Combustion
A match strikes against a rough surface. The friction generates heat that reaches the flash point of the fuel on the match head. This energy initiates a high-temperature exothermic redox chemical reaction between the fuel and atmospheric oxygen. The process produces oxidized products often in gaseous form, creating a mixture termed smoke. A flame appears only when substances undergoing combustion vaporize. Without this vaporization, fire remains invisible even if the reaction proceeds. Activation energy must be supplied to start the process, yet the heat from a flame can make it self-sustaining. Solid fuels like wood or coal first undergo endothermic pyrolysis to produce gaseous fuels. These gases then combust to supply the heat required for more production. Combustion is often hot enough to produce incandescent light as either glowing embers or a visible flame. Hydrogen burning with oxygen into water vapor releases 242 kilojoules per mole of heat. This specific reaction reduces enthalpy at constant temperature and pressure. Uncatalyzed combustion in air requires relatively high temperatures to proceed.
Fires occur naturally ignited by lightning strikes or volcanic products. Combustion was the first controlled chemical reaction discovered by humans in the form of campfires and bonfires. It continues to be the main method to produce energy for humanity today. Usually the fuel is hydrogen, hydrocarbons, or complicated mixtures such as wood containing partially oxidized hydrocarbons. The thermal energy produced from fossil fuels like coal or oil is harvested for diverse uses including cooking and electricity production. Renewable fuels such as firewood also provide this essential thermal energy. Combustion is currently the only reaction used to power rockets. It is also used to destroy both nonhazardous and hazardous waste through incineration. Oxidants for combustion have high oxidation potential including atmospheric or pure oxygen. Chlorine fluorine chlorine trifluoride nitrous oxide and nitric acid can all serve as oxidizers. For instance hydrogen burns in chlorine to form hydrogen chloride with the liberation of heat and light characteristic of combustion. Although usually not catalyzed combustion can be catalyzed by platinum or vanadium as seen in the contact process.
Complete combustion occurs when a reactant burns in oxygen producing a limited number of products. When a hydrocarbon burns in oxygen the reaction primarily yields carbon dioxide and water. Elements burned yield their most common oxides. Carbon produces carbon dioxide while sulfur yields sulfur dioxide. Iron yields iron(III) oxide. Nitrogen is not considered combustible when oxygen acts as the oxidant yet small amounts of nitrogen oxides form when air is the oxidative source. Incomplete combustion happens when there is insufficient oxygen to allow fuel to react completely. It also occurs when combustion is quenched by a heat sink like a solid surface or flame trap. Water is produced by incomplete combustion but carbon and carbon monoxide appear instead of carbon dioxide. Smoldering represents a slow low-temperature flameless form of combustion sustained by heat evolved when oxygen attacks condensed-phase fuel surfaces. Solid materials that sustain smoldering include coal cellulose wood cotton tobacco peat duff humus synthetic foams charring polymers and dust. Spontaneous combustion occurs by self-heating followed by thermal runaway and finally ignition. Phosphorus self-ignites at room temperature without applying external heat. Organic materials undergoing bacterial composting can generate enough heat to reach the point of combustion.
These oxides combine with water and oxygen in the atmosphere creating nitric acid and sulfuric acids. They return to Earth's surface as acid deposition commonly called acid rain. Acid deposition harms aquatic organisms and kills trees. Due to its formation of certain nutrients less available to plants such as calcium and phosphorus it reduces ecosystem productivity. Nitrogen oxides along with hydrocarbon pollutants contribute to ground level ozone which is a major component of smog. 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 over long periods correlates positively with heart disease risk. People surviving severe carbon monoxide poisoning may suffer long-term health problems. Carbon monoxide from air is absorbed in lungs where it binds with hemoglobin in red blood cells. This reduces the capacity of red blood cells that carry oxygen throughout the body. The formation of carbon monoxide produces less heat than carbon dioxide so complete combustion is greatly preferred especially since carbon monoxide is poisonous. When z falls below roughly 50 percent of stoichiometric value elemental carbon may become stable.
Efficient process heating requires recovery of the largest possible part of a fuel's heat of combustion into processed material. Typically dominant loss occurs as sensible heat leaving with offgas known as flue gas. Keeping offgas quantity low minimizes this heat loss. In a perfect furnace combustion air flow matches fuel flow exactly. Real world combustion does not proceed perfectly though. Unburned fuel discharged represents both heating value loss and safety hazard. Since combustibles are undesirable while unreacted oxygen presents minimal concerns the first principle provides more oxygen than theoretically needed. For methane combustion slightly more than two molecules of oxygen are required. The second principle dictates not using too much oxygen. Correct oxygen amounts require three types of measurement: active control of air and fuel flow offgas oxygen measurement and offgas combustible measurement. Each heating process exists an optimum condition of minimal offgas heat loss with acceptable combustibles concentration. Minimizing excess oxygen pays additional benefit because NOx levels are lowest when excess oxygen stays lowest. Adherence to these principles involves making material and heat balances on the combustion process. Material balance directly relates air-fuel ratio to percentage of nitrogen dioxide in combustion gas. Heat balance relates available charge heat to overall net heat produced by fuel combustion. Additional balances quantify thermal advantage from preheating combustion air or enriching it in oxygen.
Combustion instabilities manifest as violent pressure oscillations within a combustion chamber. These pressure oscillations can reach 180 decibels. Long-term exposure to cyclic pressure and thermal loads reduces engine component life. In rockets like the F1 used in the Saturn V program instabilities led to massive damage to combustion chambers. Engineers solved this problem by redesigning the fuel injector. Liquid jet engines use droplet size and distribution to attenuate instabilities. Combustion instabilities remain a major concern in ground-based gas turbine engines due to emissions. The tendency is to run lean with equivalence ratios less than one to reduce temperature and thus emissions. Running lean makes combustion very susceptible to instability. The Rayleigh Criterion forms the basis for analyzing thermoacoustic combustion instability. It evaluates the Rayleigh Index over one cycle of instability. When heat release oscillations align with pressure oscillations the Rayleigh Index becomes positive maximizing instability magnitude. If the Rayleigh Index turns negative then thermoacoustic damping occurs. Thermoacoustic instability optimally controls when heat release oscillations stay 180 degrees out of phase with pressure oscillations at same frequency. This minimizes the Rayleigh Index. Detailed descriptions from chemical kinetics perspective require large intricate webs of elementary reactions. Hydrocarbon fuels typically involve hundreds of species reacting according to thousands of reactions.
Common questions
What is the chemical reaction between a fuel and oxygen called?
Combustion is the high-temperature exothermic redox chemical reaction that occurs when a fuel reacts with atmospheric oxygen. This process produces oxidized products often in gaseous form creating a mixture termed smoke.
When does a flame appear during combustion?
A flame appears only when substances undergoing combustion vaporize. Without this vaporization fire remains invisible even if the reaction proceeds.
How much heat does hydrogen release when burning with oxygen into water vapor?
Hydrogen burning with oxygen into water vapor releases 242 kilojoules per mole of heat. This specific reaction reduces enthalpy at constant temperature and pressure.
Why does incomplete combustion produce carbon monoxide instead of carbon dioxide?
Incomplete combustion happens when there is insufficient oxygen to allow fuel to react completely or when combustion is quenched by a heat sink like a solid surface. Water is produced but carbon and carbon monoxide appear instead of carbon dioxide.
What causes acid rain from combustion oxides?
These oxides combine with water and oxygen in the atmosphere creating nitric acid and sulfuric acids. They return to Earth's surface as acid deposition commonly called acid rain which harms aquatic organisms and kills trees.