Combustion

Incineration is a redox reaction, which takes place with the release of energy in form of heat and light, so exothermic.

In common parlance is meant by combustion, the oxidation of a combustible material with oxygen flame as "fire." Burns, there are also for reactions without oxygen, the reaction of fluorine and hydrogen is one of hydrogen fluoride; Here, the fluorine replaces the oxygen as the oxidant.

• 2.1 Requirements for an internal
• 2.2 ignition
• 2.3 Anbrennvorgang and full fire

• 3.1 air ratio
• 3.2 Oxygen Demand
• 3.3 Stoichiometric concentrations
• 3.4 Example
• 3.5 Combustion calculation and exhaust gas composition

Definitions, classification

Combustion chemistry

• Combustion in the form of a fire with flames glowing appearance of volatiles. Burning solid materials with flame, so this is formed by burning gaseous pyrolysis products.
• Combustion in the form of glowing embers runs without volatiles.
• In incomplete combustion occur after the combustion combustible gases ( such as carbon monoxide, hydrogen, methane ) or solid carbon on by not arise all possible compounds to the oxidizing agent. This subheading includes the combustion of carbon to carbon monoxide and the production of charcoal, smoldering, coking.
• Slow cold oxidation can be the rusting of metals or in living organisms in the oxidation of nutrients, therefore find their " burning ".

Combustion physics

Burns, react at the premixed combustion systems with a high speed, almost abruptly and with an enormous increase in volume ( of the gaseous components ) are called blasts. These are divided in consideration of the combustion and speed of propagation.

• Detonation ( detonation velocity of the explosive technology),
• Deflagration
• And deflagration.

Useful and harmful fire

The combustion in a fire can be controlled ( friendly fire ), for example in a furnace, a boiler ( furnace ), as a campfire, or uncontrolled as malicious fire in a fire take place.

Teaching fire, fire classes

Course

During combustion, a substance, which fuel reacts chemically with oxygen or another gas. The fuel itself may be fixed (eg, wood, coal), liquid ( gasoline, ethanol ), liquid down ( wax) or gaseous ( methane gas, natural gas). Ultimately, the actual combustion starts before evaporation or cracking, allowing the resultant gases to react with the gaseous oxygen.

Requirements for combustion

For an internal combustible material in a sufficient amount necessary to react with the oxidizing agent, usually it is the oxygen (see oxygen index). In addition, the correct proportion of the combustible material with the ambient air or the reactive gas and a suitable ignition source is needed. A catalyst, the activation energy which is required to start the chemical reaction, decrease. Thus, the combustion is accelerated or the energy required for ignition are reduced.

Ignite

The introduction of the burning process, the ignition ( supplying the activation energy ) is called differently. While generally burns are ignited, fire and deflagration can especially lit detonations can be ignited ( ignition ). Vapors and gases burst into flame.

Anbrennvorgang and full fire

Once a small amount of fuel reacted, brings the heat liberated as activation energy further fuel to react, combustion is in this sense a thermal chain reaction. Released during the combustion of light is emitted from the glowing mass particles. In addition, typically the temperature greatly increased, which can be used to heat or doing work.

Currently, most hydrocarbons with oxygen in the air are reacted in plants to generate heat. The result is this exhaust gas containing nitrogen in addition to air mainly carbon dioxide ( CO) and water ( HO). Depending on the nature of the various other combustion control substances in the exhaust gas may contain common units are carbon monoxide (CO ), nitrogen oxides (NO) and unburned hydrocarbons. With rich combustion ( excess fuel ) of hydrocarbons can occur soot.

Chemistry of combustion

Air ratio

For the combustion in air, the so-called air ratio is required. This is a ratio value of the shares of the ambient air, so mainly oxygen and nitrogen:

Oxygen demand

Relative to 1 mole of the fuel is obtained the required proportion of oxygen to completely burn through:

Solving the above equation with respect to on, then one obtains:

Respectively

Where the lowercase letters indicate the number of elements contained in the fire material.

Stoichiometric concentrations

The computation required for complete combustion of fuel concentration is obtained via

Respectively

Example

One example is called the complete combustion of 1- propanol (, molar mass 60.1 g · mol -1):

Thus, for the complete combustion of 1 mol of propanol 4.5 mol of oxygen needed. It can further be of the stoichiometric concentration required for the combustion, to calculate:

Respectively

Combustion calculation and exhaust gas composition

Combustion calculations with the corresponding exhaust gas compositions are particularly efficient possible with a computational algorithm of Werner Boie for the scope of thermal engineering.

Physics of combustion

The combustible material, it may only come to oxidation, when a single atom or molecule of the fuel comes into direct contact with oxygen. Therefore, relevant to the burning rate ( burning rate ) the availability of oxygen and its intimate contact with the fuel. Some extinguishing methods are based on interrupting the oxygen supply ( fire blanket, foam, CO - extinguishing system ).

The supply of oxygen can be achieved through constant supply of fresh air by blowing in a wood fire. For wood-burning fireplace is an ideal tool. In the narrowing chimney pipe the heated exhaust gases to rise up quickly and generate a constant negative pressure around the fire. This sucks permanently zoom fresh air. An extreme form are Firestorm and forest fires, which are fanned by winds, such as the Mistral.

To make intimate contact with the surface of the fuel can be increased, the gasification of the fuel in a gas is an appropriate way. When the candle melts the wax at the bottom of the wick rises as a liquid and evaporates on the hot tip. The evaporated wax burns. An illustrative example is the flour dust explosion. If some flour blown into a candle flame, the otherwise non-combustible flour is flammable by the atomization and reacts violently. In the petrol engine takes place in the carburetor evaporation and in the diesel engine, the fuel is atomized. Liquid diesel fuel can ignite at room temperature hardly. Due to the fuel injection system and a sudden compression with the resulting heat in the combustion chamber diesel ignites and burns itself.

Over all liquids is a vapor cloud, depending on the material property of specific vapor pressure and the ambient pressure and temperature factors. If it is a flammable liquid is this vapor layer in a particular area flammable ( the lower and upper explosive limit ). The short-chain hydrocarbons, gasoline, have a high specific vapor pressure, are volatile, thus form even at low temperatures of a flammable vapor layer above the surface. The longer- diesel fires difficult since the vapor pressure is lower.

In some of the chemical compounds are the " oxidizing agent" (oxygen) and to be oxidized " material " contained in the same molecule, as in many explosives. Nitroglycerin with the formula C3H5N3O9 per molecule nine oxygen atoms (in three nitrate and nitric acid ester groups), more than enough to completely oxidize the contained in the molecule of carbon and hydrogen atoms to carbon dioxide and water. The compound is unstable and decomposes explosively even at slight shocks. The gaseous oxidation products occupy a multiple of the initial volume and produce a very high pressure which causes the snap action. Also in the propellants of rocket engines oxygen in various carrier substances is present as an oxidizing agent, as this is required in the vacuum of space.

Material Science

Burning wood begins with a heating from outside. With wet wood, the temperature increase stops at about 100 ° C depending on the boiling-point elevation by solutes. The water is evaporated as far as possible, the temperature rises, and the combustion starts. Wood can save approximately its own weight of water and the evaporation process, the latent heat is required; so is damp or wet wood hard to ignite. Dry wood ignites easily and starts at about 150 ° C. to carbonize. This is a pyrolysis of wood by heat-induced chemical decomposition processes form gaseous substances which emerge as a flame from the wood. Remaining charcoal as a mixture of carbon and ash then burn up with further zutretendem oxygen.