Why Is Fire Hot?

 

Fire is the rapid oxidation of a material in the exothermic chemical process of combustion, releasing heat, light, and various reaction products. [1] At the ignition point of the combustion reaction, it produces flames. The visible portion of the fire is flame. It consists of carbon dioxide, water vapor, oxygen, and nitrogen. Sometimes, if the fire is too hot, the gases may become ionized to produce plasma. [2] Depending on the substances we burn and any impurities outside, the color of the flame and the fire's intensity will vary. [3]

Orange and Yellow flame

 What is Ignition Temperature?

• The point where a body starts burning or catches fire is ignition. 
• The ignition temperature for Iron is 1,315 °C, and for
• Hydrogen is at 535 °C.  

 

The fire is hot because:

•  The chemical reactions involved in combustion release energy in the form of heat as they break down the molecular structure of the fuel.

• During combustion, the fuel molecules in the material break apart and recombine with oxygen molecules from the air. This process releases a significant amount of heat energy and light.

 What is Combustion?

Let us consider an example of a matchstick.

Matchstick fire

 When we rub it, its head catches fire.

 Do you know why?

Here, friction produces heat. It is enough to reach the ignition temperature of the chemical present. In this process, fire is the product of heat, and heat is the product of fire.

 

Is it fascinating?

When the bonds holding the atoms break, leading to the formation of new molecules with the release of energy.

This energy is in the form of heat, which causes the surrounding air and objects to heat up.

 The temperature of the fire depends on:

•  The type of fuel.
• The amount of oxygen available for the reaction
• The efficiency of the combustion process

 Are energy and color of flame related?

 A flame's color does not necessarily determine a temperature comparison because blackbody radiation is not the only factor that produces or determines the observed color.

It is only an estimate of temperature.

The color of a flame can provide a general indication of its temperature.

•  The energy released during combustion has a connection to the color of the flame. It has a link to the temperature of the fire. 
• Flame color depends on several factors, such as oxygen supply and the extent of fuel-oxygen mixing.

Different flame types of a Bunsen burner depend on the oxygen supply. In a laboratory under normal gravity conditions and with a closed air inlet, it burns with a yellow flame (called a safety flame) with a peak temperature of about 2,000 K. A blue-colored flame only emerges when the amount of soot decreases.

Other factors that determine its temperature are:

• Adiabatic flame (isolation), and no loss of heat to the atmosphere (may differ in certain parts).
• Atmospheric pressure.
• Percentage of oxygen content in the atmosphere
• The kind of fuel used for combustion
• Any oxidation of the fuel

The temperature of the atmospheric link to the adiabatic flame temperature (heat will transfer from a hotter to a lower temperature region more quickly).

The stoichiometric combustion process (1:1 stoichiometric), assuming no dissociation, will have the highest flame temperature. Excess air or oxygen will lower it, as will a lack of air or oxygen

The distance from the source of the flame 

In fires (particularly house fires), the low-temperature flames are often red and produce more smoke.

Do you know why?

Because of the lack of oxygen in the room, combustion is incomplete due to the presence of carbon monoxide. The temperature is only 600 to 850 °C.  

Colors and their associated temperatures:

 The highest recorded flame temperature is 4990° C. A mixture of ozone and dicyanoacetylene gases produces it. 

 

Red/Yellow flames are generally not too hot and are often associated with incomplete combustion. They are in the temperature range of 600 to 850 degrees Celsius. 

Flames of a candle or a wood fire are examples.

Candle flame

Orange flames are hotter than red or yellow flames and are in the temperature range of 850 to 1,000 degrees Celsius. 

Natural gas burners and certain stoves can produce orange flames.

Blue flames are hotter than red, yellow, or orange flames and usually indicate more complete combustion. The temperature can be around 1,026 to 1,400 degrees Celsius. 

Gas burners on stovetops and some industrial furnaces often produce blue flames.

Flames that appear white or bluish-white are hotter and are at high temperatures. These flames can exceed 1,400 degrees Celsius. 

High-temperature industrial processes and specialized burners can produce these types of flames.

Do you know that other factors can also influence the color?

•  Materials and the presence of certain chemical compounds. 
• The hottest part of a flame is the oxidizing zone, where the flame has complete combustion, and temperatures can be the highest. 

In this zone, the fuel and oxygen are thoroughly mixed and react to release the maximum heat energy.

 In a typical blue flame, you can notice three main parts:

 1The central, bright blue portion (Inner Blue Cone, hottest zone),  is where combustion occurs most efficiently. Temperatures in this area can reach around 1,400 to 1,600 degrees Celsius or even higher in some cases

2 Surrounding the inner blue cone where temperatures are lower than the inner blue cone (Luminous Zone). The color is yellow or orange. Here, combustion is incomplete. 

3 The outermost part of the flame is at lower temperatures than the inner blue cone (outer cone), Color is bluish or nearly invisible. 

Gas flame

The characteristics of different flames can vary with the type of fuel, the availability of oxygen, and the combustion conditions.

 Fires dance due to a combination of factors: 

•  Convection currents as heated air rises, causing flames to sway. 
• Uneven airflow and air pressure fluctuations make flames flicker unpredictably.
• Complex combustion dynamics and varying fuel-oxygen-heat interactions create flame instabilities and shape changes. 
• Swirling air patterns (vortices) can also influence flames.
• Oscillations in heat release result in cyclic changes in flame size and shape.

Dancing flame of a candle

In addition to these factors, the local temperature within the flame further causes expansion and contraction.

All these factors create the captivating dance of fire as they respond to the dynamic interplay of elements.

Material burned flame temperature:

Butane: ~300 °C (a cool flame in low gravity) [9]

Charcoal fire: 750–1,200 °C

Methane (natural gas):        900–1,500 °C

Bunsen burner flame: 900–1,600 °C [depending on the air valve position]

Candle flame:            ≈1,100 °C [majority]; hot spots maybe 1,300–1,400 °C

Propane blowtorch: 1,200–1,700 °C

Backdraft flame peak:         1,700–1,950 °C

Magnesium: 1,900–2,300 °C

Hydrogen torch: Up to ≈2,000 °C

MAPP gas: 2,020 °C

Acetylene blowlamp/blowtorch: Up to ≈2,300 °C

Oxyacetylene: Up to 3,300 °C

Material burner max. flame temperature (in air, diffusion flame) [7]

Animal fat: 800–900 °C

Kerosene: 990 °C

Gasoline: 1,026 °C

Wood: 1,027 °C

Methanol: 1,200 °C

Charcoal (forced draft): 1,390 °C

 Physical properties:

Fire is a chemical process in which a fuel and an oxidizing agent react, yielding carbon dioxide and water. [7] This process, known as a combustion reaction, does not proceed directly and involves intermediates. [7] Although the oxidizing agent is typically oxygen, other compounds can fulfill the role. [8]

Fires start when a flammable or combustible material, in combination with an oxidizer (oxygen gas or another oxygen-rich compound), is exposed to a source of heat or ambient temperature above the flash point. Once ignited, a chain reaction takes place. Fires can sustain their heat by further releasing the heat energy in combustion. They may propagate, providing a continuous supply of an oxidizer and fuel.

If the oxidizer is oxygen from the surrounding air, convection removes combustion products and brings oxygen to the fire due to the force of gravity. Without gravity, a fire rapidly surrounds itself with its combustion products and non-oxidizing gases from the air, which exclude oxygen and extinguish the fire. 

 Fire cannot exist without all these elements in place and in correct proportions. 

 Fire has the potential to result in conflagration, which can lead to physical and permanent damage through burning. Fire can influence ecological systems worldwide. 

 Positive effects of fire:

• Stimulate growth and maintain various ecological systems.

 Negative effects:

• Hazards to life and property, atmospheric pollution, and water contamination. [4]
• Removes protective vegetation,
• Increase in soil erosion by water during heavy rain. [5] Releases nitrogen into the atmosphere, resulting in long-term reduction in the fertility of the soil.

However, this loss of nitrogen caused by a fire can be recovered as atmospheric nitrogen and converted to ammonia by natural phenomena such as lightning or by leguminous plants such as clover, peas, and green beans.

• Burning fuel converts chemical energy into heat energy. The International Energy Agency states that nearly 80% of the world's power consistently comes from fossil fuels, such as petroleum, natural gas, and coal. [9,12] 
• The fire in a power station is used to heat water, creating steam that drives turbines. The turbines then spin an electric generator to produce electricity. [10,11] 
• Fire provides mechanical work by thermal expansion in external and internal combustion engines.

The unburnable solid remains of a combustible material left after a fire is called a clinker. If their melting point is below the flame temperature, causing it to fuse and then solidify as they cool

Highest temperature

Dicyanoacetylene is a compound of carbon and nitrogen that burns in oxygen with a bright blue-white flame at a temperature of 5,260 K (4,990 °C; 9,010 °F) and at up to 6,000 K (5,730 °C; 10,340 °F) in ozone. [14] This high flame temperature is partially due to the absence of hydrogen in the fuel (dicyanoacetylene is not a hydrocarbon). So there is no water among the combustion products.

Cyanogen, with the formula (CN)2, produces the second-hottest-known natural flame with a temperature of over 4,525 °C (8,177 °F) when it burns in oxygen. [15][16]

Cool flames

At temperatures as low as 120 °C (248 °F), fuel-air mixtures can react chemically and produce weak flames called cool flames. Humphry Davy discovered this phenomenon in 1817. The process depends on the balance of temperature and concentration of the reacting mixture. If conditions are suitable, it can initiate without any external ignition source. Cyclical variations in the balance of chemicals, particularly of intermediate products in the reaction, lead to oscillations in the flame, with a typical temperature variation of about 100 °C (212 °F) or between cool flame and full ignition. Sometimes, the variation can lead to an explosion. [13[17]

 

Woodfire

Uses of fire:

•  Fire is one of the four classical elements used by humans in rituals.
• In agriculture for clearing land
• For cooking and generating heat and light for signaling
• Used for propulsion, smelting, forging, incineration of waste, and cremation

 We have already discussed a lot about fire. Now, let me tell you the role of water in putting out the fire.

 

Why does water put out the fire?

 The primary role of water is to put out a fire by cooling it. So, there is not enough heat to sustain the fire. When you pour water onto a fire, the heat of the fire causes the water to turn into steam. It sucks away the heat (which is a form of energy) of the fire, leaving the fire lack of energy to keep burning.

References:

1.     Glossary of Wildland Fire Terminology (PDF), National Wildfire Coordinating Group, October 2007, p. 70, archived from the original (PDF) on 2008-08-21, retrieved 2008-12-18

2.    ^ Helmenstine, Anne Marie. "What is the State of Matter of Fire or Flame? Is it a Liquid, Solid, or Gas?". About.com. Archived from the original on 24 January 2009. Retrieved 2009-01-21.

3.    ^ Helmenstine, Anne Marie. "What is the State of Matter of Fire or Flame? Is it a Liquid, Solid, or Gas?". About.com. Archived from the original on 2009-01-24. Retrieved 2009-01-21.

4.    ^ Lentile, et al., 319

5.    ^ Morris, S. E.; Moses, T. A. (1987). "Forest Fire and the Natural Soil Erosion Regime in the Colorado Front Range". Annals of the Association of American Geographers. 77 (2): 245–54. doi:10.1111/j.1467-8306.1987.tb00156.x.

6.    ^ "SCIENCE WATCH; Burning Plants Adding to Nitrogen". The New York Times. 1990-08-14. ISSN 0362-4331. Retrieved 2023-11-02.

7"How electricity is generated". U.S. Energy Information Administration. Retrieved 2023-11-02.

8^ Jump up to:^a b "What is fire?". New Scientist. Retrieved November 5, 2022.

9^ Lowe, Derek (February 26, 2008). "Sand Won't Save You This Time". Science. Retrieved November 5, 2022.

 

10Christopher W. Schmidt; Steve A. Symes (2008). The analysis of burned human remains. Academic Press. pp. 2–4. ISBN 978-0-12-372510-3.

11^ Jozef Jarosinski; Bernard Veyssiere (2009). Combustion Phenomena: Selected Mechanisms of Flame Formation, Propagation and Extinction. CRC Press. p. 172. ISBN 978-0-8493-8408-0.

12  Wikipedia.

13  ^ Timmes, F. X.; Woosley, S. E. (1 September 1992). "The conductive propagation of nuclear flames. I - Degenerate C + O and O + Ne + Mg white dwarfs". The Astrophysical Journal. 396: 649–667. Bibcode:1992ApJ...396..649T. doi:10.1086/171746.

14  Kirshenbaum, A. D.; A. V. Grosse (May 1956). "The Combustion of Carbon Subnitride, NC₄N, and a Chemical Method for the Production of Continuous Temperatures in the Range of 5000–6000K". Journal of the American Chemical Society. 78 (9): 2020. doi:10.1021/ja01590a075.

15  ^ Thomas, N.; Gaydon, A. G.; Brewer, L. (1952). "Cyanogen Flames and the Dissociation Energy of N₂". The Journal of Chemical Physics. 20 (3): 369–374. Bibcode:1952JChPh..20..369T. doi:10.1063/1.1700426.

16  ^ J. B. Conway; R. H. Wilson Jr.; A. V. Grosse (1953). "The Temperature of the Cyanogen-Oxygen Flame". Journal of the American Chemical Society. 75 (2): 499. doi:10.1021/ja01098a517.

17  ^ Jones, John Clifford (September 2003). "Low temperature oxidation". Hydrocarbon process safety: a text for students and professionals. Tulsa, OK: PennWell. pp. 32–33. ISBN 978-1-59370-004-1.