Combustion

Japanese: 燃焼 - ねんしょう(英語表記)combustion
Combustion

This refers to the phenomenon in which a substance burns in air or oxygen, emitting light or a flame. However, it has come to refer more generally to the phenomenon of burning with a flame. For example, in fluorine or chlorine, various substances burn with a flame, and atomic absorption spectrometry also uses the combustion of acetylene with nitrous oxide (nitrous oxide). In chlorine, methane and phosphorus burn with a flame. In terms of the phenomenon, these are also usually treated as being included in the category of combustion.

It is used in a wide range of applications, from everyday examples such as the burning of city gas or propane gas in the home, matches, burning fallen leaves, and fireworks, to the propulsion of rockets, as a means of converting chemical energy into thermal energy.

In addition, the gradual oxidation reaction that occurs in living organisms is also sometimes called combustion.

[Yamazaki Akira]

Definition of Combustion

It is often defined as "an oxidation reaction accompanied by heat and fire," but nowadays it is more commonly viewed as a problem of the transfer of thermal energy and gas flow that accompanies a chemical reaction. When considering combustion phenomena from this perspective, they can be broadly divided into ignition and flame propagation. The latter can be divided into laminar flame propagation and turbulent flame propagation depending on the flow of unburned fuel gas entering the flame front. When the phenomenon involves a single phase, it is called a homogeneous system, and when there are two or more phases, it is called a heterogeneous system. Furthermore, it is also divided into "premixed combustion," in which the oxidizer such as air or oxygen and the fuel gas are mixed before reaching the flame front, and "diffusion combustion," in which no mixing occurs. For example, the combustion of solids such as firewood and charcoal is a heterogeneous diffusion combustion.

[Yamazaki Akira]

Oxidizer for combustion

Most combustion is a reaction with oxygen, but as mentioned above, fluorine, chlorine, nitrates, perchlorates, etc. can also be used as oxidizers. These oxidizers are called "combustion-supporting substances," and the fuel is called "combustible substances." The "combustible mixtures" often seen in newspapers are mixtures of combustion-supporting substances and combustible substances.

[Yamazaki Akira]

Combustion reaction mechanism

Generally, the main reaction of combustion occurs in the gas phase, but if there is a catalytic site on the surface of a solid, an oxidation reaction occurs on this surface, and surface combustion begins. The minimum temperature required for combustion to begin is called the ignition point. The magic trick of lighting a sugar cube on fire is nothing more than surface combustion, taking advantage of the catalytic action of potassium carbonate in the ashes, which lowers the ignition point. The amount of heat generated when a substance is completely burned is called the heat of combustion, and is usually expressed in kilocalories per gram or mole of substance. The term calorific value is also used, especially in the field of engineering.

A city gas flame is an example of homogeneous premixed combustion, but it is a steady laminar flame. This is observed when the flow rate of the fuel gas and the combustion rate of the mixed gas are well balanced. The inner flame contains unreacted fuel gas and has reducing properties, so it is called a reducing flame. The outer flame is called an oxidizing flame. This type of flame characteristic is used in dry analytical methods such as torch analysis and molten ball reaction (phosphate ball or borax ball), but the fact that oxidizing and reducing flames produce different colors is often used in ceramic glazes as well. For example, copper ions appear reddish in a reducing flame and blue in an oxidizing flame.

Although this is a familiar phenomenon and has been used by humans for over a million years, there are still many unknown aspects of its mechanism and analysis of the phenomenon. Fundamental research into it has only begun to progress in the last 20 years or so, in particular due to its connection with disasters and the increase in the number of research methods for fast reactions.

[Yamazaki Akira]

"Fire" by Seiichiro Kumagai (Iwanami Shinsho)""The Science of Candles" by Faraday (translated by Iwao Mitsuishi, Kadokawa Bunko / translated by Yuri Yajima, Iwanami Bunko)

[Reference] | Fuel | Heat value | Flame

Source: Shogakukan Encyclopedia Nipponica About Encyclopedia Nipponica Information | Legend

Japanese:

物質が空気または酸素の中で、光や炎をあげて燃える現象をいう。しかしもっと一般的に炎をあげて燃える現象をさしていうことが多くなった。たとえば、フッ素や塩素の中ではいろいろな物質が炎をあげて燃えるし、原子吸光分析などでは一酸化二窒素(亜酸化窒素)によるアセチレンの燃焼なども利用する。塩素の中ではメタンやリンなどが炎をあげて燃える。これらも現象からみてやはり燃焼のなかに含めて扱うことが普通である。

 家庭での都市ガスやプロパンガスの燃焼やマッチ、落ち葉焚(た)き、花火などの卑近な例から、ロケットの推進に至る広い範囲で、化学エネルギーを熱エネルギーに変換するための手段として利用されている。

 なお、生体内の緩やかな酸化反応も燃焼ということがある。

[山崎 昶]

燃焼の定義

よく「熱と火を伴った酸化反応」という定義がされているが、現在では化学反応に伴う熱エネルギーの移動と気体の流れの問題としての見方が主流である。この立場から燃焼現象を考えると、発火と火炎伝播(でんぱ)とに大別できる。後者は、火炎面に入ってくる燃料の未燃ガスの流れ方によって、層流火炎伝播と乱流火炎伝播とに分けられる。現象の関係する相が単一の場合は均一系、二相以上の場合は不均一系とよぶ。さらに、火炎面に到達するよりも前で、空気や酸素などの酸化剤と燃料気体が混合している「予混燃焼」と、混合が行われていない「拡散燃焼」に分けることも行われる。たとえば、薪(まき)や木炭などの固体の燃焼は、不均一系拡散燃焼である。

[山崎 昶]

燃焼のための酸化剤

ほとんどの燃焼は酸素との反応であるが、前記のようにフッ素、塩素などのほか、硝酸塩や過塩素酸塩などを酸化剤として用いることもある。これらの酸化剤を「支燃性物質」といい、燃料のほうを「可燃性物質」という。よく新聞紙上に散見する「可燃性混合物」というのは、この支燃性物質と可燃性物質との混合物のことである。

[山崎 昶]

燃焼の反応機構

一般に燃焼の主反応は気相中でおこるが、固体の表面に触媒作用をもつ箇所があると、この表面で酸化反応がおこり、表面燃焼が始まる。燃焼が始まるのに必要な最低温度は発火点とよばれている。よく手品に出てくる角砂糖に火をつけるトリックも、灰の中にある炭酸カリウム分などによる触媒作用で発火点が低下することを利用して、表面燃焼をおこさせるのにほかならない。物質が完全に燃焼するときに発生する熱量を燃焼熱heat of combustionといい、普通は物質1グラム当りあるいは1モル当りの値をキロカロリーで表す。発熱量という用語も、とくに工学の分野では用いられる。

 都市ガスの炎などは、均一系予混燃焼の例であるが、定常的な層流炎である。これが認められるのは、燃料ガスの流速と、混合気体の燃焼速度がうまくつり合ったときである。内炎は未反応の燃料気体を含み、還元性をもっているので還元炎という。外炎は酸化炎とよばれる。このような炎の特性は、吹管分析や熔球(ようきゅう)反応(リン塩球やホウ砂球)などの乾式の分析法に利用されるが、陶磁器のうわぐすりなども、この酸化炎と還元炎とで異なる色となることを利用することも少なくない。たとえば、銅イオンは還元炎では赤系統、酸化炎では青系統の色となる。

 このように身近な現象であり、100万年以上の人類による利用の歴史をもっているのだが、その機構や現象の解析にはまだ未知の点がある。基礎的な研究が進歩しだしたのはここ20年ぐらいであろう。とくに災害などとの関連もあり、また高速反応に対する研究手段も増加してきたためである。

[山崎 昶]

『熊谷清一郎著『火』(岩波新書)』『ファラデー著『ロウソクの科学』(三石巌訳・角川文庫/矢島祐利訳・岩波文庫)』

[参照項目] | 燃料 | 発熱量 |

出典 小学館 日本大百科全書(ニッポニカ)日本大百科全書(ニッポニカ)について 情報 | 凡例

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