In common parlance, the term is understood to mean the oxidation of a material with oxygen to form flames ( fire ). In chemistry, such reactions without oxygen are also referred to as burns. This includes the reaction of fluorine and hydrogen to form hydrogen fluoride ; here the fluorine replaces the oxygen as an oxidizing agent .
- The flame emits electromagnetic radiation from the ultraviolet , visible and infrared radiation range. A distinction must be made between solid-state radiation and radiation from gaseous molecules. Solid fuel or soot formed (e.g. fine dust from heating oil combustion ) is to be regarded as a solid . The solid emits radiation over the entire spectral band and the spectrum corresponds approximately to that of a black body . The gaseous molecules and atoms are heated up by the exothermic reaction of the combustion and higher energy levels of the particles are occupied, from which the particles fall back to an energetically lower level. The energy difference when the ground state is assumed is emitted as photons that form the flame . In the case of molecules, vibration and molecular bands are occupied; a line spectrum is emitted in the case of atoms. This gas radiation is selective and dependent on the components of the fuel gas and a large number of gas compounds, some of which only exist as an intermediate product until complete combustion. In contrast to solid-state radiation, the emitted radiation is not evenly distributed over the spectrum.
- In the case of "incomplete combustion", flammable gases (for example carbon monoxide, nitrogen oxides, hydrogen, methane) or solid carbon occur after combustion, in which not all possible bonds to the oxidizing agent are formed. This subheading includes the combustion of carbon to carbon monoxide or the production of charcoal, smoldering fire , coking .
- Slow "cold oxidation" can be the rusting of metals or in living organisms in the oxidation of nutrients , so their 'incineration' notice.
In technology, controlled combustion is usually aimed for, which is gradually adapted to the heat demand. This is known as stabilized burning . As a reference variable z. B. the temperature of a heat carrier or the vapor pressure of a boiling liquid is used and the mass flow of the fuel and the combustion air is adapted to the heat demand. The mass flow is regulated within a framework that only slight pressure increases occur in the combustion chamber and exhaust gas path due to the thermal expansion of the combustion gases. The technical combustion is regulated and monitored in such a way that the fuel burns in a controlled manner with the supplied combustion air.
Combustions in which a larger volume of the fuel-air mixture has formed and is then ignited is characterized by the rapid spread of the combustion in the space filled with the mixture. In closed rooms, a large increase in pressure and temperature occurs in a very short time interval, which is known as an explosion . The maximum explosion pressure of fuels containing carbon and hydrogen in closed rooms with the original ambient pressure (volume deflagration) is 10 bar; the flame speed is in the range of 0.5 m / s ( hydrocarbons ) to 2.5 m / s / hydrogen). Explosions with a slight increase in pressure (atmospheric deflagration) are known as deflagration .
There are two types of explosions in closed systems.
- Deflagrations are explosions that propagate at subsonic speeds.
- Detonations are explosions that propagate at supersonic speed and form a shock wave . With the stable detonation speeds of approx. 2000 m / s are reached. The fuel-air mixture is ignited by adiabatic compression. Explosion pressures can be significantly higher with detonation than with deflagration.
A distinction is made with regard to the space taken up during combustion (see flame characterization )
- extensive combustion at interfaces of the reactants in the flame front of a flame, for example
- volumetric combustion after premixing gaseous or vaporous components, for example
Useful and harmful fire
Fire theory, fire classes
During combustion, a substance, the fuel , reacts chemically with oxygen or with another gas. The fuel itself can be solid (e.g. wood , coal ), liquid ( gasoline , ethanol ), becoming liquid ( wax ) or gaseous ( methane gas , natural gas ). Ultimately, before the actual combustion, evaporation or cracking begins, so that the gases produced react with the gaseous oxygen in the air.
Conditions for incineration
A sufficient amount of combustible material that reacts with the oxidizing agent is required for combustion , usually oxygen (see oxygen index ). In addition, the correct proportions of the combustible substance with the ambient air or the reactive gas and a suitable ignition source are required. A catalyst can reduce the activation energy required to start the chemical reaction. This can accelerate combustion or reduce the energy required for ignition.
The initiation of the burning process, the ignition (supply of activation energy ), is called differently. While generally burns ent will ignite, particularly fire and deflagration can to ignited explosions can ge be ignited ( ignition ). Vapors and gases ignite .
Burning process and full fire
As soon as a small amount of fuel has reacted, the heat released as activation energy causes further fuel to react. In this sense, combustion is a thermal chain reaction . The light released during combustion is emitted by the glowing mass particles. In addition, the temperature typically rises very sharply, which can be used for heating or doing work.
At the moment, hydrocarbons are mostly made to react with the oxygen in the air in heat generation plants . The result is exhaust gas that, in addition to atmospheric nitrogen, mainly contains carbon dioxide (CO 2 ) and water (H 2 O). Depending on the type of combustion, the exhaust gas can contain various other substances, the most common components being carbon monoxide (CO), nitrogen oxides (NO x ) and unburned hydrocarbons. Soot can be produced when hydrocarbons are burned richly (excess fuel) .
The so-called air ratio is required for combustion in air. This is a ratio of the proportions of the ambient air, i.e. oxygen to nitrogen and argon:
In relation to 1 mol of fuel, the proportion of oxygen required for complete combustion is obtained from:
Solving the above equation , one obtains:
- , where the lower case letters indicate the number of elements contained in the fuel.
The computational concentration of fuel required for complete combustion is obtained from
An example is the complete combustion of 1-propanol ( , molar mass 60.1 g mol −1 ):
Thus, 4.5 mol of oxygen are required for complete combustion of 1 mol of propanol. In addition, the stoichiometric concentration required for combustion can be calculated:
Combustion calculation and exhaust gas composition
In the case of combustible material, oxidation can only occur when a single atom or molecule of the fuel comes into direct contact with oxygen. The availability of oxygen and its intimate contact with the fuel are therefore decisive for the rate of combustion ( burning rate ). Some extinguishing methods are based on interrupting the oxygen supply (fire blanket, foam, CO 2 extinguishing system).
The supply of oxygen can be achieved by constantly supplying fresh air by blowing into a wood fire. The fireplace is an ideal aid for wood fires . The heated flue gases rise quickly in the narrowing chimney pipe and create a constant negative pressure around the fire. This constantly draws in fresh air. Firestorms and forest fires, which are fanned by winds such as the mistral , are extreme forms .
In order to establish intimate contact, the surface area of the fuel can be increased; gasifying the fuel into a gas is a suitable option. In the case of the candle, the wax melts at the bottom of the wick, rises as a liquid and evaporates at the hot tip. The evaporated wax burns. A vivid example is the flour dust explosion . If some flour is blown into a candle flame, the otherwise incombustible flour becomes flammable due to the atomization and reacts violently. In the gasoline engine , vaporization takes place in the carburettor and the fuel is atomized in the diesel engine . Liquid diesel fuel can hardly be ignited at room temperature. Due to the injection system and a sudden compression with the resulting heating in the combustion chamber, diesel ignites itself and burns.
Depending on the substance properties of the specific vapor pressure and the environmental factors pressure and temperature, there is a vapor cloud above all liquids . If it is a flammable liquid, this vapor layer is flammable in a certain range (between the lower and upper explosion limit). The short-chain hydrocarbons, gasoline, have a high specific vapor pressure and are highly volatile, so even at low temperatures they form a flammable layer of vapor over the surface. The longer-chain diesel ignites more difficult because the vapor pressure is lower.
In some chemical compounds, the “oxidizing agent” (oxygen) and the “material” to be oxidized are contained in the same molecule , as in many explosives . Nitroglycerin with the empirical formula C 3 H 5 N 3 O 9 contains nine oxygen atoms per molecule (in three nitrate and nitric acid ester groups) and therefore more than enough to completely oxidize the carbon and hydrogen atoms contained in the molecule to carbon dioxide and water. The connection is unstable and explosively disintegrates even with slight shocks. The gaseous oxidation products take up many times the original volume and generate a very high pressure, which causes the explosive effect. In the propellants of rocket engines , oxygen is also present in various carrier substances as an oxidizing agent, as this is necessary in the vacuum of space.
The combustion of wood begins with external heating. In the case of wet wood, the temperature increase at around 100 ° C interrupts further heating of the wood before the ignition temperature is reached. Once the water has largely evaporated, the temperature rises and the escaping gases begin to burn. Wood can store approximately its own weight in water and the latent heat is necessary for the evaporation process; so damp or wet wood can hardly be ignited. Dry wood ignites more easily and begins to char from around 150 ° C. This is a pyrolysis of the wood through heat-induced chemical decomposition. Gaseous substances are formed which ignite after mixing with air. The remaining charcoal consists of the degassed wood and consists essentially of carbon, which burns off last.
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- Rodewald: fire theory . 6th edition, W. Kohlhammer, Stuttgart 2007. ISBN 978-3-17-019129-7 .
- M. Lackner, F. Winter, AK Agarwal: Handbook of Combustion . Wiley-VCH, Weinheim 2010. ISBN 978-3-527-32449-1 .
- Drysdale: An Introduction to Fire Dynamics . Second Edition 1998, John Wiley & Sons, Ltd, The Atrium, Southern Gate, Chichester West Sussex PO19 8SQ, England, ISBN 978-0-471-97291-4 .
- Gerhard Hausladen: Script heating technology. University of Kassel, February 1998, (PDF; 2.2 MB), online at delta-q.de, accessed on December 22, 2016.
- EN 1127-1 Explosive atmospheres - Explosion protection - Part 1: Fundamentals and methodology, 2011
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