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Combustion: the story on HearLore | HearLore
Combustion
The first controlled chemical reaction discovered by humans was not a complex synthesis or a precise measurement, but the simple, terrifying act of starting a fire. Before the 1st of May 1536, humanity had no concept of the chemical reaction that would eventually power the modern world, yet the instinct to harness fire for warmth and cooking predates written history. This reaction, known today as combustion, is a high-temperature exothermic redox process where a fuel reacts with an oxidant, usually atmospheric oxygen, to produce oxidized gaseous products. While the colloquial meaning of burning implies the presence of flames, combustion does not always result in fire because a flame is only visible when substances undergoing combustion vaporize. The study of this phenomenon, known as combustion science, began with the observation of campfires and bonfires, which became the main method to produce energy for humanity for millennia. The thermal energy produced from the combustion of fossil fuels such as coal or oil, or from renewable fuels such as firewood, was harvested for diverse uses such as cooking, production of electricity, or industrial and domestic heating. Combustion is also currently the only reaction used to power rockets, a fact that underscores its fundamental role in human expansion into the cosmos.
The Hidden Chemistry Of Smoke
A simple example of this reaction can be seen in the combustion of hydrogen and oxygen into water vapor, a reaction which is commonly used to fuel rocket engines. This reaction releases 242kJ/mol of heat and reduces the enthalpy accordingly at constant temperature and pressure. However, complete combustion is almost impossible to achieve, since the chemical equilibrium is not necessarily reached, or may contain unburnt products such as carbon monoxide, hydrogen, and even carbon soot or ash. Thus, the produced smoke is usually toxic and contains unburned or partially oxidized products. Any combustion at high temperatures in atmospheric air, which is 78 percent nitrogen, will also create small amounts of several nitrogen oxides, commonly referred to as NOx, since the combustion of nitrogen is thermodynamically favored at high, but not low temperatures. Since burning is rarely clean, fuel gas cleaning or catalytic converters may be required by law. The formation of carbon monoxide produces less heat than formation of carbon dioxide, so complete combustion is greatly preferred, especially as carbon monoxide is a poisonous gas. When breathed, carbon monoxide takes the place of oxygen and combines with some of the hemoglobin in the blood, rendering it unable to transport oxygen. Breathing carbon monoxide causes headache, dizziness, vomiting, and nausea, and if levels are high enough, humans become unconscious or die.
When was the first controlled chemical reaction discovered by humans?
The first controlled chemical reaction discovered by humans was not a complex synthesis or a precise measurement, but the simple, terrifying act of starting a fire. Before the 1st of May 1536, humanity had no concept of the chemical reaction that would eventually power the modern world, yet the instinct to harness fire for warmth and cooking predates written history.
What is the chemical definition of combustion?
Combustion is a high-temperature exothermic redox process where a fuel reacts with an oxidant, usually atmospheric oxygen, to produce oxidized gaseous products. While the colloquial meaning of burning implies the presence of flames, combustion does not always result in fire because a flame is only visible when substances undergoing combustion vaporize.
How does combustion behave in microgravity environments?
In the microgravity environment of space, the thermal and flow transport dynamics behave quite differently than in normal gravity conditions, fundamentally altering the shape and behavior of a flame. A candle's flame takes the shape of a sphere in such an environment, as the influence of buoyancy on physical processes is considered small relative to other flow processes that would be present at normal gravity.
What causes combustion instabilities in rocket engines?
Combustion instabilities are typically violent pressure oscillations in a combustion chamber that can reach as high as 180dB, threatening the structural integrity of engines. In rockets, such as the F1 used in the Saturn V program, instabilities led to massive damage to the combustion chamber and surrounding components until the problem was solved by re-designing the fuel injector.
Why is complete combustion preferred over incomplete combustion?
Complete combustion is greatly preferred because the formation of carbon monoxide produces less heat than formation of carbon dioxide, and carbon monoxide is a poisonous gas. When breathed, carbon monoxide takes the place of oxygen and combines with some of the hemoglobin in the blood, rendering it unable to transport oxygen.
While flames capture the imagination, a slow, low-temperature, flameless form of combustion known as smoldering is often more dangerous because it is harder to detect. Smoldering is sustained by the heat evolved when oxygen directly attacks the surface of a condensed-phase fuel, and it is a typically incomplete combustion reaction. Solid materials that can sustain a smoldering reaction include coal, cellulose, wood, cotton, tobacco, peat, duff, humus, synthetic foams, charring polymers, and dust. Common examples of smoldering phenomena are the initiation of residential fires on upholstered furniture by weak heat sources, such as a cigarette or a short-circuited wire, and the persistent combustion of biomass behind the flaming fronts of wildfires. Incomplete combustion will occur when there is not enough oxygen to allow the fuel to react completely to produce carbon dioxide and water, or 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 and carbon monoxide are produced instead of carbon dioxide. Partially oxidized compounds are also a concern, as partial oxidation of ethanol can produce harmful acetaldehyde, and carbon can produce toxic carbon monoxide.
The Physics Of Fire In Space
In the microgravity environment of space, the thermal and flow transport dynamics behave quite differently than in normal gravity conditions, fundamentally altering the shape and behavior of a flame. A candle's flame takes the shape of a sphere in such an environment, as the influence of buoyancy on physical processes is considered small relative to other flow processes that would be present at normal gravity. Microgravity combustion research contributes to the understanding of a wide variety of aspects that are relevant to both the environment of a spacecraft, such as fire dynamics relevant to crew safety on the International Space Station, and terrestrial conditions. Droplet combustion dynamics assist in developing new fuel blends for improved combustion, materials fabrication processes, thermal management of electronic systems, and multiphase flow boiling dynamics. The term microgravity refers to a gravitational state that is low, such that the influence of buoyancy on physical processes may be considered small relative to other flow processes that would be present at normal gravity. In such an environment, the thermal and flow transport dynamics can behave quite differently than in normal gravity conditions, making the study of combustion in space a critical component of aerospace engineering and safety.
The Violent Instability Of Engines
Combustion instabilities are typically violent pressure oscillations in a combustion chamber that can reach as high as 180dB, threatening the structural integrity of engines. These pressure oscillations can be as high as 180dB, and long-term exposure to these cyclic pressure and thermal loads reduces 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. When the heat release oscillations are in phase with the pressure oscillations, the Rayleigh Index is positive and the magnitude of the thermoacoustic instability is maximised. On the other hand, if the Rayleigh Index is negative, then thermoacoustic damping occurs.
The Quantum Dance Of Oxygen
Combustion in oxygen is a chain reaction in which many distinct radical intermediates participate, driven 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, 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, possible with a very low probability. 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 from the fuel to oxygen, to give a hydroperoxide radical. 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.
The Environmental Cost Of Burning
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 ground level ozone, a major component of smog. The amount of air required for complete combustion is known as the theoretical air or stoichiometric air. The amount of air above this value actually needed for optimal combustion is known as the excess air, and can vary from 5% for a natural gas boiler, to 40% for anthracite coal, to 300% for a gas turbine. 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. Such devices are required by environmental legislation for cars in most countries. They may be necessary to enable large combustion devices, such as thermal power stations, to reach legal emission standards.