When you cook food over a campfire, you’re burning a biomass fuel, a combustible fuel derived from biological materials. Wood is certainly the most common biomass fuel used today, but it’s not the only one. In places without forests, local people have long burned dung from livestock and wild animals as fuel for cooking and for warmth (Figure 10.20). In addition, plant oils (e.g., olive oil) and animal fats (e.g., whale blubber) have also been used as sources of light and heat.

Increasingly, biomass is becoming an important renewable source of fuel for transportation and for generating electricity. It may be used directly in relatively raw form, such as when wood is burned for heat, or it may be converted to a variety of liquid or gaseous fuels known as biofuels, the production of which has grown rapidly (Figure 10.21).

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Ethanol, one of the common biofuels, is widely used as a gasoline additive. The process of manufacturing ethanol from grains, including corn, barley, and rice, has been known for thousands of years—since humans first learned to make alcoholic beverages. Although modern techniques to produce ethanol for biofuel have progressed far beyond those traditional practices, most ethanol is still made by using yeast to ferment starches from grains and simple sugars from sugarcane (Figure 10.22).

Using whole plants to produce ethanol is more challenging. The process has been limited by the difficulty and cost of converting large cellulose molecules, the main chemical component of woody plant material, to simpler sugar molecules that are easily fermented to ethanol. These technical barriers are rapidly falling, however, in the face of intensive research and development. This research has provided the technical basis for opening commercial-scale biomass refineries that convert wood and other cellulose-rich materials to ethanol, a product referred to as cellulosic ethanol (Figure 10.23).

In addition to producing ethanol, used mainly as an additive to gasoline, several methods are under development to convert wood and other plant material into gasoline, diesel, and jet fuels, using genetically modified bacteria. One advantage of converting terrestrial plant biomass to these types of fuels is that they are more energy-dense: They have one-third higher energy content than ethanol per unit volume. Consequently, fuel mileage is about 30% higher when using these alternative fuels.

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Today, soybean oil, canola oil, and palm oil represent the primary source materials for the biofuel known as biodiesel, a fuel made from vegetable oils and animal fats. However, there are other sources of biodiesel under development. The algae that float in swimming pools, ponds, and the open ocean are remarkably efficient at capturing sunlight and turning it into the energy and materials they need for growth. Some algae can double in number in just 12 hours. Their exponential growth means that, compared with corn, they are able to produce at least 10 times more fuel in the same amount of time.

Moreover, they can also be grown in brackish waters and in marginal conditions that do not support food crops. Perhaps their best attribute is that they naturally produce and store oils that are similar to the vegetable oil you can buy at the grocery store. Oil can be more readily harvested and converted into fuels we already use, including biodiesel, gasoline, and even jet fuel. That’s one reason why the major oil companies—ExxonMobil, Shell, and Chevron—are funding research efforts to develop algal fuels.

Scientists are studying thousands of algal strains around the world to find those that grow the fastest and have the greatest potential for biofuel production. For instance, a group of researchers at the Scripps Institute of Oceanography have managed to genetically modify diatom algae to produce more oils than they normally would, without negatively affecting their growth. In 2009 Synthetic Genomics in La Jolla, California, which was founded by the human genome sequencing pioneer J. Craig Venter, received a $600 million commitment from ExxonMobil to build an algae biofuel plant. Nevertheless, substantial research and development will be required before algae-derived biofuels can compete with petroleum. Recently, ExxonMobil CEO Rex Tillerson told PBS that the company is still 25 years away from creating a commercially viable product: “We’ve come to understand some limits of that technology, or limits as we understand it today, which doesn’t mean it’s limited forever.”