What is rapid oxidation called? — Answers

Bibliography

Borman, G. L., and Ragland, K. W. (1998). Combustion Engineering. Boston: McGraw-Hill.

Chomiak, J. (1990). Combustion: A Study in Theory, Fact, and Application. New York: Abacus Press.

Combustion and Flame(series). New York: Elsevier.

Combustion Science and Technology (series). New York: Gordon and Breach.

Glassman, I. (1996). Combustion, 3rd ed. San Diego, CA: Academic Press.

International Symposium on Combustion (series). Pittsburgh: The Combustion Institute.

Progress in Energy Combustion Science(series). New York: Pergamon.

Turns, S. R. (2000). An Introduction to Combustion, 2nd ed. Boston: McGraw-Hill.

Williams, F. A. (1985). Combustion Theory, 2nd ed. Menlo Park, CA: Benjamin/Cummings.

Combustion management

Efficient process heating requires recovery of the largest possible part of a fuel’s heat of combustion into the material being processed.[16][17] There are many avenues of loss in the operation of a heating process.

Typically, the dominant loss is sensible heat leaving with the offgas (i.e., the flue gas). The temperature and quantity of offgas indicates its heat content (enthalpy), so keeping its quantity low minimizes heat loss.

In a perfect furnace, the combustion air flow would be matched to the fuel flow to give each fuel molecule the exact amount of oxygen needed to cause complete combustion. However, in the real world, combustion does not proceed in a perfect manner.

Unburned fuel (usually CO and H2) discharged from the system represents a heating value loss (as well as a safety hazard). Since combustibles are undesirable in the offgas, while the presence of unreacted oxygen there presents minimal safety and environmental concerns, the first principle of combustion management is to provide more oxygen than is theoretically needed to ensure that all the fuel burns.

The second principle of combustion management, however, is to not use too much oxygen. The correct amount of oxygen requires three types of measurement: first, active control of air and fuel flow; second, offgas oxygen measurement; and third, measurement of offgas combustibles.

For each heating process, there exists an optimum condition of minimal offgas heat loss with acceptable levels of combustibles concentration. Minimizing excess oxygen pays an additional benefit: for a given offgas temperature, the NOx level is lowest when excess oxygen is kept lowest.[3]

Adherence to these two principles is furthered by making material and heat balances on the combustion process.[18][19][20][21] The material balance directly relates the air/fuel ratio to the percentage of O2 in the combustion gas.

The heat balance relates the heat available for the charge to the overall net heat produced by fuel combustion.[22][23] Additional material and heat balances can be made to quantify the thermal advantage from preheating the combustion air,[24][25] or enriching it in oxygen.[26][27]

Complete

In complete combustion, the reactant burns in oxygen and produces a limited number of products. When a hydrocarbon burns in oxygen, the reaction will primarily yield carbon dioxide and water. When elements are burned, the products are primarily the most common oxides.

Carbon will yield carbon dioxide, sulfur will yield sulfur dioxide, and iron will yield iron(III) oxide. Nitrogen is not considered to be a combustible substance when oxygen is the oxidant. Still, small amounts of various nitrogen oxides (commonly designated NOx species) form when the air is the oxidative.

Combustion is not necessarily favorable to the maximum degree of oxidation, and it can be temperature-dependent. For example, sulfur trioxide is not produced quantitatively by the combustion of sulfur. NOx species appear in significant amounts above about 2,800 °F (1,540 °C), and more is produced at higher temperatures.

In most industrial applications and in fires, air is the source of oxygen (O2). In the air, each mole of oxygen is mixed with approximately 3.

71 mol of nitrogen. Nitrogen does not take part in combustion, but at high temperatures some nitrogen will be converted to NOx (mostly NO, with much smaller amounts of NO2).

On the other hand, when there is insufficient oxygen to combust the fuel completely, some fuel carbon is converted to carbon monoxide, and some of the hydrogens remain unreacted. A complete set of equations for the combustion of a hydrocarbon in the air, therefore, requires an additional calculation for the distribution of oxygen between the carbon and hydrogen in the fuel.

The amount of air required for complete combustion to take place is known as pure air[citation needed]. However, in practice, the air used is 2-3 times that of pure air.

Droplet and spray combustion and pool fires

The combustion of liquids is a fertile area for further study. Kowledge of the combustion science of individual droplets as well as groups of droplets helps improve performance of devices that rely on spray burning, particularly diesel engines. Understanding of the science of liquid pool fires potentially effects safety during spills.

Elementary reaction kinetics, kinetic mechanisms, models, and experiments

Examining the details involved in the oxidation and pyrolysis (thermal decomposition) of fuel molecules is very important. The results of these research activities will permit predictions about the chemicals emitted during incomplete combustion because reaction rate constants and chemical pathways will be evaluated and determined.

Extinction

Once propagating, flames will continue to propagate unless they are extinguished or quenched. An obvious cause of extinction is the depletion or cessation of fuel flow. Flames can be extinguished by heat loss (e.g., by passage through very small passageways that accentuate heat loss), through smothering by water or chemical fire extinguishers that slow the combustion process, or blowing the flame away with high velocity flows. Flames can also be extinguished by removing one of the reactants, such as air.

Fire safety research

Research is conducted to increase understanding of the science of combustion specifically as it relates to fires involving homes and plastics, wood, and large-scale spills. This is helpful in the development of fire prevention and extinction techniques.

Fluidized beds, porous media fixed bed combustion, and furnaces.

Specialized practical configurations for combustion have a number of practical applications such as coal burning for energy production. The study of these specialized combustion setups is necessary for better application.

Kenneth Brezinsky

See also: Catalysts; Coal, Production of; Conservation of Energy; Explosives and Propellants; Heat Transfer.

Further reading

Poinsot, Thierry; Veynante, Denis (2021). Theoretical and Numerical Combustion (3rd ed.). European Centre for Research and Advanced Training in Scientific Computation.Lackner, Maximilian; Winter, Franz; Agarwal, Avinash K., eds. (2021). Handbook of Combustion, 5 volume set. Wiley-VCH. ISBN 978-3-527-32449-1.Baukal, Charles E., ed. (1998). Oxygen-Enhanced Combustion. CRC Press.Glassman, Irvin; Yetter, Richard. Combustion (Fourth ed.).Turns, Stephen (2021). An Introduction to Combustion: Concepts and Applications.Ragland, Kenneth W; Bryden, Kenneth M. (2021). Combustion Engineering (Second ed.).Baukal, Charles E. Jr, ed. (2021). «Industrial Combustion». The John Zink Hamworthy Combustion Handbook: Three-Volume Set (Second ed.).Gardiner, W. C. Jr (2000). Gas-Phase Combustion Chemistry (Revised ed.).

Gas turbines, diesel combustion, and spark ignition engines

Application of combustion science to practical power source devices is one of the ultimate aims of developing a fundamental understanding of combustion.
Using combustion science to improve performance through design changes and engineering techniques is an ongoing research subject.

Gaseous fuels

Combustion of gaseous fuels may occur through one of four distinctive types of burning: diffusion flame, premixed flame, autoignitive reaction front, or as a detonation.[15] The type of burning that actually occurs depends on the degree to which the fuel and oxidizer are mixed prior to heating: for example, a diffusion flame is formed if the fuel and oxidizer are separated initially, whereas a premixed flame is formed otherwise.

Similarly, the type of burning also depends on the pressure: a detonation, for example, is an autoignitive reaction front coupled to a strong shock wave giving it its characteristic high-pressure peak and high detonation velocity.[15]

High-speed combustion, metals combustion, and propellants

Research in these specialized areas is aimed at developing improved and new methods for advanced propulsion. For example, an understanding of high-speed combustion is used in the development of supersonic ram jet engines, which are simple alternatives to conventional turbojet engines.

Knowledge of metals combustion is relevant to improving the use of metal additives in solid propellants to increase impulse and stability. In general, the study of propellant combustion aids in the development of more stable and longer range rocket engine performance.

Incineration, nox (no and no2) formation and control, soot formation and destruction

Environmental consequences of combustion are still a high priority requiring investigation of the chemistry and process effects on the emissions. Effective means of eliminating the pollutants is also a subject of further research.

Incomplete

Incomplete combustion will occur when there is not enough oxygen to allow the fuel to react completely to produce carbon dioxide and water. It also happens when the combustion is quenched by a heat sink, such as a solid surface or flame trap. As is the case with complete combustion, water is produced by incomplete combustion; however, carbon, carbon monoxide, and hydroxide are produced instead of carbon dioxide.

For most fuels, such as diesel oil, coal, or wood, pyrolysis occurs before combustion. In incomplete combustion, products of pyrolysis remain unburnt and contaminate the smoke with noxious particulate matter and gases. Partially oxidized compounds are also a concern; partial oxidation of ethanol can produce harmful acetaldehyde, and carbon can produce toxic carbon monoxide.

The designs of combustion devices can improve the quality of combustion, such as burners and internal combustion engines. Further improvements are achievable by catalytic after-burning devices (such as catalytic converters) or by the simple partial return of the exhaust gases into the combustion process.

The degree of combustion can be measured and analyzed with test equipment. HVAC contractors, firemen and engineers use combustion analyzers to test the efficiency of a burner during the combustion process.

Incomplete combustion of a hydrocarbon in oxygen

The incomplete (partial) combustion of a hydrocarbon with oxygen produces a gas mixture containing mainly CO2, CO, H2O, and H2.

Such gas mixtures are commonly prepared for use as protective atmospheres for the heat-treatment of metals and for gas carburizing.[11] The general reaction equation for incomplete combustion of one mole of a hydrocarbon in oxygen is:

Incomplete combustion produced carbon monoxide

Carbon monoxide is one of the products from incomplete combustion.[4] Carbon is released in the normal incomplete combustion reaction, forming soot and dust. Since carbon monoxide is a poisonous gas, complete combustion is preferable, as carbon monoxide may also lead to respiratory troubles when breathed since it takes the place of oxygen and combines with hemoglobin.[5]

Instabilities

Combustion instabilities are typically violent pressure oscillations in a combustion chamber. These pressure oscillations can be as high as 180 dB, and long-term exposure to these cyclic pressure and thermal loads reduce the life of engine components.

In rockets, such as the F1 used in the Saturn V program, instabilities led to massive damage to the combustion chamber and surrounding components. This problem was solved by re-designing the fuel injector. In liquid jet engines, the droplet size and distribution can be used to attenuate the instabilities.

Combustion instabilities are a major concern in ground-based gas turbine engines because of NOx emissions. The tendency is to run lean, an equivalence ratio less than 1, to reduce the combustion temperature and thus reduce the NOx emissions; however, running the combustion lean makes it very susceptible to combustion instability.

The Rayleigh Criterion is the basis for analysis of thermoacoustic combustion instability and is evaluated using the Rayleigh Index over one cycle of instability[51]

where q’ is the heat release rate perturbation and p’ is the pressure fluctuation.[52][53]
When the heat release oscillations are in phase with the pressure oscillations, the Rayleigh Index is positive and the magnitude of the thermo acoustic instability is maximised.

On the other hand, if the Rayleigh Index is negative, then thermoacoustic damping occurs. The Rayleigh Criterion implies that a thermoacoustic instability can be optimally controlled by having heat release oscillations 180 degrees out of phase with pressure oscillations at the same frequency.[54][55] This minimizes the Rayleigh Index.

Kinetic modelling

The kinetic modelling may be explored for insight into the reaction mechanisms of thermal decomposition in the combustion of different materials by using for instance Thermogravimetric analysis.[48]

Laminar diffusion flames

Here, too, computer based predictions about the nature of these flames require information about the chemicals and science of diffusion flames for the predictions to be accurate. The predictions are made accurate by comparison with measured chemical species concentrations, measured temperatures, and flow characteristics.

Laminar premixed flames

Research in this area focuses on understanding the chemical, thermal, and fluid-mechanical (behavior of fluids) structure of these types of flames. Recent advances in computer based modeled flames requires the knowledge developed in this type of research for calibration, validation, and prediction.

Liquid fuels

Combustion of a liquid fuel in an oxidizing atmosphere actually happens in the gas phase. It is the vapor that burns, not the liquid. Therefore, a liquid will normally catch fire only above a certain temperature: its flash point.

Microgravity combustion

Microgravity refers to the environment of extremely low gravity commonly known as a weightless environment. Under microgravity conditions, combustion phenomena that are affected by gravity, such as flames, behave differently than at Earth gravity conditions.

Premixed turbulent combustion and nonpremixed turbulent combustion

Because many practical flames are turbulent (spark ignited engine flames, oil field flares), an understanding of the interaction between the complex fluid dynamics of turbulence and the combustion processes is necessary to develop predictive computer models.

Once these predictive models are developed, they are repeatedly compared with measurements of species, temperatures, and flow in actual flames for iterative refinement. If the model is deficient, it is changed and again compared with experiment. The process is repeated until a satisfactory predictive model is obtained.

Problems associated with incomplete combustion

Environmental problems:^[6]

These oxides combine with water and oxygen in the atmosphere, creating nitric acid and sulfuric acids, which return to Earth’s surface as acid deposition, or «acid rain.» Acid deposition harms aquatic organisms and kills trees.

Due to its formation of certain nutrients that are less available to plants such as calcium and phosphorus, it reduces the productivity of the ecosystem and farms. An additional problem associated with nitrogen oxides is that they, along with hydrocarbon pollutants, contribute to the formation of tropospheric ozone, a major component of smog.

Human health problems:^[6]

Breathing carbon monoxide causes headache, dizziness, vomiting, and nausea. If carbon monoxide levels are high enough, humans become unconscious or die. Exposure to moderate and high levels of carbon monoxide over long periods is positively correlated with risk of heart disease.

People who survive severe carbon monoxide poisoning may suffer long-term health problems.[7] Carbon monoxide from air is absorbed in the lungs which then binds with hemoglobin in human’s red blood cells. This would reduce the capacity of red blood cells to carry oxygen throughout the body.

Rapid

Rapid combustion is a form of combustion, otherwise known as a fire, in which large amounts of heat and light energy are released, which often results in a flame. This is used in a form of machinery such as internal combustion engines and in thermobaric weapons.

Such a combustion is frequently called a Rapid combustion, though for an internal combustion engine this is inaccurate.[disputed – discuss] An internal combustion engine nominally operates on a controlled rapid burn.

When the fuel-air mixture in an internal combustion engine explodes, that is known as detonation.[disputed – discuss]

Rapid combustion is called what? — answers

Reaction mechanism

Combustion in oxygen is a chain reaction in which many distinct radical intermediates participate. The high energy required for initiation is explained by the unusual structure of the dioxygen molecule. The lowest-energy configuration of the dioxygen molecule is a stable, relatively unreactive diradical in a triplet spin state.

Bonding can be described with three bonding electron pairs and two antibonding electrons, with spins aligned, such that the molecule has nonzero total angular momentum. Most fuels, on the other hand, are in a singlet state, with paired spins and zero total angular momentum.

Interaction between the two is quantum mechanically a «forbidden transition», i.e. possible with a very low probability. To initiate combustion, energy is required to force dioxygen into a spin-paired state, or singlet oxygen.

Combustion of hydrocarbons is thought to be initiated by hydrogen atom abstraction (not proton abstraction) from the fuel to oxygen, to give a hydroperoxide radical (HOO). This reacts further to give hydroperoxides, which break up to give hydroxyl radicals.

There are a great variety of these processes that produce fuel radicals and oxidizing radicals. Oxidizing species include singlet oxygen, hydroxyl, monatomic oxygen, and hydroperoxyl. Such intermediates are short-lived and cannot be isolated.

However, non-radical intermediates are stable and are produced in incomplete combustion. An example is acetaldehyde produced in the combustion of ethanol. An intermediate in the combustion of carbon and hydrocarbons, carbon monoxide, is of special importance because it is a poisonous gas, but also economically useful for the production of syngas.

Solid and heavy liquid fuels also undergo a great number of pyrolysis reactions that give more easily oxidized, gaseous fuels. These reactions are endothermic and require constant energy input from the ongoing combustion reactions. A lack of oxygen or other improperly designed conditions result in these noxious and carcinogenic pyrolysis products being emitted as thick, black smoke.

The rate of combustion is the amount of a material that undergoes combustion over a period of time. It can be expressed in grams per second (g/s) or kilograms per second (kg/s).

Detailed descriptions of combustion processes, from the chemical kinetics perspective, requires the formulation of large and intricate webs of elementary reactions.[28] For instance, combustion of hydrocarbon fuels typically involve hundreds of chemical species reacting according to thousands of reactions.

Inclusion of such mechanisms within computational flow solvers still represents a pretty challenging task mainly in two aspects. First, the number of degrees of freedom (proportional to the number of chemical species) can be dramatically large; second, the source term due to reactions introduces a disparate number of time scales which makes the whole dynamical system stiff.

Therefore, a plethora of methodologies has been devised for reducing the complexity of combustion mechanisms without resorting to high detail level. Examples are provided by:

References

Research activities

The following synopsis of current research activities in the field of combustion is organized around the list of papers presented at the 27th International Symposium on Combustion (1998) under the auspices of The Combustion Institute.

Smouldering

Smouldering is the slow, low-temperature, flameless form of combustion, sustained by the heat evolved when oxygen directly attacks the surface of a condensed-phase fuel. It is a typically incomplete combustion reaction. Solid materials that can sustain a smouldering reaction include coal, cellulose, wood, cotton, tobacco, peat, duff, humus, synthetic foams, charring polymers (including polyurethane foam) and dust.

Common examples of smoldering phenomena are the initiation of residential fires on upholstered furniture by weak heat sources (e.g., a cigarette, a short-circuited wire) and the persistent combustion of biomass behind the flaming fronts of wildfires.

Solid fuels

The act of combustion consists of three relatively distinct but overlapping phases:

Spontaneous

Spontaneous combustion is a type of combustion which occurs by self-heating (increase in temperature due to exothermic internal reactions), followed by thermal runaway (self-heating which rapidly accelerates to high temperatures) and finally, ignition.

For example, phosphorus self-ignites at room temperature without the application of heat. Organic materials undergoing bacterial composting can generate enough heat to reach the point of combustion.[8]

Stoichiometric combustion of a hydrocarbon in air

If the stoichiometric combustion takes place using air as the oxygen source, the nitrogen present in the air (Atmosphere of Earth) can be added to the equation (although it does not react) to show the stoichiometric composition of the fuel in air and the composition of the resultant flue gas.

CxHy zO2 3.77zN2⟶ xCO2 y2H2O 3.77zN2{displaystyle {ce {C}}_{x}{ce {H}}_{y} z{ce {O2}} 3.77z{ce {N2 ->}} x{ce {CO2}} {frac {y}{2}}{ce {H2O}} 3.77z{ce {N2}}}

where z=x 14y{displaystyle z=x {frac {1}{4}}y}.
For example, the stoichiometric combustion of propane (C3H8{displaystyle {ce {C3H8}}}) in air is:

Stoichiometric combustion of a hydrocarbon in oxygen

Generally, the chemical equation for stoichiometric combustion of a hydrocarbon in oxygen is:

CxHy zO2⟶xCO2 y2H2O{displaystyle {ce {C_{mathit {x}}H_{mathit {y}}{} {mathit {z}}O2->{mathit {x}}CO2{} {frac {mathit {y}}{2}}H2O}}}

where z=x y4{displaystyle z=x {frac {y}{4}}}.

For example, the stoichiometric burning of propane in oxygen is:

Temperature

Assuming perfect combustion conditions, such as complete combustion under adiabatic conditions (i.e., no heat loss or gain), the adiabatic combustion temperature can be determined. The formula that yields this temperature is based on the first law of thermodynamics and takes note of the fact that the heat of combustion is used entirely for heating the fuel, the combustion air or oxygen, and the combustion product gases (commonly referred to as the flue gas).

In the case of fossil fuels burnt in air, the combustion temperature depends on all of the following:

The adiabatic combustion temperature (also known as the adiabatic flame temperature) increases for higher heating values and inlet air and fuel temperatures and for stoichiometric air ratios approaching one.

Most commonly, the adiabatic combustion temperatures for coals are around 2,200 °C (3,992 °F) (for inlet air and fuel at ambient temperatures and for λ=1.0{displaystyle lambda =1.0}), around 2,150 °C (3,902 °F) for oil and 2,000 °C (3,632 °F) for natural gas.^[49][50]

In industrial fired heaters, power stationsteam generators, and large gas-fired turbines, the more common way of expressing the usage of more than the stoichiometric combustion air is percent excess combustion air.

Trace combustion products

Various other substances begin to appear in significant amounts in combustion products when the flame temperature is above about 1600 K. When excess air is used, nitrogen may oxidize to NO and, to a much lesser extent, to NO2. CO forms by disproportionation of CO2, and H2 and OH form by disproportionation of H2O.

For example, when 1 mol of propane is burned with 28.6 mol of air (120% of the stoichiometric amount), the combustion products contain 3.

3% O2. At 1400 K, the equilibrium combustion products contain 0.03% NO and 0.

002% OH. At 1800 K, the combustion products contain 0.17% NO, 0.05% OH, 0.

What does a rapid combustion produce? — answers

What is rapid oxidation called? — answers