5.1: Solid Fuels
- Page ID
- 66704
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As stated in Unit 2, a solid is a state of matter that has a fixed volume and fixed shape.1
In order for a solid fuel to be utilized in the combustion reaction, the fuel must be converted to a gas. In an example such as wax from a candle, this is done through simple phase changes. The wax is converted to a liquid due to the radiative heat transfer, then once wicked up into the reaction zone, a second phase change converts the liquid to a gas. However, with most fuels such as wood and many plastics, the solid does not go through a phase change, rather, it is chemically broken down in a process called pyrolysis.
Pyrolysis is a process in which a material is decomposed or broken down into simpler molecular compounds by the effects of heat alone. Pyrolysis often precedes combustion.
Char is a carbonaceous material that has been burned or pyrolyzed and has a blackened appearance.2
Copyright: "charred wood" by siaronj, licensed CC BY 2.0.
Melting is the phase change that occurs when a substance converts from a solid to a liquid. Because a liquid has more energy than a solid, that means that energy must be provided to a substance to get it to change its phase. In the melting process, molecules in their solid state absorb energy. That energy loosens the tight bonds between the molecules, allowing them to move about more freely. Heat is required to convert a solid to a liquid. That heat must come from somewhere. When melting occurs, the surrounding environment cools down after it provides that heat.
Dehydration is the process of water being removed. This can happen in the early stages of pyrolysis during the combustion process.
Smoldering combustion is self-sustained glowing combustion without flame, usually with incandescence and smoke.
Copyright: "Smoldering Fire" by pfly, licensed CC BY-SA 2.0.
References
1. Brainard, J. (2014). 4.1 Solids, liquids, gases, and plasmas. In CK-12 Physical Science For Middle School. https://www.ck12.org/book/ck-12-physical-science-for-middle-school/r2/section/4.1/. Licensed CC BY-NC.
2. National Fire Protection Association. (2024). NFPA 921: Guide for fire and explosion investigations. https://link.nfpa.org/free-access/publications/921/2024