Full chain energy analysis of fuel ethanol from cane molasses in Thailand

Available online at www.sciencedirect.com

APPLIED ENERGY Applied Energy 85 (2008) 722–734 www.elsevier.com/locate/apenergy

Full chain energy analysis of fuel ethanol from cane molasses in Thailand Thu Lan T. Nguyen, Shabbir H. Gheewala *, Savitri Garivait The Joint Graduate School of Energy and Environment, King Mongkut’s University of Technology Thonburi, Bangkok, Thailand Received 24 September 2007; received in revised form 28 December 2007; accepted 1 February 2008 Available online 18 March 2008

Abstract An analysis of energy performance and supply potential was performed to evaluate molasses utilization for fuel ethanol in Thailand. The Thai government recently has set up a production target of 1.925 million litres a day of sugar-based ethanol. The molasses-based ethanol (MoE) system involves three main segments: sugar cane cultivation, molasses generation, and ethanol conversion. Negative net energy value found for MoE is a consequence of not utilizing system coproducts (e.g. stillage and cane trash) for energy. Taking into account only fossil fuel or petroleum inputs in the production cycle, the energy analysis provides results in favour of ethanol. A positive net energy of 5.95 MJ/L which corresponds to 39% energy gain shows that MoE is efficient as far as its potential to replace fossil fuels is concerned. Another encouraging result is that each MJ of petroleum inputs can produce 6.12 MJ of ethanol fuel. Regarding supply potential, if only the surplus molasses is utilized for ethanol, a shift of 8–10% sugar cane produce to fuel ethanol from its current use in sugar industry could be a probable solution. Ó 2008 Elsevier Ltd. All rights reserved. Keywords: Molasses-based ethanol; Life cycle assessment; Net energy value; Energy ratio; Thailand

1. Introduction After China and India, Thailand is considered another market for fuel ethanol in Asia. At present, the 10% ethanol in gasohol available at the Thai gas stations is mainly a fermentation product of molasses. According to [1], by 2008, 12 sugar-based ethanol plants with the total output of 1.925 million litres (ML) a day will come on stream in Thailand. In line with rapid development of process technologies involved in ethanol production from biomass, evaluations of ethanol’s advantages over gasoline through intensive life cycle assessment (LCA) studies have been conducted by groups of researchers throughout the world. Through LCA procedure, all exchanges of ethanol system with the environment and their potential impacts are examined. One instrument conventionally used to evaluate ethanol system’s efficiency is net energy value which weighs energy output against energy input. *

Corresponding author. Tel.: +66 2 4708309 10; fax: +66 2 8729805. E-mail address: [email protected] (S.H. Gheewala).

0306-2619/$ - see front matter Ó 2008 Elsevier Ltd. All rights reserved. doi:10.1016/j.apenergy.2008.02.002

Thu Lan T. Nguyen et al. / Applied Energy 85 (2008) 722–734

723

However, most assessments conducted to date focus mainly on ethanol from starch-based feedstocks [2–6] whereas just a few are available to evaluate ethanol from sugar-based feedstocks, either directly as sugar cane [7] or indirectly, as the by-product from sugar industry, e.g. molasses [8]. Having high content of fermentable sugars, molasses is considered no less important than any other feedstocks in utilization for ethanol production, especially in tropical and sub-tropical regions where the climate favours cane growth [9]. The reason for conducting this analysis is to better understand the energy performance of ethanol from molasses in Thailand. Supply potential versus production target is also an important issue to be considered. 2. Methodology 2.1. Goal and scope definition This paper aims to (1) present a full chain energy analysis of fuel ethanol from molasses in Thailand to evaluate whether the production and use of the fuel can help can help reduce fossil imports and (2) assess supply potentials of molasses for ethanol in the country based on the government target. 2.2. Molasses-based ethanol production cycle: System boundary and data sources The system boundary of the MoE life cycle is shown in Fig. 1. Major operating units located inside this boundary are sugar cane farming, molasses generation, and ethanol conversion. Transportation is a component of all operating units. Also included is the production of various items which are energy or energy-related material inputs in sugar cane farming, e.g. fertilizers, herbicides, diesel fuel, and labour. 2.2.1. Sugar cane farming Sugar cane crop rotation generally covers a two to five year period: one new planting followed by one to four ratoons. Steps involved at this stage include land preparation, planting, crop maintenance (fertilizing, weeding, watering), and harvesting. Background information on this sub-segment is available from [10–12]. Detailed information on fuel, labour and material inputs was reviewed and verified based on data collection in the central region of Thailand (CRTh), which is the largest sugar cane producing area of the country [13].

Life-style support Distribution

labour

Sugar cane farming - Land preparation - Planting - Crop maintenance - Harvesting

Transportation

gasohol

Open burning Low-end applications

surplus electricity sugar to market to grid

Sugar milling (Molasses generation) cane stalks

molasses

diesel Crude oil extraction/refining

Blending with gasoline

ethanol

Ethanol conversion

Fig. 1. Life cycle scheme for the studied system.

coal

biomass fuels Coal mining/cleaning

fertilizers herbicides

cane trash

Agrochemical production

2.2.2. Sugar milling (molasses generation) Around one year after new crop cultivation, cane stalks are cut and ready for sugar milling whereas the remaining parts, e.g. leaves and tops (termed cane trash) are either open burned or used for low-end applications. Sugar milling involves a series of process stages, e.g. crushing, clarification, boiling, seeding and centrifuging to extract sugar crystals from the cane. The process leaves behind two key co-products: one is a sticky black syrup termed molasses, the other, a straw-like residue termed bagasse. Sugar mills commonly burn bagasse to produce steam and electricity for their operation and export excess electricity to the grid.

724

Thu Lan T. Nguyen et al. / Applied Energy 85 (2008) 722–734

Energy data from a typical Thai sugar mill (with reference to technology, material use, energy use as well as product outputs) were used for the analysis. 2.2.3. Ethanol conversion The process consists of two main steps. First, molasses upon fermentation with yeast (in the presence of nutrients) yields dilute alcohol. Second, the fermented mash is passed through the distillation and then dehydration system to produce anhydrous alcohol, i.e. having 99.5% strength. The residue mash, called stillage, can be used as fertilizer, animal feed, or further refined into biogas for energy production. Data on this segment were obtained from an MoE factory in Thailand, representing one of the main suppliers to Thai oil companies. The factory employs cogeneration systems to produce both steam and electricity to meet plant energy demand. Coal represents a major part of the distillery energy requirement (59%), followed by rice husk (39%) and biogas (2%). The 2% biogas component is produced from anaerobic digestion of 12% stillage in UASB (Upflow Anaerobic Sludge Blanket) reactors. The remaining stillage is stabilized in an anaerobic pond. 2.2.4. Transportation All materials, fuels and products involved in the MoE life cycle (see Fig. 1) are hauled by different transport facilities through varying distances. Data were obtained mainly via personal communications. Educated assumptions/estimations were made where necessary. A simplified flowchart of the MoE production process in Thailand is shown in Fig. 2. 2.3. Direct energy and material inputs Direct energy and material inputs in the system are summarized in Table 1 and briefly explained as follows. – To favour crop growth, sugar cane farmers in Thailand use a modest application of mixed fertilizers, e.g. 15-15-15, 46-0-0, 21-0-0, 16-16-8. The two most common herbicides used are atrazine and ametryne. – Land preparation by farm tractors and transportation by trucks are the two main sources of diesel consumption in sugar cane farming.

Sugar Sugar milling

cane stalks

Sugar cane farming

Molasses Bagasse

Yeast, nutrients CO2 Fermentation

Supplemental biomass fuels

Surplus electricity

Steam and electricity co-generation

Distillation

Stillage

Dehydration

99.5% ethanol

Steam and electricity co-generation Biogas Fossil/biomass fuels

Anaerobic digestion Anaerobic pond stabilization

Fig. 2. Simplified flowchart of molasses-based ethanol production process.

Thu Lan T. Nguyen et al. / Applied Energy 85 (2008) 722–734

725

Table 1 Direct material and energy-related inputs/outputs associated with MoE system Segment Sugar cane farming Fertilizers Nitrogen as N Phosphorous as P2O5 Potassium as K2O Herbicides Diesel used for farm machinery Diesel used for transport Labour Sugar cane Cane trash Sugar milling Bagasse Rice husk Wood waste/bark Diesel used for transport Sugar Molasses Surplus electricity Ethanol conversion Molasses Coal Rice husk Biogas Stillage stabilized in anaerobic pond Stillage treated to produce biogas for plant use Diesel used for transport

Unit

Input

Output

kg/ha

kg active ingredient/ha L/ha L/ha man-hours/ha tonnes/ha tonnes/ha kg/tonne cane

L/tonne cane kg/tonne cane

128 37 28 10.6 63 113 518

282 7 4.4 0.11

247

103.6 45.2 16.94

kW h/tonne cane kg/L EtOH kg/L EtOH kg/L EtOH L/L EtOH L/L EtOH L/L EtOH L/L EtOH

57 11.4

4.44 0.37 0.53 18.46 1.31 0.03

18.46 9.21 1.31

– Though Thailand has been changing from a traditional agricultural-based economy to a semi-industrialized economy, human labour still represents an important input in all steps in sugar cane farming, e.g. planting, fertilizer and herbicide application, harvesting, and loading. – The solar energy stored in the biomass was not counted, since it is considered free. However, it should be noted that without this input, no other forms of energy including biomass fuels would exist. – So far bagasse is the main energy source of the sugar mill. Rice husk represents a supplemental energy source in both sugar milling and ethanol conversion. Besides, wood waste is another nationwide bio-energy resource utilized for industrial processes. – A traditional source of energy, coal is burned to produce both steam and electricity used in ethanol conversion process. Table 1 also summarizes various outputs from the system. Note that the output from a segment may become the input to another, e.g. sugar cane harvest, which is the output from sugar cane farming is an input to the sugar mill to produce sugar and molasses; the molasses output, in turn, becomes the input to ethanol conversion and so on.

3. Energy analysis 3.1. Primary energy estimation By definition, primary energy is an aggregation of heat of fuel combustion (direct energy use) and the energy used for the process of fuel extraction, refining/conversion and delivery (indirect energy use).

726

Thu Lan T. Nguyen et al. / Applied Energy 85 (2008) 722–734

(a) For diesel, fuel energy content value is obtained from a national study report [14]. Input efficiency coefficients of fossil energy sources, adapted from [15], are then used to estimate primary energy values. By the same calculation procedure, the primary energy value of coal is estimated from [2,16]. (b) A majority of the agrochemicals (e.g. fertilizers, herbicides) in Thailand are imported. It is thus reasonable to take data on energy costs of these chemical products from foreign databases [17]. (c) The energy equivalent of agricultural labour in Thailand is estimated based on ‘‘Life Style Support Energy” method [18]. Detailed derivation of the value can be found in [19]. (d) For the surplus electricity produced in the sugar mill, a factor of 10 [19] is used to convert the amount of electrical energy (kW h) to thermal energy (MJ).

3.2. Fossil energy and petroleum energy use estimation The procedure for further segregation of primary energy use/credit into fossil and petroleum energy use/ credit is summarized in Table 2 below. 3.3. Allocation for molasses use in ethanol production This study employed economic allocation to estimate the energy sequestered in the molasses input for ethanol production. The allocation ratio was derived based on the contribution of each product, sugar and molasses, to the economy. Relevant information (price and yield of sugar and molasses in Thailand) is available from [23]. 3.4. Energy performance of fuel ethanol In assessing ethanol’s energy performance, net energy value is conventionally a key indicator to identify whether ethanol production and use results in a gain or loss of energy. It weighs the energy content of ethanol against the energy inputs in the fuel production cycle. More specifically, there are three ways in which net energy issue (concerned with ethanol) is being addressed. The first one defines net energy value as follows. NEV = Energy content of ethanol Net energy inputs (total fossil and non-fossil energy inputs, excluding energy recovered from system co-products, e.g. biogas).

Table 2 Procedure for estimating fossil and petroleum energy use/credit Item

Steps of estimating

Fossil fuels Diesel, coal

Fossil energy use (MJ/L or MJ/kg) (1) Indirect energy use = primary energy – direct energy use (heat of combustion) (2) Fossil energy use in manufacturing = indirect energy use  fraction of fossil energy use in manufacturing (3) (Total) Fossil energy use = direct energy use + fossil energy use in manufacturing Petroleum energy use (1) Petroleum energy use in manufacturing = indirect energy use x fraction of petroleum energy use in manufacturing (2) (Total) Petrol. energy use = direct energy use + petrol. energy use in manufacturing (2’) (Total) Petrol. energy use = petroleum energy use in manufacturing Fossil/petroleum energy use (MJ/kg) = energy cost in manufacturing  fraction of fossil/petroleum energy use in manufacturing Fossil/petroleum energy use (MJ/man-hr) = energy cost in life style support for labour  fraction of fossil/petroleum energy in total primary energy supply in Thailand Fossil/petroleum energy credit (MJ/kWh) = Energy cost in the production of electricity in Thailand  fraction of fossil/petroleum energy in Thailand’s electricity sector

Diesel, coal

Diesel Coal Fertilizers Herbicides Human labour Surplus electricity

Information source [2,14–16] [20]

[20]

[17,20] [19,21] [19,22]

Thu Lan T. Nguyen et al. / Applied Energy 85 (2008) 722–734

727

Although energy performance has conventionally been considered using NEV, it may be more meaningful to evaluate a biofuel’s contribution to fossil energy use reduction. Such an evaluation should address how much energy is gained when non-renewable fossil fuel energy is expended to produce renewable biofuels. The equation of net (renewable) energy value, thus, takes the following form. NRnEV = Energy content of ethanol Fossil energy inputs. Net energy addressed in this way is displayed more frequently in the form of energy ratio. ER = Energy outputs/Fossil energy inputs. As far as reducing oil use (and consequently, imports) is increasingly a critical issue, the third way considers only the petroleum inputs to evaluate ethanol’s net energy gain. Energy credits of co-products, e.g. surplus electricity, animal feed, etc. are either dealt with as system energy outputs [7] or process energy savings [2]. 4. Results and discussion 4.1. Full chain energy analysis 4.1.1. Segmental energy analysis: sugar cane farming and sugar milling The analysis starts with sugar cane farming and sugar milling. As shown in Table 3, in sugar cane farming, the largest contribution comes from the energy inputs in fertilizer and herbicide manufacture at 45.6% of the total energy input, and the smallest is due to diesel fuel used for farm machinery (10.7%). The energy cost associated with one tonne cane reaching the sugar mill gate amounts to 465.4 MJ. To process sugar cane to sugar and molasses, it takes another 2263 MJ, 82% of which is provided by the bagasse as an internal energy source. Subtracting this amount of energy from the total gross energy input yields a net energy input in sugar milling of only 410.2 MJ. An energy credit has also been given to the surplus electricity produced from bagasse and other biomass fuels. At this point, the energy inputs associated with one tonne cane total 706.2 MJ. As a whole, the petroleum and fossil energy inputs contribute roughly 29.6% and 39.9% of the net energy inputs in sugar cane farming and sugar milling, respectively. 4.1.2. Life cycle energy performance of MoE It is convenient to display net energy value per litre of biofuel. Taking into account price and yield of sugar and molasses in 2006, 10.4% of the energy use in sugar cane farming and sugar milling (Table 3) are allocated to molasses. Given the conversion rate of 225 L MoE per tonne molasses or 10.17 L per tonne cane, the steps Table 3 Energy inputs in sugar cane farming and sugar milling Item

Fossil and renewable energy inputs (MJ/t cane)

Sugar cane farming Diesel fuel (farming operation) Fertilizer, herbicide Human labour Diesel (transportation) Sugar milling (Molasses generation) Bagasse Rice husk, wood waste and bark Diesel fuel (transportation) Bagasse as internal energy source Electricity output from sugar milling Net inputs

465.4 49.6 (10.7%)

441.1 49.4

212.9 44.8

212.5 (45.6%) 113.4 (24.4%) 89.9 (19.3%) 410.2

208.3 93.8 89.6 4.7

35.2 51.7 81.2 4.2

2117.6 140.8

0 0

0 0

4.7 1852.9

4.7 0

4.2 0

164.3

8.0

281.5

209.1

169.4 706.2

Fossil energy inputs (MJ/t cane)

Petrol. energy inputs (MJ/t cane)

728

Thu Lan T. Nguyen et al. / Applied Energy 85 (2008) 722–734

of calculation to get NEV and NRnEV are presented in Table 4. For a better understanding of the system’s energy performance, the energy inputs in transportation are separated and summed into one energy expenditure sub-system. The NEV result shows that MoE requires more energy to make than it is able to produce. Two possible factors contributing to this unfavourable result. First, the ethanol conversion rate as reported by the factory is relatively low compared to Refs. [8,24]. Second, a low contribution (2%) of the energy recovered from stillage treatment to the total energy requirements of the distillery. Among all sub-systems of the MoE production cycle, ethanol conversion is the most energy-consuming one, amounting up to 68.6% of the total energy inputs. The next are sugar cane farming and molasses generation at 16.7% and 14.7%, respectively (Fig. 3a). If transportation is counted as another sub-system, breakdown of the contribution made up by MoE sub-systems is as follows: ethanol conversion 63.9%; molasses generation 14.6%; sugar cane farming 13.5%; transportation 8.0% (Fig. 3b). The NRnEV result is more favourable than NEV since only fossil energy inputs are subtracted from the energy content of ethanol. As indicated in the table, the production of MoE can result in a net renewable energy gain of 5.95 MJ/L. Displayed in the form of energy ratio, the result shows that MoE is more efficient than gasoline and diesel. Each MJ of energy in MoE consumes 0.72 MJ of fossil energy, whereas each MJ of energy in gasoline and diesel consumes 1.24 and 1.19 MJ [2] of fossil energy, respectively. Even better, the estimate of petroleum inputs reveals that only a small portion (22.7%) of fossil energy inputs is petroleum-based.

Table 4 Energy performance of molasses-based fuel ethanol Item

Fossil and renewable energy inputs (MJ/L)

Molasses input as raw material Sugar cane farming Molasses generation Electricity output from sugar milling Process fuels used for MoE conversion Rice husk Coal Biogas as an energy input Biogas as internal energy source Diesel fuel used for transport activities In sugar cane farming In sugar milling In ethanol conversion and distribution Net inputs NEV / NRnEV / ER MJ ethanol produced/MJ petroleum inputs

a

8.01 3.85 4.16 1.74 18.28 7.25 11.03 0.42 0.42 2.30 0.92 0.05 1.33 26.85

Fossil energy inputs (MJ/ L)

Petrol. energy inputs (MJ/ L)

3.61

1.34

1.69 11.02

0.07 0.11

2.29

2.08

15.23

3.46

5.67 / +5.95 / 1.39 6.12

b

16.7% 14.7%

8.0%

Sugar cane farming

13.5% 14.6%

Molasses generation Ethanol conversion Transportation

68.6% 63.9%

Fig. 3. Energy consumption by segment in MoE system with transportation (a) included in each of the three main segments, and (b) separated as another segment.

Thu Lan T. Nguyen et al. / Applied Energy 85 (2008) 722–734

729

In other words, one MJ of petroleum inputs, combined with other forms of energy, can produce 6.12 MJ of ethanol fuel. It is expected that if by-products of the system, e.g. cane trash in sugar cane farming and stillage in ethanol conversion are utilized for energy, energy performance of MoE would improve significantly. This is examined in Section 4.1.4. 4.1.3. Comparison with MoE production in India A comparable case study, energy analysis of ethanol from molasses in India, was carried out by Prakash et al. [8]. The system boundary as defined in [8] includes only ethanol conversion phase. Energy consumed for the process is derived from bagasse and biogas recovered from stillage. Subtracting the amount of energy from biogas, 11.27 MJ/L, from the total process energy consumption, 21.1 MJ/L, gives a net energy input of 9.83 MJ/L. Thus, a positive NEV of 11.37 MJ/L (21.2–9.83) was evaluated. The figure of 18.7 MJ (18.28 + 0.42) presented in Table 4 shows that MoE in Thailand consumes less energy in conversion than that in India. However, 53% of energy consumption for MoE in India is derived from biogas as an internal energy source. This makes NEV of MoE in India more favourable than in Thailand where a large portion of energy consumption in ethanol conversion is derived from external sources, e.g., coal and rice husk. If the UASB reactor in the MoE plant in Thailand can treat 100% stillage to produce biogas for plant use with the rate of 35 m3 gas/m3 stillage (same as that in India), the NEV of the fuel (considering only conversion phase) becomes comparable to that of the Indian case, achieving a positive value of about 10.23 MJ/L. A lower rate of stillage produced in Thailand compared to that in India, 10.5 versus 14.0 L/L MoE, explains for its slightly lower NEV. With zero fossil energy inputs, MoE conversion in India gains more renewable energy than that in Thailand, 21.2 versus 10.18 MJ/L. 4.1.4. Sensitivity analysis Considering the segment having relatively high contribution to total energy consumption, a sensitivity analysis was conducted to find out the effect of the following factors on the base case NEV estimate: (1) energy consumption in ethanol conversion and (2) energy recovered from biogas. This can be used to estimate trends of improvement in using energy resources efficiently no matter what type of energy resource is exploited. A reduction in energy consumption in ethanol conversion would improve NEV. As shown in Fig. 4, the break-point at which NEV changes from negative to positive corresponds to about 30% reduction in the energy inputs per litre MoE. Further analysis (Fig. 5) shows that this break-point reduction could be achieved from various combinations of different ethanol conversion rates and % reduction in process energy use. In fact, based on Refs. [8,24], ethanol conversion rate would be some value not more than 285 L/tonne molasses.

3 1.5

% reduction in energy consumption in ethanol conversion NEV (MJ/L MoE)

0 0

5

10

15

20

25

30

35

40

-1.5 -3 -4.5 -6

base case

-7.5

Fig. 4. Effect of energy consumption in ethanol conversion on NEV.

45

730

Thu Lan T. Nguyen et al. / Applied Energy 85 (2008) 722–734

Evidently, bioprocess engineering nowadays makes it possible to reduce process energy demand via increasing ethanol productivity. One advanced method is cell immobilization which has been proved to bring promising results [25–28]. Various supports for cell immobilization that can be used to improve alcoholic fermentation are alumina, mineral kissiris, alginate, delignified cellulosic materials, brewer’s spent grains, carrageenan, and siliceous materials. The other important factor in determining a positive or negative NEV is the magnitude of the energy recovered from stillage treatment via biogas. As shown in Fig. 6, if more than 34.4% of the energy requirement of the distillery can be satisfied by using its own source of energy derived from biogas, NEV would become positive. A supporting analysis, summarized in Fig. 7, indicates that the 34.4% contribution of biogas to energy demand of ethanol conversion process can be achieved through combinations of varying proportion of spent wash treated to produce biogas and biogas conversion rate. Notable in the figure is the combination of 95– 100% stillage treated to produce biogas and a conversion rate of about 26–30 m3/m3 stillage; it is feasible and practical given a biogas recovery potential in the range of 22–30 m3/m3 stillage [29]. Considering all opportunities of by-product utilization to optimize MoE energy performance, effect of energy salvaged from cane trash together with 100% stillage treatment on NEV has also been examined (Fig. 8). In sugar cane production, per tonne of cane stalk harvested, there is about 200 kg cane trash left

325 30.2% reduction in the energy input per litre ethanol

(L/t molasses)

ethanol conversion rate

300 285 275 250 base case

225 200 175 5

0

10

15

20

25

30

35

40

% energy reduction in ethanol conversion Fig. 5. Break-point reduction in energy inputs per litre ethanol resulting from different combinations of % energy reduction in ethanol conversion and ethanol conversion rate.

5

NEV (MJ/L MoE)

3

1

% contribution of biogas to MoE energy consumption -1 0

5

10

15

20

25

30

35

-3

-5

base case -7

Fig. 6. Effect of biogas recovered on NEV.

40

45

50

Thu Lan T. Nguyen et al. / Applied Energy 85 (2008) 722–734

731

60 55

34.4% contribution of biogas to MoE energy consumption

45

(m3/m3 stillage)

Biogas conversion

50

40 35 30 25 20 50

60

70

80

90

100

% stillage treated to produce biogas Fig. 7. Break-point energy contribution of biogas recovered to ethanol conversion resulting from different combinations of % stillage treated and biogas conversion rate.

16 14

NEV (MJ/L MoE)

12 10 8 6 4 2 0 -2

0

10

20

30

40

50

% cane trash utilized for energy Fig. 8. Effect of energy salvaged from cane trash on NEV.

in fields [30]. Some percentage of this biomass resource is traditionally open burned. Such disposal practice is being disfavoured in the sense of energy wastage and air pollution creation. The proposed measure is not to burn cane trash in fields but instead collect it for fuel, ensuring that about 50% is reserved to secure soil quality [31]. As seen, with the contribution of energy secured from stillage only, net energy inputs in the production of ethanol and its heating value are nearly balanced, i.e. NEV approaches zero. Trends of improvement are observed with more energy salvaged from another system by-product, e.g. cane trash. The more cane trash utilized for energy, the higher the NEV of MoE. About 43% of the amount of cane trash available for MoE according to the allocation scheme, i.e. 8.9 kg/tonne cane is required to fill the gap between MoE energy requirement and the energy recovered from 100% stillage. After this point, the excess 7% trash is assumed to be utilized for electricity production for export to the grid resulting in an energy credit of 1.4 MJ/L MoE. Net energy value and oil use reduction potential of MoE considering both opportunities of utilization of stillage and cane trash for energy are presented in Table 5.

732

Thu Lan T. Nguyen et al. / Applied Energy 85 (2008) 722–734

Table 5 Energy performance of MoE with assumption that 100% stillage and 50% cane trash are utilized for energy Items Molasses input as raw material Electricity output from sugar milling Process fuels used for ethanol conversion and electricity export Cane trash as an energy input Cane trash as internal energy source Biogas as an energy input Biogas as internal energy source Energy used for cane trash collecting, baling and transport Diesel fuel for transport In sugar cane farming and sugar milling In ethanol conversion and distribution Electricity output from excess cane trash Net inputs NEV / NRnEV / ER MJ ethanol produced/MJ petroleum inputs

Fossil and renewable energy inputs (MJ/L)

Fossil energy inputs (MJ/L)

Petrol. energy inputs (MJ/L)

8.01 1.74 0

3.61 1.69 0

1.34 0.07 0

15.83 15.83 6.29 6.29 0.30

0.28

0.23

2.11

1.91

1.38 2.93

0.06 3.35

2.11 0.97 1.14 1.42 7.26 +13.92 / +18.25 / 7.22 6.32

Again, the NRnEV is more favourable than NEV result with the same reason as mentioned before. As indicated in the table, full utilization of system by-products, e.g. biogas and cane trash for energy results in an NRnEV of 18.25 MJ/L. A relatively high energy ratio of 7.2 which corresponds to an energy gain of 620% was also evaluated. Considering ethanol’s net energy gain from petroleum energy inputs, no significant improvement is observed over the base case since the scenario substitute biogas and cane trash not for petroleum but coal and rice husk. 4.2. Potentials of molasses utilization for fuel ethanol in Thailand Although molasses is a common feedstock for ethanol production in tropical countries, there is a constant risk of shortage due to high demands for this commodity in both domestic and international markets resulting in large fluctuations in molasses price. Another disadvantage lies in the fact that molasses is a by-product of the sugar industry; hence, its production rates are strongly governed by sugar cane and sugar production conditions. Total Thai sugar cane production averaged 50–60 million tonnes (Mt) a year from a planted area of 1 million hectares [13]. From this harvest, approximately 2.5–3 Mt of molasses is produced, 60–70% of which is used for liquor and animal feed, and the surplus is for export. This surplus (around 1 Mt a year) is reasonable to be used for ethanol fuel production, but 0.7–0.8 ML of MoE a day just meets about 40% of the national production target. To meet the target, a shift of sugar cane into ethanol fuel rather than its current use in sugar industry could be a probable solution. Based on a conversion rate of 70 L ethanol per tonne of sugar cane [24], it is estimated that about 5 Mt or 8–10% of the annual production of sugar cane must be diverted to ethanol production. 5. Conclusions Though the net energy (NEV) analysis provides results not in favour of the fuel, molasses ethanol has high potential to improve if co-products, e.g. stillage, cane trash are utilized for process energy in place of fossil fuels. Moreover, there are opportunities for adopting new technologies in ethanol conversion to reduce energy demand by raising ethanol productivity. Net renewable energy (NRnEV) analysis addressing how much energy is gained when non-renewable fossil fuel energy is expended to produce renewable biofuels gives results in favour of molasses ethanol. For each MJ

Thu Lan T. Nguyen et al. / Applied Energy 85 (2008) 722–734

733

of fossil energy inputs to produce molasses ethanol, there is a 39% energy gain compared to 19.5% and 15.7% loss for gasoline and diesel fuels, respectively. Even more remarkable is the figure of ethanol energy gain from petroleum energy use, 6.12 MJ/MJ. The findings highlight the positive effect of renewable fuel production in helping to reduce the dependence on non-renewable energy resources, notably petroleum the reserve of which is very near exhaustion. As per the assessment of supply potential from the surplus molasses, to meet sugar-based ethanol production target, a shift of 8–10% sugar cane produce from its current use in sugar industry to new use for fuel ethanol appears to be a feasible solution. The use of a relatively small portion of the national sugar cane production for ethanol fuel is expected to have minor impact on sugar industry. What is in need to push integrated sugar and ethanol production in Thailand is an appropriate policy favouring a flexible use of sugar cane for ethanol production either directly from sugar juice or indirectly via molasses as a by-product of sugar production. The decision on the degree of substitution between the two commodities, driven by market dynamics, would help farmers and sugar millers get reasonable prices for their produces thus stabilizing their income. Acknowledgements The authors would like to thank all officers of the Department of Agricultural Extension in the central region of Thailand for their collaboration in collecting sugar cane farming data. Thanks are also given to the sugar mill and the alcohol factory for providing relevant data on molasses generation and ethanol conversion process. The financial support of the Joint Graduate School of Energy and Environment, King Mongkut’s University of Technology Thonburi is highly appreciated. References [1] Sukphisal B. Market development of biofuels in Thailand. In: Asia biomass conference, Bangkok, Thailand; December 2005. [2] Shapouri H, Duffield JA, McAloon A, Wang M. The 2001 net energy balance of corn ethanol. US Department of Agriculture; 2004. [3] Pimentel D, Patzek TW. Ethanol production using corn, switchgrass, and wood; biodiesel production using soybean and sunflower. The limits of biomass energy. Natl Resour Res 2005;14(1):65–76. [4] Kim S, Dale BE. Allocation procedure in ethanol production system from corn grain: I. System expansion. Int J Life Cycle Assess 2002;7(4):237–43. [5] Dai D, Hu Z, Pu G, Li H, Wang CT. Energy efficiency and potentials of cassava fuel ethanol in Guangxi region of China. Energ Convers Manage 2006;47:1686–99. [6] Hu Z, Fang F, Ben D, Pu G. Net energy, CO2 emissions, and life-cycle cost assessment of cassava-based ethanol as an alternative fuel in China. Appl Energ 2004;78:247–56. [7] Macedo IC, Leal MRLV, Silva JEAR. Assessment of greenhouse gas emissions in the production and use of fuel ethanol in Brazil. Secretariat of the Environment, Government of the State of Sa˜o Paulo; 2004. [8] Prakash R, Henham A, Bhat IK. Net energy and gross pollution from bio-ethanol production. Fuel 1998;77(14):1629–33. [9] Thomas V, Kwong A. Ethanol as a lead replacement: phasing out leaded gasoline in Africa. Energ Policy 2001;29:1133–43. [10] Ando M. Present situations and improvements of upland farm management – Focusing on sugar cane production – JIRCAS Working Report 2002;30:95–101. [11] Matsuo K, Wonwiwatchai C, Yashiro M. Alternative tillage system for soil conservation, enhancement of crop growth and increasing working efficiency. JIRCAS Working Report 2002;30:111–119. [12] Srijantr T, Molle F, Chompadist C. Profitability and yield gap of sugar cane cultivation in the Mae Klong region. Thai Agri Econ J 2002;18:53–69. [13] OAE (Office of Agriculture Economics). Agricultural Statistics of Thailand. Center for agricultural information, Office of Agricultural Economics, Ministry of Agriculture and Cooperatives, Agricultural Statistics; 2006. [14] TEI (Thailand Environment Institute). Fossil fuel production and refinery LCI (2001). Bangkok, Thailand; 2001. [15] IFAS (Institute of Food and Agricultural Sciences). The energy information handbook. Florida Cooperative Extension Service. University of Florida; 1991. [16] American Embassy Jakarta. Coal Report Indonesia 2000. Coal Sector Report Indonesia 2000. Embassy of the United States of America Jakarta; 2000. . [17] Helsel ZR. Energy and alternatives for fertilizer and pesticide use. In: Fluck RC, editor. Energy in farm production. Energy in world agriculture, vol. 6. Amsterdam: Elsevier; 1992. p. 177–201. [18] Odum HT. Systems ecology. New York: Wiley; 1983, p. 476–507. [19] Nguyen TLT, Gheewala SH, Garivait S. Full chain energy analysis of fuel ethanol from cassava in Thailand. Environ Sci Technol 2007;41(11):4135–42.

734

Thu Lan T. Nguyen et al. / Applied Energy 85 (2008) 722–734

[20] Wang M. Greet 1.7 Beta – Transportation fuel cycle model. US Department of Energy, Argonne National Laboratory, Argonne, Illinois, 2006. [21] IEA (International Energy Agency). Share of Total Primary Energy Supply in 2003. Thailand – International Energy Agency Energy Statistics; 2005. . [22] DEDE (The Department of Alternative Energy Development and Efficiency) Thailand Energy Statistics 2004. Bangkok, Thailand; 2004. . [23] Prasertsri P. Thailand Sugar Semi-Annual 2006. USDA Foreign Agricultural Service – Gain Report – Global Agriculture Information Network; 2006. . [24] DEDE (The Department of Alternative Energy Development and Efficiency). Renewable Energy in Thailand: Ethanol and Biodiesel. The Department of Alternative Energy Development and Efficiency, Ministry of Energy; 2004. [25] Loukatos P, Kanellaki M, Komaitis M, Athanasiadis I, Koutinas A. A new technological approach proposed for distillate production using immobilized cells. J Biosci Bioeng 2003;95(1):35–9. [26] Kopsahelisa N, Kanellakia M, Bekatorou A. Low temperature brewing using cells immobilized on brewer’s spent grains. Food Chem 2007;104(2):480–8. [27] Kazuo M, Michiko K. Ethanol fermentation of immobilized yeast cells in a rotating basket bioreactor. Bulletin of Tokyo Kasei University. Nat Sci 2000;40:167–72. [28] Karandikar S, Prabhune A, Kalele SA, Gosavi SW, Kulkarni SK. Immobilization of thermotolerant Kluyveromyces marxianus on silica aerogel for continuous production of invert syrup. Res J Biotechnol 2006;1(2):16–9. [29] Gupta BP. Status of biomethanation development in India. World J Microbiol Biotechnol 1998;4:95–8. [30] Methacanon P. Raw materials available in Thailand. InnoBioPlast, Bangkok, Thailand; 21–24 September 2006. [31] Gabra M. Sugarcane residual fuels – a viable substitute for fossil fuels in the Tanzanian sugar industry. Renew Energ Develop 1995;8(2):5–6.