Gas combustion method
According to the mixing situation of gas and air before combustion, gas combustion is divided into three basic methods, namely: diffusion combustion, partial premixed combustion and completely premixed combustion.
Diffusion burning
If the gas and air are not in contact before ignition, and the oxygen required for combustion is obtained from the surrounding atmosphere entirely by diffusion, the gas and air are mixed and burned at the contact surface, this combustion method is called diffusion combustion.
The flow patterns are different and the diffusion methods are also different. In a laminar flow state, diffusion combustion relies on molecular diffusion to allow surrounding oxygen to enter the combustion zone; while in a turbulent flow state, it mainly relies on turbulent diffusion to obtain the oxygen required for combustion. Due to the different diffusion methods, the flame structures in the two flow states are very different.
> Laminar diffusion flame structure
The total time required for fuel combustion usually consists of two parts, the time required for physical contact between the oxidizer and the fuel and the time required for the chemical reaction to occur. Since molecular diffusion proceeds relatively slowly, its speed is much lower than the chemical reaction speed of combustion, so the speed of laminar diffusion combustion is determined by the diffusion speed of oxygen. Since the combustion reaction only takes place on the flame surface where the fuel and oxygen are in effective contact, and the chemical reaction of combustion proceeds very quickly, the flame surface is very thin.
The distance between the top of the flame cone and the nozzle is called the flame length or flame height. For laminar diffusion flames, the flame height is directly proportional to the gas flow rate and inversely proportional to the diffusion coefficient of the gas.
>Transition from laminar diffusion flame to turbulent diffusion flame
When the gas flow gradually increases, the air flow speed in the center of the flame also gradually increases. However, the diffusion of oxygen to the flame surface is still molecular diffusion, and the diffusion speed remains basically unchanged. This makes the shrinkage point of the flame surface farther and farther away from the nozzle, and the length of the flame continues to increase. At this time, the surface area of the flame increases, and the amount of gas burned per unit time also increases. However, when the air flow velocity increases to a certain critical value, the gas flow state transitions from laminar flow to turbulent flow, and the flame apex begins to beat. If the air velocity increases further, the flame itself begins to disturb. At this time, the diffusion process changes from molecular diffusion to turbulent diffusion, the mixing of gas and air intensifies, the combustion process is strengthened, the combustion speed is accelerated, and the length of the flame is shortened accordingly. As the degree of air flow disturbance intensifies, the flame begins to lose stability, and the flame becomes intermittent or even completely separates from the nozzle.
In a turbulent diffusion flame, the flame surface and other parts cannot be distinguished. The mixing, preheating and chemical reaction of gas and air are carried out throughout the torch. The shape and length of this flame completely depend on the flow direction (intersection angle) and flow characteristics of the gas and air. The flame length is independent of the airflow velocity.
>Multiphase processes in diffusion flames
Generally speaking, gas flames are transparent flames that do not emit light. But in practice, you will find that diffusion flames often appear bright yellowish in color. This is not caused by the combustion of the gaseous fuel itself, but by the combustion of solid carbon particles produced by the thermal decomposition of the fuel gas in a high-temperature, oxygen-deficient environment. When hydrocarbons undergo diffusion combustion, two different combustion zones may appear: one is a true diffusion flame, which is a very thin reaction layer extending upward from the burner outlet and does not emit light; the other is a light flame zone, in which Solid carbon particles burn, showing a bright yellowish flame.
By analyzing the changes in gas concentration and temperature in laminar diffusion flames, we can find out the reason for the appearance of the light flame: the thickness of the flame is very small. The mixed gas and oxygen are consumed in the reaction zone, the oxygen concentration drops to zero at the inner surface of the flame, and the gas concentration drops to zero at the outer surface of the flame. The temperature inside the flame surface where the combustion reaction is taking place is the highest, and it drops rapidly toward the inner and outer sides due to heat dissipation. If a point on the ordinate is equivalent to the temperature at which gas begins to decompose, then the isotherm of this temperature intersects with the gas temperature curve at point a. Between the right side of point a and the inner surface of the flame will be a high-temperature zone with only gas and no oxygen. The temperature of the gas in this area is higher than its thermal decomposition temperature (higher than its ignition temperature), but it does not get the oxygen necessary for combustion and cannot burn. Only thermal decomposition can occur.
The dehydrogenation process of hydrocarbons and the accumulation process of carbon atoms occur in the decomposition zone of hydrocarbons. Finally, quite a lot of solid carbon particles are generated, which are dispersed in the gas like mist. Once carbon particles in a diffusion flame come into contact with oxygen, a combustion process occurs between solid and gas. When these carbon particles burn, they show a bright yellowish flame, which is a characteristic of hydrocarbons during diffusion combustion. If the carbon particles are not burned out in time and are taken away by the combustion products, soot is formed.