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Fire Science: Fuel
The substance that is ignited and burned during the process of combustion is called a fuel. Fuels contain carbon (C) and hydrogen (H). Trees and other vegetation are fuels for wildland fires. These plants absorb carbon dioxide (CO2) from the atmosphere and produce sugars (C6H12O6) that contain C and H, as well as oxygen (O). This process is called photosynthesis. There are two main steps to photosynthesis. First, plants trap the sun's light energy in a compound called chlorophyll. Then, plants use the energy to produce sugar (C6H12O6). The process of photosynthesis requires water (H2O). Photosynthesis also produces water, as well as oxygen (O2). The net chemical reaction for the process of photosynthesis is:

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When fuels react with oxygen (O2) in the air during burning, carbon dioxide (CO2) and water (H2O) are produced. Combustible (burnable) gases such as carbon monoxide (CO), hydrogen (H), and methane (CH4) are also produced. Energy is released in the form of heat and light. This process is called combustion.

A similar process occurs inside plants and animals. It is called respiration. Inside plant and animal tissue, sugars (fuel) are broken down to release the stored energy. The energy is needed by the organism to perform life functions (cell growth, heart beat, etc.). During this process, carbon dioxide is released to the atmosphere. The net chemical reaction for respiration is nearly the exact opposite of photosynthesis:

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Flammability is the term used to describe how easily a fuel burns. In other words, flammability is how easily a fuel ignites and how quickly the flames spread to nearby fuel. Many factors determine the flammability of fuel. For example, the flammability of fuel in a forest--trees and grasses--can vary from season to season. During some seasons, forests experience low temperatures and large amounts of precipitation. These conditions keep the vegetation cool and moist and decrease their flammability. That is to say, the vegetation become less likely to ignite and burn. However, seasons of high temperatures and little precipitation can reduce the amount of moisture in the forest fuels and increase their flammability. This allows them to burn more quickly and intensely once ignited.

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