Hemp as A potential bio-ethanol feedstock.

Abstract

The European Union directives mandating the use of 10% biofuels by 2020 makes it essential to find suitable agricultural crops and cropping techniques for compelling with such directives. Moreover, such crops should fit specific soil and climatic conditions. Keeping this goal in mind, it is worth considering traditional crops in a modern perspective, e.g. with alternative end uses than the traditional ones. In this context, the potential of different hemp genotypes has been studied for the supply of feedstock for bioethanol production.
Postponing hemp harvest until the beginning of seed formation, in contrast with the traditional harvest at full female flowering, produced around 15 Mg ha-1 of stems with a high content of cellulose (60% ca.) and hemicellulose (15% ca.). No significant differences between monoecious and dioecious genotypes were found. The calculated bioethanol production with such biomass yield can reach up to 4500 litres ha-1. At this ethanol yield, hemp could be competitive with the most acclaimed lignocellulosic crops, that, although achieving higher biomass yields, have lower cellulose content. The high ethanol yield potential along with the environmental benefits associated to the hemp cultivation, make this crop an interesting candidate for bioethanol production.

Full text

HEMP AS A POTENTIAL BIO-ETHANOL FEEDSTOCK
A. ZATTA1, G. VENTURI1
1Department of Agroenvironmental Science and Technologies, phone: +39 0512096692 / fax: +39 0512096241
e-mail: alessandro.zatta2@unibo.it
University of Bologna, Viale Fanin 44, 40127 Bologna, Italy
ABSTRACT: The European Union directives mandating the use of 10% biofuels by 2020 makes it essential to find
suitable agricultural crops and cropping techniques for compelling with such directives. Moreover, such crops should
fit specific soil and climatic conditions. Keeping this goal in mind, it is worth considering traditional crops in a
modern perspective, e.g. with alternative end uses than the traditional ones. In this context, the potential of different
hemp genotypes has been studied for the supply of feedstock for bioethanol production.
Postponing hemp harvest until the beginning of seed formation, in contrast with the traditional harvest at full female
flowering, produced around 15 Mg ha-1 of stems with a high content of cellulose (60% ca.) and hemicellulose (15%
ca.). No significant differences between monoecious and dioecious genotypes were found. The calculated bioethanol
production with such biomass yield can reach up to 4500 litres ha-1. At this ethanol yield, hemp could be competitive
with the most acclaimed lignocellulosic crops, that, although achieving higher biomass yields, have lower cellulose
content. The high ethanol yield potential along with the environmental benefits associated to the hemp cultivation,
make this crop an interesting candidate for bioethanol production.
Keywords: Biomass, energy, harvest, lignocellulosic.
1 INTRODUCTION
The recent directives from the European Union [1; 2]
mandate the use of 10% biofuels by 2020. The aims of
this provision are to guarantee lower greenhouse gases
(GHG) emissions and less dependency from fossil fuels
imports, an improvement on supply security,
technological development and innovation, as well as
providing jobs and regional development, especially in
rural and isolated areas. Biofuels use in road haulage is
considered one of the most effective tools for reducing
the greenhouse effect and the dependency on oil imports,
which currently account for around 99% of the energy
used in this sector.
Among the alternatives to fossil fuels for road
haulage, are the biofuels from energy crops that could be
suitable for specific areas and particular environmental
conditions. An increase of 14.7 million hectares of
energy crops is foreseen by 2010 and up to 25.1 million
hectares by 2030 [3].
Producing ethanol from ligno-cellulosic crops - the so
called 2nd generation biofuels - seems to ensure a higher
yield per surface area than 1st generation biofuels, thus
requiring less lands than starch or sugar crops to produce
the same amount of bioethanol. In the last years many
studies have been conducted on annual [4; 5; 6; 7] and on
perennial ligno-cellulosic crops [8; 9], yet the most
suitable combination of crop/management/environment is
still uncertain. There is therefore the need of further
studies to support decision makers in their choices of
energy crops. In this context, the aim of this study was to
evaluate the potential of different hemp genotypes to
produce feedstock for bioethanol in Northern Italy.
2 MATERIAL AND METHODS
2.1 Field trial
The potential of three hemp genotypes, one
monoecious (Futura 75) and two dioecious (C.S. and
Fibranova), for bioethanol production was studied during
three years 2006-2008. In the first year hemp was
harvested only at the beginning of the female flowering
(31 July), which corresponds to growth code 2102 [10],
while in the last two years hemp was harvested at two
different growth stages; one harvest was at full male
flowering stage (H1), code 2102 [10]; and the other one
was at beginning of seed maturity (H2), code 2203 [10].
The field trials were carried out at the experimental
station of the University of Bologna (32 m a.s.l.; 44°33’
lat.; 11°21’ lon.). In 2006, only one factor, the genotype,
was compared, and plots were arranged according to a
complete randomized design. In 2007 and 2008 a split-
plot design with three replicates was adopted to compare
two factors: genotype (main) and harvest time (sub).
Soil was udifluventic Haplustepts fine silty, mixed,
superactive, mesic in all three trial years (USDA
classification). Sowing was done in 05/04/06, 10/04/07,
and 19/03/08, at a row distance of 20 cm; seeds were
sown 3-4 cm deep. Nitrogen fertilization (60 kg ha-1) was
applied before sowing [11].
Table I: Main soil chemical and physical
characteristics during the three-year trial
Soil characteristics 2006 2007 2008
Sand (%) 36 26 42
Silt (%) 41 49 37
Clay (%) 23 25 21
Organic Matter (%) 1.6 1.4 1.5
pH (in H
2
O) 7.3 7.9 7.1
In the first year, plots were 490 m2 (70 m x 7.3 m).
From the second year onwards the main plots had the
same dimensions as in the first year with the two harvests
set as the sub plots. Two square meters per plot were
harvested in order to estimate the total biomass and plant
density. Thirty representative plants per plot were
selected, twenty of which were separated into stems and
leaves (when present, inflorescences were added to the
leaf part). The samples were oven dried at 105 °C and
then weighed in order to determine the dry matter
content. The remaining ten plants were oven dried at
60°C and conserved for later chemical analysis.
2.2 Chemical composition
The stem chemical composition in cellulose,
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hemicellulose and lignin was determined. Samples were
first grounded (1 mm grids) with a hammer mill, and then
cellulose, hemicelluloses and lignin content were
determined though acid detergent fibre (ADF), acid
detergent lignin (ADL) and neutral detergent fibre (NDF)
as given by Goering and Van Soest [12].
3 RESULTS
Fresh harvested hemp biomass averaged around 45
Mg ha-1 with no significant differences among years,
harvesting times, genotypes, and their interactions (Table
II).
Table II: Main production parameters: Total Fresh
Weight (TFW), Total Dry weight (TDW), Stem Dry
Weight (SDW) and Leaf Dry Weight (LDW) measured in
two harvest times (H1 and H2) and three years.
2006 2007 2008
parameters H1 H1 H2 H1 H2
TFW (Mg ha-1) 44,1 42,7 45,8 46,1 42,1
TDW (Mg ha-1) 15,3 13,6 15,8 13,5 18,5
SDW (Mg ha-1) 12,4 11,2 14,0 11,8 16,5
LDW (Mg ha-1) 3,0 2,3 1,7 1,8 2,0
Stems (%) 80,4 82,7 89,1 87,0 88,7
Leaves (%) 19,6 17,3 10,9 13,0 11,3
Height (cm) 193,8
247,3
288,1
294,3
336,0
Diameter (mm) 6,0 9,2 10,1 11,8 13,2
In the first two years, dry biomass accounted for
almost 35% of total fresh biomass and it was significantly
lower in H1 than H2 (30% and 45%, respectively). The
time between full male flowering and beginning of seed
maturity in 2008 was 19 longer than in 2007 and
therefore allowed hemp to accumulate more biomass.
Weight increase between the two harvests (H1 and
H2) was 16% in 2007 and 37% in 2008. Leaf weight was
between 2 and 3 Mg ha-1 representing 13-20% of total
biomass yield for H1 and slightly more than 10% for H2.
Stems (the profitable part in agronomic terms) yielded
around 12 Mg ha-1 when hemp was harvested at full male
flowering (H1). Delayed harvest (H2) allowed a yield
increase from 2.8 in 2007 to 4.7 Mg ha-1 in 2008, that is
25-40% of total biomass. For each day of delay in
harvesting, stem weight increase was 78 grams d-1 in
2007 and 70 grams d-1 in 2008, due to both an increase in
stem height (around one centimetre per day) and stem
diameter. These results confirm those previously obtained
by Venturi [13]. However, yield increases did not
generally offset the lower fibre quality [13], so from the
textile industry point of view the best harvest time should
be when female plants reach the fully blooming stage
[14; 15; 16]. But there are still many challenges ahead to
considered before hemp is fully utilized as an energy
crop. For bioethanol production, for example, the high
level of cellulose and hemicellulose present in hemp are
of interest, whereas the lignin content represents a
negative element since it decelerates the fermentation
process [17; 18]
Importantly, this study showed that cellulose content
can strongly increase by delaying the time of harvest.
Specifically, cellulose yield was about 40% higher (3 Mg
ha-1) in H2 compared to H1. According to this, and taking
ethanol conversion coefficients given by Badger [19],
bioethanol yield were 2000-2500 l ha-1 in H1 and 3000-
3500 l ha-1 in H2. Considering also hemicellulose the
total amount of bioethanol would be 2500-3200 l ha-1 in
H1 and 3800-4500 l ha-1 in H2. With delayed harvest the
increase was around 35-45%.
Table III: Stems chemical composition and
potential ethanol yield at two harvesting periods (H1 and
H2). Since genotype X harvest time interaction was not
significant, average values of monoecious and dioecious
genotype are presented
2006
2007 2008
Parameters H1 H1 H2 H1 H2
Cellulose (%) 63 58 65 61 62
Hemicellulose (%) 17 13 12 16 16
Lignin (%) 9 10 9 9 10
Cellulose (Mg ha-1) 8 7 9 7 10
Hemicellulose (Mg ha-1) 2 1 2 2 3
Lignin (Mg ha-1) 1 1 1 1 2
Ethanol from cellulose (l ha-1) 2710
2299
3164
2494
3544
Ethanol from hemicellulose (l ha-1)
732 501 602 650 959
Total ethanol (l ha-1) 3442
2799
3766
3144
4503
4 DISCUSSIONS
Though delaying harvest can significantly affect the
fibre quality for textile use [20; 21; 16], this could be of
secondary importance for bioethanol end use. Therefore,
late harvest time could be worthwhile when hemp is
processed for ethanol as late harvest is generally
associated to a higher biomass production. In this study,
irrespective of genotype used, about 40% higher biomass
yield was reached through a late harvest, a result which is
also corroborated by previous findings [13]. This biomass
production seems still not to be competitive with other
biomass crops for thermo-electrical conversions [22], but
it would be very competitive for 2nd generation ethanol,
due to the higher cellulose content of hemp compared to
ligno-cellulose crops [6; 23; 24]. But delayed harvesting
also brings to an increase in lignin content. The presence
of lignin and hemicellulose it may create difficulties
during the processing phase because makes the access of
cellulose enzymes to cellulose difficult, thus reducing the
efficiency of the hydrolysis [23]. Nevertheless, the non
carbohydrate components of lignin have potential for use
in value-added applications such as organosol lignin used
to produce PF resin [25].
Late harvest time could be also desirable for the
lower biomass moisture content and the less presence of
leaves, which is considered useful either for stalk
chipping and drying. This is considered as an advantage
both in the case of direct chipping the standing plants and
in the case of mowing and leaving the stalks to dry in the
field for further windrowing and chipping, or even in the
case that the stems are mowed, dried in the field and
subsequently baled. The last two options allow to reduce
the time necessary for stems to dry up. Moreover such
harvesting methodologies favours the ratio of dry matter
over the total biomass volume, increasing the energy
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value and reducing the volumes and transportation costs.
During the three-year trial, no significant differences
in biomass production have been found between
monoecious and dioecious genotypes that have medium
and late growing cycles respectively. However, under the
environmental conditions where the present study has
been carried out, it may be advisable to use late growing
cycle genotypes that would allow delayed harvest until
the first half of September when autumn rains still have
not started and thus leaving enough time for soils
preparation for the subsequent cereals crops. The local
genotypes used in this study do not show pre-flowering
phenomena which induces to earlier harvest and lower
yields [26; 27; 28; 16].
The other important agronomic and environmental
aspects related to hemp cultivation must also be
highlighted. Hemp has been traditionally used as a crop
capable of enhancing soil condition and therefore it is
advisable to include it in crop rotations with cereals
cultivated either for food or fuel production. Hemp can
improve soil structure, thanks to its deep root system
[29], and above all, it can help to the control of weeds in
subsequent crops [30; 31]. Moreover, the characteristic
fast growing of hemp at early growth stages gives hemp
the advantage over weeds to fully utilize the light and soil
resources [32; 33]. Hemp cultivation needs little chemical
inputs since it does not have high fertilisation
requirements [34; 6]. Moreover, pesticides are not
normally used in hemp cultivation because the low
incidence of insects and disease [13; 35]. This latter
aspect, added to those previously described in this paper
makes hemp a very sustainable crop. The main constrain
of this crop still remains the uneconomical extraction
process and conversion (e.g. pre-treatment process and
hydrolitc enzymes) of cellulose and hemicellulose into
ethanol [23], however recent results would suggest that
this issue could be overcome soon [36].
5 ACKNOWLEDGEMENTS
This study was founded by the “Interregional
Programs” (L. 499/1999), under the approval of the
interregional committee lead by the Friuli Venezia Giulia
Region.
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