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Name: Jen
Status: Student
Grade: 6-8
Location: Jen
Country: United States
Date: March 2009


Question:
What is the temperature that grass chatches on fire? What is the temperature that wood catches on fire?



Replies:
Jen (or Brian),

The problem with both of these questions deals with the amount of moisture content that is commonly found in both grass and wood, the density of the substance and the air supply available. Water content found in wood and grass will allow for much higher temperatures to be reached than if they were dried. Different species of trees and grasses will ignite at different temperatures due to density (wood in particular) and the previously mentioned conditions. The ability for air (oxygen) to allow for combustion is also an important factor. Once ignited, the temperatures produced by wood and grasses is much higher that the temperature needed if all the above conditions are adequate. That is why fire fighters add water to a fire to put it out by requiring much higher temperatures for burning. You might want the local fire department to give a talk on this topic.

Completely dry paper (paper is a wood product and dry grass would be the same consistency) has the famous 451 F combustion temperature. Books will burn at 451 F for example. The "famous" being the book by Ray Bradbury entitled Fahrenheit 451.

Resources:

www.fpl.fs.fed.us/tmu/wood_for_energy/wood _combustion.html

Google wood ignition temperatures and grass ignition temperatures.

Steve Sample


Before the question of ignition temperature is answered, it is important to recognize what exactly is burning.

When wood or grass catches fire, the wood or grass is not burning. Rather, it is gas that is given off by the material (not gasoline, but molecules that are in a gas phase) when the wood or grass gets hot that is actually burning. Technically, this thermal or heat decomposition is called pyrolysis.

If you put a log into a closed oven and get it hot, the log will be pyrolyzed and the oven will fill up with combustible gases that will easily burn. The chemical compounds in the log are being broken apart; some form gases that burn, some form ash, tar, and all sorts of other things. Common gases are carbon monoxide and methane, and these burn easily. The ash and residue that is left over does not burn.

A commonly accepted temperature when the wood starts to pyrolyze to form burnable gases is 250C. If you light those gases with a flame they will burn. But if you heat the wood up in a closed box with no flame to start it, it may require a temperature of 450C before it spontaneously bursts into flame. Between 100C and 200C carbon dioxide may be produced. Carbon dioxide will not burn.

When a log is put on a fire, the fire heats the log, some gases are released, and those gases start to burn. This gets the log hotter and it keeps burning. It is difficult to heat a log all the way through quickly, and thus log fires burn slowly for a long time as they slowly pyrolyze. It is easy to heat individual blades of dry grass, so grass fires can spread rapidly.

Wet wood and wet grass is difficult to heat to the temperatures where gases come out, so it is difficult to get fires going with wet materials because you have to boil off the water first. Wood and grass like to hold moisture like a sponge, so even if the wood doesn't have actual liquid water on it, a lot of heat needs to be added to the wood or grass on a humid day before the wood or grass gets hot enough to be able to have self-sustaining combustion.

Another question is "flame temperature." That is, when you see a flame, how hot is it? Flame temperatures are usually around 2,000 C for a yellow flame. A bright bluish flame might be near 3,000C.

Robert A. Erck



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