtoe Million tons of oil equivalents
NMMO N-Methylmorpholine-N-oxide
NREU Non-renewable energy use
SR Substrate reactivity
TRAID Textile recycling for aid and international development
tVFA Total volatile fatty acids
UASB Up flow anaerobic sludge bed
58
59
Acknowledgments
The completion of this thesis would not have been possible without support, help,
guidance, and encouragement by several people and I am now taking the opportunity to put
my thanks into words.
First of all, I would like to express my deepest and sincerest gratitude to my supervisor
Prof. Mohammad Taherzadeh for his invaluable support and encouragement throughout my
studies. I always, clearly, was feeling his kindness and patience towards me even during our
strict and serious discussions during this study. He really did his best to teach me a lot about
the research.
I wish to express my sincere thanks to my examiner, Prof. Claes Niklasson for all his
support and help during my work on this thesis.
My warm appreciation is due to Dr. Julie Gold, the director of studies at Chalmers
Graduate School in Bioscience, for her concern and help about my PhD studies.
My deepest gratitude goes to my best friend Dr. Akram Zamani, for being warmhearted
and helpful, and for being always there for me. We have a very warm and stable friendship,
dating back since the start of our bachelor studies. Her insight helped me to be able to see, her
kindness helped me to be able to feel and her support helped me to be able to overcome the
difficulties. All these things make her a very special person, and I am proud to call her my
best friend.
I owe my sincere thanks to Dr. Keikhosro Karimi for his invaluable guidance and
contribution to some parts of this thesis and for being always kindly available to support me
throughout my studies.
I also wish to state my warm gratitude to Dr. Ilona Sarvari Horvath for her scientific
support and friendliness throughout the years and Dr. Thomas Brandberg for his valuable
advice and pleasant attitude.
My very special thanks go to Dr. Dag Henriksson for his unlimited social support during
my PhD studies.
I sincerely acknowledge the people at School of Engineering, University of Borås, for
creating a pleasant environment. Thanks also to the head of the department, Dr. Hans Björk,
and the director of studies, Dr. Peter Therning, for their concerns regarding my studies. My
sincere gratitude goes to Prof. Mikael Skrifvars for our fruitful and stimulating discussions
during part of my research. I am also very grateful to Jonas and Louise for always being
available when I needed help.
60
I appreciate my wonderful teachers Dr. Magnus Lundin and Dr. Elisabeth Feuk-Lagerstedt
at University of Borås, Prof. Lisbeth Olsson, Dr. Carl-Johan Franzen, and Dr. Britt-Marie
Steenari at Chalmers, Dr. Tomas Ekvall at IVL, and Dr. Anna Schnürer, and Dr Martin Weih
at SLU, for the PhD courses and research related discussions.
I am very grateful to Marianne and Margareta at the administration office of the Chemical
and Biological Engineering Department at Chalmers, for their help.
I would also like to thank my fellow PhD student friends as well as my colleagues at the
department. Farid, Mohammad, Azadeh, Anna, Patrik, Solmaz, Gergely, Päivi, Johan,
Supansa, Abbas, Farzad, Kamran, Anita, Haike, Kayode, Sung-Woo, and Dan: Thank you
very much!
I would like to thank the MSc and exchange students who did their best during the time
that we were working together: Marzieh, Zurima, Hosein, Mehdi, Sarah, Maryam, Kambiz,
Sara, Vahid, Karthik, Gopinath, and Arash: Thank you!
The financial support of this thesis was provided by Swedbank Stiftelsen in Sjuhärad
(Sweden) and is greatly acknowledged.
Finally, and most importantly, none of my achievements would have been possible
without the love and support of my family. My parents (Maman&Baba), to whom this thesis
is dedicated, have been an unweaving source of love, concern, support, and strength during
my life. I would like to express my deepest heart-felt appreciation to them.
61
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CHALMERS UNIVERSITY OF TECHNOLOGY SE‐41296 Göteborg, Sweden Telephone: +46‐(0) 317721000 www.chamlers.se
Abstract
In the world today, the need for sustainable processes is increasing. The work of the present thesis has been focused on conversion of the cellulosic part of waste textiles into biogas and ethanol, and its challenges. In 2009, the global annual fiber consumption exceeded 70 Mt, of which around 40% consisted of cellulosic material. This huge amount of fibers is processed into apparel, home textiles, and industrial products, ending up as waste after a certain time delay. Regretfully, current management of waste textiles mainly comprises incineration and landfilling, in spite of the potential of cellulosic material being used in the production of different biofuels. The
volume of cellulose mentioned above would be sufficient for producing around 20 billion liters of ethanol or 11.6 billion Nm3 of methane per year. Nevertheless, waste textiles are not yet accepted as a suitable substrate for biofuel production, since their processing to biofuel presents certain difficulties and challenges, e.g. high crystallinity of cotton cellulose, presence of dyes, reagents and other materials, and being textiles as a mixture of natural and synthetic fibers. High crystallinity of cotton cellulose curbs high efficient conversion by enzymatic or bacterial hydrolysis, and the presence of non-cellulosic fibers may create several processing problems. The work of the present thesis centered on these challenges.
Cotton linter and blue jeans waste textiles, all practically pure cellulose, were converted to ethanol by SSSF, using S. cerevisiae, with a yield of about 0.14 g ethanol/g textile, only 25% of the theoretical yield. To improve the yield, a pretreatment process was required and thus, several methods were examined. Alkaline pretreatments significantly improved the yield of hydrolysis and subsequent ethanol production, the most effective condition being treatment with a 12% NaOH-solution at 0 °C, increasing the yield to 0.48 g ethanol/g textile (85% of the theoretical yield).
Waste textile streams, however, are mixtures of different fibers, and a separation of the cellulosic fibers from synthetic fibers is thus necessary. The separation was not achieved using an alkaline pretreatment, and hence another approach was investigated; pretreatment with N-methyl-morpholine-N-oxide (NMMO), an industrially available and environment friendly cellulose solvent. The dissolution process was performed under different conditions in terms of solvent concentration, temperature, and duration. Pretreatment with 85% NMMO at 120 °C under atmospheric pressure for 2.5 hours, improved the ethanol yield by 150%, compared to the yield of untreated cellulose. This pretreatment proved to be of major advantage, as it provided a method for dissolving and then recovering the cellulose. Using this method as a foundation, a novel process was developed, refined and verified, by testing polyester/cellulose-blended textiles, which predominate waste textiles. The polyesters were purified as fibers after the NMMO treatments, and up to 95% of the cellulose content was regenerated. The solvent was then recovered, recycled, and reused. Furthermore, investigating the effect of this treatment on anaerobic digestion of cellulose disclosed a remarkable enhancement of the microbial solubilization; the rate in pretreated textiles was twice the rate in untreated material. The process developed in the present thesis is promising for transformation of waste textiles into a suitable substrate, to subsequently be used for biological conversion to ethanol and biogas.
