What is Combustion?

Combustion is a type of chemical reaction between a fuel and an oxidant, usually oxygen, that produces energy in the form of heat and light, most commonly as a flame. Because it produces more heat energy than it consumes, combustion is an exothermic reaction. Since it involves reduction (gain of electrons) and oxidation (loss of electrons), it is also classified as a redox reaction. 

Most combustion reactions require energy from a spark or flame to start the combustion process. If the chemical reaction produces enough energy to continue the oxidation process, it is referred to as self-sustaining combustion. 

Combustion reactions heat buildings, cook food, power cars, propel aircraft, launch rockets, and generate electricity. The increasing human use of hydrocarbon-based combustion, especially of fossil fuels, is also responsible for the rise in heat-trapping carbon-containing molecules in the earth’s atmosphere. 

That’s why the study of combustion — combustion science — is an endeavor that becomes increasingly more important. Scientists and engineers work to create more efficient reactions with various fuels and oxidants that produce fewer emissions, reduce harmful byproducts, and use sustainable or less expensive flammable materials as fuel sources. Optimizing the combustion process can significantly improve performance, cost, and emissions.

A redox reaction is a reaction in which electrons transfer between two materials. The number of electrons in a given atom or molecule is referred to as its oxidation number. Oxidation-reduction reactions are fundamental to the basic functions of life, including photosynthesis, respiration, corrosion or rusting, and combustion. 

Before we look at some common combustion reactions, here are some key terms used to describe the chemistry of combustion:

• Oxidizing agent: an ion or molecule that accepts electrons. An oxidizing agent oxidizes, or removes electrons from, the molecules in other materials.
• Reducing agent: an ion or molecule that donates electrons. A reducing agent reduces, or adds electrons to, the molecules in other materials. 
• Fuel: a material that consists of reduction agents. The most basic fuel is a hydrogen molecule. Most fuels are hydrocarbons, although some metals and highly reactive elements like phosphorus serve as fuel in combustion reactions. 
• Emissions: the ions and molecules that are the products of combustion. Although heat and light are the desired result of combustion, much of combustion science focuses on understanding and minimizing emissions. 
• Hydrocarbon: a molecule containing hydrogen and carbon, often combined with other organic and inorganic compounds. Hydrocarbons can be produced by organic processes in objects such as wood. Fossil fuels are the most common form of hydrocarbons used in combustion. They are organic hydrocarbons subjected to millions of years of heat and pressure to form complex molecules such as petroleum, coal, and natural gas. 
• Carbon oxides: molecules containing only carbon and oxygen atoms, usually carbon monoxide (CO) or carbon dioxide (CO2). Carbon oxides are the most common emission from the combustion of fuels containing carbon. 
• Nitrogen oxides: molecules containing only nitrogen and oxygen atoms. The two most common nitrogen oxides resulting from combustion in air are nitric oxide (NO) and nitrogen dioxide (NO2). Any combination of NO and NO2 is referred to as NOx. NOx is a significant source of air pollution. 
• Flame: a heated combination of gasses that are undergoing combustion. The inside or core of a flame consists of a mixture of gaseous oxidant and fuel, and the outside, or flame front, is where the combustion reaction takes place. The heat produced by the reaction excites the electrons in the gas, and when those excited electrons collapse to lower energy levels, they release energy in the form of photons. 
• Catalyst: a material that increases the rate of a chemical reaction. Catalytic materials are used in combustion to make the combustion reaction more efficient, occur at lower temperatures, and reduce unwanted emissions like NOx. 
• Pyrolysis: the decomposition of a material due to heat that does not involve oxidation. In combustion, liquid and solid fuel converts to a gas that then combusts using pyrolysis.

Chemical Equations for Combustion

The simplest form of combustion is the burning of hydrogen. It combines two hydrogen molecules and one oxygen molecule to create water vapor:

2H2 + O2 → 2H2O + 286 kJ/mol of heat

Energy in the form of heat is produced because oxygen molecules are made up of two atoms with double bonds. When heat is added, the bonds break, releasing more energy. 

The simplest hydrocarbon reactant is methane, CH4:

CH4 + 2O2 →  CO2 + 2H2O + 890 kJ/mol of heat

The combustion of methane produces more heat per mole because the methane molecule has four single bonds between the carbon atom and each hydrogen atom. 

Propane, which is C3H8, has two carbon-carbon bonds and eight hydrogen-carbon bonds:

C3H8 + 5O2 →  3CO2 + 4H2O + 2,220 Kj/mol of heat

Gasoline is a complex fuel, but the primary reductant is octane, where eight carbon atoms are bonded to 18 hydrogen atoms. That results in seven carbon-carbon bonds and 18 hydrogen-carbon bonds:

2C8H18 + 25O2 → 16CO2 + 18H2O + 5,483 kJ/mol of heat

A stoichiometric combustion reaction is theoretically ideal, in which the amount of fuel and oxygen are matched exactly, resulting in the most heat possible and maximum combustion efficiency. 

Emissions

In complete combustion, the emissions of combustion are water or, when carbon is present, water and carbon dioxide. However, most combustion involves other molecules, incomplete reactions, and secondary reactions that produce additional emissions. Any unwanted additional emissions are what we refer to as pollutants, and much of combustion science focuses on reducing these unwanted emissions.

In most cases, the nitrogen in air is inert and does not participate in combustion. However, oxygen can form bonds with nitrogen at high combustion temperatures to produce NOx. Also, when the amount of oxygen available is too low to react fully with the fuel, carbon monoxide can form instead of carbon dioxide. Volatile organic compounds can also form at low temperatures during combustion. These compounds with low boiling points easily react with other organic chemicals and produce unwanted pollutants. 

Factors Impacting Combustion Efficiency

A wide variety of characteristics drive the efficiency of combustion. Engineers can design with these factors to increase the efficiency of the thermodynamics of a given combustion application:

• Fuel chemistry: The most significant determination of the energy and emissions released in a combustion reaction is the chemistry and molecular makeup of the fuel being oxidized. The molecular bonds determine the energy needed to start combustion and the heat produced. The elemental composition, especially the nonhydrocarbon elements in the fuel, drives the types of emissions produced. Significant work is being done in fuel chemistry to design and improve new fuel solutions such as synthetic gasses, biofuels, and renewable jet fuel and to explore what additives can improve fuel chemistry. Engineers count on the Ansys Model Fuel Library to give them easy access to the properties of older fuels as well as the newest alternatives. 
• Ratio of fuel and oxygen: The oxidation reaction in combustion is driven by the amount of oxygen available to react with the fuel. The goal is to get just the right mixture so the fuel fully burns and no unwanted reactions take place. 
• Temperature: The chemical kinetics of combustion are driven by the temperature at which the reaction takes place. If the temperature is too low, only part of the combustible substance in the fuel oxidizes, and if the temperature is too high, unwanted reactions that produce NOx can occur. 
• Pressure: The higher the pressure of the gasses in a combustion reaction, the faster the reaction takes place, and more heat is generated. That is why you see a compression stage in many combustion applications like internal combustion engines and turbine engines. 
• Mixing: For combustion to take place, the molecules involved in the chemical reaction need to be in close physical proximity. Because of this, the design of how the two mix is critical to efficiency. Turbulence, gas velocity, and flame shape all drive how this mixing takes place. 
• Flame shape and stability: Because the combustion reaction takes place on the flame front, the shape and stability of the flame itself is a critical part of designing a combustion system. Heat transfer in the flame between the flame and air drives the temperature and efficiency of the combustion process. 

Designers face many challenges in balancing these different factors. As an example, the chemical kinetics of combustion are driven by a combination of mixing, fuel ratios, temperature, and pressure. Engineers often use tools like Ansys Chemkin-Pro™ software to simulate reacting flows and optimize their designs for higher efficiency and minimal byproducts and waste. Chemkin-Pro software models the chemical reaction independent of geometry. 

A general-purpose CFD software program that also contains accurate turbulence, combustion modeling, and multispecies flow, like Ansys Fluent software, is critical for capturing all of the factors in one virtual environment. Each factor can be explored, understood, and optimized. A tool like Fluent software models the three-dimensional aspects of fluid flow and combustion. 

Although every combustion reaction involves a similar chemical reaction, how that reaction takes place depends on the type of combustion and the efficiency and emissions of the reaction. 

Here are the most common types of combustion: 

• Complete combustion: When all of the fuel is fully consumed in a combustion reaction, it is called complete combustion. This full use of fuel is ideal, and the design of combustion systems concentrates on achieving as close to complete combustion as possible. 
• Incomplete combustion: When there is not enough oxygen in a combustion reaction, it is referred to as incomplete combustion. Soot and ash, along with carbon monoxide, are the byproducts of incomplete combustion. It is also sometimes called charring because carbonized fuel is left. 
• Spontaneous combustion: Some oxidation reactions generate enough heat to start combustion without the addition of outside energy. For example, phosphorus spontaneously combusts in air, as do some oils and varnishes when left on a rag. Some bacterial fermentation processes can also generate enough heat to start combustion. 
• Smoldering: Slow, flameless combustion is referred to as smoldering. Smoldering occurs when the oxidation process takes place on the surface of the solid fuel rather than gaseous fuel. Smoldering can also be referred to as slow combustion. 
• Rapid combustion: When combustion takes place in a flame that gives off heat and light, it is referred to as rapid combustion. Most industrial applications of combustion are considered rapid combustion. 
• Explosive combustion: When combustion occurs quickly enough to cause a rapid, forceful expansion of gases, it is called explosive combustion. Explosive combustion is usually achieved by igniting chemicals that contain both hydrocarbons and oxidizing molecules. 

Combustion has many uses and applications. Most applications of combustion use heat for further chemical reactions like cooking, or the heat is used to expand gases that are then used to do mechanical work, such as in an internal combustion engine. Until the introduction of electric light, combustion was the only source of artificial light. Electricity is also replacing many heat-based and pressure-producing combustion applications. 

Here are the most common ways combustion is used:

• Lighting: Since prehistory, humans have been using the light given off in combustion to provide light. It is still used in propane and oil lamps where electricity is not available or when the romance of a candle flame is desired. 
• Heating and cooking: Humans have also been using combustion to provide heat and to cook their food since prehistory. With the growth of technology and populations, combustion-based heating and cooking has moved from burning wood or peat to coal and then natural gas. 
• Natural fires: Combustion in nature, in the form of forest fires, wildfires, and brush fires, is an important part of biological systems. When not manmade, natural fires are usually started by lightning. 
• Internal combustion engines (ICEs): When combustion occurs inside a piston, the expanding gas caused by heat generation can be converted into mechanical energy. The same piston is also used to compress the air-fuel mixture before combustion. ICEs are highly optimized devices that continue to improve. Many engineers use Ansys Forte software to optimize the unique and difficult-to-simulate combustion taking place in ICEs. 
• Turbomachinery for aero engines, power, and pumping: The expanding gas from combustion can also be converted into mechanical energy through the use of a rotating turbine. The rotational energy is also used to compress air before combustion. Turbomachinery is used to power aircraft, run pumps, and generate electric energy.
• Rotating detonation engine (RDE): Instead of mechanically compressing the fuel-air mixture in an engine with mechanical work, RDEs use a supersonic wavefront traveling in an annulus to highly compress the air-fuel mixture without moving parts. 
• Rocket propulsion: When explosive combustion is uncontained on one side of the combustion chamber, it becomes rocket propulsion. Liquid fuel rockets use liquid oxygen and a liquid fuel, usually liquid hydrogen or liquid methane. Solid fuel rocket engines use propellant, which is a mixture of oxidizer and reactant, and hybrid rocket motors combine a solid hydrocarbon polymer with a liquid oxidizer, such as nitrous oxide or liquid oxygen. 
• Industrial burners: The heat from combustion can also be used in industrial applications for other chemical processes, such as distillation or to melt materials. Industrial burners can also be used to boil water to produce steam that can be converted into mechanical energy or as a way to conduct heat for long distances, such as steam heating. 

Even though combustion was one of the first technologies developed by humans, it is still undergoing rapid advances with significant R&D and breakthroughs around fuels, combustion kinetics, and new applications. These efforts combine chemistry, physics, fluid mechanics, and mechanical engineering. 

Artificial intelligence (AI) is also making its way into optimizing the thermochemistry of combustion and helping engineers develop new ways to deal with the high temperatures needed for cleaner and more efficient combustion. 

Much of the research on fuels is focused on the use of hydrogen and biofuels, especially sustainable aviation fuels (SAFs). Although aviation only contributes 2.4% of carbon emissions, the amount of commercial flights is expected to triple by 2050. So, now is the time to find fuel sources that are sustainable and have a lower impact on climate change. 

To be considered a SAF, the fuel must reduce emissions by 50% or more when compared to traditional jet fuel. Work is also ongoing on adding hydrogen to fuel or using hydrogen directly for industrial burners, ICEs, and turbine engines. 

An increase of only a few percentage points can make a huge difference in the cost of power generation as well as long-term emissions. Scientists and engineers are constructing complex simulations looking at flame shape, flame stability, and the exit profile of combustion flow to produce greater energy and lower emissions. 

Other groups are also working to improve the audible noise created by combustion and how better fluids modeling, and especially turbulence simulation, can improve efficiency. 

As these improvements are being made to fuels and the combustion process itself, teams across industries are working on new applications for combustion. The race around achieving faster aircraft is pushing new advances in ramjet and scramjet designs, in which the forward velocity of the airframe is used to compress air for combustion. Workaround RDEs point towards major efficiency improvements for natural gas turbines used for electric energy production. Enhancements to diesel engines are showing the value of highly efficient ICE power plants coupled with electric drive trains to reduce emissions in locomotives and large trucks. 

All of these efforts are increasing performance and working toward a more sustainable future where the byproduct of combustion — greenhouse gases — are reduced. 

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