Course: BFM 101 Biofuels Fundamentals

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Energy from Plants and Climate Change

Plants capture carbon dioxide (CO_2) during photosynthesis and store it in their tissues in a variety of carbon compounds (including both carbohydrates and oils). These are often energy stores for the plant but they can also be harvested and processed to provide energy for human use.

Table 1 Comparison of the energy content of plant products

Energy store | Energy (megajoules per kilogram)
Plant oil | 37
Coal | 24
Carbohydrates (including sugars) | 17
Wood (dry) | 16

1 megajoule is roughly the amount of energy that a one-bar electric fire would emit in 15 minutes.
(Note that if wood has not been seasoned to remove excess water, less energy is obtained because some of the energy is used to heat and evaporate the water.)

At present, most of our energy comes from fossil fuels which originate from CO_2 ‘locked up’ in plants by photosynthesis millions of years ago. When fossil fuels are burnt, CO_2 is released into the atmosphere adding to the levels already present. Since the industrial revolution, there has been an acceleration in the burning of fossil fuels. The levels of CO_2 in the atmosphere are monitored by the Mauna Loa Observatory in Hawaii and are estimated to have risen from 280 parts per million (p.p.m.) in 1800 to 387 p.p.m. today (i.e. in 2009). The increase in CO_2in the atmosphere has been linked to global warming (the increase, of around 0.5°C, in the average temperature of the Earth since around 1920). CO_2 is known as a ‘greenhouse’ gas and it acts with other greenhouses gases in the atmosphere (water vapour, ozone, nitrous oxide and methane) to insulate the Earth. Increases in these greenhouse gases increase this insulation and result in rises in the Earth’s temperature. The predicted dire consequences include extreme weather conditions and melting of ice sheets and glaciers, resulting in rising sea levels and hence coastal flooding.

Figure 2 Simple representation of the carbon cycle.

As Figure 2 shows, all living organisms, not just plants, have structural components that are based on carbon. The pictorial presentation of a carbon cycle illustrates how carbon moves between the reserves found in living things (plants and animals), soil (decomposing organic matter), rocks (including fossil fuels), the atmosphere and the oceans.

In Figure 2 there are many arrows depicting movement of carbon compounds from one place to another. For instance, when plants and animals die they may get buried in soil and eventually as more material is deposited on top of them, they are buried deep enough to be considered part of the rocks beneath our feet. Thus the carbon has been transferred from the atmosphere to the rock, via animals, plants and soil. This is exactly what happened when fossil fuels such as oil, coal and gas were formed millions of years ago.

The Kyoto Protocol from the United Nations Framework Convention on Climate Change, which came into force in 2005, legally commits countries that signed the protocol to reduce emissions of four greenhouse gases, including CO_2. One of the ways in which this could occur is by a shift away from fossil fuels to biofuels, although this view is contentious for reasons indicated shortly. Another is by the process known as carbon offsetting, whereby the amount of carbon released into the atmosphere by burning fossil fuels is balanced, for example by planting a certain number of trees to take up the equivalent amount of CO_2 by photosynthesis – so-called carbon sequestration. This has led to the currently used terms ‘carbon neutral’ (where the amount of CO_2 that is produced by an individual or a population is balanced by the amount of CO_2 that can be absorbed through various measures taken) and ‘achieving a zero carbon footprint’ (where the net amount of CO_2 released by a person, or by an object such a house, is zero).

This unit is adapted from Open Learn, The Open University, Biofuels course CC BY-NC-SA
www.open.edu/openlearn/science-maths-technology/science/biofuels/

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