Carbon Balance of Sugarcane Bioenergy Systems

Biomass and Bioenergy 20 (2001) 361–370

Carbon balance of sugarcane bioenergy systems Revin Panray Beeharry ∗ Department of Chemical and Sugar Engineering, Faculty of Engineering, University of Mauritius, Reduit, Mauritius Received 10 April 2000; received in revised form 12 December 2000; accepted 13 December 2000

Abstract An important criterion for bioenergy systems evaluation is their greenhouse gas mitigation potential. Sugarcane bioenergy systems are able to produce grid-bound surplus electricity but also have net CO2 emissions associated with the upstream fossil-fuel consumption for plantation management, transportation and processing of the +brous biomass. However, when compared to coal-based power generation systems, sugarcane bioenergy systems are able to avoid CO2 emissions at rates that range between 1.081 and 1:137 kg CO2 =k Wh depending on the cane-residue utilisation strategy adopted. As a consequence, sugarcane bioenergy systems stand out as promising energy projects for funding under the Kyoto Protocol’s proposed clean c 2001 Elsevier Science Ltd. All rights reserved. development mechanism and joint implementation.
Keywords: Sugarcane; Bioenergy; Greenhouse gas; Clean development mechanism

1. Introduction 1.1. Background It is widely believed that there are strong linkages between fossil energy utilisation and global environmental problems such as global warming. Cellulosic energy crops such as sugarcane biomass are one alternative to fossil fuel combustion for power generation and are part of a larger bioenergy strategy for coping with mitigation of greenhouse gas (GHG) emissions. It has been a popular misconception that bioenergy systems have no net CO2 emissions. Bioenergy strategies when operated on a sustainable basis were thought to reabsorb the equivalent amount of CO2 which is released by oxidation of the biofuels and contribute no net CO2 to the atmosphere. From previous work on ∗

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the subject [1], it has been established that sugarcane bioenergy systems require a net fossil-fuel contribution that enables them to produce surplus electricity. This upstream fossil-fuel contribution to the bioenergy strategy then amounts to net releases of CO2 . Since the net CO2 released per unit energy produced is signi+cantly lower compared to fossil fuels, sugarcane bioenergy systems stand out as promising candidates for GHG mitigation. 1.2. Flexibility mechanisms The clean development mechanism (CDM) and joint implementation (JI) are Cexibility mechanisms that have been proposed in the Kyoto Protocol [2] for GHG mitigation. They are intended to: (1) assist developing countries in achieving sustainable development and in contributing to the ultimate objective of the United Nations Framework Convention on

c 2001 Elsevier Science Ltd. All rights reserved. 0961-9534/01/$ - see front matter  PII: S 0 9 6 1 - 9 5 3 4 ( 0 0 ) 0 0 0 9 4 - 5

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Climate Change Climate (UNFCCC); and (2) to assist developed countries in achieving compliance with emission reduction targets. These mechanisms aim to accomplish the above objectives through projects (such as for power generation) that reduce GHG emissions in developing countries (non-Annex 1 countries) and provide development bene+ts to host countries. For example, the CDM allows Annex 1 countries (those with an obligation to cut GHG emissions) to use certi+ed emission reductions to show compliance with the requirements of the Kyoto Protocol. An understanding of the GHG balances of sugarcane bioenergy systems is hence of capital importance if such projects are to be considered under the CDM. Mauritius oIers an interesting case to study. 1.3. Sugarcane and the Mauritian economy Mauritius is an island situated in the south-west Indian Ocean and has a land area of just over 2000 km2 . The population is below 1.2 million. Mauritius has experienced a consistent real GDP growth of 5 –7% since 1984. Until fairly recently, Mauritius was to a large extent a mono-crop economy, dominated by sugar production. The GDP share of sugar has declined from over 25% in 1970 to just under 10% in 1997. Mauritius derives almost 25% of its exports earnings from sugar and the sugar industry provides close to 12% of employment on the island. During the last two decades the average yield of sugarcane plantations has remained quasi-stable at around 66 t cane=ha. Current annual cane production is around 5:8 M t of cane from which a total output of 580;000 t of sugar is produced. There are 14 sugar factories operating in Mauritius and during the crushing season they are all energy self-suKcient since bagasse-derived steam and factory-generated electricity are used to power the whole sugar mill. Total energy demand in Mauritius was closely coupled to gross domestic product over the last two decades. Demand for electricity has been growing at the rate of 8–10% per annum [3]. Mauritius imports almost 70% of its energy requirements. Locally available energy resources are bagasse from the sugar industry, hydro and woody biomass. In 1998, for example, the sugar factories produced 195 GWh of surplus electricity that is almost 14% of the island’s total electricity production. The potential to further

increase the share of sugarcane biomass derived power in Mauritius exists as cane residues (cane tops and leaves as well as cane trash) are not currently exploited for energy purposes. 1.4. Greenhouse gas analysis of bioenergy systems Greenhouse gas (GHG) analyses of fossil fuel chains have been carried out by several authors and published data on the GHG transactions of various commercial fuels such as diesel and coal are readily available [4,5]. Similar GHG studies have been carried out on bioenergy options such as forestry residues, miscanthus [6], hybrid poplar [7], sorghum [7] and other cellulosic energy crops [8]. Ribeiro and Rosa [9] have studied the CO2 abatement potential of fuel alcohol in Brazil. However, the GHG implications of sugarcane bioenergy systems for producing electricity have not been studied in detail. 2. Methodological framework 2.1. Life cycle assessment The methodological framework that has been used in this paper is based on life cycle assessment (LCA) methodology. Ideally, the methodology makes it possible to account for all material and energy Cows associated with the bioenergy system while keeping track of outputs (products, energy, waste materials) all along the production cycle, that is from “craddle to grave”. The technique adopted in this paper does not go as far as an LCA might. Emissions associated with construction and decommissioning of processing plants (sugarcane, fertiliser) and power generation facilities have not been accounted as these were judged to be insigni+cant in comparison with environmental eIects associated with the manufacture of fertilisers for example. 2.2. Goal and scope of the GHG analysis The goals of the GHG analysis are to evaluate and compare the GHG implications of four sugarcane bioenergy options shown in Table 1. The reference bioenergy system is based on the current practice in Mauritius. The other three alternatives have been

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363

Table 1 System de+nitions Electricity outputa (k Wh=t cane)

Sugarcane bioenergy option Yield = 66;000 kg cane=ha

Characteristics

Reference

Based on current practice in Mauritius. Only bagasse (30% of cane mass) is exploited for surplus power production.

41

Composted bagasse option

Rigorous steam management to promote bagasse savings. 25% of bagasse used for compost making. Remaining 75% of bagasse used for factory steam and surplus power generation. Cane yield=ha is 30% higher due to the eIect of compost [1]. Cane tops and leaves are included in the fuel chain. Bagasse production is 53% of cane mass processed. Both the cane tops and leaves and cane trashb are included in the fuel chain. Bagasse production is 71% of cane mass processed.

58

Whole cane option Baled residue option a From b Dead

158 276

Beeharry [10]. and dry, partially detached leaves.

Fig. 1. Analytical framework for CO2 balance and comparison.

constructed on the basis of improved utilisation of sugarcane biomass for enhancing surplus power production. The system boundary (see Fig. 1) in the case of the bioenergy system encompasses (1) culti-

vation and harvesting of sugarcane, (2) transport of the cane biomass and (3) conversion of the biomass into products (sugar) and by-products (electricity). All material and energy Cows that cross the system

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boundary were recorded as annual inputs and outputs. Since CO2 emissions are of an order of magnitude larger than any other greenhouse gas and account approximately for half of the anticipated global warming [7], the analysis that follows focuses on the CO2 implications of the sugarcane bioenergy strategies. The hard-coal-fuel chain is the alternative to bagasse-based power generation in Mauritius and has thus been retained for comparison purposes. 2.3. Comparative analysis For a bioenergy strategy to be an eligible climate-change mitigation option and for such a strategy to qualify under the CDM, it should emit less CO2 compared to the fossil-fuel-based alternative that it is meant to replace (hard coal in Mauritius). A direct comparison can be made between two alternatives that produce the same amount of useful energy (see Fig. 1). The fossil-fuel chain converts fossilised carbon into electricity while emitting CO2 during each of the steps (mining and processing, transport and conversion). That is, in addition to the combusted carbon released as CO2 , the fossil system also emits CO2 as a result of the use of fossil fuels in each of the stages. In the case of the sugarcane bioenergy fuel chain (see Fig. 1), atmospheric carbon is absorbed by the growing biomass and this carbon is ultimately sequestered in the biofuel (bagasse) as well as in the residues, stubble, roots (ultimately in the soil) and the products. When combusted, the bagasse releases the carbon to the atmosphere as CO2 mainly. Each

stage of the transformation is responsible for CO2 emissions that are associated with the use of fossil fuels in each of the processes shown in Fig. 1. For the purposes of the analysis, the carbon contained in plant matter found underground (in the soil) as well as the carbon sequestered in the products have been considered as closed cycles. 3. Energy analysis of sugarcane bioenergy systems 3.1. Fossil fuel inputs The upstream energy (fuel and electricity) consumed for each of the four sugarcane bioenergy systems has been investigated by the author [10]. Natural gas, diesel, coal and electricity (see Table 2) are directly and indirectly used in upstream processes for the manufacture and application of fertilisers, irrigation of plantations, harvest, transportation and for processing of the sugarcane. In contrast with the reference system, recovery of the cane tops and leaves (whole cane option) for example, requires additional diesel for transportation and additional electricity for processing the extra +bre into bagasse. Likewise, the other options require diIerent levels of the fossil-fuel input depending on the energy intensity of the respective upstream processes. It is clear that the higher output of surplus electricity that is achievable by the three improvement options can only take place at the expense of higher levels of fossil-fuel usage in upstream processes.

Table 2 Fossil-fuel inputs Inputa

Option

Natural gas (m3 ) Diesel (l) Coal (kg) Electricity (k Wh) Cane output (t) Surplus electricity output (k Wh=t cane) a Based b Yield

Reference

Composted bagasse

Whole cane

Baled residue

127 93 84 750 66 41

127 138 127 847 86b 58

127 147 84 1409 66 158

127 208 84 2389 66 276

on 1 ha of sugarcane plantation exploited for 1 year. with compost application is 30% higher.

R.P. Beeharry / Biomass and Bioenergy 20 (2001) 361–370 Table 3 CO2 coeKcients applicable in Mauritius

365

Table 4 Results of the allocation procedure

Fuel=energy carrier

CO2 coeKcients

Natural gas (kg CO2 =m3 ) Diesel (kg CO2 =l) Coal (kg CO2 =kg) Electricity (kg CO2 =k Wh)

2.11 2.91 2.85 0.55

3.2. CO2 coe4cients

Option

Burden sharing factor for surplus electricity (%)

Reference Composted bagasse Whole cane Baled residue

20 26 50 64

one hand and the amount of bagasse that is used for surplus electricity production on the other. In the case of the reference system, only 20% of the bagasse is used for surplus power production and hence only 20% of the total CO2 burden is allocated to the surplus electricity produced. In contrast, 64% of the total bagasse produced by the Baled Residue Option is turned into surplus electricity and hence the CO2 burden-sharing factor is 64%.

CO2 coeKcients from Table 3 have been used to convert the energy transactions of the sugarcane bioenergy system into emissions of CO2 . The detailed derivation of the coeKcients can be found in the author’s publication [10]. CO2 emissions accruing from fuel input to machinery manufacture and building construction and decommisionning are assumed to be negligible and have not been accounted for. However, the CO2 emissions associated with the energy used for producing fossil fuels have been factored into the accounting.

4. Results of the GHG analysis

3.3. CO2 burden sharing

4.1. CO2 emissions

The sugarcane system produces sugar, molasses and surplus electricity while consuming bagasse as the sole fuel at the factory level. It is thus important to allocate a certain CO2 burden to sugar and molasses on the one hand and to the surplus electricity on the other. The calculation of the CO2 burden sharing has been based on the principle of resource consumption [10,11]. The allocation factor (see Table 4) for each option has been calculated on the basis of the amount of bagasse that is used for sugar and molasses manufacture on the

The CO2 emissions associated with upstream fossil-fuel utilisation for each option has been calculated using the data presented in Tables 2– 4. For each option, the amount of each fossil fuel (and electricity) consumed is multiplied by the appropriate CO2 coeKcient (see Table 3) to get the total CO2 emitted by the system. Finally, the CO2 attributable to surplus electricity is obtained by multiplying the total CO2 by the appropriate burden-sharing factor from Table 4. The results expressed on a k Wh basis are given in Table 5.

Table 5 Comparative CO2 emission

Electricity output Eout (k Wh=t cane)

Reference

Whole cane

Baled residue

Composted bagasse

41

158

276

58

0.035 1.137 0.97

0.083 1.082 0.93

0.084 1.081 0.93

0.078 1.087 0.93

GHG performance CO2 emission (kg CO2 =k Wh) Avoided CO2 (kg CO2 =k Wh) Carbon neutralitya a See

Eq. (2).

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4.2. Avoided CO2 emission Avoided carbon emission is used as a measure of the GHG mitigation potential of the sugarcane bioenergy option. Avoided CO2 (kg CO2 =k Wh) is calculated as follows: Avoided CO2 = Cfos − Cbio ;

(1)

where, Cbio (kg=k Wh) is the CO2 emission from the bioenergy option, Cfos (kg=k Wh) is the CO2 emission from the fossil-fuel-based system that the bioenergy system is meant to replace. The avoided CO2 that can be achieved per unit electricity produced for each option is given in Table 5. The results indicate that on a per k Wh basis, the avoided CO2 is between 1.081 and 1:137 kg of CO2 . 4.3. Carbon neutrality An alternative measure is the carbon neutrality of the bioenergy option which is de+ned as Carbon neutrality (CN) = 1 − (Cbio ÷ Cfos )

(2)

For purposes of analysis, the fossil system retained coal with 1:165 kg CO2 emitted for every k Wh

produced. A truly carbon neutral bioenergy strategy would have a CN value of 1. On the other hand, systems that emit the same amount of CO2 , compared to their fossil-fuel alternatives, will have a CN value of 0. Negative carbon neutrality would imply that the bioenergy strategy is more CO2 intensive than the fossil alternative. The carbon neutrality of the reference system is the highest while those of the other options are close to 0.93; reCecting the higher fossil-fuel input that is required to collect and process additional sugarcane biomass. 4.4. Carbon closure The carbon closure [12] of a bioenergy system has been de+ned as Carbon closure = 100 × [1 − (CNet –CAbs )];

(3)

where CNet is the amount (kg) of CO2 released from the system as a result of the fossil fuel used in upstream processes, CAbs is the amount (kg) of CO2 absorbed by the biomass during growth. Since fossil fuel use is the only source of CO2 that is not counterbalanced by what is absorbed by the biomass, a process that does not use any fossil fuel will have a 100% carbon closure. In other words,

Table 6 Total annual carbon uptake for 1 ha of sugarcane plantation Component

Sugar Molasses Filter cake Water Bagasse CT&L Trash Root system a Cane

Share of millable cane

Amounta

Moisture

Dry biomass

Carbon content (dry basis)

Carbon sequestered

Equivalent CO2 uptake

(%)

(kg)

(%)

(kg)

(%)

(kg)

(kg)

6600 1584 528 0 9900 6611 5433 2970

0.420b

0.357c 0.400d 0.000 0.499e 0.493e 0.490e 0.490f

2772 565 211 0 4940 3259 2662 1455

10,164 2073 774 0 18,114 11,950 9761 5336

Total

15,865

58,172

10 3 4 52 30 31 10 15

6600 1980 2640 34,320 19,800 20,658 6732 9900

0 20 80 100 50 68 19 70

yield = 66 000 kg=ha=yr. b Assuming sugar is pure sucrose [C H O ]. 12 22 11 c Brix = 0:85 (sucrose=dry solids). d From Paturau [13]. e Based on ultimate analysis [10]. f Assuming carbon content comparable to the other +brous components. Net soil carbon sequestration is assumed to be zero.

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Table 7 Carbon closure of the sugarcane bioenergy optionsa

Net CO2 released (kg) CO2 absorbed (kg) Carbon closurec

Reference

Composted bagasseb

Whole cane

Baled residue

452 58,172 99

1496 75,624 98

1711 58,172 97

2427 58,172 96

a Based

on 1 ha of plantation over 1 year. 30% higher cane yield=ha is achieved. c The carbon sequestered in the root system is assumed to be released to the atmosphere as a result of decay. b Note

all CO2 produced will be re-absorbed by the system resulting in a zero net carbon system. The total annual CO2 uptake by 1 ha of sugarcane plantation with an average yield of 66;000 kg of sugarcane has been estimated in Table 6. The computation is based on the CO2 equivalent to the carbon contained in each component of the sugarcane plant. It is estimated that a total of 58:2 t of CO2 (CAbs ) is absorbed annually by the growing sugarcane biomass. The results shown in Table 7 indicate that the reference bioenergy system operates with an almost closed carbon cycle as only a relatively small amount of CO2 (CNet ) associated with upstream processes is released over the fuel cycle. As more of the biomass is recovered and processed for enhancing the electricity output of the bioenergy system, the carbon closure is reduced to 97% for whole cane option and further down to 96% for the baled residue option. In the case of the composted bagasse option, carbon closure is at an intermediate value of 98%. 4.5. Carbon 9ows The carbon Cows (expressed as kg CO2 ) associated with each of the four sugarcane bioenergy options are presented in Figs. 2–5. If only bagasse is combusted for power generation, almost 32% of the carbon is immediately released in the fuel gas (and a minute quantity as ash) at the power plant. Molasses and sugar carry around 21% of the equivalent CO2 while the remaining 47% remain lodged in the CT&L, trash and root system. In the case of the composted bagasse option, only 23% of the carbon is released at the power plant and around 9% becomes part of the compost that is applied to the soil. As more of the biomass is combusted (the whole cane and baled residue option),

the amount of carbon that is released at the power plant increases from 31 to 52% and to 57%.

5. Conclusions The analysis has con+rmed the potential for using sugarcane bioenergy systems for surplus electricity generation, while helping to reduce the amount of CO2 that is emitted to the atmosphere. Using data from Mauritius, it has been possible to show that depending on the degree of cane residue utilisation, the avoided CO2 emissions (compared to the hard coal fuel chain) would range between 1.081 and 1:137 kg CO2 =k Wh in Table 5. In eIect, the analysis has been able to track energy use and hence CO2 emissions throughout the entire fuel cycle to con+rm the candidature of sugarcane bioenergy systems for consideration under the proposed Clean Development Mechanism. Higher output of surplus electricity from sugarcane bioenergy systems is possible if otherwise unused parts of the sugarcane plants are recovered for energy production. The analysis has shown that the whole cane option is able to produce bagasse at the rate of 53% (compared to 30% for the reference system) of cane yield resulting in a surplus electricity output of 158 k Wh=t cane. The baled residue option would produce bagasse at the rate of 71% and produce 276 k Wh of electricity=t cane. An intermediate strategy, the composted bagasse option enables modest enhancement in surplus electricity production with around 30% increase in cane and sugar yield. The data also made an indepth analysis of the Cow of carbon in such systems possible. Net CO2 emissions are associated with fossil-fuel utilisation in upstream processes and as a consequence sugarcane bioenergy

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Fig. 2. Carbon Cows associated with the reference option.

Fig. 3. Carbon Cows associated with the composted bagasse option.

R.P. Beeharry / Biomass and Bioenergy 20 (2001) 361–370

Fig. 4. Carbon Cows associated with the whole cane option.

Fig. 5. Carbon Cows associated with the baled residue option.

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