How is heat generated during combustion
Lexicon> Letter V> Combustion
Definition: a chemical reaction between a fuel or fuel and oxygen
More general term: chemical reaction
Categories: Basic Terms, Physical Basics, Heat and Cold
Author: Dr. Rüdiger Paschotta
How to quote; suggest additional literature
Original creation: 02/27/2013; last change: 03/14/2020
Combustion processes are among the most technically important chemical reactions. B. in internal combustion engines, thermal power plants and boilers. The vast majority of the energy that mankind uses today comes from the combustion of fossil fuels. During combustion, a fuel or fuel reacts with oxygen, which is usually supplied as a component of air, releasing heat and, to a lesser extent, light. The fuel is oxidized, i.e. H. it forms chemical compounds with oxygen:
- From the carbon content z. B. from hydrocarbons becomes carbon dioxide (CO2) and in the case of incomplete combustion, sometimes also the poisonous carbon monoxide (CO).
- The hydrogen part turns into water vapor (H.2O) implemented.
- Other components of the fuel can also be oxidized; z. B. sulfur dioxide (SO2), partly also sulfur trioxide (SO3).
Most fuels consist primarily of carbon and hydrogen. This is especially true for hydrocarbon fuels such as B. heating oil, diesel fuel, gasoline and kerosene. Some fuels such as ethanol and methanol also have a significant amount of oxygen. They are almost already partially oxidized and therefore typically have a reduced calorific value.
The air supplied consists of a good 20% oxygen, which is required for combustion. The rest is mainly nitrogen, which ideally does not take part in the reaction, but simply remains in the exhaust gas. At high combustion temperatures, however, nitrogen also reacts with oxygen to form various nitrogen oxides. This is undesirable because nitrogen oxides are poisonous and at the same time the heat yield is somewhat reduced.
If a burn occurs very quickly, it is also called one Deflagration, Deflagration or Detonation, colloquially often also from explosion.
Oxidation in a broader sense is also possible with chemical substances other than oxygen, for example with halogens such as chlorine. However, combustion usually takes place with oxygen from the air.
In rare cases, combustion is carried out with pure oxygen, e.g. B. is obtained by air separation. This leads to much higher combustion temperatures and still avoids the formation of nitrogen oxides, as long as nitrogen is not introduced via the fuel.
Roughly speaking, the oxygen index of a material means the minimum oxygen content in the air that is necessary for combustion. So it is a measure of the flammability of a material.
Combustion air ratio
An important parameter of any combustion is the combustion air ratio. A combustion air ratio of approx. 1 (stoichiometric combustion) or slightly larger, d. H. a small excess of air. With solid fuels such as coal or biomass, however, a higher excess of air is often necessary in order to avoid local oxygen deficiency, which would otherwise lead to incomplete combustion.
Generation of heat
The purpose of most technical combustion processes is to release energy in the form of heat, although sometimes the focus is also on the disposal of waste materials (e.g. rubbish or scrap wood). Often the heat is partially converted into mechanical energy, e.g. B. in internal combustion engines, steam turbines or gas turbines. The fuel or fuel is used as an energy carrier. Because of the relatively high energy density of fuels compared to other energy storage systems such as B. rechargeable batteries, these are also well suited for carrying in vehicles and airplanes.
The amount of heat generated during combustion is determined by the calorific value when the water content in the exhaust gas is removed in gaseous form (i.e. as water vapor). However, if the water vapor is condensed, the heat of condensation is also obtained, and the total heat yield is determined by the (higher) calorific value.
The release of heat during combustion is of course expressed in a strong increase in the temperature of the substances involved. However, the combustion temperature in no way only depends on the amount of heat released. She takes z. B. from when there is a significant excess of air, since the excess air also has to be heated. The flames can also be cooled by emitting thermal radiation, in particular if there are soot particles in the flame. The combustion temperature in turn has an effect on the combustion: If the temperature is too low, the combustion is often incomplete, while if the temperature is too high, the formation of nitrogen oxides increases (see above). However, nitrogen oxides can break down again immediately if the exhaust gas is cooled down slowly enough.
Loss of heat generation (energy loss) can occur in various ways, e.g. B. through incomplete combustion, through the exhaust gases (see below) or through other heat losses z. B. a boiler.
Flames are a typical, but not always occurring, side effect of a combustion. These are hot gases, e.g. T. mixed with small solid particles that emit light through glow emission.
When burning carbonaceous fuels, flames are often yellow and have a relatively strong glow due to small soot particles. (Since these particles can still burn afterwards, this does not necessarily have to lead to strong soot emissions.) Blue flames, on the other hand, indicate higher combustion temperatures and low soot contents; the blue light comes from various excited molecules and radicals, some of which are only intermediate products of combustion. With some fuels such as B. methanol, the flames are very weak. There are also processes for flameless oxidation for fuels that otherwise burn with a flame.
All flames also emit infrared (invisible light, thermal radiation), which can significantly contribute to the removal of the generated heat.
The exhaust gas from combustion can contain various substances:
- the oxidized substances such as carbon dioxide, which is non-toxic in low concentrations but harmful to the climate, and water vapor
- Incomplete combustion products such as toxic carbon monoxide (CO), unburned hydrocarbons and soot particles
- toxic nitrogen oxides (especially at high combustion temperatures and rapid cooling)
- chemically aggressive sulfur dioxide if the fuel contains sulfur
- in very small amounts highly toxic dioxins and furans (e.g. when burning wood or garbage)
- small particles, e.g. B. Sulphate particles, which can arise from undesirable components of the fuel
- remaining oxygen and the nitrogen introduced
Ideally, an exhaust gas consists only of non-toxic substances such as CO2, Water vapor, oxygen and nitrogen. However, this is often not the case and the exhaust gas has to be post-treated, e.g. B. with a catalytic converter or a particle filter.
Even non-toxic exhaust gases are ecologically questionable because of the climate-damaging carbon dioxide. One solution would be the CO2-Deposition and storage.
Since exhaust gases usually leave an incineration system at a higher temperature (the exhaust gas temperature), certain energy losses occur, which are referred to as exhaust gas loss.
Solid products of combustion
If solid particles such. B. from carbon black or sulfates, you can use this z. B. deposit in a boiler or arise in the aftertreatment of the exhaust gas in a filter. Unburned hydrocarbons can attach to the particles. The particles are often small enough to be respirable and are poisonous due to various substances adhering to them.
Especially with solid fuels, ash is also formed from minerals that are contained in the fuel. Wood ash contains various plant nutrients, which in principle make the ash interesting as a fertilizer, but unfortunately also various naturally occurring toxic heavy metals.
Combustion process; Combustion technology problems
Depending on the fuel and application, very different combustion processes are used:
- In boilers, combustion is carried out by a burner, and details of the boiler are also very important for the quality and efficiency of the combustion.
- Combustion engines are either fed with a fuel-air mixture (for gasoline engines), or the fuel is fed separately, e.g. B. directly into the combustion chamber via an injection system (for direct-injection petrol and diesel engines). Usually one takes place internal combustion in a combustion chamber, in a few cases (e.g. with Stirling engines and Ericsson engines) also an external combustion.
- For large firings z. B. in power plants there are various systems, z. B. grate furnaces and fluidized bed furnaces.
In particular, gaseous fuels such as natural gas can be burned completely and cleanly with simple technical means. One problem, however, can be the formation of nitrogen oxides, which can be greatly reduced by a sophisticated combustion technique.
Premature ignition can be a problem in internal combustion engines (especially gasoline engines), especially when the compression ratio is high. The use of a fuel with high knock resistance is helpful here. Premature ignition is not possible with diesel engines, but a sufficiently high ignition ability is important, which is ensured by the Cetane number of the fuel is specified. A sufficiently high flame speed is important in all high-speed internal combustion engines, which is why z. B. Heavy oil in small diesel engines would not be used even with strong preheating.
The combustion of solid fuels such as coal, wood or other biomass is relatively complicated and correspondingly more difficult to optimize:
- The conditions in the combustion chamber are usually very inhomogeneous, i. H. inconsistent, mainly because a complete mixing of fuel and air is not possible.
- The combustion can also take place in several phases; z. For example, wood is first dried, then flammable substances are degassed (→ biomass gasification) and finally the charred wood is burned off.
- An optimal air supply is made difficult by the solid fuel itself, e.g. B. if air supplied from below is deflected on a log that burns on the top and needs more oxygen there. However, incompletely burned wood gas in the flames above can still burn out completely if more air flows in from the side. Under certain circumstances, ash can also obstruct the air supply, e.g. B. if a grate is covered too much.
- When flames meet much cooler solid fuel, the combustion temperature drops sharply there. B. can lead to a strong soot formation.
- The composition of biomass is often highly variable. For example, the composition of wood varies depending on the type of tree and degree of drying, and the bark components are different from the heartwood. Biowaste is usually particularly heterogeneous.
- A variable performance of the furnace is more difficult than z. B. to achieve with natural gas or fuel oil, since the fuel supply cannot be changed quickly. Controlling the output via the air supply (e.g. with a wood stove) is problematic as this can lead to incomplete combustion.
- Starting the combustion process is particularly difficult. When firing, the combustion temperature is often too low for complete combustion and it is more difficult to meter the air supply.
The combustion technology for many different fuels has been optimized over a long period of time. However, many more developments are needed to improve the quality of the combustion e.g. B. with regard to the exhaust gases to make further fuels (also inhomogeneous) usable well, or to enable combustion in other performance classes (z. B. also in smaller systems).
Pre-treatment of fuels
Sometimes a pre-treatment of the fuel is also very helpful:
- Liquid fuels e.g. B. from petroleum can be desulphurized in a petroleum refinery, i. H. their sulfur content is greatly reduced. This reduces the formation of toxic and aggressive sulfur dioxide during combustion.
- Petroleum products are also heavily optimized in other ways in refineries, e.g. B. by fractionation (narrowing the range of different substances) and adding additives, e.g. B. to increase the knock resistance or the ignitability or to reduce the formation of soot.
- Firewood can be processed into smaller logs or into wood pellets, which can be burned much more easily and cleanly - even in small automatic furnaces. Pellets are also advantageous for transport.
- In burners for heavy fuel oil, the oil is often first heated up so that it is sufficiently thin for atomization and more volatile.
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See also: fuel, solid fuel, fuel, heat, burner, boiler, combustion engine, calorific value, calorific value, enthalpy, oxygen index, combustion air ratio, stoichiometric combustion, flameless oxidation, lambda probe, exhaust gas, emissions and immissions, exhaust gas loss, soot, mono-combustion, co-combustion, flaring
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