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Trang 4Sugarcane and Ethanol Production and Carbon Dioxide Balances
Marcelo Dias De Oliveira
Abstract Ethanol fuel has been considered lately an efficient option for reducing
greenhouse gases emissions Brazil has now more than 30 years of experience withlarge-scale ethanol production With sugarcane as feedstock, Brazilian ethanol hassome advantages in terms of energy and CO2balances The use of bagasse for en-ergy generation contributes to lower greenhouse gases emissions Although, whencompared with gasoline, the use of sugarcane ethanol does imply in reduction ofGHG emissions, Brazilian contribution to emission reductions could be much moresignificant, if more efforts were directed for reduction of Amazon deforestation Thetrend however is to encourage ethanol production
Keywords Sugarcane ethanol · CO2 mitigation · CO2 balances · bagasse ·
Co-generation
9.1 Introduction
When the oil crisis hit Brazilian economy, and raised concerns about nationalsovereignty in the mid-70’s, sugarcane industrialists were quick to perceive in thescenario an opportunity to avoid bankruptcy After some ups and downs of theBrazilian ethanol program the same sector is taking advantage of another scenario,this time related to growing environmental concerns regarding global warming.Brazil now has jumped on the bandwagon of the environmentally friendly fuel al-ternative, and is experiencing a revival of the ethanol program, the Pr´o-alcool, firstestablished in the mid 70’s
Government incentives and subsides established by the Pr´o-alcool program, letthe country to experience a considerable increase of ethanol production and ethanol-fueled automobile passenger fleet By 1984, 94.4% of the passenger cars in Brazilwere fuelled by ethanol Posterior decline in oil prices associated with increase of
M.D De Oliveira
Avenida 10, 1260, Rio Claro - SP - Brazil, CEP 13500-450
e-mail: dias oliveira@msn.com
D Pimentel (ed.), Biofuels, Solar and Wind as Renewable Energy Systems,
C
Springer Science+Business Media B.V 2008
215
Trang 5Brazilian domestic production and high prices of sugar contributed to an expressivereduction of ethanol production in the country By 1999, ethanol-fueled cars fell toless of one percent of total sales (Rosa and Ribeiro, 1998).
Current enthusiasm with Brazilian biofuels, particularly sugarcane ethanol, ismotivated by increasing worldwide concerns with climate change Government, so-ciety and scientists talk passionately about the benefits of a “green” energy sourceand possible Brazilian contributions for the reducing of greenhouse gases (GHG)emissions The ethanol industry is quickly capitalizing the benefits of these cir-cumstances, and Brazilian government is clearly willing to encourage increases forethanol production
The present study analyses the CO2balance for Brazilian sugarcane ethanol andits possible contributions for GHG mitigation
9.2 The “Green” Promise
Biofuels are frequently portrayed as “clean fuel” (Moreira and Goldemberg, 1999;Macedo, 1998) and considered to be carbon neutral, since CO2 emitted throughcombustion of motor fuel is reabsorbed by growing more sugarcane rendering thebalance practically zero (Rosa and Ribeiro, 1998) Numerous articles advocate for
an increase in biofuels production and consumption as an environmentally friendlyoption (Macedo, 1998; Moreira and Goldemberg, 1999 and Farrel et al., 2006).Sugarcane ethanol is considered and efficient way of reducing CO2 emissions
of energy production According to Rosa and Ribeiro (1998), the use of ethanolfuel can have a significant contribution to greenhouse gas mitigation Moreira andGoldemberg (1999), consider the main attractiveness of the Brazilian ethanol pro-gram, the reduction of CO2 emissions compared with fossil fuels, as a solutionfor industrialized countries to fulfill their commitments with the United NationsFramework Climate Change Convention (UNFCCC) Beeharry (2001), points outthat since the net CO2 released per unit of energy produced is significantly lowercompared to fossil fuels, sugarcane bioenergy systems stand out as promising candi-dates for GHG mitigation Feedstock for ethanol production, in this particular case,sugarcane, grows by transforming CO2 from atmosphere and water into biomass,which is, as mentioned before the reason why such fuel is called carbon neu-tral Nonetheless, fossil fuel emissions are always associated with any agriculturalactivity
It has been a popular misconception that bioenergy systems have no net CO2sions (Beeharry, 2001) Considerable amounts of fossil fuel inputs are required forplant growth and transportation, as well as for ethanol distribution, therefore CO2emissions are present during the process of ethanol production Fertilizers, herbi-
Trang 6emis-Table 9.1 Carbon Dioxide emissions from the agricultural phase of Brazilian sugarcane
1 Grupo Cosan – Brasil.
2 Pimentel and Pimentel – 1996.
3 Based on Pimentel and Pimentel – 1996.
4 West and Marland (2002).
␣values correspondent to oil consumption of all agricultural activities and transport of sugarcane
to distilleries.
cides and insecticides have net CO2 emissions associated with their production,distribution and application CO2 emissions from agricultural inputs of sugarcaneproduction are represented on Table 9.1
Sugarcane production also results in emissions of other GHG, namely methaneand nitrous oxide Based on Lima et al (1999), CH4 and N2O emissions fromsugarcane correspond to 26.9 and 1.33 kg per hectare respectively Such emissionscorrespond to, based on Schlesinger (1997), 672 kg and 399 kg respectively of CO2equivalent
As for its distribution, based on Shapouri et al (2002), 0.44 GJ are requiredper m3 of ethanol, assuming diesel fuel is the source of this energy, and based onWest and Marland (2002) CO2emissions associated with ethanol distribution are of
227 kg Therefore net CO2emissions from ethanol production is 2926 kg CO2/ha ofsugarcane (Table 9.2)
Theoretically, there are no GHG emissions associated with distillery operations.All the energy required comes from the burning of bagasse, which is a residue ofthe milled sugarcane In fact the burning of bagasse generates more energy than thedistillery requires, resulting in some surplus of energy Conceptually CO2emissionsassociated with bagasse burning are not accounted for, since where sequestered
Table 9.2 Carbon dioxide emissions from Brazilian ethanol production
Process CO 2 equivalent emissions per ha
Trang 7during sugarcane growth and will be re-absorbed in the next season The samerationale applies to the ethanol burning in mother vehicles For accounting purposes
a complete combustion is assumed in both cases
Based on an average production of 80 tons per ha which is representative of theState of S˜ao Paulo, (Braunbeck et al., 1999), and ethanol conversion efficiency of
80 L per ton of sugarcane processed (Moreira and Goldemberg, 1999); the amount
of ethanol resulting from one ha or sugarcane plantations is 6.4 m3 Consequentlyfor production of one m3 of ethanol, GHG emissions account to 457 kg of CO2eqproduction and distribution, this corresponds to approximately 19 kg of CO2 pergigajoule (kg/GJ) of fuel Comparative values of CO2emission of other fuel sourcesare indicated on Table 9.3
Estimating the potential for GHG reduction from the use of ethanol derived fromsugarcane requires a comparison with the fossil fuel displaced In Brazil the auto-mobile fleet has basically three fuel options, natural gas, ethanol and gasoline, thelast option is actually a mixture of gasoline and ethanol The proportion of eachfuel varies slightly according to government decisions, currently is 75% gasolineand 25% ethanol Natural gas running automobiles are not manufactured in Brazil,but automobiles can be converted to natural gas at a price ranging from US$ 1200
to US$ 2100.1Although conversion to natural gas continues to rise in Brazil ulated by its fuel economy, currently such vehicles represent only about 5% of theautomobile fleet The main attention in this work will be devoted to the impacts ofethanol substitution for gasoline
stim-In 2003, Brazil began to produce flex fuel cars, which can run with both gasolineand ethanol in any proportion using the same tank In that year about 40 thousand
of such automobiles were produced, corresponding to only 2.6% of the new cars In
2006, flex fuel cars corresponded to almost 60% of the new cars with 1.25 millionunits (Anfavea, 2007) This augment is directly related with a strategy for increasingbiofuel consumption in Brazil, where the consumer is stimulated to use ethanol as
an environmental responsible option The differences in price between ethanol andgasoline also contribute for the scenario Presently in Brazil, ethanol is about 49%cheaper than gasoline, mostly due to heavier incidence of taxes over gasoline The
Table 9.3 Comparative emissions of different fuels
Sugarcane ethanol (Brazil) 19
␣Dias de Oliveira et al (2005).
West and Marland (2002).
1 Based on Dondero and Goldemberg (2005) and considering 1 US$= 2 reais
Trang 8advantage of flex fueled cars is that owners can trade back and forth between ethanoland gasoline according to the prices at the pump.
9.4 Gasoline Versus Ethanol
To estimate the effectiveness that ethanol fuel has on reducing GHG emissions forBrazilian conditions, a comparison is made considering the fuel economy of flexfuel automobiles when using ethanol or gasoline
As mentioned before the production and distribution of one m3of ethanol results
in emissions of 457 kg of CO2eq Assuming a kilometerage for Brazilian flex elled cars of 11.78 km/L for gasoline and 8.92 km/L for ethanol.2A flex fuelled carusing one m3of pure ethanol can run for 8920 km, to travel the same distance usinggasoline as fuel 757 L are necessary Given that gasoline in Brazil is actually sold as
fu-a mixture of 75% gfu-asoline fu-and 25% ethfu-anol, such volume of gfu-asohol corresponds
to 568 L of gasoline and 189 L of ethanol According to West and Marland (2002),production, distribution and combustion of one m3 of gasoline result in emissions
of 2722 kg of CO2, therefore the 568 L of gasoline will result in 1546 kg CO2 Forthe 189 L of ethanol, the amount of CO2emitted correspond to 86 kg, consequentlytotal CO2emissions add up to 1632 kg Hence ethanol option represents 1175 kg of
CO2 emissions avoided per m3produced In the hypothesis of pure gasoline beingused instead of gasohol, to substitute one m3of ethanol used, approximately 673 L
of gasoline are required, resulting in total emissions of 1832 Kg, that is, 1375 Kg
CO2more than the ethanol being replaced
9.5 Bagasse as a Source of Energy
The bagasse, is the residue of sugarcane after the same is milled It has mately 50% humidity and results in amounts of 280 kg/t of sugarcane (Beeharry,2001)
approxi-The burning of bagasse provides heat for boilers that generate steam and producethe energy required for distillery operations Since the energy generated surpassdistillery necessities, this surplus of electricity has potential for being exported,which is usually known as cogeneration, and according to Beeharry (1996), of-fers the opportunity to increase the value added while diversifying revenue sourcesfor distilleries According to Rosa and Ribeiro (1998), the utilization of sugar-canebagasse for electricity generation may become the great technological breakthroughfor Pr´o-´alcool in the context of sustained economic development while conservingthe environment They point out that the period of harvest of the sugar cane corre-sponds to the “dry period” in the Brazilian hydroelectric system, thus making the
2 Average values based on three of the most sold cars in Brazil, Volkswagen Gol, Fiat Palio, and Celta-Chevrolet, according to Paulo Campo Grande - Quatro Rodas.
Trang 9use of bagasse in the area particularly attractive for complementing hydroelectricitygeneration.
Brazilian distilleries generate an average surplus of 1.54 GJ (428 kWh) per
ha or sugarcane processed (Dias de Oliveira, 2005) This corresponds to boilersproducing steam operating at pressures of 20 bar generating small amounts ofelectricity (15–20 kWh/ton of cane) enough for the needs of the unit (Moreira andGoldemberg, 1999)
According to Beeharry (1996), advanced technologies could result in the tion of 0.72 GJ (200 kWh) per ton of sugarcane milled Such scenario would result in
genera-a vgenera-alue of energy surplus per hgenera-a or suggenera-arcgenera-ane of genera-approximgenera-ately 54 GJ (15000 kWh)
or 8.43 GJ (2342 kWh) per m3of ethanol Intermediate values indicated by Beeharry(1996), result in the generation of 0.45 GJ (125 kWh) of electricity per ton of sug-arcane milled, representing a surplus of 32.4 GJ (9000 kWh) per ha of sugarcane or5.06 GJ (1406 kWh), per m3of ethanol
According to personal communication in a visit to the Center for SugarcaneTechnology (CTC) – Piracicaba, boilers operating with pressures of 20 bars are sofar the standard in Brazilian operating distilleries, with new plants being equippedwith boilers that work at pressures of 60 bars, and are capable of generating a surplus
of 0.14 GJ (40 kWh) of energy per ton of sugarcane milled Still according to CTC,advanced technologies are yet economically unfeasible
To better illustrate the impacts that the conditions mentioned above would have
in terms of CO2emissions, a comparison will be made with current Brazilian tem of electricity generation According to Brazilian National Agency of ElectricityEnergy (ANEEL), electricity generation in Brazil comes from the sources indicated
sys-on Table 9.4
With the dominance of hydroelectricity generation, Brazilian electricity matrix
is responsible for relatively low CO2 emissions per kWh of electricity produced(kWhel) Compared with other sources, hydroelectricity has low carbon dioxide in-tensity (Krauter and Ruthers, 2004; Weisser, 2007; van de Vate, 1997) An importantpoint though, made by Rosa and Schaeffer (1995) and Fearnside (2002), is thatemissions from hydroelectric dams can be much higher than usually attributed forthis source, mostly owning to methane emissions resulting from anaerobic decom-position of organic matter of the inundated areas in hydroelectric reservoirs.Considering Brazilian electric energy matrix and based on West and Marland(2002), Krauter and Ruthers (2004), and van de Vate (1997), each kWhelgenerated
Table 9.4 Brazilian electricity energy matrix
Trang 10Table 9.5 Estimated avoided emissions resulted from the use of ethanol as fuel instead of gasoline,
and the surplus of electricity generated by distilleries∗
Avoided emissions (kg) per ha surplus electricity
∗Values calculated do not account for energy losses associated with electricity transmission
in Brazil corresponds to net CO2emissions of approximately 139 grams, comparedwith the to 660 g per kWhelof US calculated by West and Marland (2002) or the
530 kg/kWhel and 439 Kg/kWhelof Germany and Japan respectively, as calculated
by Krauter and Ruthers (2004)
Consequently the surplus of electricity per ha of sugarcane is responsible for
59 kg of avoided CO2 emissions per ha of sugarcane or 9 kg per m3 of ethanolproduced With current Brazilian ethanol production of 16 million m3, total avoided
CO2 emissions due to electricity generation correspond to 144,000 tons of CO2kg/year
In the hypothesis that advanced technologies usually referred to as biomass grated gasifier/gas turbine (BIG/GT) were the standard in Brazilian distilleries, theamount of CO2 emissions avoided per ha of sugarcane would be of approximately
inte-2085 kg or 326 kg per m3 of ethanol Intermediate technologies would representavoided emissions of 1251 kg of CO2 per ha or 195 kg CO2 per m3 of ethanol.Nevertheless, as mentioned before, advanced technologies are not yet economicallyfeasible
Considering differences in emissions from use of ethanol and gasoline, and thepotential electricity generation of distilleries, avoided emissions for the possiblescenarios of ethanol production in Brazil are summarized on Table 9.5
The results above indicated that consumption of ethanol, produced with currentpractices in Brazil, reduces CO2atmospheric emissions by 1184 kg/m3, when com-pared with gasoline use Cardenas (1993), cited by Weir (1998), reports reduction
in CO2emission of 1594 kg/m3of ethanol used in Argentina
According to Beeharry (2001), the use not only of the bagasse, but also sugarcanetops and leaves can contribute to distilleries potential for electricity exportation;such option however, would imply the elimination of pre-harvest burning and theuse of cane residues that would otherwise be left on the soil, contributing to reducesoil erosion
9.6 Pre-Harvest Burning of Sugarcane and Mechanical Harvest
One aspect very criticized of sugarcane production is its pre-harvest burning, whichhas a series of negative impacts The practice is adopted in order to facilitate themanual cut of the sugarcane According to Kicrkoff (1991), pre-harvest burning is
Trang 11responsible for increasing the levels of carbon monoxide and ozone in areas where it
is planted Godoi et al (2004) and Cancado (2003), report increases during the vest season, of respiratory problems in cities neighboring sugarcane plantations In
har-2002, legislation was passed in the state of S˜ao Paulo aiming to a gradual elimination
of the pre-harvest burning; it established a period of 30 years to its complete ination (Sirvinskas, 2003) Dias de Oliveira et al (2005) mentions that pre-harvestburning usually reaches native vegetation surrounding sugarcane crops Criticismand restrictions to the practice keep mounting and the government of Sao Paulo isworking an agreement with the distilleries to completely eliminate the practice bythe year of 2014
elim-With elimination of pre-harvest burning, sugarcane harvest will be made ically instead of manually, resulting in increase of the fossil fuel use on agriculturalphase of ethanol production, and additional CO2emissions
mechan-According CTC- Piracicaba, the harvester machines performances account for1.045 L of diesel per ton of sugarcane harvested As a result, mechanical harvestwould imply in additional use of diesel fuel in a volume of approximately 84 L/haresulting in an increase of 259 kg of CO2released per ha
9.7 Distillery Wastes
One aspect usually not addressed in energy balances and thus, GHG emissions isthe treatment of distillery wastes, the stillage, a liquid that in Brazil is usually calledvinasse Ethanol production results in vinasse amounts of 10–14 times the volume
of ethanol The characteristics of vinasse are its high concentration of nutrients andhigh biological oxygen demand (BOD), which ranges from 30 to 60 g/l, according
to Navarro et al (2000) The common destiny of this liquid is its application as a tilizer in the sugarcane plantations According to Moreira and Goldemberg (1999),the recommended rate of application is 100 m3/ha
fer-Such practices raise concerns about possible infiltration of vinasse resulting ingroundwater contamination Hassuda (1989) reports changes in groundwater qualitydue to vinasse infiltration in the Bauru aquifer localized in the state of Sao Paulo.Gloeden (1994), in another study area also report problems of groundwater con-tamination due to vinasse infiltration According to Macedo (1998), transport andapplication of vinasse requires 41.5 L of diesel per ha, resulting in emissions of
128 kg of CO2
An alternative is its treatment, which would require one kWh (3.6 MJ) per kg
of BOD removed, according to Trobish (1992), cited by Giampietro et al., 1997.Assuming the BOD values cited by Navarro et al (2000), and the production of
12 L of vinasse per liter of ethanol, between 8.3 and 16.6 GJ (2304–4608 kWh) ofenergy is required for BOD clean up, leading up to emissions ranging from 320
to 640 kg of CO2per ha of sugarcane used for ethanol production or 50–100 kg of
CO2/m3of ethanol
Another destiny for the vinasse could be its use for biogas production Besidesreducing an environmental problem, biogas production from vinasse is portrayed as
Trang 12an approach to increase the energy efficiency of ethanol production, contributing tomitigation of CO2emissions and environmental pollution load of distilleries.Based on personal communication with CTC, the process of biogas productionwould result in an energy surplus equivalent of 3.9 GJ (1082 kWh) per ha of ethanolproduced However, according to Cortez et al (1998), vinasse is not completelytransformed in the process and still has high concentration of organic material afterbiogas production Treatment of the remaining organic matter would require all theadditional energy generated by the biogas, practically reducing to zero any benefit
in terms of energy or CO2 emissions An study conducted by Granato (2003) at
a distillery in the state of Sao Paulo reports a much lower potential of electricitygeneration from anaerobic decomposition of vinasse, about 47 MJ (13 kWh) per m3
of ethanol produced, resulting in 299 MJ (83 kWh) of surplus per ha of sugarcanedevoted to ethanol production
The use of vinasse as fertilizer implies in additional use of fossil fuel and tion of N, P, K and lime in the traditional way The fossil fuel used for vinasse ap-plication results in additional emissions 128 kg CO2per ha of sugarcane Reduction
reduc-of fertilizer applied in the traditional way results also in reduction reduc-of CO2 emissions
in the amount of 204 kg, based on Azania et al (2003) The net result is a reduction
in emissions of 76 kg of CO2 There is also little variation regarding the net energy
in both options, with or without vinasse application, corresponding to a reduction ofjust 3.7% in the last option
9.8 Possible Additional Sources of Methane
As already mentioned before, common practice is the application of vinasse as a tilizer in sugarcane crops, there is currently little information about CH4 emissions
fer-to the atmosphere resulted from vinasse decomposition, which might significantlyaffect GHG balances
The increase of mechanical harvest, will result in a significant amount of residues(sugarcane tops and leaves), that would be otherwise burned in pre-harvest, to be left
on the field, which can also become a source of methane emissions A more detailedGHG balance would have undoubtedly to consider such aspects; therefore moreresearch on these issues is essential
9.9 CO2Mitigation
For the different alternative scenarios described above, avoided CO2emissions resented by the use of ethanol are summarized on Table 9.6
rep-Currently Brazil produces 4.2 billion gallons of ethanol or approximately
16 million m3per year, requiring around 3 million hectares of land (Goldemberg,2007) Assuming ethanol conversion efficiency of 80 L per ton of sugarcane, the val-ues above suggest an average yield for Brazil of approximately 67 tons of sugarcaneper ha