|
|
| (One intermediate revision by one other user not shown) |
| Line 1: |
Line 1: |
| {{hatnote|"Burning" redirects here. For combustion without external ignition, see [[spontaneous combustion]]. For the vehicle engine, see [[internal combustion engine]]. For other uses, see [[Burning (disambiguation)]] and [[Combustion (disambiguation)]].}}
| | In a quest to get rid of weight, there are a range of foods which are not merely healthy and nutritious, however aid the body lose weight naturally and heighten the bodys rate of fat loss. Incorporating several or all of these foods into a daily diet may speed up fat reduction, build a healthier body, plus place you on the road to a healthier lifestyle.<br><br>Drink green tea to feel full between food. The phytonutrient called EGCG (epigallocatechin gallate) in green tea increases the hormone responsible for making we feel satiated. Drinking green tea daily has long-term benefits also. It could enable prevent storage of excess fat, improve a appetite-regulating hormones and change how the body handles cravings plus metabolizes food.<br><br>We have to take two pills daily with water. It is advised which you take the pill after you have had the food. A little of exercise is always necessary to keep the body fit plus for greater blood circulation. The increase flow of blood means more vitality plus better health. You must furthermore maintain a good diet without junk food plus oily food. So, it's said that in the event you take the raspberry ketone pluswith a little of exercise along with a superior diet then the results can be much better and quicker. Even after this if you see you are not gaining anything with this pill then return it plus receive the full cash back.<br><br>Additionally a ski helmet will protect your head within the snow plus the melted cold water. At freezing conditions this protection could save a life! And and how rock hard these ski helmets are against collisions!<br><br>Many treatments claim that they may boost the metabolism. But, Dr. Oz recommends [http://safedietplansforwomen.com/raspberry-ketones raspberry ketones] because the most efficient choices.It helps to control adiponectin, that is a hormone which stimulates the body to improve the metabolism.<br><br>Higher degrees of ketones inside the blood can plus can make we feel sick plus raspberry ketone diet lead to vomiting. If left untreated, you will suffer from more ketones, dehydration and an imbalance of alternative chemicals that when left inside the blood might lead to unconsciousness plus eventually be fatal.<br><br>The hoodia gordonii plant has qualities inside the core. There are many pills inside the marketplace that boasts of containing this property nevertheless either having small quantities or they do not have any. Unique Hoodia has been one of the ideal supplements inside the slimming market for a good amount of years. According to the Hoodia reports, it contains the real property of the plant plus it also contains a good amount of the property in every pill about five 100 milligrams. This really is the appropriate amount for a human body. So, should you go for the hoodia fat loss program we would definitely see a change in your weight in a week or two.<br><br>The most important thing to know is that there are no shortcuts to weight reduction. Slimming pills are not because damaging to a health as they were ten years ago, plus countless are incredibly healthy plus can boost a vitality degrees and fat loss results greatly. With a small research, there are the medications that support your body, very than carrying it back. |
| [[File:Et baal.jpg|thumb|200px|The [[flame]]s caused as a result of a [[fuel]] undergoing combustion (burning)]]
| |
| {{Inadequate lead|date=April 2013}}
| |
| '''Combustion''' {{IPAc-en|k|əm|ˈ|b|ʌ|s|.|tʃ|ən}} or '''burning'''<ref>colloquial meaning of burning is combustion accompanied by flames</ref> is the sequence of [[exothermic]] chemical reactions between a [[fuel]] and an [[oxidant]] accompanied by the production of [[heat]] and conversion of chemical species. The release of heat can produce [[light]] in the form of either [[Smouldering|glowing]] or a [[flame]].
| |
| | |
| In a complete combustion reaction, a compound reacts with an [[Oxidizing agent|oxidizing element]], such as [[oxygen]] or [[fluorine]], and the products are compounds of each element in the fuel with the oxidizing element. For example: | |
| | |
| :CH<sub>4</sub>(g) + 2O<sub>2</sub>(g) → CO<sub>2</sub>(g) + 2H<sub>2</sub>O(g)
| |
| | |
| The [[standard enthalpy of reaction]] for [[methane]] combustion at {{val|298.15|ul=K}} and {{val|1|ul=atm}} is {{val|-802|ul=kJ|upl=mol}}.<ref name="ReacWeb">[http://www.crct.polymtl.ca/reacweb.htm Reaction-Web]</ref>
| |
| A simple example can be seen in the combustion of [[hydrogen]] and [[oxygen]], a reaction commonly used to fuel [[rocket engine]]s:
| |
| | |
| :2H<sub>2</sub>(g) + O<sub>2</sub>(g) → 2H<sub>2</sub>O(g)
| |
| | |
| The result is [[water]] vapor, with a standard enthalpy of reaction at {{val|298.15|ul=K}} and {{val|1|ul=atm}} of {{val|-242|ul=kJ|upl=mol}}.<ref name="ReacWeb" />
| |
| | |
| Complete combustion is almost impossible to achieve. As actual combustion reactions come to [[Chemical equilibrium|equilibrium]], a wide variety of major and minor species will be present, such as [[carbon monoxide]], [[hydrogen]] and even [[carbon]] ([[soot]] or ash). Additionally, any combustion at high temperatures in [[Atmosphere|atmospheric]] [[air]], which is 78 percent [[nitrogen]], will also create small amounts of several nitrogen oxides, commonly referred to as [[NOx|{{chem|NO|''x''}}]].
| |
| | |
| Combustion need not always involve oxygen; e.g., hydrogen burns in chlorine to form hydrogen chloride with the liberation of heat and light characteristic of combustion.
| |
| | |
| ==Types==
| |
| | |
| ===Complete vs. incomplete===
| |
| {{see also|pyrolysis}}
| |
| | |
| ====Complete====
| |
| [[File:Combustion reaction of methane.jpg|thumb|370px|The combustion of [[methane]], a hydrocarbComplete[edit]
| |
| | |
| The combustion of methane, a hydrocarbon, is a complete combustion reaction, as the products are carbon dioxide and water.
| |
| In complete combustion, the reactant burns in oxygen, producing 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, but small amounts of various nitrogen oxides (commonly designated NO
| |
| x species) form when air is the oxidant.
| |
| 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. The amount of NOx is also a function of oxygen excess.[3]
| |
| In most industrial applications and in fires, air is the source of oxygen (O
| |
| 2). In 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 NO
| |
| x (mostly NO, with much smaller amounts of NO
| |
| 2). On the other hand, when there is insufficient oxygen to completely combust the fuel, some fuel carbon is converted to carbon monoxide and some of the hydrogen remains unreacted. A more complete set of equations for the combustion of a hydrocarbon in 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 theoretical air. However, in practice the air used is 2-3x that of theoretical air.on, is a complete combustion reaction, as the products are carbon dioxide and water.]]
| |
| In complete combustion, the reactant burns in oxygen, producing 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 [[Oxidizing agent|oxidant]], but small amounts of various nitrogen oxides (commonly designated [[NOx|{{chem|NO|''x''}}]] species) form when air is the oxidant.
| |
| | |
| 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 {{convert|2800|F|C}}, and more is produced at higher temperatures. The amount of NOx is also a function of oxygen excess.<ref name="NOx formation">[http://www.alentecinc.com/papers/NOx/The%20formation%20of%20NOx_files/The%20formation%20of%20NOx.htm The formation of NOx]. Alentecinc.com. Retrieved on 2010-09-28.</ref>
| |
| | |
| In most industrial applications and in [[fire]]s, [[air]] is the source of oxygen ({{chem|O|2}}). In air, each mole of oxygen is mixed with approximately {{val|3.71|ul=mol}} of nitrogen. Nitrogen does not take part in combustion, but at high temperatures some nitrogen will be converted to [[NOx#Thermal|{{chem|NO|''x''}}]] (mostly [[Nitric oxide|{{chem|NO}}]], with much smaller amounts of [[Nitrogen dioxide|{{chem|NO|2}}]]). On the other hand, when there is insufficient oxygen to completely combust the fuel, some fuel carbon is converted to [[carbon monoxide]] and some of the hydrogen remains unreacted. A more complete set of equations for the combustion of a hydrocarbon in 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 theoretical air. However, in practice the air used is 2-3x that of theoretical air.
| |
| | |
| ====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.
| |
| | |
| 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 quality of combustion can be improved by the designs of combustion devices, such as [[Oil burner|burner]]s and [[internal combustion engines]]. Further improvements are achievable by [[catalytic]] after-burning devices (such as [[catalytic converter]]s) or by the simple partial return of the [[exhaust gas]]es into the combustion process. Such devices are required by [[environmental legislation]] for cars in most countries, and may be necessary to enable large combustion devices, such as [[thermal power station]]s, to reach legal [[emission standards]].
| |
| | |
| The degree of combustion can be measured and analyzed with test equipment. [[HVAC]] contractors, [[Firefighter|firemen]] and [[engineers]] use combustion analyzers to test the [[Fuel efficiency|efficiency]] of a burner during the combustion process. In addition, the efficiency of an internal combustion engine can be measured in this way, and some U.S. states and local municipalities use combustion analysis to define and rate the efficiency of vehicles on the road today. | |
| | |
| ===Smouldering/Slow===
| |
| [[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 smouldering 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.
| |
| | |
| ===Rapid===
| |
| [[Image:Water cooler bottle ignition.jpg|thumb|alt=Container of ethanol vapour mixed with air, undergoing rapid combustion|Container of ethanol vapour mixed with air, undergoing rapid combustion]]
| |
| 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 engine]]s and in [[thermobaric weapon]]s. Such a combustion is frequently called an [[explosion]], though for an internal combustion engine this is inaccurate. 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 [[Engine knocking|detonation]].
| |
| | |
| ===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.
| |
| | |
| ===Turbulent===
| |
| Combustion resulting in a turbulent flame is the most used for industrial application (e.g. gas turbines, gasoline engines, etc.) because the turbulence helps the mixing process between the fuel and oxidizer.
| |
| | |
| ===Microgravity===
| |
| [[Image:Microgravity Burning.jpg|thumb|Colourized gray-scale composite image of the individual frames from a video of a backlit fuel droplet burning in microgravity.]]
| |
| | |
| Combustion processes behave differently in a [[microgravity environment]] than in Earth-gravity conditions due to the lack of [[buoyancy]]. For example, a [[candle]]'s flame takes the shape of a sphere.<ref>[http://spaceflight.nasa.gov/history/shuttle-mir/science/mg/nm21460011.htm Shuttle-Mir History/Science/Microgravity/Candle Flame in Microgravity (CFM) – MGBX]. Spaceflight.nasa.gov (1999-07-16). Retrieved on 2010-09-28.</ref> Microgravity combustion research contributes to understanding of spacecraft fire safety and diverse aspects of combustion physics.
| |
| | |
| ===Micro-combustion===
| |
| 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.
| |
| | |
| ==Chemical equations==
| |
| | |
| ===Stoichiometric combustion of a hydrocarbon in oxygen===
| |
| | |
| Generally, the [[chemical equation]] for [[Stoichiometry|stoichiometric]] combustion of a [[hydrocarbon]] in oxygen is:
| |
| <div align="center"><math>C_xH_y + zO_2 \to xCO_2 + \frac{y}{2}H_2O</math></div>
| |
| where ''z'' = ''x'' + ¼''y''.
| |
| | |
| For example, the stoichiometric burning of [[propane]] in oxygen is:
| |
| <div align="center"><math>C_3H_8 + 5O_2 \to 3CO_2 + 4H_2O</math></div>
| |
| | |
| The simple word equation for the stoichiometric combustion of a hydrocarbon in oxygen is:
| |
| <div align="center"><math>\textrm{fuel} + \textrm{oxygen} \to \textrm{water} + \textrm{carbon\ dioxide}</math></div>
| |
| | |
| ===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 can be added to the equation (although it does not react) to show the composition of the resultant flue gas:
| |
| <div align="center"><math>C_xH_y + zO_2 + 3.71zN_2 \to xCO_2 + \frac{y}{2} H_2O + 3.71zN_2</math></div>
| |
| where ''z'' = ''x'' + ¼''y''.
| |
| | |
| For example, the stoichiometric combustion of propane in air is:
| |
| <div align="center"><math>C_3H_8 + 5O_2 + 18.55N_2 \to 3CO_2 + 4H_2O + 18.55N_2</math></div>
| |
| | |
| The simple word equation for the stoichiometric combustion of a hydrocarbon in air is:
| |
| <div align="center"><math>\textrm{fuel} + \textrm{oxygen} + \textrm{nitrogen} \to \textrm{water} + \textrm{carbon\ dioxide} + \textrm{nitrogen}</math></div>
| |
| | |
| ===Trace combustion products===
| |
| | |
| Various other substances begin to appear in significant amounts in combustion products when the [[Adiabatic flame temperature|flame temperature]] is above about {{val|fmt=commas|1600|ul=K}}. When excess air is used, nitrogen may oxidize to [[Nitric oxide|{{chem|NO}}]] and, to a much lesser extent, to [[Nitrogen dioxide|{{chem|NO|2}}]]. [[Carbon monoxide|{{chem|CO}}]] forms by [[disproportionation]] of [[Carbon dioxide|{{chem|CO|2}}]], and [[Hydrogen|{{chem|H|2}}]] and [[Hydroxyl radical|{{chem|OH}}]] form by disproportionation of [[Water|{{chem|H|2|O}}]].
| |
| | |
| For example, when {{val|1|ul=mol}} of propane is burned with {{val|28.6|ul=mol}} of air (120% of the stoichiometric amount), the combustion products contain 3.3% {{chem|O|2}}. At {{val|fmt=commas|1400|ul=K}}, the [[Chemical equilibrium|equilibrium]] combustion products contain 0.03% {{chem|NO}} and 0.002% {{chem|OH}}. At {{val|fmt=commas|1800|ul=K}}, the combustion products contain 0.17% {{chem|NO}}, 0.05% {{chem|OH}}, 0.01% {{chem|CO}}, and 0.004% {{chem|H|2}}.<ref name="EquiWeb">[http://www.crct.polymtl.ca/equiweb.php] Equilib-Web</ref>
| |
| | |
| [[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 [[NOx|nitrogen oxide]] emissions. Both the United States and European Union [[Emission standard|enforce limits]] to vehicle nitrogen oxide emissions, which necessitate the use of special [[catalytic converter]]s or treatment of the exhaust with [[urea]] (see [[Diesel exhaust fluid]]).
| |
| | |
| ===Incomplete combustion of a hydrocarbon in oxygen===
| |
| | |
| The incomplete (partial) combustion of a hydrocarbon with oxygen produces a gas mixture containing mainly {{chem|CO|2}}, {{chem|CO}}, {{chem|H|2|O}}, and {{chem|H|2}}. Such gas mixtures are commonly prepared for use as protective atmospheres for the [[Heat treating|heat-treatment]] of metals and for [[Carburizing|gas carburizing]].<ref>ASM Committee on Furnace Atmospheres, ''Furnace atmospheres and carbon control'', Metals Park, OH [1964].</ref> The general reaction equation for incomplete combustion of one [[Mole(unit)|mole]] of a hydrocarbon in oxygen is:
| |
| | |
| <div align="center"><math>C_xH_y + zO_2 \to aCO_2 + bCO + cH_2O + dH_2</math></div>
| |
| | |
| The simple word equation for the incomplete combustion of a hydrocarbon in oxygen is:
| |
| | |
| <div align="center"><math>\textrm{fuel} + \textrm{oxygen} \to \textrm{water} + \textrm{hydrogen} + \textrm{carbon\ dioxide} + \textrm{carbon\ monoxide}</math></div>
| |
| | |
| For stoichiometric (complete) combustion, ''z'' = ''x'' + ¼''y''. When ''z'' falls below roughly 50% of the stoichiometric value, [[Methane|{{chem|CH|4}}]] can become an important combustion product; when ''z'' falls below roughly 35% of the stoichiometric value, elemental [[carbon]] may become stable.
| |
| | |
| The products of incomplete combustion can be calculated with the aid of a [[material balance]], together with the assumption that the combustion products reach [[Chemical equilibrium|equilibrium]].<ref>{{cite journal | title = Exothermic atmospheres | journal = Industrial Heating | pages = 22 | date = June 2013 | url = http://www.industrialheating.com/articles/91142-exothermic-atmospheres | accessdate = 5 July 2013}}</ref><ref name="ExoCalc">[http://www.industrialheating.com/ExoCalc] ExoCalc</ref> For example, in the combustion of one [[Mole (unit)|mole]] of propane ({{chem|C|3|H|8}}) with four moles of {{chem|O|2}}, seven moles of combustion gas are formed, and ''z'' is 80% of the stoichiometric value. The three elemental balance equations are:
| |
| | |
| <div align="center"><math>\mathrm{Carbon:} \ a + b = 3</math></div>
| |
| <div align="center"><math>\mathrm{Hydrogen:} \ 2c + 2d = 8</math></div>
| |
| <div align="center"><math>\mathrm{Oxygen:} \ 2a + b + c = 8</math></div>
| |
| | |
| 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:
| |
| | |
| <div align="center"><math>CO + H_2O \to CO_2 + H_2; K_{eq} = \frac{a \times d}{b \times c}</math></div>
| |
| | |
| For example, at {{val|fmt=commas|1200|ul=K}} the value of ''K<sub>eq</sub>'' is 0.728.<ref name="ReacWeb" /> Solving, the combustion gas consists of 42.4% {{chem|H|2|O}}, 29.0% {{chem|CO|2}}, 14.7% {{chem|H|2}}, and 13.9% {{chem|CO}}. Carbon becomes a stable phase at {{val|fmt=commas|1200|ul=K}} and {{val|1|ul=atm}} pressure when z is less than 30% of the stoichiometric value, at which point the combustion products contain more than 98% {{chem|H|2}} and {{chem|CO}} and about 0.5% {{chem|CH|4}}.
| |
| | |
| ==Fuels==
| |
| Substances or materials which undergo combustion are called fuels. The most common examples are natural gas, propane, kerosene, diesel, petrol, charcoal, coal, wood, etc.
| |
| | |
| A good fuel is one which is readily available, is cheap, burns easily in air and at a moderate rate, has a high calorific value and is environment friendly. There is probably no fuel which can be considered an ideal fuel, but natural gas (mostly methane) comes the closest{{Citation needed|date=July 2013}}.
| |
| | |
| ===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]]. The flash point of a liquid fuel is the lowest temperature at which it can form an ignitable mix with air. It is also the minimum temperature at which there is enough evaporated fuel in the air to start combustion.
| |
| | |
| ===Solid fuels===
| |
| | |
| The act of combustion consists of three relatively distinct but overlapping phases:
| |
| * '''Preheating phase''', when the unburned [[fuel]] is heated up to its flash point and then [[fire point]]. Flammable gases start being evolved in a process similar to [[dry distillation]].
| |
| * '''Distillation phase''' or '''gaseous phase''', when the mix of evolved flammable gases with oxygen is ignited. Energy is produced in the form of heat and light. [[Flame]]s are often visible. Heat transfer from the combustion to the solid maintains the evolution of flammable vapours.
| |
| * '''Charcoal phase''' or '''solid phase''', when the output of flammable gases from the material is too low for persistent presence of flame and the [[charring|charred]] fuel does not burn rapidly and just glows and later only [[Smouldering|smoulder]]s.
| |
| | |
| [[File:disfig1.svg|thumb|centre|450px|A general scheme of [[polymer]] combustion]]
| |
| | |
| ==Combustion management==
| |
| Efficient [[Furnace#Industrial process furnaces|process heating]] requires recovery of the largest possible part of a fuel’s [[heat of combustion]] into the material being processed.<ref>{{cite journal | title = Calculating the heat of combustion for natural gas | journal = Industrial Heating | pages = 28 | date = September 2012 | url = http://www.industrialheating.com/articles/90561-calculating-the-heat-of-combustion-for-natural-gas | accessdate = 5 July 2013}}</ref><ref name="HeatCalc">[http://www.industrialheating.com/HeatCalc] HeatCalc</ref> There are many avenues of loss in the operation of a heating process. Typically, the dominant loss is [[sensible heat]] leaving with the [[Exhaust gas|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 {{chem|CO}} and {{chem|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 ({{chem|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 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.<ref name="NOx formation" />
| |
| | |
| Adherence to these two principles is furthered by making material and heat balances on the combustion process.<ref>{{cite journal | title = Making a material balance | journal = Industrial Heating | pages = 20 | date = November 2012 | url = http://www.industrialheating.com/articles/90764-making-a-material-balance | accessdate = 5 July 2013}}</ref><ref name="MatBalCalc">[http://www.industrialheating.com/MatBalCalc] MatBalCalc</ref><ref>{{cite journal | title = Making a heat balance | journal = Industrial Heating | pages = 22 | date = December 2012 | url = http://www.industrialheating.com/articles/90812-making-a-heat-balance | accessdate = 5 July 2013}}</ref><ref name="HeatBalCalc">[http://www.industrialheating.com/HeatBalCalc] HeatBalCalc</ref> The [[material balance]] directly relates the [[air/fuel ratio]] to the percentage of {{chem|O|2}} in the combustion gas. The heat balance relates the heat available for the charge to the overall net heat produced by fuel combustion.<ref>{{cite journal | title = Available combustion heat | journal = Industrial Heating | pages = 22 | date = April 2013 | url = http://www.industrialheating.com/articles/91014-available-combustion-heat | accessdate = 5 July 2013}}</ref><ref name="AvailHeatCalc">[http://www.industrialheating.com/availheatcalc] AvailHeatCalc</ref> Additional material and heat balances can be made to quantify the thermal advantage from preheating the combustion air,<ref>{{cite journal | title = Making a system balance (Part 2) | journal = Industrial Heating | pages = 24 | date = March 2012 | url = http://www.industrialheating.com/articles/90954-making-a-system-balance-part-2 | accessdate = 5 July 2013}}</ref><ref name="SysBalCalc2">[http://www.industrialheating.com/SysBalCalc2] SysBalCalc2</ref> or enriching it in oxygen.<ref>{{cite journal | title = Making a system balance (Part 1) | journal = Industrial Heating | pages = 22 | date = February 2012 | url = http://www.industrialheating.com/articles/90897-making-a-system-balance-part-1 | accessdate = 5 July 2013}}</ref><ref name="SysBalCalc">[http://www.industrialheating.com/sysbalcalc] SysBalCalc</ref>
| |
| | |
| ==Reaction mechanism==
| |
| {{Unreferenced section|date=January 2014}}
| |
| Combustion in oxygen is a chain reaction in which many distinct [[Radical (chemistry)|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 oxygen|triplet spin state]]. Bonding can be described with three bonding electron pairs and two antibonding electrons, whose [[Spin (physics)|spin]]s are 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]]. 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 radical]]s. 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 [[Poison|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 poorly 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).
| |
| | |
| ==Temperature==
| |
| [[File:Zoom lunette ardente.jpg|thumb|[[Antoine Lavoisier]] conducting an experiment related combustion generated by amplified sun light.]]
| |
| 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 [[Heat of combustion|heating value]];
| |
| * the [[air-fuel ratio|stoichiometric air to fuel ratio]] <math>{\lambda}</math>;
| |
| * the [[specific heat capacity]] of fuel and air;
| |
| * the air and fuel inlet temperatures.
| |
| | |
| 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 {{convert|2200|C|0|abbr=on}} (for inlet air and fuel at ambient temperatures and for <math>\lambda = 1.0</math>), around {{convert|2150|C|0|abbr=on}} for oil and {{convert|2000|C|0|abbr=on}} for [[natural gas]].<ref>{{cite journal | title = Adiabatic flame temperature | journal = Industrial Heating | pages = 20 | date = May 2013 | url = http://www.industrialheating.com/articles/91062-adiabatic-flame-temperature | accessdate = 5 July 2013}}</ref><ref name="AFTCalc">[http://www.industrialheating.com/AFT-Calc] AFTCalc</ref>
| |
| | |
| In industrial [[furnace|fired heaters]], [[power station]] steam generators, and large [[gas turbine|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.
| |
| | |
| ==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 reduces the life of engine components. In rockets, such as the F1 used in the Saturn V program, instabilities led to massive damage of 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 [[Thermoacoustic hot air engine|Rayleigh Criterion]] is the basis for analysis of thermoacoustic combustion instability and is evaluated using the Rayleigh Index over one cycle of instability<ref>John William Strutt, 3rd Baron Rayleigh, Sc.D., F.R.S., Honorary Fellow of Trinity College, Cambridge; "The Theory of Sound", §322h, 1878:</ref>
| |
| | |
| <div align="center"><math>G(x)=\frac{1}{T}\int_{T}q'(x,t)p'(x,t)dt</math></div>
| |
| | |
| where q' is the heat release rate perturbation and p' is the pressure fluctuation.<ref>A. A. Putnam and W. C. Dennis (1953) "Organ-pipe oscillations in a flame-filled tube," ''Fourth Symposium (International) on Combustion'', The Combustion Institute, pp. 566–574.</ref><ref>E. C. Fernandes and M. V. Heitor, [http://books.google.com/books?id=Je_hG6UfnogC&printsec=copyright&dq=rayleigh+thermoacoustic+&ie=ISO-8859-1&source=gbs_toc_s&cad=1#PPA4,M1 “Unsteady flames and the Rayleigh criterion”] in F. Culick, M. V. Heitor, and J. H. Whitelaw, ed.s, ''Unsteady Combustion'' (Dordrecht, the Netherlands: Kluwer Academic Publishers, 1996), p. 4</ref>
| |
| 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.<ref>Dowling, A. P. (2000a). "Vortices, sound and flame – a damaging combination". ''The Aeronautical Journal of the RaeS''</ref><ref>{{cite journal |doi=10.1080/00102202.2012.714020 |title=Temperature Response of an Acoustically Forced Turbulent Lean Premixed Flame: A Quantitative Experimental Determination |year=2013 |last1=Chrystie |first1=Robin S. M. |last2=Burns |first2=Iain S. |last3=Kaminski |first3=Clemens F. |journal=Combustion Science and Technology |volume=185 |pages=180}}</ref> This minimizes the Rayleigh Index.
| |
| | |
| ==See also==
| |
| {{col-begin}}
| |
| {{col-break}}
| |
| ;Related concepts
| |
| * [[Air-fuel ratio]]
| |
| * [[Autoignition temperature]]
| |
| * [[Chemical looping combustion]]
| |
| * [[Deflagration]]
| |
| * [[Detonation]]
| |
| * [[Explosion]]
| |
| * [[Fire]]
| |
| * [[Flame]]
| |
| * [[Markstein number]]
| |
| * [[Phlogiston theory]] (historical)
| |
| * [[Spontaneous combustion]]
| |
| | |
| {{col-break}}
| |
| | |
| ;Machines and equipment
| |
| * [[Boiler]]
| |
| * [[Bunsen burner]]
| |
| * [[External combustion engine]]
| |
| * [[Furnace]]
| |
| * [[Gas turbine]]
| |
| * [[Internal combustion engine]]
| |
| * [[Rocket engine]]
| |
| | |
| {{col-end}}
| |
| | |
| == References ==
| |
| {{reflist}}
| |
| | |
| ==Further reading==
| |
| {{Wiktionary}}
| |
| * {{cite book | last = Poinsot | first = Thierry | last2 = Veynante | first2 = Denis |url=http://elearning.cerfacs.fr/combustion/onlinePoinsotBook/buythirdedition/index.php |title=Theoretical and Numerical Combustion |edition=3rd |publisher=European Centre for Research and Advanced Training in Scientific Computation |year=2012}}
| |
| * {{cite book | editor1-last = Lackner | editor1-first = Maximilian | editor2-last = Winter | editor2-first = Franz | editor3-last = Agarwal | editor3-first = Avinash K. | url=http://as.wiley.com/WileyCDA/WileyTitle/productCd-3527324496.html |title=Handbook of Combustion, 5 volume set | publisher = [[Wiley-VCH]] | year = 2010 | isbn = 978-3-527-32449-1}}
| |
| * {{cite book| editor-last = Baukal | editor-first = Charles E. | title = Oxygen-Enhanced Combustion | publisher = [[CRC Press]] | year = 1998}}
| |
| * {{cite book | last = Glassman | first = Irvin | last2 = Yetter| first2 = Richard | title = Combustion | edition = Fourth}}
| |
| * {{cite book | last = Turns | first = Stephen | title = An Introduction to Combustion: Concepts and Applications | year = 2011}}
| |
| * {{cite book | last = Ragland | first = Kenneth W | last2 = Bryden | first2 = Kenneth M. | title = Combustion Engineering | edition = Second | year = 2011}}
| |
| * {{cite book | editor-last = Baukal| editor-first = Charles E. Jr | title = The John Zink Hamworthy Combustion Handbook: Three-Volume Set | edition = Second | contribution = Industrial Combustion | year = 2013}}
| |
| * {{cite book | last = Gardiner | first = W. C. Jr | title = Gas-Phase Combustion Chemistry | year = 2000 | edition = Revised}}
| |
| | |
| {{Fire}}
| |
| | |
| [[Category:Combustion| ]]
| |
| | |
| {{Link GA|ru}}
| |