Combustion systems are chemical reactions between a material that can be burned as a fuel and the chemical compounds found in oxidants, substances that transfer oxygen atoms. The main purpose behind combustion is to create some sort of energy to produce the physical science definition of work. Essentially, it is the production of a certain amount of energy transferred by the force of heat and fire. Common examples of combustion systems include boilers, fireplaces, furnaces, gas burners and other heating systems.
Household appliances are one of the most common uses of combustion systems. In a boiler, a closed unit is filled with water or another type of fluid and heated to vaporize the fluid to supply heat to other devices. A fireplace, which generally burns wood or gas, is placed within an architectural construct for the purpose of heating or cooking. Furnaces, traditionally designed to manufacture ceramics or metals, use a fuel source to heat materials or the air itself. Likewise, gas burners use natural gas or propane mixed with the air to generate a flame, which can be used to cook or burn substances.
There are a number of different types of combustion techniques used in these systems, each requiring different levels of fuel and oxidants. Complete combustion essentially burns the fuel, usually a hydrocarbon, and produces very little secondary material. Pyrolysis, or incomplete combustion, occurs when the oxidant is not readily available or limited, causing the combustion system to be partially quenched. Low-temperature combustion without a flame is called smoldering, like a burnt out campfire. On the opposite end of the spectrum, rapid combustion is a situation in which heat and light are produced very quickly, such as in the case of thermobaric weapons, like bunker-busters.
Two different types of fuels can be used in combustion systems: liquids or solids. In the case of liquids, combustion occurs during the gas phase. Liquid is heated into a gas, which catches fire when mixed with oxidants. Solid fuels need three distinct phases to successfully work. The fuel needs to be preheated, followed by the production of gases which burn and eventually the solid fuel turns into charcoal.
Perhaps the most common form of combustion systems is the engine. This is designed in two different formats: the internal and external combustion engine. Internal combustion engines contain a chamber in which fuel and the oxidizer are mixed to produce energy. External combustion engines involve the heating of a fluid through a heat exchanger, producing usable work.