It has been a popular misconception that bioenergy systems have no net CO2 emission. Considerable amounts of fossil fuel inputs are required for plant growth and transportation, as well as for ethanol distribution, and therefore CO2 emissions are present during the process of ethanol production. Fertilizers, herbicides and insecticides have net CO2 emissions associated with their production, distribution and application. Sugarcane production also results in emissions of other GHG, namely methane and nitrous oxide. Based on Lima et al. (1999), CH4 and N2O emissions from sugarcane correspond to 26.9 and 1.33 kg per hectare respectively. Such emissions correspond to 672 kg and 399 kg respectively of CO2 equivalent.
Theoretically, there are no GHG emissions associated with distillery operations. All the energy required comes from the burning of bagasse, which is a residue of the milled sugarcane. In fact the burning of bagasse generates more energy than the distillery requires, resulting in some surplus of energy. Conceptually CO2 emissions associated with bagasse burning are not accounted for, since where sequestered during sugarcane growth they will be re-absorbed in the next season. The same rationale 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 Brazil (In Guyana it ranges from 60 – 82 t/ha) and ethanol conversion efficiency of 80 L per ton of sugarcane processed, the amount of ethanol resulting from one ha of sugarcane plantations is 6.4 m3. Consequently for production of one m3 of ethanol, GHG emissions account to 457 kg of CO2eq production and distribution; this corresponds to approximately 19 kg of CO2 per gigajoule (kg/GJ) of fuel. Estimating the potential for GHG reduction from the use of ethanol derived from sugarcane requires a comparison with the fossil fuel displaced. In Brazil the automobile fleet has basically three fuel options, natural gas, ethanol and gasoline, the last option is actually a mixture of gasoline and ethanol. The proportion of each fuel varies slightly according to government decisions; currently it is 75% gasoline and 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. Although conversion to natural gas continues to rise in Brazil stimulated by its fuel economy, currently such vehicles represent only about 5% of the automobile fleet.
In 2003, Brazil began to produce flex fuel cars, which can run with both gasoline and ethanol in any proportion using the same tank. In that year about 40 thousand 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 million units. This increase is directly related to a strategy for increasing biofuel consumption in Brazil, where the consumer is stimulated to use ethanol as an environmentally responsible option. The differences in price between ethanol and gasoline also contribute to the scenario. Presently in Brazil, ethanol is about 49% cheaper than gasoline, mostly due to heavier taxes on gasoline. The advantage of flex fuelled cars is that owners can trade back and forth between ethanol and gasoline according to the prices at the pump.
It is undeniable that the use of ethanol from sugarcane represents a reduction in CO2 emissions when compared with gasoline. Nevertheless, the importance of such an option in mitigating global warming has been over-emphasized and has led to the neglect of important environmental and social aspects. Biomass plantations can produce carbon neutral fuels for power plants or transportation, but photosynthesis has too low a power density for biofuels to contribute significantly to climate stabilization. Based on UNFCCC, GHG emissions in the tropics are mainly related to deforestation and agricultural intensification, while in temperate regions GHG comes from the combustion of fossil fuel in the transportation and industrial sector. Agricultural intensification and deforestation are exactly the possible outcomes from significant increases of ethanol production in Guyana. The idea of reducing fossil fuel consumption from temperate areas by using sugarcane ethanol is impractical. In order to contribute to a reduction of fossil fuels used in developed countries, the amount of ethanol that Guyana would have to produce would require a significant increase in the agricultural area devoted to such crops.