When a wooden log is burnt, which of the following statements show the influence ofsurface area,
a.High supply of air.
b. Well-dried logs.
c. Logs cut into small pieces.
d. Removing some logs from the furnace.
Answers
Answer:
removing some logs from the furnace.
Explanation:
The reduction of ignitability and combustibility of wood products is based on chemical and physical means that have an effect on different stages of ignition and burning, for example:
heat induced changes of the internal structure of wood in molecular level;
physical and chemical processes of compounds produced in these changes, both inside the wood and in the gases formed above it;
transfer of heat in a wood product;
transfer of oxygen into reaction areas.
Following topics are covered in this section:
1. Ignition and combustion of wood
2. Fire performance of wood
2.1 Ignitability
2.2 Heat release and fire spread
2.3 Charring
2.4 Smoke production and toxicity
1. Ignition and combustion of wood
Many materials in our environment, including wood products, burn “indirectly” in the sense that the materials do not actually burn, but combustion takes place as a reaction between oxygen and the gases released from a material (an exception from this rule is the glowing combustion of charred wood where oxygen reacts directly with carbon). Under the influence of heat, wood produces easily substances that react eagerly with oxygen, leading to the high propensity of wood to ignite and burn.
Ignition and combustion of wood is mainly based on the pyrolysis (i.e. thermal decomposition) of cellulose and the reactions of pyrolysis products with each other and with gases in the air, mainly oxygen. When temperature increases, cellulose starts to pyrolyse. The decomposition products either remain inside the material or are released as gases. Gaseous substances react with each other and oxygen, releasing a large amount of heat that further induces pyrolysis and combustion reactions. Pyrolysis and burning processes are illustrated in Figure 1.
Figure 1. A schematic picture of pyrolysis and combustion of wood: a) External heating increases the temperature of wood. b) Pyrolysis starts and the chemical structure of wood is decomposed. Light pyrolysis products volatilize from the surface. c) Combustion starts. Pyrolysis products react with oxygen and produce more heat, causing a strongly growing chain reaction.
Depending on environmental conditions (such as temperature, oxygen concentration, moisture, fire retardants, pH etc.), the pyrolysis of wood can proceed mainly on two pathways presented in Figure 2a. The tar forming pathway, taking place in a temperature of approximately 300 °C, is related to the “normal” burning of wood. In this case, pyrolysis produces a lot of tar including levoglucosan that decomposes easily into burning gases under the influence of heat (see Figure 2b). Thermal decomposition can take place also through char forming pathway. In this process, cellulose is first transformed to unstable, “active” cellulose that further decomposes so that reaction products are mainly carbon dioxide and water, and the “backbone” of cellulose containing a lot of carbon (see Figure 2c).
Figure 2. a) The two main reaction pathways of thermal decomposition of wood. b) Splitting of cellulose molecules in the tar-forming reaction (normal combustion). c) Splitting of cellulose molecules in the char-forming reaction.
The pyrolysis of wood is dependent on external factors, such as the way of heating, warming-up rate of the material, etc. Therefore, wood products do not have an explicit ignition temperature, but ignition takes place on a certain temperature range where the probability of ignition becomes large enough. The temperature for the piloted ignition of wood is typically about 350 °C, whereas the spontaneous ignition requires a temperature of approximately 600 °C.
2. Fire performance of wood
The reaction-to-fire properties, such as ignitability, heat release and flame spread, are most relevant for fire retardant wood products. Charring as a fire resistance characteristic property may also be influenced especially by surface protective layers.
2.1 Ignitability
In order that wood can ignite, its temperature must rise so high that pyrolysis takes place strongly enough and the chemical reactions of combustion start. Therefore, the ignition of a wood product is dependent on the way of heating, that is, the thermal properties of the material, and the way of heat attack on the material.
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Answer:
THE CORRECT ANSWER IS OPTION D) REMOVING SOME LOCKS FROM THE FURNACE