AGRICULTURAL
SYSTEMS
Agricultural Systems 94 (2007) 586–592
www.elsevier.com/locate/agsy
Energy from agricultural residues and consequences
for land requirements for food production
Sanderine Nonhebel
*
Center for Energy and Environmental Studies, University of Groningen, Nijenborgh 4, 9747 AG Groningen, The Netherlands
Received 8 June 2006; received in revised form 21 December 2006; accepted 2 February 2007
Abstract
Using biomass as an energy source is often mentioned as an option to mitigate the enhancing greenhouse effect. Biomass for energy
purposes can be obtained from dedicated energy crops and/or from agricultural residues. The available amount of residues is large and
suggests a significant energy potential. However, most of these residues are currently used as livestock feed, which forms the basis for
important proteins in the human diet. Use of residues for energy generation is likely to affect the supply of proteins in human diet,
and therefore adaptations in the food system are required to compensate for this loss.
The purpose of this paper is to indicate the adaptations required in the food system, if agricultural residues are used for energy generation instead of livestock feed. For this, three production systems that generate both protein and energy are compared. The difference
consists in their production routes followed: (i) energy crops for energy and agricultural residues for feed, (ii) use of agricultural residues
for energy and growing protein crops (e.g. beans and pulses) for a vegetarian human diet and (iii) use of agricultural residues for energy
and growing feed crops (e.g. wheat) for livestock. The land requirements of the three systems are calculated.
The system in which energy crops were used for energy generation and agricultural residues as feed required the smallest amount of
land. From a land use perspective, therefore, it is better to produce energy from dedicated energy crops and use agricultural residues for
livestock feed.
Ó 2007 Elsevier Ltd. All rights reserved.
Keywords: Agricultural residues; Bio-energy; Food production; Protein production; Land use
1. Introduction
Using biomass as an energy source is one of the options
to mitigate the enhancing greenhouse effect. Biomass for
energy can be obtained via different routes, most significantly from energy crops and agricultural residues. Use
of energy crops implies use of plant material obtained from
crops specially grown for this purpose, such as short rotation forestry systems and rape seed crops for bio-diesel.
Use of agricultural residues implies use of plant material
generated as by-products in the food production system,
that is in agricultural production as well as in the food
industry. The total volume of these agricultural residues
*
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0308-521X/$ - see front matter Ó 2007 Elsevier Ltd. All rights reserved.
doi:10.1016/j.agsy.2007.02.004
is large (Elzenga et al., 1996). Potential energy production
from these residues is estimated at 190 PJ (PetaJoule = 1015 J) in the Netherlands (Faaij et al., 1997), whilst
on a global scale values of over 12 EJ (ExaJoule = 1018 J)
are mentioned (Hall et al., 1993).
However, agricultural residues are usually not neglected,
but reused as livestock feed. In the Netherlands about 70%
of the concentrates fed to pigs, cattle and poultry originate
from residues generated by the food processing industry
(LEI, 1996). Comparable values are found on a global scale
(Fadel, 1999). It can be said that this livestock is (more or
less) upgrading a waste stream unsuitable for human consumption into highly valued food commodities such as
meat, milk and eggs. Therefore, agricultural residues do
not represent a worthless waste stream but an important
source for the food production system (as a basis for
S. Nonhebel / Agricultural Systems 94 (2007) 586–592
protein). This protein is essential in the human diet and
cannot be left out without affecting the food quality of
the human population.
Using residues for energy generation is likely to affect
the protein supply in the human diet, thus the food system
requires adaptations in order to compensate for this loss.
The magnitude of such adaptations has a significant impact
on the overall performance of the system. If large amounts
of energy are gained from residues and protein loss can easily be compensated for, use of residues for energy generation will have a large potential. However, if the impact of
adaptations overrules the energy gain, using residues for
energy generation is not a realistic option. The purpose
of this paper is to determine the adaptations required in
the food system if agricultural residues are used for energy
generation instead of livestock feed.
2. Methods
2.1. System description
The present food production system is shown in Fig. 1.
Primary production is responsible for growth of food
crops. Harvests from primary production are converted
into food items in the food industry (e.g. wheat into bread
and pasta, soybeans into soy oil, sugar beet into sugar),
which are then consumed by the human population. Since
most crops cannot be consumed as a whole residues are
generated in this production process. These originate in
various parts of the production chain: during primary production itself, where only a part of the crop is sold to the
food industry (e.g. wheat is sold, straw remains), and in
the food industry, where only a part of the harvested product is used (e.g. sugar is sold, beet pulp remains). At present
remains of the human food production process are used as
a basis for livestock feed, fed as unprocessed residue
directly to livestock or processed (in the livestock feed
industry) into concentrates. Livestock production results
in meat, milk and eggs, which are important sources of pro-
tein in the human diet. The present food production system
shows no interference with the energy system (Fig. 1). In
the energy system primary energy sources (e.g. coal, oil,
gas) are converted into energy carriers (electricity and petrol) used in societies to fulfil the needs for heat and power.
In Western societies fossil fuels provide over 95% of total
energy requirements. The remainder is fulfilled by renewable energy sources such as solar energy, wind energy,
hydropower and biomass. When biomass is used as a
renewable energy source it is obtained from forest residues
or from (annual) crops grown for this purpose (rape seed).
In the discussion on use of agricultural residues for
energy generation only a small part of the food system
(the residues and meat production) and energy system
(energy obtained from biomass) are affected. In this paper
we only quantify changes in the affected part of the system,
and assume that the rest of the system remains the same.
Fig. 1 also shows the parts of the system studied in detail
in this paper. The research focuses on residues in the food
system, determines gains in the energy system and then
compares it with losses in the food system. This is accompanied by complications however, as gains in the energy
system are calculated in GJ energy and losses in the food
system in kg protein. Therefore, comparison of both systems is based on land required to compensate for not using
the residues. In the energy systems non-use of residues
implies that land is required for growing energy crops,
whereas in the food system it implies that extra land is
needed for the production of protein lost in the human diet.
Here two options are analysed: in the first, meat is replaced
by vegetable products and extra land is required to grow
protein crops for human consumption. In the second meat
remains in the diet and extra land is needed for the production of livestock feed crops.
Agricultural
Residues
Food
Food system
587
Energy
Energy system
Energy crops
Primary
production
Renewable
energy sources
Fossil fuels
Residues
Food
industry
Energy conversion
Conversion into
meat
Conversion into
energy carriers
Protein
Energy
Livestock
Food Consumption
Energy Consumption
Fig. 1. Schematic presentation of the present food and energy system. The
oval shows the part of the system studied in the paper.
Fig. 2. Use of agricultural residues in the energy crops system and
adaptation in the energy system.
588
S. Nonhebel / Agricultural Systems 94 (2007) 586–592
Fig. 2 shows the present situation with respect to residue
use in more detail (energy crop system). Agricultural residues are used as livestock feed and contribute to meat production. The energy system and the food system are
independent. When biomass is used as energy source, the
biomass is obtained from energy crops.
Fig. 3 shows the vegetarian system, in which residues are
removed from the food system for energy generation purposes. Meat is replaced by vegetable products based on
beans. In this system less land is required to produce bioenergy, but extra land is required for growing protein crops
(e.g. beans and pulses).
In Fig. 4 the livestock feed system is represented. Residues are used for energy generation and extra land is
required for the production of livestock feed. The human
diet remains unaffected.
Comparison of the three systems reveals that the food
and energy system are no longer independent. In the vegetarian and livestock feed system savings are achieved in the
energy system, as less forests and/or energy crops are
required for bio-energy. However, compensation in the
food system is needed to fulfil the food consumption
requirements of the population. Hence gains in the energy
system (less energy crops) involve losses in the food system
(more feed crops). Analysis of the magnitude of gains and
losses in the total system is essential for evaluation of the
sustainability of the options studied.
2.2. Quantifying the flows in the three systems
A wide variety of agricultural residues are used as livestock feed. In this paper only the largest waste-streams
from the food industry are analysed. In the Netherlands
these are oilseed cakes (from vegetable oil production)
Agricultural
Residues
Food
Energy
Protein crops
Conversion into
energy carriers
Protein
Energy
Fig. 3. Use of agricultural residues in the vegetarian system and
adaptation in the food system.
Agricultural
Residues
Food
Energy
Feed crops
Conversion
into meat
Conversion into
energy carriers
Protein
Energy
Fig. 4. Use of agricultural residues in the feed crop system and adaptation
in the food system.
and molasses from sugar production, representing 34%
and 25% of the total agricultural residues, respectively
(Meeusen-van Onna et al., 1998). Oil seed cakes and molasses are presently in use as livestock feed. Using these residues for energy production implies trade-offs in the meat
production system, since a source of livestock feed disappears. Using oil seed cakes and molasses for energy generation therefore requires growth of additional protein crops
or an increase in feed crop production.
In this paper we compare the area required for these
additional protein crops and/or feed crops with the area
reduction in energy crops in the energy system. This is
accomplished on a per capita basis. It is assumed that residues are fed to pigs.
The available residues per capita are determined. When
these are fed to pigs they result in a certain amount of pork.
When residues are used for energy generation they result in
a certain amount of energy, but then beans have to be
grown to compensate for protein losses in the human diet,
or extra feed crops to compensate for loss of livestock feed.
The use of residues for energy, however, implies that less
land is needed for growing energy crops.
2.2.1. Magnitude of available residues
The food industry sector in the Netherlands is large, it
imports crops from all over the world and convert them
into food products (vegetable oil etc.). A large share of
these food products is exported again, but residues remain
in the Netherlands. The available residues on the market
have therefore little to do with the Dutch consumption.
To avoid double counting only residues generated for
Dutch consumption are taken into account.
In the Netherlands 40 kg sugar and 30 kg vegetable oil
are consumed per person per year (Catsberg and Kem-
S. Nonhebel / Agricultural Systems 94 (2007) 586–592
589
pen-van Dommelen, 1997). Sugar is obtained from sugar
beet, and vegetable oil is mainly extracted from soybean
(LEI, 2000). At present 1 Mg sugar beet produces 140 kg
sugar, 58 kg dried pulp and 40 kg molasses (Maassen and
van Swaaij, 1999). Thus consumption of 40 kg sugar results
in 11 kg molasses and 17 kg dried pulp (or 68 kg pressed
pulp). As the oil content of soybean is 20%, consumption
of 30 kg oil generates a waste-stream of 120 kg soybean
cake.
scentrum, 1997), 1 kg meat therefore includes 0.22 kg protein. These protein can be provided by beans and pulses, as
the protein content of these crops is similar at about 20%
(Voedingscentrum, 1997). Hence, with respect to protein
content, 1.1 kg beans are equivalent to 1 kg pork. This
implies that 36 kg beans have to be produced in order to
compensate for the loss of animal protein in the diet. Average yield of beans is 3.0 Mg/ha (LEI, 2000), therefore
120 m2 are required for 36 kg beans.
2.2.2. The amount of meat that can be produced from
these residues
These residues can be used as feed for pigs. The amount
of feed required for production of a certain amount of
meat depends on the nutritional value of feed. When low
quality feed is provided, more kgs of feed are required than
with high quality feed. In the Netherlands the quality of
feed is expressed in so-called Ew-values. This value indicates to what extent the material is suitable as pig feed
and whether the composition of the material (notably carbohydrates, amino acids, fats, micronutrients) is in accordance with the nutritional requirements of pigs. For all
residues presently used as feed their nutritive value
expressed in Ew/kg can be obtained from ‘Dutch livestock
feeding tables’ (CVB, 2001). High quality feed has values
close to 1 Ew/kg and low quality feed has values around
0.3 Ew/kg. In CVB (2001) the feed requirements (expressed
in Ew) over the total lifespan of a pig are given. When
losses in the production process (slaughtering) are
included, an average of about 4 Ew are needed for each
kg of pork (Elferink, 2000). Table 1 shows the quality of
residues considered as feed for pigs and the total amount
of feed (expressed in Ew) that can be produced from agricultural residues that accompany sugar and vegetable oil
consumption. The production of 30 kg vegetable oil and
40 kg sugar results in 17 kg dried pulp (or 68 kg pressed
pulp), 11 kg molasses and 120 kg soybean cake with a total
‘feeding value’ of 132 Ew, which is sufficient to produce
33 kg pork.
2.2.4. Wheat as compensation for feed loss
For the feed crop system, feed crops worth 132 Ew must
be grown. The most frequently used feed crop is wheat,
which has a feeding quality of 1.1 Ew/kg wheat (CVB,
2001). Therefore 120 kg wheat must be grown to make
the systems comparable. Wheat yields are 7.0 Mg/ha
(LEI, 2000), requiring an area of 170 m2 for livestock feed
production.
2.2.3. Beans as compensation for protein losses in the human
diet
For the vegetarian system, nutrients in meat have to be
replaced by vegetable alternatives to make the systems
comparable. The protein content of pork is 22% (Voeding-
2.2.5. The value of residues as an energy source
Agricultural residues can also be used for energy generation. We assume that the material is combusted and heat
is used for electricity generation. This implies that the residues replace part of the coal and gas presently used as
feedstock for electricity generation. Meeusen-van Onna
et al. (1998) gives an overview of heating values of agricultural residues available in the Netherlands. Dried beet pulp
is not used as an energy source, since it requires energy to
dry it. Hence in this paper, data for pressed beet pulp is
used. As pressed beet pulp contains more water, 1 kg sugar
beet results in more kg pressed beet pulp than dried beet
pulp. Table 1 (also) shows the quality of residues as an
energy source. Total heat energy obtainable from these residues is 2.2 GJ per person.
2.2.6. Wood as energy source
In the energy crop system energy is obtained from willow short rotation forestry systems. The heating value of
willow wood is 18 MJ/kg (Hall et al., 1993). To make systems comparable, 2.2 GJ biomass heat energy has to be
produced, which would imply production of 121 kg wood.
Since short rotation forestry crops are not yet grown on a
large scale, no statistical data on yields are available. Estimates in literature show an enormous variation, ranging
from 2 Mg/ha to over 60 Mg/ha (Berndes et al., 2003;
Table 1
The amount of residues resulting from the production of 30 kg vegetable oil and 40 kg sugar
Residue
Quantity (kg)
Quality as feed (Ew/kg)
Dried beet pulp
Pressed pulp
Molasses
Soybean-cake
17
68
11
120
1.0
–
0.7
0.9
Total on waste-streams
Amount of feed (Ew)
17
7.7
108
132
Quality as energy source (MJ/kg)
Amount of energy (MJ)
–
2
12
16
136
132
1920
2188
The quality of these residues as feed and as energy source and the total amount of livestock feed and energy that can be obtained from these residues.
590
S. Nonhebel / Agricultural Systems 94 (2007) 586–592
Schelhaas and Nabuurs, 2001). Potential production of
these crops is calculated to be 10–20 Mg/ha in the temperate climate regions (Nonhebel, 2002; Hoogwijk et al.,
2003). In this paper we assume willow yield to be 15 Mg/
ha, implying that 80 m2 are needed for growing 2.2 GJ.
3. Results
With the data derived above land requirements in the
systems can be calculated (Table 2). The three systems all
produce 7.3 kg of protein and 2.2 GJ bio-energy each. In
the energy crops system (Fig. 2) 80 m2 willow in a short
rotation system is grown for bio-energy, and residues are
used as livestock feed resulting in 33 kg pork. In the vegetarian system (Fig. 3) 33 kg meat is replaced by 36 kg
beans, which requires 120 m2. Residues are used as a bioenergy source capable of generating 2.2 GJ energy. In the
livestock feed system (Fig. 4) residues are again used for
energy generation (2.2 GJ), but now additional feed crops
are grown (120 kg wheat on an area of 170 m2).
Table 2 shows the area required for production of protein and energy in the three systems. No land is attributed
to agricultural residues, since it is assumed that they are byproducts of sugar and vegetable oil consumption. This
implies that in the vegetarian and feed crop system no land
is attributed to energy and in the energy crop system no
land is attributed to meat production. The production of
36 kg beans in the vegetarian system requires 120 m2. In
the energy crops system 80 m2is needed for the production
of 2.2 GJ energy (121 kg wood), and the production of
120 kg wheat in the feed crop system requires 170 m2.
The differences that occur between the systems are large:
the energy crop system and the feed crop system produce
the same amounts of commodities (energy and pork), but
the feed crop system requires nearly 100 m2 more to do
so. The vegetarian system also requires a larger area than
the energy crops system (40 m2).
4. Discussion
Results show that when dedicated energy crops are
grown, less land is needed to produce bio-energy than the
additional area of land needed to compensate for protein
losses in the food system. The value of the area needed,
Table 2
Comparison of area required for producing proteins (33 kg pork or 36 kg
beans) and 2.2 GJ energy in the three different food-energy production
systems
Energy crop system
Vegetarian system
Feed crops system
33 kg pork
121 kg wood
RES
80 m2
2.2 GJ energy
36 kg beans
RES
120 m2
2.2 GJ energy
120 kg wheat
RES
170 m2
Total
80 m2
Total
120 m2
Total
170 m2
In the energy crops system residues (RES) are fed to pigs and energy is
grown, while in the vegetarian and feed crop system residues (RES) are
used for energy generation and beans and wheat are grown for the protein
production.
however, is strongly dependent on the assumed yields per
hectare. When yields are lower, a larger area is needed
and vice versa. Yields for wheat and beans are obtained
from agricultural statistics. Yield for the willow crop is
derived from crop growth simulation models. The estimate
for the willow crop, however, plays a crucial role in this
analysis. When yields are 50% of the value assumed here
(7.5 Mg/ha), the area needed to grow energy doubles
(160 m2). In this case using residues for energy generation
in combination with a vegetarian diet requires the smallest
amount of land. The value of the willow crop used here
needs to be discussed with respect to yields of the wheat
crop and bean crop.
Wheat yield is much higher than the yield of the bean
crop. This is caused by their different crop characteristics.
First, the growing season of a bean crop is shorter: it
requires only 5 months from sowing to harvest, while a
wheat crop requires over 8 months. A shorter growing season implies that less solar radiation is intercepted, which
results in a smaller yield. Second, the chemical composition
of the harvested plant material is different, wheat grains
mainly consist of carbohydrates, while beans also contain
protein and fats. The production of protein and fats in
the growth respiration process requires up to 3 times as
much glucose as the production of carbohydrates. The
potential production of a wheat crop is therefore higher
than of bean crop. The harvest of a willow crop (wood)
consists of cellulose and this crop has a long growing season. Willow crop characteristics illuminate that wood production requires similar amounts of glucose as for wheat.
On basis of its crop characteristics, therefore, one would
expect that willow will reach higher yields. The estimated
15 Mg in this paper seems very large in comparison with
the wheat yield of 7 Mg/ha. However, one should realize
that an additional 7 Mg of straw is generated from this system. The total biomass production (grains and straw) of a
wheat crop is therefore 14 Mg/ha. This falls in the same
range as the willow crop yield used for this analysis
(15Mg/ha). Values used in this paper involve production
of biomass (food/energy) in intensive production systems
in Western Europe (using fertilizer, pesticides and other
external inputs). In other systems different yields will be
obtained, but the relative differences between the crops
(wheat, beans, willow) remain the same.
4.1. Scaling up: consequences of findings for the
food-energy-system enlarge
The purpose of this paper was to analyse whether use of
agricultural residues as an energy source has an impact on
the food system, and to estimate the magnitude of this
impact. This was done by analysing a very small part of
the system, for use in a general analysis the data found
needs to be put into a national context.
First it is important to realize that for clarity purposes
values were calculated on a per capita basis, resulting in
small values, e.g. 33 kg, 120 m2 and 2.2 GJ. These small
S. Nonhebel / Agricultural Systems 94 (2007) 586–592
values per capita, however, distort from the fact that at
national level amounts are huge. Differences between
80 m2 and 170 m2 per capita corresponds on a national
level to nearly 144 000 ha, The same accounts for meat
and energy, where 33 kg meat and 2.2 GJ per person equals
530 000 Mg meat and 35 PJ on the national level.
Next to this only a part of all residues from the food
industry is studied, namely the two largest shares: pulp
and molasses from the sugar industry and soy bean cake
from the margarine, oil and fat industry, accounting for
more than 50% of total residues from the food industry.
Smaller quantities of residues are generated in bakeries,
breweries and the potato chips industry. The nutritional
value of these residues falls in the same range as residues
analysed in this paper, i.e. 0.9–1.0 Ew/kg (CVB, 2001).
The total amount of high nutritive value residues is twice
as high as calculated in this paper. Furthermore, residues
also occur in the agricultural sector itself. These include
straw from wheat, barley and beans. The volume of these
residues is considerably smaller, accounting for only 15%
of residues from the food industry. In general terms residues from the agricultural sector are not suitable as feed,
as their Ew value is low at 0.4 Ew/kg (Duynie, 2003).
When these residues are used as an energy source, it is
expected that the food system remains unaffected. Since
their volume is small, however, the total amount of
energy that can be obtained from them will also be
limited.
Since the actual residue stream is twice as high the
potential meat production on this resource also doubles.
This implies that 66 kg meat per capita can be produced
from food industry residues. This largely exceeds annual
human meat consumption (44 kg/capita/y). In practice a
large share of residues is fed to cows and contributes to
protein supply in the human diet via milk and cheese.
Therefore it can be suggested that residues from the food
industry play a major role in human protein supply.
With respect to energy another picture emerges. Present
energy use (including fossil fuels) is 200 GJ per capita.
The 2.2 GJ of bio-energy calculated in this paper only
accounts for 1% of the total. Bio-energy production therefore seems of limited importance. When all available agricultural residues (including straw) are included the value
would rise to about 5.0 GJ/capita, i.e. 3% of total energy
use per capita, which also seems a negligible value. On a
national scale this results in 80 PJ, which is less that
50% of the value mentioned by Faaij et al. (1997), however their analysis also included other waste streams such
as paper, household wastes and forestry residues. The
Dutch energy policy goals are to achieve a 9% share of
renewable energy sources by 2015, mainly from wind
and biomass energy. As current energy use in the Netherlands is 3200 PJ/year, 300 PJ needs to be generated from
renewables. At present only 3% is obtained from such
sources of which only 15 PJ is provided by biomass. The
potential production of 80 PJ from residues calculated in
this paper would significantly contribute to the policy
591
goals for 2015. From a renewable energy perspective,
therefore, these residues have a large potential and consequently obtain much attention.
In this paper only a small part of the food system is
analysed: the part that was affected by residue use. It is
interesting to compare results found here with the land
requirements for complete food system. Gerbens-Leenes
and Nonhebel (2002) determined the land requirement
for the total food package in The Netherlands at
1460 m2 per capita. This value is based on the present system where residues are used as livestock feed. When the
land requirements for the replacements calculated in here
are compared to this 1460 m2, the importance of residues
as livestock feed becomes clear. When all residues available (so twice the amount as calculated in 2.1) are used
as energy source an additional 340 m2 is needed for growing feed crops to compensate for the loss of livestock feed.
A 25% increase in the area currently required for food
production. This increase is partly compensated by the
smaller amount of land needed for energy crop cultivation. The net result is an increase in land required for
food and energy of 180 m2 per capita, i.e. a 12% increase
from present requirements.
When the population changes to a vegetarian diet a
smaller increase of the land is needed: only 80 m2 extra,
but it still implies an increase of 5% of the arable land area.
This change to a vegetarian diet, however, is not likely to
occur in the near future. Studies of consumer preferences
with respect to food show that meat is a highly valued food
commodity and the largest share of the consumers is not
willing to change their diet with respect to meat (Nonhebel
and Moll, 2001).
The values used in this paper to analyse the consequences of using residues for energy generation are based
on the Dutch circumstances (consumption pattern, food
industry etc.). However, the conclusion that, from a land
requirement perspective, it is better to use biomass from
dedicated energy crops instead of residues also suitable
for livestock feed holds for all food systems in the world.
The reason for this is that the crop characteristics of willow
in short rotation systems makes that this crop has a higher
yield potential (Mg/ha) than food crops.
5. Conclusion
The quantity of residues originating from the food system is large, and used for energy generation they can provide a substantial amount of renewable energy. However,
in the present system these residues are used as livestock
feed, contributing to the production of highly valued meat
and dairy products. These commodities are important
sources of protein in the human diet, and cannot be left
out without affecting the quality of food consumption.
Using residues for non-feed purposes therefore requires
adaptations in the food system to compensate for protein
losses, i.e. growing beans or supplementary livestock feed
crops. Land requirements for such adaptations are sub-
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S. Nonhebel / Agricultural Systems 94 (2007) 586–592
stantial and are larger than the area needed for energy
crops that produce equivalent amounts of energy, leading
to a net increase of the land requirements. From a land
use perspective, therefore, using residues for livestock feed
and generating bio-energy from dedicated energy crops is
the most preferable option.
Acknowledgement
The author thanks two anonymous reviewers of this
journal for helpful comments on the earlier version of the
paper.
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