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.
Why don't metals burn? | Science Questions with Surprising Answers
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 an explosion , though for an internal combustion engine this is inaccurate. When the fuel-air mixture in an internal combustion engine explodes, that is known as detonation. 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. Combustion resulting in a turbulent flame is the most used for industrial application e. The term 'micro' gravity refers to a gravitational state that is 'low' i.
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In such an environment, the thermal and flow transport dynamics can behave quite differently than in normal gravity conditions e. Microgravity combustion research contributes to the understanding of a wide variety of aspects that are relevant to both the environment of a spacecraft e. Combustion processes which happen in very small volumes are considered micro-combustion. The high surface-to-volume ratio increases specific heat loss. Quenching distance plays a vital role in stabilizing the flame in such combustion chambers.
Generally, the chemical equation for stoichiometric combustion of a hydrocarbon in oxygen is:. For example, the stoichiometric burning of propane in oxygen is:. 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.
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Note that treating all non-oxygen components in air as nitrogen gives a 'nitrogen' to oxygen ratio of 3. When excess air is used, nitrogen may oxidize to NO and, to a much lesser extent, to NO 2. Diesel engines are run with an excess of oxygen to combust small particles that tend to form with only a stoichiometric amount of oxygen, necessarily producing nitrogen oxide emissions.
Both the United States and European Union enforce limits to vehicle nitrogen oxide emissions, which necessitate the use of special catalytic converters or treatment of the exhaust with urea see Diesel exhaust fluid. Such gas mixtures are commonly prepared for use as protective atmospheres for the heat-treatment of metals and for gas carburizing. The products of incomplete combustion can be calculated with the aid of a material balance , together with the assumption that the combustion products reach equilibrium. The three elemental balance equations are:.
These three equations are insufficient in themselves to calculate the combustion gas composition. However, at the equilibrium position, the water-gas shift reaction gives another equation:. Substances or materials which undergo combustion are called fuels. The most common examples are natural gas, propane, kerosene, diesel, petrol, charcoal, coal, wood, etc. Combustion of a liquid fuel in an oxidizing atmosphere actually happens in the gas phase.
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It is the vapor that burns, not the liquid. Therefore, a liquid will normally catch fire only above a certain temperature: its flash point. The flash point of a liquid fuel is the lowest temperature at which it can form an ignitable mix with air. It is the minimum temperature at which there is enough evaporated fuel in the air to start combustion. 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.
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. Efficient process heating requires recovery of the largest possible part of a fuel's heat of combustion into the material being processed. Typically, the dominant loss is sensible heat leaving with the offgas i. 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 H 2 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.
For methane CH 4 combustion, for example, slightly more than two molecules of oxygen are required. The second principle of combustion management, however, is to not use too much oxygen.
The Fire Triangle
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. Adherence to these two principles is furthered by making material and heat balances on the combustion process.
The heat balance relates the heat available for the charge to the overall net heat produced by fuel combustion. 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.
Why don't metals burn?
Interaction between the two is quantum mechanically a " forbidden transition ", i. To initiate combustion, energy is required to force dioxygen into a spin-paired state, or singlet oxygen. This intermediate is extremely reactive.
The energy is supplied as heat , and the reaction then produces additional heat, which allows it to continue. 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.
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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. Detailed descriptions of combustion processes, from the chemical kinetics perspective, requires the formulation of large and intricate webs of elementary 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.
As a result, the direct numerical simulation of turbulent reactive flows with heavy fuels soon becomes intractable even for modern supercomputers. 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:. 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.
Assuming perfect combustion conditions, such as complete combustion under adiabatic conditions i. 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.
In industrial fired heaters , power station steam 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. For example, excess combustion air of 15 percent means that 15 percent more than the required stoichiometric air is being used. Combustion instabilities are typically violent pressure oscillations in a combustion chamber. 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 .