4.1 Conventional bioethanol production pathways
Bioethanol produced from sugar cane, corn, sugar beet, wheat and other crops with high sugar or starch contents is the most common biofuel produced today. The production process is well understood and commercially deployed around the world at small- and large-scale. The liquid biofuels can then be blended with gasoline in a variety of proportions and can be used by conventional or flex-fuel vehicles (Box 4.1).
The production of bioethanol from crops high in sugar or starch is often referred to as a biological conversion route. This is because a biological process is used to convert the sugar or starch into ethanol. Depending on the feedstock, the main components that need to be extracted are sucrose or starch. For sugar cane or sugar beet crops, the sucrose is first mechanically pressed from the raw feedstock that has been heated (milling). It is then fractionated, after which the extracted sucrose is metabolised through yeast cells fermenting the hexose. The ethanol itself is then recovered through distillation (Figure 4.1).
In contrast, starch crops must first be hydrolysed into glucose and only after this process can the yeast cells convert the carbohydrates into ethanol. The process for starchy crops starts with a similar pre-treatment process that consists of milling the grains of corn, wheat or barley, followed by liquefaction and fractionation. At this point an acidic or enzymatic hydrolysis process is required, unlike for sugar cane ethanol, that will yield hexose that can then be sent to fermentation (Figure 4.2). This process is highly efficient, although more energy-intensive than the sucrose-based route.
Figure 4.1: Simplified sugar cane to ethanol production process
Source: Based on Dias, 2013.
Figure 4.2: Simplified corn-to-ethanol production process
The process for producing ethanol from sugar or starchy crops is almost identical from the fermentation process onwards. Both processes yield residues and by-products that typically have some value. For sugar cane, bagasse is left over that can be used to fire CHP plants to provide process heating and electricity needs for the biofuels plant and potential exports to the grid.16 With starchy crops, dried distiller grain can be produced and sold as feed to various livestock industries.
Box 4.1: Anhydrous ethanol, hydrous ethanol, blending and flex-fuel vehicles
Ethanol typically contains 7% to 4% water and this is referred to as hydrous ethanol. Anhydrous ethanol is ethanol that has been dehydrated to achieve at least 99% purity.
Ethanol can be blended with gasoline, with typical low-level blends varying from 10-25% and this is typically set by legislation to standardise fuel supplies. Blends of ethanol and gasoline are known according to the percentage of ethanol in the blend. For instance, E15 is a 15% ethanol and 85% gasoline blend. Many modern vehicles17 can typically run on E10 blends without modification.
However, flex-fuel vehicles are required for higher blends and hydrous ethanol. These vehicles have fuel systems, engines, sensors and management systems (Figure 4.3) designed to run on any blend of gasoline and ethanol from 100% gasoline to E8518 or even E100 ethanol. Flex-fuel vehicles have oxygen sensors in the exhaust system that identify the fuel composition and adjust the fuel/air ratio appropriately to ensure optimal combustion on varying ethanol-gasoline blends.
In Brazil, flex-fuel vehicles that can be run on any blend of anhydrous ethanol, from E18 to E25 to 'neat' E100 hydrous ethanol have been on sale since 2003. The hydrous ethanol on sale has a maximum water content of around 5%, which is achievable by distillation alone and does not require the additional cost of dehydrating to anhydrous standards. For cold weather, where the lower evaporative pressure of ethanol is a problem, a small gasoline reservoir is incorporated to allow cold starts on gasoline before switching over to ethanol. Dual-fuel vehicles (e.g. gasoline-biomethane) are also available.
Figure 4.3: Flex-fuel vehicle characteristics
Source: U.S. DOE.
The additional costs of modifying a vehicle to take E85 blends of anhydrous ethanol and become a flex-fuel vehicle are modest. They are estimated to be between USD 85 and USD 150/vehicle for mass production (van Mensch, 2011). The incremental costs of making a car compatible with E85 blends if it is already designed to use E10 blends are very modest and between USD 10 and USD 30/vehicle for mass production. In the North American market these costs are not typically passed on to the consumer now that these modifications are widespread.19
16 Integrated advanced ethanol plants will be able to produce conventional biofuels from the sugarcane juice, while the bagasse and other cuttings from the cane fields can be used to produce advanced lignocellulosic biofuels in a separate part of the plant.
17 Different manufacturers and countries have introduced vehicles modified to run on ethanol blends at different points in time. Typically only a proportion of the vehicle fleet has these modifi-cations.
18 An E85 limit helps to cut emissions and reduce cold start problems during cold weather.