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Comparison of the energy performance of fibre sorghum, sweet sorghum and wheat monocultures in northern Italy
European Journal of Agronomy 19 (2003) 35 – 43 www.elsevier.com/locate/eja
Comparison of the energy performance of fibre sorghum, sweet sorghum and wheat monocultures in northern Italy Andrea Monti *, Gianpietro Venturi Department of Agroen6ironmental Science and Technologies, Via Filippo Re 6 /8, 40126 Bologna, Italy Received 12 April 2001; received in revised form 5 February 2002; accepted 12 February 2002
Abstract Four monocultures (fibre sorghum, sweet sorghum and wheat at high and low nitrogen doses) were compared at a field-scale over 3 years (1997–1999) in terms of net energy, net energy ratio and energy use efficiency. Two nitrogen fertilisation levels were assessed for wheat (80 and 120 kg ha − 1 of N) to evaluate the energy use efficiency of the applications. For all the crops, fuels, lubricants and farm inputs together formed around 92% of the total input, mainly due to nitrogen fertilisation. A year ×crop significant interaction was found for all considered parameters. In fact the monoculture did not lower the biomass yield of both sorghum types, while it involved a drop in wheat yield starting from the second year (third considering that wheat was also cultivated in the same field in 1996). The average (1997–1999) net energy supplied by the monoculture of sweet sorghum was 17, 40 and 50% higher than those of fibre sorghum and wheat at high and low nitrogen doses respectively. The energy use efficiency (EUE, i.e. the energy (MJ) requirement to produce a kg of dry matter) ranged from 0.78 to 0.96 for fibre sorghum, from 0.69 to 0.85 for sweet sorghum and from 1.00 to 1.23 and 0.91 to 1.33 for wheat at low and high nitrogen levels respectively. With ethanol as the end-product of the 3-year monoculture of sweet sorghum, the production process would be a bit less favourable in term of energy balance: the net energy yield would be 90% of that obtained by dry matter gasification (with an efficiency of 50%). If straw was not processed, ethanol from wheat was generally unfavourable. On average of the two nitrogen doses, net energy ratios were 0.99, 1.05 and 0.97 in the first second and third year respectively. If bagasse was not considered also sweet sorghum had a very low net energy ratio, but always higher than wheat (1.14, 1.12 and 1.24 over the 3 years). © 2002 Elsevier Science B.V. All rights reserved. Keywords: Outputs; Energy; Sweet sorghum; Fibre sorghum; Wheat
1. Introduction In the future renewable resources can increasingly contribute towards meeting energy requirements (El Bassam, 1998) with the added * Corresponding author. Tel.: + 39-051-209-1534; fax: + 39-051-209-1545. E-mail address: [email protected] (A. Monti).
advantage of greater environmental protection, especially in terms of lower CO2 emissions. Among the renewable energy sources, those from agricultural crops could play an important role in terms of environmental effects as it is believed that the emissions of SO2, NH4, NO2 and other substances responsible for the greenhouse effect are also lower (El Bassam, 1998).
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A. Monti, G. Venturi / Europ. J. Agronomy 19 (2003) 35–43
In particular, compared to the use of fossil fuels, that involves the freeing of carbon compounds that have remained fixed for millions of years, using biofuels there is a balancing out between the levels of CO2 fixed by the crops through photosynthesis and CO2 released in the entire production process and use of the energy (Hall et al., 1993; Venturi et al., 1992). The balance of the cropping stage alone favours the environmental effects for the net accumulation of CO2 in the soil (i.e. in the root). Anyway it should be pointed out that fossil fuels are used for many other purposes in society than simply providing energy and also the energy quality of fossil fuels is much more than that of solar energy (2000 vs. 1) and biomass crops (20 vs. 1); for example to do the same work as fossil fuel, solar energy should be improved 2000 times (Odum, 1984, 1983). Therefore fossil fuels should not be used for poor energy quality applications (for example heating), but only when high quality energy is needed. To make energy development in different parts of the world more rational, numerous international assemblies have set precise time limits and many states have undertaken to respect them. The European Union, in the White Book 1997 DG XVII, sets 2010 as the limit for obtaining 12% of the circa 2000 Mtoe (84 EJ) forecast as the total energy consumed from renewable energy sources. Biomass in general should supply 135 Mtoe (56% of renewable energy) and crops specifically dedicated to energy production 45 Mtoe (18%). To obtain these yields it would be necessary to dedicate around 10 million ha in the EU to energy crops and solve various problems such as crop choice and distribution in the different areas. This as a function of both the environmental effects and economic value. It is therefore necessary to identify the yield level and economic and energy costs of possible energy crops, compared to those of traditional crops, using management techniques compatible with sustainable agriculture. Agricultural crops can produce fuel for biodiesel, fermentable carbohydrates for ethanol, or dry matter for combustion, pyrolysis, gasification or liquefaction, the latter only when technology is able to render them operational and economic.
Among the annual energy crops able to produce stable amounts of biomass, sorghum has a significant yield potential, even in conditions of limited water availability because of their capacity to overcome periods of scarcity by only temporarily slowing down growth and development (Cosentino, 1996; Foti et al., 1996). Sorghum can have two energy destinations: the production of electricity or heat through direct combustion of the biomass or indirect by from gas and oils derived from it, and, for the sweet types with a high yield of fermentable carbohydrates, the production of ethanol (Gosse, 1996). In the latter case, the use of the by-products (bagasse) would lead to an increase in the energy efficiency of the production chain. Within the above mentioned scenario, the objective of this work is to evaluate at a field-scale the energy yields of sweet and fibre sorghum monocultures and to compare them to energy yield of a traditional crop (wheat) at two different nitrogen levels.
2. Materials and methods
2.1. Agronomic technique Two monocultures of fibre (cv. H128) and sweet (cv. Keller) sorghum (Sorghum bicolor Moench) and two monocultures of wheat (var. Centauro) at high (120 kg N ha − 1) and low (80 kg N ha − 1) nitrogen levels were compared over the 3 years 1997–2000, at Cadriano, Bologna (44° 33% lat. N, 11° 21% E, 32 m a.s.l.), in a loam silty soil (Haplic Calcisol, USDA), sub-alkaline (pH 7.4), with high potassium and average phosphorus and nitrogen contents. A randomised block design with four replications was used. The plots were of 120 m2, 20 m long, comprised of 12 rows for sorghum and 36 for wheat. The wheat (variety Centauro) was sown each year in the second 2 weeks of October and harvested towards the end of June. Both types of sorghum (cultivar H128 of fibre and Keller of sweet) were always sown in the first 10 days of May, with an average daily temperature of around 15 °C (Pinthus and Rosemblum, 1961);
A. Monti, G. Venturi / Europ. J. Agronomy 19 (2003) 35–43
both genotypes were harvested at the end of September in the 3 years. The planting density of sorghum, thinned at the 4– 6 leaf stage, was 11 plants m − 2, that of wheat 350 plants m − 2. None of the crops were irrigated. Weed control was done for each crop every year. During seedbed preparation all plots were fertilised with 100 kg ha − 1 of P2O5. Nitrogen fertilisation (urea) on fibre and sweet sorghum (100 kg ha − 1 of N) was applied about 20 days after emergence; in wheat, the single reduced dose was applied during the tillering phase, the high one in two applications, at emergence (40 kg ha − 1 of N) and at tillering (80 kg ha − 1 of N). The following were determined at harvest: biomass and dry matter production and their repartition in stems, leaves and panicles (fibre and sweet sorghum); grain, straw and dry matter production, weight of 1000 seeds and per hectolitre, straw and grain moisture content (wheat).
2.2. Calculation method of the energy inputs and outputs The inputs and outputs were converted from physical to energy unit measures through coefficients of transformation found in the literature (Table 1), adopting in preference those found in
situations similar to those where the work was done. A field scale diagram of the energy flows is shown in Fig. 1. The inputs for each cropping operation were calculated separately for tractors, equipment, fuels, lubricants and farm inputs (seeds, herbicides, fertilisers, etc.). The list of the energy inputs is shown briefly in Table 2. The energy costs for construction, depreciation and maintenance were calculated taking into account the average life-span, time of use for each crop and multiplying by the energy coefficients. Two differently powered tractors were used, one for ploughing or clod breaking (90 kWh) and one for the other cropping operations (60 kWh). To calculate the energy consumption of fuel it was presumed that 0.3 kg of fuel is necessary for each kWh of power used (Bolli and Scotton, 1987). The energy coefficients of fuel and lubricants were increased of 12.8% to take transport into account (Marland, 1983). The energy quota attributed to the herbicides was obtained by multiplying the amount of active ingredient used by the relative energy coefficients found in the literature. When calculating the inputs for ethanol production from sweet sorghum the simplified harvesting method was not taken into account,
Table 1 Energy coefficients for selected agricultural inputs Input
Energy coefficient
Unit of measure
Tractors Equipment Machinery shed Fuel Lubrificants Seeds
158.9 69.0 76.5 44.6 43.8 54.0
MJ MJ MJ MJ MJ MJ MJ
Fertilisers N P2O5 Herbicides Trifluralin MCPA Glyphosate Chlorsulfuron
42.5 12.6 150 130 454 365
kg−1 kg−1 m−2 d−1 kg−1 kg−1 kg−1 kg−1
MJ kg−1 MJ kg−1 MJ MJ MJ MJ
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kg−1 kg−1 kg−1 kg−1
Data source Blankenhorn et al., 1982 Pari and Ragno, 1998 Boerma et al., 1980 Cavazza et al., 1983 Southwell and Rothwell, 1977 Bacchiet et al., 1992 Bonari et al., 1992 Combes, 1998 Cavazza et al., 1983 Green, 1987 Green, 1987 Green, 1987 Turnhollow and Perlack, 1991
580 2700 705 160 180 680 600 3800
Mach. (kg)
4000 4000 3000 2400 1000 2400 3000 1800
Lifespan (h) 30 70 13 5 19 19 18 437 611
Mach. (MJ ha−1) 229 114 44 55 83 55 72 – 652
Tract.
Fibre and sweet sorghum
2832 1214 432 450 674 450 818 1619 8490
Fuels
– – – 399 5510 243 – – 6152
FESb
30 70 13 5 37 19 18 437 629
Mach.
229 114 44 55 165 55 72 – 735
Tract.
Wheat high N
b
Self-propelled. F, fertilisers; E, herbicides; S, seeds. A 90 kWh tractor for Plough and Milling and a 60 kWh tractor for the other operations were used.
a
Plough Milling Harrowing Weeding Manuring Sowing Howing Harvesta Total
Operation
Table 2 Inputs for sorghums (sweet and fibre) and wheat (high and low nitrogen doses) monocultures
2832 1214 432 450 1349 450 818 1619 9165
Fuels
6994
317 6677 –
FESb
30 70 13 5 19 19 18 437 611
Mach.
229 114 44 55 83 55 72 – 652
Tract.
Wheat low N
2832 1214 432 450 674 450 818 1619 8490
Fuels
4977
317 4660 –
FESb
38 A. Monti, G. Venturi / Europ. J. Agronomy 19 (2003) 35–43
A. Monti, G. Venturi / Europ. J. Agronomy 19 (2003) 35–43
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Fig. 1. Diagram showing the inputs, the outputs and the state variables at a field-scale.
therefore the energy input is slightly overestimated. The energy from human labour, held to be less than 0.2% of the total energy input, was never assessed (Zentner et al., 1998a,b; Borin et al., 1997), nor that related to heating and electricity in the houses, shelters, personal use or any cereal drying. The Gross Energy Requirement method (Biondi et al., 1987) of the International Federation of Institutes for Advanced Study of Stockholm (IFIAS) was used to calculate the energy balance. For this reason only non-renewable energy sources were taken into account. The energy outputs were calculated separately for the different biomass components. For fibre and sweet sorghum, the energy obtained from the combustion process of the total biomass and stems alone was examined. The energy value of the total dry matter was taken as 16 MJ kg − 1 (Bonari et al., 1996). The energy value of grain was taken as 13.8 MJ kg − 1 (Southwell and Rothwell, 1977; Stout, 1979) and that of the straw as 18.5 MJ kg − 1 (Ivarsson and Nilsson, 1988). Sweet sorghum and wheat were also evaluated taking ethanol as the end-product of the production process. Reference was made to dry processing that has lower plant costs compared to damp processing. For wheat, it was assumed that 3.5 tonnes of grain are necessary to obtain 1 tonne of anhydrous ethanol and that 1 tonne of anhydrous ethanol would generate 26.8 GJ (Parisi, 1988).
The energy input for processing into ethanol was calculated taking the following processes into account (Parisi, 1988): reception and grinding (0.23 MJ kg − 1); baking and saccharification (2.65 MJ kg − 1); fermentation (0.37 MJ kg − 1); distillation and anhydridisation (5.08 MJ kg − 1); borland concentration and drying (10.0 MJ kg − 1). For sweet sorghum an extraction plant was used with Tilby separators that offer the advantage of separating the cane into cortical fibre and pith soaked in sugary juice, retrieving around 95% of the sugars. It was assumed that from 1 tonne of fresh biomass (22.5% dry matter) 47 kg of anhydrous ethanol are obtained (Venturi et al., 1993), to which 3.4 kg of bagasse are added (at 50% moisture content), per kg of ethanol produced (Parisi, 1988). The energy value of the fresh bagasse (50% dry matter) was taken as 7 MJ kg − 1 (Shewale, 1986), i.e. 23.8 MJ kg − 1 (7×3.4). The possibility of extracting ethanol from the bagasse was not examined, as it is held to be uneconomical energy wise (3.16 MJ kg − 1) in comparison to its use as fuel (Parisi, 1988). The energy yield of the monocultures is expressed as net energy (NE, energy output less input); net energy ratio (NER, ratio between output and input); energy use efficiency (EUE), i.e. quantity of total dry matter or of grain or stems alone (EUEs) for wheat and sorghum respectively per unit of input energy. The rate of net energy yield (MJ ha − 1 day − 1) was used to compare the monocultures taking into account the different crop cycle lengths of the species.
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A. Monti, G. Venturi / Europ. J. Agronomy 19 (2003) 35–43
For wheat, the energy efficiency of the nitrogen fertilisation was also referred to, expressed as: (output N120 − output N80)/(input N120 −input N80), where N120 and N80 represent the two nitrogen fertilisation levels.
2.3. Statistical analysis Statistical analysis was by analysis of variance of the energy values of each monoculture. In the choice of the analysis model of the variance on the multi-year data, the constraint relating to monoculture involving plot randomisation only in the first year was taken into account (Satterthwaite, 1946). The effect of the interaction of year × crop was assessed by comparison of the three monocultures where each crop in each crop system was present in every year. Separation of means was obtained by Tukey’s test.
to the lowest yield of wheat. This is also in accordance with other experiences in the same area (Toderi and Cacchi, 1973). Monoculture of both sorghum types did not depress net energy yields (NE) and also the net energy ratios (NER) that actually increased over the years (Fig. 2a and b). The energy use efficiency (EUE, i.e. the energy (MJ) requirement to produce a kg of dry matter) ranged from 0.78 to 0.96 for fibre sorghum, from 0.69 to 0.85 for sweet sorghum and from 1.00 to 1.23 and 0.91 to 1.33 for wheat at low and high nitrogen level, respectively (Fig. 3a). The differences among crops were larger if only the commercial part of biomass (i.e. grain for wheat and stem for sweet and fibre sorghum) was considered (EUEs). Wheat always had higher values (lower efficiency) than both sorghum types (Fig. 3b).
3. Results and discussion For all the crops, fuels, lubricants and farm inputs together formed around 92% of the total input. In particular, nitrogen fertilisation was responsible for 27% on the total energy cost in sorghums; around 23 and 30% in wheat, with 80 and 120 kg ha − 1 of nitrogen, respectively. Generally the inputs of sorghum and wheat were broadly similar, while the outputs of both sorghum types were sharply higher than wheat. The highest energy inputs were for wheat with the high level of nitrogen fertiliser (17.1 GJ ha − 1), the lowest for wheat with low fertilisation (14.7 GJ ha − 1). A significant interaction (P 5 0.01) between year and crop was found for the energy production (Fig. 2a and b). This might be due to the depressive effects of continuous wheat system mainly on the third year (four considering that wheat was cultivated in the same field in 1996). In fact in 1999 the energy yields of both wheat monocultures were significantly (P 50.05) lower than in soybean (1997)– fibre sorghum (1998)– wheat (1999) system in the same field and conditions (unpublished data). Therefore the effect of monoculture more than year could be responsible
Fig. 2. Net energy yield (a) and net energy ratio (b) of sweet sorghum ( ), fibre sorghum () and wheat (2) at low (blank symbol) and high (filled symbol) nitrogen levels. Different letters beside each symbol indicate different means for P5 0.05 (Tukey’s test).
A. Monti, G. Venturi / Europ. J. Agronomy 19 (2003) 35–43
Fig. 3. Energy use efficiency (EUE; a) and specific energy use efficiency (EUEs; b) of sweet sorghum ( ), fibre sorghum () and wheat (2) at low (blank symbol) and high (filled symbol) nitrogen levels. Different letters signify different means for P 5 0.05 (Tukey’s test).
Sweet sorghum had very lower values compared to fibre sorghum especially for the tiny panicle production. It should be pointed out that the sweet sorghum variety had a longer growth cycle than fibre sorghum variety and this was the main reason to clarify the incidence of panicles on total dry biomass of sweet sorghum. Because of the different biomass distribution the difference between the two types of sorghum was clearer by examining stems alone. The sweet sorghum dry matter was made up of 70% stems, 22% leaves and 8% panicles, on average; in fibre sorghum the stems represented 64%, the leaves 13% and panicles 23%. Consequently the production of 1 tonne of fibre sorghum stems (d.m.) required 38% more energy than that of sweet sorghum (1.49 against 1.08 MJ kg − 1), while only 13% more energy was needed considering the total dry matter production.
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In wheat the higher nitrogen dose significantly increased the outputs only in the first year (+ 36%), with also significant (P5 0.05) higher net energy yield (+9 GJ ha − 1) than those of fibre sorghum. In the following 2 years the nitrogen doses did not differ in the net energy produced, net energy ratio and energy use efficiency, while significant differences were found for the grain energy use efficiency (EUEs) (Fig. 3b). The nitrogen energy efficiency (i.e. the output units more every extra inputs of nitrogen applied with the high dose) was relevant only in the first year (Fig. 2a and b). With reference to the average of the 3 years (Fig. 4) sweet sorghum gave the best performance with 14, 38 and 26% of net energy more than fibre sorghum and wheat at low and high nitrogen doses, respectively. The net energy ratio of sweet sorghum was more than 13% higher than fibre sorghum one and 30% higher than wheat monocultures at both nitrogen levels. The energy use efficiency was much lower for sorghum types compared to wheat. In fact sweet sorghum did need 0.74 MJ to produce a kg of dry matter, that is 15 and 47% less than fibre sorghum and wheat respectively. The differences were enlarger if the commercial products (stems or grain) were con-
Fig. 4. Relative (to sweet sorghum) values of net energy (NE), net energy ratio (NER), energy use efficiency (EUE) and specific energy use efficiency (EUEs) of sweet sorghum, fibre sorghum and wheat (average of 1997 – 1999). Standard deviations are indicated. Labels on each bar are the real values (GJ ha − 1 of NE; MJ MJ − 1 for NER; MJ kg − 1 for EUE and EUEs).
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A. Monti, G. Venturi / Europ. J. Agronomy 19 (2003) 35–43
Taking ethanol as energy product a significant year per crop interaction was found (Fig. 5a–c). If bagasse and straw were processed for energy, wheat supplied better net energy yields than sorghum in the first 2 years (around 140 and up to 170 GJ ha − 1, average of 1998 and 1999 for low and high nitrogen levels, respectively), while sweet sorghum was better, but not significant, in the last year, probably for its large stems productivity and the corresponding lower productivity of wheat (Fig. 5a). Because of the much lower energy inputs of wheat, the outputs/inputs ratios and also the energy use efficiency of wheat were always favourable than sorghum both in low and high nitrogen doses (Fig. 5b and c). The production of 1 tonne of ethanol required from 24.9 to 27.8 GJ for wheat with reduced and high fertilisation and from 21.5 to 23.8 GJ for sweet sorghum (Fig. 5c). If straw was not processed, ethanol from wheat was energetically unprofitable. On average of the two nitrogen doses, net energy ratios were 0.99, 1.05 and 0.97 in the first second and third year, respectively. If bagasse was not considered also sweet sorghum had a very low net energy ratio, but always positive and higher than wheat (1.14, 1.12 and 1.24 over the 3 years).
Acknowledgements Fig. 5. Net energy yield (NE; a), net energy ratio (NER; b) and energy use efficiency (EUE; c) of sweet sorghum and wheat (at low and high nitrogen levels) for ethanol production. Different letters on top of each bar signify different means for P50.05 (Tukey’s test). Standard deviations are indicated.
This research was supported by the Commission of the European Communities; Agriculture and Fisheries (FAIR) program, CT96-1913 co-ordinated by Professor Salvatore Foti.
sidered. On average more than 2.3 MJ were needed to produce 1 kg of wheat grain, while 1.1 and 1.5 MJ needed to produce 1 kg of stems dry matter of sweet and fibre sorghum, respectively. The average (1997– 1999) net energy rate of sweet sorghum was 2125 MJ ha − 1 day − 1, 15% more than that of fibre sorghum, more than double that of the wheat monoculture (922 MJ ha − 1 day − 1 average of low and high nitrogen doses). This can be attributed to both the higher energy yields and shorter cropping cycle of the two types of sorghum compared to wheat.
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Comparison of the energy performance of fibre sorghum, sweet sorghum and wheat monocultures in northern Italy Andrea Monti *, Gianpietro Venturi Department of Agroen6ironmental Science and Technologies, Via Filippo Re 6 /8, 40126 Bologna, Italy Received 12 April 2001; received in revised form 5 February 2002; accepted 12 February 2002
Abstract Four monocultures (fibre sorghum, sweet sorghum and wheat at high and low nitrogen doses) were compared at a field-scale over 3 years (1997–1999) in terms of net energy, net energy ratio and energy use efficiency. Two nitrogen fertilisation levels were assessed for wheat (80 and 120 kg ha − 1 of N) to evaluate the energy use efficiency of the applications. For all the crops, fuels, lubricants and farm inputs together formed around 92% of the total input, mainly due to nitrogen fertilisation. A year ×crop significant interaction was found for all considered parameters. In fact the monoculture did not lower the biomass yield of both sorghum types, while it involved a drop in wheat yield starting from the second year (third considering that wheat was also cultivated in the same field in 1996). The average (1997–1999) net energy supplied by the monoculture of sweet sorghum was 17, 40 and 50% higher than those of fibre sorghum and wheat at high and low nitrogen doses respectively. The energy use efficiency (EUE, i.e. the energy (MJ) requirement to produce a kg of dry matter) ranged from 0.78 to 0.96 for fibre sorghum, from 0.69 to 0.85 for sweet sorghum and from 1.00 to 1.23 and 0.91 to 1.33 for wheat at low and high nitrogen levels respectively. With ethanol as the end-product of the 3-year monoculture of sweet sorghum, the production process would be a bit less favourable in term of energy balance: the net energy yield would be 90% of that obtained by dry matter gasification (with an efficiency of 50%). If straw was not processed, ethanol from wheat was generally unfavourable. On average of the two nitrogen doses, net energy ratios were 0.99, 1.05 and 0.97 in the first second and third year respectively. If bagasse was not considered also sweet sorghum had a very low net energy ratio, but always higher than wheat (1.14, 1.12 and 1.24 over the 3 years). © 2002 Elsevier Science B.V. All rights reserved. Keywords: Outputs; Energy; Sweet sorghum; Fibre sorghum; Wheat
1. Introduction In the future renewable resources can increasingly contribute towards meeting energy requirements (El Bassam, 1998) with the added * Corresponding author. Tel.: + 39-051-209-1534; fax: + 39-051-209-1545. E-mail address: [email protected] (A. Monti).
advantage of greater environmental protection, especially in terms of lower CO2 emissions. Among the renewable energy sources, those from agricultural crops could play an important role in terms of environmental effects as it is believed that the emissions of SO2, NH4, NO2 and other substances responsible for the greenhouse effect are also lower (El Bassam, 1998).
1161-0301/02/$ - see front matter © 2002 Elsevier Science B.V. All rights reserved. PII: S 1 1 6 1 - 0 3 0 1 ( 0 2 ) 0 0 0 1 7 - 5
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A. Monti, G. Venturi / Europ. J. Agronomy 19 (2003) 35–43
In particular, compared to the use of fossil fuels, that involves the freeing of carbon compounds that have remained fixed for millions of years, using biofuels there is a balancing out between the levels of CO2 fixed by the crops through photosynthesis and CO2 released in the entire production process and use of the energy (Hall et al., 1993; Venturi et al., 1992). The balance of the cropping stage alone favours the environmental effects for the net accumulation of CO2 in the soil (i.e. in the root). Anyway it should be pointed out that fossil fuels are used for many other purposes in society than simply providing energy and also the energy quality of fossil fuels is much more than that of solar energy (2000 vs. 1) and biomass crops (20 vs. 1); for example to do the same work as fossil fuel, solar energy should be improved 2000 times (Odum, 1984, 1983). Therefore fossil fuels should not be used for poor energy quality applications (for example heating), but only when high quality energy is needed. To make energy development in different parts of the world more rational, numerous international assemblies have set precise time limits and many states have undertaken to respect them. The European Union, in the White Book 1997 DG XVII, sets 2010 as the limit for obtaining 12% of the circa 2000 Mtoe (84 EJ) forecast as the total energy consumed from renewable energy sources. Biomass in general should supply 135 Mtoe (56% of renewable energy) and crops specifically dedicated to energy production 45 Mtoe (18%). To obtain these yields it would be necessary to dedicate around 10 million ha in the EU to energy crops and solve various problems such as crop choice and distribution in the different areas. This as a function of both the environmental effects and economic value. It is therefore necessary to identify the yield level and economic and energy costs of possible energy crops, compared to those of traditional crops, using management techniques compatible with sustainable agriculture. Agricultural crops can produce fuel for biodiesel, fermentable carbohydrates for ethanol, or dry matter for combustion, pyrolysis, gasification or liquefaction, the latter only when technology is able to render them operational and economic.
Among the annual energy crops able to produce stable amounts of biomass, sorghum has a significant yield potential, even in conditions of limited water availability because of their capacity to overcome periods of scarcity by only temporarily slowing down growth and development (Cosentino, 1996; Foti et al., 1996). Sorghum can have two energy destinations: the production of electricity or heat through direct combustion of the biomass or indirect by from gas and oils derived from it, and, for the sweet types with a high yield of fermentable carbohydrates, the production of ethanol (Gosse, 1996). In the latter case, the use of the by-products (bagasse) would lead to an increase in the energy efficiency of the production chain. Within the above mentioned scenario, the objective of this work is to evaluate at a field-scale the energy yields of sweet and fibre sorghum monocultures and to compare them to energy yield of a traditional crop (wheat) at two different nitrogen levels.
2. Materials and methods
2.1. Agronomic technique Two monocultures of fibre (cv. H128) and sweet (cv. Keller) sorghum (Sorghum bicolor Moench) and two monocultures of wheat (var. Centauro) at high (120 kg N ha − 1) and low (80 kg N ha − 1) nitrogen levels were compared over the 3 years 1997–2000, at Cadriano, Bologna (44° 33% lat. N, 11° 21% E, 32 m a.s.l.), in a loam silty soil (Haplic Calcisol, USDA), sub-alkaline (pH 7.4), with high potassium and average phosphorus and nitrogen contents. A randomised block design with four replications was used. The plots were of 120 m2, 20 m long, comprised of 12 rows for sorghum and 36 for wheat. The wheat (variety Centauro) was sown each year in the second 2 weeks of October and harvested towards the end of June. Both types of sorghum (cultivar H128 of fibre and Keller of sweet) were always sown in the first 10 days of May, with an average daily temperature of around 15 °C (Pinthus and Rosemblum, 1961);
A. Monti, G. Venturi / Europ. J. Agronomy 19 (2003) 35–43
both genotypes were harvested at the end of September in the 3 years. The planting density of sorghum, thinned at the 4– 6 leaf stage, was 11 plants m − 2, that of wheat 350 plants m − 2. None of the crops were irrigated. Weed control was done for each crop every year. During seedbed preparation all plots were fertilised with 100 kg ha − 1 of P2O5. Nitrogen fertilisation (urea) on fibre and sweet sorghum (100 kg ha − 1 of N) was applied about 20 days after emergence; in wheat, the single reduced dose was applied during the tillering phase, the high one in two applications, at emergence (40 kg ha − 1 of N) and at tillering (80 kg ha − 1 of N). The following were determined at harvest: biomass and dry matter production and their repartition in stems, leaves and panicles (fibre and sweet sorghum); grain, straw and dry matter production, weight of 1000 seeds and per hectolitre, straw and grain moisture content (wheat).
2.2. Calculation method of the energy inputs and outputs The inputs and outputs were converted from physical to energy unit measures through coefficients of transformation found in the literature (Table 1), adopting in preference those found in
situations similar to those where the work was done. A field scale diagram of the energy flows is shown in Fig. 1. The inputs for each cropping operation were calculated separately for tractors, equipment, fuels, lubricants and farm inputs (seeds, herbicides, fertilisers, etc.). The list of the energy inputs is shown briefly in Table 2. The energy costs for construction, depreciation and maintenance were calculated taking into account the average life-span, time of use for each crop and multiplying by the energy coefficients. Two differently powered tractors were used, one for ploughing or clod breaking (90 kWh) and one for the other cropping operations (60 kWh). To calculate the energy consumption of fuel it was presumed that 0.3 kg of fuel is necessary for each kWh of power used (Bolli and Scotton, 1987). The energy coefficients of fuel and lubricants were increased of 12.8% to take transport into account (Marland, 1983). The energy quota attributed to the herbicides was obtained by multiplying the amount of active ingredient used by the relative energy coefficients found in the literature. When calculating the inputs for ethanol production from sweet sorghum the simplified harvesting method was not taken into account,
Table 1 Energy coefficients for selected agricultural inputs Input
Energy coefficient
Unit of measure
Tractors Equipment Machinery shed Fuel Lubrificants Seeds
158.9 69.0 76.5 44.6 43.8 54.0
MJ MJ MJ MJ MJ MJ MJ
Fertilisers N P2O5 Herbicides Trifluralin MCPA Glyphosate Chlorsulfuron
42.5 12.6 150 130 454 365
kg−1 kg−1 m−2 d−1 kg−1 kg−1 kg−1 kg−1
MJ kg−1 MJ kg−1 MJ MJ MJ MJ
37
kg−1 kg−1 kg−1 kg−1
Data source Blankenhorn et al., 1982 Pari and Ragno, 1998 Boerma et al., 1980 Cavazza et al., 1983 Southwell and Rothwell, 1977 Bacchiet et al., 1992 Bonari et al., 1992 Combes, 1998 Cavazza et al., 1983 Green, 1987 Green, 1987 Green, 1987 Turnhollow and Perlack, 1991
580 2700 705 160 180 680 600 3800
Mach. (kg)
4000 4000 3000 2400 1000 2400 3000 1800
Lifespan (h) 30 70 13 5 19 19 18 437 611
Mach. (MJ ha−1) 229 114 44 55 83 55 72 – 652
Tract.
Fibre and sweet sorghum
2832 1214 432 450 674 450 818 1619 8490
Fuels
– – – 399 5510 243 – – 6152
FESb
30 70 13 5 37 19 18 437 629
Mach.
229 114 44 55 165 55 72 – 735
Tract.
Wheat high N
b
Self-propelled. F, fertilisers; E, herbicides; S, seeds. A 90 kWh tractor for Plough and Milling and a 60 kWh tractor for the other operations were used.
a
Plough Milling Harrowing Weeding Manuring Sowing Howing Harvesta Total
Operation
Table 2 Inputs for sorghums (sweet and fibre) and wheat (high and low nitrogen doses) monocultures
2832 1214 432 450 1349 450 818 1619 9165
Fuels
6994
317 6677 –
FESb
30 70 13 5 19 19 18 437 611
Mach.
229 114 44 55 83 55 72 – 652
Tract.
Wheat low N
2832 1214 432 450 674 450 818 1619 8490
Fuels
4977
317 4660 –
FESb
38 A. Monti, G. Venturi / Europ. J. Agronomy 19 (2003) 35–43
A. Monti, G. Venturi / Europ. J. Agronomy 19 (2003) 35–43
39
Fig. 1. Diagram showing the inputs, the outputs and the state variables at a field-scale.
therefore the energy input is slightly overestimated. The energy from human labour, held to be less than 0.2% of the total energy input, was never assessed (Zentner et al., 1998a,b; Borin et al., 1997), nor that related to heating and electricity in the houses, shelters, personal use or any cereal drying. The Gross Energy Requirement method (Biondi et al., 1987) of the International Federation of Institutes for Advanced Study of Stockholm (IFIAS) was used to calculate the energy balance. For this reason only non-renewable energy sources were taken into account. The energy outputs were calculated separately for the different biomass components. For fibre and sweet sorghum, the energy obtained from the combustion process of the total biomass and stems alone was examined. The energy value of the total dry matter was taken as 16 MJ kg − 1 (Bonari et al., 1996). The energy value of grain was taken as 13.8 MJ kg − 1 (Southwell and Rothwell, 1977; Stout, 1979) and that of the straw as 18.5 MJ kg − 1 (Ivarsson and Nilsson, 1988). Sweet sorghum and wheat were also evaluated taking ethanol as the end-product of the production process. Reference was made to dry processing that has lower plant costs compared to damp processing. For wheat, it was assumed that 3.5 tonnes of grain are necessary to obtain 1 tonne of anhydrous ethanol and that 1 tonne of anhydrous ethanol would generate 26.8 GJ (Parisi, 1988).
The energy input for processing into ethanol was calculated taking the following processes into account (Parisi, 1988): reception and grinding (0.23 MJ kg − 1); baking and saccharification (2.65 MJ kg − 1); fermentation (0.37 MJ kg − 1); distillation and anhydridisation (5.08 MJ kg − 1); borland concentration and drying (10.0 MJ kg − 1). For sweet sorghum an extraction plant was used with Tilby separators that offer the advantage of separating the cane into cortical fibre and pith soaked in sugary juice, retrieving around 95% of the sugars. It was assumed that from 1 tonne of fresh biomass (22.5% dry matter) 47 kg of anhydrous ethanol are obtained (Venturi et al., 1993), to which 3.4 kg of bagasse are added (at 50% moisture content), per kg of ethanol produced (Parisi, 1988). The energy value of the fresh bagasse (50% dry matter) was taken as 7 MJ kg − 1 (Shewale, 1986), i.e. 23.8 MJ kg − 1 (7×3.4). The possibility of extracting ethanol from the bagasse was not examined, as it is held to be uneconomical energy wise (3.16 MJ kg − 1) in comparison to its use as fuel (Parisi, 1988). The energy yield of the monocultures is expressed as net energy (NE, energy output less input); net energy ratio (NER, ratio between output and input); energy use efficiency (EUE), i.e. quantity of total dry matter or of grain or stems alone (EUEs) for wheat and sorghum respectively per unit of input energy. The rate of net energy yield (MJ ha − 1 day − 1) was used to compare the monocultures taking into account the different crop cycle lengths of the species.
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A. Monti, G. Venturi / Europ. J. Agronomy 19 (2003) 35–43
For wheat, the energy efficiency of the nitrogen fertilisation was also referred to, expressed as: (output N120 − output N80)/(input N120 −input N80), where N120 and N80 represent the two nitrogen fertilisation levels.
2.3. Statistical analysis Statistical analysis was by analysis of variance of the energy values of each monoculture. In the choice of the analysis model of the variance on the multi-year data, the constraint relating to monoculture involving plot randomisation only in the first year was taken into account (Satterthwaite, 1946). The effect of the interaction of year × crop was assessed by comparison of the three monocultures where each crop in each crop system was present in every year. Separation of means was obtained by Tukey’s test.
to the lowest yield of wheat. This is also in accordance with other experiences in the same area (Toderi and Cacchi, 1973). Monoculture of both sorghum types did not depress net energy yields (NE) and also the net energy ratios (NER) that actually increased over the years (Fig. 2a and b). The energy use efficiency (EUE, i.e. the energy (MJ) requirement to produce a kg of dry matter) ranged from 0.78 to 0.96 for fibre sorghum, from 0.69 to 0.85 for sweet sorghum and from 1.00 to 1.23 and 0.91 to 1.33 for wheat at low and high nitrogen level, respectively (Fig. 3a). The differences among crops were larger if only the commercial part of biomass (i.e. grain for wheat and stem for sweet and fibre sorghum) was considered (EUEs). Wheat always had higher values (lower efficiency) than both sorghum types (Fig. 3b).
3. Results and discussion For all the crops, fuels, lubricants and farm inputs together formed around 92% of the total input. In particular, nitrogen fertilisation was responsible for 27% on the total energy cost in sorghums; around 23 and 30% in wheat, with 80 and 120 kg ha − 1 of nitrogen, respectively. Generally the inputs of sorghum and wheat were broadly similar, while the outputs of both sorghum types were sharply higher than wheat. The highest energy inputs were for wheat with the high level of nitrogen fertiliser (17.1 GJ ha − 1), the lowest for wheat with low fertilisation (14.7 GJ ha − 1). A significant interaction (P 5 0.01) between year and crop was found for the energy production (Fig. 2a and b). This might be due to the depressive effects of continuous wheat system mainly on the third year (four considering that wheat was cultivated in the same field in 1996). In fact in 1999 the energy yields of both wheat monocultures were significantly (P 50.05) lower than in soybean (1997)– fibre sorghum (1998)– wheat (1999) system in the same field and conditions (unpublished data). Therefore the effect of monoculture more than year could be responsible
Fig. 2. Net energy yield (a) and net energy ratio (b) of sweet sorghum ( ), fibre sorghum () and wheat (2) at low (blank symbol) and high (filled symbol) nitrogen levels. Different letters beside each symbol indicate different means for P5 0.05 (Tukey’s test).
A. Monti, G. Venturi / Europ. J. Agronomy 19 (2003) 35–43
Fig. 3. Energy use efficiency (EUE; a) and specific energy use efficiency (EUEs; b) of sweet sorghum ( ), fibre sorghum () and wheat (2) at low (blank symbol) and high (filled symbol) nitrogen levels. Different letters signify different means for P 5 0.05 (Tukey’s test).
Sweet sorghum had very lower values compared to fibre sorghum especially for the tiny panicle production. It should be pointed out that the sweet sorghum variety had a longer growth cycle than fibre sorghum variety and this was the main reason to clarify the incidence of panicles on total dry biomass of sweet sorghum. Because of the different biomass distribution the difference between the two types of sorghum was clearer by examining stems alone. The sweet sorghum dry matter was made up of 70% stems, 22% leaves and 8% panicles, on average; in fibre sorghum the stems represented 64%, the leaves 13% and panicles 23%. Consequently the production of 1 tonne of fibre sorghum stems (d.m.) required 38% more energy than that of sweet sorghum (1.49 against 1.08 MJ kg − 1), while only 13% more energy was needed considering the total dry matter production.
41
In wheat the higher nitrogen dose significantly increased the outputs only in the first year (+ 36%), with also significant (P5 0.05) higher net energy yield (+9 GJ ha − 1) than those of fibre sorghum. In the following 2 years the nitrogen doses did not differ in the net energy produced, net energy ratio and energy use efficiency, while significant differences were found for the grain energy use efficiency (EUEs) (Fig. 3b). The nitrogen energy efficiency (i.e. the output units more every extra inputs of nitrogen applied with the high dose) was relevant only in the first year (Fig. 2a and b). With reference to the average of the 3 years (Fig. 4) sweet sorghum gave the best performance with 14, 38 and 26% of net energy more than fibre sorghum and wheat at low and high nitrogen doses, respectively. The net energy ratio of sweet sorghum was more than 13% higher than fibre sorghum one and 30% higher than wheat monocultures at both nitrogen levels. The energy use efficiency was much lower for sorghum types compared to wheat. In fact sweet sorghum did need 0.74 MJ to produce a kg of dry matter, that is 15 and 47% less than fibre sorghum and wheat respectively. The differences were enlarger if the commercial products (stems or grain) were con-
Fig. 4. Relative (to sweet sorghum) values of net energy (NE), net energy ratio (NER), energy use efficiency (EUE) and specific energy use efficiency (EUEs) of sweet sorghum, fibre sorghum and wheat (average of 1997 – 1999). Standard deviations are indicated. Labels on each bar are the real values (GJ ha − 1 of NE; MJ MJ − 1 for NER; MJ kg − 1 for EUE and EUEs).
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A. Monti, G. Venturi / Europ. J. Agronomy 19 (2003) 35–43
Taking ethanol as energy product a significant year per crop interaction was found (Fig. 5a–c). If bagasse and straw were processed for energy, wheat supplied better net energy yields than sorghum in the first 2 years (around 140 and up to 170 GJ ha − 1, average of 1998 and 1999 for low and high nitrogen levels, respectively), while sweet sorghum was better, but not significant, in the last year, probably for its large stems productivity and the corresponding lower productivity of wheat (Fig. 5a). Because of the much lower energy inputs of wheat, the outputs/inputs ratios and also the energy use efficiency of wheat were always favourable than sorghum both in low and high nitrogen doses (Fig. 5b and c). The production of 1 tonne of ethanol required from 24.9 to 27.8 GJ for wheat with reduced and high fertilisation and from 21.5 to 23.8 GJ for sweet sorghum (Fig. 5c). If straw was not processed, ethanol from wheat was energetically unprofitable. On average of the two nitrogen doses, net energy ratios were 0.99, 1.05 and 0.97 in the first second and third year, respectively. If bagasse was not considered also sweet sorghum had a very low net energy ratio, but always positive and higher than wheat (1.14, 1.12 and 1.24 over the 3 years).
Acknowledgements Fig. 5. Net energy yield (NE; a), net energy ratio (NER; b) and energy use efficiency (EUE; c) of sweet sorghum and wheat (at low and high nitrogen levels) for ethanol production. Different letters on top of each bar signify different means for P50.05 (Tukey’s test). Standard deviations are indicated.
This research was supported by the Commission of the European Communities; Agriculture and Fisheries (FAIR) program, CT96-1913 co-ordinated by Professor Salvatore Foti.
sidered. On average more than 2.3 MJ were needed to produce 1 kg of wheat grain, while 1.1 and 1.5 MJ needed to produce 1 kg of stems dry matter of sweet and fibre sorghum, respectively. The average (1997– 1999) net energy rate of sweet sorghum was 2125 MJ ha − 1 day − 1, 15% more than that of fibre sorghum, more than double that of the wheat monoculture (922 MJ ha − 1 day − 1 average of low and high nitrogen doses). This can be attributed to both the higher energy yields and shorter cropping cycle of the two types of sorghum compared to wheat.
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