Chapter 5, Problem 1DID

Biofuels

A lot of energy is locked up in the chemical bonds of molecules made by plants. That energy can fuel consumers, as when an animal cell powers ATP synthesis by aerobic respiration. It can also fuel our cars, which run on energy released by burning biofuels or fossil fuels. Both processes are fundamentally the same: They release energy by breaking the bonds of organic molecules. Both use oxygen to break those bonds, and both produce carbon dioxide. Unlike fossil fuels, biofuels are a renewable source of energy: We can always make more of them simply by growing more plants. Also unlike fossil fuels, biofuels do not contribute to global climate change, because growing plant matter for fuel recycles carbon that is already in the atmosphere.

Corn, soy, sugarcane, and other food crops are rich in oils, starches, and sugars that can be easily converted to biofuels. The starch in corn kernels, for example, can be enzymatically broken down to glucose, which is fermented to ethanol by bacteria or yeast. However, growing food crops for biofuel production typically requires a lot of energy (in the form of fossil fuels) and it damages the environment. Making biofuels from other plant matter such as weeds or agricultural waste requires additional steps, because these materials contain a higher proportion of cellulose. Breaking down this tough carbohydrate to its glucose monomers adds cost to the biofuel product.

In 2006, David Tilman and his colleagues published the results of a 10-year study comparing the net energy output of various biofuels. The researchers made biofuel from a mixture of native perennial grasses grown without irrigation, fertilizer, pesticides, or herbicides, in sandy soil that was so depleted by intensive agriculture that it had been abandoned. The energy content of this biofuel and the energy it took to produce it were measured and compared with that of biofuels made from food crops (Figure 5.16).

About how much energy did ethanol produced from one hectare of corn yield? How much energy did it take to grow and produce that ethanol?

Summary Introduction

To determine: How much energy did the ethanol produced from one hectare of corn yield and how much energy did it took to grow the corn.

Concept introduction:The plants and other organic material other than fossils can also be used as a source of energy. The oils, gases, and alcohols made from these materials are called biofuels. Corn and other food crops are rich in oils, starches, and sugars that can be easily converted into biofuels.

Explanation of Solution

The researcher D and his colleagues studied for 10 years and compared the net energy output of various biofuels. The researcher grew a mixture of native perennial grasses, corn, and soy. The grasses grew without irrigation, fertilizer, and pesticides in sandy soil. The usable energy in biofuel (grasses, corn, and soy) is measured along with the energy it took to grow.

Refer to Fig. 6.1 “Energy inputs and outputs of biofuels made from three different crops” in the question. The graph plot shows the energy per hectare versus the ratio of energy output to input of the three different biofuels.

The graph shows the ethanol obtained from one hectare of corn produced $\text{23}\text{×}{\text{10}}^{\text{6}}\text{kcal}$ of energy and it required $18\text{×}{\text{10}}^{\text{6}}\text{kcal}$ growing the corn.

Conclusion

The ethanol from one hectare of corn produced approximately 23 × 10⁶ kcal. It took approximately 18 × 10⁶ kcal energy to grow the corn that was used to make the ethanol.