Sugar to Ethanol Production
Brazil produced about 18.5 million metric tones (20.4 tons) of processed sugar in the 2001/2002 harvest, with approximately 9.45 million metric tones (10.4 tons) used domestically and the rest exported. Brazilian sugar is mostly derived from sugarcane, a drought-tolerant tropical and subtropical crop containing about 12 to 17% sugars (90% sucrose, 10% glucose) and 68 to 72% moisture. Brazil grew about 272 million metric tones (300 million tons) of sugarcane in the 2001/2002 season, making it second to India in world cane production that year. In 1975, Brazil adopted a Pro-Alcool Program to convert sugar to ethanol to reduce dependence on petroleum imports that were damaging the economy.
In dedicated ethanol plants where sugar extraction and fermentation approach, cane juice is heated to about 105 to 1101C (221 to 230F) to reduce microbial contamination and then fermented. In combined sugar to ethanol production facilities, sucrose is removed by crystallization and the remaining molasses containing up to about 65% sugar fermentation. In either approach, sugar fermentation, C6H12O6, is readily fermented to ethanol, C2H5OH, by common yeast such as Saccharomyces cerevisiae according to a reaction carried out for millennia:
C6H12O6 › 2C2H5OH + 2CO2
in which CO2 is carbon dioxide released during this process. About 70% of Brazilian ethanol facilities, often called distilleries, use batch operations in which sugar, nutrients, and other ingredients are added at the beginning of sugar fermentation or fed batch processes in which one or more of these ingredients are added as fermentation proceeds. However, continuous processes in which ingredients are constantly added and product removed from fermentation vessels are also used. Yeast is typically concentrated and separated from the fermentation effluent, washed with sulfuric acid to reduce bacterial contamination, and recycled to fermentors, resulting in high cell densities that shorten fermentations to 6 to 10 h at temperatures of about 33 to 351C with high ethanol yields (90 to 92% of theoretical) and concentrations (8 to 11% on a volume basis).
According to equations above, no more than 51.1 kg of ethanol can be obtained from 100 kg of sugar fermentation, with yeast achieving about 90 to 92% of this. Thus, about 460 to 470 kg (920 to 940 pounds) or 580 to 590 liters 139 to 142 gallons ethanol result from a metric tons of sugar. Following yeast removal, the sugar fermentation broth containing ethanol, water, nutrients, and some yeast passes to distillation to make 95.5% ‘‘hydrous’’ ethanol, which can be taken to 99.6% ‘‘anhydrous’’ ethanol in a dehydration section, if desired. The remaining dissolved solids, called vinasse, are used as a fertilizer and in irrigation. About 5170 liters of ethanol could be obtained from a hectare of land in Brazil in 1996, and 13.9 billion liters or 3.7 billion gallons ethanol in addition to 13.5 million metric tonnes (14.9 million tons) of sugar were made from 273 million metric tonnes (300 million tons) of sugarcane that year.
Costs of Sugar to Ethanol Production
The costs of making ethanol in Brazil are estimated to be about $0.006 to 0.007/liter ($0.023 to 0.026/ gallon) for labor, $0.004 to 0.006/liter ($0.015 to 0.023/gallon) for maintenance, $0.002/liter ($0.008/ gallon) for chemicals, $0.002 to 0.003/liter ($0.008 to 0.011/gallon) for energy, $0.004/liter ($0.015/ gallon) for other items, and $0.127 to 0.134/liter ($0.481 to 0.507/gallon) for sugar. Thus, the total cash cost of ethanol is on the order of $0.167 to 0.185/liter ($0.632 to 0.700/gallon). The overall capital investment is about $0.52/annual liter ($2.00/gallon) of capacity for a process that operates for 150 days/year making about 240,000 liters or 900,000 gallons ethanol per day. Allowing another $0.04 to 0.06/liter ($0.15 to 0.23/gallon) for interest on capital, capital recovery, and other fixed costs, the total cost including return on capital as applied in Brazil is about $0.21 to 0.25/liter ($0.79 to 0.95/gallon) at the plant gate without transportation, distribution, taxes, and other costs. More on-stream time would greatly reduce the unit capital cost charged to ethanol. Hydrous costs about 7% less than anhydrous ethanol because less energy and capital are needed to meet final product specifications. Costs in Brazil dropped at about a 30% rate with each doubling of experience through 1990 and continue to decline at about a 10% rate due to learning curve effects resulting from commercial experience. Opportunities have been identified to reduce costs by about 23%, mostly by reducing the intensity of growing cane but by also improving conversion operations.
Energy Balance Resulted from Sugar Extraction
About 197 to 222 mega joules of fossil energy are used for providing seeds, labor, power for agricultural operations, agricultural equipment, fertilizers, lime, herbicides, insecticides, and cane transportation to grow a metric tonne of sugarcane (169,000 to 191,000 Btu/ton). However, much of the energy for sugar extraction and fermentation is provided by burning the bagasse left after processing, but some fossil energy is used for electricity generation, chemicals and lubricants manufacture, erection of buildings, and manufacture of equipment, a total of about 40 to 70 mega joules/metric tonne (34,000 to 60,000 Btu/ton) of cane. Overall, about 237 to 292 mega joules of fossil energy are consumed (204,000 to 251,000 Btu/ton) compared to 1707 to 1941 mega joules of energy in the ethanol from a metric tonne (1.470_106 to 1.669_106 Btu/ton). Based on these values, the energy balance ratio of ethanol energy output to fossil fuel input is between about 5.9 to 8.2, not including the 175 to 328 mega joules/metric tonne of cane (150,000 to 282,000 Btu/ton) left from burning bagasse after sugar extraction, a very favorable energy balance.
Greenhouse Gas Emissions from Sugar Extraction Process
In simple terms, accumulation of methane, carbon dioxide, nitrous oxide, and other gases that trap infrared radiation are predicted to cause global climate change, and carbon dioxide has the greatest impact because of the huge quantities released when fossil fuels are burned. Burning coal, petroleum, and natural gas, in order of decreasing impact, release previously trapped carbon into the atmosphere, resulting in a net accumulation of carbon dioxide. On the other hand, if fuels are made with renewable energy biomass resources such as wood, carbon dioxide released during processing and burning can be recaptured by photosynthesis when more biomass is grown to replace that harvested. While biomass is carbon dioxide neutral, fossil inputs to plant, grow, harvest, transport, convert, and distribute biomass and its products contribute to greenhouse gas accumulation and must be considered. Burning bagasse can provide 100% of the thermal energy and 92% of the electricity for processing of sugar to ethanol production, and less than about 8%comes from fossil sources. Based on Brazilian experience that 1.2 liters of ethanol can replace 1.0 liters of gasoline, the net contribution to carbon dioxide accumulation by vehicles running on hydrous ethanol is less than about 10% of gasoline, and substituting surplus bagasse or other renewables for fossil fuel to generate electricity could improve the balance further. However, intensive sugarcane agricultural practices raise serious concerns about its sustainability, and low and no-till and other practices will be needed for cane sugar to become more viable.