




































African Journal of Environmental Economics and Management Vol. 1 (5), pp. 126-143, December, 
2013. Available online at www.internationalscholarsjournals.org © International Scholars Journals 

 

 
 

Review 
 

Economics and environmental impact of bioethanol 

production technologies: an appraisal 
 

Anuj Kumar Chandel
1,2

, Chan ES
3
, Ravinder Rudravaram

1
, M. Lakshmi Narasu

2
, L. 

Venkateswar Rao
1
 and Pogaku Ravindra

3
* 

 
1
Department of Microbiology, Osmania University, Hyderabad-500 007, India.  

2
Department of Biotechnology, Jawaharlal Nehru Technological University, Hyderabad-500 007, India. 

3
School 

of Engineering and Information Technology, Universiti Malaysia Sabah,Kotakinabalu, 88999, Malaysia. 
Accepted 25 May, 2013 

 
Contemporary industrial developments and rapid pace of urbanization have called for an environ-
mentally sustainable energy sources. Ethanol made from biomass provides unique environmental, 
economic strategic benefits and can be considered as a safe and cleanest liquid fuel alternative to 
fossil fuels. There is a copious amount of lignocellulosic biomass worldwide that can be exploited for 
fuel ethanol production. Significant advances have been made at bench scale towards the fuel ethanol 
generation from lignocellulosics. However there are still technical and economical hurdles, which make 
the bioethanol program unsuccessful at commercial scale. This review provides a broad overview on 
current status of bioethanol production technologies in terms of their economic and environmental 
viability. These technologies include pretreatment of biomass, the use of cellulolytic enzymes for 
depolymerisation of carbohydrate polymers into fermentable constituents and the use of robust 
fermentative microorganisms for ethanol production. Among all the available technologies, dilute acid 
hydrolysis followed by enzymatic hydrolysis by less expensive and more efficient cellulases has been 
found more promising towards the potential economics and environmental impact. 

 
Key words: Bioethanol, lignocellulosic feedstock, ethanol fermentation, economics, environmental impact. 

 
 
Table of contents 
 
1. Introduction  
2. Lignocellulosic biomass   
2.1. Composition  
3. Ethanol Production Technologies  
3.1. Pretreatment  
3.1.1. Hydrolysis  
3.2.1. Acid hydrolysis  
3.2.1.1 Dilute acid hydrolysis   
3.2.1.1.1 First-stage dilute acid hydrolysis   
3.2.1.1.2 Two-stage dilute acid hydrolysis  
3.2.1.2. Concentrated acid hydrolysis  
3.2.2 Enzymatic hydrolysis  
3.2.2.1 Separate hydrolysis and fermentation (SHF)  
3.2.2.2 Simultaneous saccharification and fermentation (SSF)  
3.3. Direct microbial conversion (DMC)  
3.4 Fermentation  
3.4.1. Batch fermentation  
3.4.2 Fed batch fermentation  
3.4.3. Continuous fermentation  
3.4.3.1. Immobilized cells  
3.4.3.2 Recycling of process stream  
4. Economics of ethanol production technologies   
5.1. Ethanol and Environment   
5.2. Environmental impact of bioethanol production technologies and their Life-cycle assessment (LCA)  



Chandel  et al.               126 

 
 
 
6. Conclusion   
7. References  
 
 
INTRODUCTION 
 
In 1925, Henry Ford had quoted ethyl alcohol, ethanol, as 
"the fuel of the future.” He furthermore stated, "The fuel of 
the future is going to come from apples, weeds, sawdust  
– almost anything. There is fuel in every bit of vegetable 
matter that can be fermented." Today Henry Ford’s 
futuristic vision significance can be easily understood.  

In the current time, the importance of alternative energy 
source has become even more necessary not only due to 
the continuous depletion of limited fossil fuel stock but 
also for the safe and better environment, with an inevi-
table depletion of the world’s energy supply, there has 
been an increasing worldwide interest in alternative 
sources of energy (Wyman, 1999; Lynd, 2004; Herrera, 
2004; Herrera, 2006; Lin and Tanaka, 2006, Schubert, 
2006; Chandel et al., 2006a; Vertes et al., 2006, Dien et 
al., 2006). Keeping in view all the above said advantages, 
biomass based fuel development technologies should 
rapidly gain momentum and the barriers imposed earlier 
should be removed for successfully attempting the 
production of bioethanol at the commercial level.  

It is welcome to understand that the use of bioethanol 
as a source of energy would be more than just comple-
mentting for solar, wind and other intermittent renewable 
energy sources in the long run (Lin and Tanaka, 2006). 
During the last two decades, advances in technology for 
ethanol production from biomass have been developed to 
the point that large-scale production will be a reality in 
next few years (Yu and Zhang, 2004; Moiser et al., 2006). 
Ethanol production from biomass can be summarized 
briefly into following steps: depolymerization of holocellu-
lose polymer into monomeric fermentable substrate, fer-
mentation of depolymerized substrates, and the distilla-
tion of the fermentation broth to obtain dehydrated 
ethanol.  

The ethanol yields and processes economics along 
with the technical maturity and environmental benefits of 
using ethanol blend fuel are the key parameters that 
determine the feasibility of bioethanol production (Nguyen 
and Saddler, 1991).  

The burning fossil fuel at the current rate is likely to 
create an environmental crisis globally. Use of fossil fuel 
generates carbon dioxide, methane and a significant 
quantity of nitrous oxide. Most of these harmful gases are 
formed due to incomplete combustion of fossil fuel; since 
ethanol contains 35% oxygen that may result in a more 
complete combustion of fuel and thus reduces tailpipe 
emissions. 
 
 
*Corresponding author. E-mail: dr_ravindra@hotmail.com. 

Phone: 006 088 320000 ext 3048. Mobile: 006 013 87666634. 

Fax: 006 088 320348. 

 
 
 

 
Moreover, biomass energy can play an important role in 

reducing green house gas emissions. Ethanol production 
process only uses energy from renewable energy 
sources. Hence no net carbon dioxide is added to the 
atmosphere, making ethanol an environmentally bene-
ficial energy source (Bull et al., 1992; Kheshgi et al., 
2000). Furthermore, fuel ethanol from lignocelluloses may 
also open new employment opportunities in rural areas, 
and thus make a positive socio-economic impact 
(Wyman, 2003; Bevan and Franssen, 2006). Developing 
ethanol as fuel, beyond its current role as fuel oxy-
genates will require developing lignocellulosic biomass as 
a feedstock because of its abundantly available and low 
cost.  

The world ethanol production in 2004 was estimated to 
be 40 giga litres (GL) (Berg, 2004; Kim and Dale, 2004). 
Brazil and the US are the world leaders, which together 
accounted for about 60% of the world ethanol production 
exploiting sugarcane and corn respectively. In India, 
lignocellulosic biomass (crop residues, forestry and fruit 
and vegetable waste and weeds) is available in plenty. 
Renewable fuels particularly ethanol should get more and 
more attention all over the world.  

The important issue that we wish to address affirm-
atively here is that the bioethanol production, without 
doubt, needs an economical approach to address the 
global fuel needs. Research efforts are needed to design 
and improve the process, which would produce sustaina-
ble and economically feasible transportation fuel. Impro-
vement in process economics using new designed cellu-
lases enzyme cocktail are important factors in estab-
lishing a cost effective technology, besides the low cost of 
feedstock (Mojovic et al., 2006; Gray et al., 2006).  

For the long haul, it is very important to understand bio-
ethanol production technologies in terms of their econo-
mic viability, environmental feasibility and empowering 
employment opportunities before implementing a fuel 
ethanol policy.  

The choice of the best technology for lignocellulose to 
bioethanol conversion should be decided on the basis of 
overall economics (lowest cost), environmental (lowest 
pollutants) and energy (higher efficiencies)that is, comp-
rehensive process development and optimization are still 
required to make the process economically viable. 

In reality, environmental considerations, energy and tax 
policies will determine the extent of fuel ethanol utilization 
in the future (Keim and Venkatasubramanian, 1989) and 
therefore the role of one and all is very crucial to identify 
the gravity of the situation associated with bioethanol pro-
duction and use of it as an alternative fuel. 

The focus of this review is on the current status of 

available ethanol production technologies in terms of their 

  



127       Afr. J. Environ. Econ. Manage. 
 
 

 
practical cost economics and the desirable environmental 

impact they have for a whole generation of commuters 

across the globe. 

 
Lignocellulosic biomass 
 
Composition 
 
Basically, the lignocellulosic biomass comprises of cellu-
lose, hemicellulose and lignin (Hayn et al., 1993). Cellu-
lose is a linear, crystalline homopolymer with a repeating 
unit of glucose strung together beta-glucosidic linkages. 
The structure is rigid and harsh treatment is required to 
break it down (Gray et al., 2006). Hemi-cellulose consists 
of short, linear and highly branched chains of sugars. In 
contrast to cellulose, which is a polymer of only glucose, 
a hemicellulose is a hetero-polymer of D-xylose, D-
glucose, D-galactose, D-man-nose and L-arabinose 
(Saha et al., 2003). The composi-tion of holocellulose 
(cellulose + hemicellulose) varies with the origin of the 
lignocellulosic material. Table 1 shows the composition of 
the selected crop residues, woody materials, vegetable 
and fruit waste and municipal solid waste and their 
simulated ethanol production.  

Ethanol production has been taken into estimation dep-
ending upon the ratio of hexosans (glucan, galactan and 
mannan) and pentosans (xylan, arabinan) in each bio-
mass source. The ethanol production from each bio-mass 
sources was calculated / ascertained from US Depart-
ment of Energy website, which provide “Theoretical etha-
nol yield calculator” at http://www.eere.ene-rgy.gov/bio-
mass/ethanol_yield_calculator.html. 

 
Ethanol production technologies 
 
Bioconversion of lignocellulosics to ethanol consists of 

four major unit operations: pretreatment, hydrolysis, 

fermentation and product separation/ distillation. 

 
Pretreatment 
 
Pretreatment is required to alter the biomass macrosco-
pic and microscopic size and structure as well as its 
submicroscopic chemical composition and structure so 
that hydrolysis of carbohydrate fraction to monomeric 
sugars can be achieved more rapidly and with greater 
yields (Sun and Cheng, 2002; Moiser et al., 2005). 
Pretreatment affects the structure of biomass by 
solubilizing hemicellulose, reducing crystallinity and incre-
ase the available surface area and pore volume of the 
substrate. Pretreatment has been considered as one of 
the most expensive processing step in biomass to 
fermentable sugar conversion with cost as high as 30 
cents/gallon ethanol produced (Moiser et al., 2005). To 
asses the cost and performance of pretreatment met-
hods, technoeconomic analysis have been made recently 
(Eggerman and Elander, 2005). There is huge scope in 

 
 
 

 
lowering the cost of pretreatment process through exten-
sive R&D approaches. Pretreatment of cellulosic biomass 
in cost effective manner is a major challenge of cellulose 
to ethanol technology research and development.  

Native lignocellulosic biomass is extremely recalcitrant 
to enzymatic digestion. Therefore, a number of thermo-
chemical pretreatment methods have been developed to 
improve digestibility (Wyman et al., 2005). Recent studies 
have clearly proved that there is a direct correlation bet-
ween the removal of lignin and hemi-cellulose on cellu-
lose digestibility (Kim and Holtzapple, 2006). Thermo-
chemical processing options appear more promising than 
biological options for the conversion of lignin fraction of 
cellulosic biomass, which can have a detrimental effect 
on enzyme hydrolysis. It can also serve as a source of 
process energy and potential co-products that have 
important benefits in a life cycle context (Sheehan et al., 
2003). Pretreatment can be carried out in different ways 
such as mechanical combination (Cadoche and Lopez, 
1989), steam explosion (Gregg and Saddler, 1996), 
ammonia fiber explosion (Kim et al., 2003), acid or 
alkaline pretreatment (Damaso et al., 2004; Kuhad et al., 
1997) and biological treatment (Keller, et al., 2003). 
 
 
Hydrolysis 
 
After pretreatment there are two types of processes to 
hydrolyze the feed stocks into monomeric sugar consti-
tuents required for fermentation into ethanol. The hydroly-
sis methods most commonly used are acid (dilute and 
concentrated) and enzymatic. To improve the enzymatic 
hydrolytic efficiency, the lignin- hemicellulose net work 
has to be loosened for the better amenability of cellulases 
to residual carbohydrate fraction for sugar recovery. 
Dilute acid treatment is employed for the degradation of 
hemicellulose leaving lignin and cellulose network in the 
substrate. Other treatments are alkaline hydrolysis or 
microbial pretreatment with white-rot fungi (Phaenero-
chate chrysosporium, Cyathus stercoreus, Cythus bulleri 
and Pycnoporous cinnabarinus etc.) preferably act upon 
lignin leaving cellulose and hemicellulose network in the 
residual portion. However during both treatment proces-
ses, a considerable amount of carbohydrates are also 
degraded, hence the carbohydrate recovery is not satis-
factory for ethanol production. 

 
Acid hydrolysis 
 
There are two types of acid hydrolysis process commonly 
used - dilute and concentrated acid hydrolysis. The dilute 
acid process is conducted under high temperature and 
pressure and has reaction time in the range of seconds or 
minutes. The concentrated acid process uses relatively 
mild temperatures, but at high concentration of sulfuric 
acid and a minimum pressure involved, which only crea-
tes by pumping the materials from vessel to vessel. Re-
action times are typically much longer than for dilute acid



        

Table 1. Chemical composition of raw materials and simulated ethanol production     
         

Raw material Cellulose/ Hemicellulose / Lignin  Ethanol yield Reference   
 Hexosans (H) Pentosans (P)   /kg dry mass    

Sugarcane baggase 33 (H) 30 (P) 29  0.279 Kuhad and Singh, 1993   

Wheat straw 30 (H) 24 (P) 18  0.239 Kuhad and Singh, 1993   

Sorghum straw 33 (H) 18 (P) 15  0.240 Kuhad and Singh, 1993   

Rice straw 32 (H) 24 (P) 13  0.248 Kuhad and Singh, 1993   

Oat straw 41 (H) 16 (P) 11  0.252 Kuhad and Singh, 1993   

Corn cob 42 (H) 39 (P) 14  0.358 Kuhad and Singh, 1993   

Corn stalks 35 (H) 15 (P) 19  0.221 Kuhad and Singh, 1993   

Barley straw 40 (H) 20 (P) 15  0.265 Kuhad and Singh, 1993   

Ground nut shell 38 (H) 36 (P) 16  0.327 Kuhad and Singh, 1993   

Alfalfa stalks 48.5 6.5 16.6  0.209 Shleser, 1994   

Rice hulls 36 (H) 15 (P) 19  0.265 Kuhad and Singh, 1993   

Eucalyptus grandis 38 13 37  0.225 Shleser, 1994   

Eucalyptus saligna 45 12.0 25.0  0.252 Shleser, 1994   

Pine 44.0 26.0 29.0  0.310 Olsson and Hagerdal, 1996   

Poplar 47.6 27.4 19.2  0.332 Olsson and Hagerdal, 1996   

Saw dust 55.0 14.0 21.0  0.305 Olsson and Hagerdal, 1996   

Willow 37.0 23.0 21.0  0.265 Olsson and Hagerdal, 1996   

Aspen 51 29.0 16.0  0.354 Olsson and Hagerdal, 1996   

Spruce 43.0 26.0 29.0   Olsson and Hagerdal, 1996   

Birch 40.0 23.0 21.0  0.305 Olsson and Hagerdal, 1996   

Lantana camara 42.50 22.70 22.88  0.288 Chandel (Unpublished work)   

Prosopis juliflora 45.5 20.38 24.65  0.291 Chandel (Unpublished work)   

Saccharum spontaneum 45.10 22.70 24.56  0.300 Gupta, 2006   

Eicchornia crassipis 18.2 48.7 3.50  0.296 Nigam, 2002   

Paja brava 32.2 28.1 24.0  0.267 Sanchez et al., 2004   

News Paper 61 16 21  0.341 Olsson and Hagerdal, 1996   

Processed Paper 47 25 12  0.318 Olsson and Hagerdal, 1996   

Paper-based   municipal 43 13 6  0.248 Olsson and Hagerdal, 1996   
solid waste         

 

 
process 

 
Dilute acid hydrolysis 
 
In dilute acid hydrolysis, the hemicellulose fraction is 
depolymerized at lower temperature than the cellulosic 
fraction. Dilute sulfuric acid is mixed with biomass to 
hydrolyse hemicellulose to xylose and other sugars. 
Dilute acid is interacted with the biomass and the slurry is 
held at temperature ranging from120 - 220°C for a short 
period of time. Thus hemicellulosic fraction of plant cell 
wall is depolymerised and will lead to the enhancement of 
cellulose digestibility in the residual solids (Nigam, 2002; 
Sun and Cheng, 2002; Dien et al., 2006; Saha et al., 
2005). Dilute acid hydrolysis has some limitations. If 
higher temperatures (or longer residence time) are 
applied, the hemicelluosic derived monosaccharides will 
degrade and give rise to fermentation inhibitors like furan 
compounds, weak carboxylic acids and phenolic 
compounds (Olsson and Hahn- Hagerdal, 1996; Klinke et 

 

 
al., 2004; Larsson et al., 1999). These fermentation 
inhibitors are known to affect the ethanol production 
performance of fermenting microorganisms (Chandel et 
al., 2006b). In order to remove the inhibitors and increase 
the hydrolysate fermentability, several chemicals and 
biological methods have been used. These methods 
include overliming (Martinez et al., 2000), charcoal 
adsorption (Chandel et al., 2006b), ion exchange 
(Nilvebrant, 2001), detoxification with laccase (Martin et 
al., 2002; Chandel et al., 2006b), and biological 
detoxification (Lopez et al., 2004). The detoxification of 
acid hydrolysates has been shown to improve their 
fermentability; however, the cost is often higher than the 
benefits achieved (Palmqvist and Hahn- Hagerdal, 2000; 
von Sivers and Zacchi, 1996). Dilute acid hydrolysis is 
carried out in two stages- First-stage and two-stage. 

 
First-stage dilute acid hydrolysis 
 
The lignocellulosic material is first contacted with dilute 

Chandel  et al.               128 
 



129       Afr. J. Environ. Econ. Manage. 
 
 
 
sulfuric acid (0.75%) and heated to approximately 50°C 
followed by transferring to the first stage acid impregnator 
where the temperature is raised to 190°C. Approximately, 
80% of the hemicellulose and 29% of cellulose are 
hydrolyzed in the first reactor. The hydrolysate is further 
incubated at a lower temperature for a residence time of 
2 h to hydrolyse most of the oligosaccharides into mono-
saccharides followed by the separation of solid and liquid 
fractions. The solid material again washed with plentiful of 
water to maximize sugar recovery. The separated solid 
material is sent to second stage acid hydrolysis reactor 
(Figure 1). 
 
 
Two-stage dilute acid hydrolysis 
 
In two-stage dilute acid hydrolysis process, first, biomass 
is treated with dilute acid at relatively mild conditions dur-
ing which the hemicelluose fraction is hydrolyzed and the 
second stage is normally carried out at higher tempera-
ture for depolymerisation of cellulose into glucose. The 
liquid phase, containing the monomeric sugars is remo-
ved between the treatments, thereby avoiding degrada-
tion of monosaccharides formed (Figure 1). It is very 
important to avoid monosaccharide degradation products 
for improving the ethanol yield. Sanchez et al. (2004) 
carried out the two-stage dilute acid hydrolysis using Boli-
vian straw material, Paja brava. In first stage, P. brava 
material was pretreated with steam followed by dilute sul-
furic acid (0.5 or 1.0% by wt) hydrolysis at temperatures 
between 170 and 230°C for a residence time between 3 
and 10 min. The highest yield of hemicellulose derived 
sugars were found at a temperature of 190°C, and a 
reaction time of 5 – 10 min, whereas in second stage 
hydrolysis considerably higher temperature (230 °C) was 
found for hydrolysis of remaining fraction of cellulose. 

 
Concentrated acid hydrolysis 
 
This method uses concentrated sulfuric acid followed by 
a dilution with water to dissolve and hydrolyse the subs-
trate into sugar constituents. This process provides com-
plete and rapid conversion of celluose to glucose and 
hemicellulose to xylose with a little degradation. The 
concentrated acid process uses 70% sulfuric acid at 40 - 
50°C for 2 to 4 h in a reactor. The low temperatures and 
pressure will lead to minimize the sugar degradation. The 
hydrolyzed material is then washed to recover the sugars. 
 

In the next step, the cellulosic fraction has to be deploy-
merized. The solid residue from first stage is de-watered 
and soaked in 30 - 40% sulfuric acid for 50 min. at 100°C 
for further cellulose hydrolysis. The resulting slurry mix-
ture is pressed to obtain second acid–sugar stream 
(approximately 18% sugar and 30% acid). Both the sugar 
steams from two hydrolysis steps are combined and may 
be used for subsequent ethanol production. Iranmahboob 

 
 
 
 
et al. (2002) performed the concentrated acid hydrolysis 
of mixed wood chips and found that maximum sugar 
recovery (78 - 82% of theoretical yields) was achieved at 
sulfuric acid concentration (26%) for 2 h of residence 
time.  

The primary advantage of the concentrated acid pro-
cess is the potential for high sugar recovery efficiency, 
about 90% of both hemicellulose and cellulose fraction 
gets depolymerized into their monomeric fractions. The 
acid and sugar syrup are separated via ion exchange and 
then acid is reconcentrated through multiple effect 
evaporators. The remaining lignin rich solids are collected 
and optionally palletized for fuel generation (Figure 2). 
 

 
Enzymatic hydrolysis 
 
The acid, alkaline or fungal pretreated lignocellulosics 
can be saccharified enzymatically to get fermentable 
sugars (Ghose and Bisaria, 1979; Kuhad et al., 1997; Itoh 
et al., 2003; Tucker et al., 2003). Bacteria and fungi are 
the good sources of cellulases, hemicellulases that could 
be used for the hydrolysis of pretreated lignocellulosics. 
The enzymatic cocktails are usually mixtures of several 
hydrolytic enzymes comprising of cellulases, xylanases, 
hemicellulases and mannanases. In the last decade, new 
cellulases and hemicellulases from bacterial and fungal 
sources have continued been isolated and regular efforts 
have been made for the improved production of enzyme-
tic titers (Aro et al., 2005; Foreman et al., 2003). How-
ever, the cellulases were produced at a concentration too 
low to be useful. There is a group of microorganisms 
(Clostridium, Cellulomonas, Tricho-derma, Penicillium, 
Neurospora, Fusarium, Aspergillus etc.) showing a high 
cellulolytic and hemicellulolytic activity, which are also 
highly capable of fermenting monosaccharides. Genetic 
engineering is used to produce super strains, which are 
capable of hydrolysing cellulose and xylan along with fer-
mentation of glucose and xylose to ethanol (Aristidou and 
Penttila, 2000; Lin and Tanaka, 2006). The utilization of 
cellulose by microorganisms involves a substantial set of 
fundamental phenomena beyond those associated with 
enzymatic hydrolysis of cellulose (Lynd et al., 2002). 
 

 
Separate hydrolysis and fermentation (SHF) 
 
Enzymatic hydrolysis performed separately from fermen-
tation step is known as separate hydrolysis and fermen-
tation (SHF) (Sreenath et al., 2001; Wingren et al., 2003). 
The separation of hydrolysis and fermentation offers 
various processing advantage and opportunities. It ena-
bles enzymes to operate at higher temperature for increa-
sed performance and fermentation organisms to operate 
at moderate temperatures, optimizing the utilization of su-
gars (Figure 3). 



Chandel  et al.               130 

 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 

 
Figure 1. Dilute acid hydrolysis (First-stage and Two-stages) and separate fermentation of pentose and hexose sugars. 

 
 
 
Simultaneous saccharification and fermentation 

(SSF) 
 

The most important process improvement made for the 
enzymatic hydrolysis of biomass is the introduction of 
simultaneous saccharification and fermentation (SSF), 
which has been improved to include the co-fermentation 
of multiple sugar substrates (Figure 4) (Sreenath et al., 
2001; Wingren et al., 2003). This approach combined the 
cellulase enzymes and fermenting microbes in one 
vessel. This enabled a one-step process of sugar 
production and fermentation into ethanol. Simultaneous 
saccharification of both carbon polymer, cellulose to 
glucose; and hemicellulose to xylose and arabinose; and, 
fermentation will be carried out by recombinant yeast or 

the organism which has the ability to utilize both C5 and 

C6 sugars. According to Alkasrawi et al. (2006) the mode 
of preparation of yeast must be carefully considered in 
SSF designing. A more robust strain will give substantial 
process advantages in terms of higher solid loading and 
possibility to recirculate the process stream, which results 
in increased energy demand and reduced fresh water 
utilization demand in process. Adaptation of yeast to the 

 
 
 
inhibitors present in the medium is an important factor for 
consideration in the design of SSF process. More 
recently, Kroumov et al. (2006) demonstrated an 
unstructured model of SSF of starch to ethanol by 
genetically modified strain Saccharomyces cereviseae 
YPB-G, using two hierarchic levels of concept. In first 
concept, a mechanism of enzymatic hydrolysis of starch 
to glucose by combined action of two enzymes (alpha-
amylase and glucoamylase) secreted by recombinant 
yeast and the second concept was the enzymatic 
degradation of starch to glucose and simultaneous 
utilization of glucose to ethanol by microorganisms. SSF 
combines enzymatic hydrolysis with ethanol fermentation 
to keep the concentration of glucose low. The 
accumulation of ethanol in the fermenter does not inhibit 
cellulase action as much as high concentration of 
glucose; so, SSF is good strategy for increasing the 
overall rate of cellulose to ethanol conversion (Lin and 
Tanaka, 2006) . SSF gives higher ethanol yield while 
requiring lower amounts of enzyme because end- product 
inhibition from cellobiose and glucose formed during 
enzymatic hydrolysis is relieved by the yeast fermentation 
(Banat et al., 1998). However, it is not feasible for SSF to 

  



131       Afr. J. Environ. Econ. Manage. 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 

 
Figure 2. Concentrated acid hydrolysis and separate pentose and hexose sugars fermentation. 

 

 
meet all the challenges at industrial level due to its low 
rate of cellulose hydrolysis and most microorganisms 
employed for ethanol fermentation can not utilize all 
sugars derived after hydrolysis. To overcome of this 
problem, the cellulolytic enzyme cocktail should be more 
stable in wide range of pH and temperature. Also the 
fermenting microorganisms (yeasts or bacteria) should be 

able to ferment a wide range of C5 and C6 sugars. 

Recently Matthew et al. (2005) has found some promi-
sing ethanol producing bacteria viz. recombinant E. coli  
K011, Klebsiella oxytoca, and Zymomonas mobilis for 

industrial exploitation. SSF process has now improved 

after including the co-fermentation of multiple sugar subs- 

 

 
trates present in the hydrolysate. This new variant of SSF 
is known as simultaneous saccharification and co- fer-
mentation (SSCF) (Wilke et al., 1976; Patel et al., 2005; 
Wyman et al., 2005). SSF and SSCF are preferred over 
SHF, since both operations can be performed in the 
same tank resulting in lower cost, higher ethanol yield 
and shorter processing time (Wright et al., 1988). The 
most upgraded form of biomass to ethanol conversion is 
consolidated bioprocessing (CBP) - featuring cellulose 
production, cellulose hydrolysis and fermentation in one 
step-is a highly integrated approach with outstanding 
potential (Lynd et al., 2005). It has the potential to provide 
the lowest cost route for biological conversion of cellulo- 



  
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
Figure 3. SHF with separate pentose and hexose sugars and combined sugar fermentation. 

 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 

 
Figure 4. SSF with combined sugars (pentoses and hexoses) fermentation. 

Chandel  et al.               132 



133       Afr. J. Environ. Econ. Manage. 
 
 
 
losic biomass to ethanol with high rate and desired yields. 

 
Direct microbial conversion (DMC) 
 
DMC is a method of converting cellulosic biomass to 
ethanol in which both ethanol and all required enzymes 
are produced by a single microorganism. The potential 
advantage of DMC is that a dedicated process step for 
the production of cellulase enzyme is not necessary. Cel-
lulase enzyme production (or procurement) contributes 
significantly to the cost involved in enzymatic hydrolysis 
process. However, DMC is not considered the leading 
process alternative. This is because there is no robust 
organism available that can produce cellulases or other 
cell wall degrading enzymes in conjunction with ethanol 
with a high yield. Singh and Kumar (1991) found that 
several strains of Fusarium oxysporum have the potential 
for converting not only D-xylose, but also cellulose to 
ethanol in a one-step process. Distinguishing features of 
F. oxysporum for ethanol production in comparison to 
other organisms are identified. These include the advan-
tage of in situ cellulase production and cellulose fermen-
tation, pentose fermentation, and the tolerance of sugars 
and ethanol. The main disadvantage of F. oxysporum is 
its slow conversion rate of sugars to ethanol as compared 
to yeast. 

 
Fermentation 
 
Bioconversion of various biomass sources into ethanol by 
different microorganisms has been summarized in Table 
2. The sugar syrup obtained after cellulosic hydrolysis is 
used for ethanol fermentation. The ability to ferment pen-
toses along with hexoses is not widespread among 
microorganisms (Toivolla et al., 1984), S. cereviseae is 
capable of converting only hexose sugars to ethanol. The 

most promising yeasts that have the ability to use both C5 

and C6 sugars are Pichia stipitis, Candida shehatae and  
Pachysolan tannophilus. However, ethanol production 
from sugars derived from starch and sucrose has been 
commercially dominated by the yeast S. cereviseae (Lin 
and Tanaka, 2006). Thermotolerant yeast could be more 
suitable for ethanol production at industrial level. In high 
temperature process energy savings can be achieved 
through a reduction in cooling costs. Considering this ap-
proach, Sree et al. (1999) developed solid state fermen-
tation system for ethanol production from sweet sorghum 
and potato employing a thermotolerant S. cereviseae 
strain (VS3).  

Researches are now focusing on developing recombi-
nant yeast, which can greatly improve the ethanol prod-
uction yield by metabolizing all form of sugars, and redu-
ce the cost of operation. In this contention the resear-
chers have made efforts by following two approaches. 
The first approach has been to genetically modify the 
yeast and other natural ethanologens additional pentose 
metabolic pathways. The second approach is to improve 

 
 
 
ethanol yields by genetic engineering in microorganisms 
that have the ability to ferment both hexoses and pento-
ses (Jeffries and Jin, 2000; Dien et al., 2003; Katharia et 
al., 2006). Jeffries and Jin (2004) compiled the recent 
developments happened towards the genetic engineering 
of yeast metabolism and concluded that strain selection 
through mutagenesis, adaptive evolution using quantita-
tive metabolism models may help to further improve their 
ethanol production rates with increased productivities. 
Piskur et al., (2006) showed the recent developments in 
comparative genomics and bioinformatics to elucidate the 
high ethanol production mechanism from Saccharomyces 
sp.  

Though new technologies have greatly improved 
bioethanol production yet there are still a lot of problems 
that have to be solved. The major problems include main-
taining a stable performance of genetically engineered 
yeast in commercial scale fermentation operation (Ho et 
al., 1998, 1999), developing more efficient pre-treatment 
technologies for lignocellulosic biomass, and integrating 
optimal component into economic ethanol production 
system (Dien et al., 2000). Sridhar and co-workers (2002) 
made an effort to improve the thermo tolerance of yeast 
isolates by treating them with UV radiation.  

Fermentation can be performed as a batch, fed batch 
or continuous process. The choice of most suitable pro-
cess will depend upon the kinetic properties of micro-
organisms and type of lignocellulosic hydrolysate in addi-
tion to process economics aspects. 

 
Batch fermentation 
 
Traditionally, ethanol has been produced batch wise. At 
present, nearly, all of the fermentation ethanol industry 
uses the batch mode. In batch fermentation, the micro-
organism works in high substrate concentration initially 
and a high product concentration finally (Olsson and Han-
Hagerdal, 1996). The batch process is a multi-vessel pro-
cess, allows flexible operation and easy control over the 
process. Generally batch fermentation is characterized by 
low productivity with an intensive labour (Shama, 1988). 
For batch fermentation, elaborate preparatory procedu-
res are needed; and because of the discontinuous start 
up and shut down operations, high labour costs are incur-
red. This inherent disadvantage and the low productivity 
offered by the batch process have led many commercial 
operators to consider the other fermentation methods. 

 
Fed batch fermentation 
 
In fed batch fermentation the microorganism works at low 
substrate concentration with an increasing ethanol conc-
entration during the course of fermentation process. Fed 
batch cultures often provide better yield and productivities 
than batch cultures for the production of microbial meta-
bolites. For practical reasons, therefore, some continuous 
operations have been replaced by fed batch process 



Chandel  et al.               134 

 
 
 

Table 2. Various raw materials for ethanol production. 
 

Raw Material Pretreatment and Fermentation   Microorganism Reference 
  Saccharification conditions        

Sugarcane baggase Dilute acid hydrolysis Batch   C. shehatae NCIM3501  Chandel et al., 2006b 
Wheat straw  Dilute acid, Enzymatic SSF, SHF  E. coli FBR5   Saha et al., 2005 

  hydrolysis           

Rice straw  Auto hydrolysis Batch   C. shehatae NCIM3501  Abbi et al., 1996 
Sorghum straw Steam explosion, SSF   Kluyveromyces  marxianus Ballesteros et al., 2004 

  enzymatic     CECT10875     

Corn stover  Steam, enzymatic Fed- batch  S. cereviseae TMB3400  Ohgren et al., 2006 
Barley husk  Steam, enzymatic SSF   S. cereviseae   Palmarola et al, 2005 
Sun flower stalk Steam, enzymatic Batch   S. cereviseae var ellipsoideus Sharma et al., 2002 
Sugarcane leaves Alkaline H2O2 SSF   S. cereviseae NRRL-Y-132 Krishna et al., 2001 
Wheat bran  Dilute acid, Enzymatic Batch   S. cereviseae   Palmarola et al, 2005 

  hydrolysis           

Ground nut shell Acid hydrolysis Batch   S. cereviseae   Akpan et al., 2005 
Alfalfa fibers  Liquid hot water SSF, SHF  C. shehatae FPL-702  Sreenath et al., 2001 
Aspen  Acid hydrolysis Continuous, Immobilized P. stipitis R   Parekh et al., 1987 

    cells         

Saw dust  Acid hydrolysis Batch, Continuous up- Clostridium   Liu et al., 1988 
    flow reactors  thermosaccharolyticum ATCC   

       31925      

Pine  Acid hydrolysis Continuous  stirred tank P. stipitis NRRL-1724  Qureshi et al., 1991 
    reactor, Immobilized       

    cells         

Poplar  Steam explosion, SSF, SHF  S. cerevisiae   Cantarella et al., 2004 
  Enzymatic           

Birch  Acid hydrolysate Batch   S. cerevisiae   Johanssen et al., 2001 
Spruce  Dilute acid hydrolysis Fed batch  S. cerevisiae   Taherzadeh., 1999 
Willow  Steam  Batch   E. coli K011   Olsson et al., 1995 
Paja brava  Dilute acid hydrolysis, Batch   C. shehatae, P. stipitis, Sanchez et al., 2004 

  Two stage     Pachysolen tannophilus    

Eicchornia crassipes Dilute acid hydrolysis Batch   P. stipitis   Nigam, 2002  
Saccharum  Dilute acid hydrolysis, Batch, Fed Batch  P. stipitis NCIM 3498   Gupta, 2006  

spontaneum  Enzymatic           

Cassava starch Starch liquifaction Batch, Continuous Co- S. diastaticus Zymomonas mobilis Amutha and 
    immobilized cells      Gunashekhran, 2001 

Apple pomace   SSF   S. cerevisiae ATCC 24702 Ngadi and Correial, 1992 
Pine  apple canary Juice Extraction Continuous, Immobilized S. cereviseae ATCC 24553 Nigam, 2000  
waste    cells         

Banana pulp waste Juice Extraction Continuous,  S. uvarum NCIM culture 3528 Joshi et al., 2001 
    Cell recycles.        

Finger Millet   High gravity fermentation S. cerevisiae   Reddy and Reddy, 2006 
(Eleusine corcana            

flour )             

Municipal solid Acid pretraetment  Batch  S. cerevisiae   Mtui   and Nakamura, 
waste (MSW)          2005  

News print  Acid hydrolysate  Batch  E. coli B (pLOI297)   Lawford and Rousseau, 
           1993  

Industrial waste    SSF  K. marxianus, S. cereviseae Kadar et al., 2004 
 

 
(Schugerl, 1987). Keeping the low feed rate of substrate 
solution containing high concentration of fermentation 

inhibitors such as furfural, hydroxymethyl furfural and 

phenolics, the inhibitory effect of these compounds to 

 

 
yeast can be reduced. Complete fermentation of an acid 
hydrolysate of spruce, which was strongly inhibiting in 

batch fermentation, has been achieved without any deto-

xification treatment (Taherzadeh, 1999). The productivity 

  



135       Afr. J. Environ. Econ. Manage. 
 
 
 
in fed batch fermentation is limited by the feed rate which, 
in turn, is limited by the cell mass concentration. The 
specific ethanol productivity has also been reported to 
decrease with increasing cell mass concentration (Lee 
and Chang, 1987; Palmqvist et al., 1996). Ideally, the cell 
density should be kept at a level providing maximum 
ethanol productivity and yield. 
 
 
Continuous fermentation 
 
Continuous fermentation can be performed in different 
kind of bioreactors – stirred tank reactors (single or 
series) or plug flow reactors. Continuous fermentation 
often gives a higher productivity than batch fermentation, 
but at low dilution rates which offers the highest pro-
ductivities. Alexender et al., (1989) studied the effect of 
shift in temperature and aeration in steady state con-
tinuous culture of C. shehatae to determine the effects of 
ethanol on xylose metabolism. The accu-mulation of 
ethanol exerted a delayed inhibitory effect on the specific 
rate of substrate utilization. Continuous operation offers 
ease of control and is less labor intensive than batch ope-
ration. However contamination is more serious in this 
operation. Since the process must be interrupted, all the 
equipments must be cleaned, and the operation started 
again with the growth of new inoculum. The continuous 
process eliminates much of the unproductive time asso-
ciated with cleaning, recharging, adjustment of media and 
sterilization. A high cell density of microbes in the 
continuous fermenter is locked in the exponential phase, 
which allows high productivity and overall short process-
sing of 4 - 6 h as compared to the conventional batch 
fermentation (24 - 60 h). This results in substantial sav-
ings in labour and minimizes investment costs by achie-
ving a given production level with a much smaller plant. 
 
 
Immobilized cells 
 
A limitation to continuous fermentation is the difficulty of 
maintaining high cell concentration in the fermenter. The 
use of immobilized cells circumvents this difficulty. Immo-
bilization by adhesion to a surface (electrostatic or 
covalent), entrapment in polymeric matrices or retention 
by membranes has been successful for ethanol produc-
tion from hexoses (Godia et al., 1987). The applications 
of immobilized cells have made a significant advance in 
fuel ethanol production technology. Immobilized cells 
offer rapid fermentation rates with high productivity - that 
is, large fermenter volumes of mash put through per day, 
without risk of cell washout. In continuous fermentation, 
the direct immobilization of intact cells helps to retain 
cells during transfer of broth into collecting vessel. More-
over, the loss of intracellular enzyme activity can be kept 
to a minimum level by avoiding the removal of cells from 
downstream products (Najafpour, 1990). Immobilization 
of microbial cells for fermentation has been developed to 

 
 
 
eliminate inhibition caused by high concentration of subs-
trate and product and also to enhance ethanol produc-
tivity and yield. Abbi et al., (1996) observed that the rate 
of sugar consumption by immobilized cells of C. shehatae 
NCL-3501 was slightly lower than that of free cells, thus 
leading to higher ethanol production. When microorg-
anisms are attached to solid supports, fluid viscosity is 
lower which contributes to better mixing and mass 
transfer in the system. The work on ethanol production in 
an immobilized cell reactor (ICR) showed that ethanol 
production using Z. mobilis was doubled. Amutha and 
Gunasekaran (2001) reported ethanol production, 46.7 g/l 
from 150 g/l liquefied cassava starch from co-immobilized 
cells of Saccharomyces diastaticus and Z. mobilis. Yama-
da et al. (2002) successfully used recombinant Z. mobilis 
with high sugar concentration (12-15%) and further 
observed the significant role of increased biomass con-
centration in bioreactor performance for the improved 
ethanol production. A repeated batch fermentation 
system was used to produce ethanol using an immobili-
zed osmotolerant S. cereviseae, in which ethanol conc-
entration as high as 93 g/l was recorded at 200 g/l 
glucose concentration (Sree et al., 2000). Nigam (2000) 
has reported that the ethanol production rate as high as 
42.8 g/l/h was achieved from the fermentation of 
pineapple canary derived sugars by S. cereviseae ATCC 
24553. 
 
Recycling of process stream 
 
In an environmentally sustainable process, the use of 
fresh water, the amount of wastewater and the energy 
consumption must be minimized. The water consumption 
is decreased by recirculating process streams for use in 
the washing and hydrolysis steps (Palmqvist and Hahn-
Hagerdal, 2000). Recirculating part of the dilute ethanol 
stream from the fermenter can increase the ethanol 
concentration in the feed to the distillation stage. 
However, computer simulations have shown that 
recirculation of streams leads to the accumulation of non-
volatile inhibitory compounds (Galbe and Zacchi, 1992; 
Palmqvist et al., 1996). 

To increase the ethanol productivity, cell recycling has 
been employed by several workers (Fein et al., 1984; 
Maleszka et al., 1981), while retaining the simplicity of the 
batch process. Cell recycling generally does not increase 
the sugar consumption or ethanol production but the time 
required for the fermentation can be reduced by 60 - 
70%. Schneider (1989) observed a reduction in ethanol 
production after third cell cycle and suggested the 
decrease in ethanol production was due to the limitations 
of oxygen and sugar as a result of an increase in cell 
density. 
 
Economics of ethanol production technologies 
 
To be competitive, and find economic acceptance, the 

cost for bioconversion of biomass to liquid fuel must be 



Chandel  et al.               136 
 
 

 
lower than the current gasoline prices (Wayman and Par-
ekh, 1990; Subramanian et al., 2005). It seems how-ever; 
now much more attainable because of increasing efforts 
of researchers working towards improvisation in the 
efficiency of biomass conversion technologies (Vertes et 
al., 2006). 

However there is still huge scope to bring down the 
cost of biomass-to-ethanol conversion. The cost of feed-
stock and cellulolytic enzymes are two important parame-
ters for low cost ethanol production. Biomass feedstock 
cost represents around 40% of the ethanol production 
cost (Hamelinck et al., 2005). An analysis of the potential 
of bioethanol in short and long term (2030) in terms of 
performance, key technologies and economic aspects 
such as cost per kilometer driven has been conducted 
recently by Hamelinck and Faaij (2006). In this analysis, 
the production cost of bioethanol was found to be within 

the range of 16-22 /GJHHV (Euro/Giga Joules High  
Heating Value) at present and down to 13 /GJHHV in 
future (2030). The feedstock cost is major parameter  
influencing the ethanol production cost at a rate of 2-3 

/GJ fuel. Employing integrated approaches using the 
larger industrial facilities by integrated action plan along 
with cheap feedstock and potent cellulases could make 
the process more economically viable (Sun and Cheng, 
2002; Dien et al., 2006)  

Securing long supply of energy sources requires not 
only that existing fuel resources be utilized as economi-
cally as possible along with their diversification uses. 
Keeping this point of view, bioethanol is identified and 
portrayed as the entity for ensuring energy security in 
future fuel interests and global requirements. 

The choice of feedstock for ethanol production depends 
upon its availability and the ongoing uses. For example, 
agroresidues such as wheat straw, sorghum and barley 
straw are not preferable for bioethanol production due to 
their use as animal fodder. On the contrary, agroresidues 
like sugarcane bagasse, rice straw, rice bran, groundnut 
shell, corn stover, Brassica carniata stalks and soyabean 
stalks etc can be used directly because these sources 
are not preferably used as fodder for livestock. Some 
dedicated energy sources like damaged rice and sorg-
hum grains (Suresh et al., 1999), sunflower stalks and 
hulls (Sharma et al., 2000), Eicchornia crassipies (Nigam, 
2003), P. brava (Sanchez et al., 2004), alfalfa fibers 
(Sreenath et al., 2001), residual starch and crushed 
wheat grains (Davis et al., 2006), agro waste (Campo et 
al., 2006) and Saccharum spontaneum (Gupta, 2006) are 
more feasible sources for bioethanol production. Also, 
organic waste and municipal solid waste (MSW), which 
contain significant amount of cellulose could be cost eco-
nomic friendly and solve the problem of solid waste stora-
ge and management.  

In India, Department of Biotechnology, Govt. of India 

funded a nationwide research project towards the use of 
selective weeds like Lantana camara, Prosopis juliflora 

and fruit or vegetable waste into ethanol conversion due 

 

  
 
 
 
to their vast abundance, low cost and rich in fermentable 
carbohydrates.  
An important factor for reducing the cost of bioethanol 
production is to use larger industrial facilities rather than 
smaller ones. Ward and Singh (2002) also suggested the 
integrated approach (Process engineering, fermentation 
and enzyme and metabolic engineering) could improve 
the ethanol production economics. By increasing the 
plant size, the investment per unit output of product falls 
off, a ten-fold increase in size reducing the unit cost to 
less than one-half and thereby reducing unit capital cost 
charges and conversion cost reducing profitability (Way-
man and Parekh, 1990; Henke et al., 2006).  

To further improve the economy of ethanol production, 
energy integration of the ethanol production, to already 
existing plants such as pulp and paper plants is 
necessary. O’ O’Boyle et al. (1991) reported that the cost 
of producing ethanol from pine with a diluted acid hydro-

lysate process, was estimated to be 3.22 SEK L
-1

 in a 

stand alone plant in comparison to 2.54 SEK L
-1

 with an 
integrated plant. They projected that the cost of bioetha-
nol can be reduced from US$ 1.22 per liter to about US $ 
0.31 per liter on the basis of continuous improvement in 
pretreatment of biomass, enzyme application and fer-
mentation.  

Aristidou and Penttila (2000) reported that the total cost 
of ethanol will be dropped from more than $1.0 per litre to 
~ $0.3-0.5 per litre, with a projected cost of less than 
$0.25 per litre in the near future.  

Wilke et al. (1981) has made the first effort to analyse 
the cost of the conversion of biomass to ethanol process 
based on a SHF operation and concluded that neat etha-
nol could compete with gasoline at the oil prices at $20 to 
$30 per barrel. Foody (1988) has emphasized the impor-
tance of improvement in cost effective cellulases produc-
tion and outlined the potential for bringing down the price 
of bioethanol from 25-55 cents to 10-28 cents per liter. 
Subsequently, Wright (1988) and Hinman et al. (1992) 
evaluated the benefits of SSF process over to SHF 
technology and reported the cost based on economic 
evaluation of bioethanol keeping view the SSF mode of 
operation. Wooly et al. (1999) explained the further eco-
nomic analysis of bioethanol ( $ 0.78 per gallon) and 
suggested a projected cost of as low as $ 0.20 per liter by 
2015 if enzymatic processing and biomass improvement 
targets are met. The projected cost of ethanol production 
from cellulosic biomass as per the earlier estimates 
($4.63/gallon in 1980) has been reduced by almost a fac-
tor of four ($1.22/gallon) over the last 20 years (Wyman, 
1999; Wyman, 2001).  

However, Kadam et al. (2000) reported that ethanol 
production cost can be achieved at $ 1.20 per liter by us-
ing two-stage dilute acid hydrolysis process. Krishnan et 
al. (2000) worked out on economics of ethanol produc-
tion from dry-milled corn starch in fluidized–bed reactors 
using immobilized Z. mobilis cells. They analysed the 

cost of ethanol for 15 million gal/year production plant 



137       Afr. J. Environ. Econ. Manage. 
 
 

 
using Aspen Plus (Aspen Technology, Cambridge; MA) 
process simulation software and concluded that the 
operating cost savings of ethanol production in the range 
of 1.1 - 3.1 cents/gallon. Kwaiatkowski and Co- workers 
(2006) developed a model for cost evaluation of ethanol 
production from 40 million gal/year ethanol producing 
facility using corn dry-grind process technology. The 

model was developed using Super Pro Designer 
R

 soft-

ware and data collected from ethanol producers, techno-
logy suppliers, equipment manufactures and engineers 
working in the different industries. The cost of ethanol 
was found to be increased from US$ 0.235/L to US$ 
0.365/L as the price of corn increased from US$ 0.071 to 
US$ 0.125 /kg.  

The bioconversion of pentosans and hexosans from 
lignocellulosic biomass to ethanol need to be achieved 
with the maximum efficiencies in terms of higher yield and 
improved productivity to make the biomass-to-etha-nol 
process economical. The economics of the ethanol 
process is determined by the cost of sugar. The average 
biomass cost amounts to ~$0.06 per kg of sugar, or a 
contribution to the feedstock costs for ethanol production 
of as low as $0.10 per liter (Aristidou and Penttila, 2000).  

Wingren et al. (2003) evaluated the SHF and SSF 
economic using cellulase enzymes in both configurations 
with SSF being less expensive by about 10%; and esti-
mated the ethanol production cost of 0.56 – 0.67 $/L. 
Later, Wingren et al., 2004 studied the effect of reduction 
in yeast and enzyme concentration in SSF process and 
concluded final ethanol production cost 4.80 SEK/L (2.34 
US$/gal). According to the National Renewable Energy 
Laboratory (NREL, Colarado, USA) estimations, ethanol 
production cost of 20 cents per litre is possible in another 
15 years from lignocellulose biomass employing designer 
cellulases and SSCF (simultaneous saccharification and 
co-fermentation) process (Ghose and Ghose, 2003). 
However, in both the process, the use of cellulase makes 
the process cost effective (Mielenz, 2001; Alzate et al., 
2005; Gray et al., 2006). According to the analysis of US 
DOE (US Department of energy), if the enzyme cost can 
be brought for less than 10 cents per gallon of ethanol the 
cost of making ethanol could drop as low as 75 cents / 
gallon (Griffith and Atlas, 2005). Apart from focusing the 
economics of cellulase production cost, several studies 
are being carried out to improve the ethanol production 
by improving acid hydrolysis process. Luong and Tseng 
(1984) evaluated the technoeconomics of ethanol produ-
ction under continuous culture using immobilized cells of 
Z. mobilis using plug-flow reactor. They found this 
technology economically attractive and finally concluded 
that at least 4 cents/gal of ethanol could be saved using 
immobilized cells rather than the conventional batch sys-
tem. They further suggested that by switching from batch 
to immobilized processing; the fixed capital investment is 
substantially reduced, thus increasing the profitability of 
ethanol production by fermentation. Von Sivers et al. 
(1994) analysed the cost economics of ethanol producti- 

 
 
 

 
on (48 cents/gallon) from detoxified willow hemicellulosic 
hydrolysate using recombinant E. coli K011. Zacchi and 
Axelsson (1989) suggested preconcentrating dilute sugar 
solution using reverse osmosis is economically feasible to 
getting high ethanol concentration in fermented broth. 
Cysewki and Wilke (1976) described cell recycle and 
vacuum fermentation processes for continuous ethanol 
production on a production capacity of 78,000 gal ethanol 
/ day employing molasses as the fermentation substrate 
and estimated ethanol production cost 82.3 and 80.6 
cent/gal, for the cell recycle and vacuum processes, 
respectively. 

More recently Zhang (2006) analyzed the cost econo-
mics of ethanol production from a small- size lignocellu-
lose refinery with a capacity of 100 tones per day produ-
cing approximately 3 million gallons of ethanol plus co-
products. He estimated the cost of ethanol production 
~$1.00-1.20 per gallon. 

The distillation cost of per unit amount of ethanol 
produced is substantially higher at low ethanol concentra-
tions, the researchers have dealt with the idea of concen-
trating sugar solutions prior to fermentation (Cyweski et 
al., 1976; Oh et al., 2000; Iraj et al., 2002). Ethanol distil-
lation cost can be further improved by using membrane 
distillation process. It has the lowest operational cost, 
flexible, simple to use and is easy to maintain. It is most 
efficient and cost effective option among the available 
distillation processes (Gonsalves, 2006). 

 
Ethanol and Environment 
 
Ethanol represents closed carbon dioxide cycle because 
after burning of ethanol, the released carbon dioxide is 

recycled back into plant material because plants use CO2 

to synthesize cellulose during photosynthesis cycle 
(Wyman, 1999; Chandel et al., 2006a). Ethanol produc-
tion process only uses energy from renewable energy 
sources; no net carbon dioxide is added to the atmos-
phere, making ethanol an environmentally beneficial ene-
rgy source (Figure 5). In addition, the toxicity of the 
exhaust emissions from ethanol is lower than that of 
petroleum sources (Wyman and Hinman, 1990). Ethanol 
derived from biomass is the only liquid transportation fuel 
that does not contribute to the green house gas effect 
(Foody, 1988).  

As energy demand increases the global supply of fossil 
fuels cause harm to human health and contributes to the 
green house gas (GHG) emission. Hahn-Hagerdal (2006) 
alarmed to the society by seeing the security of oil supply 
and the negative impact of the fossil fuel on the environ-
ment, particularly on GHG emissions. The reduction of 
GHG pollution is the main advantage of utilizing biomass 
conversion into ethanol (Demirbas, 2007). Ethanol con-
tains 35% oxygen that helps complete combustion of fuel 
and thus reduces particulate emission that pose health 
hazard to living beings. A study conducted by Bang-Quan 
et al. (2003) on the ethanol blended diesel (E10 and E30) 



  
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 

 
Figure 5. Ethanol represents closed CO2 cycle. 

 

 
combustion at different loads found that addition of 
ethanol to diesel fuel simultaneously decreases cetane 
number, high heating value, aromatics fractions and 
kinematic viscosity of ethanol blended diesel fuels and 
changes distillation temperatures. These factors lead to 
the complete burning of ethanol and less emissions. With 
its ability to reduce ozone precursors by 20 - 30%, bio-
ethanol can play a significant role in reducing the harmful 
gasses in metro cities worldwide. Ethanol blended diesel 
(E-15) causes the 41% reduction in particulate matter and 

5% NOx emission (Subramanian et al., 2005; Chan-del et 

al., 2006a). One of the disadvantage in using ethanol as 
fuel is that aldehyde predominantly acetalde-hydes 
emissions are higher than those of gasoline. How-ever 
acetaldehydes emissions generate less adverse health 
effects in comparison to formaldehydes emitted from 
gasoline engines (Gonsalves, 2006). 

 
Environmental impact of bioethanol production 

technologies and their life-cycle assessment (LCA) 
 
Life-cycle assessment (LCA) is a conceptual framework 

and methodology for the assessment of environmental 
impacts of product systems on a cradle-to-grave basis 

(Graedel, 1999; Tan et al., 2002). Analysis of a system 
under LCA encompasses the extraction of raw materials 

 

 
and energy resources from the environment, the conver-
sion of these resources into the desired products, the 
utilization of the product by the consumer, and finally the 
disposal, reuse, or recycle of the product after its service 
life (Tan et al., 2002). The LCA approach is an effective 
way to introduce environmental considerations in process 
and product design or selection (Azapagic, 1999). Based 
on life cycle assessment (LCA) studies, ethanol produc-
tion technologies can be compared. Energy production 
and utilization cycles based on cellulosic biomass have 
near- zero green house gas emissions on a life cycle 
basis (Lynd et al., 1991). Biomass utilization into ethanol 
production offer environmental benefits in terms of nonre-
newable energy consumption and global warming impact. 
Kim and Dale (2005) studied LCA emphasizing corn and 
soyabean production and their utilization into bioethanol 
and biodiesel production and concluded that both the 
biofuels have environmental benefits in terms of nonre-
newable energy consumption and global warming impact. 
However biomass utilization into ethanol also tends to 
increase acidification and eutrophication, primarily becau-
se large nitrogen, phosphorus are released after cultiva-
tion of crops. Lechon et al. (2005) studied the LCA of 
ethanol production from wheat and barley grain and 
found that barley is a better option than wheat in terms of 
fulfillment of the green house gas emissions. 

Chandel  et al.               138 



139       Afr. J. Environ. Econ. Manage. 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 

 
Figure 6. Coordinated action for improvement of biomass to ethanol and its long term benefits. 

 

 
In an extensive study by Kadam (2000), LCA of acid 

and enzymatic hydrolysis was compared. All environ-
mental flows were examined from the product life cycle, 
its production and extraction from raw materials through 
intermediate conversion process, transportation, distribu-
tion and use. Dilute acid process was found better than 
the enzyme process in terms of greenhouse gas poten-
tial, natural resource depletion, acidification potential and 
eutrophication potential (Kadam, 2002). The reason is 
dilute acid process sends a much higher proportion of 
biomass to the boiler for electricity production, which in 
turn offsets large amounts of emissions (Kadam, 2000). 
Conversely the concentrated acid process is a net 
consumer of energy in terms of high acid load and reac-
tion temperature, results in very high values for green 
house gas (GHG) effect and other impact parameters 
(Kadam, 1999). Kemppainen and Shonnard (2005) com-
pared the energy consumption and environmental impact 
for ethanol production using timber wood and recycled 
news print. The news print conversion into ethanol has a 
slightly lower overall composite environmental index com-
pared to the timber process. However ethanol production 

 

 
from timber takes less energy, electricity and produces 
fewer emissions.  

A study conducted by Hu et al. (2004) revealed that the 
E-85 fueled vehicle is better vehicle than the gasoline 
fueled car by balancing of all the 3E”s the energy, envi-
ronmental and economic aspects. E-85 fuelled FFV 
(fossil fuelled vehicle) is about 15% higher efficient when 
compared to gasoline fuelled car. It also lowers the pollu-

tant emission viz. particulate matter, CO2, CO, NOx emis-

sion than gasoline fuelled car. E-85 fuelled vehicle is 
higher in total energy consumption and a good combined 
energy indicator. This was also in agreement with the 
recent life cycle-based (well to wheel) studies of fuel / 
propulsion alternatives for light–duty vehicles with a focus 
on lignocelluloses derived fuel ethanol by reducing 86% 
lower life cycle green house gas emissions as compared 
to the gasoline (Fleming et al., 2006). 

Tonan et al. (2006) has discussed the integrated asse-

ssment of energy conversion processes by evaluating the 
thermodynamic, economic and environmental parameters 
and found that water and air emissions of the plant 
producing ethanol are relatively low. The tansformity of 



Chandel  et al.               140 
 
 
 
ethanol (1.32 x 10

5
 seJ/J) is quite high if compared to that 

of fossil fuel (5.4 x 10
4
 seJ/J). This is even when 

bioethanol production is driven by a large amount of non-
renewable inputs (fertilizer, fuel, machinery). 
 
Conclusion 
 
In spite of laboratory based bioethanol success stories, 
the production of fuel ethanol at plant scale still remains a 
challenging issue. A positive solution to this issue could 
bring economic advantage not only for fuel and power 
industry, but also benefit the environmental rehabilitation 
and balance issues and cause. 

Worldwide, there is only one company, Iogen Corpora-
tion, Canada (http://www.iogen.ca), produce bio- ethanol 
at commercial scale using wheat straw and corn stover. 
In India, despite plentiful availability of biomass, there is 
no commercial ethanol production plant from lingocellulo-
ses. The key to the establishment of a commercial 
bioethanol production facility and the reduction in capital 
thereof, resulting in lessening of operating costs from 
each of the units of operations will be an achievement par 
standards of excellence and utility! Industry attention, not 
just the accolades is required for searching the answers 
to the fast paced fuel drain phenomena threatening to 
takeover into as a major crisis or even worse an econo-

mic depression by the end of 21
st

 century!  
For a flourishing bioethanol industry, government sup-

port is critical in correcting tax anomalies, exemption from 
excise and sales tax, deregulation of feedstock and its 
pricing and encouraging pilot projects and R&D work on 
bioethanol. Advances in pretreatment by acid catalyzed 
hemicellulose hydrolysis or employing an integrated appr-
oach in the form of consolidated bioprocessing with 
application of novel, tailored cocktails of enzymes for the 
cellulose breakdown coupled with the recent develop-
ment of genetically engineered microorganism those fer-
ment all possible sugars in biomass to ethanol at high 
productivity are the major key factors to make bioethanol 
program successful at commercial scale (Figure 6).  

The other important aspect by deploying the bioethanol 
option is its benefit to the environment. Ethanol is one of 
the best tools to fight vehicular pollution; its clean burning 
reduces the harmful gasses and particulate emissions 
that pose health hazard. The implementation of bioetha-
nol policy can be helpful in improving in envi-ronment and 
rural economic development with sustainable agricultural 
practices and enhancement of biomass feedstock cons-
cious usage towards the bioethanol industry will bring up 
the new age farmer into the limelight and horizon of 
activities and threshold of business to become renewed 
with options to deal better in life! A better farmer will 
ultimately usher in a better livelihood for one and all! 
 
ACKNOWLEDGEMENT 
 
We gratefully acknowledge the financial support from 

Universiti Malaysia Sabah,Kotakinabalu, Malaysia. 

 

  
 
 
 
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