What is burn

Combustion (chemistry)

A combustion is a redox reaction that takes place with the release of energy in the form of heat and light, i.e. exothermic.

In common parlance, combustion is understood to mean the oxidation of a combustible material with oxygen with the formation of flames as "fire". But there are also burns in reactions without oxygen, including the reaction of fluorine and hydrogen to form hydrogen fluoride; here the fluorine replaces the oxygen as an oxidizing agent.

Terms, classification

Fuel chemistry

  • Combustion in the form of a fire with the appearance of flames from glowing volatile substances. If solid substances burn with a flame, this is formed by burning gaseous pyrolysis products
  • Combustion in the form of embers takes place without glowing volatile substances.
  • At a incomplete combustion combustible gases (such as carbon monoxide, hydrogen, methane) or solid carbon occur after combustion[1]by not creating all possible bonds to the oxidizing agent. This subheading includes the combustion of carbon to carbon monoxide or the production of charcoal, smoldering fire, coking.
  • Slow cold oxidation can be determined when metals rust or when nutrients are oxidized in living beings, ie when they are "burned".

Combustion physics

Burns in which premixed systems have a high Burn rate, react almost suddenly and with an enormous increase in volume (of the gaseous components) are called explosions. These are subdivided taking into account the speed of combustion and propagation.

Useful and harmful fire

Combustion in a fire can take place in a controlled manner (useful fire), for example in a furnace, a steam boiler (furnace), as a campfire, or uncontrolled as a harmful fire in the event of a fire.

Fire theory, fire classes

course

When burned, a substance reacts fuel, 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 begins, vaporization or cracking begins, so that the gases produced react with the 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 flammable substance with the ambient air or the reactive gas and a suitable ignition source are necessary. A catalyst can reduce the activation energy required to start the chemical reaction. This can accelerate the combustion or reduce the energy required for ignition.

Ignite

The initiation of the burning process, the Ignite (Supplying the activation energy) is called differently. During general burns entcan ignite, especially fire and deflagration atignited, detonations can gebe ignited (detonator). Vapors and gases entFlames.

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 comes from 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 which, 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. When hydrocarbons are burned richly (excess fuel), soot can be produced.

Combustion chemistry

Air ratio

The so-called air ratio is required for combustion in air. This is a ratio of the proportions of the ambient air, mainly oxygen and nitrogen: $ c_ \ mathrm {O_2} = 21 \,%; c_ \ mathrm {N_2} = 79 \,% \ rightarrow {79 \ over 21} = 3 {,} 76 \, {\ mathrm {mol N_2} \ over \ mathrm {mol O_2}} $

Oxygen demand

In relation to 1 mol of fuel, the proportion of oxygen $ \ nu_ \ mathrm {O2} $ required for complete combustion is obtained from:

$ \ mathrm {C_a H_b O_c N_d F_e Cl_f Br_g J_h P_i S_j} + \ nu_ {O 2} (O_2 + 3 {,} 76 \, \ mathrm {N_2}) \ Rightarrow $

$ \ mathrm {aCO_2 + jSO_2 + {i \ over 4} P_4O_ {1 0} + eHF + fHCl + gHBr + hHJ + {b- (e + f + g + h) \ over 2} H_2 O + \ left ({d \ over 2} +3 {,} 76 \ times \ nu_ {O 2} \ right) \ times N_2} $

Solving the above equation for $ \ nu_ {O 2} $, one obtains:

$ \ nu_ {O 2} = C + S + {5 \ over 4} P + {\ mathrm {H- (F + Cl + Br + J)} \ over 4} - {O \ over 2} $ respectively

$ \ nu_ {O 2} = a + j + {5 \ over 4} i + {b- (e + f + g + h) \ over 4} - {c \ over 2} $, where the lower case letters represent the number of im Specify the elements contained in the fuel.

Stoichiometric concentrations

The computational concentration of fuel required for complete combustion is obtained from

$ \ mathrm {c_ {stoech} = {100 \ over 1+ (1 + 3 {,} 76) \ times \ nu_ {O 2}} \, [Vol%]} $

respectively


$ \ mathrm {c_ {stoech} = {Vol% \ times M \ times10 ^ {- 2} \ over (0 {,} 02405)}} \, [g / m ^ 3] $

example

An example is the complete combustion of 1-propanol ($ C_3H_8O $, molar mass 60.1 g mol−1) called:

$ \ nu_ {O 2} = 3C + 0S + {0 \ over 4} 0P + {\ mathrm {8H- (0F + 0Cl + 0Br + 0J)} \ over 4} - {1O \ over 2} $

$ \ nu_ {O 2} = 3 + 0 + 0 + {8- (0 + 0 + 0 + 0) \ over 4} - {1 \ over 2} \ = \ 3 + 2-0 {,} 5 = 4 {,} 5 $


Thus, 4.5 mol of oxygen are required for complete combustion of 1 mol of propanol. The stoichiometric concentration required for combustion can also be calculated:

$ \ mathrm {c_ {stoech} = {100 \ over 1+ (1 + 3 {,} 76) \ times 4 {,} 5} = 4 {,} 46 \, Vol%} $
respectively

$ \ mathrm {c_ {stoech} = {4 {,} 46 \ times 60 {,} 1 \ times 10 ^ {- 2} \ over (0 {,} 02405)}} = 111 {,} 45 \, g / m ^ $ 3

File: ErstickteKerze.ogv

Combustion calculation and exhaust gas composition

Combustion calculations with the corresponding exhaust gas compositions are particularly efficient for the application area of ​​heat engineering using a calculation algorithm according to Werner Boie.[2]

Combustion physics

In the case of combustible material, oxidation can only occur if a single atom or molecule of the fuel comes into direct contact with oxygen. Therefore, for the Burn rate (Burn rate) the availability of oxygen and its intimate contact with the fuel are decisive. 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 exhaust 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 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.

Above all liquids there is a dependence on the substance properties specific vapor pressure and the environmental factors pressure and temperature a cloud of steam. 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, petrol, have a high specific vapor pressure and are highly volatile, so even at low temperatures they form a flammable vapor layer 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 C3H5N3O9 contains nine oxygen atoms per molecule (in three nitrate and nitric acid ester groups) and thus 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 disintegrates explosively even with slight shocks. The gaseous oxidation products take up a multiple of 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.

Material science

The combustion of wood begins with external heating. With wet wood, the temperature increase stops at around 100 ° C, depending on the increase in the boiling point caused by dissolved substances. Once the water has largely evaporated, the temperature rises and combustion begins. 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, in some cases gaseous substances are formed that emerge from the wood as a flame. The remaining charcoal as a mixture of carbon and ash is then incinerated with additional oxygen.

See also

literature

  • J. Warnatz, U. Maas, R. W. Dibble: combustion. Springer, Berlin, 2001 ISBN 3-540-42128-9.
  • Rodewald: Fire theory. 6th edition, W. Kohlhammer, Stuttgart, 2007. ISBN 978-3-17-019129-7.
  • M. Lackner, F. Winter, A. K. 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.

Web links

Individual evidence

  1. ↑ Gerhard Hausladen: Lecture script heating technology, University of Kassel, 1992, (pdf file), last accessed October 2012
  2. ↑ B. Glück: "Material values ​​and combustion calculation".