combustion properties of fuel

The combustion properties of the fuel affect the flame temperature, flammability boundaries, ignitability, chemical reaction rates, and propensity to generate smoke particles.
1. Calorific value
Calorific value is the most important property of fuel. The heat released by the complete combustion of a unit mass or volume of fuel is called the gravimetric calorific value or volumetric calorific value. The heat of combustion released when the combustion product of unit weight of fuel (temperature 25°C) and air (temperature 25°C) cools down and the final temperature returns to 25°C (under normal pressure) (the water vapor in the combustion product condenses into water at this time) is called high calorific value. Deducting the heat released by the condensation of water vapor in the high calorific value is called the low calorific value. In the lower heating value it is assumed that the combustion products are all gaseous.
2. Spontaneous ignition temperature
Spontaneous ignition means that when there is no external ignition source, the temperature of the fuel is raised by heating so that the fuel is automatically ignited. The autoignition ignition temperature can be determined as follows:
A small amount of oil is placed in a crucible that has been heated at a high temperature, and the time delay for reaching ignition is measured. Then lower the temperature and repeat the test. At this time, the ignition time delay increases until a certain minimum ignition temperature. If it is lower than this temperature, no matter how long the delay time is, there will be no ignition. The ignition temperature increases with decreasing pressure.
3. Flash point
Flash point or flash point refers to the temperature at which the mixture of oil vapor and air burns briefly (within 5 seconds) when it is close to the flame. From the point of view of the physical and chemical nature of the flame, it is a very small explosion of a mixture of combustible gas and air. Like all mixed gas explosions, flash fire can only be produced under a certain mixture composition. When the combustible gas is too much or too little, the explosion cannot occur. Therefore, it is related to the evaporability of the flammable liquid and the minimum content in the air mixture.
At room temperature, the vapors of most liquid fuels cannot flash fire with the oxygen in the air. In order to determine the flash point of the oil, it is necessary to heat the oil, and test whether the flash point can occur at regular intervals during the heating process. The assay is performed under strictly defined conditions. It is closely related to every detail of the instruments and experimental methods used. So the flash point is also a condition constant.
The lower the flash point, the greater the fire hazard.
4. Flammable concentration limit
Combustibles (such as fuel vapor) mixed with air can only be burned within a certain concentration range. Beyond this concentration (too thin or too thick) it will not burn. Within this concentration range, once the flame is initiated, it can spread out from the ignition source, and as long as the concentration is appropriate, it can spread indefinitely. Usually define a fuel-rich limit and a fuel-lean limit (also called fuel-rich, fuel-lean limit).
To be precise, these two limits should be called non-combustible boundaries rather than flammable boundaries. Because beyond these two boundaries, it must be non-flammable, but not necessarily flammable within this range. The lean limit and the flash point are linked. The incombustible boundaries of kerosene fuels at room temperature are approximately 0.035 and 0.28 in oil-gas ratio (mass).
5. Carbon generation
The char formation of a fuel represents the propensity to generate smoke particles when burned in a combustion chamber. Carbon generation is closely related to the properties of the fuel, such as specific gravity, distillation range, viscosity, aromatic hydrocarbon content, carbon-hydrogen ratio (hydrogen content), etc.
Fuel carbon generation is the most obvious example of how fuel properties and composition affect combustion performance and combustor life. High carbon generation will cause more smoke in the exhaust, high concentration of smoke particles in the combustion area, high radiation blackness of the flame, high radiation heat transfer, high chamber wall temperature, causing deformation and cracks of the flame tube, and reducing the life of the flame tube; High temperature is likely to cause carbon deposits on the chamber wall and nozzles. The latter will affect the atomization quality of the fuel, resulting in low combustion efficiency, reduced outlet temperature distribution, and even unstable combustion.