Biofuels are derived from biological materials such as vegetable oils, corn or sugar canes. They are mainly used as an alternate fuel replacing fossil fuels, which are rapidly diminishing.
The most common types of biofuels are biodiesel, from oil-rich plants, and bioethanol from starch, which are referred to as first generation biofuels.
First-generation biofuels have the benefit of reducing the overall greenhouse gas emissions and zero carbon production in some cases as the carbon emitted during burning of biofuels is compensated by the carbon absorbed by the plants during growth.
Second-generation biofuels, on the other hand, are technologies that are still in development. This new technology focuses on producing second-generation biofuels by microbial fermentation of non-food crops.
Other sources include woody biomass from forestry, paper pulp, sawdust, bagasse, straw, stover, household food and garden waste.
The carbon footprint and overall energy yield will be drastically improved when compared to first generation biofuels as there is no need for cultivating or harvesting the crops.
Bioethanol is conventionally produced through the enzymatic conversion of starchy biomass into sugars or fermenting 6-carbon sugars. Ethanol can also be produced from feedstocks like cassava, sorghum, potatoes, sugar beets, sugar cane, corn (maize) and cereal crops.
Researchers are now focusing on using ligno-cellulosic materials for advanced bioethanol production. This includes pre-treating biomass to release hemicellulose and cellulose, hydrolysis to release fermentable 5- and 6-carbon sugars, separation of non-hydrolysed cellulose and solid residues followed by distillation.
Biodiesel is produced by trans-esterification of fats and vegetable oils via the addition of methanol and a catalyst, yielding glycerol as a by-product.
Other feedstocks include palm oil seeds, soy seeds, sunflower seeds and rapeseeds. Advanced methods include replacing methanol of fossil fuels with bioethanol, which yields fatty acid ethyl ester rather than fatty acid methyl ether.
Recent advancements in the production methods include hydrogenation of fats and oils for producing biodiesel that can blend with fossil diesel up to 50% without modifying engine efficiency.
Performance and Emission Characteristics
Ethanol produced from sugarcane is highly efficient as the crop produces high yields per hectare. The fossil energy input required for producing ethanol can be lowered if bagasse is used to provide power and heat for the process. Hence, CO2 emissions could be as low as 0.2-0.3 kg CO2/lt ethanol when compared to 2.8 kg CO2/lt for conventional gasoline.
Similarly, CO2 emissions may be reduced by 15%-25% in case of ethanol produced from corn and cereals when compared to gasoline. However, current research work involving the use of ligno-cellulosic feedstock from ethanol reduces CO2 emissions by 40 to 60% when compared other fossil fuels like diesel.
Autoignity is one of the most important characteristics of diesel fuel, which is quantified by the cetane number or index of the fuel, where the fuel ignites more rapidly with higher cetane number or index.
Researchers state that the cetane number of biodiesel ranges from 45.8 to 56.9 for soybean oil methyl esters with an average of 50.9 when compared to the cetane number of petroleum diesel that ranges from 40 to 52. In addition, biodiesel also has some drawbacks related to its performance.
Biodiesel tends to form wax crystals at low temperatures, which can clog filters and fuel lines in a vehicle. Moreover, oxygen present in the biodiesel may improve combustion thereby reducing particulate emissions, carbon monoxide and hydrocarbon.
Cost of Producing Biofuel
Costs of biofuels mainly depend on labor and land costs, feedstock, oil market and agricultural subsidies. Ethanol from sugar cane costs more than $0.40- $0.50/l of gasoline equivalent (lge) when compared to gasoline prices ranging from $0.3- $0.4/lge. Ethanol from sugar-beet, maize or wheat may cost around $0.6-$0.8/lge which can be further reduced to $0.4-$0.6/lge.
Current rate of ligno-cellulosic ethanol is around $1.0/lge at pilot scale. The cost is predicted to halve in the next decade with further process development, low-cost waste feedstock, co-production of other by-products and scaling up of plants.
Biodiesel produced from animal fat is the cheapest option as of now, which costs $0.4- $0.5/l of diesel equivalent (lde). Cost reductions of biodiesel to a price range of $0.1-$0.3/lde are expected with the scaling up of new processes.
Biofuels are important for a number of reasons. Transportation depends on the fossil fuels like petroleum and oil for its energy needs and hence it is necessary to move towards more sustainable and renewable fuels. Recent report from Pike Research, a private research firm stated that the market value of biofuels used in transportation may reach US$185 billion by 2021.
The use of biofuels would indeed result in net fuel cost savings of $890 billion when compared to fossil fuels. The key issue to take into consideration is that higher amount of biofuels may make the overall fuel costs more expensive than the fossil fuels on a short-term horizon. However, those initial costs could be compensated by the fuel cost savings in the longer-run.
In addition, several favorable government policies also exist to encourage biofuel technologies, which include quotas for renewable fuels in various countries. However, the industry also faces many challenges like allegations by development organizations and environmentalists that farmers are making use of the land to grow biofuel feedstock alone rather than food whereby driving up food prices globally.
Sources and Further Reading