We often hear a quite a bit about biofuels on this website. Officially, I'm agnostic on the subject but I think this review article in the scientific journal
Chemical Reviews gives a good overview. The article has not been formally published yet, and will come out in the next issue, but if you are interested in this topic, and have access to a science library at a university or other institution, you may wish to check it out.
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1.0. Introduction
Prior to the discovery of inexpensive fossil fuels, our society was dependent on plant biomass to meet its energy demands. The discovery of crude oil, in the 19th century, created an inexpensive liquid fuel source that helped industrialize the world and improved standards of living. Now with declining petroleum resources, combined with increased demand for petroleum by emerging economies, and political and environmental concerns about fossil fuels, it is imperative to develop economical and energy-efficient processes for the sustainable production of fuels and chemicals. In this respect, plant biomass is the only current sustainable source of organic carbon,1-3 and biofuels, fuels derived from plant biomass, are the only current sustainable source of liquid fuels. Biofuels generate significantly less greenhouse gas emissions than do fossil fuels and can even be greenhouse gas neutral if efficient methods for biofuels production are developed.4-7...
...The worldwide raw biomass energy potential in 2050 has been estimated to be between 150 and 450 EJ/ year, or 25 109 to 76 109 boe.11 Biofuels also can have a positive effect on agriculture, and the USDA recently estimated that the net farm income in the U.S. could increase from $3 billion to 6 billion annually if switchgrass became an energy crop.12 (Note: All costs in this review are reported in U.S. dollars with a conversion from U.S. dollars to Euros of 1.0 to 1.2. For the purposes of this review, we assume that the energy content of 1.00 metric ton of dry lignocellulosic biomass is equivalent to 3.15 barrels of oil, and 1 barrel of oil has 5.904 GJ as reported by Klass.2) The current cost of delivered biomass is significantly...
...In the mid-1800s, biomass supplied more than 90% of U.S. energy and fuel needs. 1,2 In the late 1800s to early 1900s, fossil fuels became the preferred energy resource.1,2 In many developing countries, biomass is still a major energy source.1,2 Other countries that use biomass to meet a large percentage of their energy demands include Sweden, 17.5%; Finland, 20.4%; and Brazil, 23.4%.1 The Roadmap for Biomass Technologies,15 authored by 26 leading experts from academia, industry, and government agencies, has predicted a gradual shift back to a carbohydrate-based economy, such that by 2030 20% of transportation fuel and 25% of chemicals in the U.S. will be produced from biomass. As discussed in this review, the transition to the carbohydrate economy is already occurring with many companies, including traditional oil and chemical companies, such as Shell,16 UOP,17 Petrobras, Conoco-Phillips,18 Dupont,19,20 Dow and BP, developing the technology and infrastructure for biofuels and biochemicals production. Governmental leaders are also recognizing the importance of this fledgling industry by providing tax breaks, money, and mandates. The European Commission has set a goal that by 2010, 5.75% of the transportation fuels in the EU will be biofuels....
...2.3. Algae
Aquatic algae are another source of triglycerides as well as carbohydrates and lignin. The advantage of using microalgae is that they have very high growth rates, utilize a large fraction of the solar energy (up to 10% of the solar energy), and can grow in conditions that are not favorable for terrestrial biomass growth. From 1978 to 1996, the U.S. Department of Energy funded a program to develop renewable transportation fuels from algae, and the results of this program are reported by Sheehan et al.38 Over 3000 strains of microalgae were collected as part of this program, and according to Sheehan et al. currently 300 species, mostly green algae and diatoms, are still housed at the University of Hawaii in a collection available to researchers.38 Microalgae are one of the most primitive forms of plants and are microscopic photosynthetic organisms. While the photosynthesis mechanism in algae is similar to other plant material, they can convert more of their solar energy into cellular structure. Macroalgae are commonly known as seaweed. Both microalgae and macroalgae are fast-growing marine and freshwater plants. Commercial production of triglycerides from microalgae has been estimated to be 72 000 L/ha-year (390 boe/ha-year), and it has been estimated that rates as high as 130 000 L/ha-year (700 boe/ha-year) could be accomplished with continued research.39...
Microalgaes can contain from 7 to 60 dry wt % triglycerides.2 Pilot plant tests, conducted over a six-year period, demonstrated that microalgae could be produced at productivity rates as high as 500 kgalgae/ha-year in a 1000 m2 pond for a single day.38 The ponds were an open face shallow water design where the water and algae are circulated around the pond. Nutrients and CO2 were continually added to the algae pond. The productivity was dependent on temperature and sunlight, which varied over the course of the experiments. Ideally, algae could be produced on algae farms in open, shallow ponds where a waste source of CO2, for example, from a fossil fuel power plant, could be efficiently bubbled into the ponds and captured by the algae.
The current limitation of microalgae is the high production cost.38 Table 6 shows the production cost of algae on a large algae farm of 400 ha.38 Two scenarios were used for cost estimation with algae growth rates of 112 and 224 metric tons/ha-year. The total biomass algae cost was $273 and $185/metric ton, which is considerably higher than the cost of lignocellulosic biomass (less than $40/metric ton). The cost for CO2 is 20-30% of the total cost, and using waste CO2 from fossil fuel power plants would decrease the cost of algae production. One of the conclusions from the cost analysis is that alternative engineering designs for microalgae production would not significantly reduce the cost of microalgae production.38 The limiting factor in cost analysis is microalgae cultivation issues, and according to Sheehah future research work should focus on the biological issues regarding microalgae production.38 Microalgae cultivation issues are limited by the availability of water, CO2, sunlights, and flat land. triglyceride production rates 45-220 times higher than terrestrial biomass (Table 2)...
...Bray analyzed the economics of biodiesel production using two different processes: the CD process which consists of transesterification in three packed column reactors in series and an alkaline catalyzed process from Lurgi PSI where the transesterification reaction occurs in two agitated reactors in series.308 The Biodiesel production costs are made up of three major components: feedstock costs, capital costs, and byproduct credits. As shown in Table 26, the biodiesel feedstock (soybean oil, methanol, and catalyst) is the single largest cost for biodiesel production representing over 70% of the biodiesel cost in this analysis. The refined production cost of biodiesel ($0.49-0.50/L) is close to the feedstock costs ($0.46-0.47/L) because glycerol currently has a high value ($1.10-2.20/kg for refined glycerol), which Bray assumed was $1.21/kg. It has been projected that increased biodiesel production will significantly decrease glycerol prices. Bray estimates that the refined glycerol price could drop to $0.77/kg, which would decrease the credit given for glycerol production to $0.07/L. Haas et al., who also modeled the biodiesel production cost, had similar economic results as those of Bray.326 Haas et al. estimated the biodiesel production cost to be $0.53/L when the feedstock soybean cost was $0.52/kg, and with a glycerol credit of $0.33/kg for an 80 wt % glycerol-water solution. The FOB cost of diesel fuel from crude oil is 44¢/L when oil prices are $55/bbl.9 Thus, biodiesel is currently slightly more expensive than petroleum diesel. However, biodiesel can be cost competitive with petroleum-derived diesel fuel, in many countries, if it is exempt from taxes. In the EU, the tax on diesel ranges from $0.34-0.99/L, which is higher than the tax in the U.S. of $0.13/L. Since feedstock is the primary cost of biodiesel, decreasing the feedstock cost will significantly decrease the biodiesel cost. Figure 40 shows...
10.5. Glycerol Utilization
Glycerol is currently too expensive to be used as a fuel; however, as biodiesel production increases the price of glycerol will decrease. Hydrogen can be produced from glycerol by aqueous-phase reforming25,160,271 (Section 8.3) or gas-phase steam reforming.342 Another process for fuel production from glycerol is the etherification of glycerol with isobutylene and ion-exchange resin catalyst to produce butyl ethers of glycerol, which could be used as an oxygenated diesel fuel additive.343,344...
...9.2. Levulinic Acid Conversion
Levulinic acid can be converted into fuels by dehydration/ hydrogenation or esterification as shown in Figure 35. Methyl tetrahydrofuran (MTHF), which has an octane number of 87 and can be blended with gasoline up to 70%, can be produced by a dehydration/hydrogenation pathway of levulinic acid. MTHF (20 wt % oxygen content) can increase the oxygen content of gasoline and has been approved by the USDOE as a component of P Series fuel. Levulinic acid is separated from other reaction products (water, formic acid, and furfural) by vacuum distillation at 160 °C and 10-50 mmHg to produce angelica lactone (the dehydration product). This reaction is reversible, and water addition will promote levulinic acid formation. Hydrogenation of angelica lactone with PdRe/carbon catalysts at 200-250 °C and 100 atm H2 produces first ç-valerolactone and then 1,4-pentanediol, which dehydrates to form MTHF in yields up to 90%.294 1-Pentanol and 2-pentanol are also produced in this process in lower yields. Reaction of angelica lactone with an alcohol in the presence of an acid or base catalyst yields levulinic esters.20 The base-catalyzed reaction is carried out at 100-150 °C...
...Triglycerides are the major component of vegetable oils and animals fats (Section 2.3). They also can be produced from aquatic biomass such as algae (Section 2.4). Vegetable oils can be used directly in diesel engines; however, there are a number of disadvantages of pure vegetable oils, including high viscosity, low volatility, and engine problems (including coking on the injectors, carbon deposits, oil ring sticking, and thickening of lubricating oils).34,307 These problems require that vegetable oils be upgraded if they are to be used as a fuel. The most common way of upgrading vegetable oils to a fuel is transesterification of triglycerides into alkyl-fatty esters (bio-diesel). Waste vegetable oils, like frying oils, can be used as feedstocks; however, changes in the process need to be made as waste vegetable oils contain free fatty acid (FFA) and water impurities. Vegetable oils can also be blended with diesel fuel or upgraded by other methods including zeolite upgrading...
11.1. Ethical Considerations
Biofuels should and can be produced sustainably with food and animal feed as coproducts.8 Ethical and moral questions arise when edible biomass products are converted into biofuels. Regions where malnutrition exists due to low levels of food production should first focus on producing edible biomass for food use before they produce biofuels from edible biomass. Shell Oil has a policy to only produce biofuels from nonedible fractions of biomass. However, as discussed in this review the largest fraction of biomass is nonedible lignocellulose such as straw, grasses, corn stover, wood, forest products, etc. Edible biomass is coproduced in plant material with lignocellulose biomass. Currently, the economics are more favorable for conversion of edible biomass into fuels due to their chemical structure, which can be more efficiently converted. Therefore, it is vital to continue to develop processes for conversion of lignocellulosic biomass into fuels. Agricultural practices in industrialized countries are very advanced, and most industrialized regions produce more than enough food for domestic food consumption. Farmers do not pick the crops based on how efficiently they produce edible food products. Instead farmers’ goals are to grow crops that maximize their income, even though more efficient crops can be grown. Biofuels also can provide farmers in developing countries another market for their products, which could improve the economies of developing countries.
The EU has established as the main area of research second-generation biofuels from various biomass resources and wastes. As has been shown in this review, it is technically possible to convert cellulose materials and organic wastes into biofuels. However, costs have to be lowered...
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