Over the last 40 years, the world has experienced several oil crises. The last decade brought four waves of oil price growth. Thus, the demand for the alternative energy sources has increased. These possess several advantages, namely, renewability, environmental sustainability, cost-effectiveness, and approved manufacturing technologies. Biofuel is a prospective alternative, so series of governmental programs, both state and federal, support production and consumption of biofuel. The Congress has established a mandatory minimum volume of biofuels to be used in the national transportation fuel supply. One needs to discuss the advantages in details if they wish to consider the countrywide use of biofuel. Furthermore, biofuel has to prove competitiveness, if tax support is annulled.
Biofuel is a fuel made from natural substances of bioethanol, biodiesel and biomethane. Biofuel is a substitute for liquid fossil fuels, precisely oil. Bioethanol is produced by conversion of starch or sugar-rich biomass into sugars via fermentation process. Biodiesel is a fuel made from oils or animal fats involving the processes of extraction, esterification, and catalytic conversions. Biomethane is a gas produced during the anaerobic decomposition of landfill materials.
The process of ethanol production realizes via biochemical and thermochemical routes. The biochemical route of biofuel production is cellulose deconstruction to base polymer and further break of cellulose and hemicellulose to sugars (glucose and xylose) which ferment into ethanol. The feedstock passes several stages. The pretreatment stage allows receiving the properly sized materials and reducing density, as well as opening the fiber structure of the cellulose. The pretreatment methods can be physical, chemical or biological. The process involves a combination of chemical treatment (treatment by acid, water or alkaline agent) and physical treatment (ball milling). Enzyme treatment proves promising outlook, though enzyme cost is a key factor for its application. The cost of enzyme for cellulose decomposition was $5 in 1999, and plummeted to $0.2 in 2005. The stage is one of the cost consuming, so it requires optimization.
Fermentation process goals to convert all sugars released at pretreatment stage to ethanol. The process is well researched and shows no difficulties. Newly developed yeast strains are able to ferment glucose and xylose to ethanol effectively. If fermentation inhibitors (furfural, hydroxyl methyl furfural) are absent, conversion process is successful. The fermentation produces 8% ethanol solution that requires ethanol recovery in distillation. The distillation is a commercial-scale process, and its only drawback is energy consumption.
The recent research proves the need for application of existing technologies for coal-to-liquid conversion for biomass-to-liquid (BTL) conversion thorough thermochemical route. The thermochemical route produces synthesis gas (syngas), which is a mixture of carbon oxide and hydrogen. The gas passes through classical Fisher-Tropsh process, widely used commercially. The process results in production of diesel, naphtha, aviation fuel, hydrogen, and methanol. These products are BTL-derived fuel products. Their quality is the same as of their mineral analogues and in some cases better. For example, the maximum cetane number of mineral diesel is 53, whereas for BTL diesel it is 80; the energy content for BTL-fuel is 34 MJ/l, while for ethanol 21 MJ/l. The challenge of thermochemical route is averting of co-products formation and catalyst sensitivity. These issues are common with classical Fisher-Tropsh process.
The use of fossil fuel is responsible for the greenhouse effect. The burning of oil and coal emits carbon and sulfur dioxides, nitrogen oxides. The usage of oil fuel results in organic substances emission. The crude oil distillation process, which results in petroleum and other oil products production, accumulates high volumes of acid sludge. Today, there is no promising cost-effective and environmental-friendly method for acid sludge processing, so it is stored in ponds. The solid waste of coal burning is ash kept at gob piles near thermal power stations.
Apparently, biofuel does not produce carbohydrates and other organic substances, ash, acid sludge, or sulfur dioxide. The formation of carbon dioxide is inevitable for every fuel. Bent, Orr, & Baker state that biofuel allows 15-70% reduction of carbon dioxide emissions for ethanol and 40-60 for biodiesel. If biofuel reduces greenhouse gasses emission by 20%, it can qualify as a renewable fuel. Most types of biofuel qualify for this mandate.
Fuel combustion requires the certain amount of oxygen. If the combustion process lacks oxygen, it emits highly toxic carbon oxide. Furthermore, any combustion process generates little quantities of carbon oxide. Bioethanol combustion typically abates carbon oxide emission. If we compare biodiesel and gasoline, the first does not form sulfur oxide and particles of petroleum black. Biofuel combustion emits fewer gases than fossil fuel combustion.
However, considering the environmental impact of any fuel usage, one needs to take into account all stages of energy production, from fuel formation to combustion emissions. Corn, cereal, maize, sugar beets, sugar cane, switchgrass, and wood are raw materials for biofuel. Since agricultural plants are vulnerable to nutrients deficiency and activity of injurious organisms, they require irrigation, fertilizing and usage of herbicides and insecticides to harvest rich crop. For example, corn requires more nitrogen fertilizers, and it causes more soil erosion than any other plant. The application of nitrogen fertilizers, herbicides and insecticides results in gradual pollution of groundwater and rivers. Some states (e.g. Arizona) use groundwater for irrigation, and pumping off the groundwater is 10 times faster than the natural recharge of aquifer. Wood cellulose use as feedstock brings deforestation and increase of biodiversity. The environmental systems used for cultivating the biofuel plants will be degrading, so the agriculture of biofuel plants is not environmentally sustainable.
Chemical plants that produce ethanol are associated with air and water pollution as any other plant. Environmental Protection agency issued warnings for air emission reductions. Fermentation process produces about 8% ethanol and 92% water, which means that 1 l of ethanol produces 13 l of wastewater. Wastewater pollution is due to organic substances and has biological oxygen demand (BOD) of 18,000 – 37,000 mg/l. BOD for residential wastewaters is around 400 mg/l and clean river has BOD of 4 mg/l. The cost of wastewater treatment has to be included in the cost of biofuel.
The municipal wastewater treatment sludge is a feedstock for biomass recovery and biofuel production. This feedstock is waste and strategy for its utilization is of greatest interest for many countries. Further studies are required for cost-effectiveness of technological principles of biofuel from municipal wastewater sludge.
When we consider agricultural and chemical impacts of biofuel production, the environmental safety and sustainability of biofuel is doubtful. The environmental impact of wastewater sludge as biomass feedstock needs further study.
If we analyze the feasibility of biofuel as a US national energy source, we need to review the benefits and cost-effectiveness of biofuel plants. Nowadays, there are pre-commercial, commercial and large commercial biofuel plants. Pre-commercial plants operate 4000 hours a year and produce 1-4 Ml of ethanol; large-scale commercial plants operate 7000 hours a year and produce 150-250 Ml of ethanol.
The price for bioethanol is $0.8-1 /l of gasoline equivalent, for biodiesel it is $1/l. If tax subsidies had been excluded, the total cost would be $1.24. The analysis of energy and costs input for ethanol production from corn, switchgrass, sunflower, wood cellulose proves negative energy return, for instance, 1MJ of soybean biodiesel requires 1.24 MJ of energy. The main energy inputs are steam and electricity required for distillation of 8% ethanol to receive 99.5% (Pimentel & Patzek, 2005). Almost all states support biofuel energy production by tax credits or direct payments. For example, the tax subsidy can be $1 for 1 liter of ethanol. The US Department of Energy invested 40% to commercial 2nd generation plants that use switchgrass, municipal solid waste, corn stover, corn cobs, wood waste, etc. as feedstock. In some cases, state provides support by investment in infrastructure that enables use of biofuels. Biofuel production certainly has perspectives for development though today its price cannot compete the gasoline.
US national gasoline use is approximately 138 billion gallons a year. US imports about 60% of crude oil. The government stimulates the development of biofuel usage in the national transportation fuel supply to 36 billion of gallons a year until 2022. In 2012, biofuel plants produced 15.2 billion of gallons, mostly cornstarch derived ethanol. The biofuel is intended for use in highway motor vehicles, locomotive, and marine diesel by 2022.
Further growth of biofuel production is expected through the use of new biofuel categories, namely non-corn starch ethanol, cellulosic and biodiesel. Whereas in 2011 about 40% of US corn was used for ethanol production, it displaced only 7% of gasoline. Schnepf & Yacobucci estimated that use of entire US corn crop as a feedstock for ethanol would bring only 17% of national gasoline. Supposing US gasoline consumption remains stable the country will need approximately 205 billion of ethanol to replace all gasoline. Thus, the overall effect of biofuel for national energy supply is insignificant, and ethanol is infeasible for total gasoline substitute.
Since new large commercial ethanol plants are under construction, we should consider such social impacts as health issues and influence on rural communities. Biofuel production does not emit any harmful substances, so no negative health impact is expected. Biofuel feedstock occupies agricultural lands that can be used for food crops. This brings the ethical issue of food crop use as biofuel feedstock. Today many people are malnourished, and US export of food corn or other crops may be more effective than biofuel production. US Department of Agriculture predicts that the production of million liters of ethanol will create 4,500 jobs. The people will be employed at the plants, farms and equipment supply enterprises. Department of Energy claims that about 100,000 jobs will become available for rural economy. Development of biofuel energy production provides sustainable development for the region.
Although the government positions biomass energy as renewable and sustainable, it has severe drawbacks. Today the total production price of biofuel is high, if government annuls subsidies. Biofuel cannot provide the necessary quantity of liquid fuel, so it is an infeasible option for US energy security. Since biofuel can replace only 15% of US liquid fuel, no substantial greenhouse gases reduction is expected. Large commercial plants require high volumes of feedstock, which typically cannot be grown sustainably without pesticides, fertilizers and irrigation. Expansion of feedstock to wood will result in deforestation and influence biodiversity.