Environmental Engineering Reference
In-Depth Information
an area of 2.2 million ha with production of 1.50 million t in India in 1998. In the Punjab State of
this country, approximately 67,000 ha of land was under sunflower cultivation with a production
of 90,000 t in 1999 (Anonymous 1999). Sunflower seed is the third-largest source of vegetable
oil worldwide, followed by soybean and palm. Sunflower oil, extracted from the commercially
available sunflower varieties containing 39-49% oil in the seed, is used for cooking, as carrier
oil, and for production of biodiesel. Among oil crops, sunflower has the highest yield of 33% that
has been exploited for biodiesel production as B100 diesel that has been claimed to have numerous
advantages over petroleum diesel: it is renewable, nontoxic, biodegradable, and produced by U.S.
and Canadian farmers. B100 biodiesel is 100% biodiesel fuel and reduces greenhouse gas emissions
by 78.3%, particulate matter by 55.4%, hydrocarbons by 56.3%, mutagenicity by 80-90%, and sulfur
by 100% (Biofuel Industries 2002). On December 20, 2008, Nishi-Nippon Railroad Co. Ltd. held
successful trial rides of a bus powered by biodiesel fuel (BDF) derived from sunflower seeds (Asia
Biomass Office 2008). After extraction of oil, a huge quantity of sunflower stalks (after harvesting
of seeds) and sunflower hulls (during the industrial processing of sunflower seeds) are generated
that do not find any suitable end use and are generally burnt in the fields, causing environmental
pollution (Sharma et al. 2002a; Okur and Saracoglu 2006). Therefore, sunflower stalks and hulls,
as lignocellulosics, afford a renewable and low-cost raw material for the production of bioethanol.
30.3 lIGnocellulosIc BIomass conversIon
The research work on the bioconversion of lignocellulosics to liquid fuels has attained new
dimensions today. The structure of these materials is very complex, and native biomass is resistant
to enzymatic hydrolysis. The steps for the production of fuel ethanol from lignocellulosic biomass
involve feedstock preparation, pretreatment, cellulase production, acid/enzymatic hydrolysis,
ethanol fermentation, and ethanol recovery. Although each of the above steps has been extensively
studied, the processing techniques required for ethanol production from lignocellulosic materials
are presently extensive and costly. Currently, the utilization of cellulosic biomass to produce fuel
ethanol presents significant technical and economic challenges, and the success of the process
depends largely on the development of environmentally friendly pretreatment procedures, highly
effective enzyme systems for conversion of pretreated biomass to fermentable sugars, and efficient
microorganisms to convert sugars to ethanol (Gray et al. 2006; Mojovic et al. 2006).
30.4 Pretreatment
In general, among lignocellulosics cellulose is a linear polymer of glucose associated with
hemicellulose and surrounded by a lignin seal. A lignin seal around cellulose microfibrils and its
limited covalent association with hemicellulose prevents enzymes and acid from accessing some
regions of the cellulose polymers. The potential formation of six hydrogen bonds (because of the
β-1,4 orientation of glucosidic bonds in cellulose) adds to its crystallinity and further impedes acid
or enzymatic hydrolysis (Weil et al. 1994)
.
The goal of any pretreatment technology is to alter or
remove structural and compositional impediments to hydrolysis to improve the rate of enzyme
hydrolysis and increase yields of fermentable sugars from cellulose or hemicellulose. These methods
cause physical and/or chemical changes in the plant biomass to achieve this result. Experimental
investigation of physical changes and chemical reactions that occur during pretreatment is required
for the development of effective and mechanistic models that can be used for the rational design of
pretreatment processes. Furthermore, pretreatment processing conditions must be tailored to the
specific chemical and structural composition of the various (and variable) sources of lignocellulosic
biomass (Mosier et al. 2005). Recent studies indicate that cellulose digestibility is directly correlated
with lignin and hemicellulose removal (Kim and Holtzapple 2006).
Several pretreatment techniques have been reported. These include physical methods such as ball
milling, hammer milling, boiling, high-pressure steam, electron irradiation, wetting, γ-irradiation,
