Bioenergy systems for the future 3 production of bioalcohol and biomethane

Production of bioalcohol and
biomethane

3

K. Ghasemzadeh*, E. Jalilnejad*, A. Basile†
*Urmia University of Technology, Urmia, Iran, †Institute on Membrane Technology
(ITM-CNR), Rende, Italy

Abbreviations
AD
BOD
COD
COx
DME
LHW
MF
MD
MBR
NF
NOx
PSA
RO

3.1

anaerobic digestion
biochemical oxygen demand
chemical oxygen demand
carbon oxides
dimethyl ether

liquid hot water
microfiltration
membrane distillation
membrane bioreactor
nanofiltration
nitrate oxides
pressure swing adsorption
reverse osmosis

Introduction

Energy plays a significant role in the economic growth of any country. Based on
statistics, global production of oil and gas is approaching its maximum, and the world
is now finding one new barrel of oil for every four it consumes (Aditiya et al., 2016;
Delfort et al., 2008; Nigam and Singh, 2011). The increasing energy crisis and growing
environmental concerns in recent years have driven the development of technologies to
allow for the substitution of fossil fuels with renewable energy (Aditiya et al., 2016;
Delfort et al., 2008; Nigam and Singh, 2011; Koh and Ghazoul, 2008). Several alternatives are currently being explored, including a range of carbon-free and renewable
sources (photovoltaics, wind and nuclear power, and hydrogen) in an attempt to replace
natural gas, coal, and oil in the electricity generation sector. However, there is no such
equivalent in transportation, since fuel cell, electric/hybrid, and natural gas-based cars
are still a long way from becoming mainstream vehicles (Murray, 2005). Among the
explored alternative energy sources, considerable attentions have been focused on
Bioenergy Systems for the Future. />© 2017 Elsevier Ltd. All rights reserved.


62

Bioenergy Systems for the Future

biofuels (bioalcohol and biomethane) because it is widely available from inexhaustible
feedstocks that can effectively reduce its production cost.
Indeed, biofuel can be produced from various kinds of renewable materials such as
corn, sorghum, cellulose, and algae biomass. However, among all biofuels, bioalcohol
such as bioethanol and biomethane is more productable than other types. On the basis
of the raw material used for its production, bioethanol and biomethane are divided into
various types (Aditiya et al., 2016). However, their production involves many processes such as pretreatment, fermentation, recovery, and refining (Thomson, 2008).
To the best of our knowledge, the largest energy demand in biofuel production is
for the steam and electricity used in the fermentation/distillation process. Hence, bioalcohol and biomethane will not be significant without improvements in this process
and reduced energy requirements.
Membrane separation technologies have gained more and more attention due to
their reduced energy requirements, lower labor costs, lower floor space requirements,
and wide flexibility of operation (Suresh et al., 1999). This technology has been
applied in many processes of bioalcohol and biomethane production instead of the
traditional process (Stevens et al., 2004; Larson, 2008; Noraini et al., 2014;
Bergeron et al., 2012; Galanakis, 2012). Therefore, the main aim of this chapter is
to present a state-of-the-art review on the bioalcohol and biomethane production processes and also on the applications of membrane technologies for their production.

3.2

Biofuels

In general, biofuels are referred to liquid, gas, and solid fuels, such as ethanol, methanol, biodiesel, Fischer-Tropsch diesel, hydrogen, and methane, which are predominantly produced from biomass. Renewable and carbon-neutral biofuels are necessary
for environmental and economic sustainability. Between 1980 and 2005, worldwide
production of biofuels increased significantly by an order of magnitude from 4.4 to
50.1 billion liters, with further dramatic increases in the next years (Koh and
Ghazoul, 2008; Murray, 2005; Licht, 2008). The economics of each fuel vary with
location, feedstock, fermentation technology, and several other factors. Political
agendas and environmental concerns also play a crucial role in the production and utilization of biofuels. The challenging point on use of these fuels is fitting biofuels into
the enormous current fuel distribution and vehicle infrastructure (Nexant, Inc, 2006).

Major benefits of biofuels are summarized in Table 3.1.
With regard to the presented studies (Nigam and Singh, 2011; Alam et al., 2012),
biofuels can broadly be classified as primary and secondary biofuels based on their source
and type. The primary biofuels, also named as natural biofuels, such as vegetables, animal
waste, landfill gas, fuelwood, wood chips, and pellets, are used in an unprocessed form,
primarily for heating, cooking, or electricity production. The secondary biofuels like
ethanol, methane, biodiesel, and DME are produced by processing of biomass and can
be used in vehicles and various industrial processes.
The secondary biofuels are modified primary fuels, which are further divided into
first-, second-, and third-generation biofuels on the basis of raw material and


Production of bioalcohol and biomethane

Table 3.1

63

Major benefit of biofuels

Economic consequences
Sustainability
Fuel diversity
Increased number of rural manufacturing jobs
Increased income taxes
Increased investments in plant and equipment
Agriculture development
International competitiveness
Reducing the dependency on imported petroleum
Environmental consequences

Greenhouse gas reduction
Reducing of air pollution
Biodegradability
Improved land and water use
Carbon sequestration
Higher combustion efficiency
Energy security
Domestic targets
Supply reliability
Reducing use of fossil fuels
Ready availability
Domestic distribution
Renewability

technology used for their production. They are produced in the form of solids (e.g.,
charcoal) liquids (e.g., ethanol, biodiesel, pyrolysis oils, and bio-oil), or gases (e.g.,
biogas (methane), synthesis gas, and hydrogen) and can be used in transport and
high-temperature industrial processes (Thomson, 2008; Hoekman, 2009). A tee diagram for classification of biofuel is shown in Fig. 3.1.
The first-generation liquid biofuels are generally produced from sugars, grains, or
seeds like wheat, palm, corn, soybean, sugarcane, rapeseed, oil crops, sugar beet,
and maize and require a relatively simple process to produce the finished fuel product.
Ethanol is the most well-known first-generation biofuel produced by fermenting sugar
extracted from crop plants and starch contained in maize kernels or other starchy crops
(Nigam and Singh, 2011; Hoekman, 2009; Suresh et al., 1999; Stevens et al., 2004). Due
to the increasing growth in production and consumption of biofuels, first-generation
fuels are being produced in significant commercial quantity in a number of countries.
However, the first generation is claimed to be not very successful because of the conflict
with food supply and high production cost due to competition with food; thus, it affects
food security and global food markets. These limitations favor the search of nonedible
biomass for the production of biofuels (Larson, 2008; Noraini et al., 2014).

Second-generation liquid biofuels are generally produced by two different
approaches, that is, biological or thermochemical processing, from agricultural lignocellulosic biomass, which are either nonedible residues of food crop production or


64

Bioenergy Systems for the Future

Biofuels

Secondary biofuels

First generation

Source: Seeds, grains,
or sugars
– Bioethanol or butanol by
fermentation of sugars
(sugars cane, sugars
beet, etc) or starch
(wheat, potato, corn, etc)
– Biodiesel by
transesterification of plant
oils (sunflower, palm,
soybean, etc)

Primary biofuels

Second generation

Third generation

Natural biofuels

Source: Lignocellulosic
biomass

Source: Algae, sea weeds

– Bioethanol or butanol by
enzymatic hydrolysis

– Biodiesel from algae

Firewood, wood chips,
pellets, animal waste,
and landfill gas

– Biomethane by anaerobic
digestion

– Bioethanol from algae
and sea weeds

– Methanol, DME, etc by
thermochemical processes

– Hydrogen from green algae
and microbes

Fig. 3.1 Classification of biofuels.

nonedible whole plant biomass (e.g., grasses or trees specifically grown for production
of energy). This generation has more advantages compared with the first generation
due to higher production yield and lower land requirement and also using nonedible
feedstocks that limits the direct food versus fuel competition associated with firstgeneration biofuels. Feedstock involved in the process can be bred specifically for
energy purposes, enabling higher production per unit land area, and a greater amount
of above-ground plant material can be used to produce biofuels. It appears evident
from the literature (Bergeron et al., 2012; Galanakis, 2012) that production of
second-generation biofuel requires most sophisticated processing production equipment, more investment per unit of production, and larger-scale facilities to confine
and curtail capital cost scale economies, which are discussed in future sections. To
achieve the potential energy and economic outcome of second-generation biofuels,
further research, development and application are required on feedstock production
and conversion technologies (Prado et al., 2016).
As indicated in Fig. 3.1, third-generation biofuels use microbes and macro- and
microalgae feedstocks as a very promising source for renewable energy production
since it can fix the greenhouse gas (CO2) by photosynthesis and does not compete with


Production of bioalcohol and biomethane

65

the production of food; hence, it is devoid of the major drawbacks associated with firstand second-generation biofuels. On the basis of current scientific knowledge and technology projections, some microorganisms like yeast, fungi, and microalgae can be used
as potential sources for biofuel as they can biosynthesize and store large amounts of fatty
acids in their biomass (Alam et al., 2012; Noraini et al., 2014). Microalgae can produce
lipids, proteins, and carbohydrates in large amounts over short periods of time, and these
products can be processed into both biofuels and valuable coproducts (Costa et al.,
2012). Fuel production from algae has various advantages such as high growth rate,
capability of growing under several conditions including in wastewater, high-efficiency

CO2 mitigation, less water demand than land crops, and more cost-effective farming
(Noraini et al., 2014).

3.2.1

Bioalcohol production

The alcohols are oxygenated fuels in which the alcohol molecule has one or more oxygen that decreases the combustion heat. The alcohols used for motor fuels are methanol (CH3OH), ethanol (C2H5OH), propanol (C3H7OH), and butanol (C4H9OH), but
among them, only methanol and bioethanol fuels are technically and economically
suitable for internal combustion engines (ICEs) (Demirbas, 2007; Ishola et al.,
2013). Bioethanol is a liquid biofuel that can be produced from several different biomass feedstock and conversion technologies. It contains 35% oxygen, which reduces
particulate and NOx emissions from combustion and also reduced hydrocarbons, carbon monoxide, and particulates in exhaust gases. Currently, ethanol is the available
commercial biofuel and the most modern biomass-based transportation fuels, which
sticks out as the most important liquid biofuel with a global production of 88.7 billion
liters in 2011 (Balat, 2011). Bioethanol has been focused as a high potential alternative
to substitute liquid fossil fuels due to its eco-friendly characteristics and relatively low
production cost when compared with other biobased fuels. Bioethanol has a higher
octane number (108), low cetane number, broader flammability limits, higher flame
speeds, and higher heats of vaporization than gasoline. These properties allow for a
higher compression ratio, shorter burn time, and leaner burn engine, which lead to theoretical efficiency advantages over gasoline in an ICE (Aditiya et al., 2016; Demirbas,
2007). Adding ethanol to gasoline has a positive effect on the air quality in every polluted urban areas. It has been stated that 10% blend of bioethanolS with gasoline would
reduce the carbon dioxide emission by 3%–6%, which makes bioethanol a cleaner fuel
in addition to being a renewable alternative to petroleum (Demirbas, 2007; Hansen
et al., 2005).
Bioethanol can be used as a 5% blend with petrol under the EU quality standard EN
228. This blend requires no engine modification and is covered by vehicle warranties.
With engine modification, it can be used at higher levels, for example, E85 (85% bioethanol) (Balat et al., 2008). Bioethanol is widely used in the United States and in Brazil.
The United States and Brazil remain the two largest producers of ethanol accounting
together for 90% of the global bioethanol production. In 2010, the United States generated 49 billion liters, or 57% of global output, and Brazil produced 28 billion liters, or
33% of the total output. Corn is the primary feedstock for US ethanol, and sugarcane is

66

Bioenergy Systems for the Future

the dominant source of ethanol in Brazil (Balat, 2011; Balat et al., 2008). Table 3.2
shows rate of ethanol production from different agro-waste, at various countries of
the world during 2010.

3.2.1.1

Production feedstocks

Biological feedstocks that contain appreciable amounts of sugar—or materials that
can be converted into sugar, such as starch or cellulose—can be fermented to produce
bioethanol (Demirbas, 2007). Feedstocks of bioethanol can be conveniently classified
into three categories of agricultural raw materials: (i) sucrose-containing feedstocks,
(ii) starchy materials, and (iii) lignocellulosic materials. Fig. 3.2 indicates the feedstock classification for bioethanol production. The availability of these materials
for bioethanol can vary considerably from season to season and depends on geographic locations.
For a given production line, the comparison of the feedstocks includes several
issues (Balat, 2011): (1) chemical composition of the biomass, (2) cultivation practices, (3) availability of land and land use practices, (4) use of resources, (5) energy
balance, (6) emission of greenhouse gases, acidifying gases and ozone depletion
gases, (7) absorption of minerals to water and soil, (8) injection of pesticides, (9) soil
erosion, (10) contribution to biodiversity and landscape value losses, (11) farm
gate price of the biomass, (12) logistic cost (transport and storage of the biomass),
(13) direct economic value of the feedstocks taking into account the coproducts,
(14) creation or maintenance of employment, and (15) water requirements and water
availability. Since feedstocks typically account for greater than one-third of the
production costs, maximizing bioethanol yield is imperative. Table 3.3 illustrates

the various feedstocks that can be utilized for bioethanol production and their comparative production potential (Balat, 2009, 2011).

Sucrose containing feedstocks
Feedstock for bioethanol is essentially composed of sugarcane and sugar beet.
Two-thirds of world sugar production is from sugarcane and one-third is from
sugar beet (Prado et al., 2016; Koc¸ar and Civaş, 2013). Sugarcane as a biofuel crop
has much expanded in the last decade, yielding anhydrous bioethanol (gasoline
additive) and hydrated bioethanol by fermentation and distillation of sugarcane
juice and molasses (Hartemink, 2008). Sugarcane is grown in tropical and subtropical countries, while sugar beet is only grown in temperate-climate countries.
Brazil is the largest single producer of sugarcane with about 31% of global sugarcane production, average sugarcane yield of about 82.4 tons/ha, and bioethanol
yield per hectare of around 6650 L/ha (Koc¸ar and Civaş, 2013; Hartemink,
2008; Gauder et al., 2011). In Asia (India, Thailand, and Philippines), sugarcane
is produced on small fields owned by small farmers. For example, India has around
7 million small farmers with an average of around 0.25 ha sugarcane fields (Linoj
Kumar et al., 2006).
Sugar beet, a cultivated plant of Beta vulgaris, is a plant whose tuber contains a
high concentration of sucrose. In 2009, France, the United States, Germany, Russia,


Rate of bioethanol production from agro-waste at various countries/continents

Countries/
continents

Wheat wastes (Tg)/
total bioethanol (GL)

Sugarcane waste
(Tg)/total bioethanol
(GL)

Rice wastes (Tg)/
total bioethanol
(GL)

Barley waste (Tg)/
total bioethanol
(GL)

Corn waste (Tg)/
total bioethanol
(GL)

Iran
Asia
Africa
Europe
America
World

7.5/3
16/50
7/3
140/42
115/20
382/119

4.3/0.63
77/23
13/4

0.01/0.004
181/26
187/55

1.05/0.378
690/202
22/7
4/2
88/12
758/223.5

0.6/0.21
3.5/2
0.5/0.5
47/15.5
19.75/3.5
64.5/22.5

0.5/0.2
45/20
3.5/2.5
31/10
375/45
230/22.5

Production of bioalcohol and biomethane

Table 3.2

67

68

Bioenergy Systems for the Future

Bioethanol feedstocks

Starch

Sugar

Root crops

Stalk crops

Root

Cereals

Cellulose

Forest
residues

Energy
crops

MSW

Agriculture
wastes

Paper
wastes

Fig. 3.2 Feedstock classification of bioethanol production.

Table 3.3

Comparison of various bioethanol feedstocks

Biomass
type
Sugarcane
Cassava
Sweet
sorghum
Corn
Wheat
Miscanthus
Switchgrass
Rice
Maize

Yield
(ton/
ha/
year)

Conversion
rate to sugar
or starch
(%)

Conversion
rate to
bioethanol
(L/ton)

Bioethanol
yield (ton/
ha/year)

Cost
($/m3)

70
40
35

12.5
25
14

70
150
80

4.9

6
2.8

160
700
200–300

5
4
80
20
30
45

69
66
–
–
–
–

410
390
–
–
–
–

2.05
1.56

8
5
1.5
1.5

250–420
380–480
–
–
–
–

and Turkey were the world’s five largest sugar beet producers. In European countries,
beet molasses are the most utilized sucrose-containing feedstock. The advantages with
sugar beet are a lower cycle of crop production, higher yield, high tolerance of a wide
range of climatic variations, and low water and fertilizer requirement. Compared with
sugarcane, sugar beet requires 35%–40% less water and fertilizer (Demirbas, 2007;
Koc¸ar and Civaş, 2013).
Sweet sorghum (Sorghum bicolor L.) is one of the most drought-resistant agricultural crops as it has the capability to remain dormant during the driest periods and
requires fewer inputs to achieve its maximal production. Of the many crops being
investigated for energy and industry, sweet sorghum is one of the most promising candidates, particularly for bioethanol production principally in developing countries
(Stevens et al., 2004; Whitfield et al., 2012).


Production of bioalcohol and biomethane

69

Starchy materials
Another type of feedstock used for bioethanol production are starch-based materials.

Starch is a biopolymer and defined as a homopolymer consisting only one monomer,
D-glucose. To produce bioethanol from starch, it is necessary to break down the chains
of this carbohydrate for obtaining glucose syrup through hydrolysis, which can be
converted into bioethanol by yeasts. This type of feedstock is the most utilized for
bioethanol production in North America and Europe. Corn/maize and wheat are mainly
employed with these purposes. The United States is predominantly a producer of bioethanol derived from corn, and production is concentrated in Midwestern states with
abundant corn supplies (Cardona and Sanchez, 2007; Shapouri et al., 2006). Maize is
increasingly used as a feedstock for the production of ethanol fuel. It is widely cultivated
throughout the world, and a greater weight of maize is produced each year than any other
grain. The United States produces 40% of the world’s harvest; other top producing countries include China, Brazil, Mexico, Indonesia, India, France, and Argentina. Worldwide
production was 817 million tons in 2009, more than rice (678 million tons) or wheat (682
million tons) (Koc¸ar and Civaş, 2013; Shapouri et al., 2006). The starch-based bioethanol
industry has been commercially viable for about many years; in that time, tremendous
improvements have been made in enzyme efficiency, reducing process costs and time
and increasing bioethanol yields.

Lignocellulosic biomass
Globally, many lignocellulosic agro-residues such as rice straw, wheat straw, sugarcane bagasse, sugarcane tops, cotton stalk, soft bamboo, and switchgrass have been
used to produce bioethanol as abundantly available feedstocks (Ravindranath et al.,
2011). Lignocellulosic materials can be classified in four groups based on the type
of resource: (1) forest residues, (2) municipal solid waste, (3) waste paper, and (4) crop
residue resources. These materials could produce up to 442 billion liters per year of
bioethanol (Balat, 2011; Gupta and Verma, 2015).
The production cost of bioethanol from food crops is very high as the raw materials
(maize or sugarcane) constitute about 40%–70% of the production cost. As a result,
promoting the use of second-generation bioethanol from lignocellulosic biomasses
such as nonfood crops, crop residues, and food/crop waste is an alternative way to
alleviate the cost and the land use conflict between food needs and fuel needs (Sun
and Cheng, 2002).
Chemical composition of lignocellulosic materials is a key factor affecting efficiency of biofuel production during conversion processes. As the lignocellulosic

complex is made up of a matrix of cellulose and lignin bound by hemicelluloses,
the main challenge in this case is to reduce the degree of crystallinity of the
cellulose and increase the fraction of amorphous cellulose by the process of pretreatment, the most suitable form for the hydrolysis step (Tye et al., 2016). Lignin
is one of the drawbacks of using lignocellulosic biomass materials in fermentation,
as it makes lignocellulose resistant to chemical and biological degradation. Sugars
in lignocellulosics are not easily available, due to this tight structure, and require a
previous pretreatment to make the hydrocarbon polymers available to


70

Bioenergy Systems for the Future

saccharification and fermentation. In general, the difficulties of using lignocellulosic materials are their poor porosity, high crystallinity, and lignin contents. The
cost of bioethanol production from lignocellulosic materials is relatively high when
based on current technologies, and the main challenges are the low yield and high
cost of the hydrolysis process. In spite of this matter, by considering these materials as a cheap and abundant feedstocks, they can be a promising renewable
resource for bioethanol production at reasonable costs (Balat, 2011; Sun and
Cheng, 2002; Tye et al., 2016).

Macro/Microalgea
Interest has now been diverted to the third-generation biomass like algae, since the
first-generation feedstock (edible crops, sugars, and starches) are under serious controversy considering the competition between food and fuel and the second-generation
biomass (lignocellulosic biomass) is limited by the high cost for lignin removal as its
incredible resistance to degradation and makes biomass saccharification costly.
Microalgae and macroalgae are the two groups of algae investigated as potential fuel
sources. Algal biofuels, also called advanced biofuels, are seen as one of the most
promising solutions of global energy crisis and climate change for the years to come
(Alam et al., 2012; Noraini et al., 2014). The production of third-generation biofuels
has many advantages over the plants used for producing first- and second-generation

biofuels due to their faster growth; capability of growing under several conditions,
including in saline, brackish, and wastewater; reduced need for water and other
resource inputs; the possibility of not occupying arable lands for their cultivation,
greenhouse gas fixation ability (net zero emission balance), and high production
capacity of lipids (Costa et al., 2012).
Cell wall composition of algae differs from those of terrestrial plants. The key
difference is low content or the absence of lignin in macro- and microalgal feedstocks,
which make them less resistant to conversion into simple sugars. The biochemical
composition of microalgae grown under normal conditions primarily encompasses
proteins (30%–50%), carbohydrates (20%–40%), and lipids (8%–15%). The types
and quantities of basic monosaccharide components such as mannose, galactose,
and arabinose are potentially very suitable for conversion into bioethanol
(de Farias Silva and Bertucco, 2016).
Ethanol production from microalgae has been reported using the main classes of
microalgae, that is, green (Ulva lactuca and U. pertusa), red (Kappaphycus alvarezii,
Gelidium amansii, G. elegans, and Gracilaria salicornia), and brown (Laminaria
japonica, L. hyperborea, Saccharina latissima, Sargassum fulvellum, Undaria
pinnatifida, and Alaria crassifolia). According to the results reported in many
researches, the standard strains of Saccharomyces cerevisiae with only a few that have
assessed ethanologenic or solventogenic bacterial strains such as Clostridium
pasteurianum and recombinant E. coli were utilized for optimal utilization and conversion of such diverse carbohydrates to bioethanol (Costa et al., 2012; de Farias Silva
and Bertucco, 2016; Doan et al., 2012).


Production of bioalcohol and biomethane

71

3.2.1.2 Production methods
Biofuels are made from biobased materials through different processes like fermentation and thermochemical processes. The type of feedstock chosen for ethanol

production has a significant impact on the design of fermentation process. Ethanol
is produced from a variety of sugar- or starch-containing crops, with modifications
in the design of the feedstock preparation processes. The modifications are required
to accommodate the physical properties of the feedstock and the nature of the carbohydrate (i.e., sugar vs starch) (Nigam and Singh, 2011). Bioethanol production
processes for different resources are discussed in detail in this section.

Bioethanol from sugar-/starch-containing feedstock
Sucrose-containing feedstocks, as the most utilized substrate for bioethanol production,
are mostly preferred because of the easier conversion of sucrose into ethanol compared
with starchy and lignocellulosic biomass. No conversion of the feedstock is required for
this substrate, as the disaccharides can be directly broken down by the microorganisms
during bioethanol production. In spite of this simple procedure, the processing of these
substrates is generally too expensive for bioethanol production, because of the availability and transport costs of its feedstocks. First-generation bioethanol is mostly produced
from corn and sugarcane using a well-established technology as shown in Fig. 3.3.
Fermentable sugars are extracted by grinding or crushing, followed by fermentation
to ethanol. Further, ethanol is separated from the product stream by distillation, followed
by dehydration (Suresh et al., 1999; Larson, 2008; Balat, 2011).
Starch is a high-yielding feedstock for bioethanol production. Grains such as corn
and wheat contain starch, which is a polysaccharide of glucose units linked by α (1–4)
and α (1–6) glycosidic bonds. Starch is not directly fermented by microorganisms like
yeast. Production of ethanol from starch is performed by either dry grind or wet milling process, which are different in the extraction method of glucose and coproducts
formed. As indicated in Fig. 3.3, the extracted starch is hydrolyzed into glucose by
using the specific enzymes like α-amylase and glucoamylase, and glucose is then fermented to ethanol (Thatoi et al., 2016; Chen and Fu, 2015).

Sugar Feedstock

Milling

Distillation and dehydration

Liquefaction and
fractionation

Fermentation

Milling

Enzymatic
hydrolysis

Starch Feedstock

Bioethanol

Fig. 3.3 Process diagram for bioethanol production from sugar and starch feedstocks.


72

Bioenergy Systems for the Future
Simultaneous saccharification
and fermentation (SSF)

Bioethanol

Fermentation
(Conversion of
sugars to
bioethanol)

Distillation
and
evaporation

Waste
management

Filter wash

Lignin

Enzymatic
hydrolysis
(Convers
enzymatic
hydrolysis
(Conversion of
cellulose to sugar)

Biomass

Pretreatment
(Solubilisation
of hemicellulose)

Recirculation of process streams

Residue-to-power production

Fig. 3.4 Schematic flow sheet for the bioconversion of bioethanol from biomass.

Bioethanol from lignocellulosic materials
Four process steps for ethanol production from lignocellulosic materials are possible,
namely, as pretreatment, hydrolysis, fermentation, and product separation/distillation.
Schematic flow sheet for the bioconversion of biomass to bioethanol is shown in
Fig. 3.4. The main challenge for this material is to reduce the degree of crystallinity
of the cellulose and increase the fraction of amorphous cellulose by the process of pretreatment as the most suitable form for the hydrolysis step. After pretreatment, the
cellulose undergoes enzymatic hydrolysis in order to obtain glucose that is converted
to ethanol by microorganisms (Sun and Cheng, 2002; Tye et al., 2016; Chen and
Fu, 2015).

Pretreatment process
The most important processing challenge in the production of biofuel is pretreatment
of the biomass for further chemical or biological treatment. Pretreatment methods
change the native properties of the substrate by solubilization and separation of
one or more of components of biomass and making the remaining solid biomass more
accessible to further treatment. For instance, starchy substrates can be fermented after
breaking starch molecule into simpler glucose molecules, and this can be done by a
pretreatment step. For lignocellulosic biomass, the pretreatment is done to break the
matrix in order to reduce the degree of crystallinity of the cellulose, increase the fraction of amorphous cellulose, and convert it to the most suitable form for enzymatic
attack (Balat, 2011; Chen and Fu, 2015).
Pretreatment methods can be categorized into physical, chemical, biological, and
physicochemical methods, which are discussed in detail below. The goals of an effective pretreatment process are (i) to form sugars directly or subsequently by hydrolysis


Production of bioalcohol and biomethane

73

(ii) to avoid loss and/or degradation of sugars formed, (iii) to limit formation of inhibitory products, (iv) to reduce energy demands, and (v) to minimize costs (Balat, 2011;

Tye et al., 2016).
Physical pretreatment The primary steps for ethanol production from agroresidues are combination of methods like mechanical milling, grinding, and chipping
that are used to diminish the particle size, increase the surface area, and improve the
mass transfer characteristics. Also these methods reduce the cellulose crystallinity and
improve the efficiency of downstream processing. Besides the mechanical combination, the physical pretreatment technology also includes uncatalyzed steam explosion,
liquid hot water pretreatment, and high-energy radiation, of which steam explosion
loosen the recalcitrant structure of plant cell wall by increasing surface area and
removes pentose sugar, but the major drawback of steam treatment during enzymatic
hydrolysis is generation of some cellulose inhibitory compounds that hamper the
enzymatic hydrolysis of the cellulose substrates (Tye et al., 2016; Chen and Fu, 2015).
Chemical and physicochemical pretreatment Chemical pretreatment methods
include ozonolysis, acid hydrolysis, and alkali hydrolysis that involve the usage of
dilute acid, alkali, ammonia, organic solvent, sulfuric and formic acids, SO2, CO2,
or other chemicals. The physicochemical pretreatment methods include ammonia
fiber explosion and steam pretreatment. These methods are easy in operation and have
good conversion yields in short span of time. But the major impedance of chemical
pretreatment is that the utilization of such chemicals affects the total economy of bioconversion of cellulosic biomass. Shenoy et al. (Balat, 2011) showed that among
chemical treatments, the dilute sulfuric acid-based pretreatment is most popular by
means of enzymatic hydrolysis using agricultural biomasses.
Biological pretreatment The biological pretreatment methods mainly involve
utilizing different fungal species like brown rot, white rot, and soft rot fungi for
degradation of the lignocellulosic complex to liberate cellulose. Biological pretreatment renders the degradation of lignin and hemicellulose, and white rot fungi
seem to be the most effective microorganism. Brown rot attacks cellulose, while white
and soft rots attack both cellulose and lignin. Celluloseless mutant was developed for
selective degradation of lignin and to prevent the loss of cellulose, but in most cases of
biological pretreatment, the rate of hydrolysis is very low. The advantages of biological pretreatment include low energy requirement and mild environmental conditions
(Tye et al., 2016; Mosier et al., 2005). The major effects of pretreatment on lignocellulosic feedstocks are illustrated in Fig. 3.5.
Hydrolysis process As the pretreatment is finished, the material is prepared for
hydrolysis, which means the process of cleavage of a molecule by adding a water molecule as shown below:
ðC5 H10 O5 Þn + nH2 O ! nC6 H12 O6

(3.1)

Generally, conversion of starchy materials to glucose monomers by saccharification
process (above reaction) is catalyzed by acids or enzymes. The enzymatic hydrolysis


74

Bioenergy Systems for the Future

Major changes
– Lignin redistribution.
– Increased porosity of
the lignified cell-wall.
– Size reduction.

Major targeted
Components
– Lignin
– Hemicellulose
– Cellulose

Pretreatment
Mechanical and/or
Thermo-chemical
and/or Biological
methods

Lignocellulosic

feedstock

– Increased surface
area of cellulose for
% of toxic by-products
released depends on
pretreatment type

greater enzymes

Energy consumption

accessibility.

– Hydrolysis
– Fermentation
– Distillation

Bioethenol

Fig. 3.5 Pretreatment upstream process, major effects.

is a long-timed process with many advantages like very mild conditions (pH 4.8 and
temperature 318–323 K) and high yields. Its maintenance costs are low compared with
alkaline and acid hydrolysis due to no corrosion problems, but the initial cost of
enzymes is too high (Prado et al., 2016; Mosier et al., 2005).
There are two basic types of acid hydrolysis processes, dilute acid and concentrated
acid, each with variations. Acids used for chemical hydrolysis include H2SO4, HCl,
H2O2, phosphoric acid, and nitric acid. Dilute acid hydrolysis processes are conducted
under high temperature (473–513 K) and pressure and have reaction times in the range

of seconds or minutes. In the process, the acid breaks down the matrix structure, and
the released polymer sugars, cellulose, and hemicellulose are hydrolyzed into free
monomer molecules readily available for fermentation and conversion to bioethanol.
It is followed by hexose and pentose degradation and formation of high concentrations
of toxic compounds including HMF and phenolics detrimental to an effective saccharification (Sun and Cheng, 2002; Chen and Fu, 2015).
Concentrated acid hydrolysis, the more prevalent method, has been considered to be
the most practical approach. Unlike dilute acid hydrolysis, concentrated acid hydrolysis
is not followed by high concentrations of inhibitors and produces a high yield of free
sugars (90%); however, it requires large quantities of acid and costly acid recycling,
which makes it commercially less attractive (Tye et al., 2016; Mosier et al., 2005).
Fermentation process Fermentation is one of the oldest processing technologies in
the world, and today, the technological advancements have made this process more
efficient and valuable. Pretreatment and hydrolysis processes are designed to optimize
the fermentation process. This natural biological pathway requires the presence of
microorganisms to ferment sugar into alcohol, lactic acid, or other end products


Production of bioalcohol and biomethane

75

depending on the conditions and raw material used. Moreover, an optimal fermentative microorganism should be tolerant to a high ethanol concentration and to chemical
inhibitors formed during pretreatment and hydrolysis process. Microorganisms, termed ethanologens, presently convert an inadequate portion of the sugars from biomass
to bioethanol. Saccharomyces cerevisiae has also been utilized for corn-based and
sugar-based biofuel industries as the primary fermentative strain for bioethanol production (Balat, 2011; Gupta and Verma, 2015; Sun and Cheng, 2002).
The fermentation of different substrates for bioethanol production can be achieved
by three processes, namely, batch, fed-batch, and continuous fermentation. Batch culture can be considered as a closed-loop, discontinuous system that contains an initial,
limited amount of nutrient, which is inoculated with microorganisms to allow the
fermentation.
Fed-batch fermentation is a production technique in between batch and continuous

fermentation in which one or more feed input, with the right component constitution is
required. Fed-batch process is a more efficient cultivation strategy than the batch process in which microorganisms work at low substrate concentration with an increasing
ethanol concentration. This system often provides better yield and productivities than
batch cultures by preventing contaminations. In continuous fermentation, the substrate is added to the fermenter continuously at a fixed rate. This maintains the microorganisms in the logarithmic growth phase, and the fermentation products are taken
out continuously. The advantage of continuous culture technique is higher percentage
of end product in comparison with batch and fed-batch systems. Also, the continuous
process eliminates much of the unproductive time associated with cleaning,
recharging, adjustment of media, and sterilization (Suresh et al., 1999; Balat, 2011;
Gupta and Verma, 2015; Chen and Fu, 2015).
Separation process Bioethanol obtained from a fermentation conversion requires
further separation and purification of ethanol from water through a distillation process. Fractional distillation is a process implemented to separate ethanol from water
based on their different volatilities. Distillation process consumes a great deal of
energy for providing heat to change liquid to vapor and condense the vapor back
to liquid at the condenser and ethanol distillate recaptured at a concentration of about
95%. With rising energy awareness and growing environmental concerns, there is a
need to reduce the energy use in industry (Suresh et al., 1999; Larson, 2008).

3.2.2

Biomethane production

As mentioned in the previous section, the demand for renewable fuels is increasing
with growing concern about climate change, air quality, energy import dependence,
and depletion of fossil fuels. The biogas is a carbon-neutral source of renewable
energy, and it is a competitive alternative in energy production in both its energy efficiency and its environmental impact. As biogas replaces fossil fuel, there is an accompanying reduction of greenhouse gas, particles, and nitrogen oxide emissions (Koc¸ar
and Civaş, 2013). Biogas production is an efficient way to dispose of organic waste,
while extracting energy, fertilizer, and valuable by-products. Biogas, a methane-rich


76

Bioenergy Systems for the Future

gas produced by anaerobic treatment of any biomass, is a multibenefit, flexible technology that can be applied on household scale, village scale, or industrial scale. Biogas
production from energy crops is a long-established technology that represents a more
thermodynamically efficient option than converting plant matter into liquid fuels.
Biogas typically contains (v/v) 50%–75% methane, 25%–50% carbon dioxide,
1%–5% water vapor, 0%–5% nitrogen, smaller amounts of hydrogen sulfide
(0–5000 ppm), ammonia (0–500 ppm), and trace concentrations of hydrogen and
carbon monoxide. Biogas is a versatile renewable fuel that can be used for power
and heat/cool production, or it can be upgraded to biomethane to be used as vehicle
fuel. Various technologies, such as water scrubbing, pressure swing adsorption, and
membrane technologies, are currently applied to upgrade biogas to biomethane for use
as vehicle fuel or to be injected into a natural gas grid (Koh and Ghazoul, 2008;
Murray, 2005; Licht, 2008; Nexant, Inc, 2006; Alam et al., 2012; Thomson, 2008;
Koc¸ar and Civaş, 2013). The anaerobic conversion of crops into methane is now viable
in some countries, and the most spectacular example of this trend is the construction
and operation of over 5000 anaerobic digesters in Germany during the past 20 years
(Koc¸ar and Civaş, 2013).

3.2.2.1

Production feedstocks

Feedstock composition is one of the major factors that affect the production of biogas.
Methane yields are related to the substrates used as feeding material, ash content, and
the level of storage sugars, and also it is linked with the type of interaction between
different wastes that interfere with digestibility of wastes in anaerobic digestion (AD)
processes. A variety of feedstocks can be used for biomethanation; however, the
conventional materials for biogas production are agricultural crops, animal wastes,

sewage sludge, woody biomass, grass, terrestrial weed, marine biomass, and freshwater biomass. The sugar and starch crops are the main energy crops currently used on a
commercial scale for the production of biomethane (i.e., 5300–12,390, 6604, and
5400 m3 CH4/ha “methane yield per hectare” for corn, triticale, and sugar beets,
respectively). For instance, with wheat or maize, up to three times more net energy
yield can be obtained per hectare by making methane instead of biodiesel or bioethanol. Although these crops generate high yields of methane, they also have other
uses as food and/or feed, which may often compete with biofuel production
(Sreekrishnan et al., 2004; G€
ubitz et al., 2010).
Germany, Austria, and Denmark produce the largest share of their biogas in
agricultural plants using energy crops, agricultural by-products, and manure
(Alam et al., 2012). Cellulosic or lignocellulosic crops are represented by different
grasses containing small percentage of lignin (hay, clover, and reed canary grass),
while other energy crops such as Panicum virgatum, Miscanthus, or switchgrass are
containing higher levels of lignin (12%–20%). Switchgrass was chosen as the model
lignocellulosic crop by the US Department of Energy in the 1990s and is believed to
return 540% more renewable energy than fossil fuel consumption (G€ubitz
et al., 2010).


Production of bioalcohol and biomethane

77

3.2.2.2 Production methods
Methane fermentation technology is considered as the most efficient form of handling
and energy generation from biomass in terms of energy input/output ratio. In biomethanation insoluble complex compounds like cellulose, proteins and fats are converted
to methane by anaerobic microbes. But the degradation of lignin is slow and incomplete, and hence, the lignocellulosic materials should be better pretreated to expose
other components (Rasi et al., 2011).
Methane fermentation is a complex biological process that needs a variety of different microbes at different stages. It involves four phases of biomass degradation and
conversion-hydrolysis, acidogenesis, cetogenesis, and methanation. Enzymes like

hydrolases and exoenzymes of facultative or obligatory anaerobic bacteria hydrolyze
cellulose, proteins, and fats to simpler monomers. During acidogenesis, these monomers
are converted to short-chain organic acids, alcohols, hydrogen, and carbon dioxide by
different sets of facultative and obligatory anaerobic bacteria. Acetogenic microorganisms reduce hydrogen and carbon dioxide to acetic acid. During this acetogenic phase,
organic acids and alcohols may get converted to acetate. These products can serve as
substrates for methane-forming bacteria in strict anaerobic conditions. The produced
methane, being a gas, is separated by itself from the liquid making the product removal
easy (Sreekrishnan et al., 2004; G€
ubitz et al., 2010).
Methane production in anaerobic fermentation system is not constant but may
decrease over time, and the rate depends on the substrate hydrolysis. The
premixing of feedstock with lignin-degrading enzymes (lignin peroxidase,
manganese peroxidase, and soybean peroxidase) or cellulose prior to AD improves
the lignocellulosic degradation and enhances methane production. The methane
production stages are described in detail in the next section (B€orjesson and
Mattiasson, 2008).

Pretreatment process
Cost competitiveness, cost-effectiveness, and ease of downstream processing are the
factors to be considered for selecting the most desirable pretreatment method.
Pretreatments are directed more specifically to enhanced methane production from
lignocellulosic biomass since the rate-limiting step in AD of lignocellulosic crops
is the hydrolysis of complex polymeric substances and, in particular, the cross-linking
of lignin that is nonbiodegradable with the cellulose and hemicellulose. Physical, thermochemical, and biochemical (enzymatic) methods have been employed to increase
biomass digestibility. In physical methods, a large portion of hemicelluloses and
lignin can be removed by methods like milling, homogenization, and flow-through.
In thermochemical methods, various inorganic salts or alkalies (lime, sodium hydroxide, ammonia, sulfuric acid, etc.) can be used, but the use of strong chemicals may
have various disadvantages like reactor corrosion, extensive washing requirement
for treated solids, and thus production of large volumes of waste streams. Also high
cost and production of undesirable products like organic acids, furan derivatives, and

phenolic compounds make it problematic. Therefore, the optimal pretreatment
method should require limited capital and operating and maintenance costs and be


78

Bioenergy Systems for the Future

Cellulose

Pretreatment

Hemicellulose

Fig. 3.6 Schematic representation of pretreatment process on lignocellulosic material.

sufficiently fast leading to reduced volumes of pretreatment units (Mosier et al., 2005;
Sreekrishnan et al., 2004; B€
orjesson and Mattiasson, 2008). Fig. 3.6 shows the
pretreatment process on lignocellulosic material.

Main production stages
Extraction of biofuels can be carried out by two methods, namely, anaerobic and thermal conversion methods, which are discussed in the following, comprehensively.
Thermal conversion method involves pyrolysis, partial combustion, and reduction
or gasification reactions. The whole process is termed as gasification, which converts
the available biomass into biofuels like product gas, syngas, and biogas. The main
constituents present in the product gas are carbon monoxide, hydrogen, carbon dioxide, methane, water vapor, nitrogen oxides, tar (heavy hydrocarbons), sulfur compounds, and particulate matter (Kumar and Shukla, 2016).
Thermal conversion method
l

l

Gasification step
Gasification technology is basically a process for converting solid or liquid feedstock into
a gaseous or liquefied fuel that can be burned to release energy. Generally, gasification process is completed in four stages: drying, pyrolysis, partial combustion, and reduction. Gasifiers are mainly classified into three types according to the type of feedstock used: fixed bed
type, fluidized bed type, and entrained flow type. The feedstock is feed from the top of the
gasifier through hopper, and the gasifying agent like air, oxygen, or steam is introduced from
the sides of the reactor at combustion zone. Gasification process involves complex combination of pyrolysis and oxidation where the biomass starts converting gaseous form and
releases CO, H2O, CH4, CO2, H2, NOx, and tar (Basu, 2010).
Methanation step
Methanation is the last stage process in the purification of the product gas. The product
gas has to be tar-free and has less concentration of CO2. The remaining CO and CO2 left in
the gas will undergo catalytic reaction with hydrogen to form methane:


Production of bioalcohol and biomethane

79

CO + 3H2 O ! CH4 + H2 O, ΔH ¼ 51:8kcal=mol

(3.2)

CO2 + 4H2 O ! CH4 + 2H2 O, ΔH ¼ 41:9kcal=mol

(3.3)

According to the literature, nickel-based catalyst is used at operating temperature of
about 300°C, which converts all oxides of carbon to methane and water (Basu, 2010).

AD method One such well-known and widely used method for bioconversion of
wastes into fuel in the absence of oxygen is AD, which is regarded as the simplest
technique due to its very limited environmental impact and high energy recovery
potential (G€
ubitz et al., 2010; B€
orjesson and Mattiasson, 2008; Kumar and Shukla,
2016; Basu, 2010; Edelmann et al., 2005). Other advantages of AD include dilution
of the toxic substances coming from any of the substrates involved, an improved nutrient balance, synergistic effects on microorganisms, a high digestion rate, and possible
detoxification based on the cometabolism process. The dilution of toxic substance can
reduce GHG emission, thus improving air quality (Edelmann et al., 2005).
The metabolic reactions that occur during AD of substrates using microorganisms
for methane formation involve four important stages: hydrolysis, acidogenesis,
acetogenesis, and methanogenesis. In addition, the structure of microbial community
presented in the system, environmental factors such as temperature and PH play a significant role in determining the performance and fate of the microbial community in
anaerobic digesters (Basu, 2010).
Researchers have reported that methanogenesis is the rate-limiting step for
easily biodegradable substances, whereas hydrolysis is the rate-limiting step for
complex organic wastes due to formation of toxic by-products like complex
heterocyclic compounds and undesirable volatile fatty acids (Lam and Lee,
2011). Fig. 3.7 depicts the main pathways of an AD, and the descriptions for each
stage are given in the following section.
l

l

l

Hydrolysis step
Hydrolysis is the first stage of the organic waste decomposition process involving the
breakdown of large organic polymer chains into smaller molecules such as simple sugars,

amino acids, and fatty acids. Other products such as hydrogen and acetate maybe used by
methanogens later in the process. Saccharolytic and proteolytic microorganisms break down
sugars and proteins, respectively. This is carried out by several hydrolytic enzymes such as
celluloses, cellobiase, xylanase, amylase, lipase, and protease secreted by hydrolytic
microbes (Basu, 2010; Demirel and Scherer, 2008).
Acidogenesis step
The second step is acidogenesis (also referred to as fermentation), in which the hydrolyzed products are degraded further to simpler organic products such as ammonia, acetate,
hydrogen, carbon dioxide, and hydrogen sulfide. These final products of fermentation will
eventually become the precursors of biomethane formation (Demirel and Scherer, 2008).
Acetogenesis step
During this step, acetogens produce intermediary products such as propionate, butyrate,
lactate, ethanol, and energy sources. Close cooperation is required between oxidative organisms and methane-producing organisms that are active during methanogenesis. This process
consumes hydrogen gas, and intermediary products will be converted to simpler organic
acid, CO2, and H2 by acetogenic bacteria (Lam and Lee, 2011; Demirel and Scherer, 2008).


80

Bioenergy Systems for the Future

Organic polymers

Carbohydrates

Proteins

Lipids

Sugars, alcohols

Amino acids

Fatty acids

Hydrolysis

Acidogensis

Intermediary products
acetate, propionate,
ethanol, lactate
Acetogensis

CH4, CO2

H2, CO2

Methanogensis
CH4, CO2

Fig. 3.7 Diagram of anaerobic digestion process.

l

Methanogenesis step
Methanogenesis is the final stage of AD methane production stage, where methanogens
produce methane from hydrogen, carbon dioxide, and acetate, and intermediates products from
hydrolysis and acidogenesis. Methane is produced by two groups of bacteria (methanogens),
namely, acetotrophic methanogens and hydrogenotrophic methanogens. Acetotrophic methanogens convert acetate to biomethane and CO2, whereas hydrogenotrophic methanogens
use H2 as electron donor and CO2 as electron acceptor to produce biomethane (Demirel

and Scherer, 2008).

3.3

Membrane processes for biofuels production

Nowadays, one of the main challenges to the large-scale industrial production of
biofuels is the lack of cost-effective separation methods for the isolation and purification of biobased chemicals and fuels (Wickramasinghe and Grzenia, 2008). Indeed, the
separation operations account for 65%–85% of the processing costs of most mature
chemical processes (Ragauskas et al., 2006). Hence, membrane separation technologies have gained more attention owing to their reduced energy requirements, lower
labor costs, lower floor space requirements, and wide flexibility of operation. This


Production of bioalcohol and biomethane

Table 3.4

81

Membrane technology application in biofuel production

Applications
Upstream from the fermentation process
Clarification or fractionation of feedstock material going to the
fermenter
Acid and alkali recovery and reuse; separation of lignin from
hydrolyzed biomass
Protein recovery/removal from hydrolyzed prepared biomass
Concentration of sugars to enable product yield enhancement in the
fermentation

Continuous enzyme reactors retain enzyme and substrate,
permitting removal of reaction-inhibiting components

Membrane
process types

UF and/or MF
UF and/or NF
UF
NF
UF and/or NF

Downstream from the fermentation process
Biomass/microbial cell retention that enables continuous recovery
of the target product component or removal of fermentation
inhibitor molecules
Concentration of organic acids with water recovery for reuse
Evaporator condensate treatment for water recovery and reuse
enabling environmental compliance
Amino acid concentration and desalting

UF and/or NF

RO
RO
NF

promising technology has been applied in many processes of bioethanol production
instead of the traditional process (Chapman et al., 2008; Vane, 2005; Qureshi and
Manderson, 1995; He et al., 2012; Lipnizki, 2010; Rothman et al., 1983).

In general, membrane filtration technologies, microfiltration (MF), ultrafiltration
(UF), nanofiltration (NF), and reverse osmosis (RO), are proving to be effective procedure to achieve optimum yields and reduce energy costs for biofuels production.
However, it should be noted that membrane filtration technology shows promise to
improve “second-generation” and “third-generation” bioethanol processes. As a general point of view, Table 3.4 presents various applications of membrane technologies
in biofuel production.
With regard to literatures (Sadati et al., 2014; Wei and Cheng, 2014; Shah and Sen,
2011), in biofuel production, most applications of membrane technology are devoted
to bioethanol production. Therefore, an overview of bioethanol production with
potential membrane applications is depicted in Fig. 3.8. Indeed, the first potential
membrane application is the harvesting of microalgae for third-generation bioethanol
synthesis. By using MF/UF, it is possible to recover microalgae. For second- and thirdgeneration bioethanol, pretreatment is a necessary step to make the carbohydrates in
the biomass available for conversion. The second potential membrane application is
the purification and concentration of prehydrolyzates after pretreatment and before
fermentation. Membrane distillation (MD), NF, and RO can concentrate the sugar
solution and remove inhibitors to fermentation process.


82

Bioenergy Systems for the Future

Starched-based
materials

Microalgae

Cellulosie material

MF/UF
Membrane process

(Harvesting)

Pretreatment

Scarification

MF/UF/MD
Membrane process
(Concentration of sugar solution)

Fermentation-pervaporation
(Hybrid process)

Ethanol fuel

Fig. 3.8 Application of membrane process for three generation types of bioethanol production.

With regard to enzyme and other value-added production recovering, an NF
process has been combined with UF process. After fermentation, low concentration
bioethanol is sent for pervaporation and preconcentration.
On the other hand, fermentation and pervaporation have been integrated to perform
continuous fermentation. During the hybrid process, UF and NF process can be
applied to remove fermentation inhibitors and yeast. However, the various fermentation systems used are batch, continuous, fed-batch and semicontinuous, immobilized
systems, and membrane bioreactor (MBR) systems. Indeed, the MBR systems, in
addition to energy and water savings, can continuously remove ethanol from the fermentation process, thereby accelerating fermentation and increasing product capacity
without the installation of additional tank. It should be noted that high alcohol-water


Production of bioalcohol and biomethane

83

selectivities are critical to the energy efficiency of pervaporation. There are some
reports in the literature (Chen et al., 2012; Ding et al., 2011; Nomura et al., 2002;
Ikegami et al., 1997; Schmidt et al., 1997) discussing about continuous ethanol production by pervaporation using different cultures, membranes, and configurations.

3.4

Conclusion and future trends

The chapter has discussed about concept of biofuels, different types of biofuels, and
various production methods of biofuels. Regarding literatures, various types of
biofuels could be produced; among them, bioalcohol and biomethane were considered
in this work. According to studies, biological feedstocks that contain appreciable
amounts of sugar or materials that can be converted into sugar, such as starch or cellulose, can be fermented to produce bioethanol, while the conventional feedstocks for
biomethane production are agricultural crops, animal wastes, sewage sludge, marine
biomass, and freshwater biomass. As a general consequence, to improve bioprocesses
for biofuel production, lower overall energy costs, and increase valuable product
recovery, water removing is necessary. Membrane technology in bioprocesses has
the potential to greatly reduce operating costs compared with the traditional processes
using an evaporator to recover or remove water, which requires very high energy use.
The RO process can offer about 75% lower capital and operational costs compared
with a five multieffect evaporator with thermal vapor recompression. The NF process
allows the recovery of proteins, peptides, amino acids, and sugars while allowing salts
to pass the membrane. On the wastewater side, AD is often used to remove BOD and
COD from the waste stream. The UF process may be used to concentrate the biosludge, followed by a water recovery and reuse RO system. This is especially important in areas where water is limited.

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